Introduction

Research on amorphous oxide-based semiconductors (AOSs) have attracted attentions as a leading candidate for flexibility, large scale, and transparent electrical devices due to high mobility, high optical transparency, and low temperature deposition compared to amorphous silicon1,2,3. Recently, these AOSs were used in various flexible devices4 and a sensor array5 as the active layer. However, the AOSs devices still suffer from instability issues such as illumination, bias, and temperature stress. In general, the origin of the instability of AOSs comes from carrier trapping and injection, ambient gas interaction, and oxygen vacancy (V0)6,7,8,9. The fundamental studies regarding first and second origins showed that the instability can be improved using a high quality gate dielectric and an appropriate passivation layer, respectively10,11. On the other hand, the instability problem related to V0 is difficult to address because V0 is involved in the intrinsic property of AOSs. To solve this issue, AOSs are deposited under the oxygen-rich condition in order to reduce oxygen vacancy, but it inevitably accompanies a decrease in mobility. Thus, many studies regarding the reduction of Vo without mobility deterioration, such as UV annealing12 and high pressure oxygen annealing13, are reported. These methods can increase the metal oxide bond and decrease Vo by chemical oxidation but it requires a prolonged time (1–2 hours) and additional external energy source. These studies are relatively difficult to apply in the manufacturing process because a huge amount of time and expensive equipments are required for them to be implemented on the large-scale substrates. Thus, it is necessary to develop an effective, largely practical, and simple method to improve the stability of AOSs. In this respect, the strong oxidizer, which is a hydroxyl radical, can preferentially react with oxygen defects without prolonged treatment and an additional energy source. The hydroxyl radical (OH*) obtained from the decomposition of hydrogen peroxide (H2O2), is generally used to eliminate organic components in the solution process due to its high oxidation potential compared to oxygen and ozone14,15,16. According to the previous reports, the H2O2 is decomposed to OH* by photolysis17, pyrolysis18, catalytic pyrolysis, and catalysis on the surface of the metal oxide particle19,20. In the case of catalytic pyrolysis and catalysis, the decomposition rate depends on the surface area of the oxide particles. This decomposition behavior in catalytic pyrolysis and catalysis has been taken into account for this defective oxide surface because the increase in surface area is accompanied by an increase in the number of defect sites on the surface. In general, this defective oxide surface, induced by UV irradiation, is called “hydrophilic surface”. According to the Sun, R. –D. et al.21, the degree of the defective oxide surface is proportional to the degree of the hydrophilic surface which is defined as the contact angle. Therefore, the strong oxidation of OH* can be used on the oxide surface by appropriate UV irradiation and thermal H2O2 solution treatment. In addition, the UV irradiation can also lead to the transition of V0 to V0 2+ states as releasing the free electrons in the oxide films and this V0 2+ tends to get back to the ground state after a certain amount of time22. Normally, these metastable defects cause persistent photoconductivity (PPC) and negative bias-illumination stress (NBIS) degradation in most of the AOSs22,23. In this respect, these metastable oxygen defects can easily react with OH*. On the basis of these background examples, we suggest the SRD method using the spontaneous decomposition of the H2O2 solution and radical reaction on the highly hydrophilic oxide surface to decrease defects related to oxygen in AOSs film. We also investigated this SRD effect for radical oxidation through electrical characteristics and stability, and conducted chemical analysis using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR).

Results and Discussion

Figure 1 shows the overall process adapted for the fabrication of SRD a-IGZO TFTs. The SRD was carried out by three steps that are comprised of UV irradiation to generate a defective surface (highly hydrophilic surface), H2O2 treatment for radical doping, and thermal annealing. First of all, it is necessary to verify the highly hydrophilic surface for radical generation. According to Sun, R. –D. et al.21, the contact angle represents the degree of hydrophilic and it depends on the UV irradiation time. Then, we measured the contact angle of the a-IGZO surface as increasing UV irradiation time in order to investigate the minimal time for the highly hydrophilic surface. Figure 2(a) shows that the UV irradiated a-IGZO surface has 7° in a UV illumination time of 15 mins. This is similar to the super-hydrophilic surface condition, which is defined as being under 5°. These results showed that the defective a-IGZO surface was formed by our UV irradiated condition.

