Accepted Manuscript

Realization of Na-doped p-type non-polar a-plane Zn1-xCdxO films by pulsed

laser deposition

Y Li XH Pan J Jiang HP He JY Huang CL Ye ZZ Ye

PII S0925-8388(13)02210-X

DOI httpdxdoiorg101016jjallcom201309071

Reference JALCOM 29425

To appear in

Received Date 18 July 2013

Revised Date 7 September 2013

Accepted Date 11 September 2013

Please cite this article as Y Li XH Pan J Jiang HP He JY Huang CL Ye ZZ Ye Realization of Na-doped

p-type non-polar a-plane Zn1-xCdxO films by pulsed laser deposition (2013) doi httpdxdoiorg101016

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1

Realization of Na-doped p-type non-polar a-plane Zn1-xCdxO films by

pulsed laser deposition

Y Li XH Pan J Jiang HP He JY Huang CL Ye ZZ Ye

State Key Laboratory of Silicon Materials Department of Materials Science and

Engineering Zhejiang University Hangzhou 310027 Peoplersquos Republic of China

Corresponding authors Tel + 86 571 87952187 fax + 86 571 87952124 E-mail

addresses panxinhuazjueducn (XH Pan) yezzzjueducn (ZZ Ye)

2

ABSTRACT

Na-doped non-polar Zn1-xCdxO thin films with different Cd content were grown

on r-plane sapphire substrates by pulsed laser deposition The Cd content in the

Zn1-xCdxO thin films was adjusted via controlling substrate temperature Based on the

X-ray diffraction analysis Na-doped Zn1-xCdxO films with Cd content below 53 at

exhibit unique non-polar lt1120

gt orientation while the films with Cd content above

53 at present lt0001gt and lt 1120

gt mixed orientations With an effective

incorporation of Na Na-doped non-polar Zn1-xCdxO films exhibit p-type conductivity

as confirmed by rectification behavior of n-ZnOp-Zn0947Cd0053ONa homojunction

An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of 028 cm2Vmiddots

and hole concentration of 331times1017

cm-3

is achieved and electrically stable over

several months The chemical states of Na were analyzed by X-ray photoelectron

spectroscopy Room-temperature photoluminescence measurements exhibit redshift

of the near-band-edge emission by alloying Cd

Keywords

Na-doped non-polar p-Type Zn1-xCdxO thin films

3

1 Introduction

ZnO has attracted much attention as a promising semiconductor material for

applications in blue and ultraviolet (UV) light-emitting diodes (LEDs) and laser

diodes (LDs) owing to its large exciton binding energy (60 meV) and direct wide

band gap (337 eV) at room temperature [1-3] Unfortunately ZnO with the c-axis as

natural growth direction like GaN due to the lack of symmetry along the lt0001gt

direction its optical properties are affected by both spontaneous and piezoelectric

polarization effects [4] The polarization induced built-in electric fields make negative

effects on device properties such as a decrease in the overlapping of the electron and

hole wave functions in the quantum well and consequently a decrease of internal

quantum efficiency of the emitting devices [5-8] Thus the growth of non-polar ZnO

films such as a-plane (1120

) or m-plane (1010

) has been proposed to avoid any

built-in electric fields which has already been demonstrated in GaN-based non-polar

quantum structures [5] Recently much effort has been devoted to the preparation and

investigation of non-polar ZnO-based thin films and heterostructures [9-15] On the

other hand in order to construct non-polar ZnO-based light-emitting device one of

the important issues is to obtain stable p-type non-polar ZnO-based thin films Yet

there have been few researches dealing with the p-type doping of non-polar ZnO

[16-18] despite the suggested advantages of non-polar ZnO thin films to obtain

p-type behavior over polar films [19] Therefore the fabrication of reliable stable

and reproducible p-type ZnO need to extensively studied in non-polar ZnO films for

its potential applications in non-polar ZnO-based optoelectronic devices

In this work we report on the growth of Na-doped non-polar a-plane Zn1-xCdxO

films on r-plane sapphire substrates by pulsed laser deposition (PLD) The effects of

4

Cd content on structural and electrical properties of Na-doped Zn1-xCdxO films are

discussed Here we choose r-plane sapphire as substrates The reason we do not

choose m-plane sapphire substrates is that it is difficult to obtain unique non-polar

m-plane Other planes such as )3110(

are included in minor with the major (1010)

planes when growing on m-plane sapphire substrates [12 20] In the case of a-plane

films pure a-plane films have been easily grown on r-plane sapphire substrates

Therefore r-plane sapphire might be more suitable substrate for growth of non-polar

ZnO film Recently our group has fabricated p-type polar ZnO films by taking

advantage of properties of Na acceptor [21-23] Na has been considered as an

effective p-type dopant in ZnO Moreover alloying ZnO with CdO will narrow the

band gap and shift the valence-band edge to higher energy [24-25] thus decreasing

the activation energy of the defect acceptor states which exhibits advantages to obtain

p-type behavior

2 Experimental details

A series of Na-doped Zn1-xCdxO thin films were prepared on r-plane sapphire

substrates by the PLD method The ceramic target ZnO-CdO-Na2CO3 (9999 purity)

with Cd content of 10 at and Na content of 2 at was used as the source material

A KrF excimer laser (248 nm 5 Hz 25 ns) was employed to ablate the target The

r-plane sapphire substrates were cleaned in successive baths of acetone ethanol and

deionized water for 30 min at room temperature respectively Prior to deposition the

growth chamber was evacuated to a base pressure of 30times10-4

Pa and then high-purity

O2 (9999 purity) was introduced as working gas Due to the different vapor

5

pressure of Cd and Zn different Cd content can be obtained by adjusting appropriate

growth temperature During deposition the growth pressure was maintained at 20 Pa

The growth temperature was adjusted to desired value of 200 250 300 and 350 oC

corresponding to 75 6 53 and 38 at Cd content in the films measured by X-ray

photoelectron spectroscopy (XPS) respectively Accordingly the Na-doped

Zn1-xCdxO thin films are labeled as samples A B C and D

The crystalline structure was analyzed by X-ray diffraction (XRD) with a Cu Ka

radiation source (λ=154056 Aring) The Cd content in the Zn1-xCdxONa films was

determined by energy-dispersive XPS The morphology of Zn1-xCdxONa films was

characterized by field-emission scanning electron microscope (FESEM)

