Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1029−1037
pubs.acs.org/acsaelm
Hole Transport Modulations in Low Dimensional γ‑CuI Films: Implication for High Figure of Merit and Thin Film Transistors Anil Annadi,† Nengduo Zhang,†,‡ David Boon Kiang Lim,† and Hao Gong*,† †
Department of Materials Science and Engineering, National University of Singapore, 117576 Singapore, Singapore NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 117456 Singapore, Singapore
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S Supporting Information *
ABSTRACT: Development of efficient transparent and conducting functionalities of p-type materials at low dimensions are extremely vital for thin film transistor (TFT) electronics and hole transport applications in solar cells. In this work, we report dimensionality effects on functionalities of p-type CuI films prepared from room temperature sputter deposited Cu3N and Cu precursors. For film thickness ranging from 300 to 35 nm, it is found that the hole density increasing with decreasing film thickness is corroborated by relative defect photo luminescence associated with the Cu vacancy shallow level, further endorsing the origin of p-type conductivity in CuI to Cu vacancies. These dimensionality dependent hole transport modulations associated with hole density enable to achieve transparent conducting properties with high light transmission >82% and figure of merit (FOM) over 2250 MΩ−1, the best performing among p-type films. At low dimensions with a film thicknesses below 35 nm, hole density shows a significant drop of two orders with transport being found to be mediated by a percolation mechanism. Structural and morphology studies reveal the formation of faceted two-dimensional nanoplates with growth preferred along (111) of the γ-phase and indeed interconnected to form percolation. CuI TFT devices fabricated in percolated nanoplate regions show a good p-type transistor performance with a current on/off ratio of 102 for a 25 nm channel, and the linear field effect mobility reached 4.8 cm2 V−1 s−1 for a 30 nm channel, being the best so far in CuI TFTs. Development of high mobility CuI TFTs using sputtering at room temperature as reported here offer great advantages for design of large area complementary devices. Further exploitation of twodimensional nanoplates of CuI for construction of p−n junctions is potential for nanoelectronics. KEYWORDS: p-type transparent semiconductor, figure of merit, two-dimensional nanoplates, thin film transistor, high mobility
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INTRODUCTION Over the past decades, several inorganic materials have been testified as transparent conductors, such as n-type-doped In2O3 and ZnO for transparent electronic applications.1,2 Whereas semiconducting n-type InGaZnO has been the best performing thin film transistor (TFT) for transistor applications.3 On the other hand, its counterpart, p-type transparent materials, has been elusive of research, owing to difficulties in achieving hole transport with high conductivity and appreciable mobility. For instance, in the case of ZnO, a conventional semiconducting oxide, hole dopant, such as Cu, would expect to generate a ptype conduction; however, in reality, simultaneous oxygen deficiency, formation, and/or unintentional H doping kill the p-type conductivity.4 A variety of oxide-based p-type materials, such as Cu2O, NiO, and Mg:Cr2O3, have been investigated as potential p-type transparent semiconductors.5−8 However, the fact is that the hole transport paths in p-type oxides mainly comprise of O 2p orbitals that are strongly localized, typically leading to a large hole effective mass, thus resulting in a low hole mobility
