Electron and Ion-beam Characterization of Nitride Semiconductor Devices.

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Title: Electron and Ion-beam Characterization of Nitride Semiconductor Devices.
Author: Parish, Chad Michael
Advisors: Phillip Russell, Committee Chair
Dieter Griffis, Committee Member
Gerd Duscher, Committee Member
Michael Escuti, Committee Member
Abstract: Gallium nitride (GaN) and its alloys are used to manufacture green-to-ultraviolet range light emitting diodes (LEDs) for the solid-state lighting industry. However, heteroepitaxial growth on substrates such as 6H-SiC or -Al2O3 results in LEDs with large densities of crystal defects; the optical and electronic properties of these defects, and their influences on LED device performance, are not yet well understood. Controlling and optimizing processing in modern optoelectronic materials, such as GaN, requires a high-resolution characterization technique to probe the localized bandstructure of the materials and their defects in order to relate properties to processing. The beam-injection modes of scanning electron microscopy (SEM) fulfill this need. When an SEM is used to examine a semiconductor, the electron beam injects electron-hole pairs (EHPs) into the semiconductor's band structure. Cathodoluminescence (CL) and electron-beam-induced current (EBIC) are SEM techniques that take advantage of this localized beam-injection of EHPs, and are used to directly probe the optoelectronic behavior of semiconductor materials, devices, and defects. This work examines the optoelectronic properties of defects in GaN-based LED devices. First, computer modeling of the polarization fields in quantum wells was performed, and quantitative predictions of cathodoluminescence peak shifts during electron injection, under varying conditions of electrical bias, were made. Experimental conditions and mathematical treatments for accurate EBIC quantification of the minority carrier diffusion length in GaN light-emitting diodes (LEDs) were developed and refined, as were combined CL and EBIC techniques for the study defect populations in GaN LEDs. Additionally, the effects of focused-ion-beam milling as a cross-sectional sample preparation technique for GaN were studied by CL and EBIC. Limitations and possible extensions to these techniques are also be discussed. By using SEM-EBIC/CL to pinpoint defects in LED devices, site-specific FIB microsampling has been used to prepare samples of defected areas for transmission electron microscopy (TEM). Analyses of these samples have shown how the identity of crystal defects within the devices directly relates to the optoelectronic behavior observed in EBIC and CL.
Date: 2006-12-06
Degree: PhD
Discipline: Materials Science and Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/3722

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