Enhanced photoluminescence emission from two-dimensional silicon photonic crystal nanocavities

We present a temperature dependent photoluminescence study of silicon optical nanocavities formed by introducing point defects into two-dimensional photonic crystals. In addition to the prominent TO phonon assisted transition from crystalline silicon at ~1.10 eV we observe a broad defect band luminescence from ~1.05-1.09 eV. Spatially resolved spectroscopy demonstrates that this defect band is present only in the region where air-holes have been etched during the fabrication process. Detectable emission from the cavity mode persists up to room-temperature, in strong contrast the background emission vanishes for T>150 K. An Ahrrenius type analysis of the temperature dependence of the luminescence signal recorded either in-resonance with the cavity mode, or weakly detuned, suggests that the higher temperature stability may arise from an enhanced internal quantum efficiency due to the Purcell-effect.

Due to its indirect electronic bandgap, crystalline silicon is rarely used as the active emitter in semiconductor optics [1]. Interband light emission is predominantly a phononassisted process and silicon has, therefore, a very poor internal quantum efficiency. However, the development of efficient silicon based light emitters would pave the way toward CMOS compatible monolithic optical interconnects and, therefore, signal processing speeds much higher than currently provided by silicon micro-electronics [2,3,4,5,6].
Recently, enhancement of the photoluminescence (PL) intensity from crystalline silicon at room-temperature has been observed using two-dimensional (2D) silicon photonic crystals (PhCs) with photonic point defect nanocavities [7,8]. Such defect PhC nanocavities modify the spatial emission profile of light; due to the in-plane photonic band gap a much larger fraction of luminescence is emitted perpendicular to the slab than parallel to it [9]. In addition, the high Q-factors that are attainable using PhC nanocavities [10,11,12] together with their small mode volumes may lead to an enhancement of the internal quantum efficiency of the active material due to an increase of the radiative emission rate via the Purcell-effect [13]. Other work demonstrates enhanced PL emission from internal light emitters embedded in photonic crystal slabs [14,15].
In this letter, we present a detailed investigation of the spectrum and temperature stability of the PL emission from crystalline silicon PhC nanocavities. Analysis of our results indicates that both the phonon satellites of the interband silicon emission and surface defect states are responsible for the luminescence of PhC cavity modes. Most interestingly, we can detect luminescence from the cavity mode up to room temperature whilst the background emission intensity rapidly reduces below our detection threshold for T ≥ 150 K. An Arrhenius type analysis of the temperature dependent data, either in resonance with the cavity mode or spectrally detuned from it, indicates that the local radiative emission rate is enhanced in resonance via the Purcell-effect.
The samples investigated were fabricated from silicon-on-insulator (SOI) wafers with a d = 250 nm thick active silicon layer on top of a 3 µm thick layer of SiO 2 . A 2D PhC is patterned into the upper silicon layer as illustrated in fig. 1 (a). We defined a triangular lattice of air holes with a period of a = 275 nm in the silicon slab as shown by the scanning electron microscope (SEM) image in the left panel of fig. 1 (b). This was done using electronbeam-lithography and subsequent SF 6 /C 4 F 8 reactive-ion-etching. These techniques allow us to control the radius r of the air holes with a precision of ±2 nm. As a final fabrication step the underlying SiO 2 is removed by hydrofluoric acid. Low mode-volume nanocavities (V M ode ≈ 1 · (λ/n) 3 ) were realized by omitting three air-holes in a row and by shifting the lateral holes away from the cavity center by 0.15 lattice constants to form modified L3 cavities [16]. A typical scanning electron micrograph (SEM) image of the samples investigated is shown in the right panel of fig. 1 (b).
Spatially-resolved optical measurements were performed using a micro-photoluminescence (µPL) spectroscopy setup. The sample was excited by a diode-pumped CW solid-state-laser emitting at λ Laser = 532 nm. We focussed the laser beam using a 50× microscope objective (N A = 0.5) to a spot size of ≈ 1 µm. The resulting PL signal was collected through the same objective and dispersed by a 0.32 m imaging monochromator equipped with a 600 lines/mm grating and a liquid nitrogen-cooled InGaAs linear diode array.
In fig. 2 (a) we present room-temperature µPL spectra recorded from a series of L3 nanocavities as the normalized radius r/a is increased from 0.28 (bottom) to 0.34 (top).
