William D. Mitchell

An electrical characterization of a two-dimensional electron gas in GaN/AlGaN on silicon substrates

We present results of transport measurements performed on AlGaN/GaN heterostructures grown on silicon substrates. Variable temperature Hall effect measurements revealed that the temperature dependence of the carrier density and mobility were characteristic of a two-dimensional electron gas (2DEG). Carrier densities greater than 1×1013 cm−2 and Hall mobilities in excess of 1500 cm2/V s were measured at room temperature. Variable field Hall measurements at low temperatures, and in magnetic fields up to 6 T, indicated that conduction is dominated by a single carrier type in these samples. Shubnikov–de Haas (SdH) measurements were also performed, but no oscillations were observed in fields up to 8 T and at temperatures as low as 1.2 K. Illuminating some of the samples with a blue (λ=470 nm) light emitting diode (LED) induced a persistent increase in the carrier density. SdH measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from an UV (λ=395 nm) LED induced well-defined SdH oscillations in fields as low as 4 T. The observation of SdH oscillations confirmed the presence of a 2DEG in these structures. It is hypothesized that small angle scattering suppressed the oscillations before exposure to UV light. This conclusion is supported by the observed increase in the quantum scattering time, τq, with the carrier density and the calculated quantum to transport scattering times ratio, τq/τc. For instance, in one of the samples the τq increased by 32% while the τc changed by only 3% as the carrier density increased; an indication of an increase in the screening of small angle scattering. The absence of SdH oscillations in fields up to 8 T and at temperatures as low as 1.2 K is not unique to AlGaN/GaN on silicon. This behavior was observed in AlGaN/GaN on sapphire and on silicon carbide. SdH oscillations were observed in one AlGaN/GaN on silicon carbide sample following exposure to radiation from an UV LED.We present results of transport measurements performed on AlGaN/GaN heterostructures grown on silicon substrates. Variable temperature Hall effect measurements revealed that the temperature dependence of the carrier density and mobility were characteristic of a two-dimensional electron gas (2DEG). Carrier densities greater than 1×1013 cm−2 and Hall mobilities in excess of 1500 cm2/V s were measured at room temperature. Variable field Hall measurements at low temperatures, and in magnetic fields up to 6 T, indicated that conduction is dominated by a single carrier type in these samples. Shubnikov–de Haas (SdH) measurements were also performed, but no oscillations were observed in fields up to 8 T and at temperatures as low as 1.2 K. Illuminating some of the samples with a blue (λ=470 nm) light emitting diode (LED) induced a persistent increase in the carrier density. SdH measurements were repeated and again no oscillations were present following illumination. However, exposing the samples to radiation from...

Vanadium donor and acceptor levels in semi-insulating 4H- and 6H-SiC

The electronic levels of vanadium in semi-insulating 4H- and 6H-SiC have been reinvestigated using temperature dependent Hall effect and resistivity measurements at temperatures up to 1000K in conjunction with electron paramagnetic resonance (EPR) and optical absorption measurements which were used to identify the charge state of vanadium in the material. Two distinct thermal activation energies were found for each polytype. The shallower of the two levels correlated with the presence of both V3+ and V4+ in the EPR and absorption experiments, demonstrating that this level is the vanadium acceptor level while the deeper level is the donor level for which the V4+ charge state was observed. The results for the V4+∕5+ donor level, EC−1.57±0.09eV for 4H-SiC and EC−1.54±0.06eV for 6H-SiC, are in agreement with the generally accepted values. However, the results for the V3+∕4+ acceptor level, EC−0.85±0.03eV in 6H-SiC and EC−1.11±0.08eV in 4H-SiC, are significantly higher than previously assumed. Variations in cr...

Compensation mechanism in high purity semi-insulating 4H-SiC

A study of deep levels in high purity semi-insulating 4H-SiC has been made using temperature dependent Hall effect (TDH), thermal and optical admittance spectroscopies, and secondary ion mass spectrometry (SIMS). Thermal activation energies from TDH varied from a low of 0.55eV to a high of 1.65eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Fits of the TDH data to different charge balance equations and comparison of the fitting results with SIMS measurements indicated that the deep levels are acceptorlike even though they are in the upper half of the band gap. Carrier concentration measurements indicated that the deep levels are present in concentrations in the low 1015cm−3 range, while SIMS results demonstrate nitrogen and boron concentrations in the low to mid-1015-cm−3 range. The results suggest that compensation in this material is a complex process involving multiple deep levels.

Study of the effects of an AlN interlayer on the transport properties of AlGaN/AlN/GaN heterostructures grown on SiC

Transport measurements were used to characterize AlGaN∕AlN∕GaN∕SiC. While the carrier concentration, n=9.2×1012cm−2, remained relatively unchanged from 300 down to 1.2K, the mobility increased from 2100 to over 3×104cm2∕Vs. Shubnikov-de Haas oscillations were observed in fields as low as 2T. Despite the high n, quantum Hall plateaus, which are rarely reported in conventional AlGaN∕GaN with comparable n, were observed. The calculated quantum scattering time of 0.28ps is longer than what is typically reported for conventional AlGaN∕GaN. This improvement is believed to be due to a reduction in alloy scattering.

