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Thin Solid Films
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Preparation of Na delta-doped p-type ZnO thin films by pulsed laserdeposition using NaF and ZnO ceramic targets
H.B. Liu, X.H. Pan ⁎, J.Y. Huang, H.P. He, Z.Z. Ye ⁎State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China
Na δ-doped p-type ZnO thin films were fabricated on quartz substrates with the structure of ZnO/Na (δ-layer)multi-layers by pulsed laser deposition. NaF ceramic targetwas used asNa source. The effects of oxygenpressureand substrate temperature on the electrical properties of Na δ-doped ZnO thin films are discussed. An optimizedresultwith a resistivity of 29.8Ω · cm,Hallmobility of 0.263 cm2/V · s, andhole concentration of 7.9 × 1017 cm−3
is achieved, and electrically stable over several months. The present work is of interest for developing a method torealize p-type ZnO thin films doping with Na.
ZnO is an attractive wide-band-gap semiconductor with a directband gap of 3.37 eV at room temperature [1]. The large exciton bindingenergy (60 meV) and well-developed bulk, film growth processespromise its optoelectronic applications, such as light-emitting diodes(LEDs), transparent conductive layer, photodetector and gas sensor[2–7]. Due to the asymmetric doping limitations, n-type ZnO can beeasily obtained via intrinsic or extrinisic dopants, whereas it has thus-far proven very difficult to obtain high quality and stable p-type ZnO[8–10].
Recently, Na has been considered as an effective p-type dopant inZnO [11,12]. There have been many reports on p-type Na-doped ZnOusing various methods [13–16]. Among these techniques, pulsed laserdeposition (PLD) is popular for its controllable, simplicity and low cost.Lin et al. reported p-type ZnO thin films via doping of Na acceptor byPLD [15,17,18]. In this work, we develop a method to insert Na δ-layerin ZnO thin film to fabricate p-type Na-doped ZnO by PLD, which isenlightened by the δ-doping. Here, we define above method as Naδ-doping.
As a doping approach, δ-doping has gained much attention as animportant technique both for fundamental studies and applications insemiconductor devices [19]. In the early 1980s, Wood et al. initiatedthis technique to synthesize Ge doped n-type GaAs with complex pro-file by molecular beam epitaxy (MBE) [20]. Then δ-doping was appliedinmany other semiconductors. Miguel et al. fabricated Ga δ-doped ZnSethin films byMBE, and the electrical and optical results were compared
to the samples obtained for uniform doping [21]. N δ-doped and N-Teδ-codoped p-type ZnSe were reported, and it was also proved that δ-doping dramatically increased the hole concentration [22,23]. Recently,Liu et al. suggested that δ-doping technique combinedwith low growthtemperature was beneficial for obtaining highly conductive p-typeMgZnO:N thin film [24]. Research activity on δ-doping has extensivelyincreased over the past few years. However, there have been fewreports on Na δ-doped ZnO thin film so far.
In this work, we report on Na δ-doped p-type ZnO thin films pre-pared by PLD. The effects of oxygen pressure and substrate temperatureon the electrical properties of δ-doped ZnO thin films are readilypresented.
2. Experimental details
Na δ-doped ZnO thin films were fabricated on quartz by the PLDmethod. Two ceramic targets, ZnO (99.99% purity) and NaF (99.99%purity) were used. A KrF excimer laser (λ = 248 nm) was employedto ablate the target. The laser repetition rate was 5 Hz and the pulseenergy was 300 mJ. Before deposition, the chamber was evacuated toa base pressure of 8.0 × 10−4 Pa, and then high-purity O2 (99.99%)was introduced as working gas. The growth pressure and substratetemperature were adjusted to desired value, respectively. After growth,ZnO thin filmswere cooled to room temperature at a rate of 3 °C/min. Aschematic of the Na δ-doped ZnO thin film is shown in Fig. 1.
