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The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey aims to characterise the physical and chemical evolution of high-mass star-forming clumps. Exploiting the unique broad frequency range and on-the-fly mapping capabilities of the Australia Telescope National Facility Mopra 22 m single-dish telescope1, MALT90 has obtained 3′ × 3′ maps towards ~2 000 dense molecular clumps identified in the ATLASGAL 870 μm Galactic plane survey. The clumps were selected to host the early stages of high-mass star formation and to span the complete range in their evolutionary states (from prestellar, to protostellar, and on to
regions and photodissociation regions). Because MALT90 mapped 16 lines simultaneously with excellent spatial (38 arcsec) and spectral (0.11 km s−1) resolution, the data reveal a wealth of information about the clumps’ morphologies, chemistry, and kinematics. In this paper we outline the survey strategy, observing mode, data reduction procedure, and highlight some early science results. All MALT90 raw and processed data products are available to the community. With its unprecedented large sample of clumps, MALT90 is the largest survey of its type ever conducted and an excellent resource for identifying interesting candidates for high-resolution studies with ALMA.
Submillimeter dust continuum emission traces high molecular column densities and, thus,
dense cloud regions in which new stars are forming. Surveys of the Galactic plane in such
emission have the potential of delivering an unbiased view of high-mass star formation
throughout the Milky Way. The location of the APEX telescope on the Chajnantor plateau in
Chile is ideally suited for mapping the inner Galaxy. ATLASGAL, The APEX Telescope Large
Area Survey of the Galaxy at 870 μm, is a survey of the Galactic plane
using the Large APEX Bolometer Camera (LABOCA), in the Galactic longitude and latitude
ranges of ±60 and ±1.5°, respectively. This survey is providing an unbiased sample of
cold dusty clumps in the Galaxy at submillimeter wavelength and a variety of molecular
line follow-up observations have been started to characterize the physical and chemical
conditions in the newly found clumps. Here, first results from this survey and its
follow-up programs are described.
The heat treatment conditions are a key factor in fabricating zirconolite ceramics and glass-ceramics following high-temperature melting. An oxide mixture melted at 1450°C and subsequently heat-treated at 1200°C yielded a glass-ceramic containing crystallized zirconolite–2M. The silica-enriched residual glass represented about 60-70 vol% of the total; the actinide surrogates (Nd, Ce)were equally distributed between the residual glass and the zirconolite crystals. Zirconolite ceramics obtained after melting an oxide mixture at 1600–1700°C consisted of zirconolite, perovskite and rutile. Rapid cooling rates (> 100°#x00B0;··min-1) were obtained by pouring the melt into ingot molds; the resulting zirconolite ceramics were characterized by crystals of zirconolite-2M ranging from 1 to no more than 20 μm. Slow cooling (< 25°C#x00B0;··min-1 produced ceramics with crystals several hundred micrometers long. Despite the microstructural differences, the chemical durability of the zirconolite ceramics was identical. The initial alteration rates r0 were about two orders of magnitude lower than those measured for the residual aluminosilicate glass of the zirconolite glass-ceramics. Moreover, during long-term leach tests at high S/V ratios to obtain advanced degrees of reaction progress, the alteration rates of all the materials diminished by over 3 to 4 orders of magnitude below r0.
Cantilever-beam measurements of ion-implantation induced stress in (InGa)As/GaAs, Ga(AsP)/GaP, and Ga(AsP)/GaAs strained layer superlattices (SLSs), grown either by molecular-beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), have shown that a mechanism for precipitous stress-relief can be operative, f or room-temperature damage -energy deposition values above - 2 × 10 keV/cm. This phenomenon is correlated with the initial residual compressive stress on the composite structure and is determined by the differences in lattice parameter between the substrate and the buffer alloy-layer.
The physical nature of the InP near-surface defect acceptor and donor states are studied by using photoemission spectroscopy. It is found that the In/n-InP(110) interface band bending does not start until the In coverage reaches about 0.3 monolayer (ML), while the In/p-InP(110) band bending is almost saturated at 0.3 ML. The annealing effect on the band bending of clean cleaved n-and p-type InP(llO) surfaces is also studied. It is found that annealing of the clean surface creates an irreversible band bending effect on the p-InP(110), but the n-InP(110) almost does not show any band bending after low temperature annealing. Based on these two striking differences in the band bending behavior of n- and p-type InP, it is proposed that the physical nature of InP near-surface defect acceptor and donor levels may be different and that phosphorus vacancies are the cause of p-InP surface Fermi level pinning.
