Streaky Reflection High-energy Electron Diffraction Research Articles

Topological insulator Bi2Se3 films on rare earth iron garnets and their high-quality interfaces

The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here, we report excellent crystallinity of c-axis oriented epitaxial TI films of Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG), by molecular beam epitaxy using Se-buffered low-temperature growth technique. Strained-TmIG films with robust perpendicular magnetic anisotropy were deposited by off-axis sputtering. We demonstrated a streaky reflection high-energy electron diffraction pattern starting from the very first quintuple layer of Bi2Se3, indicating the high-quality interface between TmIG and Bi2Se3, a prerequisite for studying interfacial exchange coupling effects. The strong interfacial exchange interaction was manifested by the observation of an anomalous Hall effect in the Bi2Se3/TmIG bilayer and a shift of the ferromagnetic resonance field of TmIG induced by Bi2Se3. We have reproducibly grown high-quality Bi2Se3/ReIG and interfaces using this TI growth method, which may be applied to grow other types of van der Waals hetero-structures.

Indium-Rich InGaN Films Grown on Ge Substrate by Plasma-Assisted Molecular Beam Epitaxy for Solar Water Splitting

Indium-rich InGaN films were grown on Ge(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The influence of the indium flux on the structural properties, surface morphology, and photocurrent for water splitting has been investigated. Before the InGaN growth, 20 nm of GaN was deposited as a buffer layer. A streaky reflection high-energy electron diffraction (RHEED) pattern was observed for the GaN buffer growth. At the onset of InGaN growth, the streaks became spotty, indicating roughening of the surface and three-dimensional (3D) growth due to the lattice mismatch between GaN and InN. The indium composition in the InGaN structure was roughly fit to be around 50% from x-ray diffraction (XRD) ω–2θ measurements. Growth with excess indium supply led to segregation of metal indium on the surface. During cooling down, this metal indium transformed partially into InN. The crystal quality of the InGaN film decreased with increase of the indium flux. The photocurrent of the InGaN films used as photoelectrodes for water splitting also decreased with increase of the indium flux.

GaN‐based UV/blue electroluminescent devices deposited on Si at low temperature

GaN layers were grown on (111)Si substrates at 450 °C by compound source molecular beam epitaxy (CS-MBE). Streaky reflection high-energy electron diffraction (RHEED) patterns of the GaN layers were observed after the growth. The results indicate the growth of a hexagonal GaN crystal on a Si substrate and the effectiveness of CS-MBE in the fabrication of GaN layers at low temperature. GaN-based electroluminescent devices (ELDs) using CS-MBE were fabricated on the Si substrates. Purplish blue emission was observed from the ELDs under the AC operation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of epitaxial FexAl1−x/(In,Al)As/InP and III–V/FexAl1−x/(In,Al)As/InP structures

We report on the growth of high quality epitaxial FexAl1−x on (In,Al)As/InP with x=0.45–0.80 and thicknesses up to 1000 Å, and on overgrowth of III–V semiconductor on the metal. Buffer layers of (In,Al)As were grown on InP substrates in a III–V growth chamber, then transferred to an attached ultrahigh vacuum chamber with dual e-beam evaporators, where FexAl1−x layers were grown by co-evaporation. Despite a large lattice mismatch of ∼1%–2% between the metal and the semiconductor, high quality layers of FexAl1−x with thicknesses of up to 1000 Å were grown. The metal layers showed bright, streaky reflection high-energy electron diffraction patterns, were specular under Nomarski microscopy, and had roughnesses of ∼1–2 monolayers as measured by atomic force microscopy. High resolution x-ray diffraction showed that even at 1000 Å, the layers were fully strained, and Pendellosung oscillations were observed, indicating high crystalline quality. Metal layers over ∼200 Å thick had resistivities in the range of those expected of bulk layers (50–100 μΩ cm). Reflectivities greater than 90% for λ=4–20 μm were observed for metal layers 1000 Å thick. Overgrowth of (In,Ga)As and (In,Al)As on the metal layers shows a marked sensitivity to As overpressure, possibly due to an In surface-riding layer on top of the metal.

