Effect of the Deposition Temperature and Thermal-Annealing on the I-V Characteristics of Low-Temperature-Processed Polycrystalline ZnO and Amorphous InGaZnO TFTs
Authors: Tari Alireza , Wong William S. 
Published: February 2020

Transition metal oxide thin-film transistors (TMO-TFTs) have attracted remarkable attention both in academia and industry during the past few years. ZnO and ZnO-based multicomponent amorphous metal oxides, especially InGaZnO (IGZO), have opened a new window to large-area flexible transparent electronic devices due to their unique properties such as high carrier transport and optical transparency.

For large-area flexible transparent electronics based on the transition metal oxides are to be fully implemented, it will be critical to tune the crystalline structure and chemical composition of the metal-oxide channel layers using low-temperature-process methods. During the past few years, several research groups have reported the effects of plasma treatment, annealing process, deposition techniques, and deposition recipes on the electrical performance of the polycrystalline ZnO and amorphous IGZO TFTs; these studies have correlated the electrical performance of the fabricated TFTs to the material characteristics of the channel layer. However, the significant effect of the low-temperature vacuum-annealing and the channel layer deposition temperature on the TFT current-voltage (I-V) characteristics and material properties of the polycrystalline ZnO and amorphous IGZO channel layers has not been well studied.

In this research, electrical dependence on the materials properties, surface topography, and chemical composition of the polycrystalline ZnO and amorphous IGZO channel layers are investigated by varying the channel layer deposition temperatures (from room-temperature to  150°C) and post processing low-temperature (150°C) vacuum-annealing. In order to exclude the effect of surface roughness of the gate dielectric and the out-diffusion of hydrogen (H) atoms from plasma-enhanced chemical vapor deposition (PECVD) of conventional gate dielectric layers to the TMO channel layer during the fabrication processes, bottom gate TMO-TFTs were fabricated on thermally grown SiO2.

A slight improvement in the threshold voltage (VT) of  150°C processed ZnO TFTs after vacuum-annealing at 150°C was observed. On the other hand, 150°C IGZO TFTs with µ=8.8 cm2/V.s, VT= 4.2 V, S.S.= 0.42 V/decade, and Ion/off >106 became highly conductive and did not show switching behavior after vacuum-annealing. In order to determine the effect of the low-temperature vacuum-annealing on the oxygen vacancy concentration, Vo, the chemical composition of the ZnO thin-films was characterized using X-ray photoelectron spectroscopy (XPS). The O 1s spectra of the ZnO film was fitted into three peaks, one located at low-binding energy (OL), the medium-binding energy (OM), and the high-binding energy (OH) through a Gaussian profile. The OL peak is attributed to the oxygen atoms in the oxide lattice without oxygen vacancies (Vo), OM is attributed to the Vo, and the OH peak is surface oxygen. The OM value in the O 1s XPS spectra showed almost the same amount of Vo, with 18.75 % and 19.2% as the as-deposited and the vacuum-annealed ZnO films, respectively. However, a significant increase in Vo concentration (23.06% vs. 28.71%) was observed in the IGZO channel layers upon annealing in vacuum. The improved I-V characteristics of ZnO TFTs after vacuu-annealing was correlated to the better film crystallinity, larger grain size (25 nm vs. 23 nm), and smoother interface roughness (0.615 vs. 0.814 nm), obtained using X-ray diffraction (XRD), scanning electron microscope (SEM), and atomic force microscopy (AFM) measurements, respectively. ZnO TFTs also exhibited a large negative-shift in VT and increased off-current when the deposition temperature of the ZnO channel layer increased from 150°C to 220°C. These findings were correlated to changes in the crystalline structure and the grain size (29 nm vs. 23 nm) rather than a change in the Voconcentration (19.04% vs. 18.75 %).

The I-V characteristics of room-temperature (RT) processed ZnO and IGZO TFTs exhibited a similar trend as the 150°C processed TMO-TFTs. The as-deposited RT ZnO and RT IGZO TFTs showed high resistivity and no switching behavior. After vacuum annealing at 150°C, the RT ZnO TFTs did not show any improvements in the I-V characteristics while the RT IGZO TFTs, displayed semiconducting behavior having I-V characteristics with µ=8.1 cm2/V.s, VT=3.5 V, S.S.=0.51 V/decade, and an Ion/off >106 due to an increase in the Vo concentration from 14.68% to 23.42%,. Lastly, the electrical stability of the drain current under dc bias will be presented.