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Photo-Thermally Activated Nanocrystalline Oxynitride With Superior Performance in Flexible Field-Effect Transistors Kyung-Chul Ok, Jun Hyung Lim, Hyun-Jun Jeong, Hyun-Mo Lee, You Seung Rim, and Jin-Seong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16046 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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ACS Applied Materials & Interfaces

Photo-Thermally Activated Nanocrystalline Oxynitride With Superior Performance in Flexible Field-Effect Transistors

Kyung-Chul Ok,†,1 Jun-Hyung Lim,†,3 Hyun-Jun Jeong,1 Hyun-Mo Lee,1 You Seung Rim,*,2 Jin-Seong Park*,1

1

Division of Materials Science and Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu,

Seoul, 04763, Republic of Korea 2

School of Intelligent Mechatronic Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul

05006, Republic of Korea 3

Display Research and Development Center, Samsung Display Company, Ltd., Yongin 17096, Republic of

Korea

Abstract Photochemical reactions in inorganic films, which can be promoted by the addition of thermal energy, enable significant changes in the properties of the films. Metaphase films depend significantly on introducing external energy, even at low temperatures. We performed thermalinduced, deep ultraviolet-based, thermal-photochemical activation of metaphase ZnOxNy films at low temperature, and we observed peculiar variations in the nanostructures with the phase transformation and densification. The separated Zn3N2 and ZnO nanocrystalline lattice in amorphous ZnOxNy was stabilized remarkably by the reduction of oxygen defects and by the interfacial atomic rearrangement without breaking N-bonding. Based on these approaches, we successfully demonstrated highly-flexible, nc-ZnOxNy thin-film transistors on polyethylene naphthalate films, and the saturation mobility showed over 60 cm2 V-1 s-1.

Keywords: zinc-oxynitride, photochemical reaction, low temperature process, thin-film transistors, flexible electronics 1

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INTRODUCTION The intrinsic properties in the deep level defects of oxygen vacancies (Vo) in metal oxide semiconductors and the properties associated with restricted percolation conduction behavior in an existing multi-cation barrier are still being challenged in the fields of device applications.1-5 2To overcome these issues, heavy transition metal cation-based semiconductors combined with multianion, metal oxynitride (MxOyNz) structures were proposed recently.6-11 Zinc-oxynitride (ZnOxNy), as a representative material composition, has high electron mobility (> 40 cm2 V-1 s-1) due to its lower effective mass (me*/me ~ 0.19) compared to metal oxide semiconductors (i.e., ZnO (0.27), In2O3 (0.22), and IZO (0.2)).6 ZnOxNy also has an excellent photostability because N 2p orbital occupies the deep-level (~2.3 eV) defects based on oxygen vacancies over the valence band maximum (VBM).6,8 Although ZnOxNy is very fascinating in device applications, the detailed nanostructures, formation mechanisms, and transformations of ZnOxNy are still questionable in various processing conditions. The processing window of stabilized O-Zn-N bonding formation as high-quality ZnOxNy film is also quite narrow for obtaining good device performance with highly compensative parameters (e.g., Ion/off ratio, threshold voltage (Vth), sub-threshold voltage swing (SS), and bias stability). This issue is due to the stabilization of the Zn-O bonding (standard enthalpy of formation, ∆Hf0 = -350 kJ mol-1 at 298 K) at relatively high temperature (over 250 oC), while meta-stable Zn-N bonding (standard enthalpy of formation, ∆Hf0 = -26.2 kJ mol-1 at 298 K) can be deformed easily as the broken bond and/or gas phase in this temperature range.12-13 In order to stabilize the bonding of heavy metal cations (In, Ga, Zn) and oxygen anions in the oxide semiconductors, it has been proposed to effectively reduce oxygen vacancies even at low temperatures via simultaneous method of ultraviolet (UV) and thermal annealing process.14-15 This process reported that the metal-oxygen (M-O) bonds could be broken by the light irradiation with high energy and then structural rearrangement would be promoted by the thermal energy, effectively reducing oxygen defects. 2


