RHEED Patterns Research Articles

AFM, SEM andin situRHEED study of Cu texture evolution on amorphous carbon by oblique angle vapor deposition

The evolution of the crystal orientation of a Cu film grown on an amorphous carbon substrate without intentional heating under 75±6° oblique angle vapor deposition was investigated ex-situ by atomic force microscopy AFM and scanning electron microscopy SEM, and in-situ by reflection high-energy electron diffraction RHEED. At the initial stage of growth 12 nm thick the diffraction patterns showed uniform rings indicating random nucleations of crystals. With more Cu depositions 100 nm thick the diffraction pattern started to break symmetrically from the middle of the 111 and 200 rings representing the absence of 111 and 200 planes parallel to the substrate. However, after this transition stage, at the thickness of 410 nm, the intensity distribution of diffraction patterns appeared asymmetric about the middle of the rings, which is interpreted as the appearance of a tilted 111 texture. Finally the diffraction patterns developed into separated short arcs and showed only a 1111 ¯10 II-O two-orientation texture. By comparing RHEED patterns with the SEM and AFM images of the final film, we argue that the tilted columns having tilted 111 top faces dominate in the later stage of growth. Furthermore, considering the geometry of crystals and shadowing effects, we argue that the vertices of columns having the highest growth velocity normal to the substrate and therefore receiving the maximum flux will dominate the film growth and determine the tilt angle of the texture and the preference of the azimuthal angle orientation.

MOVPE InN on a 3c‐SiC/Si(111) template formed by C + ‐ion implantation into Si(111)

This paper reports a new interlayer for the epitaxial growth of InN on Si. A 3c-SiC layer as an interlayer is formed by carbon (C+)-ion implantation into Si(111) wafer. After the C+-ion implantation with an energy 180 keV at 600 °C, the Si(111) wafer is annealed in O2 atmosphere at 1250 °C for 2 h. Layers of SiO2, Si and polycrystalline SiC are successively removed and a single-crystalline 3c-SiC layer is exposed successfully on the surface. MOVPE growth of InN is performed at 600 °C on a 3c-SiC/Si(111) template with a low-temperature (550 °C) GaN buffer. The grown film is confirmed to be single-crystalline wurtzite InN by the RHEED patterns. The film shows an intense photoluminescence with peak energy of 0.73 eV at room temperature. Carrier concentration and Hall mobility are 1.3 × 1019 cm–3 and 490 cm2/Vs, respectively. They are slightly inferior to those for InN films grown on sapphire. The crystalline quality of InN is found to be improved with increasing the crystalline quality of the 3c-SiC layer formed by the implantation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of initial nucleation in molecular‐beam epitaxy of GaN on lattice‐matched ZrB 2 substrates

Nitrogen-polar GaN was grown on lattice-matched ZrB2 substrates by a two-step rf-MBE process with conventional low-temperature GaN (LT-GaN) used as the nucleation layer. For nucleation in slightly Ga-rich conditions, a streaky RHEED pattern was maintained from the very beginning and persisted throughout subsequent high-temperature growth. However, despite the streaky RHEED pattern the initial nucleation was evidently not 2D as inferred from AFM and x-ray results. For nucleation in slightly N-rich conditions, spots appeared in the RHEED in the early stages of growth but a streak pattern was restored with high-temperature GaN growth. Tradeoffs were identified in optimizing the LT-GaN nucleation process. Slightly Ga-rich nucleation conditions offered the best x-ray characteristics with ω-scan FWHM values of 400-500 and 720-800 arcsec for the (0002) and (104) reflections, respectively. Short-length atomic steps (up to ∼200 nm) were present on the surfaces of such layers, but the morphology showed a sub-grain structure. Slightly N-rich nucleation conditions yielded remarkably smooth films surfaces (rms 0.5 nm over 10 µm2) with uninterrupted atomic steps up to several µm long, but inferior x-ray ω-scans (∼50% larger FWHM values). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin single-crystal Sc 2O 3 films epitaxially grown on Si (1 1 1)—structure and electrical properties

Single-crystal single-domain Sc 2O 3 films have been epitaxially grown on Si (1 1 1) using molecular beam epitaxy (MBE) techniques. The Sc 2O 3 films have the bulk bixbyite cubic phase with a very uniform thickness, a structural perfection, and a sharp interface with Si. The thin oxide films exhibit bright, streaky, and reconstructed RHEED patterns. The high-intensity oscillation in the reflectivity, the strong Pendollusung fringes around the Sc 2O 3 (2 2 2) diffraction peak, and their narrow rocking curves are observed using the high-resolution X-ray diffraction (XRD). The (1 1 1) axis of the oxide films is parallel to the (1 1 1) axis of the Si substrate. The cone scans of the Sc 2O 3 {4 4 0} and Si {2 2 0} diffraction peaks about the surface normal find a 60° symmetry rotation of the film with the respect to the substrate. The Sc 2O 3 films exhibit low electrical leakage currents and a breakdown field of more than 5 MV/cm.

