bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of the Properties of Electrochemically Deposited Semiconductor Materials for Thermoelectric Applications
Authors: Huang C.-K. , Herman J.A. , Myung N. , Lim J. R. , Fleurial J.-P.
Published: April 2011

At JPL, it is our desire to fabricate thermoelectric micro-devices for power generation and cooling applications using an electrochemical deposition (ECD) technique. We believe that the performance of our current micro-device developed is limited by the properties of the ECD materials. Therefore, the objective of this study is to develop ECD methods for obtaining n-type Bi2Te3and p-type Bi2-xSbxTe3thermoelectric materials with near bulk properties, as well as optimizing morphology and transport properties. The films of Bi2Te3and Bi2-xSbxTe3were initially obtained under various ECD conditions. Seebeck coefficients and transport properties were then measured along the direction parallel to the substrates before and after annealing at 250°C for 2hrs. From the data obtained, ECD n-Bi2Te3material can achieve a high Seebeck coefficient (-189 μV/K) when it is deposited at –200 mV vs. SCE. The in-plane resistivity, in-plane mobility, and carrier concentration are 3.0 mohm-cm, 31 cm2V−1S−1, and 6.79 × 1019cm−3, respectively. As for the p-type Bi2-xSbxTe3, it is possible to achieve a high Seebeck coefficient (+295 μV/K) when it is deposited at 0.3 mA/cm2. The in-plane resistivity, in-plane mobility, and carrier concentration are 9.8374 mohm-cm, 66.58 cm2V−1S−1, and 9.54 × 1018cm−3, respectively. From the results of our preliminary study, we have found the conditions for depositing high quality Bi2Te3and Bi2-xSbxTe3materials with thermoelectric properties comparable to those of their state-of-the-art bulk samples.
Theoretical Study of Electronic Structures of Bi2Te3/Sb2Te3 Superlattices
Authors: Li Hong , Bilc Daniel , Mahanti S. D.
Published: April 2011

To understand thermoelectric properties of multiplayer Bi2Te3/Sb2Te3 superlattices, especially their charge transport properties, electronic structure calculations were carried out using ab-initio gradient corrected density functional theory. The superlattice structures of (1,1) and (1,2) Bi2Te3/Sb2Te3 multilayers were optimized and their band structures were compared with each other. Different lattice relaxation effects are observed for the two structures. The cross-plane and inplane effective masses for both these systems are found to be comparable, consistent with experimental mobility measurements.
Synthesis and Thermoelectric Properties of AgBi3S5
Authors: Kim Jun-Ho , Bilc Daniel , Loo Sim , Short Jarrod , Mahanti S. D. , Hogan Tim , Kanatzidis Mercouri G.
Published: April 2011

The known mineral Pavonite AgBi3S5 shows a complex structure composed of NaCl type fragments but has not been studied from the thermoelectric point of view. We present initial results on the synthesis and themoelectric properties of synthetic AgBi3S5, which shows n-type metallic conductivity. In addition, the examination of the solid solutions AgSbxBi3-xS5 (x=0.3) is reported.
Calculating Seebeck Coefficients for Arbitrary Temperature Gradients
Authors: Radkowski Peter P. F. , Sands Timothy D.
Published: April 2011

A novel computational scheme has been used to predict the electric potentials generated by arbitrary temperature gradients in semiconductor materials. Written in object-oriented code, the Discrete State Simulation (DSS) is a coupled cellular automata 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. By tracking the energy-momentum-position coordinates of the individual particles that define the vibronic state at a node, the DSS undercuts equilibrium concepts such as temperature. Consequently, the DSS can represent physical systems that are described by more than one temperature or that contain physical features that defy definitions of temperature. Using modified bootstrap sampling algorithms, the DSS depicted (1) shifts in distribution functions induced by external fields and temperature gradients, (2) field-dependent transitions from linear mobility to non-linear mobility, (3) saturation velocities, (4) non-exponential decay functions generated by multiple phonon scattering modes, and (5) charge separations and electric potentials generated by temperature gradients. Ensemble averages were sensitive to the structure of dispersion relations, to the energy of the system, and to quantum coupling strengths. Seebeck coefficients were sensitive to the features of the electronic and the vibrational band structures, and their associated coupling coefficients.
X-ray Microbeam Diffraction Measurements in Polycrystalline Aluminum and Copper Thin Films
Authors: Moyer L. E. , Cargill G. S. , Yang W. , Larson B. C. , Ice G. E.
Published: April 2011

Thermally induced residual strains in polycrystalline Cu and Al films on single crystal Si and glass substrates, respectively, have been examined on a grain-by-grain basis by x-ray microbeam diffraction. The crystallographic orientation and the deviatoric strain tensor, εij*, are determined for each grain by white beam Laue diffraction. From grain orientation mapping and strain tensor measurements, information is obtained about the distributions of strains for similarly oriented grains, about strain variations within single grains, and about grain-to-grain correlations of strains. This type of information may be useful in developing and testing theories for intergrain effects in strain evolution in polycrystals.
Early yielding and stress recovery in (111) and (100) texture components in Cu thin films determined using synchrotron x-ray diffraction
Authors: Nowak D. E. , Baker S. P.
Published: April 2011

