Silicon Light Research Articles

Efficient Er/O‐Doped Silicon Light‐Emitting Diodes at Communication Wavelength by Deep Cooling

AbstractA silicon light source at the communication wavelength is the bottleneck for developing monolithically integrated silicon photonics. Doping silicon with erbium and oxygen ions is considered one of the most promising approaches to produce silicon light sources. However, this method suffers from a high concentration of defects in the form of nonradiative recombination centers at the interface between the crystalline silicon and large Er2O3/ErSi1.7 precipitates during the standard rapid thermal treatment. Here, a deep cooling process is applied to suppress the growth of these precipitates by flushing the high‐temperature Er/O‐doped silicon substrates with helium gas cooled in liquid nitrogen. The resultant light‐emitting efficiency at room temperature is enhanced by two orders of magnitude in comparison with that of the sample treated via standard rapid thermal annealing. The deep‐cooling‐processed Si samples are further processed into light‐emitting diodes. Bright electroluminescence with a main spectral peak at 1536 nm is also observed from the silicon‐based diodes with the external quantum efficiency reaching ≈0.8% at room temperature. Based on these results, the development of electrically driven silicon optical amplifiers or even lasers at communication wavelengths is promising for monolithically integrated silicon photonics.

All-silicon polarized light source based on electrically excited whispering gallery modes in inversely tapered photonic resonators

As a result of its indirect bandgap, emitting photons from silicon in an efficient way remains challenging. Silicon light emitters that can be integrated seamlessly on a CMOS platform have been demonstrated; however, none satisfies an ensemble of key requirements such as a small footprint, room-temperature operation at low voltages, and emission of narrow and polarized lines with a high spectral power density in the near-infrared range. Here, we present an all-silicon electrically driven light emitting diode that consists of an inversely tapered half-ellipsoidal silicon photonic resonator containing a p–n junction used to excite whispering gallery modes (WGMs) inside the resonator. Under low voltage operation at room temperature, such a photonic silicon light-emitting diode exhibits a band-edge emission (900–1300 nm) with a wall-plug efficiency of 10−4. The emitted spectrum is amplified in multiple WGMs and shows peaks that are polarized and have linewidths Δλ as narrow as 0.33 nm and spectral power densities as high as 8 mW cm−2 nm−1. Considering its small footprint of ∼1 µm and remarkable emission characteristics, this silicon light source constitutes a significant step ahead toward fully integrated on-chip silicon photonics.

Influence of radiation damage on the absorption of near-infrared light in silicon

The absorption length, λabs, of light with wavelengths between 0.95 and 1.30 μm in silicon irradiated with 24 GeV/c protons to 1 MeV neutron equivalent fluences up to 8.6×1015 cm−2 has been measured. It is found that λabs decreases with fluence due to radiation-induced defects. A phenomenological parametrization of the radiation-induced change of λabs as a function of wavelength and neutron equivalent fluence at room temperature is given. The observation of the decrease of λabs with irradiation is confirmed by edge-TCT measurements on irradiated silicon strip detectors. Using the measured wavelength dependence of λabs, the change of the silicon band-gap with fluence is determined.

Feasibility of underwater radiation detector using a silicon photomultiplier (SiPM)

In this paper, the development of an underwater radiation detector is presented using a silicon photomultiplier (SiPM), CsI(Tl) scintillator, and light guide. The detector characteristics were evaluated. The detector has good energy resolution characteristics in which the gamma-ray emission energy 122 keV of 57Co had an energy resolution of 18.12%, 356 keV of the 133Ba spectrum had 10.91%, 662 keV of the 137Cs spectrum had 8.84%, and 1332 keV of the of the 60Co spectrum had 4.55%. Further, in the case of mixed sources, the gamma radiation peaks were readily distinguishable, and the R-squared value in the gamma-ray energy of 122–1332 keV for energy linearity was calculated to be 0.99937, demonstrating an exceptional energy linearity. In addition, radiative contaminated water was prepared as a liquid radiation source and characterized. The measurement results showed 0.61 ± 0.0046 cps in the background and 0.74 0± 0.007 cps at the minimum concentration of 38.71 Bq/L . The R-squared value for concentrations in the range of 38.71–4955 Bq/L, which indicates the linearity of a detector based on the signal intensity, was determined to be 0.99710, indicating a good linear responsivity. The characterization results suggest that the radiation detectors based on SiPM- and CsI(Tl) can replace the currently used PMT-based radiation detector in an underwater environment.

Imaging System Based on Silicon Photomultipliers and Light Emitting Diodes for Functional Near-Infrared Spectroscopy

We built a fiber-less prototype of an optical system with 156 channels each one consisting of an optode made of a silicon photomultiplier (SiPM) and a pair of light emitting diodes (LEDs) operating at 700 nm and 830 nm. The system uses functional near-infrared spectroscopy (fNIRS) and diffuse optical tomography (DOT) imaging of the cortical activity of the human brain at frequencies above 1 Hz. In this paper, we discuss testing and system optimization performed through measurements on a multi-layered optical phantom with mechanically movable parts that simulate near-infrared light scattering inhomogeneities. The baseline optical characteristics of the phantom are carefully characterized and compared to those of human tissues. Here we discuss several technical aspects of the system development, such as LED light output drift and its possible compensation, SiPM linearity, corrections of channel signal differences, and signal-to-noise ratio (SNR). We implement an imaging algorithm that investigates large phantom regions. Thanks to the use of SiPMs, very large source-to-detector distances are acquired with a high SNR and 2 Hz time resolution. The overall results demonstrate the high potentialities of a system based on SiPMs for fNIRS/DOT human brain imaging applications.

Biological verification of the long-range effect for silicon light irradiation for planaria

The effect of hypersonic waves excited by a heterostructure based on a silicon wafer with natural oxide on the survival of planaria after their decapitation has been experimentally investigated. The aim of the work was to study the physical and biological factors affecting the regeneration of planarians. The main object of study was a model organism – planaria Dugesia Tigrina. The planaria were decapitated, and then they were monitored by their habitat for a week. Identification and counting of microorganisms, bacterial microflora inoculation, temperature, and pH control were carried out. To explain the mechanisms that occur under the influence of hypersound, a mathematical model of the passage of hypersonic waves through a thin layer of water near the glass-liquid interface was developed. In the process of regeneration after decapitation of the experimental group, it was found that in the experimental group exposed to hypersound, the survival of planaria was 60% higher than in the control. It was shown that in an aqueous medium along the glass-water interface, hypersonic propagation occurs with less attenuation than in the water column. This leads to a waveguide effect and improved transmission of hypersonic effects to the studied organisms.

Transmission electron microscope imaging of plasma grown electroformed silicon nitride-based light emitting diode for direct examination of nanocrystallization

We report for the first time a direct transmission electron microscope (TEM) imaging of a cross-section of a silicon nitride-based light emitting diode (LED), produced via a method patented by our research group. Grown by plasma enhanced chemical vapor deposition (PECVD) technique the LED structure (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO) was then subjected to a high forward voltage stress for one time only, i.e. electroforming process. After electroforming the LED exhibited a boosted visible light emission and memory effect. To study the structural effect of the electroforming on the as-deposited LED the cross-section was extracted by focused ion beam (FIB) technique directly from the electroformed diode and thus prepared for TEM imaging. Since the electroforming process caused crystallization of ITO and its breakup in some parts of the diode surface, the FIB was conducted for the cross-section containing some regions with ITO layer and some without ITO. TEM examination revealed the nanocrystalline phase formation within the intrinsic layer (i-SiNx:H) caused by the electroforming process. The average size and distribution of Si nanocrystallites formed inside i-SiNx:H was determined. The Si nanocrystallization within i-SiNx:H was compared for the regions with and without ITO layer. The previously proposed model describing the changes taken place in the diode during electroforming process was reconsidered in the light of this TEM analysis.

