Thin films of germanium-tin (GeSn) alloy with Sn content well above its equilibrium solubility limit in Ge are produced using both remote plasma-enhanced chemical vapor deposition (RPECVD) directly on silicon substrates and ion implantation of Sn into Ge. For RPECVD, the growth temperature of 302 °C resulted in fully relaxed GeSn alloys with high defect density, principally threading dislocations related to the large lattice mismatch between Si and GeSn. For the implantation case, pulsed laser melting was used to melt and crystallize the GeSn layer on a time scale of a few tens of nanoseconds. The resulting GeSn layers were also relaxed and defective, presumably again as a result of lattice mismatch with the underlying Ge lattice. However, the nature of the defects was quite different to the RPECVD method, whereby the line defects were not threading dislocations but stackinglike defects, which developed into arrays of these defects in the high Sn content region close to the surface. For the purpose of comparing RPECVD and ion-implantation methods, alloy films of similar thickness (400–450 nm) and Sn content (4.5–6.5 at. %) were examined. Film parameters (thickness, Sn content, Sn solubility, and segregation), as well as film quality and defect structures, were examined for both fabrication methods using several analytical techniques. This comparison provided us with a better physical understanding of our GeSn films and will help inform future growth/fabrication strategies targeted at minimizing defects formed in the GeSn films for the realization of optoelectronic devices.

Hyperdoped silicon is a promising material for near-infrared light detection, but to date, the device efficiency has been limited. To optimize photodetectors based on this material that operate at room temperature, we present a detailed study on the electrical nature of gold-hyperdoped silicon formed via ion implantation and pulsed-laser melting (PLM). After PLM processing, oxygen-rich and gold-rich surface layers were identified and a wet etch process was developed to remove them. Resistivity and Hall effect measurements were performed at various stages of device processing. The underlying gold-hyperdoped silicon was found to be semi-insulating, regardless of whether the surface gold was removed by etching or not. We propose a Fermi level pinning model to describe the band bending of the transformed surface layer and propose a promising device architecture for efficient Au-hyperdoped Si photodetectors.

Hyperdoping Si with transition metals to form intermediate bands for infrared absorption has attracted attention recently for producing sub-bandgap photoconductivity. In particular, Si hyperdoped with Au has been demonstrated to exhibit optoelectronic response at 1550 nm. However, the reported external quantum efficiencies (EQEs) are low, and the device fabrication processes had not been optimized. In this paper, we demonstrate a significant improvement in sub-bandgap EQE through modification of the material and device fabrication processes. By increasing the Si:Au layer thickness, modification of device design, and formation of Ohmic contacts, the EQE was measured to be as high as 0.44% at 1550 nm, nearly two orders of magnitude higher than previous reports from similar devices. Additionally, the EQE was measured to be in the 10-3 range for wavelengths as long as 2.4 μm. The EQE spectrum showed features that were attributed to defect levels from a substitutional Au acceptor defect. The above bandgap EQE showed gain in one device. Thermal annealing at 300 °C does not improve the efficiency of Si:Au photodiodes. These results demonstrate the viability of Au-hyperdoped Si for infrared detection below the bandgap of Si.

In recent years, infrared photodetectors using silicon hyperdoped with deep-level dopants started to demonstrate extended light detection beyond the silicon’s absorption edge. The reported responsivities or external quantum efficiencies, however, are typically low. Focusing on gold-hyperdoped silicon and using time-resolved terahertz spectroscopy, a non-contact photoconductivity measurement, we investigated how hyperdoping parameters affect charge carrier lifetimes. Correlating the observed lifetime characteristics with dopant distribution profiles, we identify factors that impact carrier lifetime most significantly. Specifically, the charge carrier lifetime reduces with increasing gold concentrations, increasing ion implantation energies, and increasing pulsed-laser melting fluences. Both ion implantation energy and laser fluence affect the dopant incorporation depths. The total gold dose implanted and laser fluence affect the carrier distribution profile, particularly the concentration spike toward the surface. Oxide passivation and the number of laser pulses do not impact the carrier lifetime significantly. Our findings benefit future device developments.

Germanium–tin (GeSn) alloys at sufficiently high Sn concentration, above several atomic percent, are the only group IV semiconductor exhibiting a direct bandgap and have generated much recent interest for optoelectronic applications into the mid-infrared region. Because the large lattice mismatch between GeSn and Si results in considerable strain for thin layers and a high defect density for thicker strain-relaxed layers, most reported GeSn growths incorporate a Ge buffer layer rather than depositing directly on Si substrates. Published reports of GeSn growth directly on Si utilize specialized precursors such as higher order germanes (Ge2H6, Ge3H8, or Ge4H10) or SnD4. In this paper, we report GeSn films with up to 10.6% Sn grown directly on Si substrates by remote plasma-enhanced chemical vapor deposition using GeH4 and SnCl4 precursors. These alloys have been characterized in detail using x-ray diffraction (XRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry with channeling (RBS-C), as well as Raman spectroscopy (RS) and optical microscopy. The films studied are almost fully relaxed, with small residual strain observed, particularly in thinner films, and contain a high interface density of misfit dislocations that increases with Sn concentration. The defect density decreases toward the surface. Good agreement is found between the various characterization methods for the Sn content (XRD and RBS-C), lattice parameter measurement (XRD and TEM), and defect characterization (RBS-C, TEM, and RS). Such characterization of GeSn grown directly on Si substrates is essential to allow growth parameters to be optimized for the realization of the attractive optoelectronic properties of these alloys.

Over the past decade, cavity solitons have attracted substantial attention for their rich dynamics and their myriad potential applications. Recently, there has been growing interest in understanding cavity solitons in systems of coupled resonators, where both new physics and applications can emerge. While numerous works have theoretically studied the interplay between cavity solitons and lattice topology, experimental demonstrations of cavity solitons in topological lattices remain elusive. Here, we experimentally realize cavity solitons in a Su-Schrieffer-Heeger (SSH) lattice and illustrate that the synergy between topology and soliton formation dynamics can induce soliton formation at the boundaries of a topological SSH lattice. Our work illustrates the rich physics of cavity solitons in topological lattices and demonstrates a flexible approach to study solitons in large-scale coupled resonator arrays.

Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to be by the hopping mechanism through localized states. The carrier mobility of these materials is usually very low, in the order of ~10^-3 - 10^-2 cm^2/(Vs) at room temperature. In this study, we present the Hall effect characterization of a-Ge prepared by self-ion implantation of Ge ions. The a-Ge prepared by this method is highly homogenous and has a mass density within 98.5% of the crystalline Ge. The material exhibits an exceptionally high electrical conductivity and carrier mobility (~100 cm^2/(Vs)) for an amorphous semiconductor. The temperature-dependent resistivity of the material is very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results are direct evidence for a largely-preserved band structure and non-localized states of the valence band in a-Ge, as proposed by Tauc et al. from optical characterization alone. This finding is not only significant for the understanding of electrical conductivity in covalent disordered semiconductors, but the exceptionally high mobility we have observed in amorphous Ge opens up device applications not previously considered for amorphous semiconductors.

Abstract The TOTEM collaboration at the CERN LHC has measured the differential cross-section of elastic proton–proton scattering at $$\sqrt{s} = 8\,\mathrm{TeV}$$ s = 8 TeV in the squared four-momentum transfer range $$0.2\,\mathrm{GeV^{2}}< |t| < 1.9\,\mathrm{GeV^{2}}$$ 0.2 GeV 2 < | t | < 1.9 GeV 2 . This interval includes the structure with a diffractive minimum (“dip”) and a secondary maximum (“bump”) that has also been observed at all other LHC energies, where measurements were made. A detailed characterisation of this structure for $$\sqrt{s} = 8\,\mathrm{TeV}$$ s = 8 TeV yields the positions, $$|t|_{\mathrm{dip}} = (0.521 \pm 0.007)\,\mathrm{GeV^2}$$ | t | dip = ( 0.521 ± 0.007 ) GeV 2 and $$|t|_{\mathrm{bump}} = (0.695 \pm 0.026)\,\mathrm{GeV^2}$$ | t | bump = ( 0.695 ± 0.026 ) GeV 2 , as well as the cross-section values, $$\left. {\mathrm{d}\sigma /\mathrm{d}t}\right| _{\mathrm{dip}} = (15.1 \pm 2.5)\,\mathrm{{\mu b/GeV^2}}$$ d σ / d t dip = ( 15.1 ± 2.5 ) μ b / GeV 2 and $$\left. {\mathrm{d}\sigma /\mathrm{d}t}\right| _{\mathrm{bump}} = (29.7 \pm 1.8)\,\mathrm{{\mu b/GeV^2}}$$ d σ / d t bump = ( 29.7 ± 1.8 ) μ b / GeV 2 , for the dip and the bump, respectively.

