High-Performance InTiZnO Thin-Film Transistors Deposited by ...

ISSN: 0256 - 307 X

中国物理快报

Chinese Physics Letters

Volume 30 Number 12 December 2013

A Series Journal of the Chinese Physical Society Distributed by IOP Publishing Online: http://iopscience.iop.org/0256-307X http://cpl.iphy.ac.cn

C HINESE P HYSICAL S OCIET Y Institute of Physics PUBLISHING

CHIN. PHYS. LETT. Vol. 30, No. 12 (2013) 127301

High-Performance InTiZnO Thin-Film Transistors Deposited by Magnetron Sputtering * LIU Ao(刘奥)1,2 , LIU Guo-Xia(刘国侠)1,2 , SHAN Fu-Kai(单福凯)1,2,3** , ZHU Hui-Hui(朱慧慧)1,2 , B. C. Shin3** , W. J. Lee3 , C. R. Cho4 1

2

College of Physics Science, Qingdao University, Qingdao 266071 Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071 3 Electronic Ceramics Center, DongEui University, Busan 614714, South Korea 4 College of Nanoscience and Nanotechnology, Pusan National University, Busan 609735, South Korea

(Received 15 April 2013) InTiZnO thin-film transistors (ITZO TFTs) with Al2 O3 gate dielectrics are fabricated by magnetron sputtering at room temperature. The bottom-gate-type ITZO TFTs with amorphous Al2 O3 gate dielectrics are operated in the enhancement mode and exhibit a mobility of 50.4 cm2 /V·s, threshold voltage of 1.2 V, subthreshold swing of 94.5 mV/decade, and on/off-current ratio of 7 × 106 . We believe that ITZO deposited at room temperature is an appropriate semiconductor material to produce high-mobility TFTs for developing flexible electronic devices.

PACS: 73.61.Ng, 73.40.Qv, 78.40.Fy

DOI: 10.1088/0256-307X/30/12/127301

Interest in thin-film transistors (TFTs) based on oxide semiconductors has been increasing owing to their unique properties such as high electron mobility, high optical transparency, low temperature, and low-cost fabrication processes.[1−3] Among these materials, InGaZnO (IGZO) has been studied most extensively because of its high performance. However, recent studies have pointed out their inherent problem of device instability, which deteriorates their electrical characteristics.[4−7] Light exposure and bias stressing could lead to device instabilities such as charge trapping and defect formation in the semiconductor layer or at the semiconductor-insulator interface. To address such issues, several research groups have explored the possibility of employing alternative elements (particularly, IV–B elements: Ti, Zr, and Hf) in place of Ga.[8−10] This incorporated element plays an important role in the suppression of carrier generation and improves the device stability by controlling oxygen vacancies. Among these candidates, TiO2 is considered an attractive choice for TFT channels because of its high mobility and similarity to ZnO in terms of the energy band gap.[11,12] In addition, Ti has a lower standard electrode potential of −0.86 V than −0.52 V for Ga. This indicates that Ti is a stronger suppressor of oxygen deficiencies generation than Ga originating from the shallow donor level in the subgap density of states of amorphous oxide thin films.[13] Thus, the InTiZnO (ITZO) channel layer is a more attractive material than IGZO for realizing high-performance and high-stability TFTs. In order to obtain a high on-current at low bias

