Separation of Mid-IR Light Trapping and Sulfur-Donor Absorption in Nanosecond-Laser Sulfur-Hyperdoped Silicon: a Way to Spatially- and Spectrally-Engineered Interband IR-Absorption

We perform IR nanosecond laser microtexturing and hyperdoping of commercial silicon wafers under a 5 mm thick carbon-disulfide liquid layer at broadly variable laser exposures from incomplete to multi-shot irradiation per spot. All these samples demonstrate that the mid-IR-range optical densities and sulfur-donor dopant vary concentrations nonmonotonously versus laser intensity and scanning speed due to surface ablation and its vapor bubble screening. This enabled us, for the first time, to distinguish the predominating MIR-light trapping in microstructured Si at high exposures and a characteristic sulfur-donor MIR-absorption at lower exposures at rather constant sulfur concentrations. The characteristic interband sulfur-donor IR-absorption exhibits increasing magnitude for larger dissolved sulfur clusters versus increase in total sulfur concentration, paving the way for spatially- and spectrally-engineered MIR-absorption in hyperdoped Si for thin-film photovoltaics.     

飞秒纳秒脉冲激光微构造和掺杂单晶硅

在SF6气氛下，分别利用钛宝石飞秒脉冲激光与掺钕钇铝石榴石纳秒脉冲激光对单晶硅表面进行了微构造和重掺杂，以用于光伏材料。对制备的单晶硅表面微结构的形貌、结晶性和硫元素杂质含量与分布进行了研究。实验结果表明纳秒脉冲激光制备的单晶硅表面微结构的薄层电阻较小，缺陷密度较低（结晶性高），硫元素杂质含量较高且在表面分布的范围较广，深度较大（约1 m）。此外，材料的可见-近红外波段吸收率可接近80%。基于纳秒脉冲激光微构造的单晶硅的优异性能，在样品表面制备了有效光照面积达8 cm2的太阳能电池。其中，最佳太阳能电池的串联电阻、开路电压、短路电流密度分别为0.5 , 503 mV, 35 mA/cm2，转换效率约12%。上述太阳能电池性能还可通过优化制备工艺进一步提高。     

One-Step Nanosecond-Laser Microstructuring,Sulfur-Hyperdoping,and Annealing of Silicon Surfaces in Liquid Carbon Disulfide

We perform a single-shot IR nanosecond laser processing of commercial silicon wafers in ambient air and under a 2 mm thick carbon disulfide liquid layer. We characterize the surface spots modified in the liquid ambient and the spots ablated under the same conditions in air in terms of its surface topography, chemical composition, band-structure modification, and crystalline structure by means of SEM and EDX microscopy, as well as of FT-IR and Raman spectroscopy. These studies indicate that singlestep microstructuring and deep (up to 2–3% on the surface) hyperdoping of the crystalline silicon in its submicron surface layer, preserving via pulsed laser annealing its crystallinity and providing high (10⁴ cm^?1) spectrally flat near- and mid-IR absorption coefficients, can be obtained in this novel approach, which is very promising for thin-film silicon photovoltaic devices.     

Boron‐ and Phosphorus‐Hyperdoped Silicon Nanocrystals

Hyperdoping silicon nanocrystals (Si NCs) to a concentration exceeding the solubility limit of a dopant may enable their novel applications. Here, the successful hyperdoping of Si NCs with boron (B) and phosphorus (P) is demonstrated, which are the most important dopants for Si. Despite the hyperdoping, the diamond structure of Si NCs is hardly modified. There are both electrically active B and P in hyperdoped Si NCs. It is proposed that the hyperdoping is made possible mainly by the kinetics in the nonthermal plasma synthesis of Si NCs. Collision between Si NCs and B or P atoms and the binding energy of B or P at the NC surface are critical to the understanding on the differences in the doping efficiency and dopant distribution between B and P. B‐hyperdoping‐induced tensile stress needs to be taken into account in the investigation on the doping and oxidation of Si NCs.     

