Stable p-Type Doping of ZnO Film in Aqueous Solution at Low

May 6, 2010 - Zinc oxide (ZnO) is a wide band-gap material with excellent optical properties for optoelectronics applications. However, device fabrica...
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J. Phys. Chem. C 2010, 114, 9981–9987

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Stable p-Type Doping of ZnO Film in Aqueous Solution at Low Temperatures Chuan Beng Tay,*,† Soo Jin Chua,*,† and Kian Ping Loh‡ Department of Electrical and Computer Engineering, National UniVersity of Singapore, 4 Engineering DriVe 3, Singapore 117576, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: April 12, 2010

Zinc oxide (ZnO) is a wide band-gap material with excellent optical properties for optoelectronics applications. However, device fabrication has been hampered by difficulties in obtaining a stable p-type doping. Here, we present the first report on the growth and doping of ZnO film through the incorporation of potassium (K) from group I in aqueous solution at 90 °C to yield a stable p-type doping. The contribution of potassium toward p-type conductivity is confirmed using Hall effect measurements and SIMS. A new growth strategy was introduced to obtain a good film coverage with a lower native defect density without the use of surfactants. Photoluminescence measurements confirmed the reduction of defect-related emissions and enhancement of UV band-edge emissions. Variation of carrier concentrations with temperature points to the presence of unstable hydrogen donors that can be removed by annealing at temperatures above 400 °C for extended durations. The instability of these hydrogen defects is attributed to the low growth temperatures. Finally, a p-ZnO/nGaN junction is demonstrated to have a rectifying I-V characteristic and two dominant electroluminescence peaks in the UV range of 370-390 nm, as well as a broad yellow-orange peak. 1. Introduction ZnO is a potential replacement for GaN in optoelectronic applications. It has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Furthermore, band-gap tailoring of ZnO by alloying with Cd, Mg, and Be results in less lattice distortion compared with that of GaN over the same energy range. Despite its advantages over GaN, the application of ZnO in optoelectronics is hampered by the lack of stable p-type doping due to self-compensation by donor-like native defects, low solubility of p-type dopants, and formation of deep acceptor levels.1 Group I and V elements are potential candidates for p-type dopants. Although, in theory, lithium from group I has the shallowest ionization energy, whereas nitrogen has the most compatible substitutional bond length,1 only nitrogen doping has been successfully demonstrated by various groups using MBE,2 hydrid beam deposition,3 MOCVD,4 and RF sputtering methods5 with hole concentrations in the range of (1-4) × 1018 cm-3. In contrast, there are only a few reports utilizing group I elements.6 Furthermore, to the best of our knowledge, all reports of reliable p-type doping using either group I or group V elements have been achieved using gas-phase transport growth methods. Hydrothermally grown ZnO, which has a high level of Li and K incorporation from high concentrations of LiOH and KOH (1-3 M) in the growth solutions, does not show p-type conductivity. Instead, it is typically n-type and highly insulating due to compensation by interstitial lithium and potassium, which act as donors. With the exception of Hsu et al., who exploited the use of acceptor-type intrinsic defects of the seeding layer to achieve intrinsic p-type conductivity,7 there are no other reports of extrinsic p-type doping of films using aqueous solution methods. * To whom correspondence should be addressed. E-mail: [email protected] (C.B.T.), [email protected] (S.J.C.), [email protected] (K.P.L.). † Department of Electrical and Computer Engineering. ‡ Department of Chemistry.