Figure 1
figure 1

Schematic illustration of SRD a-IGZO TFTs process.

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Figure 2
figure 2

Variations of (a) contact angle of a-IGZO surface with illumination time and transfer characteristics with different conditions: (b) Only-UV, (c) SRD, (d) H2O2 without UV irradiation.

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Figure 2(b–d) shows transfer characteristics with different treatment conditions, including only-UV, H2O2 without UV and SRD. First, the only-UV treated device exhibits a large increase in off current, as shown in Fig. 2(b). This is because UV irradiation causes the generation of electron-hole pairs, ionization of Vo, and photo-reduction of weak chemical bonds in the a-IGZO films. Second, in the H2O2 without UV samples, non-UV treated IGZO surface is a strictly hydrophilic surface, which is defined as being under 90°, in the above contact results. However, as shown in Fig. 2(c), there is no remarkable change of transfer characteristics in the only H2O2 treated device without UV irradiation. On the other hand, the SRD a-IGZO TFTs distinctly exhibit much improved transfer characteristics compared to other treated devices, as shown in Fig. 2(d). These results indicated that the combined UV irradiation and H2O2 treatment are essential for improving electrical characteristics. Table 1 summarizes the electrical parameters of a-IGZO TFTs with different treatments, including field-effect mobility (μFET), on/off ratio, and sub-threshold swing (S.S).

Table 1 Summary of the electrical parameters including μFET, on/off ratio, and S.S for different treatments.
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The stability test is crucial for estimating structural defect sites related to Vo in amorphous oxide films. In particular, hydrogen in incorporated OH*, and Vo is associated with stability degradation in NBTS and NBIS tests24,25. Accordingly, the NBTS test was first performed in Fig. 3(a) and (b), showing that the Vth of the SRD device rarely shifts (0.4 V) whereas that of the pristine device shifts 2.4 V with increased off current. This result indicated that the hydrogen incorporated OH* does not work as interstitial states which cause to negative Vth shift26. To further examine the SRD effect regarding Vo, we evaluated the NBIS test. The Vth shift of the non-treated device and SRD device are 4.6 V and 9.6 V, respectively, as shown in Fig. 3(c) and (d). Figures S1 and S2 show the results of NBTS and NBTIS tests for non-treated and SRD treated devices for 10000 s at 80 and 100°C, respectively. The Vth shift of the SRD device is much less than that of non-treated device. This result showed that the SRD method is effective in improving the stability of NBTS and NBIS caused by interstitial atom and oxygen vacancy ionization. Additionally, we conducted a PBS test. As shown in Figure S3 (a) and (b), the Vth shift of the SRD device is less than that of the pristine. This result also showed that the SRD method can enhance the stability of PBS is originated from carrier trapping caused by oxygen defects and ambient gas interaction in the non-passivation oxide device10,27. We also re-measured the electrical performance of the SRD a-IGZO TFTs after 3 months because it is possible that they would be affected by the photo-generated carrier recombination and ionized Vo neutralization. As shown in Figure S4, the enhanced transfer characteristics of SRD a-IGZO TFTs were retained even after 3 months. Therefore, these outstanding stability results have significant implications for a explaining the SRD effect, which can decrease the defect sites that are related to Vo. This is not a temporary effect.

Figure 3
figure 3

Variations of negative Vth shift under NBTS and NBIS stability of SRD a-IGZO TFTs with stress times: (a) NBTS and (c) NBIS stability of pristine, (b) NBTS and (d) NBIS stability of SRD a-IGZO TFTs.

