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We have investigated the intense infrared photoluminescence (PL) from several dozen molecular-beam-epitaxy Si1-xGex layer samples. For one sample the PL was imaged with an infrared camera with the emission integrated from 650 to 1000 meV. This sample was a SiGe/Si multiple quantum well (MQW) structure, with 40 Si0.7Ge0.3 layers 7.6 nm thick separated by 20 nm of Si on a Si(001) substrate. For excitation at normal incidence, the PL was brightest at the excitation point, but similarly intense at the sample edge. With focused excitation, the edge emission spot had the same diameter as the emission at the excitation spot. Thus the emission was much more collimated in the MQW waveguide than normally in bulk luminescence. Analysis of the luminescence spectral linewidth was performed to estimate the lateral dimensions of the Ge nanocrystals, values which were compared with plan-view TEM results.

Optoelectronics and photonics are playing an essential role in many aspects of daily life, including information and communication technologies, environmental and green technologies, mechanical and chemical sensing, consumer electronics, and biomedicine. So far, the use of optical components in communication systems has been mainly limited to direct replacement of electrical cables by optical cables. With the continual increase in link bit rates, optical cables are now replacing electrical cables for shorter and shorter interconnect lengths. Optoelectronic and photonic technologies are becoming less costly and more integrated, and there is currently an opportunity for optics to move “inside the box” and change the interconnect topology at all levels. Optical interconnects are required these days for on-chip technology as an alternative to metal wires, because of data transmission bottlenecks introduced by their unavoidable delay times, significant signal degradation, problems with power dissipation, and electromagnetic interference. Such optical interconnects will help in extending the life of Moore’s Law [1]. Currently, most optoelectronic devices are fabricated as discrete components. This approach is based on serial (e.g., step-by-step) fabrication and packaging, and it makes optoelectronic technology drastically different compared to microelectronics, where the domination of parallel fabrication made possible ultra large scale integration. Also, discrete assembly reduces the optoelectronic system reliability and decreases the manufacturing yield. Additional complications arise due to materials issues: in microelectronics the major material is elemental silicon, while traditional semiconductor materials for optoelectronics are III-V compound alloys with much more complex technological requirements. Finally, optical waveguides and waveguide based devices are very bulky compared to electron devices; thus, the densities of electron devices in integrated circuits are many orders of magnitude greater compared to that in integrated optoelectronic systems. Silicon photonics, where photonics devices are fabricated by using silicon or silicon compatible materials and where the manufacturing is based on the available microelectronics infrastructure, is emerging as the technology that can face all these challenges [2]. Silicon photonics is booming and growing at an incredible pace with many breakthroughs appearing day by day [3,4]. Speed, integration density, active components, logic, nonlinear optics, etc., are all surpassed frontiers, which silicon photonics has continuously moved apart. Many devices enabled by silicon photonics are already on the market and new ones are emerging continually [5]. Despite these advances, the major deficiency in such optoelectronic and photonic devices remains the lack of suitable silicon-based light emitters and especially lasers, which would enable a fully integrated silicon platform. In order to be commercially viable, these light emitters need to be efficient, fast, operational at room temperature, and, perhaps most importantly, be compatible with mainstream CMOS technology. Another important requirement is in the emission wavelength, which should match the optical waveguide low-loss spectral region of 1.3–1.6 μm. The main problem in utilizing silicon as a light emitter is its indirect electronic band gap that results in inefficient carrier recombination and a long radiative lifetime. Many quite different approaches to alleviating the miserable light emission in bulk silicon (~10-4 quantum efficiency at 300 K) have been proposed and are actively being explored [6]. Some, such as Si1-xGex quantum well or Si/SiO2 superlattice structures, rely on band structure engineering, while others rely on quantum confinement effects in lower dimensional structures, as typified by silicon quantum dots or porous silicon. Still another approach is impurity-mediated luminescence from, for example, isoelectronic substitution or by the addition of rare earth or transition metal ions to silicon. In this presentation, the use of the quantum confinement approach we have employed to producing efficient light emission in silicon and germanium will be reviewed. Nanostructured systems that will be covered include porous silicon, silicon quantum wells and wires, super unit cells, and arrays of silicon-germanium quantum dots. https://en.wikipedia.org/wiki/Moore%27s_law L. Pavesi and D.J. Lockwood, Silicon Photonics (Springer, Berlin, 2004). Special issue on Silicon Photonics, Proc. IEEE 97(7), July (2009). D.J. Lockwood and L. Pavesi, Silicon Photonics II: Components and Integration (Springer, Berlin, 2011). L. Pavesi and D.J. Lockwood, Silicon Photonics III: Systems and Applications (Springer, Berlin, 2016). D.J. Lockwood, Light Emission in Silicon (Academic, New York, 1998).

