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Optical characterization by spectroscopic ellipsometry of polycrystalline Si1−xGex of variable Ge composition up to 100% Ge
Thin Solid Films 315 Ž1998. 316–321
Optical characterization by spectroscopic ellipsometry of polycrystalline Si 1yxGe x of variable Ge composition up to 100% Ge F. Ferrieu ) , C. Morin, J.L. Regolini France Telecom CNET, Centre Norbert Segard, 28 ch du Õieux Chene France ˆ BP98, F38243 Meylan-Cedex, ´ Received 20 January 1997; accepted 23 April 1997
Abstract Polycrystalline Si 1yxGe x , has been recently shown as a favorable alternative to the poly-Si gate electrode for CMOS technology: the optical properties of this new material poly-Si 1yxGe x alloys layers are investigated here. Spectroscopic ellipsometry ŽSE. analysis on samples with different Ge contents up to 100%, demonstrates both the composition dependency and the morphological change in the amorphous character of the material, while the x parameter increases. The technique is thus an easy and non-destructive method for layer composition as well as for material morphology control. These are important parameters for sub-micron Ž- 0.18 m m. gate stacks material optimization. Layer morphology observation using XTEM and AFM are correlated here with SE, which in turn, would certainly be, in the near future, a routine control tool during the fabrication of the poly-SiGe gate structure. q 1998 Elsevier Science S.A. Keywords: Optical characterization; Spectroscopic ellipsometry; Polycrystalline Si 1y x Ge x
1. Introduction Single crystal Silicon–Germanium alloys for strained layer heterostructures have been extensively studied for devices applications. More recently, polycrystalline Si 1y xGe x has been demonstrated as being a promising alternative to the poly-Si gate electrode for CMOS technology w1x. This is because as grown poly-Si 1y xGe x gate material, has a lower resistivity and a tunable work function due to its variable Ge content. Moreover, the use of a so-called mid-gap material, such as poly-Ge may allow the use of a single pq doped polycide gate instead of the traditional dual pqrnq doped material. After early device applications w1,2x of the Si 1y xGe x alloys gate material, several studies followed to have a better understanding the growth kinetic aspects w3,4x: the layer nucleation on silicon oxide, the grain size and the associated incubation times w5x for the appropriated models w6x. Among the material characterization methods, spectroscopic ellipsometry of Si 1y xGe x alloys has been extensively detailed already w7x for epitaxial layers. When considering the optical properties of these alloys w8,9x, several
authors have described the change in the dielectric function ´ Ž E, x . vs. photon energy E Žev. as the x Ge content vary from zero Žpure silicon., to 100% Ge: these works have been done with unconstrained alloy bulk material by Kline et al. w10x with electroreflectance data collected within the entire range of Ge contents and later Humlicek w11x by SE. The 3rd Differentiated Spectroscopic Ellipsometry w12,13x and Electroreflectance studies w14x described the observed Ei interband transitions associated to the Critical Points ŽCP. transitions shifts, as a function of the alloy composition which behave also quasi the same for both strained or unstrained materials. Unfortunately, due to the strains relaxation mechanism, only composition range up to 30% Ge can be prepared by epitaxy. The optical properties of the chemical vapor deposition ŽCVD. materials with Ge contents ranging from 0% poly-crystalline silicon up to 100% poly-Ge, are discussed here in front of the results of other characterizations techniques such as the high resolution Transmission Electron Microscopy ŽXTEM. and the Atomic Force Microscopy ŽAFM..
2. The poly-Si 1I x Ge x layers preparation
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 2 6 2 - 9
These layers were obtained with a single wafer reactor. The used wafers were first coated with thermal SiO 2 oxide
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
˚ thickness. The oxide was then chemically cleaned of 50-A followed by DI water rinse and N2 dry. The reactant gases were SiH 4 , GeH 4 Ž10% in H 2 ., B 2 H 6 Ž1% in H 2 . and H 2 as the carrier gas. A typical gate poly layer Si 1y x Ge x stack is 200-nm high, then followed by a poly-Si capping layer about 100 nm. The Ge content of the grown bilayers were controlled by X-ray diffraction XRD w1x. The diffraction angle differences between the poly-Si peaks and the
317
alloy peaks are proportional to the Ge content, thus, by using the Vegard’s law, the x parameter is obtained.
