Publications (26)44.05 Total impact
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Article: Effects of Illumination on Ar+-Implanted n-Type 6H-SiC Epitaxial Layers
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ABSTRACT: Argon ions were implanted into n-type 6H-SiC epitaxial layers at 600°C. Postimplantation annealing was carried out at 1,600°C for 5min in an Ar ambient. Four implantation-induced defect levels were observed at EC-0.28eV, EC-0.34eV, EC-0.46eV, and EC-0.62eV by deep level transient spectroscopy. The defect center at EC-0.28eV is correlated with ED1/ED2 and with ID5. The defect at EC-0.46eV with a capture cross section of 7.8×10−16cm2 is correlated with E1/E2, while the defect at EC-0.62eV with a capture cross section of 2.6×10−14cm2 is correlated with Z1/Z2. Photo deep level transient spectroscopy was also used to study these defects. Upon illumination, the amplitudes of the deep level transient spectroscopy (DLTS) peaks increased considerably. Two emission components of Z1/Z2 were revealed: one fast and the other slow. The fast component could only be observed with a narrow rate window. In addition, a new defect was observed on the low-temperature side of the defect at EC-0.28eV when the sample was illuminated.Journal of Electronic Materials 04/2012; 36(4):340-345. · 1.47 Impact Factor -
Article: Electric field enhancement of electron emission rates from Z1/2 centers in 4H-SiC
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ABSTRACT: Z<sub>1/2</sub> defect centers were produced by irradiating 4H -SiC bulk samples with 1 MeV electrons at room temperature. The emission rate dependence on the electric field in the depletion region was measured using deep level transient spectroscopy and double-correlation deep level transient spectroscopy. It is found that the Z<sub>1/2</sub> defect level shows a strong electric field dependence with activation energy decreasing from E<sub>c</sub>-0.72 eV at zero field to E<sub>c</sub>-0.47 eV at 6.91×10<sup>5</sup> V / cm . The phonon assisted tunneling model of Karpus and Perel [Sov. Phys. JETP 64, 1376 (1986)] completely describes the experimental data. This model describes the dependence of the emission rate on electric field F as e<sub>n</sub>(F)=e<sub>no</sub> exp (F<sup>2</sup>/F<sub>c</sub><sup>2</sup>) , where F<sub>c</sub> is the characteristic field that depends on the phonon assisted tunneling time τ<sub>2</sub> . The values of F<sub>c</sub> and τ<sub>2</sub> were determined and the analysis of the data leads to the suggestion that Z<sub>1/2</sub> may be a substitutional point defect.Journal of Applied Physics 10/2009; · 2.17 Impact Factor -
Article: Electrical properties of unintentionally doped semi-insulating and conducting 6H-SiC
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ABSTRACT: Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0 eV. LTPL lines near 1.00 and 1.34 eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being intermediate between those two. OAS and TSC experiments confirmed the high purity of this material. The results suggest that the relative concentrations of a dominant deep level and nitrogen and boron impurities can explain the electrical properties in this material.Journal of Applied Physics 08/2006; 100(4):043706-043706-5. · 2.17 Impact Factor -
Article: Capacitance dispersion in ion implanted 4H and 6H-silicon carbide
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ABSTRACT: Nitrogen doped 4H–SiC and 6H–SiC epitaxial layers with net doping concentration of 1.5×1015 cm−3 were implanted with either Al, B, or Ar ions at 600 °C. The energy of the ions was 160 keV and at a dose of 2×1016 cm−2. After annealing at 1600 °C for 5–60 min, Schottky diodes were fabricated on the samples. The measured junction capacitance of ion implanted samples varies with both temperature and frequency. The thermal activation energy determined from the frequency break ωB ranges from 183 to 202 meV. The Ar implanted samples remain n-type and show no capacitive dispersion. Similarly, the junction capacitance of samples doped with Al does not exhibit dispersion. It is suggested that the implantation induced defects play a role in the observed dispersion. The data show that the series resistance is insignificant and does not account for the observed dispersion. © 2002 American Institute of Physics.Journal of Applied Physics 10/2002; 92(8):4465-4469. · 2.17 Impact Factor -
Article: Ion Implantation Induced Deep Defects in n-type 4H-Silicon Carbide
