Search

Comprehensive experimental and theoretical studies are reported to assess the vibrational and structural properties of 3C-SiC/Si (001) epilayers grown by chemical vapor deposition in a vertical reactor configuration. While the phonon features are evaluated using high resolution infrared reflectance (IRR) and Raman scattering spectroscopy (RSS) – the local inter-atomic structure is appraised by synchrotron radiation extended x-ray absorption fine structure (SR-EXAFS) method. Unlike others, our RSS results in the near backscattering geometry revealed markedly indistinctive longitudinal- and transverse-optical phonons in 3C-SiC epifilms of thickness d

This paper reports on the dependence of cycling or endurance in flash memory with the stress induced by low-pressure chemical vapor deposition (LPCVD) process. The deposited 12.5nm tetraethyl orthosilicate (TEOS) layer served as sacrificial oxide during source/drain (S/D) implantation, which was then subsequently removed. However, this deposited TEOS layer has been found to induce a stress on the channel which affects the flash memory cycling or endurance performance.

The synthesis of nano-crystalline Mg–Mn ferrites by a solution combustion method using citric acid and ammonia was investigated by varying the pH of the precursor solution, which played an important role in controlling the morphology of the synthesized powders. The phase formation, microstructure and electromagnetic properties were studied using X-ray diffraction, scanning electron microscopy, impedance analyzer and vibrating sample magnetometer. Single phase pure spinel Mg–Mn ferrite powders were obtained for all the samples at different pH (

Directing the morphological evolution of one-dimensional materials in order to tune their properties for a variety of practical applications in optical sensing and solar cells is an ongoing effort. Here, we establish a systematic method for exerting control over the morphology of nanowires (NWs) grown via a vapour–solid–solid (VSS) process from different metal catalysts. We use germanium, a technologically important material, to demonstrate how catalysts influence the axial growth rate of a crystalline core against the lateral vapour deposition of an amorphous shell which in turn deforms the NWs into straight, tapered or spiral geometries due to interfacial stress. Finite element method (FEM) and molecular dynamic (MD) simulations are further utilized to confirm the proposed mechanism of deformation in crystalline–amorphous core–shell NWs.

Magnesium silicates (MgxSiOy) and in particular forsterite (Mg2SiO4) owing to their low thermal expansion mismatch with metals are promising materials for bioactive coating of implants. Here, we report the electrophoretic deposition (EPD) of forsterite onto titanium substrates using different precursors. Unlike bulk samples which achieve full stoichiometry only beyond 1400 °C, non-stoichiometric magnesium silicate rapidly decomposes into magnesium oxide nanowires during sintering. Elemental mapping and X-ray diffraction suggest that oxygen diffusion followed by ion exchange near the substrate leads to formation of an interfacial Ti5Si3 layer. Pre-annealed forsterite powder on the other hand shows a comparatively lower diffusion rate. Overall, magnesium silicate coatings do not exhibit thermally induced microcracks upon sintering as opposed to calcium phosphate bioceramics which are currently in use.

Solid state thermal diffusion is not a common method of impurity doping in silicon carbide (SiC) device fabrication due to the extremely high temperatures required for such a process to occur. We have recently reported that solid state impurity doping by thermal diffusion in SiC is possible if there is a parallel mechanism, such as oxidation or silicidation that creates silicon or carbon vacancies, which then allows dopant impurities to diffuse into these vacancies. This paper describes the experimental procedures by which oxidation and silicidation can be used to generate vacancies and enhance impurity doping at temperatures below 1400 oC‎

FTIR, Visible and UV Raman scattering, as well as synchrotron radiation X-ray absorption, in combination, have been employed to investigate a series of CVD grown 3C-SiC/Si (100). Significant results on the optical and atomic bonding properties are obtained from these comparative studies.

An electroless nickel film contains 5-14% by weight of phosphorus. Because of the presence of such a high concentration of phosphorus, electroless nickel can be a useful and convenient source of phosphorus dopant in the fabrication of n-type ohmic contacts for SiC. This paper describes the successful deposition of a Ni:P layer on 4H-SiC through electroless nickel plating followed by a discussion of the results of surface science and electrical measurements. Specific contact resistivity on lightly-doped samples with carrier concentration of 2.5 ´ 1016 cm-3 has been found to be about 4.8 ´ 10-6 Ωcm2 without any need for ion implantation. This metallization technique is especially useful in broad area ohmic contact formation on the back of n-type SiC substrate.

The low temperature diffusion of Boron in bulk SiC crystals is investigated and simplified model of such diffusion is presented. The method of UV stimulated etching by aqueous solution of KOH is proposed and some experimental data on influence of defects on quality of prepared p-n junctions are presented.

The characteristics of boron diffusion in 3C-SiC at low temperature have been measured using spreading resistance technique and electroluminescence spectroscopy. The coefficient of boron diffusion in the temperature range of 1150 –1250°С has been found to be about 5.5 x 10-11–5.0 x 10-10 cm2/sec and the activation energy of boron diffusion was determined to be about 0.9 –1.15 eV. Electroluminescence spectra of 3C-SiC p-n junction structures showed peaks at 750 and 630 nm due to growth defects and carbon-silicon divacancies respectively.

Thermal oxides on the Ga-face of low defect density bulk gallium nitride (GaN) were controllably produced under varying conditions and subsequently analyzed. The thermal oxidation was performed in a dry oxygen atmosphere at different temperatures and different oxidation times. Each oxide layer was identified as the monoclinic β-Ga2O3 by a θ–2θ X-ray diffraction (XRD) scan. Complementary Auger electron spectroscopy (AES) and high-resolution X-ray photoelectron spectroscopy (XPS) were also performed to study the chemical and electronic states of the oxide. The surfaces of the as-grown oxide and the GaN surfaces (after oxide removal) were studied using scanning electron microscopy (SEM). The roughness of both surfaces was found to increase with increasing oxidation temperature. Schottky diodes were fabricated on the GaN surfaces after oxide removal to further examine the surface quality. An excess reverse leakage current was found for Schottky diodes fabricated on GaN surfaces that were oxidized at 950 °C or 1000 °C, indicating possible surface decomposition at these two temperatures. The characteristics of wet and dry etching of the gallium oxide layer were also investigated. Finally, metal–oxide–semiconductor (MOS) capacitors were fabricated using thermally oxidized bulk GaN substrates. Current–voltage (I–V) measurements showed a low reverse current of 1.5 pA at −10 V. The oxide breakdown field was determined to be 0.65 MV/cm. Capacitance–voltage (C–V) measurements displayed a deep depletion feature, and the interface trap density near the conduction band was estimated to be in the low 1011 eV−1 cm−2 range.

Vertical Schottky rectifiers have been fabricated on a free-standing n-GaN substrate. Circular Pt Schottky contacts with different diameters (50 μm, 150 μm and 300 μm) were prepared on the Ga-face and full backside ohmic contact was prepared on the N-face by using Ti/Al. The electron concentration of the substrate was as low as ∼7 × 1015 cm−3. Without epitaxial layer and edge termination scheme, the reverse breakdown voltages (VB) as high as 630 V and 600 V were achieved for 50 μm and 150 μm diameter rectifiers, respectively. For larger diameter (300 μm) rectifiers, VB dropped to 260 V. The forward turn-on voltage (VF) for the 50 μm diameter rectifiers was 1.2 V at the current density of 100 A/cm2, and the on-state resistance (Ron) was 2.2 mΩ cm2, producing a figure-of-merit (VB)2/Ron of 180 MW cm−2. At 10 V bias, forward currents of 0.5 A and 0.8 A were obtained for 150 μm and 300 μm diameter rectifiers, respectively. The devices exhibited an ultrafast reverse recovery characteristics, with the reverse recovery time shorter than 20 ns.

Silicon carbide (SiC) field plate terminated Schottky diodes using silicon dioxide (Si02) dielectric experience high electric field in the insulator and premature dielectric breakdown, attributed to the lower dielectric constant of the oxide. This problem can be addressed by using high-k dielectrics such as silicon nitride (Si3N4) that will reduce the field, increase the breakdown voltage and consequently improve the lifetime of the devices. While the advantages of single step field-plate terminated diodes are well-known, the breakdown voltage can be improved even further using a dual-step field-plate termination. Our 2D-numerical simulations using MEDICI have shown an improvement in breakdown voltages in excess of 25% compared to the traditional single-step field-plate terminated diodes.

