bluuebunny/crossref_metadata_2025 huggingface dataset:
Hot-electron-induced hydrogen redistribution and defect generation in metal-oxide-semiconductor capacitors
Authors: Buchanan D. A. , Marwick A. D. , DiMaria D. J. , Dori L.
Published: July 2002

Redistribution of hydrogen caused by hot-electron injection has been studied by hydrogen depth profiling with 15N nuclear reaction analysis and electrical methods. Internal photoemission and Fowler–Nordheim injection were used for electron injection into large Al-gate and polysilicon-gate capacitors, respectively. A hydrogen-rich layer (∼1015 atoms/cm2) observed at the Al/SiO2 interface was found to serve as the source of hydrogen during the hot-electron stress. A small fraction of the hydrogen released from this layer was found to be retrapped near the Si/SiO2 interface for large electron fluences in the Al-gate samples. Within the limit of detectability, ∼1014 cm−2, no hydrogen was measured using nuclear reaction analysis in the polysilicon-gate samples. The buildup of hydrogen at the Si/SiO2 interface exhibits a threshold at ∼1 MV/cm, consistent with the threshold for electron heating in SiO2. In the ‘‘wet’’ SiO2 films with purposely introduced excess hydrogen, the rate of hydrogen buildup at the Si/SiO2 interface is found to be significantly greater than that found in the ‘‘dry’’ films. During electron injection, hydrogen redistribution was also confirmed via the deactivation of boron dopant in the silicon substrate. The generation rates of interface states, neutral electron traps, and anomalous positive charge are found to increase with increasing hydrogen buildup in the substrate and the initial hydrogen concentration in the film. It is concluded that the generation of defects is preceded by the hot-electron-induced release and transport of atomic hydrogen and it is the chemical reaction of this species within the metal-oxide-semiconductor structure that generates the electrically active defects.
Strain relaxation in [001]- and [111]-GaAs/CaF2 analyzed by Raman spectroscopy
Authors: Puech P. , Landa G. , Carles R. , Pizani P. S. , Daran E. , Fontaine C.
Published: July 2002

Study is devoted to a complete characterization of GaAs/CaSrF2/CaF2 heterostructures. Due to the transparency of CaF2 substrate to visible light, Raman spectra have been obtained at both interface and surface sides of the 2 μm GaAs layer. Moreover, penetration depth of light varying with wavelength allows one to perform a tomography of this layer. The crystalline quality at the vicinity of the surface is analyzed through Raman selection rules for both [001] and [111] growth directions. In the latter case, a stress profile has been realized in order to determine its relaxation into the GaAs layer: It occurs in the first 40 nm from the interface. Finally, this methodology is applied to optimize growth conditions in order to obtain stable highly strained systems. By comparison with photoluminescence data, the Raman probe is shown to be very efficient for this purpose.
Dislocation multiplication mechanisms in low-misfit strained epitaxial layers
Authors: Beanland R.
Published: July 2002

The origin of misfit dislocations in significantly relaxed low-misfit strained layers is considered. The characteristics of strain relief due to heterogeneous dislocation nucleation at particles, nucleation of dislocations at the free surface of the layer, and multiplication mechanisms are considered and compared with the observed relaxation behavior of constant composition layers of InxGa1−xAs layers on (001) GaAs. It is proposed that dislocation multiplication is consistent with a wide range of experimental results. The observation that 60° and edge dislocations are often seen lying above, but parallel to, the interface in significantly relaxed layers has prompted a study of the possible multiplication mechanisms that they may form. It is shown that one in four reactions between 60° dislocations may lead to a pair of spiral dislocation sources, and that a single spiral source that operates only once may form at the end of edge dislocations.
A comparative study on the thermal characteristics of vertical-cavity surface-emitting lasers
Authors: Chen G.
Published: July 2002

This paper presents a comparative study of the thermal characteristics of four types of vertical-cavity surface-emitting lasers (VCSELs). They include the dielectric-isolated top-emitting p-type substrate VCSEL, the ion-implantation isolated top-emitting n-type substrate VCSEL, the dielectric-isolated bottom-emitting n-type substrate VCSEL, and the junction-isolated etched-well n-type substrate VCSEL. Microscale electrical and heat conduction effects are taken into account by employing reduced and anisotropic electrical and thermal conductivities for the Bragg reflectors. Solutions for both the electrical potential and temperature fields are obtained numerically. The calculated device temperature rise compares well with available experimental data. The simulation results show that among the studied lasers the p-type substrate laser has the best intrinsic thermal performance. Local temperature distributions in these lasers reveal that a large temperature drop occurs across the Bragg reflectors. This study demonstrates that the AlAs concentration and the structure of the Bragg reflectors strongly affect the device temperature rise due to the difference in thermal conductivity. The study also suggests that a thicker top contact should be used to reduce the thermal resistance of these devices.
Visible light emission from thin films containing Si, O, N, and H
Authors: Augustine B. H. , Irene E. A. , He Y. J. , Price K. J. , McNeil L. E. , Christensen K. N. , Maher D. M.
Published: July 2002

