G. Braithwaite

SiGe-free strained Si on insulator by wafer bonding and layer transfer

SiGe-free strained Si on insulator substrates were fabricated by wafer bonding and hydrogen-induced layer transfer of strained Si grown on bulk relaxed Si0.68Ge0.32 graded layers. Raman spectroscopy shows that the 49-nm thick strained Si on insulator structure maintains a 1.15% tensile strain even after SiGe layer removal. The strain in the structure is thermally stable during 1000 °C anneals for at least 3 min, while more extreme thermal treatments at 1100 °C cause slight film relaxation. The fabrication of epitaxially defined, thin strained Si layers directly on a buried insulator forms an ideal platform for future generations of Si-based microelectronics.

Scalability of strained-Si nMOSFETs down to 25 nm gate length

Strained-Si nMOSFETs with a standard polysilicon gate process were fabricated down to 25 nm gate length with well-behaved characteristics and small difference in short channel effects. The performance enhancement degrades linearly as the gate length becomes shorter, due to not only the parasitic resistance but also heavy halo implant. Thus the key integration issues are how to manage threshold difference and As diffusion without excess doping. With comparable doping and well controlled parasitic resistance, up to 45% improvement in drive current is predicted for sub-50 nm gate length strained-Si nMOSFETs on the Si/sub 0.8/Ge/sub 0.2/ substrate. In this work approximately 45% enhancement is in fact demonstrated for 35 nm gate length devices, through advanced channel engineering and implementation of metal gates.

Effective mobilities in pseudomorphic Si/SiGe/Si p-channel metal-oxide-semiconductor field-effect transistors with thin silicon capping layers

The room-temperature effective mobilities of pseudomorphic Si/Si0.64Ge0.36/Si p-metal-oxidesemiconductor field effect transistors are reported. The peak mobility in the buried SiGe channel increases with silicon cap thickness. It is argued that SiO2/Si interface roughness is a major source of scattering in these devices, which is attenuated for thicker silicon caps. It is also suggested that segregated Ge in the silicon cap interferes with the oxidation process, leading to increased SiO2/Si interface roughness in the case of thin silicon caps.

Technique for producing highly planar Si/SiO0.64Ge0.36/Si metal–oxide–semiconductor field effect transistor channels

Si/Si0.64Ge0.36/Si heterostructures have been grown at low temperature (450 °C) to avoid the strain-induced roughening observed for growth temperatures of 550 °C and above. The electrical properties of these structures are poor, and thought to be associated with grown-in point defects as indicated in positron annihilation spectroscopy. However, after an in situ annealing procedure (800 °C for 30 min) the electrical properties dramatically improve, giving an optimum 4 K mobility of 2500 cm2 V – 1 s – 1 for a sheet density of 6.2 × 1011 cm – 2. The low temperature growth yields highly planar interfaces, which are maintained after anneal as evidenced from transmission electron microscopy. This and secondary ion mass spectroscopy measurements demonstrate that the metastably strained alloy layer can endure the in situ anneal procedure necessary for enhanced electrical properties. Further studies have shown that the layers can also withstand a 120 min thermal oxidation at 800 °C, commensurate with metal–oxide–semiconductor device fabrication.

Indication of velocity overshoot in strained Si0.8Ge0.2 p-channel MOSFETs

A velocity-field study of several Si0.8Ge0.2/Si p-channel MOSFETs with self-aligned poly-Si gates, thick gate oxides and effective channel lengths ranging from 1.5 to 8.5 µm, was carried out at room temperature. Comprehensive two-dimensional simulations of devices using drift-diffusion (DD), and bulk Monte Carlo calibrated hydrodynamic (HD) and energy transport (ET) models have revealed enhanced high-field hole transport in strained-channel MOSFETs. A close agreement is obtained between higher-level (HD/ET) models and DD model with calibrated high-field mobility parameters. It is found that the relatively low value of extracted saturation velocity in long-channel Si0.8Ge0.2 p-MOSFETs increases considerably as the gate length is decreased. The increase in short-channel samples is attributed to non-equilibrium transport effects in the region near the source, resulting from higher mobility and longer relaxation times of holes in the strained SiGe layer. Our results not only confirm the expected advantage of strained SiGe p-MOSFETs in low-field transport, but also indicate that this is accompanied by an early onset of velocity overshoot, which may be beneficial in aggressively scaled devices.

Si/Si(0.64)Ge(0.36)/Si pMOSFETs with Enhanced Voltage Gain and Low 1/f Noise

Si/Si0.64Fe0.36/Si p MOSFETs with written gate lengths in the range 0.5mu micron to 10mu micron have been fabricated in a reduced thermal budget variant of a standard CMOS process. The devices exhibit enhanced maximum voltage-gains and reduced 1/f noise as compared to silicon controls.

Energy loss rates of two-dimensional hole gases in inverted Si/Si0.8Ge0.2 heterostructures

We have investigated the energy loss rate of hot holes as a function of carrier temperature TC in p-type inverted modulation-doped (MD) Si/SiGe heterostructures over the carrier sheet density range (3.5–13)×1011 cm–2, at lattice temperatures of 0.34 and 1.8 K. It is found that the energy loss rate (ELR) depends significantly upon the carrier sheet density, n2D. Such an n2D dependence of ELR has not been observed previously in p-type SiGe MD structures. The extracted effective mass decreases as n2D increases, which is in agreement with recent measurements on a gated inverted sample. It is shown that the energy relaxation of the two-dimensional hole gases is dominated by unscreened acoustic phonon scattering and a deformation potential of 3.0±0.4 eV is deduced.

Enhanced Velocity Overshoot and Transconductance in Si/Si(0.64)Ge(0.36)/Si pMOSFETs - Predictions for Deep Submicron Devices

Electrical measurements have been carried out on Si/Si 0.64 Ge 0.36/Si pMOS devices and it is demonstrated that enhanced low field carrier mobilities lead to concomitant and substantial enhancements in velocity overshoot and transconductance at deep submicron channel lengths. This provides considerable motivation for incorporating SiGe into Si MOS technology.

Observation of piezoelectric-like behaviour in coherently strained B-doped (100)SiGe/Si heterostructures

In the present work the hybrid acoustic spectroscopy technique has been used to demonstrate the conversion of a high frequency (HF) electric field into acoustic waves and to provide the first direct observation of the piezoelectric effect in the SiGe/Si strained layer system. The sample was a p-type modulation doped Si 0.88 Ge 0.12 /Si heterostructure containing a two-dimensional hole gas with carrier sheet density 2 X 10 11 cm -2 and a 4.2 K mobility of 10500 cm 2 V -1 s -1 . It was excited with a 225.7 MHz high frequency pulse of 1 μs duration and 0.5 W peak power. The amplitude of the output acoustic signal was about 110 dB below the electromagnetic input signal at 77 K. We believe that the observed electric field-acoustic conversion is associated with the non-centrosymmetric structure of the ordered unit cell of the strained SiGe alloy. Additionally we report on phonon Raman scattering at T = 300 K and hot hole Shubnikov-de Haas and zero magnetic field resistivity behaviour in the same heterostructures in the temperature range of 0.35 to 1.4 K. A broad band near 255 cm -1 and a peak near 435 cm -1 have been attributed to a particular Si-Ge ordering within the alloy layer. The energy relaxation of the carriers has been measured and is found to be dominated by a weakly screened piezoelectric coupled acoustic-phonon mechanism, thereby providing further evidence of ordering in the SiGe alloy.

Observation of piezoelectric-like behaviour in coherently strained B-doped heterostructures

In the present work the hybrid acoustic spectroscopy technique has been used to demonstrate the conversion of a high frequency (HF) electric field into acoustic waves and to provide the first direct observation of the piezoelectric effect in the SiGe/Si strained layer system. The sample was a p-type modulation doped Si 0.88 Ge 0.12 /Si heterostructure containing a two-dimensional hole gas with carrier sheet density 2 X 10 11 cm -2 and a 4.2 K mobility of 10500 cm 2 V -1 s -1 . It was excited with a 225.7 MHz high frequency pulse of 1 μs duration and 0.5 W peak power. The amplitude of the output acoustic signal was about 110 dB below the electromagnetic input signal at 77 K. We believe that the observed electric field-acoustic conversion is associated with the non-centrosymmetric structure of the ordered unit cell of the strained SiGe alloy. Additionally we report on phonon Raman scattering at T = 300 K and hot hole Shubnikov-de Haas and zero magnetic field resistivity behaviour in the same heterostructures in the temperature range of 0.35 to 1.4 K. A broad band near 255 cm -1 and a peak near 435 cm -1 have been attributed to a particular Si-Ge ordering within the alloy layer. The energy relaxation of the carriers has been measured and is found to be dominated by a weakly screened piezoelectric coupled acoustic-phonon mechanism, thereby providing further evidence of ordering in the SiGe alloy.