Diode Laser Annealing Research Articles

High‐Performance Full‐Solution‐Processed Oxide Thin‐Film Transistor Arrays Fabricated by Ultrafast Scanning Diode Laser

AbstractThe full‐solution‐processed oxide thin‐film transistor (TFT) array is successfully realized by using ultrafast scanning diode laser annealing (USDLA). It integrates all solution‐processed functional thin films well, including low‐electrical‐resistivity transparent conductive indium‐tin‐oxide (ITO) thin film, high‐k dielectric hafnium‐aluminum‐oxide (HAO) thin film, and high‐quality semiconductor tungsten‐zinc‐tin‐oxide (WZTO) thin film. And the stacked multilayer thin films as a whole exhibit a smooth surface with the root mean square (RMS) roughness of 0.293 nm. The full‐solution‐processed TFT array exhibits a uniform overall TFT device performance, the average values including subthreshold swing of 118 mV dec−1, threshold voltage of −0.45 V, and mobility of 12 cm2 V−1 s−1. Moreover, the full‐solution‐processed TFTs have good bias illumination stability, especially the VT shift of the PBIS and NBIS are only 0.09 and 0.4 V, respectively. The result implies that the USDLA technique can meet the large‐scale fabrication of full‐solution‐processed TFTs and provide a solid path to low‐cost high‐performance and large‐scale electronics.

Comprehensive analysis of blue diode laser-annealing of amorphous silicon films

The low temperature polycrystalline silicon (LTPS) method has proved to be a technical breakthrough, accomplishing semiconductor thin films with remarkable mobility for a range of high-performance displays, including liquid crystal display and organic light emitting diodes. However, utilizing a conventional excimer laser source for LTPS incurs high cost. In this paper, we demonstrate a comprehensive analysis of the crystallization mechanism of a-Si film (94 nm) and the thermal deformation of the glass substrate induced by Blue diode Laser Annealing (BLA). BLA provides high quality laterally grown crystals over 4 µm × 10 µm on glass substrates, which were examined by optical microscopy, scanning electron microscopy, and Raman spectroscopy. In addition, the permanent deformation introduced by the annealing process is numerically modeled, instantiating how to control the heat conduction from the thin film that affects the substrate. Our findings reveal that the permanent thermal deformation depth that can be obtained is comparable to the roughness of the silicon film for the optimum scanning speed and laser power. The combination of both experimental and numerical results elucidates the manifested physical mechanisms during the BLA process and provides the guidelines to improve the experimental parameters of this process.

An open-source CFD model for computing thermal effect in the context of laser-induced semiconductor processing in photovoltaic applications

Thermal treatment by means of lasers enables to process industry compatible photovoltaic solar cells by improving its materials’ physical, optical, and chemical properties. In order to achieve a high-quality processing result as well as optimisation and characterisation, a detailed understating on the laser-induced photovoltaic solar cells processing is absolutely necessary, which arguably may not be possible using the experimental techniques. Hence, this contribution presents a numerical model within the open-source CFD platform, known as OpenFOAM, for simulating laser-induced heat transfer and phase change phenomena in the context of photovoltaic solar cells processing. Upon development, the model was implemented to simulate three relevant and available experimental cases of photovoltaic solar cells processing: (1) laser annealing of silicon wafer with fixed continuous wave (CW) laser beam, (2) CW diode laser annealing of silicon thin-film deposited on glass substrate, and (3) nanosecond pulse laser-induced melting and solidification of silicon wafer. In all of the cases, an excellent agreement was observed between the numerical and experimental results, which not only demonstrate the accuracy but also the capability of the model to capture and explain the required thermal phenomena in processing photovoltaic solar cells. Therefore, the present contribution may simply be used as a state-of-the-art of modelling and optimisation of the considered process, and indeed other related laser-processing fabrication steps, when the correct model options are chosen.

