Structure characterization and photon absorption analysis of carbon-doped β - Fe Si 2filmXiaona Li, Dong Nie, Chuang Dong, Lei Xu, and Ze Zhang Citation: Journal of Vacuum Science & Technology A 22, 2473 (2004); doi: 10.1116/1.1795832 View online: http://dx.doi.org/10.1116/1.1795832 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/22/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Optical Investigations of Semiconducting FeSi2 using Photoreflectance Spectroscopy AIP Conf. Proc. 772, 109 (2005); 10.1063/1.1994017 Optical and structural properties of β - FeSi 2 precipitate layers in silicon J. Appl. Phys. 94, 207 (2003); 10.1063/1.1576902 Formation of light-emitting FeSi 2 in Fe thin films on ion-implanted (111)Si J. Appl. Phys. 93, 1468 (2003); 10.1063/1.1534379 Investigation of direct and indirect band gaps of [100]-oriented nearly strain-free β- FeSi 2 films grown bymolecular-beam epitaxy Appl. Phys. Lett. 80, 556 (2002); 10.1063/1.1432755 Growth of β-FeSi 2 films via noble-gas ion-beam mixing of Fe/Si bilayers J. Appl. Phys. 90, 4474 (2001); 10.1063/1.1405818

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Structure characterization and photon absorption analysisof carbon-doped b-FeSi2 film

Xiaona Lia)

State Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Dalian Universityof Technology, Dalian 116024, China and Beijing Laboratory of Electron Microscopy, Chinese Academy ofSciences, P. O. Box 2724, Beijing 100080, China

Dong Nie and Chuang DongState Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Dalian Universityof Technology, Dalian 116024, China

Lei XuState Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Fudan University,Shanghai 200433, China

Ze ZhangBeijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, P. O. Box 2724, Beijing 100080,China

(Received 7 July 2003; accepted 26 July 2004; published 28 October 2004)

Carbon-dopedb–FeSi2 films synthesized by ion implantation are investigated with the aim tofabricate high-quality semiconductingb–FeSi2 layer on silicon substrate. According to transmissionelectron microscopy(TEM) cross-section observations, carbon-doped films with homogeneousthickness and smoothb–Si interface, have higher quality than binary Fe–Si films. In particular,annealing at 500°C–700 °C leads to the formation of a flat and continuousb-type silicide layer.Improved stability of theb phase is also found. Optical emission spectroscopy measurements showthat the carbon doping influences only slightly the band gap values.© 2004 American VacuumSociety.[DOI: 10.1116/1.1795832]

I. INTRODUCTION

Metal silicides have drawn enormous attention in recentyears due to their compatibility with the present well-founded Si-based technology. Among the many metal sili-cides, only a few are semiconductors andb–FeSi2 is themost important one. It has a direct band gap ofEg

d

=0.85–0.89 eV and exhibits strong fluorescence signals near1.54mm wavelength.1–3 Many methods have been employedto synthesizeb–FeSi2 films, such as solid phase epitaxy,reaction deposition epitaxy, molecular beam epitaxy, and ionbeam synthesis(IBS). The last technique can be used directlyto fabricate heterojunction light-emitting diodes. Besides,IBS is a matured technique to dope semiconductors. Most ofthe previous work has been directed towards synthesis andcharacterization of pureb–FeSi2 films.4–7 Doping of theb–FeSi2 films have also been attempted with mainly metalelements to substitute Fe, for example, Mn or Al can act asan acceptable impurity in the silicide layer.8 According to ourprevious investigation,9–11 there are several competing andcomplex orientation relationships betweenb–FeSi2 and Simatrix, and this is an intrinsic factor that deteriorates thequality of the films. In the present work, we dopedb–FeSi2with carbon (C), with the aim to fabricate films with im-proved quality by adjusting lattice matching betweenb–FeSi2–Si. Since carbon and silicon belong to the same

group in the periodic table, we expect that such a dopingwould not change the conducting type(n- or p-type) of bphase and its optoelectronics properties would not changesignificantly. To verify this latter point, we carried out anoptical emission spectroscopy investigation.

II. EXPERIMENT

Si wafer ofn-typed(100) was selected as substrate with aresistivity of 4–8Vcm. Ion implantations of Fe and C wereconducted consecutively on MEVVA(Metal Vapor VacuumArc Ion Source) 80–10 system with the implantation param-eters as listed in Table I. The vacuum was maintained at 2310−6 Torr. The as-implanted samples were annealed invacuum at 500 °C, 600 °C, 700 °C, and 850 °C for 1 h.Transmission electron microscope(Philips-CM12) was usedto characterize the microstructure. Grazing angle x-Ray dif-fraction (GAXRD) was carried out for phase identificationwith incident angle fixed at 1°. Photon absorption was mea-

a)Author to whom correspondence should be addressed; electronic mail:meili@dlut.edu.cn

TABLE I. Implantation parameters.

