Oxidation Of InAs Research Articles

Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy

AbstractThe electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle‐resolved photoemission spectroscopy (ARPES). Large‐scale first principles simulations are enabled by using DFT calculations with a machine‐learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.

Features of the Growth and Properties of Dielectric Layers and Metal−Insulator−Semiconductor Structures Obtained via the Anodic Oxidation of InAs in an Electrolyte Containing Fluorine Ions

The fluorine-atom profiles over the dielectric-layer thickness, as well as the electrophysical parameters of metal–insulator–semiconductor structures obtained when InAs crystals are anodically oxidized under galvanostatic conditions at two anodizing current densities in an electrolyte containing fluoride ions, are investigated. The features of variations both in the fluorine-atom distribution and in the effective surfacestate charge on the InAs–layer interface, which are observed during layer growth, are discussed.

Oxidation and etching behaviors of the InAs surface in various acidic and basic chemical solutions

Indium arsenide (InAs) is the candidate of choice as a new channel material for application in future technologies beyond the Si–based electronic devices because it has a much higher electron mobility than silicon. In this study, the oxidation and etching behaviors of InAs (100) in various acidic and basic solutions, such as HF, HCl, H2SO4, NaOH, KOH, and NH4OH, were investigated. In addition, the effect of pH on the oxidation and etching reactions taking place on the InAs surface was studied using solutions with a pH ranging from 1 to 13. It was observed that the oxidation of the InAs surface was hindered in acidic solutions, which was attributed to the dissolution of the oxidized surface layer. In particular, the treatment of the InAs surface using a strongly acidic solution with a pH of less than 3 produced an oxide–free surface due to the predominant etching of the InAs surface. The addition of H2O2 to the acidic solutions greatly increased the etching rate of the InAs surface, which suggests that the oxidation process is the rate–limiting step in the sequence of reactions that occur during the etching of the InAs surface in acidic solutions. The etching of InAs was suppressed in neutral solutions, which resulted in the formation of a relatively thicker oxide layer on the surface, and mild etching of the InAs surface took place in basic solutions. However, in basic solutions, the addition of H2O2 did not significantly contribute to the increase of the oxidation state of the InAs surface; thus, its effect on the etching rate of InAs was smaller than in acidic solutions.

Effect of fluorine on the electrical properties of anodic oxide/InAs(111)A interface

The electrical properties of metal-insulator-semiconductor structures based on InAs(111)A with thin anodic insulator layers of various thicknesses (7–20 nm) are investigated. It is established that the oxidation of InAs in a fluorinated acid electrolyte results in decreasing density of surface states and fixed charge in the anodic layer to values of < 2 × 1010 cm−2 eV−1 and ∼3 × 1011 cm−2, respectively. Comparison of the electrical parameters with the chemical composition of the layers shows that an improvement in the parameters of the fluorinated anodic oxide/InAs(111)A interface is caused by the substitution of oxygen atoms for fluorine in the anode layers with the formation of indium and arsenic oxifluorides and In-F bonds on the InAs surface.

Formation of anodic layers on InAs (111)III. Study of the chemical composition

The chemical composition of ∼20-nm-thick anodic layers grown on InAs (111)III in alkaline and acid electrolytes containing or not containing NH4F is studied by X-ray photoelectron spectroscopy. It is shown that the composition of fluorinated layers is controlled by the relation between the concentrations of fluorine and hydroxide ions in the electrolyte and by diffusion processes in the growing layer. Fluorine accumulates at the (anodic layer)/InAs interface. Oxidation of InAs in an acid electrolyte with a low oxygen content and a high NH4F content brings about the formation of anodic layers with a high content of fluorine and elemental arsenic and the formation of an oxygen-free InF x /InAs interface. Fluorinated layers grown in an alkaline electrolyte with a high content of O2− and/or OH− groups contain approximately three times less fluorine and consist of indium and arsenic oxyfluorides. No distinction between the compositions of the layers grown in both types of fluorine-free electrolytes is established.

