Search

We fabricated carbon doped InAs/InxGa1−xAs/InxAl1−xAs heterostructures, which show p-type and n-type conductivity for different In contents. Two-dimensional hole gas in a structure with x=0.75 has been prepared in the ternary compound, despite the fact that carbon as an n-type dopant in InAs exhibits electron conductivity in InxGa1−xAs and InxAl1−xAs compounds with high indium content. A special doping design has been employed to obtain hole conductivity. As a result, the conductivity can be tuned from n-type to p-type with the In content and with different doping profiles in these structures.

The longitudinal magnetoresistance of a two- dimensional electron gas is shown to be strongly dependent on the presence of ohmic contacts between the voltage probes. By connecting or disconnecting contacts with additional gates across the potential probes it is possible to drastically change the amplitude of Shubnikov-de Haas oscillations.

An accurate determination of the valence band discontinuity of the lattice matched GaAs/InGaP heterostructure is performed by admittance spectroscopy measurements in p(+)/MQW/n(+) structures grown by metal-organic vapour phase epitaxy (MOVPE) using tertiarybutylarsine and tertiarybutylphosphine as alternative precursors for the V group elements. Through the analysis of the temperature dependence of the resonance frequency (omega) over bar at which the isothermal curves of the conductance over frequency G(omega)/omega have the maximum, the energy separation of 336 +/- 5 meV between the lowest heavy hole quantum level in the GaAs wells and the InGaP valence band top is obtained. A valence band discontinuity of Delta E-V = 346 +/- 5 meV is then derived by accounting for the calculated confinement energy of heavy holes (E-1(hh) = 10 meV). Experimental values of Delta E-V previously reported in the literature spread over the wide range of 300-400 meV. (C) 2007 Elsevier B.V. All rights reserved.

We have performed magnetotransport experiments on p+-(Ga,Mn)As/n+-GaAs Esaki diode devices. The spin-valve-like signal was observed in these devices in an in-plane magnetic field configuration due to Tunneling Anisotropic Magnetoresistance effect. The pattern of the observed magnetic reversal process strongly depends on the observed magnetic anisotropy of the (Ga,Mn)As layer - depending on its type the sign of the spin-valve-like signal can be changed by a simple rotation of the magnetic field by 90° or not. The type of the anisotropy is found to be strongly shaped, in a random way, during processing of the wafer.

Carbon-doped high mobility two-dimensional hole gases grown on (110) oriented GaAs substrates have been grown with hole mobilities exceeding 10^6 cm^2/Vs in single heterojunction GaAs/AlGaAs structures. At these high mobilities, a pronounced mobility anisotropy has been observed. Rashba induced spin-splitting in these asymmetric structures has been found to be independent on the transport direction.

Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs/AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106 cm2/V s at a density of 2.3×1011 cm–2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.

Photoluminescence and magnetotransport measurements have been performed to assess the quality of modulation doped GaAs/AlGaAs heterostructures. The temporal evolution of the low-temperature electron mobility of samples prepared in a molecular beam epitaxy chamber containing manganese as a source material was studied. Mn contamination was identified to be responsible for the reduction of the electron mobility to 1 x 10^{6} cm^{2}/Vs and the appearance of a distinct photoluminescence band. In contrast, structures in which this signal is absent reach mobility values of 5.4 x 10^{6} cm^{2}/Vs. This directly proves that the epitaxy of high-mobility electron systems and structures containing GaMnAs layers, in principle, can be combined in one growth chamber., Comment: 5 pages, 4 figures

We present fabrication and measurements of gated suspended low-dimensional electron systems. The core component of the device is a low-dimensional electron gas embedded in a free-standing beam processed from a GaAs/AlGaAs heterostructure. The dimensionality of the electronic system is fully controlled by a number of gating electrodes on the suspended membranes. Operation in the quantum Hall regime, in the one-dimensional case, and as zero-dimensional quantum dots is demonstrated. The resulting devices can be applied as ultrasensitive bolometers and as nanoelectromechanical circuits that reach the ultimate limits of displacement detection.

It is well-established that the electron–electron interaction strongly affects the compressibility of a two-dimensional electron system at zero magnetic field and even leads to the negative sign of the compressibility. Surprisingly, we find that its contribution is almost identical for the metallic states of electrons at zero magnetic field and of composite fermions at Landau level filling 1/2.

ZnO/InGaN/GaN heterostructured light-emitting diodes (LEDs) were fabricated by molecular beam epitaxy and atomic layer deposition. InGaN films consisted of an Mg-doped InGaN layer, an undoped InGaN layer, and a Si-doped InGaN layer. Current-voltage characteristic of the heterojunction indicated a diode-like rectification behavior. The electroluminescence spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, the heterostructured LEDs had potential application in white LEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias.

Commensurability effects between the period of an artificial array of scatterers (antidots) in a two-dimensional electron gas and the classical cyclotron radius are found to depend strongly on the size of the antidots in comparison with the period. Magnetotransport experiments on tunable antidot arrays are reported and analyzed. In the region of large depletion zones we find strong deviations of the Hall effect from the Drude slope in magnetic fields up to several teslas. From this, we deduce undepleted regions of less than 60 nm in typical 500-nm period structures. The suppression of the commensurability motion around several antidots observed in the longitudinal magnetoresistance in the case of strong depletion has the same origin and allows us to determine independently and consistently the depletion radius of the antidots.

The authors have measured the density of states in the tails of the Landau levels of GaAs-AlxGa1-xAs heterostructures by analysing the variation of the Fermi level position as a function of the magnetic field. The experimental data are best described by a gaussian-like density of states superimposed on a constant background density of states., Letter