Effects of Nitrogen and Hydrogen Codoping on the Electrical

Mar 7, 2017 - Department of Microelectronics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics a...
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Effects of Nitrogen and Hydrogen Codoping on the Electrical Performance and Reliability of InGaZnO Thin-Film Transistors Ablat Abliz,† Qingguo Gao,‡ Da Wan,† Xingqiang Liu,§ Lei Xu,† Chuansheng Liu,*,† Changzhong Jiang,† Xuefei Li,‡ Huipeng Chen,∥ Tailiang Guo,∥ Jinchai Li,† and Lei Liao*,†,§ †

Department of Microelectronics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China ‡ Wuhan National High Magnetic Field Center and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China § Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China ∥ Institute of Optoelectronic Display, Fuzhou University, Fuzhou 350002, China S Supporting Information *

ABSTRACT: Despite intensive research on improvement in electrical performances of ZnO-based thin-film transistors (TFTs), the instability issues have limited their applications for complementary electronics. Herein, we have investigated the effect of nitrogen and hydrogen (N/H) codoping on the electrical performance and reliability of amorphous InGaZnO (α-IGZO) TFTs. The performance and bias stress stability of α-IGZO device were simultaneously improved by N/H plasma treatment with a high field-effect mobility of 45.3 cm2/(V s) and small shifts of threshold voltage (Vth). On the basis of Xray photoelectron spectroscopy analysis, the improved electrical performances of α-IGZO TFT should be attributed to the appropriate amount of N/H codoping, which could not only control the Vth and carrier concentration efficiently, but also passivate the defects such as oxygen vacancy due to the formation of stable ZnN and NH bonds. Meanwhile, low-frequency noise analysis indicates that the average trap density near the α-IGZO/SiO2 interface is reduced by the nitrogen and hydrogen plasma treatment. This method could provide a step toward the development of α-IGZO TFTs for potential applications in nextgeneration high-definition optoelectronic displays. KEYWORDS: thin-film transistors, InGaZnO, low-frequency noise, plasma treatment, reliability

1. INTRODUCTION

In addition, plasma treatment has been demonstrated as an efficient method to modulate the carrier concentration (Ne) and reduce the defect states associated with oxygen vacancies (VO) in the metal oxide thin films.10 Thus, many studies have been presented on the electrical properties of metal oxide TFTs, such as mobility, channel resistance, and stability, have been improved by using different plasma treatment.11−13 Especially for hydrogen plasma treatment, in which H can act as a shallow n-type donor in an α-IGZO thin film when it occupies interstitial positions (Hi) or VO sites (VOH complexes or HO),7,14 and exhibits strong bonding with oxygen (OH) in ZnO-based thin films owing to the chemical natures of both elements.15−17 Recently, various groups have demonstrated that hydrogen plasma treatment could increase the Ne and improve the performance of α-IGZO TFTs, which indicated that hydrogen could act as a shallow donor in α-

The increasing demand for large-area, transparent, flexible, and low-cost electronics for utilization in high-resolution and fastspeed optoelectronic displays requires development of highperformance thin-film transistors (TFTs).1,2 In this regard, metal oxide TFTs have attracted much attention because of their desirable electron mobility, good optical transparency, and mechanical flexibility compared with hydrogenated amorphous silicon based TFTs.3,4 Specifically, amorphous InGaZnO (αIGZO) have been widely used as the active channel layer in TFTs for optoelectronic displays.5,6 However, the mobility is insufficient to satisfy the latest demands for high-resolution and high-definition displays. Moreover, the high density trap states and native point defect states have existed at the channel/ insulator layer interfaces and the channel layer, which may lead to the insufficient mobility and instability problems of α-IGZO TFTs.7−9 Therefore, it is a challenging issue to improve the interface quality and reduce the native defects states for enhancing the mobility and stability of α-IGZO TFTs. © 2017 American Chemical Society

Received: November 28, 2016 Accepted: March 7, 2017 Published: March 7, 2017 10798

DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic device structures of different plasma treatment in α-IGZO TFTs using Si bottom gate. (b) Transfer log scale curves of the αIGZO TFTs with different N plasma treatment time and VDS = 1 V. (c) Transfer log scale curves of the α-IGZO TFTs with different H plasma treatment time and VDS = 1 V. (d) Transfer log scale curves (left) and field-effect mobility (right) of the α-IGZO TFTs with different N/H plasma treatment time at VDS = 1 V. (e) Carrier concentration of the α-IGZO thin films as a function of different N, H, and N/H plasma treatment time. (f) Output characteristic of the α-IGZO and α-IGZO:N/H (200 s) TFTs, respectively.

