Electron tunneling through thin barrier with smooth potential at GaAs/AlAs(001) heterointerfaces

The influence of actual microscopic potential on the characteristics of resonant electron tunneling from the Γ valley in GaAs through the AlAs barrier with thickness of one lattice constant has been investigated by the methods of pseudopotential and scattering matrix factorized by the irreducible representations of the symmetry group of the heterostructure. The transition regions between the potentials of components and the barrier region are treated as the components of the Ga₂Al₂As₄ superlattice spacing to provide the continuity of the crystal potential at the boundaries of the matching of wave functions. It is demonstrated that, compared to the results obtained in the abrupt-interface model, the inclusion of the actual potential in the calculation leads to changes in the number and location of the Fano resonances, an enhancement in the localization of electron density within the barrier, and a drastic increase in the tunneling time.     

Magnetism of the Fe/ZnSe(001) interface

The magnetism of epitaxial ultrathin films of Fe on ZnSe(001) has been investigated by x-ray magnetic circular dichroism down to the submonolayer regime. In contrast to other metallic ferromagnet/semiconductor interfaces, no reduction of the Fe magnetic moment was found at the Fe/ZnSe(001) interface. Furthermore, a significant enhancement of the Fe magnetic moment compared to the bulk value was observed for coverages up to one monolayer in agreement with theoretical predictions. We also demonstrate that the magnetic properties of the Fe/ZnSe(001) interface remain stable against thermal annealing up to 300 degrees C, a prerequisite for the future development of efficient spintronics devices.     

Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

ZnSe/GaAs(100)界面电子结构的计算

利用形式散射理论的格林函数方法，采用紧束缚最近邻近似下的sp³s模型，计算了ZnSe/GaAs(100)两类界面(Se/Ga和As/Zn界面)的电子结构.分别给出了两类界面的界面带结构和波矢分辨的层态密度及其分波态密度.计算结构表明，在ZnSe/GaAs(100)两类界面的禁带隙中均无界面态，而在其价带区均存在三条束缚的界面带和四条半共振带；通过比较，分析了两类界面的界面态特征及其来源. 关键词：     

Depth-resolved cathodoluminescence characterization of buried InGaP/GaAs heterointerfaces

As a new approach for contactless and non-destructive characterization method of buried multi-layer heterointerfaces, acceleration voltage-dependence of cathodoluminescence (CL) spectra is investigated for various InGaP/GaAs multi-layer heterostructures. The plot of CL intensity vs. acceleration voltage for a multi-layer heterostructure is defined as the cathodoluminescence in-depth spectrum (CLIS). Experimental CLIS spectra on InGaP/GaAs single heterostructures and quantum well structures grown on GaAs by MOVPE and by GSMBE using TBP as the P source demonstrate that CLIS technique is very powerful to obtain depth-resolved information on multi-layer heterostructures.     

Photoluminescence and the structure of heterointerfaces of (311)A-and (311)B-oriented GaAs/AlAs superlattices

GaAs/AlAs superlattices grown simultaneously on GaAs substrates with the (311)A and (311)B orientations have been studied by photoluminescence and high-resolution transmission electron microscopy with a Fourier analysis of images. A periodic interface corrugation is observed for (311)B superlattices. A comparison of the structure of (311)A and (311)B superlattices indicates that the corrugation occurs in both cases and its period along the $[01\overline 1 ]$ direction is equal to 3.2 nm. The corrugation is less pronounced in (311)B superlattices, wherein it exhibits an additional modulation (long-wavelength disorder) with the characteristic lateral size exceeding 10 nm. The vertical correlation of regions rich in GaAs and AlAs, which is well observed in (311)A superlattices, is weak in (311)B superlattices due to the occurrence of long-wavelength disorder. The optical properties of (311)B superlattices are similar to those of (100) ones and differ radically from those of (311)A superlattices. As distinct from (311)B, strong photoluminescence polarization anisotropy is observed for (311)A superlattices. It is shown that it is the interface corrugation rather than the crystallographic (311) surface orientation that determines the optical properties of (311)A corrugated superlattices with thin GaAs and AlAs layers.     

Uniaxial magnetocrystalline anisotropy of metal/semiconductor interfaces: Fe/ZnSe(001)

A theoretical study of the magnetic moments and the in-plane magnetic anisotropy of an interface between a cubic ferromagnet and a cubic semiconductor, Fe/ZnSe(001), is presented. Theory confirms the observed, much debated, uniaxial anisotropy of the iron film. This result is important since the calculations are for perfect interfaces with squarelike environments, proving that the fourfolded symmetry of the interface Fe atoms is broken beyond the nearest neighboring semiconducting layer, effects that are usually assumed small. It is demonstrated how the uniaxial anisotropy is produced by the directional covalent bonds at the interface, even without atomic relaxations.     

In-situ optical spectroscopy and electronic properties of pyrrole sub-monolayers on Ga-rich GaAs(001)

We report on the characterization of sub-monolayers of pyrrole adsorbed on Ga-rich GaAs(001) surfaces. The interfaces were characterized by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and reflectance anisotropy spectroscopy (RAS) in a spectral range between 1.5 and 8 eV. The adsorption of pyrrole on Ga-rich GaAs(001) modifies the RAS spectrum of the clean GaAs surface significantly at the surface transitions at 2.2 and 3.5 eV indicating a chemisorption of the molecules. By the help of transients at these surface transitions during the adsorption process, we were able to prepare different molecular coverages from a sub-monolayer up to a complete molecular layer. The different coverages of pyrrole were imaged by STM and electronically characterized by STS. The measurements reveal that the adsorbed molecules electronically insulate the surface and indicate the formation of new interface states around −3.5 and +4.2 eV. The RAS measurements in the UV region show new anisotropies in the spectral range of the optical transitions of the adsorbed pyrrole molecules. Our measurements demonstrate the potential of optical and electronic spectroscopy methods for the characterization of atomically thin molecular layers on semiconductor surfaces allowing a direct access to the properties of single adsorbed molecules.     

X-ray photoelectron spectroscopy study of GaAs (001) surface thermocleaning prior to molecular beam epitaxy

X-ray photoelectron spectroscopy has been performed in order to investigate the encapsulent effect of an oxide layer, grown at the end of the chemical etching procedure, on the annealing behaviour of GaAs (001) substrates prior to M.B.E. growth. The oxide layer makes the surface unable to react with arsenic molecules until the desorption of the gallium oxide around 570 °C, preventing the surface against arsenic desorption below this temperature. Otherwise, if unoxidized substrates are used, annealing must be performed under an arsenic flux from 150 °C upwards to avoid the formation of an arsenic depleted surface.