Nitrogen function of aluminum-nitride codoped ZnO films deposited using cosputter system

Li-Wen Lai and Jheng-Tai Yan

Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan, Republic of China Chia-Hsun Chen

Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan, Republic of China

Li-Ren Lou and Ching-Ting Lee

Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan, Republic of China (Received 16 January 2009; accepted 31 March 2009)

AlN codoped ZnO films were deposited on sapphire substrates at low temperature using a cosputter system under various N

/(N

+ Ar) flow ratios. To investigate the nitrogen function, the ratio of nitrogen ambient was varied during cosputtering. AlN codoped ZnO films with various crystallographic structures and bonding configurations were measured.

With an adequate nitrogen atmosphere deposition condition and postannealing

temperature at 450

C, the p-type conductive behaviors of AlN codoped ZnO films were achieved due to the formation of Zn–N bonds. According to the low-temperature photoluminescence spectra, the binding energy (E

) of 0.16 eV for N acceptors can be calculated. Using time-resolved photoluminescence measurement, the carrier lifetime in AlN codoped ZnO films increases due to the reduction of oxygen vacancies caused by the occupation of adequate nitrogen atoms.

I. INTRODUCTION

Group V elements, such as N, P, As, and Sb, are evi- dently the most suitable sort of dopants for forming shal- low acceptors in ZnO films.

Among them, nitrogen, because of its ionic radius similar to oxygen atoms, is thought of as a promising candidate for substituting oxy- gen atoms in ZnO. Many researchers have made efforts in doping N into ZnO using different nitrogen sources and various deposition methods. However, because of the higher chemical activity of oxygen atoms,

Zn atoms prefer to bond with O atoms than N atoms, which results in a low solubility of N in ZnO. Moreover, there exist various unintentional donors in ZnO. Hence, N-doped p-type ZnO is difficult to produce. To overcome the diffi- culty, the N-III codoping method, such as N–In, N–Ga, and N–Al, has been reported and demonstrated to be a promising technique for the formation of p-type ZnO.

In this work, ZnO–AlN films were cosputtered on sap- phire substrates using ZnO and AlN targets in a mixed Ar and N

atmosphere, in which AlN was used as the source of N acceptors and Al reactive codopants. The films de- posited with different N

/(N

+ Ar) flow ratios and post-

annealed at different temperatures were characterized.

The electrical and optical properties of the resulted films were analyzed, from which we deduced an adequate de- position condition and postannealing temperature for get- ting stable p-type ZnO film.

II. EXPERIMENTAL PROCEDURE

The radio frequency (rf) magnetron cosputtering system equipped with a dual rf power supply was used to deposit Al, N codoped ZnO films on sapphire substrates. Pure AlN (99.99%) and pure ZnO (99.99%) were used as the target materials and the corresponding rf powers were fixed at 25 and 100 W, respectively. The N

/(N

+ Ar) flow ratios of 0%, 4%, 8%, and 12% with the total flow rate kept at 50 sccm were used in the deposition of Al, N codoped ZnO films with various N contents. The substrate holder was rotating during the deposition to improve the uniformity of the thickness and the doping content of the deposited films. All the samples studied here were deposited at a low temperature and without heating the substrates. The base pressure and working pressure of the chamber of the mag- netron cosputtering system were kept at 2.0  10

Torr and 10 mTorr, respectively. To activate the doping impu- rities, the samples were postannealed at 400, 450, and 500

C for 10 min in a N

ambient using a rapid-thermal annealing (RTA) system.

a)

Address all correspondence to this author.

e-mail: [email protected]

DOI: 10.1557/JMR.2009.0265

The electrical properties of the resulting films, includ- ing carrier mobility, carrier concentration, and resistivity, were determined by Hall measurement. The crystallo- graphic structures of the samples were characterized by x-ray diffraction (XRD) with Cu K

radiation ( l = 0.154 nm). The low temperature photoluminescence (LTPL) spectra of the samples were measured at 10 K by using the He-Cd laser ( l = 325 nm) as an excitation light source. A picosecond time-resolved photoluminescence (TRPL) system was applied to determine the carrier life- time of the ZnO and ZnO–AlN films at room temperature under the excitation of a wavelength of 266 nm from a Ti: sapphire laser system. The optical transmittance of the deposited ZnO and ZnO–AlN films was measured at room temperature using an ultraviolet/visible (UV/VIS) 4100 spectrophotometer (Hitachi, Tokyo, Japan).