In situ XPS study of InAs oxidation in glow-discharge plasma

This is the first in situ XPS study of the InAs oxidation kinetics in glow-discharge plasma in the atmosphere of O2 and CO2 gases and in a mixture of O2 and NF3 gases. Chemical composition of the oxide films produced by cathodic and anodic polarization of samples was examined. Main regularities and features of the oxide film formation on the InAs surface in the normal and dark glow discharge modes were revealed. Normal glow discharge in oxygen-containing plasma was shown to form bilayer oxide films on the InAs surface. The bottom layer with thickness of some nanometers, which consists of arsenic and indium oxides, forms at the initial oxidation steps, its thickness remaining virtually unchanged. The upper layer consists of Al2O3 produced by sputtering of cathode material; it serves as a barrier to oxygen diffusion, its thickness building up linearly with the treatment time in glow-discharge plasma. Chemical composition of the growing proper InAs oxide film and the stoichiometry of subsurface region of a semiconductor substrate strongly depend on the oxidation process parameters. The obtained regularities are discussed.

Band-Gap Determination of the Native Oxide Capping Quantum Dots by Use of Different Kinds of Conductive AFM Probes: Example of InAs/GaAs Quantum Dots

In most of quantum-dot (QD) systems, the electrical behavior of a single QD is directly linked to the native oxide grown on their surface. Obtaining quantitative electrical measurements requires identifying this oxide well, which is not a trivial task. Due to the use of two conductive atomic force microscopy (C-AFM) probes of different behaviors, C-AFM experiments and local electrical measurements allow one to determine the barrier heights at the interface between InAs QD and the native oxide and then to deduce the oxide band gap. In the case of InAs/GaAs QDs, based on our work and on literature results, it may be assumed that the capping oxide is an InAs oxide enriched in indium.

Etching and oxidation of InAs in planar inductively coupled plasma

The surface of InAs (1 1 1)A was investigated under plasmachemical etching in the gas mixture CH 4/H 2/Ar. Etching was performed using the RF (13.56 MHz) and ICP plasma with the power 30–150 and 50–300 W, respectively; gas pressure in the reactor was 3–10 mTorr. It was demonstrated that the composition of the subsurface layer less than 5 nm thick changes during plasmachemical etching. A method of deep etching of InAs involving ICP plasma and hydrocarbon based chemistry providing the conservation of the surface relief is proposed. Optimal conditions and the composition of the gas phase for plasmachemical etching ensuring acceptable etch rates were selected.

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/ c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/ c(8 × 2) surface by molecular oxygen (O 2) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O 2 dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21–24 Å in length at ∼25% ML coverage, representing 2–3 unit cell lengths in the [1 1 0] direction (the length of ∼5–6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10 −4) of O 2 on the InAs(0 0 1)-(4 × 2)/ c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.

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Electron interference effects at 4.2K were studied in nano-scale open dots fabricated in InAs surface inversion layers and in an InAs/AlGaSb quantum well. These structures were fabricated using AFM oxidation. We have observed periodic oscillations and aperiodic fluctuations on both systems. We investigated oscillation period using Fast Fourier Transform analysis. We found that for the InAs surface inversion layers the oscillation periods correspond to that of Aharonov-Bohm effect (AB effect) in agreement with the dot size. In contrast, for the InAs/AlGaSb quantum well, the deduced oscillation period did not correspond to the dot size. These results suggest that the observed AB oscillations are caused by the channel formed along the dot periphery due to the electron transfer from the InAs oxide to the InAs. Aperiodic fluctuations were characterized using correlation field analysis. For InAs/AlGaSb quantum well, correlation field was increased due to smaller skipping radius, as magnetic field increased. For InAs inversion layers, the correlation fields were not changed because the dot size is smaller than the cyclotron radius. The estimated electron phase breaking times tf were 1 psec and 10 psec for the InAs surface inversion layers and quantum well, respectively. The longer tf in InAs than that of GaAs systems at 4.2K indicates the importance of electron interference in InAs even at higher temperatures.

Aharonov–Bohm oscillations observed in nanoscale open-dot structures fabricated in an InAs surface inversion layer

We fabricated nanoscale open-dot structures in an InAs surface inversion layer using an atomic-force-microscope oxidation process. Due to its superior nanofabrication capability, small open-dot structures with the feature size ranging between 100 and 300 nm were successfully fabricated. The magnetoresistance signal measured at 4.2 K showed reproducible fluctuations and a periodic oscillation component that varies in both amplitude and periodicity depending on the dot size. We show that the period of the oscillations corresponds to that of the Aharonov–Bohm effect and propose that the possible mechanism for the oscillations is due to the formation of a one-dimensional electron channel enclosing the open-dot structure as a result of the electron transfer from the InAs oxide to InAs.

