Pulsed Laser Crystallization Research Articles

Pulsed laser crystallization of sol-gel derived cerium oxide thin films

AbstractCerium oxide sol-gel-derived thin films were crystallized by pulsed excimer laser annealing (ELA). X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman Spectroscopy (RS), and Fourier transform infrared (FTIR) were employed to analyze the effect of laser fluence and number of pulses on the structural and morphological properties of the irradiated films. XRD and RS results, supported by thermal simulations, confirmed crystallization into cubic cerium oxide starting from 40 mJcm−2. SEM micrographs showed that an increased number of pulses induced the formation of porous and complex nanostructured surfaces, different from the morphology obtained on thermally annealed films. Finally, water contact angle measurements revealed that the films showed a characteristic hydrophobic “petal effect”. Graphical Abstract

Single-shot selective femtosecond and picosecond infrared laser crystallization of an amorphous Ge/Si multilayer stack

Pulsed laser crystallization is an efficient annealing technique to obtain polycrystalline silicon or germanium films on non-refractory substrates. This is important for creating “flexible electronics” and can also be used to fabricate thin-film solar cells. In this work, near- and mid-infrared femtosecond and picosecond laser pulses were used to crystallize a Ge/Si multilayer stack consisting of alternating amorphous thin films of silicon and germanium. The use of infrared radiation at wavelengths of 1030 and 1500 nm with photon energies lower than the optical absorption edge in amorphous silicon allowed obtaining selective crystallization of germanium layers with a single laser shot. The phase composition of the irradiated stack was investigated by the Raman scattering technique. Several non-ablative regimes of ultrashort-pulse laser crystallization were found, from partial crystallization of germanium without intermixing the Ge/Si layers to complete intermixing of the layers with formation of GexSi1-x solid alloys. The roles of single- and two-photon absorption, thermal and non-thermal (ultrafast) melting processes, and laser-induced stresses in selective pico- and femtosecond laser annealing are discussed. It is concluded that, due to a mismatch of the thermal expansion coefficients between the adjacent stack layers, efficient explosive solid-phase crystallization of the Ge layers is possible at relatively low temperatures, well below the melting point.

Yb:Ca9Gd(VO4)7, a potential ultrafast pulse laser crystal with promising spectral properties

The Yb3+:Ca9Gd(VO4)7 crystal with dimensions of Φ19 × 25 mm3 was grown by the Czochralski method for the first time. The polycrystalline materials, crystal growth, absorption and fluorescence spectra, energy level schema, fluorescence lifetime and laser performance parameters were investigated in detail. For π-polarization, the absorption coefficients, cross-sections, maximal full width at half maximum (FWHM) at around 976 nm are 1.05 cm−1, 1.27 × 10−20 cm2 and 33 nm, respectively. For σ-polarization, the relevant absorption spectral parameters at around 981 nm are 1.58 cm−1, 1.90 × 10−20 cm2 and 28 nm, respectively. At the laser wavelength around 1023 nm, the FWHM and emission cross-sections are 57 nm and 0.94 × 10−20 cm2 for π-polarization, 54 nm and 2.18 × 10−20 cm2 for σ-polarization, respectively. The results reveal that Yb3+:Ca9Gd(VO4)7 crystal can be regarded as a potential candidate for efficient LD-pumping and ultrashort pulse laser crystal.

A novel solid-solution garnet Yb:YAG-MnASG with enhanced spectral properties

A novel solid-solution garnet Yb3+ doped Y3Al5O12Mn3Al2(SiO4)3 (Yb:YAG-MnASG) has been designed and grown for the first time. The polycrystalline materials, crystal growth, crystal structures, absorption and emission spectra, energy level schema, fluorescence lifetime and laser performance parameters were investigated in detail. Owing to the inhomogeneity in the local environment of Yb3+ caused by the cationic substitutions of Mn2+ for Y3+, Si4+ for Al3+ and high crystal-field strengths caused by distorted host sites, both of the absorption and emission spectral bandwidth of Yb:YAG-MnASG crystal are broadened compared with Yb:YAG. The maximal full width at half maximum (FWHM) of absorption spectra around 940 nm and emission spectra around 1030 nm for Yb:YAG-MnASG crystal are almost 1.2 and 2.5 times larger than those of Yb:YAG crystal, respectively. The results reveal that the novel solid-solution garnet Yb:YAG-MnASG is an excellent candidate for ultrafast pulse laser crystal.

