Showing papers on "Full width at half maximum published in 2001"

The Oxygen Vacancy as the Origin of a Green Emission in Undoped ZnO

Abstract: We have investigated undoped ZnO single crystals, which are commercially available from Eagle-Picher, by photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectroscopy. The electrical properties of this material are very similar to the samples investigated. The total residual shallow donor concentration is about 1 x 10{sup 17} cm{sup -3}. The low temperature emission is dominated by the donor bound exciton (D{sup 0}X) at 3.366 eV. At 2.45 eV the broad, unstructured ''green'' emission is located, its full width at half maximum is 320 meV. The temperature dependence of the PL reveals that this green band maintains its peak energy up to 450 K, which is a feature typical of a recombination within a localised defect, while the D{sup 0}X emission follows the shrinkage of the bandgap with increasing temperature. (orig.)

Stress control in GaN grown on silicon (111) by metalorganic vapor phase epitaxy

Abstract: The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5 μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.

Over 1.5 μm light emission from InAs quantum dots embedded in InGaAs strain-reducing layer grown by metalorganic chemical vapor deposition

Abstract: We demonstrated the 1.52 μm light emission at room temperature from self-assembled InAs quantum dots embedded in the In0.45Ga0.55As strain-reducing layer. By capping InAs quantum dots with an InGaAs strain-reducing layer instead of GaAs, the photoluminescence peak of InAs quantum dots can be controlled by changing the indium composition of the InGaAs strain-reducing layer. The full width at half maximum is as narrow as 22 meV. The wavelength of 1.52 μm is the longest wavelength so far achieved in self-assembled InAs quantum dots, which would be promising to quantum-dot lasers on GaAs substrate for application to light sources in long-wavelength optical communication systems.

Resolution limits for infrared microspectroscopy explored with synchrotron radiation

Abstract: The spatial resolution for infrared microspectroscopy is investigated to determine the practical limits imposed by diffraction or optical aberrations. Quantitative results are obtained using high brightness synchrotron radiation, which serves as a diffraction-limited infrared “point source” for the microscope. The measured resolving power is in good agreement with diffraction theory, including a ∼ 30% improvement for a confocal optical arrangement. The diffraction calculation also shows how the confocal setup leads to better image contrast. The full width at half maximum of the instrument’s resolution pattern is approximately λ/2 for this arrangement. One achieves this diffraction limit when the instrument’s apertures define a region having dimensions equal to the wavelength of interest. While commercial microspectrometers are well corrected for optical aberrations (allowing diffraction-limited results), the standard substrates used for supporting specimens introduce chromatic aberrations. An analysis of ...

Highly conductive and transparent aluminum-doped zinc oxide thin films prepared by pulsed laser deposition in oxygen ambient

Abstract: Highly conducting and transparent aluminum-doped zinc oxide films were prepared on quartz and corning glass 7059 substrate by ablating the sintered ZnO target containing 2 wt % Al2O3 with a XeCl excimer laser (λ=308 nm). To grow the films, a repetition rate of 5 Hz and energy density of 1.5 J/cm2 was kept. The effect of substrate temperature from room temperature to 400 °C and oxygen pressure (0.1–5 mTorr) have been investigated by analyzing the optical and electrical properties of these films. The average transmittance was found to be in the range of 86%–92%, and a variable resistivity (ρ) 3.56×10−3–7.0×10−3 Ω cm have been obtained. The lowest resistivity was found to be 1.4×10−4 Ω cm at 300 °C in 1 mTorr of oxygen pressure. Structural changes in the films were also investigated by determining the full width at half maximum of (002) x-ray diffraction peak. These results show improvement in the crystallinity of films, which support our conductivity and transmittance data. The sharp decrease in the transmi...

Red organic light emitting device made from triphenylene hexaester and perylene tetraester

Abstract: Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.

