Silicon Nanocrystal Formation upon Annealing of SiO 2 Layers Implanted with Si Ions

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1 Semiconductors, Vol. 6, No. 6, 00, pp Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 6, No. 6, 00, pp Original Russian Text Copyright 00 by Kachurin, Yanovskaya, Volodin, Kesler, Leier, Ruault. SEMICONDUCTOR STRUCTURES, INTERFACES, AND SURFACES Silicon Nanocrystal Formation upon Annealing of SiO Layers Implanted with Si Ions G. A. Kachurin*, S. G. Yanovskaya*, V. A. Volodin*, V. G. Kesler*, A. F. Leier*, and M.-O. Ruault** * Institute of Semiconductor Physics, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent eva, Novosibirsk, Russia kachurin@isp.nsc.ru ** CSNSM CNRS/INP, 9045 Orsay, France Submitted August, 00; accepted for publication October 9, 00 Abstract The formation of silicon nanocrystals in SiO layers implanted with Si ions was investigated by Raman scattering, X-ray photoelectron spectroscopy, and photoluminescence. The excess Si concentration was varied between and 4 at. %. It was found that Si clusters are formed immediately after implantation. As the temperature of the subsequent annealing was raised, the segregation of Si accompanied by the formation of Si Si 4 bonds was enhanced but the scattering by clusters was reduced. This effect is attributed to the transformation of loosely packed clusters into compact, separate-phase nanoscale Si precipitates, with the Raman peak observed at 490 cm being related to surface scattering. The process of Si segregation was completed at 000 C. Nevertheless, characteristic nanocrystal photoluminescence was observed only after annealing at 00 C. Simultaneously, scattering in the range cm, typical of nanocrystals, appeared; however, the surface-related peak at 490 cm persisted. It is argued that nanocrystals are composed of an inside region and a surface layer, which is responsible for their increased formation temperature. 00 MAIK Nauka/Interperiodica.. INTRODUCTION Currently, considerable attention is being given to the investigation of nanometer-size silicon crystals. This is related, on the one hand, to a steady reduction in the dimensions of microelectronic elements and, on the other hand, to the observation of intense visible-light emission from nanocrystalline Si (nc-si). Usually, nc- Si is obtained upon decomposition of the supersaturated solid solution that takes place under the annealing of SiO layers containing a certain amount of excess Si [ 4]. Strong photoluminescence (PL) in the red near- IR spectral range is only observed when the concentration of excess Si is higher than ~0 cm (~ at. %) and the annealing temperature is as high as 00 C [5 7]. In this context, a number of questions about the mechanism of nc-si formation arise. For instance, one can argue that, in the case of diffusion-controlled growth of Si nanoprecipitates, the growth should proceed at lower Si concentrations, and lower annealing temperatures would be sufficient (although at the expense of a longer annealing duration). To reveal the mechanism of the nc- Si formation, one apparently needs to use techniques that can probe the effect of annealing on excess Si at earlier stages of the process, preceding the formation of light-emitting crystallites. In this paper, we report the results of studying the behavior of excess Si in SiO in a wide range of annealing temperatures. In addition to the PL, we use Raman scattering and X-ray photoelectron spectroscopy (XPS) methods, which satisfy the above-mentioned requirements.. EXPERIMENTAL Two types of samples were investigated: SiO layers of ~ 80 nm thickness thermally grown on Si(00) and plates of silica glass polished on both sides. The advantage of the latter type of sample is the absence of strong scattering from the crystalline Si substrate that hinders the Raman measurements. The SiO plates were implanted with 50-keV Si ions at a dose of 5 0 6, 0 7, or 0 7 cm ; this corresponds to the mean excess Si concentration of about, 6, and at. %, respectively. To examine the role of defects in the scattering, SiO plates implanted with 50-keV Ar ions to a dose of 0 7 cm were also studied. The samples with thermally grown SiO layers were implanted with Si ions to yield an excess Si concentration of about 4 at. %. The Raman spectra were recorded in the quasi-backscattering configuration with an Ar laser (λ = 54.5 nm) as the pump source; polarization was disregarded. The PL was excited by a nitrogen laser operating at 7 nm. Both the PL and the Raman spectra were measured at 0 C. To prepare the samples for the XPS measurements, the upper half of the implanted layer was removed by etching in a : 8 aqueous solution of HF. A photoelectron escape depth of about nm ensures that the most important region of the layer is /0/ $ MAIK Nauka/Interperiodica