Engineered Stresses for a Functional Si Light Emitter at Bandgap: An Overview

Interfacing various materials results in stresses and strains that typically alter the properties of either or both interfaced surfaces. In this work, we focus on enhanced light emission properties at the bandgap for silicon interfaced with silica or layers of silica and silicon nitride. This discovery is corroborated by several evidences of engineered stresses and strains at the bulk or interface such as implantation and/or diffusion of dopants in silicon and rough silicon surfaces formed by wafer cutting or etching and patterning. Silicon light emitters are being eagerly sought, and upon achieving reasonable efficiencies, they may be at hand soon. The significant investment in Si-photonics by dominant semiconductor players implies that such devices will be welcomed particularly in applications such as optical emitters for next-generation integrated optical circuits and interconnects; all-Si transceivers and cross connects for integrated circuit chips; light-wave components and high-power discrete and array emitters. Engineered stresses and strains modulate the indirect Si bandgap, resulting in enhanced radiative electron–hole recombination, which competes more effectively with non-radiative recombination.

Towards integrated tunable all-silicon free-electron light sources

Extracting light from silicon is a longstanding challenge in modern engineering and physics. While silicon has underpinned the past 70 years of electronics advancement, a facile tunable and efficient silicon-based light source remains elusive. Here, we experimentally demonstrate the generation of tunable radiation from a one-dimensional, all-silicon nanograting. Light is generated by the spontaneous emission from the interaction of these nanogratings with low-energy free electrons (2–20 keV) and is recorded in the wavelength range of 800–1600 nm, which includes the silicon transparency window. Tunable free-electron-based light generation from nanoscale silicon gratings with efficiencies approaching those from metallic gratings is demonstrated. We theoretically investigate the feasibility of a scalable, compact, all-silicon tunable light source comprised of a silicon Field Emitter Array integrated with a silicon nanograting that emits at telecommunication wavelengths. Our results reveal the prospects of a CMOS-compatible electrically-pumped silicon light source for possible applications in the mid-infrared and telecommunication wavelengths.

Improved light extraction efficiency of complementary metal-oxide semiconductor hot carrier lights sources with the use of improved back-end-of-line light directing structures

Previous research has shown that the use of back-end-of-line (BEOL) light directing structures with silicon hot carrier light sources in a complementary metal-oxide semiconductor results in improved light extraction efficiency. This work focuses on the design of an improved back-end-of-line structure for improving light extraction efficiency when using substrate-based silicon light emitters. With the use of FRED optical engineering ray-tracing software, it was found that a significant amount of generated light is lost at the material interfaces of the optical structure, including losses due to significant internal reflections. Therefore, an optimized optical structure was designed to reduce internal reflections at the base of the structure. Simulation results show a 33.6% improvement in light extraction efficiency over the previously designed parabolic optical structure, over the visible spectrum. The light sources were tested using a parameter analyzer, radiometer, spectrometer, and goniometer. It was calculated that the luminance exiting the optimized optical structure had a 55.66-factor improvement over the control structure and a 1.35-factor improvement over the parabolic structure. Furthermore, the optimized structure had a 1.38-factor improvement in light extraction efficiency over the parabolic structure. Overall, the improved designed pipe-like BEOL light directing structure helped to improve the device luminescence and light emission direction from the light source, which invariably increased the light extraction efficiency.

Effects of Photonic Band Structure and Unit Super-Cell Size in Graded Photonic Super-Crystal on Broadband Light Absorption in Silicon

The newly discovered graded photonic super-crystal (GPSC) with a large size of unit cell can have novel optical properties that have not been explored. The unit super-cell in the GPSC can be designed to be large or small and thus the GPSC can have no photonic band gap or several gaps. The photonic band structures in Si GPSC can help predict the light absorption in Si. Photonic resonance modes help enhance the absorption of light in silicon; however, photonic band gaps decrease the absorption for light with a large incident angle. The Si device patterned in GPSC with a unit super-cell of 6a × 6a (a is a lattice constant in traditional photonic crystal) has a broadband high absorption with strong incident-angular dependence. The device with the unit super-cell of 12a × 12a has relatively low light absorption with weak incident-angle dependence. The Si GPSC with a unit super-cell of 8a × 8a combines both advantages of broadband high absorption and weak dependence of absorption on the incident angle.

Nano-antenna enhanced waveguide integrated light source based on an MIS tunnel junction.

