Reflection High-energy Diffraction Research Articles

Investigation of the initial stage of GaN epitaxial growth on 6H-SiC (0001) at room temperature

We have investigated the initial stage of GaN epitaxial growth on 6H-SiC (0001) at low substrate temperatures by pulsed laser deposition (PLD). We found that GaN grows epitaxially even at room temperature (RT) on atomically flat 6H-SiC (0001) surfaces, which can be explained by the enhanced surface migration of film precursors due to the use of PLD and atomically flat substrates. In situ reflection high-energy election diffraction observations have revealed that GaN films grown at above 300°C proceed in a three-dimensional mode, while those at RT proceed in a layer-by-layer growth mode with atomically flat terraces and steps. The step height turned out to be 1.5nm, which is the same height as the steps on the SiC substrates. This result indicates that the step height on the SiC surface is retained as the GaN grows.

Interface formation during molecular beam epitaxial growth of neodymium oxide on silicon

The Si/dielectric interface properties influence device performance significantly. Often the interface is not stable and changes during and/or after the growth. For a better understanding of the interface and layer formation processes of Nd2O3 on Si(001), as an example for the lanthanide oxides, well-defined experimental studies by reflection high-energy diffraction and x-ray photoelectron spectroscopy were performed under ultraclean ultrahigh vacuum conditions of molecular beam epitaxy. Complementary investigations were performed by transmission electron microscopy. We found that Nd2O3 is a candidate for replacing silicon dioxide as gate dielectric in future Si devices with suitable band gap and offset with respect to silicon. However, under ultrahigh vacuum conditions, silicide formation occurs in the initial stage of growth, which can result in large silicide inclusions and hole formation during further growth. This effect can be completely prevented by modifying the oxygen partial pressure during the interface formation and layer growth.

All-epitaxial growth of Ba0.6Sr0.4(Ti0.94Al0.06)O3–Si heterostructures and their leakage current characteristics

6 at. % Al-doped Ba0.6Sr0.4TiO3 (BSTA) thin films were grown on Ir∕MgO-buffered Si(001) substrates by in situ pulsed-laser deposition techniques. All-epitaxial growth of BSTA∕Ir∕MgO∕Si(001) heterostructures with layer-by-layer mode was evidenced by in situ reflection high-energy electron-diffraction observation and x-ray diffraction. The epitaxy relationship was determined as: BSTA⟨100⟩‖Ir⟨100⟩MgO⟨100⟩‖Si⟨100⟩ (in-plane) and BSTA(001)‖Ir(001)MgO(001)‖Si(001) (out-of-plane). The BSTA thin films exhibit an extremely smooth surface with a roughness of RMS=0.89nm. The largely reduced leakage current for BSTA thin films, which was dominated by the Schottky emission mechanism, might be attributed to combination effects of the crystal-structure-perfection and acceptor Al doping. Moreover, the BSTA thin films show good dielectric properties at low-frequency regime.

Characterization of bicrystalline epitaxial LaNiO3 films fabricated on MgO (100) substrates by pulsed laser deposition

A series of metallic LaNiO3 (LNO) thin films were deposited on MgO (1 0 0) substrates by pulsed laser deposition (PLD) under the oxygen pressure of 20 Pa at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD) was used to characterize the crystal structure of LNO films. θ–2θ scans of XRD indicate that LNO film deposited at a substrate temperature of 700 °C has a high orientation of (l l 0). At other substrate temperatures, the LNO films have mixed phases of (l l 0) and (l 0 0). Furthermore, pole figure measurements show that LNO thin films, with the bicrystalline structure, were epitaxially deposited on MgO (1 0 0) substrates in the mode of LNO (1 1 0)//MgO (1 0 0) at 700 °C. Reflection high-energy electric diffraction (RHEED) and atomic force microscopy (AFM) were also performed to investigate the microstructure of LNO films with the high (l l 0) orientation. RHEED patterns clearly confirm this epitaxial relationship. An atomically smooth surface of LNO films at 700 °C was obtained. In addition, bicrystalline epitaxial LNO films, fabricated at 700 °C, present a excellent conductivity with a lower electrical resistivity of 300 μ Ω cm. Thus, the obtained results indicate that bicystalline epitaxial LNO films could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.

