bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficiency and transient time studies of an electron cyclotron resonance ion source for radioactive ion beam production at ISAC/TRIUMF
Authors: Jayamanna K. , Yuan D. , Bishop D. , Dale D. , Dombsky M. , Kuo T. , Kadantsev S. , Keitel R. , Louie D. , McDonald M. , Olivo M. , Schmor P. , Stuber E.
Published: July 2002

A 2.45 GHz electron cyclotron resonance (ECR) ion source with a single mode resonator is being developed to produce high efficient single charged ion beams from exiguous gaseous elements. The source is intended to produce short and long half-life radioactive ion beams as well as stable ion beams for low and high energy experiments at ISAC [J. M. Poutissou, Proceedings of the ISAC Workshop (1994)]. It is obvious that for the radioactive ion beam production, the gas and ion transient time and the overall ionization efficiency are the most important parameters. The transient time is measured using ultrafast peizoelectric gas valve which could operate up to a frequency of 2 kHz. A unique feature of the source is that the plasma chamber is considerably smaller (∼170 times) than its resonance cavity in order to minimize the transient time. Quartz tubes with various diameters (5–20 mm) and 80 mm long are tested as the plasma chamber and the results are discussed. The effect of the transfer tube length, which links the target and the ion source on the transient time is also described. An axially symmetric five electrode extraction system containing three multiaperture electrodes was used to extract the beam. The source, including ECR coils and extraction system is placed in the middle of a 60 cm×60 cm×90 cm vacuum box to simulate the ISAC target module conditions. The preliminary results of the molecular and ion transient time studies, beam efficiency studies are also presented in this article.
Design of a small electron cyclotron resonance ion source with partially pulsed magnetic field
Authors: Oyaizu M. , Tojyo E. , Jeong S. C. , Ishiyama H. , Miyatake H. , Ishida Y. , Kawakami H. , Katayama I. , Nomura T.
Published: July 2002

A small size electron cyclotron resonance ion source to be operating in a pulsed mode has been designed. The main static magnetic field for the confinement is formed by permanent magnets. In addition to these magnets, two small coils are introduced for controlling the magnetic field with a repetition rate synchronized with the pulsed operation of microwave power feeding. One is installed in the middle of the plasma chamber, the other is near the anode electrode.
Comparison of H−/D− measurement in the magnetically filtered region of the large negative ion source
Authors: Hamabe M. , Oka Y. , Osakabe M. , Takeiri Y. , Tsumori K. , Kaneko O.
Published: July 2002

Characteristics of D− production are experimentally compared with that of H− production, both in extracted negative ion beam and in negative ion density in the extraction region, which is the magnetically filtered region in front of the plasma grid. Extracted D− beam current reduces to 1/2–1/3 in the case of H− beam and electron current extracted simultaneously with D− beam increases more than twice as the case of H2 discharge at the same filling gas pressure. Variation of electron density in the extraction region is similar to extracted electron current. Small reduction of D− density in the extraction region is observed, compared to H− density at the same position, but the ratio of reduction differs from the case of extracted ion beam. When the filling gas pressure is raised, D− density and beam current almost linearly increase, similar to the case of H−.
Negative hydrogen ion source development for large helical device neutral beam injector (invited)
Authors: Takeiri Y. , Kaneko O. , Tsumori K. , Oka Y. , Osakabe M. , Ikeda K. , Asano E. , Kawamoto T. , Akiyama R.
Published: July 2002

Large-scaled hydrogen negative ion source development is reviewed for a negative ion based neutral beam injector (NBI) in the large helical device (LHD) fusion machine. The target performance of the ion source is characterized by a high current of 30–40 A with a relatively low energy of 120–180 keV. A series of negative ion source development is conducted with a one-dimensionally reduced size of ion sources which still have a large beam area of 25 cm×26 cm or 50 cm with multi apertures. We employed a cesium-seeded volume production source with an external magnetic filter for the source development. Improvement of the arc plasma confinement is effective to produce a high-current negative ion of 16 A with a current density of 31 mA/cm2 at a low operational gas pressure below 0.4 Pa. Suppression of the accelerated electrons is achieved both by strengthening the magnetic field at the extraction grid and by shaping the inside of the extraction grid aperture to shield the secondary electrons against the acceleration electric field. Multi beamlets delivered from a large area are finely focused with the aperture displacement technique applied to the grounded grid. Based on these results, the LHD-NBI negative ion source was designed and fabricated with a beam area of 25 cm×125 cm. The LHD-NBI source produced 25 A of negative ions with an energy of 104 keV at a low gas pressure of 0.3 Pa. A long-pulse negative ion beam of 81 keV–1.3 MW was produced for 10 s. Four sources were installed to the LHD-NBI system, and around 4 MW of neutral beams were injected into the LHD plasmas with an energy of 100–110 keV in the first period for the NBI experiments. The LHD-NBI ion source is still being developed to improve its performance, and the key issues for the improvement are discussed.
A highly sensitive photoacoustic spectrometer for near infrared overtone
Authors: Hao Lu-yuan , Han Jia-xiang , Shi Qiang , Zhang Jin-hui , Zheng Jin-jin , Zhu Qing-shi
Published: July 2002

