bluuebunny/crossref_metadata_2025 huggingface dataset:
Interdiffusion measurements in asymmetrically strained SiGe/Si superlattices
Authors: Prokes S. M. , Wang K. L.
Published: July 2002

Interdiffusion measurements are reported for Si0.65Ge0.35/Si asymmetrically strained superlattices grown by molecular beam epitaxy at 530 °C. The temperature-dependent interdiffusion coefficient obtained from x-ray diffraction can be described by D=675 exp(−4.4 eV/kT)cm2/s in the temperature range 700–880 °C. Initially, an enhanced diffusion was observed, especially near the superlattice surface. This is attributed to the presence of nonequilibrium defects. Bulk interdiffusion measurements were made only after isoconfigurational conditions were attained. The diffusion analysis first formulated by J. W. Cahn [Acta Metall. 9, 795 (1961)] is applied here, and the relative importance of both gradient energy effects and coherency strain effects will be discussed.
Accumulation capacitance for GaAs-SiO2 interfaces with Si interlayers
Authors: Freeouf J. L. , Buchanan D. A. , Wright S. L. , Jackson T. N. , Robinson B.
Published: July 2002

We have studied the properties of metal-oxide-semiconductor (MOS) structures fabricated by remote plasma-enhanced chemical vapor deposition of SiO2 upon GaAs substrates. For n-type GaAs, a silicon interlayer has been found to improve the interface properties. For our samples and this interlayer, integration of the quasi-static capacitance curve indicates a band-bending range of about 0.8 eV. For these samples, we observe a hysteresis of ∼0.6 V, and shifts of only 0.2 V in the midpoint of the rise from minimum to maximum capacitance upon changing frequency from 10 to 200 kHz at room temperature. Similar measurements for temperatures down to 80 K establish that even at such low temperatures an accumulation capacitance is observed. This sets an upper limit of about 70 meV for the separation between the interface Fermi level and the conduction-band minimum. This limit is a factor of two smaller than the best previously reported limit for approach to the conduction band of GaAs in a MOS structure. Spectroscopic ellipsometry establishes that nearly 2 Å equivalent thickness of unoxidized silicon is at the SiO2/GaAs interface.
Effect of growth rate on the band gap of Ga0.5In0.5P
Authors: Kurtz Sarah R. , Olson J. M. , Kibbler A.
Published: July 2002

The band gap of Ga0.5In0.5P is reported as a function of growth rate and growth temperature. The Ga0.5In0.5P is grown lattice matched to 2°-off (100) GaAs substrates by atmospheric pressure organometallic chemical vapor deposition using an inlet group V/III ratio of 65. The variation of the band gap is surprisingly complex, taking five different functional forms within the two-dimensional parameter space. These include regions in which the band gap (1) increases with growth rate, (2) decreases with growth rate, (3) is independent of both growth rate and temperature, (4) is independent of growth rate, but dependent on growth temperature, and (5) is not measurable since three-dimensional, instead of two-dimensional, growth is observed. The behavior can only be explained by a theory involving competing processes. One such theory is described.
High-power cw vertical-cavity top surface-emitting GaAs quantum well lasers
Authors: Tell B. , Lee Y. H. , Brown-Goebeler K. F. , Jewell J. L. , Leibenguth R. E. , Asom M. T. , Livescu G. , Luther L. , Mattera V. D.
Published: July 2002

We have devised a novel vertical-cavity top surface-emitting GaAs quantum well laser structure which operates at 0.84 μm. The laser combines peripheral current injection with efficient heat removal and uses only the epitaxially grown semiconductor layers for the output mirrors. The structure is obtained by a patterned deep H+ implantation and anneal cycle which maintains surface conductivity while burying a high resistance layer. Peripheral injection of current occurs from the metallized contact area into the nonimplanted nonmetallized emission window. For 10-μm-diam emitting windows, ∼4 mA thresholds with continuous-wave (cw) room-temperature output powers ≳1.5 mW are obtained. Larger diameter emitting windows have maximum cw output powers greater than 3 mW. These are the highest cw powers achieved to date in current injected vertical-cavity surface-emitting lasers.
Measurement of nonlinear refractive index and transmission in polydiacetylene waveguides at 1.319 μm
Authors: Rochford K. , Zanoni R. , Stegeman G. I. , Krug W. , Miao E. , Beranek M. W.
Published: July 2002

Nonlinear optical measurements in strip-loaded channel waveguides of poly(4-BCMU) at 1.319 μm are reported. Using a pulse-modulated interferometry method, the intensity-dependent refractive index is found to have an electronic contribution, n2=(4.8±2.7)×10−8 cm2/MW, and an estimated thermal contribution, n2=−(7.9±4.5)×10−11 cm2/MW, for 60 pS laser pulses at 76 MHz. The two-photon absorption coefficient β is less than 0.25 cm/GW. Figures-of-merit suggest that poly(4-BCMU) has properties desirable for all-optical devices, but that fabrication losses and thermal effects due to absorption will limit device performance.
Large-area mosaic diamond films approaching single-crystal quality
Authors: Geis M. W. , Smith Henry I. , Argoitia A. , Angus J. , Ma G.-H. M. , Glass J. T. , Butler J. , Robinson C. J. , Pryor R.
Published: July 2002

The seeding for large-area mosaic diamond films approaching single-crystal quality is described. The technique includes patterned etching of relief structures in Si substrates, deposition from a slurry and orientation of macroscopic diamond seed crystals in the structures, and chemical vapor deposition overgrowth of the diamond seeds to form a continuous film. The film comprises ∼100 μm single crystals, which are separated by low-angle grain boundaries of a few degrees or less. We believe that these low-angle grain boundaries will not affect the electrical properties of majority-carrier devices.
Spin relaxation and thermalization of excitons in GaAs quantum wells
Authors: Damen T. C. , Leo Karl , Shah Jagdeep , Cunningham J. E.
Published: July 2002