IGZO thin films.12,18,19 However, some researchers have also observed that heavy hydrogen doping undesirably leads to photobias instability and large negative Vth shift of the α-IGZO TFTs.20,21 However, N can not only can act as an acceptor, but also serve as a defect binder because the ionic radius of N is close to O, thus N could effectively substitute for O (NO) and reduce VO in the ZnO-based thin films.22 So the N plasma treatment could improve the bias stability of the α-IGZO TFTs.23,24 However, the degradation of mobility and reduction of optical band gap energy of the N doped α-IGZO TFT were also observed.24,25 As an alternative approach, a strategy of cation and anion codoping can provide an effective solution to the adverse tradeoff between electrical performance and reliability of α-IGZO TFTs. In this work, we have demonstrated that both the electrical performance and the reliability of α-IGZO TFTs were simultaneously improved by the proper amount of N/H plasma treatment because of the reduction of VO defects and interface trap density (Dit). Furthermore, to understand the charge trap mechanisms with variations in VO and distribution of trap sites in the α-IGZO TFTs, X-ray photoelectron spectroscopy (XPS), and low-frequency noise (LFN) analyses were carried out.

80 nm Al source and drain electrodes were sputtered by a direct current sputtering process at a power of 70 W. All devices were not subjected to any post-thermal annealing. 2.2. Film and Device Characterization. Electrical characteristics of the TFTs were extracted by using a keysight Semiconductor Parameter Analyzer B1500A, B2912, and Lake Shore TTPX Probe Station at room temperature. To realize the chemical compositions and bonding states, the XPS spectra measurements were performed with a Thermo Fisher ESCALAB 250Xi instrument equipped with a monochromatic Al Kα (1486.68 eV) X-ray source, and the binding energies were calibrated using the C (1s) carbon peak (284.8 eV). The low-frequency 1/f noise (LFN) measurements were carried out by Agilent E4725A 1/f noise system with an Agilent E5052B.

3. RESULTS AND DISCUSSION Schematic diagram in Figure 1a illustrates the proposed scheme of α-IGZO bottom-gate and top contact device structure with different plasma treatments. First of all, to understand the effect of N in α-IGZO TFTs, the transfer characteristics of the αIGZO TFT after different N plasma treatment time of 0, 100, 200, and 300 s have been measured, respectively, as shown in Figure 1b. It is clear that a small amount of N doped in α-IGZO TFT could improve the subthreshold slope (SS) and Vth value than that of the undoped α-IGZO TFTs. These results indicate that N doping could passivate the defects such as VO and trap density at the α-IGZO/SiO2 interfaces.23 However, the fieldeffect mobility (μFE) is slightly decreased with increasing N plasma treatment time. In addition, the Ne is obviously decreased from 3.5 × 1018 cm−3 to 5.2 × 1017 cm−3 for the α-IGZO and α-IGZO:N (300 s) device, as shown in Figure 1e. The Ne was estimated by the microexpression of maximum current at the maximum μFE for all devices.26 The degradation of electrical properties was mainly originated from the generation of tail states related to N vacancies (due to excess of N atoms) in the α-IGZO films.23,24 It should be noted that the N vacancies form a positive charge and thus act as an electron trap center in the α-IGZO/SiO2 interface, which also cause the degradation of μFE and Vth instabilities.23,24 On the contrary, after H plasma treatment, the conductivity of α-IGZO

2. EXPERIMENTAL SECTION 2.1. Device Fabrication. The α-IGZO TFTs were fabricated on SiO2/Si substrates, where the highly doped p-type silicon wafer was used as the gate electrode and the thermally oxidized (100 nm) SiO2 layer as the gate insulator. First, a 25 nm α-IGZO thin film was deposited on the SiO2/Si substrate by (RF) magnetron sputtering using the ceramic target (In2O3:Ga2O3:ZnO = 1:1:1 in mole ratio, 99.99%). During the sputtering deposition, the RF power was 50 W, the gas mixing ratio was fixed at Ar:O2 = 8.9:0.1, the total gas pressure was maintained at 0.55 Pa, and the substrate temperature was set as 150 °C, respectively. Then, the sample was directly treated in the aforementioned RF magnetron sputtering system with a closed shutter by incorporating N2, H2, and N2/H2 plasma with gas mixing ratio of N2:Ar = 10:40, H2:Ar = 10:40 and N2:H2:Ar = 10:10:40 in the sputtering system. When treated N2, H2, and N2/H2 plasma, the plasma treatment time was 0, 100, 200, and 300 s, respectively. Finally, 10799

DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

Research Article

ACS Applied Materials & Interfaces

Table 1. Extracted Electrical Characteristics of the α-IGZO and α-IGZO:N/H TFTs with Different N/H Plasma Treatment Time α-IGZO:N/H 0 100 200 300

s s s s

IDS (μA)

Vth (V)

μFE (cm2/(V s))

26.4 51.1 83.2 42.1

± ± ± ±

± ± ± ±

2.8 2.1 1.7 2.5

0.4 0.2 0.1 0.3

15.5 31.3 45.3 23.2

Ion/Ioff

0.5 0.6 0.8 0.7

4 2 3 1

× × × ×

7

10 108 108 108

SS (V/dec.)

Dit (cm−2/eV)

± ± ± ±

× × × ×

0.53 0.28 0.21 0.36

0.06 0.04 0.03 0.05

1.7 7.5 4.5 1.1

12

10 1011 1011 1012

Nt (cm−3/eV) 3.5 8.7 5.6 1.5

× × × ×

1019 1018 1018 1019

Figure 2. Evolution of the transfer curves as a function of NBS time for the (a) α-IGZO and (b) α-IGZO:N/H (200 s) TFTs with VG = −20 V and VDS = 1 V. (c) The detailed plots of ΔVth as a function of stress time of the α-IGZO and α-IGZO:N/H (200 s) TFTs.

μm) and W (150 μm) are the channel length and width, respectively. Moreover, in view of the performance variance is associated with the nonuniformity of a-IGZO film thickness as well as device fabrication, a statistical study of the key parameter of devices is necessary to gain a comprehensive understanding of the electrical properties. Figure S3 show the distributions of μFE with different devices. A total of 40 devices are studied for each type of devices. The average and standard deviation of the calculated mobility are 11.4 ± 4.1 and 35.2 ± 10.1 cm2/(V s) for the pristine a-IGZO and a-IGZO:N/H (200 s) devices, respectively. Therefore, the nonuniformity of devices is caused by the device fabrication process, size, quality of aIGZO ceramic target, and nonuniform gas distribution during the deposition films.10 Besides, the SS value is significantly decreased from 0.53 to 0.21 V/decade for the α-IGZO and αIGZO:N/H (200 s) TFT, as shown in Figure S2 and Table 1, respectively. For all devices, the SS value can be obtained by the following:3

