Hot-electron effects in two-dimensional hopping with a large localization length

We have studied nonlinear effects in the resistance of a two-dimensional system with a large localization length on both sides of the crossover from weak to strong localization. It is shown that nonlinearity in the hopping regime is due to electron overheating rather than the field effects. This qualitatively new behavior is a signature of a two-dimensional hopping transport with a large localization length.

Fabrication of a quantum point contact by the dynamic plowing technique and wet-chemical etching

We have fabricated extremely confined ballistic constrictions using a nanolithography technique based on an atomic force microscope. Vector-scan controlled dynamic plowing with the vibrating tip enables to plastically indent a thin resist layer along a prearranged path. Transfer of the resist pattern into the semiconductor substrate is achieved by a strongly diluted aqueous etchant. In this way approximately 30 nm deep gooves were etched in the channel area of a modulation-doped GaAs/GaA1As field-effect transistor. The quantum point contacts were defined by a broken line whose 60 nm width represents the length and the sub-100 nm gap determines the width of the constriction. At liquid-helium temperature the conductance as a function of gate voltage shows a stepwise increase in units of 2e2/h. Signatures of the conductance quantization persist up to 50 K, which indicates a large subband spacing.

Nanoscale devices fabricated by direct machining of GaAs with an atomic force microscope

We demonstrate a lithography wherein the tapping mode of an atomic force microscope the Si tip is used as a chiseling tool for direct machining of a GaAs surface. Single-groove drawing movements in a vector-scan mode result in approximately 3-4 nm deep and 30 nm wide furrows, which can be combined to arbitrary noncontiguous polygon patterns. Beneath such a groove a barrier arises in the electron channel of a GaAs/A1GaAs modulation-doped field effect transistor (MODFET). Using appropriate sub-100 nm line patterns we prepared quantum point contacts and single electron devices. At T = 4.2 K the transconductance characteristics of these nanoscale MODFETs exhibit structures, which represent signatures of either the quantized conductance or Coulomb-blockade effects.