Reflection High-energy Electron Diffraction Study Research Articles

Influence of a compressive strain on the stoichiometry of the (0 0 1)CdTe surface during molecular beam epitaxy

We present here a reflection high-energy electron diffraction study of the (0 0 1)CdTe growing surface during molecular beam epitaxy. The presence of additional weakly bound Te atoms adsorbed on top of the Te dimers terminating the surface during the growth is demonstrated. The coverage of these weakly bound Te atoms is strongly dependent on the growth conditions (temperature and strain). The resulting surface stoichiometry variations have a significant effect on the CdTe growth rate and on the incorporation mechanisms of Cd and Te atoms.

Reflection high-energy electron diffraction and low energy electron diffraction studies of the homoepitaxially grown diamond (111) and (001) surfaces

Abstract We have investigated the surface morphology and crystallinity of homoepitaxially grown (111) and (001) diamond by reflection high-energy electron diffraction (RHEED) and low energy electron diffraction (LEED). In the smoothest C(001)-2×1/1×2 surface, both the RHEED and LEED patterns show half-order diffraction beams which are characteristically observed as the reconstructed surface structures. Caution has to be exercised when judging the structure of a C(111) surface from a LEED (1×1) pattern, because its observation can encompass surface morphologies ranging from single-crystal to polycrystalline diamond. In contrast, surface roughness has a dramatic effect on the RHEED pattern. The RHEED patterns of the smoothest C(111)-1×1 surface, grown at 690 °C, indicate that the surface smoothness is thought to be the closest to the ideally flat surface compared with previously reported homoepitaxial C(111) surfaces.

RHEED and AFM studies of homoepitaxial diamond thin film on C(001) substrate produced by microwave plasma CVD

Abstract A surface of a diamond thin film grown homoepitaxially on a C(001) substrate by microwave plasma chemical vapor deposition (CVD) has been studied using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The RHEED pattern showed the C(001)2×1/1×2 double-domain structure. The AFM images taken from the same sample in air showed 1×1 but locally 2×1 structure, which was confirmed by the Fourier transformed pattern of the AFM image.

Fabrication and properties of heteroepitaxial magnetite (Fe3O4) tunnel junctions

Micron-size magnetic tunnel junctions consisting of ferromagnetic Fe3O4 electrodes, with MgO as a barrier layer, have been fabricated on (100) MgO substrates. Reflection high-energy electron diffraction and transmission electron microscopy studies reveal that the Fe3O4/MgO/Fe3O4 trilayers grown by pulsed laser deposition are heteroepitaxial with abrupt interfaces. To achieve different coercivities for the top and bottom Fe3O4 layers, the trilayers are grown on MgO substrates with a CoCr2O4 buffer layer. The junctions exhibit nonlinear current–voltage characteristics and changes in junction resistance with applied field corresponding to the coercivities of the two magnetic layers. However, the observed magnetoresistance (∼0.5% at 300 K, ∼1.5% at 150 K) is much lower than would be expected for a highly spin-polarized system. Possible reasons for the reduced magnetoresistance are discussed.

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.

Atomic force microscopy studies of ZnSe self-organized dots fabricated on ZnS/GaP

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. In situ reflection high-energy electron diffraction studies reveal that growth interruption is required for the formation of the dot structure. Atomic force microscopy (AFM) images of the dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. A density of 18 μm−2 was achieved with a coverage of 8.0 ZnSe monolayers. AFM images taken at later times (up to six months later) show ripening effects. The average dot size measured at various times after growth is consistent with the prediction of the Ostwald ripening model with a growth time constant of 4±1 days for the structure with a coverage of 8.0 ZnSe monolayers. The dot size and density in the fully ripened state are essentially independent of the initial ZnSe coverage.

