bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface damage of polymethylmethacrylate plates by ice and nylon ball impacts
Authors: Wang Li-Lih , Field J. E. , Sun Q. , Liu J.
Published: July 2002

A study is described of the impact process and the corresponding surface damage to PMMA (polymethylmethacrylate) targets impacted by ice and nylon spheres. A gas gun system was used to project the spheres and the impacts were recorded using a high-speed image converter camera (Imacon 792). Special attention was paid to the conditions causing projectile failure and the surface damage to the target. The experimental results show that although the deformation and failure modes for ice and nylon are different, the surface damage to the PMMA target has a similar failure pattern, i.e., a central, circular undamaged region surrounded by an annular damaged region containing many short circumferential cracks. Analysis shows that the diameter d of the central undamaged region and the annular damaged area Am can be used to characterize the surface damage which depends on the projectile material, sphere radius R, as well as the impact velocity V0. For a given projectile, two critical impact velocities exist: One is the threshold velocity (V0)th, below which no visible surface damage is observed, and another is the fracture velocity (V0)f, above which a plate target is broken. Damage in the annular region is caused by the Rayleigh surface wave in the present experiments. The conditions for the Rayleigh surface wave to pass ahead of the expanding edge contact velocity are analyzed. Comparisons between the theoretical predictions and experimental results, including the present study and earlier data, are made for PMMA plates impacted by projectiles of different materials (ice, nylon spheres, and water drops) and at different impact angles.
Effects of oxygen-argon mixing on the electrical and physical properties of ZrTiO4 films sputtered on silicon at low temperature
Authors: Chang De-An , Lin Pang , Tseng Tseung-Yuen
Published: July 2002

Zirconium titanate (ZrTiO4) thin films (200–260 nm) on p-type (100) Si substrates were prepared using radio-frequency sputter deposition at room temperature in atmosphere of various O2/Ar mixing ratio (from 0/100 to 20/80). All films exhibited a (020) preferred orientation and small grain size with increasing O2/Ar ratio, as shown by x-ray diffraction. The variations of dielectric constant, fixed oxide charge, and interface trapped charge with O2/Ar ratio are studied with an Al/ZrTiO4/Si structure. The refractive index and dielectric constant varied in ranges 2.34–2.26 and 20–16 for the as-deposited films. Within the range investigated, crystallinity appears to have a stronger influence on the densities of charge-trapping centers in the films than oxygen stoichiometry, and the latter may determine the leakage current density. A remarkable two orders of magnitude decrease in leakage current to 5.6×10−9 A/cm2 at 0.1 MV/cm and an increase in breakdown field beyond 1.2 MV/cm are observed in the film deposited without oxygen after a 400 °C annealing in air for 1 h. The current transport in ZrTiO4 film is dominated by the Poole–Frenkel mechanism.
Giant magnetoresistance in Co/Cu multilayers with very thin Co layers: Reduced hysteresis at the second antiferromagnetic maximum
Authors: Kubinski D. J. , Holloway H.
Published: July 2002

We describe the dependence of magnetoresistance and magnetoresistive hysteresis upon the Co layer thickness in Co/Cu multilayers with Cu layer thickness equal to 20 Å, i.e., at the second antiferromagnetic maximum. When the Co layer thickness is reduced to 3 Å the magnetoresistive hysteresis disappears. The elimination of hysteresis is accompanied by a decrease of magnetoresistance, but the sensitivity to changes in the magnetic field is only reduced by a factor of 2. This result makes available the larger sensitivity of the second antiferromagnetic maximum to applications that are adversely affected by hysteresis.
Giant magnetoresistance in Co/Cu multilayers with Co layers of alternating thicknesses: Reduction of magnetoresistive hysteresis
Authors: Holloway H. , Kubinski D. J.
Published: July 2002

A description is given of the magnetoresistive properties of Co/Cu multilayers in which the thicknesses of antiferromagnetically coupled Co layers are alternated between two values. With such an arrangement, the thicker Co layers are coupled more strongly to applied magnetic fields than are the thinner Co layers. Consequently, one may attain conditions where the magnetization direction of the thinner Co layers is the result of a balance between the applied magnetic field and an antiferromagnetic coupling to the adjacent thicker Co layers, whose magnetization direction is dominated by the applied magnetic field. With specimens whose Cu spacer-layer thickness is 9 Å (i.e., at the first antiferromagnetic maximum) the dependence of total magnetization on applied magnetic field reveals the different behavior of the two types of Co layer. This behavior may be exploited to obtain materials with reduced magnetoresistive hysteresis at the second antiferromagnetic maximum (i.e., with Cu spacer thickness of 20 Å) by combining the large magnetoresistance of Co layers with thicknesses ∼15 Å with the small hysteresis of Co layers with thicknesses ∼3 Å.
Mictomagnetism of ε -MnZn alloys
Authors: Hori T. , Shiraishi H. , Nakagawa Y.
Published: July 2002

