bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification of room temperature photoluminescence in pseudomorphic modulation-doped AlGaAs/InGaAs/GaAs quantum wells
Authors: Lu Wu , Ng Geok-Ing , Jogai B. , Lee Jin-Hee , Park Chul-Soon
Published: July 2002

Room temperature photoluminescence in δ-doped pseudomorphic AlGaAs/InGaAs/GaAs quantum wells is investigated. The electron and hole subband structure and optical transition matrix elements are calculated by a self-consistent theoretical method. Separations of the calculated conduction subband levels and valence subband levels reveal that the dominant emissions are due to the transitions from the second electron subband to the first heavy-hole subband, the first electron subband to the first heavy-hole subband, and the first electron subband to the second heavy-hole subband. The calculation also predicts that the transition energies shift to lower energies with the increase of the δ-doping level. This is accounted for by the lowering of the confined levels originating from electrostatically induced band bending. The calculated transition matrix elements demonstrate that transitions with different electron and hole subband indices, i.e., forbidden pairs, are preferred. The relative magnitudes of the squared optical matrix elements for the transitions from the first and second electron subbands to the first heavy-hole subband are analyzed as functions of separations of the first two electron subbands and the first two heavy-hole subbands, respectively. The results show that the 2-1 pair has a larger oscillator strength than the 1-1 and 1-2 pairs in agreement with observations.
The effects of rapid thermal process on the properties of sputtered SrS:CeF3 thin films
Authors: Horng R. H. , Wuu D. S.
Published: July 2002

A Y2O3/SrS:Ce/BaTiO3 electroluminescent (EL) structure has been deposited by rf-magnetron reactive sputtering and, subsequently, submitted to various thermal treatments. The effects of the rapid thermal process (RTP) on the properties of SrS:Ce thin films have been investigated, and compared with the results obtained by the conventional furnace. After postdeposition annealing, the SrS film has the tendency to recrystallize preferentially in the (200) plane. The higher RTP temperature also results in the larger grain size of SrS. Auger measurements show that the RTP method can overcome the nonstoichiometry and interdiffusion problems encountered in the furnace-treated sample. It was found that the EL peaks gradually recovered to the band transitions of Ce3+ ions (475 and 530 nm) in the SrS crystal field as the RTP temperature increased from 650 to 850 °C. The threshold voltage can be further reduced to 150 V, and the brightness increases to eight times, as compared with that of the optimum furnace-treated device. The improvements in EL performance are owing to the facts that the RTP can enhance the crystallinity of SrS, alleviate oxygen-induced defect centers from interdiffusion, activate Ce3+ ions in phosphor, and yield a blueshift in emission colors.
Electronic and optical properties of periodically Si δ-doped InP grown by low pressure metalorganic vapor phase epitaxy
Authors: Souza P. L. , Yavich B. , Pamplona-Pires M. , Henriques A. B. , Gonçalves L. C. D.
Published: July 2002

A series of periodically Si δ-doped InP samples with 5 and 10 periods varying from 92 to 278 Å has been investigated in terms of the transport and optical properties. A reduction in mobility with decreasing period was observed due to the increasing overlap of the electronic wavefunction with the various Si planes. A broad band emission was detected for the periodic structures at energies higher than the InP band gap. The cutoff energy for this band decreases with the period and this behavior can be described by a d−2/3 decay. The results are discussed and compared with the ones for GaAs available in the literature.
Are equilibrium fluctuations detectable in diffracted intensities?
Authors: Tringides Michael C. , Larsson Mats I. , Pfnür Herbert , Frischat Hannes , Budde Knut , Henzler Martin
Published: July 2002

The capability of diffraction intensity fluctuations to probe dynamic phenomena at equilibrium is studied. The summation of fluctuations over different domains that scatter incoherently, the role of statistically “white-noise” generated fluctuations and the time necessary to collect a sufficiently long signal are investigated. The factors that determine the number of incoherent regions, and maximize the relative fluctuation without reducing the count rate to such low values that impractically long acquisition times are needed are discussed. Monte Carlo simulations modeling thermally generated step fluctuations are used to illustrate the results.
Structural evaluation of InAsP/InGaAsP strained-layer superlattices with dislocations as grown by metal-organic molecular beam epitaxy
Authors: Nakashima Kiichi , Sugiura Hideo
Published: July 2002

The relaxation process in InAsP/InGaAsP strained-layer superlattices (SLSs) with interfacial misfit dislocations has been investigated systematically by transmission electron microscopy (TEM) and x-ray analyses. The TEM analysis reveals that dislocations locate a little inside the buffer layer near the interface between the buffer and first well layer in the SLS. The x-ray analysis of (400) azimuthal angle dependence indicates the buffer layer has a large macroscopic tilt. Using a curve fitting analysis of various (hkl) x-ray profiles and reciprocal lattice mapping measurements, residual strain was determined quantitatively, i.e., Δa∥ and Δa⊥, in the SLS and buffer layer. These results reveal that the dislocations mainly cause lattice distortion of the buffer layer rather than relaxation of the SLS layer. The most remarkable result is that the change of a∥ is not equal to that of a⊥ in the buffer layer. This phenomenon strongly suggests that microplastic domains are generated in the buffer layer.
Hydrogen bonding in plasma-deposited amorphous hydrogenated boron films
Authors: Saß M. , Annen A. , Jacob W.
Published: July 2002

