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Rapid thermal oxidation of epitaxial SiGe thin films
Materials Science and Engineering B89 (2002) 269– 273
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Rapid thermal oxidation of epitaxial SiGe thin films A. Terrasi a,b,*, S. Scalese b, R. Adorno a, E. Ferlito a, M. Spadafora a, E. Rimini a,b a
Dipartimento di Fisica, Uni6ersita` di Catania, Corso Italia 57, 95129 Catania, Italy b INFM, Corso Italia 57, 95129 Catania, Italy
Abstract The oxidation of epitaxial thin Si1 − x Gex layers (0.06 B x B0.30) at 1000 °C in dry O2, for times between 20 and 240 s, has been investigated. The analysis of the thin oxides ( :4– 20 nm) has been performed using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Although most of the Ge is found to pile-up at the oxide/Si1 − x Gex interface, our data indicate the formation of both SiO2 and GeO2 for all investigated samples and oxidation times. Moreover, the oxidation rate, enhanced with increasing the Ge concentration in the alloy, is reported. To our knowledge, this is the first experimental evidence of GeO2 formation and rate enhancement in the regime of high temperature oxidation in dry O2 and Si1 − x Gex alloys with xB0.5. The differences could be peculiar to the initial stage of oxidation, as well as to the rapid thermal process used in our case, but a clear answer is currently unavailable. © 2002 Elsevier Science B.V. All rights reserved. Keywords: SiGe; Oxides; Epitaxy; Rapid thermal processes; Segregation; Diffusion
1. Introduction The use of Si:Ge-based materials in microelectronics (e.g. wireless communications and infrared detectors) has produced an increasing effort to study and control the properties of these compounds. In particular, since the 1970s, several studies were performed on the oxidation of Si1 − x Gex alloys [1], in order to determine the mechanisms and kinetics of this process, which is, in many aspects, very different from the oxidation of pure Si. Although many results have been explained by theoretical models [2,3], an overall insight into this subject is still lacking because of the difficulty in clarifying the role of several parameters: crystal structure of the alloy (strained or unstrained layers), Ge concentration, oxidation ambient and temperature. For example, it is well known that the oxidation of Si induces interstitial injection into the substrate [4], while it is still under debate
* Corresponding author. Tel.: + 39-095-7195-422; fax: + 39-095383-023. E-mail address: [email protected] (A. Terrasi).
what kind of point defects (interstitial or vacancy) are injected during Si1 − x Gex oxidation [3]. The main results reported in the literature show a strong dependence on the experimental conditions [5]: SiO2 is preferentially formed and Ge segregates underneath the oxide for high temperatures and low (B50%) Ge concentration [6]; mixed SiO2 + GeO2 oxides form, with low or no Ge pile-up at the interface, for low temperature and assisted oxidation or for high temperatures and Ge concentration \ 50% in wet or fluorinated ambient [3,5,7 –11]. Moreover, the oxide growth rate enhancement (GRE), with respect to pure silicon, is never reported in the case of standard thermal oxidation in dry O2 [6,12]. Most of these results can be explained in the framework of the models of Hellberg et al. [2] or Kilpatrick et al. [3], the former based on the simultaneous oxidation of both Si and Ge and the instantaneous reduction of GeO2 by silicon, as long as there is free silicon available, the latter based on the competition between oxidant and silicon fluxes at the oxide interface. It is worth noting that published data and calculations mainly deal with long oxidation times and, consequently, thick oxides, while only a few results [9,12] are
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 7 9 7 - 8
270
A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
available on thin oxides, usually obtained by rapid thermal oxidation (RTO). In this work, we investigated epitaxial Si1 − x Gex films processed by RTO in dry O2 to obtain oxide layers a few nanometers thick. Our experimental results confirm the preferential formation of SiO2 along with the segregation of Ge underneath the oxide film. Nevertheless, we also found the presence of GeO2 and an evident GRE effect (varying with the Ge concentration in the alloy).
