Compound Semiconductor Surfaces Research Articles

Nano-scale properties of defects in compound semiconductor surfaces

The present work reviews atomic-scale properties of point defects and dopant atoms exposed on and in cleavage surfaces of III–V and II–VI semiconductors. In particular, we concentrate on the identification of the types of defects and dopant atoms, the determination of the localized defect states, the electrical charge, and lattice relaxation, as well as the measurement of the interactions between different defects and/or dopant atoms. The physical mechanisms governing the formation of defect complexes, the compensation of dopant atoms, the pinning of the Fermi level, and the stability of defects are discussed in the light of the available theoretical information and experimental results obtained mostly by scanning tunneling microscopy.

A machine condition transfer function approach to run-to-run and machine-to-machine reproducibility of III-V compound semiconductor molecular beam epitaxial growth

A concept of machine condition transfer function (MCTF) based on the use of in-situ sensor response to intrinsic surface property of the material is proposed to meet the challenge of run-to-run and machine-to-machine reproducibility of optimized growth conditions for III-V compound semiconductor molecular beam epitaxical growth. The variation in the intensity of the specular beam in reflection high-energy electron diffraction from compound semiconductor surface, measured as a function of the temperature and anion pressure prior to growth, is used as the intrinsic surface property for the generation of MCTF's. The mathematical methodology for realizing MCTF's, including the sensor response surface reconstruction and optimal minimization in combination with a composite statistical design for initial estimation, is presented. Numerical evaluation of the developed methodology shows its promise in practice.

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.

Structure of Prototypical Semiconductor Surfaces and Interfaces Investigated by Photoemission Extended X-Ray Absorption Fine Structure (PEXAFS)

Extended X-ray absorption fine structure (EXAFS) has been known for half a century. However, using synchrotron radiation, it has developed into a powerful tool for determining the atomic structure of a wide variety of surfaces and interfaces. The power of this technique lies in its sensitivity to the local environment of a particular element. Photoemission extended X-ray absorption fine structure (PEXAFS) is a new variation of electron detection surface EXAFS (SEXAFS) using photoemission spectroscopy in the constant initial state mode, Due to small escape depths, a very high surface sensitivity is achieved. Other major advantages of this new technique include (i) an improved signal/noise ratio allowing very short data collection times, which is an especially useful feature for short lifetime surfaces, and (ii) double-checking interatomic distances. Combined with core level and valence band photoemission spectroscopies. PEXAFS provides the exceptional ability to probe the atomic geometry and the electronic structure at the same time and for the same surface. It thus gives access to important issues, such as (i) surface reconstruction and/or relaxation, (ii) bonding nature, (iii) adsorption site and (iv) initial interface formation. Furthermore, it could be used to clarify photoemission core level shift origin by allowing one to discriminate structural changes from other causes as initial or final state effects. This article reviews the latest PEXAFS investigations for model elemental (silicon) and compound (indium phosphide) semiconductor surfaces and their interfaces with alkali metals, antimony, aluminum, bismuth and silver.

Computer Analysis of Surface Recombination Process at Si and Compound Semiconductor Surfaces and Behavior of Surface Recombination Velocity

A theoretical analysis of the surface recombination is performed for n-Si, GaAs and InP surfaces under photo-excitation in terms of the so-called effective surface recombination velocity Seff. A very strong dependence of Seff both on the excitation light intensity and the surface fixed charge density has been found. Seff reaches its maximum when ns≈ps, independently on the light intensity. Seff can be significantly reduced by shift of the surface Fermi level towards band edges by means of the surface fixed charge QFC or by appropriately reducing or reshaping the surface state density distribution.

Optical measurement system for characterizing compound semiconductor interface and surface states

An optical measurement system that is useful for the characterization of interface and surface states on a wide variety of compound semiconductor material systems has been developed. The PC-based measurement system, using argon ion laser excitation from 10/sup -1/ to 10/sup 4/ W/cm/sup 2/ power density and a light spot focused to a FWHM of 35 /spl mu/m, acquires photoluminescence (PL) spectra from a semiconductor material system over a dynamic intensity range of more than eight orders of magnitude. The measured PL intensities that are evaluated using an extended, numerical self-consistent drift-diffusion model provide properties of interface and surface states such as density and capture cross section as well as derived quantities including nonradiative interface and surface recombination velocity.

Electronic structure of wurtzite II-VI compound semiconductor cleavage surfaces studied by scanning tunneling microscopy

We report atomically resolved scanning tunneling microscopy (STM) images of cleavage surfaces of wurtzite II-VI compound semiconductors. CdSe(112\ifmmode\bar\else\textasciimacron\fi{}0), CdSe(101\ifmmode\bar\else\textasciimacron\fi{}0), and CdS(101\ifmmode\bar\else\textasciimacron\fi{}0) were investigated. The STM images confirm a $1\ifmmode\times\else\texttimes\fi{}1$ reconstruction for all surfaces. At negative and positive sample voltages the occupied and empty dangling-bond states above anions and cations, respectively, dominate the contrast of the STM images. No states in the band gap were found. The electronic structure of the surface permits the observation of dopant atoms in subsurface layers and thus also cross-sectional scanning tunneling microscopy studies of point defects and heterostructures.

