Laser annealing of semiconductors

Conclusions The previous exposition should allow one to understand the basic mechanisms involved in laser annealing. Many aspects of it have not been treated due to lack of space, e. g., the defects associated with the laser irradiation and the technological implication of the method, which may be found in recent literature [64].Intituto di Fisica Ingegneria, Università, Roma, Italy, and GNEOP of CNR, Italy. Published in Primenenie Lazerov v Atomnoi, Molekulyarnoi i Yadernoi Fizike-Trudy II Vsesoyuznoi Shkoly, pp. 165–179, 1981.     

Laser induced superconducting properties in semiconductors

We show that an amorphous semiconductor irradiated by a laser light exhibits a sort of the Meissner effect. A guided light in a thin film of an amorphous semiconductor is stopped or modified in its intensity when the semiconductor is illuminated by another light.     

Electronic theory of irradiation-induced disoerdering and annealing in semiconductors

A new model of irradiation-induced disordering of semiconductors is proposed. According to this model, the disordered regions capable of self-annealing are stabilized by self-localized electronic excitations (electrons, holes, excitons). Pulsed annealing of these regions occurs through recombination of the electronic stoppers and dispersal of disordered regions thus takes place. This model agrees well with the experiments on amorphisation and laser pulse annealing. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 67–72, April 2006.     

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.     

Laser pulse design using optimal control theory-based adaptive simulated annealing technique: vibrational transitions and photo-dissociation

We have designed and optimised a combined laser pulse using optimal control theory-based adaptive simulated annealing technique for selective vibrational excitations and photo-dissociation. Since proper choice of pulses for specific excitation and dissociation phenomena is very difficult, we have designed a linearly combined pulse for such processes and optimised the different parameters involved in those pulses so that we can get an efficient combined pulse. The technique makes us free from choosing any arbitrary type of pulses and makes a ground to check their suitability. We have also emphasised on how we can improve the performance of simulated annealing technique by introducing an adaptive step length of the different variables during the optimisation processes. We have also pointed out on how we can choose the initial temperature for the optimisation process by introducing heating/cooling step to reduce the annealing steps so that the method becomes cost effective.     

Laser technology for submicron-doped layers formation in semiconductors

A p–n junctions formed by means of laser stimulated diffusion of dopants into semiconductors (Si, GaAs, GaP, InP) were investigated. SIMS and AES spectroscopy methods were used to measure the depth profiles of the incorporated impurities: B into Si, Zn into GaAs, GaP and InP. The volt-capacity method using an electrochemical profilometer was used for the charge carrier concentration distribution in the doped layer. Spectroscopy investigations have shown that during solid phase diffusion locally doped regions almost exactly reproduce the shape and size of the windows in the dielectrics. The lateral diffusion of the dopant is about 0.01μm. The concentration profiles of charge carrier distribution in the doped layers clearly show the specific processes of dopant diffusion and evaporation at laser solid-phase doping of semiconductors. The comparative analysis of parameters of formed semiconductor structures shows that the procedure of laser solid-phase doping can stand the comparison with technology of implantation and conventional diffusion technology. Since the laser solid-phase doping ensures also a high degree of reproducibility of p–n junction parameters, it can be effectively used for electronic devices fabrication.     

Crystallographic changes in electron pulse annealing of Ti-implanted GaP

ABSTRACT

Gallium phosphide can be considered as a prospective material for impurity band solar cell (IBSC), if sufficient amount of an appropriate impurity (Ti in our case) is introduced to the material by e.g. ion implantation without distorting the crystallographic order, necessary to maintain the semiconducting properties of the host. In our experiments, the crystallographic order (damaged by ion implantation) is restored by electron pulse annealing (EPA). When the EPA process using subthreshold electron pulse energy density is studied by RBS technique, a peculiar electron-pulse induced lattice reconstruction is observed, consisting in growth of the damaged region beyond the range observed after ion implantation. This phenomenon is confirmed by transmission electron microscopy (TEM) results and interpreted in terms of melting nuclei formed around the implanted ions in a material with high sublimation pressure.     