Hydrogenated amorphous boron (a-B:H) thin films were prepared by radio-frequency plasma deposition using B2H6 (10%) in H2 as precursor gas. The influence of the substrate temperature and self-bias on the a-B:H film structure was investigated. The boron and hydrogen atom densities were determined by ion-beam analysis. The film structure, especially the bonding of hydrogen to boron, was investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR data were quantified by using a new formalism which allows a proper calculation of the extinction coefficient from the FTIR spectra. The intensities of the different boron-hydrogen absorption bands were compared with the ion-beam analyzed hydrogen atom densities to determine the absorption strength of the B–H terminal and B–H–B bridge bonds. A non-negligible fraction of hydrogen is shown to be bonded to boron in a B–H–B bridge bond.
A study on interface and charge trapping properties of nitrided n-channel metal-oxide-semiconductor field-effect transistors by backsurface argon bombardment
Authors: Lai P. T. , Xu J. P. , Lo H. B. , Cheng Y. C.
Published: July 2002

A low-energy (550 eV) argon-ion beam was used to directly bombard the backsurface of nitrided n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) after the completion of all conventional processing steps. The interface and oxide-charge trapping characteristics of the bombarded MOSFETs were investigated as compared to nonbombarded and reoxidized-nitrided n-MOSFETs. It was found that after bombardment, interface state density decreases and interface hardness against hot-carrier bombardment enhances, and oxide charge trapping properties were also improved. The improvements exhibit a turnaround behavior depending on bombardment conditions and could be attributed to stress compensation in the vicinity of the Si/SiO2 interface and an annealing effect.
Minority-carrier lifetime damage coefficient of irradiated InP
Authors: Keyes B. M. , Ahrenkiel R. K. , Shaw G. J. , Summers G. P.
Published: July 2002

Minority-carrier lifetime damage coefficients for 1 MeV electron, 3 MeV proton, and 6 MeV alpha particle irradiation of n-type (4.5×1015 and 1.3×1017 cm−3) and p-type (2.5×1017 cm−3) InP have been measured using time-resolved photoluminescence. These values are relatively insensitive to carrier type and show a slight increase with increasing carrier concentration. Evidence of comparable electron and hole capture lifetimes is found for the dominant recombination defect. The effect of 3 MeV proton and 6 MeV alpha particles relative to 1 MeV electrons is an increase in the lifetime damage coefficient by factors of about 104 and 105, respectively.
Electron emission and structure properties of cesiated carbon films prepared by negative carbon ion beam
Authors: Ko Y. W. , Kim S. I.
Published: July 2002

The work function, field emission property (turn-on field), and Auger electron spectroscopy of cesiated carbon films on Si (100) have been investigated for codeposition of Cs neutral and C− ion beams at different energies (25–150 eV). The higher energy (150 eV) C− ion beam produces the lower work function surface (1.1 eV) as well as sp3 rich carbon film. The work function depends both on cesium concentration as well as on the sp3 fraction in the carbon films. The turn-on field of the film can be as low as 7 V/μm. The thermal stability of the low work function surface has been investigated for postdeposition annealing up to 600 °C. An extremely high stability cold cathode has been made by forming cesium carbide.
Laterally oxidized GaInP/AlGaInP quantum well visible laser diodes
Authors: Floyd P. D. , Sun D. , Treat D. W.
Published: July 2002

High efficiency, low threshold visible AlGaInP/GaInP laser diodes using a buried AlAs native oxide for carrier and optical confinement are described. The lasers incorporate a thin AlAs layer in the upper cladding region, which when laterally wet oxidized, forms a narrow aperture. The lasers exhibited modest performance under continuous wave (cw) operation. Low temperature (400 °C) post-fabrication annealing was shown to dramatically improve the device characteristics. The lasers operate with room temperature cw threshold currents of 20 mA with external differential quantum efficiencies of 27% per facet (0.25 W/A per facet) for an uncoated 625-μm-long, 3.5-μm-wide device.
Temperature induced blue shift and broadening of intersubband absorption and photocurrent spectra in GaAs/Al0.3Ga0.7As multiple quantum wells
Authors: Huang X. L. , Shin Y. G. , Suh E.-K. , Lee H. J. , Hwang Y. G. , Huang Q.
Published: July 2002

The temperature induced blue shift of the intersubband absorption peak in GaAs/Al0.3Ga0.7As multiple quantum wells has been observed and calculated as a function of well thickness (Lw). As the temperature is lowered from 290 to 10 K, the absorption peak shifts to the higher energy by 7.1 meV. Calculation shows that the change in the barrier height dominates the blue shift at small Lw while exchange interaction is important for large Lw. At 80 K, the photocurrent spectrum (PCS) at 0 V bias (Vb) is gradually reduced and cut off by the barrier. The bias induced forbidden transition from the ground state, E1, to the second excited state, E3, is observed in PCS. The broadening of full width at half maximum of PCS is related to the bias induced forbidden transition at small bias (0<Vb<0.2 V) and the reduction of the barrier height and tunneling lifetime at large bias (0.2<Vb<2 V).
Kinetics and mechanisms of TiN oxidation beneath Pt thin films
Authors: McIntyre Paul C. , Summerfelt Scott R.
Published: July 2002

