Effects of Growth Temperature and Annealing on Propertiesof Zn3Sn2O7 Thin Films and Application in TFTs

YONG-YUE CHEN,1 XI-KUN CAI,1 ZHEN-YU YE,1 XIONG WANG,1

BING-PO ZHANG,1 and HUI-ZHEN WU1,2

1.—Department of Physics, State Key Laboratory of Silicon Materials, Zhejiang University,Hangzhou 310027, People’s Republic of China. 2.—e-mail: hzwu@zju.edu.cn

The effects of growth temperature and annealing on the physical properties ofZn3Sn2O7 thin films were investigated in this work. The Zn3Sn2O7 thin filmswere deposited on glass substrates by radio frequency (rf) magnetron sput-tering. It is found that the films are amorphous regardless of the growthtemperature. The film grown at room temperature shows the highest mobilityof 8.1 cm2 V�1 s�1 and the lowest carrier concentration of 2.0 9 1015 cm�3.The highest carrier concentration of 1.6 9 1019 cm�3 is obtained at the growthtemperature of 250�C. Annealing treatment of the Zn3Sn2O7 thin filmsresulted in increases of carrier concentration and mobility. The averagetransmittance of the as-deposited and annealed films reaches 80%. By using aZn3Sn2O7 thin film as the channel and a Ta2O5 thin film as the insulatinglayer, we fabricated transparent Zn3Sn2O7 thin-film transistors with field-effect mobility of 21.2 cm2 V�1 s�1, on/off current ratio of 105, threshold volt-age of 0.8 V, and subthreshold swing of 0.8 V/decade.

Key words: XRD, electrical properties, optical properties, annealing,thin-film transistor (TFT)

INTRODUCTION

Thin-film transistors (TFTs) have been widelyused in many fields, such as flat-panel displays,computers, and sensors. TFTs based on transparentconducting oxides (TCOs) such as ZnO,1 SnO2,2 andmulticomponent alloy oxides such as Zn-Mg-O,3

Zn-Sn-O,4 Zn-In-O,5 and In-Ga-Zn-O6 haveattracted much attention in recent years because oftheir high electron mobility and high optical trans-parency in the visible region compared with amor-phous silicon. Among these oxides, Zn3Sn2O7 is apromising channel material for high-performanceTFTs. The advantages of Zn3Sn2O7 include its lowsensitivity to visible light, physical robustness, andextremely high resistance to scratching, as well asthe abundance of Zn and Sn on Earth.7 Zn3Sn2O7

thin films have been realized by many methods,such as spray pyrolysis, the sol–gel method, pulsed

laser deposition, sputtering, and filtered vacuumarc deposition. In this work, Zn3Sn2O7 thin filmswere deposited by rf magnetron sputtering using analloy target. The effects of growth temperature andannealing on the structural, electrical, and opticalproperties of Zn3Sn2O7 thin films were investigated.We also fabricated fully transparent Zn3Sn2O7 TFTdevices on glass substrates using a Zn3Sn2O7 film asthe channel and a Ta2O5 film as the insulatinglayer. The TFT devices exhibit good satura-tion characteristics and gate voltage control.

EXPERIMENTAL PROCEDURES

Zn3Sn2O7 Film

Zn3Sn2O7 thin films were deposited on glasssubstrates by rf magnetron sputtering using analloy target of 50 wt.%ZnO-50 wt.%SnO2. High-purity (99.999%) argon was used as sputtering gasin the deposition process. The flow rate of Ar gaswas 20 sccm, and the working pressure was 3.0 Pa.The target power was fixed at 300 W. The glass

(Received November 14, 2012; accepted March 21, 2013;published online April 30, 2013)

Journal of ELECTRONIC MATERIALS, Vol. 42, No. 8, 2013

DOI: 10.1007/s11664-013-2589-9� 2013 TMS

2459

substrates were cleaned in an ultrasonic bath withacetone, alcohol, and deionized water, and thendried by blowing high-purity nitrogen gas. Thechamber base pressure was approximately6.5 9 10�5 Pa, and the target was presputtered for20 min before deposition was initiated. As shown inTable I, the films were deposited at room tempera-ture (RT), 55�C, 100�C, 160�C, 200�C, and 250�C,respectively. The RT-deposited samples were thenannealed at 55�C, 100�C, 160�C, and 250�C in vac-uum for 30 min, respectively. The growth tempera-tures given in this paper have been calibrated totemperatures of the growth surface.

