Xiaoman Duan

Bio: Xiaoman Duan is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Photonic crystal & Silicon. The author has an hindex of 13, co-authored 45 publications receiving 1374 citations. Previous affiliations of Xiaoman Duan include National Semiconductor.

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

Abstract: An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.

Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector

Abstract: Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 μm thick monocrystalline thin film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.

Design of Highly Efficient Light-Trapping Structures for Thin-Film Crystalline Silicon Solar Cells

Abstract: We present a design optimization of a highly efficient light-trapping structure to significantly increase the efficiency of thin-film crystalline silicon solar cells. The structure consists of an antireflection (AR) coating, a silicon active layer, and a back reflector that combines a diffractive reflection grating with a distributed Bragg reflector. We have demonstrated that with careful design optimization, the presented light-trapping structure can lead to a remarkable cell-efficiency enhancement for the cells with very thin silicon active layers (typically 2.0-10.0 mum) due to the significantly enhanced absorption in the wavelength range of 800-1100 nm. On the other hand, less enhancement has been predicted for much thicker cells (i.e.,>100 mum) due to the limited absorption increase in this wavelength range. According to our simulation, the overall cell efficiency can be doubled for a 2.0-mum-thick cell with light-trapping structure. It is found that the improvement is mainly contributed by the optimized AR coating and diffraction grating with the corresponding relative improvements of 36% and 54%, respectively. The simulation results show that the absolute cell efficiency of a 2.0-mum-thick cell with the optimal light-trapping structure can be as large as 12%.

Photon band gap properties and omnidirectional reflectance in Si∕SiO2 Thue–Morse quasicrystals

Abstract: Aperiodic one-dimensional Si∕SiO2 Thue–Morse (T–M) multilayer structures have been fabricated in order to investigate both the band gap properties with respect to the system size (band gap scaling) and the omnidirectional reflectance at the fundamental optical band gap. Variable angle reflectance data have experimentally demonstrated a large reflectance band gap in the optical spectrum of a T–M quasicrystal, in agreement with transfer matrix simulations. We explain the physical origin of the T–M omnidirectional band gap as a result of periodic spatial correlations in the complex T–M structure. The unprecedented degree of structural flexibility of T–M systems can provide an attractive alternative to photonic crystals for the fabrication of photonic devices.

Optical transmission losses in polycrystalline silicon strip waveguides: effects of waveguide dimensions, thermal treatment, hydrogen passivation, and wavelength

Abstract: Signal propagation delays dominate over gate delays in the ever-shrinking ultra large scale integrated (ULSI) circuits. Consequently, silicon-based monolithic optoelectronic circuits (SMOE) with their light speed signal propagation can provide unique advantages for future generations of microprocessors. For such SMOE circuits, we need optical interconnects compatible with silicon technology. Strip waveguides consisting of polycrystalline silicon (polySi) clad with SiO2 offer excellent optical confinement and ease of fabrication that are ideal for such interconnect applications. One major challenge with using this material system, however, is its insertion loss. In this paper we provide techniques for minimizing optical transmission losses in polySi strip waveguides. Our previous work using polySi strip waveguides, showed an optical transmission loss of 15 dB/cm at λ=1.55 µm, which is a communication wavelength of choice in optical fibers because it represents an absorption minimum. Similar measurements in crystalline silicon strip waveguides1 yielded transmission losses of less than 1 dB/cm. Hitherto, in decreasing loss from 77 dB/cm to 15 dB/cm, we had minimized loss from surface scattering by improving the film surface morphology, and decreased bulk absorption with hydrogen passivation. In this paper we report a further reduction in the residual bulk loss from 15 dB/cm to 9 dB/cm. By experimenting with different waveguide core dimensions, we find that the contribution of bulk loss towards net transmission loss decreases with waveguide core thickness. Additionally, high temperature treatment provides strain relief in the polySi, decreasing transmission loss. Annealing in an oxygen ambient is not recommended because it always increases transmission loss. Hydrogen passivation improves transmission, attributable to passivation of light-absorbing dangling bond defect sites present at polySi grain boundaries. Together, these methods have resulted in the lowest measured loss value of 9 dB/cm at λ=1.55 µm. Since integrated SiGe and Ge photodetectors are more efficient at shorter wavelengths like λ=1.32 µm, transmission loss is also measured at λ=1.32 µm. Losses at the two wavelengths (1.32 µm and 1.55 µm) are similar when defects and stress in the waveguides are minimized.

A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor

Abstract: Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only ∼20 MHz (refs 10, 11), although it has been predicted theoretically that a ∼1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

Rationale and challenges for optical interconnects to electronic chips

Abstract: The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.

Light management for photovoltaics using high-index nanostructures.

Abstract: High-performance photovoltaic cells use semiconductors to convert sunlight into clean electrical power, and transparent dielectrics or conductive oxides as antireflection coatings. A common feature of these materials is their high refractive index. Whereas high-index materials in a planar form tend to produce a strong, undesired reflection of sunlight, high-index nanostructures afford new ways to manipulate light at a subwavelength scale. For example, nanoscale wires, particles and voids support strong optical resonances that can enhance and effectively control light absorption and scattering processes. As such, they provide ideal building blocks for novel, broadband antireflection coatings, light-trapping layers and super-absorbing films. This Review discusses some of the recent developments in the design and implementation of such photonic elements in thin-film photovoltaic cells.

Guiding, modulating, and emitting light on Silicon-challenges and opportunities

Abstract: Silicon photonics could enable a chip-scale platform for monolithic integration of optics and microelectronics for applications of optical interconnects in which high data streams are required in a small footprint. This paper discusses mechanisms in silicon photonics for waveguiding, modulating, light amplification, and emission. These mechanisms, together with recent advances of fabrication techniques, have enabled the demonstration of ultracompact passive and active silicon photonic components with very low loss.