Low-temperature Molecular Beam Research Articles

Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E g ) and the transition between the conduction band and spin-orbit-split valence subband (E g + Δ SO ) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).

Internal rotation in trifluoromethylsulfur pentafluoride: CF3SF5 by Fourier transform microwave spectroscopy

Trifluoromethylsulfur pentafluoride CF3SF5, which has been attracting much attention because of its unusually large global warming potential, was investigated by Fourier transform microwave spectroscopy in order to determine the twelve-fold potential barrier to internal rotation in this molecule. We have found the V12 value to be close to zero. Relaxation among internal-rotation and overall-rotation levels was found inhomogeneous, resulting in distributions quite different from thermal in low-temperature molecular beam, which might affect significantly thermodynamic properties of the molecule. Rotational spectra of the 13C species and the 34S species were also observed in natural abundance, leading to the rs CS bond length of 1.8808 (7) Å.

Folded acoustic phonons in Si/Ge superlattices with Ge quantum dots

The spectra of Raman scattering by folded acoustic phonons in Si/Ge superlattices with pseudomorphic layers of Ge quantum dots (QDs) grown by low-temperature (T = 250°C) molecular beam epitaxy are studied. New features of the folded phonon lines related to the resonant enhancement and unusual intensity ratio of the doublet lines that cannot be explained by the existing theory have been observed. The observed modes are shown to be related to the vibrations localized to the QDs and induced by the folded phonons of the Si spacer layers. The calculations performed in the model of a one-dimensional chain of atoms have allowed the nature of the localization of acoustic phonons attributable to a modification of the phonon spectrum of a thin QD layer to be explained. The observed intensity ratio of the folded phonon doublet lines is caused by asymmetry of the relief of the QD layers.

Hydration of OCS with One to Four Water Molecules in Atmospheric and Laboratory Conditions

Carbonyl sulfide is the most abundant sulfur gas in the atmosphere. We have used MP2 and CCSD(T) theory to study the structures and thermochemistries of carbonyl sulfide interacting with one to four water molecules. We have completed an extensive search for clusters of OCS(H2O)n , where n = 1-4. We located three dimers, two trimers, five tetramers, and four pentamers with the MP2/aug-cc-pVDZ method. In each of the complexes with two or more waters, OCS preferentially interacts with low-energy water clusters. Our results match current theoretical and experimental literature, showing correlation with available geometries and frequencies for the OCS(H2O) species. The CCSD(T)/aug-cc-pVTZ thermochemical values combined with the average amount of OCS and the saturated concentration of H2O in the troposphere, lead to the prediction of 10(6) OCS(H2O) clusters x cm(-3) and 10(2) OCS(H2O)2 clusters x cm(-3) at 298 K. We predict the structures of OCS(H2O)n , n = 1-4 that should predominate in a low-temperature molecular beam and identify specific infrared vibrations that can be used to identify these different clusters.

InAs/InGaP/GaAs heterojunction power Schottky rectifiers

A low-temperature (LT) grown InAs epi-layer has been applied as the gate to the dual-material structure of lattice-matched InGaP on GaAs, to make a high-temperature power rectifier. The LT molecular beam epitaxy technique enables the formation of an abrupt interface between InAs and InGaP. This heterojunction rectifier utilises the strong thermal stability of the InAs/InGaP heterojunction and the high figure-of-merit of the InGaP/GaAs dual-material structure for power devices. The LT-InAs/InGaP/GaAs heterojunction rectifier demonstrates lower on-state resistance, lower off-state leakage current, and higher breakdown voltage, than metal/GaAs Schottky rectifiers.

Origination of misfit dislocations at the surface during the growth of GeSi/Si(001) films by low-temperature (300–400°C) molecular-beam epitaxy

The method of the molecular-beam epitaxy, at comparatively low temperatures (300–400°C), was used to grow GexSi1 − x /Si(001) films with a constant composition (x = 0.19–0.32) across a film and as well as two-layer heterostructures with the Ge content at the upper layer no lower than 0.41. Using transmission electron microscopy, it is shown that the main cause of an increase in the density of threading dislocations with increasing Ge fraction in the plastically relaxed films is the origination of the dislocation half-loops at the film surface; in turn, these dislocation half-loops are generated owing to the formation of a three-dimensional profile at the surface of the growing or annealed film.

