Ion-beam induced epitaxy of silicon

Volume 71A, number 2,3

PHYSICS LETTERS

30 April 1979

ION-BEAM INDUCED EPITAXY OF SILICON I. GOLECKI, G.E. CHAPMAN

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S.S. LAU, B.Y. TSAUR and J.W. MAYER

California Institute of Technology, Pasadena, CA 91125, USA Received 20 November 1978

Epitaxial regrowth of an amorphous Si layer on a (100) Si crystal held at 200—400°Cis achieved under bombardment with Si, Kr, or Xe ions. Channeling measurements with MeV He ions show the regrowth proceeds from the amorphous— crystalline interface, and has an initially linear dose dependence. The annealing beam, however, introduces additional damage centered at or beyond the ion range. Amorphous layers obtained by low-temperature self-ion bombardment regrow much more readily than amorphous deposited layers.

Epitaxial regrowth of an amorphous semiconductor layer on a single-crystal substrate can be induced by maintaining the sample at an appropriate temperature in in a furnace [1]. Recently, it has been shown that similar epitaxy can be achieved by irradiation with a laser pulse or an electron beam pulse [2] of an appropriate energy density and duration. Here we report on the use of ion beams of Si, Kr, or Xe to achieve epitaxial regrowth of amorphous silicon layers on [100] silicon single-crystal substrates. Silicon (100) wafers were uniformly amorphized to a depth of 2300 A by random-incidence Si implants at T —70°C[3]. The energies and doses used were 100 keV (2 X 1015 Si~/cm2)and 80 keV (I X 1015 S181cm2), with a dose rate of 0.5 pA/cm2. The width and non-crystalline character of the surface region were determined by means of 1.5 MeV He+ channeling analysis. The samples were then heated to temperatures in the range 200—400°Cand bombarded in the random direction (i.e. at off the [100] axis) with Si ions to various doses. Similar irradiations were carried out at 300°Cwith Kr and Xe ions on samples having a l200 A thick amorphous surface layer. The dose rates used in the “hot” implants were in the range 0.5—2.0 pA/cm2, and did not cause any beam heating of the target. The ion energies were chosen so as to vary the position of the ion distribution peak 70

Present address: School of Applied Science, Riverina College of Advanced Education, Wagga Wagga, NSW 2650, Australia.

(i.e. the projected range) from inside the amorphous region through to the underlying crystal, past the amorphous—crystalline interface. The temperature range studied was well below that where purely thermal epitaxial growth (SPEG) takes place (i.e. ‘-~500°C for the time of the present experimental runs [1]). For comparison, we also bombarded a Si wafer having a deposited amorphous surface layer. The e-gun evaporation was done at a rate of -‘30 A/s in an ionpumped vacuum system, the pressure during evaporation being in the low lO~Torr range. Prior to evaporation, the wafer was degreased in organic solvents in an ultrasonic bath, and dipped in a dilute HF solution. We first consider the regrowth of amorphous layers originally produced by low-temperature self-ion bornbardment. The results of a typical experiment are shown in fig. 1. A “hot” implant of 1 X 1016 Si! cm2 of 250 keV is seen to induce an epitaxial regrowth of 885 A at 300°C. The growth front proceeds from the amorphous—crystalline interface, as in furnace and pulse annealing. The deep, broad maximum appearing in the regrown portion (b) and in the crystalline part (d) after the hot implant is due to disorder caused by elastic collisions at the end of the ion range. It is centered at the projected range of the 250 keV Si ions (“-4000 A) [4]. This disordered region has been shown previously [3] to consist of defect clusters and dislocation loops, and can be partially annealed at T> 800°C.The channel267

Volume 71A, number 2,3

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PHYSICS LETTERS

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30 April 1979

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Energy (MeV) Fig. 1. Ion-beam induced epitaxial regrowth (IBIX) of a selfion implanted amorphous layer of Si on [100] Si. Channeling analysis by means of 1.5 MeV He+ ions: (a) original amorphous layer (2300 A), (b) after “hot” implant (regrown to 1415 A), (c) virgin crystal, (d) hot implant into virgin crystal, (e) ran~ dom yield. The projected range of the “hot” implant was ‘—4000 A.

ing yields behind the amorphous layers in spectra (a) and (b) agree with the values calculated from Meyer’s multiple scattering theory [5], as applied by Campisano et al. [6]. This indicates that the deep disorder does not reach the regrown interface (b) in this case, The surface region of the virgin crystal is not affected by the hot implant as far as channeling is concerned; see spectra (d) and (c) in fig. 1. Thus the epitaxy may be related to that part of the energy loss of the mcident Si ions that is dissipated in electronic excitation. The original amorphous—crystalline interface (a) is at a distance of ~—1 .4 times the projected range straggling, ~ [4], from the peak of the deep damage (towards the surface). Irradiations performed with Kr and Xe ions resulted in similar regrowth of the amorphous zone. The higher dose “hot” implants with the latter ions, however, caused in addition an increase in the surface peak in the virgin crystal. This was probably due to the larger portion of energy dissipated in nuclear collisions at the surface, as compared to the Si implants. The deep damage peak was also higher for Kr and Xe for comparable regrowth thicknesses. The position of the 268

