Transient Photoconductivity of GaN Thin Film on Sapphire Substrate

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INTRODUCTION GaN and its alloys with Al and In, have recently attracted considerable interest. They are important candidate materials for blue light emitting diodes(LEDs)[1], and high temperature, high power electronics[2-3]. It was found as early as in 1970's that the native defects of GaN played an important role in the light emitting[4]. However, the device fabrication requires these defects as few as possible. Due to the scarcity of bulk single crystalline GaN and suitable substrates for GaN epitaxy, the minimum dislocation density in GaN thin films are as high as 108 cm -2. It is significant for high quality materials and devices to study the formation mechanism, specific properties and control methods of GaN native defects. Although the electronic structure of GaN induced by impurities and defects, such as "yellow band"[5-8] and exciton states near the band edge[9-1 1], has been studied extensively, it is not very clear yet. The processes of photogenerated carrier generation, recombination and motion are all related to the band structure of GaN. The photoconductivity (PC) of GaN has been investigated by several groups[12-15], and some information related to the forbidden band have been obtained. However, as a way to study the lifetime and defects of undoped GaN , transient PC has not been paid enough attention to. In this work, the transient PC of GaN thin film has been studied as a function of temperature and optical power. We have also developed a recombination model that may account for all the experiment observations reported in this paper.

EXPERIMENTS The GaN sample was grown by metalorganic chemical vapor deposition (MOCVD), on (0001) 303 Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society

sapphire substrate at about 1050°C, with a 50 nm-thick GaN buffer layer grown at 550°C. The epilayer growth rate is about I jim/h. Van der Paul measurement shows that the sample is unintentionally doped n-type, and the carrier concentration is obtained as 1017 cm 3 . The full width of half maximum (FWHM) of X-ray diffraction rock curve is 170", indicating that the sample is high-quality single crystal. The thickness of GaN film is about 1 ýim calculated by the transmission spectrum[16], which confirms the designed one of growth. Aluminum electrodes were evaporated on GaN thin film, and then annealed at 200-300°C to form ohmic contacts. For the transient PC measurement, a YAG:Nd pulsed laser was used as the light source. The wavelength of the incident pulsed laser light was 355nm. The width and frequency of the pulse are 20 ns and 10 Hz, respectively. The voltage outputs from across the load resistor were measured with a HP54510B digitizing oscilloscope and recorded by a computer. The time-resolution was 1 ps.

Fig.1 PC decay curves of undoped GaN

02

M015

300°C(solid line). The fitting curve by 2-

Fitting curveby 2-exponent Experimental curve at 300-Cfunction

0.010 B

film at room temperature(' spots) and

curve RT Experimental - ---

•"

exponent function is described with dash

a

line.

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