Schottky Barriers on p-GaN
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ABSTRACT Schottky barriers were formed on p-GaN. p-GaN layers doped with Mg were grown by metalorganic chemical vapor deposition (MOCVD). 6H-SiC wafers were used as substrates. The barriers were made by vacuum thermal evaporation of Au. Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the barriers were investigated. The concentration of the ionized acceptors in the p-layers was measured to be about -1017 cm- 3 . The barrier height was determined to be 2.48 eV by C - V measurements at room temperature. The forward current flow mechanism through the barriers is discussed. INTRODUCTION Recently extensive study has been done on Schottky barriers formed on group III nitrides [1-3]. The parameters of Au Schottky barriers to n-GaN were studied in detail [1,2]. In paper [2] we reported the characteristics of Au barriers formed on n-GaN layers grown on SiC substrates: barrier height was determined to be 1.03 eV using both C-V and I - V characteristics. The electron affinity for GaN was calculated to be 4.1 eV using the Au-n-GaN barrier characteristics (both from C - V and I - V data). Much less information is available on Schottky barriers to p-type A3 N materials. M.A. Khan et al [4] used Ti/Au Schottky barriers formed on p-GaN to fabricate a photodetector, but barrier characteristics were not published. In this paper, the electrical characteristics of Au Schottky barriers formed on p-GaN layers are reported. EXPERIMENTAL PROCEDURE GaN layers were grown by MOCVD on SiC substrates at Cree Research, Inc. [5]. The substrates used were n-type 6H-SiC wafers. GaN was deposited on the (0001)Si face of the substrate. First a Si-doped n-GaN layer about 2 gm thick was grown. The donor concentration Nd in the layer was -2x10 18 cm-3 . Then a Mg-doped GaN p-layer about 1 gim thick was grown on top. Schottky barriers were formed by vacuum thermal evaporation of high purity Au on samples which were previously etched in a KOH-water solution for 2 min and then boiled in deionized water. Before evaporation, the gold was cleaned in organic solvents and acids. The gold was evaporated in a vacuum of -3x10- 3 Pa through shadow masks. The area of the Schottky contact was 1x10- 3 cm 2 and the thickness of the evaporated metal ranged from 0.1 to 0.15 gim. The thickness of evaporated metal and the effective area of the Schottky contact were measured by electron microscopy using electron beam induced current (EBIC) and back scattered electron modes. Pd was evaporated on the 837
Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
perimeter of the samples to form an ohmic contact to p-GaN. A planar device geometry (Fig. 1) was used. The C-V measurements were performed using a computer controlled system at a test frequency of 1 MHz and a test signal of 20 mV rms. The I-V characteristics were measured in the temperature range from 300 to 600 K. The measurements were done in a cryostat where the sample temperature was maintained with an accuracy of 0.1 K. Au
Pd p
-
Pd
GaN
n+- GaN n - 6H-SiC Fig. 1. Cross s
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