Excitonic transitions in silicon nanostructures probed by time‐resolved photoluminescence spectroscopy

Abstract

Time‐resolved photoluminescence spectroscopy has been utilized to reveal excitonic transitions in silicon nanowires and silicon nanocrystals. While previous works on porous silicon and silicon nanocrystals have shown a two‐level splitting, e.g., singlet‐triplet states, our measurements reveal the fine structure of the excitons including three semi‐bright states and a ground dark excitonic state. Surprisingly, for silicon nanowires we have found the slowest semi‐dark exciton to fall above the faster semi‐bright excitonic state as opposed to silicon nanocrystals. The results are analyzed in terms of spin and orbital selection rules showing that the interchange in the level's hierarchy corresponds to a swap between spin‐forbidden and orbitally‐allowed states. We assign this surprising phenomenon to the relationship between the exchange interaction and the direct Coulomb interaction, which is affected by the dimensionality of the exciton. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Exciton Dynamics in Silicon Nanowires

“…However, passivation of the surface of the SiNWs with an oxide layer will lead to a decreased density of DT states, resulting in a net decrease in trap state PL intensity. Nevertheless, the PL that is observed (modeled in Figure ) is believed to arise from the recombination from the Si/SiO 2 interface states, which act as DTs, for which this assignment has been made before. , SiNW samples with lower relative porosity will thus have low overall PL, yet will exhibit characteristic SiO 2 peaks (Figure ).…”

“…Bulk silicon is known to be an indirect bandgap semiconductor with weak PL. Because of this, quantum confinement effects are thought to be the cause of PL in silicon nanocrystals. − Surface effects have also been suggested to play an important role in the PL of nanostructured Si. − For instance, cathode luminescence and time-resolved PL spectroscopy have shown that luminescence in silicon nanowires (SiNWs) is due to interface states and surface radiative recombination centers located at the interface of the silicon core and the SiO 2 cladding layer. , …”

“…15−18 For instance, cathode luminescence and time-resolved PL spectroscopy have shown that luminescence in silicon nanowires (SiNWs) is due to interface states and surface radiative recombination centers located at the interface of the silicon core and the SiO 2 cladding layer. 19,20 Ultrafast dynamics of excitons and charge carriers in nanostructured silicon have been studied, particularly in relation to its PL properties. Fast carrier thermalization times of ∼150 fs via phonon emission have been seen before 21 along with intraband relaxation times of ∼240 fs.…”

“…Nevertheless, the PL that is observed (modeled in Figure 5) is believed to arise from the recombination from the Si/SiO 2 interface states, which act as DTs, for which this assignment has been made before. 19,20 SiNW samples with lower relative porosity will thus have low overall PL, yet will exhibit characteristic SiO 2 peaks (Figure 2).…”

Glassy-like Transients in Semiconductor Nanomaterials

“…In particular, for the electrical systems, the “sluggishness” in manifested by a characteristic time constant τ of the rise and/or the decay of σ(t) which is much longer than any experimental macroscopic or intrinsic microscopic time of the system such as carrier scattering or recombination times [ 20 ] (which are less than a msec. in Si NCs [ 39 , 40 ]).…”