The ε-MnZn alloy (disordered hcp structure) containing 15 at. % Mn seems to be ferromagnetic below about 200 K; the magnetization σ in a field H of 9.5 kOe amounts to 26 emu/g at a temperature T of 4.2 K. The σ vs T curve in lower H, however, shows a maximum, suggesting mictomagnetism. This is verified by the existence of a hump on the ac susceptibility χac vs T curve. The σ versus concentration curve shows a maximum at 15 at. % Mn. The χac vs T curves for the 12 at. % Mn alloy show a cusp at 45 K in addition to the hump between 80 and 180 K, suggesting a spin glass state. Both the cusp and the hump are suppressed by a dc magnetic field applied parallel to the ac field. The micromagnetic behavior becomes more pronounced for the Mn richer alloys which show only the hump in the χac vs T curve. The σ vs T curves exhibit a magnetic field cooling effect below a freezing temperature Tf. The values of Tf for the alloys containing 20, 30, and 45 at. % Mn are 38, 50, and 75 K, respectively. Although the antiferromagnetic interaction predominates in the Mn rich alloys, a long-range antiferromagnetic order may not be expected because of the random distribution of Mn moments. The 25 at. % Mn alloy shows strong ferromagnetism, if the atomic ordering in the hcp lattice is realized.
Ultraviolet and electron radiation induced fragmentation of adsorbed ferrocene
Authors: Welipitiya Dulip , Green A. , Woods J. P. , Dowben P. A. , Robertson Brian W. , Byun Dongjin , Zhang Jiandi
Published: July 2002

From thermal desorption spectroscopy we find that ferrocene, Fe(C5H5)2, adsorbs and desorbs associatively on Ag(100). Photoemission results indicate that the initially adsorbed surface species closely resembles that of molecular ferrocene. The shift in photoemission binding energies relative to the gas phase is largely independent of the molecular orbital. We find that ultraviolet light does lead to partial fragmentation of the ferrocene and that the molecular fragments are much more strongly bound to the surface than the associatively adsorbed ferrocene. Since fragmentation occurs only in the presence of incident radiation, selective area deposition from this class of molecules is possible. Using a focused electron beam in a scanning transmission electron microscope, we show that selective area deposition of features with resolution of a few hundred angstroms is readily achieved.
Formation of two-dimensional electron and hole gases in undoped AlxGa1−xAs/GaAs heterostructures
Authors: Hirayama Y.
Published: July 2002

Two-dimensional electron or hole gases (2DEG or 2DHG) are confined at the same interface in an undoped heterostructure by an electric field generated by a top gate. The combination of ion-implanted ohmic regions, an undoped heterostructure with superlattice barriers, and a metal gate is used to fabricate structures by a conventional process without self-alignment. High-quality 2DEG and 2DHG with a carrier density up to 8×1011 cm−2 are formed with a small gate leakage current. Switching between 2DEG and 2DHG at the same heterointerface is achieved by changing the sign of the gate voltage.
Evolution of surface roughness in epitaxial Si0.7Ge0.3(001) as a function of growth temperature (200–600 °C) and Si(001) substrate miscut
Authors: Lee N.-E. , Cahill David G. , Greene J. E.
Published: July 2002

The evolution of surface roughness in epitaxial Si0.7Ge0.3 alloys grown on Si(001) as a function of temperature (200–600 °C), thickness (t=7.5–100 nm), and substrate miscut were investigated by atomic force microscopy and quantified in terms of the height-difference correlation function G(ρ), in which ρ is lateral distance and [G(ρ→∞)]1/2 is proportional to the surface width. The films were deposited by ultrahigh vacuum ion-beam sputter deposition at 0.1 nm s−1. Strain-induced surface roughening was found to dominate in alloys grown on singular Si(001) substrates at Ts≳450 °C where [G(ρ→∞)]1/2 initially increases with increasing t through the formation of coherent islanding. The islands are preferentially bounded along 〈100〉 directions and exhibit 105 faceting. This tendency is enhanced, with much better developed 〈100〉 islands separated by deep trenches—of interest for growth of self-assembled nanostructures—in films grown on Si(001)-4°[100]. Increasing the film thickness above critical values for strain relaxation leads to island coalescence and surface smoothening. At very low growth temperatures (Ts≤250 °C), film surfaces roughen kinetically, due to limited adatom diffusivity, but at far lower rates than in the higher-temperature strain-induced regime. Si0.7Ge0.3 alloy surfaces are smoother, while the films exhibit larger critical epitaxial thicknesses, than those of pure Si films grown in this temperature regime. There is an intermediate growth temperature range, however, over which the alloy film surfaces remain extremely smooth even at thicknesses near critical values for strain relaxation. This latter result is of potential importance for device fabrication.
Capacitance and conductance of semiconductor heterojunctions with continuous energy distribution of interface states
Authors: Sakhaf M. , Schmeits M.
Published: July 2002

Electrical conduction in semiconductor heterojunctions with interface states presenting a continuous density of states distribution inside the gap is studied theoretically. The defect states are given with their density of states function and capture cross sections for transitions to the valence and conduction band. Conduction is considered under steady-state dc applied voltage V and under small-signal ac conditions. The formal developments are based on the resolution of Poisson equation and the continuity equations for electrons, holes, and occupied defect states. The numerical integration yields the position dependent dc and ac components of the concentrations of electrons, holes, occupied defect states, and of the current densities and recombination rates. For any value of x, one obtains the occupation function ft as function of the defect energy Et and the transition rates to the valence and conduction band. This allows a detailed analysis of the response of the interface state system to the external applied voltage. The main electrical characteristics obtained in the final analysis are the dc current–voltage and the ac capacitance–frequency and capacitance–voltage curves. The method is applied to a InGaAs/InP heterojunction with interface DOS functions being either constant inside the gap or having gaussian shapes with a given mean value and standard deviation. Discrete levels are treated as a particular case. The I(V) curve has an ideality factor n different from one, reaching the theoretical value of 2 for discrete midgap states. Comparison of C(f ) curves for different DOS functions allows us to show the appearance of characteristic features depending on the explicit form of the DOS function and different from those of the discrete level case. The C(f ) curves, computed for different values of applied voltage V, show cutoff frequencies whose values increase with V. It is shown that this is due to the fact that different defect states are involved in the dynamical response of the system. The low frequency C−2(V) curves are not linear, as for the ideal junction case, and present structures which are correlated with the slope of the interface charge density are represented as a function of applied voltage V.
Analysis of geometrical effects on the behavior of transverse and longitudinal modes of amorphous silicon compounds
Authors: Moreno J. A. , Garrido B. , Samitier J. , Morante J. R.
Published: July 2002

