Homogeneous AlGaN/GaN superlattices grown on free-standing (1(1)over-bar00) GaN substrates by plasma-assisted molecular beam epitaxy
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77 NANOSCIENCE AND NANOTECHNOLOGY↗
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Photoluminescence lineshape and dynamics of localized excitonic transitions in InAsP epitaxial layers
The excitonic radiative transitions of InAs{sub x}P{sub 1−x} (x = 0.13 and x = 0.40) alloy epitaxial layers were studied through magnetic field and temperature dependent photoluminescence and time-resolved photoluminescence spectroscopy. While the linewidth and lineshape of the exciton transition for x = 0.40 indicate the presence of alloy broadening due to random anion distribution and the existence of localized exciton states, those of x = 0.13 suggest that this type of compositional disorder is absent in x = 0.13. This localization is further supported by the behavior of the exciton transitions at low temperature and high magnetic fields. InAs{sub 0.4}P{sub 0.6} exhibits anomalous “S-shaped” temperature dependence of the excition emission peak below 100 K as well as linewidth broadening at high magnetic fields due to the compression of the excitonic volume amid compositional fluctuations. Finally, photoluminescence decay patterns suggest that the excitons radiatively relax through two channels, a fast and a slow decay. While the lifetime of the fast decay is comparable for both compositions (∼30 ps), that of the slow decay increases from 206 ps to 427 ps as x increases from 0.13 to 0.40, attributable to carrier migration between the localization states of InAs{sub 0.4}P{sub 0.6}.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
AlGaN composition dependence of the band offsets for epitaxial Gd{sub 2}O{sub 3}/Al{sub x}Ga{sub 1−x}N (0 ≤ x ≤ 0.67) heterostructures
Gd{sub 2}O{sub 3} films were prepared on (0001)-oriented Al{sub x}Ga{sub 1−x}N (0 ≤ x ≤ 0.67) thin film substrates via reactive molecular-beam epitaxy. X-ray diffraction revealed that these films possessed the cubic bixbyite structure regardless of substrate composition and were all 111-oriented with in-plane rotations to account for the symmetry difference between the oxide film and nitride epilayer. Valence band offsets were characterized by X-ray photoelectron spectroscopy and were determined to be 0.41 ± 0.02 eV, 0.17 ± 0.02 eV, and 0.06 ± 0.03 eV at the Gd{sub 2}O{sub 3}/Al{sub x}Ga{sub 1−x}N interfaces for x = 0, 0.28, and 0.67, respectively.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Perspective: Extremely fine tuning of doping enabled by combinatorial molecular-beam epitaxy
Chemical doping provides an effective method to control the electric properties of complex oxides. However, the state-of-art accuracy in controlling doping is limited to about 1%. This hampers elucidation of the precise doping dependences of physical properties and phenomena of interest, such as quantum phase transitions. Using the combinatorial molecular beam epitaxy, we improve the accuracy in tuning the doping level by two orders of magnitude. We illustrate this novel method by two examples: a systematic investigation of the doping dependence of interface superconductivity, and a study of the competing ground states in the vicinity of the insulator-to-superconductor transition.
