K. Y. Lim

Conductive layer near the GaN/sapphire interface and its effect on electron transport in unintentionally doped n-type GaN epilayers

Temperature-dependent Hall effect measurements on unintentionally doped n-type GaN epilayers show that, above room temperature, the Hall-mobility values of different samples vary parallel with each other with temperature. We demonstrate that this anomaly is mainly due to a conductive layer near the GaN/sapphire interface for thin samples with low carrier density. Through trapping electrons, threading edge dislocations (TEDs) debilitate the epilayer contribution in a two-layer mixed conduction model involving the epilayer and the near-interface layer. The trapping may, in part, explain low mobility and anomalous transport in pure GaN layers. Scattering by TEDs is important only at low temperatures.

Bias-assisted photoelectrochemical oxidation of n-GaN in H2O

Growth of gallium oxide on n-GaN was realized in H2O by bias-assisted photoelectrochemical (PEC) oxidation using Al as a counterelectrode instead of a Pt commonly used in the PEC process. Although the growth of the oxide was not observed at below 2 V, the initial oxide growth rate of 8.7 nm/min was shown at a bias of 15 V and ultraviolet light intensity of 300 mW/cm2. However, the growth rate lowered and oxide thickness was saturated to 340 nm. The saturated oxide thickness and initial growth rate were increased with the applied bias. The homogeneous oxide growth and near stoichiometric composition of Ga2O3 were observed in Auger electron spectroscopy analysis results.

AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor with oxidized Ni as a gate insulator

We fabricated the AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) using the oxidized Ni(NiO) as a gate oxide and compared electrical properties of this device with those of a conventional AlGaN∕GaN heterostructure field-effect transistor (HFET). NiO was prepared by oxidation of Ni metal of 100A at 600°C for 5min in air ambient. For HFET and MOSHFET with a gate length of 1.2μm, the maximum drain currents were about 800mA∕mm and the maximum transconductances were 136 and 105mS∕mm, respectively. As the oxidation temperature of Ni increased from 300 to 600°C the gate leakage current decreased dramatically due to the formation of insulating NiO. The gate leakage current for the MOSHFET with the oxidized NiO at 600°C was about four orders of magnitude smaller than that of the HFET. Based on the dc characteristics, NiO as a gate oxide is comparable with other gate oxides.

Metal-organic chemical vapor deposition growth of GaN thin film on 3C-SiC/Si(111) substrate using various buffer layers

Abstract GaN films were grown on 3C-SiC/Si(111) substrate using GaN, AlN, and GaN/AlN superlattices as the buffer layer in a metal–organic chemical vapor deposition (MOCVD) system. The surface morphology and structural and optical qualities of the GaN films were greatly improved by employing the buffer layers in the growth of GaN film on the 3C-SiC/Si substrate. The use of the superlattice buffer layer produced well-oriented GaN growth along the c-axis with good surface morphology. Low-temperature photoluminescence (PL) measurements showed that peaks associated with band-edge emission and donor–accepter pair recombination (D0A0) were observed from GaN films grown with and without GaN or AlN buffer layers, whereas GaN films grown with the superlattice buffer layer exhibited a strong band-edge peak with very weak D0A0 emission. The surface morphology and structural and optical properties of the GaN films were well correlated for the evaluation of GaN crystal quality.

Thermal distributions of surface states causing the current collapse in unpassivated AlGaN∕GaN heterostructure field-effect transistors

The dc characteristics of the AlGaN∕GaN heterostructure field-effect transistors were examined at temperatures ranging from 25 to 260 °C under white light illumination. Drain current collapse measured was defined by the difference of drain current between light on and light off at Vgs=1V and Vds=5V. The surface-passivated device showed no drain current collapse, but the unpassivated device showed severe drain current collapse at 25 °C. Drain current and drain current collapse with an increase in temperature reduced, which resulted from the reduction of the electron mobility or saturation velocity and the thermal activation of the trapped electrons, respectively. Eventually, drain current collapse disappeared completely above 250 °C. The behavior of the temperature-dependent drain current collapse showed that the surface states for trapping electrons were continuously distributed with the temperature not having specific energy states.

