Realization of In2O3 thin film transistors through reactive evaporation process

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Realization of In 2 O 3 thin film transistors through reactive evaporation process

Dhananjay and Chih-Wei Chu

Citation: Applied Physics Letters 91, 132111 (2007); doi: 10.1063/1.2789788

View online: http://dx.doi.org/10.1063/1.2789788

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/13?ver=pdfcov Published by the AIP Publishing

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Realization of In

₂

O

₃

thin film transistors through reactive evaporation

process

Dhananjay

Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 11529, Taiwan

Chih-Wei Chua兲

Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 11529, Taiwan and Department of Photonics, National Chiao-Tung University, Hsinchu 30013, Taiwan

共Received 11 July 2007; accepted 5 September 2007; published online 27 September 2007兲 In₂O₃ thin films have been grown by reactive evaporation of indium in ambient oxygen. The films were structurally characterized by x-ray diffraction共XRD兲 and atomic force microscopy techniques. The results of XRD revealed that the films were polycrystalline in nature with preferred 共222兲 orientation. The as-grown films were subjected to various annealing treatments to modulate the conductivity of the films for thin film transistors 共TFTs兲. TFTs fabricated on SiO2 gate dielectric

exhibited an on/off ratio of 104 _{and a field-effect mobility of 27 cm}2_{/ V s. High on-state current}

Physics. 关DOI:10.1063/1.2789788兴

Indium oxide 共In2O3兲 is a n-type oxide semiconductor

with a wide band gap共⬃3.6兲 at room temperature. Thin films of In₂O₃are superior to other transparent oxide counterparts such as ZnO, CdO, and SnO2, largely due to their high

mo-bility of 10– 75 cm2_V−1_s−1 _{at a carrier density of}

⬃1019_{– 10}20_cm−3_.1

It belongs to a complex cubic bixbyte structure with a lattice parameter a = 10.11 Å.2 The coexist-ence of high optical transmittance in the visible region and high electrical conductivity makes them potential candidate as active layers in thin film solar cells.3Moreover, the elec-trical property of this material could easily be controlled by simple heat treatments.4 Such a heat treatment changes the composition of the film due to the annihilation of the oxygen vacancy, thereby leading to stoichiometric films.

Due to the large mobility and low processing tempera-tures, transparent amorphous oxide semiconductors have re-ceived much attention in the current research field of thin film transistors. Combinatorial approach5 has been utilized by the earlier researchers to find a suitable material for the active channel layer, for example, In–Ga–Zn–O,6In–Zn–O,7 Zn–Sn–O,8 and In–Sn–O.9 However, the different vapor pressures of the individual element lead to the difficulty in tuning the optimum composition and the reproducibility. Hence, finding a best composition becomes difficult in such a multicomponent systems. According to the earlier research-ers, such a multicomponent system was necessary to reduce the electron background concentration and to enhance the channel resistivity. For example, to suppress the channel con-ductivity in In2O3, divalent Zn partially replaces In, i.e.,

90 wt % In2O3– 10 wt % ZnO and were reported in the

literature.10 Moreover, most of these multicomponent oxide TFTs reported in the literature were amorphous in nature. Perhaps it will be more interesting if undoped In2O3could be

used as a channel layer in its crystalline form. So the moti-vation of the present work is to utilize In2O3 as a channel

layer in its undoped form. In the present article, we demon-strate the fabrication of undoped In2O3 thin film transistors

without any external doping. This was feasible with a simple thermal evaporation technique in high purity oxygen ambient and the subsequent annealing treatments. In₂O₃thin films are shown to be promising candidates as channel material with high mobility for the TFTs.

