Multiatomic Steps Research Articles

MBE growth of self-assembled InGaAs quantum dots aligned along quasi-periodic multi-atomic steps on a vicinal (1 1 1)B GaAs surface

Growth mechanisms of InGaAs three-dimensional (3D) islands formed on vicinal (1 1 1)B GaAs surfaces with multi-atomic steps are studied by examining surface morphologies of In x Ga 1− x As layers having various In contents x and/or nominal thickness d. When an InGaAs layer with x = 0.29 and d = 3 nm was deposited by molecular beam epitaxy (MBE), 3D islands with lateral sizes of about 40 nm, almost twice as much as the average period (∼20 nm) of underlying GaAs steps, were formed and densely aligned along the steps. A nearly flat surface was formed, when an InGaAs layer with x = 0.10 and d = 3 nm was deposited. In case of d = 2 nm and x = 0.30 , smaller 3D islands were densely formed and their lateral size perpendicular to the steps was typically about 20 nm. Such 3D islands were hardly found when d = 2 nm and x = 0.55 . These results support our hypothetical model that the 3D island formation is induced by the abrupt increase in strain energy resulting from the coherent connection of a pair of InGaAs wire-like structures on a pair of adjacent GaAs multi-atomic steps.

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520°C, and then exhibits a sudden decrease at 535°C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates.

Formation of self-assembled InGaAs quantum dot arrays aligned along quasiperiodic multiatomic steps on vicinal (111)B GaAs

Dense and highly ordered arrays of self-assembled InGaAs quantum dots are formed by molecular beam epitaxy along multiatomic steps on vicinal (111)B GaAs. This unique structure has been synthesized by depositing a nominally 3-nm-thick In0.3Ga0.7As layer onto a periodically corrugated surface prepared on a GaAs substrate tilted 8.5° from (111)B. Each dot is typically 30–50nm in lateral size and about 4nm in height. Accumulation and release processes of strains in InGaAs layers deposited on stepped surfaces are discussed to suggest a possible mechanism for the aligned dot formation.

Dependence of bimodal size distribution on temperature and optical properties of InAs quantum dots grown on vicinal GaAs (100) substrates by using MOCVD

Self-assembled InAs quantum dots (QDs) are grown on vicinal GaAs (100) substrates by using metal-organic chemical vapour deposition (MOCVD). An abnormal temperature dependence of bimodal size distribution of InAs quantum dots is found. As the temperature increases, the density of the small dots grows larger while the density of the large dots turns smaller, which is contrary to the evolution of QDs on exact GaAs (100) substrates. This trend is explained by taking into account the presence of multiatomic steps on the substrates. The optical properties of InAs QDs on vicinal GaAs(100) substrates are also studied by photoluminescence (PL) . It is found that dots on a vicinal substrate have a longer emission wavelength, a narrower PL line width and a much larger PL intensity.

Temperature-dependent conductance of quasi-one-dimensional electrons in a novel constricted [formula omitted] channel with corrugated interfaces

Transport properties of a novel quasi-ballistic quantum wire field-effect transistor are studied experimentally and then discussed in relation to a theory for dirty Tomonaga–Luttinger (T–L) liquids. The sample was prepared by constricting lithographically an epitaxially grown In 0.1 Ga 0.9 As / GaAs quantum well channel, whose bottom interface is corrugated by a quasi-periodic array of multi-atomic steps of 20 nm in periodicity. A quasi-one-dimensional channel of about 200 nm in metallurgical width and 2 μ m in length was formed and its conductance parallel to the steps was measured at temperatures between 4 and 0.3 K as a function of gate voltage. Plateau-like structures substantially lower than 2 e 2 / h were observed. The conductance at each gate voltage decreases sensitively as temperature lowers until it gets nearly constant below a critical temperature. These tendencies are found to be qualitatively consistent with the theory of Ogata and Fukuyama for dirty T–L liquids. The temperature dependence above the critical temperature is found to fit quantitatively with the formula of Ogata and Fukuyama, if the parameters are suitably chosen.

