Open Access
Add to BibSonomyAbstract
A silicon light emitter in telecom-band based on a single germanium quantum dot precisely embedded in a silicon photonic crystal nanocavity is fabricated by a scalable method. A sharp resonant luminescence peak is observed at 1498.8 nm, which is enhanced by more than three orders of magnitude. The Purcell factor for the fundamental resonant mode is estimated from enhancement factor and increased collection efficiency. The cavity modes coupled to the ground state and excited state emission of germanium quantum dot are identified in the luminescence spectrum. Our devices provide a CMOS-compatible way of developing silicon-based low-power consuming light emitters, and are promising for realizing on-chip single photon sources.
© 2015 Optical Society of America
1. Introduction
The coupling of photons and matter [1] in semiconductor nanocavities has been a platform for cavity quantum electrodynamics [2–5 ] (QED) research. A monolithically integrated cavity QED system, consisting of a self-assembled quantum dot (QD) embedded in a nanocavity, could fix the emitter location with respect to the cavity mode electric filed maximum and significantly enhance the emitter-cavity coupling by optical resonance in the cavity. A number of experiments have demonstrated the potential of QD-based solid-state cavity QED in applications such as single photon sources [6, 7 ], on demand polarization–entangled photons source [8], low threshold laser oscillation [4, 9, 10 ] and strong coupling effect such as vacuum Rabi splitting [11, 12 ]. While cavity QED applications have been performed using various material systems [6, 13 ], Single quantum dot (SQD)-cavity systems on the silicon-on-insulator (SOI) platform have not, to our knowledge, been reported. SOI chip have a number of advantages, such as advanced fabrication technology for scalability, low-cost and high density integration, low transmission loss in telecom-band, and on-chip integration of electronics. SOI chips are also used for applications such as optical interconnections for high-performance computing system [14, 15 ]. The realization of QD-cavity systems on the SOI platform will pave the way to scalable quantum information applications.
Germanium (Ge) QDs are a promising candidate due to their compatibility with silicon (Si) technology and ability of emitting light in the telecom-band. Self-assembled Ge QDs can emit light between 1.3 and 1.6 μm in the telecom-band, but have poor spectral purity and low luminescence intensity. Various approaches have been introduced to enhance the Ge QD’s light emission efficiency and select the emission wavelength [16–19 ]. Recently, R. Jannesari et al. implemented on the SOI photonic crystal slabs with commensurately embedded Ge QD emitters for near-infrared light emission [20]. Enhanced photoluminescence (PL) output is observed due to the coupling of the QD emitters to leaky modes of the photonic crystal slabs. Our group reported a strong telecom-band light emission from multilayer Ge QDs embedded in modified photonic crystal (PhC) L3 cavities with an estimated Purcell factor of 6.7 [21]. Since the locations of self-assembled QDs grown on flat substrate are random, the difficulty of achieving site-controlled SQD in optical nanocavity has partially hindered the research of SQD-cavity systems. Progress have been achieved in the growth of site-controlled low density Ge SQDs [22]. Our group has recently demonstrated the growth of self-assembled Ge SQDs array with an ultra-low density on nanohole-patterned Si substrate via molecular beam epitaxy (MBE) [23, 24 ].
In this paper, we demonstrate a process to locate the position of buried Ge SQD at the cavity mode electric filed maximum: the position of SQDs is predetermined by growing in an array of nanoholes that is aligned to a set of markers, allowing precise overlay between optical cavities and the SQDs array. This scalable fabrication method is highly suitable for large-volume fabrication of Si nanocavities with precisely embedded Ge SQDs. Strong resonant luminescence at telecom wavelengths from Ge SQD embedded in a modified PhC L3 cavity is observed up to room temperature. The enhancement factor of fundamental resonant mode is ~1300, with an estimated Purcell factor ~60. The achieved high luminescence enhancement is owing to the good spatial and spectral overlap between the SQD and the PhC cavity. In addition, different recombination mechanisms of excited carriers through ground and excited states in Ge SQD are identified by the resonant luminescence spectra.