Phonons in {110} surfaces of III-V compound semiconductors

High-resolution electron energy-loss spectroscopy was applied to determine phonon dispersions of cleaved AlSb(110), GaP(110), GaSb(110), and InAs(110) surfaces. Besides the acoustic surface waves and the optical Fuchs-Kliewer modes additional flat phonon bands were detected in all materials. The experimental results agree very well with recent ab initio calculations of surface phonon dispersion curves. For GaP(110), several phonon modes are observed within the fundamental gap between acoustic and optical bulk phonon bands. In AlSb(110) a gap mode is detected very close to the bulk optical band edge whereas the GaSb gap is free of surface-localized states. The measurements confirm theoretical predictions that significant gap modes exist at these surfaces if the anion mass is smaller than the cation mass. For the acoustic and the Fuchs-Kliewer modes chemical trends are demonstrated. The differences of the excitation energies in the materials considered depend only on the anion and cation masses as well as the nearest-neighbor distances.

Photochemistry of dimethylcadmium on compound semiconductor surfaces

We present dynamical studies of the surface photochemistry of dimethylcadmium (DMCd) adsorbed on GaAs(110) and CdTe(110) surfaces. This important precursor for chemical vapor deposition is studied using time-of-flight (TOF) spectroscopy in conjunction with temperature programmed desorption (TPD). For the case of greater than one monolayer coverages, on these surfaces, both photofragmentation and photodesorption are observed following irradiation. The relative probabilities of these two processes are found to be strongly dependent on adsorbate coverage and incident photon energy. Direct photoabsorption by the adsorbed molecule is found to dominate the photofragmentation process, with the substrate strongly reducing the probability of photofragmentation at low coverages due to the operation of substrate mediated channels for excited state relaxation. The CH3 fragments observed following photodissociation are found to have kinetic energies that are invariant with the energy of the exciting photon (193 and 248 nm), an effect attributed to an efficient curve crossing between excited states in this molecule. The molecular desorption process is found to exhibit an unusual coverage dependence which is linked to the probability of deposition of a significant amount of vibrational excitation in the overlayer via substrate mediated de-excitation processes. Following the adsorption of one monolayer on the GaAs substrate, the photochemistry observed is significantly different due to the thermal dissociation of DMCd to form adsorbed methylgallium moieties. In this case, multiple features are observed in the TOF spectrum with kinetic energies significantly different to those observed for the gas phase or physisorbed molecule. Correlation of the TOF spectral features with thermal desorption data allows these TOF features to be assigned to specific adsorbed intermediates.

Adatom Densities on GaAs: Evidence for Near-Equilibrium Growth

We examine the equilibrium of a compound semiconductor surface under molecular beam epitaxy (MBE) conditions, both theoretically and experimentally. For GaAs, the Ga chemical potential and adatom density depend sensitively on As pressure as well as temperature. Our results suggest that MBE growth may take place under conditions much closer to equilibrium than has been believed. We also show that standard one-component models cannot, even in principle, reproduce both the adatom density and its temperature dependence.

Formation of Self-Assembled Monolayer on Cleaved Compound Semiconductor Surfaces and Its Nano-structure on AlGaAs/GaAs Heterostructure.

Formation of Self-Assembled Monolayer on Cleaved Compound Semiconductor Surfaces and Its Nano-structure on AlGaAs/GaAs Heterostructure.

Method for determining the composition and orientation of III–V {001} semiconductor surfaces

A method for determining the composition and orientation of III–V {001} semiconductor surfaces is presented and applications are described. The information is obtained from the techniques of time-of-flight scattering and recoiling spectrometry (TOF-SARS), using the composition from azimuth-specific elemental accessibilities (CASEA) method, and low energy electron diffraction (LEED). The azimuth-specific elemental accessibilities (ASEA) are measured experimentally and calculated from the number of accessible atoms in the unit cell and from three-dimensional trajectory simulations using the SARIC program. The in situ analyses identify the 1st-layer elemental species and determine the orientation of the reconstructed surface symmetry elements with respect to the bulk crystallographic directions. This is demonstrated for the III–V {001} compound semiconductor surfaces of GaAs and InAs in the (4 × 2) and (4 × 2) phases and InP in the (4 × 2) phase. The analyses confirm the missing-row-dimer (MRD) structure for GaAs and InAs in which the missing row direction is parallel to the direction of the 1st-layer multimers (dimers) and the missing-row-trimer-dimer (MRTD) structure for InP in which the missing row direction is perpendicular to the direction of the 1st-layer multimers (trimers).

