Anjana Bagga

Dark and bright excitonic states in nitride quantum dots

Formation of excitonic states in quantum dots of nitride based III-V semiconductors GaN and AlN including coulomb and exchange interactions are investigated. Dark exciton formation is found to occur for both GaN quantum dots(QDs) with wurtzite structure having positive crystal field splitting and GaN and AlN QDs with zero crystal field splitting with a transition from dark to bright exciton at about 40{\AA}. In wurtzite AlN QDs with negative crystal field splitting the splitting between the dark and bright excitonic states is very small and vanishes at about 15{\AA}.

Spin hot spots in vertically coupled few-electron isolated quantum dots

The effects of spin-orbit (SO) coupling arising from the confinement potential in single and two vertically-coupled quantum dots have been investigated. Our work indicates that a dot containing a single electron shows the lifting of the degeneracy of dipole-allowed transitions at B=0 due to the SO coupling which disappears for a dot containing two electrons. For coupled dots with one electron in each dot, the optical spectra is not affected by the coupling and is the same as the dot containing one electron. However, for the case of two coupled dots where one partner dot has two interacting electrons while the other dot has only one electron, a remarkable effect is observed where the oscillator strength of two out of four dipole-allowed transition lines disappears as the distance between the dots is decreased.

Controlling wave function localization in a multiple quantum well structure

The dynamics of a wave function describing a particle confined in a multiple quantum well potential is studied numerically. In particular, the case of four wells and six wells has been studied for the first time. As a consequence of quantum mechanical tunneling, an initial wavefunction designed to be localized in one well can localize in the others after a certain time and hop between wells at times which depends on the height and width of the barriers separating the wells. This control over the evolution of the wavefunction with time has direct implications in applications based on carrier dynamics in multiple quantum well nanostructures and can also provide novel mechanisms in solid state quantum computation for information storage and processing. The ability to include any number of wells and control the carrier dynamics in them through easily accessible parameters in our study makes this a particularly attractive system from the point of view of applications.

Tailoring of Seebeck coefficient with surface roughness effects in silicon sub-50-nm films

The effect of surface roughness on the Seebeck coefficient in the sub-50-nm scale silicon ultra thin films is investigated theoretically using nonequilibrium Greens function formalism. For systematic studies, the surface roughness is modelled by varying thickness periodically with square wave profile characterized by two parameters: amplitude (A0) and wavelength (λ). Since high Seebeck coefficient is obtained if the temperature difference between the ends of device produces higher currents and higher induced voltages, we investigate how the generated current and induced voltage is affected with increasing A0 and λ. The theoretical investigations show that pseudoperiodicity of the device structure gives rise to two effects: firstly the threshold energy at which the transmission of current starts is shifted towards higher energy sides and secondly transmission spectra of current possess pseudobands and pseudogaps. The width of the pseudobands and their occupancies determine the total generated current. It is found that current decreases with increasing A0 but shows a complicated trend with λ. The trends of threshold energy determine the trends of Seebeck voltage with roughness parameters. The increase in threshold energy makes the current flow in higher energy levels. Thus, the Seebeck voltage, i.e. voltage required to nullify this current, increases. Increase in Seebeck voltage results in increase in Seebeck coefficient. We find that threshold energy increases with increasing A0 and frequency (1/λ). Hence, Seebeck voltage and Seebeck coefficient increase vice versa. It is observed that Seebeck coefficient is tuneable with surface roughness parameters.

Stokes shift in quantum dots: Origin of dark exciton

A very interesting feature of semiconductor quantum dots(QDs) is the redshift of emission peaks with respect to absorption spectra and its size dependence. The red shift of the emission spectra with respect to absorption spectra is known as Stokes shift, which is commonly observed in semiconductor quantum dots (QDs) and is one of the most important quantities that determine the optical properties of QDs. As the radius increases the redshift decreases and disappears beyond a certain radius. The mechanism of Stokes shift in semiconductor quantum dots is investigated by calculating the energy of the excitonic states. We have taken into account all possible contributions to the total electronic energy in the dot, i.e, dielectric mismatch between dot and surrounding medium, the effects of finite barrier height and electron-hole exchange interaction. The Stokes shift is calculated as a function of radius of dot and compared with experimental data on two different semiconductor based quantum dots. These results provide evidence for exchange splitting of excitonic states, as the mechanism of Stokes shift in QD.

Surface roughness effects on seebeck coefficient in silicon ultra thin films

Surface roughness effects play a crucial role in determining the performance of a device with dimensionality in the sub-50nm scale. In this work we theoretically investigate surface roughness effects on Seebeck coefficient of ultrathin silicon film using non equilibrium Greens function technique. To systematically study surface roughness effects, thickness of the film is varied periodically with square wave profile characterized by two parameters: amplitude(A0) and wavelength(λ). The results show that Seebeck coefficient increases with increasing roughness amplitude and frequency(1/λ). It is found that current which flows through the device, due to temperature gradient, is reduced due to surface roughness. It is interesting to note that, though the current decreases, the voltage required to nullify this current increases. Due to this increase in voltage Seebeck coefficient increases.

Magnetic field effects on intersubband transitions in quantum cascade structures

We report on our study of the effects of external magnetic field on the intersubband optical transitions in quantum cascade systems. We address the properties of two types of cascade structures: terahertz quantum cascade lasers and quantum dot infrared photodetectors. In both cases we study the optical properties of active regions of the systems: optical emission in the case of quantum cascade lasers and optical absorption in the case of photodetectors. The new features and new peaks in the optical spectra appear in the tilted magnetic field for quantum cascade lasers and in the parallel magnetic field for quantum dot photodetectors. In the relation to the possible spintronic application of cascade structures we study interplay of spin-orbit and magnetic field effects in cascade systems. If spin-orbit coupling is strong enough then at finite parallel magnetic field the optical emission spectra of quantum cascade lasers have two-peak structure.

Spin-orbit interaction in a quantum cascade transition

We have investigated the effect of spin-orbit (SO) coupling on the emission spectra of a quantum cascade laser. In an externally applied magnetic field parallel to the electron plane, the SO coupling would result in a double-peak structure of the optical spectra. This structure can be observed within some interval of magnetic fields and only for diagonal optical transitions when the SO coupling is different in different quantum wells.