In order to study diffusion behavior of Na and F from δ-layer toZnO layer, the depth profile of the film was performed by secondaryion mass spectroscopy (SIMS). One δ-layer (ZnO/Na/ZnO) ZnO thinfilm was grown at the substrate temperature of 550 °C and oxygenpressure of 2 Pa for 30, 5, and 30 min, respectively. Fig. 2 illustrates
Fig. 1. A schematic of the Na δ-doped ZnO thin film.
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the depth profiles of Na and F in the Na δ-doped ZnO thin film. Thethickness of first grown ZnO layer is approximately 100 nm, and thetop ZnO layer is approximately 200 nm. Notably, Na with a sharpdiffusion edge and F with broad plateau are observed, indicating thatNa and F have diffused into the ZnO layers during growing and coolingprocesses and F behaves larger migration rate. Based on the SIMSdepth profile, the diffusion rate of Na in ZnO is calculated to be about2 nm/min.
Considering uniform distribution of Na in our samples, Na δ-dopedZnO thin films were fabricated by ablating ZnO target 90 s followedby NaF target 10 s in the following experiments. This structure wasrepeated for 20 periods. Here, two kinds of ZnO films were fabricated:oxygen pressure ranging from 0.02 to 45 Pa by maintaining substratetemperature at 550 °C; and substrate temperature ranging from 450to 700 °C by maintaining oxygen pressure at 2 Pa. The film thicknesswas about 300 nm.
The electrical properties were carried out by Hall-effect measure-ments using the van der Pauw configuration (BIO-RAD HL5500PC) atroom temperature. The crystal structure and surface morphologywere characterized by X-ray diffraction (XRD) (Bede, U.K.) with CuKα radiation (λ = 0.15406 nm) (Configuration = Bracket Flat SampleStage) and field-emission scanning electron microscopy (FE-SEM)(ULTRA55) operated at 5 kV. The depth profile of the samplewas inves-tigated by a CAMECA IMS-6 F SIMS using O2+ ion at 15 kV. The chemi-cal states of Na were analyzed by x-ray photoelectron spectroscopy(XPS) (Thermo ESCALAB 250 using amonochromatized Al Kα radiationsource of energy value 1486.6 eV, and the beam current density is20 mA) and calibrated by the C 1 s peak (285.0 eV). Na 1 s core levelspectra were fitted to Gaussian line shape by adopting a linear back-ground. The current–voltage (I–V) characteristics were measured atroom temperature using Agilent E5270B parameter analyzer.
Fig. 2. Depth profiles of Na and F in one δ-layer (ZnO/Na/ZnO) ZnO thin film grown atthe substrate temperature of 550 °C and oxygen pressure of 2 Pa for 30, 5, and 30 min,respectively. The inset shows a schematic of the ZnO film.
3. Results and discussion
After the growth of Na δ-doped ZnO thin films, the Ni (10 nm)/Au(90 nm) contact metal layers were deposited by electron-beam evapo-ration. Though the resistivity of Na δ-doped p-type ZnO thin films ishigh, the contacts show good ohmic behavior during the Hall-effectmeasurements. Fig. 3(a) shows the resistivity, and carrier concentrationof Na δ-doped ZnO thin films as a function of substrate temperaturevaried from 450 °C to 700 °C. It is seen that p-type conduction can beachieved in narrow substrate temperature ranging from 550 °C to650 °C. The optimal result is realized at a medium substrate tempera-ture of 550 °C,with a resistivity of 444Ω · cm, and a hole concentrationof 1.45 × 1015 cm−3. However, hole concentration is too low. An effec-tively functional ZnO LED requires at least high 1017 to low 1018 cm−3
range hole concentration in the p-layer, based on the observationfromGaN LEDs. Therefore, moreworkwill be done to improve the elec-trical properties of Na δ-doped p-type ZnO thin films. Fig. 3(b) showsthe resistivity, and carrier concentration of Na δ-doped ZnO thin filmsas a function of oxygen pressure varied from 0.02 Pa to 45 Pa. It isseen that Na δ-doped ZnO thin films show p-type conduction whenoxygen pressure is above 2 Pa. It is worth noting that the film preparedat 15 Pa gives the lowest resistivity of 29.8Ω · cmwith a hole concen-tration of 7.97 × 1017 cm−3. To investigate the stability of the p-typeconduction, this sample was reexamined six months later. It is foundthat Na δ-doped ZnO thin film keeps the p-type conduction withoutany obvious degradation in electrical properties. However, Hallmobilityof optimized sample is only 0.263 cm2/V · s, which is lower than thatreported by Lai [13] and Lin [15]. In view of the above fact, it could beunderstood that Na δ-doped ZnO thin films are polycrystalline, whichinduces many defects and grain boundaries.