Experimental tests for boson condensation and superconductivity in semiconductors during pulsed beam annealing

Van Vechten and Compaan and, independently, Nagy and Noga have proposed that the high reflectivity state observed in pulsed laser/electron/ion beam annealing of Si and other semiconductors results when the electron-hole plasma created by the initial absorption of the ionizing radiation undergoes a boson condensation. The condensed state is described in the same form as the BCS superconductor and has many of the properties of the superconducting state. Lattice temperatures for the material during this phase have beeb measured by Raman scattering to lie in the range from 500 to 1000 K. At this writing no magnetic experiment has been completed to test this proposition further. In addition to the practical problems associated with such a test, there are theoretical questions such as whether or not the bipolar plasma ought to be able to expel a magnetic field within the 100 ns or so that it persists in the proposed boson condensed phase. If not, it does not exhibit perfect diamagnetism and the Meissner effect. An experiment is proposed that is argued to be both easy to perform and to offer a conclusive demonstration of boson condensation. The proposed experiment involves observation of the single plasmon mode by diffraction or from the appearance of linear ripples left on the solid surface and the rotation of this pattern. Preliminary observations seem to confirm the condensation.     

Laser annealing of radiation defects in low disordered layers

Abstract

Carrier concentration and mobility dependences on the annealing temperature have been compared for the cases of thermal (10 min) and laser (8 ms) annealing of silicon, implanted with low doses of P⁺ ions. It was found that the recovery of concentration and mobility depended on the heating time in different ways. The laser annealing requires higher temperatures for mobility recovery then the thermal treatment. The same temperature shift was observed for increase of carrier concentration but only after doses less than 3.10¹²m^?2. When the doses exceeded 3.10¹² cm^?2 laser and thermal annealing resulted in equal electron concentrations providing the equality of heating temperatures. Annealing of disordered regions was accounted for the mobility recovery. The increase of electron concentration was explained as an instantaneous decay of defect-impurity complexes. To check the validity of the assumptions laser annealing of electron and light ions irradiated materials was investigated.     

S=1/2体系的EPR波谱模拟

用一个以Fortran语言编制的S=1/2体系的EPR模拟谱程序.并引入了弛豫项,图谱线型得到了改善,使模拟谱与实验谱更为接近;对超超精细分裂的考虑,使该程序可适合于复杂图谱的模拟;求积分方法的近似处理,缩短了模拟谱的耗时,使复杂图谱的模拟在微机上成为现实.     

Laser pulse reflection by anisotropic plasma

The effect of anomalous amplification of test laser pulse reflected by a nonequilibrium plasma formed under atom ionization by a strong pulse of circularly polarized wave is described. It is shown that the gain in radiation intensity may reach ten orders of magnitude. The most effective amplification takes place for frequencies comparable with the Weibel instability growth rate.     

Reactor scale modeling of multi-walled carbon nanotube growth

As the mechanisms of carbon nanotube (CNT) growth becomes known, it becomes important to understand how to implement this knowledge into reactor scale models to optimize CNT growth. In past work, we have reported fundamental mechanisms and competing deposition regimes that dictate single wall carbon nanotube growth. In this study, we will further explore the growth of carbon nanotubes with multiple walls. A tube flow chemical vapor deposition reactor is simulated using the commercial software package COMSOL, and considered the growth of single- and multi-walled carbon nanotubes. It was found that the limiting reaction processes for multi-walled carbon nanotubes change at different temperatures than the single walled carbon nanotubes and it was shown that the reactions directly governing CNT growth are a limiting process over certain parameters. This work shows that the optimum conditions for CNT growth are dependent on temperature, chemical concentration, and the number of nanotube walls. Optimal reactor conditions have been identified as defined by (1) a critical inlet methane concentration that results in hydrogen abstraction limited versus hydrocarbon adsorption limited reaction kinetic regime, and (2) activation energy of reaction for a given reactor temperature and inlet methane concentration. Successful optimization of a CNT growth processes requires taking all of those variables into account.     

Nonlinear pulse propagation in semiconductors: hole burning within a homogeneous line

Features reminiscent of spectral hole burning in a homogeneous line are predicted to result from the interaction of small area pulses with the semiconductor exciton resonance. The small area pulses may be designed through pulse shaping or evolve naturally in bulk semiconductors via polaritonic effects. The spectral features exhibit signatures that are characteristic for the underlying material nonlinearity and should occur in any system with isolated spectral resonances and coherent nonlinearities.     