3. Composition morphology and roughness Film composition and crystallinity were observed by ŽXRD. on poly-Si capped, poly-Si 1y xGe x layers, in order
Fig. 1. XTEM observation of poly-Si Ža., 55% Ge Žb. and 100% poly-Si capped Si 1y x Ge x layer Žc..
318
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
to have simultaneously the Si and the alloy diffraction peaks. Si 1y xGe x diffraction peak position relative to Si peak is a measurement of the Ge content into the alloy. These measurements were complemented by SIMS profiles which show a high uniformity of the in-depth of the Ge contents profile ŽG. Gauneau, CnetrLannion, unpublished results. where Si 1y xGe x alloys percentage is known with an accuracy better than 2 or 3%. Cross-section transmission electron microscopy ŽXTEM. was used to study layer morphology and corroborate the layer thickness measurements obtained by differential weighing. These analysis indicate a clear change between the poly-Si, with a quasi-amorphous morphology and the lowest roughness and the 55% or the 100% Ge films which have a highest roughness and already an oriented texture. To study the microstructure of the polycrystalline films, we used XTEM in order to elucidate the grain structure and orientation. Fig. 1a shows a XTEM micrograph of 200-nm thick poly-Si film. Grains are small and randomly oriented giving as a result a non-columnar growth in contrast with what is normally observed on furnace grown poly-Si or poly-Si 1y xGe x w4x. The same random structure is observed in the alloy poly-Si 1y xGe x Ž55%. as observed in Fig. 1b Žthe alloy is on the bottom and the poly-Si cap on the top. and for poly-Ge as observed in Fig. 1c. The XTEM micrographs also show the top surface roughness as complemented then by the AFM measurements: there is no visible poly-Si layer, even under high resolution, between the poly-Si 1y xGe x and the oxide. In Fig. 2, the two layers have a total thickness of 200 nm. In this same figure ŽFig. 2a., a poly-Si layer and Fig. 2b corresponds to a poly-Si 1y xGe x Ž55%.. In Fig. 2a, the poly-Si has a roughness Žrms. of about 2 nm, Fig. 2b shows a stack structure of 120 nm of poly-Si 1y xGe x Ž55%. capped with 80 nm of poly-Si. The measured roughness is below 10 nm and increasing the Ge content to a 100%, the roughness does not change as observed on the rms values. We did not observe any particular effect in grain size when poly-SiGe is doped with boron from B 2 H 6 Ž1% in H 2 . up to 1 = 10 20 rcm3.
4. Spectroscopic ellipsometry measurements A rotating polarizer spectroscopic ellipsometer in the visible range, i.e., between 250 nm and 800 nm, and with a 708 incidence angle is used. Si 1y xGe x samples were optically analyzed. The x compositions were ranging from x s 0, i.e., the poly-Si Ž200-nm thick poly-Si on top of a 5-nm SiO 2 . and respectively 25, 55, 75 and 100% Ge, undoped Si 1y xGe x alloys, 120-nm thick on top of a 5-nm gate SiO 2 . A so thin gate oxide layer Ž4.6 nm as measured
319
prior the Si 1y xGe x film processing. lowers the interference contrast within the Si 1y xGe x layer Ža very weak oscillation can only be detected in the cos D spectra in this case, usually polycrystalline samples are classically studied in reflectometry or ellipsometry w15x with a 100-nm SiO 2 thickness.. However, SE is able to extract the material dielectric properties of the poly-Si layer as well as in the case of the Si 1y xGe x alloys samples: deduced from our measurements and shown in the Fig. 3, the real and imaginary parts of the dielectric function e im are obtained here in the case of poly-Si sample. The processing of the ellipsometry measurements, i.e., tanC and cos D vs. photons wavelengths Žin eV or nm., have been done using a multilayer optimization software. In most cases, an additional oxide overlayer has to be added in order to account for a possible native oxide or even of the residual surface roughness. It was accomplished introducing an additional layer with an approximative of 50% mixing of amorphous silicon, Ža-Si. and SiO 2 . In the multistack, all the materials