36 MATERIALS SCIENCE↗
Structural properties of Bi{sub 2−x}Mn{sub x}Se{sub 3} thin films grown via molecular beam epitaxy
The effects of Mn doping on the structural properties of the topological insulator Bi{sub 2}Se{sub 3} in thin film form were studied in samples grown via molecular beam epitaxy. Extended x-ray absorption fine structure measurements, supported by density functional theory calculations, indicate that preferential incorporation occurs substitutionally in Bi sites across the entire film volume. This finding is consistent with x-ray diffraction measurements which show that the out of plane lattice constant expands while the in plane lattice constant contracts as the Mn concentration is increased. X-ray photoelectron spectroscopy indicates that the Mn valency is 2+ and that the Mn bonding is similar to that in MnSe. The expansion along the out of plane direction is most likely due to weakening of the Van der Waals interactions between adjacent Se planes. Transport measurements are consistent with this Mn{sup 2+} substitution of Bi sites if additional structural defects induced by this substitution are taken into account.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Direct growth of graphene on in situ epitaxial hexagonal boron nitride flakes by plasma-assisted molecular beam epitaxy
Hexagonal boron nitride (h-BN) single-crystal domains were grown on cobalt (Co) substrates at a substrate temperature of 850–900 °C using plasma-assisted molecular beam epitaxy. Three-point star shape h-BN domains were observed by scanning electron microscopy, and confirmed by Raman and X-ray photoelectron spectroscopy. The h-BN on Co template was used for in situ growth of multilayer graphene, leading to an h-BN/graphene heterostructure. Carbon atoms preferentially nucleate on Co substrate and edges of h-BN and then grow laterally to form continuous graphene. Further introduction of carbon atoms results in layer-by-layer growth of graphene on graphene and lateral growth of graphene on h-BN until it may cover entire h-BN flakes.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
A study on the epitaxial Bi{sub 2}Se{sub 3} thin film grown by vapor phase epitaxy
We report the growth of high quality Bi{sub 2}Se{sub 3} thin films on Al{sub 2}O{sub 3} substrates by using chemical vapor deposition. From the atomic force microscope, x-ray diffraction and transmission electron microscope measurements we found that the films are of good crystalline quality, have two distinct domains and can be grown epitaxially on the Al{sub 2}O{sub 3} substrate. Carrier concentration in the sample is found to be 1.1 × 10{sup 19} cm{sup −3} between T = 2 K to T = 300 K, and electron mobility can reach 954 cm{sup 2}/V s at T = 2 K. Weak anti-localization effect is observed in the low temperature magneto-transport measurement for the sample which indicates that the thin film has topological surface state.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Semi-adsorption-controlled growth window for half-Heusler FeVSb epitaxial films
The electronic, magnetic, thermoelectric, and topological properties of Heusler compounds (composition XYZ or X 2 YZ) are highly sensitive to stoichiometry and defects. Here we establish the existence and experimentally map the bounds of a semi-adsorption-controlled growth window for semiconducting half-Heusler FeVSb films, grown by molecular beam epitaxy (MBE). We show that due to the high volatility of Sb, the Sb stoichiometry is self-limiting for a finite range of growth temperatures and Sb fluxes, similar to the growth of III-V semiconductors such as GaSb and GaAs. Films grown within this window are nearly structurally indistinguishable by x-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED). Here, the highest electron mobility and lowest background carrier density are obtained towards the Sb-rich bound of the window, suggesting that Sb vacancies may be a common defect. Similar semi-adsorption-controlled bounds are expected for other ternary intermetallics that contain a volatile species Z = {Sb, As, Bi}, e.g., CoTiSb, LuPtSb, GdPtBi, and NiMnSb. However, outstanding challenges remain in controlling the remaining Fe/V (X/Y) transition metal stoichiometry.
36 MATERIALS SCIENCE↗
Photoelectrochemical etching of epitaxial InGaN thin films: self-limited kinetics and nanostructuring
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solar (photovoltaic), solid state lighting, phonon↗
Low temperature epitaxial growth of Ge on Ca F2 buffered cube-textured Ni
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Gaire, C↗
Surface Morphology and Transport Studies of Epitaxial Graphene on SiC(000-1)
Annealing conditions are critical to the properties of epitaxial graphene formed by thermal decomposition of silicon carbide. Here, we report the evolution of coherent electronic transport with increasing anneal temperatures, combined with low energy electron micrographs of equivalent surfaces showing corresponding structural coherence. Ultrahigh vacuum conditions and temperatures in the range of 1250-1300 C produce granular films with a lateral grain size of {approx}20 nm, while temperatures of 1400 C or higher result in grains with progressively larger lateral dimensions in the micron range. Transport measurements show how the electronic coherence length increases as a result of the more coherent physical structure, with a crossover from two-dimensional variable range hopping to the weak localization regime. Here, we show that while the duration of the anneal affects coverage and clustering of grains, the size of individual grains is determined by anneal temperature, with evidence of coalescence of smaller grains into larger domains, suggesting that multistage anneals at different temperatures may yield high-quality graphene.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