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We investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using both optical experiments and hybrid density functional theory calculations. Previous work has established that carbon acceptors $({\mathrm{C}}_{\mathrm{N}})$ give rise to a yellow luminescence band near 2.2 eV along with a blue luminescence band near 2.9 eV. Photoluminescence measurements show the yellow band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid density functional theory we calculate the electrical and optical behavior of carbon centers containing multiple carbon impurities, which may arise in heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor $({\mathrm{C}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{C}}_{\mathrm{N}})$ complex is a candidate to explain the shift in the yellow luminescence peak. Tricarbon complexes have high formation energies and modest binding energies, and also give rise to optical transitions that are inconsistent with the observed spectra. We also identify the split dicarbon interstitial on the gallium site as a low-energy species with a large binding energy that may act to compensate carbon acceptors. Local vibrational modes are calculated for carbon impurity centers, and we compare these results to recent experiments. Dicarbon and tricarbon complexes involving ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$ exhibit modes that are only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes much higher than ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$.

Carbon doping is a viable approach for compensating the unintentional donors in GaN and achieving semi-insulating substrates necessary for high-frequency, high-power devices. In this work, bulk material properties and point defects are studied in mm-thick free-standing carbon-doped GaN to understand the efficacy of the carbon dopant. Temperature-dependent Hall measurements reveal high resistivity and low carrier concentrations at temperatures as high as 560°C in a 6 × 1017 cm−3 C-doped sample, and electron paramagnetic resonance (EPR) indicates that carbon acts as the compensating defect. Photoluminescence, in agreement with photo-EPR, suggests that the compensating center is CN; however, additional defects, which possibly limit compensation, are formed at carbon concentrations greater than 5 × 1017 cm−3.

Gallium oxide (Ga2O3), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga2O3 crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga2O3. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies.

Gallium oxide (Ga2O3), a promising candidate for high-power electronics, may be doped with Fe to control the native n-type conductivity and produce semi-insulating single-crystalline substrates. Here, intentionally as well as unintentionally Fe-doped β-Ga2O3 crystals grown by the Czochralski (Cz) and the floating zone (FZ) methods are studied. While the Cz samples contain Ir on the order of 1017 cm−3 due to the Ir crucible, the FZ samples are free of this contaminant. Photoinduced electron paramagnetic resonance (EPR) is performed by illuminating the samples with light emitting diodes from 0.7 to 4.7 eV while monitoring the intensity of the EPR signal. The following optical transitions associated with Fe are observed: one between 1 and 2 eV occurs in both Cz and FZ samples, another near 2.5 eV is unique to Cz Ga2O3, and a third near 3 eV appears in FZ material. The transition below 2 eV is the well-studied Fe2+-to-Fe3+ transition, intrinsic to Fe. Near 2.5 and 3 eV, the amount of Fe3+ decreases by capturing electrons excited from other defects present near or below the middle of the bandgap. By comparing Cz and Ir-free FZ crystals, we show that the 2.5 eV transition is primarily caused by Ir in Fe-doped Cz samples, whereas the ∼3 eV transition observed in FZ samples could be caused by isolated or complex intrinsic defects. By differentiating transitions between Cz and FZ material, the results will be relevant to epitaxial films, which do not contain the Ir present in the commonly studied Cz material.

Fe acts as an electron trap in gallium oxide (Ga2O3), thereby producing a semi-insulating material that can be used in device fabrication. However, such trapping can lead to negative effects when Fe is unintentionally incorporated into bulk crystals or thin films. In this work, photoinduced electron paramagnetic resonance (photo-EPR) is used to investigate carrier capture at Fe in β-Ga2O3. Two crystals doped with 8 × 1017 cm−3 and 5 × 1018 cm−3 Fe and one Mg-doped crystal containing 7 × 1016 cm−3 unintentional Fe are studied by illuminating with LEDs of photon energies 0.7–4.7 eV. Steady state photo-EPR results show that electrons excited from Ir, an unintentional impurity in bulk crystals, are trapped at Fe during illumination with photon energy greater than 2 eV. Significantly, however, trapping at Fe also occurs in the crystals where Ir does not participate. In such cases, we suggest that excitation of intrinsic defects such as oxygen or gallium vacancies are responsible for trapping of carriers at Fe. The results imply that the investigation of intrinsic defects and their interaction with Fe is necessary to realize stable and reliable Ga2O3:Fe devices.

Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm−3 and 1019 cm−3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor–shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm−3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

Impurities such as Fe and Mg are intentionally incorporated into Ga2O3 to control the intrinsic n-type conductivity. This work examines the defect level of the intentional and unintentional impurities in β-Ga2O3 substrates grown for electronic applications. First, several dominant impurities, Fe3+, Mg0, and Ir4+, are identified using electron paramagnetic resonance spectroscopy. Then, by illuminating the crystals with selected wavelengths between 1500 and 300 nm, the photothreshold for excitation or quenching of each impurity is examined. Defect levels are estimated from the photothreshold after considering interdefect transitions and lattice relaxation. Thresholds for Ir4+ (2.0–2.3 eV) and Fe3+ (1.2–1.5 eV) support the reported values of the Ir3+/4+ and Fe2+/3+ defect levels, but the photothreshold of 1.6–1.9 eV observed for the quenching of neutral Mg requires further consideration.

High hole concentrations in AlxGa1-xN become increasingly difficult to obtain as the concentration of Al increases. It is well known in GaN and related alloys that hole concentration is directly affected by compensation and extended defects. Using electron paramagnetic resonance (EPR) spectroscopy, we studied the amount of neutral Mg in AlxGa1-xN with x = 0 to 0.28. 0.4-0.9 μm thick Mg-doped AlxGa1-xN films were grown by metal-organic chemical vapour deposition and annealed at 900 °C anneal in N2. EPR measurements indicate that the amount of neutral Mg decreased by 60% in AlxGa1-xN films for x = 0.18 and 0.28 as compared to x=0.00 and 0.08. Experiments also showed that the lower neutral Mg for higher Al compositions trend did not depend on threading dislocation densities in the range of 3-20x109 cm-2, capping the surface with 5 nm of P+ GaN, or detailed annealing conditions. Additional studies show that oxygen and carbon concentrations are insufficient to account for the decrease in neutral Mg observed in the samples. Although the study cannot isolate the cause for the decrease in neutral Mg, the results clearly demonstrate that the acceptor concentration decreases with increasing Al, providing an additional limitation to achieving high hole densities. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A point defect in acceptor-doped bulk free standing GaN grown by the high nitrogen pressure solution method (HNPS) was investigated using electron paramagnetic resonance (EPR) spectroscopy. A nearly isotropic spectrum with g-value of 1.984 is observed after illumination with 2.8 eV light and is stable after removal of the light. Comparison with earlier work on p-type GaN films and SIMS data on the HNPS substrates suggests that the defect is related to an acceptor, heavily compensated during growth by oxygen. Time-dependent photo-EPR data are well modeled as a single defect-to-band transition where the defect level is 0.67 eV above the valence band edge. Although Mg- and Be-doped samples were studied, only those samples containing Be produced the EPR signal. This observation, coupled with the similarity of the defect level measured to that predicted from the theory of others, suggests that the defect is related to a Be impurity.

High hole concentrations in Al x Ga1−x N become increasingly difficult to obtain as the Al mole fraction increases. The problem is believed to be related to compensation, extended defects, and the band gap of the alloy. Whereas electrical measurements are commonly used to measure hole density, in this work we used electron paramagnetic resonance (EPR) spectroscopy to investigate a defect related to the neutral Mg acceptor. The amount and symmetry of neutral Mg in MOCVD-grown Al x Ga1−x N with x = 0 to 0.28 was monitored for films with different dislocation densities and surface conditions. EPR measurements indicated that the amount of neutral Mg decreased by 60% in 900°C-annealed Al x Ga1−x N films for x = 0.18 and 0.28 as compared with x = 0.00 and 0.08. A decrease in the angular dependence of the EPR signal accompanied the increased x, suggesting a change in the local environment of the Mg. Neither dislocation density nor annealing conditions contribute to the reduced amount of neutral Mg in samples with the higher Al concentration. Rather, compensation is the simplest explanation of the observations, because a donor could both reduce the number of neutral acceptors and cause the variation in the angular dependence.

