Clean GaAs Surface Research Articles

A 2D Bismuth-Induced Honeycomb Surface Structure on GaAs(111).

Two-dimensional (2D) topological insulators have fascinating physical properties which are promising for applications within spintronics. In order to realize spintronic devices working at room temperature, materials with a large nontrivial gap are needed. Bismuthene, a 2D layer of Bi atoms in a honeycomb structure, has recently attracted strong attention because of its record-large nontrivial gap, which is due to the strong spin-orbit coupling of Bi and the unusually strong interaction of the Bi atoms with the surface atoms of the substrate underneath. It would be a significant step forward to be able to form 2D materials with properties such as bismuthene on semiconductors such as GaAs, which has a band gap size relevant for electronics and a direct band gap for optical applications. Here, we present the successful formation of a 2D Bi honeycomb structure on GaAs, which fulfills these conditions. Bi atoms have been incorporated into a clean GaAs(111) surface, with As termination, based on Bi deposition under optimized growth conditions. Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/S) demonstrates a well-ordered large-scale honeycomb structure, consisting of Bi atoms in a √3 × √3 30° reconstruction on GaAs(111). X-ray photoelectron spectroscopy shows that the Bi atoms of the honeycomb structure only bond to the underlying As atoms. This is supported by calculations based on density functional theory that confirm the honeycomb structure with a large Bi-As binding energy and predict Bi-induced electronic bands within the GaAs band gap that open up a gap of nontrivial topological nature. STS results support the existence of Bi-induced states within the GaAs band gap. The GaAs:Bi honeycomb layer found here has a similar structure as previously published bismuthene on SiC or on Ag, though with a significantly larger lattice constant and only weak Bi-Bi bonding. It can therefore be considered as an extreme case of bismuthene, which is fundamentally interesting. Furthermore, it has the same exciting electronic properties, opening a large nontrivial gap, which is the requirement for room-temperature spintronic applications, and it is directly integrated in GaAs, a direct band gap semiconductor with a large range of (opto)electronic devices.

Revisiting heat treatment and surface activation of GaAs photocathodes: In situ studies using scanning tunneling microscopy and photoelectron spectroscopy

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as Cs2Te, with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with Cs2Te activation on GaAs used heat cleaning temperatures around 400°C to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around 580°C, which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for 580°C heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a 580°C heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as Cs2O or Cs11O3.

Preparation of Ga-terminated negative electron affinity-GaAs (100) surface by HCl-isopropanol treatment for nanoanalysis by scanning tunneling microscopy

Negative electron affinity (NEA) surfaces can emit electrons by low-energy light illumination that is nearly equal to the bandgap energy of a semiconductor because NEA surfaces lower the vacuum level to below the conduction-band minimum. In particular, NEA-GaAs surfaces show distinct characteristics such as high spin polarization, low emittance, short pulsed operation, and high intensity. NEA surfaces are formed by alternating application of Cs and O2 on a clean GaAs surface. Scanning tunneling microscopy (STM) was used to investigate the surface states of NEA-GaAs (100) surfaces prepared using HCl-isopropanol treatment followed by annealing in an ultrahigh vacuum. The results indicated remarkable improvement in the surface quality of the GaAs (100). The authors have been studying the relationship between electron emission properties and the adsorption structures of Cs on Ga-terminated GaAs surfaces. Here, they report the first observation of NEA-Ga-terminated surfaces with Cs adsorption using STM.

NEA surface activation of GaAs photocathode with different gases

The GaAs photo-cathode is a unique device which is able to generate highly polarized electron beam up to 90% and an extremely small emittance beam. The photo-electron emission is possible with IR (Infrared Red) and visible light and the quantum efficiency can be more than 20%. These unique features depend on the negative electron affinity (NEA) surface made by adsorption of Cs and O2/NF3, but this surface is easily damaged by residual gas adsorption, ion back-bombardment, etc. The exact structure of the NEA surface is not known, even there are several hypotheses. In this study, we performed the NEA activation on a cleaned GaAs surface with CO2, CO, N2, and O2 gases and compared the results to improve our understanding on the NEA surface. We found that the NEA activation with CO2 is similar to that with O2, but the maximum QE value is significantly lower than that with O2. We also found that N2 and CO gases did not contribute to NEA activation at all. By analyzing CO2 activation, we found that atomic oxygen activates the NEA surface and CO degrades the NEA surface simultaneously. We found that the NEA activation ability of atomic oxygen is almost a half of that of O2 molecule.

