Scanning Microprobe Reflection High-energy Electron Research Articles

Electromigration and phase transformation of Ag on a Cu-precovered Si([formula omitted]) surface

Electromigration and phase transformation of an ultrathin-Ag film patch deposited on a Cu-precovered incommensurate “5×5” phase on Si(111) surface were investigated by in situ ultrahigh-vacuum scanning electron microscopy and microprobe reflection high-energy electron diffraction, and compared with the previous results on the clean and Au-precovered Si(111) surfaces. The film patch spread quite slowly towards the cathode by current apply (and resultant heating), at the initial stage of which the patch area showed a 3×3 phase, consisted of Ag–Cu surface alloy, with three-dimensional (3D) islands in it. Later, a phase transformation from the 3×3 into a 21×21 structure proceeded, accompanied with the 3D islands dissolving. These surface alloy formations play a key role in the migration, which is similar to the case of on Au-precovered Si(111) surfaces.

Real-time scanning microprobe reflection high-energy electron diffraction observations of the cleaning process of GaAs substrates

GaAs surface cleaning using atomic hydrogen (H·) prior to molecular beam epitaxy has been compared to the conventional thermal treatment of GaAs surfaces. The surface morphology was observed in real time using in situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). GaAs surfaces have been found to be uniformly cleaned at temperatures of about 400°C using the H· treatment. On the other hand, a local initiation of the desorption of the oxide layer has been observed during the conventional thermal treatment at about 580°C.

Real-time scanning microprobe reflection high-energy electron diffraction observations of the cleaning process of GaAs substrates

GaAs surface cleaning using atomic hydrogen (H·) prior to molecular beam epitaxy has been compared to the conventional thermal treatment of GaAs surfaces. The surface morphology was observed in real time using in situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). GaAs surfaces have been found to be uniformly cleaned at temperatures of about 400°C using the H· treatment. On the other hand, a local initiation of the desorption of the oxide layer has been observed during the conventional thermal treatment at about 580°C.

Surface diffusion length of Ga adatoms in molecular-beam epitaxy on GaAs(100)–(110) facet structures

By the molecular-beam epitaxial (MBE) growth of GaAs on [001]-mesa stripes patterned on GaAs(100) substrates, (110) facets were formed on the mesa edges defining (100)–(110) facet structures. The surface diffusion length of Ga adatoms along the [010] direction on the mesa stripes was obtained for a variety of growth conditions by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Using these values and the corresponding growth rate on the GaAs(110) facets, the diffusion length on the (110) plane was estimated. We found that the Ga diffusion length on the (110) plane is longer than that on the (100) and (111)B planes. The long diffusion length on the (110) plane is discussed in terms of the particular surface reconstruction on this plane.

Surface diffusion length of Ga adatoms in molecular-beam epitaxy on GaAs(100)-(110) facet structures

By the molecular-beam epitaxial (MBE) growth of GaAs on [001]-mesa stripes patterned on GaAs(100) substrates, (110) facets were formed on the mesa edges defining (100)–(110) facet structures. The surface diffusion length of Ga adatoms along the [010] direction on the mesa stripes was obtained for a variety of growth conditions by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Using these values and the corresponding growth rate on the GaAs(110) facets, the diffusion length on the (110) plane was estimated. We found that the Ga diffusion length on the (110) plane is longer than that on the (100) and (111)B planes. The long diffusion length on the (110) plane is discussed in terms of the particular surface reconstruction on this plane.

Real-time scanning microprobe reflection high-energy electron diffraction observations of III–V growth during molecular-beam epitaxy

Scanning microprobe reflection high-energy electron diffraction is a powerful technique to investigate the growth mechanism through microscopic surface observations during molecular-beam epitaxy (MBE) or metalorganic MBE. The Ga droplet formation and annihilation were observed in real time during GaAs growth with an alternative source supply. Droplets with a round shape whose diameter was the order of a micrometer were formed when only Ga was supplied over an amount of monolayer of GaAs on the surface. They were annihilated when As was supplied to grow a GaAs layer. At high substrate temperatures, the droplets on the vicinal GaAs surface moved upward to atomic steps before its annihilation. Anomalous features of droplet formation were observed during AlAs growth with an alternative source supply on the GaAs substrate. This is interpreted by the intermixing between Al and Ga atoms on the surface during growth. Indium droplets were also observed during InAs MBE growth at high temperatures, which were revealed to be the origin of the InAs hillocks.

