Fabrication and doping of poly-SiGe using excimer-laser processing

Pulsed Laser-Induced Epitaxy (PLIE)/Gas Immersion Laser Doping (GILD) offers several advantages over alternative epitaxial processes, especially because the process can be made spatially selective. Here, a pulsed XeCl excimer laser is used to grow poly-Si_1–xGe_x layers with Ge fractions up to 30% by intermixing a structure of electron beam-evaporated a-Ge on poly-Si deposited on quartz. Arsenic or boron dopant is incorporated during the melt process by using, respectively, an AsF₅ or BF₃ gas ambient. RBS and SIMS analysis reveal that the Ge metallurgical depth, the dopant junction depth and the incorporated dopant dose scale with the laser energy density and the number of laser pulses. The sheet resistance values reached after GILD process are low enough to be suitable for the fabrication of source and drain for poly-SiGe TFTs.     

Optoacoustic laser spectroscopy of excited vibrational molecular states

A new detection method for absorption from excited vibrational states is suggested, based on optoacoustic detection of weak absorption in a heated gas. Using this method CO₂ laser radiation (λ=9.6 μm) absorption was investigated from excited vibrational states of CO₂, BCl₃, and BF₃ molecules.     

Photoabsorption of BCl3 Gas under pulsed ArF excimer laser irradiation

The gas immersion laser doping (GILD) technique requires the measurement of the fraction of incident light absorbed in the gas phase during the irradiation with a pulsed laser. Here we report the absorption of boron trichloride (BCl₃) gas at the wavelength of a pulsed ArF excimer laser (=193 nm). We have determined the one-photon (₁) and two-photon () absorption cross sections of this dopant gas for 193 nm. The values of ₁ and are 3.6×10^–20 cm² and 9×10^–45 cm⁴·s, respectively. However, the distinction between simultaneous and sequential absorption has not been possible. Based on these results, we have established a relationship which allows the calculation of the fraction of incident light absorbed as a function of incident intensity and gas pressure.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Lewis acid-base surface interaction of some boron compounds with N-doped graphene; first principles study

We studied density functional theory (DFT) calculations in terms of energetic and electronic properties toward adsorption of some boron compounds (B(OCH₃)₃, BF₃ and BC1₃) on the surface of pristine as well as N-doped graphene using WB97XD/6-31 + G(d,p) level of theory. The net charge transfer of mentioned molecules on the surface of pristine and N-doped graphene was calculated with above-mentioned basis set using natural bond orbital and Mulliken charge analysis during complex formation. The computed dipole moment shows when above-mentioned molecules approach to the surface of N-doped graphene, the amount of the dielectric (μD) will change depending on the kind of molecule. Our calculations reveal that N-doped graphene system has much higher adsorption energy, higher net charge transfer value than pristine graphene due to Lewis acid-base interaction. Comparing B(OCH₃)₃ as an organic boron derivative with boron trihalides (BF₃ and BCl₃), the Lewis acidity increases in the order of BF₃ < BC1₃< B(OCH₃)₃ with adsorption energies (E_ads) of −8.7, −18.3 and −26.5 kJ/mol (BSSE) respectively, while low adsorption energies were calculated on pristine graphene for mentioned molecules.     

Basic features of boron isotope separation by SILARC method in the two-step iterative static model

In this paper we develop a new static model for boron isotope separation by the laser assisted retardation of condensation method (SILARC) on the basis of model proposed by Jeff Eerkens. Our model is thought to be adequate to so-called two-step iterative scheme for isotope separation. This rather simple model helps to understand combined action on boron separation by SILARC method of all important parameters and relations between them. These parameters include carrier gas, molar fraction of BCl₃ molecules in carrier gas, laser pulse intensity, gas pulse duration, gas pressure and temperature in reservoir and irradiation cells, optimal irradiation cell and skimmer chamber volumes, and optimal nozzle throughput. A method for finding optimal values of these parameters based on some objective function global minimum search was suggested. It turns out that minimum of this objective function is directly related to the minimum of total energy consumed, and total setup volume. Relations between nozzle throat area, IC volume, laser intensity, number of nozzles, number of vacuum pumps, and required isotope production rate were derived. Two types of industrial scale irradiation cells are compared. The first one has one large throughput slit nozzle, while the second one has numerous small nozzles arranged in parallel arrays for better overlap with laser beam. It is shown that the last one outperforms the former one significantly. It is argued that NO₂ is the best carrier gas for boron isotope separation from the point of view of energy efficiency and Ar from the point of view of setup compactness.     

Diagnostics of a Compact Discharge-Pumped XeCl Laser with BCl3 Halogen Donor

The results of a comprehensive study of a compact UV-preionized XeCl laser are presented. The subjects of this study were: discharge voltage and current measurements, dye laser probing of the active medium, and the mass spectrometry of gas mixture degradation products. It is shown that the gas lifetime was significantly improved when the laser was operated with BCl₃ as a halogen donor instead of commonly used HCl. By the dye laser absorption and gain probing, the temporal and spatial dependences of the densities for several plasma components, Ne*, Xe*, Xe⁺*, Cl^–, XeCl* and of ground state boron atoms were measured. Some aspects of plasma kinetics for uniform and constricted phases of the discharge are discussed. By the mass spectrometry of gas mixture degradation products on long-term operation of the laser device several gaseous (N₂, O₂, CO₂, H₂O, C₂H₄) and solid (NiCl₂, H₃BO₃) products were detected in the laser chamber. NH₄Cl was determined to be a stable fraction of the deposits on optics surfaces. The reasons for the improvement of gas lifetime for BCl₃-containing gas mixtures are discussed.     