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ABSTRACT: Aluminum (Al) and argon (Ar) ions were implanted into n-type 4H-SiC epitaxial layers at 600 °C. The energy of the ions was 160 keV at a dose of 2 × 1016 cm-2. After annealing at 1600 °C for 5–60 minutes, Schottky diodes were fabricated on the ion implanted samples. Deep Level Transient Spectroscopy (DLTS) was used to characterize ion implantation induced defects. A defect at EC-0.18 eV was observed in the Al+ implanted devices annealed for five and fifteen minutes. However, annealing for 30 minutes produced an additional deeper defect at EC -0.24 eV. This defect annealed out after a sixty minute anneal. DLTS studies of Ar+ implanted devices showed six defect levels at EC -0.18 eV, EC -0.23 eV, EC -0.31 eV, EC -0.38eV, EC -0.72 eV, and EC -0.81eV. These defects are attributed to intrinsic-related defects. It is suggested that “hot” implantation of Al+ inhibits the formation of intrinsic-related defects. While “hot” implantation of Ar+ into 4H-SiC does not reduce the concentration of the vacancies and interstitials.MRS Proceedings. 12/2001; 742. -
Article: Fermi level control and deep levels in semi-insulating 4H–SiC
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ABSTRACT: Temperature dependent Hall effect, optical admittance spectroscopy, and optical absorption measurements of semi-insulating bulk 4H–SiC are reported. Both intentionally vanadium doped material and commercial grade semi-insulating material were investigated. The carrier concentration versus inverse temperature results from Hall effect measurements up to 1000 K indicated the samples were dominated by one of two deep levels near midgap. In addition to the deep donor level of substitutional vanadium, Ec−1.6 eV, we observed another level at Ec−1.1 eV in some samples, indicating that levels other than the vanadium donor can pin the Fermi level in semi-insulating SiC. Optical admittance measurements on the semi-insulating material indicate the presence of levels at Ec−1.73 and 1.18 eV that were previously observed in conducting samples with this technique and we attribute these levels to the same defects producing the 1.1 and 1.6 eV levels seen by Hall effect. Secondary ion mass spectroscopy measurements of dopant and impurity concentrations are reported. Even though vanadium is present in all of these samples, along with other impurities we are at present unable to definitively identify the 1.1 eV level. © 1999 American Institute of Physics.Journal of Applied Physics 10/1999; 86(9):5040-5044. · 2.17 Impact Factor -
Conference Proceeding: The comparative studies of chemical vapor deposition grown epitaxial layers and of sublimation sandwich method grown 4H-SiC samples
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ABSTRACT: Thermal admittance spectroscopy was used to characterize the shallow dopants in chemical vapor deposition (CVD) grown thin films and in sublimation sandwich method (SSM) grown 4H-SiC layers. The values of the activation energy levels of E{sub c}-0.054 eV for Nitrogen at the hexagonal site and of E{sub c}-0.10 eV for Nitrogen at the cubic site were indices of comparison. The net carrier concentrations (N{sub D}-N{sub V}) of the films were determined by capacitance-voltage measurements. The net carrier concentrations for the SSM films ranged from 2 x 10{sup 17} to 7 x 10{sup 17} cm{sup {minus}3}. The two Nitrogen levels were observed in the CVD films. Hopping conduction with an activation energy of E{sub c}-0.0058 eV was observed in one SSM sample having N{sub D}-N{sub V} = 7 x 10{sup 17} cm{sup {minus}3}.06/1999 -
Article: Shallow acceptor levels in 4H- and 6H-SiC
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ABSTRACT: Shallow impurities are the principal means of affecting the electrical properties of semiconductors in order to induce desired characteristics. They can be used to isolate a region by introducing carriers of opposite charge, or they can be used to enhance the number of carriers of a particular type. Thermal admittance spectroscopy has been used to determine the activation energies of the principal p-type dopants, Al and B, in 4H and 6H-SiC, and temperature dependent Hall effect measurements were used to study the shallow B acceptors in 6H-SiC. The accept or species B and Al occupy inequivalent lattice sites in the Si sublattice, and would be expected to exhibit distinct energy levels for each site in analogy to the well known donor energy levels of N. Activation energies for B in 6H-SiC were found to be Eh=Ev+0.27 eV, Ek1=Ev+0.31 eV, and Ek2=Ev+0.38 eV. Al acceptors in 4H-SiC were found to exhibit two energy levels at Eh=Ev+0.212 eV and Ek=Ev+0.266 eV.Journal of Electronic Materials 01/1999; 28(3):190-195. · 1.47 Impact Factor -