Silicon has been the semiconductor of choice for microelectronics largely because of the unique properties of its native oxide (SiO2) and the Si/SiO2 interface. For high-temperature and/or high-power applications, however, one needs a semiconductor with a wider energy gap and higher thermal conductivity. Silicon carbide has the right properties and the same native oxide as Si. However, in the late 1990’s it was found that the SiC/SiO2 interface had high interface trap densities, resulting in poor electron mobilities. Annealing in hydrogen, which is key to the quality of Si/SiO2 interfaces, proved ineffective. This paper presents a synthesis of theoretical and experimental work by the authors in the last six years and parallel work in the literature. High-quality SiC/SiO2 interfaces were achieved by annealing in NO gas and monatomic H. The key elements that lead to highquality Si/SiO2 interfaces and low-quality SiC/SiO2 interfaces are identified and the role of N and H treatments is described. More specifically, optimal Si and SiC surfaces for oxidation are identified and the atomic-scale processes of oxidation and resulting interface defects are described. In the case of SiC, we conclude that excess carbon at the SiC/SiO2 interface leads to a bonded Si-C-O interlayer with a mix of fourfold- and threefold-coordinated C and Si atoms. The threefold coordinated atoms are responsible for the high interface trap density and can be eliminated either by H-passivation or replacement by N. Residual Si-Si bonds, which are partially passivated by H and N remain the main limitation. Perspectives for the future for both Si- and SiC-based MOSFETs are discussed.

Silicon carbide (SiC) field-plate terminated Schottky diodes using silicon dioxide (SiO 2 ) dielectric experience high electric field in the insulator and premature dielectric breakdown, attributed to the lower dielectric constant of the oxide. To alleviate this problem we explore the use of high- k dielectrics, silicon nitride (Si 3 N 4 ) and sapphire (Al 2 O 3 ), on 4H-SiC by numerical simulations using Medici. The simulation results show significant improvement in blocking voltages by as much as 30% and much lower electric field within the dielectrics. There is also a slight reduction in the specific-on resistance ( R sp-on ) and a small increase in the forward current density due to the formation of an accumulation layer in SiC where the metal overlaps the dielectric. This effect is enhanced with increasing dielectric constant and decreasing dielectric thickness for a given dielectric.

Reactive ion etching of 4H-SiC was performed using a CHF"3-O"2 plasma. The etch rate and mean surface roughness were investigated as a function of the ratio of the O"2 flow rate to the total gas flow rate. It was found that oxygen plays an indirect role in contributing to the etching of SiC. An optimum O"2 fraction of 20% was found to give a maximum etch rate of 35nm/min. On the other hand, the root-mean-square (RMS) surface roughness was found to increase from 1.31 to 2.34nm when the O"2 fraction increased from 0% to 80%. Auger electron spectroscopy results for the samples etched at the optimum condition of 20% O"2 fraction revealed the presence of oxygen on the etched surface in a form of an oxide-like layer (SiO"x). No carbon residue (carbon rich-layer) and aluminum were found. Based on our results, the role of O"2 in the reactive ion etching of 4H-SiC will be presented.

4H-SiC Schottky barrier diodes (SBDs) were fabricated and characterized from room temperature to 573K using HfN"xB"y as Schottky electrodes. The results are compared to SBDs fabricated using other electrodes that include Ni, Pt, Ti and Au. The Schottky barrier height @F"b for as-deposited HfN"xB"y/n-/n+ diodes, determined from room temperature current-voltage characteristics, is 0.887eV. This is lower than those of SBDs fabricated using other metals such as Au, where @F"b is 1.79eV. The HfN"xB"y/n-/n+ diodes studied have a much higher on-resistance R"o"n of around 38.24m@W-cm^2, which is about four times that of Au/n-/n+ diodes, due to the higher sheet resistance of the sputtered HfN"xB"y electrode layer. The barrier height @F"b and ideality factor @h of the HfN"xB"y/n-/n+ diodes remain almost unchanged after 400 and 750^oC anneal in N"2. This suggests excellent thermal and chemical stability of HfN"xB"y in contact with 4H-SiC.