Laser pulse modulation instabilities in partially stripped plasma

The laser pulse modulation instabilities in partially stripped plasma were discussed based on the phase and group velocities of the laser pulse and the two processes that modulation instabilities excited. The excitation condition and growth rate of the modulation instability were obtained. It was found that the positive chirp and competition between normal and abnormal dispersions play important roles in the modulation instability. In the partially stripped plasma, the increased positive chirp enhances the modulation instability, and the dispersion competition reduces it.     

核电堆内构件围板螺栓超声检测优化有限元仿真研究

围板螺栓是核电堆内构件的关键紧固部件,典型的围板螺栓主要为外六角形式。在长期辐照和震动等恶劣环境服役过程中,其断裂韧性下降,脆性增加,最终导致螺栓退化开裂或断裂脱落,需要对其进行定期无损检测。然而由于其结构的复杂性,现有超声检测方法仍难以对其进行有效检测。为进一步提高超声检测方法对于围板螺栓内部缺陷的检测能力,针对堆内构件围板螺栓典型缺陷失效形式,通过有限元数值仿真技术开展适用于围板螺栓不同区域(螺栓过渡区域、螺杆区域和螺纹区域)的超声检测方法和工艺研究,对超声探头激励频率、晶片尺寸及超声波入射角度等进行优化,实现对不同区域15%缺陷横截面积占比的检测目标。为研发具有高度匹配性和高灵敏度的堆内构件围板螺栓超声传感器、实现对围板螺栓敏感区域的高精度检测提供理论指导。     

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO₂ laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.     

Laser short pulse heating of metal nano-wires

Non-equilibrium energy transfer takes place in a solid substrate during a short-pulse laser irradiation and temperature field can be obtained analytically in the irradiated region. In the present study, laser short-pulse heating of metal nano-wire is considered and the analytical solution for two-dimensional axisymmetric nano-wire is presented. Since the absorption of the incident beam takes place in the skin of the irradiated surface, a volumetric heat source resembling the absorption process is incorporated in the analysis. Three different nano-wire materials are introduced in the analysis for the comparison reason. These include silver, chromium, and copper. It is found that temperature decay is gradual on the surface vicinity and temporal variation of the surface temperature follows almost the laser pulse intensity profile at the irradiated center.     

磁脉冲压缩系统的仿真研究

 分析了传统磁脉冲压缩系统的原理及缺点,对基于省去磁芯复位电路的磁脉冲压缩系统进行了介绍,为了更加深入地分析这种磁脉冲压缩系统,应用Pspice仿真分析软件建立了省去磁芯复位电路的磁脉冲压缩系统模型,计算出各级压缩过程中各元件的电压波形,及饱和变压器和磁开关的磁芯感应强度变化曲线;经测试,当负载为500 Ω纯阻性负载时,系统两端输出的负极性脉冲峰值约-27 kV,半高宽约为70 ns,下降沿约为40 ns。通过分析仿真结果与实验结果,仿真中需考虑测量探头引入的并联电容对电路的影响。     

Laser annealing of silicon layers amorphized by molecular ions

The behaviour, after laser beam annealing, of heavily doped silicon layers obtained by a high current density atomic and molecular ion bombardment is investigated. The ion beam is realized by glow discharge of a gas containing the dopant, acceleration towards the sample, without any magnet.

The annealing is performed by using a high power (3.5 J/cm²) pulsed laser and the surfaces are studied by Rutherford backscattering, secondary ions mass spectroscopy and conductivity measurements. Comparison with a classical thermal annealing shows the advantage of the laser pulse to restore completely the original cristallinity, even if the layer is im-planted at doses in excess the solubility limit of the dopant, leading to a full surface amorphization.     

激光脉冲波形优化与材料压缩特性

 结合快点火靶丸预压缩涉及的基本物理问题,分析了驱动激光脉冲波形对材料压缩过程的影响,并利用辐射流体力学程序MULTI模拟了不同激光波形下的材料压缩情况。结果表明:通过调节激光脉冲波形,优化激光初始强度、激光强度开始上升时间和上升速度,控制压缩过程中的熵增,可以获得更高的材料压缩度。