Oxidation kinetics are reported for TiN thin film diffusion barriers beneath polycrystalline Pt films of 50–200 nm thickness annealed in dry O2/N2 ambients of varying oxygen partial pressure and total pressure near 1 atm. Oxygen resonance backscattering spectrometry was used to detect thin oxide layers at the Pt/TiN interface produced by oxidation annealing at 475–650 °C. Over part of this temperature range, a linear oxidation rate law was observed for the buried TiN film, indicating the oxidation rate was independent of average titanium oxide thickness. Possible rate-controlling oxidation mechanisms are analyzed critically in light of the kinetic data and results from microstructural studies of the Pt/TiN specimens. We conclude that the rate-limiting step in the Pt/TiN oxidation process is diffusion of oxygen through the Pt grain boundaries.
Internal photoluminescence and lifetime of light-emitting diodes on conductive ZnSe substrates
Authors: Wenisch H. , Fehrer M. , Ohkawa K. , Hommel D. , Prokesch M. , Rinas U. , Hartmann H.
Published: July 2002

We report on the molecular beam epitaxial growth of green (508 nm) and blue (489 nm) light-emitting diodes (LEDs) on conductive ZnSe substrates. The resistivity of the (001)-oriented ZnSe wafers was drastically reduced by a zinc extraction treatment to typical values of 1×10−1 Ω cm. The intensity of an additional orange band around 600 nm observed in electroluminescence depends strongly on the wavelength of the multi-quantum-well emission. This can be explained by absorption and effective re-emission in the substrate material named internal photoluminescence and was confirmed by transmission and photoluminescence experiments with the bare substrates. The LEDs with lifetimes up to 100 hours proofed to be surprisingly stable compared to the structures on undoped ZnSe substrates grown before.
Phenomenological model of ion mixing by thermal spikes and activated processes
Authors: Park Byungwoo
Published: July 2002

In a dense collision cascade of energetic ions with a solid, energy is deposited instantaneously in a very small region, leading to a disordered liquid-like state. An activated process for diffusion of energized atoms is assumed with a temperature distribution T(r,t), considering the effects of thermal conduction into a substrate and temperature-dependent thermal conductivity. A phenomenological model is developed that the mixing rate Dt is correlated with a heat of mixing (analogous to Darken’s relation), and is linearly dependent with nuclear stopping power, instead of a power-law dependence. This resolves the problem of Seitz and Koehler’s 1956 model, and many succeeding publications.
Estimation of effective conductivity of random heterogeneous media by diffusion processes
Authors: Grigoriu Mircea
Published: July 2002

A new Monte Carlo (MC) simulation method is developed for estimating the effective conductivity of random heterogeneous media. The method is based on the Itô formula for differentiable functions of diffusion processes, the similarity of the functional form of the generators of these processes and the operators of the steady state diffusion equation, smooth approximations of the conductivity field of multiphase heterogeneous media, and MC simulation. The article establishes the theoretical framework of the proposed method and gives estimates of the effective conductivity for a one-dimensional random heterogeneous medium with two phases.
Matrix isolation Fourier transform infrared spectroscopic study of energetic nitrogen fluxes applied to fabrication of nitride thin films. Observation of N3 radical and quantitative estimation of matrix-isolated N atoms
Authors: Khabashesku V. N. , Margrave J. L. , Waters K. , Schultz J. A.
Published: July 2002

The formation of stable molecules such as NH3, NO, N2O, and HCN as well as reactive transient species, e.g., NH, NH2 and N3, resulting from the reactions of energetic N atoms with a residual background of H2O and hydrogen in cryogenic solid N2 matrices at temperatures of 12–27 K has been studied in situ by Fourier Transform Infrared spectroscopy (FTIR). Bombardment of the matrix by N and N2 neutrals with estimated energies between 20 and 120 eV produces a concentration of N3 radicals linearly proportional to the beam flux. The lower limit concentration of active N atoms stored in a cryomatrix has been derived from FTIR spectroscopic measurements.
Inverse problem in planar magnetron sputtering
Authors: Kusumoto Yoshiro
Published: July 2002

Mathematical consideration is given to the optimal source distribution which produces a film thickness profile having the least deviation from the flat in axisymmetric planar magnetron sputtering. When the definition domain of the solution is expanded to the Schwartz distribution space, the optimal solution is found to be represented by a finite number of Dirac’s δ-functions, that is, discretized erosion rings of zero width. Numerical analysis yields the bifurcation diagram of the optimal solution. With increasing target radius for a fixed target-substrate distance, the number of δ-functions increases infinitely. A newly bifurcated δ-function first appears at the center of the target remains there for a time, and then begins to move outwards.
Pulsed laser deposition of optical waveguiding strontium barium niobate films
Authors: Zhu Y. Y. , Xiao R. F. , Wong G. K. L.
Published: July 2002

Strontium barium niobate waveguiding thin films were prepared by pulsed laser deposition. The relationship between the film structure and the deposition temperature were analyzed by x-ray diffraction and atomic force microscopy. Waveguiding properties such as refractive index and mode dispersion were examined and a dispersion relation was obtained for the (001)-oriented films.
Structural and electrical properties of p+n junctions in Si by low energy Ga+ implantation
Authors: Parry C. P. , Whall T. E. , Parker E. H. C.
Published: July 2002

