Diamond films synthesized using Microwave Plasma Chemical Vapor Deposition (MWCVD) were evaluated for use as dielectric material for high power and high temperature capacitors. The effect that the deposition parameters and annealing have on the frequency and temperature stability of the electronic properties was investigated. Dielectric constants ranging between 8.0 and 4.2 and resistivities between 1× 108 ohm-cm and 5×1014 ohm-cm were obtained. Diamond produced using less than 6.6% methane had very stable dielectric constants over the frequency range of 100 Hz to IMHz, and the loss tangent was less than 0.01. Adding oxygen to the precursor gas increased the dielectric constant and lowered the loss tangent of CVD diamond, but the resistivity was also lowered. As the temperature increased to 300°C, the dielectric constant and loss tangent increased. However, when diamond was annealed to 700°C, there was less than a 5% change in the dielectric constant from 23°C to 300°C.

Annealing of defects in as-grown and proton irradiated bulk n- and p-type 6H-SiC has been investigated by positron lifetime spectroscopy and Doppler-broadening measurements. The grown-in defects do not anneal out until 1600 °C, whereas the radiation induced defects show three main annealing stages well below this temperature. During annealing, the formation of larger defect complexes can be observed.

We have systematically studied the optical properties of InGaN/GaN multiple quantum wells (MQWs) at 10 K under different excitation conditions using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The MQWs were grown on 1.8-μm-thick GaN on c-plane sapphire films by metal-organic chemical vapor deposition. Structures consist of 12 MQWs with 3-nm-thick In0.2Ga0.8N wells, 4.5-nm-thick GaN barriers, and a 100-nm-thick A10.07Ga0.93N capping layer. We found that the PL emission consists of a strong main peak at 2.80 eV and a much weaker and broader secondary peak at ˜ 2.25 eV. We observed that both the peak position and the spectral width of the main peak vary with the excitation energy. The PL peak blueshifts and narrows when the excitation energies are varied from 3.81 eV (above the bandgap of the AlGaN capping layer) to 2.99 eV (below the bandgap of the GaN barrier layers). The intensity ratio of the main peak to the secondary peak also varied with excitation energy. The PLE studies, combined with the excitation dependent PL emission studies, reveal that the two observed emission peaks originate from different layers of the MQWs. In addition, we found that the InGaN-related spontaneous emission processes are strongly affected by different carrier generation conditions with further complications associated with compositional fluctuations and possible carrier localization. The differences in the observed emission processes under varying excitation conditions can be explained by considering the transfer of carriers between the different layers of the MQW structures.

The growth modes and resulting microstructures of GaN films grown by Metalorganic Vapor Phase Epitaxy (MOVPE) on (0001) sapphire with either a simple nitridation of the surface and N2 as carrier gas or with a treatment of the nitridated surface under silane and ammonia with a mixture of H2 and N2 as carrier gas have been investigated. The second process results in a three dimensional first stage of the growth before a coalescence of the islands and the smoothing of the surface. Using this growth process, the defects density can be decreased down to 3.108 cm−2. This reduction in defect density has a drastic effect on photoluminescence properties of the material, and the luminescence efficiency can be increased by a factor of 20. It is also observed that the strain of this material is much higher as compared with material grown with the first process.

The barrier heights of Ni/Ga/Ni and Re Schottky contacts to n-GaN were investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The elemental Ni contacts were also investigated for comparison. The Ni/Ga/Ni and Re diodes were stable on short term annealing up to 700 °C. It was also shown that the Ni/Ga/Ni (84 at% Ni, 16 at% Ga) diodes were more stable than Ni diodes. This work provides the first demonstration of Schottky diodes to n-GaN that are stable upon annealing at 700 °C.