2 648 KACHURIN et al. XPS signal, arb. units Binding energy, ev Fig.. The XPS spectra of thermal SiO on Si: () as-grown layer; () layer implanted with Si ions; and implanted layer after annealing at () 500, (4) 650, (5) 000, and (6) 50 C, respectively. probed. The XPS measurements were carried out using a Riber MAC- spectrometer with 0.5-eV resolution. We employed a nonmonochromatic Mg K α X-ray source; the line energy is 5.6 ev. Anneals were carried out in a nitrogen atmosphere or in a vacuum, with their duration being up to h.. RESULTS The XPS spectra of thermally grown SiO layers contain a single line at ~ 0 ev, which corresponds to 6 a Si atom bonded to four O atoms. After Si-ion implantation, this band broadens and a tail appears on its lowenergy side, where the bands corresponding to Si Si (0. ev), Si Si (0. ev), Si Si (99.9 ev), and Si Si 4 (99.4 ev) are known to be located. This indicates that Si atoms with various degrees of oxidation are present in the implanted layers. The effect of subsequent annealing was the following: as the temperature increases, the width of the Si O 4 band returns to its initial value and the band of 4-coordinated Si at 99.4 ev (whose intensity may be considered as a measure of Si-phase precipitation) becomes more pronounced (Fig. ). One can see from Fig. that Si precipitation is nearly completed at 000 C, and raising the annealing temperature to 50 C does not lead to any changes in this respect. The Raman spectra of SiO plates after implantation with Si and Ar ions are shown in Fig.. Si implantation results in the appearance of new spectral features whose intensity depends on the dose; they do not appear upon Ar implantation. The spectra of all Si-implanted samples contain a peak at ~ 490 cm and a weaker, very broad band at ~ cm peaked around 480 cm. The scattering intensity increases as the Si dose is raised, with the increase being more rapid in the low-frequency portion of the broad-band feature. The effect of annealing at 800 C is illustrated in Fig.. In spite of the fact that, according to the XPS data (Fig. ), the Si segregation increases with the annealing temperature, we observed a weakening of the broad band in the Raman spectra of all samples. In the samples with a low implantation dose, the scattering Raman shift, cm Raman shift, cm Fig.. The Raman spectra of SiO plates implanted with Ar ions (dashed line) and Si ions (solid lines); the doses are () 5 0 6, () 0 7, and () 0 7 cm. Fig.. The Raman spectra of Si-implanted SiO plates annealed for 0 min at 800 C; the implantation doses are () 5 0 6, () 0 7, and () 0 7 cm. SEMICONDUCTORS Vol. 6 No. 6 00

3 SILICON NANOCRYSTAL FORMATION UPON ANNEALING OF SiO LAYERS 649 intensity in the region above ~500 cm and below ~470 cm virtually dropped to the background level. Simultaneously, in the samples with low and medium doses, a step formed near 480 cm, where scattering by the amorphous-si phase takes place. The peak at 490 cm persisted in all spectra; in the medium-dose sample, its intensity even increased, with the spectral position unchanged. In the samples with an implanted dose of 0 7 cm, the scattering against both lowand high-frequency regions of the spectrum remained considerable and the peak at 490 cm was less pronounced against the background of the broad band. Figure 4 represents the spectra of the samples annealed at 00 C. The most important feature of these spectra is that the peak at 490 cm is still present, but the scattering at its low-frequency side disappeared and steps appeared on the high-frequency side, where scattering by nanocrystals is expected to occur. In the samples with