2-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 (Si0.70Ge0.30) 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 mu m relax the strain of the Si0.70Ge0.30 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 10(7) cm(-2) determined from transmission electron microscopy (TEM) measurements. These structures have low n-type background impurity concentrations (<10(16)cm(-3)) and the Si0.70Ge0.30 barriers can be successfully doped with a unity activation ratio in the 10(17) to 10(20) cm(-3) range. At present, we obtain 300 K (0.4 K) electron mobilities and sheet densities in our 2DEGs of 10(3) (5.3 x 10(4)) cm(2)/Vs and 3 x 10(12) (5.2 x 10(11)) 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.