Room-temperature photoluminescence (PL) measurements were carried out with

excitation by a 325 nm line of a He-Cd laser to evaluate the optical property of the

films The electrical properties were investigated by Hall-effect measurements with

indium contacts and a magnetic field of 032 T using the Van der Pauw configuration

(BID-RAD HL5500PC) at room temperature

3 Results and discussion

Fig 1(a) shows the XRD patterns of Na-doped Zn1-xCdxO samples grown on

r-plane sapphire substrates in θ-2θ configuration It can be seen that the samples have

a typical hexagonal wurtzite structure of Zn1-xCdxO films (JCPDS card number

36-1451) No peaks related to CdO or Na2O are detected in the patterns implying that

all the four samples are single-phase Zn1-xCdxO For the samples C and D only ZnO

)0211(

peak appears in XRD patterns suggesting that the two films have a pure

6

a-plane orientation However for the samples A and B polar (0002) peak is also

observed Sample B is grown as (1120

) a-plane in major volume portions with minor

plane of (0001) While it is contrary in sample A which is grown as (0001) c-plane

in major volume portions It is worth pointing out that growth condition such as

temperature pressure dopant et al may play an important role in the growth of

non-polar ZnO-based film In this work pure a-plane orientation non-polar

Zn1-xCdxONa film with Cd content up to 53 at was achieved on r-plane sapphire

substrate

Fig 1(b) shows the XRD ω-rocking curve for (1120 ) diffraction peak of sample

C The full width at half maximum (FWHM) value of (1120 ) ω-rocking curve is 087

degree It is accepted that the line broadening of XRD ω-rocking curve is due to the

local tilt of crystal planes by the extended defects such as defect conglomerates and

the localized defects such as point defects The FWHM values of (1120

) ω-rocking

curves and the peak positions for the four samples are shown in Fig 1(c) The ion

radius of Cd2+

(097 Ǻ) is larger than that of Zn2+

(074 Ǻ) with the assumption that

all Cd2+

ions substitute for Zn2+

the (1120

) peak of Zn1-xCdxO film would move to

lower angle As can be seen in Fig 1(c) the peak positions shift to lower angle with

the increasing of Cd content confirming the alloying behavior of Cd It can also be

found that the FWHM increases as Cd content increases which is attributed to the

degradation of the film crystallinity caused by alloying more Cd in ZnO

In order to investigate the surface morphology of the Zn1-xCdxONa films

FESEM analysis was performed on these samples At the Cd content of 53 at and

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

3

1 Introduction

ZnO has attracted much attention as a promising semiconductor material for

applications in blue and ultraviolet (UV) light-emitting diodes (LEDs) and laser

diodes (LDs) owing to its large exciton binding energy (60 meV) and direct wide

band gap (337 eV) at room temperature [1-3] Unfortunately ZnO with the c-axis as

natural growth direction like GaN due to the lack of symmetry along the lt0001gt

direction its optical properties are affected by both spontaneous and piezoelectric

polarization effects [4] The polarization induced built-in electric fields make negative

effects on device properties such as a decrease in the overlapping of the electron and

hole wave functions in the quantum well and consequently a decrease of internal

quantum efficiency of the emitting devices [5-8] Thus the growth of non-polar ZnO

films such as a-plane (1120

) or m-plane (1010

) has been proposed to avoid any

built-in electric fields which has already been demonstrated in GaN-based non-polar

quantum structures [5] Recently much effort has been devoted to the preparation and

investigation of non-polar ZnO-based thin films and heterostructures [9-15] On the

other hand in order to construct non-polar ZnO-based light-emitting device one of

the important issues is to obtain stable p-type non-polar ZnO-based thin films Yet

there have been few researches dealing with the p-type doping of non-polar ZnO

[16-18] despite the suggested advantages of non-polar ZnO thin films to obtain

p-type behavior over polar films [19] Therefore the fabrication of reliable stable

and reproducible p-type ZnO need to extensively studied in non-polar ZnO films for

its potential applications in non-polar ZnO-based optoelectronic devices

In this work we report on the growth of Na-doped non-polar a-plane Zn1-xCdxO

films on r-plane sapphire substrates by pulsed laser deposition (PLD) The effects of

4

Cd content on structural and electrical properties of Na-doped Zn1-xCdxO films are

discussed Here we choose r-plane sapphire as substrates The reason we do not

choose m-plane sapphire substrates is that it is difficult to obtain unique non-polar

m-plane Other planes such as )3110(

are included in minor with the major (1010)

planes when growing on m-plane sapphire substrates [12 20] In the case of a-plane

films pure a-plane films have been easily grown on r-plane sapphire substrates

Therefore r-plane sapphire might be more suitable substrate for growth of non-polar

