Experimental study and computer simulations of the implantation of laser plasma ions in pulsed electric fields

Results are presented from experimental studies of pulsed plasma flows generated by nanosecond laser pulses with an intensity of 7 × 10⁸ W/cm² from a solid-state target in a strong electric field. The current pulses through the laser target and the depth distributions of the iron ions implanted in a silicon substrate to which a negative high-voltage pulse was applied are measured. The physical processes occurring in laser plasma with an initial iron ion density of 6 × 10¹⁰ cm⁻³ are simulated numerically by the particle-in-cell method for different delay times and different shapes of the accelerating high-voltage pulse. The model developed allows one to calculate the ion flows onto the processed substrate, the electron flows onto the target, and the energy spectra of the implanted ions. The results from computer simulations are found to be in good agreement the experimental data.     

纳秒激光电离分子团簇产生高价离子实验研究

 利用飞行时间质谱仪，研究了功率密度为10⁹~10¹¹ W/cm²，波长为532 nm 的纳秒激光对苯、呋喃、甲醇及碘甲烷分子团簇的激光电离过程。实验观察到了高平动能的高价离子C^q+(q≤3)，O^q+(q≤3)和I^q+(q≤4)，该过程经历了以“初始的多光子电离引发-逆轫致吸收加热-电子碰撞电离模式”为主的激光团簇作用过程，后期经历了团簇的库仑爆炸过程。实验发现：即使激光能量变化一个量级以上时，主要高价离子的种类及占全部离子产物的比率也没有明显的变化，但是高价离子的初始平动能随激光强度的增大而增加；分子中含有较多个外壳层电子的氧、碘原子更容易电离产生高价离子，而碳离子的价态和强度相对较低。     

Hyperdoping of silicon with deep-level impurities by pulsed YAG laser melting

Hyperdoping with deep-level impurity is a promising method to prepare intermediate band semiconductors. We prepared silicon hyperdoped with deep-level impurities, sulfur and titanium, by ion implantation followed by pulsed YAG laser melting. The processes of sulfur and titanium hyperdoping are comparatively studied. The amorphous sulfur and titanium ion-implanted layers changed to monocrystal by following pulsed laser melting. The depth profile of sulfur impurity after pulsed laser melting is similar to that of ion-implanted sample, while large segregation is observed for titanium hyperdoping. The crystallinity and degree of segregation depend on the laser shot number and initially implanted titanium dose. There is a trade-off between crystallinity and depth profile of impurity for titanium hyperdoping. From a viewpoint material processing, formation of high-quality silicon monocrystal hyperdoped with sulfur is easier than that with titanium. Correlation between the mid-infrared optical absorption and photoconductivity is also discussed for sulfur-hyperdoped sample.     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

Radiation effects and pulsed laser deposition of titanium by Nd:Yag laser

A study of Ti laser irradiation and thin film deposition produced by an Nd:Yag pulsed laser is presented. The laser pulse, 9?ns width, has a power density of the order of 10¹⁰?W/cm². The titanium etching rate is of the order of 1?µg/pulse, it increases with the laser fluence and shows a threshold value at about 30?J/cm² laser fluence. The angular distribution of ejected atoms (neutrals and ions) is peaked along the normal of the target surface. At high fluence, the fractional ionization of the plasma produced by the laser is of the order of 10%. Time-of-flight measurements demonstrate that the titanium ions, at high laser fluence, may reach kinetic energies of about 1?keV. Obtained results can be employed to produce energetic titanium ions, to produce coverage of thin films of titanium and to realize high adherent titanium-substrate interfaces. The obtained results can be employed to produce energetic titanium ions, to produce a coverage of thin titanium films on polymers, and to realize highly adherent titanium–substrate interfaces.     

Recombination processes in atomic clusters irradiated by superintense femtosecond laser pulses

Various mechanisms of recombination of electrons with multiply charged atomic ions in atomic clusters irradiated by superintense femtosecond laser pulses are discussed. All of the recombination mechanisms are shown to take a time considerably longer than the laser pulse duration and, hence, they can develop only in a homogeneous, fairly rarefied cluster plasma after pulse termination. All autoionization states of multiply charged ions in a dense cluster plasma have been found to be destroyed by the Holtsmark electric field.     

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)³He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD₂ foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and ¹¹B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.     

Creating Ti:Al<Subscript>2</Subscript>O<Subscript>3</Subscript> media with pinched beams of electrons and Ti<Superscript><Emphasis Type="Italic">n</Emphasis>+</Superscript> ions

The collective acceleration of multiply charged titanium ions on pinched electron beams and the mechanisms of titanium ion implantation into sapphire crystals are studied. Efficient Ti³⁺ ion implantation is achieved by irradiating sapphire crystals with high current picosecond beams of electrons and Ti ⁿ⁺. The ions are shock-implanted into regular positions of the crystalline lattice, replacing Al³⁺ ions.     

