High-performance p-channel transistors with transparent Zn doped-CuI

‘Ideal’ transparent p-type semiconductors are required for the integration of high-performance thin-film transistors (TFTs) and circuits. Although CuI has recently attracted attention owing to its excellent opto-electrical properties, solution processability, and low-temperature synthesis, the uncontrolled copper vacancy generation and subsequent excessive hole doping hinder its use as a semiconductor material in TFT devices. In this study, we propose a doping approach through soft chemical solution process and transparent p-type Zn-doped CuI semiconductor for high-performance TFTs and circuits. The optimised TFTs annealed at 80 °C exhibit a high hole mobility of over 5 cm2 V−1 s−1 and high on/off current ratio of ~107 with good operational stability and reproducibility. The CuI:Zn semiconductors show intrinsic advantages for next-generation TFT applications and wider applications in optoelectronics and energy conversion/storage devices. This study paves the way for the realisation of transparent, flexible, and large-area integrated circuits combined with n-type metal-oxide semiconductor.

The chemical components of different CuI:Zn thin films were clarified using X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2c and supplementary Fig. 3a, the normalized Zn 2p peak intensity linearly increases with the Zn 2+ content, which confirms the increased dopant concentration. No satellite peak is observed between Cu 2p peaks, indicating that only Cu + exists in the CuI:Zn films. The N signal cannot be detected for all samples. As for C 1s spectra, only one single peak at ~284.3 eV was observed with low intensity of ~300 cps, indicating carbon mainly originates from the adsorbed adventitious contaminants on film surface. For pristine CuI, the strong diffraction peak located at 2θ = 25.5 o is assigned to CuI (111) plane (JCPDS card No. 06-0246). The peak intensity is significantly decreased after Pb 2+ or Bi 3+ doping, which means the CuI film crystallinity was effectively suppressed. Due to the larger ionic radii of Bi 3+ (108 pm) and Pb 2+ (120 pm) compared to those of Cu + (96 pm), the substitution at Cu sites leads to an increased lattice constant, resulting in the diffraction peak shifts toward lower angles.
The smaller shift caused by the addition of Pb 2+ can be understood by the low doping efficiency.
Although the Ga 3+ (62 pm) have a smaller size than Cu + , the Ga 3+ doping in CuI slightly shifts the CuI (111) plane towards a lower diffraction angle. This effect can be also understood by the Cu vacancies getting filled by Ga 3+ leading to a slightly expanded lattice. However, owing to the limited solubility in CuI matrix (large ionic radii), separated PbI 2 and BiI 3 phases appear at lower angle and the intensities increase with higher doping contents. As for the Ga 3+ doped CuI, the slightly shift of CuI (111) plane shift toward lower diffraction angle was also observed, which can be understood by the Ga 3+ filling on Cu vacancies and thus the slightly expanded lattice. For Bi-doped CuI thin films, the optical transmittance and bandgap values reduced remarkably with increasing doping content. In addition, the obvious extra absorption was noted in the (αhν) 2 to hν curves, which could be attributed to segregated BiI 3 phase. This also implies the poor doping efficiency of Bi 3+ in CuI matrix. For the CuI thin films incorporated with other dopants (e.g., Ni 2+ , Pb 2+ , and Sn 4+ ), the optical transmittance reduced significantly, just like Bi-doped ones (data not shown here). The optimisation of channel thickness is important to achieve high performance TFTs. Due to the high hole concentration in CuI:Zn channel layers, their thickness should be thinned to deplete excessive holes for the high I on /I off . We investigated the thickness-dependent device performance through adjusting CuI solution concentration while fixing the Zn 2+ doping amount, i.e., 5 mol%.
The monotonically decreased μ sat and negative V TH shift along with the thinner film thickness could be mainly attributed to the reduced absolute hole amounts, which is similar to our previous observation on CuI TFTs 2 . The μ sat of CuI:Bi 5 mol% TFTs is 0.45 cm 2 V -1 s -1 , which is slightly higher than those of Pb-doped devices (0.34 cm 2 V -1 s -1 with 5 mol% Pb 2+ ). Given a larger ion radius of Pb 2+ (120 pm) than Bi 3+ (108 pm), the lower doping efficiency is expected. Despite the notable current inhibition for Ga 3+doped CuI TFTs, all the devices exhibit poor current modulation behavior without reliable OFF state. The small ionic radius of Ga 3+ makes it prefer to act as interstitial impurities rather than fill in copper vacancies or substitute Cu + . The statement is confirmed by the clockwise hysteresis in double-sweep transfer curves, revealing that mobile impurity ions exist in the CuI:Ga channel layer. 4 As for the Ni 2+ -doped devices, no ideal transistor was observed. Considering the improper octahedral geometry and facile hydrate formation tendency in NiI 2 , the results indicate that Ni 2+ is not suitable as CuI dopant. We also tried Sb 3+ and In 3+ as dopants in CuI matrix. However, it is unsuccessful to prepare the mixed solution no matter in air or inert environment.  As for the Zn-doped CuI, due to high doping efficiency of Zn 2+ and low valence state compared with Bi 3+ , the polarization phenomenon occurred in a very short time even with high Zn 2+ doping contents (Fig.3e). When the CuI:Zn TFT was exposed in air for short term, the obvious p-doping effect was observed with higher current level and right-shift threshold voltage. The p-doping originates from the absorption of O 2 from air. During the fabrication and annealing process of CuI:Zn thin films, the evaporation of a trace amount of iodine could occur, leaving iodide vacancies (electron donors) inside the films. Upon exposing the device in air, O 2 molecules can occupy iodide vacancies, resulting in acceptor-like electronic states. 5,6 During this period, the moisture can also diffuse into the crystal lattice of materials, resulting in the hole interaction with polar H 2 O at grain boundaries.

Supplementary
In addition, the adsorbed H 2 O at grain boundaries can increase the energy barrier for hole transport. 7,8 Due to the impressionable feature for CuI:Zn to O 2 , the short-term explosion could lead to the pdoping phenomenon. Therefore, the I DS showed abnormal increase under bias stress test in ambient condition.  Note: "--" means not mentioned in the literature; "×" means not demonstrated in the paper.