Reactive ion etching (RIE) of bulk 4H-SiC based on CHF"3-O"2 plasma was studied by varying the rf power and process pressure. The elements on the etched surface and the surface roughness were characterized by Auger electron spectroscopy and atomic force microscopy, respectively. It was found that the surface roughening is mainly caused by Al contamination and C rich layer (C residues) induced micromasking effect. The micromasking effect is in turn determined by the dc self-bias developed at the substrates. A threshold dc self-bias exists at around -320 to -330V, beyond which no micromasking effect was observed. This observation is explained in terms of physical ion bombardment and sputtering in the RIE process.

Thermally grown oxide on 4H-SiC has been post-annealed in diluted N"2O (10% N"2O in N"2) at different temperatures from 900 to 1100^oC. The quality of the nitrided oxide and the SiO"2/4H-SiC interface was investigated by AC conductance and high frequency C-V measurements based on Al/SiO"2/4H-SiC metal-insulator-semiconductor (MOS) structure. It is found that N"2O annealing at 1000^oC produces the lowest interface state density, though the difference is not so significant when compared to the other samples annealed at 900 and 1100^oC. These results can be explained by the high temperature dynamic decomposition process of N"2O. By fitting the AC conductance data, it is found that higher temperature nitridation increases the capture cross-section of the interface traps.

Reactive ion etching of SiC induced surface damage, e.g., micromasking effect induced coarse and textured surface, is one of the main concerns in the fabrication of SiC based power devices [1]. Based on CHF3 + O2 plasma, 4H-SiC was etched under a wide range of RF power. Extreme coarse and textured etched surfaces were observed under certain etching conditions. A super-linear relationship was found between the surface roughness and RF power when the latter was varied from 40 to 160 W. A further increase in the RF power to 200 W caused the surface roughness to drop abruptly from its maximum value of 182.4 nm to its minimum value of 1.3 nm. Auger electron spectroscopy (AES) results revealed that besides the Al micromasking effect, the carbon residue that formed a carbon-rich layer, could also play a significant role in affecting the surface roughness. Based on the AES results, an alternative explanation on the origin of the coarse surface is proposed.

In this work, we present results on the study of bonding and concentration of carbon in 4H-SiC MOS structure by x-ray photoelectron spectroscopy (XPS). The XPS spectra were fitted by several Gaussian lineshape functions. It is found that the so-called carbon clusters (C-C bonds) appear at the interface of SiO2/SiC, but are not seen in the oxide bulk. However, there are still some SiOxCy and Si-C bonds inside the oxide and the integrated area ratio of SiOxCy/Si-C bonds increases when further away from the SiO2/SiC interface. These observations can be interpreted in terms of the dynamic oxidation process that transforms Si-C bonds into SiOxCy bonds, which are then further oxidized to form SiO2 bonds.

The relationship between nitrogen content and interface trap density (Dit) in SiO2/4H–SiC near the conduction band has been quantitatively determined. Nitridation using NO significantly reduces Dit near the conduction band, but the effect saturates after ≈2.5×1014 cm−2 of nitrogen. These results are consistent with a model of the interface in which defects such as carbon clusters or silicon suboxide states produce traps with energies corresponding to the sizes of the defects. Nitrogen passivation results in the dissolution of the defects, which then lowers the energies of the traps in the band gap.

A graphite mask was used to realize selective doping of aluminum/boron in 4H–SiC by thermal diffusion at a temperature range of 1800–2100 °C. The doping profiles investigated by secondary ion mass spectrometry show that a high aluminum concentration of 5×1019 cm−3 near the surface and linearly graded boron profile up to several micrometers in depth can be obtained. Hall effect measurement was also employed to obtain the electrical characteristics of the diffused region, from which the carrier concentration (1×1019 cm−3) and hole mobility (7 cm2/V s) at room temperature were extracted. Room temperature photoluminescence indicates that the dominant luminescence is attributed to the donor acceptor pair recombination, in which boron D complex is the prevailing center rather than Al and boron shallow acceptors. Cathodoluminescence micrographs clearly illustrate a pattern with the locally diffused regions. To confirm the viability of the diffusion process, planar p-n diodes with a fairly low forward voltage dro...

