Doo-Cheol Park

Self-organized CdSe quantum dots onto cleaved GaAs (110) originating from Stranski–Krastanow growth mode

Self-organized CdSe/ZnSe quantum dots (QDs) have been fabricated on GaAs (110) crystal surfaces, which were obtained by cleaving GaAs (100) wafers in ultrahigh vacuum. CdSe showed a conventional Stranski–Krastanow growth mode on the ZnSe (110) lower cladding layer, whose surfaces are atomically flat. The wetting layers, which are compose of quantum wells with well widths of 1, 2, and 3 monomolecular layers, showed sharp photoluminescence (PL). The fabricated CdSe QDs showed intense green PL spectra, whose peak is located at 2.192 eV, with a linewidth of 0.24 eV. The state filling effect in CdSe QDs was also observed by employing excitation power dependence of the PL intensity.

Fabrication and optical properties of ZnCdSe/ZnSe single quantum wells on GaAs(110) surfaces cleaved in UHV by molecular beam epitaxy

High-quality single quantum wells (SQWs) have been fabricated on GaAs(110) crystal surfaces which were obtained by cleaving GaAs(100) wafers in ultra high vacuum (UHV). The growth interruption was found to prevent the growth of high-quality quantum well structures. The (110)-oriented SQWs grown under appropriate conditions show a very sharp photoluminescence spectrum whose full width at half-maximum (FWHM) is as small as 4.9 meV. The sample showed a strong in-plane polarization angle dependence of the emission intensity, due to the compressive strain in the well layer. The ratio of maximum to minimum values of the polarized PL intensity was 1.6.

LOCALIZED EXCITONIC EMISSIONS OF ZNCDSE/ZNSE QUANTUM WELLS GROWN ON A GAAS(110) CLEAVED SURFACE

Anomalous optical properties of Zn1−xCdxSe/ZnSe strained-layer single quantum wells (SQWs) fabricated on cleaved GaAs(110) surfaces in ultrahigh vacuum by molecular beam epitaxy have been investigated. From the temperature-dependent photoluminescence measurement, the origins of localized excitonic emissions of two kinds of SQW structures with different Cd compositions are suggested. Localization centers of SQWs with low Cd composition (14%) are mainly originated by the well thickness fluctuation. However, SQWs with high Cd composition (27%) showed localized excitonic emissions at low temperature due to the Cd composition fluctuations rather than the lack of thickness uniformity of the well layer. Estimated averaged depths of localization of excitons by model calculations are 9 and 14 meV for the wells with 14% and 27% Cd compositions, respectively.

Optimization of ZnSe growth on the cleavage-induced GaAs (1 1 0) surface by molecular-beam epitaxy

Optimum conditions were investigated for the growth of high-quality ZnSe epitaxial layers on GaAs (1 1 0) cleaved surfaces in molecular-beam epitaxy (MBE) through the surface morphology, structural and optical characterizations. Most of the layers showed poor characteristics with bad surface morphology. However, the layer, which was grown at a substrate temperature of 250°C and with a beam-equivalent pressure ratio, i.e. p(Se)p(Zn) of 1.0, showed a strong and sharp photoluminescence spectrum. A free exciton peak was located at 2.807 eV indicating a coherent growth of the layer. Moreover, the layer showed a clear X-ray diffraction curve with high-order interference fringes and a mirror-like surface morphology.

Self-aligning phenomena of ZnCdSe islands grown by molecular beam epitaxy on GaAs(110) surface cleaved in ultra high vacuum

Abstract Self-organized formation of islands has been investigated for the growth of Zn0.7Cd0.3Se on the GaAs(110) surface cleaved in ultra high vacuum by molecular beam epitaxy. We found, for the first time, phenomena that the self-organized Zn1 − xCdxSe islands aligned in long-range up to several tens of μm on GaAs(110) surface. Two types of surface structures were observed, pyramidal-shaped islands and elongated triangular ridges. The islands were selectively grown on the top of the ridges along [1 1 0] direction. The ridges were presumably caused by the anisotropic in-plane strain relaxation of the epilayers on the (110)-oriented substrate. The periodic strain distributions on the ridges are suggested to account for the arrangement of the islands.

