Boosting Carrier Mobility in Zinc Oxynitride Thin-Film Transistors via

Jun 2, 2019 - (11,14) Owing to these attractive benefits, the effects of anion .... The Ta-capped ZnON films exhibited better uniformity in terms of t...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22501−22509

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Boosting Carrier Mobility in Zinc Oxynitride Thin-Film Transistors via Tantalum Oxide Encapsulation Taeho Kim,† Min Jae Kim,† Jiwon Lee, and Jae Kyeong Jeong* Department of Electronic Engineering, Hanyang University, Seoul 133-791, South Korea

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ABSTRACT: Novel TaOx encapsulation was presented to enhance the field-effect mobility (μFE) of ZnON thin-film transistors (TFTs) consisting of a metallic Ta film deposited onto the ZnON surface followed by a modest annealing process. The resulting TaOx/ZnON film stack exhibited a more uniform distribution of nanoscale ZnON crystallites with increased stoichiometric anion lattices compared to the control ZnON film. The control ZnON TFTs exhibited a reasonable μFE, subthreshold gate swing (SS), and ION/OFF ratio of 36.2 cm2/V·s, 0.28 V/decade, and 2.9 × 108, respectively. A significantly enhanced μFE value of 89.4 cm2/V·s was achieved for ZnON TFTs with a TaOx encapsulation layer, whereas the SS of 0.33 V/ decade and ION/OFF ratio of 8.6 × 108 were comparable to those of the control device. This improvement could be explained by scavenging and passivation effects of the TaOx film on the ZnON channel layer. Density of states (DOS)-based modeling and simulation were performed to obtain greater insight with regard to increasing the performance of the ZnON TFTs with a TaOx encapsulation layer. A smaller number of subgap states near the conduction band (CB) minimum and a higher net carrier density for the TaOx-capped device increased the Fermi energy level toward the CB edge under thermal equilibrium conditions, leading to efficient band conduction and fast carrier transport under the on-state condition. KEYWORDS: zinc oxynitride, tantalum oxide, thin-film transistor, encapsulation, scavenge effect, device stability effective mass and high carrier mobility.10−12 Optoelectronic properties such as the optical band gap (Eg), effective carrier mobility, and density of the ZnON thin films can be tailored by controlling the anion ratio of wurtzite ZnO (Eg ≈ 3.2 eV) and cubic Zn3N2 (Eg ≈ 1.0 eV).11 In ZnO, persistent photoconductivity (PPC) after prolonged light exposure is attributed to the photoionization of neutral VO to metastable VO2+.13 The substitution of oxygen anions with nitrogen anions in ZnO caused the band gap to narrow gradually, which could be explained by the superposition of the nitrogen 2p band above the oxygen 2p band at the valence band (VB) edge.11 Thus, neutral VO defects at ∼1.0 eV above the VB edge of ZnO could be buried in ZnON, which strongly mitigated the PPC effect and negative bias illumination stress (NBIS)-induced VTH instability in ZnO and AOS TFTs.11,14 Owing to these

1. INTRODUCTION Amorphous oxide semiconductor (AOS) materials such as zinc oxide (ZnO) and indium gallium zinc oxide (IGZO) have attracted considerable attention as an alternative to current silicon-based semiconductors for use in liquid crystal (LC) and organic light-emitting diode (OLED) screen applications due to their intriguing properties including a high mobility, good switching modulation ratio, transparency, and excellent uniformity.1 Rapid advances with regard to IGZO materials and processes since its discovery in 2004 have led to the commercialization of IGZO thin-film transistors (TFTs) for ultralow-power LC mobile phones and large-size highresolution OLED televisions.2−4 However, the notorious instability of AOS TFTs under gate bias and illumination stress conditions is still a big concern where oxygen vacancy defects (VO) in AOS materials are responsible for a huge negative shift in the threshold voltage (VTH).5−9 Recently, zinc oxynitride (ZnON) materials were proposed as promising n-type semiconductors due to their low electron © 2019 American Chemical Society

