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We have observed electron-hole droplet (EHD) emission enhanced by silicon photonic crystal (Si PhC) nanocavities with a surface oxide. The EHD is employed as a massive emitter that remains inside the nanocavity to achieve efficient cavity-emitter coupling. Time-resolved emission measurements demonstrate that the surface oxide greatly reduces the nonradiative annihilation of the EHDs and maintains them in the PhC nanocavities. It is found that the surface-oxidized Si PhC nanocavity enhances EHD emission in addition to the Purcell enhancement of the resonant cavity, which will contribute to works on Si light emission and the cavity quantum electrodynamics of electron-hole condensates.
© 2016 Optical Society of America
1. Introduction
The control of spontaneous emission (SE) has been a fundamental issue for the physics of light-matter interaction and the development of practical light emitting devices. Recently, photonic nanostructures including photonic crystals (PhCs) and plasmonic materials have been fabricated to change the SE rate of photon emitters by modifying the local density of photon states (LDOS) of the nanostructures [1, 2 ]. The PhC nanocavity is an attractive device for controlling the emission of photon emitters. The SE rate of the emitters is generally increased by the resonant PhC nanocavity thanks to its high Q and small mode volume, and this is called the Purcell effect [3]. In contrast to the resonant nanocavity, the photonic band gap (PBG) formed by PhC structures suppresses the SE of the emitters because of their small LDOS [4, 5 ].
The PhC nanocavity has been used for the emission enhancement of silicon (Si), which is well known as an inefficient light emitting semiconductor due to the intrinsic small SE rate of electron-hole (e-h) pairs. It is reported that Si PhC nanocavities increase the intensity of Si emissions from hot-carrier plasma and free excitons created by optical excitation [6–8 ]. However, there are two possible issues limiting the efficiency of cavity-enhanced Si emissions. The first is the large amount of surface recombination in Si PhC samples. The large side walls of many air holes in the PhC structures quickly annihilate electrons and holes as a result of nonradiative recombination at their air-Si interfaces [9]. In bare Si PhC samples, the emission efficiency is greatly reduced because the nonradiative recombination rate is larger than the SE rate of e-h pairs. The second issue is that it is possible that the hot e-h plasma and free excitons move rapidly away from the small nanocavity due to their fast diffusion. This diffusion is observed as a nonradiative process that reduces the number of e-h pairs remaining inside the nanocavity. The suppression of both the nonradiative surface recombination and the diffusion of the e-h pairs is the key to achieve efficient cavity-emitter coupling and further emission enhancement of Si PhC nanocavities in addition to the Purcell effect.
In this study, we employed the electron-hole droplet (EHD) as a photon emitter and surface oxidation of Si. The EHD is a condensate of electrons and holes excited in Si, which has a large mass of M ~107 me - h, where the effective mass of an exciton is me - h = 0.45 m 0 and m 0 is the rest mass of an electron [10]. The large mass of the EHD contributes to its small diffusion constant of DEHD ~10−4 cm2/s at a temperature of 4 K, which is estimated as DEHD = τpk B T/M, where τp, k B, and T are the momentum relaxation time of 1 ns [11], the Boltzmann constant, and temperature, respectively. This diffusion constant of the EHD is six orders of magnitude smaller than that of 150 cm2/s for a free exciton at 4 K [12]. The EHD diffusion length is calculated to be 40 nm by , where the EHD lifetime τ is typically 140 ns in bulk Si. The EHD diffusion length is two orders of magnitude smaller than 60 μm for a free exciton with a lifetime of 230 ns [13]. The small diffusivity of the massive EHD enables e-h pairs to remain inside the PhC nanocavity for a longer time than free excitons. This diffusion suppression is an alternative way of confining the e-h pairs in a nanostructure that is different from quantum confinements by Si nanocrystals and trap potentials of defects or impurities in Si [14–17 ].