Synchrotron x-ray diffraction experiments were used to study the thermomechanical behavior of individual texture components in passivated Cu thin films. Films were deposited to a thickness of 500 nm on SiNx barrier layers on Si substrates and then passivated with SiNx. The films were highly textured with grains having (111) or (100) planes parallel to the plane of the film. In-plane film stresses were determined separately in the two texture components as a function of temperature during thermal cycles and also during isothermal holds at 140°C. The results are compared to models of yield behavior and anelastic recovery.
Thermal-Mechanical Evaluation of Plated Electro-Magnetic NiFe for MEMS Generators
Authors: Xue Yibin , Hillman Keithan , Veazie David
Published: April 2011

In micro-electro-mechanical system (MEMS) devices that contain multi-layers of metallic, ceramic, and polymeric materials, acquiring a complete understanding of the mechanical properties and interfacial strength are critical in designing the devices. In general, these layered materials are specially fabricated with thickness of each layer on submicron and/or micron scale. Particularly, the properties of these layered materials strongly depend on the fabrication processes and subsequent treatments. In this paper, we study the magnetic metallic films, NiFe (80/20 in weight), for MEMS generator/motor applications. A set of tensile tests was conducted on the plated nickel-iron metallic films of various thicknesses and subjected to various annealing conditions. Distinctive differences in mechanical behaviors between the electroplated films and the corresponding bulk alloys were observed. Furthermore, the NiFe film as-deposited exhibits almost pure elastic deformation and brittle fracture at room temperature; whereas the NiFe film subjected to annealing exhibits strain hardening and ductile fracture when tested at room temperature. X-ray diffraction was used to capture the variation of crystalline structure and the residual stresses in the electroplated films as deposited and after annealing. An atomic force microscope was used to obtain the surface morphology of the films as deposited and after annealing.
Design Maps for the Tensile Yield Strength of Nanoscale Metallic Multilayers
Authors: Lamm Adrienne V. , Anderson Peter M.
Published: April 2011

Metallic nanolayered composite materials can exhibit yield strengths one and a half to two times that of the constituents from which they are constructed. However, experimental data frequently show that there is a critical bi-layer period Λ below which the strength no longer increases with decreasing Λ To help understand the origins of this behavior and to guide future design of multilayers, maps of the internal stress and overall tensile macroyield stress are calculated as functions of the volume fractions of the two alternating constituents and bi-layer period, for a given lattice parameter ratio and elastic modulus ratio. Adopted here is a premise suggested by embedded atom simulations of Cu/Nb multilayers and recent experimental work on γ -Ni/γ-Ni3Al multilayers that the overall tensile strength is determined by the applied stress needed to eliminate the compressive bi-axial stress in the alternating layers. The results indicate that indeed, there is a critical bi-layer period below which the strength is independent of bi-layer period. In this regime, multilayer tensile strength is most effectively improved by increasing the stored compressive stress. This is achieved by decreasing the volume fraction of the compressively stressed phase. This manuscript extends previous work by providing closed-form expressions for the macroyield strength of free-standing multilayered thin films.
Thin Film Herringbone Buckling Patterns
Authors: Chen Xi , Hutchinson John W.
Published: April 2011

Thin metal film deposited on compliant substrate undergoes equi-biaxial compression and buckles into a highly ordered herringbone pattern [1,2]. In this study, it is shown that compare with the bifurcation competing modes, the herringbone mode has the lowest strain energy and therefore it is the preferred buckling pattern in the thin film.
The Effect of Oxygen on Adhesion of Thin Copper Films to Silicon Nitride
Authors: Pang Mengzhi , Backhaus-Ricoult Monika , Baker Shefford P.
Published: April 2011

The effect of oxygen on adhesion and chemistry at interfaces between Cu thin films and SiNx barrier layers on Si substrates was investigated. Films were deposited in an ultra-high-vacuum sputter deposition chamber with good control of oxygen content. Adhesion was measured using a high throughput driver film method. Microstructure and bonding were investigated using transmission electron microscopy and electron energy loss spectroscopy, respectively, on sample cross sections. For films to which a small amount of oxygen was added during deposition, oxygen segregated to Cu/SiNx interfaces during thermal cycling, where it induced charge transfer and the presence of Cu+ interfacial states, and significantly reduced the work of adhesion.
Toughness and Contact Behavior of Conventional and Low-k Dielectric Thin Films
Authors: Cook Robert F. , Morris Dylan J. , Thurn Jeremy
Published: April 2011

A comprehensive indentation fracture mechanics framework is established that allows the fracture properties of thin films to be determined. The framework is composed of four stress-intensity factors characterizing the stress fields arising from (i) elastic contact, (ii) wedging, (iii) residual elastic-plastic mismatch and (iv) pre-existing film stress. The amplitudes of the stress-intensity factors depend on the deformation properties of the film and vary throughout the indentation cycle. The toughness values of a PVD alumina, for which (iii) and (iv) are dominant, and a low-k film, for which (i), (ii) and (iv) are dominant, are evaluated.
Formability of ultra-thin plasma-polymer films deposited on metal sheet: mesoscopic and nanoscopic aspects of defect formation
Authors: Baumert B. , Stratmann M. , Rohwerder M.
Published: April 2011

