Silicon has long been established as the material of choice for the microelectronics industry. This is not yet true in photonics, where the limited degrees of freedom in material design combined with the indirect bandgap are a major constraint. Recent developments, especially those enabled by nanoscale engineering of the electronic and photonic properties, are starting to change the picture, and some silicon nanostructures now approach or even exceed the performance of equivalent direct-bandgap materials. Focusing on two application areas, namely communications and photovoltaics, we review recent progress in silicon nanocrystals, nanowires and photonic crystals as key examples of functional nanostructures. We assess the state of the art in each field and highlight the challenges that need to be overcome to make silicon a truly high-performing photonic material.

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Silicon nanostructures for photonics and photovoltaics

Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal pla...

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications

In this paper we review the theory of silicon nanowires. We focus on nanowires with diameters below 10 nm, where quantum effects become important and the properties diverge significantly from those of bulk silicon. These wires can be efficiently treated within electronic structure simulation methods and will be among the most important functional blocks of future nanoelectronic devices. Firstly, we review the structural properties of silicon nanowires, emphasizing the close connection between the growth orientation, the cross-section and the bounding facets. Secondly, we discuss the electronic structure of pristine and doped nanowires, which hold the ultimate key for their applicability in novel electronic devices. Finally, we review transport properties where some of the most important limitations in the performances of nanowire-based devices can lay. Many of the unique properties of these systems are at the same time defying challenges and opportunities for great technological advances.

Colloquium: Structural, electronic, and transport properties of silicon nanowires

Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.

Graphene Coupled with Silicon Quantum Dots for High-Performance Bulk-Silicon-Based Schottky-Junction Photodetectors

We perform ab initio calculations within the Landauer formalism to study the influence of doping on the conductance of surface-passivated silicon nanowires. It is shown that impurities located in the core of the wire induce a strong resonant backscattering at the impurity bound state energies. Surface dangling bond defects have hardly any direct effect on conductance, but they strongly trap both p- and n-type impurities, as evidenced in the case of H-passivated wires and Si/SiO2 interfaces. Upon surface trapping, impurities become transparent to transport, as they are electrically inactive and do not induce any resonant backscattering.

/pdf/conductance-surface-traps-and-passivation-in-doped-silicon-3kqfz1zvyk.pdf

Conductance, surface traps and passivation in doped Silicon Nanowires

The paramagnetic defects in and on Si nanowires (SiNWs) obtained by oxide-assisted growth were studied by conventional electron spin resonance spectroscopy. For the as-grown nanowires, three different defects were found: Dangling bonds or Pb-centers with g=2.0065, located at the interface of the crystalline core to the surrounding oxide, E′-centers with g=2.0005 and EX-centers with g=2.00252, located in the oxide. For the EX-centers, the characteristic hyperfine lines separated by 16.4G were detected. The as-grown SiNWs showed a spin density of about 1018cm−3. H termination of the nanowires via hydrofluoric acid decreases the spin density drastically to 3×1016cm−3. The optical absorption spectra of SiNWs determined by photothermal deflection spectroscopy are comparable to those of microcrystalline silicon and show a similar decrease of defect density upon H termination.

Paramagnetic defects of silicon nanowires

Freestanding silicon nanocrystals (Si-ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si-ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge-trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low-cost post-growth treatment routes based on wet-etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H-terminated Si-ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si-ncs, which retain a defect density below that of untreated Si-ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si-ncs are extremely stable against oxidation and maintain a very low defect density after a long-term storage in air, opening the possibility of device processing in ambient atmosphere.

Low-Cost Post-Growth Treatments of Crystalline Silicon Nanoparticles Improving Surface and Electronic Properties

Nanoscale silicon particles, essentially hydrogen terminated nanoscale silicon particles, essentially alkyl terminated nanoscale silicon particles, partially alkyl terminated nanoscale silicon particles, methods for producing the particles, and methods for forming electrical components, electronic circuits, and electrochemically active fillers with the particles.

Nanoscale silicon particles

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Paramagnetic defects of silicon nanowires

Low-Cost Post-Growth Treatments of Crystalline Silicon Nanoparticles Improving Surface and Electronic Properties

Nanoscale silicon particles