RHEED observation of the growth of chalcopyrite-type MnGeP 2 on GaAs(0 0 1) substrate using Ge–buffer layer

Epitaxial growth of a novel chalcopyrite-type MnGeP 2 has been investigated using an MBE technique. In order to improve the surface morphology of the films, an effect of introduction of a Ge buffer layer was investigated. This results in dramatic change of the RHEED pattern from spotty to streaky ones. Improvement of surface morphology was confirmed by SEM observation. We attribute the improvement to the crystallographic affinity of Ge with II–IV–V 2 compounds and atomically flat surface of the buffer layer.

3C-SiC/Si template growth and atomic layer epitaxy of cubic GaN by RF-MBE

Growth of 2 in cubic gallium nitride (c-GaN) film on a cubic silicon carbide on Si template (3C-SiC/Si) using Si (0 0 1) and Si(1 1 1) substrates was performed by using a radio frequency molecular beam epitaxy (RF-MBE) system, which was equipped with a jet nozzle. The 3C-SiC/Si template was prepared using alternating exposure of acetylene ( C 2 H 2 ) gas introduced by the jet nozzle under ultra high vacuum and Si molecular beam from a Si effusion cell. Very smooth 3C-SiC/Si templates with RMS roughness by atomic force microscopy of about 0.2–0.4 nm were grown here. Good crystal quality of low-temperature buffer (LTB) layer was obtained on a good 3C-SiC/Si template and confirmed through the observation of RHEED pattern. A grown c-GaN film's X-ray diffraction full-width at half-maximum of (0 0 2) peak was 13.2 arcmin ( 2 θ ) for a 544 nm-thick film. c-GaN was also grown using an atomic layer epitaxy (ALE) method. The ALE method proved to be necessary for a uniform and homogeneous atomic nitrogen flux under slightly Ga excess condition to grow the uniform c-GaN using rf-MBE.

Two-dimensional growth of ZnO epitaxial films on c-Al2O3 (0 0 0 1) substrates with optimized growth temperature and low-temperature buffer layer by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were deposited on c-plane sapphire substrates with the low-temperature (LT) ZnO homo-buffer layer by plasma-assisted molecular beam epitaxy. LT ZnO buffer layer with the thickness of 15nm was grown at 500°C. After high-temperature annealing at 800°C for 30min, the growth of ZnO with about 800nm thickness was restarted at different temperatures from 680 to 800°C. Although the surface of the LT-buffer layer was three-dimensional, appropriate subsequent growth temperature facilitated two-dimensional growth. The smallest full-width at half-maximum (FWHM) of X-ray ω-rocking for ZnO(0002) diffraction was 85arcsec and then slightly increased with the increament of the deposition temperature. The RHEED pattern over the surface of ZnO film grown at 720°C showed very streaky lines, while streaky lines superimposed with spotty patterns were obtained at other temperatures. From the Hall measurement, the mobility values for the ZnO films deposited at 720 and 760°C were 103 and 105cm2/Vs, and the carrier concentration was 2.45×1017 and 2.21×1017/cm3, respectively. In low-temperature photoluminescence measurement at 10K, most of the ZnO thin films showed neutral donor-bound exciton, I4(D0, X) at 3.362eV and acceptor-bound exciton, I10(A0, X) at 3.3497eV were clearly observed with the phonon replica at 3.308eV, and the lowest FWFM of I10 peak was found to be 8.4meV for the ZnO grown at 720°C.

Reconstruction of the [formula omitted]-Ga[formula omitted]O[formula omitted] (1 0 0) cleavage surface to hexagonal GaN after NH[formula omitted] nitridation

The potential use of β-Ga2O3 wafers as transparent conductive substrates for compounds with hexagonal structure, in particular the GaN-system, is shown. Nitridation of the main cleavage plane under high temperature and NH3 gas results in the substitution of O by N on the surface, and a simultaneous surface reconstruction. The 2-fold symmetry of the initial (100) plane changes to a 6-fold symmetry, keeping the [010] azimuth as a symmetry direction. The RHEED pattern suggests the formation of GaN on the β-Ga2O3 surface, with the b-axis of β-Ga2O3 parallel to the 〈112¯0〉 direction of GaN, so that β-Ga2O3 can be used as a substrate with no lattice mismatch to GaN.

