bluuebunny/crossref_metadata_2025_split huggingface dataset:
Gas-Source Molecular Beam Epitaxy Growth and Characterization of GaNP/GaP Structures
Authors: Xin H. P. , Tu C. W. 
Published: April 2011

ABSTRACTGaNP 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

ABSTRACTThe 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

ABSTRACTUsing 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

ABSTRACTThermal 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

ABSTRACTStrain 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

ABSTRACTUsing 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

ABSTRACTUsing 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

ABSTRACTFor 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

ABSTRACTDetailed 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

ABSTRACTStructural 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

ABSTRACTThe 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

ABSTRACTThis 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

ABSTRACTAn 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

ABSTRACTA 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

ABSTRACTWe 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

ABSTRACTTwo 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

ABSTRACTWe 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

ABSTRACTTogether 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

ABSTRACTVickers 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

ABSTRACTLow-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.