are known except the Si xGe x Ži.e., we use tabulated values for Si and SiO 2 , and amorphous silicon a-Si.. The Effective Medium Approximation mixing theory ŽEMA. describes now the poly-silicon case: in this latter case Ži.e., the poly-silicon sample for the optical indices w15x., a mixing of only amorphous silicon ŽE. Palik data w16x. with voids Ž6.84%. yields to a very low goodness of fit value and confirmed by the amorphous structure of the sample observed also by XTEM. Other analytical model such as the Bloomer Forohoui model, has been also used in this case. It leads to an a-Si energy gap Eg s 0.98 ev w17x. Nevertheless, the analytical models are only here to provide close starting n, k values: after this first approach, both optical indices, n and k values Ž n˜ s n q ik ., are taken as the two parameters to be adjusted within the visible range for the tanC and cos D data. The samples with different Si 1y x Ge x compositions can be compared only in term of their imaginary part of the dielectric function ´ im Ž E . rather than the optical indices n and k. Ž ´ Ž E . s n˜ 2 Ž E . where E is the photon energy Žev. or expressed in wavelengths nanometers Žnm... In Fig. 3, the maximum of ´ im is found at 3.8 ev Ž326 nm. which is a close value to these corresponding also to maximum of the dielectric function of amorphous Si w17x. Starting from an approximate dielectric function associated to Si 1y xGe x material Že.g., an ema material or from data already known from the Si 1y xGe x optical indices database w10x., the final dielectric function is obtained similarly. Our results are reported in the case of the poly-Si xGe x samples on Fig. 4. The resulting ´ im Ž E . curves Žsee Fig. 4. are definitely different from the result of the poly-Si sample. The dielectric functions ´ im Ž E . of Si 1yxGe x films are
˚ 55% Ge Fig. 2. AFM atomic Force Microscopy of the surface in the case of Ža. poly-Si with a rms 2.23 nm and Pekrvalley of 20.4 nm and Žb. 1200 A capped by 800 nm, Si peak valley is 72.4 nm and rms 6.2 nm.
320
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
Fig. 3. As deduced from the ellipsometry measurements, the real and imaginary part of the dielectric function ´ im in the case of poly silicon sample. The maximum of ´ im is found at 3.8 eV Ž326 nm. which is a close value to these corresponding also to maximum of the dielectric function of amorphous Si w16,17x.
more similar to the case of a polycrystalline materials w17x than to the case of amorphous materials, where most singularities should be absent. This is the first time here, to our knowledge, that the optical properties of these poly-alloys are reported and in such a wide range of x composition. Only a slight widening is noted when compared to the results with the curves which can be calculated from the Humlicek approximation Žsee figure of Ref. w11x.. For low Ge concentration, 25% sample the shape of ´ im is slightly
shifted to the red side of the spectral range with two singularities at 3.2 and 4.3 ev but with a wider and a lower value for the first observed maximum. These two maxima are the critical points associated to the interband transitions Ei1 w10x. If we consider these curves with increasing Ge contents, as shown in Fig. 4, one finds a composition dependent shift of the transition energies Ei corresponding roughly to the shift diagram obtained from ER measurements w10x. In the same time, the crystalline morphology is
Fig. 4. Measured ´ im Ž E . following the SiGe layer composition together with the dielectric function corresponding to crystalline silicon Žc-Si. and Žc-Ge.. Respective compositions 25, 55, 75 and 100% Ge correspond to the studied samples.
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
revealed and can be clearly observed. Quantitative analysis would go well beyond the scope of this paper but it is clearly shown here that the dielectric function is affected by these both effects composition and morphology.
321
vations. This work has been carried out within the GRESSI consortium between CEA-LETI and FRANCE TELECOM-CNET.
