Thin Solid Films 526 (2012) 195–200

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Double layer structures of transparent conductive oxide suitable for solar cells:Ga-doped ZnO on undoped ZnO

Housei Akazawa ⁎NTT Microsystem Integration Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

⁎ Tel.: +81 46 240 2659; fax: +81 46 270 2372.E-mail address: akazawa.housei@lab.ntt.co.jp.

0040-6090/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2012.10.111

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 July 2012Received in revised form 29 October 2012Accepted 30 October 2012Available online 14 November 2012

Keywords:Zinc oxideGa-doped zinc oxideTransparent conductive oxideSputteringInfrared transmittanceSolar cell

We investigated the transparent conductive properties of hetero-double layers: Ga-doped ZnO (GZO) overlaidon undoped ZnO. We prepared five samples for given unit thicknesses h: ZnO and GZO films with a thicknessof h to characterize the hybrid structure, ZnO and GZO films with a thickness of 2h, and a GZO/ZnO doublelayer with a thickness of h for each layer (h=50, 100, 150, and 200 nm). If we assumed that the upper andbottom half of the 2h-thick GZO films as well as those of GZO/ZnO were connected in parallel in terms of theequivalent electric circuit, the calculated sheet resistance of the upper GZO layer scarcely depended on whetherthe bottom layer was GZO or ZnO. Hence, the bottom layer played the role of providing a crystalline template forthe upper layer that actually governed electrical transport. Also, the infrared transmittance of the upper GZOlayer was immune to what the bottom layer consisted of. While GZO/ZnO had 1.1–1.5 times higher sheet resis-tance than 2h-thick GZO, the optical transmittance of GZO/ZnO in the near-infrared region was 20–40% higher,demonstrating that the GZO/ZnO double layer structure is suitable for transparent electrodes in solar cells.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conductive oxide (TCO) films are nowadays widelyemployed in various kinds of electrical and optical devices. The onlyconcern about optical properties regarding their use in flat paneldisplays is the transmittance level in the visible wavelength range.The requirement for solar cells is somewhat different; transmittancein the infrared range should be high enough as well because the sun-light spectrum has a substantial fraction of photons with wavelengthsbetween 1000 and 2500 nm. If infrared light is efficiently utilized withadditional contrivances such as surface-plasmon-resonance-enhancedphoto-absorption, the photovoltaic efficiencies will be dramaticallyimproved. Up to now, SnO2 [1], Sb-doped SnO2 [2,3], and F-dopedSnO2 [4,5] have been proposed and actually used as electrodes forsolar cells. Because even lower resistivities can be achieved by usingZnO-based materials [6], the feasibility of Ga-doped ZnO (GZO) orAl-doped ZnO (AZO) has also been investigated by focusing on solarcells [7–10].

GZO films are generally more conductive than ZnO films but do nottransmit light in the infrared range, which renders GZO rather useful forsolar blind windows [11]. This is due to their higher plasma frequencyresulting from their higher carrier concentration. ZnO films havelower carrier concentration and thus have higher optical transmittance.Boron-doped ZnO, which is characterized by intermediate resistivityand transmittance between ZnO and GZO, has been expected to be a

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candidate material for solar cells [12–14]. However, typical resistivitiesof ZnO:B films in the 10−3 Ω·cm range can easily be obtained withundoped ZnO films. There is still a debate as to whether or not dopedboron atoms really behave as extrinsic donors.

It is well known that when materials from the ZnO family aredirectly deposited on glass substrates, their crystallinity is the worstat the initial stage of growth. This makes electron mobility near theinterface region extremely low. Furthermore, carriers are not gener-ated if crystallinity is so poor that even small crystallites cannot becreated. When the crystal quality increasingly improves fartherfrom the interface [15], both carrier concentration and Hall mobilityincrease [16–19]. Since poor crystallinity in the interfacial region isthe primary bottleneck to achieve ultra-thin TCO films, how to securesuperior crystallinity at an early stage of growth is a critical issue toachieve high performance of TCO films.