Transition intensity calculation of Yb:YAG**Project supported by the National Natural Science Foundation of China (Grant Nos. 61405206, 51502292, and 51702322), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant Nos. CXJJ-16M251 and CXJJ-15M055), and the National Key Research and Development Program of China (Grant No. 2016YFB0402101)

The Yb:YAG is an excellent high-average power and ultra-short pulse laser crystal. Transition intensity parameters and Huang–Rhys factors are fitted to its emission spectrum by the full-profile fitting method. Calculated results indicate that the emission spectrum of Yb:YAG at cryogenic temperature consists of three pure electron state transitions and two phonon-assisted transitions, one vibronic transition releases one-phonon of 3 cm−1, and the other vibronic transition absorbs one-phonon of 22 cm−1. At 300 K, the phonon assisted transition of 3 cm−1 turns into two- or more-phonon assisted transitions. The procedure absorbing phonon can reduce the thermal load of Yb:YAG and improve the laser efficiency, which may be one of the reasons why Yb:YAG has excellent performance. The emission bands of Yb:YAG are broadened thermally, and the peak values decrease by several times. The emission cross sections of Yb:YAG determined by Fuchtbauer–Ladenburg (F–L) formula are remarkably different from those calculated with , which indicates that it is necessary for a laser material to determine its transition intensity parameters in order to reasonably evaluate the laser performance.

Visualization of nanosecond laser-induced dewetting, ablation and crystallization processes in thin silicon films

In the present work, nanosecond pulsed laser crystallization, dewetting and ablation of thin amorphous silicon films are investigated by time-resolved imaging. Laser pulses of 532 nm wavelength and 7 ns temporal width are irradiated on silicon film. Below the dewetting threshold, crystallization process happens after 400 ns laser irradiation in the spot central region. With the increasing of laser fluence, it is observed that the dewetting process does not conclude until 300 ns after the laser irradiation, forming droplet-like particles in the spot central region. At higher laser intensities, ablative material removal occurs in the spot center. Cylindrical rims are formed in the peripheral dewetting zone due to solidification of transported matter at about 500 ns following the laser pulse exposure.

A promising ultrafast pulse laser crystal with a disordered structure: Yb3+:Sr3Gd2(BO3)4

A Yb3+-doped Sr3Gd2(BO3)4 (Yb:SGB) crystal was grown using the Czochralski method. A systematic investigation of the disordered structure characteristics, polarized spectral properties and continuous wave (CW) laser performances of this crystal was reported for the first time. Due to the statistical distributions of Gd3+ and Sr2+ ions, the absorption and emission spectra are strongly inhomogeneously broadened when introducing Yb3+ in the host. This feature of the broad spectra facilitates efficient pumping of InGaAs diodes and production of ultrafast pulse lasers. A maximum CW laser output power of 2.82 W under an absorbed pump power of 6.4 W was achieved with a central wavelength of 1055 nm, corresponding to a slope efficiency of 54% and an optical conversion efficiency of 44%. The results reveal that Yb:SGB is an excellent candidate for an ultrafast pulse laser crystal.

Effect of medium range order on pulsed laser crystallization of amorphous germanium thin films

Sputter deposited amorphous Ge thin films had their nanostructure altered by irradiation with high-energy Ar+ ions. The change in the structure resulted in a reduction in medium range order (MRO) characterized using fluctuation electron microscopy. The pulsed laser crystallization kinetics of the as-deposited versus irradiated materials were investigated using the dynamic transmission electron microscope operated in the multi-frame movie mode. The propagation rate of the crystallization front for the irradiated material was lower; the changes were correlated to the MRO difference and formation of a thin liquid layer during crystallization.

Ultraviolet pulsed laser crystallization of Ba0.8Sr0.2TiO3 films on LaNiO3-coated silicon substrates

In this work, Ba0.8Sr0.2TiO3 (BST) films on LaNiO3-buffered SiO2/Si (LNO/SiO2/Si) substrates were crystallized by pulsed laser irradiation. Solution-derived amorphous barium–strontium–titanate precursor layers were crystallized with a KrF excimer laser in oxygen ambient at fluences ranging from 50 to 75mJcm−2. With the substrate temperature set to 500°C, the number of pulses and film thickness were varied until high-quality crystallinity could be achieved. It was found that films with a thickness of 40nm are fully crystallized with a uniaxial {00l} orientation which is predetermined by the LaNiO3 orientation. On the other hand, for 160nm thick films, crystallization was observed after 12,000 pulses in the 70nm close to the surface, while the rest of the film remained amorphous. The large temperature difference between the film surface and interface due to the low thermal conductivity of the amorphous BST is suggested as the origin of this behavior. Films thicker than 80nm cracked on crystallization due to the stress caused by the different thermal expansion coefficients of film and substrate, as well as the large temperature variations within the BST film.