Uniform design and regression analysis of LPCVD boron carbide from BCl3-CH4-H2 system

Boron carbide was prepared by low pressure chemical vapor deposition (LPCVD) from BCl₃-CH₄-H₂ system. The deposition process conditions were optimized through using a uniform design method and regression analysis. The regression model of the deposition rate was established. The influences of deposition temperature (T), deposition time (t), inlet BCl₃/CH₄ gas ratio (δ), and inlet H₂/CH₄ gas ratio (θ) on deposition rate and microstructure of the coatings were investigated. The optimized deposition parameters were obtained theoretically. The morphologies, phases, microstructure and composition of deposits were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman micro-spectroscopy, transmission electron microscopy (TEM), energy dispersive spectra (EDS), and Auger electron spectra (AES), the results showed that different boron carbides were produced by three kinds of deposition mechanisms.     

Quadratic potential functions for the boron trihalides

General quadratic compliance constants have been evaluated for the boron trihalides using frequency data and Coriolis coupling constants obtained from a variety of sources. Coriolis data from band contour analysis are too imprecise to help determine the E′ symmetry potential constants. Solutions obtained using Cyvin's Coriolis constants appear to be reasonable on the basis of trends in the valence compliants and interaction displacement coordinates. The semiempirical SCF-MO scheme MNDO has been employed to establish prior constraints of the interaction displacement coordinates in the estimate of the quadratic potential constants for BF₃ and BCl₃.     

Influence of laser fluence and dopant gas pressure on properties of photochemical doping of Si into GaAs using XeCl excimer Laser

Silicon doping into GaAs has been performed with the combination of pulsed XeCl excimer laser (wavelength: 308 nm) and silane gas (SiH₄). Sheet resistances and depth profiles of the Si-doped GaAs as the functions of laser fluence, the number of laser pulses and gas pressure have been measured in order to make clear the relation between properties of doped GaAs and irradiation conditions. The secondary ion mass spectroscopy (SIMS) has revealed that the depth of Si in GaAs is limited in such a very shallow region (30–110 nm) that might be controlled easily by irradiation conditions. The efficiency for carrier generation of Si in GaAs with laser fluence is discussed.     

Two processes that lower the emission energy of an electric-discharge KrF laser

Results are reported of an experimental investigation of the processes that lower the emission energy of an electric-discharge excimer KrF laser operating on mixtures containing F₂ and NF₃. The existence is demonstrated of two processes, reversible and irreversible, that lower the KrF-laser emission energy as the number of excitation pulses is increased (without continuous replenishment of the mixture) and as the pulse repetition frequency is increased. The irreversible process is connected with the decrease of the concentration of initial halogen-containing gas in the mixture as a result of interaction between the halogen atoms and the chamber material. The reversible process is due to the long reduction time of the halogen-containing molecule (~ 1 sec for F₂) and influences the laser emission energy only at pulse repetition frequencies that exceed the reciprocal time of reduction of these molecules. If complex halogen-containing molecules (NF₃, SF₆, ...) are used, the pulse-repetition regime is realized because of the radicals that are produced. The use of such molecules, however, affects adversely the service life of the excimer gas mixture.Translated from Lazernye Sistemy, pp. 46–57, 1982.     

Ultra-violet laser doping of silicon

In situ generation of boron from triethyl boron has been used in the ultra-violet laser doping of silicon, over a range of dopant concentrations. The quality of the doped material has been investigated using Auger electron analysis and electrical probes, and indicates that high activated dopant concentrations are readily achieved, although the purity of the material is degraded by unwanted alkyl derivatives.     

Formation of p-type layer by KrF excimer laser doping of carbon into GaAs in CH4 gas ambient

Carbon doping of GaAs using a KrF excimer laser to form a p-type active layer is described. Methane gas (CH₄) was used as a source of the C acceptor. Various quantities such as sheet resistance, surface carrier density, Hall mobility, and depth profile of C-doped GaAs are measured as the functions of laser fluence and laser pulse. It is shown that C atoms are doped only within a limited depth as shallow as 50 nm or less and with extremely high concentration exceeding 1×10²¹ cm^–3. The maximum activation efficiency is found to be 69.0%. Laser induced changes of surface morphologies and electron diffraction patterns are also discussed. Furthermore, non-alloyed ohmic contacts using laser-doped p-type GaAs are demonstrated.     

