P.C. Taylor

Disorder-induced far infrared absorption in amorphous materials

Abstract In a variety of amorphous materials, the far infrared and microwave absorption (3–3000 GHz or 0.1–100 cm−1) is orders of magnitude greater than that observed in corresponding crystalline materials. This absorption is temperature independent and rises with frequency as νβ (β ≲ 2). The magnitude of the absorption correlates with the density of states inferred from the T3 term in specific heat, and is interpreted as due to a disorder-induced coupling of the far infrared radiation to a density of low frequency Debye modes in the amorphous solids.

Photostructural effects in glassy As2Se3 and As2S3

Abstract Photostructural effects are investigated in 300K substrate evaporated films of As2S3 and As2Se3 using nuclear quadrupole resonance (NQR) techniques. NQR results confirm the existence of As2S4 (or As4Se4) molecular units in the films and suggest the presence of AsS3 (or AsSe3) pyramidal units which lack the longer range (two-dimensional) correlations present in the bulk. Structural differences between film and bulk are greater (and photostructural changes are easier to induce) in As2S3 than in As2Se3.

The coordination of boron in aSi: (B,H)

Abstract Pulsed 11 B NMR measurements performed on an aSi: (B,H) film containing ∼10 atomic % B reveal the presence of two distinct boron sites. The spin-spin relaxation time T 2 is frequency independent and of a magnitude which indicates that the boron is clustered rather than uniformly distributed. The large quadrupolar coupling inferred from the NMR linewidth strongly suggests that most of the boron in aSi: (B,H) does not exist in tetrahedral sites but is threefold coordinated.

Thermally generated paramagnetism in amorphous arsenic

Abstract A density of paramagnetic states which increases with temperature has been observed and characterized in amorphous As using ESR and optical absorption techniques. Unlike the chalcogenide amorphous semiconductors, these localized paramagnetic gap states are characterized by small effective electronic correlation energies (U) and are interpreted as resulting from thermal excitation of strained bonds at As vacancies.

Optically induced localized paramagnetic states in amorphous As

Abstract Photoluminescence and optically induced ESR and absorption due to localized states in the forbidden gap have been observed in amorphous As below 77 K. Analysis of the ESR spectru, indicates that these centers are highly localized and in orbitals which are predominantly p-like.

Iron impurities as non-radiative recombination centres in chalcogenide glasses

Abstract Photoluminescence and E.S.R. studies have been carried out on pure and iron-doped As2S3 glass. The luminescence spectra, photoluminescence excitation spectra, temperature dependence of the luminescence efficiency, and luminescence fatiguing curves demonstrate that the iron impurities quench the luminescence efficiency by introducing competing non-radiative recombination centres. The E.S.R. measurements reveal that iron is a pervasive inadvertent impurity even in nominally pure chalcogenide glasses. It is suggested that non-radiative recombination occurs by transfer of an excitons recombination energy to localized d-band excited states of an iron ion; the excited iron ion then relaxes non-radiatively by phonon emission.

Far infrared and microwave optical properties of vitreous As2Se3

Abstract Optical experiments (30–600 cm −1 ) in vitreous As 2 Se 2 show absorption peaks centered at 45, 102, 156 and 237 cm −1 . These peaks were best fit by Gaussian lineshapes. Dielectric loss experiments yielded a conductivity consistent with an ω S ( s = 0.9) dependence in the range 1 to 6 GHz. The observed conductivity spectrum satisfactorily accounts for the difference between high and low frequency dielectric constants.

On the absence of photodarkening in pnictide amorphous semiconductors

Abstract Parallel photodarkening experiments have been performed at 77 K on two glassy chalcogenides (As 2 Se 3 and As 2 Se 1.5 Te 1.5 and one amorphous pnictide (α-As). Thermally reversible photodarkening effects are observed in both chalcogenides, but no photodarkening is observed in α-As even though α-As and As 2 Se 1.5 Te 1.5 have very similar band gaps. The negative result in α-As confirms the importance of non-bonding valence band electrons in the photodarkening process.

Experimental investigation of defect states in amorphous chalcogenide glasses

Abstract Our current understanding of the nature of defects in pure and doped amorphous semiconductors has been greatly improved with the advent of several recent models which predict the influence of such defects on the optical and transport properties. We shall describe an experimental characterization of the important defect states in the model binary chalcogenide semiconductor (As2Se3) which has been systematically doped (to levels of 1017−1020 cm−3) with a variety of predominantly metallic impurities (Mn, Fe, Ni, Cu, Zn, Ga, In, Tl, Li, Na, K, I). Several complementary experimental techniques have been employed including transient hole transport, dc conductivity, electron spin resonance and photoluminescence (PL) measurements. Dramatic changes of the hole drift mobility are observed with the addition of low concentrations of most metallic impurities, but the PL and photo-induced ESR results are generally much less sensitive to these dopants. These findings will be discussed in the context of current models of defects which associate transport traps, paramagnetic and recombination centers with bonding anomalies on the major constituent (As or Se) atoms.

The effects of impurities upon photoluminescence and optically induced paramagnetic states in chalcogenide glasses

Abstract Photoluminescence (PL) and optically induced electron spin resonance (ESR) have been studied in As2Se3 glasses doped with Cu, Tl, I, Ag, In, and K and in B-doped As2S3 glass. In all cases there is no significant change in PL efficiency or intensity of induced ESR until dopant concentrations exceed ∼1 at.%. These results are in marked contrast the strong dependence of transient hole transport upon dopant concentration in the same glasses. These parallel but contrasting observations are discussed in terms of the predicted effects of dopants on defects in chalcogenide glasses postulated by Mott on the basis of the charged defect model of Mott, Davis and Street (MDS). PL and ESR studies of the AsSe glass system have revealed that oxygen contamination can severely quench PL efficiency and ESR intensity for As concentrations