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The infrared (IR) reflectivity of the cubic metal oxides MgO, MnO, and NiO has been measured at room temperature using the technique of oblique incidence. The use of this technique at three angles of incidence provides multiple sets of spectra (including both s- and p-polarizations) to analyze when compared to the standard normal incidence case, which has been used extensively in the past for these compounds. It is shown that the transverse optic (TO) and longitudinal optic (LO) mode phonon parameters can be determined with greater accuracy by using the factorized model for the fits, as compared with the popular classical model used previously, and by fitting the derivative of the reflectivity. Our results for the phonon mode parameters are similar to those found earlier, as could be expected, but are generally more precise. An analysis of the difference in frequency of the TO mode in antiferromagnetic NiO at room temperature for the two polarizations of reflected light revealed a TO mode splitting of about 4 cm−1, with the p-polarized light TO mode having the higher frequency: This small splitting is in agreement with theoretical predictions. This more precise IR method for revealing the phonon mode behavior in such magnetically ordered cubic metal oxides, which are prototypes of strongly correlated electronic systems and have been found to be Mott-Hubbard insulators, may be readily applied to any similar antiferromagnetic system.

Following up on some recent work that has been presented (Orapunt et al., J Appl Phys 119:065702-1-12, 2016), we report on the optical properties associated with a unique form of thin-film silicon that has been deposited onto optical quality fused quartz substrates through ultra-high-vacuum evaporation. For the purposes of this particular analysis, we focus on how the growth temperature influences the spectral dependence of the optical functions associated with these thin silicon films, for growth temperatures ranging from 98 to 572 °C. Through measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence, we determine the spectral dependence of the refractive index, the extinction coefficient, the real and imaginary parts of the dielectric function, and the optical absorption coefficient for the 11 thin silicon films considered in this analysis. We find that generally the refractive index increases in response to increases in the growth temperature. The optical absorption spectral dependence is also observed to exhibit a fundamental transition in its functional behavior accompanying increases in the growth temperature. Some details, related to recently developed methods employed for the determination of the optical functions from measurements of the reflectance and transmittance spectra, are provided as a complement to this analysis.

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.

This Chapter discusses the phenomena associated with germanium (Ge) nanocrystals emitting near infrared radiation under optical excitation. We describe how the emission properties are influenced by various effects, including those of strain and particle confinement, as well as excitation mechanisms. Two example systems are discussed, namely one of isolated Ge quantum dots and another of Ge nanocrystals coherently imbedded in SiGe alloy layers, where both systems were grown by molecular beam epitaxy (MBE) on Si substrates. For the Ge dot ensembles, we show how particle size information can be derived from the emission spectrum. For the Ge nanocrystals, the emission spectra are analyzed for the effects of strain and particle confinement over a wide range of Ge fractions in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape, which were a 1.4 nm thickness in the MBE growth direction and a 7 nm lateral dimension. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures., Series: Topics in Applied Physics

The determination of the spectral dependence of thin-film optical functions from experimental measurements of the optical response, performed on thin-film/substrate structures, represents a problem of general interest to the thin-film community. In this paper, we present details related to thin-film optical function acquisition from experimental measurements of the reflectance and transmittance spectra, these being worked out through a case study performed on the optical functions associated with a series of thin-films of silicon that had been grown through ultra-high vacuum evaporation deposition on optical-quality-fused quartz substrates, this study being performed by Moghaddam et al. (J Mater Sci: Mater Electron 31:13186–13198, 2020). Following the presentation of our approach to deposition, details related to our optical response measurements, the uncertainty in our reflectance and transmittance results, the analytical framework within which the optical functions were acquired, how the ambient and the optical-quality-fused quartz substrate optical functions were modeled, and means through which unphysical solutions were identified and eliminated, are presented. We believe that the collection of these results will be of interest to those within the broader thin-film community.

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).

