Crystallization Of Amorphous Si Research Articles

Electric-field-enhanced aluminum-induced crystallization of amorphous silicon thin film using decreasing stepwise current method

Electric-field-enhanced aluminum-induced crystallization (AIC) was applied to the crystallization of amorphous Si (a-Si) using a stepwise current method with two to four decreasing steps. In AIC, Si diffuses into an Al layer, which generates a layer exchange from an a-Si/Al layer to an Al/polycrystalline Si (p-Si) layer. This increases the electrical resistivity of the Al/p-Si layer. The stepwise decreasing current was an attempt to overcome the limitations of electric-field-enhanced AIC using a constant current. The samples were fabricated by depositing a 200 nm-thick a-Si layer and sputtering a 300 nm-thick Al layer onto an Eagle XG glass substrate. In-situ resistance and reflectivity were measured to monitor the AIC mechanism and layer exchange. The reflectivity measurements were compared with thin-film optics calculations on the a-Si/Al and Al/p-Si layers to indicate the growth of p-Si. The temperature variations during the heating process were supported by a numerical analysis. The crystallinity of produced polycrystalline silicon (p-Si) was verified by a Raman peak at around 519–520 cm−1. The stepwise current supply increased the crystallization time and improved the crystallinity of the produced p-Si compared to the constant current method.

Femtosecond Laser-Induced Crystallization of Amorphous Silicon Thin Films under a Thin Molybdenum Layer.

A new process to crystallize amorphous silicon without melting and the generation of excessive heating of nearby components is presented. We propose the addition of a molybdenum layer to improve the quality of the laser-induced crystallization over that achieved by direct irradiation of silicon alone. The advantages are that it allows the control of crystallite size by varying the applied fluence of a near-infrared femtosecond laser. It offers two fluence regimes for nanocrystallization and polycrystallization with small and large crystallite sizes, respectively. The high repetition rate of the compact femtosecond laser source enables high-quality crystallization over large areas. In this proposed method, a multilayer structure is irradiated with a single femtosecond laser pulse. The multilayer structure includes a substrate, a target amorphous Si layer coated with an additional molybdenum thin film. The Si layer is crystallized by irradiating the Mo layer at different fluence regimes. The transfer of energy from the irradiated Mo layer to the Si film causes the crystallization of amorphous Si at low temperatures (∼700 K). Numerical simulations were carried out to estimate the electron and lattice temperatures for different fluence regimes using a two-temperature model. The roles of direct phonon transport and inelastic electron scattering at the Mo–Si interface were considered in the transfer of energy from the Mo to the Si film. The simulations confirm the experimental evidence that amorphous Si was crystallized in an all-solid-state process at temperatures lower than the melting point of Si, which is consistent with the results from transmission electron microscopy (TEM) and Raman. The formation of crystallized Si with controlled crystallite size after laser treatment can lead to longer mean free paths for carriers and increased electrical conductivity.

Continuous-Wave Laser Lateral Crystallization of A-Si Thin Films on Polyimide Using a Heatsink Layer Embedded in the Buffer SiO2

Continuous-wave laser crystallization of amorphous Si (a-Si) thin films on a polyimide (PI)-coated glass substrate is studied by using a single scan of a highly uniform top-flat line-beam with a 123 nm SiO2 cap layer and a thin buffer layer at room temperature in air. The total buffer layer thickness is reduced to as thin as 1.55 μm including the heatsink a-Si layer and two SiO2 layers. Damage to the polyimide during the crystallization is successfully suppressed by the heatsink layer. An 88.3% {100} surface fraction is obtained for a 60-nm-thick Si thin film within 15° on polyimide even with a low scan velocity of 15 mm/s.

Crystallization of Optically Thick Amorphous Silicon Films by Near-IR Femtosecond Laser Processing

We demonstrated efficient crystallization of amorphous Si films induced by their direct irradiation with near-IR femtosecond laser pulses coming at sub-MHz repetition rate. Comprehensive analysis of morphology and composition of the laser-annealed film by atomic-force microscopy, Fourier-transform IR, Raman and energy dispersive X-ray spectroscopy as well as numerical modeling of optical spectra confirmed efficient crystallization of amorphous Si and high-quality of the obtained films opening pathway for applications in thin-film solar cells, transistors and displays.