ZnO film Recently our group has fabricated p-type polar ZnO films by taking

advantage of properties of Na acceptor [21-23] Na has been considered as an

effective p-type dopant in ZnO Moreover alloying ZnO with CdO will narrow the

band gap and shift the valence-band edge to higher energy [24-25] thus decreasing

the activation energy of the defect acceptor states which exhibits advantages to obtain

p-type behavior

2 Experimental details

A series of Na-doped Zn1-xCdxO thin films were prepared on r-plane sapphire

substrates by the PLD method The ceramic target ZnO-CdO-Na2CO3 (9999 purity)

with Cd content of 10 at and Na content of 2 at was used as the source material

A KrF excimer laser (248 nm 5 Hz 25 ns) was employed to ablate the target The

r-plane sapphire substrates were cleaned in successive baths of acetone ethanol and

deionized water for 30 min at room temperature respectively Prior to deposition the

growth chamber was evacuated to a base pressure of 30times10-4

Pa and then high-purity

O2 (9999 purity) was introduced as working gas Due to the different vapor

5

pressure of Cd and Zn different Cd content can be obtained by adjusting appropriate

growth temperature During deposition the growth pressure was maintained at 20 Pa

The growth temperature was adjusted to desired value of 200 250 300 and 350 oC

corresponding to 75 6 53 and 38 at Cd content in the films measured by X-ray

photoelectron spectroscopy (XPS) respectively Accordingly the Na-doped

Zn1-xCdxO thin films are labeled as samples A B C and D

The crystalline structure was analyzed by X-ray diffraction (XRD) with a Cu Ka

radiation source (λ=154056 Aring) The Cd content in the Zn1-xCdxONa films was

determined by energy-dispersive XPS The morphology of Zn1-xCdxONa films was

characterized by field-emission scanning electron microscope (FESEM)

Room-temperature photoluminescence (PL) measurements were carried out with

excitation by a 325 nm line of a He-Cd laser to evaluate the optical property of the

films The electrical properties were investigated by Hall-effect measurements with

indium contacts and a magnetic field of 032 T using the Van der Pauw configuration

(BID-RAD HL5500PC) at room temperature

3 Results and discussion

Fig 1(a) shows the XRD patterns of Na-doped Zn1-xCdxO samples grown on

r-plane sapphire substrates in θ-2θ configuration It can be seen that the samples have

a typical hexagonal wurtzite structure of Zn1-xCdxO films (JCPDS card number

36-1451) No peaks related to CdO or Na2O are detected in the patterns implying that

all the four samples are single-phase Zn1-xCdxO For the samples C and D only ZnO

)0211(

peak appears in XRD patterns suggesting that the two films have a pure

6

a-plane orientation However for the samples A and B polar (0002) peak is also

observed Sample B is grown as (1120

) a-plane in major volume portions with minor

plane of (0001) While it is contrary in sample A which is grown as (0001) c-plane

in major volume portions It is worth pointing out that growth condition such as

temperature pressure dopant et al may play an important role in the growth of

non-polar ZnO-based film In this work pure a-plane orientation non-polar

Zn1-xCdxONa film with Cd content up to 53 at was achieved on r-plane sapphire

substrate

Fig 1(b) shows the XRD ω-rocking curve for (1120 ) diffraction peak of sample

C The full width at half maximum (FWHM) value of (1120 ) ω-rocking curve is 087

degree It is accepted that the line broadening of XRD ω-rocking curve is due to the

local tilt of crystal planes by the extended defects such as defect conglomerates and

the localized defects such as point defects The FWHM values of (1120

) ω-rocking

curves and the peak positions for the four samples are shown in Fig 1(c) The ion

radius of Cd2+

(097 Ǻ) is larger than that of Zn2+

(074 Ǻ) with the assumption that

all Cd2+

ions substitute for Zn2+

the (1120

) peak of Zn1-xCdxO film would move to

lower angle As can be seen in Fig 1(c) the peak positions shift to lower angle with

the increasing of Cd content confirming the alloying behavior of Cd It can also be

found that the FWHM increases as Cd content increases which is attributed to the

degradation of the film crystallinity caused by alloying more Cd in ZnO

In order to investigate the surface morphology of the Zn1-xCdxONa films

FESEM analysis was performed on these samples At the Cd content of 53 at and

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

4

Cd content on structural and electrical properties of Na-doped Zn1-xCdxO films are

discussed Here we choose r-plane sapphire as substrates The reason we do not

choose m-plane sapphire substrates is that it is difficult to obtain unique non-polar

m-plane Other planes such as )3110(

are included in minor with the major (1010)

planes when growing on m-plane sapphire substrates [12 20] In the case of a-plane

films pure a-plane films have been easily grown on r-plane sapphire substrates

Therefore r-plane sapphire might be more suitable substrate for growth of non-polar

ZnO film Recently our group has fabricated p-type polar ZnO films by taking

advantage of properties of Na acceptor [21-23] Na has been considered as an

effective p-type dopant in ZnO Moreover alloying ZnO with CdO will narrow the

band gap and shift the valence-band edge to higher energy [24-25] thus decreasing

the activation energy of the defect acceptor states which exhibits advantages to obtain

p-type behavior

2 Experimental details

A series of Na-doped Zn1-xCdxO thin films were prepared on r-plane sapphire

substrates by the PLD method The ceramic target ZnO-CdO-Na2CO3 (9999 purity)

with Cd content of 10 at and Na content of 2 at was used as the source material

A KrF excimer laser (248 nm 5 Hz 25 ns) was employed to ablate the target The

r-plane sapphire substrates were cleaned in successive baths of acetone ethanol and

deionized water for 30 min at room temperature respectively Prior to deposition the

growth chamber was evacuated to a base pressure of 30times10-4

Pa and then high-purity

O2 (9999 purity) was introduced as working gas Due to the different vapor

5

pressure of Cd and Zn different Cd content can be obtained by adjusting appropriate

growth temperature During deposition the growth pressure was maintained at 20 Pa

The growth temperature was adjusted to desired value of 200 250 300 and 350 oC

corresponding to 75 6 53 and 38 at Cd content in the films measured by X-ray

photoelectron spectroscopy (XPS) respectively Accordingly the Na-doped

Zn1-xCdxO thin films are labeled as samples A B C and D

The crystalline structure was analyzed by X-ray diffraction (XRD) with a Cu Ka

radiation source (λ=154056 Aring) The Cd content in the Zn1-xCdxONa films was

determined by energy-dispersive XPS The morphology of Zn1-xCdxONa films was

characterized by field-emission scanning electron microscope (FESEM)

