CHIN. PHYS. LETT. Vol. 27, No. 12 (2010) 128504
ZnO-Based Transparent Thin-Film Transistors with MgO Gate Dielectric Grown by in-situ MOCVD * ZHAO Wang(赵旺), DONG Xin(董鑫), ZHAO Long(赵龙), SHI Zhi-Feng(史志锋), WANG Jin(王瑾), WANG Hui(王辉), XIA Xiao-Chuan(夏晓川), CHANG Yu-Chun(常玉春)** , ZHANG Bao-Lin(张宝林), DU Guo-Tong(杜国同) State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012
(Received 7 Feburay 2010) ZnO transparent thin-film transistors with MgO gate dielectric were fabricated by in-situ metal organic chemical vapor deposition (MOCVD) technology. We used an uninterrupted growth method to simplify the fabrication steps and to avoid the unexpectable contaminating during epitaxy process. MgO layer is helpful to reduce the gate leakage current, as well as to achieve high transparency in visible light band, due to the wide band gap (7.7 eV) and high dielectric constant (9.8). The XRD measurement indicates that the ZnO layer has high crystal quality. The field effect mobility and the on/off current ratio of the device is 2.69 cm2 V−1 s−1 and ∼ 1 × 104 , respectively.
PACS: 85. 30. Tv, 81. 15. Gh
DOI: 10.1088/0256-307X/27/12/128504
Traditionally, thin-film transistors (TFTs) are fabricated with the polycrystalline or amorphous silicon as the active channel layer. However, due to the light sensitivity of silicon, optical shield layers should be inserted in these types of TFTs. Therefore, TFTs based on the transparent films may be a better solution. ZnO thin films have been intensively studied and fabricated by various methods.[1−3] Recently, ZnO is used as the active layer in the transparent TFTs[4−10] due to its prominent advantages in photoelectric properties, such as wide band gap (3.3 eV[11] ) and large electron channel mobility, resulting in visible-region transparent and high on/off ratio. The insulating layer of TFTs is also important for improving the devices’ performance. Compared with conventional gate dielectric materials such as SiO2 , MgO is a highly ionic insulating crystalline rock salt-structure with good chemical/physical stability. Wide bandgap of MgO (7.7 eV[12] ) is beneficial to a higher transparent rate for visible light. Moreover, the dielectric constant of MgO (9.8)[13] is much higher than SiO2 (3.9),[14] which means that MgO may offer high breakdown voltage and low gate leakage current. The most important benefit is that high-quality MgO thin layers can be grown by metal organic chemical vapor deposition (MOCVD). Thus it becomes possible to prepare ZnO/MgO transparent TFTs through subsequential growth processes. The unexpectable contaminating will be avoided and the quality of the interface will be improved. The fabrication steps can also be significantly simplified. In this Letter, we demonstrate an enhancement mode ZnO-TFT with an MgO-gate dielectric layer,
prepared by the MOCVD technology. Both crystal qualities and electrical characteristics of the ZnO-TFT are measured and discussed in detail. S
D Al (100 nm)
G
ZnO (100 nm) MgO (200 nm) ITO Glass substrate
Fig. 1. Schematic diagram of the ZnO-TFT structure.
Figure 1 shows the device structure schematically. In the figure, glass substrates are applied and 170-nmthick indium tin oxide (ITO) films (8.2 ± 0.4 Ω·cm−2 ) are coated on the glass substrates. The TFT is then bottom gated and the ITO film serves as the gate electrode. Before depositing the MgO and ZnO layers, the ITO substrate was ultrasonically cleaned by sequential treatment with acetone, methanol and deionized water for 5 min, respectively. Bis(cyclopentadienyl) magnesium [(C5 H5 )2 Mg] was introduced into the reaction chamber using a high-purity (99.9999%) carrier Ar gas. High-purity O2 as oxygen source was introduced into the reaction chamber directly. The temperature of MgO growth process was 500∘ C. When the thickness of the MgO layer reached 200 nm, the MgO growth was terminated and the deposition of ZnO was started. ZnO was deposited on MgO by using diethyl zinc [Zn(C2 H5 )2 ] and high-purity O2 as forming gases. The substrate temperature was also changed to 400∘ C. The thickness of the ZnO active
* Supported
by the National Natural Science Foundation of China under Grant Nos 60877020 and 60976010. whom correspondence should be addressed. Email:
[email protected] c 2010 Chinese Physical Society and IOP Publishing Ltd ○ ** To
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CHIN. PHYS. LETT. Vol. 27, No. 12 (2010) 128504
diffraction peak corresponding to (111) plane of MgO films.[16]
Transmittance (%)
100
Type n
ITO(622)
ITO(440)
MgO(111)
ITO(400) ZnO(101)
1000 0 30
40
50
60
MgO
60 50
500
70
600
700
800
Wavelength (nm)
Fig. 3. The optical transmission spectrum of the ZnO/MgO and MgO thin films.
(mA)
0.3
(a) =0
=12 V
~ 12 V
10-1
step=2 V
0.2
=10 V
0.1
=8 V =6 V =0 V
0 5 10 15 20 25 (V)
(mA)
0.4
(b)
VDS=24V
10-2 10-3 10-4 -5 0
5 10 15 GS
(V)
Fig. 4. The output (a) and transfer (b) characteristics of the ZnO TFT.