High quality, β-SiC (111) monocrystalline films have been epitaxially grown via chemical vapor deposition at 1683K on hexagonal 6H αc-SiC (0001) using a (SiH4 + C2H4)/(H2) gas flow rate ratio of 1:3000. Cross-sectional transmission electron microscopy showed almost no line or planar defects at the film/substrate interface and a low density within the bulk of the film. Furthermore, high resolution transmission electron microscopy revealed a coherent β-SiC/α-SiC interface. Secondary ion mass spectrometry indicated that the diffusion of the Al in the substrate into the as-grown film was negligible. Unintentionally doped films are n-type with the carrier concentrations virtually always in the range of 1016-1017cm3, as determined by capacitance-voltage measurements. A gold-β-SiC Schottky diode having an ideality constant of approximately 1.6 was fabricated on the β-SiC epilayer.
We have employed photoluminescence spectroscopy to determine the interdiffusion of Al and Ga in (Al,Ga)As/GaAs quantum wells. The luminescence due to the n=l electron to heavy hole transition in these wells before and after anneal was measured. A variational calculation was employed to determine the expected position of this luminescence peak both before and after the anneal. A single diffusion coefficient, D, was used to model the interdiffusion of the Al and Ga and from the shifted position of the luminescence peaks under various anneal conditions of time and temperature its value was determined. These measurements were performed as a function of temperature to yield ΔE where D = D0e -ΔE/KT. This diffusion coefficient was also studied as a function of the initial Al composition in the cladding layers which ranged from 0.3 to 1.0.
The lifetime of photogenerated electrons in a-Ge:H/a-Si:H multilayer structures with layers about 100Å thick, is enhanced by two orders of magnitude above the value in bulk a-Ge:H. This lifetime enhancement effect is explained by charge separation produced by layers, due to the assymmetry in the conduction and valence band offsets at the interfaces. The peak in the electron lifetime as a function of superlattice periodicity is determined by a trade-off between the electron tunneling rate into the a-Si:H barriers and electrostatic repulsion of holes due to photo-induced space charge.
We report on measurements of ultrafast relaxation processes in transmission and reflection in amorphous multilayer structures consisting of a-Si:H, a-SiNx:H, a-SiOx:H, and a-Ge:H. The decays recorded in transmission in the a-Si:H/a-SiNx:H and a-Si:H/a-SiOx:H multilayers depend strongly on the silicon sublayer thickness and are interpreted in terms of carrier transport to and trapping at interfacial defects. In the a-Si:H/a-Ge:H multilayers we observe oscillations in reflectivity due to standing acoustic waves with a frequency that depends on the repeat distance of the multilayer.
Abrupt, epitaxial silicide/silicon heterostructures may be grown so that, for the first time, the physics of electron transport across near perfect, single crystal, metal/semiconductor interfaces may be probed experimentally. Transport measurements through type-A and -B oriented NiSi2 layers on Si(111) substrates have revealed Schottky barrier heights differing by 140 meV. In this paper we present results of experiments designed to explore the possible role of bulk and interface defects in determining the potential barrier at these near ideal epitaxial metal-semiconductor contacts. We have found little evidence for the presence of defects and the Schottky barrier is insensitive to details of the microscopic interfacial perfection. By contrast we find that both the electrical quality and magnitude of the barrier occurring at the NiSi2 /Si(100) heterojunction are dependent upon details of the microscopic interfacial perfection.
Comparative thin film adhesion studies were performed on GaAs substrates using Au or Au-Ni-Ge, metallization materials. The influence of parameters such as crystal orientation, substrate surface preparation, deposition vacuum conditions, thickness and composition of films, and post-evaporative treatment such as ion-beam mixing and thermal annealing, on film adhesion was considered. The quality of the adhesion bond was measured using Scotch tape tests and a Sebastian adhesion tester. Film interfaces were characterized using AES, XPS, and RBS techniques. The results indicate that the most important factors dominating the quality of adhesion were surface preparation and the deposition vacuum conditions. Films deposited under optimum conditions were found to adhere so well that the GaAs crystal would fracture before the film would pull free of the substrate. The influence of ion beam mixing on the quality of adhesion was tested and only in the cases of depositions under the less optimum conditions, where the adhesion was poor, could an improvement be made in the adhesive properties.
Effects of the doping level of indium impurity on electrical and optical properties of the CdS thin films, prepared with solution-sprayed method have been studied. The density of free carriers can be raised to an order of magnitude higher than that of non-doped case by increasing doping level of indium up to 0.01mole-% while indium doping does not significantly change the mobility from its value in non-doped samples. The optical energy gap is decreased with increasing indium content while the thermal activation energy remains unchanged. The experimental results are discussed in terms of the self compensation and radiative recombination phenomena.