Room-temperature photoluminescence (PL) measurements were carried out with

excitation by a 325 nm line of a He-Cd laser to evaluate the optical property of the

films The electrical properties were investigated by Hall-effect measurements with

indium contacts and a magnetic field of 032 T using the Van der Pauw configuration

(BID-RAD HL5500PC) at room temperature

3 Results and discussion

Fig 1(a) shows the XRD patterns of Na-doped Zn1-xCdxO samples grown on

r-plane sapphire substrates in θ-2θ configuration It can be seen that the samples have

a typical hexagonal wurtzite structure of Zn1-xCdxO films (JCPDS card number

36-1451) No peaks related to CdO or Na2O are detected in the patterns implying that

all the four samples are single-phase Zn1-xCdxO For the samples C and D only ZnO

)0211(

peak appears in XRD patterns suggesting that the two films have a pure

6

a-plane orientation However for the samples A and B polar (0002) peak is also

observed Sample B is grown as (1120

) a-plane in major volume portions with minor

plane of (0001) While it is contrary in sample A which is grown as (0001) c-plane

in major volume portions It is worth pointing out that growth condition such as

temperature pressure dopant et al may play an important role in the growth of

non-polar ZnO-based film In this work pure a-plane orientation non-polar

Zn1-xCdxONa film with Cd content up to 53 at was achieved on r-plane sapphire

substrate

Fig 1(b) shows the XRD ω-rocking curve for (1120 ) diffraction peak of sample

C The full width at half maximum (FWHM) value of (1120 ) ω-rocking curve is 087

degree It is accepted that the line broadening of XRD ω-rocking curve is due to the

local tilt of crystal planes by the extended defects such as defect conglomerates and

the localized defects such as point defects The FWHM values of (1120

) ω-rocking

curves and the peak positions for the four samples are shown in Fig 1(c) The ion

radius of Cd2+

(097 Ǻ) is larger than that of Zn2+

(074 Ǻ) with the assumption that

all Cd2+

ions substitute for Zn2+

the (1120

) peak of Zn1-xCdxO film would move to

lower angle As can be seen in Fig 1(c) the peak positions shift to lower angle with

the increasing of Cd content confirming the alloying behavior of Cd It can also be

found that the FWHM increases as Cd content increases which is attributed to the

degradation of the film crystallinity caused by alloying more Cd in ZnO

In order to investigate the surface morphology of the Zn1-xCdxONa films

FESEM analysis was performed on these samples At the Cd content of 53 at and

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

5

pressure of Cd and Zn different Cd content can be obtained by adjusting appropriate

growth temperature During deposition the growth pressure was maintained at 20 Pa

The growth temperature was adjusted to desired value of 200 250 300 and 350 oC

corresponding to 75 6 53 and 38 at Cd content in the films measured by X-ray

photoelectron spectroscopy (XPS) respectively Accordingly the Na-doped

Zn1-xCdxO thin films are labeled as samples A B C and D

The crystalline structure was analyzed by X-ray diffraction (XRD) with a Cu Ka

radiation source (λ=154056 Aring) The Cd content in the Zn1-xCdxONa films was

determined by energy-dispersive XPS The morphology of Zn1-xCdxONa films was

characterized by field-emission scanning electron microscope (FESEM)

Room-temperature photoluminescence (PL) measurements were carried out with

excitation by a 325 nm line of a He-Cd laser to evaluate the optical property of the

films The electrical properties were investigated by Hall-effect measurements with

indium contacts and a magnetic field of 032 T using the Van der Pauw configuration

(BID-RAD HL5500PC) at room temperature

3 Results and discussion

Fig 1(a) shows the XRD patterns of Na-doped Zn1-xCdxO samples grown on

r-plane sapphire substrates in θ-2θ configuration It can be seen that the samples have

a typical hexagonal wurtzite structure of Zn1-xCdxO films (JCPDS card number

36-1451) No peaks related to CdO or Na2O are detected in the patterns implying that

all the four samples are single-phase Zn1-xCdxO For the samples C and D only ZnO

)0211(

peak appears in XRD patterns suggesting that the two films have a pure

6

a-plane orientation However for the samples A and B polar (0002) peak is also

observed Sample B is grown as (1120

) a-plane in major volume portions with minor

plane of (0001) While it is contrary in sample A which is grown as (0001) c-plane

in major volume portions It is worth pointing out that growth condition such as

temperature pressure dopant et al may play an important role in the growth of

non-polar ZnO-based film In this work pure a-plane orientation non-polar

Zn1-xCdxONa film with Cd content up to 53 at was achieved on r-plane sapphire

substrate

Fig 1(b) shows the XRD ω-rocking curve for (1120 ) diffraction peak of sample

C The full width at half maximum (FWHM) value of (1120 ) ω-rocking curve is 087

degree It is accepted that the line broadening of XRD ω-rocking curve is due to the

local tilt of crystal planes by the extended defects such as defect conglomerates and

the localized defects such as point defects The FWHM values of (1120

) ω-rocking

curves and the peak positions for the four samples are shown in Fig 1(c) The ion

radius of Cd2+

(097 Ǻ) is larger than that of Zn2+

(074 Ǻ) with the assumption that

all Cd2+

ions substitute for Zn2+

the (1120

) peak of Zn1-xCdxO film would move to

lower angle As can be seen in Fig 1(c) the peak positions shift to lower angle with

