Extruded Bismuth-Telluride-Based <Emphasis Type="Italic">n</Emphasis>-Type Alloys for Waste Heat Thermoelectric Recovery Applications

Thermoelectric (TE) generator modules for a number of waste heat recovery applications are required to operate between room temperature and 500 K, a temperature range for which the composition of bismuth-telluride-based alloys needs to be adjusted to optimize performance. In particular n-type alloys do not perform as well as p-type and require a more systematic study. We have produced, by mechanical alloying followed by hot extrusion, alloys, within the range with fixed carrier concentration () to optimize their TE performance in the temperature range 300 K to 420 K. The optimum composition has been identified to be and which is very close to the composition that also maximizes the ratio of the electron mobility to the lattice component of the thermal conductivity. The optimized alloy performance can be further increased by adjusting the carrier concentration.     

Creep Life Assessment of Tin-Based Lead-Free Solders Based on Imaginary Initial Strain Rate

The creep properties of tin-based, lead-free solders, Sn-3.0Ag-0.5Cu and Sn-7.5Zn-3.0Bi, were investigated for the temperature range from 298 K to 398 K. The creep rupture time decreases with increasing initial stress and temperature. The Omega method is applied to the analysis of the solder creep curves. The creep rate is expressed by the following formula: , where and Ω are experimentally determined. The parameter , the imaginary initial strain rate, increases with increasing initial stress and temperature. The parameter Ω is temperature dependent, but less dependent on the initial stress. The apparent activation energy for is 108 kJ/mol in Sn-3.0Ag-0.5Cu and 83 kJ/mol in Sn-7.5Zn-3.0Bi. These values are close to the activation energy for the lattice diffusion of tin. The creep rupture time is calculated using the parameters and Ω. The calculated creep rupture time is in good agreement with the measured creep rupture time.     

Planar defects and double-domain epitaxy in epitaxial YSi<Subscript>2−x</Subscript> and ErSi<Subscript>2−x</Subscript> thin films on Si substrates

Interfacial reactions of Y and Er thin films on both (111)Si and (001)Si have been studied by transmission electron microscopy (TEM). Epitaxial rare-earth (RE) silicide films were grown on (111)Si. Planar defects, identified to be stacking faults on planes with 1/6 displacement vectors, were formed as a result of the coalescence of epitaxial silicide islands. Double-domain epitaxy was found to form in RE silicides on (001)Si samples resulting from a large lattice mismatch along one direction and symmetry conditions at the silicide/(001)Si interfaces. The orientation relationships are [0001]RESi_2−x// Si, RESi_2−x//(001)Si and [0001]RESi_2−x/ Si, RESi_2−x//(001)Si. The density of staking faults in (111) samples and the domain size in (001) samples were found to decrease and increase with annealing temperature, respectively.     

Nanotrenches Induced by Catalyst Particles on ZnSe Surfaces

Nanotrenches are induced by thermal annealing Au droplets on ZnSe surfaces. High-resolution scanning electron microscopy studies of the nanotrench structures reveal that the preferred migration directions of the catalyst droplets are along the direction family. On a ZnSe(111)B surface, each of the trenches is along one of the six directions while on a nonvicinal ZnSe(100) surface, the trenches are along a pair of antiparallel directions. Based on the results obtained from atomic force microscopy surface profiling and electron energy-loss spectroscopy chemical analysis techniques, a model is proposed to describe the possible formation mechanisms of the␣observed nanotrenches. The highly parallel nanotrenches induced on the Au/ZnSe(100) structure as revealed in this study are potentially useful as a template for in situ fabrication of ordered one-dimensional nanostructures (such as nanowires) of many materials.     

How r-Plane Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Surface Modifications Impact the Growth of Epitaxial (001) CeO<Subscript>2</Subscript> Thin Films

We present evidence that it is the presence or absence of atomic terraces with a specific crystallographic orientation on the (102) Al₂O₃ surface that promotes growth of single-crystal (001) CeO₂ films over polycrystalline (111) CeO₂ films. The CeO₂ film nucleates so that the [010] and [100] directions of the film align parallel and perpendicular to the terrace edges. In the absence of terraces, multidomain (111) CeO₂ films result in which the in-plane orientation of the two domains are rotated by 85.71°, so that a [110] CeO₂ direction aligns parallel to either the or Al₂O₃ direction.     