Sample

Implantation parametersAcceleration voltage(kV)/dosagesions/cm2d

C–Fe dosageratioFe C

No. 1, Si (100) 60/431017¯ ¯

No. 2, Si (100) 60/431017 20/431015 1%No. 3, Si (100) 60/431017 20/231016 5%

2473 2473J. Vac. Sci. Technol. A 22 (6), Nov/Dec 2004 0734-2101/2004/22 (6)/2473/6/$19.00 ©2004 American Vacuum Society

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sured on a UV-3101PC visible-infrared spectrometer wherethe reference sample was a raw Si wafer without ion implan-tation.

III. RESULTS AND DISCUSSIONS

A. Structure characterization

In our previous work, it was found that the implantationof Fe under low acceleration voltages and low dosages re-sulted in the formation ofg phase in as-implanted state.9–11

To form b phase directly, at least 50 kV and 431017 ions/cm2 would be required. Therefore, we selected60 kV as the implantation voltage while maintaining a con-stant dosage for Fe. Since the carbon ions are implantedmuch more deeply than the Fe ions, we chose a low voltagefor carbon, 20 kV, which is the lowest voltage possible for

our equipment. According to the calculations withTRIM96,the projection range of Fe in Si matrix isRp=78.7 nm withDRp=24.7 nm, and the projection range of C in the doublelayers of FeSi2 s100 nmd and Si substrate isRp=39.9 nmwith DRp=22.7 nm.

Two typical samples, sample 1(without carbon doping)and sample 2(Carbon-doped), were analyzed by TEM. Fig-ure 1 shows cross-sectional TEM images of sample 1 an-nealed at different temperatures. The viewing direction isperpendicular to Si(110). Two amorphous layers are formedin the as-implanted state[Fig. 1(a)]. According to the plane-view observation illustrated in Fig. 2(a), the outer layer is infact mixed with primary crystallites. The grain size of thecrystallites is in nanometer scale and the nanograins agglom-erate into large clusters several micrometers in size imbed-ded in the amorphous matrix. The electron diffraction pattern[Fig. 2(b)] indicates that these tiny crystallites are all of theb–FeSi2 structure. The thickness of this layer is about50 nm. Beneath this surface layer exists another buriedamorphous layer of thickness about 40 nm, as proved bySAED pattern in Fig. 2(c). In the substrate adjacent to thislayer, there is an implantation-damaged zone containingimplantation-induced structure defects andb-type precipi-tates. For more details about the damaged zone, the readersare referred to our previous works.9,10

After annealing at 500 °C, 600 °C, 700 °C, and 850 °C,respectively, for 1 h, theb–FeSi2 phase is formed from theamorphous state. Their cross-section TEM images are shownin Figs. 1(b)–1(e). Upon annealing, the amorphous film crys-tallizes into theb–FeSi2 phase, starting from the outer layer.At 500 °C, the outer layer has anb-amorphous mixed struc-ture and the buried amorphous layer maintains amorphous.At 600 °C, both layers crystallize into theb phase. However,even after the complete crystallization, the original interfacebetween the two amorphous layers still persists but movesupwards towards the surface. At higher temperatures, theouter layer becomes thinner and the inner layer thicker. At850 °C, the interface disappears. The grains grow with in-creasing temperatures, destroying the originally flat

FIG. 1. Cross-section TEM images taken from sample 1(Fe: 60 kV, 431017 ions/cm2). The viewing direction is along Si[110]. (a) As-implanted,(b) 500 °C/1 h, °C) 600 °C/1 h,(d) 700 °C/1 h,(e) 850 °C/1 h.

FIG. 2. Plane view image of Fig. 1(a), (a) and corresponding electron dif-fraction pattern of amorphous matrix(c) and silicides clusters(b). So thebphase has already been formed in isolated clusters in the as-implanted state.

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amorphous–Si interface. At 850 °C, theb phase layer,originally continuous, shrinks into separated islands.Implantation-induced damage is recovered gradually as tem-perature rises, as visible in the cross-sectional images from500 °C to 850 °C.

Sample 1 is not doped with carbon. Sample 2 was pre-pared under the same implantation parameters for Fe ele-ment, while with the subsequent implantation of carbon. Fig-ure 3 illustrates a cross-sectional morphology evolutionversus annealing temperature for this sample. The viewingdirection is also perpendicular to Si(110). The carbon dos-age relative to that of Fe is 1%.

