G. Kioseoglou

Efficient electrical spin injection from a magnetic metal/tunnel barrier contact into a semiconductor

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology.

Local structure and chemical valency of Mn impurities in wide-band-gap III–V magnetic alloy semiconductors Ga1−xMnxN

Local structure and effective chemical valency of Mn impurity atoms incorporated in wide-band-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques. The x-ray results provide direct evidence for the substitution of majority Mn atoms for the Ga sites in GaN, with an effective valency close to Mn(II), up to a rather high Mn concentration about 2 at. %. A small fraction of the impurity atoms could also form Mn clusters.

Local environment surrounding magnetic impurity atoms in a structural phase transition of Co-doped TiO2 nanocrystal ferromagnetic semiconductors

The local environment surrounding magnetic impurity atoms and the host crystal structure of codoped TiO2 (TiO2:Co) nanocrystal ferromagnetic semiconductors have been investigated using the x-ray absorption fine structure and powder diffraction techniques. It has been found that the magnetic Co impurity atoms substitute for the Ti sites in an anataselike local environment through a structural phase transition when the material changes from an amorphous phase to a mixture of anatase and rutile crystal structures and then to a rutile structure as a result of increasing the anneal temperature. This result reveals an interesting feature that the local structure around magnetic impurity atoms can remain practically unchanged while the material undergoes drastic structural variations and a loss of room-temperature ferromagnetism.

Electrical spin injection across air-exposed epitaxially regrown semiconductor interfaces

We have fabricated spin-polarized light-emitting diode structures via epitaxial regrowth of Zn1−xMnxSe on air-exposed surfaces of AlyGa1−yAs/GaAs quantum wells. No passivation procedures were used to protect or prepare the III–V surface. The electroluminescence is strongly circularly polarized due to the electrical injection of spin-polarized electrons from the ZnMnSe contact into the GaAs quantum well. An analysis of the optical polarization yields a lower bound of 65% for the spin injection efficiency. These results demonstrate the robustness of the spin injection process in the diffusive transport regime, and attest to the practicality of manufacturing semiconductor-based spin injection devices.

Studies of Mn/GaAs digital alloys using x-ray absorption fine structure and x-ray diffraction methods

Local structure and effective chemical valency of Mn atoms in Mn/GaAs digital alloys have been investigated using the x-ray absorption fine structure techniques. The samples were prepared by molecular-beam epitaxy with different thickness of GaAs layers separating the nominal Mn monolayers. Lattice constants of the digital alloys are found by x-ray diffraction to increase linearly in a very narrow range (about 0.3%) with the Mn/GaAs ratio in the samples. Our data show that Mn atoms in the nominal Mn monolayers actually combine with GaAs to form (Ga, Mn)As alloys with Mn atoms substituting for the Ga sites in GaAs. This result clearly rules out the possibility of dominant MnAs formation.

“Inverted hut” structure of Si–Ge nanocrystals studied by extended x-ray absorption fine structure method

Local structure around Ge in Si/Ge superlattices containing the “inverted hut” nanocrystals has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. In contrast to the usual nanometer-sized Ge “hut clusters” commonly grown on top of Si layers using the conventional Stranski–Krastanow self-organized growth mode, SiGe-alloy nanocrystals can be formed beneath the Ge wetting layer and grown into the Si layer in Si/Ge superlattices prepared in a low-temperature molecular beam epitaxy growth mode, and exhibit inverted hut nanocrystal structures regularly spaced along the Si/Ge interface. The EXAFS results obtained with varying Ge wetting layer thickness provide a direct evidence that intermixing of Ge and Si atoms takes place in a zone of about 1–3 monolayers on each side of the Si/Ge interface. The intermixing of constituent atoms allows a mechanism other than the usual formation of misfit dislocations to release the strain energy resulted from lattice mismatch between Si ...

Investigation of nanoscale structure in digital layers of Mn/GaAs and MnGa/GaAs

Grazing incidence x-ray scattering (GIXS) and x-ray diffraction (XRD) techniques have been employed to study the microscopic structure of magnetic digital layers of Mn/GaAs and MnGa/GaAs. Samples with various GaAs layer thickness (8 to 16 monolayers) and a half monolayer of either Mn or MnGa were prepared by low-temperature molecular-beam epitaxy. All digital alloys consist of 50 periods of magnetic layers separated by GaAs. High crystalline quality was verified and the periodicity and layer thickness were determined from the GIXS and XRD data. In order to investigate the magnetic properties, we performed magnetization measurements on all samples using superconducting quantum interference device magnetometry (SQUID).

Effects of heat treatment on diffusion of Cu atoms into CdTe single crystals

Angular dependence of x-ray fluorescence and x-ray absorption fine structure techniques have been used to study the diffusion of Cu atoms into the photovoltaic material CdTe. Depth profile, effective valency, and local structure of Cu atoms in a Cu-doped single crystal of CdTe were investigated before and after a second heat treatment. Enhanced Cu diffusion into the CdTe single crystal was observed as a result of heating at a moderate temperature around 200 °C, resulting in a redistribution of the Cu impurities through a broader depth profile. Some of the Cu atoms are believed either to form small complexes with Te or occupy interstitial sites in the host but accompanied by a large local lattice distortion while others substitute for Cd on the cation sites. The results thus demonstrate that these nondestructive x-ray characterization methods are useful for probing microstructural changes in CdTe photovoltaic materials/devices in which some Cu-containing compounds are used as back contacts.

Internal transitions of two-dimensional charged magneto-excitons X−: theory and experiment

Abstract Internal spin-singlet and spin-triplet transitions of charged excitons X− in magnetic fields in quantum wells have been studied experimentally and theoretically. The allowed X− transitions are photoionizing and exhibit a characteristic double-peak structure, which reflects the rich structure of the magnetoexciton continua in higher Landau levels (LLs). We discuss a novel exact selection rule, a hidden manifestation of translational invariance, that governs transitions of charged mobile complexes in a magnetic field.

Migration of Te atoms and structural changes in CdS/CdTe heterojuctions studied by x-ray scattering and fluorescence

X-ray reflectivity and angular dependence of x-ray fluorescence (ADXRF) techniques have been employed for a quantitative study of the Te depth profile and structural changes in a series of CdS/CdTe heterojuctions annealed at various temperatures. The temperature dependence of surface roughening and Te migration is observed in both reflectivity and fluorescence experiments. Changes in the interface morphology and Te distribution are quantified by detailed analysis of the ADXRF data with the aid of reflectivity measurements. The results show that a large amount of Te up to 50% could migrate into the CdS layer and suggest that an extra layer of compounds can be formed near the CdS top surface. We have thus demonstrated that the x-ray reflectivity and ADXRF methods can be used as effective tools for nondestructive characterization of the concentration depth profile and interface morphology in layered structures on a nanometer scale.