High-energy Electron Diffraction Oscillations Research Articles

Strained SrTiO3/BaTiO3 Superlattices Formed by Laser Ablation Technique and Their High Dielectric Properties

Artificial dielectric superlattices of SrTiO3/BaTiO3 (STO/BTO) have been formed by a pulsed laser ablation technique with in situ monitoring of reflection high-energy electron diffraction (RHEED) oscillation. The superlattices with a stacking periodicity of a few unit cells show higher dielectric constant than that of (Sr0.5Ba0.5)TiO3 films with change of temperature or applied frequency. The tetragonal structures of the superlattices are maintained at temperatures of more than 200° C owing to a large stress at the interfaces between STO and BTO layers.

Influence of the growth mode on the quality of metallic superlattices: The case of the (100) Felr system

It is shown that it is possible to grow FeIr metallic superlattices where the growth of Fe on Ir as well as Ir on Fe is two dimensional. The two-dimensional growth is shown by the occurrence of reflection high-energy electron-diffraction (RHEED) oscillations during the growth of Fe on Ir (100) up to 4–7 planes, and during the Ir growth on the Fe surface for an unlimited thickness. Transmission electron microscopy thus shows that superlattices with flat interfaces on a large scale are obtained. The RHEED oscillations during Fe growth disappear after 4–7 deposited planes and the Fe growth becomes three dimensional. The quality of the superlattices is thus damaged by this three-dimensional growth.

Differential photo-voltage spectroscopy for characterizing epitaxial multilayered and quantum well structures

An improved photo-voltage spectroscopy (PVS) technique, capable of accurately characterizing multilayered and quantum well structures, is presented. The technique is developed by noise reduction and differentiation of the photovoltage signal which improves the accuracy and sensitivity of the standard photo-voltage spectroscopy dramatically. The resulting differential-photo-voltage spectroscopy is capable of measuring the energy gap (Eg) and hence the composition, of ternary and quaternary compounds in multilayered structures at room temperature, with a substantially higher degree of confidence than the standard PVS technique. Several structures have been examined successfully. Two representative AlGaAs/GaAs structures are reported here to establish the capabilities of this technique. These structures were found to provide values for Eg and Al mole fraction, in excellent agreement with both the targeted values based on reflective high-energy electron diffraction oscillations during molecular beam epitaxial growth, and the measured values from variable angle ellipsometry. Numerous exciton transitions from quantum wells are clearly resolved and shown to be in excellent agreement with the theoretically predicted ones.

Surfactant-Mediated Growth of Aigaas by Molecular Beam Epitaxy

ABSTRACTAntimony was used as a surfactant during solid-source molecular beam epitaxy of AIGaAs layers. A steady-state surface-segregated population of Sb was maintained at the AIGaAs growth surface by providing a continuous Sb2 flux to compensate for loss due to thermal desorption. Above ∼ 650 °C, the incorporation rate of Sb was negligible, thereby allowing the deposition of AlGaAs layers despite the presence of Sb at the surface. A significant improvement in the optical quality of Al0.24Ga0 76As layers was observed by photoluminescence. In addition, extended reflection high energy electron diffraction oscillations and a reduction in Al0.24Ga0.76As surface roughness was observed when Sb was employed as a surfactant.

Low temperature limits to molecular beam epitaxy of GaAs

We report reflection high-energy electron diffraction oscillations during molecular beam epitaxy of GaAs at temperatures as low as 125 °C using As2 as the arsenic source. At a substrate temperature of 200 °C, the As:Ga ratio is shown to be the critical parameter which determines the amplitude of these oscillations. The largest amplitude oscillations are obtained when the As:Ga ratio is about 1:1. In order for these oscillations to occur, the arriving atoms must have some surface mobility. Monte Carlo simulations show that step density oscillations with a period corresponding to one monolayer are expected if the incident atoms are allowed to move as little as one lattice site upon arrival at the surface.

Low-temperature growth on Si(111) substrates.

Low-temperature (T150 K) reflection high-energy electron-diffraction (RHEED) oscillations during the growth of ultrathin films suggest the presence of nonzero adatom mobility. In most systems thermal diffusion cannot account for the observed oscillations, and the origin of the oscillations is still unclear. Experiments on Ag/Si(111) at 150 K demonstrate the absence of thermal diffusion based on the scaling of the RHEED intensity at different flux rates (i.e., the diffracted intensity is only a function of the total deposited amount independent of the flux rate). We have performed molecular-dynamics simulations to understand possible low-temperature mechanisms responsible for the oscillations. Classical two- and three-body Si potentials were used together with an adatom mass that is 3.84 times the Si mass to account for the Ag/Si mass ratio. Results indicate that the landing site of the adatom is important in predicting the subsequent lateral motion of the adatom. A majority (about 75%) of the deposited atoms adsorb within a lattice spacing of the landing site. However, a fraction (25%) of the deposited atoms eventually adsorb a few lattice spacings away from the landing site before equilibration to the substrate, but it is not clear if this is sufficient to account for the oscillations. The energy transfer to the substrate is much less efficient than in fcc/fcc systems, because of the stiffness of the Si-Si potentials.

