Development of a cathodoluminescence system for TEM and its applications to the investigation of bandgap-transition characteristics of GaN and MoS2

Abstract

Cathodoluminescence (CL) is an optical and an electrical phenomenon arising from interactions between accelerated electrons and materials. When the highly energetic electrons exposed on a luminescent material, the primary electrons excite the electrons in the matter, and the excited electrons release its excess energy as photons in accordance with their internal transitions. The CL spectroscopy provides invaluable information on electronic structure such as bandgap energy, excitonic states, impurity-, and dopant-levels. In this thesis, the developments of the CL system for transmission electron microscopy (TEM) and its applications to semiconductor research were presented. Firstly, it was developed that the novel TEM-CL holder with a retractable light-collecting optics, which is composed of miniaturized optical components and integrated on head part of the holder. By moving the whole optics isolated from built-in cryostat, large CL observation area and successive measurement of EDS signal were guaranteed while maintaining accurate alignment of the optics. Performance and applicability of the system were demonstrated by analyzing ZnO nanowires. Secondly, the TEM-CL analysis results of polar (0001) GaN light-emitting diodes (LEDs) were discussed. The luminescence properties of V-shape pits, which are the characteristic defects in c-plane InGaN/GaN quantum well structures, were directly correlated with its microstructure. It was clearly revealed that potential barriers, formed by discontinuous QWs around the hexagonal pit, can effectively block the diffusion of carriers into the {10-11}-faceted quantum well structures (QWs) and threading dislocations (TDs), which in turn increases the effective carrier density usable for radiative recombination. Thirdly, the emission properties of the semi-polar (11-22)-GaN LEDs were analyzed. The CL maps clearly identify the type of the extended defects and visualize their distribution at a glance. In addition, it was directly observed that the influence of substrate pattern on the defect reduction. The characteristic surface undulations of the semi-polar (11-22) GaN epi were also investigated. It was clearly observed that the undulations are also present in QWs composed of the flat (11-22) facets and the sidewall {01-11} facets. The high In-incorporation efficiency of the sidewall {01-11} facets caused high In composition and long wavelength emission, resulting in non-uniform luminescence and anisotropic emission of LED. It is expected that this result will assist the analysis on the characteristics of the semi-polar GaN epi and the development of the high efficiency GaN LEDs. Lastly, the luminescence properties of few-layered MoS2 were investigated with the optical excitation and the electron beam excitation. As a result, it was observed the peak A and B due to direct excitonic transitions at K point in both photoluminescence (PL) and CL. The large peak C, which is more close to the quasiparticle bandgap energy, is appeared only in CL. Based on previously reported calculation works, the peak C may correspond to nearly degenerate excitonic transitions near Γ point. The luminescence of the high-energy exciton in MoS2 was firstly observed and investigated. I believe that these results provide insight into understanding the excitonic behavior of MoS2 and exploring novel optoelectronic applications.