References 5. Conclusions We have obtained poly-SiGe with up to 100% Ge. This is the first time here, to our knowledge that the optical properties of these poly-alloys are reported and in such a wide range of x composition. The XTEM observation shows Žand spectroscopic ellipsometry confirms. that the poly Si 1y xGe x samples are more and more polycrystalline as the x composition increases. This is seen by the fact that the critical points of the dielectric function as observed in Si 1y xGe x crystalline alloys become more and more visible and that the associated CP singularities shifts follow the law as it can be expected for unstrained Si 1y xGe x alloys. We conclude that the materials morphology and the effect of its x Ge fraction agree with the previous spectroscopic ellipsometry analysis. So far, proof has been made that SE is a good tool for quality control for such new materials even in an industrial lines. Structural characterization have been yet correlated with spectroscopic ellipsometry taken in the clean room environment in order to follow process reproducibility as well as spatial homogeneity.
Acknowledgements The authors are grateful to Mrs. E. Jourde and Dr. E. Andre´ of CNET DcfrCap for the XTEM and AFM obser-
w1x T.-J. King, J.R. Pfiester, J.D. Shott, J.P. McVittie, K.C. Saraswat, IEDM Technol. Digest 90 Ž1990. 253. w2x N. Kistler, J. Woo, IEDM Technol. Digest 93 Ž1993. 727. w3x J. Holleman, A.E.T. Kuiper, J.F. Verweij, J. Electrochem. Soc. 140 Ž1993. 1717. w4x T.-J. King, K.C. Saraswat, J. Electrochem. Soc. 141 Ž1994. 2235. w5x H.C. Lin, C.Y. Chang, W.H. Chen, W.C. Tsai, T.C. Chang, T.G. Jung, H.Y. Lin, J. Electrochem. Soc. 141 Ž1994. 2559. w6x M. Cao, A. Wang, K.C. Saraswat, J. Electrochem. Soc. 142 Ž1995. 1566. w7x J.L. Regolini, D. Bensahel, J. Mercier, J. Electron. Mater. 19 Ž1990. 1075. w8x P.M. Garone, J.C. Sturm, P.V. Schwartz, S.A. Schwarz, B.J. Wilkens, Appl. Phys. Lett. 56 Ž1990. 1275. w9x M.L. Green, Y.S. Ali, D. Brasen, S. Nakahara, J. Electron. Mater. 17 Ž1988. 229. w10x J.S. Kline, F.H. Pollak, M. Cardona, Helv. Phys. Acta 41 Ž1968. 968–977. w11x J. Humlicek, M. Garriga, M.I. Alonso, M. Cardona, J. Appl. Phys. 65 Ž1989. 2832–2877. w12x F. Ferrieu, F. Beck, D. Dutartre, Solid State Commun. 82 Ž1992. 427–430. w13x C. Pickering, R.T. Carline, D.J. Robbins, W.Y. Leong, S.J. Barnett, A.D. Pitt, A.G. Cullis, J. Appl. Phys. 73 Ž1. Ž1993. 239–250. w14x Y. Yin, F.H. Pollak, P. Auvray, D. Dutartre, R. Pantel, J.A. Chroboczek, Thin Solid Films 222 Ž1992. 85–88. w15x P. Montaudon, M.H. Debroux, F. Ferrieu, A. Vareille, Thin Solid Films 125 Ž1985. 235–241. w16x E. Palik, Handbook of the Optical Constants of Solids, Academic Press, Vol. I and II, 1985–1991. w17x G.E. Jellison Jr., F.A. Modine, Appl. Phys. Lett. 69 Ž3. Ž1996. 371–373, and references therein.