The effects of nitridation on properties of GaN grown on sapphire substrate by metal-organic chemical vapour deposition

The material qualities of GaN overlayers grown on (0001) sapphire substrates by metal-organic chemical vapour deposition were investigated as a function of nitridation time. The nitridation of sapphire surface notably affects on the properties of the GaN overlayers, such as surface morphology and structural, optical and electrical properties. The GaN overlayer with a short nitridation time of 30 s shows a good surface morphology, a relatively low background Hall carrier concentration and a high electron mobility, but the defect-related emission around 550 nm increased. Furthermore, for the GaN overlayer with nitridation for 180 s, even though it has a low (102) full width at half-maximum of the high-resolution x-ray diffraction rocking curve, other material properties were degraded owing to the long nitridation time. It was found that material properties have a trade-off relationship with the variation of nitridation time.

Thermal treatment effect of the GaN buffer layer on the photoluminescence characteristics of the GaN epilayer

Photoluminescence properties of undoped wurtzite GaN epilayers grown on sapphire substrates with different buffer layer treatment conditions in metalorganic chemical vapor deposition (MOCVD) growth have been studied as a function of temperature. At low temperatures, very well resolved spectral features associated with the GaN band structure were observed. From the photoluminescence (PL) data for free excitons, an accurate value of the A exciton binding energy was found. The localization energies of the excitons bound to neutral acceptor are found to agree with Haynes’ rule with the proportionality factor close to 0.1. The longitudinal optical (LO) phonon assisted photoluminescence associated with both the bound and free excitons has been observed. The characteristics of free excitons and their LO phonon replica have been studied in detail with the temperature variation and related to the point defects. The behavior of the peak energy and the full width at half maximum of the exciton band as a function of ...

Characteristic properties of Raman scattering and photoluminescence on ZnO crystals doped through phosphorous-ion implantation

P-doped ZnO was fabricated by means of the ion-implantation method. At the Raman measurement, the blue shift of the E2high mode and A1(LO) phonon of the inactive mode were observed after the P-ion implantation. It suggested to be caused by the compressive stress. Thus, Hall effect measurement indicates that the acceptor levels exists in P-doped ZnO while still maintaining n-type ZnO. From the X-ray photoelectron spectroscopy, the chemical bond formation of the P2p3/2 spectrum consisted of 2(P2O5) molecules. Therefore, the implanted P ions were substituted to the Zn site in ZnO. From the photoluminescence (PL) spectra, P-related PL peaks were observed in the energy ranges of 3.1 and 3.5 eV, and its origin was analyzed at PZn-2VZn complexes, acting as a shallow acceptor. With increasing temperatures, the neutral-acceptor bound-exciton emission, (A0, X), shows a tendency to quench the intensity and extend the emission linewidth. From the relations of the intensity and the linewidth as a function of temperatu...

Growth and characterization of GaN epilayers on chemically etched surface of 3C-SiC intermediate layer grown on Si(111) substrate

High-quality GaN films were grown on Si(111) substrate using 3C-SiC intermediate layer by metal-organic chemical vapor deposition. The 3C-SiC intermediate layer was grown on the Si(111) substrate by chemical vapor deposition using tetramethylsilane as a single source precursor. We have investigated the effect of chemical etching of SiC intermediate layer surface under different conditions. SiC layer was etched using potassium carbonate (K 2 CO 3 ), hydrochloric acid (HCl), and hydrofluoric acid (HF), respectively. The surface roughness of 3C-SiC intermediate layer decreased as the chemical etching time was increased. GaN films with local atomically flat surfaces were obtained and the X-ray diffraction full-width at half maximum of (0002) peak was 664.5 arcsec for a 1.56 μm thick film. The reduced SiC surface roughness decreased the defect density in GaN epilayers. In the photoluminescence spectra at room temperature, the yellow band emission peak at around 2.2 eV disappeared.

Co-doping characteristics of Si and Zn with Mg in p-type GaN

The authors investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn co-doped Gan films grown by metalorganic chemical vapor deposition. They have grown p-GaN film with a resistivity of 1.26 {center_dot} cm and a hole density of 4.3 x 10{sup 17} cm{sup {minus}3} by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 {center_dot} cm) and a high hole concentration (8.5 x 10{sup 17} cm{sup {minus}3}) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 x 10{sup {minus}4} {center_dot} cm{sup 2}, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 x 10{sup {minus}3} {center_dot} cm{sup 2}).