2. Device fabrication and characterization setup
The fabrication processes of our devices are as follows. As shown in Fig. 1(a) , alignment markers were etched into SOI substrate consisting of a 70 nm thick Si (001) layer on a 3 μm thick buried oxide (BOX). The metal mask (e.g., Al mask) of alignment markers were formed by UV lithography, subsequent metal deposition and lift off. Anisotropic etching through the BOX was carried out in an Oxford 100 ICP plasma etcher using SF6/C4F8 and O2/C4F8 mixture for Si and SiO2 etching, respectively. The inset of Fig. 1(a) shows the optical microscope image of alignment marker after the ICP etching and chemical removal of metal mask. In Fig. 1(b), ordered nanohole patterns were defined by E-beam lithography (Vistec EBPG 5000 Plus) using the ZEP 520A resist. These patterns were then transferred to top Si by reactive ion etching (RIE) using O2 and CF4 gases. The nanoholes are periodically aligned along two orthogonal Si <110> directions with a period of 2 μm, and show cylindrical shape after RIE etching (inset of Fig. 1(b)). The mean width and depth of the nanoholes are 50 and 25 nm, respectively. The templates were cleaned by the standard RCA method and passivated with H by dip in dilute HF solution before loading into a solid source MBE chamber (Riber Eva-32). The growth procedure (Fig. 1(c)) of the Ge QDs consists in turn of a thermal degasing at 820 °C, a 40 nm Si buffer layer growth, and a 3.6 monolayer of Ge deposition. The substrate temperature was kept at 450 °C during the buffer layer growth and was ramped from 450 to 550 °C during the deposition of Ge. The samples were then capped with a 130 nm thick Si layer at 450 °C. The inset of Fig. 1(c) shows a three-dimensional (3D) AFM micrograph of an uncapped Ge SQD. Finally, the PhC L3 cavity patterns were defined on ZEP 520A resist relative to alignment markers. These patterns were etched down to the BOX using anisotropic ICP process, as shown in Fig. 1(d). In order to increase the symmetry of the cavity structure and strengthen optical confinement along the surface normal direction, the BOX layer was removed by dipping in a dilute HF solution. The schematic structure of the finished device is shown in Fig. 1(e).
Fig. 1 Process flow for the fabrication of PhC cavitiy with commensurably embedded Ge SQD. (a-d) Schematic structures of the sample during the fabrication process are shown. The inset of (a) shows the optical microscope image of alignment marker on SOI substrate. The inset of (b-c) shows the AFM image of a nanohole and a Ge SQD. The inset of (d) shows a SEM image of two PhC cavities along with four alignment markers. (e) The schematic structure of the device. The red dot represents Ge SQD in the top Si/Ge layer.
The micro-photoluminescence (μ-PL) measurements were performed in a temperature-controlled liquid-helium cryostat with internal x–y nanopositioners, essential for stability and the ability to re-find a given nanocavity. The sample was excited with a 532 nm diode laser with variable pumping intensity using neutral density filters, which was focused to a 2-μm spot on the center of the PhC cavity by a high NA microscope objective (NA = 0.85). The μ-PL signal was collected by the same objective, then dispersed by a monochromator with a 500 mm focus length and recorded by a liquid-nitrogen-cooled InGaAs detector array.
3. Results and discussion
Figure 2(a) shows the surface morphology of the site-controlled Ge SQDs grown on the nanohole patterned SOI substrate. The average size of Ge QDs is about 97 nm in width and 6.7 nm in height. A scanning electron microscope (SEM) image of the fabricated PhC L3 cavity with embedded Ge SQD is shown in Fig. 2(b). The lattice constant a is 395 nm and the radius of air holes r is 0.30a. The positions of the three holes adjacent to the cavity are optimized to obtain high quality factor (Q factor) [25]. Radius of the holes around the cavity, which is denoted in Fig. 2(b), is enlarged by a quantity of Δr = + 0.03a to optimize the far fields of the cavity modes and obtain stronger vertical radiation [26, 27 ]. The small white dot in the center membrane of L3 cavity shows the location of the buried Ge SQD.