Optical characterization by spectroscopic ellipsometry of polycrystalline Si 1yxGe x of variable Ge composition up to 100% Ge F. Ferrieu ) , C. Morin, J.L. Regolini France Telecom CNET, Centre Norbert Segard, 28 ch du Õieux Chene France ˆ BP98, F38243 Meylan-Cedex, ´ Received 20 January 1997; accepted 23 April 1997
Abstract Polycrystalline Si 1yxGe x , has been recently shown as a favorable alternative to the poly-Si gate electrode for CMOS technology: the optical properties of this new material poly-Si 1yxGe x alloys layers are investigated here. Spectroscopic ellipsometry ŽSE. analysis on samples with different Ge contents up to 100%, demonstrates both the composition dependency and the morphological change in the amorphous character of the material, while the x parameter increases. The technique is thus an easy and non-destructive method for layer composition as well as for material morphology control. These are important parameters for sub-micron Ž- 0.18 m m. gate stacks material optimization. Layer morphology observation using XTEM and AFM are correlated here with SE, which in turn, would certainly be, in the near future, a routine control tool during the fabrication of the poly-SiGe gate structure. q 1998 Elsevier Science S.A. Keywords: Optical characterization; Spectroscopic ellipsometry; Polycrystalline Si 1y x Ge x
1. Introduction Single crystal Silicon–Germanium alloys for strained layer heterostructures have been extensively studied for devices applications. More recently, polycrystalline Si 1y xGe x has been demonstrated as being a promising alternative to the poly-Si gate electrode for CMOS technology w1x. This is because as grown poly-Si 1y xGe x gate material, has a lower resistivity and a tunable work function due to its variable Ge content. Moreover, the use of a so-called mid-gap material, such as poly-Ge may allow the use of a single pq doped polycide gate instead of the traditional dual pqrnq doped material. After early device applications w1,2x of the Si 1y xGe x alloys gate material, several studies followed to have a better understanding the growth kinetic aspects w3,4x: the layer nucleation on silicon oxide, the grain size and the associated incubation times w5x for the appropriated models w6x. Among the material characterization methods, spectroscopic ellipsometry of Si 1y xGe x alloys has been extensively detailed already w7x for epitaxial layers. When considering the optical properties of these alloys w8,9x, several
authors have described the change in the dielectric function ´ Ž E, x . vs. photon energy E Žev. as the x Ge content vary from zero Žpure silicon., to 100% Ge: these works have been done with unconstrained alloy bulk material by Kline et al. w10x with electroreflectance data collected within the entire range of Ge contents and later Humlicek w11x by SE. The 3rd Differentiated Spectroscopic Ellipsometry w12,13x and Electroreflectance studies w14x described the observed Ei interband transitions associated to the Critical Points ŽCP. transitions shifts, as a function of the alloy composition which behave also quasi the same for both strained or unstrained materials. Unfortunately, due to the strains relaxation mechanism, only composition range up to 30% Ge can be prepared by epitaxy. The optical properties of the chemical vapor deposition ŽCVD. materials with Ge contents ranging from 0% poly-crystalline silicon up to 100% poly-Ge, are discussed here in front of the results of other characterizations techniques such as the high resolution Transmission Electron Microscopy ŽXTEM. and the Atomic Force Microscopy ŽAFM..
2. The poly-Si 1I x Ge x layers preparation
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 2 6 2 - 9
These layers were obtained with a single wafer reactor. The used wafers were first coated with thermal SiO 2 oxide
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
˚ thickness. The oxide was then chemically cleaned of 50-A followed by DI water rinse and N2 dry. The reactant gases were SiH 4 , GeH 4 Ž10% in H 2 ., B 2 H 6 Ž1% in H 2 . and H 2 as the carrier gas. A typical gate poly layer Si 1y x Ge x stack is 200-nm high, then followed by a poly-Si capping layer about 100 nm. The Ge content of the grown bilayers were controlled by X-ray diffraction XRD w1x. The diffraction angle differences between the poly-Si peaks and the
317
alloy peaks are proportional to the Ge content, thus, by using the Vegard’s law, the x parameter is obtained.
3. Composition morphology and roughness Film composition and crystallinity were observed by ŽXRD. on poly-Si capped, poly-Si 1y xGe x layers, in order
Fig. 1. XTEM observation of poly-Si Ža., 55% Ge Žb. and 100% poly-Si capped Si 1y x Ge x layer Žc..