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Unlike metal oxides, metal oxynitride has complicated metal cation with anions and shows meta stable properties. It is noted that we focused on this point in terms of O-O, O-N, and gaseous N2. We explored the low-temperature, photochemical reaction effect of ZnOxNy films by deep ultraviolet (UV, the wavelength of 185 and 254 nm) with thermal annealing process at 175 oC. We observed that the localized bonded states, such as Zn+, O-O, N-O, and N-N related bondings, were reduced remarkably via photo-dissociation and thermal photochemical reaction (TPC). The bonding reaction-associated phenomenon could affect nanocrystalline phase separation and grain growth of Zn3N2 and ZnO in the amorphous ZnOxNy (a-ZnOxNy) structure, resulting in high-quality ZnOxNy films. Based on TPC treatment, we successfully demonstrated flexible ZnOxNy thin-film transistors (TFTs) that were formed at the low temperature of 175 oC and had ultrahigh mobilities, exceeding 60 cm2 V-1 s-1, and excellent mechanical stability even after 10,000 cycles.

METHODS Film and device fabrication. Direct current (DC), reactive sputtered amorphous ZnOxNy films were used as an inverted, staggered, bottom-gate structure. Thermally-grown, 100 nm-thick SiO2 and p++-Si substrates were used as the gate insulator and the gate electrode, respectively. The 3-inch pure Zn metal target and mixed Ar/O/N (5/1.2/40 sccm) gases were used as the DC-reactive sputtering sources. The base pressure and the working pressure were 5 x 10-7 and 5 x 10-3 Torr, respectively. The DC power was kept on 100 W, and the final thickness of the amorphous ZnOxNy film was 15 nm. As-deposited ZnOxNy films were treated in air at 175 oC by LT and TPC methods. The source/drain (S/D) electrode of the 100 nm-thick, Sn-doped In2O3 (ITO) film was deposited using the sputtering method. The width (W) and the length (L) of the channel were 800 and 200 µm, respectively. The detailed methods used to fabricate the flexible devices are provided in the supporting information section. Film analysis. The optical transmittance and electronic band structures were studied by UV3



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visible spectroscopy (Shimadzu, UV 3600) and spectroscopic ellipsometry (Woollam, RC2), respectively. The crystalline structures of the ZnOxNy thin films were analyzed by grazing incidence angle X-ray diffraction (GIAXRD, Rigaku, Smart-lap). The fixed incident beam angle (ω) was 1o during two theta (2θ) measurement. The fine crystalline and lattice structures of Zn-O and Zn-N were analyzed by high resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F). The chemical bonding properties and compositions of ZnOxNy were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC, PHI 5000 Versa probe II) with depth profiling. The ion gun conditions in the XPS facilities, such as gas source, power, and angle were controlled to prevent preferential behavior. The sputtering gas source and power were neon (Ne) gas and0.5 kV, respectively. The electronic structures of a-ZnOxNy with the K-edge of oxygen and nitrogen were analyzed by X-ray absorption near edge spectroscopy (XANES) using the 10D1 line in the Pohang accelerator laboratory (PAL). Device characterization. The electrical performance of the a- ZnOxNy TFT devices were measured by a semiconductor parameter analyzer (HP4145A). The electrical transfer characteristics and gate bias instabilities (VG = ±20 V) were evaluated in the vacuum probe station (~ 3 mTorr) to avoid the chemisorption of ambient gases (e.g., H2O and O2)

RESULTS AND DISCUSSION Figure 1 shows the photochemical reaction from the surface to the bulk region in the low temperature (LT) + ultraviolet (UV)/ozone process. The UV spectrum shows a broad Gaussian distribution with the range of 170-300 nm.16 Both high energy and resonant absorption in target molecules can be associated with the bond dissociation energy (BDE).12, 17 In particular, O-O and N-O related bonds were broken easily, forming oxygen and nitrogen anions, while, as shown in Table I, Zn-O, Zn-N, and N-N bonds did not participate in these reactions. Then, broken oxygen and nitrogen anions could be strongly bonded with ionized zinc cations. The strong oxidative 4


