bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropy in Atomic-Scale Interface Structure and Mobility in Inas/Ga1_Xinxsb Superlattices
Authors: Lew A. Y. , Zuo S. L. , Yu E. T. , Miles R. H.
Published: September 2012

We have used cross-sectional scanning tunneling microscopy to study the atomic-scale interface structure of InAs/Ga, _In.xSb superlattices grown by molecular-beam epitaxy. Detailed, quantitative analysis of interface profiles obtained from constant-current images of both (110) and (1ī0) cross-sectional planes of the superlattice indicates that interfaces in the (1ī0) plane exhibit a higher degree of interface roughness than those in the (110) plane, and that the Ga1-xln xAs interfaces are rougher than the InAs-on-Gal1-xInxSb interfaces. The roughness data are consistent with anisotropy in interface structure arising from anisotropic island formation during growth, and in addition with a growth-sequence-dependent interface asymmetry resulting from differences in interfacial bond structure between the superlattice layers. Roughness data are compared with measurements of anisotropy in low-temperature Hall mobilities of the samples.
Surface Reconstruction and Morphology of Hydrogen Sulfide Treated GaAs (001) Substrate
Authors: Suda Jun , Kawakami Yoichi , Fujita Shizuo , Fujita Shigeo
Published: September 2012

We report several new results in hydrogen sulfide (H2S) treatment of a GaAs (001) substrate. Surface reconstruction and morphology were investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) in terms of the annealing temperature and the H2S irradiation sequence. A (4 × 3) GaAs surface was obtained by annealing the substrate under H2S irradiation (4 × 10-7 Torr). The surface was atomically flat, i.e., large terraces with monolayer steps were clearly observed. A (2 × 6) S-terminated GaAs surface was obtained by irradiation H2S at 300°C on a Ga-terminated surface, which was formed by annealing at 580°C in high vacuum. The molecular beam epitaxy (MBE) growth of ZnSSe-based semiconductors on the (4 × 3) surface results in high quality structures such as a novel ZnSSe/ZnMgSSe tensile-strained quantum well (QW).
Surface Roughening and Composition Modulation of ZnSe-related II-VI epitaxial films
Authors: Tomiya Shigetaka , Tsukamoto Hironori , Itoh Satoshi , Nakano Kazushi , Morita Etsuo , Ishibashi Akira
Published: September 2012

We have investigated ZnSSe and ZnMgSSe epitaxial layers lattice-matched to GaAs (001) substrates grown by molecular beam epitaxy using atomic force microscopy and transmission electron microscopy. Under II-rich conditions with c(2x2) surface reconstruction, surface morphology exhibited corrugation aligned in the [1ī0] direction and composition modulation was observed in the same [1ī0] direction. Under VI-rich condition with (2x1) surface reconstruction, the surface morphology becomes rounded grain-like and composition modulation was not observed. The formation of composition modulation is associated with the surface corrugated structures.
Direct Bandgap Quantum Wells on GaP
Authors: Lee Jong-Won , Schremer Alfred T. , Fekete Dan , Shealy James R. , Ballantyne Joseph M.
Published: September 2012

Currently, there are no direct-bandgap alloy semiconductors that can be grown lattice-matched to GaP substrates. A strained layer of GaInP can be grown on GaP, however, with difficulties. First, GaInP is an indirect-bandgap material for In concentrations up to ∼30%. Second, the band alignment between GaInP and GaP is type-II for In concentrations up to ∼60%. The Mathews-Blakeslee critical thickness of GaInP layer on GaP is prohibitively small in the useful In concentration range. GaInP is known to grow in an ordered phase in certain growth conditions. By changing the growth conditions, a heterojunction of ordered GaInP and disordered GaInP can be grown. The conduction band offset going from a disordered GaInP phase to an ordered GaInP phase has been reported to be about 150 meV. Using a layer of ordered GaInP, a QW with type-I band alignment may be grown on GaP for a wider range of composition.

We have grown a series of approximately 60 Å thick GaP/GaInP/GaP strained quantum wells of various compositions using OMVPE. Strong photoluminescence, which exhibited an unusual temperature dependence, has been observed on many samples. A study of the QW’s using X-ray diffraction, TEM, and variable temperature PL reveals behaviors consistent with direct bandgap GaInP quantum wells containing ordered and disordered domains.
Formation of Large Conduction Band Discontinuities of Heterointerfaces Using CdF2 AND CaF2 on Si(111)
Authors: Izumi Akira , Matsubara Noriyuki , Kushida Yusuke , Tsutsui Kazuo , Sokolov Nikolai S.
Published: September 2012

We proposed use of a new CdF2/CaF2 heterointerface for the formation of large conduction band discontinuities to apply quantum effect devices fabricated on Si substrates. Resonant tunneling diodes using this heterointerface on Si were fabricated and negative differential resistance whose P/V current ratio of 24 at highest was observed at room temperature.
Comparison of the Morphological and Optical Characteristics of InP Islands on GalnP/GaAs (311)A and (100)
Authors: Pelzel R. I. , Reaves C. M. , DenBaars S. P. , Weinberg W. H.
Published: September 2012

