Chae-Deok Lee

Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

Formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature epitaxy

We report the direct formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature molecular beam epitaxy. To drive a three dimensional growth mode, the (1×1) AlGaAs surface was exposed alternately to the Ga and As sources. The resulting GaAs nanocrystals having {111} facets were clearly identified by high-resolution transmission electron microscopy. The emission spectra also confirmed the formation of dots. The transition to a three-dimensional growth mode is attributed to the limited surface migration of Ga adatoms on the AlGaAs surface, which has excess As at low substrate temperature.

Magnetophotoluminescence investigations of charged excitons in GaAs/(Al,Ga)As quantum wells

Abstract We have observed charged excitons in undoped GaAs/Al0.25Ga0.75As multiple quantum wells of differing well sizes. Photoluminescence(PL) originated from each well shows a double peak associated with the neutral exciton and the charged exciton whose existence is due to a charge imbalance of photogenerated carriers in each well. The laser-power dependence and circular polarization characteristics of PL readily identifies the negatively charged exciton(X−) in wide wells where an excess electron can exist. The binding energy of electron in the singlet state of X−, the so-called trion binding energy, increases with the magnetic field. For a 19nm quantum well, it is 1.2 meV and 1.9 meV at B=0 T and ∼ 9T, respectively.

Influence of thin AlAs interface layers on exciton transitions in InGaAs/GaAs quantum wells

We have optically determined carrier effective masses for a series of InGaAs/GaAs quantum wells with 0 or 2 monolayer AlAs sandwiched between InGaAs and GaAs. Photoluminescence investigations of exciton ground state were carried in the presence of a magnetic field up to 50 T. The reduced mass of heavy-hole exciton was determined by theoretical fits to its diamagnetic shifts. The heavy-hole in-plane mass increased when ultra-thin AlAs layers were inserted at the both interfaces of a quantum well.

Origin of short period oscillation near GaAs band-gap energy in photoreflectance

Abstract We investigated the origin of the short-period oscillation (SPO) in photoreflectance (PR) spectra of selectively doped GaAs samples. In the PR spectra of the samples the SPO are separated into two distinct signals. The intensity of the lower-energy signal decreases rapidly as the temperature is lowered. Therefore, we conclude that the short-period oscillation is due to the hole-ionized acceptor ( h - A − ) pair modulation and the free exciton modulation in the cap layer.