Yb3+:Sr3Y2(BO3)4: A potential ultrashort pulse laser crystal

A Yb3+:Sr3Y2(BO3)4 crystal was grown successfully by the Czochralski method. The polarized spectral properties and continuous wave laser output of this crystal were investigated in detail. The crystal has larger absorption and emission cross sections compared with many mature Yb3+-doped borate crystals. The full width at half maximum of the emission bands around 1023nm are 69nm (E//a), 61nm (E//b) and 65nm (E//c). 3.47W continuous wave laser output with a slope efficiency of 29% and an optical conversion efficiency of 24% was obtained. The results reveal that Yb3+:Sr3Y2(BO3)4 crystal is an excellent candidate for ultrashort pulse laser crystal.

Crystalline photoactive copper indium diselenide thin films by pulsed laser crystallization of nanoparticle-inks at ambient conditions

Direct pulsed laser crystallization (DPLC) is explored to rapidly crystallize large area coated copper indium diselenide (CIS) nanoparticle-inks.

Charge carrier transport and collection enhancement of copper indium diselenide photoactive nanoparticle-ink by laser crystallization

There has been increasing needs for cost-effective and high performance thin film deposition techniques for photovoltaics. Among all deposition techniques, roll-to-roll printing of nanomaterials has been a promising method. However, the printed thin film contains many internal imperfections, which reduce the charge-collection performance. Here, direct pulse laser crystallization (DPLC) of photoactive nanoparticles-inks is studied to meet this challenge. In this study, copper indium selenite (CIS) nanoparticle-inks is applied as an example. Enhanced crystallinity, densified structure in the thin film is resulted after DLPC under optimal conditions. It is found that the decreased film internal imperfections after DPLC results in reducing scattering and multi-trapping effects. Both of them contribute to better charge-collection performance of CIS absorber material by increasing extended state mobility and carrier lifetime, when carrier transport and kinetics are coupled. Charge carrier transport was characterized after DPLC, showing mobility increased by 2 orders of magnitude. Photocurrent under AM1.5 illumination was measured and shown 10 times enhancement of integrated power density after DPLC, which may lead to higher efficiency in photo-electric energy conversion.

Growth, thermal and spectral characteristics of Yb3+:Sr6YSc(BO3)6 crystal

Abstract A crystal of Yb 3+ :Sr 6 YSc(BO 3 ) 6 was grown successfully from Li 6 B 4 O 9 flux by the top-seeded solution growth method. The crystal's thermal, mechanical and spectral characteristics were investigated in detail. It possesses small thermal expansion coefficients, moderate thermal conductivities, and large hardness. The crystal has a strong absorption band at 967 nm with a full width at half-maximum of about 3.4 nm. The crystal has a broad emission band at 1016 nm with the full width at half-maximum of about 64 nm. The emission cross sections were calculated by reciprocity method and Fuchtbauer-Ladenburg formula. The fluorescence lifetime is 5.98 ms. The results reveal that Yb 3+ :Sr 6 YSc(BO 3 ) 6 crystal is a new promising tunable and ultrashort pulse laser crystal.

Single-Grain Si Thin-Film Transistors for Monolithic 3D-ICs and Flexible Electronics

We review our recent achievements in monolithic 3D-ICs and flexible electronics based on single-grain Si TFTs that are fabricated inside a single-grain with a low-temperature process. Based on pulsed-laser crystallization and submicron sized cavities made in the substrate, amorphous-Si precursor film was converted into poly-Si having grains that are formed on predetermined positions. Using the method called µ-Czochralski process and LPCVD a-Si precursor film, two layers of the SG Si TFT layers with the grains having a diameter of 6µm were vertically stacked with a maximum process temperature of 550°C. Mobility for electrons and holes were 600cm2/Vs and 200cm2/Vs, respectively. As a demonstration of monolithic 3D-ICs, the two SG-TFT layers were successfully implemented into CMOS inverter, 3D 6T-SRAM and single-grain lateral PIN photo-diode with in-pixel amplifier. The SG Si TFTs were applied to flexible electronics. In this case, the a-Si precursor was prepared by doctor-blade coating of liquid-Si based on pure cyclopentasilane (CPS) on a polyimide (PI) substrate with maximum process temperature of 350°C. The µ-Czochralski process provided location-controlled Si grains with a diameter of 3µm and mobilities of 460 and 121cm2/Vs for electrons and holes, respectively, were obtained. The devices on PI were transferred to a plastic foil which can operate with a bending diameter of 6mm. Those results indicate that the SG TFTs are attractive for their use in both monolithic 3D-ICs and flexible electronics.