Conference Proceeding: The 1.1 eV deep level in 4H-SiC
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ABSTRACT: Temperature dependent Hall effect and optical absorption measurements of vanadium doped semi-insulating 4H-SiC samples are reported along with optical admittance spectroscopy measurements of related material. In addition to the deep donor and acceptor levels of substitutional vanadium, E<sub>C</sub>-1.6 eV and E<sub>C</sub>-0.7 eV respectively, we report an additional level at 1.1 eV. This level has been seen in undoped 4H-SiC by optical admittance spectroscopy which has also detected a similar level at E<sub>C</sub>-1.0 eV in 6H-SiC, and by temperature dependent Hall effect in vanadium doped material. Optical absorption measurements of the vanadium intra-center absorption show that this level is not either of the two substitutional vanadium levels. SIMS measurements support the hypothesis that the 1.1 eV level is a complex of vanadium and another impurity, possibly titaniumSemiconducting and Insulating Materials, 1998. (SIMC-X) Proceedings of the 10th Conference on; 02/1998 -
Article: Boron acceptor levels in 6H-SiC bulk samples
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ABSTRACT: Thermal admittance spectroscopy has been used to determine the ground-state energies of the boron impurity in 6H -SiC. The background doping, N<sub>A</sub>-N<sub>D</sub>, of the samples used in this study ranged from 3×10<sup>16</sup> to 1×10<sup>18</sup> cm <sup> -3 </sup>. 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 E<sub>v</sub>+0.27 eV , E<sub>v</sub>+0.31 eV , and E<sub>v</sub>+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. © 1997 American Institute of Physics.Applied Physics Letters 10/1997; · 3.84 Impact Factor -
Article: Hopping conduction in heavily doped bulk n-type SiC
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ABSTRACT: The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 1017 cm−3 or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε3 obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε3 value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy Ea for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.Journal of Electronic Materials 02/1997; 26(3):113-118. · 1.47 Impact Factor -
Article: Shallow and deep levels in n‐type 4H‐SiC
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ABSTRACT: The nitrogen levels in 4H‐SiC have been determined using thermal admittance spectroscopy. The values of E c -0.053 eV for nitrogen at the hexagonal site and E c -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 E c -1.73 eV. The photoconductance build up transients of the E c -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 E c -1.73 eV level follow the stretched exponential form. The potential barrier against recapture of photoexcited carriers was determined to be 18 meV for the vanadium donor level in 4H‐SiC. © 1996 American Institute of Physics.Journal of Applied Physics 06/1996; · 2.17 Impact Factor -
Article: Determination of the activation energy ϵ3 for impurity conduction in n‐type 4H–SiC
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ABSTRACT: 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 ϵ 3 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 N D –N A ≪10<sup>18</sup> cm<sup>-3</sup>. The thermal admittance spectrum of the lightly n‐type sample showed the two nitrogen levels at 53 and 100 meV.Applied Physics Letters 06/1996; · 3.84 Impact Factor -
Article: Electronic properties of boron in p-type bulk 6H-SiC
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ABSTRACT: The electronic properties of boron in bulk 6H-SiC have been studied by temperature dependent Hall effect, thermal admittance spectroscopy, and optical absorption. A single acceptor level located between 0.27 and 0.35 eV above the valence band is associated with boron on a silicon lattice site. The deep nature of this acceptor level prevents complete thermal activation of the level at room temperature and thus carrier concentration measurements at this temperature will not give the total boron concentration. A spread in the measured activation energy for boron is reported. Measurement of optical absorption is suggested as a nondestructive measure of boron concentration. No evidence for the D-center was observed in this material.Journal of Electronic Materials 04/1996; 25(5):863-867. · 1.47 Impact Factor -
