H. Kostial

HIGH P-TYPE CONDUCTIVITY IN CUBIC GAN/GAAS(113)A BY USING BE AS THE ACCEPTOR AND O AS THE CODOPANT

P‐type room‐temperature conductivities as high as 50/Ω cm are achieved in cubic GaN layers by the concept of reactive codoping. We use Be as the acceptor species and O as the reactive donor to render isolated Coulomb scatterers into dipole scatterers. This concept allows us to achieve high hole mobilities and thus p‐type conductivities.

Spin injection from Fe3Si into GaAs

We demonstrate room-temperature spin injection from the epitaxially grown ferromagnetic metal Fe3Si into the semiconductor GaAs. The injection efficiency is comparable to values previously obtained for the Fe∕GaAs and MnAs∕GaAs hybrid systems using the emission of similar (In,Ga)As∕GaAs light-emitting diodes for the detection of spin polarization. The temperature dependence of the detected polarization is explained by taking into account spin relaxation inside the semiconductor device.

Controlled n-type doping of AlN:Si films grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

We study the properties of Si-doped AlN films grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy. Whereas nominally undoped AlN films are invariably insulating in nature, Si-doped films are found to be semiconducting with an electron concentration up to 7.4×1017cm−3, and a resistivity approaching 1 Ω cm at room temperature. Even heavy Si-doping (1×1020cm−3) does not degrade the structural properties of the AlN films. The morphology of these films is characterized by Si-induced step-bunching, but remains smooth with a rms roughness of about 1 nm.

Properties of InN layers grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

We study the impact of different buffer layers and growth conditions on the properties of InN layers grown on 6H–SiC(0001) by plasma-assisted molecular beam epitaxy. Both GaN and AlN buffer layers result in a significant improvement of the structural quality compared to InN layers grown directly on the SiC substrate. However, to obtain layers exhibiting a high structural integrity, smooth surface morphology, high mobility and strong band-to-band photoluminescence, contradicting growth conditions are found to be required. Furthermore, since InN(0001) dissociates already at temperatures below the onset of In desorption, it is difficult to avoid In accumulation and inclusions of crystalline In in the layer under In-rich conditions.

Lasing properties of GaAs/(Al,Ga)As quantum-cascade lasers as a function of injector doping density

The lasing properties of GaAs/Al0.33Ga0.67As quantum-cascade lasers are investigated as a function of injector doping concentration ns between 2×1011 and 1×1012 cm−2 per period. Lasing is observed for ns⩾3.5×1011 cm−2, with optimal lasing properties (minimum of the threshold current and maximum of the modified characteristic temperature) for nopt≈6×1011 cm−2. With increasing ns up to nopt, the lasing energy of 115 meV exhibits first a blueshift to 135 meV, followed by a redshift to 120 meV for higher doping levels. This shift of the lasing energy as a function of ns is discussed in terms of changes in the field distribution, occupation of additional levels above the upper laser level, and electron–electron interactions.

Crack-free and conductive Si-doped AlN∕GaN distributed Bragg reflectors grown on 6H-SiC(0001)

We demonstrate Si-doped n-type AlN∕GaN distributed Bragg reflectors grown on 6H-SiC(0001). The structures are crack-free and have a stopband centered around 450nm with a full width at half maximum between 40 and 50nm. The maximum measured reflectance is ⩾99%. A comparison between Si-doped and undoped structures reveals no degradation of the reflectance due to the Si doping. Vertical conductance measurements at room temperature on the samples show an ohmic I–V behavior in the entire measurement range. The measured resistivity at 77K is only a factor of 2 larger than the resistivity measured at room temperature.

Reduction of Remote Impurity Scattering in Heavily Modulation-Doped GaAs and (GaIn)As Quantum Wells with AlAs/GaAs Type-II-Superlattice Barriers

We present a new semiconductor heterostructure to reduce impurity scattering in remotely doped GaAs and (GaIn)As single quantum wells. By using heavy-mass X-electrons in the short-period AlAs/GaAs superlattice barriers, the potential fluctuations of the ionized Si dopants are smoothed. In 10 nm GaAs and (InGa)As single quantum wells, respective densities of about 2×1016 m-2 and 3 ×1016 m-2 can be achieved with a low level of remote impurity scattering. For (InGa)As single quantum wells, the reduction of the impurity scattering manifests itself as an increase of the single particle relaxation time. Structure design and growth parameters for achieving ultrahigh conductivities are discussed.

Self-organized etching technique for fabricating a quasiregular array of MnAs nanoislands

We present a method to fabricate arrays of MnAs islands having diameters of less than 100 nm without using nanofabrication techniques. The strain balance in the MnAs layer grown epitaxially on GaAs substrates collapses when the heterostructure is immersed in a wet-chemical etch solution. As a consequence, the MnAs layer develops a regular row of cracks and submicron-wide strips are carved from it. The strips are additionally sliced into islands during etching since two phases of MnAs, α-MnAs and β-MnAs, which alternate in a periodic way along the strips, are etched at different rates. The period of the quasiregular array of MnAs islands can be adjusted through the thickness of the MnAs layer. We also show that MnAs islands can serve as a nearly ideal etch mask to create GaAs columns by dry etching.

Impact of N-induced potential fluctuations on the electron transport in Ga(As,N)

We investigate the electron transport in Ga(As,N) layers focusing on the influence of potential fluctuations. With increasing electron concentration, a metal-insulator transition is observed in the temperature dependence of the resistivity for a series of samples containing 0.8% of N. The observed behavior is discussed in the frame of Anderson transition. By increasing the N concentration up to 2.2%, we observe an increase of the potential fluctuations’ amplitude. Mean-square values for the intrinsic N-induced fluctuation γN are obtained from percolation theory to be larger than 30 meV. Rapid thermal annealing reduces γN significantly for samples with higher N concentration. These large potential fluctuations lead to electron localization and induce thermally activated conductivity which is observable up to 300 K.

Conductance anisotropy of high-mobility, modulation-doped GaAs single quantum wells

Abstract The conductance in high-mobility and high-density, modulation-doped GaAs single quantum wells on GaAs (0 0 1) substrates with thin spacer layers is strongly anisotropic with a 2–3 times higher mobility in the [ 1 1 0] than that in the [1 1 0] direction. We show that the anisotropic scattering potential is strongly influenced by additional X-like electrons in the barriers formed by short-period superlattices. The X-electrons are able to considerably smooth the fluctuations of the potential; thus, increasing the correlation length of the fluctuations. We investigate the correlation length of the potential fluctuations by oscillations in the low-field magnetoresistance.