4000
2000
ZnO/MgO
DS
5000
3000
70
0.0
ZnO(002)
ITO(222)
Intensity (arb. units)
6000
Carrier concentration (cm−3 ) 3.09 × 1017
80
400
Table 1. Electrical properties of the ZnO films. Hall mobility (cm2 V−1 s−1 ) 3.4
90
40
DS
layer was 100 nm, measured by a stylus profilometer. Aluminum was then evaporated on ZnO surface under temperature of about 700∘ C, patterned by a shadow mask to form the TFT’s source and drain electrodes. The channel between source/drain electrodes is designed into 𝑊/𝐿 = 1000 µm/50 µm. Lastly the device was annealed at 300∘ C for 2 min in N2 ambient to reduce the contact resistance. The ITO bottom gate contact was formed by wet etching to remove the MgO/ZnO film with HCl aqueous solution. The electrical properties of the ZnO films were measured by a Hall system (Accent HL5500) with the van der Pauw configuration. The electrical characteristics of the ZnO-TFT devices were measured using a KEITHLEY 4200 semiconductor characterization system at room temperature. The crystal structure and orientation of the ZnO/MgO layers were measured by x-ray diffraction(XRD) using a SIEMENSD 5005 xray diffractometer in a 𝜃 − 2𝜃 configuration with Cu 𝐾𝛼 radiation. The optical transmittance of the composite ZnO/MgO layer in visible region was measured by a Shimadzu UV/VIS 3100 PC double beam spectrophotometer. The Hall measurement results are listed in Table 1. The measurements are carried out at room temperature. It can be seen that the ZnO film exhibits n-type conductivity.
80
2 (deg)
Fig. 2. The XRD pattern of the TFT device.
Figure 2 shows the XRD pattern of the TFT device. The diffraction peak is centered at 34.5∘ , corresponding to the (002) plane of the ZnO films. It indicates that the films are c-axis oriented polycrystalline and with a hexagonal structure. The average crystallite size was about 30 nm, estimated by the fullwidth at half-maximum of the diffraction peak and the Scherrer formula.[15] From Fig. 2, we also find a weak
Figure 3 shows the optical transmittances of the composite ZnO/MgO layer. It is seen that the stacked ZnO/MgO layer has excellent optical transmission in the visible region. In order to investigate the optical loss of the MgO layer, the transmittances of the MgO film have also been measured. The average transmittances of both composite ZnO/MgO layer and single MgO layer are approximately 85% in the spectrum region of 400–800 nm, indicating that transmittance loss induced by the MgO layer can be neglected. The output and transfer characteristics of the MgO dielectric based ZnO TFT are shown in Fig. 4. As seen in Fig. 4(a), the source-to-drain 𝐼DS current changes as a function of the gate voltage 𝑉GS . Obviously, the device operates in enhancement mode, which is more preferable than depletion mode for the TFT applications. The ZnO/MgO-TFT exhibits hard saturation evidenced by the flatness of the 𝐼DS curves at high 𝑉DS region. Figure 4(b) shows the transfer curve measured at a fixed drain voltage of 24 V. The off-current is less than 45 nA, and the saturated 𝐼DS is 0.4 mA when 𝑉DS is 24 V. The calculated on/off current ratio is about 104 .
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CHIN. PHYS. LETT. Vol. 27, No. 12 (2010) 128504
The field effect mobility 𝜇FE can be obtained in the saturation region using the equation[17] 𝐼DS =
1 𝑊 𝜇𝐶OX (𝑉GS − 𝑉TH )2 , 2 𝐿
𝑉DS > 𝑉GS − 𝑉TH ,
where 𝑊 and 𝐿 are the channel width and length, respectively; 𝐶OX is the capacitance per unit area of the gate insulator; 𝑉DS and 𝑉GS are the drain-source voltage and gate-source voltage, respectively. The threshold voltage 𝑉TH and the field effect mobility 𝜇FE are determined by plotting the square root of drain current measured at 𝑉DS = 𝑉GS in the saturation region, as shown in Fig. 5. Threshold voltage 𝑉TH is 5.1 V, obtained in the figure. The calculated 𝜇FE is around 2.69 cm2 /V·s with the gate capacitance of a 200-nm MgO layer.
0.20 DS=
24 V
)
0.010
0.05
th
1/2
0.005
(A
1/2
0.10
D
(mA)
0.15
= 5.1 V
0.000
0.00 -5
0
5
10
GS
15
(V)
1/2
Fig. 5. The drain current 𝐼𝐷 and 𝐼𝐷 gate voltage 𝑉GS .
20
as functions of the
High performance ZnO-TFTs have been prepared with MgO as the gate dielectric layer grown by the MOCVD method. The uninterrupted growth method, as utilized in this study, not only simplifies the fabrication steps, but also achieves good crystal and optical qualities. As the gate dielectric, the MgO layer contributes to reducing the gate leakage current because of the large band offset of the ZnO channel
layer. Experimental results show that our ZnO-TFTs operate in enhancement mode with an on/off ratio about 104 . The field effect mobility is 2.69 cm2 /V·s and the threshold voltage is around 5.1 V. The electrical performances of ZnO/MgO-TFTs are expected to improve further by optimizing growth conditions.
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