Gallium Arsenide Surface Research Articles

Stable gallium arsenide MIS capacitors and MIS field effect transistors by (NH/sub 4/)/sub 2/S/sub x/ treatment and hydrogenation using plasma deposited silicon nitride gate insulator

The beneficial effects of sulfur passivation of gallium arsenide (GaAs) surface by (NH/sub 4/)/sub 2/S/sub x/ chemical treatment and by hydrogenation of the insulator-GaAs interface using the plasma-enhanced chemical vapor-deposited (PECVD) silicon nitride gate dielectric film as the source of hydrogen are illustrated by fabricating Al/PECVD silicon nitride/n-GaAs MIS capacitors and metal insulator semiconductor field effect transistors (MISFET). Post metallization annealing (PMA) at temperatures in the range 450-550/spl deg/C is shown to be the key process for achieving midgap interface state density below 10/sup 11//cm/sup 2//eV and maximum incremental transconductance, which is about 75% of the theoretical maximum limit. MIS capacitors are fabricated on (NH/sub 4/)/sub 2/S/sub x/ treated GaAs substrate using gate dielectrics such as PECVD SiO/sub 2/ and silicon oxynitride to demonstrate that the PMA is less effective with these dielectrics because of their lower hydrogen content. The small signal AC transconductance, g/sub ms/ measurements on MISFETs fabricated using silicon nitride, have shown that the low-frequency degradation of g/sub ms/ is almost absent in the devices fabricated on (NH/sub 4/)/sub 2/S/sub x/-treated GaAs substrates and subjected to PMA. The drain current stability in these devices is demonstrated to be excellent, with an initial drift of only 2% of the starting value. The dual role of silicon nitride layer, namely, protection against loss of sulfur and an excellent source of hydrogen for additional surface passivation along with sulfur is demonstrated by comparing the transconductance of MISFETs fabricated on GaAs substrates annealed without the nitride cap after the (NH/sub 4/)/sub 2/S/sub x/ treatment.

The role of atomic surface structure in the metalorganic chemical vapor deposition of III-V compound semiconductors

The atomic structure of gallium arsenide and indium phosphide (001) surfaces in the metalorganic chemical vapor deposition (MOCVD) environment has been characterized in situ by scanning tunneling microscopy and infrared spectroscopy. During growth at V/III ratios above 10, these films exhibit similar surface structures. They are terminated with alkyl groups, hydrogen atoms and group V dimers (As or P) adsorbed on top of a complete layer of group V atoms. As the V/III ratio decreases, the exposed arsenic and phosphorous atoms desorb from the surface. On gallium arsenide, this occurs through a phase transition, involving gallium out-diffusion and surface roughening. By contrast, on InP, the underlying phosphorous atoms simply form rows of dimers, yielding a smooth, continuous layer.

Atomic force microscopy on bare and thiol monolayer covered gallium arsenide

In this work, we study the functionalized gallium arsenide (GaAs) surface with self-assembled monolayer (SAM) by means of atomic force microscopy (AFM). First, we study the flatness of the dummy GaAs surface through AFM in electrolyte. Second, with the same technique, we study the effect of SAMs of octadecanethiol (C 18H 37SH) and alcohol-thiol (HOCH 2(CH 2) 10SH) on GaAs surface. Finally, for GaAs treated with octadecanethiol monolayer, we observe a hysteresis—demonstrating that the surface is hydrophobic and that the Van der Waals interaction between the tip and the sample is very high. GaAs treated with alcohol-thiol shows hysteresis, which shows that the surface is hydrophilic and uncharged.

Variational quantum Monte Carlo calculations for solid surfaces

Quantum Monte Carlo methods have proven to predict atomic and bulk properties of light and non-light elements with high accuracy. Here we report on the first variational quantum Monte Carlo (VMC) calculations for solid surfaces. Taking the boundary condition for the simulation from a finite layer geometry, the Hamiltonian, including a nonlocal pseudopotential, is cast in a layer resolved form and evaluated with a two-dimensional Ewald summation technique. The exact cancellation of all Jellium contributions to the Hamiltonian is ensured. The many-body trial wave function consists of a Slater determinant with parameterized localized orbitals and a Jastrow factor with a common two-body term plus a new confinement term representing further variational freedom to take into account the existence of the surface. We present results for the ideal (110) surface of Galliumarsenide for different system sizes. With the optimized trial wave function, we determine some properties related to a solid surface to illustrate that VMC techniques provide standard results under full inclusion of many-body effects at solid surfaces.

