(Invited) Better GaN Substrates Are the Key to Advancing GaN Device Technology
Authors: Freitas Jaime A. , Culbertson James C. 
Published: February 2024

Gallium nitride (GaN) is a semiconductor that has an unusual combination of extreme values of fundamental physical and chemical properties. Its wide bandgap (3.4 eV at room temperature) results in a low intrinsic carrier density and thus low leakage and dark currents. These properties combined with its direct gap are crucial requirements for photo-detectors and high-temperature electronic applications. Despite the relatively large carrier effective masses, which lead to lower carrier mobilities, the high electron saturation velocity and high breakdown field make possible the fabrication of high frequency electronic devices. In addition, its relatively high thermal conductivity opens the possibility to realize a number of high-power device applications. These unique characteristics have made possible the fabrication of a variety of optoelectronic and electronic devices capable of performing at extreme operation conditions of current, voltage, temperature, and in harsh environments. GaN and its alloys and heterostructures with InN and AlN have resulted in devices with superior performance for a variety of applications. Despite this unprecedented and not fully developed semiconductor materials success, the realization of high-performance and high-yield devices will require native GaN substrates with well-controlled physical properties.
Presently, Hydride Vapor Phase Epitaxy (HVPE) and Ammonothermal (Ammono-GaN) bulk growth are the only two methods that have successfully demonstrated the manufacture of GaN boules. The first, a fast-growth non-equilibrium process, only reproduces the starting substrate dimensions, while the latter, a slow-growth equilibrium process, allows increasing boule dimensions under controlled growth conditions. Despite the impressive accomplishments recently achieved by these growth methods, much work still needs to be done to establish a fully satisfactory bulk growth method for this material. Heteroepitaxial HVPE-GaN suffers from high dislocation densities, while Ammono-GaN suffers from high oxygen incorporation. Recent results suggest that Ammono-GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by HVPE [1]. This approach resulted on HVPE-GaN wafers with orders of magnitude reduced extended defect and lower oxygen impurity concentrations. However, the relatively high background concentration of pervasive impurities is still preventing the realization of high performance devices. In this work, we illustrate how defect-sensitive optical techniques can be used to assist the optimization procedures at various stages of growth [2]. A brief review of defect-sensitive optical spectroscopic techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology.
References

J. A. Freitas, Jr., J.C. Culbertson, N.A. Mahadik, T. Sochacki, M. Iwinska, and M.S. Bockowski, J. Crystal Growth 456 (2016) 113.

J. A. Freitas, Jr., J.C. Culbertson, and E. R. Glaser, Crystals, 12 (2022) 1294.