voltage, it is crucial to improve the capacitive coupling between the dielectric and channel layers. These high𝑘 dielectric materials are thought to address this requirement; at the same time, the thickness of the gate dielectric layer must also be reduced. Among the wellknown high-𝑘 materials, amorphous aluminum oxide (Al2 O3 ) is a very promising candidate due to its high dielectric constants (𝑘 = 7.0–9.0) and large bandgap (𝐸𝑔 = 8.45–9.9 eV), which are significantly better than those of SiO2 (𝑘 = 3.9, 𝐸𝑔 = 8.0–8.9).[14] In this study, we fabricate TFTs using amorphous ITZO as the channel layer and Al2 O3 as the gate dielectric layer. The high field-effect mobility and low subthreshold slope are obtained in our devices, which are much higher than that of the ITZO-TFTs with traditional thermal-oxidized SiO2 gate dielectric.[8] ITZO TFTs were fabricated on a heavily doped ptype Si substrate (resistivity ∼0.0015 Ω·cm) with an 80-nm-thick Al2 O3 dielectric layer, which is shown schematically in Fig. 1. The substrate was first cleaned using the standard process of washing in an ultrasonic bath with acetone and methanol, followed by deionized water. An Al2 O3 dielectric layer was deposited on the Si substrate at room temperature (RT) using pulsed laser deposition (PLD). During deposition, an Al2 O3 ceramic target was used to produce a plasma plume. An excimer laser (248 nm) was used to ablate the ceramic target with a repetition rate of 5 Hz and an energy density of 1 J/cm2 . The ITZO channel layer was fabricated by cosputtering a ZTO (10mol% TiO2 -doped ZnO) target and an In2 O3 target by an rf magnetron sputtering

* Supported by the Research Fund of DongEui University (2012AA189), and the Natural Science Foundation of Shandong Province under Grant Nos ZR2011FM010 and ZR2012FM020. ** Corresponding authors. Email: [email protected]; [email protected] © 2013 Chinese Physical Society and IOP Publishing Ltd

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technique. Both targets were 99.99% pure. The experimental conditions were as follows: The rf power densities for both targets were kept at 4.5 W/cm2 , the working pressure was 0.5 Pa, the deposition time was 20 min, and the gas flow rate for Ar and O2 were 60 and 3 SCCM, respectively. Under these conditions, the thickness of the ITZO channel layer was approximately 40 nm. The ITO source and drain electrodes, with a thickness of 100 nm, were deposited on the channel layer by ion beam sputtering, and the length and width of the channel were 250 and 1000 µm, respectively.

ITO

ITO

ITZO Al2O3

measured in the wavelength range 200–800 nm. It was found that the average transmittances of ITZO and Al2 O3 films are over 80% in the visible range (400– 700 nm). This result indicates that the ITZO-TFT device is fully transparent in the visible range. The band gap of the semiconductor channel is an important parameter in the development of TFTs. The electrical properties of the channel layer are sensitive to photons having energy values larger than its band gap. The band gap of the ITZO thin film was extracted by plotting (𝛼ℎ𝜈)2 against the photon energy ℎ𝜈 and extrapolating to the energy axis. The inset of Fig. 2 shows that the value of the calculated optical band gap is approximately 3.92 eV. This value is much larger than the photon energy of visible light; thus the ITZO TFT is not sensitive to visible light. RMS=0.93 nm

Silicon

(a)

RMS=0.53 nm

(b)

Cu gate

Fig. 1. Schematic diagram of ITZO TFT with Al2 O3 gate dielectric. 500 nm

100

500 nm

Al2O3

Fig. 3. AFM images of (a) Al2 O3 thin film and (b) ITZO thin film on sapphire substrates.

=3.92 eV

2

60

(arb. units)

ITZO

h

)

40

(

Transmittance (%)

80

20 3.4

3.6

3.8

4.0

Energy (eV) 0

300

400

500

600

700

800

Wavelength (nm)

Fig. 2. Transmittances of ITZO and Al2 O3 thin films on sapphire substrates. Inset: calculation method for the band gap of an ITZO film.

The transmittances of the as-grown ITZO and Al2 O3 thin films on sapphire substrates were investigated by UV-Vis spectrophotometry (UV-2550, Japan). The surface morphologies were investigated by atomic force microscopy (AFM, Seiko, SPA400). To evaluate the electrical properties of the ITZO TFTs, the capacitance of the dielectric layer in the structure Al/Al2 O3 /FTO was examined, and the capacitance-voltage (𝐶–𝑉 ) characteristic of the Al2 O3 dielectric layer was measured at 1 MHz by an impedance analyzer (Agilent 4294 A). The electrical properties of the device were measured in a dark box by a semiconductor parameter analyzer (Keithley 2612 A). Figure 2 shows the transmittances of ITZO and Al2 O3 thin films on sapphire substrates, which were

