A. O. Evwaraye

Shallow and deep levels in n‐type 4H‐SiC

The nitrogen levels in 4H‐SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H‐SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H‐SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential for...

Examination of electrical and optical properties of vanadium in bulk n‐type silicon carbide

Deep level transient spectroscopy (DLTS) has been used to characterize deep impurity levels in n‐type 6H‐SiC single crystals. A defect level at Ec−0.71 eV with an electron capture cross section σ=5.63×10−20 cm2 was observed. Defect concentration profiles confirm that the defect is a bulk defect. Infrared absorption measurements in the spectral range of 7000–7700 cm−1 were made using these samples. The infrared absorption spectrum characteristic of vanadium in silicon carbide is composed of a group of three absorption lines in the spectral range of 7000–7700 cm−1. This infrared signature was seen in the specimens in which the DLTS spectrum revealed the presence of deep traps. This signature was absent in those specimens where no deep traps were indicated by DLTS. Correlating these facts, we have concluded that the observed peak at Ec−0.71 eV was due to vanadium atoms in silicon carbide.

Observation of surface defects in 6H‐SiC wafers

A broad peak was observed in commercially available single‐crystal 6H‐SiC material. The samples were nitrogen doped, n type with free carrier concentration (ND−NA) of 1.3×1018 cm−3 that was determined from capacitance‐voltage (C‐V) measurements. The defect concentration profile showed that the defect was spatially localized and had a maximum concentration of 2.5×1014 cm−3 at 570 A from the semiconductor‐metal interface. The activation energy varied with applied voltage from Ec−0.40 eV at VR=−7 V to Ec−0.54 eV at VR=−5 V. This can be explained qualitatively in terms of the Poole–Frenkel effect. The defect was removed by the growth and subsequent removal of an oxide layer. Therefore, we conclude that the defect was caused by residual damage from the polishing process.

Shallow levels in n‐type 6H‐silicon carbide as determined by admittance spectroscopy

Admittance spectroscopy has been used to study shallow levels in n‐type 6H‐SiC single crystals. A total of eight unintentionally doped n‐type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n‐type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n‐type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).Admittance spectroscopy has been used to study shallow levels in n‐type 6H‐SiC single crystals. A total of eight unintentionally doped n‐type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n‐type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n‐type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).

Boron acceptor levels in 6H-SiC bulk samples

Thermal admittance spectroscopy has been used to determine the ground-state energies of the boron impurity in 6H-SiC. The background doping, NA−ND, of the samples used in this study ranged from 3×1016 to 1×1018 cm−3. From electron spin resonance studies, it is known that boron substitutes for silicon in the silicon carbide lattice occupying three inequivalent sites. Using admittance spectroscopy the ground state energies of Ev+0.27 eV, Ev+0.31 eV, and Ev+0.38 eV were determined for the shallow boron acceptor in 6H-SiC. The free carrier concentration does not appear to be the only determining factor for which the boron acceptor level is observed.

Persistent photoconductance in n‐type 6H‐SiC

Defects in n‐type 6H‐SiC have been studied using optical admittance spectroscopy. Six conductance peaks, which correspond to photoexcitation of electrons into the conduction band from defects and the valence band at different wavelengths, were clearly identified. Persistent photoconductance (PPC) due to a defect 1.07 eV below the conduction band was studied. The decay kinetics of the PPC follow the stretched exponential form. The relaxation time constant τ and the stretching factor β were systematically measured as functions of temperature. The thermal capture barrier of 61 meV was determined from these results. It was also found that the PPC can be quenched optically by illumination with sub band gap radiation. This is the first reported observation of optical quenching of PPC in n‐type 6H‐SiC. The lattice relaxation model is used to qualitatively explain these experimental results.

OPTICAL ADMITTANCE STUDIES OF VANADIUM DONOR LEVEL IN HIGH-RESISTIVITY P-TYPE 6H-SIC

The vanadium donor level in high‐resistivity p‐type 6H‐SiC has been studied using optical admittance spectroscopy. Besides the conductance peak due to the band to band transitions, there are three conductance peaks in the spectra of most of the samples. These peaks correspond to photoexcitation of electrons from the valence band to the defect levels. The conductance peak due to the vanadium donor [V4+(3d1)] level at Ev+1.55 eV is identified. The build up of the photoconductance at this peak was studied and it was found that the conductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1, α2, Gmax(1) and Gmax(2), were determined as functions of temperature. The persistent photoconductance (PPC) due to this defect was also studied. The decay kinetics of the PPC follow the stretched exponential form. The potential barrier against recapture of carriers was determined to be 220 meV for the vanadium donor level.

Determination of the band offsets of the 4H–SiC/6H–SiC heterojunction using the vanadium donor (0/+) level as a reference

Optical admittance spectroscopy has been used to study defects in 4H–SiC; the vanadium donor level at EC‐1.73 eV has been identified. The optical admittance spectrum of 4H–SiC is similar to that of 6H–SiC, where the vanadium donor level is at EC‐1.59 eV. The band gaps of 6H–SiC and 4H–SiC were measured. The values of 3.10±0.03 eV for 6H–SiC and 3.41±0.03 eV for 4H–SiC are in reasonable agreement with reported values. Using the vanadium donor level in both 4H–SiC and 6H–SiC as a common reference, the band offsets for 6H–SiC/4H–SiC heterojunction are estimated to be ΔEC=0.14 eV and ΔEV=0.17 eV.

Determination of the activation energy ε3 for impurity conduction in n‐type 4H–SiC

Impurity conduction (or hopping conduction) has been observed in the more heavily n‐type 4H–SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies e3 for hopping were 4–5 meV and 2.3–3.0 meV, respectively. No evidence of hopping conduction was seen by either method in the sample where ND–NA<1018 cm−3. The thermal admittance spectrum of the lightly n‐type sample showed the two nitrogen levels at 53 and 100 meV.

KINETICS OF SLOW BUILDUP OF PHOTOCONDUCTANCE IN N-TYPE 6H-SIC

The slow buildup of photoconductance due to a peak centered at 1.13 eV below the conduction band in n‐type 6H–SiC samples has been studied using optical admittance spectroscopy. The background doping of the samples ranges from 3.2×1015 cm−3 to 2×1018 cm−3. The results indicate that the carriers responsible for the observed conductance are excited from defect levels that are energetically close. The photoconductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1,α2,Gmax(1), and Gmax(2), are determined as functions of temperatures. The background doping ND‐NA has no effect on the conductance buildup process, this indicates that the nitrogen donors play no role either in the formation of the defect or in the buildup process of the photoconductance.