Ultra-Narrow Linewidth Photo-Emitters in Polymorphic Selenium Nanoflakes.

Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τc ) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of -0.45% cm-1 in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics.

Angular Dependence of Solar Cell Parameters in Crystalline Silicon Solar Cells Textured with Periodic Array of Microholes.

Surface texturing is an indispensable way of increasing absorption in solar cells. In order to properly characterize the effect of texturing, the angular dependence of the incidence light should be addressed. This is particularly important when the actual application where the incidence angle of the sunlight varies during the day is considered. This study presents the angular dependence of solar cell parameters in the case of periodically textured crystalline silicon (c-Si) solar cells with microholes. A standard solar cell with pyramid texturing is also studied for comparison. It is shown that the incidence angle for the highest efficiency depends on the surface structure. While a standard pyramid-textured surface performs best at the zero angle of incidence, it is needed to tilt the sample with microholes textures 15° with respect to the surface normal. This is also confirmed by the simulation study performed for the structures presented in this study.

In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon.

Silicon is an excellent material for microelectronics and integrated photonics1-3 with untapped potential for mid-IR optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realised with techniques like reactive ion etching. Embedded optical elements, like in glass7, electronic devices, and better electronic-photonic integration are lacking8. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1 µm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has a different optical index than unmodified parts, which enables numerous photonic devices. Optionally, these parts are chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface, i.e., "in-chip" microstructures for microfluidic cooling of chips, vias, MEMS, photovoltaic applications and photonic devices that match or surpass the corresponding state-of-the-art device performances.

Nanowire decorated, ultra-thin, single crystalline silicon for photovoltaic devices.

Reducing silicon (Si) wafer thickness in the photovoltaic industry has always been demanded for lowering the overall cost. Further benefits such as short collection lengths and improved open circuit voltages can also be achieved by Si thickness reduction. However, the problem with thin films is poor light absorption. One way to decrease optical losses in photovoltaic devices is to minimize the front side reflection. This approach can be applied to front contacted ultra-thin crystalline Si solar cells to increase the light absorption. In this work, homojunction solar cells were fabricated using ultra-thin and flexible single crystal Si wafers. A metal assisted chemical etching method was used for the nanowire (NW) texturization of ultra-thin Si wafers to compensate weak light absorption. A relative improvement of 56% in the reflectivity was observed for ultra-thin Si wafers with the thickness of 20 ± 0.2 µm upon NW texturization. NW length and top contact optimization resulted in a relative enhancement of 23% ± 5% in photovoltaic conversion efficiency.

Investigation of Photoluminescence Mechanisms from SiO2/Si:SiO2/SiO2 Structures in Weak Quantum Confined Regime by Deconvolution of Photoluminescence Spectra.

Si nanocrystals embedded in SiO2 matrix were prepared by co-sputtering method followed by a post annealing process in N2 ambient. By fixing sputtering parameters, the effects of annealing time and annealing temperature on the optical properties of Si nanocrystals are investigated. Origin and evolution of the photoluminescence (PL) in weak quantum confinement regime are discussed in the light of X-ray diffraction, Fourier transform infrared, and temperature dependent photoluminescence measurements. For all samples, the PL peaks tend to decompose to four Gaussian peaks in which attributed to the radiative defects in SiO2 matrix, nc-Si/SiO2 interface related localized defects, localized states in the amorphous Si band gap and quantum confinement of excitons in smaller nanocrystals. Considering the observation of luminescence and its decomposition tendency in nanocrystals with average sizes larger than exciton's Bohr radius the necessity to distinguish between the role of smaller and larger nanocrystals in the PL mechanisms is discussed. Furthermore, possible origin of the interface related localized states in particular Si=O double bonds in the nc-Si/SiO2 interface and that of radiative defects in SiO2 matrix are discussed.

Multiscale Self-Assembly of Silicon Quantum Dots into an Anisotropic Three-Dimensional Random Network.

Multiscale self-assembly is ubiquitous in nature but its deliberate use to synthesize multifunctional three-dimensional materials remains rare, partly due to the notoriously difficult problem of controlling topology from atomic to macroscopic scales to obtain intended material properties. Here, we propose a simple, modular, noncolloidal methodology that is based on exploiting universality in stochastic growth dynamics and driving the growth process under far-from-equilibrium conditions toward a preplanned structure. As proof of principle, we demonstrate a confined-but-connected solid structure, comprising an anisotropic random network of silicon quantum-dots that hierarchically self-assembles from the atomic to the microscopic scales. First, quantum-dots form to subsequently interconnect without inflating their diameters to form a random network, and this network then grows in a preferential direction to form undulated and branching nanowire-like structures. This specific topology simultaneously achieves two scale-dependent features, which were previously thought to be mutually exclusive: good electrical conduction on the microscale and a bandgap tunable over a range of energies on the nanoscale.

Effect of surface type on structural and optical properties of Ag nanoparticles formed by dewetting.