The surface morphologies of the Al2 O3 and ITZO thin films are shown in Figs. 3(a) and 3(b), respectively. The scanning areas were 2 µm × 2 µm. It was found that the Al2 O3 thin film deposited by PLD has a smooth surface with an rms roughness of 0.93 nm, and the thin film does not contain any pin holes. This smooth surface of the dielectric layer plays an important role in realizing high-performance TFTs, which can effectively reduce carrier scattering and obtain high field-effect mobility. In addition, the ITZO channel layer deposited by magnetron sputtering has a smoother surface with an rms roughness of 0.53 nm. The small surface roughness is mainly related to the amorphous nature of ITZO thin film fabricated at RT and will definitely benefit the operation of the ITZO TFTs based on Al2 O3 dielectrics. To measure the dielectric properties of the Al2 O3 thin film, circular-top Al electrodes with a diameter of 1 mm were deposited on the Al2 O3 layer through a shadow mask. The unit-area capacitance of the Al/Al2 O3 /FTO structure was measured to be 59.8 nF/cm2 at a frequency of 1 MHz, as shown in Fig. 4. This value is much higher than that of thermal-oxidized SiO2 (𝐶SiO2 = 6.9 nF/cm2 ).[15] The dielectric constant was calculated from the equation 𝐶𝑖 = 𝜀0 𝜀𝑟 /𝑑, where 𝐶𝑖 is the capacitance per area, 𝜀0 is the permittivity of vacuum, and 𝑑 is the thickness of the dielectric layer. The dielectric constant was calculated to be approximately 9.1, which is similar to

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=1 MHz

2

Areal capacitance (nF/cm )

59.6

Leakage current (A)

the previously reported value.[16] The inset of Fig. 4 shows the leakage current of the Al2 O3 thin film; a leakage current of 10 nA at 2 V was obtained. These results indicate that the Al2 O3 thin film deposited by PLD could be applied as the dielectric layer for high performance oxide TFTs.

59.4 59.2

10-7 10-8 10-9 10-10 10-11 10-12 0 1 2 3 4 5 Voltage (V)

59.0 58.8

-4

-2

0

Voltage (V)

2

4

DS

(A)

Fig. 4. Typical 𝐶–𝑉 characteristics of Al2 O3 thin film with structure Al/Al2 O3 /FTO. Inset: the leakage current of the Al2 O3 thin film.

10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10

DS=0.5

V

DS=5.0

V

TH=1.2

V 2

FE

cm /Vs

SS=94.5 mV/dec

𝐿 is the channel length. The calculated 𝜇FE value is as high as 50.4 cm2 /V·s. The 𝜇FE obtained in this study is much higher than those ITZO TFTs based on other dielectrics[8,17] and is also much higher than those TFTs based on Al2 O3 dielectrics in previous reports.[14,18] One of the main advantages exhibited by the as-fabricated TFTs lies in the magnitude of the electron channel mobility leading to higher drive currents and fast device operating speeds. In the saturation region, the threshold voltage (𝑉TH ), which was extracted from the square-root plot of the drain current, was calculated to be 1.2 V. This indicates that the TFT operates in fully enhanced mode. An on/off current ratio (𝐼on /𝐼off ) of 7 × 106 with a high on-current of 1 mA was also obtained. Thus, the as-fabricated ITZO/Al2 O3 TFT is a good candidate to work as the driven switch in OLEDs. In addition, if we want to turn on the transistor quickly, we must obtain a small subthreshold swing (SS) value. The SS value, which is defined as the 𝑉DS required to increase the 𝐼DS by one decade, was calculated to be 94.5 mV/dec in the ITZO/Al2 O3 TFT. The small value of SS was attributed to both the high gate capacitance density and the good interface charge density.[17] This result indicates that the slopes in the subthreshold region are very steep, suggesting that the ITZO/Al2 O3 TFT exhibits excellent switching characteristics at low operating voltage. From SS we can infer the maximum area density of states (𝑁Smax ) at the interface between the ITZO channel layer and the Al2 O3 dielectric by using the following equation: 𝑁Smax =

-2

0

2

GS

(V)

4

6

Fig. 5. Typical transfer characteristics of ITZO TFT based on Al2 O3 gate dielectric.

