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hotoinduced Transient Spectroscopy of Defect Centers in GaN and SiC1

. P. Kami n´skia, R. Koz l owskia, M. Kozubala, J. Zelazkoa, M. Miczugab, and M. Paw l owskib a

Institute of Electronic Materials Technology, 01-919 Warszawa, Poland b Military University of Technology, 00-908 Warszawa, Poland e-mail: [email protected] Submitted September 12, 2006; accepted for publication October 3, 2006

Abstract—The potentialities of photoinduced transient spectroscopy in terms of investigation of defect centers in wide-band-gap semiconductors are presented. An experimental system dedicated to measurements of the photocurrent transients at temperatures 20–800 K is described and a new approach to extraction of trap parameters from the photocurrent relaxation waveforms recorded in a selected temperature range is presented. The approach is based on the two-dimensional analysis of the waveforms as a function of time and temperature using the correlation procedure. As a result, three-dimensional images showing the temperature changes of the emission rate for detected defect centers are produced and a neural network method is applied to determine the parameters of defect centers. The new approach is exemplified by studies of defect centers in high-resistivity GaN : Mg and semi-insulating 6H-SiC : V. PACS numbers: 71.55.Eq, 71.55.Ht, 72.40.+w DOI: 10.1134/S1063782607040100 1

1. INTRODUCTION Quality assessment of epitaxial GaN and semi-insulating (SI) SiC wafers is of great importance in terms of manufacturing advanced monolithic microwave integrated circuits (MMICs) operating at millimeter-wave frequencies. In particular, the development of broadband satellite communication systems has involved a substantial demand for high-quality power devices based on these materials. The power densities recorded for GaN-based high-electron mobility transistors (HEMTs) at 10–100 GHz are far superior to those for GaAs HEMTs. The technology allowing the best device performance today involves the growth of GaN HEMT structures on wafers made from SI SiC. This is because of the excellent thermal conductivity and smaller lattice mismatch of SiC compared to sapphire substrate. Semi-insulating SiC substrates are required in order to reduce connection capacitance and dielectric losses. The objective of this paper is to show the potentialities of the photoinduced transient spectroscopy (PITS) as a powerful tool for investigation of defect levels in wide band gap semiconductors with high resistivity. A new approach to visualization of the measurement results is presented. It is based on creating images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for detected defect centers. For this purpose, a two-dimensional 1 The

text was submitted by the authors in English.

analysis of the temperature-induced changes in the photocurrent relaxation waveforms is performed and the fringes are obtained through projecting the spectral surfaces on the plane given by the axes for the temperature and the emission ra