Improvedp/n junctions in Ge-doped GaAs grown by molecular beam epitaxy

The effects of single-pulse ruby laser irradiation have been investigated in Si samples with disorder layers located at a depth of 2000 Å from the crystal surface and extending up to 8000 Å. This disorder was obtained by implantation with 350 keV N⁺ to a fluence of 2×10¹⁶/cm². Channeling, diffraction and transmission electron microscopy were used to characterize the structure of the irradiated layers. After 1.5 J/cm² irradiation the damaged layer reorders partially, while for about 2.0J/cm² the surface single crystal becomes polycrystalline. At a higher energy density all the material undergoes the transition to single crystal. Calculations based on the liquid model accounts in part for the experimental results.     

Lasing of optically pumped XeF and analysis of the spectrum of the laser transitionB^2 \sum _{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^ + - X^2 \sum _{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^ +

The operating principle of a photodissociation XeF laser (λ?350 nm) is discussed. A laser operating on the gas mixture XeF₂:N₂(SF₆):Ar pumped by radiation from an open high-current discharge is described. The maximum lasing duration is 5 μsec, the maximum output energy is 0.15 J. The vibrational structure of the XeF-molecule transition B²Σ_1/2 ⁺ ? X²Σ_1/2 ⁺, observed in absorption, luminescence, and stimulated emission, is investlgated. The vibrational quantum and the anharmonicity in the upper and lower states are measured: ω_e ^' = 308.7 cm^-1, ω_e ^'x_e ^' = 1.44 cm^-1, ω_e ^'' = 225.7 cm^-1, ω_e ^''x_e ^'' = 11.0 cm^-1. The difference between the equilibrium internuclear distances of the XeF molecule is determined for the excited (B) and ground (X) states, namely r_e ^' - r_e ^'' = 0.33 +- 0.01 Å.     

The geometry and forcefield of acetylene

The variational method has been used to determine the geometry and ground state potential surface of acetylene. All the parameters were refined through a least-squares fit to J = 0, 1 levels for C₂H₂ and C₂D₂. A new program was written to evaluate the rovibrational energy levels; in particular, primitive basis sets were developed for all values of J taking into account the singularity for linear geometries. Thus Σ, II, δ states can be refined. The full theory for tetraatomic linear molecules is presented. In this refinement 150 observed levels were used as data, below 10 000cm^?1. The geometry was refined and gives R_e(CC) = 1.2028 Å, R_e(CH) = 1.0618 Å, to be compared with the best experimentally derived values of 1.2027 ± 0.0005 Å, 1.062 ± 0.001 Å, respectively. The zero point energies are 5771.1cm^?1 for C₂H₂ and 4571.1cm^?1 for C₂D₂.     

Caries resistance of lased human root surface with 10.6 μm CO2 laser-thermal,morphological, and microhardness analysis

Although the cariostatic effects of CO₂ laser on enamel have been shown, its effects on root surface demineralization remains uncertain. The objectives of this in vitro research was to establish safe parameters for a pulsed 10.6 μm CO₂ laser and to evaluate its effect on morphological features of the root surface, as well as on the reduction of root demineralization. Ninety-five human root surfaces were randomly divided into five groups: G1-No treatment (control); G2—2.5 J/cm²; G3—4.0 J/cm²; G4—5.0 J/cm²; and G5—6.0 J/cm². Intrapulpal temperature was evaluated during root surface irradiation by a thermocouple and morphological changes were evaluated by SEM. After the surface treatment, the specimens were submitted to a 7-day pH-cycling model. Subsequently, the cross-sectional Knoop microhardness values were measured. For all irradiated groups, intrapulpal temperature changes were less than 1.5°C. Scanning electron microscopy images indicated that fluences as low as 4.0 J/cm² were sufficient to induce morphological changes in the root surface. Additionally, for fluences reaching or exceeding 4.0 J/cm², laser-induced inhibitory effects on root surface demineralization were observed. It was concluded that laser energy density in the range of 4.0 to 6.0 J/cm² could be applied to a dental root to reduce demineralization of this surface without compromising pulp vitality.     

