Epitaxial Growth of Highly Strained SiGe Layers Directly on Si(001) Substrate

摘要

It has been observed that at lower growth temperatures, epitaxial layers of Si1-xGex on Si(001) can be grown beyond the predicted critical thickness[1] for plastic relaxation [2]. These layers are known as metastable, since if raised to a temperature higher than their growth temperature, relaxation will occur. A number of comprehensive studies have been published for Si1-xGex epilayers with a low Ge composition, including the recent work with a Ge content up to 52% [2]. However, no work has been carried out on higher Ge composition layers i.e. above 60% . Since the strain in the layer is proportional to its lattice mismatch with the Si substrate, layers with higher Ge composition will exhibit a higher degree of compressive strain. This higher strain ought to enhance a room-temperature two-dimensional hole gas (2DHG) mobility in a Si0.4Ge0.6 quantum well (QW) grown pseudomorphically on a Si(001) substrate. This property is very important for application of such materials in electronic devices like Field Effect Transistors (FETs) and others. It is also very important to maintain the SiGe QW thickness around or above 10 nm in order to minimize negative effect of interface scattering on 2DHG mobility. In this work, we demonstrate that highly strained epilayers of Si0.4Ge0.6 can be grown pseudomorphically on a Si (001) substrate with thicknesses up to ~25 nm, which is is significantly above the thickness usually accepted as being stable or even metastable. Epitaxial layers were grown in an industrial reduced pressure chemical vapour deposition (RP-CVD) system. Standard, commercial available disilane and germane precursors were used. The epilayer thicknesses were obtained by XTEM analysis and high resolution X-ray diffraction HR-XRD. The epilayer surface morphology was analyzed by atomic force microscopy (AFM) and defects observed in the relaxed epilayers were analyzed by combination of plan view transmission electron microscopy PV-TEM and AFM. We report that it wa- possible to grow up to 24nm of Si0.4Ge0.6 before strain relaxation occurred. For epilayers thinner than 24 nm a surface roughness comparable to that of the Si substrate was observed, for example the 21 nm layer showed an RMS roughness of 0.08 nm (Figure 2). These thin layers produced rocking curves with clearly defined thickness fringes (Figure 1) that indicate no relaxation by defect formation. Above 24 nm, the layers showed a high surface roughness with the characteristic crosshatch pattern (Figure 3). This indicates non-uniform elastic strain fields from dislocations [3] , suggesting strain relaxation. Above 24 nm the thickness fringes seen in the rocking curves rapidly reduced in intensity compared to the thinner layers, which is also indicative of strain relaxation. A thickness of 24 nm of strained Si0.4Ge0.6 on Si (001) is higher than any attempts at growth of this strained alloy directly on Si reported previously. The results obtained show very high potential for application of highly strained Si0.4Ge0.6/Si(001) structures in a variety of electronic and photonic Si based devices. Detailed characterisation will be presented along with growth details. [1]R. People and J. C. Bean,'Calculation of critical layer thickness versus lattice mismatch for GexSi1?x/Si strained-layer heterostructures', Appl. Phys. Lett., vol. 47, no. 3, p. 322, 1985. [2] J. M. Hartmann, A. Abbadie, and S. Favier,'Critical thickness for plastic relaxation of SiGe on Si(001) revisited', Journal of Applied Physics, vol. 110, no. 8, p. 083529, 2011. [3] H. Chen, Y. Li, C. Peng, H. Liu, Y. Liu, Q. Huang, J. Zhou, and Q.-K. Xue,'Crosshatching on a SiGe film grown on a Si(001) substrate studied by Raman mapping and atomic force microscopy',Physical Review B, vol. 65, no. 23, May 2002.