GaN-Rich Side of GaNAs Grown by Gas Source Molecular Beam Epitaxy

A large variation in wavelength from the ultraviolet to longer than 2 µm could be achieved in the GaN-rich side of the GaN1-xAsx alloy due to the large bowing of bandgap energy. Layers of GaN1-xAsx are grown on (0001) sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ion-removed ECR radical cell after the growth of GaN buffer layers. During the growth of GaN1-xAsx layers, a streaky reflection high-energy electron diffraction (RHEED) pattern was observed. The excitonic photoluminescence (PL) peak from the GaN-rich side of the GaN1-xAsx layer shows a large red shift as the As content changes. When an As content of up to x=0.009 is attained, a bandgap bowing parameter of 19.6 eV is experimentally obtained. Such a large value of the bowing parameter is promising for applications to optical devices operating over wide range of wavelength.

Gas-source molecular beam epitaxial growth and characterization of InNxP1−x on InP

We report a study of N incorporation into InP using a radio frequency (rf) N plasma source. Very streaky reflection high-energy electron diffraction patterns are observed for InNxP1−x (x <1%) grown on InP, indicating layer-by-layer growth of the film. The sharp x-ray diffraction peak and the clear Pendelloesung fringes in the high-resolution x-ray rocking curves reveal the high crystalline quality and uniformity of the film. They also suggest the smoothness of the interface between InP and InNxP1−xand of the surface of the InNxP1−x layer. This is further confirmed by scanning electron microscopy on these samples, where featureless surface is obtained. The formation of an InNP alloy is confirmed by x-ray θ-2θ diffraction measurement where no phase separation is observed. Different ways to increase the N composition in InNP were explored. At a fixed N2 flow-rate fraction, lowering the growth temperature increases the N composition in InNP. Raising the rf power or using a larger beam exit aperture will also increase the N incorporation as a result of the availability of more active N species.

Epitaxial growth of γ - Al2O3 layers on Si(111) using Al solid source and N2O gas molecular beam epitaxy

High crystalline quality γ-Al2O3 films were epitaxially grown on Si(111) substrates at low growth temperatures from 750 to 900 °C by Al solid source and N2O gas molecular beam epitaxy. Very thin γ-Al2O3 films grown at 850 °C showed streaky reflection high-energy electron diffraction patterns. By in situ x-ray photoelectron spectroscopy measurements, carbon contamination, as is seen in the films grown with Al(CH3)3 source, was not detected within the sensitivity. The stoichiometry of the growth films was found to be similar to that of Al2O3. Growth rates of epitaxial γ-Al2O3 layers were found to decrease with increasing growth temperatures.

Heteroepitaxy and characterization of CuInSe 2 on GaAs(001)

CuInSe 2 (CIS) films with Cu/In ratios of γ = 0.81–1.81 have been grown on (001)-oriented GaAs substrates by molecular beam epitaxy at substrate temperatures of T s = 350–550° C. Film properties were found to be substantially different for Cu- and In-rich regions. Cu-rich films were p-type, and streaky reflection high-energy electron diffraction (RHEED) patterns and sharp photoluminescence (PL) emissions were observed, suggesting high quality epitaxial films. In-rich films were highly resistive, and contained a large number of twins formed on {112} planes. A broad and strong PL emission, a donor-acceptor pair emission, was observed, which blue-shifted with increasing excitation power, indicating heavy compensation.