Photoemission study of GaAs(100) grown at low temperature

GaAs(100) layers grown by low-temperature (LT) molecular beam epitaxy were studied by means of valence-band and core-level photoelectron spectroscopy. Small differences were found between valence-band spectra from the LT layers and corresponding layers grown at high temperature. In the Ga 3d spectra a new component was found in the LT-GaAs. The relative intensity of this component was found to be practically constant with varying probing depth. It is proposed that this component represents sites coordinated to the five-atom As clusters formed at As-Ga antisites. This interpretation implies a higher density of antisite defects in the near-surface region than typically found in the bulk.

The Effect of SiGe Barriers on the Thermal Stability of Highly B-Doped Si Surface Layers

ABSTRACTWe have investigated the use of low-temperature (320 °C) molecular-beam epitaxy (MBE) to form highly conductive, p+, ultra-shallow layers in Si. Although the as-grown B-doped Si is electrically active, in a practical application the doped layers may be exposed to high temperature during post-growth device processing. To minimize the B diffusion, we investigated the use of SiGe diffusion barrier layers. In this work we demonstrate there is less B redistribution with the SiGe diffusion barriers. The use of SiGe diffusion barriers may prove to be critical in the activation of B implants for the formation of ultra-shallow junctions.

Polycrystal isolation of InGaP/GaAs HBTs to reduce collector capacitance

InGaP/GaAs heterojunction bipolar transistors (HBTs) with polycrystalline GaAs buried under the base electrode have been fabricated using low-temperature gas-source molecular beam epitaxy on SiO/sub 2/-patterned substrates. A cutoff frequency of 120 GHz and a maximum oscillation frequency of 230 GHz were obtained for three parallel 0.7/spl times/8.5 /spl mu/m HBTs. Compared to HBTs without the polycrystal, the collector capacitance was reduced by 28% and the maximum stable gain was improved by 1.2 dB due to complete carrier depletion in the polycrystal under the base electrode. These results show the high potential of the proposed HBTs for high-speed digital and broadband-amplifier applications.

Evidence for Substitutional C, Ordering Effects and Interdiffusion in Epitaxial GE-C and GE-RICH GE-SI-C Alloys

AbstractWe report on the epitaxial growth of Ge-C and Ge-Si-C alloys (C<10%) grown on Si(100) and Ge(100) substrates using low temperature (∼200°C) molecular beam epitaxy. Thin films (50–70 nm) were characterized in-situ by RHEED and ex-situ by transmission electron microscopy, xray diffraction, and Raman spectroscopy. The films were annealed at 750°C and 850°C in an Ar atmosphere to study interdiffusion effects.Raman spectroscopy of Ge-C on Ge indicates the existence of a Ge-C local mode at 530cm−1 and is direct evidence for the presence of substitutional C in Ge. The GeSiC alloys grown on Ge do not show the Ge-C local mode, consistent with preferential Si-C bonding. There is evidence for strain enhanced solubility of C based on a comparison of the substitutional C content in Ge-C films on Si (∼1 at %) and on Ge substrates (∼0.1 at %). Silicon interdiffusion in annealed Ge-C samples is strongly suppressed by the presence of C. A simple diffusion model is used to illustrate Si indiffusion in Ge.

Molecular Beam Epitaxy of Nonstoichiometric Semiconductors and Multiphase Material Systems

Abstract When arsenides are grown by molecular beam epitaxy at low substrate temperatures, as much as 2% excess arsenic can be incorporated into the epilayer. This excess arsenic is in the form of antisites, but there is also a substantial concentration of gallium vacancies. With anneal, there is a significant decrease in the arsenic antisite and gallium vancancy concentrations as the excess arsenic precipitates. With further anneal, the arsenic precipitates coarsen. This combination of low substrate temperature molecular beam epitaxy and a subsequent anneal results in a broad spectrum of materials, from highly defected epilayers to a two-phase system of semimetallic arsenic precipitates in an arsenide semiconductor matrix. These materials exhibit some very interesting and useful electrical and optical properties.

