Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Electrical Engineering

Sub-Department

Electrical Engineering

First Advisor

Goutam Koley

Abstract

The field of group III-nitride semiconductors has seen incredible developments during last couple of decades. They are recognized as the most promising materials for a wide field of optoelectronics and electronic devices. Their bandgap ranges from 6.2 eV for AlN to 0.7 eV for InN, covering a wide spectral range from infrared (1.77 mm) to deep ultraviolet (200 nm). Their direct bandgap makes them useful for fabricating optoelectronic devices such as light emitting diodes (LEDs), laser diodes (LDs), and photodetectors. III-nitride semiconductor materials also possess strong bond strengths and exhibit good structural, chemical and thermal stability. These properties make it possible for III-nitride based devices to operate in the high-temperature environments and also make them compatible with high processing temperatures. They also have high electron saturation velocity and high breakdown field. Such unique material properties also make III-nitride semiconductor very popular for the applications in high power, high frequency devices such as high electron mobility transistors (HEMTs).

AlGaN is the semiconductor materials of choice for optoelectronic devices in the ultra-violet (UV) spectral range and high power, high frequency electronic devices. Significant advances have been made in AlGaN based UV light emitting diodes high electron mobility transistors during the last decade. Performance and reliability of these devices strongly depend on the electronic properties of epitaxial layers which are critically affected by structural defects and unintentional doped impurities. As the bandgap of AlGaN increases with its Al composition, the ionization energies for silicon (n-type dopant) and magnesium (p-type dopant) increase too, resulting in a lower ionization efficiency. Therefore, both n-type and p-type doping of AlGaN layers is much more difficult than GaN. Very high Si doping concentration is needed to achieve low resistivity AlGaN layers.

Effective doping without compromising material quality is the key to high efficiency of AlGaN based devices. This thesis is focused on optimization of AlGaN epitaxial growth and the effects of n-type doping on AlGaN epilayer quality. AlGaN films were grown on c-plane sapphire substrates using metal organic chemical vapor deposition. First growth optimization of AlGaN layer was carried out. A systematic study of a series of Si-doped AlxGa1-xN layers with three different x = 50%, 65% and 72% was accomplished. Detailed material characterization including x-ray diffractometry, atomic force microscopy, hall-effect measurement, sheet resistance mapping, and transmission line measurement was carried out. As the doping concentration increased, carrier concentration was found to monotonically increase whereas doping efficiency reduced. Hall mobility and carrier concentrations were found to reduce with increased Al concentration, as expected due to increased ionization energy. Calculated doping efficiency was found to be consistent with doping efficiency equation. The comprehensive results, relevant conclusions and future trends are included.

Rights

© 2013, Devendra Diwan

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