Boron nitride (BN) and gallium nitride (GaN) are known as superior semiconductor materials for high power and high temperature applications. Undoped BN layers grown using ion beam and electron cyclotron resonance (ECR) assisted physical deposition on conductive GaN films have demonstrated good insulating properties. These films are thus good candidates as thin insulating layers in high temperature GaN-based device structures such as MIS diodes and MISFETs due to their close thermal expansion coefficient. In order to address the device processing issue, reactive ion etching (RIE) tests were performed on these films. Using Cl2/Ar chemistry, etch rates up to 600 Å/min were measured. These rates were found to increase linearly with increasing rf power and Cl2 flow rate. GaN layers grown by gas source MBE were also dry etched, resulting in smooth sidewalls. Etch rates up to 1,400 Å/min were achieved at 200 W rf power (-280 V d.c. bias) in a BCl3/Cl2/Ar chemistry; this is the highest RIE rate reported up to now for GaN. Using Cl2/Ar and BCl3/Cl2/Ar for BN and GaN respectively, etch selectivities in excess of 5:1 can be obtained. Finally, preliminary Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) results on residue deposition and surface composition changes as a function of the different etch conditions are presented.

The InGaN ternary system, which is useful for blue and green light emitting or laser diodes, is studied with respect to an unstable mixing region in the phase field. The unstable region is analyzed using a strictly regular solution model. The interaction parameter used in the analysis is obtained from a strain energy calculation using the valence force field model, modified for both wurtzite and zinc-blende structures to avoid overestimation of the strain energy. The structural deviation from an ideal wurtzite structure in GaN and InN is also taken into account in our model. The interaction parameters of InGaN obtained by our analysis for the wurtzite and zinc-blende structures are 7.81 kcal/mol and 6.63 kcal/mol, respectively. According to the calculated results of the interaction parameters, the critical temperature for wurtzite InGaN and zinc-blende InGaN are found to be 1967 K and 1668 K, respectively. This suggests that, at a typical growth temperature of 800°C, a wide unstable mixing region exists in both wurtzite and zinc-blende structures. In order to show the validity of our calculation results, we compare the calculated results and the experimental results using the calculation of the interaction parameter for the InGaAs system. The calculated results agree well with the experimental results.

Raman spectroscopy has been used to investigate wafers of both 4H-SiC and 6H-SiC. The wafers studied were semi-insulating and n-type (nitrogen) doped with concentrations between 2.1 × 1018 cm−3 and 1.2 × 1019 cm−3. Significant coupling of the A1 longitudinal optical (LO) phonon to the plasmon mode was observed. The position of this peak shows a direct correlation with the carrier concentration. Examination of the Raman spectra from different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. This data is compared with a resistivity map of the wafer. These results suggest that Raman spectroscopy of the LO phonon-plasmon mode can be used as a noninvasive, in situ diagnostic for SiC wafer production and substrate evaluation.

We studied the transient photoconductivity(PC) properties of GaN thin film on (0001) sapphire substrate. The decay curves of PC obtained by YAG:Nd pulsed laser are divided into two special regions, which are the beginning and tail decay regions, corresponding to two time constants, 0.1 ms and 1.0 ms, respectively. Keeping the same light intensity and bias voltage, when the sample was heated to 300°C, the time constants are both reduced. It shows that there are a large number of traps at the position of tens of meV above the valence band edge. The PC ascends S-shaped when the data were collected during the first 200 ns. As the sample is not shined, the PC sharply reduces and oscillates, indicating that the carriers redistributed between the deep traps and the valence band. The experiment results are explained by the model that involves two hole trapping levels, the shallow trapping level and the deep trapping level. Moreover, the mechanism of the curves varied as the light intensities are also explained by the model.

A significant source current generated by a carrier multiplication process is observed at large drain voltages in the subthreshold regime, along with simultaneous increase of the gate current and light emission signal. Provided no on-surface premature breakdown takes place, a bulk channel avalanche breakdown process is proposed as the dominant breakdown mechanism for a large range of gate-to-source dc voltages. This process in the GaN channel is responsible for the excess source and drain currents, light emission, and excess gate current beyond its normal value measured in a gate-to-drain diode configuration. The role of the gate bias in controlling the channel vs. the gate breakdown mechanisms is described.

