Intrinsic p -type ZnO films fabricated by atmospheric pressure metal organic chemical
vapor deposition
Yen-Chin Huang, Zhen-Yu Li, Li-Wei Weng, Wu-Yih Uen, Shan-Ming Lan, Sen-Mao Liao, Tai-Yuan Lin, Yu-Hsiang Huang, Jian-Wen Chen, and Tsun-Neng Yang
Citation: Journal of Vacuum Science & Technology A 28, 1307 (2010); doi: 10.1116/1.3484138 View online: http://dx.doi.org/10.1116/1.3484138
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/28/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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organic chemical vapor deposition
Yen-Chin Huanga兲
Department of Electronic Engineering, College of Electrical Engineering and Computer Science, Chung Yuan Christian University, Chung-Li 32023, Taiwan
Zhen-Yu Li
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 TA Hsueh Road, Hsinchu 30010, Taiwan
Li-Wei Weng, Wu-Yih Uen,b兲Shan-Ming Lan, and Sen-Mao Liao
Department of Electronic Engineering, College of Electrical Engineering and Computer Science, Chung Yuan Christian University, Chung-Li 32023, Taiwan
Tai-Yuan Lin
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 222, Taiwan
Yu-Hsiang Huang, Jian-Wen Chen, and Tsun-Neng Yang
Institute of Nuclear Energy Research, P.O. Box 3-11, Lungtan 32500, Taiwan
共Received 5 February 2010; accepted 9 August 2010; published 23 September 2010兲
The structural, electrical, and optical properties of ZnO films fabricated by atmospheric pressure metal organic chemical vapor deposition 共AP-MOCVD兲 under various gas flow ratios of 关H2O兴/关DEZn兴 共VI/II ratio兲 ranging from 0.55 to 2.74 were systematically examined. Hall effect
measurements exhibited an evident effect of the VI/II ratio on the conduction type of the intrinsic films. An n-type film was fabricated at the VI/II ratio= 0.55; however, p-type ZnO films with the hole concentration of the order of 1017 cm−3 could be achieved at VI/II ratios higher than 1.0. In particular, the highest mobility of 91.6 cm2/V s and the lowest resistivity of 0.369 ⍀ cm have been achieved for the specimen fabricated at the VI/II ratio= 1.10. Moreover, room-temperature photoluminescence共PL兲 measurements demonstrated an interstitial Zn 共Zni兲 donor defect related
emission at 2.9 eV for the n-type film, while a Zn vacancy共VZn兲 acceptor defect related one at 3.09
eV for the p-type films. The existence of material intrinsic defects was further confirmed by low temperature PL measurements conducted at 10 K. Conclusively, the conduction type of undoped ZnO films deposited by AP-MOCVD is resolved by the VI/II ratio used, which causes the formation of various kinds of intrinsic defects, Zni otherwise VZn. p-type ZnO films with the hole
concentration in the range of共1.5–3.3兲⫻1017 cm−3 can be achieved with good reproducibility by modulating a VI/II ratio the range 1.0–2.2 for the AP-MOCVD process. © 2010 American Vacuum Society. 关DOI: 10.1116/1.3484138兴
I. INTRODUCTION
Zinc oxide共ZnO兲 is a wide band gap semiconductor with a direct band gap of 3.37 eV at room temperature and a large exciton bind energy of 60 meV, which makes it a good can-didate for applications in highly efficient and stable room-temperature ultraviolet lasers and light emitting diodes.1–3To achieve such goals, the growth of high-quality p-type ZnO is necessary. However, the fabrication of p-type ZnO films by doping is difficult due to the compensation effect of native n-type carriers released by the donor-type defects such as oxygen vacancies and zinc interstitials.4,5
Recently, several groups have reported the growth of p-type ZnO by doping group V elements: As,6 P,7 and N,8,9
but an explicit mechanism justifying these results is still lacking and the issue of reproducibility becomes the bottle-neck in the development of related devices. In addition, some groups showed intrinsic p-type behavior in the ZnO films fabricated by different methods, such as pulsed laser deposition,10,11magnetron sputtering,12,13low pressure metal organic chemical vapor deposition,14 plasma-assisted low pressure metal organic chemical vapor deposition,15 and in-ductively coupled plasma enhanced chemical vapor deposition.16 For all the intrinsic films mentioned in Refs.