Epitaxial Growth of γ -Al2O3 Insulator Films on Si by Molecular Beam Epitaxy Using an Al Solid Source and N2O Gas

AbstractHigh quality crystalline γ -Al2O3 films were epitaxially grown on Si(111) substrates at growth temperatures from 750 to 900°C by molecular beam epitaxy using an Al solid source and N2O gas. Very thin γ -Al2O3 films grown at a growth temperature of 850°C showed streaky reflection high-energy electron diffraction patterns. By in situ x-ray photoelectron spectroscopy measurements, carbon contamination, as is seen in the films grown with a Al(CH3)3 source, was not detected within the measurement sensitivity. The stoichiometry of the grown film was found to be similar to that of Al2O3. Growth rates of epitaxial γ -Al2O3 layers decreased with increasing growth temperatures. The predominant growth of the γ -Al2O3(111) crystal orientation was confirmed on Si(110) and Si(100) substrates.

High-indium modulation doped field effect transistors on GaAs substrates

We have grown InxGa1−xAs modulation doped field effect transistors (MODFETs) using novel buffer layer schemes on GaAs substrates with x values up to 0.5. The MODFET active layers were grown at substrate temperatures between 500 and 520 °C and characterized by x-ray diffraction, photoluminescence, and Hall effect measurements. MODFET devices with 0.15 μm gate length were fabricated, and dc and rf tested. Our work has indicated that the electron mobility in the MODFETs is a function of the growth mode, i.e., two-dimensional (2D) layer-by-layer versus three-dimensional island growth. MODFET performance, the ability to maintain a streaky reflection high-energy electron diffraction pattern, as well as morphology had a very strong correlation. With these observations in mind we used a buffer layer scheme to give 2D growth fronts for InxGa1−xAs with x values up to 0.5, which is a modification of the buffer scheme described in K. Maezawa and T. Mizutani, IEEE Trans. Electron Devices 37, 1416 (1990). For In0.34Ga0.66As channel MODFETs we observed a room-temperature electron mobility of 6620 cm2/V s with a 2D carrier density of 5.9 × 1012 cm−2 (double-doped channel), a dc transconductance of 800 mS/mm and a rf gain of 13.5 dB at 35 GHz for a 0.15 μm gate length.

High-quality ZnSe/GaAs superlattices: MEE growth, and structural and optical characterization

High-quality ZnSe/GaAs superlattices were grown by migration-enhanced epitaxy (MEE) and characterized using X-ray diffraction, electron microscopy and photoluminescence. The streaky reflection high-energy electron diffraction (RHEED) pattern and strong, persistent RHEED oscillations observed during the MEE growth of the superlattices indicate a smooth growing surface. The sharp satellite peaks observed clearly in the double-crystal X-ray diffraction rocking curve of a 21-period ZnSe/GaAs superlattice confirm the excellent crystalline and interfacial quality of the superlattice. Cross-section transmission electron microscopy (TEM) shows flat, abrupt heterointerfaces. Superlattice photoluminescence spectra show that both photo-excited electrons and holes are confined in GaAs wells; also seen are the sharp excitonic features from the coherently strained ZnSe cap and barrier layers of the superlattice.

Migration-enhanced epitaxy growth and characterization of high quality ZnSe/GaAs superlattices

We report the growth of high quality ZnSe/GaAs superlattices by migration-enhanced epitaxy (MEE) and their characterization using x-ray diffraction, electron microscopy, and photoluminescence. A streaky reflection high-energy electron diffraction (RHEED) pattern and strong, persistent RHEED oscillation during the MEE growth of the superlattices indicate a smooth growing surface. The sharp satellite peaks observed clearly in the double-crystal x-ray diffraction rocking curve of a 21-period ZnSe/GaAssuperlattice confirm the excellent crystalline and interfacial quality of the superlattice. Cross-section high-resolution electron microscopy indicates coherent lattice arrangement and abrupt interface at ZnSe-on-GaAs as well as GaAs-on-ZnSe heterointerfaces. Photoluminescence spectra from a superlattice with a periodicity of 40 nm show a strong peak at a wavelength of 829.5 nm with a linewidth of 6.3 meV at a temperature of 10 K; we believe this to be the first observation of photoluminescence from any multilayer structure involving GaAs-on-ZnSe growth.