We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is ≊50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in ≊50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.
Epitaxial silicon deposition at 300 °C with remote plasma processing using SiH4/H2 mixtures
Authors: Hattangady S. V. , Posthill J. B. , Fountain G. G. , Rudder R. A. , Mantini M. J. , Markunas R. J.
Published: July 2002

Epitaxial Si films have been deposited on Si(100) at 300 °C by remote plasma-enhanced chemical vapor deposition using SiH4/H2 mixtures with deposition rates as high as 25 Å/min at these low temperatures. Hall measurements of the film show an unintentional doping level of about 1×1017 cm−3 with electron mobilities of 700 cm2 V−1s−1. Critical to the process is the in situ cleaning of the silicon substrate surface prior to deposition.
YBa2Cu3O7−δ thin-film gradiometers: Fabrication and performance
Authors: Eidelloth W. , Oh B. , Robertazzi R. P. , Gallagher W. J. , Koch R. H.
Published: July 2002

We have fabricated and tested superconducting first-order gradiometers made from YBa2Cu3O7−δ (YBCO) thin-film flux transformers coupled to YBCO dc SQUIDs. The flux transformers consisted of 10-turn input coils with 5-μm linewidth and two pick-up loops with 600-μm linewidth, each of which surrounded an area of 2.1 by 2.8 mm. All levels were patterned by wet etching. A magnetic field gradient of 1 nT/m resulted in a flux of 5.8×10−4 Φ in the SQUID which corresponds to a responsivity of 1.21×10−9 m3. The 1/f noise level at 77 K and 10 Hz was 0.6 nT/m√Hz and was dominated by SQUID noise.
ZnSe-ZnSSe electro-optic waveguide modulators
Authors: Haase M. A. , Cheng H. , Misemer D. K. , Strand T. A. , DePuydt J. M.
Published: July 2002

Rib waveguides have been fabricated in the ZnSe-ZnSSe material system, grown by molecular beam epitaxy on GaAs substrates. Using Schottky barrier contacts, rib-waveguide optical phase modulators have been demonstrated and the strength of the linear electro-optic (Pockels) effect has been measured. A change in the index of refraction of Δn/ΔF=±1.7×10−9 cm/V has been attained for 633 nm light. This translates to a maximum Δn of 0.0008. Rib waveguides with losses as low as 0.66 cm−1 have been measured.
Quantized conductance in ballistic constrictions at 30 K
Authors: Snider Gregory L. , Miller Mark S. , Rooks Michael J. , Hu Evelyn L.
Published: July 2002

Quantized conductance in a split-gate structure is observed at a temperature of 30 K. Plateaus in the conductance as a function of gate voltage are observed at 30 K, while traces of the first step exist as high as 44 K. Plateaus are also seen with a bias of up to 12 mV across the constriction. This performance is made possible by placing the two-dimensional electron gas (2DEG) 30 nm below the surface, and a gate separation of 100 nm. Simulations of this structure, using two-dimensional, self-consistent solutions of Schrödinger’s and Poisson’s equations, suggest subband separations of 10 meV.
Fully connected bulk Pb1−xSnxMo6S7.6 samples made by hot isostatic pressing
Authors: Le Lay L. , Willis T. C. , Larbalestier D. C.
Published: July 2002

High-density bulk Pb1−xSnxMo6S7.6 (0≤x≤1) samples have been made by hot isostatic pressing at the relatively low temperatures of 800 and 900 °C. Critical current densities, computed from magnetization data taken at various magnetic fields and temperatures, show excellent temperature scaling indicating that a single pinning mechanism is present. The magnetization of all samples scales with size, showing that full density samples are not granular. Samples sintered without pressure at higher temperatures did not achieve full density and did not exhibit good size scaling of the magnetization, thus indicating only percolation connections across the sample in this case. We conclude that there is no intrinsic granularity in these Chevrel phases even though their coherence length is short.
Synthesis of Si1−yCy alloys by molecular beam epitaxy
Authors: Iyer S. S. , Eberl K. , Goorsky M. S. , LeGoues F. K. , Tsang J. C. , Cardone F.
Published: July 2002

We have synthesized pseudomorphic Si1−yCy (y≤0.05) alloys and strained layer superlattices on silicon by molecular beam epitaxy using solid sources for carbon and silicon. The introduction of C into substitutional sites in the silicon lattice is kinetically stabilized by low-temperature growth conditions (500–600 °C) and relatively high Si fluxes, against extremely low C solubility (10−6 at 1420 °C) and the thermodynamically favored silicon carbide phases. Higher temperature growth leads to an islanded morphology. At lower temperatures, disruption of epitaxy occurs via the formation of highly twinned layers or even amorphous growth. The temperature window for alloy growth is reduced as the C concentration is increased. X-ray diffraction, transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy confirm the growth of pseudomorphic, tetragonally strained alloy layers with no detectable silicon carbide precipitation. These alloy layers allow for the engineering of Si-based lattice constants less than that of bulk silicon using isovalent elements. They have potential application in band-engineered Si devices and may provide band gaps wider than that of Si, and perhaps, larger band offsets than that provided by the Si1−xGex system.
Raman scattering from H or O terminated porous Si
Authors: Tsang J. C. , Tischler M. A. , Collins R. T.
Published: July 2002