脉冲激光辐照p型硅单晶向n型转化

本文研究了p型硅单晶用红宝石脉冲激光辐照之后向n型转化的效应。这个效应与补偿杂质有关。实验结果表明:脉冲激光辐照之后补偿杂质浓度增加,补偿杂质磷分布发生变化,杂质补偿度大于7％的样品在被激光辐照的区域较容易转变为n型导电。这个效应与硅单晶中氧含量、晶面和晶体生长方式无关。 关键词：     

Phase transitions in erbium-doped silicon exposed to laser radiation

The dynamics of phase transitions induced by nanopulsed ruby laser radiation (80 nsec, 2 J/cm²) both in silicon layers doped with erbium ions and in those containing doped erbium and oxygen have been studied by an optical probing method. It is shown that the reflectivity behavior of structures under pulsed irradiation is governed by phase transitions (melting and crystallization) of implanted silicon and also by interference effects at the interfaces of the resulting phases. It is established that the profiles of erbium distribution change under nanosecond laser irradiation and that the dopant is forced out to the surface due to a segregation effect at small implantation doses. As the implanatation dose increases, diffusion deep into the sample tends to prevail over segregation. A considerable increase in the photoluminescence peak intensity at 0.81 eV is found after both the pulsed laser processing and thermal post-annealing of doped samples as opposed to spectra of samples subjected either to thermal annealing or to pulsed laser irradiation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 225–231, March–April, 2009.     

Investigation of an ion-implanted semiconductor layer by X-ray fluorescence analysis and ellipsometry

A method for determining the depth distribution of ion-implanted impurity atoms in semiconductors is developed. The method consists in measuring the concentration of impurities by X-ray fluorescence analysis upon the ellipsometry controlled removal of thin semiconductor layers. It is found that the prolonged low-energy X-ray radiation exposure of an ion-implanted semiconductor layer leads to a change in the distribution profile of the ion-implanted impurity atoms.     

Raman scattering study of ion implanted and C.W.-Laser annealed polycrystalline silicon

Recovery of crystallinity in ion-implanted and c.w.-laser annealed polycrystalline silicon films has been examined by the Raman spectroscopy. The variation of the Raman frequency and width with laser annealing and thermal annealing shows the presence of uniform strain in the laser annealed polysilicon. It is found that the subsequent thermal annealing releases the strain.     

Structural and electrical characteristics of a hyperdoped silicon surface layer with deep donor sulfur states

Inhomogeneous hyperdoping of a 100-nm-thick silicon surface layer with sulfur atoms at concentrations above 2 × 10²¹ cm^?3 was obtained via its femtosecond laser ablation in a sulfur-containing organic solvent. Infrared transmission spectroscopy reveals distinct interband absorption peaks of donor sulfur states, which are absent in the initial crystalline silicon, and a broad absorption band of free carriers with a concentration of ～10¹⁸ cm^?3. The rather low free-carrier concentration is related to equilibrium room-temperature ionization of localized donor sulfur states, preserving their nondegenerate character owing to the strong electronion binding in the donor states.     

Mathematical modeling of laser induced heating and melting in solids

An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.     

Range distributions for 25–200 keV 14N+ ions

Many features involved in the problem of heat flow through a semifinite metal sample irradiated with nanosecond laser pulses are discussed with reference both to the underlying physics and to the numerical solution of the heat diffusion equation.

Analytical expressions for several quantities, such as average velocities for the liquid-solid interface motion, maximum melted depth, and maximum surface temperature are presented. Experimental impurity profiles detected in Al samples after laser irradiation are analyzed on the basis of a diffusion-segregation hypothesis. The effects of ruby laser irradiation on virgin and ion-implanted Al single crystals, studied by using Rutherford backscattering and channelling are discussed in terms of epitaxial lattice regrowth, defect formation and metastable solid solutions.     