Incorporating ultrahigh concentrations of deep-level dopants in silicon drastically alters silicon’s optoelectronic properties. Photodiodes built from silicon hyperdoped with gold extend light sensitivity into the shortwave infrared region, far beyond the absorption edge of a pristine silicon sample. Deep-level dopants, however, also enhance carrier recombination; even though hyperdoped silicon has great light absorption properties, short charge carrier lifetime limits its applications. In this work, using terahertz spectroscopy, we investigate the charge carrier lifetime of gold–hyperdoped silicon, where the gold dopants are introduced by either film deposition or ion implantation, followed by pulsed laser melting. Using reactive ion etching, we measure how carrier lifetime changes when dopant concentration profiles are altered. Furthermore, using a 1D diffusion and recombination model, we simulate carrier dynamics when electrons are excited by sub-bandgap light. Our results show that the dopant distribution profile heavily influences excited carrier dynamics. We found that etching improves the half-life by a factor of two. In the short-wave-infrared range, the gold dopants are both light absorption centers and recombination centers. Focusing on optoelectronic properties in the short-wave-infrared region, our results suggest that these samples are over doped—etching much of the gold dopants away has little impact on the number of excited electrons at a later time. Our results suggest that dopant profile engineering is important for building efficient optoelectronic devices using hyperdoped semiconductors.

Ion implantation of transition metals into Si, followed by pulsed laser melting and rapid solidification, shows promise for making Si devices with sub-band gap optoelectronic response. We study Si implanted with Au at doses ranging from 1015–1016 at cm−2, with all but the lowest dose exhibiting interface breakdown during solidification, resulting in heavily defected layers. Terahertz photocarrier lifetime measurements confirm that layers with breakdown show recombination lifetimes of about 100 ps, compared to 800 ps for a layer with no breakdown. Device measurements, however, show more photoresponse at 1550 nm in a layer with breakdown than in a layer without. The results suggest that avoiding breakdown may be desirable but might not necessarily be imperative for making a useful device.

We present a study of the sub-bandgap photoresponse and leakage current in gold-hyperdoped silicon photodiodes prepared using pulsed laser melting (PLM) of sub-nanometer gold films on n -type silicon substrates. Variable-temperature photo- and dark-current analysis provide insight into the role of PLM conditions on device performance. In general, we find photocurrent activation energies comparable to room temperature, suggesting a weak thermally-assisted optical photoresponse mechanism. Additionally, we establish a connection between repetitive PLM pulsing and increased device leakage current, which originates from electrically-active defects. Finally, we propose an explanation for the limited sub-bandgap external quantum efficiencies reported for hyperdoped silicon devices on the basis that the depletion layer largely does not encompass the hyperdoped layer where absorption occurs.

Hyperdoping germanium with gold is a potential method to produce room-temperature short-wavelength-infrared radiation (SWIR; $1.4--3.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium ($\mathrm{Ge}$:$\mathrm{Au}$) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that $\mathrm{Ge}$:$\mathrm{Au}$ carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-band-gap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of $\mathrm{Ge}$:$\mathrm{Au}$ for SWIR photodetection applications.

As discussed in the previous chapters, ion implantation followed by pulsed laser melting is a well-studied process for achieving localized heavy doping in semiconductors, where the dopant concentration can exceed the equilibrium solid solubility limit by orders of magnitude. In addition to the useful electrical properties exhibited by semiconductors hyperdoped with p- and n-type dopants, novel material properties have also been observed over the recent years in single-crystalline semiconductors containing a nonequilibrium concentration of various nondopant impurities. In this chapter, we focus on the intentional introduction of impurities into semiconductors as a method to modify their electronic band structure and, therefore, optical properties, a process which has recently become known as "optical hyperdoping." The research effort in optical hyperdoping has almost exclusively focused on Si and most of this chapter is dedicated to reviewing the literature on optical hyperdoping in Si. However, there has been an emerging work on hyperdoped Ge which we summarize. In addition, a logical extension of the Ge work to dilute GeSn alloys, also formed by implantation and pulsed laser melting, has received attention recently and a brief overview of this body of research is provided.

