Accepted Manuscript

P-type ZnO materials: theory, growth, properties and devices

J.C. Fan, K.M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao

PII: S0079-6425(13)00024-8

DOI: http://dx.doi.org/10.1016/j.pmatsci.2013.03.002

Reference: JPMS 314

To appear in: Progress in Materials Science

Please cite this article as: Fan, J.C., Sreekanth, K.M., Xie, Z., Chang, S.L., Rao, K.V., P-type ZnO materials: theory,

growth, properties and devices, Progress in Materials Science (2013), doi: http://dx.doi.org/10.1016/j.pmatsci.

2013.03.002

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1

P-type ZnO materials: theory, growth, properties and devices 2

J.C. Fan1,2*, K. M. Sreekanth1,3, Z. Xie4, S.L. Chang2, K.V. Rao1,* 3 1Department of Materials Science, Tmfy-MSE, The Royal Institute of Technology, 4 SE100 44Stockholm, Sweden 5 2Science School, National University of Defense Technology, Changsha, 410073, 6 China. 7

3Department of Physics, Amrita Vishwa Vidyapeetham University, AmritapuriCampus, 8 Kollam 690 525, Kerala, India 9 4College of Physics and Microelectronics Science, Hunan University, Changsha, 10 410082, China 11

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The parameters of the manuscript are as follows: 14

The number of manuscript folios: 351 pages including figures 15

The number of figures: 127 figures 16

The number of tables: 22 tables 17

The number of references: 449 references 18

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Keywords: p-Type ZnO, Growth techniques, Native defects, Doping, 20

Self-compensation, XZn-2VZn acceptor model 21

22 Author information: 23 24 *Corresponding author: 25 K.V. Rao 26 Department of Materials Science, Tmfy-MSE, The Royal Institute of Technology, 27 SE100 44 Stockholm, Sweden 28 Tel: +46(0)8 7908406 ; fax: +46(0)8 790 7771 29 E-mail: rao@kth.se 30 31

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1 J.C. Fan 2 1Department of Materials Science, Tmfy-MSE, The Royal Institute of Technology, 3 SE100 44 Stockholm, Sweden 4 Tel: +46(0)8 7908406; fax: +46(0)8 790 7771 5 2Science School, National University of Defense Technology, Changsha, 410073, 6 China 7 E-mail: fanjincheng2006@yahoo.com.cn 8 9 Z. Xie 10 College of Physics and Microelectronics Science, Hunan University, Changsha, 11 410082, China 12 Tel/fax: +86-731-88822858 13 E-mail: xiezhong@hnu.cn 14 15 S.L. Chang 16 Science School, National University of Defense Technology, Changsha, 410073, 17 China 18 Tel/fax: +86-731-84573255 19 E-mail:slchang@nudt.edu.cn 20 21 K. M. Sreekanth 22 1Department of Materials Science, Tmfy-MSE, The Royal Institute of Technology, 23 SE100 44 Stockholm, Sweden 24 Tel: +46(0)8 7908406; fax: +46(0)8 790 7771 25 3Department of Physics, Amrita Vishwa Vidyapeetham University, Amritapuri 26 Campus, Kollam 690 525, Kerala, India 27 E-mail: sreekanth@mse.kth.se 28 29 The authors declare no conflict of interests. 30 31 32

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Outline 1

ABSTRACT 2

1. Introduction 3

2. Native defects in ZnO 4

3. Intrinsic n-type and p-type doping asymmetry in ZnO 5

4. Selection of p-type dopant for ZnO 6

5. Co-doping method 7

6. XZn-2VZn acceptor model 8

7. Growth techniques for p-type ZnO 9

7.1 Pulse laser deposition (PLD) 10

7.2 Molecular-beam epitaxy(MBE) 11

7.3 Chemical vapor deposition (CVD) 12

7.3.1.CVD 13

7.3.2 metal-organic chemical vapor deposition (MOCVD) 14

7.4. Magnetron Sputtering 15

7.5. Sol-gel 16

7.6. Ultrasonic spray pyrolysis (USP) 17

7.7. Ion implantation 18

7.8. Hydrothermal method 19

7.9. Hybrid beam deposition (HBD) 20

7.10. Other techniques 21

7.10.1. Oxided method 22

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7.10.2. Atomic layer deposition (ALD) 1

7.10.3. E-beam deposition (EBM) 2

7.10.4. Filtered cathodic vacuum arc technique (FCVA) 3

8. Undoped p-type ZnO materials 4

9. p-type doping of ZnO materials 5

9.1. Group-V acceptors 6

9.1.1 Nitrogen 7

9.1.2. Phosphorous 8

9.1.3. Arsenic 9

9.1.4. Antimony 10

9.2. Group-IV (Carbon) 11

9.3. Group-I acceptor 12

9.3.1 Group-Ia (Li, Na and K) 13

9.3.2 Group-Ib (Ag, Cu and Au ) 14

10. Band-gap engineering and p-type ZnMgO materials 15

11. Ferromagnetism in p-type ZnO (MgxZn1-xO) 16

12. p-type ZnO-based devices 17

12.1 Light emission diodes (LED) 18

12.1.1. Homojunction LED 19

12.1.2. Heterojunction LED 20

12.2. Laser diodes (LDs) 21

12.3. Photodetector 22

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12.4. Field-effect transistor (FET) 1

12.5. Sensors 2

12.6. Piezoelectric nanogenerator 3

13. Summary and outlook 4

Acknowledgements 5

References 6

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Notation 1

A0X: Neutral acceptor-bound excitons 2

AFM: Atomic force microscopy 3

CL: Cathodeluminescence 4

CVD: Chemical vapor deposition 5

DLTS: Deep-level transient spectroscopy 6

DAP: Donor acceptor pair 7

DL: Deep level 8

D0X: Neutral donor-bound excitons 9

DAP: Donor-acceptor pair 10

DBX: Donor- bound-exciton 11

DOS: Density of state 12

DC: Direct current 13

DMS: Diluted magnetic semiconductor 14

EA: Acceptor activation energy 15

EL: Electroluminescence 16

Ef : Formation energy 17

EF : Fermi energy 18

FA: Free electron to acceptor level 19

FCVA: Filtered cathodic vacuum arc technique 20

FWHM: Full width at half maximum 21

FET: Field effect transistor 22

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FTO: Fluorine-doped tin oxide 1

HBD: Hybrid beam deposition 2

kB: Boltzmann constant 3

LED: Light emission diode 4

LDs: Laser diodes 5

LDA: Local density approximation 6

LE: Localized exciton 7

MBE: Molecular-beam epitaxy 8

MOCVD: Metal-organic chemical deposition 9

MOVPE: Metal-organic vapor phase epitaxy 10

MQW: Multiple quantum well 11

MESFET: Metal-semiconductor field effect transistor 12

NBE: Near band edge 13

NR: Nanorod 14

NW: Nanowire

15

OMVPE: Organometallic vapor phase epitaxy 16

PL: Photoluminescence 17

PLD: Pulse laser deposition 18

RT: Room temperature 19

RF: Radio frequency 20

RTA: Rapid thermal annealing 21

SEM: Scanning electron microscope 22

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SCCM: Standard-state cubic centimeter per minute 1

SIMS: Secondary-ion-mass spectroscopy 2

SQUID: Superconducting quantum interference devices 3

T: Temperature 4

Tc: Curie temperature 5

TM: Transition-metal 6

USP: Ultrasonic spray pyrolysis 7

UV: Ultraviolet 8

VBM: Valence-band maximum 9

XA : A free excitons 10

XRD: X-ray diffraction 11

XPS: X-ray photoelectron spectroscopy 12

XANES: X-ray absorption near-edge structure 13

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ABSTRACT 1

In the past ten years, ZnO as a semiconductor has attracted considerable attention 2

due to its unique properties, such as high electron mobility, wide and direct band gap 3

and large exciton binding energy. ZnO has been considered a promising material for 4

optoelectronic device applications, and the fabrications of high quality p-type ZnO 5

and p-n junction are the key steps to realize these applications. However, the reliable 6

p-type doping of the material remains a major challenge because of the 7

self-compensation from native donor defects (Vo and Zni) and/or hydrogen 8

incorporation. Considerable efforts have been made to obtain p-type ZnO by doping 9

different elements with various techniques. Remarkable progresses have been 10

achieved, both theoretically and experimentally. In this paper, we discuss p-type ZnO 11

materials: theory, growth, properties and devices, comprehensively. We first discuss 12

the native defects in ZnO. Among the native defects in ZnO, VZn and Oi act as 13

acceptors. We then present the theory of p-type doping in ZnO, and summarize the 14

growth techniques for p-type ZnO and the properties of p-type ZnO materials. 15

Theoretically, the principles of selection of p-type dopant, codoping method and 16

XZn-2VZn acceptor model are introduced. Experimentally, besides the intrinsic p-type 17

ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by 18

various techniques using Group-I, IV and V elements. We pay a special attention to 19

the band gap of p-type ZnO by band-gap engineering and room temperature 20

ferromagnetism observed in p-type ZnO. Finally, we summarize the devices based on 21

p-type ZnO materials. 22

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1. Introduction 1

ZnO has received much attention over the past years because it has a wide range 2

of properties depending on doping, including a range of conductivity from metallic to 3

insulating, high transparency, piezoelectricity, wide-band gap semiconductivity, 4

room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing 5

effects, as seen from a surge of a relevant number of publications [1-9]. Figure1 6

depicts the number of ZnO related publications per anno from 1920 to 2011 [10]. It 7

can be seen that the present renaissance in ZnO research started in the mid 1990s. 8

More than 5000 publications containing ZnO in the title, abstract or keywords were 9

published in 2009 and even higher numbers in 2010. 10

The wide range of useful properties displayed by ZnO has been recognized for a 11

long time. The research on ZnO started gradually in the 1930s [11]. For example, the 12

lattice parameters and optical properties of ZnO were studied for many decades 13

[11-13]. Figure2 shows ZnO wurtzite structure and the theoretical band gap of ZnO. 14