表面层对1 064 nm高反射镜损伤阈值影响

 采用电子束蒸发的方法制备了３种具有不同表面层材料及结构的中心波长为1 064 nm的零度高反镜,３种膜系表面层分别为1/4波长光学厚度的HfO₂,1/2波长光学厚度的SiO₂,以及1/4波长光学厚度的SiO₂。光谱测试表明：三者在1 064 nm处均有较高的反射率（高于99.8%）,利用热透镜的方法测量得到３个膜系辐照激光正入射情况下,薄膜对光的吸收比例分别为3.0×10^-6,5.0×10^-6和6.5×10^-6,其损伤阈值分别为32.5,45.2和28.4 J/cm²。并在膜层内部电场分布和膜层材料物理特性的基础上分析了３种不同表面层膜系吸收和损伤阈值差别的原因。     

Laser induced reorder in Pb implanted Ge

Ge samples implanted with 40 keV Pb at a fluence of 3×10¹⁵/cm² were irradiated with ruby laser single pulses of 15ns duration. Reordering of the damaged layer occurs for energy density irradiation above 0.6 J/cm². The Pb atoms redistribute with a large component at the sample surface which is easily etched off. The remaining part of Pb impurities is substitutionally located, and the concentration exceeds the solid solubility limit by three order of magnitude. The formation of the metastable solution is explained in terms of a transient liquid layer produced during laser irradiation.     

Quenching kinetics and small signal gain spectrum of the ZnI photodissociation laser

By photodissociation ZnI₂ with 193 nm (ArF) laser radiation, the rate constants for quenching of the upper and lower energy levels of the ZnI (B → X) laser by ZnI₂ have been measured to be (1.7 ± 0.2) × 10^-9 and (1.4 ± 0.4) × 10^-9 cm³ s^-1, respectively. Although the former rate constant was found to be laser intensity-dependent for I ? 10⁵ W cm^-2, the ZnI(B) state radiative lifetime was determined to be 26 ± 4 ns. Also, the small signal gain coefficient, g₀, of this molecular laser has a peak value of ? 15% cm^-1 at λ ? 602 nm and exceeds 5% cm^-1 for 591 nm ≤ λ ≤ 608 nm for a potential tuning range of at least 170 Å.     

Structure of 4,4′-butane-(1,4,7-three-p-tolylsulphonyl-1,4,7-three amine) diphthalonitrile

Abstract

The crystal structure of the title compound, C₄₁ H₃₅ N₇ O₆ S₃ was determined as monoclinic by single crystal X-Ray diffraction technique. The molecular structure was identified by IR, ¹H-NMR, ¹³C-NMR and elemental analysis. The crystal parameters of this compound are as follows: monoclinic P 2 1/n, a = 12.694(2) Å, b = 26.204(2) Å, c = 13.005(2) Å, β = 102.95(2)°, V = 4216.02(1) Å.³, Z = 4, Dx = 1.289 g/cm³, F(000) = 1704, λ (MoKα) = 0.71070 Å, μ = 0.2 mm^?1. The structure was solved by SHELXS-97 and refined by SHELXL-97. R = 0.06 for 3178 observed reflections with I > 2σ (I).     

The influence of the implantation temperature on the generation of dislocations in antimony-implanted and annealed silicon

We report the first observation of non-equilibrium redistribution effects during laser treatment of a binary system having equilibrium segregation coefficient, k₀, much greater than unity. Polycrystalline aluminium samples implanted with 30 keV Sb⁺ ions to a dose of 1.7 × 10¹⁷ ions/cm² were irradiated with single pulses (7 ns FWHM) from a Nd: glass laser operating in TEM₀₀ mode. The peak energy density (at the centre of the laser spot) varied from 2.0 to 5.7 J/cm². A detailed liquid phase diffusion analysis, explicitly incorporating rapid melt front motion and interfacial segregation, is performed to fit the Sb depth profiles measured with a nuclear microprobe. An effective distribution coefficient k = 1 (as compared to equilibrium value of k₀～7) is obtained in agreement with the theoretical limiting value for large melt front velocities.     

Structure of bis(4-Methyl Pyridine) Cadmium Tetracyanonickelate

Abstract

The structure, C₁₆H₁₄CdN₆Ni, consist of corrugated polymeric networks made up of tetracyanonickelate ions coordinated to Cd. The 4-methyl pyridine molecules bound to Cd in trans positions are located on both sides of the network. The bonding in the networks occurs because of a departure of the Ni-C-N-Cd sequence of atoms from linearity at the C and N positions. The crystal structure of the title compound was determined as monoclinic by single crystal X-Ray diffraction technique. The crystal parameters of this compound are as follows: monoclinic C2/m, a=18.156(2) Å, b=7.581(2) Å. c= 6.983(2) Å, β = 110.09(2)°, V = 902.6(5) ų Z=2, Dx = 1.698 g/cm³, F(000) - 456, λ (MoKα) = 0.71070 Å, μ = 2.121 mm^?1. The structure was solved by SHELXS-86 and refined by SHELXL-93. R = 0.02 for 1074 observed reflections with I > 2[sgrave] (I).     

