Composition Dependence of the Band Gap and Doping inCu2O-Based Alloys as Predicted by an Extension of the Dilute-Defect Model

Abstract

Tuning the opto-electronic properties through alloying is essential for semiconductor technology. Currently, mostly isovalent and isostructural alloys are used (e.g., group-IV and III-V), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. We developed an approach to predict compositional dependence of both band-structure and electrical properties from ab-initio calculations by extending conventional dilute defect model to higher (alloy) concentrations. Considering alloying of aliovalent (Mg, Zn, Cd) cations and isovalent anions (S, Se) into Cu2O, we predict tunability of band-gap energies and doping levels over a wide range, including the type conversion from p-to n-type. Initial synthesis and characterization of Zn and Se substituted Cu2O support the defect model, suggesting these alloys as promising novel oxide semiconductor materials. Cu-poor (Cu 2 O/CuO)Cu-rich (Cu 2 O/Cu)

Non-equilibrium alloying controls optoelectronic properties in Cu2O thin films for photovoltaic absorber applications

“…31 Upon Zn and Se substitution the band gap of the alloys shifts slightly to lower energies, as reported before. 17 This decrease of E g by À0.7x eV and À1.0y eV for Zn and Se, respectively, further aids the desired reduction of the effective absorption threshold of Cu 2 O (Fig. 1(c)).…”

“…16 In our recent computational study backed by preliminary experiments, we considered an alloying approach with aliovalent group IIA (Mg, Zn, Cd) and isovalent group VIA (S, Se) elements for tuning both the electrical and optical properties of Cu 2 O. 17 It was found that Zn Cu þ donors pair with V Cu À acceptors 18 to form 4-fold coordinated electrically neutral Zn 2Cu defect complexes that reduce the band gap of the alloy. Alloying with S/Se leads to binding of S O /Se O and V Cu þ , lowering the formation energy of V Cu þ and increasing the p-type doping, which is an unusual effect for isovalent alloying.…”

“…These Cu 2 O-based alloys are predicted to have a considerable mixing enthalpy and hence a limited solubility of the substitution elements in thermodynamic equilibrium. Nevertheless, synthesis of Cu 2 O 1Ày Se y , 17 Cu 2À2x Zn x O, 17,19 Cu 2 O, 20 and Cu 2 O 1Àx S x , 21 alloy thin films has been reported in literature by non-equilibrium physical vapor deposition (PVD) techniques. Using the similar synthesis approach, in this study we demonstrate how co-alloying Cu 2 O with Zn and Se independently controls its optical and electrical properties.…”

“…The hole concentration in the alloys has been predicted to increase with increasing Se content and to remain largely unchanged with increasing Zn/Cd content, except for very high levels of Zn/Cd substitution. 17 The experiments in this paper were performed using highthroughput combinatorial approach. The Cu 2À2x Zn x O 1Ày Se y thin films libraries of $1 lm thickness with slightly varying xand y-composition were deposited using co-sputtering from ceramic Cu 2 O, ZnO, and Cu 2 Se targets on stationary 50.8 Â 50.8 mm.…”

“…More details on composition and structural properties of the ZnO and Cu 2 Se alloyed Cu 2 O thin films had been reported. 17 The experimentally measured electrical properties of the Cu 2À2x Zn x O 1Ày Se y and the baseline thin film alloys are summarized in Fig. 2(b).…”