The more information about ZnO basic properties can be found in Ref.[2-4]. 15

As a significant technological material, ZnO are considered as a promising 16

material in various fields, such as optoelectronics, sensors actuators, energy, 17

biomedical sciences and spintronics, as shown in Figure3 [2-8,14]. 18

The renewed interest in ZnO, starting from 1990s is trigged by the commercial 19

success of GaN-based optoelectronic and electronic devices [15]. GaN and ZnO have 20

similar properties, as summarized in Table1 [6,7,16,17]. Furthermore, ZnO has a 21

number of advantages properties compared to GaN rendering it even more suitable for 22

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optoelectronic device technology [2-8,18-24]: 1

ZnO has a exciton binding energy of 60meV at room temperature (RT), which is 2

higher than one of GaN (24meV), resulting in enhanced luminescence efficiency 3

and allows further operation of devices based on excitonic transitions at RT and 4

above (the thermal energy at room temperature is about 25 meV). 5

The band gap of ZnO ( Eg =3.4eV) can be effectively modulated (controled) in 6

3- 4.5eV by doping Cd or Mg. 7

ZnO film can be fabricated with large area and good uniformity on various 8

substrates, leading to the application in a wider field, however, GaN film is 9

prepared on some limited substrates ( SiC, Sapphire, Si ). Moreover, the 10

availability of native substrates for ZnO is not a case for GaN. 11

The growth temperature for high quality ZnO film is about 5000C, which is much 12

lower than that for GaN film (10000C). 13

ZnO has much simpler crystal-growth technology, and can be easily wet- 14

chemically etched, which is much cheaper than dry-etching methods that have to 15

be performed to GaN, resulting in a potentially lower cost for ZnO-based devices. 16

For space application, the higher resistance to radiation induced damage of ZnO 17

compared to GaN. 18

Figure4a shows the schematic structure of a typical homostructural pin 19

junction prepared by Tsukaza et al [25]. The I-V curve of the device displayed the 20

good rectification with a threshold voltage of about 7V (Figure4b). The 21

electroluminescence (EL) spectrum from the pin junction (blue) and 22

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photoluminescence (PL) spectrum of a p-type ZnO film at 300K were shown in 1

Figure4c, which indicated that ZnO was a potential material for making 2

short-wavelength optoelectronic devices, such as light emission diodes (LEDs) for 3

display, solid-state illumination and photodetector. 4

2. Native defects in ZnO 5

The control of defects and associated charge carriers is of paramount importance in 6

applications that exploit the wide range of properties of ZnO materials since the 7

defects have great effects on doping, minority carrier lifetime and luminescence 8

effeiciency, and may be directly involved in the diffusion mechanisms connected to 9

growth, processing and devices degradation. There are a number of intrinsic defects in 10

ZnO with different ionization energies: O vacancy (Vo), Zn vacancy (VZn), Zn 11

interstitial (Zni), O interstitial (Oi) and antisite Zn (ZnO). Zn interstitials and oxygen 12

vacancies are known to be the predominant ionic defect types [26-29]. The energy 13

levels of the native defects in ZnO film were calculated, as shown in Figure5 [30]. 14

The atomic and electronic structures of the native defects in ZnO have been 15

extensively investigated, both theoretically and experimentally [26-34]. However, 16

there is no one widely accepted model to understand them. To better understand the 17

p-type behaviors in intrinsic ZnO materials and self-compensation in undoped and 18

doped ZnO materials, here, we introduce two typical results about native defects in 19

ZnO materials. Janotti et al calculated the formation energy of native defects in ZnO 20

based on the density functional theory within the local density approximation (LDA) 21

as well as the LDA+U approach for overcoming the band-gap problem [26]. In this 22

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theory, the concentration of a defect in a crystal depends on its formation energy (Ef) 1

in the following form: 2

c =Nsites exp[-Ef/(kBT)] (1) 3

Where Ef is the formation energy, Nsites is the number of sites the defect can be 4

incorporated on, kB is the Boltzmann constant, and T is the temperature. Equation (1) 5

reveals that a low formation energy implies a high equilibrium concentration and a 6

high formation energy means that defect are unlikely to form. 7

The formation energies of the defects can be determined by : 8

Ef(defect) = Etotal(defect) - Etotal(ZnO) + (defect) + q(EF+EV) (2) 9

Where Etotal(defect) is the total energy of a supercell containing the defect, such as 10

VO and VZn, Etot (ZnO) is the total energy of a ZnO perfect crystal in the same 11

supercell, is the defect chemical potential. The formation energies of native defects 12

in ZnO were summarized in Table2. 13

Figure6 shows the formation energies for the relevant native point defects in ZnO 14

as a function of Fermi-level position [26]. In Zn-rich conditions, Vo has lower 15

formation energy than Zni (Figure6, Zn-rich), and hence should be more abundant in 16

ZnO. In O-rich conditions, the formation energy of VZn is lower and VZn should be 17

dominant. 18

They also studied the atomic structure of the native defects in ZnO, the local 19

atomic relaxations around the native defects is shown in Figure7 [26]. 20

Vidya et al studied the energies of intrinsic defects and their complexes in ZnO by 21

density functional calculations and found that Zn-interstitial and Zn-antisite defects 22

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were shallow donors and the acceptor levels originated from VZn were relatively 1

localized, resulting in it was rather difficult to achieve p-type conductivity with 2

sufficient hole mobility [29]. The observed luminescence peaks in ZnO could be 3

contributed to intrinsic defect complexes. 4

3. Intrinsic n-type and p-type doping asymmetry in ZnO 5

To realize the various applications of ZnO in optoelectronic devices, it is essential 6

to fabricate high quality p-n homojunction, in other words, both high quality n- and 7

p-types doping are required for ZnO. As-grown ZnO frequently shows high levels of 8

unintentional n-type conductivity due to native donor defect and/or hydrogen 9

incorporation and it is easy to obtain high quality n-type ZnO by doping group III 10

elements, such as Al and Ga. However, ZnO exhibits an asymmetry in its ability to be 11

doped p-type because of self-compensation caused by native defects: for instance, in 12

an attempt to dope the material p type, certain native defects which acts as donors 13

may spontaneously form and compensate the deliberately introduced acceptors 14

[35-37]. In the case, EF moves close to the valence-band maximum (VBM), 15

spontaneously, the formation energy of the charge donor defects decreases because 16

they will donate the electrons into Fermi reservoir, as shown in Figure8 [35]. 17

Zhang et al studied the microscopic equilibrium mechanisms of intrinsic doping 18

asymmetry in ZnO [36]. They imagined ZnO to be in equilibrium with a reservoir of 19

Zn and O, and calculated the formation enthalpy of defect of charge q by equation 20

(3) and defect transition energy by equation (5) : 21

H (q,) = E(q,) + n + qEF (3 ) 22

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Where, E(q,) =E(q,) ( defect +host) - E (host only) + n (solid) + qEv (4 ) 1

Here, E(q,) (defect + host) is the total energy of a cell including the host material and 2

defect in charge state q. E (host only) is the total energy of the cell containing just 3

the host. EF is the Fermi energy, Ev is the valence-band maximum of the host crystal. 4

n is the number of atom being removed during the defect formation from the host 5

crystal to the atomic reservoir. 6

(q/q')= [E(q, ) - E(q', ) ]/(q' - q) (5) 7

Where q and q' are two different states of the defect . If EF

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(2) Donors must have low formation enthalpy H, even if EF is high in the gap, so 1

that the donors become abundant. It is true for Vo and Zni in Zn-rich condition 2

[Figure9(a)]. 3

(3) Electron-killer centers (Oi,VZn) must have high formation enthalpy even if EF is 4

high in the gap, so that they do not form. It is approximately true for Zn-rich but not 5

O-rich conditions. 6

Therefore, the intrinsic n-type in ZnO is due to Zni in Zn-rich condition, which is 7

in good agreement with experiment results. 8

They also considered that the conditions for p-type doping via native defects are 9

the following [36]: 10

(a) Acceptors (Oi, VZn) must have shallow levels (0/-), (-/2-), or (0/2-) with respect 11

to VBM, so that they readily produce holes. VZn is a shallow acceptor, but not 12

Oi (Figure9 and Table 4). 13

(b) Acceptors must have low formation enthalpy H even if EF is low in the gap, 14

so that the acceptors become abundant. It is true only for O-rich, but not for 15

Zn-rich conditions (Figure9). 16

(c) Hole-killer centers (Vo, Zni, ZnO) must have high formation enthalpy, even if EF 17

is low in the gap, so that they do not form. The condition is not true for both 18

Zn-rich and O-rich conditions. 19

Therefore, they concluded that ZnO could not be doped p-type under thermal 20

equilibrium because (i) the formation enthalpies of the hole-killers Vo, Zni and Zno 21

are low and (ii) in Zn-rich conditions, it was rather difficult to form hole producing 22

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acceptors. 1

4. Selection of p-type dopant for ZnO 2

To produce shallow defect levels, there are two rules for choosing an appropriate 3

dopant [35, 37-40]: 4

(a), an appropriate dopant should favor the growth conditions that will suppress the 5

formation of compensation defects, because the solubility of dopants and 6

concentration of intrinsic defects are depended on the growth conditions. 7

Yan et al studied the formation energies of charge neutral defects as a function of 8

the O chemical potential, as shown in Figure10 [35]. In Figure10, it can be seen that at 9

O-poor conditions, the formation energies for "acceptor-killer" defects (Zni,Vo and 10

Zno) are decreased and formed, easily, therefore, to suppress the formation of 11

"acceptor-killer" defects, an O-rich conditions is preferred to fabricate p-type ZnO. 12

(b), Dopants at cation sites in compound semiconductors generally produce 13

shallower acceptor levels than dopants at anion sites. Dopant-substituting cations can 14

cause smaller perturbation in VBM than dopants at anion sites. For group-I impurities, 15

the VBM state mainly consists of the anion p orbitals with small mixing of the cation 16

p and d orbitals, and then replacing Zn by group-I impurities lead to small 17

perturbations at VBM. Therefore, group-I elements as p type dopant in ZnO have 18

shallower acceptor level compared to the group-V elements, especially to P and As, as 19

shown in Table5 [37]. 20

Theoretically, group-I elements as p-type dopants in ZnO are far better than 21

group-V elements. However, for group-I elements doping, the efficiency is generally 22

