Reflection High-energy Electron Diffraction Oscillations Research Articles

Role of the Step Density in Reflection High-Energy Electron Diffraction: Questioning the Step Density Model

The step density model of reflection high-energy electron diffraction oscillations is investigated. Within this model, the temporal evolution of the specular beam intensity during growth by molecular beam epitaxy represents the evolution of the step density during deposition. This is found to be inconsistent with diffraction theory. In particular, when the concentration of atoms in the deposited layer is fixed, an increase of the step density causes an increase of the specular beam reflectivity, contrary to the prediction of the step density model.

Homoepitaxy of silicon at low temperature on clean and Ga-covered substrates

We present a high resolution transmission electron microscopy (HRTEM) and reflection high energy electron diffraction (RHEED) study of the molecular beam homoepitaxy of Si(111) with and without Ga as surfactant. Cross-sectional images show that the growth at low temperature was associated with a periodic stacking fault (ABC/BCA fault type), leading to a high surface roughness. If the Si film was grown with 1/3 monolayer of Ga added prior to the growth, cross-sectional images show a perfectly flat surface, even at growth temperatures as low as 300 °C. According to a systematic study of the damping of the RHEED oscillations during the growth with and without Ga and the determination of the 2D nucleation/step-flow transition, no obvious kinetic effect could be attributed to the surfactant action. © 1997 Elsevier Science S.A.

Direct evidence for the step density model in the initial stages of the layer-by-layer homoepitaxial growth of GaAs(111)A

Scanning tunnelling microscopy (STM) has been used to investigate the morphological basis of the specular beam intensity oscillations observed in reflection high-energy electron diffraction (RHEED) studies during the initial stages of GaAs(111)A homoepitaxy. Analysis of STM images after the deposition of controlled amounts of GaAs up to a coverage of 2 monolayers show a strong relationship between the step density and the RHEED specular beam intensity. It is shown that the RHEED oscillations observed during the initial stages of growth reflect the temporal variation in surface step density.

Statistical multiple diffuse scattering from rough surfaces in RHEED — beyond the distorted-wave Born approximation

In reflection high-energy electron diffraction (RHEED) of growing surfaces in molecular beam epitaxy (MBE), diffuse scattering is generated by atom vibrations, point vacancies and growth islands (or surface roughness). Most of the existing RHEED theories have been developed under the first-order diffuse scattering approximation, and thus they are restricted for surfaces whose roughness is relatively low. In fact, crystal surfaces grown by MBE are usually rough; the change of surface coverage from 0 to 1 monolayer accounts for the observed RHEED oscillation. In this paper, a formal dynamical theory of RHEED has been developed to calculate the diffuse scattering produced by both atom vibrations and point vacancies at surfaces. The theory is aimed at recovering the multiple diffuse scattering that has been dropped by the distorted-wave Born approximation (DWBA). With the inclusion of a complex potential in the dynamical calculation, a rigorous proof is given to show that the high-order diffuse scattering terms are recovered in the calculation using the equation originally derived under the DWBA. This conclusion establishes the basis for expanding the RHEED theories developed under the first-order diffuse scattering to cases where the degree of surface roughness is high, allowing dynamical calculation of RHEED rocking curves for any growing surface. The statistical time and structure averages over the distorted crystal potential are evaluated analytically before numerical calculation. The dynamic form factor is calculated with consideration of anisotropic surface atom vibration and point vacancies at a growing surface.

Epitaxial growth of silver on an Si( 111) √3 × √3 Au surface at room temperature

Growth of Ag on an Si(111) √3 × √3 Au surface was investigated by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). At an initial stage of Ag deposition, the √3 × √3 structure fades into the diffuse background. Subsequently, a faint pattern from Ag(111) appears at 0.8 monolayers (ML, 1 ML = 1.39 × 10 15 atoms cm −2). At 7 ML growth the RHEED pattern shows a flat Ag(111) surface, although RHEED oscillations are hardly observed during growth. For Ag deposition onto an amorphous-like surface covered by 0.6 ML Au on the √3 × √3 surface, RHEED intensity oscillations are observed during growth. At an initial stage of Ag growth on the surface, extra streaks are observed in the RHEED pattern, continuing up to about 6 ML deposition. During the appearance of the extra streaks, the Auger spectrum of Si-LVV splits into two peaks at 88 and 92 eV. Small Si-LVV, 92 eV, and Au-NOO peaks remain up to 10 ML and the oscillation amplitude of the RHEED intensity is not damped up to 10 ML coverage. In the case of Ag deposition onto the √3 × √3 Au surface, since no segregation of Si and Au atoms is observed, it is concluded that the silicide and Si segregations induce two-dimensional nucleation on the surface.