318
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
to have simultaneously the Si and the alloy diffraction peaks. Si 1y xGe x diffraction peak position relative to Si peak is a measurement of the Ge content into the alloy. These measurements were complemented by SIMS profiles which show a high uniformity of the in-depth of the Ge contents profile ŽG. Gauneau, CnetrLannion, unpublished results. where Si 1y xGe x alloys percentage is known with an accuracy better than 2 or 3%. Cross-section transmission electron microscopy ŽXTEM. was used to study layer morphology and corroborate the layer thickness measurements obtained by differential weighing. These analysis indicate a clear change between the poly-Si, with a quasi-amorphous morphology and the lowest roughness and the 55% or the 100% Ge films which have a highest roughness and already an oriented texture. To study the microstructure of the polycrystalline films, we used XTEM in order to elucidate the grain structure and orientation. Fig. 1a shows a XTEM micrograph of 200-nm thick poly-Si film. Grains are small and randomly oriented giving as a result a non-columnar growth in contrast with what is normally observed on furnace grown poly-Si or poly-Si 1y xGe x w4x. The same random structure is observed in the alloy poly-Si 1y xGe x Ž55%. as observed in Fig. 1b Žthe alloy is on the bottom and the poly-Si cap on the top. and for poly-Ge as observed in Fig. 1c. The XTEM micrographs also show the top surface roughness as complemented then by the AFM measurements: there is no visible poly-Si layer, even under high resolution, between the poly-Si 1y xGe x and the oxide. In Fig. 2, the two layers have a total thickness of 200 nm. In this same figure ŽFig. 2a., a poly-Si layer and Fig. 2b corresponds to a poly-Si 1y xGe x Ž55%.. In Fig. 2a, the poly-Si has a roughness Žrms. of about 2 nm, Fig. 2b shows a stack structure of 120 nm of poly-Si 1y xGe x Ž55%. capped with 80 nm of poly-Si. The measured roughness is below 10 nm and increasing the Ge content to a 100%, the roughness does not change as observed on the rms values. We did not observe any particular effect in grain size when poly-SiGe is doped with boron from B 2 H 6 Ž1% in H 2 . up to 1 = 10 20 rcm3.
4. Spectroscopic ellipsometry measurements A rotating polarizer spectroscopic ellipsometer in the visible range, i.e., between 250 nm and 800 nm, and with a 708 incidence angle is used. Si 1y xGe x samples were optically analyzed. The x compositions were ranging from x s 0, i.e., the poly-Si Ž200-nm thick poly-Si on top of a 5-nm SiO 2 . and respectively 25, 55, 75 and 100% Ge, undoped Si 1y xGe x alloys, 120-nm thick on top of a 5-nm gate SiO 2 . A so thin gate oxide layer Ž4.6 nm as measured
319
prior the Si 1y xGe x film processing. lowers the interference contrast within the Si 1y xGe x layer Ža very weak oscillation can only be detected in the cos D spectra in this case, usually polycrystalline samples are classically studied in reflectometry or ellipsometry w15x with a 100-nm SiO 2 thickness.. However, SE is able to extract the material dielectric properties of the poly-Si layer as well as in the case of the Si 1y xGe x alloys samples: deduced from our measurements and shown in the Fig. 3, the real and imaginary parts of the dielectric function e im are obtained here in the case of poly-Si sample. The processing of the ellipsometry measurements, i.e., tanC and cos D vs. photons wavelengths Žin eV or nm., have been done using a multilayer optimization software. In most cases, an additional oxide overlayer has to be added in order to account for a possible native oxide or even of the residual surface roughness. It was accomplished introducing an additional layer with an approximative of 50% mixing of amorphous silicon, Ža-Si. and SiO 2 . In the multistack, all the materials are known except the Si xGe x Ži.e., we use tabulated values for Si and SiO 2 , and amorphous silicon a-Si.. The Effective Medium Approximation mixing theory ŽEMA. describes now the poly-silicon case: in this latter case Ži.e., the poly-silicon sample for the optical indices w15x., a mixing of only amorphous silicon ŽE. Palik data w16x. with voids Ž6.84%. yields to a very low goodness of fit value and confirmed by the amorphous structure of the sample observed also by XTEM. Other analytical model such as the Bloomer Forohoui model, has been also used in this case. It leads to an a-Si energy gap Eg s 0.98 ev w17x. Nevertheless, the analytical models are only here to provide close starting n, k values: after this first approach, both optical indices, n and k