Se-treated GaAs Research Articles

Atomic structure of the Se-passivated GaAs(001) surface revisited

We present a combined experimental and theoretical study of the Se-treated GaAs(001)-(2times 1) surface. The (2times 1) structure with the two-fold coordinated Se atom at the outermost layer and the three-fold coordinated Se atom at the third layer was found to be energetically stable and agrees well with the experimental data from scanning tunneling microscopy, low energy electron diffraction, and x-ray photoelectron spectroscopy. This atomic geometry accounts for the improved stability of the Se-treated surface against the oxidation. The present result allows us to address a long-standing question on the structure of the Se-passivated GaAs surface, and will leads us to a more complete understanding of the physical origin of the electrical and chemical passivation of Se-treated GaAs surface.

Atomic structure and passivated nature of the Se-treated GaAs(111)B surface

We have systematically studied the atomic structure and electronic properties of the Se-treated GaAs(111)B surface using scanning tunneling microscopy, reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and first-principles calculations. We have found that Se atoms substitute frac{3}{4} monolayer of As atoms at the outermost layer of the ideal (111)B surface. Charge transfer from Se to As eliminates all of unsaturated dangling bonds, so that the surface is electronically stabilized, leaving no surface states in the mid-gap region.

Evolution of Surface and Interface Structures in Molecular-Beam Epitaxy of MoSe2 on GaAs(111)A and (111)B

We have systematically studied the atomistic growth processes of monolayer MoSe2 on GaAs(111)A and (111)B substrates. A combination of complementary techniques of reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy allows us to study the evolution processes of surface and interface structures during the MoSe2 growth. Highly oriented MoSe2 films are epitaxially grown in two steps: the amorphous Mo islands are initially formed on Se-treated GaAs surfaces, which is followed by the crystallization into MoSe2 under the supply of a Se molecular beam. While the initial Mo deposition leads to the segregation of Se atoms from the Se-treated GaAs surface, the subsequent supply of the Se beam induces the reconstruction of the Se-terminated GaAs structure beneath the MoSe2 film.

Vertical twinning of the Dirac cone at the interface between topological insulators and semiconductors.

Topological insulators are a new class of matter characterized by the unique electronic properties of an insulating bulk and metallic boundaries arising from non-trivial bulk band topology. While the surfaces of topological insulators have been well studied, the interface between topological insulators and semiconductors may not only be more technologically relevant, but the interaction with non-topological states may fundamentally alter the physics. Here, we present a general model to show that this type of interaction can lead to vertical twinning of the Dirac cone, whereby the hybridized non-spin-degenerate interfacial states cross twice as they span the bulk bandgap. This hybridization leads to spin-momentum locking of non-topological states with either helical (clockwise or anticlockwise) or even anti-helical (negative winding number) spin orientation depending on the parametization of the interaction. Model results are corroborated by first-principles calculations of the technologically relevant Bi2Se3 film van der Waals bound to a Se-treated GaAs substrate.

The interface formation of PTCDA on Se-modified GaAs(100) surfaces

The interface formation between the molecular beam deposited molecular semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and Se-treated GaAs(100)-(2×1) surface was investigated by soft X-ray photoemission spectroscopy (SXPS) using synchrotron radiation. For room temperature growth, no chemical bond is observed. At elevated sample temperature up to 350°C after the growth, all PTCDA except one monolayer desorbs from the surface. A loss of Se dimer atoms on top of the surface, which is in accordance with the number of anhydride oxygen atoms of PTCDA in a unit cell, indicates the reaction with PTCDA molecules upon annealing. The growth mode and the band banding upon PTCDA deposition on Se-treated GaAs(100) are also discussed.

Epitaxy, Modification of Electronic Structures, Overlayer-Substrate Reaction and Segregation in Ferromagnetic Co Films on Se-Treated GaAs(001) Surface

The electronic structure and chemical states of the overlayer and substrate in epitaxially grown Co films on Se-treated GaAs(001) 2×1 are investigated by photoemission spectroscopy (PES) using synchrotron radiation. Adatom (Co)-substrate (Ga) intermixing is found at the initial stages of Co deposition. The intermixed layer is then buried and epitaxial growth is achieved following further deposition. During the intermixing, Se is floated over the surface of Co overlayers. A small amount of As-segregation is also observed in the thicker Co films.

