Grain growth kinetics during ion beam irradiation of chemical vapor depositedamorphous siliconC. Spinella, S. Lombardo, and S. U. Campisano Citation: Applied Physics Letters 57, 554 (1990); doi: 10.1063/1.103644 View online: http://dx.doi.org/10.1063/1.103644 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/57/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Kinetic roughening of amorphous silicon during hot-wire chemical vapor deposition at low temperature J. Appl. Phys. 101, 024915 (2007); 10.1063/1.2424527 Grain boundary mediated amorphization in silicon during ion irradiation Appl. Phys. Lett. 56, 30 (1990); 10.1063/1.102637 Grain growth and size distribution in ionirradiated chemical vapor deposited amorphous silicon Appl. Phys. Lett. 55, 109 (1989); 10.1063/1.102118 A model for the discharge kinetics and plasma chemistry during plasma enhanced chemical vapor deposition ofamorphous silicon J. Appl. Phys. 63, 2532 (1988); 10.1063/1.340989 Kinetics and mechanism of amorphous hydrogenated silicon growth by homogeneous chemical vapor deposition Appl. Phys. Lett. 39, 73 (1981); 10.1063/1.92521

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Grain growth kinetics during ion beam irradiation of chemical vapor deposited amorphous sincon

C. Spinella and S. Lombardo htituta di Metodalogle e Teenalogle per ta A,ficraelettronicu, CNR, corso [talia 57.1 95129 Catania, Italy

S. U. Campisano Dipartimento di Fisica dell'Universita, corso Ita/ia 57.195129 Catania, lta(v

(Received 5 March 1990; accepted for pUblication 27 May 1990)

The amorphous to polycrystal transition during Kr ion beam irradiation of chemical vapor deposited silicon layers has been studied in the temperature range 320-480 0c. At each irradiation temperature the average grain diameter increases linearly with the Kr dose, while the grain density remains constant within the experimental accuracy. The growth rate follows a complex behavior which can be described by dynamic defect generation and annihilation. The absolute value of the grain growth rate is equal to that ofthe ion-assisted epitaxial layer by layer crystallization in the silicon (111) orientation. This result can be related to the crystal grain structure and morphology.

Amorphous silicon layers deposited onto single-crystal material undergo a phase transformation by ion-assisted processes at relatively low temperatures. For a truly amor­phous layer deposited onto single crystal silicon epitaxial growth was observed. U Polycrystalline material is formed during 600 ke V Kr ions bombardment at T - 450 eel for silicon oxide substrates. The grains growing under ion beam irradiation are characterized by a high degree of symmetry (spheres or cylinders) and by a very narrow size distribu­tion.

cmZ are reported in Fig. 1 (b). As it appears the diameter distribution is very narrow; the fun width at half maximum is of the order of20% the average diameter. It must be noted that a similar sample whose grains grow by a pure thermal treatment at 700 °C has a broader size distribution3 due to the occurrence of nucleation and thus one observes an in­crease in both the average diameter and the density of crystal grains. Grain density and crystalline fraction must be mea-

In this letter we report detailed measurements of the growth kinetics of isolated crystal grains surrounded by amorphous material. The shape of these symmetrical grains is also discussed.

Silicon layers 90 nm thick were deposited at 540 "C in a low-pressure chemical vapor deposition (LPCVD) appara­tus. The pressure and the deposition rate were 250 mTon and 3 nm!min, respectively. Before deposition the single­crystal silicon substrate was coated with a thick Si02 1ayer in order to allow transmission electron microscopy (TEM) ob­servations after a lift-off process. The samples were implant­ed at room temperature by 130 ke V Ge + ions at a dose of 8 X 1013!cm2

• Such pre-irradi.ation procedure is needed to reduce the density of growing grains with respect to the as­deposited material, allowing us to observe the growth of iso­lated grains." The amorphous to polycrystal transition was induced by 600 ke V Kr ion irradiati.ons at ftuences in the 1015_2 X 1016/cm2 range and dose rates of about 1 X 10 12

ions/cm2 s. During these irradiations the samples were held within ± 3 ac at a temperature in the range 320-480 ac. A portion of each sample was shadowed from the beam so that it could experience the same thermal history aft he irradiated portion. No sign of crystallinity was observed in these shad­owed areas.

