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

Abstract

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.