bluuebunny/crossref_metadata_2025 huggingface dataset:
Active mode locking at 50 GHz repetition frequency by half-frequency modulation of monolithic semiconductor lasers integrated with electroabsorption modulators
Authors: Sato Kenji , Kotaka Isamu , Kondo Yasuhiro , Yamamoto Mitsuo
Published: July 2002

Active mode locking achieved at a 50 GHz repetition frequency by modulation at half (25 GHz) the cavity resonance frequency using a monolithic mode-locked InGaAsP laser integrated with an electroabsorption modulator is described. A pulse width of around 3 ps and a high suppression ratio of more than 33 dB of the intensity modulation at the driving frequency are obtained.
Actinometric thermoluminescence response of KCl1−xBrx:Eu2+ mixed crystals
Authors: Castañeda B. , Aceves R. , Piters T. M. , Barboza-Flores M. , Meléndrez R. , Pérez-Salas R.
Published: July 2002

Europium-doped KCl1−xBrx mixed crystals have been studied as a selective ultraviolet (UV) detector material in the actinometric region (200–300 nm). Thermoluminescence glow curves of room-temperature UV-irradiated samples were analyzed as a function of composition x and irradiation wavelength. A thermoluminescence enhancement, relative to the pure KCl and KBr end components is found; for 210–300 nm irradiation the KCl40Br60:Eu2+ solid solution presents a significant increase in thermoluminescence efficiency.
Epitaxial superlattices of TiAg alloy/MgO grown on MgO(001)
Authors: Kado Tetsuo
Published: July 2002

High crystalline quality epitaxial metal/insulator superlattices composed of Ti1−xAgx (0.03<x<0.06) and MgO have been grown on MgO(001) substrates at 273 K by electron beam evaporation in ultrahigh vacuum. X-ray diffraction measurements and cross-sectional high-resolution transmission electron microscopy measurements revealed that the crystal structures of the TiAg alloy and MgO in the superlattices were tetragonal and that the superlattices were so-called strained-layer superlattices. The x-ray rocking curve was measured around one of satellite peaks in the middle angle range, and showed the full width at half-maximum of the curve was 0.04°–0.05°.
Twinning orientation in YBa2Cu3O7−x films deposited on YAlO3 substrates
Authors: Dubourdieu C. , Sénateur J. P. , Thomas O. , Weiss F. , Thrane B. P. , Brunel M.
Published: July 2002

An x-ray diffraction study of YBa2Cu3O7−x (YBCO) films deposited on YAlO3 (001) substrates by metal organic chemical vapor deposition is reported. The twinning orientation of YBCO on YAlO3 is compared to the one observed for films deposited on MgO (001), SrTiO3 (001), and LaAlO3 (012) substrates. The φ scans performed on hhl type reflections and the grazing incidence x-ray diffraction on the 020/200 reflections of YBCO reveal a particular epitaxial relationship on YAlO3: there is only one twinning direction relatively to the substrate. The [110] and [11̄0] directions of YBCO are aligned with the [010] direction of YAlO3. This twinning geometry may be explained by coherency strains.
Relaxation charge anomalies in the charge–voltage characteristics of alternating-current thin-film electroluminescent devices
Authors: Keir P. D. , Le H. , Thuemler R. L. , Hitt J. , Wager J. F.
Published: July 2002

Anomalous charge–voltage (Q–V) characteristics are observed for several types of alternating-current thin-film electroluminescent (ACTFEL) devices. These Q–V curves are anomalous because conduction charge flows in these devices exclusively during the portion of the wave form in which the applied voltage is constant, at its maximum value; this kind of conduction charge is denoted relaxation charge. In a normal ACTFEL device, most of the conduction charge flows during the portion of the wave form in which the applied voltage increases with time. The anomalous Q–V characteristics are attributed to insulator leakage for the devices tested. Simulation shows that such anomalous behavior may arise from either insulator or phosphor leakage.
Optical effects of interdiffusion in GaAs/AlAs heterostructures: Atomic scale calculations
Authors: Koiller Belita , Martins A. S. , Chacham H.
Published: July 2002

We investigate the effects of atomic interdiffusion in the optical properties of AlAs/GaAs quantum wells through tight-binding model calculations. Ensembles of supercells, each cell containing up to ∼104 atoms with periodic boundary conditions, are used to simulate the heterostructures. The oscillator strength f of optical transitions at the absorption threshold is calculated as a function of the quantum-wells width W and the diffusion length L. As L increases, f undergoes a discontinuous transition to zero, indicating indirect-gap behavior for L larger than a critical value of the diffusion length. A unified behavior of f as a function of L/W1.7 is found. This permits relating optical properties to structural properties through simple fitting formulas.
N-type zinc phosphide grown by molecular beam epitaxy
Authors: Suda Toshikazu , Kakishita Kazuhiko , Sato Hiroyuki , Sasaki Keisuke
Published: July 2002

We report on the results of Hall effect and photoluminescence (PL) in n-type Zn3P2 grown by molecular beam epitaxy. The Zn3P2 thin films indicated n-type conductivity instead of the usual p-type conductivity due to a strong self-compensation effect with Hall mobility and carrier concentration of 3–7×103 cm2/Vs and 3–9×1010 cm−3 at room temperature, respectively. Donor levels of 0.01 and 0.73 eV from the conduction band were identified by resistivity and Hall effect measurements. The PL spectra show donor-acceptor pair emission near 1.41 eV at 20 K ascribed to an acceptor level of 0.26 eV from the valence band.
Properties of GaN grown at high rates on sapphire and on 6H–SiC
Authors: Fischer S. , Wetzel C. , Hansen W. L. , Bourret-Courchesne E. D. , Meyer B. K. , Haller E. E.
Published: July 2002

