bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric properties and complex defect in (Sr1−xBi2/3x)TiO3 ceramics
Authors: Chen Ang , Zhi Yu
Published: July 2002

The positron annihilation technique was adopted to study the defect structure of the dielectric ceramics with the nominal composition (Sr1−xBi2/3x) TiO3, where x was 0, 0.06, 0.11, 0.16, 0.21, 0.25, 0.29, 0.33, 0.36, 0.40, and 0.43, respectively. A two phase field occurs above 0.36. The results show that the dielectric properties are closely related to the defect structure which varies as bismuth content changes. It is clear that the complex defect was formed in the materials, where x ranged from 0.11 to 0.36. The existence of the complex-defect in the perovskite structure results in the typical ‘‘hopping ions’’ relaxation polarization.
Optical studies of PdO thin films
Authors: Remillard J. T. , Weber W. H. , McBride J. R. , Soltis R. E.
Published: July 2002

Palladium oxide thin films are studied by ellipsometry, optical transmission, and Raman scattering. The PdO films are made by completely oxidizing Pd films sputtered onto fused silica substrates, and their optical constants determined using a combination of ellipsometry and transmission measurements. Oxidation kinetics experiments are performed on thick Pd substrates at temperatures of 300–500 °C. The optical properties of the films show a thickness dependence suggesting an increased absorption at the PdO-Pd interface. Ellipsometry on oriented single crystals of PdO yields the anisotropic optical constants at 632.8 nm. These results are combined with an effective medium theory to show that the oxidized films have a void density of 20–25%. Raman scattering is demonstrated to be a very sensitive probe for detecting the presence of PdO on Pd. An oxide only a few Å thick is readily detectable, despite the absence of any surface enhanced Raman effect.
Interaction between copper and point defects in proton-irradiated silicon
Authors: Tamulevicius S. , Svensson B. G. , Aboelfotoh M. O. , Hallén A.
Published: July 2002

Schottky barrier structures have been formed by the deposition of Cu and Pt on n-type Si(100) at room temperature. The structures were irradiated by 300 keV protons or 1.3 MeV alpha particles to doses between 109 and 1010 cm−2. Deep level transient spectroscopy measurements performed in the temperature range 80–290 K revealed a new level ∼0.31 eV below the conduction band edge in the proton-bombarded Cu/Si(100) samples. The level exhibits metastable properties, and reversible cycling of its strength can be accomplished by a procedure where thermal annealing, forward biasing (hole injection), and white light excitation are undertaken. Evidence is presented showing that the level is associated with both Cu and H. Furthermore, the production rate of electrically active defects, e.g., divacancy and vacancy oxygen centers, is found to be substantially lower in the Cu/Si(100) samples compared with Pt/Si(100) samples irradiated under identical conditions. This is attributed to passivation of the irradiation-induced acceptor centers by fast diffusing interstitial Cu+, and in particular, the effect is more pronounced at shallow depths close to the Cu/Si interface than in the deep tail beyond the implantation peak.
Characterization of GaAs layers grown by low temperature molecular beam epitaxy using ion beam techniques
Authors: Yu Kin Man , Kaminska M. , Liliental-Weber Z.
Published: July 2002

The stoichiometry, crystallinity, defect concentration, and the excess As lattice location in GaAs layers grown by molecular beam epitaxy at low growth temperatures (≤300 °C) were studied using ion beam techniques. The excess As concentration in the layers was measured by particle induced x-ray emission and was found to increase as the growth temperature was lowered. Excess As concentrations up to 1.5 at. % were measured in layers grown at 190 °C. After annealing at temperatures higher than 400 °C under As overpressure, the excess As atoms coalesce to form As precipitates as revealed by transmission electron microscopy. Ion channeling on the unannealed layers grown at 200 °C revealed that they have good crystalline quality with a large fraction of the excess As atoms sitting at interstitial sites close to the normal As sites in the lattice. The rest of the excess As atoms are believed to be in an AsGa antisite position.
High-power single-mode AlGaAs lasers with bent-waveguide nonabsorbing etched mirrors
Authors: Gfeller F. R. , Buchmann P. , Epperlein P. W. , Meier H. P. , Reithmaier J. P.
Published: July 2002

A nonabsorbing etched mirror structure for AlGaAs single-quantum-well graded-refractive-index separate-confinement heterostructure ridge lasers is analyzed with respect to mirror coupling coefficient, threshold current penalty, and far-field pattern. Measurements of the mirror temperature showed a reduction from 50 to 20 K at 30 mW optical power depending on the degree of overlap of the optical intensity with the absorbing bent-waveguide profile. Pulsed catastrophic optical damage power levels up to 400 mW and a thermally saturated cw power of 165 mW with single-mode operation up to 80 mW have been achieved. Lifetime measurements at 40 mW constant optical power indicated degradation rates ≤10−5/h comparable to AlGaAs lasers with cleaved and coated mirrors.
Schlieren and dye laser resonance absorption photographic investigations of KrF excimer laser-ablated atoms and molecules from polyimide, polyethyleneterephthalate, and aluminum
Authors: Ventzek Peter L. G. , Gilgenbach Ronald M. , Ching Chi Hong , Lindley Roger A.
Published: July 2002

