bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of guided polaritons in electro-optic LiTaO3 probes
Authors: Hollricher O. , Rüders F. , Buchal Ch.
Published: July 2002

LiTaO3 is a widely used material for electro-optic sampling of electrical pulses. Many groups now use thin sensor crystals of 20 μm thickness, which is an optimum thickness for best measurement accuracy and minimum invasiveness of the probe. We demonstrate that by using these crystals of 20 μm thickness, a severe distortion of the measured signal can occur, if the bandwidth of the probed electrical signal exceeds about 700 GHz. In this case, polaritons are generated during reflection of the electrical signal at the LiTaO3-to-air interface in the rear of the crystal. Because the thin LiTaO3 crystal acts as a planar waveguide for the polaritons, not only can the electrical signal be seen during the measurement, but also these guided polaritons. We propose the use of longer crystals of at least 1 mm length to overcome this problem.
Effect of structure and morphology on resistive loss at 10 GHz of the large-area laser-deposited YBa2Cu3O7 thin films
Authors: Liu J. Z. , Tian Y. J. , Li L. , Guo L. P. , Zhao Z. X. , Xu S. F. , Lu H. B. , Zhou Y. L. , Chen Z. H. , Cui D. F. , Yang G. Z. , Zhang P. C. , Bai C.
Published: July 2002

Three large-area YBa2Cu3O7(YBCO) superconducting thin films have been laser-deposited under almost identical conditions. However, the microwave surface resistance Rs of the films deviated greatly, they are 280 μΩ, 3.78 mΩ, and 97 mΩ, respectively. It was found that the structure and morphology of the films greatly influence the resistive losses at 10 GHz and 77 K of the YBCO thin films. Different loss mechanisms were discussed. For high Rs, the large angle grain boundaries were the dominate defect in the thin films and increased the Rs of the film markedly. For low Rs, it was mainly due to the misaligned ‘‘123’’ grains in the thin films and the intrinsic loss. For Rs up to the mΩ range, besides the misaligned 123 grains, domain boundaries and nonsuperconducting outgrowths of different sizes appeared and this caused the rise of Rs value. All these results were given experimentally and discussed theoretically.
Critical currents versus normal-state resistivity in granular BiPb-based copper-oxide superconductors
Authors: Díaz A. , Pomar A. , Domarco G. , Maza J. , Vidal Félix
Published: July 2002

Both the critical current intensity at zero applied magnetic field Ic(T), from nitrogen temperature up to the critical temperature, and the normal-state resistivity ρn(T) have been measured in granular Bi1.5Pb0.5Sr2Ca2Cu3Oy (BiPbSCCO) superconductors, with different granular characteristics. It is found that the combination [Ic(T)/P][ρn(0)/ρ′n], where ρ(0) is the residual normal-state resistivity, ρ′n is the temperature derivative and P is the sample’s perimeter, is essentially constant from sample to sample at any temperature. It thus constitutes a universal temperature-dependent critical current for the polycrystalline BiPbSCCO family.
Photoluminescence spectroscopy and the effective mass theory of strained (In,Ga)As/GaAs heterostructures grown on (112)B GaAs substrates
Authors: Henderson R. H. , Sun D. , Towe E.
Published: July 2002

The photoluminescence characteristics of pseudomorphic In0.19Ga0.81As/GaAs quantum well structures grown on both the conventional (001) and the unconventional (112)B GaAs substrate are investigated. It is found that the emission spectra of the structures grown on the (112)B surface exhibit some spectral characteristics not observed on similar structures grown on the (001) surface. A spectral blue shift of the e→hh1 transition with increasing optical pump intensity is observed for the quantum wells on the (112) surface. This shift is interpreted to be evidence of a strain-induced piezoelectric field. A second spectral feature located within the band gap of the In0.19Ga0.81As layer is also observed for the (112) structure; this feature is thought to be an impurity-related emission. The expected transition energies of the quantum well structures are calculated using the effective mass theory based on the 4×4 Luttinger valence band Hamiltonian, and a related strain Hamiltonian.
Adhesion between Te-based alloy films and fluorocarbon sublayers during the ablative hole opening process
Authors: Horie M. , Tamura T. , Ohgaki M. , Yoshida H. , Kobayashi T. , Kisaka Y. , Kobayashi Y.
Published: July 2002

Adhesion between thin Te-based alloy films and fluorocarbon polymer sublayers, prepared by sputtering or plasma polymerization, was investigated by observing the 1 μm-sized ablative hole opening process with a focused laser beam. Interpretations of the mechanisms for the change in energy required for the hole opening and pit geometry were based on the framework of studies of the ablative hole opening process for optical recording. Observations suggest that the molten material flow during the hole opening includes a ductile fracture and a viscous flow of the molten sublayer material as well as of active layer material. Adhesion acts as an energy barrier against the above mentioned flow of molten material during the hole opening process. Since the fluorocarbon films used in the present work had highly cross-linked structures, the adhesion was mainly dominated by the dynamic force of adhesion. Therefore, the hole opening process was mainly affected by the dynamic force of adhesion rather than the static force, which is dominated by the surface energy of the sublayer. There was a good correlation between the dynamic force of adhesion estimated by the peel-off strength and the concentrations of the -CF- and -C-CF- structures estimated from C1s spectra obtained by x-ray photoelectron spectroscopy.
Plasma enhanced chemical vapor deposited SiO2 layers for passivation of InGaAs:Fe metal-semiconductor-metal photodetectors
Authors: Ambrée P. , Wandel K. , Böttcher E. H. , Bimberg D.
Published: July 2002

