Influence of Substrate Annealing Temperature upon Deep Levels in n-Type 4H SiC
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Influence of Substrate Annealing Temperature upon Deep Levels in n-Type 4H SiC Martin E. Kordesch1 , Florentina Perjeru1 and R. L. Woodin2 Department of Physics and Astronomy, Athens OH 45701 2 Extreme Devices, Inc., Austin TX 78744
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ABSTRACT The evolution of deep levels that depend upon annealing temperature is investigated for n-type 4H SiC-Ni Schottky barriers. Several samples, cut from the same wafer, have been left unheated or annealed at 400°C, 700° C and 900° C, in air. Deep level transient spectroscopy (DLTS) has been used to investigate deep levels in all four sets of samples. Electron traps with activation energies EC-ET = 0.19 to 0.5 eV are observed, as well as a hole trap in the sample annealed at 900 C at energy ET-EV = 0.14 eV.
INTRODUCTION Silicon carbide (SiC) based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power and high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide is expected to enable significant improvements in applications and systems such as improved high-voltage switching [1-3], energy savings in public electric power distribution and powerful microwave electronics for radar and communications [4]. Doping the material with Al or N during growth is used to obtain p-type and n-type material, respectively, resulting in defect creation. A resulting defect, N residing at the C lattice site, has activation energies in the range 0.08-0.150 eV [5]. Residual impurities present in the growth systems create defects as well. Independent of the polytype, B residing at a Si lattice site forms a deep level at approximately EV+ 0.30 eV [6,7]. Vanadium forms deep levels at 0.35eV, 0.7eV and near midgap in 4H- and 6H-SiC material [8]. We have performed Deep level transient spectroscopy on 4H SiC after heating to different temperatures between 400 and 900 C, with nickel Schottky barriers formed after heating. EXPERIMENTAL The material used in this study, 4H-SiC acquired from Cree Corp., doped with N (n=9 x10 cm-3) has been cut in four pieces. Three of these have been annealed in air for 30 min at 400º, 700º, and 900º C, respectively, while on the fourth piece no heat treatment has been applied. After annealing, all samples have been cleaned in HF: deionized water (1:2) for 5 min, then rinsed in deionized water and dried in inert gas. After cleaning, the samples have been placed in a standard DC sputtering system for Ni Schottky contact deposition through masks with circular dots with diameters of 0.5 mm. On each sample, 12 dots have been deposited resulting in the formation of 12 Schottky barriers (thickness of the contact 400 nm). Ohmic 17
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contacts have been formed by evaporation of Al on the back of the samples (contact thickness of approximately 1000 nm). To test the properties of the Schottky barriers thus obtained, current-voltage measurements (I-V) were performed at 300 K using a Keithley 236 source measure unit. Capacitance-voltage (C-V) and deep level transient spec
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