Ballistic transport of AlAs two-dimensional electrons

The success of our modern electronics age stems from our advanced technology to process information by manipulating electrons in solid-state devices. Among the few fundamental ways to manipulate the electrons, such as using their charge and spin, the control and manipulation of the electron's valley degree of freedom in semiconductors remains practically unexplored. In this thesis we focus on some basic aspects of ballistic transport in a two-valley two-dimensional electron system (2DES), realized in high quality AlAs quantum wells.; We start by demonstrating valley-resolved ballistic transport in an experiment using a Hall bar device with a surface grating. From the analysis of the frequencies of the commensurability oscillations in the magnetoresistance at various densities we deduce the mass anisotropy factor, namely the ratio of the longitudinal and the transverse effective masses, ml/ mt = 5.2 +/- 0.5, a fundamental parameter for the anisotropic conduction bands in AlAs. We then present results from similar experiments in devices with antidot lattices that reveal peaks in magnetoresistance. Through an analysis of the positions of the peaks associated with the smallest commensurate orbit, we obtain a value for the mass anisotropy factor, ml/mt = 5.2 +/- 0.4, consistent with the value deduced from the surface-grating samples.; The anisotropy of the effective mass can be exploited to realize a simple "valley filter" device using a quantum point contact (QPC), a one-dimensional quantum ballistic channel. This device may play an important role in "valleytronics" or valley-based electronic applications. Our experiments on the QPC in the AlAs 2DES reveal that the conductance of this system is nearly quantized at multiples of 2e2/h, instead of 4e2/h as expected from a valley and spin degenerate system. This observation indicates a broken valley degeneracy due to the mass anisotropy as well as residual strain in the QPC.; Finally, we demonstrate a novel giant piezoresistance effect in an AlAs 2DES with an antidot lattice. Such a device may have potential applications as an ultra-sensitive strain sensor. It exemplifies one of the many uses of manipulating the electron valley degree of freedom in a solid-state device.