Laser-induced Defects Research Articles

High resolution structural characterisation of laser-induced defect clusters inside diamond

Laser writing with ultrashort pulses provides a potential route for the manufacture of three-dimensional wires, waveguides, and defects within diamond. We present a transmission electron microscopy study of the intrinsic structure of the laser modifications and reveal a complex distribution of defects. Electron energy loss spectroscopy indicates that the majority of the irradiated region remains as sp3 bonded diamond. Electrically conductive paths are attributed to the formation of multiple nano-scale, sp2-bonded graphitic wires and a network of strain-relieving micro-cracks.

Study on optical and electrical properties of gold-doped silicon fabricated by femtosecond laser

Microstructured silicon (Si) materials have been fabricated by femtosecond (fs) laser ablation and have been hyperdoped with gold (Au) impurities. The ablated Si materials showed large and thermostable infrared absorption at 1.1–2.5 μm wavelengths, which was contributed by sub-band absorption and laser-induced defects absorption. The Au–Si alloy was formed after laser irradiation onto the Au-coated Si surface, which was determined by XRD characterization. Using N-type Si substrate, the fabricated Au-doped Si performed lower sheet carrier density due to the self-compensation effect between deep donor and acceptor energy lever of Au in bulk Si material. From Hall measurement, both the p- and n- types of Au-doped Si samples can be obtained by controlling the type of Si substrate.

Selective emitter solar cell through simultaneous laser doping and grooving of silicon followed by self-aligned metal plating

Both buried contact solar cells (BCSC) and laser doped selective emitter (LDSE) solar cells have achieved considerable success in large-scale manufacturing. Both technologies are based on plated contacts. High metal aspect ratios achieved by BCSC allow low shading loss while the buried metal contacts in the grooves provide good contact adhesion strength. In comparison, although the LDSE cell achieves significantly higher efficiencies and is a much simpler approach for forming the selective emitter region and self-aligned metal plating, the metal adhesion strength falls well short of that achieved by the BCSC. Recent studies show that plated contacts based on the latter can be more durable than screen-printed contacts. This work introduces a new concept of laser doping with grooving to form narrow grooves with heavily doped walls in a simultaneous step, with the self-aligned metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The laser-doped grooves are only 3–5µm wide and 10–15µm deep; the very steep walls of these grooves remain exposed even after the subsequent deposition of the antireflection coating (ARC). This unique feature significantly reduces the formation of laser-induced defects since the stress due to the thermal expansion mismatch between the ARC and silicon is avoided. Furthermore, the exposed walls allow nucleation of the subsequent metal plating. This novel structure also benefits from greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon surface in the same way as the BCSC. Cell efficiencies over 19% are achieved by using this technology on p-type Czochralski (Cz) wafers with a full area aluminum (Al) back surface field (BSF) rear contact. It is expected that much higher voltages and consequently higher efficiencies could be achieved if this technology is combined with a passivated rear approach.

Controlled energy deposition and void-like modification inside transparent solids by two-color tightly focused femtosecond laser pulses

We report a bulk void-like micromodification of fused silica using two-color μJ-energy level tightly focused (NA = 0.5) co-propagating seeding (visible, 0.62 μm) and heating (near-IR, 1.24 μm) femtosecond laser pulses with online third harmonic diagnostics of created microplasmas as well as subsequent laser-induced void-like defects. It has been shown experimentally and theoretically that production of seeding electrons through multiphoton ionization by visible laser pulses paves the way for controllability of the energy deposition and laser-induced micromodification via carrier heating by delayed infrared laser pulses inside the material. Experimental results demonstrate wide possibilities to increase the density of energy deposited up to 6 kJ cm−3 inside the dielectric by tight focusing of two color fs-laser pulses and elliptical polarization for infrared heating fs-laser pulses. The developed theoretical approach predicts the enhancement of deposited energy density up to 9 kJ cm−3 using longer (mid-IR) wavelengths for heating laser pulses.