To form and maintain the EHD in the Si PhC nanocavities, the e-h pair concentration should exceed the critical concentration of 3 × 1018 cm−3 or the temperature should be lower than the critical temperature of 25 K [18]. Such dense or cold e-h pairs need less nonradiative recombination, which limits the increase in the e-h pair concentration and heats Si locally through thermal relaxation. To reduce the nonradiative recombination, we thermally oxidized the samples. Sample oxidation coats the surface of Si PhC structures with SiO2, which reduces the nonradiative surface recombination at air-Si interfaces [19]. This process makes it possible to obtain a sufficiently high e-h pair concentration to maintain the EHD in Si PhC nanocavities while suppressing unintentional local heating, and then contributes to the intense and efficient emission of the EHD.
In addition, previous studies on the emission enhancement by Si PhC nanocavities are based on only photoluminescence (PL) intensity data because time-resolved measurements are mostly hard for weak Si emissions. However, the intense EHD emission enhanced by the surface oxide enables us to simultaneously obtain PL spectra and time-resolved PL data with a high signal-to-noise ratio which allow a discussion about dynamics of the e-h pairs and the Purcell enhancement of the EHD emission induced by Si PhC nanocavities.
2. Experimental
PhC samples were fabricated on a silicon-on-insulator (SOI) wafer. The dimensions of a PhC cavity are shown in Fig. 1(a) . The top layer of the SOI wafer was a 160-nm-thick n-type Si film. Fine electron-beam lithography and dry etching processes were used to fabricate periodic air-hole arrays in the top Si film. The SiO2 layer underneath the PhC patterns was removed with hydrofluoric acid solution. An optical cavity called an L3 cavity was formed in the area of three missing holes as seen in the center of Fig. 1(b). Two end holes of the L3 cavity were shifted by 0.15a to increase the Q value [20], where the lattice period a is specified later. The electric field distribution of the fundamental cavity mode was calculated with the finite-difference time-domain (FDTD) method. The cavity mode is dominated by an electric field polarized perpendicular to the long axis of the L3 cavity, shown by Ey in Fig. 1(c). The FDTD calculation estimated a modal volume, Q value (Qv) determined by the optical out-of-plane loss, and emission detection efficiency with an objective at the resonant wavelength of the fundamental mode [6, 7 ].
Fig. 1 (a) Dimensions of a fabricated L3 cavity. The yellow and blue balls indicate a simplistic picture of the EHD formed by electrons and holes. (b) Top view of a typical L3 cavity taken with a scanning electron microscope. This is a sample without surface oxide. The scale bar indicates 1 μm. (c) Spatial distribution of the cavity-confined electric field directed to the y-axis (Ey). This mode is a fundamental cavity mode. (d) Set up for photoluminescence measurements. DM: dichroic mirror, L: objective, LPF: optical low-pass filter, P: polarizer, BPF: optical band-pass filter.
The PhC structures were fabricated and then the sample was cut into two pieces with exactly the same structure. One of the pieces was used in PL measurements of the emission from the sample without thermal oxidation. The other piece was placed in a furnace filled with a mixture of oxygen and nitrogen, and oxidized at 900 °C for 25 minutes to form a thin SiO2 layer over the entire Si surface. The SiO2 layer was 10 nm thick.
In the PL measurements, the samples were cooled to 4 K with a liquid helium cryostat. To create EHDs only in the top Si film, a pulsed ultraviolet laser light (Spectra Physics, Tsunami) with a spot diameter of 3 μm was focused on the center of the cavity using a 50x objective. The wavelength and pulse width of the laser light were 373 nm and 10 ps, respectively. The top Si film absorbs 93% of the incident light power at this wavelength [21]. The numerical aperture (NA) of the objective was 0.42. The same objective simultaneously collected the EHD emission from the samples, where the observation area was adjusted by a confocal setup to be the same as the area of the excitation spot. The emission was guided to a spectrometer attached to a liquid-nitrogen-cooled InGaAs detector array. A polarizer was placed in front of the spectrometer to detect the component of the electric field of the emission (Ey) coupling to the fundamental cavity mode as shown in Fig. 1(c). When we measured the spectra of the samples, the average incident power and the pulse repetition rate of the ultraviolet laser were 30 or 120 μW and 8 MHz, respectively. In PL lifetime measurements, the pulse train was selected by an acoustic modulator to reduce the repetition rate to 4 MHz because the EHD emissions from some samples had lifetimes longer than the repetition period of the laser pulse. The emission guided to the spectrometer was switched to a band-pass filter with a bandwidth of 1 nm to select a wavelength. After the filter, the emission was detected with a superconducting single photon detector (SSPD) immersed in liquid helium. The photon-counting signal from the SSPD was fed into a time-to-amplitude converter and the PL decay was recorded. The time resolution of the system was 0.14 ns.