Pre-avalanche Ultraviolet Photoconduction Properties of Transitional-Metal-Doped ZnO Nanowires

The ultraviolet (UV) photoelectric characteristics of transitional metal (Cu) doped ZnO nanowires produced by the self-catalytic vapor–liquid–solid (VLS) method were investigated by performing a series of photoconduction and time-resolved measurements. The photocurrent voltage characteristics obtained on the nanowires configured as two-terminal metal–semiconductor–metal photodetectors exhibited a nonmonotonic behavior attributed to the interplay of several limiting mechanisms: Schottky contacts and trapping/detrapping effects that take place at low and intermediate (pre-avalanche) bias regimes, respectively. In the intermediate biases, the photocurrent was power-law dependent, i.e., changed with voltage as and for excitation wavelengths of 365 nm, 302 nm, and 254 nm, respectively. The dependence of the exponent on the wavelength of the light is analyzed and explained based on the detailed consideration of the contribution of different deep-defect Cu levels formed within the band gap of ZnO. The study will be important to those working in the area of ZnO-based nanophotodetectors, optical switches, and sensors.     

Fabrication of Mesa Shaped InGaN-Based Light-Emitting Diodes Through a Photoelectrochemical Process

InGaN-based light-emitting diodes (LEDs) were fabricated to have a higher light extraction through the photoelectrochemical (PEC) mesa shaping process. After the PEC selective wet oxidation and wet etching processes, stable and controllable crystallographic etching planes were formed as p-type GaN {} planes and n-type GaN {} planes included at an angle of 27 deg. The ever-present cone-shaped structure of a PEC-treated LED has a larger light scattering area and higher light extraction cones on the mesa sidewall, as analyzed by microphotoluminescence and light output power measurement. This cone-shaped-sidewall LED has a higher light output power and a larger divergence angle compared with a conventional LED measured in an LED chip form.     

Discommensuration of Doped [Ca<Subscript>2</Subscript>CoO<Subscript>3</Subscript>]<Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>CoO<Subscript>2</Subscript>

Single-crystal x-ray diffraction and high-resolution electron microscopy studies were carried out for Co-121 ([Ca₂CoO₃] _p CoO₂) and Sr-doped Co-121 grown in a KCl flux at 810°C. Typically, the samples were 2 mm × 2 mm × 0.02 mm in size. The single-crystal diffraction intensities were measured by the use of a four-circle diffractometer. Twinned super reflections, e.g., and were observed in the electron diffraction patterns. These super reflections were not observed in the end-member. Diffuse scattering was observed in the same reciprocal space by the single-crystal x-ray diffraction study. A discommensurate crystal model is proposed for the Sr-doped system.     

Surface Morphology and Defect Formation Mechanisms for HgCdTe (211)B Grown by Molecular Beam Epitaxy

The surface morphology and crystallinity of HgCdTe films grown by molecular beam epitaxy (MBE) on both CdZnTe and CdTe/Si (211)B substrates were characterized using atomic force microscopy (AFM), as well as scanning (SEM) and transmission (TEM) electron microscopy. Crosshatch patterns and sandy-beach-like morphologies were commonly found on MBE (211) HgCdTe epilayers grown on both CdZnTe and CdTe/Si substrates. The patterns were oriented along the , , and directions, which were associated with the intersection between the (211) growth plane and each of the eight equivalent HgCdTe slip planes. This was caused by strain-driven operation of slip in these systems with relative large Schmid factor, and was accompanied by dislocation formation as well as surface strain relief. Surface crater defects were associated with relatively high growth temperature and/or low Hg flux, whereas microtwins were associated with relatively low growth temperature and/or high Hg flux. AFM and electron microscopy were used to reveal the formation mechanisms of these defects. HgCdTe/HgCdTe superlattices with layer composition differences of less than 2% were grown by MBE on CdZnTe substrates in order to clarify the formation mechanisms of void defects. The micrographs directly revealed the spiral nature of growth, hence demonstrating that the formation of void defects could be associated with the Burton, Cabrera, and Frank (BCF) growth mode. Void defects, including microvoids and craters, were caused by screw defect clusters, which could be triggered by Te precipitates, impurities, dust, other contamination or flakes. Needle defects originated from screw defect clusters linearly aligned along the directions with opposite Burgers vector directions. They were visible in HgCdTe epilayers grown on interfacial superlattices. Hillocks were generated owing to twin growth of void or needle defects on (111) planes due to low growth temperature and the corresponding insufficient Hg movement on the growth surface. Therefore, in addition to nucleation and growth of HgCdTe in the normal two-dimensional layer growth mode, the BCF growth mode played an important role and should be taken into account during investigation of HgCdTe MBE growth mechanisms.     