Fig. 2 (a) Atomic force microscope (AFM) micrograph of the Ge SQDs grown on the nanohole patterns with a period of 2 μm. The dotted PhC cavity pattern schematically shows the relative position between cavity center and SQD. (b) The scanning electron microscope (SEM) image of fabricated PhC L3 cavity with embedded Ge SQD. The three holes adjacent to the cavity are laterally shifted by 0.2 a, 0.025 a, 0.2 a, respectively, shown with orange arrows. Radius of the holes around the cavity, which are labeled by the red ovals, is enlarged by a quantity of Δr = + 0.03a to optimize the far fields of the cavity modes for stronger vertical radiation. (c) The simulated electric field intensity profile (|E|2) at the plane of z = 0 (the center of the membrane) for the fundamental mode of the L3 cavity. (d) The simulated far-field pattern (electric filed intensity profile, |E|2) for the fundamental mode of the L3 cavity. White concentric circles correspond to θ = 30°, 45°, 60°, 90° from the inner one to the outer one, respectively. (e) The statistical distribution of alignment error between SQD and the cavity center. The x axis represents alignment error between SQDs and the target position, and the y axis shows the number of devices within a certain misalignment range. (f) The μ-PL spectrum for the unprocessed Ge SQD measured at 7 K with an excitation power of 460 μW at λ = 532 nm. The QD and wetting layer (WL) related emission is indicated in the figure. Si related peaks are observed at approximately 1088 nm (TA phonon), 1127 nm (TO phonon), and 1195 nm (TO + Γ phonon).
Three-dimensional finite-difference time-domain (3D-FDTD) simulation is carried out to calculate the fundamental cavity mode of L3 cavity with geometry parameters measured from the SEM image, shown in Fig. 2(c). Strong light field is confined within an small mode volume of V c~0.67(λ/n)3~0.0645 μm3, where λ is the resonance wavelength of light in vacuum. The Q factor of this design is ~20000. The simulated far-field pattern (electric filed intensity profile, |E|2) for the fundamental mode of the L3 cavity is shown in Fig. 2(d). It is concentrated along the vertical direction and will produce high collection efficiency for a limited objective numerical aperture (NA). Figure 2(e) shows the statistical distribution of alignment error between SQDs and the center position of PhC cavities. About 80 SQD-cavity systems are fabricated on one wafer and the alignment error is measured from high resolution SEM images (e.g., Fig. 2(b)). This off-center distribution can be fitted by Gaussian function with mean overlay error of 22 nm. The inhomogeneous sidewall deposition during MBE growth reduces the alignment precision a little bit. Note that we only consider the absolute alignment error between SQD and the cavity center without including the direction. Figure 2(f) shows the measured μ-PL spectrum for the unprocessed Ge SQD at T PL = 7 K. There is no clear QD PL peak in the spectral range [22], indicating that the PL signal of Ge SQDs is too weak to be identified. The weak signal of unprocessed Ge SQD is mainly caused by it low quantum efficiency.
As seen in Fig. 3 , black and red curves represent the μ-PL spectrum of in the L3 cavity and Si/Ge membrane with unprocessed Ge SQD array, respectively. Note that the PL spectrum of unprocessed region is multiplied by a factor of 25 for better viewing. Several sharp resonant peaks are observed to dominate the spectrum over an almost flat and weak background emission in the μ-PL spectrum. These resonant peaks are identified as the PhC cavity modes, corresponding to the four theoretically predicted cavity modes noted as M0 to M3, respectively [21]. The highest emission peak at 1,498.8 nm represents luminescence from the fundamental cavity mode (M0 mode). Compared with the unprocessed region, the PL intensity is significantly enhanced. The enhancement factor, which is defined as the ratio of the resonant peak intensity to the PL intensity of the unprocessed Si/Ge membrane at the peak wavelength, is ~1318. Figures 3(b) and 3(c) show the magnified graph of the PL spectrum for the emission peak M0 and M3 in Fig. 3(a), respectively. The full-width-half-maximum (FWHM) of peak M0 is fitted to be 349 pm using a Lorentz function. The corresponding Q factor is 4300. This experimental Q factor is reduced by the air holes’ fabrication errors and the free-carrier absorption of the photogenerated carriers. The peak M3 is located at 1334.1 nm with a FWHM of 1051 pm.
Fig. 3 (a) PL spectra of Ge SQD in a unprocessed membrane and a L3 nanocavity measured at T = 7 K with an excitation power of 460 μW. (b) and (c) show the magnified graph of the PL spectra for the emission peak labeled as “M0” and “M3” in (a), respectively.
