Y.-J. Chang

High-performance, spin-coated zinc tin oxide thin-film transistors

We have developed a general and low-cost, solution-based process that is suitable for the deposition of transparent conducting oxides through spin-coating or inkjet printing under ambient conditions. Highly transparent (-95% in the visible portion) zinc tin oxide semiconducting thin films were deposited by spin coating. The deposited films were found to be smooth and uniform with an amorphous structure. Enhancement-mode metal-insulator-semiconductor field-effect transistors were fabricated showing a field-effect mobility (μ FE ) as high as 16 cm 2 /V s, a turn-on voltage of 2 V, a current on-to-off ratio greater than 10 5 , and a high on-current of 2.25 mA.

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.

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.

Growth Kinetics of Thin-Film Cadmium Sulfide by Ammonia-Thiourea Based CBD

In this work, chemical bath deposition (CBD) of CdS was monitored using a quartz crystal microbalance as a function of time, temperature, reactant concentration (ex situ atomic absorption measurement), and pH level. It is found that the reaction could be limited by mass transport at certain concentrations and temperatures. The measurements also indicate the total Cd concentration in the solution changes dramatically in the linear growth regime with a constant deposition rate. One possible explanation is that the linear growth regime is actually a combination of molecule-by-molecule growth and cluster-by-cluster growth mode. This hypothesis is further supported by real time dynamic light scattering and transmission electron microscopy measurements.

Functional Porous Tin Oxide Thin Films Fabricated by Inkjet Printing Process

The highly transparent SnO 2 thin films were deposited through solution-based inkjet printing using a simple precursor solution. We were able to fabricate porous tin oxide thin film that has a thin nanoporous layer on top and a thicker meso- and macroporous layer beneath the top layer. The thin-film transmittance is over 98% in the visible wavelength range. A mechanism based on gas evolution was proposed to explain the formation of porous structure. A depletion-mode thin-film transistor using the porous tin oxide channel layer was fabricated with a field-effect mobility of 3.62 cm 2 /V s.

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.

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.