Young Ju Park

Reversible transition between InGaAs dot structure and InGaAsP flat surface

We have studied the in situ modification of coherently grown InGaAs dots by interaction with phosphorus. By monitoring the intensity of reflection high-energy electron diffraction transmission spot, the in situ phosphorus (precracked PH3) supply on the InGaAs dots was examined at 480 °C. It was found that the phosphorus exposure induces a surface structure change from a dot structure to a flat surface. The change is caused by the replacement of arsenic in the dots by phosphorus, which reduces the strain between the InGaAs(P) dots and the GaAs substrate. By switching AsH3/PH3 beams in situ, a reversible transition of the surface structure between the InGaAs dot structure and the InGaAsP flat surface was observed. A transitional state between the dot structure and the flat surface was metastabilized by tuning the AsH3/PH3 beam ratio. The metastabilized surface was observed ex situ using a high-resolution scanning electron microscope.

Fabrication of wirelike InAs quantum dots on 2°-off GaAs (100) substrates by changing the thickness of the InAs layer

Wirelike InAs quatum dots (QDs) grown on 2°-off (100) GaAs substrates by changing the thickness of the InAs layer were successfully fabricated. The sizes of the InAs QDs along the step lines increased with increasing the thickness of the InAs layer, and their increases were attributed to transform of the InAs QDs into the wirelike InAs QDs. The optimal thicknesses of the InAs layers for the wirelike QDs and the interval of the wirelike QDs were significantly affected by the terrace width resulting from the bunching effect due to the thickness variations of the GaAs buffer layers grown on 2°-off (100) GaAs substrates. These results indicate that these wirelike InAs QDs are useful for applications in nanoelectronic devices, such as wrap gate single electron transistors.

Selective formation of one- and two-dimensional arrayed InGaAs quantum dots using Ga2O3 thin film as a mask material

We report on the selective formation of InGaAs quantum dots (QDs) by molecular beam epitaxy. Nanoscale patterned Ga2O3 thin film deposited on the GaAs (100) substrate was employed as a mask material. Due to the enhanced migration effect of the group-III adatoms, such as Ga and In on Ga2O3 mask layer, the InGaAs QDs formed on the patterned substrate results in coalesced islands unlike those formed on the nonpatterned substrate. The estimation of the relative volume of the islands per unit area revealed that the desorption process as well as the migration of the Ga and In adatoms might occur on the Ga2O3 layer during the growth process, providing a good selective growth of self-assembled QDs.

Shape and Interband Transition Behavior of InAs Quantum Dots Dependent on Number of Stacking Cycles

InAs/GaAs multistacked quantum dot (QD) layers were grown by using molecular beam epitaxy with various numbers of stacking cycles to investigate the shape and the interband transition in the InAs QDs. The appearance of another photoluminescence (PL) peak on InAs/GaAs QDs with more than six stacking cycles originated from the change of the QDs from an isotropic pyramidal shape to an elongated anisotropic pyramidal shape. Dislocation lines along the [110] direction existing on the InAs/GaAs QDs with more than six stacking cycles were attributed to the existence of excessive strain fields. Scanning transmission electron microscope and atomic force microscope images showed that the QD shape in the [110] direction was elongated without any remarkable change in the volume of the QDs. These results indicate that the shape of the InAs/GaAs QDs was strongly affected by the number of the stacking cycles and that the appearance of another PL peak is related to the change of the QD shape.

Electrical and optical characterizations of self-assembled quantum dots formed by the atomic layer epitaxy technique

We investigated the electrical and optical properties of InGaAs self-assembled quantum dots grown using the atomic layer epitaxy (ALE) technique. Dots–in–a–well structures were grown by alternately supplying InAs and GaAs sources on an InGaAs layer and covering with another InGaAs layer. Three samples produced with different numbers of cycles of alternate InAs/GaAs supply were characterized by capacitance-voltage and photoluminescence (PL) measurements. For the ten cycle dots–in–a–well structure, a strong zero-dimensional electron confinement was observed even at room temperature. On the other hand, for the five-cycle structure, the PL results indicate that the InGaAs quantum well structure coexists unstably with premature quantum dots. By comparing the results for samples with different numbers of cycles, we suggest that an ALE dots–in–a–well structure can be formed by the aggregation of In and Ga atoms incorporated into the InGaAs quantum well layer when the number of cycles exceeds the critical number ...