The PL enhancement is attributed to the increased collection efficiency and the Purcell effect in the PhC cavity. So, we can estimate the value of the experimental Purcell factor using the following equation:
where γ is the experimentally measured PL enhancement factor, Fp is the Purcell factor, while η cavity and η membrane are the light collection efficiencies from the resonance cavity and un-patterned Si membrane, respectively.The collection efficiency of unprocessed Si/Ge membrane can be given by η membrane = 1-cos[sin−1(NA/n)]~4.51%, where n~2.863 is the effective refractive index of Si/Ge membrane and NA = 0.85 is the numerical aperture of the objective. We calculate the collection efficiency (η cavity) into the objective lens placed above the photonic crystal membrane by simulating the far-field emission patterns of the resonant modes with the aid of a standard near-to-far-field projection [28], shown in Fig. 2(d). The calculated collection efficiency η cavity for the M0 mode is 90.5% when the NA = 0.85. The enhancement of collection efficiency is given by η cavity/η membrane~20. Then, the estimated Purcell factor for M0 mode is Fp = 1318/20 = 65.9. Compared to the previously reported L3 nanocavity embedded with dense Ge QDs [21], our SQD sample achieved an estimated Purcell factor about 10 times higher than the dense QDs sample.
The Purcell factor Fp which indicates the enhancement ratio of spontaneous emission rate of a emitter coupling with an optical cavity, can be expressed by the following equation [29] for a single emitter with dipole moment 'd':
where 1/Qeff = 1/Qc + 1/Qe is the effective quality factor, taking into account the cavity Q-factor as well as the finite linewidth of the emitters. λc is the wavelength of the resonant mode, Vc is the mode volume. The subscripts c and e stand for cavity and emitter, respectively. The first term in Eq. (2) shows that nanocavity with a very high Q-factor and a small mode volume is the precondition for achieving high Purcell factor. In order to observe the Purcell effect in its full magnitude, the emitter should be well matched with the frequency, spatial distribution and polarization of the mode. The second term in Eq. (2) refers to the degree of spatial overlap between the emitter and cavity mode. Because the electric field of x-polarized M0 mode at cavity center is very weak, only y-polarized dipole could have strong interaction with y-polarized M0 mode. Here we present a simulated “spatial map” of the normalized Purcell factor for y-polarized dipole as a function of QD position in a L3 cavity (considering the M0 mode). As can be seen in Fig. 4(a) and 4(b) , if the dipole is placed away from the antinode position, the Purcell factor would reduce substantially as it scales with the field strength squared. If the overlay misalignment of y-polarized dipole emitter is less than 90nm, normalized Purcell factor will obtain its half maximum.Fig. 4 (a) Normalized Purcell factor as a function of QD position on the z = 0 plane for the M0 mode. (b) Normalized Purcell factor along the white dot lines in (a) for both x & y directions.
For the dense QDs devices, the spatial positions of QDs in the cavity region are random [30]. The chance of finding a QD lay in the cavity mode electric filed maximum is rather low. In contrast, for the site-controlled SQD devices, the position of the SQD is precisely located at the cavity center. If the SQD’s diploe orientation is parallel with Ey component of the fundamental mode, the Purcell effect will achieve its full magnitude.
As shown in Fig. 2(f), the μ-PL peak of Ge SQDs is too weak to be clearly identified. However, based on the third term in Eq. (2), the Purcell factor will reach its maximum when the cavity mode wavelength meets the emitter’s PL peak position, resulting a maximum PL enhancement. Figure 5(a) shows the PL enhancement factor of M0 mode from L3 cavities with different lattice constants (a = 378~420 nm). Some devices show a higher enhancement factor than others, indicating one emission peak of Ge SQDs is in the range of 1498~1518 nm. Figure 5(b) shows the PL enhancement factor of M3 mode from a series of L3 cavities. Some devices show a higher enhancement factor than others, which means another peak position of Ge SQDs is in the range of 1338~1360 nm. The energy difference between the two peaks is about 92 meV. P. Boucaud et al. has reported that the first excited state of Ge QDs in the z direction (h001) is calculated at 82 meV above the ground state [31]. Therefore, we suggest that the M0 mode light emission correspond to coupling between the cavity fundamental mode and light emission through ground state of QD and the M3 mode light emission is attributed to the coupling between the M3 cavity mode and the light emission through the excited state of the holes.
Fig. 5 (a) The PL enhancement factor of M0 mode from L3 cavities with different lattice constants (a = 378~420 nm). (b) The PL enhancement factor of M3 mode from a series of L3 cavities.
