Anomalous near-surface effects in room temperature implanted germanium

Nuclear Instruments and Methods 209/210 (1983) 303-307 North-Holland Publishing Company

303

A N O M A L O U S NEAR-SURFACE EFFECTS IN R O O M T E M P E R A T U R E I M P L A N T E D GERMANIUM E.M. LAWSON AAEC, Private Mail Bag, Sutherland 2232, Australia

K.T. S H O R T and J.S. W I L L I A M S RMIT, Melbourne 3000, Australia

B.R. APPLETON, O.W. H O L L A N D and O.E. SCHOW III ORNL, Oak Ridge TN 37830, U.S.A.

Rutherford backscanering and channeling analysis of high dose, room temperature implanted Ge has revealed an anomalous near-surface yield deficit. The effect is attributed to the absorption of oxygenand other light mass contaminants into a highlyporous implanted layer upon exposure to air. The effect is not observed during liquid nitrogen implantation. Implant dose and species dependences, and the effect of annealing are examined.

1. Introduction Ion implantation is an important technique for doping Si and GaAs in the semiconductor industry. Consequently, implantation damage and its subsequent removal has been relatively well characterised in these semiconductors [1]. In contrast, Ge, being of less technological importance, has been not been as extensively studied. However, recent investigations [2,9] have shown that room temperature implantation into Ge can result in near-surface structural modifications which differ markedly from the implant damage typically observed in Si and GaAs. In particular, Rutherford backscattering and channeling spectra exhibited a large near-surface yield deficit which could be correlated with an anomalously high incorporation of light mass impurities into room temperature implanted Ge. Furthermore, channeling analysis revealed that the yield deficit persisted over the depth of implantation damage, and TEM investigations have shown that the near-surface region is comprised of a highly porous amorphous and part polycrystalline layer [2,9]. Interestingly, implants carried out into Ge held at liquid nitrogen temperatures did not exhibit any such unusual surface effects, and the amorphised layers were similar in nature to those produced in Si.

This paper reports on more detailed investigation of the unusual surface effects produced in room temperature implanted Ge. In particular, the dose and species dependence, loss of implant species and the annealing behaviour have been studied.

2. Experimental Bi, T1, Sb, In and Ge ions were implanted into (100) Ge (p-type, 5 × l0 II cm 3) at doses in the range 5 × 1014 cm 2 to 1 × 1016 cm -2. The measurements of the dose and species dependence were carried out on samples where the range distributions were roughly similar. For example, 90 keV implants of Bi and T1, and 70 keV implants of In and Sb give Rp of - 230 A and ARp of 80-100 ~, [31. Implanted samples were analysed by 2 MeV He + backscattering and (100) channeling using 170 ° and grazing-exit-angle geometries [4] for optimum mass and depth resolution, respectively. Oxygen ( 16O( c~, a') 16O) and carbon ( 12C(p, p') 12C) profiles were measured [5] using nuclear reactions, and transmission electron microscopy [2] was employed to monitor structural modifications to the implanted Ge.

0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

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E.M. Lawson et al. / Anomalous near-surface effects

3. Results and discussion

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Upon examination of the implanted Ge by RBS, a distinct yield deficit was noted from the nearsurface Ge. The series of spectra shown in fig. 1 illustrate the dose dependence of the effect for 90 keV TI implantation into (100) Ge. The yield deficit clearly increases with increasing dose. Moreover, it is m a x i m u m at the surface and decreases with depth over the extent of the implantation damage, as indicated by the channeling spectra. If the near-surface is assumed to be amorphous and have a density close to that of crystalline Ge, the " a m o r p h o u s " layer corresponding to the spectrum in fig. l(c) is about 700-800 ,& thick, or roughly 3 4 times R p for 90 keV T1. However, a depth scale has not been given for fig. 1 since, as we illustrate below, considerable variation in the structure, composition and density of the implanted layer can result in drastic changes to the depth scale. The dose dependence of the yield deficit was observed with all implant species examined. Furthermore, room temperature implants had a distinctly black appearance following exposure to air. Another feature of fig. 1 is the apparent loss of T1 with i n c r e a s i n g dose (figs. l(b) and (c)). This effect is characterised in table 1, where implanted and measured doses (from 170 ° RBS spectra) are listed for the most heavily implanted samples. A striking loss of implant material (greatly in excess of that calculated from sputtering [6]) is observed for the highest doses of all species. Figs. 2 and 3 show RBS spectra which illustrate

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of incorporation of large concentrations of In and Bi into the near-surface Ge. However, the expected magnitude of the yield deficit based upon - 5 at.% Bi and In in Ge would be less than 10% compared with a value of > 20% observed. Furthermore, fig. 3 clearly shows a large yield deficit for Ge implantation into Ge, where no such composition change is expected. (Note that the depth scale in fig. 3 is based upon a near-surface density equivalent to that of crystalline Ge.) A plausible explanation for the yield deficit observed in RBS spectra is the incorporation of large concentrations of light contaminants into the near-surface of implanted Ge. This has been confirmed by nuclear reaction analysis [5] which has indicated substantial concentrations of both oxygen and carbon distributed over the damage depth. An example of the oxygen profile measured in implanted and unimplanted Ge is shown in fig. 4. Other low mass contaminants (e.g. N and H) may also be present but have not, as yet, been investigated. The magnitude of the RBS yield deficit and measured oxygen profiles suggest that up to 50 light mass atoms can be incorporated per incident ion, corresponding to about 25 at.% at the near-surface for a 1 x 1016 c m 2 implant. TEM analysis [2] has suggested a possible mechanism for light mass incorporation into im-