By optimizing the size of a photoacoustic cell for decreasing acoustic loss and noise, and using a proper multipass arrangement for increasing light power in the cell and optical coupling to the acoustic wave, a high-resolution external laser photoacoustic spectrometer with a detection sensitivity of 6.35×10−9 cm−1 (1:1 signal to noise level) has been developed. Using this apparatus, the high quality Doppler-limited overtone spectra of AsH3 (600) and H2Se (60) have been observed at room temperature in the wave number regions 11 500–11 650 and 12 600–12 925 cm−1, respectively. Results are presented which highlight the applicability of this apparatus to the spectroscopy of weak optical transitions.
GaN grown by molecular beam epitaxy at high growth rates using ammonia as the nitrogen source
Authors: Yang Z. , Li L. K. , Wang W. I.
Published: July 2002

The 9.5 eV bond energy of the nitrogen molecule makes it very difficult to break it up into atoms and incorporate in III–V nitride compounds grown by molecular beam epitaxy (MBE). By comparison, it is relatively easy to dissociate ammonia due to the existence of a catalytic effect on the GaN surface when there is Ga present. Using ammonia as the nitrogen source, we have achieved high quality GaN by MBE at a growth rate as high as 1 μm/h. This is an order-of-magnitude faster than previously reported using electron-cyclotron resonance plasma-assisted growth. Most importantly, our results indicate that there is no intrinsic limit to the growth rate of GaN using ammonia, a situation similar to that of conventional III–V MBE using gas sources. The unintentional n-type doping as low as 2×1017 cm−3 at room temperature. In addition, room-temperature hole densities of 4×1017 cm−3 in Mg-doped GaN films have been achieved without postgrowth annealing. Low-temperature photoluminescence for both undoped and Mg-doped GaN are dominated by near band-edge emissions without deep-level related luminescence, indicative of high quality materials.
Full scale prototype of high Q pendulum for interferometric gravitational wave detectors
Authors: Cagnoli G. , Gammaitoni L. , Kovalik J. , Marchesoni F. , Punturo M.
Published: July 2002

A series of measurements were undertaken to measure the quality factor of a full scale prototype of the last stage suspension system (a 21 kg pendulum) of the VIRGO gravitational wave interferometer, by using a structure with negligible recoil losses. The suspension wires consisted of two loops of C85 harmonic steel with a diameter of 200 μm that formed a pendulum with a length of 0.70 m. The measured pendulum Q varied from ∼1×104 for the wire simply cradling the mass to 8×105 for the wire being offset from the mass with a prism shaped spacer, while the theoretical value for the pendulum Q was calculated to be 6.7×106. The best measured Q was 1.1×106 for a 20 kg glass mass with metal clamps rigidly attached to it. The main factor that limits the pendulum Q is the dissipation in the wire–test mass attachment. More advanced designs using new materials are needed in order to increase the pendulum Q significantly.
High pressure reaction cell and transfer mechanism for ultrahigh vacuum spectroscopic chambers
Authors: Nelson A. E. , Schulz K. H.
Published: July 2002

A novel high pressure reaction cell and sample transfer mechanism for ultrahigh vacuum (UHV) spectroscopic chambers is described. The design employs a unique modification of a commercial load-lock transfer system to emulate a tractable microreactor. The reaction cell has an operating pressure range of <1×10−4 to 1000 Torr and can be evacuated to UHV conditions to enable sample transfer into the spectroscopic chamber. Additionally, a newly designed sample holder equipped with electrical and thermocouple contacts is described. The sample holder is capable of resistive specimen heating to 400 and 800 °C with current requirements of 14 A (2 V) and 25 A (3.5 V), respectively. The design enables thorough material science characterization of catalytic reactions and the surface chemistry of catalytic materials without exposing the specimen to atmospheric contaminants. The system is constructed primarily from readily available commercial equipment allowing its rapid implementation into existing laboratories.
Magnetic properties of radio frequency sputtered thin films of La–Pb–Mn oxides
Authors: Srinivasan G. , Babu V. Suresh , Seehra M. S.
Published: July 2002

We report the synthesis by radio frequency sputtering and characterization of thin films of La0.74Pb0.26MnOz on Si, the substrate of choice for device integration. Textured films with (110) orientation were grown on (100) Si and the films are ferromagnetic below 325 K. Resistivity ρ data show a metal-to-semiconductor transition at Tms=250 K. The ferromagnetic resonance linewidth shows a discontinuity at Tms. Data on magnetoresistance MR=[ρ(0)−ρ(H)]/ρ(0) versus temperature indicate a maximum value of 22% at Tms for a static magnetic field of 2 T and MR values of 15%–22% are observed over the entire temperature range 5–300 K.
Characterization of structural defects in wurtzite GaN grown on 6H SiC using plasma-enhanced molecular beam epitaxy
Authors: Smith David J. , Chandrasekhar D. , Sverdlov B. , Botchkarev A. , Salvador A. , Morkoç H.
Published: July 2002

Thin wurtzite GaN films have been grown by plasma-enhanced molecular beam epitaxy on the basal plane of 6H SiC, with and without AlN buffer layers. Threading defects, identified from high-resolution electron micrographs as double-positioning boundaries (DPBs), originate at the substrate–buffer and/or buffer–film interfaces. The density of these faults seems to be related to the smoothness of the substrate, so that their occurrence emphasizes the importance of adequate substrate preparation. Stacking faults within the GaN are often visible parallel to the SiC substrate basal plane, sometimes terminating at the DPBs. These faults are related to the particular growth conditions, with greatly decreased density obtained for lower plasma power during GaN deposition. Growth of high quality GaN without stacking faults was achieved without using AlN buffer layers by deposition directly onto a vicinal SiC surface having a miscut angle of 4°. Such stepped substrates represent a potentially useful means for controlled growth of the DPBs, which could then serve as suitable stress-relieving defects in lieu of misfit dislocations.
Performance of microstrip bandpass filters using high-T
c superconducting YBa2Cu3O7−δ thin films on LaAlO3
Authors: Hong J. P. , Lee J. S.
Published: July 2002

