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Effect of NH3 plasma treatment on the device performance of ZnO based thin film transistors
Vacuum 85 (2011) 904e907
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of NH3 plasma treatment on the device performance of ZnO based thin film transistors R. Navamathavan a, *, R. Nirmala b, Cheul Ro Lee a a
Semiconductor Materials and Processes Laboratory, School of Advanced Materials Engineering, Engineering College, Chonbuk National University, Chonju 664-14, Chonbuk 561-756, South Korea b Department of Bionano System Engineering, Chonbuk National University, Chonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2010 Received in revised form 16 November 2010 Accepted 17 January 2011
We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH3 plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH3 plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm2/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 1011 A and on to off ratio higher than 105. These results demonstrate that NH3 plasma treatment could effectively enhance the performance of the ZnO based TFT device. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Thin film transistor NH3 plasma treatment Electrical properties
1. Introduction Wide bandgap oxide-based thin film transistors (TFTs) have attracted considerable attention, because of their superior properties that include transparency, and high field effect mobility compared to that of the conventional a-Si TFTs [1,2]. In particular, ZnO based TFTs are very attractive as pixel drivers in active-matrix liquid crystal display (AMLCD) with high filling factors or even entirely transparent displays [3e9]. For these reason, several groups have focused on ZnO based TFTs for use in invisible display devices and to improve the device performance. The majority of these studies have been based on various substrates, gate and gate insulator layers in order to find a suitable device structure for ZnO TFTs that meets specific application requirements and to enhance device performance [10e14]. However, little information is available on the performance of ZnO TFTs with plasma passivation treatment. Recently, some studies reported the effect of N2O, Ar and H2 plasma treatment on the performance of amorphous indium gallium zinc oxide (a-IGZO) TFTs [15e18]. In order to improve the performance of ZnO based TFTs more intensive research works to be required. In this paper, we report on improvement of electrical
* Corresponding author. Tel.: þ82 63 270 2304; fax: þ82 63 270 2305. E-mail address: [email protected] (R. Navamathavan). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.01.008
properties of the ZnO TFTs upon NH3 plasma treatment during the device fabrication. 2. Experimental A commercially sintered undoped ZnO target (99.999%) with a diameter of 200 was used for the sputtering process. The substrate was a Corning 1737 glass coated with a 180 nm indium tin oxide (ITO) film. Before depositing the ZnO active layer, substrates were ultrasonically degreased by sequential treatment with trichlorethylene, acetone and methanol for 6 min each and then rinsed with deionized water to remove organic impurities. Magnetron sputtering was carried out in a mixed atmosphere of oxygen and argon (ratio of 1:3) by supplying an rf power of 100 W. Pre-sputtering was maintained for 10 min in order to clean the target surface. Before starting the deposition, the chamber was evacuated to a base pressure of less than 106 mbar. The sputtering pressure for the ZnO film was maintained at 13 mbar. The ITO layer was used as the bottom gate contact for the ZnO based TFT device. The gate insulator material was an SiNx film, deposited using a mixture of SiH4, NH3, and N2 gas by plasma enhanced chemical vapor deposition (PECVD) at a temperature of 300 C with an rf power of 60 W. The deposition conditions for the gate insulator layer SiNx to prepare the ZnO TFTs was SiH4/NH3/ N2 ¼ 195/35/800 sccm at a working pressure of 1.3 mbar. To improve the ZnO TFT performance, the NH3 plasma passivation
R. Navamathavan et al. / Vacuum 85 (2011) 904e907 -7
2.5x10
(b)
W/L = 500/15
VG = 8 V
-7
2.0x10
Drain current, ID (A)
(60 sccm with a power of 30 W at 300 C for 2 min) was performed before and after depositing the ZnO channel layer. Undoped ZnO thin films with a thickness of 200 nm were grown by an rf magnetron sputtering technique at a substrate temperature of 300 C. After that, source and drain electrodes were patterned using standard photolithography, Ti (30 nm) and Au (50 nm) metal layers were then deposited at room temperature by e-beam evaporation and patterned by means of a lift-off process. The detailed structural characteristics and fabrication of the ZnO TFTs were appeared elsewhere [19,20]. The channel width (W) and the channel length (L) used in the study were 500 mm and 15 mm, respectively. The currentevoltage (IeV) characteristics and transfer characteristics were measured by means of a semiconductor parameter analyzer (HP 4155A).