An overall comparative study is made of the spin-phonon interactions in several rutile-structure transition-metal difluorides, specifically FeF₂, MnF₂, NiF₂, and CoF₂, in terms of recent developments obtained experimentally using inelastic light scattering spectroscopy and theoretically using a modified mean-field approach to estimate spin-pair correlation functions. New experimental data are presented here and interpreted within an extended and comprehensive theoretical treatment to yield estimates for the spin-phonon coupling coefficients and the relative magnitudes of the magneto-optical coupling coefficients. С использованием последних результатов экспериментально с применением спектроскопии неупругого рассеяния света и теоретически методом модифицированного среднеполевого подхода для оценки корреляционных функций спиновой пары проведено полное сравнительное исследование спин-фононных взаимодействий в нескольких дифлуоридах переходных металлов рутиловой структуры, в частности FeF₂, MnF₂, NiF₂ и CoF₂. С целью получения оценок коэффициентов спин-фононной связи и относительных величин коэффициентов магнитооптической связи представлены новые экспериментальные данные, которые интерпретируются в рамках всестороннего расширенного теоретического описания З використанням останніх результатів експериментально зі застосуванням спектроскопії непружного розсіяння світла та теоретично методом модифікованого середньопольового підходу для оцінки кореляційних функцій спінової пари проведено повне порівняльне дослідження спін-фононних взаємодій в декількох діфлуоридах перехідних металів рутилової структури, зокрема FeF₂, MnF₂, NiF₂ та CoF₂. З метою отримання оцінок коефіцієнтів спін-фононного зв’язку та відносних величин коефіцієнтів магнітооптичного зв’язку представлено нові експериментальні данні, які інтерпретуються в рамках всебічного розширеного теоретичного опису.

In this article we review in detail the magnetic properties of the simple cubic oxides NiO, MnO, and CoO, and also discuss the properties of the more complicated, but related, transition-metal oxides CuO and FeO. The magnetic ordering scheme and the several competing magnetic interactions strongly affect the lattice structure (giving rise to distortions) and hence the vibrational properties. The various interactions of the magnetic moments plus the spin-phonon coupling, which heavily influence the physical properties of these metal monoxides, are discussed thoroughly.

We show that photoluminescence properties of PbS nanocrystal thin films are directly related to film morphology and nanocrystal density. In densely packed PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is mainly responsible for the photoluminescence spectra narrowing and shift toward longer wavelengths. At elevated temperatures, back energy transfer is proposed to be responsible for an unusual photoluminescence intensity temperature dependence. In thin films with a low PbS nanocrystal density, the energy transfer is suppressed, and the effect is dramatically reduced., Comment: (8 pages, 7 figures)

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).

Morphological information has been obtained using the strong near-infrared photoluminescence emitted by germanium (Ge) nanocrystals (NCs) coherently imbedded in SiGe alloy layers, grown by molecular beam epitaxy on Si substrates. The emission spectra are analyzed for the effects of strain, carrier confinement, and disorder over a wide range of Ge concentrations in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures. We indicate how direct gap behavior might be achieved for Ge NCs lattice matched within dilute Ge1-ySny alloys, where tensile strain would be present in the NCs in all three directions.

Using ultra-high-vacuum evaporation, thin-films of silicon were grown. This paper starts with a placement of our thin-films into the overall thin-film silicon continuum. Then, our thin-film’s Raman spectra are examined. In particular, two short-range order Raman related measures, namely the breadth and position of the Raman spectrum’s transverse-optic-peak, and an intermediate-range order Raman related measure, namely the ratio of the integrated transverse-acoustic and transverse-optic-peak intensities, are plotted as functions of the growth temperature. We then correlate these changes found in the nature of the order found within our thin-films of silicon with the changes observed in the spectral dependence of the refractive index, focusing on the maximum of the refractive index over the spectral range under consideration in this analysis. We find that the maximum of the refractive index increases in response to increased ordering. Finally, the potential device implications of these results are explored.

Alloy semiconductor magic-size clusters (MSCs) have received scant attention and little is known about their formation pathway. Here, we report the synthesis of alloy CdTeSe MSC-399 (exhibiting sharp absorption peaking at 399 nm) at room temperature, together with an explanation of its formation pathway. The evolution of MSC-399 at room temperature is detected when two prenucleation-stage samples of binary CdTe and CdSe are mixed, which are transparent in optical absorption. For a reaction consisting of Cd, Te, and Se precursors, no MSC-399 is observed. Synchrotron-based in-situ small angle X-ray scattering (SAXS) suggests that the sizes of the two samples and their mixture are similar. We argue that substitution reactions take place after the two binary samples are mixed, which result in the formation of MSC-399 from its precursor compound (PC-399). The present study provides a room-temperature avenue to engineering alloy MSCs and an in-depth understanding of their probable formation pathway., Alloy magic-size clusters (MSCs) are difficult to synthesize, in part because so little is known about how they form. Here, the authors produce single-ensemble alloy CdTeSe MSCs at room temperature by mixing prenucleation-stage solutions of CdTe and CdSe, uncovering a formation pathway that may extend to the synthesis of other alloy MSCs.

We analyze the intense photoluminescence (PL) observed at energies from 600 to 1100 meV for a large number of Si₁₋ₓGeₓ epitaxial layers grown by molecular beam epitaxy. In the present work we show that this previously unexplained broad PL peak can be assigned to Ge nanocrystals (NCs) self-assembled within the SiGe layers. These NCs are assumed to be lattice matched to the SiGe in the vertical, growth direction. A consequence of this assignment is that as the Ge-fraction in the SiGe layer increases the vertical strain in the NCs changes from compressive to tensile at x ~ 0.36, lowering the NC bandgap (BG) below that of bulk Ge. We examine the PL results for more than 60 samples exhibiting this broad PL peak by examining how it follows the strained Ge BG for x from 0.05 to 0.53. The PL is resolvable as two narrower peaks separated by the momentum conserving phonon energy for Ge. Strain and confinement shifted NC bound exciton energies calculated numerically agree well with the measured ones. When Raman scattering results were examined for some of the same samples, the phonon mode frequencies obtained provided valuable corroborative evidence for the presence of the Ge NCs.

Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface. It is also shown that the nanowire surface states act as shunt resistors and considerably degrade the Si-Ge nanowire device characteristics.

A number of thin silicon films are prepared through ultra-high-vacuum evaporation on optical quality fused quartz substrates with different growth temperatures. Through an analysis of grazing incidence X-ray diffraction results, a phase transition, from amorphous-to-crystalline, is found corresponding to increases in the growth temperature. The corresponding Raman spectra are also observed to change their form as the films go through this phase transition. Using a Raman peak decomposition process, this phase transition is then quantitatively characterized through the determination of the amount of intermediate-range order and the crystalline volume fraction for the various growth temperatures considered in this analysis. The possible device consequences of these results are then commented upon.

Silicon-Germanium strain engineering has been used for more than two decades in silicon based devices and has contributed to the scaling down of a transistor’s size and to significant improvements in device performance. However, while conventional silicon-germanium based electronics has experienced rapid and steady growth, thanks to this continuous miniaturization of transistors, this trend cannot continue indefinitely. Industry has already moved to alternate methods such as FinFET devices, in which a thin silicon channel is placed vertically, and the FD-SOI (FD-SGOI) design consisting of a thin film Si(SiGe) channel placed horizontally. For nodes scaled down below 28 nm, low power operation will be inherently hindered by both the imperfect interface, non-uniformity of ultra-thin films and quantum confinement effects, which increase the effective bandgap. In these devices, despite the intense research activity on the strained SiGe ultra-thin body, which accounts for a large portion of such microelectronic devices (below the 45 nm node), we still fail to properly understand the mechanisms that limit hole and electron mobilities in SGOI layers. In addition, one of the main challenges for Si based devices remains the fabrication of efficient group-IV photon sources / photon detectors compatible with the microelectronic industry, which would usefully replace the integration of III-V heterostructures on Si. The major bottleneck is that group-IV semiconductor elements have indirect bandgaps, but with possibilities of being transformed to direct bandgaps using strain engineering strategies. In this presentation, we will review the formation mechanism of Ge-rich layers on SOI by condensation at different temperatures. TEM cross-section and GPA analysis of the heterostructures will be presented. We will also report the physical and optical properties of these heterostructures. Special attention is devoted to the influence of the SiGe thickness reduction (up to few MLs), where quantum confinement is prominent in the optical properties of the layers. Raman and PL results will be presented to better explain such confinement behavior. We show that novel SGOI substrates could represent a key strategy for the fabrication of future photonic devices.

A comprehensive experimental and theoretical study of indirect gap optical absorption in bulk Ge is presented. While this topic has been the subject of intense studies from the early days of semiconductor physics, the resonant aspect of the absorption received very little attention until now. This is a unique property of Ge related to the proximity of the direct and indirect gaps. The absorption coefficient was measured over the entire spectral range between the two gaps for comparison with theory. It is shown that the standard textbook expressions, obtained by assuming intermediate states with constant energy, are in very poor agreement with experiment. A theory first proposed by Hartman [R. L. Hartman, Phys. Rev. 127, 765 (1962)], which takes into account the energy dependence of the intermediate states, provides a much better account of the photon-energy dependence of the absorption, but the prediction of the experimental absorption strength requires the incorporation of excitonic effects. The latter, however, have only been considered by Elliott [R. J. Elliott, Phys. Rev. 108, 1384 (1957)] in the limit of constant intermediate state energy. A generalization to the case of energy-dependent intermediate states, consistent with Hartman's theory, is presented here. The basic qualitative difference with the classical Elliott theory is that the excitonic character of the intermediate states affects the computed absorption, generating an additional resonant enhancement that is confirmed by the experimental data. The generalized theory presented here agrees very well with the experimental absorption using independently determined band structure parameters.

The optical absorption coefficient of pure Ge has been determined from high-accuracy, high-precision optical measurements at photon energies covering the spectral range between the indirect and direct gaps. The results are compared with a theoretical model that fully accounts for the resonant nature of the energy denominators that appear in perturbation-theory expansions of the absorption coefficient. The model generalizes the classic Elliott approach to indirect excitons, and leads to a predicted optical absorption that is in excellent agreement with the experimental values using just a single adjustable parameter: the average deformation potential ${D}_{\mathrm{\ensuremath{\Gamma}}L}$ coupling electrons at the bottom of the direct and indirect valleys in the conduction band. Remarkably, the fitted value, ${D}_{\mathrm{\ensuremath{\Gamma}}L}=4.3\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/\mathrm{cm}$, is in nearly perfect agreement with independent measurements and ab initio predictions of this parameter, confirming the validity of the proposed theory, which has general applicability.

Light emission from Si and Ge nanostructures (NSs) has been of great interest in recent years (1) owing to the need for silicon-based light sources for applications in silicon optoelectronics and photonics (2). 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. Two major avenues toward optical interconnects on a chip include a hybrid approach with III-V densely packaged optoelectronic components and the all-group-IV approach (mainly Si, Ge and SiGe), where the all major components, e.g., light emitters, modulators, waveguides and photodetectors, are monolithically integrated into the CMOS environment. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters (1). Band gap engineering employing quantum wells, quantum wires or quantum dots has been proposed as one way to overcome this limitation and Si/Ge or Si/SiGe-alloy thin multilayer quantum well structures grown on Si have been produced on this principle, 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 (1–3). Recently, through employing novel band-gap engineering strategies, we have prepared several different entirely new bright light-emitting Si/Ge NSs including one possessing a direct gap. The latter structure is based on constructing a new super unit cell comprised of multiple planar epitaxial layers of Si and Ge grown on (001) Si0.4Ge0.6 (4). Others are based on silicon-germanium layers grown epitaxially on silicon in such a way as to form multiple layer three-dimensional NSs (quantum dots) (5, 6). We have also developed a simple and efficient electrochemical process that combines galvanic reaction and focused-ion-beam lithography to selectively synthesize gold nanoparticles that are consequently used for the growth of ordered SiGe nanowire arrays with predefined diameter (200 nm) and position (7, 8). Here, the light emitting properties of these and other recent similar Si/Ge NSs that have been found to luminesce efficiently at wavelengths in the important spectral range of 1.1–1.6 μm are compared for possible applications in optoelectronics and photonics. D. J. Lockwood, Light Emission in Silicon, Academic Press, San Diego (1998). L. Pavesi and D. J. Lockwood, Silicon Photonics, Springer, Berlin (2004). D. J. Lockwood and L. Tsybeskov, IEEE J. Sel. Top. Quantum Electronics 20(4), 8200807 (2014). D. J. Lockwood, N. L. Rowell, A. Gouyé, L. Favre, A. Ronda and I. Berbezier, ECS Trans. 61(5), 31 (2014). S. A. Mala, L. Tsybeskov D. J. Lockwood, X. Wu and J.-M. Baribeau, Appl. Phys. Lett. 103, 033103 (2013). S. A. Mala, L. Tsybeskov, D. J. Lockwood, X. Wu and J.-M. Baribeau, Physica B 453, 29 (2014). A. Benkouider, A. Ronda, A. Gouye, C. Herrier, L. Favre, D. J. Lockwood, N. L. Rowell, A. Delobbe, P. Sudraud, and I. Berbezier, Nanotechnology 25(33), 335303 (2014). D. J. Lockwood, N. L. Rowell, A. Benkouider, A. Ronda, L. Favre and I. Berbezier, Beilstein J. Nanotechnol. 2014(5), 2498 (2014).

We report on the optical properties of SiGe nanowires (NWs) grown by molecular beam epitaxy (MBE) in ordered arrays on SiO2/Si(111) substrates. The production method employs Au catalysts with self-limited sizes deposited in SiO2-free sites opened-up in the substrate by focused ion beam patterning for the preferential nucleation and growth of these well-organized NWs. The NWs thus produced have a diameter of 200 nm, a length of 200 nm, and a Ge concentration x = 0.15. Their photoluminescence (PL) spectra were measured at low temperatures (from 6 to 25 K) with excitation at 405 and 458 nm. There are four major features in the energy range of interest (980–1120 meV) at energies of 1040.7, 1082.8, 1092.5, and 1098.5 meV, which are assigned to the NW-transverse optic (TO) Si–Si mode, NW-transverse acoustic (TA), Si–substrate–TO and NW-no-phonon (NP) lines, respectively. From these results the NW TA and TO phonon energies are found to be 15.7 and 57.8 meV, respectively, which agree very well with the values expected for bulk Si1−xGex with x = 0.15, while the measured NW NP energy of 1099 meV would indicate a bulk-like Ge concentration of x = 0.14. Both of these concentrations values, as determined from PL, are in agreement with the target value. The NWs are too large in diameter for a quantum confinement induced energy shift in the band gap. Nevertheless, NW PL is readily observed, indicating that efficient carrier recombination is occurring within the NWs.

Results obtained from photoluminescence (PL) measurements performed on Si/Si 1-xGe x nanostructures with a single Si 1-xGe x nanometer-thick layer (NL) incorporated into Si/Si 0.6Ge 0.4 cluster multilayers (CMs) are reviewed. By employing depth-dependent pulsed laser excitation, the SiGe NL PL decay is found to be nearly a 1000 times faster compared to that in CM PL. A physical model taking into consideration the Si/SiGe hetero-interface composition is proposed to explain the fast and slow time-dependent recombination rates.

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.

The applications of in situ vibrational spectroscopy for identifying bulk and surface Ni(OH)2 are discussed. Raman spectra from α- and β-Ni(OH)2 samples immersed in water are generally similar to those collected from comparable dry samples. However, the Raman scattering intensities vary, and dry β-Ni(OH)2 additionally exhibits a surface O-H stretching mode at 3690 cm-1. Using in situ Raman spectroscopy, the spontaneous transformation of α-Ni(OH)2 to β-Ni(OH)2 in room-temperature water was monitored. Such transformations are conventionally performed in high-temperature alkaline media. An intralayer OH-diffusion model is proposed. Internal stresses at the α/β-phase boundary caused shifted peaks, higher order vibrational modes, and a new water peak at 3520 cm-1. We conclude that Raman spectroscopy may be applied to observe Ni(OH)2 materials in situ during chemical and electrochemical treatments. Such measurements provide information on the proportions of α- and β-Ni(OH)2 and their fine structural details with high sensitivity. © Published 2014 by the American Chemical Society.

We analyzed structural and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts. The observed nonlinear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20–30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections, as revealed by transmission electron microscopy.We analyzed structural and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts. The observed nonlinear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20–30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections, as revealed by transmission electron microscopy.

Experimental data are reported for the temperature and polarization dependence of the oneand two-magnon Raman light scattering in the rutile-structure antiferromagnet CoF2. The correspondingn Stokes and anti-Stokes Raman spectra are measured, emphasizing the temperature and polarization variations of the one-magnon frequency, line width, and integrated intensity. A Green's function method is employed to derive the excitation energies and their intensities over a broad range of temperatures (below the Néel temperature = 38 K) for a spin S = 3/2 anisotropic antiferromagnet with strong spin-orbit coupling. The exchange terms are analyzed using the random-phase approximation while single-ion anisotropy terms are treated exactly by generating a closed set of coupled Green's function equations. Good overall agreement between theory and experiment is obtained. © 2013 The Korean Physical Society.

HgTe plates have been grown by vapor phase epitaxy on (111) SrTiO3 substrates with a preferred orientation in the (111) crystalline direction, as indicated by x-ray diffraction. Examination of the plates using the micro-Raman mapping shows that the HgTe plates exhibit unusual strain patterns: the Raman peaks from the transverse-optical and longitudinal-optical phonons for the thicker (central) parts of the HgTe plates are at the same frequency as that of the bulk HgTe, while the Raman peaks for the thinner parts of the HgTe plates, which surround the thicker parts and can hardly be seen in a scanning electron microscope, are significantly larger in frequency. The full width at half maximum is smaller in the thinner areas than in the thicker parts. Theoretical analysis shows that the HgTe plates on SrTiO3 substrates suffer from compressive stress, and this may be sufficient to induce the three-dimensional topological insulator behavior in HgTe.

The development of a light emitter compatible with Si based complementary metal-oxide- semiconductor (CMOS) circuit technology and fast optical interconnects is important for the new generations of microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures (NSs) with light emission in the important optical communication wavelength range of 1.3 – 1.55 μm are compatible with conventional CMOS processes. However, the predicted and experimentally confirmed long carrier radiative lifetimes in Si and Si/Si1-xGex NSs impede the demonstration of efficient light-emitting devices and lasers. Thus, engineering of Si/Si1-xGex heterostructures with controlled composition and interface abruptness is critical in producing the desired fast and efficient photoluminescence (PL) peaked at around 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one dimension (corresponding to the case of quantum wells), two dimensions (quantum wires), and three dimensions (quantum dots). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic decay time as much as 1000 times shorter than that observed in conventional Si/SiGe NSs. The experimentally observed non-exponential PL decay in Si/SiGe NSs is explained as being due to variations of the distances separating electrons and holes at the Si/SiGe heterointerface. The results demonstrate that an abrupt Si/SiGe heterointerface reduces the carrier radiative recombination lifetime and increases the PL quantum efficiency making these SiGe NSs promising candidates for applications in CMOS compatible light-emitting devices.