Electrical Characterization of MOVPE-GROWN P-Type GaN:Mg Against Annealing Temperature

Hall effect measurements have been applied for the electrical characterization of p-type Mgdoped GaN grown by metalorganic vapor-phase epitaxy on sapphire substrates in terms of annealing temperature for dehydrogenation (N2 annealing) and hydrogenation (H2 annealing) of the acceptors. With the N2 annealing temperature from 600 to 900 °C for dehydrogenation, both hole concentration and mobility increases, showing more activation of acceptors and less incorporation of unfavorable scattering centers probably originating from Mg-H bondings. The N2 annealing at higher than the growth temperature results in reduced hole concentration, but the mobility gets higher. Some defects compensating acceptors may be induced at high temperature annealing, but they seem to be no scattering centers and be inactivated by successive hydrogenation and re-dehydrogenation at the optimum dehydrogenation temperature 900 °C. The electrical degradation of GaN due to thermal damage is not very destructive and can be well recovered by annealing treatments. INTRODUCTION It is now a common understanding that Mg acceptors doped in GaN by metalorganic vaporphase epitaxy (MOVPE) are severely passivated with hydrogen in as-grown layers and can be activated to achieve p-type conductivity by post-growth thermal annealing in N2 atmosphere resulting in dehydrogenation. With referring the earliest reports by Nakamura et al. [1], the thermal annealing has been considered to be effective at a temperature sufficiently lower than the growth temperature, e.g., at 600 °C or slightly higher. Although the doped Mg acceptors are expected to be activated almost completely at the present annealing conditions, there have not been a sufficient number of reports investigating the activation processes and the possible defects induced by thermal damage during the annealing. Youn et al.[2] showed that there was an optimum annealing temperature resulting in the highest hole concentration, dependent on the existing defect structures in Mg-doped GaN (GaN:Mg) layers. The hole concentration decreased at higher annealing temperatures, different from the data in ref.[1] showing the constant hole concentration with annealing temperature above 650 °C, and this phenomenon was interpreted by generation of nitrogen vacancies compensating Mg at higher annealing temperatures. As is demonstrated in this work, the possibility of defect generation or other thermal effects may be an important factor to be taken into account in the discussion of the activation process of Mg acceptors. In this study, aiming at detailed understanding of activation and degradation mechanism in GaN:Mg layers, characterization of electrical properties has done in terms of wide variety of the annealing temperature, including that higher than the growth temperature, for dehydrogenation in N2 atmosphere. Effects of successive hydrogenation in H2 atmosphere and redehydrogenation were also investigated. https://doi.org/10.1557/S1092578300003227 Downloaded from https://www.cambridge.org/core. IP address: 54.70.40.11, on 01 Apr 2019 at 08:49:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.

In Situ Monitoring And Control For MBE Growth Of Optoelectronic Devices

Self-organized CdSe/ZnSe quantum dots (QDs) were fabricated on the cleavage-induced GaAs (110) surface in ultra high vacuum (UHV) by molecular beam epitaxy (MBE). CdSe layer showed the Stranski¿Krastanow (S-K) growth mode. QWs and QDs emissions originated from the wetting layer and island structures, respectively, were observed in photoluminescence (PL) spectra. This is a evidence of S-K type where island structures are self-formed on the two-dimensional wetting layer as a result of the transition of the growth mode. The state filling effect in the QDs was also observed by employing excitation power dependence on the PL intensity. By using the microscopic PL spectroscopy, the broad PL peak of QDs was resolved into a number of sharp peaks. These peaks are attributed to the recombination of excitons localized at the individual QDs indicating that the fabricated CdSe islands have quasi-zero-dimensional δ-function like density of states.

Growth of InxGa1−xN thin films on indium tin oxide/glass substrates by RF plasma enhanced chemical vapor deposition

Abstract InxGa1−xN thin films with GaN buffer layers (200°C) were grown on indium tin oxide (ITO)/glass substrates at 500°C by RF plasma enhanced chemical vapor deposition (PECVD). Preferred (0002) and (10 1 1) directions of the samples and successful incorporation of In with TMIn flow rate were characterized by X-ray diffraction (XRD). Results of the optical absorption measurements showed that the absorption band edges shifted to the lower energy with increasing In solid compositions. Atomic force microscopy (AFM) showed the lumps of grains tended to be larger with In solid composition although the surface deteriorated.

Fabrication of Zn 1- x Cd x Se quantum dots by molecular beam epitaxy on the GaAs (110) cleaved surface

Abstract The Growth Interruption (GI) process during the growth of Zn 1− x Cd x Se/ZnSe Single Quantum Wells (SQWs) on the GaAs (110) cleaved surfaces by Molecular Beam Epitaxy (MBE) was found to reduce the transition thickness of strained Zn 1− x Cd x Se active layer from two- to three-dimensional growth, thus it gives the formation of quantum dots (QDs) with Stranski–Krastanow mode. The GI effect is stronger for the Zn 1− x Cd x Se active layer with a higher Cd composition. The fabricated SQW samples showed Photoluminescence (PL) spectra originating from both QD and QW, and these emissions had nonlinear properties for the excitation power dependence of PL intensity.