Remarkable Improvement in Foldability of Poly‐Si Thin‐Film Transistor on Polyimide Substrate Using Blue Laser Crystallization of Amorphous Si and Comparison with Conventional Poly‐Si Thin‐Film Transistor Used for Foldable Displays

Highly robust poly‐Si thin‐film transistor (TFT) on polyimide (PI) substrate using blue laser annealing (BLA) of amorphous silicon (a‐Si) for lateral crystallization is demonstrated. Its foldability is compared with the conventional excimer laser annealing (ELA) poly‐Si TFT on PI used for foldable displays exhibiting field‐effect mobility of 85 cm2 (V s)−1. The BLA poly‐Si TFT on PI exhibits the field‐effect mobility, threshold voltage (VTH), and subthreshold swing of 153 cm2 (V s)−1, −2.7 V, and 0.2 V dec−1, respectively. Most important finding is the excellent foldability of BLA TFT compared with the ELA poly‐Si TFTs on PI substrates. The VTH shift of BLA poly‐Si TFT is ≈0.1 V, which is much smaller than that (≈2 V) of ELA TFT on PI upon 30 000 cycle folding. The defects are generated at the grain boundary region of ELA poly‐Si during folding. However, BLA poly‐Si has no protrusion in the poly‐Si channel and thus no defect generation during folding. This leads to excellent foldability of BLA poly‐Si on PI substrate.

An assessment on crystallization phenomena of Si in Al/a-Si thin films via thermal annealing and ion irradiation.

In the present study, crystallization of amorphous-Si (a-Si) in Al/a-Si bilayer thin films under thermal annealing and ion irradiation has been investigated for future solar energy materials applications. In particular, the effect of thickness ratio (e.g. in Al : a-Si, the ratio of the Al and a-Si layer thickness) and temperature during irradiation on crystallization of the Si films has been explored for the first time. Two sets of samples with thickness ratio 1 : 1 (set-A: 50 nm Al/50 nm a-Si) and thickness ratio 1 : 3 (set-B: 50 nm Al/150 nm a-Si) have been prepared on thermally oxidized Si-substrates. In one experiment, thermal annealing of the as-prepared sample (of both the sets) has been done at different temperatures of 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C. Significant crystallization was found to initiate at 200 °C with the help of thermal annealing, which increased further by increasing the temperature. In another experiment, ion irradiation on both sets of samples has been carried out at 100 °C and 200 °C using 100 MeV Ni7+ ions with fluences of 1 × 1012 ions per cm2, 5 × 1012 ions per cm2, 1 × 1013 ions per cm2, and 5 × 1013 ions per cm2. Significant crystallization of Si was observed at a remarkably low temperature of 100 °C under ion irradiation. The samples irradiated at 100 °C show better crystallization than the samples irradiated at 200 °C. The maximum crystallization of a-Si has been observed at a fluence of 1 × 1012 ions per cm2, which was found to decrease with increasing ion fluence at both temperatures (i.e. 100 °C & 200 °C). The crystallization of a-Si is found to be better for set-B samples as compared to set-A samples at all the fluences and irradiation temperatures. The present work is aimed at developing the understanding of the crystallization process, which may have significant advantages for designing crystalline layers at lower temperature using appropriate masks for irradiation at the desired location. The detailed mechanisms behind all the above observations are discussed in this paper.