Modelling laser-induced heat transfer phenomena of thin-films using OpenFOAM

• A Finite Volume (FV) Model for simulating laser annealing of thin-film based structure. • Experimental validation of the OpenFOAM-based conjugate heat transfer model. • Detailed understanding of the diode laser-induced thermal and optical phenomena of the multilayer structures. • A guideline for optimising and designing of thin-film based architecture. This paper presents a numerical simulation model for analysing diode laser-induced thermal phenomena on high absorbing thin-films. Continuous wave (CW) diode laser, in the form of millisecond annealing, is a promising tool for processing of the high absorbing thin-film based solar cells. The localised nature of laser processing, in one hand, gives an opportunity to heat the film to a higher temperature over a much shorter time than the conventional methods, while, on the other hand, allowing the substrate to remain at low temperature with a low ramp-up rate. Also, unlike the laser processing in pulse mode, the CW diode laser can heat the high absorbing thin-films over a longer period without initiating melting and dwetting. Since the CW diode laser-induced thermal treatment is entirely characterised by its parameters, the inappropriate setting of its parameters may lead to a negative effect on the absorber layer . In view of these, being highly a thermal process, the basic understating of the CW diode laser annealing of high absorbing thin-film is necessary, which arguably may not be possible using the experimental techniques. Numerical simulations, in this case, offer the possibility to acquire knowledge of the process; hence, in this contribution, a numerical model has been developed in an open-source based computational fluid dynamics (CFD) platform, known as OpenFOAM. Upon development of the model, its prediction has been compared with the experimental results of the CW diode laser annealing of silicon thin-film on the glass substrate , in which an excellent agreement with the experimental results has been observed. In the second part of this study, the developed numerical model has been implemented to reveal the CW diode laser-induced thermal phenomena of the high absorbing CZTS thin-films, which is considered as deposited on Molybdenum (Mo) coated glass substrate, a typical structure used for designing of CZTS thin-film based solar cells. Although CZTS thin-film is considered as the ideal material for using as an absorber in the next generation thin-film based solar cells, its behaviour in the case of CW diode laser-induced thermal treatment has not been understood yet. Hence, the present study may likely to contribute to the processing and optimising of the CZTS thin-film based structure.

Thermal Effects on the Hydrogen Passivation of Silicon Wafers During Diode Laser Annealing

This paper presents a study of the laser induced thermal effects on the hydrogen passivation of silicon wafers. Thermal treatment, due to laser annealing, can be responsible for the release of atomic hydrogen from the dielectric layers, the diffusion of hydrogen into the silicon wafers and the passivation of defects. The whole process results in a reduction of active defects, and hence an increase of the minority carrier lifetime of the silicon wafers. Understanding of the mechanisms of heat transfer during the laser annealing is essential for obtaining an optimal hydrogenation effect. In this paper, a numerical model for evaluation of temporal and spatial temperature distributions in a silicon wafer during laser annealing is developed. The model is developed using the open‐source C + + framework known as OpenFOAM. A continuous wave (CW) diode laser of 808 nm wavelength is used for laser annealing of the silicon wafers containing dielectric layers of hydrogenated silicon nitride. The simulations are carried out at the experimental conditions and the impact of key laser annealing parameters, such as scanning speeds, laser power densities and substrates initial temperatures, are carefully investigated. The simulated results are then correlated with the experimental results of photoluminescene (PL) imaging and injection level dependent effective lifetime data. Our results show that the localized hydrogen passivation of the silicon wafers using laser annealing depends on the peak temperature and duration of heat. Additionally, it is found that the preheating of the wafer has a substantial effect on the reduction of thermal stress, while leading to a good degree of localized hydrogen passivation of the wafers. A correlation is also developed for the maximum spot temperature which will serve as a guideline for selecting optimum operating conditions.

35.1: Blue-Diode Laser Annealing of Amorphous Silicon Films for Low-Cost, Large-Scale Manufacturing of Advanced Displays

Blue diode laser annealing (BLA) systems have been successfully developed for low-temperature crystallization from amorphous silicon film on glass substrates. It is demonstrated that it can achieve high quality poly-Si film with large grain sizes, as studied by optical microscopy, Raman spectroscopy and AFM measurements. The p-channel poly-Si TFT on glass using BLA exhibited the field-effect mobility of 168.05 cm2/Vs and subthreshold swing of 220 mV/dec. BLA is scalable to large-scale display manufacturing and significantly lowers the capital expense and operating cost, since diode lasers are highly reliable and rugged, remarkably efficient, and maintenance-free, allowing 24/7 operations without the need of consumables. The BLA system consists of an array of high-power blue diode lasers (λ ~ 450 nm) and beam-shaping optics to produce a line-shaped beam which can be scanned at high speed to anneal the Si film in a single scan, where the high scan speed minimizes thermal stress. The scan-to-scan is seamless to cover a large panel such as G6 or G8 without glass damage. The physical phenomena responsible for melting and recrystallization are explained by heat transfer simulations for process optimization.