TFT was dramatically increased and became very conductive due to the increasing Ne, as shown in Figure 1c. Specifically, the Ne of α-IGZO TFT is obviously increased from 3.5 × 1018 cm−3 to 2.3 × 1019 cm−3 after the 300 s hydrogen treatment, as shown in Figure 1e. Although the α-IGZO:H (300 s) TFT exhibited was very conductive, the TFT lost its gate control ability and showed large negative shift Vth because of the excessive H plasma treatment. These results indicate that the H can act as a shallow donor and increase the Ne in α-IGZO films, which agrees with the previous studies.18−20 In addition, we have figured out the effect of hydrogen on a-IGZO TFTs, and our results demonstrated that the conduction band energy level offset (ΔECB) is decreased after H plasma treatment.19 On the bases of semiconductor physics, the smaller ΔECB in α-IGZO:H leads to easier activation of electrons compared with the αIGZO film, which indicates that the Ne is increased by H plasma treatment. To overcome the disadvantages of the α-IGZO:N and αIGZO:H TFTs, the N/H codoping α-IGZO TFTs were studied systematically. Figure 1d,f shows the typical transfer and output curves of the α-IGZO and α-IGZO:N/H devices with the different plasma treatment times, and these devices exhibit the typical n-channel enhancement mode. Also, the transfer linear curves of the α-IGZO and α-IGZO:N/H devices are shown in Figure S1 of the Supporting Information. Furthermore, the extracted electrical characteristics of the both α-IGZO and α-IGZO:N/H devices, including Vth, SS, IDS, on/off current ratio (Ion/Ioff), Dit, and μFE, are summarized in Table 1. In all devices, the Vth was determined from the horizontal intercept of a linear part in IDS1/2 versus VGS plot. It can be seen that the IDS, μFE of α-IGZO:N/H TFTs were apparently increased with an increase of N/H plasma treatment time, as shown in Figures 1d and S2, respectively. Specifically, the μFE is obviously increased from 15.5 to 45.3 cm2/(V s) for the α-IGZO and α-IGZO:N/H (200 s) TFT. The μFE can be obtained from the following equation:3 μFE =

⎛ d log(IDS) ⎞−1 SS = ⎜ ⎟ ⎝ dVGS ⎠

where IDS is the drain to source current, and VGS is the gate bias. It is well-known that the Vth and SS of metal oxide TFTs are related to the Dit and the number of deep states.3,8 On the basis of the above SS value, the Dit could be obtained from the following equation:8 Dit =

⎞ Cox ⎛ qSS ⎜ − 1⎟ 2 ⎝ ⎠ q kT ln 10

(3)

where q is the elementary electron charge, kT is the thermal energy, and Cox is the gate dielectric capacitance per unit area. Therefore, the Dit value is decreased from 1.7 × 1012 cm−2/eV to 4.5 × 1011 cm−2/eV for α-IGZO and α-IGZO:N/H (200 s) TFTs, respectively. From Figure 1e, we can see that the Ne of α-IGZO:N/H device has not any change obviously, because the NO acceptor has been passivated by hydrogen.27,28 It should be noted that the device exhibits the degraded performance such as μFE (23 cm2/(V s)), SS (0.36 V/decade), Dit (1.7 × 1012 cm−2/eV), and Vth (2.5 V) when N/H treatment time is increased to 300 s, which mainly originates from the heavy N/

gmL WVDSCox

(2)

(1)

where gm is transconductance and can be obtained by deriving the IDS−VGS curve, Cox is capacitance per unit area, and L (120 10800

DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

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ACS Applied Materials & Interfaces

oxygen on the surface of the films due to specific species (absorbed H2O, O2, and CO3).32 Thus, the relative quantity of VO is expressed by the area ratio of the O2 peak to the total O 1s peak (O2/Ototal). With increasing N/H treatment time from 0 to 200 s, the area ratio of peak O2/Ototal decreases from 32.7% to 25.3%, which indicating that VO can be effectively suppressed by N/H plasma treatment. This result also agrees with the improvement of stability for α-IGZO:N/H TFTs. Furthermore, the N 1s spectra of α-IGZO and α-IGZO:N/H (200 s) films were also analyzed, respectively. It can be seen that there is no obvious broad peak in N 1s spectrum of αIGZO film without N/H treatment in Figure 3a. Interestingly,