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that the yield deficit is not significantly dependent on the implanted species. In fig. 2, similar doses of In and Bi result in a similar magnitude and distribution of the yield deficit. In these cases it is tempting to account for the yield deficit in terms 15OC

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plantation-damaged Ge layers. Samples implanted at room temperature were found to contain a complex damage layer consisting of (i) a highly porous, amorphous/polycrystalline near-surface layer [2,9] containing many craters, (ii) an underlying, more uniform amorphous layer, and (iii) a deeper crystalline layer containing dislocations. The total thickness of this damaged layer was 2-3 times larger than that expected from the channeling spectra, suggesting the implanted layer to be considerably less dense than crystalline Ge. It is suggested that the oxygen, carbon and other contaminants are absorbed into the porous implanted Ge layers following exposure to air. In this regard, it is significant to note that room temperature implantation under UHV also resulted in a yield deficit as did implants carried out with different Ge starting material. We speculate that the marked loss of implant species may result from increased sputtering yield, attributable, in part, to an increased surface area. The observed temperature dependence of the near-surface structure is intriguing. Liquid nitrogen implants exhibit the expected uniform amorphous layer and RBS spectra show no yield deficit, whereas elevated temperature implants show a somewhat diminished yield deficit. We suggest that the generation of the porous structure

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is related to the mobility of implantation-generated defects at room temperature and above. We have not as yet identified the defect mechanism which might lead to such a damage structure. Preliminary annealing measurements have been carried out to ascertain the effect of the anomalous near-surface structure on damage removal. Fig. 5 shows that furnace annealing at 350°C for 30 min and then 400°C for 30 min does not significantly reduce the magnitude and distribution of the yield deficit (low mass contamination). This annealing sequence should be sufficient to completely recrystallise the amorphous Ge [7]. The damage layer has indeed recrystallised epitaxially but only over roughly half its thickness. Considerable residual damage is present in the near-surface region. Indeed, the channeling spectra following 350 and 400°C anneals were identical, indicating dramatic retardation of the growth process in the surface region corresponding to the yield deficit. We suggest that only the more uniform, deeper amorphous layer can grow epitaxially, and that the porous near-surface (contaminated) layer recrystallises in a polycrystalline manner. This porous, contaminated layer has also been observed [8] to hinder liquid phase epitaxial growth induced by pulsed laser annealing.

E.M. Lawson et el. / Anomalous near-surface effects

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structures identifiable as yield deficits in RBS spectra. The effect appears to result from absorption of oxygen and other light mass contaminants into a highly porous damage layer following exposure to air. The effect is not observed for liquid nitrogen implants suggesting that the structure of the implanted Ge surface is strongly influenced by defect migration during room temperature implantation. Once formed, the porous, contaminated surface layer cannot be fully regrown (epitaxially) upon annealing up to 400°C. These high dose effects may have deleterious consequences for device structures fabricated in Ge. Further, it is clearly important to investigate the existence and extent of such effects in the more important semiconductors, Si and GaAs.

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Two of us (JSW and KTS) acknowledge financial support from ARGS, AINSE and the Australian Special Research Centres Scheme.

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4. Conclusion We have shown that room temperature implantation into Ge can given rise to anomalous surface

[1] J.W. Mayer, L. Eriksson and J.A. Davies, Ion implantation in semiconductors (Academic Press, New York, 1970). [2] B.R. Appleton, O.W. Holland, J. Narayan, O.E. Schow III, J.S. Williams, K.T. Short and E.M. Lawson, Appl. Phys. Lett. 41 (1982) 711. [3] J.F. Gibbons, W.S. Johnson and S.W. Mylorie, Projected range statistics (Halsted Press, Pennsylvania, 1975). [4] J.S. Williams, Nucl. Instr. and Meth. 149 (1978) 207. [5] O.W. Holland, B.R. Appleton, J. Narayan and O.E. Schow III, unpublished. [6] H.H. Andersen and H.L. Bay, in: Sputtering by ion bombardment, ed., R. Behrisch (Springer-Verlag, Berlin, 1982). [7] S.S. Lau, J.W. Mayer and W. Tseng, in: Handbook of semiconductors, ed., S.P. Keller (North-Holland, Amsterdam, 1980) Vol. 3, Ch. 8. [8] B.R. Appleton, O.W. Holland, O.E. Schow III, C.W. White, J.S. Williams, K.T. Short and E.M. Lawson, unpublished• [9] I.H. Wilson, J. Appl. Phys. 53 (1982) 1698.

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