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radicals (O*) from ozone (O3) also acting as an oxidizing agent to the ZnOxNy film.18-20 The excess nitrogen regarding the impurity as the volatile N2 molecules would diffuse out of the gas phase (which was confirmed by Figure 3c). These complex photochemical reaction routes could influence the change in the electrical and structural properties of the ZnOxNy film. This photochemical reaction of the ZnOxNy structure also can affect phase separation, phase transformation, or crystallization. LT and TPC treated films had broad peaks in two theta (2θ) ranging from 33o to 37o, which were a mixed phase of ZnO (002), (101) and Zn3N2 of (321), (400) (Figure S1 in the Supporting Information).21 The as-deposited ZnOxNy film had an amorphous-like phase, because the hexagonal and cubic structures of ZnO and Zn3N2 were difficult in those crystallizations to make new phases. However, the LT- and TPC-treated ZnOxNy films showed the growth of broad peaks of the ZnO and Zn3N2 phases, and the intensity of the mixed phase in the TPC-treated films was stronger than that of the LT process. Note that the TPC treatment resulted in greater energy transfer into the amorphous ZnOxNy films than the LT process. To understand the phase transformation mechanism in the TPC process as well as weak peak changes in XRD, we performed high-resolution TEM (HR-TEM) and the fast Fourier transform (FFT) analysis of the ZnOxNy films for the LT and the TPC processes. We focused especially on nanocrystalline (nc)-ZnOxNy structures that could confirm the mechanism of phase transformation, atomic rearrangement, and defects-related localized bonding variations. As shown in Figures 2a and 2c, the TPC-treated ZnOxNy film was thinner than the film with the LT treatment (changed from 15 to 12.5 nm). The density of the film was estimated by analyzing the intensity of the transmitted electrons, which was extracted by TEM cross-sectional image contrast. The darker regions represented lower intensities of the transmitted electrons than the gray regions, which could be correlated with the phase transformation and densification of the films (Figure S2 in the Supporting Information).22 The LT-treated ZnOxNy film showed incomplete phase transformation compared to the TCP process. In addition, the partially-oxidized nc-ZnO structure with (002) lattice parameters 5



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of 2.58-2.62 Å was observed at the surface and upper region of the ZnOxNy film (Figure S3 in the Supporting Information), whereas a relatively small fraction of the nanocrystalline (002) ZnO and (400) Zn3N2 phases was embedded in the amorphous ZnOxNy, as shown in Figure 2b. This was attributed to the insufficient energy supply for the growth of nc-ZnO through the phase transformation and oxidation of a-ZnOxNy and oxygen defects. However, the lower density, unstable amorphous ZnOxNy phase, which contained the localized oxygen- or nitrogen-related defects, clearly was removed by the TPC process. In addition, as shown in Figure 2d, a large fraction of ZnO with (002) lattice parameter of 2.57-2.62 Å and a large fraction of Zn3N2 with (004) lattice paramter of 2.47-2.50 Å coexisted without significant loss of the Zn-N structure, even when subjceted to a strong, photochemical oxidation process. Figure 2e shows the proposed mechanism of the thermal-induced, photochemical reaction of the phase transformation and densification on ZnOxNy films. The localized bonding states of ZnO and Zn3N2 in the amorphous phase can be activated by photolysis, which results separately in the enhanced growth and densification of the Zn-O and Zn-N phases. That is, a-ZnOxNy could be transferred to stabilized structures with the formation of c-axis oriented columnar Zn-O structure and excess N2 gas molecules, which could be diffused out through the TPC process. Figure 3a shows representative XPS O 1s and N 1s spectra with deconvoluted curves (A and B) and (C, D, and E) for LT- and TPC-treated ZnOxNy films, respectively7. The Gaussian fitted energy positions of A (529.8 eV) and B (531.1 eV) represent the metal-oxygen bonds (Zn-O) and oxygenrelated defect states, respectively. N 1s spectrum was also deconvoluted with these peak positions of C (395.3 eV), D (396.3 eV), and E (403.5 eV). The binding energies of the D and E peaks indicated the formation of nitrogen molecular state (N2) bonds and nitrogen oxide (Nx-Oy) bonds, respectively. Figure 3b shows the qualitative comparison of each of the chemical bondings. The TPC treatment reduced the oxygen deficiencies (peak B) due to the photochemical reaction, and it was highly related with the growth of the stoichiometric ZnO phase. In the case of the N 1s 6