The shape of self-assembled InAs islands grown by molecular beam epitaxy

We have determined the shape of InAs quantum dots using reflection high energy electron diffraction. Our results indicate that self-assembled InAs islands possess a pyramidal shape with {136} bounding facets. This shape is characterized by C_2v symmetry and a parallelogram base, which is elongated along the direction. Cross-sectional transmission electron microscopy images taken along the [110] and directions as well as atomic force microscopy images strongly support the {136} shape. Furthermore, polarization-resolved photoluminescence spectra show strong in-plane anisotropy, with emission predominantly polarized along the direction, consistent with the proposed quantum dot shape.     

Predicting and Understanding Order of Heteroepitaxial Quantum Dots

Heteroepitaxial self-assembled quantum dots (SAQDs) will allow breakthroughs in electronics and optoelectronics. SAQDs are a result of Stranski–Krastanow growth, whereby a growing planar film becomes unstable after an initial wetting layer is formed. Common systems are and For applications, SAQD arrays need to be ordered. The roles of crystal anisotropy, random initial conditions and thermal fluctuations in influencing SAQD order during early stages of SAQD formation are studied through a simple stochastic model of surface diffusion. Surface diffusion is analyzed through a linear and perturbatively non-linear analysis. The role of crystal anisotropy in enhancing SAQD order is elucidated. It is also found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical wetting surface height that marks the onset of non-planar film growth.     

Polytype Stability and Microstructural Characterization of Silicon Carbide Epitaxial Films Grown on [ {\hbox{11}}\overline{{\hbox{2}}} {\hbox{0}} ]- and [0001]-Oriented Silicon Carbide Substrates

The polytype and surface and defect microstructure of epitaxial layers grown on 4H(), 4H(0001) on-axis, 4H(0001) 8° off-axis, and 6H(0001) on-axis substrates have been investigated. High-resolution x-ray diffraction (XRD) revealed the epitaxial layers on 4H() and 4H(0001) 8° off-axis to have the 4H-SiC (silicon carbide) polytype, while the 3C-SiC polytype was identified for epitaxial layers on 4H(0001) and 6H(0001) on-axis substrates. Cathodoluminescence (CL), Raman spectroscopy, and transmission electron microscopy (TEM) confirmed these results. The epitaxial surface of 4H() films was specular with a roughness of 0.16-nm root-mean-square (RMS), in contrast to the surfaces of the other epitaxial layer-substrate orientations, which contained curvilinear boundaries, growth pits (∼3 × 10⁴ cm⁻²), triangular defects >100 μm, and significant step bunching. Molten KOH etching revealed large defect densities within 4H() films that decreased with film thickness to ∼10⁶ cm⁻² at 2.5 μm, while cross-sectional TEM studies showed areas free of defects and an indistinguishable film-substrate interface for 4H() epitaxial layers.     

Structural Analysis of CdTe Hetero-epitaxy on (211) Si

X-ray diffraction full-width at half-maximum (XRD FWHM), reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM) indicate a mosaic structure for molecular-beam epitaxy (MBE) (211)B CdTe/Si. AFM measurements indicate long, thin, small-angle-disoriented grains for CdTe/Si epilayers. These disoriented grains are ~1 μm in the [] direction and are ~40 nm in the [] direction. The RHEED pattern in the [] direction depicts nearly ideal single-crystal periodicity. The RHEED pattern in the [] direction is indicative of small-angle-disoriented crystalline grains. Scanning electron microscopy (SEM), AFM, and XRD measurements all indicate an approximate factor of 10 increase in the Everson etch pit density (EPD) over standard Nomarski microscopy Everson EPD determination.     

