Chemical vapour etching of Si, SiGe and Ge with HCl; applications to the formation of thin relaxed SiGe buffers and to the revelation of threading dislocations

We have studied the etching of silicon, SiGe and germanium layers with gaseous HCl in reduced pressure-chemical vapour deposition (RP-CVD). We have observed the occurrence of two etch regimes depending on the etching temperature. The first regime takes place at high temperatures and is characterized by low activation energies (~7 kcal mol−1), this whatever the germanium content of the etched layer. The other regime occurs at low temperatures and has associated high activation energies (which strongly depend upon the germanium concentration of the etched layer: 86 kcal mol−1 for pure Si versus 28 kcal mol−1 for pure Ge). Modifying the HCl partial pressure has different effects depending on the regime. In the high temperature regime, increasing the HCl partial pressure will almost quadratically increase the etch rate (ER ∝ PHCl1.76), this both for Si and Si0.67Ge0.33. Meanwhile, the dependence is sub-linear in the low temperature regime (Si ER ∝ PHCl0.53 and Si0.67Ge0.33 ER ∝ PHCl0.82). The temperature where the regime shifts from one to the other decreases when the Ge concentration increases. To illustrate the added value of the chemical vapour etching, we have demonstrated two possible applications. The first one is the realization of SiGe thin strain relaxed buffers (TSRBs) in the active areas of shallow trench isolation (STI) patterned wafers after etching away the silicon with HCl. We have observed the occurrence of some etching loading effects when moving from a blanket to a patterned wafer. The SiGe TSRBs exhibit some good structural properties (rms roughness of 0.12 nm, no defects observed in cross-sectional transmission electron microscopy). However, they are not fully relaxed and facets are present at the STI/epitaxial stack boundary, signifying they are still not mature enough to be integrated in a metal oxide semiconductor technology. Another possible application is to decorate through some in situ HCl etching the dislocations threading through SiGe relaxed thick layers, with some significant advantages over commonly used wet etching solutions such as the Secco and the Schimmel ones.

Source:IOPscience

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