IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 2, FEBRUARY 2013
Transparent Junctionless Thin-Film Transistors With Tunable Operation Mode Gengming Zhang, Qing Wan, Jia Sun, Guodong Wu, and Liqiang Zhu
Abstract—Junctionless low-voltage transparent indium-zincoxide (IZO) thin-film transistors (TFTs) gated by SiO2 -based solid electrolyte films are fabricated on glass substrates by a full room-temperature process. The attractive feature of such TFTs is that the channel and source/drain electrodes are the same ultrathin IZO film without any source/drain electrodes. The operation mode of such devices can be tuned from depletion mode to enhancement mode when the thickness of the IZO film is reduced from 30 to 10 nm. Devices operated in both modes show a small subthreshold swing of < 120 mV/dec and a large current on/off ratio of > 106 . Index Terms—Indium-zinc-oxide (IZO), junctionless transparent transistors, operation mode modulation.
I. I NTRODUCTION
ECENTLY, novel junctionless nanowire transistors have been proposed, in which all of the channel and source/drain electrodes are shared by a single heavily doped Si nanowire without any source/drain junction formation , . However, the fabrication process of such a device usually needs sophisticated photolithography for patterning the nanowire . Oxide-based thin-film transistors (TFTs) have recently attracted a great deal of attention due to their wide applications. However, the fabrication of oxide-based TFTs with junctionless structure is rarely reported. Transparent indiumzinc-oxide (IZO)-based homojunction TFTs with an IZO active layer and IZO source/drain electrodes were reported, but twostep sputtering deposition and two photolithography processes were needed , . Recently, Ahn et al. reported the fabrication of IGZO-based junctionless TFTs by a selective H2 plasma treatment of the source/drain area . However, complicated multistep photolithography process was still adopted. In our previous reports, we developed a self-assembled sputtering method for low-voltage oxide-based homojunction TFT fabrication , . At the same time, IZO film was demonstrated to be a good candidate for the active channel layer Manuscript received October 31, 2012; revised November 24, 2012; accepted December 2, 2012. Date of publication January 9, 2013; date of current version January 23, 2013. This work was supported in part by the National Program on Key Basic Research Project under Grant 2012CB933004, by the National Natural Science Foundation of China under Grant 11174300, and by the Fok Ying Tung Education Foundation under Grant 121063. The review of this letter was arranged by Editor S. J. Koester. The authors are with the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China, and also with the Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China (e-mail: [email protected]
nimte.ac.cn). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2012.2232277
because of its high electronic mobility –. In this letter, fully transparent IZO-based junctionless TFT arrays in which the channel and source/drain electrodes are based on the same IZO film are fabricated on glass substrates at room temperature with only one shadow mask. Moreover, the operation mode of such junctionless TFTs can be tuned from depletion mode to enhancement mode by reducing the IZO film thickness from 30 to 10 nm. II. E XPERIMENTAL D ETAILS The entire fabrication process of such junctionless devices was performed at room temperature. First, SiO2 -based solid electrolyte films with a thickness of ∼2.0 μm were deposited on conducting ITO glass substrates by plasma-enhanced chemical vapor deposition method. Then, patterned IZO films with different thicknesses (10, 20, 30, and 60 nm) were deposited by radiofrequency magnetron sputtering with a metal shadow mask. The length and width of the patterned IZO films are 1000 and 150 μm, respectively. The structure and surface roughness of the room-temperature deposited IZO films were investigated by X-ray diffractometer (Bruker AXS D8 Advance) and SPM (Veeco Dimension3100V). The optical transmittance of the TFT arrays on glass substrates was investigated by a spectrometer (Perkin Elmer Lambda 950). The electrical characterizations are performed by an impedance analyzer (Solartron 1260) and a semiconductor parameter analyzer (Keithley 4200 SCS) at room temperature with a relative humidity of 60%. For transfer curve measurements, two tungsten (W) probes are directly connected to the same patterned IZO film with a probe distance of 250 μm. III. R ESULTS AND D ISCUSSION Fig. 1(a) shows the schematic image of the IZO-based junctionless TFT arrays on glass substrates. Fig. 1(b) shows the optical transmittance spectra in the wavelength range from 200 to 1000 nm. An average transmittance of 82% was obtained in the visible region (380–800 nm). The inset is an optical image of the junctionless transparent TFT arrays, and the background is clearly visible. Fig. 2(a) shows the XRD pattern of the IZO film, and the inset is the surface AFM image. XRD results indicate that the IZO film deposited at room temperature is amorphous, which is favorable for the high-mobility TFT channel layer . The average surface roughness (rms, the root-mean-square roughness) is 2.2 nm (2 μm × 2 μm). Fig. 2(b) shows the frequency-dependent specific gate capacitance and phase angle (θ(f )) of the SiO2 -based solid electrolyte film. A maximal
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IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 2, FEBRUARY 2013
Fig. 1. (a) Schematic image of the IZO-based junctionless TFT arrays on glass substrates. (b) Optical transmittance spectra of the junctionless transparent TFT arrays. Fig. 3. (a and b) Transfer characteristics (Ids –Vg ) of the junctionless IZObased TFTs at Vds = 2.0 V with different film thicknesses.
