Aluminum-induced Crystallization Process Research Articles

Influence of current density on constant current electric field enhanced aluminum induced crystallization of amorphous silicon thin films

In this study, electric field-enhanced aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) thin films was investigated considering various current densities. Constant electric currents of 4, 5, and 6 A were applied to assess the Joule heating characteristics during specific durations (40, 15.5, and 9 s). The samples were prepared by depositing a 200-nm thick a-Si layer on a glass substrate and sputtering a 300-nm thick aluminum (Al) layer onto the a-Si layer. During the AIC process, layer exchange occured via the diffusion of silicon (Si) into the Al layer. This phenomenon was verified by the in-situ reflectivity, which was in excellent agreement with thin-film optics calculations. During the annealing, processing temperatures were lower than 470 °C, which is less than the Al-Si eutectic temperature of 577 °C. The Raman peak observed near a wavenumber of 519 cm−1 revealed the formation of polycrystalline silicon films through Joule heating processes. Field-effect scanning electron microscopy images were captured and analyzed to investigate the surface morphology of the samples, specifically focusing on grain growth.

Rapid electric field-enhanced crystallization of amorphous silicon thin films with an aluminum layer

A rapid aluminum induced crystallization (AIC) method of amorphous silicon (a-Si) thin film was suggested. A constant electrical current is supplied to an aluminum (Al) layer deposited on the a-Si thin film and anneals the Al/Si stack fast by Joule-heating. The Joule heating is presumed to expedite a-Si thin film crystallization. To investigate the crystallization mechanism, an in-situ temperature and reflectivity measurement was adopted during the process. The maximum crystallization temperature was 744.15 K, which was found to be similar to the typical furnace AIC process temperature and less than the Al/Si eutectic temperature of 850 K. The measured and calculated surface reflectivity demonstrated the rapid Al induced layer exchange behavior between Si and Al layers. The entire process was accomplished for only a few dozens of seconds. This rapid layer exchange is caused by sudden increase of temperature-dependent Al diffusion coefficient in a Si layer and additional electric field enhancement. The Raman peak of 519 cm−1 verified the formation of polycrystalline silicon (p-Si) after the Al etching process. Scanning electron microscopy images showed that the resulting p-Si structure was porous with an average pore size of around 1.2 μm.

Влияние температуры отжига на кинетику процесса алюминий-индуцированной кристаллизации тонких пленок аморфного субоксида кремния

In this work, the kinetics of aluminum-induced crystallization (AIC) of non-stoichiometric silicon oxide a-SiO0.25 was investigated for annealing temperatures of 370, 385 and 400 °C, as a result of which thin films of polycrystalline silicon were obtained. It is shown that for low annealing temperatures, the surface morphology of the crystalline material is represented by dendric structures corresponding to the growth model with diffusion-limited aggregation. In addition, with an increase in the annealing temperature, the nucleation density increases from 3 to 53 mm–2. From the Arrhenius plot, the activation energy of the AIC process of a-SiO0.25 was obtained for the first time, which was 3.7±0.4 eV.

Solid phase epitaxial thickening of boron and phosphorus doped polycrystalline silicon thin films formed by aluminium induced crystallization technique on glass substrate