A soft x‐ray standing wave measurement system for analyzing compound semiconductor surfaces prepared by molecular beam epitaxy

A soft x‐ray standing wave (soft‐XSW) measurement system has been developed for analyzing III–V compound semiconductor surfaces prepared by molecular beam epitaxy (MBE). It consists of a fast entry chamber ( a load‐lock chamber), a transfer chamber, a MBE growth chamber, and an analysis chamber. It is connected to NTT’s synchrotron radiation beam line at the Photon Factory. Samples prepared by MBE can be transferred to the analysis chamber through ultrahigh vacuum. The goniometer in this analysis system was shown to be suitable for soft‐XSW triangulation studies. Both backreflection and angle‐scan soft‐XSW experiments for adsorbate/GaAs(001) systems prepared by MBE were demonstrated.

Charged steps on III-V compound semiconductor surfaces.

The electrical charge of steps on (110) surfaces of InP, GaP, and GaAs is probed by scanning tunneling microscopy. It is demonstrated that step edges, with and without kinks, are charged and have localized defect states in the band gap. The charge of indium-terminated step edges on p-type doped InP(110) after annealing is found to be independent of the step orientation, while a strong orientation dependence is observed for phosphorus-terminated steps. This is explained by a partial compensation of the charge due to rebonding after phosphorus desorption. The magnitude of charge is estimated to be in the range of +1e to +3e per lattice spacing. \textcopyright{} 1996 The American Physical Society.

Atomically resolved image of cleaved surfaces of compound semiconductors observed with an ultrahigh vacuum atomic force microscope

An atomically resolved image of a cleaved InP(110) surface with an ultrahigh vacuum atomic force microscope (UHV AFM) was obtained under the contact mode for the first time. The rectangular lattice could be clearly observed. The distances and corrugation amplitudes between the protrusions along the [001] and [11̄0] directions are estimated to be 5.8±0.6 and 4.1±0.4 Å, and 1.3±0.2 and 0.7±0.2 Å, respectively. These results suggest that the UHV AFM has potential for investigating III–V compound semiconductor surfaces on an atomic scale.

In situ observation of phase transition and the transition-induced step bunching on InAs(001) surfaces by scanning electron microscopy

The reconstruction and the step structure change during the phase transition on an InAs(001) surface under As pressure is observed directly by using scanning electron microscopy. The domain formation corresponding to the first-order phase transition is clearly seen as predicted by Monte Carlo simulation with a lattice gas model. It is also seen that the step structure largely depends on the surface reconstruction and the misorientation direction. This gives the first observation of phase-transition-induced step bunching on a compound semiconductor surface.

Accounting for stoichiometry changes on compound semiconductor surfaces

We discuss how to self-consistently account for stoichiometry changes on surfaces with complex reconstructions, e.g. polar compound semiconductor surfaces. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The method is specifically applied to molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) results for GaAs(100) surfaces. The method easily demonstrates why only ∼ 12 monolayer (ML) of As is needed to convert from the Ga-rich surface to the (2 × 4) As-rich surface (as observed experimentally), even though the latter surface has an As coverage of 34 ML in the top layer. We also demonstrate how to convert from a more complex reconstruction, having two incomplete layers, to a simpler reconstruction having only one incomplete layer. The methodology also shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because any such transition would not yield the observed 1 ML per cycle growth rate. We briefly discuss how adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 × 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 12 ML CH3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The ideal termination of the Ga-rich GaAs(100)-(1 × 2)-CH3 surface now allows for a plausible ALE mechanism which yields 1 ML deposition per cycle.

Atomic resolution image of cleaved surface of compound semiconductors observed with ultrahigh-vacuum atomic force microscope in contact and noncontact modes

Atomic resolution image of cleaved surface of compound semiconductors observed with ultrahigh-vacuum atomic force microscope in contact and noncontact modes

非接触モード超高真空原子間力顕微鏡による化合物半導体表面の原子分解能観察

非接触モード超高真空原子間力顕微鏡による化合物半導体表面の原子分解能観察

Surface stoichiometry and the role of adsorbates during GaAs atomic layer epitaxy

Several questions regarding the stoichiometry of atomic layer epitaxy (ALE) arise because most polar compound semiconductor surfaces reconstruct into structures terminated with less than monolayer coverage at the outermost layer. In this paper we briefly discuss how to self-consistently account for stoichiometry changes on surfaces that are not ideally terminated. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The methodology shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because no such transition would yield the observed 1 monolayer (ML) per cycle growth rate. In fact, ideal ALE (1 ML/cycle) must involve at least one adsorbate induced surface reconstruction. Adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces because of their ability to passivate dangling bond states. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 x 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 1 2 ML CH 3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The GaAs(100)-(1 x 2)-Ch 3 surface was also examined using surface infrared spectroscopy (SIRS) using a multiple internal reflection geometry. This surface exhibits relatively sharp infrared linewidths suggestive of a well ordered structure. The polarization dependence of the symmetric stretching and bending CH 3 modes also supports our proposed structure of the GaAs(100)-(1 x 2)-Ch 3 surface. The ideal termination of the Ga-rich GaAs(100)-(1 x 2)-CH 3 surface for a plausible ALE mechanism which yields 1 ML deposition per cycle.