We report the fabrication, chemical, optical, and photoluminescence characterization of amorphous silicon-rich oxynitride (SiOxNy:H) thin films by plasma-enhanced chemical-vapor deposition. The film compositions were followed by changes in the refractive index. X-ray photoelectron and Fourier transform infrared spectroscopy indicate that the chemical composition is dominated by silicon suboxide bonding with N present as a significant impurity. A broad tunable photoluminescence (PL) emission is visible at room temperature with a quantum efficiency of 0.011% at peak energies to 3.15 eV. The radiative lifetimes are less than 10 ns, and there is nearly no temperature dependence of the PL intensity down to 80 K. Ex situ annealing at temperatures above 850 °C results in an increase in PL efficiency by nearly three orders of magnitude, and the PL intensity is independent of the annealing ambient. The PL results are remarkably similar to literature results in oxidized porous silicon and oxidized nanocrystalline Si thin films, and suggest that the radiative center is due to the defect structure in the silicon suboxide moiety.
Mechanism of the formation of hydrogen-induced interface states for Pt/silicon oxide/Si metal–oxide–semiconductor tunneling diodes
Authors: Kobayashi H. , Iwadate H. , Kogetsu Y. , Nakato Y.
Published: July 2002

The mechanism of the formation of hydrogen-induced interface states at the Si/silicon oxide interface for metal–oxide–semiconductor tunneling diodes has been investigated by conductance measurements as well as current–voltage measurements. It is found that the diffusing species through the silicon oxide layer to form the interface states is protons, not hydrogen atoms. A conductance peak due to the interface states is present at the reverse bias voltage of −0.3 V. The density of the interface states increases nearly exponentially with time t after the introduction of hydrogen in the air. The time constant of the interface state density versus time curve increases with the hydrogen concentration, in contrast to usual chemical reactions in which the reaction time constant decreases with an increase in the concentration of reactants. This unusual result can be explained by the mechanism that the interfacial reaction sites located adjacent to the interface states react with protons more easily than the other sites, resulting in the formation of two-dimensional aggregations of the interface states. The bias voltage at the constant forward current density is shifted slowly only when a forward bias is applied throughout the measurements, while such a shift does not occur when a reverse bias voltage is applied during the intervals of the current–voltage measurements. The density of the interface states is high in the presence of hydrogen in the air, but the density decreases markedly after evacuating hydrogen-containing air, indicating that the interface states equilibrate with hydrogen in the air.
Temperature dependence of the barrier height of metal-semiconductor contacts on 6H-SiC
Authors: Smith S. R. , Evwaraye A. O. , Mitchel W. C.
Published: July 2002

We have examined the temperature dependence of the barrier height of Al, Ag, Au, and W metal-semiconductor contacts on n-type 6H-SiC, and Al and Ag metal-semiconductor contacts on p-type 6H-SiC. The barrier height was determined from the reverse biased capacitance-voltage characteristics of the contacts at temperatures ranging from 300 to 670 K. The measurements were made at 1 MHz. These measurements were compared to the behavior predicted by standard models. All the diodes displayed a negative temperature dependence on n-type SiC, and a positive temperature dependence on p-type SiC. The temperature coefficient is related to the electronegativity of the metal by linear expression.
Anisotropic oxidation of 6H-SiC
Authors: Christiansen K. , Helbig R.
Published: July 2002

The anisotropy of the thermal oxidation of 6H-SiC has been investigated. Spheres of Acheson and modified Lely single crystals were prepared and subsequently oxidized under wet oxidation conditions. Afterwards the spheres showed an impressive image of interference colors in different crystal orientations due to different oxide thicknesses. The star-shaped structure on the (0001) Si face and the regular structure in the plane perpendicular to the c axis with six maxima and six minima is remarkable. The orientation of one maximum of oxide thickness could be assigned from Laue diffraction patterns to the (11̄00) and the orientation of one minimum of oxide thickness to the (112̄0) orientation. The oxide thicknesses were determined by Rutherford backscattering spectrometry and subsequently fitting of the measured Rutherford backscattering spectra by a spectrometry simulation program on various points on the spheres. The oxide thickness was determined as a function of the azimuth angle of the sphere. Different behavior at the interface SiC/SiO2 for the (0001) Si face and the (0001̄) C face was observed and is discussed.
High field magnetization measurements of SmFe11Ti and SmFe11TiH1−δ
Authors: Isnard O. , Guillot M. , Miraglia S. , Fruchart D.
Published: July 2002

The magnetic properties of the ferromagnetic compounds, SmFe11Ti and SmFe11TiH1−δ, which are potential candidates for high performance permanent magnets are investigated. At room temperature both compounds exhibit a uniaxial anisotropy, with the easy direction of magnetization parallel to the tetragonal axis. High field magnetization measurements up to 240 kOe in continuous fields are reported. The major changes upon hydrogen insertion are increases of the Curie temperature and of the saturation magnetization. The magnetic anisotropy constants, K1 and K2, are determined and the insertion of hydrogen is found to increase K1. If the external field is applied perpendicular to the alignment direction, anomalous magnetization behaviors are observed in both compounds below about 150 K. The shape of this anomaly is strongly modified by the hydrogen insertion.
Cathodoluminescence study of the spatial distribution of electron-hole pairs generated by an electron beam in Al0.4Ga0.6As
Authors: Bonard Jean-Marc , Ganière Jean-Daniel , Akamatsu Brigitte , Araújo Daniel , Reinhart Franz-Karl
Published: July 2002

