Suppression of Negative Gate Bias and Illumination Stress Degradation by Fluorine-Passivated In-Ga-Zn-O Thin-Film Transistors

A bottom-gate IGZO TFT with a SiO_X etch-stop-layer (ESL) was fabricated. Two kinds of passivation of SiO_X (designated "IGZO TFT with SiO_X-Pa") and SiN_X:F (designated "IGZO TFT with SiN_X:F-Pa") were deposited on the IGZO TFT. The TFT structure used in this experiments was shown in Fig. 1, and detail fabrication process of the IGZO TFTs was reported in Ref. 19. The SiO_X passivation was deposited at 170°C by conventional plasma-enhanced chemical vapor deposition with a fluorine-free gas chemistry of SiH₄/N₂O/N₂. In contrast, the SiN_X:F passivation was deposited at 200°C by inductively coupled plasma-enhanced chemical vapor deposition with a hydrogen-free gas chemistry of SiF₄/N₂. The F content in the SiN_X:F passivation was evaluated to be 11 at.% by Rutherford backscattering spectrometry. Both TFTs were post-annealed in N₂ ambient at 350°C for 1 and 3 h, respectively, before electrical measurements. Channel length (L) and width (W) of the TFTs were 20 and 50 μm, respectively.

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All the electrical measurements were carried out in the dark in ambient air using an Agilent 4156C precision semiconductor parameter analyzer. During the NBIS test, gate voltage (V_GS) of –20 V and blue light (λ = 460 nm, 0.2 mW·cm⁻²) were simultaneously applied to the TFTs with grounded source and drain electrodes at room temperature. The NBIS was interrupted temporarily when the transfer characteristics were measured by a double-sweeping V_GS mode at a drain voltage (V_DS) of 0.1 V in darkness, and then NBIS was re-applied up to an accumulated stress time of 10⁴ s. For the double-sweeping V_GS mode, transfer characteristics were measured with V_GS from −10 to 20 V (denoted hereafter as forward measurement), and then scanned instantly back to −10 V (denoted hereafter as reverse measurement). For the photo-response measurements, the TFTs were illuminated for 2 min by a monochromatic light from the top side of the TFT at an intensity of 0.2 mW·cm². Then, transfer characteristics were measured immediately after turning off the monochromatic light. The wavelength of monochromatic light was varied from 630 to 400 nm.

All TFTs exhibited good transfer characteristics (refer to Ref. 19), and the initial properties of the TFTs with either SiO_X or SiN_X:F passivation after 1 h and 3 h annealing, respectively, were summarized in Table I. Both subthreshold swing (S) and hysteresis (ΔV_H) were improved for the TFT with SiN_X:F-Pa when the post-annealing time was increased from 1 to 3 h. When the annealing time further increased to 5h, electrical properties of the TFT with SiO_X-Pa did not change significantly, whereas that of the TFT with SiN_X:F-Pa became conductive and did not exhibit switching properties. We have reported in Ref. 19 that, for the TFT with SiN_X:F passivation, F was incorporated into the SiOx-ESL during the SiN_X:F passivation deposition, and the F in SiOx-ESL diffused into the IGZO channel when the post-annealing time was increased. The results indicated that appropriate amount of F passivated effectively the electron traps in the IGZO bulk or at a GI/IGZO interface; however, excess F in the IGZO increased carrier concentration since F also acted as a shallow donor in IGZO.^20,21 We also discussed in detail the relationship between PBTS stability of the IGZO TFT, F and hydrogen content in the IGZO channel in Ref. 19. It was confirmed by secondary ion mass spectrometry (SIMS) that the PBTS stability drastically improved for the TFT with SiN_X:F passivation when the F diffused into the IGZO channel through post-annealing of 3 h.

To further investigate the F passivation effects on the near-VBM state and its influence on NBIS stability, changes in the transfer characteristics of the TFTs with either SiO_X or SiN_X:F passivation were evaluated under blue light illumination (λ = 460 nm, 0.2 mW·cm⁻²) with a V_GS stress of −20 V. It was confirmed that the shift in V_th under a V_GS stress of −20 V without light (NBS) was negligible for both of the TFTs with either SiO_X or SiN_X:F passivation after 1 h or 3 h (data not shown here). However, when a V_GS stress of −20 V was combined with blue light (NBIS), both instability of V_th and on-current degradation were observed from both TFTs after post-annealing of 1 h, as shown in Fig. 2. The degradation tendency is very similar for both TFTs with either SiO_X or SiN_X:F passivation after 1 h annealing.

