Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures

Abstract

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/neff)3 < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

Germanium Template Assisted Integration of Gallium Arsenide Nanocrystals on Silicon: A Versatile Platform for Modern Optoelectronic Materials

“…To get a better insight into the mechanism which causes the intense and discrete sub‐band gap emission, the mode formation inside a regular GaAs rhombic‐dodecahedron was studied using finite difference time domain (FDTD) simulations (Lumerical) . Therefore, spectra of the relative mode emission intensities for GaAs rhombic‐dodecahedra were calculated.…”

“…While optical devices based on silicon (Si) photonics are already well established and approach market maturity, fully integrated Si‐based optoelectronic devices for on‐chip optical interconnects, data processing, and sensing would require a small near‐infrared light source because the Si host medium is strongly absorbing at visible wavelengths . However, since the band‐to‐band transition in Si does not simultaneously obey the laws of energy and momentum conservation, spontaneous emission rates are low as compared with those of direct semiconductors and efficient Si‐based solid state lightning remains challenging . Numerous options to improve the efficiency of Si‐based light emission have been investigated, most of which rely on quantum confinement in integrated nanocrystals, quantum wells or superlattices and the introduction of efficient (quasi‐direct) transitions by sophisticated strain‐ and materials‐engineering methods .…”

Fundamentals of the nanowire solar cell: Optimization of the open circuit voltage

“…Another way of increasing the external photoluminescence of a nanowire with low is by using the Purcell effect, as has recently been demonstrated by Schmitt et al 163 . By tailoring the local density of optical states around the emitter, the spontaneous emission rate can be enhanced by the Purcell effect into an enhanced emission rate / , in which ∝ is the Purcell factor, Q the cavity quality factor and V the cavity mode volume.…”

“…In a solar cell, the Purcell effect can be very useful to enhance the radiative rate relative to the nonradiative loss. Schmitt et al 163 demonstrated whispering gallery modes in an inverted nanocone, with a Q-factor of 1300, and observed a 200x enhancement of the external photoluminescence by this nanophotonic engineering approach.…”

Free chargesversusexcitons: photoluminescence investigation of InGaN/GaN multiple quantum well nanorods and their planar counterparts

“…Previously, researchers argued that these improvements were the result of a combination of in-plane strain relaxation, a decrease in the quantum-confined Stark effect (QCSE), light extraction efficiency enhancement and lateral carrier confinement. 6,[9][10][11] However, detailed analyses of carrier dynamics are still unable to explain the change in luminescence properties of MQW nanorods over planar layers with respect to their carrier density dependence. [12][13][14][15] In particular, the impact of the significant increase of surface states in nanorods is not fully understood.…”