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Ge MOS capacitors with tri-layer gate dielectric are proposed by using GeON interlayer, TaON sandwich layer, and HfTiON high-k dielectric. Very small capacitance equivalent thickness (0.79~0.91 nm) is achieved. Experimental results show that the NO pretreated sample exhibits the best electrical properties, such as low interface-state density (5.4 × 1011 cm-2 eV-1), low gate leakage current density (~ 3.16 × 10-4 Acm-2 at Vg - Vfb = 1 V) and high device reliability. All of these should be attributed to the facts that the NO nitridation could form a GeON interlayer with suitable N content and thus provide an excellent GeON/Ge interface with strong Ge-N bonds, while the TaON sandwich layer could separate Hf and Ge, thus effectively preventing the reaction between them and improving the interface quality and electrical properties of the devices.