Aluminium induced crystallization (AIC) technique can be used to form the high-quality and large-grained polycrystalline silicon (poly-Si) thin films, which are with the thickness of ~200 nm and used as a seed layer, on silicon nitride coated glass substrate. Thanks to aluminium metal in AIC process, the natural doping of AIC thin films is p+ type (~2 × 1018 cm−3). On the other hand, recombination of carriers can be controlled by partial doping through the defects that may have advantages to improve the thin film quality by the overdoping induced passivation. In this study, boron (B) and phosphorus (P) doped AIC seed layers were thicken to ~2 μm by solid phase epitaxy (SPE) technique at 800 °C for 3 h under nitrogen flow in a tube furnace. During the crystallization annealing, exodiffusion of dopants was formed through the SPE film from the AIC seed layer. Optical microscope and electron back scattering diffraction technique (EBSD) were used to analyse the structural quality of the Si films. The poly-Si layer with an average grain size value of ~32 μm was formed by AIC + SPE technique for P doped samples while EBSD analysis gave no results for B doped samples due to the quite deterioration on the surface of the films. AIC + SPE films were analyzed in terms of structural properties by using micro-Raman Spectroscopy and X-ray diffraction systems. The results showed that the crystallinity of compressive stress formed AIC + SPE films reached up to 98.55%. Additionally, the Raman analysis pointed out that no temperature-induced stress were generated in the AIC + SPE films while compressive stress was induced by increasing the annealing duration for doped AIC film. For all samples, the preferred orientation was 〈100〉, and the crystallite size up to 44.4 nm was formed by phosphorus doping of AIC films. The doping efficiency was determined by time-of-flight secondary ion mass spectroscopy for doped samples. A graded n+n doping profile was obtained by exo-diffusion of phosphorus from the overdoped seed layer during the epitaxial thickening while boron doping of SPE film has failed with exo-diffusion of boron from AIC seed layer into SPE film. Finally, high-quality n+n type poly-Si films were fabricated on glass substrate by using AIC + SPE technique.

Study on silicon crystallization with aluminum deposition temperature in the aluminum-induced crystallization process using silicon oxide

Aluminum-induced crystallization (AIC) is one process which increases silicon grain size at low temperatures. In this study, we analyzed the effect of silicon crystallization according to the aluminum deposition conditions in the AIC process using silicon oxide. The initial aluminum layer was analyzed using a field emission-scanning electron microscopy (FE-SEM) after cutting the samples with a focused-ion-beam (FIB). Through FE-SEM, we observed that the aluminum grain size of the original aluminum layer increased in proportion to the aluminum deposition temperature. However, not only aluminum grain size but also surface roughness and porosity of the initial aluminum layer were increased. The initial aluminum layer, according to the deposition temperature, significantly affected the crystallized silicon grain size. The silicon grain size was decreased from 16.97 μm to 7.81 μm according to the increase of the aluminum deposition temperature. This was because the Si diffusion area was increased by the increase of the aluminum surface roughness.

Preparation of poly-Si films by inverted AIC process on graphite substrate

Graphite/a-Si/Al laminated structures were deposited on graphite substrate using magnetron sputtering technology. The substrate temperature of amorphous silicon (a-Si) deposition and the Al/Si thickness ratio have strong influences on a-Si crystallization, various conditions were used to investigate the polycrystalline silicon (poly-Si) thin films grown by inverted aluminum-induced crystallization (AIC) for process optimization. In this paper, we established the AIC model to explain the effects of the two factors in the inverted AIC process. The poly-Si thin films were characterized By means of Scanning electron microscope (SEM), X-ray diffraction (XRD) and Raman spectroscopy (Raman), which showed that the samples with strong preferred (111) orientation and high crystallization quality that were favorable for epitaxially growing poly-Si thick film cells.

Effects of the Si/Al layer thickness on the continuity, crystalline orientation and the growth kinetics of the poly-Si thin films formed by aluminum-induced crystallization

The joint impact of the Si/Al layer thickness on the growth kinetics, the crystalline orientation and the size of the poly-Si grains resulting from aluminum-induced crystallization process is analyzed. It is shown that the surface coverage of resulting poly-Si layers rapidly decreases together with annealing temperature and the Si/Al ratio. The surplus of a-Si over the Al needed to ensure continuity of the poly-Si thin film is in the range of 35%–50% for Al layers thicker than 225nm, but rapidly goes up to 200% as the thickness of the Al layer decreases below 50nm. It is demonstrated that the angular distribution of grain orientations is discrete and shifts towards the {111} direction as the Si/Al increases. It is reported that during an isothermal annealing, the nucleation of Si grains occurs in two steps. Finally, a simple model of the aluminum-induced crystallization process explaining the two-step nucleation is proposed.