Results on the passivation and antireflection coating of InGaAs:Fe metal-semiconductor-metal photodetectors using remote plasma-enhanced chemical vapor deposited SiO2 layers are reported. The deposition of SiO2 on the detector surface leads to a reduction of the dark current by nearly two orders of magnitude at 5 V bias. Temperature-dependent measurements of the leakage current characteristics indicate that the Schottky barrier height is substantially lowered near the metallization edges of the reversed biased contact fingers. The effective barrier height in the edge region, which controls the magnitude of the leakage current is determined by activation energy plots to be 0.14 eV for nonpassivated and 0.20 eV for passivated structures, respectively. Apart from the improvement of the dark current characteristics, the SiO2 coating results in a drastic reduction of the photocurrent gain. The long-term stability of the passivation is proved.
Solid-state reactions in binary mixtures of nanometer-sized particles
Authors: Dickenscheid W. , Birringer R.
Published: July 2002

Solid-state reactions in mixtures of nanometer-sized Cu and Zr as well as Ni and Zr crystallites—produced by inert-gas condensation followed by in situ compaction—have been investigated by x-ray diffraction and thermal analysis. Nanocrystalline particle mixtures and multilayered samples showed qualitatively the same behavior: A solid-state amorphization reaction occurs in NixZr1−x samples but not in CuxZr1−x. The results are discussed in the framework of a heterogeneous nucleation and growth process for the amorphous phase. Comparison with results from ball-milling experiments and mechanically prepared multilayers reveals the significance of additional external driving forces.
Scanning tunneling microscope crystallography of titanium silicide on Si(100) substrates
Authors: Stephenson A. W. , Welland M. E.
Published: July 2002

A scanning tunneling microscope (STM) in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(100) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Methods for STM crystallography have been developed and used to identify possible epitaxial silicide/silicon relationships based on morphological considerations. Atomic resolution images of a titanium silicide crystallite have identified a 2×2 silicon termination of a C54-TiSi2(111) surface. It is shown that unambiguous identification of epitaxial relationships requires images of the atomic structure of the silicide crystallite surfaces in addition to morphological information.
Highly textured PbTiO3 thin films through a sol-gel process
Authors: Guzman G. , Barboux P. , Perrière J.
Published: July 2002

PbTiO3 thin films have been deposited by a sol-gel method onto various crystalline substrates and their texture has been investigated. The perovskite phase crystallizes directly from an amorphous mixture above 350 °C. Films have preferential orientation along the (001) axis on (100) MgO, are strongly oriented along both the (110) and (101) axis on (1102) sapphire, and along the (001) axis on (100) SrTiO3. High texturing is only observed on this last substrate as evidenced by rocking curves and channeling effects in Rutherford backscattering spectrometry. The origin of the strain observed along the c axis is also discussed. The thickness of the films has a strong effect on the texturing. Thin films crystallize as well textured, whereas orientation appears in thick films only after heating at high temperatures.
Study on symmetry forbidden transitions in an InxGa1−xAs/GaAs single quantum well by temperature dependence
Authors: Wang D. P. , Chen C. T. , Kuan H. , Shei S. C. , Su Y. K.
Published: July 2002

The photoreflectance (PR) spectroscopy of the single quantum well InxGa1−xAs/GaAs system has been measured at various temperatures. The selection rules for the interband transitions are Δn=0, where n is the quantum number of the nth subband in the quantum well. The symmetry forbidden transitions (Δn≠0), such as 12H (where mnH denotes transition between the mth conduction to nth valence subband of heavy hole), were often observed in the experiments and it was attributed to the existence of the built-in electric field in the quantum well. In this work, we change the strength of the built-in electric field by varying the temperatures of the samples. By varying the temperatures of the samples, the strength of the field can be changed by the effect of photo-induced voltages. The measured ratios of the intensities of 12H to 11H transitions decrease as the temperatures are lowered. Therefore, the existence of the built-in electric field may account for the observations of the symmetry forbidden transition 12H in the experiments.
Preparation and structural analysis of nickel/nickel oxide multilayers
Authors: Kado Tetsuo , Yamada Toshikazu
Published: July 2002

Metal/oxide multilayers composed of nickel and nickel oxide were prepared on silicon, fused quartz, and sapphire substrates in an ultrahigh vacuum deposition system using two different methods for growing oxide layers: an oxidation method of metal layering using pulsed oxygen molecular beam, and an evaporation method of NiO with an electron beam evaporator. With x-ray diffraction and calculations using dynamical theory of x-ray reflection, the films prepared via oxidation with more than 60 pulses of oxygen beam at 298 K, and the films prepared by evaporation below 473 K revealed multilayers of Ni/NiOx with artificial periodicity. Multilayers prepared with the oxidation method on silicon and fused quartz have homogeneous bilayers composed of polycrystalline Ni and amorphous-like NiO. In contrast, multilayers prepared by the evaporation method on a sapphire substrate are structurally graded, with layer structures which change gradually from single crystals (1st layer) to polycrystals (last layer).
The wavelength shift in GaInAsSb photodiode structures
Authors: Ru Guoping , Zheng Yanlan , Li Aizhen
Published: July 2002