Ultrashallow p+n junctions have been formed in silicon by low energy (5.5 keV) Ga+ implantation into n-type substrates. This avoids the use implantation of molecular species such as BF2+ or preamorphization with Ge+ or Si+, which degrade the integrity of p+n junctions in metastably strained SixGe1−x layers. High resolution secondary ion mass spectroscopy measurements indicate an implant peak at less than 10 nm, except for postanneal temperatures above 800 °C, for which severe loss of profile control was observed. Electrical characteristics of the implanted junctions were determined from diode current–voltage measurements and Hall data. At low anneal temperatures, these showed good rectification behavior, with an ideality factor of 1.1±0.1 and a reverse bias leakage of ≈3 μA cm−2 in a relatively large junction area of 5×10−2 cm2. The electrical properties of the p+n junctions were found to be sensitive to implant dose, improving with increasing dose. At 580 °C, implant doses were achieved that were completely activated at levels above previously published Ga equilibrium solubility data. For temperatures of 800 °C, reverse annealing occurred, observed as a reduction in carrier concentration with increasing anneal time and severe profile broadening.
Deep states in silicon on sapphire by transient-current spectroscopy
Authors: Sadoh T. , Matsushita A. , Zhang Y.-Q. , Bai D.-J. , Baba A. , Kenjo A. , Tsurushima T. , Mori H. , Nakashima H.
Published: July 2002

It is demonstrated that deep states in silicon on sapphire (SOS) films can be evaluated by transient-current spectroscopy (TCS). In the TCS spectra, a broad peak extending over 100–200 K was observed for the 6000-Å-thick n-type SOS film. Assuming the value of capture cross section to be 10−15 cm2 and independent of temperature, the density distribution of deep states was estimated. The density distribution shows a peak of 1.2×1012 cm−2 eV−1 at EC−0.25 eV. Raman backscattering spectroscopy was also performed to evaluate the stress in the silicon film. It was concluded that the defects detected by TCS should be caused by the compressive stress of 6.2×108 Pa in the silicon film.
Alternating current susceptibility study of a two-phase nanocomposite Fe88Nd6B6 alloy in the temperature range 77–990 K
Authors: Fang J. S. , Leu M. S. , Chin T. S.
Published: July 2002

Alternating current-initial susceptibility (χac) studies on as-spun amorphous and rapid-thermal-annealed two-phase nanocomposite Fe88Nd6B6 alloy ribbons are performed at 77–990 K. A novel means of quantitatively estimating the amount of the residual amorphous phase in heated ribbons is also derived from the χac measurement. Experimental results indicate that the amount of the residual amorphous phase decreases with an increasing heating temperature. The residual amount is 28.87 vol % after the amorphous ribbons are heated to 773 K, and then becomes undetectable when the temperature reaches heating to 923 K. The amount of the residual amorphous phase can be described as follows: R (in vol %)=0.857T/(T–753)–4.392, where T denotes the heating temperature above 753 K. In addition, the full width at half-maximum of the χac peak corresponding to the Curie temperature of the Nd2Fe14B phase correlates well with the extent of exchange coupling interaction between the hard and soft nanophases. Furthermore, the spin reorientation transition temperature of the Nd2Fe14B nanophase in the two-phase nanocomposite Fe88Nd6B6 alloy shifts to 110 K, i.e., significantly lower than 135 K for the Nd2Fe14B single crystal. This shift is attributed to the exchange coupling effect between the nanophases.
Hydrogenation and annealing behaviors of deep levels in strained CdTe(111)/GaAs(100) heterostructures
Authors: Kim M. D. , Kang T. W. , Kim T. W.
Published: July 2002

Photo-induced-current transient spectroscopy measurements on CdTe/GaAs strained heterostructures grown by molecular beam epitaxy were carried out to investigate the hydrogenation and the annealing behaviors of the deep levels due to the crystal defects. The six hole-traps of the as-grown CdTe epilayer were observed. As the growth temperature and the rapid thermal annealing (RTA) temperature increased, the intensities of the H5 and the H6 peaks due to the crystal defects decreased, and that of the peak related to the shallow impurities increased. After annealing, the H5 and the H6 deep levels did not disappear in CdTe/GaAs heterostructures. These results indicate that the H5 and the H6 deep levels are related to the crystal defects due to the lattice mismatch. While the H6 signal was remarkably affected by the growth temperature of the CdTe epilayer, it was independent of the thickness of the CdTe. These results indicate that the crystallinity of a CdTe epilayer grown on GaAs(100) is improved by RTA, and that the improvement of the CdTe crystallinity is considered to be due to the elimination of the defects which were formed at heterointerface during the growth of the CdTe epilayer.
Magnetic properties of YBa2Cu3Oy crystal with various oxygen annealing conditions
Authors: Watahiki Masaya , Jang Wen-Jye , Tajima Setuko , Murakami Masato
Published: July 2002

It is known that the superconducting properties of YBa2Cu3Oy are strongly dependent on the oxygen content, which affects the anisotropy and the doping level. We measured the magnetic properties of the same YBa2Cu3Oy single crystal in order to systematically study the effect of oxygen annealing condition. The doping level could be controlled simply by changing the annealing temperature. The overdoped samples exhibited a very high irreversibility field of >7 T at 77 K and for the H∥c, although the critical temperature was slightly lower than that of the optimum doped sample. The secondary peak effects are observed for all the samples, however, the source of the peak effect was rather complicated and depended on the annealing conditions.
An atomic force microscopy study of the surface morphology of InP/GaAs heteroepitaxial layers subjected to rapid thermal annealing
Authors: Riesz Ferenc , Vignali C. , Pelosi C. , Rakennus K. , Hakkarainen T.
Published: July 2002

