The various origins of a frequency-dependent conductivity in amorphous semiconductors are reviewed, stressing particularly recent advances and the influences that factors such as correlation and non-random spatial distributions of electrically active centres can have on the a.c. conductivity. A comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials. It is concluded that the a.c. behaviour at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.

A.c. conduction in amorphous chalcogenide and pnictide semiconductors

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

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)

Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

The present paper reviews a quarter of a century of electron spin resonance (ESR) studies in glasses. Specific subtopics addressed include transition-group ions, radiation-induced defect centers, ferromagnetic precipitates, and photo-induced centers. For brevity, most of the examples in the first three categories are confirmed to the familiar oxide glass systems and only the most concrete conclusions are elaborated. The section treating radiation-induced defects is further abridged in light of several reviews of this topic in the current literature. Even within these limitations of scope, a great diversity of material is summarized. Most of the studies cited provide information concerning the atomic scale structure of the host glasses and several have immediate impact on practical problems ranging from glass technology to geochemistry.

Electron spin resonance in glasses

Mid-gap optical absorption and electron spin resonance (ESR) attributed to optically induced localized paramagnetic states have been studied in several chalcogenide glasses and amorphous arsenic for $T\ensuremath{\le}80$ K. The observation of these optically induced metastable paramagnetic centers provides the most direct evidence yet obtained for the existence of localized gap states in these amorphous semiconductors. Irradiation with light whose energy corresponds to the Urbach tail of the absorption edge ($\ensuremath{\alpha}\ensuremath{\simeq}100$ ${\mathrm{cm}}^{\ensuremath{-}1}$) excites photoluminescence (PL) which fatigues (decays) with continuing excitation. The fatigue of the PL is accompanied by the appearance of a growing ESR signal which is not present before illumination. The occurrence of an associated induced optical absorption demonstrates that the paramagnetic states are located in the forbidden gap of these amorphous semiconductors. Subsequent prolonged irradiation of the sample with infrared light in the mid-gap induced absorption band reduces the strength of or "bleaches" both the optically induced ESR signal and the induced absorption and restores the fatigued PL to its cold dark (initial) efficiency. These facts suggest a mechanism whereby radiative recombination centers which become inactive during PL fatigue are closely associated with the production of the metastable paramagnetic centers. The optically induced ESR centers are apparently unique to the amorphous phase and their density (${N}_{s}$) saturates at or below ${10}^{17}$ spins per ${\mathrm{cm}}^{3}$ in all materials measured to date regardless of light intensity. Analysis of the ESR spectra has led to the identification of a hole center which consists of an electron missing from a nonbonding lone pair chalcogen orbital and a center in As-containing glasses and amorphous arsenic which is localized in an As $p$ orbital. A detailed account of the ESR spectral analysis is presented. Changes in the local structural order as a function of composition appear as changes in the $g$ values and width of the optically induced ESR spectrum. The value of ${N}_{s}$ is also a function of composition with the highest values occurring in glasses containing substantial amounts of As such as ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The addition of Ge depresses ${N}_{s}$ to levels as low as \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in Ge${\mathrm{Se}}_{2}$, indicating that the structural differences which accompany the introduction of tetrahedrally coordinated Ge reduce the density of the specific structural anomaly which determines the value of ${N}_{s}$. Possible mechanisms for the generation of the optically induced states are presented, and the relationship between the experimental results and various models proposed to explain the properties of localized gap states is discussed.