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spectrum, the nitrogen-related chemical bondings remained unchanged irrespective of the processing methods that were used. That is, the growth of nc-ZnO from meta-stable ZnOxNy was confirmed by analysis of the chemical bonding, while the Zn3N2 phase did not change. As the results of O 1s and N 1s analysis, TPC process can effective in the reduction of oxygen related defects including oxygen vacancies (VO) accompanied with phase transformation, as shown in TEM. Typically, oxygen vacancies are well known as the electron donor (VO → VO++ + 2e-) and play a role of the charge trapping sites.23 It means that the reduction of oxygen-related defects and imperfect structures of the ZnOxNy can influence both the densified films and improve electrical modulation. Figure 3c shows the O and N K-edges of the X-ray near edge spectroscopy (XANES) spectra in the ZnOxNy films for the different treatments. The measured O- and K-edge absorption spectra reflected the electronic structure between Zn and the anions (O and N), which was related to the crystal structure and the bonding symmetry.24-25 The featured peaks of P1 and P2 were proportional to the electron excitations from O 1s to O 2pσ (along the bi-layer) and O 2pπ (along the c-axis). The relative P2 peak of the TPC-treated ZnOxNy film was increased, which was related to the increase in the Zn-O bonds according to formation of the c-axis oriented, wurzite-like ZnO structure. This agreed with the variations of the chemical and structural properties. The positions of the P3, P4, and P5 peaks related to the resonance of Zn-N hybridization were indicated Zn 3d-N 2pσ, mixed Zn 3dN 2pπ and N2 molecules (vibration mode) and N-O state, respectively.26 This result also confirmed that the Zn3N2 structure could not be transferred to other phases. As a result, the oxygen-related defects could be reduced with the growth of nc-ZnO phase. The chemical and structural properties of the films are critical parameters because of their effect on the performance of the semiconductor's field-effect mobility and electrical stability. Based on our experiments, the TPC-treated ZnOxNy structure had more highly-packed Zn3N2-ZnO nanocystalline structures than the LT-reated ZnOxNy. This phenomenon showed good agreement with other analyses, and they showed the phase 7



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stabilization of both the Zn-O and Zn-N lattices. In the comprehensive studies of the physical and chemical properties of ZnOxNy films, TPC treatment was effective for fabricating stable nc-ZnOxNy semiconducting films. Figure 4a shows the transfer characteristics of LT- and TPC-treated nc-ZnOxNy TFTs. The TPCtreated devices had different characteristics for the threshold voltage (Vth) and sub-threshold slope (SS). The value of Vth was changed significantly, from -8.84 to -1.65 V, and the value of SS was improved from 0.63 to 0.25 V dec.-1. The saturation mobilities (µsat) of the devices were similar (LT: 55.7 cm2 V-1 s-1, and TPC: 54.8 cm2 V-1 s-1) irrespective of the increment of nc-ZnO that resulted in the expansion of the optical bandgap caused by reducing the concentration of the carrier (Figure S4 in the Supporting Information). The positive shift of Vth between LT- and TPC-treated ZnOxNy TFTs could be attributed from the decrease of net electron charges (∆Qinduced = Ci∆Vth/q, where Ci is gate capacitance per unit area and q is quantity of electric charge) in the whole channel when applying the gate electric field.27 The ∆Qinduced,LT-TPC value was 1.01 × 1018 cm-3 (1.51 × 1012 cm-2) and the possible trapping charges can be negligible due to the small amount of ∆Qinduced,Hysteresis value (~ 1016 cm-3) during the gate bias sweep from -15 to 25V. Besides, the electrical device instabilities have been discussed about formation of oxygen related defects as the role of charge trapping sites. The calculated trap densities of the bulk sites with respect to the reduced thickness and SS values were 1.41 × 1018 eV-1 cm-3 (LT) and 7.06 × 1017 eV-1 cm-3 (TPC), and which reflected the above results.28 Furthermore, the reduced trap densities also affected the improvement of the devices' stabilities, and the variations of Vth were reduced significantly for the positive and negative gate bias stress test, as shown in Figure 4b (Table S1 in the Supporting Information). These improvements of the performance of the device could be beneficial for use in lowtemperature processing of polymer substrates for flexible applications. We constructed flexible ncZnOxNy TFTs with the TPC process at the temperature of 175 oC on polyethylene naphthalate 8