implanted doses of and 0 7 cm, the steps can be seen at 495 and 500 cm, respectively; the scattering around 50 cm is at the background level in both cases. The high-frequency scattering was observed (a broad line at about (55 ± 0) cm ) only in the samples with an implanted dose of 0 7 cm. Recall that bulk crystalline Si exhibits a line at 50 cm with a width of ~.6 cm. An intense PL band in the long-wavelength region of the visible spectrum, which is commonly attributed to the emission from nanometer-sized Si crystals, appeared after annealing at 00 C (see Fig. 5). As the Si concentration is raised, the PL peak shifts to longer wavelengths (from ~ 750 to ~ 850 nm). It should also be noted that an increase in the mean Si concentration from to 4 at. % leads to a decrease in the luminescence intensity. Anneals at 00 C did not result in the appearance of emission bands at shorter wavelengths (in the spectral region to 450 nm). 4. DISCUSSION The data obtained give insight into the process of nc-si formation under annealing and allow an understanding of its special features in relation to the Si concentration and the annealing temperature. The XPS spectra in Fig. indicate that variously coordinated atoms of excess Si (from Si Si to Si Si 4 ) can be found in implanted layers. Thus, in as-implanted material, the excess Si already forms clusters. The wide Raman band between 50 and 450 cm (see Fig. ) most probably originates from the Si clusters. The following are arguments in favor of this assignment. This band is not related to radiation defects in SiO, since it does not appear upon implantation with heavier Ar + ions, in which case the defect production rate is much higher. Next, the band intensity increases as the Si dose is raised, and it is centered around 480 cm, where scattering by Si Si bonds in amorphous silicon takes place. Experimental data [8, 9] and calculations of the possi Raman shift, cm Fig. 4. The Raman spectra of Si-implanted SiO plates annealed for 0 min at 00 C; the implantation doses are () 5 0 6, () 0 7, and () 0 7 cm. PL intensity, arb. units Wavelength, nm Fig. 5. The photoluminescence spectra of SiO plates (curves, ) and a thermal SiO layer (curve ) implanted with Si ions and annealed at 00 C. The mean Si concentrations are (), (), and () 4 at. %. SEMICONDUCTORS Vol. 6 No. 6 00

4 650 KACHURIN et al. ble Raman lines corresponding to different types of clusters [0] agree with the assumption on the cluster origin of this band. Comparing the samples with different Si implantation doses, we note that nc-si formation is possible in the material with an excess Si concentration on the order of 0 cm. When the average distance between the excess Si atoms becomes ~ nm, the probability of the fluctuation-assisted formation of Si clusters in as-implanted layers (prior to any annealing) is greatly enhanced. We believe that the presence of such clusters, acting as nucleation centers, is a necessary condition for the homogeneous decomposition of the solid solution under annealing and for the subsequent formation of nc-si. In the opposite case, heterogeneous decomposition takes place and the excess Si atoms simply diffuse to random sinks. Sample annealing results in rather unusual behavior. On the one hand, the XPS data indicate that the segregation of excess Si from the oxide is enhanced as the annealing temperature is raised (see Fig. ). On the other hand, annealing at 800 C leads to a weakening of the cluster-related Raman scattering (see Fig. ). This means that segregating Si appears in some new state. Those clusters where Si atoms have both Si and O as their neighbors are chainlike or fractal Si inclusions in SiO, which, in fact, do not have distinct boundaries. One can see from Fig. that, as the annealing temperature increases, 4-coordinated atoms (which form Si Si 4 bonds) become the most abundant. Apparently, loosely packed clusters transform into more compact Si precipitates possessing a phase boundary. It is reasonable to attribute the peak at 490 cm to the scattering by the surface of nanoscale Si inclusions. The nature of this peak has not been discussed previously, although some indications of its existence can be found in the literature [,, ]. Our assumption