III. EXPERIMENTAL RESULTS AND DISCUSSION Table I lists the cosputtering deposition conditions [ZnO and AlN targets, N

/(N

+ Ar) flow ratio], postannealing temperatures, and electrical properties of the deposited ZnO and ZnO–AlN samples. For the purpose of easy iden- tification, the samples were divided into five groups in accordance with the deposition conditions, which were referred to as sample group A, B, C, D, and E, respectively (Table I). For the samples of group A, after thermal annealing, Ti/Au (30/100 nm) circular contact metal pat- terns were deposited by electron-beam evaporation and then annealed at 300

C in N

ambient for 1 min to form ohmic contact. However, for the AlN and ZnO cosputtered films with unknown conduction types, two kinds of con- tact metals of Ti/Au (30/100 nm) and Ni/Au (30/100 nm) were deposited and then annealed at 300 and 400

C for 1 min in N

ambient, respectively. It was checked to find out which one formed an ohmic contact.

Compared with the other sample groups, the samples of group B exhibited the highest electron concentration, which would be attributed to the effective substitution of Zn sites by Al atoms provided by cosputtering AlN. It is known that the substitutional Al atoms form shallow donors and contribute conduction electrons.

The de- crease in resistivity of the samples of B group with the increase of annealing temperature was attributed to the activation of the effective donor impurity Al in AlN codoped ZnO films. However, the high electron concen- tration also implies that at the deposition condition of B group (under pure Ar ambient), not enough N accep- tors were formed. In other words, the formation of Zn–N bonds is not preferential in an insufficient N atmosphere during the deposition process.

The sample with a lower annealing temperature of 400

C in group C exhibited n-type conductive behavior.

With the same annealing temperature, the samples in groups D and E, deposited under higher N

/(N

+ Ar) flow ratios, exhibited a high resistivity and ambiguous carrier type. However, with the intermediate postannealing tem- perature at 450

C, the deposited films of groups C, D, and E were converted from n-type into p-type conduction, indicating that the N-related acceptor dopants were acti- vated properly by the annealing treatment at this tempera- ture. According to the reported theoretical calculation,

the formation of III-2N complex shallow acceptors was more favorable than that of 2N complex in codoped ZnO films due to the decrease of Madelung energy. Our experi- mental results show that the nitrogen ambient in the de- position process affects the formation of III-2N complex in ZnO of samples C, D, and E. In the previous study, concerning N-doped ZnO deposited with various N

flow ratios in the sputtering process,

it was shown that the rf source can activate nitrogen gas into mixtures of molecu- lar and atomic N. The amount of (N)

acceptors due to

TABLE I. Resistivity, Hall mobility, carrier concentration, and carrier type measured by Hall measurement for films deposited at various conditions.

Sample group

ID Target

Gas (sccm)

Postannealing temperature (

C)

Resistivity

(O-cm) Hall mobility (cm

/V-s)

Carrier concentration (cm

)

