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Z.C.HUANG*, J.C. CHEN** AND D.B. MOTT*** Hughes STX Corporation, 7701 Greenbelt Road, Suite 400, Greenbelt, MD 20770 Department of Computer Science and Electrical Engineering, University of Maryland Baltimore County, Baltimore, MD 21228 *** Goddard Space Flight Center, Code 718.1, NASA, Greenbelt, MD 20771 *

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ABSTRACT Deep levels in insulating GaN grown by metalorganic chemical vapor deposition have been studied using thermally stimulated current (TSC) and photocurrent (PC) spectroscopies. Five main traps were observed by TSC measurement in the as-grown undoped GaN in the range of 0-0.75 eV below the conduction band edge or above the valence band edge. Their activation energies were 0.11, 0.24, 0.36, 0.53 and 0.62 eV, respectively. PC measurements showed three deep levels located within the bandgap at 1.32, 1.70 and 2.36 eV, respectively. Furnace annealing was carried out on GaN for identifying all the observed deep levels. We have found that the 0.24, 0.36 and 0.53 eV traps were eliminated by annealing at 1000'C under N2 for six hours, whereas the 0.62 eV trap density increased after annealing. The three deep levels detected by the PC measurement were not affected by annealing. The 1.70 eV trap, which is located at the midgap, does not seem to compensate with narrow donors. We attribute the 0. 11 eV trap to surface states, and the 0.62 eV trap to nitrogen vacancies. INTRODUCTION Gallium nitride has been considered to be one of the most promising materials for the

development of high responsivity and solar blind ultraviolet (UV) detectors due to its wide bandgap (3.4 eV). This allows instruments utilizing GaN UV detector arrays to operate in an environment with significant visible radiation without expensive visible blocking filters and extensive stray light baffling. One of the central problems in GaN UV detectors is the slow response to the UV radiation. Khan et al' reported that the response time for their metalsemiconductor-metal (M-S-M) GaN detectors was about Ims. We have found that the M-S-M detectors made from insulating GaN epilayers grown by different groups (Cree Research Inc., Applied Physics Lab. of John Hopkins University, and Naval Research Lab. etc,) have a response time ranging from 0.25 to 10ms. It is suggested that the high density of defects in GaN was the cause for the slow response. These defects are mainly stacking faults and dislocations in the materials grown by metalorganic chemical vapor deposition (MOCVD). Lester et al.2 and Qian et al.3 reported that the dislocation density in GaN was as high as 10910 10 cm -24. According to Kurtin et al. 4, GaN is an ionic material, in which the electronic properties are fundamentally different from those of more covalent materials such as Si, GaAs, or InP. In view of the electrical properties of the defects in undoped insulating GaN, almost no results have been reported to date. Its insulating nature and its low carrier mobilities make it difficult to carry out conventional electrical measurements 5'6 . In this paper, we studied the deep levels i