Prospects

In the development of ZnO-based optoelectronic devices, it is necessary to synthesize both high-quality n- and p-type ZnO films. The n-type ZnO is easily available even without any doping; however the fabrication of p-type ZnO is difficult due to the self-compensation effect from native defects. Known acceptors in ZnO include group-I elements such as lithium (Li) [3–5], Na and K, copper (Cu) [6], silver (Ag) [7], Zn vacancies and group-V elements such as N, P and As [8]. It has been believed that the most promising dopants for p-type ZnO are the group-V elements, although theory suggests some difficulty in achieving a shallow acceptor level.

Recently, another p-type doping mechanism was proposed for group-V elements (P and As) [9]. P and As substitute Zn sites, forming a donor, then it induces two Zn-vacancy acceptors as complex form (PZn–2VZn or AsZn–2VZn) [10, 11]. However, the choice of p-type dopant and growth technique remains controversial and the reliability of p-type ZnO and the doping mechanism are still a subject of debate.

Recently, we found that as time elapsed the electric characteristics of impurity (Li and N) doped p-type ZnO films with hole carriers gradually changed to n-type with electron carriers, the same as that of intrinsic ZnO. The mechanism of this electrical transition is crucial to the fabrication of p-type ZnO layer and still unknown.

Therefore, it’s important to understand the mechanism driving the decay of

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hole-carriers, which is crucial for the growth of stable p-type ZnO layer.

Bose-Einstein condensation (BEC) of an ideal gas of bosons has been the subject of intense study in excitonic and atomic systems. Excitons and biexcitons are bosons at low densities. Ideal bosons exhibit a so-called Bose-Einstein condensation at sufficiently low temperature and high density. BEC is a macroscopic population of one state in k -space, generally k = 0. The appearance of an excitonic condensed phase in bulk crystals and quantum-well (QW) structures has been the major subject:

Experiments on Cu2O are considered to be quite promising [12-14]. In GaAs/AlAs coupled QW’s, an anomalous transport behavior of indirect excitons under high magnetic fields, which suggests excitonic superfluidity, was reported [15, 16]. A possibility of the condensation of weakly localized excitons in GaAs/Al

xGa

1-xAs double QW’s was experimentally proposed [17]. With the advent of semiconductor QWs, the possibility of observing the quantum statistics of bosons in two-dimensional systems has been raised. An interesting situation of Bose-Einstein statistics in a QW was reported by Kim and Wolfe [18: a two-component gas system of excitons and biexcitons. They showed theoretically and experimentally that a well-known square law of the density relation between excitons and biexcitons is modified by Bose-Einstein statistics. Assuming thermal equilibrium between excitons and biexcitons, there exists a situation, in which the equilibrium chemical potential (μ)

135

comes close to the biexciton energy per electron-hole pair E

BEX/2, i.e., E

BEX/2 - μ ≤ kT, where E

BEX/2, is lower than the exciton energy (E

EX) by a half of the biexciton binding energy. In such a situation, the biexciton density is governed by the strongly increasing part of the Bose-Einstein distribution function, while a saturation of the exciton density occurs, leading to the appearance of a threshold-like increase of the biexciton density. This behavior can be understood from Bose-Einstein statistics of the exciton-biexciton system. It was experimentally demonstrated from time-resolved PL spectra in the decay processes of excitons and biexcitons in a GaAs QW [18] and GaAs/AlAs superlattice [19] at a bath temperature of 2 and 5 K, respectively. Therefore, time-resolved PL measurements are capable of investigating BEC effect for the bosons. However, there are no reports to our knowledge on the phenomena with a BEC of exciton-biexciton in ZnO-based structures. The precise information of the exciton-biexciton density relationship at various excitation powers and bath temperatures estimated from the line-shape analysis of time-resolved PL spectra will be a long-term goal.

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References

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P. Ibbetson, S. Keller, S. DenBaars, J. S. Speck, and U. K. Mishara, MRS Internet J. Nitride Semicond. Res. 4, 2 (1999).

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Batchelor, and R. Davis, Appl. Phys. Lett. 75, 196 (1999).

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Cantwell, Appl. Phys. Lett. 81, 1830 (2002).

[9] M. Joseph, H. Tabata, H. Saeki, K. Ueda, and T. Kawai. Physica B 302–303 140 (2001).

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[12] D. W. Snoke, J. P. Wolfe, and A. Mysyrowicz, Phys. Rev. B 41, 11 171 (1990).

[13] E. Fortin, S. Fafard, and A. Mysyrowicz, Phys. Rev. Lett. 70, 3951 (1993).

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73, 304 (1994).

[16] L. V. Butov and A. I. Filin, Phys. Rev. B 58, 1980 (1998).

[17] E. S. Moskalenko, V. V. Krivolapchuk, and A. L. Zhmodikov, Fiz. Tverd. Tela (St.

