Thermally assisted tunneling processes in InxGa1-xAs/GaAs quantum-dot structures

An ensemble of self-assembled InxGa1−xAs quantum dots has been grown in n-conducting GaAs with a low dislocation density by molecular beam epitaxy using the so-called activated alloy phase separation growth approach. In the quantum-dot ensemble the average dot diameter amounts to 9±1 nm and electron ground and first excited states are formed. The dots exhibit a high photoluminescence efficiency in the technologically important 1.3 _m spectral range. Using photocurrent measurements and deep-level transient spectroscopy, we observed thermally assisted tunneling processes out of the quantum-dot electron ground state 190 meV below the GaAs conduction-band minimum. Even at room temperature the intersubband relaxation of photogenerated electrons from the first excited state into the electron ground state is faster than thermally assisted tunneling out of the first excited state; thus electrons always leave the dot from the ground state. We use the structural and optical properties of the InGaAs quantum dots to establish the energy diagram by self-consistent band-structure calculations. Moreover, by relating theoretical and experimental capacitance-voltage data, we show that below 75 K every quantum dot can be charged with up to three electrons, whereas at room temperature only two electrons are confined in every such quantum dot.