Ryo Oga

Atomic-scale observation of interfacial roughness and As–P exchange in InGaAs/InP multiple quantum wells

Cross-sectional scanning tunneling microscopy (XSTM) has been used to study interfacial properties of InP-on-InGaAs interfaces in InGaAs/InP multiple quantum wells grown by metalorganic vapor phase epitaxy with a growth interruption. XSTM has enabled us to separately identify step-like roughness and distributions of As atoms incorporated in the InP layer near the interface. The As composition profile along the growth direction analyzed from distributions of As atoms in XSTM images shows an exponential variation with distance from the InP-on-InGaAs interface. It is found that the growth interruption of 30 s reduces considerably the roughness amplitude to 0.45 nm from 1.1 nm and increases the coherent length from 22 to 27 nm.

Reactor structure dependence of interface abruptness in GaInAs/InP and GaInP/GaAs grown by organometallic vapor phase epitaxy

Abstract We have grown Ga 0.47 In 0.53 As/InP and Ga 0.51 In 0.49 P/GaAs heterostructures by organometallic vapor phase epitaxy (OMVPE) using a multi-barrel reactor and compared their interface abruptness with that using a conventional single-barrel reactor. The multi-barrel reactor had a vertical four-barrel structure with five gas inlets, i.e. one inlet at the center in addition to one inlet for each barrel of four. In the growth, TBAs and TBP were used as group-V sources and supplied separately to barrels opposite to each other, while TEGa and TMIn as group-III sources were supplied from a center inlet to all the barrels. Secondary ion mass spectroscopy (SIMS) measurements on Ga 0.47 In 0.53 As/InP heterostructures reveled excellent abruptness of P distribution across the interface and negligible P contamination in the Ga 0.47 In 0.53 As layer. Such advantages have also been obtained in the Ga 0.51 In 0.49 P/GaAs system.

White light source in infrared region from InAs quantum dots grown on (001) InP substrates by droplet heteroepitaxy

We have observed extremely wide spectral range electroluminescence (EL) from InAs quantum dots (QDs) on (001) InP substrates at room temperature. The InAs QDs were grown by droplet heteroepitaxy using a low-pressure organometallic vapor phase epitaxial system. Room-temperature EL in a very wide wavelength range from 950 to 2200 nm was observed from InAs QDs embedded in InP matrix. The wide range emission indicates that the QDs have white optical gain in the infrared region at room temperature, which can be applied to efficient optical amplifiers for 1.0–1.6 μm fiber communication.

Perturbation method of analysis of crystal truncation rod data

A new method of direct inversion of crystal truncation rod (CTR) data is demonstrated for the analysis of layered semiconductor heterostructure materials. This method is based on approximations that are valid when the electron density deviations and lattice strain are small in the regions of the sample close to a well defined surface. The CTR diffraction pattern can then be regarded as a perturbation with respect to that of an ideal surface. The direct inversion method is shown to work for the analysis of a series of InP/GaInAs/ InP heterostructures. The ability to recover strain information is demonstrated with a model calculation. The beginning of breakdown of the perturbation approximation is seen and explained in both cases.

Cross-Sectional Scanning Tunneling Microscopy Study of Interfacial Roughness in an InGaAs/InP Multiple Quantum Well Structure Grown by Metalorganic Vapor Phase Epitaxy

Interfacial roughness of InxGa1-xAs/InP (x=0.53) multiple quantum wells (MQW) grown by metalorganic vapor phase epitaxy has been investigated by cross-sectional scanning tunneling microscopy (STM). The MQW structure is composed of 125 periods of 11-nm-wide well layers and 44-nm-wide barrier layers on an InP (001) substrate. The observed STM images have revealed that the InGaAs-on-InP interface is extremely sharp compared to the InP-on-InGaAs interface; the roughness and terrace size on the InGaAs-on-InP interface are 1–2 monolayers (ML) and 31 nm, respectively, while they are 3–4 ML and 9 nm on the InP-on-InGaAs interface. The large roughness observed for the InP-on-InGaAs interface is interpreted in terms of the incorporation of residual As atoms on the InGaAs surface.