Reflection high-energy electron diffraction and atomic force microscopy studies on homoepitaxial growth of SrTiO 3(001)

This paper reports a systematic study on the homoepitaxial growth of SrTiO 3(001) conducted using reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) in order to improve our understanding of the basic processes in the epitaxial growth of perovskite thin films. Under certain growth conditions, the homoepitaxial growth of SrTiO 3 essentially proceeds in the layer-by-layer (2D nucleation) growth mode with the basic unit of a molecular layer. This cyclic process due to unit-cell-by-unit-cell (uc-by-uc) growth is confirmed by undamped RHEED oscillations and the corresponding temporal evolution of AFM images of the growth front. Furthermore, the surface mobility is controlled by growth temperature and oxidation condition. By changing growth temperature or oxidation condition, crossovers from layer-by-layer growth to step-flow like growth and from layer-by-layer growth to 3D island growth through Stranski–Krastanov growth are observed. On the basis of these results, a universal phase diagram is proposed for the growth mode of the epitaxial film growth of complex oxides.

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si 1− xGe x morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si 1− x Ge x /Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si 1− x Ge x (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

Reflection high-energy electron diffraction studies of wurtzite GaN grown by molecular beam epitaxy

We report a comprehensive reflection high-energy electron diffraction study of the surface structure of GaN as a function of substrate temperature and III–V ratio for growth using elemental gallium and active nitrogen derived from a rf plasma source. An emission spectroscopy analysis of the composition of the nitrogen plasma showed that the neutral atomic species dominated the growth process. The effect of substrate pretreatment is also discussed. It was found that good quality growth accompanied by a reconstructed surface are only obtainable during growth under a slightly Ga-rich regime and after pretreatment by nitridation and the growth of a low temperature buffer layer. Reconstruction mode diagrams are presented both for layers during growth and also for layers which have been cooled after growth. The implications of these plots are discussed in terms of surface vacancy densities.

Low-temperature GaAs grown by molecular-beam epitaxy under high As overpressure: a reflection high-energy electron diffraction study

Reflection high-energy electron diffraction (RHEED) was used for in situ monitoring of the growth behavior of GaAs at various temperatures. The growth was performed on both singular and 2° off (001) GaAs substrates. The temperature dependence as well as the As/Ga beam equivalent pressure ratio dependence of the RHEED specular beam intensity oscillation characteristics were studied. Strong RHEED oscillations were observed at low temperatures under As/Ga ratios far from the stoichiometric condition.

Interface between a III–VI intercalation compound and a solid electrolyte: XPS and RHEED study

An X-ray Photoelectron Spectroscopy (XPS) and Reflection High Energy Electron Diffraction (RHEED) study of interfaces between B 2O 3–0.5Na 2O and InSe has been performed as a function of the thickness of the InSe film. From RHEED and XPS, the InSe film grows mainly in a two-dimensional mode along the smooth polished borate substrate and the interface between InSe and borate glass can be considered as abrupt. The RHEED patterns indicate an in-plane misorientation of the domains. The semi-quantitative analysis of the intensities of photoelectron peaks leads to substrate attenuation and deposit uptake curves. They follow mainly an exponential law and allow an estimation of the escape depth of the photoelectrons in agreement with the calculated ones and with the flux determined from interferometry measurements although a good definition and determination of the monolayer thickness is required. Nevertheless, the phenomenological average `monolayer' defined by Seah and Dench agrees with the experimental estimation of the escape depth, even in the case of these strong anisotropic materials. We suggest that a sodium insertion in the layered compound is plausible.

RHEED study of Nb thin film growth on α-Al 2O 3 (0001) substrate

The Nb films with thickness from several tens to 500 Å were grown by means of a special evaporation source (NbEBES) on (0001) alumina substrates in ultra high vacuum (UHV) conditions. The quality of the substrate surface and epitaxial parameters of the Nb over layers were examined by Reflection High Energy Electron Diffraction (RHEED). The two epitaxial orientations of Nb film with respect to the (0001) surface plane of alumina substrate were observed in the dependence of the substrate temperature during the Nb deposition. In the range between 250 and 400°C the (110) epitaxial orientation was found. For the substrate temperature higher than 400°C the (111) epitaxial orientation took place. Also the influence of the substrate temperature on the surface roughness of the resulted Nb film was studied. Very low evaporation rate (several Å/min) at the substrate temperature range between 350 and 400°C produced a relatively smooth surface of the Nb thin film with the (110) epitaxial orientation.

