Liquid-Phase Etching and Chemical Passivation of III-V Semiconductors

Abstract

The development of metal-oxide-semiconductor field effect transistor (MOSFET) technology relies on new channel materials with higher carrier mobilities that allow faster switching but at lower voltages. III-V semiconductors are suitable for channel materials in n-type MOSFETs due to their higher electron mobility. However, the interface between the gate dielectric and the III-V surface shows defects that detriment the electrical performance of the transistor. These defects are attributed to interfacial oxides that create energy states in the band gap. Therefore, III-V oxides must be removed and the surface must be protected from reoxidation for the deposition of other functional layers. In this work, oxide etching and passivation of III-V semiconductors were studied to understand the oxide etching mechanism and to develop passivation techniques that allow the integration of these materials in device manufacturing. The etching of GaAs(100) was studied using aqueous HCl and H₂O₂ mixtures with and without the addition of alpha-hydroxy acids. Oxide etching depends on the strength of the acid. Without the addition H₂O₂, acetic, glycolic, tartaric and hydrochloric acids (pKₐ lower than 5) are able to remove oxides. Upon the addition of H₂O₂, only the stronger acids (glycolic, tartaric and hydrochloric) with a pKₐ lower than 4 are able to compete with H₂O₂ and etch the oxides. Oxide removal leaves an As-rich surface, and in the case of HCl, etching leaves a surface terminated with As-Cl species. As-As dimers are formed when oxides are etched with HCl and organic acids. After oxide removal with HF or HCl, the fresh GaAs and InP surfaces were passivated with a series of alkanethiols (C(n)H(2n+1)SH) to assess their effectiveness in protecting the substrate from reoxidation. Longer C chains provided increased protectiong due to their increased chain-chain interactions that allow them to form a denser and well-ordered monolayer. The surface is chemically passivated through S-X (where X = As, Ga for GaAs, and In for InP) bonding between the alkanethiolate layer and the surface. A layer formed by 1-eicosanethiol protected GaAs for 30 min, but prevented reoxidation of InP for at least 5 hours. Since the thickness of the alkanethiol layer is the same, the difference in protection is a result of the density of the layer and S bonding with the substrate.