Investigation of deep levels in bulk GaN
Abstract: The first gallium nitride (GaN) crystal was grown by hydride vapor phase epitaxy in 1969 by Maruska and Tietjen and since then, there has been an intensive development of the field, especially after the ground breaking discoveries concerning growth and p-type doping of GaN done by the 2014 year Nobel Laureates in Physics, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura. GaN and its alloys with In and Al belong to a semiconductor group which is referred as the III-nitrides. It has outstanding properties such as a direct wide bandgap (3.4 eV for GaN), high breakdown voltage and high electron mobility. With these properties, GaN is a promising material for a variety of applications in electronics and optoelectronics. The perhaps most important application is GaN-based light-emitting-diodes (LED) which can produce a highbrightness blue light. Since the bandgap of GaN can be controlled by alloying it with aluminium (Al) or indium (In) for a larger or smaller bandgap, respectively, GaN is very important for optoelectronic applications from infrared to the deep ultraviolet region. There are other semiconductors with bandgap similar to GaN such as SiC, and the first commercially blue light emitting LEDs where manufactured in SiC, however, SiC has an indirect bandgap with a low efficiency of emitting photons, and today, the SiC based LEDs have been completely replaced by the considerable more efficient GaN based LEDs.One problem, which has hampered the development of GaN based devices, is the lack of native substrate of GaN. Due to that, most of the GaN based devices are fabricated on foreign substrates such as SiC or Al2O3. Growing on a foreign substrate results in high threading dislocation (TD) densities (~109 cm-2) and stress in the GaN layer due to lattice mismatch and difference of thermal expansion coefficient between GaN and the substrate. The high TD density and the stress influence the performance of the devices.Another important aspect related to GaN which has attracted manystudies is how defects affect the efficiency of GaN-based devices.Therefore, it is necessary to understand the properties and to identifythem. When we know there properties, one can estimate how they willinfluence the behavior of devices, and thereby, optimize the performance of the device for its application. Basically, a fundamental knowledge of defect properties, and how to introduce them in a controlled manner, or to avoid them, is important in order to optimize the performance of devices. Defects can be introduced both intentionally and unintentionally into semiconductors during the growth process, during processing of the device or from the working environment, for example, devices working in a radioactive ambient are more likely to have defects induced by irradiation.This thesis is focused on electrical characterization of defects in bulk GaN grown by halide vapor phase epitaxy (HVPE) by using deep level transient spectroscopy. Other measurement techniques like currentvoltage measurement (IV), capacitance-voltage measurement (CV) and Hall measurement were also been used. Defects related to the growth process and the polishing process are discussed in Paper 1. In Paper 2 and Paper 3, we focus on intrinsic defects in GaN introduced intentionally by electron irradiation. This type of defects are important since they can be unintentionally introduced during growth of the material, in the fabrication process of devices or if it is exposed to a radioactive environment. By electron irradiation, we can in a controlled manner introduce intrinsic defects for studies and by varying the electron beam energy and doses we can judge the nature of them. After electron irradiation, we observed several electrically active defects. These defects were characterized by DLTS to get important parameters such as activation energy, trap concentration, trap profile and capture cross-section. Especially, from temperature-dependent capacitance transient studies, we have determined the mechanism of the electron capturing process for some of them.
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