Raman spectroscopy is a standard characterization technique for any carbon system. Here we review the Raman spectra of amorphous, nanostructured, diamond-like carbon and nanodiamond. We show how to use resonant Raman spectroscopy to determine structure and composition of carbon films with and without nitrogen. The measured spectra change with varying excitation energy. By visible and ultraviolet excitation measurements, the G peak dispersion can be derived and correlated with key parameters, such as density, sp(3) content, elastic constants and chemical composition. We then discuss the assignment of the peaks at 1150 and 1480 cm(-1) often observed in nanodiamond. We review the resonant Raman, isotope substitution and annealing experiments, which lead to the assignment of these peaks to trans-polyacetylene.

Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond

Recent developments in the theoretical analysis and experimental study of frequency-dependent loss by relaxation in amorphous semiconductors are reviewed. A unified theoretical treatment of the complex a.c. conductivity is given, within the pair approximation, for single electron tunnelling and hopping in both uncorrelated and strongly correlated cases, and the discussion is then extended to pair processes and to atomic relaxation. The problems associated with measuring the frequency dependent conductivity of amorphous samples are considered, and relevant measurements reported for the different classes of amorphous semiconductors, tetrahedral and group V materials and chalcogenides are reviewed in the light of the available theoretical models. The similarity in the magnitudes and frequency, temperature and electric field dependences of the losses observed in many different systems at liquid helium temperatures is noted, and the possible physical reasons for this are examined.

Frequency-dependent loss in amorphous semiconductors

Hard amorphous (diamond-like) carbons

Characteristic order over distances of 15–30 A is indicated by various experiments on covalent non-crystalline solids. The data are reviewed and detailed structural models are developed containing 30–1000 atoms.

Topology of covalent non-crystalline solids II: Medium-range order in chalcogenide alloys and ASi(Ge)

A review of the properties of hydrogen in crystalline semiconductors is presented. The equilibrium lattice positions of the various states of hydrogen are detailed, together with the reactions of atomic hydrogen with shallow and deep level impurities that passivate their electrical activity. Evidence for several charge states of mobile hydrogen provides a consistent picture for both the temperature dependence of its diffusivity and the chemical reactions with shallow level dopants. The electrical and optical characteristics of hydrogen-related defects in both elemental and compound semiconductors are discussed, along with the surface damage caused by hydrogen bombardment. The bonding configurations of hydrogen on semiconductor surfaces and the prevalence of its incorporation during many benign processing steps are reviewed. We conclude by identifying the most important areas for future effort.

Hydrogen in crystalline semiconductors

The ir and Raman spectra of $a$-Si: H alloys produced by plasma decomposition of Si${\mathrm{H}}_{4}$ are studied for a wide range of deposition conditions. The vibrational spectra display modes which can be characterized as predominantly hydrogen motions. Analysis of these modes shows four types of local Si-H bonding environments which are identified as SiH, Si${\mathrm{H}}_{2}$, Si${\mathrm{H}}_{3}$, and coupled Si${\mathrm{H}}_{2}$ or ${(\mathrm{Si}{\mathrm{H}}_{2})}_{n}$ units. On the basis of these identifications, it is found that samples produced on high-temperature (above 200\ifmmode^\circ\else\textdegree\fi{}C) substrates have SiH, Si${\mathrm{H}}_{2}$, and ${(\mathrm{Si}{\mathrm{H}}_{2})}_{n}$ groups with very little Si${\mathrm{H}}_{3}$. In contrast, the ir and Raman spectra of samples produced on room-temperature or cooled substrates are dominated by vibrational modes of Si${\mathrm{H}}_{3}$ and ${(\mathrm{Si}{\mathrm{H}}_{2})}_{n}$. The relative concentrations of these hydrogen-containing groups are not simply proportional to the total hydrogen concentration in a given sample.

Structural interpretation of the vibrational spectra of a-Si: H alloys

The photoluminescence of plasma-deposited amorphous silicon is investigated. The luminescence intensity and spectral line shape are shown to be sensitive to many deposition variables, in particular the power coupled into the discharge, the concentration of silane in the gas stream, and the deposition substrate temperature. Maximum intensity is obtained in samples deposited with low power (\ensuremath{\sim}1 W), a silane concentration of \ensuremath{\gtrsim} 10% and a deposition temperature of 200-300\ifmmode^\circ\else\textdegree\fi{}C. ESR studies show that the luminescence intensity is determined by competing nonradiative transitions to localized defect states whose density varies with deposition conditions. The presence of defect states is related to the way hydrogen is incorporated into the samples, but the details of the defect structure are not yet clear. Oxygen impurities are observed to give a broad, weak luminescence peak centered near 1.1 eV. It is suggested that the active oxygen centers are similar to the charged defects postulated for chalcogenide glasses.

Luminescence studies of plasma-deposited hydrogenated silicon

Luminescence, electron spin resonance (ESR), optical absorption, conductivity, and composition data are measured on doped and compensated hydrogenated amorphous silicon. In material singly doped with boron or phosphorus, a variety of experiments indicates the introduction of a large defect density (up to ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) of the dangling-bond type. Compensation increases the luminescence efficiency, but the luminescence peak shifts strongly to lower energy. Compensation reduces the ESR resonance at $g=2.0055$, but a broad resonance characteristic of a hole trap remains. We deduce that compensation reduces the dangling-bond density, but introduces a new band of localized states above the valence-band edge. We associate these new states with boron-phosphorus complexes whose origin is a chemical interaction occurring during deposition. Changes in the dangling-bond density with doping and compensation lead us to propose an autocompensation mechanism of defect formation. Also reported is the first observation of a metastable light-induced ESR signal in $a$-Si: H.

Defect states in doped and compensated a -Si: H

The relationships between hydrogen vibrational spectra, electron spin densities and refractive index are investigated for a range of plasma-deposited amorphous silicon-hydrogen alloys. Results are also presented on the morphology of thick films as shown by scanning electron microscopy. A model is proposed for the structural origin of defects in these alloys based on voids that grow perpendicular to the film surface and are associated with coupled SiH2 units.

John C. Knights

Papers

Structural interpretation of the vibrational spectra of a-Si: H alloys

Luminescence studies of plasma-deposited hydrogenated silicon

Defect states in doped and compensated a -Si: H

Defects in plasma-deposited a-Si: H

Hydrogen evolution and defect creation in amorphous Si: H alloys