A study on Electrophoretic Fabrication of Pd/InP Schottky Diode Hydrogen Sensors

• Main Authors: Cheng-Po Chang, 張政柏
• Other Authors: Huey-Ing Chen
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/84830908803440063920

碩士 === 國立成功大學 === 化學工程學系碩博士班 === 95 === In this study, the electrophoretic deposition (EPD) combining with Pd nanoparticles was employed to fabricate Pd/InP Schottky diodes as hydrogen sensors. Firstly, the effects of EPD variables including deposition time and applied voltage on the resulting surface morphologies and current-voltage (I-V) characteristics were investigated. Secondly, hydrogen sensing performances of the EPD Pd/InP (denoted as MS device) and Pd/oxide/InP (denoted as MOS device) were investigated under hydrogen concentrations of 50 ppm- 1% H2/air at 313 K. Assuming that the hydrogen sensing behavior could be described by the Temkin adsorption model, the thermodynamic and kinetic parameters were then estimated from the steady-state and transient detection analyses. From experimental results, it revealed that as the deposition time increased, not only the particle size of deposited Pd increased but also the size distribution of Pd particles became broader. From I-V characteristics analyzed by using the thermionic emission model, it was found that the Schottky barrier heights (SBHs) in air for three devices, 30V-1(EPD at 30V for 1 h), 30V-2 (EPD at 30V for 2 h), and 30V-3(EPD at 30V for 3 h) were very close (about 650meV). Especially, the 30V-1 device exhibited the highest hydrogen sensitivity, attributing from the largest effective surface area. Besides, as increasing the EPD applied voltage from 10V to 20V, the hydrogen sensitivity increased, whereas it was contrast decreased as the applied voltage was raised to 30V. To further observe the surface morphologies of the Pd gates, it illustrated the Pd layer became denser as the EPD voltage increased from 10Vto 20V. It resulted in the increase of number of active sites available for hydrogen adsorption. However, as the EPD voltage increased to 30V, the Pd grains enlarged which would result in the decrease of the number of active sites and then lowering the sensing sensitivity. Accordingly, tuning an appropriate conditions in the EPD process including deposition time and applied voltage were really essential for achieve an excellent sensing device. Furthermore, from results of Temkin model analyses, the numbers of adsorption sites for 10V-2, 20V-2, and 30V-2 devices were estimated as 6.6x1013, 3.3x1014, and 9.8x1013, respectively. It was noted that the 20V-2 device exhibited the highest sensitivity which showed a good agreement with that observed from the SEM observation. Moreover, from the results of transient detection, it revealed that the initial rate for hydrogen adsorption on the Pd surface obeyed first-order kinetic model, the activation energies for different studied devices were estimated around 20.6 kJ mol-1. As compared with the MS device, the MOS device exhibited higher hydrogen sensing sensitivity. This was inferred that the existence of oxide interlayer could prevent the formation of Pd-InP compounds and eliminate the Fermi-level pinning effect. However, due to the short thickness and poor denseness of oxide layer, the MOS device fabricated in this work did not show a large enhancement in hydrogen sensitivity.