Hydrodynamic phenomena from KrF excimer laser ablation (10−3–20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by schlieren photography, shadowgraphy, and dye laser resonance absorption photography (DLRAP). Experiments were performed both in vacuum and gaseous environments (10−5–760 Torr air, nitrogen, and argon). In vacuum, ablation plumes are observed to expand like a reflected rarefaction wave. As the background gas pressure is increased, shock waves and reduced-density ablation plumes become visible. Below 10 Torr, the ablation plume follows closely behind the shock wave. Between 20 and 100 Torr, the plume recedes behind the shock wave. Below 10 Torr and above about 200 Torr, both the plume and the shock expand with the same temporal power law dependence. Agreement is found between these power law dependences and those predicted by ideal blast wave theory. The DLRAP diagnostic clearly shows that the ablated material (CN molecule from polyimide and ground state neutral aluminum atoms from laser-ablated aluminum) resides in the ablation plume. CN molecules are detected in both argon and air environments proving that CN is generated as an ablation product and not by reaction with the background gas. As the background gas pressure and the time after ablation is increased, the film darkening due to the laser-ablated material begins to fade leaving only the nonresonant shadowgraphy component of the plume. The plume dynamics observed by DLRAP are discussed in terms of gas dynamics, plume chemical kinetics, material diffusion in the plume, and cluster/particulate formation.
The plasma properties of laser-ablated SiO2
Authors: Wolf Paul J.
Published: July 2002

The optical emission from laser-produced plasmas generated by 1.06 μm irradiation of SiO2 targets at a flux of 7×1010 W cm−2 was recorded and analyzed between 250 and 800 nm. The ionization states of Si and O were mapped as a function of both time from the incident laser pulse and location from the front surface of the target. Electron temperatures were calculated using the relative emission intensities of Si(II) and O(II) ionization states (Te=3.4 eV), and an electron number density was determined from the Stark-broadened linewidths of five Si(II) emission lines. The ablated material was collected on Si substrates to examine the particulate nature of the plasma. Thin films were grown in the process and properties of these films were examined using IR reflectance and transmittance spectroscopy, scanning electron microscope analyses, and Auger electron spectroscopy.
Electrical determination of band offsets in a p-Ga0.77In0.23As0.20Sb0.80/ n-GaSb type-II heterojunction
Authors: Mebarki M. , Boukredimi D. , Sadik S. , Lazzari J. L.
Published: July 2002

Conduction- and valence-band discontinuities in lattice-matched p-Ga0.77In0.23As0.20Sb0.80/n-GaSb heterojunctions grown by liquid-phase epitaxy have been determined using the capacitance-voltage intercept method. The deduced energy-band diagram shows a staggered-lineup (type II) structure with conduction- and valence-band offsets ΔEC=(330±50) meV and ΔEV=(120±50) meV.
Dielectric breakdown of polyvinylidene fluoride and its comparisons with other polymers
Authors: Jow T. R. , Cygan P. J.
Published: July 2002

Using ac and dc methods, we have investigated the dielectric strengths of polyvinylidene fluoride (PVDF)—a ferroelectric/piezoelectric polymer, and some nonferroelectric polymers. The ac to dc ratio of breakdown strengths for PVDF is significantly lower than for other polymers. This observation is discussed in terms of the specific dielectric properties of PVDF, namely, its ferroelectricity and piezoelectricity.
Physical properties of semi-insulating polycrystalline silicon. I. Structure, electronic properties, and electrical conductivity
Authors: Brüesch P. , Stockmeier Th. , Stucki F. , Buffat P. A.
Published: July 2002

Semi-insulating polycrystalline silicon (SIPOS) are thin SiOx films (0≤x≤2), deposited by means of low pressure chemical vapor deposition on suitable substrates (silicon or sapphire). Although SIPOS has important applications in the semiconductor and solar cell technology, its physical properties which depend strongly on the oxygen content x, are not well known. In the present contribution, SIPOS as deposited at 660 °C in the range 0≤x≤1 is investigated by using different and complementary methods, namely nuclear reaction analysis, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and electrical conductivity measurements. On the basis of these experiments it is found that SIPOS consists of a nanometer-scale mixture containing Si, SiO2, and at least one suboxide (SiO1−Δ with Δ≊0.14). SIPOS with x≤0.034 is polycrystalline, while SIPOS with x≥0.4 is completely amorphous. In the range 0.034≤x≤0.4 there is a transition from crystalline Si grains to amorphous Si grains which is accompanied with a drastic decrease in grain size. The microstructure of SIPOS is consistent with a shell model, in which the Si grains are embedded in an amorphous oxide matrix consisting of SiO2 and SiO1−Δ. The latter oxide is located at the grain boundaries of the Si grains, i.e., it forms the transition oxide between the Si grains and the SiO2 matrix. Within the shell model, conduction proceeds by tunneling of thermally activated carriers through the oxide barriers separating adjacent grains. The model is able to qualitatively reproduce the observed low-bias conductivity σ(x,T) in the high-temperature regime.
Surface recombination in GaAs PN junction diode
Authors: Mazhari B. , Morkoç H.
Published: July 2002

