Hiroaki Tao

Effects of thermal annealing on variations of electron traps in the channel region of amorphous In-Ga-Zn-O thin film transistor

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.

Photoelectron emission yield experiments on evolution of sub-gap states in amorphous In-Ga-Zn-O thin films with post deposition hydrogen treatment

Total photoyield emission spectroscopy (TPYS) was applied to study the evolution of sub-gap states in hydrogen-treated amorphous In-Ga-Zn-O (a-IGZO) thin films. The a-IGZO thin films were subjected to hydrogen radicals and subsequently annealed in ultra-high vacuum (UHV) conditions. A clear onset of the electron emission was observed at around 4.3 eV from the hydrogen-treated a-IGZO thin films. After successive UHV annealing at 300 °C, the onset in the TPYS spectra was shifted to 4.15 eV, and the photoelectron emission from the sub-gap states was decreased as the annealing temperature was increased. In conjunction with the results of thermal desorption spectrometer, it was deduced that the hydrogen atoms incorporated in the a-IGZO thin films induced metastable sub-gap states at around 4.3 eV from vacuum level just after the hydrogenation. It was also suggested that the defect configuration was changed due to the higher temperature UHV annealing, and that the hydrogen atoms desorbed with the involvement of Zn atoms. These experiments produced direct evidence to show the formation of sub-gap states as a result of hydrogen incorporation into the a-IGZO thin films.

Correlation of trap states with negative bias thermal illumination stress stabilities in amorphous In–Ga–Zn–O thin-film transistors studied by photoinduced transient spectroscopy

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.

Effects of etch stop layer deposition conditions on stress stabilities in amorphous In-Ga-Zn-O thin film transistors

Effects of etch stop layer (ESL) deposition conditions on stress stabilities in amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) were investigated. The performances of the a-IGZO TFTs were varied depending on the flow rate of SiH4/N2O in chemical vapor deposition (CVD) of ESL. The threshold voltage shift after positive bias thermal stress test was decreased with increasing the flow rate of SiH4/N2O. The power-law time dependence on the value of the threshold voltage shift suggested the reduction of the density of the interfacial states between the ESL and the a-IGZO layers. The features were will correlated with the sheet resistances of the films, which is consistent with the above argument.

Effect of Mg or Ag addition on the evaporation field of Al.

It is known that the distribution of the charge-states as well as the evaporation field shift to higher values as the specimen temperature is decreased at a constant rate of evaporation. This study has explored the effect of Mg or Ag addition on the evaporation field of Al in terms of the charge state distribution of the field evaporated Al ions. The fractional abundance of Al(2+) ions with respect to the total Al ions in Al-Mg alloy is lower than that in pure Al, whereas it shows higher level in the Al-Ag alloy at lower temperatures. The temperature dependence of the fractional abundance of Al(2+) ions has been also confirmed, suggesting that Al atoms in the Al-Mg alloy need lower evaporation field, while higher field is necessary to evaporate Al atoms in the Al-Ag alloy, compared with pure Al. This tendency is in agreement with that of the evaporation fields estimated theoretically by means of measurements of the work function and calculations of the binding energy of the pure Al, Al-Mg and Al-Ag alloys.

Comparative study on light-induced negative-bias stress stabilities in amorphous In-Ga-Zn-O thin film transistors with photoinduced transient spectroscopy

A comparative study on light-induced negative-bias stress stabilities in amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) was performed by means of photoinduced transient spectroscopy (PITS). When the a-IGZO thin films were deposited with 4% O2 partial pressure (P/P), a dominant peak with a maximum of around 100 K was clearly observed from the sample. There was a flow rate of SiH4/N2O of 4/100 sccm for the ESL deposition, while the PITS spectra from the sample with a flow rate of SiH4/N2O of 6/150 sccm possessed a broader peak of around 115 K and an apparent shoulder of around 200-280 K was observed. This shoulder of around the 200-280 K was clarified when the a-IGZO thin film was deposited with an O2 P/P of 20 % In accordance with the changes in the electronic structures in the a-IGZO thin films due to the ESL deposition, the stability of the TFTs against the negative bias thermal Illumination stress (NBTIS) was degraded; the value of the Vth shift after the 2h-NBTIS test was increased from 2.5 to 6.0 V The decreasing the compensating acceptors and/or the increasing the hydrogen-related donors could be the origin of the negative Vth shift during the NBTIS test.

Evolution of hydrogen-related defect states in amorphous In-Ga-Zn-O analyzed by photoelectron emission yield experiments

Total photoyield spectroscopy (TPYS) was applied to investigate the evolution of hydrogen-related defect states in amorphous In-Ga-Zn-O (a-IGZO) thin films. It was found that the defect states located at around 4.3 eV from the vacuum level were formed as a result of hydrogenation. After thermal annealing at 300 °C, the onset of the TPYS spectra shifted to 4.15 eV. As the annealing temperature was increased, the photoyield from the defect states decreased, implying that there was outdiffusion of hydrogen with Zn from the a-IGZO. These experiments produced direct evidence which shows the formation of defect states as a result of hydrogen incorporation into the a-IGZO thin films.