The excitation power density used in these measurements was 550 kWcm −2 . Using these measurement conditions the intensity of the emission from the unpatterned region of the device was below our detection sensitivity. In strong contrast, each of the spectra recorded from the PhC nanocavities clearly reveals five distinct peaks (marked with arrows on fig. 2 (a)) that shift to higher energy with increasing r/a ratio [19]. The extracted peak energies are plotted in fig. 2 (b), clearly demonstrating that all peaks (labeled M1 to M6) shift in a similar way to higher energies with increasing r/a, as previously reported in refs. [7,8]. and experiment for M5, which is not observed in the spectra, probably due to a higher sensitivity of mode Q and frequency to structural disorder. This tends to lead to a low-Q and, thus, broad emission from M5. The comparison between simulation and experiment clearly shows that we observe PhC mode-emission in the spectral vicinity of the silicon interband luminescence and that we are able to clearly distinguish the cavity mode emission from the background. defined [17]. We investigated the intensity of the broad emission band as a function of the excitation position on the sample. Selected results of these measurements are presented in fig. 3 (b) where we plot the integrated PL intensity from 1.06 eV to 1.08 eV (highlighted orange in (a)) for a spatial scan across the PhC (compare with fig. 3 (a) fig. 3 (c), where we plot the integrated intensity of M3 as a function of peak position (red data points). For comparison, we plot the PL spectrum recorded from the r/a = 0.25 sample (black line). The mode intensity clearly tracks the spectrum of the TO replica and follows 6 also the intensity trend of the surface defect band. These observations indicate that the cavity modes are pumped predominantly via the phonon replica and also more weakly via the surface defects introduced by the fabrication process. This arises from the fact that the luminescent defects are close to the air-silicon interface, where the electric field amplitude of the eigenmodes as shown in fig. 2 (c) are weak, leading to a weak coupling between mode and surface defects.
We continue to analyze the temperature stability of the PL spectrum recorded from the PhC cavity site. Typical temperature dependent data recorded from the M1 mode emission for a PhC with r/a = 0.34 from 24 K to room-temperature (295 K) is presented in fig. 4 (a).
We observe a clear and rapid reduction in the intensity of the TO-phonon replica, the M1 mode emission and the surface defect band, respectively. This is caused by an increasing importance of non-radiative decay channels, like Auger and free carrier recombination [18].
For further quantitative analysis we describe the temperature dependent PL intensity I P L (T ) as a function of the external photon detection probability P , the excitation rate R and the internal quantum efficiency η int (T ): In these equations Γ Rad is the interband radiative recombination rate and Γ N R (T ) is the non-radiative recombination rate. The temperature-dependence of Γ N R (T ) is described by distributing excitation amongst the two levels, |2 and |1 separated by E A according to Boltzmann statistics. A schematic few level-diagram of the system is presented in fig. 4 (b).
State |1 is pumped at an excitation rate R and decays either via a radiative channel to |0 or non-radiatively over |2 . Eqn. 1a to eqn. 1c are directly obtained from rate equations governing the steady state populations of |1 and |2 using the approximation Γ 0 N R Γ * N R (T ). This condition ensures that the non-radiative channel dominates carrier recombination at elevated temperature as expected for silicon. By reformulating eqn. 1a we obtain ln I P L (T 0K ) where I P L (T 0K ) is the extrapolated PL intensity for T → 0 K [20]. In fig. 4  replica (black squares), the surface defect band PL from 1.07 meV to 1.08 meV (blue circles) and the M1 cavity mode emission (red triangles) [21]. For all datasets we observe a straight line as predicted by eqn. 2 supporting the vailidity of our analysis. From the slopes of these curves we extract an activation energy E A of 8 ± 2 meV (surface defects), 10 ± 2 meV (TOreplica) and 10 ± 2 meV (M1 mode) for non-radiative recombination processes. To obtain these fits we only took into account I P L (T ) for T < 100 K where sufficiently intense signal from mode, defect band and TO-replica are available to make a valid comparison. From the similar values of E A for the cavity mode and TO-replica we conclude that the mode is predominantly excited via the TO-replica, as already deduced from the data shown in fig. 3 (c).
Comparing mode and TO-replica in fig. 4 (c), we see from eqn. 2 that the lower values of ln(I P L (T 0K )/I P L (T ) − 1) from the mode emission can only be explained via the term Γ Rad /Γ 0 N R , since both emission peaks exhibit the same activation energy. Hence, the data suggests that the presence of the cavity mode results in a ratio of Γ Rad /Γ 0 N R which is larger than for the TO-replica. As shown in refs. [7,9], an enhanced PL signal can also be caused by a redistribution of the spatial emission profile via the photonic crystal structure. However, this effect does not increase the radiative recombination rate. Therefore, the results indicate that the enhanced photoluminescence from the cavity mode may be caused by an enhanced internal quantum efficiency η int (T ) due to a larger radiative carrier recombination rate caused by the Purcell-effect [22]. This would explain the observed temperature stability of the mode emission up to room-temperature, in strong contrast to the vanishing background emission for T ≥ 150 K.
In conclusion, we presented a temperature dependent investigation of PL in Si PhC nanocavities. We suggest two mechanisms being responsible for the luminescence of cavity modes, namely phonon-mediated recombination from charge carriers in the electronic band states and recombination from charge carriers trapped in surface defect states. Furthermore, we observed an enhanced internal quantum efficiency in spectral resonance with the cavity mode emission.
We acknowledge financial support from the German Excellence Initiative via the Nanosystems Initiative Munich (NIM), the TUM International Graduate School of Science and En-