Surface recombination in GaAs PN junction is investigated. It is argued that the commonly used expression for surface recombination current proposed by Henry et al. represents the current due to injection of carriers from the bulk to the surface from only one site within the junction depletion region where electron and hole densities are equal. This current with an ideality factor of 2 represents drift diffusion of carriers in the surface channel within the junction depletion region. At low forward bias, the neglect of injection from other sites results in little error but at higher forward bias the recombination current resulting from injection outside the junction becomes more important. It is shown that the latter may not always have an ideality factor of 1 as is commonly assumed.
Deposited silicon nitride with low electron trapping rates
Authors: Park Y. C. , Jackson W. B. , Smith D. L. , Johnson N. M.
Published: July 2002

In this article, nitrogen-rich silicon nitride plasma deposited under conditions minimizing Si—H bonding is shown to possess extremely low bulk electron trapping rates which are as low or lower than plasma-deposited oxides produced using He dilution. The bulk trap density, measured by avalanche injection decreases as the rf power is decreased. The total charge trapped within these silicon nitrides reaches a saturation value determined by high field detrapping in thick nitride films.
Effect of rapid thermal annealing on both the stress and the bonding states of a-SiC:H films
Authors: El Khakani M. A. , Chaker M. , Jean A. , Boily S. , Pépin H. , Kieffer J. C. , Gujrathi S. C.
Published: July 2002

The stress evolution of plasma enhanced chemical vapor deposition a-SiC:H films was studied by increasing the annealing temperature from 300 to 850 °C. A large stress range from −1 GPa compressive to 1 GPa tensile was investigated. Infrared absorption, x-ray photoelectron spectroscopy, and elastic recoil detection analysis techniques were used to follow the Si-C, Si-H, and C-H absorption band evolutions, the Si2p and C1s chemical bondings, and the a-SiC:H film hydrogen content variations with the annealing temperatures, respectively. It is pointed out that the compressive stress relaxation is due to the hydrogenated bond (Si—H and C—H) dissociation, whereas the tensile stress is caused by additional Si—C bond formation. At high annealing temperatures, a total hydrogen content decrease is clearly observed. This total hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. The results are discussed and a quantitative model correlating the intrinsic stress variation to the Si—H, C—H, and Si—C bond density variations is proposed.
Improved photovoltaic properties for Au/AlPcCl/n-Si solar cells with morphology-controlled AlPcCl deposition
Authors: Yanagi Hisao , Kataura Hidenori , Ueda Yasukiyo
Published: July 2002

Photovoltaic solar cells were constructed by successive vacuum deposition of chloroaluminum phthalocyanine (AlPcCl) and Au on a single-crystal n-Si wafer. By controlling the thickness and morphology of the AlPcCl deposits in the Au/AlPcCl/n-Si cells, an improved photovoltaic efficiency was obtained as compared to the Au/n-Si Schottky-type cell. The cell with a homogeneous, ultrathin (10 nm thickness) AlPcCl film, which was prepared by deposition on an n-Si substrate kept at −20 °C, exhibited a high photovoltage (Voc=0.45 V) and strongly sensitized photocurrents in the Q-band absorption region (600–800 nm) of AlPcCl chromophore. This improved photoresponse was attributed to a space-charge layer generated inside the AlPcCl film, where the intervening AlPcCl layer prevented the n-Si surface from forming surface states because of noncontact with the Au top electrode. When discontinuous island crystallites of AlPcCl were formed on the n-Si surface kept at 200 °C, the Au/AlPcCl/n-Si cell also exhibited an improved efficiency with a high photovoltage (Voc=0.42 V).

The discontinuous AlPcCl coverage, providing both the Au/n-Si Schottky contact and AlPcCl/n-Si heterojunction, modulated a diffusion potential at the space-charge region to give rise to a steep barrier gradient at the edges of the AlPcCl island. Its high photovoltaic response was attributed to an efficient charge-carrier separation from excitons that were generated on the uncoated n-Si area and diffused at the AlPcCl-island edge.
Comparison of covariance linear predictive modeling to the modulation function method for use in deep level transient spectroscopy
Authors: Doolittle W. A. , Rohatgi A.
Published: July 2002

This article provides a quantitative understanding of the merits and disadvantages of various methods of analyzing the deep level transient spectroscopy (DLTS) transients and spectrums. The modulation function method is compared to the covariance method of linear predictive modeling (CMLPM) transient analysis technique, and conventional DLTS spectrum methods. It is found that for high signal to noise ratio transients (high trap concentration), the modulation function method is superior to all other methods. As the signal to noise ratio decreases, CMLPM resolves closely spaced defect levels better than any other method examined. For very low signal to noise ratios, the effects of noise can only be overcome by the inherent averaging present in the more conventional DLTS spectrum analyses. The proper choice of modulation functions is also discussed, and a modified modulation function is proposed for higher resolution of closely spaced traps. Finally, DLTS transients from an electron irradiated sample containing five traps, are analyzed by conventional DLTS spectrum methods, the modulation function method and CMLPM to experimentally support the theoretical predictions.
Limit of normal incident absorption in quantum well detectors
Authors: Xu Wenlan , Willander M.
Published: July 2002

In this paper the conditions to detect normal incident radiation for n-type indirect-gap semiconductor quantum well detectors have been analyzed. Using the concept of invariable quantities of ellipsoidal constant energy surface under the coordinate transformation, we investigate the variation rules of the oscillator strength and absorption coefficient with various growth directions of the quantum well. The limit of normal incident absorption in quantum well detectors is given for different materials.
Physico-chemical changes in a-C:H by MeV He ion irradiation
Authors: Paterson Melissa J. , Orrman-Rossiter Kevin G. , Bhargava Suresh , Hoffman Alon
Published: July 2002