Here, we report a simple, reliable, and low-cost method for growing and doping p-type ZnO films in aqueous solution, with potassium from group I as the p-type dopant. Unlike the case of hydrothermal growth, aqueous solution growth methods do not require a high concentration of LiOH and KOH to act as “mineralizers”. By keeping the potassium concentration below 0.24 M, the problem of interstitial potassium acting as donors can be avoided. A multistep film growth strategy was used, beginning with a seed layer growth at pH 10.9, followed by three successive film layer growth cycles at pH 7.5. The chosen ranges of pH of the seeding and film layer growth solutions in this approach are reverse of those reported by Andeen,8 Kim,9 and Sim,10 who have reported epitaxial film on spinel and GaN substrates. There are several motivations for this approach. First, by starting at pH 10.9, the combination of a positively charged polar Zn surface and the negatively charged majority carriers gives rise to fast anisotropic growth along the c-axis direction and produces a dense array of ZnO nanorods with very good surface coverage across the entire substrate in a very short growth duration. Second, the subsequent film growth cycles at pH 7.5 have a slow growth rate due to the interaction between the positively charged substrate surface and majority growth units in the solution. This slow growth rate improves the quality of film and reduces the concentration of native defects, as demonstrated by our earlier characterization results using photoluminescence and Raman scattering spectroscopy.11 Lastly, the tips of rods grown at pH 7.5 tend to have a flat morphology that facilitates the coalescence of the rods to form a smooth and continuous film. This eliminates the need for surfactants, such as sodium tricitrate, which are usually employed at pH 10-11 where the negatively charged citrate ions adsorb onto the positively charged polar surfaces and reduce the growth rate along the c direction. Although the citrate ions enhance the lateral growth and coalescence of the film, their incorporation inside the crystal bulk during growth creates discontinuities in the perfect lattice.

10.1021/jp101039s  2010 American Chemical Society Published on Web 05/06/2010

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Figure 1. Morphology evolution on a Si(100) substrate from (a) the as-grown seed layer grown with 0.03 M ZnAc2, 0.4 M NH4OH, and 0.07 M KAc to the (b) first and (c) third as-grown film layers grown with 0.03 M ZnAc2 and 0.07 M KAc.

Although existing methods for film growth in aqueous solution require a closely lattice-matched substrate, such as ScMgAlO4,12 MgAl2O4,8a,10 and GaN,9 our proposed approach can be applied to any substrate regardless of its lattice matching with ZnO. For substrates with large lattice mismatch, only one additional step of spin-coating a thin layer of ZnO nanoparticles onto the substrate is required before commencing the seed layer growth process. More details on the preparation of the ZnO nanoparticles, coating, and growth process are provided in the Experimental Methods section of this paper. 2. Experimental Methods ZnO film growth on substrates with large lattice mismatch begins with spin-coating a thin layer of ZnO nanoparticles on the substrate. The ZnO nanoparticles, with diameters typically ranging from 10 to 20 nm, were prepared by stirring a solution containing 0.02 M potassium hydroxide (KOH) and 0.01 M zinc acetate dihydrate (ZnAc2) in methanol at 60 °C for 2 h. The presence of KOH in the alcoholic solution expedites the hydrolysis and condensation of Zn2+ to form ZnO nanoparticles. The substrate surface is then covered with a suspension of ZnO nanoparticles in methanol, which is dropped through a 0.2 µm PTFE membrane and spun at 3000 rpm for 30 s. This spincoating step is repeated three times before the substrate is annealed at 300 °C in an oven for 10 min. For substrates with good lattice matching, such as ScMgAlO4, MgAl2O4, and GaN, this spin-coating step is not required. Next, the seed layer growth is carried out by placing the substrate, facing downward, in a growth solution consisting of 0.03 M ZnAc2, 0.4 M ammonium hydroxide (NH4OH), and a certain concentration of potassium acetate (KAc) for 30 min at 90 °C. After the seed layer growth is completed, the substrate is removed from the solution and rinsed thoroughly with deionized water before it is immersed into the film layer growth solution consisting of 0.03 M ZnAc2 and a specified concentration of KAc for 3 h at 90 °C. The film layer growth step is performed three times to allow the rods to coalesce and form a continuous film. Between each growth step, the substrate is removed and rinsed thoroughly with deionized water. The concentration of KAc that is added into the seed and film layer growth solution is varied in order to study the effect of KAc concentration on the incorporation of potassium in ZnO. For Hall measurements, ZnO films were grown on Al2O3(0001) in order to withstand the high annealing temperatures. Aluminum contacts with a thickness of 1 µm were evaporated on the four corners of the sample for Hall measurements. The carrier concentration and mobility were measured using an Accent HL5500 Hall measurement system after growth and after annealing in nitrogen ambient at temperatures ranging from 200 to 700 °C. Annealing was performed using an ULVAC RTA system. The annealing chamber was pumped down to a