H treatment and carrier scattering events for channel electrons. Therefore, the improved electrical performance can be attributed to the fact that the appropriate amount of N/H treatment could suppress the natural intrinsic defects states in α-IGZO films and reduce trap density at the α-IGZO/SiO2 interfaces. Additionally, the AFM results showed that the rootmean-square roughness is obviously decreased from 0.76 nm of a-IGZO to 0.23 nm of a-IGZO:N/H (200 s) film, as shown in Figure S4, which indicated that the surface morphology of the a-IGZO film became smoother by N/H plasma treatment. After the optimization, the α-IGZO:N/H (200 s) device exhibits high electrical performance, including a high Ion/Ioff of 108, a high μFE of 45.3 cm2/(V s), a small SS of 0.21 V/decade, and a reasonable Vth value of 1.7 V compared to that of the α-IGZO TFTs. To characterize the effects of N/H codoping on stability of α-IGZO TFT, the negative gate bias stress (NBS) and positive gate bias stress (PBS) stability of α-IGZO and α-IGZO:N/H (200 s) TFTs were analyzed, where both of the devices were stressed under a VG of ±20 V and stress duration of 3600 s at room temperature. Figure 2a,b show the evolution of transfer characteristics as a function of NBS time for the a-IGZO TFT and α-IGZO:N/H (200 s) TFT, respectively. The α-IGZO TFT exhibits a large negative Vth shift (ΔVth) of −9.2 V under the NBS with stress time of 3600 s. However, the α-IGZO:N/ H (200 s) TFT exhibits a small negative ΔVth of −1.6 V under the identical NBS condition. Meanwhile, Figure S5 shows the evolution of transfer characteristics as a function of PBS time for the α-IGZO and α-IGZO:N/H (200 s) TFT, respectively. The α-IGZO TFT showed a large positive ΔVth of 9.7 V under the PBS with stress time of 3600 s. Nevertheless, the αIGZO:N/H (200 s) TFT showed a small positive ΔVth of 1.8 V under the identical PBS condition. Furthermore, the ΔVth as a function of bias stress time for the α-IGZO and α-IGZO:N/H (200 s) devices are shown in Figure 2c. It was found that the αIGZO:N/H (200 s) device exhibits apparently better bias stress stability, which shows much smaller ΔVth without degradation μFE and SS value than that of the α-IGZO device. It should be noted that the negative and positive ΔVth after NBS and PBS for both of the devices can be explained by the following mechanisms: the field-induced charge trapping, the absorption/ desorption of oxygen-related molecules, and the creation of ionized VO defects at the channel/dielectric interface.8,9,29−31 According to the above experimental results, we have assumed that the improved stability of α-IGZO TFT is related to the reduction of VO defects and trap density at the near α-IGZO/ SiO2 interfaces. In addition, the Dit value of α-IGZO:N/H (200 s) device showed lower than that of the α-IGZO device, which is consistent with the change in the Vth under gate bias stress. In order to understand the reason and mechanism, XPS measurement was carried out to realize the qualitative chemical properties of the α-IGZO and α-IGZO:N/H (200 s) films. The relative O 1s spectra are shown in Figure S6. Therefore, the O 1s spectrum can be deconvoluted into three different peaks by using Gaussian fitting with the subtraction of a Shirley type background. The O 1s region of the XPS spectrum can be divided into three regions: low binding energy (O1, 530 eV), medium binding energy (O2, 531 eV), and high binding energy (O3, 532 eV). The O1 peak is attributed to the bonding between oxygen atoms and metal atoms (Zn, Ga, and In) without oxygen deficiency.19 Similarly, the O2 peak is related to the oxygen deficient region like VO in the α-IGZO film.31 The O3 peak is associated with the presence of loosely bound

Figure 3. (a) N 1s spectra of the α-IGZO and (b) α-IGZO:N/H (200 s) films.

