Monolayer Steps Research Articles

Step Structures on Vicinal InAs (001) under ( 2×4) and ( 4×2) Surface Reconstructions

Step structures on InAs (001) vicinal surfaces under two different surface reconstructions were investigated by scanning tunneling microscopy. On an InAs surface misoriented by 1° toward the [110] direction, relatively straight monolayer steps along the [11̄0] direction were observed under an As-stabilized (2×4) reconstruction. On the other hand, step-bunching of about 10 monolayers was detected under an In-stabilized (4×2) reconstruction. On an InAs surface misoriented by 1° toward the [11̄0] direction, ragged monolayer steps roughly running parallel to the [110] direction were seen under the (2×4) reconstruction. However, relatively straight steps along the [110] direction with step-bunching of 2-3 monolayers were observed under the (4×2) reconstruction. These results indicate that thermodynamically favorable step structures are different between the (2×4) and (4×2) reconstructions.

The origin of surface roughening in lattice-mismatched Frank van der Merwe type heteroepitaxy

Heteroepitaxial growth of 2% lattice-mismatched EuTe on PbTe(111) by molecular beam epitaxy is investigated in the two-dimensional layer-by-layer growth regime (Frank van der Merwe growth mode) combining scanning tunnelling microscopy and in-situ reflection high-energy electron diffraction. The injection of misfit dislocation at the critical thickness of the epitaxial layer causes the appearance of monolayer step lines on the surface along certain crystallographic directions. These are formed by the glide of the grown-in threading dislocations in the layer at the onset of strain relaxation according to the (100) [110] and, among others, the (3̄1̄1) [110] glide systems. Because of the initially very sluggish strain relaxation process, at first the epitaxial growth is not influenced by the formation of the surface step lines. However, at a layer thickness of 40 ML the strain relaxation rate strongly increases due to high excess stress in the layer and a dense network of surface step lines is formed, resulting in an abrupt surface roughening transition with an increase of the root mean square roughness by a factor of four.

Surface processes responsible for reflectance-anisotropy oscillations during molecular beam epitaxy growth of GaAs(001)

Reflectance anisotropy spectroscopy (RAS) oscillations during molecular beam epitaxy (MBE) growth of singular GaAs(001) were studied at substrate temperatures between 500°C and 610°C. RAS spectra were measured and related to surface reconstructions determined by reflection high-energy electron diffraction (RHEED). The RAS signal sensitively monitors surface changes when growth is initiated. With opening of the Ga shutter an immediate change in the RAS response indicates the modified As adsorption/desorption equilibrium due to the presence of Ga atoms on the surface. During growth of the first GaAs monolayers RAS oscillations mirror the oscillation of the monolayer step density in island growth mode. From all experimental evidence it must be concluded that the step density modulates the As dimer coverage because As desorbs predominantly in regions close to the island boundaries.

Effects of interface flatness and abruptness on optical and electrical characteristics of GaAs/AlGaAs quantum structures grown by metalorganic vapor phase epitaxy

Heterointerface flatness and abruptness of GaAs/AlGaAs quantum wells are controlled during metalorganic vapor phase epitaxy (MOVPE). Various growth modes such as two-dimensional (2D) nucleation, step flow, and step bunching are achieved by changing the growth temperature and substrate surface misorientation angle. The degree of monolayer step smoothness and ordering are also changed. The effects of these nanometer-scale morphologies and interface abruptness on the full width at half maximum (FWHM) of 4.2-K photoluminescence spectra from quantum wells (QWs) are individually determined. The FWHM is increased in order by degraded interface abruptness, step roughness, multisteps, and 2D islands. Effective thickness variations introduced by these features are roughly ≳2.0, 1.0, 0.6, and 0.2 monolayers (ML), respectively, for 7-ML-thick QWs. Some of these effects on current–voltage characteristics of double barrier resonant tunneling diodes (DBRTDs) are also investigated. The factor most strongly affecting the characteristics of DBRTDs is step disordering, that is, a difference in step edge location between the top and the bottom surfaces of barriers. By making this difference smaller than the electron’s de Broglie wavelength using vicinal substrates, we obtain the best negative resistance characteristics ever reported for MOVPE growth.