3. Surface oxidation
First, we demonstrate the effect of surface oxidation on EHD emission from unpatterned SOI wafers. Figure 2 shows the PL spectra and PL decays for the SOI wafers with and without thermal oxidation of the top surface when the average excitation laser power is 120 μW. In Fig. 2(a), there are three broad emissions at wavelengths of 1105, 1147, and 1217 nm. As in previous studies, these emissions are assigned to EHD emissions mediated with transverse acoustic (TA), transverse and longitudinal optical (TO/LO) phonons, and two optical phonon emissions, respectively [22]. This study focused only on the EHD line accompanied by the TO/LO phonon emission because of its large intensity. Figure 2(a) shows that the EHD is formed in both the SOI wafers with and without the oxidation at the high excitation laser power and a low temperature. However, the surface oxidation definitely increases the intensity of the EHD emissions. The enhancement ratio is estimated to be 6 for the intensity integrated from 1130 to 1170 nm. The difference induced by the oxidation is also found in the PL decays of the EHD(TO/LO) emission at 1149 nm as shown in Fig. 2(b). The PL decay of the EHD emission for the oxidized sample is elongated as compared with the sample without oxidation. The exponential curve fittings show PL lifetimes of 146 and 37 ns for samples with and without oxidation, respectively. The surface oxidation increases the PL lifetime fourfold.
Fig. 2 (a) PL spectra for unpatterned SOI samples with and without surface oxide. The average excitation laser power is 120 μW. (b) Normalized PL decays of the EHD emission mediated with TO and LO phonon emission with and without surface oxide. The detection wavelength and bandwidth are 1149 nm and 1 nm, respectively. The integration time is 3600 s. The broken white lines are the single exponential lines fitted to the measured data.
The PL decay shows the sum of the radiative decay of the SE and nonradiative decays, which means ΓPL = ΓR + ΓNR where ΓPL, ΓR and ΓNR are the PL decay rate, SE rate and nonradiative decay rate, respectively. It is theoretically predicted that the SE rate of the EHD is less than one order of magnitude larger than that of free excitons due to the correlated electrons and holes in the condensate [23–25 ]. However, the PL decay of the EHD is often determined by nonradiative decays resulting from the surface recombination, Auger process, and thermal evaporation of e-h pairs because these nonradiative decay rates are of the order of μs−1, which is still two or three orders of magnitude larger than the SE rate that is of the order of ms−1 [26, 27 ]. In the case of ΓNR >> ΓR, the PL intensity (I PL) is proportional to the emission quantum efficiency (QE) defined by ΓR/(ΓR + ΓNR) = ΓR/ΓPL ΓR/ΓNR. Thus, the PL intensity in the samples should increase linearly as the PL lifetime increases (I PL 1/ΓPL). Figure 2(b) shows that the PL decay is exactly elongated after the surface oxidation. Since the oxidation does not change radiative recombination of the EHD in general, it can be assumed that the PL decay is determined by a nonradiative surface recombination in the samples. This consideration is supported by the fact that the PL intensity enhancement ratio of 6 agrees well with the PL lifetime elongation ratio of 4. This correlation between the PL intensity and its decay rate enhancements proves that the surface oxidation of the SOI samples increases the EHD emission intensity by reducing the nonradiative surface recombination at the top Si surface.
The surface recombination velocities (SRVs) are estimated from the PL decays with relations of for bare samples and for oxidized samples, where ΓA is the Auger recombination rate, t is the thickness of the Si films, vair and vox are the SRVs at Si-air and Si-SiO2 boundaries, respectively [28]. If it is assumed the Auger recombination lifetime of 150 ns (ΓA = 6.7 μs−1) [25], vair = 160 cm/s and vox = 0.7 cm/s are calculated from the two equations. The SRV at the Si-SiO2 boundary in our samples is close to that of oxidized Si substrates [28].
