Study Of The Sulfur Passivation In GaAs Microwave Power Mesfet

The surface passivation of GaAs is an unsolved problem for a long time. The new sulfur passivation technique developed in late 80's injects new life into the study of the passivation although the chemical stability of the passivation layer is not so satisfactory. In this paper, the theory of negatively charged surface states is used to investigate dynamic breakdown characteristics and the increase of gate-drain breakdown voltage as well as the reduction of saturated drain-source current after sulfur passivation. The measure which can improve the stability of sulfur passivation is proposed.The result of the test for dynamic breakdown characteristics reveal that breakdown voltage increases as the lengths of the pulses applied to the gate and drain electrodes increase. This could be mainly due to the influence of surface states.The (NH4)2Sx wet passivation treatment effectively removes the oxide and excess As from the surface and thereby suppresses the donor type antisite defects AsGa formation. In other words, the (NH4)2Sx treatment reduces the Nd and the ratio of Nd/Na, where Na is the concentration of acceptor-type defects, and Nd is the concentration of donor-type defects. The increase of acceptor states in the surface increases the density of the negative charge. The negative charge in the surface can reduce the density of electric field lines at the drain-side edge of the gate. Namely, the electric field at the drain-side edge of the gate decreases with the increasing of negative charge density in the surface, so the breakdown voltage of GaAs MESFET's will increase.The mechanism for the decrease of saturated drain-source current (IDSS) due to sulfur passivation was also investigated. Oxygen atoms in the air are known to actively react with the fresh GaAs. It was observed that the Ga-O bond is stronger than that of As-O and that Ga atoms preferentially migrate towards the surface leaving vacancies behind in the subsurface region. This behavior can convert the subsurface layer into an As-enriched one. Namely, the excess elemental As could promote the creation of donor-type AsGa defects at the subsurface of GaAs. The (NH4)2Sx solution can remove the oxide and excess As from the surface and thereby suppress the AsGa defects formation. It is likely that the sulfur passivation process reduces Nd andthe ratio of Nd/Na. As a result, the Fermi level at the surface will shift towards the valence band maximum (VBM). Accordingly the band bending increases, and the surface depletion layer thickness enhances, therefore, the channel thickness reduces. This is the main factor resulting in the decrease of saturated drain-source current.The (NH4)2Sx treatment combined with SiNx passivation by PECVD is used to study the improvement of stability of GaAs surface using (NH4)2Sx treatment. The influence of SiNx deposited by PECVD in different condition, especially changing deposition temperature, on the GaAs surface after sulfur passivation is measured by SIMS analysis combined with the test for direct current breakdown characteristics. As a protective film, the passivation technique which consists of (NH4)2Sx solution as pretreatment, and then low-temperature(80℃) SiNx film ,finally high-temperature SiNx film should be adopted. The breakdown characteristic basically maintain after the annealing at 200℃ for 24 hours. The (NH4)2Sx treatment combined with SiNx passivation is a promising passivation process in the fabrication of GaAs devices.