Thick GaN films were deposited with growth rates as high as 250 μm/h by the direct reaction of ammonia and gallium vapor at 1240 °C. The characteristics of our films are comparable to those of typical thin films grown by metal organic chemical vapor deposition or molecular beam epitaxy. Grown under identical conditions, films on (0001) sapphire and on (0001) 6H–SiC were compared in terms of their structural and optical properties. Considering x-ray rocking curve full width at half-maximum (FWHM: 420 arcsec), photoluminescence linewidths of the excitons (FWHM: 3 meV at 6 K and 100 meV at 300 K), free electron concentration, defect related luminescence, and the homogeneity of these properties, we find superior values for films grown on SiC. For both substrate materials we find an optimum growth rate window of 40–80 μm/h.
Novel class of low molecular-weight organic resists for nanometer lithography
Authors: Yoshiiwa Motoko , Kageyama Hiroshi , Shirota Yasuhiko , Wakaya Fujio , Gamo Kenji , Takai Mikio
Published: July 2002

A novel class of low molecular-weight organic resist materials for nanometer lithography, 1,3,5-tris[4-(4-toluenesulfonyloxy)phenyl]benzene (TsOTPB) and 4,4′,4″-tris(allylsuccinimido) triphenylamine (ASITPA), was designed and synthesized. TsOTPB with a glass-transition temperature (Tg) of 64 °C and ASITPA with a Tg of 80 °C were found to function as positive and negative resists, respectively, enabling the fabrication of 150 and 70 nm line patterns on exposure to an electron beam at 50 keV.
Effect of thin HgTe layers on dislocations in HgCdTe layers on Si substrates
Authors: Okamoto T. , Saito T. , Murakami S. , Nishino H. , Maruyama K. , Nishijima Y. , Wada H. , Nagashima M. , Nogami Y.
Published: July 2002

We have found the effects of HgTe layers on dislocations of (111)B HgCdTe layers grown on Si substrates by metalorganic vapor phase epitaxy. The dislocations in HgCdTe layers were reduced by inserting thin HgTe layers between HgCdTe and CdTe buffer layers. Using this method, the dislocation density of 2.3×106 cm−2 was obtained, which is less than quarter that of HgCdTe layers without HgTe.
The minority carrier lifetime of n-type 4H- and 6H-SiC epitaxial layers
Authors: Kordina O. , Bergman J. P. , Hallin C. , Janzén E.
Published: July 2002

The minority carrier lifetime has been measured on n-type 6H- and 4H-SiC epitaxial layers. We observe inherently longer lifetimes in 4H layers compared to 6H-SiC layers. A value as high as 2.1 μs has been measured at room temperature in 4H-SiC, however, large variations may be observed over the surface. The lifetime increases with temperature and at a typical operating temperature of a device the lifetime is close to 5 μs. The lifetime appears to be correlated with the morphology of the epitaxial film showing that the lifetime limiting defect may be related to a crystalline imperfection. A strong correlation can also be seen with the thickness of the epitaxial layers.
Including getter effect in a numerical contrast calculation for micrographs: A numerical contrast calculation for electron beam induced current at gettered dislocations
Authors: Mohr H.
Published: July 2002

Electron beam induced current (EBIC) microscopy is a very powerful technique for revealing the electrical activity of defects in semiconductors. Gettering of impurities at defects has been observed previously after certain sample heat treatments, which resulted in altered contrast patterns and line scan profiles. Getter effects have been included in a numerical or analytical contrast simulation which employ the Monte Carlo method for generating the carrier distribution. We compare the findings with an observed white contrast at misfit dislocations in EBIC micrographs.
Vertical-cavity surface-emitting lasers with low-ripple optical pump bands
Authors: Knopp K. J. , Christensen D. H. , Hill J. R.
Published: July 2002

We have used multilayer mirror optimization methods to enhance the coupling of pump light into vertical-cavity surface-emitting lasers (VCSELs). With previously reported devices, pump light was coupled into VCSEL cavities through interference notches in the mirror reflectance spectrum. This approach is sensitive to temperature dependent reflectance spectrum shifts. We have created devices with a wide pump-band window of low reflectance. We report the simulation, growth, and optically pumped lasing of such optimized low-ripple VCSELs. Further, broadband pump windows open the possibility of spectrally-broad optical pumps and they eliminate need for costly tunable pump lasers.
Surface-normal 3×3 non-blocking wavelength-selective crossbar using polymer-based volume holograms
Authors: Zhou Charles , Chen Ray T.
Published: July 2002

We present a 3×3 surface-normal wavelength-selective crossbar using polymer-based volume holograms. A prototype device is demonstrated using the center wavelength of 775 nm and Δλ=10 nm. Employment of 1/4-pitch graded-index rod lenses reduces the required nine wavelengths to three while maintaining the 3×3 interconnects. The diffraction efficiencies of 75%, 83%, and 75% are experimentally confirmed for wavelength 765, 775, and 785 nm, respectively. Surface-normal configuration eliminates the conventional edge-coupling scheme which is vulnerable in a harsh environment. A 3×3 crossbar is demonstrated with a two-way system insertion loss less than 3 dB and channel-to-channel cross talk less than 20 dB.
Preparation of cubic boron nitride thin film by the helicon wave plasma enhanced chemical vapor deposition
Authors: Kim Seon-Hyo , Kim Ig-Hyeon , Kim Ki-Sung
Published: July 2002