Step dynamics on highly oriented and vicinal surfaces of Si(111)

By using high-temperature scanning tunneling microscopy we have measured step fluctuations on highly oriented and vicinal surfaces of Si(111) in the temperature range up to the (7 × 7) (1 × 1) phase transition temperature (1100 K). On mono-layer steps these fluctuations mostly occur in a width corresponding to a 7 × 7 unit cell half. By an Arrhenius plot of the fluctuation rate an activation energy for 7 × 7 kink diffusion of 1.8 ± 0.3 eV was estimated. On vicinal surfaces and just below the phase transition temperature fluctuations of entire 7 × 7 domains are observed.

Optical investigations in (In,Ga)As/GaAs quantum wells grown by metalorganic molecular-beam epitaxy.

Photoluminescence, reflectivity, and thermally detected optical-absorption (TDOA) experiments have been performed at liquid-helium temperatures on two strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum-well (QW) structures grown by metalorganic molecular-beam epitaxy (MOMBE). The QW thicknesses vary from 3--15 ML and 4--16 ML by two monolayer (ML) steps. It is demonstrated that the MOMBE technique allows the thickness to be controlled with an accuracy of 1 ML. The electron--heavy-hole excitonic transitions (${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$) are detected for all the QW's. TDOA enables the light-hole excitonic transition to be observed for the thinnest well in the second sample. The QW excitonic absorption energies are compared with calculations within the framework of the envelope-function approximation by taking into account the strain effects and the indium segregation phenomenon. An accurate determination of the strain conduction-band offset is derived (${\mathit{Q}}_{\mathit{c}}$=0.64\ifmmode\pm\else\textpm\fi{}0.01) and it is found that the indium segregation is very weak at the QW interfaces compared to structures grown by MBE. The light-hole band configuration is type I for the evaluated alloy composition (x=0.21--0.22). Under low excitation intensity, intermediate photoluminescence emissions appear between the ${\mathit{e}}_{1}$${\mathrm{hh}}_{1}$ lines corresponding to nominal thicknesses; they can be associated with fluctuations of 1 ML thickness, which are reported, to our knowledge, for the first time, in this type of QW system. Efficient thermally activated interwell transfer of excitons is also in evidence from photoluminescence experiments. A simple model is used to account for the transfer from narrower to broader wells when the temperature is increased.

Generation mechanism of CuAu-I type ordered structures in InGaAs crystals grown on (110) InP substrates by molecular beam epitaxy

The generation mechanism for CuAu-I type ordered structures in InGaAs grown on (110) InP substrates by molecular beam epitaxy is discussed. On the basis of previous work together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. This model have been experimentally proven by the analyses of electron diffraction patterns from different crystals grown on different growth surfaces.

Generation mechanism of CuAu-I type ordered structures in InGaAs crystals grown on (110) InP substrates by molecular beam epitaxy

The generation mechanism for CuAu-I type ordered structures in InGaAs grown on (110) InP substrates by molecular beam epitaxy is discussed. On the basis of previous work together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. This model have been experimentally proven by the analyses of electron diffraction patterns from different crystals grown on different growth surfaces.

Transmission electron microscopic evaluation of InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110)InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) consisting of InAs/GaAs and InAs/AlAs monolayer superlattices grown on slightly misoriented (110)InP substrates by molecular beam epitaxy have been structurally evaluated by transmission electron microscopy. We used (110)InP 3° tilted toward the [001] direction. The ISPLs were fabricated by an alternative growth of half monolayers of AlAs and GaAs and one monolayer of InAs while maintaining regular arrays of one monolayer steps on the growth surface. In electron diffraction patterns from the (110) cross-section, two types of superstructure spots double-positioned in the 〈001〉 direction are observed, consistent with the existence of the IPSLs. Dark-field imaging from the superstructure spots reveal a periodic diffraction contrast with an average lateral periodicity of about 4 nm, i.e., one terrace width. However, meandering of the vertical interface and partial disordering in the IPSLs are often observed. From high resolution (110) cross-sectional TEM images, the presence of IPSLs was also confirmed with an atomic scale resolution, although their vertical interface are meandering. In electron diffraction patterns from the (110) plan-view, extra-spots similar to those observed in the (110) cross-section were seen. Dark-field images from the superstructure spots indicated that the IPSLs were formed almost exactly along the 〈110〉 direction, suggesting that the steps on the growth surface are very straight along the 〈110〉 direction.

