Advanced Processing of GaN for Electronic Devices

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Abstract
This review focuses on understanding and optimization of several key aspects of GaN device processing. A novel rapid thermal processing up to 1500°C, in conjunction with AlN encapsulation, has been developed. The activation processes of implanted Si or Group VI donors, and common acceptors in GaN by using this ultrahigh-temperature annealing, along with its effects on surface degradation, dopant redistribution, and damage removal have been examined. 1400 degrees has proven to be the optimum temperature to achieve high activation efficiency and to repair the ion-induced lattice defects. Ion implantation was also employed to create high-resistivity GaN. Damage-related isolation with sheet resistances of 1012 Ω/square in n-GaN and 1010 Ω/square in p-GaN has been achieved by implant of O and transition metal elements. The effects of surface cleanliness on characteristics of GaN Schottky contacts have been investigated, and the reduction in barrier height was correlated with removing the native oxide that forms an insulating layer on the conventionally cleaned surface. W alloys have been deposited on Si-implanted samples and Mg-doped epilayers to achieve ohmic contacts with low resistance and better thermal stability than the existing non refractory contact schemes. Dry etching damage in GaN has been studied systematically using Schottky diode measurements. Wet chemical etching and thermal annealing processes have been developed to restore the ion-degraded material properties. Based on these technical improvements, attempts have been made to demonstrate GaN-based bipolar transistors. The devices operated in common base mode at current densities up to 3.6 kAcm−2and temperatures up to 300°C. The key issues that currently limit the device performance, such as high base resistance, poor impurity control, and defects resulting from the heteroepitaxial growth, have been addressed. A physically based simulation suggested that GaN bipolar devices may still suffer from small minority-carrier lifetime in the absence of aforementioned processing problems.

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