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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).

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)

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy (TRTS) is an all-optical, contact-free technique that directly measures the transient mobile carrier dynamics and terahertz conductivity in materials over picosecond time scales, and is uniquely suited as a probe of conductivity in nanomaterials. Using low temperature MBE-grown silicon films as an example, we show how TRTS can be used to probe microscopic photoconductivity as well as obtain crucial insights into sample morphology. The thin silicon films consist of a mixture of amorphous and crystalline phases, and their relative content changes drastically with growth temperature. Photoexcited carrier dynamics in these films are determined by film crystallinity: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. The complex THz conductivity spectra obtained from the TRTS measurements show that the long range conductivity is significantly higher in films grown at higher temperatures that contain a larger fraction of crystalline material with larger crystal grain sizes.

Light emission from Si and Ge nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon opto-electronics and photonics. Optical interconnects are required now 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. 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. Recently, through employing novel band-gap engineering strategies, we have prepared several different entirely new bright light-emitting Si/Ge nanostructures 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. Others are based on silicon-germanium layers grown epitaxially on silicon in in such a way as to form multiple layer three-dimensional nanostructures (quantum dots). Lastly, we have 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. Here we report on the optical properties of such Si/Ge nanostructures, which are found to luminesce efficiently at wavelengths in the important spectral range of 1.1–1.6 μm. This work has been carried out in collaboration with J.-M. Baribeau and N.L. Rowell, National Research Council, Ottawa, Canada; L. Tsybeskov, New Jersey Institute of Technology, Newark, USA; and I. Berbezier, Institut Matériaux Microélectronique Nanosciences de Provence, Marseille, France.

Photoluminescence (PL) measurements were performed in Si/Si1−xGex nanostructures with a single Si0.92Ge0.08 nanometer-thick layer incorporated into Si/Si0.6Ge0.4 cluster multilayers. Under pulsed laser excitation, the PL decay associated with the Si0.92Ge0.08 nano-layer is found to be nearly a 1000 times faster compared to that in Si/Si0.6Ge0.4 cluster multilayers. A model considering Si/SiGe hetero-interface composition and explaining the fast and slow time-dependent recombination rates is proposed.

We review recent advances in the growth of Si1−xGex islands and Ge dots on (001) Si. We first discuss the evolution of the island morphology with Si1−xGex coverage and the effect of growth parameters or post-growth annealing on the shape of the islands and dots. We outline some of the structural and optical properties of Si1−xGex islands and assess progress in the determination of their composition and strain distribution. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and in particular to control their size, density, and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence the nucleation and growth of Ge islands.

The transverse (TO) and longitudinal (LO) optical phonons in AlAs, GaP, GaAs, InP, InAs and InSb have been measured at room temperature by infrared spectroscopy using an oblique incidence reflectance method. The spectra obtained were then fitted using a novel approach to determine the TO and LO phonon frequencies and damping. The results obtained are found to be more precise than in earlier reflectivity measurements using near-normal angles of incidence and provide information on the damping of both phonons. Apart from the GaAs LO mode, the observed damping parameters are found to be quite different from those predicted by theory. From these results the Lowndes condition governing the relative magnitudes of the TO and LO phonon line widths is found to be violated for all these zincblende semiconductors.

Brillouin scattering has been performed to study the effects of natural and electrochemical oxidation on the surface and bulk acoustic mode properties of porous silicon films. The acoustic mode frequencies are observed to decrease with increasing oxidation time and this is attributed to the progressive transformation of silicon–silicon bonds on the inner pore walls into SiO2. The dependence of the surface and bulk acoustic mode frequencies on the natural oxidation time allows the evaluation of these wave frequencies for a porous-SiO2 glass film, and hence the estimation of its bulk modulus. Both Brillouin and Raman data confirm the expectation that oxidation leads to a reduction in silicon nanoparticle size, and provide a correlation between the redsifted acoustic mode frequency and the oxidation period.

Luminescence can be broadly defined as the generation of light in excess of that radiated by thermal means. Since ancient times, human interest in and a long fascination with phosphorescence or luminescence originated from the mysterious fact that an apparently invisible power source could somehow produce observable light. Today, the phenomenon is generally well understood and the commercial applications of luminescence are ubiquitous, with light emitting materials being widely deployed in common commodities such as lamps, television screens, and computer and cell phone displays.

