Photoinduced transient spectroscopy (PITS) was applied to study the effects of thermal annealing in the thin-film transistor (TFT) fabrication process on the variations of the electron traps in the channel region of amorphous In-Ga-Zn-O (a-IGZO). A dominant peak with a maximum of around 130 K was observed in the PITS spectra, but the detailed features were varied depending on the annealing conditions. The six particular temperatures corresponding to the trap states were extracted at about 100, 140, 150, 210, 320, and 390 K from the differential PITS spectra, showing good correlation with the trap states observed in ZnO. The results of thermal desorption spectrometry suggested that the variation of electron traps in the a-IGZO thin films has its origin in the decomposition of O and Zn during the annealing process. The annealing after the etch-stop layer deposition was also examined. The peak at about 150 K extracted from the differential PITS spectra before and after the annealing was markedly decreased. The activation energy of the corresponding trap states was estimated to be around 0.3 eV, which was close to those known as the E3 center in ZnO. Secondary ion mass spectroscopy analysis suggested that the reduction of trap density was mainly due to a decrease in the number of defects which involve hydrogen atoms in their configuration. Considering these results, the variations in the electron traps in the a-IGZO thin films during the TFT fabrication process should be attributed to the introduction of Zn, O, and/or H-related defects into tetrahedra consisting of Zn-O bonds.
Negative bias thermal illumination stress (NBTIS) stabilities in amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) were studied by photoinduced transient spectroscopy (PITS). The degradation of TFT performance correlated with trap states in the channel region of a-IGZO TFTs with an etch stop layer (ESL). A prominent peak at approximately 100 K was observed in a-IGZO formed under a partial pressure (p/p) of 4% O2. With increasing O2 p/p, an apparent shoulder of around 230 K appeared in PITS spectra. A higher flow rate of SiH4/N2O for the ESL deposition induced trap states associated with the 230 K peak. The peak at approximately 100 K could originate from the depletion of Zn by preannealing, while the peak at approximately 230 K should be attributed to the oxygen-deficient and/or Zn-rich defects due to the formation of OH in a-IGZO. The trap states in a-IGZO TFTs gave rise to degradation in terms of NBTIS. The threshold voltage shift (ΔV
th) was 2.5 V, but it increased with the O2 p/p as well as the flow rate of SiH4/N2O for ESL deposition. The time dependence of ΔV
th suggested that hydrogen from the ESL and/or in the a-IGZO thin films was incorporated and modified the trap states in the channel region of the a-IGZO TFTs.
The electronic structures and electronic states near the back channel surfaces in amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) were investigated. The X-ray photoelectron spectroscopy (XPS) analysis revealed that, due to insufficient wet-etching of source/drain (S/D) of Mo metal, continuous sub-gap states formed throughout the bandgap, which induced degradation of the subthreshold swing of the TFTs with a back channel etch (BCE) structure. The stability under the negative bias thermal illumination stress (NBTIS) in the a-IGZO BCE-type TFTs was worse than that of conventional etch stop layer (ESL)-type TFTs. The Vth shift (ΔVth) increased from −2.5 V to −4.0 V with a hump-shaped variation. According to photoinduced transient spectroscopy (PITS) analysis, a broader shoulder of around 160 K clearly appeared in PITS spectra from the a-IGZO thin films with the BCE structure. The effect of post process annealing applied as the final process step of the TFT fabrication (post-annealing) was also examined against the a-IGZO BCE-TFTs. As the post-annealing temperature was increased, the peak at around 160 K which is associated with the hydrogen-related traps in a-IGZO thin film was markedly decreased. The results for the post-annealing at 300°C suggested a decrease in density of trap states which originated from depletion of Zn. A marked improvement of the stress stabilities under the positive bias, thermal stress (PBTS) was also observed as well as NBTIS.
The physical properties of amorphous In–Ga–Zn–O (a-IGZO) films deposited by DC sputtering under various sputtering pressures were investigated. The sputtering pressure was found to influence various physical properties. Lower sputtering pressures resulted in film densification and decreased both surface roughness and hydrogen concentration. In addition, transistor performance characteristics such as saturation mobility and sub-threshold swing improved as the sputtering pressure decreased. These results yield insight into the correlation between thin film transistor (TFT) performance and deposition conditions.
SUMMARYWe have investigated the microwave-detected photoconductivity responses from the amorphous In-Ga-Zn-O (a-IGZO) thin films. The time constant extracted by the slope of the slow part of the reflectivity signals are correlated with TFT performances. We have evaluated the influences of the sputtering conditions on the quality of a-IGZO thin film, as well as the influences of gate insulation films and annealing conditions, by comparing the TFT characteristics with the microwave photoconductivity decay (µ-PCD). It is concluded that the µ-PCD is a promising method for in-line process monitoring for the IGZO-TFTs fabrication. key words: oxide semiconductor, thin film transistor, microwave-detected photoconductivity decay, in-line process monitoring
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