The effect of proximity-cap rapid thermal annealing on the surface morphology of (100) InP epitaxial layers grown on GaAs substrates is studied by atomic force microscopy. Only small roughening was found beside the macroscopic thermal etch pits up to annealing at 940 °C for 10 s. Artifacts in the image formation are identified.
Optimum doping level in a-Si:H and a-SiC:H materials
Authors: Hadjadj A. , St’ahel P. , Roca i Cabarrocas P. , Paret V. , Bounouh Y. , Martin J. C.
Published: July 2002

The changes in the optical and electrical properties of thick a-Si:H and a-SiC:H films doped with diborane are investigated. In situ spectroscopic ellipsometry measurements reveal that, at a ratio of diborane to silane Cg=[B2H6]/[SiH4]<10−3, the optical properties of both materials are not strongly modified by boron doping. However, in the case of a-Si:H films, an improvement of the morphological and optical properties is observed at Cg=0.45×10−3. The existence of an optimum doping level at Cg<10−3 in the case of an a-Si:H p layer is confirmed by the dependence of the open-circuit voltage of a-Si:H based solar cells on the doping level of the p layer.
A simple method for the determination of superlattice band structures
Authors: Maiz F. , Hfaiedh A. , Yacoubi N.
Published: July 2002

A simple method is used in order to calculate the band structure of a nonperiodic multilayer semiconducting heterostructure. It may be used in order to determine the band energy levels and the wave function of each energy, hence the probability density. The thicknesses of the layers may be different and the potential is neither periodical nor symmetrical.
Hole transport across the (Al,Ga)(As,Sb) barrier in InAs–(Al,Ga)(As,Sb) heterostructures
Authors: Brar Berinder , Kroemer Herbert
Published: July 2002

We investigate the current transport across the (Al,Ga)(As,Sb) barrier in InAs–(Al,Ga)(As,Sb) heterostructures and show that the transport mechanism consists of thermionic emission of holes over the valence band barrier. Temperature-dependent I-V measurements are performed on diodes fabricated from three samples with different compositions of the top (Al,Ga)(As,Sb) barrier. Consistent with picture of thermionic hole transport in the valence band, we find that compared to a sample with binary AlSb barriers, the addition of Ga to the barrier leads to an increase in the current flowing across the barrier, while the addition of As causes a decrease in the current across the barrier. From temperature-dependent Hall measurements performed on the samples we extract the pinning position of the Fermi-level at the surface of the diodes and are therefore able to obtain a zero-bias energy band diagram for the diodes. From the temperature dependence of the thermionic currents, we extract energy barrier heights that agree well with the expected energy barrier for thermionic hole transport in the valence band of the three (Al,Ga)(As,Sb) barriers investigated. The voltage dependence of both the diode current and the extracted energy barrier height is shown to be consistent with the thermionic emission of holes over the valence band.
Ablation of volatile films by laser heating of substrates
Authors: Ellegaard Ole , Schou Jo/rgen
Published: July 2002

Thermal ablation of frozen layers of solid nitrogen were studied experimentally by irradiating condensed layers on a quartz crystal microbalance with a nitrogen laser at the standard wavelength 337.1 nm. The nitrogen layer is transparent to the incoming laser light and the ablation (vaporization) is caused by heat transfer from the underlying silver substrate layer. Experimental data are compared to calculated data from a three-layer finite-difference heat conduction model. The model takes into account the strong dependence of material parameters on the temperature. Calculated data are presented for water ice as well. The consequences of thermal ablation of volatile films by laser heating of substrates in existing experiments and methods are discussed.
Improvement of the stability under illumination of a-Si:H films elaborated by ion-beam-assisted evaporation using a hydrogen–argon plasma
Authors: Rinnert H. , Vergnat M. , Marchal G. , Burneau A.
Published: July 2002

Hydrogenated amorphous silicon films were deposited by ion-beam-assisted evaporation using a hydrogen–argon plasma. The influence of the substrate temperature was studied. Light induced photoconductivity decay measurements showed that high stability materials can be obtained under well defined conditions. By combined infrared spectrometry and thermal desorption spectrometry experiments, it was demonstrated that microstructure has a great influence on the stability against light induced defects.
Stoichiometry of the diamond/silicon interface and its influence on the silicon content of diamond films
Authors: Sharda T. , Vaidya A. , Misra D. S. , Bhargava S. , Bist H. D. , Veluchamy P. , Minoura H. , Selvam P.
Published: July 2002

Thin films of diamond were grown by microwave plasma chemical vapor deposition at growth pressures of 10, 20, 40, and 60 Torr keeping the substrate temperature constant at 975 °C. Increase in the growth pressure reduced the size of the plasma ball resulting in an increase in the microwave power density (MPD). The films were characterized by scanning electron microscopy, micro-Raman, and photoluminescence (PL) spectroscopy. A systematic variation was observed in surface morphology and quality of the films. The intensity of the peak at 1.68 eV in the PL spectra of the films, which is assigned to Si impurities was also observed to increase consistently with the MPD. The stoichiometry of the diamond/silicon interface was studied by x-ray photoelectron spectroscopy (XPS) and found to be a sensitive function of the MPD. XPS results showed the formation of nonstoichiometric SiC along with other carbon phases in the initial stages of the growth. A correlation was observed between the composition of the interface and the intensity of the 1.68 eV peak. The above results are explained in terms of the increase in the impingement flux density of atomic hydrogen with the MPD.
Electrical properties of the positive temperature coefficient of resistivity materials with 490 °C critical temperature
Authors: Lin Yen-Yi , Hu Chen-Ti , Chang Horng-Yi , Lin I-Nan
Published: July 2002