Pulsed nanosecond annealing of magnesium-implanted silicon

Single-crystalline silicon is implanted by magnesium ions at room temperature and then subjected to pulsed ion-beam annealing. The surface morphology, crystallinity, and optical properties of the implanted silicon are studied before and after annealing. It is shown that ion implantation makes a near-surface layer of silicon about 0.1 m thick amorphous. Pulsed nanosecond ion-beam annealing results in silicon recrystallization and the formation of crystalline magnesium silicide precipitates. Optimal values of the implantation dose and pulse energy density for the formation of magnesium silicide precipitates in the near-surface layer of silicon are found.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Laser plasma acceleration of quasi-monoenergetic beams of light multicharged ions

Experimental results on the spectra of residual impurity ions from a plasma created on the surface of a tungsten target that is irradiated by a femtosecond laser pulse and is subjected to preliminary pulsed laser cleaning are reported. It is shown that deep modulation corresponding to the formation of a quasi-monoenergetic ion beam appears in the energy spectra of protons and light multicharged carbon ions in this case. The position of a “dip” at the scale of velocities closely correlates with the position of the front of heavier ions.     

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

Charge composition of metal ions versus laser radiation density in laser mass spectrometry

From experimental data for the yield of differently ionized ions at laser mass spectrometry, the dependence of the charge composition of the ions on the laser radiation density is found for a number of metals. It is established that the amount of multiply charged ions predominates over low-charged ones when the radiation density grows. An explanation is given for the fact that singly charged ions prevail over multiply charged ones at the end of laser-induced plasma spread. The influence of recombination on the ion distribution in the mass spectrum is estimated.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

Ion acceleration in a solitary wave by an intense picosecond laser pulse

Acceleration of ions in a solitary wave produced by shock-wave decay in a plasma slab irradiated by an intense picosecond laser pulse is studied via particle-in-cell simulation. Instead of exponential distribution as in known mechanisms of ion acceleration from the target surface, these ions accelerated forwardly form a bunch with relatively low energy spread. The bunch is shown to be a solitary wave moving over expanding plasma; its velocity can exceed the maximal velocity of ions accelerated forward from the rear side of the target.     

A hard-tube pulser of 60 kV, 10 A for experiment and modeling inplasma immersion ion implantation

This paper describes an experiment and modeling in plasma immersion ion implantation using a high-voltage pulsed power system. This consists of a high-voltage pulse generator that uses a hard tube switch. The reason for using this type of circuit category in the Plasma Immersion Ion Implantation (PIII) facility rather than a previously used pulse-forming network (PFN) circuit configuration is stated. The experimental results of the application of this device to a glow discharge PIII are also discussed. In order to assess these results, a simple electrical model describes the plasma as a resistive load in parallel with a capacitance taking into account the pulse rise-time distortion caused by a long connecting coaxial cable. Plasma parameters for PIII processing, such as ion average implantation current and plasma sheath thickness, are calculated from the experimental settings     

Multiple charge states of titanium ions in laser produced plasma

An intense laser radiation (10¹² to 10¹¹ W/cm⁻²) focused on the solid target creates a hot (≥1 keV) and dense plasma having high ionization state. The multiple charged ions with high current densities produced during laser matter interaction have potential application in accelerators as an ion source. This paper presents generation and detection of highly stripped titanium ions (Ti) in laser produced plasma. An Nd:glass laser (KAMETRON) delivering 50 J energy (λ=0.53 μm) in 2.5 ns was focused onto a titanium target to produce plasma. This plasma was allowed to drift across a space of ∼3 m through a diagnostic hole in the focusing mirror before ions are finally detected with the help of electrostatic ion analyzer. Maximum current density was detected for the charge states of +16 and +17 of Ti ions for laser intensity of ∼10¹¹ W/cm⁻².     

Doubly charged ions resulting from the multiphoton atomic ionization of alkaline-earth metals

The study of the creation of doubly charged ions upon the laser-induced multiphoton ionization of alkaline-earth metals is reviewed. The results show that the features of the creation of the doubly charged ions depend on the spectral range of the ionizing laser radiation. In the ionization of the alkaline-earth atoms by the visible laser radiation, the cascade creation of the doubly charged ions is predominantly implemented when the ions result from the multiphoton ionization of singly charged ions created in the presence of the same laser pulse. In the multiphoton ionization of the alkaline-earth atoms by the IR laser radiation, the two-electron process occurs when the doubly charged ions are directly created from neutral atoms owing to the ionization via induced resonances with strongly perturbed states of these atoms.     

高压耦合高功率脉冲磁控溅射的增强放电效应

等离子体源离子注入与沉积技术作为一种可生产高结合力、高致密度涂层的真空镀膜技术,具有广阔的应用前景,尤其适用于高载荷工况下服役的功能涂层制备.该技术中金属等离子体源是关键,而现有的脉冲阴极弧源结构复杂,且由于伴随"金属液滴"而需要增加过滤装置.本文研究了另一种简单结构的金属等离子体源备选一高功率脉冲磁控溅射源(HPPMS)的放电特性,采用等离子体发射光谱仪探索了不同的耦合高压对HPPMS放电靶电流特性和等离子体特性的作用.发现耦合高压对HPPMS放电有明显的促进作用,相同靶电压下的放电强度大幅增加,相对于金属放电,耦合高压对气体放电的促进作用更加明显,但在自溅射为主的高压放电阶段对金属放电的促进作用明显增强.讨论了耦合高压对HPPMS放电的增强机制,发现耦合高压自辉光放电、耦合高压和HPPMS电压构成双向负压形成的空心阴极效应,以及耦合高压鞘层改善的双极扩散效应都对HPPMS放电的增强有明显作用.