M.Sc Thesis

M.Sc StudentRuslan Presman
SubjectOptical Control of the Charge State of Semiconductor
Quantum Dots
DepartmentDepartment of Physics
Supervisor Full Professors Gershoni David
Full Thesis textFull thesis text - English Version


Self-assembled semiconductor quantum dots are nanometer size droplets of semiconductor of one type embedded in a semiconductor of another type. They localize charge-carriers in three dimensions. This localization results in a discrete set of energy levels for charge carriers (electrons and holes), resembling in many ways spectra of natural atoms and molecules. This “atomic-like” spectral feature of QDs, together with their compatibility with modern semiconductor-based microelectronics and optoelectronics, make QD-based devices particularly promising as building blocks for future technologies involving quantum information processing.

In the current work, we use our detailed understanding of the many-carrier confined states of single QDs and the various optical transitions between these states to demonstrate that the charge state of QDs can be determined almost at will by very weak intensities of light. The light itself is too weak to generate photoluminescence (PL) emission on its own, but its photon energy determines the average charge in the QDs. By changing the light color from red (700 nm) to violet (406 nm), we succeeded in varying the QD charge state from, on average, doubly positively charged to negatively charged, respectively. For a given excitation wavelength, the average charge was different, as judged by the intensity of the emission from various charged exciton states. By continuously controlling the wavelength, any desired average charge state can be obtained.

We qualitatively understand this charging control mechanism in terms of deionization of ionized impurity centers in the vicinity of the QD, preferentially by electrons, which have a much higher kinetic energy under these above-bangap excitation energies.

We performed a detailed study of the PL spectra of single QDs as a function of excitation energy and intensity. Under these conditions, we also performed polarization-sensitive auto- and cross- correlation measurements of the emission intensity from various well-identified spectral lines. All these measurements are favorably compared with a relatively simple rate equations model, which takes into account light-induced charging and discharging, spin-conserving and non-conserving relaxations, and optical recombinations. The PL spectra are obtained from the steady state solutions of our rate equation model, and the correlation measurements are obtained by setting the initial and final states to the identified polarized emission lines. We succeed in obtaining quantitative agreement between all these measurements using a small number of adjustable parameters. In particular, our model reveals the role and importance of dark states in the PL spectrum of semiconductor quantum dots.