International Journal of Hydrogen Energy 32 (2007) 2726–2729www.elsevier.com/locate/ijhydene

Novel photosensitive materials for hydrogen generation throughphotovoltaic electricity

K. Yamaguchia,∗, H. Udonob

aQuantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai-mura, Ibaraki-ken 319-1195, JapanbDepartment of Electrical and Electronic Engineering, College of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki-ken 316-8511, Japan

Available online 3 January 2007

Abstract

The present paper is concerned with the research and development activities on processing technology of novel silicide-based materials forthe production of low cost photovoltaic electricity that can be used for hydrogen generation through water electrolysis. The paper discussessome of the properties of semiconducting iron disilicide (�-FeSi2) essential for photovoltaic applications. It is shown that highly oriented�-FeSi2 film of 100 nm in thickness can be deposited on silicon (Si) substrates, while a bulk single crystal of about 3–4 mm can be grownfrom solution containing garium (Ga; p-type), zinc (Zn; p-type) and tin (Sn; n-type).� 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Iron disilicide (�-FeSi2); Single crystal; Thin film; Iron beam sputter deposition

1. Introduction

Hydrogen production via the photovoltaic route will not di-rectly result in carbon dioxide emissions and, therefore, maybe considered as environmentally friendly technology. For thispurpose, titanium dioxide (TiO2) is considered to be suitabledue to its high corrosion resistance in aqueous solutions. How-ever, it is known that the energy conversion efficiency of solarenergy using a photo-electrochemical cell equipped with un-doped TiO2 is very small [1], owing to its relatively large bandgap (3.2 eV).

Among the semiconducting silicides known to date, � iron(Fe) disilicide, �-FeSi2, has received much attention due to thefact that it consists of the elements that are naturally abun-dant and less toxic as compared to currently available semi-conductors that are composed of III–V or II–VI compoundsemiconductor materials. For these reasons, such a group ofsemiconductors is called “ecologically friendly” semiconduc-tors [2]. Historically, �-FeSi2 first emerged as a candidate mate-rial for thermoelectric applications owing to its relatively largeSeebeck coefficient [3]. Then, it was reported that �-FeSi2 is a

∗ Corresponding author. Tel.: +81 29 282 5754; fax: +81 29 284 3822.E-mail address: yamaguchi.kenji@jaea.go.jp (K. Yamaguchi).

0360-3199/$ - see front matter � 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2006.09.014

“direct” semiconductor with an energy gap of 0.87 eV [4] (seethe comment in Table 1), while its absorption coefficient isfound to be quite large [5]. In addition, as common to most sili-cides, it has high thermal stability and resistance to oxidation,and is expected to be also resistant to irradiation as well [6].It is worthwhile to mention that it is the only semiconductingsilicide for which light emission has been clearly observed sofar. Consequently, considerable efforts have been undertaken inthe recent years to fabricate this material for optoelectronic orphotovoltaic application [7].

The paper will consider the properties of �-FeSi2 comparedto that of Si in terms of application to photovoltaics. Then, thepaper will describe the fabrication processes of both bulk crys-tal and thin film of �-FeSi2. It will be reported that high qualitysingle crystal of several mm’s in size can be fabricated by meansof solution growth method, while highly sophisticated process-ing procedure of �-FeSi2, based on the application of ion beamtechnique and nano-structural characterization, enabled the for-mation of high quality film with a thickness of about 100 nm.

2. �-FeSi2 as photovoltaic material

The semiconducting phase of �-FeSi2 has an orthor-hombic crystalline structure with the lattice constants of

K. Yamaguchi, H. Udono / International Journal of Hydrogen Energy 32 (2007) 2726–2729 2727

Fig. 1. Absorption coefficient of �-FeSi2, together with some other semicon-ducting materials, as a function of photon energy.

a = 0.9863 nm, b = 0.7791 nm and c = 0.7833 nm, and is ther-modynamically stable up to 1210 K. As mentioned in a previoussection, in contrast to TiO2 (3.2 eV), �-FeSi2 is a narrow-band-gap (0.87 eV) semiconductor, so that application of anodic biaspotential is essential to elevate the Fermi level (EF) cathodeabove the H+/H2 energy level, thus making the process of wa-ter decomposition possible. On the other hand, it has a verylarge absorption coefficient of 105 cm−1 at 1 eV as shown inFig. 1. This means that thin films can be applied for solar cells,so that the use of silicon (Si) can be significantly reduced. Inaddition, it will be explained in Section 3.2 that �-FeSi2 canbe grown epitaxially on Si substrates. Therefore, thin film of�-FeSi2 on Si substrate can be employed to use the infrared rayside of sun optical spectrum that cannot be used by Si.