Real-time scanning microprobe reflection high-energy electron diffraction observations of InGaAs surfaces during molecular-beam epitaxy on InP substrates

The microscopic surface features were observed during the molecular-beam epitaxy of InxGa1−xAs on InP (100) substrates by scanning microprobe reflection high-energy diffraction in real time as a function of x from 0.41 to 0.96. In the case of In0.41Ga0.59As (the lattice mismatch of the strained layer was f=−0.77%, where the minus sign represents a smaller lattice constant of an epitaxial layer than that of a substrate), three-dimensional island growth was observed from the start of growth. On the other hand, two-dimensional layer-by-layer growth was maintained during growth of InxGa1−xAs with x=0.45–0.75 (f=−0.47 to +1.48%, where the plus sign represents a larger lattice constant of an epilayer than that of a substrate). While the growth of compressive epilayers with f between +0.36% and +1.48% proceeded, a surface crosshatched morphology was observed after the growth of certain film thicknesses, which were dependent on the lattice mismatch. A rough textured morphology was observed instead of a crosshatched morphology for those epilayers with a lattice mismatch greater than +2.29% (In0.87Ga0.13As), where three-dimensional island growth was favored over the layer-by-layer growth mode. Transmission electron microscope observations showed that the presence of a surface crosshatched pattern was correlated with the presence of 60° mixed misfit dislocations. The result indicates that the thickness at which the crosshatched line appears represents the critical layer thickness corresponding to misfit dislocation generation.

Application of real-time low-energy ion scattering spectroscopy to heterointerface formation processes of molecular beam epitaxially grown III–V compound semiconductors

Coaxial impact-collision ion scattering spectroscopy (CAICISS) has been applied for the in situ analysis of heterointerface formation processes and related phenomena of III–V compound semiconductors in a CAICISS/molecular beam epitaxy (MBE) system where CAICISS was directly combined with an MBE chamber. We first demonstrate that CAICISS is useful for studying surface step structures by taking vicinal GaAs (100) surfaces as an example. The initial growth stages are then investigated regarding various lattice-mismatch (f) systems of AlAs on GaAs (f = 0.14%) and vice versa, GaAs on InP (f ≈ 4%), and GaAs on InAs (f ≈ 7%) and vice versa, in terms of the variations in the scattering intensity of incident ions. The obtained results are discussed especially from the view point of the growth mode (two-dimensional (2D) or 3D). During these processes, the exchange of atoms between the deposited films and the substrates is shown to be clearly observed for the AlAs/GaAs and GaAs/InAs systems based on the scattering intensity variation. These results are also discussed on the basis of other supporting data for the same phenomena observed by scanning tunneling microscopy and scanning microprobe reflection high-energy electron diffraction.

Lateral growth of GaAs on patterned {-1-1-1}B substrates for the fabrication of nano wires using metalorganic molecular beam epitaxy

We have grown GaAs nano wire structures (45 × 20 nm2) buried in AIAs layers by lateral metalorganic molecular beam epitaxy on the terraced sidewalls of mesa-grooved (-1-1-1)B substrates. The growth of GaAs occurred primarily on the sidewall of the mesa-grooves and not on the (-1-1-1)B surface for arsenic pressures greater than 2.0 × 10−3 Pa at a substrate temperature of 480°C. An (0-1-1) facet formation during the lateral epitaxy at the intersection region between the bottom (-1-1-1)B surface and the (1-2-2)A sidewall has been directly observed by real-time scanning microprobe reflection high-energy electron diffraction. The growth rate on the (0111) facet was estimated from the variation of its width with growth time.

Arsenic Pressure Dependence of Surface Diffusion of Ga on Nonplanar GaAs Substrates

The arsenic pressure dependence of Ga adatom surface diffusion in molecular beam epitaxy (MBE) on nonplanar substrates was investigated. By using in situ scanning microprobe reflection high-energy electron diffraction (µ-RHEED), the distribution of the growth rate of GaAs on the (001) surface near the edge of the (111)A or (111)B sidewall was measured under various arsenic pressures. The surface diffusion length of Ga adatom incorporation on the (001) surface derived from the distribution is on the order of micrometers and it shows a strong dependence on arsenic pressure. A simple model based on one-dimensional surface diffusion was proposed. With this theory, the lifetime of Ga adatom incorporation on other surfaces is obtained.

Real-time μ-RHEED observations of droplets on GaAs (111) B surfaces during growth with alternating source supply

Detailed observations were carried out on the microscopic surface features of GaAs and AlAs on GaAs (111)B surfaces during molecular beam epitaxial growth with an alternating source supply by scanning microprobe reflection high-energy electron diffraction in real time. It was found that Ga (or Al) droplets are formed during growth under excess Ga (or Al) supply; the droplets migrate on the surfaces during and after Ga (or Al) supply in the homoepitaxial growth.

Real-time observations of III-V growth on patterned substrates by μ-RHEED

Scanning microprobe reflection high-energy electron diffraction (μ-RHEED) reveals microscopic surface features during MBE growth on a non-planar substrate. The Ga droplet formation and their movement were observed in GaAs growth with an alternating source supply on a vicinal plane. Appearance and development of additional facets on the edge of the mesa stripe were monitored during growth in real time. The growth front of the lateral epitaxy was observed, and it was found that the (311) facet was formed as a sidewall of a mesa stripe along the [0 11 direction on the (111)B substrate. InAs hillock formation which depended on the line spacing of the stripe pattern of the substrate was observed on high-temperature substrates, and the real-time observation showed that the hillocks originated in the droplets on the surface.

