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Efficient room-temperature luminescence at optical telecommunication wavelengths and originating from direct band-to-band recombination has been observed in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction, micro-Raman and optical-absorption spectroscopy measurements indicate high tensile-strains while combined photoluminescence spectroscopy, excitation-power evolution and time-resolved measurements suggest direct band-to-band recombination. Such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb light-emission properties, facile fabrication and compatibility with standard microelectronic processes.
©2010 Optical Society of America
1. Introduction
Several approaches have been previously explored to provide efficient light emission in silicon and demonstrate laser action at room temperature and under electrical injection. Chemical anodization in porous silicon [1], quantum-size effects in silicon nanocrystals [2,3] or superlattices [4], Raman amplification [5,6], rare-earth elements [7,8] and point-defect centers [9–11] have allowed some progress towards reaching this goal. More recently, hybrid structures including III-V active layers bonded on silicon [12] and combinations of tensile strains and n-type doping in bulk germanium grown on silicon [13] have also led to most promising results.
Meanwhile, optically-active germanium (Ge) nanocrystals have been synthesized by magnetron cosputtering [14–17], ion implantation [18–21], isotropic chemical etching into porous germanium [22], thermal decomposition [23], solid-phase epitaxy [24], sol-gel chemistry [25] ultra-high vacuum chemical vapor deposition [26,27], Stranski-Krastanov growth by molecular-beam epitaxy [28] and, more recently, using ball-milling techniques [29]. Indeed, Ge can also provide stronger confinement than silicon nanocrystals due to a higher dielectric constant and a lighter carrier effective mass [22], while large nanocrystals can be made to emit efficiently in a spectral range more suited for optical telecommunications on chip. Moreover, band-engineered germanium nanocrystals would constitute a material of choice for silicon-photonics integration. Indeed germanium’s near-direct band structure can lead to efficient light emission at telecommunication wavelengths [13] and Ge is already known to be fully-compatible with conventional CMOS processes [30].
In this letter, we report on the efficient light-emission at room-temperature originating from direct band-to-band recombination in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Mechanical grinding techniques provide a convenient alternative to more conventional synthesis approaches. Indeed, we found that mechanical-grinding schemes have been sparsely used in the past to synthesize large quantities of nanocrystals made of heteroclite metal-semiconductor compounds [31–35]. Moreover, some of these nanocrystalline materials have displayed unique physical properties [35–37].
2. Fabrication
High-purity nanocrystalline germanium powder such shown in Fig. 1 is obtained through mechanical mortar-grinding of a bulk undoped (100) germanium wafer. A first 10 min. grinding was performed using an agate mortar and pestle to obtain a coarse powder. Then, a subsequent 25 min. grinding was performed to obtain the fine nanocrystalline powder shown in Fig. 1.
Fig. 1 (a) The nanocrystalline powder synthesized by mechanical grinding and shown in the inset can also be pressed into laminates using a 12-ton press. (b,c) TEM micrographs of the Ge nanocrystals. (d) Typical selected area electron diffraction (SAED) patterns show the sound crystalline structure of the nanocrystals. The lattice constants measured from several electron diffraction patterns suggest a 2.2 ± 1.1% tensile strain compared to the bulk. (e) HRTEM micrograph showing the strain-induced deformations seen in the largest Ge nanocrystals.
As shown in Fig. 1(a), this nanocrystalline powder can also be pressed into polycrystalline laminates using a 12-ton press. While mechanical grinding provides un-paralleled flexibility, low-cost and high yield, it still offers a limited level of control, especially on the grain size. As shown in Fig. 1(b,c), Ge nanocrystal diameters vary from 7 nm to hundreds of nanometers. Indeed, dynamic light scattering analysis suggests a very broad size distribution with an average nanocrystal diameter of 140 ± 20 nm and less than 10% of the nanocrystals being smaller than 20 nm, which is consistent with our SEM/TEM observations. As displayed in Fig. 1(d), typical selected area electron diffraction (SAED) patterns show the sound crystalline structure of the nanocrystals. However, the lattice constants calculated from several diffraction patterns suggest a 2.2 ± 1.1% tensile strain compared to the bulk germanium used to synthesize the powder. Indeed, HRTEM measurements performed on the largest nanocrystals and shown in Fig. 1(e) provide clear evidences of high strain-induced deformations that are consistent with previous results obtained for ball-milled nanocrystals [29].