We use the cathodoluminescence mode of a scanning electron microscope to investigate the depth and lateral dependencies of the electron-hole pairs generation by the electron beam in Al0.4Ga0.6As semiconducting material. A multiquantum well structure acts as a detector to measure the relative number of generated minority carriers by their radiative recombination, allowing a direct assessment of the generation volume in the sample. In contrast to electron-beam induced current which was used in former studies, the method avoids the effect of carrier diffusion for direct band gap materials. This novel technique can be readily applied to other III-V and II-VI semiconductors. The results may be used for the quantitative interpretation of cathodoluminescence and electron-beam induced current measurements.
Magnetic properties and microstructure studies of Sm–Fe magnetic thin films
Authors: Sun H. , Tomida T. , Hirosawa S.
Published: July 2002

Binary SmxFe100−x(2<x<12) thin films were made by rf sputtering of the target onto preheated quartz glass substrates. Structural characterization, magnetic measurement, and microstructure observation were carried out by x-ray diffractometer, vibrating sample magnetometer, scanning electron microscope, and transmission electron microscope (TEM), respectively. Directly crystallized films were synthesized at substrate temperature higher than 400 °C and the main phase of the films is the ThMn12-type tetragonal phase with lattice parameters a=8.65 Å and c=4.78 Å. Films have the predominant [111] and [001] crystallographic texture when the substrate temperature is lower and higher than 600 °C, respectively. The perpendicular magnetization measured under the applied field of 22 kOe is about 30% higher than the saturation magnetization of bulk SmFe11Ti. Coercivities of about 4.0 kOe can be obtained in the substrate temperature range between 430 and 550 °C with grain size of 0.2–0.3 μm. Cross-sectional TEM microstructure observations show significant columnar structures in the films. The [001] texture formed in the films is also clearly reflected by the planar and cross-sectional TEM diffraction patterns.
Analysis of dark-line defect growth suppression in InxGa1−xAs/GaAs strained heterostructures
Authors: Wang H. , Hopgood A. A. , Ng G. I.
Published: July 2002

The driving force of 〈100〉 dark-line defect (DLD) climbing growth based on vacancy unsaturation is discussed. In InxGa1−xAs/GaAs strained structures, it is found that compressive strain can reduce the osmotic (climb) force and can suppress the climb of DLDs in 〈100〉 direction. The percentage of indium in InxGa1−xAs/GaAs strained heterostructures for the suppression of 〈100〉 DLD propagation is calculated under different material growth temperatures and doping levels. For an n-type doping level higher than 5×1016 cm−3, an indium percentage less than approximately 9% in InxGa1−xAs/GaAs heterostructures is sufficient to stop the 〈100〉 DLDs growth and agrees well with the experimental observation. These results are useful for the design and fabrication of high reliability strained heterostructure devices.
Exchange anisotropy in NiFe/Fe-doped NiO bilayers
Authors: Lai Chih-Huang , Bailey William E. , White Robert L. , Anthony Thomas C.
Published: July 2002

Fe-doped NiO polycrystalline and {111} epitaxial films were grown by solid-source metalorganic chemical vapor deposition. The exchange field for NiFe on the polycrystalline Fe-doped NiO films increased with Fe content up to 18 at. % Fe, and then decreased with further increases in Fe content; the coercivity, blocking temperature, and grain size monotonically decreased with increasing Fe content. Similar results were observed in {111} films. The increase of the exchange field and the reduction of the blocking temperature are attributed to dissolved Fe atoms in the NiO. Low exchange coupling between NiFe and α-Fe2O3 may be responsible for the decrease of the exchange field in highly Fe-doped NiO films. Furthermore, 18% Fe-doped NiO showed an enhancement of exchange fields at thicknesses between 450 and 950 Å, and a considerable large training effect was observed.
Observation of non-trigonal lattice distortion in pseudomorphic InGaAs/GaAs superlattices grown on misoriented (111)B GaAs
Authors: Sanz-Hervás A. , Aguilar M. , Sánchez-Rojas J. L. , Sacedón A. , Calleja E. , Muñoz E. , Villar C. , Abril E. J. , López M.
Published: July 2002

We demonstrate the asymmetric lattice distortion of pseudomorphic superlattices grown on misoriented substrates. High-resolution x-ray diffractometry has been employed to study InGaAs/GaAs superlattices grown by molecular beam epitaxy on (111)B GaAs with a miscut angle of 1°. We observe the nontrigonal deformation of the InGaAs unit cells, while the GaAs cells retain the cubic symmetry. This result is in accord with the behavior predicted by the anisotropic elasticity theory. Consequently, the threefold symmetry of the [1̄1̄1̄] axis is lost and, therefore, the interpretation of the diffraction profiles must take account of the actual Miller indices of the substrate surface and reflecting planes used during each measurement. Otherwise, a non-negligible error in the determination of the period thickness and indium fraction could be introduced.
Optimizing the two-coil mutual inductance measurement of the superconducting penetration depth in thin films
Authors: Claassen J. H. , Wilson M. L. , Byers J. M. , Adrian S.
Published: July 2002

When a pair of coils is positioned on opposite sides of a superconducting thin film, measurement of their mutual inductance may in principle be used to infer the penetration depth λ in the superconductor. We have studied how to optimize this measurement with respect to coil design, and have found that the approach that has been generally used is far from the optimum. Useful simplifications to the expression relating mutual inductance to penetration depth are derived. An analysis of the sources of uncertainty in determining λ is presented. For an optimized coil set, the major source of uncertainty often is uncertainty of the thickness of the film. The sensitivity to changes in λ is also studied; it is shown that this can approach 1 pm for a typical high temperature superconductor sample. Finally, it is shown that the analysis may be extended to normal metal films, with the skin depth playing a role similar to that of the penetration depth in superconductors. Measurement of a high conductivity normal metal foil can be used to check calculated calibration factors for a coil set or to determine the skin depth.
Novel AlN/GaN insulated gate heterostructure field effect transistor with modulation doping and one-dimensional simulation of charge control
Authors: Imanaga Syunji , Kawai Hiroji
Published: July 2002

