Wan-Shick Hong

Amorphous silicon pixel layers with cesium iodide converters for medical radiography

We describe the properties of evaporated layers of cesium iodide (thallium activated) deposited on substrates that enable easy coupling to amorphous silicon pixel arrays. The CsI(Tl) layers range in thickness from 65 to 220 /spl mu/m. We used the two-boat evaporator system to deposit CsI(Tl) layers. This system ensures the formation of the scintillator film with homogenous thallium concentration which is essential for optimizing the scintillation light emission efficiency. The Tl concentration was kept to 0.1-0.2 mole percent for the highest light output. Temperature annealing can affect the microstructure as well as light output of the CsI(Tl) film. 200-360/spl deg/C temperature annealing can increase the light output by a factor of two. The amorphous silicon pixel arrays are p-i-n diodes approximately 1 /spl mu/m thick with transparent electrodes to enable them to detect the scintillation light produced by X-rays incident on the CsI(Tl). Digital radiography requires a good spatial resolution. This is accomplished by making the detector pixel size less than 50 /spl mu/m. The light emission from the CsI(Tl) is collimated by techniques involving the deposition process on patterned substrates. We have measured MTF of greater than 12 line pairs per mm at the 10% level. >

Improved electrical and transport characteristics of amorphous silicon by enriching with microcrystalline silicon

The authors have deposited n-i-p diodes with microcrystalline intrinsic layers for radiation detection applications. The diodes show interesting electrical characteristics which have not been reported before. From TOF measurement for their best samples, the authors obtained {mu}{sub e} values which are about 3 times larger than their standard a-Si:H. for {mu}{tau} values approximately a factor of 2 improvement was observed. The N*{sub D} values derived from hole-onset measurements show lower ionized dangling bond density than normal a-Si:H material. The authors propose a simple model which can very well explain the experimental results.

Utilization of a-Si:H Switching Diodes for Signal Readout from a-Si:H Pixel Detectors

Two-dimensional arrays of amorphous silicon photodiodes can be used as position-sensitive radiation detectors when they are coupled to an appropriate phosphor. We have developed signal readout schemes from amorphous silicon photodiode arrays utilizing one or two switching diodes attached to each pixel photodiode. Individual cells and prototype arrays of amorphous silicon photodiodes with single- and double-diode switching readout were fabricated and tested. A charge storage time and a readout time were measured. The measurement results were analyzed by simple circuit theory.

Defect Equilibration and Intrinsic Stress in Undoped Hydrogenated Amorphous Silicon

Relaxation data for the thermal equilibrium defect densities in undoped a-Si:H are obtained by time-of-flight (TOF) measurement in the temperature range of 160° to 250° C. The internal stress in the material is also measured. The mobility-lifetime product of electrons (µτ) increases from 0.50×10-7 to its equilibrium value of 2.24×10-7 cm2/V during the 160° C annealing. The equilibrium value of µτ is equivalent to the spin density (N s) of 1.12×1015 cm-3. The N s curves have a minimun value just before their equilibrium. The time dependence of the N s relaxation follows a two-term stretched exponential form which corresponds to two metastable states, and each relaxation time is activated with activation energies of 1.10 to 1.20 eV. The thermal equilibrium N s increases with temperature with an activation energy of 0.20 to 0.30 eV. The data for the second annealing at 160° C after the first long annealings at 200° and 250° C also follows the two-term stretched exponential form derived from the first annealing data. The result suggests the presence of a multivalley energy configuration diagram at metastable states. The drift mobility of electrons (µ) increases slightly compared with the µτ changes, and no stress change is observed during the various annealing steps. It is concluded that the structural change is much smaller than the change in metastable-state densities during annealing in the temperature range of 160° to 250° C.

Improvement of Electronic Transport Characteristics of Amorphous Silicon by Hydrogen Dilution of Silane

We have investigated the electrical and material properties of intrinsic amorphous silicon deposited with hydrogen dilution of silane. The hydrogenated material was used as intrinsic layers of n-i-p diodes, which showed interesting electrical characteristics. From time of flight (TOF) measurement for our best samples produced at hydrogen to silane ratio of 15, we obtained mobility (µ) values about 3-4 times larger than our standard amorphous silicon (a-Si:H). Approximately a factor of 2 improvement was observed for µτ values. The N D* values of the hydrogen diluted a-Si:H were measured for the first time and show lower ionized dangling bond density than the normal a-Si:H material. At a hydrogen to silane gas flow ratio of 20, some microcrystalline formation was observed in the deposited material. We propose a simple macroscopic model to assess the effect of microcrystals and grain boundaries on the electronic properties of mixed amorphous and microcrystalline material.