10–16, the conduction type was changed from n-type to p-type by varying the oxygen pressure in the growth environment10,12,14,16 or by conducting the film growth or postannealing at suitable temperatures11,13,15 in O2 environ-ment.
In this work, we report on the fabrication of intrinsic p-type ZnO films by atmospheric pressure metal organic chemical vapor deposition 共AP-MOCVD兲. We examine the effect of gas flow ratio of关H2O兴/关DEZn兴 on the conduction
a兲Electronic mail: [email protected]
b兲Author to whom correspondence should be addressed; electronic mail: [email protected]
type of the ZnO films by comparing their electrical and op-tical properties systemaop-tically and by analyzing the possible mechanisms responsible for the results.
II. EXPERIMENT
Undoped ZnO thin films were deposited using a custom-made, one-flow AP-MOCVD system. The substrates are 2 in. 共111兲-oriented, p-type Si wafers with a resistivity of 1–3 k⍀ cm. The growth chamber is a water-cooled vertical reactor. The substrate susceptor is made of graphite, 2 in. in diameter and coated with a SiC film on the top surface by a chemical vapor deposition technique. Diethylzinc 共DEZn兲 and de-ionized water共H2O兲 were used as the sources of Zn
and O, respectively, and N2was used as the carrier gas. The
growth of ZnO layer was conducted at 400 ° C with the gas flow ratio of关H2O兴/关DEZn兴 共VI/II ratio兲 being varied from
0.55 to 2.74. The growth time duration for all specimens was set for 40 min to achieve a thickness of about 500–600 nm, recognized by their cross-sectional scanning electron micros-copy共SEM兲 images.
The crystal structure of the ZnO thin films was analyzed by powder X-ray diffraction using Cu K␣ line as the x-ray source 共=1.540 56 Å兲. Electrical resistivity, carrier con-centration, and mobility of the ZnO thin films were evaluated by Hall effect measurements performed at room temperature using the four-probe van der Pauw configuration. The optical properties were examined by photoluminescence共PL兲 mea-surements performed at room temperature and 12 K. PL spectra were excited by the 325 nm line of a He–Cd laser. III. RESULTS AND DISCUSSION
All the films produced were found to be of polycrystalline wurtzite structure with a dominant grain orientation of共101兲 regardless of the various VI/II ratios used for film deposition. However, there were evident effects of VI/II ratio on the electrical and optical properties of film as described below.
Before the Hall measurement, the current-voltage 共I-V兲 characteristics of our specimens had been measured with two Ohmic contacts formed on the top ZnO film and the bottom p-Si substrate, respectively, to gain initiative knowledge of the conduction type of ZnO films fabricated. Figure 1 dis-plays the I-V characteristics of the specimens examined and the inset in the lower right shows the schematic of the mea-surement structure. As shown, a rectifying behavior is found only for the specimen fabricated with the VI/II ratio of 0.55, demonstrating an n-type conductivity of the deposited film. However, nonrectifying behavior is observed for the speci-mens fabricated with the VI/II ratio equal to and higher than 1.1. On the same figure, the I-V characteristics of the p-Si substrate are also displayed for a reference. Obviously, the p-type conductivity has been achieved for the specimens fab-ricated with the VI/II ratio higher than 1.1. Note that all the nonrectifying I-V characteristics except that from p-Si sub-strate demonsub-strate a slight deviation from a straight line, manifesting that a weakly conducting heterojunction has been formed between the ZnO film and Si substrate. Hall effect measurements were then carried out to characterize in
