C. C. Tin

Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide

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.

Characterization and modeling of the nitrogen passivation of interface traps in SiO2/4H–SiC

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.

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.

High electrical resistivity diamond films deposited from an oxyacetylene flame

Diamond films with electrical resistivity as high as 1014 Ω cm have been deposited on silicon, molybdenum, and other foreign substrates from an oxyacetylene flame. Exposure of the highly resistive diamond films to a hydrogen plasma leads to the decrease of the electrical resistivity by several orders of magnitude. Low level incorporation of atomic hydrogen in the oxyacetylene flame into the diamond film under the flame deposition conditions is believed to be the cause of the high electrical resistivity of the flame‐grown diamond films. In contrast to the relatively low resistivity of diamond films deposited in a hydrogen/methane plasma, flame deposition provides a means of growing electrically insulating diamond thin films.

Raman characterization of electronic properties of self-assembled GaN nanorods grown by plasma-assisted molecular-beam epitaxy

We have investigated the Raman scattering of the aligned gallium nitride (GaN) nanorods grown by plasma-assisted molecular-beam epitaxy. It was determined by Raman spectroscopy that the GaN nanorods are relatively strain free. The free carrier concentration, as well as electron mobility of the GaN nanorods, was obtained by the line shape analysis of the coupled A1longitudinal-optical(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017cm−3 and 140cm2Vs, respectively. The local temperature of the nanorod sample was estimated based on the ratio of Stokes to anti-Stokes Raman peak intensity. Since the position of the LO phonon peak was found to be dependent on both the temperature and the LO phonon-plasmon coupling, it is crucial to consider the temperature effect in determining the frequency of the uncoupled LO phonon mode for the line shape analysis. The frequency of the A1(LO) mode of an undoped bulk GaN was used as a reference to dete...

Nitrogen Passivation of the Interface States Near the Conduction Band Edge in 4H-Silicon Carbide

This paper describes the development of a nitrogen-based passivation technique for interface states near the conduction band edge [D it (E c )] in 4H-SiC/SiO 2 . These states have been observed and characterized in several laboratories for n - and p -SiC since their existence was first proposed by Schorner, et al. [1]. The origin of these states remains a point of discussion, but there is now general agreement that these states are largely responsible for the lower channel mobilities that are reported for n -channel, inversion mode 4H-SiC MOSFETs. Over the past year, much attention has been focused on finding methods by which these states can be passivated. The nitrogen passivation process that is described herein is based on post-oxidation, high temperature anneals in nitric oxide. An NO anneal at atmospheric pressure, 1175°C and 200–400sccm for 2hr reduces the interface state density at E c -E ≅0.1eV in n -4H-SiC by more than one order of magnitude - from > 3×10 13 to approximately 2×10 12 cm −2 eV −1 . Measurements for passivated MOSFETs yield effective channel mobilities of approximately 30–35cm 2 /V-s and low field mobilities of around 100cm 2 /V-s. These mobilities are the highest yet reported for MOSFETs fabricated with thermal oxides on standard 4H-SiC and represent a significant improvement compared to the single digit mobilities commonly reported for 4H inversion mode devices. The reduction in the interface state density is associated with the passivation of carbon cluster states that have energies near the conduction band edge. However, attempts to optimize the the passivation process for both dry and wet thermal oxides do not appear to reduce Dit(Ec) below about 2×1012cm −2 eV −1 (compared to approximately 10 10 cm −2 eV −1 for passivated Si/SiO 2 ). This may be an indication that two types of interface states exist in the upper half of the SiC band gap – one type that is amenable to passivation by nitrogen and one that is not. Following NO passivation, the average breakdown field for dry oxides on p -4H-SiC is higher than the average field for wet oxides (7.6MV/cm compared to 7.1MV/cm at room temperature). However, both breakdown fields are lower than the average value of 8.2MV/cm measured for wet oxide layers that were not passivated. The lower breakdown fields can be attributed to donor-like states that appear near the valence band edge during passivation.

The Response of High Voltage 4H-SiC P-N Junction Diodes to Different Edge Termination Techniques

Edge termination is an important aspect in the design of high power p-n junction devices. In this paper, we compare the breakdown characteristics of 4H-SiC p + -n diodes with oxide passivation and with edge termination using either low or high energy ion implantations. N- and p-type epilayers of 4H-SiC were grown by chemical vapor deposition on n + 4H-SiC wafers. Circular mesa structures of different diameters were patterned and isolated by reactive ion etching. Four types of samples were fabricated. The first group was not implanted or passivated and was left for control. The second type consisted of oxide-passivated diode structures while the third and fourth types were ion implanted with 30 keV Ar + and 2.2 MeV He + ions, respectively. The time dependent breakdown characteristics were determined using a fast voltage ramp technique. The reverse bias breakdown voltages and leakage currents of these diodes were different for the different types of the edge termination. Diodes terminated using 2.2 MeV ion implantation yielded the best breakdown characteristics. A majority of the diodes exhibited abrupt breakdown.

Electrical and Optical Properties of Diamond Films Deposited from an Oxy-Acetylene Flame

Diamond exhibits remarkable physical and chemical properties that have attracted attention from scientists and engineers for a long time. The invention of chemical vapor deposition (CVD) techniques for the deposition of diamond on foreign substrates at low pressures, i.e., atmospheric or subatmospheric pressures, and low temperatures, i.e., below 1000°C, has further stimulated a great deal of interest for researchers to optimize the deposition processes for practical application of the remarkable diamond thin films.

Raman scattering of self-assembled gallium nitride nanorods synthesized by plasma-assisted molecular beam epitaxy

Raman scattering experiment was carried out to study material and electronic properties of the aligned GaN nanorods grown by plasma-assisted molecular beam epitaxy. Due to the small size of the nanorods and the relatively high power density of the laser illumination used in this investigation, local heating of the sample cannot be neglected. Special care has been taken to identify the frequency shift in the optical phonon peaks induced by local heating prior to any spectral analysis. The residual stress in the GaN nanorods was estimated by analyzing the frequency shift of its E2 Raman mode. It was found that the frequency of E2 mode is shifted by only 0.1 cm -1 comparing with the stress-free frequency, indicating a negligible residual stress in nanorods. Owing to aligned geometry of the nanorods and the back-scattering scattering geometry used in the experiment, A 1 (LO) Raman mode was collected. The free carrier concentration as well as electron mobility of the GaN nanorods were obtained by the line shape analysis of the coupled A 1 (LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017 cm -3 and 140 cm 2 /Vs, respectively. We presented a general method of better determining the electronic parameters of the GaN nanostructures via Raman scattering.