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We performed an XPS depth analysis to confirm the chemical characteristics and the SRD effect. Figure 4 shows the O1s spectra for a-IGZO film with a different treatment condition. The O1s spectra were deconvoluted into three different peaks that were centered at 530.1 ± 0.2, 531.0 ± 0.2, and 532.0 ± 0.2 eV. The first peak represents lattice oxygen with a low binding energy related to In, Zn, and Ga metal-oxide (M-O) bonds. The second peak with middle binding energy and the high binding energy peak correspond to Vo and weakly bonded hydroxyl groups (-OH), respectively28. Figure 4(a–c) show that the SRD a-IGZO films have a higher M-O bond and the lowest oxygen vacancies compared with those of non-treated and H2O2 w/o UV treated devices. This SRD effect was maintained with film depth, as shown in Figure (d–f). In addition, it was found that the oxygen vacancies rapidly decrease in the area near the back-channel region by the radical oxidation, as shown in Fig. 4(e). These findings appear to demonstrate the reasons for the improved stability results by the decrease in V0 and reduced V0 layer effect, which is similar to the self-passivation effect, at the back-channel surface region29. Additionally, to investigate the OH* bond with metal cation, we analyzed Zn 2p3/2 XPS spectra because Zn atoms can react well with oxygen species compared to In and Ga, due to their lower bond energy30. As a result, Fig. 5 shows that the SRD a-IGZO films have an increase in Zn-O (Zn2+) and Zn-OH bond, and a decrease in Zn interstitials (Zn+) compared to those of the pristine and H2O2 without UV treated device. This means that OH* has preferentially bonded with V0 and metal cation defects in a-IGZO films for the increase in Zn-O (Zn2+) and decrease in Zn+. We additionally investigated the location of the Fermi energy level of SRD a-IGZO films in valence band offsets derived from XPS data to verify the reason for the improvement of μFET despite the decreasing V0 concentration31,32. For the SRD a-IGZO films, the Fermi level was located relatively close to the conduction band, as shown in Figure S5. The Fermi energy level shift can be explained by the change in carrier concentration.

Figure 4
figure 4

XPS results from deconvolution of O 1 s spectra for the a-IGZO film under different conditions: (a) Pristine (b) SRD, (c) H2O2 w/o UV at 20 secs etched region, and area percentage of XPS depth profile with etch time: Variations of (d) M-O, (e) Vo, (f) -OH.

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Figure 5
figure 5

Zn 2p3/2 XPS spectra of a-IGZO film with different conditions: (a) Pristine, (b) SRD, and (c) H2O2 w/o UV at 20 secs etched region.

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As shown in the XPS depth results, the SRD can affect a-IGZO films with depth. In order to confirm the SRD effect, we re-investigated the electrical transfer characteristics with decreasing channel thickness for the reduction of the radical inter-diffusion distance from the back-channel surface. Figure 6 shows the transfer characteristics with decreasing channel thickness and statistical parameters, including μFET, S.S, and maximum trapped charge density (Nmax). Nmax was extracted from the transfer characteristics using the following relationship:

$${N}_{\max }=(\frac{S.S\,\bullet \,\mathrm{log}(e)}{kT/q}-1)\frac{{C}_{i}}{q}$$
(1)

where k is the Boltzmann constant, T is the absolute temperature, Ci is the gate capacitance per unit area, and q is the elementary charge. The SRD, the thinner a-IGZO TFTs, have a superior transfer characteristic. The μFET is additionally improved from 16.1 to 17.5 cm2/V•s, S.S is decreased from 0.35 to 0.32 V/decade, and Nmax is decreased from 8.67 × 1011 to 7.3 × 1011 cm−2.

Figure 6
figure 6

Transfer characteristics of pristine and SRD a-IGZO TFTs with decreasing channel thickness: (a) log scale, (b) statistical parameters including μFET, S.S, and Nmax, and (c) Linear scale.

