Adatoms of indium on Si(111) surfaces: Application of reflection high energy electron diffraction to desorption experiments

An attempt is made to derive thermodynamic quantities which characterize the initial stages of film growth using reflection high energy electron diffraction. Critical temperatures for the appearance of superstructures (√3 × √3, √31 × √31 and 4 × 1) are determined for Si(111) surfaces which were bombarded with constant-flux molecular beams of indium (J = (0.05?3.5) × 10^-2 monolayers s^-1). Equilibrium between adsorption and desorption was observed at temperatures above 450 °C. From the J dependence of the critical temperatures, the sojourn time of indium adatoms on Si(111) 7 × 7 is estimated to be $\text{τ}{}_7\text{(T) = 9 × 10}{}^{-13}\text{exp}\text{(}\text{238}\text{kJ mol}{}^{-1}\text{RT}\text{)s}$.     

Structural properties of the PdSi interface: An investigation by reflection high energy electron diffraction

The first stages of the formation of silicide were studied at various temperatures during palladium deposition onto an Si(111) 7 × 7 surface using the reflection high energy electron diffraction technique.For temperatures below 200°C, the interaction of palladium with silicon leads to the formation of three-dimensional Pd₂Si crystallites.At more elevated temperatures (300–400°C) two-dimensional phases were distinguished that are denoted as follows: Si(111) √3 R(30°) Pd and Si(111) 2√3 R(30°) Pd.     

Chemical vapor deposition of tungsten oxides: A comparative study by X-ray photoelectron spectroscopy,X-ray diffraction and reflection high energy electron diffraction

Thin films of tungsten compounds have been deposited by pyrolysis of W(CO)₆ on aluminium plates held at 400°C in oxygen or a neutral environment. Such films have been analysed by X-ray photoelectron spectroscopy X-ray diffraction and reflection high energy electron diffraction. Different compounds have been revealed ranging from metalliic tungsten to fully oxidized tungsten. The X-ray photoelectron spectroscopy study of the W_4f doublet has enabled the determination of various oxidation states of tungsten. A rather good agreement is found with the X-ray diffraction and reflection high energy electron diffraction measurements.     

The structural characteristics of radiation damage produced by high energy (2.7 MeV) ion implantation in GaAs

The radiation damage induced by the implantation of 2.7 MeV P⁺ and N⁺ ions with a dose of 6.4 × 10¹⁶ ions cm^–3 into GaAs at room temperature has been studied by transmission electron microscopy. The as-implanted material was found to consist of a buried amorphous layer which was sandwiched between a heavily damaged but crystalline cover layer exhibiting a high density of black dot defects, microtwins and dislocation loops and a less damaged substrate region. Post-implantation annealing of the specimens at 250° C for 6 h resulted in the recrystallization of the amorphous and cover layers by random nucleation of grains producing a polycrystalline region on the single crystal substrate. However, a second stage annealing of these samples at 400° C for 2 h caused an epitaxial regrowth of the implanted layer on the undamaged substrate producing single crystal regions which were heavily twinned on all {111} planes. The results of the present microstructural analyses have been compared with the previous infra-red reflectivity studies on identically implanted GaAs samples to determine the effects of structural changes on the dielectric properties. The two studies are found to be in reasonable agreement. The present results are also compared with those from previous lower energy-lower dose implantations.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

Reflection high energy electron diffraction and X-ray studies of AlN films grown on Si(111) and Si(001) by organometallic chemical vapour deposition

The crystallinity of AlN films on silicon substrates grown by organometallic chemical vapour deposition was investigated using X-ray diffraction and reflection high energy electron diffraction (RHEED). Single-crystal films of good quality with atomically smooth surfaces can be epitaxially grown on Si(111) substrates. Epitaxial films can also be grown on Si(001) substrates. These films have previously been reported to have a fibre structure. Different RHEED patterns were observed from the films on Si(111) and Si(001). It is established that the films grown on Si(001) consist of two types of crystallite with the following orientations: [11$\text{2}$0]_AlN//[110]_Si and [11$\text{2}$0]_AlN//[1$\text{1}$0]_Si The thickness dependence of the crystallinity was also investigated. The standard deviation σ of the X-ray rocking curve for the films grown on Si(111) is less than that for the films on Si(001) and is independent of the film thickness. The σ values for the films on Si(001) decrease markedly with increasing film thickness. On the basis of these observations, the growth mechanism of AlN epitaxial films on Si(111) and Si(001) is discussed.     

