Engineering the electronic band structure for multiband solar cells

Using the unique features of the electronic band structure of GaN(x)As(1-x) alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the band anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.     

Conductance peak splitting in charge polarized coupled quantum dots

We report a measurement of linear conductance through a series double dot as a function of the total double dot charge and the charge difference for interdot tunnel conductances between zero and one mode. The dots are defined by ten independently tunable electrostatic gates on the surface of a GaAs/AlGaAs heterostructure to allow separate adjustment of dot charge and interdot conductance. For weak interdot tunneling the measured double dot conductance agrees with a transport model in which each dot is individually governed by Coulomb blockade theory. As interdot tunnel conductance increases toward one mode, the measured conductance peak positions and shapes indicate both a relaxation of the charge quantization condition for individual dots and quantum mechanical charge sharing between dots. The results are in quantitative agreement with many body charge fluctuation theory.     

A concept for a tunable antenna-coupled intersubband terahertz (TACIT) detector

Detectors based on intersubband transitions in quantum wells have great potential for use between one and several terahertz. We propose a tunable, antenna-coupled, intersubband terahertz (TACIT) detector that is both sensitive and fast, with a speed limited only by the intersubband relaxation rate (1 ns at at ). The detector is sensitive over a narrow range of frequencies, and the frequency of peak absorption can be tuned by applying a bias voltage to the device.     

Effect of a charged scanned probe microscope tip on a subsurface electron gas

Using a cryogenic scanned probe microscope (SPM) one can locally modify the sheet density of a two-dimensional electron gas (2DEG), and image the ballistic flow of electrons through a point contact in the 2DEG located beneath the surface of a GaAs/AlGaAs heterostructure. We calculate the capacitively induced change in sheet density when a charged SPM tip is brought into contact with a semiconductor heterostructure containing a two-dimensional electron gas. A simple scattering model, based on a local change in sheet density, is used to analyze experimental data taken with the SPM method described above.     

Manifestation of the bulk phase transition in the edge energy spectrum in a two-dimensional bilayer electron system

We use a quasi-Corbino sample geometry with independent contacts to different edge states in the quantum Hall effect regime to investigate the edge energy spectrum of a bilayer electron system at a total filling factor of ν=2. By analyzing nonlinear I–V curves in normal and tilted magnetic fields, we conclude that the edge energy spectrum is in a close connection with the bulk one. At the bulk phase transition spin-singlet-canted antiferromagnetic phase, the I–V curve becomes linear, indicating the disappearance or strong narrowing of the ν=1 incompressible strip at the edge of the sample.     

Spin-orbit coupling,antilocalization, and parallel magnetic fields in quantum dots

We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantum dots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization.