Muhammad Fahad Bhopal

Aluminum Induced Crystallization of Amorphous Silicon Dependent on Annealing Conditions with Graphite Plate

Aluminum induced crystallization (AIC) of amorphous silicon was studied for thin-film solar cell. The AIC have been usually researched on glass substrate which has smooth surface. However, in this paper, the graphite plate was used as a substrate for using thin-film solar cell which has 1μm roughness. The growth silicon layer characteristic could be relatively different with that using glass substrate by the surface roughness. Therefore, the properties of crystallized silicon layer were studied for grain size analysis with variation in temperature and time during the AIC annealing process. The crystalline fraction and crystallinity was analyzed by Optical microscope, X-ray diffraction (XRD), and Raman spectrometer measurement methods. Additionally, the grain size was also relatively analyzed with FWHM results. As a result of measurements, crystalline fraction of grown silicon was increased with the increasing of temperature and time. The maximum crystalline fraction of grown silicon was 92.85% for 2400 minutes of annealing duration at 500°C.

Poly-crystalline thin-film by aluminum induced crystallization on aluminum nitride substrate

Thin-film polycrystalline silicon (pc-Si) on foreign (non-silicon) substrates has been researched by various research groups for the production of photovoltaic cells. High quality pc-Si deposition on foreign substrates with superior optical properties is considered to be the main hurdle in cell fabrication. Metal induced crystallization (MIC) is one of the renowned techniques used to produce this quality of material. In the current study, an aluminum induced crystallization (AIC) method was adopted to produce pc-Si thin-film on aluminum nitride (AlN) substrate by a seed layer approach. Aluminum and a-Si layer were deposited using an e-beam evaporator. Various annealing conditions were used in order to investigate the AIC grown pc-Si seed layers for process optimization. The effect of thermal annealing on grain size, defects preferentially crystallographic orientation of the grains were analyzed. Surface morphology was studied using an optical microscope. Poly-silicon film with a crystallinity fraction between 95-100% and an FWHM between 5-6 cm−1 is achievable at low temperatures and for short time intervals. A grain size of about 10 micron can be obtained at a low deposition rate on an AIN substrate. Similarly, Focused ion beam (FIB) also showed that at 425 °C sample B and at 400 °C sample A were fully crystallized. The crystalline quality of pc-Si was evaluated using μ-Raman spectroscopy as a function of annealed conditions and Grazing incidence X-ray diffraction (GIXRD) was used to determine the phase direction of the pc-Si layer. The current study implicates that a poly-silicon layer with good crystallographic orientation and crystallinity fraction is achievable on AIN substrate at low temperatures and short time frames.

Ni/Cu/Ag plated contacts: A study of resistivity and contact adhesion for crystalline-Si solar cells

Ni/Cu/Ag plated contacts were examined as an alternate to Ag screen printed contacts for silicon (Si) solar cell metallization. To realize a reliable contact for industrial applications, the contact resistance and its adhesion to Si substrates were evaluated. Si surface roughness by picosecond (ps) laser ablation of silicon-nitride (SiNx) antireflection coating (ARC) was done in order to prepare the patterns. The sintering process after Ni/Cu/Ag full metallization in the form of the post-annealing process was applied to investigate the contact resistivity and adhesion. A very low contact resistivity of approximately 0.5 mΩcm2 has been achieved with measurements made by the transfer length method (TLM). Thin finger lines of about 26 μm wide and a line resistance of 0.51 Ω/cm have been realized by plating technology. Improved contact adhesion by combining the ps-laser-ablation and post-annealing process has been achieved. We have shown the peel-off strengths >1 N/mm with a higher average adhesion of 1.9 N/mm. Our pull-tab adhesion tests demonstrate excellent strength well above the wafer breakage force.