Photo-EPR and Hall Measurements on Undoped High Purity Semi-Insulating 4H-SiC Substrates

EPR measurements at 37 GHz and 77K have been made on undoped High Purity SemiInsulating 4H-SiC in the dark and after illumination. Two intrinsic defects with donor-like behavior have been detected by EPR. Temperature dependent Hall effect measurements confirm that the conduction is n-type in these samples. It is suggested that that the deep donor-like defects with S =1/2 are the VSi 3− and probably VC 1− defects and that they might be responsible for the compensation of shallow nitrogen and boron in SI4H SiC. Introduction Undoped high purity semi-insulating (HPSI) physical vapor transport (PVT) grown 4H SiC material shows a variety of activation energies ranging in our measurements from 0.95 to 1.4 eV. But the deep level or levels responsible for compensation in this material is not known at present. Recent EPR measurements at several laboratories have detected intrinsic defects in undoped semiinsulating (s.i.) SiC. Photo-EPR experiments in these laboratories suggest that these defects have energy levels in the bottom half of the bandgap [1-3]. Here we report the results of a photo-EPR study of HPSI s.i. 4H-SiC samples along with high temperature Hall effect (HTHE) measurements. Experimental Details A series of undoped PVT HPSI 4H-SiC samples were investigated by EPR and HTHE measurements. EPR measurements were performed on Q-band EPR spectrometer. The photo-EPR experiments were performed using a 250 W high-pressure mercury vapor lamp and a 100 W halogen lamp combined with interference filters.

Cr:ZnSe laser incorporating anti-reflection microstructures exhibiting low-loss, damage-resistant lasing at near quantum limit efficiency

We report demonstration of efficient continuous-wave lasing from chromium-doped zinc selenide using anti-reflection microstructures (ARMs) in place of thin-film AR coatings or Brewster angle cavity geometries. ARM textures are more resistant to laser-induced damage than coatings, exhibit low-loss, wide angular acceptance, broad wavelength effectiveness, and are not susceptible to water absorption. Slope-efficiencies of 68% were achieved, which compares favorably to the thin-film control samples at 58% for the same cavity. ARMs hold promise for near-term power scaling and wavelength agility of transition-metal-ion doped II-VI lasers.

Deep Level Point Defects in Semi-Insulating SiC

The high resistivity of SiC required for many device applications is achieved by compensating residual donors or acceptors with vanadium or intrinsic defects. This work addresses the defect levels of substitutional vanadium and the positively charged carbon vacancy (VC +) in semiinsulating (SI) SiC. After reviewing the earlier studies related to both defects, the paper focuses on temperature-dependent Hall measurements and photo-induced electron paramagnetic resonance (EPR) experiments of 4H and 6H SI SiC. In vanadium-doped samples, a V3+/4+ level near Ec-1.1 eV (4H) and Ec-0.85 eV (6H) is estimated by a comparison of dark EPR spectra and the activation energy determined from the Hall data, assuming that vanadium controls the Fermi level. In high purity semiinsulating substrates, analysis of time-dependent and steady-state photo-EPR data suggests that the plus-to-neutral transition of the carbon vacancy involves a structural relaxation of about 0.6 eV.

Deep Level near EC – 0.55 eV in Undoped 4H-SiC Substrates

A variety of 4H-SiC samples from undoped crystals grown by the physical vapor transport technique have been studied by temperature dependent Hall effect, optical and thermal admittance spectroscopy and thermally stimulated current. In most samples studied the activation energies were in the range 0.9 - 1.6 eV expected for commercial grade HPSI 4H-SiC. However, in several samples from developmental crystals a previously unreported deep level at EC-0.55 ± 0.01 eV was observed. Thermal admittance spectroscopy detected one level with an energy of about 0.53 eV while optical admittance spectroscopy measurements resolved two levels at 0.56 and 0.64 eV. Thermally stimulated current measurements made to study compensated levels in the material detected several peaks at energies in the range 0.2 to 0.6 eV.

Intrinsic deep levels in semi-insulating silicon carbide

High temperature Hall effect and resistivity measurements have been made on undoped, high purity semi-insulating (HPSI) 4H SiC samples. Both physical vapor transport and high temperature chemical vapor deposition grown samples have been investigated. Resistivity measurements before and after annealing at temperatures up to 1800°C are also reported. Hall and resistivity results are compared with low temperature photoluminescence results. The thermal activation energies for HPSI material taken from temperature dependent resistivity measurements varied from 0.9 to 1.5 eV. Hall effect measurements were made on several HPSI. In all cases the material was found to be n-type and the measured carrier concentration activation energies agreed within a few tens of percent with the resistivity activation energies. Mixed conduction analysis of the data suggests that the hole concentration was negligible in all of the samples studied. This suggests that the defects responsible for the semi-insulating properties have deep levels located in the upper half of the bandgap.

Optically Induced Transitions among Point Defects in High Purity and Vanadium-Doped Semi-Insulating 4H SiC

We use electron paramagnetic resonance (EPR) spectroscopy and photo-induced EPR to discern the compensating species associated with the Fermi level extracted from temperaturedependent Hall measurements of vanadium-doped 4H SiC and high purity semi-insulating 4H SiC. The results demonstrate that the V 4+ /V 5+ transition is responsible for the 1.6 eV carrier activation energy (Ea) measured in some of the vanadium-doped samples. In other vanadium-doped wafers, photo-EPR data reveal transitions for V 3+ at energies between 1.1 and 1.25 eV, consistent with the activation energy. However, the relationship between V 3+ and Ea is not clear. The carbon vacancy (VC), which is detected in all HPSI wafers, exhibits a range of photo-thresholds similar to the varying values measured for Ea. Therefore, although EPR measurements show that the number of uncompensated carbon vacancies is below that thought necessary to neutralize the residual shallow donors and acceptors, VC likely participates in compensation.