We have fabricated a microstrip three-pole bandpass filter with a fractional bandwidth of 5% at a center frequency of 10.5 GHz utilizing laser ablated high- Tc superconducting YBa2Cu3O7−δ thin films on LaAlO3 substrates. Microwave performance of the filter was carried out as a function of temperature. The filter exhibited low insertion losses of about 0.077 and 0.304 dB at 20 and 77 K, respectively. More than 15 dB in return losses was obtained at the same temperatures. Intermodulation distortion measurements were performed to evaluate power-handling capacity of the filter, which results from the nonlinear effects of surface resistance. Third-order intercept points were 68 dBm at 20 K and 58 dBm at 77 K. The nonlinear performance in the passband was also measured using a group delay method. The superconducting filter was compared with an equivalent gold filter which showed the insertion loss of more than 3.5 dB at 77 K.
Surface nucleation of Ti silicides at elevated temperatures
Authors: Tung R. T.
Published: July 2002

The nucleation of Ti silicide at the surfaces of Si was studied. Deposition of Ti and codeposition of TiSix at elevated temperatures on single crystal and amorphous Si led to the direct growth of silicides. The temperature and composition of the deposition and the crystallinity of the substrate were found to have a strong effect on the phases(s) of the silicide layer. A remarkably low nucleation temperature, ∼500 °C, for the low-resistivity C54-TiSi2 phase was observed on amorphous Si. Stoichiometric and uniform TiSi2 layers were grown with the depositions of pure Ti. On crystalline Si, uniform TiSi2 layers were also grown at ∼500 °C with the deposition of essentially Ti. The significant difference between silicide formation in the present scheme and that under conventional silicide processing was discussed in terms of a possible circumvention of precursor amorphous salicide phases during surface nucleation.
Effect of facet roughness on etched-facet semiconductor laser diodes
Authors: Francis D. A. , Chang-Hasnain C. J. , Eason K.
Published: July 2002

We calculate the effects of facet roughness on laser performance of etched-facet semiconductor diode lasers. Facet roughness can be caused by the finite pixel size, used in photolithographic mask fabrication, or in the facet etching process. We consider various sizes of roughness and show that appreciable effects can result from roughness levels previously considered optically flat. Far-field shifts and modal coupling caused by facet roughness are also calculated. Results are highly useful for designing lasers with curved or arbitrarily oriented facts such as unstable resonators or beam steering fan laser diodes.
C-axis oriented (Hg,Ti)-based superconducting films with T
c≳125 K
Authors: Foong F. , Bedard B. , Xu Q. L. , Liou S. H.
Published: July 2002

Thin films with mostly (Hg,Tl)1Ba2Ca3Cu2O8+δ [(Hg,Tl)-1223] phase have been fabricated by radio frequency magnetron sputtering of precursor films and post-annealing method. The doping of a small amount of thallium in the film is helpful to the formation of the three-layer CuO2 compound. These films have a highly oriented structure with the c-axis perpendicular to the film surface. Resistivity measurements show that the films after annealing at 300 °C for 1 h in O2 have the superconducting transition temperature of Tc(onset)=133 K and Tc(zero)=127 K. Scanning electron micrographs of the film reveal platelike micrometer-size grains coalesce to cover the substrate surface.
Persistent spectral hole burning at liquid nitrogen temperature in Eu 3+-doped aluminosilicate glass
Authors: Mao Yalan , Gavrilovic P. , Singh S. , Bruce A. , Grodkiewicz W. H.
Published: July 2002

The observation of persistent spectral hole burning (PSHB) in Eu3+-doped aluminosilicate glass at 77 K is reported in this letter. The homogeneous linewidth of the 7F0→ 5D0 transition is measured to be ∼0.5 Å by both PSHB and fluorescence line narrowing techniques. The ratio of inhomogeneous-to-hole widths at 77 K is 45. The 7F0→ 5D0 transition is visible directly in transmission due to the high concentration of Eu3+.
Driving force behind voltage shifts in ferroelectric materials
Authors: Warren W. L. , Al-Shareef H. N. , Dimos D. , Tuttle B. A. , Pike G. E.
Published: July 2002

By systematically poling Pb(Zr,Ti)O3 capacitors to different extents, we observe a linear relationship between the remanent polarization and the magnitude of voltage offsets in the hysteresis curve. This result directly shows that the polarization is the impetus behind voltage shifts and, thus, imprint in ferroelectric capacitors. It is proposed that the increased polarization lowers the electrostatic potential well for the trapping of electrons thereby leading to greater voltage shifts. We also find that the remanent polarization and defect occupancy are temperature dependent which collectively impact the observed voltage offsets measured at elevated temperature.
Characteristics of carbon nitride films synthesized by single-source ion beam enhanced deposition system
Authors: Wu Zhaocu , Yu Yuehui , Liu Xianghuai
Published: July 2002

Carbon nitride films on silicon (111) and (100) wafers and Ti/C substrates with nitrogen concentration up to 50 at. % have been prepared by ion beam enhanced deposition carried out in a single ion source system. The possibility of synthesis of carbon nitride films was investigated. The nitrogen concentration and the compositions of the films were analyzed by Rutherford backscattering which gave out the nitrogen concentration of the films ranging from 37 to 50 at. %, Fourier transform infrared spectroscopy, and x-ray photoelectron spectra. The characteristics of C 1s peaks at 284.6, 285.9, 287.5, 289.5, eV and N  1s peaks at 396.0, 398.4, 399.9, 402.1 eV enabled us to analyze the compositions of the films. The results of the analyses indicate that various phases such as different carbon phases, a tetrahedrally bonded phase (β-C3N4) and other carbon nitrides (the ratio of N/C varied from 0.5 to 1.1) are contained in the films. Furthermore the concentration of the tetrahedrally bonded phase increases with the increment of the nitrogen concentration.
Extraordinary thermopower in magnetoresistive (La1−xYx)0.67Ca0.33MnO3 oxides
Authors: Fontcuberta J. , Seffar A. , Granados X. , García-Muñoz J. L. , Obradors X. , Piñol S.
Published: July 2002