This work successfully obtains a positive temperature coefficient of resistivity characteristics with critical temperature (Tc) at around 490 and 420 °C for PbTiO3 (PT), and (Pb0.9Sr0.1)TiO3 (PST) materials, respectively. The tetragonality of these materials are (c/a)PT=1.065 and (c/a)PST=1.046. In addition, the donor level of Y3+ ions in PT materials [(Ed)PT=0.09–0.11 eV] is smaller than that in PST materials [i.e., (Ed)PST=0.29–0.40 eV]. The resistivity jumping ratios of the two materials are similar, that is, (ρmax/ρmin)PT≅102.2 and (ρmax/ρmin)PST≅101.6. Multiple-Tc resistivity-temperature (ρ-T) characteristics are observed, that is ascribed to the formation of an incomplete core-shell microstructure with the Pb-deficient shell not entirely surrounding the stoichiometric core.
Effect of inhomogeneity on quantum well far-infrared lasers
Authors: Xin Zhi-Jun , Rutt H. N.
Published: July 2002

The effect of inhomogeneity on quantum well far-infrared lasers caused by quantum well thickness and composition fluctuations is discussed. Assuming a linear distribution corresponding to systematic shift and a Gaussian distribution corresponding to random deviation and for a modest deviation σ=±3.4 meV (approximately equivalent to a 1 monolayer fluctuation), we have found that the gain is reduced by 20% and 37% and the gain full half width is broadened by 46% and 85%, respectively. Because the broadening mechanism is inhomogeneous and it is much wider than the cavity mode separation, this will lead to multi-mode operation unless selection is provided. It also indicates that if the structural inhomogeneity is small enough the broadening is dominated by the Lorentzian broadening and it is still possible to obtain single mode action due to mode competition. The calculation is based on a rectangular quantum well, but the conclusion is also applicable to all other semiconductor lasers based on multi-quantum well structures.
Stress effects induced in SiGe strained layers by low-temperature ultraviolet-assisted oxidation
Authors: Craciun Valentin , Boyd Ian W. , Andreazza Pascal , Boulmer-Leborgne Chantal
Published: July 2002

Low-temperature (550 °C) ultraviolet-assisted oxidation of epitaxially grown Si0.8Ge0.2 strained layers on (100) Si substrates leads to the formation of nanocrystalline Ge (nc-Ge) particles embedded in a SiO2 layer which exhibit visible photoluminescence with an emission maximum situated at around 2.18 eV. Grazing incidence x-ray diffraction investigations of the remaining SiGe layer which was continually rejected by the advancing oxide layer have shown the presence of the (311) diffraction peak. This implies that the unoxidized SiGe layer has been fractured in many regions misoriented with respect to the initial layer. The displacement of the SiGe (311) peak position recorded for samples which had their grown oxide layer stripped away when compared with as-grown samples indicates the presence of a significant stress in these low-temperature grown oxide layers. This stress is believed to explain both the survival of the nc-Ge particles during prolonged oxidation and the fracturing of the buried SiGe layer.
Studies of two-subband occupied electron gas in modulation-doped In0.52Al0.48As/In0.53Ga0.47As single quantum well by far-infrared modulated photoluminescence
Authors: Dai Y. T. , Fan J. C. , Chen Y. F.
Published: July 2002

We report an investigation of electronic and optical properties of two-subband occupied electron gas confined in Al0.48In0.52As/Ga0.47In0.53As quantum well by far-infrared (FIR) modulated photoluminescence (PL) measurements. We show that, in addition to the characterization of the effective mass and Fermi energy, this technique is a useful tool for the understanding of the luminescence processes. Well-resolved Landau-level splittings were observed in the FIR modulated PL spectrum at a fixed magnetic field. It is found that the PL spectrum involves the transitions between localized holes and electrons at all occupied k states. For the FIR modulated spectrum monitored at the peak position as a function of magnetic field, a strong double cyclotron-resonance signal was observed. The double structure of cyclotron resonance is attributed to the cyclotron transitions in the first and second subbands. The obtained effective masses are found to be considerably heavier than the bulk value by 22% and 46% for the first and second subband, respectively. The heavier effective mass of the second subband is opposite to the behavior measured in the triangular quantum well. We point out that, in order to explain the enhanced masses, both the effects of band nonparabolicity and wavefunction penetration into the barrier material have to be taken into account.
Surface dopant concentration monitoring using noncontact surface charge profiling
Authors: Roman P. , Staffa J. , Fakhouri S. , Brubaker M. , Ruzyllo J. , Torek K. , Kamieniecki E.
Published: July 2002

This study is concerned with variations of the concentration of active boron dopant in the near surface region of silicon wafers. Boron can be deactivated by pairing with hydrogen or metals, particularly Cu and Fe, all of which may originate from the surface polishing process. The temperature dependence of boron activation is studied using the surface charge profiling method. Based on the determined activation energy of 1.28 eV it was concluded that in the p-type wafers used in this study initially observed boron deactivation was dominated by its interaction with hydrogen introduced during wafer polishing.
The coexistence of pressure waves in the operation of quartz-crystal shear-wave sensors
Authors: Reddy S. M. , Jones J. P. , Lewis T. J.
Published: July 2002

