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Materials Chemistry and Physics 124 (2010) 704–708

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

eterojunction bipolar assembly with CrxTi1−xO2 thin films and vertically alignednO nanorods

oumen Das, Sang-Hoon Kim, Yong-Kyu Park, Cheol-Min Choi, Dae-Young Kim, Yoon-Bong Hahn ∗

chool of Semiconductor and Chemical Engineering, Dept. of BIN Fusion Technology, and Nanomaterials Processing Research Center, Chonbuk National University, 664-14, Duckjin-ong 1-Ga, Jeonju 561-756, South Korea

r t i c l e i n f o

rticle history:eceived 16 January 2010eceived in revised form 7 June 2010ccepted 17 July 2010

a b s t r a c t

Polycrystalline and homogeneous CrxTi1−xO2 thin films were deposited on silicon (Si) substrates and onindium doped tin oxide (ITO) coated glass substrates by spin coating technique. We report the p-typeconductivity in CrxTi1−xO2 thin films (x = 0.005, 0.05, 0.1, 0.15, 0.2) and variable turn-on voltages (VO) inheterojunction ZnO-nanorod/CrxTi1−xO2/ITO bipolar device. Results showed that VO varies substantially

eywords:hin filmsiO2

nOipolar devicePS

from ∼0.8 V (x = 0.005) to ∼0.53 (x = 0.2) for the bipolar assembly. X-ray photoelectron spectroscopy (XPS)showed that chemical state of Ti is the +4 valence state and Cr remains in three different oxidation statesof +3. XPS in the valence band region showed a shift in the binding energy towards the lower energy sidewith increasing Cr intake confirming more p-type conductivity in CrxTi1−xO2 thin films.

© 2010 Elsevier B.V. All rights reserved.

lectrical study

. Introduction

The nano-TiO2 electrode is one of the most significant compo-ents used in dye sensitized solar cells [1,2]. It is also used as annode buffer layer in bipolar junction devices in order to improvehe turn-on voltage (VO) [3]. Moreover, self-assembled TiO2 struc-ures have been fabricated and used as a hole-injecting layer inybrid electroluminescent devices [4]. It is reported that the designnd fabrication of the TiO2 electrode substantially influence theapability of electron conductivity [5,6].

TiO2 is an n-type semiconductor, while p-type TiO2 semicon-uctor can be prepared by doping Cr+3, or Fe+3 [7]. Ruiz et al. [8]eported that with increasing Cr content, the electronic conductionhanges from n-type to p-type. Liau and Lin [9] showed the recti-ying property of the p–n homojunction device using commercialano-TiO2 (P25) (n-type) and Cr+3 doped TiO2 (p-type). In recentimes, researchers employ vertically aligned n-type ZnO nanorodsn hybrid photovoltaic cells and in organic light emitting devicess preferred medium for easy transportation of charge carriers

10–12]. Reported photovoltaic cell employing one-dimensionalnO nanorods in fact recorded higher efficiency than blending typeaterials used for the same purposes [13].

∗ Corresponding author. Tel.: +82 63 270 2439; fax: +82 63 270 2306.E-mail address: ybhahn@chonbuk.ac.kr (Y.-B. Hahn).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.07.040

In the present work, a CrxTi1−xO2/ZnO nanorods (p–n) hetero-junction bipolar device was fabricated via wet chemical route. Theeffects of the Cr ion doping on the structural and electrical proper-ties of the assembly were also studied.

2. Experimental details

2.1. Wet chemical synthesis of Cr+3 doped TiO2

TiO2 nanoparticles were produced by mixing 0.001 mol titanium (IV) isopropox-ide (TIPO) with 5 mL ethanol. The mixture immediately produced white precipitateof titanium hydroxide. In another glass beaker, required amount Cr(NO3)3 was dis-solved in 2 mL ethanol. These two solutions were mixed together and 2 mL aceticacid was added. The solution was stirred slowly (300 rpm) for 6 h. After that a cleargreenish solution was produced. The concentrations of Cr+3 were varied as 0.5, 5,10, 15, 20 mol% (x = 0.005, 0.05, 0.1, 0.15, 0.2) in the total 100% Cr+3:Ti+4 (CrxTi1−xO2)intake.

