Kinetic aspects of solid-phase epitaxial growth of amorphous Si

NUCLEAR INSTRUMENTS

AND METHODS

149 ( 1 9 7 8 )

623-627

; ©

NORTH-HOLLAND

P U B L I S H I N G CO.

KINETIC ASPECTS OF SOLID-PHASE EPITAXIAL GROWTH OF A M O R P H O U S Si Z. L LIAU, S. S. LAU, M-A. NICOLET, J. W. MAYER

California Institute of Technology, Pasadena, California 91125, U.S.A. R. J. BLATTNER, P. WILLIAMS and C. A. EVANS, Jr.

Materials Research Laboratory and School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A.

We have studied solid-phase transport phenomena in the thin-film system which consists of a Pd 2Si layer sandwiched between an underlying Si single crystal substrate and a top layer of amorphous Si. After a 475 °C annealing, the Pd-silicide layer moves toward the surface on the sample as the single crystal substrate grows with a corresponding consumption of the amorphous Si layer. Using MeV 4He+ backscattering spectrometry, we find that the time dependence of the transport process has rather different rates for samples prepared at different times, varying by up to three orders of magnitude. By monitoring the rate under various modified experimental conditions, we have established that the transport rate depends on the vacuum deposition of the amorphous Si layer during sample preparations. Specifically, the rate was found to correlate with the carbon contamination ( ~ 3 at.%) which was detected in the amorphous Si layers by Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS). Relatively "carbon-free" depositions were carried out and unique features were indeed observed on these samples.

1. Introduction It has recently been demonstrated that electrically active epitaxial layers of silicon can be grown from an amorphous source by solid-state reactions involving a palladium silicide (pd2Si) layer1-4). Samples initially consist of a single crystal Si substrate, a Pd layer ( - 1 0 0 0 A thick), a thin dopant layer (typically N5 A of antimony) and a surface layer of about 1/zm of amorphous Si. The processing steps include reacting the layered structure at 280 °C to form Pd2Si followed by annealing of the resultant structure at about 500°C to promote the epitaxial growth of the amorphous Si layer onto the single crystal substrate. The mechanism of this solid-phase epitaxial growth (SPEG) has been investigated previously. It was found that the mechanism depends on Pd2Si dissociationS). A preliminary study of the kinetics of SPE showed the time dependence on (100) substrates to be characterized by at least two distinct stages2,3). In the first stage, the rate is relatively high and essentially constant. However, after a certain critical time and thickness the rate is observed to change discontinuously. In this second stage, the rate is appreciably lower than before. The ratio of these two characteristic rates was found to be quite reproducible with a value of about 8. While these growth characteristics are general and applicable to all (100) samples, the values of

the transport rates were observed to vary by as much as three orders of magnitude between samples prepared using nominally the same methods and apparatus but, at different times. In particular, rates were observed to be reproducible only for samples prepared simultaneously. Samples prepared at different times had different (and seemingly random) transport rates, yet the general growth features remained unchanged. The objective of this study was to explore the cause of this behavior.

2. Sample preparation The procedure of sample preparation has been described in detail elsewhere~-4). Briefly, (100) Si substrates were cleaned in organic solvents, RCA solution, dilute HF solutioh and high-purity water. After cleaning, the samples were immediately loaded into an ion-pumped vacuum chamber. The Pd was e-gun evaporated at a rate of -.~ 10 A/s, up to a thickness of 1000/k. The Si evaporation charge was usually contained in a vitreous carbon liner; the deposition rate was about 30-100 A/s; the thickness ranged from 3000 to 10 000 A. All evaporations were done at a pressure of < 1 × 10 -6 torr, and annealing was performed in a vacuum of -~ 1 × 10 -6 torr.

3. Backscattering spectrometry The formation of the palladium silicide and the IX. C O M B I N E D

TECHNIQUES

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rate at which the thickness of the amorphous Si film decreased were studied by MeV 4He+ backscattering. This technique has been described in detail elsewhere6). By comparing backscattering spectra of unannealed and annealed samples, the decrease in the amount of amorphous Si can be deduced from an energy shift in the low-energy edge of the amorphous Si signal as shown in fig. la. Fig. lb is a plot of the sonsumption of amorphous Si as a function of annealing time measured from the same sample as in fig. la.

4. Auger electron spectroscopy (AES) A Physical Electronics Industries (PHI) model 545 Scanning Auger Microprobe was used to characterize unannealed samples chemically. A high purity Ta foil mask (2 mm diameter hole) was placed over each sample to limit the area consumed during 2 keV Ar + sputter depth profiling. Analyses were performed with an initial residual vacuum of < 1 x 10 -9 torr backfilled with research2.0 MeV 4He+ Backscattering cm .J LH m

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a sequence of annealingsat 475 °C. The sampleveforeSPEG has an amorphous Si layer [indicated Si(a)] of 6300 ~ on the surface and a Pd2Si layer of 1800A under Si(a). (b) Time dependence of the consumption of the amorphous Si layer. The SPEG rate is determined from the slope of the straight (solid) line.

