The Crystalline Structure of Thin Bismuth Layers Grown on Silicon (111) Substrates

Abstract

Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — α-Bi and β-Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. β-Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2θ values showed the biaxial compressive strain. For comparison, α-Bi layers are misoriented in six in-plane directions and have β-Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest α-Bi layers due to higher compression.

Phase-Selective Epitaxy of Trigonal and Orthorhombic Bismuth Thin Films on Si (111)

“…It is due to the fact that the projected values of both sets of orthorhombic lattices at 8 nm (Figure 8a) and 25 nm (Figure 8d) remain almost identical (Equation ( 3)). The acquired lattices in a 25 nm-thick Bi ORTH epilayer are found to be in close agreement with earlier reports on Si (111), although the buckling height is still much smaller than the reported value of 0.5 Å [21,26,27,29,41].…”

“…The measured value provided = 2 × (3.046) ± 0.002 nm −1 = 1.914 ± 0.002 Å −1 , which results in an out-of-plane lattice "b" = 6.57 ± 0.01 Å. The acquired value is in good agreement with earlier reported results on Si (111) substrates [21,26,27]. The symmetric RSM also confirms the absence of any tilted or heavily misaligned grains, pointing to a high-quality epilayer.…”

“…The RC measurement at the Bi ORTH peak, depicted as inset in Figure 6 g, has provided the FWHM value of 58 ± 10″, which is comparable to the crystal quality of the Bi TRIG , depicted in Figure 3 c. To evaluate the out-of-plane lattice parameters and to identify the presence of any tilted grains, a symmetric RSM across the Bi ORTH peak is acquired and depicted in Figure 6 h. The measured value provided = 2 (3.046) ± 0.002 nm −1 = 1.914 ± 0.002 Å −1 , which results in an out-of-plane lattice “b” = 6.57 ± 0.01 Å. The acquired value is in good agreement with earlier reported results on Si substrates [ 21 , 26 , 27 ]. The symmetric RSM also confirms the absence of any tilted or heavily misaligned grains, pointing to a high-quality epilayer.…”

“…A recent study has reported the presence of 60-nm-wide Bi ORTH crystal grains reaching the thickness of approx. 25 nm in polycrystalline films [ 26 ]. Thus, the possibility of growing thick Bi ORTH crystallites is addressed, while the uncertainty of achieving phase-pure, fully coalesced, and high-crystal-quality Bi ORTH epilayers still remains.…”

“…While growing on Si substrates, all reported growths were conducted on either [ 19 , 20 ] or [ 21 , 22 , 23 , 24 , 25 ] reconstructed surfaces. Both reconstructions facilitate epitaxy in the early stages of growth by providing a high density of nucleation sites; however, both result in an increased number of rotational domains with relatively small grain sizes, reducing the overall quality of epilayers [ 26 , 27 , 28 ]. Additionally, the maximum height before leaving the topological phase, approx.…”