We have observed self-organization in Si nanocrystals fabricated by high temperature annealing of nanometer-thick layers of amorphous Si confined between thin SiO 2 layers. Silicon nanoparticles larger than ∼8 nm spontaneously form brick-shaped crystallites oriented along the [1 1 1] direction while smaller nanocrystals adopt a more spherical geometry. The observed ordering and self-organization are important steps towards reproducible maskless Si nanofabrication and the consequent construction of Si/SiO 2 quantum devices.

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy (MBE). From measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence we have determined the spectral dependence of the refractive index, the extinction coefficient, the optical absorption coefficient, and the real and complex components of the dielectric function. These optical dispersion relationships are contrasted with those corresponding to other forms of a-Si.

Raman spectroscopy has been used to assess the concentration dependence of optical phonons in In1−xGaxAs epilayers grown by chemical beam epitaxy on InP(100). The alloy composition was varied from x=0.325 to x=0.55 to cover the technologically important and physically interesting range where the strain changes sign at x=0.468 from negative to positive. The Raman spectra were curve resolved to reveal the GaAs-like longitudinal optic (LO), disorder induced, InAs-like LO, and InAs-like transverse optic phonons. An examination of the concentration dependence of the phonon frequencies showed that the GaAs-like LO mode varied as ω(cm−1)=252.77+58.643x−50.108x2 for 0.325⩽x⩽0.55. A comparison of these results with previous infrared and Raman work on In1−xGaxAs has revealed that the concentration dependence of the optical phonon frequencies in the unstrained system is still not accurately known.

The fabrication of size-controlled Si nanocrystals encapsulated by SiO2 is complicated by the amorphous nature of SiO2 and the mismatch in their thermal expansion coefficients. A preferred crystallographic orientation is especially important for Si quantum dots because of the Si indirect-gap band structure and a strong anisotropy in the energy-momentum dispersion. We report self-organization in Si nanocrystals fabricated by the crystallization of nanometer-thick layers of amorphous Si confined between SiO2 layers. Si nanoparticles larger than 8 nm spontaneously form brick-shaped crystallites oriented along the [111] direction. The preferred orientation and distinct shape result in a relatively narrow luminescence. The observed ordering and self-organization is an important step toward reproducible maskless Si nanofabrication and the consequent construction of Si/SiO2 quantum devices., American Physical Society, Annual March Meeting, 12-16 March 2001, Seattle, Washington, USA

A nanocrystalline silicon superlattice (nc-Si SLs) is a structure consisting of Si nanocrystal layers separated by nanometer-thick SiO2. A long range order in the nc-Si SL is obtained along the direction of growth by periodically alternating layers of Si nanocrystals and SiO2. A number of characterization techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), Auger elemental microanalysis. X-ray diffraction and X-ray small angle reflection have proved that the nc-Si SL exhibits a very narrow nanocrystal size distribution (less than 5% in average) and very abrupt and flat nc-Si/SiO2 interfaces with a roughness of

In this work we report a principle that allows one to write visible light emitting silicon patterns of arbitrary shape down to the sub-micrometer scale. We demonstrate that porous Si growth can electrochemically be initiated preferentially at surface defects created in an n-type Si substrate by Si++ ion bombardment. Using a focused ion beam (FIB) as a source of ions, arbitrary defect patterns can be written into a substrate. The growth of light emitting porous silicon is then selectively achieved by an electrochemical treatment which triggers Si dissolution only at these defect sites. The selectivity of the electrochemical dissolution reaction can be attributed to a facilitated Schottky barrier breakdown at the implanted surface defects which leads to the desired pore formation in confined surface areas.

Brillouin spectroscopy of porous Si $(\ensuremath{\pi}$-Si) formed on p-type (100) Si has revealed two acoustic-phonon peaks associated with the surface and film modes of the thick porous slab. They exhibit relatively large peak widths that are attributed to fractons. Longitudinal wave guiding is enhanced in high porosity $\ensuremath{\pi}$-Si due to its very low sound velocities compared with Si. Unlike opaque semiconductors where ripple scattering is dominant, the elasto-optic contribution to the scattering mechanism is found to be significant in $\ensuremath{\pi}$-Si. Both phonon velocities show appreciable negative dispersion due to a porosity gradient.