Low energy electron diffraction,Auger electron spectroscopy and angle-resolved photoemission from silver films on Si(100) and Si(111)

We studied the growth of thin silver films on Si(100)2 × 1 and Si(111) 7 × 7 between room temperature and 300 °C. At room temperature and with a silver flux of 2.4 × 10¹² atoms s^-1 cm^-2 a nearly exponential dependence of the Si L_2,3 VV and Ag M₄ VV Auger intensities indicates layer-by-layer (Frank-van der Merwe) growth up to a film thickness of a few monolayers. With increasing coverage θ silver grows in form of three-dimensional islands even at room temperature. In spite of the existence of the well-known Si(111)?(√3 × √3)R30° Ag surface structure after annealing (which we observe for θ = 1) we did not find an ordered Si-Ag interface layer for silver on Si(100). For elevated temperatures the growth of this system may therefore be described by the Volmer-Weber (pure three-dimensional island growth) mechanism, whereas silver condenses on Si(111) according to the Stranski-Krastanov (interface layer plus three-dimensional islands) growth mode.     

Measurement of lattice parameters of AIBIIICVI2 chalcopyrite-type epitaxial layers using reflection high energy electron diffraction

It is shown that the c/a ratio of epitaxial layers of I-III-VI₂ compounds can be obtained from reflection high energy electron diffraction measurements with no knowledge of the lattice parameters c and a provided that the c/a ratio is sufficiently different from 2.Results are given for CuGaSe₂. In special cases it is also possible to obtain the lattice parameters a and c.     

Oscillation of the mirror and fractional RHEED reflections during homoepitaxy on the (2×4)-reconstructed GaAs(001) surface

Oscillations of the intensity of mirror and fractional RHEED reflections during homoepitaxy on the GaAs(001)-(2×4) reconstructed surface were studied. A considerable difference was observed in the patterns of intensity variation for the mirror and the fractional (0 1/4) and (0 3/4) reflections corresponding to the α and β phases on the reconstructed surface. A kinetic scheme of elementary processes occurring on the Ga(001) surface upon the homoepitaxial growth initiation is proposed. The activation energy for the nucleation process was experimentally determined (5-eV). It is shown that the temperature dependence of the probability of critical nucleus formation is determined by the desorption of As₂ dimers.     

Low energy electron diffraction and surface conductivity studies of the indium antimonide (110) surface during gas adsorption

InSb (110) surfaces which had been cleaned by argon bombardment and annealing were investigated by low energy electron diffraction and surface conductivity measurements during the adsorption of different gases. The diffraction patterns before and during exposure to oxygen and nitrogen only exhibit diffraction spots which are compatible with the bulk periodicity. Exposure resulted in a gradual decrease in the relative intensity of all beams. The changes in the surface conductivity during exposure are determined quantitatively by the coverage and the type of adsorbed gas. The results imply that the ion-bombarded and annealed (110) InSb surface of p-type InSb shows hole conduction.     

X-ray diffraction technique to observe magnetostriction and magnetization at coextensive volumes of iron

We present a new technique for observing both magnetostriction and magnetization as a function of applied magnetic field strength. They are simultaneously measured by the same X-ray probes with a goniometer at exactly coextensive specimen volumes. The measurements yield experimental magnetostriction curves during a cyclic magnetization process in iron (100) single-crystal specimen at room temperature. The technique is a new tool to investigate magnetic properties within X-ray extinction depth from the surface.     

Development of multilayer laminar-type diffraction gratings to achieve high diffraction efficiencies in the 1-8 keV energy region

W/C and Co/SiO(2) multilayer gratings have been fabricated by depositing a multilayer coating on the surface of laminar-type holographic master gratings. The diffraction efficiency was measured by reflectometers in the energy region of 0.6-8.0 keV at synchrotron radiation facilities as well as with an x-ray diffractometer at 8.05 keV. The Co/SiO(2) and W/C multilayer gratings showed peak diffraction efficiencies of 0.47 and 0.38 at 6.0 and 8.0 keV, respectively. To our knowledge, the peak efficiency of the W/C multilayer grating is the highest measured with hard x rays. The diffraction efficiency of the Co/SiO(2) multilayer gratings was higher than that of the W/C multilayer grating in the energy range of 2.5-6.0 keV. However, it decreased significantly in the energy above the K absorption edge of Co (7.71 keV). For the Co/SiO(2) multilayer grating, the measured diffraction efficiencies agreed with the calculated curves assuming a rms roughness of approximately 1 nm.     

