Microprobe Reflection High-energy Electron Diffraction Research Articles

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.

Study of Si(001) 2×1 domain conversion during direct current and radiative heatings

Si(001) 2×1 domain conversions induced by sample electric current are observed using microprobe reflection high-energy electron diffraction. Samples are heated by using both direct current and a radiative heater. It is found that diffusion anisotropy of Si adatoms exists on 2×1 surfaces and that the domain conversion velocity is proportional to a voltage applied between the sample edges. This indicates that the conversion process is induced by the diffusion anisotropy and the electric force acting on positively charged adatoms. Minor 2×1 domain terraces always spread when the sample is heated radiatively. This preserves narrow 2×1 and 1×2 domain terraces after the conversion process and during Si growth process.

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.

Microprobe RHEED study of electromigration effect on Si MBE growth

Si(001)2×1 domain conversion induced by a sample current and its effect on Si MBE growth are studied using microprobe reflection high-energy electron diffraction. Samples are heated by using both a direct current and a radiative heater. It is found that diffusion anisotropy of Si adatoms and the electric force acting on positively charged adatoms cause the domain conversion. Minor 2 × 1 domain terraces always spread by a long-range repulsive interaction between atomic steps when the sample is heated radiatively. The domain conversion phenomenon and the repulsive interaction cause biatomic step flow growth during MBE growth. Atom migration induced by the sample current promotes Si MBE growth.

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.

Observation of electromigration effect upon Si-MBE growth on Si(001) surface

Si(001) surface topography changes during annealing, and Si molecular beam epitaxial (MBE) growth is observed by reflection electron microscopy using microprobe reflection high-energy electron diffraction. When the sample is heated above 600°C, one of the monoatomic steps to which the 2 × 1 dimer axis is normal or parallel becomes less stable than the other, depending on the direction of the sample electric current used for heating. Si atoms are detached from the unstable steps and captured by the stable steps. This causes selective growth of one of the two 2 × 1 domains on the surface and biatomic step growth during Si MBE at high substrate temperature (∼900°C).

I n s i t u scanning microprobe reflection high-energy electron diffraction observation of GaAs surfaces during molecular-beam epitaxial growth

A specially designed molecular-beam epitaxy (MBE) system with scanning microprobe reflection high-energy electron diffraction (RHEED) has been developed. The incident electron beam is designed to focus down to 10 nm in diameter and scan the surface of the substrate under MBE growth conditions. Using the RHEED intensity variation produced by the scanning of the incident beam, we obtain a microscopic image, i.e., which is the scanning reflection electron microscope (SREM) image, highly sensitive to the surface structure. The SREM images of GaAs(001) using the specular beam spot revealed granular features over the entire surfaces of MBE-grown GaAs layers which come from undulations of the surface. The undulation wavelength was typically 0.1–0.5 μm and developed along [1̄10] direction during MBE growth at 570 °C. At higher substrate temperatures, little development of the undulation was observed. As an overall feature of MBE growth, two-dimensional layer-by-layer growth was observed over the whole surface having undulations.

Ultrathin film growth of silicides studied using microprobe reflection high-energy electron diffraction and Auger

The generic parameters and processes involved in solid phase epitaxial growth of ultrathin films by contact reaction are discussed. The particular strengths of quantitative reflection high-energy electron diffraction (RHEED) and Auger line-shape analyses for separating coexisting phases are demonstrated with examples from the Ni/Si(111) system. Using RHEED Kikuchi patterns and rocking curves, we identify the ordered structures NiSi2 (mixed A and B type) and Ni2Si-θ. From the Auger line shape we find that the low-coverage room-temperature material is disordered Ni2Si, the NiSi2 epitaxial layers are capped with a layer of excess silicon, and the surface layers pass through a NiSi-like phase in the transformation from Ni2Si to NiSi2.

Microscopic observation of Si MBE on Si(001) surface using microprobe RHEED

Si(001) surface topography changes during Si molecular beam epitaxy (MBE) were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (450 °C), it was found that alternate growth of 2×1 and 1×2 surface reconstruction domains occurred with the half period of the intensity oscillations, and the shape of the domains was preserved during MBE. When almost no oscillations were observed at higher substrate temperature (>750°C), atomic step migration occurred during MBE. These results provide direct evidence that RHEED intensity oscillations occur by layer-by-layer two-dimensional nucleation growth.

Low-temperature SOI (Si-on-insulator) formation by lateral solid-phase epitaxy

Low-temperature (550–600 °C) formation of a Si-on-insulator structure by a solid-phase process is investigated. A microprobe (μ) reflection high-energy electron diffraction observation reveals that oriented crystal growth propagates from the seeding area in solid-phase epitaxy (SPE). The effects of random nucleation, epitaxial alignment, and local doping on lateral (L)-SPE are examined. As a result, a relatively large L-SPE area, 14 μm from the seeding area, is achieved on insulating regions. Crystal quality and electrical properties of L-SPE layers are examined using μ-Raman spectroscopy and field-effect transistor fabrication. A small stress field, 2.5×109 dyn/cm2, and high electron mobility, 720 cm2/V s, comparable to that of bulk Si are obtained.

マイクロプローブ反射電子回折法による表面研究

Si (111) surface topography changed during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED) . When RHEED intensity oscillations were observed at low substrate tempe-rature (350°C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500°C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer-by-layer two-dimensional nucleation growth.

Observation of Si(111) surface topography changes during Si molecular beam epitaxial growth using microprobe reflection high-energy electron diffraction

Si(111) surface topography changes during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (350 °C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500 °C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer-by-layer two-dimensional nucleation growth.

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [1̄1̄2] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3 R30°, sharp √3 × √3 R30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.