The structural properties of the Zn3Sn2O7

thin films were analyzed by a D/max-rA x-raydiffractometer (XRD) using Cu Ka radiation(k = 1.5406 A). The optical transmittance was mea-sured by a UV-3150 spectrophotometer. Filmthicknesses were measured by an XP-1 stylus pro-filometer. The resistivity, electron mobility, andcarrier concentration of the films were obtained bymeasurement of the Hall effect using the van derPauw configuration with a HL5500 apparatus inwhich indium electrodes were used for ohmic con-tacts.

Zn3Sn2O7 TFT Fabrication

Figure 1 shows a schematic diagram of theZn3Sn2O7 TFT structure. We fabricated theZn3Sn2O7 TFT devices on glass substrates using aZn3Sn2O7 film as the channel and a Ta2O5(200 nm)film as the gate insulator, which is a bottom-gateTFT structure. The procedure for TFT fabricationwas as follows: First, an indium tin oxide (60 nm)film as the optically transparent and electricallyconductive electrode of the gate was deposited on aglass substrate by rf magnetron sputtering andsubsequently patterned by photolithography andwet etching. Next, a Ta2O5 insulating layer(200 nm) deposited by rf magnetron sputtering waspatterned by a lift-off process. Then, a Zn3Sn2O7

film (60 nm) as the channel was deposited by rfmagnetron sputtering and patterned by photoli-thography. Finally, an In2O3 thin film (200 nm) assource and drain electrodes was deposited by rfmagnetron sputtering and patterned by a lift-offprocess. Thus, the transparent Zn3Sn2O7 TFTdevices were fabricated. The output and transfercharacteristics of the Zn3Sn2O7 TFTs were mea-sured by a Keithley 4200 semiconductor character-ization system.

RESULTS AND DISCUSSION

Structural Properties

Figure 2a shows the XRD patterns for three typ-ical Zn3Sn2O7 thin-film samples deposited atRT and 250�C, and the film deposited at RT thenannealed at 250�C in vacuum. The films eitherdeposited or annealed at different temperatures areall amorphous. In the XRD results, the broad peakslocated between 2h values of 20� and 30� arisefrom diffraction from the glass substrate. Similarresults have been reported in previously publishedpapers.8–10 Tadatsugu Minami et al. reported thatZn3Sn2O7 films prepared at 100�C and 200�C withdifferent Sn contents [Sn/(Zn + Sn) atomic ratio]varying from �20 at.% to 80 at.% were amor-phous.11 Young et al. reported that polycrystallinefilms could be prepared at substrate temperaturesabove 550�C.8 Figure 2b shows the energy-dispersivex-ray spectroscopy (EDX) results for Zn3Sn2O7 filmdeposited at RT. The Sn content for the film is 40%,and the crystal expression is Zn3Sn2O7, in agreementwith results reported in Ref. 11.

Electrical Properties

The resistivity of the Zn3Sn2O7 films was deter-mined from the carrier concentration and carriermobility as shown in the following equation:

q¼ 1

qlnn; (1)

where q, q, ln, and n represent the resistivity,electron charge, carrier mobility, and carrier con-centration, respectively. Figure 3 shows the variationof resistivity, mobility, and carrier concentration withgrowth temperature. The resistivity of the Zn3Sn2O7

thin films decreases from 399.0 X cm to 0.057 X cm,while the carrier concentration increases by fourorders of magnitude, as the growth temperature risesfrom RT to 250�C. The film grown at RT has thehighest mobility of 8.1 cm2 V�1 s�1 and the lowest

Table I. Growth and annealing parameters of the Zn3Sn2O7 thin films

Sample Number 1 2 3 4 5 6 7 8 9 10

Growth temperature RT 55�C 100�C 160�C 200�C 250�C RT RT RT RTAnnealing temperature – – – – – – 55�C 100�C 160�C 250�C

Fig. 1. Schematic diagram of the Zn3Sn2O7 TFT structure.