We present a review of recent studies of inelastic light scattering spectroscopy in two types of Si/SiGe nanostructures: planar superlattices and cluster (dot) multilayers including first- and second-order Raman scattering, polarized Raman scattering and low-frequency inelastic light scattering associated with folded acoustic phonons. The results are used in semi-quantitative analysis of chemical composition, strain and thermal conductivity in these technologically important materials for electronic and optoelectronic devices.

Fast optical interconnects together with an associated light emitter that are both compatible with conventional Si-based complementary metal-oxide-semiconductor (CMOS) integrated circuit technology is an unavoidable requirement for the next-generation microprocessors and computers. Self-assembled Si/Si1−xGex nanostructures (NSs), which can emit light at wavelengths within the important optical communication wavelength range of 1.3–1.55 μm, are already compatible with standard CMOS practices. However, the expected long carrier radiative lifetimes observed to date in Si and Si/Si1−xGex NSs have prevented the attainment of efficient light-emitting devices, including the desired lasers. Thus, the engineering of Si/Si1−xGex heterostructures having a controlled composition and sharp interfaces is crucial for producing the requisite fast and efficient photoluminescence (PL) at energies in the range of 0.8–0.9 eV. In this paper, we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in three dimensions (corresponding to the case of quantum dots), two dimensions (corresponding to quantum wires), and one dimension (corresponding to quantum wells). The interface sharpness is influenced by many factors, such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50–120 nm produce a clear PL signal associated with band-to-band electron–hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1−xGex NSs incorporated into a Si0.6Ge0.4 wavy superlattice structure display PL of high intensity while exhibiting a characteristic decay time that is up to 1000 times shorter than that found in conventional Si/SiGe NSs. The non-exponential PL decay found experimentally in Si/SiGe NSs can be interpreted as resulting from variations in the separation distance between electrons and holes at the Si/SiGe heterointerface. The results demonstrate that a sharp Si/SiGe heterointerface acts to reduce the carrier radiative recombination lifetime and increase the PL quantum efficiency, which makes these SiGe NSs favorable candidates for future light-emitting device applications in CMOS technology.

A number of thin silicon films are deposited on crystalline silicon, native oxidized crystalline silicon, and optical quality fused quartz substrates through the use of ultra-high-vacuum evaporation at growth temperatures ranging from 98 to 572 °C. An analysis of their grazing incidence X-ray diffraction and Raman spectra indicates that a phase transition, from amorphous-to-crystalline, occurs as the growth temperature is increased. Through a peak decomposition process, applied to the Raman spectroscopy results, the crystalline volume fractions associated with these samples are plotted as a function of the growth temperature for the different substrates considered. It is noted that the samples grown on the crystalline silicon substrates have the lowest crystallanity onset temperature, whereas those grown on the optical quality fused quartz substrates have the highest crystallanity onset temperature; the samples grown on the native oxidized crystalline silicon substrates have a crystallanity onset temperature between these two limits. These resultant dependencies on the growth temperature provide a quantitative means of characterizing the amorphous-to-crystalline phase transition within these thin silicon films. It is noted that the thin silicon film grown on an optical quality fused quartz substrate at 572 °C, possessing an 83% crystalline volume fraction, exhibits an optical absorption spectrum which is quite distinct from that associated with the other thin silicon films. We suggest that this is due to the onset of sufficient long-range order in the film for wave-vector conservation to apply, at least partially. Finally, we use a semi-classical optical absorption analysis to study how this phase transition, from amorphous-to-crystalline, impacts the spectral dependence of the optical absorption coefficient.

From ancient times, people have gazed up at the sky in daylight and asked the perennial question "Why is the sky blue?" [1]. Other similar and related questions are "Why is the night sky black?", "Why are sunrises and sunsets red?", and "Why are the clouds white?" . Rayleigh [2-5] and Mie scattering [6] lie behind the long sought answers to all such questions about the colors seen in the sky.

For Ge nanodots ∼20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.

