Hideyuki Takagishi

Single-grain Si thin-film transistors on flexible polyimide substrate fabricated from doctor-blade coated liquid-Si

Solution process of silicon will provide high-speed transistor fabrication with low-cost by, for example, roll-to-roll process. In this paper, a low-temperature process (350 °C) is reported for fabrication of high-quality Si devices on a polyimide substrate from doctor-blade coated liquid-Si. With this method, different semiconductor devices have been fabricated, reporting a carrier mobility of 460 cm2/V s and 121 cm2/V s for electrons and holes, respectively. The devices were peeled off and transferred onto a polyethylene naphthalate foil to achieve flexible devices. CMOS inverters were also fabricated and show full output swing.

Amorphous silicon carbide films prepared using vaporized silicon ink

The deposition of wide-band-gap silicon films using nonvacuum processes rather than conventional vacuum processes is of substantial interest because it may reduce cost. Herein, we present the optical and electrical properties of p-type hydrogenated amorphous silicon carbide (a-SiC:H) films prepared using a nonvacuum process in a simple chamber with a vaporized silicon ink consisting of cyclopentasilane, cyclohexene, and decaborane. The incorporation of carbon into the silicon network induced by the addition of cyclohexene to the silicon ink resulted in an increase in the optical band gap (Eg) of films from 1.56 to 2.11 eV. The conductivity of films with Eg 1.9 eV show lower conductivity than expected because of the incorporation of excess carbon without the formation of Si–C bonds.

Polymeric precursor for solution-processed amorphous silicon carbide

Amorphous silicon carbide (a-SiC) films are deposited via solution-based processes using a polymeric precursor solution consisting of polydihydrosilane with pendant hexyl groups. Unlike conventional polymeric precursors, this polymer neither requires catalysts nor oxidation for its synthesis and cross-linkage. Therefore, our polymeric precursor provides sufficient purity for the fabrication of solution-processed semiconducting a-SiC. Polymer-to-ceramic conversion is systematically investigated under various pyrolysis temperatures ranging from 320 °C to 420 °C. The polymer primarily undergoes cross-linking at temperatures above 150 °C with increasing polymer fraction; this cross-linking is followed by carbon atoms being incorporated into an amorphous network at 380 °C. The incorporated carbon atoms in the film are predominantly in the sp3-bonding state with almost no amorphous graphite-like sp2 CC clusters, leading to marked changes in the films properties. The conductivity values of the resulting a-SiC films are comparable with those of semiconducting a-SiC films prepared using vacuum-based deposition.

Effects of catalyst-generated atomic hydrogen treatment on amorphous silicon fabricated by Liquid-Si printing

The film property distributions along the thickness direction of the catalyst-generated atomic hydrogen (Cat-H*) treatment effects on hydrogenated amorphous silicon (a-Si:H) fabricated by plasma-enhanced chemical vapor deposition (plasma-CVD) and Liquid-Si printing (LSP) were systematically investigated. The a-Si:H films fabricated by LSP (L-a-Si:H) had nanosize voids; however, these films showed a decrease in void size around the surface region after Cat-H* treatment, in contrast to stable plasma-CVD films without voids. The decrease in nonaffected area by Cat-H* treatment in L-a-Si:H films improved the performance of a-Si:H solar cells with L-a-Si:H. Additionally, we achieved a 3.1% conversion efficiency for a-Si:H solar cells with L-a-Si:H as the active layer by stacking nondoped a-Si:H, fabricated by plasma-CVD, on the active layer.

Fabrication of interdigitated back-contact silicon heterojunction solar cells on a 53-µm-thick crystalline silicon substrate by using the optimized inkjet printing method for etching mask formation

Inkjet-printing-based fabrication process of the interdigitated back-contact silicon heterojunction solar cells has the potential to reduce the manufacturing costs because of its low machine and material costs and its applicability to thinner fragile silicon substrates than 100 µm. In this study, ink and printing parameters were investigated to obtain the desirable fine patterns and the resultant accuracy of the linewidths was less than ±0.05 mm on a flat surface. The completed cells using inkjet-printing showed almost the same performance of that fabricated by photolithography. In addition, flexible and free-standing cell on a 53-µm-thick Si substrate has been successfully fabricated.

Well-defined silicon patterns by imprinting of liquid silicon

Well-defined silicon patterns with a high aspect ratio and sharp edges were directly formed by imprinting of liquid silicon which was synthesized via the photopolymerization of cyclopentasilane (CPS: Si5H10) with UV light. After the patterns were formed, they were converted into amorphous silicon patterns by post-annealing at 400 °C. Both dotted and lined patterns whose size ranges from 1 μm to 100 nm were obtained, suggesting that their size could be further reduced. It is very noteworthy that all the obtained patterns demonstrated a good aspect ratio and sharp edges, despite a large shrinkage of 70–80% during the process. By conducting solid-phase crystallization at 800 °C, the pattern portion was converted into polycrystalline pure silicon, whereas the residual film region remained in an amorphous state containing large amounts of oxygen and carbon atoms. Based on the experimental results, the relationship between the decomposition and solidification processes of liquid silicon and its imprinting behavior was clarified and the mechanism of impurity condensation in a residual film region at the annealing temperature of 800 °C and that of well-defined shape formation were discussed. The developed method can be expected to be used in the fabrication of micro-silicon devices because of its production simplicity.