more detail the electrical properties of the films deposited at various VI/II ratios and the results are depicted in Fig. 2. Generally, the p-type substrate used would influence the Hall measurement of the p-type deposited film. However, this in-fluence seems negligible here for the relatively low resistiv-ities obtained in Fig.2, probably because of the presence of the weakly conducting heterojunction mentioned above. The intrinsic p-type conductivity of ZnO was initially found in 1992 by Butkhuzi et al.17and confirmed later by some other groups.12–16All these findings have been completed through the adjustment of oxygen content in the growth/heat-treatment environment, leading to the conversion of conduc-tion type. As is known, the n-type conductivity for ZnO can readily be achieved if the intrinsic 共undoped兲 materials are deposited under zinc-rich conditions, or if they are extrinsi-cally doped with group-III elements such as Al and Ga. However, it also depends on the deposition temperature and/or VI/II ratio for the ZnO films prepared. It is quite difficult for the undoped films to have definite p- or n-type conductivity. Usually, undoped ZnO films show n-type elec-trical properties due to the native point defects of oxygen vacancies共VO兲 and Zn interstitials 共Zni兲, which may be
pro-duced due to a low oxygen content in the film deposition
-10 -5 0 5 10 -0.002 -0.001 0.000 0.001 0.002
Voltage (V)
p-Si sub only [H2O]/[DEZ] ratio 0.55 1.1 1.64 2.19 2.74 ZnO contact contact p-Si(111)
FIG. 1. 共Color online兲 I-V characteristics taken from the Ohmic contacts to p-Si共111兲 and ZnO films produced with different VI/II ratios.
0.4 0.6 0.8 1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.1 1 10 100 [H2O]/[DEZn] ratio Carrier concentration (x10 1 7 cm -3 ) 0.1 1 10 100 Resistivity (ohm-cm) Mobility (cm 2 /V-s) n n type p type µ ρ
FIG. 2.共Color online兲 Carrier concentration, mobility, and resistivity of ZnO films deposited at various VI/II ratios.
1308 Huang et al.: Intrinsic p-type ZnO films fabricated 1308
process. On the contrary, the intrinsic p-type conductivity of ZnO films can be expected for the film deposition to be conducted in an oxygen-rich condition because the sufficient content of oxygen included might produce native Zn vacan-cies共VZn兲, which would act as acceptors. It should be men-tioned that the p-type doping effect of nitrogen originated from the deposition atmosphere of N2 can be excluded
ac-cording to the results of the specimen with the lowest VI/II ratio共0.55兲 and our previous study on glass substrate.18It is therefore considered that both VOand Zniare the main
na-tive donor defects for the ZnO films deposited at VI/II ratio= 0.55, which consequently featured the n-type conduc-tivity for the ZnO films deposited. On the other hand, when the VI/II ratio was increased to higher than 1.0, namely, an oxygen-rich condition for the film deposition, the ZnO film converted its conduction to p type due to an increase in the concentration of the acceptor type defect VZn. The increase
of hole concentration from 1.85⫻1017 to 3.33⫻1017 cm−3
for the films deposited at the VI/II ratios varying from 1.10 to 2.19 might suggest that more and more VZndefects have
been produced in the films fabricated. It should be noted that the resistivity of the films produced at VI/II ratio = 1.10– 2.19 also increases from 0.369 to 0.491 ⍀ cm. This variation tendency is considered to be dominated by the Hall mobility, which decreases from 91.6 to 38.2 cm2V−1s−1 with increasing the VI/II ratio from 1.10 to 2.19. However, the hole concentration of the film deposited at the highest VI/II ratio of 2.74 decreased. This is probably because the introduction of too many excess oxygen atoms might cause the formation of some complex defects involving VZn, the divacancy, VZnVO, and the O-related defects, which either
reduce the concentration of VZn itself or behave as a deep
donor to compensate the effect induced by VZn.19
Figure 3共a兲 displays the room-temperature PL spectra of ZnO films with the PL intensity being normalized. Two emis-sion lines are observable in the PL spectrum of the ZnO film fabricated at VI/II ratio= 0.55. The near band edge emission at 3.33 eV is ascribed to free exciton recombination20and the deep level emission band at 2.9 eV is attributed to Zni.