Chen, Cai, Ye, Wang, Zhang, and Wu2460

carrier concentration of 2.0 9 1015 cm�3. The highestcarrier concentration is obtained from the samplegrown at 250�C with the value of 1.65 9 1019 cm�3.The electron mobility versus growth temperature firstdecreases from RT to 160�C, then increases andreaches 6.7 cm2 V�1 s�1 at 250�C. The decrease inelectron mobility for the samples grown from RT to160�C can be explainedby ionized impurity scattering.The main ionized impurities here are oxygen vacan-cies. On the one hand, as the growth temperature ri-ses, more oxygen atoms evaporate from the growthsurface of the film, thus the density of ionized impuritycenters increases.9 On the other hand, with furtherincrease of the growth temperature, the adatoms of Znand Sn obtain enough thermal energy to move on thefilmsurfaceand findmorestable sites,which improvesthe film crystalline quality.10 Therefore, the measuredelectron mobilities for the alloy thin films grown at200�C and 250�C are higher than those grown at 50�Cto 160�C.

Figure 4 shows the variation of resistivity, mobil-ity, and carrier concentration for the Zn3Sn2O7 thinfilms with annealing temperature. The resistivity ofthe Zn3Sn2O7 films decreases from 213.0 X cm to46.9 X cm while the carrier concentration increasesby a factor of 2 as the annealing temperature risesfrom 55�C to 250�C, while the mobility increases from3.4 cm V�1 s�1 to 7.9 cm V�1 s�1. Compared withthe as-deposited films (RT grown), the annealed films

show lower resistivity and higher carrier concentra-tion. Increase of the annealing temperature in vac-uum leads to an increase of the density of oxygenvacancies in the oxide films, which causes an increaseof the carrier concentration and a decrease of the filmresistivity. The increase of electron mobility withincreasing annealing temperature from 55�C to250�C can be explained by noting that the internalstress in the films is relaxed and the density ofintrinsic defects is decreased in the annealing pro-cess, as demonstrated experimentally in Ref. 12.

The carrier concentration is closely related to thedensity of oxygen vacancies in the films. At lowergrowth temperatures, fewer oxygen atoms evapo-rate from the growth surface and a low density ofoxygen vacancies is produced. Thus, low carrierconcentration and high resistivity can be expectedin the samples grown at low temperatures. It isnoted from Figs. 3 and 4 that, at low temperatures(such as 50�C), both the as-deposited and annealedsamples show high resistivity of �350 X cm and�230 X cm, respectively. This difference couldresult from variation of the Zn-Sn-O target betweendifferent sputtering batches. However, the sampledeposited at high temperature (250�C) has evidently

0 10 20 30 40 50 60 70 80 90

250 °C vacuum annealed film

Inte

nsity

/(a.

u.)

2θ/degree

250 °C deposited film

RT deposited film

(a)

(b)

Fig. 2. (a) XRD patterns of Zn3Sn2O7 films deposited at RT and250�C, and the film deposited at RT then annealed at 250�C invacuum. (b) EDX results of Zn3Sn2O7 film deposited at RT.

0 50 100 150 200 2500

100

200

300

400

500

Res

isti

vity

/⋅cm

Growth Temperature/°C

Resistance

0

3

6

9

12

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ilit

y/cm

2 ⋅V-1

⋅s-1

Mobility

1015

1016

1017

1018

1019

1020

Car

rier

con

cent

ratio

n/cm

-3

Carrier concentration

Fig. 3. Variation of resistivity, mobility, and carrier concentration withgrowth temperature.

