Gye Mo Yang

Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions

InGaN/GaN multiple-quantum-well light-emitting-diode structures utilizing tunnel contact junctions grown by metalorganic chemical vapor deposition have been demonstrated. The p+/n+ GaN tunnel junctions are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. Thus, metal ohmic contacts are done at the same time on the top and the lower contact layers. The reverse-biased tunnel contact junction provides lateral current spreading without semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. The tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a small penalty in voltage drop compared to conventional devices.

Formation mechanism of V defects in the InGaN/GaN multiple quantum wells grown on GaN layers with low threading dislocation density

V-defect formation of the InxGa1−xN/GaN multiple quantum wells (MQWs) grown on GaN layers with different threading dislocation (TD) densities was investigated. From cross-sectional transmission electron microscopy, we found that all V defects are not always connected with TDs at their bottom. By increasing the indium composition in the InxGa1−xN well layer or decreasing the TD density of the thick GaN layer, many V defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation. Also, TD density in the thick GaN layer affects not only the origin of V-defect formation but also the critical indium composition of the InxGa1−xN well on the formation of V defects.

Selective etching of AlGaAs/GaAs structures using the solutions of citric acid/H2O2 and de-ionized H2O/buffered oxide etch

Etching results using the solution system of citric acid/H2O2 and de-ionized H2O/buffered oxide etch are shown to provide good selective wet etching of AlGaAs/GaAs structures. For AlxGa1−xAs (x 0.7) quickly decreases with decreasing Al composition in de-ionized H2O/buffered oxide etch solution, providing a high degree of etching selectivity. These simple selective etching processes have been applied to define AlAs/GaAs distributed Bragg reflector mesas in a vertical-cavity surface-emitting laser structure.

Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum wells (MQWs) grown with various growth interruptions between the InxGa1−xN well and GaN barrier by metalorganic chemical vapor deposition were investigated using photoluminescence (PL), high-resolution transmission electron microscopy, and energy filtered transmission electron microscopy (EFTEM). The integrated PL intensity of the MQWs with growth interruptions is abruptly reduced compared to that of the MQW without growth interruption. Also, as the interruption time increases the peak emission shows a continuous blueshift. Evidence of indium clustering is directly observed both by using an indium ratio map of the MQWs and from indium composition measurements along an InGaN well using EFTEM. The higher-intensity and lower-energy emission of light from the MQW grown without interruption showing indium clustering is believed to be caused by the recombination of excitons localized in indium clustering regions and the increased indium composition in these recombination centers.

Microstructural characterization of InGaN/GaN multiple quantum wells with high indium composition

Abstract The microstructural study of InGaN/GaN multiple quantum well (MQW) structures with high In (indium) composition (>30%) has been performed using transmission electron microscopy (TEM). The increased strain in InGaN/GaN MQWs by high In composition is relaxed by the formation of several defects such as dislocations, stacking faults, V-defects, and tetragonal shape defects. High-resolution TEM (HRTEM) measurement shows a new formation mechanism of V-defects, which is related to the stacking mismatch boundary induced by stacking faults. These V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice. In addition, evidence of In clustering is directly observed both by using an In ratio map of the MQWs and from In composition measurements along an InGaN well using energy filtered TEM (EFTEM).

Effect of buffer layers and stacking faults on the reduction of threading dislocation density in GaN overlayers grown by metalorganic chemical vapor deposition

We have studied the effect of the trimethylgallium (TMGa) flow rate in the GaN buffer layer on the optical and structural quality. From low temperature photoluminescence measurements, a GaN overlayer grown on a buffer layer with the TMGa flow rate of 80 μmol/min shows the intense donor-acceptor pair transition peak at 3.27 eV and the weak yellow band emission at 2.2 eV, which are related with stacking faults and threading dislocations from transmission electron microscopy images, respectively. As the TMGa flow rate of the GaN buffer increases, the threading dislocation density rapidly decreased and stacking faults increased in the GaN overlayers. Also, a total threading dislocation density at the optimum condition of the buffer layer is the very low 1×108 cm−2, which is due to the interaction of stacking faults with the vertical threading dislocations and the bending of threading dislocations near the stacking faults. High-resolution x-ray diffraction results show that a high density of stacking faults is...

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.

Influence of strain relaxation on structural and optical characteristics of InGaN/GaN multiple quantum wells with high indium composition

Influence of strain relaxation on structural and optical properties of the InGaN/GaN multiple quantum wells (MQWs) with high indium composition grown by metalorganic chemical vapor deposition was investigated. From photoluminescence and transmission electron microscopy (TEM), we found that within the MQWs, the formation of misfit dislocation affects the degradation of optical properties more than the formation of stacking faults. For the MQWs with indium composition above the critical indium composition on the formation of misfit dislocation, the position of the main emission peak is significantly affected by the increase of quantum well numbers compared to samples with indium composition below the critical indium composition. The origins of redshift by the increase of quantum well numbers is believed to be caused by the increase of indium segregation in the MQWs using high-resolution TEM and energy dispersive x-ray spectroscopy.

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...

GaN-Based Light-Emitting Diodes on Micro-Lens Patterned Sapphire Substrate

GaN-based light-emitting diodes (LEDs) were fabricated on a micro-lens patterned sapphire substrate (ML-PSS). ML patterning on the sapphire substrate was carried out by using photolithography with photo-resist reflow technique and dry etching process using chlorine based inductively coupled plasma. The ML-PSS was prepared using a periodic ML pattern with diameters of 3 µm and spacing of 2, 4, and 5 µm, respectively, on the c-plane sapphire substrate. The leakage current of the LEDs fabricated on the ML-PSS greatly decreased compared to that of a conventional LED and it decreases with increasing ML-pattern spacing; it decreases from 1.8 to 0.2 µA at reverse voltage of 15 V as the ML-pattern spacing is increased from 2 to 5 µm. The output power of the LED with 5 µm spacing was about 155% higher than that of a conventional LED and about 10% higher than that of the LED on the PSS with spacing of 2 µm. This improvement of the output power is contributed not only by reduction of dislocation density depending on spacing of patterning but also by the enhancement of light extraction efficiency with outcoupling via the ML patterned facets on sapphire substrate.