The effect of He ion irradiation on the hydrogen content and structure of a-C:H films has been examined. Comparison of elastic recoil detection and Fourier transform infrared spectroscopy results have shown that significant hydrogen depletion occurs in conjunction with an equally significant change in the structure of the films. Examination of the films’ hydrogen profiles also indicates that the diffusion length of molecular hydrogen in these materials during bombardment may be between 160–320 nm.
An assessment of the roles of climb and glide in misfit strain relief
Authors: Jesser W. A. , van der Merwe J. H.
Published: July 2002

In order to assess the relative contributions of glide and climb processes in the relaxation of misfit strain in heteroepitaxial layers, the glide and climb velocities of dislocations are compared at several temperatures. It is shown that the glide velocity is much greater than the climb velocity under normal conditions. For copper and silicon, about four orders of magnitude can exist at 3/4 of the melting temperature with lower temperatures leading to larger differences. One therefore expects relaxation of misfit strain to proceed primarily by glide mechanisms. Two cases are addressed here in which climb processes can be important: (i) by-passing of an obstacle and (ii) redistribution of an irregular array of interfacial misfit dislocations into a regular array of lower energy.
Spectroscopic investigation of electroluminescent porous silicon
Authors: Pavesi L. , Ceschini M. , Mariotto G. , Zanghellini E. , Bisi O. , Anderle M. , Calliari L. , Fedrizzi M. , Fedrizzi L.
Published: July 2002

Light-emitting porous silicon films have been obtained by anodic etching p-type Si samples in a HF-ethanol solution. Porous Si samples efficiently luminesce at room temperature in the visible region. A degradation of the luminescence intensity with time is observed. Micro-Raman spectroscopy of free-standing porous silicon layers indicates phonon confinements as well as a strong laser heating effects. The surface chemical composition and the effect of electron-beam irradiation has been investigated through Auger spectroscopy. The Si LVV Auger transition dominates the spectrum, even in aged samples. The Si line shape gives evidence of a covalent bond between the porous Si surface atoms and some adsorbed species. A prolonged electron irradiation results in a strong variation of the surface chemical composition, with an anomalous carbon accumulation. Gold thin films have been deposited on the porous Si surface to form metal-semiconductor junctions. Schottky diodes with large rectifying ratio, ideality factor, and series resistance are obtained. When the junction is forward biased, electroluminescence is observed. Electroluminescence degrades with time while the current does not. When the junction is reverse biased a significant photocurrent is obtained. The results are discussed in the framework of the surface state emission model for the luminescence.
Binary information storage at zero bias in quantum-well diodes
Authors: Buot F. A. , Rajagopal A. K.
Published: July 2002

It is argued, based on the intrinsic time-dependent behavior of double-barrier structures, that a modification of a conventional quantum-well diode with special spacer-layer structure in the source and/or the drain region will lead to two stable current-voltage and charge state behaviors all the way down to zero bias. This viewpoint explains the salient features of a recent experimental observation on quantum-well diodes with n−-n+-n− spacer layers. We substantiate this with a simple theory of self-consistent charge buildup and bistability, and show that a limited supply or highly altered distribution of electrons from the emitter at high bias leads to fractional recharging of the quantum well and fractional current values, during the decreasing voltage sweep portion of a ‘‘closed-loop’’ voltage sweep. This is in contrast with previous theories based on numerical simulations which allow for more than two current states, by virtue of the use of time-independent analysis and/or the use of ‘‘open-multibranch’’ voltage sweep which do not correspond to the ‘‘closed-loop’’ voltage sweep in the actual experiments mentioned above. This two charge state phenomenon then is the basis for a feasible binary-information storage device at zero bias without dissipation.
Thermal characteristics of blue-green II-VI semiconductor lasers
Authors: Drenten R. R. , Haberern K. W. , Gaines J. M.
Published: July 2002

Threshold current densities and wavelengths of gain maximum and longitudinal modes have been determined as a function of temperature for various laser structures. The onset of intrapulse heating has been studied and interpreted. In Zn0.92Mg0.08S0.12Se0.88/ZnS0.06Se0.94 /Zn0.8Cd0.2Se lasers, thermal resistances have been measured, using substrate-up and substrate-down mounting. From these, continuous-wave lasing regimes have been determined.
Vacancy-controlled model of degradation in InGaAs/AlGaAs/GaAs heterostructure lasers
Authors: Hopgood A. A.
Published: July 2002

A model is proposed for dark line defect (DLD) growth in semiconductor lasers by dislocation climb. The model assumes that climb occurs by the migration of randomly distributed vacancies toward existing dislocations. The effects of a strained layer on the number of vacancies and their stability are investigated. It is concluded that vacancies can act as strain relievers, thereby reducing their energy of formation. In strained-layer lasers this has the effect of reducing the driving force for DLD growth by climb but increasing the pool of vacancies available for the process to occur. These findings are offered as an explanation for the anomalous behavior of DLDs in strained-layer lasers.
Theoretical performance of InAs/ InxGa1−xSb superlattice-based midwave infrared lasers
Authors: Grein C. H. , Young P. M. , Ehrenreich H.
Published: July 2002