Polysilicon Thin Film Developed on Flexible Polyimide for Biomedical Applications

Flexible pressure and thermal sensors are the critical parts of the functional electronic skin of prostheses and robots, as well as flexible catheter/devices for multimodal biomedical monitoring. In this paper, a polysilicon thin film was developed on a flexible polyimide substrate using aluminum induced crystallization process for biomedical pressure and temperature sensing applications. The formation of polycrystalline structure was verified from the developed polysilicon film. Long term stability and real-time temperature tests were performed to evaluate potential application to the in vivo monitoring of brain or body temperature. The linear real-time change of polysilicon resistance with temperature was attained with the temperature coefficient of the resistance of −0.0027/°C and the resolution of 0.05 °C. With a gauge factor of 10.3, the polysilicon film developed in this paper represents a promising material for the development of high-sensitivity pressure sensors on flexible polyimide substrates. [2015-0106]

The effect of high concentration of phosphorus in aluminum-induced crystallization of amorphous silicon films

Polycrystalline silicon (poly-Si) film prepared by aluminum-induced crystallization (AIC) of highly phosphorus (P)-doped amorphous silicon has been proposed for suppressing Si islands and voids. Si islands on the poly-Si surface and voids at the interface between the poly-Si and the glass are caused by Si atoms' out-diffusion toward the top surface during the AIC process, which also degrades the electrical characteristics of the film. High-P dopants can form P precipitations and retard the out-diffusion of Si atoms, hence suppressing Si islands and voids. Also, P precipitations can enhance the crystallization rate and grain growth. P dopants act as donors and compensate for the p-type poly-Si film, which is formed by Al doping during the AIC process. The hole concentration of undoped poly-Si is reduced by approximately three orders to 7×1012cm−3. Such near-neutral poly-Si films could be applied as intrinsic (i) layers in p-i-n structures to explore solar cell applications. Also, the enhanced quality of P-doped AIC-Si increases the mobility by five times owing to reduced scattering and trap density.

Fabrication of Si(111) crystalline thin film on graphene by aluminum-induced crystallization

We report the fabrication of a Si(111) crystalline thin film on graphene by the aluminum-induced crystallization (AIC) process. The AIC process of Si(111) on graphene is shown to be enhanced compared to that on an amorphous SiO2 substrate, resulting in a more homogeneous Si(111) thin film structure as revealed by X-ray diffraction and atomic force microscopy measurements. Raman measurements confirm that the graphene is intact throughout the process, retaining its characteristic phonon spectrum without any appearance of the D peak. A red-shift of Raman peaks, which is more pronounced for the 2D peak, is observed in graphene after the crystallization process. It is found to correlate with the red-shift of the Si Raman peak, suggesting an epitaxial relationship between graphene and the adsorbed AIC Si(111) film with both the graphene and Si under tensile strain.

Crystallized Si layer properties of novel aluminum-induced crystallization

Aluminum-induced crystallization (AIC) is usually used for making silicon seed layer. In this paper, we investigated the AIC process varied with different diffusion barrier materials. The barrier materials were native Al oxide, directly deposited Al2O3, and SiO2 in AIC process. The effects of these diffusion barrier materials were analyzed by using electron backscatter diffraction (EBSD), Raman spectroscopy, field emission scanning electron microscope (FE-SEM). The results showed that the case of native Al oxide showed diffusion-limited aggregation which usually resulted in polycrystalline structure. On the other hand, the case of SiO2 layer showed kinetic-limited aggregation, which generally resulted in the mono-crystalline structure. Therefore, we introduced the novel AIC process with SiO2 layers.

Consideration of a Local Aluminum-Induced Crystallization Process Guided along the µ-Holes Fabricated with fs Laser Pulses

Consideration of a Local Aluminum-Induced Crystallization Process Guided along the µ-Holes Fabricated with fs Laser Pulses

Silicon–hydrogen bond effects on aluminum-induced crystallization of hydrogenated amorphous silicon films

The effects of hydrogen dilution on aluminum-induced crystallization (AIC) of hydrogenated amorphous silicon (a-Si:H) films have been studied. The Raman spectra showed that the short-range order (SRO) and the intermedium-range order (IRO) of the as-deposited a-Si films increased with the increase of the H2 dilution from 0% to 20%. The optical microscope (OM) and X-ray diffraction (XRD) observation revealed that, compared to the a-Si:H film deposited in pure Ar, the a-Si:H films deposited with H2 dilution in the range of 3–8% possessed a lower crystallization rate while the a-Si:H films deposited with high H2 dilution in the range of 15–20% possessed a faster crystallization rate. It was found that majority of the hydrogen existed in the form of monohydride (SiH) bond in the a-Si:H films with H2 dilution ratio of 3–8%, the bonding energy of which was higher than that of Si–Si bond, leading to a lower crystallization rate of a-Si:H films. While the dihydride (SiH2) bond became dominant in the a-Si:H films with high H2 dilution of 15–20%, the bonding energy of which was lower than that of Si–Si bond, thus accelerating the crystallization rate. Therefore, it was illustrated that not the hydrogen concentration but the form of silicon–hydrogen bond determined the AIC process of a-Si:H films.

Aluminium Induced Crystallization of Amorphous Silicon via Solution Derived Catalyst

In this paper, aluminum induced crystallization (AIC) was studied by examining the effect of using solution derived AlCl3 catalyst. Such catalyst preparation method offers possibility of low-cost, non-vacuum solution process and allows examination of the role of alumina on the AIC process. The deposited AIC films were examined by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray diffraction (XRD) and four probe measurements. It was found that AIC process is highly dependent on annealing temperature and can occur at annealing temperatures above 500°C through Al2O3 formation. Based on the presented data, a possible growth model is proposed to clarify AIC mechanism.

Aluminum-induced crystallization for thin-film polycrystalline silicon solar cells: Achievements and perspective

Thin-film polycrystalline silicon (pc-Si) solar cell technology tries to combine the cost benefit of thin-film technology and the quality potential of crystalline Si technology. For this type of solar cells the challenge is to fabricate high quality coarse-grained (grain size of 1–100 µm) layers on non-silicon substrates in combination with superb light management. One possible material fabrication approach is metal induced crystallization (MIC). Many metals lower the crystallization temperature of a-Si but for photovoltaic (PV) solar cell applications aluminum seems to be the most interesting. Approximately a decade after the suggestion of Nast et al. to use aluminum induced layer exchange for thin-film solar cells, this paper will discuss the aluminum induced crystallization process itself as well as the implementation into workable solar cells. The record aluminum induced crystallization (AIC) solar cell today features an energy conversion efficiency of 8.5%, too low to be competitive with other PV technologies. The main problems and limitations as encountered today will be described. We believe that an energy conversion efficiency of 8.5% is not the limit of AIC solar cells. Different possible solutions to improve the AIC process as well as the resulting solar cells will be discussed together with our vision on the direction and future of AIC solar cell research.

Piezoresistive pressure sensor using low-temperature aluminium induced crystallization of sputter-deposited amorphous silicon film

In the present work, we have investigated the piezoresistive properties of silicon films prepared by the radio frequency magnetron sputtering technique, followed by the aluminium induced crystallization (AIC) process. Orientation and grain size of the polysilicon films were studied by x-ray diffraction analysis and found to be in the range 30–50 nm. Annealing of the Al–Si stack on an oxidized silicon substrate was performed in air ambient at 300–550 °C, resulting in layer exchange and transformation from amorphous to polysilicon phase. Van der Pauw and Hall measurement techniques were used to investigate the sheet resistance and carrier mobility of the resulting polycrystalline silicon film. The effect of Al thickness on the sheet resistance and mobility was also studied in the present work. A piezoresistive pressure sensor was fabricated on an oxidized silicon substrate in a Wheatstone bridge configuration, comprising of four piezoresistors made of polysilicon film obtained by the AIC process. The diaphragm was formed by the bulk-micromachining of silicon substrate. The response of the pressure sensor with applied negative pressure in 10–95 kPa range was studied. The gauge factor was estimated to be 5 and 18 for differently located piezoresistors on the diaphragm. The sensitivity of the pressure sensor was measured to be ∼ 30 mV MPa−1, when the Wheatstone bridge was biased at 1 V input voltage.

