Photocurrent Response in Mg-Doped GaN
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ABSTRACT The photoconductivity response of Mg-doped GaN thin films was studied in the time domain of 50 nanoseconds to a few milliseconds in the temperature range of 100K to 390K. The response time, defined as the time when the photocurrent decreased to half its maximum value, is in the sub-microseconds at room temperature, but increased to a few microseconds at low temperatures. The contact capacitance is suspected for this behavior. Slower decay components due to trapping at defect states were also observed.
INTRODUCTION Ill-nitride semiconductors are promising materials for UV and x-ray radiation detection. In such applications, the photocurrent response is of primary interest. In the past, we reported the photoconductivity spectra and rlgt product for both n-type and p-type GaN, as well as the photocurrent decay in n-type GaN[1,2]. We also reported the photocurrent decay in the long time domain (the persistent photoconductivity) in undoped n-type and Mg-doped p-type GaN thin films[3]. Recently, we further found that some relatively resistive Mg-doped GaN films have high photoconductivity to dark conductivity ratio[4], a characteristics desirable for detector applications. It is interesting to see how fast is the photocurrent response of these films. In this paper, we present our study of the photocurrent response of Mg-doped GaN thin films in the short time domain. The photocurrent response in the milliseconds domain was previously measured at room temperature by Stevens et al[5]. Here we extended the measurements down to 50 nanoseconds and to a wide temperature range of 100K390K. Photocurrent response time, defined as the time when the photocurrent decreased to half its maximum value, was in sub-microseconds at room temperature. However, the data exhibited complicated non-exponential behavior in the time domain of nanoseconds to milliseconds, reflecting possible effect of contact capacitance and effect of a broad distributions of defect states across the bandgap of GaN.
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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
EXPERIMENTAL DETAILS The GaN thin films were grown on sapphire substrate at Astralux in a hot-wall atmospheric-pressure metalorganic vapor phase epitaxy system. Before deposition, the basal plane sapphire substrates were cleaned in organic solvents and were pretreated at 1100 oC in hydrogen. The GaN films were grown at about 1030 oC on a GaN buffer layer deposited at about 600 oC. Mg-doping was provided by transporting cp 2Mg into the growth chamber with ammonia and trimethylgallium. To make sure that the measured data are of general relevance, we also measured the photocurrent response of high quality Mg-doped p-type GaN films grown at Meijo University using atmospheric pressure MOCVD[6]. The photoconductivity was excited by a pulsed nitrogen laser at 337 nm (PTI Inc. model PL2300), and was measured in a coplanar geometry with two indium contacts soldered to the GaN surface. The gap between the electrodes was about 1 mm. A DC voltage was applied to the sample through
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