905
-7
1.5x10
6V -7
1.0x10
4V -8
5.0x10
2V 0V
0.0
0
10
20
30
40
50
Drain voltage, VDS (V)
3. Results and discussion
Fig. 2. Output characteristics of ZnO TFT without NH3 plasma passivation for various gate bias 0e8 V in steps of 2 V.
current at reverse gate voltages, but also in reducing the contact resistance of the source and the drain of the ZnO TFTs. Fig. 3(a) and (b) show the transfer characteristics of ZnO TFTs with and without NH3 plasma treatment, respectively. The transfer
-5
a
VDS = 10 V
10
-6
10
Drain Current, Log (ID) (A)
Fig. 1 shows the typical drain current (ID) as a function of sourcedrain voltage (VDS) of ZnO TFT with NH3 plasma passivated for a constant gate voltage (VG) in the range of 0e8 V. A drastic increase of the drain current was observed for the ZnO TFTs with different gate biasing. The saturation current was about 4800 nA under a gate bias of 8 V. Fig. 2 shows the drain current (ID) as a function of source-drain voltage (VDS) of ZnO TFT without NH3 plasma passivated for a constant gate voltage (VG) in the range of 0e8 V. The saturation current was about 225 nA under a gate bias of 8 V. The drain current increased significantly (by about one order of magnitude) for the NH3 plasma treated ZnO TFT than that of without plasma treatment. The enhancement of drain current as a result of NH3 plasma treatment can be ascribed to the improvement of surface states of interface between ZnO and SiN layers. One likely reason is that the NH3 plasma passivation can induce hydrogen doping into the ZnO film because hydrogen atom is well known to be a shallow donor located in some interstitial sites within ZnO film [21]. Moreover, this device shows hard saturation as shown in Figs. 1 and 2, demonstrating that the ZnO TFT operates in the enhancement mode (normally-off characteristics) because the drain current is zero at zero gate bias and a positive gate bias is required for the onset of conduction. In addition, no current crowding effect was observed, which implies that the Ti/Au ohmic contact seems to be very effective, not only blocking the hole
-7
10
-8
10
-9
10
-10
10
-11
10
-10
0
10
20
30
20
30
Gate Voltage, VGS (V) -5
10
VG = 8 V
-6
5.0x10
b
VDS = 10 V
6V
-6
Drain current, ID (A)
4.0x10
4V
-6
3.0x10
2V -6
0V
2.0x10
-6
1.0x10
0.0
Drain Current, Log(ID) (A)
-6
10
-7
10
-8
10
-9
10
-10
10
-11
0
10
20
30
40
50
Drain voltage, VDS (V) Fig. 1. Output characteristics of ZnO TFT with NH3 plasma passivation for various gate bias 0e8 V in steps of 2 V.
10
-10
0
10
Gate Voltage, VGS (V) Fig. 3. Transfer characteristics of ZnO TFT, (a) with NH3 plasma passivation and (b) without NH3 plasma passivation.
