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In the present study, the sub-gap states of amorphous In-Ga-Zn-O (a-IGZO) thin films treated with various process conditions have been evaluated by means of capacitance-voltage (C-V) characteristics and isothermal capacitance transient spectroscopy (ICTS). It was found that the space-charge densities of the a-IGZO decreased as the oxygen partial pressure was increased during the sputtering of a-IGZO thin films. The ICTS spectra for the 4, 8, and 12 % samples were similar and the peak positions were found to be around 1 × 10-2 s at 180 K. On the other hand, the peak position for the 20 % sample shifted to a longer time regime and was located at around 2 × 10-1 s at 180 K. The total densities of the traps for the 4, 8, and 12 % samples were calculated to be 5−6 × 1016 cm-3, while that for 20 % was one order of magnitude lower than the others. From Thermal desorption spectrometer, it was found that desorption of Zn atoms started at a temperature higher than 300 °C for the 4 % sample, while desorption of Zn was not observed for the 20 % sample. The introduction of the sub-gap states could be attributed to oxygen-rich and/or Zn-deficient defects in the a-IGZO thin films formed during thermal annealing.

SUMMARY We 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 ( -PCD). It is concluded that the -PCD is a promising method for in-line process monitoring for the IGZO-TFTs fabrication.

In the present study, the electronic structure and film composition at the back channel surface of amorphous In-Ga-Zn-O (a-IGZO) thin films were characterized to discuss the origin of the threshold voltage shift in the a-IGZO TFTs under light negative bias and temperature stress (LNBTS) test. Photoelectron yield spectroscopy revealed that surface gap states were observed near the valence band maximum, but were not responsible for the hump observed in the transfer curve. By using photoelectron spectroscopy for chemical analysis (ESCA), it was found that the film composition was changed as it was approaching to the surface; the content of In increased while that of Ga decreased. A increase in oxygen deficiency was also observed near the back channel surface. In accordance with the change, downward band bending was detected by ultraviolet photoelectron spectroscopy near the surface (up to 5 ~ 7 nm) region. According to cathodoluminescence (CL) measurements, two broad bands were observed at around 1.8 and 2.4 eV. The gap states causing the luminescence were mainly located near the back channel surface. It is considered that the trap states due to the oxygen deficiency and/or the Zn interstitials could cause the hump effect in the transfer curve of a-IGZO TFTs.

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% O 2 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 SiH 4 /N 2 O of 4/100 sccm for the ESL deposition, while the PITS spectra from the sample with a flow rate of SiH 4 /N 2 O 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 O 2 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 V th 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 V th shift during the NBTIS test.

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.

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.

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.

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.

Metal-oxide-semiconductor (MOS) diodes with various gate insulators (G/Is) were characterized by capacitance–voltage characteristics and isothermal capacitance transient spectroscopy (ICTS) to evaluate the effect of H and OH desorption and diffusion on the electronic structures in amorphous In–Ga–Zn–O (a-IGZO) thin films. The density and the distribution of the space charge were found to be varied depending on the nature of the G/I. In the case of thermally grown SiO2 (thermal SiO2) G/Is, a high space-charge region was observed near the a-IGZO and G/I interface. After thermal annealing, the space-charge density in the deeper region of the film decreased, whereas remained unchanged near the interface region. The ICTS spectra obtained from the MOS diodes with the thermal SiO2 G/Is consisted of two broad peaks at around 5 × 10−4 and 3 × 10−2 s before annealing, while one broad peak was observed at around 1 × 10−4 s at the interface and at around 1 × 10−3 s in the bulk after annealing. Further, the trap densi...

Electron traps in amorphous In–Ga–Zn–O (a-IGZO) thin films were studied using isothermal capacitance transient spectroscopy (ICTS). Broad peaks that shifted toward a longer elapsed time with an increase in the filling pulse width were detected from the ICTS spectra for metal-oxide-semiconductor diodes consisting of a Mo/SiO2/a-IGZO structure. The time constant of the peak position at 180 K was found to be from ∼1 m to ∼100 ms, corresponding to a range of energy level from ∼170 to ∼230 meV below the conduction band edge. The total trap density around the peak was estimated to be ∼1 × 1016 cm−3·eV−1. Further, according to the biasing voltage dependence of the ICTS signal, the density of the trap states increases by about three orders of magnitude near the interface between the a-IGZO and the gate dielectric layers. The electron transport in electronic devices using an a-IGZO could be influenced by the trap states detected in the present study.