Lyman alpha (1216 Å) generation in krypton

We measured the nonlinear susceptibilities of Kr up to fifth order via nonresonant third-harmonic generation (THG) of a frequency-doubled dye laser at 3648 Å. In agreement with theory the optimum intensity conversion efficiency saturates at 10^-5 in pure Kr. The phase matching of the third harmonic and fundamental wave breaks up owing to intensity-dependent changes of refractive indices. Phase matching in high-density Kr/Ar mixtures raised the 3648 Å to 1216 Å third harmonic conversion obtained in pure Kr by more than two orders of magnitude, as predicted by theory but hitherto never experimentally confirmed. With laser intensities of 5 × 10⁹ W/cm² at 3648 Å we measured conversion efficiencies of 1.4 × 10^-4 limited by gas breakdown. Taking into account the Lyman alpha absorption of Kr₂ and KrAr molecules in the high-density regime, a conversion efficiency of 10^-3 is expected from extrapolating the experimental results to Kr/Ar mixtures of up to 3 × 10³ torr. It is limited by self-focusing, assuming the minimum threshold intensity of gas breakdown is 10¹¹ W/cm², as predicted by theory.     

Silicon substrate temperature effects on surface roughness induced by ultrafast laser processing

We report the effect of substrate temperature (T_sub) in the range 300-900 K on the surface roughness of silicon wafer resulted from femtosecond laser ablation. The surface roughness observed at the laser fluences less then 0.3 J/cm² increases with increasing T_sub. However, the surface roughness decreases with increasing T_sub for the laser fluences between 0.5 and 1.0 J/cm². If the laser fluence is higher than 2.0 J/cm², the surface roughness is independent of T_sub. The effect of T_sub on the surface roughness can be understood in terms of the temperature dependence of optical absorption coefficient of silicon substrate, which eventually alters a mechanism underlying the fs-laser-material ablation process between optical penetration and thermal diffusion processes.     

Excimer-laser ablation and micro-patterning of ceramic Si3N4

3 N₄ has been investigated. The ablation threshold in air, Φ_th, is around 0.3±0.1 J/cm² with ArF- and 0.9±0.2 J/cm² with KrF-laser radiation. With fluences Φ_th<Φ<4 J/cm² the irradiated surface is either very flat or it exhibits a cone-type structure, depending on the number of laser pulses employed. With fluences of 5 to 10 J/cm², the sample surface becomes very smooth, much smoother than the original mechanically polished surface. Pores, scratches, and cracks observed on the non-irradiated surface are absent within the illuminated area. In this regime, the ablation rates are typically 0.1 to 0.2 μm/pulse. Received: 10 April 1997/Accepted: 11 April 1997     

Surface modulation of silicon surface by excimer laser at laser fluence below ablation threshold

Controlled single step fabrication of silicon conical surface modulations on [311] silicon surface is reported utilizing KrF excimer laser [λ=248 nm] at laser fluence below ablation threshold laser fluence. When laser fluence was increased gradually from 0 to 0.2 J/cm² for fixed 200 numbers of shots; first nanopores are observed to form at 0.1 J/cm², then very shallow nanocones evolve as a function of laser fluence. At 0.2 J/cm², nanoparticles are observed to form. Up to 0.15 J/cm² the very shallow nanocone volume is smaller but increases at a fast rate with laser fluence thereafter. It is observed that the net material volume before and after the laser irradiation remains the same, a sign of the melting and resolidification without any ablation.     

Absorption of CO2 laser pulses at different wavelengths by ground-state and vibrationally heated SF6

The absorption of CO₂ laser pulses by low pressure SF₆ gas has been investigated over a wide range of energy fluxes. For laser energy fluxes of 0.01–1 J cm^-2 the effective absorption cross section varies between 0.2 and 2 × 10^-18 cm². For each laser line an individual dependence on the energy is found and in some cases minor changes in the absorption behaviour seem to occur around 0.1 J cm^-2. SF₆ excited with an average vibrational energy content of up to 20 photons/molecule does not absorb measurable amounts of 9.4 μm laser light. The influence of various SF₆ and Ar pressures on the temporal shape of the transmitted pulses has been investigated.     