Growth of poly-crystalline silicon–germanium on silicon by aluminum-induced crystallization

The formation of poly-crystalline silicon–germanium films on single-crystalline silicon substrates by the method of aluminum-induced crystallization was investigated. The aluminum and germanium films were evaporated onto the single-crystalline silicon substrate to form an amorphous-germanium/aluminum/single-crystalline silicon structure that was annealed at 450°C–550°C for 0–3h. The structural properties of the films were examined using x-ray diffraction, Raman spectroscopy and Auger electron spectroscopy. The x-ray diffraction patterns confirmed that the initial transition from an amorphous to a poly-crystalline structure occurs after 20min of aluminum-induced crystallization annealing process at 450°C. The micro-Raman spectral analysis showed that the aluminum-induced crystallization process yields a better poly-crystalline SiGe film when the film is annealed at 450°C for 40min. The growth mechanism of the poly-crystalline silicon–germanium by aluminum-induced crystallization was also studied and is discussed.

Direct growth of large grain polycrystalline silicon films on aluminum-induced crystallization seed layer using hot-wire chemical vapor deposition

Large grain polycrystalline silicon (poly-Si) films on glass substrates have been deposited on an aluminum-induced crystallization (AIC) seed layer using hot-wire chemical vapor deposition (HWCVD). A poly-Si seed layer was first formed by the AIC process and a thicker poly-Si film was subsequently deposited upon the seed layer using HWCVD. The effects of AIC annealing parameters on the structural and electrical properties of the poly-Si seed layers were characterized by Raman scattering spectroscopy, field-emission scanning electron microscopy, and Hall measurements. It was found that the crystallinity of seed layer was enhanced with increasing the annealing duration and temperature. The poly-Si seed layer formed at optimum annealing parameters can reach a grain size of 700nm, hole concentration of 3.5×1018cm−3, and Hall mobility of 22cm2/Vs. After forming the seed layer, poly-Si films with good crystalline quality and high growth rate (>1nm/s) can be obtained using HWCVD. These results indicated that the HWCVD-deposited poly-Si film on an AIC seed layer could be a promising candidate for thin-film Si photovoltaic applications.

Structural and Electrical Properties of Polysilicon Films Prepared by AIC Process for a Polycrystalline Silicon Solar Cell Seed Layer