Received: March 3, 2019 Accepted: June 3, 2019 Published: June 2, 2019 22501

DOI: 10.1021/acsami.9b03865 ACS Appl. Mater. Interfaces 2019, 11, 22501−22509

Research Article

ACS Applied Materials & Interfaces

Hillsboro, Oregon, USA). The chemical states of the ZnON films were examined via X-ray photoelectron spectroscopy (XPS, SIGMA PROBE, Thermo VG, Waltham, Massachusetts, USA). For XPS analysis, ZnON films were inserted into the XPS chamber within 2 h after PDA processing at 250 °C. The back surface region (∼5 nm) of the ZnON films was plasma-etched in situ within the XPS chamber to remove surface contamination. The N and O 1s XP spectra were deconvoluted into three asymmetric Lorentzian−Gaussian peaks with a Shirley background. Curve fitting for all spectra was performed with fixed full width at half-maximum and constant Gaussian-to-Lorentzian ratio parameters while allowing variability with regard to the peak binding energies and areas. The electrical characteristics of the TFTs were measured using a Keithley 2636 source parameter at room temperature under ambient air, and a two-dimensional device simulator ATLAS (Silvaco, Silvaco International, Santa Clara, CA, USA) was used to evaluate the density-of-state (DOS) distribution under the conduction band minimum (EC) and to calculate the energy band diagram of the ZnON and Ta-capped ZnON TFTs.

attractive benefits, the effects of anion composition and annealing temperature on the structural, chemical, and electrical properties of ZnON thin films have been intensively studied, rendering remarkably high field-effect mobilities (>50 cm2/V·s) and excellent ION/OFF ratios (>107) in the ZnON TFTs.15−17 Despite these merits, the chemical instability of ZnON materials is still an open question to be resolved prior to their facile implementation within commercial electronics where nitrogen-related defects and out-diffusion due to rather weak Zn−N chemical bonding invoke the deterioration of electrical properties and long-term stability.18 In this study, a new scavenge route was suggested to improve the chemical and electrical stabilities of n-channel ZnON thin films, consisting of the deposition of metallic tantalum films onto sputtered ZnON followed by subsequent thermal annealing. ZnON TFTs fabricated by this novel encapsulation method exhibited a high field-effect mobility of 89.4 cm2/V·s, low subthreshold gate swing of 0.33 V/decade, good ION/OFF ratio of 8.6 × 108, and long-term storage stability.

3. RESULTS AND DISCUSSION Figure 2 shows the GIXRD patterns of the control and Tacapped ZnON films on SiO2/Si substrates. With regard to the

2. EXPERIMENTAL SECTION ZnON TFTs with an inverted staggered bottom-gate and top-contact structure were fabricated on Si substrates. A 100 nm-thick SiO2 gate insulator layer was grown via thermal oxidation onto a heavily doped ptype Si wafer. The heavily doped Si substrate acted as the gate electrode. A 20 nm-thick ZnON channel layer was deposited via a reactive sputtering process where the Ar/O2/N2 reactive gas ratio, dc power, and chamber pressure during channel preparation were 5/2.5/ 30, 100 W, and 5 mTorr, respectively. An ITO film was used as the source/drain (S/D) electrode and was deposited onto the ZnON/ SiO2/Si substrate via magnetron sputtering. The dc power and chamber pressure under an Ar atmosphere were 50 W and 5 mTorr, respectively. The ZnON channel island and ITO S/D electrode were patterned through a shadow mask during sputter deposition. The width (W) and length (L) values of the ZnON TFTs were 1000 and 300 μm, respectively. The devices were then contact-annealed in an electric furnace for 1 h at 250 °C under an air atmosphere (hereafter referred to as the control device). A 10 nm-thick Ta film was selectively deposited via dc sputtering through a shadow mask onto the ZnON film between the source and drain electrode with dimensions of W/L = 2300 μm/150 μm (Figure 1). A final postdeposition annealing (PDA) step was performed at 200 °C for 1 h under ambient O2, which will be referred to as Ta-capped ZnON TFTs. The structural properties of the ZnON films were evaluated via grazing incidence X-ray diffraction (GIXRD, X’Pert PRO, PANalytical, Egham, Surrey, U.K.) using Cu Kα radiation and high-resolution transmission electron microscopy (HRTEM, Tecnai F20, FEI,

Figure 2. GIXRD spectra of the ZnON/SiO2/Si stack with and without a Ta layer. A Ta layer film was annealed at various temperatures under atmospheric O2.