Cubic boron nitride (c-BN) film was deposited on Si(100) substrate using the chemical vapor deposition process assisted by high density plasma of Helicon wave with Borazine (B3N3H6) precursor. It was found that the bombardment of ions with high flux and energy onto the film was necessarily required for synthesizing a c-BN film. Increasing a negative rf bias on the substrate increased the formation fraction of c-BN in the film. A nearly pure c-BN phase was synthesized at the conditions of plasma density in the reactor and rf substrate bias, above 1011 cm−3 and −350 V, respectively. The phase identification of BN film was carried out by the transmission electron microscopy as well as Fourier transformed infrared spectroscopy. The infrared spectra for c-BN film synthesized at the rf bias of −350 V appeared at 1093 cm−1 with a strong single peak, which is close to a value for the characteristic vibration mode of bulk c-BN (1065 cm−1). The c-BN in the film was also confirmed and found to be a fine poly-crystalline with the grain sizes ranging from 200 to 400 Å.
Heteroepitaxial growth of β-SiC thin films on Si(100) substrate using bis-trimethylsilylmethane
Authors: Bahng Wook , Kim Hyeong Joon
Published: July 2002

A non-toxic and non-flammable organosilicon source having alternate Si–C bonding structure, bis-trimethylsilylmethane [C7H20Si2], was first used to deposit epitaxial β-SiC films at low growth temperature by chemical vapor deposition. Epitaxial β-SiC films were successfully grown on carburized Si(100) substrates at temperatures as low as 1100 °C, although the carburized buffer layer was a well-oriented, (100) polycrystalline film. Transmission electron microscopy analysis revealed that the films contain twins, stacking faults, and antiphase boundaries. Without the carburized buffer layer, highly (111) preferred oriented β-SiC films were grown by increasing the growth temperature.
Application of an InSb flat crystal with elliptically shaped modulated structures to x-ray point focusing
Authors: Chang W. Z. , Uschmann I. , Förster E. , Rothuizen H.
Published: July 2002

Obtained experimental results demonstrate that x rays can be focused to a point by using one flat InSb crystal with elliptically shaped modulated structures based on the Bragg and Fresnel diffraction principles. Ti Kα radiation and InSb (111) reflections are used to focus a 20-μm source size to a point of 9 μm, which is in good agreement with predictions from the wave-optics approach. The quality of the modulated structure fabrication is judged by the shape of the focal spot, which indicates no visible aberration effects.
An optical heterodyne experiment for sensitive detection of laser induced photocarrier gratings in semiconductors
Authors: Strauss J. , Hundhausen M. , Ley L.
Published: July 2002

We measure the diffraction of a cw probe laser beam from an absorption index grating set up in a thin film semiconductor by illumination with a moving optical interference fringe pattern. Taking advantage of the heterodyne detection principle whereby the weak diffracted laser beam is brought to interfere with the probe beam we are able to work with diffraction efficiencies as low as 10−14. When applied to amorphous hydrogenated silicon the diffraction efficiency depends in a characteristic way on an applied external electric field. We identify two physical mechanisms for the diffraction, namely a periodic temperature profile related to the absorption of the laser grating and the quadratic electro-optical effect.
Optimization of power extraction in a high-power soliton fiber ring laser containing a chirped fiber grating
Authors: Tamura K. , Komukai T. , Nakazawa M.
Published: July 2002

Power extraction from a high-power soliton fiber ring laser containing a chirped fiber grating is investigated with a variable output coupler. Comparisons are made between the intracavity pulse and the rejection port output pulse. It is shown that a good design for extracting high quality pulses at high power is to insert an intracavity filter to suppress spectral sideband generation and to take the output from the rejection port. The results should apply in general to soliton fiber lasers. Gain saturation is shown to be the main mechanism in the laser that compensates for changes in cavity loss.
Structural investigation of short period GaInAs/InP superlattices
Authors: Emerson D. T. , Shealy J. R.
Published: July 2002

Structural properties of short period GaInAs/InP superlattices prepared by organometallic vapor phase epitaxy are investigated with Raman scattering. The observation of as many as five orders of folded longitudinal acoustic doublets attests to the structural quality of the superlattices. Well and barrier relative layer thicknesses and superlattice period are estimated from analysis of doublet intensity and position, respectively. Examination of the optic modes reveals that the well is comprised of two separate layers: a GaInAsP layer under tension near the barrier/well interface and a GaInAs layer under compression near the well/barrier interface. Finally, the Raman results are confirmed by simulation of double crystal x-ray diffraction spectra from the short period structures.
Scanning tunneling spectroscopy studies of the Au–H-terminated Si interface
Authors: Gheber L. A. , Hershfinkel M. , Gorodetsky G. , Volterra V.
Published: July 2002

The interface of hydrogen-terminated Si(111) and evaporated Au overlayers was investigated by scanning tunneling spectroscopy. It is shown that the first monolayers of Au atoms chemically bind to the silicon surface and no Schottky barrier is formed at this initial stage of gold growth on silicon. Deposited Au films of higher thickness present rectification in the I–V characteristics. Evidence for migration of Si atoms up to the surface of a 50 Å Au film is presented.
In situ growth and superconducting properties of YNi2B2C thin films
Authors: Andreone A. , Iavarone M. , Vaglio R. , Manini P. , Cogliati E.
Published: July 2002