Epitaxial regrowth and defect formation during pulsed nanosecond annealing of amorphous layers. Part 1: Experimental results

The results of structure investigations of antimony ion-implanted silicon layers after the pulsed nanosecond annealing by ruby laser have been presented. Formation of micro-twins and tetrahedrons of stacking faults has been found to take place on the initial amorphous/crystalline Si interface for various degrees of liquid Si overcooling.     

TiN growth on Si by hybrid radical beam PLD

High-quality (good crystallinity and stoichiometry) titanium nitride (TiN) thin films were grown on Si(100) substrates by pulsed laser deposition (PLD) using a high-purity titanium target (99.99%) and nitrogen radical beam. The crystallinity, chemical composition, and depth profiles of the grown films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS), respectively. The XRD pattern indicated that the preferred growth of TiN(200) with an orientation parallel to the Si(100) direction was obtained and the nitrogen radical drastically improved crystallinity compared with that grown in ambient nitrogen gas. RBS spectra indicated that the combination of PLD and the nitrogen radical beam suppressed silicidation at the interface between the Si substrate and TiN thin film during growth. The XPS analysis revealed that this method achieved the synthesis of stoichiometric TiN films.     

Halfway-plane and continuum calculations of blocking dips for displaced atoms

Abstract

Redistribution of phosphorus dopant in 1N2175 phototransistors damaged by a pulsed neodynium laser has been investigated using the electron microprobe technique. Four different effects observed in the concentration profdes were explained by the action of diffusion and segregation, in the presence of temperature and compositional gradients. One specific effect was analyzed using a simplified model consisting of diffusion in the melt followed by the segregation upon refreezing. It was estimated that certain areas of the phototransistor surface remained above the melting point for times of the order of 10^?4 sec. Correlation was made between the results of these microprobe measurements and previously observed laser-damage effects in the electrical characteristics. It is suggested that impurity migration may be an important mechanism for damage to junction devices by pulsed lasers in an intermediate energy range.     

Studies of pulsed laser melting and rapid solidification using amorphous silicon

Pulsed laser melting of ion implantation-amorphized silicon layers, and the subsequent solidification of undercooled liquid silicon, have been studied experimentally and theoretically. Measurements of the time of the onset of melting of amorphous silicon layers, during an incident laser pulse, have been combined with measurements of the duration of melting, and with modified melting model calculations to demonstrate that the thermal conductivity, K_a, of amorphous silicon is very low (K_a0.02 W/cm K). K_a is also found to be the dominant parameter determining the dynamical response of amorphous silicon to pulsed laser radiation; the latent heat of fusion and melting temperature of amorphous silicon are relatively unimportant. Transmission electron microscopy indicates that bulk (volume) nucleation occurs directly from the highly undercooled liquid silicon that can be prepared by pulsed laser melting of amorphous silicon layers at low laser energy densities. A modified thermal melting model has been constructed to simulate this effect and is presented. Nucleation of crystalline silicon apparently occurs at a nucleation temperature, T_n, that is higher than the temperature, T_a, of the liquid-to-amorphous phase transition. The model calculations demonstrate that the release of latent heat by bulk nucleation occurring during the melt-in process is essential to obtaining agreement with experimentally observed depths of melting. These calculations also show that this release of latent heat accompanying bulk nucleation can result in the existence of buried molten layers of silicon in the interior of the sample after the surface has solidified. It is pointed out that the occurrence of bulk nucleation implies that the liquid-to-amorphous phase transition (produced using picosecond or ultraviolet nanosecond laser pulses) cannot be explained by purely thermodynamic considerations.     

第一性原理计算绝缘体-金属转变临界掺杂浓度:Co重掺杂Si体系

基于密度泛函理论的第一性原理方法,本文旨在探索确定绝缘体-金属转变临界浓度的理论计算方法.以Co重掺杂Si为研究对象,构建并计算了10个Co不同掺杂浓度模型的晶体结构、杂质形成能及其电子性质.发现在Co掺杂Si体系的带隙中形成了杂质能级,杂质能级的位置和宽度随着Co浓度的增加呈线性变化.当Co掺杂浓度较高时杂质形成能逐渐稳定,且杂质能级穿过费米能级使体系表现出金属性.综合杂质形成能的变化趋势,以及杂质能级极小值与费米能级间的距离条件,可预测出发生绝缘体-金属转变的Co掺杂浓度为2.601Wingdings 2MC@10~(20) cm~(-3),与实验结果相一致.上述两条依据应用于S重掺杂Si体系和Se重掺杂Si体系同样成立.     