Transmission electron microscopic evaluation of InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110) InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) consisting of InAs/GaAs and InAs/AlAs monolayer superlattices grown on slightly misoriented (110)InP substrates by molecular beam epitaxy have been structurally evaluated by transmission electron microscopy. We used (110)InP 3° tilted toward the [00 1 ] direction. The ISPLs were fabricated by an alternative growth of half monolayers of AlAs and GaAs and one monolayer of InAs while maintaining regular arrays of one monolayer steps on the growth surface. In electron diffraction patterns from the ( 1 10) cross-section, two types of superstructure spots double-positioned in the 〈001〉 direction are observed, consistent with the existence of the IPSLs. Dark-field imaging from the superstructure spots reveal a periodic diffraction contrast with an average lateral periodicity of about 4 nm, i.e., one terrace width. However, meandering of the vertical interface and partial disordering in the IPSLs are often observed. From high resolution ( 1 10) cross-sectional TEM images, the presence of IPSLs was also confirmed with an atomic scale resolution, although their vertical interface are meandering. In electron diffraction patterns from the (110) plan-view, extra-spots similar to those observed in the ( 1 10) cross-section were seen. Dark-field images from the superstructure spots indicated that the IPSLs were formed almost exactly along the 〈110〉 direction, suggesting that the steps on the growth surface are very straight along the 〈110〉 direction.

InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110) InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlatices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3° toward the [00 1 ] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ( 1 10) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick) superlattice in addition to a peak corresponding to the In 0.5Al 0.25Ga 0.25As alloy.

High pressure studies of negative differential resistance in InAs/GaSb resonant conduction heterostructures

A single heterojunction of n-InAs/p-GaSb behaves as a two dimensional electron gas (2DEG) back to back with a two dimensional hole gas (2DHG), due to overlap between the GaSb valence band and the InAs conduction band. An unusual feature is the possibility of growing a ‘ light’ GaAs or a ‘ heavy’ InSb interface. The two interface types are predicted to give rise to different electronic and vibrational properties, while the existence of monolayer steps or interdiffusion could give rise to a mixed interface character. In this paper we demonstrate that vertical transport measurements as a function of pressure are a sensitive way of probing the electronic structure of the interface. On biasing InAs negative with respect to GaSb, electron transfer takes place from the 2DEG to the 2DHG, and the current initially increases with bias. At a critical bias, quantum confinement raises the energy of the 2DEG above that of the 2DHG and a region of Negative Differential Resistance (NDR) then occurs. The size of the NDR feature and the critical bias are both reduced by hydrostatic pressure, showing that the band overlap decreases. For a number of samples grown with a heavy interface by atmospheric pressure metalorganic vapour phase epitaxy the NDR vanishes at a threshold, P c ~ 12 ± 2 kbar. This threshold is consistent with a band overlap of 120 ± 20 meV and a shift of −10 meV/kbar. The NDR properties are found to be very sensitive to the detailed sequence of switching of the precursors during interface growth, as well as to the interface type.

Photoluminescence spectroscopy of growth-interrupted GaAs/AlAs single quantum wells subjected to hydrogenation

Photoluminescence excitation and emission spectroscopy (PLE and PL) have been used to investigate growth interrupted GaAs/AlAs single quantum wells (SQW). Evidence that growth interruption leads to smooth interfaces which vary in height by integer monolayer steps is provided by changes in the PLE and PL spectra from a broad single band for noninterrupted SQW to narrow multiple peak features for growth interrupted samples. However, growth interruption results in a dramatic reduction in the intensity of the photoluminescence signal above 30 K. Hydrogenation treatment has been found to recover the photoluminescence intensity in the growth-interrupted samples which we attribute to passivation of nonradiative centers incorporated during the growth interruption process. Similar, though less dramatic, effects have been observed for noninterrupted samples. Time-resolved photoluminescence spectroscopy and decay time measurements show that hydrogen passivates carbon acceptors also incorporated during interruption, and in addition, demonstrate the migration process of excitons from the narrower to the wider well widths in these SQW. This latter exciton kinetic information fully supports the view that growth interruption results in large lateral islands with monolayer fluctuations in well width.