Thin films of the recently discovered intermetallic superconducting compound YNi2B2C have been grown in situ using a magnetron sputtering technique. High quality, c-axis oriented, films were obtained on MgO substrates held at about 800 °C during the sputtering process. The best films showed maximum Tc=15.25 K (onset), a transition width of ≊0.3 K, a room-temperature resistivity of 53±3 μΩ cm, a residual resistivity ratio up to ≊5.4, a low-temperature critical current Jc≊105 A/cm2 and Bc2≥6 T. Superconducting films were also obtained on sapphire and LaAlO3 substrates.
GaAs/AlGaAs quantum wires fabricated by SiO2 capping-induced intermixing
Authors: Pépin A. , Vieu C. , Schneider M. , Planel R. , Bloch J. , Assayag G. Ben , Launois H. , Marzin J. Y. , Nissim Y.
Published: July 2002

We demonstrate that selective intermixing of GaAs/AlGaAs quantum well heterostructures induced by SiO2 capping and subsequent annealing can be spatially localized on a length scale compatible with the lateral confinement of carriers into quantum wires. Low temperature optical spectroscopy measurements including linear polarization anisotropy analysis show evidence of the formation of one-dimensional subbands. A mechanism involving the ability of the thermal stress field generated in the heterostructure by the patterned SiO2 film to pilot the diffusion of the excess Ga vacancies, which are responsible for the enhanced interdiffusion under SiO2 is suggested to account for the high lateral selectivity achievable with this novel process.
Blue photoluminescence of CdAl2S4 single crystal
Authors: Jin Moon-Seog , Kim Wha-Tek
Published: July 2002

The CdAl2S4 single crystal grown by the chemical transport reaction method in an excess sulfur atmosphere showed the intensive blue photoluminescence (PL) peaked at 462 nm at 280 K. The CdAl2S4 single crystal had the defect chalcopyrite structure, and the optical energy gap of the single crystal with the direct band structure was found to be 3.398 eV at 280 K, and to be 3.577 eV at 17 K.
Stimulated emission at 300 K from photopumped GaN grown by plasma-assisted molecular beam epitaxy with an inductively coupled plasma source
Authors: Gluschenkov O. , Myoung J. M. , Shim K. H. , Kim K. , Figen Z. G. , Gao J. , Eden J. G.
Published: July 2002

GaN epilayers have been grown on basal plane (0001) sapphire by plasma-assisted molecular beam epitaxy (MBE) with a novel, inductively coupled nitrogen plasma source. Films grown at 700 °C generate stimulated emission at 300 K when optically pumped in vertical geometry with ∼3.5 eV (λ=355 nm) photons. The extrapolated pump power threshold is ∼3.6 MW cm−2 which corresponds to an absorbed value of 700 kW cm−2 and a peak carrier number density of ∼4×1019 cm−3.
Room temperature electro-optic effect in CdHgTe multiple quantum well heterostructures at 1.5 μm
Authors: Mula Guido , Pelekanos N. T. , Gentile P. , Magnea N. , Pautrat J. L.
Published: July 2002

We present a study of the room temperature electromodulation properties of CdHgTe heterostructures as a base component for a photorefractive device operating at 1.5 μm. We reach absorption modulation as high as 200%, in the near-gap transparent region of the spectrum, by application of a reverse bias of the order of 100 kV/cm. We observe the transition from the Franz–Keldysh effect in 200 Å wide wells to the quantum confined Stark effect in 90 Å wells. The calculated diffraction efficiency in a suitable photorefractive device would be of the order of 8×10−4 in both cases.
Surface processing with non-Gaussian beams
Authors: Kaplan A. F. H.
Published: July 2002

The heat transfer problem of a moving source of heat scanning over a semi-infinite solid is of considerable importance for surface processing with high energy beams. The analytical solution of a moving Gaussian beam is extended by superposition of several Gaussian beams in order to approximate real beam shapes, thus providing high flexibility and accuracy. By superimposing two concentric beams, a real intensity profile with a central minimum was approximated with an accuracy of 13% compared to 35% for the conventional Gaussian beam approach. For the corresponding temperature field, an error of 57% for the depth and 17% for the width of the melt pool was eliminated.
Measuring electron transfer in real space in biased asymmetric double quantum wells using far-infrared cyclotron resonance
Authors: He X.-F. , Ryu S. R. , Li W. J. , Haetty J. , Petrou A. , McCombe B. D. , Schaff W.
Published: July 2002

We report measurements of electron transfer in real space in GaAs/AlxGa1−xAs asymmetric double quantum wells under an electric field by far-infrared cyclotron resonance (CR). Due to nonparabolicity, the asymmetric quantum structure results in well-resolved CR lines, which allow contactless measurements of the electron density in each well. Our results show that electrons tunnel through the barrier from one well to the other at the level anticrossing of their ground states.
Spectrally and temporally resolved gain dynamics in mode locked semiconductor quantum well lasers
Authors: Miltényi P. , Koch M. , Hofmann M. , Jung H. , Elsässer W.
Published: July 2002

We perform measurements of the differential gain of modelocked semiconductor quantum well lasers over a wide spectral range employing a modified “electrical pump-optical probe” technique. We find that the two-dimensional density of states expresses itself significantly in the spectral dependence and value of the differential gain. Contributions of the second electronic level are observed in the high-energy range with an increasing in the differential gain, whereas a slightly decreasing differential gain with decreasing wavelength is observed in spectral regimes, where only the first quantized level contributes.
Speed photodetectors based on amorphous and microcrystalline silicon p–i–n devices
Authors: Vieira M.
Published: July 2002

