Study of germanium MOSFETs with ultrathin high-k gate dielectrics
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The continued scaling of Si CMOS devices has led to an increased attention to high-k gate dielectrics as replacements for SiO2, where eventually the physical thickness of SiO2 cannot be scaled further before gate oxide leakage becomes prohibitively large. However, a major challenge of replacing SiO2 with a high-k gate dielectric is that high-k Si MOSFETs exhibit degraded channel mobility. It is for this reason that Ge has recently received renewed attention as a possible replacement for Si in high-k CMOS devices, because its higher electron (2.5X) and hole (4X) bulk mobility relative to that of Si allows for the prospect of improved MOSFET channel mobility, while maintaining the potential to continue aggressive device scaling. Two high-k dielectric materials – Al2O3 and HfO2 – were studied . These films were deposited on bulk Ge using reactive atomic-beam deposition. Germanium MOS capacitors, p-MOSFETs and n-MOSFETs were fabricated and characterized. Ultrathin Al2O3 films with equivalent oxide thickness, teq, of 23Å were deposited on surface-nitrided Ge, with gate leakage currents 3 orders of magnitude lower than a SiO2 film of equivalent thickness, and hysteresis of less than 5mV. Significantly thinner films were achieved with HfO2, with the most aggressive HfO2/Ge gate stack having a teq ~ 11Å and gate leakage current that was 6 orders of magnitude less than that for a SiO2 film of equivalent thickness. Surface nitridation and post-deposition anneals were important in improving the quality of the high-k/Ge gate stacks. Surface nitridation reduced leakage current density, C-V hysteresis, charge trapping, interface state density, and inhibited interfacial layer formation which reduced the film’s equivalent oxide thickness. Germanium p-MOSFETs with ultrathin HfO2 films of varying thicknesses were characterized, with the thinnest HfO2 film having a teq of 10.5Å, and excellent gate leakage characteristics. For Ge p-MOSFETs with thicker HfO2 films (teq ~ 32Å), we observed a clear enhancement in hole mobility of about 1.4-1.6 times that a Si MOSFET (with HfO2 gate dielectric) control. However, hole mobility decreased with decreasing gate dielectric thickness, demonstrating that at ultrathin film thicknesses, additional mobility degradation mechanisms become significant. Germanium n-MOSFETs with ultrathin HfO2 films of varying thicknesses, ranging from teq ~ 16Å to 36Å, were studied. Extracted electron mobility of the Ge nMOSFETs were low, possibly due to high interface charge densities as well as high series resistances arising from incomplete source/drain phosphorus dopant activation.