Silicon Quantum Dot Superlattice Research Articles

Quantum Dot Gate (QDG) Quantum Dot Channel (QDC) Multistate Logic Non-Volatile Memory (NVM) with High-K Dielectric HfO2 Barriers

This paper presents the theory, fabrication and experimental testing results for a multiple state Non-Volatile Memory (NVM), comprised of hafnium oxide high-k dielectric tunnel and gate barriers as well as a Silicon Quantum Dot Superlattice (QDSL) implemented for the floating gate and inversion channel (QDG) and (QDC) respectively. With the conclusion of Moore’s Law for conventional transistor fabrication, regarding the minimum gate size, current efforts in memory cell research and development are focused on bridging the gap between the conventions of the past sixty years and the future of computing. One method of continuing the increasing chip density is to create multistate devices capable of storing and processing additional logic states beyond 1 and 0. Replacing the silicon nitride floating gate of a conventional Flash NVM with QDSL gives rise to minibands that result in greater control over charge levels stored in the QDG and additional intermediate states. Utilizing Hot Carrier Injection (HCI) programming, for the realized device, various magnitudes of gate voltage pulses demonstrated the ability to accurately control the charge levels stored in the QDG. This corresponds to multiple threshold voltage shifts allowing detection of multiple states during read operations.

Influence of barrier layer’s height on the performance of Si quantum dot solar cells

We investigated the influence of the barrier layer height in silicon quantum dot superlattice (Si-QDSL) solar cells on the cell performance by two-dimensional device simulation taking into account the quantum size effect. Although at Si-QD diameters (d) of 5 and 10 nm better cell performance was obtained as the barrier height increased, in the case of d = 3 nm, the efficiency decreased as the barrier height increased. Further investigation showed that the band mismatch at the interface between Si-QDSL and the n-type layer degraded open-circuit voltage (Voc) at d = 3 nm. After the optimization of the physical properties of the n-type layer, Voc was improved to 1.43 V and the bandgap-Voc (Woc) offset became about 0.4 V, suggesting that the band mismatch is the significant factor for the Voc loss. From these results, it was found that careful selection of n- or p-type materials is necessary when the strong quantum confinement effect occurs in Si-QDSL.

Design and Simulation of Intermediate Band Solar Cell With Ultradense Type-II Multilayer Ge/Si Quantum Dot Superlattice

We studied the miniband dependence on the structural parameters and shape in type-II multilayer germanium (Ge)/silicon (Si) quantum dot superlattice (QDSL) solar cell. A maximum tunable range of ground-state energy is 19% by tuning layer distance down to 0.5 nm, whereas 24.5% is achieved by adjusting the horizontal dot-to-dot spacing down to 0.3 nm. The reduction of effective bandgap is severe for cylindrical QDs than ellipsoidal and conical QDs in the ultradense QDSL, thus leading to a relatively lower conversion efficiency. On average, the thickness of QD shows a negative correlation to conversion efficiency. We observed a high conversion efficiency of 27.22% in a bilayer conical QDSL under an AM1.5 spectral irradiance and one sun illumination.

Effect of ALD-Al2O3 Passivated Silicon Quantum Dot Superlattices on p/i/n+ Solar Cells

The photovoltaic (PV) nature of the silicon (Si) quantum dot super lattice (QDSL) is studied with an atomic-layer-deposited aluminum oxide film (ALD-Al2O3) and a conventional sputtered-grown amorphous silicon carbide film (a-SiC). The QDSL structures act as an intermediate layer in a p/i/n+ Si solar cell. The QDSL consists of 4-nm Si on 2-nm SiC nanodisks (NDs) arrayed in an ALD-Al2O3 and a-SiC passivation matrix. Formation of Si-NDs was confirmed by bright field scanning transmission electron microscope. A significant PV response in generating a high photocurrent density $\mathit {J_{sc}}$ of 30.15 mA/cm $^{2}$ , open circuit voltage $\mathit {V_{oc}}$ of 0.50 V, fill factor FF of 0.61, and efficiency $\eta $ of 9.12% was observed in ALD-Al2O3/QDSL solar cell with respect to a-SiC/QDSL solar cell with $\mathit {J_{sc}}$ of 26.94 mA/cm $^{2}$ , $\mathit {V_{oc}}$ of 0.50 V, FF of 0.47, and $\eta $ of 6.42%. A wide range of photo-carrier transports by the ALD-Al2O3/QDSL structure is possible in the external quantum efficiency spectra with respect to a-SiC/QDSL solar cell. The enhanced PV performance of the QD solar cells was clarified in terms of simulating the absorption contributions for all possible transitions in the nanostructure with different passivation films.

Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

AbstractThe photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density Jsc of 29.24 mA/cm2, open circuit voltage Voc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with Jsc of 25.27 mA/cm2, Voc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with Jsc of 27.63 mA/cm2, Voc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.

Silicon quantum dot superlattice solar cell structure including silicon nanocrystals in a photogeneration layer.