We report that the thermopower Q(T) of the (La1−xYx)0.67Ca0.33MnO3 (x=0–0.25) magnetoresistive oxides, displays a pronounced peak at temperatures Ts close to the ferromagnetic ordering temperature. The temperature Ts decreases monotonously when increasing x whereas the maximal Q(Ts) is found to increase with the Y contents. The huge enhancement of Q at Ts is found to be as high as 460% for x=0.25. It is shown that the extraordinary thermopower observed at Ts results from the reduced charge carrier mobility when approaching Ts and the variation of Q(T) with the Y contents reflects the accompanying band narrowing.
Metallic particle sizing on flat surfaces: Application to conducting substrates
Authors: Moreno F. , Saiz J. M. , Valle P. J. , González F.
Published: July 2002

A fast and accurate microsizing method is introduced and analyzed for metallic protuberances on flat substrates. It is based on the measurement of the minima angular positions of the S-polarized far field scattering patterns at normal incidence. The proposed method has been theoretically and experimentally checked for both cylindrical and spherical metallic protuberances on conducting flat substrates. The excellent agreement between theory and experiment proves the efficiency of the method. We also comment on the application of this method for other protuberance geometries and different substrates other than metallic.
Evolution of the crystallographic position of As impurities in heavily doped Si crystals as their electrical activity changes
Authors: Herrera-Gómez A. , Rousseau P. M. , Materlik G. , Kendelewicz T. , Woicik J. C. , Griffin P. B. , Plummer J. , Spicer W. E.
Published: July 2002

Arsenic impurities in silicon can be electrically activated beyond their electrical solubility to as high as 4×1021/cm3 by ion implantation and laser melting; further annealing decreases this activity to its equilibrium saturation level. To characterize the deactivation process, we used x-ray standing-wave spectroscopy. Hall effect, and secondary-ion-mass spectroscopy. Our results indicate that the As impurities remain in substitutional positions even after 85% of the activation has been lost, so deactivation cannot be due to As migration to interstitial sites or to large precipitates.
Study of deep level defects in n-GaN by the optical transmission method
Authors: Balagurov L. , Chong P. J.
Published: July 2002

Absorption coefficient spectra are determined by optical transmission of undoped gallium nitride films, which is used for the density of states distribution. The absorption of n-type GaN in the region of photon energy lower than the band edge is regarded as attributable to the local deep defect levels. For the photoemission of electrons to the conduction band, two optical threshold energies were detected, approximately at 2.1 eV from one deep level and near 1.1 eV from one or several deep levels. Discussion is centered on the possible participation of either level in the commonly observed 2.2 eV defect luminescence transitions.
Direct determination of carrier capture times in low-dimensional semiconductor lasers: The role of quantum capture in high speed modulation
Authors: Wang Jian , Griesinger Uwe A. , Schweizer Heinz
Published: July 2002

The effective carrier capture time, which limits the maximum modulation bandwidth of low-dimensional semiconductor lasers, can be determined by a simple technique as shown in this letter. For demonstration, the technique is applied to InGaAs/InGaAsP multi-quantum well, InGaAs/GaAs single quantum well and InGaAs/InGaAsP wire lasers, respectively, at low temperature (2 K). Moreover, by comparing quantum-well lasers with wire lasers, which have different packing densities of the active regions but approximately the same carrier diffusion length before capture, we found that the effective carrier capture times for these two laser types are quite different. This shows the significant role of the scale-up factor in the contribution of quantum capture time to the total effective carrier capture time, as discussed by S. C. Kan et al. [Appl. Phys. Lett. 62, 2307 (1993)].
Determining the absolute value of penetration depth of large area films
Authors: Lee Ju Young , Kim Young Hwan , Hahn Taek-Sang , Choi Sang Sam
Published: July 2002

We report a method that allows the measurement of the absolute size of the magnetic penetration depth λ(T) of large area YBa2Cu3O7−δ films using the two-coil method where a YBa2Cu3O7−δ film is overlapped with a 1-μm-thick Pb film equivalent in size placed in the gap between the drive and receive coil. The drive coil has fewer counter turns on the top section than the bottom, which is closer to the film, so the film’s edge current would be suppressed and the accuracy that depends on geometry is greatly improved. The transition of the Pb film causes a steplike portion in the mutual inductance M(T) graph and the resultant step height leads to the absolute value of λ.
Back-gating effect of low-temperature GaAs on a pseudomorphic modulation-doped field-effect transistor
Authors: Folkes P. A. , Smith Doran , Lux R. A. , Zhou W. , Thompson R. , Moerkirk R. , Lemeune M. , Cooke P. , Brown K.
Published: July 2002