Infrared absorption spectra of molecular beam epitaxial grown GaInAsSb alloys, GaInAsSb p-i-n and p-n photodiode structures are reported. It is found that the absorption edge of the N-type GaInAsSb has a blue shift compared with that of the unintentionally doped GaInAsSb, which is thought to be the Moss–Burstein shift. On the other hand, the absorption edges of the p-i-n and p-n photodiode structures have red shifts compared to the respective layers, which results from the built-in field induced Franz–Keldysh effect. By using the WKB method, we have calculated the absorption spectra for photons of energy less than the energy gap. The theoretically predicted red shift is in good agreement with the experimental results.
Critical current and its determination on TlSr2Ca2Ba2Cu3Oy by ac susceptibility measurement
Authors: Ding S. Y. , Lin J. W. , Wang G. Q. , Ren C. , Jin X. , Yao X. X. , Peng H. T. , Peng Q. Y. , Zhou S. H.
Published: July 2002

A method is proposed according to which critical current density Jc at various temperatures T and dc fields B is estimated on (Tl0.5Pb0.5)(Ba0.2Sr0.8)2Ca2Cu3Oy based upon the flux diffusion barrier heights U(J,B,T) determined by means of ac susceptibility measurement at different ac field amplitudes, frequencies, T, and B. Jc is also estimated by the dc hysteresis measurement and compared with the former. The high Jc(B,T) and Tc show that this Tl-based superconductor is one of the more promising high-Tc materials from a practical standpoint.
Optimization and analysis of solar selective surfaces with continuous and multilayer profiles
Authors: Yin Y. , Collins R. E.
Published: July 2002

Both continuous and multilayer profiles of solar selective surfaces have been analyzed and optimized. The structure having the highest photothermal efficiency has two uniform absorbing layers with two antireflection layers of different refractive index materials. Interference effects were analyzed in terms of optical path length of the sublayers and their combination. The phase cancellation positions for the optimized multilayer profiles are well distributed in the solar radiation range but do not occur at wavelengths above about 2.5 μm. A study of the electric-field distribution in these optimized profiles has shown that the sharp transition at above about 2.5 μm from low to high reflectance is due to the absence of high filling factor sublayers which can act as an effective reflecting substrate. The selective properties of the optimized multilayer structures are not particularly sensitive to the optimized graded profile for thickness variations of ±20%.
Interfacial structure of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 studied by positron annihilation
Authors: Liu T. , Zhao Z. T. , Xu Z. X. , Ma R. Z. , Guo Y. H. , Cao H. M. , Wang Y. Y.
Published: July 2002

Nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy prepared by the crystallization of amorphous alloy has been studied by using the positron annihilation technique. Positron parameters, i.e., lifetime τ1, τ2, and line shape parameter S are obtained as a function of the annealing temperature. The results show that there exist two types of defects at the interfaces of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy: vacancy-like and vacancy-like group microvoids characterized by the lifetime τ1 and τ2. The former is in overwhelming majority. The changes of the structural defects corresponding to different stages, structural relaxation, and crystallization are discussed.
Study of Pt diffusion in thin copper films under two kinetic regimes
Authors: Aleshin A. N. , Egorov V. K. , Bokstein B. S. , Kurkin P. V.
Published: July 2002

The diffusion of platinum in thin copper films has been studied by Rutherford backscattering spectrometry (RBS) under the kinetic conditions B (within the temperature range of 200–290 °C) and C (room temperature). The RBS spectra were modified to concentration curves for both bulk and grain boundary (GB) diffusion. Under the kinetic regime B the triple products KδDb (Db is the GB diffusion coefficient, δ is the GB width, K is the enrichment ratio) were obtained using Whipple’s model. Under the kinetic regime C the absolute value of the GB diffusion coefficient of Pt in Cu films was obtained. A comparison between the data on GB diffusion for the kinetics B extrapolated to room temperature and the data on GB diffusion for the kinetics C enables one to derive the product Kδ and to separate the contribution of segregation into the GB diffusion of Pt in Cu films.
Formation of Ti silicides by metal-vapor vacuum arc ion source implantation
Authors: Zhu D. H. , Liu B. X.
Published: July 2002

Metal-vapor vacuum arc ion source was employed to synthesize Ti silicides by Ti implantation directly into Si or through a deposited titanium film on Si wafers. The implantation was conducted at room temperature at an extracted voltage of 40 kV. In the directly implanted Si wafers, the transition of Ti disilicides from a metastable C49-TiSi2 to an equilibrium phase C54-TiSi2 was observed when the current density was of 125 μA/cm2 at a nominal dose range of 3–5×1017/cm2, while in the Si wafers with a deposited Ti film, C54-TiSi2 was formed when the current density was of 125 μA/cm2 at a fixed nominal dose of 5×1017/cm2. The temperature rise caused by ion implantation was calculated by solving a differential thermal conduction equation and the results were employed to discuss the formation mechanism of Ti silicides.
Quantum confined Lorentz effect in a quantum wire
Authors: Balandin A. , Bandyopadhyay S.
Published: July 2002

