Investigation on solid-phase crystallization techniques for low temperature polysilicon thin-film transistors

Low-temperature polysilicon (LTPS) has emerged as a dominant technology for high performance thin-film transistors (TFTs) used in mobile liquid crystal display (LCD) and organic light emitting diode (OLED) display products. As users demand higher quality in flat panel displays with a larger viewing area and finer resolution, the improvement in carrier mobility of LTPS compared to that of hydrogenated amorphous silicon (a-Si:H) makes it an excellent candidate as a channel material for TFT. Advantages include improvements in switching speed and the ability to incorporate peripheral scan and data driver circuitry onto a low cost display substrate. Solid-phase crystallization (SPC) is a useful technique to realize polysilicon films due to its simplicity and low cost compared to excimer-laser annealing (ELA),which has many challenges in back-plane manufacturing on large glass panels.Metal induced crystallization (MIC) results in polycrystalline silicon films with grain size as large as tens of microns. Flash-lamp annealing (FLA) is a new and novel method to crystallize a-Si films at high temperature without distortion of the glass substrate by performing an annealing within millisecond range.This work investigates SPC, MIC and FLA techniques to realize LTPS films. In addition, TFTs were designed and fabricated to characterize the device quality of the semiconductor layer, and to compare the performance of different structural arrangements

Laser Surface Texturing, Crystallization and Scribing of Thin Films in Solar Cell Applications

Thin films have been considered for use in terrestrial solar cell applications because of their significantly reduced cost compared with bulk crystalline silicon. However, their overall efficiency and stability are less than that of their bulk crystalline counterpart. The work presented in this thesis seeks to investigate these issues via a series of experimental and numerical analysis of the influences of laser processing on microstructure, optical and electrical properties of two absorber materials, a-Si:H (hydrogenated amorphous silicon) and CdTe (cadmium telluride). a-Si:H thin film solar cells suffer from disadvantages of low efficiencies and light induced degradation. A one-step laser processing is investigated for introducing light-trapping structure and crystallization on a-Si:H thin films, which can potentially simultaneously alleviate the two weaknesses of a-Si:H. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by irradiation of both femtosecond (fs) infrared and nanosecond (ns) excimer lasers enhanced light absorption, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially improve. It is shown that growth is a more dominant spike formation mechanism in excimer laser processing, rather than ablation which is dominant during fs laser texturing. Experimental and analytical approaches are also developed revealing the effect of hydrogen on texturing behavior and crystallization during excimer laser irradiation, and a step-by-step crystallization process is proposed to prevent the hydrogen from diffusing out in order to reduce the defect density. In addition, a comparison of absorptance spectra for various surface morphologies and crystallinity is developed and the absorptance across the solar spectrum shows that the combination of surface texturing and crystallization induced by laser processing is very promising for a-Si:H thin film solar cell applications. CdTe thin-film solar cells are the basis of a significant technology with major commercial impact on terrestrial photovoltaic production, since CdTe leads to substantial cost reduction. Laser scribing is a key process used to increase thin-film solar panel efficiency through the formation of serial interconnections to reduce photocurrent and resistance losses. Currently, scribing is performed using glass-side laser processes which have led to increased scribe quality. Defects formed during scribing such as micro cracks, film delamination, thermal effect and tapered sidewall geometries, however, still keep solar panels from reaching their theoretical efficiencies. In this study, a ns Nd:YAG laser operating at the fundamental (1064nm) or frequency-doubled (532nm) wavelengths is employed for pattern 1 (P1) and 2 (P2) scribing on CdTe thin-film solar cells. The experimental investigation shows that film removal mechanisms for different materials are due to laser-induced ablation, thermal stress and micro-explosion processes. The formation mechanisms and mitigation techniques of the defects during micro-explosion process are studied. A fully-coupled thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress distribution responsible for SnO2:F film removal, and a plasma expansion model is also investigated to simulate the film removal of CdTe/Cds multilayer due to the micro-explosion process. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency

Thin film transistors fabricated by in-situ doped unhydrogenated polysilicon films obtained by solid phase crystallization

International audienceHigh mobility low temperature (≤ 600°C) unhydrogenated in-situ doped polysilicon thin film transistors are made. Polysilicon layers are grown by a LPCVD technique and crystallized in vacuum by a thermal annealing. Source and drain regions are in-situ doped. Gate insulator is made of an APCVD silicon dioxide. Hydrogen passivation is not performed on the transistors. One type of transistors is made of two polysilicon layers, the other one is constituted of a single polysilicon layer. The electrical properties are better for transistors made of single polysilicon layer: a low threshold voltage (1.2 V), a subthreshold slope S = 0.7 V/dec, a high field effect mobility (≈ 100 cm2/Vs) and a On/Off state current ratio higher than 107 for a drain voltage Vds = 1 V. At low drain voltage, for both transistors, the Off state current results from a pure thermal emission of trapped carriers. However, at high drain voltage, the electrical behavior is different: in the case of single polysilicon TFTs, the current obeys the field-assisted (Poole-Frenkel) thermal emission model of trapped carriers while for TFTs made of two polysilicon layers, the higher Off state current results from a field-enhanced thermal emission

Ultrafast epitaxial growth kinetics in functional oxide thin films grown by pulsed laser annealing of chemical solutions

The crystallization process and physical properties of different functional oxide thin films (CeZrO, LaNiO, BaSrTiO, and LaSrMnO) on single crystal substrates (YO:ZrO, LaAlO, and SrTiO) are studied by pulsed laser annealing (PLA). A Nd:YAG laser source (λ = 266 nm, 10 Hz and τ ∼3 ns) is employed to crystallize chemical solution deposited (CSD) amorphous/nanocrystalline films under atmospheric conditions. We provide new insight on the influence of photochemical and photothermal interactions on the epitaxial crystallization kinetics of oxide thin films during the transformation from amorphous/polycrystalline material (i.e., atomic diffusion, epitaxial growth rates, and activation energies of nucleation and crystallization). The epitaxial growth is investigated by varying the laser fluence and the applied number of pulses. The morphology, structure, and epitaxial evolution of films are evaluated by means of atomic force and transmission electron microscopies and X-ray diffraction. Highly epitaxial oriented films of 20-40 nm in thickness are obtained by PLA. The crystallization kinetics of laser treatments is determined to be orders of magnitude faster than thermal treatments with similar activation energies (1.5-4.1 eV), mainly due to the large temperature gradients inducing modified atomic diffusion mechanisms derived mainly from photothermal interactions, as well as a minor contribution of photochemical effects. The fast heating rates achieved by PLA also contribute to the fast epitaxial growth due to reduced coarsening of polycrystalline material. The measurement of the physical properties (electrical resistivity and magnetism) of laser processed CSD films has revealed significantly good functionalities, close to those of thermally grown films, but with much shorter processing times.We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2013-0295 and SEV-2015- 0496), CONSOLIDER Excellence Network (MAT2015-68994-REDC), COACHSUPENERGY project (MAT2014-51778-C2-1-R, co-financed by the European Regional Development Fund), and the projects MAT2011-28874-C02-01, ENE2014-56109-C3-3-R and Consolider Nanoselect CSD2007-00041, and from the Catalan Government with 2014-SGR-753, 2014-SGR-1638 and Xarmae. AQ and MdlM are also grateful to CSIC and European Social Fund program for JAE-Predoc fellowships (E-08-2012-1321248 and E-08-2013-1028356).Peer Reviewe