bluuebunny/crossref_metadata_2025 huggingface dataset:
Phonon Thermal Conductivity of Superlattice Nanowires for Thermoelectric Applications
Authors: Dames C. , Dresselhaus M. S. , Chen G.
Published: April 2011

An incoherent particle model has been developed to predict the phonon thermal conductivity of nanowires and superlattice nanowires. It is argued that the surface roughness of most real nanowires prevents the formation of idealized confined dispersion relations for typical temperatures and diameters. Instead, the three-dimensional bulk dispersion is used, thus addressing only classical size effects. Four adjustable parameters capture the effects of diameter, superlattice, Umklapp, impurity, and alloy scattering. Predictions are compared with experimental data for nanowires and superlattice nanowires down to 22 nm diameter and 20 K, and are in good agreement above ∼40 nm diameter. The analysis suggests that ideal low thermal conductivity nanowires for thermoelectric applications would have small-diameter, alternating alloy segments that are acoustically dissimilar but electrically similar.
Influence of Stress on Thermoelectric Properties of Antimony Telluride
Authors: Mahan Gerald D. , Jeon Gun Sang , Thonhauser T.
Published: April 2011

We present first-principles calculation for the behavior of antimony telluride under stress. We focus on the calculation of transport properties for pure Sb2Te3and the investigation of stress-induced defects. Our transport calculations were done within first-principles using the linearized-augmented plane-wave method, where we first calculated the transport distribution for several stress conditions. Next, using these stress dependent transport distributions, we derived the stress dependence of the electrical conductivity, the Seebeck coefficient, and the power factor. Our calculations for the stress-induced defects were done utilizing a pseu-dopotential method and confining ourselves to the antisite defect. In all cases, our results are in good agreement with experimentally obtained data. Furthermore, we found that hydrostatic pressure does not improve the power factor, while, on the other hand, a large increase under uniaxial stress can be observed. Also, both hydrostatic pressure and uniaxial stress are found to lower the formation energies of the antisite defects, suggesting a structural transition at high pressures.
Thermoelectric properties of Ni-based oxides
Authors: Funahashi R. , Mikami M. , Urata S. , Kouuchi T. , Mizuno K. , Chong K.
Published: April 2011

A high-throughput screening technique has been developed and was utilized in the discovery of a new n-type oxide possessing good thermoelectric properties. Screening of metal binary systems consisting of 3d transition metals using this technique showed LaNiO3 to possess the desired n-type properties. Electrical resistivity (ρ) of this oxide is favorably quite low, however, the Seebeck coefficient (S) is as small as –25 μV/K. To enhance the thermoelectric properties of LaNiO3, high-throughput screening was employed to examine candidates from the metal ternary La1–xMxNiO3 and LaNi1–xNxO3 systems. Bi substitution in the La1–xMxNiO3 systems and Cu substitution in the LaNi1–xNxO3 systems were found to be effective for improvement of S and ρ respectively.
Effect of Ni in new skutterudite compounds CaxCo4Sb12
Authors: Puyet Matthieu , Lenoir Bertrand , Dauscher Anne , Scherrer Hubert , Dehmas Moukrane , Hejtmanek Jiri , Stiewe Christian , Müller Eckhard
Published: April 2011

The transport properties of the partially filled CaxCo4-yNiySb12 skutterudite compounds have been investigated in the 300 – 800 K temperature range. We underline the positive influence of the Ni substitution on the electrical resistivity and thermopower while the thermal properties – thermal conductivity – remains almost unaffected. These results suggest again a beneficial effect of Ni atoms on the dimensionless figure of merit in CoSb3 based compounds.
New Transition-Metal Doped Germanium Clathrates
Authors: Li Yang , Ross Joseph H.
Published: April 2011

We have investigated transition-metal substitutions into germanium clathrates, and describe the properties of two different Fe-substituted Ge clathrates, one with the chiral Ba6Ge25-type clathrate structure, and one based on the Ba6Ga16Ge30clathrate with the type-I structure. In both cases Fe exhibits a high-spin local moment, with 5.5 μBand 5.6 μBper Fe. We observe ferromagnetic ordering, withTcup to 278 K in the most heavily substituted Ba6Ga16Ge30clathrate. X-ray powder diffraction and electron microprobe measurements confirmed the formation of these substituted phases. Ni-doped type I clathrates were found to be diamagnetic. Strong competition from Mn11Ge8makes it difficult to produce pure Mn-doped clathrates, however a composite material with partially substituted chiral structure is described. Similar efforts to produce Co, Cr, and V-substituted clathrates have not been successful.
Thermoelectric Properties of NaZn13-type Intermetallic Compounds
Authors: Amagai Y. , Yamamoto A. , Lee C. H. , Takazawa H. , Noguchi T. , Obara H. , Iida T. , Takanashi Y.
Published: April 2011