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limited by the formation of compensating interstitial. Vidya et al performed 1

density-functional calculations for Li-doped ZnO using large supercells and studied 2

the Li doping, Li-pairs, and complexes between Li and intrinsic defects in ZnO [41]. 3

They found that Li at the octahedral interstitial site was rather stable at Zn-rich and 4

equilibrium conditions, while the acceptor-type LiZn became stable at oxygen-rich 5

conditions. Li at oxygen site (donor) was not stable compared to Lii and LiZn. For Li 6

pair defects, the double-donor 2Lii were stable only at extreme Zn-rich condition, the 7

double-acceptor 2LiZn pair were stable at extreme O-rich condition. However, the 8

(Lii-LiZn) which was stable in neutral state was dominant in equilibrium condition. 9

They also considered Li -impurities + intrinsic defects. Among LiZn+ intrinsic defects, 10

the donor-type (LiZn-VO) was stable under Zn-rich condition and acceptor type 11

(LiZn-VZn) was stable under O-rich condition. The results indicated that an O-rich 12

condition was preferred to obtain p-type Li-doped ZnO. Lee et al proposed a method 13

to fabricate the low-resistivity p-type ZnO with group-I elements: codoping with 14

group-I elements and H in ZnO [42]. The formation of compensating interstitials was 15

severely suppressed, and the acceptor solubility was greatly enhanced by forming 16

H-acceptor complexes. After driving H from the sample by annealing, the 17

low-resistivity p-type ZnO could be achieved. Lin et al prepared Na-H codoping ZnO 18

films by PLD [43]. The film exhibited a high resistivity near 105 cm, due to the 19

domination of NaZnH over NaZn. After rapid thermal annealing at 550C in nitrogen, 20

the resistivity decreased to 196.7cm and the film exhibited p-type conduction 21

(carrier concentration: 2.481016-1.11017cm-3), indicating that Na interstitials were 22

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suppressed by codoping H and Na. 1

Yan et al considered that Group-Ib elements (Cu, Ag and Au) may be better 2

candidates than group-Ia elements for p-type ZnO doping, because the 3

self-compensation was very small for Group-Ib elements in ZnO. Yan et al also 4

calculated the stability of AX center for group-Ib as p-type dopant in ZnO, revealing 5

that AX centers (a deep defect complex which may compensates for acceptors) for 6

group-Ib impurities were only metastable in ZnO [44]. Low resistivity p-type ZnO 7

films with Ag dopant have been prepared by PLD and the films exhibited a hole 8

concentration of 2.29 1018cm-3, a resistivity of 0.9.cm and a mobility of 9

3.03cm2/Vs [45]. 10

Interestingly, Hapiuk et al studied p-doping in ZnO sodalite by state-of-the-art 11

calculations and proposed that endohedral doping of cagelike structures was a 12

promising method for the achievement of p doping in ZnO, even up to degenerate 13

levels and O, F, Cl, Br, Te, and I were good possible candidates [46]. The results need 14

to be proved by the experiments, further. 15

5. Co-doping method 16

Although N is considered as a good candidate to obtain p-type ZnO, the solubility 17

of N in ZnO is low, resulting in the low hole concentration. To fabricate high 18

conducting p-type ZnO, based on ab initio electronic band-structure calculations, the 19

co-doping method was suggested by Yamamoto et al [47-49]. Yamamoto considered 20

that acceptor (A) and donor (D) as codopants in a ratio of A/D=2/1 enhanced the 21

incorporation of acceptor, lowing the acceptor levels and raising the donor levels. 22

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Figure11 shows the schematic energy diagram for p-type codoped semiconductor [49]. 1

The acceptor (A) level is lowered and the donor (D) level is raised with the formation 2

of acceptordonoracceptor complexes upon codoping, which is caused by the strong 3

interaction between acceptor and reactive donor codopants. 4

Figure12 shows the crystal structure of supercells for ZnO:(Ga, 2N), crystals, and 5

the formation of complexes, III-N pairs (III = In, Al and Ga, labeled "Ga"), occupy the 6

nearest-neighbor sites and a more distant N, located at the next-nearest-neighbor site in 7

a layer closest to the layer including the IIIN pair [49]. Figure13 shows the 8

site-decomposed density of state (DOS) of p states at the N sites for ZnO:N and 9

ZnO:(III,N). Compared with Figure13(a), the DOS peaks in Figure13(b)- 13(d) shift 10

to the top of the valence band, indicating the N acceptor levels are decreased [47]. 11

With Li as an acceptor and F as reactive donor codopants, Yamamoto et al also 12

proposed codoping approach to obtain low resistivity p-type ZnO [48]. They 13

considered that p-type doping using Li impurity caused a remarkable increase in the 14

Madelung energy, resulting in the instability of ionic charge distributions in ZnO:Li 15

and the formation of Vo in the vicinity of the Li sites (Figure14). To suppress the 16

formation of donor defect (Vo) induced by Li doping and enhance the solubility of Li, 17

F as reactive donor codopant was introduced into the structure of ZnO, forming 18

Li-F-Li complexes (Figure15), which caused lowered (raised) acceptor (donor) levels 19

in the band gap. Total energy calculations showed that codoping with Li species as 20

acceptor and F as reactive-codopant donor was very effective method to fabricate 21

low-resistivity p-type ZnO. 22

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Based on the work of Yamamoto et al, many groups have carried out codoping 1

using different dopants, such as N-Al [50, 51], N-Ag [52], N-P [53], P-In [54], N-Zr 2

[55], N-B [56], Fe-N [57] and N-Mg [58] to obtain p-type ZnO, theoretically and 3

experimentally. 4

In addition, for N-doping in ZnO, the coexistence of H can enhance the solubility 5

and suppress the formation of compensating defects by passivating N dopant, forming 6

NH complexes [(NH)O] on O sites [35]. The (NH)O complexes electronically mimic O 7

sites, resulting in the higher concentration of the complexes. Figure16 shows the 8

calculated formation energy for (NH)O complexes as a function of O chemical 9

potential, indicating that the formation energy of (NH)O complexes is lower than that 10

of other defects in O-poor condition and the formation of compensating defects is 11

suppressed. The p-type conduction can be obtained after driving out H from the 12

sample by annealing. Lee et al considered that codoping with H element, the group-I 13

acceptor solubility in ZnO could be greatly enhanced [42]. In Li-H codoping ZnO, the 14

solubility limit of Li was greatly improved, with the maximum Li concentration of 15

about 1020cm-3(Figure17). 16

6. XZn 2VZn acceptor model (X= P, As and Sb) 17

First-principles calculations shows that XO (PO, AsO and SbO) are deep acceptors 18

and have high acceptor-ionization energies, owing to their large ionic radii as 19

compared to O, which make it impossible for XO to dope ZnO efficiently p-type [37, 20

59]. However, recent experiments showed that p-type ZnO could be prepared with X 21

as the dopant by various techniques [60-62]. Therefore, we can not contribute the 22

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p-type behaviors in X doped ZnO to XO, simply. Based on first-principle calculation, 1

Limpijumnong et al proposed XZn2VZn acceptor model about the 2

large-size-mismatched impurities in ZnO [63]. In this model, X atom occupies Zn 3

antisite, not O sites, forming XZn2VZn acceptor. For example, AsZn2VZn acceptor is 4

formed as following reaction: 5

AsZn3+ + VZn2- ( AsZn VZn)+ (1) 6

and ( AsZn VZn)+ + VZn2- ( AsZn 2VZn)- (2) 7

The formation of AsZn 2VZn complexes is a complicated process, involving both 8

the optimization of Madelung energy and the transformation of the As atom into a 9

new fivefold coordination. The detail described about the process can be found in 10

Ref.[63]. Figure18 shows the atomic structure of AsZn 2VZn complexes. 11

They also calculated the defect formation energy under the oxygen- and 12

arsenic-rich conditions, as shown in Figure19. AsZn - 2VZn complexes had lower 13

formation energy and could be formed easily, acting as acceptor in As-doped ZnO. 14

The ionization energy of AsZn - 2VZn complexes was calculated to be 0.15eV (0.16eV 15

for SbZn 2VZn ). 16

7. Growth techniques for p-type ZnO 17

To realize the application of ZnO in optoeletronic devices, many growth 18

techniques, such as Pulse laser deposition (PLD), molecular-beam epitaxy (MBE), 19

metal-organic chemical deposition (MOCVD), magnetron sputtering, are performed 20

to obtain low resistivity p-type ZnO materials. 21

7.1. Pulse laser deposition (PLD) 22

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PLD is now widely used deposition approach for film deposition, particularly in 1

oxide research. The system consists of a vacuum chamber equipped with pumps, a 2

target holder and rotator, substrate heater, and is typically equipped with various 3

pressure gauges, controllers, and other instruments to control the deposition 4

environment of the system, as shown schematically in Figure20 [64]. In PLD method, 5

a pulsed-laser is focused onto a target of the material and locally heats and vaporizes 6

the target surface, producing an ejected plasma or plume of atoms, ions, and 7

molecules. The plume expands away from the target with a strong forward-directed 8

velocity distribution of different particles and deposits on the substrate placed 9

opposite to the target. PLD method has several attractive features, including 10

stoichiometric transfer of material from the target, generation of energetic species, 11

hyperthermal reaction between the ablated cations and molecular oxygen in the 12

ablation plasma, and compatibility with background pressures ranging from ultra-high 13

vacuum to 100Pa [2, 64, 65]. 14

Oh et al reported the growth of nominally undoped p-type ZnO films on Si(111) 15

substrates by pulsed-laser deposition [66]. In their experiments, the chamber was 16

evacuated by a turbo-molecular pump down to a base pressure of 1.510-6Torr, the 17

substrate temperature was 500C, and the laser energy density was fixed at 1.7J /cm2. 18