Enhanced performance of a dielectric barrier discharge lamp using short-pulsed excitation

Abstract: We observe marked increases in the time-averaged intensity, peak intensity, efficiency and spectral purity of the VUV output from an Xe excimer barrier discharge lamp when using short-pulse (~150 ns FWHM (full width half maximum)) excitation. Intensity increases with Xe pressure up to 600 Torr with a maximum output 2.6 times higher and an efficiency 3.2 times higher than the same lamp excited by conventional ac excitation (i.e. sinusoidal voltage waveform). The output occurs in regular short pulses (<300 ns FWHM) with a peak intensity more than six times the peak intensity typically obtained using ac. The spectral characteristics are similar to that observed using ac excitation except that the ratio of VUV to visible Xe* emission increases by a factor of three. The pulsed discharge appears diffuse (i.e. glow-like), even at the higher pressures at which the ac discharge is filamentary. It is concluded that the enhanced performance results largely from the ability for pulsed excitation to generate a discharge at near atmospheric pressures with a much lower electron density than that possible using ac.

Time-of-flight and emission spectroscopy study of femtosecond laser ablation of titanium

Abstract: Femtosecond laser ablation of titanium has been studied via time-of-flight (TOF) and emission spectroscopy measurement. Laser pulses of 80 fs full width at half maximum at λ=800 nm were delivered by a Ti:sapphire femtosecond laser system. A vacuum chamber with a base pressure of 10−7 Torr was built for ion TOF measurement. These ion TOF spectra were utilized to determine the velocity distribution of the ejected ions. While nanosecond laser ablation typically generates ions of tens of eV, femtosecond laser irradiation even at moderate energy densities were found to produce energetic ions with energies in the range of a few keV. Two ablation regimes, exhibiting different laser fluence dependence of the total ion yields, and the corresponding percentage of energetic ions and the crater depth, were identified and explained on the basis of the two-temperature model. Clean craters were observed by interferometric microscope measurements, indicating the advantages of and potential for applying femtosecond lasers...

New picosecond laser system for easy tunability over the whole ultraviolet/visible/near infrared wavelength range based on flexible harmonic generation and optical parametric oscillation

Abstract: A new laser-based and time-correlated single photon counting (TCSPC) detection system which allows easy and fast tuning of excitation wavelengths over a broad range from 240 to 1300 nm, with small gaps from 335 to 360 nm and 660 to 720 nm, has been built. The unique combination of a mode-locked Ti:sapphire laser, an optical parametric oscillator, pulse selectors, and harmonic generators delivers ultrafast laser pulses (1–2 ps) with variable repetition rates and excitation wavelengths. Performance characteristics of the laser system at different excitation wavelengths are reported and the TCSPC setup, which is characterized by a total instrument response function of 25 ps full width at half maximum, is described. Typical TCSPC measurements demonstrate the capability of the system of deriving decay or species associated excitation spectra.

Epitaxial growth of Pr2O3 on Si(111) and the observation of a hexagonal to cubic phase transition during postgrowth N2 annealing

Abstract: We demonstrate the epitaxial growth of hexagonal Pr2O3(001) on Si(111), with x-ray ω-scan full width at half maximum values as low as 0.06°, which is comparable with Si substrates. We find that a phase transition takes place during the anneal of the as-grown films in N2 below the growth temperature. The annealed films display a cubic structure isomorphic to manganese oxide, (111) oriented but 180° rotated about the Si(111) surface normal. The phase transition can be due to nitrogen incorporation.

Interlevel Ge/Si quantum dot infrared photodetector

Abstract: A self-assembled Ge/Si quantum dot interlevel infrared photodetector operating at room temperature and at normal incidence is demonstrated. The spectral response exhibits two peaks in the 58–82 and 132–147 meV energy regions with full width at half maximum linewidths as narrow as 25 meV. The two photocurrent maxima are ascribed to transitions from the hole ground state to the excited states in the dots. The peak detectivity and responsive quantum efficiency are 1.7×108 cm Hz1/2/W and 0.1% for the transition from the ground state to the first excited state and 7×107 cm Hz1/2/W and 0.08% for the transition from the ground state to the second excited state. At large dc bias, a redshift in the transition energies is observed. We argue that the resonance shifts are due to suppression of the depolarization field effect, representing the experimental manifestation of dynamic screening associated with collective electron–electron interaction in the dots.

