Study of electrical properties of Al/Si3N4/n-GaAs MIS capacitors deposited at low and high frequency PECVD

Abstract

As for silicon, surface passivation of GaAs and III-V semiconductors using silicon nitride (Si3N4) deposited by plasma enhanced chemical deposition (PECVD) is widely used to improve devices and circuits stability, reliability and for encapsulation. In this work, the effect of plasma excitation frequency in the PECVD reactor on the surface passivation efficiency of GaAs during Si3N4 deposition was investigated. Metal-Insulator-Semiconductor (Al/Si3N4/n-GaAs) capacitors are fabricated and characterized using capacitance–voltage (C–V), and conductance–voltage (G–V) to compare electronic properties of GaAs/Si3N4 interfaces depending on the use of a high frequency PECVD (HF-PECVD) or low frequency (LF-PECVD) process. The drastic advantage of using the LF-PECVD technique for the passivation of GaAs is clearly demonstrated on the characteristic C–V at 1 MHz where a good surface potential was observed, while a quasi-pinned surface Fermi level was found when HF-PECVD was used. To unpin Fermi level, a sulfur pre-treatment prior before HF-PECVD deposition and post-metallisation annealing were necessary. A lower frequency dispersion and a lower hysteresis indicating low densities of slow traps were observed for MIS devices fabricated by LF-PECVD. The advantage of having an efficient passivation without sulfur treatment is important since ammonium sulfide used for this purpose is corrosive and difficult to adapt in industrial environment. The better electronic properties of GaAs/Si3N4 interface were found for silicon nitride layers using LF-PECVD deposition. This can probably be associated with the high-level injection of H+ ions on the semiconductor surface reducing thus the native oxides during the initial steps of dielectric deposition.