We have studied the effect of surface orientation on the optical and morphological characteristics of coherently-strained InP islands grown on GaInP/GaAs. The differences between islands grown on the (100) orientation and the (311)A orientation are studied. Islands grown on the (311)A orientation are more dense than the islands grown on the (100) orientation. For the (100) orientation, the island height distribution is bimodal peaked at 20 Å and 220 Å. For the (311)A orientation, the island height distribution is also bimodal peaked at 15 Å and 60 Å. Photoluminescence measurements for the (311)A orientation show a peak at 1.9 eV attributed to small islands. This peak is shifted to higher energies in comparison to the corresponding peak for the (100) orientation which is at 1.77 eV. This peak shift is due to the fact that the small islands on the (311)A orientation are smaller than the corresponding islands on the (100) orientation.
Structural Investigations of Self-Assembled Ge-Dots by X-Ray Diffraction and Reflection
Authors: Darhuber A.A. , Holy V. , Stangl J. , Bauer G. , Schittenhelm P. , Abstreiter G.
Published: September 2012

Self-organized Ge-dots on (001)-oriented Si-substrates have been studied using two-dimensionally resolved high resolution x-ray diffraction and reflectivity. The degree of the vertical correlation of the dot positions ("stacking") has been derived as well as a lateral ordering of the dots in a (disordered) square array with main axes parallel to ]100] and ]010].
Improvement of InGaP/GaAs Heterointerface Quality by Controlling AsH3 Flow Conditions
Authors: Fukai Yoshino K. , Hyuga Fumiaki , Nittono Takumi , Watanabe Kazuo , Sugahara Hirohiko
Published: September 2012

This paper examines the conduction band offset, ∆Ec, and interface charge density, σ, of disordered InGaP and GaAs heterointerfaces by controlling the AsH 3 cover time and flow rate at the growth interval from GaAs to InGaP. Short AsH, cover time (0.05 min) creates high ΔEc of 0.2 eV and low σ of 6.3×l0 10cm −2. Extending the AsH 3 cover time by 50 times cuts ΔEc to almost 0 eV and increases σ by one order. Interface morphology for long-AsH3 -cover-time samples observed by atomic force microscopy shows a terrace structure on GaAs surface, which means the surface is As rich. These results suggest that an As-poor GaAs surface is essential to achieving high-quality InGaP/GaAs heterointerfaces.
Direct Formation of Fine Structure by Low Energy Focused Ion Beam
Authors: Chikyow T. , Shikanai A. , Koguchi N.
Published: September 2012

GaAs micro crystals in line were grown on a sulfur-terminated GaAs surface by low energy focused ion beam. Ga ions, picked out from a liquid Ga ion source, were accelerated up to 10 KV to obtain a focused ion beam. The ions were given a positive bias to reduce their kinetic energy by retarding lens.The Ga ions landed on the surface softly and formed a series of Ga droplets. By subsequent As molecule supply to the Ga droplet, GaAs micro crystals in line were grown. This method was found to be useful to make fine structures directly on the semiconductor materials.
Surface Adsorption Kinetics of Ga Wire Arrays on Si(112)
Authors: Prokes S.M. , Glembocki O.J.
Published: September 2012

The growth and energetics of Ga chains on a Si(112) facet surface have been investigated. Low energy electron diffraction (LEED) and Reflectance Difference Anisotropy (RDA) experiments have been performed in UHV to examine the kinetics of the Ga atom chain formation process along the Si steps. This is a self-limiting process for substrate temperatures in the 400°C to 500°C range, since the binding energy of the metal atoms at the step edges is greater than that on the (11l)-type terraces. At saturation coverage, the Ga chains exhibit sixfold periodicity along the step edges, but below saturation, these chains exhibit fivefold periodicity. A model has been suggested from these results in which the periodicities of the chains are determined by the metal atom vacancies, which balance the tensile stress created by the larger metal atoms. We have used RDA to study the time evolution of the surface kinetics and changes in the surface reconstruction of Ga deposited on these facet (112) Si surfaces, as a function of substrate temperature and Ga deposition rate. The RDA measurement is shown to be sensitive to the formation of both the 5×1 as well as 6 × 1 reconstructions and to island formation at low substrate temperatures. The diffusion-controlled Ga line formation process at low temperatures has also been characterized as well as the evaporation controlled regime, which is evident at substrate temperatures over 400°C.
Ultra Thin SiO2 Mask Layer for Nano-Scale Selective-Area Pecvd of Si
Authors: Park J. W. , Yasuda T. , Ikuta K. , Kuo L.H. , Yamasaki S. , Tanaka K.
Published: September 2012

We discuss the applicability of ultrathin SiO2 layers as a mask for low-temperature selective-area deposition of Si. Thin oxide layers with estimated thickness ranging from 4 to 20 Å were formed by oxidizing H-terminated Si(100) surfaces by a remote plasma exposure at room temperature. Low-temperature selective-area deposition was carried out using two different techniques: flow-modulated plasma-enhanced chemical vapor deposition (FM-PECVD) using SiH4 and H2, and very low pressure CVD (VLPCVD) using Si2H4. We show that the ultra-thin plasma oxide layers exhibit good properties for a use as a passivating mask layer, and that the oxide layer can be patterned directly by E-beam irradiation. These results open up a possibility to realize Si-nanostructures formation by selective-area processing. Degradation of the oxide layer by plasma processing is also discussed.
Interfacial Arsenic from Wet Oxidation of AlxGa1-xAs/GaAs: Its Effects on Electronic Properties and New Approaches to Mis Device Fabrication
Authors: Ashby Carol I.H. , Sullivan John P. , Missert Nancy A. , Newcomer Paula P. , Hou Hong Q. , Hammons B.E. , Hafich Michael J. , Baca Albert G.
Published: September 2012

