Thin Solid Films 518 (2010) 3000–3003

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Steady-state photoconductivity of amorphous In–Ga–Zn–O

Dong Hee Lee a,⁎, Ken-ichi Kawamura b, Kenji Nomura b, Hiroshi Yanagi a, Toshio Kamiya a,b,Masahiro Hirano b, Hideo Hosono a,b,c

a Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japanb ERATO-SORST, JST, in Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japanc Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

⁎ Corresponding author. Tel.: +81 45 924 5628; fax:E-mail address: donghee@lucid.msl.titech.ac.jp (D.H

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.10.129

a b s t r a c t

a r t i c l e i n f o

Available online 28 October 2009

Keywords:Amorphous oxide semiconductorMobility–lifetime productPhotoresponse

Photoresponse was investigated for an amorphous oxide semiconductor, In–Ga–Zn–O, by the steady-statephotoconductivity (SSPC) method. All the films exhibited extremely slow reversible photoresponses.Analysis of the transient photocurrent at varied temperatures provided similar activation energies of ~0.5 eVfor both the time constants and the photoconductivity. Mobility–lifetime (μτ) products were estimated fromthe photoconductivity spectra measured at the sweep rate of 2 nm/s, which monotonically increased withincreasing dark conductivity σD (i.e. the Fermi level EF becomes shallower). The obtained μτ values arelarger than those of hydrogenated amorphous silicon even if the EF dependence is considered.

+81 45 924 5855.. Lee).

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Amorphous In–Ga–Zn–O (a-IGZO), a representative amorphousoxide semiconductor (AOS), is expected as a channel material of thinfilm transistors (TFTs) inflat-panel displays (FPDs) because a-IGZO TFTsexhibit large electron mobilities over 10 cm2(Vs)−1 even deposited atroom temperature (RT) [1]. Therefore, several prototype next-genera-tion displays using a-IGZO TFTs have rapidly been developed [2,3].

Although high-performance a-IGZO TFTs [4] and their electricaltransport mechanisms have been reported from several groups [5,6],there is still a lack of understanding of their basic characteristics, suchas the defect states, photoresponse, and generation/recombinationprocesses of photoexcited carriers. At present, stability of a-IGZO TFTagainst light exposure is an important issue to be controlled forpractical applications and mass production of a-IGZO based FPDs.

Photoconductivity measurements are powerful methods to probethe dynamics of photoexcited carriers. In addition, these directlyprovide important information such as a recombination mechanismand distribution of subgap states in semiconductors; therefore, it isexpected that detailed investigation of photoresponsewill lead to awayto fabricate improved quality films/TFTs and to control the operationcharacteristics of optoelectronic devices. To quantitatively evaluatephotoresponse, mobility–lifetime (μτ) product is one of the mostimportant parameters because it is a material property independent of(i.e., normalized by) photon flux, film thickness, absorption coefficientand so on. It also determines the performance of photovoltaic devicesand the structure of optoelectronic devices; therefore it is a fundamentalparameter that must be known for designing these devices.

In this paper, we evaluate the μτ products of a-IGZO by steady-state photoconductivity (SSPC) measurements.

2. Experimental details

2.1. Sample preparation

All the a-IGZO films (~50 nm in thickness)were deposited on silicaglass substrates by pulsed laser deposition with a KrF excimer laserusing a polycrystalline InGaZnO4 target at RT. The laser energy densitywas ~8 Jcm−2pulse−1 and the repetition frequency was 10 Hz. Thefilms with different dark conductivities (σD) were employed in orderto investigate the effect of the Fermi level, and σD was controlled to be10−7–10−1Scm−1 by varying the oxygen partial pressure from 6.2 to6.9 Pa. For the SSPC measurement, interdigital coplanar electroderegions (0.02 cm wide and 3.8 cm long) were defined by photoli-thography and lift-off, and the electrodes were formed by depositing5 nm-thick Ti and 30 nm-thick Au layers in this order by electron-beam evaporation at RT. The ohmic contacts were confirmed inpreliminary experiments, and the photocurrent/conductivity spectrawere measured at an applied voltage of 5 V.