We report the electrical resistivity and the Seebeck coefficient of AZn13(A = Sr, Ba, and La) and LaCo13measured over a wide temperature range and their thermal conductivity measured at room temperature. The electrical measurements of AZn13and LaCo13above room temperature reveal that the compounds show good metallic behavior. We find that the absolute value of Seebeck coefficient for AZn13(A = Sr, Ba, and La) increases with increasing temperature, which is a typical metallic behavior and the absolute value is less than 3μVK−1at room temperature. Accordingly, the power factor of AZn13is quite low. Temperature dependence of the Seebeck coefficient for LaCo13is similar to that of Co. The absolute value of the Seebeck coefficient for LaCo13is high as a metallic conductor and approaches -30μVK−1at 500K, which leads LaCo13to large power factor of 1.8 × 10−3Wm−1K−2. We obtained lattice components of the thermal conductivity by subtracting electronic contributions from the total thermal conductivity. The electronic components of the thermal conductivity were estimated using Wiedemann-Frantz law assumingL(Lorentz number) is 2.45 × 10−8V2K−2. The thermal conductivities of the lattice components for AZn13(A = Sr, Ba, and La) and LaCo13with NaZn13type structure are about 10 Wm−1K−1, respectively. These values are high as compared with other thermoelectric materials.
Crystallographic Features of Rhenium Disilicide
Authors: Tanaka K. , Inui H. , Ohba T. , Tsutsui S. , Mizumaki M.
Published: April 2011

Crystallographic features of Rhenium disilicide have been examined by powder and single-crystal X-ray diffractometry with a synchrotron radiation. The chemical composition of the compound is determined as Re4Si7 (ReSi2-x, x = 0.25) from lattice constants and specific density determined. The structure is defined as a vacancy ordered structure with the space group of Cm. The positions of the constituent atoms in the structure have been refined. The positions of silicon atoms apparently shift from the ideal positions of the C11b structure which is the structure without containing structural vacancies.
Hall Effect Measurements on New Thermoelectric Materials
Authors: Short Jarrod , Loo Sim , Lal Sangeeta , Hsu Kuei Fang , Quarez Eric , Kanatzidis Mercouri G. , Hogan Timothy P.
Published: April 2011

In the field of thermoelectrics, the figure of merit of new materials is based on the electrical conductivity, thermoelectric power, and thermal conductivity of the sample, however additional insight is gained through knowledge of the carrier concentrations and mobility in the materials. The figure of merit is commonly related to the material properties through the B factor which is directly dependent on the mobility of the carriers as well as the effective mass.

To gain additional insight on the new materials of interest for thermoelectric applications, a Hall Effect system has been developed for measuring the temperature dependent carrier concentrations and mobilities. In this paper, the measurement system will be described, and recent results for several new materials will be presented.
Effects of Sb-doping on electrical transport properties of Co-based half-Heusler compound
Authors: Ono Yasuhiro , Inayama Shingo , Adachi Hideaki , Yotsuhashi Satoshi , Miyazaki Yuzuru , Kajitani Tsuyoshi
Published: April 2011

Electrical transport properties of NbCoSn1−xSbx(x=0, 0.01, 0.02 and 0.05), a half-Heusler compound and its alloys, have been studied in the temperature range from 80 K to 850 K. As-prepared samples exhibit metallic conduction and similar Seebeck coefficients near 300 K (S= –100 μVK−1). Except for NbCoSn0.95Sn0.05, both electrical resistivity, ρ, and the absolute value ofSappreciably increase during the annealing for 6 days at 1123 K. Unusual increase in ρ of the annealed NbCoSn sample is found at about 200 K. ρ-T curves of the other annealed samples remain metallic over the measured temperature range and the ρ value noticeably decreases with increasing Sb content,x. Among the annealed samples, the high power factor, 25×10−4Wm−1K−2at 850 K, is obtained for NbCoSn0.95Sb0.05. The band structure of NbCoSn is calculated based on the determined crystal structure, indicating that NbCoSn is an indirect transition-type semiconductor with a band gap of approximately 1 eV. This is not consistent with the metallic behavior of ρ observed for the annealed NbCoSn sample above 400 K. Partial disordering of Nb and Co atoms is a conceivable answer.
Thickness Dependences of the Thermoelectric Properties in (001)KCl/PbTe/SnTe/PbTe Heterostructures
Authors: Rogacheva Elena I. , Nashchekina Olga N. , Lyubchenko Svetlana G. , Vekhov Yegor O. , Dresselhaus Mildred S. , Dresselhaus Gene
Published: April 2011

The dependences of the thermoelectric properties of (001)KCl/PbTe/SnTe/PbTe three-layer structures on the SnTe layer thickness (dSnTe = 0.5–6.0 nm) at a fixed thickness of PbTe layers were studied. It was established that the thickness dependences of the Seebeck coefficient, the Hall coefficient, electrical conductivity, charge carrier mobility, and the thermoelectric power factor are distinctly non-monotonic. Two possible reasons for this non-monotonic behavior of the thickness dependences of the thermoelectric properties are considered: the size quantization of the energy spectrum in a SnTe quantum well and / or the formation of edge misfit dislocations at the interfaces after reaching the critical thickness, which corresponds to the transition from a pseudomorphic growth to the introduction of misfit dislocations at the interfaces. It is suggested that the observed effect has a general character and should be taken into account when optimizing thermoelectric properties of superlattices.
Synthesis, Crystal Structure and Thermoelectric Properties of β-K2Bi8Se13 Solid Solutions
Authors: Kyratsi Theodora , Chung Duck Young , Dyck Jeff S. , Uher Ctirad , Lal Sangeeta , Loo Sim , Hogan Tim , Ireland John , Kannewurf Carl R. , Hatzikraniotis Evripides , Paraskevopoulos Konstantinos M. , Kanatzidis Mercouri G.
Published: April 2011