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(PEN) substrates, as shown in Figure 5a. Details concerning the fabrication methods are described in the experimental section. Figure 5b shows the electrical transfer characteristics of free-standing, flexible nc-ZnOxNy TFTs, and the characteristics of the device were the same after delamination from the carrier glass. The electrical properties of free-standing, flexible nc-ZnOxNy TFTs were µsat and sub-threshold swing (SS) values of 61.7 ± 2.7 cm2 V-1 s-1 and 0.18 ± 0.04 V dec.-1, respectively. These values were better than those of rigid substrates, which could be attributed to the use of a high-k dielectric and the small dimensions of the channel area related to the bulk resistance.1-2 Figures 5c and d show the tensile bending test and the transfer curves of the flexible nc-ZnOxNy TFTs with a curvature radius (R) of 5 mm. The transfer curves did not change significantly even after 10,000 cycles. The extracted Vth and µsat parameters and the SS values are shown in Figure 5e. Among them, the shift of Vth evidently was observed toward the negative direction, which could be attributed to the deterioration of the interfaces between the channel and the dielectric as a result of the broken M-O bonding sites due to the mechanical stress.29-30

CONCLUSION In summary, we explored the phase transformation mechanism of ZnOxNy films by using a thermalphotochemical process at low temperature. We clearly observed that the phase transformation and densification of the films with the appearance of nc-ZnO, while the fraction of nc-Zn3N2 was not changed. These processes influenced the formation of highly-densified nc-Zn3N2 structures, as shown in the cross-sectional HR-TEM analysis. These structures could improve the electrical device performance by reducing the oxygen related defects. Based on the TPC process, we successfully demonstrated the high performance and flexible nc-ZnON TFTs at 175oC. We highly expected that the TPC process can be applicable for the low temperature process in the inorganic semiconducting materials and devices.

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ASSOCIATED CONTENT

Supporting Information Description for fabrication of flexible devices. Threshold voltage shift (∆Vth) for negative and positive gate bias stress as a function of stress time (Table S1). X-ray diffraction spectra of Asdeposition, LT-, and TPC-treated ZnOxNy films on SiO2 substrate measured by the glancing incident x-ray diffraction (GIXRD) method. The fixed angle (ω) = 0.5o, and the mixed phase of ZnO (101), (002) and Zn3N2 (321) and (400) are shown in the broad range of 33 – 37o (Figure S1). Cross-sectional HR-TEM images of LT- and TPC-treated ZnOxNy films and the profiles of transmission electron intensity of the films (Figure S2). Cross-sectional HR-TEM image of the LTtreated ZnOxNy film; nanocrystalline ZnO phase oriented with (002) and (100) on the upper side (Figure S3). Optical band gap (Eg) of LT- and TPC-treated ZnOxNy films (Figure S4).

AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] (Jin-Seong Park)

*

E-mail: [email protected] (You Seung Rim)

AUTHOR CONTRIBUTIONS K.–C. O, J.–H. L, H.–J. J, H.–M. L, Y. S. R, J.-S. P. designed the ZnON TPC experiments and fabricated the thin film transistor on a flexible substrate. K.–C. O, J.–H. L. performed various film analyses (J.–H. L. did the TEM work). The manuscript was written by the contribution of all authors, who have approved the final version of the manuscript. †K.–C. O and J.–H. L contributed equally to this work.