The undoped ZnO films were deposited at different oxygen pressures of 1.510-6, 19

610-5, 310-4, and 5.610-3Torr. All the ZnO layers grown were found to be c-axis 20

oriented. Hall effect measurements showed that p-type ZnO films with a hole 21

concentration of ~1018cm-3 were prepared at oxygen pressures of 310-4 and 5.610-3 22

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Torr. 1

Lin et al studied the Na doping concentration on the structural and electrical 2

properties of ZnO films grown by PLD [67]. In the X-ray diffraction (XRD) patterns 3

of the samples, only (002) and (004) peaks of the ZnO were found, demonstrating a 4

high (0 0 0 2) preferential orientation and single-phase wurtzite of the deposited films. 5

The shifts of ZnO (002) peaks with the Na content in the targets increases, gradually 6

were ascribed to the expansion of the ZnO lattice due to the substitution of Na for Zn. 7

The electrical properties of Na doped ZnO films are summarized in Table6 [67], 8

indicating that the conduction of Na doped ZnO films transformed from n-type to 9

p-type with the Na content in the targets increases and the Na doped p-type ZnO films 10

with a resistivity of 13.8-19cm, a Hall mobility of 0.12-1.42cm2V-1s-1 and a hole 11

concentration of 4.781017 - 4.661018cm-3 were achieved, being electrically stable 12

over 9 months. 13

Vaithianathan et al prepared As-doped p-type ZnO films on Al2O3 substrates using 14

a Zn3As2 /ZnO target with pulsed laser deposition [68]. 15

Figure21 shows a typical XRD pattern of the As-doped ZnO film. Only the Bragg 16

reflections that correspond to the ZnO (002) and (004) planes appear along with the 17

Al2O3 substrate (006) reflection, definitely indicating that the film was c-axis oriented 18

with the ZnO single phase. The well-defined six poles in the pole figure obtained from 19

ZnO {101} reflections (inset in Figure21) indicate that the film is aligned to the 20

sapphire substrate in the in-plane direction. 21

Room temperature Hall measurements indicated that the As-doped ZnO films 22

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exhibited p-type conductivity after being annealed at 200C in N2 ambient (rapid 1

thermal annealing ) for 2min and the hole concentrations varied between 2.481017 and 2

1.181018 cm-3. The the neutral-acceptor bound exciton (A0X) of p-type As-doped 3

ZnO films was at 3.354eV [68]. They also performed rapid thermal annealing in 4

flowing O2 ambient to P-doped ZnO films deposited by PLD and obtained p-type ZnO 5

films with a hole concentration of 5.11014 1.51017 cm3 [69]. 6

Kim et al fabricated N-doped and ZrN codoped p-type ZnO films on sapphire 7

and Si substrates by PLD with N2O as N source [55]. A KrF excimer was operated at a 8

pulse rate of 10 Hz and the energy density of the laser beam at the target surface was 9

1.5J/cm2 with a target-substrate distance of 5.8cm. In their experiments, the ZrN 10

codoped ZnO films grown at 500C and 510-5 Torr of N2O showed p-type 11

conduction with a low resistivity of 0.026 cm, a carrier concentration of 5.491019 12

cm-3, and a Hall mobility of 4.38cm2V-1s-1. They prepared p-n heterojunctions by 13

depositing ZrN codoped p-type ZnO films on n-type Si, and the I-V curve of the p-n 14

heterojunction exhibited a typical rectifying behavior, as shown in Figrure22, further 15

confirming the p-type conduction of ZrN codoped ZnO films. 16

Lu et al prepared N-Li codoped p-type ZnO films by PLD with a KrF excimer 17

laser (Compex102, 248nm, 25ns) as the ablation source [70]. The target was a 18

high-purity ZnOLi2O ceramic disk with Li content of 0.1at.%. and N2O was N 19

source. Figure23 shows the electrical properties of ZnO:(Li,N) films as a function of 20

N2O pressure varied from 5 to 25Pa. The optimal result was realized at N2O of 15Pa, 21

with a resistivity of 0.93 cm, a mobility of ~0.75 cm2/V s, and a hole concentration 22

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of 8.921018 cm-3. 1

7.2. Molecular-beam epitaxy (MBE) 2

MBE is a process growing thin, epitaxial films of a wide variety of materials, 3

ranging from oxides to semiconductors to metals. It can generate exceedingly pure 4

and defect-free oxide films by virtue of the high purity levels available in 5

commercially available metals and O2, as well as the low energy of the incident 6

species [2, 64, 65, 71-75]. In MBE process, the growth parameter can be controlled, 7

precisely, especially, with the feedback from reflection high-energy electron 8

diffraction, the growth mode of the epilayers can be monitored in real time during 9

growth. Modern MBE system is normally consisted of several vacuum chambers 10

(Growth chamber, analysis chamber, substrate introduce chamber), each with a 11

suitable pumping system, as shown in Figure24a [74]. The growth chamber is where 12

the critical part of the process occupied (Figure24b) [75]. 13

D.C. Look et al prepared N-doped ZnO layer by MBE on Li-diffused, bulk, 14

semi-insulating ZnO substrate [76]. Zn source was provided by a solid-source 15

dual-zone effusion cell, O and N were from research-grade O2 and N2, respectively, and 16

O and N plasma were generated by a RF plasma source operated at a power of 350W. 17

The substrate temperature was varied from 450 to 700C. Room temperature Hall 18

effect measurements revealed that the N-doped ZnO layer exhibited p type conduction. 19

The sample grown at 525C had a resistivity of 40 cm, a mobility of 2cm2/Vs, and a 20

hole concentration of 91016cm-3. Low temperature PL showed that an A0X line 21

associated with NO was at 3.315eV. Sun et al studied the hole transport of p-type ZnO 22

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grown by plasma-assistant MBE [77]. The p-type N-doped ZnO films with the 1

thicknesses of 850nm, were grown on c-Al2O3 by P-MBE using radical NO as oxygen 2

and nitrogen sources. At room temperature, the film exhibited a hole concentration of 3

4.451017cm-3, a resistivity of 78cm, a mobility of 0.18cm2/Vs. Temperature-Hall 4

(T-Hall) measurements ranging from 85 to 140K were measured for the sample 5

(Figure25). The Hall mobility decreased from 9.69 to 0.18cm2/Vs, and the hole 6

concentration increased from 3.87 1014 to 5.81 1016 cm3 with the increasing 7

temperature. The activation energy of the nitrogen acceptor (EA) in p-type ZnO films 8

was about 75meV. 9

Xiu et al have grown p-type P doped ZnO films with a hole concentration of ~1018 10

cm-3 [60,78]. In their experiments, elemental zinc (5N) was evaporated with a low 11

temperature effusion cell. The oxygen (5N) plasma was generated with a 12

radio-frequency plasma source. GaP was used as P dopant source. The films were 13

grown at 720C with oxygen flow rate of 6SCCM. The growth parameters and 14

electrical properties of P-doped ZnO are summarized in Table7, and Figure26 shows 15

the growth rate as a function of Zn cell temperature, indicating that the conduction 16

type of P-doped ZnO are depended on both Zn and GaP cell temperatures [60]. 17

Xiu et al also prepared Sb -doped p-type ZnO films on n-Si substrate with a room 18

temperature mobility of 25.9cm2/Vs by MBE [79]. The details of growth conditions 19

can be found in reference [79]. Figure27 shows the Hall mobility H as a function of 20

temperature. Values of 20.0 and 1900.0cm2/Vs were measured at 300 and 40K, 21

respectively, and the resistivity of the film was about 0.2cm at room temperature 22

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(the inset of Figure27). The acceptor-bound excitons (A0X) of Sb-doped p-type ZnO 1

film were at 3.360 and 3.358eV. 2

Wang et al have reported the MBE growth of As doped p-type ZnO on Al2O3 3

substrates using 6N elemental Zn and radical O as the precursors for the ZnO film, 4

and then the as-grown ZnO films were annealed along with a GaAs wafer in air 5

ambient, and arsenic in the GaAs evaporated and entered into the ZnO films [80]. 6

When annealing at 5650C, the film exhibited p-type conduction with a hole 7

concentration of 3.71017cm-3 and the activation energy of the acceptors was about 8

164meV. X-ray photoelectron spectroscopy (XPS) result showed that As3d peak was at 9

about 44.8eV, corresponding to As-O bonding, which indicated that the arsenic atoms 10

may occupy Zn sites. 11

7.3. Chemical vapor deposition (CVD) 12

7.3.1 CVD 13

Chemical vapor deposition is a chemical process used to produce high-purity, 14

high-performance solid materials [2, 81, 82]. In a typical CVD process, the substrate 15

is exposed to one or more volatile precursors which react and/or decompose on the 16

substrate surface to produce the desired deposit. Frequently, volatile by products are 17

also produced, which are removed by gas flow through the reaction chamber. The 18

technique is widely used in the fabrication of epitaxial films and nanomaterials. 19

(There are several modifications of this method depending on plasma method, such as 20

Microwave plasma-assisted CVD (MPCVD) and plasma-enhanced CVD (PECVD). 21

Figure28 shows a typical schematic diagram of plasma CVD. 22

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Undoped p-type ZnO films were fabricated on Si(100) and quartz substrates at 1

different temperatures by plasma-assistant CVD [83]. Diethylzinc (DEZ, Zn(C2H5)2) 2

and O2 gas were used as a precursor, and Ar was used as the carrier gas of DEZ. The 3

films deposited at a very small oxygen flow rate had n-type conduction, while the 4

films grown at higher oxygen flow rates exhibited p-type conduction with a hole 5

concentration of 2.21018-6.61018cm-3, a mobility of 200-272cm2/Vs, and a 6

resistivity of 4.710-3 and 9.710-3 cm. 7

Liu et al reported Na-doped p-type ZnO microwires with a diameter of 2 - 6m on 8

irron screen using chemical vapor deposition [84]. They investigated the electrical 9

transport properties of ZnO microwire back-gated field effect transistor (FET) and 10

found that the monotonic increase of the drain current (IDS) with increasingly negative 11

gate bias (VGS), indicating p-type conduction of Na-doped ZnO (Figure29). The hole 12

concentration and mobility were estimated to be ~1.31016cm-3 and 2.1cm2V-1s-1, 13

respectively. The A0X (acceptor-bound exciton) emissions related Na were at about 14