The solar cell structure of n-type poly-silicon/5-nm-diameter silicon nanocrystals embedded in an amorphous silicon oxycarbide matrix (30 layers)/p-type hydrogenated amorphous silicon/Al electrode was fabricated on a quartz substrate. An open-circuit voltage and a fill factor of 518 mV and 0.51 in the solar cell were obtained, respectively. The absorption edge of the solar cell was 1.49 eV, which corresponds to the optical bandgap of the silicon nanocrystal materials, suggesting that it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of crystalline silicon.PACS85.35.Be; 84.60.Jt; 78.67.Bf

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 1019 cm-3 to 3.7 × 1017 cm-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.

Generation of high photocurrent in three-dimensional silicon quantum dot superlattice fabricated by combining bio-template and neutral beam etching for quantum dot solar cells

We fabricated a three-dimensional (3D) stacked Si nanodisk (Si-ND) array with a high aspect ratio and uniform size by using our advanced top-down technology consisting of bio-template and neutral beam etching processes. We found from conductive atomic microscope measurements that conductivity became higher as the arrangement was changed from a single Si-ND to two-dimensional (2D) and 3D arrays with the same matrix of SiC, i.e., the coupling of wave functions was changed. Moreover, our theoretical calculations suggested that the formation of minibands enhanced tunneling current, which well supported our experimental results. Further analysis indicated that four or more Si-NDs basically maximized the advantage of minibands in our structure. However, it appeared that differences in miniband widths between 2D and 3D Si-ND arrays did not affect the enhancement of the optical absorption coefficient. Hence, high photocurrent could be observed in our Si-ND array with high photoabsorption and carrier conductivity due to the formation of 3D minibands.

Improvement of Electrical Properties of Silicon Quantum Dot Superlattice Solar Cells with Diffusion Barrier Layers

We investigate the effects of a niobium-doped titanium dioxide (TiO2:Nb) diffusion barrier layer on the performance of silicon quantum dot superlattice (Si-QDSL) solar cells. The insertion of a 2-nm-thick TiO2:Nb layer significantly reduces phosphorus diffusion from a highly doped n-type layer into a Si-QDSL layer during thermal annealing at 900 °C. The phosphorous concentration in the Si-QDSL layer of the solar cell with the TiO2:Nb diffusion barrier layer was found to be less than 1018 cm-3, which is approximately two orders of magnitude lower than that of the solar cell without the diffusion barrier layer. The reduction in phosphorous concentration leads to the improvement of photo-generated carrier collection in the Si-QDSL layer. The short circuit current density of the solar cell with the diffusion barrier layer was dramatically improved to 1.6 mA/cm2 without the degradation of open circuit voltage and fill factor.

Numerical Approach to the Performance of Silicon Quantum Dots Superlattice Solar Cells Taking into Account the Quantum Effect

The electrical characteristics of silicon quantum dot superlattice (Si-QDSL) solar cells have been investigated using a two-dimensional (2D) device simulator, taking the quantum size effect into account. The average bandgap of a Si-QDSL embedded in an amorphous silicon oxycarbide (a-SiOxCy: Eg=2.5 eV) matrix increased from 1.21 to 1.75 eV with decreasing diameter of Si QDs from 10 to 3 nm owing to the quantum size effect. It should be noted that the Voc of Si-QDSL solar cells also increased to 1.11 V with decreasing diameter of Si QDs to 3 nm. This result indicates that it is possible to enhance Voc by the quantum size effect and that a Si-QDSL may be a promising material for all-silicon tandem solar cells.

Numerical Approach to the Performance of Silicon Quantum Dots Superlattice Solar Cells Taking into Account the Quantum Effect

The electrical characteristics of silicon quantum dot superlattice (Si-QDSL) solar cells have been investigated using a two-dimensional (2D) device simulator, taking the quantum size effect into account. The average bandgap of a Si-QDSL embedded in an amorphous silicon oxycarbide (a-SiOxCy: Eg=2.5 eV) matrix increased from 1.21 to 1.75 eV with decreasing diameter of Si QDs from 10 to 3 nm owing to the quantum size effect. It should be noted that the Voc of Si-QDSL solar cells also increased to 1.11 V with decreasing diameter of Si QDs to 3 nm. This result indicates that it is possible to enhance Voc by the quantum size effect and that a Si-QDSL may be a promising material for all-silicon tandem solar cells.

Crystallization Behavior of Silicon Quantum Dots in a Silicon Nitride Matrix

Silicon quantum dot superlattice was fabricated by alternating deposition of silicon rich nitride (SRN) and Si3N4 layers using RF magnetron co-sputtering. Samples were then annealed at temperatures between 800 and 1,100 degrees C and characterized by grazing incident X-ray diffraction (GIXRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). GIXRD and Raman analyses show that the formation of silicon quantum dots occurs with annealing above 1,100 degrees C for at least 60 minutes. As the annealing time increased the crystallization of silicon quantum dots was also increased. TEM images clearly showed SRN/Si3N4 superlattice structure and silicon quantum dots formation in SRN layers after annealing at 1,100 degrees C for more than 60 minutes. The changes in FTIR transmission spectra observed with annealing condition corresponded to the configuration of Si-N bonds. Crystallization of silicon quantum dots in a silicon nitride matrix started stabilizing after 60 minutes' annealing and approached completion after 120 minutes'. The systematic investigation of silicon quantum dots in a silicon nitride matrix and their properties for solar cell application are presented.

