Optical characteristic of epitaxial pseudomorphic FeSi2

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Solid State Communications, Vol. 86, No. 4, pp. 217-219, 1993. Printed in Great Britain.

OPTICAL CHARACTERISTIC

0038-1098/93 $6.00+.00 Pergamon Press Ltd

OF EPITAXIAL PSEUDOMORPHIC

FeSi2

G.Guizzetti and F.Marabelli

Dipartimento di Fisica "A. Volta", Uuiversitfi degli Studi di Pavia, Via Bassi 6, L27100 Pavia, Italy N.Onda and H.von K£nel

Laboratorium ffir FestkJrperphysik, ETtI Zfirich, CH-8093 Zfirich, Switzerland Received 26 January 1993 by C. Calandra Optical measurements over a large spectral range from far infrared (6 meV) up to ultraviolet (6 eV) have been performed on an epitaxial film of a pseudomorphic phase related to metastable 7-FeSi2. The sample, grown by molecular beam epitaxy, shows metallic properties in the infrared whereas at energies larger than 1 eV the optical response is similar to the one of semiconducting fl-FeSi2. The scattering mechanisms affecting the free cartier behavior have been discussed. the following MBE-growth carried out at substrate temperatures below 200 °C, no further annealing was performed. Selected area transmission electron diffraction 11 and X-ray diffraction showed that under these conditions the films crystallize in a CsCl-derived defect phase in which the Fe atoms occupy the centers of the Si cubes in a random fashion. This structure is well distinct from the 7-FeSi2 structure which is obtained upon ordering the Fe atoms in alternate planes 11. The lattice parameter of this defect-CsCl phase is close to half of that of Si. The optical measurements were performed on a 330 thick film, grown in the manner described above, from 0.006 eV (200 pm) towards about 6 eV (200 nm). Spectral ellipsometry was performed between 0.7 and 5 eV (1.75 - 0.25 pro) by using a SOPRA instrument model MOSS ES4G. Reflectance (R) and transmittamce (T) measurements were carried out with two different spectrometers: a Fourier transform spectrometer Bruker IFS 113v for photon energies up to 0.6 eV, and a Perkin Elmer 330 from 0.5 eV up to 6 eV (transmittance measurements were possible only up to 1.2 eV, due to the absorption edge of the Si substrate). For R measurements in the Fourier spectrometer (IR) a gold mirror was used as reference, whereas for near IR and vis-UV an aluminium mirror. The corresponding error bars in the determination of the absolute values were ~3 % and +5 %, respectively. Due to the limited thickness of the FeSi2 layer (330 .~) the optical response obtained by both techniques (R and T and eUipsometry) was analyzed in terms of a multilayer structure considering the Si (111) substrate too. In the case of R and T results, an auxiliary measurement was performed on the substrate alone, in order to normalize the results for spurious effects due to eventual oxide layers or light scattering on the backside of the substrate. Using the already known film thickness, we obtained from the elllpsometric results the spectra of the two components, real and imaginary, of the complex refraction index ~ = n + ik. Outside the range of the ellipsometric measurements the inversion of reflectance and transmittance data, in order to calculate n and k,

I. I N T R O D U C T I O N The study of transition metal silicides has always been important duc to their stability and the very good compatibility with the silicon technology. I The discovery of a narrow-gap semiconducting character of some silicides is a further element of interest. ~'s In the last years the attention in this field has been concentrated on the epitaxial growth of high quality samples of some specific compounds considered particularly interesting and promising in their applications.4 This is the case of iron disilicide, whose semiconducting orthorombic ~-phasc exhibits a gap of about 0.8 eV, well suitable for optoelectronic applications,5-s whereas the cubic, metallic v-phase has an excellent lattice match with Si (111). 9'1° It is thus important to investigate the structural and the electronic properties of FeSi2 with regard to the growth conditions. In this paper we present the optical characterization over a large spectral range from the far infrared (FIR) to the ultraviolet (UV) of a pseudomorphic disordered phase related to metastablc 7-FeSi2. The Fe atoms are not ordered, however, in this structure. They occupy instead 50 % of the cation sites of the CsCl structure in a random fashion. Such a pseudomorphic phase of FeSi2, or rather Fe0.sSi, with metallic properties can be considered as precursor phase of fl-FeSi2. In the following the electronic properties of this metalUc pseudomorphic phase will be discussed.