A reflection high-energy electron diffraction and atomic force microscopy study of the chemical beam epitaxial growth of InAs and InP islands on (001) GaP

We have studied the formation of strained InAs and InP island structures on GaP surfaces grown by chemical beam epitaxy. InP grows pseudomorphically for 3 ML before island crystallization is observed by reflection high-energy electron diffraction, following a typical Stranski–Krastanov growth mode. For the growth of InAs on GaP, three-dimensional diffraction peaks are observed after 0.9 ML of InAs have been deposited, indicating a Volmer–Weber growth mode. Atomic force microscopy studies of these structures are presented and the optical properties are discussed.

Visualization of precipitation induced crystallographic shear planes as one-dimensional structures on surfaces: an STM and RHEED study on TiO 2(110)

Surface sensitive techniques [scanning tunneling microscopy (STM), reflection high-energy electron diffraction (RHEED), and Auger electron spectroscopy (AES) have been used to study the effects of changes in the bulk on the surface structure in TiO 2. Annealing of TiO 2(110) at elevated temperatures leads to straight steps on the surface with lengths in excess of 500 Å. The structure of these steps has been investigated in detail by voltage dependent STM and electron diffraction. The steps are caused by crystallographic shear (CS) during annealing creating areas of different crystallographic structure. STM and RHEED studies showed that the dominant directions of the steps are along [1̄10], [1̄11̄] and [1̄11]. From these experimental finding and the tetragonal symmetry of rutile it was concluded that the CS planes belong to the close packed family {112} of planes. CS is correlated to Ca precipitation. CaO diffusion to the surface may be an important process in the creation of oxygen vacancies. The calcium precipitates induce a (1 × 3) surface reconstruction which is visible both in STM images and electron diffraction patterns.

Growth and structure of internal Cu/Al 2O 3 and Cu/Ti/Al 2O 3 interfaces

Thin Cu films and Cu/Ti bilayers were grown on (0001) α-Al 2O 3 by molecular beam epitaxy (MBE) at substrate temperatures ranging from 373 K to 473 K. Results on growth behaviour, low-energy interface orientation relationships and the nature of bonding at the interfaces are reported. Transmission electron microscopy (TEM) and in-situ reflection high-energy electron diffraction (RHEED) studies show that Cu/Ti bilayers nucleate and grow epitaxially on the Al 2O 3 substrates with an (111) Cu〈110〉 Cu∥(0001) Ti〈21̄1̄0〉 Ti∥(0001) Al 2 O 3 〈101̄0〉 Al 2 O 3 orientation relationship. In contrast, Cu films on (0001) α-Al 2O 3 grow in two stages. Initially, the Cu nuclei have random orientation but they realign with increasing film thickness into an heteroepitaxial (111) Cu〈110〉 Cu∥(0001) Al 2O 3 〈101̄0〉 Al 2O 3 orientation relationship. In high-resolution transmission electron microscopy (HRTEM) no reaction phases are observed at the atomically sharp Cu/Al 2O 3 and Ti/Al 2O 3 interfaces. Electron energy-loss spectroscopy (EELS) of the interfaces reveals bonding between copper or titanium and the oxygen sublattice of Al 2O 3, The interfacial widths of copper and titanium atoms involved in the bonding are estimated using the characteristic absorption edge data. In the case of Ti/Al 2O 3 the interfacial width is estimated to be 0.5 nm±0.1 nm. The lower affinity of Cu to oxygen results in a smaller interfacial width of 0.34 nm±0.06 nm. These values correspond to the apparent fraction of the metal layers that participate in the bonding with the oxygen sublattice of (0001) α-Al 2O 3.

