High-κ Gate Dielectric on Tunable Tensile Strained Germanium Heterogeneously Integrated on Silicon: Role of Strain, Process, and Interface States

Tensile strained germanium (& epsilon;-Ge) layers heterogeneouslyintegrated on Si substrates are of technological importance for nanoscaletransistors and photonics. In this work, the tunable tensile strained(0% to 1.2%) & epsilon;-Ge layers were grown by solid source molecularbeam epitaxy using GaAs and linearly graded In x Ga1-x As as intermediatebuffers, and their structural and metal-oxide semiconductor capacitor(MOS-Cs) properties were analyzed as a function of strain and processconditions. X-ray topography measurements displayed no visible thermalcrack and a low thermal stress of & SIM;50 MPa. Temperature dependentstrain relaxation properties, studied by X-ray and Raman analyses,confirmed that the tensile strain amount of 1.2% was well preservedwithin the & epsilon;-Ge layer when annealed up to 550 & DEG;C. Further,transmission electron microscopic study revealed a good quality 1.2%strained & epsilon;-Ge/In0.17Ga0.83As heterointerface.In addition, unstrained Ge (0% & epsilon;-Ge) MOS-Cs with atomic layerdeposited Al2O3 and thermally grown GeO2 composite gate dielectrics of varying oxidation times (0.25-7.5min) at 550 & DEG;C exhibited a low interface state density (D (it)) of & SIM;2.5 x 10(11) eV(-1) cm(-2) at 5 min oxidation duration.The minimum oxidation time needed for good capacitance-voltage(C-V) characteristics on0.2% & epsilon;-Ge is inadequate to accomplish similar C-V characteristics on 1.2% & epsilon;-Ge MOS-C,due to the higher strain field impeding the formation of the GeO2 interface passivation layer at lower oxidation duration.In addition, with the trade-off between the minimum D (it) and minimum equivalent oxide thickness values, & SIM;1.5min is found to be an optimum oxidation time for good quality 1.2%& epsilon;-Ge MOS-C. The minimum D (it) valuesof 1.36 x 10(11) and 2.06 x 10(11) eV(-1) cm(-2) for 0.2% and 1.2% & epsilon;-Ge,respectively, were determined for 4 nm Al2O3 with 5 min thermal oxidation at 550 & DEG;C. Therefore, the successfulmonolithic integration of tunable tensile strain Ge on Si with structuraldefects and MOS-Cs analyses offer a path for the development of tensilestrained Ge-based nanoscale transistors.