X-ray diffraction study of strain relaxation, spontaneous compositional gradient, and dislocation density in GeSn/Ge/Si(100) heterostructures

Abstract

The strain relaxation, depth profiles of composition and density of dislocations in GeSn epilayers were studied by using the x-ray diffraction. Regions with uniform and graded composition, different levels of strain relaxation, and dislocation density were found in the GeSn layers grown at fixed growth conditions. At the initial stage of growth, the GeSn layer is under ~8 × 10 -3 compressive strain, and the Sn composition is close to the target value of 6.6%. With increasing layer thickness, the Sn content is enhanced by 0.85 ± 0.12% due to the relief of compressive strain down to ~5.0 × 10 -4 . The plastic strain relaxation is dominated by 60 • misfit dislocations at the GeSn/Ge interface, where their density is 3 × 10 5 cm −1 . The relaxed GeSn layer serves as a buffer for the high-quality GeSn overlayer with uniform and enhanced Sn composition. The laboratory-based x-ray diffraction is shown as a non-destructive reliable technique for strain, composition, and defects characterization, as an alternative to secondary ion mass spectrometry and transmission electron microscope techniques.

Quantitative Correlation Study of Dislocation Generation, Strain Relief, and Sn Outdiffusion in Thermally Annealed GeSn Epilayers

“…The strain state and Sn composition of the GeSn samples, before and after annealing, were estimated from X-ray diffraction reciprocal space maps (RSMs) measured across the asymmetric Ge 2̅2̅4 reflection and using the following expressions where Q x and Q z are the reciprocal space coordinates; h , k , and l are Miller indices; a and c are the actual lateral and vertical lattice parameters of the GeSn layer; a s is the lateral lattice parameter of the Ge-VS; a Sn = 0.6489 nm and a Ge = 0.5658 nm are the lattice parameters of bulk α-Sn and Ge, respectively; b = 0.0041 nm is the bowing parameter; ε x = ( a – a 0 )/ a 0 is the lateral strain; ε z = ( c – a 0 )/ a 0 is the vertical strain; and C 12 and C 11 are the elastic parameters of a GeSn alloy given by the linear relation between those of α-Sn and Ge, such as C = xC Sn + (1 – x ) C Ge .…”

“…Therefore, MDs are much stronger strain-releasing defects and consequently determine the diffuse intensity of the X-ray diffraction peaks. The MD density was estimated using RSM simulations, 12 as shown in Figure 5b and 5c for samples S1 and S2 annealed for 8 h. At the same time, the MDs and TDs are components of a dislocation loop so that each loop contains a misfit segment and two threading arms. Moreover, the dislocation penetrating from the Ge-VS also contains a misfit segment and one threading arm.…”

“…Unfortunately, epitaxial growth of GeSn alloys with high Sn content is difficult due to the low-equilibrium solid solubility of Sn in Ge (∼1 atom %) and the large lattice mismatch of α-Sn with Ge (14%) and Si (17%) substrates . As a result, metastable GeSn alloys are conventionally grown under nonequilibrium conditions by chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), where the needed low-growth temperatures and strain relaxation are possible and are key factors for achieving Sn-rich epitaxial GeSn alloys. − As a consequence, investigation of the stability of these Sn-rich epitaxial GeSn alloy samples is important. The effect of thermal treatment of GeSn epitaxial layers on Sn diffusion and stain relaxation processes was studied previously over a wide temperature range.…”

Impact of Long-Term Annealing on Photoluminescence from Ge1−xSnx Alloys

“…1 − 2 12 11 (3) where the elastic constants 12 and 11 and the lattice parameter 0 of the GeSn alloy are given by linear interpolation between the parameters of Sn and Ge, such as 0 = + (1 − ) [21]. Using the RSM data of Figure 2 coupled with RSM modelling, the strain and defect density as a function of annealing, is computed for S14, S15, S29, and S32 in Figure 3.…”

Impact of defects on photoexcited carrier relaxation dynamics in GeSn thin films

“…On the other hand, a low-temperature process leads to the formation of localized or extended amorphous inclusions and a significantly roughened surface [17]. Commonly, the GeSn epitaxial layers are characterized by a high density of misfit dislocations (MDs) at the GeSn/Ge interface [18,19] and Sn segregation at the surface of GeSn films [20,21].…”