D. Heitmann

One‐dimensional electronic systems in ultrafine mesa‐etched single and multiple quantum well wires

Ultrafine mesa‐etched structures with lateral geometrical dimensions of 250 to 550 nm have been prepared in modulation‐doped AlGaAs/GaAs heterostructures and multiple quantum well systems. From magnetotransport measurements at low temperatures (T=2.2 K) on these single and multiple quantum well wire structures we find that the lateral carrier confinement leads to the formation of one‐dimensional electronic subbands of typically 2 meV energy separation. The width of the electron channel is smaller than the geometrical width, indicating a lateral depletion length of about 100 to 150 nm at the mesa edge.

One‐dimensional lateral‐field‐effect transistor with trench gate‐channel insulation

A novel unipolar transistor device has been realized starting from two‐dimensional electron systems (2DES) in modulation‐doped AlGaAs/GaAs heterostructures. A 600‐nm‐wide 1D channel is insulated laterally from 2DES regimes by 700‐nm‐wide deep mesa etched trenches. The conductivity in the quasi‐one‐dimensional channel can be tuned via the in‐plane lateral field effect of the adjacent 2DES gates where the vacuum (or air) in the etched trenches serves as the dielectric. Room‐temperature operation is demonstrated yielding a 17 μS transconductance corresponding to 170 mS/mm 2D transconductance.

Optical determination of carrier density in pseudomorphic AlGaAs/InGaAs/GaAs hetero-field-effect transistor structures by photoluminescence

A photoluminescence (PL) analysis of a highly degenerate two‐dimensional electron gas (2DEG) in pseudomorphic modulation‐doped AlGaAs/InGaAs/GaAs transistor structures is reported. The PL response from samples with one or two populated electron subbands is dominated by one or two spectral bands, respectively, with a high‐energy intensity cutoff. The spectral width varies linearly with the measured 2DEG sheet density ns or with a Schottky barrier depletion voltage, which directly reflects the two‐dimensional density of states (2DDOS) below the Fermi level. We used the effective electron mass from cyclotron resonance experiments to evaluate the 2DDOS and can thus directly determine ns from the spectral width via the 2DDOS. Independent ns values were obtained from Shubnikov–de Haas measurements and agree excellently with ns values from PL.

Preparation of one-dimensional single and multi-layered quantum wire structures by ultrafine deep mesa etching techniques

Abstract We report on optimized holographic lithography and reactive ion etching processes to prepare by deep mesa etching techniques ultrafine structures with lateral dimensions of 200 nm to 550 nm in modulation doped AlGaAs/GaAs heterostructures and multi quantum well systems. These structures exhibit a one-dimensional (1D) electronic behaviour with discrete 1D subbands of typically 1 meV to 2 meV energy separation. The lateral width of the 1D electron channels is smaller than the geometrical width, indicating a lateral edge depletion region which, however, for an optimized process is only 100 nm on either side of the 1D wires.

Observation and analysis of quantum wire structures by high-resolution X-ray diffraction

We report on the double-crystal X-ray diffraction analysis of AlGaAs/GaAs quantum wire structures. We show that X-ray scattering is very sensitive to the modulation of structural parameters parallel to the crystal surface under certain diffraction conditions. The experimental diffraction patterns exhibit quantum wire satellite peaks which allow us to determine the quantum wire period and the quantum wire width. Furthermore, we found that due to the finite lateral width of the quantum wires a partial asymmetric strain relaxation of the unit cell occurs resulting in an orthorhombic lattice deformation.

Spectroscopy of quantum dots and antidots

Abstract The remarkable progress of submicron technology has made it possible to realize man-made low-dimensional electronic systems. Starting from two-dimensional electronic systems (2DES) in semiconductor heterostructures the electrons are further confined by lateral potentials acting on a submicron scale. This induces quantum confined energy states such that, for wires, a set of one-dimensional subbands with free dispersion in only one direction is formed or, for dots, artificial “atoms” with a totally discrete energy spectrum are obtained. A reversed structure with respect to quantum dots are “antidots” where geometrical holes are “punched” into an originally 2DES. These low-dimensional systems exhibit unique properties. In this review we would like to discuss some recent results on far-infrared excitations in quantum dots and antidots.

Quantum wires prepared by molecular beam epitaxy regrowth on patterned AlGaAs buffer layers

Modulation doped GaAs quantum wires are prepared by molecular beam epitaxy regrowth on patterned AlGaAs buffer layers. The structural properties are investigated by scanning electron microscopy and by transmission electron microscopy. Far‐infrared spectroscopy provides information about the lateral confinement and the carrier density in the quantum wires. The measurements indicate a distinct dependence of the electronic width on the orientation of the quantum wires within the (100) plane. Confinement energies of 6.9, 9.3, and 11.7 meV are determined for the [011], [001], and the [011] wire orientations, respectively.

Preparation of quantum wires and quantum dots by deep mesa etching of AlGaAs-GaAs and InGaAs-InAlAs heterostructures

Abstract A1GaAs-GaAs and InGaAs-InA1As quantum wires and dots have been fabricated by ultrafine deep mesa etching through holographically defined mask. We report on the optimization of mask preparation and dry etching techniques which make it possible to realize quantum confined electronic systems of low dimensions.

Plasma and single particle excitations in quasi-one-dimensional electron systems

Abstract The equilibrium properties and dynamic response of quasi-one-dimensional electron systems (1DES) have been calculated for a confinement modelled by a harmonic oscillator potential. These calculations are compared with self-consistently calculated 1D subband separations in a split-gate configuration. It is shown that in the limit of a vanishing occupation the classical dynamic response frequency coincides with the subband separation. With increasing number of occupied 1D subbands the dynamic response frequency decreases slightly whereas the subband separation decreases drastically. Thus, for a large number of occupied 1D subbands, the dynamic response frequency is significantly higher than the 1D subband separation as is observed in experiments.

Quantum wires prepared by liquid‐phase‐epitaxial overgrowth of dry‐etched AlGaAs‐GaAs heterostructures

We have developed a technique to overgrow deep‐mesa‐etched AlGaAs‐GaAs quantum wires with AlGaAs by liquid‐phase epitaxy and achieve improved interfacial properties. In particular, we find a significant reduction of the surface‐state density and related lateral edge depletion in modulation‐doped quantum wires. We demonstrate the formation of a quasi‐one‐dimensional electron system by far infrared spectroscopy.