The peak frequency, width, and shape of the transverse-optical (TO) and longitudinal-optical (LO) infrared absorption modes of silicon oxides (SiO2, SiOx), silicon nitrides (Si3N4, SiNx), silicon oxynitrides (SiOxNy), and other silicon compounds have often been connected with stress, stoichiometry, defects, structural order, and other properties of the layers. However, certain geometrical effects strongly influence the spectral response of the material independent of its physical or structural properties. The influence of layer thickness, multilayer configuration, substrate effects, angles, and polarization on the behavior of TO and LO bands are presented and discussed. Some corrections are suggested to reduce experimental error and for the reliable measurement of stress, composition, disorder, and structure.
Nanostructure and magnetic anisotropy of Co/Au multilayers
Authors: Liu Y. , Shan Z. S. , Sellmyer D. J.
Published: July 2002

Multilayer Co/Au films were deposited on 40 nm Au underlayer by magnetron sputtering. The Co thickness is kept constant at 0.5 nm while the Au thickness is varied from 0.5 to 8 nm. Magnetic measurements show that the perpendicular anisotropy of the films changes from 3×106 to 1.5×106 erg/cm3 as the Au thickness changes from 6 to 1 nm. Two films of (Co 0.5 nm/Au 6 nm)×9/Au 40 nm (denoted Au6) and (Co 0.5 nm/Au 1 nm)×50/Au 40 nm (denoted Au1) were examined by transmission electron microscopy. For the Au6 film, a bright-field image reveals the curvature of the multilayer. High-resolution electron microscopy shows that the curvature consists of many atomic steps leading to a rough interface. The Au layer has the fcc structure (ABCABC stacking) and the Co layer of two atomic layers interrupts the ABCABC stacking of Au and could be fitted to the AB stacking. In contrast to the Au6 film, the Au layers and the Co layers in the Au1 film cannot be identified in the high-resolution electron microscopy image. Bright field transmission electron microscopy images reveal the evidence of local clustering of Au and Co. A high density of 1/3[111]a dislocations is observed which supports the clustering argument. The anisotropy of the films is discussed in connection with the observed nanostructure involving the steps and the clustering of Au and Co.
New bonded magnet materials (invited)
Authors: Coey J. M. D. , O’Donnell K.
Published: July 2002

Recent developments in rare earth intermetallic materials which may be suitable for bonded magnets are reviewed. These include isotropic coercive powder with a refined grain structure, isotropic single-phase powder showing remanence enhancement, exchange-coupled hard/soft nanostructures, anisotropic coercive powder, and two-phase dipole-coupled powders. Most of these new materials are based on Nd2Fe14B or Sm2Fe17N3.
Optical properties of hexagonal GaN
Authors: Kawashima Takahiro , Yoshikawa Hisashi , Adachi Sadao , Fuke Shunro , Ohtsuka Kohji
Published: July 2002

Single-crystalline hexagonal GaN (α-GaN) films have been grown on (0001) sapphire substrates by metalorganic chemical vapor deposition at 1040 °C. The complex dielectric functions, ε(E)=ε1(E)+iε2(E), of the epitaxial films have been measured by spectroscopic ellipsometry (SE) for E⊥c in the region between 1.5 and 5.0 eV at room temperature. Previously published ultraviolet SE spectra of α-GaN are examined by considering the effects of surface roughness using an analysis based on an effective medium model. Ex situ atomic force microscopy is used to assess independently surface flatness. By mathematically removing the effects of surface roughness, the most reliable ε(E) values for α-GaN are presented in the 1.25–10 eV photon–energy range. Theoretical dispersion analysis suggests that the E0 structure could be characterized by a three-dimensional M0 critical point and the E1α (α=A,B,C) structures by two-dimensional M1 critical points. To facilitate design of various optoelectronic devices, dielectric-function-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity of α-GaN are also presented.
Structural inhomogeneity and silicon enrichment of buried SiO2 layers formed by oxygen ion implantation in silicon
Authors: Afanas’ev V. V. , Stesmans A. , Revesz A. G. , Hughes H. L.
Published: July 2002

The microstructure and electrical properties of buried SiO2 layers produced in silicon by the implantation of oxygen ions are analyzed in terms of implantation parameters and supplemental incorporation of oxygen. The buried oxides show inhomogeneous etching in aqueous HF, revealing the presence of a crystalline oxide phase and Si-enriched regions. Silicon enrichment in SiO2 is found in the form of Si inclusions and oxygen deficient network defects. The former are found to be sensitive to the oxygen implantation profile, and may arise as a result of a blockage of Si outdiffusion by crystalline oxide inclusions. The network defects, in turn, are predominantly generated during high temperature postimplantation annealing, caused possibly by some mechanism of silicon transport from the interfaces into the bulk of oxide. The electron trapping and electrical conduction characteristics of buried oxides are found to correlate with the density and size of the inhomogeneities. By contrast, hole trapping and the generation of positive charge at the Si/oxide interfaces by exposure to hydrogen at elevated temperature are controlled by the network defects in the bulk of the oxide and in the near interfacial layers, respectively.
The elastic constants of silicon carbide: A Brillouin-scattering study of 4H and 6H SiC single crystals
Authors: Kamitani K. , Grimsditch M. , Nipko J. C. , Loong C.-K. , Okada M. , Kimura I.
Published: July 2002

