Investigation of Zinc-Tin-Oxide Thin-Film Transistors ... - Springer Link

Electron. Mater. Lett., Vol. 10, No. 1 (2014), pp. 89-94 DOI: 10.1007/s13391-013-3112-4

Investigation of Zinc-Tin-Oxide Thin-Film Transistors with Varying SnO2 Contents Po-Jui Kuo, Sheng-Po Chang,* and Shoou-Jinn Chang Institute of Microelectronics & Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center National Cheng Kung University, Tainan 70101, Taiwan (received date: 16 April 2013 / accepted date: 5 July 2013 / published date: 10 January 2014) Zinc tin oxide (ZTO) thin-film transistors (TFTs) were prepared on a glass substrate by deposition using radio frequency (RF) magnetron co-sputtering, followed by annealing at 300°C for 20 min. The properties of ZTO thin films were found to be dependent on the atomic compositional ratio of Zn:Sn; the device performance and operational stability of the fabricated ZTO TFTs, including the mobility, on-off current ratio, threshold voltage, and subthreshold slope, were strongly influenced by the Sn content. Better TFT performance was achieved when the Sn content in the ZTO thin film was low, and the optimal mobility was 18 cm2 V−1 s−1, threshold voltage was 0.5 V, and subthreshold slope was 0.227 V·dec−1. Notably, the device performance and operational stability of the RF magnetron co-sputtered ZTO TFTs could be improved by optimizing the Zn:Sn atomic compositional ratio in the films. Keywords: zinc tin oxide, thin film transistor, sputtering, tin dioxide

1. INTRODUCTION Metal oxide materials have been widely investigated for use in passive devices such as dielectrics and capacitors and as transparent conducting oxides. In particular, transparent conducting oxides such as zinc oxide (ZnO) and tin oxide (SnO2) have been extensively studied because of their wide applicability in displays, solar cells, photo-detectors, and biosensors.[1-6] In recent years, transparent metal oxide semiconductors have attracted great interest as the active layer in transparent thin-film transistors (TTFTs).[7-11] Recently, amorphous oxide semiconductors have been recognized as excellent materials for TTFTs.[12,13] These materials are highly attractive because they can be prepared at low processing temperatures and grown on plastic substrates to generate high-quality films suitable for functional devices. The mobility of conduction electrons is proportional to the width of the conduction bands, and a narrow band tends to localize carriers. Hence, a large overlap between relevant orbitals is required to achieve large mobility and degenerate conduction in amorphous semiconductors. In addition, the magnitude of the orbital overlap must not be sensitive to the structural randomness intrinsic to the amorphous state in order to reduce the formation of shallow localized states (often referred to as “tail states”). Metal oxides composed of heavy metal cations (HMCs) with an *Corresponding author: [email protected] ©KIM and Springer

electronic configuration (n−1)d10ns0 (with n ≥ 4) satisfy these requirements. A large bandgap in oxides is attained because of the low energy of the oxygen 2p orbitals, which constitute the valence band maximum region. The lower part of the conduction band in these oxides is primarily composed of the ns orbitals of HMCs. Zinc-based amorphous oxides such as indium gallium zinc oxide (IGZO),[14] indium zinc oxide (IZO),[15] zinc indium tin oxide (ZITO),[16] and zinc tin oxide (ZTO)[17] have been reported for use in the TTFT channel layer. These amorphous oxide TFTs were fabricated on various substrates, including silicon, glass, and exible polymers, and exhibited good performance. In the case of ZTO TFTs, the Zn:Sn ratio is an important parameter, as the tin content in these TFTs has a significant impact on the output characteristics such as mobility, on-off current ratio, threshold voltage, and subthreshold slope. Therefore, it is necessary to evaluate the effect of the atomic composition on the microstructure of the oxide films in order to enable them as a key element for long-term application. In this study, the correlation between the Zn/Sn atomic composition ratio and the performance of the devices fabricated with the radio frequency (RF) co-sputtering process for ZTO TFTs is investigated.

