Atomic layer deposition of Nb-doped ZnO for thin film transistors

Shaw, A.; Wrench, J.S.; Jin, J.D.; Whittles, T.J.; Mitrovic, I.Z.; Raja, M.; Dhanak, V.R.; Chalker, P.R.; Hall, S.

Abstract

We present physical and electrical characterization of niobium-doped zinc oxide (NbZnO) for thin film transistor (TFT) applications. The NbZnO films were deposited using atomic layer deposition. X-ray diffraction measurements indicate that the crystallinity of the NbZnO films reduces with an increase in the Nb content and lower deposition temperature. It was confirmed using X-ray photoelectron spectroscopy that Nb5+ is present within the NbZnO matrix. Furthermore, photoluminescence indicates that the band gap of the ZnO increases with a higher Nb content, which is explained by the Burstein-Moss effect. For TFT applications, a growth temperature of 175 °C for 3.8% NbZnO provided the best TFT characteristics with a saturation mobility of 7.9 cm2/Vs, the current On/Off ratio of 1 × 108, and the subthreshold swing of 0.34 V/decade. The transport is seen to follow a multiple-trap and release mechanism at lower gate voltages and percolation thereafter. Zinc oxide (ZnO) has received a great deal of attention over the recent years particularly for the application of transparent electronics for active matrix displays. ZnO shows advantages when compared to materials such as amorphous silicon in terms of higher saturation mobilities1,2 and a larger band gap (Eg ∼ 3.37 eV)1 enabling the potential for transparent display technology. The atomic layer deposition (ALD) of ZnO provides the potential for large area homogeneous films. Low temperature ALD (<150 °C) is known to achieve the best performance as the films are less conductive, with the reported mobilities of >10 cm2/Vs.3–5 However, indium gallium zinc oxide (IGZO) is the preferred ZnO based material for thin film transistors (TFTs)6 due to its relatively high electron mobility, stability, and good control of conductivity. However, for large scale production, non-indium based materials are desirable for cost effectiveness, hence the requirement for alternative dopants. Research into non-indium based ZnO materials for TFT application includes: gallium,7 silicon,8 and magnesium.9 These dopants act as effective oxygen vacancy (Vo) suppressors and can further increase the band-gap of the ZnO through the Burstein-Moss effect.7–9 Niobium (Nb) has the potential as a dopant, due to its high valency (Nb5+) which offers the prospect of a Vo suppressor, superior to the dopants gallium (Ga3+), silicon (Si4+), and magnesium (Mg2+). Furthermore, Nb can act as an effective substitutional dopant for Zn2+ and subsequently reduce the disorder and carrier scattering. To date, the studies of Nb-doped ZnO (NbZnO) films have been reported using pulse layer deposition (PLD) 10,11 and sputtering.12 The use of Nb-doping in transparent semiconducting oxides has been reported for TiOx TFTs.13 In this letter, we present the physical and electrical characteristics of NbZnO films for active layers in TFT applications. The NbZnO films of nominal thickness 50 nm were deposited using ALD on heavily doped, thermally oxidized (50 nm) n-type Si wafers. A capping layer of 5 nm Al2O3 was first deposited by ALD at 200 °C on the SiO2. The active layer NbZnO was then deposited at 200 °C using the precursors diethlyzinc (DEZn) and niobium pentaethoxide (Nb(OEt)5); temperatures of these precursors entering the chamber were ambient and 140 °C, respectively. The Nb ALD cycle fraction was varied from 1% to 12.5%, where the NbZnO films were made by first depositing the x-cycles of ZnO by successive steps of DEZn and then H2O vapor on the surface. After the x-cycles of ZnO, a single Nb2O5 cycle is deposited by the successive steps of Nb(OEt)5 and H2O. The process is repeated until the desired film thickness is reached. For example, a film with a Nb cycle percentage of 2% would be achieved by 49 cycles of DEZn and H2O (x = 49) followed by 1 cycle of Nb(OEt)5 and H2O repeated 6 times giving a total of 294 ZnO and 6 Nb2O5 cycles. This equates to a cycle fraction of 0.02 and a cycle percentage of 2% (6/(294 + 6) × 100% = 2%). The thickness of the films was confirmed by spectroscopic ellipsometry as 52 ± 2 nm. The effect of the ALD growth temperature was tested for 3.8% NbZnO at 150, 175, 200, and 225 °C. X-ray diffraction (XRD), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) were performed to determine the crystallographic nature, optical properties, and composition, respectively. The XRD measurements were performed on 1% NbZnO as-deposited, 1% to 12.5% NbZnO annealed for 1 hour at 300 °C air, and 3.8% NbZnO grown at 150 °C, 200 °C, and 225 °C annealed under the same conditions. The crystalline phases were identified by XRD using Cu Kα radiation (0.154051 nm, 40 kV, and 50 mA), and the diffraction patterns are shown in Figs. 1(a) and 1(b). The films are polycrystalline with peaks (100), (002), and (101). Fig. 1(a) shows the comparison between a 1% NbZnO film as-grown at 200 °C and after annealing. The act of annealing in air enhances the film crystallinity as indicated by the increase in the intensity of the (100) and (101) peaks as well as the reduction of their full width at half maxima (FWHM) from 0.50 to 0.45 and 0.51 to 0.44, respectively. In general, there is greater (002) directionality, although as the Nb content is increased, the films' crystallinity is reduced eventually becoming amorphous at 9.1% and above. For the (002) peak, the grain size is reduced from 20 to 14 nm and an increase in Nb from 1% to 6.8% is calculated using the Scherrer equation.14 This reduction in ZnO crystallinity with Nb concentration has also been reported when grown using PLD.11 Fig. 1(b) demonstrates that reducing the growth temperature serves to reduce the crystallinity of the ZnO as there are no observable (002) or (101) peaks at 150 °C.

Citation

Shaw, A., Wrench, J., Jin, J., Whittles, T., Mitrovic, I., Raja, M., …Hall, S. (2016). Atomic layer deposition of Nb-doped ZnO for thin film transistors. Applied Physics Letters, 109(22), Article 222103. https://doi.org/10.1063/1.4968194

• Journal Article Type: Article
• Acceptance Date: Nov 6, 2016
• Online Publication Date: Nov 29, 2016
• Publication Date: Nov 29, 2016
• Deposit Date: Jun 1, 2017
• Publicly Available Date: Jan 17, 2018
• Journal: Applied Physics Letters
• Print ISSN: 0003-6951
• Electronic ISSN: 1077-3118
• Publisher: American Institute of Physics
• Peer Reviewed: Peer Reviewed
• Volume: 109
• Issue: 22
• Article Number: 222103
• DOI: https://doi.org/10.1063/1.4968194