Mototaka Ochi

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.

P‐149: Anode Electrode of Al‐Ni Alloy Film Directly in Contact with ITO for Top‐Emitting OLEDs

A bottom anode with high reflectivity is one of the most important materials for top-emitting OLEDs. As an anode electrode, stacked ITO/Al-Ni alloy-based films, as opposed to conventional stacked ITO/Ag films, were investigated for the first time. It was found that the direct contact technique of the Al-Ni alloy-based film with the ITO film using a film annealing process was applicable for OLEDs, as in the case of LCD interconnections that have already been developed by the authors. It was thus concluded that the Al-Ni alloy-based films were suitable materials for the anode electrode of OLEDs. They are a better choice than Ag film, due to their low material cost and ease of production of large-area sputtering targets for large-Size OLED-TVs.

Characterization of interface of Al-Ni/a-Si for thin film transistor using high-resolution Rutherford backscattering spectrometry.

High-resolution Rutherford backscattering spectrometry (HRBS) in combination with grazing angle argon sputtering was carried out to characterize the interface of aluminum-nickel (Al-Ni) alloy and amorphous-silicon films in a thin film transistor (TFT) for liquid crystal display (LCD). After thinning the top Al-Ni layer by a 1-keV Ar sputtering, the sensitivity of the interface oxygen was improved to be twice higher than that before sputtering. The results revealed that the oxygen at the interface relates to the contact characteristics.

24.2: Single Layer Al‐Ni‐La‐Si Interconnections for Source and Drain of LTPS‐TFT LCDs Using Direct Contacts with both ITO and poly‐Si

It was demonstrated for the first time that the use of Al-Ni-La-Si alloy films for the direct contacts of interconnection lines with both ITO and poly-Si was feasible for the LTPS-TFTs. Measured contact resistivity was in the order of 10−4 Ω.cm2 for ITO and Al-0.6 at.% Ni-0.1 at.% La-0.5 at.% Si. The Al alloy films patterned on poly-Si were found to be stable below 350 °C. It was also found that Al alloy has good dry etching characteristics such as a high etching rate and good selectivity to photo-resist, suitable for high-resolution LTPS-TFT LCDs.

Evaluation of stress stabilities in amorphous In–Ga–Zn–O thin-film transistors: Effect of passivation with Si-based resin

Fabrication process conditions of a passivation (PV) layer correlated with stress stabilities of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). In etch-stop layer (ESL)-TFTs, by inserting a Si-based resin between SiN x and SiO x PV layers, the peak intensity in the photoinduced transient spectroscopy (PITS) spectrum was notably reduced. This suggested the suppression of hydrogen incorporation into a-IGZO, which led to the improvement of stability under negative bias thermal illumination stress (NBTIS). In contrast, the hydrogen-related defects in the a-IGZO were easily formed by the back-channel etch (BCE) process. Furthermore, it was found that, under NBTIS, the transfer curves of the BCE-TFTs shifted in parallel owing to the positive fixed charge located in the back channel of the a-IGZO TFTs. The hump-shaped shift increased with stress time. This is because hydrogen atoms located at the back-channel surfaces of the a-IGZO and/or PV layers were incorporated into the channel region of the BCE-TFTs and induced the hydrogen-related defects.

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.

56.4: Formation of Single‐Layer Al‐Alloy Interconnection for Source and Drain of a‐Si TFT Using One‐Wet‐One‐Dry Etching with Four‐Mask Process

Single layer Al-alloy interconnection for source and drain of amorphous Si (a-Si) thin film transistor (TFT) is demonstrated by direct-contact technology with no barrier metals at the interfaces with both ITO and a-Si. Thermally stable contacts were formed on a-Si with a buried nitridation layer while maintaining the contact resistivity as low as 0.1Ωcm2. Excessive interdiffusion between the Al-alloy electrode and a-Si that can degrade TFT characteristics was suppressed by the nitridation layer. The Al-alloy direct contact technology, combined with one-wet-one-dry etching with four-mask process, drastically simplifies the the TFT fabrication process and contributes to cost reduction of a-Si TFT LCD.

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.