In order to validate the usefulness of a high-𝑘 Al2 O3 thin film as the gate dielectric for ITZO TFTs, devices with a top contact electrode architecture were fabricated. The corresponding transfer curves were measured in the linear and saturation regions, as shown in Fig. 5. It is found that the ITZO TFT exhibits clear pinch-off and a desirable saturation behavior. No current crowding was observed, which indicates good ohmic contact between the ITZO channel and the ITO source/drain electrodes. The field-effect mobility 𝜇FE in the saturation regime √︀ (𝑉DS > 𝑉GS − 𝑉TH ) was calculated by fitting the |𝐼DS | vs 𝑉GS data to the square law: 𝐼DS =

𝑊 𝜇 𝐶𝑖 (𝑉GS − 𝑉TH )2 , 2𝐿 FE

(1)

where 𝐶𝑖 = 59.8 nF/cm2 , 𝑊 is the channel width, and

[︁ SS × log 𝑒 𝑘𝑇 /𝑞

−1

]︁ 𝐶

𝑖

𝑞

,

(2)

where 𝑘 is the Boltzmann constant, 𝑞 is the electron charge, 𝑒 is the base of natural logarithm. By considering the value of 𝐶𝑖 , an 𝑁Smax value of 6.5×1011 cm−2 is obtained for the ITZO/Al2 O3 interface. However, this value is much lower than that of Al2 O3 -based TFTs utilizing other methods such as CVD,[18] ALD,[20] and anodic process.[21] Moreover, the hysteresis behavior of the transfer characteristics of ITZO TFTs was investigated by sweeping 𝑉DS in both the forward and reverse directions, as shown in Fig. 6. It is found that a positive shift of 2.0 V in 𝑉TH was obtained. This clockwise hysteresis (i.e., positive hysteresis window, ∆𝑉TH = 𝑉TH, Reverse − 𝑉TH, Forward ) phenomenon suggests that negative charge carriers are either trapped at the interface between the channel and the insulator, or injected into the insulator from the channel.[22] The trapped charge density (𝑁𝑇 ) was estimated to be 7.5×1011 cm−2 using the equation 𝑁𝑇 = 𝐶 × ∆𝑉TH /𝑞, which is in the same order of magnitude of 𝑁Smax . The trap states greatly influence the performance of the TFTs and small trap state density between the chan-

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DS

(A)

nel/insulator interface could increase the 𝜇FE and decrease the 𝑉TH and SS. These improved characteristics are believed to arise from the application of a high-𝑘 dielectric (such as Al2 O3 ) and the smooth interface between the ITZO and Al2 O3 (as observed in the AFM investigation).

10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10

DS=0.5

V

TH=2.0

V

Sweep direction Forward Reverse

-2

0

2

GS

(V)

4

6

Fig. 6. Forward and reverse transfer characteristics of ITZO TFTs at a 𝑉DS of 0.5 V.

In summary, the high-performance ITZO-TFT with a high-𝑘 Al2 O3 gate dielectric has been realized at room temperature. The as-fabricated TFTs show excellent characteristics such as a high 𝜇FE of 50.4 cm2 /V·s, small SS value of 94.5 mV/decade, low 𝑉TH of 1.2 V, and 𝐼on /𝐼off ratio of 7 × 106 . The high capacitance of Al2 O3 dielectric, smooth insulator surface, and small trap density near the channel/insulator interface are the primary reasons for the high-performance ITZO/Al2 O3 TFT in this work. This result demonstrates that the ITZO/Al2 O3 TFT will be an ideal candidate for the commercial production of TFT sheets toward the development of future flexible electronic devices.

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Chinese Physics Letters Volume 30

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December 2013

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