2.2. CrxTi1−xO2 thin films

Three successive layers of CrxTi1−xO2 films were deposited on Si substrates andon ITO coated glass substrates by spin coating technique at 4000 rpm for 20 s. Eachintermediate layer was dried at 70 ◦C for 10 min. Finally, the films were annealed at400 and 800 ◦C for 1 h in vacuum and cooled naturally.

2.3. Growth of vertically aligned ZnO nanorod

Prior to the growth of ZnO nanorods, a thin ZnO seed layer with a thickness of∼30 nm was deposited using a ZnO target (99.999%) on the CrxTi1−xO2 thin films.The substrate temperature was maintained around ∼300 ◦C using the rf sputteringsystem. The film growth was carried out in pure Ar (20 sccm) at a constant workingpressure of 1.5 × 10−3 Torr and rf power of 60 W. The ZnO nanorods were grown on

try and Physics 124 (2010) 704–708 705

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Fig. 1. (a) The cross-sectional view of the aligned ZnO nanorods. Inset, the topview of the corss-section of the individual ZnO nanorods. (b) TEM image of theZnO nanorod. Inset, the SAED pattern reveals the single crystalline nature of the

S. Das et al. / Materials Chemis

rxTi1−xO2/ITO/glass substrates by seed-layer assisted growth technique. For this anqueous solution was prepared using 0.05 M equimolar zinc nitrate hexahydrate andexamethylene-tetramine precursors. The detail of the preparation was describedlsewhere [14–17].

.4. Fabrication of p–n junction assembly and measurement

CrxTi1−xO2 thin films (p-type) were spin coated on 1 × 1 cm2 ITO coated glassubstrates and annealed at 400 ◦C for 1 h, and vertical ZnO nanorods (n-type)ere then grown by simple low temperature solution method, as described above.

ogether this, the assembly worked as bipolar junction (p–n) device.The thickness of the CrxTi1−xO2 thin films and the cross-sectional image of the

nO-nanorod/CrxTi1−xO2/ITO/glass-substrate were studied by field emission scan-ing electron microscope (FESEM, JSM S4800) accompanied by energy dispersive-ray analysis (EDAX) attachment for the compositional analysis. The morphology of

he ZnO nanorods and the Cr+3 doped TiO2 nanoparticles were studied by transmis-ion electron microscope (TEM, JTEM 2010). X-ray photoelectron spectra (XPS) wereecorded with a Thermo K-alpha ESKA System with a monochromatic Al-k� sourcend a charge neutralizer. Fourier transform infrared (FTIR) spectroscopy (Nicoletagma 750 IR series II spectrometer) was used in the range of 400–1200 cm−1. The

lectrical (I–V) property of the samples was carried out utilizing 4-probe stationHP 4156C semiconductor parameter analyzer) in the range from −2 to 2 V at roomemperature. For this 2 mm × 2 mm Al electrode was deposited on the top of thessembly by thermal evaporation method.

In the next section the samples will be referred as follows. Sample S1 (x = 0.005),ample S2 (x = 0.05), sample S3 (x = 0.1), sample S4 (x = 0.15) and sample S5 (x = 0.2)re annealed at 400 ◦C for 1 h. Sample S1A (x = 0.005), S3A (x = 0.1) and S5A (x = 0.2)re annealed at 800 ◦C for 1 h. For FTIR measurements, these films were coated oni substrates with identical post-deposition conditions.