Solid solution series of the type K2Bi8-xSbxSe13, K2-xRbxBi8Se13 as well as K2Bi8Se13-xSx were prepared and the distribution of the atoms (Bi/Sb, K/Rb and Se/S) on different crystallographic sites, the band gaps and their thermoelectric properties were studied. The distribution Se/S appears to be more uniform than the distribution of the Sb and Rb atoms in the β-K2Bi8Se13 structure that shows preference in specific sites in the lattice. Band gap is mainly affected by Sb and S substitution. Seebeck coefficient measurements showed n-type character for of all Se/S members. In the Bi/Sb series an enhancement of p-type character was observed. The thermoelectric performance as well as preliminary high temperature measurements suggest the potential of these materials for high temperature applications.
Microstructure – Mechanical Properties Relationship of Laser Interference Irradiated Ni/Al Multi-Film
Authors: Daniel C. , Lasagni A. , Mücklich F.
Published: April 2011

Due to the corresponding intermetallic compounds, Ni/Al multi-layered thin film systems are important to protect against the mechanical and chemical impacts on the bulk component. The mechanical properties of these intermetallic compounds, NiAl, can be further improved by combining with other stiff phases. The mechanical properties would be optimized if the lateral surface composite can be made in such a way that the different phases are arranged periodically with a preferred orientation, micro-scaled period and reticulated phase interfaces. Such optimized surface composites have been achieved by laser interference irradiation in a nano-grained structure.

In this study, the thin film systems are produced by physical vapor deposition and subsequently irradiated by the interference pattern of two or more coherent laser beams. The corresponding periodical heat treatment has been analyzed by thermal simulation, and thermal simulation results are compared with the experimental results. Further, the phase transitions during laser interference irradiation are calculated. The structural investigations of irradiated films - grain sizes and deformation by TEM, stress and texture by XRD - are compared with the mechanical properties - hardness and Young's modulus by NI-AFM.
Young's Modulus, Poisson's Ratio, and Nanoscale Deformation Fields of MEMS Materials
Authors: Chasiotis I. , Cho S. W. , Friedmann T. A. , Sullivan J. P.
Published: April 2011

The mechanical properties of hydrogen-free tetrahedral amorphous diamond-like carbon have been investigated in connection with its elastic and failure properties. Micro-tension specimens of gage thickness of 1.2–1.8 μm and widths of 10 μm or 50 μm have been fabricated by the Sandia National Laboratories (SNL). The mechanical characterization has been conducted via in situ AFM measurements and Digital Image Correlation (DIC) data strain analysis and the local deformation fields of (a) uniform and (b) internally notched tension specimens with acute notches (K=27) have been experimentally obtained. Young's modulus and Poisson's ratio were measured for the first time directly from such small specimens and averaged 750 GPa and v=0.16 respectively, while the tensile strength was found to be very consistent averaging 7.1 GPa. Stressed material domains with smaller dimensions in the vicinity of micronotches exhibited even higher failure strengths reaching 11.5 GPa with limited data scatter. AFM images of in situ tested specimens have indicated sp3 to sp2 phase transformations on the film surface that was subject to ultra-high tensile stresses (>6 GPa). This is the first time these phase transformations are observed during tensile tests of brittle materials.
Effects of Thickness and Indenter Geometry in Nanoindentation of Nickel Thin Films
Authors: Parakala Padma , Mirshams Reza A. , Nasrazadani Seifollah , Lian Kun
Published: April 2011

Effects of thickness and tip geometry on Ni thin films deposited on Cu substrate were studied using nanoindenter. The deformation mechanisms in correlation to hardness measurements were discussed at various loads and depths of penetration. The Berkovich, Cube corner and Conical tips have been used in this study. Initially, the hardness and modulus of elasticity were measured at a depth of 10% of film thickness. The depth of penetration was increased to 20% to observe the depth effects. Analysis of data showed that there is an Indentation Size Effect (ISE) irrespective of indenter tip geometries.
Temperature Dependence of Biaxial Modulus and Thermal Expansion Coefficient of Thin Films Using Wafer Curvature Method
Authors: Capanu M. , Cervin-Lawry A. , Patel A. , Koutsaroff I. , Woo P. , Wu Lynda , Oh J. , Obeng J. , McClelland B.
Published: April 2011

The present work uses wafer curvature (disk) method to measure the temperature variation of the biaxial modulus and the thermal expansion coefficient TEC from 25°C to 205°C. The following thin films were measured: PolySi, low stress LPCVD SixNy and (Ba0.7, Sr0.3)TiO3 (BST). To improve the precision, perfect circular thin wafers were used: 4 inches circular 280μm thick <111>Si and 450μm thick Fused Silica. The measurements were performed using a commercial Tencor FLX2320 Stress Measurement System. The film thickness was measured with Tencor TF1 thin film optical system.
Stress-field in Sputtered Mo Thin Films and Mo/Ni Superlattices: Origin and Evolution after Ion-Irradiation
Authors: Debelle A. , Abadias G. , Michel A. , Jaouen C. , Guérin Ph. , Drouet M.
Published: April 2011