A TEM micrograph of a sample irradiated at 450°C to a fluence of 8 X 1015!cm2 is reported in Fig. 1 (a). As already reported the ion-induced grains are cylindrical in shape.3

The grain density, for the Ge pre-amorphization dose used in the present experiment, does not depend on the Kr ion dose at each irradiation temperature. The grain size distributions detennined at 450°C for doses of3 X 1015!cm2 and 8 X 1015!

Is!

30

'" ~ '<0 S-. 0 20 '0 !.. Q)

..0 S ;j 10 z

o~~~~~~~~~~~~~~~~

o 50 100 150 200 250

{til Grain Diameter (nm)

FIG. I. (a) Transmission electron micrograph of an amorphous layer irra­diated at 450 "C by 600 keY Krto a dose ofS X 10" ions/em'- (b) The grain size distribution, for doses of 3 X 10" ions/em" and 8>< 1015 iOlls/em' arc ft>l'orted in the lower part.

554 Appl. Phys. Lett. 57 (6), 6 August 1990 0003-6951/90/320554-03$02.00 (Si 1990 .American Institute of Physics 554

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sured as a function of time and aU these data must be ana­lyzed in terms of the Avrami-Mehl-Johnson description to determine both growth kinetics and nucleation rate. 4

On the other hand, under ion beam irradiation the grain density is constant and the grain size distribution is very narrow. Both observations imply that the nucleation rate is zero within our experimental accuracy. The grain growth velocity is then determined only by the time evolution of the grain radius.

The experimental data reported in Fig. 2 show that the average grain diameter increases linearly with the Kr dose so that the grain growth velocity is directly the slope of the straight lines fitting these experimental data. The growth rate increases with the irradiation temperature and the re­sults are summarized in Fig. 3 where the growth rate is re­ported versus 1! kT in a semilogarithmic plot. The growth rate temperature dependence is much weaker than that ob­served for the pure thermal transition; the apparent activa­tion energy is about 0.3 eV. In order to describe the mea­sured growth rates by a quantitative approach we have applied a phenomenological model previously developed to explain the planar crystallization of amorphous layers in­duced by ion beam irradiation. 5 This approach does not pro­vide a simple Arrhenius behavior. It i.s assumed that the crystallization process is due to the motion of kinks at the amorphous-crystal interface. At low substrate temperatures ( < 500 ·C) the generation of these kinks occurs mainly within the collisional cascade of each impinging ion during the thermal spike regimc< After the generation, kinks annihi­late in pairs as foreseen by the Jackson modeL6 In this de­scription the average growth rate at any substrate tempera­ture is given by

R = (aANoIY¢)Vi 109[sinh(yNe

) J \ No

for large values of y. In this expression a is the lattice param­eter, No is the concentration of defects generated by the im­pinging ion, Ne is the equilibrium concentration of defects thermally generated at the substrate temperature, A is the

FIG. 2. Average grain diameter as a function of the Kr iOIl dose and at different irradiation temperatures.

555 Appl. Phys. Lett., Vol. 57. No.6, 6 August 1990

--j

'" fa 10 1 u

"" I'll ~ 0 . ..,

::!:

. ~ • 0 ..... 10° • "" E f

c: ....... 090

l "0

"" 10- 1 p::: "C

14- 16 18 20

l/kT (eV- I]

FIG. 3. Semilogarithmic plot of the growth rate. The full line is the best fit to the proposed model.

