7.1 Conclusion
We have synthesized the MgZnO alloys with wurtzite structure for various Mg contents, which can be achieved simply by thermal diffusion and a sol-gel method.
The Mg compositions in MgxZn1-xO alloys were determined roughly by X-ray diffraction and photoluminescence spectroscopy. The relation between the lattice constants and the Mg concentration, i.e., the a-axis length increases while the c-axis length decreases with increasing incorporation concentration, usually is common in MgZnO systems. By adjusting the Mg compositions within the range of 0 ≤ x ≤ 0.14, the blueshift of near-band-edge emission and excitonic absorption demonstrated the band gap modulation of ZnO, achieving a band-gap enlargement of ~ 240 meV at room temperature in present work. The dependence of the lattice parameter and the near-band-edge emission on Mg incorporation indicated that Mg2+ is incorporated into the ZnO host lattice and occupies the lattice sites of Zn2+. Moreover, it is found that MgO has a solubility limit of above 10% in ZnO according to the present prepared approach, which coincides with that reported for MgxZn1-xO fabricated by a solution-based route.
Due to the differences in the ionic radii of Mg impurity and host ions, when Mg2+ ions occupy the Zn sites, some new lattice defects are introduced or intrinsic host-lattice defects become activated. As a result, some additional vibrational modes, characteristic of substitution-induced vibrations, appear in the Raman spectra. In addition to the host phonons of ZnO, we found two anomalous modes at 383 and 510 cm-1, which correlated with the Mg-related lattice vibrations. We suggested that the
383 cm-1 band is likely the local vibrational mode of Mg on substitutional O site in the ZnO lattice, and the 510 cm-1 band is presumably attributed to the host lattice defects.
One of the most important aspects of substitutional semiconductor alloys is the nature of the alloy potential fluctuations (APFs). The APFs destroy translational invariance, an effect that manifests itself as a breakdown of the usual q = 0 Raman selection rule, thus leading to the change of the Raman line shape. In Raman-scattering studies, the microscopic nature of the substitutional disorder is discussed by analyzing the compositional dependence of the E2 (high) phonon mode in MgxZn1-xO submicropowders. It is shown that the Raman spectral broadening and asymmetry induced by the APFs can be quantitatively explained in terms of the modified spatial correlation model that considers the grain size distribution. We inferred that the APFs cause a decrease in grain size with increasing Mg concentration.
Especially for x > 10%, the grain size diminishes as a result of the random distribution of aggregated Mg2+ ions in ZnO crystal lattice to form MgO clusters or nanocrystals, which partition a larger MgZnO crystal into subcrystals
Additionally, we reported a detailed investigation on the formation of biexciton assisted by acoustic and optical phonon scattering in ZnO powders. Other than the proposed exponent comes near theoretical value of 2, we observed a sublinear dependence on excitation power for biexciton at low temperature (≤ 40 K). It can be considered that the exciton kinetic energy is inefficient to couple with optical phonons but lowering the kinetic energy of the excitonic gas by emitting acoustic phonons.
Due to the small energy quanta of the acoustic phonons, the dissipation of the kinetic energy in the excitonic system is rather slow and requires many scattering events before quasi-equilibrium is reached. By elevating the temperature so that the kinetic energy of excitons reaches the energy of the lowest optical phonon, near 12 meV in
this case, the optical phonon will participate in the exciton relaxation process. The efficient cooling of exciton with assistance of optical phonon scattering allows effectively bounding exciton pairs to form biexcitons at high temperature (≥ 80 K).
Furthermore, the inelastic scattering between excitons with assistance of optical phonons ionizes one of the scattered excitons to n = ∞ state rather than n = 2 state and efficiently cools the other exciton to the lower polariton branch so that high P∞
emission was observed for high temperature. Contrarily, without assistance of optical phonon, high P2 emission was found at low temperature.
Besides, the interaction between exciton and phonon has significant influence on the optical properties of semiconductors, such as the energy relaxation rate of excited carriers and phonon replicas of excitons in the luminescence spectra. As is well known, polar semiconductors experience a strong Fröhlich interaction that gives rise to exciton-longitudinal optical (LO)-phonon interaction. It is found that the exciton binding energy of MgxZn1-xO (0 ≤ x ≤ 0.05) powders raises with increasing Mg substitution by fitting the temperature-dependent photoluminescence spectra. It was implied that exciton localization takes place and that the degree of localization increases with increasing x. The essential origin of the localization is thought to be the spatial fluctuation of the local composition of Mg in the alloys, which results in the spatial fluctuation of the potential energy for the excitons. The strength of exciton-LO-phonon coupling was deduced from the energy shift of exciton emission with temperature variation. The reduction of the coupling strength of exciton-LO-phonon may originate from a diminution in the exciton Bohr radius making the exciton less polar, which could be explained by the dopant-induced increase of the exciton binding energy.
