Enhancement of Ammonia Gas Sensing Properties of GaAs-Based Schottky Diodes Using Ammonium Sulfide Surface Passivation

In this work, enhancement of gas sensing characteristics of GaAs-based Schottky diode sensors passivated by ammonium sulfide are studied and demonstrated. The GaAs surface is passivated by saturated ammonium sulfide solution to remove its native oxide and reduction of surface state densities. The measurements indicate a significant improvement in electrical characteristics of Pt-Pd(alloy)/GaAs diode where the Schottky barrier height increases from 0.688 to 0.758eV after surface passivation. The Schottky diode with passivated GaAs surface exhibits a response of 63% upon exposure to 600ppm ammonia gas at a working temperature of 150°C, which is about 2.5 times higher than that of the non-passivated sensor. This noteworthy improvement is attributed to the decrement of metal-semiconductor interface states, preventing the Fermi level pinning. Furthermore, the response (recovery) times of 29 (138)s is obtained upon exposure to 600ppm ammonia at 150°C, showing a value of around 2 times lower than that for the non-passivated sensor.