2. Experimental Strained epitaxial Si1 − x Gex layers, with x= 0.06, 0.14, 0.18 and 0.30, were grown by molecular beam epitaxy on Si (100) 5-inch wafers. Films were deposited at T= 550 °C after a 100 nm thick Si buffer layer. The Si1 − x Gex thickness was kept below the critical thickness value of People and Bean [13,14] for metastable Si1 − x Gex layers. Details on Si1 − x Gex preparation and characterization can be found elsewhere [15]. Si1 − x Gex and pure Si samples were oxidized by RTO in dry O2 flux (92 ml min − 1, 19 psi) at 1000 °C for 20, 30, 120 and 240 s. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) were employed to investigate the oxide chemical composition, thickness and the Ge redistribution. The XPS spectra were obtained using a monochromatized AlKh photon source (1486.6 eV) and an hemispherical electron analyzer (VG CLAM4) in the integrated angle mode. Survey, Si2p, Ge2p, Ge3d, O1s and O2s spectra were recorded at :85° take-off angle, with an overall energy resolution of : 0.6 eV. RBS analysis was performed using He+ ions at 1.7 meV in both random and [100] channeled geometry, with a backscattering angle of 104° (glancing detection) to enhance the surface sensitivity.
3. Results and discussion Fig. 1 report the XPS spectra of the Si2p (Fig. 1a) and Ge3d (Fig. 1b) core levels for samples with three different percentages of Ge, processed in RTO for 20 s. The formation of SiO2, in all cases, is clearly shown by the chemical shift of the Si2p peak from −99.8 eV (unoxidized Si0 + state) to − 104.2 eV (Si4 + ) binding energy. Similarly, the Ge3d spectra reported in Fig. 1(b) show the formation of GeO2 by the presence of the Ge4 + peak shifted at about − 34.2 eV, along with the unoxidized Ge0 + contribution at about − 29.8 eV. The O2s peak is also visible in Fig. 1(b). Thus, the formation of a mixed SiO2 + GeO2 layer is clearly demonstrated by the XPS data, as well as a very low contribution of sub-stoichiometric oxides, whose signal lies in between the 0+ and the 4+ states. A further XPS analysis was performed after the oxide removal, revealing a strong Ge segregation at the (SiO2 +GeO2)/ Si1 − x Gex interface (data not shown). Data of Fig. 1(a) in particular shows the increase of the Si4 + /Si0 + ratio with the Ge percentage in the alloy. This is related to the increase of the oxide thickness, but since the quantitative analysis of XPS data can be affected by the Ge segregation below the SiO2 +GeO2 film, the calculation of the absolute amount of oxygen will be carried out using RBS data. However, the oxide composition can be determined by considering the intensities of the Si4 + , Ge4 + (in Si2p and Ge3d spectra, respectively) and the O1s peak, along with their relative sensitivity factors [8]. Results of these calculations for two of our samples are reported in Table 1 with an Si oxidized sample used as reference. In the range of the oxidation time used and within the uncertainty of this kind of calculation, we observe that the oxides are mostly SiO2, while the Ge is reduced by a factor of 10, with respect to the initial concentration in the alloy. Also, the oxide composition does not seem to depend on the oxidation time, having almost
Fig. 1. XPS spectra of Si2p (a) and Ge3d (b) after RTO at 1000 °C of 20 s for samples with different Ge concentration: 6% (thin line), 14% (dotted line), 30% (thick line). The 4 + electronic states, corresponding to the SiO2 and GeO2 chemical environment, are indicated.
