Investigations of electronic states in self-assembled InAs/GaAs quantum-dot structures

Space-charge techniques, such as capacitance-voltage (CV) spectroscopy and deep-level transient spectroscopy (DLTS), are used to examine the electronic states of ensembles of selfassembled InAs quantum dots (QDs), embedded in a GaAs matrix and grown by the 3D Stranski-Krastanow growth mode. We present direct experimental evidence of the coexistence of deep levels in the same epitaxial layer of optically active quantum dots. The InAs quantum dots show very good optical properties, as evidenced by the strong photoluminescence (PL) at room temperature at ~1.3 μm. The reverse-bias dependence of the DLTS signal together with results from the reference samples, containing thin InAs layers but no quantum dots, confirms that the deep levels coexist in the dot layer and are most likely caused during the latticemismatched growth process. Laplace deep-level transient spectroscopy (LDLTS) is a technique developed primarily to study the point defects in semiconductors, which has also recently been applied to the semiconductor quantum-dot structures. The newly developed technique can provide orders of magnitude better resolution than the conventional DLTS method. By applying the LDLTS technique, we are able to study the electronic fine structure of the deep-level states coexisting in the dot layer. As a way of tuning the electronic properties, postgrowth rapid thermal annealing (RTA) has been applied to the semiconductor quantum dots, and the induced optical and electrical changes are studied using PL and DLTS. These combined optical and electrical experiments also confirm our findings of the coexistence of the deep levels with the QDs. By a comparison of the DLTS data with the PL spectra, we find that the effects of RTA on the optical spectra are closely linked with the alternations of the electronic structures, and that a new deep level (0.62 eV) is created in the structure, which dominates the whole spectra at certain annealing temperatures. Furthermore, by combining the CV, conventional and Laplace DLTS techniques, we systematically and quantitatively investigate the underlying emission mechanisms in the QD single-level two-electron system. Electron emissions from the singly and doubly occupied QD s states can be resolved by the LDLTS technique. The emission processes are investigated in detail by the pulse-bias dependency. The electron distribution profile in quantum dots is identified by applying an appropriate set of voltage pulses across the Schottky diode structure. © 2008 by Nova Science Publishers, Inc. All rights reserved.