In the oxidized SOI samples, the EHD diffusion length is estimated to be 40 nm with an EHD lifetime of 146 ns, namely the PL lifetime obtained in Fig. 2(b). Since this diffusion length is sufficiently smaller than the observation spot diameter of 3 μm, the EHD stays in the observation area within its lifetime. This means that the nonradiative decay that originates from the diffusion of the EHD outside the observation area is negligible.
We oxidized the surfaces of samples with PhC structures. Figure 3 shows PL spectra and PL decays for Si PhC cavities with and without surface oxide when the excitation laser power is 120 μW. The broadband emission found in Fig. 3(a) can be assigned to the EHD emission mediated with TO/LO phonon emission while the peak wavelength of the EHD is shifted to 1165 nm as compared with that for the SOI wafers shown in Fig. 2(a). The red shit originates from the local strain of the air-suspended PhC membrane and the change in the Fermi level of the condensate induced by the increase in the temperature. In addition, the PL decay is also changed by the sample heating resulting from laser excitation and a low thermal conductivity of the PhC membrane. The thermal evaporation of e-h pairs from EHD surfaces can be observed as a non-exponential decay of the PL as shown in Fig. 3(b) [29].
Fig. 3 (a) PL spectra for PhC cavities with and without surface oxide on a logarithmic scale. The cavity resonance was adjusted to the peak of the EHD emission by changing the lattice period of the air-hole arrays. The lattice periods are 294 nm and 301 nm, respectively, for the samples without and with surface oxide. The average excitation laser power is 120 μW. The arrows indicate the detection wavelengths for time-resolved measurements. (b) Normalized PL decays at a wavelength far from the cavity resonance with a lattice period of 298 nm. The detection wavelength is 1150 nm for the bare sample and 1160 nm for the oxidized sample. The integration time is 3600 s. The bandwidth is 1 nm. These show the EHD emissions from the off-resonant PhC structure surrounding the PhC cavity. The PL lifetime is defined as the time it takes for the normalized PL intensity to reach e −1.
The sharp peaks around 1160 nm show the cavity-enhanced emissions from PhC cavities. The surface oxidation shifts the cavity resonance to a shorter wavelength because the effective refractive index of the sample is reduced by the transformation from Si to SiO2. Thus, after the surface oxidation we adjusted the cavity resonance to near the center wavelength of the broad EHD emission by changing the lattice period to maximize the intensity of the cavity-enhanced emission. In Fig. 3(a), the broad EHD emission from the PhC sample without surface oxide is very weak although the bare SOI sample shows EHD emissions at a same excitation laser power as seen in Fig. 2(a). However, the surface oxidation of the PhC sample greatly increases the intensity of the EHD emission. Even at an excitation laser power of 15 μW, the broad EHD emission is clearly observed for the oxidized PhC sample. Simultaneously, the oxidation increases the peak intensity of the emission from the resonant cavity forty-fold. As compared with a previous study, this peak intensity enhancement ratio is larger than ~7 for the Si PhC cavities at room temperature although the dephasing time of the emitters is different [6]. In addition, Fig. 3(a) shows that the cavity enhancement ratio stays constant regardless the surface oxide, which means that the coupling strength between the EHD and the cavity is unchanged by the surface oxide. The cavity-enhanced emission is discussed in more detail later.
The PL decays of the emission far from the cavity resonance, i.e. out of resonance, are shown in Fig. 3(b). The PL from the PhC without surface oxide shows a rapid decay with a lifetime of 0.7 ns. In contrast, the PL lifetime in the oxidized sample is 22 ns. The elongation ratio of the PL lifetime is approximately 30. The enhancement of the PL intensity and simultaneous PL lifetime elongation show that oxidation is effective in enhancing the EHD emission from the PhC samples by reducing the surface nonradiative recombination. In addition, the elongation ratio of the PL lifetime of 30 is 8 times larger than that for the SOI samples. This larger lifetime ratio of the PhC samples suggests that the reduction in the surface recombination of the PhC structure by the oxidation is more significant than that for the SOI samples because the PhC structure has many Si surfaces at the side walls of the air holes, which contribute to the dominant nonradiative recombination of the EHD.
