Mary Ellen Zvanut

Characterization of paramagnetic defect centers in three polytypes of dry heat treated, oxidized SiC

This work describes the characterization of defect centers in 3C–SiC, 4H–SiC, and 6H–SiC. The different SiC crystal structures are examined with electron paramagnetic resonance after thermal oxidation, and after dry (<1 ppm H2O) N2 or O2 heat treatment. The centers are described by g values that range from 2.0025 to 2.0029, which are typical of C dangling bonds. Because the centers are activated in ambients that eliminate H2O and are passivated in ambients that contain H2O, it is suggested that the centers are C dangling bonds created during the dry heat treatment when hydrogen or a hydrogenous species releases from C bonds. The activation characteristics for the centers is the same for both 6H and 3C polytypes; however, centers in the 6H–SiC samples are passivated at lower temperatures than the centers in the 3C–SiC samples. The passivation behavior is attributed to differences in the hydrogen diffusion rates in these materials rather than significant differences in the chemistry of the centers. Etching ...

Defect levels and types of point defects in high-purity and vanadium-doped semi-insulating 4H–SiC

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 n...

Generation of thermally induced defects in buried SiO2 films

We show that annealing of the buried oxide layer used for device isolation generates point defects in the SiO2 film and that this defect generation is independent of temperature above 1000 °C. Electron paramagnetic resonance data obtained on thermally grown buried oxides and those fabricated by ion implantation indicate that the defect is intrinsic to the structure of SiO2 and is associated with an oxygen deficient environment. The similarity in the generation of the defects studied here and the formation of SiO observed in earlier studies of low pressure high temperature oxidation suggests that the formation of the buried oxide defect is related to the reduction of SiO2 and the release of SiO.

Laser performance of Cr2+-doped ZnS

Laser properties and spectroscopic characterization of diffusion doped Cr2+: ZnS crystals synthesized by chemical transport reaction from gas phase are reported. Lasing was realized with a threshold of 170 uJ and slope efficiency of 9.5 % with respect to the 1.5607 um pump energy, in a hemispherical cavity. Low doped samples (3-4 cm-1 at 1.7 um) of 1.7 mm thickness were utilized. The 1.5607 um excitation was realized with a D2 Raman cell pumped in a backscattering geometry by the 1.064 um radiation of the single frequency Nd:YAG laser. Maximum output energy reached 100 uJ. Lasing in the hemispherical cavity was achieved with output couplers R=80 % and 90 % at 2.36 um and radius of curvature 20 cm. Absorption cross section was estimated from spectroscopic measurements and was in a good agreement with saturation data (0.80x10-18 cm2) calculated with the modified Frantz-Nodvik equation for a four level slow absorber. Findlay Clay losses were found to be about 14%. Selective cavity experiments were performed in a hemispherical cavity with a CaF2 prism as the dispersive element. A tuning range of 2.05-2.40 um was realized, limited by the spectral range of the output coupler of the selective laser cavity.

Spectroscopic characterization and laser performance of diffusion doped Cr2+:ZnS

Spectroscopic, saturation, and laser properties of diffusion doped Cr2+:ZnS crystals synthesized by chemical reaction from gas phase are reported. Lasing was realized with a threshold of 170 µJ and slope efficiency of 9.5% with respect to the 1.5607 µm pump energy, in a hemispherical cavity. Maximum output energy reached 100 µJ. A tuning range of 2.05-2.40 µm was realized, limited by the spectral range of the output coupler of the selective hemispherical laser cavity. Selective cavity experiments used a CaF2 prism as the dispersive element. Findlay Clay losses were found to be about 14%. Absorption cross section (0.80×l0−18 cm2) was estimated from spectroscopic measurements and was in good agreement with saturation data, calculated with the modified Frantz-Nodvik equation for a four level slow absorber.

Passivation and activation of Mg acceptors in heavily doped GaN

Electron paramagnetic resonance measurements are used to monitor the passivation and activation of the Mg-related acceptor in GaN doped with different concentrations of Mg, up to 2 × 1020 cm−3. Samples were annealed in either forming gas (H2:N2) or pure N2 between 200 and 900 °C. As expected, the Mg-related EPR signal is reduced by at least a factor of ten during the forming gas treatment; while the pure N2 environment revives the signal. However, the study also shows that reactions between Mg and hydrogen occur at a temperature as low as 525 °C in the 1020 cm−3 Mg doped samples; while in more lightly doped samples, temperatures greater than 700 °C are required to observe changes in the Mg signal intensity. While the observations support the model in which a hydrogen atom ionizes at the Mg impurity and the remaining proton bonds at a near neighbor, the different temperature dependence suggests that hydrogen diffusion is affected by the increased Mg concentration.

A model for Be-related photo-absorption in compensated GaN:Be substrates

A photo-induced electron paramagnetic resonance (EPR) attributed to beryllium-related acceptors was identified in GaN:Be substrates grown by the high nitrogen pressure solution technique. The acceptors, initially compensated by shallow O-related donors, were observed after illumination with photon energy greater than 2.7 eV. To adequately fit the time-dependent photo-EPR data over time periods up to 90 min, a two-defect model was developed based on three charge transfer processes: (1) photo-excitation of electrons from compensated acceptors, (2) electron capture by the positively charged donors and neutral acceptors directly from the conduction band, and (3) electron transfer from the donors to acceptors. The analysis of the spectral dependence of the optical cross section leads to the Be-related acceptor level lying 0.7 eV above the valence band maximum, consistent with the role of the acceptor as a compensating center as well as the 2.2 eV luminescence that others observed from these and other GaN:Be sa...

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.

Intrinsic point defects in oxidized 3C epitaxial layers on Si substrates

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.

Effect of local fields on the Mg acceptor in GaN films and GaN substrates

The electron paramagnetic resonance (EPR) spectrum of the Mg acceptor is studied in a variety of GaN samples, including mm-thick free-standing substrates and sub-micron heteroepitaxial films. The former allows a view of the acceptor unique for EPR—in an environment with less than 107 cm−3 dislocations and doping densities ranging from 4 × 1016 to 6 × 1018 cm−3. By probing Mg in a broad range of samples in one study a new feature of the acceptor emerges. The EPR data reveal an anisotropic line shape that reflects a change in the crystal field in the vicinity of the Mg acceptor. This feature must be accommodated by any of the models proposed for the Mg acceptor. Here, we show that one such previously proposed model agrees well with the EPR data obtained from the wide variety of samples studied. The work implies that Mg-doped GaN contains a common Mg-related defect which can be affected by local crystal fields established during growth.