Three important oxidation regimes have been identified in the temporal evolution of the wet thermal oxidation of AlxGa1-xAs (1 ≥ x ≥ 0.90) on GaAs: 1) oxidation of Al and Ga in the AlxGa1-xAs alloy to form an amorphous oxide layer, 2) oxidative formation and elimination of elemental As (both crystalline and amorphous) and of amorphous As2O3, and 3) crystallization of the oxide film. Residual As can result in up to a 100-fold increase in leakage current and a 30% increase in the dielectric constant and produce strong Fermi-level pinning and high leakage currents at the oxidized AlxGa1-xAs/GaAs interface. The presence of thermodynamically-favored interfacial As may impose a fundamental limitation on the application of AlGaAs wet oxidation for achieving MIS devices in the GaAs material system.
Nitridation of Si(111)−7×7 Surface by Low Energy Nitrogen Ions : STM Investigation
Authors: Sook Ha Jeong , Park Kang-Ho , Soo Yun Wan , Lee El-Hang , Park Seong-Ju
Published: September 2012

The surface structure of Si(111) post-annealed at 980 °C after nitrogen ion induced nitridation has been investigated by using a scanning tunneling microscope (STM) and low energy electron diffraction (LEED). The LEED and STM results indicated the formation of ordered domain of quadruplet structure in the silicon nitride layer. The LEED pattern taken from the nitrated Si(111) surface showed a coexistence of 7×7 domain with quadruplet one. In the STM image taken from the same surface, a three directional periodicity with a periodic arrangement of white protrusions was observed in the local area of silicon nitride island and its symmetry directions were rotated about 10° with respect to those of Si(111) surface. In addition to the quadruplet structure of the silicon nitride island, meta-stable structures such as 9×9, c(4×2), and 2×2 as well as 7×7 phase boundaries were observed to have been formed on the Si(111) surface during the rapid cooling of nitrated surface from the post-annealing temperature of 980 °C. The investigation of the surface structure of nitrated Si(111) showed that the surface nitrated at high temperature had better epitaxial silicon nitride layer than that post-annealed after nitridation at room temperature.
Chemically Stable Semiconductor Surface Layers Using Low-Temperature Grown GaAs
Authors: Janes D.B. , Hong S. , Kolagunta V. R. , McInturff D. , NG T.-B. , Reifenberger R. , West S.D. , Woodall J.M.
Published: September 2012

The chemical stability of a GaAs layer structure consisting of a thin (10 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a heavily n-doped GaAs layer, both grown by molecular beam epitaxy, is described. Scanning tunneling spectroscopy and X-ray photoelectron spectroscopy performed after atmospheric exposure indicate that the LTG:GaAs surface layer oxidizes much less rapidly than comparable layers of stoichiometric GaAs. There is also evidence that the terminal oxide thickness is smaller than that of stoichiometric GaAs. The spectroscopy results are used to confirm a model for conduction in low resistance, nonalloyed contacts employing comparable layer structures. The inhibited surface oxidation rate is attributed to the bulk Fermi level pinning and the low minority carrier lifetime in unannealed LTG:GaAs. Device applications including low-resistance cap layers for field-effect transistors are described.
Spectroscopic Investigation of Lithium Intercalation in Thin Films of Anatase Titanium Dioxide
Authors: Van De Krol R. , Goossens A. , Schoonman J.
Published: September 2012

Thin films of anatase TiO2 have been deposited on tin-doped indium oxide (ITO) and Sb-doped tin oxide using electron beam evaporation. Subsequently these samples have been mounted into an electrochemical cell. Assuming a composition of LixTiO2 with x being 0.5 or 1, voltammetric measurements show that all lithium is present within the first 15 or 7.5 nm of the TiO2 film, respectively. Stepped potential experiments in combination with optical transmission measurements show that de-intercalation is much faster than intercalation. Differential absorption spectra as a function of intercalation potential suggest that the observed dark coloring of intercalated anatase can be attributed to electron traps at Li+ sites. This contradicts assumptions made in literature that the coloring mechanism of anatase is based on intervalence charge transfer from Ti4+ to Ti3+.
Interface properties of Si3N4/Si/n-GaAs metal-insulator-semiconductor structures grown on GaAs(111)B substrate
Authors: Park D.G. , Diatezua D. M. , Chen Z. , Mohammad S. N. , Morkoç H.
Published: September 2012

We present characteristics of Al/Si3N4/Si/n-GaAs metal-insulator-semiconductor (MIS) interfaces grown on GaAs(111)B prepared with a combination of in situ molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) techniques . The density of the surface states in the high 1010 eV-1 cm-2 near the GaAs midgap for the GaAs grown at 575°C and 625°C was obtained. The MIS structure with GaAs homoepitaxial layer grown at 625°C, showing smoother surface morphology than the surface grown at 575 °C, exhibited small hysteresis which was as small as 30 mV under a field excursion of 1.5 MV/cm. The presence of a 1 MHz frequency response at 77 K suggests that the traps be within 60 meV of the conduction band edge of GaAs.
Characterization of GaAs Surfaces Subjected to A Cl2/Ar High Density Plasma Etching Process
Authors: Eddy C.R. , Glembocki O.J. , Shamamian V.A. , Leonhardt D. , Holm R.T. , Butler J.E. , Thoms B.D. , Pang S.W. , Ko K.K. , Berg E.W. , Stutz C.E.
Published: September 2012

High density plasma etching of III-V compound semiconductors is critically important to the development of advanced optoelectronic and high frequency devices. Unfortunately, the surface chemistry of these processes is not well understood. In an effort to monitor surface processes and their dependence on process conditions in a realistic etching environment, we have applied mass spectroscopic techniques for the study of GaAs etching in Cl2/Ar chemistry. Etch product chlorides were monitored, together with optical measurement of the surface temperature by diffuse reflectance spectroscopy, as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. Observations from the spectroscopic techniques were correlated with ex situ surface damage assessments of unpassivated surfaces by photoreflectance spectroscopy. As a result, insights are made into regions of process conditions that are well suited to anisotropic, low damage etching.
Interfacial Layer Formation of a Heat Treated Teos Based Oxide Prepared by a Pecvd Technique
Authors: Lee T.J. , Jeong D.S. , Song C.S. , Lee S.Y. , Park C.H.
Published: September 2012