3.35eV. 15

Yuan et al reported the high-quality well-aligned single-crystal N-doped p-type 16

ZnO nanowires (NWs) grown on R-sapphire substrates (Figure30) using N2O as a 17

dopant gas via a simple reactive CVD process [85]. The mixture of ZnO and graphite 18

(1:1molar) were used as source materials, Argon was acted as carrier gas, O2 and N2O 19

were used as reactive gases to grown N-doped ZnO nanowires. They fabricated N 20

doped ZnO nanowire-based FET to study the electrical transport properties of the 21

nanowire by spreading ZnO nanowires suspension in alcohol on a SiO2 (300nm)/p+-Si 22

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wafers, followed by depositing Ti/Au electrodes on individual NWs by 1

photolithography and e-beam evaporation. The IDC-VDS curve of FET based on 2

N-doped ZnO NW showed opposite dependence on Vg (Figure31), which signified 3

the p-type conductivity of the nanowires. The hole concentration and mobility were 4

estimated to be (1-2) 1016cm-3 and 10-17cm2V-1s-1, respectively. The acceptor bound 5

exciton emission was at 3.356eV. Barnes prepared N-doped p-type ZnO films with 6

typical hole concentrations of 641017/cm3 and mobilities between 0.5 and 1cm2/Vs 7

by high-vacuum plasma-assisted chemical vapor deposition [86]. 8

Xiang et al studied the influence of annealing on conduction type of P doped ZnO 9

nanowires grown on an a-plane sapphire substrate by CVD [87]. The O2/N2 mixture 10

was used as the carrier gas. A mixture of ZnO powder, Zn powder, graphite powder(a 11

molar ratio of 1:1 ZnO/C) and P2O5 powder was used as the source. After growth of P 12

doped ZnO nanowires, ZnO:P NWs were annealed in N2 gas at 850C. Characteristics 13

of ZnO NW FETs showed that both as-grown ZnO:P NWs and unintentionally doped 14

ZnO NWs had n-type behavior and the annealed ZnO:P NWs exhibited p-type 15

conduction with a mobility of ~1.7cm2/Vs (Figure32). p-type conduction from 16

annealed ZnO:P NWs was quite stable and persists for storage in air for more than 2 17

months before showing n-type behavior. In the low temperature PL spectra of 18

annealed ZnO:P NW, two neutral acceptor-bound excitons at 3.357 and 3.353meV, 19

respectively, were observed. 20

Zhang et al investigated Ultraviolet (UV) electroluminescence from As-doped 21

ZnO nanowire homojunction prepared by CVD [88]. The mixture of ZnO and 22

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graphite powders (the molar ratio, 1:1) was used materials source. The ZnO nanowire 1

arrays were grown on GaAs substrates and then the samples were annealed in O2 at 2

6500C for 15min to realize the diffusion of As from GaAs to ZnO. The I-V curve from 3

n-ZnO/p-ZnO nanowire array/GaAs structure after annealing exhibited a clearly 4

rectifying behavior, indicating the formation of ZnO homojunction. Furthermore, 5

distinct ultraviolet EL emission centered at 382nm was obtained at room temperature. 6

Figure33 shows the microstructure of Sb-doped p-type ZnO nanowires grown 7

using CVD method by Wang et al [89], revealing the single-crystalline nature of the 8

ZnO nanowires. To study the electrical properties of the ZnO nanowire, ZnO 9

nanowire FETs were prepared by standard photolithography. The Id-Vd curve of FET 10

based on Sb-doped ZnO NW showed opposite dependence on Vg, indicating the 11

p-type behavior of nanowire (Figure34). The hole concentration and mobility were 12

calculated to be 6.91017cm-3 and 0.037cm2V-1s-1, respectively. Wang et al also 13

prepared ZnO p-n homojunctions based on Sb-doped p-type nanowire arrays and 14

n-type thin films. The rectifying behavior of I-V curve obtained from the 15

homojunctions also proved the p type conduction of Sb-doped ZnO nanowire [89]. 16

7.3.2. Metal-organic chemical vapor deposition (MOCVD) 17

Metal-organic chemical vapor deposition, also known as organometallic vapor 18

phase epitaxy (OMVPE) and Metal-organic vapor phase epitaxy (MOVPE), is a 19

chemical vapor deposition method of epitaxial growth of materials [2, 6, 90-93]. It has 20

several advantages compared with other growth methods of materials, such as 21

excellent film uniformity over large areas, good conformal step coverage, easy and 22

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reproducible control of film stoichiometry via composition of gas phase [91]. It is 1

routinely utilized in the electronics industry. With MOCVD, the cations necessary for 2

film growth are delivered as constituents of organometallic molecules. For oxide film 3

growth with MOCVD, typical conditions are the following: (i) a low process pressure 4

(0.7-13mbar), (ii) a carrier gas (Ar and /or N2O) flow rate of 50-500 SCCM, (iii) a 5

rate of oxidising gas (O2 and /or N2O) of 100-2000 SCCM, and the liquid MO 6

precursor flow rate of 0.07-1ml/min [91]. In addition, substrate temperature is another 7

important parameter because it determines the deposition rate of the precursors and 8

has strong effect on the structure of the films. The liquid or solid MO precursors can 9

be dissolved in the organic liquid, mixed and introduced into evaporation cell and 10

then delivered into the reactor by carrier gas [91]. Figures 35a,b show the flow 11

diagram for the liquid-delivery technique and schematic diagram of a MOCVD 12

system, respectively. 13

Huang et al fabricated intrinsic ZnO films by atmospheric pressure MOCVD 14

under various gas flow ratios of [H2O] / [DEZn] (VI/II ratio) ranging from 0.55 to 15

2.74 [94]. Figure36 shows the electrical properties at various [H2O] / [DEZn] ratios. It 16

can be seen that p-type ZnO films with the hole concentration of the order of ~1017 17

cm-3 were achieved at VI/II ratios higher than 1.0. The neutral acceptor bound exciton 18

emissions of p-type ZnO were at 3.3503.355eV. Particularly, p-type ZnO film 19

deposited with [H2O] / [DEZn] =1.10, exhibited the highest mobility of 91.6cm2 /V s 20

and the lowest resistivity of 0.369cm. They attributed p-type conduction of undoped 21

ZnO films to the VZn defect. 22

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Ma et al prepared intrinsic p-type ZnO by MOCVD (MOVPE) by controlling the 1

oxygen partial pressure during growth. They fixed the flow rate of DEZn at 2

6.4510-5mol/min and changed the O2 partial pressure from 25 to 75Pa [95]. For ZnO 3

films with O2 partial pressure at above 55Pa, the films exhibited p-type conduction 4

with hole concentration of 1014-1016cm-3. Zeng et al reported that in intrinsic p-type 5

ZnO grown on Al2O3 substrates by plasma-assisted low pressure metal-organic 6

chemical vapor deposition with Diethyl zinc as the zinc source and oxygen plasma as 7

oxygen source [96]. In their experiments, the intrinsic p-type ZnO with a hole 8

concentration above 1017cm3 were achieved at the growth temperatures of 250 and 9

300C. 10

Tan et al reported the unintentional carbon-doped ZnO films on sapphire(0001) 11

substrate by MOCVD and then annealed in air, O2, and N2 ambient, respectively, at 12

800C with an atmospheric pressure for 1h [97]. The annealed ZnO film showed 13

p-type conduction and had fairly high Hall mobility and hole concentration varying 14

from 7.5 to 20.7cm2 /Vs and 1.41017 to 5.651018 cm-3, respectively (Figure37a). The 15

activation energy EA and compensation ratio ND/NA were estimated to be 50.2 meV 16

and 0.11, respectively (Figure37b). 17

Xu et al fabricated ZnO films by low pressure MOCVD on glass substrates with 18

diethylzinc (DEZ) as organic source and NO and N2O as oxygen and N source [98]. 19

Table8 displays the electrical properties of N-doped ZnO films grown at various 20

temperatures. When temperature

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concentrations of 1015-1017cm-3. Du et al reported the N-doped p-type ZnO films with 1

the hole concentrations of 5.51015- 8.31017 cm3 by MOVPE [99]. 2

Wang et al investigated p-type ZnO films with (Ga,N) co-doping on sapphire 3

substrates by MOVPE [100]. Diethylzinc (DEZn) was the zinc source and N2O was 4

used as the oxygen source, dimethylhydrazine (DMHy) was employed as the nitrogen 5

source, and both trimethylgallium (TMGa) and DMHy were gallium source. Hall 6

measurements showed that the (Ga,N) codoped ZnO grown at 500-5500C had p-type 7

conduction with a hole concentration of about 2.411018cm-3 and a hole mobility of 8

about 4.29cm2V-1s-1. The acceptor energy was estimated to be 160meV from 9

low-temperature photoluminescence spectra. 10

Pan et al reported that P-doped p-type ZnO thin films were grown on quartz and 11

n-type Si (100) substrates by metalorganic chemical vapor deposition [101]. In their 12

experiments, Diethylzinc was acted as the zinc source, and high-purity O2 and P2O5 13

powder were used as the oxygen source and P-dopant source, respectively. They used 14

a special thermal evaporator to evaporate P2O5. Figure38 shows the dependence of the 15

electrical properties of P-doped ZnO films on quartz on evaporating temperature. The 16

ZnO films exhibited p-type behavior and the optimal electrical properties of ZnO 17

grown at 4500C on quartz were achieved with a resistivity of 14.9cm, a mobility of 18

0.227cm2 V-1s-1, and a hole concentration of 1.841018 cm-3. 19

Huang et al prepared ZnO films on GaAs substrates by atmospheric pressure 20

MOCVD and then annealing the films at various temperatures of 500-6500C [102]. 21

Figure39 shows the electrical properties of ZnO films as a function of the film 22

