Jeong-Sik Lee

A narrow photoluminescence linewidth of 21 meV at 1.35 μm from strain-reduced InAs quantum dots covered by In0.2Ga0.8As grown on GaAs substrates

InAs quantum dots with size fluctuations of less than 4% were grown on GaAs using the self-assembling method. By covering the quantum dots with In0.2Ga0.8As or In0.2Al0.8As, strain in InAs dots can be partly reduced due to relaxation of lattice constraint in the growth direction. This results in low-energy emission (about 1.3 μm) from the quantum dots. The photoluminescence linewidth can be reduced to 21 meV at room temperature. This width is completely comparable to the theoretical limit of a band-to-band emission from a quantum well at room temperature. Because the dots can be uniformly covered by the strain reducing layers, factors that degrade size uniformity during coverage, such as compositional mixing or segregation, will be suppressed, allowing for an almost ideal buried quantum dot structure.

Light Extraction Simulation of Surface-Textured Light-Emitting Diodes by Finite-Difference Time-Domain Method and Ray-Tracing Method

The light extraction efficiency (ηl) of surface-textured light-emitting diodes (LEDs) is calculated using a simulation based on the combination of the finite-difference time-domain (FDTD) method and the ray-tracing method. Considering the transmittance and light-scattering effects, which depend on the size of the surface-textured structure (STS), we investigated the dependence of ηl on the size of the STS including the period and aspect ratio. As a result, a clear dependence on the size of the STS has been shown. Furthermore, by optimizing the size of the STS, we found ηl to be about 4 times larger than that of the surface flat structure (SFS). In addition, we demonstrated that the absorption coefficient of the GaN layer (αGaN) affected ηl of the STS more strongly than that of the SFS. We suggest that both wide light-scattering and a high GaN crystalline quality are important points for improving ηl when texturing the top surface of LEDs.

In0.5Ga0.5As quantum dot intermixing and evaporation in GaAs capping layer growth

Our study of GaAs growth over self-assembled In0.5Ga0.5As quantum dots grown by metalorganic vapor-phase epitaxy showed that GaAs capping layer surface morphology at the onset strongly depended on temperature. Incompletely capped In0.5Ga0.5As islands were elongated toward [110], indicating anisotropy in intermixing. During higher-temperature growth interruption, islands show craters in quantum dot centers. Craters become hexagonal holes whose depth matches GaAs capping layer thickness. Postannealing photoluminescence spectra show no peak corresponding to overly large quantum dot radiation, indicating that growth interruption after capping layer formation at a certain thickness eliminates overly large quantum dots.

Imaging and single dot spectroscopy of InP self-assembled quantum dots

Abstract Optical properties of InP self-assembled quantum dots (SADs) embedded in Ga0.5In0.5P were studied under various conditions by means of the micro-spectroscopy. A strong optical anisotropy of the InP SADs was found in the macro-photoluminescence spectra and micro-photoluminescence (μ-PL) images showing two-fold symmetry, which reflects the anisotropic structure of the Ga0.5In0.5P matrix. Successive red-shifts of the μ-PL peaks and a recovery of the PL intensity with the increase of the temperature were clearly observed. The fluorescence intermittency from a single quantum dot was also investigated. The on–off switching rate is drastically enhanced by a weak near-IR laser beam irradiation. The influence of the matrix on the optical properties of the SADs is discussed.

Spontaneous one-dimensional lateral alignment of multistacked InGaAs quantum dots on GaAs (n 1 1)B substrates

In0.45Ga0.55As/GaAs multistacking quantum dot (QD) structures were fabricated on a GaAs (n11)B (n=2–4) substrate by metalorganic vapor-phase epitaxy. QDs spontaneously aligned in the [0 1 1] direction were observed on stacked QDs, whereas QDs were randomly distributed in the initial In0.45Ga0.55As layer growth. The formation mechanism of this self-alignment was studied by changing the number of In0.45Ga0.55As/GaAs multilayers and crystallographic arrangement. Photoluminescence spectra showing clear polarization dependence indicate carrier coupling in the QD arrays. This growth technique results in spontaneously aligned InGaAs QDs without any preprocessing technique prior to growth.