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It can be demonstrated that the reduction effect of V0 increases in the thinner a-IGZO TFTs and it is also possible to decrease defect sites near the interface region between the channel and the gate dielectric by radical oxidation because it was closer from the back-channel to the interface region as the channel thickness decreased. Furthermore, to indirectly confirm the radical diffusion, we performed FTIR analysis with a differently treated bi-layer a-IGZO structure that was comprised of depositing sputter a-IGZO (30 nm) on the solution-processed a-IGZO, including rich-organic components. Figure S6 shows the FTIR spectra of the non-treatment, H2O2 without UV and SRD-treated bi-layer a-IGZO samples. These spectra support the diffusion of OH* during the SRD treatment because OH* is a powerful oxidizer to organic components. Thus, we can verify the diffused radical reaction through the variation of organic (C-H bond) peak in solution-processed IGZO. The C-H peak-related bending vibration at a range of 900–1370 cm−1 and stretching vibration at 2900–3100 cm−1 is evidently suppressed in the spectra of the SRD-treated sample33. This FTIR result shows OH* can moderately diffuse in the a-IGZO films. Therefore, these results indicated that the SRD is more effective as the channel thickness decreased and it can also promote the interface characteristics of a-IGZO films.

We propose the chemical mechanism of SRD, as illustrated in Fig. 7. On the basis of the above analysis data, the SRD mechanism can be demonstrated by photo-activated defects in the surface region and radical oxidation by thermally radical solution treatment, as shown in Fig. 7. The energies of UV light at 185 nm and 254 nm are 6.7 eV and 4.8 eV, respectively. This UV energy is greater than the binding energy of O2 (5.13 eV) as well as the binding energy of In-O (1.7 eV), Ga-O (2.0 eV) and Zn-O (1.5 eV)34,35. Thus, the UV light can make defect sites related to oxygen vacancies, oxygen interstitials, and interstitial metal cations in the oxide surface region by breaking the bond, and the following equations (2) and (3) generally agreed with the decomposition reaction in H2O2 kinetics according to the previous reports36,37.

$${{\rm{H}}}_{2}{{\rm{O}}}_{2}+{\rm{defect}}\,{\rm{sites}}\to {{\rm{OH}}}^{\ast }+{{\rm{OH}}}^{\ast }$$
(2)
$${{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{OH}}}^{\ast }\to {{\rm{HO}}}_{2}^{\ast }\,+{{\rm{OH}}}^{\ast }\to {{\rm{H}}}_{2}{\rm{O}}+{{\rm{O}}}_{2}$$
(3)
Figure 7
figure 7

The schematic illustration of SRD mechanism.

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The initial step of the catalytic decomposition of H2O2 on an amorphous oxide surface is homolytic cleavage of the O-O bond to form two OH*19. The further reaction of OH* with H2O2 rises to form the oxygen molecule at the surface region. This decomposition behavior would also be promoted by a thermally treated H2O2 solution (80 °C) because the activation energy of H2O2 decomposition decreases and the decomposition rate increases in accordance with an increase in the H2O2 solution temperature20. In addition, the oxygen atom can be partially generated through a second reaction of oxygen molecules and \({{\rm{OH}}}^{\ast }({{\rm{OH}}}^{\ast }+{{\rm{O}}}_{2}\to {{\rm{HO}}}_{2}^{\ast }+O)\) 38. These generated radical and oxygen atoms under thermal annealing, can preferentially bond with oxygen and metal cation defects, including ionized Vo, metal cation interstitial, and oxygen interstitial. In other words, the UV irradiation artificially ionizes V0 in deep level states, and the generated OH* through thermal radical solution treatment on the catalytic surface can make a chemical reaction with these metastable defects (ionized Vo). Moreover, this radical has a high reactivity and diffusivity, thus, it would vigorously react at the back-channel surface of a-IGZO film and this reaction would also lead to the formation of a reduced V0 layer at the back-channel surface region. This reduced V0 layer inhibits the interaction of the adsorption of molecules from ambient air39. Additionally, some diffused radical can also decrease oxygen defects near the interface region of the a-IGZO films and gate dielectric. Lastly, the μFET improvement effect can be taken into account for the change of carrier concentration as well as the decrease in interface trap sites. The change of carrier concentration can be brought about by the reaction of H2O2 and oxygen interstitials (Oi) generated by UV irradiation37. Therefore, the SRD method facilitates an effective decrease in artificially activated V0, an increase of self-passivation effect, and improvement of μFET by radical oxidation through sequential UV and thermal radical solution treatment.