Optical and EPR characterization of Cr and Fe doped ZnSe crystals annealed in Zn vapor revealed a strong bleaching of the divalent state of transition metal ions. Photo induced EPR kinetics were studied in 20–80 K temperature range. Analysis of time-dependent data reveals Cr1+ signal rise time decreases with increasing temperature. The non-exponential decay of Cr1+ concentration were analyzed using Auger-type recombination process. The photoluminescence quantum yield of Cr2+ ions at 5E(D) → 5T2(D) mid-IR transition excited via chromium ionization process was measured to be close to 100%.

InxGa1–xN and AlxGa1–xN alloys are used in many optoelectronic applications due to their tunable band gap, but p-type doping remains a challenge. To better understand the Mg acceptor in nitride alloys, we investigate the effects of In or Al mole fraction, growth temperature and sample thickness on the amount of un-ionized (neutral) Mg using electron paramagnetic resonance (EPR) spectroscopy. The results show that neither temperature nor thickness effects the concentration of the neutral Mg-related acceptor defects; however, the mole fraction of metal, In or Al, alters the behavior of the dopant. For InxGa1–xN, a broadening of the EPR linewidth is shown to be directly related to the presence of a nearby In and is consistent with a lowering of the acceptor level. Incorporation of Al into GaN, on the other hand, produces a systematic decrease in the concentration of neutral Mg-related acceptors as the amount of Al increases. Earlier studies indicate that the reduction is caused by incomplete hydrogen removal from the acceptor impurity. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This study investigates the Fe impurities believed to act as deep acceptors that contribute to electrical compensation of the n-type conductivity in as-grown Ga2O3. A variation of the traditional optical absorption measurement, photoinduced electron paramagnetic resonance (EPR) spectroscopy, is used to identify charge transitions in bulk Fe-doped and Mg-doped Ga2O3 with the support of hybrid functional calculations. Steady-state photo-EPR measurements show that the first optically induced change in Fe3+ occurs at 1.2 eV, significantly larger than the calculated defect levels for Fe. However, the optical cross section spectrum determined from time-dependent photo-EPR measurements compare well with a calculated cross section spectrum for the Fe2+-to-Fe3+ transition when the relaxation energy predicted from the density functional theory is folded into the model. This work explicitly demonstrates the need for an accurate accounting of electron-lattice coupling when interpreting optically induced phenomena.

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3-3.5 K and 9.4-130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0° to 60°. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of [Formula: see text] in free-standing samples, indicating that the local basal crystal field is non-uniform.

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T=4K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A−+hν→A0+eCB−) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A0+hν→A−+hVB+) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with CN being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral CN.

An acceptor-related impurity was studied in bulk free standing GaN using electron paramagnetic resonance spectroscopy. Both undoped and Mg-doped substrates grown by the high nitrogen pressure solution method were examined, but only the doped sample revealed the presence of an EPR-detected center. Comparison with earlier work on p -type GaN films suggests that the defect is an acceptor, heavily compensated during growth. Photo-EPR studies revealed a photo-ionization threshold of 2.8 eV, which is interpreted as excitation of an electron from the acceptor to the conduction band. Based on the concentration and photo-threshold, the EPR center detected in the HNPS doped samples is tentatively identified as Be, which SIMS measurements indicate is a trace impurity in the doped GaN sample. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron paramagnetic resonance (EPR) spectroscopy was used to study the point defects in 2 × 1017–1019 cm−3 C-doped GaN substrates grown by hydride vapor phase epitaxy. The intensity of an isotropic signal with g = 1.987 ± 0.001 increased monotonically with the carbon concentration, indicating that the EPR signal represents a carbon-related defect. In each sample, the signal intensity increased under illumination with photon energy greater than 2.75 eV, and the photo-induced signal decreased with subsequent illumination at 0.95 eV. A second signal, well-documented to be a shallow donor, appeared along with the g = 1.987 signal in the most lightly doped samples. The appearance of the donor confirms that the photo-induced increase is caused by excitation of an electron to the conduction band and implies that a defect level for the carbon-related center is about 1 eV above the valence band edge, consistent with temperature-dependent Hall measurements.

