Reduced pressure–chemical vapor deposition of Ge thick layers on Si(001) for 1.3–1.55-μm photodetection

Abstract

Ge-based photodetectors operating in the low loss windows (1.3–1.6 μm) of silica fibers are highly desirable for the development of optical interconnections on silicon-on-insulator substrates. We have therefore investigated the structural and optical properties of Ge thick films grown directly onto Si(001) substrates using a production-compatible reduced pressure chemical vapor deposition system. We have first of all evidenced a Ge growth regime which is akin to a supply-limited one in the 400–750 °C temperature range (Ea=6.9 kcal mol−1). The thick Ge layers grown using a low-temperature/high-temperature approach are in a definite tensile-strain configuration, with a threading dislocation density for as-grown layers of the order of 9×108 cm−2 (annealed: <2×108 cm−2). The surface of those Ge thick layers is rather smooth, especially when considering the large lattice mismatch between Ge and Si. The root-mean-square roughness is indeed of the order of 0.6 nm (2 nm) only for as-grown (annealed) layers. A chemical mechanical polishing step followed by some Ge re-epitaxy can help in bringing the surface roughness of annealed layers down, however (0.5 nm). The Ge layers produced are of high optical quality. An absorption coefficient alpha equal to 4300 cm−1 (3400 cm−1) has indeed been found at room temperature and for a 1.55-μm wavelength for as-grown (annealed) layers. A 20-meV band-gap shrinkage with respect to bulk Ge (0.78 eV⇔0.80 eV) is observed as well in those tensile-strained Ge epilayers.

Tensely strained GeSn alloys as optical gain media

“…We show for instance in Si 0.04 Ge 0.82 Sn 0.14 , evidences lattice relaxation. Similar to the growth of low temperature Ge buffers, 19 but different from relaxed SiGe buffers, no threading dislocations crossing the relaxed GeSn layer were observed for all investigated structures. The above results demonstrate the possibility of tuning the lattice constants and, hence, the elastic strain, especially tensile strain as will be shown below, in Ge(Sn) layers grown on top of these high Sn content (Si)GeSn buffers.…”

Experimental Demonstration of Room-Temperature Spin Transport inn-Type Germanium Epilayers

“…The structure consisted of a 1-μm-thick undoped Ge epilayer and a 50 nm heavily n-type doped Ge epilayer with a phosphorous doping concentration of ∼1.0 × 10 19 cm −3 and a degree of relaxation of 104%, calculated from the analysis of measured high-resolution x-ray diffraction symmetrical and asymmetrical reciprocal space maps. This overrelaxation of the Ge channel is attributed to the difference in the thermal expansion coefficients between Ge and Si; i.e., the Ge channel is 100% relaxed during the temperature growth, but is under slight tensile strain after cooling down to RT [46,47,49,50]. The Ge epilayers were measured to have a root mean square roughness below 1 nm by atomic force microscopy (AFM) and had a threading dislocation density of ∼5 × 10 6 cm −2 .…”

Reverse graded relaxed buffers for high Ge content SiGe virtual substrates

“…Initially, a relatively high quality 1 m Ge layer was grown directly on the Si͑001͒ substrate using the two temperature growth method. 3,4 This allows dislocations to nucleate while keeping the roughness to a minimum at the initial lower temperature and then allows the dislocations to glide during the higher temperature phase, the temperatures used were 400°C for the initial seed layer then 670°C for the high temperature layer. After this, the reverse linear graded ͑RLG͒ layer was deposited at a high growth temperature of 850°C, with the Ge content reducing linearly from x =1 to x = 0.78 over a thickness t LG , which was varied from 50 to 5000 nm corresponding to a range of effective grading rates between 440% / m and 4.4% / m. Finally, a 1.1 m constant composition Si 0.22 Ge 0.78 layer was grown.…”

“…The first is through a two temperature growth method directly on the Si͑001͒ substrate. 3,4 The second is by a slow grading of a buffer structure, 5,6 which, for such high-composition layers leads to a relatively thick structure, typically 8 -12 m, since a Ge grading rate of 10% / m and a 1-2 m Si 1−x Ge x cap layer have to be grown to obtain a root mean square ͑rms͒ surface roughness of 8 -15 nm ͑Refs. 6-8͒ and threading dislocations density ͑TDD͒ in the range 10 6 cm −2 or below.…”