Tensile-strained Si layers grown on Si_0.6Ge_0.4and Si_0.5Ge_0.5virtual substrates: I. Film thickness and morphology

Abstract

We have studied the structural properties of tensile-strained Si layers grown on polished Si 0.6 Ge 0.4 and Si 0.5 Ge 0.5 virtual substrates as a function of their thickness. Two gaseous precursor chemistries have been assessed for the reduced pressure-chemical vapour deposition of the sSi layers: SiH 2 Cl 2 at 700 • C and SiH 4 at 600 • C. We have used specular x-ray reflectivity and spectroscopic ellipsometry to gain access to the sSi layer thickness (and the associated sSi growth rate). The surfaces of sSi layers grown at 600 • C using SiH 4 are characterized by a small spatial wavelength (a few hundred nm) roughness. Meanwhile, some lines along the 1 1 0 directions can be observed for thick sSi layers grown at 700 • C using SiH 2 Cl 2 , hinting at the presence of stacking faults. We obtained (for 10 µm × 10 µm atomic force microscopy images) surface root-mean-square roughness (Z ranges) between 0.19 and 0.36 nm (1.8 and 3.9 nm). By comparison, the rms roughnesses (the Z range) associated with 360 µm × 368 µm optical interferometry images are between 0.7 and 1.4 nm (7.1 and 12.1 nm), with some small amplitude but very long spatial wavelength (tens of µm) surface cross-hatch remaining on Si 0.6 Ge 0.4 VS. The interfaces between sSi and SiGe are very abrupt, as illustrated by high-resolution transmission electron microscopy and by the Ge decay profile in secondary ions mass spectrometry: 0.73 nm/decade for sSi on Si 0.5 Ge 0.5 VS and 1.06 nm/decade for sSi on Si 0.6 Ge 0.4 VS, more or less independently of the sSi growth chemistry. The larger value for sSi on Si 0.6 Ge 0.4 VS is most probably due to some instrumental broadening linked to the small remaining cross-hatch.

Impact of the H2 bake temperature on the structural properties of tensily strained Si layers on SiGe

“…This value is slightly higher for sSi layers grown with exactly the same process parameters on top of Si 0.63 Ge 0.37 : 16.170.2 nm. The associated sSi growth rates, 2.17 nm min À1 on Si 0.73 Ge 0.27 and 2.23 nm min À1 on Si 0.63 Ge 0.37 , are quite close to the ones found for the same process conditions on (i) Si 0.52 Ge 0.48 and Si 0.60 Ge 0.40 (2.37 nm min À1 ) [7] and (ii) Si 0.71 Ge 0.29 and Si 0.79 Ge 0.21 (2.30 nm min À1 ) [9].…”

“…less than 10 mm). With this growth sequence, the top layer is expected to be almost totally relaxed, with threading dislocations densities (TDDs) of the order of 10 5 cm À2 [7][8][9]. After the use of several chemical mechanical polishing (CMP) steps to get rid of the surface cross-hatch, the surfaces of those SiGe VSs were cleaned in an automated wet bench [13].…”

“…These lines are most probably the signature of SFs (i.e. {1 1 1} planes of broken or distorted bond extending through the thickness of the sSi film) or misfit dislocations (MDs) at the interface between sSi and SiGe [7][8][9][24][25][26][27]. It seems likely that SFs would be more deleterious to device operation than MDs [28], although the latter have been shown to act as dopant diffusion pipes in bulk nMOS transistors [29].…”

“…We have quite recently investigated the structural properties of sSi layers grown on top of polished Si 1Àx Ge x VSs (with x equal either to 0.2, 0.3, 0.4 or 0.5) as a function of (i) their thickness and (ii) the growth chemistry and temperature adopted to deposit the sSi layers (either SiH 2 Cl 2 at 700 1C or SiH 4 at 600 1C) [7][8][9]. The surface preparation adopted in all cases was the following one: (i) a ''HF-last'' wet cleaning that yielded SiGe surfaces mostly free of native oxide and carbon contamination followed by (ii) a 850 1C, 120 s H 2 bake at 20 Torr in order to get rid of the remaining C and O atoms on the surface prior to sSi re-growth [10].…”

Reduced Pressure–Chemical Vapour Deposition of Si/SiGe heterostructures for nanoelectronics

“…Surfaces nevertheless remain flat irrespective of the sSi layer thickness or the Ge content of the VS underneath (root mean square roughness in-between 0.27 and 0.40 nm for Ge contents in the 20-50% range) [30][31][32]. The interfaces in-between sSi and SiGe are abrupt, as illustrated by High Resolution TEM (see Fig.…”