We show that for appropriate layer widths the performance of ideal InAs/InxGa1−xSb superlattice-based midwave injection lasers can be limited by radiative rather than Auger recombination. The threshold carrier densities and lifetimes are calculated over the 77–300 K temperature range at 3.5 μm. Lifetimes are obtained from detailed calculations of band-to-band Auger and radiative recombination rates based on realistic nonparabolic band structures. This system is therefore a promising new laser candidate.
Photoluminescence and Auger spectroscopy of porous Si: Solvent, reactive ion etching, annealing, and substrate boron level effects
Authors: Shinar R. , Robinson D. S. , Partee J. , Lane P. A. , Shinar J.
Published: July 2002

The photoluminescence (PL) at λ≤850 nm of boron-doped porous Si films anodically etched, passively etched, annealed, and reactive ion etched (RIE) under systematically varied conditions is described and discussed. As previously observed, the PL yield η of films etched from 7 to 20 Ω cm wafers in HF/H2O/C2H5OH solutions rapidly degraded during 40 mW illumination in air at 488 nm. In addition, it was totally quenched by O2 annealing or RIE but not by H2 RIE. However, the yield of films etched in HF/H2O only decreased by less than 10% following similar illumination for 7 h, and O2 annealing or RIE reduced it by 0%–50% only. The instability under illumination, the effects of oxygen annealing and RIE, Auger line shape measurements, and surface charge buildup during Auger analysis are consistent with a significantly higher density of weak Si—Si bonds in films etched in ethanol-containing solutions. These bonds are apparently broken by photoinduced oxidation, oxygen annealing, or O2 RIE, but the oxygen does not efficiently passivate the newly generated dangling bonds. While the peak emission wavelength λmax of the films etched from these wafers shifted with annealing and etching conditions, it invariably peaked in the ∼7600–8600 Å range; η reached ∼1.7%. H2 RIE had no effect on η, but λmax was blueshifted. This behavior is attributed to similar breaking of weak Si—Si bonds followed by efficient passivation of the newly generated dangling bonds.

The blueshift results from the higher H/Si ratio; the invariant yield is believed to be limited by the density of dangling bonds at the SiOxHy/Si interface which is apparently unaffected by the treatments. It is therefore believed that the PL is at least partially due to SiHx or SiOxHy complexes on the crystallite and column surfaces, which are similar to those found at the surface of some H-rich a-Si:H. Size effects, however, cannot be ruled out. In contrast to films etched from 7 to 20 Ω cm wafers, λmax of those prepared from 1 to 2 Ω cm substrates was consistently ∼7000 Å after a brief exposure to air. While η exhibited a complex dependence on the O2 RIE power, λmax was unchanged by either O2 or H2 plasmas. The emission from heavily doped ∼0.02 Ω cm films was usually undetectable. However, H2 RIE resulted in observable PL.
Anomalous photovoltaic effect and negative photoconductivity in thin, amorphous GaAs-Si films
Authors: Reuter Herbert , Schmitt Heinz
Published: July 2002

An anomalous photovoltaic effect and negative photoconductivity were observed in obliquely deposited thin, amorphous GaAs, Si, and GaAs-Si films. A model of the E(r↘,k↘)-relation of amorphous semiconductors shows that some carriers in the extended states can have negative effective masses. The number of these photocarriers is high if the energetic range of the localized states is broad. This can lead to negative photoconductivity. Additionally, gradients of the densities of carriers with positive mobilities in the localized and with negative mobilities in the extended states can lead to diffusion photovoltages of several hundred volts in a single photocell with a length of 10 mm.
New model for the thickness and mismatch dependencies of threading dislocation densities in mismatched heteroepitaxial layers
Authors: Ayers J. E.
Published: July 2002

A new model is proposed for the thickness dependence of the threading dislocation densities in mismatched heteroepitaxial semiconductor layers. This ‘‘glide model’’ has been developed based on the premise that dislocations establish mechanical equilibrium between the line tension in their misfit segments and the glide force experienced by their threading segments. The glide model correctly predicts the inverse relationship between the film thickness and the dislocation density in mismatched heteroepitaxial layers which are much thicker than the critical layer thickness. It also predicts the weak dependence of the dislocation density on the lattice mismatch. The quantitative results of the glide model are in fair agreement with published experimental results although no unknown or adjustable parameters have been incorporated.
Luminescence study of structural changes induced by laser cutting in diamond films
Authors: Cremades A. , Piqueras J.
Published: July 2002

The effect of laser cutting on the structure of a chemical vapor deposited diamond film has been investigated by cathodoluminescence (CL) in the scanning electron microscope. The variation of particle morphology and CL spectra as a function of the distance to the cutting edge is described and possible laser induced structural changes are discussed. At the damaged region total CL emission increases and nitrogen-vacancy centers are detected.
Strain and crystallographic orientation effects on interband optical matrix elements and band gaps of [11l ]-oriented III-V epilayers
Authors: Henderson R. H. , Towe E.
Published: July 2002

We report a derivation of the 4×4 Luttinger valence band and Bir–Pikus strain Hamiltonians valid for any [11l]-oriented epilayers. It is shown that epitaxial layers grown on substrates oriented in certain of the [11l] directions exhibit anisotropic optical transition matrix elements. This anisotropy is predicted to have its maximum for the (110) surface. For pseudomorphic epitaxial layers, calculations indicate that the zone center band gap of these structures is changed due to the effects of strain, with the largest changes occurring for those structures grown on the (111) surface.
Theory of optical gain in strained-layer quantum wells within the 6×6 Luttinger–Kohn model
Authors: Ahn Doyeol , Yoon Sean J. , Chuang Shun Lien , Chang Chih-Sheng
Published: July 2002

