Takahisa Tanaka

Upper limit of two-dimensional hole gas mobility in strained Ge/SiGe heterostructures

High two-dimensional hole gas (2DHG) mobility (μ2DHG>10000cm2/Vs at T 5000cm2/Vs at room temperature is presented.

Experimental and theoretical analysis of the temperature dependence of the two-dimensional electron mobility in a strained Si quantum well

The temperature dependence of the mobility of the two-dimensional electron gas (2DEG) in a silicon quantum well strained by Si0.7Ge0.3 relaxed buffer layer is determined precisely by a mobility spectrum analysis. The 2DEG mobility is 2780 cm2/V s at room temperature and, upon cooling, increases continuously to reach μ2DEG=7.4×104cm2/Vs at 7 K. A back gate installed on the sample changes the 2DEG concentration n successfully to establish μ2DEG∝n1.4 at the constant temperature T=10K, implying that the scattering at such low temperature is limited solely by the remote ionized impurity scattering. Based on this finding, theoretical analysis of the temperature dependence of μ2DEG is performed based on the relaxation time approximation using 2DEG wavefunctions and subband structures determined self-consistently and including three major scatterings; by intravalley acoustic phonons, intervalley g-processes of longitudinal optical (LO) phonons, and remote ionized impurities. The calculation included only three fi...

Experimental Study on Deformation Potential (

Deformation potential (D_ac), which is one of the most important parameters determining the rate of electron-acoustic phonon scattering, in Si around MOS interfaces is thoroughly studied with regard to the dependences on surface carrier densities, back-gate biases, and device structures. It is demonstrated that D_ac increases sharply at the MOS interface. To investigate the impact of the increased D_ac on μ_e, thick body-channel SOI MOSFETs, where drain current flows in the entire SOI layers, was fabricated. The carrier transport experiments reveal that μ_e of greater than 1100 cm² V^-1 s^-1 is obtained in body-channel SOI MOSFETs with the SOI thickness of greater than 70 nm. By taking into account the Dac profile around the MOS interface, experimental μ_e of SOI MOSFETs is numerically reproduced over a wide range of SOI thicknesses. μ_e of the body-channel SOI MOSFETs is also well reproduced using the same D_ac profile. Thus, it is concluded that D_ac increases sharply at the Si/SiO₂ interface. The accurate modeling of the increased D_ac around the Si/SiO₂ interface is indispensable for designing high-performance and/or low-power 3-D MOSFETs including FinFETs, extremely thin SOI MOSFETs, and nanowire MOSFETs, because these types of MOSFETs have greater interface-to-volume ratios.

{D}_{{{ac}}}

Ge/SiGe heterostructures are promising candidates of future p-type FETs. The two-dimensional hole gas (2DHG) formed in the strained Ge layer has high hole mobility because the modification of the band structure by the lattice mismatch leads to reduction in the effective mass and suppression of the interband scattering. Up to now, the highest 2DHG mobility obtained experimentally with the Ge/SiGe heterostructure is 3100 cm2/Vs for room temperature. This hole mobility is about 150% and 700% more than those in bulk Ge and Si, respectively. However, the theoretical limit of 2DHG mobility in the Ge/SiGe heterostructure has not been established due to the experimental challenge of measuring purely the 2DHG mobility in Ge/SiGe heterostructures and theoretical challenge of modeling it with the anisotropy and nonparabolicity of the valence band included appropriately. Here we present experimental and theoretical investigations of 2DHG mobility in Ge/SiGe heterostructures and deduce the theoretical limit of 2DHG mobility as a function of the strain in Ge.

) in MOSFETs: Demonstration of Increased

Understanding the dopant properties in heavily doped nanoscale semiconductors is essential to design nanoscale devices. We report the deionization or finite ionization energy of dopants in silicon (Si) nanofilms with dopant concentration (ND) of greater than 10(19) cm(-3), which is in contrast to the zero ionization energy (ED) in bulk Si at the same ND. From the comparison of experimentally observed and theoretically calculated ED, we attribute the deionization to the suppression of metal-insulator transition in highly doped nanoscale semiconductors in addition to the quantum confinement and the dielectric mismatch, which greatly increase ED in low-doped nanoscale semiconductors. Thus, for nanoscale transistors, ND should be higher than that estimated from bulk Si dopant properties in order to reduce their resistivity by the metal-insulator transition.

{D}_{{{ac}}}

Effects of the diameter on the drain current of uniaxially strained Si nanowire (NW) MOSFETs are investigated. Based on the deterministic solution of the multi-subband Boltzmann transport equation, the drain current is calculated considering the intravalley acoustic phonon scatterings, intervalley phonon scatterings and interface roughness scatterings. We found 3 nm diameter [110] oriented Si NW MOSFETs shows ~2X drain current enhancement by the 1% uniaxial tensile strain.