Raman scattering by phonons has been used for several decades to obtain information about the structural and electronic properties of various Si/SiGe nanostructures including quantum wells, superlattices, and dot/cluster multilayers [1–5]. A Raman scattering event arising from lattice vibrations (or phonons) in a semiconductor is described as the interaction of incoming light (or photon) of frequency ωi and wavevector qi with a phonon of frequency ωp and wavevector qp to produce scattered light of frequency ωs and wavevector qs. The scattering process is required to satisfy energy and momentum conservation (viz., ωs = ωi ± ωp). Since the wavevector of visible light is relatively small, Raman scattering involves only phonons with energies close to the center of the unit-cell Brillouin zone, where acoustic phonons have much smaller energies compared to those of optical phonons. As compared to the usual bulk acoustic phonons, mini Brillouin-zone folded acoustic phonons appear specifically in periodic structures such as superlattices and produce additional peaks in inelastic light scattering at wavenumbers between bulk acoustic phonons and optical phonons [6–8] and that are typically observed at wavenumbers less than 100 cm-1. In Si/SiGe nanostructures, as is also found in bulk SiGe alloys, the Raman spectrum comprises three major bands with a Si-like (Si) peak at ~ 520 cm-1, an alloy-like (SiGe) peak at ~ 400 cm-1, and a Ge-like (Ge) peak at ~ 300 cm-1 [9, 10], with other weaker Raman features located between the major peaks. The presence of highly-disordered (or amorphous) Si, SiGe, and Ge inclusions in a sample can be observed through the presence of broader, but otherwise similar, Raman features. These Raman peaks exhibit various dependencies on strain, temperature and chemical composition. Experiments involving Raman thermometry require a comparison of the Stokes (positive frequency shift) and anti-Stokes (negative frequency shift) Raman peak intensities. The optical polarization dependence of the Raman scattering intensity is defined by the Raman scattering tensors. In Si/SiGe nanostructures, this technique can be used to detect various imperfections in epitaxially grown samples, including inhomogeneous strain. Results obtained from inelastic light scattering spectroscopy investigations employing first- and second-order Raman scattering, polarized Raman scattering, and low-frequency light scattering associated with folded acoustic phonons of Si/SiGe nanostructures comprised of either planar superlattices or cluster (SiGe dot) multilayers separated by Si layers are used for analyzing the chemical composition, strain, and thermal conductivity in such technologically important materials as these for electronic and optoelectronic devices. References [1] F. Cerdeira, A. Pinczuk, J.C. Bean ,B. Batlogg, and B.A. Wilson, Appl. Phys. Lett. 45, 1138 (1984). [2] J.L. Liu, Y.S. Tang, K.L. Wang, T. Radetic, and R. Gronsky, Appl. Phys. Lett.74, 1863 (1999). [3] E.G. Barbagiovanni, D.J. Lockwood, P.J. Simpson, and L.V. Goncharova, J. Appl. Phys. 111, 034307 (2012). [4] J. Menéndez, A. Pinczuk, J. Bevk, and J.P. Mannaerts, J. Vac. Sci. Technol. B 6 1306 (1988). [5] B.V. Kamenev, L. Tsybeskov, J.-M. Baribeau, and D.J. Lockwood, Appl. Phys. Lett. 84 1293 (2004). [6] M. Cardona and P. Yu, Fundamentals of Semiconductors, Springer-Verlag, Berlin, Heidelberg (2005), p. 619. [7] M.I. Alonso and K. Winer, Phys. Rev. B 39, 10056 (1989). [8] P.M. Mooney, F.H. Dacol, J.C. Tsang, and J.O.Chu, Appl. Phys. Lett. 62, 2069 (1993). [9] S.A. Mala, L. Tsybeskov, D.J. Lockwood, X. Wu, and J.-M. Baribeau, J. Appl. Phys. 116, 014305 (2014). [10] F. Cerdeira, M.I. Alonso, D. Niles, M. Garriga, M. Cardona, E. Kasper, and H. Kibbel, Phys. Rev. B 40, 1361 (1989).

