Potential of Metal Reactive Oxides for Development of 4H-Silicon Carbide-based Metal-Oxide- Semiconductor Gas Sensor

The escalating demand in automotive industry nowadays has evoked civic awareness towards environmental issues brought by the exhaust emissions through fossil fuel combustion engine or hydrogen leak in fuel cell vehicles. Metal-oxide-semiconductor (MOS)-based gas sensors that were able to monitor the changes of oxygen and hydrogen leak were required. By replenishing the conventional silicon-MOS-based gas sensors with 4H-silicon carbide (4H-SiC)-based, low power consumption gas sensors were achievable. Present work summarized experimental works and findings obtained from the investigation of lanthanum cerium oxide (LaxCeyOz) as the metal reactive oxide used in 4H-SiC based MOS gas sensors. The catalytic properties of LaxCeyOz were disclosed due to the presence of bulk defect (oxygen vacancies) in the oxides. These oxygen vacancies would serve as the active sites for the adsorption of gaseous molecules, which were required in gas sensing. A mechanism associated with the oxygen vacancies formation and disappearance was schematized and summarized. Further exploration of utilizing LaxCeyOz as a potential metal reactive oxide on 4H-SiC substrate, responsive towards oxygen and hydrogen were discussed. Higher sensitivity and selectivity was demonstrated by the gas sensor towards oxygen and this prompted further research on oxygen sensing in ultra-low voltage region. Keywordsmetal reactive oxide; metal-oxide-semiconductor; oxygen; sensing; oxygen vacancy. Introduction and Experimental: Since decades ago, fossil fuel has been conquering the market of automotive industry for use in the conventional combustion vehicles. Dealing with the depletion of fossil fuel nowadays, hybrid vehicles are invented to substitute the conventional combustion vehicles. Although lesser fuels may be consumed in hybrid vehicles, it is undeniable that exhaust emission is an environmental issue that needs public awareness [1]. In order to minimize the environmental threat, oxygen sensor is installed in the combustion system of either the conventional or hybrid vehicles. However, the public is not aware of existence of the oxygen sensor, which has been used to monitor oxygen (O2) level in the exhaust and so, onboard computer can regulate air/fuel mixture to reduce the exhaust emissions [2]. A condition may happen when the oxygen sensor is malfunctioning, whereby a false voltage signal is sent and causes a rise in fuel consumption and exhaust emissions. Thereafter, Kuan Yew Cheong et al /Int.J. ChemTech Res.2014,6(3),pp 1766-1770. 1767 emergence of electric vehicles helps to reduce the exhaust emissions. However, the inconvenience associated with recharging the electric vehicles has pushed to a more recent development of electric vehicles, which utilize fuel cell stacks to convert hydrogen gas and oxygen from air to electricity [3]. The hydrogen gas can be easily refilled at any hydrogen gas station. Nonetheless, hydrogen gas leak is a major problem since it is odourless and highly inflammable [4]. Therefore, hydrogen sensor ought to be installed in the fuel cell electric vehicles to detect the gas leak. Thinking of the need for continuous monitoring of the harmful exhaust emissions and detecting hydrogen leak in automotive industry, research and developments have been performed experimentally to seek for a metal-oxide-semiconductor (MOS) based gas sensor that can withstand high temperature atmosphere. Furthermore, an ultra-low voltage operated MOS based gas sensor is encouraged to attain a low power consumption gas sensor [5]. With these being achieved, the sensor may age slower. This work utilizes 4H-silicon carbide (SiC) as the semiconductor substrate in the MOS based gas sensor, owing to its low intrinsic carrier concentration and high thermal conductivity [6], which respectively provides lower leakage current and fast heat dissipation. The attainment of these two criteria will promotes the achievement of low power consumption MOS based gas sensor. In addition, in the aspects of gate oxide materials, cerium oxide (CeO2), which has been used as a high dielectric constant (k) metal reactive oxide in 4H-SiC-based MOS gas sensor [7], is further researched by doping with trivalent lanthanum cation (La). The purpose of utilizing metal reactive oxide as the gate oxide material is attributed to its offering of higher sensitivity when compared with SiO2. Besides, its ability to deliver both insulating and catalytic properties is essential to withstand breakdown voltage of the sensor and to ensure good sensor performance. The beneficial effects of doping CeO2 with La 3+ towards the enhancement of catalytic properties in CeO2, which has led to the development of 4H-SiC-based MOS capacitor gas sensor were reported. Literatures reporting about La-doped CeO2 (LaxCeyOz) were particularly concentrating on various catalytic applications, such as soot oxidation [8], methane combustion [9], and three-way catalysts [10], yet the utilization of this material as a metal-reactive oxide remains unsound. Therefore, this work explored the utilization of LaxCeyOz in this aspect by initially preparing the precursor using metal-organic decomposition method, as conveniently described in Ref. [11]. The precursor was spun on the Si (reference) and 4H-SiC substrates at 4000 rpm for 30 s. The samples were postdeposition annealed in a horizontal tube furnace in different conditions prior to Al top electrode deposition using thermal evaporator (Auto 306). Photolithography process was carried out to pattern an array of 2.5 x 10 cm area capacitor. Lastly, Al was deposited as a back contact of the MOS structure. Results and Discussion: The experimental works were carried out by initially varying the PDA temperatures from 400 to 1000°C and then