Cost Model Developed in European Project LIMA

In this paper we show the results of the cost model developed in LIMA project (Seventh Framework Programme, CN: 248909). The LIMA project is entitled “Improve photovoltaic efficiency by applying novel effects at the limits of light to matter interaction”. The project started in January 2010 and during this year a cost model of the device developed in the project has been developed to assess the industrial viability of this innovative approach to increase the efficiency and reduce the cost of photovoltaic solar cells. During 2011 the cost model has been actualized and a new scenario has been defined. The LIMA project exploits cutting edge photonic technologies to enhance silicon solar cell efficiencies with new concepts in nanostructured materials. It proposes nanostructured surface layers designed to increase the light absorption in the solar cell while decreasing the surface and interface recombination loss. The integration on a back contact solar cell further reduces these interface losses and avoids shading. The project improves light-matter interaction by the use a surface plasmonic nanoparticle layer. This reduces reflection and efficiently couples incident radiation into the solar cell where it is trapped by internal reflection. Surface and interface recombination are minimized by using silicon quantum dot superlattices in a passivating matrix.

Cost model for LIMA device

Abstract In this paper we show the results of the cost model developed in LIMA project (FP7-248909). The LIMA project is titled “Improve photovoltaic efficiency by applying novel effects at the limits of light to matter interaction”. The project started in January 2010 and during this year a cost model of the device developed in the project has been developed to assess the industrial viability of this innovative approach to increase the efficiency and reduce the cost of photovoltaic solar cells. LIMA project exploits cutting edge photonic technologies to enhance silicon solar cell efficiencies with new concepts in nanostructured materials. It proposes nano-structured surface layers designed to increase light absorption in the solar cell while decreasing surface and interface recombination loss. Integration in a back contact design further reduces these interface losses and avoids shading. The project improves light-matter interaction by the use a surface plasmonic nanoparticle layer. This reduces reflection and efficiently couples incident radiation into the solar cell where it is trapped by internal reflection. Surface and interface recombination are minimized by using silicon quantum dot superlattices in a passivating matrix. The distance between quantum dots ensures wave-function overlap and good conductivity.

Fabrication of Silicon Quantum Dots in Si3N4 Matrix Using RF Magnetron Co-Sputtering

Films consisting of a silicon quantum dot superlattice were fabricated by alternating deposition of silicon rich silicon nitride and <TEX>$Si_3N_4$</TEX> layers using an rf magnetron co-sputtering system. In order to use the silicon quantum dot super lattice structure for third generation multi junction solar cell applications, it is important to control the dot size. Moreover, silicon quantum dots have to be in a regularly spaced array in the dielectric matrix material for in order to allow for effective carrier transport. In this study, therefore, we fabricated silicon quantum dot superlattice films under various conditions and investigated crystallization behavior of the silicon quantum dot super lattice structure. Fourier transform infrared spectroscopy (FTIR) spectra showed an increased intensity of the <TEX>$840\;cm^{-1}$</TEX> peak with increasing annealing temperature due to the increase in the number of Si-N bonds. A more conspicuous characteristic of this process is the increased intensity of the <TEX>$1100\;cm^{-1}$</TEX> peak. This peak was attributed to annealing induced reordering in the films that led to increased Si-<TEX>$N_4$</TEX> bonding. X-ray photoelectron spectroscopy (XPS) analysis showed that peak position was shifted to higher bonding energy as silicon 2p bonding energy changed. This transition is related to the formation of silicon quantum dots. Transmission electron microscopy (TEM) and electron spin resonance (ESR) analysis also confirmed the formation of silicon quantum dots. This study revealed that post annealing at <TEX>$1100^{\circ}C$</TEX> for at least one hour is necessary to precipitate the silicon quantum dots in the <TEX>$SiN_x$</TEX> matrix.

Silicon quantum dot superlattices: Modeling of energy bands, densities of states, and mobilities for silicon tandem solar cell applications

Quantum dot superlattices offer prospects for new generations of semiconductor devices. One possible recently suggested application is in tandem solar cells based entirely on silicon, using confinement in the quantum dot to control the cell band gap. In this paper, we use the effective mass approach to calculate the conduction band structure of a three-dimensional silicon quantum dot superlattice with the dots embedded in a matrix of silicon dioxide, silicon nitride, or silicon carbide. The quantum dot superlattice is modeled as a regularly spaced array of equally sized cubic dots in the respective matrix. Incorporating the effect of silicon anisotropic effective mass is shown to reduce both the degeneracies of the isotropic solutions and the energy separation between states. Electron densities of state and mobilities are derived from the band structure data. Theoretical results for the effect of dot size, interdot distance, and matrix material have been obtained. These results clarify the required design features of silicon quantum dot superlattices for the proposed all-silicon tandem solar cells.