Intraband spectroscopy of excited quantum dot levels by measuring photoinduced currents

The development of semiconductor quantum dot (QD) devices particularly for the infrared requires information on their intraband energy levels. For most of the samples FTIR spectroscopy does not provide reliable results on intraband transitions in the mid-infrared due to very low absorption signals. In recent years photocurrent spectroscopy was demonstrated as an alternative method. In this paper we present a modified photocurrent method, which does not require complicated contact structures or contact layers. The samples are laterally contacted by simply soldering wires and are biased by several 10 V. The intraband resonances are monitored via current changes induced by optical excitation with intense picosecond mid-infrared laser pulses. Both bound to bound and bound to continuum transitions enhance the current. We present data taken on two highly n-doped GaAs/InAs QD samples with different doping concentration at 77 K and at room temperature. In this way, a nearly complete picture of intraband transitions between excited levels in the conduction band is obtained. The intraband spectra obtained with this technique nicely agree with calculated QD intraband levels.