The complete sets of elastic constants of 4H and 6H silicon carbide single crystals were determined by Brillouin scattering. The elastic constants of 6H SiC are C11=501±4, C33=553±4, C44=163±4, C12=111±5, and C13=52±9 GPa; the corresponding ones of 4H SiC are the same within experimental uncertainties. The compressibility, 4.5×10−3 GPa, is about 3–5 times smaller than those reported for polycrystalline SiC materials.
Deposition of SiNx:H thin films by the electron cyclotron resonance and its application to Al/SiNx:H/Si structures
Authors: Garcia S. , Martil I. , Gonzalez Diaz G. , Castan E. , Dueñas S. , Fernandez M.
Published: July 2002

We have analyzed the electrical properties and bonding characteristics of SiNx:H thin films deposited at 200 °C by the electron cyclotron resonance plasma method. The films show the presence of hydrogen bonded to silicon (at the films with the ratio N/Si<1.33) or to nitrogen (for films where the ratio N/Si is higher than 1.33). In the films with the N/Si ratio of 1.38, the hydrogen content is 6 at. %. For compositions which are comprised of between N/Si=1.1 and 1.4, hydrogen concentration remains below 10 at. %. The films with N/Si=1.38 exhibited the better values of the electrical properties (resistivity, 6×1013 Ω cm; and electric breakdown field, 3 MV/cm). We have used these films to make metal-insulator-semiconductor (MIS) devices on n-type silicon wafers. C–V measurements accomplished on the structures indicate that the interface trap density is kept in the range (3–5)×1011 cm−2 eV−1 for films with the N/Si ratio below 1.38. For films where the N/Si ratio is higher than 1.4, the trap density suddenly increases, following the same trend of the concentration of N–H bonds in the SiNx:H films. The results are explained on the basis of the model recently reported by Lucovsky [J. Vac. Sci. Technol. B 14, 2832 (1996)] for the electrical behavior of (oxide–nitride–oxide)/Si structures. The model is additionally supported by deep level transient spectroscopy measurements, that show the presence of silicon dangling bonds at the insulator/semiconductor interface (the so-called PbN0 center). The concentration of these centers follows the same trend with the film composition of the interface trap density and, as a consequence, with the concentration of N–H bonds. This result further supports the N–H bonds located at the insulator/semiconductor interface which act as a precursor site to the defect generation of the type •Si≡Si3, i.e., the PbN0 centers. A close relation between interface trap density, PbN0 centers and N–H bond density is established.
Infrared and x-ray photoelectron spectroscopy studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films
Authors: Choi W. K. , Ong T. Y. , Tan L. S. , Loh F. C. , Tan K. L.
Published: July 2002

The effects of annealing on the structural properties of radio-frequency sputtered amorphous silicon carbide films prepared under different hydrogen partial pressures (PH) were investigated. Infrared (IR) results of the as-prepared films suggest that as PH increases, more hydrogen is incorporated into the film to form the Si–H and C–H bonds and less silicon and carbon atoms are available to form the Si–C bonds. X-ray photoelectron spectroscopy (XPS) results of the as-prepared films agree with the IR results in that the percent of Si–C decreases and the percent of Si–H and C–H increases as PH increases. IR and XPS results of the annealed films suggest that as the annealing temperature increases, the dangling Si and C bonds will combine to form the Si–C bonds for the unhydrogenated samples. The increase in Si–C bonds for the hydrogenated samples is more likely to be due to the formation of Si–C bonds from the breaking up of the Si–H and C–H bonds.
Charge injection and conduction on the surface of insulators
Authors: Pépin M. P. , Wintle H. J.
Published: July 2002

We have calculated numerically the flow of charge onto an insulator surface by injection from an electrode touching the surface, using conformal mapping in conjunction with the boundary element method. We have assumed that the driving fields are due to the electrodes and to the surface charge itself, while the natural conductivity of the surface is negligible. We have considered three geometries used experimentally. In general, we find that for strong injection, the surface charge is confined to the region close to the injecting electrode, that the absorption current behaves as I∝t−γ,γ∼1/3, and that the absorption and resorption currents do not exhibit mirror symmetry. Furthermore, if the active electrode can inject charges of either sign, then on shorting the electrodes a counter charge is injected, which leads to a more rapid discharge at early times but does not give rise to a current reversal (anomalous current). Materials of higher dielectric constant store more surface charge. We compare our results with previous calculations and with existing experimental work.
Lattice effects in ferromagnetic manganite perovskites
Authors: Singh D. J. , Pickett W. E.
Published: July 2002

Density functional calculations are used to investigate the relationships among lattice distortions, magnetic ordering, and electronic properties in manganite perovskites. Distortions open narrow pseudogaps at the middle of the majority eg manifold. The effect on transport is very dependent on doping and falls off as divalent ions replace La in LaMnO3. Distortions in the ferromagnetic ordered colossal magnetoresistance regime are weakly coupled to electronic properties relative to pure LaMnO3.
Field-induced strain under load in Ni–Mn–Ga magnetic shape memory materials
Authors: Murray S. J. , Farinelli M. , Kantner C. , Huang J. K. , Allen S. M. , O’Handley R. C.
Published: July 2002

