Spotty Reflection High-energy Electron Diffraction Research Articles

Growth Control of Twin InSb/GaAs Nano-Stripes by Molecular Beam Epitaxy

ABSTRACTInSb has been considered as a promising material for spintronic applications owing to its pronounced spin effects as a result of large intrinsic electronic g-factor. In addition, embedding InSb quantum nanostructures in a GaAs matrix could create type-II band alignment, where radiation lifetimes are longer than those of the typical type-I systems. Such characteristics are promising for memory devices and infrared photonic applications. The growth of InSb/GaAs quantum nanostructures by strain driven mechanism using molecular beam epitaxy with low growth temperature, slow growth rate, Sb soaking process prior to In deposition, and small amount of In deposition typically creates a mixture of twin and single nano-stripe structures with truncated pyramid shape. In this work, we further investigate the growth mechanism of such twin InSb/GaAs nano-stripes by controlling the growth conditions, consisting of nanostructure growth duration and growth temperature. When the growth temperature is kept to less than 300°C and In deposition is set to only a few monolayers, we found that 25-40% of formed nanostructures are twin InSb/GaAs nano-stripes. However, when the In deposition is stopped immediately after the spotty reflection high-energy electron diffraction patterns are observed, the ratio of twin nano-stripes to single ones is increased to 50-60%. We therefore describe the growth mechanism of twin nano-stripes as the early state of single nano-stripe formation, where the twin nano-stripes are initially formed during the first monolayer of InSb formation as a result of large lattice mismatch of 14.6%. When In deposition is increased to a few monolayers, the gap between twin nano-stripes is filled up and consequently forms the single nano-stripes instead. With this particular twin nano-stripe growth mechanism, the preservation of high ratio of twin nano-stripe formation can be expected by further reducing the growth temperature, i.e. less than 260°C. These twin nano-stripes may find applications in the fields of spintronics and novel interference nano-devices.

Study of Sb template for heteroepitaxial growth of GaSb thin film on Si(111)substrate

GaSb thin films have been grown on Si(111) substrates by molecular beam epitaxy (MBE), using an Sb template as an initiation layer. The film's crystal properties were investigated by in situ reflection high energy electron diffraction (RHEED), atomic force microscope (AFM), X-ray reciprocal space mapping (RSM) and ϕ-scan X-ray diffraction (XRD). RSMs around Si and GaSb 224 reciprocal lattice points indicated that the GaSb thin film grown on the Sb template is nearly unstrained, in contrast to that grown on an AlSb initiation layer which is under tensile strain. The extra peaks on the ϕ-scan XRD profiles can be assigned to {224} diffraction originating from a sub-domain region that corresponds to the region rotated by 180° with respect to the fundamental domain of epitaxial film. The very weak intensity of the extra peaks suggests that using the Sb template is an effective way to suppress sub-domain generation in the epitaxial film, resulting in a higher quality GaSb thin film on Si(111) substrates. The spotty RHEED patterns observed were analyzed as a superposition of zinc-blende type diffraction spots on the [111]–[2¯11] plane and [111]–[21¯1¯] plane, indicating that the GaSb quantum dots at the initial growth stage are not randomly orientated polycrystalline structures but a two-domain structure with zinc-blende type crystals.

Improvement in crystal quality of ZnO film on Si substrate by using a homo-buffer layer

Abstract ZnO thin films without and with a homo-buffer layer have been prepared on Si(1 1 1) substrates by pulsed laser deposition (PLD) under various conditions. Photoluminescence (PL) measurement indicates that the optical quality of ZnO thin film is dramatically improved by introducing oxygen into the growth chamber. The sample deposited at 60 Pa possesses the best optical properties among the oxygen pressure range studied. X-ray diffraction (XRD) results show that the films directly deposited on Si are of polycrystalline ZnO structures. A low-temperature (500 °C) deposited ZnO buffer layer was used to enhance the crystal quality of the ZnO film. Compared to the film without the buffer layer, the film with the buffer layer exhibits aligned spotty reflection high-energy electron diffraction (RHEED) pattern and stronger near-band-edge emission (NBE) with a smaller full-width at half-maximum (FWHM) of 98 meV. The structural properties of ZnO buffer layers grown at different temperatures were investigated by RHEED patterns. It is suggested that the present characteristics of the ZnO epilayer may be raised further by elevating the growth temperature of buffer layer to 600 °C.