vacuum pressure of less than 5 × 10-5 mTorr before nitrogen gas was flowed into the chamber with a flow rate of 100 sccm. The time taken to increase the sample temperature from room temperature to the final annealing temperature, ranging from 200 to 800 °C, is fixed at 30s. The sample temperature was then held constant for 10 min before it was allowed to cool to room temperature. The flow of nitrogen gas was maintained until the sample has cooled down completely. The film morphology was observed using a Hitachi S4100 FESEM. The photoluminescence (PL) spectra were measured using a Renishaw Ramascope 2000 micro-PL with a He-Cd laser as excitation source. The electroluminescence (EL) spectra was measured using the same micro-PL setup with the device biased by a Keithley 6430 Sourcemeter in the absence of laser excitation. Finally, the SIMS depth profile was obtained using TOF-SIMS IV, which employs a high-current Ga ion beam to sputter the sample and a low-current Ar ion beam to analyze the sputtered crater. 3. Results and Discussion 3.1. ZnO Film Growth. The evolution of the surface morphology from the nanorods after the first step of seed layer growth to a coalesced film after the final step of the third film layer growth in the presence of 0.07 M KAc is shown in Figures 1 and 2 for n-Si(100) and n-GaN substrates, respectively. The coalescence into a smooth and continuous film is much better for GaN because of the good lattice match between GaN and ZnO. For non-lattice-matched substrates, the random orientation of the nanorods requires a longer time for coalescence into a continuous film. A comparison of the various film morphologies when the concentration of KAc is varied from 0 to 0.24 M, on an Al2O3(0001) substrate, is shown in Figure 3. In the ZnAc2-NH4OH system, higher concentrations of KAc lead to larger diameters and lengths of the ZnO rods and thus thicker films with improved coalescence. The difference in the defect density of films grown at pH 7.5 and 10.9 can be seen in Figure 4, which shows the development of the photoluminescence spectra after a seed layer growth step and the subsequent film layer growth steps. The nanorods after the seed layer growth step at pH 10.9 have a low UV emission intensity and a high visible emission intensity from defects, indicating the presence of a high density of defects. The high density of defects is expected when growing at pH 10.9 because of the fast growth rate and high concentration of OH- ions.11 A sharp increase in the UV emission intensity and a slight decrease in the visible emission intensity can be observed after undergoing 30 min of film layer growth at pH 7.5. The UV emission intensity increases with film layer growth duration while the visible emission decreases. This is expected because the top film layer, which has a much lower density of defects, contributes the strong UV emission and decreases the

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Figure 2. Morphology evolution on a n-GaN epilayer from (a) the seed layer grown with 0.03 M ZnAc2, 0.4 M NH4OH, and 0.07 M KAc to the (b) first and (c) third film layers grown with 0.03 M ZnAc2 and 0.07 M KAc.

Figure 3. Morphology of the as-grown ZnO film on an Al2O3(0001) substrate when grown (a) without KAc and with (b) 0.07 M and (c) 0.24 M KAc.

Figure 4. Photoluminescence spectra of (a) the as-grown seed layer and the subsequent film growth layers after (b) 30, (c) 90, and (d) 180 min. The UV and visible photoluminescences are shown on different vertical scales to improve clarity. The film layer growth step significantly enhances the UV emission while slightly reducing the visible emissions.