Over four decades ago, pulsed-laser melting, or pulsed-laser annealing as it was termed at that time, was the subject of intense study as a potential advance in silicon device processing. In particular, it was found that nanosecond laser melting of the near-surface of silicon and subsequent liquid phase epitaxy could not only very effectively remove lattice disorder following ion implantation, but could achieve dopant electrical activities exceeding equilibrium solubility limits. However, when it was realised that solid phase annealing at longer time scales could achieve similar results, interest in pulsed-laser melting waned for over two decades as a processing method for silicon devices. With the emergence of flat panel displays in the 1990s, pulsed-laser melting was found to offer an attractive solution for large area crystallisation of amorphous silicon and dopant activation. This method gave improved thin film transistors used in the panel backplane to define the pixelation of displays. For this application, ultra-rapid pulsed laser melting remains the crystallisation method of choice since the heating is confined to the silicon thin film and the underlying glass or plastic substrates are protected from thermal degradation. This article will be organised chronologically, but treatment naturally divides into the two main topics: (1) an electrical doping research focus up until around 2000, and (2) optical doping as the research focus after that time. In the first part of this article, the early pulsed-laser annealing studies for electrical doping of silicon are reviewed, followed by the more recent use of pulsed-lasers for flat panel display fabrication. In terms of the second topic of this review, optical doping of silicon for efficient infrared light detection, this process requires deep level impurities to be introduced into the silicon lattice at high concentrations to form an intermediate band within the silicon bandgap. The chalcogen elements and then transition metals were investigated from the early 2000s since they can provide the required deep levels in silicon. However, their low solid solubilities necessitated ultra-rapid pulsed-laser melting to achieve supersaturation in silicon many orders of magnitude beyond the equilibrium solid solubility. Although infrared light absorption has been demonstrated using this approach, significant challenges were encountered in attempting to achieve efficient optical doping in such cases, or hyperdoping as it has been termed. Issues that limit this approach include: lateral and surface impurity segregation during solidification from the melt, leading to defective filaments throughout the doped layer; and poor efficiency of collection of photo-induced carriers necessary for the fabrication of photodetectors. The history and current status of optical hyperdoping of silicon with deep level impurities is reviewed in the second part of this article.

Following recent successful demonstrations of enhanced infrared absorption in Au-hyperdoped Si, there has been strong interest in fabricating other metal-hyperdoped Si systems as a highly attractive approach for Si-based infrared photodetection. In this work, we address the somewhat contentious issue in the literature as to whether it is possible, using ion implantation and nanosecond pulsed-laser melting, to achieve hyperdoping of Si with Ag and Ti at concentrations exceeding that required to form an intermediate impurity band within the Si bandgap ( N IB ∼ 6 × 10 19 cm − 3). A wide range of characterization techniques were used to investigate these material systems, especially the quality of liquid-phase epitaxy, impurity concentration distribution both in depth and laterally, and impurity lattice location. Our results indicate that the high concentrations of opto-electrically active Ag or Ti in monocrystalline Si required to form an impurity band are not achieved. In particular, the usual behavior during rapid solidification is for near-complete surface segregation of the impurity, or for it to be trapped within a highly defective subsurface layer due to filamentary breakdown. Although our measurements showed that the maximum concentration of impurities outside metal-rich filaments is comparable to N IB for both Ag and Ti, there is no preferential Ag or Ti lattice location after pulsed-laser melting anywhere in the material. Thus, the concentration of opto-electrically active Ag and Ti that can be homogeneously incorporated into Si is expected to be well below N IB, leaving Au as the only viable impurity to date for achieving the required level of hyperdoping in Si.

Hyperdoping semiconductor is one of the material candidates for intermediate band photovoltaics. We investigate implantation damage and associated defects. Time resolved terahertz spectroscopy reveals that charge carrier lifetime is hindered by defects not detected by conventional microscopic and spectroscopic techniques.

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25–165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure–temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

1 Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA 2 Departamento de Estructura de la Materia, Física térmica y Electrónica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid 28040, Spain 3 Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia 4 Department of Physics and Astronomy, Ångström Laboratory, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden 5 Robert Bosch LLC, Cambridge, Massachusetts 02138, USA 6 Dipartimento di Fisica e Astronomia, Università di Padova and CNR-IMM, Via Marzolo 8, I-35131 Padova, Italy

A search is conducted for a low-mass charged Higgs boson produced in a top quark decay and subsequently decaying into a charm and a strange quark. The data sample was recorded in proton-proton collisions at $\\sqrt{s}=$ 13 TeV by the CMS experiment at the LHC and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The search is performed in the process of top quark pair production, where one top quark decays to a bottom quark and a charged Higgs boson, and the other to a bottom quark and a W boson. With the W boson decaying to a charged lepton (electron or muon) and a neutrino, the final state comprises an isolated lepton, missing transverse momentum, and at least four jets, of which two are tagged as b jets. To enhance the search sensitivity, one of the jets originating from the charged Higgs boson is required to satisfy a charm tagging selection. No significant excess beyond standard model predictions is found in the dijet invariant mass distribution. An upper limit in the range 1.68-0.25% is set on the branching fraction of the top quark decay to the charged Higgs boson and bottom quark for a charged Higgs boson mass between 80 and 160 GeV.