In both the SOI and PhC samples, we found that the PL decay of the EHD is determined by nonradiative surface recombination while the diffusion of the massive EHD is suppressed. This result might be attributed to the large diameter of the EHD that is approximately 1 μm [10]. Such large droplets touch the surfaces in the nanometer-sized Si structures, and then the e-h pairs in the droplets recombine at surface trap centers. However, the thin surface SiO2 formed by thermal oxidation greatly reduces the surface recombination and increases the PL intensity of the EHD. This oxidation process plays an important role in obtaining the long-lived EHD in the Si PhC nanostructures and its intense emission with a high emission QE.
4. Cavity-enhanced emission
The EHD emission from oxidized PhC samples is sufficiently intense to perform time-integrated and time-resolved PL measurements with a high signal-to-noise ratio for a discussion of the Purcell effect induced by the PhC cavities. In fact, we could not discuss the precise cavity enhancement of the EHD emission intensity for PhC cavities without the surface oxide because the EHD emission intensity at an off-resonant state is close to a noise level as seen in Fig. 2(a).
Figure 4(a) shows the PL spectrum for another oxidized PhC cavity sample with a lattice period of 298 nm when the excitation laser power is 30 μW that is twice higher than the lower limit to observe the EHD emission. The sharp PL peak at 1162 nm comes from the EHD coupled with the PhC cavity. The resonant wavelength of the cavity was adjusted to the center wavelength of the broad EHD emission to increase the PL intensity of the cavity-enhanced EHD emission. The experimental Q value (Qt) of the cavity is estimated to be 22,000 from the linewidth of the cavity emission and the spectrometer resolution. The Qt value is almost half the simulated Q value of Qv = 45,000. As the excitation laser power increased from 30 to 120 μW, the Qt value decreased and the cavity resonance shifted to a shorter wavelength. These phenomena suggest that the difference between the simulated and experimental Q values results from the free-carrier absorption of the cavity-confined light [15]. The integrated intensity of the cavity-enhanced sharp EHD emission (Ic) is 9 times larger than that of the background EHD emission (Ip) from the surrounding PhC structure at an off-resonant wavelength. Figure 4(b) shows the EHD emission decays at the resonant wavelength of the cavity and at −2 nm lower than the cavity resonance to allow us to compare the EHD emissions from the resonant cavity and surrounding PhC structure. As shown in Fig. 4(c), the spectral filter for the sharp emission at the cavity resonance corresponds to the spatial filter extracting the emission from only the cavity although the excitation and observation areas are larger than the cavity. This is reasonable because the sharp PL spectrum comes from only the cavity-enhanced emission of the EHD remaining inside the PhC cavity. The off-resonant EHD emission is assumed to be the EHD emission from the surrounding PhC structure. Note that both PL decays have almost same lifetime of 23 ns and no cavity effect on the PL decay is observed. This means that the EHD diffusion from the small PhC cavity to the surrounding PhC structure, which is observed as the PL lifetime reduction, is negligible. In addition, the cavity-enhanced SE rate caused by the Purcell effect is not observed in the time-resolved PL measurement. Both in the cavity and the PhC structure, the nonradiative decay is still dominant in the PL decays. Thus, a discussion of the SE rate enhancement for the EHD is impossible with the time-resolved PL data alone. A precise Purcell enhancement in the SE rate is estimated from cavity-enhanced PL decays that are proportional to cavity Q values if the SE enhanced by the cavity is faster than any nonradiative processes as seen in an impurity-doped Si [17].
Fig. 4 (a) PL spectrum of the PhC cavity with a lattice period of 298 nm. This lattice period is different from that shown in Fig. 3(a). The cavity resonance is located at the peak of the EHD emission. The average excitation laser power is 30 μW. The two shaded areas show the spectral window for the PL decay measurements. (b) PL decays of the EHD emission at the cavity resonance (1162.1 nm) and at an out-of-resonance wavelength (1160.1 nm). The integration times for on- and off-resonant emissions are 120 and 3600 s, respectively. The bandwidth is 1 nm. (c) Excitation and detection areas on a PhC cavity sample. The spectral filter for a cavity-enhanced PL enables to detect only the emission from a cavity area. The surrounding PhC area is determined by the excitation spot. (d) Simulated cross-sectional maps of the normalized electric field intensity for the cavity (upper) and PhC slab (lower) in a logarithmic scale. The detectable portions are also displayed.