Various attempts have been devoted to solving this problem. Al-though the best crystallinity can be obtained in epitaxial growth onsingle crystal substrates such as sapphire [20] and quartz [21], thisis obviously a high-cost solution and unrealistic for producing de-vices. Another approach is to insert a nano-meter-scale inorganiccrystalline layer, i.e., a nano-sheet, between a TCO film and the glasssubstrate [22]. However, covering the macroscopic substrate surfacewhile assembling numerous nano-sheets is a hard task. This nano-sheet approach still seems to be far away from the production stage.Also, the adhesive strength of TCO films to the substrate is reduced.Another way is to deposit a thin buffer layer prior to growing thefilm for the TCO body [21]. A polycrystalline ZnON seed layer hasbeen demonstrated to be promising in promoting crystallization of

Fig. 1. (a) Schematic model of five structures for unit thickness of h along with provi-sional upper GZO layer without bottom layer, and (b) equivalent model for electricconduction and optical transmission.

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the film for the TCO body at the initial stage of growth [23]. However,once a buffer layer is deposited in the presence of O2 (and N2) gases,the ZnO target surface is oxidized (or nitrided) more than the bestconditions required to deposit the TCO film. If the buffer layer isdeposited in the same deposition chamber, it interferes with the de-position of the TCO film, which should be carried out under a reducedatmosphere.

Against this background, we propose that a double layer structure,where an upper GZO or AZO layer is overlaid on an undoped ZnO bot-tom layer, would be a competitive candidate for transparent elec-trodes in solar cells. The basic concept is as follows. If a GZO film isdivided into two parts, e.g., as bottom and upper layers, the bottomlayer is regarded as a buffer layer that leads to better crystallinityfor the upper layer. This means that the bottom layer scarcely contrib-utes to electrical conduction and the upper layer, where crystallinityhas been sufficiently improved, primarily provides the conductionpath. Nevertheless, the bottom half of GZO is more photo-absorptivethan ZnO because of its higher carrier concentration. Hence, if wesubstitute the bottom half of GZO with ZnO, the transmittance ofthe resulting GZO/ZnO double layer film would be improved in the in-frared range while electrical conductivity would be slightly degraded.We demonstrated the feasibility of this GZO/ZnO double layer struc-ture as a transparent electrode for solar cells.

2. Experimental details

The details on our dual target sputtering system have been de-scribed elsewhere [24]. The base pressure in the deposition chamberwas 5×10−5 Pa. ZnO films were deposited by electron cyclotron res-onance (ECR) plasma sputtering from a ZnO target [25]. The ECR plas-ma was generated from Ar gas without introducing O2 gas into theplasma source. The powers of the microwaves and RF applied to thetarget were both 500 W. GZO films were deposited by ECR plasmasputtering from the ZnO target and simultaneous RF magnetronsputtering from a Ga2O3 target at a fixed RF magnetron power of20 W [24]. The substrates we used were rectangular glass plateswith dimensions of 52×76 mm. Although the substrate was not in-tentionally annealed by using a heater from its rear side, impingingsputtered atoms and an Ar plasma stream warmed it up to 70 °Cduring deposition. Ga concentration in the deposited films had a con-tinuous distribution over the substrate surface, which enabled combi-natorial evaluation of sheet resistance and optical transmittance as afunction of Ga2O3 content. Regarding GZO as a ZnO–Ga2O3 compositematerial, we denoted Ga concentration in a GZO film as the weight ofGa2O3 with respect to the total weight of ZnO and Ga2O3. Ga2O3

content was determined by inductively coupled plasma atomic emis-sion spectroscopy (SPS1700, Seiko Instruments). Emission lines at213.856 nm (Zn) and 417.206 nm (Ga) were measured at an RFpower of 1.3 kW. Sheet resistance was measured by using a four-point prober and optical transmittance spectra were measured witha spectrophotometer (UV-3100, Shimadzu). Crystal structures wereevaluated with X-ray diffraction (XRD) (RINT1500, Rigaku) using aCu Kα line with the Bragg–Brentano configuration.