intensity of the excitation laser that reaches the underlying seed layer. To minimize the defects from the seed layer growth and maintain a high optical quality in the bulk of the ZnO film, the seed layer growth duration is kept short at 30 min, while the film layer growth duration is extended to three cycles of 3 h each. 3.2. Potassium as p-Type Dopant and Effects of Thermal Annealing. The results of Hall effect measurements and SIMS depth profile for the samples grown without KAc and with 0.07 and 0.24 M KAc are summarized in Table 1 and Figure 5, respectively. On comparison of the carrier concentrations in Table 1 and the depth profiles of potassium in Figure 5, a positive correlation can be established between the hole concentration and the concentration of K in the ZnO film. Without KAc, the actual potassium concentration is negligible and the film has n-type conductivity. The experimental data also

TABLE 1: Summary of Carrier Parameters Obtained from Hall Effect Measurements for Samples Grown without KAc and with 0.07 and 0.24 M KAc concentration of KAc (M)

film thicknessa (µm)

type of majority carrier

carrier concentrationb (cm-3)

carrier mobilityb (cm2/V-s)

0 0.07 0.24

1.7 1.9 2.0

n p p

1.4 × 1016 3.8 × 1017 3.7 × 1014

0.45 0.038 11.8

a The film thickness is obtained from the SEM image of the cross section of the film. b The carrier concentrations and mobilities are obtained from Hall effect measurements.

show that the amount of K incorporated in the structure cannot be simply increased by increasing the concentration of KAc in the growth solution. One possible reason is that the increased

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Figure 5. SIMS depth profile for Zn, O, and K for samples grown without KAc and with 0.07 and 0.24 M KAc in the growth solution.

Figure 6. Effect of annealing temperatures on the carrier concentration and mobility for ZnO films grown (a) without any KAc and with (b) 0.07 and (c) 0.24 M KAc. (d) The effect of annealing duration at 800 °C for samples grown in 0.24 M KAc. Annealing for all samples was done in a nitrogen ambient. Data points for as-grown samples are represented at 100 °C. The electron concentrations and mobilities are marked by “b” and “x”, respectively, whereas the hole concentration and mobility are marked by “O” and “+”, respectively.

concentration of acetate ions in the solution results in a higher adsorption of acetate ions on the positively charged faces of ZnO at pH 7.5, which, in turn, blocks the adsorption and incorporation of K. The point with the highest K and hole concentration is obtained with 0.07 M KAc. The effect of post-annealing treatments was studied using samples grown without KAc and with 0.07 and 0.24 M KAc. Annealing temperatures were varied from 200 to 700 °C in a nitrogen ambient for a fixed duration of 10 min. Figure 6a shows the results for the sample that has been grown without the presence of KAc. The as-grown ZnO film is n-type with an intrinsic carrier concentration of about 1.4 × 1016 cm-3. The

film is intrinsically n-type without the presence of any extrinsic dopants, possibly due to the presence of native defects in the structure. At 400 °C, the electron concentration increases to above 1018 cm-3. This sharp rise is attributed to activation of hydrogen donors13 that are usually present in samples grown in aqueous solution as a result of incomplete dehydration during the formation of ZnO.14 Above 400 °C, the electron concentration decreases gradually to about 3 × 1018 cm-3 at 700 °C. This gradual decrease is attributed to desorption of hydrogen from ZnO. Figure 6b,c shows the temperature dependence of the doping levels in a sample that is grown with 0.07 and 0.24 M KAc in

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Figure 7. I-V characteristic of the p-ZnO/n-GaN diode shows a rectifying characteristic with a turn-on voltage (Vt) of about 2.45 V. The inset shows the I-V plotted in logarithmic scale and the schematic diagram of the device. The logarithmic plot of the current against the voltage shows a low reverse bias current in the order of 10-5 A at a reverse bias voltage of 5 V.