Fig. 2. (a) XRD pattern of the IZO film deposited at room temperature. The inset is the surface AFM image of the IZO film. (b) Frequency-dependent specific gate capacitance and phase angle (θ(f )) of the SiO2 -based solid electrolyte film in the frequency range (1.0 Hz–1.0 MHz).
capacitance ∼6.5 μF/cm2 was measured at 1.0 Hz. The main contribution to the large capacitance at low frequencies is due to the formation of the electric double layer (EDL) at the SiO2 electrolyte/electrode interfaces , . The phase angle curve indicates a more resistive character at high frequencies
[f > 100 kHz, θ(f ) > −45◦ ], but it presents a more capacitive behavior at low frequencies [f < 100 kHz, θ(f ) < −45◦ ]. Fig. 3(a) shows the transfer characteristics (Ids –Vgs , Vds = 2.0 V) of IZO-based junctionless TFTs with different film thicknesses. When the IZO film thickness is 60 nm, the drain current is too large to be switched off. All TFTs with IZO film thickness of not thicker than 30 nm can be fully switched off. The subthreshold swing (SS) is estimated to be 120 mV/dec for device A, 90 mV/dec for device B, and 80 mV/dec for device C, respectively. The drain current on/off ratio is calculated to be 1.1 × 106 , 6.2 × 106 , and 2.2 × 107 for devices A, B, and C, respectively. A thinner channel is favorable for small SS and large current on/off ratio, which is in good agreement with the results based on In2 O3 TFTs reported by Noh et al. . By the way, small counterclockwise hysteresis windows of less than 0.2 V are observed in all transfer curves due to the mobile protons in the SiO2 electrolyte gate dielectrics. Fig. 3(b) shows the Ids –Vg curves at Vds = 2.0 V of devices A, B, and C in square root scale. The Vth values of devices A, B, and C are estimated to be −0.8, −0.10, and 0.19 V, respectively. Therefore, device A with a 30-nm IZO channel is operated in depletion mode, and device C with a 10-nm IZO channel is operated in enhancement mode. Based on the equation in the saturation region (VDS > VGS − VTH ), Ids = W Ci μ/2L(Vgs − Vth )2 , where W/L is the width/length ratio (150 μm/250 μm = 0.6), in the case of our device measurement, and Ci is the gate capacitance per unit area (6.5 μF/cm2 ); then, the field-effect mobility (μ) is calculated to be 51, 57, and 65 cm2 /V · s for devices A, B, and C, respectively. Because the source/drain geometry is not well defined and the EDL gate capacitance still changes with
ZHANG et al.: TRANSPARENT JUNCTIONLESS THIN-FILM TRANSISTORS WITH TUNABLE OPERATION MODE
obtained. Such junctionless transparent low-voltage TFTs has potential applications in low-cost portable sensors. R EFERENCES
Fig. 4. Drain current (device A) in response to the square-wave gate voltage with Vg pulses of −2.0 and 1.0 V at Vds = 2.0 V.
frequency at low frequencies, therefore, the calculated mobility has additional uncertainty and may be overestimated. It is well known that the adsorbed oxygen can capture an electron from the conduction band and result in channel surface depletion . At the same time, mobile protons in the SiO2 electrolyte film can partly deplete the IZO channel too. When IZO thickness is larger than the total thickness of the two depletion layers, the channel is only partly depleted under zero gate bias, and the device will operate in a depletion mode. When the channel layer thickness is reduced to be 10 nm, the channel layer is almost completely depleted under zero gate bias, and enhancement mode operation can be obtained, as proved by our experimental results. In order to investigate the switching reproducibility and stability, the drain current of the TFTs with a 30-nm IZO film in response to the square-wave gate voltage was measured. Fig. 4 shows the Ids with Vg pulses of −2.0 and 1.0 V at VDS = 2.0 V. An on current of ∼300 μA and an on/off current ratio of ∼1.5 × 106 are obtained and nearly unchanged after 20 repeating pulse scanning, which indicates that electrostatic modulation rather than electrochemical mechanism is dominant for such transparent junctionless device operation. IV. C ONCLUSION In conclusion, low-voltage ( 2.0 V) transparent junctionless IZO-based TFT arrays were fabricated on conducting glass substrates at room temperature. Both enhancement and depletion modes were realized by changing the thickness of the deposited IZO film layer. High performance with a small SS (< 0.2 V/dec) and a large current on/off ratio (> 106 ) was
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