It is demonstrated that an AT-cut quartz crystal driven in the thickness-shear-wave mode and typically used as a sensor to monitor the viscoelastic shear-wave properties of a fluid also produce longitudinal pressure waves. Unlike the shear wave, these waves are capable of long-range propagation through the fluid and of reflection at its boundaries, notably at an outer fluid–air interface. They introduce a component into the measured electrical impedance and resonance frequency shift of the crystal, which reflects the setting up of cyclic pressure-wave resonances in the fluid. This has important implications for the practical employment of these crystal as sensors. Under appropriate conditions, as demonstrated for water and n-octane, it is possible to determine the propagating properties of sound waves in a fluid simultaneously with the viscoelastic shear-wave properties. These experiments are supported by an analysis of the appropriate hydrodynamic equations for waves in the crystal–fluid system, which predicts electrical characteristics in close agreement with those found experimentally.
Scanning tunneling microscope induced nanostructuring of a Si(111)/Ag(3×3)R30° surface
Authors: Riehl-Chudoba M. , Richter Wo. , Gasparov V. A.
Published: July 2002

An atomically flat Si(111)/Ag(3×3)R30° surface has been modified using a scanning tunneling microscope in ultrahigh vacuum. Mesoscopic pits have been created by applying negative voltage pulses to the sample, while at opposite voltage polarity mounds were formed. Moreover, lines could be written by moving the scanner at elevated voltages. The threshold voltage for pit formation increases almost linearly with the distance of the tip to the surface and drops to a value below 2 V for the closest approach. At sufficiently high voltages the depth extends beyond the silver layer height. The lateral pit size is well below 8 nm and can be reduced to values between 2 nm and 5 nm for voltages slightly above the threshold. Even selective top layer Ag atom removal has been achieved.
Effects of discrete trap levels on carrier transport in organic electroluminescent devices
Authors: Xu Ji-Hai , Shen Jun
Published: July 2002

The doping effect on the current–voltage characteristics of organic electroluminescent devices is studied based upon the double-carrier injection theory. The equations for the currents in the trap-charge-limited region are derived by assuming a doping-induced discrete trap level in the energy gap. Possible explanations of current experimental observations are proposed based on the equations.
Magnetic force imaging of domain structures for a (Pr/Nd)FeB alloy
Authors: Al-Khafaji M. A. , Jones D. G. R. , Rainforth W. M. , Gibbs M. R. J. , Davies H. A. , Harris I. R.
Published: July 2002

Magnetic force microscopy (MFM) has been used to image the magnetic stray field from a (Pr/Nd)FeB hard magnetic alloy containing α-Fe regions. Images were acquired with three different sensing tips (CoCr, NiFe and Fe/SiO2 coated Si). The MFM images were compared to optical Kerr effect images from the same area. Kerr images of the (Pr,Nd)2Fe14B grains were similar to MFM images obtained using the CoCr tip, revealing a regular open-flux magnetic structure for those grains imaged on a plane perpendicular to the easy direction and stripe domains on those grains imaged on a plane containing the easy direction. MFM images indicated that the α-Fe particles were single domain, while Kerr images suggested a substructure within α-Fe. The low coercivity NiFe and Fe/SiO2 sensing tips provided images only of the magnetic domain boundaries. The difference in image characteristics between the CoCr and NiFe and Fe/SiO2 tips is believed to be associated with the extent to which the initial tip magnetisation is changed by the stray field from the sample.
Electro-optic and photorefractive two-beam coupling properties of lead barium niobate crystals
Authors: Liu Alice , Hesselink Lambertus , Lee Myeongkyu , Feigelson Robert S.
Published: July 2002

Lead barium niobate crystals (Pb1−xBaxNb2O6) are attractive photorefractive materials for holographic data storage. The electro-optic coefficients r33 and r13 are determined using a Mach–Zehnder interferometer setup, and are measured to be 160±20 and 32±4 pm/V, respectively. The angular dependence of the photorefractive two-beam coupling gain is also investigated, and the products of the electro-optic coefficients and the electron-hole competition factor rζ are determined and compared with the electro-optic coefficients obtained using the interferometric method. It is found that the electron-hole competition factor is at least 0.7, indicating that the photorefractive process is mostly due to electrons. The effective trap densities at several wavelengths are also determined from the gain dispersion measurements, and are found to be 1.3–2.2×1016 cm−3.
Simulation of a submicron 3C–SiC ballistic diode based on the Lei–Ting hydrodynamic balance equations
Authors: Weng X. M. , Cui H. L.
Published: July 2002

Numerical simulation of a submicron 3C–SiC n+−n−n+ ballistic diode, based upon the recently developed Lei–Ting hydrodynamic balance equations, is presented. The electron velocity, electron density, electron temperature, and the electrical potential distributed along the diode are obtained under different bias voltages.
A Monte Carlo approach for incorporation of memory effect in switched gate bias experiments
Authors: Subbaraman Shaila , Sharma D. K. , Vasi J. , Das A.
Published: July 2002

This article presents a Monte Carlo approach to simulate the dispersive transport of holes/H+ ions in silicon-dioxide using simplified formalism of continuous time random walk theory given by Montroll and Scher [E. W. Montroll and H. Scher, J. Stat. Phys. 9, 101 (1973); H. Scher and E. W. Montroll, Phys. Rev. B 12, 2455 (1975)]. It has been shown that the simulated results of this approach match quite well with the theoretical predictions for a value of dispersion coefficient, α, =0.5 which validates our Monte Carlo simulator. We have used this simulator to resolve the memory effect observed by Saks et al. [N. S. Saks, D. B. Brown, and R. W. Rendell, IEEE Trans. Nucl. Sci. 38, 1130 (1991)] in their simulations of switched gate bias experiments in context with the hydrogen ion transport model for the buildup of radiation-induced interface states. We have accounted for the memory effect, which is related with the dwell time of an ion at the last hopping site just before field switching, by treating the first hop after field switching differently from all other hops. We have shown that the memory effect can be incorporated by modifying the value of α only for the first hop after field switching depending on the dwell time at the last hopping site just before field switching. We have found that a logarithmic relationship between the two gives a very good match between the simulated and the experimental results of half-time (the time required to build 50% of saturated interface states) versus time of field switching, tswitch, relationship.
High dopant and carrier concentration effects in gallium aluminum arsenide: Densities of states and effective intrinsic carrier concentrations
Authors: Bennett Herbert S.
Published: July 2002