In the present study the ion irradiation technique is used to investigate the origin of the stress-field in Mo layers grown by ion beam sputtering. Strain measurements were performed by X- ray Diffraction (XRD) using the sin2ψ method. The evolution of the sin2ψ plots with ion irradiation shows that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. A new stress model based on a triaxial state of stress, which includes a hydrostatic component linked to point defects induced volume distortions, is derived to interpret the XRD results. For pure Mo films, the obtained a0 parameter is close to the bulk value, while for Mo sublayers in Mo/Ni superlattices, the a0 value is lower due to intermixing between Ni and Mo. These results demonstrate that ion irradiation is a powerful tool for stress relaxation, which allows to obtain additional information on the respective contribution of chemical effects and growth defects to the a0 value.
Fracture in Thin Oxide Films
Authors: Bahr D. F. , Olson A. L. , Morasch K. R. , Kennedy M. S. , Rodriguez Marek D. , Alamr A.
Published: April 2011

In thin film systems, failure often occurs via fracture mechanisms, with either through thickness cracking or interfacial delamination leading to failure of the device or layer. Measuring the stress at which fracture occurs in these thin film systems requires testing methods amicable to both the small scale of the films as well as the complex relationship between the mechanical properties of the film and the substrate. This paper will cover two oxide film systems, a piezoelectric ceramic (PZT) on platinum and passive oxide films (primarily Cr2O3) on stainless steels. Both will be tested with a bulk method (bulge testing in the case of PZT and circumferentially notched tensile bars for the stainless steel) and then with a nanoindentation method developed for testing fracture for hard coatings on soft substrates. The differences in stresses required for failure between the bulk and the nanoscale tests will be discussed in terms of differences in flaw population.
Measurement of Residual Stress in ZnO Thin Films Deposited on Silicon Wafers by the Indentation Fracture Test
Authors: Zhang Tong-Yi , Huang Bin
Published: April 2011

ABSTRCT:

The indentation fracture test was used to evaluate residual stress in ZnO thin films deposited on silicon wafers. Using the calibrated constants involved in a simple semi-empirical formula, which was developed based on fracture analysis and experimental results, we estimated the residual stress in the ZnO thin films by measuring the indentation-induced crack length as a function of the indentation load.
Fracture patterns in thin films and multilayers
Authors: Volinsky Alex A. , Meyer Dirk C. , Leisegang Tilmann , Paufler Peter
Published: April 2011

While there are many stress relief mechanisms observed in thin films, excessive residual and externally applied stresses cause film fracture. In the case of tensile stress a network of through-thickness cracks forms in the film. In the case of compressive stress thin film buckling is observed in the form of blisters. Thin film delamination is an inseparable phenomenon of buckling. The buckling delamination blisters can be either circular, straight, or form periodic buckling patterns commonly known as telephone cord delamination morphology.

While excessive biaxial residual stress is the key for causing thin film fracture, either in tension, or compression, it is the influence of the external stress that can control the final fracture pattern. In this paper we consider phone cord buckling delamination observed in compressed W/Si and TiWN/GaAs thin film systems, as well as spiral and sinusoidal though-thickness cracks observed in Mo/Si multilayers under 3-point high-temperature bending in tension.
A Membrane Deflection Fracture Experiment to Investigate Fracture Toughness of Freestanding MEMS Materials
Authors: Espinosa H. D. , Peng B.
Published: April 2011

This paper presents a novel Membrane Deflection Fracture Experiment (MDFE) to investigate the fracture toughness of MEMS and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing pre-existing cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.7 MPa m1/2. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 7.2 MPa m1/2. Comparison of these two values, using the model proposed by Drory et al. [9], provides a correction factor of 2/3, which corresponds to a mean value ofρ/2x=1/2.
Stress Relaxation during Isothermal Annealing at Elevated Temperatures in Electroplated Cu Films
Authors: Hwang Soo-Jung , Joo Young-Chang , Koike Junichi
Published: April 2011

Deformation mechanisms of electroplated Cu thin films on TaN/SiO2/Si were investigated by performing isothermal annealing above 200 °C. Stress relaxation behavior during isothermal annealing was analyzed by curve fitting using exponential decay equations. During heating, fast relaxation and subsequent slow relaxation processes were observed. In contrast, during cooling, only slow relaxation process was observed. Among possible mechanisms for stress relaxation, diffusion creep was found to be the most plausible mechanism based on the obtained values of the activation energy. It was suggested that the slow relaxation process observed both in the heating and in the cooling processes was attributed to a grain-boundary diffusion creep. On the other hand, the fast relaxation process observed during heating was attributed to a surface-diffusion controlled mechanism. The surface diffusion mechanism was considered to be characteristic to Cu thin films that did not form stable surface oxide.
Thermal Stability and Internal Stress for Strongly (111) Oriented Cu Films
Authors: Takayama Sinji , Oikawa Makato , Himuro Tokuji
Published: April 2011

Internal stresses and thermal stability of strongly (111) oriented Cu thin films, which are one of promising interconnect materials in advanced ULSI devices, have been studied comparing with those of non-oriented Cu films. Their internal stresses parallel to a film surface were measured by a conventional X-ray diffraction technique (d-spacing vs. sin2ψ method), while the strain distribution with depth by a grazing incidence X-ray scattering (GIXS) methods. Large stress relaxation in strongly (111) oriented Cu films takes place at 200°C without showing any significant grain growth and formation of thermal defects like hillocks. The residual internal stresses of highly oriented (111) Cu films increase almost linearly throughout the thickness up to the substrates. The feature of stress distribution in film depth does not change on annealing. The changes of the residual stresses at each depth are nearly the same as stresses parallel to film surface measured.
In situ investigation of growth of gold on crystalline TiO2 and amorphous Al2O3 substrates
Authors: Lauter L. , Abermann R.
Published: April 2011