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annealing temperature. The ZnO films annealed exhibited p-type behavior with the 1

hole concentrations ranging from 4.71018 to 8.71019cm-3. 2

7.4. Magnetron Sputtering 3

Magnetron sputtering (DC sputtering, RF sputtering and reactive sputtering) is one 4

popular growth technique for films studies due to its low cost, simplicity and low 5

operating temperature [2, 64, 65, 91, 103, 104]. In sputtering deposition, the target 6

material is bombarded by energetic ions, such as Ar+, to release target atoms. These 7

atoms are then deposited on a nearby substrate surface as a thin film. The plasma 8

needed for sputtering the target materials can be generated by various power sources. 9

Both DC and RF plasma sputtering are common used and combined with a bias 10

magnetic field which allows for a control of the kinetic energy of the sputtered species. 11

A typical deposition rate of 1-10nm/min is achieved for magnetron sputtering with a 12

power density of 0.5-2W/cm2 and the pressure of working gas in the range of 13

10-3mbar [91]. Currently, magnetron sputtering has been used to make semiconductor, 14

dielectric, insulating, magnetic, and superconducting oxide materials as well as 15

catalysts, protective coatings, and more. Figure40 shows a schematic diagram of the 16

magnetron sputtering system and a photograph of the typical glow from ZnO target 17

when sputtering [103]. 18

Zeng et al reported the Li-doped p-type ZnO films on glass with hole 19

concentration of 1015-1017cm-3 by DC reactive magnetron sputtering [105]. The 20

sputtering target was a disk of Zn metal mixed with 0.1at % Li. High purity O2 and Ar 21

were used as sputtering gas with a constant total pressure of 4Pa. Li-doped p-type 22

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ZnO films were deposited at the temperature range of 450-6000C. The optimized 1

result was realized at the substrate temperature of 550C with a resistivity of 16.4cm, 2

Hall mobility of 2.65cm2/Vs, and hole concentration of 1.441017cm-3. The acceptor 3

activation energy was about 110meV. Tang et al used RF magnetron sputtering to 4

fabricate Li-doped p-type ZnO thin films by sputtering Li-doped ZnO targets at room 5

temperature. When Li content in the target was 1at%, the ZnO:Li/n-Si heterojunction 6

displayed rectifying behavior, indicating that Li-doped ZnO film had p-type 7

conduction [106]. 8

Wu et al prepared the K-doped p type ZnO thin films on (0001) Al2O3 substrates 9

by RF magnetron sputtering, using ZnO target mixed with 1wt.% K2O [107]. High 10

purity Ar and O2 were working gas with a constant total pressure about 3-5Pa. Hall 11

measurements showed that the p-type ZnO were grown at substrate temperature of 12

300-7000C, with a hole concentration of 1016-1017cm-3. The films consisted of densely 13

packed columns perpendicular to the substrate and had a high smoothness of the 14

surface and a super smallness of the crystal (Figure41). 15

p-type ZnO films with Al-N codoping were deposited by the reactive magnetron 16

sputtering using 0.4 at.% Al-doped Zn metal as target and N2O as sputtering gas [108]. 17

The AlN co-doped ZnO films deposited on quartz at 5000C exhibited p-type 18

behavior with a hole concentration of 1018cm-3. AlN-co-doped p-ZnO/n-Si(100) 19

heterojunction and AlN-co-doped p-ZnO/n-type Al-doped ZnO homojunction were 20

prepared and their I-V curves showed rectifying behavior, confirming the p-type 21

conduction of AlN-co-doped ZnO films. Kumar et al prepared Ga-N codoped p-type 22

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ZnO films by sputtering ZnO:Ga2O3 target in pure N2O ambient [109]. p-Type ZnO 1

were fabricated at the temperature range of 450- 600C and the codoped film 2

deposited 5500C had a lowest resistivity of 38cm and a hole concentration of 3

3.91017 cm-3. The analysis of structure and surface morphology for the films revealed 4

that the films had high quality with a dense columnar structure and c-axis orientation. 5

Chen et al realized p-type ZnO films on various substrates with the InN codoping 6

method by DC reactive magnetron sputtering [110]. InxZn1-x metal was used as 7

codoping target, a mixture of Ar and N2O was working gas with the ratio of 1:1 and 8

total pressure of 4Pa. Table9 shows the electrical properties InN codoped ZnO films 9

on different temperatures. At lower temperature (4800C), the films shows high 10

resistivity and the conduction type is not confirmed, at intermediate temperature 11

(4900CT5800C), the codoped ZnO films exhibit p-type behavior and at high 12

temperature (T5900C), the conduction type of the films changes into n-type from 13

p-type. They considered that the conduction type of In-N codoped ZnO films could be 14

controlled by adjusting the growth conditions during film deposition. 15

BN codoped p-type ZnO thin films have been realized by radio frequency 16

magnetron sputtering using ZnO target mixed with 1at% BN and a mixture of argon 17

and oxygen as sputtering gas [111,112]. The conduction type of the films was 18

depended on the oxygen partial pressure ratios in the sputtering gas. When oxygen 19

partial pressure ratio was 70%, the codoped ZnO film showed p-type conduction with 20

a hole concentration of 1.81017cm-3 [111]. 21

Kim et al reported the realization of p-type ZnO by P doping and thermal 22

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activation of the dopant [113]. P-doped ZnO films were grown on a sapphire c plane 1

by RF magnetron sputtering, using ZnO target mixed with 1wt% P2O5. The as-grown 2

ZnO:P films exhibited n-type conductivity and these films were converted to p-type 3

ZnO by a rapid thermal annealing (RTA) process at a temperature above 800C in a N2 4

ambient. The p-type ZnO:P films had a hole conceentration of 1017-1019cm-3, a 5

mobility of 0.53-5.51cm2/Vs, a resistivity of 0.59-4.4cm. Figures42, 43 show the 6

carrier concentration and mobility of ZnO:P films treated by rapid thermal annealing, 7

respectively. 8

P-doped ZnO films were grown on n-Si (111) substrate by RF magnetron 9

sputtering using a 2at.% P2O5 mixed ZnO target [114]. A mixture of high purity Ar 10

and O2 with a gas flow ratio of 1:0.04 was used as the sputtering gas. The films were 11

grown at 3500C. The as-grown films had high resistance, however, the films annealed 12

at 7500C in an oxygen ambient within a certain pressure range (1.3103 3.9103 Pa) 13

exhibited p-type conduction with a hole concentration of 1016-1017cm-3, a mobility of 14

4-13cm2/Vs, a resistivity of 10.4-19.3cm. The I-V curve of p-type ZnO/n-Si 15

heterojunction showed a rectifying behavior, confirming p-type conduction of ZnO 16

films. 17

Wang et al prepared P-Ga codoped ZnO films on sapphire substrates by 18

magnetron sputtering with ZnO target mixed with 5wt% P2O5 and 0.2 wt% Ga2O3 19

[115]. The films were deposited at 3000C and then annealed at 6000C for 40min in 20

vacuum. The as-grown films showed n-type behavior with a electron concentration of 21

1018cm-3, however, the conduction type of codoped ZnO films was changed to p-type 22

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by annealing in vacuum. The hole concentration, carrier mobility, and resistivity of 1

P-Ga codoped p-type ZnO:P films were 1.61018cm-3, 9cm2/Vs, and 0.37cm, 2

respectively. 3

Wang et al investigated As-doped p-type ZnO films grown by sputtering and 4

thermal diffusion [116]. ZnO films were deposited on the semi-insulating GaAs (001) 5

substrate at different temperatures (400, 500 and 6000C) by RF magnetron sputtering 6

with a sintered ZnO target, and then annealing treatment was carried out in the growth 7

chamber for 60min with various annealing conditions. XRD patterns of ZnO films 8

indicated the films were a single-phase wurtzite structure and a preferred orientation 9

along the c axis. In the Secondary-ion-mass Spectroscopy (SIMS) depth profile of 10

annealed ZnO films, as shown in Figure44, As was observed clearly and its 11

concentration profile was almost flat throughout the film depth, indicating that As was 12

diffused into ZnO film with a uniform distribution. Hall effect measurements showed 13

that all as-grown films were n-type and were converted into p-type conductivity with 14

a hole concentrations of 3.61019-9.41019cm-3 by annealing at 750 C in a vacuum. 15

Fan et al prepared As-doped p-type ZnO films deposited on SiO2/Si substrates by 16

sputtering a Zn3As2//ZnO ceramic target and on glass by cosputtering Zn3As2 and ZnO 17

targets [117]. The substrate temperature was varied from 200 to 5000C. Figure45 18

shows the carrier concentrations of the As-doped ZnO film grown on the SiO2/Si and 19

the glass substrates as a function of substrate temperature. The films deposited on 20

SiO2/Si at 3500C varied from n type to p type as illustrated by the large error bar and 21

the film grown at 4000C exhibited p-type conduction with a hole concentration of 22

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31018cm-3 and a mobility of 1.0cm2/Vs. Further increasing substrate temperature 1

would decreased the hole concentration (Figure45 red circles). As-doped ZnO films 2

grown on the glass had very similar substrate temperature dependence compared to 3

those grown on the SiO2/Si substrate (Figure45 blue squares). When temperature 4

>3500C, the films grown on glass exhibited stable p-type behavior. In addition, 5

annealing may change the conduction type of As-doped ZnO films from n type to p 6

type (Figure45 inset). The acceptor binding energy EA was estimated to be 155meV. 7

Yun et al investigated Al-As codoped ZnO films fabricated on SiO2 substrates by 8

cosputtering ZnO and AlAs targets [118]. Mixtures of high purity Ar and O2 were 9

used as working gases. The power on ZnO target was fixed 240W and AlAs target 10

power was varied from 80 to 200W to change the Al and As contents. After deposition, 11

the ZnO films were annealed at 600C in constant O2 or N2 flow of 7.5 l/min for 5 12

min. Table10 shows the electrical properties of Al-As codoped ZnO films, indicating 13

that films annealed in O2 (samples2, 3, 5, and 6) exhibited p-type conductivity with a 14

hole concentration of 1017-1020cm-3, whereas those annealed in N2 (sample1) exhibited 15

n-type behavior. 16

7.5. Sol-gel 17

Sol-gel process, also known as chemical solution deposition, is a wet-chemical 18

method used widely in the field of materials science [119, 120]. The method is used 19

mainly for the preparation of materials starting from a precursor. In sol-gel process, 20

the precursor in a homogeneous solution undergoes a succession of transformations: 21

