Microprobe Reflection High-energy Electron Research Articles

Influence of Initial Amorphous Layer Deposition Temperature on Lateral solid-Phase Epitaxy of Silicon

The influence of deposition temperature (Td) of initial amorphous layer on the lateral solid-phase epitaxy (L-SPE) of silicon was investigated in a wide deposition temperature range between -20°C and 405°C. Nomarski optical microscopy, scanning electron microscopy (SEM), cross-sectional transmission microscopy (TEM) and microprobe reflection high-energy electron diffraction (µ-RHEED) techniques were used. A hitherto unnoticed markedly strong correlation was found between Td and L-SPE growth where the lowest deposition temperature of Td=-20°C gave the highest growth rate and best film quality. Detailed SEM and TEM observation of amorphous Si (a-Si) films revealed that there were many vertical pores threading through a-Si films. A new L-SPE growth model explaining Td-dependent growth rate was proposed in terms of growth delay induced by the vertical pores. Low-temperature deposition of initial amorphous layers was shown to be an effective way of achieving high-quality films in a short time.

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.

Effect of As molecular species on inter-surface diffusion in GaAs MBE for ridge structure fabrication

The faceting and the shrinkage processes of ridge structure fabricated along [1 1 0] on GaAs (0 0 1) substrate with As 2 flux and As 4 flux were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in the MBE chamber. It was found that the shrinkage of top size with As 4 flux was faster than that with As 2 flux. To analyze the result of the experiments, the calculation based on two-face inter-surface diffusion model was conducted. From this calculation, we show that the shrinkage of the top size is slower when the arsenic pressure is higher, simply because the lateral flow of Ga adatoms towards the top surface becomes smaller. Under the As 2 flux, the amount of Ga adatoms which migrate to (0 0 1) surfaces from (1 1 1)B surface becomes smaller compared with that under As 4 flux due to the higher reactivity of As 2 to Ga adatoms.

Real-time observations of mesa shrinkage process in MBE of GaAs on (1 1 1)B patterned substrates and theoretical analysis

The faceting and shrinkage processes of mesa structures on GaAs (1 1 1)B substrates were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in MBE chamber. To understand the experimental results, the calculation based on inter-surface diffusion model was conducted and the results are compared with the experiments. From the comparison we evaluated diffusion coefficient and incorporation lifetime of Ga adatoms. The lifetime and the diffusion coefficient of (1 1 1)B substrate were estimated, respectively, as 1/70 as small and 50 times as large as those of {1 1 0} side wall. The mesa shrinks faster when arsenic pressure is higher, because Ga adatoms on {1 1 0} side wall flow more towards (1 1 1)B top and less towards the bottom.

Structural transformation of hydrogen-adsorbed Si(111)-√3×√3-Ag surfaces induced by electron-stimulated desorption

Atomic hydrogen irradiated Si(111)-√3×√3-Ag surfaces, which involve nanometer size Ag clusters and hydrogen terminated regions, are studied by combined surface analysis with ultrahigh-vacuum scanning electron microscopy (UHV-SEM), scanning reflection electron microscopy (SREM), microprobe reflection high-energy electron diffration (μ-RHEED) and Auger electron spectroscopy (AES). Electron beam irradiation onto this surface results in disappearance of the Ag clusters, and subsequent low-temperature annealing at 275°C transforms the surface to the original √3×√3-Ag structure. Hydrogen removal from the Si surface induced by electron-stimulated desorption (ESD) and surface diffusion of Ag atoms on the bare regions by low-temperature annealing explain this structural transformation.

MICROPROBE RHEED/STM COMBINED MICROSCOPY

We have developed microprobe reflection high energy electron diffraction combined with scanning tunneling microscope and molecular beam epitaxy equipment. This combination makes it possible to study and control surface processes in the magnification range from several hundred micrometers to the atomic scale. An electron biprism is also attached to the incident electron beam path, which produces a new kind of scanning electron microscopy called scanning interference electron microscopy. The two coherently divided electron beams created by the biprism produce electron interference fringes. The electron interference fringes are used to form ultrafine periodic structures by electron-stimulated surface reaction and to characterize electromagnetic properties of the surfaces. The formation of periodic carbon grid lines produced by the interference fringes on a GaAs surface and the study of Ge thin film growth on a partially Ga adsorbed Si (111) surface are described for application examples of the microscopy.