50 100 150 200 25040

80

120

160

200

Resistivity

Res

isti

vity

/ Ω⋅c

m

Annealing temperature/°C

0

3

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ilit

y/cm

2 ⋅V-1

⋅s-1

Mobility

0.5

1

1.5

2

2.5

Car

rier

con

cent

ratio

n/×1

016cm

-3

Carrier concentration

Fig. 4. Variation of resistivity, mobility, and carrier concentration withannealing temperature.

Effects of Growth Temperature and Annealing on Properties of Zn3Sn2O7

Thin Films and Application in TFTs2461

lower resistivity than that of the sample annealed athigh temperature (250�C). For the sample depositedat 250�C, in the growth process hot oxygen adatomsthat arrived at the growth surface of the film wereprone to reevaporate from the surface, thus moreoxygen vacancies could be formed. The annealingprocess, however, was clearly different from thegrowth process. When a RT-grown sample wassubjected to 250�C annealing, the oxygen atomsinside the film were not so easily lost as surfaceones. Thus, its resistivity can be expected to behigher than that of the sample deposited at 250�C.

Optical Properties

Figure 5 shows the optical transmission spectrain the visible region for the Zn3Sn2O7 thin filmsdeposited at RT and 250�C, and the film deposited atRT then annealed at 250�C in vacuum. The averagetransmittance of all three samples reaches 80%. Theinset displays the curves of (ahm)2 versus photonenergy, which follows the equations below:13

a ¼ ln1

T

� �=d;

ðahmÞ2 ¼ Aðhm� EgÞ;(2)

where a, T, and d are the absorption coefficient,transmittance, and film thickness, respectively. A isa constant for the direct transition, and Eg is theoptical band gap of the crystal. By fitting thetransmission spectra, we obtained optical band gapvalues of 4.04 eV, 4.07 eV, and 4.11 eV for thesamples grown at RT then annealed at 250�C, andas-deposited samples grown at RT and 250�C,respectively. We also calculated the band gaps ofother deposited Zn3Sn2O7 samples, which variedfrom 3.89 eV to 4.11 eV. It is unexpected thatZn3Sn2O7 thin films have such wide band gaps.

Other studies have reported Zn3Sn2O7 thin filmswith band gaps of 3.53 eV to 3.72 eV.12–15 Abhijit Deet al. reported SnOx films with band gaps in therange of 3.83 eV to 4.13 eV.16 The measured largeroptical band gaps for the Zn3Sn2O7 films in thisstudy cannot be explained in terms of the alloying ofZnO and SnOx. However, there may be other factorscausing the band gap widening, such as short- andmedium-range rearrangement of disordered atommatrices in amorphous films.17 The change of theband gaps of the Zn3Sn2O7 samples may be relatedto microstructure variation in the amorphous regionof atom networks, but further work needs to be doneto understand this issue.

Application in TFTs

From the electrical properties presented in thesection of ‘‘Electrical Properties’’, it is seen that theZn3Sn2O7 thin films deposited at high temperaturehave relatively low mobility and high carrier con-centration, which is not suitable for TFTs. So, RT-deposited Zn3Sn2O7 was used as the channel of TFTs.All the materials used to fabricate the Zn3Sn2O7

TFTs are transparent in the visible region.Figure 6a shows the output characteristics of a

Zn3Sn2O7 TFT with VGS varying from 0 V to 3 V insteps of 0.5 V. The device operates well in n-channelenhancementmodewith low draincurrent at zero gatevoltage. A current crowding phenomenon near theorigin is observed. This behavior is known to be due tononohmic contact between the S/D electrodes and thechannel. In the TFTs, transparent In2O3 thin filmswith carrier density of �1019 cm�3 were used as theS/D electrodes, thus nonohmic contact may be in-volved. The staggered-electrode structure of the TFTsmay be the second factor.18,19 Figure 6b shows thetransfer characteristics of the Zn3Sn2O7 TFT. A hys-teresis phenomenon was observed when charges weretrapped within the gate insulator and at the interfacesbetween the two layers.20,21 The negative chargestrapped at the channel/insulator interface can cause apositive threshold voltage shift.22,23 The Zn3Sn2O7