906
R. Navamathavan et al. / Vacuum 85 (2011) 904e907
characteristics were observed to be almost the same for both cases, but the on-current and off-current values are slightly improved for the NH3 plasma treated TFT than that of the untreated device. Higher on/off ratio can be observed for the plasma treated ZnO TFTs. The electrical properties of NH3 plasma treated and untreated ZnO TFTs were deduced. The threshold voltage and subthreshold swing for the ZnO TFTs with and without NH3 plasma treated were observed to the 14 and 16 V, and 0.44 and 0.49 V/dec, respectively. From the transfer characteristics as shown in Fig. 3(a) and (b) revealed that a low offcurrent, of the order of 1011 A, and an on-to-off drain current ratio of 105 were achieved. When the ZnO TFT device was NH3 plasma treated, the interface trap density decreased owing to the ZnO channel/dielectric interface dangling bonds reduction. Similar results were reported in some cases of a-Si based TFTs [22e24] and also in other oxide-based TFTs upon plasma passivation [15e18]. We infer that the NH3 plasma passivation was attributed to improve the electrical properties of ZnO TFTs. It is found that the ZnO TFTs after NH3 plasma passivation exhibits significantly superior device characteristics to those with untreated samples. The field effect mobility (mFE) and the threshold voltage (VTH) were obtained in the saturation region (VDS ¼ 10 V) were calculated by fitting a straight line to the plot of the square root of ID versus VGS [as shown in Fig. 4(a) and (b)], according to the following relation for a field effect transistor [25],
a
VDS = 10 V
Square root of drain current, ID
1/2
1/2
(A )
0.0025
0.0020
0.0015
0.0010
VTh = 14 V 0.0000 -10
1/2
(A )
1/2
Where W (500 mm) and L (15 mm) are width and length of the channel, mFE is the field effect mobility, CSiNx is the capacitance per unit area, VGS is the gate-source voltage and VTH is the threshold voltage. From the intercept and the slope of a linear fit to the I1/2 D versus VGS, VTH and mFE were obtained (see Fig. 4). The field effect mobility of the NH3 plasma treated ZnO TFT is higher than that without treatment. Therefore, we infer that the electrical properties of the NH3 plasma treated ZnO TFT device are much superior to those of the untreated device. Recently, Ahn et al. [18] reported the improvement of electrical properties of the amorphous indium gallium zinc oxide (a-IGZO) TFTs with Ar and H2 plasma treatment, which suggested a significant rise in the free electron concentration due to an increased density of H related donors. Thus, the performance of TFT subjected to the plasma treatment is far superior to those of the TFT without plasma treatment. One of the possible mechanisms during the NH3 plasma passivation is that the hydrogen atom forms an interfacial dipole layer which can collapse the Schottky barrier and produce more ohmic-like behavior for the ZnO channel layer [17,18]. Also, the more interstitial hydrogen atoms as shallow donors act as a compensation of the deep acceptors in ZnO channel layers when they diffused into the ZnO crystal lattice. Thus, the shallow donor electrons remaining after compensating the acceptors contribute to channel conductivity as mobile carriers in the ZnO channel [26]. In other words, the shallow donors could serve as the origin of the ntype conductivity of the undoped ZnO channel layer. Therefore, such a plasma treatment significantly improves the electrical properties of the ZnO TFTs. The NH3 plasma treatment thus makes the SiNx dielectric layer to reduce the dangling bonds and improving the interface of the dielectric/ZnO channel layer [27,28]. From the comparison of the two TFT devices, we concluded that the plasma treatment enhances the electrical properties of the device.
We have fabricated the ZnO based thin film transistors by rf magnetron sputtering with and without NH3 plasma treatment. The electrical properties were observed to be significantly improved for the ZnO TFTs with NH3 plasma treated to those without plasma treatment. The NH3 plasma treated ZnO TFT exhibited excellent electrical characteristics: a subthreshold swing of 0.44 V/dec, a minimum off-current of 1011 A, a threshold voltage of 14 V, a field effect mobility of 34 cm2/Vs, and an on/off ratio of 105. Our result suggests that NH3 plasma passivation is also one of the methods to improve the better performance of ZnO based TFTs.
0
10
20
30
Gate voltage, VGS (V)
Square Root of Drain Current, ID
W m C ðV VTH Þ2 2L FE SiNx GS
4. Conclusions
0.0005
0.0035
ID ¼
b
VDS = 10 V
0.0030 0.0025
References
0.0020 0.0015 0.0010 0.0005 0.0000 -10
VTH = 16 V 0
10
20
30
Gate Voltage, VGS, (V) Fig. 4. The square root of drain current versus gate voltage plot for the estimation of threshold voltage, (a) with NH3 plasma passivation and (b) without NH3 plasma passivation.