Synthesis of as-grown superconducting MgB 2 thin films by molecular beam epitaxy in UHV conditions

As-grown superconducting MgB 2 thin films have been grown on SrTiO 3(0 0 1), MgO(0 0 1), and Al 2O 3(0 0 0 1) substrates by a molecular beam epitaxy (MBE) method with novel co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, electrical resistivity measurements, and SQUID magnetometer. The RHEED patterns indicate three-dimensional growth for MgB 2. The highest T c determined by resistivity measurement was about 36 K in these samples. And a clear Meissner effect below T c was observed using magnetic susceptibility measurement. We will discuss the influence of B buffer layer on the structural and physical properties.

Structure of tungsten oxide nanoclusters

Tungsten oxide clusters were prepared by oxidizing tungsten clusters deposited on sapphire. The oxidation was carried out by thermal treatment under various conditions. The morphology and the structure of the oxide clusters were investigated by AFM and RHEED. It was found that the tungsten deposit on the Al 2O 3 substrate was partially oxidized to WO 2 by annealing in vacuum. The Al 2O 3 substrate exhibited a modified surface indicating that the tungsten clusters were oxidized by the substrate oxygen. Oxidation under low oxygen pressure (10 Pa) gave rise to a WO 2 phase while oxidation in air led to WO 3. In all experiments, the epitaxial orientations of tungsten or tungsten oxide clusters were deduced from the RHEED patterns.

The effect of ZnO homo-buffer layer on ZnO thin films grown on c-Al 2O 3(0 0 0 1) by plasma assisted molecular beam epitaxy

ZnO thin films were deposited on c-plane sapphire substrates on the ZnO homo-buffer layer with different thickness and growth temperature by plasma assisted molecular beam epitaxy. The effects of buffer-layer growth variables on the properties of the ZnO films were investigated and discussed on a collective basis compared with other reports. RHEED patterns were taken over different buffer layer surfaces and the initial growth mode of ZnO buffer layer was recognized as Stranski–Krastanov mode by the direct observation of a streaky pattern superimposed with a spotty pattern of thicker than 8 nm. Through examining the XRD θ-rocking curve of ZnO (0 0 0 2) peak, it seems that the crystalline quality of the ZnO thin film grown on the ZnO buffer layer was gradually improved with the increase of the buffer layer thickness. Strong near band-edge emission at 378 nm was well observed without deep-level emission at the ZnO films grown on the 15 nm buffer layer prepared at 500–600°C, and those grown on the thicker buffer layer or prepared at 400°C or 700°C showed deep-level emission around 510 nm. In Hall measurement, the ZnO films showing deep-level emission gave also carrier concentration higher than 1×10 19/cm 3 and those with better crystalline quality seemed to have high mobility of μ=40–57 cm 2/V s.

Advanced study of various characteristics found in RHEED patterns during the growth of InAs quantum dots on GaAs (0 0 1) substrate by molecular beam epitaxy

Various characteristics of RHEED patterns were examined with respect to geometrical and crystallographic point of views including chevron patterns frequently found after the growth of self-assembled InAs quantum dots, streak lines and Laue circles from GaAs wafer surface. These three patterns offer clear understanding of surface morphology change during or after dot growth. Various patterns and geometrical relationship between those patterns, coupled with specific surface reconstruction structures of GaAs (0 0 1) surface were studied.

Cleaning effect of ZnSe surface by hydrogen treatment

It has been revealed that the oxide layer which covers ZnSe is removed by very simple hydrogen (H2) gas treatments. The H2 gas was clacked by a tungsten heater set in a ceramic tube. The apparatus of the gas irradiation is inserted into the growth chamber of MBE through a K-cell port. The RHEED patterns of oxidized ZnSe surfaces show drastic change from spotty to streaky ones after exposure to the clacked H2 gas. The photoluminescence spectra of the H2 treated ZnSe indicate (A0, x), SA- and Cu-green-like structures. The temperature of the substrate was kept at 340 °C during the H2 treatment, which is the growth temperature of MBE-ZnSe. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MBE growth and characterization of hexagonal ZnCdMgSe layers and ZnCdSe/ZnCdMgSe QW structures on GaAs (111) substrates

AbstractZnCdMgSe epilayers and ZnCdSe/ZnCdMgSe quantum well structures were grown on GaAs(111)A and B substrates by MBE using a low‐temperature‐grown CdS buffer layer. X‐ray reciprocal space maps demonstrated that ZnCdMgSe epilayers grown on CdS buffer layers were predominantly hexagonal structures. In the quantum well structures, in‐situ RHEED pattern also demonstrated the hexagonal structure during the growth. X‐ray diffraction results showed that the crystalline quality of the ZnCdMgSe epilayers grown on GaAs(111)A surface was much better than that on the (111)B surface. Low temperature photoluminescence of ZnCdMgSe epilayers were dominated by the deep and shallow donor–acceptor pair emissions for the case of GaAs(111)A and B, respectively. The peak energy positions of the photoluminescence spectra increased with increasing the growth temperature. Although strong emissions from ZnCdMgSe barriers were observed, emissions from the ZnCdSe QW structure were very weak. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure and magnetic properties of Fe/KCoF3 thin films

Abstract A molecular beam epitaxy system was used to deposit a ferro/anitiferromagnet (Fe/KCoF 3 ) structures. Depending on the deposition conditions the fluoride grew either in the single crystal or polycrystalline forms, which was initially confirmed by the RHEED patterns and X-ray studies. The crystalline form of the KCoF 3 significantly affected magnetic properties, in particular exchange bias and magnetocrystalline anisotropy. Extensive transmission electron microscopy (TEM) studies have been made in order to reveal more details on the microstructure of the fluoride and of the Fe/KCoF 3 interface.