Past and future of graphene/silicon heterojunction solar cells: a review

Graphene/silicon (Gr/Si) Schottky junction solar cells represent an alternative low-cost, easy fabrication structure in photovoltaic devices. After graphenes emergence in 2004, the first Gr/Si solar cell was fabricated in 2010, and was able to achieve upto 15% efficiency in less than a decade. This breakthrough in cell efficiency was realized by the fact that Gr has tremendous electrical and optical properties for photovoltaic applications. In this review, we highlight some of the recent progress in Gr/Si heterojunction solar cells. The growth processes of 2D graphene using the CVD process are discussed in detail. Afterwards, the key parameters that help to enhance the power conversion efficiency (PCE) of solar cells are considered. The interface of Gr/Si and the effects of chemical doping on the cell parameters were studied. Lastly, the challenges and limitations along with the future developments for Gr/Si solar cells are discussed in detail.

Study of SiOx thickness effects on aluminum-induced crystallization

The thickness effects of SiOx which was deposited as an intermediate layer between aluminum and silicon were studied on Aluminum-induced crystallization (AIC). The SiOx layer thickness varied from 2 nm to 20 nm and affected the crystallization process of the AIC. In the case of the thin SiOx layer, crystallized silicon morphology showed kinetic-limited aggregation. On the other hand, crystallized silicon processed with the thick SiOx layer showed diffusion-limited aggregation due to slow silicon diffusion velocity. Kinetic-limited aggregation showed large grain. The schematic crystallization model was used to describe the relationship between crystallization and grain size in this paper.

High Temperature Crystallization Process of a-Si Thin-films on Aluminum Nitride Substrates

Crystallization of silicon (Si) from amorphous silicon (a-Si) on foreign substrates has been studied by various research institutes. Crystallization of silicon thin-films on foreign substrates acts as an active layer in silicon thinfilm solar cells. In this research, due to the compatibility of thermal stability and expansion coefficient with Si, we used an aluminum nitride (AIN) substrate as an alternative candidate to glass and other ceramic substrates. P-type amorphous Si 5 μm thin-film was deposited using an ebeam evaporator directly on AIN substrates. The deposited layer was annealed at high temperature (°C) with N2 environment in a conventional tube furnace. Optical characterization was done using an optical microscope to investigate the surface morphology of as-grown and annealed samples. A smoother surface with an average grain size of about 3–4 μm was formed after annealing. Reflectance parameters were measured by UV-vis spectrometry. UV-vis-NIR was studied on as-grown and annealed samples to calculate the quality factor of the Si thin-film which was about 84.4 %. X-ray diffraction (XRD) was used to determine the phase direction of the Si thin-film before and after thermal annealing. It was observed that FWHM varied from 7.73 to 9.30 cm−1, Raman shift was from 520 to 522 cm−1, and the stressed level also changed from 180 to 540 Mpa after annealing at high temperature for a long time. Interestingly, a crystallinity fraction was achieved of about 90 % at 1100 °C.

A Study of Silicon Crystallization Dependence upon Silicon Thickness in Aluminum-induced Crystallization Process

Aluminum-induced crystallization(AIC) is one of the ways to improve characteristics of thin-film poly-crystalline (TFPC) silicon solar cell since it shows large grain size and good properties for TFPC silicon solar cell. In AIC process, aluminum is firstly deposited on foreign substrate. Then, silicon is deposited on aluminum. Annealing was done below the eutectic temperature of aluminum and silicon(577 oC). Afterward, amorphous silicon layer is crystallized by aluminum-inducing process. In this paper, we report amorphous silicon layer thickness effects to crystallized silicon and hillock characteristics since the amorphous silicon layer is important layer for AIC.Crystallized silicon properties were analyzed dependent upon silicon thickness (50 % 200 % than aluminum layer thickness). In case of the thick amorphous silicon (above 300 nm), hillocks grew on the crystallized silicon layer, Even though silicon hillock showed high crystallinity as a result of Raman spectroscopy, it had not enough grain size (average 4 μm). Therefore, as results, samples which had 113 % 120 % silicon and aluminum thickness ratio showed proper thickness to use as a seed layer. (Received February 19, 2018; Accepted March 16, 2018)

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.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 increa...