The aim being to fabricate (1 1 0) localized silicon-on-insulator (L-SOI) devices, we have first of all completed the Semicond. Sci. Technol. 23 105018 (2008) study of the differences between (1 1 0) and (1 0 0) surfaces in terms of (i) HCl etch kinetics and (ii) SiGe growth kinetics (with a chlorinated chemistry). The core layers of a L-SOI device are indeed obtained thanks to the in situ HCl etching (on patterned wafers) of the Si active areas followed by the selective epitaxial growth of a Si 0.7 Ge 0.3 /Si stack. Given that SiGe(1 1 0) layers grown at 650 • C in windows of patterned wafers are rough, we have first of all studied the 600 • C growth kinetics of SiGe(1 1 0). As expected, the SiGe growth rate decreases as the growth temperature decreases from 650 • C down to 600 • C (irrespective of the surface orientation). The SiGe(1 0 0) growth rate increases linearly with the germane mass flow. Meanwhile, the SiGe(1 1 0) growth rate increases in a sub-linear fashion and then saturates at much lower values than on (1 0 0). The Ge concentration x dependence on the F(GeH 4 )/F(SiH 2 Cl 2 ) mass flow ratio is parabolic on (1 0 0) and linear on (1 1 0), with lower values on the latter than on the former. We have then used those data to fabricate (1 0 0) and (1 1 0) L-SOI structures. The high HCl partial pressure recessing of the Si(1 1 0) and Si(1 0 0) active areas was performed at 675 • C and 725 • C, respectively. An increase of both the Si(1 1 0) HCl etch rate and the SiGe growth rate (be it at 650 • C on (1 0 0) or at 600 • C on (1 1 0)) was noticed when switching from blanket to patterned wafers (factors of 2.5-3 for HCI and 1.5 for SiGe). Finally, Si(1 1 0) growth times were multiplied by 4/3 compared to the Si(1 0 0) growth time in order to obtain similar thickness Si caps. Subsequent process steps were very similar on (1 0 0) and (1 1 0). Almost the same etch rates were notably obtained for the lateral etching of the (1 1 0) and (1 0 0) SiGe sacrificial layers (thanks to a CF 4 -based dry plasma), with no anisotropy. Significant hole mobility gains (electron mobility loss) compared to the universal Si(1 0 0)/SiO 2 mobility were evidenced in long, narrow (L = 10 μm; W = 0.08 μm) 'bulk-like' epitaxial Si(1 1 0) L-SOI devices (i.e. with SiGe still present under most of the Si channel). The gain (the loss) monotonically increased from 120% (11%) up to 246% (58%) when moving away from the [0 0 1] direction toward the [1 −1 0] direction (not reached, however: at most at 60 • to [0 0 1]). Vastly improved hole transport properties (a factor of 2 On current increase) were evidenced in short dimensions L-SOI devices (L = 0.35 μm; W = 0.08 μm) when switching from (1 0 0) surfaces with 1 1 0 conduction channels to (1 1 0) surfaces with a [0 0 1] propagation direction for the holes.
In this brief, we propose simple and analytical models for threshold voltage and subthreshold slope including shortchannel and quantum effects for fully depleted or undoped double-gate MOS devices.Index Terms-Double-gate device, drain-induced barrier lowering (DIBL), short-channel effects (SCEs), subthreshold slope, threshold voltage.
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