control ZnON film, thermal annealing was performed at 250 °C under an air atmosphere. The control ZnON film exhibited no noticeable peaks except for 51.4° (due to the Si substrate), suggesting its amorphous or nanocrystal nature.14 The Tacapped ZnON samples were subjected to a two-step annealing process consisting of first annealing at 250 °C under an air atmosphere prior to the deposition of the Ta film and a second PDA at different temperatures of 100, 200, 300, and 600 °C under an O2 atmosphere after the deposition of the Ta film to investigate Ta-induced microstructure evolution of the underlying ZnON films. The Ta-capped ZnON samples after the second PDA at 100 and 200 °C exhibited discernible peaks at 38.2°, which could be attributed to the (202) reflection of the metallic β-Ta (JCPDS card no. 25-1280) phase. This metallic βTa phase disappeared for the Ta-capped ZnON sample after the second PDA at 300 °C, indicating that the metallic Ta film was converted to an amorphous TaOx film via thermal oxidation.19 Interestingly, a new peak near 35.1° for this sample was

Figure 1. Schematic diagram of Ta-capped ZnON TFTs fabricated onto a highly doped p-type silicon wafer. 22502

DOI: 10.1021/acsami.9b03865 ACS Appl. Mater. Interfaces 2019, 11, 22501−22509

Research Article

ACS Applied Materials & Interfaces observed. It is well known that crystalline Zn3N2 possesses an antibixbyite structure with a lattice constant of 9.788 Å, belonging to the body-centered cubic (bcc) system (space group Ia3̅).20 In the case of a high-purity Zn3N2 crystal, a (321) peak is expected to appear at ∼34.3°, which was quite close to that observed (35.1°). Considering that the O2− anions substituted with sublattice sites in the Zn3N2 crystal, this peak could be assigned to (321) of the cubic system. A smaller lattice constant of 9.540 Å calculated from the observed (321) reflection position could be understood by the fact that the O2− ion exhibited a smaller radius than the N3− ion. This suggested that the control ZnON sample consisted of aggregated Osubstituted Zn3N2 nanocrystallites. By contrast, the ZnON sample after a high PDA temperature of 600 °C exhibited a well-defined polycrystalline wurtzite ZnO structure (JCPDS card no. 36-1451), regardless of the existence of a Ta capping layer (Figure 2). The Gibbs free energies of formation (ΔGf) for Zn3N2 and ZnO were calculated to be 156.4 and −262.3 kJ/ mol,21 respectively, at the elevated temperature of 600 °C. The negative ΔGf value of ZnO means that ZnO is a thermodynamically stable phase. In contrast, the Zn3N2 is a highly unstable state, considering its positive ΔGf value. Thus, nitrogen will be dissociated from the Zn−N lattice and subsequently completely diffused out from the ZnON channel layer through the thin TaOx layer at a high annealing temperature of 600 °C, leading to the conversion from ZnON to ZnO. Indeed, this conversion from Zn3N2 (or ZnON) to ZnO at high temperatures (>500 °C) has been frequently reported in the literature.22,23 Owing to this reason, characterization of ZnON films without/with a Ta capping layer focused on a relative low-temperature PDA at 200 °C. When the annealing process was performed at a low temperature (≤200 °C), this kind of conversion can be kinetically prevented by its low available thermal energy. Because the contribution of an amorphous region possibly existing in the ZnON films (in the case of PDA temperatures less than 250 °C) was not reflected in the XRD analysis,14,24 HRTEM analysis was performed to reveal the entire microstructure of the ZnON films. Figure 3 shows HRTEM images of the control and Tacapped ZnON samples annealed at 200 °C for 1 h under ambient O2. The periodic lattice image in Figure 3a clearly indicates that the control ZnON film is in the crystalline phase. Evolution of the selected area diffraction patterns (SADPs) yielded variations with regard to the degree of crystallization along the depth direction of the control ZnON films. ZnON films near the SiO2/Si substrate exhibited rather diffused hallow SADPs, suggesting a lack of highly ordered cations and anions. However, the SADPs of the ZnON films exhibited discernible Bragg spots with the growing direction, indicating that crystalline quality improved with an increase in deposition time. Interestingly, the depth-dependent evolution of SADPs disappeared for Ta-capped ZnON films, as can be seen in Figure 3b. The Ta-capped ZnON films exhibited better uniformity in terms of the degree of crystallization along the depth direction compared to the control ZnON films. The SADPs were identified as (400), (222), and (044) reflections of crystalline Zn3N2 with an antibixbyite structure, corresponding to interplanar spacings of 2.42, 2.79, and 1.71 Å, respectively. These values were smaller than those of conventional cubic Zn3N2 crystals by ∼0.1% because Zn−N possesses a larger bond length than Zn−O under an identical coordination number of 6 (bond lengths of Zn−N and Zn−O were 2.11 and 1.91 Å, respectively).25,26 Therefore, the Ta-capped ZnON film