Aluminum induced crystallization of amorphous Si: Thermal annealing and ion irradiation process

The Al-induced crystallization of amorphous (a)-Si under thermal annealing and swift heavy ion irradiation has been investigated. The c-Al (50 nm)/a-Si (150 nm) thin films have been prepared on thermally oxidized Si-substrates. A set of samples have been annealed at temperatures ranging from 100 °C to 500 °C to achieve crystallization. Another set of similar samples have been irradiated at an elevated temperature of 100 °C using 100 MeV Ni+7 ions at fluences of 1 × 1012 ions-cm−2, 5 × 1012 ions-cm−2, 1 × 1013 ions-cm−2, and 5 × 1013 ions-cm−2. The crystallization of a-Si is observed at annealing temperature of 200 °C. The crystallinity increases with increasing temperature. On the other hand, irradiation using swift heavy ion leads to crystallization of a-Si at significantly lower temperature of 100 °C. The irradiation induced crystallization is explained in terms of creation of vacancies, interstitials and mixing of Si and Al atoms at a-Si/c-metal interface due to energy deposited by swift Ni ions.

Dynamics of interstitial atoms and vacancies during the crystallization of amorphous Si and Ge films by flash lamp annealing

We examined the dynamics of interstitial atoms and vacancies in amorphous Si (a-Si) and a-Ge films crystallized by flash lamp annealing in consideration of the self-diffusion coefficients of Si and Ge. We found that the interstitial atoms play an important role in the liquid-phase crystallization (LPC) of a-Si films, whereas the vacancies are more important for the solid-phase crystallization (SPC) of a-Si films along with the LPC and SPC of a-Ge films. For Si, the crystal defect density of the film crystallized by LPC was higher than that of the film crystallized by SPC; the opposite result was achieved for Ge. This phenomenon is considered to be attributed to the existence of interstitial atoms introduced in Si. The thermodynamic calculated results related to the relationship between the point defect and SPC or LPC supported the crystallization mechanism.

Investigation on Crack Suppression by Thermal-Plasma-Jet Crystallization of Amorphous Silicon Films on Flexible Glass Substrate

Introduction.Ultra-thin glass substrate is attracting much attention as flexible substrates for curved display applications. In fabrication of thin-film transistors (TFTs) on flexible glass, crystallization of amorphous Si (a-Si) is one of the key process technologies [1-3]. Rapid-thermal-annealing (RTA) in microsecond regime is necessary to form high crystallinity Si without thermal damage to glass. We have proposed the application of atmospheric pressure DC ark discharge thermal-plasma-jet (TPJ) to the crystallization of a-Si films [4]. Because of its simple structure and atmospheric pressure process, TPJ enables low cost crystallization compared with excimer laser annealing (ELA) technique. High speed lateral crystallization (HSLC) is induced by sweeping a molten region at a speed as high as 4000 mm/s. However, residual tensile stress is generated because of the densification of glass surface after RTA process. As a result, significant number of cracks are generated on the surface of glass substrate in cases of sever annealing conditions. In this study, we applied µ-TPJ crystallization of a-Si films on ultra-thin glass substrate and investigated the effect of glass thickness on residual stress and crack generation. Experimental.After RCA cleaning and HF treatment of 100-µm-thick flexible glass and 500-µm-thick conventional glass substrates, 100-nm-thick a-Si films were deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 250 °C. Dehydrogenation was carried at 450 °C in N2 ambient for 1 h. TPJ irradiation set up is shown in Fig. 1. µ-TPJ was generated by DC arc discharge under atmospheric pressure with supplying power (P) of 1.7 kW between electrodes. The Ar gas flow rate (f) was 1.0 L/min. µ-TPJ was generated by blowing out the thermal plasma through an orifice with its diameter (ϕ) of 600 µm. The distances between the plasma source and glass substrates (d) were fixed at 1.0 mm and the substrates were linearly moved by a motion stage in front of the µ-TPJ with scanning speed (v) of 2200 mm/s. For the measurement of temperature profile in the glass substrate, transient reflectivity during TPJ annealing was measured by irradiating bare conventional glass substrate with a He-Ne laser (632.8nm) from the back surface. Details of the non-contact temperature measurement are described in Ref [5]. Results and discussion.The microscope images of crystallized Si films on each substrate are shown in Fig. 2. Many cracks were observed in conventional glass substrate. On the other hand, no crack was observed in flexible glass substrate, although both glass substrates were annealed at the same time. Crystallization was conducted under various µ-TPJ irradiation conditions, and almost no cracks were found in flexible glass and it showed significant resistance against RTA. This result suggests that the tensile stress at glass surface is significantly reduced in the case of ultra-thin flexible glass. Previous study calculated residual stress profile in glass substrate on the basis of multilayered beam theory [6]. To obtain stress profile, we measured transient temperature profile during TPJ irradiation by optical probe (Fig .3). We assumed surface densification of glass when the temperature exceeded softening point (1258 K). We conducted residual stress analysis of glass substrate as the residual stress is shown in Fig. 4 [6]. Strong tensile stress is introduced in glass surface due to the shrinkage by RTA. At the same time, compressive stress is introduced inside of glass to compensate the tensile stress at the surface. It should be noted that the depth of stress compensation is in the range of ~400 µm as shown in Fig. 4, which is larger than the thickness of flexible glass. It is expected that the back side of thin glass substrate is free to relax the stress and plays a roll in suppression of residual stress in the surface of glass. Conclusions.We have crystallized a-Si films on flexible glass substrate by µ-TPJ without crack generation. Flexible glass substrate is useful to relieve surface tensile stress and as the result, crack generation is significantly suppressed. Acknowledgments. A part of this work was supported by Research Institute for Nanodevice and Bio Systems (RNBS) Hiroshima University, and JSPS KAKENHI Grant Number (B)(16H0433400).