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate a type of point defect present in 1019 cm−3 carbon-doped GaN substrates grown by hydride vapor phase epitaxy. A broad, isotropic resonance at g ∼ 1.987 was observed at 3.5 K, and the EPR intensity increased with illumination at energies greater than 2.75 eV and decreased with photon energies greater than 0.95 eV. The latter is consistent with a deep level of 0.95 eV above the valence band maximum and implies that the associated defect likely participates in donor compensation. The ionization energy for this defect is close to the predicted value for the (−/0) transition level of CN and transition levels associated with Ga vacancies such as VGa and VGa-ON-2H.

Low temperature infrared photoluminescence (PL) performed on a large set of bulk SiC substrates has revealed distinct series of lines between 0.8 and 1.5 eV for samples with nitrogen levels between ~ 1016 and 1017 cm-3. Semi-insulating and intentionally N-doped wafers grown by PVT and HTCVD were investigated. Two groups of PL lines clustered near 1.0 and 1.35 eV, respectively, were observed in n-type 4H-SiC. Not surprisingly, a multiplicity of features at slightly different energy positions was found for this emission from the 6H- and 15R-SiC polytypes. Both sets of lines were not observed for substrates with N doping concentrations greater than 3x1017cm-3. Thus, it appears this IR emission can serve as optical “fingerprints” of bulk n-type substrate with moderate levels of N impurities. Models for the possible origins of these lines will also be discussed.

The understanding of the structure and associated defect level of point defects in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy-carbon antisite pair, − C Si V C . Samples containing this defect exhibit two different photo thresholds, which depend on the resistivity activation energy, Ea. For samples with Ea less than 0.8 eV, a photothreshold at 0.75+/- 0.05 eV is observed, but for those with Ea greater than 0.8 eV, the threshold is between 2 and 2.5 eV. Previous work focused on the former case. Here, the SiC substrates with the larger Ea are emphasized, showing that the photo-threshold likely measures the neutral to negative defect level, − / 0 C Si V C .

A nitrogen-related pair defect is studied as a function of doping density in 4H and 6H SiC. Electron paramagnetic resonance measurements verify that one nucleus in the pair is nitrogen, but the second part of the pair remains uncertain. The pair concentration varies monotonically with nitrogen concentration in samples with doping density 1018–1016 cm−3 and the boron concentration is an order of magnitude less than that of nitrogen. The pair center is not observed in the dark or under ultraviolet illumination when the nitrogen and boron concentrations are similar. We conclude that the pair is generated in all nitrogen-doped samples, but like the isolated nitrogen impurity, may be compensated by boron.

The understanding of the structure and associated defect level of a point defect in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy carbon antisite pair, $$V_C C_{Si}^ - .$$ The photo-threshold for detection of the signal is 0.75 eV in samples with resistivity activation energy, Ea, of 0.5 eV or smaller. For samples with larger Ea, SI-5 is detected only after irradiation with photon energy greater than 2.5 eV. The results suggest that $$V_C C_{Si}^{ - - / - }$$ is 0.75 eV below the conduction band edge and that the negative to neutral level is within 0.1 eV of $$V_C C_{Si}^{- - / -} .$$

Halide chemical vapor deposition (HCVD) allows for rapid growth while maintaining the purity afforded by a CVD process. While several shallow and deep defect levels have been identified in 6H HCVD substrates using electrical techniques, here we examine several different point defects found in 4H n-type HCVD SiC using electron paramagnetic resonance (EPR) spectroscopy. One spectrum, which exhibits axial symmetry and broadens upon heating, may represent a collection of shallow defects. The other prominent defect has the g tensor of the negatively charged carbon vacancy, but additional hyperfine lines suggest a more complex center. The role of these defects is not yet determined, but we note that the concentrations are similar to those found for the electrically detected defect levels, making them a reasonable source of electrically active centers.