Migration-assisted Si subatomic-layer epitaxy from Si2H6

Submonolayer by submonolayer Si epitaxy (subatomic-layer epitaxy, SALE) from Si2H6 on Si(001) has been successfully realized independent of the adsorption coverage by repeating self-limited Si2H6 adsorption and surface adatom migration induced by surface thermal excitation with Ar+ laser irradiation and self-resistive heating. With the self-limited Si2H6 adsorption and the migration assist, a substrate temperature window and a laser power window with a constant growth rate and an atomically flat surface have been obtained. The fact conversely indicates that the surface temperature control within the limited temperature range is important during the thermal excitation to obtain the atomical surface flattening. On the basis of the results of the reflection high-energy electron diffraction study on a Si2H6/Si(001) system together with the SALE growth experiments, models for the SALE growth mechanisms and the growth modes are proposed.

Reflection high-energy electron-diffraction study of melting and solidification of Pb on graphite

The melting and solidification of Pb thin films on pyrolytic graphite are investigated in situ by reflection high-energy electron diffraction. Thin films with thicknesses of 4--150 monolayers are investigated. The surface morphology of the thin films were studied by scanning electron microscopy. Superheating of the Pb thin films by $4\ifmmode\pm\else\textpm\fi{}2$ to $12\ifmmode\pm\else\textpm\fi{}2$ K is observed from diffraction intensity measurements. Upon cooling the substrate, the Pb on graphite is seen to supercool by $\ensuremath{\sim}69\ifmmode\pm\else\textpm\fi{}4\mathrm{K}.$

Atomic layer epitaxy of CdTe, MgTe and MnTe; growth of CdTe/MnTe tilted superlattices on vicinal surfaces

Atomic layer epitaxy (ALE) is investigated for binary semiconductors of the telluride family, namely CdTe, MgTe and MnTe. Thanks to a systematic reflection high energy electron diffraction (RHEED) study, an autoregulated growth at 0.5 monolayer/ALE cycle is obtained for CdTe in a substrate temperature range between 260°C and 290°C. RHEED studies on MgTe ALE, together with X-ray diffraction experiments on ALE grown CdTe/MgTe superlattices, reveal that an autoregulated growth at 0.7±0.1 MgTe monolayer/ALE cycle can be achieved in a substrate temperature range between 260 and 300°C. For MnTe, all deposited Mn atoms get incorporated, so that no autoregulated growth can be achieved, as illustrated by transmission electron microscopy (TEM) data on ALE grown CdTe/MnTe superlattices. Finally, all this know-how is used to epitaxy CdTe/MnTe tilted superlattices onto (001) CdZnTe vicinal surfaces.

Structure and magnetism of the Fe/GaAs interface

We study the magnetic properties of Fe thin films epitaxially grown on GaAs (001) for a large range of substrate temperature. Magnetization deficiency has been observed and studied. Its dependence on both thickness and temperature clearly show the existence of a nearly half-magnetized phase at the interface, covered by “as-bulk” Fe. Furthermore, reflection high-energy electron diffraction studies show a transition between two bcc structures with different crystalline parameters. Transmission electron microscopy confirms the formation of this interfacial phase, for which the compound Fe3Ga2−xAsx seems to be the best candidate.

Growth, doping, and etching of GaAs and InGaAs using tris-dimethylaminoarsenic

Growth, doping, and in situ etching of GaAs using cracked and uncracked tris-dimethylaminoarsenic (TDMAAs) in chemical beam epitaxy (CBE) have been studied. A reflection high-energy electron diffraction study indicates that the complete decomposition of TDMAAs occurs below 370 °C. Possible TDMAAs decomposition pathways, which are consistent with experimental data, are presented. The carbon level in undoped GaAs films and the hole concentration in carbon-doped GaAs films using uncracked TDMAAs are much lower than those using cracked TDMAAs, which may be due to atomic hydrogen released from the β-hydride elimination process of uncracked TDMAAs on the GaAs surface. The in situ GaAs etching effect was observed only when the substrate was exposed to uncracked TDMAAs. Possibly, reactive free radicals generated by uncracked TDMAAs on GaAs surfaces are responsible for the etching effect. Surface roughness in selective-area growth of GaAs and the poor interfaces in InGaAs/GaAs multiple quantum wells grown by CBE using uncracked TDMAAs may result from this etching effect.