Hyun-Chul Ko

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.

Narrow luminescence lines from self-assembled CdSe quantum dots at room temperature

Narrow luminescence lines from self-assembled CdSe quantum dots (QDs) are studied at room temperature by using time-resolved photoluminescence spectroscopy. The narrowest line of the luminescence has a full width at half maximum of 2.1 meV at room temperature. The narrow luminescence lines are mainly the contribution of CdSe QDs with a longer recombination lifetime to the luminescence.

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.

New approach to the fabrication of CdSe/ZnSe quantum dots using a cleaved-edge overgrowth technique

Abstract Spontaneous formation of CdSe/ZnSe quantum dots (QDs) in ultra high vacuum (UHV) by molecular beam epitaxy (MBE) was investigated using cleavage-induced GaAs (1 1 0) surfaces as substrates. Stranski-Krastanow (SK)-type CdSe QDs were randomly distributed on the atomically flat surface. On the other hand, QD structures were abnormally formed at the macroscopic steps produced by the cleaving process of the substrates. These QDs were naturally arranged along the edge of the steps. Microscopic photoluminescence (PL) spectra of the QDs at step regions showed a few number of sharp peaks originating from the individual QDs. Therefore, the cleaved edge overgrowth technique is a novel method for fabrication of isolated CdSe QDs which is useful to investigate optical properties of individual QD structures.

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.

Self-formed ZnCdSe ridge and island structures on GaAs(110) surface

We found new phenomena that ridge and island structures were naturally formed during the molecular beam epitaxy (MBE) of Zn0.7Cd0.3Se onto GaAs(110) surfaces obtained by cleaving in ultra high vacuum (UHV). In contrast to the mirror-like surface of ZnSe layers, Zn0.7Cd0.3Se epilayers showed two types of structures on the surface, i.e. pyramidal-shaped islands and asymmetric Δ-shaped ridges oriented the [110] direction. The ridges have strong correlation with the underlying dislocations produced by anisotropic in-plane strain relaxation. Local variation of the growth rate due to the strain field caused by the dislocations is suggested as a formation mechanism of such ridge structures.

Self-Organizing Process of Moderately Strained Zn1-xCdxSe Layer Grown on GaAs(110) by Molecular Beam Epitaxy

Self-organizing process of moderately strained Zn1-xCdxSe layer on the GaAs(110) surface was investigated during the molecular beam epitaxy (MBE). The GaAs(110) surface was prepared by cleavage in ultrahigh vacuum (UHV). In contrast to the ZnSe layers which have a mirror-like surface, two types of surface structures were observed on the Zn1-xCdxSe layer, i.e. pyramidal-shaped islands and asymmetric Δ-shaped ridges oriented parallel to the [110] direction, indicating that the strain relaxation process was not of a conventional Stranski-Krastanow type. The ridges were strong correlated with the underlying dislocations produced by anisotropic in-plane strain relaxation. Local variation in the growth rate due to periodic strain distributions caused by the dislocations is suggested to be the formation mechanism of the ridge structures.

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.