Photosensitive Capacitance Effect In High-purity Semi-insulating 4H-SiC
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Photosensitive Capacitance Effect In High-purity Semi-insulating 4H-SiC Joseph Register1, Stephen E. Saddow1 and Kevin Boulais2 1
University of South Florida, Dept. of Electrical Engineering, 4202 E. Fowler Ave., Tampa, FL 33620, USA 2
Naval Surface Warfare Center, Dahlgren, VA
Abstract We demonstrate a novel optically tunable photosensitive capacitor (PSC) made from high-purity semi-insulating 4H-SiC. Photosensitive capacitors can provide continuously variable reactive tuning in RF circuitry or enable capacitive-optical sensing applications. Unlike varactors, PSCs often do not require a DC bias voltage to operate. To demonstrate the effect, we fabricated several 1cm x 1cm square photocapacitor devices from bulk material using metal-evaporated Ti/Au contacts using a simple planar parallel-gap geometry. IV curves were taken of the devices using an HP-4145B semiconductor parameter analyzer to verify Schottky behavior as a function of DC bias. The samples were then illuminated with pulsed below-bandgap 470 nm and 590 nm high intensity LED light sources. The resulting data demonstrated an increase in capacitance, Cs, and a drop in resistance, Rs, with increasing optical intensity incident on the device. The observed shifts in both Cs and Rs were repeatable. At a measurement frequency of 33 kHz. Cs increased from its nominal value of 186.7 pF to 575.6 pF while Rs dropped from 150.0 kΩ to 22.4 kΩ. This demonstrates the existence of the photocapacitance effect in high-purity semiinsulating 4H-SiC and thus warrants further investigation. The underlying phenomenon of the effect is suspected to be light interaction with the dominant deep level traps through the Shockley–Read–Hall (SRH) recombination mechanism. Introduction Semi-Insulating silicon carbide has very low radiative and Auger recombination coefficients and is therefore dominated by SRH (Shockley Read Hall) recombination and surface effects within the material[1][2]. Deep level transient spectroscopy (DLTS) spectra of HPSI (high purity semiinsulating) 4H-SiC samples reveal two fundamental deep level electron traps with DLTS designations Z1/Z2 (Ec-0.65 eV) and EH6/EH7 (Ec-1.65eV ) [3], [4] (Fig.1).
Figure 1: Summary of major recombination centers within 4H-SiC [1][5].
In addition to these traps another trap at 1.1eV has been explored using optical absorption measurements described by Mitchel et al. as a non-vandium level likely caused by a titanium
impurity[6]. The Z1/Z2 center is still of unknown origin but is known to be an efficient recombination center and is often cited as the dominant defect level limiting the carrier lifetime in this material [3]. Work in EPR/DLTS has suggested that the double defect arises from a silicon antisite - silicon vacancy complex [7]. As such, the Z1/Z2 trap center is a combination defect and has similar equal cross section for electrons and holes (10-14cm2) making it an efficient recombination center [7]. The DLTS technique requires assumptions to be made about the temperature dependence of the capture cross section parameter
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