On the Influence of Surface Phenomena Upon Charge Transport in Te‐Based Glassy Semiconductors

• Published in: physica status solidi (b) Vol. 255; no. 6
• Main Authors: Tsiulyanu, Dumitru, Ciobanu, M.
• Format: Journal Article
• Language: English
• Published: 01-06-2018
• Subjects: charge transport; CO2; glassy chalcogenides; H2O; NO2; surfaces
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201700447

The effect of surface phenomena caused by adsorption of NO2, CO2, and H2O vapor on the charge transport in thin films of chalcogenide glassy semiconductors (ChGS) of the As2S3Ge8–Te system has been studied. The investigations have been carried out by measuring the dynamic conductivity of these films in a large (5 to 107 Hz) frequency range, in both dry air and its mixture with controlled concentrations of the above mentioned gases at different temperatures. In addition to this, the temperature dependence of DC conductivity has also been measured for an unambiguous interpretation of the results. It is shown that depending on material's composition, the AC conductivity in dry air can either be frequency independent across the full range or be frequency independent until ≈104 Hz, with increasing σ(ω) ≈ ωn (n ≈ 0.7) at higher frequencies. This behavior demonstrates that for these glassy films there is a competition of several transport mechanisms, so that depending on frequency at normal temperatures the charge is carried: by free holes or by tunneling between localized states at either valence band edge or those near the Fermi level. An estimation has been made for the density of states at the Fermi level of As2S3Ge8Te13 that appears to be N(EF) ≈ 1.3 × 1021 eV−1 cm−3. Changes of environmental conditions either in air humidification or application of even very small (ppm) concentration of NO2 dramatically influence the spectra of dynamic conductivity that, on the other hand, remain nearly unaffected upon CO2 vapor application. This is evidence that for some chalcogenide materials the surface phenomena disturb the energetic distribution of states adjacent to surface, changing the transport mechanisms.
The ambient environment differently affects the AC spectra of conductivity of chalcogenide glassy semiconductors (ChGS): addition of both NO2 and water vapor strongly influences the shape of the conductivity spectrum, while the addition of CO2 practically does not affect it. Thus, the surface phenomena of ChGS caused by gas adsorption can lead to a modification of the charge transport mechanism. Apparently, this phenomenon is typical for disordered semiconductors and is due to both disturbing of the distribution of energetic states and variation of the mobility of free carriers in the region adjacent to surface.