Improving the performance of polymer light-emitting diodes using polymer solid solutions

Abstract: Optical and electronic properties of conjugated polymers can be modified significantly by arranging the polymer chains differently. For example, it is well known that polymer solutions of lower concentrations have different photoluminescence spectra from solutions of higher concentrations. In this manuscript, we demonstrated that such effects can also be achieved in solid states, i.e., in polymer solid solutions consisting of a narrower band-gap material—poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) with a wider band-gap polymer–poly(9,9-dioctylfluorene). At very low MEH-PPV concentrations, emission spectra of these solid solutions are similar to the photoluminescence spectrum of a diluted MEH-PPV solution in an ordinary organic solvent, suggesting the absence of a significant amount of interchain species. In addition, the electroluminescence efficiency is significant enhanced. Furthermore, the full width at half maximum (FWHM) is dramatically reduced at lower MEH-PPV concentrations...

Efficient organic red electroluminescent device with narrow emission peak

Abstract: We report a red fluorescent material of 3-(dicyanomethylene)-5,5-dimethyl-1(4-dimethylamino-styryl)cyclohexene (DCDDC) and its use as emitter or dopant in tris-(8-hydroxyquinoline) aluminum based electrolumiescent (EL) devices. The DCDDC was synthesized by a simple “one-pot” reaction and shows bright red photoluminescence with a peak wavelength at 650 nm. EL properties of double-layer light-emitting diodes of indium tin oxide (ITO)/poly(N)-vinylcarbazole (PVK):N,N′-diphenyl-N,N-bis(3-methylphenyl)1,1–biphenyl-4,4′-diamine (TPD)/DCDDC/Mg:Ag and ITO/PVK:TPD/DCDDC(1%):Alq3/Mg:Ag using DCDDC as red emitters or dopants were characterized. The device with 1% DCDDC-doped Alq3 as emitting layer has a turn-on voltage of 5 V, and the highest luminance of 5600 cd/m2 is obtained at 15 V. The EL efficiency of the device at 100 cd/m2 is in the range of 1.1 l m/W, and the highest EL efficiency is 1.6 l m/W. The devices emit in the red region, and the full width at half maximum of the EL spectra is 70 nm.

1.6 μm emission from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses

Abstract: 16 μm emission originated from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses were investigated under an optical pump of a conventional 1480 nm laser diode The measured peak wavelength and full width at half maximum of the fluorescent emission were ∼1650 and >100 nm, respectively A moderate lifetime of the thermally coupled upper manifolds (∼212±5 μs) together with a high stimulated emission cross section of ∼(3±1)×10−20 cm2 promises to be useful for 16 μm band fiber-optic amplifier that can be pumped with an existing high-power laser diode Codoping of Er3+ significantly enhanced the emission intensity by way of a nonradiative Er3+: 4I13/2→Pr3+: (3F3,3F4) energy transfer

Structural and electroacoustic studies of AlN thin films during low temperature radio frequency sputter deposition

Abstract: AIN is a material used in a wide variety of applications such as electroacoustic devices, blue diodes, IR windows, thermal conductors, metal-insulator-semiconductor structures, integrated circuit packaging, etc In this work thin piezoelectric AIN polycrystalline films have been grown on Si and SiO2 using rf magnetron sputter deposition in an Ar/N-2 gas mixture The structural properties of the film have been optimized by varying the deposition parameters, such as process pressure, gas mixture, substrate temperature, discharge power, etc [K Tominaga et al, Jpn J Appl Phys, Part 1 35, 4972 (1996); H Okana et al, ibid 31, 3446 (1992); K Kazuya, T Hanabusa, and K Tominaga, Thin Solid Films 281-282, 340 (1996)] It was found that the best film texture was obtained for a particular set of parameters, namely process pressure of 4 mTorr, substrate temperature 350 degreesC, discharge power 350 W, and a gas mixture of 25% Ar and 75% N-2 The films as examined by x-ray diffraction exhibited a columnar structure with a strong (001) texture, and a fall width at half maximum (FWHM) rocking curve of 16 degrees Atomic force microscopy measurements indicated a surface roughness with a rms value of 8 Angstrom Classical nonapodized transversal surface acoustic wave filters operating at a frequency of 534 MHz were fabricated to characterize the electroacoustic properties of the films The measurements indicated a coupling coefficient of 037% and a phase velocity of 4900 m/s Further, thin epitaxial films were grown on (001)alpha -Al2O3 (sapphire) under the same deposition conditions except the substrate temperature The films exhibited a (001)AlN//(001)alpha -Al2O3 plane orientation with a (002) rocking curve FWHM value of about 04 degrees, showing a relatively good alignment of the c axis The in-plane orientation was [110]AlN//[120]alpha -Al2O3 corresponding to a rotation of the AIN film of 30 degrees with respect to the (001)alpha -Al2O3 surface Cross-sectional transmission electron microscopy studies indicated a population of both thread and edge dislocations with decreasing concentrations with film thickness (C) 2001 American Vacuum Society