Optical admittance spectroscopy (OAS), supported by electron paramagnetic resonance (EPR) measurements, is used to identify the controversial vanadium acceptor levels in vanadium-doped semi-insulating (SI) 4H-SiC and 6H-SiC. The V3+/4+ levels for the cubic site are likely located at E c − 0.67 ± 0.02 eV and E c − 0.70 ± 0.02 eV in 6H-SiC and E c − 0.75 ± 0.02 eV in 4H-SiC. A peak at 0.87 ± 0.02 eV in the 6H-SiC is tentatively assigned to the same transition at the hexagonal site and the associated transition in 4H-SiC is thought to occur near 0.94 eV. All assignments are supported by the observation of V3+ in the EPR spectrum.

Strontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity.

The purpose of this study is to determine the vanadium defect levels in semi-insulating 4H-SiC and 6H-SiC using optical admittance spectroscopy (OAS). OAS data show several distinct peaks for the vanadium-doped SI 4H-SiC and 6H-SiC. Comparison of the data for the two polytypes suggests that peaks at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV in 6H substrates and 0.75 ± 0.02 eV in 4H substrates are related to V3+/4+ levels at the cubic sites. A peak at 0.87 ± 0.02 eV in the 6H sample is assigned to the same defect level at the hexagonal site and the associated transition in 4H was observed at 0.94 ± 0.02 eV in our spectra. The donor levels are thought to be related to peaks at 1.94 ± 0.05 eV and 1.87 ± 0.05 eV in 4H and 6H samples, respectively. The differences between the values obtained from the optical admittance measurements and those reported in the literature are attributed to thermal relaxation and/or contributions from defect complexes.

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.

Electron paramagnetic resonance (EPR) and temperature-dependent Hall measurements were performed on seven different vanadium-doped semi-insulating SiC samples. Comparison of the EPR data and carrier activation energy suggests that the acceptor level for vanadium is 1.1 eV below the conduction band edge ( E c ) in 4H SiC and within 0.86 of E c in the 6H polytype. Photo-induced EPR results support the level assignments. However, analysis of the V 4+ spectra in 4H samples suggests that the dominant vanadium EPR signal monitored in the 4H samples used for this experiment does not represent a simple isolated impurity. Rather, the results reflect a strained or complex defect.

Electron paramagnetic resonance (EPR) spectroscopy and photo-induced EPR are used to examine the point defects in vanadium-doped 4H–SiC and high-purity semi-insulating (HPSI) 4H–SiC grown by physical vapor transport. Both types of samples often exhibit a 1.1-eV activation energy, Ea, as extracted from the temperature-dependent Hall/resistivity measurements; however, different defects are related to the same Ea in each case. In the vanadium-doped wafers, the EPR data reveal both V4+ and V3+ in the same sample; thus, the 1.1-eV Hall activation energy is tentatively interpreted as the V3+∕4+ acceptor level. However, this conclusion cannot be confirmed because additional defects complicate the photoresponse of vanadium. The carbon vacancy, which is detected in all the HPSI wafers, exhibits a range of photothresholds similar to the various values measured for Ea. The photo-EPR and temperature-dependent Hall/resistivity measurements made before and after a 1600°C anneal suggest that several different types of nonuniformly distributed defects participate in compensation of the HPSI material and that annealing selectively removes those with different defect levels.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool to study the physical and chemical structure of point defects in crystalline semiconductors. Investigations throughout the past few decades have provided detailed descriptions of some of the most important intrinsic defects and impurities in silicon carbide. Several reviews have summarized the significant findings. This review expands the scope of earlier work by focusing on EPR studies of as-grown electronic grade SiC. Because the most recent technological interests focus on growing high-resistivity substrates for high frequency, high power devices, many of the defects discussed here are identified in semi-insulating 4H SiC. The access to high purity material has enabled the identification of at least one new centre, the positively charged carbon vacancy, while the detailed electronic structure of the others is still uncertain. Intentional and unintentional impurities also play a role. For this reason, vanadium and some shallow impurities will be briefly reviewed. Finally, the structure and chemistry of carbon-related defects thought to be located near the surface of SiC will be summarized.