grade argon to ~ 5 × 10 -5 torr. Derivative peak to peak "intensities" for Auger transitions characteristic of the elements C (272 eV), O (510 eV), Pd (244 eV) and Si (1619 eV) were monitored. An average impurity/Si ratio normalized to pure element peak heights 7) was calculated point by point to obtain a bulk relative impurity concentration for each sample. 5. Secondary ion mass spectrometry (SIMS) A modified AEIIM-20 ion microprobe mass spectrometer was used for all SIMS measurements8). A recently developed Cs ÷ primary ion source 9) was used to facilitate carbon and oxygen analyses due to increased ion yields. Sample chamber residual vacuum was better than 3 x 10 -8 torr. Depth profiling was done by rastering the 25 keV primary beam (N 20 ~ m spot) over an area 100x 100/~m 2 while monitoring the ion intensity of a species of interest via an electronic apertureS°). Sputter rates were obtained from backscattering layer thicknesses combined with sputter times to interfaces.

6. Results and discussion The SPE rate is defined here as the rate at which the thickness of the amorphous Si layer decreases when annealed at 475 °C. As the example of fig. lb shows, there are two distinct stages. The rate in each stage is approximately constant. Such SPEG rate measurements were carried out for nearly 50 sets of samples prepared (under similar conditions) over a period of more than one year. All samples exhibited a growth behavior which was similar to that shown in fig. 1. However, samples prepared at different times had quite different SPEG rates. To determine the underlying factor or factors which causes such non-reproducible SPEG rates, the influence of various experimental conditions was investigated. For example, the possibilities of random drifts of temperature and contaminations in the vacuum annealing ambient, oxygen contamination in air before annealing, the effect of Pd2Si formation temperatures, and the influences of substrate doping and deviation from exact (100) orientation were investigated. No correlation was found. These experiments did, however, indicate consistently that the rate was determined by the film deposition procedures. Various parameters of the Pd deposition, such as the Pd thickness, the deposition rate and the substrate temperature, were

SOLID-PHASE

therefore investigated. Negative results of these tests narrowed the choice further to the Si deposition step. To ensure that the evaporated Si layer was truly amorphous, 28Si+ ions of sufficient energy and dose ~l) to ensure complete amorphotization were implanted into the deposited layer. The SPEG rate of such an implanted sample was found to be the same as that of the corresponding ,unimplanted sample. Other parameters of Si deposition, such as evaporation rate, layer thickness and substrate temperature were also checked. Since evaporation parameters or the effect of implantation on the deposited Si layer did not seem to influence the transport kinetics, chemical impurity effects were investigated. Unannealed SPEG structures exhibiting transport rate varia-

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tions over three orders of magnitude were characterized by AES in both sputter depth profiling and spectral modes. Oxygen contaminationin all samples analyzed appeared to be consistently low ( < 1 X 10 ]8 atoms/cm 3) from evaporation to evaporation and therefore seemed to be an unimportant factor. The only bulk impurity detected in the evaporated Si layer by this technique was carbon. Fig. 2 shows an AES depth profile for a typical unannealed sample. The apparent peak in the carbon distribution in the Pd region was shown to result from interference of the C(272eV) and Pd (244 eV) transitions. This interference of palladium on carbon precluded an AES study of carbon redistribution after transport due to the presence of some Pd in the grown Si layer. A preliminary study of carbon distributions in annealed samples has been done using SIMS. The indication is that carbon is distributed in the grown layer essentially in the same way and at the same levels as it appears in the deposited layer, i.e., there is no significant rejection of carbon by the Pd2Si layer. SPEG rates were observed to vary with bulk carbon concentrations determined by AES as shown in fig. 3. These data suggest a very strong inverse dependence, although no precise analytical form for this relationship can be established because of the scatter in the data. The uncertainty in the measured carbon concentrations is partly due to the fact that the carbon levels involved approach the sensitivity limit of the AES technique.

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The largest relative carbon concentration observed (lowest transport rate) was less than 3 at.%. It is interesting to note in this context that in the case of regrowth of amorphous Si layers formed by ion implantation, impurities also play an important role in the kinetics]3.J4). Certain impurities, such as P and B, increase the regrowth rate while other impurities, such as O and C, reduce the rate of regrowth. Once the presence of carbon contamination in the evaporated Si layer was established, its origin was assumed to be the carbon hearth liner used during Si evaporation. This assumption was tested by evaporating Si without a carbon liner. Auger depth profiling 'analysis of a sample prepared in this way revealed that carbon in the deposited Si layer was below the detection limit (<0.1 at.N). Quantitative studies using the ion microprobe placed the carbon level in this sample around 100 ppm atomic, determined by comparison with a ~2C+ implant standard. The SPEG behavior of these "carbon-free" samples is peculiar. An example is shown in fig. 4. The edges of the Pd signal after annealing at I