Photoemission oscillations as an in situ monitor of layer thickness with monolayer resolution

Photoemission oscillations have been observed during the growth of InAs and AlSb on (001) GaAs substrates. The ability to observe photoemission oscillations is dependent on the growth conditions in a manner similar to reflection high-energy electron diffraction oscillations. Consequently, the mechanism responsible for photoemission oscillations is believed to be related to fluctuations in surface step density during deposition. We have used the photoemission oscillation technique to measure the AlSb barrier thicknesses during the growth of InAs/AlSb resonant tunneling diodes. Measured peak current densities are clearly better correlated with barrier thicknesses measured by photoemission oscillations than by barrier thicknesses assumed by time-based ‘‘dead reckoning.’’ Calculations of current/voltage characteristics of the resonant tunneling devices indicate that a substantial amount of As is incorporated in the barrier layer as a result of the growth conditions selected.

A novel Ba-Cu-O compound synthesized by pulsed laser deposition

We have discovered a new oxide phase which may be written as Ba 3Cu 3O x (BCO). The sample was a thin film epitaxially grown on SrTiO 3(100) substrate. The BCO film was prepared using a pulsed ArF excimer laser ablation technique. X-ray analysis has revealed that the structure of the new phase, BCO, is tetragonal with the lattice parameters, a = 0.39 nm and c = 1.29 nm, with the unit cell consisting of a stack of three perovskite-type layers. During the film growth, the reflection high-energy electron diffraction (RHEED) pattern remained streaky and exhibited intensity oscillation as the film thickness increased. The RHEED oscillation periodicity was found to correspond to a single unit-cell-thick layer of BCO (1.29 nm). The BCO films do not show superconductivity but semiconductive behaviors.

Growth, structural and optical characterization of MBE [formula omitted] quantum wells

ZnCdSeZnSe quantum wells (QW) were grown with molecular beam epitaxy (MBE) on GaAs(001) substrates cleaned with hydrogen plasma. Reflection high-energy electron diffraction (RHEED) was used for in-situ growth control of the QW structures. A quantitative evaluation of the RHEED oscillations yields an exact value of the growth rate. Furthermore, in comparing the growth rate of the barrier and the well a reasonable estimate of the Cd content is possible. The in-situ RHEED measurements were supplemented by ex-situ HRXRD and HRTEM investigations. The latter method was found to be especially useful to evaluate the Cd-concentration profile of the QWs by digital analysis of lattice images (DALI). X-ray rocking curves of MQWs were recorded which show well-resolved satellite peaks. A comparison with simulations based on dynamical diffraction theory yields the structural parameters such as well width, barrier width and composition of the QWs. The XRD and TEM results are compared with the parameters determined by RHEED. In addition, we performed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy for optical characterization of the samples. The PLE spectra show an interference pattern which is explained by standing polariton waves.

Mechanisms of molecular beam epitaxial growth of (001) HgTe

We report on an investigation of the growth mechanisms of HgTe using a combination of reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The experiments were carried out on untilted (001) CdTe substrates. Growth rates were determined from RHEED intensity oscillations of the (00) specular spot reflection. The amplitude of these oscillations decrease with increasing substrate temperature. Above 178 °C no RHEED oscillations could be measured. Upon reducing the sample temperature below 178 °C these oscillations could again be observed. This cyclic behavior could be induced several times for each sample, indicating a reversible change in the growth mechanism. In order to correlate the surface structure with RHEED observations, several samples have been investigated with STM. Thus, it could be confirmed that a temperature dependent transition occurs during the MBE growth of HgTe from the island growth mode below the critical temperature of 178 °C to a step flow mode above this temperature.

