High-mobility p-type semiconducting two-dimensional β-TeO2

Latest directions in p-type transparent conductor design

“…Indeed, synthesis of b-TeO 2 bilayers has been successful from a eutectic melt (Te : Se 5% : 95%) droplet that is rolled on a silicon wafer using a van der Waals printing process. 92 Both EELS and XPS measurement of the valence band confirmed a band gap of 3.7 eV, the hole effective mass was determined to be 0.51m 0 from scanning tunnelling spectroscopy data, in good agreement with the theoretical value, and the bilayers performed well when deposited in field effect transistors. DFT calculations on the bilayer reveal hole mobility values of 436 cm 2 and 7690 V À1 s À1 in the a and b directions respectively, due to the different conduction pathways on these axes.…”

“…Of the most interest in recent years are the significant developments made in the non-oxide field, with excellent valence band dispersion achievable in phosphides, selenides, tellurides and halides. 63,80,92,94,116,120 Many of these materials have 3-dimensional crystal structures, in the sense that their conduction pathways exist in all three cartesian directions, such as BP due to its zinc blende structure, which is a significant advantage over the quasi-2-dimensional structures of delafossites and oxychalcogenides when implementing them into a heterojunction device. Overcoming stability, deposition and safety concerns in these simple non-oxide systems is a major goal in the coming years, as the allure of straightforward interfacing and potentially cheap synthesis of these basic crystal structures is attractive to industry -indeed, CuI has already generated interest as a p-type TFT.…”

“…b-TeO 2 has been recently proposed as a wide band gap semiconductor due to its direct band gap of 3.70 eV in monolayer form. 92,116 Hybrid DFT calculations show that the valence band is comprised mainly of Te 5p and O 2p, with smaller contributions from Te 5s orbitals, and that there is high, anisotropic mobility of both electrons and holes, with the latter predicted to reach mobility of 9100 cm 2 V À1 s À1 in the G -X direction for the monolayer system. 116 It was calculated to have a low cleavage energy similar to black phosphorus, so exfoliation is expected to be relatively straightforward.…”

“…Hall measurements of the mobility gave an average value of 141 cm 2 V À1 s À1 . 92 The p-type dopability remains to be tested, with the possibility of charge compensating native defects posing the biggest challenge, and there is of course the problem of Te abundance, but if a large hole concentration can be engineered in b-TeO 2 then it could be an exciting direction for future research.…”

Recent advances in the fabrication of 2D metal oxides

“…However, the required β-TeO 2 phase forms only at temperatures below 350°C, which is below the melting point of its parent metal and prevents the direct synthesis of this polymorph form. To solve these problems, the p-type β-TeO 2 nanosheets were obtained using a doped metal formed from the alloying of Te with Se ( Zavabeti et al., 2021 ). The eutectic melt contains 5 wt % Te and 95 wt % Se and exhibits a melting point much lower than 200°C.…”