relies on the following argumentation. The position of the peak is close to that of the line corresponding to Si Si bonds in amorphous silicon. At the surface, the interatomic bonds are slightly stretched in comparison with those in the bulk [ 5]. In nanoparticles, the fraction of atoms that belong to the surface layer is high [6] and the surface scattering itself is fairly intense [7, 8]. Our observation that, in the samples with an implantation dose of 0 7 cm, annealing at 800 C leads to an enhancement of the peak at 490 cm is explained by the transformation of the clusters into nanoscale precipitates with a distinct phase boundary. When the implantation dose is lower, the number and the size of the clusters decrease. When the dose is higher, the transformation of the clusters into separate-phase nanoscale precipitates is hindered by the rapid growth of the size of clusters and by their partial linking. The sharp dependence on the dose and the behavior of the clusters under annealing agree well with the results of computer simulation [9], in which percolation-type cluster formation was considered and Si diffusion in SiO was taken into account [0]. It should be noted that the surface-related peak is present even in the spectra of unannealed samples (see Fig. ). Probably, some of the Si inclusions in the asimplanted material either already have a boundary (which participates in light scattering) or are twodimensional. The XPS data indicate that the segregation of Si from SiO with the formation of separate-phase precipitates is largely completed at 000 C; however, intense PL is observed only if the annealing temperature is raised to 00 C (cf. Figs., 5). According to [, 4 6], intense red PL appears due to the formation of nc-si. Obviously, it is crystallization that brings about a drop in the scattering on the lower-frequency side of the 490-cm peak and a rise of scattering in the high-frequency region, which is characteristic of nc-si (see Fig. 4). Thus, nanoscale separate-phase Si precipitates crystallize at considerably higher temperatures than the C needed for amorphous silicon crystallization. A similar observation was reported previously []: after the amorphization of Si nanocrystals was induced by radiation, their recrystallization took place only upon annealing at temperatures higher than ~000 C. Thus, there is one more reason as to why high-temperature annealing of the samples is needed for nc-si formation (another one is the requirement for the conditions of diffusion-controlled growth of the crystal size). The thermodynamics of small particles is governed by the balance between the gain in the bulk energy and the expenditure of energy related to overcoming the surface tension. We believe that it is the counteraction of the surface that causes the high crystallization temperatures of nc-si. On the one hand, the fact that the peak at 490 cm persisted even after annealing at 00 C provides an additional argument in favor of its surfacerelated nature, and, on the other hand, it indicates that the physical properties of the surface layer are not identical to those of the bulk. It follows from the shift of the PL peak position (Fig. 5) that the average crystallite size in the sample grows from ~ to ~ 5 6 nm with an increase in the Si concentration. At the same time, calculations of the Raman shift [ 4] indicate that peaks at 495 and 500 cm (Fig. 4) correspond to a crystallite size of about nm. Evidently, the discrepancy between these two estimates is related to some difference between the properties of the inside region of the crystallite and the surface layer. Finally, it should be noted that, previously, studies of the formation of light-emitting nc-si by Raman spectroscopy encountered certain difficulties [, 8, 4, 5, 6]. On the one hand, only in samples with small amounts of excess Si did annealing result in the appearance of luminescence; however, the Raman scattering did not correspond to nc-si. On the other hand, with an increase in the Si concentration and in the precipitate sizes, the expected Raman signal appeared but the PL vanished. These problems are, apparently, related to the existence of two different scattering regions in the crystallites, the core and the surface region. SEMICONDUCTORS Vol. 6 No. 6 00