We prepared a set of five samples for unit thickness h, as schemati-cally illustrated in Fig. 1(a). They included a ZnOfilmwith a thickness ofh (ZnO(h)), a GZO filmwith a thickness of h (GZO(h)), a ZnOfilmwith athickness of 2h (ZnO(2h)), a GZO filmwith a thickness of 2h (GZO(2h)),and a double layer structure consisting of an upper GZO layer (u-GZO)and a bottom ZnO layer with a thickness of h for each layer (GZO(h)/ZnO(h)). We regarded GZO(2h) as being composed of an upper halfof GZO overlaid on a bottom half of GZO, i.e., GZO(2h)≡u-GZO(h)/GZO(h). Four sets of sample groups with h=50, 100, 150, and200 nmwere produced. We assumed that the upper and bottom layersin double layers were connected in parallel in terms of an equivalentelectrical circuit [Fig. 1(b)]. This was rationalized by the fact that elec-trons travel horizontally and those scattered at a large angle by ionized

impurities and grain boundaries do not actually contribute to electricconduction. We could thus calculate the sheet resistance of u-GZO inGZO(h)/ZnO(h) from the sheet resistance of GZO(h)/ZnO(h) and thatof ZnO(h). Similarly, we could calculate the sheet resistance of u-GZOin GZO(2h) from the sheet resistance of GZO(2h) and that of GZO(h).We also assumed that the optical transmittance of the GZO(h)/ZnO(h)doubled layer would simply be given as the product of the transmit-tance of u-GZO(h) and ZnO(h) by neglecting interference at the inter-face [Fig. 1(b)]. This approximation is valid in an infrared range wherethe effect of interference fringes has decayed. We could calculate thetransmittance spectra of u-GZO(h) in GZO(h)/ZnO(h) from the opticaltransmittance of GZO(h)/ZnO(h) and ZnO(h). Similarly, we could calcu-late the optical transmittance of u-GZO(h) in GZO(2h) from the trans-mittance of GZO(2h) and GZO(h).

3. Results and discussion

Fig. 2 plots the ZnO(002) XRD peak intensity measured in theω-2θscan mode as a function of the thickness of ZnO and GZO films. TheZnO(002) intensity can be fitted to a parabolic function with respectto the thickness, whichmeans that the crystallinity improves in an ac-celerated manner as the film thickens. The coefficient of the paraboliccurve for GZO (0.55) being larger than that for ZnO (0.27) indicatessuperior crystallinity of GZO films because they contain more oxygenatoms supplied from the Ga2O3 target.

Fig. 3 compares the calculated sheet resistance of u-GZO(h) inGZO(2h) and u-GZO(h) in GZO(h)/ZnO(h) for unit thicknesses of 50,100, and 150 nm. The sheet resistances of u-GZO(h) are similar foreach unit thickness, irrespective of what the bottom layer consistsof. We can also see that thickness is the major parameter reflectingcrystallinity. Whether Ga is doped or not in the bottom layer has littleeffect on the crystallinity of the u-GZO layer or sheet resistance. Thebottom layer can be regarded as a crystalline template to facilitate

Fig. 2. Dependence of ZnO(002) XRD intensities on film thickness of ZnO (open circles)and GZO (closed circles).

Fig. 4. Comparison of optical transmittance spectra contributed by u-GZO layer inGZO(h)/ZnO(h) (dashed line) and in GZO(2h) (solid line) for unit thicknesses of(a) 50, (b) 100, and (c) 150 nm. Ga2O3 content in GZO is 3.6 wt.%.

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the formation of high-quality crystalline domains just from the begin-ning of the upper layer.

Fig. 4 compares the calculated optical transmittance spectra of theu-GZO layer (Ga2O3 content of 3.6 wt.%) in GZO(2h) and the u-GZOlayer in GZO(h)/ZnO(h) for unit thicknesses of 50, 100 and 150 nm.The interference fringe is responsible for the unrealistic transmittanceexceeding 100% below 1000 nm. The transmittance curves overlapone another in the infrared range, where interference fringes do notappear. This is a confirmation that the optical properties of u-GZOin the infrared region scarcely depend on whether the bottom layeris ZnO or GZO. The measured transmittance spectra of ZnO(h) andGZO(h) films are compared in Fig. 5 for h=50, 100, and 150 nm

Fig. 3. Comparison of sheet resistance of u-GZO layer in GZO(h)/ZnO(h) (open circles)and in GZO(2h) (closed circles) for unit thickness of (a) 50, (b) 100, and (c) 150 nm.

with Ga2O3 content of 3.6 wt.%. Here, the two curves for each h valuediffer considerably. The transmittance of GZO in the infrared range ismore suppressed than that of ZnO, whereas GZO is more transparentbetween 400 and 800 nm. The reduced transmittance of ZnO withinthe visible wavelength is caused by oxygen deficiencies in ZnO, butthey are necessary to make ZnO conductive [26].