Sacrificial Self-Assembled Monolayers for the Passivation of GaAs (100) Surfaces and Interfaces

The use of sacrificial self-assembled monolayers (SAMs) to prepare clean n-type GaAs (100) surfaces without band bending in vacuo is demonstrated. GaAs surface passivation using octadecanethiol SAMs after HCl cleaning is shown to lead to an enhancement of the room-temperature photoluminescence intensity. Synchrotron-radiation photoelectron spectroscopy (SRPES) finds that the interfacial oxide between GaAs and the SAM remains below the detection limit. Evidence for both Ga–S and As–S bonds at the GaAs–thiolate interface is found. The limited thermal stability of the SAM allows the desorption of the alkyl chains by in situ thermal annealing at temperatures above 180 °C, leaving S bonded to Ga behind. The resulting surface contains only a very small amount of O (0.05 ML coverage) and C (about 3% of the SAM remaining) and shows no band bending with the surface Fermi level close to the conduction band. Atomic layer deposition of Al2O3 on this surface occurs via the formation of Al–S bonds without introducing any additional band bending. This indicates that the surface preparation of n-type GaAs (100) using sacrificial octadecanethiol SAMs followed by in situ thermal removal provides a route toward GaAs/oxide interfaces without interfacial oxides and without band bending.

A detailed look beneath the surface: Evidence of a surface reconstruction beneath a capping layer

Many physical effects are strongly depending on the composition of the interfaces between separating layers. Hence, the knowledge of the interfacial characteristics such as structure, chemical bonds, or magnetic properties of the corresponding materials is essential for an understanding and optimization of these effects. This study reports on a combined magnetic and structural analysis using X-ray photoelectron diffraction (XPD) and transverse magneto-optical Kerr effect (T-MOKE). The information depth of these methods is demonstrated by investigating the uppermost GaAs(001) layer beneath a Fe-film and the interfacial regimes of Fe/GaAs(001) beneath an MgO capping layer.Iron was prepared on a clean GaAs(001) surface and a GaAs(001)-(4×2)-reconstructed surface. Beneath the Fe-film, the (4×2)-reconstruction is not lifted, which is clearly shown by the diffraction pattern of the GaAs(4×2)-Fe surface. It is shown that Fe inter-diffusion, resulting in an amorphous interface, is almost prevented by the Ga-rich reconstruction. The magneto-optical measurements with T-MOKE clearly demonstrated the Fe-interlayer in a ferromagnetic state. We find no evidence for magnetic properties neither within the signal of the GaAs-substrate nor the MgO-film.

Fabrication of metallic magnetic nanostructures by argon ion milling using a reversed-polarity planar magnetron ion source

We demonstrate that a planar magnetron may be used as a source of ions for milling micro- and nanostructured devices. Reversing the polarity of the magnetron head, in combination with applying a voltage bias to the thin-film sample, allows acceleration of ions produced in the Ar glow-discharge to energies suitable for pattern transfer via etching. We have fabricated generic Hall-bar and nanowire L-bar structures from sputter deposited Ta/Ni/Ta trilayer films grown onto clean GaAs(001) surfaces. No degradation of the magnetic properties or contamination of the deposition chamber vacuum is observed, demonstrating that this method is effective for etching magnetic device structures patterned by both optical- and electron-beam lithography techniques.

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.

Interaction of 2-Propanol with the GaAs(100) Surface

Synchrotron photoemission spectroscopy is applied to study the interaction of 2-propanol with the clean GaAs(100) surface to understand the solid−solvent interaction at the semiconductor/electrolyte interface. Adsorption of 2-propanol molecules at liquid-nitrogen temperature and emersion of the semiconductor from liquid 2-propanol at room temperature are performed to gain a more detailed insight into surface reaction steps and processes involved. It is found that on adsorption at liquid-nitrogen temperature a 2-propanol molecule dissociates on a GaAs(100) surface to a hydrogen atom, which is adsorbed on an As atom that is part of an As−As dimer, and a 2-propoxy group, which adsorbs to an adjacent Ga atom. On a GaAs(100) surface emersed from liquid 2-propanol at room temperature, two different states of dissociative adsorption are observed. The first state corresponds to dissociation of a 2-propanol molecule into a hydrogen atom and a 2-propoxy group. The second state corresponds to dissociation into an OH group and a 2-propyl radical. On the GaAs(100) surface neither adsorption of 2-propanol molecules nor emersion from liquid 2-propanol causes destruction of the As−As dimer structure.