Au induced crystallization and layer exchange in a-Si/Au thin film on glass below and above the eutectic temperature

Au (metal) induced layer exchange and crystallization of amorphous Si (a-Si) on corning glass (CG) in a-Si/Au/CG thin film specimen upon vacuum annealing (~10−8mbar) is reported. The crystallization characteristics of these specimens at temperatures ranging on either side of the Au-Si eutectic temperature Te(Au-Si) are investigated. Our results shows that solid state diffusion assisted layer exchange of the a-Si and Au layer, followed by crystallization of a-Si into crystalline Si (c-Si) occur at 350°C (below the Te). The upper limit to this crystallization mechanism was observed to be above the Te(Au-Si) and this continues till ~400°C in the present study. Above this limit, the layer exchange phenomenon ceases and eutectic mixing reaction of Au-Si takes over to form diffusion limited crystalline aggregates of Si with different microstructural attributes.

Optical properties of Si nanocrystals formed with laser pulse annealing

Nano- and femtosecond laser pulse annealings were applied for crystallization of thin amorphous Si films and amorphous Si nanoclusters in silicon-rich nitride and oxide films. Laser assisted formation of amorphous Si nanoclusters in the non-stoichiometric dielectric films was observed. Non-thermal effects in crystallization of amorphous Si by femtosecond annealing were supposed. This approach is applicable for the creation of dielectric films with Si nanoclusters on non-refractory substrates.

Influences of hydrogen dilution on the growth of Si-based core–shell nanowires by HWCVD, and their structure and optical properties

Si-based core–shell nanowires were grown on Ni-coated crystal silicon substrates using a hot-wire chemical vapor deposition technique. The NiSi nanoparticles acted as catalysts that facilitated the growth of the core–shell nanowires without any hydrogen dilution as well as that ranging from 20 to 99 %. These nanowires were structured by single-crystalline NiSi cores and amorphous shells with consisting of nanocrystallites embedded within an amorphous matrix. Raman results reveal crystallization of amorphous Si to crystalline Si up to the crystalline volume fraction of 92.3 % for the nanowires grown with hydrogen dilution. An increase in hydrogen dilution enhanced the decomposition rate and the gas-phase reactions for SiC shell formation, while further increases up to 99 % suppressed the growth of the nanowires. Moreover, a phased transition from Si to SiC occurred with increases in hydrogen dilution above 20 %. The nanowires demonstrated superior optical absorption in the visible region, revealing their significant light-trapping ability. This paper discusses the influences of hydrogen dilution on the structure and optical properties of these core–shell nanowires.