Single-crystal Ni2MnGa shows a nearly 0.2% strain under a magnetic field of 8 kOe at −8 °C. Polycrystalline samples have been prepared near stoichiometry to study the composition dependence of the magnetic and elastic properties. A narrow band of compositions has been found having a range of Curie and martensitic transformation temperatures, Tc and T0, extending to above room temperature. The compressive stress–strain characteristics in variable transverse field were studied in samples selected to have T0 just below room temperature. Stress-induced martensite was observed as expected and the magnetic field was applied under fixed load for various stresses. A transverse field of 3200 Oe caused the sample to strain under load doing work that increased up to 1.3 J/kg with increasing volume fraction of stress-induced martensite.
Maximum energy product of isotropic Nd-Fe-B-based nanocomposite magnets
Authors: Kuma J. , Kitajima N. , Kanai Y. , Fukunaga H.
Published: July 2002

The maximum energy products, (BH)max, of isotropic Nd-Fe-B-based nanocomposite magnets were calculated by a three dimensional model (10×10×10) that incorporated non-uniform magnetization reversals by subdivisions of the grains. Based on the results of our simulation, the largest obtainable (BH)max for a sample with 60% soft and 40% hard grains is about 290 kJ/m3. This value is lower than the previous crude estimate of 400 kJ/m3 but is almost twice as large as the experimental measured maximum of 160 kJ/m3 suggesting that further improvement in (BH)max is possible if the nanostructure of an actual magnet can made close to the theoretically ideal structure.
Dielectric properties of Ba(Ti1−yYy)O3 ceramics
Authors: Jing Zhi , Ang Chen , Yu Zhi , Vilarinho P. M. , Baptista J. L.
Published: July 2002

Temperature and frequency dependence of the real (ε′) and imaginary (ε″) parts of the dielectric permittivity of Ba(Ti1−yYy)O3 (0.028⩽y⩽0.258) ceramic samples were studied in the temperature range 12–500 K at 102, 103, and 104, and 105 Hz. The permittivity peaks are rounded and frequency dispersive when y>0.028. The maximum of the rounded peak (εm′) decreases with an increase in the yttrium concentration. The temperature corresponding to the permittivity maximum is linearly shifted to lower temperatures at a rate of 29 K per molar percent of yttrium when y⩽0.122, but it remains constant when y⩾0.122. The solid solubility limit of 0.122 is confirmed. Typical relaxor behavior was observed and characterized with empirical parameters in Ba(Ti1−yYy)O3 solid solutions.
Deep level transient spectroscopy characterization of 1 keV He, Ne, and Ar ion bombarded, epitaxially grown n-Si
Authors: Deenapanray P. N. K. , Auret F. D. , Ridgway M. C. , Goodman S. A. , Myburg G. , Malherbe J. B.
Published: July 2002

Deep level transient spectroscopy has been used to investigate the electronic properties and isochronal annealing behavior of defects formed in epitaxially grown n-Si by 1 keV He-, Ne-, and Ar-ion bombardment. Similarities between peaks found for the different bombardment gases suggested that they were from structurally related defects. Two families of such related defects were observed in the unannealed samples. Annealing data revealed additional peaks and enabled another defect family formed above 400 °C to be identified. The energy levels and capture cross sections have been determined for three new families of related defects. The defect families were presumed to be either complex vacancy clusters or hydrogen related.
Electronic structures of cubic lattice quantum dots
Authors: Li Shu-Shen , Xia Jian-Bai
Published: July 2002

In this article, we give the electronic structure and optical transition matrix elements of coupled quantum dots (QDs) arranged as different cubic lattices: simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) superlattices. The results indicate that electron and hole energies of bcc, sc, and fcc superlattices are the lowest, the highest, and the middle, respectively, for the same subband under the same QD density or under the same superlattice constant. For a fixed QD density, the confinement effects in sc, fcc, and bcc superlattices are the strongest, the middle, and the weakest, respectively. There are only one, two, and four confined energy bands, with energies lower than the potential barrier for sc, bcc, and fcc QD superlattices, respectively. The results have great significance for researching and making semiconductor quantum dot devices.
Effect of strain on the interdiffusion of InGaAs/GaAs heterostructures
Authors: Gillin W. P.
Published: July 2002

The results of interdiffusion experiments on two multiple quantum well heterostructures of InGaAs/GaAs are presented. The two samples each had four quantum wells with indium concentrations, in order from the surface, of 5%, 15%, 20%, and 10%. The two samples also had different barrier layer thicknesses to allow any strain or depth dependence of the diffusion to be observed. No effect of strain or depth on the diffusion was observed.
Electrical and optical properties of Be-doped InP grown at low temperature by solid source atomic layer molecular beam epitaxy
Authors: Postigo P. A. , Dotor M. L. , Huertas P. , Garcı́a F. , Golmayo D. , Briones F.
Published: July 2002

Beryllium-doped InP layers have been grown by solid source atomic layer molecular beam epitaxy at low substrate temperature. The residual n-type doping was reduced by controlling both the amplitude and the length of the phosphorus pulse. We have shown a well controlled p-type doping and obtained a hole concentration in the range 4×1017–3×1019 cm−3 at room temperature. The electrical and optical properties of InP layers grown at low temperatures were investigated by Hall effect and photoluminescence (PL) measurements. PL spectra for lightly doped samples have a near band emission at 1.41 eV and Be-related emissions around 1.38 eV.
Angle dependence of the ferromagnetic resonance linewidth and two magnon losses in pulsed laser deposited films of yttrium iron garnet, MnZn ferrite, and NiZn ferrite
Authors: Srivastava Anuj K. , Hurben Michael J. , Wittenauer Michael A. , Kabos Pavel , Patton Carl E. , Ramesh R. , Dorsey Paul C. , Chrisey Douglas B.
Published: July 2002