Low-temperature (LT) photoluminescence (PL) measurements on pseudomorphic modulation-doped transistors with a (LT) GaAs layer in the GaAs substrate have revealed the existence of a substrate voltage. The substrate voltage is manifested by a decrease in the PL transition energies of the quantum-well subbands due to the quantum confined Stark effect. Our results indicate that the substrate voltage is generated by the trapping of holes from the undoped molecular-beam-epitaxy-grown GaAs at the GaAs/LT GaAs interface by the high concentration of arsenic antisite defects in the LT GaAs layer.
InGaAs metal-semiconductor-metal photodetectors with engineered Schottky barrier heights
Authors: Wohlmuth W. A. , Arafa M. , Mahajan A. , Fay P. , Adesida I.
Published: July 2002

An InAlAs/InGaAs/InP metal-semiconductor-metal photodetector with engineered Schottky barrier heights has been fabricated. A significant decrease in dark current with no change in the responsivity or the bandwidth was obtained by independently engineering the Schottky barrier heights at the anode and cathode. These photodiodes with an electrode width and spacing of 2 μm exhibited a dark current density of 20.0 fA/μm2 at an applied bias of 5 V. This dark current density is ∼6 times lower than the previously reported minimum.
Effect of interdiffusion of quantum well infrared photodetector
Authors: Lee Alex S. W. , Li E. Herbert
Published: July 2002

The intersubband infrared photodetector performance is theoretically analyzed for various stages of interdiffusion in AlGaAs/GaAs quantum well. The absorption strength and responsivity are enhanced for certain extents of interdiffusion and the peak detection wavelength red shifts continuously with a large tunable range from 7 to 38.4 μm. The dark current is at an acceptable value for small diffusion extent.
Near field scanning optical microscopy measurements of optical intensity distributions in semiconductor channel waveguides
Authors: Poweleit C. D. , Naghski David H. , Lindsay Susan M. , Boyd Joseph T. , Jackson Howard E.
Published: July 2002

We present results of near field scanning optical microscopy measurements performed on single mode AlGaAs ridge channel waveguides. The optical intensity distribution just above the surface of the waveguide structure has been measured by scanning a tapered, aluminum-coated, fiber probe transverse to the waveguide propagation direction. Experimental results are compared with model calculations performed using both the effective index and beam propagation methods. An accurate description of submicron features in the intensity profile near the ridge edges, as well as the magnitude of the field outside the channel, requires the use of the beam propagation method.
High-reflectivity, wide-bandwidth optical phase conjugation via four-wave mixing in potassium vapor
Authors: Lanzerotti M. Y. , Schirmer R. W. , Gaeta Alexander L.
Published: July 2002

We report on a high-reflectivity (up to 670%) wide-bandwidth (up to 230 MHz) phase-conjugate mirror formed using backward-four-wave mixing with continuous-wave pump beams in a 2-mm potassium vapor cell. The reflectivities and bandwidths are significantly larger than have been measured previously, and the bandwidth is ten times greater than is predicted theoretically. The reflectivity-bandwidth product is more than an order of magnitude improvement over those previously achieved with other continuous-wave phase-conjugate systems.
Electric field measurements in an argon glow discharge using laser spectroscopy
Authors: Choi Y. W. , Bowden M. D. , Muraoka K.
Published: July 2002

Direct measurements of the electric field in the sheath of a dc glow discharge were made using laser spectroscopic measurements of argon atoms. The effect of the electric field on the 4s⇒7f transitions was used to determine the electric field. Both laser opto-galvanic spectra and laser-induced fluorescence spectra were obtained. The magnitude of the electric field was determined using an experimentally obtained calibration of the change in wavelength produced by the electric field.
Diagnostics of a non-equilibrium inductively coupled plasmas in argon
Authors: Okigawa Akifumi , Lj.Petrović Zoran , Tadokoro Masahiro , Makabe Toshiaki , Nakano Nobuhiko , Itoh Akihiko
Published: July 2002

An apparatus including a robot manipulated optical system has been built to provide absolute emission data from an inductively coupled plasma operating in argon in non-equilibrium conditions. Data for the two-dimensional population densities of one of the excited states of neutral argon ArI are presented together with the Langmuir probe data for electron density and plasma potential. Measurements were performed at 500 mTorr and 50 mTorr and with 100 sccm flow of argon under the constant power condition, 100 W. The data show that excitation profile is almost identical to the radial profile of electron density. At higher pressure maximum excitation and electron density are produced close to the walls, while at low pressure maximum excitation is at the center of the vessel and a high field is formed close to the walls.
Evidence of interface trap creation by hot-electrons in AlGaAs/GaAs high electron mobility transistors
Authors: Meneghesso Gaudenzio , Paccagnella Alessandro , Haddab Youcef , Canali Claudio , Zanoni Enrico
Published: July 2002

We report on the hot-electrons induced degradation in AlGaAs/GaAs high electron mobility transistors (HEMTs), consisting of a decrease in the drain current and an increase in the parasitic drain resistance. The amount of the degradation is proportional to the impact-ionization rate which is related to the electron energy. Transconductance dispersion measurements and drain current deep level transient spectroscopy (DLTS) have been used to identify interface traps which are located at the AlGaAs/GaAs interface in the gate-drain access region and are the causes of the observed degradation.
Photoluminescence characterization of the quantum well structure and influence of optical illumination on the electrical performance of AlGaN/GaN modulation-doped field-effect transistors
Authors: Mohammad S. N. , Fan Z.-F. , Salvador A. , Aktas O. , Botchkarev A. E. , Kim W. , Morkoç Hadis
Published: July 2002