Figure 8. Electroluminescence spectra measured at room temperature in continuous current mode with forward bias currents from 20 to 70 mA. The EL spectra are similar to the PL spectra of the p-ZnO layer. The UV emission originates from bound excitons, whereas the broad yelloworange peak originates from deep level defects.

the growth solution, respectively. The as-grown films are p-type with carrier concentrations of 3.8 × 1017 and 3.7 × 1014 cm-3, respectively. Below 300 °C, the p-type doping is stable with a concentration range of 1017 to 1018 cm-3. However, when annealed at 400 °C and above, the p-type conductivity switches to n-type. Similar to that of undoped ZnO in Figure 6a, the electron concentration appears to decline gradually with higher annealing temperatures. The switch from p- to n-type at 400 °C with a sharp increase in electron concentration, followed by a gradual decrease in electron concentration with higher annealing temperatures, is observed for all samples, regardless of the presence of K. The former suggests that, although hydrogen defects are present in the as-grown ZnO, they are not located at electrically active sites. When annealed at 400 °C or higher, these hydrogen defects begin to diffuse through the lattice and move into electrically active positions, thus contributing to the sharp increase in

electron concentration to the order of 1019 cm-3. The latter suggests that these electrically active hydrogen defects are unstable at high temperatures above 400 °C, and they desorb from the sample, leading to the observed decrease in the electron concentrations. The as-grown electrical inactivity and high temperature instability of these hydrogen defects can be attributed to the low growth temperatures in aqueous solution (90 °C), which limits the kinetic energy and prevents the hydrogen defects from migrating to stable lattice sites during growth. In contrast, those hydrothermally grown ZnO synthesized at 300 °C and above have demonstrated very stable hydrogen defects, consisting of a single O-H bond aligned with the c axis, which can be destroyed only at temperatures above 1200 °C.13 The stability of this type of defect is further supported by Kresse’s theoretical calculations on the stabilization effect of hydrogen for the polar Zn face.15 The thermal behavior of the hydrogen defects in our samples is closer to that observed

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by Ip et al., who incorporated hydrogen into ZnO using ion implantation and showed that they can be completely removed by annealing at temperatures ranging from 500 to 700 °C.16 This difference in the thermal stability of the hydrogen defects between samples grown at 90 and 300 °C is crucial to the successful p-type doping of ZnO because the activated hydrogen defects can contribute electron concentrations high enough to overcompensate for the extrinsic p-type dopants. For device processing steps with temperatures above 400 °C, the instability of hydrogen defects in samples grown at low temperatures can be exploited to drive out these hydrogen defects and reduce their concentration to a level below the initial p-type doping and revert back to p-type conductivity. This is demonstrated in Figure 6d, which shows the changes in the carrier concentrations for the sample grown in 0.24 M KAc when it is annealed for different durations at 800 °C. As observed earlier in Figure 6c, a 10 min anneal at 800 °C activates the hydrogen defects and converts the p-type conductivity to n-type. This conversion can be reversed to recover the p-type conductivity when annealed at 800 °C for more than 20 min. In comparison, hydrothermal samples that are grown at 300-400 °C with a high concentration of Li or K remain highly insulating and have a slightly n-type conductivity despite undergoing thermal annealing at 1100 °C for 4 h.17 This again shows the relative stability of the hydrogen defects when grown at high temperatures and underlines the importance of low growth temperatures (90 °C) to achieve p-type conductivity. 3.3. Fabrication of a p-ZnO/n-GaN UV LED. Finally, to further confirm the p-type conductivity, a p-ZnO film is grown using 0.07 M KAc on a n-GaN epilayer that was grown by MOCVD on a sapphire substrate. An In/Zn dot and Ti (15 nm)/ Al (220 nm)/Ni (40 nm)/Au (50 nm) were used as p and n contacts, respectively. A schematic of the device is shown in the inset of Figure 7. The I-V plot in Figure 7 demonstrates p-n junction characteristics with a turn-on voltage of 2.45 V. The inset of Figure 7 plots the logarithm of the current against the voltage and shows a low reverse bias current in the order of 10-5 A at a reverse bias voltage of 5 V. The electroluminescence spectra of the device for forward bias currents ranging from 20 to 70 mA are shown in Figure 8. The UV emission at 20 mA consists of a peak centered at 372 nm with a shoulder at 378 nm and can be attributed to bound exciton emissions. As the current is increased to 70 mA, these peaks shift to 375 and 386 nm, respectively, possibly due to a higher temperature in the junction. Besides the UV emission, a broad yellow-orange luminescence is also observed and is believed to originate from the deep level defects that were introduced during the seed layer growth. Overall, the results of the I-V characteristic and electroluminescence provide further evidence of p-doping of the ZnO film as well as demonstrate the potential of aqueous solution methods as an alternative growth method for ZnO-based device fabrication. 4. Conclusions The p-type doping of ZnO film through the incorporation of potassium from group I in aqueous solution at 90 °C has been demonstrated. A new growth strategy, beginning with a short growth at pH 10-11, followed by a series of slow growth cycles at pH 7.5, was employed to obtain a good film coverage with a lower native defect density without the use of surfactants. The highest doping concentration of 3.8 × 1017 cm-3 was obtained with 0.07 M KAc. The activation of intrinsic hydrogen defects through thermal annealing at temperatures higher than 400 °C can overcompensate for the p-type doping and convert