The as-implanted film is also double layered[Fig. 3(a)],with the outer layer containing theb clusters [Figs.4(a)–4(c)]. This is similar to the undoped sample[Fig. 1(a)].Further comparison with the undoped sample reveals finergrain sizes for the primaryb phase crystallites.

Upon annealing, the effect of carbon doping is furthermanifested. In Fig. 3(b), after 500°C/1 h annealing, ahighly flat and uniformb phase film is formed with clearinterface with the Si substrate. In contrast, theb–Si interface

begin to deteriorate at this temperature in sample 1. The filmcan be divided into three layers. The outer and inner layersconsist of b-phase grains that originate mainly from theamorphous layers. A thin but definitely present amorphouslayer persists between the twob layers. Such a sandwichstructure indicates that the crystallization occurs both fromthe outer surface and from the interface with respect to the Sisubstrate.

At 600 °C, the sandwich structure is completely crystal-lized, forming twob layers. The interface between the innerb–FeSi2 layer and the Si substrate is slightly roughened butstill significantly smoother than the Fe-implanted sample.

At higher temperatures[700 °C and 850 °C, Figs. 3(d)and 3(e)], the two layers merge into a single silicide layer.The b–Si interface smoothness decreases further but thebfilm is still continuous, in contrast to the shrinkage in theFe-implanted sample. Therefore, the thermal stability of theb–FeSi2 phase is increased through carbon doping.

Difference of grain size and structure state of the films insamples 1 and 2 were further revealed by TEM observationin plan-view mode, as shown in Fig. 5. For sample 1 an-nealed at 850 °C, silicide grains grew large, while the filmshrunk into separatedb islands, as shown in Fig. 5(a), theblack particles areb–FeSi2 grains, and the white regions are

FIG. 3. Cross-section TEM images of sample 2,(Fe: 60 kV, 431017 ions/cm2, C: 20 kV, 431015 ions/cm2, C/Fe dose=1%). The viewdirection is along Si[100]. (a) As-implanted, (b) 500 °C/1 h, (c)600 °C/1 h,(d) 700 °C/1 h,(e) 850 °C/1 h.

FIG. 4. Plane view image of Fig. 3(a), (a), and corresponding electron dif-fraction pattern of silicides clusters(b) and amorphous matrix(c).

FIG. 5. Plane view image of samples 1 and 2 after annealing at 850 °C;(a)undoped sample 1(b) doped sample 2.

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silicon matrix with implantation damages moving to the top.For sample 2, the silicide layer retained its continuity[Fig.5(b)]. As compared to the undoped sample 1, theb–FeSi2grain size is rather small. The small grain size might beattributed to the change of initial amorphous structure, as thematrix phase ofb phase before crystallization. When doping

with C ions, the amorphous structure will be enriched indefects and C impurities resulting in an high nucleation rate,and as a consequence the grains are finer.

Figure 6 shows the XRD patterns of samples 2 and 3annealed at 700 °C. Only diffraction peaks ofb–FeSi2 wereobserved and there are no peaks of Fe–C and Si–C com-pounds. So it can be concluded that carbon ions exist as solidsolution state in theb phase and Si substrate.

Moreover, a remarkable feature in the as-implantedsamples is the formation of double amorphous layers. Ionimplantation often leads to amorphous films. For example,Lin et al. also obtained Fe–Si amorphous films.12 At 60 kV,in both carbon-doped and undoped samples, the amorphousstate is formed. However, the amorphous separation into twolayers is still not understood. We suppose that there might betwo favorable glass-forming compositions in the Fe–Si sys-

FIG. 7. Photon absorption spectrum ofsatd2 as a function of photon energyE measured on samples annealed at different temperatures for the Fe-implanted sample(sample 1), a being the absorption coefficient andt beingthe thickness of the silicide layer.

FIG. 8. Band gap valuesEgd obtained from Fig. 7 as a function of annealing

temperatures(sample 1).

FIG. 6. X-ray diffraction patterns of the samples 2 and 3(Fe: 60 kV, 431017 ions/cm2, C: 20 kV, 431015 ions/cm2, 231016 ions/cm2) annealed at 700 °C.

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tems, as evidenced by two eutectic points(67 at. %Si and73.5 at. %Si) near the b phase composition zones66.7 at. %Sid. This argument is still to be verified by furtherexperiments. While, the formation of the primaryb phase inthe outer amorphous layer might be related to less energydeposition in this region, just similar to the lower voltagecase (for example, 50 kV implantation leads tob phasedirectly).