Low-fluence CO2 laser irradiation decreases enamel solubility

This study investigated whether subablative-pulsed CO₂ laser (10.6 μm) irradiation, using fluences lower than 1 J/cm², was capable of reducing enamel acid solubility. Fifty-one samples of bovine dental enamel were divided into three groups: control group, which was not irradiated (CG); group laser A (LA) irradiated with 0.3 J/cm²; and group laser B (LB) irradiated with 0.7 J/cm². After irradiation, the samples were subjected to demineralization in an acetate buffer solution and were then analyzed by SEM. A finite-element model was used to calculate the temperature increase. The calcium and phosphorous content in the demineralization solution were measured with an ICP-OES. ANOVA and the t-test pairwise comparison (p < 0.016) revealed that LB showed significantly lower mean Ca and P content values in the demineralization solution than other groups. A reduction in the enamel solubility can be obtained with pulsed CO₂ laser irradiation (0.7 J/cm², 135 mJ/pulse, 74 Hz, 100 μs) without any surface photomodification and a less than 2°C temperature increase at a 3-mm depth from the surface.     

Electron paramagnetic resonance in deuterated nickel fluosilicate

Electron paramagnetic resonance measurements in single crystals of NiSiF₆. 6D₂O were made at K, Ku and Ka bands at 4.2 K and between 77 K and 300 K. The measured g values were in the range 2.23–2.26, while the zero-field splitting parameter D varied from ?(0.185 ± 0.005) cm^?1 at 4.2 K to ?(0.53 ± 0.01) cm^?1 at 298 K. The parameters of the trimolecular hexagonal unit cell were determined to be approximately a = 9.28 Å, c = 9.58 Å from powder X-ray diffraction measurements at room temperature.     

Characterization of a LiCoO2 thin film cathode grown by pulsed laser deposition

A highly (003)-oriented pure LiCoO₂ thin film cathode, without Co₃O₄ impurities, was grown on a stainless steel substrate by pulsed laser deposition and characterized by electrochemical testing, scanning electron microscopy (SEM), ex situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). The initial reversible discharge capacity of the LiCoO₂ thin film cathode reached 52.5?μAh/cm²µm and capacity loss was about 0.18% per cycle at a current density of 12.74?μA/cm². The chemical diffusion coefficient of the Li⁺ ion was estimated to be about 4.7?×?10^?11?cm²/s from cyclic voltammetric (CV) scans. Ex situ XRD revealed that the spacing of crystalline planes expanded about 0.09?Å when charged to 4.2?V, corresponding to Li_0.5CoO₂, lower than the value for composite powder LiCoO₂ electrodes. XPS results showed that the number of low-coordinated oxygen ions increased relative to the removal of Li⁺ ions.     

Infrared spectrum and vibrational potential function of the chlorite anion in the matrix-isolated M+ClO2− species

The chlorite anion, ClO₂^?, was prepared for spectral investigation by depositing alkali metal atoms and chlorine dioxide at high dilution in argon onto a 15°K optical surface. Intense infrared absorptions near 820, 790, and 420 cm^?1, which exhibited small alkali metal effects, are due to the three intraionic modes of the chlorite anion in the M⁺ClO₂^? species. The ClO force constant for ClO₂^? (4.11 mdyn/Å) is less than the value for ClO₂ (6.61 mdyn/Å), which is consistent with the antibonding character of the additional electron on ClO₂^?.     

Optical properties of laser-processed In x Ga1−x As

Various spots in GaAs, In-diffused with the 1.064 μm line of pulsed Nd:YAG laser with several energy densities, have been characterized and compared with samples prepared by the conventional rapid thermal annealing method. Of the energy densities used, the spot processed with an energy density of 7 J/cm² shows In _x Ga_1?x As phases with an indium concentration of 60% and below. An abrupt boundary in the indium concentration is observed at the edge of the laser-annealed spot. The diffusion depth is found to be less than 1000 Å. The spot processed with an energy density of 14 J/cm² shows considerable damage from the irradiation resulting in strain in the lattice. The samples prepared by the thermal annealing method show similar results to the laser-diffused samples. However, these thermally annealed samples suffer from arsenic loss unlike the laser-processed samples. It can be concluded that laser-induced alloying of indium into GaAs can be achieved with less arsenic loss than the thermal annealing method.