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ACKNOWLEDGEMENTS This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (project number 10051403 and 10052020) and KDRC (Korea Display Research Corporation). In addition, this work was partially supported by the Ministry of Science, ICT & Future Planning (MISP) of Korea, under its National Program for Excellence in Software (the SW-oriented, college-support program) (R7718-16-1005) supervised by the Institute for Information & Communications Technology Promotion (IITP).

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(28) Rolland, A.; Richard, J.; Kieider, J. P.; Mencaraglia, D. Electrical Properties of Amorphous Silicon Transistors and MIS–Devices: Comparative Study of Top Nitride and Bottom Nitride Configurations. J. Electrochem. Soc. 1993, 140, 3679-3683. (29) Heremans, P.; Tripathi, A. K.; de Jamblinne de Meux, A.; Smits, E. C. P.; Hou, B.; Pourtois, G.; Gelinck, G. H. Mechanical and Electronic Properties of Thin-Film Transistors on Plastic, and Their Integration in Flexible Electronic Applications. Adv. Mater. 2016, 28, 4266-4282. (30) Sheng, J.; Park, J.; Choi, D. W.; Lim, J.; Park, J. S. A Study on the Electrical Properties of Atomic Layer Deposition Grown InOx on Flexible Substrates with Respect to N2O Plasma Treatment and the Associated Thin-Film Transistor Behavior under Repetitive Mechanical Stress. ACS Applied Materials and Interfaces 2016, 8, 31136-31143.

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Page 16 of 24

List of Figure Captions Figure 1. Schematics of two photochemical reactions of the oxygen radical (O*) and ozone (O3) to produce byproducts in ambient air: (1) The gas out of excessive nitrogen molecules; (2) The diffusion of oxygen radicals into the O-Zn-N occurring simultaneously by the thermalphotochemical process

Figure 2. Cross-sectional HR-TEM images of a-ZnOxNy films for different post-treatments: (a), (b) LT process; (c), (d) TPC process. The fast Fourier transform patterns in the inset represent the ZnO (red circle) and Zn3N2 (white circle) phases. The thickness of the TPC-treated a-ZnOxNy films shows less than the thickness of the LT-treated films. The densified, TPC-treated a-ZnOxNy films have large fractions of (002) ZnO nanocrystalline phases than the LT-treated films; (e) Proposed transformation mechanism of highly-packed Zn3N2-ZnO nanostructures by the photochemical reaction. The nc-ZnO phases are grew by consuming a-ZnOxNy.

Figure 3. Chemical bonding analysis of ZnOxNy films for the different treatments: (a), (b) XPS O 1s and N 1s spectra with Gaussian fitted curves of LT- and TPC-treated films for the different chemical bonding states, respectively; (c) The relative peak ratio of the featured curves in XPS O 1s and N 1s spectra. Although the area of region B corresponds to the oxygen deficiencies in TPCtreated ZnOxNy film is reduced rather than the LT process, N-related bondings of ZnOxNy films does not be changed, regardless of LT and TPC treatment methods; (d) Molecular orbital ordering of XANES spectra in the O-K and N-K edges of the LT- and TPC-treated ZnOxNy films, respectively. The P2 peak corresponds to the c-axis oriented ZnO phase in TPC-treated ZnOxNy film, and it is higher than the corresponding peak for the LT-treated ZnOxNy film. 16


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Figure 4. (a) Transfer curves of LT- and TPC-treated ZnOxNy TFTs plotted by drain current (ID), gate leakage current (IG, gray color) and saturation mobility (µsat) with respect to gate voltage (VG). Compared to the LT-treated device, the electrical transfer characteristics of the TPC-treated device shows improved Vth and SS values with no significant change of mobility; (b) device instabilities of threshold voltage shift (∆Vth) under gate bias stresses (VG =±20 V) as a function of stress time (1 hr). The shift of Vth in the TPC-treated device was less than that in the LT-treated device in both NBS and PBS bias stability.

Figure 5. Demonstration of the flexible nc-ZnOxNy TFTs with the TPC process fabricated at 175 oC on PEN substrates: (a) photos of flexible nc-ZnOxNy TFT patterned by photolithography. It shows clearly patterned TFT structure and no crack damages after the cooling-off type delamination process; (b) Transfer curves of flexible nc-ZnOxNy before and after delamination; (c)-(e) picture of tensile bending test and transfer curves of flexible nc-ZnOxNy TFTs with the curvature radius (R) of 5 mm for 10,000 cycles and variations of major parameters (∆Vth, ∆µsat, ∆SS) as a function of bonding cycles.

