Seung-Yeol Han

Low-Temperature, High-Performance, Solution-Processed Indium Oxide Thin-Film Transistors

Solution-processed In(2)O(3) thin-film transistors (TFTs) were fabricated by a spin-coating process using a metal halide precursor, InCl(3), dissolved in acetonitrile. A thin and uniform film can be controlled and formed by adding ethylene glycol. The synthesized In(2)O(3) thin films were annealed at various temperatures ranging from 200 to 600 °C in air or in an O(2)/O(3) atmospheric environment. The TFTs annealed at 500 °C under air exhibited a high field-effect mobility of 55.26 cm(2) V(-1) s(-1) and an I(on)/I(off) current ratio of 10(7). In(2)O(3) TFTs annealed under an O(2)/O(3) atmosphere at temperatures from 200 to 300 °C exhibited excellent n-type transistor behaviors with field-effect mobilities of 0.85-22.14 cm(2) V(-1) s(-1) and I(on)/I(off) ratios of 10(5)-10(6). The annealing atmosphere of O(2)/O(3) elevates solution-processed In(2)O(3) TFTs to higher performance at lower processing temperature.

The Growth Mechanism of Nickel Oxide Thin Films by Room-Temperature Chemical Bath Deposition

Chemical bath deposition (CBD) is an advantageous thin film deposition technique for depositing compound semiconductors at low temperature. In this paper, nickel oxide thin films were prepared by CBD from an aqueous solution composed of nickel sulfate, potassium persulfate, and ammonia at room temperature. Thin film growth mechanisms were studied by using quartz crystal microbalance, UV-vis absorption, and photon correlation spectroscopy. The data indicate that film growth is strongly dependent upon mixing conditions and competes with homogeneous particle formation. No film formation was observed without the addition of persulfate. A growth mechanism based on the combination of particle sticking and molecule level heterogeneous growth is proposed. The as-deposited film contained α-Ni(OH) 2 and 4Ni(OH) 2 ·NiOOH·xH 2 O and was converted to nickel oxide (NiO) by thermal annealing according to thermogravimetric. X-ray diffraction and X-ray photoelectron spectroscopy measurements.

Inkjet printed high-mobility indium zinc tin oxide thin film transistors

Thin-film transistors based on inkjet printed indium zinc tin oxide (IZTO) channel layers are reported in this paper. The printed IZTO transistor has a high field-effect mobility (µFE = ∼30 cm2V−1 s−1), excellent on-to-off current ratio (>1 × 106) and behaves as an enhancement mode device (turn-on voltage = 2 V). This mobility is an order magnitude higher than previously reported for inkjet printed oxide-based transistors. The printed films are highly transparent in the UV-Visible regime with a transmittance higher than 95%. A transparent thin film transistor using a printed IZTO channel was also demonstrated for the first time.

8.01% CuInGaSe2 solar cells fabricated by air-stable low-cost inks

CuInGaSe(2) (CIGS), a promising thin film solar cell material, has gained lots of attention in decades due to its high energy conversion efficiency and potential lower manufacture cost over conventional Si solar cells. As a cheaper processing method compared to vacuum-based techniques, solution-based deposition has been successfully applied to fabricate electronic devices, such as transistors and solar cells. In this paper, we reported CIGS thin film solar cells with an energy conversion efficiency reaching up to 8.01% using air-stable, low-cost inks. The newly developed inks consist of commercially available, low-cost compounds and solvents and can be processed using a variety of printing and coating techniques. More importantly, the inks can produce CIGS films free of copper selenides and amorphous carbon, two common by-products from solution-based CIGS processes. The mechanism for the transformation from metal salt precursor films to CIGS absorber thin films and the influence of selenium vapour pressure on absorber film quality and photovoltaic device performance were investigated and discussed. High-quality CIGS films with micrometer-sized crystals were obtained by using higher selenization partial pressure.

Inkjet-Printed High Mobility Transparent–Oxide Semiconductors

In this paper, we report a general and low-cost process to fabricate high mobility metal-oxide semiconductors that is suitable for thin-film electronics. This process use simple metal halide precursors dissolved in an organic solvent and is capable of forming uniform and continuous thin films via inkjet-printing or spin-coating process. This process has been demonstrated to deposit a variety of semiconducting metal oxides include binary oxides (ZnO, In2O3 , SnO2 , Ga2O3 ), ternary oxides (ZIO, ITO, ZTO, IGO) and quaternary compounds (IZTO, IGZO). Functional thin film transistors with high field-effect mobility were fabricated successfully using channel layers deposited from this process. This synthetic pathway opens an avenue to form patterned metal oxide semiconductors through a simple and low-cost process and to fabricate high performance transparent thin film electronics via digital fabrication processes on large substrates.