Shorted waveguide ferromagnetic resonance (FMR) measurements were made at 9.5 GHz for pulsed laser deposited yttrium iron garnet (YIG), MnZn ferrite, and NiZn ferrite films with thicknesses of 1.8, 0.5, and 1.5 μm, respectively. The FMR field versus the field angle confirmed the operational assumption of a uniform mode response. For the YIG, the linewidth was 10 Oe when the external field and magnetization vectors were perpendicular to the disk (perpendicular FMR) and increased smoothly to a maximum value of 27 Oe when the field and magnetization were in the film plane (parallel FMR). For the MnZn, the linewidth was 49 Oe at perpendicular FMR, increased with the angle between the film normal and the external field to a broad maximum of 80 Oe at 35°, and then dropped to 65 Oe at parallel FMR. For the NiZn, the linewidth was 310 Oe at perpendicular FMR, increased with the field angle to a broad maximum of 1530 Oe at 45°, and then dropped to 960 Oe at parallel FMR. The linewidths were larger than predicted for reasonable values of the Landau–Lifshitz damping and showed angle dependences which indicated nonintrinsic contributions to the loss. Two magnon scattering was used to model these linewidth differences. The angle dependences of the excess linewidths show qualitative agreement with the two magnon predictions, with inhomogeneity sizes in the submicron range and volume fractions below 1%.
1/f
noise measurements on indium antimonide metal–oxide–semiconductor field-effect transistors
Authors: Liaw U. H. , Su Y. K.
Published: July 2002

We performed the 1/f noise measurements on n-channel indium antimonide (InSb) metal–oxide–semiconductor field-effect transistors (MOSFETs) biased in linear and saturation regions operated at 77 K. Through the investigation of the dependence of drain voltage noise power spectral density on gate and drain bias, we have estimated the oxide–semiconductor interface trap density as a function of energy. In this analysis, both the noise magnitude and the spectral shape were studied. In this study, the frequency span was performed from 150 Hz to 5 kHz. The noise behavior of the device was modeled using the modified McWhorter Model, originally developed for silicon FETs. The interface trap concentration values computed from the low-frequency noise measurements using the previous model were found to agree closely with InSb/SiO2 interface properties measured by capacitance and conductance methods.
Deep level analysis of radiation-induced defects in Si crystals and solar cells
Authors: Yamaguchi Masafumi , Khan Aurangzeb , Taylor Stephen J. , Ando Koshi , Yamaguchi Tsutomu , Matsuda Sumio , Aburaya Takashi
Published: July 2002

Deep level transient spectroscopy (DLTS) analysis of radiation-induced defects in p-type Si crystals and solar cells has been carried out to clarify the mechanism of the anomalous degradation of Si n+–p–p+ structure space cells induced by high-energy, high-fluence electron/proton irradiations. A large concentration of a minority-carrier trap with an activation energy of about 0.18 eV has been observed in irradiated p-Si using DLTS measurements, as well as the majority-carrier traps at around Ev+0.18 eV and Ev+0.36 eV, Correlations between DLTS data and solar-cell properties for irradiated and annealed Si diodes and solar cells have shown that type conversion of p-Si base layer from p-type to n-type is found to be mainly caused by introduction of the 0.18 eV minority-carrier trap center, that is, this center acts as a deep-donor center. The Ev+0.36 eV majority-carrier trap center is thought to also act as a recombination center that decreases minority-carrier lifetime (diffusion length). Moreover, origins of radiation-induced defects in heavily irradiated p-Si and generation of deep-donor defect has also been discussed.
Influence of postdeposition annealing on the enhanced structural and electrical properties of amorphous and crystalline Ta2O5 thin films for dynamic random access memory applications
Authors: Joshi P. C. , Cole M. W.
Published: July 2002

Tantalum oxide (Ta2O5) thin films were fabricated on Pt-coated Si, n+-Si, and poly-Si substrates by metalorganic solution deposition technique. The effects of postdeposition annealing on the structural, electrical, and optical properties were analyzed. The Ta2O5 films were amorphous up to 600 °C. A well-crystallized orthorhombic phase with strong a-axis orientation was obtained at an annealing temperature of 650 °C. The refractive index was found to increase with annealing temperature and a value of 2.08 (at 630 nm) was obtained for films annealed at 750 °C. The electrical measurements were conducted on metal–insulator–metal (MIM) and metal–insulator–semiconductor capacitors. The dielectric constant of amorphous Ta2O5 thin films was in the range 29.2–29.5 up to 600 °C, while crystalline thin films, annealed in the temperature range 650–750 °C, exhibited enhanced dielectric constant in the range 45.6–51.7. The high dielectric constant in crystalline thin films was attributed to orientation dependence of the dielectric permittivity. The dielectric loss factor did not show any appreciable dependence on the annealing temperature and was in the range 0.006–0.009. The frequency dispersion of the dielectric properties was also analyzed. The films exhibited high resistivities of the order of 1012–1015 Ω cm at an applied electric field of 1 MV/cm in the annealing temperature range of 500–750 °C. The measurement of current–voltage (I–V) characteristics in MIM capacitors indicated the conduction process to be bulk limited. The I–V characteristics were ohmic at low fields, and Poole–Frenkel effect dominated at high fields. The temperature coefficient of capacitance was in the range 52–114 ppm/°C for films annealed in the temperature range 500–750 °C. The bias stability of capacitance, measured at an applied electric field of 1 MV/cm, was better than 1.41% for Ta2O5 films annealed up to 750 °C. For a 0.15-μm-thick film, a unit area capacitance of 3.0 fF/μm2 and a charge storage density of 22.3 fC/μm2 were obtained at an applied electric field of 0.5 MV/cm.
Electronic band structure of insulating hydrogenated carbon-germanium films
Authors: Tyczkowski J. , Ledzion R.
Published: July 2002