An interfacial layer is formed between the TEOS(tetraethylorthosilicate) based oxide and silicon substrate when the oxide is thermally treated and it shows unique properties by leaving a unremovable layer in an H F(hydrofluoric acid) dipping. While the interface formed in the temperature range of 950 - 1100 °C demonstrates hydrophilic state, it renders the surface hydrophobic in the higher range of 1150 - 1200 °C. Its characteristics are analyzed by ESCA and the measurement of water contact angle on the silicon surface after stripping the oxide.
Suppression of Surface SiO2 Layer and Solid Phase Epitaxy of Amorphously Deposited Si Films using Heating-Up Under Si2H6 Environment
Authors: Choe Tae-Hee , Kim Se-June , Choi Woon , Kim Hyoung-June
Published: September 2012

A novel technique to realize a selectively grown Si epitaxy layer has been developed. This technique consists of deposition of amorphous Si with oxide-free Si surface, selective solid phase epitaxial (SPE) growth on Si windows, and etching of uncrystallized amorphous Si on SiO2 layer. Formation of surface oxide can be effectively suppressed by flowing Si2H6 gas during heating-up stage to the deposition temperature. This method enables us to grow epitaxial layer without any high temperature cleaning procedures. Substantially higher growth rate of vertical SPE on Si windows over lateral SPE on SiO2 regions allows the growth of thick SPE layer with a minimized lateral overgrowth. With a proper etching solution, the remaining amorphous Si on SiO2 layer can be readily etched to form a selectively defined epitaxial layer on Si windows.
Thin Films of CoSi2 Co-Deposited onto Si1-xGexAlloys
Authors: Goeller Peter T. , Boyanov Boyan I. , Sayers Dale E. , Nemanich Robert J.
Published: September 2012

Cobalt disilicide films have been formed on strained epitaxial Si0.80Ge0.20/Si(100) alloys via co-deposition of silicon and cobalt. Co-deposition is shown to improve the epitaxy and prevent the phase segregation commonly observed with the formation of Co/SiGe contacts using other methods such as the direct deposition of cobalt onto SiGe or the sequential deposition of a silicon sacrificial layer and cobalt onto SiGe. EXAFS measurements at the cobalt K edge indicate that co-deposited films annealed at 500–700° C are indeed crystalline CoSi2 throughout this temperature range. The XRD patterns of the co-deposited films do not exhibit any of the CoSi2 (111), (220) or (311) peaks normally associated with other preparation methods. The sheet resistance and r.m.s. roughness of the CoSi2 films increase monotonically with annealing temperature. These results indicate that co-deposited films are epitaxial to the (100)-oriented SiGe substrate and suggest that low thermal budget, low resistivity contacts to strained SiGe can be grown with this method. Issues related to the presence of Ge at the CoSi2/substrate interface will be discussed.
Investigation of Cu-Ge/Gaas Metal-Semiconductor Interfaces for Low Resistance Ohmic Contacts
Authors: Oktyabrsky Serge , Borek M.A. , Aboelfotoh M.O. , Narayan J.
Published: September 2012

Chemistry and interfacial reactions of the Cu-Ge alloyed ohmic contacts to n-GaAs with extremely low specific contact resistivity (6.5×10-7 Ω·cm2 for n~1017 cm-3) have been investigated by transmission electron microscopy, EDX and SIMS. Unique properties of the contact layers are related to the formation (at Ge concentration above 15 at.%) of a polycrystalline layer of ordered orthorhombic ε1-Cu3Ge phase. Formation of the ε1-phase is believed to be responsible for high thermal stability, interface sharpness and uniform chemical composition. The results suggest that the formation of the ζ- and ε1,-Cu3Ge phases creates a highly Ge-doped n+-GaAs interfacial layer which provides the low contact resistivity. Layers with Ge deficiency to form ζ-phase show nonuniform intermediate layer of hexagonal β-Cu3As phase which grows epitaxially on Ga{111} planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing the ζ-phase which is formed in the structures with average Ge concentration of as low as 5 at.%. These layers also exhibit ohmic behavior.
Effects of the Substrate Pretreatments on the Leakage Current in the Low-Temperature Poly-Si TFTs
Authors: Kim Tae-Kyung , Lee Byung-Il , Ihn Tae-Hyung , Joo Seung-Ki
Published: September 2012

Poly-Si TFT’s were fabricated on the glass substrate by the Metal Induced Lateral Crystallization(MILC). Before deposition of the active a-Si thin films(1000 Å), the glass substrate was pretreated in three different ways such as oxidation of a-Si(100 Å), oxide buffer layer deposition(1000 Å), and ion mass doping of the glass substrates. The leakage current at reverse bias could be reduced by one order of magnitude by the substrate pretreatment. Field effect mobility of n-channel TFT’s was 108cm2/Vsec, subthreshold slope and on/off current ratio were 0.7 V/dec. and about 106, respectively.
A Study of the Interaction between Cu3Ge and (100) Si, and its Effect on Electrical Properties
Authors: Borek M.A. , Oktyabrsky S. , Aboelfotoh M.O. , Narayan J.
Published: September 2012

