Kee-Youn Jang

Two-Dimensional In0.4Ga0.6As/GaAs Quantum Dot Superlattices Realized by Self-Organized Epitaxial Growth

We report on the realization of two-dimensional (2D) In0.4Ga0.6As/GaAs quantum dot superlattices (QDSLs) by self-organized epitaxial growth. The conditions for the formation of extended states or minibands are analyzed by treating QD arrays as disordered systems. Ordered quantum dot (QD) arrays are fabricated on GaAs (311)B substrates. High density and small size are achieved by decreasing the growth temperature. A large red shift of the photoluminescence (PL) peak energy and a dramatic narrowing of the linewidth are found when the dots become smaller and closer. The exciton coherence length in the high-density ordered QD array is confirmed to be much larger than the QD diameter by PL decay time measurements and by using beryllium impurities as scattering centers. As a comparison, the incoherent exciton motion dominated by nonresonant tunneling is discussed. The transition from coherent to incoherent, including the intermediate state, and the localization of excitons are demonstrated by various mechanisms.

Isolated and close-packed In0.4Ga0.6As/GaAs (311)B quantum dots

Abstract Remarkably different characteristics were found on isolated and close-packed self-assembled In 0.4 Ga 0.6 As/GaAs (311)B quantum dots (QDs). Sub-peaks superimposed on the regular Gaussian photoluminescence (PL) peak were observed in a close-packed QD array, in contrast to the single Gaussian PL peak in an isolated one. A tunneling-current peak in an isolated QD array changes into several equidistant features in a close-packed one. The possible mechanisms of the fine structures are discussed.

Effect of Charge Distribution in Quantum Dots Array on Two-Dimensional Electron Gas

With InGaAs quantum dots (QDs) embedded in the vicinity, GaAs/AlGaAs two-dimensional electron gas (2DEG) was studied by transport measurements and by calculations with QDs charges as Coulomb scattering centers. The mobility of 2DEG decreases with increasing QDs density up to 5×1010 cm-2, which can be fitted by considering the finite spatial extension of QD-charge distribution. However, two distinct effects are observed as the QDs density increases from 5×1010 cm-2 to 2×1011 cm-2. One is that the 2DEG mobility is saturated at a low level, and the other is that it gradually approaches the high value of 2DEG without QDs. The former can be reproduced by taking into account the interdot overlapping of electron probability distributions, while the latter is argued to be the result of interdot coupling through wavefunction overlapping. These explanations are supported further by the temperature dependence of the conductance.

Effect of hydrogen in AlGaAs grown by atomic hydrogen-assisted molecular beam epitaxy

Abstract We have found that the photoluminescence (PL) peak position of Al 0.25 Ga 0.75 As notably changes with atomic hydrogen (H) supply in molecular beam epitaxy. The PL peak position for the sample grown with atomic H at 580°C is shifted toward lower energy compared to that of the `without H by as much as ∼80xa0meV. From secondary ion mass spectroscopy (SIMS) measurements, it is also found that the observed shifts of PL peak position for the `with H is due to changes in the Al contents, and is attributed to re-evaporation of Al in the form of Al hydrides during the growth with atomic H. The dependence of Al contents on growth temperature is also investigated by PL and reflection high-energy electron diffraction (RHEED) oscillations. The effect of atomic H on Al contents in AlGaAs are most pronounced at the growth temperature of 580°C.

Effects of atomic hydrogen in molecular beam epitaxy of Al(Ga)As

Abstract The effects of atomic hydrogen on the growth of Al(Ga)As by molecular beam epitaxy (MBE) have been investigated. A decrease in Al content was observed for AlGaAs films grown by atomic hydrogen-assisted MBE (H-MBE) compared to conventional MBE-grown films. This tendency was consistent with the results of decreased AlAs growth rates as determined by in situ reflection high-energy electron diffraction (RHEED). Further, the increase in the mass signal of AlH 2 + by quadruple mass spectrometer was observed during H-MBE growth. In H-MBE, it is thought that enhanced Al re-evaporation from the growth surface arises via formation of Al hydrides during growth. The observed difference of Al re-evaporation between vicinal A-surface and B-surface has been discussed, which is ascribed to the decreased growth rate and Al content.

Effect of Atomic Hydrogen on GaAs Growth on GaAs(311)A Substrate in Molecular Beam Epitaxy

The uniform corrugated structures along [1overline12] direction have been found in the GaAs layers grown on GaAs (311)A substrates by atomic hydrogen-assisted molecular beam epitaxy (H-MBE). On the other hand, no corrugated structures were observed in the growth by conventional MBE. The differences in the surface morphology between H-MBE and conventional MBE samples were analyzed by using an atomic force microscope. In H-MBE, atomic H was continuously supplied during the growth, and hence the dangling bonds on the surface would be terminated by the H atoms. Thus, the [1overline12] azimuth-oriented corrugations observed in H-MBE samples is thought to be due to the presence and interation of atomic H with the migrating Ga atoms, and preference of growth along the the steps where Ga atoms are terminated by H atoms.

Realization and characterization of two-dimensional quantum dot superlattices forming “artificial crystals”

The realization of two-dimensional quantum dot superlattices (2D QDSLs) is briefly reported. The formation of minibands is manifested in the large red shift of the photoluminescence (PL) peak as well as the dramatic narrowing of the PL linewidth. This is confirmed more directly by various experiments intentionally designed to localize excitons.

Band-gap energy anomaly observed in AlGaAs grown by atomic hydrogen-assisted molecular beam epitaxy

Abstract Red shifts in photoluminescence (PL) peaks have been observed for Al 0.25 Ga 0.75 As grown by atomic hydrogen (H)-assisted molecular beam epitaxy (MBE) by an amount of as large as ~80xa0meV compared to those grown without atomic H. From absorption spectra measurements, the observed red shifts for with atomic H case were due to changes in the band-gap energies, and not due to changes in Al compositions nor formation of new H-related emission centers. It is thought that possible cause of the observed band-gap anomaly is related with spontaneous formation of an ordered superlattice during atomic H-assisted MBE.