Surface morphology of metalorganic vapor phase epitaxy grown strained-layer InxGa1−xAs on GaAs observed by atomic force microscopy

We have studied the surface morphology and growth mechanism of metalorganic vapor phase epitaxy grown strained-layer InxGa1−xAs (x=0.2 or 0.5) on GaAs with atomic force microscopy. Morphological instability of monolayer steps was observed on a 10 nm thick strained-layer In0.2Ga0.8As. Three-dimensional (3D) growth was observed for x=0.5 when grown at 650 °C. By lowering the growth temperature to 600 °C, the growth mode is 2-D for 5 nm films (x=0.5). Monolayer steps and 2D islands can be seen. Increasing the layer thickness to 7.5 nm at 600 °C caused the growth of 3D islands and the generation of misfit dislocations.

Atomic Ordering of III-V Alloy Semiconductors Grown on (110)InP and its Influence on Electron Mobility

AbstractAtomic ordering in InGaAs, InAlAs, and GaAsSb crystals grown on (110) InP substrates by molecular beam epitaxy, has been studied by transmission electron microscopy. In the electron diffraction pattern from these crystals, superstructure spots associated with CuAu-I type ordered structure are found. When the tilting angle of the substrates increases, the ordering becomes stronger. The ordering is also stronger in crystals grown on substrates tilted toward the <001> or the <001> direction than those on substrates tilted toward the <110> direction. From these results, one can conclude that atomic steps on the growth surface play an important role in the formation of ordered structures. The ordering becomes stronger when the growth temperature increases. In high resolution images of the crystal, doubling in periodicity of 220 and 200 lattice fringes is found, which is associated with CuAu-I type ordered structure. Moreover, anti-phase boundaries are very often observed in the ordered regions. It is also found that ordering is not perfect, and that ordered regions are plate-like microdomains lying on planes slightly tilted from the (110) plane. On the basis of these results together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. These models have been experimentally proven by the analyses of electron diffraction patterns from different InGaAs crystals grown on different growth surfaces. We have fabricated InGaAs/N-InAlAs heterostructures with a strongly ordered InGaAs channel layer. The measured two-dimensional electron gas mobilities from these structures are found to be 100,000 cm2/Vs in the <110> direction and 161,000 cm2/Vs in the <110> direction with a sheet electron concentration (Ns) of 9.5 × 1011 cm−2 at 6 K. The latter mobility is much higher than both calculated alloy scattering limited mobility and the best experimental results for lattice-matched InGaAs/N-InA]As systems. The mobility enhancement in the <110> direction is considered to be achieved by the suppression of alloy scattering due to the occurrence of ordering in the InGaAs channel layer.