Carrier type

A ZnO Ar:50 400 3.67  10

3.66 4.61  10

n

450 2.74  10

4.71 4.83  10

n

500 2.09  10

4.79 6.57  10

n

B ZnO

AlN

Ar:50 400 1.45  10

4.02 1.07  10

n

450 2.92  10

4.42 4.85  10

n

500 2.73  10

4.64 4.95  10

n

C ZnO

AlN

Ar:48 N

:2

400 7.44 1.56 7.23  10

n

450 3.88 1.35 1.17  10

p

500 1.67  10

4.42 8.31  10

n

D ZnO

AlN

Ar:46 N

:4

400 1.39  10

3.51 1.28  10

p/n

450 6.12  10

1.24 8.21  10

p

500 1.53  10

4.13 9.92  10

n

E ZnO

AlN

Ar:44 N

:6

400 1.28  10

2.12 2.28  10

p/n

450 1.13  10

3.14 1.76  10

p

500 2.81  10

3.25 6.95  10

n

substitution of O by atomic N increases and the number of (N

)

due to substitution of O by N

decreases within ZnO films with a decreasing nitrogen glow ratio. The fact that samples D and E deposited under a higher N

/(N

+ Ar) flow ratio have a lower hole concentration was due to the self-compensation induced by the higher (N

)

concentra- tion within ZnO films. Moreover, the carrier type of sam- ples in C, D, and E, annealed at a higher temperature of 500

C, changed from p-type to n-type. This phenomenon was attributed to the dissociation of Zn–N bonds and the formation of native defects, such as oxygen vacancies.

Figure 1 shows the XRD spectra of the samples depos- ited on sapphire substrates under various deposition con- ditions and annealed at 450

C. The only diffraction peak observed was the (0002) plane of the wurtzite crystallized phase. No other peaks, corresponding to AlN, Al

O

, or Zn

N

phases, were observed in XRD spectra of these films. The lattice constant c for the pure ZnO (sample A) and ZnO–AlN [sample E, N

/(N

+ Ar) = 12%] were calculated to be about 5.218 and 5.274 A ˚ , corresponding to the XRD peaks 34.36

and 33.98

, respectively. Since the length of the Zn–N bond is slightly shorter than that of the Zn–O bond,

the associated lattice constant would not be increased by substituting N atom for O atom.

Furthermore, because the radius of N

molecule was larg- er than that of O atom, the lattice constant of the ZnO films containing (N

)

was larger.

Therefore, the larger lattice constant of samples D and E deposited at higher N

/(N

+ Ar) flow ratios was attributed to the substitution of N

for O site. This implies that part of N

molecules were incorporated into ZnO films directly. Moreover, N

on O substitution (N

)

and N on O substitution (N)

in ZnO films were considered as double shallow donors and acceptors,

respectively. The different quantities of (N)

and (N

)

were the reason why the samples C, D,

and E annealed at 450

C exhibited different carrier con- centration.

Figure 2 shows the LTPL (10 K) spectra of the undoped ZnO and the AlN codoped ZnO films, which were deposited at N

/(N

+Ar) flow ratio of 0%, 4%, 8%, and 12% and postannealed at 450

C. The emission peak at 3.362 eV of undoped ZnO (sample A), shown in Fig. 2, was assigned as the neutral donor bound exciton (D

X) in ZnO film. The peak located at 3.315 eV was labeled as donor-acceptor pair (DAP) transition.

The associated emission of sample B, deposited at an ambient without nitrogen, revealed a smaller emission intensity. However, the PL spectra of the AlN codoped ZnO films deposited at a N

containing ambient (samples C, D, and E) were apparently different from that of the pure ZnO film. A new strong peak at 3.332 eV was clearly observed for samples C, D, and E. This peak could be attributed to the neutral acceptor bound exciton (A

X) in ZnO film cosputtered with AlN under N

containing ambient. The new peak at 3.278 eV was attributed to the recombination emissions of free electron to acceptor hole level (FA) due to the nitrogen in the oxygen site (N)

.

The binding energy (E

) of N acceptors at 10 K can be estimated as

E

ðeVÞ ¼ E

E

þ k

T =2 ¼ 3:437

 3:278 þ 0:00086 ¼ 0:16 eV ; ð1Þ where E

= 3.437 eV is the intrinsic band gap of ZnO,

E

is the temperature-dependent transition energy, k

is the Boltzmann constant, and T is the absolute temperature.