Amorphous carbon-germanium films (a-GeXCY:H) produced by plasma-assisted chemical vapor deposition from tetramethylgermanium in a radio-frequency (13.56 MHz) glow discharge, were investigated. It had been found previously that these films, according to the discharge power P, could be obtained as an amorphous semiconductor or an amorphous insulator. The latter type of the films (deposited at P=5 W) is a subject of this article. Investigations on an optical absorption, bulk photogeneration, and internal photoemission were performed. On this basis the main parameters of the electronic band structure of the films, such as transport gap EG, optical gaps, Eopt and E04, electron affinity χ, and ionization potential J, were determined. These parameters change drastically as a result of an aging process. For the fresh films, EG=5.7 eV, Eopt=3.1 eV, and E04=3.6 eV, while for the aged films, EG=7.1 eV, Eopt=3.8 eV, E04=4.2 eV, χ=1.2 eV, and J=8.3 eV. It is suggested that the aging process consists in the oxidation of the films, which causes a reduction of the density of localized states in the bulk of the film as well as a drastic decrease in the surface states (from about 1018 m−2 V−1 for the fresh film to 1.6×1016 m−2 V−1 for the aged film).
Intrinsic trapping sites in rare-earth and yttrium oxyorthosilicates
Authors: Cooke D. W. , Bennett B. L. , Muenchausen R. E. , McClellan K. J. , Roper J. M. , Whittaker M. T.
Published: July 2002

Similarity among the thermally stimulated luminescence glow curves of undoped Lu2SiO5 and Ce3+-doped oxyorthosilicates possessing the monoclinic C2/c structure strongly suggests the luminescence traps are intrinsic in origin. They are most likely associated with the configuration of oxygen ions in the vicinity of not only the Ce3+ ion, as suggested in previous work, but also the host lanthanide ion. The optical absorption spectrum of pristine Lu2SiO5 shows the presence of intrinsic absorption centers that are enhanced upon x irradiation as seen in other oxides containing oxygen related point defects.
Influence of surface topography on depth profiles obtained by Rutherford backscattering spectrometry
Authors: Slotte J. , Laakso A. , Ahlgren T. , Rauhala E. , Salonen R. , Räisänen J. , Simon A. , Uzonyi I. , Kiss Á. Z. , Somorjai E.
Published: July 2002

A method for determining correct depth profiles from samples with rough surfaces is presented. The method combines Rutherford backscattering spectrometry with atomic force microscopy. The topographical information obtained by atomic force microscopy is used to calculate the effect of the surface roughness on the backscattering spectrum. As an example, annealed Au/ZnSe heterostructures are studied. Gold grains were observed on the surfaces of the annealed samples. The annealing also caused diffusion of gold into the ZnSe. Backscattering spectra of the samples were measured with a 2 MeV He+4 ion beam. A scanning nuclear microprobe was used to verify the results by measuring backscattering from grains and from areas of the samples where no grains had been formed during annealing.
Thermoelectric properties of pure and doped FeMSb (M=V,Nb)
Authors: Young D. P. , Khalifah P. , Cava R. J. , Ramirez A. P.
Published: July 2002

The thermoelectric properties of the pure and doped half-Heusler compounds FeVSb and FeNbSb are reported. The electrical resistivities are between 0.2 and 20 mΩ cm at room temperature. Thermoelectric power measurements indicate that FeVSb is an n-type material with moderate Seebeck coefficients near −70 μV/K at 300 K. The thermal conductivity at room temperature is large, approximately 0.1 W/cm K, and increases with decreasing temperature. Chemical substitutions, which have a dramatic effect on the transport properties, were performed in an effort to enhance the thermoelectric performance. Band-structure calculations are presented for the pure materials.
1/f
noise modeling in long channel amorphous silicon thin film transistors
Authors: Rhayem J. , Rigaud D. , Valenza M. , Szydlo N. , Lebrun H.
Published: July 2002

1/f noise investigations in thin film transistors with long channel and thin thickness of amorphous silicon film are presented. It is found that the noise behavior follows the mobility fluctuation model in ohmic and saturation regimes, whereas in the subthreshold conduction, a quadratic law versus the drain current is observed. The noise modeling is proposed taking into account the equations usually utilized for crystalline silicon metal–oxide–semiconductor field-effect transistors according to Hooge’s theory. Moreover, the Berkeley Short-Channel Insulated Gate Field-Effect Transistor Model is adapted to predict the noise levels. Two noise parameters have been extracted: The first is used in the subthreshold region, whereas we show that the second, directly related to Hooge’s parameter, is adequate to describe alone the noise in normal conduction up to the saturation.
Magneto-optical Kerr effects of ferromagnetic Ni-gratings
Authors: Schmitte T. , Schemberg T. , Westerholt K. , Zabel H. , Schädler K. , Kunze U.
Published: July 2002

We report measurements of the magneto-optical Kerr effect (MOKE) on an optical grating of Ni-stripes on Si(111) and a grating of Al-stripes on a Ni thin film. The analysis of the MOKE signal at different orders of diffraction n reveals a change of the Kerr signal amplitude, which is periodic in n having the same periodicity as the total intensity of the diffracted light. For the grating of Ni-stripes we observe a definite change of the shape of the magnetic hysteresis loops also varying systematically with the order of diffraction n.
Faraday effect enhancement in Co–ferrite layer incorporated into one-dimensional photonic crystal working as a Fabry–Pérot resonator
Authors: Takeda E. , Todoroki N. , Kitamoto Y. , Abe M. , Inoue M. , Fujii T. , Arai Ken’ichi
Published: July 2002