Work Function Changes of GaAs Surfaces Induced by Se treatment

Effects of a Se treatment on the work function of epitaxially grown GaAs surfaces were measured by photoemission spectroscopy. Although the Se treatments reduced the band bending of n-type GaAs, the work function increased. This result indicates that the surface component of the work function increased and that the surface dipole moment, whose direction was from the surface to the bulk (outer layer: -, inner layer: +), was formed at the Se-treated GaAs surface. On the other hand, qualitative analysis based on the electron counting model revealed that the direction of the surface dipole in the previously proposed structural model of Se/GaAs(001) was completely opposite to that obtained in our experimental results. It is suggested that this discrepancy is due to the vacancies in the internal Ga layer in the structural model.

The effect of surface modification on the formation of quantum structures in highly mismatched heterostructures: InAs on GaAs(100)

The effect of interface modification by insertion of Se layer on the formation of InAs dots on GaAs(001) is investigated by means of reflection high energy electron diffraction (RHEED) and atomic force microscope. The incorporation of a Se single layer into the interface is found to give rise to an extended critical thickness, presumably due to reduction of interface energy by the formation of Se–Ga/Se–In bonds. The reorganization of InAs supercritical 2D layers to form InAs dots on Se-treated GaAs(001) surface is reviewed by AFM images.

Surface structure of Zn- or Se-treated GaAs(001) and its influence for ZnSe heteroepitaxy

The surface structures of Se- or Zn-treated GaAs(001) have been investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and reflection high-energy electron diffraction connected to a dual chamber MBE system. The influence of controlled GaAs surface for the initial stage of ZnSe heteroepitaxy has also been studied. A (2 × 1) structure was generated by Se-exposure on both As- and Ga-stabilized GaAs(001) surface. This is recognized as a result of the reaction between Se and Ga. In the case of Zn-exposure on (2 × 4) surfaces, the change of RHEED intensity and XPS measurement suggested Zn deposition but there were no obvious changes in the STM images. During the ZnSe growth on a Se-terminated GaAs-(2 × 1) surface, RHEED patterns immediately became spotty and a disordered rough surface was observed by STM. On the other hand, during ZnSe growth on the Zn-treated GaAs-(2 × 4) surface, RHEED patterns were streaky from the very beginning. We could also successfully obtain STM images of a Se-stabilized ZnSe(001)-(2 × 1) reconstruction and this means that well-ordered surfaces were obtained as a result of layer mode growth using the Zn-treatment.

Superstructures of Se-Treated GaAs(001) Surface Studied by Scanning Tunneling Microscopy.

Under the condition of low chemical potential, a 2×3 and other new superstructures, the axes of which are in directions different from crystal axes, were observed for Se-treated GaAs(001) surface by scanning tunneling microscopy (STM). These structures were explained by the dimer model where buckling structures of the Se dimers play important roles. The controversial 2×1 dimer structure was also observed; however, the corrugation in the 2×1 cluster was very small (-0.01nm), which is considered to have caused difficulties in the observation of this surface by STM.

Surface structure of Se-treated GaAs(001) from angle-resolved analysis of core-level photoelectron spectra.

Angle-resolved core-level photoelectron spectroscopy using a monochromatized synchrotron radiation source was used to study the structure of the Se-treated GaAs(001) surface. Depth profiling was achieved by varying the photoelectron angle with respect to the sample surface. The Se 3d spectrum was found to consist of two components. The higher-kinetic-energy component is assigned as the surface-sensitive component, which is opposite to the conventional assignment based on Ga coordination. The intensity ratios of the two Se components are calculated with a layer attenuation model, and the polar angle dependence shows that the two components cannot simply be assigned as ``surface'' and ``inner'' Se but are two different chemical states that exist at both the first and second Se layers, with only one chemical state existing at the third Se layer. Based on these results, we derived a Ga-vacancy zinc-blende structure where the first Ga layer has a vacancy occupancy of approximately 25%.

High-quality molecular-beam epitaxial regrowth of (Al,Ga)As on Se-modified (100) GaAs surfaces

It is shown that high-quality molecular-beam epitaxial (MBE) regrowth of (Al,Ga)As on GaAs can be achieved by chemically passivating the GaAs surface ex situ prior to regrowth with aqueous selenium reagents. Reflection high-energy electron diffraction intensity oscillations show the bidimensional character of the regrowth and high-resolution transmission electron microscopy reveals defect-free regrown interfaces. Photoluminescence intensity from the Se-treated GaAs surfaces on which Al0.5Ga0.5 As is regrown rivals that from an all in situ grown AlGaAs/GaAs interface. The high quality of these regrown interfaces could be attributed to the thermally and chemically stable selenium and oxygen phases that remain bound to GaAs under MBE conditions.