II. EXPERIMENTAL

LAYOUT

AND

RESULTS

Pseudomorphic FeSi2 films were grown by molecular beam epitaxy (MBE) onto clean S i ( l l l ) surfaces with parallel monolayer steps due to the unintentional wafer misorientatlon. In a first step a 10 t[ thick template was grown by codeposltlng Fe and Si in a stoichiometric ratio and subsequent annealing at 450 °C. After 217

218

EPITAXIAL PSEUDOMORPHIC FeSi2 40

i

[

i

i

FeSi2 30 I1

-

20 10

i

0.1

I

i

I

0.2 0.3 photon energy (eV)

0.4

0.5

Fig.1 ReM (n) and imaginary (k) part of the complex refractive index of pseudomorphlc FeSi2 in the infrared. gave two couples of possible solutions. Furthermore, the results are highly sensitive to the absolute value of the measured R and T and a very small variation in R (or T) can produce an appreciable change of n and k. This implies the possibility of a shift or a tilt of the obtained n and k spectra, but the spectral features are conserved anyway. Then we chose a couple of solutions matching the ellipsometric results in the spectral range where both sets of data were available and consistent with the measured static conductivity ~rdc = 1.6 + 2.0 × 1015s-a in the zero frequency limit as well. 1° A further check of the physical consistency of the resulting data was done by controlling that Kramers-Kronig relations are verified. Finally, the uncertainty in the dc-conductivity value and in the measured R and T in the IR could affect the absolute values of the derived n and k values shown in the Fig.l, but these are fully consistent with all experimental observations and physically meaningful. Fig.2 shows the high energy part of the spectra.

IIl. DISCUSSION The IR results for FeSi2, with the n and k values i

i

i

Vol. 86, No. 4

increasing for decreasing energies, confirm the m'etallic character of this compound. Nevertheless, due to the small thickness of the sample, it was possible to detect the vibrational spectrum in the far IR. It consists of three broad bands centered at about 32, 44 and 54 meV respectively. A complete and detailed analysis of this vibrational part, both theoretical and experimental, will be given in a further publication. As much as concern our case, anyway, such part, due to its relativdy low oscillator strength, is related only to small perturbations in the n and k spectra, which don't affect the general trend dominated by the electronic contribution. In order to discuss the electronic behavior, it can be useful to look at the optical conductivity (Fig.3) spectrum tropt - ~ - ~ . The integral of the intraband part of the optical conauctivity is directly proportional to the square of the plasma frequency wp2 = 4'~c~N ,~. (N is the free carrier density). This part is the more broadened in energy the larger the scattering; in effect the limit at zero frequency (or trdc) is proportional to -~', where 7 is the scattering rate. For the interband excitations ~ropt is roughly proportional to the Joint Density of States (JDOS). The behavior of the optical conductivity in the IR (and of all the other optical functions, e.g.: n values are larger than k values) at the low energies, where the intraband transitions should dominate, gives evidence of a strong scattering mechanism acting on the free carriers. This can be related to the following different processes. i) One is alloy scattering and scattering by magnetic impurities. A confirmation of the importance of such phenomena is given in the transport measurements by the relatively high resistivity value which is temperature independent up to high temperatures, where a decrease occurs and the resistivity of the Si substrate enter the measurementsJ 0 ii) The instability of the related 7-phase with respect to lattice distortions, toghether with some observations done on ;5-FeSi2 suggest a strong electronphonon interaction occurring in FeSi2. 5,9 By the way, one can notice that in our "disordered" system the whole vibrational spectrum is well visible, even though the compound is metallic (usually free carrier absorption screens the lattice modes) and without regard to the selection rules (in the CaF2 structure only one IR

i

7

FeSi2

~.

6

~

5

FeSi2

v

?

~

4

"2

g

3

f-

~ 0

I

1

I

I

I

2

3

4

5

photon energy (eV)

2

O 0

, 10 .2

Fig.2 ReM (n ) and imaginary (k ) part of the complex refractive index of pseudomorphic FeSi2. A bore O.75 e V the spectra correspond to elllpsometric data.

