A Study of V3+/4+ Levels in Semi-insulating 6H-SiC using Optical Admittance and Electron Paramagnetic Resonance Spectros
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0911-B06-05
A Study of V3+/4+ Levels in Semi-insulating 6H-SiC using Optical Admittance and Electron Paramagnetic Resonance Spectroscopies Wonwoo Lee1, and Mary E Zvanut2 1 University of Alabama at Birmingham, 1530 3RD Ave S., Birmingham, 35294 2 University of Alabama at Birmingham, Birmingham, 35294 ABSTRACT The purpose of this study is to identify the vanadium acceptor levels in semiinsulating (SI) 6H-SiC using optical admittance spectroscopy (OAS) and electron paramagnetic resonance (EPR) spectroscopy. OAS conductance peaks near at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV are identified as V3+/4+ levels at the quasi-cubic sites. An OAS peak at 0.87 eV is assigned to the same transition at the hexagonal site. EPR measurements before illumination revealed the characteristic spectrum of V3+. The presence of the V3+ signal supports the identification of the OAS peaks as transitions from the V3+/4+ level to the conduction band. Photo-induced EPR measurements reveal a change in the intensity of V3+ and V4+ at 0.8 ± 0.1 eV, where the amplitude of the V3+ charge state decreases and that of V4+ increases by approximately equal amounts. Although the individual sites are not resolved in the photo-induced EPR data, the 0.8 eV feature strongly supports the assignment of the three OAS peaks as acceptor levels.
INTRODUCTION Silicon carbide compared to other semiconductor materials has relatively unique electrical and physical properties, such as high electric field breakdown, high electron mobility, and high thermal conductivity. Thus, silicon carbide is a good candidate for a new generation of electronic devices with high power, high frequency, and high temperature-resistant properties. Doping with an electrically active impurity, such as vanadium, is one way to make a semi-insulating material with resistivity as high as 1010 ohm-cm at room temperature [1]. SI SiC polytypes may be used as a substrate in fabrication of electronic devices such as MESFETs to reduce dielectric loss and device parasitics. For these reasons, many different techniques have been used to investigate the vanadium acceptor levels in 6H-SiC, such as magnetic circular dichroism (MCD) and electron spin resonance (ESR), radiotracer deep level transient spectroscopy (DLTS), photoluminescence (PL), deep level optical absorption (DLOA), and Hall measurements [2-8]. For example, Achtziger and coworker suggested that the vanadium acceptor level is located 0.71 eV (quasi-cubic) and 0.75 eV (hexagonal) below the conduction band edge in n-type 6H-SiC using radiotracer DLTS [5]. Lauer and coworker reported that the vanadium acceptor levels in 6H-SiC are located at Ec-0.74 eV (hexagonal) and Ec-0.68 eV (quasi-cubic) based on PL and DLOA studies [6]. A significant feature of all of these studies is that there is no direct evidence of the charge states of species assigned to the transitions.
OAS enables the measurement of high resistivity samples while avoiding the difficulties encountered when using techniques such as DLTS and thermal admittance spectroscopy [9]. The main
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