Lee E. Rumaner

Vacuum sublimation of GaSe : a molecular source for deposition of GaSe

Abstract The layered structure of GaSe makes it suitable for the technique of Van der Waals epitaxy, a molecular beam deposition technique where heterostructures are fabricated based on a property of interest and are not limited by lattice and thermal expansion mismatch. Single crystal GaSe has been grown on GaAs(111)A by vacuum sublimation from a single GaSe source without terminating the GaAs surface dangling bonds prior to deposition. Mass spectroscopy was used to determine the composition of the molecular source. It was found that GaSe sublimates stoichiometrically to form Ga2Se and Se2. This results in free selenium (Se2) in the molecular beam which can terminate the dangling bonds at the GaAs surface prior to bulk GaSe growth. X-ray photoelectron spectroscopy and reflection high energy electron diffraction studies of the deposition confirm this termination process and the structure of subsequent GaSe growth.

Nucleation and growth of GaSe on GaAs by Van der Waal epitaxy

Nucleation and growth of GaSe on GaAs(1 1 1)B substrate was studied to develop a better understanding of the Van der Waals epitaxy (VDWE) process. A complementary combination of RHEED, XPS, AFM and HREM were used. The initial GaAs(1 1 1)B surface reacts with incoming flux, forming a compound similar to GaSe at the surface. After one monolayer coverage, further growth of GaSe is on GaSe; the system changes from a heteroepitaxial to homoepitaxial system. Nucleation in the VDWE process is characterized by the formation of small oriented clusters with the weak Van der Waals bond dictating the orientation relationship. Further growth is found to be limited by surface kinetics, with a large diffusion barrier at the growing step edge. This barrier leads to islanding with the island shape dictated by the substrate temperature. A modified deposition procedure was developed to obtain uniform layer-by-layer growth of well-aligned flat GaSe domains without islanding.

The role of oxygen and zirconium in the formation and growth of Nb3sn grains

One method of producing Nb3Sn is to react a molten tin alloy with a solid niobium alloy. Using this process, the addition of zirconium and oxygen to the niobium foil has been found to dramatically reduce the Nb3Sn grain size and affect the Nb3Sn superconducting critical current properties. Nb3Sn grains grow semicoherently on the niobium alloy foil. The initial grain size is about 50 nm. These initial Nb3Sn grains coarsen rapidly to become equiaxed grains about 0.2 μm in diameter. The equiaxed Nb3Sn grains away from the Nb/Nb3Sn interface are completely surrounded by a tin alloy phase that would have been liquid at the reaction temperature. Based on transmission electron microscopy observation and electrical property characterization, it is concluded that ZrO2 clusters, less than 10 Å in size, form in the niobium alloy foil during processing. These clusters combine at the Nb/Nb3Sn interface to form ZrO2 precipitates. The ZrO2 precipitates are found in all of the Nb3Sn grains that have formed from a reaction between the liquid tin and the solid niobium at the Nb/Nb3Sn interface. The precipitates are coherent with their host Nb3Sn grains. During Nb3Sn grain growth, the ZrO2 precipitates dissolve in shrinking grains and reprecipitate in growing grains, as the migrating grain boundary intersects the precipitate. This dissolution/reprecipitation process slows the growth of Nb3Sn grains.

Microstructure evolution of GaSe thin films grown on GaAs(100) by molecular beam epitaxy

GaSe thin films were grown on a GaAs(100) substrate by molecular beam epitaxy. Microstructures of the thin films and interface were characterized by transmission electron microscopy. The dominant polytype formed in the GaSe thin films was a γ type, which has a 3R-rhombohedral structure with R3m space group. Predominant crystallographic orientation between the GaSe thin films and the GaAs substrate was characterized as: [1100]GaSe‖[011]GaAs/(0001)GaSe‖(100)GaAs. In addition, GaSe thin films with orientation of [1210]GaSe‖[011]GaAs/(0001)GaSe‖(100)GaAs can also grow in some local areas. The interface between GaSe thin films and GaAs substrate constitutes thin intermediate layers of a vacancy ordered β-Ga2Se3, the structure of which inherits the crystallographic features of the GaAs(100) surface. Mechanisms responsible for formation of the preferable crystallographic orientation in the GaSe thin films in the initial growth stage are suggested.

Interaction of GaSe with GaAs(111): Formation of heterostructures with large lattice mismatch

We have studied the epitaxial growth of GaSe, a layered van der Waals material, on GaAs, a zinc-blende-structure semiconductor. This heterostructure exhibits a 6% lattice mismatch, and is a prototypical example of van der Waals epitaxy, where the weak van der Waals interaction allows the misfit to be accommodated without the formation of electronically active defects. GaSe was supplied to the growing surface from a single GaSe Knudsen cell. Reflection high energy electron diffraction and x-ray photoemission spectroscopy studies of the nucleation of GaSe indicate Se reacts with the GaAs surface to remove the surface dangling bonds prior to GaSe formation. This is followed by the oriented growth of stoichiometric GaSe layers, that are rotationally aligned with the underlying GaAs substrate. The termination of the GaAs dangling bonds most likely occurs by Se substitution for As in the surface layer of GaAs(111) B and by direct bonding of Se to surface Ga on GaAs(111) A surfaces. In addition, photoemission me...

Interface Structure, Grain Morphology, and Kinetics of Growth of the Superconducting Intermetallic Compound Nb 3 Sn Doped with ZrO 2 and Copper

One method for the fabrication of the superconducting compound Nb 3 Sn involves interdiffusion of a surface coating of Sn alloyed with Cu on Nb containing Zr and O. In this study, the kinetics and microstructure associated with this reaction have been studied in detail. The results show that small Nb3Sn grains nucleate at the Nb 3 Sn/Nb interface, and that the Nb 3 Sn grains experience grain growth immediately after they are formed. ZrO 2 precipitates are observed in the Nb 3 Sn at the Nb 3 Sn/Nb interface and throughout the Nb 3 Sn. The ZrO 2 precipitates occur in the form of small partially-coherent spheres in the Nb 3 Sn. No ZrO 2 precipitates are observed by TEM in the unreacted Nb. The grain boundaries in the Nb 3 Sn region are coated with a Sn-Nb-Cu alloy which would have been liquid at the diffusion/reaction temperature. The thickness of the Nb 3 Sn reaction layer formed during the isothermal diffusion anneal is proportional to time to the first power, indicating “reaction”-controlled rather than “diffusion”-controlled kinetics. The absence of diffusion-controlled kinetics can be explained by the presence of the liquid coating on the Nb 3 Sn grains. Diffusion of Sn in this liquid layer is apparently fast enough to not be the limiting kinetic step.