Peculiarities of Estimating the Optical Band Gap of Thin Films of Phase Change Memory Materials

Periodic Relief Fabrication and Reversible Phase Transitions in Amorphous Ge2Sb2Te5 Thin Films upon Multi-Pulse Femtosecond Irradiation

“…where ω is the incident laser radiation frequency, γ = e 0 /(m * µ) is the carrier collision frequency, e 0 and m * are the electron charge and effective mass, respectively, as well as µ = 52 cm 2 /(V•s) is the charge carrier mobility for GST225 [34]. The static dielectric permittivity of ε ∞ = 18.2 and ε ∞ = 44.2 correspond [27] to the non-excited amorphous and crystalline GST225, respectively. In this case, n e is the value of non-equilibrium charge carrier density, which can be estimated from the following differential equation:…”

“…where the absorption and reflection constants of the amorphous GST225 thin films for the wavelength of 1250 nm are taken as α = 24,500 cm −1 and R = 0.39 [27], respectively. Laser pulse intensity I(t) is defined as follows [35]:…”

“…This reversible phase transition is possible in GST225 due to the metastability of its FCC crystalline lattice [45,46]. In the case of laser action on the GST225 film surface, the FCC phase transformation into the amorphous one can probably be achieved during the subsequent processes of material melting due to the strong light absorption of crystalline GST225 (α ≈ 2 × 10 5 cm −1 for the λ = 1250 nm) [27] and subsequent rapid quenching of the melt [47]. In this case, the rapid cooling of the film, which is necessary for the amorphous phase formation can be provided by the efficient heat removal to the substrate [48,49].…”

“…Previously, LIPSS with the wavelength period were fabricated on amorphous GST225 thin films deposited on conductive substrates [14][15][16]. In addition, it was revealed that the treatment by near-infrared femtosecond pulses facilitates the effective penetration of radiation into the used material, which exhibits relatively small absorbance and reflection in this spectral range [27]. However, any quantitative explanation of ripple formation in these films has not been made yet.…”

Specific Features of Formation of Laser‐Induced Periodic Surface Structures on Ge2Sb2Te5 Amorphous Thin Films under Illumination by Femtosecond Laser Pulses

“…For GST225 this value is varied in the range from 0.63 to 0.75 eV for amorphous state depending on the conditions of thin films preparation and the calculation algorithm; for crystalline state, the value is less and varies from 0.29 to 0.62 eV. [ 22 ] In this study, we analyze how a change in femtosecond laser wavelength affects the ripple parameters at constant parameters of thin films. The knowledge of specific features of LIPSS formation under various femtosecond light conditions allows one to shed light into the mechanism of fine surface structuring of PCMs.…”