Figures 6(a) and 6(b) show the temperature dependence for the M0 and M3 modes of L3 cavity with lattice constant a = 395 nm. Strong resonant luminescence peaks are observed up to room temperature. The red shift of PL peaks is caused by the increase of refractive index when temperature increases. The peak intensity of M0 mode increases as the T PL increases and reaches to a maximum when T PL = 160 K. We roughly calculate the emitted optical power to be about 1.87 pW based on the integrated counts (22910 per second) at 160 K. Similar trend is also applied for M3 mode, but the peak intensity of M3 mode reaches to a maximum at a lower temperature (T PL = 49.5 K). Strong intensity roll off is observed for T PL>160 K (M0 mode) and T PL>49.5 K (M3 mode), which can be fitted by an activation energy, Ea, characterizing the barrier against thermalization of excitons from QD structure into the outer wetting layer or Si matrix [32]. The integrated PL intensities of M0 and M3 modes as the function of measurement temperatures are shown in Fig. 6(c). The activation energy of M0 and M3 modes are fitted to be 151 and 83 meV, respectively. It is worth noting that the activation energy difference (68 meV) between M0 and M3 modes is close to the reported energy difference between ground state and the first exited state (82 meV).
Fig. 6 (a-b) show measured PL peaks of M0 and M3 modes at different temperatures. Strong resonant luminescence peaks are observed up to room temperature. The excitation power is 460 μW. (c) The integrated PL intensity of M0 & M3 modes as the function of T PL. The solid lines are the fits of experimental data. (d) The schematic picture of the type-II energy band lineup in the Si/Ge QD system along
Figures 6(d) shows schematic picture of the type-II energy band lineup in the Si/Ge QD system along the growth direction. Interband optical transitions are classifed as four kinds of recombination processes. As illustrated in Fig. 6(d), there are spatially indirect across the Si/Ge interface [33, 34 ], spatially direct transitions inside the Ge QD through both the ground state and the exited state of confined holes [31]. When the temperature increases, holes and electrons which are previously trapped in the wetting layer at low temperature could be thermally activated to move to the QD. Thus more holes will be trapped in the QD and attract more electrons near the QD, resulting in an increase of PL intensity. On the other hand, a redistribution of the emission from the spatially indirect band to the spatially direct band will happen due to the increase of the temperature. Since the energy band lineup is of type-II, the dominating optical transition at low temperature is in the spatially indirect transition across the interface. Raising the sample temperature increase the probability for electrons to populate the higher energy level (Δ(2)) inside the Ge QD, which opens the possibility for the spatial direct transition [33]. When the electron is inside the QD, the overlap of electron and hole wave function is increased since the electron and hole are located in the same volume, resulting in higher oscillator strength and higher PL intensity. When the temperature goes even higher (>160K for M0 mode, >50K for M3 mode), the thermalization of excitons causes less holes participating the QD’s interband optical transition, leading to the significant dropping of PL intensity.
Figure 7(a) shows the power dependences of integrated PL intensities for M0 and M3 mode, respectively. The power dependence is fitted by I~Pm, where I is the integrated intensity and P the excitation power. For M0 mode, the integrated intensity shows a super-linear dependence (m = 1.18~1.39) at weak excitation power (P<92 μW) and a sub-linear power dependency (m = 0.3~0.35) on higher excitation powers (P>345 μW). We attribute the unusual super-linear dependence to the enhancement of QD’s spontaneous emission rate caused by Purcell effect and the possible stimulated emission in the Ge SQD due to the strong photon-QD interaction in the high-density localized carriers and photon in the nanocavity. The sublinear power dependence of PL intensity is due to a typical saturation effect of the type-II alignment. The ground state density in the SQD is limited, and the probability for nonradiative Auger recombination will increase when the number of electron-hole pairs in the dot is increased [35]. The direct competition of Auger recombination and radiative recombination leads to an exponent m<1 at high excitation [36]. For M3 mode, the integrated intensity shows a super-linear dependence (m = 1.16) at larger excitation power range (P<575 μW) and then saturates at even higher excitation power. At low excitation power, the intensity of M0 mode is larger than that of M3 mode, due to the fact that the recombination possibility of ground state is higher than that of excited state. When increasing excitation power, the ground state emission gradually saturates while the excited state emission keep its super-linear power dependence until P = 575 μW. This can be attributed to the state density of excited state is larger than that of ground state in Ge QDs [31]. Figure 7(b) shows the power dependence of linewidth for M0 mode. The linewidth is governed by the cavity property. As pumping power increasing, the linewidth of M0 mode increases due to the free-carrier absorption of the photogenerated carriers. The linewidthes show a higher increasing speed at larger excitation power range (P>100 μW), which is also due to the saturation effect of Ge SQD.