SR-stimulated etching and OMVPE growth for semiconductor nanostructure fabrication

Abstract Synchrotron radiation- (SR-)stimulated etching and selective area growth by organometallic vapor phase epitaxy were performed to form an ordered array of InP crystals on SiO 2 -patterned InP (001) substrate. The SR-stimulated etching was used to pattern the SiO 2 film, because photochemical reaction using SR was expected to provide smooth surfaces, vertical side walls and fine patterning. In the first place, we investigated the basic properties of the SR-stimulated etching by using a mm-size pattern of SiO 2 mask. The etched depth was observed to increase linearly with the irradiation dose. It was found that the etching depth was controlled very accurately. Next, we used μm-size patterns of SiO 2 masks for fabricating the ordered array of InP crystals. In a atomic force microscope image of the sample after etching, a steep side wall was observed. However, the etched surface was not smooth, contrary to our expectation. Moreover, some dust were observed on the surface. From this dust it was found that the SR-stimulated etching had a resolution of ≤100 nm at most.

Droplet Hetero-Epitaxy of InAs Quantum Structures on InP Nanopyramids Formed by Selective-Area Flow Rate Modulation Epitaxy

We have grown InAs quantum structures by droplet hetero-epitaxy on InP nanopyramids and investigated their luminescence property. InP nanopyramids with improved size control were formed successfully by selective-area flow rate modulation epitaxy (FME). The standard deviation of the bottom size of the nanopyramids decreases by means of selective-area FME. Droplet hetero-epitaxy was carried out on InP nanopyramids to grow InAs quantum structures. The shape of the nanopyramids becomes sharper with longer TMIn supply time. In photoluminescence (PL) measurements at 4.2 K, characteristic luminescence is clearly observed. The mapping measurement of the PL intensities reveals that the luminescence originates from the patterned area with the nanopyramids. The PL peak positions shift slightly to a longer wavelength region with increasing supply time. These results suggest that InAs quantum structures are formed on InP nanopyramids.

Control of group-III atoms distribution in thin quantum wells analyzed by X-ray CTR scattering measurement

The InP/Ga/sub 0.47/In/sub 0.53/As(3ML)/InP quantum well structure samples were grown by organometallic vapor phase epitaxy at different growth temperatures to find the dependence of the composition profiles on the growth temperature. The samples were investigated by the X-ray crystal truncation rod scattering measurement. The distribution of Ga could be controlled by the change of the growth temperature, which had never be controlled by the change of the source-gas-flow sequence. In order to make the distributions of atoms as designed, the growth temperature should be selected at first to control the distribution of Ga, then the source-gas-flow sequence should be tuned to control the distribution of As.

Droplet hetero-epitaxy of InAs quantum dots on InP nanopyramids formed by selective-area flow rate modulation epitaxy

We have grown InAs quantum structures by droplet hetero-epitaxy on InP nanopyramids and investigated their luminescence properties. The nanopyramids with improved size control are formed successfully by selective-area flow rate modulation epitaxy (FME). In photoluminescence (PL) measurements at 4.2 K, characteristic luminescence due to InAs quantum structures is observed clearly, depending slightly on TMIn supply time for InAs growth. We have also compared the luminescence in the samples with differently patterned areas.

Analytical evaluation of growth process in a sub-micron scale selective-area growth by OMVPE

We investigated the effect of the mask pattern size on the source supply process of selective-area growth (SAG) by organometallic vapor phase epitaxy, especially focusing growth with a sub-micrometer mask pattern. We considered two theoretical models; a vapor phase diffusion model and a surface diffusion model, and solved the analytical expressions of the growth rate due to the models, individually. Comparison of the theoretical models shows that the predominant process changes from the surface diffusion model to the other with increasing the mask pattern size. The SAG experiments with sub-micrometer mask pattern were performed. The predominant process was found to be the surface diffusion model. In contrast, the other studies considering mask pattern over several- or several ten-micrometers often conclude the vapor phase diffusion process is dominant. The comprehensive conclusion agrees with the prediction of the size dependence indicated by the present theoretical consideration.