Electrical characteristics of high performance GaN modulation-doped field-effect transistors (MODFETs) grown by reactive molecular beam epitaxy method are studied experimentally both in dark and under white light illumination. The maximum measured drain-source current is 626 mA/mm in dark and 695 mA/mm under illumination, which saturates at a relatively low drain-source voltage VDS. The transconductance increases with decreasing gate length reaching a value of 210 mS/mm in dark and 222 mS/mm under illumination for devices with a gate length of LG=1.5μm. Breakdown voltages of about 90 V have also been exhibited by devices with gate length LG=1.5 μm and drain-gate distance LDG=1 μm. To our knowledge, these are the best values obtained so far from GaN MODFETs which we attribute to the suppression of leakage currents and improved materials structure of the devices.
Growth and properties of InxGa1−xN/AlyGa1−yN multiquantum wells developed by molecular beam epitaxy
Authors: Singh R. , Doppalapudi D. , Moustakas T. D.
Published: July 2002

In0.09Ga0.91N/GaN and In0.35Ga0.65N/Al0.1Ga0.9N multiquantum well structures (MQW) were grown by molecular beam epitaxy on (0001) sapphire substrates. The thickness of the wells and the barriers were in the range of 80–120 Å. The microstructure of these MQW structures was investigated by transmission electron microscopy. The room temperature photoluminescence spectra in these MQW structures peak at 387 and 463 nm with full width at half maximum of 16 and 28 nm, respectively.
A niobiumnitride mixer with niobium tuning circuit
Authors: Plathner B. , Schicke M. , Lehnert T. , Gundlach K. H. , Rothermel H. , Aoyagi M. , Takada S.
Published: July 2002

This letter reports a low noise submillimeter-wave mixer using NbN tunnel junctions integrated in Nb matching circuits. The double side band receiver noise temperature was 245 K at 345 GHz. Plasma conditions for NbN film deposition on quartz substrates at room temperature are created by using a second Nb target as a selective nitrogen pump. Electrodes for tunnel junctions with critical temperatures above 15 K and normal state resistivities in the range from 130 to 160 μΩ cm were obtained. This permits integrating NbN junctions into normal metal or non-NbN superconducting matching circuits, which is of great interest for THz mixers.
Atomic force microscopy of selenium sulfide passivated GaAs (100) surface
Authors: Kuruvilla Beena Annie , Datta A. , Shekhawat G. S. , Sharma A. K. , Vyas P. D. , Gupta R. P. , Kulkarni S. K.
Published: July 2002

Surface structure studies of GaAs(100) with and without chemical passivation have been made using atomic force microscopy (AFM). Passivation was carried out using a solution of SeS2 which has proved to be a successful passivating agent as seen from the increase in the photoluminescence (PL) intensity. Atomic force microscopy results indicate that it is possible to obtain ordered surface layers on GaAs using SeS2.
Strong instantaneous contribution in femtosecond degenerate four-wave mixing from 350 μm InP due to virtual excitation
Authors: Yu Sungkyu , Chu Jang Hee , Lee Joo In , Kim Dongho , Yee Y. H. , Kim D. S. , Leem Jae-Young , Lee Cheul-Ro , Lee Jong Hyun
Published: July 2002

We have observed strong four-wave mixing (FWM) signals from the third up to the seventh order for a 350 μm undoped InP at 10 K with an excitation at far below the band gap. The third order spectrally resolved FWM signal shifts continuously to blue as time delay moves away from positive to negative. The peak intensities of the third and fifth order time-integrated FWM signals decrease rapidly as the detuning increases, as (detuning)−2 and (detuning)−6, respectively. The FWM signal at far below the band gap is attributable to the strong instantaneous contribution of the excitation pulse.
Reduced temperature sensitivity of the wavelength of a diode laser in a stress-engineered hydrostatic package
Authors: Cohen Daniel A. , Heimbuch Mark E. , Coldren Larry A.
Published: July 2002

By using the effects of increasing hydrostatic pressure to counteract the effects of rising temperature, we demonstrate a technique to stabilize the wavelength of an uncooled diode laser. We use the differential thermal expansion between various materials incorporated into the laser package to automatically generate a temperature-dependent pressure, and obtain a 50% reduction in the temperature sensitivity of the wavelength of a 1.55 μm GaInAsP/InP laser.
Improved atomic force microscopy resolution using an electric double layer
Authors: Sokolov I. Yu. , Henderson G. S. , Wicks F. J. , Ozin G. A.
Published: July 2002

High resolution (“atomic”) images of clinochlore and muscovite have been obtained in aqueous solution by inducing an electric double layer between the atomic force microscope tip and the sample surface. The electric double layer is created by the addition of a surfactant to water and greatly improves image resolution. A theoretical model is proposed to explain the improved resolution.
Analysis of temperature sensitivity in semiconductor lasers using gain and spontaneous emission measurements
Authors: Fang W. , Hattendorf M. , Chuang S. L. , Minch J. , Chang C. S. , Bethea C. G. , Chen Y. K.
Published: July 2002

A consistent method to characterize the temperature dependence of bulk InGaAsP semiconductor laser diodes is presented. Independent measurements of the gain and spontaneous emission spectra are conducted, and the spectra are calibrated using their fundamental relationship. This procedure will provide a unique approach to extract precise values for laser diode parameters such as quasi-Fermi level separation, peak modal gain, and total loss. The radiative and nonradiative current densities can then be calculated as a function of temperature and injection current. By comparing the measured data with a theoretical model, the carrier density is calculated. Important phenomena contributing to the strong temperature dependence of long-wavelength bulk InGaAsP/InP lasers are highlighted.
Optical properties of two-dimensional photonic crystals with graphite structure
Authors: Cassagne D. , Jouanin C. , Bertho D.
Published: July 2002