We study the effects of the spin-orbit (SO) split-off band coupling with the heavy- and the light-hole bands on the band structure, density of states, dipole moment, and the linear and nonlinear optical gains of strained-layer quantum wells by comparing the 6×6 and the 4×4 Luttinger–Kohn models. First, a unitary transformation is found that block diagonalizes the 6×6 Hamiltonian into two 3×3 blocks that are real symmetric in the finite difference formulation. We find that the peak gains are overestimated, especially in the case of a tensile quantum well, in the 4×4 model, for the InGaAs–InP quantum-well systems which show weak SO coupling. The change of the density of states is suggested to be the dominant factor for the discrepancy of the results between the 6×6 and the 4×4 models in the linear gain. For nonlinear gain the discrepancy between the two models is smaller than that of the linear gain because the change of the fourth-order dipole moment in the nonlinear gain compensates that of the density of states. We also considered InGaP–In(AlGa)P quantum wells which have very narrow SO split-off energy. In this case, because of the strong coupling between the SO bands and the hole bands, the InGaP–In(AlGa)P quantum-well systems show more complicated behavior. The peak gain of strained quantum well is overestimated in the low carrier injection region and is underestimated in the high injection, in the 4×4 model. On the other hand, the peak gain of an unstrained quantum well is overestimated in the 4×4 model over the wide range of carrier densities.
Radiative and nonradiative rates and deep levels in zinc selenide grown by molecular-beam epitaxy
Authors: Allen J. W. , Reid D. T. , Sibbett W. , Sleat W. , Zheng Jia-Zhen , Hommel D. , Jobst B.
Published: July 2002

A series of chlorine-doped epitaxial layers of ZnSe have been grown by molecular-beam epitaxy. Measurements have been made of the relative intensity of the blue photoluminescence at room temperature, of the decay time of this emission, and of the concentrations of deep levels determined by photocapacitance techniques. A GaAs buffer layer improves the light emission and decreases the deep-level concentrations. Over the uncompensated donor concentration range 2×1016 to 3×1018 cm−3 the photoluminescence intensity, decay rate, and deep-level concentrations only vary by one order of magnitude. The results are consistent with the room-temperature blue emission resulting in recombination of a free hole with electrons on relatively isolated donors, this radiative recombination competing with a strong nonradiative Hall–Shockley–Read recombination via deep levels in the lower half of the energy gap. The significance of these results for the making of light-emitting diodes and lasers is discussed.
Mechanism of laser ablation for aqueous media irradiated under confined-stress conditions
Authors: Oraevsky Alexander A. , Jacques Steven L. , Tittel Frank K.
Published: July 2002

Pulsed laser ablation of aqueous medium irradiated under conditions of temporal confinement of thermal stress is described. Time-resolved measurements of laser-induced transient stress waves with simultaneous imaging of ablation process by laser-flash photography were performed. Stress transients induced in aqueous solution of K2CrO4 by ablative nanosecond laser pulses at 355 nm were studied by a broad-band lithium niobate acoustic transducer. Recoil momentum upon material ejection was measured from the temporal profiles of the acoustic transducer signal as a function of incident laser fluence. Cavitation bubbles produced in the irradiated volume during the tensile phase of thermoelastic stress were shown to drive material ejection at temperatures substantially below 100 °C. Experimental data are evident that nanosecond-pulse laser ablation of aqueous media (when temporal stress-confinement conditions are satisfied) include the following two main stages of material ejection: (1) ejection of water microdroplets due to expansion and rupture of subsurface cavitation bubbles; (2) ejection of liquid streams with substantial volume upon collapse of initial crater and large cavitation bubbles in the depth of irradiated volume (after coalescence of smaller bubbles).
Low-temperature buffer layer for growth of a low-dislocation-density SiGe layer on Si by molecular-beam epitaxy
Authors: Chen H. , Guo L. W. , Cui Q. , Hu Q. , Huang Q. , Zhou J. M.
Published: July 2002

A method using a low-temperature Si (LT-Si) buffer layer is developed to grow a SiGe epilayer with low density of dislocations on a Si substrate by molecular-beam epitaxy. In this method, a LT-Si layer is used to release the stress of the SiGe layer. The samples have been investigated by x-ray double-crystal diffraction and transmission electron microscopy. The results indicate that the LT-Si is effective to release the stress and suppress threading dislocations.
Effective mass theory for III-V semiconductors on arbitrary (hkl) surfaces
Authors: Henderson R. H. , Towe E.
Published: July 2002

