Robin Nicholas

Abstract:

Thermodynamic arguments are presented for the formation of atomic order in heteroepitaxially grown semiconductor quantum dots. From thermodynamics several significant properties of these systems can be derived, such as an enhanced critical temperature of the disorder-order transition, the possible co-existence of differently ordered domains of varying size and orientation, the possible existence of structures that have not been observed before in semiconductors, the occurrence of atomic order over time, and the occurrence of short range order when the growth proceeds at low temperatures. Transmission electron microscopy results support these predictions. Finally, we speculate on the cause for the observed increase in life time of (In, Ga)As/GaAs quantum dot lasers [H-Y. Liu et al., Appl. Phys. Lett. 79, 2868 (2001)].

Abstract:

We have measured the current-voltage (I-V) of type-II InAs/GaSb/InAs double heterojunctions (DHET's) with "GaAs like" interface bonding and GaSb thickness between 0 and 1200 Å. A negative differential resistance (NDR) is observed for all DHET's with GaSb thickness >60 Å below which a dramatic change in the shape of the I-V and a marked hysteresis is observed in some samples. The temperature dependence of the I-V is found to be very strong below this critical GaSb thickness. The I-V characteristics of selected DHET's are also presented under hydrostatic pressures up to 11 kbar. Finally, a midinfrared electroluminescence is observed at 1 bar with a threshold at the NDR valley bias. The band profile calculations presented in the analysis are markedly different to those given in the literature and arise due to the positive charge that it is argued will build up in the GaSb layer under bias. We conclude that the dominant conduction mechanism in DHET's is most likely to arise out of an inelastic electron-heavy-hole interaction similar to that observed in single heterojunctions with "GaAs like" interface bonding, and not out of resonant electron-light-hole tunneling as proposed by Yu et al. A Zener tunneling mechanism is shown to contribute to the background current beyond NDR.

Abstract:

Measurements of the reflectivity of a two-dimensional electron gas are used to deduce the polarization of the composite fermion of hole system formed for Landau level occupancies in the regime 1 < v <2. The measurements are consistent with the formation of a mixed spin CF system and allow the density of states or 'polarization' effective mass of the CF holes to be determined. The mass values at v = 3/2 are found to be ∼ 1.9me for electron densities of 4.4x1011 cm-2, which is significantly larger than those found from measurements of the energy gaps at finite values of effective magnetic field.

Abstract:

We report measurements of photoluminescence (PL) from self-assembled InSb quantum dots (QD's) grown by metal-organic vapor-phase epitaxy in a matrix of GaSb as a function of excitation power, temperature, and magnetic field. PL is observed in the region 1.7-1.8 μm from InSb quantum dots. For low excitation power the PL is dominated by the lowest quantum dot transition energy. When the excitation power is increased the quantum dot transition increases in energy by ∼ 11 meV, and further transitions are observed from the wetting layer, bulk acceptor, and free excitons. Magneto-PL is used to calculate the in-plane dot confinement energies by fitting the data to the ground state of a Fock-Darwin set of energy levels. The in-plane confinement energy deduced increases from ∼6 to ∼18 meV as the excitation power is increased. This is similar to the increase in the quantum dot transition energy, and suggests that this is due to a progressive population of a distribution of strongly communicating dots with decreasing lateral sizes. Further support for this picture comes from the temperature dependence of the quantum dot transition energy, which is also found to increase by a relatively similar amount as the temperature is raised from 11 to 50 K, following a correction for the temperature dependence of the bulk energy band gaps.