GaN Based Heterostructure Growth and Application to Electronic Devices and Gas Sensors.

Abstract

This thesis focuses on III-Nitrides for electronic and gas sensing devices. Systematic material growth optimization was performed by using various characterization techniques in order to improve device performance. For high frequency device operation, resistive bulk GaN is preferable to prevent parasitic leakage through the layer. Growth conditions for bulk GaN were optimized for this purpose using an in-house Metal Organic Vapor Phase Epitaxy system. The pinch-off current of AlGaN/GaN Heterostructure Field Effect Transistors based on these layers was reduced by a factor of twenty in comparison with devices fabricated with non-optimized bulk GaN. The best results obtained from this heterostructure were a room temperature Hall mobility and sheet charge density of about 1270cm2/Vs and 7.0•10^12cm−2, respectively. AlN/GaN HFETs were investigated because of the possibility of reducing gate leakage current, operating at higher temperature and higher power in comparison with AlGaN/GaN HFETs. In-situ SiNx was employed for surface passivation. SiNx improved the ohmic quality of this device and the RF characteristics were also improved; fT and fmax were enhanced by a factor of two and three, respectively in contrast to devices without the passivation. To address the issue pertaining to the polarization fields of c-plane GaN, non-polar GaN growth was investigated using r-plane sapphire substrates. The RMS surface roughness and full width at half maximum were improved to 2nm and 1000", respectively, using high temperature AlN buffer layers. This shows a quality improvement by a factor of two to three compared to layers grown using low temperature GaN NLs in this work. Diodes were fabricated for gas sensing using c-plane GaN and AlGaN/GaN heterostructures. Optimization of size and thickness of the Pt Schottky contact improved CO sensitivity by a factor of six compared to non-optimized sensors. Fabry-Pérot filters with GaN/air gap based distributed Bragg reflectors were designed at 450nm as a detector in optical gas sensing systems. Simulations of their optical and mechanical properties showed the feasibility of this device design. The growth and etching study of AlN as a sacrificial layer manifested that reasonable etching rate can be obtained when the layer was grown at 800C.