In-situ reflection high-energy electron diffraction study of epitaxial growth of Cu on NaCl (100) under oblique angle vapor deposition

Epitaxial growth of copper on annealed NaCl(100) surface was carried out using thermal evaporation at an oblique angle of incidence (75 ± 5)° with respect to the substrate normal. The substrate was kept at a temperature of (150 ± 5)°C. The crystalline structure of the Cu film was studied in situ by reflection high energy electron diffraction at various deposition times. We observed that the film grows through nucleation of epitaxial islands followed by coalescence and then flattening of the film. The chevron shaped diffraction patterns formed by the refraction effect of electrons were used to identify the crystal facets. With longer deposition times, instead of columnar structures, a continuous epitaxial film was formed despite the oblique angle incidence of the vapor. The morphology of the final film was characterized ex situ by atomic force microscopy and shows L-shaped pores asymmetric with respect to the vapor incident direction.     

Review: Scanning high-energy electron diffraction (SHEED) in materials science

The SHEED technique enables direct and rapid quantitative measurement of energyfiltered electron-diffraction intensities to be made. it may be used for structural studies of single crystals, polycrystalline films and amorphous materials. These may be examined in transmission if sufficiently thin or, otherwise, by reflection electron diffraction. In addition to conventional passive observation, structural studies of growing thin and thick films may be made during deposition. Work on all these topics is reviewed and the advantages of SHEED in each case clearly demonstrated.     

Electrical, structural and morphological characteristics of rapidly annealed Pd/n-InP (100) Schottky structure

The electrical and structural properties of the Pd/InP (100) Schottky barrier diodes have been investigated as a function of annealing temperature by current–voltage (I–V), capacitance–voltage (C–V) and X-ray diffraction (XRD) measurements. The Schottky barrier height of the as-deposited, 100 and 200°C annealed contacts determined from the I–V and C–V measurements are 0.56 and 0.81 eV, 0.57 and 0.81 eV, and 0.58 and 0.82 eV, respectively. However, both the measurements showed that the Schottky barrier height of the Pd/n-InP Schottky contact is increased to 0.59 eV (I–V) and 0.83 eV (C–V) when the contact is annealed at 300°C for 1 min in nitrogen atmosphere. Further Schottky barrier height decreases to 0.57 eV (I–V), 0.71 eV (C–V) and 0.53 eV (I–V), 0.67 eV (C–V) after annealing at 400 and 500°C samples. The result shows that the optimum annealing temperature for the Pd/InP Schottky diode is 300°C. Norde method is also used to determine the barrier height of Pd Schottky contacts and the values are 0.56 eV for the as-deposited contact, 0.57, 0.57, 0.58, 0.57 and 0.54 eV for contacts annealed at 100, 200, 300, 400 and 500°C which are consistent with the values obtained by the I–V measurements. From the atomic force microscopy results, it is evident that the overall surface morphology of the Pd/InP Schottky diode is fairly smooth. Based on the XRD results, the formation of phosphorus-oxygen compounds at the interface may be responsible for the variation in barrier heights observed in Pd/InP Schottky contacts with annealing temperature.     

An investigation of epitaxial films by means of high energy electron diffraction: II. A study of film morphology

In the present work film morphology was studied by an electron diffraction method. The electron microscopy replica method seems to be insufficient to give unambiguous information concerning film morphology and the growth process, but a combination of diffraction data and electron microscope observations gives much more complete information. We applied these methods to investigations of Ge epitaxial films on GaAs and Si substrates at different stages of growth (at thicknesses of 20 Å upwards).     

Analysis of structural distortion in Eshelby twisted InP nanowires by scanning precession electron diffraction

Transmission electron microscopes (TEM) are widely used in nanotechnology research. However, it is still challenging to characterize nanoscale objects; their small size coupled with dynamical diffraction makes interpreting real- or reciprocal-space data difficult. Scanning precession electron diffraction ((S)PED) represents an invaluable contribution, reducing the dynamical contributions to the diffraction pattern at high spatial resolution. Here a detailed analysis of wurtzite InP nanowires (30–40 nm in diameter) containing a screw dislocation and an associated wire lattice torsion is presented. It has been possible to characterize the dislocation with great detail (Burgers and line vector, handedness). Through careful measurement of the strain field and comparison with dynamical electron diffraction simulations, this was found to be compatible with a Burgers vector modulus equal to one hexagonal lattice cell parameter despite the observed crystal rotation rate being larger (ca. 20%) than that predicted by classical elastic theory for the nominal wire diameter. These findings corroborate the importance of the (S)PED technique for characterizing nanoscale materials.