J. R. Williams

Improved inversion channel mobility for 4H-SiC MOSFETs following high temperature anneals in nitric oxide

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.

HIGH-TEMPERATURE OHMIC CONTACT TO N-TYPE 6H-SIC USING NICKEL

Specific contact resistances measured at elevated temperatures for Ni ohmic contacts to 6H‐SiC were reported. The specific contact resistances were measured with the linear transmission line method at both room temperature and at 500 °C and yielded values <5×10−6 Ω cm2 at both temperatures. The trend shows a decreasing contact resistance at higher temperatures. The annealed metal film is a nickel silicide with substantial mixing of C throughout the silicide layer.

Density of interface states, electron traps, and hole traps as a function of the nitrogen density in SiO2 on SiC

Nitridation of the SiO2/SiC interface yields a reduction in interface state density, immunity to electron injection, as well as increased hole trapping. It is shown that the accumulation of nitrogen at the oxide/semiconductor interface is solely responsible for these three effects. The evolution of the density of interface states, electron traps, and hole traps is measured in metal-oxide-semiconductor capacitors as a function of the nitrogen content which is varied by adjusting the gate oxide NO annealing time. A rate equation is derived to model the change in the interface state density, observed at various energy levels, in terms of nitrogen binding cross-sections. While the generation of acceptor interface states upon electron injection is suppressed after minimum N incorporation, the density of oxide hole traps appears to scale linearly with the amount of nitrogen. The origin and the properties of the N-induced hole traps resembles those of the defects responsible for enhanced negative bias temperatur...

Lasing in whispering gallery mode in ZnO nanonails

We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be 17MWcm−2. Very sharp emission peaks (full width at half maximum of 0.08nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

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.

Effect of nitric oxide annealing on the interface trap density near the conduction bandedge of 4H–SiC at the oxide/(112̄0) 4H–SiC interface

Nitric oxide postoxidation anneal results in a significant decrease of defect state density (Dit) near the conduction bandedge of n-4H–SiC at the oxide/(1120) 4H–SiC interface. Comparison with measurements on the conventional (0001) Si-terminated face shows a similar interface state density following passivation. Medium energy ion scattering provides a quantitative measure of nitrogen incorporation at the SiO2/SiC interface.

Finding the optimum Al–Ti alloy composition for use as an ohmic contact to p-type SiC

Abstract Alloys of aluminum and titanium have been successfully used to form low resistance ohmic contacts to p-type SiC. While the 90 wt.% Al alloy has been studied extensively, the literature does not reveal any work which indicates whether the 90/10 or any other alloy composition is the best alloy for use as an ohmic contact material to p-SiC. This work systematically looks at four different Al–Ti alloy compositions in an attempt to decide which alloy if any is superior as an ohmic contact material. The alloy compositions that were studied were chosen by examining the binary Al–Ti phase diagram and choosing specific phases prior to reaction with the SiC. It will be shown that only alloys which have some amount of a liquid phase present at the anneal temperature will form an ohmic contact to p-type SiC.

Interface trap passivation for SiO2/(0001) C-terminated 4H-SiC

Interface trap passivation at the SiO2∕carbon-terminated (0001¯) face of 4H-SiC utilizing nitridation and hydrogenation has been evaluated. The SiO2∕SiC interface, created by dry thermal oxidation on the C face, shows appreciably higher interface state density near the conduction band compared to the (0001) Si face. A postoxidation anneal in nitric oxide followed by a postmetallization anneal in hydrogen results in dramatic reduction of the trap density by over an order of magnitude near the conduction band. The electrical measurements have been correlated with the interfacial chemistry.Interface trap passivation at the SiO2∕carbon-terminated (0001¯) face of 4H-SiC utilizing nitridation and hydrogenation has been evaluated. The SiO2∕SiC interface, created by dry thermal oxidation on the C face, shows appreciably higher interface state density near the conduction band compared to the (0001) Si face. A postoxidation anneal in nitric oxide followed by a postmetallization anneal in hydrogen results in dramatic reduction of the trap density by over an order of magnitude near the conduction band. The electrical measurements have been correlated with the interfacial chemistry.

High-Mobility Stable 4H-SiC MOSFETs Using a Thin PSG Interfacial Passivation Layer

Phosphorous from P₂O₅ is more effective than nitrogen for passivating the 4H-SiC/SiO₂ interface. The peak value of the field-effect mobility for 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) after phosphorus passivation is approximately 80 cm²/V·s. However, P₂O₅ converts the SiO₂ layer to phosphosilicate glass (PSG)-a polar material that introduces voltage instabilities which negate the benefits of lower interface trap density and higher mobility. We report a significant improvement in voltage stability with mobilities as high as 72 cm²/V·s for MOSFETs fabricated with a thin PSG gate layer ( ~ 10 nm) capped with a deposited oxide.

Etching of 6H‐SiC and 4H‐SiC using NF 3 in a Reactive Ion Etching System

The use of pure NF 3 source gas in reactive ion etching of bulk and epitaxy Si-face, 6H-SiC, and 4H-SiC is reported. The effects of RF power and chamber pressure on etch rate and surface morphology are discussed. A process developed for a smooth, residue-free etch, with a relatively high etch rate of ∼1500 A/min is examined using scanning electron microscopy and Auger electron spectroscopy surface analysis. The process developed had a self-induced dc bias ranging from 25 to 50 V, a forward RF power of 275 W (1.7 W/cm 2 ), chamber pressure of 225 mT, and a NF 3 flow rate between 95 and 110 sccm. No chemical residue or aluminum micromasking was observed on any of the samples etched with the above process.