We propose a novel AlN/GaN insulated gate heterostructure field effect transistor (FET) with modulation doping. The vertical structure of the FET was AlN(1)/AlGaN(2)/InGaN(3)/AlGaN(4)/AlGaN(5)/GaN(substrate)(6). The typical widths of gate insulator (1) and channel (3) are 4 and 5 nm, respectively. Charge control in the FET was simulated in one dimension by solving Poisson and Schrödinger equations self-consistently. The dependence of transconductance (Gm) and the cutoff frequency (fT) on the gate voltage (Vgs) was obtained, then the optimum structure was determined. We found: (i) In the above structure, without the electron supplying layer AlGaN(2) in the gate side, the FET has high Gm (max=2.9 S/mm) and fT (max=120 GHz; Lg (gate length)=2.5 μm) values in the broad Vgs region (about 3 V) in Gm−Vgs and fT−Vgs characteristics. (ii) Both Gm−Vgs and fT−Vgs characteristics show high values in the Vgs region, which becomes broader as the conduction band discontinuity between the channel (3) and electron supplying layers, (2) and (4), increases. (iii) The optimum channel width (w) is 2 nm⩽w⩽10 nm for the structure with only an electron supplying layer (4). This condition prevents lowering of Gm in the low Vgs region, and keeps the parasitic resistance between gate and source/drain low. (iv) There is an optimum doping concentration and an optimum width of the electron supplying layer, which depend on the conduction band discontinuity between the channel and the electron supplying layer. (v) Channel doping reduces intrinsic Gm and fT in the low Vgs region in Gm−Vgs and fT−Vgs characteristics and does not necessarily increase significantly the equilibrium two-dimensional electron gas concentration.
Impact of surface properties on the dielectric breakdown for polycrystalline and multilayered BaTiO3 thin films
Authors: Oh Jeong-Hoon , Lee Y. H. , Ju B. K. , Shin D. K. , Park Chang-Yub , Oh M. H.
Published: July 2002

The dielectric reliability for polycrystalline and multilayered BaTiO3 thin films has been evaluated using time-zero and time-dependent dielectric breakdown techniques. The histogram of dielectric breakdown for multilayered BaTiO3 thin films showed a typical Weibull distribution in contrast to a random distribution when compared with polycrystalline BaTiO3 thin films. The measurement resulted in that the 400 nm-thick multilayered BaTiO3 thin film sustained about 105 hour-long operation at 1 MV/cm, showing superior properties when compared with polycrystalline. The smaller leakage current level was obtained for a multilayered BaTiO3 film having a relatively thick underlayer of polycrystalline BaTiO3 film. The value of the breakdown field was smaller at the thicker multilayered BaTiO3 while the distribution of the breakdown widened for thicker film. Analysis of the roughness for the films confirmed that the field breakdown mechanism (e.g., lowering and spreading) is related to the surface roughness of the topmost layer which varies with the thickness of underlying polycrystalline BaTiO3. The reduced leakage current at the thick multilayered BaTiO3 was due to the presence of a wide transition layer between polycrystalline and amorphous BaTiO3.
Analysis of Fermi level pinning and surface state distribution in InAlAs heterostructures
Authors: Chou W. Y. , Chang G. S. , Hwang W. C. , Hwang J. S.
Published: July 2002

The band gaps, built-in electric field, and surface Fermi level of a series In1−xAlxAs surface-intrinsic-n+ (SIN+) structures have been studied by photoreflectance at room temperature. The samples were grown by molecular beam epitaxy with an undoped layer thickness of 1000 Å. Our study indicates that, in contrast to GaAs and AlGaAs, the surface Fermi level is not pinned at midgap over aluminum concentration of 0.42–0.57. The pinning position is composition dependent. The undoped layer was subsequently etched to 800, 600, 400, and 200 Å. Different chemical solutions were used in the etching process and the built-in electric field is found independent of the etching process. Although the surface Fermi level, in general, varies with the undoped layer thicknesses, there exists, for each Al concentration, a certain range of thicknesses within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we conclude that the surface states distribute over two separate regions within the energy band gap and the densities of surface states are as low as 1.36±0.05×1011 cm−2 eV−1 for the distribution near the conduction band and 4.38±0.05×1011 cm−2 eV−1 for the distribution near valence band.
Observation of resonant optical–phonon assisted tunneling in asymmetric double quantum wells
Authors: Ozaki Shunji , Feng Jie Ming , Park Jae Hyun , Osako Shin-ichi , Kubo Hitoshi , Morifuji Masato , Mori Nobuya , Hamaguchi Chihiro
Published: July 2002

Laser Raman microscope measurements in asymmetric double quantum wells with coupled narrow and wide quantum wells were performed to observe the nonequilibrium longitudinal-optical (LO) phonons that are generated by electrons during the phonon assisted tunneling. Both the Stokes and the anti-Stokes intensities show maxima at a certain applied voltage, where the calculated subband spacing between the wide and the narrow quantum well states is found to be equal to the LO phonon energy. This fact indicates that the population of nonequilibrium LO phonons becomes maximum when resonant LO phonon scattering occurs. A strong reduction in the photoluminescence intensity for the narrow quantum well is also observed at the same bias condition.
Insulating properties of tantalum pentoxide capacitor films obtained by annealing in dry ozone
Authors: Moon B. K. , Isobe C. , Aoyama J.
Published: July 2002

