Physico-Chemical nanomaterials science

Features of the electrical properties of ZnO:Al/ZnS/n-CdTe heterojunctions
Maistruk E.V., Orletskyi I.G., Ilashchuk M.I., Koziarskyi I.P.,Koziarskyi D.P.
Yuriy Fedkovych Chernivtsi National University, 2 Kotsubynsky st., 58002 Chernivtsi, Ukraine.
E-mail: i.koziarskyi@chnu.edu.ua

ZnS thin films are promising semiconductor materials for use in solar cells as a transparent front window or buffer layer. A high transmittance (T = 85%) in the visible and near infrared ranges of the optical spectrum, a wide band gap (Eg ≈ 3.4 eV), and better crystal lattice matching compared to CdS [1] with A2B6 substrates contribute to their use in buffer layer heterostructures based on CdTe.
ZnO:Al/ZnS/n-CdTe heterostructures were fabricated using freshly split crystalline CdTe substrates doped with Cl impurity (Cl concentration in the melt was N0Cl ≈ 51018 cm–3). The structures were fabricated by sequential deposition of thin films of ZnS (~ 0.2 µm) and ZnO:Al (~ 0.1–0.15 µm) by high-frequency magnetron sputtering of zinc sulfide and aluminum-doped zinc oxide targets.
Analysis of the I-V-characteristics of the studied heterojunctions at forward and reverse voltages in the temperature range Т = 296–338 K showed the presence of the current rectification effect due to the potential barrier qk = 0.72 eV. The mechanism of charge carriers passing through the barrier at forward biases is well explained on the basis of the theory of space-charge-limited currents, which arises due to the presence of shallow monoenergetic (Nt ≈ 1014 cm-3) and deep (with exponential energy distribution) traps in the CdTe near-surface layer. The reverse currents are generated with the participation of the deep energy level ∆E = 0.76 eV in the region of the base material depleted on the main charge carriers. A feature of the C-V-characteristics of ZnO:Al/ZnS/n-CdTe heterojunctions in the reverse bias region (at frequencies f = 10–1000 kHz) is the independence of the measured capacitance from voltage and its change with a change in the frequency of the excitation signal. The electrical properties of ZnO:Al/ZnS/n-CdTe heterojunctions are explained by the presence of a high-resistance CdSxTe1-x layer in the near-surface region of cadmium telluride, which is formed due to the diffusion of sulfur atoms during the fabrication of the structure. 

1.	Hwang D. H., Ahn J. H., Hui1K. N., Hui K. S.,  Son Y. G. Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering // Nanoscale Research Letters.-2012.-26.-P. 7.