The characterization of the growth of sub-monolayer coverages (1/200th to 1 monolayer) of Si and Be on GaAs(001): A reflectance anisotropy spectroscopy and reflection high-energy electron diffraction study

The techniques of reflectance anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) have been employed, for the first time in concert, to characterize the growth of sub-monolayer coverages of both Si and Be deposited onto the GaAs(001)-c(4×4) surface. The RHEED observations enabled the RAS spectra collected for a series of Si and Be coverages, in the range 0.005 to 1.000 monolayer (ML), to be interpreted in terms of changes in the sample surface structure. For the case of Si/GaAs(001) the following series of surface reconstructions were observed with increasing Si coverage: c(4×4), c(4×4)/(1×2), (1×2), (1×2)/(3×1) and, (3×1). During the deposition of Be/GaAs(001), c(4×4), c(4×4)/(1×2), c(4×4)/(1×3), (1×2)/(1×3), (1×3), and (1×2) reconstructions were noted to evolve. The fact that unique, but highly reproducible, RAS signatures were obtained for each of these surface phases demonstrates the applicability of a combined RHEED/RAS system for monitoring sub-monolayer heteroepitaxial growth with a surface sensitivity of the order of 1/200th of a monolayer.     

Atomic-scale controlled incorporation of ultrahigh-density Si doping sheets in GaAs

Delta-doped GaAs:Si with doping densities up to 4×10¹⁴ cm⁻² has been grown by molecular beam epitaxy (MBE) at a substrate temperature of 590°C. To promote an ordered incorporation and thus avoid clustering of Si atoms, vicinal GaAs(001) surfaces 2° misoriented towards (111)Ga were used and Si was supplied in pulses. As evidenced by real-time reflection high-energy electron diffraction (RHEED) measurements an ordered incorporation of Si atoms on Ga sites along the step edges takes place. Although the ordered (3×2) structure degrades at high coverages, unusual high sheet carrier concentrations are obtained by pulsed delta-doping for doping concentrations >10¹³ cm⁻², as revealed by Hall measurements. The surface conditions during GaAs overgrowth have a strong influence on the free electron concentration, too. Raman scattering by local vibration modes and secondary ion mass spectrometry (SIMS) measurements are used to show that this is related to segregation effects as well as to a modification of the site occupancy.     

Surface reconstruction of sulfur-terminated GaAs(001) observed during annealing process by scanning tunneling microscopy

The surface reconstruction of in-situ prepared sulfur-terminated (S-terminated) and sulfur-protected (S-protected) GaAs(001) is studied by scanning tunneling microscopy (STM) at high temperatures of up to 260°C. We demonstrate a new reordering process from the (4×6) Ga-stabilized surface to the (2×6) S-stabilized surface and a novel method of air protection using a S passivation layer. Moreover, the in-situ observation of the annealing process of this S-protection layer is performed by high-temperature STM, avoiding the adsorption from environments and verifying that the (2×6) structure still becomes dominant on the S-terminated GaAs(001) surface without the effects of As atoms.     

Negative magnetoresistance in Si atomic-layer-doped GaAs

Magnetoresistance of a Si atomic-layer-doped (δ-doped) GaAs is measured to study the transport properties of the δ-doped structure. The magnetoresistance decreases as the temperature is lowered and the depth of the δ-doped layer, i.e., the distance between the surface and the δ-doped layer, becomes less than 300 nm. Negative magnetoresistance is observed below 80 K in a sample having a doping density of 1.67×10¹² cm⁻² and a δ-doped layer depth of 40 nm. The conduction channel in the δ-doped structure consists of two parts, the quantized states in the conduction band and the impurity band. The origin of the negative magnetoresistance is suggested to be the conduction in the inhomogeneous potential due to localized impurity potential.     