. Results and discussion

.1. Morphology

The FESEM image in Fig. 1(a) shows the cross-sectional view ofhe vertical ZnO nanorods (n-type) with length ∼1 �m and diame-er around ∼30 nm (inset, Fig. 1(a)). Fig. 1(b) shows the TEM imagesf these nanorods along with the selected area diffraction (SAED)attern which shows diffraction spots indicating single crystallineature of the obtained nanorods. Fig. 1(c) shows the high resolu-ion TEM image of Cr doped colloidal TiO2 nanoparticles annealedt 400 ◦C for 1 h. The lattice fringes were calculated for the anatasehase of the TiO2 (1 0 1) plane with lattice spacing d ≈ 3.46 nm. Insethows the selected area diffraction pattern (SAED) showing the1 0 1), (0 0 4) and (2 0 0) plane of the anatase phase of polycrys-alline TiO2 [18]. It was pointed out in the earlier reports [8,19]hat CrxTi1−xO2 exhibit p-type conductivity. It is also known thathe incorporation of Cr does not affect the crystallography of TiO2

aterial, [20] and that it crystallizes at high annealing tempera-ures.

.2. XPS and FTIR spectra

X-ray photoelectron spectra are shown in Fig. 2. Fig. 2(a) indi-ated the Ti 2p doublet for samples S2, S3, S4 and S5 consist ofwo wide peaks of Ti 2p3/2 and Ti 2p1/2 with binding energies ∼458nd ∼464 eV, respectively, both belong to Ti–O bonds [21]. Inter-stingly, the high resolution XPS spectra of Ti2p3/2 for sample S5n the inset of Fig. 2(a) indicated that the chemical states of Ti con-ains two kinds of valence including Ti3+ and Ti4+. Fig. 2(b) showshe XPS spectra of the same samples (S2–S5) in the valence bandegion. The spectra indicate a downward shifting of the Fermi leveloward the valence band edge, and the electronic structure gradu-lly acquires more p-type conductivity with increasing Cr content.ig. 2(c) and (d) shows a comparative XPS analysis of the CrxTi1−xO2amples (x = 0.1 and 0.2) annealed at 400 ◦C (S3, S5) and 800 ◦C (S3A,

5A). The spectra showed insignificant shifts in the Ti 2p3/2 and Tip1/2 peaks for S3 and S3A samples, whereas for S5 and S5A sam-les a shift of 0.208 eV (for Ti 2p3/2) and 0.233 eV (for Ti 2p1/2) wasbserved in the lower binding energy side. The valence edge alsohifts more to lower binding energy side for sample S5A compared

nanorods, (c) the high resolution TEM images show the TiO2 colloidal particleannealed at 400 ◦C for 1 h. The fringes reveal (1 0 1) lattice plane of anatase TiO2.Inset of (c), the SAED also confirms the diffraction rings from (1 0 1), (0 0 4) and(2 0 0) lattice planes of anatase TiO2.

to that for sample S3A. Thus the effects of Cr doping was more obvi-ous for samples treated at higher annealing temperature due tobetter crystallinity and better rearrangement of the Cr atoms in theTi lattice site. We observed (by X-ray diffraction study, not included

in the report. See supplementary file) that at 800 ◦C temperaturethe CrxTi1−xO2 samples get complete phase transformation fromanatase (obtained at 400 ◦C) to rutile structures. The XPS spectra ofCr2p3/2 in Fig. 2(e) and (f) for sample S3 and S5 show that two dif-

706 S. Das et al. / Materials Chemistry and Physics 124 (2010) 704–708

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ig. 2. XPS of Ti2p level for (a) CrxTi1−xO2 films (x = 0.05, 0.1, 0.15 and 0.2), anneaf) show the Cr2p level for CrxTi1−xO2 films (x = 0.1 and 0.2). The valence band edge00 ◦C and (d) for for CrxTi1−xO2 films (x = 0.1 and 0.2) annealed at 400 ◦C and 800 ◦

i3+ and Ti4+ for S5 (x = 0.2).