The fabrication of Si-based light emitting materials is a very active area of research. We have pursued two avenues of research that have provided interesting prospects for practical applications. 1. Bright quantum-confined luminescence can be obtained from Si/SiO2 superlattices. Placing them in an optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength. The superlattice PL efficiency can be considerably enhanced and its peak wavelength and bandwidth modified selectively. 2. For porous semiconductors, a method that allows the writing of visible light emitting patterns of arbitrary shape down to the nanometer scale is described. The process relies on selective electrochemistry for pore formation at surface defects created by focused ion beam implantation of Si.

The optical properties of porous GaAs formed electrochemically on n- and p-type GaAs in HCl electrolyte are reported. The porous structure comprises GaAs crystallites ranging in size from micrometers to nanometers and under certain chemical conditions other transparent crystallites of As 2 O 3 and Ga 2 O 3 form. Photoluminescence (PL) measurements at 295 K reveal an “infrared” PL at ∼840 nm and a “green” PL at ∼ 540 nm, which could easily be seen by the naked eye in some samples. The infrared and green PL peak wavelength and intensity varied from sample to sample consistent with an assignment to quantum confinement effects in GaAs micro- and nanocrystallites, respectively.

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO2 superlattices. Placing them in a one-dimensional optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and top interfaces. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of a quartz substrate case. The addition of a broad-band high reflector to the superlattice surface results in enhanced narrow-band emission. These results show that a suitably designed planar microcavity can not only considerably increase the external efficiency of luminescence in Si/SiO2 superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.

We report a principle that allows one to write visible light emitting silicon patterns of arbitrary shape down to the submicrometer scale. We demonstrate that porous Si growth can electrochemically be initiated preferentially at surface defects created in an $n$-type Si substrate by ${\mathrm{Si}}^{++}$ focused ion beam bombardment. For $n$-type material in the dark, the electrochemical pore formation potential (Schottky barrier breakdown voltage) is significantly lower at the implanted locations than for an unimplanted surface. This difference in the threshold voltages is exploited to achieve the selectivity of the pore formation process.

Diamond-like carbon films were deposited by excimer laser ablation of graphite on Zr-2.5Nb pressure tube material at substrate temperatures of 25 and 200 °C. Characterizations of the films by nanoindentation, Raman spectroscopy, and Fourier transform infrared reflection spectroscopy show that the films have a very high hardness, are transparent to infrared light, and exhibit Raman spectra typical of diamond-like carbon films. The films also exhibit a high refractive index in the infrared spectrum. Increasing the substrate temperature from 25 to 200 °C results in a 25% increase in the deposition rate, but produces lower mechanical properties for the DLC films. The high refractive index was attributed to a lack of incorporation of hydrogen into the films and the lower mechanical properties of the films deposited on the heated substrate were explained by an increase in the sp 2 sp 3 bonds ratio.

The inelastic light scattering spectra of acoustic modes have been measured for a series of CdTe Cd 1−x Mn x Te superlattices with x = 0.1. Analysis of the Brillouin to folded acoustic mode intensities yields a photoelastic coupling coefficient ratio of P alloy P CdTe = 0.18 for excitation at 1.648 eV (752.5 nm). The results indicate that the superlattice photoelastic modulation contribution dominates that of the acoustic modulation in the Raman scattering mechanism.

