Defect physics of $\text{BaCu}Ch\text{F}$ ($Ch=\text{S}$, Se, Te) $p$-type transparent conductors

Native point defects, defect complexes, and oxygen impurities in $\text{BaCu}Ch\text{F}$ were studied using density functional theory calculations, self-consistent thermodynamic simulations, and various experimental techniques. Unintentional $p$-type conductivity in $\text{BaCu}Ch\text{F}$ is explained by the presence of copper vacancies with transition levels in the valence band. These acceptor-like defects are partially compensated by donor-like chalcogen vacancies with transition levels deep in the gap. Chalcogen vacancies also cause the experimentally observed subgap photoluminescence, optical absorption, and persistent photoconductivity in BaCuSF and BaCuSeF. In thermodynamic equilibrium, both copper and chalcogen vacancies have low formation enthalpies and are likely to form defect complexes among themselves and with fluorine interstitials. The calculated Fermi level pinning range in $\text{BaCu}Ch\text{F}$ is narrow and located close to the valence band maximum. It makes $\text{BaCu}Ch\text{F}$ a suitable transparent $p$-type contact layer for optoelectronic applications but hinders attempts to fabricate transparent thin-film transistors using this material. Oxygen-related defects do not affect bulk $\text{BaCu}Ch\text{F}$ properties but surface oxidation decreases the mean free path of free holes by almost an order of magnitude.

Figure 1

(Color online) Crystal structure of $\text{BaCu}Ch\text{F}$ ($Ch=\text{S}$, Se, Te), $Ln\text{CuO}Ch$ ($Ln=\text{La}$, Pr, Nd), and $\text{LaFeO}Pn$ ($Pn=\text{P}$, As, Sb).

Figure 2

(Color online) (a) BaCuSF stability polyhedron. (b) Cross ection of the stability polyhedron by $\Delta {\mu }_{\text{Ba}}=0.25\Delta H^{\text{BaCuSF}}$ plane.

Figure 3

(Color online) Formation enthalpies of the lowest charge state of $V_{\text{Cu}}$ and $V_{Ch}$ as a function of Fermi energy for (a) BaCuSF, (b) BaCuSeF, and (c) BaCuTeF. The empty circles at the kinks of the lines correspond to defect transition levels. The Fermi energy pinning range is shadowed. The vertical dashed-dotted lines are the equilibrium Fermi energies at $700°\text{C}$. Insets: Experimental Fermi energies on the surface (XPS), in the bulk (Hall), and theoretical equilibrium Fermi energies (DFT) at $300°\text{C}$.

Figure 4

(Color online) Differential optical absorption spectra of ${\text{BaCu}}_{1+x}Ch\text{F}$ thin films for (a) $Ch=\text{S}$, (b) $Ch=\text{Te}$. (c) Optical absorption spectra of a ${\text{BaCuS}}_{1-x}\text{F}$ thin film measured 3 h and 3 days after removal from vacuum. Insets: (a) absorption measurement setup, (b) proposed ${\text{BaCu}}_{1+x}Ch\text{F}$ absorption model, (d) proposed ${\text{BaCuS}}_{1-x}\text{F}$ absorption model.

Figure 5

(Color online) Photoluminescence spectra of (a) BaCuSeF thin film, (b) BaCuSF thin film, and (c) BaCuSF powder aged for 5 years. Insets: (a) normalized conductivity of BaCuSF thin film after light exposure, (b) PL measurement setup, and (c) proposed PL model.