Sublimation mechanisms of (100) and (111) CdTe

The sublimation activation energies of (100) and (111)B CdTe have been determined by reflection high energy electron diffraction oscillation measurements. An interpretation of the data is proposed, assuming that the sublimation can be described in both cases by a two-step mechanism. In the explored temperature range, the sublimation kinetics are governed by a common process, leading to a unique activation energy value of 1.90±0.05 eV.

An investigation of MBE growth of (Zn, Mg)Se using RHEED oscillations

We have used reflection high-energy electron diffraction (RHEED) oscillations to measure the growth rates of Zn 1− x .Mg x Se and ZnSe as functions of substrate temperature, II/VI flux ratio, and electron-beam filament current. Although the ZnSe and Zn 1− x Mg x Se growth rates vary with these growth parameters, the growth rate difference attributable to the addition of magnesium does not depend on them, if growth is done with a selenium excess. Thus, unlike the other common components of II/VI materials (Zn, Cd, S, Se and Te), the sticking coefficient of magnesium is independent of substrate temperature and II/VI flux ratio. Electron-beam-induced desorption of magnesium is also negligible. For growth with a selenium excess, the composition of Zn 1− x Mg x Se films determined by RHEED oscillations agrees well with X-ray fluorescence measurements. However, if growth is done without an excess of selenium, then a simple composition calculation using the Zn 1− x Mg x Se and ZnSe growth rates indicates a magnesium content which is lower than the true magnesium content. This occurs because of a decrease in the sticking coefficient of zinc induced by the presence of magnesium.

On the mechanism of reflection high-energy electron diffraction oscillations studied by phase-locked epitaxy of ZnSe

Molecular beam epitaxy (MBE) is one of the most promising methods for growth of sophisticated device structures. Starting the growth on a flat surface, reflection high-energy electron diffraction (RHEED) oscillations occur. The question which phase of the oscillations corresponds to lattice plane completion and the most flat surface morphology is not yet fully resolved. There is hardly a direct access to the answer. Phase-locked epitaxy (PLE), however, appears to be a tool for studying this phase problem. PLE permits the growth of layers without losing RHEED oscillations due to damping the thickness of which is great enough to become measurable by common techniques. This enables to compare the directly determined layer thickness with that obtained by counting the number of RHEED oscillation periods. Results are presented that show the phase relation between RHEED oscillations and lattice plane completion.

Wide gap Cd 1−xMg xTe: molecular beam epitaxial growth and characterization

We have grown the ternary alloy Cd 1- x Mg x Te as well as Cd 1- x Mg x Te/CdTe quantum well structures by molecular beam epitaxy - to our knowledge for the first time. Cd 1- x Mg x Te exhibits some very interesting features: The band gap has been determined as a function of Mg concentration, and a band gap of 3.0 eV was found for zincblende MgTe at room temperature. Cd 1- x Mg x Te thin films with Mg concentrations of up to 0.75 were fabricated, which corresponds to a band gap of 2.8 eV at low temperatures. Therefore, the whole visible band gap range (at room temperature) can be covered with Mg concentrations between 0.30 (red) and 0.75 (blue). Bulk MgTe crystallizes in the wurtzite structure, but zincblende MgTe could be grown on (100) oriented CdTe substrates up to a layer thickness of approximately 500 nm. The lattice mismatch between zincblende MgTe and CdTe was found to be as small as 1.0%. The growth of cubic MgTe could be followed by reflection high energy electron diffraction (RHEED) oscillations. In general, excellent structural quality could be reached, which is demonstrated by the FWHM of 22 arc sec for a Cd 1- x Mg x Te thin film with 0.44 Mg concentration on a Cd 1- x Zn x Te nearly lattice matched substrate. The Poisson number of Cd 1- x Mg x Te has been determined by X-ray diffraction as a function of Mg concentration. Cd 1- x Mg x Te/CdTe single quantum well structures have been fabricated with a large confinement energy of up to 0.8 eV. The photoluminescence spectra show exciton lines with very narrow linewidths. We are able to observe excited exciton states, and from the energetic difference between 1s and 2s heavy hole exciton lines we deduce exciton binding energies. Very bright luminescence could be seen even at room temperature, which is an indication of a large exciton binding energy and an effective radiative recombination.

