Manipulation of Surface Charge on GaN
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1202-I04-01
Manipulation of Surface Charge on GaN J. D. Ferguson1, M. A. Foussekis1, M. D. Ruchala1, J. C. Moore2, M. A. Reshchikov1, and A. A. Baski1 1 Department of Physics, Virginia Commonwealth University, Richmond, VA, U.S.A. 2 Department of Chemistry and Physics, Longwood University, Farmville, VA, U.S.A. ABSTRACT We have characterized the surface charge on a variety of GaN samples using two surface potential techniques, conventional Kelvin probe and scanning Kelvin probe microscope (SKPM). Kelvin probe was primarily used to measure the change in surface potential under UV illumination, otherwise known as the surface photovoltage (SPV). Due to band bending near the semiconductor surface of about 1 eV in dark conditions, the SPV signal for n-type GaN typically reaches 0.5 to 0.6 eV upon switching on UV light. This value can slowly decrease by up to 0.3 eV during UV illumination in air ambient for 2-3 hours. We report that samples with many hours of ambient UV exposure do not show this slow decrease during SPV measurements, consistent with the UV-induced growth of a thicker surface oxide that limits charge transfer. In addition to prolonged UV exposure, the surface contact potential was also manipulated by local charge injection. In this procedure, the surface is charged using a metallized atomic force microscope tip which is scanned in contact with the sample. Subsequent SKPM measurements indicate an increase or decrease in the surface contact potential for the charged region, depending on the applied voltage polarity. Measurements of the discharge behavior in dark for these regions show a logarithmic time behavior, similar to the decay behavior during our observations of SPV transients after switching off the light. As expected, illumination of the surface increases the discharge rate and restores the charged area to its original state. INTRODUCTION While GaN is widely used in optoelectronic applications in the blue and UV spectral regions, an understanding of its surface-related electrical and optical properties remains limited. The presence of surface charge can be an important factor for the proper functioning of GaN devices; however, little is known about the origin of the surface charge in n- and p-type GaN. It is well established that n-type GaN shows an upward band bending of about 1 eV due to negative charge at the surface [1,2,3,4], while p-type GaN exhibits even larger downward band bending due to positive charge at the surface [3,5,6,7]. The origin of such surface charges may be due to surface states or adsorbed species on the surface. The band bending in n-type GaN can be reduced by 0.3 to 0.9 eV via the surface photovoltage (SPV) effect, where UV illumination causes photogenerated holes to reach the surface [3,8,9,10]. The SPV and presence of charge at the surface can be determined using Kelvin probe measurements. Previously, we reported preliminary results on local charging of the n-type GaN surface with negative charge [11]. In this work we present results on charging and its dynamics for n- an
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