Degradation of gallium arsenide under irradiation with an excimer laser

The results of a study of degradation of the surface of gallium arsenide resulting from irradiation with a power excimer laser at power densities ranging from the threshold power to the power level causing local melting of the surface are presented. Two degradation mechanisms have been identified, one of which causes the formation of a thin near-surface layer of modified nonstoichimetric gallium arsenide at a power level higher than 1×107 W/cm2 and the other of which causes the formation of a separate gallium phase. The formation of the separate gallium phase can be produced either by a single pulse of laser radiation with a power density exceeding 2.7×1011 W/cm2 or by a few less powerful pulses. An empirical relationship has been established between the power density and the number of pulses causing the formation of the separate gallium phase. It has also been established that as a result of laser irradiation at the boundary of “cold” and “hot” gallium arsenide, periodically ordered defects in the form of blocks aligned along the [100] directions emerge.

The Stability of Lysozyme Adsorbed on Silica and Gallium Arsenide Surfaces: Preferential Destabilization of Part of the Lysozyme Structure by Gallium Arsenide

Changes in the structural stability of lysozyme, upon adsorption to silica and gallium arsenide (GaAs) surfaces, are studied using a combination of hydrogen/deuterium exchange and matrix-assisted laser desorption/ionization mass spectrometry. This relatively new method offers a tool for directly monitoring the structural stability of adsorbed proteins and generates values for the stabilization energy of proteins and their domains in the adsorbed state. The adsorption and desorption kinetics of lysozyme on silica and GaAs surfaces are monitored with ellipsometry. From the adsorption kinetics it can be inferred that lysozyme adsorbs somewhat faster and in slightly higher amounts onto GaAs than on silica. The average Gibbs free energy required to open the lysozyme structure in solution (5.7 kcal/mol) is only slightly reduced upon adsorption onto silica, resulting in a Gibbs free energy of 5.4 kcal/mol. Adsorption onto GaAs surfaces results in a larger decrease in the stability of lysozyme. Moreover, a distinct difference in the stability within the lysozyme molecule is observed. Whereas one part of lysozyme adsorbed onto GaAs has an average structural stability of −5.5 kcal/mol, an approximately equally large part of the molecule has a stability of −3.7 kcal/mol.

Electrochemical passivation of gallium arsenide surface with organic self-assembled monolayers in aqueous electrolytes

Self-assembled monolayers of octadecylthiol (ODT) were reconstituted on freshly etched gallium arsenide (n-GaAs) for the electrochemical stabilization against decomposition of the surfaces (passivation) in aqueous buffers. The surface composition was evaluated by x-ray photoelectron spectroscopy to optimize the surface treatment before ODT deposition. Electrochemical properties of the monolayers were monitored by cyclic voltammetry and impedance spectroscopy. The impedance spectrum of the photoetched n-GaAs after the deposition of the ODT monolayer was stable in an aqueous electrolyte at pH=7.5 for more than 24 h within the sensitivity of our experimental technique. The effective passivation of GaAs surfaces is an essential step towards biosensor applications.

Mechanism of Arsine Adsorption on the Gallium-Rich GaAs(001)−(4 × 2) Surface

The kinetics and mechanism of arsine adsorption on the (4 × 2) surface of gallium arsenide (001) has been studied by scanning tunneling microscopy, infrared spectroscopy, and ab initio quantum chemistry calculations. Arsine forms a dative bond to a gallium dimer. Then, this species either desorbs from the surface or decomposes to an AsH2 or AsH fragment with hydrogen transfer to an arsenic site. Finally, desorption of hydrogen leaves arsenic dimers on the surface. The energy barriers for arsine desorption and dissociation into AsH2 are estimated to be 9.3 and 16.5 kcal/mol, respectively. Gallium hydride is not produced upon dissociation of AsH3 because this process is not energetically favorable.