Fractal analysis of the Rayleigh Photoinduced Light Scattering Pattern from LiNbO3:Zn Crystals

Fractal analysis was used to study Rayleigh photoinduced light scattering (PILS) patterns in a series of LiNbO3:Zn single crystals (0.018–0.88 mass%) that were grown from the congruent melt and were promising as nonlinear optical materials with low photorefraction and coercive-field values. Results from fractal analysis and Raman light-scattering spectroscopy were compared. Extremes found on the time dependence of the fractal dimension of various layers of the PILS pattern speckle structure indicated that the concentration of laser-induced defects in the photorefractive crystal changed. The rate of change of the concentration of laser-induced defects depended non-linearly on the crystal Zn concentration. The form of congruent Zn-doped LiNbO3 crystals with the most ordered structure was identified.

飞秒激光在不同羟基浓度纯石英玻璃内部诱导缺陷研究

飞秒激光在不同羟基浓度纯石英玻璃内部诱导缺陷研究

One-step selective formation of silver nanoparticles on atomic layered MoS2 by laser-induced defect engineering and photoreduction

This work developed a simple, rapid approach to controllable and selective decoration of noble metal nanoparticles on atomic layered MoS2via a single-step micro-power laser-induced defect engineering and photo-reduction technique.

A laser induced forward transfer process for selective boron emitters

In this work, we present a novel technological approach to form highly boron-doped selective emitters. The selective emitters are formed by using a Laser Induced Forward Transfer Doping (DLIFT) process, which allows precise adjustment of the doping profile by tuning the laser parameters and choosing an appropriate doping source. Surface dopant concentrations of up to Ns≈1×1021cm−3 were achieved by using DLIFT. Subsequent BBr3 tube furnace diffusion was performed to form a low surface concentration (Ns≈1×1019cm−3) homogeneous emitter. In comparison to the conventionally applied BBr3 tube diffusion, an increase in open circuit voltage of up to 6mV is predicted based on the photoluminescence measurements performed after passivation, screen-printing and firing processes. Simulations suggest that this voltage gain is most possibly due to a lower emitter saturation current density (j0e) of the highly doped DLIFT samples compared to the BBr3 diffusion samples. Moreover, it is observed that the laser-induced defects are reduced successfully after a subsequent boron tribromide (BBr3) tube diffusion, which is used to form the homogeneous emitter after the DLIFT process. The micro Raman spectroscopy results indicated that the compressive stress of −200MPa in the silicon crystal lattice after DLIFT process is released by the subsequent BBr3 tube diffusion. These results demonstrate that the DLIFT process in combination with the conventionally applied BBr3 tube diffusion can be an effective approach to form selective boron emitters in high efficiency n-type crystalline silicon (c-Si) solar cells.

Study of laser-induced defects in LiNbO3:Zn crystals via fractal analysis

Fractal analysis is used to study patterns of photo-induced light scattering in stoichiometric LiNbO3 single crystals and in a series of LiNbO3:Zn (0.018–0.88 wt %) single crystals. It is found that the concentration of laser-induced defects depends on that of Zn, and that energy is redistributed between the layers of laser-induced scattering.

Ligand-Driven and Full-Color-Tunable Fiber Source: Toward Next-Generation Clinic Fiber-Endoscope Tomography with Cellular Resolution.

In many biomedical applications, broad full-color emission is important, especially for wavelengths below 450 nm, which are difficult to cover via supercontinuum generation. Single-crystalline-core sapphires with defect-driven emissions have potential roles in the development of next-generation broadband light sources because their defect centers demonstrate multiple emission bands with tailored ligand fields. However, the inability to realize high quantum yields with high crystallinity by conventional methods hinders the applicability of ultra-broadband emissions. Here, we present how an effective one-step fiber-drawing process, followed by a simple and controllable thermal treatment, enables a low-loss, full-color, and crystal fiber-based generation with substantial color variability. The broad spectrum extends from 330 nm, which is over 50 nm further into the UV region than that in previously reported results. The predicted submicrometer spatial resolutions demonstrate that the defect–ligand fields are potentially beneficial for achieving in vivo cellular tomography. It is also noteworthy that the efficiency of the milliwatt-level full-color generation, with an optical-to-optical efficiency of nearly 5%, is the highest among that of the existing active waveguide schemes. In addition, direct evidence from high-resolution transmission electron microscopy together with electron energy loss spectroscopy and crystal-field ligands reveals an excellent crystalline core, atomically defined core/cladding interfacial roughness, and significant enhancements in new laser-induced electronic defect levels. Our work suggests an inexpensive, facile, and highly scalable route toward achieving cellular-resolution tomographic imaging and represents an important step in the development of endoscope-compatible diagnostic devices.

