Kee Young Lim

Determination of the band-gap energy of Al1−xInxN grown by metal–organic chemical-vapor deposition

Ternary AlInN was grown by metal–organic chemical-vapor deposition in the high Al composition regime. The band-gap energy of AlInN ternary was measured by optical absorption spectroscopy at room temperature. The band-gap energy of Al0.92In0.08N is 5.26 eV. The potential application of AlInN as a barrier material for GaN is also discussed.

Reduction of dislocations in GaN epilayers grown on Si(111) substrate using SixNy inserting layer

High-quality GaN films have been grown on Si(111) substrate by metalorganic chemical vapor deposition using a SixNy inserting layer. Due to the large difference of lattice constant and thermal expansion coefficient between GaN and Si,GaN growth on Si(111) substrate usually leads to an initially high dislocation density and cracks. It is found that the SixNy inserting layer plays a very important role in the enhancement of crystal quality and surface morphology of GaN films. The crystalline quality of overlying GaN layer grown on SixNy inserting layer depends on the deposition time of SixNy inserting layer. The high-resolution x-ray diffraction results show that the dislocation density in GaN epilayer decreases with increasing SixNy growth time. It was confirmed that the misfit dislocations in the GaN films with 5min deposition time for SixNy inserting layer almost stop at the SixNy inserting layer by transmission electron microscope measurements.

Al concentration control of epitaxial AlGaN alloys and interface control of GaN/AlGaN quantum well structures

AlxGa1−xN alloys were grown by metalorganic chemical vapor deposition on c-plane sapphire substrates under various growth conditions. Both the Al concentration and the growth rate of AlGaN are strongly affected by gas-phase parasitic reaction between ammonia and group-III sources. As the ammonia flow rate increases with fixed flow rate of group-III sources, both the Al concentration in the solid and the growth rate of AlGaN are decreased. It was also found that the Al concentration in the solid saturated as increasing the Al gas composition increased by increasing the flow rate of the Al source precursor. In addition, the influence of growth interruption when group-III sources are temporarily shut off on the optical quality of GaN/AlGaN single quantum well (QW) structures was studied to realize high-quality QWs. Photoluminescence measurements revealed that the emission peak is blueshifted as the interruption time increases and the emission intensity is maximized at a several-second short interruption time...

Effects of growth rate of a GaN buffer layer on the properties of GaN on a sapphire substrate

We studied the effects of the growth rate of a GaN buffer layer grown on a GaN epilayer. It was found that this growth rate plays a key role in improving the quality of the GaN film on a sapphire substrate and an optimum growth rate exists that yields the best crystal quality. A GaN film grown on a buffer layer with the optimum growth rate of 18.3 nm/min has an electron Hall mobility of 539 cm2/V s and a dislocation density of approximately 2×108 cm−2. These improvements of GaN film qualities are illustrated by the promotion of the lateral growth mode.

Effects of Initial Thermal Cleaning Treatment of a Sapphire Substrate Surface on the GaN Epilayer

We investigated the effects of the in situ thermal cleaning treatment of a (0001) sapphire substrate surface in hydrogen ambient on the structural, optical, and electrical properties of epitaxial GaN films grown by metalorganic chemical vapor deposition (MOCVD). Hall effect, X-ray diffraction (XRD), and photoluminescence measurements of GaN films grown at 1040°C clearly indicate that the film quality is strongly affected by the thermal cleaning treatment of the substrate surface. GaN films under the optimized thermal cleaning treatment at 1070°C for 10 min showed minimum full-widths at half maximum (FWHMs) of 273 arcsec and 728 arcsec for (002) and (102) XRD peaks, respectively. In addition, the FWHM of the band-edge emission peak was as narrow as 28.3 meV, and the intensity ratio between the band edge emission and the yellow band emission was as high as 100 at room temperature. It was also found that the roughness of sapphire surface was reduced after the thermal treatment.