Polycrystalline silicon (pc-Si) films are produced by aluminum-induced crystallization (AIC) process for a polycrystalline silicon solar cell seed layer, and the structural and electrical properties of the films are analyzed. The used structure is glass/Al/ Al<sub >2</sub>O<sub >3</sub>/a-Si, and the thickness of Al<sub >2</sub>O<sub >3</sub> layer was varied from 2&#x2009;nm to 20&#x2009;nm to investigate the influence of the Al<sub >2</sub>O<sub >3</sub> layer thickness on the formation of the polycrystalline silicon. The annealing temperature and annealing time were fixed to <svg style="vertical-align:-0.17555pt;width:28.025px;" id="M1" height="11.0375" version="1.1" viewBox="0 0 28.025 11.0375" width="28.025" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,11.0375)"> <g transform="translate(72,-63.17)"> <text transform="matrix(1,0,0,-1,-71.95,63.39)"> <tspan style="font-size: 12.50px; " x="0" y="0">4</tspan> <tspan style="font-size: 12.50px; " x="6.2515001" y="0">0</tspan> <tspan style="font-size: 12.50px; " x="12.503" y="0">0</tspan> </text> <text transform="matrix(1,0,0,-1,-53.2,68.56)"> <tspan style="font-size: 8.75px; " x="0" y="0">∘</tspan> </text> </g> </g> </svg>C and 5 hours, respectively, for the AIC process conditions. As a result, it is observed that the average grain size of the pc-Si films is rapidly smaller with increasing the thickness of Al<sub >2</sub>O<sub >3</sub> layer, whereas the film quality, as defects and Hall mobility, was gradually degraded with only small difference. We obtained the maximum average grain size of 15&#x2009;<i >&#x3bc;</i>m for the pc-Si film with the thickness of Al<sub >2</sub>O<sub >3</sub> layer of 4&#x2009;nm. The best resistivity and the Hall mobility was <svg style="vertical-align:-0.3135pt;width:65.824997px;" id="M2" height="14.375" version="1.1" viewBox="0 0 65.824997 14.375" width="65.824997" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,14.375)"> <g transform="translate(72,-60.5)"> <text transform="matrix(1,0,0,-1,-71.95,60.86)"> <tspan style="font-size: 12.50px; " x="0" y="0">6</tspan> <tspan style="font-size: 12.50px; " x="6.2515001" y="0">.</tspan> <tspan style="font-size: 12.50px; " x="9.3772497" y="0">1</tspan> <tspan style="font-size: 12.50px; " x="18.404415" y="0">×</tspan> <tspan style="font-size: 12.50px; " x="29.182001" y="0">1</tspan> <tspan style="font-size: 12.50px; " x="35.433502" y="0">0</tspan> </text> <text transform="matrix(1,0,0,-1,-30.26,66.03)"> <tspan style="font-size: 8.75px; " x="0" y="0">−</tspan> <tspan style="font-size: 8.75px; " x="5.99512" y="0">2</tspan> </text> </g> </g> </svg>&#x2009;<svg style="vertical-align:-0.12538pt;width:42.262501px;" id="M3" height="10.85" version="1.1" viewBox="0 0 42.262501 10.85" width="42.262501" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,10.85)"> <g transform="translate(72,-63.32)"> <text transform="matrix(1,0,0,-1,-71.95,63.5)"> <tspan style="font-size: 12.50px; " x="0" y="0">Ω</tspan> <tspan style="font-size: 12.50px; " x="12.077898" y="0">⋅</tspan> <tspan style="font-size: 12.50px; " x="18.429422" y="0">c</tspan> <tspan style="font-size: 12.50px; " x="23.980755" y="0">m</tspan> </text> </g> </g> </svg> and 90.91&#x2009;cm<sup >2</sup>/Vs, respectively, in the case of 8&#x2009;nm thick Al oxide layer.

Intragrain defects in polycrystalline silicon layers grown by aluminum-induced crystallization and epitaxy for thin-film solar cells

Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason for the grain size independent device performance. Besides dislocations and stacking faults, also Σ3 boundaries were electrically active as shown by combining electron backscattered diffraction with EBIC measurements. The electrical activity of the defects is probably triggered by impurity decoration. Plasma hydrogenation changed the electrical behavior of the defects, as seen by photoluminescence, but the defects were not completely passivated as shown by EBIC measurements. In order to reveal the origin of the defects, cross section transmission electron microscopy measurements were done showing that the intragrain defects are already present in the AIC seed layer and get copied into the epitaxial layer during epitaxial growth. The same types of intragrain defects were found in layers made on different substrates (alumina ceramic, glass ceramic, and oxidized silicon wafer) from which we conclude that intragrain defects are not related to the relatively rough alumina ceramic substrates often used in combination with high temperature epitaxy. Further improvement of the material quality, and hence device performance, is therefore not simply achieved by increasing the grain size, but the intragrain quality of the material also needs to be taken into account. For pc-Si layers based on AIC and epitaxial growth, the seed layer has a crucial impact on the intragrain defect formation.