H. Künzel

Simultaneous modulation of electron and hole conductivity in a new periodic GaAs doping multilayer structure

A new periodic n‐p doping multilayer structure in GaAs grown by molecular beam epitaxy is presented, in which the concentration of electrons and holes in the respective doping regions and thereby the conductivity can be varied simultaneously by more than 1012 carriers cm−2 per layer by an external potential difference eUnp applied between the constituent n‐ and p‐type layers. The intriguing properties of this prototype of a bulk multijunction field‐effect transistor structure result from a suppression of the direct electron‐hole recombination by a space‐charge potential periodically modulated in the direction of layer growth. The observed data are in excellent agreement with the calculated characteristics using the preselected design parameters of the structure.

Photoluminescence study of electron-hole recombination across the tunable effective gap in GaAs n-i-p-i superlattices

Abstract The excitation-intensity dependence of photoluminescence from the tunable band gap in GaAs n-i-p-i superlattices has been studied as a function of laser frequency, and of thickness and doping concentration of the constituent layers of the superlattice. The results demonstrate that with increasing doping concentration the luminescence frequency may be tuned even over a wider range than previously reported. This increased tunability range and excitation-intensity dependence of the luminescence agrees with our theoretical expectations.

Tunable electroluminescence from GaAs doping superlattices

The first observation of strongly tunable electroluminescence from a new type of semiconductor superlattice is reported. We achieved a shift of the electroluminescence peak energy by several hundred millielectron volts below the gap of pure GaAs, when electrons and holes are injected over long distances via selective electrodes. This directly reflects the tunability of the indirect gap in real space, which is a unique feature of doping superlattices. The results further demonstrate that with increasing doping concentration of the constituent superlattice layers the luminescence frequency can be tuned over an even wider range. This behavior agrees well with our calculations.

Investigation of persistent photoconductivity in Si-dopedn-Al x Ga1?x as grown by molecular beam epitaxy

The persistent photoconductivity effect in Si-dopedn-AlxGa1−xAs layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10–300 K. In the alloy composition range 0.25<x <0.40 the electrical properties ofn-AlxGa1−xAs are governed by a deep electron trap having an emission barrier of 0.34–0.40 eV (depending on the doping concentration), as determined by admittance measurements. The concentration of deep electron traps, deduced from low-temperature capacitance measurements, is found to coincide with the amount of persistent photoconductivity observed in the material. Consequently, the earlier proposed population of two-dimensional subbands at the AlxGa1−xAs/GaAs-substrate hetero-interface, i.e. charge separation bymacroscopic barriers, can not account for the measured high overall number of persistent photoexcited carriers. Instead, the vanishing small capture rates of photoexcited electrons result frommicroscopic capture barriers. The dominant deep electron trap, which we attribute to deep donor-type (DX) centers, is found to be homogeneously distributed throughout the AlxGa1−xAs layer depth. From our Hall effect measurements a trap depth of 0.05–0.12 eV (depending on the doping concentration) below the conduction band is derived. The capture barrier is thus in the order of 0.30 eV. This value is in excellent agreement with data obtained from liquid phase epitaxially grown Si-dopedn-AlxGa1−xAs.

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦Ts≦600[°C]) practicable forn-type layer growth. Using a constant As4 to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingTs. Growth at 500° ≦Ts≦600[°C] produces high-qualityn-GaAs: Ge films (ND/NA≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [GeAs] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [CAs] decreases thus leading to the well-establishednonlinear behaviour of the (NA/ND vs. 1/Ts plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([GeAs]/[GeGa]) vs. 1/Ts plot which has exactly the same slope as the (PAs2/PGa) vs. 1/Ts plot derived from vapour pressure data of As2 and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.

Improvedp/n junctions in Ge-doped GaAs grown by molecular beam epitaxy

Abstractp/n junctions in Ge-doped GaAs consisting ofp-type layers with extremely smooth surfaces and low compensationn-type layers were fabricated by molecular beam epitaxy (MBE). During growth the site occupancy of Ge was controlled by varying the substrate temperature fromTs<500°C (n-type layers) toTs>600°C (p-type layers) at aconstant As4 to Ga flux ratio of 2. This yields stable growth conditions for the generation of Ga and As vacancies, respectively.

Quantum transport in GaAs doping superlattices

The resistance of the n channels in GaAs doping superlattices shows clear oscillations as a function of the magnetic field, and varies with the voltage applied between the n and the p layers and with the orientation of the magnetic field with respect to the doping layer plane. These experimental results are analyzed in terms of the two‐dimensional conductivity of the system, and the subband separation is derived from a comparison between the measurements and model calculations.

Quantum transport of electrons confined in a thin GaAs layer by an impurity space charge potential in high magnetic fields

Abstract The resistance of a thin, n-doped, GaAs layer sandwiched between p-doped layers shows clear quantum oscillations as a function of the magnetic field which depend on the orientation of the magnetic field with respect to the layer plane. The data for parallel and perpendicular magnetic fields are analyzed in terms of two dimensional conductivity using a simple model and the subband structure is deduced consistently from both experiments.

Determination of photoexcited carrier concentration and mobility in GaAs doping superlattices by hall effect measurements

The photo-Hall effect in a new type of periodicp-n doping multilayer structures (superlattices) of GaAs grown by molecular beam epitaxy has been investigated. In these space charge systems electrons and holes are separated in real space. As a consequence, large deviations from thermal equilibrium become quasi-stable. Carrier generation by optical absorption occurs in these doping superlattices even at photon energies far below the gap of the homogeneous semiconductor material. The photoexcitation results in a strong enhancement of the conductivityparallel to the layers and in a substantial photovoltaic response. An increase in carrierconcentration as well as an increase in carriermobility both contribute to the observed enhancement of the conductivity under excitation. The absolute values of changes in free-carrier concentration are very large due to the manyfold active layers of the structure. The measured free-carrier mobilities depend on the population of the multilayer system. A reduction in mobility as compared to bulk material is found to be more pronounced in weakly populated systems. This finding is caused by the larger weight of the boundary regions of the total active layers where the free-carrier density is lower than the density of ionized impurities resulting in an enhanced impurity scattering.

Modulation of two‐dimensional conductivity in a molecular beam epitaxially grown GaAs bulk space‐charge system

The electron channel conductivity σ(2)nn in a molecular beam epitaxially grown structure consisting of an ultrathin n‐doped GaAs layer sandwiched between two thicker p‐type layers has been studied in detail. The free carrier concentration n(2) in the channel can be varied continuously over a wide range by an external potential difference betweeen the active n layer and the p‐type material. The observed characteristics demonstrate that this three‐layer pnp doping structure in GaAs represents a special version of the junction field effect transistor and is an ideal system for studying the two‐dimensional conduction near the mobility edge.