Recent advances in forming semiconductor heterojunctions within spatially confined nanoscale objects, including nanowires (NWs), show that the traditional limitations in the lattice-mismatched hetero-growth can be challenged. In various III-V semiconductor NWs, abrupt heterojunctions have been successfully demonstrated using the vapor-liquid-solid (VLS) growth for GaAs/InAs (7% mismatch) and InAs/InP (3% mismatch) heterojunctions. In group IV semiconductors, the approach is complicated not only by the 4.2% lattice mismatch between Si and Ge but also because Si and Ge both have a quite high solubility in the Au-Si catalyst. During chemical vapor deposition (CVD) based VLS growth using SiH4 and GeH4 (or similar gases), a supply of Si remains effectively “on” in the catalyst, and Si effectively intermixes with the arriving Ge even if the SiH4 flow is already switched “off”. One way to address this problem is to choose a catalyst with a lower Si solubility, e.g. AlAu2 and AgAu. Another possibility is to significantly reduce growth temperature before turning a GeH4 source “on”. Using the latter technique, we fabricated Si-Ge heterojunction NWs with nearly ideal interface and only an 8 nm thick SiGe transition layer between straight and nearly micron-long Si and Ge NW segments and analyzed their structural and electrical properties. The observed changes in the photoluminescence and Raman spectra as function of temperature, non-linear and rectifying current-voltage characteristics, strong flicker noise and damped current oscillations with frequencies of 20-30 MHz are explained using the proposed SiGe heterojunction NW energy band diagram including the energy states associated with the NW surface (and near-surface) structural imperfections revealed by transmission electron microscopy and a mismatch in Si and Ge coefficients of thermal expansion.

A silicon-compatible group IV coherent light source remains the missing link in Si-based photonics, despite much effort. Recently lasing and high efficiency photoluminescence (PL) have been shown for germanium (Ge), a material with a relatively small indirect to direct bandgap (BG) difference. We previously had observed a very intense, low temperature PL in dozens of samples for MBE-grown Si1-xGex epitaxial layers with x from 0.05 to 0.53, with quantum efficiencies up to 5%. This PL was neither defect nor dislocation related, but appeared to be strong with a large lifetime due to localization effects. As shown in Fig. 1, the PL consisted of a broad peak with asymmetry to low photon energies. With Ge fraction this peak was constant in shape and tracked the BG variation, but was ~80 meV below the indirect BG for strained SiGe. The width of this peak at ~50 meV was too small to be due to a no phonon (NP) line with its TO phonon replica if the material were Si or SiGe as the NP-TO spacing is about 58 meV for those materials. Fig. 1 shows that for higher Ge fractions the PL is emitted at energies significantly below those for bulk Ge, with its BG of 744 meV. This broad, intense PL peak has been unexplained, although a morphological origin was suggested by TEM. Here we will show from PL data that the peak is due to Ge nanocrystals (NCs) imbedded in SiGe layers (see Fig. 2). Epitaxy requires that the SiGe epilayers and the Ge NCs be lattice matched to Si (001) in the x-y plane, so that both the SiGe and NCs are under compression in that plane. Unconstrained in the vertical (z) direction, the epilayer is under tensile strain, leading to a vertical lattice constant that increases with Ge fraction and is larger than that for unstrained SiGe. Here the volume of the unit cell for the epitaxial SiGe is assumed to be the same as that for unstrained SiGe. The lattice of the Ge NCs is constrained to match the SiGe epilayer vertically. For relatively Si-rich SiGe, the Ge NCs are under compression vertically, but for more Ge-rich SiGe the vertical lattice constant of the strained SiGe exceeds that of bulk Ge. At this point the vertical strain in the Ge NC becomes tensile, which first occurs for a Ge fraction in the SiGe of 0.36 (Fig. 3 - red trace). The BG variation in Ge with uniaxial strain can be computed using deformation potential theory. As the strain becomes more strongly tensile both the direct and indirect gap energies decline, with the direct energy decreasing more rapidly than the indirect one. As shown in the blue traces of Fig. 3, the direct gap crosses the indirect at a tensile uniaxial strain of 4.4%, resulting in a direct gap semiconductor, a highly desirable outcome. Our results with PL point the way to this transition point, but the maximum vertical tensile strain for the present Ge NCs is not much greater than 2.1%. Nonetheless, the fact that we see PL below the indirect BG of bulk Ge is explained by the Ge NCs being under tensile strain vertically, reducing their BG. The broad peak width and shape can be curve resolved with two peaks, each ~30 meV wide, separated by ~35 meV, i.e., very near the momentum conserving TO phonon energy for Ge.The NP peak is wide (25-30 meV) due in part to confinement shift variations from size variability and to alloy disorder broadening in the SiGe. To test our hypothesis further, we have calculated the emission energy in a numerical model that includes the effects of strain on the Ge BG and of the confinement blue shifts on exciton energy for both the SiGe layers and the imbedded NCs, the latter assumed to be of a single vertical size (2.5 nm). The calculated energies for over 45 samples of varying composition are compared with the measured PL energies in Fig. 4, where the slope is somewhat different from the expected value of unity due to possible lower order influences, such as variations in the NC size and deviations from the linear deformation potential model for the strained BG. However, we do have very good general agreement between our computed emission energies and those observed in PL, which provides validation for our theory that imbedded Ge NCs have given rise through carrier localization in three dimensions to the intense, broad PL observed in MBE grown SiGe. Figure 1