Low Temperature Chemical Vapor Deposition of 3C-SiC on Si Substrates

In the present work an UHVCVD method was developed which allows the epitaxial growth of 3C-SiC on Si substrates at temperatures below 1000°C. The developed method enable the growth of low stress or nearly stress free single crystalline 3C-SiC layers on Si. The influence of hydrogen on the growth process are be discussed. The structural properties of the 3C-SiC(100) layers were studied with reflection high-energy diffraction, atomic force microscopy, X-ray diffraction and the layer thickness were measured by reflectometry as well as visible ellipsometry. The tensile strain reduction at optimized growth temperature, Si/C ratio in the gas phase and deposition rate are demonstrated by the observation of freestanding SiC cantilevers.

RHEED study of GaAs(3 3 1)B surface

In this paper, the growth dynamics and the surface morphology of the GaAs(3 3 1)B surface have been studied by both in situ reflection high-energy election diffraction (RHEED) and in situ scanning tunneling microscopy (STM). For the first time, a RHEED oscillation is reported on high index GaAs(3 3 1)B faceted surface with (1 1 0) and (1 1 1)B facets. The RHEED oscillation was observed only along the [ 1 ¯ 1 ¯ 6 ] direction. Absence of any RHEED oscillations along [ 1 1 6 ¯ ] , [ 1 ¯ 1 0 ] , and [ 1 1 ¯ 0 ] indicates a possible growth model in which the GaAs(3 3 1)B surface is moving frontward through fractional growth of its (1 1 1)B facets. These results help us to better understand the nature of RHEED oscillation on high index GaAs.

Self-assembled InAs quantum dots on GaSb/GaAs(0 0 1) layers by molecular beam epitaxy

Self-assembled InAs quantum dots (QDs) were grown on one-monolayer-thick GaSb/GaAs(0 0 1) layer by molecular beam epitaxy using Stranski–Krastanov mode. High dot density of about 1 × 10 11 cm - 2 was demonstrated on the GaSb/GaAs buffer layer. In spite of high dot density, coalescence of neighboring dots was effectively suppressed on the GaSb surface. InAs coverage dependence of the dot structure was studied by using atomic force microscopy and reflection high-energy electron-beam diffraction. For large InAs coverage, the {1 1 0} facet appeared on the side wall of the large dots, and, a narrow PL linewidth of 33 meV could be obtained from highly dense InAs QDs.

Atomically-controlled GaSb-termination of GaAs surface and its properties

An atomically-controlled GaSb layer was fabricated on GaAs(001) substrates by using the surface exchange reaction between arsenic and antimony atoms. The exchange reaction process was controlled by molecular-beam irradiation of Sb4 and was monitored by using reflection high-energy electron-beam diffraction (RHEED). One-cycle oscillation of RHEED specular-beam intensity during the Sb4 irradiation was explained by the change of surface structure from GaAs to GaSb. From transmission electron microscopy and photoluminescence (PL) measurements, it was found that fabricated GaAs/GaSb quantum well (QW) structures have an abrupt heterointerface and high crystal quality. In scanning tunneling microscopy measurements, tunneling spectra obtained from the GaSb-terminated GaAs sample revealed suppression of density of surface states.

Studies of the electronic structure at the Fe3O4–NiO interface

The interfacial electronic structure between the metallic ferrimagnet Fe3O4 and the insulating antiferromagnet NiO is investigated in the lattice matched heteroepitaxial system Fe3O4 (100)–NiO (100) by growing ultrathin NiO films on single-crystal Fe3O4 (100) substrates. The Fe3O4 (√2×√2)R45° surface is characterized prior to growth by low-energy electron diffraction, reflection high-energy diffraction, scanning tunneling microscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy. UPS and XPS, which sample several monolayers in the substrate–overlayer structure, are used to monitor near-surface electronic properties versus NiO overlayer thickness. Comparison of experimental He II UPS spectra of the valence band electronic structure with a simple model of substrate–overlayer emission indicates that the electronic transition from Fe3O4 to NiO is nearly atomically sharp.

Two-magnon scattering in a self-assembled nanoscale network of misfit dislocations

The static and dynamic properties of magnetic Pd/Fe(001) ultrathin film crystalline structures prepared on GaAs(001) templates were investigated by ferromagnetic resonance (FMR) from 10 to 73 GHz. It will be shown that the formation of a self-assembled nanoscale network of misfit dislocations in crystalline structures can be detected during the growth by fan-out diffraction features in reflection high electron energy diffraction. This network of defects leads to a strong extrinsic magnetic damping. The out-of-plane measurements of the FMR linewidth have revealed that the extrinsic damping is caused by two-magnon scattering. The contribution to the FMR linewidth from two-magnon scattering is strongly anisotropic and follows the rectangular symmetry of the glide planes of the misfit dislocation network. It will be shown that the observed strong anisotropy in two-magnon scattering can be interpreted by Fourier components of magnetic defects. The angular dependence of the Fourier components results in an effective channelling of scattered spin waves.

