Band bending near the surface in GaN as detected by a charge sensitive probe
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Band bending near the surface in GaN as detected by a charge sensitive probe S. Sabuktagin, M. A. Reshchikov, D. K. Johnstone, and H. Morkoç Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284 ABSTRACT We measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 - 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects. INTRODUCTION GaN-based optical and electronic devices have progressed rapidly in recent years. However, little attention has been given to the effect of the surface on the properties of GaN and related devices. Surface states due to Ga or N termination, structural defects, adsorbates, and oxidation play an important role in semiconductor device performance. The surface of undoped atomically clean GaN shows an upward band bending of 0.9 eV, while that which is chemically cleaned and air-exposed exhibits an upward band bending of 0.4 eV [1]. It is also known that a thin (~9 Å) oxide layer [2], as well as chemisorbed oxygen atoms (up to one monolayer) [1,3], are present on the GaN surface. The chemisorbed oxygen apparently removes (passivates) the intrinsic surface states in GaN and reduces the band bending by ~0.15 eV [1]. Other measurements showed an upward band bending of 0.75 eV in n-type GaN and a downward band bending of 0.75 eV in p-type GaN [4]. The band bending at the Ga-face is expected to be much larger as compared to that at the N-face surface due to strong spontaneous polarization in GaN, which can have a large influence on Schottky barrier heights [5]. The reported difference in the Schottky barrier heights between Ga- and N-faces varies from 0.2 eV [5] to 1.4 eV [6]. Illumination of a semiconductor with ultraviolet (UV) light causes a reduction of the band bending due to partial screening of the electric field by photogenerated electrons and holes (photo-voltage effect) [7]. The light-induced variation of the band bending up to ~0.35 eV has been observed for the clean surface of n-type GaN, decreasing with increasing sample temperature or with exposure to oxygen [8]. In this work, we have determined surface band bending and studied photo-voltage effect on different GaN samples using the atomic force microscopy (AFM) in the surface potential mode (also referred to as Kelvin probe method). EXPERIME
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