The dependence of the crystalline structure of porous Si on the bonding of surface Si atoms to either H or O has been studied by Raman spectroscopy. H terminated porous Si shows microcrystalline character while O terminated porous Si shows atomic disorder within the Si particles.
Raman study of low growth temperature GaAs
Authors: Gant T. A. , Shen H. , Flemish J. R. , Fotiadis L. , Dutta M.
Published: July 2002

A Raman study has been made of GaAs grown by MBE at low temperature, which is known to produce nonstoichiometric (As-rich) material. The epilayer has a slightly larger lattice constant than the GaAs substrate, as measured by x-ray diffraction. Raman measurements show that the LO phonon of LT-GaAs is shifted down in frequency from that of bulk GaAs. The shift is related to the presence of AsGa antisite defects, which cause a reduction in the electric field associated with the LO phonon. After the sample was annealed, the epilayer had the same lattice constant as the substrate, and the LO phonon moved closer to that of bulk GaAs.
Role of electrorefraction in quantum-well Fabry–Perot modulators
Authors: Livescu G. , Boyd G. D. , Morgan R. A. , Chirovsky L. M. F. , Fox A. M. , Leibenguth R. E. , Asom M. T. , Focht M. W.
Published: July 2002

The effects of electrorefraction in quantum-well Fabry–Perot (FP) modulators are discussed theoretically and illustrated experimentally. The large electrorefraction at the zero-field heavy-hole exciton is shown to produce strong modulation in FP devices operated at that wavelength. At longer operating wavelengths, the weakening electrorefraction plays a diminishing role. By taking into account the field dependent absorption α(λ,E) and dispersion n(λ,E) of the quantum-well material, the measured reflectivity spectra are successfully modeled.
Observation of growing kinetics of particles in a helium-diluted silane rf plasma
Authors: Watanabe Y. , Shiratani M. , Yamashita M.
Published: July 2002

Growing process and behavior of particles in a helium-diluted silane rf plasma are studied using two kinds of laser light scattering methods. The region where particles began to be observed appears in a sheath edge region near the periphery of the rf electrode, and then tends to extend inward along the rf electrode and also toward the grounded electrode. From this result, it can be concluded that particles are mainly produced around the sheath edge of the rf electrode. In the sheath of the rf electrode and its neighborhood, larger particles tend to reside closer to the rf electrode, which suggests that they are suspended by an electrostatic force due to the time-averaged sheath electric field and some force increasing with particle size. The size and density of particles around the sheath edge of the rf electrode are ≳60 nm and ≲109 cm−3 under our experimental conditions.
Transient recovery of minority-carrier lifetime in silicon after ultraviolet irradiation
Authors: Zhong L. , Buczkowski A. , Katayama K. , Shimura F.
Published: July 2002

A recovery process of minority-carrier recombination lifetime after ultraviolet (UV) irradiation has been investigated with a laser-microwave photoconductance technique for silicon wafers with native oxide. It is found that the effective lifetime which greatly increases after UV irradiation recovers to the initial value primarily with an exponential law characterized by a specific time constant called recovery time. The recovery time depends on experimental conditions where, for example, an accumulation effect of UV irradiation is observed. A mechanism of the effective lifetime recovery process is correlated mainly with the behavior of slow states associated with the silicon/native oxide interface.
Lateral motion of terrace width distributions during step-flow growth
Authors: Chalmers S. A. , Tsao J. Y. , Gossard A. C.
Published: July 2002

We have observed two phenomena that occur during (Al,Ga)Sb lateral superlattice (LSL) growth which have implications on our understanding of adatom/step edge interactions on vicinal surfaces: shifts of the lateral distribution of terrace widths in the direction of step propagation, and lateral variations in the superlattice tilt angle that are correlated with the terrace width distribution. Both phenomena can be explained with a model of step-flow growth that includes both asymmetric adatom attachment at step edges and anisotropic adatom crossing over multiple step edges. A comparison between numerical simulations of this model and experimentally observed (Al,Ga)Sb LSL terrace width distributions leads to quantitative estimates of adatom migration characteristics. We find that at least one type of adatom, probably Ga, has a migration length equivalent to several terrace widths, and moves up and down step edges nearly isotropically. This method of determining adatom migration characteristics can be extended to any material that LSL layers can be grown above and below as terrace width markers.
Thermal stability of Si/Si1−xGex/Si heterostructures deposited by very low pressure chemical vapor deposition
Authors: Jang Syun-Ming , Kim Hyoun-Woo , Reif Rafael
Published: July 2002

We have studied the thermal stability of metastable Si/Si1−xGex/Si strained structures deposited by very low pressure chemical vapor deposition at 620 °C. Samples after furnace anneals at 800–1100 °C for 30 min were characterized by double-crystal x-ray diffraction and plan-view transmission electron microscopy to determine residual strain and misfit dislocation structure. It is found that strains in ∼1400 Å Si/520–700 Å Si1−xGex/Si structures with Ge contents of 0.09–0.16 relax gradually at temperatures <950 °C but relax rapidly at temperatures ≳950 °C, showing a transition and different mechanisms in the relaxation process. The process, involving a single-kink dislocation mechanism as well as Si-Ge interdiffusion, has been investigated for Si0.87Ge0.13.
Ultraviolet photochemical nitridation of GaAs
Authors: Zhu X.-Y. , Huett T. , Wolf M. , White J. M.
Published: July 2002