the broad peak was observed after the N/H treatment, which can be deconvoluted to two peaks, one peak at 396.4 eV from NZn bond and the other peak at 398.2 eV from NH bond are shown in Figure 3b. This result is consistent with the previous studies.13 The low binding energy peak at 396.4 eV related to N ions corresponding to nitrogen and high binding energy peak at 398.2 eV may be derived from H passivated NO acceptor.33 Thus, the N 1s spectrum of α-IGZO:N/H (200 s) thin film indicated that N and H atoms are successfully incorporated in α-IGZO film. Recently, the theoretical calculations results reveal that H atom in N-doped ZnObased films favorably connected with the NO at an antibond site and the formation of more stable NOH complex bound, and the dissociation of NO−H complexes bound needs activation energy of at least 1.25 eV.28,34 Moreover, the hydrogenation can greatly enhance the binding between NO and VZn,35 subsequently forming (NOHVZn) more stable complex bound than (NOVZn) in N doped ZnO-based films.35,36 And also, the evidence of NH complex existing in ZnO-based films in deed has been found.37 The above results could indicate that the N/H plasma treatment passivated many defects and formation of stable NH complex resulting in improvement electrical properties of α-IGZO TFTs. In order to further investigate the effect of N/H plasma treatment on the interface quality of α-IGZO TFTs, the LFN was measured for α-IGZO TFTs with different N/H plasma treatment times of 0, 100, 200, and 300 s in linear operation at VDS = 1 V, respectively. A group of LFN noise plots measured with the gate voltage VGS stepped from weak to strong inversion and linear region of the TFTs (VGS − Vth = 3 V and VDS = 1 V) in Figure 4a (the LFN spectrum data of 100 s is not shown). The standard method is Hooge’s empirical relationship by the following:38,39 SID = 10801

AIDα fβ

(4) DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

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at the SiO2/α-IGZO interface.40−43 Moreover, the information on the dominant noise mechanism can also be obtained by the input-referred noise spectral density (SVG). As shown in Figure 4c, the SVG = SID/gm2 at f = 20 Hz has negligible dependence on gate-voltage overdrive for all devices, which further confirm that CNF cause SVG to be independent of (VGS − Vth).42,43 Furthermore, according to the CNF model, the effective average interfacial trap density (Nt) could be obtained from the SVG.45 For a fair comparison between the α-IGZO and the αIGZO:N/H TFTs, the Nt can be extracted using the following relationship:45,46 S VG =

q2kTNt 2 WLγfCox

(5)

where q is the elementary electron charge, f is the noise frequency, kT is the thermal energy, and Cox is the gate dielectric capacitance per unit area. Finally, γ is the tunneling attenuation coefficient of the electron wave function within the SiO2 dielectric. Here, the γ can be further described by the following:47,48

Figure 4. (a) Typically normalized LFN spectrum of the α-IGZO and α-IGZO:N/H TFTs with VGS − Vth = 3 V and VDS = 1 V. (b) Normalized SID/ID2 as a function of VGS − Vth log−log plots of the αIGZO and α-IGZO:N/H TFTs with 20 Hz and VDS = 1 V. (c) Inputreferred noise versus gate-voltage overdrive for the α-IGZO and αIGZO:N/H TFTs with 20 Hz and VDS = 1 V. (d) Calculated average interface trap density as a function of different overdrive gate voltage.

γ=

4π 2m*Φit h

(6)

where m* is the electron effective mass in SiO2 dielectric, h is the Planck’s constant, and Φit is the tunneling barrier height for SiO2/α-IGZO interface. The lowest Nt for the α-IGZO:N/H TFTs can be attributed to the lowest SVG because we used the same gate oxide thickness (Cox). It is found that the Nt of αIGZO:N/H device can be significantly reduced compared with α-IGZO TFT device, which are calculated to be 3.5 × 1019 cm−3/eV and 5.6 × 1018 cm−3/eV for α-IGZO TFTs and αIGZO:N/H (200 s) TFTs. The Nt was in good agreement with Dit estimated from SS in the α-IGZO TFTs and α-IGZO:N/H TFTs, as shown in Figure 4d and Table 1. However, the excessive N/H (300 s) plasma treatment can generate more interstitial defects rather than passivate trap density at the SiO2/α-IGZO interface, thus inducing high noise level and high Nt, which can lead to the degradation of electrical performances of α-IGZO:N/H (300 s) device. On the basis of the above XPS and LFN results, the proper amount of N/H incorporation could effectively reduce the trap density at the SiO2/α-IGZO interface and passivate the VO related defects of the α-IGZO TFTs, which could be responsible for the improvement in electrical performance and reliability of the α-IGZO:N/H devices.