the increasing of Cd content confirming the alloying behavior of Cd It can also be

found that the FWHM increases as Cd content increases which is attributed to the

degradation of the film crystallinity caused by alloying more Cd in ZnO

In order to investigate the surface morphology of the Zn1-xCdxONa films

FESEM analysis was performed on these samples At the Cd content of 53 at and

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

6

a-plane orientation However for the samples A and B polar (0002) peak is also

observed Sample B is grown as (1120

) a-plane in major volume portions with minor

plane of (0001) While it is contrary in sample A which is grown as (0001) c-plane

in major volume portions It is worth pointing out that growth condition such as

temperature pressure dopant et al may play an important role in the growth of

non-polar ZnO-based film In this work pure a-plane orientation non-polar

Zn1-xCdxONa film with Cd content up to 53 at was achieved on r-plane sapphire

substrate

Fig 1(b) shows the XRD ω-rocking curve for (1120 ) diffraction peak of sample

C The full width at half maximum (FWHM) value of (1120 ) ω-rocking curve is 087

degree It is accepted that the line broadening of XRD ω-rocking curve is due to the

local tilt of crystal planes by the extended defects such as defect conglomerates and

the localized defects such as point defects The FWHM values of (1120

) ω-rocking

curves and the peak positions for the four samples are shown in Fig 1(c) The ion

radius of Cd2+

(097 Ǻ) is larger than that of Zn2+

(074 Ǻ) with the assumption that

all Cd2+

ions substitute for Zn2+

the (1120

) peak of Zn1-xCdxO film would move to

lower angle As can be seen in Fig 1(c) the peak positions shift to lower angle with

the increasing of Cd content confirming the alloying behavior of Cd It can also be

found that the FWHM increases as Cd content increases which is attributed to the

degradation of the film crystallinity caused by alloying more Cd in ZnO

In order to investigate the surface morphology of the Zn1-xCdxONa films

FESEM analysis was performed on these samples At the Cd content of 53 at and

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

7

38 at (samples C and D) an anisotropic morphology with stripes elongated along

the c-axis is clearly seen which is a typical surface of non-polar ZnO [18] as shown

in Fig 2(c) and (d) The average size of anisotropic stripes for the sample C is about

300times30 nm and the long sides of these stripes are consistent with the c-axis Fig 2(a)

and (b) are the SEM images of Zn1-xCdxONa films with Cd content of 75 at and 6

at (samples A and B) The hexagonal ZnO nanostructures with an average size of

~100 nm appear and the densities of the nanostructures increase as Cd content

increases which might be caused by the mixed orientations of lt0001gt and lt1120

gt

in the films

The electrical properties were investigated by Van der Pauw Hall measurements

at room temperature Table I summarizes the electrical properties of Zn1-xCdxONa

films grown at different temperature All the films show p-type conductivities The

resistivity decreases from 7307 to 6743 Ω cm as growth temperature increases from

200 to 300 oC However there is little change in resistivity for the film grown at

higher temperature of 350 oC The different resistivity for the films grown at different

temperature could be interpreted as follows At low temperature more Cd would be

incorporated into the Zn1-xCdxONa films which leads to form more interstitial metal

atom and oxygen vacancy defects compensating NaZn acceptor On the other hand the

Zn1-xCdxONa films grown at 200 and 250 oC have polycrystalline hexagonal wurtzite

structures which possess a large number of grain boundaries induced by

incorporation of more Cd atoms as shown in Fig 2 The grain boundaries would

scatter the carriers and have an impact on hole mobility causing higher resistivity It

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

8

is therefore concluded that non-polar orientation and p-type conductivity are

intensively influenced by Cd content controlled by growth temperature Proper

growth temperature should be chosen to obtain pure non-polar films with good p-type

conductivity An optimized result with a resistivity of 6743 Ωmiddotcm Hall mobility of

028 cm2Vmiddots and hole concentration of 331times10

17 cm

-3 is achieved at the Cd content

of 53 at and electrically stable over several months

To investigate the conduction mechanism of Zn1-xCdxONa films XPS spectra of

sample C are shown in Fig 3 Fig 3(a) shows a typical Zn 2p32 core level spectrum

of ZnO film The peak at ~10225 eV is attributed to Zn-O bond Fig 3(b) shows O 1s

core level spectrum of the film Gaussian fitting to the O 1s core level spectrum

reveals two peaks centered at ~5309 eV and ~5332 eV corresponding to oxygen ions

in the fully oxidized surrounding and oxygen adsorption [26] The Na 1s core level

spectrum is shown in Fig 3(c) The Na peak located at ~10708 eV is associated to

Na-O bond The content of Na is calculated to be about 16 at The XPS result

indicates that Na atoms substitute Zn atoms in non-polar Zn0947Cd0053O film and the

formation of NaZn acceptor is the origin of the p-type conductivity Fig 3(d) illustrates

core level spectrum of Cd 3d52 Only one Cd 3d52 peak at ~4054 eV is found which

can be attributed to Cd2+

state [27] No evidence of the metallic Cd peak or other

valence state is observed which confirms that Cd exists only in the oxidized state