We have studied band-to-band optical magnetoabsorption in a semiconductor quantum wire subjected to a transverse magnetic field. The magnetic field induces a blueshift in the absorption peaks and makes the linewidth narrower. Furthermore, it quenches photoluminescence and absorption in much the same way as an electric field in the quantum confined Franz–Keldysh effect (QCFKE). We call this the quantum confined Lorentz effect (QCLE), since it is the Lorentz force skewing the electron and hole wavefunctions in the quantum wire that causes the quenching. The QCLE has an advantage over the QCFKE in that it may be observed even in quantum wires with relatively leaky barriers. The other important difference is that while the QCFKE is accompanied by a redshift in the absorption or photoluminescence peak, the QCLE is accompanied by a blueshift.
In situ x-ray photoelectron spectroscopy study of aluminum/poly (p-phenylenevinylene) interfaces
Authors: Konstadinidis K. , Papadimitrakopoulos F. , Galvin M. , Opila R. L.
Published: July 2002

The interface between evaporated aluminum and poly(p-phenylenevinylene) (PPV) was studied in situ using x-ray photoelectron spectroscopy (XPS). PPV has potential for use in electroluminescent devices, and Al can be used as the electron-injecting electrode. Aluminum was deposited on the PPV surface and the evolution of the chemical interactions at the interface was monitored using XPS. It was observed that the evaporated aluminum atoms react with the oxygen-containing groups present as impurities on the surface of PPV to form Al-O-C linkages. The Al atoms also interact with the π system of the polymer, which, based on the changes in the valence-band spectrum, results in the disruption of the conjugation along the polymer chain in the near surface region. In contrast to recently published work by E. Ettedgui, H. Razafitrimo, K. T. Park, Y. Gao, and B. R. Hsieh [J. Appl. Phys. 75, 7526 (1994)], no polymer band bending is observed. Shifts of the Al 2p metallic peak as a function of coverage are attributed to cluster growth of the deposited film.
Thermal annealing of light-induced defects in p-i-p and n-i-n hydrogenated amorphous silicon structures: Influence of hole and electron injection
Authors: Meaudre M. , Meaudre R. , Vignoli S.
Published: July 2002

The thermal annealing of light-induced defects in p-i-p and n-i-n hydrogenated amorphous silicon (a-Si:H) structures has been studied at 140 °C under ohmic and single-carrier injection conditions. The influence of hole or electron injection on steady state defect density and relaxation time of the isothermal defect annealing has been studied. All the experimental results can be qualitatively explained by a simple model in which the creation rate of the defects in intrinsic a-Si:H is proportional to the n-p product of the carrier densities, and the annealing rate is proportional to the product of defect density and hole density.
Limit of validity of the thermionic-field-emission treatment of electron injection across emitter-base junctions in abrupt heterojunction bipolar transistors
Authors: Kumar T. , Cahay M. , Shi S. , Roenker K. , Stanchina W. E.
Published: July 2002

A hybrid model is developed to simulate electron transport through the emitter-base heterojunction and the base region of abrupt heterojunction bipolar transistors. The energy distribution of the injected electron flux through the emitter-base junction is calculated using a rigorous quantum-mechanical treatment of electron tunneling and thermionic emission across the spike at the emitter-base junction. The results are compared with those predicted by the conventional thermionic-field-emission model. For both models, the electron fluxes injected across the emitter-base junction are used as initial energy distributions in a regional Monte Carlo calculation to model electron transport through the base. The average base transit times are calculated using the impulse response technique as a function of the emitter-base voltage. The differences between the thermionic-field-emission model and the rigorous quantum-mechanical approaches to model electron transport through abrupt heterojunction bipolar transistors are pointed out.
Effects of an impurity sheet in the well on resonant tunneling in double-barrier structures
Authors: Xu Huaizhe , Chen Guanghua
Published: July 2002

Effects of an impurity sheet in the middle of the well on resonant tunneling are theoretically analyzed in double-barrier structures. It was found that the energies of the even-resonance states shift toward higher levels and the states get broader as the height of the perturbation potential strength increases. There are no changes in the odd-resonance states because of the nature of their wave functions.
Comment on ‘‘Anisotropy studies of molecular-beam-epitaxy-grown Co(111) thin films by ferromagnetic resonance’’ [J. Appl. Phys. 75, 6492 (1994)]
Authors: Artman J. O.
Published: July 2002

The magnetic free energy expression E used to calculate ferromagnetic resonance frequencies by F. Schreiber et al., J. Appl. Phys. 75, 6492 (1994) is examined. The expression is correct for hexagonal site symmetry films but not for any type of cubic symmetry film. The correct expression, including both K1c and K2c anisotropy contributions, for E with H in the basal plane of a (111) film is given in the text.
Collection of athermal phonons into doped Ge thermistors using quasiparticle trapping
Authors: Grannan S. M. , Young B. A. , Lange A. E. , Labov S. , Sadoulet B. , Emes J. , Haller E. E.
Published: July 2002

We have developed a low-temperature particle detector that uses a novel quasiparticle trapping mechanism to funnel athermal phonon energy from an 80 mg Ge absorber into a 1.6 mg doped Ge thermistor via a superconducting Al film. We report on pulse height spectra obtained at 320 mK by scanning a 241Am alpha source along the device, and show that up to 20% of the energy deposited in the Ge absorber by a 5.5 MeV alpha particle interaction can be collected into a thermistor via quasiparticle trapping. We show that this device is sensitive to the position of an alpha particle interaction in the Ge absorber for interaction distances of up to 5 mm from a quasiparticle trap.
Application of intermixing to p-type GaAs/AlAs distributed Bragg reflectors for series resistance reduction in vertical cavity devices
Authors: Khan A. , Woodbridge K. , Ghisoni M. , Parry G. , Beyer G. , Roberts J. , Pate M. , Hill G.
Published: July 2002