The growth of thin gold films on highly crystalline TiO2 and amorphous Al2O3 substrates and its dependence on substrate temperature was investigated under UHV-conditions by in situ internal stress measurements. Deposition of gold on amorphous Al2O3 at substrate temperatures between 27°C and 300°C shows a stress vs. thickness curve which indicates island growth at first and the formation of a polycrystalline film at higher thickness. A comparable stress vs. thickness curve is found for the growth of gold on the highly crystalline TiO2 substrate at substrate temperatures below 200°C, again indicating island growth. At higher temperatures, however, a new tensile stress feature at low gold coverage is interpreted to indicate the formation of a strained interface layer. This strain in the gold film is eliminated after deposition of a few monolayers most likely through incorporation of dislocations and defects. The growth stress at higher film thickness is indicative of island film growth.
Thickness Dependent Stress Relaxation with the Onset of L10 Ordering in FePt Thin Films
Authors: Wierman K. W. , Platt C. L. , Howard J. K.
Published: April 2011

During the chemical ordering of the high anisotropy FePt L10 phase a fcc-→fct structural distortion in the unit cell occurs. Monitoring the stress relaxation time dependencies during isothermal annealing cycles may lead to a better understanding of the L10 ordering kinetics thickness dependencies in thin FePt films. During the isothermal portion of the anneal cycle all films showed an exponentially decaying stress relaxation with time to lower compressive stress states for all temperatures and thickness. The stress relaxation rate increased significantly with an increase in Fe51Pt49 film thickness and annealing temperature. Stress relaxation activation energies of 2.21 eV, 2.40 eV, and 2.96 eV for the 50nm, 25nm, and 10nm Fe51Pt49 films, respectively, were extracted. This correlated well with the observed trends in the post annealed film coercivity, increase of the (001) superlattice L10 peak, and the shifting of the (111) peak position, all indicators of L10 chemical ordering transformation.
PROBING STRAIN FIELDS ABOUT THIN FILM STRUCTURES USING X-RAY MICRODIFFRACTION
Authors: Murray C. E. , Noyan I. C. , Mooney P. M. , Lai B. , Cai Z.
Published: April 2011

The transfer of strain between thin film features and the underlying substrate represents an important factor in the performance and reliability of semiconductor devices, particularly as the distances between these structures decrease. In order to characterize the interaction regions produced in the substrate due to strained thin film structures, we employed synchrotron-based x-ray diffraction techniques to map the enhanced diffracted intensity of the single-crystal Si substrate at sub-micron resolution. The dynamic-to-kinematic transition observed in the scattering of x-rays from deformed crystals makes this technique extremely sensitive to the amount of substrate deformation as a function of position. Measurements were conducted on 1 μm thick Ni dots evaporated onto Si (111) substrates and 0.24 μm thick, heteroepitaxially grown SiGe strips of various widths on Si (001). The interaction field resolved by the enhanced Si diffracted intensity in the substrate extended up to 100 times the thickness of these features. Although the boundary of the interaction field varied as a function of feature width, a characteristic curve was generated to describe the decay rate of enhanced Si diffracted intensity when the distance from the feature edge is normalized by a mean interaction distance (MID). The rate of decay of the strain fields predicted by traditional treatments of the mechanical interaction between the thin film and substrate did not correspond to the measured decay rates.
Stress and Strength of free-standing 2-dimensional tetrahedral amorphous carbon bridge arrays
Authors: Chua Daniel H.C. , Tsai T. H. , Milne W. I. , Sheeja D. , Tay B. K.
Published: April 2011

The fabrication of ultrathin (25nm) 2-dimensional free-standing arrays of tetrahedral amorphous Carbon (ta-C) microbridges is reported for the first time. The ta-C films were deposited by a Filtered Cathodic Vacuum Arc (FCVA) deposition system where the sp3 content in the film was measured to be in excess of 90% by high resolution XPS. Continuous arrays of free standing taC bridges whose length/width ratios ranged from 1:1 to 12:1 were successfully fabricated while maintaining the same thickness. Due to the naturally high compressive stress of ta-C films, the buckling of films was perpendicular to the length of the beam. The displacement of curvature obtained was in good agreement with FEM simulation results. Moreover, the curvature or arch of these ultrathin films, coupled with a high Young's modulus (750GPa) and Hardness (60GPa), meant they could withstand a vertical force in excess of 8000μN without breaking.
Dislocation Dynamics in Semiconductor Thin Film-Substrate Systems
Authors: Tan E. H. , Sun L. Z.
Published: April 2011

A discrete dislocation dynamics model is developed to establish the equations of motion for three-dimensional interacting dislocation loops in the semiconductor thin film – substrate system. The film is assumed to be an elastic layer and is perfectly bonded with another elastic substrate. Dislocation loops are discretized into segments, each of which is represented by a parametric space curve of specific shape functions and associated degree of freedom. The dislocation stress field is calculated as an essential ingredient in the dislocation dynamics method. Dislocation dynamics and interaction with film surface/interface are simulated.
A Coupled Cellular Automata Representation of Nanoscale Transport Across Semiconductor Interfaces
Authors: Radkowski Peter P. F. , Sands Timothy D.
Published: April 2011

