Dielectric spectroscopic investigation of reversible photo-induced changes in amorphous Ge2Sb2Se5 thin films

Abstract

Broadband dielectric spectroscopy (BDS) was used to study thin films of Ge2Sb2Se5 exposed to low fluence UV-irradiation that resulted in thermally reversible photo-induced changes (PICs) on the film surfaces. Changes were characterized by reversible changes in film morphology and changes in the microwave scattering signature prior to and following irradiation. The PIC resulting from irradiation formed a low modulus and electrically resistive layer at the film surface. Modest heating of the photo-exposed material to about 60 °C anneals out the photo-formed layer to leave behind a very thin layer of smooth, thermally stable material at the film surface. Re-exposure of the material to UV-light recreated a foamy layer, which anneals away with modest heating over several cycles. The broadband microwave insertion loss (S21) increased with the film morphology changes after UV light exposure and decreased with thermal anneal over several cycles. These changes are correlated with transformations in the film's surface morphology and possibly structural modifications in the amorphous film. Thus, the BDS analysis provided interesting new insights into the nature of photo-induced processes in chalcogenide films, such as the electrical resistance consequences of morphological/structural changes.

Time-dependent laser irradiation-induced kinetics of changes in linear–nonlinear optical properties of Bi15In20Se65 thin films for IR applications

“…This could be due to the laser-induced local structural change that increases carrier concentration. Figure 8(a) depicts the observed "χ (3) " with "E g " variation, which clearly showed the opposite pattern in "E g " with "χ (3) " with different exposure durations. "χ (3) " at different wavelength regions show the same trend as the refractive index, as shown in the inset Fig.…”

“…where "χ (1) " represents the linear part of polarization and "χ (2) " and "χ (3) " represent the second-order and third-order non-linear susceptibilities, respectively. Second-order non-linear susceptibility vanishes in centrosymmetric media while possessing a non-zero value in the systems lacking inversion symmetry.…”

“…Second-order non-linear susceptibility vanishes in centrosymmetric media while possessing a non-zero value in the systems lacking inversion symmetry. Thus, the majority of non-linear effects in amorphous semiconductors, such as four-wave mixing, third harmonic generation, and non-linear refraction, are contributed from one single non-zero term, thirdorder non-linear susceptibility (χ (3) ). 51 According to Miller's rule, the linear and non-linear optical susceptibility (χ (1) ) and (χ (3) ) can be estimated as follows: 64,65…”

“…where "n o " is the static refractive index for hν → 0 and "A" is a constant with a value of ∼1.7 × 10 −10 (for χ (3) measured in esu). Table II displays the calculated "χ (1) " and "χ (3) " values which showed improvement in both "χ (1) " and "χ (3) " as the laser exposure duration increased. This could be due to the laser-induced local structural change that increases carrier concentration.…”

“…Recently, there has been much interest in the study of external energy-induced material characteristics from the standpoint of both basic physics and device technology. Several external energy inputs such as foreign elements doping, 1 ion irradiation, 2 laser irradiation, 3 and thermal annealing 4 impact on various properties of thin films, opening up new pathways for many applications in photonics and optoelectronic devices. Among them, laser-induced modifications in amorphous materials are considered an important aspect that includes a better understanding of the mechanism in amorphous materials and corresponding practical applications.…”