Ti-adsorption on Se-prereacted GaAs (110) surfaces studied by SXPS

This study is embedded in the broader context of reactive metal overlayers and passivation on GaAs. High resolution synchrotron-radiation photoemission experiments for Ti coverages on Se-reacted GaAs (110) surfaces show that, contrary to clean Ti-reacted GaAs (110) interface, there is no initial disruption (submonolayer reaction) of the surface involving both Ga and As atoms during early stage of interface formation. However, a delayed Ti involved reaction appears at a trigger coverage of about 1 ML involving only As atoms from the prereacted As Se interface configuration continued by Stranski-Krastanov growth mode. A second reacted phase starts to form two Ga Ti involved configurations replacing Ga Se bonds near a Ti coverage of 4 ML. The preferential chemical trapping of Ti by Se atoms is associated with a smaller interface thickness very likely <10 ML instead of 50 ML in the case of clean Ti/GaAs (110) interface caused by mobile As atoms.     

Scanning tunneling microscopy of the GaAs (311)A surface reconstruction

Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the reconstruction of the GaAs (311)A surface. During and after molecular-beam epitaxy, in-situ RHEED of the surface shows a lateral periodicity of 3.2 nm perpendicular to the [ 33] direction. This periodicity is confirmed employing in-situ STM. The height of this periodic surface structure is two monolayers, i.e., 0.34 nm. We show how this reconstruction, characterized by a dimerization of the surface As atoms, can be formed using a simple electron counting model. The excellent agreement with the experimental results further support this model, that was already found to explain the reconstructions of the (100) and (111) surfaces.     

Facets formation of pyramidal Si nanocrystals selectively grown on Si(0 0 1) windows in ultrathin SiO2 films

We have used in situ scanning tunneling microscopy (STM) to study the facet formation in the selective growth of pyramidal Si nanocrystals on Si(0 0 1) windows in ultrathin 0.3-nm-thick SiO₂ films. Broad (0 0 1) surfaces developed as the top of the crystals, and {1, 1, (2n+1)} (n=1–6) facets formed the sidewalls. As growth continued, the slope angle of sidewall facets increased, and {1, 1, 9} and {1, 1, (2m+1)} (0 <m < 4) facets often came to coexist on the sidewalls. On well-oriented Si(0 0 1) surfaces, layer-by-layer growth in the [0 0 1] direction was dominant. On vicinal Si(0 0 1) surfaces, lateral step growth took place in the initial stage, and the layer-by-layer growth was suppressed until after a large (0 0 1) surface had formed as the top of the crystal.     

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si₂H₆) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si₂H₆. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.     

Surface reconstructions and growth mode transitions of AlAs(100)

The surface reconstructions of AlAs(100) layers grown by molecular beam epitaxy (MBE) on GaAs(100) were mapped as a function of substrate temperature and arsenic flux. Three main reconstructions were observed - a c(4×4) at lower temperatures and higher arsenic fluxes, a (2×4) at middle temperatures, and a (3×2) at higher temperatures and lower arsenic fluxes. Growth of AlAs on AlAs(100) is layer-by-layer for the high temperature and low temperature reconstructions. In the mid-temperature region, AlAs grows rough on (2×4) reconstructed AlAs(100) as indicated by rapidly damped reflection high-energy electron diffraction (RHEED) intensity oscillations and the appearance of three-dimensional (3D) features. The addition of fractional layers of Ga enhances the smooth growth of AlAs. A metastable (5×2) reconstruction was observed when a fraction of a layer of Ga was present on the surface. The results indicate that Ga segregates during the growth of AlAs on GaAs(100) at temperatures at least as low as 500°C, and that annealing at temperatures above 700°C removes most of the Ga from the surface.     