We present a study of the transmission coefficients of two-dimensional photonic band gap materials consisting of dielectric cylinders in graphite arrangement. By the study of the attenuation versus slab thickness, we determine the most efficient graphite configuration. We show how uncoupled modes create opaque regions for plane waves propagating along the Γ-P direction and widen the gap originating from the existence of forbidden photonic bands. Our results demonstrate that graphite structure is a promising geometry yielding an attenuation as strong as triangular structure with greater convenience in the fabrication at the submicronic scale.
Over 30% efficient InGaP/GaAs tandem solar cells
Authors: Takamoto Tatsuya , Ikeda Eiji , Kurita Hiroshi , Ohmori Masamichi
Published: July 2002

A two-terminal monolithic InGaP/GaAs tandem solar cell with a new efficiency record of 30.28% is realized with a practical large area of 4 cm2 under one-sun air-mass 1.5 global illumination. We report improvements of the tandem cell performance by introducing a double-hetero (hereafter DH) structure InGaP tunnel junction, in which the InGaP layers are surrounded by high band gap AlInP barriers. The DH structure by AlInP barriers increase the peak current of InGaP tunnel junction. The AlInP barrier directly below the InGaP top cell, which takes the part of a back surface field (hereafter BSF) layer, is found to be considerably effective in reflecting minority carriers in the top cell. The AlInP BSF layer does not only form a high potential barrier but also prevents the diffusion of zinc from a high doped tunnel junction toward the top cell during epitaxial growth. Furthermore, an InGaP tunnel junction reduces the absorption loss, which exists in a GaAs tunnel junction, and increases the photogenerated current in the GaAs bottom cell.
Power-dependent microwave properties of superconducting YBa2Cu3O7−x films on buffered polycrystalline substrates
Authors: Findikoglu A. T. , Arendt P. N. , Foltyn S. R. , Groves J. R. , Jia Q. X. , Peterson E. J. , Bulaevskii L. , Maley M. P. , Reagor D. W.
Published: July 2002

We have studied the microwave properties of 0.4 μm thick YBa2Cu3O7−x (YBCO) films on polycrystalline substrates with ion-beam-assisted-deposited yttria-stabilized zirconia buffer layers using a parallel-plate resonator technique at 10 GHz. The YBCO films with similar in-plane texture grown on both forsterite and Ni-based alloy substrates show similar microwave properties. We measure low-power surface resistance Rs values of about 0.5 mΩ at 76 K and 0.15 mΩ at 4 K for films with an in-plane mosaic spread of about 7°. Single-tone power-dependence measurements show that the surface resistance and the surface reactance increase linearly and by the same amount with increasing microwave field level. At intermediate power levels, the intermodulation measurements show odd-order intermodulation products that increase quadratically with two-tone input power. These results indicate a hysteretic vortex penetration mechanism in the weak links as the most plausible source of the observed nonlinearities in these films.
Loss of electrical conductivity in boron-doped diamond due to ion-induced damage
Authors: Kalish R. , Uzan-Saguy C. , Philosoph B. , Richter V. , Prawer S.
Published: July 2002

The response of B-ion-implanted type-IIa diamond to light ion (H, He) irradiation is investigated by monitoring the sample resistance as a function of dose. It is found that the resistivity of the layer increases rapidly with increasing dose, and reaches the resistivity of the undoped diamond for irradiation doses much less than those required for the onset of damage related electrical conductivity in pristine diamond. It is shown that defects created by the nuclear stopping process act as compensating centers for the B acceptors. The present findings are of importance for the design of radiation hard diamond based electronic devices and suggests a method for the isolation of B-doped devices on a diamond chip. The results of the present work also explain why the collection distance in intrinsic nondoped diamond radiation detectors actually increases with increasing ion dose.
Carrier concentration enhancement of p-type ZnSe and ZnS by codoping with active nitrogen and tellurium by using a δ-doping technique
Authors: Jung H. D. , Song C. D. , Wang S. Q. , Arai K. , Wu Y. H. , Zhu Z. , Yao T. , Katayama-Yoshida H.
Published: July 2002

We have studied the p-type doping of ZnSe and ZnS grown by molecular beam epitaxy using active nitrogen and tellurium. Highly conductive p-type ZnSe and ZnS films have been achieved by inserting heavily N-doped ZnTe layers into each layer. A hole concentration of 7×1018 cm−3 in a ZnSe/ZnTe:N δ-doped layer and a [Na-Nd] value of 5×1017 cm−3 in a ZnS/ZnTe:N δ-doped layer were obtained. Excitonic emission associated with the N acceptor at 2.792 eV in ZnSe was dominant in the photoluminescence spectra at 14 K and the emission related to the Te isoelectronic deep centers was negligibly weak. These results indicate that the incorporation of ZnTe:N single layers by this δ-doping method dramatically increases the hole concentration, and the optical properties are consistent with this improvement.
Biphotonic self-diffraction in azo-doped polymer film
Authors: Wu Pengfei , Zou Bingsuo , Wu Xiaochun , Xu Jiren , Gong Xiong , Zhang Guilan , Tang Guoqing , Chen Wenju
Published: July 2002

A new nonlinear optical phenomenon, biphotonic self-diffraction (BSD), is reported in azo dye doped in polymer matrix. The first-order and the second-order BSD signals are observed in the experiment. It is demonstrated that the dynamic behaviors of the BSD signals are dependent on the cis-trans isomerization of azo molecules by two color lights.
Si/SiO
2
interface roughness: Comparison between surface second harmonic generation and x-ray scattering
Authors: Cundiff S. T. , Knox W. H. , Baumann F. H. , Evans-Lutterodt K. W. , Tang M.-T. , Green M. L , van Driel H. M.
Published: July 2002