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S O L I D - P H A S E E P I T A X I A L GROWTH

idea of a stabilizing effect of carbon on evaporated amorphous Si. A recrystallization of the amorphous Si also explains the reduction of the SPEG rate observed in "carbon-free" samples as the growth process proceeds. We have also investigated the effect of substrate orientation on the transport kinetics in SPEG for (100), (111) and near (110) wafers, as shown in fig. 5. Samples with (100)-oriented substrates were observed to be the fastest, with (110) and {111) samples having rates of 76% and 51% of the (100) rate, respectively. The substrate orientation appears to have no effect on the growth rate of the second stage. The reason for this behavior is not clear. The orientation dependence of the regrowth rate of amorphous Si layers obtained by ion implantation 18) is included in fig. 5 for comparison. In that case, amorphous Si layers, approx. 4000/1, thick, were formed on single crystal Si samples by implantation of 28Si ions at LN2 substrate temperatures. The comparison shows that the orientation dependence of the growth rate is much more pronounced for the regrowth of implanted amorphous layers than for SPEG through Pd-silicide. The overall dependence is the same in both cases, though. 7. Concluding remarks We have described a series of experiments which point out that the deposition process of the amorphous Si layer and the orientation of the substrate strongly influence the transport kinetics of SPEG. In particular, there is a inverse correlation between the SPEG rate and the concentration of carbon as an impurity in the amorphous Si layer. Carbon thus tends to slow down the SPEG process of amorphous Si. In addition, carbon also seems to stabilize the amorphous Si phase by delaying or eliminating spontaneous crystallization of the amorphous Si, thus allowing the SPEG process to take place. We believe that these two effects of carbon (i.e. inverse correlation and structural stabilization) are interrelated. Indeed, carbon-free samples exhibit high initial SPEG rates, but the rate decreases rapidly with time. Evidence points towards an early spontaneous crystallization of the amorphous Si layer, which reduces the driving force for SPEG. In view of the pronounced effects observed in the present study, it is likely that other structural or physical properties (such as electrical or optical) of the amorphous Si may also

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be modified by the doping of carbon. Experiments employing ion implantation of carbon into carbonfree amorphous Si layers would allow direct and quantitative investigation of the influence of carbon. Such experiments are in the planning stages. The authors would like to thank C. Canali for his participation in the early stage of this work. The technical assistance of R. Gorris and J. J. Mallory is also gratefully acknowledged. This research was supported in part by National Science Foundation Grants NSF-DMR-76-01058 at the University of Illinois and NSF-CHS-76-03694 at the University of Illinois and the California Institute of Technology. The initial phase of this study was supported by the Office of Naval Research (L. Cooper). References 1) C. Canali, S. U. Campisano, S. S. Lau, Z. L. Liau and J. W. Mayer, J. Appl. Phys. 46 (1975) 2831. 2) Z. L. Liau, S. U. Campisano, C. Canali, S. S. Lau and J. W. Mayer, J. Electrochem. Soc. 122 (1975) 1696. 3) Z. L. Liau, S. S. Lau, M-A. Nicolet and J. W. Mayer, Thin Solid Films 44 (1977) 149. 4) S. S. Lau, C. Canali, Z. i. Liau, K. Nakamura, M-A. Nicolet, J. W. Mayer, R. J. Blattner and C. A. Evans, Jr., Appl. Phys. Lett. 28 (1976) 148. 5) R. Pretorius, Z. L. Liau, S. S. Lau and M.-A. Nicolet, Appl. Phys. Lett. 29 (1976) 598. 6) W. K. Chu, J. W. Mayer, M-A. Nicolet, T. M. Buck, G. Amsel and F. Eisen, Thin Solid Films 17 (1973) 1. 7) p. W. Palmberg, G. E. Riach, R. E. Weber and N. C. MacDonald, Handbook of Auger electron spectroscopy (Physical Electronics Industries, Inc., Edina, Minnesota, 1972). 8) D. K. Bakale, B. N. Colby and C. A. Evans, Jr., Anal. Chem. 47 (1975) 1532. 9) p. Williams, R. K. Lewis, C. A. Evans, Jr. and P. R. Hanley, submitted to Anal. Chem. 10) p. Williams and C. A. Evans, Jr., J. Mass Spec. Ion Phys. 22 (1976) 327. IJ) See, for example, J. W. Mayer, L. Eriksson and J. A. Davies, Ion implantation in semiconductors (Academic Press, New York, 1970). 12) C. A. Evans, Jr., Anal. Chem. 47 (1975) 818A. i3) L. Csepregi, E. F. Kennedy~, T. J. Gallagher, J. W. Mayer and T. W. Sigmon, Appl. Phys. (to be published). 14) E. F. Kennedy° L. Csepregi, J. W. Mayer and T. W. Sigmort, Appl. Phys. (to be published). 15) K. Nakamura, M-A. Nicolet, J. W. Mayer, R. J. Blattner and C. A. Evans, Jr., J. Appl. Phys. 46 (1975) 4678. J6) A. K. Sinha, B. C. Giessen and D. E. Polk, in Treatise on solid state chemistry vol. 3 (ed. N. B. Hannay; Plenum Press, N.Y.,1976). 17) A. Barna, P. B. Barna and J. F. Pozca, J. Non-Crystal. Solids 8-10 (1972) 36. 18) L. Csepregi, E. F. Kennedy, J. W. Mayer, T. W. Sigmon and T. R. Cass, to be published. IX. COMBINED T E C H N I Q U E S