Effects of Doping Profile on Characteristics of InAs Quantum Dots

Capacitance-voltage measurements were carried out to investigate the effects of the doping profile on characteristics of self-assembled InAs quantum dots. Using this technique, we observed features of zero-dimensional electron confinement indicating the presence of quantum dots. However, the number of confined states differed depending on the doping profile, and this fact was confirmed by photoluminescence measurements. The equation in the depletion approximation led us to calculate the distribution of carriers as a function of depth from the sample surface, and the results are in agreement with the depth of the quantum dot layer.

Alignment of InAs quantum dots on a controllable strain-relaxed substrate using an InAs/GaAs superlattice

We fabricated InAs self-assembled quantum dots on a strained layer using molecular beam epitaxy. The controllable strained layer consisted of an InAs/GaAs superlattice and a GaAs spacer layer on a GaAs (001) substrate. We formed two-dimensional arrays of quantum dots along the 〈110〉 directions on the partially strain-relaxed layer that is formed using the superlattice system. The increase in the thickness of the partially strain-relaxed layer resulted in stronger alignment of the quantum dots. The aligned quantum dots are applicable to quantum devices, because they confine carriers well, in spite of the existence of dislocation networks. Strongly aligned quantum dots have a lower carrier transition energy because of their larger size and increased relaxation.

β-SiC Thin film growth using microwave plasma activated CH4-SiH4 sources

Abstract The activation of a source gas by plasma is used to decrease the growth temperature of a highly oriented epitaxial SiC film on a Si(100) substrate. A mixed source gas of 1% CH 4 –0.5% SiH 4 –H 2 is activated by a microwave plasma at 100 Torr with the power between 400 and 900 W. No pre-deposition processes such as carbonization is applied. A substrate is immersed in the plasma, whose temperature is controlled between 795 and 1000°C by varying the microwave power. Polycrystalline SiC films, whose grains appear equiaxed and randomly oriented, forms under substrate temperatures below 900°C. However, the grains begin to change their shape into elongated ones above 935°C, whose longitudinal axes are aligned to the 〈011〉 direction of the Si substrate. The growth rate of the SiC film decreases with increasing temperature. The stoichiometry, C/Si concentration ratio, is maintained as 1.1 throughout the temperature range.

The facet evolution during metalorganic vapor phase epitaxial growth on V-grooved high Miller index GaAs substrates

Abstract The facet evoluting during metalorganic vapor phase epitaxial (MOVPE) growth on high Miller index V-grooved GaAs substrates with (1311)A, (511)A, (311)A, and (211)A as well as (100) orientations, has been investigated. The {433}A and (100) facets evolved on the as-etched V-grooved substrate having {111}A side walls, regardless of substrate orientation. As the substrate orientation is tilted toward (211)A, the (433)A facet on the long-side wall is extended, while the length of the newly formed (100) facet on the other side is reduced. The (4⦶3⦶3) facet on the short-side wall formed in the early stage of growth diminishes more rapidly. The direction of the locus line of the intersection points between the two facets is initially [100]. However, this direction eventually changes after the (4⦶3⦶3)A facet on the short-side wall disappears. The growth rate properties of the facets can be explained by channel effect and different surface mobilities of different species.

Interdiffusion and structural change in an InGaAs dots-in-a-well structure by rapid thermal annealing

Post-growth rapid thermal annealing (RTA) has been used to investigate an interdiffusion and the structural change in an InGaAs dots-in-a-well (DWELL) structure grown by molecular beam epitaxy using an alternately supplying InAs and GaAs sources. In the case of the as-grown sample, which has a metastable quantum structure due to an intentional deficit of source materials, it is found that an InGaAs quantum well (QW) coexists with the premature quantum dots (QDs), and an intermediate layer exists between the QW and the QDs. Through the RTA process at 600 and 800°C for 30s, metastable structure changes into a normal DWELL structure composed of QDs and QW as a result of the intermixing of premature QDs and the intermediate layer.