A sensitive optical speed sensor has been developed utilizing the transient lateral photoeffect induced in a Si thin-film p–i–n photodetector by a moving light spot. The local time dependent illumination changes the lateral photocurrent and modulates the potential barrier across the junction allowing the sensing of the light spot velocity. The performances of an amorphous (a-Si:H) and a microcrystalline (μc-Si:H) silicon-based sensor will be compared and discussed taking into account the differences in material properties and device geometry. The μc-p–i–n structure has the advantage of faster response, high current capability, and excellent stability under illumination when compared with the amorphous counterpart.
Noise in wavelength conversion using four-wave mixing in semiconductor optical amplifiers
Authors: Martelli F. , Mecozzi A. , D’Ottavi A. , Scotti S. , Spano P. , Dall’Ara R. , Eckner J. , Guekos G.
Published: July 2002

We measure the pump power dependence of efficiency and signal-to-background ratio in wavelength conversion using four-wave mixing in semiconductor optical amplifiers, for different lengths of the device. We show that the efficiency for a 1-mm-long amplifier is about 40 dB larger and the signal-to-noise ratio is about 23 dB larger than for a 0.25 mm amplifier. We also show that, unlike the efficiency, for constant ratio between pump- and signal-power, the signal-to-background ratio monotonically increases with the pump power. We show theoretically that this behavior holds for any range of pump-power values.
Oxide precipitation at silicon grain boundaries
Authors: Schroer E. , Hopfe S. , Werner P. , Gösele U. , Duscher G. , Rühle M. , Tan T. Y.
Published: July 2002

Oxygen precipitates at various grain boundaries in crystalline silicon, formed after prolonged high temperature annealing, grow within a narrow size distribution. This narrow size distribution appears to depend on the specific grain boundary. On the basis of this observation a model is derived which is based on the energy balance between grain boundary energy, Si/SiO2 interface energy, and an additional term describing the energy of the ledges of the faceted precipitates. This model predicts an energy minimum for a defined size of the precipitates.
Laser induced implanted oxide (Ll2Ox) and polycrystalline silicon film simultaneously fabricated by excimer laser irradiation
Authors: Park Cheol-Min , Min Byung-Hyuk , Yoo Juhn-Suk , Jun Jae-Hong , Choi Hong-Seok , Han Min-Koo
Published: July 2002

A method to form the gate oxide and recrystallize the polycrystalline silicon (poly-Si) active layer simultaneously is proposed. During the irradiation of excimer laser, the poly-Si film is recrystallized while the oxygen ion impurities injected into the amorphous silicon (a-Si) film are activated by laser energy and react with silicon atoms to form SiO2. Our experimental results show that a high quality oxide and a poly-Si film with fine grain have been fabricated successfully by the proposed method. High quality interface between oxide and poly-Si films has also been obtained.
A substrate-mode holographic collimating and beam shaping element for laser diodes
Authors: Chang Jen-Tsorng , Su Der-Chin , Huang Zhi-Xian , Huang Yang-Tung
Published: July 2002

A new type of substrate-mode holographic collimating and beam shaping element for laser diodes is presented. Techniques and design considerations are described. A sample is fabricated and its function is demonstrated. It has many merits, such as easy fabrication, low cost, compactness of monolithic structure, and is easily used.
Novel post-etching treatment of small windows in oxide for selective epitaxial growth
Authors: Maes J. W. H. , Caro J. , Visser C. C. G. , Zijlstra T. , van der Drift E. W. J. M. , Radelaar S. , Tichelaar F. D. , Fakkeldij E. J. M.
Published: July 2002

A novel post-etching treatment of small windows in oxide for selective epitaxial Si growth is presented. The treatment is a short reactive ion etch in a Cl2 plasma, applied after window etching. Transmission electron microscopy (TEM) analysis of grown structures shows that high quality epitaxy is obtained in windows as small as 46 nm, indicating effective removal of fluorocarbon contamination. X-ray photoemission spectroscopy (XPS) analysis of the Si surface confirms this. From electrical measurements on small Si point contacts fabricated with this treatment, we deduce that the constriction is high quality single-crystalline Si, in agreement with the TEM and XPS results.
Energy enhancement of harmonic photons with a half-period-delayed pulse
Authors: Hu Suxing , Xu Zhizhan
Published: July 2002

We present a method to enhance the energy of the harmonic photon, namely using an additional half-period-delayed pulse. Because of the interference effect of two pulses, the interaction time between the external field and the atomic system is shortened. Thus, the atomic system can sustain a more intense laser pulse. It means that the actual intensity which contributes to harmonic emission will be increased. Therefore, the harmonic is extended to higher order. Our numerical simulations indicate the substantial energy increment of harmonic photons.
Electrical conduction of buried SiO2 layers analyzed by photon stimulated electron tunneling
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

The conductivity of buried SiO2 layers produced by implantation of oxygen ions into silicon was compared with the photon stimulated electron tunneling from Si into SiO2. The latter process is found to originate from defects located in interfacial SiO2 layers with an electron energy level 2.8 eV below the oxide conduction band. The dark and photon induced currents show a correlated dependence on the electric field strength in the oxide, and both increase with the Si enrichment of the oxide, thus advancing the isolated defects as the common origin of both currents. The same defects were also observed in thermally grown and deposited oxides, so they appear as intrinsic imperfections related to the excess silicon in SiO2.
Role of oxygen in the formation of voids at the SiC–Si interface
Authors: Leycuras A.
Published: July 2002

