Fast and Slow UV-Photoresponse in n-Type GaN
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(1) Physics Department, Instituto Superior Trcnico, P-1096 Lisbon, Portugal (2) Instituto de Engenharia de Sistemas e Computadores, P-1000 Lisbon, Portugal (3) I. Physics Department, University of Giessen, D-35392 Giessen, Germany
ABSTRACT The photocurrent decay in n-type GaN films prepared by low-pressure chemical vapor deposition (LPCVD) was measured in the ms-to-s time range using steady-state UV light and in the ýis time regime using short high-power pulses from higher harmonics of a Nd:YAG laser. A power law time dependence is observed with exponents ranging from -0.1 to -0.3, which is an indication of a broad distribution of trapping states inside the band gap. Combining Hall effect results and the magnitude of the initial slope of 2 4 the photocurrent decay we estimate a mobility-lifetime product of 2.lxlO- cm /V for photogenerated electrons at times below a few jts. Slow transients might be a handicap for applications of GaN in UV detectors.
1. INTRODUCTION Applications of GaN-related materials in devices like UV-detectors or high-temperature transistors might be handicapped by slow current transients, as seen, for example, in the persistent photoconductivity effect (PPC) [1]. The PPC effect, which manifest itself as a prolonged non-exponential conductivity decay after exposure to sub-bandgap light, was attributed to internal strain or to charging and discharging of deep traps in AlGaN alloy films. Metastable changes in conductivity were observed up to several thousand seconds [2]. The samples we used for this study were prepared by low-pressure chemical vapor deposition 8 3 without intentional doping. Hall measurements show that the carrier concentration is about 101 cm- and the samples are n-type. Therefore the measured photocurrents were well below the dark current levels. Nevertheless, effects similar to those encountered in highly resistive AlGaN alloy films occur, like frequency-dependent photoconductivity and non-exponential current decay after pulsed excitation. We discuss whether both the slow (s) and the fast (pts) photocurrent decay can be described by changes of occupation of deep traps.
2. SLOW PHOTOCURRENT DECAY WITH CHOPPED LIGHT The GaN samples were prepared by low-pressure chemical vapor deposition (LPCVD) from GaCI3 and NH 3 sources on A120 3 substrates [3]. The deposition temperature and process pressure were 965 OC and 0.5 mbar, respectively. GaCI 3 was transported in a 50 sccm N2 carrier gas heated to 70 °C. The NH 3 flow was set at 100 sccm. This resulted in a growth rate of about 3 A/s.
395 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
Photocurrent decays were measured with coplanar Cr contacts. The 1 mm gap was illuminated either with chopped light from a 50 W Xe-lamp or with 5 ns pulses of the 532 nm line of a frequencydoubled Nd:YAG laser, or the fourth harmonic line at 266 nm. The applied voltage was kept between 1 and 3 V where ohmic behavior of the contacts was assured. The relatively large dark currents were compensated for electronically since the
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