ErSb/GaSb(001) and GaSb/ErSb/GaSb(001) heterostructures and [ErSb,GaSb] superlattices: Molecular beam epitaxy growth and characterization

Successful growth of ErSb(001) single crystal layers on GaSb(001) substrates has been demonstrated. The reflection high-energy electron diffraction patterns show a (4×4) surface reconstruction. Reflection high-energy electron diffraction oscillations, x-ray diffraction, and Rutherford backscattering with channeling indicate single crystal monolayer-by-monolayer growth and continuity of the Sb sublattice at the ErSb/GaSb interface. The ErSb has a low room temperature resistivity equal to 30 μΩ cm but may be used as a metallic reflector only for wavelengths greater than 2.4 μm. The overgrowth of GaSb on ErSb leads to mirrorlike surfaces but the overlayers contain symmetry-related defects. On the contrary, nearly perfect GaSb overlayers were grown on [ErSb,GaSb] superlattices which exhibit metallic behavior.

Epitaxial solid-solution films of immiscible MgO and CaO

Metastable solid solutions in the MgO-CaO system grow readily on MgO at 300 °C by molecular beam epitaxy. The epitaxy displays both the reflection high-energy electron diffraction oscillations characteristic of layer-by-layer growth and the lattice rotations which have been related to island nucleation. Mg1−xCaxO solid solutions grow despite a larger miscibility gap than in any system for which epitaxial solid solutions have been grown. Epitaxial quenching of nonequilibrium solid solutions is possible because of the very low growth temperatures for rock-salt oxides.

Determination of the sticking coefficient of disilane on Si(001) using the first reflection high energy electron diffraction oscillation period

We have investigated the growth of Si from disilane on Si(001) in GSMBE. The growth rates were determined from reflection high energy electron diffraction (RHEED) intensity oscillations. Close inspection of the oscillation traces revealed a rapid decrease in intensity after beginning of growth for the first half period. This decrease was always faster than half the period of the remaining oscillation trace. The decrease does not depend on growth temperature but does depend on the incident flux. To interpret this effect, we assume that the surface is initially hydrogen-free. Under these circumstances, a maximum amount of disilane will quickly adsorb and decompose on the Si surface, which accounts for the sharp decrease in the beginning for high incident fluxes. Further adsorption (and thus regular growth) is only possible after subsequent desorption of hydrogen. Thus, in this time, the hydrogen surface coverage develops towards the equilibrium value, which is constant during the subsequent growth. It is assumed that during the time to reach the equilibrium (τ i), 0.5 ML of Si are deposited, which in turn corresponds to a minimum in RHEED intensity. Thus, by injecting a certain flux and measuring τ i, the determination of the reactive sticking coefficient α r, which is defined as the ratio between the amount of chemisorbing flux and the incoming flux on a bare surface, is possible. This way, the sticking coefficient of disilane on the Si(001) surface was found to be independent of substrate temperature in the investigated range between 464 and 580°C.

Gas source molecular beam epitaxy and X-ray absorption fine structure characterization of InGaAs/InP short period superlattices

InGaAs/InP short period superlattices (SPSLs) were grown using gas source migration enhanced epitaxy (GSMEE) and using gas source molecular beam epitaxy (GSMBE) on (001)InP substrates. The local structure around the interfaces was investigated by Ga K-edge X-ray absorption fine structure (XAFS) using synchrotron radiation (SR). It was confirmed by the observation of persistent reflection high-energy electron diffraction (RHEED) oscillations that layer-by-layer nucleation takes place even during the growth of the heterointerfaces by precise control of migration enhanced epitaxy (MEE) sequences. XAFS analysis showed that the first nearest neighbor around Ga in the GSMEE grown sample with In-As interfaces is As, while those in the GSMBE grown sample are As and P. These results suggest that the atomically-controlled abrupt heterointerfaces can be obtained by GSMEE, whereas some intermixing of atoms at the heterointerfaces takes place in samples grown by GSMBE.