In this paper we experimentally investigate the application of selective interdiffusion to p-type (Zn doped) distributed Bragg reflectors, as employed within a range of vertical cavity devices, as a means of lowering the series resistance. Various rapid thermal anneal temperatures and times are studied, both with and without silica encapsulants. The degree of intermixing, and hence series resistance reduction, is found to be cap dependent and this is verified both by secondary ion mass spectrometry and electrical resistance measurements. Both these techniques suggest that the intermixing, due primarily to Zn enhanced interdiffusion, is increased when no encapsulant is used. In this case a series resistance reduction approaching 50% is achieved within the 14 period GaAs/AlAs Bragg reflector. In the silica encapsulated case Ga vacancies are injected into the structure and these suppress the movement of Zn. The resulting Ga diffusion coefficient in this case is found to decrease by a factor of 3 relative to the uncapped case. The effects of interdiffusion on the reflection properties of the reflector, under various anneal conditions, are also described. This has previously been studied theoretically [Floyd et al., J. Appl. Phys. 75, 7666 (1994)] but here we offer experimental evidence on the optical effects of intermixing. We show that the peak reflectivity is found to decrease only slightly with temperature and time, again depending on capping conditions. The major effect contributing to the reflectivity decrease however is surface degradation due to As out-diffusion. The reflectivity does decrease appreciably (∼10%) when the interdiffusion length of the group III atoms becomes a significant fraction of the quarter-wave thickness of the GaAs/AlAs layers, as demonstrated in samples annealed at high temperatures and for long times (960 °C/360 s). These issues, along with that of planar integration of vertical cavity lasers and other devices such as modulators and detectors, are of importance for future applications.
Measurements of reduced-density air channels produced by a double-pulsed electron beam
Authors: Bieniosek F. M. , Cartier S. L.
Published: July 2002

Single-pulse and noncollinear double-pulse channels formed by intense relativistic electron beams were studied by using a laser deflection probe for times up to 100 ms. The first of two pulsed electron beams propagating in a 30-cm-diam tube was magnetically deflected to generate an off-axis neutral-density channel of ∼40% depth in moist air at 250 Torr. Subsequently after a time delay of 1.36 ms, a similar electron beam pulse (1.2 MeV, 13 kA, 10 ns full width at half maximum) was injected along the tube axis. After injection of the second pulse, the rate of channel decay was enhanced, and a large-scale convective motion of the hot channel gas toward the second pulse ensued. Measurements of the channel motion are compared with the predictions of a simple hydrodynamic model.
Shock wave mechanics in porous ceramic assemblies
Authors: Boogerd P. , Verbeek H. J. , Stuivinga M. , Van der Steen A. C. , Schoonman J.
Published: July 2002

The propagation of shock waves in ceramic particle assemblies is described with a simple mechanical model. Shock wave velocities in particle assemblies of BeO, SiC, and UO2 are predicted within 9% from measurements at a different density, and solid material properties. Assembly properties of several BeO, SiC, and UO2 porous assemblies are compared. The results indicate that it is possible to discriminate between material effects and assembly effects. This yields the possibility of calculating shock waves in a porous ceramic assembly using generally applicable assembly properties, and the solid material properties of the ceramic.
Photoablation: Schottky barriers on patterned Si surfaces
Authors: Grebel H. , Fang K. J.
Published: July 2002

Easy to make, patterned, Schottky barriers are investigated. In particular, the following aspects have been considered: the patterning technique, the electrical barrier height, and potential usage as solar cells. Patterning of the Si surfaces was achieved by photoablation process using an UV excimer laser in a presence of various solutions. Using a 5 mW red HeNe laser launched at various angles on the Si surface we have found that patterned solar cells ablated with 2:3:100 of HF:HNO3:H2O were as much as 23% more efficient than nonpatterned cells.
Long pulse electron beam pumped molecular F*2 laser
Authors: Lankhorst F. T. J. L. , Bastiaens H. M. J. , Botma H. , Peters P. J. M. , Witteman W. J.
Published: July 2002

An electron beam pumped molecular F*2 laser with optical pulse widths up to 160 ns, and an output energy of 1.7 J (optical flux of 4.6 MW/cm2) has been realized. The widths of the laser pulses seem only limited by the duration of the excitation pulse (160 ns). For specific output powers up to 100 kW/cm3 no signs of self-terminating laser pulses due to bottlenecking in the lower laser level have been observed.
Calculation of semiconductor intrinsic properties using a nonparabolic three-band model
Authors: Ariel V. , Schacham S. E. , Bahir G.
Published: July 2002

The intrinsic carrier concentration and the Fermi level are calculated using an approximation of the three-band k⋅p model. The approach is suitable for temperatures from 0 to 400 K and includes the contribution of electrons in the lowest conduction band, heavy holes, and light holes. An approximation for the nonparabolic Fermi–Dirac integral is used in the calculation of carrier concentrations. The resulting expressions are simple and depend explicitly on temperature, semiconductor band gap, carrier effective masses, and nonparabolicity factors. Consequently, the present approach seems suitable for the calculation of intrinsic properties of any semiconductor material for which the three-band k⋅p model is applicable. The model is compared with a numerical integration of the k⋅p model for Hg1−xCdxTe (0.17<x<1) and with experimental measurements (0.2<x<0.29). We also present a comparison between the model and a numerical evaluation of the three-band model for InGaAs. Because of their simplicity the new expressions seem suitable for both numerical and analytical modeling of semiconductor devices and materials.
Origin of unknown x-ray diffraction peaks from incommensurate superlattices
Authors: Kim S. K. , Chang C. H. , Lee Y. P. , Koo Y. M.
Published: July 2002