A Comparison of Chemical Bath Deposition of CdS from a Batch Reactor and a Continuous-Flow Microreactor

cSeagate Technology, Minneapolis, Minnesota 55435-5489, USA In this paper, we report a comparison between CdS deposition by a conventional batch reactor and a newly developed continuousflow microreactor. This microreactor setup makes use of a micromixer for efficient mixing of the reactant streams and helps in controlling the homogeneous reaction before the solution impinges on a substrate. Transmission electron microscopy analysis indicated that an impinging flux without the formation of nanoparticles could be obtained from this reactor at a short residence time. The surface morphology of the deposited films clearly indicated an improvement of film smoothness and coverage over films deposited from a batch process. Highly oriented nanocrystalline CdS films were obtained from the continuous-flow microreactor in contrast to poor crystalline films from the batch process. This new approach could be adopted for the deposition of other compound semiconductor thin films at low temperatures using a solution-based chemistry with improved control over the processing chemistry.

Nanocrystalline CdS MISFETs Fabricated by a Novel Continuous Flow Microreactor

In this work, we developed a continuous flow microreactor that is capable of overcoming the drawbacks associated with chemical bath deposition. Uniform, smooth, and highly oriented nanocrystalline CdS semiconductor thin films were successfully deposited on oxidized silicon substrates at low temperature (80°C) using this microreactor. Functional thin-film transistors with an effective mobility of 1.46 cm 2 /V s were fabricated from the as-deposited films without any postannealing process. This process is a potentially low-cost avenue for the fabrication of thin-film electronics on flexible polymeric substrates.

Fabrication of high-performance, low-temperature solution processed amorphous indium oxide thin-film transistors using a volatile nitrate precursor

In this study, we fabricate amorphous indium oxide thin film transistors (TFTs) on a display glass substrate at various annealing temperatures from 200 °C to 300 °C. Using a volatile nitrate precursor, we were able to fabricate TFTs with excellent device performance within this annealing temperature range. Amorphous In2O3 films could be obtained by carefully controlling the film thickness and annealing temperature. TFTs based on amorphous In2O3 channel layers with an average mobility as high as 7.5 cm2 V−1 s−1, an Ion/Ioff ratio of 107, and Von = −5 V could be fabricated at 300 °C annealing temperature in air. The devices prepared at 200 °C still exhibit transistor characteristics with an average mobility of 0.04 cm2 V−1 s−1, an Ion/Ioff ratio of 105, and Von = 0 V. The temperature effects on the device performances are elucidated based on X-ray photoelectron spectroscopy and thermal gravimetric analysis characterization results of precursors and the resulting amorphous In2O3 thin films.

Nanostructured ZnO as Biomimetic Anti-reflective Coatings on Textured Silicon Using a Continuous Solution Process

A novel table-top, microreactor-assisted nanomaterial deposition (MAND™) process, which combines the merits of microreaction technology with solution-phase nanomaterial synthesis and film deposition, was used to grow a nanostructured ZnO anti-reflective coating on a textured silicon substrate from aqueous solution. The subwavelength, anti-reflective nanostructures mimicked the structure and performance of the surface of the eye from a night-flying moth. Solution-processed Ag nanoparticles were applied as a seed layer on the textured silicon surface leading to preferred heterogeneous nucleation and good area coverage. Preferential growth of the nanostructured ZnO was controlled by changing residence time, reaction temperature, and concentration of precursor solution without the use of a buffer reagent (e.g. HMTA). Well-aligned ZnO nanorod arrays were fabricated by MAND at a very high deposition rate (i.e. 125 nm min−1) compared to batch hydrothermal method. The surface reflection of the polished silicon was suppressed from an average of 30.8% between wavelengths of 400 and 900 nm to 10.6% after micro-scale pyramidal surface texturing to 3.4% after application of the ZnO nanostructure on the textured silicon. The results provide a potential economical path to broadband anti-reflection (AR) for silicon wafers and solar cell substrates.

Chemical nanoparticle deposition of transparent ZnO thin films

A novel approach to deposit transparent ZnO thin films is reported. This approach uses a continuous-flow microreactor to generate a flux of nanoparticles which then impinge on a heated substrate. The as-deposited film consists of highly transparent nanocrystalline ZnO with a dilated optical bandgap of 4.35 eV. Functional ZnO metal-insulator-semiconductor field-effect-transistors (MISFETs) were successfully fabricated using this technique after a post-air-annealing process. A MISFET with an effective mobility of 0.16 cm 2 /V s and a current on-to-off ratio of ∼10 4 was produced. This approach is promising as a low-cost technique for fabricating nanostructured thin films.