Fig. 7 (a) The power dependence of integrated PL intensity of M0 and M3 modes at 30 K. The solid lines are the fits of experimental data. (b) The power dependence of linewidth for M0 mode at 30 K.
4. Summary
In summary, we demonstrate the feasibility of controllable coupling of a Ge SQD with a PhC nanocavity on SOI substrate. The technique of Ge QD ordering and alignment is based on standard Si processes and can be applied in massive fabrication. The average misalignment between site-controlled Ge SQD and cavity center is about 22 nm. Strong resonant luminescence from Ge SQD embedded in PhC L3 cavity is observed up to room temperature. The strongest resonant luminescence peak is obtained at 1498.8 nm, with an enhancement factor of over 1300. A Purcell factor ~60 is estimated from the PL enhancement, which is about much higher than the estimated Purcell factor of the L3 cavity with Ge QDs growth by traditional self-assembled epitaxy. The theoretical analysis shows a pronounced influence of Purcell factor on the location of the QD emitter. We attribute the accurate spatial and spectral overlap between SQD and PhC cavity to be the main reason for the high Purcell enhancement. Four different kinds of transition processes are coexisting in Ge SQD, which consequently results in a broad band PL emission. The PL peak of M0 mode and M3 mode are assigned to be light emission from QD ground state and excited state recombination, respectively. The difference of activation energy and photon energy between M0 and M3 mode are close to the energy difference between ground state and excited state. Both M0 and M3 modes show super-linear power dependence at low excitation power. At high excitation, the direct competition of Auger recombination and radiative recombination leads to sub-linear power dependence. The dot ordering and alignment approach provides a scalable, low cost and CMOS compatible way to fabricate Si-based SQD-cavity systems, showing a promising candidate for Si-based non-classical devices. Further efforts are necessary to realize telecom-band Si single photon sources and Si-based integrated cavity QDE systems.
Acknowledgment
This work was partly supported by the Major State Basic Research Development Program of China (grant 2013CB632104, 2011CB925601, 2013CB933303 and 2012CB922103), and National Natural Science Foundation of China (grant 61177049 and 61335002). We thank all the engineers in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in device fabrication. We also acknowledge Xuetao Gan for discussions.
References and links
1. E. M. Purcell, “Spontaneous emission probabilities at radio frequencies,” Phys. Rev. 69, 681 (1946).
2. D. Englund, A. Faraon, I. Fushman, N. Stoltz, P. Petroff, and J. Vucković, “Controlling cavity reflectivity with a single quantum dot,” Nature 450(7171), 857–861 (2007). [CrossRef] [PubMed]
3. A. Faraon, I. Fushman, D. Englund, N. Stoltz, P. Petroff, and J. Vučković, “Coherent generation of non-classical light on a chip via photon-induced tunnelling and blockade,” Nat. Phys. 4(11), 859–863 (2008). [CrossRef]
4. M. Nomura, N. Kumagai, S. Iwamoto, Y. Ota, and Y. Arakawa, “Laser oscillation in a strongly coupled single-quantum-dot–nanocavity system,” Nat. Phys. 6(4), 279–283 (2010). [CrossRef]
5. D. Englund, B. Shields, K. Rivoire, F. Hatami, J. Vučković, H. Park, and M. D. Lukin, “Deterministic coupling of a single nitrogen vacancy center to a photonic crystal cavity,” Nano Lett. 10(10), 3922–3926 (2010). [CrossRef] [PubMed]