We fabricated a one-dimensional magnetophotonic crystal (MPC), in which a magnetic layer of Co–ferrite (∼40 nm in thickness) is sandwiched by a couple of dielectric multilayer reflectors with (SiO2/TiO2)×7 structure. The Co–ferrite layer was synthesized by ferrite plating from aqueous solution at 90 °C. The MPC is so designed to enhance magneto-optical Faraday rotation θF at the Fabry–Pérot resonance wavelength λ of the multilayer structure. θF was observed to increase by factor of ∼5.4, but at λ=∼620 nm which is deviated from the target resonance (λ=740 nm) and at very weak transmissivity. This may be because the Co–ferrite layer is rough in surface and smaller in thickness than the target thickness at which Fabry–Pérot resonance is designed to occur.
Effect of Zn substitution on para- to ferromagnetic transition temperature in La0.67Ca0.33Mn1−xZnxO3 colossal magnetoresistance materials
Authors: Awana V. P. S. , Schmitt E. , Gmelin E. , Gupta Anurag , Sedky A. , Narlikar A. V. , de Lima O. F. , Cardoso Claudio A. , Malik S. K. , Yelon W. B.
Published: July 2002

Structural, magnetic, and thermal measurements are carried out on the La0.67Ca0.33Mn1−xZnxO3 system with x=0.0–0.50. The structural characterization of the samples is done by Rietveld analysis of their neutron diffraction patterns. Zn substitutes at Mn site isostructurally until x=0.30. Oxygen content remains nearly invariant with x. Magnetic and thermal measurements as well as the electrical resistance show a para- to-ferromagnetic transition at Tp, which decreases with an increase of x. For low Zn concentration (until x=0.075) the decrease dTp/dx is smaller than for the larger concentrations of Zn. Relative decrease dTp/dx at higher concentrations (x>0.10) is similar to that observed earlier for the La0.67Ca0.33Mn1−xFexO3 system. For the transition at Tp, the related change of magnetic entropy (ΔStrs) are calculated from the heat capacity data and indicate that for x=0 the expected value ΔStrs=12.8 J/mol K is recovered.
Epitaxial metastable Ge1−yCy (y⩽0.02) alloys grown on Ge(001) from hyperthermal beams: C incorporation and lattice sites
Authors: D’Arcy-Gall J. , Desjardins P. , Petrov I. , Greene J. E. , Paultre J.-E. , Masut R. A. , Gujrathi S. C. , Roorda S.
Published: July 2002

Epitaxial metastable Ge1−yCy alloy layers with y⩽0.02 were grown on Ge(001) at temperatures Ts=200–550 °C using hyperthermal Ge and C beams with average energies of 16 and 24 eV, respectively, in order to investigate C incorporation pathways in the Ge lattice. High-resolution reciprocal lattice maps show that all as-deposited alloy layers are fully coherent with the substrate. Layers grown at Ts⩽350 °C are in compression due to higher C concentrations in interstitial than in substitutional sites. The compressive strain decreases (i.e., the substitutional C concentration increases) with increasing Ts within this temperature range. At higher growth temperatures, as-deposited alloys are nearly strain free since the majority of the incorporated C is trapped at extended defects. Annealing the Ge1−yCy layers at Ta=450 and 550 °C leads to a significant increase, proportional to the strain in the as-deposited films, in compressive strain. Further annealing at Ta=650 °C results in the formation of dislocation loops which act as sinks for interstitial and substitutional C atoms and thus relieves residual macroscopic strain. Finally, we show that the large compressive strain associated with interstitial C atoms must be accounted for in order to determine the total incorporated C fraction from diffraction analyses.
Continuous fluorescence microphotolysis to observe lateral diffusion in membranes. Theoretical methods and applications
Authors: Brünger Axel , Peters Reiner , Schulten Klaus
Published: July 2002

On the basis of solutions to two-dimensional diffusion-reaction equations the fluorescence signal for photobleaching experiments with arbitrary time-dependent and time-independent irradiation profiles is evaluated. The solutions involve spatial discretization and spectral expansion, spatial and temporal discretization employing the Crank–Nicholson integration scheme, generalized moment expansion, and an approximation yielding an analytical expression. The algorithms developed can be installed on small computers to determine lateral diffusion coefficients from observed fluorescence signals. The theory developed is applied to photobleaching with constant irradiation profiles and the resulting diffusion coefficients, in the range 0.001 to 10.0 μm2 s−1 for the systems investigated, are compared to the results of conventional photobleaching experiments. Applications of the theory compare further the signals resulting from Gaussian and rectangular irradiation profiles, the influence of the membrane geometry, i.e., planar or spherical, on the fluorescence signal, and the effect of finite diffusion spaces.
Statistical errors in molecular dynamics averages
Authors: Schiferl Sheila K. , Wallace Duane C.
Published: July 2002

A molecular dynamics calculation produces a time-dependent fluctuating signal whose average is a thermodynamic quantity of interest. The average of the kinetic energy, for example, is proportional to the temperature. A procedure is described for determining when the molecular dynamics system is in equilibrium with respect to a given variable, according to the condition that the mean and the bandwidth of the signal should be sensibly constant in time. Confidence limits for the mean are obtained from an analysis of a finite length of the equilibrium signal. The role of serial correlation in this analysis is discussed. The occurence of unstable behavior in molecular dynamics data is noted, and a statistical test for a level shift is described.