Monolayer nitridation of Ga-rich GaAs(100) is achieved at 100 K by simultaneous exposure to ammonia and 6.4 eV photons in a vacuum environment. This process is a result of nonthermal photodissociation of adsorbed ammonia. Surface NH2 is identified as an important intermediate in nitridation.
Electromigration induced transgranular slit failures in near bamboo Al and Al-2% Cu thin-film interconnects
Authors: Sanchez John E. , Kraft Oliver , Arzt Eduard
Published: July 2002

We report the morphology of electromigration induced transgranular slit-like voids in Al and Al-2% Cu thin-film interconnects with near bamboo microstructures. The grain structure surrounding fatal voids was determined from images produced by scanning electron and focused ion beam microscopy. Fatal voids were seen to traverse grains near to, but apart from, bamboo boundaries or grain triple points, and often appear to emanate from small wedge-shaped voids at the interconnect edge. The small volume slit voids are consistent with an observed decrease in fatal void volume with reduced linewidth/grain size ratio. However, the transgranular void pathway represents a new failure mechanism responsible for reduced reliability in the narrow interconnects which are more relevant for advanced integrated circuit technologies. The conditions for stress-induced transgranular slit voiding are briefly discussed.
Local intermixing of GaAs/GaAlAs quantum structures by individual ion implant tracks
Authors: Kalish R. , Kramer L.-Y. , Law K.-K. , Merz J. L. , Feldman L. C. , Jacobson D. C. , Weir B. E.
Published: July 2002

The intermixing of GaAs/AlGaAs quantum wells due to the passage of single energetic heavy ions (8 MeV Bi) is deduced from photoluminescence measurements after rapid thermal annealing (900 °C, 4 min). It is found that while the structural damage radius around an ion track due to pure kinematic mixing is very small (1.0–3.0 nm), thermal annealing increases this mixing radius by more than an order of magnitude (to ∼20 nm). This finding has important implications for ion-beam definition of quantum dots or wires in quantum well structures. Our results also clarify the proposed explanation of particle channeling as the cause of ion induced mixing far in excess of the amorphous range.
Deep levels induced by high-energy boron ion implantation into p-silicon
Authors: Sayama H. , Takai M. , Yuba Y. , Namba S. , Tsukamoto K. , Akasaka Y.
Published: July 2002

Defects induced by B+ implantation at 0.7 MeV into n+p diodes were investigated using leakage current and deep level transient spectroscopy (DLTS). Leakage current increases drastically by implantation to a dose of more than 3×1013 cm−2. DLTS spectra reveal two hole trap levels in the shallower region than the projected range of B+. One level at 265 K (Ev+0.65 eV) is associated with point defects around dislocation kinks formed by B+ implantation to doses of more than 3×1013 cm−2. Another level at 290 K (Ev+0.67 eV) is mainly responsible for the excess leakage current. This level was, for the first time, found in p-Si after high-energy ion implantation followed by annealing.
Room temperature reactions involving silicon dangling bond centers and molecular hydrogen in amorphous SiO2 thin films on silicon
Authors: Conley J. F. , Lenahan P. M.
Published: July 2002

Exposing thin films of amorphous SiO2 to molecular hydrogen at room temperature converts some silicon dangling bond defects, E′ centers, into two hydrogen coupled complexes. These reactions may play important roles in radiation and hot carrier instabilities in metal/oxide/silicon devices.
Transmission and reflection peaks in ballistic transport
Authors: Price Peter J.
Published: July 2002

An analytical treatment of quantum ballistic transport, for electrons in nanostructures, shows that Lorentzian energy-dependence peaks of transmission probability and of reflection probability are equally inherent. For both cases the resonance is associated with a local quasilevel state, having a decay time that is related to the Lorentzian energy half-width by the Breit-Wigner formula. For a reflection resonance, the peak value of reflection probability is shown to be 1 regardless of the symmetry of the system, in contrast to the transmission resonance case.
Measurement of conduction band offsets through Schottky diode transport measurements
Authors: Morris I. L. , Williams R. H. , Davies J. I. , Clarke G. J.
Published: July 2002

The charge transport properties of the Schottky diode structure Au/In0.52Al0.48As/In0.53Ga0.47As have been computer modeled by employing Poisson’s equation and the thermionic emission theory. From the unusual form of the current-voltage (I-V) relationship, it was predicted that the conduction band offset of the lattice matched InAlAs/InGaAs system could be measured. Experimental results confirm the form of the I-V relationship and an excellent estimate of the conduction band offset, ΔEc=0.53 eV, was obtained. Further low temperature I-V measurements are provided as confirmation of our conduction band profiles.
Kinetics of hydrogen desorption in surface-limited thin-film growth of SiGe alloys
Authors: Sharp J. W. , Eres Gyula
Published: July 2002

The kinetics of hydrogen desorption in surface-limited thin-film growth of SiGe alloys from binary mixtures of disilane and digermane was investigated by surface differential reflectance. The hydrogen desorption process from the alloy surface was found to consist of two components. Both components are thermally activated, but the activation energies appear to equal neither the hydrogen desorption energy from pure silicon nor that from pure germanium surfaces. We suggest that the two components represent Ge- and Si-mediated hydrogen desorption, with the former being more rapid than the latter.
Paramagnetic resonance of platinum ions in PbTiO3 single crystals
Authors: Warren W. L. , Tuttle B. A. , Sun B. N. , Huang Y. , Payne D. A.
Published: July 2002

Optical excitation of trivalent platinum (5d7, t6e) ions in lead titanate single crystals have been observed for the first time by electron paramagnetic resonance (EPR). The g and hyperfine coupling A tensors were found to be axial: g∥=1.938, g⊥=2.478, A∥=0.0164 cm−1, and A⊥=0.0324 cm−1. Analysis of the EPR spectra using crystal field theory indicates that the Pt+3 ions are in tetragonally distorted (elongated) octahedral sites, strongly suggesting that they substitute for the central Ti+4 ions in the perovskite lattice.
High-power InGaN/GaN double-heterostructure violet light emitting diodes
Authors: Nakamura Shuji , Senoh Masayuki , Mukai Takashi
Published: July 2002