To investigate the conduction mechanism of Na δ-doped ZnO thinfilm, XPS was performed to examine the chemical bonding states ofNa and F in the ZnO films. Fig. 4(a)-(d) illustrate Na 1 s and F 1 score level spectra of δ-doped ZnO film fabricated at oxygen pressureof 0.02 Pa (sample A) and 15 Pa (sample B), respectively. For bothsamples, a peak attributed to Na 1 s is observed. Gaussian fitting tothe Na 1 s spectrum of sample A reveals two peaks centered at 1072.4and 1070.8 eV, as shown in Fig. 4(a). Gaussian fitting to the Na 1 s spec-trum of sample B reveals one peak centered at 1070.6 eV, as shown inFig. 4(c). In addition, a peak attributed to F 1 s is observed in sampleA, while the F 1 s spectrum of sample B is near the noise level due tothe detection limit of XPS. The result indicates that Na δ-doped ZnOfilm deposited at low oxygen pressure is codoped by Na and F, whileNa-mono-doped ZnO is obtained at high oxygen pressure. This differ-ence may be responsible for the controlled carrier type. Consideringneglectable F doped into sample B, the bonding state of Na 1 s at1070.6 eV is probably associated to Na-O. For sample A, the peak at1070.8 eV is associated toNa-O,while the peak at 1072.4 eV is probablyassociated to Na-F based on the fact that F is also doped into sample A.Note that the peak at 1070.6 eV is related to Na-O bonds (ZnNa), whichare believed to contribute to the p-type behavior as demonstrated by
Fig. 3. Carrier concentration and resistivity of Na δ-doped ZnO thin films as a function of (a) substrate temperature at 450, 500, 550, 600, 650 and 700 °C and (b) oxygen pressure at0.02, 0.2, 2, 10, 15, 30 and 45 Pa.
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Hall-effect measurement. It should also be noted that there are twocompetitive reactions (Na reacts with O or Na reacts with F) duringthe deposition of δ-layer. At low oxygen pressure of 0.02 Pa, Na mightprefer to react with F, then little F dopes into ZnO films. While at highoxygen pressure, the quantity of O is much more than F, resulting inNa having more chance to react with O. As the increasing of oxygenpressure, Na reacting with F becomes more difficult, therefore F isexcluded from the ZnO films. On the other hand, the scattering effectbecomes intense as the increasing of oxygen pressure, which resultsin the decreasing of F concentration.
Together with previous Hall effect results, the conduction mecha-nism of Na δ-doped ZnO thin film is proposed. The dopants of Na andF are sensitive to oxygen pressure. High oxygen pressure is propitious
Fig. 4. XPS core level spectra of Na δ-doped ZnO thin films fabricated at oxyg
to obtain Na-mono-doped ZnO. This is consistent with the results thatNa δ-doped ZnO thin films show p-type conduction when oxygen pres-sure is above 2 Pa. From XPS analysis, Na substitutes Zn to form NaZn-O,which acts as acceptor in Na δ-doped ZnO thin film. As oxygen pressuredecreases, n-type dopant F is also doped into the ZnO film. The relativeconcentration of Na and F determines the conductivity of films. Whenoxygen pressure is 2 Pa, Na δ-doped ZnO exhibits weak p-type conduc-tivity. With further decreasing in oxygen pressure, n-type dopant Fbecomes the majority, so the ZnO film turns to n-type conductivity.