A process is described to enhance the diffusion rate of impurities in silicon carbide so that doping by thermal diffusion can be done at lower temperatures. This process involves depositing a thin film consisting of an oxide of the impurity followed by annealing in an oxidizing ambient. The process uses the lower formation energy of silicon dioxide relative to that of the impurity-oxide to create vacancies in silicon carbide and to promote dissociation of the impurity-oxide. The impurity atoms then diffuse from the thin film into the near-surface region of silicon carbide.

An optimized multiple floating guard ring structure is investigated for the first time as an edge termination method for high voltage 4H-SiC planar devices. Simulations were performed to investigate SiC guard ring termination, and determine the optimum guard ring spacing for planar diodes with up to four floating rings. Simulated optimized designs predicted breakdown values from 40% of the ideal breakdown with a single ring, to 84% of the ideal value for diodes with four rings. Implanted 4H-SiC pn diodes with optimized guard ring designs were fabricated and results correlated to simulation. Experimental breakdown values of 1.2–1.3 kV for guard ring structure with four rings were in good agreement with simulated results.

The orientation dependence of lateral overgrowth and vertical growth of InP deposits selectively grown through stripe openings in a phosphosilicate glass mask was investigated using halide vapor-phase epitaxy. The lateral overgrowth was observed to be strongly dependent on the orientation of stripe openings, and the vertical growth was shown to be inversely correlated to the lateral overgrowth. For a sufficiently long growth time, the growth fronts of lateral overgrowth layers from adjacent stripe openings merged together to form a completely overgrown layer. The morphology of the region where the growth fronts are merged was shown to be dependent on the orientation of stripe openings through which the growth was seeded. We also investigated a method to separate the completely overgrown layer from the substrate. This mechanical epitaxial lift-off technique may be used for hybrid integration of optoelectronic and electronic devices.

A series of 4H-silicon carbide (SiC) epilayers grown on 4H-SiC substrates by low pressure chemical vapor deposition (LPCVD) with different silane (SiH4) to propane (C3H8) gas flow ratios were studied by angle resolved X-ray photoelectron spectroscopy (ARXPS) and atomic force microscopy (AFM). ARXPS revealed that the surfaces of the samples consisted of elemental Si, Si oxides (SiO2 and SiOx where x

Tantalum pentoxide (Ta"2O"5) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current-voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta"2O"5 of around 4.3eV, resulting in a small band offset between the 4H-SiC and Ta"2O"5. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta"2O"5 and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta"2O"5 at 900^oC in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C-V measurements of the Ta"2O"5/SiO"2/4H-SiC capacitors.

Raman microprobe measurements have been made on several 4HSiC epilayers grown by low-pressure chemical vapor deposition (LPCVD). The peak intensity of the TO phonon at 795.7 cm −1 normalized by that of the TO phonon at 775.0 cm −1 was found to vary with the growth conditions as well as localized microstructural disorder. The relative intensity of the LO peak at 961.6 cm −1 was also affected by localized microstructural disorder. The dependence of the relative intensities of these two Raman scattering peaks on the morphology as well as the growth conditions provides important insight into the application of Raman scattering measurements to evaluating the quality of 4HSiC epilayers.

The formation of etch pits during heteroepitaxial growth of 3CSiC on silicon is a common occurrence. A comparison of the morphology of 3CSiC epilayers grown on silicon at both atmospheric and low pressures shows that the problem is more severe when the epilayers are grown at low reactor gas pressure. It is found that the density of etch pits is affected by the condition of the susceptor for both atmospheric and low pressure processes. At low pressure, the composition of the process gas during buffer layer growth plays an important role in suppressing the formation of etch pits. The nature of etch pits and various attempts to suppress the formation of etch pits are discussed.

Synchrotron radiation X-ray absorption and UV 325 nm excitation Raman scattering- photoluminescence (PL) have been employed to investigate a series of 4H-SiC wafers, including bulk, epitaxial single or multiple layer structures by chemical vapor deposition. Significant results on the atomic bonding and PL-Raman properties are obtained from these comparative studies.