Conclusions

In this study, we investigated self-activated radical doping (SRD), which consists of UV irradiation and thermal radical solution treatment. The SRD a-IGZO TFTs exhibited remarkably improved electrical performances. The μFET and on/off ratio were improved from 9.1 to 17.5 cm2/Vs and from 8.9 × 107 to 7.96 × 109, respectively. The negative and positive Vth shift evidently decreased under bias, temperature, and illumination stress and the enhanced transfer characteristics were retained after 3 months. In addition, we confirmed that the SRD mechanism demonstrated radical oxidation with defects related to oxygen from XPS depth and FTIR analysis. Therefore, we concluded that the SRD method can effectively decrease V0 and induce self-passivation at the back-channel surface of a-IGZO film. Moreover, the SRD showed it is possible to control not only the Vo but also the interface property of amorphous oxide and gate dielectric using the non-vacuum method.

Methods

Fabrication of thin film transistors (TFTs)

First, we fabricated the bottom-gate a-IGZO TFTs structure by depositing a-IGZO on the heavily doped p+-Si wafer with thermally oxidized SiO2 of 120 nm. Then, 40 nm a-IGZO was deposited by RF magnetron sputtering as the active layer using 3-inch IGZO target (In2O3:Ga2O3:ZnO = 1:1:1 mol%) under rf power of 150 W, working pressure of 5 mTorr, and total deposition time of 5 mins at room temperature. Following the active layer deposition, the samples were annealed at 300 °C for 1 hour in ambient air. And then, 100 nm Al source and drain electrodes were deposited by a radio frequency sputter system with a shadow mask. The channel region of TFTs was defined with a width of 1000 μm and a length of 150 μm.

The SRD treatments

The SRD was performed on a-IGZO TFTs by a three-step process as shown in Fig. 1. First, these samples were irradiated for 15 mins by mercury lamp-based UV light, where there was a wavelength of 185 nm and 256 nm and a photon flux density of 60 mW/cm2. Then, these samples were dipped in the 30% H2O2 (80 °C) solution for 20 secs. Finally, these samples were thermally annealed at 120 °C for 5 mins. We also fabricated only-UV and H2O2 without UV treated samples for verification of the SRD effect.

Electrical and chemical measurements

The electrical characteristics of the SRD TFTs were measured in the dark at room temperature using an HP4156C semiconductor parameter analyzer. To evaluate negative bias- temperature stress (NBTS) and NBIS stability, VGS = −20 V and VDS = 10.1 V at 50 °C, and under 5700 lux of white LED were applied for 3600 secs in air, respectively. Moreover, positive bias-stress (PBS) at VGS = 20 V and VDS = 10.1 V were applied for 3600 secs in ambient air. To investigate the chemical characteristics after SRD, we performed a depth profile XPS (Thermo Scientific. K-alpha) and Attenuated Total Reflection FTIR (ATR-FTIR, Bruker Vertex 70). XPS analyses were used to monitor the variation of composition, chemical structure, and valence band offset in oxide films.

Fabrication of bi-layered a-IGZO structure for FTIR analysis

We additionally fabricated a bi-layered IGZO structure with solution-processed IGZO and sputtered IGZO on a p+-Si substrate with thermally oxidized SiO2. We prepared 0.3 M IGZO solutions with an In:Ga:Zn molar ratio of 5:1:2 by dissolving indium nitrate hydrate (In(NO3)3·xH2O), gallium nitrate hydrate (Ga(NO3)3·xH2O), and zinc acetate hydrate (Zn(CH3COO)2·2H2O) into 2-methoxyethanol (CH3OCH2CH2OH) solvent. The IGZO solutions were spin-coated at 3000 rpm for 30 secs. This coated film was baked for 20 min at 100 °C and then 30 nm-a-IGZO was deposited by sputtering with the above mentioned same condition.