Optical Spectroscopy of Self-Assembled In(Ga)As Quantum Dots
Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH
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A reliable knowledge of the shape and compositional profile of buried InAs / GaAs self-assembled quantum dots (QDs) is of utmost importance for making reliable comparisons of experimental data with theoretical calculations of the electronic and optical properties. We demonstrate that photocurrent spectroscopy (PC) provides a simple means with which the absolute absorption spectra and fundamental inter-band optical properties of QDs can be investigated directly. Studies in high external electric field (Fz) reveal microscopic information regarding the magnitude and sign of the electron-hole vertical (z-axis) alignment (excitonic dipole). Surprisingly, the sign of the observed dipole for our QDs is found to be opposite to many recent calculations, an observation that can only be reconciled with theory if the dots have a severely truncated shape and the indium composition of the dots increases strongly towards the apex. Complementary, high-resolution cross-sectional STM measurements performed on the same QD material confirm these predictions, demonstrating that the QDs have a truncated pyramidal cross sectional shape and an increasing In content towards QD apex.
Temperature dependent PC and photoluminescence (PL) measurements enable us to investigate carrier capture and escape timescales for the dots. We find that carrier capture from the barrier material is extremely sensitive to the external electric field, being completely suppressed for Fz>20kV/cm. In contrast, carriers resonantly excited into the QDs are much less sensitive to Fz, radiative recombination dominating up to ~100kV/cm.
Spectral resolution limitations associated with inhomogeneous broadening can be completely circumvented by optically probing individual QDs. Photoluminescence investigations of neutral and charged exciton and bi-exciton states in single In(Ga)As QDs will be presented. These measurements enable us to measure directly the influence of Coulomb interactions on the optical properties of a single QD containing up to four (Ne=4) quasi-equilibrium electrons. By carefully controlling the experimental conditions, single dot PL spectra are dominated by charge neutral, single exciton (X0) recombination. Following electron charging of the dot using a gate electrode, X0 quenches and is replaced by the negatively charged single exciton (X-). For a range of dots investigated, the X0®X- energy shift between two (X0 = e + h) and three (X- = 2e + h) multi-particle configurations is –5.5±0.8meV. Subsequent electron charging events (X2- etc.) result in the sequential appearance of characteristic emission multiplets arising from distinct multi-particle configurations of the many body system. In addition to charging of single exciton species, bi-exciton recombination (2X0) is observed (DE[X0®2X0]=-2±0.3meV) at higher excitation levels. The charging behaviour of 2X0 is found to be quite different to X0, arising from the different multi-particle configurations in the QD. In particular, the 2X0®2X- energy splitting is small (DE[2X0®2X-]=-0.7±0.2meV) demonstrating that the excess (p-level) electron interacts rather more weakly with the completely filled (2e + 2h) s-levels. These observations are indicative of a shell-like energy level structure for the many particle dot.