Highly strained quantum structures grown on GaAs (0 0 1) vicinal substrate by MOCVD

In this paper, we report experimental results regarding highly strained InP and InAs/InP quantum nanostructures grown on vicinal GaAs (0 0 1) substrate with 7° miscut towards (1 1 0) orientation by MOCVD using TBA and TBP as group V sources. Formation of well-aligned wire nanostructures has been observed, which may be attributed to the effects of multi-atomic steps formed during GaAs buffer layer growth. We also observed strong photoluminescence from these nanostructures even for the type II band gap alignment of the InP/GaAs quantum wires. Our results show a possible way for fabricating low-dimensional quantum nanostructures with band gap engineering.

Mechanically Biased Self-Assembly of Quantum Dots by Nanoindentation

AbstractNanoindentations were created in the GaAs(100) surface to act as strain centers to bias the nucleation of self-assembled InAs quantum dots providing for patterned growth. Indents were generated using loads below 450 μN with a sharp cube corner indenter. Growth of InAs quantum dots on indent patterns is performed using molecular beam epitaxy (MBE). The effect of indent spacing and size on the patterned growth is investigated. The structural analysis of the quantum dots including spatial ordering, size, and shape are characterized by ex-situ atomic force microscopy (AFM). Results reveal that the indent patterns clearly bias nucleation with dot structures selectively growing on top of each indent. It is speculated that the biased nucleation is due to a combination of favorable surface strain and multi-atomic step formation at the indent sites, which leads to increased adatom diffusion on the patterned area.

Growth mode transitions induced by hydrogen-assisted MBE on vicinal GaAs(110)

The homoepitaxial growth of GaAs by hydrogen-assisted molecular beam epitaxy (H-MBE) on (110) substrates vicinal to (111)A has been studied by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) for different kinetic regimes. When the GaAs growth rate is limited by the kinetics of adatom incorporation to steps, the presence of chemisorbed H on the surface after oxide removal promotes the incorporation of adatoms to steps from the lower terraces, leading to the formation of multiatomic step arrays or ridge patterns by a combination of step propagation and two-dimensional layer-by-layer growth. Supply of atomic H during epitaxy favours three-dimensional growth, leading to Ga-induced surface roughening or mound formation. At high temperatures, the Ga–As interactions at step edges are faster and stable growth of GaAs occurs by step propagation, leading to a faceted surface when H is used both during oxide removal and/or MBE growth.

Comparative study of InAs quantum dots grown on different GaAs substrates by MOCVD

We present a comparative study of InAs quantum dots grown on Si-doped GaAs (1 0 0) substrates, Si-doped GaAs (1 0 0) vicinal substrates, and semi-insulating GaAs (1 0 0) substrates. The density and size distribution of quantum dots varied greatly with the different substrates used. While dots on exact substrates showed only one dominant size, a clear bimodal size distribution of the InAs quantum dots was observed on GaAs vicinal substrates, which is attributed to the reduced surface diffusion due to the presence of multiatomic steps. The emission wavelength is blueshifted during the growth of GaAs cap layer with a significant narrowing of FWHM. We found that the blueshift is smaller for QDs grown on GaAs (1 0 0) vicinal substrates than that for dots on exact GaAs (1 0 0) substrates. This is attributed to the energy barrier formed at the multiatomic step kinks which prohibits the migration of In adatoms during the early stage of cap layer growth.

Kinetically driven self-organization during hydrogen-assisted MBE growth on GaAs(110)

Self-organized growth of GaAs on (110) substrates vicinal to (111)A by hydrogen-assisted molecular beam epitaxy (H-MBE) has been studied for different kinetic regimes using atomic force microscopy (AFM). When GaAs growth is limited by kinetics of adatom incorporation to steps, the presence of chemisorbed H on the surface after oxide removal promotes the incorporation of adatoms to steps from the lower terraces, leading to the formation of laterally ordered multiatomic step arrays by step bunching in the Ga-supply limited regime and ridge patterns in the As-limited regime with improved uniformity and reduced dimensions with respect to those formed by conventional MBE growth. We attribute these changes in pattern morphology to a rapid self-organization of the surface associated with a reduction of the Ehrlich–Schwoebel (E–S) barrrier to As from the lower terraces being incorporated at steps via dissociation of an AsH2* intermediate.