High temperature Hall effect and resistivity measurements have been made on semi-insulating 4H–SiC samples. Both vanadium doped and undoped materials have been studied. Resistivity measurements before and after annealing up to 1800 °C are also reported. The thermal activation energy of the resistivity in vanadium doped samples has one of two values, 1.5 and 1.1 eV, due, respectively, to the vanadium donor level and an as yet unidentified defect. The activation energies for high purity semi-insulating material (HPSI) varied from 0.9 to 1.5 eV. Hall effect measurements were made on several HPSI and 1.1 eV V-doped samples. In all cases the material was found to be n-type. Mixed conduction analysis of the data suggests that the hole concentration is negligible in all samples studied. This suggests that the defects responsible for the semi-insulating properties have deep levels located in the upper half of the bandgap. The resistivity of V-doped samples were unaffected by anneals up to 1800 °C. The annealing results for HPSI samples were mixed.

The potential of using ternary defect chalcopyrites as laser active materials is investigated by performing absorption, photoluminescence, and EPR studies of as-grown and vacuum annealed Cr-doped CdGa2S4. Although no consistent affect of annealing is observed, an increased concentration of Cr in the melt during growth is shown to broaden an absorption band at 1288 nm. Several of the absorption bands could be attributed to multiple charge states of Cr thought to be present in the crystal. A broad fluorescence was observed at visible wavelengths; however, no emission was detected in the middle-infrared region, contrary to expectation. The lack of infrared fluorescence may be attributed to an exchange interaction involving Cr impurities.

The microscopic process involved with thermal activation of the Mg acceptors in GaN epitaxial films is investigated using electron paramagnetic resonance (EPR) spectroscopy. Samples were heat treated in dry N2 for 30 min at temperatures between 200 and 1000 °C. Below 850 °C, the data are consistent with release of hydrogen from a Mg complex and corroborate the initial assignment of the EPR signal to the p-type acceptor. However, the Mg-related EPR signal is permanently quenched for anneals above 850 °C suggesting that additional point defects are created during high temperature treatment.

This work investigates the thermal stability of point defects in high-purity SiC. Electron paramagnetic resonance measurements made after Ar annealing between 600°C and 1700°C reveal changes in at least two defects, a carbon vacancy related center (VC) and the shallow boron acceptor. Between 1000°C and 1400°C, the number of carbon vacancies decreased and the number of boron acceptors increased in about the same proportion. Above 1400°C, the concentration of B continued to grow, while the number of carbon vacancies fell below the detection limit. After the highest temperature anneal, the initial preannealed concentration of VC was restored by illumination with sub-bandgap 578-nm light. The effects of heat treatment between 1000°C and 1400°C are interpreted in terms of the charge exchange between a compensated B acceptor and VC. The observation of optically induced carbon vacancies after the final anneal suggests that they are thermally stable in these samples up to 1700°C.

