Jayita Kanungo

Fabrication and Characterization of ZnO/p-Si and TiO 2 /p-Si Heterojunctions for Hydrogen Detection—Influence of Pd Functionalization

Nanocrystallyne n-ZnO and p-TiO2 thin films were deposited on crystalline Si substrate by the simple and low-cost sol-gel method. The surfaces of nanocrystalline ZnO and TiO2 were modified by PdCl2 solution to passivate the defect states and to improve the gas sensitivity. Both unmodified and Pd modified sensors with Pd-Ag/n-ZnO/p-Si/Al and Pd-Ag/p-TiO2/p-Si/Al device structures were exposed to different hydrogen concentrations (0.01%, 0.05%, 0.1%, 0.5%, and 1%) at the optimum biasing voltage and temperature. The effect of gas response was investigated using nitrogen as well as synthetic air as the carrier gas. Both the sensor configurations showed the improved gas response after surface treatment with Pd ions. Surface modified p-TiO2 sensors recorded higher gas response (78%), response time (3 s), and recovery time (74 s) compared with ZnO sensors under the similar conditions.

Nanocrystalline Porous Silicon

Porous silicon can be considered as a silicon crystal having a network of voids in it. The nano sized voids in the bulk silicon result in a sponge-like structure of pores and channels surrounded by a skeleton of crystalline Si nano wires. Porous silicon (PS) is gaining scientific and technological attention as a potential platform mainly for its multifarious applications in sensing and photonic devices (Canham, 1997a; Pavesi & Dubos;1997; Dimitrov,1995; Tsamis et al., 2002; Archer & Fauchet, 2003; Barillaro et al.,2003). The extremely large surface to volume ratio (500m2/cm3) of PS, the ease of its formation, control of the surface morphology through variation of the formation parameters and its compatibility to silicon IC technology leading to an amenability to the development of smart systems-on-chip sensors have made it a very attractive material. Due to these multi functional applications of PS, recently it has been proposed to be an educational vehicle for introducing nanotechnology and inter-disciplinary material science by eminent scientists working in this field. But in order to develop porous silicon based devices and their integration to electronic circuits the low resistance stable electrical contacts are necessary. However, unlike crystalline silicon the outstanding problem with PS is the instability of its native interface with a metastable Si–Hx termination (Tsai et al.,1991). The metastable hydrosilicon can undergo spontaneous oxidation in ambient atmosphere and results in the degradation of surface structures. This also creates problems to get a stable Ohmic contact (Deresmes et al.,1995; Stievenard & Deresmes, 1995) which is again a very important factor regarding its commercial applications. Therefore passivation of surface is necessary to make stable porous silicon based devices. For that purpose substituting surface hydrogen by another chemical species has appeared desirable. Oxidations (Rossi et al, 2001; Bsiesy, et al 1991; Petrova-Koch et al., 1992) nitradation (Anderson et al., 1993) and halogenetion (Lauerhaas & Sailor, 1993) are found to be useful for PS surface passivation. Derivatisation by organic groups and polymer (Lees et al. 2003; Mandal et al. 2006), offers an alternative possibility to stabilize the material. Metals like Cu, Ag, In etc. were also used to modify the porous silicon surface to stabilize its photoluminescence properties (Andsager et al 1994; Steiner et al., 1994). Surface modification of PS using noble metals like Pd and Pt has also been studied recently (Kanungo et al. 2009a). The details on PS are given in a comprehensive review published by Cullis et al. (Cullis et al., 1997) and in the handbook on Porous Silicon properties edited by Canham (Canham, 1997a). H. Foll et al. (Foll et al. 2002) and V. Parkhutik (Parkhutik, 1999) also elaborately reviewed the formation and applications of porous silicon.

The Effect of Different Metal Electrodes on the Performance of ZnO/p-Si/Al Hetero-Structure for Hydrogen Detection

Nanocrystalline n-ZnO was prepared by the simple low-cost sol-gel method and was deposited on crystalline Si substrate. Sensors with Al/n-ZnO/p-Si/Al, Ni/n-ZnO/p-Si/Al, Au/n-ZnO/p-Si/Al, and Pd/n-ZnO/p-Si/Al device structures were exposed to different hydrogen concentrations (0.01%, 0.05%, 0.1%, 0.5%, and 1.0%) at the optimum biasing voltage and temperature. The effect of gas response was investigated using nitrogen as the carrier gas. Pd/n-ZnO/p-Si/Al sensor gives the best results (response 37% and response time 174 s) due to the high absorption and dissociation power of Pd toward hydrogen. A possible gas sensing mechanism was suggested with a qualitative energy band diagram.

P2.5.3 SiC-FET Sensors for Methanol Leakage Detection

Pt and Ir SiC based Field Effect Transistor sensors were tested to detect low concentration of methanol (<200 ppm) for both process control and leak detection applications. Pt sensors gave good and very fast response at 200°C, while Ir sensors gave larger but much slower response. The presence of oxygen improved the response of the sensor which was favorable for the leak detection application. The influence of hydrogen and propene to the sensor response was also studied. Beside the experimental work, the detection mechanism and different sensing behavior of Pt and Ir were studied by quantum chemical calculations.

Surface Modified Nanoporous Materials for Hydrogen Sensing

Nanoporous silicon and nanoporous ZnO were prepared by the electrochemical anodization of crystalline Si and Zn substrate. To passivate the defect states (arise due to the nano structure of the PS and ZnO thin film and introduce a barrier for the current conduction) and to improve the gas sensitivity the porous silicon and nano crystalline ZnO surfaces were modified by PdCl2 solution. The Pd modified sensors having Pd-Ag (26%)/PS/Si/Al (MIS) and Pd-Ag (26%)/ZnO/Zn (MIM) device structures were investigated in different hydrogen concentrations (0.01, 0.05, 0.1, 0.5 and 1.0%) regarding optimum biasing voltage and temperature. Both the sensor showed superior performance in terms of operating temperature, response magnitude, response time and recovery time.