A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
271
Table 1 Oxide composition obtained by Si4+, Ge4+ and O1s photoelectron intensities. The first column reports the oxidation time. The second, third and fourth columns refer to different samples, while the relative sub-columns indicate the percentage of each element into the oxide. tox (s)
20 120 240
Si0.70 Ge0.30+RTO
Si0.94Ge0.06+RTO
Si+RTO (SiO2)
% Si
% Ge
%O
% Si
% Ge
%O
% Si
%O
36.4 9 3 36.8 9 3 36.5 93
3.19 0.3 3.19 0.3 2.99 0.3
60.5 9 6 60.1 9 6 60.6 9 6
36.9 9 3 39.4 9 3 39.7 9 3
0.76 90.1 0.56 90.1 0.61 90.1
62.3 9 6 60.09 6 59.7 9 6
34.99 3 38.59 3 39.69 3
65.09 6 61.49 6 60.49 6
the same composition for all the times, at least in the probed thickness (: 4 – 20 nm). Further information has been obtained by RBS analyses. In particular, the evolution of the channeling yield with the oxidation time for the sample with the highest Ge content (30%), is shown in Fig. 2, where the arrows indicate the species signal at the surface. The area of the O peaks gives a direct measurement of the amount of this element in the oxide, while that of Si and Ge peaks is the convolution of several contributions: atoms into the oxide, atoms at the Si1 − x Gex surface (surface peak), atoms displaced from the crystal lattice because of the Ge segregation and Si1 − x Gex film relaxation. The main features of Fig. 2 are: (1) the increasing of oxygen with the RTO time; (2) the rise of the RBS yield due to the de-channeling effect with the oxide growth, Ge segregation and film relaxation; (3) the shift of the main Ge peak towards the low channel side (Ge pile-up at the interface); and (4) the small Ge shoulder at the surface energy for t =240 s, signature of the GeO2. The total amount of oxygen atoms obtained by RBS analysis could be converted into oxide thickness, once the density of the oxide itself is known but, due to the co-presence of Si and Ge, we prefer to show our data in terms of atomic densities (atoms/cm2). Results versus the oxidation time are reported in Fig. 3(a) for pure Si and two Ge concentrations (6 and 30%) and in Fig. 3(b) versus the Ge percentage in the Si1 − x Gex alloy for a single RTO process of 20 s. The graphics indicate a consistent GRE effect (up to a factor of 2.2) for all samples and oxidation times. Moreover, the GRE seems to have a linear dependence on the Ge percentage, at least in this regime. To further show the behavior of the Ge in our samples, we report in Fig. 4 the channeling RBS spectra relative to the same sample of Fig. 2 (Ge 30%) for the as grown film (dotted line), after oxidation for 240 s (thick line) and after oxide etching (thin line). It is important to note the shift of the Ge peak after the oxidation with respect to the as grown sample, indicating that most of the Ge is rejected from the growing oxide. After etching the oxide, the peak moves back to the surface channel, while the small shoulder
(GeO2) on the right side disappears. Similar effects are observed also for samples with lower Ge percentage. On the base of our XPS and RBS results, we can state the formation of mixed oxides for all the Ge percentages and oxidation times studied in the present work. The effect of the Ge in GRE is also clear: about a factor of 2 with respect to pure Si in the case of Si0.7Ge0.3. These results are unexpected in the framework of the existing models, for which samples having Ge concentration 5 30% and processed in dry O2 at 1000 °C, should have produced neither GeO2 nor GRE phenomena. Theoretical models, in fact, suggest that GeO2 formation is not thermodynamically favored in these conditions. The instability of GeO2 species at high temperatures (\700 °C) in presence of Si is considered the main reason against the formation of mixed oxides for dry oxidation. In particular, as long as the silicon flux to the oxidizing front is greater than the oxidant flux, only SiO2 should be formed, since the reaction Si+ GeO2 SiO2 + Ge should be instantaneous [2]. Furthermore, equilibrium between the oxidant flux from the surface to the bulk and the silicon flux in the opposite direction must be kept into account. Thus, highly reactive oxidation conditions (e.g. wet instead of dry ambient, atomic instead of molecular oxygen) are reported to induce mixed oxide formation and GRE, since the Ge is rapidly incorporated during the reaction. Disagreement with our results might be attributed to
Fig. 2. RBS spectra recorded in channeling geometry along the [100] direction for the sample with 30% of Ge concentration and oxidized for 30 s (dots), 120 s (thin line) and 240 s (thick line).
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A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
Fig. 3. Oxygen amount calculated using the RBS data versus the oxidation time for three different samples (a) and versus the Ge concentration for a single RTO process (b).
shown mixed oxides SiO2 + GeO2 and a strong oxidation rate enhancement. The differences with data and models reported in the literature for low Ge concentrations, dry O2 ambient and high oxidation temperatures have been clearly highlighted, putting in evidence the limits of the existing models on the one hand and the peculiarity of the RTO process and thin oxide structure on the other hand. No final conclusions can be drawn on the bases of this evidence and further investigations are necessary for a deeper understanding of the involved phenomena.