Optoelectronic device performance on reduced threading dislocation density GaAs/Si

Abstract: A technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer–Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the “monolithic thickness,” hm. Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than hm possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum (FWHM) of 110 arcsec with a dislocation density at the film surface of 3×106 cm−2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes (LEDs) that were grown on the reduced threading dislocation density GaAs/Si and by means of forward- and revers...

Growth of high-quality GaN on Si(111) substrate by ultrahigh vacuum chemical vapor deposition

Abstract: An ultrahigh vacuum chemical vapor deposition (UHVCVD) system was employed to grow high-quality GaN on a Si (111) substrate using a thin AlN buffer layer. X-ray diffraction, high-resolution electron microscopy (HREM), and photoluminescence (PL) data indicate that a single crystalline GaN layer with a wurtzite structure was epitaxially grown on a silicon substrate. The full width at half maximum (FWHM) of the x-ray rocking curve for the GaN (0002) diffraction was 16.7 arc min. A cross-sectional HREM image showed an amorphous SiNx layer at the Si/AlN interface, as well as stacking faults and inversion domain boundaries in the GaN epilayer. An intense PL emission line, which is associated with the recombination of the donor bound exciton, was observed at 10 K PL spectra (FWHM=6.8 meV) and a strong band edge emission was obtained (FWHM=33 meV) as well, even at room temperature. These results indicate that high-quality GaN can be grown on Si (111) substrates using a UHVCVD growth method.

High‐Efficiency GaN/AlxGa1—xN Multi‐Quantum‐Well Light Emitter Grown on Low‐Dislocation Density AlxGa1—xN

Abstract: Crack-free and low-dislocation-density Al x Ga 1-x N was achieved by low-temperature-deposited interlayer technique in combination with a lateral seeding epitaxy. We found that there was a strong correlation between the threading dislocation density and the PL intensity in the GaN/AIGaN MQWs. This new UV light emitting diode exhibits strong UV light output, having peak wavelength of 352 nm, a full width at half maximum as narrow as 6 nm and output power of 0.6 mW at 50 mA dc current injection.

Crack‐Free Thick GaN Layers on Silicon (111) by Metalorganic Vapor Phase Epitaxy

Abstract: The tensile strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy was investigated. Thick (0.9-2.5 μm) GaN layers without any crack were deposited on Si(111) using AIN/GaN superlattices as templates. X-ray diffraction and photoluminescence measurements were used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layer. Evolution of strain as a function of the GaN layer thickness is also determined. Dislocation reduction (from 10 10 to 2.5 x 10 9 cm -2 ) is observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed, with full width at half maximum of the I 2 line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) X-ray rocking curve attests the high crystalline quality of GaN on Si(111 ), when AlN/GaN superlattices are used.

Characterization of free-standing hydride vapor phase epitaxy GaN

Abstract: A free-standing GaN template grown by hydride vapor phase epitaxy has been characterized by transmission electron microscopy (TEM). The TEM investigation was augmented by x-ray diffraction, defect delineation etching process followed by imaging with atomic force microscopy and variable temperature photoluminescence. The density of dislocations near the N face was determined to be, in order, 3±1×107, 4±1×107, and about 1×107 cm−2 by cross-sectional TEM, plan-view TEM, and a defect revealing etch, respectively. The same methods on the Ga face revealed the defect concentration to be, in order, less than 1×107 cm−2 by plan-view TEM, less than 5×106 cm−2 by cross-sectional TEM, and 5×105 cm−2 by defect revealing hot H3PO4 acid, respectively. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (101_4) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth was a...

