Electrical Characterization of Sputter Deposition Induced Defects in n-GaN
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G 6.13 Mat. Res. Soc. Proc. Vol. 537 ©1999 Materials Research Society
opposite was found. Using deep level transient spectroscopy (DLTS) [9], it was shown that this barrier height alteration was accompanied by the introduction of sputter deposition induced defects at and below the semiconductor surface, thought to be the cause of the barrier alteration [4]. No studies have yet been reported for GaN where the contact quality and metallization induced defects are investigated as function of the metallization process. In this paper we report the current-voltage (I-V) characteristics of resistively evaporated and sputter deposited Au Schottky contacts on epitaxially grown GaN. We also report the properties of the defects, determined by DLTS, present in epitaxially grown GaN before and after sputter deposition of Au Schottky contacts thereon. We show that sputter deposition introduces at least four electron traps, two of which have not previously been observed in as-grown or in processed epitaxial GaN. EXPERIMENTAL PROCEDURE For this study we used epitaxial GaN with a free carrier density of (2-3)x10 16 crm-3 , grown by organo-metallic vapor phase epitaxy (OMVPE). Before contact fabrication, the samples were cleaned [10] by first boiling them in aqua-regia and rinsing in de-ionised water, and then degreasing them by boiling in trichloroethylene followed by rinsing in boiling isopropanol and thereafter in de-ionised water. Finally, the samples were dipped in HCI:H 20 (1:1) for 10 seconds. After this cleaning, Ti/A1/Ni/Au (150 A/2200A//400 k/500 A) ohmic contacts [11] were fabricated on the GaN and annealed at 500 °C for 5 minutes in Ar. Prior to Schottky barrier diode (SBD) fabrication, the samples were again degreased and dipped in an HCI:H 20 (1:1) solution. Following this, circular Au Schottky contacts, 0.6 mm in diameter and I ýtm thick, were sputterdeposited on the GaN through a metal contact mask, as close as possible to the ohmic contact to minimise the diode series resistance. Sputter deposition was performed in DC mode at a power of 0.141 kW in an Ar pressure of 4.8x10-3 mbar at a rate of 4.5 nm s-'. For control purposes, Au SBDs were resistively deposited next to the sputter deposited SBDs. Room temperature current-voltage (I-V) measurements were used to assess the quality of the Schottky contacts. The sputter deposition induced defects were characterised by DLTS using a Stanford Research lock-in amplifier (model SR830), which facilitates transient analysis at pulse frequencies of as low as 1 mHz. The energy level, ET, in the bandgap and apparent capture cross section, aa, of a defect, the combination of which is referred to as its DLTS "signature", were determined from Arrhenius plots of T le vs IIT, where e is the emission rate at a temperature T. RESULTS AND DISCUSSION I-V Measurements I-V measurements showed that resistively deposited Au SBDs exhibited excellent rectification properties (Fig. 1). The forward I-V curves of these diodes are linear for at least seven decades of current and the ideality factor
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