Electrical Transport Properties Of The Amorphous Carbon/Silicon Heterojunctions Fabricated By Magnetron Sputtering

Amorphous carbon film/n-Si (a-C/n-Si) heterojuctions were fabricated by magnetron sputtering, and their electrical characteristics, gas pressure sensitivity and ammonia gas sensitivity have been investigated.The current-voltage (I-V) characteristics and temperature dependence of the a-C/n-Si heterojunctions have been investigated. The results show that these junctions have good rectifying properties in the temperature range 80³00 K. The interesting result is that the current-voltage curve changes dramatically with increasing applied voltage and temperature. For the forward bias voltages, the junction shows Ohmic mechanism characteristic in the temperature range 240³00 K. However, the conduction mechanism changes from Ohmic for the low bias voltages to space charge limited current for the high bias voltages in the temperature range 80²40 K. While for the reverse bias voltages, it changes from Schottky emission to breakdown with increasing voltage. Another important phenomenon is that the temperature dependence of the junction resistance shows a metal-insulator transition, whose transition temperature can be controlled by the bias voltage. A possible mechanism is given to understand the abnormal electrical properties.The results show that the gas pressure has a large effect on the reverse bias I-V characteristics of the junctions. For a given reverse bias voltage, the reverse current can increase by 3300 % when the gas pressure decreases from 100000 Pa to 100 Pa. Moreover, the deposition temperature and gas pressure have a large effect on the a-C/Si junctions'sensitivity. The results indicate that the sample deposited at 4 Pa and 300 K which has the largest gas pressure sensitivity. The effect of gas pressure may be attributed to the physisorption process of gas molecules which increases the space charge width and changes the surface states of the junction. This study shows that the a-C/Si junctions have potential application as gas pressure sensors. An approach to detect NH₃ gas at room temperature was demonstrated by using simple a-C/Si junctions. The results show that the a-C/Si junctions have immediate response, high sensitivity and good reproducibility for NH₃ gas detection. For a given reverse bias voltage,the resistance of the junction can increase by 100 times rapidly when exposed to NH₃ gas. Moreover, the resistance of the junction recovers rapidly when the junction is transferred from NH₃ gas to air. The results show that these junctions have immediate response, high sensitivity and good reproducibility for NH₃ gas detection. This phenomenon may be attributed to the change of the space charge width of the junction, which is caused by the adsorption of NH₃ gas molecules. This study shows that these a-C/Si junctions have potential application as NH₃ gas detect sensors.