Fig 4 shows room-temperature PL spectra of the four samples All films exhibit

a dominant near-band-edge (NBE) emission The NBE peaks located at 298 305

315 and 322 eV for samples A B C and D show a redshift compared with the PL

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

9

energy of non-polar ZnO film (~330 eV at room temperature) Such redshift behavior

is believed to be the result of the alloying Cd Besides the films show a deep level

emission centered at ~22 eV which is closely related to the oxygen-related intrinsic

defects such as oxygen vacancies [28] It should be noted that the oxygen-related

intrinsic defects emission decreases gradually from sample A to sample D suggesting

that the formation of oxygen vacancies increases as Cd content increases As is well

known oxygen vacancies acting as donor will compensate NaZn acceptor Thus the

intensively compensation effect for samples A and B with higher Cd concentration

will lead to high resistivity which agrees well with the Hall measurements

To further confirm the p-type conduction in Na-doped non-polar Zn1-xCdxO thin

films an undoped n-type non-polar ZnO layer was deposited on sample C to form a

ZnO homojunction diode TiAu and NiAu electrodes were used to form ohmic

contacts in the n-type and p-type layers respectively A schematic structure of the p-n

homojunction is shown in Fig 5 and the current-voltage (I-V) measurement was

performed at room temperature As shown in Fig 5 the I-V curve of the diode

displays a good rectification characteristic with a threshold voltage of ~2V at forward

bias at room temperature This result could be considered as a proof of the p-type

behavior for Na-doped non-polar Zn1-xCdxO thin films Due to ex-situ deposition

process of the p-n junction the reduction of the turn-on voltage might be attributed

both to the high defect concentration in the interface and to the low carrier

concentration in the n-type side of the junction [29-30] The large leakage current for

this p-n junction may result from the film quality which should be improved in the

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

10

future

4 Conclusions

In summary we have fabricated a-plane non-polar Na-doped Zn1-xCdxO thin

films on r-plane sapphire substrates by PLD method The structural morphological

electrical and optical properties are revealed from XRD SEM Hall and PL The XRD

results reveal that unique non-polar lt1120

gt orientation of Na-doped Zn1-xCdxO films

can be obtained with Cd content below 53 at Increasing Cd content may lead to

polar growth With an effective incorporation of Na the Na-doped non-polar

Zn1-xCdxO thin films exhibit p-type conductivity XPS spectra have confirmed that the

Na incorporated in the film should exist as NaZn which acts as acceptor in Na-doped

non-polar Zn1-xCdxO thin films ZnO p-n junction demonstrates the firm p-type

conductivity in Na-doped non-polar Zn1-xCdxO thin films The results will represent

meaningful steps toward the applications in non-polar ZnO-based optoelectronic

devices

Acknowledgements

This work was supported by National Natural Science Foundation of China

under Grant No 51302244 and 51172204 Zhejiang Provincial Public Technology

Research of China under Grant No 2012C21114 Zhejiang Provincial Natural

Science Foundation of China under Grant No LQ13E020001 and Doctoral Fund

of Ministry of Education of China under Grant No 2011010110013

11

References

[1] DC Look Mater Sci Eng B 80 (2001) 383

[2] A Tsukazaki A Ohtomo T Onuma M Ohtani T Makino M Sumiya K

Ohtani SF Chichibu S Fuke Y Segawa H Ohno H Koinuma M Kawasaki Nat

Mater 4 (2005) 42

[3] XH Pan J Jiang YJ Zeng HP He LP Zhu ZZ Ye BH Zhao XQ Pan J

Appl Phys 103 (2008) 023708

[4] C Noguera J Phys Condens Mat 12 (2000) R367

[5] P Waltereit O Brandt A Trampert HT Grahn J Menninger M Ramsteiner

M Reiche KH Ploog Nature 406 (2000) 865

[6] M Leroux N Grandjean M Lauumlgt J Massies B Gil P Lefebvre P Bigenwald

Phys Rev B 58 (1998) R13371

[7] F Bernardini V Fiorentini D Vanderbilt Phys Rev B 56 (1997) R10024

[8] SH Park D Ahn Appl Phys Lett 87 (2005) 253509

[9] P Pant JD Budai R Aggarwal Roger J Narayan J Narayan Acta Mater 57

(2009) 4426

[10] JM Chauveau C Morhain B Lo B Vinter P Vennacuteegu`es M Lauumlgt D

Buell M Tesseire-Doninelli G Neu Appl Phys A 88 (2007) 65

[11] J Elanchezhiyan KR Bae WJ Lee BC Shin SC Kim Mater Lett 64

(2010) 1190

[12] WH Lin JJ Wu MMC Chou L Chang Cryst Growth Des 9 (2009) 3301

[13] JM Chauveau M Teisseire H Kim-Chauveau C Deparis C Morhain B

Vinter Appl Phys Lett 97 (2010) 081903

12

[14] Y Li XH Pan YZ Zhang HP He J Jiang JY Huang CL Ye ZZ Ye J

Appl Phys 112 (2012) 103519

[15] G Tabares A Hierro B Vinter JM Chauveau Appl Phys Lett 99 (2011)