Figure 3 shows the LTPL (10 K) spectra of the samples of C annealed at 400, 450, and 500

C. The PL peaks of A

X and FA are evident for all three samples. According to previous reports,

the broad blue-green emission band around 2.4–2.8 eV were assigned as a deep level

FIG. 1. XRD spectra of ZnO and AlN codoped ZnO films deposited on sapphire substrates under various N

/(N

+ Ar) flow ratios and postannealed at a temperature of 450

C.

FIG. 2. LTPL in UV emission range of ZnO, AlN–ZnO [N

/(N

+ Ar) =

0%], AlN–ZnO [N

/(N + Ar) = 4%, 8%, 12%] films annealed at 450

C.

emission caused by the dopant-induced defects. The other broad deep level emission at 2.15 eV was regarded as the oxygen-related emission.

As shown in Fig. 3, the emis- sion induced by dopant increased and the oxygen-related emission decreased apparently, when the annealing tem- perature increased from 400 to 450

C for sample C. The decrease of the oxygen-related emission can be consistent- ly attributed to the occupation of the oxygen vacancy site by nitrogen atoms. This implies that more (N)

acceptors were formed, which interprets the stable p-type behavior for the sample in group C annealed at 450

C. The degra- dation of the p-type behavior for samples annealed at higher temperatures was attributed to the dissociation of Zn–N bonds and Zn–O bonds. The oxygen-related emis- sion evidently increased and a high electron carrier con- centration was obtained for the sample in C annealed at 500

C, as listed in Table I.

For samples of pure ZnO and AlN codoped ZnO with the postannealing temperature fixed at 450

C, the LTPL spectra were measured at 10 K and are shown in Fig. 4.

The green-band emission (2.35 eV) and orange-band emission (1.89 eV) of the undoped ZnO film (sample A) were attributed to oxygen vacancies and oxygen intersti- tials, respectively.

As shown in Fig. 4, the intensity of the two corresponding emission bands of sample B was smaller than that of sample A, which implies that less oxygen vacancies existed in sample B than in sample A.

This implies that the higher electron concentration of sample B (listed in Table I) might be attributed to the substitutional Al atoms on Zn sites instead of oxygen vacancies. Compared to the case of the sample C, more N

on O substitution sites (N

)

simultaneously existed in samples D and E, which resulted in the larger lattice constants as identified by the XRD experimental results.

This is consistent with the fact that samples D and E were deposited under higher N

/(N

+ Ar) flow ratios. Further- more, the fact that the green-band emission of samples D and E is larger than that of sample C implies more O

vacancies existed in samples D and E, which can be ten- tatively interpreted as the result of compensation for the existence of more (N

)

in the film. Because both O vacancies and (N

)

serve as donors in ZnO film, the N acceptors formed in samples D and E were partly com- pensated by them and resulted in a lower hole concentra- tion as seen in Table I.

Figure 5(a) shows PL spectra of the neutral acceptor bound exciton (A

X) of the sample annealed at 450

C in group C, measured at temperatures from 10 to 300 K. The spectra exhibit a redshift of the peak position of A

X emission, which is due to the decrease of the ZnO band gap energy with the temperature.

The intensity of the A

X emission peaks decreases with the increase of the temperature, but at the same time, the free exciton emis- sion increases. The transition from bound exciton emis- sion to free excition (FX) emission occurred due to the thermal dissociation of the A

X at a higher temperature.

Figure 5(b) plotted the emission intensity of the neu- tral acceptor bound exciton (A

X) as a function of recip- rocal temperature. This behavior can be expressed by the following equation

:

I ¼ I

=½1 þ a expðE

=kTÞ ; ð2Þ where I

is the emission intensity at 0 K, a is the fitting parameter, E

is the activation energy from the acceptor- bound exiton to free exciton, k is the Boltzmann con- stant, and T is the absolute temperature. The fitting parameter a = 138.9 and activation energy of 15 meV were obtained from the fitting of the experimental data above 70 K. Using the Haynes rule that E

/E

is approx- imately equal to 0.1 for ZnO material system,

the ac- ceptor binding energy (E

) of 0.15 eV was derived. This result is quite in agreement with the acceptor binding energy derived from Eq. (1).