Impact of Laser-Induced Oxidation on Silicon Wafer Solar Cells’ Performance

Lasers are frequently used to locally open dielectric layers for contacting the front or rear side of advanced silicon wafer solar cell architectures. For reasons of cost, laser processes are usually carried out in ambient environment, which inevitably leads to the oxidation of silicon. In order to achieve maximum solar cell efficiency, the laser processing has to be optimized thoroughly. The current research only focuses on minimizing laser-induced defects in silicon. However, our investigation shows another side effect of laser ablation, i.e., laser-induced oxidation, which is often neglected and undesirable for the subsequent metallization process. In this paper, we investigated the impact of laser-induced oxidation, which is caused by laser ablation in air, on the performance of silicon wafer solar cells. We show that by applying a postlaser etching step, laser-induced defects and oxide film can be removed effectively, providing improved conditions for the subsequent metallization process.

Analysis of defects patterned by femtosecond pulses inside KBr and SiO2 glass

Colour centres in KBr and defects in silica glass were formed by focused femtosecond laser pulses. It is shown that under simple laser exposure, KBr develops a similar colouration as that achieved with electron and ion bombardment or high-energy X-ray irradiation. The three-dimensional (3D) character of direct laser writing in the volume of KBr allows a new level of control in the spatial arrangement of colour centres and defects. Five different colour centres were identified in KBr through the absorption spectrum; they have different charge and vacancy distribution configurations. The densities of the V- and F-centres were estimated to be 3.9 × 1019 and 3.4 × 1019 cm−3 using Smakula’s formula. In silica, a high density of paramagnetic E′ centres ~1.9 × 1020 cm−3 was determined by quantitative electron spin resonance spectroscopy. Birefringence due to colour centres and laser-induced defects was measured using Stokes polarimetry. In the case of colour centres in KBr, retardation in excess of 0.05 $$\pi$$ was determined throughout the visible spectrum from 400 to 800 nm. The use of polariscopy for analysis of high-pressure and high-temperature phase formation induced by 3D laser structuring is discussed.

Fabrication of two-dimensional periodic structures on silicon after scanning irradiation with femtosecond laser multi-beams

Two-dimensional (2D) periodic structures were fabricated on silicon surfaces by femtosecond laser irradiation in air and water, with the assistance of a microlens array (MLA) placed in the beam's path. By scanning the laser beam along the silicon surface, multiple grooves were simultaneously fabricated in parallel along with smaller laser-induced ripples. The 2D periodic structures contained long-periodic grooves and perpendicular short-periodic laser-induced ripples, which had periods of several microns and several hundred nanometers, respectively. We investigated the influence of laser power and scanning velocity on the morphological evolution of the 2D periodic structures in air and water. Large-area grid-like structures with ripples were fabricated by successively scanning once along each direction of the silicon's surface, which showed enhanced optical absorption. Hydrofluoric acid was then used to remove any oxygen and laser-induced defects for all-silicon structures.

Defect-assisted local field rearrangement during nanograting formation with femtosecond pulses

Defect-assisted local field rearrangement during nanograting formation is demonstrated by simply writing a nanograting perpendicularly across another one. It is shown that pre-distributed nanogrooves and laser-induced defects have a great impact on local field arrangement. Depending on the writing parameters, the nanograting period could be significantly altered as a consequence of this local field rearrangement. Accordingly, precise control of the overlap between neighboring passes during a sequential scan is achieved and well-shaped large-area nanogratings can be produced.

The Impact of N2 Anneal on Laser Processed Silicon

In this paper, the impact of the N2 anneal on laser processed silicon was investigated using photoluminescence (PL) imaging through comparing the PL signal of laser processed samples before and after N2 anneal. The samples capped with different dielectrics or without any dielectric (bare surface) prior to the laser processing were used, enabling the evaluation of the influence of the N2 anneal on the defects caused both by laser thermal effect and by the existence of dielectrics. Generally, it was found that N2 anneal is an effective way to reduce both the laser and dielectric induced defects.