Figure 3. Cross-sectional TEM images of the (a) control ZnON film and (b) Ta-capped ZnON film. Fast Fourier transform (FFT) patterns of the selected area are shown in the right, obtained from the (A, C) center and (B, D) bottom regions of the control ZnON film (Tacapped ZnON film), respectively.

can be regarded as an aggregate of O-substituted Zn2N3 nanocrystallites. In our previous research, a chemical reaction with a transition Ta layer enabled low-temperature crystallization of the amorphous IGZO film.19 The reduction in the onset crystallization temperature of IGZO from >600 °C (without a catalytic layer) to 300 °C was attributed to the donation of electrons from Ta into IGZO and subsequent facile rearrangement due to the change in the bonding characteristics of the IGZO. Similar Ta-induced crystallization can also occur in the ZnON system. Indeed, the metallic Zn and VO were found to exist near the TaOx/ZnON interface after the second PDA at 200 °C, as shown in the XP spectra of Zn 2p (Figure S1) and O 1s (Figure S2) (also see Table S1), indicating that the Zn−O bond is dissociated by the oxidation of the Ta layer during the PDA step. In the case of nitrogen, it was difficult to analyze its chemical states near the TaOx/ZnON interface because of the overlap between Ta 4p3/2 and N 1s (see Figure S3). Once the nuclei were formed near the TaOx/ZnON interface, the crystalline region grew and propagated in the depth direction. It constitutes the reason for the better uniformity and crystallinity of Ta-capped ZnON compared to the control ZnON films. Considering that carrier transport under the TFT on-state mainly occurred in the ZnON region near the ZnON/SiO2 interface, it was anticipated that the Tacapped ZnON films would exhibit promising carrier mobilities due to enhanced ordering of ZnON lattices. The disparity of the structural analysis result between XRD and TEM can arise from the different scattering power of two methods. The discernible peak for the control and Ta-capped ZnON films annealed at 200 °C could not be observed in XRD results because (i) the thickness of ZnON films is so thin (∼20 nm) and (ii) the ZnON films had either a poorly crystallized structure (in the case of the control ZnON film) or a rather random orientation with nanoscale grain size (the Ta-capped ZnO film). On the other hand, the scattering power of an electron is ∼104 times larger than that of an X-ray,27 which is the reason for the better resolution of TEM compared to XRD. 22503

DOI: 10.1021/acsami.9b03865 ACS Appl. Mater. Interfaces 2019, 11, 22501−22509

Research Article

ACS Applied Materials & Interfaces Figure 4 shows the depth profiles of zinc, oxygen, and nitrogen in the control and Ta-capped ZnON films annealed at

Figure 5. N 1s XP spectra of the (a) control ZnON film and (b) Tacapped ZnON film. O 1s XP spectra of the (c) control ZnON film and (d) Ta-capped ZnON film.

Figure 4. XPS depth profile of the (a) control ZnON film and (b) Tacapped ZnON film.

into three bases centered at 530.5, 531.8, and 532.8 eV. The peaks could be attributed to oxygen bonded to fully coordinated Zn ions (Zn−O lattice), undercoordinated Zn ions (oxygen vacancy, VO), and impurity-related oxygen such as hydroxyl groups (OH), respectively.29,34 Compared to the control ZnON film, Ta-capped ZnON films exhibited less OH fractions as summarized in Table 1. It was noted that oxygen

200 °C for 1 h under ambient O2. In the control ZnON film, constituent elements such as zinc, oxygen, and nitrogen were uniformly distributed along the depth direction except for the surface area. It could also be seen that the anion compositions of oxygen and nitrogen were ∼30 and 10 atom %, respectively. By contrast, the Ta-capped ZnON film exhibited interdiffusion of tantalum and zinc atoms, whereas the distribution of oxygen and nitrogen anions was similar to that of the control ZnON film. Oxygen composition of ∼10 atom % in the Ta film indicated that the metallic tantalum film was partially oxidized to TaOx during the second PDA at 200 °C under an O2 atmosphere. Observation of a metallic Ta-related peak in the XRD analysis suggested that the partial oxidation occurred effectively near a grain boundary of the Ta film and not the entire Ta region due to greater diffusivity of oxygen species along two-dimensional defects. The rapid concentration gradient of oxygen near the TaOx film suggested a supply of oxygen molecules from atmospheric O2. The chemical states of the different ZnON films were examined via XPS. Figure 5a,b shows the N 1s XP spectra of the control and Ta-capped ZnON films. Photoelectron binding energies were calibrated to the C 1s peak for C−C bonds at 284.5 eV. The N 1s peak was deconvoluted into three peaks at 395.2, 396.0, and 397.4 eV. The N 1s peaks centered at 395.2 and 396.0 eV were assigned to the Zn−N bonding energies for stoichiometric Zn3N2 and nonstoichiometric ZnxNy, respectively. The peak at 397.4 eV originated from N−N chemical bonds.12−20,24,28−31 The increase (decrease) in intensity of stoichiometric Zn3N2 (nonstoichiometric ZnxNy) for the Ta-capped ZnON films suggested that the electrical transport properties could be substantially improved for transistors with this channel layer because point defects such as VN in nonstoichiometric ZnxNy could act as potential electron-trapping centers and disconnect an electron conduction pathway.11,24,32,33 The O 1s XPS spectra for both ZnON films were also compared and depicted in Figure 5c,d. The asymmetric O 1s peaks were deconvoluted