Growth and properties of Al-rich InxAl1−xN ternary alloy grown on GaN template by metalorganic chemical vapour deposition

An Al-rich InxAl1−xN ternary alloy was grown on a GaN template by metal–organic chemical vapour deposition (MOCVD). The GaN template was fabricated on a c-plane sapphire with a low temperature GaN nucleation layer. The growth of the 300 nm thick InxAl1−xN layer was carried out under various growth temperatures and pressures. The surface morphology and the InN molar fraction of the InxAl1−xN layer were assessed by using atomic force microscopy (AFM) and high resolution x-ray diffraction, respectively. The AFM surface images of the InxAl1−xN ternary alloy exhibited quantum dot-like grains caused by the 3D island growth mode. The grains, however, disappeared rapidly by increasing diffusion length and mobility of the Al adatoms with increasing growth temperature and the full width at half maximum value of ternary peaks in HR-XRD decreased with decreasing growth pressure. The MOCVD growth condition with the increased growth temperature and decreased growth pressure would be effective to grow the InxAl1−xN ternary alloy with a smooth surface and improved quality. The optical band edge of InxAl1−xN ternary alloys was estimated by optical absorbance and, based on the results of HR-XRD and optical absorbance measurements, we obtained the bowing parameter of the InxAl1−xN ternary alloy at b = 5.3 eV, which was slightly larger than that of previous reports.

Mechanistic Study and Characterization of 3C-SiC(100) Grown on Si(100)

Single crystal cubic SiC(100) thin films were grown on Si(100) from thermal cracking of tetramethylsilane (TMS) as a function of growth temperature reactor pressure, source gas flow rate, and Si/C atomic ratio in a radio frequency-inductive chemical vapor deposition reactor. The orientation of as-grown SiC films mostly followed that of the Si substrate. However, the growth of SiC( 111) was detected in the films grown at high TMS flow rates and high Si/C atomic ratios. The thickness of the SiC film was increased with the growth temperature, reactor pressure, TMS flow rate, and Si/C atomic ratio. Voids were formed in the silicon side of the SiC/Si interface, hut disappeared when the reactor pressure TMS flow rate, and Si/C ratio were increased. The formation of voids was attributed to the outdiffusion of Si atoms from the Si substrate. The stresses and strains generated in the grown films were also investigated in terms of the growth conditions. The formation mechanism of voids at the SiC/Si interface was discussed on the basis of the experimental observations.

Sealing of AlAs against wet oxidation and its use in the fabrication of vertical-cavity surface-emitting lasers

We have studied the process of sealing an exposed AlAs to prevent further wet oxidation. A critical step in this sealing process consists of the first wet oxidation for a short time at 408 °C in a steam environment of a previously room-ambient exposed AlAs surface. During this brief wet oxidation, a dense oxide surface barrier with a thickness of 1.1 μm is formed, which further blocks diffusing oxygen species during the second wet oxidation. The effectiveness of the sealing is demonstrated through its use as a mask against wet oxidation in the fabrication of oxide-confined vertical-cavity surface-emitting lasers.

Thermally induced capacitance and electric field domains in GaAsAl0.3Ga0.7As quantum well infrared photodetector

The capacitance (C) of a GaAsAl0.3Ga0.7As quantum well infrared photodetector (QWIP) was measured as a function of temperature and bias voltage. Thermally generated electrons in the excited sub-band lead to the high capacitance because of the reduction of effective dielectric layer thickness as a result of the wave function extension to the barrier layer. Peaks and valleys in the capacitance-voltage (C-V) curves have been attributed to the on- and off-resonant states in QWIP. The formation and evolution of bias induced electric field domains (EFDs) were analyzed from the C-V line shapes, which depend on electron distributions in sub-bands, the resonant coupling between sub-bands in the neighboring wells, and the transport stability in QWIP. Fine structures in the capacitance-temperature (C-T) curves show that the formation of EFDs is thermally induced. We show the onset temperature of the capacitance can be used to determine the maximum operating temperature of a QWIP.

Delta-Doping of Si in GaN by Metalorganic Chemical Vapor Deposition

Si delta-doping in the GaN layer has been successfully demonstrated by metalorganic chemical vapor deposition. From the capacitance-voltage measurement, a sharp carrier concentration profile with a full width at half-maximum of 4.1 nm has been achieved with a peak concentration of 9.8×1018 cm-3. Si delta-doping concentration increases and then decreases with an increase in delta-doping time, and the use of a post-purge step in the ammonia ambient reduces Si delta-doping concentration. This indicates that delta-doping concentration is limited by the Si desorption process due to high growth temperatures.