In-situ characterization of II/VI molecular beam epitaxy growth using reflection high-energy electron diffraction oscillations

The paper reviews the use of reflection high-energy electron diffraction (RHEED) oscillations to characterize molecular beam epitaxy (MBE) growth of II/VI wide-bandgap semiconductor materials. RHEED oscillations are routinely used in III/V crystal growth to measure growth rates and determine compositions. Their use in II/VI growth has been hampered by the difficulty in observing RHEED oscillations. However, with proper preparation of the crystal surface, oscillations are readily observable, and can be used to control growth and to investigate the physics of growth on the atomic layer scale. Several examples for II/VI MBE growth are examined in detail. Measurements of oscillations during the growth of ZnSe on deliberately misoriented substrates have led to an upper bound on surface diffusion of the II/VI atoms. Measurements of the oscillations occuring during migration enhanced epitaxy (MEE) have been used to determine the amount of material deposited in each MEE cycle. Investigations of the characteristics shape of the oscillations have permitted development of a method to control composition of ZnS x Se 1− x .

Effects of As4 flux on reflection high-energy electron diffraction oscillations during growth of GaAs at low temperatures

This article reports on the observation of reflection high-energy electron diffraction oscillations during growth of GaAs by molecular beam epitaxy at temperatures as low as 60 °C. At low temperatures (<300 °C), the amplitude of the oscillations is shown to be sensitive to the As:Ga flux ratio. The largest amplitude oscillations are observed under stoichiometric conditions.

Effect of atomic oxygen on the initial growth mode in thin epitaxial cuprate films.

The basic growth mode of a thin epitaxial cuprate film (200 A\r{}) on a given substrate depends sensitively on the balance between various thermodynamic and kinetic factors related to the high-${\mathit{T}}_{\mathit{c}}$ phase formation and the surface microstructure at the growth front of the deposited film. Under the standard optimized growth conditions for high-quality epitaxial films, the deposition of a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ film on an atomically smooth (110) ${\mathrm{SrTiO}}_{3}$ substrate, for example, is characterized by a strong damping in the reflection high-energy electron diffraction (RHEED) oscillation suggesting a predominant island growth mode. We have demonstrated that with an atomic oxygen and the technique of RHEED-controlled growth interruption it is possible to minimize surface roughness and to fabricate unit-cell smooth ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films over a large area (\ensuremath{\sim}0.5 cm\ifmmode\times\else\texttimes\fi{}1 cm). The results of this study suggest that two-dimensional layer growth can be induced by the combined use of atomic oxygen and growth conditions, such as low deposition rate, low oxygen partial pressure (2 mTorr), that produce low supersaturation at the growth front.

Growth mode evolution during homoepitaxy of GaAs (001)

Scanning tunneling microscopy studies have been performed on GaAs homoepitaxial films grown by molecular-beam epitaxy. After an initial transient regime, indicated by reflection high-energy electron diffraction oscillations, the system evolves to a dynamical steady state. This state is characterized by a constant step density and as such the growth mode can be termed generalized step flow.

Tailored heterointerface formation in InGaAs/InP superlattices by gas source migration-enhanced epitaxy

InGaAs/InP short period superlattices (SPSLs) with different interface atomic arrangements were grown by gas source migration-enhanced epitaxy (GSMEE) on (001)InP substrates. The local structure found around interfaces was investigated by P K-edge X-ray absorption fine structure (XAFS) using synchrotron radiation (SR), and by highly sensitive Raman scattering spectroscopy. Persistent reflection high-energy electron diffraction (RHEED) oscillations reveal that layer-by-layer nucleation can take place even during the growth of heterointerfaces by precise control of migration-enhanced epitaxy (MEE) sequences. The RHEED intensity traces of InGaAs/InP SPSLs without special characteristic shapes at the heterointerfaces were observed for the first time. XAFS analysis shows that the first nearest neighbors to P in the sample with InGa-P interfaces are In and Ga and that the first nearest neighbor in the sample with In-As interfaces is In. No clear GaP longitudinal optical (LO) phonon line is observed in the Raman scattering spectrum for SPSL with In-As interfaces, while a sharp and strong GaP LO phonon line is detected in SPSL with InGa-P interfaces.

Scanning tunneling microscopy study on YBa 2Cu 3O 7−x films with growth stop by observing reflection high-energy electron diffraction oscillations

The growth of YBa 2Cu 3O 7−x thin films was stopped on a maximum or a minimum of an intensity oscillation in reflection high- energy electron diffraction (RHEED). The resulting film surfaces were observed by scanning tunneling microscopy. Films stopped on a maximum have smoother surfaces, closer to the completion of a monolayer of unit cells, than those on a minimum. The width of monolayer terraces was almost the same as that of the substrates determined from the misorientation. On the terraces, islands of one unit cell height were observed, so that the diffusion length of YBa 2Cu 3O 7−x must be shorter than the terrace size.