TFT device in our study shows hysteresis with ananticlockwise loop, indicating that it originates frompositive charges trapped at the Ta2O5/Zn3Sn2O7

interface. The mismatching Ta2O5/Zn3Sn2O7 inter-face induced trapping centers which trap typicalmobile ion charges such as Na+ or K+ from the Ta2O5

films. The hysteresis width is measured to be 2.4 V atIDS = 10�7 A. The saturation field-effect mobility ln

and thresholdvoltage VTH werecalculated by straight-line fitting of the square root of IDS versus VGS,according to the equation for the saturation region:24

IDS ¼W

2L

� �lnCi VGS � VTHð Þ2; (3)

where Ci is the capacitance per unit area of the gateinsulator, 11.4 nF/cm2. The width-to-length ratio(W/L) of the Zn3Sn2O7 TFT is 200 lm/50 lm. The

Fig. 5. Optical transmission spectra of Zn3Sn2O7 thin films depos-ited at RT and 250�C, and the film deposited at RT then annealed at250�C in vacuum. The inset shows the curves of (ahm)2 versusphoton energy for the three samples.

Chen, Cai, Ye, Wang, Zhang, and Wu2462

calculated ln is 21.2 cm2 V�1 s�1 and VTH is 0.8 V,showing that the Zn3Sn2O7 TFT operates inenhancement mode. The on/off current ratio is �105.The subthreshold swing can be derived from theIDS–VGS curves through the following formula:

S ¼ dVGS

d log IDSð Þ ; (4)

where S is related to the density of states at thechannel/insulator interface by the following relation:

Nt ¼S log eð ÞkT=q

� 1

� �Ci

q: (5)

The values of S and Nt are obtained as0.8 V/decade and 8.8 9 1011 cm�2, respectively. Thevalue of S is lower than previously reported,25

indicating a relatively low trap density at theTa2O5/Zn3Sn2O7 interface.

CONCLUSIONS

Zn3Sn2O7 thin films were deposited on glasssubstrates by rf magnetron sputtering at differenttemperatures. The film grown at room temperatureshows the highest mobility of 8.1 cm V�1 s�1 andthe lowest carrier concentration of 2.0 9 1015 cm�3.The highest carrier concentration is obtained at thegrowth temperature of 250�C with the value of1.65 9 1019 cm�3. RT-deposited Zn3Sn2O7 thin

films were subjected to annealing treatment, whichcaused increases of both carrier concentration andmobility. Zn3Sn2O7 TFTs made using high-k Ta2O5

as an insulator demonstrated field-effect mobility of21.2 cm2 V�1 s�1 and on/off current ratio of 105.This work indicates that Ta2O5 film synthesized bymagnetron sputtering is a promising insulator forTFTs with low drive voltage and high mobility.

ACKNOWLEDGEMENTS

This work was sponsored by the Natural ScienceFoundation of China under Grant Nos. 61290305and 91021020 and the Natural Science Foundationof Zhejiang Province under Grant Nos. Z6100117and Z1110057.

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0 2 4 6 8 100.0

3.0x10-6

6.0x10-6

9.0x10-6

1.2x10-5I D

S/A

0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V

VDS

/V

VGS=(a)

-10 -5 0 5 1010-10

10-9

10-8

10-7

10-6

10-5

VGS

/V

I DS/A

sweep from -10V to 10V(b)

0.000

0.002

0.004

0.006

(ID

S)1/2 /A

1/2

VDS

=10V

sweep from 10V to -10V

Fig. 6. (a) Output characteristics of a Zn3Sn2O7 TFT; (b) Transfercharacteristics of the Zn3Sn2O7 TFT.

Effects of Growth Temperature and Annealing on Properties of Zn3Sn2O7

Thin Films and Application in TFTs2463