Dynamic study of the surfaces of (001) gallium arsenide in metal-organic vapor-phase epitaxy during arsenic desorption

We have investigated by reflectance anisotropy spectroscopy the arsenic desorption from GaAs (001) at various temperatures in metal–organic vapor-phase epitaxy to obtain reaction orders and activation energies. The highest arsenic coverage, found at low temperatures with arsine stabilization, corresponds to a (4×3) reconstruction. Without arsine, arsenic starts to desorb and less arsenic-rich reconstructions are observed, depending on temperature: c(4×4) (below 800 K), β2(2×4) (below 920 K), α(2×4), and only with hydrogen carrier gas finally (4×2) (above 950 K). Above 920 K the reaction order differs in hydrogen and nitrogen atmosphere, probably due to an etching effect of hydrogen radicals. The five different desorption processes show either a first- or zero-order time dependence. First order is related to the desorption from the terraces and zero order to desorption from the steps (or kinks) on the surfaces. The activation energies for all processes are around 2.5 eV. This energy is, therefore, assumed to be the activation energy for the removal of an arsenic dimer from the surface.

Electron spectroscopy of objects for nanoelectronics

An electron-spectroscopic analysis is made of layered nanostructures and clusters at the surface and in the bulk of a solid. A new method of forming metal/insulator/semiconductor (superconductor) nanostructures is proposed based on ion-stimulated metal segregation effects at the surface of low-temperature gallium arsenide and a 123 high-temperature superconductor. The geometric parameters and electronic structure of these nano-objects are studied. It is shown that their electronic properties can be controllably varied in situ by acting on the surface. The dimensional transformation of the electronic properties of metal clusters is studied for clusters in the insulator SiO2, in the superconductor LTMBE-GaAs, and on silicon and graphite surfaces. The nature of this transformation is clarified. A diagnostics for cluster ensembles is developed by which one can determine the parameters needed to describe singleelectron transport: the average number of atoms per cluster, the average distance between clusters and isolated atoms, and the chemical state of the atoms. Ensembles of silver clusters with specified parameters are obtained on a silicon surface. It is shown that these ensembles are potentially useful for developing single-electron devices.

Reconstruction and electron states of a Ga2Se3-GaAs heterointerface

It is established by electron microscopy and electron diffraction analysis that the formation of Ga2Se3(110) layers on GaAs(100) and (111) surfaces during heat treatment of the latter in selenium vapor is accompanied by the formation of transition regions with crystallographic orientations [310] and \([\bar 2\bar 1\bar 1]\), respectively. It follows from an investigation of the spectrum of surface electron states in the resulting heterostructures that a reduction in the density of surface electron states is achieved only after selenium vapor treatment in a narrow interval of treatment durations (from 5 min to 30 min under the conditions established in the present study). All the results are discussed on the basis of concepts involving reconstruction of the gallium arsenide surface during chalcogen treatment.

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.

Effects of octa decyl thiol (ODT) treatment on the gallium arsenide surface and interface state density

The ability of Octa Decyl Thiol (ODT) compound in passivating Gallium Arsenide (GaAs) surfaces is studied by comparing the XPS spectra on the ODT treated and untreated GaAs samples and by comparing the electrical characteristics of Schottky diodes and MIS capacitors fabricated on them. A detailed study on MIS capacitors and an evaluation of the interface state density (Dit) has clearly demonstrated that even though the ODT treatment reduces the interface state density in the upper half of the band gap more than an order of magnitude compared to untreated samples, it is not effective in reducing the midgap Dit values. From the XPS spectra taken on the ODT treated GaAs surfaces, this is attributed to an island-like ODT coating and to the inability of this sulphur compound treatment to remove the native oxide present on the GaAs surface.

Effect of ion sputtering on the statistical properties of a surface

The modification of a gallium arsenide surface during irradiation by heavy cesium ions Cs+ is investigated by measuring the surface height distribution with an atomic force microscope. Both increases and decreases in the rms height σ, an integral parameter of the surface, are observed to occur. It is established that for all experimental samples the roughness of the gallium arsenide surface increases in a 1–100 nm lateral range. Analysis of the structure function yields an estimate of the characteristic lateral dimensions of the surface structures arising during ion etching.

Influence of the step height of the vicinal surface of germanium on the formation of antiphase boundaries in a gallium-arsenide-germanium-gallium-arsenide(001) system

An investigation was made of the formation of antiphase boundaries in a GaAs/Ge/GaAs(001) system using accurately oriented substrates and substrates misoriented by 3° and 5° in the [110] direction. It was shown that growth of germanium on a misoriented gallium arsenide surface leads to the formation of diatomic steps of height a0/2 and therefore results in the absence of any antiphase boundaries in a GaAs film grown on this surface. Conditions required to obtain a vicinal Ge surface consisting of monatomic steps of height a0/4, whose presence leads to the formation of antiphase boundaries during GaAs growth, are determined.