(a) hydrolysis of the molecular precursor; (b) polymerization via successive 22

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bimolecular additions of ions, forming oxo-, hydroxyl, or aquabridges; (c) 1

condensation by dehydration; (d) nucleation; and (e) growth [120]. Currently, two 2

Sol-gel routes are used: metal alkoxides in organic solvents or metal salts in aqueous 3

solutions. Figure46 shows the main steps of fabrication of thin films and powder by 4

sol-gel method. 5

Wang prepared Li-doped ZnO films on n-type Si substrate by Sol-gel method 6

using lithium nitrate (LiNO3) as Li source [121]. XRD patterns of ZnO showed the 7

films were polycrystalline and the size of crystal grain was depended on Li 8

concentration in the films. Hall effect measurements revealed that Li-doped p-type 9

ZnO films were prepared and the hole concentration, mobility and resistivity were 10

3.981016-5.321018cm-3, 2.87-155cm2/Vs and 1.10-12.34cm, respectively. 11

Dutta et al reported p-ZnO/n-Si heterojunction achieved by depositing Al-N 12

codoped p-type ZnO film on n-Si by low-cost sol-gel technique [122]. In their 13

experiments, Ammonium acetate [CH3COONH4] and aluminium nitrate 14

[Al(NO3)39H2O], as nitrogen and aluminium sources, were added to 0.4mol sol of 15

zinc acetate 2-hydrate [Zn(CH3COO)22H2O] with the atomic ratio of Zn/N/Al of 16

1:1:0.01, and the sol was spin coated on the cleaned n-Si(100) and dried at 120C in 17

air followed by heating at 550C in oxygen. The I-V characteristic of the p-ZnO/n-Si 18

heterojunction in Figure47 exhibits a good rectifying behavior with IF/IR~10 at 4V in 19

the dark, indicating that the p-type conduction of N-Al codoping ZnO films. Under 20

UV (350nm) and visible (450nm) illumination, the turn-on voltage of the junction 21

became 0.67 and 0.39eV, respectively. 22

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Leung et al reported the fabrication of ZnO p-n homojunction which consisted of 1

n-type ZnO nanowires array by a hydrothermal method covered with p-type Al, N 2

co-doped ZnO film by a sol-gel method, as shown in Figure48 [123]. The p-type ZnO 3

films had a hole concentration of ~1016cm-3, a mobility of 125-217cm2/Vs. The clear 4

rectifying behavior of I-V curve obtained from the homojunction (Figure49) 5

confirmed the p-type conduction of Al,N codoped ZnO films. 6

7.6. Ultrasonic spray pyrolysis (USP) 7

Ultrasonic spray pyrolysis is based on thermal deposition of the source solution 8

spayed by an ultrasonic nozzle onto the surface of a heated solid substrate [124-127]. 9

Every ultrasonic nozzle is operated at a specific resonance frequency determined by 10

the length of the nozzle. The technique can be used to prepare the stoichiometoric and 11

homogeneous compounds instantaneously by spraying the solutions with the desired 12

amounts of cations into the hot zone of an electric furnace. Figure50 shows the 13

schematic diagram of spray pyrolysis reactor system [124]. 14

Wang et al fabricated intrinsic p-type ZnO thin film on sapphire substrate by 15

ultrasonic spray pyrolysis and investigated the grain boundaries related p-type 16

conductivity [128]. The aqueous zinc nitrate solution (0.1M) was used as a precursor. 17

High purity N2, Air and O2 were chosen as carrier gases, respectively. They found 18

that ZnO grown with various carrier gases exhibited different conduction types. The 19

film deposited with N2 had n-type conduction (resistivity: 3.53102cm, mobility: 20

13.2cm2/Vs, and electron concentration: 1.341015cm-3), the ZnO film prepared with 21

air as carrier gas exhibited p-type conduction with the high resistivity 22

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(resistivity:1.92104cm, mobility: 127cm2/Vs, and hole concentration: 1

2.571012cm-3 ) and the sample obtained with O2 as carrier gas showed p-type 2

conduction with low resistivity (resistivity: 2.18cm, mobility: 261cm2/Vs, and hole 3

concentration: 1.101016cm-3 ). They considered the O2 absorbed in grain boundary 4

caused p-type conduction of ZnO films and the inverse layer near the grain boundary 5

induced the quasi-two-dimensional hole gas, resulting in high Hall mobility of the 6

p-type ZnO film. 7

Bian et al reported the Nitrogen-doped p-type ZnO films grown on Si(100) 8

substrates by ultrasonic spray pyrolysis with Zn(CH3COO)22H2O and CH3COONH4 9

as the sources of zinc and nitrogen, respectively[129]. The films showed 10

polycrystalline with a hexagonal wurtzite structure. Hall-effect and Seebeck-effect 11

measurements indicated that the ZnO:N exhibited p-type conduction with a hole 12

concentration of 8.591018cm-3, a mobility of 24.1cm2/Vs, and a resistivity of 13

3.0210-2cm and seebeck coefficient of 408.2mV/K. The rectifying behavior of I-V 14

curve obtained from ZnO p-n homojunction further indicated the success of 15

synthesizing p-type ZnO. Similarly, Bian et al also fabricated N-In p-type ZnO by 16

ultrasonic spray pyrolysis using In(NO3)3 as In source [130]. The hole concentration, 17

mobility, resistivity and seebeck coefficient of the p-type ZnO films were 2.441018 18

cm-3, 155cm2/Vs, and 1.710-2cm and 1477.1mV/K, respectively. Figure51 shows 19

the dependence of Hall mobility and resistivity of NIn codoped ZnO films on the 20

temperature [130]. 21

7.7. Ion implantation 22

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Ion implantation is a materials engineering process by which ions of a material are 1

accelerated in an electrical field and impacted into another solid [131, 132]. In the 2

process, dopant atoms are volatilized, ionized, accelerated, separated by the 3

mass-to-charge ratios, and directed at a target. The atoms enter the crystal lattice, 4

collide with the host atoms, lose energy, and finally come to rest at some depth within 5

the solid. Figure52 shows the schematic of a typical implantation system [131]. 6

Currently, ion implantation is used widely to change the physical, chemical and 7

electrical properties of materials, including the introduction of dopant in 8

semiconductors due to its advantages, such as the selective implanting of certain areas 9

on the sample, the high concentration of the implanting elements, and the precise 10

control of implant concentration and depth distribution. 11

Gu et al reported N-implanted ZnO p-n homojunction. In their experiments, 12

N+-ion was implanted into the polished front side of ZnO single crystal at 300C, and 13

then annealed at different temperatures (650-1200C) in air [133]. The I-V curve of 14

the homojunction is shown in Figure53, indicating that the conduction type of 15

N+-implanted layer was n-type and was changed to p-type after annealing. The hole 16

concentration of p-type layer was estimated to be ~1017 cm-3. 17

Braunstein et al investigated As+-implanted ZnO films grown on SiO2/Si by RF 18

magnetron sputtering [134]. Implantations were performed with the samples held at 19

liquid-nitrogen temperature, followed by an in situ rapid heating to 500560C, and 20

then annealed the samples at 9000C in the following oxygen, or in air. The As+- 21

implanted ZnO films, using the cold implantation rapid annealing process, turned p 22

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type upon annealing in oxygen atmosphere, at 900C, with a carrier concentration 1

slightly larger than 1013cm2. Yang et al prepared vertically aligned ZnO nanorods on 2

n-type Si(111) substrate by vapor-phase transport method, and then performed As+- 3

implantation perpendicularly to the ZnO nanorods with 50 and 180keV As+ ions at a 4

dosage of 11014 or 11015cm2 [135]. The as-implanted ZnO nanorods were annealed 5

for 2h at 750C under vacuum (510-2Torr) with an O2 flow, forming ZnO:As 6

nanorods/n-ZnO nanorods homojunction (Figure54). The rectifying behavior of 7

typical p-n junction was clearly detected in the I-V curve obtained from nanorods 8

homojunction (Figure55), indicating that p-type conduction of As+-implanted ZnO 9

nanorods. 10

7.8. Hydrothermal method 11

Hydrothermal method is a good approach for synthesis of ZnO bulk, film and 12

nanowires with the use of ZnO seeds in the forms of thin films or nanoparticles 13

[136-138]. Hsu et al reported that the conduction type of ZnO nanorods grown by a 14

hydrothermal method could be controlled by simple changes in the seed layer 15

preparation [139]. They prepared p-type ZnO nanorod on ZnO single crystal with hole 16

concentration of the order 1017cm-3 by adjusting the properties of the seed layer (zinc 17

acetate derived seed, 2000C; zinc acetate derived seed, 3500C and electrodeposited 18

seed) (Figure56). The I-V curves obtained from ZnO nonorod/ZnO crystal 19

homojunction revealed clearly rectifying behavior (Figure57b), indicating that p-type 20

conduction of ZnO nonorod. p-Type conductivity in ZnO nanorods was attributed to 21

increased concentration of zinc vacancies and decreased concentration of donor 22