High resolution x-ray diffraction and x-ray topography study of GaN thin films, grown on sapphire (00.1) substrate by reduced pressure metalorganic vapor phase epitaxy under various conditions, were performed. An attempt was made to correlate the mobility in films with similar carrier concentration with the strain and dislocation density. X-ray topography revealed the defects present in the film.

Influence of C, N and O ion-implantation on the yellow luminescence (YL) of halide vapor phase epitaxy (HVPE) and metal-organic vapor phase epitaxy (MOVPE) grown GaN has been studied by photoluminescence (PL) spectroscopy. For the HVPE-grown samples, only C implantation produces a significant enhancement of YL while its corresponding overall PL integrated intensity is only ˜72% of its original value. Implantation of O or N do not appreciably change the YL but decrease the BE integrated intensities by a factor of ten. The full-width-halfmaximum (FWHM) of the BE band expanded to about 1.4 × of the original value in all three cases. These results indicate that one source of the YL is strongly correlated to incorporation of C into the GaN film. The C-induced defect complexes in GaN are optically active while O or N does not lead to any new luminescence features. The variable-temperature PL measurements on the C-implanted HVPE-grown GaN reveal that the FWHM of the YL band increases linearly and the peak energy of the YL changes very little with the increasing measurement temperature, while the integrated intensity of the YL band decreases monotonically. This trend of the YL FWHM and peak energy with the measurement temperature can be explained by a two-channel transition model. For MOVPE-grown GaN films., all ion-implanted samples exhibit a significant loss of both the YL and overall integrated PL intensity. Residual C contamination in the MOVPE samples could be large compared to the additional ion-implanted carbon leading to a reduced impact of the ion-implanted carbon.

The incorporation of Er into GaN by in-situ doping during halide vapor phase epitaxy has been investigated. The NH3, HCl, metallic Ga and Er were used as source materials, while N2 was employed as a carrier gas. The GaN:Er films were obtained at different Ga/Er source temperatures. The SIMS analysis shows that the steady state Er doping concentration can be as high as 2×1018 cm−3. All in-situ Er-doped samples luminescence at 1.54 μm due to the 4I13/2 → 4I15/2 transition of Er3+ at both low (11K) and room temperature. The higher the Ga/Er boat temperature, the stronger the 1.54 μm luminescence, implying a higher incorporation rate of Er into the GaN. The 4I11/2 → 4I15/2 transition luminescence, centered around 980 nm, can also be detected at both low and room temperature. The broad-spectrum PL measurements exhibited sharp bandedge luminescence without the presence of the yellow luminescence band.

P doping of gallium nitride by incorporation of magnesium in the layers was controlled recently. Only Popovici et al. [1] have published the results of a Raman study on p type GaN. In the present communication, we report on the interaction of the free hole gas with the axial A1(LO) or planar E1(LO) phonon modes, evidenced by Raman scattering: the observed coupled phononplasmon mode is found very different from the corresponding one evidenced in silicon doped (n type) GaN. We compare the experimental data with the lineshape calculated within a dielectric model, using the results of electrical measurements. These results are also compared with infrared reflectivity spectra.

Pd/SiC has been used as a high temperature hydrocarbon and hydrogen gas sensor in environmental and aeronautical applications. In this work, the relationships between diffusion, reaction, and interfacial chemical composition with electrical properties for Pd ultra-thin films on 6H-SiC (∼< 30Å) are studied at different annealing temperatures.

