Excitation wavelength dependent photoluminescence in structurally non-uniform Si/SiGe-island heteroepitxial multilayers

Abstract

In nanometer-size Si/SiGe-island heteroepitxial multilayers grown on Si(001), low temperature photoluminescence spectra are observed that strongly depend on the excitation wavelength and show a strong correlation with structural properties revealed by transmission electron microscopy. These experimental results can be explained by assuming that the optically created carriers are strongly localized at Si/SiGe island heterointerfaces. We show that electron-hole pairs are generated and recombine within spatial regions mainly defined by the photoexcitation penetration depth, and that the estimated exciton diffusion length is notably short and comparable with the SiGe-island average size.

Structural and optical properties of axial silicon-germanium nanowire heterojunctions

“…This process is even more efficient at the Si/Ge hetero-interface, where the $8 nm thick SiGe alloy layer is found. In SiGe alloys and nanostructures, compositional fluctuation is known to be responsible for low-temperature exciton localization; 43 therefore, exciton diffusion toward the surface is suppressed. Also, SiGe compositional fluctuation is responsible for the reduction of the exciton radiative lifetime.…”

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Impact on SiGe Light Emission

“…At an excitation wavelength of 325 nm with a penetration depth of only ~10 nm, the PL spectrum is shifted slightly toward lower photon energy and has a significantly reduced FWHM (~100 meV). This result demonstrates that the cluster composition and Si/SiGe interface abruptness, as illustrated in Figures 11 and 12, are influencing the PL behavior: the more abrupt the Si/SiGe interface, the smaller the FWHM of the PL peak, and, in the topmost SiGe layer, a higher Ge composition results in the PL peak being shifted toward lower photon energy (Modi et al, 2012a). In other similar samples, it has been shown that continuous optical excitation at shorter wavelengths produces substantial PL fatigue after a delay of about 10 min due to the accumulation of charge within the SiGe clusters (Modi et al, 2012b).…”

“…Single Nanometer-Thick SiGe Nanocluster Sample A TEM micrograph of a specially fabricated sample containing a very thin Si1−xGex cluster layer above a regular stack of such cluster layers that was grown using Si/SiGe MBE is shown in Figure 10A (Modi et al, 2012a). The sample is grown on a Si substrate and comprises two SiGe cluster layers with a cluster height of ~6-8 nm separated by a Si layer of ~8 nm thickness between the SiGe clusters (i.e., there are two SiGe/Si pairs); a 30-nm-thick Si spacer layer; another five SiGe/Si layer pairs to initiate 3D growth; a layer of SiGe clusters with a height of approximately 18 nm; a thinner separating Si layer; a final SiGe cluster layer; and a thin Si cap.…”

“…At low temperatures, carrier diffusion is known to be negligible in Si/SiGe NSs (Modi et al, 2012a). Hence, to account for the difference between the PL spectra obtained using shorter (325 nm) and longer (365 nm) wavelength excitation (see Figure 15A), one must allow for the difference in photoexcitation penetration depth, which is ~10 −6 cm at 325 nm and ~10 −5 cm at 365 nm (Paul, 2004;Modi et al, 2012a).…”

“…Hence, to account for the difference between the PL spectra obtained using shorter (325 nm) and longer (365 nm) wavelength excitation (see Figure 15A), one must allow for the difference in photoexcitation penetration depth, which is ~10 −6 cm at 325 nm and ~10 −5 cm at 365 nm (Paul, 2004;Modi et al, 2012a). Thus, the PL at ~ 0.9 eV is mostly associated with the 4-5-nm-thick Si1−xGex NL with x = 8 at.%, while the PL at ~ 0.8 eV is related to SiGe CMs with x reaching a maximum of 40 at.%.…”