Acknowledgements Fig. 4. Channeling RBS along the [100] direction for the Si0.7Ge0.3. Spectra are relative to the sample after MBE growth (dots), after 240 s RTO process (thick line) and after oxide removal by HF etching (thin line).
the fact that models and data so far reported deal almost exclusively with long oxidation times and thick oxides, while we focused our work on very thin oxides and on the initial oxidation stage. Furthermore, during RTO processes, the diffusion of Ge is reduced compared to a standard furnace oxidation at the same temperature. We can suppose that oxygen at the surface reacts with Si and Ge, forming SiO2 and GeO2. At the same time, Si is consumed reducing GeO2 into SiO2 and the free Ge can now diffuse and pile-up at the oxide/ substrate interface. However, once the oxidation front moves, the oxide maintains its composition and the GeO2 still present will remain stable with the oxidation time, as suggested by our XPS data.
4. Conclusions Our data of RTO processed Si1 − x Gex layers have
The authors are grateful to Francesco Priolo for stimulating discussions and input. This work was supported by INFM and ST-Microelectronics (Contract No. 6486000).
References [1] S.C. Jain, P. Balk, Thin Sol. Films 223 (1993) 348 and references therein. [2] P.-E. Hellberg, S.-L. Zhang, F.M. d’Heurle, C.S. Petersson, J. Appl. Phys. 82 (1997) 5773. [3] S.J. Kilpatrick, R.J. Jaccodine, P.E. Thompson, J. Appl. Phys. 81 (1997) 8018. [4] S.M. Hu, J. Appl. Phys. 45 (1974) 1567. [5] L.S. Riley, S. Hall, J. Appl. Phys. 85 (9) (1999) 6828. [6] H.K. Liou, P. Mei, U. Gennser, E.S. Yang, Appl. Phys. Lett. 59 (1991) 1200. [7] C. Te´ telin, X. Wallart, J.P. Nys, L. Vescan, D.J. Gravesteijn, J. Appl. Phys. 83 (5) (1998) 2842. [8] J.M. Madsen, Z. Cui, C.G. Takoudis, J. Appl. Phys. 87 (4) (2000) 2046. [9] D.K. Nayak, K. Kamjoo, J.S. Park, J.C.S. Woo, K.L. Wang, Appl. Phys. Lett. 57 (4) (1990) 369. [10] W.S. Liu, E.W. Lee, M.-A. Nicolet, V. Arbet-Engels, K.L. Wang, N.M. Abuhadba, C.R. Aita, J. Appl. Phys. 71 (8) (1992) 4015. [11] J. Euge`ne, F.K. LeGoues, V.P. Kesan, S.S. Iyer, F.M. d’Heurle, Appl. Phys. Lett. 59 (1991) 78.
A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273 [12] D.K. Nayak, K. Kamjoo, J.S. Park, J.C.S. Woo, K.L. Wang, IEEE Trans. El. Dev. 39 (1992) 56. [13] R. People, J.C. Bean, Appl. Phys. Lett. 47 (3) (1985) 322. [14] R. People, IEEE J. Quant. Electron. 22 (9) (1986) 1696.
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[15] M. Re, S. Scalese, S. Mirabella, A. Terrasi, F. Priolo, E. Rimini, M. Berti, A. Coati, A. Drigo, A. Carnera, D. De Salvador, C. Spinella, A. La Mantia. J. Cryst. Growth 227 – 228 (2001) 749. (Proceedings of the Eleventh International Conference on MBE, Beijing, 10 – 15 September, 2000).