Cr3+-doped CaMgSi2O6crystal: a promising tunable laser and ultrashort laser crystal

This paper reports the growth and spectral properties of the Cr3+:CaMgSi2O6 (Cr3+:CMS) crystal. The Cr3+:CMS crystal was grown by the Czochralski method. Investigation of its spectral properties shows that the Cr3+:CMS crystal has broad, strong absorption and emission cross-sections. It has two broad absorption bands which center at 460 nm and 652 nm. The emission cross-section σe at 986.1 nm is 14.39 × 10−20 cm2 at 300 K. The emission FWHM is up to 212 nm. Therefore, the Cr3+:CMS crystal can be considered as a potential tunable and ultrashort pulse laser crystal.

Quasi-equilibrium size distribution of subcritical nuclei in amorphous phase change AgIn-Sb2Te

We investigate the effect of low temperature annealing or of extended storage at room temperature on the subsequent nucleation behavior of amorphous AgIn-incorporated Sb2Te (AIST), a material for phase change memories. Time-resolved reflectivity measurements during pulsed laser crystallization reveal the rates of solid-phase transformation, while fluctuation transmission electron microscopy detects the nanoscale order in the amorphous phase prior to crystallization. The nanoscale order is postulated to consist of subcritical nuclei that coarsen upon annealing at temperatures ranging from 25 °C (for months) or 100 °C (for hours). Samples that have been annealed remain fully amorphous as evaluated by conventional diffraction experiments. Shorter nucleation times are consistently associated with the observation of increased nanoscale order. The effect of annealing is observed to saturate: there is no further reduction in nucleation time or increase in nanoscale order for annealing at 100 °C beyond three hours. This result supports the general prediction of classical nucleation theory that the size distribution of subcritical nuclei increases from the as-deposited state to a quasi-equilibrium.

Direct pulsed laser crystallization of nanocrystals for absorbent layers in photovoltaics: Multiphysics simulation and experiment

Direct pulsed laser crystallization (DPLC) of nanoparticles of photoactive material—Copper Indium Selenide (nanoCIS) is investigated by multiphysics simulation and experiments. Laser interaction with nanoparticles is fundamentally different from their bulk counterparts. A multiphysics electromagnetic-heat transfer model is built to simulate DPLC of nanoparticles. It is found smaller photoactive nanomaterials (e.g., nanoCIS) require less laser fluence to accomplish the DPLC due to their stronger interactions with incident laser and lower melting point. The simulated optimal laser fluence is validated by experiments observation of ideal microstructure. Selectivity of DPLC process is also confirmed by multiphysics simulation and experiments. The combination effects of pulse numbers and laser intensity to trigger laser ablation are investigated in order to avoid undesired results during multiple laser processing. The number of pulse numbers is inversely proportional to the laser fluence to trigger laser ablation.

Nanoscale quantification of charge injection and transportation process in Si-nanocrystal based sandwiched structure

Si nanocrystals are formed by using KrF pulsed laser crystallization of an amorphous SiC/ultrathin amorphous Si/amorphous SiC sandwiched structure. Electrons and holes are injected into Si nanocrystals via a biased conductive AFM tip and the carrier decay and transportation behaviours at the nanoscale are studied by joint characterization techniques of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM). Quantification of the surface charge density is realized by solving the Poisson equation based on KPFM measurements. Besides, the asymmetric barrier height for electrons and holes is considered to play a dominant role in controlling the charge retention and transportation characteristics. The methodology developed in this work is promising for studying the charge injection and transportation process in other materials and structures at the nanoscale.

Large area crystallization of amorphous Si with overlapping high repetition rate laser pulses

This paper presents a pulsed laser crystallization technique, enabling large area crystallization of amorphous Si to produce grains having well-defined size and orientation. The method is developed by first determining the parameters influencing crystallization induced by single laser pulses of circular cross-sectional profile. In a second step, crystallization by overlapping round spots is examined. The experiments reveal three zones characterized by distinctly different crystallized morphologies following the laser irradiation. One of these zones corresponds to the regime of lateral crystal growth, wherein grains are driven towards the center of the spot by the radial temperature gradient. These findings are then applied to processing via line beam profiles that facilitate large area crystallization upon rapid translation of the specimen. Crystallization of extended areas hinges on the determination of the crystal growth length for a single spot. The pitch between successive pulses is then set on the basis of this information. It is shown that the pulse energy has only a weak effect on the crystal growth length.

Dynamical process of KrF pulsed excimer laser crystallization of ultrathin amorphous silicon films to form Si nano-dots

Molecular dynamics (MD) simulations based on the Tersoff potential have been developed to study the laser-induced crystallization of amorphous silicon (a-Si) film with ultrathin thickness to form size-controllable Si nano-dots. The influences of laser fluence and a-Si film thickness on the crystallization process were discussed. Classic nucleation theory was used to explain the results of the MD simulations. The constrain effect of a-Si films thickness on the formation of Si nano-dots was evaluated accordingly.