Thermal shift of the resonance between an electron gas and quantum dots: What is the origin?

Abstract

The operation of quantum dots at highest possible temperatures is desirable for many applications. Capacitance-voltage spectroscopy (C(V)-spectroscopy) measurements are an established instrument to analyze the electronic structure and energy levels of self-assembled quantum dots (QDs). We perform C(V) in the dark and C(V) under the influence of non-resonant illumination, probing exciton states up to $X^{4+}$ on InAs QDs embedded in a GaAs matrix for temperatures ranging from 2.5 K to 120 K. While a small shift in the charging spectra resonance is observed for the two pure spin degenerate electron s-state charging voltages with increasing temperature, a huge shift is visible for the electron-hole excitonic states resonance voltages. The $s_2$-peak moves to slightly higher, the $s_1$-peak to slightly lower charging voltages. In contrast, the excitonic states are surprisingly charged at much lower voltages upon increasing temperature. We derive a rate-model allowing to attribute and value different contributions to these shifts. Resonant tunnelling, state degeneracy and hole generation rate in combination with the Fermi distribution function turn out to be of great importance for the observed effects. The differences in the shifting behavior is connected to different equilibria schemes for the peaks; s-peaks arise when tunneling-in- and out-rates become equal, while excitonic peaks occur, when electron tunneling-in- and hole-generation rates are balanced.