Control of GaSb/GaAs Quantum Nanostructures by Molecular Beam Epitaxy

GaSb quantum wells (QWs) and quantum dots (QDs) were fabricated on GaAs(001) surfaces by molecular beam epitaxy. By scanning transmission electron microscopy and photoluminescence (PL) measurements, their nanostructures were evaluated. One-monolayer-thick GaSb was precisely formed by the surface exchange reaction between As and Sb atoms, which was monitored by reflection high-energy electron-beam diffraction. In the Stranski-Krastanov growth of the GaSb QDs, low growth rate and low Sb4 pressure were effective conditions for suppressing size fluctuation. As a result, we could obtain the narrowest PL linewidth of 67 meV for GaSb/GaAs QDs.

Controlled stacking growth of uniform InAs quantum dots by molecular beam epitaxy

Double-stacked InAs quantum dots (QDs) were grown by molecular beam epitaxy via Stranski–Krastanov growth mode. Transition of the facet formation from {1 3 6} plane to {1 1 0} plane was observed during the stacking growth of InAs QDs by reflection high-energy electron-beam diffraction. The enhanced growth rate and the different facet formation in the stacking growth were caused by tensile strain of the GaAs underlying layer. Low arsenic pressure and low growth rate conditions played an important role for a perfect coupling and uniformity in the size of the stacked QDs. The narrow photoluminescence line width of 17.6 meV was successfully obtained from the stacked InAs QDs.

Morphology, Desorption Energy and Mean Residence Time of Vanadyl-Phthalocyanine Molecules Deposited on Two Kinds of Substrates by Molecular Beam Epitaxy

Vanadyl-phthalocyanine (VOPc) thin films were prepared on two kinds of substrates by molecular beam epitaxy (MBE). Their characterization was carried out by scanning electron microscopy (SEM), reflection high-energy diffraction (RHEED) and atomic force microscopy (AFM). The desorption energy and mean residence time of VOPc molecules deposited on two kinds of substrates were also investigated using a Vis/UV spectroscope. The relationship between the morphology of VOPc thin film and the desorption energy of VOPc molecules deposited was discussed.

Formation and optical properties of InAs/GaAs quantum dots for applications as infrared photodetectors operating at room temperature

The formation and optical properties of lateral quantum-dot infrared photodetector (QDIP) structures utilizing uniform-sized InAs QDs obtained by controlling the intensity of the reflection high-energy diffraction pattern and the As/In flux ratio were investigated. The thickness of the pseudomorphic, strained InAs wetting layer increased slightly with decreasing As/In flux ratio. The characteristic peaks corresponding to the interband and the intersubband transitions were observed, respectively, in the photoluminescence and Fourier transform infrared spectra. The absorption spectrum showed an absorption peak corresponding to the intersubband transition at around 10 μm. These results indicate that InAs/GaAs QDs obtained by controlling growth conditions hold promise for potential applications in QDIP devices operating at room temperature.

Ga-rich GaAs(0 0 1) surfaces observed by STM during high-temperature annealing in MBE

Ga-rich GaAs (0 0 1) surfaces are successfully observed by scanning tunneling microscopy (STM) during high-temperature annealing in molecular beam epitaxy. With a substrate temperature of 550°C, reflection high-energy diffraction patterns and reflectance anisotropy spectra confirm a (4×2) Ga-stabilized surface. STM images clearly show alteration of the surface reconstructions while scanning. It is postulated that detaching and attaching of Ga adatoms may be the cause of this surface dynamics. For these conditions it is determined that ζ(4×4), ζ2(4×4) and ζ(4×6) reconstructions co-exist on the surface. The ζ2(4×4) reconstruction contains a Ga tetramer cluster.