Interfacial reaction of atomic-layer-deposited HfO2 film as a function of the surface state of an n-GaAs (100) substrate

The characteristics of interfacial reactions and the valence band offset of HfO2 films grown on GaAs by atomic layer deposition were investigated by combining high-resolution x-ray photoelectron spectroscopy and high-resolution electron transmission microscopy. The interfacial characteristics are significantly dependent on the surface state of the GaAs substrate. Polycrystalline HfO2 film on a clean GaAs surface was changed to a well-ordered crystalline film as the annealing temperature increased, and a clean interface with no interfacial layer formed at temperatures above 600°C. The valence band offset of the film grown on the oxidized GaAs surface gradually increased with the stoichiometric change in the interfacial layer.

Theoretical study of the optical response of the adsorption of Sb on the GaAs(110) surface

AbstractA theoretical study of the optical response of the adsorption of Sb atoms on the clean GaAs(110) surface is presented. The reflectance anisotropy spectroscopy (RAS) spectrum is calculated and analysed as a function of Sbcoverage. To capture some of the possible dynamical processes of Sb sub‐monolayer growth, a 1 × 3 unit cell is considered whose equilibrium geometry has been obtained by performing an ab‐initio pseudopotential calculation with the use of density functional theory within the local density approximation. The RAS is calculated via the ab‐initio scheme and also by using a semiempirical tight binding formalism. Some trends in the RAS spectra are identified as the clean surface is partially covered by Sb until the one monolayer coverage is achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultra-high vacuum direct bonding of a p–n junction GaAs wafer using low-energy hydrogen ion beam surface cleaning

Low-energy hydrogen ion bombardment is used to clean GaAs surfaces. The hydrogen ions produce contamination-free surfaces without changes in surface composition (stoichiometry) and surface roughness. The wafers were brought into contact at room temperature after cleaning under ultra-high vacuum (UHV), and bonded over the whole area (2 inches) without application of external mechanical pressure. After bonding, the p-GaAs/n-GaAs wafer pair was annealed at 200 °C for 30 min under UHV conditions (<5×10 −10 mbar) to improve the interface bonding strength and to achieve a full-area wafer bonding. Infrared (IR) imaging of the as-bonded wafers directly reveal the real bonding behaviour. High-resolution transmission electron microscopy images reveal that the wafers have been directly bonded without damage of the crystal lattice or intermediate layer and the interface is smooth. Current–voltage characterization shows near-ideal forward characteristics and the recombination in p–n junction space charge region.

Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and∕or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal.

Reconfiguration of GaAs(110) surface with the adsorption of Xe atoms

First-principle calculations in the frame of density-functional theory (DFT) in the general gradient approximation (GGA) are performed by using the augmented plane wave plus local orbital (APW+lo) method for pure GaAs(110) surface and the adsorptions of Xe atoms on it. A supercell consisting of five atomic layers is constructed to simulate the geometical configuration of clean GaAs(110) surface and the adsorption of Xe atoms. The Newton dynamics method is used to relax the atomic positions. Initiating with the Xe atom on top of Ga atom, As atom, and at the bridge site, respectively, it is found that the total energy of the supercell reaches the minimum when the Xe atoms are located at the bridge site. Additionally, it is shown that the adsorption of Xe atoms make the relaxed GaAs(110) surface to return to the ideal crystal geometrical configuration as generally expected.

Structural and optical properties of InAs quantum dots regrown on atomic hydrogen-cleaned GaAs surface

We report the structural and optical properties of InAs quantum dots (QDs) directly regrown on air-exposed and subsequent atomic hydrogen-cleaned (AHC) GaAs (001) surface. The average size of InAs QDs on the AHC GaAs surface is 29nm, which is larger than 22nm for the conventionally grown InAs QDs on GaAs. The integrated photoluminescence intensity of the InAs QDs on the AHC GaAs measured at room temperature is larger than that of the reference sample by two orders, even though the cleaned GaAs surface directly faced the base of the InAs QDs. The decrease in the interface states between the wetting layer and AHC GaAs was confirmed by Franz–Keldysh oscillations of the photoreflectance spectra.

