Investigation of Hole Dopability in Oxygen $2p$-Dominated Bands

TL;DR

This work reveals the intrinsic difficulty of hole dopability in oxides whose valence bands are primarily $O$-$2p$ in character. Through a 9-step high-throughput workflow applied to 845 oxides, the study filters materials by $V_O$-driven hole compensation, $w_{ m O-2p}$ at the VBM, cation-dopant acceptor levels, and final $HSE06$ defect energetics, concluding that only CaCdO$_2$—with a slight Cd-$3d$ admixture at the VBM—shows realistic potential for hole doping into $O$-$2p$-dominated bands. The findings emphasize the extreme challenge of achieving true $O$-$2p$ hole conductivity and corroborate the enduring usefulness of the conventional VBM-raising strategy, while also highlighting the critical role of accurate defect energetics and the limitations of band-edge alignment alone as predictors of dopability. The work further demonstrates that even promising candidates require nontrivial $d$-orbital or lone-pair contributions to enable shallow acceptor behavior, guiding future design toward oxides that deliberately incorporate such hybridizations. Limitations include focusing on thermodynamically stable, 0 K oxides and hydrogenic dopants, suggesting that exploration of metastable phases or alternative dopant mechanisms could reveal additional p-type oxides.

Abstract

The development of $p$-type oxide semiconductors remains impeded by the inherently low-lying valence-band maximum (VBM) dominated by O-2$p$ states. A prevailing approach to mitigate this limitation is to elevate the VBM by introducing cation states that hybridize with O-2$p$ orbitals or lie energetically above the O-2$p$ level. Nevertheless, the $p$-type oxides reported to date exhibit limited hole mobilities. To expand the search space, it is essential to accurately understand the intrinsic difficulty of introducing holes into O-2$p$-dominated bands. Accordingly, we evaluated 845 oxides to identify those in which holes can be doped into O-2$p$-dominated bands. Our high-throughput screening revealed CaCdO$_2$ as the only promising exemplar, in which the VBM is slightly hybridized with deep-lying Cd-3$d$ states. Our screening suggests that hole doping into O-2$p$-dominated bands is extremely difficult and thus reinforces the effectiveness of the traditional ``VBM-raising strategy.''

Figures (6)

• Figure 1: Flow to screen oxides that can be hole doped into the O-2$p$-dominated bands in this research. The remaining numbers of oxides in each step are also shown. The oxides enclosed in the dashed box were excluded from the screening based on a detailed analysis of their VBM character (see text). The acceptors calculated in the third step are listed in Table S1 of the Supporting Information, and the 152 oxides considered in the second step are listed in Tables S2–S4, categorized according to the O-2$p$ contribution at the VBM.
• Figure 2: (a) Number of oxides containing each element among the 152 oxides retained after screening based on oxygen vacancy formation energies. The percentages in parentheses indicate the survival rates after screening. The number of oxides containing each element before the screening are shown in the Supporting Information. (b, c) Histogram of the O-2$p$ contribution at the VBM ($w_{\mathrm{O\text{-}2}p}$) for (b) 845 target oxides and (c) 152 oxides remaining after screening.
• Figure 3: The O-2$p$ contributions at the VBM ($w_{\mathrm{O\text{-}2}p}$) are shown in descending order. Oxides are categorized into four classes, as indicated in the legend (see text for details). For oxides with $w_{\mathrm{O\text{-}2}p}$ greater than that of ZnO, the acceptor levels introduced by dopants are also classified into three categories. Gray symbols indicate oxides with deep acceptor levels in PBEsol calculations. Blue symbols represent oxides that have shallow acceptor levels in PBEsol but become deep in HSE06. Orange symbols denote oxides with shallow acceptor levels even when using HSE06. Isosurface plots of representative oxides are shown at 5% of the maximum electron density.
• Figure 4: Crystal structures with squared wavefunctions at the VBM, electronic band structures, and densities of states for (a) CrCdO$_4$, (b) BaHgO$_2$, (c) ZnO, (d) Zn$_2$PtO$_4$, (e) L1$_0$--CaCdO$_2$, and (f) SQS model for CaCdO$_2$. The O-2$p$ contributions at the VBM ($w_{\mathrm{O\text{-}2}p}$) and the dopants confirmed to behave as shallow acceptors are also indicated in parentheses.
• Figure 5: Formation energies of native defects (solid lines) and Na dopants (dashed lines) as a function of the Fermi level in (a) CrCdO$_4$ (O-rich, Cr-poor), (b) L1$_0$--CaCdO$_2$, and (c) SQS model for CaCdO$_2$. The chemical potentials are set at oxygen-rich conditions (see the Supporting Information). $V_X$ denotes a vacancy at the $X$ site, while $X_i$ denotes an interstitial at the $X$ position. The atomic sites and interstitial sites are shown in the crystal structures in Figure \ref{['fig:banddos']}. $X_Y$ denotes an antisite defect where $X$ is substituted at the $Y$ site. In (c), only the lowest formation energies are shown for each defect type except for the oxygen vacancies.