Integration of an array of Ag nanoparticles in solar cells is expected to increase light trapping through field enhancement and plasmonic scattering. Requirement of Ag nanoparticle decoration of cell surfaces or interfaces at the macro-scale, calls for a self-organized fabrication method such as thermal dewetting. Optical properties of a 2D array of Ag nanoparticles are known to be very sensitive to their shape and size. We show that these parameters depend on the type of the substrate used. We observe that the average nanoparticle size decreases with increasing substrate thermal conductivity and nanoparticle size distribution broadens with increasing surface roughness.

Understanding the plasmonic properties of dewetting formed Ag nanoparticles for large area solar cell applications.

The effects of substrates with technological interest for solar cell industry are examined on the plasmonic properties of Ag nanoparticles fabricated by dewetting technique. Both surface matching (boundary element) and propagator (finite difference time domain) methods are used in numerical simulations to describe plasmonic properties and to interpret experimental data. The uncertainty on the locations of nanoparticles by the substrate in experiment is explained by the simulations of various Ag nanoparticle configurations. The change in plasmon resonance due to the location of nanoparticles with respect to the substrate, interactions among them, their shapes, and sizes as well as dielectric properties of substrate are discussed theoretically and implications of these for the experiment are deliberated.

Silicon nanowire-silver indium selenide heterojunction photodiodes.

Structural and optoelectronic properties of silicon (Si) nanowire-silver indium selenide (AgInSe2) thin film heterojunctions were investigated. The metal-assisted etching method was employed to fabricate vertically aligned Si nanowire arrays. Stoichiometric AgInSe2 films were then deposited onto the nanowires using co-sputtering and sequential selenization techniques. It was demonstrated that the three-dimensional interface between the Si nanowire arrays and the AgInSe2 thin film significantly improved the photosensitivity of the heterojunction diode compared to the planar reference. The improvements in device performance are discussed in terms of interface state density, reflective losses and surface recombination of the photogenerated carriers, especially in the high-energy region of the spectrum.

Exploration of the horizontally staggered light guides for high concentration CPV applications.

The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (PV) technology. The wafer cost comprises up to 35-40% of the total module cost. New approaches and system designs are needed in order to reduce the share of the wafer cost in photovoltaic energy systems. Here we explore the horizontally staggered light guide solar optics for use in Concentrated Photovoltaic (CPV) applications. This optical system comprises a lens array system coupled to a horizontal light guide which directs the incoming light beam to its edge. We have designed and simulated this system using a commercial ray tracing software (Zemax). The system is more compact, thinner and more robust compared to the conventional CPV systems. Concentration levels as high as 1000x can easily be reached when the system is properly designed. With such a high concentration level, a good acceptance angle of + -1 degree is still be conserved. The analysis of the system reveals that the total optical efficiency of the system could be as high as %94.4 without any anti-reflection (AR) coating. Optical losses can be reduced by just accommodating a single layer AR coating on the initial lens array leading to a %96.5 optical efficiency. Thermal behavior of high concentration linear concentrator is also discussed and compared with a conventional point focus CPV system.

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires.

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 µm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles.

The resonance behavior of localized surface plasmons in silver and gold nanoparticles was studied in the visible and near-infrared regions of the electromagnetic spectrum. Arrays of nano-sized gold (Au) and silver (Ag) particles with different properties were produced with electron-beam lithography technique over glass substrates. The effect of the particle size, shape variations, period, thickness, metal type, substrate type and sulfidation were studied via transmission and reflectance measurements. The results are compared with the theoretical calculations based on the DDA simulations performed by software developed in this study. We propose a new intensity modulation technique based on localized surface plasmons in nanoparticles with asymmetric shapes.

A figure of merit for optimization of nanocrystal flash memory design.

Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media.

Evaluation of the radiative recombination mechanism in Si nanocrystals embedded in Silica matrix.

We report measurements of the temperature dependence of photoluminescence (PL) life-time and efficiency of Si nanocrystals (Si-Nc) embedded in silica matrix. We use a practical technique based on lock-in acquisition that allows us to simultaneously evaluate, at each emission-energy, intensity and decay-time of the detected signal. Samples are prepared by Silicon-ion implantation in a SiO2 layer followed by thermal annealing. The implantation dose of Si ions ranges between 2 x 10(16) cm-2 and 2 x 10(17) cm(-2). Intensity of Si-Nc PL shows the characteristic rising by increasing the temperature up to approximately 100 K followed by a flattening or a weak reduction up to room temperature. This behaviour reveals a population of radiative states built up by a thermally activated process. Similarly, the measured PL decay-rate is not constant with temperature but shows evidence of a thermal activation. By measuring on different samples the activation energies Ea involved in the temperature dependence of PL intensity and decay time we verify that in all these processes Ea is a decreasing function of implantation dose (i.e., of crystallite size). This result is consistent with models connecting radiative recombination to excitons confined inside Si-Nc, in seeming contrast with the common attribution of PL of non-passivated Si-Nc to the recombination from surface/interface states. To verify the consistency of this statement, we have compared our experimental data with the predictions of quantum confinement theory obtaining an excellent agreement.

The quantum confined Stark effect in silicon nanocrystals.

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model.