Youngboo Moon

Effects of As/P exchange reaction on the formation of InAs/InP quantum dots

InAs self-assembled quantum dots (SAQDs) were grown on InP at various temperatures and V/III ratios by metalorganic chemical vapor deposition. The density, size distribution, and shape of the InAs SAQDs changed significantly with temperature and V/III ratio. Careful analysis of the total volume of the dots grown at various conditions showed that the volume far exceeded the amount of deposition supplied from the gas-phase sources. The amount of excess InAs and the aspect ratio (height/lateral size) of the SAQD increased with temperature and V/III ratio, strongly suggesting that the As/P exchange reaction at the surface played an important role in the kinetics of SAQD formation. Insertion of a lattice-matched InGaAs buffer layer suppressed the excess InAs formation, and lowered the aspect ratio, confirming the effect of the As/P exchange reaction.

Experimental determination of current spill-over and its effect on the efficiency droop in InGaN/GaN blue-light-emitting-diodes

We report the experimental determination of current spill-over in InGaN/GaN blue light emitting diodes by measuring the change in the forward current generated by a resonant excitation. To quantify accurately, the absorption of the laser as a function of the forward current was also determined. Two samples that have clearly different behavior of efficiency droop were compared to clarify the relationship between the current spill-over and the efficiency droop. We conclude that the carrier spill-over does occur and can be a significant cause for the efficiency droop but cannot single-handedly account for the efficiency droop.

Optimization of InGaN/GaN multiple quantum well layers by a two-step varied-barrier-growth temperature method

We report the effect of different temperature profiles on the quality of the InGaN/GaN multiple quantum well (MQW) structures by employing photoluminescence (PL) and atomic force microscopy (AFM). We adopted a two-step varied-barrier-growth temperature method to improve the structural and optical properties of the InGaN/GaN MQW layers. The low-temperature GaN barrier layer was introduced to reduce the desorption rate of the indium atoms of the InGaN well, and then the high-temperature GaN barrier was grown to reduce the defects of InGaN/GaN MQWs. When the width of the low-temperature GaN barrier was 50 A and the high-temperature GaN barrier was grown at 1000 °C, the defect and surface roughness were significantly reduced, especially with a reduction in the depth of V-defect as low as 20 A.

Well-to-well non-uniformity in InGaN/GaN multiple quantum wells characterized by capacitance-voltage measurement with additional laser illumination

We experimentally investigated well-to-well non-uniformity in InGaN/GaN multiple quantum well (MQW) structures by using capacitance-voltage measurements with additional laser illumination. By varying the illuminating power of the resonant excitation, well-to-well non-uniformity through the MQWs was clearly revealed. The quantum wells (QWs) close to the n-GaN side show higher carrier accumulations and larger position shift as the excitation power is increased, relative to the p-side QWs. Both results were attributed to the existence of stronger piezoelectric fields in the n-side QWs induced by subsequent partial relaxation of strain through the MQWs.

Role of photovoltaic effects on characterizing emission properties of InGaN/GaN light emitting diodes

Strong photovoltaic effects on photoluminescence (PL) spectra in InGaN/GaN blue light emitting diodes were investigated. Due to severe carrier escape from quantum wells, significant photovoltaic effects occur in PL measurement in open-circuit condition, which strongly affect the PL peak position and intensity. We reveal that proper correlation between electroluminescence and PL peak positions cannot be obtained without proper consideration of the photovoltaic effects. By changing sample temperature and the PL excitation power, the generated photovoltage varies in the range of 2.0 to 2.6 V. We show that in the open-circuit condition, which is the usual case, the determination of radiative efficiency by measuring the PL intensity ratio of low-and high-temperature cannot be accurate, and the excitation intensity dependent PL cannot be solely intrinsic either. Both the absorption of incident laser and the carrier escape from the quantum well are bias-sensitive. By a simple and straightforward method, we deter...