Thin-film devices pose unique challenges to device simulators. Relaxation approximations to the Boltzmann Transport Equation may be vitiated as the width and length of the thin-film components approach the mean free path lengths of electrons and phonons. Concomitant with these reduced lengths, surface and other interface scattering mechanisms exert greater influences on the transport processes. Consequently, it may not be effective to use transport models that depend upon analytic or other pre-ordained representations of electron, phonon, and photon distribution functions. Finally, thin-film simulations must account for tunneling processes, local defects, nanoscale dopant gradients, nanoscale roughness, and nanoscale variations in local geometries. Consequently, it is desirable to develop a nanoscale simulation technique that (1) can arbitrarily vary material properties in real space while (2) tracking reciprocal space scattering for arbitrary and volatile distributions of electrons, phonons, and photons. The Discrete State Simulation (DSS) forms the backbone of such a simulation. Written with the run-time flexibility of object-oriented code, the Discrete State Simulation (DSS) is a coupled cellular automata (CA) simulator that builds upon the objects and rules of quantum mechanics. The DSS represents global non-equilibrium processes as patterns that emerge through an ensemble of scattering events that are localized at vibronic nodes. The run-time flexibility of the DSS enables dynamic rule construction - computational algorithms that evolve in response to the conditions that are being simulated. The reported DSS effort simulated nanoscale transport processes at dopant-defined junctions by coupling electron-phonon and electron-photon scattering mechanisms. Using electronic band structures, phonon band structures, and deformation potentials analogous to silicon <100> material properties, the DSS generated data depicted the dynamic band bending induced by femtosecond variations in applied electric fields.
Photochemical Bonding of Fluorocarbon and Fused Silica Glass for Ultraviolet Ray Transmitting
Authors: Asano Kenji , Murahara Masataka
Published: April 2011

Fluorocarbon was photo-chemically combined to a fused silica glass with the silicon oil used as a bonding agent. Balsam, unsaturated polyester resins and UV hardening adhesives have been generally used for joining two optical glasses together. They, however, have a strong absorption band in the UV region. Therefore, a new bonding method was developed for optical materials to allow UV rays to pass through using silicone oil and excimer- lamp. This new method requires the fluorocarbon-polishing pad employed in our PCP (Photo-Chemical Polishing) method in hydrofluoric acid ambience, which is bonded with the silica glass. The silicone oil was put between the fused silica glass and the fluorocarbon (FEP), and an excimer- lamp was irradiated. When the excimer lamplight was irradiated vertically, the silicon oil ((-O-Si(CH3)-O-)n) was photo-dissociated and reacted with the oxygen adsorbed on the silica glass surface to produce a SiO2. On the other hand, the H atoms photo-dissociated from the silicon oil pulled out the F atoms of the FEP. As a result, the FEP and the silica glass were combined. The tensile strength of the sample bonded by the photo-chemical reaction was evaluated. The tensile strength of 5.4 [kgf/cm2] was obtained, whereas that of the non-treatment sample was nil. Moreover, the transmittance of the vitrified silicone oil was measured at the 193 nm of ArF laser wavelength. It increased by 90.6% from 29.2% without the UV photon irradiation. The results showed that the silicon oil changed to silica glass by the excited oxygen, which improved the UV rays under 200nm transmittance.
Engineering CuInGaSSe2 Surface Properties to Enhance Device Performance
Authors: Menezes Shalini , Li Yan , Menezes Sharmila J.
Published: April 2011

The CuInGaSSe2/CdS heterostructure interface has a special effect on the performance of an important thin film photovoltaic device. The CdS buffer layer is essential to stabilize the performance of CuInGaSSe2 based devices. It adjusts the lattice mismatch at the absorber/window interface, repairs CuInGaSSe2 surface defects and protects it from air oxidation. Unfortunately, the CdS material has many environmental issues. This paper reports an alternate chemical approach to engineer the interface defects in CuInGaSSe2 and maximize its PV output. It describes a simple processing step to manipulate the defect density. This step could potentially reduce sensitivity to the ambience, widen the surface bandgap and replace the current hazardous processes used in state-of-the-art CuInGaSSe2 modules. Photocurrent and spectral response measurement in an electrolytic medium monitor the effects of surface modification, specific metal ions and time. The CuInGaSSe2 films respond easily to a number of external stimuli with either positive or negative changes in the electro-optic properties. Strong time dependence of the photocurrent suggests a dynamic equilibrium of point defects in the CuInGaSSe2 film. The results provide new insights into the effects of stoichiometry, deposition methods and oxide formation, on the defect chemistry. They also provide directions for reconfiguring the deep defects for enhanced device performance without the need for toxic etchants or buffer layers, and the environmental hazards associated with these steps.
Design and Fabrication of GaN-based Permeable-Base Transistors
Authors: Cabalu Jasper S. , Gunter Liberty L. , Friel Ian , Bhattacharyya Anirban , Fedyunin Yuri , Chu Kanin , Bellotti Enrico , Eddy Charles , Moustakas Theodore D.
Published: April 2011

