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Fabrication and Characterization of Methanol and Hydrogen Sensors Based on Porous Silicon

Razi Astaraie, Fatemeh | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40765 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Iraji Zad, Azam
  7. Abstract:
  8. We studied porous silicon as a resistive gas sensor at room temperature. Meso- and macro- porous silicon were formed by electrochemical etching of single crystal silicon wafers in different conditions. For Meso porous Si we used p+-type wafers as substrate and for macro porous Si formation n- and p-type wafers were used. The mechanism of methanol vapor sensing was studied by Scanning tunneling spectroscopy and Fourier transform infrared spectroscopy. The results showed that methanol increases partial oxidation of surface and causes increase of defect density. These defects play important role to trap electrons and create “acceptor surface states”. These acceptor surface states in p+-type (n-type) porous silicon increase (decrease) the free charge carriers and consequently increase (decrease) electrical conductivity. Macroporous Si made on p-type wafers were used as substrates for Pd growth by two electroless and electrodeposition methods for hydrogen sensing. In electroless, Pd concentrations, process duration and specially sample treatment (rinsing) before procedure affect on the growth and amount of the deposited Pd on the surface. But electrodeposition method causes Pd particles permeate further inside the pores. The sensitivity of Pd deposited samples to hydrogen at room temperature was measured in two ways: planar and diodic methods. The results showed that highest response to 2500 ppm hydrogen in diodic and planar method is 8.3 and 21, respectively. A qualitative model was suggested to explain aging and annealing effects on the sensing properties by surveying the consequences of X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) techniques. We also studied growth of multi-walled carbon nanotubes (MWCNTs) on the surface of macroporous silicon coated by Pd catalyst and their hydrogen sensing properties. The scanning electron microscopy revealed that the macroporous silicon which was coated by electrodeposition has highest density of MWCNTs even inside the pores but they are less sensitive to hydrogen. In addition, the hydrogen sensing is an irreversible process in these samples which is a problem in fabrication of CNTs based sensors.
  9. Keywords:
  10. Porous Silicon ; Fourier Transform Infra Red (FT-IR)Spectroscopy ; Carbon Nanotubes ; Electrodeposition ; Organic Vapor Sensor ; Hydrogen Sensor

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