tailieunhanh - Crystalline Silicon Properties and Uses Part 11

Tham khảo tài liệu 'crystalline silicon properties and uses part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Nanocrystalline Porous Silicon 239 pedrero et al. Pedrero et al. 2004 reported on PdO PS structure for sensing ammonia and other reducing gases. As already mentioned instability is the most important problem related to porous silicon. As a chemically active high surface volume ratio material it can be oxidized easily. Because of this chemical instability the physical properties of PS may change with time. However the oxidized PS may be more stable and may contain less interface states responsible for the Fermi-level pinning. The oxygen adsorption may also result in an increase of conductance Khoshnevis et al. 2006 . The authors reported porous silicon based room temperature hydrogen sensor Kanungo et al. 2009b . The Metal-Insulator-Semiconductor MIS sensors were fabricated using both unmodified and surface modified porous silicon. Pd-Ag 26 was chosen as the top noble metal electrode to fabricate the Pd-Ag PS Si Al sensor structure. The junctions were characterized by I-V studies and were confirmed to behave as Schottky devices. They were subsequently used for hydrogen sensing at room temperature. At higher temperatures the junction deteriorated most probably due to the damage of PS surface. The modified sensors showed improvements over the unmodified samples in terms of response time of response time of recovery and stability as shown in the Table 5. The superior performance was observed for Pd modified sensors showing 84 response 8 sec response time and 207 sec recovery time on exposure to 1 hydrogen in nitrogen carrier gas. Fig. 13. Band diagram not to scale of Pd-Ag PS junction a in absence and b in presence of hydrogen. A decrease in metal work function due to the formation of a dipole layer at the interface by the diffused hydrogen increases the barrier height at the metal PS p-type junction. The mechanism of hydrogen sensing was attributed to the dipole formation at the metal-PS interface in presence of the reducing gas that decreases the work function of the