Unexpected x-ray diffraction peaks have been observed in some thin film modulated structures in which each constituent element has a nonintegral number of atomic layers. The origin of these peaks has not been clearly identified. The positions and intensities of these peaks were analyzed by numerical calculation from a model superlattice. The results indicate that the positions of the anomalous peaks are caused by a new long range periodicity due to the nonintegral number of atomic layers of each constituent element and that the intensities of the anomalous peaks are determined by the interfacial structure between the two different kinds of atomic layers.
New potentiometry method in scanning tunneling microscopy: Exploiting the correlation of fluctuations
Authors: Koslowski B. , Baur C.
Published: July 2002

We developed a new scanning tunneling microscopy technique to measure the surface potential. The new method exploits the tunneling voltage dependence of the tip-sample separation. The indirect measurement of the potential together with a differential measurement technique makes the new potentiometry insensitive to errors of the electronic setup and provides submicrovolt sensitivity limited by approximately thermal noise of the tunneling resistance. We illustrate the new technique by basic measurements performed under ultrahigh-vacuum conditions. In addition we present the coherence of tunneling current fluctuations and potential fluctuations which underlines the quality of the new technique: the coherence differs significantly from unity. We conclude that the tunneling resistance does not have thermal voltage fluctuations predicted by Nyquist’s formula. Possible sources of residual voltage fluctuations such as single electron tunneling effects are discussed.
Optical and electrical investigation of semiconducting amorphous Si:P alloy thin films
Authors: Li X.-H. , Carlsson J. R. A. , Gong S. F. , Hentzell H. T. G. , Liedberg B.
Published: July 2002

Measurements are reported on the infrared (IR) absorption, the optical band gap, and the dark conductivity of amorphous silicon-phosphorus alloy thin films (a-Si:P) with 20–44 at. % P prepared by coevaporation of Si and P. The results show that the optical band gap can be tailored in a range of 1.5–2.15 eV by varying the P concentration and the annealing temperature. The band gap for the sample with 20 at % P is the widest (1.70–1.82 eV) when annealed at temperatures ≤600 °C. From the IR-absorption study, a stretching mode associated with Si—P bonds at 465–474 cm−1 has been found for all alloy films. An evolution (shift or sharpening) of the Si—P absorption band with annealing temperature and P concentration have also been observed. Based on the IR-absorption study the change of bonding structures in the alloy films is discussed. Conductivity measurements show that two electron conduction processes mainly exist in the investigated temperature range: extended-state conduction in the conduction band at high temperatures and hopping conduction in the band tail at low temperatures. The transition temperature is around room temperature (14–55 °C), indicating that the carriers may mainly drift by hopping even at room temperature. The correlation between the optical band gap and the bonding structure is discussed.
Entropy of melting: Its effect on the liquid-liquid phase change of carbon
Authors: van Thiel Mathias , Ree Francis H.
Published: July 2002

Graphite melting experiments reported recently suggest that the enthalpy of melting of graphite decreases slowly and linearly with pressure, unlike our previous suggestion, which included a liquid-liquid phase change like those observed in other systems. We have investigated the possibility that this smooth pressure dependence could eliminate the liquid-liquid phase change. Our analysis shows that the available experimental enthalpies of melting cannot rule out such a phase change. An accurate determination of the (P,T) melting line curvature can shed light on the presence or absence of such a phase change.
Longitudinal-mode switching in a two-segment multiquantum-well distributed-feedback laser
Authors: Li G. P. , Makino T.
Published: July 2002

Two types of wavelength switching are observed experimentally in two-segment 1.55 μm InGaAsP/InP ridge waveguide multiquantum-well distributed-feedback (DFB) lasers. The first type occurs between the longitudinal modes on the opposite sides of the DFB stop band, while the second type occurs on the same side of the stop band. The physical mechanism is correlated to a slight difference of the effective grating period between the two inhomogeneously injected laser segments.
Texture analysis of chemical vapor deposited diamond films on silicon by the component method
Authors: Helming K. , Geier S. , Schreck M. , Hessmer R. , Stritzker B. , Rauschenbach B.
Published: July 2002

Oriented diamond films have been grown on Si(001) and Si(111) substrates by microwave plasma chemical vapor deposition. X-ray diffractometry has been employed for pole figure measurements which have been evaluated by the component method. This technique can be applied to multiphase materials with overlapping pole figures. It decomposes the texture into components by identifying preferred directions in the pole density distribution. Thereby the textures of both diamond on Si(001) and on Si(111) have been reproduced quantitatively elucidating the heteroepitaxial orientational relationship and the occurrence of twinning. The volume fractions of both epitaxially oriented diamond crystallites and their twins of first order have been determined. It is shown that under the employed nucleation and growth conditions twinning is more pronounced for diamond on Si(111) than on Si(001). Furthermore, the fraction of randomly oriented crystallites in both textures has been determined.
Properties of metal-organic chemical-vapor-deposition mercury telluride contacts on p-type cadmium telluride
Authors: Asa G. , Nemirovsky Y.
Published: July 2002