Theoretical model for adsorption of Sb on the GaAs(110) surface

We present a model for studying the adsorption of Sb atoms on the clean GaAs(110) surface and analyse the consequences of the adsorption on the reflectance anisotropy spectroscopy (RAS). A 1 × 8 unit cell is taken as a basis of the Sb-covered GaAs(110) structure. We allow the Sb-covered surface to be disordered by letting every surface atom move freely around its equilibrium position. In order to obtain a representative RAS spectrum of the surface we generate an ensemble with N different structural realizations of the surface and the ensemble RAS average is performed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of Cs adsorption on clean and Sb-covered GaAs surfaces

A full-potential linearized augmented-plane-wave (FLAPW) method is used for the investigation of the electronic structure of a series group III–V semiconductors. The electronic structure of low-index clean GaAs surfaces and with thin overlayers of cesium is studied. We analyze Cs(O) coadsorption on the GaAs(0 0 1) surface as well as the influence of Sb on its electron and adsorption properties.

Optimized cleaning method for producing device quality InP(100) surfaces

A very effective, two-step chemical etching method to produce clean InP(100) surfaces when combined with thermal annealing has been developed. The hydrogen peroxide∕sulfuric acid-based solutions, which are successfully used to clean GaAs(100) surfaces, leave a significant amount of residual oxide on the InP surface which cannot be removed by thermal annealing. Therefore, a second chemical etching step is needed to remove the oxide. We found that strong acid solutions with HCl or H2SO4 are able to remove the surface oxide and leave the InP surface passivated with elemental P which is, in turn, terminated with H. This yields a hydrophobic surface and allows for lower temperatures to be used during annealing. We also determined that the effectiveness of oxide removal is strongly dependent on the concentration of the acid. Surfaces cleaned by HF solutions were also studied and result in a hydrophilic surface with F-terminated surface In atoms. The chemical reactions leading to the differences in behavior between InP and GaAs are analyzed and the optimum cleaning method for InP is discussed.

Surface Reactions of 1-Propanethiol on GaAs(100)

The adsorption and decomposition pathways of 1-propanethiol on a Ga-rich GaAs(100) surface have been investigated using the techniques of temperature programmed desorption, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). 1-Propanethiol adsorbs dissociatively on a clean GaAs(100) surface to form propanethiolate and hydrogen. Further reactions of these species to form new products compete with the recombinative desorption of molecular propanethiol. The C-S bond scission in the propanethiolate results in the formation of propyl species and elemental sulfur. The generation of propene via beta-hydride elimination then follows. In addition, propane and hydrogen form via reductive elimination processes. A recombinative high-temperature propanethiol desorption state is also observed. XPS and TOF-SIMS analyses confirm the presence of sulfur on the GaAs(100) surface following thermal decomposition. This paper discusses the mechanisms by which these products form on the GaAs(100) surface.

Surface passivation and morphology of GaAs(1 0 0) treated in HCl-isopropanol solution

A promising chemical surface preparation technique, which consists in the treatment of GaAs(1 0 0) in HCl-isopropyl alcohol (HCl-iPA) solution under nitrogen atmosphere, is further developed. It was shown earlier [Tereshchenko et al., J. Vac. Sci. Technol. A 17 (1999) 2655] that HCl-iPA treatment and subsequent anneals in vacuum yielded atomically clean GaAs(1 0 0) surface with the whole range of surface reconstructions characteristic of this crystal face. In the present work the mechanisms of the passivation of GaAs(1 0 0) surfaces by arsenic overlayers as a result of HCl-iPA treatment are experimentally studied by X-ray photoelectron spectroscopy, low-energy electron diffraction and atomic force microscopy. The HCl-iPA treatment of clean As-stabilized GaAs(1 0 0) surfaces results in chemical passivation of the surface by submonolayer amount of excess arsenic. For the initially oxidized surfaces the treatment leads to the formation of 1–3 monolayers of amorphous arsenic on the surface, with the major part of the arsenic originating from the surface oxides dissolved in HCl-iPA. The HCl-iPA treatment preserves the atomic flatness of the GaAs(1 0 0) surface, keeping the mean roughness on a very low level of approximately ∼0.1 nm.