Electron paramagnetic resonance (EPR) has been employed to examine thermo- and photo-activated interactions between as-grown intrinsic defects and nitrogen/boron impurities in nominally high-resistivity 4H SiC produced without intentional vanadium doping. The EPR spectra, recorded in the dark at 4 K, revealed either a low concentration of paramagnetic boron, nitrogen, or a carbon-vacancy-related defect (ID-1). The photo-induced charge transfer between ID-1 and both boron and nitrogen has been detected. In some samples, illumination with 0.9-1.6-eV light at 4.2 K quenched the ID-1 signal and produced an approximately equal amount of shallow paramagnetic boron. Exposure to 1.9-2.7-eV light restored the ID-1 center and produced shallow paramagnetic nitrogen. A model based on excitation from the band edges and the threshold energies obtained from the photo-EPR data placed the ID-1 level 0.9 ± 0.1 eV above the valence-band edge. Comparison of these data with that obtained from other samples that exhibited excess boron or nitrogen impurities suggested the presence of an additional defect no more than 0.9 eV below the conduction-band edge. This defect can exchange charge with the nitrogen donor.

Although several magnetic resonance studies address the Mg acceptor in GaN, there are few reports on Mg doping in the alloys, where hole production depends strongly on the Al or In content. Our electron paramagnetic resonance (EPR) measurements of the p-type alloys suggest that the Mg impurity retains the axial symmetry, characteristic of a p-type dopant in both alloys; however, In and Al produce additional, different characteristics of the acceptor. In InGaN, the behavior is consistent with a lowering of the acceptor level and increasing hole density as In concentration increases. For AlGaN, the amount of neutral Mg decreases with increasing Al content, which is attributed to different kinetics of hydrogen diffusion thought to occur in samples with higher Al mole fraction.

Several nitride-based device structures would benefit from the availability of high quality, large-area, freestanding semi-insulating GaN substrates. Due to the intrinsic n-type nature of GaN, however, the incorporation of compensating centers such as Fe is necessary to achieve the high resistivity required. We are using electron paramagnetic resonance (EPR) to explore charge transfer in 450 um thick GaN:Fe plates to understand the basic mechanisms related to compensation so that the material may be optimized for device applications. The results suggest that the simple model based on one shallow donor and a single Fe level is insufficient to describe compensation. Rather, the observation of the neutral donor and Fe3+ indicates that either the two species are spatially segregated or additional compensating and donor defects must be present.

An intrinsic defect (ID) has been identified in as-grown 4H–SiC by electron paramagnetic resonance (EPR). The EPR parameters of an ID measured in our nominally semi-insulating material are similar to the literature data of the EI5 defect produced in p-type 4H and 6H–SiC by 2.5 MeV electrons and assigned to the carbon vacancy (N.T. Son, P.N. Hai, E. Jansen, Phys. Rev. B 63 (2000) 201201). However, comparison of the ID and EI5 centers reveals that the as-grown and radiation-induced centers exhibit different annealing behavior. Photo-induced EPR locates the ID level 0.9 eV above the valence band edge.

We have used electron paramagnetic resonance to study two intrinsic defects in oxidized epitaxial layers of 3C SiC, a potential substitute for Si in high speed, high power electronics. One center can be described by an isotropic g value of 2.0044. The defect is distinguished by a strong temperature dependence and is located throughout the 3C epitaxial layer. Although the detailed structure has not been determined, the g value is consistent with theoretical predictions for a Si dangling bond surrounded by carbon atoms. The second Si-related center, induced by post oxidation dry heat treatment, is characterized by an axial g tensor with g∥=2.0023 and g⊥=2.0082. The spectroscopic and chemical properties of the latter defect are consistent with those of the Si Pb center, a Si dangling bond located at a Si/insulator interface. Experiments show that in our 3C samples, the Pb signal is probably located on the Si side of a SiO2/Si interface between the SiC epilayer and the Si substrate.

Thickness and etch rate of SiO2 films thermally grown on hexagonal SiC substrates were compared to results obtained from SiO2/Si samples. The data confirm that profilometry and ellipsometry yield the same thickness values for oxides grown on Si and SiC. Within the accuracy of our measurements, oxides grown on different polytypes and faces of SiC etch at the same rate in a HF acid solution. The etch rate using a 50:1 H2O:HF(50%) solution at room temperature is 0.1 nm/s and is uniform throughout the thickness of the SiO2 films. The rate is the same as that obtained for SiO2 grown on Si.