The semimetal-semiconductor HgTe/CdTe (p-type) graded heterostructures were formed by the epitaxial growth of HgTe on 〈111〉-oriented CdTe substrates using metal-organic chemical-vapor-deposition. These graded heterostructures are lattice matched, have a high degree of structural perfection, and the semimetallic HgTe provides both the high electron affinity and high carrier concentration required for ohmic contacts. Unlike the abrupt HgTe/CdTe heterostructure which is rectifying due to the valence-band offset and interface charges that reside on the polar surfaces of 〈111〉-oriented CdTe, the graded heterostructures form ohmic contacts. The contact specific resistance can be as low as 0.4 Ω cm2 and is determined by the transmission line method on HgTe/Ti/Au stripes processed by ion-beam milling (on CdTe bulk with resistivity of 10 Ω cm). The contacts are characterized electrically by current-voltage and dynamic resistance-voltage characteristics. Auger depth profile and morphology of contacts grown on the A and B faces are compared. Double-crystal rocking curves characterize the structural perfection of the heterostructure. Numerical calculation of the band diagram of the graded heterostructure, which is reported, indicates that the contact resistance may be strongly affected by the grading width as well as the exact distribution of the doping level across the heterostructure.
Characterization of low-temperature AlxGa1−xAs lattice properties using high resolution x-ray diffraction
Authors: Verma A. K. , Smith J. S. , Fujioka H. , Weber E. R.
Published: July 2002

Using high resolution x-ray diffraction techniques, we have studied the lattice parameter behavior of low-temperature (LT) AlxGa1−xAs as a function of annealing temperature and aluminum content. Similar to LT GaAs, the as-grown LT AlxGa1−xAs layers exhibit a dilated lattice constant which, upon annealing, contracts to that of ‘‘normal’’ material. The onset of this contraction in LT Al0.3Ga0.7As, however, is found to occur at an annealing temperature nearly 100 °C higher than that required for LT GaAs. In addition, the relative lattice expansion in the as-grown LT layer is found to be a decreasing function of Al content, ranging from 0.099% for LT GaAs to 0.059% for LT Al0.3Ga0.7As. This is attributed to lower than expected As incorporation in the LT AlxGa1−xAs during growth.
A comparison of models for phonon scattering in silicon inversion layers
Authors: Gámiz F. , López-Villanueva J. A.
Published: July 2002

Two-dimensional (2D) and quasi-two-dimensional (Q2D) models for phonon scattering in Si-(100) inversion layers are compared. An analytical expression for an electron mobility component limited by phonons, recently reported with the 2D model, is modified by using the Q2D approach.
Quantum beats of excitons in Zn1−xCdxSe/ZnSe multiple quantum well structures
Authors: Weckendrup D. , Saschek M. , Neukirch U. , Gutowski J. , Ferreira S. O. , Sitter H.
Published: July 2002

We investigate the coherent dynamics of excitons in Zn1−xCdxSe/ZnSe multiple quantum wells by means of spectrally integrated and spectrally resolved transient four-wave mixing. When simultaneously exciting the heavy- and light-hole exciton resonance, a beating of the spectrally integrated four-wave mixing signal is observed. The measured beating period does not exactly correspond to the energetic split of the involved free-exciton resonances. By including a bound-exciton state in the calculation of the four-wave mixing signal, quite a reasonable fit can be achieved yielding the dephasing times of the excitonic transitions. The spectrally resolved signal allows to determine the origin of the oscillation as a quantum beat between the light- and heavy-hole exciton.
Empirical formula for the spin-orbit splitting in semiconductors
Authors: Nag B. R.
Published: July 2002

The spin-orbit splittings in elemental and compound semiconductors are shown to be linearly related with the atomic ionization potential. The relation is used to judge the acceptability of the different reported values of the splitting for various semiconductors.
Refractory metal silicides synthesized by metal vapor vacuum arc ion source implantation
Authors: Zhu D. H. , Liu B. X.
Published: July 2002

Refractory metal silicides, namely NbSi2, TaSi2, WSi2, and MoSi2, were successfully synthesized by using a metal vapor vacuum arc (MEVVA) ion source to implant the respective metal ions with high current density into Si(111) and Si(100) wafers. The implantation was conducted at room temperature with an extracted voltage of 40 kV. When the current densities of the refractory metal ions were up to 65 μA/cm2, the equilibrium hexagonal NbSi2 and TaSi2 phases were formed at an implantation dose of 3×1017 ions/cm2, while the hexagonal WSi2 and MoSi2 phases were formed at a dose of 5×1017 ions/cm2. With increasing the current density up to 90 μA/cm2, the transition of the hexagonal WSi2 and MoSi2 phases to their most stable tetragonal structures was observed. Postannealing at 750 and 950 °C resulted in the formation of the unique tetragonal WSi2 and MoSi2 phases, respectively. The electrical property of the MEVVA-synthesized refractory metal silicides was measured for both as-implanted and postannealed wafers. In addition, the formation of the refractory metal silicides by MEVVA implantation is discussed in terms of the beam heating effect caused by high current ion implantation.
Theory of signal and noise performance of superconducting quantum interferometers with two quantization loops
Authors: Greenberg Y. S. , Krivoy G. S. , Koch H.
Published: July 2002