The purpose of this letter is to observe voids at the SiC–Si interface beneath the SiC layers grown by chemical vapor deposition at high temperature. It is shown in this letter that the volume of the voids per unit area is proportional to the oxygen concentration in the Si substrate over seven orders of magnitude. In situ dynamical reflectivity measurements show that the voids are formed during the carbonization step and especially when the carbon, which has diffused deeply into the Si substrate, diffuses back toward the SiC layer just completed at the substrate surface. This back diffusion is due to the inversion of the carbon concentration gradient sign at that moment. It is accompanied by the formation of CO, resulting either from the reduction of SiO or SiO2 dissolved in the Si substrate. Diffusion of carbon in silicon might improve the methods of purification for the removal of oxygen which remains the main impurity of the purest silicon material.
Spatial resolution of the molecular alignment in electroclinic liquid crystals
Authors: Lindle J. R. , Bartoli F. J. , Flom S. R. , Harter A. T. , Ratna B. R. , Shashidhar R.
Published: July 2002

Field induced deformations of the bookshelf geometry in electroclinic liquid crystals are investigated by means of spatially resolved phase retardation experiments. It is found that the triangular deformation model does not adequately describe the optical data, and underestimates the achievable device contrast ratios. To more correctly model the smectic layer deformation, it is necessary to consider a distribution of molecular directors within a stripe domain.
Spin relaxation of electrons in p-doped GaAs quantum wells under applied voltage
Authors: Maialle M. Z. , Degani M. H.
Published: July 2002

The spin-relaxation times of the electrons excited in p-doped quantum wells are calculated with the spin-flip mechanism being the exchange interaction between the excited electrons and the Fermi sea of holes. The spin mixing of the valence-hole states is included in the calculation. On one hand, this mixing enhances the spin-flip scattering due to an increase of the hole density of states. On the other hand, the exchange loses strength, resulting in spin-relaxation times similar to the ones calculated when neglecting valence spin mixing. The effects of an applied gate voltage to the system are also investigated, since it varies the exchange strength, by breaking the mirror symmetry of the potential, and changes the hole concentration in the well.
Sequential excited-to-excited states resonant tunneling and electric field domains in long period superlattices
Authors: Mityagin Yu. A. , Murzin V. N. , Efimov Yu. A. , Rasulova G. K.
Published: July 2002

The sequential excited-to-excited state resonant tunneling effect was observed in weakly coupled long period superlattices resulting in additional negative differential conductivity resonances in multistable current–voltage characteristics. The results obtained show the evidence of the highly nonequilibrium distribution of carriers over subbands with energies below the optical phonon energy in SL. A new type of electric field domain due to resonant tunneling between excited subbands in adjacent wells is considered, and experimental evidence of such a domain existence is given.
Surface morphology and optical characterization of GaN grown on α-Al2O3 (0001) by radio-frequency-assisted molecular beam epitaxy
Authors: Yeadon M. , Hamdani F. , Xu G. Y. , Salvador A. , Botchkarev A. E. , Gibson J. M. , Morkoç H.
Published: July 2002

Wurtzitic GaN epilayers have been grown on sapphire (0001) substrates by radio-frequency atomic nitrogen plasma-assisted molecular beam epitaxy. Atomic force microscopy, reflectivity, and photoluminescence measurements have been performed in order to investigate the influence of in situ and ex situ annealing on the surface morphology and optical properties. We demonstrate a significant improvement in both the structural and optical quality of our GaN epilayers using in situ annealed low-temperature-deposited GaN buffer layers.
Impact of electron confinement on the lasing properties of ZnS/ZnSe superlattices
Authors: Cingolani R. , Prete P. , Lomascolo M. , Coli’ G. , Calcagnile L. , Lovergine N. , Salviati G. , Lazzarini L.
Published: July 2002

We report a detailed spectroscopic study of the electronic states in ultranarrow ZnS/ZnSe superlattices. The conduction to valence band offset ratio is found to be around 8:92 in nearly pseudomorphic structures. The impact of the weak electron confinement on the optical and electronic properties is discussed, with special attention to laser applications.
Spatial gain profiles of a continuous wave radio-frequency pumped atomic xenon laser
Authors: Tskhai S. N. , Blok F. J. , Udalov Yu. B. , Peters P. J. M. , Witteman W. J. , Ochkin V. N.
Published: July 2002

Spatially dependent small signal gain measurements in a continuous wave rf excited Ar–He–Xe (59.5/40/0.5) gas discharge are presented. Maximum values for the small signal gain of the 2.03 and 2.65 μm xenon transitions of about 22% and 37%/cm, respectively, were obtained at a total pressure of 120 mbar, an input power per unit electrode area of 20 W/cm2 and at a rf driving frequency of 115 MHz. With a spatial resolution better than 0.5 mm, a strongly inhomogeneous gain distribution in the transverse direction was measured.
Decomposition of amorphous Zr41Ti14Cu12.5Ni10Be22.5 alloy: As investigated by small angle neutron scattering
Authors: Liu Jun-Ming
Published: July 2002

In situ small angle neutron scattering has been applied to investigate in details the structural relaxation of amorphous Zr41Ti14Cu12.5Ni10Be22.5 alloy between 620 and 673 K and its effect on subsequent crystallization at 673 K. We demonstrate that crystallization in the decomposed alloy is significantly suppressed. It is revealed that spinodal decomposition of the alloy generates a second supercooled liquid phase embedded in the similarly disordered matrix. The second phase achieves a volume fraction of about 10% and exhibits a barlike pattern. Anomalous kinetics of the microstructural coarsening is observed.
Temperature evolution of excitonic luminescence in nitrogen doped zinc selenide
Authors: Kothandaraman C. , Neumark G. F. , Kash J. A.
Published: July 2002