Forming of pre-coated metal sheet becomes of increasing importance in various fields of industrial application. For instance, the trend in automotive industry goes for ready coated steel sheet that can be formed and cut without loss of performance. This so called “finish first, fabricate later” concept increases the value of the steel sheet supplied by the steel industry and releases the automotive industry from the burden of the coating process. Consequently, studies on the effect of forming on the performance of corrosion protective coating systems adhering to metals sheet are carried out in order to determine the forming limiting curves for existing coatings [1]. New coating systems are sought in order to improve the formability of coating/metal sheet composites. As a very promising step into this direction, in the recent years ultra-thin plasma-polymer films have emerged as candidates for substituting phosphatation and the environmentally critical chromatation as pre-treatments for steel sheet. These Si-containing plasma polymers provide good corrosion protection properties to steel as well as to zinc-coated steel [2–3].
Scratch Test Measurements on CrNx Coatings
Authors: Hoy R. , van Essen P. , Kamminga J.-D. , Janssen G. C. A. M. , Ehiasarian A. P.
Published: April 2011

Hard CrNx coatings of various composition were deposited on hot work tool steel by means of reactive sputter deposition in an industrial PVD reactor. All deposited coatings (thickness 3–4 micron) were under compressive stress. The coatings were subjected to scratch testing with a diamond stylus. Two critical loads were determined. The first critical load, Lc1, indicates the onset of chipping and/or spalling. The second critical load, Lc2, indicates complete coating removal inside the track. Micrographs showing the coating failure are presented. Stoichiomeric CrN coatings outperform understoichiometric CrN0.6 coatings in scratch tests. It was found that for all specimens the coating-to-substrate adhesion is very good, independent on composition. Although the values of Lc1 and Lc2 are distinctly different, their dependence on composition is similar.
Resistance to cracking of a stretchable semiconductor: Speed of crack propagation for varying energy release rate
Authors: Liu Sheng , Lim Hee C. , Qu Min , Federici John F. , Thomas Gordon A. , Gleskova Helena , Wagner Sigurd
Published: April 2011

We have measured and calculated the propagation velocity of successive cracks in a single sample of amorphous SiNx as a function of energy release rate. We have obtained the conditions for controlled, repetitive crack formation by using a substrate of compliant plastic that survives the cracking of a thin film formed on it. We have recorded the crack velocity curves using high-speed micro-photography using dark field illumination. Under uniform, uniaxial tensile strain, the films crack in an array of essentially straight, parallel lines, if the increase of the strain density is slow. We find reasonable agreement in the comparison of theory and experiment and find a linear relationship between the initial velocity and energy release rate threshold. Consequently, in cases where the theoretical agreement with the data is reasonable, the successive cracks show velocity curves that scale with each other.
Computational and Experimental Characterization of Indentation Creep
Authors: Dao Ming , Takagi Hidenari , Fujiwara Masami , Otsuka Masahisa
Published: April 2011

ABSTRACT:Detailed finite-element computations and carefully designed indentation creep experiments were carried out in order to establish a robust and systematic method to accurately extract creep properties during indentation creep tests. Finite-element simulations confirmed that, for a power law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent, n, the activation energy for creep, Qc and so on can be evaluated robustly. Samples made from an Al-5.3mol%Mg solid solution alloy were tested at temperatures ranging from 573 K to 773 K. The results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime.
Preparation of Oxygen Ion Conducting Doped Lanthanum Gallate Thin Films on Amorphous and Single Crystal Substrates by Pulsed Laser Deposition
Authors: Mitsugi Fumiaki , Kanazawa Seiji , Ohkubo Toshikazu , Nomoto Yukiharu , Takita Yusaku , Ishihara Tatsumi
Published: April 2011

An La1-xSrxGa1-y-zMgyCozO3-(x+y+z)/2 (LSGMCO) has attracted much attention because it can be useable as an electrolyte of a solid oxide fuel cell due to its high oxide ion conductivity. We prepared LSGMCO thin films on silica glass and LaAlO3 single crystal substrates by pulsed laser deposition and evaluated their properties. LSGMCO thin films deposited at 800°C were poly-crystal and the deposition pressure affected their surface morphologies. In the case of the LaAlO3 single crystal substrate, a c-axis oriented LSGMCO thin film was obtained. DC conductivity and complex impedance of LSGMCO thin films were measured in vacuum atmosphere to investigate the effect of the crystal orientation on the oxide ion conductivity. It was revealed that resistance at a grain boundary of films is more dominant compare with the grain interior.
Electrical Properties of β-FeSi2 Thin Films on Insulating Substrates
Authors: Akiyama Kensuke , Kimura Takeshi , Nishiyama Shin , Hattori Takeo , Ohashi Naoki , Funakubo Hiroshi
Published: April 2011

Iron silicide thin films were prepared on insulating substrates using RF magnetron sputtering method. Amorphous, polycrystalline and epitaxial β-FeSi2 were obtained on MgO(001), Al2O3(110) and Al2O3(001) substrates, respectively. Electrical conductivities of these films showed similar temperature dependence. Intrinsic band conduction and hopping conduction mechanism were predominant above and below 600K, respectively. The localized ordering in the polycrystalline and epitaxial films that controled the movement of carriers were as low as in the amorphous film. For the epitaxial β-FeSi2 film, electrical conductivity below 600K were affected by atomic ratio of silicon to iron (Si/Fe) in the films, because the localized ordering in the films decreased as Si/Fe atomic ratio decreased.
Role of Interface Quality and Film Doping Density in Amorphous Si / Crystalline Si Heterojunctions for Photovoltaic Applications
Authors: Farrokh Baroughi M. , Sivoththaman S.
Published: April 2011