A series of 3C-SiC/Si(100) films grown by chemical vapour deposition (CVD) with different growth times and hence film thicknesses, are studied by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). XPS showed that the surfaces of the samples consist of Si oxides (SiO2 and SiO2 where x

Interface state density profiling of the thermal oxide / n-type 4H-SiC interface which underwent post-oxidation nitric-oxide (NO) annealing showed that an interface state density of approximately 1×1011 cm−2eV−1 could be achieved at around 0.2 eV below the conduction band. It decreased exponentially by two orders to 1×109 cm-2eV-1 at around 0.9 eV from the conduction band. The values are comparable or better than other published work. The low interface state density achieved near the conduction band is important towards improved channel carrier mobility in SiC MOSFETs. A positive flat-band voltage shift of the SiC based MOS capacitor was also observed. The shift reduced under UV illumination. It could be attributed to slow acceptor-like (negatively-charged) traps, which may have contributed to the instabilities observed in drain current and threshold voltage suffered by SiC MOSFETs.

Results presented in this letter demonstrate that the effective channel mobility of lateral, inversion-mode 4H-SiC MOSFETs is increased significantly after passivation of SiC/SiO/sub 2/ interface states near the conduction band edge by high temperature anneals in nitric oxide. Hi-lo capacitance-voltage (C-V) and ac conductance measurements indicate that, at 0.1 eV below the conduction band edge, the interface trap density decreases from approximately 2/spl times/10/sup 13/ to 2/spl times/10/sup 12/ eV/sup -1/ cm/sup -2/ following anneals in nitric oxide at 1175/spl deg/C for 2 h. The effective channel mobility for MOSFETs fabricated with either wet or dry oxides increases by an order of magnitude to approximately 30-35 cm/sup 2//V-s following the passivation anneals.

Phosphorus is an important n-type dopant for both silicon and silicon carbide. Although solid-state diffusion of phosphorus in silicon has been well documented and experimentally proven, not much is known about phosphorus solid-state diffusion in silicon carbide, especially at lower temperatures. A convenient source of phosphorus for solid-state diffusion in silicon carbide is phosphorus oxide. The possibility of using phosphorus oxide as a dopant source for silicon carbide is investigated by considering the probable reactions between silicon carbide and phosphorus oxide at temperatures below 1700 K using published thermodynamic data. By considering the standard free energies of reactions, it can be shown that phosphorus can be introduced in silicon carbide at temperatures below 1700 K using phosphorus oxide. A successful development of low temperature dopant incorporation in silicon carbide would reduce the need for high temperature processes and prevent process-induced thermal degradation of critical device structures such as the oxide-semiconductor interface. Experimental results showing phosphorus impurity incorporation and activation in 4H-SiC are presented.

Results of room temperature capacitance–voltage measurements are reported for SiO2/4H–SiC (n and p type) metal-oxide-semiconductor capacitors annealed in ammonia following oxide layer growth using standard wet oxidation techniques. For n-SiC capacitors, both the interface state density near the conduction band edge and the effective oxide charge are substantially reduced by the ammonia anneals. For 2 h anneals, the oxide charge appears to have a minimum value for an anneal temperature of approximately 1100 °C. However, for p-SiC, anneals in ammonia produce no improvement in the interface state density near the valence band edge, and the effective oxide charge is not reduced compared to samples that were not annealed. Results are compared to those reported previously for anneals in nitric oxide. Ion beam analyses of the oxide layers show substantially more nitrogen incorporation with the ammonia anneals, although for n-SiC, the decrease in Dit is comparable for both nitric oxide and ammonia anneals. Result...

Results of capacitance–voltage measurements are reported for metal–oxide–semiconductor capacitors fabricated using the 4H polytype of silicon carbide doped with either nitrogen (n) or aluminum (p). Annealing in nitric oxide after a standard oxidation/reoxidation process results in a slight increase in the defect state density in the lower portion of the band gap for p-SiC and a significant decrease in the density of states in the upper half of the gap for n-SiC. Theoretical calculations provide an explanation for these results in terms of N passivating C and C clusters at the oxide–semiconductor interface.