B. Photon absorption measurement

The band structure ofb–FeSi2 was calculated by Chris-tensen, who found that beside the direct bandgap there is alsoan indirect gap at about 0.80 eV at room temperature, thedifference between the two bandgaps being about 20 meV.13

This result was later confirmed by experiments.14–16 Forpractical reasons, more attention was paid to the direct gapEg

d in the range of 0.85–0.89 eV.1–3 Yanget al.measured theb–FeSi2 films synthesized by IBS and pointed out a directband gap at temperature range of 10–300 K,Eg

d=0.904 eV

at 10 K andEgd=0.847 eV at room temperature.4,5

Figure 7 shows the absorption indexsatd2 as a function ofphoton energyE for sample 1 annealed at different tempera-tures,a being the absorption coefficient of the silicide layer(the Si substrate contribution has been subtracted using thereference sample), t the thickness of the silicide layer. Theslopes of the curves increase with increasing annealing tem-peratures. We know from aforementioned TEM results thatthe surface layer in the as-implanted and low-temperatureannealed samples contains amorphous phase. Such an in-crease in slope reflects the increase in theb content in thefilms. A direct band gap is revealed for all theb phasesamples, the value of which,Eg

d, being determined by

extrapolating the curves toa2=0.4,5

There are nonzero backgrounds in all these curves. Ionimplantation certainly brings damage to the Si substrate. Theg phase precipitates also contribute to the absorption behav-ior. During the annealing, damages are gradually recovered,and theg silicide transforms intob–FeSi2, all decreasing thebackgrounds in these curves. Another factor contributing tothe nonzero background is related to the reference sample,which is a raw Si wafer, whose surface state is different fromthe implanted samples.

The bandgap values are summarized in Table II. Figure 8shows the band gap changes as a function of annealing tem-perature as obtained from Fig. 7. After the 500 °C annealing,theb–FeSi2 layer appears but the crystallization is not com-plete and amorphous absorption still exerts its influence. This

is the reason why the 500 °C curve has a higher backgroundand lower Eg

d value, which does not truly reflectEgd of

b–FeSi2. With the increase of temperature, theb–FeSi2layer is formed and residual stress is decreased. The reducedstress increasesEg

d value according to Ref. 17. At 600 °C,crystallization is complete but the defect structure is not fullyrecovered as compared with the 700 °C and 850 °C anneal-ing, so a high background is still present, thus decreasing theEg

d value. At 850 °C,Egd decreases as compared with the

700 °C annealing due to shrinkage of theb–FeSi2 film intoseparated islands. Therefore, the optimum state is the filmannealed at 700 °C.

Figure 9 shows the photon absorption curves for samples1–3 (Fe: 60 kV, 431017 ions/cm2, C: 20 kV, 0, 431015 ions/cm2, 231016 ions/cm2) annealed at700 °C/1 h. TheEg

d value decreases for sample 2(1% car-bon) and sample 3(5% carbon) by a small amount(about0.015 eV), as summarized in Table II.b–FeSi2 thin-filmsgrown on Si substrates by ion-beam synthesis(IBS) and

TABLE II. Summary of bandgap valuesEgd.

Egd (eV) of SampleNo. 1, annealed

Egd (eV) as are influenced by C-doping in 700°C/1 h annealed

samples

500 °C/1 h600 °C/1 h700 °C/1 h850 °C/1 h

0.6850.8500.8650.835

No1: Fe 60 kV, 431017 ions/cm2

No2: Fe 60 kV, 431017 ions/cm2; C 20 kV, 431015 ions/cm2

No3: Fe 60 kV, 431017 ions/cm2; C 20 kV, 231016 ions/cm2

0.8650.8620.858

FIG. 9. Absorption curves for samples 1–3(Fe: 60 kV, 431017 ions/cm2,C: 20 kV, 0, 431015 ions/cm2, 231016 ions/cm2) annealed at 700°C/1 h.

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other deposition techniques such as molecular beam epitaxy(MBE) and chemical vapor deposition(CVD), have so farshown p-type conductivity.18 The current carrier of p-typesemiconductor is hole, when doping the C+ with positive-charge intob–FeSi2, it increases the hole density in thesemiconductor. The increasing of current carrier induces thebandgap to be narrower. It is the reason that the bandgapturns to be narrower after carbon doping.

In samples annealed at 700 °C, we found that theEgd

values are all in the range of reported values in the litera-tures. Therefore, we confirm that carbon doping only slightlyaffects the band structure of theb phase.

IV. CONCLUSIONS

The microstructure and photon absorption of binaryb–FeSi2 and carbon-dopedb–FesSi, Cd2 films synthesizedby ion implantation were investigated. Carbon-dopedb–FesSi, Cd2 films are proved to have higher quality thanbinary b–FeSi2 films as manifested by smoother interface,higher thermal stability, and finer grains. The optimumbfilms are obtained at 700 °C annealing. Carbon doping doesnot influence significantly the band gap values.

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