21
This suggests that donors were formed in the ZnO film, as dem-onstrated in the Hall effect measurement above, an n-type conductive behavior. However, the PL spectra of the speci-mens fabricated with the VI/II ratio over 1.0 changed evi-dently as indicated in the same figure. The deep level emis-sion was absent and a much sharper band with the energy ranging from 3.0 to 3.5 eV appeared instead. The lumines-cence peak energies involved in this emission band could be assigned after one of the spectra共VI/II ratio=2.19兲 was con-voluted with Gaussian fittings. As illustrated in Fig. 4共b兲, three emission lines are resolved at around 3.28, 3.25, and 3.09 eV, which signify the emissions from free exciton,22 free electron neutral acceptors,22and VZn,14respectively. The
presence of the weak VZn-related peak on the low-energy
side of the PL spectra might suggest that VZndefects exist in
the ZnO films fabricated with the VI/II ratio over 1.0, which played the role of acceptors and resulted in a compensation
effect for electrons. This result is consistent with what have been obtained from the Hall effect measurements described above.
To investigate in some more detail the influence of VI/II ratio on the optical properties of the films fabricated, low temperature PL measurements were performed at 12 K. As demonstrated in Fig. 4共a兲, the PL spectrum of the ZnO film deposited at the VI/II ratio= 0.55 is dominated by a broad line at 3.368 eV, probably due to the high concentration of electron concentration 共⬎1018 cm−3兲. This emission was
re-ported to feature a neutral donor bound exciton 共D0X兲 emission.23In addition to the D0X line, there are also emis-sion lines related to the two-electron satellite 共TES兲 transi-tions of D0X and the donor-acceptor-pair 共DAP兲 transition peaking at 3.332 eV24 and 3.230 eV,25 respectively. In the effective-mass approximation, the difference between the en-ergy of D0X line共ED0X兲 and that of TES related emission
共ETES兲 can be used to obtain the donor ionization energy
共ED兲 from ED= 4/3共ED0X− ETES兲.23 Using this equation, the
EDwas estimated to be 48 meV, which is close to the energy
level presented by donor energy Zni共50 meV兲.26 Therefore,
the ZnO film fabricated at the VI/II ratio= 0.55 exhibits n-type conductivity originated from Znidonor-type defects,
as regularly recognized. On the other hand, for the films deposited at the VI/II ratios ranging from 1.0 and 2.19, the
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 0.0 0.2 0.4 0.6 0.8 1.0 [H2O]/[DEZ] ratio 0.55 1.10 1.64 2.19 2.74 No rma li ze d PL intens ity
Photon Energy (eV)
2.90 3.09 3.33 3.28 3.25 (a) 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 0 10000 20000 30000 40000 50000 60000 PL intensity (a. u.)
Photon energy (eV) 3.284 3.252
3.09
(b)
FIG. 3. 共Color online兲 共a兲 Room-temperature PL spectra from ZnO films fabricated at various VI/II ratios.共b兲 Gaussian fitting to the PL emission of ZnO film produced at the VI/II ratio= 2.19.
PL spectra 关Figs.4共b兲and4共c兲兴 were dominated by the line at 3.350–3.355 eV, characteristic of the exciton bound to neutral acceptor 共A0X兲 emission.23,25,27,28 In addition, the peak situated at 3.306–3.322 eV is assigned to the free elec-tron to neutral acceptor 共eA0兲 transition.25,29 The line at
3.249–3.234 eV is considered to be due to the DAP transition.23,25,29Also, the line at 3.180–3.164 eV is assigned to the longitudinal optical共LO兲 phonon replica of DAP tran-sition since a deviation between them is about 69–70 meV, approximating the energy of a LO phonon for ZnO共about 72 meV兲.23,29
Moreover, it can be seen that when the VI/II ratio= 2.19, the intensity of the A0X line comes to a maxi-mum and the Hall measurement also demonstrates a highest hole concentration. Then, both the A0X emission line inten-sity and hole concentration decrease once the VI/II ratio is increased to 2.74. On the other hand, the eA0line intensifies all the way with the VI/II ratio and ultimately even surpasses the A0X line. The results described above might manifest that modulating the VI/II ratio to an oxygen-rich condition for the AP-MOCVD process would essentially produce large quantities of native defect VZn, and VZn-related acceptors
consequently result in the p-type conductivity of the undoped ZnO films. However, too heavily the excess oxygen atoms introduced into the ZnO film might somewhat cause oxygen interstitial atoms, oxygen antisite atoms beside the VZn.
