Direct Bandgap Silicon: Tensile‐Strained Silicon Nanocrystals

Abstract

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect‐gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect‐gap nature. Here, we propose a solution to this long‐standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E(k) band‐structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole‐allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.

Bright trions in direct-bandgap silicon nanocrystals revealed by low-temperature single-nanocrystal spectroscopy

“…Tensile strain induced by the methyl capping then further up-shifts the D 1 minimum and forms a dip in the C 15 maximum, thereby transforming methylcapped SiNCs into a direct-bandgap material (the black curve; note the good correspondence with the bandstructure of an example of a direct-bandgap material GaAs on the right). This transformation is supported both experimentally by optical measurements and theoretically by density functional theory calculations, both of which have been published elsewhere 5 . We should stress here that the computed bandstructure of NCs is "fuzzy" due to the Heisenberg uncertainty relations and contains sharp, level-like states separated by minigaps of forbidden energies, especially close to band edges.…”

“…Details of the preparation procedure and the investigation of the direct-bandgap switch-over in this material were published elsewhere 5,24,29 . The strain-engineered direct-bandgap SiNCs under study are characterized by short radiative lifetime 5,24 of 10 ns and a mean SiNC diameter of 2.523 nm 24,30 .…”

“…At RT, the relaxation rate amounts to ,10 11 s 21 , which translates into the relaxation "decay time" of ,10 ps. This is a very fast process compared with the photoluminescence (PL) decay time in the SiNCs under study 5,24 (2 ns), which implies that the high-energy wing should not be observed. However, as a result of the strong size dependence, the relaxation time can change by several orders of magnitude with only a 10% change in size and can reach values as high as nanoseconds 32,33 .…”

“…5, connected with the influence of the naturally straining methyl capping (see also Supplementary Section S5). We want to note here that the same phonon sub-structure also exists in the high-energy wing, as discussed in Supplementary Section S2.7.…”

“…The use of SiNCs instead of the bulk form of silicon, however, can alleviate the problem of poor light emission only partly, because SiNCs preserve the indirect nature of the bandgap 1,2 , and the light-emission rate thus remains low, on the order of 10 4 s 21 or less 3 . However, when passivated by a "suitable" organic material, SiNCs can have fast macroscopic radiative lifetime [4][5][6][7] and can actually be transformed into a direct-bandgap material 5,[8][9][10] . In particular, we have recently shown both theoretically and experimentally that properly capped SiNCs can be strain-engineered into a material with fundamental direct C 15 -C 25 ' bandgap 5 and the corresponding fast electron-hole radiative recombination rate of 10 8 s 21 (see also Figure 1a and the section on "Results overview").…”