Defect States in SiC/GaN-and SiC/AlGaN/GaN-Heterostructures Characterized by Admittance and Photocurrent Spectroscopy
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Germany of Physics, University of Stuttgart, D- 70550 Stuttgart, Germany
Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.71 (1999) ABSTRACT Deep defect levels in n-type GaN/AIN/6H-SiC- and GaN/AJGaN/6H-SiC- heterostructures grown by Metallorganic Vapor Phase Epitaxy were analyzed by Thermal and Optical Admittance and Photocurrent Spectroscopy. The various thermal and optical transitions in the spectra originating from both the Schottky contact as well as the GaN/SiC- and AIGaN/GaNheterojunctions were separated. This was achieved by variation of the modulation frequency, the use of different contact arrangements and by comparison with reference spectra from GaN/Sapphir samples and SiC substrates. In the GaN/AIGaN/SiC structures a bias voltage dependent peak shift was found which is correlated to an interface related defect distribution. In additionally to, SIC related defects, defect-band-transitions involving defects with transition energies at 2.2eV, 1.85eV, Ec-(470±40) meV and Eo-(65-95) meV were found for the GaN layer. INTRODUCTION Besides their optoelectronic potential, GaN and their related compounds are very important for high-frequency, high-power and high- temperature electronic devices /1/. Especially, heterostructures based on GaN and A1GaN on SiC- substrate are main parts of some novel devices such as MESFET's, MODFET's, bipolar transistors /1/, UV-detectors /2/, LED's and laser diodes /1/. Defects states in the layers and in the interface regions influence both optical /3/ and electrical /4/ properties. For example, in GaN / p-type SIC -structures the photoluminescence and electroluminescence spectra show defect related transitions /3/ which cause tunnelingassisted currents described in /4/. Furthermore, in GaN / p-type SiC heterojunctions, indications on interface defect states were found by C-V-characteristics /5/ and by admittance spectroscopy /6/. In GaN/AIGaN pin-structures on SiC-substrates, Mg-related recombination centers were found with temperature dependent dark current measurements and with current-DLTS having activation energies of 0.191eV and 0.207eV, respectively /7/. The aim of this paper is to present evidences for defects and localize these traps in the layers and interfaces using Thermal and Optical Admittance Spectroscopy. We show, that defect spectra of heterostructures can be led back to those of the corresponding single layers on sapphire. We were able to separate the different heterojunctions by variation of the modulation frequency and the contact arrangement of the samples. These investigations provide the for the determination of the capacitance and conduction values of the individual parts of the heterostructure. Only with these knowledge an calculation of the concentrations of the different traps would be possible. The quantitative analysis will be the aim of future work. G 3.71 Mat. Res. Soc. Proc. Vol. 537 0 1999 Materials Research Society
EXPERIMENTAL The first group of samples consist of a n-type 6H-SiC- substrate (carrier concentration of about 1
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