K. Wongchotigul

Near band-edge transition in aluminum nitride thin films grown by metal organic chemical vapor deposition

Cathodoluminescence measurements were performed for carbon doped and undoped aluminum nitride thin films in the temperature range from liquid helium to room temperature. The AlN films were grown on three different substrates: 6H–SiC, 4H–SiC, and sapphire. From these samples, a strong luminescence peak surrounded by two weaker peaks in the near band-edge region, near 6 eV, was observed. For AlN on sapphire, this near band-edge transition can be further resolved into three peaks at 6.11, 5.92, and 5.82 eV. These peaks are believed to be due to exciton recombination. The effects of substrate materials and carbon doping on the exciton peak were discussed. The temperature dependence of the peak position and line width of this transition was also studied. The temperature coefficient of the band-gap energy is estimated to be 0.51 meV/K.

Dispersion properties of aluminum nitride as measured by an optical waveguide technique

An aluminum nitride thin film on sapphire substrate was prepared by metal–organic chemical-vapor deposition for optical waveguide study. A rutile prism coupler was employed to display the waveguide modes (N lines) with wavelengths of 632.8, 532.1, 514.5, and 488.0 nm. The refractive index and thickness of the waveguide material were obtained by prism–coupler measurement. The parameters in the dispersion equation were determined by curve fitting. The attenuation in the waveguide was evaluated by scattering loss measurements using a fiber probe. The attenuation coefficient α is in the range of 1.5–2.1 cm−1 (5.4–7.6 dB cm−1), depending on the surface roughness of the sample and different orders of waveguide modes.

Low resistivity aluminum nitride: carbon (AlN : C) films grown by metal organic chemical vapor deposition

Abstract Low resistivity, p-type, single crystal aluminum nitride-carbon (AlN : C) films were grown by metal organic chemical vapor deposition (MOCVD). Films produced with high partial pressure of propane during growth exhibited high conductivity. Van der Pauw measurements indicated that the resistivity of the as-grown films changed dramatically from 108 Ω cm for unintentionally doped samples to less than 0.1 Ω cm for partial pressures of propane greater than 0.5 × 10−3 Torr. Reflection high energy electron diffraction (RHEED) measurements performed “in situ” just after film growth indicated that the material is single crystal up to a propane partial pressure of 2.5 × 10−3 Torr. P-n junctions of n-type 6H-SiC and p-type AlN : C were fabricated, and blue emission (centered at 490 nm) was observed from the heterojunction under forward bias.

An investigation of 3C-SiC photoconductive power switching devices

Abstract Photoconductive semiconductor switches (PCSS) have unique advantages such as high power, high speed, negligible time jitter and long lifetime. Silicon carbide (SiC), due to its high dielectric strength and other desired physical properties, is an excellent material for PCSS. However, no result has been reported on cubic silicon carbide (3C-SiC) PCSS. In this work, PCSS were fabricated on the following three types of 3C-S1C material; (i) boron doped, (ii) unintentionally doped single crystals, and (iii) polycrystalline. The PCSS were investigated using ArF and XeCl excimer lasers. Practical switches with many potential applications were successfully fabricated. The best results were obtained from the PCSS made from polycrystalline material. The dark resistivity of the material was as high as 10 6 Ω cm. The operating breakdown field was 250 kV cm −1 , which is the highest reported for all lateral geometry PCSS and was limited by the surface flashover effect. The highest peak photocurrent density through the PCSS was greater than 10 kA cm −2 . The ratio of the off-state resistance on the on-state resistance, R off / R on , was ~ 10 5 , and the lowest on-state resistance was 45 Ω. The width of the photocurrent pulse was 15–30 ns, which was limited by the laser pulse width, indicating that the PCSS can operate in the megahertz range. The trigger gain of the polycrystalline 3C-SiC PCSS was 4.7 and the switching efficiency was 52%.

Optical waveguide formed by cubic silicon carbide on sapphire substrates

Optical confinement in beta silicon carbide (β‐SiC) thin films on sapphire substrate is demonstrated. Measurements are performed on waveguides formed by the mechanical transfer of thin β‐SiC films to sapphire. Recent results of epitaxial films of SiC on sapphire substrates attest to the technological viability of optoelectronic devices made from silicon carbide. Far‐field mode patterns are shown. We believe this is the first step in validating a silicon carbide optoelectronic technology.

Observation of Near Band Edge Transition in Aluminum Nitride Thin Film Grown by MOCVD

The Cathodoluminescence (CL) measurements of undoped and carbon doped aluminum nitride (AlN) thin films near the band-edge region were performed at 300, 77 and 4.2 K, respectively. These films were grown on three different substrates: 6H-SiC, 4H-SiC and sapphire. A dominant peak was observed in undoped samples around 5.9 eV. This peak can be further resolved into three distinct peaks at 6.05, 5.85, and 5.69 eV for AlN on sapphire. The temperature dependence of the peak positions and line widths were investigated. These peaks are believed to be due to exciton recombination. Also, the absorption spectra of carbon doped AlN on sapphire were analyzed to study the Urbach tail parameters which play an important role in near band-edge transitions.

Crystal structure of (SiC)x(AlN)1-x grown on 6H-SiC by MOCVD

Abstract (SiC) x (AlN) 1− x films were grown on (1000) 6H-SiC substrates by MOCVD. The crystal structures of alloys containing 20 and 80 mol% of AlN were observed using TEM. The electron diffraction pattern of the former sample shows 2H clusters mixed with cubic structure while the latter sample shows clearly separated columnary grains of 2H structure.

Anodic etching of p-type cubic silicon carbide

Abstractp-Type cubic silicon carbide was anodically etched using an electrolyte of HF:HCl:H2O. The etching depth was determined versus time with a fixed current density of 96.4 mA cm−2. It was found that the etching was very smooth and very uniform. An etch rate of 22.7 nm s−1 was obtained in a 1:1:50 HF:HCl:H2O electrolyte.

Growth Mechanism and Structure of Ain Films Grown on Sapphire by MOCVD

AIN films grown on sapphire by MOCVD with different V/IAR ratios were investigated by XRD, TEM and AFM. The AIN films show single crystalline character as well as columnar structure. The growth of AIN has three zones: (1) high-density nucleation layer (2) fine columnar growth and (3) grain merging and lateral growth. The films grown at intermediate V/III ratio have the maximum value of [0002] FWHM. When the V/HI ratio increases, the thickness of the nucleation zone and the film misorientation increase, but when the ratio is increased further, the nucleation zone decreases and the AIN film has a more highly oriented growth. These results suggest that the AIN films with optimum crystalline quality can be obtained by varying the V/III ratio during growth.

Ion-beam milling of cubic silicon carbide (3C-SiC)

Abstract Cubic silicon carbide (3C-SiC) was etched by utilizing an Ar+ ion-beam. The etching depth was determined versus angle, ion source current, anode voltage, and time. It was found that the maximum etching rate occurred when the ion-beam was incident at an angle of 45°. Constant etching rates were determined with respect to ion source current, anode voltage, and time.