Ashtosh Ganjoo

A model for the photostructural changes in amorphous chalcogenides

Prolonged photoirradiation induces a volume expansion and causes a decrease in the optical bandgap (photodarkening) in well annealed amorphous chalcogenides. A microscopic mechanism for the origin of these changes is proposed in terms of the repulsive Coulomb force between layered clusters which can be negatively charged by electron accumulation in the conductionband tails. It is also proposed that the photoinduced interlayer Coulomb repulsion increases the interlayer separation, which is responsible for the volume expansion, and it also induces an in-plane slip motion which causes the reduction in the bandgap.

Transient photodarkening in amorphous chalcogenides

Abstract Illumination of amorphous arsenic chalcogenides with band-gap light induces photodarkening and volume expansion. In addition to metastable changes, which persist after the light is switched off, transient effects occur during and after illumination. We present measurements of these metastable and transient effects in evaporated films of a-As 2 S 3 , a-As 2 Se 3 and a-Se. The results are discussed in terms of a model in which, on photoexcitation, layer- or chain-like clusters of atoms expand and slip relative to each other.

Photoinduced structural changes in obliquely deposited As- and Ge-based amorphous chalcogenides: correlation between changes in thickness and band gap

Abstract The effect of band gap illumination and annealing below the glass transition temperature on the thickness and the optical band gap of As-based (As 2 Se 3 , As 2 S 3 ) and Ge-based (GeSe 2 , GeS 2 ) obliquely deposited chalcogenide films has been studied. It is observed that in the case of arsenic (As)-based glasses, illumination increases the thickness (expansion) and the band gap decreases (darkening), while as for germanium (Ge)-based glasses, both thickness and band gap show an opposite behavior to that of As-based glasses. By annealing the samples, before and/or after illumination, the trends of the changes in thickness and band gap are reversed. A strong correlation between the changes in thickness and band gap has been established for the first time.

Photoluminescence lifetime distribution of a-Si:H and a-Ge:H expanded to nanosecond region using wide-band frequency-resolved spectroscopy

Abstract We present the first survey on quadrature frequency-resolved spectroscopy (QFRS) spectra in the nanosecond (ns) region of photoluminescence (PL) of a-Si:H and a-Ge:H. Our QFRS study reveals that there is no evidence of a peak or shoulder for the ns PL lifetime component, and thus the PL lifetime distribution has basically a double-peak structure with two components of μs and ms lifetimes in both materials. The μs component in the lifetime distribution is enhanced at higher PL photon energy, which favors the exciton model. Generation rate ( G ) dependence of QFRS spectra shows a common feature, for both a-Si:H and a-Ge:H, where excitonic emission shifts to non-geminate radiative recombination as G increases.

Photoluminescence lifetime distributions of chalcogenide glasses obtained by wide-band frequency resolved spectroscopy

Photoluminescence (PL) lifetime distributions for amorphous arsenic chalcogenides g-As2Se3 and g-As2S3, and amorphous selenium a-Se, are obtained for the first time using a quadrature frequency resolved spectroscopy (QFRS) technique modified for nanosecond resolution. The g-As2S3 and a-Se chalcogenides exhibit double-peak lifetime distributions, whereas the lifetime distribution of g-As2Se3 peaks uniquely at around 10−4 s, which is consistent with earlier results. PL fatigue is found to reduce the intensity of PL but not affect the observed PL lifetimes. A self-trapped exciton model is adopted to explain the experimental results, providing reasonable mechanisms for the two-component lifetimes and associated phenomena. For a-Se, singlet–triplet exchange energy of ≈160 meV is estimated.

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si : H) and amorphous As2Se3 (a-As2Se3) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As2Se3 than for a-Si : H has been observed and this is interpreted in terms of the higher structural flexibility of a-As2Se3. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.

In situ measurements of photo-induced volume changes in amorphous chalcogenide films

Abstract Photo-induced thickness changes in obliquely deposited amorphous As 2 S 3 and GeSe 2 films have been measured to determine the mechanisms of changes during illumination . For As 2 S 3 , the thickness increases and then decreases and saturates after some time. There is a decrease followed by saturated change even after the illumination is switched off. On the other hand, GeSe 2 has a continuous decrease of thickness with time, which remains constant after the illumination is turned off. The difference in these properties between As 2 S 3 and GeSe 2 films is discussed in terms of different structural units for these films (layered for a-As 2 S 3 and three-dimensional for a-GeSe 2 ).

Estimation of density of charged defects in amorphous chalcogenides from a.c. conductivity: Random-walk approach for bipolarons based on correlated barrier hopping

Abstract Although the pair approximation (PA) based on correlated bamer hopping (CBH) is now fairly well established to explain the a.c. conductivity of amorphous chalcogenides, the density of charged states deduced from PA is large compared with those estimated from other studies, namely light-induced electron spin resonance and drift mobility. We have used the continuous-time random-walk approximation based on CBH to estimate the density of charged defects and have applied it to experimental data for amorphous As2Se3, and a density of charged defects consistent with other measurements has been estimated.

Photoconductivity of macroscopically inhomogeneous amorphous semiconductors: case example for a-Si:H

Abstract The behavior of dc and ac photoconductivities observed in a-Si:H in the temperature range 20–300 K, as a case example, is discussed in the context of potential fluctuations. A model is proposed for the non-equilibrium carrier transport in a macroscopic inhomogeneous system and comparison of its results with the experimental data produces physical parameters related with the potential fluctuations.

Light-induced annealing of dangling bonds in He-diluted glow discharge a-Si:H films

We report the results of a study of light-induced annealing of dangling bonds, as well as their light-induced creation in He-diluted glow discharge a-Si:H with a large amount of hydrogen at room temperature (RT) and at 75°C, by using an electron spin resonance (ESR) technique. Under illumination 0.7 W/cm2 at room temperature, dangling bonds are created by prolonged illumination, and their density tends to saturate. On the other hand, under strong illumination (1.6 W/cm2) at room temperature, dangling bonds are initially created and then photoannealed after 5 min of illumination. These results for He-diluted glow discharge samples are discussed and compared with those for standard and H2-diluted glow discharge samples.