The nature of island formation in the homoepitaxial growth of GaAs(110)

The initial growth stages of GaAs(110) thin films by molecular beam epitaxy (MBE) have been studied by scanning tunnelling microscopy (STM). For depositions at 520°C up to one monolayer, STM images confirm the layer-by-layer nature of growth as indicated by reflection high energy electron diffraction (RHEED) intensity oscillations. The dynamics of island nucleation and coalescence is similar to that previously observed on GaAs(001) surfaces, with the temporal variation in surface step density giving rise to the specular RHEED oscillations. However, surfaces obtained after the deposition of sub-monolayer coverages of GaAs at 520°C show a striking growth morphology. The islands adopt a characteristic triangular shape, with {113}- and {115}-type steps and the apex directed along the [001] direction. This highly specific crystallographic shape suggests that nucleations occur at the base of the triangles.

Stoichiometric low-temperature GaAs and AlGaAs: A reflection high-energy electron-diffraction study

By careful control of the arsenic beam supply during the growth of GaAs and Al0.42Ga0.56As at low temperatures (∼200 °C), very strong and sustained reflection high-energy electron-diffraction (RHEED) oscillations have been observed. Both the period and intensity of the RHEED oscillations are shown to be a strong function of the arsenic overpressure with the former increasing with increase arsenic supply, reflecting a decrease in the number of atoms taking part in the two-dimensional (2D) growth mode, and the latter decreasing with increasing arsenic supply, reflecting the creation of a barrier to 2D growth by the excess arsenic. Under exact stoichiometric conditions, the quality of the GaAs and AlGaAs is comparable to those grown at high temperatures. It is therefore surmized that nonstoichiometry in low-temperature-grown GaAs can be overcome leading to the growth of stoichiometric low temperature materials possessing properties similar to those of conventional high-temperature-grown layers.

Manipulation of nucleation by growth rate modulation

We report on manipulation of nucleation by means of periodic modulation of the growth rate R, both during molecular beam epitaxial growth of Si on Si(111) and using a simulated bilayer structure. We have found that the bilayer-by-bilayer growth can be improved and that the surface roughness can be reduced by applying the R modulation synchronized with the deposition of a multiple of full bilayers. For nonsynchronized growth the phase of reflection high-energy electron diffraction (RHEED) intensity oscillations could be shifted relative to the bilayer deposition period and a beating phenomenon of the RHEED oscillations was observed, which can be attributed to the superimposed effects of the R modulation and the regular bilayer-by-bilayer growth. We have used the phenomenological correspondence between the surface step density and the RHEED oscillations as a basis to discuss the growth. By employing a kinetic solid-on-solid Monte Carlo model without vacancies and overhangs all significant experimental features could be simulated.

Double period RHEED oscillations during MBE growth of GaAs and AlAs on the GaAs(110) surface

Molecular beam epitaxy (MBE) of GaAs and AlAs on the GaAs(110) surface has been studied in situ using reflection high-energy electron diffraction (RHEED). Additionally ex situ atomic force microscopy (AFM) has been employed to investigate the surface topography after the growth. During the growth complex RHEED oscillations with a single and a double period can be measured, depending on the diffraction feature that is monitored. The AFM images of the surface reveal mono- and bilayer high steps and islands, reflecting a layer-by-layer growth with the participation of both at the same time. A simple model that leads to a qualitative understanding of the experimental observation is presented.

Measurement of photoemission oscillations during molecular beam epitaxial growth of (001) GaAs, AlAs, AlGaAs, InAs, and AlSb

Photoemission oscillations are observed from (001) GaAs, AlAs, AlGaAs, InAs, and AlSb. The periodicity of the oscillations corresponds to that of reflection high-energy electron diffraction oscillations measured using the same growth conditions. For both methods, the oscillation amplitude is strongly dependent on the substrate temperature, III/V flux ratio, and the nature of the surface on which growth is nucleated for the oscillation measurement.

Sputter cleaning and smoothening of GaAs(001) using glancing-angle ion bombardment

Flat, clean, essentially defect-free GaAs(001) surfaces were produced at 570 °C in an As4 overpressure using 1 keV Ar ion bombardment at an impingement angle φ of 15° from the surface plane and a dose of 2.3×1016 ions/cm2. Ion bombardment smoothened the surfaces leading to minimum roughness values of ≊0.3 nm and reflection high-energy electron diffraction (RHEED) patterns that showed streaks with a 2×4 reconstruction. GaAs films grown by molecular beam epitaxy on the sputter cleaned surfaces exhibited strong RHEED oscillations. Cross-sectional transmission electron microscope images showed that the epitaxial layers and substrates were defect-free except for 2–3-nm diam dislocation loops observed 10–20 nm below the substrate surface, separated by ≳100 nm along the interface.