where SID is the current noise spectral density, f is the frequency, ID is the current through the device channel, and A is the noise amplitude. The exponent α and β are ideally close to 2 and 1, respectively. Therefore, the measured normalized drain-current noise spectral density (SID/ID2) fits well to the 1/f β noise theory (the f is ranging from 5 to 1000 Hz) and the SID/ ID2 is good agreement with classical 1/f noise theory (the β is ranging from 0.95 to 1.07) for all devices under all bias condition in Figure 4a. It is clear that the SID/ID2 of αIGZO:N/H device was decreased compared with the α-IGZO device, indicating that the average trap density within the device active channel region is decreased. From the study of 1/f noise in metal oxide TFTs for the past few decades, two mechanisms have been generally considered to explain the origin of 1/f noise in metal oxide TFTs, namely, carrier number fluctuation (CNF) and correlated mobility fluctuation (CMF).40,41 In CMF theory, the observed 1/f noise in the conductance is caused by fluctuations in the mobility of the free carriers in the conducting channel of the metal oxide TFTs.40,41 Moreover, in CNF theory, the 1/f noise is caused by generation− recombination noise in the electron transitions between the conduction band of the channel material and the traps in the oxide layer.40−42 One method of finding the dominant mechanism of 1/f noise is to investigate the dependence of SID/ID2 on gate over drive voltages (VGS − Vth)−m at a fixed frequency.42,43 Generally, a slope of m = −2 indicates that CNF in the dielectric/channel interface region is the main origin of the generated LFNs, whereas a slope of m = −1 indicates that CMF is their main origin of 1/f noise, respectively.41−44 In order to verify origin of the LFN, SID/ID2 is measured at f = 20 Hz, VDS = 1 V, and (VGS − Vth) ranges from 1 to 5 V in Figure 4b. The slopes of the all curves corresponding to the α-IGZO (black), α-IGZO:N/H (200 s) (red) and α-IGZO:N/H (300 s) (blue) device are all around m ≈ −2. According to the established LFN theory, a slope of m ≈ −2 indicated that the main source of LFN can be attributed to the CNF, which is caused by the tunneling of free-charge carriers into oxide traps

4. CONCLUSIONS This work investigated the effects of N/H plasma treatment on the electrical characteristics of α-IGZO TFTs. The results indicated that the electrical properties were significantly improved, including the μFE of 45.3 cm2/(V s), Ion/Ioff of 108, Vth of 1.7 V, and SS of 0.21 V/decade. In addition, the αIGZO:N/H (200 s) devices have also shown better stability than that of the α-IGZO devices. According to the XPS results, these characteristics are mainly attributed to the N/H plasma treatment, which could effectively passivate the VO defects in the α-IGZO film. Furthermore, on the basis of low-frequency noise analysis, the SID/ID2 of α-IGZO:N/H devices are decreased compared with the α-IGZO device, which indicated that the average trap density near the α-IGZO/SiO2 interface is reduced by the N/H plasma treatment. Overall, the αIGZO:N/H devices with high electrical performance and 10802

DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

Research Article

ACS Applied Materials & Interfaces

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reliability could be used for various applications in future high resolution flat panel displays.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15275. The transfer linear curves of the α-IGZO and αIGZO:N/H TFTs. Maximum IDS, Vth, μFE, and SS value of the α-IGZO device as a function of N/H plasma treatment time. The statistical studies on electrical proprieties for a-IGZO and α-IGZO:N/H TFTs. AFM characterization of a-IGZO and a-IGZO:N/H films. The evolution of the transfer curves as a function of PBS time for the α-IGZO and α-IGZO:N/H TFTs. O 1s XPS spectra of the α-IGZO and α-IGZO:N/H films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (L.C.). *E-mail: [email protected]. (L.L.). ORCID

Huipeng Chen: 0000-0003-1706-3174 Lei Liao: 0000-0003-1325-2410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported in part by National Key Research and Development Program of China (2016YFB0401103 and 2016YFF0203600), the NSFC grants (Nos. 61376085, 11575132, and 11574083), and Ten Thousand Talents Program for Young Talents.

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REFERENCES

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DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804

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DOI: 10.1021/acsami.6b15275 ACS Appl. Mater. Interfaces 2017, 9, 10798−10804