5 SILICON NANOCRYSTAL FORMATION UPON ANNEALING OF SiO LAYERS CONCLUSION Thus, analysis of the XPS and Raman spectra of SiO layers implanted with Si to concentrations of several atomic percent indicates that silicon clusters are formed even in the as-implanted material and grow as the implantation dose is increased. Upon subsequent thermal treatments, two concurrent processes take place: segregation of excess Si from SiO, which brings about the dominance of Si Si 4 bonds, and weakening of the broad scattering band that originates from Si clusters. The latter effect is attributed to the transformation of loosely packed non-separate-phase Si precipitates into more compact separate-phase ones, which have a distinct boundary with SiO. This boundary is responsible for the observed peak at 490 cm in the Raman spectra. The segregation of excess Si is almost complete at 000 C, but intense photoluminescence that is characteristic of nanometer-sized silicon crystallites appears only after annealing at 00 C. Under these conditions, the surface-related peak at 490 cm persists, while the peak related to scattering by amorphous Si precipitates around 480 cm is replaced by a high-frequency peak in the range between 495 and 50 cm, where scattering by Si nanocrystals occurs. Estimates of the nanocrystal size based on the luminescence and Raman-scattering data indicate that the crystallites are composed of a core and a surface layer, whose thickness is about one nanometer. Apparently, it is the surfacelayer counteraction that is responsible for the high values of the crystallization temperature of Si nanoprecipitates. ACKNOWLEDGMENTS We are grateful to V.D. Akhmetov for his help in carrying out the experiments. This study was supported by the Russian Foundation for Basic Research (project no ) and by the Interdisciplinary Program Physics of the Solid- State Nanostructures. REFERENCES. T. Shimizu-Iwayama, K. Fujita, S. Nakao, et al., J. Appl. Phys. 75 (), 7779 (994).. U. Herrmann, H. H. Dunken, E. Wendler, and W. Wesch, J. Non-Cryst. Solids 04, 7 (996).. E. Wendler, U. Herrmann, W. Wesch, and H. H. Dunken, Nucl. Instrum. Methods Phys. Res. B 6, (996). 4. G. A. Kachurin, I. E. Tyschenko, K. S. Zhuravlev, et al., Nucl. Instrum. Methods Phys. Res. B, 57 (997). 5. P. Mutti, G. Ghislotti, S. Bertoni, et al., Appl. Phys. Lett. 66, 85 (995). 6. G. A. Kachurin, A. F. Leier, K. S. Zhuravlev, et al., Fiz. Tekh. Poluprovodn. (St. Petersburg), 7 (998) [Semiconductors, (998)]. 7. S. P. Withrow, C. W. White, A. Meldrum, et al., J. Appl. Phys. 86, 96 (999). 8. S. Hayashi, T. Nagareda, Y. Kanazawa, and K. Yamamoto, Jpn. J. Appl. Phys., 840 (99). 9. T. Ehara and S. Machida, Thin Solid Films 46, 75 (999). 0. K. Jackson, M. R. Pederson, D. Porezag, et al., Phys. Rev. B 55, 549 (997).. Y. Guyot, B. Champagnon, M. Boudeulle, et al., Thin Solid Films 97, 88 (997).. V. G. Baru, M. I. Elinson, V. A. Zhitov, et al., Mikroélektronika 7, 456 (998).. S. Veprek, F.-A. Sarrot, and Z. Iqbal, Phys. Rev. B 6, 44 (987). 4. Ch. Ossadnik, S. Veprek, and I. Gregora, Thin Solid Films 7, 48 (999). 5. R. Z. Valiev and I. V. Aleksandrov, Nanostructural Materials Produced by Severe Plastic Deformation (Logos, Moscow, 000). 6. H. H. Andersen and E. Johnson, Nucl. Instrum. Methods Phys. Res. B 06, 480 (995). 7. Yu. I. Petrov, Clusters and Small Particles (Nauka, Moscow, 986). 8. S. Hayashi and H. Kanamori, Phys. Rev. B 6, 7079 (98). 9. A. F. Leier, L. N. Safronov, and G. A. Kachurin, Fiz. Tekh. Poluprovodn. (St. Petersburg), 89 (999) [Semiconductors, 80 (999)]. 0. L. A. Nesbit, Appl. Phys. Lett. 46, 8 (985).. G. A. Kachurin, S. G. Yanovskaya, M.-O. Ruault, et al., Fiz. Tekh. Poluprovodn. (St. Petersburg) 4, 004 (000) [Semiconductors 4, 965 (000)].. T. R. Guilinger, M. J. Kelly, D. R. Tallant, et al., Mater. Res. Soc. Symp. Proc. 8, 5 (99).. J. Zi, H. Buscher, C. Falter, et al., Appl. Phys. Lett. 69, 00 (996). 4. P. Mishra and K. P. Jain, Phys. Rev. B 6, 4790 (000). 5. T. Okada, T. Iwaki, K. Yamamoto, et al., Solid State Commun. 49, 809 (984). 6. T. Inokuma, Y. Wakayama, T. Muramoto, et al., J. Appl. Phys. 8, 8 (998). Translated by M. Skorikov SEMICONDUCTORS Vol. 6 No. 6 00