Jung-Fu Hsu

Silicon Nanowires as pH Sensor

Silicon nanowires (SiNWs) have been used as a sensing layer in an extended-gate field-effect transistor (EGFET) for the measurement of solution pH. SiNWs were synthesized directly from silicon substrates via a catalytic reaction in N2 atmosphere at 955°C. The SiNWs have an average diameter of approximately 30–50 nm and a length of up to a few tens of micrometers. Here, silicon bulk materials and SiNWs were used alternately as the sensing layer in an extended-gate field-effect transistor in measuring solution pH. Experimental results showed that the pH sensitivity of silicon bulk materials is poor. However, good pH sensing properties of SiNWs, with a sensitivity of 58.3 mV/pH, was observed. Therefore, it was suggested that the pH sensitivity of silicon bulk materials was greatly improved by downsizing them to the nanoscale.

The growth of silicon nanowires using a parallel plate structure

Silicon nanowires (SiNWs) were synthesized on both Si and Si/sub 3/N/sub 4//Si substrates via an catalytic reaction in N/sub 2/ atmosphere at 1000/spl deg/C using a parallel plate structure. The thickness of the Au catalyst layer varied from 6 nm to 12 nm. Resulting materials were characterized by field-emission scanning electron microscopy (FESEM). The number density and length of nanowires decreased as the thickness of Au layer increased. Moreover, SiNWs could be grown on both the cap and the bottom of the parallel plate structure. Therefore, the parallel plate structure can be used to improve the production of SiNWs.

Successive current-voltage measurements of a thick isolated diamond film

Polycrystalline diamond films were deposited on p-type (1 0 0) silicon substrate using a methane/hydrogen gas mixture in a microwave plasma-assisted chemical vapor deposition system. After the back-etched process, the Al contacts were evaporated on both sides of a 150 m thick isolated diamond film for consecutive high-voltage measurements. It was found that the current–voltage ( I–V) characteristics of the Al/diamond/Al structure exhibited two Schottky barrier diodes in a back-to-back configuration. Since the top diamond surface possessed better diamond quality than the bottom surface, the top Schottky diode with a breakdown voltage of 897 V and a lower breakdown voltage of −515 V for the bottom Schottky diode was observed for the first I–V measurement. However, the breakdown voltage was decreased by 37 and 140 V for the top and bottom Schottky diodes after the consecutive sixth repeated measurements. It was found that the oxygenated phenomenon was more prominent; in addition, the quality of the isolated diamond film was also degraded after the consecutive high-voltage measurements. It was indicated that decrease of the breakdown voltage was due to the oxidation layer and the non-diamond components on both surfaces of the isolated diamond film.