2. EXPERIMENTAL PROCEDURE Glass substrates were ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water. The bottom gate, comprising a 100-nm-thick layer of Al, was deposited via

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Fig. 1. Structure of ZTO thin-film transistor.

thermal evaporation, and Plasma-enhanced chemical vapor deposition (PECVD) was used to grow a 200-nm-thick SiO2 layer as the dielectric layer. The ZTO thin films, used as the active layers, were then deposited via the RF co-sputtering technique. A high purity SnO2 target (99.99%, UMAT) was used as the RF gun 1 and a ZnO target (99.99%, UMAT) as the RF gun 2 in a sputter-up configuration. The distance between the substrate holder and the RF guns was 9 cm, and an angle of 30° was maintained between each gun and the substrate holder. The substrates were rotated at a speed of 20 rotations per minute (rpm). Initially, the deposition chamber was evacuated to a base pressure of 5 × 10−6 Torr. The working gases, argon and oxygen, were then released into the chamber through a mass ow controller. The flow rate of oxygen and argon were controlled to 2 sccm and 48 sccm, respectively, and the working pressure was maintained at 3 mTorr by throttling the bae valve. At first, the power applied to ZnO was maintained at 90 W and that applied to SnO2 was varied from 50 to 90 W, thus changing the Sn content in the active layer. Prior to the deposition of the drain and the source, the samples were annealed at 300°C under ambient pressure for 20 min. Finally, 100-nm-thick Al electrodes were deposited as the drain as well as source electrodes via thermal evaporation. Figure 1 shows the structure of the fabricated devices. The channel length and width of the ZTO TFTs were 100 µm and 1 mm, respectively. The physical characteristics of the ZTO thin film and the device output performance were evaluated by analyzing the phase and crystalline orientation of the ZTO thin film using x-ray diffraction (XRD), measuring the optical transmittance, and determining the atomic composition using energydispersive x-ray spectroscopy (EDS). The current-voltage (I-V) characteristics of the ZTO TFTs were measured at room temperature in dark, using a B1500 (Agilent Technologies) semiconductor parameter analyzer.

Fig. 2. X-ray diffraction profiles of the ZTO film with various radio frequency powers used for deposition of SnO2.

Fig. 3. Optical transmission spectra of co-sputtered ZTO thin film.

3. RESULTS AND DISCUSSION Figure 2 shows the XRD spectrum of the ZTO thin film. The RF power applied to the ZnO target during the growth of the film was fixed at 90 W and the power applied to the SnO2 target was varied from 50 to 90 W. In spite of the systematic increase in the applied RF power to the SnO2

Fig. 4. Sn/(Zn + Sn) ratio of ZTO thin film with various radio frequency powers applied for deposition of SnO2.

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during the growth, no defined peaks were apparent in the spectrum, indicating that the fabricated ZTO thin film was amorphous. Figure 3 shows the transmission spectrum of the ZTO thin film deposited on a glass substrate. The ZTO thin films were highly transparent with 80% transmission in the visible range (380 - 780 nm). The chemical compositions of the films were determined via EDS. The films grown by application of up to 50 W RF power to the SnO2 target do

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not have significant tin content. However, when higher RF powers are used, the tin content in the film increased slightly. Figure 4 demonstrates that the Sn/(Sn + Zn) atomic ratio in the film increased with increasing RF power. Figures 5(a)-(c) show the drain current versus drain-tosource voltage (ID-VDS) output characteristics of the ZTO TFT at various gate voltages (VG). The results show that when the SnO2 deposition power is increased, ID reaches saturation slowly; however, with lower SnO2 deposition power, ID approaches saturation more quickly. This indicates that the fabrication of the ZTO TFT with a lower SnO2 deposition power improves the output characteristics. Figure 6 shows the transfer characteristics of ID versus VG at VDS = 2 V for ZTO TFTs fabricated with a fixed ZnOdeposition power of 90 W while varying the SnO2 deposition power (50 W, 70 W, and 90 W). The electrical parameters, including the saturation mobility and threshold voltage, were derived from a linear fitting to the plot of the square root of ID versus VG using the following equation for the saturation region [18]: WC 2 ID = ----------i µ SAT ( VG −Vth ) 2L

(1)

where W and L are the channel width and length, respectively; µSAT is the saturation mobility; Ci is the capacitance per unit area of the SiO2 gate insulator (dielectric constant 3.9); and Vth is the threshold voltage. The ZTO TFT fabricated with a SnO2 deposition power of 90 W exhibited a saturation mobility of 6.29 cm2 V−1 s−1 and a threshold voltage of 2.6 V. The saturation mobility of the device employing a SnO2 deposition power of 70 W was 9.25 cm2 V−1 s−1 and the threshold voltage was 1.5 V. The TFT employing SnO2 deposited at 50 W showed high performance with a saturation mobility of 18 cm2 V−1 s−1 and a threshold voltage of 0.5 V. The variations in the mobility and shift of the subthreshold slope of the ZTO TFTs with variations in the SnO2 deposition

Fig. 5. ID-VDS curves of ZTO TFTs fabricated using various SnO2 deposition powers (a) ZnO 90 W and SnO2 50 W, (b) ZnO 90 W and SnO2 70 W, and (c) ZnO 90 W and SnO2 90 W.

Fig. 6. Transfer characteristics of ZTO TFTs with various SnO2deposition powers.


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power are shown in Fig. 7. As the SnO2 deposition power increases, the mobility is gradually reduced, whereas the subthreshold slope increases as the SnO2 deposition power increases. Figure 8 shows the dependence of the threshold voltage (Vth) on the SnO2 deposition power. The Vth shifts from negative to positive with increasing Sn content. These observations indicate enhancement of the operation of the ZTO TFTs on a positive gate bias. The results suggest that higher SnO2 power increased the Sn content; at the same time, the higher power might damage the sample or cause

Fig. 7. Dependence of subthreshold slope and mobility on deposition power of SnO2.

Fig. 8. Relationship between Vth and SnO2 deposition power.

higher interface trap density. The relationship between the TFT characteristics and the Sn content is shown in Table 1, which also indicates the variations in the parameters with different SnO2 deposition power. The output performance of the ZTO TFTs is evidently dependent on the Sn content, where a lower SnO2 deposition power is associated with enhanced device characteristics. On the other hand, the ZTO films with high Sn concentrations (ZnO:SnO2 power ratio of 90 W:90 W and 90 W:80 W) demonstrated an almost conducting behavior in the measurement range, which is possibly due to high carrier concentrations in the channels. The mobility degrades rapidly outside the optimum composition ratio, and the subthreshold slope increases when the Sn/(Zn + Sn) ratio is higher. The extremely low mobility and high subthreshold slope of the Sn-rich ZTO TFTs (ZnO:SnO2 power ratios of 90 W:90 W) can be attributed to a high carrier concentration and a high density of surface states at the interface between the channel and SiO2 gate dielectric, respectively. The interface trap density can be determined from the subthreshold slope. The density of the interface states (Nt) at the ZTO interface can be calculated by using the following equation: C SSlog ( e ) Nt = --------------------- −1 -----i . q ( kT/q )

(2)

The Nt values are shown in Table 1. It was observed that higher SnO2 power not only increased the Sn content but also caused more interface traps. Moreover, increasing the Sn content enhances the conductivity of the ZTO thin film because of the higher carrier concentration. Furthermore, the high oxidation potential of Zn (0.76 V) compared to that of Sn (0.14 V) may also increase the conductivity of the film. Higher Zn concentration will lead to a higher density of Zn atoms at the surface, and because Zn has a higher oxidation potential than Sn, more oxygen molecules can be adsorbed, thereby trapping more electrons,[19] with a consequent reduction in the carrier concentration. From Figure 6, it is apparent that the ratio of Sn/(Zn + Sn) has a significant effect on the transfer characteristics, i.e., when the Sn content is lower, the TFT has a lower leakage current, and the turn-on and turn-off ratios approach saturation more quickly. In contrast, when the carrier concentration