071108

[16] S Gangil A Nakamura M Shimomura J Temmyo Jpn J Appl Phys 46

(2007) 23

[17] D Taiumlnoff M Al-Khalfioui C Deparis B Vinter M Teisseire C Morhain

JM Chauveau Appl Phys Lett 98 (2011) 131915

[18] P Ding XH Pan ZZYe JY Huang HH Zhang W Chen CY Zhu Mater

Lett 71(2012)18

[19] S Lautenschlaeger S Eisermann MN Hofmann U Roemer M Pinnisch A

Laufer BK Meyer H von Wenckstern A Lajn F Schmidt M Grundmann J

Blaesing A Krost J Cryst Growth 312 (2010) 2078

[20] CC Kuo BH Lin S Yang WR Liu WF Hsieh CH Hsu Appl Phys Lett

101 (2012) 011901

[21] SS Lin JG Lu ZZ Ye HP He XQ Gu LX Chen JY Huang BH Zhao

Solid State Commun 148 (2008) 25

[22] SS Lin ZZ Ye JG Lu HP He LX Chen XQ Gu JY Huang LP Zhu

BH Zhao J Phys D Appl Phys 41 (2008) 155114

[23] HP He SS Lin GD Yuan LQ Zhang WF Zhang LB Luo YL Cao ZZ

Ye ST Lee J Phys Chem C 115 (2011) 19018

[24] JJ Chen F Ren YJ Li DP Norton SJ Peartona A Osinsky JW Dong

13

PP Chow JF Weaver Appl Phys Lett 87 (2005) 192106

[25] J Jiang LP Zhu Y Li YM Guo WS Zhou L Cao HP He ZZ Ye J

Alloys Comp 547 (2013) 59

[26] SS Lin HP He YF Lu ZZ Ye J Appl Phys 106 (2009) 093508

[27] DW Ma ZZ Ye HM Lu JY Huang BH Zhao LP Zhu HJ Zhang PM

He Thin Solid Films 461 (2004) 250

[28] K Vanheusden WL Warren CH Seager DR Tallant JA Voigt BE Gnade

J Appl Phys 79 (1996) 7983

[29] WZ Xu ZZ Ye YJ Zeng LP Zhu BH Zhao L Jiang JG Lu HP He

SB Zhang Appl Phys Lett 88 (2006) 173506

[30] KH Bang DK Hwang MC Park YD Ko I Yun JM Myoung Appl Surf

Sci 210 (2003) 177

14

Table

Table I Electrical properties of Na-doped Zn1-xCdxO films deposited on r-plane

sapphire substrates

Sample Resistivity

(Ω cm)

Mobility

(cm2V s)

Concentration

(cm-3

)

Conduction type

A 7307 047 182times1016

p

B 2925 023 929times1016

p

C 6743 028 331times1017

p

D 7579 038 217times1017

p

15

Figure captions

Fig 1 (a) XRD patterns of Na-doped Zn1-xCdxO samples in θ-2θ geometry (b) XRD

ω-rocking curve for (1120 ) diffraction peak of sample C (c) the FWHM values of

(1120

) ω-rocking curves and the peak positions as a function of growth temperature

Fig 2 SEM images of Na-doped Zn1-xCdxO films with different Cd concent (a) 75

at (b) 6 at (c) 53 at (d) 38 at

Fig 3 XPS core level spectra of sample C (a) Zn 2p32 (b) O 1s (c) Na 1s and (d)

Cd 3d52

Fig 4 Room-temperature PL spectra of the four samples

Fig 5 I-V characteristics of the non-polar n-ZnOp-Zn0947Cd0053ONa homojunction

at room temperature The inset shows the schematic structure of the non-polar

ZnO-based p-n junction

Fig1

12

[14] Y Li XH Pan YZ Zhang HP He J Jiang JY Huang CL Ye ZZ Ye J

Appl Phys 112 (2012) 103519

[15] G Tabares A Hierro B Vinter JM Chauveau Appl Phys Lett 99 (2011)

071108

[16] S Gangil A Nakamura M Shimomura J Temmyo Jpn J Appl Phys 46

(2007) 23

[17] D Taiumlnoff M Al-Khalfioui C Deparis B Vinter M Teisseire C Morhain

JM Chauveau Appl Phys Lett 98 (2011) 131915

[18] P Ding XH Pan ZZYe JY Huang HH Zhang W Chen CY Zhu Mater

Lett 71(2012)18

[19] S Lautenschlaeger S Eisermann MN Hofmann U Roemer M Pinnisch A

Laufer BK Meyer H von Wenckstern A Lajn F Schmidt M Grundmann J

Blaesing A Krost J Cryst Growth 312 (2010) 2078

[20] CC Kuo BH Lin S Yang WR Liu WF Hsieh CH Hsu Appl Phys Lett

101 (2012) 011901

[21] SS Lin JG Lu ZZ Ye HP He XQ Gu LX Chen JY Huang BH Zhao

Solid State Commun 148 (2008) 25

[22] SS Lin ZZ Ye JG Lu HP He LX Chen XQ Gu JY Huang LP Zhu

BH Zhao J Phys D Appl Phys 41 (2008) 155114

[23] HP He SS Lin GD Yuan LQ Zhang WF Zhang LB Luo YL Cao ZZ

Ye ST Lee J Phys Chem C 115 (2011) 19018

[24] JJ Chen F Ren YJ Li DP Norton SJ Peartona A Osinsky JW Dong

13

PP Chow JF Weaver Appl Phys Lett 87 (2005) 192106

[25] J Jiang LP Zhu Y Li YM Guo WS Zhou L Cao HP He ZZ Ye J

Alloys Comp 547 (2013) 59

[26] SS Lin HP He YF Lu ZZ Ye J Appl Phys 106 (2009) 093508

[27] DW Ma ZZ Ye HM Lu JY Huang BH Zhao LP Zhu HJ Zhang PM

He Thin Solid Films 461 (2004) 250

[28] K Vanheusden WL Warren CH Seager DR Tallant JA Voigt BE Gnade

J Appl Phys 79 (1996) 7983

[29] WZ Xu ZZ Ye YJ Zeng LP Zhu BH Zhao L Jiang JG Lu HP He

SB Zhang Appl Phys Lett 88 (2006) 173506

[30] KH Bang DK Hwang MC Park YD Ko I Yun JM Myoung Appl Surf

Sci 210 (2003) 177

14

Table

Table I Electrical properties of Na-doped Zn1-xCdxO films deposited on r-plane

sapphire substrates

Sample Resistivity

(Ω cm)

Mobility

(cm2V s)

Concentration

(cm-3

)