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impurities. 1

Lee et al reported Li-doped ZnO nanowire with a hole concentration of 2

1.681011cm-3 and a mobility of 2.52cm2/Vs prepared by hydrothermal method using 3

lithium nitrate as Li source [140]. Two acceptor-bound exciton peaks at 3.282 4

3.294eV were observed in low temperature PL of p-type Li-doped ZnO nanowire. The 5

p-type conduction of Li-doped ZnO nanowire were confirmed by field-effect 6

transistor with a single annealed ZnO:Li nanowire as a p-channel. Figures58a,b show 7

typical source-to-drain current (IDS)-voltage (VDS) p-type output at different gate 8

voltages ( VG ) and transfer characteristics from the device. 9

Ding et al investigated N-doped p-type ZnO films were achieved by a 10

hydrothermal treatment method [141]. The n-type undoped ZnO film with an electron 11

concentration of 31016cm-3 was grown on a c-plane Al2O3 substrate using the 12

plasma-assisted molecular beam epitaxy technique and was cut into four pieces. One 13

piece of them was acted as as-grown sample, the other three pieces were treated by 14

different hydrothermal process. The ZnO films were conversed to p-type conduction 15

with a hole concentration of 2.21015- 4.41016cm-3, a mobility of 0.8-8.6cm2/Vs. The 16

I-V characteristics of ZnO p-n homojunction showed the rectifying behavior, 17

confirming p-type conduction of ZnO:N films. The acceptor-bound exciton was at 18

3.353eV and the acceptor energy level was calculated to be about 112 meV. 19

Fang et al reported Phosphorus-doped p-type ZnO nanorods and ZnO nanorod p-n 20

homojunction LED fabricated by hydrothermal method [142]. They prepared ZnO 21

film with the thickness of 100nm on Si(100) substrates by magnetron sputtering as a 22

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seed layer, and then synthesized P- doped ZnO nanorod by hydrothermal process 1

using zinc acetate [Zn(Ac)22H2O] and NH4H2PO4 as Zn and P source, respectively. 2

Finally, P-doped ZnO nanorods were annealed at 800C for 1h under ambient Ar. The 3

undoped ZnO nanorods were also grown following the same procedure without 4

adding NH4H2PO4. The ZnO (002) diffraction peak is dominant in XRD spectra, 5

which means the samples are the c-axis orientation, as shown in Figure59. The shift of 6

(002) peak to larger-angle side after P doping indicated that P ion successfully 7

diffused into the ZnO crystal lattice (Figure59 inset). The p-n homojunction LED was 8

fabricated by growing P-doped ZnO nanorods on undoped ZnO nanorods. The 9

observation of peaks related to acceptor at 3.316 and 3.324eV in PL spectra of 10

P-doped ZnO nanorods and the rectifying behavior of p-n homojunction showed p 11

type conduction of P-doped ZnO nanorods. 12

7.9. Hybrid beam deposition (HBD) 13

Hybrid beam deposition is a materials (metal oxide) growth technique that is 14

developed by modified techniques out of PLD, MBE and CVD. It has wide 15

application in fabricating and doping metal oxide, such as ZnO and MgO [143]. 16

Compared with PLD, MBE and CVD system, HBD is very flexible in choosing 17

source materials. Figure60 shows the schematic of a HBD system. 18

As-doped p-type ZnO films were grown on ZnO, SiC, and sapphire substrates by 19

HBD method [144-147]. The (Zn,O) plasma was produced by ablating ZnO target 20

with laser beam and an As molecular beam for As doping was supplied with an 21

effusion cell [144]. Hall measurements showed that the conduction type of As-doped 22

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ZnO film converted from intrinsic n-type to highly conductive p-type with increased 1

As dopant concentration to above 11018cm-3. The optimal electrical properties of the 2

film had a hole concentration of 41017cm-3, a mobility of 35cm2/Vs. The results from 3

PL of p-type ZnO revealed that two different energy levels related As existed, in the 4

range from 110 to 170meV, above the maximum of the ZnO valence band. In addition, 5

p-type ZnO:As-based optoelectronic devices, such as light emission diodes, 6

photodetector and field-effect transistor, were prepared by HBD [146,147]. 7

7.10. Other techniques 8

Beside above mentioned methods, there are other techniques to be used to produce 9

p-type ZnO materials, such as Oxided [148-152], atomic layer deposition [153-155], 10

e-beam deposition [156-158] and filtered cathodic vacuum arc technique [159-161]. 11

7.10.1 Oxided method 12

Nakano et al investigated N-doped ZnO films prepared by thermal oxidation of 13

sputtering Zn3N2 films [148]. The Zn3N2 films were fabricated on fused quartz 14

substrates by radio-frequency magnetron sputtering of a metallic Zn target in a 15

mixture of Ar and N2, and then were annealed in following O2 at different 16

temperatures of 500-8000C for 60min. The XRD results showed that the Zn3N2 films 17

were converted to polycrystalline ZnO phases with a hexagonal structure after 18

annealing. Hall-effect measurements revealed that the ZnO:N films oxided at 500 and 19

6000C had subtle behavior between n and p types and the ZnO:N films annealed at 20

700 and 8000C exhibited stable p-type behavior with a hole concentration of 21

~1017cm-3 ( Figure61). 22

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Przedziecka et al reported p-type ZnO:Sb prepared by thermal oxidation of the 1

Zn-Sb films [151]. The Zn-Sb source material was deposited by magnetron sputtering 2

on c-oriented sapphire substrates, and then the samples were oxidized by annealing in 3

a flowing O2 at a various temperatures of 600-9000C. The Sb-doped ZnO films 4

obtained by this way had p-type conduction with a hole concentration of ~1017cm-3, a 5

mobility of ~7cm2/Vs. In the low-temperature PL spectra of Sb-doped p-type ZnO 6

films, the A0X emission was at about 3.311eV and the acceptor binding energy EA was 7

estimated to be 137meV. 8

7.10.2. Atomic layer deposition 9

Atomic layer deposition is an important film deposition technique, and it is used 10

to prepare p-type ZnO films. 11

Dunlop et al studied N-doped ZnO films deposited by atmospheric atomic layer 12

deposition between 100 and 300C using Diethlyl zinc, water and ammonia as Zn, O 13

and N source, respectively [154]. The as-grown samples exhibited n-type conduction. 14

After a low temperature dark annealing, the conduction type of the samples grown at 15

lower temperature (1500C) became to p type behavior with a hole concentration of 16

~1015cm-3, a mobility of 0.2-0.4cm2/Vs. While the samples deposited at higher 17

temperatures (2000C), the samples had higher mobilities (up to 6cm2/Vs). Lee et al 18

prepared N-doped p-type ZnO films on sapphire (0001) substrates by atomic layer 19

epitaxy with Zn(C2H5)2 H2O and NH3 as a zinc precursor, an oxidant and a doping 20

source gas, respectively [153]. All as-grown samples showed n-type characteristics, 21

and after annealing at 1000C in an oxygen atmosphere of 1atm for 1h, all of them 22

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were changed to p-type conduction with a hole concentration of 2.43-3.411016cm-3, a 1

mobility of 0.81-1.01cm2/Vs and a resistivity of 210-235cm. 2

p-Type ZnO films were fabricated by atomic layer deposition on semi-insulating 3

GaAs substrates and followed by rapid thermal annealing in oxygen ambient [155]. 4

Figure62 shows the electrical properties of the samples at different temperature [155]. 5

It can be seen that the conduction type was changed from n-type to p-type by rapid 6

thermal annealing at temperatures of above 6000C and a hole concentration of 7

3.441020cm-3 was achieved for the sample treated by RTA at 700 C. 8

7.10.3. E-beam deposition 9

E-beam deposition is an important technique to fabricate film materials. In the 10

process, a target anode is bombarded with an electron beam given off by a charged 11

tungsten filament under high vacuum, and the atoms from the target are transformed 12

into the gaseous phase and then deposit on the substrate, forming film [156]. 13

Kim et al reported Ag-doped ZnO thin films were grown on glass substrates at 14

1500C by e-beam evaporation technique using ZnO:Ag2O target [157]. The as-grown 15

samples were annealed at different temperatures of 350-6500C. Hall-effect 16

measurements showed that the as-grown and annealed at 3500C samples exhibited 17

p-type conduction with a hole concentration of (3.98-5.09)1019cm-3. For the samples 18

annealed at above 3500C, the resistivity was so high that it was beyond the measurable 19

range of Hall effect measurements, which was caused by the formation of much Agi in 20

the films annealed at higher temperature. The acceptor states observed at 3.311eV in 21

low temperature PL spectrum of the ZnO:Ag films was attributed to Ag. 22

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Kumar et al studied the As-doped p-type ZnO thin films using As2O3 as doping 1

source material by e-beam evaporation [158]. The As-doped ZnO films were 2

deposited on glass substrate and then annealed at 400-6000C in Ar ambient for 60min. 3

The as-grown films had n type conduction with a electron concentration of 1019cm-3, 4

however, the annealed samples showed p-type behavior with a hole concentration of 5

3.701011-3.631017cm-3. 6

7.10.4. Filtered cathodic vacuum arc technique (FCVA) 7

Cathodic vacuum arc is a deposition technique that has unique characteristics, 8

such as high ion energy, high ionization rate and multiple ion charge states, depending 9

on the cathodic materials [159,160]. The filtered cathodic vacuum arc (FCVA) refers 10

to the combined cathodic arc with the magnetic filters. The details of the technique 11

can be found in Ref [159] and interested readers are also directed to comprehensive 12

review article on metal oxide grown by the technique [160]. 13

Yuen et al reported high quality As-doped p-type ZnO films on quartz substrate at 14

room temperature using Zn3As2 as As source by FCVA [161]. The As-doped ZnO 15

films showed a good c-axis orientation and high-crystal quality. The electrical 16

properties of the p-type films were depended on the distance between substrate and 17

target (Figure63). The high quality p-type As-doped ZnO film was achieved , which 18

had a hole concentration of 41019cm-3, a mobility of 2cm2/Vs and a resistivity of 19

0.05cm. The rectifying behavior of I-V curve obtained from the ZnO p-n 20

homojunction on plastic substrate confirmed the p type conduction of As-doped ZnO 21

films. 22

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In addition, p-type ZnO materials were fabricated by excimer laser doping [162], 1

radical beam gettering epitaxy [163], and electrochemical route [164,165], although 2

the electrical properties of the samples need to be studied, further. 3

The growth method of p-type ZnO have been summarized in above contents, here, 4

to better understand the effects of growth method on the electrical properties of p-type 5

ZnO, the typical electrical properties of p-type ZnO:N grown on