Effect of adding Ar gas on the pulse height distribution of BF<Subscript>3</Subscript>-filled neutron detectors

Boron trifluoride (BF₃) proportional counters are used as detectors for thermal neutrons. They are characterized by high neutron sensitivity and good gamma discriminating properties. Most practical BF₃ counters are filled with pure boron trifluoride gas enriched up to 96% ¹⁰B. But BF₃ is not an ideal proportional counter gas. Worsening of plateau characteristics is observed with increasing radius due to impurities in gas. To overcome this problem, counters are filled with BF₃ with an admixture of a more suitable gas such as argon. The dilution of BF₃ with argon causes a decrease in detection efficiency, but the pulse height spectrum shows sharper peaks and more stable plateau characteristics than counters filled with pure BF₃. The present investigations are undertaken to study the pulse height distribution and other important factors in BF₃+Ar filled signal counters for neutron beam applications. Tests are performed with detectors with cylindrical geometry filled with BF₃ gas enriched in ¹⁰B to 90%, and high purity Ar in different proportions. By analysing pulse height spectra, a value of 6.1 ± 0.2 has been obtained for the branching ratio of the ¹⁰B(n,α) reaction.      

Target density and surface state effects on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and the gas condensation process. Sintered iron oxide (-Fe₂O₃) targets were ablated in oxygen ambient by an ArF excimer laser. The product of ablation comprised Fe₂O₃ at lower pressure and a mixture of Fe₂O₃ and FeO at higher pressure by X-ray-diffraction measurements. The maximum ambient oxygen pressure, P_S(O₂), at which the product composition was still a single Fe₂O₃ phase was higher for the higher-density target than for its lower-density counterpart. The target surface state affected the product composition only if the pressure was set to the pressure P_S(O₂) of 40 Pa for a high-density target. When the fluence was high (200 mJ/pulse, 3.3-mm² spot size), the product composition varied at the initial stage of laser irradiation with the number of laser pulses from a mixture of Fe₂O₃ and FeO to only Fe₂O₃ along with the target surface morphological change from a grooved structure to a smooth surface. Product composition was practically independent of the number of pulses by low-fluence laser irradiation even at this particular pressure of 40 Pa. PACS 81.07.Bc; 81.15.Fg; 61.10.Nz     

Performance study of the KrCl discharge laser with liquid chlorine donors

The effects of liquid chlorine donors (BCl₃ and CCl₄) on the output energy and the efficiency of a KrCl laser (222 nm) are investigated. Maximum laser energies of 230 mJ (3.3 J/l) in pulses of 16 ns duration were obtained from a 0.035% BCl₃/10.1% Kr/89.8% Ne mixture at a total pressure of 2600 Torr, and at a specific power loading of about 24 MW/cm³, a power conversion efficiency of 0.86% was obtained. These figures are reduced by a factor of two when He is used as buffer gas. Much weaker laser emissions were obtained when CCl₄ was used as a chlorine donor.     

First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

强红外场作用下BCl3振动态传能动力学研究

基于文献[1],本工作在较高泵浦激光能通量范围,测量并研究了BCl₃分子振动激发v_3吸收谱及时间演变,观察了v₃激发弛豫的几种能量转移过程,以及对泵浦激光能量BCl₃气压等参数的依赖关系。表明泵浦光脉冲产生一个振动态非热分布的系综,转动能量转移对引起这种非热分布有重要作用。用简化碰撞动力学模型讨论了BCl₃振动激发吸收谱的演变过程,得到振动态再分布的简单关系;Pτ_v-v≌c/K′(T_v,T₀,q)和等效振动温度、平均吸收光子数的分析表达式,与实验结果定性地符合。 关键词：     

The role of diffusion in broadband infrared absorption in chalcogen-doped silicon

Sulfur doping of silicon beyond the solubility limit by femtosecond laser irradiation leads to near-unity broadband absorption of visible and infrared light and the realization of silicon-based infrared photodetectors. The nature of the infrared absorption is not yet well understood. Here we present a study on the reduction of infrared absorptance after various anneals of different temperatures and durations for three chalcogens (sulfur, selenium, and tellurium) dissolved into silicon by femtosecond laser irradiation. For sulfur doping, we irradiate silicon in SF₆ gas; for selenium and tellurium, we evaporate a film onto the silicon and irradiate in N₂ gas; lastly, as a control, we irradiated untreated silicon in N₂ gas. Our analysis shows that the deactivation of infrared absorption after thermal annealing is likely caused by dopant diffusion. We observe that a characteristic diffusion length—common to all three dopants—leads to the reduction of infrared absorption. Using diffusion theory, we suggest a model in which grain size of the resolidified surface layer can account for this characteristic diffusion length, indicating that deactivation of infrared absorptance may be caused by precipitation of the dopant at the grain boundaries.     

Large-scale synthesis of neodymium hexaboride nanowires by self-catalyst

Large scale NdB₆ nanowires have been successfully fabricated for the first time using a self-catalyst method with Nd powders and boron trichloride (BCl₃) gas mixed with hydrogen and argon. X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the samples. Transmission electron microscopy (TEM) reveals that the NdB₆ nanowires are single crystals with cubic structure. Our investigation forms part of a series of studies for finding comparatively inexpensive methods to prepare RB₆ nanomaterials.