In this study, to estimate the Purcell enhancement factor in the SE rate of the EHD, both PL intensities and its decays data are used. Here, note that the enhancement factor calculated from PL intensities still includes a potential large uncertainty because the measured PL intensities are variable by a small misalignment of the objective. However, the time-resolved PL data obtained in our experiments improves the precision of the estimated enhancement factor as compared with the studies discussing the Purcell enhancement only with PL intensities. The relation between the integrated PL intensity and its decay rate is described by
where η, V, and ζ are the emission QE defined by ΓR/(ΓR + ΓNR), emission volume, and the detection efficiency with our objective, respectively. The suffixes, c and p indicate the resonant cavity and surrounding off-resonant PhC structure, respectively. Vc is equal to the modal volume of the PhC cavity, which is 0.03 μm3, and Vp is defined by the excitation-detection spot on the PhC structure and estimated to be approximately 1.1 μm3. Vc/Vp is the excitation efficiency for the cavity because the excitation-detection spot size is larger than the cavity size. Qt/Qv is a fraction of the cavity emission extracted to out of plane. From the FDTD simulation, we calculated the detection efficiencies of our objective to be approximately 0.1 for both the resonant cavity and the off-resonant PhC. Figure 4(d) shows simulated cross-sectional maps of the electric field intensity for the cavity and PhC slab. The objective detects the vertically-traveling emission from the samples, which is not confined by total internal reflection in the PhC slab. The side-lobe emission from the cavity mode is not detected by the NA of our objective. Since the PL decays shown in Fig. 4(b) tell us that the experimental nonradiative lifetime of the EHD is 23 ns and remains unchanged whether the PhC cavity resonates, the QE enhancement ratio is simplified to . Thus, Eq. (1) is modified to . In our sample, Qt/Qv = 0.5, Vc/Vp = 0.03, ζc/ζp ~1 and Ic/Ip = 9 are provided by the PL measurements and FDTD simulations, and then ~600 is derived. This SE rate enhancement includes the Purcell enhancement of the PhC cavity and the SE suppression by the PBG of the PhC structure. If we assume that the SE suppression factor of the PBG is approximately 0.1 [5], the Purcell enhancement factor of the PhC cavity is approximately estimated to be 60. The Purcell enhancement factor may depend on 1/Vc, not Qt/Vc, when the spectral linewidth of the photon emitter is larger than the cavity linewidth [30, 31 ]. Since the linewidth of the EHD is larger than the cavity linewidth, we speculate that the Purcell factor estimated here is attributed to the small Vc of the PhC cavity.It is also considerable that the spatial and spectral redistributions of the EHD population take place at the PhC nanocavity due to the hole burning induced by the cavity-enhanced SE of the EHD [32]. However, since we found that the enhanced SE rate of less than 1 μs−1 is still two orders of magnitude smaller than the nonradiative recombination rate of 43 μs−1 (23 ns), the EHD populations inside and outside the nanocavity are uniformly decayed by the dominant nonradiative recombination, and then there is no hole burning and no redistributions of the EHDs.
5. Summary
We have demonstrated the intense Si emission from the surface-oxidized Si PhC nanocavities coupled with a massive EHD. The thermal oxidation of the PhC samples increases the EHD emission intensity because the surface oxide greatly reduces the nonradiative surface recombination of the EHD in the air-hole arrays of the PhC structures. The enhanced EHD emission enables us to perform PL measurements with a high signal-to-noise ratio and discuss the Purcell effect of the PhC nanocavities. The estimated change in the SE rate by on- and off-resonant nanocavities shows that the PhC nanocavity concentrates the broad EHD emission into a sharp spectral line as a result of SE enhancement at the cavity resonance and SE suppression at an out-of-resonance wavelength. In addition, it is found that the EHD remains in the PhC nanocavity because no significant diffusion of the EHD from the nanocavity to outside is observed in the time-resolved PL measurements.
