In-Ku Kang

Bolometric properties of reactively sputtered TiO2−x films for thermal infrared image sensors

A heat-sensitive layer (TiO2−x ) was successfully deposited by RF reactive magnetron sputtering for infrared (IR) image sensors at different relative mass flow of oxygen gas (R O2) levels. The deposition rate was decreased with an increase in the percentage of R O2 from 3.4% to 3.7%. TiO2−x samples deposited at room temperature exhibited amorphous characteristics. Oxygen deficiency causes a change in the oxidation state and is assumed to decrease the Ti4+ component on the surfaces of TiO2−x films. The oxygen stoichiometry (x) in TiO2−x films decreased from 0.35 to 0.05 with increasing the R O2 level from 3.4% to 3.7%, respectively. In TiO2−x −test-patterned samples, the resistivity decreased with the temperature, confirming the typical semiconducting property. The bolometric properties of the resistivity, temperature coefficient of resistance (TCR), and the flicker (1/ f) noise parameter were determined at different x values in TiO2−x samples. The rate of TCR dependency with regard to the 1/ f noise parameter is a universal bolometric parameter (β), acting as the dynamic element in a bolometer. It is high when a sample has a relatively low resistivity (0.82 Ωcm) and a lower 1/ f noise parameter (3.16 × 10−12). The results of this study indicate that reactively sputtered TiO2−x is a viable bolometric material for uncooled IR image sensor devices.

Enhanced bolometric properties of TiO2−x thin films by thermal annealing

The effect of thermal annealing on the bolometric properties of TiO2−x films was investigated. The test-patterned TiO2−x samples were annealed at 300 °C temperature in order to enhance their structural and electrical properties for effective infrared image sensor device applications. The crystallinity was changed from amorphous to rutile/anatase in annealed TiO2−x films. Compared to the as-deposited samples, a decrement of the band gap and a decrease of the electrical resistivity were perceived in annealed samples. We found that the annealed samples show linear current-voltage (I−V) characteristic performance, which implies that ohmic contact was well formed at the interface between the TiO2−x and the Ti electrode. Moreover, the annealed TiO2−x sample had a significantly low 1/f noise parameter (1.21 × 10−13) with a high bolometric parameter (β) value compared to those of the as-deposited samples. As a result, the thermal annealing process can be used to prepare TiO2−x film for a high‐performance bolometr...

Effect of sputtering pressure on microstructure and bolometric properties of Nb:TiO2−x films for infrared image sensor applications

This study aims to investigate the influence of the sputtering pressure (PS) on Nb:TiO2−x films to enhance the bolometric properties. A decrease in the growth rate with the sputtering pressure was perceived in amorphous Nb:TiO2−x films. The incorporation of oxygen with PS was confirmed in an X-ray photo electron spectroscopy analysis. The electrical resistivity was increased with an increase in PS due to a decrease in the number of oxygen vacancies. The linear I-V characteristics confirmed the ohmic contact behavior between the Nb:TiO2−x layer and the electrode material. The present investigation finds that the sample with lower resistivity has good bolometric properties with low noise and high universal bolometric parameters. Finally, the Nb:TiO2−x sample deposited at a sputtering pressure of 2 mTorr shows better bolometric properties than other materials for infrared image sensor applications.

Systematic Investigation on Deposition Temperature Effect of Ni 1– x O Thin Films for Uncooled Infrared Image Sensor Applications

Nickel oxide (Ni_1-xO) films deposited by RF reactive magnetron sputtering were investigated at various substrate temperatures ranging from room temperature to 250 °C for uncooled infrared image sensor applications. The structural properties measured by X-ray diffraction and X-ray photoelectron spectroscopy showed that the deposited films had a Ni-deficient structure and that films deposited at a higher temperature showed more crystallized structures with fewer microstructural defects. These defects had an effect on several properties influencing the sensing performance. The conductivity was found to decrease from 9.47 to 0.10 S/cm with the deposition temperature. In addition, with an increase in the deposition temperature, both the absolute temperature coefficient of resistance and the normalized Hooge parameter, representing the magnitude of 1/f noise, increased from 1.56%/K to 2.76%/K and from 9.12×10^-28 to 2.40×10^-27 m³, respectively. (α_H/n)^1/2/|β|, a useful figure of merit determining the performance of infrared sensor, was varied in the range of 1.77×10^-14 and 2.06×10_-14 m^3/2K/% with the deposition temperature, and the best performance was obtained from the film deposited at 250 °C. Consequently, the nickel oxide film is deemed to be a good potential candidate for uncooled infrared sensor applications.

Microstructural Effects on Nickel Oxide Film Properties in an Infrared Electrochromic Window for Shutter-Less Infrared Sensor Application

Infrared electrochromic devices (ECDs) were fabricated and characterized in mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) ranges. They consisted of three layers with two metal electrodes. The layers were an ion storage layer, an ion conducting layer, and an electrochromic layer. As an ion storage layer, NiO film was used. To ensure higher performance of the ECDs, the microstructural and electrochromic properties of NiO films deposited at various sputtering pressures were investigated using SEM, X-ray photoelectron spectroscopy, and cyclic voltammetry. In addition, WO3 film and Ta2O5 film were used in the two types of fabricated devices as a cathodic electrochromic layer and an ion conducting layer, respectively. The transmittance values in the MWIR and LWIR ECDs were measured in colored and bleached states. The measured transmittance ratios of the colored state relative to the bleached state were 0.66 and 0.72 in the MWIR and LWIR ranges, respectively.

Influence of deposition temperature on TiO2−x films for infrared image sensor applications: TiO2−x films: Infrared image sensor applications

The present study explores the improved sensor device properties of sputtered TiO_2-x films at different deposition temperatures (Td=25-250°C) by means of test pattern device. All the T_d test pattern samples show linear I-V characteristic performance which implies that ohmic contact was well formed at the interface between the TiO_2-x and the Ti electrode. The resistivity, activation energy (Ea) and the temperature coefficient of resistance (TCR) values of the device samples were decreased up to 200°C of T_d. The dependence of the 1/f noise and the TCR on resistivity in TiO_2-x has been measured. The sample deposited at 200°C had a significantly low 1/f noise parameter and a high universal bolometric parameter (β). However, at T_d of 250°C the Ea, TCR and the 1/f noise values were increased due to increase of the resistivity. The TCR and 1/f noise values are proportional to the resistivity of TiO_2-x films. As a result, the low resistivity of TiO_2-x sample sputtered at 200°C is a viable bolometric material for uncooled IR image sensors.

Solvent-Tolerant Patterning of Poly(3-hexylthiophene) Film by Subtractive Photolithography

This study investigates how the fringing field affects the total current flow within a conducting polymer. In order to extract the fringing field component of bar pattern resistors, a solvent-assisted patterning method using subtractive photolithography was successfully established for the conducting polymer poly(3-hexylthiophene). By comparing the current quantities of unpatterned and patterned resistors, a conductance factor for the fringing field was calculated, proving to be almost constant regardless of the resistor length. It is therefore concluded that the length as well as the width of the conducting polymer film need to be suitably patterned for the precise operation of organic electronic devices. In this regard, the patterning method developed will be useful for the fabrication of micro-scale devices.