,

h i J,,,I

, 10-1

,

,

,,Lt,I

, 10 o

,

,

, ,,, 101

photon energy (eV)

Fig.3 Optical conductivity of pseudomorphic FeSi2.

Vol. 86, No. 4

EPITAXIAL PSEUDOMORPHIC FeSi2

made is active instead of the broad bands observed in our measurements). iii) A contribution difficult to evaluate can be played by magnetic interactions, as observed for example, in the Hall effect measurements, a° iv) A last scattering process can be ascribed to interband absorption occurring already at very low energies (down to 0.1-0.2 eV). In effect, band structure calculations, performed on the fluorite structure of FeSi2, give a high density of (essentially) Fe-d states just below the Fermi level,s This could imply either an enhancement of the effective mass (and a low plasma frequency) and the possibility of interband transitions at low energies, due to the complex band structure. Unfortunately the presence of a monotonic behavior (no minima) of the absorption in the IR (related to disorder and scattering processes), together with the uncertainty in the absolute determination of the optical functions by inversion of R and T, makes very hard to decouple the intra- and the inter-band contributions and to fix the value of the plasma frequency wp. A reasonable estimate gives a wp value between 2 and 4 eV assuming the energy limit of free carrier absorption to be well larger than the highest energies of the vibrational

219

excitations (~0.1 eV) and lower than the onset of the first well distinguishable interband transitions (~0.8-1.0 eV). These values, by using a carrier density corresponding to 1 carrier per Fe atom, N ~ 2.5 x 1022 carriers/cm*, would mean an effective mass ranging between 3 and 12 m

e .

Above about 0.7 eV the conductivity begins to increase rapidly and it shows a first well defined shoulder at about 1.4 eV. A second, enhanced structure occurs at 3.5 eV. It is interesting to compare this spectral behavior with the spectrum of an epitaxial layer of the semiconducting phase B-FeSi2.5'a~ The energy of the observed transitions in the optical conductivity are about the same in the two cases, but the strengths are higher in the semiconductor than in the metal where the structures are smaller and broader. The large broadening can be easily ascribed to disorder effects too. The real part of the dielectric function ~1 = n 2 - k ~ crosses the zero line (n and k are equal) a bit above 4 eV in our measurements and at about 4.4 eV in the B-phase results. Anyway, it seems that the electronic band structure is not drastically changed when passing from the semiconductor to the metal, even if a fraction of the spectral density of states straddles the Fermi level covering the gap.

REFERENCES Appl.Phys.Lett. 56, 2126 (1990).

1. see for example the Proceeding of the European Workshop on Refractory Metals and Silicides (Saltjobaden, March 1991) published on Appl.Surface Science, Vol.53 (1991).

7. A.Charalabos, A.Dimitriadis and J.H.Werner, J. Appl. Phys. 68, 93 (1990).

2. M.C.Bost and J.E.Mahan, J.Appl.Phys. 63, 839 (1988).

8. A.Borghesi, A.Piaggi, A.Stella, G.Guizzetti, F. Nava and G.Santoro, Proc. 20th ICPS, ed. by E.M.Anastassakis and J.D.Joannopoulos (World, Singapore, 1990) p.2001.

3. M.C.Bost and J.E.Mahan, J.Appl.Phys. 64, 2034 (1988).

9. N.E.Christensen, Phys.Rev.B 42, 7148 (1990)

4. L.J.Chen and K.N.Tu, Material Science Reports 6, N.2,3 (1991) p.53.

10. N.Onda, J.Henz, E.MiiUer, K.A.Mgder and H.von KKnel, Appl.Surf.Sci. 56-58, 421 (1992).

5. C.A.Dimitriadis, J.E.Werner, S. Logothetidis, M. Stutzmann, J.Weber and R.Nesper, J.Appl.Phys. 68, 1726 (1990).

11. H. yon KKnel, K.A. MKder, E. Mfiller, N. Onda, and H. Sirringhaus, Phys. Rev. B 45, 13807 (1992).

6. J.E.Mahan, K.M.Geib, G.Y.Robinson, R.G.Long, Y.Xinghua, G.Bal, M.A.Nicolet and M.Nathan,

12. F.Marabelli, M.Amiotti, V.BeUani, G.Guizzetti, A.Borghesi, S.Lagomarsino and F.Scarinci, Proceeding of the ICPS 21 (Peking 1992).