The cross-sectional view of the device structure used in the present work is presented in Fig.1. Bottom gate TFTs were fabricated on heavily doped n-type silicon substrates with 300 nm of thermally oxidized silicon dioxide. A 20 nm In2O3 thin film was then deposited on SiO2 gate dielectric

through thermal evaporation process. Prior to the deposition, the chamber was pumped down to⬃10−6_{Torr. Indium and}

oxygen of high purity were used as evaporation source and reactive gas, respectively. The flow rate of the gas was con-trolled by means of a mass flow controller. Subsequently, deposition was carried out at a partial pressure of ⬃2 ⫻10−4_{Torr. The substrate temperature was fixed at 100 ° C.}

The deposition rate共0.2 Å/s兲 and thickness 共⬃20 nm兲 of the film were in situ monitored by means of quartz crystal moni-tor. The as-deposited films had a sheet resistance of 50 k⍀. The films were subjected to air annealing in the temperature range of 200– 300 ° C for 2 h to modulate the conductivity. Finally, silver source and drain electrodes were deposited onto the In2O3channel layer by thermal evaporation through

a兲_{Author to whom correspondence should be addressed. Electronic mail:}

[email protected] FIG. 1. 共Color online兲 Schematic diagram of the In₂O₃thin film transistor.

APPLIED PHYSICS LETTERS 91, 132111共2007兲

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a mechanical shadow mask. The typical channel length and width were 100 and 2000␮m, respectively. To ensure better source and drain contacts on the In₂O₃active channel layers, air annealing of the device was carried out at 200 ° C for 15 min. The structure details of the In2O3 thin films were

determined by x-ray diffraction using Cu K␣ radiation. Atomic force microscopy was used to get an insight into the surface morphology and roughness of the films. A semiquan-titative analysis of the film composition was carried out us-ing energy-dispersive analysis of x rays. Electrical character-ization of the film was carried out using an HP 4156 semiconductor parameter analyzer. Device testing was real-ized in a glovebox under N2environment.

Figure 2 shows the x-ray diffraction 共XRD兲 pattern of the In2O3 thin films annealed at two different temperatures,

namely, 200 and 300 ° C for 2 h. The semitransparent as-deposited films were found to be amorphous in nature and hence, not displayed in Fig. 2. The air annealing of the samples at 200 and 300 ° C induces crystallinity in the films and the films exhibited the preferred共222兲 orientation, con-firming bixbyite structure.11The full width at half maximum 共FWHM兲 of 共222兲 diffraction peak was 0.48° and 0.32° for the films annealed at 200 and 300 ° C, respectively. Such a small FWHM ensures the high quality of the films. More-over, the composition of the films was measured using the energy dispersive analysis of x rays and it was found that the ratio of O / In is well above the stoichiometric value 共=1.5兲共Ref.12兲 for In2O3, indicating excess of oxygen in the films.

Such a composition共In=27 and O=73 at. %兲 was desirable for getting the proper channel resistivity for the TFTs.

Atomic force microscopy 共AFM兲 image of the In₂O₃ thin films that were annealed at 200 and 300 ° C for the TFT fabrication are displayed in Figs.3共a兲and3共b兲, respectively. To compare the smoothness of the films annealed at different temperatures, their root-mean-square 共rms兲 roughness was analyzed. The films were atomically smooth and dense grain morphology was observed. The surface roughness plays a major role in the device performance and it has been reported

in the literature.13 It has been found that the active channels with minimal surface roughness leads to better device performance.14,15 The rms roughness of the films derived from AFM was around 1.4 and 4.2 nm for the films annealed at 200 and 300 ° C, respectively. It was found that the thin film transistors fabricated with In2O3 thin films grown at

