Atomic Layer Deposition of Undoped TiO₂ Exhibiting p-Type Conductivity

Abstract

With prominent photocatalytic applications and widespread use in semiconductor devices, TiO2 is one of the most popular metal oxides. However, despite its popularity, it has yet to achieve its full potential due to a lack of effective methods for achieving p-type conductivity. Here, we show that undoped p-type TiO2 films can be fabricated by atomic layer deposition (ALD) and that their electrical properties can be controlled across a wide range using proper postprocessing anneals in various ambient environments. Hole mobilities larger than 400 cm(2)/(V·s) are accessible superseding the use of extrinsic doping, which generally produces orders of magnitude smaller values. Through a combination of analyses and experiments, we provide evidence that this behavior is primarily due to an excess of oxygen in the films. This discovery enables entirely new categories of TiO2 devices and applications, and unlocks the potential to improve existing ones. TiO2 homojunction diodes fabricated completely by ALD are developed as a demonstration of the utility of these techniques and shown to exhibit useful rectifying characteristics even with minimal processing refinement.

Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy conversion

“…52 Most importantly, the formation of the n-p junction is the critical step towards the development of solar cells based on oxide semiconductors, such as TiO 2 . 52 Most importantly, the formation of the n-p junction is the critical step towards the development of solar cells based on oxide semiconductors, such as TiO 2 .…”

Rutile phase n- and p-type anodic titania nanotube arrays with square-shaped pore morphologies

“…p-Type TiO 2 films are exciting because while difficult to obtain, they extend the application spectrum of n-type TiO 2 by enabling transparent and high power homojunction diodes, new photocatalytic configurations to exploit the reducing behavior of excess electrons and heterojunction solar cells based on n-type absorbers and a p-type TiO 2 scaffold. 43 This work demonstrates a solution-based synthetic route for p-type TiO 2 nanostructures that does not involve doping by metal ions. Characteristics of the three types of nanotubes are summarized in Table S1 in the ESI.…”

Atomic Layer Deposition of p‐Type Semiconducting Thin Films: a Review

“…[184] Very recently TiO2 -a well-known intrinsically n-type TCO material due to oxygen vacancies and/or titanium interstitials [241,242] -was fabricated as p-type films by ALD both without external doping and with nitrogen doping. [115,116] However, the origin of the uncommon p-type conductivity in the two cases seemed not to be the same. In the former un-doped case the p-type doping was assigned to be due to excess oxygen, [116] while in the latter N-doped case due to titanium vacancies.…”

“…[115,116] However, the origin of the uncommon p-type conductivity in the two cases seemed not to be the same. In the former un-doped case the p-type doping was assigned to be due to excess oxygen, [116] while in the latter N-doped case due to titanium vacancies. [115] Nonetheless, these films showed promising electrical and optical properties ( Table 6), and even homodiodes were fabricated using them (Figure 6).…”