Petersburg) 42, 1492 (2000) [Phys. Solid State 42, 1535 (2000).]

[18] J. C. Kim and J. P. Wolfe, Phys. Rev. B 57, 9861 (1998).

[19] H. Ichida and M. Nakayama, Phys. Rev. B 63, 195316 (2001).

劉維仁簡歷 (Vita)

基本資料

姓名: 劉 維 仁 (Wei-Rein Liu) 性別: 男

出生年月日: 1972 年 11 月 09 日 籍貫: 花蓮縣

永久通訊處: 桃園縣八德市建國路 471 巷 2 弄 2 號 Email: wereinliu.eo92g@nctu.edu.tw

liouforrest@hotmail.com 學歷:

1997.9-2001.6 國立成功大學材料科學及工程系 學士 2001.6-2003.9 國立交通大學光電工程研究所 碩士 2003.9-2009.2 國立交通大學光電工程研究所 博士 博士論文題目:

在氧化鋅磊晶薄膜物理特性中晶體缺陷結構的角色

The role of crystal defect structures in the physical properties of

ZnO epitaxial films

Publication list

I. Refereed Journal Publications:

1. W.-R. Liu, W.F. Hsieh, C.-H. Hsu, K.S. Liang, and F.S.-S. Chien, “Influence of the threading dislocations on the electrical properties in epitaxial ZnO thin films”

J. Cryst. Growth 297, 294-299 (2006).

2. W.-R. Liu, W. F. Hsieh, C.-H. Hsu,b, Keng S. Liang and F. S.-S. Chien,

“Threading dislocations in domain-matching epitaxial films of ZnO,” J. Appl.

Cryst. 40 924-930 (2007).

3. Song Yang, Hsu-Cheng Hsu, W.-R. Liu, Hsin-Min Cheng, and Wen-Feng Hsieh,

“Correlation between photoluminescence and varied growth pressure of well-aligned ZnO nanorods on fused silica substrate,” Optical Materials 30, 502-507 (2007).

4. W.-R. Liu, Y.-H. Li, W. F. Hsieh, C.-H. Hsu, W. C. Lee, M. Hongand J. Kwo,

“Correlation between crystal structure and photoluminescence for epitaxial ZnO on Si (111) using a γ-Al2O3 buffer layer,” J. Phys. D: Appl. Phys. 41, 065105-1 - 065105-5 (2008).

5. W.-R. Liu, Y.-H. Li, W. F. Hsieh, C.-H. Hsu, W. C. Lee, Y. J. Lee, M. Hong and J. Kwo, “Domain Matching Epitaxial Growth of High-Quality ZnO Film Using a Y₂O₃ Buffer Layer on Si (111),” Crystal Growth & Design 9,239-242(2009).

6. Jun-Rong Chen, Tien-Chang Lu, Yung-Chi Wu, Shiang-Chi Lin, Wei-Rein Liu, Wen-Feng Hsieh, Chien-Cheng Kuo, and Cheng-Chung Lee, “Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature,” Appl. Phys. Let. 94, 061103-1 - 061103-3 (2009).

7. Chia-Lung Tsai, Yow-Jon Lin, Yi-Min Chin, W-R Liu, W. F. Hsieh, C-H Hsu and Jian-An Chu, “Low-resistance nonalloyed ohmic contacts on undoped ZnO films grown by pulsed-laser deposition,” J. Phys. D: Appl. Phys. 42, 095108-1 - 065105-6 (2009)

II. Conference

1. W.-R. Liu, W. F. Hsieh, C.-H. Hsu, K. S. Liang, and F. S. -S. Chien, “Role of the threading dislocation on domain-epitaxially grown ZnO films using XRD and TEM,” Spring Meeting of Material Research Society (2006).

2. S.Y.Huang, W.-R. Liu, F. S. -S. Chien, C.-H. Hsu, K. S. Liang, and W. F. Hsieh,

“Correlated scanning capacitance and conductive atomic force microscopy studies of dislocation in ZnO film,” Spring Meeting of Material Research Society (2006).

3. F. S. -S. Chien, W.-R. Liu, C. C. Tsai, C. Y. Li, C. C. Hsu, C.-H. Hsu,C. S.

Chang, and C.-H. Hsu, “Effect of Dislocations on contact potential in epitaxial ZnO thin flims studies by electricostatic force microscopy,” Spring Meeting of Material Research Society (2006).

4. W.-R. Liu, Y.-H. Li,W. F. Hsieh, C.-H. Hsu, W. C. Lee, Y. J. Lee, M. Hong, and J. Kwo, “Y2O3 buffer layer for high-quality ZnO epitaxial growth on Si(111),”

International Conference on Solid State Devices and Materials, Japan, Tsukuba, Sep. (2008).