The temperature evolution of the luminescence due to excitons was studied in nitrogen doped zinc selenide using time-resolved and time-integrated photoluminescence (PL) techniques. The luminescence decay time of the acceptor bound exciton was found to be constant at temperatures below 14 K with a value of ∼430 ps and then decreased with temperature, with an activation energy of 13 meV. By comparison with the PL spectrum, the decrease was identified as due to the thermalization of the bound exciton into a free exciton and a neutral acceptor. The temperature evolution of the PL data shows that the donor bound exciton luminescence increases with reference to the deeper acceptor bound exciton luminescence, and this is explained using a kinetic model.
Laser thermoacoustic modulation for space charge measurement
Authors: Franceschi J. L. , Haas V.
Published: July 2002

A new method for measuring the space charge distribution in polymer insulators has been developed. This frequency method is based on the thermoacoustic signal which propagates into the insulator when a chopped laser beam is modulated in intensity and focused on the surface of the sample. This method has been used for measuring space charge distribution in a polyethyleneterephtalate sample.
Ferroelectric domain gratings and Barkhausen spikes in potassium lithium tantalate niobate
Authors: Tong Xiaolin , Yariv Amnon , Zhang Min , Agranat Aharon J. , Hofmeister Rudolf , Leyva Victor
Published: July 2002

The observation of Barkhausen current spikes during the recording of volume phase holograms in potassium lithium tantalate niobate is reported on. These spikes are due to the ferroelectric domain reversal induced by photorefractive space charge fields. Both “small” (1 nA) and “large” (100 nA) spikes are observed, which correspond to micro and macro domain reversal, respectively. The diffraction efficiency can change as much as 50% during a single macrodomain switching.
Secondary photon emission in plasma processing
Authors: Moshkalyov Stanislav , Machida Munemasa , Campos Delton , Dulkin Alexander
Published: July 2002

Optical emission spectroscopy with high spatial resolution was applied for the study of plasma–material interaction in low-pressure reactive ion etching. Atomic and molecular emission by sputtered material has been found to be strongly localized near the surface. Excited particles are produced during sputtering by energetic ions, with the mechanisms being different for atoms and molecules. In atomic secondary photon emission, a cascade from highly excited levels is shown to be important. This method can be used as a probe during plasma processing.
Dry etch damage in inductively coupled plasma exposed GaAs/AlGaAs heterojunction bipolar transistors
Authors: Ren F. , Lee J. W. , Abernathy C. R. , Pearton S. J. , Constantine C. , Barratt C. , Shul R. J.
Published: July 2002

The dc current gain and emitter and base sheet resistance of C-doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) have been used to measure damage introduced by exposure to Ar inductively coupled plasmas (ICP). As the ICP source power is increased at fixed rf chuck power, the damage-induced changes in device characteristics are reduced due to a reduction in ion energy. Beyond a particular ICP source power (∼1000 W for 50 W rf chuck power), the damage increases due to the increase in ion flux, even though the ion energy is low (<30 eV). These results are a clear demonstration of the advantage of high ion density plasmas for pattern transfer in damage-sensitive minority carrier devices such as HBTs.
Negative differential conductance in lateral double-barrier transistors fabricated in strained Si quantum wells
Authors: Koester S. J. , Ismail K. , Lee K. Y. , Chu J. O.
Published: July 2002

The operation of a laterally patterned negative differential conductance (NDC) device based on hot-electron effects in strained Si quantum wells is demonstrated. The device consists of a small, etch-defined dot region connected to narrow leads on either side via point-contact tunnel barriers. The entire device is fabricated on a high-mobility Si/Si1−xGex heterostructure wafer using electron-beam lithography and low-damage reactive-ion etching. At T=0.4 K (T=1.3 K), the drain characteristic of this device shows a pronounced NDC region with a peak-to-valley ratio (PVR) greater than 600 (100). The PVR is reduced with increasing temperature, with remnants of the NDC observable to 30 K. The NDC is attributed to phonon emission by hot electrons injected into the dot region.
Excited state absorption of Cr4+ ion in forsterite
Authors: Yumashev K. V. , Kuleshov N. V. , Prokoshin P. V. , Malyarevich A. M. , Mikhailov V. P.
Published: July 2002

Polarized differential absorption spectra of Cr4+-doped forsterite have been studied using a picosecond pump–probe technique. For pump beam and probe beam polarization parallel to the a axis, an excited state absorption in the 450–800 nm region is observed and assigned to the T23→T13(P3) transition of Cr4+ ion. The lifetime of the T13(F3) state is estimated to be less than 15 ps.
Observation of triatomic ionic excimers: Emission spectra of Kr2+Cs
Authors: Delaporte Ph. , Voitik M. , Sentis M.
Published: July 2002

Triatomic ionic excimers have now been observed. Alkali rare-gas triatomic ionic molecules have been produced by electron beam excitation. Emission spectra centered at ∼159 nm due to Kr2+Cs radiative decay were recorded. Lifetime and binding energy of the lowest excited state are estimated; the ground state is repulsive. Two formation channels for these triatomic ions are proposed and their rate coefficients estimated.
Random telegraph signal amplitude in submicron n-channel metal oxide semiconductor field effect transistors
Authors: Godoy A. , Gámiz F. , Palma A. , Jiménez-Tejada J. A. , Carceller J. E.
Published: July 2002

Random telegraph signal (RTS) amplitude has been studied in a submicron n-channel metal oxide semiconductor field effect transistor as a function of gate voltage. To do so, we have employed a complete simulator of metal oxide semiconductor devices where the effect of a single acceptor trap placed in the silicon oxide was taken into account. The dominant role played by the screening of the charged trap due to free channel carriers has been demonstrated. Furthermore, the effect of the mobility and carrier number fluctuations on the normalized drain current fluctuations were separated, revealing the importance of mobility variations on the RTS amplitude.
Positron annihilation in ZnSe layers grown on GaAs: Zinc vacancies and drift in the electric field at the ZnSe/GaAs interface
Authors: Liszkay L. , Corbel C. , Hautojärvi P. , Aulombard R. , Cloı̂tre T. , Griesche J. , Kiessling F.
Published: July 2002