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In contrast, it was found that NBIS degradation exhibited a large difference between the TFT with SiO_X-Pa and that with SiN_X:F-Pa when the post-annealing time was increased from 1 to 3 h, as shown in Fig. 3. For the former, the more serious degradation was observed after 3 h annealing as shown in Figs. 3a and 3b; however, the degradation tendency was similar to that of the TFT after 1 h annealing. In contrast to the TFT with SiO_X-Pa, it should be noted that the transfer curves of the TFT with SiN_X:F-Pa hardly shifted at all under NBIS after post-annealing for 3 h as shown in Figs. 3c and 3d.

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To describe the improvement in NBIS reliability more clearly, the change of transfer characteristics before and after NBIS of 10⁴ s are summarized in Figs. 4a and 4b for the TFTs with either SiO_X or SiN_X:F passivation, respectively. For the former, hysteresis in the transfer characteristics increased from 0.38 V (initial) to 4.31 V after NBIS of 10⁴ s, as shown in Fig. 4a. A negative shift in V_th, accompanied by on-current degradation, was observed in the forward measurement, and a positive shift in V_th without subthreshold degradation was observed in the reverse measurement. A detailed mechanism for NBIS degradation of the TFT with SiO_X-Pa was reported in Ref. 22 and 23. For the IGZO TFT with SiN_X:F-Pa, in contrast, the V_th shift under NBIS was suppressed after NBIS of 10⁴ s. An increase in hysteresis of the TFT with SiN_X:F-Pa was negligible even after the NBIS of 10⁴ s.

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Figure 5 shows the SIMS depth profile of mass-to-charge ratio (m/z) of 19 (F or OH) ions in the SiO_X-Pa(200 nm)/SiO_X-ES(200 nm)/IGZO(100 nm)/Si and SiN_X:F-Pa(200 nm)/SiO_X-ES(200 nm)/IGZO(100 nm)/Si stacked films. Due to the saturation of fluorine ion count in the SiN_X:F-Pa film, depth profiles was recorded after in-situ sputtering of the passivation layer during the SIMS measurements. It was confirmed no difference of hydrogen profile in the IGZO channel between the TFTs with SiOx and SiN_x:F passivation¹⁹ (Data not shown here). Figures 5a and 5b respectively show the comparison of the ion ratios of m/z = 19 normalized by ¹⁸O in the SiO_X-ES/IGZO stacks between SiN_X:F-Pa and SiO_X-Pa after 1 h and 3 h annealing. As shown in Fig. 5a, it was found large difference of m/z = 19 content in the SiO_X-ES layers between the SiO_X-Pa and SiN_X:F-Pa samples even after 1 h annealing. This result indicates that fluorine, which was generated during the SiN_X:F deposition, was introduced in the SiO_X-ES for the case of SiN_X:F-Pa. Although the SIMS result revealed the existence of F in the SiO_X-ES layer with SiN_X:F-Pa, the F cannot be diffused in the IGZO layer after 1 h annealing. When the annealing period increased to 3 h, as shown in Fig. 5b, F diffusion in an IGZO layer can be detected for the case of the SiN_X:F passivation. Thus, the results indicated that a long annealing time enhanced F diffusion from SiO_X-ES layer to an IGZO channel when F existed in the SiO_X-ES layer. Moreover, the results shown in Figs. 4b and 5b suggests that the near-VBM state would be passivated effectively by F when the F diffused into the IGZO channel.