followed by PDA time from 15 to 120 min at an optimum temperature determined beforehand. In LaxCeyOz/Si system, an unintentionally growth of 15 nm thick SiO2 IL at 1000°C (Fig. 1 (a)) [12] with the presence of Si nanocrystals was detected using cross-sectional high resolution transmission electron microscopy (HRTEM) analysis. The SiO2 growth was accelerated as PDA time was prolonged to 120 min through the attainment of 42 nm thick of the SiO2 IL (Fig. 1 (b)) [12]. The SiO2 growth of this thick was considerably thicker than the usual 5 nm thick SiO2 and this suggested the likelihood of catalytic oxidation [12] of the Si substrate surface, owing to the oxygen vacancies present in the LaxCeyOz to facilitate oxygen dissociation from the environment. This idea was claimed according to the theoretical defect reaction (Equation 1) that describes the formation of oxygen vacancies in LaxCeyOz when foreign cation like La 3+ is doped into CeO2 [13]. La2O3 + 6Ce x Ce 2La ’ Ce + 6Ce ’ Ce + 4Vö + 3/2 O2 (g) (1) where CeCe is neutral Ce on Ce lattice; Vö is oxygen vacancy on oxygen site with charge of +2, Ce ’ Ce and La ’ Ce denote Ce on Ce site and La on Ce site, respectively. A more concrete evidence to prove the presence of oxygen vacancies in LaxCeyOz have been done through the detection of Ce 3+ concentration in LaxCeyOz by X-ray photoelectron spectroscopy (XPS) narrow scans on a depth profiling basis that was carried out in 4H-SiC system (Fig. 1 (c)-(f)) [14]. The XPS core level spectra labeled as uo, vo, u’, and v’ denote the Ce 3+ while u, v, u”, v”, u’”, and v’” represent the Ce. As the PDA time was increased from 15 to 120 min at constant temperature of 1000°C [15], re-oxidation occurred through the emergence of more Ce-related spectrum from XPS analysis, through a calculation of Ce concentration as shown in Fig. 2. This finding signified the occurrence of redox in LaxCeyOz and suggested that the de-activation of CeO2 happening at 800°C [16], as reported in literatures was eliminated in LaxCeyOz. As an overview of the aforesaid research works that were done, summarizing for the significant role taken by oxygen vacancies present in LaxCeyOz onto the catalytic property of LaxCeyOz, a redox mechanism Kuan Yew Cheong et al /Int.J. ChemTech Res.2014,6(3),pp 1766-1770. 1768 elaborating the oxygen vacancy formation and annihilation was important. Thus far, studies regarding the mechanism in LaxCeyOz are strewn, reporting about oxygen vacancy formation and oxygen anion migration using computational methods. The contribution of La in LaxCeyOz was rarely discussed. A mechanism in association with the role played by La as the dopant was therefore proposed using findings from XPS and Scanning TEM-Energy dispersive spectroscopy (EDS) line scan analysis for the LaxCeyOz/4H-SiC samples, which were post-deposition annealed in a reducing and an oxidizing ambient [17]. Figure 3 (a) illustrates the reduction mechanism happening in LaxCeyOz, which initiates with substitution of Ce 4+ in CeO2 by La 3+ (Step 1), leading to the release of Ce interstitial, formation of oxygen vacancies (Step 2), and reduction of neighbouring Ce cations to Ce (Step 3). The process is reversed when the released O2 (Equation 1) is captured by the oxygen vacancies (Step 4) or in the presence of oxygen (Fig. 3 (b)), whereby the adsorption of oxygen gas from the ambient happens at the oxygen vacancies (Step 4’), followed by oxidation of Ce back to Ce (Step 3’) and filling of oxygen vacancies (Step 2’), as well as the displacement of substitutional La to the interstitial sites (Step 1’) via a kick-out mechanism. In alliance with the catalytic property of LaxCeyOz in catalyzing the aforementioned reactions as a function of PDA temperature and time, further research studies were devoted to application of the oxide in 4HSiC based MOS capacitors for gas sensing. Amongst the MOS capacitors fabricated using different PDA temperature and time, the one post-deposition annealed at 1000°C for 15 min offered the greatest sensitivity, stability, and recovery on O2 and H2 (not shown). This is owing to the presence of the highest Ce 3+ concentration in the oxide annealed for 15 min (Fig. 2), whereby the highest number of oxygen vacancies that serve as the adsorption sites of the gas molecules is present. The sensor was also determined to feature higher selectivity towards O2 than H2 sensing, owing to the acquisition of higher sensitivity on O2 sensing. It is believed that the effect of oxide quality improvement after filling of the oxygen vacancies by O2 and/or atomic O is more prominent than the nitridation and passivation effects by N2, H2, N, and/or H during H2 sensing. Further investigation was carried out to study the ability of the MOS capacitors to operate in ultra-low voltage regime for O2 sensing, eligible for future low power consumption gas sensing technology. Results indicated that the capacitor gas sensors were best performing at gate bias of 0.3 V (Fig. 4), attributed to the attainment of 97.71% of sensitivity, response time of 0.37 s, and recovery time of 1.14 s [18]. Figure 1. Cross-sectional high resolution transmission electron microscopy (HRTEM) images of sample postdeposition annealed at (a) 15 min and (b) 120 min [14]. Figure (c)-(f) represents Ce 3d X-ray photoelectron spectroscopy (XPS) core level spectra of sample post-deposition annealed at 15, 30, 60, and 120 min, respectively [15]. Kuan Yew Cheong et al /Int.J. ChemTech Res.2014,6(3),pp 1766-1770. 1769 Figure 2. Concentration of Ce(III) in LaxCeyOz from Ce 3d XPS core level spectra analysis [15]. Figure 3. Schematic diagrams showing (a) reduction and (b) oxidation process happening in forming gas and oxygen ambient, respectively [17]. Figure 4. Ultra-low voltage (0.3 V) operated Al/LaxCeyOz/4H-SiC gas sensor towards oxygen gas sensing at different temperatures (25, 75, and 125°C) [18].