Ultrafast electro-optical conversion at nanoscale is of fundamental interest for information transfer and optical interconnects. Light emission from a quantum tunnel junction provides an opportunity owing to its unique capability of ultrafast response and small footprint. However, the main challenge to the wide adoption of the tunnel junction is its low emission efficiency caused by the low inelastic electron tunneling proportion and radiation efficiency. In this Letter, an electrically driven silicon light source with its efficiency enhanced by using a nano-antenna in a metal-insulator-semiconductor junction is proposed. Strong plasmon confinement in the nano-antenna provides large local density of optical states and bridges the wave vector mismatch between nanoscale volume field confinement and far-field radiation. Two orders of magnitude of emission enhancement are achieved over typical planar MIS junctions.

Optical absorption beyond the Yablonovitch limit with light funnel arrays

Light trapping in the spectral range of visible to the near infrared is important for a plethora of energy-related photonic devices. The study numerically examines light trapping in arrays of subwavelength silicon light funnels (LF arrays) realized on silicon-on-insulator (SOI) wafers. We demonstrate the possibility of light trapping beyond the Yablonovitch limit and ~5% enhancement beyond the limit is shown. The SOI wafers are used for two reasons: firstly, SOI wafers introduce two bottom interfaces which allow efficient optical coupling between the LF arrays and the underlying substrates and, secondly, the potential for the realization of energy harvesting photonic devices realized on SOI wafers. Strong light trapping and high absorption in LF array-substrate complexes is shown for relatively short LF arrays. The strong absorption peaks correspond to a high optical intensity in the arrays which is 3–4 orders of magnitude higher than in ambient. Subsequently, these absorption peaks conclude strong excitations in the substrates. We show that the overall transmission is low on account of the two bottom interfaces of the SOI geometry. Specifically, the strong absorption peaks are due to low transmission coupled with low reflectivity which suggests forward scattering by the arrays into the substrates. Next, light trapping in LF array-substrate complexes is examined for higher LF arrays, and the dependency of the LF bottom diameter on the overall absorption of the complex is studied. We show that for small bottom diameter the excitation of the substrate is poor possibly due to lack of photonic states at the LF bottom. The excitation of the substrates increases as the LF bottom diameter is increased, however dramatic decrease in absorption is recorded for a bottom diameter that reflects the geometry of a nanopillar array. We show that the substrate excitation by a LF array is more efficient than the excitation by a nanopillar array as LFs provide a homogenous power spread in the substrate, whereas the substrate excitation by nanopillars is governed by an anisotropic power spread.

A dependency of emission efficiency of poly-silicon light-emitting device on avalanching current

A novel polysilicon light emitting device (LED) was realized in a standard complementary metal oxide semiconductor (CMOS) process that is based on a p-n junction reverse bias configuration. This LED can emit visible light based on the reverse bias p-n junctions in the dark. The central emitting doped structure element of this LED is n+-p-n+-p-n+ stacked layout, which is similar to two reverse bias p-n junctions and two forward bias p-n junctions connected in series. The device mechanism for the visible light emitting process is defined by means of an avalanche breakdown process that occurs between the highly doped n+ region and the lightly doped p region in the reverse bias p-n junctions. By using hot carriers generated in avalanche process, the emission spectrum from the device exhibits a wide spectrum whose wavelength range is from 400 nm to 900 nm at an operating voltage of 16 V. We compare the designed light intensities at the different wavelengths in other to obtain the dependency of the light emission at various wavelengths under different currents conditions. From the experimental observations, and based on the calculation of the quantum efficiency and power conversion efficiencies as related to the light emission, we confirmed that this particular LED has a better efficiency than three-terminal gated diode. The silicon light source could find some applications in on-chip optical interconnect and in electro-optical conversions in future all-silicon integrated photonic circuitry.

Application prospect of metal-oxide-semiconductor silicon light emitting devices in integrated circuits

Photonic interconnects have potentials to break increasingly severe energy efficiency and bandwidth density bottlenecks of electrical interconnect in scaled complementary metal oxide semiconductor (CMOS) integrated circuits, leading to the emergence of optoelectronic integrated circuits (OEICs) that utilize electronic and photonic devices together in a synergistic way to achieve better performance than those based on pure electronic device technology. By reviewing the progresses of Si-based light-emitting device, the schematic of MOS-like light source integrated with waveguides and the following photodetector is analyzed for its availability. It is believed that on-chip optical interconnects could be achieved by standard CMOS technology successfully with the speed as fast as the velocity of light, supplying propulsions for nest-generation OEICs.