2.2. Derivation of μτ product from SSPC measurement

The SSPC measurements were performed by focusing continuous-wave monochromated light on the sample. In this case, the conditionfor the steady-state is given by Eq. (1)

dndt

= ηF−Δnτ

= 0; ð1Þ

3001D.H. Lee et al. / Thin Solid Films 518 (2010) 3000–3003

where η is the quantum efficiency (for rough discussion, η is oftenassumed to be unity), F the absorbed photon flux in cm−3s−1, Δn thedensity of photocarriers in cm−3, and τ the lifetime of thephotocarriers. Then, net photoconductivity σph, which is defined asthe difference in the conductivities under light illumination σph,raw

and in dark σD, is given by

σph≈eμΔn = eημτF =IphdLE

; ð2Þ

where d is the film thickness, L the electrode length, and E the appliedelectric field. F is estimated as F=F0 (1−T−R)/d, where F0 is thephoton flux emitted from themonochromated light source in cm−2s−1,T and R the measured transmittance and reflectance, respectively. Theημτ product is calculated by Eq. (3).

ημτ = Iph = ðeLEF0ð1−T−RÞÞ: ð3Þ

Hereafter, we simply refer to the ημτ product as μτ product byequating η to be unity. We employed a simple SSPC measurementsystem as follows. The light source was a 400 W xenon lamp equippedwith a gratingmonochromator. F0 spectra weremeasured using a pre-calibrated Si photodiode. The electrical current was measured by adigital picoammeter. T and R spectra of the films were also measuredusing a UV–vis spectrophotometer.

3. Results and discussion

3.1. μτ products

Fig. 1(a) shows an incident photon flux F0 spectrum, which is neededfor evaluating μτ products. A typical photon flux is 5×1013 cm−2s−1 atthe photon energy of 3.1 eV. Figure 1(b) shows absorbed coefficient αspectra of the a-IGZOfilmswith theσD values from10−5 to 10−3Scm−1.The α spectra show almost constant values <2000 cm−1, which are thedetection limit of our measurement system [7]. Tauc plots provided thatthe optical bandgaps are Eg ~3.06 eV. It is observed that non-negligibletail-like optical absorption exists below Eg down to ~2.9 eV, but these arenearly independent of σD. We reported that the subgap tail-like states

Fig. 1. (a) Incident photon flux spectrum of light source. (b) Optical absorption spectra. (conductivities.

originate largely from deep occupied states distributing from the valenceband maximum [8].

Fig. 1(c and d) shows (c) photoconductivity σph and (d) μτ productspectra of the a-IGZO films used in Fig. 1(b). The σph=σph,raw−σD

were estimated from the σD values and the conductivity measuredunder the light illumination of Fig. 1(a) [σph,raw]. The σph spectra weremeasured at the scanning rate of 2 nm/s. Here we should note that theσD value is affected significantly by the history of light illumination andchanges with time, which is due to the very slow photoresponse,which will be discussed later. Therefore, it is necessary to fix themeasurement process in order to obtain reliable and reproducible σD

values. In this study, we measured the dark current–voltagecharacteristics more than 1h after the sample was placed in a darkmeasurement box. The σph spectra thus obtained (Fig. 1(c)) show thatthe photoconductivity depends strongly on σD (i.e. the Fermi level),and is roughly proportional to σD. As a matter of course expected fromEq. (3), the μτ product also have the same dependency onσD as seen inFig. 1(d). For all the samples, small but non-negligible photocarriersare generated even by subgap energy photons below Eg, which reflectsthe subgapdensity of states [9,10] similar to the constant photocurrentmethod [11]. It should be noticed that the μτ products are almostconstant at photon energies >Eg, which indicate that the recombina-tion process does not depend on the excitation energy. Hereafter, wefocus on the photoconductivity and μτ product at 3.1 eV, which is justabove Eg.