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Rapid thermal oxidation of epitaxial SiGe thin films A. Terrasi a,b,*, S. Scalese b, R. Adorno a, E. Ferlito a, M. Spadafora a, E. Rimini a,b a
Dipartimento di Fisica, Uni6ersita` di Catania, Corso Italia 57, 95129 Catania, Italy b INFM, Corso Italia 57, 95129 Catania, Italy
Abstract The oxidation of epitaxial thin Si1 − x Gex layers (0.06 B x B0.30) at 1000 °C in dry O2, for times between 20 and 240 s, has been investigated. The analysis of the thin oxides ( :4– 20 nm) has been performed using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Although most of the Ge is found to pile-up at the oxide/Si1 − x Gex interface, our data indicate the formation of both SiO2 and GeO2 for all investigated samples and oxidation times. Moreover, the oxidation rate, enhanced with increasing the Ge concentration in the alloy, is reported. To our knowledge, this is the first experimental evidence of GeO2 formation and rate enhancement in the regime of high temperature oxidation in dry O2 and Si1 − x Gex alloys with xB0.5. The differences could be peculiar to the initial stage of oxidation, as well as to the rapid thermal process used in our case, but a clear answer is currently unavailable. © 2002 Elsevier Science B.V. All rights reserved. Keywords: SiGe; Oxides; Epitaxy; Rapid thermal processes; Segregation; Diffusion
1. Introduction The use of Si:Ge-based materials in microelectronics (e.g. wireless communications and infrared detectors) has produced an increasing effort to study and control the properties of these compounds. In particular, since the 1970s, several studies were performed on the oxidation of Si1 − x Gex alloys [1], in order to determine the mechanisms and kinetics of this process, which is, in many aspects, very different from the oxidation of pure Si. Although many results have been explained by theoretical models [2,3], an overall insight into this subject is still lacking because of the difficulty in clarifying the role of several parameters: crystal structure of the alloy (strained or unstrained layers), Ge concentration, oxidation ambient and temperature. For example, it is well known that the oxidation of Si induces interstitial injection into the substrate [4], while it is still under debate
* Corresponding author. Tel.: + 39-095-7195-422; fax: + 39-095383-023. E-mail address: [email protected] (A. Terrasi).
what kind of point defects (interstitial or vacancy) are injected during Si1 − x Gex oxidation [3]. The main results reported in the literature show a strong dependence on the experimental conditions [5]: SiO2 is preferentially formed and Ge segregates underneath the oxide for high temperatures and low (B50%) Ge concentration [6]; mixed SiO2 + GeO2 oxides form, with low or no Ge pile-up at the interface, for low temperature and assisted oxidation or for high temperatures and Ge concentration \ 50% in wet or fluorinated ambient [3,5,7 –11]. Moreover, the oxide growth rate enhancement (GRE), with respect to pure silicon, is never reported in the case of standard thermal oxidation in dry O2 [6,12]. Most of these results can be explained in the framework of the models of Hellberg et al. [2] or Kilpatrick et al. [3], the former based on the simultaneous oxidation of both Si and Ge and the instantaneous reduction of GeO2 by silicon, as long as there is free silicon available, the latter based on the competition between oxidant and silicon fluxes at the oxide interface. It is worth noting that published data and calculations mainly deal with long oxidation times and, consequently, thick oxides, while only a few results [9,12] are
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 7 9 7 - 8
270
A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
available on thin oxides, usually obtained by rapid thermal oxidation (RTO). In this work, we investigated epitaxial Si1 − x Gex films processed by RTO in dry O2 to obtain oxide layers a few nanometers thick. Our experimental results confirm the preferential formation of SiO2 along with the segregation of Ge underneath the oxide film. Nevertheless, we also found the presence of GeO2 and an evident GRE effect (varying with the Ge concentration in the alloy).
2. Experimental Strained epitaxial Si1 − x Gex layers, with x= 0.06, 0.14, 0.18 and 0.30, were grown by molecular beam epitaxy on Si (100) 5-inch wafers. Films were deposited at T= 550 °C after a 100 nm thick Si buffer layer. The Si1 − x Gex thickness was kept below the critical thickness value of People and Bean [13,14] for metastable Si1 − x Gex layers. Details on Si1 − x Gex preparation and characterization can be found elsewhere [15]. Si1 − x Gex and pure Si samples were oxidized by RTO in dry O2 flux (92 ml min − 1, 19 psi) at 1000 °C for 20, 30, 120 and 240 s. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) were employed to investigate the oxide chemical composition, thickness and the Ge redistribution. The XPS spectra were obtained using a monochromatized AlKh photon source (1486.6 eV) and an hemispherical electron analyzer (VG CLAM4) in the integrated angle mode. Survey, Si2p, Ge2p, Ge3d, O1s and O2s spectra were recorded at :85° take-off angle, with an overall energy resolution of : 0.6 eV. RBS analysis was performed using He+ ions at 1.7 meV in both random and [100] channeled geometry, with a backscattering angle of 104° (glancing detection) to enhance the surface sensitivity.
3. Results and discussion Fig. 1 report the XPS spectra of the Si2p (Fig. 1a) and Ge3d (Fig. 1b) core levels for samples with three different percentages of Ge, processed in RTO for 20 s. The formation of SiO2, in all cases, is clearly shown by the chemical shift of the Si2p peak from −99.8 eV (unoxidized Si0 + state) to − 104.2 eV (Si4 + ) binding energy. Similarly, the Ge3d spectra reported in Fig. 1(b) show the formation of GeO2 by the presence of the Ge4 + peak shifted at about − 34.2 eV, along with the unoxidized Ge0 + contribution at about − 29.8 eV. The O2s peak is also visible in Fig. 1(b). Thus, the formation of a mixed SiO2 + GeO2 layer is clearly demonstrated by the XPS data, as well as a very low contribution of sub-stoichiometric oxides, whose signal lies in between the 0+ and the 4+ states. A further XPS analysis was performed after the oxide removal, revealing a strong Ge segregation at the (SiO2 +GeO2)/ Si1 − x Gex interface (data not shown). Data of Fig. 1(a) in particular shows the increase of the Si4 + /Si0 + ratio with the Ge percentage in the alloy. This is related to the increase of the oxide thickness, but since the quantitative analysis of XPS data can be affected by the Ge segregation below the SiO2 +GeO2 film, the calculation of the absolute amount of oxygen will be carried out using RBS data. However, the oxide composition can be determined by considering the intensities of the Si4 + , Ge4 + (in Si2p and Ge3d spectra, respectively) and the O1s peak, along with their relative sensitivity factors [8]. Results of these calculations for two of our samples are reported in Table 1 with an Si oxidized sample used as reference. In the range of the oxidation time used and within the uncertainty of this kind of calculation, we observe that the oxides are mostly SiO2, while the Ge is reduced by a factor of 10, with respect to the initial concentration in the alloy. Also, the oxide composition does not seem to depend on the oxidation time, having almost
Fig. 1. XPS spectra of Si2p (a) and Ge3d (b) after RTO at 1000 °C of 20 s for samples with different Ge concentration: 6% (thin line), 14% (dotted line), 30% (thick line). The 4 + electronic states, corresponding to the SiO2 and GeO2 chemical environment, are indicated.
A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
271
Table 1 Oxide composition obtained by Si4+, Ge4+ and O1s photoelectron intensities. The first column reports the oxidation time. The second, third and fourth columns refer to different samples, while the relative sub-columns indicate the percentage of each element into the oxide. tox (s)
20 120 240
Si0.70 Ge0.30+RTO
Si0.94Ge0.06+RTO
Si+RTO (SiO2)
% Si
% Ge
%O
% Si
% Ge
%O
% Si
%O
36.4 9 3 36.8 9 3 36.5 93
3.19 0.3 3.19 0.3 2.99 0.3
60.5 9 6 60.1 9 6 60.6 9 6
36.9 9 3 39.4 9 3 39.7 9 3
0.76 90.1 0.56 90.1 0.61 90.1
62.3 9 6 60.09 6 59.7 9 6
34.99 3 38.59 3 39.69 3
65.09 6 61.49 6 60.49 6
the same composition for all the times, at least in the probed thickness (: 4 – 20 nm). Further information has been obtained by RBS analyses. In particular, the evolution of the channeling yield with the oxidation time for the sample with the highest Ge content (30%), is shown in Fig. 2, where the arrows indicate the species signal at the surface. The area of the O peaks gives a direct measurement of the amount of this element in the oxide, while that of Si and Ge peaks is the convolution of several contributions: atoms into the oxide, atoms at the Si1 − x Gex surface (surface peak), atoms displaced from the crystal lattice because of the Ge segregation and Si1 − x Gex film relaxation. The main features of Fig. 2 are: (1) the increasing of oxygen with the RTO time; (2) the rise of the RBS yield due to the de-channeling effect with the oxide growth, Ge segregation and film relaxation; (3) the shift of the main Ge peak towards the low channel side (Ge pile-up at the interface); and (4) the small Ge shoulder at the surface energy for t =240 s, signature of the GeO2. The total amount of oxygen atoms obtained by RBS analysis could be converted into oxide thickness, once the density of the oxide itself is known but, due to the co-presence of Si and Ge, we prefer to show our data in terms of atomic densities (atoms/cm2). Results versus the oxidation time are reported in Fig. 3(a) for pure Si and two Ge concentrations (6 and 30%) and in Fig. 3(b) versus the Ge percentage in the Si1 − x Gex alloy for a single RTO process of 20 s. The graphics indicate