(A) study on un-doped/Li-doped nickel oxide thin films for micro-bolometer sensing materials

Abstract

In this dissertation, we studied undoped and Li-doped nickel oxide films for use as an infrared sensing material of bolometer. Although vanadium oxide ($VO_x$) and amorphous silicon (a-Si) are now mainly used as a sensing material, $VO_x$ is not CMOS compatible material and has a problem of reproducibility. And, a-Si offers a low sensing performance due to its relatively large noise characteristic. Therefore, this research aimed to form a new sensing material which can overcome aforementioned drawbacks of convention materials while providing excel-lent sensing performance. Undoped and Li-doped nickel oxide films were deposited by the RF reactive magnetron sputtering method hav-ing advantages of not only easy controllability of the film properties, but also large area uniformity and high deposition rate. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma optical emission spectroscopy (ICP-OES), atomic force microscopy (AFM), and secondary electron microscopy (SEM) were used to investigate the structural properties of the deposited films, and optical characteristics were analyzed by using spectroscopic ellipsometer. In addition, to evaluate the bolometric performance, the bar pat-tern device were designed and fabricated and electrical conductivity, temperature coefficient of resistance (TCR) and noise characteristics were measured. First, the structural characteristics of the nickel oxide film were observed by changing the deposition parameters, such as, the deposition temperature, the process pressure, the power, and the oxygen fraction, and the effect of these changes on the sensing performance was examined. As a result, all the thin films except for the thin film formed at low oxygen fraction (< 20%) exhibits a Ni-deficient polycrystalline NiO structure (NaCl-type). It can also be confirmed that at higher deposition temperature, higher process pressure and lower oxygen fractions, a nickel oxide film with better crystallinity and fewer structural defects is formed. Since the structural defects in nickel oxide film acts as acceptors providing hole carriers, the bolometric properties are influenced by the struc-tural characteristics. Therefore, the fewer structural defects in the deposited film, the lower electrical conductivity while both the normalized Hooge parameter indicating magnitude of 1/f noise and the TCR values increased. Therefore, as the number of structural defects in nickel oxide film decreases, the electrical conductivity of the film decreases, while both the TCR and the normalized Hooge parameter (indicating the magnitude of 1/f noise) increase. Since the change of TCR and 1/f noise due to the change of structural defect have opposite effects on the sensing performance, we introduced the $(α_H/n)^{1/2}/ |β| mid$ considering both factors to evaluate the sensing perfor-mance. As a results, it was found that the nickel oxide film with -2.76 %/K of the TCR and $2.40×10^{-27} m^3$ of the $α_H/n$ deposited at 250℃, 1 mTorr, 500 W, and 80% oxygen fraction revealed the best sensing performance. However, as mentioned above, the variations in TCR and 1/f noise caused by the change in structural defects in the case of the nickel oxide film revealed a trade-off relationship that has opposite effects on the sensing per-formance. Therefore, it is found that there is a limit to improve the sensing performance even if the structural defects are largely changed. Thus, we prepared a nickel oxide film containing a large number of hole carriers while having excellent film quality with less structural defects by doping the monovalent lithium ions into the nickel oxide film. It has been confirmed that the change of the structural characteristics due to the doping of lithium remarkably reduces the magnitude of the 1/f noise arisen from the nickel oxide film by more than 100 times. Moreover, it was possible to form a Li-doped nickel oxide film having the TCR value of -2.85 %/K and the $α_H/n$ of $2.45×10^{-29} m^3$ by improving the crystallinity via the change of the deposition temperature. Li-doped nickel oxide film reveals a 10 times higher sensing performance compared to the aforementioned nickel oxide film. And it implies that the sensing performance of the Li-doped nickel oxide film is comparable to that of the vanadium oxide film. Based on these experimental results, it can be concluded that the Li-doped nickel oxide film has high feasibility as a new alternative sensing material for bolometers that can provide superior sensing performance while overcoming the drawbacks of conventional materials.