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List of Tables Table I. Bond dissociation energy (BDE) related to photolysis. Bonding

BDE (KJ mol-1)

Wavelength (nm)

Bonding

BDE (KJ mol-1)

Wavelength (nm)

N-O

631.62

189.5

Zn-Zn

22.2

5390.9

N≡N

946

126.5

O-Zn

250

478.7

N-N

945.33

126.6

N-Zn

22.6

5295.5

493.3

242.6

ON-N

480.7

249.0

O-O ( D)

682.8

175.3

ON-O

305

392.4

O3

493.8

242.4

3

O-O ( P) 1

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Figure 1. Schematics of two photochemical reactions of the oxygen radical (O*) and ozone (O3) to produce byproducts in ambient air: (1) The gas out of excessive nitrogen molecules; (2) The diffusion of oxygen radicals into the O-Zn-N occurring simultaneously by the thermalphotochemical process

19



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Figure 2. Cross-sectional HR-TEM images of a-ZnOxNy films for different post-treatments: (a), (b) LT process; (c), (d) TPC process. The fast Fourier transform patterns in the inset represent the ZnO (red circle) and Zn3N2 (white circle) phases. The thickness of the TPC-treated a-ZnOxNy films shows less than the thickness of the LT-treated films. The densified, TPC-treated a-ZnOxNy films have large fractions of (002) ZnO nanocrystalline phases than the LT-treated films; (e) Proposed transformation mechanism of highly-packed Zn3N2-ZnO nanostructures by the photochemical reaction. The nc-ZnO phases are grew by consuming a-ZnOxNy.

20


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Figure 3. Chemical bonding analysis of ZnOxNy films for the different treatments: (a), (b) XPS O 1s and N 1s spectra with Gaussian fitted curves of LT- and TPC-treated films for the different chemical bonding states, respectively; (c) The relative peak ratio of the featured curves in XPS O 1s and N 1s spectra. Although the area of region B corresponds to the oxygen deficiencies in TPCtreated ZnOxNy film is reduced rather than the LT process, N-related bondings of ZnOxNy films does not be changed, regardless of LT and TPC treatment methods; (d) Molecular orbital ordering of XANES spectra in the O-K and N-K edges of the LT- and TPC-treated ZnOxNy films, respectively. The P2 peak corresponds to the c-axis oriented ZnO phase in TPC-treated ZnOxNy film, and it is higher than the corresponding peak for the LT-treated ZnOxNy film.

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Figure 4. (a) Transfer curves of LT- and TPC-treated ZnOxNy TFTs plotted by drain current (ID), gate leakage current (IG, gray color) and saturation mobility (µsat) with respect to gate voltage (VG). Compared to the LT-treated device, the electrical transfer characteristics of the TPC-treated device shows improved Vth and SS values with no significant change of mobility; (b) device instabilities of threshold voltage shift (∆Vth) under gate bias stresses (VG =±20 V) as a function of stress time (1 hr). The shift of Vth in the TPC-treated device was less than that in the LT-treated device in both NBS and PBS bias stability.

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Figure 5. Demonstration of the flexible nc-ZnOxNy TFTs with the TPC process fabricated at 175 oC on PEN substrates: (a) photos of flexible nc-ZnOxNy TFT patterned by photolithography. It shows clearly patterned TFT structure and no crack damages after the cooling-off type delamination process; (b) Transfer curves of flexible nc-ZnOxNy before and after delamination; (c)-(e) picture of tensile bending test and transfer curves of flexible nc-ZnOxNy TFTs with the curvature radius (R) of 5 mm for 10,000 cycles and variations of major parameters (∆Vth, ∆µsat, ∆SS) as a function of bonding cycles

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Table of Contents

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Photothermally Activated Nanocrystalline ... - ACS Publications

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