Boosting the kesterite Cu2ZnSnS4 solar cells performance by diode laser annealing

In this work, a novel diode laser annealing was performed as a post-sulfurization heat treatment on pure-sulfide Cu2ZnSnS4 thin film solar cells fabricated by sputtering deposition. The effect on both film quality and device performance after annealing at various laser doses was investigated. After receiving an ultra-fast laser scan treatment, the crystallinity of the CZTS film can be improved and the Cu/Zn disordering is reduced. The power conversion efficiency of the laser annealed device was boosted to 7.33%, compared with the reference cell efficiency of 6.72%. The major increase in the short circuit current is driving force of such improvement, from 18mA/cm2 to 19.31mA/cm2 after laser treatment. These results indicate that the quality of CZTS thin films and the device performance can be effectively improved by the diode laser annealing.

Diode laser annealing of epitaxy Ge on sapphire (0 0 0 1) grown by magnetron sputtering

In this work, magnetron sputtering was used to epitaxially grow Ge films on sapphire at low temperatures (400°C–500°C) and diode laser annealing was employed to improve the crystallinity of the Ge films. The deposition temperature might slightly influence the crystallinity of the Ge film through affecting the diffusion rate of Ge. After laser scans of milliseconds exposure time, significant defect density reduction and removal of polycrystalline portion are achieved. Magnetron sputtering combined with diode laser annealing offers a low-cost and fast method in fabricating high quality single crystalline Ge on sapphire which is potentially capable of large-scale production.

Diode laser annealing on sputtered epitaxial Cu2ZnSnS4 thin films

In this work, epitaxial Cu2ZnSnS4 (CZTS) films grown by magnetron sputtering were annealed by continuous wavelength diode laser as an alternative post‐deposition heat treatment replacing conventional post‐annealing within sulfur atmosphere to improve the film quality. Upon ultrafast millisecond diode laser annealing, improvement of crystallinity and reduction of Cu/Zn disordering were observed in the CZTS thin films. The results indicate that the diode laser treatment can enable fast, effective, and efficient annealing of CZTS thin films.

Diode laser annealing on Ge/Si (100) epitaxial films grown by magnetron sputtering

In this work, epitaxial Ge films grown on Si by magnetron sputtering were annealed by diode laser as a replacement for conventional thermal annealing to reduce the threading dislocation density (TDD). After laser scans of millisecond exposure time, improvement of crystallinity and increase in tensile strain are observed in the Ge films. TDD of the Ge film is reduced by two orders of magnitude from 1010cm−2 to 108cm−2 after only three laser scans. Diode laser annealing is a fast and low-cost method to effectively reduce TDD in epitaxial Ge films on Si.

Defect elimination in solid-phase crystallised Si thin films by line-focus diode laser annealing

A high density of intragrain defects in solid-phase crystallised Si thin films results in poor electronic properties and impedes their use for thin-film solar cell or thin-film transistor applications. This paper demonstrates that a high-power line-focus diode laser can eliminate intragrain defects (microtwins and dislocations) in polycrystalline Si films while maintaining the smooth defect-free surface. Improved electronic properties of ultra-thin polycrystalline Si thin films are thus achieved. To alleviate crack formation during diode laser annealing, a rapid-thermal pre-treatment at 800°C for 60s is introduced since it effectively relieves the tensile stress in the films and thus generates a more stable precursor material for subsequent laser annealing. The film thickness plays an important role in diode laser annealing. The films thinner than 100nm show relatively smaller improvement due to the limited absorption of 808nm laser radiation.

Effect on structural, optical and electrical properties of aluminum-doped zinc oxide films using diode laser annealing

Abstract This study investigated the laser annealing characteristics of aluminum-doped zinc oxide (AZO) films using a diode laser source (808 nm) combined with moving stage with varying parameters, including laser fluence and speed of moving stage in air atmosphere. The commercial AZO thin films were prepared by RF magnetron sputtering on glass substrates. The films characteristics were systematically analyzed using a field emission scanning electron microscope, an atomic force microscope (AFM), an X-ray diffraction (XRD) equipment, an ultraviolet–visible–near-infrared (UV–vis–NIR) spectrophotometer, a four points probe instrument, and a Hall effect measurement system. The experimental results indicate that varying the laser fluence and annealing speed affected the optical, electrical, and structural characteristics of the AZO films. After annealing, approximately 90% of transmittance spectra exhibited slight changes in the visible region. All resistivity values of the laser-annealed AZO films decreased substantially from 4×10 −2 Ω cm to 2.8×10 −2 Ω cm. The absorption band edge moved toward shorter or longer wavelengths, depending on the annealing laser fluence and annealing speed. The optical energy band gap of the annealed AZO films increased because the carrier concentration of the annealed AZO films increased. The grain size increased in conjunction with the annealing speed. The AFM-derived root mean square (RMS) values decreased as the annealing speed increased, and the corresponding RMS values ranged from 1.4 to 1.9 nm.

