O. Maksimov

Exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAs

We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1−xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (TB=48±2K) and the Curie temperature of the ferromagnet (TC=55.1±0.2K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnetization hysteresis loop when the bilayer is cooled in the presence of an applied magnetic field and an enhancement of the coercive field.

Stoichiometric, nonstoichiometric, and locally nonstoichiometric SrTiO3 films grown by molecular beam epitaxy

SrTiO3 films were grown by reactive molecular beam epitaxy to have varying degrees of both global and local cationic nonstoichiometries (with stoichiometry defined as a 1:1 ratio of Sr:Ti). Slight global excesses of Sr and Ti resulted in two-fold reconstructions in the reflection high-energy electron diffraction patterns along the [110] and [100] azimuths, respectively. Larger global nonstoichiometries (2:1 and 1:2 ratios) were also accommodated into the film’s crystalline structure and affected the long-range crystalline order as observed in the x-ray diffraction patterns, both of which were related to the parent perovskite pattern. Local nonstoichiometries were introduced by depositing multiple monolayers (MLs) (from 2 to 33) of SrO and TiO2 in an alternating fashion, while maintaining the global SrTiO3 stoichiometry. These layered structures of SrO and TiO2 blocks inter-reacted during growth to form highly crystalline epitaxial SrTiO3. Films grown in this manner with blocks thicker than 8 MLs were full...

Nanoengineered Curie temperature in laterally patterned ferromagnetic semiconductor heterostructures

We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.

Effects of Be on the II–VI/GaAs interface and on CdSe quantum dot formation

The effects of Be on the II–VI/GaAs interface and on CdSe quantum dot (QD) formation were investigated. A (1×2) surface reconstruction was observed after a Be–Zn coirradiation of the (001) GaAs (2×4) surface. ZnBeSe epilayers grown after the Be–Zn coirradiation show very high crystalline quality with x-ray rocking curve linewidths down to 23 arcsec and a low etch pit density of 4×104 cm−2, and good optical quality with a band-edge photoluminescence (PL) emission peak linewidth of 2.5 meV at 13 K. However, ZnBeSe epilayers grown after Zn irradiation alone have poor crystalline quality and poor optical properties. Atomic force microscopy measurements show that CdSe QDs grown on ZnBeSe have higher density and smaller size than those grown on ZnSe. A narrower PL emission peak with higher emission energy was observed for the CdSe QDs sandwiched by ZnBeSe. These results indicate that the formation of CdSe QDs as well as the II–VI/GaAs interface are modified by the presence of Be.

Exchange biasing of the ferromagnetic semiconductor (Ga,Mn)As by MnO (invited)

We provide an overview of progress on the exchange biasing of a ferromagnetic semiconductor (Ga1−xMnxAs) by proximity to an antiferromagnetic oxide layer (MnO). We present a detailed characterization study of the antiferromagnetic layer using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray reflection. In addition, we describe the variation of the exchange and coercive fields with temperature and cooling field for multiple samples.

A series of layered intergrowth phases grown by molecular beam epitaxy: SrmTiO2+m(m=1–5)

SrmTiO2+m phases having one TiO2 layer sandwiched between m SrO layers were grown using molecular beam epitaxy. The out-of-plane (in-plane) lattice parameters determined by x-ray diffraction were c(a)=9.14A (3.78A), 23.55A (3.75A), and 14.60A (3.75A) for Sr3TiO5, Sr4TiO6, and Sr5TiO7, respectively. Both lattice parameters change abruptly on going from the m=2 Ruddlesden-Popper phase to m=3 phase, indicating a significant change in the bond lengths (or strain states) on transitioning from the known members to the higher order members of this structural family. Electron microscopy confirmed the artificially layered structures.

Structural properties of SrO thin films grown by molecular beam epitaxy on LaAlO3 substrates

SrO films were grown on LaAlO3 substrates by molecular beam epitaxy and characterized using reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD). The evolution of the RHEED pattern is discussed as a function of film thickness. 500A thick SrO films were relaxed and exhibited RHEED patterns indicative of an atomically smooth surface having uniform terrace heights. Films had the epitaxial relationship (001)SrO‖(001)LaAlO3; [010]SrO‖[110]LaAlO3. This 45° in-plane rotation minimizes mismatch and leads to films of high crystalline quality, as verified by Kikuchi lines in the RHEED patterns and narrow rocking curves of the (002) XRD peak.

Width dependence of annealing effects in (Ga,Mn)As nanowires

We study the time dependence of annealing on a series of GaAs-capped (Ga,Mn)As nanowires of varying widths. For different annealing times, our measurements indicate that decreasing the wire width monotonically increases the Curie temperature enhancement associated with annealing, as well as the drop in resistivity. These results are consistent with the lateral diffusion of interstitial Mn ions, which constitute an important source of defects in these materials. Furthermore, the thinner wires show a higher rate of change of conductivity with annealing time, suggesting a more efficient removal of Mn interstitials in thinner wires.

Luminescent properties of BexCd1−xSe thin films

Abstract We report photoluminescence (PL) study of Be x Cd 1− x Se epitaxial layers ( x

Growth of GaN films on GaAs substrates in an As-free environment

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth.