Near Infrared Light Detectors Based on Uhvcvd Epitaxial Ge on Si (100)
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SiGe structures on silicon represent an alternative solution for near infrared photodetection, owing to their narrow band gap and full compatibility with silicon technologies. SiGe alloy heterostructures, multi-quantum-wells and superlattices have been used as active layers of p-i-n photodetectors and their operation at 1.3jim has been successfully demonstrated for both normal incidence [2] and waveguide configurations [3]. The fundamental limitation of SiGe-based heterostructures is the large lattice mismatch that implies a small critical layer thickness for strain relaxation. Actually, pure germanium represents the best candidate as absorbing material in 1.31.55 gim photodetectors, due to its direct bandgap of 0.85eV. However, owing to the high lattice mismatch with Si, it is not easy to obtain Ge films with characteristics suitable for the integrated electronics: thick (efficiency of the device), flat (realization of sub-micron lithography), defect-free (high-speed). Thick graded buffer layers can be used to partially relax the strain. However, although the buffer can solve the problem of the lattice mismatch [4], the MBE growth of a thick layer is long and expensive. In the present work we have investigated the feasibility of the simplest fabrication approach, i.e., the CVD epitaxial growth of thick layers (well over the critical layer thickness) of pure germanium on silicon. The use of pure Ge on Si for efficient photon detection in the range 1.3-1.5 gim, firstly attempted by Luryi et al.[5], requires thicknesses far beyond the critical thickness for strain relaxation and, consequently, the large lattice mismatch (4 %) is expected to generate a high density of dislocations. To limit and confine the dislocations we introduce a thin Ge buffer layer grown at low temperature, where the surfactant action of hydrogen [6] allows the deposition of flat layers far thicker than the wetting layer in the normal Stranski-Krastanov growth mode. In such a way we were able to grow flat films of thickness comparable with the penetration depth of the near infrared light. 193
Mat. Res. Soc. Symp. Proc. Vol. 486 01998 Materials Research Society
EXPERIMENTAL The Si(100) substrates used in this study were cleaned by heating up to 1100 °C in H2 atmosphere in a UHV growth chamber, whose basic pressure was in the high 10-11 Torr range. As determined by XPS, RHEED, and AFM, respectively, the surfaces were clean (C and 0 contamination below 0.1 %),2xl reconstructed and flat ( roughness of- 1 monolayer). The Ge films were grown by Low Pressure Chemical Vapour Deposition using high purity germane without carrier gas with a pressure in the mtorr range. The growth proceeded in two steps. In the first step the substrate temperature was T= 330 oC in order to grow a flat and relaxed epitaxial layer -50 nm thick. Due to the surfactant action of hydrogen at such low temperature, in the first step of the growth the nucleation of the 3D Ge islands is inhibited and the strain is relaxed via the insertion of misfit dislocations. Once the elastic
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