These defects may weaken the interaction between the free excitons and the neutral acceptors, and also possibly form some complex defects as described above, which therefore impede a further elevation of hole concentration by VI/II ratio. Nevertheless, there are still more efforts required to
clarify the details of defect structures and their connections to the VI/II ratio used for the ZnO deposition.
IV. CONCLUSIONS
The effects of VI/II ratio on the conduction type of un-doped ZnO films fabricated by AP-MOCVD have been in-vestigated. The ZnO film fabricated at the VI/II ratio= 0.55 exhibits n-type conductivity. However, the ZnO film fabri-cated at a VI/II ratio over 1.0 demonstrates p-type conduc-tivity. In particular, an intrinsic p-type ZnO film with the highest mobility of 91.6 cm2/V s, the lowest resistivity of
0.369 ⍀ cm, and a hole concentration of 1.85⫻1017 cm−3
have been achieved at the VI/II ratio= 1.10. The origin of the conversion of conduction type for ZnO films produced was confirmed by PL measurements. Room-temperature PL mea-surements demonstrated a Znidonor defect related emission
at 2.9 eV for the n-type film, while a VZn acceptor defect
related emission at 3.09 eV for the p-type films. Moreover, the low temperature measurements performed at 10 K exhib-ited that the PL spectrum of the n-type film was dominated by the neutral donor bound exciton emission at 3.368 eV while those of p-type films were dominated by the neutral acceptor bound exciton emissions at 3.350–3.355 eV. Con-clusively, the conduction type of undoped ZnO films depos-ited by AP-MOCVD is resolved by the VI/II ratio used, which causes the formation of various kinds of intrinsic de-fects, Zniotherwise VZn. Intrinsic p-type ZnO films with the
hole concentration in the range of共1.5–3.3兲⫻1017 cm−3can
be fabricated by AP-MOCVD by modulating the VI/II ratio in the range 1.0–2.2.
ACKNOWLEDGMENTS
The authors earnestly appreciate the Institute of Nuclear Energy Research共INER兲 for all the technical assistance con-cerned with this work. The authors are also grateful to the National Science Council of Taiwan, for financially support-ing the research under Contract Nos. NSC 99-2221-E-033-030 and NSC 99-2632-E-033-001-MY3.
1X. L. Guo, J. H. Choi, H. Tabata, and T. Kawai, Jpn. J. Appl. Phys., Part 2 40, L177共2001兲.
2A. Tsukazaki, M. Kubota, A. Ohtomo, T. Ohura, K. Ohtani, H. Ohno, S. F. Chichibu, and M. Kawasaki, Jpn. J. Appl. Phys., Part 2 44, L643 共2005兲.
3J. D. Ye, S. L. Gu, S. M. Zhu, W. Liu, S. M. Liu, R. Zhang, Y. Shi, and Y. D. Zheng, Appl. Phys. Lett. 88, 182112共2006兲.
4S. B. Zhang, S. H. Wei, and A. Zunger, Phys. Rev. B 63, 075205共2001兲. 5F. Oba, S. Nishitani, S. Isotani, H. Adachi, and I. Tanaka, J. Appl. Phys.
90, 824共2001兲.
6Y. R. Ryu, S. Zhu, D. C. Look, J. M. Wrobel, H. M. Jeong, and H. W. White, J. Cryst. Growth 216, 330共2000兲.
7K. K. Kim, H. S. Kim, D. K. Hwang, J. H. Hong, and S. J. Park, Appl. Phys. Lett. 83, 63共2003兲.