InGaN/GaN double-heterostructure light-emitting diodes were fabricated. The output power was 90 μW and the external quantum efficiency was as high as 0.15% at a forward current of 20 mA at room temperature. The peak wavelengths of the electroluminescence (EL) varied between 411 and 420 nm with changes in the growth temperatures of an InGaN active layer between 820 and 800 °C. The full widths at half maximum of EL were between 22 and 25 nm.
Spontaneous spin polarization of ballistic electrons in single-mode quantum wires due to spin splitting
Authors: Fasol Gerhard , Sakaki Hiroyuki
Published: July 2002

We show that a quantum-wire device with spin splitting can work as an active spin polarizer. Hot electrons in one ‘‘spin’’ subband (e.g., ‘‘spin up’’) may pass such a device with weak electron pair scattering, while electrons in the opposite subband (‘‘spin down’’) may have high conversion probability into the spin-up subband, resulting in spin polarization of a hot electron beam. Under different circumstances a hot electron beam passing through a single-mode quantum wire may induce a steady state magnetization of the background electron gas in a section of the wire weakly coupled to the environment.
Valence band offset of GaAs/GaAs0.68P0.32 multiple quantum wells
Authors: Shan W. , Hwang S. J. , Song J. J. , Hou H. Q. , Tu C. W.
Published: July 2002

Low-temperature photoluminescence measurements have been performed under hydrostatic pressure on GaAs/GaAs0.68P0.32 strained multiple quantum well samples grown by gas-source molecular beam epitaxy. The pressure induced crossover of the first confined electron state in the GaAs wells against the conduction band (001) X minima in the GaAs0.68P0.32 barriers has been observed, which allows a direct spectroscopic determination of the valence band offset for the heterostructure. As the result we obtain the unstrained valence band offset as 0.09±0.02 eV, which corresponds to an approximate 77:23 distribution of the energy gap difference in the conduction and valence bands, respectively, for the GaAs/GaAs0.68P0.32 system.
Reevaluation of blueshifts introduced by lateral confinement in quantum-well wire structures
Authors: Gustafsson A. , Liu X. , Maximov I. , Samuelson L. , Seifert W.
Published: July 2002

The luminescence from etched quantum-well wire structures often exhibits an unexpectedly large blueshift of the peak energy position. Further, the shift is usually quite independent of the width of the wires. In this letter we show that this blueshift can be explained as a decreased transfer of excitons between areas of different monolayer thicknesses within the single quantum wells, caused by a change in the exciton diffusion from being two-dimensional in the quantum wells to one-dimensional in the wires. This reduced transfer will result in a blueshift of the peak energy position if the monolayer splitting is unresolved.
Strain studies of silicon-germanium epilayers on silicon substrates using Raman spectroscopy
Authors: Lu F. , Perry C. H. , Namavar F. , Rowell N. L. , Soref R. A.
Published: July 2002

Atmospheric pressure chemical vapor deposition techniques have been used to grow electronic quality Si-Ge epilayers on Si substrates. The degree of tetragonal strain in the layers has been determined using Raman spectroscopy. The relative energy shift of the Si-Si phonon line associated with the Si1−xGex epilayers from a pseudoalloy of the same composition was used as a quantitative measure of the strain. Layer growth was found to be almost commensurate with the Si substrates for thicknesses in the region of ∼100 nm. For x≂0.1 the resulting films were highly strained and homogeneous. The strain diminished with increasing thickness and it was estimated that a layer would be fully relaxed when the thickness exceeded 3000 nm. The phonon linewidths provided information on the epilayer and interface quality.
Time dependence and optical quenching of photoluminescence in porous silicon
Authors: Laiho R. , Pavlov A. , Hovi O. , Tsuboi T.
Published: July 2002

We have investigated time dependence and light-induced quenching of photoluminescence (PL) in porous Si samples. After excitation with a N2 laser pulse, a shift of the maximum of the emission band from 630 to 680 nm within a time interval of 3≤t≤100 μs was observed. A consistent description of the PL data is obtained by using a model based on three different electron-hole recombination processes in the sample. Measurements of light-induced quenching of PL show great similarities with fatigue of photocurrent in illuminated hydrogenated amorphous silicon.
Low temperature InAlAs buffer layers using trimethylarsenic and arsine by metalorganic chemical vapor deposition
Authors: Pan N. , Carter J. , Elliott J. , Hendriks H. , Brierley S. , Hsieh K. C.
Published: July 2002

Low temperature (LT) InAlAs buffer layers grown lattice matched to InP substrates using a combination of trimethylarsenic and arsine were demonstrated. The LT InAlAs buffer layer showed excellent surface morphology with a maximum resistivity of 2×105 Ω cm at a growth temperature of 475 °C. Low temperature photoluminescence and Hall-effect measurements confirming the high quality of epitaxial layers grown on top of the LT InAlAs buffer layer. Electrochemical capacitance voltage measurements consistently confirmed the absence of conductive impurity spikes at the epitaxial/substrate interface.
Efficient tuning of the carrier capture efficiency of quantum wells by introducing a barrier asymmetry
Authors: Gerard J. M. , Deveaud B. , Regreny A.
Published: July 2002