Spectroelectrochemistry was used to study the electron trapping and rectifying behavior in several diphenylamine endgroup polymeric bilayers. Various combinations of the following monomers were pairwise sequentially electropolymerized onto ITO transparent electrodes: FD, DNTD, DPTD and Cl4DPTD. Poly(FD) is a p-type material while poly(DNTD), poly(DPTD) and poly(Cl4DPTD) are bipolar materials being both n-type and p-type. Bilayers of ITO|poly(FD)|poly (DNTD) or ITO|poly (FD)|poly (DPTD), block electrons from reducing the outer layer even at -1.0 V vs Ag/AgCl, yet holes effectively oxidize both layers. The LUMO differences between poly(DNTD) and poly(Cl4DPTD) or poly(DPTD) and poly(Cl4DPTD) provide a large enough electronic barrier that electron trapping can occur between these n-type materials. The visible spectra results imply that these polymers, poly(DPTD) or poly(Cl4DPTD) can be used as photovoltaic materials.

An attempt has been made to use RBS techniques for a quantitative analysis of the transport of Au in GaAs over the temperature range 300–450 ° C. Two components appear to be involved in the mass transport, which has been treated as a diffusion process. The temperature dependence of the diffusion coefficient for the faster of the two components can be described approximately by the equation D( cm 2 / s ) = D 0 exp(− Q kT ) with D 0 = 8.9 × 10 −9 cm 2 /s and Q = 0.65 eV. The limitations of this approach are discussed, and results are compared with those presented previously by other authors.

The interface passivation process based on post-oxidation, high temperature anneals in nitric oxide (NO) is well established for SiO2 on (0001) 4H-SiC. The NO process results in an order of magnitude or more reduction in the interface state density near the 4H conduction band edge. However, trap densities are still high compared to those measured for Si / SiO2 passivated with post-oxidation anneals in hydrogen. Herein, we report the results of studies for 4H-SiC / SiO2 undertaken to determine the effects of additional passivation anneals in hydrogen when these anneals are carried out following a standard NO anneal. After NO passivation and Pt deposition to form gate contacts, post-metallization anneals in hydrogen further reduced the trap density from approximately 1.5 × 1012 cm−2eV−1 to about 6 × 1011 cm−2eV−1 at a trap energy of 0.1 eV below the band edge for dry thermal oxides on both (0001) and (11–20) 4H-SiC.

Silicon carbide (SiC) is an excellent semiconductor for the fabrication of high power and high temperature electronic devices. SiC pn junctions are critical components of SiC high power devices and circuits. However, the high electric field behavior of SiC p-n junction structures is not well characterized. The study of the high field breakdown mechanisms of SiC p-n junction plays an important role in determining the proper design of SiC high power p-n junction-based devices. We have determined the high field breakdown behaviors of several types of 4H-SiC epitaxial p-n junction diodes of different design. In our efforts to increase the breakdown voltage, we have found that oxide passivation did not substantially affect the breakdown voltage but edge termination using argon ion implantation is effective in improving the breakdown voltage of SiC-p-n junction diodes.

We report the effect on SiO/sub 2//SiC interface state density and effective oxide charge of different Si:C ratios (0.12 to 0.55) used during the growth of n-4H and n-6H-SiC epitaxial layers. We also report the effects of post-growth re-oxidation anneals and post- metalization anneals on the interface state density for both n- and p-4H-SiC. The interface trap density near the conduction band and the effective oxide charge increase with increasing Si:C ratio for both polytypes; however, the n-4H polytype is found to have an order of magnitude higher interface trap density near the conduction band compared to n-6H-SiC. The effective oxide charge is also higher for n-4H polytype. The distribution of interface states for 4H-SiC (measured using n- and p-type material) is asymmetric, with a higher trap density near the conduction band. Post-growth re-oxidation in wet O/sub 2/ at 950/spl deg/C increases the interface trap density near the conduction for n-4H-SiC. Post-metalization annealing at 450/spl deg/C in Ar for an Al gate metal results in a reduction of the effective oxide charge from 9.5/spl times/10/sup 11/ cm/sup -2/ to 3.5/spl times/10/sup 11/ cm/sup -2/. The effective oxide charge for n-4H samples with Mo gates decreases with increasing post metalization annealing temperature. Interface state densities are not affected by the post-metalization anneals in Ar.