A new method of flux calibration for gas source molecular beam epitaxy of InP and its application to migration enhanced epitaxy

Reflection high-energy electron diffraction (RHEED) oscillations in gas source molecular beam epitaxy (MBE) of InP consist of a P-limited region (region I), In-limited region (region II) and a P-desorption limited region (region III). Precise calibration of In and P fluxes from RHEED oscillations can be made by finding, on a singular substrate at a low growth temperature, the intersection between region II and the linear portion of region I. It is shown that this new method can be applied to low temperature (300°C) migration enhanced epitaxy (MEE) growth of InP, and layers with much improved optical and electrical properties were obtained.

A new method of flux calibration for gas source molecular beam epitaxy of InP and its application to migration enhanced epitaxy

Reflection high-energy electron diffraction (RHEED) oscillations in gas source molecular beam epitaxy (MBE) of InP consist of a P-limited region (region I), In-limited region (region II) and a P-desorption limited region (region III). Precise calibration of In and P fluxes from RHEED oscillations can be made by finding, on a singular substrate at a low growth temperature, the intersection between region II and the linear portion of region I. It is shown that this new method can be applied to low temperature (300°C) migration enhanced epitaxy (MEE) growth of InP, and layers with much improved optical and electrical properties were obtained.

Gold growth on Si(111) √3 × √3 Ag and √3 × √3 Au surfaces

Epitaxial growth of gold on Si(111) √3 × √3 Ag and √3 × √3 Au surfaces is investigated by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). A stable RHEED oscillation is observed during the epitaxial growth of Au(111) on both surfaces at room temperature after 2 monolayer (ML) deposition (1 ML = 1.39 × 10 15 atoms/ cm 2). Before the oscillation, at about 0.5 ML, a diffuse RHEED pattern due to formation of a disordered structure appears for both surfaces. During RHEED oscillations, gold silicide segregates on the grown surfaces. The roles of the disordered structure formation and the silicide segregation in the layered epitaxial growth are discussed.

In situ monitoring of reflection high-energy electron diffraction oscillation during the growth of gallium nitride films by gas-source molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) specular intensity oscillations have been observed during the growth of gallium nitride (GaN) films by gas-source molecular beam epitaxy (GS-MBE) for the first time. The GaN films were grown on sapphire substrates using metal gallium and ammonia as source materials, following the deposition of a GaN buffer layer at lower temperature. The incident electron beam was parallel to the GaN [1 1 00] azimuth direction and the signal intensity was measured with a photo-multiplier via an optical fiber fixed on the specular spot. The total thickness of the film was 1700 Å which includes the 180 Å buffer layer. RHEED oscillations during growth were observed on the sapphire (0001) substrate. Comparing the period of the RHEED intensity oscillations with those calculated from the growth rate, we found that a single period of the RHEED intensity oscillation corresponds to a monolayer growth of GaN.

Simultaneous reflection high-energy electron diffraction oscillations and mass spectroscopy investigations during molecular beam epitaxy growth of (001) GaAs - smooth surfaces or stoichiometric films?

Film composition and surface morphology of molecular beam epitaxy (MBE)-grown GaAs(001) surfaces were investigated in situ as a function of flux ratio J Ga J As 4 . The flux of As 4 molecules desorbing from the sample surface was measured with a quadrupole mass spectrometer (QMS) simultaneously with the observation of reflection high energy electron diffraction (RHEED) intensity oscillations. The incorporation ratio, given by the number of incorporated Ga atoms per As atom, was calculated independent of both the QMS data and the decreasing growth rate for growth conditions with As deficiency, as obtained from the RHEED oscillation frequency. Stoichiometric growth was found up to a flux ratio J Ga J As 4 ≈ 0.9 . At flux ratios of 1.1 to 1.2, a minimum of the damping of the RHEED oscillation amplitude indicates a very smooth growth front profile, but Ga excess appears to be incorporated mainly in As sites without disturbing the crystal lattice. This assumption was confirmed by photoluminescence (PL) spectroscopy of films grown at a flux ratio J Ga J As 4 of 1.2. An additional PL peak was observed, which indicates the incorporation of Ga atoms on As sites.