Yu. A. Danilov

Using laser sputtering to obtain semiconductor nanoheterostructures

Laser sputtering of solid-state targets in a hydrogen atmosphere has been used to form semiconductor nanoheterostructures. Impurities (Te or Mn) in the form of delta-doped layers obtained by laser sputtering of the corresponding targets in the process of vapor-phase epitaxy using organometallic compounds can be introduced into light-emitting structures based on the InGaAs/GaAs system to make it possible to control the spectrum and the electroluminescence intensity. Reducing the hydrogen pressure in the reactor to 25-50Torr allows laser deposition to be carried out at reduced temperatures of the epitaxial layers of the base material and makes it possible to obtain GaAs and InAs semiconductors with a high manganese-doping level that demonstrate ferromagnetic properties at room temperature.

The special features of the Hall effect in GaMnSb layers deposited from a laser plasma

Epitaxial GaMnSb films with Mn contents up to about 10 at. % were obtained by deposition from a laser plasma in vacuum. The growth temperature Ts during deposition was varied from 440 to 200°C, which changed the concentration of holes from 3 × 1019 to 5 × 1020 cm−3, respectively. Structure studies showed that, apart from Mn ions substituting Ga, the GaMnSb layers contained ferromagnetic clusters with Mn and shallow acceptor defects of the GaSb type controlled by the Ts value. Unlike single-phase GaMnSb systems studied earlier with negative anomalous Hall effect values and Curie temperatures not exceeding 30 K, the films obtained in this work exhibited a positive anomalous Hall effect, whose hysteresis character manifested itself up to room temperature and was the more substantial the higher the concentration of holes. The unusual behavior of this effect was interpreted in terms of the interaction of charge carriers with ferromagnetic clusters, which was to a substantial extent determined by the presence of Schottky barriers at the boundary between the clusters and the semiconducting matrix; this interaction increased as the concentration of holes grew. The absence of this effect in semiconducting compounds based on III–V Group elements with MnSb or MnAs ferromagnetic clusters was discussed in the literature; we showed that this absence was most likely related to the low hole concentrations in these objects.

Emission properties of InGaAs/GaAs heterostructures with δ⟨Mn⟩-doped barrier

Light-emitting device heterostructures with a ?Mn-doped layer inserted between the Schottky contact and near-surface InGaAs/GaAs quantum well (QW) have been fabricated. The ?Mn-doped layer facilitates hole tunnelling from the Schottky contact to the QW and impedes that of QW electrons in the opposite direction. It leads to a highly enhanced electroluminescence signal from the InGaAs QW. An effective p?d exchange interaction of holes with magnetic moments of Mn ions is found to strongly enhance the effective hole g-factor and the circular polarization of the low (~2?K) temperature emission up to 20% in magnetic fields of 1?2?T.

Dynamic spin polarization by orientation-dependent separation in a ferromagnet–semiconductor hybrid

Integration of magnetism into semiconductor electronics would facilitate an all-in-one-chip computer. Ferromagnet/bulk semiconductor hybrids have been, so far, mainly considered as key devices to read out the ferromagnetism by means of spin injection. Here we demonstrate that a Mn-based ferromagnetic layer acts as an orientation-dependent separator for carrier spins confined in a semiconductor quantum well that is set apart from the ferromagnet by a barrier only a few nanometers thick. By this spin-separation effect, a non-equilibrium electron-spin polarization is accumulated in the quantum well due to spin-dependent electron transfer to the ferromagnet. The significant advance of this hybrid design is that the excellent optical properties of the quantum well are maintained. This opens up the possibility of optical readout of the ferromagnets magnetization and control of the non-equilibrium spin polarization in non-magnetic quantum wells.

Anomalous hall effect in Mn δ-doped GaAs/In0.17Ga0.83As/GaAs quantum wells with high hole mobility

Magnetic and magnetotransport properties of GaAs(δ〈Mn〉)/In0.17Ga0.83As/GaAs quantum wells with different Mn concentrations are studied. The delta-doped manganese layer has been separated from the GaAs quantum well with a spacer with an optimal thickness (3 nm), which has provided a sufficiently high hole mobility (≥103 cm2V−1 s−1) in the quantum wells and their effective exchange with Mn atoms. It is found that the anomalous Hall effect (AHE) is exhibited only in a restricted temperature range above and below the Curie temperature, while the AHE is not observed in quantum wells with quasi-metallic conductivity. Thus, it is shown that the use of the AHE is inefficient in studying magnetic ordering in semiconductor systems with high-mobility carriers. The features observed in the behavior of the resistance, magnetoresistance, and Hall effect are discussed in terms of the interaction of holes with magnetic Mn ions with regard to fluctuations of their potential, hole transport on the percolation level, and hopping conduction.