Vacuum ultraviolet hole injection was used to study the initial trapping properties of three different types of buried oxides, SIMOX, Unibond, and polySi/SiO2/Si samples. After accounting for the charge centroid, the initial hole trapping efficiency for SIMOX samples is shown to be at least a factor of four greater than that measured for Unibond buried oxides. We suggest that the difference is due to the post fabrication anneal temperature and the lack of moisture in the ambient during the anneal.

We have studied the photoluminescence (PL) mechanism of photo- and electroluminescent amorphous silicon oxynitride films grown by plasma enhanced chemical vapor deposition. The composition of the films was determined by Rutherford backscattering spectrometry and monitored by the index of refraction with single-wavelength ellipsometry. Two sets of samples were grown, each with different reactant gas residence times in the deposition chamber. For samples grown with a residence time of about 5 s, the energy of the PL peak for 2.54 eV excitation is 2.3 eV for stoichiometric films and redshifts with increasing silicon content to 1.7 eV for the most silicon-rich films. The energy of the PL peak for 3.8 eV excitation is 2.8 eV for stoichiometric films and 2.1 eV for the most silicon-rich films. For stoichiometric films, the PL intensity is independent of temperature between 80 and 300 K using 2.54 eV excitation, but the PL intensity decreases by a factor of two over the same temperature range for 3.8 eV excitation. The authors interpret these aspects of the PL as consistent with tail-state recombination. Other results imply the PL is due to a specific luminescence center related to Si–Si or Si–H bonding. A 450 °C anneal reduces the paramagnetic defect density in the films, as detected by electron paramagnetic resonance, by an order of magnitude, but does not increase the PL intensity, while a 950 °C anneal increases both the defect density and the PL intensity. In addition, films in a second set of samples, grown with a residence time of 1.8 s, display very different PL behavior than samples in the first set with the same composition. Samples near stoichiometry in the second set have a PL peak at 2.06 eV and are 20 times less intense than stoichiometric samples in the first set. Optical absorption measurements indicate both types of samples contain Si–Si bonds, with the second set containing many more Si–Si bonds than the first. Fourier-transform infrared measurements indicate the presence of a Si–H bond that is stable at temperatures of 950 °C in the first set, but not in the second set. Thus, the study as a whole suggests a complete picture of luminescence in our silicon oxynitride films must incorporate elements of both tail-state and luminescence center models. The relation of the results to other PL studies in silicon alloys and porous silicon is discussed.

Electron paramagnetic resonance is used to study a near surface defect in oxidized 4H 6H SiC substrates and 3C SiC epitaxial layers. The defect is observed after wet oxidation and a 900° C dry thermal treatment, but the defect is not located in the oxide. The heat treatment activates the EPR center by removing a hydrogen-related species from a C bond. Oxidation studies suggest that reaction of H 2 O with SiC creates the defect. However, an alternative theory in which the defect is intrinsic to SiC cannot be disregarded.

Electron paramagnetic resonance studies show that defects are activated in oxidized 3C-SiC and 6H-SiC by heat-treatments at temperatures greater than 800°C in dry (

The interaction of two diamond powders and synthetic diamond single crystals with H2O has been studied in the temperature range of 650–850 °C under pressures up to 500 MPa. The reaction products were characterized using electron paramagnetic resonance (EPR), Raman and Fourier transform infra-red spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM) and selected area diffraction. A noticeable interaction of fine grain diamond powders with H2O starts at approx. 700 °C while single crystals are stable up to 800 °C. At 800–850 °C, etching pits appear on the single crystal diamond surface. Thermodynamic calculations predict the formation of hydrogen, carbon oxides and hydrocarbons. Formation of graphitic carbon via the fluid is observed. Redeposition of nanocrystalline diamond and healing of crystal defects are suggested by TEM and EPR investigations.