The use of hexachlorodisilane (Si2Cl6) as an alternative to silane for growth of polycrystalline silicon films has been investigated. Films were grown at atmospheric pressure in both hydrogen and nitrogen carrier gases over a temperature range of 450–900°C. Deposition rate data indicate the existence of two growth regimes at high and low temperatures and in the presence or absence of hydrogen. The change from amorphous to polycrystalline growth takes place at 600–650°C. At 600°C deposited films are amorphous but crystallize during the growth process. The chlorine content of high-temperature films was found to be less than 0.01 at.%.
Self-interstitial transport kinetics in float-zone and Czochralski silicon was studied during thermal oxidation of silicon membranes. Bulk recombination of interstitials is higher in the CZ than in the FZ silicon. The low apparent interstitial diffusivity obtained in this study is explained by a bulk effect.
The perfection of epitaxial nickel films grown on the (00.ℓ) or basal plane of heated sapphire (A1203) single crystals were studied with X-ray diffraction techniques. Nickel films approximately 700 Å thick formed by vapor deposition increased in perfection as the temperature of the sapphire approached 1400°C. Although the nickel atom distances are 10.3% smaller than those of the closed-packed direction in sapphire, the strain was accommodated at the interface rather than being distributed through the thickness of the nickel film. Diffuse rods of X-ray scattering which are associated with diffraction from the interface gave information about the nature of the roughness at the interface.
The measurement of Shubnikov-de Haas(SdH) oscillation is proposed as a new technique for evaluating the quality of a heterointerface. The first excited state of 2-dimensional electron energy levels is determined for several samples using the measurements of SdH oscillation. Lower values of the first excited state energy are found for the samples with a low mobility. The low value can be approximately explained in terms of graded interface model.
The intermixing of AlAs/GaAs superlattices has been investigated as a function of Si concentration following anneals in the range of 500 to 900 C. The superlattice samples were grown by molecular beam epitaxy(MBE) and the near surface layers were doped with silicon at concentrations of 2×10 to 5×1018 cm-3. Si and Al depth profiles were measured with secondary ion mass spectrometry (SIMS).The diffusion length and activation energy of Al as a function of silicon dopant concentration are derived from the SIMS data. In the temperature range studied an activation energy for the Al interdiffusion of -4eV is observed with the diffusion coefficients increasing rapidly with Si concentration.
The reaction of monolayers of (metal) absorbâtes with clean (semiconductor) surfaces has long been studied by surface scientists using spectroscopie probes.  Recently it has been demonstrated that the knowledge acquired from these studies has led to new techniques for the fabrication of novel epitaxial structures which are not possible by any other means. The orientation of NiSi2, as well as the layer uniformity, was found to be sensitive to slight variations in growth parameters. This discovery has led to the fabrication of single crystal NiSi2 layers of either type A or type B orientation on Si(111). Type A NiSL has the same orientation as the silicon substrate. Type B NiSi2 shares the surface normal <111> axis with the Si, but is rotated 180δ about this axis with respect to the Si. The variation of NiSi2 orientation depends primarily on the deposited nickel thickness and is among the most intriguing phonomena in heteroepitaxial growth. Its significance became even more appreciated when it was discovered that the Schottky barrier height for type A NiSi2 differed significantly from that for type B NiSi2. These results and the prospect of using thin epitaxial suicides in high speed devices have recently attracted a lot of attention as well as triggered investigations of other properties of this material system. Those who followed closely the experimental conditions as described in the original paper have had no trouble in obtaining high quality suicide layers of pure orientation. At the same time, many research groups have used slightly different conditions and, as a result, have not been able to grow single-orientation NiSi2 layers.
We have employed a nozzle jet expansion technique to deposit Al thin film on chemically cleaned Si(n) surface. Pure Al is evaporated in a graphite crucible at a temperature of 15 50°C and is then ejected through a small nozzle into a vacuum region of 10-6 Torr. The Schottky barrier height of the as-deposited films is measured (using the J-V technique) to be 0.77eV, which is substantially higher than that obtained by conventional evaporation-deposition techniques(≤0.68eV). Our result suggests that an intimate Al/Si(n) contact has been formed during the jet expansion deposition of Al films.
During the deposition, the Al jet beam can be partially ionized by electron bombardment. It is shown that the Schottky barrier height remain unchanged if a bias potential of V s0.5KeV is applied to the substrate during deposition. For Va >0.5 KeV, the diode became leaky and the barrier height was reduced. The energetic of the jet beam, with and without post ionization and acceleration, is discussed with respect to thin film and interface formation.
Fe-doped semi-insul ati ng layers of InP, In0.48A10.52As,and In0.49Ga0.51P have been grown by liquid phase epitaxy for the first time. Behaviors of Fe doping in these materials have been well explained by the solubility of Fe in the growth solution and the temperature dependent distribution coefficients of Fe. It has been found that the distribution coefficients of Fe in the ternary alloys are much greater than those in InP.