Highly insulating tantalum pentoxide (Ta2O5) capacitor films were obtained by dry O3 annealing at low temperatures ranging from 350 to 500 °C. Typical leakage current density and resistivity of a 10-nm-thick Ta2O5 film measured at 2 MV/cm were 2.5×10−8 A/cm2 and 4.8×1013 Ω cm, respectively, obtained by dry O3 annealing at 450 °C, while leakage current density of the as-deposited film was about 10−1 A/cm2. It was confirmed that the mechanism which improves the insulating properties after dry O3 annealing involves the reduction of the leakage current due to the reduction of the concentration of such impurities as hydrogen and carbon, and the reduction of oxygen vacancies in Ta2O5 films. In addition, amorphous Ta2O5 films annealed in dry O3 show much better insulating properties than polycrystalline films. This can be attributed to the suppression of leakage current at the grain boundaries. Regarding dielectric properties, Ta2O5 films have an average dielectric constant of 28, and the typical SiO2 equivalent thickness of a 10-nm-thick Ta2O5 film was 2.4 nm. Results of the time dependent dielectric breakdown measurement, the calculation of the storage node height of a capacitor, and the frequency dependence of the capacitance value suggest that the above Ta2O5 film is applicable to an integrated capacitor for next generation devices, replacing conventional capacitor films.
Infrared p-n-junction diodes in epitaxial narrow gap PbTe layers on Si substrates
Authors: John J. , Zogg H.
Published: July 2002

The characteristics of p-n+ junctions in PbTe layers on Si(111) grown by molecular beam epitaxy are described. The temperature dependence of the leakage currents and ideality factors show that the junctions are generation-recombination limited over the 300–100 K range. The lifetimes deduced for the minority carriers (about 0.1 ns) suggest that their diffusion length is limited by the density of the threading dislocations, which was about 108 cm−2 for these heavily lattice mismatched layers. The theoretical diffusion limit at 200 K would be attained by reducing the dislocation density by a factor of 100. Such low densities have already been obtained in lead–chalcogenide layers on Si substrates by temperature cyclings.
Epitaxial growth of gallium nitride thin films on A-Plane sapphire by molecular beam epitaxy
Authors: Doppalapudi D. , Iliopoulos E. , Basu S. N. , Moustakas T. D.
Published: July 2002

In this article, we propose a crystallographic model to describe epitaxy of GaN on (112̄0) sapphire (A plane). The (11̄02) cleavage plane in sapphire is shown to extend to the GaN lattice as the (112̄0) plane, facilitating the formation of cleaved facets. It is shown that, although the lattice mismatch is much smaller than in the case of epitaxy on (0001), the difference in the planar symmetry in this case results in high-strained bonds near the interface. The use of nitridation and a low temperature buffer is therefore necessary. A systematic study of GaN growth on the A-plane sapphire by plasma-assisted molecular beam epitaxy was carried out to study the effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films. Transmission electron microscopy (TEM) studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientation relationship with the substrate. On the contrary, nitridation leads to films with superior structural and optoelectronic properties. The low temperature GaN buffer, grown on nitridated substrates, was found to also have a pronounced effect on the optoelectronic properties of the GaN films, especially in those with low carrier concentrations. The correlation between TEM and photoluminescence studies suggests that the transition at 3.27 eV can be attributed to the cubic domains in the films.
Nitrogen bonding environments and local order in hydrogenated amorphous silicon nitride films studied by Raman spectroscopy
Authors: Bandet J. , Despax B. , Caumont M.
Published: July 2002

Hydrogenated amorphous silicon nitride (a-SiNx:H) films prepared by plasma-enhanced chemical vapor deposition in silane-ammonia radio frequency glow discharge were studied around the stoichiometry by Raman spectroscopy. The properties of these films were investigated using a micro-Raman setup and a reflector substrate to enhance the recorded signal. A spectroscopic signature that discriminates among the different amorphous films showed a gradual building up of the phonon density of states when increasing the level of nitrogenation up to N/Si=1.4. For this composition, a-SiN:H keeps the memory of the corresponding Si3N4 crystal modes. Besides, the Si–N stretching modes of the Nn–Si–H(4−n) evidenced the stages of the substitution of H by NH bonds allowing to ascribe the lines located at 1050, 1015, and 820 cm−1 to SiN4, N3–Si–H, and N2–Si–H2 groups, respectively.
Determination of surface state density for GaAs and InAlAs by room temperature photoreflectance
Authors: Chang G. S. , Hwang W. C. , Wang Y. C. , Yang Z. P. , Hwang J. S.
Published: July 2002

Room temperature photoreflectance (PR) was used to investigate the surface state densities of GaAs and In0.52Al0.48As surface intrinsic-n+ structures. The built-in electric field and thus the surface barrier height are evaluated using the observed Franz–Keldysh oscillations in the PR spectra. Based on the thermionic emission theory and current-transport theory, the surface state density as well as the pinning position of the Fermi level can be determined from the dependence of the surface barrier height on the pump beam intensity. Even though this method is significantly simpler, easier to perform, and time efficient compared with other approaches, the results obtained agree with the literature.
Defects in GaAs grown by molecular-beam epitaxy at low temperatures: stoichiometry, doping, and deactivation of n-type conductivity
Authors: Laine T. , Saarinen K. , Hautojärvi P. , Corbel C. , Missous M.
Published: July 2002

