Bruno Lépine

Measurement of the valence-band offset at the epitaxial MgO-GaAs(001) heterojunction by x-ray photoelectron spectroscopy

The electronic band structure at the interface of the MgO-GaAs(001) tunnel contact has been experimentally studied. X-ray photoelectron spectroscopy has been used to measure the valence-band offset at the MgO-GaAs(001) heterojunction interface. The valence-band offset ΔEV is determined to be 4.2±0.1eV. As a consequence, a nested “type-I” band alignment with a conduction-band offset of ΔEC=2.2±0.1eV is found. The accurate determination of the valence and conduction band offsets is important for the fundamental understanding of the tunnel spin injection in GaAs.

Work function shifts, Schottky barrier height, and ionization potential determination of thin MgO films on Ag(001)

The electronic band structure and the work function of MgO thin films epitaxially grown on Ag(001) have been investigated using x-ray and ultraviolet photoelectron spectroscopy for various oxide thicknesses. The deposition of thin MgO films on Ag(001) induces a strong diminution in the metal work function. The p-type Schottky barrier height is constant at 3.85+/-0.10 eV above two MgO monolayers and the experimental value of the ionization potential is 7.15+/-0.15 eV. Our results are well consistent with the description of the Schottky barrier height in terms of the Schottky-Mott model corrected by an MgO-induced polarization effect.

Transport property study of MgO-GaAs(001) contacts for spin injection devices

The electrical properties of Au∕MgO∕n-GaAs(001) tunnel structures have been investigated with capacitance-voltage and current-voltage measurements at room temperature with various MgO thicknesses between 0.5 and 6.0nm. For an oxide thickness higher than 2nm and for low bias voltages, the voltage essentially drops across the oxide and the structure progressively enters the high-current mode of operation with increasing reverse bias voltage, the property sought in spin injection devices. In this mode, we demonstrate that a large amount of charge accumulates at the MgO∕GaAs interface in interface traps located in the semiconductor band gap.

Band structure of the epitaxial Fe∕MgO∕GaAs(001) tunnel junction studied by x-ray and ultraviolet photoelectron spectroscopies

The electronic band structure in the epitaxial Fe∕MgO∕GaAs(001) tunnel junction has been studied by x-ray and ultraviolet photoelectron spectroscopy measurements. The Schottky barrier height (SBH) of Fe on MgO∕GaAs heterostructure is determined to be 3.3±0.1eV, which sets the Fe Fermi level at about 0.3eV above the GaAs valence band maximum. This SBH is also exactly the same as that measured from Fe on MgO monocrystal. After Fe deposition, no band bending change is observed in MgO and GaAs underlayers. On the contrary, Au and Al depositions led to clear variation of the band bending in both MgO and GaAs layers. This effect is analyzed as a fingerprint of defect states at the MgO∕GaAs interface.

Angle-resolved x-ray photoemission study of GaAs(001) surfaces

Angle-resolved X-ray photoelectron spectroscopy (XPS) has been used to characterise chemically etched GaAs(001) versus epitaxially grown surfaces. The measured amplitude of X-ray photoelectron diffraction (XPD) modulation is then an excellent probe of the local structural environment around the emitter atom in the near-surface substrate. The observed modulations are here compared to the results of single- and double-scattering cluster calculations with a fine agreement. Moreover, because of our choice of the emission plane and taking into account the similarity in the scattering factors of Ga and As atoms, XPD effects can be almost eliminated from the angular variation of the fractional peak ratio IAs(IAs + IGa) which is then easily related to the arsenic concentration profile CAs(z). A best-fitting procedure is used to select the experimental concentration depth profile which characterises the surface under study. Samples exposed to different technological treatments, like ion implantation, chemical etching and annealing have been studied and compared. The concentrations of As atoms at the surface for As-capped, MBE-grown (001) samples after annealing around 350°C and for chemically etched GaAs(001) wafers indicate the same type of surface (As-terminated) as far as composition and structure are concerned.

Spatially resolved electronic properties of MgO/GaAs(001) tunnel barrier studied by ballistic electron emission microscopy

The spatially resolved electronic structure of the epitaxial Au/MgO/GaAs(001) tunnel junction has been studied by ballistic electron emission microscopy. The Schottky barrier height of Au on the MgO/GaAs heterostructure is determined to be 3.90 eV, in good agreement with spatially averaged x-ray photoelectron spectroscopy measurements. Locally, two well-defined conduction channels are observed for electrons energies of 2.5 and 3.8 eV, i.e., below the conduction band minimum of the oxide layer. These conduction channels are attributed to band of defect states in the band-gap of the tunnel barrier related to oxygen vacancies in the MgO layer. These defect states are responsible for the low barrier height measured on magnetic tunnel junctions with epitaxial MgO(001) tunnel barriers.

X-RAY PHOTOELECTRON DIFFRACTION STUDY OF YBAS/GAAS(001) AND SCAS/GAAS(001)HETEROSTRUCTURES

Polar angle distributions of core level photoemission intensities recorded on YbAs/GaAs(001) and ScAs/GaAs(001) heterostructures are presented. They allow first the surface roughness of thin YbAs overlayers to be seen, second to estimate the tetragonal distortion of a strained ScAs film and, third, the most interesting point, to demonstrate in a direct fashion that the mixed (Yb‐As) (010) planes of YbAs grow in the prolongation of the As planes of GaAs. The results are compared to those obtained by other authors with various techniques. The main advantage of the photoelectron diffraction method over the other techniques is that it can be performed on very thin epitaxial films (some monolayers) directly in situ under ultrahigh vacuum.

Layer-resolved study of Mg atom incorporation at the MgO/Ag(001) buried interface.

By combining x-ray excited Auger electron diffraction experiments and multiple scattering calculations we reveal a layer-resolved shift for the Mg KL23L23 Auger transition in MgO ultrathin films (4-6 Å) on Ag(001). This resolution is exploited to demonstrate the possibility of controlling Mg atom incorporation at the MgO/Ag(001) interface by exposing the MgO films to a Mg flux. A substantial reduction of the MgO/Ag(001) work function is observed during the exposition phase and reflects both band-offset variations at the interface and band bending effects in the oxide film.

Tuning the Schottky barrier height at MgO/metal interface

We present an experimental investigation of the interface electronic structure of thin MgO films epitaxially grown on Ag(001) by x-ray and ultraviolet photoemission spectroscopy as a function of the oxide growth conditions. It is shown that the Schottky barrier height at MgO/metal interface can be tuned over 0.7 eV by a modification of the oxygen partial pressure or the sample temperature. These experimental results are explained in the framework of the extended Schottky-Mott model and the MgO-induced polarization effect by Mg enrichment of the silver surface region.

Formation of a body-centered-cubic Fe-based alloy at the Fe/GaAs(001) interface

The room temperature epitaxial growth of Fe films on the As-rich GaAs(001)-(2×4) surface is studied using x-ray photoelectron spectroscopy as well as reflection high-energy electron diffraction and photoelectron diffraction. Interdiffusion mechanisms take place between Fe and GaAs during the deposition of the first 4 ML (0.7nm) Fe. The authors find that an Fe-based substitutional alloy with a body-centered-cubic structure confined on several atomic planes and containing 30% of foreign species (Ga and As atoms) sits at the Fe∕GaAs(001) interface. This intermixed layer is then buried by an almost pure Fe layer.