Optically induced metastable paramagnetic states in amorphous semiconductors

Microwave and far-infrared absorption have been studied in several chalcogenide and oxide glasses (${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, ${\mathrm{Tl}}_{2}$Se${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$, 3Si${\mathrm{O}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} ${\mathrm{Na}}_{2}$O) at frequencies between 0.1 and 100 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 300 K and at 0.8 ${\mathrm{cm}}^{\ensuremath{-}1}$ between 10 and 300 K. In addition, previously published data for Si${\mathrm{O}}_{2}$ and other amorphous materials are analyzed. The results exhibit comparable frequency- and temperature-dependent behavior for all amorphous materials. The magnitude of the absorption in the amorphous solids over the entire frequency range is strongly enhanced (by an order of magnitude) over the absorption in the corresponding crystalline solids. The enhanced absorption is interpreted in terms of disorder-induced optical coupling to Debye-like as well as to non-Debye-like modes. The non-Debye-like modes dominate the absorption at low frequencies $\frac{\ensuremath{\omega}}{2\ensuremath{\pi}c}\ensuremath{\ll}10$ ${\mathrm{cm}}^{\ensuremath{-}1}$ at temperatures $10lTl300$ K. It is shown that multiphonon difference processes as well as resonant optical excitations of two-level modes do not account for the experimental observations.

Temperature and frequency dependences of the far-infrared and microwave optical absorption in amorphous materials

Results of pulsed NMR studies of hydrogen in $a$-Si: H prepared at several laboratories by glow discharge of silane are presented. The origins of the two $^{1}\mathrm{H}$ NMR lines seen in almost all samples of $a$-Si: H are discussed. Solid-echo measurements are presented which indicate that these two components are due to spatially isolated groups of protons. We attribute the narrow line to protons slightly clustered in the bulk of the material and the broad line to protons distributed on internal surfaces. The spin-lattice relaxation time shows a minimum at $T\ensuremath{\approx}30$ K which is interpreted as due to relaxation via spin diffusion to a small number of ${\mathrm{H}}_{2}$ molecules acting as relaxation centers. Annealing results suggest that all the hydrogen molecules are trapped in very similar sites.

H1NMR ina-Si

The pure ${\mathrm{As}}^{75}$ nuclear quadrupole resonances (NQR) of crystalline and amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ have been investigated and compared. The temperature dependence of the resonance frequencies, the asymmetry parameters $\ensuremath{\eta}$, and the orientation of the principal axes of the electric field grandient with respect to the crystal axes have been obtained for crystalline ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ (orpiment). We obtain ${\ensuremath{\eta}}_{\mathrm{I}}=0.343$ and ${\ensuremath{\eta}}_{\mathrm{II}}=0.374$, where I and II refer to the two inequivalent As sites in the unit cell. A Townes-Dailey calculation for the NQR frequency of the asymmetric As${\mathrm{S}}_{3}$ pyramidal unit has also been performed. The NQR spectrum of crystalline ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ consists of two narrow lines approximately 50 kHz wide occuring at 70.38 and 72.86 MHz at 4.2 \ifmmode^\circ\else\textdegree\fi{}K, while the spectrum of amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ consists of a broad line with 3.5-MHz halfwidth. This is explained by a distribution of apex pyramidal bonding angles in the glass with a halfwidth of \ensuremath{\sim}1\ifmmode^\circ\else\textdegree\fi{}. The nuclear transverse relaxation times ${T}_{2}$ have been measured by the spin-echo technique in both the glass and the crystal. In the crystal, ${T}_{2}=0.6$ msec for site II, and ${T}_{2}\ensuremath{\cong}1$ msec for site I; while we obtain ${T}_{2}=0.6$ msec for the glass. The close agreement between the glass and crystal transverse relaxation indicates the retention of some crystalline correlation in the glass. The spin-lattice relaxation time ${T}_{1}$ has been measured as a function of temperature in the crystal and can be fit by the relation ${T}_{1}=30.64{sinh}^{2}(\frac{14.5}{kT})$, indicating that the nuclei relax by Raman scattering of optical phonons. The spin-lattice relaxation time measured in the glass is much faster, obeying the relation ${T}_{1}=125{T}^{\ensuremath{-}2}$. The faster relaxation in the glass indicates the presence of extra low-frequency modes which are absent in the crystal.

P. C. Taylor

Papers

Optically induced metastable paramagnetic states in amorphous semiconductors

Temperature and frequency dependences of the far-infrared and microwave optical absorption in amorphous materials

H1NMR ina-Si

Nuclear quadrupole resonance in amorphous and crystalline As 2 S 3

Molecular hydrogen in a-Si: H