Effect of Channel Layer Thickness on The Device Characteristics of Room Temperature Fabricated In2O3Thin-film Transistors

JIAO Yang，ZHANG Xin-an，ZHAI Jun-xia，YU Xian-kun，DING Ling-hong，ZHANG Wei-feng

(School of Physics and Electronics，Institute of Microsystem，Key Laboratory of Photovoltaic Materials，Henan University，Kaifeng 475004，China)

*Corresponding Author，E-mail:xinanzhang@henu.edu.cn

1 Introduction

During the recent years，transparent oxide semiconductors(TOSs)，such as ZnO，ZnMgO，SnO2，Ga2O3，In-Sn-O，In2O3，In-Ga-Zn-O［1-5］have been widely explored for their wide applications.They can be used as active channel layer for high performance thin-film transistors.TOS-TFTs can meet the needs of fast response，high light sensitivity and high definition flat panel display(FPDs)［6-7］，It can also improve the aperture ratio，the brightness and thus lower the power consumption.In2O3is one of potential candidates used as the active layer，because of its excellent optical transmission，high mobility，chemical stability，thermal stability，and smooth surface.In2O3channel layer in oxide TFTs is usually prepared by magnetron sputtering.

In this study，we investigate the characteristics of In2O3TFT with respect to the thickness of the channel layers，and try to find the optimum thickness of the In2O3channel layer.Difference thickness In2O3active layers in the range of 10～100 nm were deposited using DC magnetron sputtering at room temperature.It is found that the In2O3TFT with the optimized channel thickness exhibites excellent electrical performance.

2 Experiments

The In2O3TFTs with a bottom gate structure were fabricated on Si substrate with 300 nm thick SiO2formed via thermal oxidation.The bottom gate TFTs were prepared with the structure of Al(source and drain electrodes)/In2O3(channel layer)/SiO2(dielectric layer)/Si/In(gate electrode).The In2O3films were deposited on SiO2/Si substrates by DC magnetron sputtering，using a metallic indium target(4N)and keeping a distance of 70 mm between the target and substrates.The base pressure was less than 5×10-4Pa，the flow rate of Ar and O2were 10 and 30 cm3/min，respectively，and the total working pressure was 2 Pa.The sputtering power was kept at 38 W.The ratio of channel width to length was 600/100 μm，which was defined by a shadow mask.The thickness of In2O3films patterned by the first shadow mask varied as a function of the deposition time and was about 10，40，70，100 nm，respectively.Then，Al deposited as source and drain electrodes on the In2O3channel layer by DC magnetron sputtering through the second shadow mask.The schematic cross-sectional diagram of In2O3TFT was illustrated in Fig.1.

Fig.1 Schematic cross-sectional diagram of the In2O3TFT structure

The crystallization property of In2O3thin film was evaluated by X-ray diffraction(XRD，DX-2700)measurement in θ-2θ scan mode.The electrical transport measurements of In2O3TFTs were measured in air ambient using a microprobe station connected to a semiconductor parameter analyzer(Keithley 4200CS).

3 Results and Discussion

Fig.2 XRD spectra of the In2O3films with different thicknesses deposited at room temperature

Fig.2 shows the XRD spectra of the In2O3films with different thicknesses.For In2O3films with thicknesses of 10 nm and 40 nm，no X-ray peaks could be detected.For In2O3films with 70 nm thickness，slight broad peak at 2θ=30.29°is observed，indicating that the In2O3film began change to crystalline.When the thickness increased to 100 nm，dominant peak at 2θ=30.29°that corresponds to the(222)orientation of the In2O3cubic bixbyitc structure is observed，indicating formation of crystalline In2O3and preferential orientation of the(111)along the surface normal.

Fig.3 Output characteristics of In2O3TFTs with different channel thicknesses.(a)10 nm.(b)40 nm.(c)70nm.(d)100 nm.