Atomic Layer Deposited TiO2–IrOx Alloys Enable Corrosion Resistant Water Oxidation on Silicon at High Photovoltage

We synthesized by atomic layer deposition (ALD) TiO2–IrOx alloys that enable high photovoltages and catalyze water oxidation on silicon metal–insulator–semiconductor (MIS) photoanodes. The ratio of TiO2 to IrOx was precisely controlled by varying the number of ALD cycles for each precursor. Silicon with a 2 nm surface SiO2 layer was coated with TiO2–IrOx alloys ranging in composition from 18 to 35% iridium relative to the sum of titanium and iridium concentrations. IrOx catalyzed oxygen evolution and imparted a high work function to the TiO2–IrOx alloys, enabling photovoltages during water oxidation that exceeded 600 mV. TiO2 imparted stability and inhibited corrosion of the underlying silicon light absorber. After annealing in forming gas (5% H2/95% N2), TiO2–IrOx alloys were stable for 12 h of continuous water oxidation in 1 M H2SO4. Key properties of the MIS junction affecting electrochemical operation were also extracted by electrochemical impedance spectroscopy. This work demonstrates that alloying b...

Broadband absorption of the solar radiation with surface arrays of subwavelength light funnels

Surface arrays of subwavelength silicon light funnels (LFs) were recently introduced as a promising new avenue towards efficient light trapping and the broadband absorption of the solar radiation. In the current work we use finite-difference time-domain calculations to understand the nature of the optical coupling between silicon LF arrays and underlying silicon substrates. We start by examining the broadband absorption dependency of free-floating LF arrays on the LF bottom diameter, and we show that light trapping at the Yablonovitch limit can be realized by tuning the LF bottom diameter. Next we examine the optical coupling between LF arrays and underlying substrates. We show that the array-substrate complexes exhibit strong absorption peaks at the near infrared. The origin of the absorption peaks is traced to either strong optical excitation of the LF arrays (occupation of Mie modes), strong excitations of the substrates (guided modes, Fabry-Perot and mode hybridization), or excitations of both the arrays and the substrates. Finally, we demonstrate light trapping at the Yablonovitch limit with the LF array-substrate complex.

Status of the CALDER project: Cryogenic light detectors for background suppression

The development of large area cryogenic light detectors is one of the priorities of next generation bolometric experiments searching for neutrinoless double beta decay. The simultaneous read-out of the heat and light signals enables particle identification, provided that the energy resolution and the light collection are sufficiently high. CALDER (Cryogenic wide-Area Light Detectors with Excellent Resolution) is developing phonon-mediated silicon light detectors using KIDs , with the goal of sensing an area of 5 × 5 cm 2 with a resolution of 20 eV RMS. We present the latest results obtained with aluminum chips and with newly developed multi-layer titanium–aluminum chips featuring a remarkable sensitivity.

Tuning Electroluminescence from a Plasmonic Cavity-Coupled Silicon Light Source

The combination of Moore's law and Dennard's scaling rules have constituted the fundamental guidelines for the silicon-based semiconductor industry for decades. Furthermore, the enormous growth of global data volume has pushed the demand for complex and densely packed devices. In recent years, it has become clear that wired interconnects impose increasingly severe speed and power limitations onto integrated circuits as scaling slows toward a halt. To overcome these limitations, there is a clear need for optical data processing. Despite significant progress in the development of silicon photonics, light sources remain challenging owing to the indirect bandgap of group IV materials. It is therefore highly desirable to develop new concepts for a silicon light source that meets efficiency and footprint requirements similar to their electronic counterparts. Here, we demonstrate an electrically driven and tunable silicon light source by matching the resonant modes of a silver nanocavity with the hot luminescence spectrum of an avalanching p-n junction. The cavity significantly enhances phonon-assisted recombination of hot carriers by tailoring the local density of states at the size-tunable resonance. Such tunable nanoscale emitter may be of great interest for short-reach communications, microdisplays or lab-on-chip applications.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths.

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 μm ( Eg = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.