3. Tensile-strains in bulk Germanium
Band structure calculations shown in Fig. 2(a) were obtained using an open-source software to illustrate the effect of tensile strains on the band-structure of bulk Ge [38,39]. Indeed, it illustrates how tensile strains bring the gamma (г)-point of bulk Ge closer to the L-valley minimum. For moderate tensile-strains, it has been shown that n-type doping can be used to fill the L-valley up to the gamma-point and favor direct band-to-band recombination in bulk Ge [13,40]. However, moderate levels of biaxial tensile strains can still require very high doping levels [13]. As an indicator, Fig. 2(b) shows the n-type doping concentration required to fill the L-valley as a function of the level of tensile-strains for the biaxial (2D) and hydrostatic (3D) cases calculated using a first-order band-narrowing model for bulk Ge. Yet, the results from this simple model are consistent with previous reports obtained from 0.25% biaxially tensile-strained Ge using ~7.6E + 19 cm−3 donor-impurity concentrations [13]. This model suggests that the tensile-strains measured from the SAED pattern would be sufficient to favor direct band-to-band recombination in such Ge nanocrystals.
Fig. 2 (a) Electronic bandstructure calculated for fully-relaxed and 0.5% tensile-strained bulk Ge [38,39]. (b) Estimate of the donor concentration required to fill the L-valley as a function of the level of tensile-strains for the biaxial and hydrostatic cases obtained using a first-order band-narrowing model.
4. Optical and structural properties
Raman and optical-absorption spectroscopy have also been used to confirm the presence of high built-in tensile strains in the nanocrystals. The clear downshift of the Ge-Ge vibrational peak shown in Fig. 3(a) is also indicator of tensile strains [41–44]. Indeed, Raman width and mean roughness are both known to originate from inhomogeneous strains and dislocations [44]. While Fano resonance artifacts in the Raman spectra are most unlikely in undoped germanium [45], thermal and phonon-confinement effects were also discarded by measuring similar Ge-Ge vibrational peak’s shift and width while varying the excitation power from 1,000 to 15,000 W/cm2, as shown in Fig. 3(b). Indeed, pronounced phonon-confinement, size or thermal effects would translate into a clear linear (thermal effects) or non-linear (phonon-confinement or size-effects) variation in the Ge-Ge peaks’ shift and width when varying the excitation power over such a broad range [46,47].
Fig. 3 (a) Raman spectroscopy of the Ge-Ge vibration measured for the germanium nanocrystals and the bulk material they were synthesized from. (b) Ge-Ge vibration peak shift and full-width at half-maximum (FWHM) for the nanocrystals and the bulk Ge measured at 5,000, 10,000 and 15,000 W/cm2 excitation intensities.
Optical absorption spectroscopy measurements shown in Fig. 4 also indicate a much more pronounced absorption tail compared with the bulk [47], most likely originating from the downshift of the gamma-point expected from the tensile-strained germanium. Both results are consistent with the tensile strains measured from the SAED pattern.
Fig. 4 Optical absorption spectroscopy measurements showing the enhanced low-energy tail for the Ge nanocrystals compared with the bulk.
5. Room-temperature light-emission properties
The room-temperature photoluminescence from the nanocrystals shown in Fig. 5(a) displays a broad emission spectrum at telecommunication wavelengths, which is more than 2 orders of magnitude stronger than the bulk germanium it was synthesized from. This drastic enhancement is expected from a direct recombination-mediated regime. Most importantly, we also know that tensile strains should lower the bandgap as we have shown in Fig. 2(a). Yet, the photoluminescence spectra indicate that the quantum-confinement effect is sufficient to bring the emission from those strained nanocrystals back into the optical telecommunication range.
Fig. 5 (a) Photoluminescence spectroscopy of the Ge nanocrystals (○) and bulk Ge (●) at room temperature. (b) Ge nanocrystals room-temperature integral photoluminescence intensity as a function of the excitation intensity. (c) Transient-photoluminescence measurements. We measured the detection limit of the entire system and found it better than 25 μs (highlighted region). The plain lines indicate the fits of the photoluminescence transients.
The superlinear dependence of the integral photoluminescence emission as a function of the excitation power shown in Fig. 5(b) is also suggestive of direct recombination [40]. The time-resolved photoluminescence measurements obtained at room-temperature from the germanium nanocrystals and from the bulk provide more supporting evidences [48]. As shown in Fig. 5(c), the bulk germanium transients indicate that the emission possesses a fast and a slow component originating from the phonon-mediated recombination. In the Ge nanocrystals, the disappearance of the long-lived transient combined with the strongly-enhanced luminescence confirms that the emission originates mostly from direct band-to-band recombination. Although surface states might also play a role, they are highly unlikely to affect the transients measured here since their recombination lifetimes would be far below the detection limit of the setup.
6. Conclusion
In summary, we report efficient light-emission at room-temperature originating from direct band-to-band recombination in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction (SAED), HRTEM, Raman & optical-absorption spectroscopy measurements point to high tensile-strains while combined photoluminescence spectroscopy, power-evolution and time-resolved measurements suggest direct band-to-band recombination. These results are also consistent with theoretical expectations for bulk germanium under high tensile strains. We believe that such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb room-temperature light-emission in the optical telecommunication range, facile fabrication and compatibility with standard microelectronic processes.
Acknowledgements
We are most thankful to the DARPA-MTO Young Faculty Award program and the University of Delaware Research Foundation for their support of this project.
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