Physical and optical properties of bulk and thin films of a–Ge–Sb–Te lone-pair semiconductors

ABSTRACT

Physical parameters and their correlation with optical band gap and refractive index for amorphous Ge₂₀Te_80-xSb_x (x = 0, 2, 4, 6, 8, 10) lone-pair semiconductors were discussed. Using average coordination number $\left(⟨r⟩\right),$ lone-pair electrons (L) were calculated and found to decrease with an increase in the Sb content. However, L value greater than 3 indicated that Ge₂₀Te_80-xSb_x alloys can retain their vitreous nature. The density of alloys was measured both experimentally and theoretically. Increased density values with the increase of Sb content further accounts for the concurrent increase in the refractive index values. The results were explained on the basis of increased polarizability and larger Bohr radius of Sb. Compactness of the structure was found to decrease with the alterations of the atomic arrangements. Molar volume (calculated from the measured density values) found to decrease with Sb content. The number of atoms per unit volume (N) was calculated, using molar volume and average coordination number and were found to increase. This increase in N accounts for the decrease in the optical band gap. Other optical parameters viz. optical density, penetration depth and Urbach energy were also calculated. An increase in Urbach energy showed the reduction in band tail width with increased defect concentration. Positions of the conduction bands and valence bands were found to shift towards the Fermi edge, which accounts for the reduction in the optical band gap. Chemical bond approach (CBA) model was applied to estimate the cohesive energy of the system and found to decrease with the Sb content. A linear relation is found between the behaviour of cohesive energy and band gap (calculated both experimentally and theoretically). Deviation of stoichiometry confirmed chalcogen-rich region for all the compositions. The mean bond energy was found to be proportional to the glass transition temperature and showed maxima at the chemical threshold. The obtained results were discussed in terms of average coordination number or equivalently structure of the glassy matrix, decrease in average stabilization energy, electronegativity and average heat of atomization of the system. In amorphous materials, maximum optical non-linearity has been predicted. To open the prospects of Ge–Te–Sb vitreous system for non-linearity, Sheikh and Bahae relationship was applied, using experimentally obtained parameters, at a telecommunication wavelength. Obtained results showed a decent agreement with values available in the literature at 0.8 eV or 1550 nm. Non-linear refractive indices, almost three orders higher than silica glass, were obtained. These results may lead to yielding more sensitive optical limiting devices, and these glasses may be used as an optical material for a high-speed communication fibre.

KEYWORDS:

• Glass forming ability
• Urbach edge
• band gap
• cohesive energy
• glass transition temperature
• non-linearity

Disclosure statement

No potential conflict of interest was reported by the authors.