Light emission from Si nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon photonics. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering has been proposed as one way to overcome this limitation and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recently, through employing novel band gap engineering computations, entirely new Si/Ge [M. d’Avezac, J.-W. Luo, T. Chanier, and A. Zunger, Phys. Rev. Lett. 108, 027401 (2012)] and Si [H. J. Xiang, B. Huang, E. Kan, S.-H. Wei, and X. G. Gong, Phys. Rev. Lett. 110, 118702 (2013)] supercell structures possessing direct gaps have been proposed. According to d’Avezac et al., a SiGe2Si2Ge2SiGen superstructure should have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber data transmission applications. Here we report on the growth of such a structure and its optical properties. Two similar samples were prepared by different growth methods: molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). In both samples the superstructure (terminated with n = 12 monolayers of Ge) was grown on a 30-nm thick buffer layer of Si0.4Ge0.6 on a 5 µm thick relaxed buffer layer of Si0.4Ge0.6on a 750 µm thick (001) Si substrate. The photoluminescence (PL) spectra were measured using a Bomem DA3 FTIR spectrometer, with the samples excited at low temperatures with either 70 mW of 405 nm or 35 mW of 458 nm laser light. Photoluminescence (PL) spectra obtained at 6 K with excitation at 405 nm are shown in Fig. 1. Similar spectra were obtained with 458 nm excitation. No PL was detected from the two samples in the energy range 1000-1850 meV or at room temperature. The sharp drop at low energy near 700 meV is due to the cut-off in the instrumental response. A strong low-energy PL doublet is seen, with peaks near 780 and 820 meV, together with a much weaker peak at 872 meV. The ratio of intensities of the strong and weak peaks is the same in both samples. The intensities of all three PL peaks decrease with increasing temperature up to 25 K, but the weak peak decreases in intensity faster than that of the strong peaks. The weak peak at 872 meV is most likely the dipole-allowed direct-gap transition expected at 0.863 eV in the superstructure. The small difference in energy between theory and experiment could be the result of a difference in strain within the layer in the sample compared with the ideal (perfect) modeled structure or from assumptions in parameter values in the model. The strong peaks at 820 and 780 meV are assigned to the no-phonon and transverse-optic-phonon emission lines, respectively, of the Si0.4Ge0.6 buffer layer. The ~40 meV separation between the two strong peaks is characteristic of the phonon energies in SiGe alloys. The energies of the peaks, however, are much lower than that expected for a bulk Si0.4Ge0.6 alloy (~0.97 eV). The energies and general appearance of these peaks is reminiscent of what has been obtained from PL studies of SiGe nanostructures imbedded in Si. It is therefore likely that this PL arises predominately at the Si0.4Ge0.6/superstructure interface where there is type-II band alignment. In conclusion, we have obtained experimental evidence of the predicted direct-gap optically-allowed transition in a special supercell comprised of a number of ultrathin layers of Si and Ge.

Self-assembled Ge nanodots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 or TiO2 layer on Si(001). The dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. Using both the k•p and tight binding theoretical models, we have analyzed the PL spectrum in terms of the dot size distribution required to reproduce the observed asymmetric band shape. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL quantum efficiency with decreasing dot diameter. In addition, we show it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.