After intensive and long illumination of undoped a‐Si:H samples the dark ESR signal is considerably enhanced. The g value of 2.0055 and the linewidth DHpp = 6–7 G leads to the conclusion that the light‐induced defects are single dangling bonds. They disappear again by thermal anneal at about 220 °C.

Light-induced dangling bonds in hydrogenated amorphous silicon

Luminescence and related investigations of recombination in hydrogenated amorphous silicon prepared by glow discharge and sputtering are described. Emphasis is given to a detailed discussion of the various competing radiative and non-radiative recombination mechanisms. The evidence of the luminescence data is compared to other measurements that involve recombination processes. The experiments demonstrate that the dominant luminescence is a tunnelling transition between band tail electrons and holes. Other luminescence transitions involving defects and impurities, and the role of phonon interactions in the transition are discussed. Experiments in which luminescence is used as a probe of the properties of a-Si:H and related materials are also discussed.

Luminescence and recombination in hydrogenated amorphous silicon

Infrared absorption spectra are presented of amorphous Si1−xCx films made by the glow discharge process. Four bands, representing the stretching modes of SiH, CH, and SiC, and the wagging mode of SiCH3, were followed over the entire alloy composition range. It is shown that the compositional dependence of these bands varies in a manner predicted by a model in which Si and C atoms are randomly distributed throughout the film without chemical ordering. The bonded hydrogen content in the films is estimated to be of the order of 20 to 30 at%, depending on composition, with the silicon atoms bonding to a single hydrogen and the carbons bonding predominantly as CH2 and CH3 groups.



Infrarotabsorptionsspektren von amorphen Si1−xCx-Schichten werden mitgeteilt, die durch einen Glimmentladungsprozes hergestellt wurden. Vier Banden, die die Streckungsmoden von SiH, CH, SiC und die „wagging”-Mode von SiCH3 darstellen, lassen sich uber den gesamten Zusammensetzungsbereich der Legierung verfolgen. Es wird gezeigt, das die Zusammensetzungsabhan gigkeit dieser Banden durch ein Modell vorhergesagt wird, in dem die Si- und C-Atome statistisch in der gesamten Schicht ohne chemische Ordnung verteilt sind. Es wird gefunden, das der Anteil an gebundenem Wasserstoff in den Schichten die Grosenordnugn von 20 bis 30 At% hat, je nach Zusammensetzung, wobei die Siliziumatome an isoliertem Wasserstoff gebunden und die Kohlenstoffatome vorwiegend als CH2− und CH3−Gruppen gebunden sind.

Vibrational spectrum of hydrogenated amorphous SiC films

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed. >

Amorphous silicon solar cells

Characterized of glow-discharge deposited a-Si:H

Hydrogen evolution and infrared vibrational absorption have been interpreted to show (1) the existence of weakly bonded H in $a$-Ge: H and Ge-rich $a$-Si-Ge: H alloys, and (2) a preference ratio in excess of 5 for the attachment of H to Si over Ge in the formation of such alloys. It is suggested that this may account for the inferior photoresponse of hydrogenated binaries compared to elements, and also for the superiority of $a$-Si: H over $a$-Ge: H. Some ideas on better compensators than H are advanced.

Preferential Attachment of H in Amorphous Hydrogenated Binary Semiconductors and Consequent Inferior Reduction of Pseudogap State Density

Photoelectron Spectra of Hydrogenated Amorphous Silicon

Photoemission studies involving the valence and core levels of doped and undoped amorphous hydrogenated silicon (a-Si: H) prepared by the glow-discharge method are reported The valence-band spectra show states identified as hydrogen ls/silicon 3p—3s bonding orbitals which are sensitive to the bonding configuration of hydrogen Singly and multiply bonded hydrogen have been distinguished unambiguously A recession of the valence-band edge (E v) of up to 1 eV is observed for the highest hydrogen concentrations (∼50%) The amounts of boron and phosphorus incorporated into a-Si: H as dopants have been estimated through the corresponding core-level intensities The incorporation efficiencies are 80% (P) and 70% (B) respectively However, only 10% of the incorporated atoms act as dopants The position of the Fermi level relative to E v as a function of dopant concentration has also been determined By combining these positions with activation energies obtained from conductivity measurements, the locati

Photoemission studies on in situ prepared hydrogenated amorphous silicon films

Results on the optical absorption, photoluminescence, and photoconductivity spectra and on electrical-transport data of amorphous ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$:H alloys prepared from the plasma decomposition of silane and germane are presented An attempt is made to find a self-consistent interpretation of the data in terms of changes in the optical energy gap and in the gap-state densities with mixtures of different ratios of Si${\mathrm{H}}_{4}$ to Ge${\mathrm{H}}_{4}$ It is found that the photoconductivity and the photoluminescence intensity are much higher than would be expected from the previously deduced values of the ratio of Si\char22{}H to Ge\char22{}H bonds from infrared absorption spectra The principal conclusion of our results is that the changes in the intrinsic properties of a random tetrahedrally coordinated alloy as a function of alloy composition $x$ are often masked by the extrinsic effects of changes with $x$ in H content, defect density, and microstructure; the latter changes depend sensitively on the preparation method

Optical absorption, photoconductivity, and photoluminescence of glow-discharge amorphous Si 1-x Ge x alloys

Extended x-ray-absorption fine-structure (EXAFS) measurements were made on three samples of germanium films that were sputtered on substrates at temperatures of 175\ifmmode^\circ\else\textdegree\fi{}C, intermediate between 175 and 250\ifmmode^\circ\else\textdegree\fi{}C, and 325\ifmmode^\circ\else\textdegree\fi{}C. Analysis of the second coordination shell of these samples gives clear evidence of incipient heterogeneous crystallization from an amorphous matrix. Although x-ray diffraction and optical-absorption measurements indicated that the low- and intermediate-${T}_{s}$ samples were amorphous, EXAFS indicated that the intermediate-${T}_{s}$ sample was heterogeneous, 20% of which consisted of microcrystallites approximately 10 \AA{} in size embedded in an amorphous matrix. The fact that EXAFS can distinguish between microcrystalline and amorphous states gives the most direct experimental evidence to date that the amorphous state is a continuous random network.

B. von Roedern

Papers

Preferential Attachment of H in Amorphous Hydrogenated Binary Semiconductors and Consequent Inferior Reduction of Pseudogap State Density

Photoelectron Spectra of Hydrogenated Amorphous Silicon

Photoemission studies on in situ prepared hydrogenated amorphous silicon films

Optical absorption, photoconductivity, and photoluminescence of glow-discharge amorphous Si 1-x Ge x alloys

Incipient amorphous-to-crystalline transition in Ge