Exciton–polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO
Exciton–polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO
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DOI：
10.1088/0268-1242/20/4/009
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发表时间：
2005-04
影响因子：
1.9
通讯作者：
S. Chichibu;A. Uedono;A. Tsukazaki;T. Onuma;M. Zamfirescu;A. Ohtomo;A. Kavokin;G. Cantwell;C. Litton;T. Sota;Masashi Kawasaki
中科院分区：
文献类型：
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作者：
S. Chichibu;A. Uedono;A. Tsukazaki;T. Onuma;M. Zamfirescu;A. Ohtomo;A. Kavokin;G. Cantwell;C. Litton;T. Sota;Masashi Kawasaki
本文综述了ZnO晶体中激子的静态和动态响应，为利用半导体微腔中激子-极化激元的玻色凝聚实现相干单色光源极化激元激光器提供了理论依据。为了解决目前存在的问题，并为获得质量更好的ZnO外延层铺平道路，本文进行了以下四个方面的研究：（1）在8 K下记录了体相和外延层ZnO的偏振光反射谱（OR）、光反射谱（PR）和光致发光谱（PL）; PR共振的能量对应于上、下激子-极化激元分支的能量，其中A-、B-和C-激子同时耦合到电磁波。由于相应的极化激元分支的PL峰进行了观察。相应激子的纵向-横向分裂（ωLT）分别为1.5、11.1和13.1 meV。后两个值比GaAs的0.08 meV大两个数量级以上。(ii)使用这些值和材料参数，相应的真空场拉比分裂的激子极化激元耦合到一个模型MC模式计算为191毫电子伏，这是有史以来报道的最高值为半导体MC和满足要求，观察强激子光耦合制度所需的极化激元激光高于室温。(iii)测量了激光辅助分子束外延（L-MBE）生长的非极性ZnO外延层的偏振OR和PR谱，因为在非极性取向生长的ZnO量子威尔斯（QW）由于消除了垂直于QW平面的自发极化场和压电极化场而有望显示出更高的发射效率。根据各向异性应变纤锌矿材料的极化选择规则，它们表现出面内各向异性激子共振。(iv)结合正电子湮没测量结果，用时间分辨光致发光技术研究了L-MBE ZnO中点缺陷对无辐射过程的影响。室温下自由激子的PL强度随无辐射寿命（τnr）的增加而自然增加。在ScAlMgO 4衬底上生长的异质外延薄膜中，随着生长温度（Tg）的升高，τnr值增大，Zn空位（VZn）密度或尺寸减小，而使用同质外延衬底进一步降低了VZn密度。τnr值随带正、负电荷和中性点缺陷（包括复合物）总密度的减小而增大，而不随VZn密度的减小而增大。结果表明，非辐射复合过程不是由单点缺陷控制的，而是由VZn引入的某些缺陷，如VZn-缺陷复合物。由于在高Tg下生长薄膜，随后进行原位退火，消除了缺陷，结合使用高温退火的ZnO自缓冲层，在室温下获得了3.8 ns的自发辐射τnr。通过采用不断改进的外延生长方法，有望在不久的将来在室温下观察到极化激元激光效应。
Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton–polaritons in semiconductor microcavities (MCs). To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K. Energies of PR resonances corresponded to those of upper and lower exciton–polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave. PL peaks due to the corresponding polariton branches were observed. Longitudinal–transverse splittings (ωLT) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 meV. (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton–polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton–light coupling regime necessary for polariton lasing above room temperature. (iii) Polarized OR and PR spectra of an out-plane nonpolar ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane. They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material. (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement. Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr). The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density. The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes. As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 3.8 ns was obtained at room temperature. By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future.