Epitaxy of carbon-rich silicon with MBE

At high concentrations, carbon in silicon shows some properties of technological interests like gap engineering, lattice engineering or the reduction of the so-called ‘Transient Enhanced Diffusion’ of some dopants. The carbon incorporation in concentrations above 1% is nevertheless a difficult task due to the low carbon solubility in silicon. Here, we present a systematic approach of the Si 1− x C x layer growth by Molecular Beam Epitaxy (MBE). To study the influence of the growth temperature on the carbon incorporation, we deposit 100 nm thick silicon layers with different carbon concentrations separated by a 100 nm silicon spacer for different temperatures, ranging from 370 up to 800 °C. In-situ Reflection High Energy Diffraction (RHEED), Secondary Ion Mass Spectroscopy (SIMS) and cross-sectional Transmission Electron Microscopy (TEM) have been performed to characterize the as-grown structures. We show the presence of two distinct growth phenomena, a ‘classical’ one, around 450 °C, where carbon is mainly substitutionally incorporated, defect free until 2%; and a second one, above 550 °C, showing an agglomeration of carbon in thin layers with presumably a very high local concentration, without any planar defects. We propose in this work a diagram of Si:C growth by MBE.

Growth of epitaxial CoSi2 on 6H-SiC(0001)Si

Epitaxial growth of (111)-oriented CoSi2 has been achieved on a scratch-free 6H-SiC(0001)Si substrate. The surface was prepared using atmospheric hydrogen etching and ultrahigh vacuum Si cleaning. A high-quality CoSi2 thin film was obtained by a modified template method and co-deposition of Co and Si at 550 °C. The structure and morphology of the film is studied by means of reflection high electron energy diffraction, x-ray absorption fine structure, x-ray diffraction, and atomic force microscopy.

Optical Characterization of MBE Grown Zinc-Blende AlGaN

Zinc-blende AlGaN epilayers were grown by plasma-assisted MBE on thick (001) SiC deposited by CVD on Si substrates. Alloy compositions were controlled in situ by reflection high energy diffraction (RHEED) oscillations, and confirmed by Rutherford backscattering (RBS) post growth analysis. The zinc-blende nature of our samples was assessed by RHEED analysis. From reflectivity measurements at room temperature, we deduce the variation of the lowest direct absorption edge of zinc-blende AlGaN as a function of the Al content x, E0(x) = 3.25 (1 — x) + 6.05x — 1.4x (1 — x), E0 in eV. The dispersion of the refractive index n of cubic AlN was also extracted from our reflectivity and RBS data, and fitted by a Sellmeier-type relation, n2 = 3.05 + 1.38 λ2/(λ2—1802), λ being the wavelength in nm.

Nucleation and growth of ZnO on [formula omitted] sapphire substrates using molecular beam epitaxy

It has been recently demonstrated that it is possible to eliminate 30° rotation twins in c-oriented epitaxial ZnO films by growing on the (1 1 ̄ 2 0) face of sapphire despite the apparent symmetry mismatch between the (6-fold) epilayer and the (2-fold) substrate. To further elucidate the details of the growth process, we have investigated the initial stages of growth of ZnO on (1 1 ̄ 2 0) sapphire using molecular beam epitaxy. Films of approximately 3, 5, 8, 10, 20, 100, and 600 nm thickness were investigated by in situ reflection high-energy diffraction, high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and extended X-ray absorption (XAFS). XAFS indicates that there is no discernible change in nearest-neighbor distance with film thickness. On the other hand, HRXRD measurements indicate an ∼1% increase in the c-lattice constant with decreasing film thickness. XAFS measurements of the second nearest-neighbor structural disorder were observed to increase sharply for the thinnest films implying that distortion of bond angles is responsible for the increase in the c-axis observed by HRXRD. HRXRD reciprocal area maps of the symmetric ZnO (0 0 0 2) reflection indicate the onset of diffuse scattering from 20 nm, while AFM indicates a concurrent change in surface morphology suggesting a change in the growth process for t>∼20 nm .

Self Size-Limiting Process of InAs Quantum Dots Grown by Molecular Beam Epitaxy

In the molecular beam epitaxy (MBE) growth of InAs quantum dots (QDs), we investigated the dot formation via the self size-limiting process. Accumulation of size-limited InAs QDs resulted in narrow inhomogeneous broadening of photoluminescence (PL) linewidth and low energy shift of PL peak position. Atomic force microscopy (AFM) and reflection high-energy electron-beam diffraction (RHEED) observations revealed the appearance of {136} facets on the side wall of size-limited InAs dots. It was proposed that the facet formation played an important role in the self size-limiting behavior.