In this work, we present the first successful fabrication of an etched grooved GaN-based permeable-base transistor structure. Growth of the device structures was done by Molecular Beam Epitaxy (MBE) on thick HVPE GaN quasi-substrates. The fabrication process took advantage of isolations pads via He implantation and SiN deposition, as well as submicron ICP etching of collector fingers patterned via e-beam lithography. SEM of the finished devices shows smooth etched finger structures and base layer surface with finger sidewall angles of ∼85° for 1:1 and 1:3 finger spacing. Specific contact resistivities of ∼ 3 × 10-6 Ω•cm2 for the ohmic contacts were achieved with Ti/Al/Ni/Au metallization scheme. Preliminary DC testing of the devices show a collector current IC = 140 mA/mm at VCE of 5V and VBE of +0.5V. The maximum transconductance gm is ∼111 mS/mm in the measured collector-emitter bias range. These values are comparable, within the measurement tolerance, to physics-based modeling results.
MOCVD AlGaN/GaN HFET's Material Optimization and Devices Characterization
Authors: Demchuk Alexander , Olson Don , Olson Dan , Shin Minseub , Munns Gordon
Published: April 2011

An optimization of growth parameters of AlxGa1*xN/AlN/GaN heterostructure field effect transistors (HFET) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique on SiC and sapphire substrates with relatively high Al mole fraction in the barrier layer (0.3 < x < 0.5) has been presented. The properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1-xN/GaN heterojunction can be tuned by careful adjustments of AlxGa1-xN barrier layer thickness and Al mole fraction, x. The 2DEG sheet conductivities (μ ns) as high as 2.6 × 1016 V-1s-1 at μ ∼ 2200 cm2/Vs and ns ∼ 1.2 × 1013 cm-2 has been achieved on AlxGa1-xN/AlN/GaN HFET structures on SiC substrate at x = 0.47. HFET devices processed on these structures exhibited improved low field conductivities and DC and high frequency performance. Saturation currents above 1.2 A/mm at 0 V gate bias, transconductance as high as 340 mS/mm at Lg = 0.25 μm and FT × Lg > 20 GHz × μm were demonstrated on HFET structure grown on SiC substrates.
Recombination Related to Two-Dimensional Electron Gas of AlxGa1-xN/GaN Single Heterostructures Studied with Picosecond Time-Resolved Photoluminescence
Authors: Yang Qing , Armitage Rob , Weber Eicke R. , Birkhahn Ronald , Gotthold David , Guo Shiping , Albert Brian
Published: April 2011

Nearbandgap radiative recombination in undoped AlxGa1-xN/GaN (x = 0.28 to 0.35) single heterostructures grown by metalorganic chemical vapor deposition is investigated using picosecond time-resolved photoluminescence at 11K.Room temperature Hall effect measurements show both high sheet carrier concentrations (∼1×1013 cm-2) and high mobilities (∼1000 cm2/Vs), suggesting the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The luminescence transient of the 3.481 eV emissions, which is usually assigned to donor-bound exciton emission of GaN, consists of an initial fast component and a slow second component. Samples with higher aluminum concentrations show broad luminescence peaks in the spectral range of 3.41—3.45 eV with long decay lifetimes over 1 ns. In addition, the below bandgap emissions saturate at high excitation power and shift toward lower energies with increasing time delay. The observed emissions are explained by the recombination processes involving the 2DEG at the heterointerface.
Effect of Buffer Design on AlGaN/AlN/GaN Heterostrucutres by MBE
Authors: Namkoong Gon , Alan W. , Brown A. S. , Losurdo M. , Giangregorio M. M. , Bruno G.
Published: April 2011

The effect of the buffer layers on the subsequent GaN epitaxial layers and electrical properties of AlGaN/AlN/GaN heterojunction structures nitrided at various temperatures was investigated. For AlN buffer layers, two different growth conditions of AlN buffer layers were introduced to avoid Al droplets. We found that etch pit density and structural quality of GaN epitaxial layer strongly depends on the growth conditions of AlN buffer layers. When using a double buffer layer (low temperature GaN on high temperature AlN) for 200 °C nitridation, the etch pit density was measured to high 107 cm-2 in GaN epitaxial layers. Furthermore, we observed that electrical properties of AlGaN/AlN/GaN heterostructures depend on growth conditions of buffer layers and nitridation temperatures. The mobility in Al0.33Ga0.67N/AlN/GaN structures grown on single AlN buffer layers for 200 °C nitridation were 1300 cm2/Vs at a sheet charge of 1.6×1013 cm-2. Using the double buffer layer for 200 °C nitridation, the mobility increased to 1587 cm2/Vs with a sheet charge of 1.25×1013 cm-2.
Optical and Microstructural Characterisation of InN Grown by PAMBE on (0001) Sapphire and (001) YSZ
Authors: Anderson P. A. , Kendrick C. E. , Lee T. E. , Diehl W. , Reeves R. J. , Kennedy V. J. , Markwitz A. , Kinsey R. J. , Durbin S. M.
Published: April 2011

InN continues to be a topic of great interest, particularly with respect to the issues surrounding its bandgap energy. To further explore this material and its properties, we have grown 200–300 nm InN films by a plasma-assisted molecular beam epitaxy (PAMBE) technique on a variety of substrates, including (0001) sapphire, (100) InAs, and both (100) and (111) YSZ. Single-crystal films regardless of quality all show the commonly reported broad luminescence feature in the range of 0.7 to 0.8 eV, although we have also observed this feature in polycrystalline films. Growth on (100) InAs and (100) YSZ was motivated by a desire to explore cubic InN; in both cases growth appears to be initially cubic, but a mixture of hexagonal and cubic phases is detected in the final layer.
Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature
Authors: D'Evelyn M. P. , Narang K. J. , Park D.-S. , Hong H. C. , Barber M. , Tysoe S. A. , Leman J. , Balch J. , Lou V. L. , LeBoeuf S. F. , Gao Y. , Teetsov J. A. , Codella P. J. , Tavernier P. R. , Clarke D. R. , Molnar R. J.
Published: April 2011