The Effect of Si Additions on the Epitaxial Growth of Gel−x Cx Alloys on Si(100)

AbstractWe report on the differences in the epitaxial growth mechanisms between Ge1−xCx (O<x<0.1) and Ge1−x−ySixCy (x=0.2, 0<y<0.05) alloys grown on Si(100) using low temperature( 200°C) molecular beam epitaxy. Thin films (50˜65nm) were characterized in situ by RHEED and ex situ by transmission electron microscopy and x-ray diffraction. With increasing C concentration, the microstructure of both Ge and GeSi alloys changes from 2D layer growth to 3D islanding. The d400 spacing of the relaxed alloys decreases marginally, with a maximum of 1at.% C being substitutionally incorporated. Ge-C films with higher C content have a high density of planar defects, typically twins and stacking faults. The addition of 20% Si does not appear to increase the amount of substitutional C in the films. Rather, the additions of 20% Si to Ge-C alloys somehow seems to enhance the tendency for the formation of planar defects.

Interface Phenomena in very thin Si-Ge Heterostructures

ABSTRACTInterfacial properties of thin Si/Ge heterostructures are discussed. It is shown that (SimGen) p superlattices with well defined periodicity can be synthesized on both (100) Si and Ge using low temperature (350 °C) Molecular beam epitaxy. Multilayer structures are however intermixed over one or two monolayers and exhibit a roughness with lateral scale > 100 nm and an amplitude of the order of two Monolayers. On Si the roughness is strongly correlated from interface to interface, possibly due to small surface mobility during growth. Segregation of Ge during growth has also been evidenced by x-ray photoelectron spectroscopy and may explain some of the interface Mixing. The strong composition dependence of Ge diffusion in Si1−xGex alloys causes anisotropie diffusion upon annealing whereby a large composition gradient can be preserved at the heterointerfaces due to the very slow diffusion rate of Ge atoms in pure Si. This effect is important in structures which alternate thin Ge and thick Si layers and is thus more prominent in superlattices grown on Si substrates.

High quality AlGaAs layers grown by molecular beam epitaxy at low temperatures

Low-temperature (∼600 °C) molecular beam epitaxy (MBE) growth of AlGaAs has been studied. It was found that the quality of AlGaAs grown at low temperatures can be as good as that grown at high temperatures (>700 °C) if the source materials and the growth chamber are very clean. The threshold currents of Al0.6Ga0.4As/Al0.15Ga0.85As/Al0.6Ga0.4As double heterostructure (DH) lasers grown at low temperatures and high temperatures are almost the same. The material quality can be further improved with a proper amount of indium doping. Photoluminescence (PL) linewidths of 3.1 meV and 1.7 meV have been measured for Indoped Al0.42Ga0.58As and Al0.18Ga0.82As at 4 K, respectively. They are the narrowest linewidths for the MBE-grown AlGaAs with comparable Al contents at any growth temperature. With a proper amount of In doping, double-barrier resonant tunnelling diodes have also shown improved peak-to-valley current ratios.

Electrical Characterization of Low Temperature GaAs Layers, and Observation of the Extremely Large Carrier Concentrations in Undoped Material

ABSTRACTWe present here the results of a study on the effect of substrate temperature, Ts, on the electrical and physical characteristics of low temperature (LT) molecular beam epitaxy GaAs layers. Hall measurements have been performed on the asgrown samples and on samples annealed at 610 °C and 850 °C. Si implantation into these layers has also been investigated.

Experimental investigation of small He clusters

The question of the existence of a bound state for He2 has been investigated by observing ionized Hen+ clusters produced in a low temperature molecular beam. The results agree with the prediction that there is no bound state for He2. (AIP)