FIG. 3. LTPL of the AlN–ZnO [N

/(N

+Ar) = 4%] films annealed at 400, 450, and 500

C, respectively.

FIG. 4. LTPL in deep level emission range of ZnO, AlN–ZnO [N

/

(N

+ Ar) = 0%], AlN–ZnO [N

/(N

+ Ar) = 4%, 8%, 12%] films

annealed at 450

C.

To investigate the carrier lifetime ( t) of the pure ZnO and AlN codoped ZnO films annealed at 450

C, the decay of the near band edge emission (380 nm) at room temperature was measured using a TRPL system with a picosecond Ti: sapphire laser of a wavelength 266 nm as an excitation source. The results are shown in Fig. 6. It was found that the 1/e carrier lifetime of the n-type pure ZnO films was 0.25 ns. When the N

/(N

+ Ar) flow ratio used in the deposition of the AlN codoped ZnO films increased from 0% to 4%, the carrier lifetime in- creased from 0.25 to 0.69 ns. However, the carrier life- time decreased with further increasing N

/(N

+ Ar) flow ratio from 8% to 12%. The increase of carrier lifetime can be attributed to the decrease of bypass proc- esses, such as recombination caused by oxygen vacan- cies.

This is consistent with the fact that (N)

formation is preferential to the formation of O vacancies for the sample annealed at 450

C of group C.

Figure 7 shows the optical transmittance spectra mea- sured at room temperature for the pure ZnO and AlN codoped ZnO films deposited on sapphire substrates and annealed at 450

C. For samples A, B, and C, the sharp absorption edge and the optical transmittance of above 90% in the visible region were observed. On the contrary, the absorption edge of AlN codoped ZnO films deposited under higher nitrogen ambient [N

/(N

+ Ar) = 8%, 12%]

shifted to a longer wavelength. Compared with sample A, the blue shift of absorption edge of the sample B was due to the higher electron carrier concentration, which was similar to the result of Al-doped ZnO(AZO) thin films.

As a direct band gap semiconductor, the optical energy gap can be obtained from the measured absorption coef- ficient a

:

a ¼ ðhn  E

Þ

=hn ; ð3Þ where h n and E

are the photon energy and the absorp- tion edge, respectively. The absorption coefficient a is determined from a = ln(1/T)/d, where T and d are the measured optical transmittance and the film thickness, respectively. The optical band gaps shown in Fig. 8 are determined from Eq. (3) by plotting the (ahn)

versus h n and then extrapolating the linear part to the energy axis.

The blue shift from 3.289 eV (sample A) to 3.301 eV (sample B) is attributed to the Burstein–Moss effect.

The room temperature optical band gap thus derived is 3.278, 3.222, and 3.210 eV for the AlN codoped ZnO films deposited under N

/(N

+ Ar) flow ratio of 4%, 8%, and 12%, respectively. The decrease of the optical band gap is ascribed to the smaller ionicity of the Zn–N bond than that of the Zn–O bond, because the electron negativity of N(3.0) is smaller than that of O(3.5).

FIG. 5. (a) PL spectra of neutral acceptor bound exciton (A

X) emis- sion at temperatures from 10 to 300 K. (b) Intensity of the neutral acceptor bound exciton (A

X) emission as a function of reciprocal temperature.

FIG. 6. Time-resolved PL spectra of the ZnO and AlN–ZnO films

annealed at 450

C.