PEDOT:PSS emitters on multicrystalline silicon thin-film absorbers for hybrid solar cells

We fabricated an efficient hybrid solar cell by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar multicrystalline Si (mc-Si) thin films. The only 5 μm thin Si absorber layers were prepared by diode laser crystallization of amorphous Si deposited by electron beam evaporation on glass. On these absorber layers, we studied the effect of SiOx and Al2O3 terminated Si surfaces. The short circuit density and power conversion efficiency (PCE) of the mc-Si/Al2O3/PEDOT:PSS solar cell increase from 20.6 to 25.4 mA/cm2 and from 7.3% to 10.3%, respectively, as compared to the mc-Si/SiOx/PEDOT:PSS cell. Al2O3 lowers the interface recombination and improves the adhesion of the polymer film on the hydrophobic mc-Si thin film. Open circuit voltages up to 604 mV were reached. This study demonstrates the highest PCE so far of a hybrid solar cell with a planar thin film Si absorber.

Effect of the crystallization-induction layer of yttria-stabilized zirconia on the solid state crystallization of an amorphous Si film

We investigated the crystallization-induction (CI) effect of yttria-stabilized zirconia (YSZ) on the solid phase crystallization of amorphous Si (a-Si) films. The incubation time τi for crystallization on a polycrystalline YSZ layer was shorter than that on a glass substrate. From the result of Arrhenius plots of 1/τi, it is suggested that the CI effect is not due to the difference in the activation energy Ei but to a higher nucleation site area density on the YSZ layer. Also, preheating the YSZ layer prior to a-Si film deposition was effective to shorten the incubation time τi because Ei was reduced.

Femtosecond laser-irradiated crystallization of amorphous Si_2Sb_2Te_3 films and its in-situ characterization by coherent phonon spectroscopy

Femtosecond laser-irradiation-induced phase change of a new amorphous Si(2)Sb(2)Te(3) film with a good thermal stability and low reset current is studied by coherent phonon spectroscopy. New coherent optical phonons (COP) occur as laser irradiation fluence reaches some threshold, implying laser-induced phase change emerged. The compositions in phase-changed area revealed by COP modes agree well with ones in reported annealed crystallized film, implying laser-induced phase change as crystallization. Pump fluence dependence of COP dynamics reveals good crystallization quality of the phase-changed film, exhibiting promising application of Si(2)Sb(2)Te(3) films in optical phase change memory. Acoustic phonons are also found and identified.

Layer Transfer and Simultaneous Crystallization of Amorphous Si Films with Mid-Air Structure Induced by Near-Infrared Semiconductor Diode Laser Irradiation

Layer transfer and simultaneous crystallization of amorphous Si (a-Si) films induced by near-infrared semiconductor-diode-laser (SDL) irradiation have been investigated. The a-Si films supported by columns on a starting substrate (quartz) and a counter substrate (glass and polyethylene terephthalate) were face-to-face contact, and an SDL irradiated the a-Si films. After SDL irradiation, the Si films of 6 × 6 mm2 area were completely transferred and crystallized simultaneously on the counter substrates. In-situ monitoring revealed that the layer transfer took place either in the solid phase or the liquid phase followed by phase transformation in the cooling period.

Influence of rapid thermal annealing on the process of aluminum induced crystallization of amorphous Si

The effects of annealing methods on the crystallization process and microstructure of polycrystalline silicon (poly-Si) films obtained by aluminum-induced crystallization (AIC) of amorphous Si (a-Si) films were comparatively investigated. Glass/Al/a-Si structures were annealed by rapid thermal annealing (RTA) and conventional furnace at 500 °C for different times in Ar. As compared to furnace annealing, AIC of a-Si films annealed by RTA possesses a shorter period of nucleation time, a higher nucleation density and reduces the process time to form continuous poly-Si films. It is revealed that the continuous Si films obtained by both RTA and conventional furnace annealing are polycrystalline in nature, exhibiting good microstructures with Raman peaks at 518 cm−1 and full-width at half-maximums of 6.43–6.48 cm−1.

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.

In-situ TEM probing of laser-induced crystallization of amorphous Si

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi2 and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi2 and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.