A study on the improvement of GaAs MOS devices with high-k gate dielectric

Abstract

Since silicon-based Metal-Oxide-Semiconductor (MOS) technology is approaching its physical limit, high-mobility semiconductors (Ge, III-V compounds, etc.) have been proposed to replace silicon. Among them, GaAs has attracted much attention recently as the channel material for MOSFETs with high speed and low power due to its much higher electron mobility (8500 cm2/Vs) and larger bandgap than Si. Unfortunately, lacking a stable high-quality native oxide, GaAs-based MOS devices cannot achieve adequately high performance because of poor oxide/GaAs interfacial properties that severely pin the Fermi-level at the GaAs surface. Especially when using high-k gate dielectrics, remote Coulomb scattering and interface-roughness scattering significantly reduce the carrier mobility. Therefore, aiming at these problems, this thesis focuses on the performance improvements of GaAs MOS devices by using high-k gate dielectrics, interfacial passivation layers (IPL), and fluorine treatments. Experimental results indicate that NbAlON gate dielectric, La-based IPL and fluorine incorporation are capable of achieving high-performance GaAs MOS devices for future high-speed electronics applications. Firstly, GaAs MOS capacitors with NbAlON gate dielectric having different Nb contents are studied. The dielectric constant (k) and crystallization temperature of the AlON dielectric can be improved by Nb incorporation. However, as the Nb content increases further, the dielectric/GaAs interface quality and dielectric leakage become poorer due to the oxygen vacancies in Nb2O5. Therefore, an optimal Nb/(Al+Nb) atomic ratio of 62.5% is obtained for the NbAlON dielectric. By comparing with reported GaAs MOS devices with different high-k dielectrics, it can be suggested that NbAlON is a promising dielectric for high-performance GaAs MOS applications. Then, since the NbAlON/GaAs interfacial properties are still not satisfactory, LaAlON inserted between NbAlON and GaAs as IPL is proposed and investigated, with improvements in electrical properties and reliability achieved: relatively high k value (25.5), small flat-band voltage (0.67V), small hysteresis (45mV), small frequency dispersion, low gate leakage (6.18×10-6 A/cm2 at Vfb+1 V) and low interface-state density (Dit) (6.8×1011cm-2/eV, record low value for gate dielectrics prepared by sputtering). These should be attributed to suppressed growth of GaAs native oxides and reduced in-diffusion of elements from the dielectric to the substrate by the LaAlON IPL. Next, the effects of fluorine-plasma treatment before or after the deposition of gate dielectric are studied on the most mature Hf-based dielectric for GaAs MOS capacitor. Electrical characterizations and XPS analysis show that fluorine atoms can passivate the oxygen vacancies in the dielectric and defects at/near the high-k/GaAs interface, and the growth of unstable GaAs native oxides can be suppressed by strong fluorine-related bonds. Moreover, fluorine can be incorporated more effectively by post-deposition treatment, and thus achieves more improvements on the performance of GaAs MOS devices. Finally, the combined effects of LaTaON IPL and fluorine incorporation are studied. Experimental results show that the device exhibits low Dit (8×1011cm-2/eV), small flatband voltage (0.69V), small frequency dispersion and low gate leakage (6.35×10-6A/cm2 at Vfb +1V). These indicate that the fluorine atoms can enhance the effects of the LaTaON IPL by further passivating the oxygen vacancies in the dielectric stack and further suppressing the growth of GaAs native oxides.