Fig. 2. Dose dependence of the epitaxially regrown layer thickness at 300°Cfor the following implantation conditions: 250 keV Si (R~= 4000 A) into 2300 A a-Si/[100J Si (+); 120 keY

Si (R~= 1800 A) (v), 300 keV Kr (R~= 1400 A) (A), 460 keY Kr (R~= 2500 A) (.), 650 keV Kr (Rp = 3900 A) (•), 400 key Xe (R~= 1600 A) (o) into 1200 A a-Si/[100] Si. The solid straight line is an aid to the eye. The original a-Si

layer was produced by low-temperature self-ion bombardment in all cases (see text).

damage peak was at (1.3—1.5) R~in the case of Kr,

as compared to -‘Rn for Si. This is in contrast to the predictions of Brice’s calculations [7] ,in which the energy deposited into nuclear processes peaks at (0.72—0.76) R~.However, the latter calculations are only valid at low temperature, where all annealing effects can be neglected. The position of the deep damage peak remained constant from 200°C to 400°C for the Si implants. The dose dependence of the regrowth at 300°Cis shown in fig. 2. For doses lower than ‘—1 X 1016 cm2 there was a linear dependence, as verified by plotting the data on logarithmic scales. For higher doses the regrowth increased less rapidly. The exact position of the projected range with respect to the amorphous— crystalline interface was not critical for the regrowth, as long as it was not at the interface itself. Thus the interface appears to act as a sink for vacancies originating along the ion tracks. The number of vacancies and/or their mobility are enhanced by the excitation caused by the incoming ions. Preliminary measurements of the temperature dependence of the regrowth for Si implants showed a sixfold increase between 200 and 300°C,and another 1.5-fold increase between 300 and 400°C.No simple thermally activated process

Volume 7lA, number 2,3

PHYSICS LETTERS

could be associated with the regrowth from these data. For comparison, we repeated one of the experiments (1016 Si/cm2, 250 keY, 300°C)on a 2600 A thick deposited amorphous Si layer. The observed regrowth was half that obtained with an amorphous layer produced by a low-temperature self-ion implant. We speculate that an oxide and other contaminants at the interface play a role here, as in the case of furnace annealing [8]. Beam-annealing effects somewhat similar to the ones observed here had been reported by Holmén et al. [9] for 40 keV self-ion irradiation of Ge. Sputtering had been excluded as a possible mechanism in that case (with the sputtering yield S 8) [10], as it is here (S < I for the Si implants, and S< 5 for the Kr and Xe implants). To summarize, we have observed epitaxial regrowth of amorphous Si layers on [100] silicon, induced by implants of Si, Kr, or Xe ions at 200—400°C.The regrowth depends linearly on dose up to ~1016 cm2, with a slope of -9 X 10—14 A cm2 or equivalently 45 monolayers/monolayer. Thus the same ion which can be used to create an amorphous layer in silicon below —70°C,can also be used to epitaxially regrow an amorphous layer at T ~ 200°C,along with the introduction of some stable disorder. We would like to thank M.A. Nicolet for useful discussions, and D.K. Brice for providing us with calcu-

30 April 1979

lations of energy deposition profiles for several cases of interest to this study. We gratefully acknowledge the technical assistance of J. Mallory, R. Gorris, and D. Tonn, and the financial assistance provided by the Office of Naval Research (L. Cooper).

References [1] L. Csepregi, J.W. Mayer and T.W. Sigmon, Phys. Lett. 54A (1975) 157. [2] See e.g. the review by J.W. Mayer and P.T. Clogston, in: Proc. Laser annealing workshop (Catania, Italy, 1978)

pp. 1—12. [3]L. Csepregi, E.F. Kennedy, S.S. Lau, J.W. Mayer and T.W. Sigmon, App!. Phys. Lett. 29 (1976) 645. [4]J.E. Gibbons, W.S. Johnson and S.W. Mylroie, Projected range statistics, 2nd ed. (Dowden, Hutchinson, Ross, Stroudsburg, PA, 1975). [5] L. Meyer, Phys. Stat. Sol. (b) 44, 253. [61 S.U. Campisano, G. Foti, F. Grasso and E. Rimini, Phys. Rev. B8 (1973) 1811. [7]D.K. Brice, Ion implantation range and energy deposition distributions, Vol. 1 (IFI/Plenum, New York, 1975); and private communication (1978). [8] L. Csepregi, E.F. Kennedy, J.W. Mayer and T.W. Sigmon, J. Appi. Phys. 49 (1978) 3906. [9] G. Holmén, S. Peterström, A. Burén and E. B~gh, Radiat. Eff. 24 (1975) 45;

G. Holmén, A. Burén and P. Hogberg, Radiat. Eff. 24 (1975) 51. [10] G. Holmén, Thesis (Chalmers Univ. of Technology, GotebOrg, 1974).

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