We use a low-energy positron beam to study the influence of doping and stoichiometry on the native defects in GaAs grown by molecular-beam epitaxy at 250 °C. Ga vacancies are identified in all samples by measuring the momentum distribution of annihilating core electrons. The charge of VGa is negative in Si-doped samples but neutral in undoped and Be-doped material. We propose that the Ga vacancies are complexed with As antisites in undoped and Be-doped samples and with Si impurities in n-type material. The concentration of Ga vacancies depends on the doping and stoichiometry of growth conditions. It follows generally the trends in the VGa formation energy as a function of the Fermi level position and stoichiometry. The strong loss of free carriers in the As-rich Si-doped samples is attributed to the formation of Ga vacancy complexes, negative ion defects and inactive clusters of Si atoms.
Self-organized growth of zero-, one-, and two-dimensional nanoscale SiC structures by oxygen-enhanced hydrogen plasma sputtering
Authors: Sun Yong , Miyasato Tatsuro , Wigmore J. K.
Published: July 2002

A variety of nanoscale SiC structures, in particular the grain, whisker, and flake, displaying, respectively, zero, one, and two dimensions, has been grown by hydrogen plasma sputtering of a SiC target in the presence of a small amount of oxygen. Growth of the different nanoscale structures takes place by various mechanisms. The SiC whiskers are initiated by SiO2 seed crystals in the SiC film matrix at temperatures around 700 °C. On the other hand, the grains and flakes are separated by insertions of amorphous and graphitic carbon in the films at higher temperatures, around 950 °C. Both these processes result from the reaction of oxygen with the growing SiC film.
Optical properties of submonolayer InAs/InP quantum dots on vicinal surfaces
Authors: Paki P. , Leonelli R. , Isnard L. , Masut R. A.
Published: July 2002

We have investigated the effect of the misorientation (001) InP substrates on the optical properties of submonolayers of InAs in InP grown by metalorganic chemical vapor deposition. InAs submonolayers were systematically studied using low temperature photoluminescence (PL), photoluminescence excitation spectroscopy and temperature-dependent, excitation density PL. For submonolayer samples with oriented substrates, the observed PL linewidths and energies are satisfactorily explained within a two-dimensional (2D) quantum well picture. The formation of InAs isolated quantum dots which is found in the submonolayer samples with misoriented substrates towards (110) orientations, however, results in 0D exciton localization.
Magnetization curves of plastically deformed Fe metals and alloys
Authors: Takahashi S. , Echigoya J. , Motoki Z.
Published: July 2002

The hysteresis loop of plastically deformed Fe metals and alloys was measured for single crystals, polycrystalline iron, and A533B steel samples, and the dislocation density and structure of these samples were observed by electron microscopy. The relation between structure-sensitive properties and applied stress σ was analyzed in connection with grain boundaries and dislocations. The coercive force Hc increases with σ and is represented by one curve in all the samples. The magnetic susceptibility χc above the coercive field can be described by a simple relation to the magnetic field H. χc=c/H3 in a limited region of H. The parameter c depends only on lattice defects such as dislocations and the grain size and has a simple relation to them, but is independent of the kinds of samples or of the process of plastic deformation. The susceptibility χc is explained in comparison with the susceptibility χr in the range of approach to saturation. The magnetic properties c and Hc are discussed considering the dislocation density and its distribution including the effect of grain boundaries. These magnetic properties are useful for nondestructive tests of metal fatigue.
Enhanced optical gain in InGaN–AlGaN quantum wire and quantum dot lasers due to excitonic transitions
Authors: Huang W. , Jain F.
Published: July 2002

This article presents computation of optical gain and threshold current density in InGaN–AlGaN quantum wire and dot lasers in the presence of dislocations and surface states. The exciton binding energy including the effect of strain induced piezoelectric field is calculated to be 10–80 meV in InGaN–AlGaN quantum wires and dots, depending on the lateral and transverse dimensions. In contrast to the conventional GaAs or InP based quantum wires, these high binding energy results in large exciton densities, making optical transitions due to excitons dominant over free electrons and holes. Optical gain and threshold current density in InGaN–AlGaN based multiple quantum wire and dot lasers are computed including the effect of dislocation-induced traps. The calculated threshold current density Jth for defect free compressive-strained InGaN quantum wire (50 Å×50 Å) and dot (50 Å × 50 Å × 50 Å) lasers, realized on sapphire or SiC substrates, are shown to yield ultralow threshold current density of 233 and 88 A/cm2, respectively. In the presence of dislocations (1×1010 cm−2), the threshold current densities only increase to 924 and 623 A/cm2 for the same wire and dot, when we include the contribution of excitonic transitions. However, the corresponding values increase significantly to 30 838 and 11 647 A/cm2 if the exciton enhancement is not included.
On the reduction of direct tunneling leakage through ultrathin gate oxides by a one-dimensional Schrödinger–Poisson solver
Authors: Cassan Eric
Published: July 2002

A full self-consistent one-dimensional Schrödinger–Poisson model is reported in this article, which is specifically dedicated to the study of direct tunneling current through ultrathin gate oxide of metal-oxide-semiconductor (MOS) structures. The gate current is obtained by estimating the quasibound state lifetimes within the formalism of the formal reflection delay time of wave packets using the transfer-matrix method. As an alternative design to conventional MOS structures, two strategies are investigated in this work to scale oxide thickness in the sub 1.5 nm range while keeping an acceptable gate current leakage of some A/cm2. These include nitride/oxide stacked gate dielectrics used to increase the insulator thickness, and heterostructure MOS capacitors to confine electrons in a buried quantum well. Tensile strained Si1−yCy/Si and Si/Si1−xGex heterostructures that provide a convenient conduction band offset are proposed in this order. A conduction band offset of 0.19 eV is shown to yield nearly the same but limited improvement than the stacked gate dielectrics structure. Compared with the conventional MOS device of equivalent oxide thickness, a gate current reduction by more than two orders of magnitude is reached by using a heterostructure with a conduction band offset of 0.31 eV. For MOS transistor application this significant gain may be in addition to the driving current increase that can be expected from the strain-induced improvement of electron transport properties.
Modeling of the spectral response of AlxGa1−xN Schottky ultraviolet photodetectors
Authors: Bouhdada A. , Hanzaz M. , Gibart P. , Omnès F. , Monroy E. , Muñoz E.
Published: July 2002

Schottky AlxGa1−xN photodetectors were developed for solar ultraviolet A and B detection. Modeling is performed by developing programs of simulation leading to the most suitable device structure such as doping density, semiconductor thickness, etc. Simulations allow us to determine the most appropriate parameters
Monte Carlo evaluations of degeneracy and interface roughness effects on electron transport in AlGaN–GaN heterostructures
Authors: Li T. , Joshi R. P. , Fazi C.
Published: July 2002

Calculations of the electronic mobility and drift velocity have been carried out for bulk GaN and AlGaN–GaN heterojunctions based on a Monte Carlo approach. The bulk calculations were intended to serve as a validity check of the simulation model. For the heterojunction electron mobility calculations, polarization effects, degeneracy, and interface roughness scattering were all taken into account. Degeneracy is shown to play an important role, especially at large gate bias. Very good agreement with available experiments has been obtained, and yields a set of best-fit transport parameters. Our results underscore the dominance of interface roughness scattering, and demonstrate that a parameterized model based on weak-perturbation, Born approximation theory can yield sufficiently accurate results.
The impact of higher polarization basis functions on molecular ab initio results. I. The ground state of F2
Authors: Jankowski Karol , Becherer Ralf , Scharf Peter , Schiffer Heinz , Ahlrichs Reinhart
Published: July 2002

The F2 ground state potential curve has been computed near re by means of the MCSCF, CI(SD), MR-CI(SD) and the recently proposed CPF method. Extensive sets of polarization functions including up to five d sets, three f sets, and one g set have been used to study the convergence of the total energy of De, and re for these methods on basis set extension. SCF and MCSCF show a similar behavior and are practically converged on the 2d1f level. Convergence is much slower for methods which include external correlation, such as CI(SD), MR-CI(SD), CPF. The largest calculation performed—MR-CI(SD) for a [6 4 2 1] basis—included 280 076 configuration state functions, and yields De and re with an accuracy of 0.07 eV and 0.015 a.u. These remaining errors are attributed mainly to the neglect of g functions. The present results provide detailed information concerning the relative importance of polarization functions which can be ordered as: d(1)>d(2)≳f(1)>d(3) for SCF and MCSCF and d(1)>f(1)≳d(2)>g(1)≳f(2)≳d(3) for methods including external correlation.
The observation by EPR of trivalent hafnium in LuPO4, YPO4, and ScPO4
Authors: Abraham M. M. , Boatner L. A. , Ramey J. O.
Published: July 2002

Recent EPR studies of the 4d1 configuration ion Zr3+ in the tetragonal-symmetry orthophosphate hosts LuPO4, YPO4, and ScPO4, have led to additional investigations of d1 configuration ions associated with unusual valence states of other elements. Single crystals of LuPO4, YPO4, and ScPO4 doped with hafnium were grown and the electron paramagnetic resonance (EPR) spectrum of the 5d1 ion Hf 3+ was observed in all three cases. EPR signals due to trivalent hafnium were only observed, however, when the doped samples were gamma irradiated at 77 K and maintained at 77 K during the EPR observations. Since there are two odd isotopes of hafnium with different nuclear spins and different natural abundances, identification of the spectrum of Hf 3+ was straightforward. Since Hf 3+ represents such an unusual valence for hafnium in the solid state, however, this identification was unequivocally verified by preparing crystals doped with isotopically enriched 179Hf. The spectroscopic splitting factors, g∥ and g⊥, and the parallel magnetic hyperfine constants A∥ of 177Hf and 179Hf were obtained for Hf 3+ in all three hosts. A determination of the hyperfine parameters with the applied magnetic field oriented in the perpendicular direction was not carried out due to quadrupole effects. As in the case of Zr3+, the observed electronic g values for Hf 3+ could not be accounted for by the standard second-order equations which included the spin-orbit-induced admixture of some other excited orbital levels of the d1 configuration into the ground orbital level. The ratio of the nuclear magnetic moments 177μ/179μ, was determined to be 1.2361(4) and 1.2362(3) for hafnium in ScPO4 and LuPO4, respectively.
Time resolved resonance Raman spectra of anilino radical and aniline radical cation
Authors: Tripathi G. N. R. , Schuler R. H.
Published: July 2002