IV. CONCLUSIONS

High quality p-type ZnO films can be obtained by cosputtering of ZnO and AlN targets under an adequate N

/(N

+ Ar) flow ratio of 4% and postannealing at 450

C. According to the LTPL emission spectra, we deduced that the binding energy E

of N acceptor is about 0.16 eV. Degradation of the p-type behavior for films deposited under a higher nitrogen ambient and annealed at a higher temperature is caused by the forma- tion of the (N

)

shallow donor level and the formation of native defects. The carrier lifetime of AlN codoped ZnO films was longer than that of the pure ZnO. This is consistent with the observation that more (N)

were formed, which results in good p-type ZnO films. This study is expected to provide a reproducible and control- lable method to deposit p-type ZnO films using a mag- netron cosputtering system.

ACKNOWLEDGMENT

This work was supported by the National Science Council of Taiwan, Republic of China (NSC97-2623-7- 006-002-NU).

REFERENCES

1. K. Minegishi, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, and A. Shimizu: Growth of p-type zinc oxide films by chemical vapor deposition. Jpn. J. Appl. Phys. 36, L1453 (1997).

2. V. Vaithianathan, B.T. Lee, and S.S. Kim: Pulsed-laser-deposited p-type ZnO films with phosphorus doping. J. Appl. Phys. 98, 043519 (2005).

3. D.C. Look, G.M. Renlund, R.H. Burgener II, and J.R. Sizelove:

As-doped p-type ZnO produced by an evaporation/sputtering proc- ess. Appl. Phys. Lett. 85, 5269 (2004).

4. L.J. Mandalapu, F.X. Xiu, Z. Yang, D.T. Zhao, and J.L. Liu:

P-type behavior from Sb-doped ZnO heterojunction photodiodes.

Appl. Phys. Lett. 88, 112108 (2006).

5. D.S. Liu, C.S. Sheu, and C.T. Lee: Aluminum-nitride codoped zinc oxide films prepared using a radio-frequency magnetron cosputtering system. J. Appl. Phys. 102, 033516 (2007).

6. L.L. Chen, Z.Z. Ye, J.G. Lu, and P.K. Chu: Control and improve- ment of p-type conductivity in indium and nitrogen codoped ZnO thin film. Appl. Phys. Lett. 89, 252113 (2006).

7. M. Kumar, T.H. Kim, S.S. Kim, and B.T. Lee: Growth of epitaxi- al p-type ZnO thin films by codoping of Ga and N. Appl. Phys.

Lett. 89, 112103 (2006).

8. G.D. Yuan, Z.Z. Ye, L.P. Zhu, Q. Qian, B.H. Zhao, R.X. Fan, C.L. Perkins, and S.B. Zhang: Control of conduction type in Al- and N-codoped ZnO thin films. Appl. Phys. Lett. 86, 202106 (2005).

9. R. Konishi, K. Noda, and H. Sasakura: Heat treatment effect of ZnO:Al thin films prepared by facing-target-type sputtering method. Jpn. J. Appl. Phys. 30, 567 (1991).

10. T. Yamamoto and H.K. Yoshida: Solution using a codoping method to unipolarity for the fabrication of p-type ZnO. Jpn. J.

Appl. Phys., Part 2 38, L166 (1999).

11. C.L. Perkins, S.H. Lee, X. Li, S.E. Asher, and T.J. Coutts: Identi- fication of nitrogen chemical states in N-doped ZnO via x-ray photoelectron spectroscopy. J. Appl. Phys. 97, 034907 (2005).

12. C.H. Park, S.B. Zhang, and S.H. Wei: Origin of p-type doping difficulty in ZnO: The impurity perspective. Phys. Rev. B 66, 073202 (2002).

13. B. Yao, D.Z. Shen, Z.Z. Zhang, X.H. Wang, Z.P. Wei, B.H. Li, Y.M. Lv, X.W. Fan, L.X. Guan, G.Z. Xing, C.X. Cong, and Y.P. Xie: Effects of nitrogen doping and illumination on lattice constants and conductivity behavior of zinc oxide grown by mag- netron sputtering. J. Appl. Phys. 99, 123510 (2006).