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH 3). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.

Observations of facet formation during MOMBE of GaAs on a patterned substrate

We report on the first real time observation of facet formation during metalorganic molecular beam epitaxy (MOMBE) of GaAs on a mesa-etched GaAs(100) surface by in situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Besides the initial (100) surface and (111)A sidewall, a (411)A facet was formed at the lower sidewall and (311)A and (5 11 )A facets were formed at the upper sidewall after growth initiation. Good agreement was obtained between the shape of the epitaxial layer determined using μ-RHEED and that observed using a cross-sectional secondary electron microscope (SEM) after growth. The results indicate that the shape of the growing surface is exactly monitored by μ-RHEED in real time.

ＩＩＩ‐Ｖ族化合物半導体成長のμ‐ＲＨＥＥＤ実時間観測

Scanning microprobe reflection high-energy electron diffraction (μ-RHEED) reveals microscopic surface features during MBE growth. The Ga droplet formation and annihilation were observed in GaAs growth with alternative source supply. The droplet movement was also observed on a vicinal plane of a GaAs (111) substrate. InAs hillock formation was observed on high temperature substrates, and the real-time observation showed that the hillocks originated in the droplets on the surface. The surface diffusion length of adatoms was estimated from the distribution of growth rates near the edge of a mesa stripe on the substrate. The obtained values of the diffusion length were much longer than the averaged distance between atomic steps on the surface. This means that the incorporation ratio of the adatom into an atomic step is much less than unity.

Surface diffusion length during MBE and MOMBE measured from distribution of growth rates

The growth rates of the layers, which were grown both by usual solid-source molecular beam epitaxy (MBE) and by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMGa) and metal As source materials, on a mesa-etched (001) GaAs surface, were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion length of the surface adatoms was determined by the gradient of the variation of the growth rate. The obtained values of the diffusion lengths were of the order of a micrometer under the usual growth conditions both in MBE and in MOMBE. The diffusion length in the case of MOMBE was larger than that of a Ga atom in the case of MBE.

Lateral Metalorganic Molecular Beam Epitaxy of GaAs on Patterned (1̄1̄1̄)B Substrates

GaAs layers were grown on patterned (1̄1̄1̄)B substrates having (1̄2̄2̄)A sidewalls with various arsenic fluxes at a fixed temperature of 480°C by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMGa) and metal arsenic. Vertical growth rate on the top and bottom (1̄1̄1̄)B surfaces decreased rapidly as the arsenic flux was increased. For arsenic fluxes of 2.0×10-3 Pa and more, only lateral epitaxy on the (1̄2̄2̄)A sidewall was achieved. Real-time scanning microprobe reflection high-energy electron diffraction (µ-RHEED) observations showed that the surface smoothness of the epitaxial layer was maintained throughout the growth time under the optimized condition.

Surface diffusion length observed by in situ scanning microprobe reflection high-energy electron diffraction

The dependence of surface diffusion phenomena of Ga adatoms on As flux during molecular beam epitaxy is investigated. Variations of growth rates of GaAs layers grown on the (001) surfaces adjacent to the (111) surfaces were measured by scanning microprobe reflection high-energy electron diffraction. The surface diffusion length is derived from the variations of the growth rates. It is found that the surface diffusion length of the Ga adatoms becomes larger under lower As flux.

Real-time μ-RHEED observations of GaAs surfaces during growth with alternating source supply

The microscopic surface features of GaAs and AlAs during growth with an alternating source supply were observed by scanning microprobe reflection high-energy electron diffraction in real time. Scanning reflection electron microscope images were obtained in the TV scan mode (50 frames per second). The creation of Ga droplets was observed when Ga atoms over the critical amount depending on the substrate temperature were supplied. They disappeared after a sufficient amount of As was supplied. When 5 monolayers of Ga atoms were supplied per cycle, small islands with bright contrast appeared due to three-dimensional nucleation after As was supplied.

Distributions of growth rates on patterned surfaces measured by scanning microprobe reflection high-energy electron diffraction

Microscopic distributions of growth rates on GaAs(001) layers next to (111)A and (111)B surfaces were measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high-energy electron diffraction. The increase in the growth rate on the GaAs(001) surface near the edge of (111)A surfaces is observed and the decrease in the growth rate on the GaAs(001) surface near the edge of the (111)B surfaces is found out. The exponential variation of the growth rate as a function of the distance from the edge, reflects surface diffusion of Ga atoms. The diffusion lengths along the [110] and [1̄10] directions are estimated to be about 1 and 8 μm at 560 °C, respectively.