The reduction in the dimensions of advanced semiconductor devices has brought about the need for new metallization materials with low resistivity and high electromigration resistance. The Cu3Ge, has been suggested as a contact and metallization material due to its low resistivity (6 μΩ-cm), high electromigration resistance, and high chemical stability. We have grown thin films of Cu3Ge on (100) Si by the sequential e-beam deposition of an amorphous Ge layer then a Cu layer, followed by a thermal anneal to crystallize the Cu3Ge film. The Cu-Ge films maintain their low resistivity (10-15 μΩ cm) over a range of anneal temperatures, up to an anneal temperature of 600°C, where a significant increase in resistivity is observed. We have shown that this increase in resistivity is directly related to the structure of the Cu3Ge film and interface. We have observed by cross-sectional transmission electron microscopy (TEM), that films of Cu3Ge form a smooth, atomically sharp interface with (100) Si, up to an anneal temperature of 600°C, where the film agglomerates, and additional compounds are observed. In this paper, we discuss the correlation between microstructure, interface structure and electrical properties of these novel thin film structures.
Electrical Characterization of In Schottky Contacts to Epitaxial n-In0.46Ga0.54P Grown on n+-GaAs by Mocvd
Authors: Marcano N. , Singh A.
Published: September 2012

In/n-In0.46Ga0.54P Schottky diode was fabricated by thermal evaporation of In on chemically etched surface of In0.45Ga0.54P:Si epitaxial layer grown on highly doped n type GaAs. The In metal formed a high quality rectifying contact to In0.46Ga0.54P:Si with a rectification ratio of 500. The direct current-voltage/temperature (I-V/T) characteristics were non-ideal with the values of the ideality factor (n) between 1.26-1.78 for 400>T>260 K. The forward I-V data strongly indicated that the current was controlled by the generation-recombination (GR) and thermionic emission (TE) mechanisms for temperature in the range 260-400 K. From the temperature variation of the TE reverse saturation current, the values of (0.75±0.05)V and the (4.5±0.5)×10-5 Acm-2K-2 for the zero bias zero temperature barrier height (φoo) and modified effective Richardson constant were obtained. The 1 MHz capacitance-voltage (C-V) data for 260 K < T < 400 K was analyzed in terms of the C-2-V relation including the effect of interface layer to obtain more realistic values of the barrier height (φbo). The temperature dependence of φbo was described the relation φbo =(0.86±10.03) - (8.4±0.7)×l0-4T. The values of φoo, obtained by the I-V and C-V techniques agreed well.
Conversion Tunneling in Non-Ideal Schottky Barriers: Virtual Resonance Manifestation and Interface States Influence
Authors: Romanov D.A. , Kalameitsev A.V. , Kovchavtsev A.P. , Subbotin I.M.
Published: September 2012

We have investigated experimentally and theoretically the elastic conversion tunneling of charge carriers in MOS structures Au on p+-InAs with superthin (10-20 Å) oxide film, the structures used in infrared photodetectors. In these structures the Schottky barrier provides near-surface inversion layer. The tunnel current-voltage (I-V) curves obtained at helium temperatures demonstrate the negative differential resistance region (NDR). We develop semiclassical two-band transfer matrix approach to the conversion tunneling analysis in a multilayer structure and calculate on its base the I-V curves dependence on the structure parameters. The NDR occurs to be caused by the motion of the remote quantum level in the inversion layer. The calculated I-V characteristics agree with the experimental ones quite well. The very existence of NDR and the shape of I-V curves depends strongly on the nature of localized electron states at the semiconductor interface. The characteristics of these electron states are used in the calculations as fitting parameters. Therefore, we suggest a new method for the interface states diagnosis.
Theory of Reactive Adsorption on Si(100)
Authors: Doren D. J. , Brown A. Robinson , Konecny R.
Published: September 2012

Density functional calculations on cluster models of Si(100)-2xl have been used to predict reaction mechanisms and energetics for several reactive adsorption processes. Dissociative adsorption of H2, H20, BH3, and SiH4 will be described and compared to available experimental data. Based on these examples, a qualitative theory of mechanisms for dissociative adsorption of hydrides on silicon surfaces will be proposed. These reactions can largely be understood in terms of the electron density distributions in the molecule and surface dimer. On a buckled dimer, there are both electron-rich and electron-deficient sites, which have different chemical interactions with adsorbates. The role of this difference is illustrated in a novel surface Diels-Alder reaction, for which symmetric addition to an unbuckled surface dimer is allowed by orbital symmetry. This reaction creates new reactive surface sites that may be useful for subsequent chemical surface modification.
Multilevel Approaches Toward Monitoring and Control of Semiconductor Epitaxy
Authors: Aspnes D. E. , Dietz N. , Rossow U. , Bachmann K. J.
Published: September 2012

Various optical techniques have been developed over the last few years to allow real-time analysis of regions of importance for semiconductor epitaxy, in particular the unreacted and reacted parts of the surface reaction layer (SRL) and the near-surface region of the sample. When coupled with emerging microscopic methods of calculating optical properties, these approaches will allow several levels of control beyond that which has been currently demonstrated.
Investigation of electric fields, interface charges, and conduction band offsets at ZnSe/GaAs heterojunctions with a novel photoreflectance technique
Authors: Dougherty D. J. , Fleischer S.B. , Warlick E. L. , House J. L. , Petrich G. S. , Ho E. , Kolodziejski L. A. , Ippen E. P.
Published: September 2012