The effects of arbitrary substrate orientation on the electronic and optical properties of III-V zinc-blende semiconductors are considered. A unitary transformation matrix is used to rotate the 4×4 Luttinger valence band Hamiltonian, and the Bir-Pikus strain Hamiltonian from the conventional (001) surface to any arbitrary (hkl) surface of interest. The effects of strain on several electronic and optical properties are examined. It is found that the strain-induced change in the forbidden gap is largest for the (111) plane and other equivalent planes. Furthermore, the strain is also found to induce both a longitudinal and a transverse piezoelectric field. The longitudinal field reaches a maximum for the (111) surface and its other equivalent planes, while the transverse field reaches a maximum for the (110) surface and its other equivalent planes. The orientation-dependence of the hole effective masses is also examined; it is found that the (111) surface, and other equivalent planes, exhibits the largest heavy-hole mass among all possible planes. Finally, this article examines the effects of orientation on the optical transition matrix elements. For incident light with in-plane polarization vectors, the matrix elements are, in general, anisotropic—with the largest anisotropies predicted for two new surfaces: the (√310) and the (1√30) surface.
Determination of the relative densities of high-voltage stressed-generated traps near the anode and cathode in 10-nm-thick silicon oxides
Authors: Hughes T. W. , Dumin D. J.
Published: July 2002

The relative densities of high-voltage stress-generated traps near the anode and cathode in 10-nm-thick silicon oxides have been measured after both positive and negative gate voltage stressing. The density of traps near the stress anode and cathode were approximately equal after negative gate voltage stressing. The density of traps near the stress cathode was higher than near the stress anode after positive gate voltage stressing. These results implied that impact ionization near the anode was not a significant generator of traps in 10-nm-thick oxides.
Calculations of lattice parameters and reaction-induced volume and stress changes for Al(Ti) alloys
Authors: Tseng Wei-Tsu
Published: July 2002

First-principles calculations of the lattice parameter and its temperature variation for (Al), Ti, and TiAl3 are conducted based upon the linear muffin-tin orbitals method with the atomic sphere approximation in conjunction with the Debye–Grüneisen approximation and cluster variation method. The calculated lattice parameters are in close agreement with those determined by x-ray diffraction over a wide temperature spectrum except for slight deviation at low temperatures. Stress and volume changes induced by the formation of TiAl3 compound from both Al(Ti) solid solution and from interactions at (Al)/Ti interfaces are derived using the calculated lattice parameters. The effect of an external stress field on the precipitation-induced volume and stress changes is also investigated. Comparisons of the calculated stress changes in this study with experimental data taken from other work show excellent agreement. The results suggest that the stress alterations as a result of TiAl3 compound formation during annealing may raise serious reliability concern of metallizations for microelectronics.
Space-charge associated with Fano-type resonances in nanostructures
Authors: Price Peter J.
Published: July 2002

It is shown, by analysis in terms of the eigenvalues of the S matrix, that the Fano type of transmission resonance in nanostructures has the same space-charge property as applies to the simple Lozentzian resonances. The total space charge in the structure enclosed by the leads, due to filling all electron states (of one spin state) over the energy range of a narrow resonance, is equal to just one electron charge.
Capacitance-voltage profiling of quantum well structures
Authors: Tschirner B. M. , Morier-Genoud F. , Martin D. , Reinhart F. K.
Published: July 2002

Capacitance-voltage (C-V) analysis of high quality MBE grown quantum well samples shows that carrier distributions are averaged over the scale of the Debye length. This averaging process results in strongly temperature dependent C-V-deduced doping distributions that can be very different from the actual ones. The doping distribution of the structure is obtained by fitting numerically simulated curves to the measured C-V curves and doping profiles, respectively. Although the calculations do not take quantum size effects into account they show good agreement with the measured data.
Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots
Authors: Adler F. , Geiger M. , Bauknecht A. , Scholz F. , Schweizer H. , Pilkuhn M. H. , Ohnesorge B. , Forchel A.
Published: July 2002

We present experimental results concerning optical transitions and carrier dynamics (capture and relaxation) in self assembled InAs/GaAs quantum dot structures grown by metalorganic vapor phase epitaxy. Photoluminescence (PL) measurements at high excitation level reveal optical transitions above the ground state emission. These transitions are found to originate from occupied hole states by solving the quantum dot eigenvalue problem. Time-resolved studies after non-resonant pulse excitation exhibit a relaxation ladder of the excited carriers from the GaAs barrier down to the ground state of the quantum dots. From both the continuous-wave measurements and the PL-decay curves we conclude that the carrier relaxation at non-resonant excitation is mediated by Coulomb interaction (Auger effect). PL-decay curves after resonant pulse excitation reveal a longer rise time compared to non-resonant excitation which is a clear indication of a relaxation bottleneck inside the quantum dots. We interpret the rise time (≊ 400 ps) in this case to originate from relaxation via scattering by acoustic phonons. The PL-decay time of the ground state emission ≊700 ps is interpreted as the excitonic lifetime of the quantum dot.
Structural and optical characterization of InP grown on Si(111) by metalorganic vapor phase epitaxy using thermal cycle growth
Authors: Ababou Y. , Desjardins P. , Chennouf A. , Leonelli R. , Hetherington D. , Yelon A. , L’Espérance G. , Masut R. A.
Published: July 2002

