Light emission from porous silicon and related materials

Hydrogenated amorphous silicon (a-Si:H) has been used for decades—doped and as intrinsic absorber layers—in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that a-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

Review: Progress in solar cells from hydrogenated amorphous silicon

We review the bonding in a wide range of non-crystalline materials, such as the elemental and compound amorphous semiconductors, liquid semiconductors and, briefly, some oxide glasses and amorphous silicon dioxide. The review concentrates on the experimental determination of electronic structure from photoemission, optical spectra and core spectra and on its theoretical description, particularly using tight-binding methods. The review also describes the bonding at defects in amorphous semiconductors and in particular that of dopants and hydrogen in hydrogenated amorphous silicon (a-Si:H).

Electronic structure of amorphous semiconductors

Silicon nanowires were fabricated for the first time by electrochemical template synthesis at room temperature. This innovative, cheap, and simple process consists of electroreduction of Si ions using a nonaqueous solvent and insulating nanoporous membranes with average pore diameters from 400 to 15 nm which fix the nanowires diameters. Characterization techniques such as scanning and transmission electron microscopies, infrared absorption measurements, X-ray diffraction experiments, energy dispersive X-ray, and Raman spectrometries show that the as-deposited silicon nanowires are amorphous, composed of pure Si and homogeneous in sizes with average diameters and lengths well matching with the nanopores' diameters and the thicknesses of the membranes. Thanks to annealing treatments, it is possible to crystallize the Si nanowires, demonstrating the potentiality for this innovative electrochemical process to obtain a wide range of Si nanowires with well controlled diameters and lengths.

Growth of silicon nanowires of controlled diameters by electrodeposition in ionic liquid at room temperature.

Monohydride and dihydride formation at Si(100) 2x1: A high resolution electron energy loss spectroscopy study

A study of the vibrational properties of polysilanelike conformations in amorphous hydrogenated silicon ($a$-Si: H) is presented. With the use of the valence force-field representation of the near-neighbor interactions, the generalized cluster-Bethe-lattice method, and a chemical bonding model for obtaining realistic force constants, the one-phonon densities of states for various polysilanelike conformations [e.g., Si${\mathrm{H}}_{2}$, ${(\mathrm{S}\mathrm{i}{\mathrm{H}}_{2})}_{3}$, ${(\mathrm{Si}{\mathrm{H}}_{2})}_{n}$] in an amorphous silicon matrix are calculated. Our results indicate that all of the features which have been reported in the 800- to 900-${\mathrm{cm}}^{\ensuremath{-}1}$ frequency regime of the infrared absorption and Raman scattering spectra of $a$-Si: H alloys can be attributed to the vibrational modes of polysilanelike conformations.

Phonons in polysilane alloys

The effects of bulklike and surfacelike surroundings on the electronic density of states of a variety of Si-H bonding conformations in hydrogenated amorphous silicon are examined using the cluster Bethe-lattice approach. Firstly, we discover that two fundamentally different bonding patterns, with different consequences for the doping mechanism, are consistent with ultraviolet photoemission spectroscopy (UPS) data. These are (1) H atoms bonded in microcrystalline regions and (2) clusters of monohydrides (SiH) in a continuous random network. Our results suggest an experiment in which x-ray photoemission spectroscopy and UPS taken together should distinguish between (1) and (2) and hence contribute toward understanding doping. Secondly, by using the calculated densities of states, the energies of a number of conformations and dehydrogenation reactions are calculated with the use of an empirical bond-strength total-energy scheme. Our results agree with results from annealing experiments. We introduce an improvement in the Bethe-lattice method which permits efficient solution of a second-neighbor tight-binding Hamiltonian, and which is valid for $N\mathrm{th}$-neighbor interactions. We also estimate the Hubbard $U$, Stokes shifts, and electronic states associated with neutral and charged dangling bonds.

William B. Pollard

Papers

Phonons in polysilane alloys

Electronic states and total energies in hydrogenated amorphous silicon