However, there are some properties of �-FeSi2 that are lessfavorable for photovoltaic application in comparison with Si.As it has been found that the energy gap of �-FeSi2 at zero tem-perature determined from optical absorption measurement onbulk single crystal is 0.814 eV [8], the maximum solar energyconversion efficiency would be 17%, which is smaller than thatof crystalline Si. Using the same data, the intrinsic carrier con-centration of �-FeSi2 at 300 K is estimated to be 5×1013 cm−3

[9]. This value is about 3 orders of magnitude larger than thatof bulk crystalline Si at the same temperature. The origin of thisintrinsic conduction or the identification of carriers is yet to beclarified. These facts may pose some difficulty in fabricatingp–n junction for the effective charge separation of the carriers.

Little is known as to the corrosion resistance of iron silicides,although it is reported that �-FeSi2 single crystals are not etchedby hydrochloric acid (HCl; 36%), nitric acid (HNO3; 60%) oraqua regia [10]. Furthermore, least is known as to irradiationbehavior of semiconducting silicides in general, while some lit-erature data indicate that both crystalline and amorphous Si maybe sensitive to irradiation by light [11] or ion beams [12]. Pho-toconductivity of non-doped crystalline Si starts to decrease atvery low incident ion (17 MeV H+) fluence, while the radiationtolerance of doped Si is prolonged to some extent. Amorphous

Si (a-Si) is somewhat more radiation-resistant to ion bombard-ment. Together with other data reported in literature [13–16],Table 1 summarizes the main features of �-FeSi2 and Si (in-cluding a-Si) relevant to photovoltaic application.

3. Fabrication of highly quality �-FeSi2 bulk crystal andthin film

3.1. Fabrication of bulk crystals

There are two phases for disilicide composition in the Fe–Siphase diagram. The high temperature phase �-FeSi2, which isthermodynamically metastable at ambient temperature, trans-forms into the low-temperature phase, �-FeSi2, which is sta-ble below 1210 K. Therefore, ordinary bulk growth techniquesfrom the melt such as the Bridgemann, Czochralski (CZ), float-ing zone (FZ) and horizontal gradient freeze (HGF) methodsare not used for single crystal growth of �-FeSi2. The firstpreparation of single crystalline �-FeSi2 is reported in 1968 bythe chemical vapor transport (CVT) method using iodine as atransport agent [17]. However, the crystals grown by CVT areneedlelike with the size of 0.5 × 1 × 10 mm3, and usually havetwins. Temperature gradient solution growth (TGSG) [18–20]is an alternative method, which has succeeded in growing sin-gle crystalline �-FeSi2. Main advantages of TGSG can be sum-marized as follows:

(1) Since crystals are grown under near equilibrium condition,lattice defects such as point defects, line defects and planedefects can be greatly reduced.

(2) Doping control can be easily achieved by changing thesolvent composition.

Typical growth conditions are as follows: growth temperature;TG = 1153–1173 K, temperature difference between a sourceand a growth seed; �T = 40–60 K, temperature gradient of thefurnace; 40–60 K cm−1.

p-Type �-FeSi2 single crystals were grown using garium (Ga)or zinc (Zn) solvent, in which the largest sized crystals weregrown from Ga solvent. Fig. 2(a) shows a �-FeSi2 plate-likecrystal grown on a carbon seed using Ga solvent [20]. The sizeis 10 mm in diameter. The crystal was highly conductive withthe hole concentration of (1–2) × 1019 cm−3 at room temper-ature (RT). On the other hand, low conductive crystals withthe hole concentration of (2–4) × 1017 cm−3 were grown fromZn solvent (Fig. 2(b)) [3,21]. n-Type single crystals were alsogrown using tin (Sn) solvent [22]. Typical electron concentra-tion was (0.3–5) × 1018 cm−3 for Sn–�-FeSi2.

3.2. Fabrication of highly oriented thin film of �-FeSi2

The lattice parameters of �-FeSi2 acceptably match thoseof Si (1 0 0) and Si (1 1 1) substrates; where the epitaxial re-lationships can be expressed as �-FeSi2 (1 0 0)//Si (1 0 0) and�-FeSi2 (1 0 1) or (1 1 0) // Si (1 1 1), with the lattice mismatchbeing 1.4–2.0% and < 5.5%, respectively. This fact indicatesthat epitaxial growth of �-FeSi2 film on Si substrate is possible.

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Table 1Comparison of the fundamental material properties of �-FeSi2 and bulk Si in connection with photovoltaic applications

c-Si a-Si �-FeSi2

Resource and Si: resourceful and harmless Si: resourceful and harmless Fe and Si: resourceful and harmlessenvironmental issues Cell; bulk Cell; film is possible Cell; thin filmAbsorption coefficienta > 103 cm−1 > 104 cm−1 at 2 eV > 105 cm−1 at 1 eVBand gap (300 K) 1.1 eV [13] (indirect) 1.7–1.8 eV [14] (Tauc gap) 0.87 eVb

Intrinsic carrier concentration 1 × 1010 cm−3 at 300 K [13] —c 5 × 1013 cm−3 at 300 K (calc.)Resistance to irradiation Sensitive [12] More resistant to ion bombardment Unknown (probably superior to c-Si)

than c-Si [12]

aSee Fig. 1.bIt has been found that bulk �-FeSi2 single crystal is an indirect semiconductor [5], but the strained crystal is expected to have a direct band gap [15].cThe electrical properties are known to be influenced by the presence of dangling bonds, the density of which can be reduced by hydrogenation (a-Si:H) [16].

Fig. 2. Examples of high quality �-FeSi2 single crystal; (a) Plate-like �-FeSi2grown on carbon seed by TGSG method using Ga solvent (b) p-type �-FeSi2single crystals grown by TGSG method using Zn solvent.

One of the methods which has been proven to be successfulin obtaining epitaxial thin film of �-FeSi2 is ‘ion beam sput-ter deposition’ (IBSD) technique, in which Fe atoms sputteredfrom Fe-containing solid target are deposited onto Si substrateheated at elevated temperature [23]. Main features of IBSDprocess can be summarized as follows:

(1) Since ion sources and deposition chamber are pumped in-dependently, ultra-high vacuum condition can be achievedduring deposition. Therefore, the content of impuritiesfrom the residual gas can be greatly reduced.

(2) Although the incident energy bombarding the solid targetexceeds several tens of keV, the energy of sputtered atomsis in the order of several tens of eV, so that formation ofirradiation-induced defects are negligible.

Typical deposition conditions are as follows: incident ion;argon (Ar), incident energy; 35 keV, ion current density; ap-prox. 0.2 mA cm−2, target materials; Fe or Fe–Si compounds,deposited Fe thickness; 8–30 nm, substrate temperatures;773–1073 K, whereas the base pressure of the deposition

0

100

200

300

400

10 20 30 40 50 60 70 80 90

2θ (degree)

Si (

200)

Si (

400)

(400

)

(600

)

(800

)

Spec

tral

Int

ensi

ty (

a. u

)

Fig. 3. XRD pattern showing highly oriented �-FeSi2 film formed on Si(1 0 0) substrate. The film was formed at 973 K.

chamber was less than 8.0 × 10−8 Pa, the residual gas pressureduring deposition was < 2.7 × 10−5 Pa.

The crystal orientation of �-FeSi2 was found to be quite sen-sitive to the preparation methods of substrate surface prior todeposition [24]. Among the various methods tried, “in situ”sputter cleaning of the substrate surface by 1–3 keV neon ions(Ne+), followed by thermal annealing at 1073 K, was not onlyconvenient, but also resulted in the formation of highly oriented�-FeSi2 (1 0 0) film on Si (1 0 0) [25]. As shown in the XRDpattern of Fig. 3, only the peaks of � (1 0 0) phases are ob-served, indicating clearly the epitaxial relationship of � (1 0 0) //Si (1 0 0). In addition, a clear-cut interface of silicide/substrateis visible in the cross-sectional TEM image of Fig. 4. At theFe deposition rate of 0.5 nm min−1, the film with highest de-gree of crystal orientation was obtained at 973 K with the de-posited Fe thickness of 30 nm (equivalent to �-FeSi2 thicknessof about 100 nm), whereas similar degree of crystal orientationwas obtained at 873 K with the deposited Fe thickness of 15 nm[26]. The above results are considered to be the consequenceof success to control the inter-diffusion of Si and Fe atoms atthe silicide–silicon interface during film formation through op-timization of the deposited thickness and the substrate temper-ature [26], coupled with the careful treatment of the substratesurface [24,25]. All these procedures are done within the pro-cesses of IBSD method.

K. Yamaguchi, H. Udono / International Journal of Hydrogen Energy 32 (2007) 2726–2729 2729

Fig. 4. High resolution cross-sectional TEM image of �-FeSi2/Si interface.The substrate surface was treated by “in situ” sputter cleaning by 1 keV Ne+and post-irradiation thermal annealing.

4. Summary

It was shown that an “ecologically friendly” semiconductor,�-FeSi2, has some attractive features that are more suitablefor a solar cell material, and since its fabrication process iscompatible with current Si technologies, it can be used togetherwith Si as well. We have succeeded in growing high quality�-FeSi2 single crystals by the TGSG method using Ga, Zn andSn solvent. The crystals grown from Ga and Zn solvent werep-type conduction, whereas ones grown from Sn were n-type.Highly oriented �-FeSi2 (1 0 0) film with the thickness of about100 nm can be formed on Si (1 0 0) by appropriately combiningsurface cleaning of the substrate and the deposition conditionsof IBSD (ion beam sputter deposition) process.

Acknowledgments

The authors would like to thank Dr. M. Sasase of Wakasawan Energy Research Center for providing the cross-sectionalTEM image of �-FeSi2 thin film, and also, Drs. H. Yamamoto,K. Shimura, S. Shamoto and K. Hojou of Japan Atomic EnergyAgency for fruitful discussion.

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