RHEED studies of MOMBE growth using TMGa or TEGa with As2

MOMBE and CBE growth has until recently been based on largely empirical studies of the epitaxial process. We have used reflection high energy electron diffraction (RHEED), previously applied to the study of MBE, to study the growth GaAs using TMGa and As₂. In this work we have extended our previous studies to include a detailed study of the effect of As₂ flux on growth rate and to compare data on singular and vicinal plane surfaces cut off orientation in two orthogonal {110} directions. Clear evidence for site blocking mechanisms is observed together with an indication that the concentration of elemental Ga present on the surface during growth is negligible even under conditions where the arrival rate of TMGa exceeds that As₂. We have compared this behaviour with that observed using TEGa and As₂ under identical conditions. Using TEGa a conversion from a (2×4) to (4×2) reconstructed surface is observed under As deficient conditions indicating the presence of elemental Ga on the surface. This is accompanied by an abrupt change in growth rate similar to that secn in MBE.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Structural transformation of As-stabilized surfaces caused by Ga-deposition detected by time-resolved surface photo-absorption

Evolution of surface structures during conversion from various As-stabilized surfaces to Ga-stabilized ones is observed by surface photo-absorption. Desorption of As dimers parallel to [110] is observed during the structural change of c(4×4) and (2×4)γ. The evolution of Ga- and As-related peaks is isolated in surface dielectric anisotropy spectra, which are determined using p- and s-polarized surface photo-absorption spectra during the conversion of (2×4)β to a Ga-stabilized (4×2) surface.     

Simultaneous reflection high-energy electron diffraction oscillations and mass spectroscopy investigations during molecular beam epitaxy growth of (001) GaAs - smooth surfaces or stoichiometric films?

Film composition and surface morphology of molecular beam epitaxy (MBE)-grown GaAs(001) surfaces were investigated in situ as a function of flux ratio J_Ga/J_As4. The flux of As₄ molecules desorbing from the sample surface was measured with a quadrupole mass spectrometer (QMS) simultaneously with the observation of reflection high energy electron diffraction (RHEED) intensity oscillations. The incorporation ratio, given by the number of incorporated Ga atoms per As atom, was calculated independent of both the QMS data and the decreasing growth rate for growth conditions with As deficiency, as obtained from the RHEED oscillation frequency. Stoichiometric growth was found up to a flux ratio J_Ga/J_As4 ≈ 0.9. At flux ratios of 1.1 to 1.2, a minimum of the damping of the RHEED oscillation amplitude indicates a very smooth growth front profile, but Ga excess appears to be incorporated mainly in As sites without disturbing the crystal lattice. This assumption was confirmed by photoluminescence (PL) spectroscopy of films grown at a flux ratio J_Ga/J_As4 of 1.2. An additional PL peak was observed, which indicates the incorporation of Ga atoms on As sites.     

Nucleation and ripening of seeded InAs/GaAs quantum dots

We have investigated the nucleation and ripening of pairs of InAs/GaAs quantum dot layers separated by thin (2–20 nm) GaAs spacer layers. Reflection high energy electron diffraction (RHEED) measurements show that the 2D–3D transition in the second layer can occur for less than 1 monolayer deposition of InAs. Immediately after the islanding transition in the second layer chevrons were observed with included angles as low as 20° and this angle was seen to increase continuously to 45±2° as more material was deposited. Atomic force microscopy showed the dot density in both layers to be the same. It is proposed that surface morphology can radically alter processes that determine the nucleation and ripening of the 3D islands.     

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02×10¹¹ cm⁻² of InAs islands with In_0.15Ga_0.85As underlying layer has been achieved on GaAs (1 0 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 μm photoluminescence (PL) from InAs islands with In_0.15Ga_0.85As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dots devices.     

As-rich reconstruction stability observed by high temperature scanning tunnelling microscopy

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in-situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As₄ flux. The reconstructions c(4×4), (2×4) and (3×1) are long established for GaAs(0 0 1) between 400 and 600 °C for varying Ga and As flux, however the stoichiometry of incommensurate transient reconstructions is still uncertain. By performing high temperature STM on an initial (2×4) surface between 250 and 450 °C in the absence of an As flux, small domains with varying reconstruction are observed in a similar manner to the InAs/GaAs(0 0 1) wetting layer. The local storage of excess Ga in Ga-rich domains could provide insight into sub ML homo- and hetero-epitaxial growth.     