Further study of crystal orientation of Na δ-doped p-type ZnO thinfilm was carried out by XRD. Fig. 5 shows the XRD patterns of ZnO andNa δ-doped ZnO thin films fabricated at oxygen pressure of 0.02 Paand 15 Pa. Only two peaks corresponding to the (0002) and (0004)
en pressure of 0.02 Pa (a) Na 1 s, (b) F 1 s and 15 Pa (c)Na 1 s, (d) F 1 s.
Fig. 5. XRD patterns of ZnO and Na δ-doped ZnO thin films fabricated at oxygen pressureof 0.02 Pa and 15 Pa. The inset shows the spectra of (0002) peak.
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plane of ZnO are observed in the pattern, suggesting a high (0002)preferential orientation for all films. Due to the incorporation ofNa or F atoms, the intensity of the (0002) peak decreases evidently,indicating the degrading crystallinity. Furthermore, the diffractionangle of the (0002) peak of sample grown at 15 Pa shifts to thelower diffraction angle side by 0.15°, while 0.05° for sample grownat 0.02 Pa, as shown in the inset of Fig. 5. Based on the discussion ofXPS results, Na-mono-doped ZnO is obtained at oxygen pressure of15 Pa. As the radius of Na is larger than that of Zn, the NaZn wouldcause the expansion of ZnO lattice, which is consistent with thedecrease of 2θ value. Yet, the sample deposited at low oxygen pressureof 0.02 Pa is codoped by Na and F. Since the radius of F is smaller thanthat of O, FO would compensate the lattice expansion caused by Na-doping, which results in relatively smaller angle shift.
The surface morphology of Na δ-doped ZnO thin films was obtainedusing FE-SEM. Fig. 6(a)-(c) show the SEM top views of ZnO and Naδ-doped ZnO fabricated at oxygen pressure of 0.02 Pa and 15 Pa,respectively. As shown in Fig. 6, the smooth surface is composed ofdense grains. Moreover, as the increasing of oxygen pressure, Naδ-doped ZnO thin film shows distinct degradation in morphology
Fig. 6. SEM top views of (a) ZnO and Na δ-doped ZnO thin film
compared to that of ZnO film. Together with XRD results, it is very rea-sonable to conclude that the degrading crystallinity and morphologyare caused by doping.
To further confirm the p-type conduction in Na δ-doped ZnO thinfilms, ZnO homojunction diodewith the size of 0.25 cm × 0.25 cmwasfabricatedwith the structure of Na δ-doped ZnO thin films deposited onsingle-crystal ZnO substrate at 550 °C with oxygen pressure of 15 Pa.The single-crystal ZnO serves as n-type layer, and Na δ-doped ZnOserves as p-type layer. Ti/Au and Ni/Au layers deposited by electron-beam evaporation were used as n-type and p-type electrodes, showinglinear I–V characteristics indicative of ohmic behavior. The deviceexhibits rectification for repeated measurements, as shown in Fig. 7.
4. Conclusions
In summary, we have presented a method to realize p-type ZnOfilms via Na δ-doping by PLD. Oxygen pressure together with substratetemperature plays an important role in determining the conductivity offilms. The Na δ-doped ZnO thin films obtained show good p-typeconduction at room temperature as well as acceptable crystallinityand surface morphology. XRD analysis and XPS spectra have confirmedthat the Na incorporated in the film should exist as NaZn, which acts asacceptor in Na δ-doped p-type ZnO thin film. Hall and ZnO p-n junctiondemonstrate the firm p-type conductivity in Na δ-doped ZnO thin films.
Acknowledgements
This work was supported by National Natural Science Foundationof China under Grant No. 51172204, Zhejiang Provincial PublicTechnology Research of China under Grant No. 2012C21114, andDoctoral Fund of Ministry of Education of China under Grant No.2011010110013.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tsf.2013.05.133.
s grown at oxygen pressure of 0.02 Pa (b) and 15 Pa (c).
Fig. 7. I–V characteristics of a typical ZnO p-n junction on single-crystal ZnO substrate.Na δ-doped ZnO thin film grown at 550 °C with oxygen pressure of 15 Pa serves asp-type layer.
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