Fig.3 shows the source-to-drain current(IDS)curves as a function of the source-to-drain voltage(VDS)for different gate voltages(VG)of In2O3TFTs with the channel thickness between 10 and 100 nm.The gate voltage(VG)is increased from 0 to 20 V in steps of 5 V.The In2O3TFT with the 10 nm thick channel layer shows in Fig.3(a)exhibited typical field effect transistor characteristics.The channel conductance increased conspicuously as VDSincreased under the positive VG.It is noted that the channel layer of the In2O3TFT is n-type，electron carriers are generated by the positive VG.The channel conductance curves showed a clear pinch-off and current saturation，implying that the entire thickness of the In2O3channel can be completely depleted of free electrons［8-9］.Moreover，this device operates in the enhancement mode indicating normally off characteristics are exhibited.In general，operation in the enhancement mode is preferable compared to depletion mode behavior because it is unnecessary to apply a gate voltage to switch off the transistor［10］.The In2O3TFT with the 40 nm thick channel layer exhibited the depletion mode as shown in Fig.3(b).It is noticed that the saturation IDSof about 240 μA was obtained and increased compared to that of the In2O3TFT with the 10 nm thick channel layer.The In2O3TFTs with the 70 and 100 nm thick channel layer shown in Fig.3(c，d)exhibited the high off-current due to the thicker channel layer.It indicates that this device had normally-on characteristics due to high electrical conductivity［11］.Consequently，the TFT channel thickness should be considered to control the electrical properties.

Fig.4 displays the IDS1/2-VGScharacteristics of the devices.The threshold voltage(VT)and saturation channel mobility can extract by fitting straight lines into the plots of the square root of drain current vs.gate voltage，according to the equation［12］:

Where Ciis the capacitance per unit area of the insulator layer，W and L are the channel width and length，VDSand VGSare the drain-source voltage and gate-source voltage，respectively.Here，we extracted the values of 6.2，14.9，30.4 cm2·V-1·s-1for devices with 10，40，70 nm In2O3channels，respectively.

Fig.4 Plot of I1D/S2vs.VGSof the TFTs at a fixed VDS=25 V with the thicknesses of 10 nm(a)，40 nm(b)，70 nm(c)，100 nm(d)，respectively.

Fig.5 shows the transfer characteristics of the In2O3TFTs.For the In2O3TFT with 10，40，70 nm thick channel layer shown in Fig.5(a)，(b)，(c)，the drain current curves indicate the off-state current is as low as about 1.0×10-10A.The channel is thin enough to allow the devices to be turnedoff by depleting the entire channel.The drain current on/off ratio calculated from the drain current at maximum and minimum values is estimated to be about 106.The sub-threshold voltage swing(S)is defined as the voltage required increasing the drain current by a factor of 10［13］.The smaller the S the easier it is to switch the transistor to an off state.From transfer characteristic，we can also determine the gate voltage swing，S，through the relation:

Fig.5 Transfer characteristics of In2O3TFTs with different channel thicknesses.(a)10 nm.(b)40 nm.(c)70 nm.(d)100 nm.

Here，we extracted the values of 2.7，5.3，3.2 V/dec for devices with 10，40，70 nm In2O3，respectively.For the In2O3TFT with the 100 nm thick channel layer shown in Fig.4(d)，the poor transfer characteristics with the higher off current was observed.It is considered that the grain boundaries are important electron scattering centers in oxide semiconductor.Thicker channel layer have greater grain size and decreased grain boundaries，so cause higher electron mobility and higher off current.Therefore，it is concluded that a reasonable channel thickness deposited at room temperature was found to be 10 nm for applying to the In2O3TFT，which operates in the enhancement mode with low subthreshold swing，high on/off ratio and low off current.

4 Conclusion

Bottom-gate structure In2O3thin-film transistors were fabricated by DC magnetron sputtering.The electrical characteristics of In2O3thin film transistors with the different channel thicknesses were investigated.The In2O3TFT with an appropriate channel thickness of 10 nm exhibited a good transistor performance with the field-effect mobility of 6.2 cm2·V-1·s-1，the threshold voltage of 2.5 V，the current on/off ratio of 106and the subthreshold swing of 4 V/dec.Further increasing the channel thickness exhibited depletion mode characteristics with a high off-current.These developments shall be useful to further advance the TFT technology for real applications.

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