values Ž n˜ s n q ik ., are taken as the two parameters to be adjusted within the visible range for the tanC and cos D data. The samples with different Si 1y x Ge x compositions can be compared only in term of their imaginary part of the dielectric function ´ im Ž E . rather than the optical indices n and k. Ž ´ Ž E . s n˜ 2 Ž E . where E is the photon energy Žev. or expressed in wavelengths nanometers Žnm... In Fig. 3, the maximum of ´ im is found at 3.8 ev Ž326 nm. which is a close value to these corresponding also to maximum of the dielectric function of amorphous Si w17x. Starting from an approximate dielectric function associated to Si 1y xGe x material Že.g., an ema material or from data already known from the Si 1y xGe x optical indices database w10x., the final dielectric function is obtained similarly. Our results are reported in the case of the poly-Si xGe x samples on Fig. 4. The resulting ´ im Ž E . curves Žsee Fig. 4. are definitely different from the result of the poly-Si sample. The dielectric functions ´ im Ž E . of Si 1yxGe x films are
˚ 55% Ge Fig. 2. AFM atomic Force Microscopy of the surface in the case of Ža. poly-Si with a rms 2.23 nm and Pekrvalley of 20.4 nm and Žb. 1200 A capped by 800 nm, Si peak valley is 72.4 nm and rms 6.2 nm.
320
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
Fig. 3. As deduced from the ellipsometry measurements, the real and imaginary part of the dielectric function ´ im in the case of poly silicon sample. The maximum of ´ im is found at 3.8 eV Ž326 nm. which is a close value to these corresponding also to maximum of the dielectric function of amorphous Si w16,17x.
more similar to the case of a polycrystalline materials w17x than to the case of amorphous materials, where most singularities should be absent. This is the first time here, to our knowledge, that the optical properties of these poly-alloys are reported and in such a wide range of x composition. Only a slight widening is noted when compared to the results with the curves which can be calculated from the Humlicek approximation Žsee figure of Ref. w11x.. For low Ge concentration, 25% sample the shape of ´ im is slightly
shifted to the red side of the spectral range with two singularities at 3.2 and 4.3 ev but with a wider and a lower value for the first observed maximum. These two maxima are the critical points associated to the interband transitions Ei1 w10x. If we consider these curves with increasing Ge contents, as shown in Fig. 4, one finds a composition dependent shift of the transition energies Ei corresponding roughly to the shift diagram obtained from ER measurements w10x. In the same time, the crystalline morphology is
Fig. 4. Measured ´ im Ž E . following the SiGe layer composition together with the dielectric function corresponding to crystalline silicon Žc-Si. and Žc-Ge.. Respective compositions 25, 55, 75 and 100% Ge correspond to the studied samples.
F. Ferrieu et al.r Thin Solid Films 315 (1998) 316–321
revealed and can be clearly observed. Quantitative analysis would go well beyond the scope of this paper but it is clearly shown here that the dielectric function is affected by these both effects composition and morphology.
321
vations. This work has been carried out within the GRESSI consortium between CEA-LETI and FRANCE TELECOM-CNET.
References 5. Conclusions We have obtained poly-SiGe with up to 100% Ge. This is the first time here, to our knowledge that the optical properties of these poly-alloys are reported and in such a wide range of x composition. The XTEM observation shows Žand spectroscopic ellipsometry confirms. that the poly Si 1y xGe x samples are more and more polycrystalline as the x composition increases. This is seen by the fact that the critical points of the dielectric function as observed in Si 1y xGe x crystalline alloys become more and more visible and that the associated CP singularities shifts follow the law as it can be expected for unstrained Si 1y xGe x alloys. We conclude that the materials morphology and the effect of its x Ge fraction agree with the previous spectroscopic ellipsometry analysis. So far, proof has been made that SE is a good tool for quality control for such new materials even in an industrial lines. Structural characterization have been yet correlated with spectroscopic ellipsometry taken in the clean room environment in order to follow process reproducibility as well as spatial homogeneity.
Acknowledgements The authors are grateful to Mrs. E. Jourde and Dr. E. Andre´ of CNET DcfrCap for the XTEM and AFM obser-
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