Article: Temperature dependence of the barrier height of metal‐semiconductor contacts on 6H‐SiC
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ABSTRACT: We have examined the temperature dependence of the barrier height of Al, Ag, Au, and W metal‐semiconductor contacts on n‐type 6H‐SiC, and Al and Ag metal‐semiconductor contacts on p‐type 6H‐SiC. The barrier height was determined from the reverse biased capacitance‐voltage characteristics of the contacts at temperatures ranging from 300 to 670 K. The measurements were made at 1 MHz. These measurements were compared to the behavior predicted by standard models. All the diodes displayed a negative temperature dependence on n‐type SiC, and a positive temperature dependence on p‐type SiC. The temperature coefficient is related to the electronegativity of the metal by linear expression.Journal of Applied Physics 02/1996; · 2.17 Impact Factor -
Article: Optical admittance studies of vanadium donor level in high‐resistivity p‐type 6H‐SiC
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ABSTRACT: 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 [V<sup>4+</sup>(3d<sup>1</sup>)] level at E v +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, α 1 , α 2 , G max (1) and G max (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.Journal of Applied Physics 02/1996; · 2.17 Impact Factor -
Article: Determination of the band offsets of the 4H–SiC/6H–SiC heterojunction using the vanadium donor (0/+) level as a reference
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ABSTRACT: 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. © 1995 American Institute of Physics.Applied Physics Letters 11/1995; 67(22):3319-3321. · 3.84 Impact Factor -
Article: Persistent photoconductance in n‐type 6H‐SiC
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ABSTRACT: 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. © 1995 American Institute of Physics.Journal of Applied Physics 06/1995; · 2.17 Impact Factor -
Article: Electrical and Optical Properties of Defects in N-Type 4h-Sic
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ABSTRACT: Several n-type 4H-SiC samples grown by the physical vapor transport technique were studied using both thermal and optical admittance spectroscopy. The ionization energy ED for nitrogen donors in 4H-SiC was determined to be 0.053 eV and 0.10 eV below the conduction band. This agrees reasonably well with the values of 0.052 eV and 0.092 eV determined by IR absorption measurements. It is believed that EC-0.053 eV is the ionization energy of the nitrogen atom occupying the hexagonal site (h). The 0.10 eV activation energy has been attributed to the nitrogen atoms occupying the cubic sites(k).The optical admittance studies reveal five conductance peaks. One of these corresponds to the band-to-band transitions from which the bandgap of 3.41 eV was determined at 40K. The other four conductance peaks correspond to transitions from defect levels to the conduction band. These defects are located at EC-1.73 eV, EC-1.18 eV, EC-0.87 eV, and EC-0.72 eV.MRS Proceedings. 12/1994; 410. -
Article: Shallow levels in n‐type 6H‐silicon carbide as determined by admittance spectroscopy
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ABSTRACT: 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 E C -0.04 eV and E C -0.03 eV were detected in the more heavily n‐type (N D -N A =10<sup>18</sup> cm<sup>-3</sup>) samples. These defects may be due to contaminants other than nitrogen. A defect level at E C -0.08 eV as detected in a sample with N D -N A =8.9×10<sup>17</sup> cm<sup>-3</sup>. This level is associated with nitrogen at the hexagonal site (h). An electron trap at E C -0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k 1 k 2 ). This level was observed only in the lightly n‐type samples (N D -N A =4.7 ×10<sup>15</sup>–6.4×10<sup>17</sup> cm<sup>-3</sup>).Journal of Applied Physics 05/1994; · 2.17 Impact Factor
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1984–2012
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University of Dayton
- Department of Physics
Dayton, OH, USA
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1993–2002
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Wright-Patterson Air Force Base
Dayton, OH, USA
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