Gallium arsenide and indium arsenide surfaces produced by metalorganic vapor-phase epitaxy

Thin films of GaAs and InAs were deposited on GaAs(0 0 1) substrates by metalorganic vapor-phase epitaxy (MOVPE), and their surfaces were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). Gallium arsenide surfaces produced in the MOVPE reactor at 570°C and a V/III ratio of 50 exhibit a (1×2) reconstruction, and are covered with weakly bound alkyl groups. Heating this material in flowing hydrogen in the reactor produces a variety of surface phases, depending on the sample temperature. These phases include (2×4) and (4×2) reconstructions, all of which are terminated with As or Ga dimers. The surfaces of InAs films grown by MOVPE exhibit these same phases. This study demonstrates that compound semiconductor surfaces formed in the MOVPE environment are nearly the same as those produced by molecular-beam epitaxy under ultrahigh vacuum conditions.

Ab initiocalculation of the reflectance anisotropy of GaAs(110)

We compute the optical properties of the (110) surface of gallium arsenide within the first-principles density-functional theory local-density approximation scheme, using norm-conserving pseudopotentials. Starting from the surface electronic structure calculation, we analyze the imaginary part of the theoretical dielectric function, separating surface and bulk contributions. The effects of the nonlocality of the pseudopotential are studied, by working both in the transverse gauge (neglecting them) and in the longitudinal gauge (where they are automatically included). The two calculations, although giving different dielectric functions, yield the same reflectance anisotropy, which compares well with experimental data and with previous theoretical results.

Structure and composition of the c(4×4) reconstruction formed during gallium arsenide metalorganic vapor-phase epitaxy

Surfaces of GaAs (001) were prepared by metalorganic vapor-phase epitaxy and characterized by scanning tunneling microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy, and low-energy electron diffraction. Upon removal from the reactor, the gallium arsenide surface exhibits a (1×2) reconstruction, which is a disordered variant of the c(4×4). The disorder arises from the presence of adsorbed alkyl groups. Heating the sample to 350 °C desorbs the hydrocarbons and produces a well-ordered c(4×4) structure. A model is proposed for the alkyl-terminated (1×2) reconstruction.

Creation of vicinal facets on the surface of gallium arsenide with orientations close to (100) under conditions of nonequilibrium mass transfer

The method of low-energy electron diffraction is used to investigate surfaces of GaAs misaligned by 3° from (001) as they are heated in vacuum to a temperature of T=550 °C in the field of a temperature gradient ▽T⋍50 deg/cm. When no ▽T is present, a (1×4) structure forms at the surface, which is preserved under annealing for 1 hour. In the presence of ▽T, reflection doublets appear along the direction [110], indicating a faceted surface. Analogous behavior is observed after the crystals are annealed with no ▽T at T=650–700 °C, when the evaporation of arsenic becomes noticeable. The calculated value of the atomic heat of transport Q* of 2.3 eV is close to Q* for transition metals, where it is associated with phonon drag by the atoms.

Gallium arsenide surface chemistry and surface damage in a chlorine high density plasma etch process

In an effort to monitor ion-driven surface chemistry in the high density plasma etching of GaAs by Cl2/Ar plasma chemistries, we have applied mass spectrometry and careful substrate temperature control. Etch product chlorides were mass analyzed while the substrate temperature was monitored by optical bandgap thermometry and as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. By ensuring that the substrate temperature does not deviate during process variations, the changes in product mass peak intensities are a direct measure of changes in the ionassisted surface chemistry which promotes anisotropic etching. Experimental results show that ion-assisted surface chemistry is optimum when sufficient Cl and Cl+ are present in the incident plasma flux. These conditions are met at low coupled microwave powers ( 200 eV. Photoreflectance measurements of the surface Fermi level show significant damage for ion energies >75 eV. However, in situ and ex situ surface passivations can recover the surface Fermi level for up to 200 eV ion energies, in good correlation to the onset of sputtering and subsurface damage. Thus, anisotropic, low damage pattern transfer is possible for ion energies between 50 and 200 eV.