Effects of oxygen pressure on the microstructure of LaNiO3 conductive thin film monitored byin situreflection high energy diffraction

LaNiO3 (LNO) conductive thin films with different thicknesses were deposited on SrTiO3 (100) substrates in different oxygen pressures. Effects of oxygen pressure on microstructure of LNO conductive film have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray photoelectron spectroscopy (XPS). In the relatively low oxygen pressure, LNO film displays spotty RHEED pattern. When the thickness increases up to a critical value, about 30nm, the spotty RHEED pattern gradually changes to streaky pattern, and the RHEED oscillation curve appears. The streaky RHEED pattern of LNO film deposited in the relatively high oxygen pressure can be observed at the initial growth. With pumping the oxygen pressure to a relatively low value, the RHEED pattern gradually changes to spotty one. When the oxygen pressure is increased back to a high value, the RHEED pattern changes to streaky one again. This RHEED pattern transformation induced by the oxygen pressure is reversible. Ex situ XPS results indicate that the element Ni of LNO film deposited in the relatively low oxygen pressure with a thickness below the critical value exists in the form as Ni2+ while as Ni3+ in the relatively high oxygen pressure. When the thickness increases to the critical value, the chemical valence of element Ni in the top layers of LNO film deposited in the relatively low oxygen pressure is +3. A mechanism of effects of oxygen pressure on microstructure of LaNiO3 conductive thin film is given. This sensitivity to oxygen for LNO thin film can be used in sensor devices.

Infrared and Ultraviolet Raman Spectra of AlN Thin Films Grown on Si(111)

AbstractInfrared (IR) spectroscopy and ultraviolet near-resonance enhanced Raman scattering have been used as alternative techniques to characterize the AlN films grown on Si(111). Epitaxial AlN films have been prepared by plasma source molecular beam epitaxy (PSMBE) at growth temperatures of 400 and 650 oC. The AlN/Si grown at 650°C shows distinct reflection high-energy electron diffraction (RHEED) patterns with the following epitaxial relations: AlN [0001] ‖ Si [111], and AlN < 011 0 > ‖ Si < 112 >. This is in sharp contrast with the observation of a spotty RHEED pattern for AlN/Si sample grown at 400°C, which appeared the same when viewed along any Si azimuth. Furthermore, the X-ray diffraction (XRD) rocking curve width showed a significant reduction from 5.7ofor sample grown at 400°C to 2° for the one grown at 650°C. The observation of Raman active A1(LO) and E2, and IR active E1(TO) zone center phonons, in both the films, is in accordance with the c-axis orientation of the films. However, the observed phonon mode frequencies and line widths indicate that the AlN samples grown at 400°C have less strain but more disorder compared to the ones grown at 650°C in agreement with RHEED and XRD data.

Improvement in the crystal quality of ZnSe films on Si(111) substrates with a nitrogen surface treatment

We have achieved a significant improvement in the crystal quality of ZnSe films grown by pulsed molecular beam epitaxy (MBE) on Si(111) by irradiating the substrates with a plasma of nitrogen (N-plasma) prior to the deposition. Reflection high-energy electron diffraction (RHEED) patterns during the pulsed MBE growth on the N-plasma-treated Si surface showed very well defined streaks with a twofold reconstruction indicating an atomically flat surface. In sharp contrast spotty RHEED patterns with a diffuse background were observed during the initial stages of the conventional MBE growth of ZnSe on untreated substrates, indicating an initial three-dimensional growth mode. Atomic force microscopy confirmed a smoother surface for the samples grown on the N-plasma-treated Si substrates. Moreover, transmission electron microscopy revealed a decrease in the density of crystal defects in the ZnSe epilayers by the use of the N-plasma treatment.

Growth mode of AlN epitaxial layers on 6H-SiC by plasma assisted molecular beam epitaxy

AlN layers were grown on 3.5° off 6H-SiC substrate by plasma-assisted molecular beam epitaxy (MBE). A study of the growth mechanism has been carried out by varying the growth time and the III/V ratio, at a constant growth temperature of 800°C. The layers were characterized by means of Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that nitrogen-rich conditions always give rise to a three-dimensional growth with spotty RHEED patterns and rough surface. In Al-rich conditions, the growth is much more two dimensional like with streaky RHEED patterns. By AFM, step bunching growth is observed. The size of the steps depends on the thickness of the layer as well as on the III/V ratio. All the samples grown in Al-rich conditions showed better crystalline quality than those grown in N-rich conditions.

Two-dimensional growth mode promotion of ZnSe on Si(1 1 1) by using a nitrogen substrate surface treatment

The growth mode of ZnSe epilayers prepared by pulsed molecular beam epitaxy (MBE) on Si(1 1 1) substrates, irradiated with a plasma of nitrogen (N-plasma) prior to the deposition, was compared to that of ZnSe epilayers prepared by conventional MBE directly on untreated Si substrates. Spotty reflection high-energy electron diffraction (RHEED) patterns with a diffuse background were observed during the initial stages of the conventional MBE growth. In sharp contrast, the RHEED patterns during the pulsed MBE growth on the N-plasma treated Si surface showed very well-defined streaks, and moreover a two-fold reconstruction was observed indicating an atomically flat surface. A strong evidence of the improved epitaxy, and the two-dimensional nucleation obtained by this technique was the clear presence of large amplitude RHEED oscillations. AFM images confirmed a flatter ZnSe surface for this sample.