volume crystallized in a single defect jump, Vj is the jump frequency, Va is the volume amorphized by one ion, if; is the dose rate, and r is given by

r = (Nocralrrtt¢)vj (2)

with Yo equal to the radius of the collision cascade, The tem­perature dependence of the growth rate R is contained in Vj:

v) = Va exp( - ElkT) , (3)

where E is the activation energy for kink migration. This model explains the experiments on the transition

between amorphization and crystallization,? the orientation dependence, and the effects of impurities.s In particular, si.nce kinks are thermally generated in the thermal spike re­gime, their number is proportional to the number of genera­tion sites at the crystal-amorphous interface and it will de­pend on the substrate orientation.8

The full line reported in Fig. 3 represents the best fit of the above equation for the growth rate R to the experimental data, The fitting parameters are if = 6.3 X 10- 14 cm2

,

It = 2x 10- 23 cm3, (that is one atomic volume),

vo =1.7X1014 S-l, E=1.14 eV, ro=6 nm, Va =3.3

X 10- 22 cm3, and No = 6.5X lOH'/cm3

• These values are in good agreement with the values used to describe the planar crystallization, It must be noted the resulting No value is that required to tit the experiments for the planar motion of ( 111) interface, It seems then that the grain growth is con­trolled by the motion of ( 111) interfaces.

To confirm this hypothesis we have performed detailed analysis by using the dark field techni.que and the results are illustrated in Fig. 4 for simple grains. As it appears the sim­plest grain (upper part) is formed by two halves separated by a (f III twin boundary. The upper right of the figure reports the three-dimensional sketch of the grain structure. The growth occurs by nucleation and moti.on of terraces onto (111) planes which determine the morphology of the grain. The angle between these planes is - 70° that is in ex­cellent agreement with that measured in the TEM micro­graph. The edges A, D, C along (112) directions have been

Spinella, Lombardo, and Campisano 555

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FIG. 4. Dark field micrographies of two selected grains showing the pres­ence of twin boundaries. On the right-hand side the grain structure is sche­matically depicted.

experimentally observed too. In the lower part of Fig. 4 is reported the TEM micrograph showing a grain containing a double twin along the [T 11 J and [1 I 1} axes and the corre­sponding edges. The two-dimensional (2-D) grain crystal­lography is sketched on the corresponding right hand. Of course the 3-D representation of this grain is much more complex and it has not been attempted here. It is however evident that planes defining the crystal-amorphous i.nter­faces are still (111) planes.

This description of the grain topography directly ex-

556 Appl. Phys. Lett., Vol. 57, No.6, 6 August 1990

plains why the grain growth kinetics is equal to that of the planar ( 111) crystallizati.on.

In conclusion we have shown that the ion beam assisted growth rate of single-crystal grains surrounded by amor­phous material in pure silicon is governed by a complex tem­perature dependence which can be accounted for by an intra­cascade model. The absolute value of the growth rate and the grain shape are controlled by (111) crystal-amorphous in­terfaces.

This work was supported in part by Progetto Finaliz­zato Materiali e Dispositivi per l'Elettronica a Stato Solido (Consiglio N azionale delle Ricerche) .

'A. LaFerla, E. Rimini, and G. Feria, AppL Phys. Lett. 52,712 (l988). 2C. Spinella, F. Priolo, E. Rimini, and G. Feria, N llOVO Cimento D 11, 1805 ( 1989).

'C. Spinella, S. Lombardo, and S. U. Campisano, App!. Phys. Lett. 55,109 (1989).

4R. B.lverson and R. Reif, J. Appl. Phys. 62,1675 (1987). sF. Priolo, C. Spinella, and E. Rimini, Phys. Rev. B 41,5235 (1990). 'K. A. Jackson, J. Mater. Res. 3,1218 (1988). 7W. L. Brown, R. G. Elliman, R. V. Knoell, A. Leiberich, J. Linnros, D. M. Maher, and J. S. Williams, in Microscopy 0/ Semiconductor Materials, In­stitute of Physics Conference Series, edited by A. G. Cullis (The Institute of Physics, Bristol, 1987), p. 61.

'F. Spaepen and D. Turnbull, in Laser Annealing o/Semiconductors, edited by J. M. Poate and J, W. Mayer (Academic, New York, 1982), p. 15.

Spinella, Lombardo. and Campisano 556

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