6. A. J. Shields, “Semiconductor quantum light sources,” Nat. Photonics 1(4), 215–223 (2007). [CrossRef]
7. M. D. Birowosuto, H. Sumikura, S. Matsuo, H. Taniyama, P. J. van Veldhoven, R. Nötzel, and M. Notomi, “Fast Purcell-enhanced single photon source in 1,550-nm telecom band from a resonant quantum dot-cavity coupling,” Sci. Rep. 2, 321 (2012). [CrossRef] [PubMed]
8. A. Mohan, M. Felici, P. Gallo, B. Dwir, A. Rudra, J. Faist, and E. Kapon, “Polarization-entangled photons produced with high-symmetry site-controlled quantum dots,” Nat. Photonics 4(5), 302–306 (2010). [CrossRef]
9. S. Strauf, K. Hennessy, M. T. Rakher, Y.-S. Choi, A. Badolato, L. C. Andreani, E. L. Hu, P. M. Petroff, and D. Bouwmeester, “Self-tuned quantum dot gain in photonic crystal lasers,” Phys. Rev. Lett. 96(12), 127404 (2006). [CrossRef] [PubMed]
10. B. Ellis, M. A. Mayer, G. Shambat, T. Sarmiento, J. Harris, E. E. Haller, and J. Vučković, “Ultralow-threshold electrically pumped quantum-dot photonic-crystal nanocavity laser,” Nat. Photonics 5(5), 297–300 (2011). [CrossRef]
11. K. Hennessy, A. Badolato, M. Winger, D. Gerace, M. Atatüre, S. Gulde, S. Fält, E. L. Hu, and A. Imamoğlu, “Quantum nature of a strongly coupled single quantum dot-cavity system,” Nature 445(7130), 896–899 (2007). [CrossRef] [PubMed]
12. T. Yoshie, A. Scherer, J. Hendrickson, G. Khitrova, H. M. Gibbs, G. Rupper, C. Ell, O. B. Shchekin, and D. G. Deppe, “Vacuum Rabi splitting with a single quantum dot in a photonic crystal nanocavity,” Nature 432(7014), 200–203 (2004). [CrossRef] [PubMed]
13. S. Buckley, K. Rivoire, and J. Vučković, “Engineered quantum dot single-photon sources,” Rep. Prog. Phys. 75(12), 126503 (2012). [CrossRef] [PubMed]
14. J. Mower and D. Englund, “Efficient generation of single and entangled photons on a silicon photonic integrated chip,” Phys. Rev. A 84(5), 052326 (2011). [CrossRef]
15. G.-H. Duan, S. Olivier, S. Malhouitre, A. Accard, P. Kaspar, G. de Valicourt, G. Levaufre, N. Girard, A. Le Liepvre, A. Shen, D. Make, F. Lelarge, C. Jany, K. Ribaud, F. Mallecot, P. Charbonnier, H. Gariah, C. Kopp, and J.-L. Gentner, “New Advances on Heterogeneous Integration of III–V on Silicon,” J. Lightwave Technol. 33(5), 976–983 (2015). [CrossRef]
16. J. Xia, Y. Ikegami, Y. Shiraki, N. Usami, and Y. Nakata, “Strong resonant luminescence from Ge quantum dots in photonic crystal microcavity at room temperature,” Appl. Phys. Lett. 89(20), 201102 (2006). [CrossRef]
17. J. Xia, Y. Takeda, N. Usami, T. Maruizumi, and Y. Shiraki, “Room-temperature electroluminescence from Si microdisks with Ge quantum dots,” Opt. Express 18(13), 13945–13950 (2010). [CrossRef] [PubMed]
18. X. Xu, S. Narusawa, T. Chiba, T. Tsuboi, J. Xia, N. Usami, T. Maruizumi, and Y. Shiraki, “Silicon-based light-emitting devices based on Ge self-assembled quantum dots embedded in optical cavities,” IEEE J. Sel. Top. Quantum Electron. 18(6), 1830–1838 (2012). [CrossRef]
19. Y. Zhang, C. Zeng, D. Li, X. Zhao, G. Gao, J. Yu, and J. Xia, “Enhanced light emission from Ge quantum dots in photonic crystal ring resonator,” Opt. Express 22(10), 12248–12254 (2014). [CrossRef] [PubMed]
20. R. Jannesari, M. Schatzl, F. Hackl, M. Glaser, K. Hingerl, T. Fromherz, and F. Schäffler, “Commensurate germanium light emitters in silicon-on-insulator photonic crystal slabs,” Opt. Express 22(21), 25426–25435 (2014). [CrossRef] [PubMed]