The superconducting quantum interferometer with two quantization loops, the so-called double SQUID, or D-SQUID, is a modification of the well-known rf SQUID. The modification consists of a replacement of the one junction of the rf SQUID by a superconducting loop with two Josephson junctions. This device can be used to transform the weak magnetic flux applied to the loop with junctions to a voltage across the tank circuit inductively coupled to the other loop. As has previously been shown in experiments [G. S. Krivoy and H. Koch, J. Appl. Phys. 74, 2925 (1993)], D-SQUIDs have an extremely large output signal swing up to 1 mV and a transfer function of several mV/(h/2e). In this paper, a D-SQUID theory using a numerical and an analytical approach is developed. The theoretical results are compared with the above-mentioned experiments.
Analysis of optical interferometric measurements of guided acoustic waves in transparent solid media
Authors: Jia X. , Matteï Ch. , Quentin G.
Published: July 2002

Guided acoustic waves propagating in transparent and isotropic solids are studied by optical interferometry via the photoelastic effect. Unlike the photoelastic technique, the interferometric method permits the measurement of the phase shift rather than the polarization change of the light passing through an acoustic field. By analyzing the acoustically induced change in the index ellipsoid of refraction, it is demonstrated that the optical phase shift is proportional to the dilatation or the relative change in volume of the material. The dilatation fields of the symmetric and antisymmetric Lamb modes S0 and A0, as well as that of the Rayleigh wave, were calculated. Experiments performed in fused quartz by the interferometric method are in good agreement with theoretical predictions. Compared to the conventional photoelastic technique, the interferometric measurement of acoustic wave is phase sensitive and quantitative.
Nucleation of crystalline phases in plastically deformed amorphous NiZr
Authors: Mazzone G. , Vittori Antisari M.
Published: July 2002

The thermodynamic and kinetic aspects of the nucleation of crystalline phases in plastically deformed a-NiZr layers have been examined considering the effect of a concentration gradient and of an advancing interface on the suppression of both the homogeneous and heterogeneous nucleation process. In establishing the effect of a concentration gradient, the possibility of limited atomic mobility has been explicitly taken into account. An extension to the case of plastic deformation of the concept of a critical velocity of the advancing interface has allowed us to derive a simple expression for the critical thickness of the amorphous layer which can be tested experimentally. Contrary to the case of thermally induced growth of the amorphous layer it is shown that in the case of plastic deformation, because of the combined effect of thermodynamic and kinetic constraints, there is only a finite range of thickness of the amorphous phase which allows heterogeneous nucleation of a crystalline phase. The theoretical predictions on the critical thickness of the amorphous layer and on the critical size of the crystalline nuclei have been compared with the experimental observation that, during plastic deformation, homogeneous nucleation of a metastable crystalline phase is followed at a slightly higher thickness of the amorphous phase by heterogeneous nucleation of a different crystalline compound at the a-NiZr/Zr interface. The theoretical predictions are compatible with the experimental observations and allow us to interpret both the observed sequence of nucleation events and the different growth behavior of the crystalline phases formed by homogeneous and heterogeneous nucleation.
Electrodeposition and characterization of magnetic Ni-Fe thin films on InP(100) surfaces
Authors: Gao L. J. , Anderson G. W. , Norton P. R. , Lu Z-H. , McCaffrey J. P. , Graham M. J.
Published: July 2002

Electrodeposited magnetic Ni-Fe films are used in storage devices and are applicable as magnetic sensors. In this work, we demonstrate the electrochemical conditions for deposition of permalloy Ni-Fe nanocrystalline films onto InP(100) surfaces. The prepared Ni-Fe films were analyzed by scanning electron microscopy to determine surface morphology and by Auger electron spectroscopy for compositional depth profiling. Permalloy films with bulk composition of 81% Ni and 18% Fe were obtained by electrodeposition at −1.2 V (versus standard calomel electrode) in a bath of 0.5 M NiSO4, 0.02 M FeSO4, 0.4 M H3BO3, pH=3. Transmission electron microscopy measurements show that these films consist of fcc Ni-Fe nanocrystallites embedded in an amorphous matrix. The films also show good magnetic hysteresis loops, with low coercivity. The magnetic properties of these films are improved by an extended anneal at 100 °C. Interdiffusion occurred between Ni-Fe and the InP substrate after the sample was heated to 300 °C, and consequently a loss of ferromagnetic behavior was observed.
Effects of carrier mass differences on the I–V characteristics of resonant interband tunneling structures in the presence of parallel magnetic field
Authors: Davidovich Maria A.
Published: July 2002

We investigate the effect of differences between the quantum well and the electrode carrier effective masses on the current–voltage characteristics of double-barrier interband tunneling structures under an applied magnetic field parallel to the current. The system is described by a two-band tight-binding Hamiltonian that incorporates electron and light-hole interaction and the current is calculated using the Keldysh nonequilibrium Green’s-function diagrammatic technique. The formalism is applied to InAs/AlSb/GaSb double-barrier structures considering either the InAs or the GaSb as the quantum well. The behavior of the I–V characteristics as a function of the magnetic-field strength is strongly influenced by the mass differences and opposite curvatures of the conduction and the valence bands. The features we obtain are quite different for InAs and GaSb. They are both also different from the more traditional magneto-tunneling results for GaAs/AlGaAs, where the electrodes and the quantum well electronic masses are equal. The behavior of the I–V characteristics with the applied magnetic field is in good qualitative agreement with available experimental data.