Tay et al. the film to n-type with an electron concentration of 1 × 1019 cm-3. By extending the annealing time, the concentration of hydrogen defects and electrons can be reduced, and the p-type conductivity recovered. The low growth temperature employed in aqueous solution growth methods is crucial for successful p-type conductivity because it minimizes the formation of stable hydrogen defects and facilitates their removal when annealed at extended durations above 700 °C. Finally, a p-ZnO/n-GaN junction was demonstrated to have p-n junction characteristics with UV and yellow-orange electroluminescences originating from bound excitons and deep level defects. References and Notes (1) Park, C. H.; Zhang, S. B.; Wei, S.-H. Origin of p-type doping difficulty in ZnO: The impurity perspective. Phys. ReV. B 2002, 66, 073202. (2) Tsukazaki, A.; Kubota, M.; Ohtomo, A.; Onuma, T.; Ohtani, K.; Ohno, H.; Chichibu, S. F.; Kawasaki, M. Blue light-emitting diode based on ZnO. Jpn. J. Appl. Phys. 2005, 44, L643–L645. (b) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Repeated temperature modulation epitaxy for p-type doping and light emitting diode based on ZnO. Nat. Mater. 2005, 4, 42–46. (3) (a) Ryu, Y.; Lee, T.-S.; Lubguban, J. A.; White, H. W.; Kim, B.-J.; Park, Y.-S.; Youn, C.-J. Next generation of oxide potonic devices: ZnO-based ultraviolet light emitting diodes. Appl. Phys. Lett. 2006, 88, 241108. (b) Ryu, Y. R.; Lubguban, J. A.; Lee, T. S.; White, H. W.; Jeong, T. S.; Youn, C. J.; Kim, B. J. Excitonic ultraviolet lasing in ZnO-based light emitting devices. Appl. Phys. Lett. 2007, 90, 131115. (4) (a) Liu, W.; Gu, S. L.; Ye, J. D.; Zhu, S. M.; Liu, S. M.; Zhou, X.; Zhang, R.; Shi, Y.; Zheng, Y. D.; Hang, Y.; Zhang, C. L. Blue-yellow ZnO homostructural light emitting diode realized by metalorganic chemical vapor deposition technique. Appl. Phys. Lett. 2006, 88, 092101. (b) Pan, M.; Rondon, R.; Cloud, J.; Rengarajan, V.; Nemeth, W.; Valencia, A.; Gomez, J.; Spencer, N.; Nause, J. ZnO based light emitting diodes growth and fabrication. Proc. SPIE 2006, 6122, 61220M. (c) Xu, W. Z.; Ye, Z. Z.; Zeng, Y. J.; Zhu, L. P.; Zhao, B. H.; Jiang, L.; Lu, J. G.; He, H. P.; Zhang, S. B. ZnO light-emitting diode grown by plasma-assisted metalorganic chemical vapor deposition. Appl. Phys. Lett. 2006, 88, 173506. (5) Lim, J.-H.; Kang, C.-K.; Kim, K.-K.; Park, I.-K.; Hwang, D.-K.; Park, S.-J. UV electroluminescence emission from ZnO light-emitting diodes grown by high-temperature radio frequency sputtering. AdV. Mater. 