Carrier Emission and Electronic Properties of Self-Organized Semiconductor Quantum Dots

Abstract

In this work, the carrier dynamics and electronic properties of self-organized semiconductor quantum dots are studied by depletion-layer capacitance transient spectroscopy (or deep level transient spectroscopy – DLTS). The first experimental investigations of carrier escape from InAs/GaAs and Ge/Si quantum dots by this method are reported. In the emission of electrons from a triple-layer of InAs/GaAs quantum dots, thermal activation and tunnel escape from the ground state are clearly identified. The thermally activated process with an activation energy of 94 meV is attributed to the transition from the electron ground state to the first excited state. For single-layer samples an activation energy of 82 meV for electron escape for the same transition is determined. Hole emission from such quantum dots is found to be solely due to thermal activation from the quantum dot hole ground state to the matrix valence band. The activation energy of 194 meV is hence much larger than in the case of electrons. The absence of tunneling in the hole escape process is explained by the larger effective mass, which leads to a decreased tunneling probability. The derived quantum dot energy level scheme agrees well with results from optical investigations, admittance spectroscopy, and predictions based on eight-band k·p theory including strain and piezoelectricity. Furthermore, the influence of the electric field and the position of the Fermi level on the emission process is studied. From the investigation of hole escape from multiply-charged Ge/Si quantum dots, a ground state activation energy of 350 meV was obtained, in good agreement with results from photoluminescence and admittance spectroscopy. Furthermore, hole emission with an activation energy of about 100 meV is observed, which is attributed to carrier escape from the Ge wetting layer. It is demonstrated, that by adjusting filling pulse bias and reverse bias, the quantum dots can be partly filled or emptied. This is reflected also by the activation energy. From these observations it is concluded that many-particle effects dominate the electronic structure, and thus the carrier capture and emission processes in selforganized quantum dots. By simulating DLTS spectra of carrier escape from quantum dots with help of a microstate model developed in this work, a strong dependence on the electronic structure is revealed. The presented experimental and theoretical results demonstrate the capabilities of capacitance techniques for investigating quantum dot systems.