300 ° C exhibited a large leakage current and hence poor on/off ratio共data not shown兲 compared to the films grown at 200 ° C. This is mainly due to the higher surface roughness of the samples grown at 300 ° C compared to the films grown at 200 ° C and earlier researchers have observed such a phe-nomena previously.14The rms roughness and the relative in-tensity of the共222兲 peak 共Fig.2兲 have shown a similar

varia-tion with annealing temperature and such phenomena were observed in Sn doped In2O3 systems by the earlier

researchers..16 Hence, the surface roughness plays a major role in the performance of the device and the annealing tem-perature of 200 ° C was found to be optimal for the better performance of the device. Moreover, increase in grain size with increase in annealing temperature was observed. In other words, there was a decrease in the grain boundary den-sity due to the agglomeration of the grains, which leads to decrease in grain boundary scattering of the carriers. This, in turn, results in increase in the conductivity. Hence, the over-all carrier concentration of the films increases drasticover-ally with the increased grain size and the channel becomes too conductive, thereby resulting in a poor on/off ratio and a large leakage current of the device. Hence, from the AFM investigations, it infers that air-annealing temperature of 200 ° C is desirable for the In₂O₃thin films for getting better performance of the device.

Figure 4 shows the representative drain-source current 共ID兲 as a function of drain to source voltage 共VDS兲 at various

gate voltages of the In2O3 based TFTs with SiO2 gate

insu-lators. Typical channel width共W兲 to length 共L兲 ratio used in the present study, i.e., W / L was 20. The ID-VDS curve

indi-cates that the drain current increases with increase in gate FIG. 2. 共Color online兲 X-ray diffraction pattern of the In2O3 thin films

annealed at different temperatures. FIG. 3.共Color online兲 AFM images of the In2O3thin films annealed for 2 h

at共a兲 200 °C and 共b兲 300 °C.

132111-2 Dhananjay and C.-W. Chu Appl. Phys. Lett. 91, 132111共2007兲

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voltage, exhibiting the apparent field effect. The transistor operates in n-type depletion mode and exhibits a well-defined pinch-off and saturation region. The channel resistiv-ity of⬃10 ⍀ cm was calculated from the linear slope of the output characteristics at zero gate voltage. Similar to most of the inorganic oxide TFTs, such as ZnO,17 the fabricated In2O3 TFTs obey the standard field-effect transistor theory,

with the Fermi level in the channel fully modulated by the gate and the drain voltages. Moreover, the device had a high output current of the order of milliamperes, which makes it a promising candidate for devices such as smart cards, electric motors and light emitting diodes.

Figure5shows the variation of IDSas a function of VGS

at a fixed drain voltage. The typical on/off ratio of 104 _was

estimated. The threshold voltage and field-effect mobility in the saturated regime were estimated using the following re-lation:

ID=

WCi␮FE

2L 共VGS− VT兲

2_, _共1兲

where Ciis the capacitance per unit area of the gate

dielec-tric, VT is the threshold voltage, and ␮FE is the field-effect

mobility. The threshold voltage and the field effect mobility computed from the x-axis intercept of the square root of ID

versus VGS plot and the slope of the plot were found to be

10.5 V and 27 cm2_{/ V / s, respectively. The obtained value of}

mobility was one order magnitude order higher than those obtained for indium-zinc-oxide systems reported in the literature.18This could be mainly attributed to共i兲 the crystal-line phase of the In2O3 thin films used in the present study

and共ii兲 reduced scattering centers for the carriers across the drain and source, since there are no external dopants in un-doped In2O3 films.

In conclusion, we demonstrate that In₂O₃thin films form a promising n-channel material for transparent TFTs. The devices were fabricated at low substrate temperature by re-active evaporation technique. The variation of surface rough-ness with annealing temperature was analyzed and it was observed that the film with minimal surface roughness is desirable for the TFTs. Transistors utilizing SiO2gate

dielec-tric demonstrate good operating characteristics with an on/ off ratio of 104_{, a high field-effect mobility, and an output}

current. Thus, In2O3 seems to be a viable active channel

material for the emerging field of transparent thin film tran-sistors.

The authors are grateful to the National Science Council 共NSC兲, Taiwan 共96-2221-E-001-017-MY2兲 and the Thematic Project of Academia Sinica, Taiwan for financial support.

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FIG. 5. Source to drain current as a function of gate voltage共V_GS兲 at a fixed drain voltage of 70 V.

132111-3 Dhananjay and C.-W. Chu Appl. Phys. Lett. 91, 132111共2007兲