We used a slow positron beam to investigate the depth dependence of the positron–electron pair momentum distribution in ZnSe layers grown on a GaAs substrate. We report evidence that positrons annihilate in lattice in undoped ZnSe and at vacancies in heavily n-type ZnSe. It is also demonstrated that positrons in semi-insulating ZnSe are drifted to GaAs by fields of 1–3 kV/cm.
Dielectric-base transistors with doped channel
Authors: Hato Tsunehiro , Yoshida Akira , Yoshida Chikako , Suzuki Hideo , Yokoyama Naoki
Published: July 2002

The dielectric-base transistor (DBT) is expected to be coupled with various functional oxides such as high-temperature superconductors and ferroelectrics. We experimented with lowering the conduction band of the channel to reduce the operating voltage. LaTiO3 deposited on SrTiO3 supplies carriers in the SrTiO3 substrate by displacing Sr2+ and La3+. With this technique, we fabricated a YBa2Cu3O7−x/In2O3/SrTiO3/LaTiO3/SrTiO3 transistor with a partially doped channel. The transistor operates at under 1 V while maintaining a voltage amplification factor of 2, which is one order smaller than the 15 V operating voltage of a transistor with an undoped channel. The base potential relative to the emitter conduction band has been reduced to 0.3 eV.
Be–Zn interdiffusion and its influence on InGaAsP lasers fabricated by hybrid growth of chemical beam epitaxy and metalorganic vapor phase epitaxy
Authors: Sugiura Hideo , Kondo Susum , Mitsuhara Manabu , Matsumoto Shinichi , Itoh Masayuki
Published: July 2002

We have studied the dopant redistribution between a chemical beam epitaxy (CBE) grown InGaAsP laser structure and a metalorganic vapor phase epitaxy overgrown InP layer. Secondary ion mass spectroscopy analysis reveals that Zn and Be atoms deeply interdiffuse in the adjacent InP layers for a Zn doping of 1018 cm−3 and that a fraction of Zn atoms go through the CBE InP and penetrate the laser structure guide layer. We have found that the Zn outdiffusion is significantly suppressed by reducing the Be doping concentration from 1018 to 5×1017 cm−3. As a result, for tensile-strained InGaAsP multiquantum well (MQW) buried-heterostructure (BH) lasers, the threshold current and internal loss decrease from 13 to 9 mA and 15 to 10 cm−1 by lowering the Be doping, respectively. InAsP MQW BH lasers have an internal loss of 5.5 cm−1.
A broadband antireflection coating for enhanced holographic recording and readout in bismuth silicon oxide
Authors: Karim Z. , Kyriakakis C. , Tanguay A. R.
Published: July 2002

We demonstrate a high-quality double-layer antireflection coating for high index (n=2.61 at 514 nm) photorefractive and electro-optic bismuth silicon oxide (Bi12SiO20) crystals. The antireflection coating comprises two electron-beam-deposited quarter-wave dielectric layers of MgF2 and ZrO2, and increases the beam throughput by as much as 20% per interface at normal incidence. For holographic recording applications, the antireflection coating eliminates multiple internal reflections that produce extraneous gratings. The combination of these two factors significantly increases the diffraction efficiency and the two-beam coupling gain. Key characteristics of the double-layer coating include a broadband minimum that encompasses typical write and read wavelengths for Bi12SiO20 with normal-incidence reflectivities of less than 0.2% at 514 nm and 1% at 633 nm, respectively, and a forgiving angular dispersion for both TE and TM polarized waves with reflectivities of less than 2% for angles of incidence up to 45°.
The effect of implantation temperature on the surface hardness, elastic modulus and Raman scattering in amorphous carbon
Authors: Lee Deok-Hyung , Lee Hyukjae , Park Byungwoo , Poker David B. , Riester Laura
Published: July 2002

Nitrogen implantation into amorphous carbon has been studied at various implantation temperatures by using 100 keV N+ at 5 μA and fluences of 2×1017 ions/cm2. The apparent surface hardness and elastic modulus from nanoindentation are well correlated with an asymmetric diffuse peak at around 1500 cm−1 and a broad band at ∼700 cm−1 in the Raman spectra. Both the enhanced strengths and Raman characteristics show very weak Arrhenius-type implantation-temperature dependence with activation enthalpies of approximately 20 meV in the temperature range 223−1073 K.
Ion milling damage and regrowth of oxide substrates studied by ion channeling and atomic force microscopy
Authors: Takeuchi I. , Sharma R. P. , Choopun S. , Lobb C. J. , Venkatesan T.
Published: July 2002

We have studied the effect of ion milling on the surface crystallinity of the metal oxide substrates LaAlO3, SrTiO3, and NdGaO3 which are used for fabrication of high-Tc Josephson junctions and circuits. Ion channeling of the milled substrates reveals a damage-induced peak corresponding to a disordered layer of ≈60 Å at the surface. Annealing the substrates in oxygen ambient at various temperatures ranging from 600 to 1100 °C resulted in the regrowth of the damaged layer at the surface of the substrates as was indicated by the reduction in size of the surface peak observed in the channeled spectrum, as well as by formation of lattice steps as seen by atomic force microscopy. A significant reduction in the damage peak size and the formation of smooth completed lattice steps are seen only after annealing at temperatures ⩾950 °C.