We report, in this paper, submicrosecond time resolved resonance Raman spectra of anilino radical and its radical cation as observed in pulse radiolytic studies of the oxidation of aniline in aqueous solution. By excitation in resonance with the broad and weak electronic transition of anilino radical at 400 nm (ε∼1250 M−1 cm−1) we have observed, for the first time, the vibrational features of this radical. The Wilson ν8a ring stretching mode at 1560 cm−1 is most strongly resonance enhanced. The ν7a CN stretching band at 1505 cm−1, which is shifted to higher frequency by 231 cm−1 with respect to aniline, is also prominent. The frequency of this latter mode indicates that the CN bond in the radical has considerable double bond character. The Raman spectrum of aniline radical cation, excited in resonance with the ∼425 nm electronic absorption (ε∼4000 M−1 cm−1), shows features which are similar to phenoxyl radical. Most of the observed frequencies of this radical in solution are in good agreement with vibrational energies determined by recent laser photoelectron spectroscopic studies in the vapor phase. The bands most strongly enhanced in the resonance Raman spectrum are, however, weak in the photoelectron spectrum. While the vibrational frequencies observed for anilino radical and its isoelectronic cation are quite similar, the resonance enhancement patterns are very different. In particular the ν14 b2 mode of anilino radical observed at 1324 cm−1 is highly resonance enhanced because of strong vibronic coupling between the 400 nm 2A2–2B1 and the higher 2B1–2B1 electronic transitions.
Fourier transform NMR at low temperatures
Authors: Waugh J. S. , Gonen O. , Kuhns P.
Published: July 2002

In NMR at low temperatures the steady-state absorption/dispersion spectrum in general differs from the Fourier transform of the free induction decay. The nature of this difference is discussed and an experimental example is given.
Nonperturbative theory of temperature-dependent optical dephasing in crystals. IV. Microscopic model for pseudolocal phonons
Authors: Hsu D. , Skinner J. L.
Published: July 2002

We continue our study of the optical dephasing of impurities by phonons. The pure dephasing contribution to the homogeneous linewidth is calculated to all orders in the excitation–phonon interaction. We introduce a microscopic model for the pseudolocal phonon spectral density when the impurity is in the ground electronic state. This microscopic model improves upon our previous phenomenological model. The model is applied to several experimental systems—in all cases, the theoretical results provide a reasonably accurate description of the temperature-dependent linewidth with no adjustable parameters.
Nonlinear electron spin resonance techniques for the study of inhomogeneously broadened spectra
Authors: Giordano M. , Leporini D. , Martinelli M. , Pardi L. , Santucci S. , Umeton C.
Published: July 2002

The application of nonlinear multiple irradiation electron spin resonance techniques to the case of inhomogeneously broadened spectra is studied. A detailed theoretical analysis within a unified method shows that the longitudinally detected electron spin resonance (LODESR) and the double modulation electron spin resonance (DOMESR) techniques represent two different aspects of the same physical effect and, under the same conditions, both give information on the longitudinal relaxation time T1 of the single spin packet. Previous papers, giving different interpretation for the double modulation spectrum, are critically reviewed. The usefulness of the two techniques in the case of inhomogeneously broadened lines is put into evidence by experiments with dextrose chars pyrolyzed at different temperatures. The results are in excellent agreement with theoretical results. The optimum application ranges of these nonlinear techniques are discussed and compared.
The adsorption and reactions of NO on Ag(111) at 80 K
Authors: So S. K. , Franchy R. , Ho W.
Published: July 2002

The adsorption of NO on Ag(111) at 80 K has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). At low exposures (≤0.05 L) NO is adsorbed in part dissociatively and in part molecularly in two different threefold bridge states (in upright and bent or tilted orientation with respect to the surface normal). The NO molecules in the threefold bridge position are desorbed at 100 K. With increasing exposure the desorption temperature shifts gradually to 110 K. At medium exposures (∼0.13 L) additional NO is adsorbed molecularly in an atop position with an upright orientation in admixture with atomic N and O and molecular NO adsorbed in threefold bridge states. The NO molecules adsorbed in atop position are weakly bonded on the surface and are desorbed at about 90 K. Simultaneously, N2O is formed and adsorbed on the surface. At saturation a new bent or tilted NO species in atop position appears on the surface and the amount of N2O significantly increases. The N2O molecules are desorbed at 90, 110, and 190 K. At saturation all the NO and NO-derived species (the threefold bridge and atop states, N2O and atomic oxygen and nitrogen) coexist on Ag(111).
Electrical properties of polarizable ionic solutions. I. Theoretical aspects
Authors: Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

We generalize previous work [J. Chem. Phys. 85, 6645 (1986)] on the relation between the frequency-dependent dielectric constant and conductivity and time correlation functions of electrical current and polarization in electrolyte solutions by allowing the ions and solvent molecules to be polarizable. Detailed results are given for the infinite system (no boundary), spherical system embedded in a continuum and periodic boundary conditions. The Stillinger–Lovett (SL) sum rules are derived for these geometries. It is shown, in particular, that they provide a means of calculating the high frequency dielectric constant in a molecular dynamics simulation. A test of the phenomenological coefficient-susceptibility relations and the SL conditions is presented in part II by performing molecular dynamics simulations on a model electrolyte solution with different boundary conditions.