Si heterostructures

Superlattices and Microstructures, Vol. 2, No. 5, 1986

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PARALLEL AND PERPENDICULAR TRANSPORT IN Si/CoSi 2 AND Si/CoSi2/Si HETEROSTRUCTURES P.A. Bedoz, E. Rosencher, A. Briggs" and F. Arnaud d'Avitaya CNET, Chemin du Vieux Chine B.P. 98, 38243 Meylan Cedex, France *CRTBT, BP 166X, 38042 Grenoble Cedex, France Received August 17,1986

It has recently become possible to grow epitaxial Si/CoSi2/Si semiconductor - metal - semiconductor heterostructures of almost perfect crystalline quality. Electronic transport in the plane of the metal film (parallel transport) is investigated by the extensive studies of resistivity and superconducting properties of these films. The sharp influence of film thickness on both phenomena is presented and its physical origin is briefly discussed. The transfer of hot electrons emitted by the top Sieoi/CoSi2diede to the Sibulk/COSi 2 through the metal film (perpendicular transport) is studied. Experimental data strongly favor the hypothesis of ballistic transport with a ballistic mean free path close to the one deduced from resistivity measurements.

Thanks to advances in ultra high vacuum and growth technologies, it has now become possible to realize ultra- thin and epitaxial metallic CoSi 2 films on top of Si substrates. It has been shown that strained and continuous CoSi 2 films can be grown with thicknesses ranging from 3.5 up to 20 nmf,2. For thicknesses above 20 nm, the films are found to be polycrystalline and to present a rather milky aspect. The realization of these films is a first step in the fabrication of the epitaxial SilCoSi2/Si Semicon~Juctor - Metal - Semiconductor (SMS) hetero structure". The availability of such well controlled systems offers the opportunity to study beth parallel and perpendicular transport in thin metallic films. 1. Parallel transport Recently, Hensel et al 4 have shown that down to very low thicknesses i.e. 10 nm, the film resistivity exhibits very little dependence on the Co~i 2 film thickness. Using the Fuchs-Sondheimer theory~', which introduces a specularity parameter p which is the fraction of electrons specularly reflected from the interfaces, they showed that the carriers reflect from the boundaries with a very high degree of specularity i.e. with little loss of phase coherence. However, they also mentioned an increase of the low temperature limit resistivity P0 for thicknesses below 10 nm. We have extended their measurements down to lower film thicknesses, i.e. from 14 down to 3.5 rim. The resulting resistivity versus temperature p(T) curves are presented in Fig. 1. Experimental data clearly show that, as the film thickness is decreased down to 8 nm, the p(T) curves are translated as a whole towards higher resistivity. For thicknesses lower than 8 nm, however, the slope of the p(T) curves increases up to 30% while the limit

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resistivity is multiplied by a factor of four compared to the bulk one. In order to emphasize this unusual behavior, the slope of the linear part of the p(T) curves, /)p/0T, is plotted in the inset as a function of film thickness. Concerning this figure, a few points should be addressed in details: a) Film thicknesses are first determined in the growth chamber using a quartz microbalance, calibrated by Rutherford backscattering measurements. The values obtained are then checked with those deduced from ex - situ resistance and Hall effect measurements: indeed, assuming a hole carrier density in the metal layer independent of the film thickness i.e. ~ 3x1022 cm - 3 , both sets of values agree within 10% for all metal thicknesses. b) For film thicknesses above 20 nm as well as for bulk CoSi2, the resistivity versus temperature curves are almost identical with a limit resistivity o f P0 2.5 i~C},cm, and a room temperature resistivity of about 14 t~.em. As these curves are very close to the 14 nm one, they are not presented in the figure. c) One may wonder whether the presence of pinholes in the metal film may account for the observed increase in the resistivity. However, because of the conservation of the total quantity of CoSi 2, the current density in the metal does not depend on the presence of discontinuities in the film, up to the percolation threshold. Thus, the resistivity and Hall effect measurements are not affected by the eventual presence of pinholes in the CoSi 2. d) p(T) measurements have also been performed in the metal base of a SilCoSi2/Si heterostructure. Small openings are etched in the Si overlayer in order to

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Superlattices and Microstructures, Vol. 2, No. 5, 1986

426

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make electrical contacts on the 9 nm thick CoSi 2 film, while the major part of the CoSi 2 film remains sandwiched betwen the two Si layers. This film exhibits a rather high limit resistivity (P0 ~ 5,5 p~.cm) and a high slope 0pIOT as shown in the inset. Here again the film thickness and the absence of discontinuities in the film have been checked by electrical and Hall measurements. Since CoSi 2 has been found to be superconducting, 6,7 we have then studied the influence of film thickness W on the superconducting critical temperature T c. The experimental T c versus W curve is shown in Fig. 2. The experimental data clearly show a very sharp reduction in T c as the film thickness is decreased under 10 nm. Let us note that: a) the 3.5 nm thick sample did not show a superconducting transition down to 18 mK, the lowest attainable temperature in our dilution cryostat and b) the Si/CoSi2/Si sample exhibits a "high" T c very close to the bulk one i.e. 1.22 K. As both resistivity and superconductivity data show a strong deviation from bulk and thick film behavior for thicknesses lower than 10 nm, we are led