In-situ and ex-situ characterization of GaAs/AlAs quantum well structures using spectroscopic ellipsometry

Spectroscopic ellipsometer is used to monitor the MBE growth of quantum well structures. Real time monitoring of the growth enabled the measurement of growth rate and correlation with RHEED oscillations. The growth of a single GaAs/AlAs quantum well is also monitored in real time using multiple wavelengths. Interface roughness of the interrupted “inverted” AlAs/GaAs interface was also monitored with SE. Under our growth conditions, we measure approximately a 2 ML interfacial region at the inverted interface. A correlation with photoluminescence is also discussed.     

In situ monitoring of CdTe nucleation on GaAs (100) using spectroscopic ellipsometry

In situ spectroscopic ellipsometry was used to monitor the nucleation behavior of CdTe grown on vicinal GaAs (100) substrates by organometallic vapor phase epitaxy. CdTe was grown on GaAs (100) substrates of exact and 2° off towards 110 orientations. A spectroscopic ellipsometer was used to collect in situ data at 44 wavelengths from 4000–7000 Å. The Bruggeman's effective medium approximation was employed to determine the variation of the epilayer volume fraction with thickness, which was an indirect way of monitoring the expected island growth behavior. The Stranski-Krastonov (layer plus island) mode of growth was clearly observed for CdTe growth. The growth on the 2° off substrate was also “denser” than that on exact (100), which implied that coalescence of the islands occurred at lower thickness. This was expected since island nucleation is most favored along the ledges on the surface whose spacing decreases with increasing misorientation. A simple nucleation model, assuming cylinder-like islands, was able to fit the experimental data quite well, lending support to the island growth model.     

InxGa1−xAs/GaAs quantum wire structures grown on GaAs (100) patterned substrates with [001] ridges

In_xGa_1−xAs/GaAs (x = 0.12-0.23) quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on [001] ridges with various widths (1.1-12 μm) of patterned GaAs (100) substrate. The smallest lateral width of the InGaAs/GaAs quantum wire (QWR) structures was estimated to be about 0.1 μm by high-resolution scanning electron microscope (SEM). The In contents of the grown InGaAs/GaAs QWs on the ridges were studied as a function of ridge top width (ridge width of the MBE grown layer) by cathodoluminescence (CL) measurements at 78 K. Compared to the InGaAs QW grown on a flat substrate, the In content of the InGaAs/GaAs QW on the ridge increases from 0.22 to 0.23 when the ridge top width decreases to about 2.9 μm, but it decreases steeply from 0.23 down to 0.12 with a further decrease of the ridge width from 2.9 to 0.05 μm. A simulation of MBE growth of InGaAs on the [001] ridges shows that this reduced In content for narrow ridges is due to a large migration of Ga atoms to the (100) ridge top region from {110} side facets.     

Chemical trend observed in anisotropic surface reflectance spectra of MOVPE by surface photoabsorption

This paper investigates the origin of the surface reflectance spectrum for the group-V-stabilized III–V surface during MOVPE by using surface photoabsorption. A chemical shift is observed for the stoichiometry sensitive peak in the anisotropic spectra of arsenides and phosphides. The peaks observed in the phosphides are located at higher energies than the arsenides, besides the peak in each compound shows a red-shift as the lattice constant increases. To investigate the possibility of the critical point of the bulk energy state appearing in the reflectance spectrum induced by surface modification, the anisotropic spectrum during InAs-on-GaAs heteroepitaxy are measured. One monolayer InAs growth on GaAs results in a drastic change that a peak sign is reversed, accompanied by a red-shift. This can be interpreted by the optical transition change corresponding to the surface conversion from a two-As-layer c(4 × 4)-like surface in GaAs to a one-As-dimer layer having a bond axis perpendicular to the c(4 × 4) As dimer. The contribution of the GaAs bulk electronic state in the reflectance spectrum is not observed. These results support the model that the anisotropic peak originates from an optical transition of the group-V dimer. The anisotropic spectrum measurement also makes it possible to monitor the P/As surface exchange and the As-atom segregation during the InP-on-InAs heteroepitaxy.