Two dimensional electron gases in SiGe/Si heterostructures grown by gas source molecular beam epitaxy

We have realized two dimensional electron gases (2DEGs) in tensile strained silicon (Si) channels between strain relaxed silicon germanium (Si_0.70Ge_0.30) barriers grown on Si(100) substrates by Gas Source Molecular Beam Epitaxy (GSMBE). Disilane (Si₂H₆), germane (GeH₄), and arsine (AsH₃) are used as the source gases. Compositionally graded buffer layers with a linear gradient of 30%; Ge/1 Μm relax the strain of the Si_0.70Ge_0.30 barrier layers by an amount greater than 95%; as determined from X-ray diffraction (XRD) rocking curves. Dislocation densities in the vicinity of the active strained Si channels are below 10⁷ cm^-2 as determined from transmission electron microscopy (TEM) measurements. These structures have low n-type background impurity concentrations ( < 10¹⁶ cm^-3) and the Si_0.70Ge_0.30 barriers can be successfully doped with a unity activation ratio in the 10¹⁷ to 10²⁰ cm^-3 range. At present, we obtain 300 K (0.4 K) electron mobilities and sheet densities in our 2DEGs of 10³ (5.3 × 10⁴) cm²/Vs and 3 × 10¹² (5.2 × 10¹¹) cm^-2, respectively. A discussion of the requirements for growing these structures by GSMBE and the modifications needed to improve the transport properties of the 2DEGs is presented.