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to think of a common origin for both phenomena. We shall review in the following some possible explanations. Behavior of p(T): the enhancement of the 0p/aT values with increasing P0 is a striking result in opposition with usual behavior. Indeed, as the disorder is increased in a metallic film for instance by ion implantation 8, the limit resistivity PO is increased while the p(T) curve tends to flatten. Under sufficiently disordere(~t conditions, the saturation regime is attained °, i.e. the resistivity becomes almost independent of temperature. We then think that this phenomena is not tied to the degradation of the material quality but that it has a more intrinsic nature: inded, the film thickness W could affect either the electron - phonon interaction constant or the density of states at the Fermi level N(EF), this influence being either due to strain in the metal film or bidimensional effects. Behavior of T^ vs. W: the decrease of T c with decreasing metal ~lm thickness has been the subject of extensive studies (see for instance M. Strongin et all0). Commonly invoked origins are the following: a) localization effects 11, b) proximity effects, i.e. the decrease of T c is attributed to the reduction of the effective attractive interaction experienced by the electrons 12, c) magnetic impurities at the metal - semiconductor interface due to incompletely bounded Co atoms, as shown by high resolution transmission electron microscopy and d) strain

427

Superlattices and Microstructures, Vol. 2, No. 5, 1986 2. Perpendicular transport

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We have studied the transfer of electrons injected from the forward biased Sie_i/CoSi 2 junction into the reverse biased Sibulk/COSi 2 diode, t~hrough

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the CoSi2 metal base. The transfer ratio c( i.e. the collector to emitter current ratio is plotted in Fig. 3 as a function of the metal film thickness W, at 77 and 300 K. The exponential dependence of c( with respect to W is in strong favor of the ballistic transport of electrons through the CoSi 2 film. The ballistic mean free path, i.e. the mean distance travelled by a hot electron between two scattering events is found to be of 8±1.5 nm at 300 K and 35+5 nm at 77 K. These values are in good agreement with the mean free paths deduced from parallel resistivity measurements in bulk material. This indicates that the same scattering mechanisms control both the electron transport close to the Fermi level and the hot electron relaxation in the 0.7 eV range above E F, i.e. the SilCoSi 2 Schottky barrier height.

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Acknowledgement- The authors are indebted to Dr. M. Gurvitch from Bell Laboratories for fruitful discussions and to Dr. C. d'Anterroches from CNET for high resolution transmission electron microscopy observations.

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effects 13. Interpretation a) clearly does not hold since the sheet resistivity of our film~, is far below the localization threshold i.e. R = l~/e~ 4000 C}o Interpretation b) leads to a rather smooth decrease of T c with decreasing W, which does not account for our experimental data. Interpretation c) and d) are not consistent with the higher T c observed in the Si/CoSi2/Si heterostructure, Here again we are led to think that a fundamental phenomena affects the electron - phonon interaction and/or the Fermi level density of states. However, an increase of the strength of the electron - phonon interaction would lead to the simultaneous increase of ~)p/ST and of T c, in the frame of the BCS theory. This is thus in contradiction with our experimental results. On the other hand, a decrease in the density of states at the Fermi level would increase ap/ST and decrease T c thus being a good candidate to explain our data. The physical origin of a such decrease in N(EF) with decreasing film thickness remains to be found.

REFERENCES 1. F. Arnaud d'Avitaya, S. Delage, E. Rosencher and J. Derrien, J. Vac. Sci. & Tecnol. B3, 770 (1985). 2. C. d'Anterroches and F. Arnaud d'Avitaya, Thin Solid Films 137, 351 (1985). 3. E. Rosencher, S. Delage, Y. Campidelli and F. Arnaud d'Avitaya, Electron. Lett. 20, 762 (1984). 4. J.C. Hensel, R.T. Tung, J.M Poate and F.C. Unterwald, Phys. Rev. Lett. 54, 1840 (1985). 5. See for instance C.R. Tellier and A.J. Tosser Size effect in this films (Elsevier, Amsterdam, 1982). 6. B.T. Matthias and J.K. Hulm, Phys.Rev. 89, 439 (1953). 7. P.A. Badoz, A. Briggs, E. Rosencher and F. Arnaud d'Avitaya, J. Physique Left. 46, 979 (1985). 8. JC. Hensel, R.T. t~ng, J.M. Poate and F.C. Unterwald, Appl. Phys. Lett. 44, 913 (1984). 9. M. Gurvitch, Phys. Rev. B 28, 544 (1983). 10. M. Strongin, R.S. Thompson, O.F. Kammerer and J.E. Crow, Phys. Rev. B 1, 1078 (1970). 11. D.V. Borodin, Yu. I. Latyshev and F.Ya. Nad', Pis'ma Zh. Eksp. Teor. Fiz. 35, 201 (1982); JETP Lett. 35, 249 (1982). 12. M.G. Karkut, D. Ariosa, J.M. Triscone and O. Fischer, Phys. Rev. B 32, 4800 (1985). 13. R. Meservey and B.B. Schwartz, Superconductivity, Ed. R.D. Parks (Marcel Dekker Inc., New York), 169 (1969). 14. E. Rosencher, P.A. Badoz, J.C. Pfister, F. Arnaud d'Avitaya, G. Vincent and S. Delage, AppL Phys. Lett. 49, 271 (1986).