离子注入硅片快速退火后的红外椭偏光谱研究

离子注入硅片经高温退火后晶体结构缺陷会被修复,其在可见光波段下的光学性质趋于单晶硅,常规的可见光椭偏光谱法对掺杂影响的测量不再有效.本研究将测量波段扩展到红外区域(2—20μm),报道了利用红外椭偏光谱法测量经离子注入掺杂并高温退火的硅片掺杂层光学和电学性质的方法和结果.通过建立基于Drude自由载流子吸收的等效光学模型,得到了杂质激活后掺杂层的杂质浓度分布、电阻率和载流子迁移率等电学参数,以及掺杂层的红外光学常数色散关系,分析了这些参数随注入剂量的关系并对其物理机理给予了解释.研究表明,中远红外椭偏测量是表征退火硅片的有效方法,且测量波长越长,所能分辨的掺杂浓度越低.     

Simulation of the melting and crystallization processes in monocrystalline silicon exposed to nanosecond laser radiation

Numerical simulation of the melting and crystallization processes of monocrystalline silicon exposed to the nanosecond radiation of a ruby laser was carried out with the kinetics of the phase transformations accounted for on the basis of Kolmogorov equations. A two-dimensional mechanism of nucleation and growth of the new phase was invoked to describe the phase transitions. It was shown that the temporal dependences of monocrystal overheating and liquid phase supercooling in the melting and crystallization stages, respectively, are nonmonotonic and determined by the kinetics of the phase transitions. The maximum values of the overheating and supercooling were ∼100 K.     

Peculiarities of pulsed ion implantation from a laser plasma containing multiply charged ions

A mathematical model describing the dynamics of a pulsed laser plasma with multiply charged ions, as well as the formation of the accelerated ion flow in an external magnetic field, is developed. Experimental studies and mathematical simulation by the particle-in-cell method are used to determine the role of multiply charged ions in the process of ion implantation into a silicon substrate from the pulsed plasma containing singly and doubly charged titanium ions. The plasma spreads between parallel-plate electrodes (Ti target and Si substrate) along the normal to the surface of the target. Ions are accelerated by high-voltage negative pulses applied to the substrate. It is found that doubly charged ions effectively participate in the implantation process when an external electric field is applied very soon after the laser action on the target. The application of a high-voltage pulse with an amplitude of 50 kV 0.5 μs after a laser pulse leads to ion implantation with an energy close to 100 keV. With increasing delay in the application of the high-voltage pulse, the upper boundary of the energy spectrum of implanted ions is displaced towards lower energies. Comparison of the depth profiles of titanium distribution in silicon calculated from the results of simulation are compared with the experimental profiles shows that the model developed here correctly describes the formation of the high-energy component of the ion flow, which is responsible for defect formation and doping of deep layers of the substrate.     

激光辐照下薄膜镜面波纹损伤机理研究

 用透镜将脉冲激光束聚焦在铝膜镜面上，在部分样品的激光损伤边缘区，出现波纹状损伤形貌。大多数情况下，有多套波纹同时存在，在镜面形成花纹，或者其中有一两套波纹占优势。提出了脉冲激光辐照下光学元件损伤区边缘产生波纹的一种光干涉模型：认为样品表面本身存在微米级杂质颗粒或者表面缺陷点在激光辐照下首先产生鼓包变形，鼓包或杂质颗粒将激光反射（散射）在未发生变形的区域，与直接辐照在该区域的激光束产生干涉，使得变形区周围的光强呈周期性分布，当变形区进一步损伤后，则在损伤区周围留下了波纹状图案。模拟实验验证了这种设想。