A GaAs quantum well (QW) placed in an AlGaAs barrier layer of linearly graded composition has been studied by photoluminescence, in order to estimate experimentally the carrier capture efficiency of a QW in an electric field. The introduction of a slight compositional asymmetry (3% to 6% Al) between both sides of the QW allows to enhance or drastically reduce the capture probability. This tuning is efficient for temperatures as high as 77 K. Our experimental results, supported by a quantum mechanical calculation of the capture probability, suggest novel routes for optimizing QW infrared photodetectors and QW lasers.
Scanning surface harmonic microscopy of self-assembled monolayers on gold
Authors: Mizutani W. , Michel B. , Schierle R. , Wolf H. , Rohrer H.
Published: July 2002

Adsorption of self-assembled monolayers on gold surfaces greatly enhances the local probe-induced third-harmonic (TH) generation at microwave frequency. At a given location the TH signal is, as a function of gap distance, proportional to the tunneling current. A constant TH signal image of self-assembled monolayers made from 11-mercaptoundecanol shows the same depression pattern with similar resolution as that obtained with tunneling microscopy. The tunneling current remains constant across the depressions at constant TH signal. However, the TH signal is enhanced on certain protrusions of the monocomponent layers and on net-like patterns on the layers made from mixtures of perfluorononanoyl-2-mercaptoethylamide and 1-mercaptododecane.
Calculation of InAs/AlSb(001) band offsets: Effect of strain and interfacial atomic structure
Authors: Dandrea R. G. , Duke C. B.
Published: July 2002

The band offsets of strained InAs grown on an AlSb(001) substrate are calculated for both the In-Sb and Al-As interfaces. A valence band offset of 0.19 eV (AlSb valence max higher) is calculated within first-principles local density functional theory, independent of the type of interfacial bonding. Short-period superlattices with identical InAs and AlSb lengths but differing only in the type of interfacial bond exhibit, however, quantum-confined states that can differ by 0.25 eV, and this is explained in terms of the superlattice wave functions. The 0.13 eV strain-induced decrease in the InAs gap leads to an extremely small energy difference of 0.04–0.10 eV between the InAs conduction minimum and the AlSb valence maximum, and presents a possible explanation for the increase in the InAs quantum well charge seen with increasing temperature.
Dependence of pretilt angle on the topography of substrate in liquid crystal alignment brought about by rubbing technique
Authors: Hatoh Hitoshi , Yamamoto Takeshi , Morizumi Yasue , Okamoto Masumi , Kubo Akira
Published: July 2002

It was found that the pretilt angle of a liquid crystal alignment brought about using the rubbing technique depends upon the surface topography of substrate. The presence of small projections on the substrate was found to increase the pretilt angle of a liquid crystal alignment. It was found that the pretilt angle depends upon the projection shape and its relationship with the rubbing direction.
Origin of nonuniform properties of YBa2Cu3 O7−x/CaRuO3/YBa2Cu3O7−x Josephson edge junctions
Authors: Olsson E. , Char K.
Published: July 2002

The fine scale microstructure of YBa2Cu3O7−x (YBCO)/CaRuO3 (CRO)/YBCO edge junctions has been characterized using transmission electron microscopy. The results showed that the damages due to ion beam milling during fabrication of the YBCO edges annealed out during heating to higher temperatures for deposition of additional layers. No contamination due to the ex situ processing of the edges was detected. The CRO grew epitaxially on the YBCO edge and small insulating Y2O3 particles nucleated on the CRO giving rise to inhomogeneous electrical properties along the junction. The nucleation of Y2O3 particles was enhanced by the microscopically rough morphology of the YBCO edge. The results stress the importance of the morphology of the YBCO edge and show that general conclusions about the interface structure cannot be drawn from corresponding planar geometries.
Photoluminescence properties of nitrogen-doped ZnSe grown by molecular-beam epitaxy
Authors: Zhu Ziqiang , Takebayashi Kazuhisa , Tanaka Kiyotake , Ebisutani Takashi , Kawamata Junji , Yao Takafumi
Published: July 2002

This letter reports the photoluminescence (PL) properties of N-doped ZnSe epilayers grown by molecular-beam epitaxy using a microwave plasma source for N doping. The temperature dependence of PL spectra from N-doped ZnSe epilayers with different net acceptor concentrations is described in detail. The emission due to transition from deep donors to free holes (DdF) is observed from highly N-doped ZnSe epilayers at high temperatures, which gives a direct evidence for the formation of a deep N-associated donor. The ionization energy for a deep N-associated donor is estimated as 55±5 meV from comparison of the temperature dependence of the DdF emission energy with that of the free exciton energy.
Spatial uniformity of minority-carrier lifetime in polycrystalline CdTe solar cells
Authors: Ahrenkiel R. K. , Keyes B. M. , Levi D. L. , Emery K. , Chu T. L. , Chu S. S.
Published: July 2002

Time-resolved photoluminescence was used to map recombination lifetimes in polycrystalline CdS/CdTe solar cells. Typical lifetime profiles indicated spatial variation by factors of 2–3 across 1 cm dimensions. Correlated device efficiency measurements indicated that spatial regions of lifetime minima dominated the open-circuit voltage.
Fabrication of a diamond field emitter array
Authors: Okano Ken , Hoshina Kimihiko , Iida Masamori , Koizumi Satoshi , Inuzuka Tadao
Published: July 2002