Interaction between oxygen and InAs(1 1 1) surfaces, influence of the electron accumulation layer

The oxidation of InAs(111) surfaces has been studied by photoelectron spectroscopy. Both the InAs(111)A and the InAs(111)B surfaces are studied and it is found that the initial oxidation follows different paths for the two surfaces. At low oxygen exposures of the A face the Fermi level structure, which is due to the electron accumulation layer, increases in intensity. On the B-side the intensity of the lone pair surface state decreases with increasing oxygen exposure. For larger exposures significant changes can be observed in the line shape of the In 4d and the As 3d core levels.

Effect of surface reconstruction on the low-temperature oxidation of InAs(100): Optical investigations

Using reflectance anisotropy spectroscopy (RAS), after identifying the main spectral lines, for the InAs(001) surfaces, we demonstrate that low-temperature (140 K) oxygen adsorption proceeds in a completely different way for the As-rich and for the In-rich surfaces. Oxygen is chemisorbed on the In-rich (4\ifmmode\times\else\texttimes\fi{}2) surface, as found from the disappearance from the RA spectrum of the optical transitions due to surface dimers. The adsorbed oxygen cannot be removed by annealing, even up to 670 K. On the As-rich ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\alpha}}_{2}$ surface reconstructions, adsorption is limited to molecular physisorption: oxygen is desorbed by annealing at a significantly lower temperature of \ensuremath{\approx}300 K, and only weakly perturbs the signal related to surface dimers.

A Single-Electron Transistor Produced by Nanoscale Oxidation of InAs

Atomic force microscope (AFM) oxidation was applied for the direct oxidation of a thin InAs layer in order to fabricate a single-electron transistor (SET) from an InAs/AlGaSb heterostructure. By employing the oxidized InAs as an insulator to form an in-plane-gate structure, SET structures were fabricated. For the improvement of the electrical properties of the InAs oxide as an insulator, a thinner InAs channel layer (10 nm) was introduced. This layer enables the AFM oxidation process at relatively low tip voltages with reduced tip wear. An improved breakdown voltage of as high as 1 V with a negligible leakage current of less than 1 nA was obtained. By using the 10-nm-thick InAs channel layer, SETs with a one- or two-isolated island structure were fabricated. The device characteristics measured at 4.2 K show SET characteristics confirming the successful formation of the gate insulator. While the SET with one island showed reproducible Coulomb oscillations, the other showed unstable oscillatory behavior due to the sensitivity of the structure to background charges. The improved SET characteristics evidenced by the clear Coulomb diamond structure indicate that the introduction of the thinner InAs channel layer is critical.

UV/ozone-activated growth of oxide layers on InAs(001) surfaces and oxide desorption under arsenic pressure

The oxidation of InAs(001) surfaces under UV-generated ozone and thermal desorption of the oxide layers under an As pressure was investigated by in situ reflection high-energy electron diffraction (RHEED), XPS, and STM, as well as by ex situ TEM and AFM. Amorphous oxide layers of up to 3 nm thickness were produced at room temperature, and consisted in As 2O 3, As 2O 5, and In 2O 3, as well as some atomic As at the interface between oxide and substrate. During heating to about 300°C, the As-oxides desorbed, together with the initially present carbonaceous contamination, while In 2O 3 recrystallized, partly in epitaxial orientation on the substrate. Most of the In-oxide islands evaporated upon heating to 480°C. A short time heat pulse to about 580°C resulted in complete desorption of the remaining oxide islands, while more gradual heating caused decomposition of the InAs and formation of In droplets. It is demonstrated that UV/ozone oxidation and controlled oxide desorption yields clean, stoichiometric and reasonably flat surfaces, which are suitable for molecular beam epitaxy (MBE) growth of InAs layers and heterostructures.

Quantitative ARXPS depth profiling characterisation of native oxides grown on In 0.53Ga 0.47As(100) single crystals

