Terahertz Ionization of Highly Charged InGaAs Quantum Posts

Over the last decade, the optoelectronic properties of self-assembled semiconductor quantum dots (QDs) have been studied intensely. Quantum dots confine carriers in the conduction and valence band three dimensionally, acting as artificial atoms with energy levels that are tunable based on the dot’s geometry and material parameters. We have recently introduced a novel quantum dot based nanostructure grown by molecular beam epitaxy [1] called the quantum post (QP). These structures - whose height can be controlled with nanometer precision - form short cylindrical structures (average composition In.4Ga.6As) aligned along the growth direction. The QPs are embedded in an In.1Ga.9As quantum well (QW) of the same height as the QPs. The three dimensional confinement of the post structures produces quantized states. The energies of these states relative to the Fermi level of the structure are strongly dependent on the amount of charge in the QPs. This strong dependence is due to the similarity of the electron-electron Coulomb repulsion energies in the QPs and the spacing between their electronic states (~10 meV).
Here we report absorption from electrons transitioning from three dimensionally confined QP states to one dimensionally confined states of the two dimensional electron gas in the surrounding QW. For the absorption experiments, a single QP layer was embedded between an n-doped back- gate and a Schottky-contact, allowing for controllable loading of electrons into the QPs and surrounding quantum well. The electronic loading behavior was studied by capacitance- voltage spectroscopy, which shows distinct loading-features of electrons into the QPs and the QW. The absorption measurements were performed at a voltage where the posts are completely filled (~6 electrons per post) and the well is filled to a sheet charge density of ~2.4x1011/cm². Absorption measurements were taken in an FTIR with the THz light polarized parallel to the growth direction to couple only to states arising from vertical confinement. Comparison of spectra from a sample containing quantum posts and a reference quantum well sample show an absorption feature due to the QPs entirely absent in the QW sample.
Temperature dependent absorption measurements show that this absorption is due to a transition that begins in the QP. Using 8 band k.p calculations of post and well energies as a function of the number of electrons in the posts, the absorption is determined to be from an ionizing transition from the posts to the well. The highest filled state in the posts absorbs a terahertz photon, making a transition to a weakly bound unfilled post state ~20 meV higher. From this state, the electron quickly scatters into the quantum well matrix. The Coulomb repulsion of the QP electrons locally depopulates the quantum well states, leaving open states for the electrons to scatter into despite the well’s large average charge density. These results represent a promising structure for investigation of Coulomb blockade physics, as well as the physics of ionizing transitions that occur when Coulomb effects are on the same energy scale as the ionizing transition, a regime only accessible in these artificial atom systems. Additionally, these ionizing transitions hold promise for use in terahertz infrared photodetectors with their three dimensional confinement giving reduced sensitivity to temperature compared to their quantum well based equivalents.
This work is supported by the NSF NIRT grant No. CCF 0507295 and the Alexander von Humboldt Foundation.
References [1] Krenner, H. J. & Petroff, P. M., Sol. St. Comm. 149, 1386 (2009).