Optical characterisation of III-V semiconductor quantum dots and quantum dot structures
This thesis describes an extensive study of the optical properties of In(Ga)As/Ga(Al)As quantum dots, both singularly and in laser devices. For the optical characterisation, the spectroscopic techniques of photoluminescence (PL), photoluminescence-excitation (PLE) and electroluminescence (EL) have been used. Additionally, for studying submicron structures, sophisticated micro-PL techniques allowed excitation, and detection, of a single quantum dot. A variety of special growth and fabrication techniques were used to isolate a single quantum dot from the rest of the ensemble. Firstly, a submicron mesa that isolated a single quantum dot was fabricated using e-beam lithography and dry etching techniques. Excitation intensity and wavelength dependence of the emission provided information about the nature of the exciton complexes in the dot and the absorption and relaxation of carriers in the structure. The linewidth of the single exciton, and the variation with temperature, was measured. Secondly, samples consisting of a single layer of InGaAs quantum dots incorporated within the intrinsic region of a GaAs/AlGaAs Metal-Insulator-Semiconductor type Schottky structure allowed sequential dot charging via variation of the gate bias. Both n- and p- type samples, grown under similar conditions, were investigated. Single quantum dots were probed through submicron apertures opened in the top metal contact using electron beam lithography and dry etching. The emission undergoes several pronounced changes as additional carriers are sequentially loaded into the dot. The results reveal direct information regarding Coulomb interaction and correlation effects in dots containing a controlled number of excess electrons (up to four) or holes (up to two). Magneto- optical results reveal further information about the few-particle wave functions. A detailed study of the device performance and physical processes in In(Ga)As- Ga(Al)As self-assembled quantum dot lasers showed that these lasers offer several potential advantages over conventional quantum well lasers including low threshold current density, temperature insensitive threshold current density and high differential gain. Device performance (threshold current density and its variation with temperature) has been studied as a function of a number of parameters, including dot density, dot composition and dot confinement potential. The application of large (<14T) magnetic fields has been used to study carrier transport and dot carrier capture effects. When applied along the growth axis, such fields result in an increase in the threshold current density and a decrease in the external quantum efficiency. These effects are attributed to inhibited inplane carrier transport resulting in an increase in the carrier capture efficiency by non-lasing dots. Spontaneously emitted light, recorded via small windows formed in the top metallic contact, has been studied as a function of injection current and temperature. Analysis of this data provides information on non-radiative loss mechanisms and dot carrier dynamics.