7.2 Prospective
Due to numerous unique properties expected in the low-dimensional system, nanometer-scale materials promise to be important in the next-generation optoelectronic devices. Low-dimensional ZnO nanostructures, such as quantum dots (QDs) [1,2], nanoparticles (NPs) [3,4], nanobelts [5], nanowires [6], and quantum wells [7], have been widely investigated for the feasible requirement. In particular, ZnO QDs and NPs are of great interest because of the three-dimensional confinement of carrier and phonon leads not only continuous tuning of the optoelectronic properties but also improvement in device performance. Nevertheless, the surface of QDs is usually composed of uncoordinated atoms, which make the QDs highly active and quench the PL emission. Recently, Demangeot et al. [4] have reported the low-temperature PL from ZnO NPs with different particle sizes which were synthesized by a RT organometallic method. The study showed no shift from the low-temperature PL measurement indicated that UV emission was most likely dominated by weakly bound localized defects, which could come from the surface-bound ionized acceptor-exciton complexes, rather than the size-dependent quantum confinement effect. It is therefore important to note that the nanocrystals synthesized by chemical methods indeed occasionally cause the product suffering the active surround, which could intensely transform the intrinsic properties of the core.
Accordingly, the demand for surface passivation of the NPs and the QDs is significant.
Overcoating the QDs with higher bandgap inorganic materials has been shown to improve the luminescence by suppressing surface nonradiative recombination centers.
In these regards, considerable researches have been devoted to the core-shell structure QDs, including CdSe/ZnSe [8], InAs/CdSe [9], Si/SiO2 [10], and PbSe/PbS QDs [11].
It is known that the band gap of ZnO can be tuned by alloying with MgO; we have
discussed the detailed characteristics of ZnO:Mg alloys in this dissertation.
Therefore, similar efforts for the ZnMgO QDs and ZnO/MgO core-shell QDs are strongly demanded from both the fundamental scientific research and photonic application points of view.
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 BEC 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/AlxGa1-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) comes close to the biexciton energy per electron-hole pair EBEX/2, i.e., EBEX/2 - μ ≤ kT, where EBEX/2 is lower than the exciton energy (EEX) 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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潘晴如簡歷 (Vita)
基本資料
姓名：潘 晴 如 (Ching-Ju Pan) 性別：女
出生年月日： 1977 年 02 月 13 日 籍貫: 高雄市
永久通訊處：(804) 高雄市鼓山區大順一路 432 巷 46 號 email:chingju.eo90g@nctu.edu.tw;
chingjupan@yahoo.com.tw.
學歷：
1995.9 – 1999.6 私立中原大學物理系 學士 1999.9 – 2001.6 國立交通大學電物所 碩士 2002.9 – 2008.1 國立交通大學光電所 博士
博士論文題目 :
掺鎂氧化鋅材料之激子–聲子交互作用研究
Influence of phonons on excitonic characteristics in
Mg-incorporated ZnO
Publication list
I. Refereed Journal Publications:
Ching-Ju Pan
1. , Kuo-Feng Lin, Wei-Tse Hsu, and Wen-Feng Hsieh, “Reducing exciton-longitudinal optical phonon coupling with increasing Mg incorporation in MgZnO powders”, Journal of Applied Physics, 102, 123504 (2007).
Ching-Ju Pan
2. , Kuo-Feng Lin, and Wen-Feng Hsieh, “Acoustic and optical phonon assisted formation of biexcitons”, Applied Physics Letters, 91, 11907 (2007).
Ching-Ju Pan
3. , Kuo-Feng Lin, Wei-Tse Hsu, and Wen-Feng Hsieh, “Raman study of alloy potential fluctuations in MgxZn1−xO nanopowders”, Journal of Physics-Condensed Matter, 19, 186201 (2007).
Ching-Ju Pan
4. , Hsu-Cheng Hsu, Hsin-Ming Cheng, Chun-Yi Wu, and Wen-Feng Hsieh, “Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires”, Journal of Solid State Chemistry, 180, 1188 (2007).
II. Conference:
Ching-Ju Pan
1. , Kuo-Feng Lin, Wei-Tse Hsu and Wen-Feng Hsieh, “Abnormal phonon modes and alloy potential fluctuations in MgxZn1−xO nanostructures”, in Conference of Year 2007 Annual Meeting of Chinese Physical Society, Chunli, TAIWAN, oral paper (2007).
Ching-Ju Pan
2. , Hsu-Cheng Hsu, and Wen-Feng Hsieh, “Stimulated emission of ZnMgO sponges formed by simple diffusion MgO capped ZnO nanowires”, in Proceedings of Optics and Photonics Taiwan'05, Tainan, TAIWAN, oral paper (2005).
Ching-Ju Pan
3. , Wei-Hao Chiu, Hsin-Min Cheng, Hsu-Cheng Hsu, Chun-Yi Wu, and Wen-Feng Hsieh, “Artificial Structure Color”, the design image for the cover competition of Materials Today (2005).
Ching-Ju Pan
4. , Hsu-Cheng Hsu, and Wen-Feng Hsieh, “Structural effect on optical properties of ZnO inverse opal photonic crystals”, Taiwan Nano Tech, post paper (2005).