Fig. 6 plots the sheet resistance (R) of ZnO(h), ZnO(2h), GZO(2h),GZO(h)/ZnO(h), and u-GZO(h) in GZO(h)/ZnO(h) as a function of theGa2O3 content for a unit thickness of h=50 nm. ZnO(50 nm) has thehighest sheet resistance for the five samples. The sheet resistanceof u-GZO(50 nm), GZO(50 nm)/ZnO(50 nm), and GZO(100 nm) arecomparable. The sheet resistance of ZnO(100 nm) is still higherthan these three. The sheet resistance at Ga2O3 content lower than6 wt.% follows the order:

R GZO 2 hð Þð ÞbR GZO hð Þ=ZnO hð Þð ÞbR u−GZO hð Þð ÞbR ZnO 2 hð Þð ÞbR ZnO hð Þð Þ:

We confirmed this order for all h between 50 and 200 nm. Thesheet resistance of u-GZO(h) higher than that of GZO(h)/ZnO(h)means that the bottom ZnO(h) contributes somewhat to electric con-duction. The sheet resistance of GZO(h)/ZnO(h) higher than that ofGZO(2h) means that the bottom part of GZO(2h), i.e., GZO(h), ismore conductive than ZnO(h).

Substantial fractions of Ga atoms are non-activated at Ga2O3 con-tent above 10 wt.%, and such impurity atoms prevent crystallization,resulting in changes in the crystal structure of GZO [24,27]. Wefound the sheet resistance at h=50 and 100 nm followed the orderat 19 wt.%:

R ZnO 2 hð Þð ÞeR GZO hð Þ=ZnO hð Þð ÞbR GZO 2 hð Þð ÞbR u−GZO hð Þð ÞbR ZnO hð Þð Þ:

The carrier concentration was high for GZO film with high Ga2O3

content, but Hall mobility was low. However, the carrier concentration

Fig. 5. Comparison of optical transmittance spectra of ZnO(h) (dashed line) andGZO(h) (solid line) for unit thicknesses of (a) 50, (b) 100, and (c) 150 nm. Ga2O3 con-tent in GZO is 3.6 wt.%.

Fig. 7. Comparison of optical transmittance spectra of GZO(h)/ZnO(h) (dashed line)and GZO(2h) (solid line) for h values of (a) 50, (b) 100, and (c) 200 nm.

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of ZnO was low but its Hall mobility was high. As a result of these fac-tors, the sheet resistance of ZnO(2h) took values comparable to thoseof GZO(h)/ZnO(h). The optical transmittance spectra of GZO(50 nm)/ZnO(50 nm) and GZO(100 nm) are compared in Fig. 7(a). While theirtransmittance levels are similar at wavelengths shorter than 1000 nm,the transmittance of GZO/ZnO is higher by 10–15% in an infraredrange between 1000 and 3000 nm.

Fig. 8 compares the sheet resistance of the five samples witha unit thickness of 100 nm. In this case, the sheet resistance ofGZO(200 nm), GZO(100 nm)/ZnO(100 nm), and u-GZO(100 nm)in GZO(100 nm)/ZnO(100 nm) also take similar values. At a Ga2O3

Fig. 6. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h)layer in GZO(h)/ZnO(h) for h=50 nm.

content of 3.6 wt.%, for instance, R(GZO(200 nm))=46 Ω/□, R(GZO(100 nm)/ZnO(100 nm))=52 Ω/□, and R(u-GZO(100 nm))=59 Ω/□.These values indicate that the sheet resistance of the bottom halfof GZO(200 nm), i.e., GZO(100 nm), and that in GZO(100 nm)/ZnO(100 nm) are much higher than that of u-GZO(100 nm), and electricconduction mostly relies on u-GZO(100 nm). The optical transmittanceof GZO(200 nm) and GZO(100 nm)/ZnO(100 nm) are compared inFig. 7(b). Their transmittance at wavelengths shorter than 1000 nmare at similar levels, whereas the transmittance of GZO(100 nm)/ZnO(100 nm) in the infrared range is higher than that of GZO(200 nm)by 20–40% at each wavelength. In summary, using GZO(100 nm)/

Fig. 8. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h)layer in GZO(h)/ZnO(h) for h=100 nm.