We report the growth and characterization of bulk GaN single crystals by temperature-gradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm−2 have been achieved. Additional characterization of the GaN crystals is also presented.
Electron Field Emission from GaN Nanotip Pyramids
Authors: Ng Hock M. , Shaw Jonathan , Chowdhury Aref , Weimann Nils G.
Published: April 2011

Electron field emission was measured from GaN nanotip pyramids formed by polarity-selective chemical etching in KOH solution. The GaN samples were grown by plasma-assisted molecular beam epitaxy and consisted of regions of Ga- and N-polar GaN grown at the same time. The pyramids were formed only in the N-polar regions and have extremely sharp tips with diameters estimated to be less than 20 nm. Field emission measurements showed a characteristic Fowler-Nordheim behavior. The average turn-on field was 1.6 V/μm with a corresponding normalized field enhancement factor of about 1500.
Optimization of GaN/AlGaN Quantum Wells for Ultraviolet Emitters
Authors: Hangleiter A. , Fuhrmann D. , Greve M. , Rossow U.
Published: April 2011

We have studied GaN/AlGaN and GaInN/GaN quantum well structures grown on sapphire and SiC substrates using low-pressure MOVPE. The emission wavelength of the GaN/AlGaN QW's was varied in the 320–400 nm range and that of the GaInN/GaN QW's in the 390–480 nm range, by adjusting the QW width. Using temperature dependent resonant-excitation photoluminescence measurements we have determined the internal quantum efficiency (IQE) and analyzed its temperature dependence. For the GaN/AlGaN QW's we achieve a reasonable IQE of in excess of 5 % only at fairly high excitation power, while GaInN/GaN QW's provide IQE's of up to 46 % at rather low power density, indicating a much stronger role of defects for the former. While thermal activation energies are dominated by intrinsic mechanisms for GaInN/GaN QW's, lower activation barriers evident from the GaN/AlGaN data also hint at a more import role of defects.
Structural Units and Energy of Grain Boundaries in GaN
Authors: Chen Jun , Ruterana Pierre , Nouet Gerard
Published: April 2011

The energy of coincidence grain boundaries in wurtzite structure was investigated with the Stillinger-Weber potential after modification for GaN. A boundary can have two interfaces with a different period corresponding to the edge and diagonal of the unit cell of coincidence site lattices for rotations around [0001] in the range 0–60°. Their energy depends mainly on the atomic structure of the dislocation cores used for the boundary plane reconstruction. As a function of the structure units in the period, the energy of the edge and diagonal shows two minima which are observed for the edge boundaries.
Band bending near the surface in GaN as detected by a charge sensitive probe
Authors: Sabuktagin S. , Reshchikov M. A. , Johnstone D. K. , Morkoç H.
Published: April 2011

We measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 – 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects.
Processing of rare earth doped GaN with ion beams
Authors: Lorenz K. , Wahl U. , Alves E. , Wojtowicz T. , Ruterana P. , Dalmasso S. , Martin R. W. , ÒDonnell K. P. , Ruffenach S. , Briot O. , Vantomme A.
Published: April 2011

GaN epilayers grown by MOCVD were implanted with different fluences of thulium at room temperature and at 500 °C in order to find the optimum implantation conditions. Rutherford backscattering spectrometry in the channeling mode was used to monitor the damage evolution in the Ga-sublattice and to establish the lattice site location of the thulium ions. The nature of structural defects was studied with transmission electron microscopy and the optical properties of the samples with room temperature cathodoluminescence. The introduced damage could be significantly reduced by implantation at high temperature for fluences up to 5×1015 Tm/cm2. Annealing was necessary for optical activation of the implanted samples, in all cases. After annealing, sharp rare earth related emissions were observed in the blue and in the near infra-red spectral region.
Photoluminescence from freestanding GaN with (1010) orientation
Authors: Reshchikov M. A. , Teke A. , Maruska H. P. , Hill D. W. , Morkoç H.
Published: April 2011

Freestanding GaN templates with (1010) orientation (M-plane) were obtained by halide vapor phase epitaxy (HVPE) on nearly lattice-matched LiAlO2and subsequent removal of the substrate by wet chemical etching. Photoluminescence (PL) spectrum from both sides of the GaN template investigated is dominated by peaks at 3.47, 3.42 and 3.36 eV, tentatively attributed to an exciton bound to the neutral shallow donor and two unidentified structural defects, respectively. The quantum efficiency of the exciton-related emission exceeds 10%, whereas that of the combined emission from the defect-related bands (red, yellow and blue) is below 0.1%. The evolution of the PL spectrum with temperature and excitation intensity is analyzed in detail. Effects of polishing and etching on the PL properties are also discussed.
Confocal Photoluminescence and Cathodoluminescence Studies of AlGaN
Authors: Dierolf V. , Svitelskiy O. , Cargill G. S. , Nikiforov A. Yu. , Redwing J. , Acord J.
Published: April 2011

AlxGa1-xN films grown by MOCVD on sapphire and SiC substrates have been investigated by spatially resolved confocal photoluminescence microscopy and cathodoluminescence spectroscopy and mapping. The sample on SiC has a rougher topography, but it is much more uniform in emission intensity and wavelength than the sample on sapphire.