Defect annealing in ultra-thin polycrystalline silicon films on glass: Rapid thermal versus laser processing

Conventional thermal and line-focus diode laser annealing was applied to ultra-thin, 160nm, solid-phase crystallized phosphorous-doped Si film on glass to improve their crystal and electronic quality. Electronic quality improvement is reflected in higher dopant activation and lower sheet resistance. The active dopant concentration increases from 5.6×1019cm−3 in as-crystallized film to 7.6×1019cm−3 and 1.0×1020cm−3 after thermal and laser annealing respectively. Sheet resistance decreases from 75Ω/□, to 54Ω/□ and 45.1Ω/□ after thermal and laser annealing respectively. However, electron mobility after laser annealing (86.6cm2/Vs) is lower than mobility in as-crystallized (92.5cm2/Vs) films and in the films after thermal annealing (95.3cm2/Vs). The lower mobility is likely due to scattering effects by higher concentration of ionized dopant. Crystal quality improvement is evidenced by TEM imaging. The observed intragrain defect (microtwins and dislocations) density is reduced after either thermal or laser annealing, the latter resulting in the lowest defect density. Thus, laser annealing allows achieving better crystal and electronic poly-Si film quality compared to conventional thermal annealing. Such a laser annealed poly-Si film can be used as a high quality seed layer for epitaxial thickening for solar cell and TFT applications.

Comparative study of microstructure and corrosion resistance of electroless Ni-W-P coatings treated by laser and furnace-annealing

A comparative study of amorphous electroless Ni-W-P coatings on mild steel substrate treated by a high power diode laser and furnace annealing was presented. Effects of different laser operating parameters and furnace-annealing conditions on microstructures, in terms of crystallisation, pores formation and grain growth, were investigated using SEM/EDX and XRD. Corrosion behaviours of these coatings before and after various treatments were evaluated with anodic polarisation in 0.5 mol/L H 2SO 4 solution. The results show that the furnace-annealing produces either a mixture of nanocrystallined Ni and amorphous phases or precipitated Ni 3P phase distributed in nanocrystallined Ni-based matrix, depending on annealing temperatures, whilst the laser treatment under the operating conditions only produces nanocrystallined Ni-based matrix with Ni 3P precipitates. Corrosion performance of the coatings treated by both the laser and the furnace-annealing is dependent on the annealing temperature and laser operating conditions. Corrosion mechanisms of various treated-coatings were discussed in the consideration of phase constitutes and proportion, grain sizes of both Ni and Ni 3P phases, pores formation and residual stresses.

Deactivation of submelt laser annealed arsenic ultrashallow junctions in silicon during subsequent thermal treatment

The use of nonequilibrium annealing approaches can produce very high levels of arsenic electrical activation in Si. However, subsequent thermal treatments between 500 and 800°C easily deactivate the dopant to a level one order of magnitude below the solid solubility. In this work, the authors study the deactivation of laser annealed (LA) ultrashallow arsenic distributions in silicon using Hall effect measurements, extended x-ray absorption fine structure spectroscopy, and secondary ion mass spectrometry. Single crystal Si (100) wafers implanted with As ions at 2keV energy and different doses were activated with a millisecond LA at 1300°C using a scanning diode laser annealing system under nonmelt conditions. The samples were then thermally treated in a furnace at 300–900°C in a N2 atmosphere for 10min. Electrical deactivation has been observed for all the implanted doses but for the lowest one. In particular, it was observed that the higher the As dose the easier the deactivation, in particular, after the 700°C post-LA treatment. At 900°C, in-depth diffusion and a resulting reactivation has been observed for samples implanted with 1×1015 and 3×1015cm−2.

Crystallization of amorphous Ge films induced by semiconductor diode laser annealing

Crystallization of amorphous Ge (a-Ge) films induced by semiconductor diode laser (SDL) irradiation has been investigated. 500-nm-thick a-Ge films are crystallized by scanning 807 nm CW SDL light focused to 4.0 × 0.1 mm 2 with the scanning speed ( v) ranging from 100 to 350 mm/s. From X-ray diffraction and Raman scattering spectra, it is confirmed that the films crystallized at v = 100 mm/s show [220] preferential orientation with no residual amorphous component. On the basis of in-situ monitoring, it has been confirmed that a-Ge films annealed at a v higher than 200 mm/s are transformed to crystalline phase via solid phase crystallization, while films annealed with v lower than 170 mm/s are melted and recrystallized.