ZnSe/GaAs heterojunctions were investigated by contactless electroreflectance and photoreflectance techniques. Negative surface charge densities on the order of 1012 cm-2 were observed for films grown on n-type GaAs indicating a large contribution to the conduction band barrier between the materials due to band bending. The conduction band offset was also measured using a new photoreflectance technique involving a tunable pump laser.
Optical Investigations of InAs Growth on GaAs and Lasing in Singly and Multiply Stacked Island Quantum Boxes
Authors: Kalburge A. , Ramachandran T. R. , Heitz R. , Xie Q. , Chen P. , Madhukar A.
Published: September 2012

We report on the optical investigations of InAs growth on GaAs. In-situ STM/AFM studies show the presence of features 2-4 ML high, which we call quasi-3D (Q3D) clusters, well in advance of 3D island formation. Though the photoluminescence (PL) emission from these Q3D clusters is in the same wavelength regime as that from well developed 3D islands, they show characteristic differences in the PL excitation spectra and temperature dependence of PL, distinguishing them clearly from the 3D islands. Finally, we discuss the lasing observed from lasers containing single and five sets of InAs layers grown with conditions in which the in-situ STM/AFM studies show only 3D islands.
Metrology of Very Thin Silicon Epitaxial Films using Spectroscopic Ellipsometry
Authors: Chen Weize , Reif Rafael
Published: September 2012

Non-destructive silicon epitaxial film thickness measurements have been performed using spectroscopic ellipsometry (SE) in the visible to near infrared range. The epitaxial films were grown by CVD at 700 – 900°C. It is shown that SE can simultaneously determine the epitaxial film thickness, the native oxide thickness, and the substrate dopant concentration. The epitaxial film thicknesses measured by SE are in excellent agreement with results of secondary ion mass spectrometry (SIMS). The dopant concentrations measured by SE also agree well with SIMS results for n-type substrates, but are consistently higher than SIMS values for p-type substrates. This study shows that spectroscopic ellipsometry could be used in semiconductor manufacturing environments as a viable non-destructive technique for sub-0.5 μm silicon epitaxial film thickness measurement.
Nonradiative recombination on Si surfaces during anodic oxidation in fluoride solution
Authors: Rappich J. , Timoshenko V. Yu. , Dittrich Th.
Published: September 2012

The anodic oxidation of p-Si(100) in aqueous NH4F solution is investigated in-situ by photoluminescence (PL) with short N2-laser pulses as a function of pH at constant potential. The PL intensity depends sensitively on the modification of the Si surface and on the change of the oxidation rate during current oscillations. The interruption of the anodic oxidation at the maximum of a current oscillation peak leads to a current transient which shows typical features of oxide removal and hydrogenation for two different oxide thicknesses. This makes it possible to realize an intermediate state of the Si surface with a coexisting hydrophilic (oxide) and hydrophobic (hydrogenated) part.
Advanced Lithography for Nanofabrication
Authors: Hui Frank Y.C. , Eres Gyula
Published: September 2012

A novel method for generating lateral features by patterning the naturally forming surface hydride layer on Si is described. Because of the relatively strong chemical bonding between silicon and hydrogen, the hydride layer acts as a robust passivation layer with essentially zero surface mobility at ordinary temperatures. A focused electron beam from a scanning electron microscope was used for patterning. Upon losing the hydrogen passivation the silicon surface sites become highly reactive. Ideally, the lifetime of such a pattern in a clean environment should be infinite. Deliberate exposure of the entire wafer to a suitable gas phase precursor results in selective area film growth on the depassivated pattern. Linewidths and feature sizes of silicon dioxide on silicon below 100 nm were achieved upon exposure to air. The silicon dioxide is robust and allows effective pattern transfer by anisotropic wet-chemical etching. In this paper, the mechanism of hydrogen desorption and subsequent pattern formation, and the factors that govern the ultimate pattern resolution will be discussed.
Chemical Characterization by FT-IR Spectrometry and Modification of the Very First Atomic Layer of A TiO2 Nanosized Powder
Authors: Baraton M.-I. , Merhari L. , Chancel F. , Tribout J.
Published: September 2012

Nanosized powders exhibit high specific surface areas resulting in enhanced reactivities. Surface tailoring by controlled adsorption of molecules can thus be conveniently performed and more easily monitored by surface-sensitive techniques. In situ and ex situ grafting procedures of hexamethyldisilazane (HMDS) on nanosized titania (n-TiO2) powder were carried out and studied by Fourier transform infrared spectrometry (FT-IR). In addition to a decrease of the hydrophilic OH groups, the vibration analysis revealed hydrophobic CH3 groups on the grafted samples. Co-adsorption of CO and H2O on the differently grafted samples showed a large reduction of water effect compared to the as-received n-TiO2 powder. Modulation of infrared transmitted energy by controlled adsorption of O2 and CO made it possible to qualitatively compare the electronic properties of the surface-tailored samples.
In-Situ Etch to Improve Chemical Beam Epitaxy Regrown AlgaAs/GaAs Interfaces for HBT Applications
Authors: Hsin Y.M. , Li N. Y. , Tu C. W. , Asbeck P. M.
Published: September 2012

We have studied the etching effect of AlxGa1-xAs (0≤ x ≤ 0.5) by trisdimethylaminoarsenic (TDMAAs) at different substrate temperatures, and the quality of the resulting etched/regrown GaAs interface. We find that the etching rate of AlxGa1-x As decreases with increasing Al composition, and the interface trap density of the TDMAAs etched/regrown interface can be reduced by about a factor of 10 as deduced from capacitance-voltage carrier profiles. A smooth surface morphology of GaAs with an interface state density of 1.4×l011 cm−2 can be obtained at a lower in-situ etching temperature of 550°C. Moreover, by using this in-situ etching the I-V characteristics of regrown p-n junctions of Al0.35Ga0.65As/Al0.25Ga0.75As and Al0.35Ga0.65As/GaAs can be improved.
Gas Migration in MX80 Buffer Bentonite
Authors: Horseman S. T. , Harrington J. F. , Sellin P.
Published: September 2012