A diamond field emitter array has been fabricated. by Chemical vapor deposition. Diamond was grown on an inverted pyramidal-shape Si substrate followed by removal of the substrate. The fabricated array was placed in a high vacuum pumping system with the pressure of ∼10−7 Torr and the emission current as a function of the anode voltage was measured. The distance between the tungsten anode and the diamond surface was held constant at 100 μm throughout the measurement. As a result, a current larger than 10−4 A was obtained for an anode voltage of 6 kV. A linear relationship in the Fowler–Nordheim plot indicated the existence of electron field emission from the fabricated diamond field emitter array.
Generalized relationship between gain, noise, and capture probability of quantum well infrared photodetectors
Authors: Choi K. K.
Published: July 2002

We show that due to the discrete nature of multiple quantum well structures, the decay function of an energetic electron depends on the spatial extent of the electron wave function. As a result, the current gain of a quantum well infrared photodetector (QWIP) cannot be uniquely specified by the single well capture probability pc. Specifically, for a given pc the current gain of an extended state electron is found to be larger than that of a localized electron. Consequently, for a typical QWIP, it is possible that the photocurrent gain is larger than the dark current gain, resulting in a reduction of dark current induced generation-recombination noise from its expected value.
Highly charged Ta ions produced by the photodissociation iodine laser with subnanosecond pulses
Authors: Láska L. , Mašek K. , Králiková B. , Krása J. , Skála J. , Rohlena K. , Woryna E. , Wol/owski J. , Langbein K. , Haseroth H.
Published: July 2002

The results of the generation of tantalum ions with a charge state of 46+ are reported. Iodine laser pulses of about 15 J energy and 350 ps in duration were used for a Ta plasma production. Multiply charged ions were recorded by the diagnostic techniques based on the time-of-flight method in the distance of approximately 200 cm. The plasma temperature estimated from the ion measurements was found to be about 1750 eV.
Growth of (111) oriented MgO film on Si substrate by the sol-gel method
Authors: Yoon Jong-Gul , Kim Kun
Published: July 2002

Growth and structural evolution of a MgO thin film fabricated on Si substrate by using sol-gel method were investigated. Textured MgO films displaying (111) MgO∥(100)Si  and (111)MgO∥(111)Si orientations were obtained. Crystallization of (111) oriented MgO film was observed to occur above 500 °C regardless of the substrate orientations. X-ray diffraction results and surface morphology investigated by atomic force microscope show structural evolution of the film as annealing temperature is varied. Grain growth of larger size was observed for the films on Si(111). Formation of a Si-O-Mg layer caused by the reaction of the substrate surface with the metal alkoxide wet film during pyrolysis and crystallization is suggested to account for the (111) oriented MgO film on Si(100) and (111) substrates.
In situ real time studies of the formation of polycrystalline silicon films on glass grown by a layer-by-layer technique
Authors: Akasaka T. , Shimizu I.
Published: July 2002

The effect of atomic hydrogen on thin deposited layers of amorphous silicon was studied. Amorphous silicon layers less than 10 nm thick were first deposited from fluorinated precursors. These layers were then exposed to an atomic hydrogen flux. The amorphous layers quickly relaxed to a crystalline structure. Thick films of high crystalline content were prepared through sequential repetition of the deposition and hydrogen exposure process (layer-by-layer technique). The relaxation process was studied by real time in situ ellipsometry and infrared measurements. The relationship between substrate temperature, amorphous layer thickness, hydrogen exposure time, and structure was determined. A new model in which hydrogen acts to ‘‘liquify’’ the subsurface region by breaking Si–Si bonds is suggested. From the ‘‘liquidlike’’ state the subsurface relaxes to its most thermodynamically stable constituents; during relaxation, crystalline silicon is formed with effluence of SiH4, SiFxH4−x, and SiF4 vapors.
Reflection high energy electron diffraction study of nitrogen plasma interactions with a GaAs (100) surface
Authors: Hauenstein R. J. , Collins D. A. , Cai X. P. , O’Steen M. L. , McGill T. C.
Published: July 2002

Effect of a nitrogen electron-cyclotron-resonance (ECR) microwave plasma on near-surface composition, crystal structure, and morphology of the As-stabilized GaAs (100) surface is investigated with the use of digitally image-processed in situ reflection high energy electron diffraction. Nitridation is performed on molecular beam epitaxially (MBE) grown GaAs surfaces near 600 °C under typical conditions for ECR microwave plasma-assisted MBE growth of GaN films on GaAs. Brief plasma exposures (≊3–5 s) are shown to result in a specular, coherently strained, relatively stable, GaN film approximately one monolayer in thickness, which can be commensurately overgrown with GaAs while longer exposures (up to 1 min) result in incommensurate zincblende epitaxial GaN island structures. Specular and nonspecular film formations are explained in terms of N-for-As surface and subsurface anion exchange reactions, respectively. Commensurate growth of ultrathin buried GaN layers in GaAs is achieved.
Secondary ion mass spectrometric studies of SrTiO3 buffering effect on (Pb1−xLax)(Zr1−yTiy)1−x/4O3 thin films prepared by pulsed laser deposition
Authors: Ling Y. C. , Wang J. P. , Yeh M.H. , Liu K. S. , Lin I. N.
Published: July 2002

Thin films of (Pb1−xLax)(Zr1−yTiy)1−x/4O3 (5/70/30) were deposited in situ onto (100) Si, Pt/Ti/SiO2/Si, and SrTiO3/Si substrates by pulsed laser deposition from stoichiometric targets and subsequent annealing. Films grown on SrTiO3/Si exhibited desired perovskite structure. On (100) Si and Pt/Ti/SiO2/Si substrates, films exhibited a perovskite-pyrochlore mixed structure. Apparent Pb deficiency at the near-surface region was observed. Films deposited at different substrates showed variations in Si content. The buffering effect of SrTiO3 was evidenced using x-ray diffraction and secondary ion mass spectrometry analyses.
Computer simulation of an electrostatic spherical analyzer used as an energy spectrograph
Authors: Sablik M. J. , Winningham J. D. , Gurgiolo C. , Johnstone A. D.
Published: July 2002