The roughness of the Si(100)/SiO2 interface is measured using both surface second harmonic generation (SSHG) and x-ray scattering. A comparison between these techniques shows a clear correlation for typical industrial oxides, despite the techniques being sensitive to differing regions of the roughness spectrum. The SSHG measurements are made using ∼10 fs pulses centered at 850 nm and at 80 MHz repetition rate. The short pulses produce a similar signal to noise ratio as earlier measurements, but use much lower average power, thus avoiding possible artifacts such as sample heating.
Infrared ellipsometry on hexagonal and cubic boron nitride thin films
Authors: Franke E. , Neumann H. , Schubert M. , Tiwald T. E. , Woollam J. A. , Hahn J.
Published: July 2002

Infrared spectroscopic ellipsometry (IRSE) over the wavelength range from 700 to 3000 cm−1 has been used to study and distinguish the microstructure of polycrystalline hexagonal and cubic boron nitride thin films deposited by magnetron sputtering onto (100) silicon. The IRSE data are sensitive to the thin-film layer structure, phase composition, and average grain c-axes orientations of the hexagonal phase. We determine the amount of cubic material in high cubic boron nitride content thin films from the infrared optical dielectric function using an effective medium approach.
Semi-insulating In0.49Ga0.51P grown at reduced substrate temperature by low-pressure metalorganic chemical vapor deposition
Authors: Hartmann Q. J. , Gardner N. F. , Horton T. U. , Curtis A. P. , Ahmari D. A. , Fresina M. T. , Baker J. E. , Stillman G. E.
Published: July 2002

High-resistivity unintentionally-doped In0.49Ga0.51P lattice matched to GaAs has been grown via low-pressure metalorganic chemical vapor deposition at a reduced growth temperature. These layers have excellent surface quality and are single crystal. The resistivity increases exponentially as the growth temperature is decreased from 550 to 490 °C, resulting in a resistivity of ∼109 Ω cm for samples grown at 490 °C. In addition, the photoluminescence intensity decreases exponentially for growth temperatures below 550 °C, indicating an increase in nonradiative recombination related to an increasing trap concentration. For samples grown at 550 °C, constant capacitance deep level transient spectroscopy measurements show a strong broad peak at ∼200 °K with an ionization energy of 0.40±0.04 eV, verifying the presence of an electron trap. The gummel plot and I–V characteristics of an InGaP/GaAs heterojunction bipolar transistor (HBT) with a 2000-Å-thick InGaP buffer layer grown at 500 °C are identical to that of an HBT grown without the InGaP buffer layer, indicating that the semi-insulating InGaP layer is compatible with GaAs-based device epitaxy.
Field-angle dependence of alternating current loss in multifilamentary high-Tc superconducting tapes
Authors: Oomen M. P. , Rieger J. , Leghissa M. , ten Kate H. H. J.
Published: July 2002

The energy loss in multifilamentary superconducting tapes has been measured for various orientations of the external ac magnetic field. When the ac loss in parallel and perpendicular fields is known, the loss at intermediate field angles can be predicted over a wide range of field amplitudes. The prediction is based upon the following assumptions: For low-field amplitudes, the current patterns that would be induced in the tape by the parallel and perpendicular field components can be summed. For high-field amplitudes, the tape only carries the current pattern induced by the perpendicular field component. These assumptions are supported by theory.
Calculation of the Hall scattering factor using a Monte Carlo technique
Authors: Dijkstra J. E. , Wenckebach W. Th.
Published: July 2002

We present a method to calculate the Hall scattering factor using a Monte Carlo technique. The greater accuracy of the new method makes it possible to calculate rH in p-Si and Ge at room temperature much faster. The models employed in the calculation include all details of the valence band and acoustical and optical phonon scattering processes.
Enhancement on field emission characteristics of pulsed laser deposited diamondlike carbon films using Au precoatings
Authors: Chuang F. Y. , Sun C. Y. , Cheng H. F. , Lin I. N.
Published: July 2002

Using Au precoatings has been observed to significantly enhance the field emission properties of diamondlike carbon (DLC) films deposited on Si substrates. The electron emission can be turned on at a low field as 7 V/μm and a large emission current density as 2000 μA/cm2 can be obtained at 20 V/μm applied field. However, preannealing the Au-coated Si substrates at 500 °C for 30 min is necessary to achieve such a performance. Microscopic examination on surface and cross-sectional morphologies of the DLC/Au/Si films using atomic force microscopy and scanning electron microscopy, respectively, in conjunction with the elemental depth profile examination of these films using secondary ion mass spectroscopy, indicated that substantial interdiffusion between DLC, Au, and Si layers has occurred. Such kind of reaction is proposed to lower the resistance for electrons to transport across the interfaces and, thereafter, enhances the field emission properties of the DLC/Au/Si films.
Anomalous position of the maximum in magnetic hysteresis loops measured on (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes
Authors: Koblischka M. R. , Galkin L. Půst, A. , Nálevka P.
Published: July 2002

An anomalous position of the central peak of magnetic hysteresis loops is observed in (Bi,Pb)-2223/Ag tapes at positive external fields Bext for dBext/dt<0. We model the grain structure by a system of superconducting disk-shaped grains touching each other in small, but highly conducting contacts at the disk circumferences. The sign of the internal field Bi at the circumference of large, thin well oriented grains in the tape plays an important role. While in great majority of the sample volume Bi is delayed behind Bext, just at the grain circumferences Bi might be considerably ahead of Bext. Based on these considerations, a model sample which should exhibit similar features of the central peak is proposed.