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.

In this paper, we review the present status of light emitters based on SiGe nanostructures. In order to be commercially valuable, these light emitters should be efficient, fast, operational at room temperature, and, perhaps most important, compatible with the ldquomainstreamrdquo complementary metal-oxide-semiconductor (CMOS) technology. Another important requirement is in the emission wavelength, which should match the optical waveguide low-loss spectral region, i.e., 1.3-1.6 mum. Among other approaches, epitaxially grown Si/SiGe quantum wells and quantum dot/quantum well complexes produce efficient photoluminescence and electroluminescence in the required spectral range. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency and a long carrier radiative lifetime. The latest progress in the understanding of physics of carrier recombination in Si/SiGe nanostructures is reviewed, and a new route toward CMOS compatible light emitters for on-chip optical interconnects is proposed.

Photoluminescence (PL) spectroscopy has been used to study the incorporation of C in several samples consisting of strained Si 1 − x − y Ge x C y epilayers lattice matched to Si (001). To obtain the total C concentration, these samples were characterized by both SIMS and Auger emission spectroscopy, and X-ray diffraction data was analyzed to obtain the substitutional C concentration. The difference of the total and substitutional C concentrations, i.e., the non-substitutional carbon fraction, was found to be directly correlated with specific spectral lines in both the room-temperature Raman and low-temperature PL spectra.

Ge nanocrystals have been obtained from the dewetting process during thermal annealing of anamorphous Ge layer deposited by molecular beam epitaxy on a thin SiO 2 layer on Si(001). The Genanocrystals were then capped with a thin layer of amorphous Si. The mean nanocrystal size – 2.5–60nm – depends on the initial Ge layer thickness. Low-temperature photoluminescence (PL) measurementswere performed to investigate quantum confinement effects on the Ge nanocrystal energy gap and defectstates. For the present range of particle sizes, the nanoparticle PL emission appeared as a wide near-infraredband near 900 meV although a weak confined band was also observed for the smallest nanoparticles. Furtherthermal annealing of the samples increased the interband recombination by nearly two orders of magnitude.Crown Copyright c 2008 Published by Elsevier Ltd. All rights reserved. Keywords: Photoluminescence; Nanocrystals; Germanium; Silicon dioxide; Silicon 1. IntroductionAlthough achieving efficient light emission from group IV semiconducting materials, asubject of intense research activity, has proven elusive, carrier localization methods have led tosignificantly enhanced optical emission from indirect gap materials and at higher temperatures.For example, room-temperature visible photoluminescence (PL) has been observed from porousSi [1] and strong infrared PL from various island or quantum dot systems [2–9]. A particular

Three-dimensional SiGe nanostructures grown on Si (SiGe/Si) using molecular beam epitaxy or low-pressure chemical vapor deposition exhibit photoluminescence and electroluminescence in the important spectral range of 1.3–1.6 μm. At a high level of photoexcitation or carrier injection, thermal quenching of the luminescence intensity is suppressed and the previously confirmed type-II energy band alignment at Si/SiGe cluster heterointerfaces no longer controls radiative carrier recombination. Instead, a recently proposed dynamic type-I energy band alignment is found to be responsible for the strong decrease in carrier radiative lifetime and further increase in the luminescence quantum efficiency.

Infrared vibrational spectroscopy in an attenuated total reflection (ATR) geometry has been employed to investigate the presence of organic thin layers on Si-wafer surfaces. The phenomena have been simulated to show there can be a field enhancement with the presented single-reflection ATR (SR-ATR) approach which is substantially larger than for conventional ATR or specular reflection. In SR-ATR, a discontinuity of the field normal to the film contributes a field enhancement in the lower index thin film causing a two order of magnitude increase in sensitivity. SR-ATR was employed to characterize a single monolayer of undecylenic acid self-assembled on Si(1 1 1) and to investigate a two monolayer system obtained by adding a monolayer of bovine serum albumin protein.