In situ ellipsometric study of the formation process of metalorganic vapor-phase epitaxy-grown quantum dots

Our in situ ellipsometry study of metalorganic vapor-phase epitaxy-grown quantum dot (QD) structures showed that the Δ–Ψ trajectory of the ellipsometric signal in Stranski–Krastanow QD growth sharply differs from that in layer-by-layer growth. When QD formation starts, Δ rapidly decreases, inflecting the Δ–Ψ trajectory. This indicates increased scattering loss, deduced from increased surface roughness produced in the transition from two- to three-dimensional surface morphology. Ex situ atomic force microscopy and photoluminescence results correspond well to the ellipsometric signal at the start of QD formation. Based on these results, we discuss growth-dependent QD formation processes such as QD formation onset and growth mode transition.

Multiatomic step formation with excellent uniformity on vicinal (1 1 1)A GaAs surfaces by metalorganic vapor-phase epitaxy

Abstract Giant steps with high uniformity and continuity have been successfully formed by metalorganic vapor-phase epitaxy on GaAs (1 1 1)A vicinal surfaces for the application of quantum wire fabrication. The step height is more than 30 monoatomic layer at a growth temperature of 660°C. The surface morphologies obtained from scanning electron microscopy and atomic force microscopy measurements show the straight steps on (1 1 1)A substrates misoriented toward [2 1 1] direction with high uniformity. Terrace uniformity and step continuity are improved as the off-angle increases. A straight section of a step exceeding 10 μm was obtained on 4° off-substrate. Fluctuation has also been suppressed within 1 monoatomic layer height on this vicinal substrate.

Surface structure control of GaAs (111)A vicinal substrates by metal-organic vapor-phase epitaxy

Periodic structures with high uniformity have been fabricated by metal-organic vapor-phase epitaxy on GaAs (1 1 1)A vicinal surfaces. Scanning electron microscope and atomic force microscope surface images show strict dependence on substrate off-direction, growth temperature (Tg), and V/III ratio. Giant steps with long-range continuity and high uniformity have been formed on GaAs (1 1 1)A substrates misoriented towards the [112¯] direction. In addition, the heights of the giant steps show the saturation at high Tg and the value is independent of substrate off-angle. On the other hand, homogeneous zigzag steps have emerged on the substrate off toward the [0 0 1] azimuth after the growth at high Tg, whereas relatively rough steps were formed on both substrates at low Tg. The results indicate the growth-mode transition from step-flow to facet formation as growth temperature gets higher.

Atomic layer epitaxy of GaAs and GaAsxP1 − x on nominally oriented GaAs(111) substrates with high quality surface and interfaces

Abstract Layer-by-layer growth of GaAs and GaAs x P 1 − x ( x = 0.7−0.8) on nominally oriented (111) substrates has been performed by atomic layer epitaxy (ALE). The ideal growth rate of one monolayer per cycle was obtained on a (111)B substrate, but not on a (111)A substrate. Mirror-like smooth surface morphologies can be obtained on both (111)A and (111)B substrates in a wide range of growth conditions. The X-ray rocking curve of the GaAs x P 1 − x GaAs multilayer sample grown on the (111)B substrate showing clear satellite patterns indicates abrupt interfaces that are superior to those of the (100) sample. It was also found that the disorder in growth such as the generation of hillocks can be reduced drastically by the ALE method.

Self limiting growth on nominally oriented (111)A GaAs substrates in atomic layer epitaxy

Abstract Layer-by-layer growth on nominally oriented (111)A GaAs substrate has been performed by atomic layer epitaxy (ALE). Under enough AsH3 feeding, the growth rate saturation of one-fourth monolayer per cycle was observed at the Tg range between 560°C and 600°C and three-eighths of monolayer per cycle at the Tg range below 550°C on (111)A substrate. The drastic change of the growth rate saturation at around 550°C indicates some kinds of surface reconstructions or site occupation on (111)A surface during AsH3 supply.