The changes in carrier densities of states and effective intrinsic carrier concentrations due to high dopant and carrier concentration effects in Ga1−yAlyAs are calculated at 300 K for donor densities between 1016 and 1019 cm−3 and for acceptor densities between 1016 and 1020 cm−3. The mole fraction of AlAs, y, varies between 0.0 and 0.3 in these calculations. The Klauder self-energy method (fifth level of approximation) is used to describe the effects of carrier–dopant ion interactions on the conduction and valence bands and their densities of states. The effects of carrier–carrier interactions (majority-carrier exchange and minority-carrier correlation) are calculated by extending the work of Abram et al. to include both finite wave numbers and finite temperatures. This article is the first reported use of one theory to treat both sides of the Mott transition. Namely, solutions of the very nonlinear integral equations that result from the Klauder fifth-level approximation are obtained for low-dopant densities that involve both bound and continuum states and for high-dopant densities that involve primarily continuum states. The results show that quantitatively significant changes occur in densities of states, locations of band edges, and effective intrinsic-carrier concentrations from the values usually used by researchers, particularly, from those values found in most device simulators. These results are important for predictive simulations of optoelectronic, microwave, and electronic devices because such computer simulations require physically reasonable values of the changes in the locations of band edges, densities of states, and effective intrinsic carrier concentrations.
Growth mechanisms in atomic layer epitaxy of GaAs
Authors: Arès R. , Watkins S. P. , Yeo P. , Horley G. A. , O’Brien P. , Jones A. C.
Published: July 2002

We present a study of the different mechanisms governing the growth of GaAs by atomic layer epitaxy (ALE). The gallium precursors such as trimethylgallium (TMGa), triethylgallium (TEGa), and trisneopentylgallium (TNPGa) were characterized and compared during ALE by in situ reflectance difference spectroscopy (RDS). A fundamental difference is observed in the RDS behavior after the alkyl exposure between self-limiting and non-self-limiting precursors. A transient is observed during the purge following alkyl exposure for both self-limiting sources, (TMGa and TNPGa), and is absent with TEGa. This is attributed to the presence of methyl radicals when using self-limiting precursors. Atomic force microscopy measurements of the surface morphology show that a non-self-limiting surface prepared with TEGa has droplets forming on it. In contrast, TMGa and TNPGa show well-defined terraces without droplets. RDS real-time measurements show that Ga incorporating on the c(4×4) surface first reacts with the As in the top layer with a process involving both desorption of As and rearrangement of the remaining top As. We used these measurements to provide a detailed structural model of the process of Ga incorporation, and thus, solve the problem of how an As-rich surface with more than 1 ML coverage can yield 1 ML of material per ALE cycle.
Monte Carlo simulation of an amorphous hydrogenated silicon film deposition from a gas jet activated by an electron beam
Authors: Vasenkov A. V. , Sharafutdinov R. G. , Skrinnikov A. V.
Published: July 2002

A Monte Carlo hybrid model was developed to simulate film deposition from a gas jet activated by an electron beam. This model includes calculation of the interaction of an electron beam with a gas jet and the transport of particles from the activation zone to the substrate. In the first stage, the three-dimensional distribution of monosilane’s dissociation rate in a mixture of Ar with 10% SiH4 was determined. Using this distribution, absolute deposition rates for an amorphous hydrogenated silicon film were calculated and they compared favorably with experimental data.
Temperature dependence of the elastic constants of solid and liquid Cd0.96Zn0.04Te obtained by laser ultrasound
Authors: Queheillalt Douglas T. , Wadley Haydn N. G.
Published: July 2002

Laser based ultrasonic techniques provide a potential noninvasive sensing methodology to monitor the solid–liquid interface shape and position during crystal growth provided, the temperature dependence of the ultrasonic velocity for both solid and liquid phases are known a priori. A laser ultrasonic approach has been used to measure the ultrasonic velocity of single crystal solid and liquid Cd0.96Zn0.04Te as a function of temperature. The longitudinal wave velocity was found to be a strong monotonically decreasing function of temperature in the solid and liquid phases and exhibited an abrupt almost 50% decrease upon melting. Using both longitudinal and shear wave velocity values together with data for the temperature dependent density allowed a complete evaluation of the temperature dependent single crystal elastic stiffness constants (C11, C12, C44) for the solid and the adiabatic bulk modulus (KS) for the liquid. In addition, evaluation of the thermoelastic modulus (MT) has given insight on the impact of dislocation generation from excessive thermal gradient induced stresses during growth. The simplicity of making high temperature laser ultrasonic measurements together with the large longitudinal wave velocity difference between the solid and liquid phases suggests a laser based ultrasonic sensor has significant potential for sensing the solid–liquid interface during the growth of CdTe alloys.