We report the results of an angle-resolved X-ray photoelectron spectroscopy (ARXPS) quantitative study of roomtemperature native oxides on LPE grown In 0.53Ga 0.47As(100). ARXPS is a powerful non-destructive experimental technique for identifyting chemical species and following compositional variations close to the surface (less than 100 Å of thickness). Such investigations can be of relevance for understanding the growth mechanism, competition between different cations with respect to oxidation, and surface damage due to the degradation process. In this approach a model of photoemission from a planar InGaAs bulk covered with two homogeneous overlayers (representing an oxide and a degradation region) constitutes the basis of the computer-aided ARXPS analysis. The thickness and the composition of the two overlayers are the parameters of a curve fitting, based on the Laplace transform of step-like profile functions, of the experimental data taken at different photoelectron escape angles. Our results indicate the presence of non-stoichiometric oxides lacking oxygen with respect to In(OH) 3, Ga 2O 3 and As 2O 3, respectively. An oxide layer about 9 Å thick is detected on the top of the original surface and is composed of 66% indium hydroxide, 31% arsenic oxide and 3% elemental arsenic. The following inner region corresponds to a diffuse interface oxide/substrate about 12 Å thick, containing 41% GaAs, 28% gallium oxide, 28% indium hydroxide and 3% InAs. Finally, the composition of the substrate agrees with the nominal composition of the epitaxially grown crystal. These results indicate a preferential oxidation of InAs with respect to GaAs and suggests that the driving force of the oxidation process is the binding energy difference of the two binary compounds constituting the crystal.

Initial stages of InAs deposition on SrF2-coated EuBa2Cu3O7−y thin-film superconductors

Initial stages of InAs deposition on EuBa2Cu3O7−y (EBCO) thin-film superconductors with a SrF2 interlayer are investigated by using synchrotron-radiation photoelectron spectroscopy. Core-level and valence-band spectra can give imformation on both oxidation of InAs and reduction of the underlying EBCO with a very thin surface layer. These spectral evolutions demonstrate that SrF2 does not affect the underlying EBCO, and that the two-step growth procedure, comprising the initial InAs layer growth at room temperature and the top InAs layer growth at 200 °C, is effective in suppressing the interfacial redox reactions. Furthermore, cross-sectional transmission electron microscopy and x-ray-diffraction studies comfirm that SrF2 forms a uniform layer on the EBCO surface, and that the two-step growth process can crystallize an InAs top layer, which is highly (111) oriented with a full width at half-maximum of 0.25°.

Oxidation of InAs(110) and correlated changes of electronic surface properties

The adsorption of unexcited, molecular oxygen at clean, cleaved InAs(110) surfaces at room temperature was monitored by AES, EELS, UPS, and a Kelvin probe under the same experimental conditions. The main oxygen uptake was found to set in at some 106 L of O2 and to proceed in two subsequent stages, T1 and T2, which are activated and immobile adsorption followed by field-aided oxide-film growth (Cabrera–Mott mechanism). The oxygen is incorporated in the top layer rather than adsorbed on it since the unchanged ionization energy indicates the absence of adsorption dipoles with moment components normal to the surface. With increasing oxygen uptake, As(3d) and In(4d) energy losses chemically shifted by 3 and 0.6 eV as well as an O(2p)-related energy loss of 6.8 eV and a O(2p)-related valence-band feature are observed to grow to the same extent as the O(510 eV) Auger line. Already at very low exposures, which result in no oxygen uptake detectable by AES, a surface band-bending builds up which saturates at some 104 L of O2. The Fermi level is then pinned at 0.13 eV above the bottom of the conduction band. Above 108 L of O2, i.e., in adsorption stage T2, the work function was found to decrease again.

Thermal oxidation of InAs and characterization of the oxide film

The thermal oxidation of InAs and the properties of the resultant oxide films were studied. Uniform InAs oxide films with a resistivity of 10 12 Ω cm were obtained on InAs substrates by low temperature oxidation at around 400°C. The thermal oxide films are mainly composed of polycrystalline In 2O 3 and As 2O 3. Increasing oxidation temperature causes thermal decomposition of the As 2O 3 and the accumulation of elemental arsenic in the oxide film. The oxide resistivity decreases with increasing oxidation temperature, mainly as a result of the thermal decomposition of the As 2O 3. The interface state densities near the midgap of InAs are (2–5) × 10 11 cm -2 eV -1. A metamorphic layer is formed beneath the thermal oxide of InAs by high temperature oxidation above 500°C.

Thermal oxidation of InAs

The growth rate and chemical composition of thermally grown oxides on 〈111〉 InAs are presented. The oxide was found to grow very slowly at 350°C but the growth was rapid at 450 °C. The oxides grown at 350 °C contained approximately equal concentrations of As 2O 3 and In 2O 3 and no detectable elemental arsenic. The As 2O 3 concentration decreased and the elemental arsenic concentration increased with increasing growth temperature. The oxides were amorphous unless the growth temperature was 475 °C or above.