Two‐dimensional, self‐consistent solutions of the Schrodinger and Poisson equations are used to find the electron states in GaAs/AlGaAs quantum well wires. Both deep and shallow mesa structures are simulated. Our results show that while these structures are capable of providing the single occupied subband and wide energy separations needed for a true quantum wire, the process tolerances allowed are very small, on the order of 200 A of width variation. Cutoff widths calculated are 1000 A for the shallow mesa and 2100 A for the deep mesa. The agreement with experimental results is good for the shallow mesa, but poor for the deep mesa. This suggests additional process‐induced sidewall depletion mechanisms contributing to the cutoff of the deep mesa structures.

Electron states in mesa‐etched one‐dimensional quantum well wires

Reviews the transport properties of electrons in semiconductor heterojunction structures, in which the degrees of freedom for motion of the charge carriers are reduced by confining potentials, thereby producing low- (i.e. two-, one- or even zero-) dimensional electronic structures. A theoretical treatment of quantisation effects due to two-dimensional (2D) and one-dimensional (1D) confinement, with and without an applied magnetic field, will be given, and used to interpret a range of experimental results obtained from GaAs/AlxGa1-xAs 2D and 1D heterojunction structures. For 2D samples, these results cover low-field mobility measurements, which give information on the scattering mechanisms present, and high-field magnetotransport studies, including the quantum Hall effect and parallel conduction processes. In 1D structures, quantum and ballistic transport processes will be discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/52/10/002/pdf

Electronic transport in low-dimensional structures

A simple method is described for calculating electronic states in narrow semiconducting wires near threshold, where there are few electrons in the system and the potential in which the electrons move can be found purely from electrostatics. The pinning of the chemical potential on the surface of GaAs and (Al,Ga)As means that the surface can be treated as an equipotential, a great simplification that permits analytic solutions. The method is applied to wires fabricated by several techniques, either in epitaxial or modulation-doped layers. The width of the wavefunction can be reduced to about 20 nm in a typical shallow-etched wire or one 'squeezed' under a split gate. Energy levels for motion parallel to the interface are about 10 meV apart while those for motion normal to the interface are separated by about 40 meV. These relative scales are similar to those calculated by Laux and Stern (1986, 1988) for narrow channels in silicon. Confinement can be improved by putting the electrons closer to the surface. Comparison with the numerical work of Laux, Frank and Stern (1988) on squeezed wires in GaAs shows some differences which are traced to the appearance of an inversion layer of holes when there is a large negative bias on the gate. It is possible to define a depletion width for a two-dimensional electron gas with this method, as half the width of a deep etched mesa at the point where electrons just appear. Agreement with the experiments of Choi, Tsui and Alavi (1987) is acceptable, but highlights the problem of defining a unique depletion width in a two-dimensional system.

Electronic states in narrow semiconducting wires near threshold

Raman scattering from arrays of GaAs quantum cylinders with radii down to 30 nm has been observed. The spectra show an enhanced TO phonon intensity due to a change in the first-order selection rules. In addition, the smallest cylinders exhibit an extra spectral feature between the TO and LO phonons. The behaviour of this additional peak as a function of the scattering geometry, the cylinder size and the variation of the medium surrounding the cylinders is consistent with a theoretical model of the surface phonons of a cylinder.

Surface phonons in GaAs cylinders

The ability to fabricate structures with lateral dimensions in the sub-100-nm range has opened a new field of research. This paper first reviews recent advances in nanolithography techniques, with a brief discussion of their relative merits and fundamental limits. Special emphasis is given to the scanning electron-beam method, which is the most widely used nanolithography method at the present time. The two main areas of nanostructure research are device technology and basic science. Highlights of a number of exploratory programs in these two areas are presented.

Nanostructure technology

Conducting wires of widths down to 0.1 μm have been fabricated in modulation‐doped GaAs/AlGaAs material using SiCl4 reactive ion etching with a negative resist mask. The resist used was high‐resolution negative (HNR) and its applicability to microstructure fabrication is discussed. A novel dry etch for GaAs, CH4/H2 reactive ion etching, produces low damage and shows much promise for wire fabrication.

Fabrication of quantum wires in GaAs/AlGaAs heterolayers

MESFETs with 0.17μm gate length were manufactured with an n+GaAs active layer (3 × 1018cm-3) and an undoped Ga0.3Al0.7As buffer layer grown by molecular-beam epitaxy. The deives showed very high transconductance (700mS/mm) with good pinchoff characteristics. The experimental transconductance values were compared with calculated ones using a model that assumed total carrier confinement within the active layer by a barrier potential at the GaAs/GaAlAs interface. The results suggest that very high-transconductance short-gate-length MESFETs can be fabricated with a heavily doped GaAs active layer provided that the carrier density in the active layer is maintained at the doping level.

M. Al-Mudares

Papers

Fabrication of quantum wires in GaAs/AlGaAs heterolayers

Very high-transconductance short-channel GaAs MESFETs with Ga0.3Al0.7As buffer layer