Exit positions and velocities of charged particles passing through an electrostatic spherical analyzer were generated by a computer program which utilizes transformations to the trajectory plane of the particle to obtain an exact solution in the central force field of the analyzer. Entrance and exit planes were treated as a potential jump to zero potential, thus simulating grounded wire meshes at the entrance and exit. It was found that particle input energies could be determined as a function of exit position. A wide variety of designs were tried, varying plate radii, plate voltages, and incoming collimation of the particles. A power-law relationship between exit radius and input energy was found for designs in which the mean radial position between the plates corresponded to zero potential. However, the total input energy spread ΔE of the particles passed by an analyzer could never be made to approach twice the mean input energy E of the particles passed. Since a parabolic analyzer can be made to have a ΔE/E=1.92, we conclude that the parabolic analyzer makes a better spectrograph.
Automated pressure scanning of tunable dye lasers
Authors: Gottscho Richard A.
Published: July 2002

Tunable laser spectroscopy has recently been used in diagnosing the chemistry and physics of plasma, combustion, and chemical vapor deposition reactors. However, the impact of these spectroscopic techniques on real-time process monitoring has been limited to date. One of the reasons for this is the difficulty in tuning the laser frequency to a specific transition and having it remain there for more than a few minutes. I describe a simple method by which these difficulties can be circumvented and frequency tuning can be controlled remotely. A microprocessor is used to regulate the gas flow of a pressure tuned oscillator so that constant pressure (and, indirectly, freqency) is maintained (for excitation of a particular transition) or swept (to record an excitation or absorption spectrum). As a natural extension the pressure can be slaved to a wavemeter so that direct frequency control can be achieved.
Photon counting circuits
Authors: Candy B. H.
Published: July 2002

Two photon counters with variable dead times are described, one of simple design which is suited for low count rates and the other for system dead times down to 2 ns. Cheap, well-known components are used and the counters were designed by computer aid with a criterion that circuit performance be relatively insensitive to component values. An example of PCB layout is presented.
Construction techniques of a molecular hydrogen laser
Authors: Babis Jeffery S. , Huth Thomas C. , Denton M. Bonner
Published: July 2002

Simple techniques are described for construction of a discharge channel, and spark gap trigger electrode for a traveling-wave excited molecular hydrogen laser.
Raman spectroscopy study of microscopic strain in epitaxial Si1−x−yGexCy alloys
Authors: Menéndez J. , Gopalan P. , Spencer G. S. , Cave N. , Strane J. W.
Published: July 2002

Raman spectroscopy is used to characterize the microscopic bond structure of Si1−x−yGexCy alloys epitaxially grown on Si substrates by C implantation into Si1−xGex epilayers and subsequent annealing. The Ge and C concentrations in these alloys can be chosen so that its average lattice constant equals the lattice constant of Si. Our Raman results suggest that in these strain-compensated alloys the Si–Si bonds are not identical to those in bulk Si but experience a considerable local deformation.
Diamond anvil cell for cryogenic temperature with optical measurement system
Authors: Kawamura Haruki , Tachikawa Kyoji , Shimomura Osamu
Published: July 2002

A diamond anvil type high-pressure cell for a low-temperature experiment with an optical system is described. The diamond anvil type pressure cell is actuated by helium gas in the cryostat at low temperatures. The optical system is for the pressure calibration at low temperatures by the use of the ruby fluorescence method and for in situ direct observation of the sample at low temperatures. A pressure more than 40 GPa has been continuously generated and the pressure is determined in situ by the ruby scale with an accuracy of 0.3 below 35 GPa and 0.6 above 35 GPa.
Quantum-ferroelectric pressure sensor for use at low temperatures
Authors: Lawless W. N. , Clark C. F. , Samara G. A.
Published: July 2002

A small, multilayer capacitance sensor (∼70 mm3) for measuring pressure over a broad range at low temperatures is described. The dielectric material is a quantum ferroelectric in the (Cd, Pb)2(Nb, Ta)2 O7 ceramic system, and the reciprocal capacitance varies linearly with pressure into the kilobar region. The average sensitivity of the devices studied is 151 pF/kbar, and the sensor is essentially independent of both intense magnetic fields and temperature below 10 K. A calibration for the slight magnetic field effect is given, and the sensor is dielectrically stable at helium temperatures. A one-point pressure calibration is estimated to be reliable to <±0.1%, and pressure changes in the millibar range can be resolved if certain precautions are taken.
Audiofrequency measurement of JFET noise versus temperature in a high-impedance preamplifier
Authors: Klein Spencer , Innes Walter , Price John C.
Published: July 2002

We describe a high-impedance audiofrequency preamplifier and present audiofrequency noise spectra for 2N4416 and U311 JFETs in a temperature range from room temperature to 80 K. It is found that optimum noise performance is obtained at −140°C. We present an analysis of some of the sources of noise. Amplifier input capacitance is also measured and discussed.
New molecular-dissociation furnace for H and O atom sources
Authors: Van Zyl B. , Gealy M. W.
Published: July 2002

A new type of molecular-dissociation furnace has been developed for producing thermal beams of H and O atoms to be used as targets for fast atomic projectiles in various scattering experiments. Target-particle densities in the beam from the furnace of about 1011 cm−3 have been achieved with molecular-dissociation fractions in excess of 0.7 for each species. The techniques used are described and compared with more traditional furnace designs.