Compositional Ordering in GalnP for Heterostructure Formation

AbstractCompositional ordering is observed in essentially all IIIN semiconductor alloys. The mechanism leading to the specific ordered structures formed is only now beginning to be understood. It is intimately related to the atomic-scale physical processes occurring on the surface during epitaxial growth, specifically surface reconstruction and the attachment of atoms at steps. This paper will emphasize the use of surface analytic techniques to better understand the ordering processes during organometallic vapor phase epitaxial growth of GalnP. Surface photo absorption has been used to determine, for the first time, that the surface reconstruction is (2×4)-like during growth of GalnP with Cu-Pt ordering. By increasing the growth temperature and/or decreasing the V/Ill ratio the concentration of phosphorus dimers characteristic of the (2×4) structure is observed to decrease in tandem with a decrease in the degree of order measured in the epitaxial layers. This suggests that formation of the (2×4) reconstruction is necessary for formation of the Cu-Pt ordered structure.Atomic force microscopy was used to examine the step structure. For exactly (001)-oriented substrates, growth at low V/Ill ratios produces islands surrounded by single monolayer steps. Increasing the V/Ill ratio leads to the formation of bilayer steps approximately 6 Å high. A similar change in the structure of steps is observed in the vicinal regions for substrates misoriented to produce [110] steps. This phenomenon is interpreted in terms of the stabilization, at high V/Ill ratios, of the (2×2) reconstruction on the (111)B regions of the bilayer steps. This may also have an effect on formation of the Cu-Pt ordered structure.By changing the substrate temperature or the input partial pressure of the phosphorus precursor, the degree of order and, hence, the bandgap energy of GalnP, can be modulated during growth. This has allowed the production of a variety of single and double heterostructures. The heterostructures produced by changing the temperature from 740 to 620°C and from 620 to 520°C are abrupt. The heterostructures formed by changing the phosphine flow rate are not. The ordered structure in the lower layer persists into the upper layer. This may be an indication of the importance of the step structure.

Atomic force microscopic study of directional SrSO 4(001) surface and its etching property

Different step structures were observed with atomic force microscopy (AFM) on SrSO 4(001) cleaved in air in two ways. Monolayer steps were observed when the crystal was cleaved by wedging open a crack. On the other hand, bilayer steps were observed when it was cleaved by giving a blow on a knife edge placed along the a-axis on a crystal face. In the latter case, the cleavage proceeded under slightly misoriented tensile stress, favoring formation of bilayer cleavage steps due to alternating directions of the bonds connecting (001) ionic layers. Directionality in the arrangement of the ions at the (001) surface was reflected on the shape of etch pits formed by dissolving the surface in electrolyte solutions. Relative stabilities of the steps surrounding the bow-shaped pits are discussed. The directionality of the surface has potential application in constructing two-dimensional assemblies of functional molecules.

Multiple scattering calculations of step contrast in REM images of the Si(001) surface

Simulations of reflection electron microscopy (REM) images of both monolayer and bilayer steps on the bulk-terminated Si(001) surface, for the case when the primary electron beam azimuth is directed parallel to the line of the step, are presented. The simulations employ our previously reported theory of REM image formation which uses a 2D Bloch wave formulation of dynamical (multiple scattering) elastic RHEED theory to calculate diffracted amplitudes propagating from the surface. The only step contrast mechanism considered here is phase contrast and this is sufficient to produce the characteristic “black-white” appearance observed experimentally. Defocusing of the simulated images is also discussed.

Morphology on GaAs surfaces grown by metalorganic chemical vapor deposition and molecular beam epitaxy

The morphology on GaAs surfaces grown by metalorganic chemical vapor deposition (MOCVD) is observed using an atomic force microscope and the results are compared with those obtained for surfaces grown by molecular beam epitaxy (MBE). Monolayer steps on MOCVD-grown surfaces are observed for the first time. The largest terrace observed is 1 μm wide. This is one order of magnitude wider than those observed on MBE-grown surfaces under similar growth conditions. Surface undulation is less than that on the MBE-grown surface. Preliminary studies on surfaces grown by chemical beam epitaxy (CBE) show that terrace width and surface undulation are similar to those found on MBE-grown surfaces.

Study of the crystal growth of a (15,17,1) vicinal plane on a Si(110) surface using high-temperature scanning tunneling microscopy

High-temperature scanning tunneling microscopy was used to study the crystal growth of a (15,17,1) vicinal plane on a Si(110) surface, and the dynamic behavior of surface Si atoms at high temperature was directly observed. It was found that an impurity hillock, which may be SiC on the surface, pins step flows and bunches them together to form a vicinal plane such as (17,15,1), (17,15,1̄), (15,17,1), and (15,17,1̄) at 710 °C. The vicinal plane has a faceted structure and is formed by a periodic arrangement of monolayer steps and has a terrace of 2.5 nm in the 〈111〉 direction. The step-edge direction is 〈112〉. At 695 °C, it was also found that the 16 structure was formed on a flat area. It was confirmed that the 16 structure and the vicinal plane (15,17,1) coexisted in a temperature range of about 700 °C to room temperature.