Photoluminescence and photoreflectance of GaInNAs single quantum wells

Abstract: The temperature dependence of photoluminescence (PL) and photoreflectance (PR) was studied on a 10 nm GaInNAs/GaAs single quantum well prepared by molecular-beam epitaxy using a solid As source. PL was dominated by the near-band edge PL peak with a full width at half maximum (FWHM) value of 16–28 meV for 50–280 K. This indicates that the GaInNAs epilayer was of good quality. The temperature dependence of the band gap energy was studied by the PR measurements, and the dependence was comparable to that of the GaInAs. The near-band edge PL at low temperature exhibited a blueshift with an increase in excitation intensity and temperature (8–50 K). It had a large FWHM value of 24–26 meV at 8 K. These results are discussed in terms of carrier localization.

Effect of Magnetic Field on Phosphorus Centre in Diamond

Abstract: This work focuses on a sample of diamond thin film which was grown epitaxially on polished {111} surface of synthetic type Ib diamond crystals. Phosphorus doping was achieved during growth using phosphine in the reactant gas ([P]/[C] = 500 ppm, [P] = 3 × 10 18 cm -3 ). Hall effect measurements show n-type conductivity and thermal activation energy of free electrons of 610 ± 10 meV. The sample was characterised by Fourier Transformed Infrared (FTIR) spectroscopy at 1.8 K under a magnetic field (0 to 13 Tesla) parallel to the {111} direction. The evolution of the 1S → 2P +/- absorption peak shows an increase of the FWHM (full width at half maximum) proportionally to the magnetic field. The results suggest a linear Zeeman splitting. For a magnetic field of 13 T the absorption peak splits into 1S → 2P + and 2P - . This splitting confirms the attribution of this peak to a transition to the 2P +/- excited state of phosphorus. A value for the transverse effective mass of the electron m t = 0.31m 0 has been deduced from the splitting.

Time-resolved sensing of organic vapors in low modulating porous silicon dielectric mirrors

Abstract: Time-resolved vapor diffusion into low modulating porous silicon dielectric mirrors has uncovered salient information about the spatial concentration distribution of the vapor within the porous matrix. Results indicate that relatively high concentrations of vapor result in instantaneous capillary condensation in the pores and yield a uniform concentration distribution. Results are modeled for both a concentration gradient and uniform filling of the porous matrix and compared to the experimental observations. Experiments with acetone, butanone, and pentanone have shown a linear dependence of the capillary condensation saturation time on the molecular weight of the organic solvents. An indicator of a concentration gradient within the matrix has been identified in the form of the full width at half maximum of the first-order reflectivity peak.

Influence of the temperature and excitation power on the optical properties of InGaAs/GaAs quantum wells grown on vicinal GaAs(001) surfaces

Abstract: Photoluminescence experiments were performed as a function of temperature and excitation intensity in order to investigate the optical properties of In0.1Ga0.9As/GaAs quantum wells grown on vicinal GaAs(001) substrates with different miscut angles. The misorientation of the surface played an important role and influenced the intensity, efficiency, energy, and full width at half maximum of the optical emission, as well as the segregation of indium atoms. It is shown that at high temperature the optical properties of InGaAs quantum wells grown on vicinal substrates are slightly inferior to ones of the same structure grown a nominal surface because of the faster escape of the carriers.

Energy resolution of 12 eV at 5.9 keV from Al-superconducting tunnel junction detectors

Abstract: An energy resolution of ΔE=12 eV [full width at half maximum (FWHM)] has been measured for the 55Mn Kα1 line (E=59 keV) using a superconducting lead absorber (90×90×13 μm3) readout by a single aluminum superconducting tunnel junction (100×100 μm2) The total detector area has been illuminated, while a slit mask of 150 μm width was used to partially shadow the detector leads and substrate area from impinging x rays The total electronic noise contribution was measured as ΔEelec=4 eV (FWHM) The superconducting tunnel junction is located on a Si3N4 membrane of thickness 03 μm, the lead absorber is separated from the superconducting tunnel junction’s top layer by a thin layer of natural aluminum oxide No deviations from linear energy response were observed in the energy range between E=174 keV and E=649 keV The same resolution was obtained for a single aluminum superconducting tunnel junction, where the influence of substrate events was suppressed by a metallic buffer layer between tunnel junction and

Red-Light-Emitting Organic Electroluminescent Devices with Bisanil Dye as Emitter