Selective Nucleation and Band-Gap Widening of LiNbO3Nanocrystals on an α-Al2O3Substrate

Selective nucleation of LiNbO 3 nanocrystals on the steps was observed during the initial stage of LiNbO 3 film growth in an α-Al 2 O 3 (0001) substrate with a multiatomic step structure. In addition, stress stored in the nanocrystals as a result of the different thermal expansion coefficients of the film and the substrate caused band-gap widening. As the size of the nanocrystals decreased, the band gap shifted to a higher energy.

Surface morphology and photoluminescence of InAs quantum dots grown on [11~0]-oriented streaked islands under ultra-low V/III ratio

The influences of the low V/III ratio on the surface morphologies and temperature-dependent photoluminescence spectrum of InAs-GaAs quantum dots (QDs) prepared by organometallic vapor phase epitaxy are investigated. Due to the accumulation of In adatoms at the multiatomic step edge on [001] 2/sup 0/ off toward an [111] n-type GaAs substrate, InAs island growth with /spl sim/1 ML coverage takes place prior to the InAs QD formation. With increasing InAs coverage, self-assembled InAs QDs are observed near the InAs islands, which is attributed to the recapture of desorbed In atoms nucleating with supplied As atoms on the edge along [11~0]-orientation of the GaAs substrate.

Nanopatterning of GaAs(110) vicinal surfaces by hydrogen-assisted MBE

ABSTRACTThe effect of atomic hydrogen on the growth mode and surface morphology of GaAs(110) thin films grown by molecular beam epitaxy (H-MBE) has been studied for different kinetic regimes using atomic force microscopy (AFM). Growth in the Ga supply-limited regime after H-assisted oxide removal leads to the formation of multi-atomic step arrays by step bunching with a very uniform terrace size distribution in the 80 nm range. Growth under As-deficient conditions after H-assisted oxide removal induces a rapid self-organization of the GaAs(110) surface into a ridge pattern along the <001> tilt direction, which is broken down into a 3D mound morphology when H is also present during growth. A chacteristic nanofacetting of the surface with very straight <1–10> -type steps is observed at high temperatures regardless of atomic hydrogen being used during oxide desorption and/or epitaxial growth.

Electron transport and optical properties of InGaAs quantum wells with quasi-periodic ( L0∼30 nm) interface corrugation grown on vicinal (1 1 1)B GaAs

We have grown InGaAs quantum wells (QWs) with quasi-periodic interface corrugation on (1 1 1)B misoriented GaAs substrates and studied their transport and optical properties. By atomic force microscopy the corrugation is found to be multiatomic steps of about 30 nm in period and 2 nm in height which lie along the 〈1 0−1〉 direction. Electron mobilities μ across the steps at 4.2 K depend strongly on the electron concentration N s, being proportional to N s 3.3, and are far smaller than μ along the steps by a factor of 5–10. By comparing these data with theory, μ– N s characteristics are roughly explained by taking into account both periodic and random scattering components of interface corrugation. Optical properties of a QW with corrugation are also discussed.

Photoluminescence of isolated quantum dots inmetastable InAs arrays

Certain cases are studied in which quantum dots (QDs) showno collective properties. It is demonstrated that physicallyisolated QDs (IQDs) are typical of the single-layermetastable QD arrays grown by submonolayer migration enhancedepitaxy on vicinal substrates. Their formation is attributed tothe isolation of separate InAs clusters on vicinal terraces dueto the wetting layer ruptures on the edges of bunched multiatomic steps. The carrier exchange between such QDsis not observed. Coalesced QDs formed in upperlayers are typical of multi-layer metastable arrays. Thelateral carrier exchange between such QDs is inhibitedup to 300 K due to the effective electron channelling from themto underlying layers. It is demonstrated that the appearance ofsuch isolated conglomerates results in the anomalous temperature dependence of photoluminescence (PL) for the multi-layerarray. A model is proposed, which explains the influence ofIQDs on the 3D array PL.

