Optoelectronic properties of wide bandgap semiconductor under the sub-bandgap optical excitation

The gallium nitride (GaN) and zinc oxide (ZnO) are typical wide bandgap semiconductors that have been intensively investigated in past several decades. They can be potentially used in a lot of applications such as blue light-emitting diodes, laser diodes, high-power and high-frequency transistors, and spintronics. Despite the intensive research, some fundamental properties of them are still remaining unclear and even controversial. For example, the identification of deep acceptors in GaN is currently challenging for both experimental and theoretical methods. In this thesis study, the optoelectronic properties of GaN and ZnO are investigated using photoluminescence under various sub-bandgap optical excitations. Some new insights into the optoelectronic properties of the materials are obtained for the first time, to the best of our knowledge. The main results and findings of this thesis study are summarized as below. Large negative thermal quenching (NTQ) of the yellow luminescence (YL) in GaN layer of InGaN/GaN quantum well (QW) samples are observed for the first time, due to the thermal migration of carriers from the InGaN QW layers to the GaN barrier layers. Such unusual phenomenon happens only when the carriers are optically excited inside the QW layers with laser having photon-energy slightly lower than the GaN bandgap, providing a solid evidence for the occurrence of thermal transfer of photo-excited carriers from the QW layers to the GaN barrier layers. A simple model considering the thermal transfer of carriers is proposed to interpret the observed NTQ phenomenon. The thermal activation energy of the carriers is determined by fitting the reciprocal temperature dependence of the YL intensity in Arrhenius plot based on the model. Further, using the YL NTQ phenomenon the binding energies of deep acceptors in n-type GaN are investigated. The YL NTQ is also observed in Si-doped and intentionally un-doped doped n-type GaN samples grown by metal organic chemical vapor deposition. To explain the phenomenon a model is provided by considering the acceptor or donor mediated and laser-pumped electron transition process from the valance to the conduction band. The model is capable of shielding the role of shallow donors in YL NTQ. By fitting the experimental YL NTQ of Si-doped samples, binding energy of responsible deep-acceptor Si substituting for N (SiN) was obtained as 283.3 meV. For un-doped GaN samples, effective binding energies of 397.7, 447.5, 619.5 and 1100.1 meV are obtained, and are attributed as Ga vacancy (VGa) related native deep-acceptors. To have a deep insight into the below the bandgap absorption coefficients and Urbach tail depth of ZnO, a novel approach of measuring the self-absorption (SA) effect on the two-photon luminescence (TPL) spectrum of the ZnO bulk crystal rod at cryogenic temperature is proposed. Under a geometric configuration of side-excitation and front-detection, the intensities of several major spectral components of TPL spectra of ZnO can be decisively tuned by precisely varying the transmitting distance of luminescence signal, so that the absorption coefficients at different wavelengths can be determined on the basis of Beer-Lambert law. Furthermore, the peak position of donor bound exciton luminescence exhibits a unique redshift tendency with increasing the transmitting distance. Starting from the product of Lorentzian line-shape function and exponential absorption edge of Urbach tail, an analytical formula is derived to quantitatively interpret the experimental redshift characteristic with the transmitting distance. The energy depth of Urbach tail of the studied ZnO crystal is deduced to be 13.3 meV. In principle, this new approach can be used to determine absorption coefficient of any luminescent solids as long as the SA effect happens. Finally using the large ZnO crystal rod the third order optical nonlinear effect of ZnO is investigated. With the help of visible green two-photon luminescence, two focusing points are observed on the propagation axis of a converging femtosecond laser beam in ZnO single crystal rod. Numerical calculations with a well-established theory show that the self-focusing effect makes a significant contribution to the formation of the first focusing point. The second focusing point is caused by self-refocusing, and its position is smaller than value predicted by a well-established model due to the laser power attenuation during the propagation in the ZnO crystal. The experimental results are highly consistent with the prediction of the self-focusing and refocusing model for the femtosecond laser filament propagation.

氮化镓（GaN）和氧化锌（ZnO）是典型的宽带隙半导体，在过去的几十年中得到了广泛的研究。它们可以潜在地用于许多应用，如蓝光发光二极管、激光二极管、大功率和高频晶体管以及自旋电子学。尽管进行了深入的研究，但它们的一些基本性质仍然不清楚，甚至存在争议。例如，GaN中深受主的识别目前对实验和理论方法都具有挑战性。本论文利用光致发光技术研究了GaN和ZnO在不同亚带隙光激发下的光电特性。据我们所知，首次获得了对材料光电特性的一些新见解。本文研究的主要结果和发现总结如下。 由于载流子从InGaN量子阱层向GaN势垒层的热迁移，首次在InGaN/GaN量子阱（QW）样品的GaN层中观察到黄色发光（YL）的大负热猝灭（NTQ）。只有当载流子在QW层内被光子能量略低于GaN带隙的激光光激发时，这种不寻常的现象才会发生，这为光激发载流子从QW层到GaN势垒层的热迁移提供了坚实的证据。提出了一个考虑载流子热迁移的简单模型来解释观测到的NTQ现象。基于该模型，通过拟合Arrhenius图中黄光发光强度对温度倒数的依赖关系，确定了载流子的热活化能。 此外，利用YL-NTQ现象研究了n型GaN中深受主的结合能。在金属有机化学气相沉积法生长的Si掺杂和非掺杂n型GaN样品中也观察到了YL-NTQ现象。为了解释这一现象，通过考虑受主或施主介导的结合激光泵浦的电子从价带到导带的跃迁过程，提出了一个理论模型。该模型能够弱化浅层施主杂质在NTQ中的作用。通过拟合Si掺杂样品的实验YL-NTQ数据，得到深层受主Si取代N（SiN）的结合能为283.3 mev。对于未掺杂的GaN样品，获得了397.7、447.5、619.5和1100.1 mev的有效结合能，并将其归因于与Ga空位（VGa）相关的本征深受主。 为了深入了解ZnO的亚带隙吸收系数和Urbach带尾深度，提出了一种低温下测量ZnO晶体的自吸收（SA）效应的新方法。在侧面双光子激发和正面探测的几何结构下，通过精确改变发光信号的传输距离，可以调整ZnO TPL光谱的几个主要光谱成分的强度，因此，可以根据比尔-朗伯定律确定不同波长的吸收系数。此外，施主束缚激子发光的峰值位置随着传输距离的增加呈现出独特的红移趋势。从洛伦兹线型函数与Urbach指数吸收边的乘积出发，导出了定量解释实验红移特性随传输距离变化的解析式。所研究的ZnO晶体的Urbach尾的能量深度为13.3 mev。原则上，只要发生SA效应，这种新方法就可以用来确定任何发光固体的吸收系数。 最后利用大尺寸ZnO晶体棒研究了ZnO的三阶非线性光学效应。利用可见绿光双光子发光技术，在飞秒激光束在ZnO单晶棒中的传输轴上观察到两个聚焦点。理论数值计算表明，自聚焦效应对第一聚焦点的形成起着重要作用。第二个焦点是由自聚焦引起的。实验结果与飞秒激光丝状传输的自聚焦和再聚焦模型的预测高度一致。