Abstract: Double-layer organic thin film electroluminescent (EL) devices having red emission have been constructed using bisanil dyes. The basic structure of the EL device consisted of a hole-transport layer and a luminescent layer between the indium-tin-oxide (ITO) glass and magnesium electrodes. The hole-transport layer was a N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD)-doped poly(N-vinylcarbazole) (PVK) film. The luminescent layer consisted of a host material, 8-hydroxyquinoline aluminum (Alq3), and bisanil dye as the dopant. The full width at half maximum (FWHM) of the PL and EL spectra of two bisanil dyes is as narrow as 50 nm. Among the two materials studied by us, the N,N'-bis[4-(N,N-dimethylamino)-benzylidene]diaminomaleonitrile (BAM) showed a better EL performance. A bright pure red light with a peak at 620 nm and a maximum luminance of 6230 cd/m2 was observed.

Coulomb-Volkov approach of atom ionization by intense and ultrashort laser pulses

Abstract: We present a nonperturbative theoretical approach, based on Coulomb-Volkov-type states, which is able to predict both angular and energy distributions of ejected electrons when atoms interact with a very short and intense laser pulse. In a previous paper [Eur. Phys. J. D 11, 191 (2000)], it was shown that, for atomic hydrogen targets, this theory makes accurate predictions as long as the interaction time does not allow more than two optical cycles. Recently, multigigawatt laser pulses with a full width at half maximum of less than two optical cycles have been generated by Nisoli et al. [Opt. Lett. 22, 522 (1997)] at $\ensuremath{\lambda}=800\mathrm{nm}.$ In the present paper, it is shown that predictions of the Coulomb-Volkov approach for the ionization of a hydrogen atom by laser pulses similar to the ones generated by Nisoli et al. are in very good agreement with the predictions of an ``exact'' numerical treatment. Further, the domain where the Coulomb-Volkov theory applies is marked out by means of a consistent accuracy parameter and by comparison with an ``exact'' numerical treatment. It is shown that, subject to the above-mentioned condition, good predictions may always be issued as long as the interaction time does not exceed half the initial orbital period of the electron. For a given laser pulse duration, predictions are all the better that the laser field amplitude is high and the initial quantum number is large.

Internal distortion in ceria-doped hafnia solid solutions: High-resolution x-ray diffraction and Raman scattering

Abstract: High-resolution synchrotron x-ray powder diffraction and Raman scattering have indicated the existence of tetragonal ceria-doped hafnia with both an axial ratio ${c}_{f}{/a}_{f}$ of unity and internal shear defomation. The values of ${c}_{f}{/a}_{f}$ ratio in ${\mathrm{HfO}}_{2}--X\ensuremath{-}{\mathrm{mol}\mathrm{}%\ensuremath{-}\mathrm{C}\mathrm{e}\mathrm{O}}_{2}$ samples $(X=80,$ 85, and 90) are estimated to be $1\ifmmode\pm\else\textpm\fi{}0.00004$ from the estimated standard deviation of the peak positions and to be $1\ifmmode\pm\else\textpm\fi{}0.002$ from the full width at half maximum of the ${400}_{f}$ reflection peaks. However, the Raman spectra of these samples clearly demonstrate the tetragonal phase. The intensity of the Raman band around 280 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is one of the characteristic bands of the tetragonal phase, continuously decreased and approached zero between X=90 and 95, which reflects the tetragonal-cubic phase boundary is located within this compositional range. The lattice distortion does not induce the cubic-tetragonal phase transition but an internal distortion in oxygen sublattice, oxygen displacement along the ${c}_{f}$ axis from the ideal fluorite position, does.

Growth of β-FeSi2 Thin Film on Si (111) by Metal-Organic Chemical Vapor Deposition

Abstract: We succeeded for the first time in preparing a high-quality (101)-oriented epitaxial β-FeSi2 film on Si(111) wafer by metal-organic chemical vapor deposition (MOCVD) using Fe(CO)5 and SiH4 as source materials. The full width at half maximum (FWHM) of the rocking curve of the β-FeSi2 (202) peak was 0.46 degree for the film deposited at 750°C at a rate of 4 nm/min. Moreover, a smooth-surface film of up to 650 nm thickness could be deposited by this method. Carbon content in the film was less than 0.1 at%.