Controlled flow-rate gas injection experiments have been performed on
pre-compacted samples of KBS-3 specification M×801 buffer
bentonite using helium as a safe replacement for hydrogen. By simultaneously
applying a confining pressure and backpressure, specimens were
isotropically-consolidated and fully water-saturated under pre-determined
effective stress conditions, before injecting gas using a syringe pump.
Ingoing and outgoing gas fluxes were monitored. All tests exhibited a
conspicuous threshold pressure for breakthrough, somewhat larger than the
sum of the swelling pressure and the backpressure. All tests showed a
post-peak negative transient leading to steady-state gas flow. Using a
stepped history of flow rate, the flow law was shown to be nonlinear. With
the injection pump stationary (i.e. zero applied flow rate), gas pressure
declined with time to a finite value. When gas flow was reestablished, the
threshold value for gas breakthrough was found to be significantly lower
than in virgin clay. There is strong evidence to suggest that the capillary
pressure for the penetration of interparticle pore space of buffer bentonite
is of such a magnitude that normal two-phase flow is impossible. Gas entry
and breakthrough is therefore accompanied by the development of microcracks
which propagate through the clay from gas source to sink. The experiments
suggest that these pathways open under high gas pressure conditions and
partially close if gas pressure falls, providing a possible explanation of
the nonlinearity of the flow law.
Determination of Porewater Chemistry in Compacted
Bentonite
Authors: Lehikoinen J. , Muurinen A. , Melamed A. , Pitkänen P.
Published: September 2012

Laboratory experiments were performed to study the interaction between
groundwater and compacted sodium bentonite (Volclay MX-80). The solutions
used were the fresh and saline groundwater simulants. The experiments were
carried out in aerobic and anaerobic conditions at elevated temperature. Of
main interest in the present study were the chemical changes in the reacting
solution, bentonite porewater. and bentonite itself. The results for major
cations display a principal difference between the interactions with fresh
and saline solutions, while the differences between aerobic and anaerobic
conditions within each solution case seem to be minor. The experimental
results for the bentonite-water equilibria were interpreted in terms of a
multi-site surface complexation model and the computer program HYDRAQL. The
apparent diffusivities for sodium and sulphate in bentonite samples
sandwiched between two filterplates were also determined.
Experimental Study on Scale Effects of Bentonite / Sand Mixtures
on Gas Migration Properties
Authors: Hokari Toshiyuki , Okihara Mitsunobu , Ishii Takashi , Ishii Takao , Ikuse Hiroyuki
Published: September 2012

Gas migration properties of bentonite/sand mixtures for a large-scale
structure were estimated on the basis of laboratory test results obtained
with small test specimens. Water and gas migration tests were carried out to
clarify scale effects on migration properties of the mixtures.
Near-Field Thermal-Hydrological Behavior for Alternative
Repository Designs at Yucca Mountain
Authors: Buscheck Thomas A. , Nitao John J. , Ramspott Lawrence D.
Published: September 2012

Three-dimensional calculations that explicitly represent a realistic mixture
of waste packages (WPs) are used to analyze decay-heat-driven
thermal-hydrological behavior around emplacement drifts in a potential
high-level waste facility at Yucca Mountain. Calculations, using the NUFT
code, compare two fundamentally different ways that WPs can be arranged in
the repository, with a focus on temperature, relative humidity, and
liquid-phase flux on WPs. These quantities strongly affect WP integrity and
the mobilization and release of radionuclides from WPs. Point-load spacing,
which places the WPs roughly equidistant from each other, thermally isolates
WPs from each other, causing large variability in temperature, relative
humidity, and liquid-phase flux along the drifts. Line-load spacing, which
places WPs nearly end to end in widely spaced drifts, results in more
locally intensive and uniform heating along the drifts, causing hotter,
drier, and more uniform conditions. A larger and more persistent reduction
in relative humidity on WPs occurs if the drifts are backfilled with a
low-thermal-conductivity granular material with hydrologie properties that
minimize moisture wicking.
Application of Interaction Matrices in Performance Assessment -
the Fuel Matrix as an Example
Authors: Sellin Patrik , Bruno Jordi , Cera Ester
Published: September 2012

In a safety analysis of a repository for high level nuclear waste, it is of
primary importance to identify and document all processes that are acting on
the repository system. An interaction matrix methodology have been applied
to the spent fuel subsystem. The purpose of this application is to identify,
structure and rank the Process System and to discuss how the identified
processes can be treated.
Optimization of Lanthanide Borosilicate Frit Compositions for
the Immobilization of Actinides Using a Plackett-Burman/Simplex Algorithm
Design
Authors: Mesko M. G. , Meaker T. F. , Ramsey W. G. , Marra J. C. , Peeler D. K.
Published: September 2012

Immobilization by vitrification is one potential disposition option for a
portion of the United States' excess plutonium inventory. Research has been
performed at the Savannah River Site (SRS) to determine the optimum
composition of a lanthanide borosilicate frit for the vitrification of
plutonium using a Plackett-Burman design and simplex algorithm as a
statistical tool. This technique uses various response variables to rank and
optimize a composition. The variables used in this study correspond to
homogeneity, durability, actinide solubility and devitrification after
heat-treatment.

The optimized frit composition was determined using a constant
ThO2 loading of 20 wt%. No noticeable trends were followed
with respect to the individual components which may indicate a relatively
robust system able to accommodate variations in the feed.