Fig. 2 shows the μτ product at the photon energy of 3.1 eV as afunction of σD plotted in terms of a double-logarithmic scale. The μτvalues were extracted from the μτ product spectra measured by theabove procedure. Note that the Fermi level is roughly estimated fromthe relation σD=enDμ, where the drift mobility μ is assumed to be10 cm2(Vs)−1, and the electron density at the thermal equilibriumnD=NCexp[−(EC−EF) /kBT], where NC is the effective density of stateof the conduction band (estimated to be 5.2×1018cm−3 using thereported electron effective mass of 0.35 me [5]), EC−EF the Fermilevel measured from the conduction band edge energy EC, and kB theBoltzmann constant. It tells that EC−EF ranges from 0.1 to 0.5 eV asσD decreases from 10−1 to 10−7Scm−1.

We can see that log μτ increases almost linearly with log σD,indicating that μτ follows a relation μτ=AσD

β, where A is a constant

c) Photoconductivity and (d) μτ product spectra of a-IGZO films with different dark

Fig. 3. Response of transient photocurrent Iph as a function of time. The black line showsthe measured result and the red line is a fitting result to an exponential law.

Fig. 2. μτ product as a function of dark conductivity. Those of a-Si:H and μc-Si are alsoshown for comparison [14].

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and β an exponent factor. β is ~0.75 at σD>10−4Scm−1, andincreases to ~1.1 in the lower σD region. This threshold σD valuecorresponds to the nD value of 6×1013cm−3. We reported that the μvalue of a-IGZO decreases with decreasing nD especially atnD<1016cm−3 [1]. Although the agreement between these thresholdnD values is not good enough, we consider that the increase in theslope of the μτ–σD curve at σD<10−4Scm−1 is attributed in part tothe decrease in μ.

This increase in the μτ product with increasing σD is related to theup-shift of the Fermi level for n-type semiconductors. We reported

Fig. 4. (a) Photoconductivity as a function of photon flux F0 for a-IGZO films wit

that a-IGZO have subgap states [8–10], which may work asrecombination centers. The shift in the Fermi level changes thermaloccupation of the recombination centers; i.e., when σD becomeslower, the Fermi level becomes deeper and more unoccupied statesare generated in the bandgap, which results in larger indirectrecombination centers and shorter lifetime τ. Similar relations areknown also for hydrogenated amorphous silicon (a-Si:H) andmicrocrystalline silicon (μc-Si) [12–14], whose μτ–σD values areindicated by the oval regions in Fig. 2. It shows that a-IGZO have ratherlarge μτ products compared to a-Si:H and μc-Si even if the dependenceon σD is taken into consideration.

3.2. Slow photoresponse of a-IGZO

We found that all the a-IGZO films exhibited extremely slowphotoresponse in the SSPC measurements. Fig. 3 shows the timevariation of transient photocurrent Iph for the a-IGZO film withσD~4×10−5Scm−1. In this experiment, 3.1 eV monochromated lightwas illuminated on the sample for the first 600s, and then the lightwas turned off. We can see that a-IGZO shows very slow responsesboth after the light is turned on and after the light is turned off. Itshould be stressed that this change is reversible; i.e., the Iph returns tothe initial value >50min after the light is turned off. The photo-response roughly follows exponential laws, e.g. Iph= Isat[1−exp(−t /τon)], where Isat is a constant and τon is a time constant of Iph underlight illumination. The τon is as long as 368 s and the Isat is >6μA. Notethat the μτ values in Fig. 2 were extracted from the spectrummeasurements, where each data point was measured at a scanningrate of 2 nm/s. On the other hand, the slow response in Fig. 3 indicatesthat larger μτ values are obtained if measurement time is longer; i.e.,the values in Fig. 2 are underestimated significantly because themeasurement time was far shorter than τon. Indeed, the μτ productestimated from the Isat value in Fig. 3 is 5×10−1cm2V−1, which ismuch larger than that in Fig. 2 (3×10−2cm2V−1). It indicates that weshould be careful of evaluating a μτ product in a-IGZO because adifferent evaluation procedure would give an order of magnitudedifferent value if one does not consider the slow response.