erent oxidation states of +3 (∼577 eV) and +6 (∼578 eV) exist in therxTi1−xO2 thin films. The Cr2p1/2 peak observed at ∼586 eV. Thehange in oxidation state of Ti and Cr indicates that there is chargeransfer taking place from Ti to Cr metal ions [22,23]. Moreover,omparing peak positions of Cr2p3/2 for S3 (577.08, 579.41 eV) andhat for S5 (576.44, 578.36 eV) or of Cr2p1/2 as 587.07 eV (for S3)nd 586.37 eV (for S5), we see that there is a negative peak shiftin S3 andS5) of 0.64, 1.05 eV for Cr2p3/2 and 0.70 eV for Cr2p1/2.n S3 and S5, there is a positive peak shift of 0.61 eV for Ti2p3/2nd 0.65 eV for Ti2p1/2 peak. These results suggest the Cr ions areoped in the TiO2 lattice instead of the formation of Cr2O3 at theurface of TiO2. When TiO2 is doped with Cr+3 ions, the bindingnergy change in each atom occurs due to the electron transfer fromi2p to Cr2p. Finally, the opposite results in the binding energy ofi2p and Cr2p were generated as indicated in Fig. 2(a), (e), and (f).his signifies the Cr incorporation significantly influences the localhemical environment of the Ti ions [24].

The Fourier transform infrared spectroscopy in Fig. 3 showedomparative spectra of TiO2 and CrxTi1−xO2 thin films (x = 0.005,.1, 0.2) deposited on the Si substrates. It is observed that thei–O vibration absorption peak at about ∼510 cm−1 becomes more

rominent in the doped samples. In the lower wave-number regionwo prominent peaks are observed at around 424, 435 cm−1. Theseeaks are assigned to Ti–O–Ti vibration [25]. The effect of the Croping and temperatures on these bands was also obvious fromhe spectra. As is revealed, the bands at 424, 435 cm−1 show highest

400 ◦C, (c) for CrxTi1−xO2 films (x = 0.1 and 0.2) annealed at 400 ◦C and 800 ◦C. (e),Ti 2p state are shown (b) CrxTi1−xO2 films (x = 0.05, 0.1, 0.15 and 0.2), annealed at

et (a) shows that the chemical states of Ti contains two kinds of valence including

intensity for pure TiO2 annealed at 800 ◦C (rutile phase) comparedto that annealed at 400 ◦C (anatase phase). After Cr doping, the bandat 435 cm−1 becomes weaker and finally disappears in sample S3and instead another peak at 457 cm−1 appears, which initially wasnot present in pure TiO2 sample or in sample S1. These spectra alsoindicated that for doped samples (S1, S3 and S5) the band structuresfor anatase (annealed at 400 ◦C) and rutile (800 ◦C) carry identicalvibration modes. The peak at ∼1100 cm−1 can be ascribed to Ti4+

ions in C4� coordination [26]. The peaks at ∼609, 618 cm−1 are maybe due to the silicon substrates.

3.3. I–V characteristics

Fig. 4(a) on the other hand shows the cross-sectional image ofa representative ZnO nanorod/CrxTi1−xO2/ITO/glass heterojunctionbipolar device (x = 0.1). The image revealed fairly homogeneousstack of layers with ∼300 nm thick ITO layer, ∼230 nm thickCrxTi1−xO2 film and ∼1.6 �m long ZnO nanorods. The elementalanalysis in the form of EDAX is shown in Fig. 4(b). The spectrumshows the presence of In, Ti, Zn and O. The small peak around5.9 keV in the spectrum can be assigned to Cr. No other impurity

peak was observed. The effect of Cr doping in TiO2 thin films wasapparent in the electrical characterization of the samples. Fig. 5(a)shows the I–V characteristics of the junction assembly for samplesS1–S5 (x = 0.005–0.2). The rectifying characteristics of the assemblyare apparent in all of the samples though it is more obvious for Cr

S. Das et al. / Materials Chemistry and Physics 124 (2010) 704–708 707

Fig. 3. The FTIR spectra of the pure TiO2 and CrxTi1−xO2 films (x = 0.005, 0.1, 0.2)deposited on Si substrates and annealed at 400 ◦C and 800 ◦C for 1 h.

Fig. 4. (a) The cross-sectional FESEM image of the ZnOnanorod/CrxTi1−xO2/ITO/glass heterojunction assembly (x = 0.1, sample S3).(b) The energy dispersive X-ray spectrum (EDAX) showing the presence of In, Si, Ti,Zn, Cr and O in the assembly.