2006, 18, 2720–2724. (6) (a) Lin, S. S.; Lu, J. G.; Ye, Z. Z.; He, H. P.; Gu, X. Q.; Chen, L. X.; Huang, J. Y.; Zhao, B. H. p-Type behavior in Na-doped ZnO films and ZnO homojunction light-emitting diodes. Solid State Commun. 2008, 148, 25–28. (b) Jun, W.; Yintang, Y. Deposition of K-doped p-type ZnO thin films on (0001) Al2O3 substrates. Mater. Lett. 2008, 62, 1899–1901. (7) Hsu, Y. F.; Xi, Y. Y.; Tam, K. H.; Djurisic, A. B.; Luo, J.; Ling, C. C.; Cheung, C. K.; Ng, A. M. C.; Chan, W. K.; Deng, X.; Beling, C. D.; Fung, S.; Cheah, K. W.; Fong, P. W. K.; Surya, C. C. Undoped p-type ZnO nanorods synthesized by a hydrothermal method. AdV. Funct. Mater. 2008, 18, 1020–1030. (8) (a) Andeen, D.; Kim, J. H.; Lange, F. F.; Goh, G. K. L.; Tripathy, S. Lateral epitaxial overgrowth of ZnO in water. AdV. Funct. Mater. 2006, 16, 799–804. (b) Andeen, D.; Loeffler, L.; Padture, N.; Lange, F. F. Crystal chemistry of epitaxial ZnO on (111) MgAl2O4 produced by hydrothermal synthesis. J. Cryst. Growth 2003, 259, 103–109. (9) Kim, J. H.; Kim, E.-M.; Andeen, D.; Thomson, D.; Denbaars, S. P.; Lange, F. F. Growth of heteroepitaxial ZnO thin films on GaN-buffered Al2O3 (0001) substrates by low temperature hydrothermal synthesis at 90°C. AdV. Funct. Mater. 2007, 17, 463–471. (10) Sim, A. Y. L.; Goh, G. K. L.; Tripathy, S.; Andeen, D.; Lange, F. F. Photoluminescence of hydrothermally epitaxied ZnO films. Electrochim. Acta 2007, 52, 2933–2937. (11) Tay, C. B.; Chua, S. J.; Loh, K. P. Investigation of morphology and photoluminescence of hydrothermally grown ZnO nanorods on substrates pre-coated with ZnO nanoparticles. J. Cryst. Growth 2009, 311, 1278–1284. (12) Wessler, B.; Steinecker, A.; Mader, W. Epitaxial growth of ZnO thin films on ScAlMgO4 (0001) by chemical solution deposition. J. Cryst. Growth 2002, 242, 283–292. (13) Lavrov, E. V.; Borrnert, F.; Webber, J. Dominant hydrogen-oxygen complex in hydrothermally grown ZnO. Phys. ReV. B 2005, 71, 035205. (14) Brauer, G.; Anwand, W.; Grambole, D.; Skorupa, W.; Hou, Y.; Andreev, A.; Teichert, C.; Tam, K. H.; Djurisic, A. B. Non-destructive characterization of vertical ZnO nanowire arrays by slow positron implantation spectroscopy, atomic force microscopy, and nuclear reaction analysis. Nanotechnology 2007, 18, 195301.

Stable p-Type Doping of ZnO Film (15) Kresse, G.; Dulub, O.; Diebold, U. Competing stabilization mechanism for the polar ZnO(0001)-Zn surface. Phys. ReV. B 2003, 68, 245409. (16) Ip, K.; Overberg, M. E.; Heo, Y. W.; Norton, D. P.; Pearton, S. J.; Kucheyev, S. O.; Jagadish, C.; Williams, J. S.; Wilson, R. G.; Zavada, J. M. Thermal stability of ion-implanted hydrogen in ZnO. Appl. Phys. Lett. 2002, 81, 3996–3998.

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