Surface states at the clean surfaces of cleaved Si(111) and GaAs(110)

A combination of ellipsometric data on the electronic transitions from occupied to unoccupied surface states and published photoemission data on the energy distribution of the occupied surface states has been used to construct models of the surface states densities at the cleaved Si (111) and GaAs (110) surfaces.     

Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

Optical detection of surface states in GaAs(110) and GaP(110)

We present results on the optical detection of surface states in GaAs(110)and GaP(110)by the method of the fractional change of external reflectivity. Optical transitions are observed at ～3.1 eV m GaAs(110) and ～3.4 eV in GaP. A comparison with existing theories suggests a rotational relaxation model for the surface, with partial relaxation for GaAs(110) and full relaxation for GaP(110).     

Low temperature adsorption of water on cleaved GaAs(110) surfaces

The adsorption and desorption of water on UHV-cleaved GaAs(110) surfaces was studied using synchrotron-excited photoelectron spectroscopy. Water was adsorbed at T = 100 K. Desorption was studied during heating to room temperature. At low coverages, dissociated species are observed followed by molecular adsorption. Molecular water is desorbed at T = 160 K. The dissociated species are also mainly desorbed after heating to room temperature. The chemical changes are accompanied by substrate binding-energy shifts, reflecting the movement of the Fermi level at the surface.     

A trend in the energy distributions of Ga dangling-orbital surface states on GaP,GaAs and GaSb (110)

Relaxed (110) surfaces of GaP, GaAs, and GaSb are studied by cluster calculations. Strongly localized surface states near the band gap are presented. It is shown that Ga dangling-orbital surface states can occur within the band gap as a result of a strong decrease in the anti-bonding character of surface Ga orbitals. A trend in the bond length on going from GaSb to GaP explains experimental data. Surface states associated with the group-V atoms are also described.     

Surface states of GaAs (110): Pseudopotential calculations

In this letter we present calculations for surface states in high symmetry points Γ, M, X and X' of Jones' zone of GaAs (110), using a step barrier potential. It is found that only four surface states appear in the thermal gap; two in Γ point at 0.38 and 1.2 eV from the top of the valence band considered as origin of energies, one at 0.8 eV in M point and another in X′ point at 0.3 eV. Also, we obtain a low band of surface states between ?3 and ?5.5 eV.     

Very large tunneling anisotropic magnetoresistance of a (Ga,Mn)As/GaAs/(Ga,Mn)As stack

We report the discovery of a very large tunneling anisotropic magnetoresistance in an epitaxially grown (Ga,Mn)As/GaAs/(Ga,Mn)As structure. The key novel spintronics features of this effect are as follows: (i) both normal and inverted spin-valve-like signals; (ii) a large nonhysteretic magnetoresistance for magnetic fields perpendicular to the interfaces; (iii) magnetization orientations for extremal resistance are, in general, not aligned with the magnetic easy and hard axis; (iv) enormous amplification of the effect at low bias and temperatures.     

On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.     

Current-driven magnetization reversal in a ferromagnetic semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 microm2 device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2 x 10(5) A/cm2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.     

Surface states on unrelaxed GaAs (110) in the bond-orbital model

We have used the bond-orbital model of Pantelides and Harrison to compute eigenstates for a twelve layer slab of GaAs (110). Dispersion for each of the states is presented as a function of surface wave vector. Also, probability distributions for electrons in each of the surface states are given. These results are in good agreement with those of Joannopoulos and Cohen who did not make the bond-orbital approximation, thus helping to establish the usefulness of this method for surface calculations.     

Diffusion thermopower of (Ga,Mn)As/GaAs tunnel junctions

We report the observation of tunneling anisotropic magnetothermopower, a voltage response to a temperature difference across an interface between a normal and a magnetic semiconductor. The resulting voltage is related to the energy derivative of the density of states in the magnetic material, and thus has a strongly anisotropic response to the direction of magnetization in the material. The effect will have relevance to the operation of semiconductor spintronic devices, and may indeed already play a role in correctly interpreting the details of some earlier spin injection studies.     

Tight-binding calculations of (111) surface densities of states of Ge and GaAs

We have used the tight-binding method to calculate the local densities of states of unreconstructed Ge (111) and GaAs (111), ($\text{111}$) surfaces. In the unrelaxed surface configuration we find two types of states for each surface. The effects of relaxation on Ge surface states are also discussed.