a consistent GRE effect (up to a factor of 2.2) for all samples and oxidation times. Moreover, the GRE seems to have a linear dependence on the Ge percentage, at least in this regime. To further show the behavior of the Ge in our samples, we report in Fig. 4 the channeling RBS spectra relative to the same sample of Fig. 2 (Ge 30%) for the as grown film (dotted line), after oxidation for 240 s (thick line) and after oxide etching (thin line). It is important to note the shift of the Ge peak after the oxidation with respect to the as grown sample, indicating that most of the Ge is rejected from the growing oxide. After etching the oxide, the peak moves back to the surface channel, while the small shoulder
(GeO2) on the right side disappears. Similar effects are observed also for samples with lower Ge percentage. On the base of our XPS and RBS results, we can state the formation of mixed oxides for all the Ge percentages and oxidation times studied in the present work. The effect of the Ge in GRE is also clear: about a factor of 2 with respect to pure Si in the case of Si0.7Ge0.3. These results are unexpected in the framework of the existing models, for which samples having Ge concentration 5 30% and processed in dry O2 at 1000 °C, should have produced neither GeO2 nor GRE phenomena. Theoretical models, in fact, suggest that GeO2 formation is not thermodynamically favored in these conditions. The instability of GeO2 species at high temperatures (\700 °C) in presence of Si is considered the main reason against the formation of mixed oxides for dry oxidation. In particular, as long as the silicon flux to the oxidizing front is greater than the oxidant flux, only SiO2 should be formed, since the reaction Si+ GeO2 SiO2 + Ge should be instantaneous [2]. Furthermore, equilibrium between the oxidant flux from the surface to the bulk and the silicon flux in the opposite direction must be kept into account. Thus, highly reactive oxidation conditions (e.g. wet instead of dry ambient, atomic instead of molecular oxygen) are reported to induce mixed oxide formation and GRE, since the Ge is rapidly incorporated during the reaction. Disagreement with our results might be attributed to
Fig. 2. RBS spectra recorded in channeling geometry along the [100] direction for the sample with 30% of Ge concentration and oxidized for 30 s (dots), 120 s (thin line) and 240 s (thick line).
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A. Terrasi et al. / Materials Science and Engineering B89 (2002) 269–273
Fig. 3. Oxygen amount calculated using the RBS data versus the oxidation time for three different samples (a) and versus the Ge concentration for a single RTO process (b).
shown mixed oxides SiO2 + GeO2 and a strong oxidation rate enhancement. The differences with data and models reported in the literature for low Ge concentrations, dry O2 ambient and high oxidation temperatures have been clearly highlighted, putting in evidence the limits of the existing models on the one hand and the peculiarity of the RTO process and thin oxide structure on the other hand. No final conclusions can be drawn on the bases of this evidence and further investigations are necessary for a deeper understanding of the involved phenomena.
Acknowledgements Fig. 4. Channeling RBS along the [100] direction for the Si0.7Ge0.3. Spectra are relative to the sample after MBE growth (dots), after 240 s RTO process (thick line) and after oxide removal by HF etching (thin line).
the fact that models and data so far reported deal almost exclusively with long oxidation times and thick oxides, while we focused our work on very thin oxides and on the initial oxidation stage. Furthermore, during RTO processes, the diffusion of Ge is reduced compared to a standard furnace oxidation at the same temperature. We can suppose that oxygen at the surface reacts with Si and Ge, forming SiO2 and GeO2. At the same time, Si is consumed reducing GeO2 into SiO2 and the free Ge can now diffuse and pile-up at the oxide/ substrate interface. However, once the oxidation front moves, the oxide maintains its composition and the GeO2 still present will remain stable with the oxidation time, as suggested by our XPS data.
4. Conclusions Our data of RTO processed Si1 − x Gex layers have
The authors are grateful to Francesco Priolo for stimulating discussions and input. This work was supported by INFM and ST-Microelectronics (Contract No. 6486000).
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