Formation of amorphous silicon passivation films with high stability against postannealing, air exposure, and light soaking using liquid silicon

We applied liquid-source vapor deposition (LVD), thermal CVD from the vapor of cyclopentasilane (CPS), to form amorphous silicon (a-Si) passivation films on crystalline Si (c-Si) wafers, and investigated the thermal stability of the films against postannealing. LVD a-Si passivation films showed a high initial effective minority carrier lifetime (τeff) of >300 µs and a higher thermal stability than a reference plasma-enhanced chemical-vapor-deposited (PECVD) sample. The high thermal stability of LVD a-Si passivation films may be attributed to the considerably high deposition temperature of the films at 360 °C or more. LVD a-Si passivation films were sufficiently stable also against air exposure and 1-sun light soaking. We also confirmed that the epitaxial growth of Si films does not occur on c-Si even at such high deposition temperatures, and LVD could realize the simultaneous deposition of a-Si films on both sides of a c-Si wafer.

Formation of epitaxial cobalt disilicide thin film by solution processing

A precursor obtained by the reaction between cyclopentasilane and dicobaltoctacarbonyl in a toluene solution provides a method for the production of a low-resistivity thin film of cobalt disilicide CoSi2 (ca. 15 µΩ·cm) on a silicon (100) substrate by spin coating and annealing. Several types of analysis have revealed that the films formed on silicon substrates have a double-layer structure consisting of an upper layer (oxidized complex film) and an epitaxial CoSi2 layer with low resistivity along the (100) orientation of the silicon substrate. The former layer can be removed by dry etching or chemical mechanical planarization when used in actual applications.

Fabrication of n-type Si nanostructures by direct nanoimprinting with liquid-Si ink

Nanostructures of n-type amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) with a height of 270 nm and line widths of 110-165 nm were fabricated directly onto a substrate through a simple imprinting process that does not require vacuum conditions or photolithography. The n-type Liquid-Si ink was synthesized via photopolymerization of cyclopentasilane (Si5H10) and white phosphorus (P4). By raising the temperature from 160 °C to 200 °C during the nanoimprinting process, well-defined angular patterns were fabricated without any cracking, peeling, or deflections. After the nanoimprinting process, a-Si was produced by heating the nanostructures at 400°C-700 °C, and poly-Si was produced by heating at 800 °C. The dopant P diffuses uniformly in the Si films, and its concentration can be controlled by varying the concentration of P4 in the ink. The specific resistance of the n-type poly-Si pattern was 7.0 × 10−3Ω ⋅ cm, which is comparable to the specific resistance of flat n-type poly-Si films.

Si heterojunction solar cells with a-Si passivation films formed from liquid Si

We fabricated Si heterojunction (SHJ) solar cells with a-Si passivation films formed from liquid silicon, and confirmed the actual operation of SHJ solar cells with liquid-Si-based a-Si films for the first time. Cyclopentasilane (Si5H10, CPS) was used as a source material and intrinsic a-Si (i-a-Si) films were formed from CPS vapor through thermal chemical vapor deposition (CVD) at an atmospheric pressure in nitrogen atmosphere. We refer to this novel deposition technique as liquid-source vapor deposition (LVD). Interestingly, LVD can realize the simultaneous deposition of a-Si films on both sides of a c-Si wafer. i-a-Si films with a thickness of about 15 nm were formed on (100)-oriented mirror-polished n-type c-Si wafers by LVD at substrate temperatures of 360-400 °C. Their effective minority carrier lifetimes (reff) of the c-Si wafers with LVD i-a-Si films evaluated by microwave photoconductivity decay (μ-PCD) were more than 300 μs. The LVD a-Si passivation films showed high thermal stability against post-annealing at 360 °C or more. This property is particularly important to form additional films on the LVD i-aSi films by using liquid Si which requires annealing at high temperatures of around 400 °C. We also confirmed that LVD a-Si passivation films have high stability against air exposure and light soaking. The SHJ solar cells fabricated in this study consisted of LVD i-a-Si passivation layers, p- and n-type a-Si films formed by catalytic chemical vapor deposition (Cat-CVD), sputtered ITO films, and comb-shaped Ag electrodes. The SHJ cells fabricated showed actual photovoltaic property with an open-circuit voltage (Voc) of more than 0.6 V under 1-sun illumination. This result indicates that SHJ solar cells can be fabricated by using liquid Si, which does not require the usage of high-cost vacuum systems.