Abstract
Zinc oxynitride (ZnON) semiconductors are suitable for high performance thin-film transistors (TFTs) with excellent device stability under negative bias illumination stress (NBIS). The present work provides a first approach on the optimization of electrical performance and stability of the TFTs via studying the resonant interaction between anions or vacancies in ZnON. It is found that the incorporation of nitrogen increases the concentration of nitrogen vacancies (VN+s), which generate larger concentrations of free electrons with increased mobility. However, a critical amount of nitrogen exists, above which electrically inactive divacancy (VN-VN)0 forms, thus reducing the number of carriers and their mobility. The presence of nitrogen anions also reduces the relative content of oxygen anions, therefore diminishing the probability of forming O-O dimers (peroxides). The latter is well known to accelerate device degradation under NBIS. Calculations indicate that a balance between device performance and NBIS stability may be achieved by optimizing the nitrogen to oxygen anion ratio. Experimental results confirm that the degradation of the TFTs with respect to NBIS becomes less severe as the nitrogen content in the film increases, while the device performance reaches an intermediate peak, with field effect mobility exceeding 50 cm2/Vs.
Highlights
Amorphous oxide semiconductors (AOSs) such as In-Ga-Zn-O (IGZO) with high carrier mobility are good alternatives to their amorphous silicon counterparts for the fabrication of high performance thin-film transistors (TFTs)[1], especially in the field of flexible electronics or flat panel displays
From a point defect perspective, the following issues may be of interest: (i) Where is the VO level located when nitrogen is incorporated into ZnO? (ii) how do nitrogen vacancies (VN) and oxygen vacancies (VO) differ in terms of formation enthalpy and deep-to-shallow electronic transitions? (iii) How are the valence band tail (VBT) states affected? in a nitrogen-rich environment with a high probability of finding two neighboring nitrogen anions or vacancies, which of a closely-paired N-divacancy complex (VN-VN) and a nitrogen dimer (N-N) is more likely to form? Understanding the above physical phenomena is of the essence in order to successfully tailor the performance and stability of zinc oxynitride (ZnON) thin-film transistors
The results indicate that (i) shallow single-donor VN defects form more than deep double-donor VO because the Zn-N binding energy is smaller than that of a Zn-O bond, (ii) the highest occupied levels of the VN+ and VO0, which are anticipated to be photo-excited, are located near the VBT states above the valence band maximum (VBM), and (iii) the formation of O-O dimers that induce device degradation under negative bias illumination stress (NBIS) becomes less likely as the nitrogen content increases
Summary
Amorphous oxide semiconductors (AOSs) such as In-Ga-Zn-O (IGZO) with high carrier mobility are good alternatives to their amorphous silicon counterparts for the fabrication of high performance thin-film transistors (TFTs)[1], especially in the field of flexible electronics or flat panel displays. High performance AOS TFTs that exhibit high field effect mobility undergo relatively large shifts in threshold voltage when subjected to negative gate bias in the presence of visible light. The latter condition emulates the operating environment of a switching element in an operating active matrix liquid crystal display (AMLCD) with a backlight unit, or in a transparent active matrix organic light emitting diode (AMOLED) TFT array exposed to ambient illumination. It was proposed that deep VO defects may become electrically inactive if the valence band minimum of the host material were raised above the VO levels, in the presence of abundant nitrogen anions. From a point defect perspective, the following issues may be of interest: (i) Where is the VO level located (deep or shallow) when nitrogen is incorporated into ZnO? (ii) how do nitrogen vacancies (VN) and oxygen vacancies (VO) differ in terms of formation enthalpy and deep-to-shallow electronic transitions? (iii) How are the valence band tail (VBT) states (responsible for the formation of peroxides) affected? in a nitrogen-rich environment with a high probability of finding two neighboring nitrogen anions or vacancies, which of a closely-paired N-divacancy complex (VN-VN) and a nitrogen dimer (N-N) is more likely to form? Understanding the above physical phenomena is of the essence in order to successfully tailor the performance and stability of ZnON thin-film transistors
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