Ultra-thin film Pd on 6H-SiC has been prepared by the RF sputtering method. The Schottky barrier heights are measured by XPS for an unannealed sample and samples annealed from 100°C to 400°C for 30 minutes, respectively. No significant change in the Schottky barrier height of the Pd/SiC contact was found in the temperature range. The morphology from UHV-STM and AFM show that the unannealed Pd thin film had good uniformity across the SiC substrate, and the Pd has dispersed, and then partially aggregated into rounded shaped precipitates with increasing annealing temperatures. At 400°C, all Pd metal has reacted and formed to silicides. From XPS, Pd2Si was found on the surface after annealing, and almost all Pd has reacted to become Pd2Si after annealing at 400°C. No other silicide was found. The intensity of Pd on XPS decreases enormously at 400°C. This is explained if Pd has diffused into SiC. The Pd diffusion and the formation of Pd silicides do not significantly affect the Schottky barrier height. The SiO2 was found at the top of surface after annealing, and increased in amount with increasing annealing temperature. The SiO2 formation was accelerated by the presence of Pd. Pd may play a role in absorbing oxygen, and activating Si from SiC to form SiO2.

Key Words: Pd thin film, SiC, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and atomic force microscopy.

Using a fast ramp response technique, the high field characteristics, specifically the breakdown strength, of thermally grown silicon-dioxide (SiO2) and MOCVD grown aluminum-nitride (AIN), on n-type 6H-SiC epilayers is obtained as a function of three different processing conditions for the insulator growth. Significant improvement in the breakdown strength of thermally grown SiO2 after a 30 minute post annealing at 400°C in nitrogen ambient is reported. Further, the influence of temperature profile during the AIN growth on the breakdown strength is reported.

A correlation between gate oxide breakdown in MOS capacitor structures and structural defects in SiC wafers is reported. The oxide breakdown under high applied fields, in the accumulation regime of the MOS capacitor structure, is observed to occur at locations corresponding to the edge of bulk structural defects in the SiC wafer such as polytype inclusions, regions of crystallographic mis-orientation, or different doping concentration. Breakdown measurements on more than 50 different MOS structures did not indicate any failure of the oxide exactly above a micropipe. The scatter in the oxide breakdown field across a 10mm × 10mm square area was about 50%, and the highest breakdown field obtained was close to 8 MV/cm.

Photoemission spectroscopy (UPS) was used to investigate the interface properties of deposited silicon on hexagonal 6H-silicon carbide. SiC cleaned in Si flux from a molecular beam epitaxy (MBE) system was used for this study. All processes were accomplished in an ultra high vacuum integrated system that allowed all cleaning, deposition, and analysis to be completed without exposure to ambient atmosphere. Thicknesses of sub- to multiple monolayers were deposited and the valence band structure was investigated. The valence band maximum (VBM) was observed to shift for Si depositions greater than 1 monolayer. The VBM offset was determined to be 2.4eV for a layer of 60Å Si on SiC. Furthermore, the prominent surface state feature of the silicon carbide (0001)si surface is reduced after Si deposition. The results are discussed in terms of the electronic properties of the Si – SiC interface.

Convergent beam electron diffraction (CBED) was applied to study polarity of GaN heterolayers grown by MOCVD on sapphire and SiC substrates and also mechano-chemically polished bulk GaN platelet crystals grown from Ga melt under hydrostatic pressure of N. Heterolayers of GaN grown on both types of substrates showed (0001) Ga polarity, e.g. Ga to N along c-axis. Mechano-chemical etching of bulk GaN platelet crystals leaves one surface smooth and the opposite surface rough with visible damage caused by mechanical polishing. CBED studies showed that the smooth surface has (0001) N polarity, confirming our earlier studies.

In the context of spectral-hole burning experiments in 4H- and 6H-SiC doped with vanadium the energy positions of the V4+/5+ level in both polytypes were determined in order to resolve discrepancies in literature. From these numbers the band offset of 6H/4H-SiC is calculated by using the Langer-Heinrich rule, and found to be of staggered type II. Furthermore the experiments show that thermally stable electronic traps exist in both polytypes at room temperature and considerably above, which may result in longtime transient shifts of electronic properties.