Heteroepitaxial InP layers were grown on Si(111) by metalorganic vapor phase epitaxy using thermal cycle growth. The best crystallographic and optical quality was obtained when thermal cycle growth was begun after only a thin InP layer had been deposited. High resolution x-ray diffraction rocking curves of 4.8 μm thick InP layers yield full widths at half-maximum as low as 76 arc s and show that epilayers have a positive tilt with respect to the substrate. Cross-section transmission electron microscopy observations and Rutherford backscattering measurements show that thermal cycling induces a net reduction of defect density in the interfacial region. Photoluminescence (PL) measurements performed on the best quality thermal cycle grown sample show a thermal strain induced energy splitting of 3.8 meV between the free exciton emissions associated with heavy and light holes. Two other peaks in the PL spectra correspond to acceptor-bound (A0,X)mj=±3/2 and (A0,X)mj=±1/2 excitonic transitions, as confirmed by photoluminescence excitation measurements. Their full width at half-maxima are 1.4 and 0.9 meV, respectively, for the optimized samples. They may be associated with Si acting as an acceptor.
Deep level defects in electron-irradiated 4H SiC epitaxial layers
Authors: Hemmingsson C. , Son N. T. , Kordina O. , Bergman J. P. , Janzén E. , Lindström J. L. , Savage S. , Nordell N.
Published: July 2002

Deep level defects in electron-irradiated 4H SiC epitaxial layers grown by chemical vapor deposition were studied using deep level transient spectroscopy. The measurements performed on electron-irradiated p+n junctions in the temperature range 100–750 K revealed several electron traps and one hole trap with thermal ionization energies ranging from 0.35 to 1.65 eV. Most of these defects were already observed at a dose of irradiation as low as ≈5×1013 cm−2. Dose dependence and annealing behavior of the defects were investigated. For two of these electron traps, the electron capture cross section was measured. From the temperature dependence studies, the capture cross section of these two defects are shown to be temperature independent.
Photoluminescence excitation measurements on erbium implanted GaN
Authors: Torvik J. T. , Feuerstein R. J. , Qiu C. H. , Pankove J. I. , Namavar F.
Published: July 2002

The temperature dependence of the optical excitation cross section of Er implanted n-type GaN was studied using photoluminescence excitation spectroscopy. Due to the large 3.4 eV band gap of GaN, it was possible to probe two Er absorption lines using a tunable Ti:sapphire laser in the 770–1010 nm range. Photoluminescence excitation spectra exhibiting several Stark splittings revealed a complex dependence upon temperature. The largest excitation cross section in the third excited state was 1.65×10−20 cm2 at an excitation wavelength of 809.4 nm when measured at 77 K. This value is roughly three times larger than the cross section in the second excited state at 4.8×10−21 cm2 when pumping at 983.0 nm. The Er-related photoluminescence was reduced between 1.5 and 4.8 times when going from 77 K to room temperature, except when pumping around 998 nm. At this excitation wavelength the room temperature photoluminescence was stronger by a factor of 1.26 compared to that at 77 K.
Angular and energy dependence of ion bombardment of Mo/Si multilayers
Authors: Voorma H.-J. , Louis E. , Bijkerk F. , Abdali S.
Published: July 2002

The process of ion bombardment is investigated for the fabrication of Mo/Si multilayer x-ray mirrors using e-beam evaporation. The ion treatment is applied immediately after deposition of each of the Si layers to smoothen the layers by removing an additional thickness of the Si layer. In this study the parameters of Kr+ ion bombardment have been optimized within the energy range 300 eV–2 keV and an angular range between 20° and 50°. The optical performance of the Mo/Si multilayers is determined by absolute measurements of the near-normal-incidence reflectivity at 14.4 nm wavelength. The multilayer structures are analyzed further with small-angle reflectivity measurements using both specular reflectivity and diffuse x-ray scattering. The optimal smoothening parameters are obtained by determining the effect of ion bombardment on the interface roughness of the Si layer. The optimal conditions are found to be 2 keV at 50° angle of incidence with respect to the surface. These settings result in 47% reflectivity at 85° (λ=14.4 nm) for a 16-period Mo/Si multilayer mirror, corresponding to an interface roughness of 0.21 nm rms. Analysis shows that the interface roughness is determined by ion induced viscous flow, an effect which increases with ion energy as well as angle of incidence. In order to determine the effect of intermixing of the Si and Mo atoms, the penetration depth of the Kr+ ions is calculated as a function of ion energy and angle of incidence. Furthermore, the angular dependence of the etch yield, obtained from the in situ reflectivity measurements, is investigated in order o determine the optimal ion beam parameters for the production of multilayer mirrors on curved substrates.
Light-emitting diodes based on poly-p-phenylene-vinylene: II. Impedance spectroscopy
Authors: Meier M. , Karg S. , Riess W.
Published: July 2002

Electrical impedance measurements on poly-p-phenylene-vinylene (PPV) light-emitting diodes in the frequency range between 100 Hz and 10 MHz are reported. A significant difference can be revealed between the device characteristics of light-emitting diodes eliminated on indium-tin-oxide (ITO) and those of other high-work-function metals (e.g., Au). Thermal conversion of the precursor polymer on ITO substrates results in a p-type doping of the conjugated polymer PPV. Hence, devices in the configuration ITO/PPV/Al display Schottky behavior, which can be modeled by a simple equivalent circuit of two RC elements in series, representing a bulk and a junction region. The low-frequency device capacitance displays a pronounced voltage dependence and, from a detailed analysis, the ionized acceptor concentration NA, the diffusion potential VD, and the width of the space charge region w can be obtained. Typical values for NA are 1016–1017 cm−3, and for VD within the range 1–1.5 V, resulting in a width w of the space charge region at zero bias of about 50–150 nm. Via temperature-dependent investigations a transition from a p-type semiconductor Schottky diode at room temperature to a metal/insulator(polymer)/metal structure at lower temperatures is revealed.