8D. C. Look, D. C. Reynolds, C. W. Litton, R. L. Jones, D. B. Eason, and G. Cantwell, Appl. Phys. Lett. 81, 1830共2002兲.
9W. Z. Xu, Z. Z. Ye, T. Zhou, B. H. Zhao, L. P. Zhu, and J. Y. Huang, J. Cryst. Growth 265, 133共2004兲.
10M. S. Oh, S. H. Kim, and T. Y. Seong, Appl. Phys. Lett. 87, 122103 共2005兲.
11N. Brilis, D. Tsamakis, H. Ali, S. Krishnamoorthy, and A. A. Iliadis, Thin Solid Films 516, 4226共2008兲. 2.8 3.0 3.2 3.4 3.6 3.8
P h o to n en erg y (eV )
3.368 3 .332 3.2 30 3 .355 3.3 22 3.3 53 3.3 18 3.249 3 .180 3.3 51 3.3 09 3.243 3.1 73 3.35 0 3 .306 3.23 4 3 .16 4 / ratio=0.55 / ratio=1.10 / ratio=1.64 / ratio=2.19 / ratio=2.74 (a) (b) (c) (d) (e)FIG. 4. 12 K PL spectra of undoped ZnO films deposited with different VI/II ratios.
1310 Huang et al.: Intrinsic p-type ZnO films fabricated 1310
12G. Xiong, J. Wilkinson, B. Mischuck, S. Tuzemen, K. B. Ucer, and R. T. Williams, Appl. Phys. Lett. 80, 1195共2002兲.
13G. Z. Xing, B. Yao, C. X. Cong, T. Yang, Y. P. Xie, B. H. Li, and D. Z. Shen, J. Alloys Compd. 457, 36共2008兲.
14Y. Ma, G. T. Du, S. R. Yang, Z. T. Li, B. J. Zhao, X. T. Yang, T. P. Yang, Y. T. Zhang, and D. L. Liu, J. Appl. Phys. 95, 6268共2004兲.
15Y. J. Zeng, Z. Z. Ye, W. Z. Xu, J. G. Lu, H. P. He, L. P. Zhu, and B. H. Zhao, Appl. Phys. Lett. 88, 262103共2006兲.
16K. H. Nam, H. Kim, H. Y. Lee, D. H. Han, and J. J. Lee, Surf. Coat. Technol. 202, 5463共2008兲.
17T. V. Butkhuzi, A. V. Bureyev, A. N. Georgobiani, N. P. Kekelidze, and T. G. Hhulordava, J. Cryst. Growth 117, 366共1992兲.
18Y. C. Huang et al., Thin Solid Films 517, 5537共2009兲.
19J. Zhong, A. H. Kitai, P. Mascher, and W. Puff, J. Electrochem. Soc. 140, 3644共1993兲.
20B. Zhen, Z. Jingwen, Y. Xiaodong, W. Dong, Z. Xin, Z. Weifeng, and H.
Xun, J. Cryst. Growth 303, 407共2007兲.
21B. Lin, Z. Fu, and Y. Jia, Appl. Phys. Lett. 79, 943共2001兲.
22B. P. Zhang, N. T. Binh, Y. Segawa, Y. Kashiwaba, and K. Haga, Appl. Phys. Lett. 84, 586共2004兲.
23A. Teke, Ü. Özgür, S. Doğan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, and H. O. Everitt, Phys. Rev. B 70, 195207共2004兲.
24D. C. Reynolds, D. C. Look, and B. Jogai, Phys. Rev. B 57, 12151 共1998兲.
25Y. R. Ryu, T. S. Lee, and H. W. White, Appl. Phys. Lett. 83, 87共2003兲. 26D. C. Look, J. W. Hemsky, and J. R. Sizelove, Phys. Rev. Lett. 82, 2552
共1999兲.
27D. C. Kim, B. H. Kong, and H. K. Cho, J. Phys. D 42, 065406共2009兲. 28S. A. Studenikin, M. Cocivera, W. Kellner, and H. Pascher, J. Lumin. 91,
223共2000兲.