Dependence on In content of In xGa 1− xAs quantum dots grown along GaAs multiatomic steps by MOVPE

We fabricated self-organized In x Ga 1− x As quantum dots on GaAs(0 0 1) vicinal surfaces having multiatomic steps by using metalorganic vapor phase epitaxy. The dots preferentially grew along the GaAs multiatomic step edges. We investigated the density and uniformity of the dots as a function of the substrate misorientation angle, the layer thickness, the In content, and the growth temperature. Observation of grown surfaces by atomic force microscopy showed that the dot formation depended strongly on the In content and layer thickness. In general, for lower In contents and/or thinner layers, no dot formation was observed because the interface stresses were insufficient for three-dimensional island growth. For higher In content and/or thicker layers, high-density dots formed on the GaAs multiatomic steps. However, these dots were not always uniform: they included small dots (∼40 nm) and large islands (>100 nm). Under optimum growth conditions, that is, 3.2-monolayer In 0.8Ga 0.2As growth, we obtained high-density (6.2×10 10/cm 2) and highly uniform (10–25 nm) small quantum dots along the GaAs multiatomic step edges.

The initial stage of InGaAs growth by MOVPE on multiatomic-stepped GaAs structures

We investigate the formation of self-organized InGaAs quantum wires (QWRs) on coherent multiatomic-stepped GaAs that had been grown on GaAs vicinal substrates by metal-organic vapor phase epitaxy. The key issues in the formation of InGaAs QWRs are the uniformity of the multiatomic steps and the large modulation in the lateral thickness of InGaAs on multiatomic-stepped GaAs structures. We use images obtained by atomic force microscopy and cross-sectional images of InGaAs layers obtained by transmission electron microscopy to investigate the surface morphology of InGaAs layers on multiatomic-stepped GaAs structures. As a result, we found that in the initial stages of InGaAs growth, Ga and In atoms preferentially attach themselves to the bottom edges of the multiatomic steps, and multiatomic-stepped structures with their own characteristic period and then reappear. These results suggest that the Schwoebel barrier, which causes the step bunching, may be decreased by the change of surface reconstruction after the introduction of small amount of In atoms to GaAs surfaces in the initial stage of growth. Furthermore, multiatomic-stepped structures formed during the growth of InGaAs layers have different periods from those on the underlying GaAs layer. The difference between the multiatomic-stepped structures on GaAs and InGaAs is thought to be caused by the difference of Schwoebel barriers between these two stepped surfaces.

Self-assembled growth of cubic silicon carbide nano-islands on silicon

Silicon carbide(SiC) nano-crystallites are formed by the thermal reaction of C 60 molecules with silicon atoms on Si(0 0 1) and Si(1 1 1) surfaces at 900–1150°C. The surface morphologies and structures of the cubic β-SiC islands on Si(0 0 1) and Si(1 1 1) are investigated in situ using scanning tunneling microscopy. Comparing to bulk SiC crystals, distinctive atomic structures are formed on the surfaces of the SiC crystallites: a (2×3) on SiC(0 0 1) and a (2 3 ×2 3 )R30° reconstruction on SiC(1 1 1). In addition, monatomic and multiatomic steps are observed.

Anisotropic mobilities of low-dimensional electrons at stepped n-AlGaAs/GaAs interfaces with 15 nm periodicity on vicinal (111)B substrates

Laterally modulated two-dimensional electron systems with 15 nm periodicity have been formed at n-AlGaAs/GaAs heterojunctions by making use of multi-atomic steps formed on vicinal (1 1 1)B GaAs epilayers. Highly anisotropic electron mobilities have been observed, where mobility μ || parallel to the steps increases monotonically up to 280 000 cm 2/ V s with carrier density N s at 4.2 K while mobility μ ⊥ perpendicular to the steps decreases remarkably to 32 000 cm 2/ V s at high N s. By comparing this experimental result with theory, we have found that a periodic component of the in-plane potential is important for the anisotropy.

Atomic Hydrogen Induced Step Bunching on High-Index GaAs Substrates for Fabrication of Novel Quantum Wire and Quantum Dot Arrays by Molecular Beam Epitaxy

We present our recent experiments on the effect of atomic hydrogen on the growth on planar and patterned high-index GaAs substrates by molecular beam epitaxy and its application to novel quantum wire and quantum dot arrays. The promotion of step bunching on GaAs (331)A substrates by atomic hydrogen to well-ordered multiatomic step arrays is utilized for the fabrication of modulation-doped conductive quantum wires with strong anisotropy of the electron conductivity. Atomic hydrogen induced step bunching on GaAs (311)A substrates combined with lithographic patterning of the substrate prior to growth produces uniform arrays of quantum dots.