Growth and Characterization of Pulsed-Laser-Deposited Ilmenite–Hematite Thin Films

The ilmenite–hematite (1 − x) FeTiO₃ · xFe₂O₃ solid solution system is considered to be a novel material for spin-electronics, microelectronics, high-temperature electronics, and radhard electronics. This paper focuses on thin films of composition x = 0.33 grown on (100) MgO single-crystal substrates using pulsed-laser deposition (PLD) under different argon–oxygen mixtures. The surface of the MgO was found to possess MgO₂ crystals, yielding an orientation relationship, [001] MgO ∥ [011] MgO₂ and (00)MgO ∥ (10) MgO₂. The structural characterizations show that the films are crystalline and homogeneous without any secondary phase. The films show a weak and inclined (110) growth epitaxy. A bandgap of 3.4–3.7 eV was obtained for these films from optical measurements carried out in the UV–visible region. Electrical measurements confirmed the semiconducting behavior. However, the resistivity was found to increase substantially on the slightest addition of oxygen into the chamber.     

A kinetic study of structured surface relief patterning of GaP (\bar 1\bar 1\bar 1)

Orientation dependent etching of photolithographically patterned GaP was investigated using solutions of HCl:CH₃COOH:H₂O₂. The pattern was prepared using standard ultraviolet lithography and was a two-dimensional grid with an 18 μm repeat, consisting of 15 μm squares separated by 3 μm spaces. The mask sides were aligned along the and directions. Under appropriate etching conditions, high quality arrays of pyramids were prepared. These pyramids were defined by , and facets. It was shown that the etching process depended on the degree of solution aging after initial mixing. For a freshly prepared solution, the etching rate showed an inverse dependence on time. For short etching times (below 5 min), an intermediate etching profile was followed, while for long times (greater than 5 min) etching was kinetically controlled. We demonstrated that controlled etching at extremely low rates (0.1–0.5 μm/min) is feasible with this new approach.     

Characterization of Epitaxial Indium Nitride Interlayers for Ohmic Contacts to Silicon Carbide

Ohmic contacts to n-type 4H- and 6H-SiC without postdeposition annealing were achieved using an interlayer of epitaxial InN beneath a layer of Ti. The InN films were grown by reactive dc magnetron sputtering at 450°C, whereas the Ti films were deposited by electron-beam evaporation at room temperature. The InN films were characterized by x-ray diffraction (XRD), secondary electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM), and Hall-effect measurements. Both XRD and TEM observations revealed that the Ti and InN films have epitaxial relationships with the 6H-SiC substrate as follows: (0001)[]_Ti∥(0001)[]_InN∥(0001)[]_6H-SiC. The Ti/InN/SiC contacts displayed ohmic behavior, whereas Ti/SiC contacts (without an InN interlayer) were nonohmic. These results suggest that InN reduces the Schottky barrier height at the SiC surface via a small conduction-band offset and support previous reports of an electron accumulation layer at the surface of InN.     

Effect of substrate orientation on phase separation in epitaxial GaInAsSb

The effect of substrate misorientation on phase separation in Ga_1−xIn_xAs_ySb_1−y nominally lattice matched to GaSb is reported. The layers were grown at 575°C by organometallic vapor phase epitaxy on vicinal (001) GaSb substrates, miscut or (101). Ga_1−xIn_xAs_ySb_1−y (x ~ 0.1, ~ 0.09) layers, which have 300-K photoluminescence (PL) peak emission at ~2.1 μm, grow stepbunched and exhibit minimal phase separation. The full width at half maximum of 4-K PL spectra is slightly smaller at 7 meV for layers grown on substrates miscut toward compared to 9 meV for layers grown on substrates miscut toward and (101). Ga_1−xIn_xAs_ySb_1−y layers with higher alloy composition (0.16≤x≤0.19, 0.14≤y≤0.17), which have 300-K PL peak emission at ~2.4 μm, have significant phase separation. These layers are characterized by increased lattice constant variations and epitaxial tilt, broad PL spectra with significant band tailing, and strong contrast modulation in transmission electron microscopy. The degree of decomposition depends on substrate miscut direction: Ga_1−xIn_xAs_ySb_1−y layers grown on (001) substrates are more homogeneous than those grown on (001) and (001)2°→(101) substrates. The results are attributed to the smaller adatom diffusion length on substrates miscut toward .     

