Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin films

Abstract

We report on a study of the physical properties of ZnTe:Cu films with Cu content up to ∼12 at. % prepared using rf magnetron sputtering. The composition and lateral homogeneities are studied using X-ray photoelectron spectroscopy (XPS). Atomic force microscopy measurements on films deposited at different substrate temperatures (up to 325 °C) yielded activation energy of 12 kJ/mole for the grains growth. The results of XPS and electrical and optical measurements provide evidence for the formation of the ternary zinc copper telluride alloy in films containing Cu concentration above ∼4 at. %. The XPS results suggest that copper is incorporated in the alloy with oxidation state Cu1+ so that the alloy formula can be written Zn1−yCuy Te with y = 2−x, where x is a parameter measuring the stoichiometry in the Cu site. The formation of this alloy causes appreciable shift in the binding energies of the XPS peaks besides an IR shift in the energy band gap. Detailed analysis of the optical absorption data revealed the presence of two additional transitions, besides the band gap one, originating from the Γ8 and Γ7 (spin-orbit) valence bands to a donor level at ∼0.34 eV below the Γ6 conduction band. This interpretation yields a value for the valence band splitting energy Δ ≅ 0.87 eV independent of copper concentration. On the other hand, the mechanism of formation of the alloy is tentatively explained in terms of a point defect reaction in which substitutional Cu defect CuZn is also created. Assuming that substitutional Cu is the dominant acceptor in the Zn rich alloy as in ZnTe, its formation energy was determined to be 1.7 eV close to the theoretical value (1.41 eV) in ZnTe.

Reduced electrical performance of Zn enriched ZnTe nanoinclusion semiconductors thin films for buffer layer in solar cells

“…The carbon line is on 288 eV indicates the presences of CO 2 from environment. It might be due to carbonate or as an adsorbate [18][19][20]. The carbide entities were not found as we don't have any peak around 284.5 eV which might be the surface charge calibrant.…”

“…The contaminant elements like carbon and oxygen were on the surface and variation in concentration of oxygen was observed, so the existence of formation of auxiliary oxides [20].…”

Pronounced Impact of p-Type Carriers and Reduction of Bandgap in Semiconducting ZnTe Thin Films by Cu Doping for Intermediate Buffer Layer in Heterojunction Solar Cells

“…The possibility of self-compensation doping [11] by metallic dopants is one of the reasons for their extensive use in opto-electronics. Previously, ZnTe has been used in ternary compounds like MgZnTe, CdZnTe, and HgZnTe as hybrid semiconductors [12]. ZnTe growth has been well documented by electro-chemical deposition, two-source evaporation, sputtering [12], and closed space sublimation (CSS) technique [13,14,15,16,17].…”

Self-Assembled Formation of Well-Aligned Cu-Te Nano-Rods on Heavily Cu-Doped ZnTe Thin Films

“…It is well known that Cu-doped ZnTe thin films or bulk materials are p-type due to the substitutional incorporation of Cu atoms into the Zn lattice sites [24]. The observed initial rapid fall of the resistivity up to T Cu  = 870 °C is attributed to such a substitutional doping.…”

“…Among previous reports on Cu doping of ZnTe thin films, most of them mainly focus on their electrical properties as a function of the Cu concentration without performing their structural and chemical analysis. Akkad and Abdulraheem recently provided evidence for the formation of the ternary zinc-copper-telluride alloy films containing Cu concentration above ~4 at.% prepared using RF magnetron sputtering [24], indicating that a structural or phase change could occur as Cu concentration in ZnTe reaches the over-doped regime. In this study, we discovered the formation of one-dimensional (1D) surface modulation and Cu-Te nano-rods for heavily Cu-doped ZnTe thin films grown by MBE.…”