Fig. 9. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h)layer in GZO(h)/ZnO(h) for h=200 nm.

Fig. 11. Comparison of optical transmittance spectra of u-GZO(50 nm)/ZnO(150 nm)(dashed line), u-GZO(100 nm)/ZnO(100 nm) (dotted line), and u-GZO(150 nm)/ZnO(50 nm) (solid line).

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ZnO(100 nm) instead of GZO(200 nm) deteriorates the sheet resistanceby 13%, but improves the infrared transmittance by 20–40% at eachwavelength.

Fig. 9 plots the sheet resistance of the five samples with a unit thick-ness of 200 nm. The general tendency is the same as that already seen inFigs. 6 and8. At aGa2O3 content of 3.6 wt.%, R(GZO(400 nm))=14 Ω/□,R(GZO(200 nm)/ZnO(200 nm))=21 Ω/□, and R(u-GZO(200 nm))=25 Ω/□. The optical transmittance spectra of GZO(400 nm) and GZO(200 nm)/ZnO(200 nm) in Fig. 7(c) again reveal that the transmittancein the visible range is similar but differs by 20–40% at wavelengthslonger than 1000 nm.

Finally,we investigated thebestwayof sharing the thickness betweenu-GZO(h1) and ZnO(h2) in the GZO(h1)/ZnO(h2) double layers whenthe total thickness is fixed, i.e., h1+h2=200 nm. We considered threesamples: GZO(50 nm)/ZnO(150 nm), GZO(100 nm)/ZnO(100 nm), andGZO(150 nm)/ZnO(50 nm). Fig. 10 plots the sheet resistance of thethree GZO/ZnO double layer films and Fig. 11 plots their optical transmit-tance. The sheet resistance and optical transmittance of GZO(100 nm)/ZnO(100 nm) were measured but those of GZO(50 nm)/ZnO(150 nm)and GZO(150 nm)/ZnO(50 nm) were calculated. While GZO(150 nm)/ZnO(50 nm) has the lowest sheet resistance, its transmittance at wave-lengths longer than 1200 nm is the lowest. While GZO(50 nm)/ZnO(150 nm) has the highest transmittance at wavelengths longer

Fig. 10. Comparison of sheet resistance of u-GZO(50 nm)/ZnO(150 nm) (open circles),u-GZO(100 nm)/ZnO(100 nm) (open triangles), and u-GZO(150 nm)/ZnO(50 nm)(open squares).

than 1200 nm, its sheet resistance is the highest. GZO(100 nm)/ZnO(100 nm) has a medium sheet resistance and intermediate trans-mittance level above 1200 nm for the other two samples. The h1 andh2 values can be optimized depending on theweighted priority of infra-red transmittance and sheet resistance.

Apart from the requirement for high transmittance within theavailable wavelength of sunlight, three other important factors forapplication to solar cells include efficient confinement of incidentlight, efficient conversion of photon energies to electric current,and suppressed recombination between photo-generated carriers.Although it is unknown whether the GZO/ZnO double layer structureperforms satisfactorily concerning these requirements, its electricconduction parameters and optical transmittance are at least compet-itive against SnO2-based TCOs.

4. Conclusion

We demonstrated the feasibility of a GZO/ZnO double layer struc-ture for application to transparent electrodes in solar cells. Whetherthe bottom layer is GZO or ZnO did not affect sheet resistance or theinfrared transmittance of the upper GZO layer in GZO(h)/ZnO(h)and in GZO(2h). The optical transmittance of ZnO and GZO bottomlayers considerably differed in the infrared range. If we replacedGZO(2h) film with GZO(h)/ZnO(h) film that had the same total thick-ness, the optical transmittance in the infrared region increased by15–40% at each wavelength, while sheet resistance increased by1.1–1.5 times.

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