Field Emission Property of Double-walled Carbon Nanotubes Related to Purification and Transmittance

Double-walled carbon nanotubes (DWCNTs) with high purity were produced by the catalytic decomposition of tetrahydrofuran (THF) using a Fe-Mo/MgO catalyst at 800°C. The as-synthesized DWCNTs typically have catalytic impurities and amorphous carbon, which were removed by a two-step purification process consisting of acid treatment and oxidation. In the acid treatment, metallic catalysts were removed in HCl at room temperature for 5 hr with magnetic stirring. Subsequently, the oxidation, using air at 380°C for 5 hr in the a vertical-type furnace, was used to remove the amorphous carbon particles. The DWCNT suspension was prepared by dispersing the purified DWCNTs in the aqueous sodium dodecyl sulfate solution with horn-type sonication. This was then air-sprayed on ITO glass to fabricate DWCNT field emitters. The field emission properties of DWCNT films related to transmittance were studied. This study provides the possibility of the application of large-area transparent CNT field emission cathodes. (Received October 8, 2010)

Nonvolatile Memory Characteristics of Double-Stacked Si Nanocluster Floating Gate Transistor

We have studied nonvolatile memory properties of MOSFETs with double-stacked Si nanoclusters in the oxide-gate stacks. We formed Si nanoclusters of a uniform size distribution on a 5 nm-thick tunneling oxide layer, followed by a 10 nm-thick intermediate oxide and a second layer of Si nanoclusters by using LPCVD system. We then investigated the memory characteristics of the MOSFET and observed that the charge retention time of a double-stacked Si nanocluster MOSFET was longer than that of a single-layer device. We also found that the double-stacked Si nanocluster MOSFET is suitable for use as a dual-bit memory.

Gate Insulator Inhomogeneity in Thin Film Transistors Having a Polycrystalline Silicon Layer Prepared Directly by Catalytic Chemical Vapor Deposition at a Low Temperature

Polycrystalline silicon (poly-Si) films were prepared directly at a low temperature (<200 °C) by using catalytic chemical vapor deposition (Cat-CVD) technique without subsequent crystallization steps. Top-gate coplanar type thin-film transistors were fabricated using the as-deposited poly-Si films. We obtained a high mobility of ~40 cm2/(V s) and a subthreshold slope of 0.54 V/decade. Instability in threshold voltage with the drain bias could be suppressed by improving the homogeneity in the gate insulator.

Utilization of amorphous silicon carbide (a-Si:C:H) as a resistive layer in gas microstrip detectors

Thin semiconducting films of hydrogenated amorphous silicon (a-Si:H) and its carbon alloy (a-Si:C:H) were applied to gas microstrip detectors in order to control gain instabilities due to charges on the substrate. Thin ({approximately}100nm) layers of a-Si:H or p-doped a-Si:C:H were placed either over or under the electrodes using the plasma enhanced chemical vapor deposition (PECVD) technique to provide the substrate with a suitable surface conductivity. By changing the carbon content and boron doping density, the sheet resistance of the a-Si:C:H coating could be successfully controlled in the range of 10{sup 12} {approximately} 10{sup 17} {Omega}/{four_gradient}, and the light sensitivity, which causes the resistivity to vary with ambient light conditions, was minimized. An avalanche gain of 5000 and energy resolution of 20% FWHM were achieved and the gain remained constant over a week of operation. A-Si:C:H film is an attractive alternative to ion-implanted or semiconducting glass due to the wide range of resistivities possible and the feasibility of making deposits over a large area at low cost.

Annealing effect of low-temperature (<150°C) Cat-CVD gate dielectric silicon nitride films diluted with atomic hydrogen

— The effect of in-situ hydrogen pretreatment on dielectric properties of silicon nitride (SiNx) thin films for a gate dielectric layer has been studied. SiNxthin films were grown at a low temperature (150°C) by Catalytic CVD followed by conventional furnace annealing at 150°C for 2 hours. The in-situ hydrogen pretreatment was performed without vacuum break before the sample was transferred to the furnace for thermal annealing. Capacitance—voltage (C-V) and current-density—voltage (J-V) measurement showed that the hydrogen pretreatment was effective in reducing the hysteresis in the C-V curve and in increasing the breakdown voltage. Without the treatment, the 150°C annealing failed to produce reliable C-V and I-V characteristics. The C-V hysteresis and the threshold voltage shift of SiNx were improved by furnace annealing as the hydrogen dilution ratio increased. Also, addition of hydrogen to the deposition gas mixture helped to improve the dielectric properties of the SiNx films after thermal annealing. The combination of hydrogen dilution of the source gas and the in-situ hydrogen treatment was successful in producing low-temperature SiNx films applicable to a-Si TFTs. The TFT fabricated by using these films showed a field-effect mobility of 0.23 cm2/V-sec and a Vth of 3.1 V.