14. E.C. Lee, Y.S. Kim, Y.G. Jin, and K.J. Chang: Compensation mech- anism for N acceptors in ZnO. Phys. Rev. B 64, 085120 (2001).

15. Y. Zhang, D.J. Chen, and C.T. Lee: Free exciton emission and dephasing in individual ZnO nanowires. Appl. Phys. Lett. 91, 161911 (2007).

16. A.N. Georgobiani, A.N. Gruzintsev, V.T. Volkov, M.O. Vorobiev, V.I. Demin, and V.A. Dravin: P-type ZnO:N obtained by ion implantation of nitrogen with post-implantation annealing in oxy- gen radicals. Nucl. Instrum. Methods Phys. Res., Sect. A 514, 117 (2003).

17. Y.R. Ryu, T.S. Lee, and H.W. White: Properties of arsenic-doped p-type ZnO grown by hybrid beam deposition. Appl. Phys. Lett.

83, 87 (2003).

18. F.X. Xiu, Z. Yang, L.J. Mandalapu, and J.L. Liu: P-type ZnO films with solid-source phosphorus doping by molecular-beam epitaxy. Appl. Phys. Lett. 88, 052106 (2006).

FIG. 8. Optical energy band gaps of the ZnO and AlN–ZnO films annealed at 450

C.

FIG. 7. Optical transmission spectra of the ZnO and AlN–ZnO films

annealed at 450

C.

19. J.G. Lu, L.P. Zhu, Z.Z. Ye, Y.J. Zeng, F. Zhuge, B.H. Zhao, and D.W. Ma: Improved N–Al codoped p-type ZnO thin films by intro- duction of a homo-buffer layer. J. Cryst. Growth 274, 425 (2005).

20. Y.G. Wang, S.P. Lau, X.H. Zhang, H.W. Lee, H.H. Hng, and B.K. Tay: Observations of nitrogen-related photoluminescence bands from nitrogen-doped ZnO films. J. Cryst. Growth 252, 265 (2003).

21. Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, and B.K. Tay: Photo- luminescence study of ZnO films prepared by thermal oxidation of Zn metallic films in air. J. Appl. Phys. 94, 354 (2003).

22. S.A. Studenikin, N. Golego, and M. Cocivera: Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis. J. Appl. Phys. 84, 2287 (1998).

23. X.H. Pan, Z.Z. Ye, J.Y. Huang, Y.J. Zeng, H.P. He, X.Q. Gu, L.P. Zhu, and B.H. Zhao: P-type behavior in Li-doped Zn

Mg

O thin films. J. Cryst. Growth 310, 1029 (2008).

24. F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, and J.L. Liu:

Photoluminescence study of Sb-doped p-type ZnO films by mo- lecular-beam epitaxy. Appl. Phys. Lett. 87, 252102 (2005).

25. P.S. Chen, T.H. Lee, L.W. Lai, and C.T. Lee: Schottky mecha- nism for Ni/Au contact with chlorine-treated n-type GaN layer.

J. Appl. Phys. 101, 024507 (2007).

26. L.W. Lai and C.T. Lee: Investigation of optical and electrical properties of ZnO thin films. Mater. Chem. Phys. 110, 393 (2008).

27. T. Koida, S.F. Chichibu, A. Uedono, A. Tsukazaki, M. Kawasaki, T. Sota, Y. Segawa, and H. Koinuma: Correlation between the photoluminescence lifetime and defect density in bulk and epitax- ial ZnO. Appl. Phys. Lett. 82, 532 (2003).

28. J.G. Lu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, L. Wang, J. Yuan, B.H. Zhao, and Q.L. Liang: Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions. J. Appl. Phys.

100, 073714 (2006).

29. E. Burstein: Anomalous optical absorption limit in InSb. Phys.

Rev. 93, 632 (1954).

30. G. Yuan, Z. Ye, L. Zhu, Y. Zeng, J. Huang, Q. Qian, and J. Lu:

P-type conduction in Al–N co-doped ZnO films. Mater. Lett. 58,

3741 (2004).