Batches containing various loadings of ThO2 were melted to
determine if actinide solubility was improved in the optimized composition
compared to that of a similar lanthanide borosilicate glass. No noticeable
improvement in ThO2 solubility was realized as a result of using
this optimization technique.
Source-Term Analysis for Hanford Low-Activity Tank Waste Using
the Reaction-Transport Code AREST-CT
Authors: Chen Y. , McGrail B. P. , Engel D. W.
Published: September 2012

A general, integrated performance assessment code, AREST-CT, was used to
analyze the influence of various factors on the release rates of
radionuclides from a proposed facility for disposal of low-activity tank
wastes. The code couples various process models together based on the
framework of reaction-transport theory. The disposal facility was modeled as
a 1-D column surrounded by soil. A borosilicate waste glass, LD6–5412 was
the waste form considered in the analysis. Included in the simulations were
38 aqueous species, 14 minerals, 21 equilibrium reactions, and 16 kinetic
reactions. Dissolution rate of the glass and the release rates of Te, Pu, U,
Np, I, Se under different conditions were calculated for 50,000 years. The
simulations revealed that 1) open exchange between the atmosphere and
pore-water within the vault significantly improves the performance; 2) an
ion-exchange reaction between the glass and aqueous phase increases the
release rates significantly; and 3) at the hydrogeologie conditions under
consideration, variation of the pore-water velocity has little effect on the
release rate of radionuclides. These results provide a scientific basis for
formulation of waste forms and engineering design of the disposal facility.
Reaction-transport modeling can provide information on the long-term
performance of disposal systems that are not obtainable from laboratory
experiments alone or by conventional decoupled process models.
A Mathematical Model for Advective Release from Perforated Waste
Package Container Under Dripping Water
Authors: Lee Joon H.
Published: September 2012

A defense-in-depth engineered barrier system (EBS) is employed in the
current design concept for the potential high-level nuclear waste repository
at Yucca Mountain, Nevada, USA. Simplifying the geometry of the cylindrical
waste container into the equivalent spherical configuration, and
incorporating detailed analysis of the mechanics of water flow around the
waste container surface, a mathematical model is developed for advective
release from a “failed” (or perforated) waste container under dripping
water. It is shown that the advective release rates are controlled by
diffusion through the perforations in the waste container, and affected
insignificantly by the dripping flow rate for the flow
rate range considered. The release rates depend strongly on the number of
perforations (or pit penetrations) in the waste container. The insensitivity
of the release rate to the dripping flow rate is explained by the fact that
radionuclide is released from the container surface to the boundary layer of
the water film which contacts the container surface and is relatively
stagnant. Also, since a laminar flow around the waste container surface is
assumed in the model development, radionuclides transport across the water
layers in the film by diffusion only. Additionally, the insignificant effect
of the flow rate is contributed by the “short” penetration depth of
radionuclide into the water film assumed in the model development. The
analyses show that the number of perforations, the size of perforation, the
container wall thickness, and the geometry (i.e., radius) of the waste
container are important parameters that control the advective release rate.
It is emphasized that the “failed” (or perforated) waste package container
can still perform as a potentially important barrier to radionuclide
release.
Bounding Analysis for Solubility
Authors: Ohi T. , Nakajima K.
Published: September 2012

In performance assessment of geological disposal systems for High Level
Radioactive Waste (HLW), the change of environment over the long-term must
be considered. Therefore, it is necessary to consider a wide range of
parameters concerned with radionuclides migration, especially the dependence
of solubility on geochemical environment. In this study, assuming that the
release rate of the nuclides from buffer material is limited by inventory
ultimately, the relationship between the initial inventory and the
solubility that produces a solubility-invariant maximum release rate from
the buffer is examined by using a simple steady-state analytical solution
without decay. As the result, the threshold of “effective” solubility in the
performance assessment of the geological disposal systems for HLW is
obtained as a function of initial inventory, distribution coefficient (Kd),
diffusion coefficient, and thickness, porosity and density of the buffer.
Also, the threshold of “effective” steady dissolution rate corresponding to
the threshold of “effective” solubility is obtained.
Americium/Curium Extraction from a Lanthanide Borosilicate
Glass
Authors: Rudisill T. S. , Pareizs J. M. , Ramsey W. G.
Published: September 2012

A solution containing kilogram quantities of highly radioactive isotopes of
amerícium and curium (Am/Cm) and lanthanide fission products is currently
stored in a process tank at the Department of Energy's Savannah River Site
(SRS). This tank and its vital support systems are old, subject to
deterioration, and prone to possible leakage. For this reason, a program has
been initiated to stabilize this material as a lanthanide borosilicate (LBS)
glass.1 The Am/Cm has commercial value and is desired for use
by the heavy isotope programs at the Oak Ridge National Laboratory
(ORNL).

A recovery flowsheet was demonstrated using a curium-containing glass to
extract the Am/Cm from the glass matrix. The procedure involved grinding the
glass to less than 200 mesh and dissolving in concentrated nitric acid at
110°C. Under these conditions, the dissolution was essentially 100% after 2
hours except for the insoluble silicon. Using a nonradioactive surrogate,
the expected glass dissolution rate during Am/Cm recovery was bracketed by
using both static and agitated conditions. The measured rates, 0.0082 and
0.040 g/hrcm2, were used to develop a predictive model for the
time required to dissolve a spherical glass particle in terms of the glass
density, particle size, and measured rate. The calculated dissolution time
was in agreement with the experimental observation that the curium glass
dissolution was complete in less than 2 hr.