Fig. 5. The I–V characteristics of the ZnO nanorod/CrxTi1−xO2/ITO (x = 0.005, 0.05,

0.1, 0.15, 0.2) assembly show (a) rectifying characteristics with ideality factor (�)close to range 7–10, (b) shows the variations of the turn-on voltage of the bipo-lar assembly with the molar concentration of Cr. Inset (b) shows representativeFowler–Nordheim plots for samples S1 and S4.

intake in the range of 5–20 mol% (x = 0.05–0.2). It was observed thatthe forward current is comparatively larger than that in the reversebias. The current in the device can be expressed as I = I0exp(qV/�kT),where V is the voltage across the diode, I0 is the saturation current,q is the charge, k is the Boltzmann constant and T is the temperaturein Kelvin. � is the ideality factor which is determined from the slopeof the ln I–V plot. The slope gives q/�kT from which the calculated� found to be in the range of ∼7–10 for our bipolar device whichis higher than the ideal value of ∼1–2. The deviations indicate thateither there are unusual recombination mechanisms taking placeor that the recombination is variable in magnitude. Interestingly,we observed that the turn-on voltages (VO) of the junction varywith the Cr intake and in each case VO is lower than 1 V. For exactdetermination of VO most researchers use a tunneling model of theFowler–Nordheim (F–N) theory [27,28]. In this model the tunnelingcurrent can be expressed as:

I˛E2exp(

− b

E

)(1)

where E is the external electric field, b is a parameter thatdepends on the barrier shape. The data for I–V curves were used

to plot ln (I/E2) versus 1/E. The calculated turn-on voltage for theZnO nanorod/CrxTi1−xO2 bipolar assembly with x = 0.005 is ∼0.8 Vwhereas it is equal to ∼0.53 V for x = 0.2. Fig. 5(b) shows the vari-ation of VO with x (mol% of Cr into TiO2 lattice site). Inset shows arepresentative F–N plot which is close to linear, the slope of which

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ives the constant, b. Thus the incorporation of Cr in the buffer layeras an effect to improve the attribution of holes, leading to the bal-nce electron and holes within the recombination zone which inurn can improve the over all device performance of ZnO based het-rojunction structures. The decrease in the turn-on voltage can bettributed to the lowering of the valence band edge with the incor-oration of Cr into the TiO2 lattice site and on the lowering of thearrier height. If we assume that the charge is tunneling throughtriangular barrier at the interface then the constant b is given by

29]:

= 8�√

2(m∗)ϕ3/2

3qh(2)

ere ϕ is the barrier height and m* is the effective mass of the holesn TiO2. h is the Planck’s constant and q is the elementary charge.ssuming that the electric field is constant across the device and

hat the effective mass equals the free electron mass, the calculatedarrier heights are of the order of ∼10−2 eV.

. Summary

Bipolar device was fabricated with the CrxTi1−xO2 thin films (p-ype) and vertically oriented ZnO nanorods (n-type) on ITO coatedlass substrate. The rectifying characteristics indicated successfulabrication of the p–n junction device. The results were explainedn the basis of the XPS where we observed the shifting of Ti2pnd Cr2p levels signifying considerable influence on the chemi-al environment of the Ti ions due to Cr doping. The results cane utilized in fabricating useful organic light emitting diodes or

norganic–organic hybrid devices where CrxTi1−xO2 thin films cane used as anode buffer layer to control the charge recombination.

cknowledgements

This work was supported in part by the Priority Research Cen-

ers Program through the National Research Foundation of Korea2009-0094033) and the World Class University program (R31-008-000-20029-0) funded by the Ministry of Education, Sciencend Technology (MEST). Authors also thanks Jeonju branch of KBSIor SEM and TEM analysis. One of the authors (S. D.) extends special

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d Physics 124 (2010) 704–708

thanks to Mr. Kang, Jong-Gyun, EM lab, Centre for University-wideResearch Facilities, for his selfless assistance.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matchemphys.2010.07.040.

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