Dr. Mohamed Hassan Abdellatif

ABSTRACT

Optical Study of Nanostructured Semiconductor Materials

M. H. Abdellatif Nano-Photonics Center, Korea Institute of Science and Technology, Seoul 136-791, Korea.

The carrier thermodynamics of low dimensional systems is an important aspect for optical devices. The impact of small size on the charge carrier confinement has effect on the emission line and the variation of the emission energy with temperature, which are all important issues in designing optoelectronic devices. In this work, the temperature and excitation power dependence of photoluminescence (PL) on temperature of four different structures, namely InGaAs/GaAs quantum well (QW), InAs quantum dot (QD), InAs QD in asymmetric InGaAs/GaAs QW, and AlGaAs/GaAs multiquantum well (MQW), are studied in the temperature region from 16K to near room temperature, and the excitation laser power from 1 mW to 30 mW. The carrier thermodynamics was studied based on steady state model, which predicts linear dependence of the PL integrated intensity on excitation power density in the low temperature region that switches to superlinear at high temperature.

In InGaAs/GaAs QW sample, the structure has long lived linearity of the excitation power dependence of PL up to 210K, which is considered as critical exciton temperature, at which the superlinearity starts. The higher critical exciton temperature is attributed to In-atom segregation during growth, this In-atom segregation provides a QD like behaviour to the QW sample, since the increased In-content works as a three dimensional confinement potential for electron-hole pairs. This additional confinement increases the exciton binding energy in this system which allows exciton to survive longer with temperature. The excitation power dependence measurement shows that electron-hole pairs in the studied InGaAs/GaAs QW structure are weakly correlated above 210K.

The critical exciton temperature for InAs QD sample was found to be 160K, hence the carrier dynamics in InAs QD sample is dominated by uncorrelated electron-hole pair in the temperature region higher than 160K, although it is comparable with values mentioned in literature for samples prepared by conventional SK method, but the obtained SEM data shows that preparing the sample using Migration Enhanced Molecular Beam Epitaxy (MEMBE) provides better control on the QD parameters than the previously known self assembled SK growth mode.

InAs QD embedded in asymmetric InGaAs/GaAs QW is a good candidate for optical communications. The confinement potential is strongly modified by embedding QD in a QW, and hence the carrier thermodynamics is also influenced. This enhanced confinement potential increases the exciton binding energy, and hence the critical exciton temperature reaches 210K. Also the temperature dependence of the ground state peak position for QD peak was measured and fitted with Varshni relation, The bigger values of Varshni parameters may be attributed to In/Ga inter diffusion during growth.