Table 1. Relative Chemical Composition of the Control and Ta-Capped ZnON Films Obtained via XPS Analysesa element

chemical state

control ZnON

Ta-capped ZnON

N 1s

nonstoichiometric ZnxNy stoichiometric Zn3N2 N−N bond Zn−O bond VO impurity (OH)

46.8% 42.7% 10.4% 88.1% 5.7% 6.2%

38.7% 52.1% 9.2% 89.5% 7.8% 2.7%

O 1s

a

Each fraction was calculated as the area ratio of the Gaussian distribution centered on each binding energy to the total area.

substituted at nitrogen sites of stoichiometric Zn3N2 was a shallow electron donor.35 Thus, the free electron density (Ne) for the Ta-capped ZnO film was expected to be higher than that of the control ZnON film, which will be discussed later. It was obvious that the metal capping and subsequent annealing process was an effective method to reduce anion-related defect centers such as VN as well as OH-related impurities within the ZnON films. It is also noted that Ta ions significantly diffused into the ZnON film during the second PDA, as shown in Figure 4b. The Ta ions have different valence states such as +1, +2, +4, and +5 (see Figure S4). These Ta ions with different valence states can capture the free electron carriers depending on the EF level, which can degrade the gate swing capability of the fabricated transistor. 22504

DOI: 10.1021/acsami.9b03865 ACS Appl. Mater. Interfaces 2019, 11, 22501−22509

Research Article

ACS Applied Materials & Interfaces

oxidation power than ZnO. Thus, PDA of the Ta/ZnON stack at a high temperature would cause oxidation of the Ta film and simultaneous reduction of the ZnON film to metallic Zn. However, if the PDA temperature was moderate (1 eV) against the dissociation of Zn−O bonds would prevent strong oxidation of the TaOx film involving the conversion of ZnON to a metallic Zn film. Instead, weakly bonded oxygen and/or nitrogen species such as OH and N−N bonds could be selectively eliminated near the back surface of the ZnON films.19,38,39 This tentative model was consistent with the fact that the N and O 1s XP spectra for ZnON with a TaOx encapsulation film exhibited a smaller intensity for nonstoichiometric ZnxNy, N−N bonds, and OHrelated impurity peaks. Because those defects acted as scattering centers in ZnON, reductions in weakly bonded species could result in enhanced carrier mobility. The feasibility of the other transition metals such as Ti and Ni as an effective capping layer was also examined. Our finding was that the usage of TiOx and NiOx was also effective to boost the field-effect mobility of the resulting ZnON transistors, as shown in Figure S6. The comparable mobility of 75−81 cm2/V·s was obtained for these capped devices, indicating that the capping approach can be extendable to other transition metals. However, it has to be mentioned that the SS values for the Ti- or Ni-capped devices were more degraded to 0.50−0.53 V/decade, which is larger than that (0.33 V/decade) for the Ta-capped devices. Although the exact reason is beyond the scope of this paper, the lower oxidation power of Ni and Ti may be one of the possible origins compared to that of Ta. The Gibbs free energies of formation (ΔGf) of NiO, TiO2, and Ta2O5 are −211.54, −889.41, and −1911.2 kJ/mol, respectively, at the given annealing temperature.21 The capability of facilitating the lattice ordering and creating the VO should be strongly related to the oxidation power of the capping materials, which is the highest for the Ta capping. It should be also noted that the surface effect for the passivated device invokes the deterioration of transfer characteristics in terms of SS and VTH values when the ultrathin channel is used (for example,