Pyrometer unit for GaAs substrate temperature control in an MBE system

An optical pyrometer designed for precision measurement of the GaAs substrate temperature during MBE growth is considered. The pyrometer can be calibrated against a certain characteristic absolute temperature that is visually determined from a change in the RHEED pattern. This enables one to calculate the absolute temperature of the substrate with regard to its radiant emissivity and minimize the inaccuracy of radiation temperature measurement. The inaccuracy is associated with the deposition of growth products on the pyrometer window.

UHV-SEM Observation of Surface Electromigration with Silver Tip

We have observed the surface electromigration with a silver tip by UHV-SEM and micro-probe RHEED. The silver tip etched and cleaned by heating and melting in UHV was mounted on a micro-slider touching heated Si(111) samples. The bright area under the silver tip is spread to the direction of cathode, the structure of which is determined to be √3×√3 structure from RHEED patterns. We have observed different shapes depending on heating temperature: At low temperatures, the shape of spread is like a bar, because the driving force of surface electromigration is superior to that of diffusion. At high temperatures, the shape is like a circle, because the driving force of diffusion is superior to that of surface electromigration. Using the silver tip, surface electromigration can be observed at high temperatures where lots of silver atoms are evaporated. We have checked the reproducibility of surface electromigration with the silver tip which is a supply source for thin film formation.

Organometallic vapor phase epitaxial growth of GaN on a 3c-SiC/Si(1 1 1) template formed by C +-ion implantation into Si(1 1 1) substrate

This paper reports a novel method to grow a wurtzite GaN on Si(1 1 1) by using a single-crystalline 3c-SiC interlayer. The interlayer is formed by the carbon (C +)-ion implantation into Si(1 1 1) wafers. The C +-ion implantation is performed at 600°C at an accelerating voltage of 180 kV. After the implantation wafers are annealed in O 2 atmosphere at 1250°C for 2 h. Auger analysis reveals the presence of SiC at a depth of 0.3–0.5 μm from the surface. Upper layers of SiO 2, Si and polycrystalline SiC on the single-crystalline 3c-SiC are successively removed and the single-crystalline SiC layer is successfully exposed on the surface. Organometallic vapor phase epitaxial (OMVPE) growth of GaN layer is made at 1000°C on the 3c-SiC/Si(1 1 1) template with a low-temperature (550°C) GaN buffer. The grown layer is confirmed to be single-crystalline wurtzite GaN by the RHEED pattern. The layer shows an intense excitonic emission with a peak energy of 3.416 eV at room temperature. The PL peak position and Raman shift show that the GaN layer has a small tensile stress of about 0.6 Gpa, which is a half of the value reported for a film grown on Si with an AlN buffer. This is supported by the fact that no cracks are found on the GaN surface.

Size and density estimation of self‐assembled InAs quantum dots on GaAs(001) substrate through the analysis of RHEED patterns

Size and density of self-assembled InAs quantum dots grown on GaAs(001) surface were calculated through the analysis of characteristic RHEED intensity and patterns, on the assumption that the logical thickness of two dimensional wetting layer should have a discrete value. The estimated results were compared with those of AFM direct observation. Pyramidal and round island dot models were introduced for the estimation, which are consisted of continuous flat InAs layers. During dot growth, substrate was not rotated to develop dot density gradient. Estimated results were not exactly matched with AFM observed results, because the estimated results were based on in-situ RHEED intensity measurement, while AFM is ex-situ observation method. Nevertheless, estimation results showed an interesting tendency that less developed small dot had the shape closer to pyramidal model, while over developed big dot had the shape closer to round island model.

Growth of Cu(In,Ga)S 2 on Si(100) substrates by multisource evaporation

Cu(In,Ga)S 2 films were grown on Si(100) by vacuum evaporation using the constituent elementary substances as sources. It was found from X-ray diffraction studies that orientational growth occurred in a limited temperature region. The chalcopyrite structure was confirmed by studying RHEED patterns. The existence of c-axis growth, two kinds of a-axis growth and four kinds of {112} twins which can be classified as two symmetrical pairs is proposed.