We have studied some key aspects of the (n+)a-Si/(p)c-Si heterojunction structures photovoltaic applications. The importance of the hetero-interface quality has been studied by both theoretical simulations and experimental characterization. The surface treatment prior to the film deposition is very critical. Even though H-plasma passivation can help passivate bulk defects in low quality Si, extreme care should be taken to avoid plasma induced surface damage which will result in a defective hetero-interface. Furthermore, (n+) a-Si films with different doping levels have also been characterized and their influence on the device performance has been studied.
“Transport limiting mechanisms of amorphous (a-Si:H) and microcrystalline(mc-Si:H) thin film heterostructures for photovoltaic application”
Authors: Khan Shahrukh A. , Anderson W.A.
Published: April 2011

Direct conversion of sunlight into electricity employing amorphous (a-Si:H) and microcrystalline (mc-Si) silicon solar cell structures provides cheap, continuous, large-area device capability. However, the inherent instability due to light induced degradation of such cells poses a challenge for practical photovoltaic applications. To work around this problem, we have made use of a-Si:H/c-Si (crystalline) and mc-Si/c-Si heterostructures. Plasma decomposition of 2% silane gas (SiH4/He) in an ECR (electron cyclotron resonance)-CVD configuration forms the basis of the deposition technique. Electro-optical properties exhibit photoconductivity (σp) of 6.5×10−6 S/cm and a low dark conductivity (σd) of 1.4×10−9 S/cm for a-Si:H films and a relatively high σp of 2.1×10−4 S/cm and a high σd of 1.2×10−7 S/cm for mc-Si films. A possible multi-tunneling transport mechanism is evident for a low forward bias with the current conduction becoming space charge limited with increasing bias. The domination of this latter mechanism may be responsible for non-ideal junction behavior.
Assembly of CdSe/CdS Quantum Dots on Au Surfaces for Photoreception
Authors: Tang Jing , Birkedal Henrik , McFarland Eric W. , Stucky Galen D.
Published: April 2011

CdSe/CdS core/shell quantum dots have been synthesized and assembled onto pre-functionalized gold surfaces by either hydrogen bonding or covalent bonds through different functional groups. Control of the conditions during the deposition process allows producing a high coverage of quantum dots via molecular linkages. The quantum-dot surface is highly photoactive and is used in a surface sensitized Schottky barrier photovoltaic structure as the photoreception component. Atomic force microscopy (AFM) and X-ray photoelectron Spectroscopy (XPS) are used to characterize and confirm the morphology and linkage of the assemblies on Au surfaces. The electron transfer from the quantum-dot layer to the Schottky barrier device is examined by measuring the current-voltage (IV) curve of such photovoltaic devices under simulated sun light.
Enhancement of Photoelectric Effect in Organic Dye Thin Film Cells by Surface Plasmon Excitation
Authors: Kato Keizo , Hirano Masaru , Shinbo Kazunari , Kaneko Futao , Wakamatsu Takashi
Published: April 2011

Photoelectric properties have been investigated for the organic dye thin film cells utilizing surface plasmon (SP) excitation. The cells fabricated in this work had a prism/Al thin film/copper phtalocyanine (CuPc) thin film/Ag thin film structure, which was the Kretschmann configuration in the attenuated total reflection (ATR) method. Since the CuPc thin film exhibits p-type conduction, the Schottky and Ohmic contacts can be obtained at the interfaces between the CuPc and Al thin films and between the CuPc and Ag thin films, respectively, and the cells used in this work show photoelectric properties. The ATR and short-circuit photocurrent properties have been simultaneously measured as a function of the incident angles of the laser beams. The experimental and calculated results of the ATR and short-circuit photocurrent properties and the optical absorptions of the organic dye layers were discussed. It was found that the photoelectric effect in the organic dye thin film cells the Kretschmann configuration in the ATR method was enhanced by the SP excitation.
Solvent Processible Composite Carbon Nanotube Cathode for Polymer LED Applications
Authors: Liu Gao , Johnson Steve , Kerr John B.
Published: April 2011

Composites of single wall carbon nanotubes (SWNT) and conductive polymers were studied as potential cathode materials for application in polymer light emitting devices. A new conductive poly(2,7–9,9 (di(oxy-2,5,8-trioxadecane))fluorene) (PFO) possessing surfactant properties was used to stabilize SWNTs in solutions. The rigid PFO backbones act as a template while the ethyleneoxide side chains appear to wrap around the SWNTs. Up to 0.02% (by weight) of SWNTs are stabilized in the solution phase. The current vs. voltage behavior of the SWNT/PFO composite film (2% SWNT in PFO by weight) shows that most of the current is carried by SWNTs.
High Performance Solar Blind Detectors based on AlGaN grown by MBE and MOCVD
Authors: DUBOZ Jean-Yves , Reverchon Jean-Luc , Mosca Mauro , Grandjean Nicolas , Omnes Franck
Published: April 2011

Solar blind detectors based on AlGaN grown by Molecular Beam Epitaxy and Metal Organic Vapor Phase Epitaxy have been fabricated and characterized. Metal Semiconductor Metal (MSM) detectors and vertical Schottky detectors have been realized, with a design that allows back side illumination. The growth was optimized in order to improve the layer quality, avoid crack formation, and provide the best detector performance. The technological process was also optimized in order to reduce the dark currents and improve the spectral rejection ratio, which is a key factor for solar blind detection. As a result, a rejection ratio of 5 decades between the UV (below 300 nm) and 400 nm, and a steep cut off limited by alloy fluctuations have been obtained. A noise equivalent power below 10 fW is obtained in MSM detectors.
GaN quantum dot UV light emitting diode
Authors: Lee Jeong-Sik , Tanaka Satoru , Ramvall Peter , Okagawa Hiroaki
Published: April 2011

The fabrication and evaluation of a UV light-emitting diode (LED) incorporating GaN quantum dots as the active layer is demonstrated. The GaN quantum dots were fabricated on an AlxGa1-xN (x∼0.1) surface using Si as an antisurfactant. Exposing the AlxGa1-xN surface to the Si antisurfactant prior to GaN growth enabled the formation of quantum dots on a surface where growth by the Stranski-Krastanov mode would not be possible. A fairly high density of dots (1010-1011 cm-2) with controllable dot sizes was achieved. Room temperature luminescence at 360 nm was clearly observed during current injection (cw) into an LED structure including the GaN quantum dots. The origin of the electroluminescence is discussed by comparing it to photoluminescence measurements.
LP-MOCVD growth of GaAlN/GaN heterostructures on Silicon Carbide. Application to HEMT's devices.
Authors: di Forte Poisson M-A. , Magis M. , Tordjman M. , Aubry R. , Peschang M. , Delage S. L. , di Persio J. , Grimbert B. , Hoel V. , Delos E. , Ducatteau D. , Gaquiere C.
Published: April 2011

This paper reports on the LP-MOCVD growth optimisation of GaAlN/GaN heterostructures grown on Silicon Carbide substrates for HEMT applications, and on the first device performances obtained with these structures. The critical impact of some growth parameters on the physical properties of the GaAlN/GaN epilayers has been identified and studied using High Resolution X-Ray diffraction (HR-XRD), AFM, C-V and sonogauge measurements. The SiC substrate surface preparation (both ex-situ and in-situ) and the nucleation layer growth conditions (growth temperature, thickness, composition and strain) have been found to be key steps of the GaAlN/GaN/SiC growth process. SiC substrates from different suppliers have been evaluated and their influence on the physical properties of the GaAlN/GaN HEMT structures investigated. Static characteristics of the devices such as maximum drain current Idss or pinch-off voltage have been correlated with the nucleation layer composition of the HEMT structure and the defect density of the SiC substrate. First devices measured at 10 GHz using a load pull system exhibited CW output power in excess of 2.8 W/mm for a gate length of 0.5 μm. Under static measurements, we found an Idss around 1 A/mm and a pinch-off voltage of –5 V.
Thermal Conductivity of GaN Grown on Silicon Substrates
Authors: Mion C. , Chang Y. C. , Muth J. F. , Rajagopal P. , Brown J. D.
Published: April 2011

One of the principle problems of high power electronic devices is the extraction of heat from the active region of the device. The thermal conductivity of the substrate is a crucial parameter affecting the thermal dissipation capability of a device. In this study we investigated the thermal conductivity at room temperature of five GaN samples grown on silicon, employing the 3 omega thermal conductivity method. The thickness of the GaN layers varied from 0 to 700 nm. The effective thermal conductivity for the GaN layers was found to range from 130 to 140 W/m·K which is comparable to thermal conductivity of the silicon substrate. This result indicates that the heat transfer of GaN on silicon is as good as either GaN or silicon alone, and that no substantial thermal degradation attributable to the GaN/Silicon interface is observed.
Arsenic Incorporation Behavior in Nitrogen-rich GaNAs Alloys Synthesized by Metalorganic Chemical Vapor Deposition (MOCVD)
Authors: Gherasimova M. , Wheeler R. G. , Guido L. J. , Chang K. L. , Hsieh K. C.
Published: April 2011

Homogeneous GaNAs alloy films containing 3 to 4 percent of arsenic anion fraction were synthesized by metalorganic chemical vapor deposition (MOCVD). Post-growth annealing resulted in the formation of GaAs precipitates of several nanometers in size embedded in the nitrogen-rich GaN(As) matrix, while as-grown ternary films exhibited no evidence of phase segregation. Band gap reduction due to alloying was observed by the optical transmission measurements, leading to an estimate for the bowing parameter of 25 eV in the films with the arsenic content of 3.5 %.
Investigation of molecular co-doping for low ionization energy p-type centers in (Ga,Al)N
Authors: Feng Zhe Chuan , Payne Adam M. , Nicol David , Helm Paul D. , Ferguson Ian , Senawiratne Jayantha , Strassburg Martin , Dietz Nikolaus , Hoffmann Axel , Hums Christoph
Published: April 2011

This work initiates an investigation of molecular co-doping to produce p-type centers in (Ga,Al)N with ionization energies lower than Mg. Dopant complexes can be formed between a doubly ionized acceptor such as (Cu, Li or Ag) and a singly ionized donor (silicon). Ion implantation of Cu, Li and Ag into silicon doped GaN films grown by Metalorganic Chemical Vapor Deposition (MOCVD) has been performed. Secondary ion mass spectroscopy (SIMS) data confirmed the simulated depth profile. High resolution X-ray diffraction and Raman spectroscopy were used to characterize the crystalline damage and subsequent recovery upon anneal. A complete recovery was observed after high temperature (700–900°C) annealing. Low temperature (6K) photoluminescence (PL) for Cu-implanted GaN showed bands identified with crystalline lattice damage due to the Cu-implantation. The annealed samples showed recovery of standard crystalline GaN features. Additional donor-acceptor pair features are observed below 3.35 eV indicating the existence of an acceptor state.
Novel Method for the Activation of Acceptor Dopant in AlN Introducing Localized Band by Isoelectronic Dopant
Authors: Takizawa Toshiyuki
Published: April 2011

In this study we propose a novel method to increase hole concentration introducing isovalent substitutional dopant into a p-type nitride semiconductor. Acceptor dopant in nitride semiconductors makes deep acceptor level (>100 meV) and generates few hole carriers into the valence band because of large electron affinity of N.In contrast to this, substitution of isovalent group-V atoms (P, As and Sb) that has smaller affinity than N makes a localized group-V band upward the valence band maximum (VBM). When both acceptor and isovalent group-V atoms are incorporated into nitride semiconductor, holes can be drastically activated by isovalent atoms, and can easily move in the group-V band. We have also investigated this material, Mg-doped AlN:V (V=P, As or Sb), using first-principles pseudopotential method. As a calculation result, substitiution of P and As makes localized group-V band upward the VBM of AlN, and moreover this can be adjusted the VBM of GaN. The Mg incorporation into AlN:V as an acceptor dopant drastically decreases the Fermi level (ΔEF=-0.10 eV), that is, hole concentration can be drastically raised by the group-V band. Consequently novel p-type material with isovalent dopant can be a candidate to efficiently inject hole current into the VBM of GaN.
Fabrication of Thermoelectric Devices Using AlInN and InON Films prepared by reactive radio-frequency sputtering
Authors: Yamaguchi S. , Izaki R. , Kaiwa N. , Sugimura S. , Yamamoto A.
Published: April 2011

We have studied the thermoelectric properties of AlInN and InON films prepared by reactive radio-frequency (RF) sputtering. We have fabricated thermoelectric devices which are composed of 20-pair nitride or oxynitride films with Chromel metal. For a AlInN device, the maximum output power was 7.6×10-7 W at ΔT = 257K, and for a InON device, that was 6.5×10-8 W at ΔT = 214K.
Synthesis and characterization of high quality ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K
Authors: Wu Stephen Y. , Liu H. X. , Gu Lin , Singh R. K. , van Schilfgaarde M. , Smith David J. , Dilley N. R. , Montes L. , Simmonds M. B. , Newman N.
Published: April 2011

Reactive MBE growth was used to synthesize ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K. 2% Cr-doped GaN and 7% Cr-doped AlN were found to have a saturation magnetization moment of 0.42 and 0.6 μB/Cr atom, indicating that ∼14% and ∼20% of the Cr, respectively, are magnetically active. Structural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) did not find evidence of a ferromagnetic secondary phase. Electrical characterization indicate that the resistivity of the Cr-doped GaN films depends exponentially on temperature as R=Roexp[(To/T)1/2], characteristic of variable range hopping. In contrast, Cr-doped AlN films are highly resistive. Local spin density functional calculations predict that Cr forms a deep level defect in both systems and the t2 level falls approximately at midgap. Our theoretical and experimental results indicate that ferromagnetism in Cr-doped GaN and AlN arises as a result of the double exchange mechanism within the partially filled Cr t2 band.
GaN Epitaxial Growth Process at High Growth Temperature by NH3 Source Molecular Beam Epitaxy
Authors: Ohshima Naoki , Sugihara Akihiro , Yoshida Naoya , Okabe Naohiko
Published: April 2011

We have investigated in detail dependence of annealing GaN buffer layer and GaN growth processes on a sapphire substrate at a high temperature of 1000 degree C. The GaN layers are grown by NH3 gas source molecular beam epitaxy. The behavior of GaN buffer and epitaxial layer has been observed by in-situ reflection high-energy electron diffraction and the surface morphologies of as-grown and chemically etched GaN layers by atomic force microscopy. It is found that there is distinct difference in the surface morphology of epitaxial GaN layer between at growth temperatures of below 950 degree C and that of 1000 degree C. It has been considered that the growth kinetics of GaN epitaxial layer extremely depends on the growth temperature.
Structural Characterization of Low-Temperature InN Buffer Layer Grown by RF-MBE
Authors: Araki T. , Nanishi Y.
Published: April 2011

The microstructure of an InN buffer layer grown on (0001) sapphire at low temperature by radio-frequency molecular beam epitaxy (RF-MBE) is characterized by transmission electron microscopy. The low-temperature InN buffer layer is found to contain local inhomogeneous regions of island-like grains surrounded by misoriented InN grains and inclusions of cubic phase. The generation of such anti-phase InN nuclei near the island-like grains is expected to give rise to defects at the interface. It is considered that these anti-phase InN nuclei are formed by local fluctuations of stoichiometry due to inadequate surface migration during the growth of the InN buffer layer, indicating the important of controlling the surface stoichiometry during InN growth.
Synthesis, Structure and Luminescence of High Brightness Gallium Nitride Powder
Authors: Garcia R. , Thomas A. , Bell A. , Stevens M. , Ponce F. A.
Published: April 2011

Highly luminescent GaN powders have been synthesized by reacting high purity gallium metal with ultra-high purity ammonia in a horizontal quartz reactor at 1100 °C. The powders produced in this reactor consist of light grey micro-crystals with wurtzite structure. Elemental analysis indicates that the powders obtained by this method have a high nitrogen concentration (more than stoichiometric GaN, 16.73 weight %). Powder X-ray diffraction demonstrates that the material has a high purity and single crystalline structure. Electron microscopy shows that the powders consist of at least two kinds of particles, small sized platelets and large sized needles. The resulting GaN powders are thousands of times more cathode- and photo-luminescent than other GaN powders including commercially available material. Their luminescence intensities are comparable to that of GaN thin films grown by hydride vapor phase epitaxy and metal organic chemical vapor deposition.
Temperature Dependence of the Optical Properties for InN Films Grown by RF-MBE
Authors: Ishitani Y. , Xu K. , Terashima W. , Masuyama H. , Yoshitani M. , Hashimoto N. , Che S. B. , Yoshikawa A.
Published: April 2011

InN epitaxial layers are grown on sapphire substrates. The investigated samples have electron concentration in a range of 1.8 ×1018 − 1.1 ×1019 cm-3. Optical reflection and transmission measurements are performed. The plasma edge energy position in the spectra is constant in a measurement temperature range of 5 – 300 K. The reflection and transmission spectra are calculated on the basis of the LO phonon-electron coupling scheme and non-parabolic conduction band structure. From this analysis we find that the observed absorption edge is attributed to valence band to conduction band transition rather than the valence band to defect (impurity) band transition, and the intrinsic bandgap energy of 0.64 (±0.03) eV. This bandgap energy increases by 40 – 50 meV as the temperature decreases from 295 to 10 K.
Study of the growth mechanism and properties of InN films grown by MOCVD
Authors: Jain Abhishek , Redwing Joan M.
Published: April 2011

Thin films of InN were grown on (0001) Sapphire by MOCVD. The effect of growth conditions and buffer layer on the film morphology was studied. Growth temperature and TMI flow rate were important factors in the growth of InN. The use of a low temperature AlN buffer layer was also found to improve the morphology and crystal quality of the films. Thin (<40Å) AlN buffer layers produced the best results while polycrystalline InN was obtained when the buffer layer thickness exceeded 60Å. Delamination of the InN films was observed to occur at growth temperature, which limited the thickness of the films to less than 300 nm. A room temperature mobility of 792 cm2/Vs and an electron concentration of 2.1×1019 cm-3 were measured in an approximately 200 nm thick InN layer grown on sapphire.
Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method
Authors: Hanser Drew , Liu Lianghong , Preble Edward A. , Thomas Darin , Williams Mark
Published: April 2011

High quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 μm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution X-ray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×106 to 1×107 cm-2.
Growth of GaN crystals under ammonothermal conditions
Authors: Callahan Michael J. , Wang Buguo , Bouthillette Lionel O. , Wang Sheng-Qi , Kolis Joseph W. , Bliss David F.
Published: April 2011

Growth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 μm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM.
Vapor phase transport of AlN in an RF heated reactor: Low and high temperature studies
Authors: Noveski V. , Schlesser R. , Freitas J. , Mahajan S. , Beaudoin S. , Sitar Z.
Published: April 2011

AlN crystals were grown from the vapor phase in an RF heated AlN sublimation reactor. The studies were performed with the following goals: 1) to optimize the growth rate by investigating mass transfer effects, and 2) to establish a process for epitaxial growth on AlN seeds. A one-dimensional mass transfer model based on equilibrium sublimation and gas-phase diffusion was developed. Model parameters were estimated and the model was validated from growth experiments carried out in a 600 Torr nitrogen atmosphere and temperatures ranging from 2000 to 2400°C. Continuous growth on AlN seed crystals was accomplished as a result of optimizing the initial stage of growth and achieving a delicate balance between the rate of mass transfer and the rate of surface rearrangement. During this experimental study, centimeter-size single crystals of AlN were obtained within the 1.25” diameter boule that was grown at a predicted growth rate of 0.1–0.3 mm/hr, at 500 Torr of nitrogen, short source-to-seed distance, low supersaturation and growth temperatures of 2110–2140°C. Chemical analysis of impurities in the grown AlN boules confirmed a very low oxygen contamination of 100 ppm wt. Cathodoluminescence studies showed well defined near band edge emission peak located slightly above 6 eV.
HNO3 treatment of sapphire for management of GaN polarity in MOCVD method: Comparison of the properties of +c and –c GaN region
Authors: Takabe Motoki , Sumiya Masatomo , Fuke Shunro
Published: April 2011

GaN film with Ga- (+c) and N- (-c) face polarities was simultaneously grown on c-plane sapphire substrate by using only metalorganic chemical vapor deposition. Although several kinds of processes were examined, we achieved the simultaneous growth by using the patterned AlN nucleation layer or selective treatment of sapphire surface in HNO3 solution. The latter process improved not only the quality of the sample but also the boundary between +c and –c GaN region, because the treat of substrate itself for controlling the polarity of GaN was carried out at lower temperature. The PL and Raman spectra of the simultaneous growth samples indicated larger amount of impurities in –c GaN side, which was consistent with our previous work [Sumiya et al. Appl. Phys. Lett. 76, 2098 (2000)].
Analysis of InGaN-GaN quantum well chemistry and interfaces by transmission electron microscopy and X-ray scattering
Authors: Smeeton T. M. , Kappers M. J. , Barnard J. S. , Humphreys C. J.
Published: April 2011

We have studied the distribution of indium in InxGa1-xN single quantum wells (SQWs) with compositions of In0.18Ga0.82N and In0.22Ga0.78N. Based on lattice parameter maps extracted from HRTEM lattice fringe images, we do not find evidence for strong nanometre-scale fluctuations in the composition of the wells (indium “clusters”). Z-contrast (high-angle annular dark field) scanning TEM (STEM) and X-ray reflectivity (XRR) results both show that the width of the interface at the top of the quantum well is slightly greater than that at the bottom, which is quite abrupt. The quantum wells exhibit bright photoluminescence so these results suggest that the conventional explanation for the high efficiency of light emission from InGaN LEDs may not apply in these structures.
High Temperature Implantation of Tm in GaN
Authors: Lorenz K. , Wahl U. , Alves E. , Dalmasso S. , Martin R. W. , O'Donnell K. P.
Published: April 2011

Thulium ions were implanted into MOCVD grown GaN films with a fluence of 2.5×1015 at/cm2 at temperatures between 20 and 500 °C. The lattice damage introduced by the implantation and the effect of post-implant annealing were investigated using the Rutherford backscattering/channeling (RBS/C) technique. Whereas for room temperature implantation the implanted layer becomes amorphous, high temperature implantation inhibits amorphisation. For implantation temperatures higher than 300 °C the RBS/C results clearly show two different damage regions - one at the surface and the second deeper in the crystal coinciding with the Tm depth profile. Annealing causes a decrease of the surface damage as well as initiating regrowth from the unimplanted bulk GaN. For the samples that were not completely amorphous a large part of the Tm atoms were found to be incorporated in Ga-sites. The optical properties of the ion implanted GaN films have been studied by room temperature cathodoluminescence. Directly following the implantation no Tm-related luminescence was observed. Subsequent annealing of the samples achieved optical activation, revealing well-defined emissions due to intra-4f-shell transitions of the Tm3+ ions in the blue spectral range at 477 nm and in the near infra-red at 804 nm.
Reciprocal Space Mapping of X-Ray Diffraction Intensity of GaN-Based Laser Diodes Grown on GaN Substrates
Authors: Tachibana K. , Harada Y. , Saito S. , Nunoue S. , Katsuno H. , Hongo C. , Hatakoshi G. , Onomura M.
Published: April 2011

Characterization by reciprocal space mapping of x-ray diffraction (XRD) intensity was carried out for epitaxial layers of GaN-based laser structures on two GaN substrates: GaN substrate and GaN template on sapphire substrate. The difference between these two substrates was shown clearly. The distribution of XRD intensity of the epitaxial layers on GaN substrate was smaller than that of the epitaxial layers on GaN template on sapphire substrate. In the lasers with the epitaxial structure on GaN substrate, the light output power was as high as 200 mW under continuous-wave operation at room temperature. Excellent noise characteristics with relative intensity noise of -132 dB/Hz were also obtained at a low light output power of 3 mW without any high-frequency modulation. These results support that GaN substrates are promising for realizing GaN-based lasers with high performance.
Spectroscopy of Intraband Electron Confinement in Self-Assembled GaN/AlN Quantum Dots
Authors: Helman Ana , Moumanis Khalid , Tchernycheva Maria , Lusson Alain , Julien Francois , Damilano Benjamin , Grandjean Nicolas , Massies Jean , Adelmann Christophe , Fossard Frederic , Dang Daniel Le Si , Daudin Bruno
Published: April 2011

We present a detailed analysis of inter- and intraband transitions in GaN/AlN self-organized quantum dots grown on sapphire, silicon (111) and 6H-SiC substrates. Quantum dot samples have been characterized by means of transmission electron microscopy, photoluminescence and photo-induced absorption spectroscopy. Interlevel transitions in the conduction band are observed in the 0.52–0.98 eV energy range, thus covering the telecommunication band. The s-pz absorption is peaked at 0.8 eV (0.52 eV) for samples with dot height of 1.5 nm (6 nm). Calculations show that in bigger dots the transition energy is governed by the value of the internal field.
Structure and photoluminescence investigations of Er doped GaN layers grown by MBE
Authors: Wojtowicz T. , Ng H. M. , Ruterana P.
Published: April 2011

In this work, we carry out TEM analysis on GaN layers grown on sapphire and doped in situ by MBE. In parallel, photoluminescence and electroluminescence experiments are used to determine the possible emission of the grown layers. It has been shown previously that the emission peaks at Er concentrations of about 1% and that their intensity dramatically decreases with increasing Er concentration probably due to compositional quenching. We report on the evolution of the microstructure versus composition and try to explain the quenching effects that can be related to the microstructure.