Influence of delta〈Mn〉 doping parameters of the GaAs barrier on circularly polarized luminescence of GaAs/InGaAs heterostructures

The circular polarization of low-temperature electroluminescence of diodes based on heterostructures with an undoped quantum well InGaAs/GaAs and a delta〈Mn〉 layer in the GaAs barrier has been investigated. The possibility of changing the degree of circular polarization of the electroluminescence by varying the main structural parameters of diodes (spacer layer thickness, i.e., the distance between the delta〈Mn〉 layer and the quantum well, atomic concentration in the delta〈Mn〉 layer, and introduction of an additional acceptor delta layer) has been analyzed. It has been revealed that the variation in the spacer layer thickness is the most effective method for controlling the degree of circular polarization of the electroluminescence.

Ferromagnetism in epitaxial germanium and silicon layers supersaturated with managanese and iron impurities

The possibility of laser synthesis of diluted magnetic semiconductors based on germanium and silicon doped with manganese or iron up to 10–15 at % has been shown. According to data on the electronic levels of 3d atoms in semiconductors, Mn and Fe impurities are most preferable for realizing ferromagnetism in Ge and Si through the Ruderman-Kittel-Kasuya-Yosida mechanism. Epitaxial Ge and Si layers 50–110 nm in thickness were grown on gallium arsenide or sapphire single crystal substrates heated to 200–480°C. The content of a 3d impurity has been measured by x-ray spectroscopy. The ferromagnetism of layers and high magnetic and acceptor activities of Mn in Ge, as well as of Mn and Fe in Si, are manifested in the observation of the Kerr effect, anomalous Hall effect, high hole conductivity, and anisotropic ferromagnetic resonance at 77–500 K. According to the ferromagnetic resonance data, the Curie point of Ge:Mn and Si:Mn on a GaAs substrate and of Si:Fe on an Al2O3 substrate is no lower than 420, 500, and 77 K, respectively.

Effect of compressive and tensile stresses in GaMnAs layers on their magnetic properties

The effect of elastic stresses (compressive, tensile) on the magnetic properties of epitaxial GaMnAs layers prepared by laser deposition of solid-state targets in a gas atmosphere on different buffer sublayers (InxGa1 − xAs and InxGa1 − xP) and substrates (GaAs, InP) has been investigated. It has been established from the investigations of magnetic-field dependences of the Hall resistance that all layers exhibit ferromagnetic properties with the Curie temperature ∼50 K. It has been shown that, in the case of tensile stresses in GaMnAs layers (InxGa1 − xAs and InxGa1 − xP buffers and InP substrate), the anomalous Hall effect shape demonstrates a predominant orientation of the easy-magnetization axis in the growth direction, unlike the GaMnAs layers prepared on a GaAs substrate (with compressive stresses), which demonstrate the predominance of the component of the magnetization vector in the layer plane.

Formation of thin, flexible, conducting films composed of multilayer graphene

The possibility of fabricating high-quality thin (10–100 nm), flexible, semitransparent films of multilayer graphene and graphite nanoplates (ultrathin graphite) using a modified Langmuir-Blodgett method is demonstrated. The high quality of the resulting samples is confirmed via Raman spectroscopy and high-resolution scanning electron microscopy. Sheet resistance measurements for the films show values of ∼100 Ω/sq and lower.

Photoluminescence response of a quantum well to a change in the magnetic field of the Mn δ Layer in InGaAs/GaAs heterostructures

Ferromagnetic ordering of two types (depending on the sample geometry) is found to occur in a thin Ga1 − xMnxAs alloy layer (Mn δ layer) in heterostructures containing an InGaAs/GaAs quantum well. Singular samples in which the δ Mn layer is parallel to the (001) GaAs plane exhibit the “3/2” Bloch temperature dependence of magnetization, and vicinal samples in which the δ Mn layer deviates from the (001) GaAs plane exhibit a “percolation” ferromagnetic transition. The photoluminescence polarization of the quantum well is shown to follow changes in the magnetization of the Mn δ layer as a function of temperature according to the Bloch law in the singular samples and to a percolation law in the vicinal samples.