Growth of self-assembled (Zn)CdSe nanostructures on ZnSe by migration enhanced epitaxy

The growth of CdSe on ZnSe by migration enhanced epitaxy has been studied in situ by means of reflection high-energy electron diffraction (RHEED) and reflection anisotropy spectroscopy (RAS). The change from a streaky to a spotty RHEED pattern during CdSe deposition indicates the transition to a three-dimensional growth mode. This change is accompanied by a damping of RHEED intensity oscillations and a decrease in the overall intensity. RAS spectra show a pronounced change at a photon energy of about 2.2 eV with increasing CdSe thickness. To investigate the role of lattice mismatch, ternary ZnCdSe layers of different compositions have been grown. With decreasing Cd concentration the critical thickness for the growth mode transition increases and for Cd concentrations below 30% no growth mode transition could be observed.

Growth of ZnSe and ZnS films on Si(111) substrates with a nitrogen surface treatment

We have investigated the effects of substrate surface irradiation with nitrogen plasma prior to growth on the initial stages of the heteroepitaxy of ZnSe on Si(111). The reflection high-energy electron diffraction (RHEED) patterns during the pulsed molecular beam epitaxial growth of ZnSe on the N-plasma-treated Si surface showed very well-defined streaks with a twofold reconstruction indicating an atomically flat surface. In sharp contrast, during the initial stages of the ZnSe growth on untreated Si substrates, spotty RHEED patterns with a diffuse background were observed. A strong evidence of the two-dimensional ZnSe nucleation obtained on the N-plasma-treated Si surface was the clear presence of large-amplitude RHEED oscillations. Atomic force microscopy measurements confirmed that we have achieved a substantial improvement on the ZnSe heteroepitaxy on Si substrates. We show that this novel N-plasma substrate treatment is useful for the growth of other Zn-chalcogenides compounds on Si substrates.

Study on improving InxGa1−xAs/InyGa1−yP heterointerfaces in gas-source molecular beam expitaxy

The As/P exchange behavior on a GaAs and an InxGa1−xAs surface exposed to a P2 flux and on an InyGa1−yP surface exposed to an As2 flux has been studied by reflection high-energy electron diffraction (RHEED) in a gas-source molecular beam epitaxy system. We found the exposure of InxGa1−xAs to a P2 flux results in a severe substitution of As atoms by P atoms at the normal growth temperature (520 °C), forming a strained transition layer and causing a rough surface within 10 s, which is indicated by a spotty RHEED pattern. The InyGa1−yP surfaces, however, remained relatively stable under an As2 flux. Two methods (a thin protective layer and low-temperature growth) for eliminating the As/P exchange have been developed, and both result in abrupt interfaces.

GaAs heteroepitaxial growth on vicinal Si(110) substrates by molecular beam epitaxy

We report the growth mechanism of GaAs on vicinal Si(110) substrates by molecular beam epitaxy, and investigate the effects of off-angle and -direction on the crystalline quality of grown films. The quality was improved with the increase of the off-angle and strongly influenced by the off-direction. Off-direction toward the [001] was better than that toward the [11̄0] for obtaining high-quality films. The cause is related to the structure and density of the steps. We infer from reflection high-energy electron diffraction results that GaAs films on vicinal Si(110) with off-angles above 2° toward the [001] direction grew up to a thickness of 3 nm in step-flow-like mode. The spotty reflection high-energy electron diffraction patterns characteristic of three-dimensional growth in GaAs/Si(100) were not observed at a thickness above 3 nm.

Molecular beam epitaxy of superconducting (Rb,Ba)BiO3

The simple cubic perovskite (Rb,Ba)BiO3 can be grown at temperatures below 350 °C by molecular beam epitaxy using a rf plasma atomic oxygen source. Films with superconducting onsets in resistivity as high as 27 K are obtained without annealing. The epitaxy proceeds in the normal (100) orientation on {100} SrTiO3, despite a 10% lattice mismatch. (110) epitaxy and spotty reflection high-energy electron diffraction (RHEED) patterns are obtained on {100} MgO substrates, despite the good lattice match for (100) growth. Streaked and spotty RHEED patterns have been obtained on either substrate. Sticking coefficients for bismuth depend on the growth conditions, indicating that the epitaxy is partially controlled by desorption kinetics.