In this study, the PhC samples were excited by the pulsed laser. Assuming that a continuous wave laser is used for the excitation, the oxidized PhC might be able to accumulate EHDs because of less nonradiative recombination, and it is speculated that the surface oxidation shows a larger emission enhancement than that under the pulse excitation. Although we need sample cooling by liquid helium to observe the EHD emission, our results show the importance of the suppression of both nonradiative surface recombination and diffusion of e-h pairs, which is a common issue to achieve efficient emission from Si devices operating at any temperature. In addition, the cavity quantum electrodynamics of e-h condensates will expand the study for controlling the kinetics of e-h condensates with photonic and mechanical interactions in semiconductor nanostructures.
Acknowledgments
We thank Dr. K. Nishiguchi for performing the sample oxidation and for fruitful discussions, and Dr. T. Tamamura and D. Takagi for their help with sample fabrication. This work was part of a research project supported by the National Institute of Information and Communications Technology (NICT-QAT113u02) in Japan.
References and links
1. S. Noda, M. Fujita, and T. Asano, “Spontaneous-emission control by photonic crystals and nanocavities,” Nat. Photonics 1(8), 449–458 (2007). [CrossRef]
2. M. S. Tame, K. R. McEnery, S. K. Özdemir, J. Lee, S. A. Maier, and M. S. Kim, “Quantum plasmonics,” Nat. Phys. 9(6), 329–340 (2013). [CrossRef]
3. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).
4. M. Fujita, S. Takahashi, Y. Tanaka, T. Asano, and S. Noda, “Simultaneous inhibition and redistribution of spontaneous light emission in photonic crystals,” Science 308(5726), 1296–1298 (2005). [CrossRef] [PubMed]
5. B. Julsgaard, J. Johansen, S. Stobbe, T. Stolberg-Rohr, T. Sünner, M. Kamp, A. Forchel, and P. Lodahl, “Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes,” Appl. Phys. Lett. 93(9), 094102 (2008). [CrossRef]
6. S. Iwamoto, Y. Arakawa, and A. Gomyo, “Observation of enhanced photoluminescence from silicon photonic crystal nanocavity at room temperature,” Appl. Phys. Lett. 91(21), 211104 (2007). [CrossRef]
7. M. Fujita, Y. Tanaka, and S. Noda, “Light emission from silicon in photonic crystal nanocavity,” IEEE J. Sel. Top. Quantum Electron. 14(4), 1090–1097 (2008). [CrossRef]
8. N. Hauke, T. Zabel, K. Müller, M. Kaniber, A. Laucht, D. Bougeard, G. Abstreiter, J. J. Finley, and Y. Arakawa, “Enhanced photoluminescence emission from two-dimensional silicon photonic crystal nanocavities,” New J. Phys. 12(5), 053005 (2010). [CrossRef]
9. M. Fujita, B. Gelloz, N. Koshida, and S. Noda, “Reduction in surface recombination and enhancement of light emission in silicon photonic crystals treated by high-pressure water-vapor annealing,” Appl. Phys. Lett. 97(12), 121111 (2010). [CrossRef]
10. M. Capizzi, M. Voos, C. Benoit à la Guillaume, and J. C. McGroddy, “Electron-hole drops in silicon,” Solid State Commun. 16(6), 709–712 (1975). [CrossRef]
11. M. A. Tamor and J. P. Wolfe, “Electron-hole droplet transport up to near-sonic velocity in Si,” Phys. Rev. B 26(10), 5743–5755 (1982). [CrossRef]
12. M. A. Tamor and J. P. Wolfe, “Drift and diffusion of free excitons in Si,” Phys. Rev. Lett. 44(25), 1703–1706 (1980). [CrossRef]
13. W. Schmid, “Formation and decay of electron-hole drops in Si,” Solid State Commun. 19(4), 347–349 (1976). [CrossRef]
14. L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, and F. Priolo, “Optical gain in silicon nanocrystals,” Nature 408(6811), 440–444 (2000). [CrossRef] [PubMed]
