Photoinduced Volume Expansion Research Articles

The dynamics of photoinduced defect creation in amorphous chalcogenides: The origin of the stretched exponential function

The article discusses the dynamics of photoinduced defect creations (PDC) in amorphous chalcogenides, which is described by the stretched exponential function (SEF), while the well known photodarkening (PD) and photoinduced volume expansion (PVE) are governed only by the exponential function. It is shown that the exponential distribution of the thermal activation barrier produces the SEF in PDC, suggesting that thermal energy, as well as photon energy, is incorporated in PDC mechanisms. The differences in dynamics among three major photoinduced effects (PD, PVE, and PDC) in amorphous chalcogenides are now well understood.

Modelling of photoinduced changes in chalcogenide glasses: a-Se and a-As2Se3

The microscopic processes in chalcogenide glasses caused by illumination have been investigated using tight-binding molecular dynamics computer simulation. After the photon absorption we have treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. The interplay between photo-induced bond breaking and inter-chain bond formation leads to either volume expansion or shrinkage. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. The model is supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.

Modelling of photo-induced changes in chalcogenide glasses: a-Se and a-AsSe

The microscopic processes in amorphous selenium caused by light-illumination have been investigated using molecular dynamics simulation. After photon absorption we treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. The interplay between photo-induced bond breaking and inter-chain bond formation leads to either volume expansion or shrinkage. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. In the non-ideal case, a part of the microscopic processes is metastable and the total expansion includes the transient and metastable changes. We can describe this macroscopic effect, too. Our rate-equation models for a-Se and a-AsSe are supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.

Universal feature of photo-induced volume changes in chalcogenide glasses

We have investigated in chalcogenide glasses the microscopic processes caused by illumination using tight-binding molecular dynamics computer simulation. After photon absorption we treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. This model is supported by the results of recent measurements.

Photoinduced Volume Changes in Amorphous Selenium

We have modeled the photoinduced volume change in amorphous selenium. After photon absorption, we treated the excited electron and hole independently within the framework of the tight-binding formalism. We found covalent bond breaking in amorphous networks with photoinduced excited electrons, whereas excited holes contribute to the formation of interchain bonds. We also observed a correlated volume change of the amorphous samples. Our results provide a new and universal description, which can simultaneously explain the photoinduced volume expansion and shrinkage. This model is supported by very recent in situ surface height measurements for amorphous selenium.

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se 8 ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyze the photo-induced instability in amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photo-induced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination.

The role of photogenerated charges in photoinduced expansion of chalcogenide glasses

A recently formulated electrical model of photoinduced volume expansion caused by the Coulomb interaction between photogenerated carriers in chalcogenide glasses is examined quantitatively. It is shown that, depending upon photoexcitation conditions, the model predicts either expansion or contraction. However, the calculated expansion and contraction are both far below experimentally observed values. This implies an important role of charge-density fluctuations in photoinduced structural phenomena observed in chalcogenide glasses.

Intramolecular structural model for photoinduced plasticity in chalcogenide glasses

Selected spectral features of Raman spectra of glassy ${\mathrm{As}}_{2}{\mathrm{S}}_{3}$ subjected to elongation stress and sub-band-gap light illumination are analyzed and compared with polarization-dependent information obtained from the bulk glass at room temperature and near the glass transition temperature. The data are suggestive of specific structural changes which involve the transformation of atomic arrangements from realgarlike ${\mathrm{As}}_{4}{\mathrm{S}}_{4}$ molecules, originally present in virgin (untreated) fibers, into planar orpimentlike clusters. Implications of these atomic rearrangements to the incipient photoinduced fluidity---the onset of plastic deformation---in ${\mathrm{As}}_{2}{\mathrm{S}}_{3}$ glass are discussed. Kinetics of photoinduced plastic changes is compared to that of Raman spectra changes, revealing a qualitative similar behavior. An approximate estimation of the relative contribution of intermolecular rearrangements and the intramolecular structural mechanism proposed in this paper has revealed that the latter is responsible for almost 30% of the photoinduced elongation of the fiber's length at room temperature. The proposed mechanism can as well serve as rationale for understanding the photoinduced volume expansion observed in chalcogenide glasses.

Photoinduced volume changes related to photoinduced anisotropy in chalcogenide glasses

We report metastable photoinduced volume expansion (hyperexpansion) of up to 10 µm in height and several millimetres in diameter for the chalcogenide glass As 2 S 3 . This is an order of magnitude larger than reported to date in any material, including chalcogenide glasses. We suggest a model for the effect and propose that its origin is similar to the origin of photoinduced anisotropy in glassy chalcogenides proposed earlier. The model is based on the anisotropic chemical bonding found in most chalcogenide glasses; the anisotropy arises from the fact that chalcogen atoms exhibit both strong covalent bonding and weak van der Waals bonding. The origin of the photoinduced volume changes (expansion and contraction) is in a thermally induced diffusion, or athermal tunnelling, of photoexcited chalcogen atoms, with possible subsequent sublimation from the sample surface. The primary role of hyperpolarizable van der Waals bonds is supported by the absence of photoinduced volume changes in the chalcogenide glass Ga 1.4 La 0.6 S 3, in which chalcogen atoms do not bond via van der Waals bonds.

Photoinduced volume expansion and contraction in a-Si:H films

Photoinduced volume expansion and contraction in hydrogenated amorphous silicon (a-Si:H) films have been studied. Photoinduced volume contraction seems to occur in a-Si:H films having hydrogen contents less than ∼5 at.%. The wavelength dependence of the photoinduced volume expansion indicates that one of driving forces of this phenomenon may be relaxation energy of photoexcited carriers. The reciprocity relation between light intensity and illumination time does not hold both in photoinduced volume expansion and photodegradation. The mechanism of the photoinduced volume changes is also discussed.

A study on the correlation between the photoinduced volume expansion and the internal stress in hydrogenated amorphous silicon

A photoinduced volume expansion phenomenon in hydrogenated amorphous silicon (a-Si:H) films has been studied from a viewpoint of the initial stress in the film by using the optical-lever bending method. The dependence on deposition temperature for the photoinduced expansion and the initial stress is similar, that is, both have maximum stress at a deposition temperature of 300°C. On the other hand, little difference in the photoinduced expansion was observed among films prepared at the same substrate temperature which have different initial stresses. These results indicate that an internal stress does not affect the photoinduced expansion and that intrinsic stress due to the structure of a-Si:H is the origin of the phenomenon.

In situ photoexpansion measurements of amorphous As2S3 films: Role of photocarriers

To understand the dynamics of photoinduced volume expansion (PVE) in amorphous chalcogenides, in situ PVE (time evolution of thickness changes during illumination) measurements have been performed. Two distinct behaviors, transient and metastable PVE, have been observed. A strong correlation between the changes in thickness and photocurrent is reported.