Conduction type

A 7307 047 182times1016

p

B 2925 023 929times1016

p

C 6743 028 331times1017

p

D 7579 038 217times1017

p

15

Figure captions

Fig 1 (a) XRD patterns of Na-doped Zn1-xCdxO samples in θ-2θ geometry (b) XRD

ω-rocking curve for (1120 ) diffraction peak of sample C (c) the FWHM values of

(1120

) ω-rocking curves and the peak positions as a function of growth temperature

Fig 2 SEM images of Na-doped Zn1-xCdxO films with different Cd concent (a) 75

at (b) 6 at (c) 53 at (d) 38 at

Fig 3 XPS core level spectra of sample C (a) Zn 2p32 (b) O 1s (c) Na 1s and (d)

Cd 3d52

Fig 4 Room-temperature PL spectra of the four samples

Fig 5 I-V characteristics of the non-polar n-ZnOp-Zn0947Cd0053ONa homojunction

at room temperature The inset shows the schematic structure of the non-polar

ZnO-based p-n junction

Fig1

13

PP Chow JF Weaver Appl Phys Lett 87 (2005) 192106

[25] J Jiang LP Zhu Y Li YM Guo WS Zhou L Cao HP He ZZ Ye J

Alloys Comp 547 (2013) 59

[26] SS Lin HP He YF Lu ZZ Ye J Appl Phys 106 (2009) 093508

[27] DW Ma ZZ Ye HM Lu JY Huang BH Zhao LP Zhu HJ Zhang PM

He Thin Solid Films 461 (2004) 250

[28] K Vanheusden WL Warren CH Seager DR Tallant JA Voigt BE Gnade

J Appl Phys 79 (1996) 7983

[29] WZ Xu ZZ Ye YJ Zeng LP Zhu BH Zhao L Jiang JG Lu HP He

SB Zhang Appl Phys Lett 88 (2006) 173506

[30] KH Bang DK Hwang MC Park YD Ko I Yun JM Myoung Appl Surf

Sci 210 (2003) 177

14

Table

Table I Electrical properties of Na-doped Zn1-xCdxO films deposited on r-plane

sapphire substrates

Sample Resistivity

(Ω cm)

Mobility

(cm2V s)

Concentration

(cm-3

)

Conduction type

A 7307 047 182times1016

p

B 2925 023 929times1016

p

C 6743 028 331times1017

p

D 7579 038 217times1017

p

15

Figure captions

Fig 1 (a) XRD patterns of Na-doped Zn1-xCdxO samples in θ-2θ geometry (b) XRD

ω-rocking curve for (1120 ) diffraction peak of sample C (c) the FWHM values of

(1120

) ω-rocking curves and the peak positions as a function of growth temperature

Fig 2 SEM images of Na-doped Zn1-xCdxO films with different Cd concent (a) 75

at (b) 6 at (c) 53 at (d) 38 at

Fig 3 XPS core level spectra of sample C (a) Zn 2p32 (b) O 1s (c) Na 1s and (d)

Cd 3d52

Fig 4 Room-temperature PL spectra of the four samples

Fig 5 I-V characteristics of the non-polar n-ZnOp-Zn0947Cd0053ONa homojunction

at room temperature The inset shows the schematic structure of the non-polar

ZnO-based p-n junction

Fig1

14

Table

Table I Electrical properties of Na-doped Zn1-xCdxO films deposited on r-plane

sapphire substrates

Sample Resistivity

(Ω cm)

Mobility

(cm2V s)

Concentration

(cm-3

)

Conduction type

A 7307 047 182times1016

p

B 2925 023 929times1016

p

C 6743 028 331times1017

p

D 7579 038 217times1017

p

15

Figure captions

Fig 1 (a) XRD patterns of Na-doped Zn1-xCdxO samples in θ-2θ geometry (b) XRD

ω-rocking curve for (1120 ) diffraction peak of sample C (c) the FWHM values of

(1120

) ω-rocking curves and the peak positions as a function of growth temperature

Fig 2 SEM images of Na-doped Zn1-xCdxO films with different Cd concent (a) 75

at (b) 6 at (c) 53 at (d) 38 at

Fig 3 XPS core level spectra of sample C (a) Zn 2p32 (b) O 1s (c) Na 1s and (d)

Cd 3d52

Fig 4 Room-temperature PL spectra of the four samples

Fig 5 I-V characteristics of the non-polar n-ZnOp-Zn0947Cd0053ONa homojunction

at room temperature The inset shows the schematic structure of the non-polar

ZnO-based p-n junction

Fig1

15

Figure captions

Fig 1 (a) XRD patterns of Na-doped Zn1-xCdxO samples in θ-2θ geometry (b) XRD

ω-rocking curve for (1120 ) diffraction peak of sample C (c) the FWHM values of

(1120

) ω-rocking curves and the peak positions as a function of growth temperature

Fig 2 SEM images of Na-doped Zn1-xCdxO films with different Cd concent (a) 75

at (b) 6 at (c) 53 at (d) 38 at

Fig 3 XPS core level spectra of sample C (a) Zn 2p32 (b) O 1s (c) Na 1s and (d)

Cd 3d52

Fig 4 Room-temperature PL spectra of the four samples

Fig 5 I-V characteristics of the non-polar n-ZnOp-Zn0947Cd0053ONa homojunction

at room temperature The inset shows the schematic structure of the non-polar

ZnO-based p-n junction

Fig1

Fig1

Fig2

Fig3

Fig4

Fig5

gt The Cd content in Zn1-xCdxO films was adjusted via controlling

substrate temperature gt Na-doped non-polar Zn1-xCdxO films exhibit

p-type conductivity gt XPS spectra confirm that Na incorporated in the

films exists as NaZn gt Room-temperature PL measurements exhibit

redshift of the NBE emission by alloying Cd