[1] Dehuff NL, Kettenring ES, Hong D, Chiang HQ, Wager JF, Hoffman RL, et al. J Appl Phys 2005;97:064505. [2] Cross RBM, Souza MMD, Deane SC, Young ND. IEEE Trans Electron Devices 2008;55:1109. [3] Masuda S, Kitamura K, Okumura Y, Miyatake S, Tabata H, Kawai T. J Appl Phys 2003;93:1624. [4] Wager JF. Science 2003;300:1245. [5] Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosona H. Science 2003;300:1269. [6] Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosona H. Nature 2004;432:488. [7] Carcia PF, McLean RS, Reilly MH, Nunes G. Appl Phys Lett 2003;82:1117. [8] Nishi J, Hossain FM, Takagi S, Aita T, Saikusa K, Ohmaki Y, et al. Jpn J Appl Phys 2003;42:L347. [9] Bae HS, Yoon MH, Kim JH, Im S. Appl Phys Lett 2003;83:5313. [10] Hoffman RL. J Appl Phys 2004;95:5813. [11] Bae HS, Kim JH, Im S. Electrochem Solid State Lett 2004;7:G279. [12] Dimitriadis CA, Coxon PA. Appl Phys Lett 1989;54:620.
R. Navamathavan et al. / Vacuum 85 (2011) 904e907 [13] Matins R, Barquinha P, Pereira L, Ferreira I, Fortunato E. Appl Phys A 2007;89:37. [14] Fortunato E, Barquinha P, Pimentel A, Goncalves A, Marques A, Pereira L, et al. Thin Solid Films 2005;487:205. [15] Park JS, Jeong JK, Mo YG, Kim HD, Kim SI. Appl Phys Lett 2007;90:262106. [16] Park J, Song L, Kim S, Kim S, Kim C, Lee J, et al. Appl Phys Lett 2008;93:053501. [17] Park J, Kim S, Kim C, Kim S, Song I, Yin H, et al. Appl Phys Lett 2008;93:053505. [18] Ahn BD, Shin HS, Kim HJ, Park JS, Jeong JK. Appl Phys Lett 2008;93:203506. [19] Navamathavan R, Choi CK, Park SJ. J Alloys Compd 2009;475:889. [20] Navamathavan R, Choi CK, Yang EJ, Lim JH, Hwang DK, Park SJ. Solid State Electron 2008;52:813.
907
[21] Bae HS, Im S. Thin Film Solids 2004;469e470:75. [22] Bersuker G, Zeitzoff P, Brwon G, Huff HR. Mater Today 2004;7:26. [23] Jahinuzzaman SM, Sultana A, Sakariya K, Servati P, Nathan A. Appl Phys Lett 2005;87:023502. [24] Powell MJ. IEEE Trans Electron Devices 1989;36:2753. [25] Kagan CR, Andry P, editors. Thin film transistors. New York: Marcel Dekker; 2003. p. 125. [26] Van De Walle CG. Phys Rev Lett 2000;85:1012. [27] Ramashan K, Hwan DK, Park SD, Bae JW, Yeom GY, Park SJ, et al. Electrochem Solid State Lett 2008;11:H55. [28] Komsa HP, Pasquarello A. Appl Phys Lett 2010;97:191901.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of NH3 plasma treatment on the device performance of ZnO based thin film transistors R. Navamathavan a, *, R. Nirmala b, Cheul Ro Lee a a
Semiconductor Materials and Processes Laboratory, School of Advanced Materials Engineering, Engineering College, Chonbuk National University, Chonju 664-14, Chonbuk 561-756, South Korea b Department of Bionano System Engineering, Chonbuk National University, Chonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2010 Received in revised form 16 November 2010 Accepted 17 January 2011
We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH3 plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH3 plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm2/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 1011 A and on to off ratio higher than 105. These results demonstrate that NH3 plasma treatment could effectively enhance the performance of the ZnO based TFT device. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Thin film transistor NH3 plasma treatment Electrical properties
1. Introduction Wide bandgap oxide-based thin film transistors (TFTs) have attracted considerable attention, because of their superior properties that include transparency, and high field effect mobility compared to that of the conventional a-Si TFTs [1,2]. In particular, ZnO based TFTs are very attractive as pixel drivers in active-matrix liquid crystal display (AMLCD) with high filling factors or even entirely transparent displays [3e9]. For these reason, several groups have focused on ZnO based TFTs for use in invisible display devices and to improve the device performance. The majority of these studies have been based on various substrates, gate and gate insulator layers in order to find a suitable device structure for ZnO TFTs that meets specific application requirements and to enhance device performance [10e14]. However, little information is available on the performance of ZnO TFTs with plasma passivation treatment. Recently, some studies reported the effect of N2O, Ar and H2 plasma treatment on the performance of amorphous indium gallium zinc oxide (a-IGZO) TFTs [15e18]. In order to improve the performance of ZnO based TFTs more intensive research works to be required. In this paper, we report on improvement of electrical
* Corresponding author. Tel.: þ82 63 270 2304; fax: þ82 63 270 2305. E-mail address: [email protected] (R. Navamathavan). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.01.008
properties of the ZnO TFTs upon NH3 plasma treatment during the device fabrication. 2. Experimental A commercially sintered undoped ZnO target (99.999%) with a diameter of 200 was used for the sputtering process. The substrate was a Corning 1737 glass coated with a 180 nm indium tin oxide (ITO) film. Before depositing the ZnO active layer, substrates were ultrasonically degreased by sequential treatment with trichlorethylene, acetone and methanol for 6 min each and then rinsed with deionized water to remove organic impurities. Magnetron sputtering was carried out in a mixed atmosphere of oxygen and argon (ratio of 1:3) by supplying an rf power of 100 W. Pre-sputtering was maintained for 10 min in order to clean the target surface. Before starting the deposition, the chamber was evacuated to a base pressure of less than 106 mbar. The sputtering pressure for the ZnO film was maintained at 13 mbar. The ITO layer was used as the bottom gate contact for the ZnO based TFT device. The gate insulator material was an SiNx film, deposited using a mixture of SiH4, NH3, and N2 gas by plasma enhanced chemical vapor deposition (PECVD) at a temperature of 300 C with an rf power of 60 W. The deposition conditions for the gate insulator layer SiNx to prepare the ZnO TFTs was SiH4/NH3/ N2 ¼ 195/35/800 sccm at a working pressure of 1.3 mbar. To improve the ZnO TFT performance, the NH3 plasma passivation
R. Navamathavan et al. / Vacuum 85 (2011) 904e907 -7
2.5x10
(b)
W/L = 500/15
VG = 8 V
-7
2.0x10
Drain current, ID (A)
(60 sccm with a power of 30 W at 300 C for 2 min) was performed before and after depositing the ZnO channel layer. Undoped ZnO thin films with a thickness of 200 nm were grown by an rf magnetron sputtering technique at a substrate temperature of 300 C. After that, source and drain electrodes were patterned using standard photolithography, Ti (30 nm) and Au (50 nm) metal layers were then deposited at room temperature by e-beam evaporation and patterned by means of a lift-off process. The detailed structural characteristics and fabrication of the ZnO TFTs were appeared elsewhere [19,20]. The channel width (W) and the channel length (L) used in the study were 500 mm and 15 mm, respectively. The currentevoltage (IeV) characteristics and transfer characteristics were measured by means of a semiconductor parameter analyzer (HP 4155A).
905
-7
1.5x10
6V -7
1.0x10
4V -8
5.0x10
2V 0V
0.0
0
10
20
30
40
50
Drain voltage, VDS (V)
3. Results and discussion
Fig. 2. Output characteristics of ZnO TFT without NH3 plasma passivation for various gate bias 0e8 V in steps of 2 V.
current at reverse gate voltages, but also in reducing the contact resistance of the source and the drain of the ZnO TFTs. Fig. 3(a) and (b) show the transfer characteristics of ZnO TFTs with and without NH3 plasma treatment, respectively. The transfer
-5
a
VDS = 10 V
10
-6
10
Drain Current, Log (ID) (A)
Fig. 1 shows the typical drain current (ID) as a function of sourcedrain voltage (VDS) of ZnO TFT with NH3 plasma passivated for a constant gate voltage (VG) in the range of 0e8 V. A drastic increase of the drain current was observed for the ZnO TFTs with different gate biasing. The saturation current was about 4800 nA under a gate bias of 8 V. Fig. 2 shows the drain current (ID) as a function of source-drain voltage (VDS) of ZnO TFT without NH3 plasma passivated for a constant gate voltage (VG) in the range of 0e8 V. The saturation current was about 225 nA under a gate bias of 8 V. The drain current increased significantly (by about one order of magnitude) for the NH3 plasma treated ZnO TFT than that of without plasma treatment. The enhancement of drain current as a result of NH3 plasma treatment can be ascribed to the improvement of surface states of interface between ZnO and SiN layers. One likely reason is that the NH3 plasma passivation can induce hydrogen doping into the ZnO film because hydrogen atom is well known to be a shallow donor located in some interstitial sites within ZnO film [21]. Moreover, this device shows hard saturation as shown in Figs. 1 and 2, demonstrating that the ZnO TFT operates in the enhancement mode (normally-off characteristics) because the drain current is zero at zero gate bias and a positive gate bias is required for the onset of conduction. In addition, no current crowding effect was observed, which implies that the Ti/Au ohmic contact seems to be very effective, not only blocking the hole
-7
10
-8
10
-9
10
-10
10
-11
10
-10
0
10
20
30
20
30
Gate Voltage, VGS (V) -5
10
VG = 8 V
-6
5.0x10
b
VDS = 10 V
6V
-6
Drain current, ID (A)
4.0x10
4V
-6
3.0x10
2V -6
0V
2.0x10
-6
1.0x10
0.0
Drain Current, Log(ID) (A)
-6
10
-7
10
-8
10
-9
10
-10
10
-11
0
10
20
30
40
50
Drain voltage, VDS (V) Fig. 1. Output characteristics of ZnO TFT with NH3 plasma passivation for various gate bias 0e8 V in steps of 2 V.
10
-10
0
10
Gate Voltage, VGS (V) Fig. 3. Transfer characteristics of ZnO TFT, (a) with NH3 plasma passivation and (b) without NH3 plasma passivation.
906
R. Navamathavan et al. / Vacuum 85 (2011) 904e907
characteristics were observed to be almost the same for both cases, but the on-current and off-current values are slightly improved for the NH3 plasma treated TFT than that of the untreated device. Higher on/off ratio can be observed for the plasma treated ZnO TFTs. The electrical properties of NH3 plasma treated and untreated ZnO TFTs were deduced. The threshold voltage and subthreshold swing for the ZnO TFTs with and without NH3 plasma treated were observed to the 14 and 16 V, and 0.44 and 0.49 V/dec, respectively. From the transfer characteristics as shown in Fig. 3(a) and (b) revealed that a low offcurrent, of the order of 1011 A, and an on-to-off drain current ratio of 105 were achieved. When the ZnO TFT device was NH3 plasma treated, the interface trap density decreased owing to the ZnO channel/dielectric interface dangling bonds reduction. Similar results were reported in some cases of a-Si based TFTs [22e24] and also in other oxide-based TFTs upon plasma passivation [15e18]. We infer that the NH3 plasma passivation was attributed to improve the electrical properties of ZnO TFTs. It is found that the ZnO TFTs after NH3 plasma passivation exhibits significantly superior device characteristics to those with untreated samples. The field effect mobility (mFE) and the threshold voltage (VTH) were obtained in the saturation region (VDS ¼ 10 V) were calculated by fitting a straight line to the plot of the square root of ID versus VGS [as shown in Fig. 4(a) and (b)], according to the following relation for a field effect transistor [25],
a
VDS = 10 V
Square root of drain current, ID
1/2
1/2
(A )
0.0025
0.0020
0.0015
0.0010
VTh = 14 V 0.0000 -10
1/2
(A )
1/2
Where W (500 mm) and L (15 mm) are width and length of the channel, mFE is the field effect mobility, CSiNx is the capacitance per unit area, VGS is the gate-source voltage and VTH is the threshold voltage. From the intercept and the slope of a linear fit to the I1/2 D versus VGS, VTH and mFE were obtained (see Fig. 4). The field effect mobility of the NH3 plasma treated ZnO TFT is higher than that without treatment. Therefore, we infer that the electrical properties of the NH3 plasma treated ZnO TFT device are much superior to those of the untreated device. Recently, Ahn et al. [18] reported the improvement of electrical properties of the amorphous indium gallium zinc oxide (a-IGZO) TFTs with Ar and H2 plasma treatment, which suggested a significant rise in the free electron concentration due to an increased density of H related donors. Thus, the performance of TFT subjected to the plasma treatment is far superior to those of the TFT without plasma treatment. One of the possible mechanisms during the NH3 plasma passivation is that the hydrogen atom forms an interfacial dipole layer which can collapse the Schottky barrier and produce more ohmic-like behavior for the ZnO channel layer [17,18]. Also, the more interstitial hydrogen atoms as shallow donors act as a compensation of the deep acceptors in ZnO channel layers when they diffused into the ZnO crystal lattice. Thus, the shallow donor electrons remaining after compensating the acceptors contribute to channel conductivity as mobile carriers in the ZnO channel [26]. In other words, the shallow donors could serve as the origin of the ntype conductivity of the undoped ZnO channel layer. Therefore, such a plasma treatment significantly improves the electrical properties of the ZnO TFTs. The NH3 plasma treatment thus makes the SiNx dielectric layer to reduce the dangling bonds and improving the interface of the dielectric/ZnO channel layer [27,28]. From the comparison of the two TFT devices, we concluded that the plasma treatment enhances the electrical properties of the device.
We have fabricated the ZnO based thin film transistors by rf magnetron sputtering with and without NH3 plasma treatment. The electrical properties were observed to be significantly improved for the ZnO TFTs with NH3 plasma treated to those without plasma treatment. The NH3 plasma treated ZnO TFT exhibited excellent electrical characteristics: a subthreshold swing of 0.44 V/dec, a minimum off-current of 1011 A, a threshold voltage of 14 V, a field effect mobility of 34 cm2/Vs, and an on/off ratio of 105. Our result suggests that NH3 plasma passivation is also one of the methods to improve the better performance of ZnO based TFTs.
0
10
20
30
Gate voltage, VGS (V)
Square Root of Drain Current, ID
W m C ðV VTH Þ2 2L FE SiNx GS
4. Conclusions
0.0005
0.0035
ID ¼
b
VDS = 10 V
0.0030 0.0025
References
0.0020 0.0015 0.0010 0.0005 0.0000 -10
VTH = 16 V 0
10
20
30
Gate Voltage, VGS, (V) Fig. 4. The square root of drain current versus gate voltage plot for the estimation of threshold voltage, (a) with NH3 plasma passivation and (b) without NH3 plasma passivation.
[1] Dehuff NL, Kettenring ES, Hong D, Chiang HQ, Wager JF, Hoffman RL, et al. J Appl Phys 2005;97:064505. [2] Cross RBM, Souza MMD, Deane SC, Young ND. IEEE Trans Electron Devices 2008;55:1109. [3] Masuda S, Kitamura K, Okumura Y, Miyatake S, Tabata H, Kawai T. J Appl Phys 2003;93:1624. [4] Wager JF. Science 2003;300:1245. [5] Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosona H. Science 2003;300:1269. [6] Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosona H. Nature 2004;432:488. [7] Carcia PF, McLean RS, Reilly MH, Nunes G. Appl Phys Lett 2003;82:1117. [8] Nishi J, Hossain FM, Takagi S, Aita T, Saikusa K, Ohmaki Y, et al. Jpn J Appl Phys 2003;42:L347. [9] Bae HS, Yoon MH, Kim JH, Im S. Appl Phys Lett 2003;83:5313. [10] Hoffman RL. J Appl Phys 2004;95:5813. [11] Bae HS, Kim JH, Im S. Electrochem Solid State Lett 2004;7:G279. [12] Dimitriadis CA, Coxon PA. Appl Phys Lett 1989;54:620.
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