We also studied photon flux F0 dependence of photoconductivity(Fig. 4(a)), showing that σph follows well the power law σph=CF0

γ forall the a-IGZO films with different σD values. The exponent γ isplotted as a function of σD in Fig. 4(b). It shows that the γ value non-monotonically changes with σD and ranges from ~1.0 to ~0.4. Thechange in the γ value is an indication of the change in therecombination mechanism. In amorphous semiconductors, γ values

h different σD. (b) Exponent γ in the relation σph=CF0γ as a function of σD.

Fig. 5. Variation of the time constant under light illumination as a function oftemperature. That of photoconductivity is also shown.

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ranging from 0.5 to 1 are usually observed. γ=0.5 is associated with abimolecular recombination process, in which a photoexcited electronand a hole recombine directly between the conduction band (tail) andthe valence band (tail). On the other hand, γ=1 is associated with amonomolecular recombination process, which occurs when recom-bination centers (deep states) dominate the lifetime of photoexcitedcarriers. However, γ shows the complex variation; i.e., γ increaseswith increasing σD at σD<4×10−5Scm−1 but turns to decrease atlarger σD. The above standard mechanism suggests that therecombination process changes from a bimolecular one to amonomolecular one, and finally again to a bimolecular one as EFincreases from 0.5 to 0.1 eV below EC. This complex variation is notsimply explained only by the change of EF, and further study isneeded.

Finally, we measured temperature dependence of the transientphotocurrent for the film with the σD value of ~10−3Scm−1. Notethat the γ value of this sample is ~0.5 and different largely from thatin Fig. 3 (γ~0.9). Probably due to this difference, which is associatedwith a different recombination mechanism, we needed to incorporatetwo exponential terms to reproduce the transient photocurrent in thiscase; i.e. the photocurrent follows the relation Iph= Isat,1 [1−exp(−t /τon,1)]+ Isat,2 [1−exp(−t /τon,2)], where τon,1 and τon,2 are thetime constants, and Isat,1 and Isat,2 are the saturation currents thatcorrespond to τon,1 and τon,2, respectively. The values at RT wereτon,1=51.5 and τon,2=421 s. Fig. 5 shows that both the timeconstants follow a thermally-activated behavior τon,1,2=τon,1,20 exp(+Eτon,1,2 /kBT), where τon,1,20 and Eτon,1,2 are the constants andactivation energies for τon,1 and τon,2, respectively. Further, thetemperature dependence of photoconductivity is also superimposed.It is seen that all the activation energies are almost the same,Ea~0.5 eV. This result suggests that the origin of the two recombina-tion processes and the photocarrier generation process are the same.However, because the σD value of this film is ~10−3Scm−1 and theFermi level is estimated to be ~0.23 eV below EC, the above activationprocess is not explained by thermal excitation of electrons from theFermi level. In addition, the potential barrier height in the conductionband is ~0.1 eV [6] in a-IGZO. Therefore, we should consider that this

large activation energy Ea comes from anothermechanism related to adeep state or to a defect formation process having a similar formationenergy, and the latter model is more likely because the a-IGZO film isn-type and the Fermi level is shallower than Ea. That is, in the a-IGZOfilms deposited at RT, the slow photoresponse is caused by generationof some defects or meta-stable states that release mobile electrons.The formation energy of the defect/meta-stable states is as small as0.5 eV. This process is reversible and the generated mobile electronsand defects/meta-stable states are annihilated even at RT, but theannihilation process needs a long time at the order of an hour.

4. Summary

We investigate photoresponse in a-IGZO and found a-IGZO filmsexhibit very slow response both in the generation and the recombi-nation processes. It was indicated that the recombination process iscontrolled by a thermally-activation process with an activation energyof ~0.5 eV. Mobility–lifetime (μτ) products were evaluated fromphotoconductivity spectra measured at the sweep rate of 2 nm/s,which showed the μτ values of a-IGZO thus evaluated are severalorders of magnitude larger than those of a-Si:H, because they havehigh Fermi levels and long photocarrier lifetime. Finally we like topoint that the slow photoresponse should be considered in order tosystematically measure and to control the operation characteristics ofa-IGZO TFTs under light illumination.

Acknowledgement

This work is partially supported by the New Energy and IndustrialTechnology Development Organization (NEDO) under the Ministry ofEconomy, Trade and Industry.

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