Table 1. Electrical parameters and energy-dispersive x-ray spectroscopy atomic ratios for ZTO thin films. SnO2 power (W)

Sn/(Zn+Sn)

SSa (V/dec.)

Ion/Ioff

50

0.36

0.227

3.2 × 105

Nt (cm-2)

µsat (cm2V-1s-1)

0.5

2.9 × 1011

18

0.8

7.9 × 1011

10.18

60

0.38

0.51

1 × 10

70

0.58

0.54

1.2 × 10

4

1.5

8.4 × 1011

9.25

1.6 × 10

4

2.1

8.6 × 1011

8.19

6.5 × 10

3

2.6

11

6.29

80 90 a

4

VT (V)

0.59 0.6

0.55 0.6

SS, subthreshold slope.


9.5 × 10

P.-J. Kuo et al. Table 2. Comparison of our TFTs with others reported in literature. Author

a

2

SS (V/dec.)

Ion/Ioff

µsat (cm V s )

0.23

>106

2.5

C. G. Lee et al. [20]

6

S. Jeong et al. [21]

--

10

Y. Jeong et al.[22]

1.7

107

0.58

Y. H. Kim et al. [23]

1.03

>108

3.99

P. Görrn et al. [24]

0.4

106

S. J. Seo et al. [25] In Present Work a

−1 −1

1.1

10 8

0.4

3.86 × 10

0.227

3.2 × 105

15.70 18

SS, subthreshold slope.

increases, the transfer characteristics degrade gradually, the leakage current is enhanced, and the turn-on and turn-off ratios become indiscernible, and reach saturation more slowly. These results indicate that by optimizing the Sn content of the ZTO films, the charge carrier concentration, interface states, and surface morphology can be controlled, thereby resulting in the improved device performance and operational stability of the RF co-sputtered ZTO TFTs. Comparisons of our TFTs with those reported in literature are shown in Table 2. By doping SnO2 to the ZnO TFTs, we have successfully fabricated ZTO TFTs that showed enhanced performance. Use of the ZTO channel employing the optimum Zn:Sn ratio resulted in the improved device performance as well as good operational stability.

4. CONCLUSIONS In summary, ZTO thin films were fabricated using the RF co-sputtering method. To control the Sn content in the ZTO thin film, the ZnO-deposition power was fixed while the SnO2 power was varied. The Zn:Sn composition ratio of the film has a significant impact on the electrical properties and operational stability. Increasing the Sn concentration caused a significant change in the microstructure of the film; consequently, the electrical properties and operational stabilities of the ZTO TFTs were affected. The carrier concentration of the ZTO thin film increased when the SnO2 deposition power was higher, thereby degrading the performance of the TFTs. Better performance was achieved by lowering the Sn concentration in the ZTO thin films, giving rise to an optimal mobility of 18 cm2 V−1 s−1, threshold voltage of 0.5 V, and subthreshold slope of 0.227 V dec−1. Use of the ZTO channel employing the optimum Zn:Sn ratio resulted in the improved device performance as well as good operational stability.

ACKNOWLEDGMENTS The authors thank the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, R.O.C., for the financial support under Contract No. 101-

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2221-E-006-139 and 101-D0204-6, and the LED Lighting Research Center of NCKU for the assistance with device characterization. This work was also supported in part by the Center for Frontier Materials and Micro/Nano Science and Technology, the National Cheng Kung University, Taiwan, as well as by the Advanced Optoelectronic Technology Center, the National Cheng Kung University, under projects from the Ministry of Education.

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