Effects of substrate orientation and surface reconstruction on patterned substrate OMVPE of GaAs

Organometallic vapor phase epitaxial growth of GaAs on 320 nm high mesas was used to study the dependence of lateral growth upon the substrate misorientation from (100) and the mesa wall orientation on the substrate. GaAs (100) substrates were misoriented by 3° toward eight major crystallographic directions, consisting of the four nearest [111] and [110] directions. The mesa sidewalls were oriented either parallel to the 〈011〉 and 〈01 〉 directions or rotated by 45° to be parallel to the 〈001〉 and 〈010〉 directions. GaAs films were grown with TMGa and TBA at T=575°C. The lateral growth rates were up to 25 times higher than the vertical growth rate of 1.3 μm/hour. Optical microscopy and atomic force microscopy (AFM) showed that under the given growth conditions lateral growth off mesa sidewalls is most rapid in the 〈011〉 and/or 〈0 〉 directions and less in the perpendicular 〈01 〉 and 〈0 1〉 directions (lateral growth anisotropy). By raising the temperature to 625°C lateral growth in the 〈01 〉 -〈0 1〉 directions increased while it remained almost constant in the 〈011〉 -〈0 〉 directions. Published results show that the partial pressure of As also affects lateral growth. Differences in the lateral growth rates in the 〈011〉 and its opposite 〈0 〉 directions result from substrate misorientation but not from the orientation of the mesa walls on the substrate. Anisotropic lateral growth rates in different crystallographic directions appear to be caused by both, (1) 1-dimensional Ga diffusion defined by surface reconstruction, and (2) a relatively low energy barrier to atoms flowing over high-to-low terrace steps. A lateral growth model is proposed that describes anisotropic lateral growth at mesa sidewalls in terms of growth conditions and substrate misorientations. The model also explains the difference in the preferential lateral growth directions between MBE and OMVPE.     

Growth of oriented diamond film on single crystalline 6H-SiC substrates

The bias-enhanced nucleation (BEN) technique in hot-filament chemical vapor deposition (HF-CVD) has been applied to single crystalline 6H-SiC substrates for the deposition of oriented diamond. The results of scanning electron microscopy (SEM) showed that on (000 ) face not only oriented diamond with relationship (111) Dia.//(000 )6H-SiC and 〈110〉Dia.//(11 0)6H-SiC, but also high nucleation density (>10⁹ cm⁻²) have been achieved. In the case of deposition on (0001) face of 6H-SiC under the same experimental conditions, although the nucleation density of diamond was enhanced, however, oriented diamond was not found. Diamond nucleation density is higher on (0001) face than that on (000 ) face. The differences in diamond oriented nucleation and nucleation density on these two faces are attributed to the difference of their specific free surface energy. The experimental results have shown that the 6H-SiC substrate surfaces are etched by the accelerated H-ions during BEN process, and many micro-triangular crystals with the faces of the kind {01 4} are formed on the substrate surface. Diamonds nucleate on the top of the micro-triangular crystals. Micro-Raman spectrum shows a strong feature of diamond crystals at 1334 cm⁻¹.     

Multivariable backward-shift-invariant subspaces and observability operators

It is well known that subspaces of the Hardy space over the unit disk which are invariant under the backward shift occur as the image of an observability operator associated with a discrete-time linear system with stable state-dynamics, as well as the functional-model space for a Hilbert space contraction operator. We discuss two multivariable extensions of this structure, where the classical Hardy space is replaced by (1) the Fock space of formal power series in a collection of d noncommuting indeterminates with norm-square-summable vector coefficients, and (2) the reproducing kernel Hilbert space (often now called the Arveson space) over the unit ball in with reproducing kernel ). In the first case, the associated linear system is of noncommutative Fornasini–Marchesini type with evolution along a free semigroup with d generators, while in the second case the linear system is a standard (commutative) Fornasini–Marchesini-type system with evolution along the integer lattice . An abelianization map (or symmetrization of the Fock space) links the first case with the second. The second case has special features depending on whether the operator-tuple defining the state dynamics is commutative or not. The paper focuses on multidimensional state-output linear systems and the associated observability operators; followup papers Ball, Bollotnikov, and Fang (2007a, 2007b) use the results here to extend the analysis to represent observability-operator ranges as reproducing kernel Hilbert spaces with reproducing kernels constructed from the transfer function of a conservative multidimensional (noncommutative or commutative) input-state-output linear system.