Based upon graphite mask, selective aluminum/boron doping of SiC by thermal diffusion has been successfully realized in a temperature range of 1800 to 2100°C. Secondary ion mass spectrometry (SIMS) was used to identify the doping profiles, which showed very high aluminum concentration (5×1019 cm−3) near the surface and linearly graded boron profile up to several micrometers in depth. Hall-effect measurement was also employed to obtain the carrier concentration, which showed more than 1019 cm−3 carrier concentration at room temperature. Cathodoluminescence (CL) image clearly illustrated the locally diffused pattern. In addition, planar p-n diodes based upon this technique were fabricated and current-voltage (I-V) characteristics were measured. Excellent rectification property has been obtained. Built-in voltage of 2.9 V in the formed p-n junction was obtained by capacitance-voltage (C-V) measurement.

Aluminum nitride (AlN) is a potential gate insulator material for 6H-SiC metal-insulator-semiconductor (MIS) devices for high temperature and high power applications. A critical requirement for a viable gate insulator material is that the insulator/semiconductor interface must have a very low interface state density, low insulator fixed charge density, as well as a low density of interfacial trapping centers. Our capacitance-voltage (CV) measurements have shown that it is possible to obtain an AlN/6H-SiC interface with such characteristics. Although CV measurements done at 573 K have shown evidence of the activation of deeper interfacial charged centers, they still reveal the presence of a low number of interface states and low insulator charge density. The disadvantage of this material system is the high leakage current density. However, by using a stacked AlN structure, we have found that it is possible to obtain MIS structures that are able to sustain breakdown fields close to the theoretical values. The possible use of a very thin AlN layer to enhance the characteristics of a Au-6H-SiC Schottky diode will also be discussed.

The current vs voltage characteristics of 4H-SiC MOS capacitors under deep depletion are obtained by a fast ramp response technique so as to obtain the maximum field that can be applied to the MOS structure without the failure of either the semiconductor or the oxide layer. The experiments on n-type 4H SiC wafers having a 5 μm thick epilayer of 1015 – 1016 cm−3 doping concentration and an oxide layer 1200Å – 1500Å thick, indicate the significant influence of the oxide quality and defects in the semiconductor on the nature of the current response during accumulation and deep depletion measurements. The effect of the conductivity of the oxide layer is reflected clearly in the current response, even though classical C-V measurements do not indicate any abnormality. Apart from obtaining the maximum breakdown fields of the semiconductor and the oxide, the fast-ramp response technique provides useful information about the generation processes associated with defects in the MOS structure.

Polytype 6H silicon carbide samples having polar faces terminated with either carbon (000\(\bar 1\)) or silicon (0001) have been oxidized at 810°C for varying times using a microwave plasma source. Silicon dioxide layer thicknesses have been determined using Rutherford backscattering techniques, and the oxidized samples were further characterized using capacitance-voltage techniques and the methods of Raman spectroscopy. Results indicate that oxide layers of comparable quality to those produced using standard thermal oxidation methods can be obtained in less time at lower temperature.

The temperature-dependent electrical characteristics of Schottky rectifiers fabricated with a SiO2 field plate on a freestanding n− gallium nitride (GaN) substrate were reported in the temperature range of 298–473K. The Schottky barrier heights evaluated from forward current-voltage measurement revealed an increase of Schottky barrier height and series resistance but a decrease of ideality factor (n) with increasing temperature. However, the Schottky barrier heights evaluated from capacitance-voltage measurement remained almost the same throughout the temperature range measured. The Richardson constant extrapolated from ln(J0∕T2) vs 1∕T plot was found to be 0.029Acm−2K−2. A modified Richardson plot with ln(J0∕T2) vs 1∕nT showed better linearity, and the corresponding effective Richardson constant was 35Acm−2K−2. The device showed a high reverse breakdown voltage of 560V at room temperature. The negative temperature coefficients were found for reverse breakdown voltage, which is indicative of a defect-assi...

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.