15. S. G. Cloutier, P. A. Kossyrev, and J. Xu, “Optical gain and stimulated emission in periodic nanopatterned crystalline silicon,” Nat. Mater. 4(12), 887–891 (2005). [CrossRef] [PubMed]
16. R. Lo Savio, S. L. Portalupi, D. Gerace, A. Shakoor, T. F. Krauss, L. O’Faolain, L. C. Andreani, and M. Galli, “Room-temperature emission at telecom wavelengths from silicon photonic crystal nanocavities,” Appl. Phys. Lett. 98(20), 201106 (2011). [CrossRef]
17. H. Sumikura, E. Kuramochi, H. Taniyama, and M. Notomi, “Ultrafast spontaneous emission of copper-doped silicon enhanced by an optical nanocavity,” Sci. Rep. 4, 5040 (2014). [CrossRef] [PubMed]
18. J. Shah, M. Combescot, and A. H. Dayem, “Investigation of exciton-plasma Mott transition in Si,” Phys. Rev. Lett. 38(25), 1497–1500 (1977). [CrossRef]
19. S. Ibuka and M. Tajima, “Temporal decay measurement of condensate luminescence and its application to characterization of silicon-on-insulator wafers,” J. Appl. Phys. 91(8), 5035 (2002). [CrossRef]
20. Y. Akahane, T. Asano, B.-S. Song, and S. Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal,” Nature 425(6961), 944–947 (2003). [CrossRef] [PubMed]
21. M. Tajima, S. Ibuka, and S. Arai, “Condensate luminescence under ultraviolet excitation: application to the study of ultrathin SOI layers,” Mater. Sci. Eng. B 91–92, 10–15 (2002). [CrossRef]
22. M. Tajima, S. Ibuka, H. Aga, and T. Abe, “Characterization of bond and etch-back silicon-on-insulator wafers by photoluminescence under ultraviolet excitation,” Appl. Phys. Lett. 70(2), 231 (1997). [CrossRef]
23. C. Benoît à la Guillaume, M. Voos, and F. Salvan, “Condensation of free excitons into electron-hole drops in pure Germanium,” Phys. Rev. B 5(8), 3079–3087 (1972). [CrossRef]
24. J. C. Hensel, T. G. Phillips, and G. A. Thomas, “Electron-hole liquid in semiconductors: Experimental,” in Solid State Physics, Advances in Research and Applications, vol. 32, H. Ehrenreich, F. Seitz, and D. Turnbull, ed. (Academic, 1977).
25. P. L. Gourley and J. P. Wolfe, “Properties of the electron-hole liquid in Si: zero stress to the high-stress limit,” Phys. Rev. B 24(10), 5970–5998 (1981). [CrossRef]
26. J. D. Cuthbert, “Recombination kinetics of excitonic molecules and free excitons in intrinsic silicon,” Phys. Rev. B 1(4), 1552–1557 (1970). [CrossRef]
27. Y. Pokrovskii, “Condensation of non-equilibrium charge carriers in semiconductors,” Phys. Status Solidi. A 11(2), 385–410 (1972). [CrossRef]
28. M. J. Kerr and A. Cuevas, “Very low bulk and surface recombination in oxidized silicon wafers,” Semicond. Sci. Technol. 17(1), 35–38 (2002). [CrossRef]
29. C. D. Jeffries, “Electron-hole condensation in semiconductors: Electrons and holes condense into freely moving liquid metallic droplets, a plasma phase with novel properties,” Science 189(4207), 955–964 (1975). [CrossRef] [PubMed]
30. H. Y. Ryu and M. Notomi, “Enhancement of spontaneous emission from the resonant modes of a photonic crystal slab single-defect cavity,” Opt. Lett. 28(23), 2390–2392 (2003). [CrossRef] [PubMed]
31. S. Nakayama, S. Ishida, S. Iwamoto, and Y. Arakawa, “Effect of cavity mode volume on photoluminescence from silicon photonic crystal nanocavities,” Appl. Phys. Lett. 98(17), 171102 (2011). [CrossRef]
32. T. Baba, D. Sano, K. Nozaki, K. Inoshita, Y. Kuroki, and F. Koyama, “Observation of fast spontaneous emission decay in GaInAsP photonic crystal point defect nanocavity at room temperature,” Appl. Phys. Lett. 85(18), 3989 (2004). [CrossRef]