21. Y. Zhang, C. Zeng, D. Li, Z. Huang, K. Li, J. Yu, J. Li, X. Xu, T. Maruizumi, and J. Xia, “Enhanced 1524-nm Emission From Ge Quantum Dots in a Modified Photonic Crystal L3 Cavity,” IEEE Photonics J. 5(5), 4500607 (2013). [CrossRef]
22. F. Hackl, M. Grydlik, M. Brehm, H. Groiss, F. Schäffler, T. Fromherz, and G. Bauer, “Microphotoluminescence and perfect ordering of SiGe islands on pit-patterned Si(001) substrates,” Nanotechnology 22(16), 165302 (2011). [CrossRef] [PubMed]
23. Y. Ma, S. Huang, C. Zeng, T. Zhou, Z. Zhong, T. Zhou, Y. Fan, X. Yang, J. Xia, and Z. Jiang, “Towards controllable growth of self-assembled SiGe single and double quantum dot nanostructures,” Nanoscale 6(8), 3941–3948 (2014). [CrossRef] [PubMed]
24. Y. Ma, C. Zeng, T. Zhou, S. Huang, Y. Fan, Z. Zhong, X. Yang, J. Xia, and Z. Jiang, “Ordering of low-density Ge quantum dot on patterned Si substrate,” J. Phys. D Appl. Phys. 47(48), 485303 (2014). [CrossRef]
25. Y. Akahane, T. Asano, B. S. Song, and S. Noda, “Fine-tuned high-Q photonic-crystal nanocavity,” Opt. Express 13(4), 1202–1214 (2005). [CrossRef] [PubMed]
26. S. L. Portalupi, M. Galli, C. Reardon, T. F. Krauss, L. O’Faolain, L. C. Andreani, and D. Gerace, “Planar photonic crystal cavities with far-field optimization for high coupling efficiency and quality factor,” Opt. Express 18(15), 16064–16073 (2010). [CrossRef] [PubMed]
27. N.-V.-Q. Tran, S. Combrié, and A. De Rossi, “Directive emission from high-Q photonic crystal cavities through band folding,” Phys. Rev. B 79(4), 041101 (2009). [CrossRef]
28. M. Toishi, D. Englund, A. Faraon, and J. Vucković, “High-brightness single photon source from a quantum dot in a directional-emission nanocavity,” Opt. Express 17(17), 14618–14626 (2009). [CrossRef] [PubMed]
29. J.-M. Gerard, “Solid-state cavity-quantum electrodynamics with self-assembled quantum dots,” in Single Quantum Dots (Springer, 2003), pp. 269–314.
30. D. Grützmacher, T. Fromherz, C. Dais, J. Stangl, E. Müller, Y. Ekinci, H. H. Solak, H. Sigg, R. T. Lechner, E. Wintersberger, S. Birner, V. Holý, and G. Bauer, “Three-dimensional Si/Ge quantum dot crystals,” Nano Lett. 7(10), 3150–3156 (2007). [CrossRef] [PubMed]
31. P. Boucaud, S. Sauvage, M. Elkurdi, E. Mercier, T. Brunhes, V. L. Thanh, D. Bouchier, O. Kermarrec, Y. Campidelli, and D. Bensahel, “Optical recombination from excited states in Ge/Si self-assembled quantum dots,” Phys. Rev. B 64(15), 155310 (2001). [CrossRef]
32. J. Wan, G. Jin, Z. Jiang, Y. Luo, J. Liu, and K. L. Wang, “Band alignments and photon-induced carrier transfer from wetting layers to Ge islands grown on Si (001),” Appl. Phys. Lett. 78(12), 1763–1765 (2001). [CrossRef]
33. M. Larsson, A. Elfving, W.-X. Ni, G. V. Hansson, and P.-O. Holtz, “Growth-temperature-dependent band alignment in Si/ Ge quantum dots from photoluminescence spectroscopy,” Phys. Rev. B 73(19), 195319 (2006). [CrossRef]
34. B. Adnane, K. Karlsson, G. Hansson, P.-O. Holtz, and W.-X. Ni, “Spatially direct and indirect transitions of self-assembled GeSi/Si quantum dots studied by photoluminescence excitation spectroscopy,” Appl. Phys. Lett. 96(18), 181107 (2010). [CrossRef]
35. T. Tayagaki, S. Fukatsu, and Y. Kanemitsu, “Photoluminescence dynamics and reduced Auger recombination in Si 1− x Ge x/Si superlattices under high-density photoexcitation,” Phys. Rev. B 79(4), 041301 (2009). [CrossRef]
36. M. Dashiell, U. Denker, and O. Schmidt, “Photoluminescence investigation of phononless radiative recombination and thermal-stability of germanium hut clusters on silicon (001),” Appl. Phys. Lett. 79(14), 2261–2263 (2001). [CrossRef]