Under the proviso that the existing tight-binding (TB) and effective mass (EM) theoretical models provide a good description of the Ge dot energy gap versus dot diameter, this work investigates the effect of nanoparticle size and the size distribution on the near infrared PL spectrum obtained from self-assembled Ge dots grown on a thin layer of TiO2 or SiO2 on Si. For the as-grown samples, the dot PL emission occupies a wide near-infrared band between 0.8 and 1 eV. The PL efficiency versus dot size for four samples was obtained in three steps. Firstly, the PL spectrum was converted to an intensity plot versus dot diameter rather than energy by taking the PL emission from each dot to occur at the dot bandgap calculated using the TB or EM model. Secondly, a numerical form for the physical size distribution of that sample was obtained by performing a least-squares fit of a Gaussian to the dot size distribution measured by atomic force microscopy or transmission electron microscopy. Finally, the PL efficiency versus dot size was calculated using the fitted Gaussian dot size distribution to normalize the PL intensity distribution obtained in the first step. Although the absolute intensities of the PL from the samples vary, the calculated curves are all well-fitted by straight lines on a log-log plot with essentially the same slope for all samples, which indicates that under weak confinement there is a universal power-law increase in PL efficiency with decreasing dot size. © 2013 SPIE., Photonics North 2013, June 3-5, 2013, Ottawa, ON, Canada, Series: Proceedings of SPIE; no. 8915

Preface Chapter 1. Plasmonic devices for fast optoelectronics and enhanced Raman sensors Chapter 2. Recent Advances in Nanoplasmonics and Magnetoplasmonics Chapter 3. Nanoscale Photovoltaics and the Terawatt Challenge Chapter 4. Carbon Nanotube Assemblies for Transparent Conducting Electrodes Chapter 3. Nanoscale Photovoltaics and the Terawatt Challenge Chapter 4. Carbon Nanotube Assemblies for Transparent Conducting Electrodes Chapter 5. Silicon Electroplating for Low Cost Solar Cells and Thin Film Transistors Chapter 6. Resistive switching models by ion migration in metal oxides Chapter 7. Exploring Surfaces of Materials with Atomic Force Microscopy Index

Several multiquantum wells of InP/Ga x In1−x As y P1−y grown by chemical‐beam epitaxy have been studied by high‐resolution x‐ray diffraction, low‐temperature photoluminescence, and Raman scattering to characterize interfacial layers between the barriers and the wells. These interfacial layers are created during the initial stage of growth of the quaternary material as a result of the longer transient for the saturation of the group‐III elements flux. The combination of x‐ray diffraction and photoluminescence allows a precise determination of the interfacial layer thickness and composition grading and shows that interface roughness is of the order of 1 monolayer.Raman scattering confirms these results and is used to determine values of the sound velocity and of the index of refraction in the quaternary alloy material.

We have used x-ray diffraction, specular reflectivity, and diffuse scattering, complemented by Raman spectroscopy, to study the interfaces in a series of (0.5 nm Ge/2 nm Si)50 atomic layer superlattices on (001)Si grown by molecular beam epitaxy in the temperature range 150–650 °C. X-ray specular reflectivity revealed that the structures have a well-defined periodicity with interface widths of about 0.2–0.3 nm in the 300–590 °C temperature range. Offset reflectivity scans showed that the diffuse scattering peaks at values of perpendicular wave vector transfer corresponding to the superlattice satellite peaks, indicating that the interfaces are vertically correlated. Transverse rocking scans of satellite peaks showed a diffuse component corresponding to an interface corrugation of typical length scale of ∼0.5 μm. The wavelength of the undulations is a minimum along the miscut direction and is typically 30–40 times larger than the surface average terrace width assuming monolayer steps, independently of the magnitude of the wafer misorientation. The amplitude of the undulation evolves with growth temperature and is minimum for growth at ∼460 °C and peaks at ∼520 °C. Raman scattering showed the chemical abruptness of the interfaces at low growth temperatures and indicated a change in the growth mode near 450 °C

The folded acoustic modes in a series of InSb/In1-xAlxSb (0.2

We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, we first describe the successful deposition of Lanthanum Monosulfide via pulsed laser deposition on Si and MgO substrates and alumina templates. These thin films have been characterized by X-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the Scanning Anode Field Emission Microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of low work function materials having a orientation perpendicular to the surface and outcropping it are surrounded by a matrix of amorphous materials with higher work function. To date, LaS thin films have been used successfully as cold cathode emitters with measured emitted current densities as high as 50 A/cm2. Finally, we describe the successful growth of LaS thin films on InP substrates and, more recently, the production of LaS nanoballs and nanoclusters using Pulsed Laser Ablation.