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To prove the F passivation of the near-VBM state, the photo-response of the IGZO TFTs with either SiO_X or SiN_X:F passivation was measured after post-annealing for 3 h. The photo-response of the TFTs was evaluated in the photon energy range from 2.0 to 3.1 eV to investigate the photo excitation of electrons from the near-VBM state which was reported to be ∼2.3 eV from a conduction band (E_C).¹

Figures 6a and 6b summarize the change in S and turn-on voltage (V_on) of the IGZO TFTs with either SiO_X or SiN_X:F passivation, respectively, as a function of the photon energy of incident light. The subthreshold swing began to change for the TFT with SiO_X-Pa when the photon energy exceeded ∼2.5 eV, while that for the TFT with SiN_X:F-Pa did not change until the photon energy exceeded 2.9 eV. The V_on of the TFT with SiO_X-Pa initially shifted in the positive V_GS direction and then shifted in the negative V_GS direction when the photon energy exceeded ∼2.5 eV, whereas that of the TFT with SiN_X:F-Pa did not change until the photon energy exceeded 2.9 eV. These results clearly showed that the photo-response of the IGZO TFTs with SiN_X:F-Pa could be improved by F passivation of the near-VBM state.

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Figure 7 shows a schematic illustration of the degradation mechanism in IGZO TFTs with either SiO_X or SiN_X:F passivation under NBIS. It has been reported that high-density electron traps formed by oxygen vacancy (V_O) existed in an IGZO channel about 2.3 eV from E_C.¹⁸ During the NBIS tests, a photon energy of 2.7 eV was sufficient to excite trapped electron from the near-VBM (neutral V_O) state to E_C. Meanwhile, the ionized V_O⁺ and V_O²⁺ oxygen vacancies were simultaneously neutralized by photoexcitation of electrons from the valence band to V_O⁺ and V_O²⁺, which contribute to the generation of holes in the valence band. Photo-excited holes and electrons were respectively trapped at the GI/IGZO interface or in the GI and at the IGZO/ESL interface or in the ESL. In addition, donor-like defects were generated near the Fermi level (E_F) during the NBIS,^6,13 as shown in Fig. 7a. The donor-like defect creation was not observed without light (NBS), and was enhanced by decreasing negative gate bias during the NBIS.²² The result suggests that photo-excited V_O⁺ or V_O²⁺ would be an origin of the donor-like defects generated near E_F. Similar result was reported in Ref. 6.

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Both hole trapping at the GI/IGZO interface or in the GI and donor-like defect creation near E_F are responsible for the negative shift in V_th accompanied by on-current degradation in the forward measurement as shown in Figs. 2a, 2c, and 3a. However, during the forward measurement, donor-like defects generated near E_F could be stabilized by capturing electrons when electrons were accumulated in the channel by applying positive V_GS.²² Furthermore, most of all trapped holes at the GI/IGZO interface or in the GI were de-trapped by use of a positive V_GS during the forward measurement. As a consequence of the stabilization of donor-like defects near E_F and the de-trapping trapped hole at the GI/IGZO interface or in the GI, only the influence of uniformly trapped electron at the IGZO/ESL interface or in the ESL induced a parallel positive turn-on voltage shift without subthreshold degradation in the reverse measurement as shown in Figs. 2b, 2d, and 3b. An influence of electron trapping at the IGZO/ESL interface or in the ESL under NBIS on transfer characteristics was reported in Refs. 22 and 23. In contrast, when F diffused into the IGZO channel, the near-VBM state in IGZO TFTs with SiN_X:F-Pa can be passivated effectively by F, as shown in Fig. 7b, since F has a similar ionic radius to that of oxygen. As a consequence of the F passivation of the near-VBM state, the photo-response was improved for the TFTs with SiN_X:F-Pa. Thanks to the F passivation of the near-VBM state, NBIS degradation can be suppressed since photo generation of electron (2e⁻) and the V_O²⁺ pair was significantly reduced. Hence, we conclude that diffused F in an IGZO channel passivated effectively the near-VBM state, resulting in a suppression of NBIS degradation.

We investigated the NBIS stability and photo-response of IGZO TFTs with either SiO_X or SiN_X:F passivation. Improvements in NBIS stability and photo-sensitivity were achieved for the IGZO TFTs with SiN_X:F passivation when F diffused into an IGZO channel over a long period of annealing. Diffused F in an IGZO channel passivated effectively the near-VBM state, resulting in a suppression of NBIS degradation. We demonstrated that the fluorine-passivated IGZO TFT is advantageous for achieving highly reliable oxide TFTs under conditions of both NBIS and PBTS.¹⁹

The authors thank T. Takatoh, M. Fujinaga, N. Fujita, H. Ishida, and T. Satoyoshi of Tokyo Electron Co. Ltd. for their experimental supports of the SiN_X:F deposition and valuable discussions.