A Nonpolar a-Plane GaN Grown on a Hemispherical Patterned r-Plane Sapphire Substrate

A high-quality a-plane GaN layer with a pit-free, mirror-like surface was grown on a hemispherical patterned r-plane sapphire substrate (PSS) by metal–organic chemical vapor deposition. The use of the r-plane PSS reduced the density of defects such as basal plane staking faults and threading dislocations of the a-plane GaN. The low-temperature-photoluminescence intensity of the emissions at 3.25 and 3.39 eV related with the defects increased along with the near-band-edge emission at 3.46 eV. The intensity of yellow emission at 2.2 eV on the PSS was remarkably decreased, which indicates the improvement of the crystal quality because of the defect reduction. The InGaN light emitting diode grown on the PSS showed an output power of 7.4 mW at 100 mA, which was about 7 times higher than that on the planar substrate.

Structural characterization of amorphized InP: Evidence for chemical disorder

Extended x-ray absorption fine-structure measurements at the In K edge of amorphous InP are presented. The presence of chemical disorder in the form of like-atom bonding has been unambiguously demonstrated in stoichiometric InP amorphized by ion implantation. In–In bonding comprised 14%±4% of the In–atom constituent bonds. Also, relative to the crystalline value of four P atoms, an increase in the total In coordination number to 4.16±0.32 atoms was observed for the amorphous phase, as composed of 3.56±0.19 P and 0.60±0.13 In atoms. Experimental results were consistent with recent ab initio structural calculations and, furthermore, demonstrated that amorphous InP is best described by a Polk-like continuous random network, containing both even- and odd-membered rings.

Shape change of InAs self-assembled quantum dots induced by As/P exchange reaction

Abstract Self-assembled InAs quantum dots (SAQDs) were grown on InP and on lattice-matched InGaAs buffer layers by metalorganic chemical vapor deposition (MOCVD). The aspect ratio of the SAQD grown on InP increased with temperature and V/III ratio, but it reduced substantially when grown on InGaAs buffer layers. Moreover, the dots formed on InGaAs were faceted, whereas those on InP were dome-shaped, strongly suggesting that the As/P exchange reaction at the surface played an important role in the kinetics of SAQD formation. The shape of InAs SAQDs on InGaAs buffer layers was a truncated pyramid with four {136} facets and their base edges were parallel to 〈130〉 directions.

Analysis of P adsorption and desorption on the (001) InP surface using surface photoabsorption

We present an in situ study of P desorption and adsorption on the (001) InP surface using surface photoabsorption (SPA). The SPA spectra show three peaks at 2.1, 2.9, and 3.1 eV. Here, based on previous studies, we have identified them as a P dimer related and two surface In dimer related peaks, respectively. The initial sharp drop and the slow recovery of the SPA signal at 430 nm after PH3 is switched on implies that at least two steps are involved in the P desorption process. We also observed a structure at 3.1 eV in the spectrum of the In-stabilized surface. We have determined from temperature dependent desorption measurements that the activation energy for P desorption from this surface is 3.36 eV.

Raman and emission characteristics of a-plane InGaN/GaN blue-green light emitting diodes on r-sapphire substrates

Raman and emission properties of a nonpolar a-plane InGaN/GaN blue-green light emitting diode (LED) on an r-sapphire substrate are investigated and compared with a conventional c-plane blue-green LED. The output power of the a-plane LED was 1.4 mW at 20 mA. The c-plane LED has higher EQE, but it reaches the maximum at a lower forward current and the droop is faster than the a-plane counterpart. As the reverse bias increased, a blueshift in the PL spectra was not observed in the a-plane structure, which is indicative of an absence of quantum confined Stark effects. However, a strong blueshift in the electroluminescence spectra was still present, which means the In localization effects are relevant in nonpolar InGaN/GaN quantum wells. In the Raman spectra, a strong anisotropy of E2(high) phonon modes was observed. By comparing the frequency of the E2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart.