bluuebunny/crossref_metadata_2025 huggingface dataset:
ESR investigations on Ca perovskite
Authors: Chipara M. , Liou Sy-Hwang , Borca C. N. , Shoemaker R. , Adenwalla S. , Dowben P. A.
Published: April 2011

Electron spin resonance studies on fine powders of La0.65Ca0.35MnO3, performed in the X band, are reported. The coexistence of paramagnetic and ferromagnetic phases, in a narrow temperature range close to the Curie temperature, is observed. The electron spin resonance measurements do not support the presence of bipolarons above the Curie temperature. Temperature dependence of the ESR linewidth is governed by the hopping of polarons and the corresponding activation energy is about 150 meV above TC.
Structure and Magnetic Properties of Nd-Co-Fe-Si-C Intermetallics
Authors: Sorescu Monica , Grabias A. , Valeanu M.
Published: April 2011

Intermetallics of the type Nd2Co15−xFexSi2Cy (x=7; 9; 12 and y=0; 1) were prepared by arc melting in order to study the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in these systems. The investigations were performed by x-ray diffraction, Mössbauer spectroscopy and hysteresis loop measurements. Both site and average hyperfine magnetic field values were analyzed in order to discriminate between the Fe and Co contributions to the local atomic environments in these systems. While a higher Fe content as well as a higher Co content can both lead to an increase in the site magnetic fields, it was found that the model based on hyperfine field distributions provides a better description of the trends observed in hyperfine magnetic fields in these compounds.
Magnetic Behavior and Structure of Electrodeposited, Mechanically Hard Fe-C and Fe-Ni-C Alloys
Authors: Haseeb A. S. M. A. , Hayashi Y. , Masuda M.
Published: April 2011

Iron-carbon based hard, martensitic alloys are usually produced by conventional high temperature heat treatment. In the present work, the galvanostatic electrodeposition method has been employed to obtain hard Fe-0.96 mass % C and Fe-15.4 mass% Ni-0.70 mass% C alloys at around room temperature. The alloys have been investigated by SEM, XPS, XRD, and microhardness measurements, and their magnetic properties have been studied by vibrating sample magnetometer.

The as-deposited alloys were found to possess high mechanical hardness, 750-810 HV. Both alloys exhibit a smoother surface morphology as compared to a non-alloyed iron film obtained under similar electrochemical conditions. The coercive force of the as-deposited Fe-C and Fe-Ni-C alloys is 3930 and 494 A.m−1respectively. In comparison, pure iron film deposited under similar conditions possesses a coercive force of 1592 A.m−1. The Fe-Ni-C alloy has a combination of high mechanical hardness and relatively soft magnetic properties, which may be of interest in potential applications requiring both soft magnetic properties and improved tribological performance. The effects of annealing on the behavior of the alloys are discussed.
Changes in Stress and Microstructure in PtMn/CoFe Bilayers during Annealing
Authors: Bozeman S.P. , Daniels B.J. , Larson D.J.
Published: April 2011

PtMn is one of several candidate antiferromagnetic materials for biasing of spin valve giant magnetoresistive (GMR) sensors used in magnetic recording heads. The as-deposited crystal structure of PtMn is face-centered cubic, which is not antiferromagnetic, and it is commonly annealed to transform it to the ordered face-centered tetragonal structure, which is antiferromagnetic. The changes in the thin film stress during this transformation can be up to 1.5 GPa in magnitude. In this work, the stresses in sputter-deposited, PtMn/CoFe bilayers were measured during annealing using a laser-based wafer curvature technique. During annealing, the stress initially increases before approaching a constant level. This behavior was examined for samples with a range of PtMn compositions and varying degrees of PtMn {111} texture. Both the magnitude and rate of change of the stress are sensitive to the composition of the films, but are relatively insensitive to the film texture. These results are discussed in terms of the binary alloy phase diagram.
Raman Spectroscopy: A Unique Tool for the Study of Thin Films
Authors: De Wolf Ingrid
Published: April 2011

In this paper, the different applications of Raman spectroscopy for the study of thin films is briefly discussed, using examples from microelectronics. Special attention is given to the application of micro-Raman spectroscopy for the measurement of local stress in and near films.
In Situ Stress and Strain Measurements During the Growth of Cu/Ni (001) Multilayers
Authors: Bigault T. , Bocquet F. , Labat S. , Thomas O. , Marty A. , Gilles B.
Published: April 2011

Cu/Ni (001) multilayers have been grown by molecular beam epitaxy at room temperature. In-situ electron diffraction and curvature measurements performed during the growth are presented. The average lattice parameter in the equiatomic multilayers evolves gradually towards the alloy lattice parameter. The in-plane lattice parameter of both Cu and Ni evolves continuously towards the bulk lattice parameter with no evidence of pseudomorphic growth. The combination of diffraction and curvature measurements suggests that the Ni on Cu interface is diffuse. This is attributed to the surfactant behaviour of Cu. This results shed new insights into the interesting magnetic properties of Ni films on Cu (001).
Si1−xGex Island Formation by Post-Growth Anneal on Supercritical Layers Grown by RPCVD
Authors: Grimm K. , Vescan L. , Nanver L.K. , Visser C.C.G. , Lüthl H.
Published: April 2011

SiGe island layers have been created by post growth anneal on supercritical layers grown at low temperatures (500°C) and high hydrogen pressures (40 Torr). The epitaxial growth has been performed in a commercially available single wafer RPCVD reactor using SiH2Cl2 and GeH4 as precursor gasses. SiGe layers grown under these conditions exceed the critical thickness for the onset of elastic relaxation reported for lower hydrogen pressures as well as the critical thickness for plastic relaxation by more than one order of magnitude. A subsequent anneal step is used to form the islands. This procedure allows some degree of control in the formation of SiGe islands. High island densities and uniform size distributions were achieved. Photoluminescence as well as electroluminescence measurements of these layers show strong emission from SiGe.
Quantum Dot Growth in the Si-Ge-C System Through Multi-Step Procedure
Authors: Wakayama Yutaka , Gerth Gerhard , Werner Peter , Sokolov Leonid V.
Published: April 2011

To fabricate nanometer-sized Ge dots on Si(100), we have investigated multi-step procedure, involving low temperature deposition of a Ge layer, a sub-monolayer C on a Ge wetting layer, a Ge top layer for three-dimensional (3D) dot formation and post-annealing. Effects of each procedure were discussed on the basis of an atomic force microscope study. 10nm-sized Ge dots with a high number density in the order of 1011 cm−2 were grown on the Si(100) substrate by combining each procedure and optimizing experimental conditions, such as deposition temperature, the C layer thickness and post-annealing temperature.
A Quantitative Model of Surface Segregation in III-V Ternary Compounds
Authors: Karpov Sergey Yu. , Makarov Yuri N.
Published: April 2011

A quantitative model of surface segregation free from adjustable parameters is suggested for the growth of ternary III-V compounds. In contrast to previous approaches, the model considers the dynamics of surface population by the three elements producing the ternary alloy. The underlying assumption is that the atoms in the adsorption layer are in equilibrium with the crystal bulk. Elastic strain arising in the epitaxial layer due to the lattice constant mismatch with the substrate is found to be one of the key factors affecting segregation. Along with growth temperature, it controls the segregation efficiency and the composition profile evolution in a growing heterostructure. The effect of the V/III ratio, growth rate and other parameters is accounted for. Here, we apply the model to analyze the InGaAs growth by molecular beam epitaxy owing to the vast experimental data available for the model verification. The theoretical predictions show a good agreement with the experimental observations
Strong Photon-Exciton Coupling in the Near-Field Luminescence of Semiconductor Quantum Dots
Authors: Mintairov A. M. , Vlasov A. S. , Merz J. L. , Kovalenkov O. V. , Reynolds J. P. , Vinokurov D. A.
Published: April 2011

We present low-temperature near-field scanning optical microscopy (NSOM) measurements of self-organized InP quantum dots (QD) embedded in a GaInP layer. We observed an anomalously strong increase in the emission energy (∼270 meV) of a single QD by changing the tip-to-surface distance over ∼100 nm in the near-field region. The effect indicates the formation of a near-field nanocavity having extremely high photon-exciton mixing and can be interpreted as the observation of the coupled photon-exciton pair
Effect of Lattice Mismatch on the Decay of Rheed Oscillations During Growth of Strained InGaAs/GaAs Heterostructures
Authors: Nemcsics Àkos , RiEsz Ferenc
Published: April 2011

The decay of the oscillation of the intensity of the specular beam of the reflection high-energy electron diffraction pattern is analyzed during the molecular beam epitaxial growth of strained InGaAs/GaAs heteroepitaxial structures. The oscillations' amplitude was found to decrease exponentially versustime during the InGaAs growth. Further, the decay time constant decreases with InAs mole fraction, indicating that the lattice strain increases islanding during growth. A simple semi-quantitative model based on the growth front roughening is formulated to explain the results. Assuming that the oscillation decay is related partly to the strain and partly due to kinetic effects during growth, a decay component that is solely due to strain can be separated; we find that the onset of increased roughening due to misfit strain component roughly corresponds to the equilibrium critical layer thickness
Gas-Source Molecular Beam Epitaxy Growth and Characterization of GaNP/GaP Structures
Authors: Xin H. P. , Tu C. W.
Published: April 2011

GaNP bulk layers with different N concentrations and GaN0.025P0.975/GaP multiple quantum wells (MQWs) with various well thicknesses were grown on (100) GaP substrates by gas-source molecular beam epitaxy with a RF nitrogen radical beam source. Using high-resolution X-ray rocking curves, photoluminescence (PL) and absorption measurements, we have shown that incorporation of N in GaNxP1−x, alloys (x ≥0.43%) leads to a direct bandgap behavior of GaNP, and yields strong room-temperature PL from the epilayers. A large Stokes shift of 200 meV is found between the PL peak energy and the absorption edge for the GaNP epilayers, indicating a very strong carrier localization. From the PL peaks of a series of GaN0.025P0.975/GaP MQWs with different well thicknesses grown at the same growth condition, a large conduction-band effective mass mc* − 0.9 me has been obtained for the GaN0.025P0.975alloy, indicating a mixing of Γ and X band wave functions in the conduction band
Structural Investigations of (GaIn)(NAs)/GaAs Multi-Quantum-Wells by Transmission Electron Microscopy
Authors: Volz K. , Hasse A. , Schaper A.K. , Weiric T.E. , Höhnsdorf F. , Koch J. , Stolz W.
Published: April 2011

The structure of compressively strained (GaIn)(NAs)/GaAs multi-quantum wells (MQWs) grown by MOVPE is investigated using TEM. The quaternary, metastable material exhibits a high structural perfection if a N concentration of 4% is not exceeded. Phase separation or clustering effects are not observed, and the In is dispersed homogeneously throughout the quantum wells. The interface roughness of the quantum wells to the GaAs barriers is in the order of several monolayers. Increasing the N content to above 4.5% results in a deterioration of the structure and of the homogeneity of the wells
Local Ordering in GaN-Rich Ternary GaNP Alloys
Authors: Soni R.K. , Dobal P.S. , Katiyar R.S. , Asahi H. , Tampo H. , Gonda S.
Published: April 2011

Using micro-Raman scattering we have studied atomic distribution and the ordering effects in GaN-rich GaNP layers on (0001) sapphire substrates grown by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). Raman spectrum from layers grown at higher temperatures (≥700°C) shows coexistence of GaP-rich region with cubic symmetry and GaN-rich region with hexagonal symmetry. Sharp TO and LO phonon lines, indicative of {111} ordering in the GaP region are observed. Increasing phosphorous (P >1.5%) in GaNP alloy leads to phase separation that is reflected in the suppression of GaN-like Raman modes. The phase-separated region shows an additional Raman line at 384 cm−1 between the TO and LO phonon of GaP due to strongly confined LO phonon in ordered {111} (GaP)n(GaN)m nanometer size clusters. Decreasing growth temperature increases the phosphorous concentration in the GaNP layer and disorder activated acoustic modes appear in the Raman spectrum as a result of symmetry break down. When phosphorous concentration reaches 8.2 % in the layer grown at 570°C, Raman spectrum shows broad amorphous like bands indicative of short-range ordering
Thermal Stability of Si/Ge Hetero-Interface Grown by Atomic-Layer Epitaxy
Authors: Ikeda Keiji , Yanase Jiro , Sugahara Satoshi , Matsumura Masakiyo
Published: April 2011

Thermal stability has been evaluated for ALE-grown Si/Ge interfaces by co-axial impact collision ion scattering spectroscopy. The IML-thick Si layer on Ge was stable only at less than 360°C. The 2ML-thick Si layer on Ge, however, was stable up to 550°C, and Si layers could be also ALE-grown successively on the 2ML-thick Si layer on Ge, while keeping the interface abrupt, since the Si-ALE growth temperature was about 530°C.
Strain Induced Compositional Modulations in AlGaAs Overlayers Induced by Lateral Surface Gratings
Authors: Pietsch U. , Zeimer U. , Hofmann L. , Grenzer J. , Gramlich S.
Published: April 2011

Strain and compositional modulation in AlxGa1−xAs layers grown by metalorganic vapour phase epitaxy (MOVPE) over a sinusoidally shaped GaAs (001) surface grating were studied by scanning electron microscopy (SEM), X-ray grazing-incidence diffraction (GID) and photoluminescence (PL). Two growth temperatures and two compositions were chosen to realize planar overlayers. By SEM a periodic reduction in Al-content was found at the valley positions of the GaAs grating. The appearance of such vertical quantum wells (VQWs) has been explained by the growth rate anisotropy between high-index and (001) planes and a curvature-induced capillarity flow of Ga. Estimated from PL energies a larger reduction of the Al-concentration in the VQW and also at the high-index sidewall facets was found than compared to predictions from the capillarity flow theory. Using depth-resolved GID we show that the formation of VQWs is accompanied by a periodic lateral strain field. Therefore we assume, that the formation of the VQWs is influenced by strain induced diffusion due to the interaction of opposite sidewall facets.
The Importance of In Situ Monitors in the Preparation of Layered Oxide Heterostructures by Reactive MBE
Authors: Schlom D.G. , Haenit J.H. , Theis C.D. , Tian W. , Pan X.Q. , Brown G.W. , Hawley M.E.
Published: April 2011

Using a variety of in situ monitors and when possible adsorption-controlled growth conditions, layered oxide heterostructures including new compounds and metastable superlattices have been grown by reactive molecular beam epitaxy (MBE). The heteroepitaxial layers grown include Bi4Ti3 O12—SrTiO3 and Bi4Ti3O12—PbTiO3 Aurivillius phases, Srn+1TinO3n+1 Ruddlesden-Popper phases, and metastable PbTiO3 / SrTiO3 and BaTiO3 / SrTiO3 superlattices. Accurate composition control is key to the controlled growth of such structures, and to this end combinations of reflection high-energy electron diffraction (RHEED), atomic absorption spectroscopy (AA), a quartz crystal microbalance (QCM), and adsorption-controlled growth conditions were employed during growth. The structural perfection of the films has been investigated using in situ RHEED, four-circle x-ray diffraction, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (TEM).
Epitaxial Oxide Films on Silicon: Growth, Modeling and Device Properties
Authors: Droopad R. , Wang J. , Eisenbeiser K. , Yu Z. , Ramdani J. , Curless J. A. , Overgaard C. D. , Finder J. M. , Hallmark J. A. , Kaushik V. , Nguyen B. Y. , Marshall D. S. , Ooms W. J.
Published: April 2011

Using molecular beam epitaxy, thin films perovskite-type oxide SrxBa1−xTiO3 (0≤×≤1) has been grown epitaxially on Si(001) substrates. Reflection high energy electron diffraction measurements and X-ray diffraction analysis indicate that high quality heteroepitaxy on Si takes place with SrxBa1−xTiO3(001)//Si(001) and SrxBa1−xTiO3[010]//Si[10]. Extensive atomic simulations have been carried out to understand the initial growth mechanism of the oxide layers on silicon. SrTiO3 layers grown directly on Si have been used as the gate dielectric for the fabrication of MOSFET devices. By varying the growth conditions the thickness of the amorphous interfacial silicon oxide layer formed during the growth of the oxide layers has been engineered to minimize the device short channel effects. An effective oxide thickness <10 Å has been obtained for a 110 Å thick SrTiO3 dielectric film with interface state density around 6.4 × 1010 cm−2 eV-1, and the inversion layer carrier mobilities of 220 cm2 V−1 s−1 and 62 cm 2 V−ls−1 for NMOS and PMOS devices, respectively. The gate leakage in these devices is 2 orders of magnitude smaller than a comparable SiO2 gate dielectric MOSFET.
Modeling of the Transition From Active to Passive Oxidation of Si(100)
Authors: Liu Da-Jiang , Choi Cheol Ho , Gordon Mark S. , Evans J.W.
Published: April 2011

For a Si(100)2×1 surface exposed to oxygen, there is a transition from etching (“active” oxidation via removal of SiO) to “passive” oxidation (buildup of an oxide film) with decreasing surface temperature. The transition depends sensitively on a competition between SiO desorption, and oxide island formation. We analyze these processes utilizing both ab-initio quantum chemistry studies of key energetics and lattice-gas models for the cooperative behavior.
Spatial Correlations in Growing Films
Authors: Bartelt M.C.
Published: April 2011

Detailed analyses of non-equilibrium lattice-gas models of island nucleation and growth during film deposition (or etching) have been invaluable in elucidating basic issues in nucleation theory, deviations from mean-field predictions, and experimental observations. Particularly interesting and useful is the behavior of spatial correlations in the adlayer which develop during island (or pit) formation. In particular, a strong depletion in the population of island pairs at separations smaller than the average follows from depletion in the density of diffusing adspecies near islands. This feature delays percolation of clusters of coalesced islands. Another recently discovered and more subtle feature is a strong correlation between the width of the island-free region surrounding an island and the size and growth rate of that island. This direct correlation between island sizes and separations controls the shape of the island size distribution. If incorporated into rate-equation descriptions, it recovers the exact form of the scaling function for the island size distribution.
Equilibrated Surface Alloys and the Influence of Strain
Authors: Thayer G.E. , Ozolins V. , Schmid A.K. , Stevens J.L. , Chiang S. , Hwang R.Q.
Published: April 2011

Structural and thermodynamic properties of equilibrated ultrathin Pd-Au, Cu-Ag, and Co-Ag alloys on Ru(0001) were studied and are comparatively discussed. These studies provide a framework for a detailed understanding of the energetics of surface alloy formation.

Additionally, the differences between the two bulk immiscible systems (Cu-Ag and Co-Ag) and the bulk miscible Pd-Au system are discussed.
Elaboration of Single Crystalline AU/NI80FE20/CU/NI80FE20/NIO(111) Spin Valves
Authors: Mocuta C. , Barbier A. , Lafaye S. , Bayle-Guillemaud P.
Published: April 2011

The successful preparation of fully epitaxial spin-valves elaborated on NiO(111) single crystals is reported. The growth conditions of a smooth 2D permalloy (Py = Ni80Fe20) layer have been determined. A very strong magnetic exchange interaction is evidenced on the Py/NiO(111) interface. It strongly modifies the magnetic properties of Py that acquires high coercivity, leaving it as a hard magnetic material. The second Py layer has unchanged magnetic properties and plays the role of the sensing soft magnetic layer. A giant magnetoresistance (GMR) of 3.5% at room temperature was easily obtained without optimizing the thickness of the different layers. Complementarily energy-filtered (EF-) and high-resolution (HR-) transmission electron microscopy (TEM) were used to fully characterize the spin valves. Within the framework of magnetic exchange coupling, our results open the possibility to elaborate model spin valves in which the role of each interface can be investigated and controlled at the atomic level. The detrimental effect of an inter-diffusion at the Py/NiO interface is evidenced.
Investigations of Non-Micropipe X-Ray Imaged Crystal Defects in SiC Devices
Authors: Neudeck P. G. , Kuczmarski M. A. , Dudley M. , Vetter W. M. , Su H. B. , Keys L. J. , Trunek A. J.
Published: April 2011

This paper updates on-going experimental and theoretical investigations of non-micropipe defects imaged by synchrotron white beam X-ray topography (SWBXT) in SiC devices and epitaxial layers. Computer-based thermal modeling of screw-dislocation related breakdown in SiC diodes has been initiated to gain insights into internal temperature profiles as a function of microplasma power. A preliminary study of epitaxial 4H- and 6H-SiC p+n mesa diodes indicates that very low angle boundaries, whose electrical properties have not previously been reported, do not significantly impact DC I-V properties (forward and reverse) measured at biases less than 70% of the SiC breakdown field. The presence of very small growth pits on the surface of commercial 4H-SiC epitaxial layers, almost undetectable by high magnification optical microscopy, was revealed by atomic force microscopy and found to correspond to the locations of closed core screw dislocations imaged by SWBXT.
Fabrication of Enhancement-Mode GaN-Based Metal–Insulator-Semiconductor Field Effect Transistor
Authors: Chen P. , Zhang R. , Zhou Y.G. , Xie S.Y. , Luo Z.Y. , Chen Z.Z. , Li W.P. , Gu S.L. , Zheng Y.D.
Published: April 2011

An enhancement-mode GaN metal-insulator-semiconductor field-effect transistor was successfully fabricated on a GaN/AlGaN/GaN double heterojunction structure with SiO2 as insulator layer. The enhancement mode DC characteristics have been achieved in the device with a gate length of 6 μm and a gate width of 100 μm. The device exhibited a DC transconductance of 0.6 mS/mm and a maximum drain-source current of 5 mA/mm. The gate leakage current is lower than 10−6 A at a bias of -10 V and the gate breakdown voltage is higher than 20 V. The channel stands a good chance of forming by hole accumulation between the top GaN layer and the AlGaN layer. The p-channel can be attributed to the presence of a piezoelectric field in the heterojunction, and the strongly asymmetric band bending and carriers distribution induced by the piezoelectric field. High-frequency capacitance-voltage measurement also gives a circumstantial evidence of the presence of a p-channel in the device structure.
Current Gain Simulation of Npn AlGaN/GaN Heterojunction Bipolar Transistors
Authors: Monier C. , Pearton S. J. , Baca A. G. , Chang P. C. , Zhang L. , Han J. , Shul R. J. , Ren F. , LaRoche J.
Published: April 2011

A drift-diffusion model has been used to explore the performance capabilities of Npn AlGaN/GaN heterojunction bipolar transistors. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be the cause of limited current gain values for Npn devices. Several computation approaches have been considered to improve their performance. Reasonable improvement of the DC current gain β is observed by realistically reducing the base thickness in accordance with processing limitations. Base transport enhancement is also predicted by the introduction of a quasi-electric field in the base.
Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer
Authors: Kim Yihwan , Subramanya Sudhir G. , Krueger Joachim , Siegle Henrik , Shapiro Noad , Armitage Robert , Feick Henning , Weber Eicke R. , Kisielowski Christian , Yang Yi , Cerrina Franco
Published: April 2011

We demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 × 1017 cm−3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.
2DEGs and 2DHGs Induced by Spontaneous and Piezoelectric Polarization in AlGaN/GaN Heterostructures
Authors: Ambacher O. , Link A. , Hackenbuchner S. , Stutzmann M. , Dimitrov R. , Murphy M. , Smart J. , Shealy J.R. , Green B. , Schaff W.J. , Eastman L.F.
Published: April 2011

Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail
High Room-Temperature Hole Concentrations above 1019 cm−3 in Mg-Doped InGaN/GaN Superlattices
Authors: Kumakura Kazuhide , Makimoto Toshiki , Kobayashi Naoki
Published: April 2011

We achieved high hole concentrations above 1019 cm−3 at room temperature in Mg-doped InxGa1-xN/GaN (4 nm/4 nm) superlattices grown by metalorganic vapor phase epitaxy. The hole concentrations for the InxGa1-xN/GaN superlattices increased with the In mole fraction, and the maximum hole concentration reached 2.8 Å 1019 cm−3 for the In0.22Ga0.78N/GaN superlattice. The hole concentrations for the superlattices strongly depend on the structural parameters of the superlattices. The band bending due to the strain-induced piezoelectric field and the valence band structures of the InGaN/GaN heterostructures affect the hole generation in the superlattices. The weak temperature dependence of the resistivities for the InGaN/GaN superlattices with higher In mole fractions indicates highly efficient hole generation in the superlattice.
Comparison of Different Substrate Pre-Treatments on the Quality of GaN Film Growth on 6H-, 4H-, and 3C-SiC
Authors: Lee K. H. , Hong M. H. , Teker K. , Jacob C. , Pirouz P.
Published: April 2011

Together with sapphire, SiC is the most common substrate material for GaN epitaxial growth. In fact, SiC has advantages over sapphire because of its better thermal conductivity and lower film substrate lattice mismatch (∼3.5%). However, nucleation of GaN on SiC is rather difficult because of the low surface energy of SiC and the sensitivity of substrate preparation. This latter point makes it essential to use a very careful cleaning step, and also to pre-treat the substrate surface by growing a thick buffer layer of AlN at a relatively high temperature. In this study, several pre-treatment steps of SiC for GaN deposition were tested including (a) nitration with NH3 for 0.5-20 minutes, (b) pre-adsorption of trimethyl gallium (TMG) or trimethyl aluminum (TMA) for 0.5-5 minutes, and (c) deposition of an AlN buffer layer at ∼1150°C. After each pre-treatment, GaN was deposited by MOCVD using dilute H2(Ar+12%H2), NH3 and TMG. All the films were characterized by XRD and cross-sectional TEM. After nitration of SiC, the deposited GaN film was found to be polycrystalline. In case of pre-adsorption of TMG, epitaxial but island-like GaN formed on the substrate. In the third case, with an ultra-thin (∼1.5nm) coverage of AlN on SiC (by pre-adsorption of TMA or by 50 seconds deposition of AlN), GaN epilayers were successfully deposited on SiC. However, when AlN was deposited for longer than 3 minutes (up to 10 minutes), only polycrystalline GaN was obtained. With this technique of covering the surface with an ultra-thin layer of AlN, epitaxial GaN has been successfully deposited on 6H-SiC (0001), on 4H-SiC(0001), and on 3C-SiC/Si(111) substrates. The effect of the different pre-treatments of SiC on the quality of the deposited GaN films will be discussed and compared, and the optimal conditions for GaN deposition for each substrate will be presented.
Dislocations Produced by Indentation Deformation of HPVE GaN Films
Authors: Hong M. H. , Pirouz P. , Tavernier P. M. , Clarke D. R.
Published: April 2011

Vickers hardness tests on {0001} and (1120) faces of a relatively thick GaN film grown on a (0001) sapphire substrate have been performed in the temperature range 25-1200°C. The microstructure of the plastic zone around the indentation sites has been investigated by transmission electron microscopy (TEM). At room temperature, the hardness was measured to be 12.3 GPa on the basal plane, and 11.1 GPa on the prism plane. The hardness decreases gradually with increasing temperature up to ∼800°C and then shows a plateau between ∼800 and ∼1050°C after which it decreases again above ∼1100°C. In contrast to the rather straight dislocations produced by room-temperature indentation of the (0001) face, the dislocations generated by indenting the (1120) face at room temperature were curved and in the shape of half-loops emanating from the indentation sites on the prism planes. Such dislocations were not dissociated within the resolution of weak-beam TEM.
Nature of Low-Frequency Excess Noise in n-Type Gallium Nitride
Authors: Zhu C. F. , Fong W. K. , Leung B. H. , Cheng C. C. , Surya C.
Published: April 2011

Low-frequency noise is investigated in n-type GaN film grown by rf-plasma assisted molecular beam epitaxy. The temperature dependence of the voltage noise power spectra, SV(f), was examined from 400K to 80K in the frequency range between 30Hz and 100KHz, which can be modeled as the superposition of 1/f (flicker) noise G-R noise. At f > 500 Hz the noise is dominated by G-R noise with activation energies of 360meV and 65meV from the conduct band. The results clearly demonstrate the trap origin for both the 1/f noise and G-R noise. At the low-frequency range the fluctuation was dominated by 1/f noise. To determine the origin of the noise we considered both the bulk mobility fluctuation and the trap fluctuation models. Our experimental results showed that rapid thermal annealing (RTA) at 800°C resulted in over one order of magnitude decrease in the Hooge parameter. Annealing at temperatures in excess of 1000°C resulted in significant increase in the noise. Photoluminescence and x-ray diffraction measurements also showed that the crystallinity of the films improved with RTA at 800°C with an accompanying reduction in deep levels. Annealing at 900°C and 1000°C resulted in an increase in the FWHM of the x-ray diffraction indicative of thermal decomposition of the materials. The results are in excellent agreement with the trend of Hooge parameters as a function of annealing temperature, strongly indicating trap origin of the observed 1/f noise.
Long Time-Constant Trap Effects in Nitride Heterostructure Field Effect Transistors
Authors: Dang Xiaozhong , Asbeck Peter M. , Yu Edward T. , Boutros Karim S. , Redwing Joan M.
Published: April 2011

Current collapse effects in an Al0.25Ga0.75N/GaN HFET have been investigated under pulsed bias conditions, and a detailed investigation of current responses to changes in drain or gate bias voltage (drain-lag and gate-lag, respectively) has been performed. Three components of transient current response to changes in drain and gate bias voltages are distinguished. Surface treatment using KOH etching and the influence of pulsed bias conditions on threshold voltage are investigated to explore the origins of traps associated with each current transient component.
Lattice Parameters and Thermal Expansion of Important Semiconductors and Their Substrates
Authors: Reeber Robert R. , Wang Kai
Published: April 2011

Thermal expansion and lattice parameters are useful thermophysical properties of materials. A knowledge of their temperature dependence is essential for optimizing device design and crystal growth conditions while minimizing the residual stress in epitaxial films and electronic devices. Currently first principles theory can predict lattice parameters to 0.01%. Information required for device design and thin film crystal growth requires improvements two to three orders of magnitude better than this. Available experimental results from our work and the literature will be reviewed in terms of a high temperature predictive model. Tables of thermal expansion and lattice parameters for AlN, GaN, 6H-SiC, β-SiC, MgO, Al2O3, ZnO and GaAs are provided.
Electric and Morphology Studies of Ohmic Contacts on AlGaN/GaN
Authors: Tilak V. , Dimitrov R. , Murphy M. , Green B. , Smart J. , Schaff W.J. , Shealy J.R. , Eastman L.F.
Published: April 2011

AlGaN/GaN is a promising system for high power electron devices. Quality of ohmic contacts is a critical parameter in determining the performance of the device. Although we have achieved a transfer resistance (Rc) of 0.35Δmm and ρc of 9.5×10−7 Δcm−2 the morphology and edge acuity of the contacts are poor. The standard ohmic contact recipes consist of a combination of Titanium and Aluminum with Nickel and/or Gold. This is annealed at 800°C-950°C [1-5]. In this work we study ohmic contacts on unintentionally doped Al0.3Ga0.7N/GaN system. We look at ratios of Ti/Al from 0 to 2 to determine which is the optimum ratio in terms of surface morphology and electrical characteristics. From our studies we conclude that morphology of a Ti/Al contact is good over a ratio of 0.3 and the contact resistance is minimized at a Ti/Al of 0.6. The ohmic contacts are improved electrically if a layer of gold is added on top. The best electrical contacts however were obtained with a four layer recipe of Ti/Al/Ti/Au, which gave contact resistance (Rc) around 0.45Δmm, but the morphology of the contacts was poor.
Inductively Coupled High-Density Plasma-Induced Etch Damage of GaN MESFETs
Authors: Shul R. J. , Zhang L. , Baca A. G. , Willison C. G. , Han J. , Pearton S. J. , Lee K. P. , Ren F.
Published: April 2011

The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 νm/min and anisotropic etch profiles are readily achieved in Inductively Coupled Plasma (ICP) etch systems. Unfortunately, under these conditions plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, device performance was sensitive to ion bombardment energy and ion flux.
Silicon Carbide Die Attach Scheme for 500°C Operation
Authors: Chen Liang-Yu , Hunter Gary W. , Neudeck Philip G.
Published: April 2011

Single crystal silicon carbide (SiC) has such excellent physical, chemical, and electronic properties that SiC based semiconductor electronics can operate at temperatures in excess of 600°C well beyond the high temperature limit for Si based semiconductor devices. SiC semiconductor devices have been demonstrated to be operable at temperatures as high as 600°C, but only in a probe-station environment partially because suitable packaging technology for high temperature (500°C and beyond) devices is still in development. One of the core technologies necessary for high temperature electronic packaging is semiconductor die-attach with low and stable electrical resistance. This paper discusses a low resistance die-attach method and the results of testing carried out at both room temperature and 500°C in air. A 1 mm2 SiC Schottky diode die was attached to aluminum nitride (AlN) and 96% pure alumina ceramic substrates using precious metal based thick-film material. The attached test die using this scheme survived both electronically and mechanically performance and stability tests at 500°C in oxidizing environment of air for 550 hours. The upper limit of electrical resistance of the die-attach interface estimated by forward I-V curves of an attached diode before and during heat treatment indicated stable and low attach-resistance at both room-temperature and 500°C over the entire 550 hours test period. The future durability tests are also discussed.
The Effects of Post-Oxidation Anneal Conditions on Interface State Density Near the Conduction Band Edge and Inversion Channel Mobility for SiC MOSFETs
Authors: Chung G.Y. , Tin C.C. , Williams J. R. , McDonald K. , Ventra M. Di , Pantelides S.T. , Feldman L.C. , Weller R.A.
Published: April 2011

Results are reported for the passivation of interface states near the conduction band edge in n-4H-SiC using post-oxidation anneals in nitric oxide, ammonia and forming gas (N2/5%H2). Anneals in nitric oxide and ammonia reduce the interface state density significantly, while forming gas anneals are largely ineffective. Results suggest that interface states in SiO2/SiC and SiO2/Si have different origins, and a model is described for interface state passivation by nitrogen in the SiO2/SiC system. The inversion channel mobility of 4H-SiC MOSFETs increases with the NO annealing.
Deep RIE Process for Silicon Carbide Power Electronics and MEMS
Authors: Beheim Glenn , Salupo Carl S.
Published: April 2011

Reactive ion etching (RIE) of silicon carbide (SiC) to depths ranging from 10 μm to more than 100 μm is required for the fabrication of SiC power electronics and SiC MEMS. A deep RIE process using an inductively coupled plasma (ICP) etch system has been developed which provides anisotropic etch profiles and smooth etched surfaces, a high rate (3000 Å/min), and a high selectivity (80:1) to the etch mask. An etch depth of 100 μm is demonstrated.
Pyroelectric Coefficient Optimization Through Grain Size Control in (Ba, Sr)TiO3 Thin Films
Authors: Schloss Lawrence F. , Haller Eugene E.
Published: April 2011

Thin Ba0.2Sr0.8TiO3 films were pulsed laser deposited at laser pulse repetition rates of 1, 5 and 20 Hz in order to assess the effect of grain size on pyroelectric properties. Scanning electron microscopy reveals an increase in columnar grain size with decreasing laser pulse rate, as expected. Substrate bending measurements show an increase in tensile stress with increasing grain size, suggesting some form of stress relaxation has occurred. X-ray diffraction studies reveal all films to be epitaxial and oriented, while Rutherford Backscattering Spectroscopy finds no significant differences in film composition. The films grown at 5 and 20 Hz display a suppressed dielectric permittivity as a function of temperature, while the film deposited at 1 Hz displays an unusually sharp and large dielectric permittivity peak. A variation on current theory concerning phase transitions of stressed ferroelectric thin films is proposed to explain these results.
Control of Valence States in ZnO by CoDoping Method
Authors: Yamamoto T. , Yoshida H. -K.
Published: April 2011

We have investigated the electronic structures of p- or n-type doped ZnO based on ab initio electronic band structure calculations in order to control valence states in ZnO for the fabrication of low-resistivity p-type ZnO. We find unipolarity in ZnO; p-type doping using Li or N increases the Madelung energy while n-type doping using Al, Ga, In or F species decreases the Madelung energy. We have proposed materials design: codoping using N acceptors and reactive codopants, Al or Ga, enhances electric properties in p-type codoped ZnO. It has been already verified by experiments using the N acceptors and Ga reactive donor codopants. We find a very weak repulsive interaction between Li acceptors and the delocalization of the Li-impurity states for Lidoped ZnO, in contrast with the case of N-doped ZnO. On the other hand, we find the compensation mechanism by the formation of 0 vacancy in the vicinity of the Li-acceptor sites. We propose a group VII element, F species, as a promising candidate for use of the reactive codopant as for Li-doped ZnO in order to realize low-resistivity p-type ZnO.
A Study of the Crystallization of Amorphous Indium (Tin) Oxide
Authors: Paine David C. , Chason Eric , Chen Eric , Sparacin Dan , Yeorn Hyo-Young
Published: April 2011

The crystallization of amorphous indium oxide thin films with zero to 9.8 wt% SnO2, was studied using a combination of in situ MOSS (multibeam optical stress senor) and in situ resistivity measurements. We report that amorphous indium oxide deposited using electron beam evaporation undergoes crystallization in a two part process of amorphous structure relaxation followed by crystallization. MOSS measurements show that the relaxation process corresponds to a densification of the amorphous structure while crystallization results in a molar volume increase.
Low Electrically Resistive Transparent Indium-Tin-Oxide Epitaxial Film on (001) Surface of YSZ by Pulsed Laser Deposition
Authors: Ohta Hiromichi , Orita Masahiro , Hirano Masahiro , Hosono Hideo , Kawazoe Hiroshi , Tanji Hiroaki
Published: April 2011

High quality ITO thin films were grown hetero-epitaxially on extremely flat substrate of (001) YSZ by a pulsed laser deposition technique at a substrate temperature of 600°C. The crystal orientation relationship between the film and YSZ were confirmed as ITO (001) // YSZ (001) and ITO (010) // YSZ (010), respectively, by HR-XRD and HR-TEM. The carrier densities of the films were almost equal to Sn02 concentration in the films. That is, almost all the doped Sn4+ ions were activated to release electrons to the conduction band. The carrier densities of the films were enhanced up to 1.9×1021cm−3, while the Hall mobility showed a slight, almost linear, decrease from 55 to 40cm2V−1s−1 with increasing SnO2 concentration. The low resistivity is due to larger electron mobility, which most likely resulted from good crystal quality of the films. The optical transmissivity of the film exceeded 85% at wavelengths from 340 to 780nm.
Room Temperature Operation of UV-LED Composed of TCO Hetero p-n Junction, p-SrCu2O2/n-ZnO
Authors: Ohta Hiromichi , Kawamura Ken-Ichi , Orita Masahiro , Sarukura Nobuhiko , Hirano Masahiro , Hosono Hideo
Published: April 2011

Room temperature operation of UV LED is realized for the first time using a hetero p-n junction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Ni/SrCu2O2/ZnO/ITO multi-layered film was epitaxially grown on an extremely flat YSZ (111) surface by a PLD. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n junction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 1.5V. A relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO.
Oxygen Ions Diffusitives in Tin Dioxide Based Thin Films
Authors: Shishkin N.Y. , Zharsky I.M.
Published: April 2011

Oxygene ions diffusitives in tin dioxide based thin films were measured with electrochemical method. The Pd content 0-3% at. thin films were investigated. The dependencies of the diffusitives vs. content of the layer were obtained.
Pulsed Laser Deposition of Epitaxial SrVO3 Films on (100)LaAlO3 and (100)Si
Authors: Yip P.W. , Wong K.H.
Published: April 2011

Thin films of SrVO3 have been grown on (100)LaAlO3 and TiN buffered (100)Si substrates by pulsed laser deposition. The films were deposited in temperature range of 450°C – 750°C and under ambient oxygen pressure between 101−6 and 101−2 Torr. Their structural properties were characterized using a four-circle x-ray diffractometer. High quality SrVO3 films were obtained at growth temperatures above 500°C without post annealing. Heteroepitaxial relationship of < 100 >SrVo3 ∥ < 100 >LaAOl3 and <100>SrVo3∥ < 100 >TiN ∥ < 100 >Si were observed for films deposited at ≤ 550°C. X-ray photoelectron spectroscopic studies of the films suggest that the vanadium is mainly tetravalent and pentavalent. Charge transport measurements show that the films vary from semiconducting to highly conducting for different growth conditions. Resistivity of a few micro-ohm cm was recorded for some of the epitaxial SrVO3 films.