Photoluminescence Spectra of CdTe Single Crystals Subjected to Nanosecond Laser Irradiation

Low temperature photoluminescence (PL) of high-resistivity detector-grade Cl-compensated CdTe semiconductor crystals subjected to irradiation with nanosecond (τ = 7 ns) laser pulses of the second harmonic (λ = 532 nm) of a YAG:Nd laser is studied. Irradiation of CdTe crystals within the certain range of laser pulse energy densities results in a relative decrease in the emission intensity in both the deep energy level and edge regions and an increase in the exciton band intensity in the PL spectra. The evolution of the PL spectra depending on laser energy density, excitation level and temperature under excitation are analyzed. Laser-stimulated transformation of the point defect structure of the CdTe surface region and mechanisms of laser-induced defect formation are discussed. The optimal regimes of laser processing have been obtained which result in the minimum ratio of the defect and exciton bands that is an evidence of an increase in the structural perfection of the irradiated crystals.

Fractal analysis of photoinduced light-scattering patterns in stoichiometric LiNbO3 crystals

We have analyzed the fractal dimension of photoinduced light-scattering patterns in stoichiometric lithium niobate single crystals of different genesis and, based on this analysis, we have investigated the dynamics of manifestation of laser-induced defects in different layers of the photoinduced light-scattering pattern in these crystals. Energy transfer between photoinduced light-scattering layers has been revealed. We have shown that, as distinct from the method of investigation of the photoinduced light scattering by measuring the time dynamics of the opening angle of the indicatrix of the speckle structure of the photoinduced light scattering, the investigation of the time dynamics of the fractal dimension of different layers of the photoinduced light-scattering pattern makes it possible to register particular features of time changes of the order of laser-induced defects in the crystal.

Insight into the degradation mechanism resisting the laser-induced damage of optical films in vacuum by evolution of laser-induced defects

The degradation mechanism resisting the laser-induced damage of optical thin films in a vacuum may be understood by the evolution of laser-induced defects. The accumulation of these newly created defects with irradiated shot numbers results in the severe fatigue effects of multiple-pulse laser damage in the vacuum. The correlative theory was used to further explain this degradation mechanism of multi-pulse irradiation in a vacuum.

Nanosecond multi-pulse laser-induced damage mechanisms in pure and mixed oxide thin films

We report on nanosecond laser-induced damage of pure and mixed oxide thin films deposited by ion beam sputtering. Silica, hafnia and alumina as well as their binary mixtures have been tested in S-on-1 mode at 355nm and 266nm using a multiscale approach. The results were analyzed qualitatively to discuss the different fatigue behaviors observed. The absence of a multi-photon absorption step in the 1-on-1 damage thresholds as a function of the band gap indicates defect-mediated damage mechanisms. During the multi-pulse experiments we observed laser-induced defects that cause fatigue effects and preexisting low-density defects, which are insensitive to multiple pulse irradiation. Depending on material and beam size both types of defects (preexisting and light-induced) may contribute equally to the observed damage probability. Comparing the fatigue behavior of the mixtures to their constituting pure oxides, we found that, in general, the fatigue behavior of binary mixtures cannot be interpolated from the behaviors of the pure oxides.

Modeling surface defects in fused silica optics for laser wave propagation

Modulation of the laser intensity caused by surface defects on optical components is a main concern for high-power lasers. Among the consequences of this effect, the laser damage occurrence can be magnified on the downstream components in the laser chain. In order to understand the impact of defects on wave propagation, a specific numerical approach has been developed. The defects are modeled as phase perturbations that are used as inputs in either a 1D axi-symmetric code based on finite differences or a 2D code that uses fast Fourier transform. The computations are then used to evaluate the laser intensity modulations generated by CO2 laser-induced defects at the surface of fused silica optics. A dedicated damage experiment at 355 nm has been carried out for various defect dimensions. Consistent results are found between experiments and simulations, both on the quantitative values of magnitude and positions of the modulations. This study proves that, for short propagation distances, it is necessary to use the exact shape of the defect in the simulations, especially the complex rim structure characteristic of CO2 laser craters.