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 observation of In deposition on GaAs during molecular beam epitaxy by scanning electron microscopy

In-situ observation of In droplet formation and annihilation on GaAs(001) at 580°C was made by using a scanning electron microscope and microprobe reflection high-energy electron diffraction, both installed in the MBE growth chamber. It has been found that the density of In droplets depends strongly upon incident In flux and drops quickly to zero at a certain value of In flux. The effects of As pressure and growth steps on the droplet density are discussed.

Inter-surface diffusion of In on (111)A-(001) InAs nonplanar substrates in molecular beam epitaxy

The inter-surface diffusion of In adatoms between a (111)A and a (001) surface on the InAs (111)A-(001) nonplanar substrate was investigated for the first time using microprobe reflection high-energy electron diffraction (μ-RHEED). The surface diffusion of In adatoms is found to depend strongly on the growth temperature and the arsenic pressure, while it is independent of the In flux. Furthermore, it was observed that the migration direction of the In lateral flux between the (111)A and the (001) InAs surface changes depending on the growth conditions.

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.

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.

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.

Migration of Ga atoms during Si molecular beam epitaxial growth on a Ga-adsorbed Si(111) surface

Ga atom migration during Si molecular beam epitaxial growth on a Ga-adsorbed Si(111)√3 × √3 surface is investigated by microprobe reflection high-energy electron diffraction. Ga atom migration in the depth direction (surface segregation) and termination of Si dangling bonds by Ga atoms cause a reduction of the production rate of two-dimensional nuclei and an enhancement of surface diffusion of Si atoms during Si-MBE. This improves the crystallinity of Si-grown films. Ga atom migration in the lateral direction is also investigated using a partially Ga-adsorbed surface. The Ga-adsorbed areas are preserved during Si-MBE at low substrate temperature ( $ ̃ 300°C), and during annealing of the sample at high substrate temperature ( $ ̃ 600°C). The adsorbed areas clearly change during Si-MBE at high substrate temperature ( $ ̃ 600°C). This indicates that Ga atoms migrate in the lateral direction on the surface when interchange between Si atoms and Ga atoms occurs in the surface segregation process of Ga atoms during Si-MBE.

In situ observation of the growth process of the InAsGaAs heteroepitaxial system using scanning microprobe reflection high energy electron diffraction-total reflection angle X-ray spectroscopy

A scanning reflection electron microscopy-total reflection angle X-ray spectroscopy (SREMTRAXS) system has been newly developed for in situ monitoring of the crystal structure and the chemical composition of film surfaces during molecular beam epitaxy. In this system, the primary electron beam from the microprobe reflection high energy electron diffraction gun is focused on the sample down to below 1 μm in diameter. The SREM image and TRAXS image are simultaneously obtained by monitoring the diffraction spot intensity on the fluorescent screen and by detecting the X-rays emitted from the sample at the critical angle for total reflection respectively. The surfaces of InAs films on GaAs substrates grown under various conditions were observed in situ with this system. The take-off angle dependence of the characteristic X-ray intensity was closely related to the surface morphology of the grown film observed with this system.

Study of solid surfaces using micro-electron-beams

New surface-analysis apparatus and techniques have been developed which are based on a micro-electron beam. The spatial resolution of the ultra-high vacuum scanning electron microscope is about 100 Å and micro-probe reflection high-energy electron diffraction patterns can be obtained. Diffraction patterns of either Auger electrons or quasi-elastically back-scattered electrons from the probed micro-domain can be measured in combination with a specially developed “constant take-off angle electron energy analyzer”. These diffraction techniques can be used for the determination of surface structures of a region as small as 0.1 × 1 μm 2. These techniques are called μ-probe Auger electron diffraction and μ-probe grazing-incidence back-scattering medium energy electron diffraction. Study of In/Si(111) surfaces by these techniques is introduced.