Grain growth and size distribution in ionirradiated chemical vapor depositedamorphous siliconC. Spinella, S. Lombardo, and S. U. Campisano Citation: Applied Physics Letters 55, 109 (1989); doi: 10.1063/1.102118 View online: http://dx.doi.org/10.1063/1.102118 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/55/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Fundamental study of ion-irradiation effects on the columnar growth of chromium films prepared by ion-beamand vapor deposition J. Vac. Sci. Technol. A 19, 153 (2001); 10.1116/1.1335835 Positive ions as growth precursors in plasma enhanced chemical vapor deposition of hydrogenated amorphoussilicon Appl. Phys. Lett. 75, 609 (1999); 10.1063/1.124456 Grainsize distribution in ionirradiated amorphous Si films on glass substrates J. Appl. Phys. 71, 648 (1992); 10.1063/1.351349 Grain growth kinetics during ion beam irradiation of chemical vapor deposited amorphous silicon Appl. Phys. Lett. 57, 554 (1990); 10.1063/1.103644 Retardation of nucleation rate for grain size enhancement by deep silicon ion implantation of lowpressurechemical vapor deposited amorphous silicon films J. Appl. Phys. 65, 4036 (1989); 10.1063/1.343327

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Grain growth and size distribution in ion .. irradiated chemical vapor deposited amorphous sincon

C. Spinella, S, Lombardo, and S. U. Campisano Dipartimento di Fisica dell'Universita. corso Italia 57, 195129 Catania, Italy

(Received 23 January 1989; accepted for publication 2 May 1989)

The amorphous to polycrystal transition in chemical vapor deposited (CVD) amorphous silicon has been studied at 450 ·C under Kr ion beam irradiation. The average grain size increases linearly with the ion dose, and the grain size distribution is very narrow compared to thermally grown grains, These results are consistent with the presence of crystal seeds in CVD material. All these seeds can grow simultaneously under ion beam irradiation. For layers compietely preamorphized by Ge+ implantation, no ion beam induced nucleation is observed.

The amorphous to single-crystal transition in silicon has been investigated in detail. 1-3 Solid phase epitaxial growth rate in ion-implanted material has been measured in the 500-1200 °C temperature range l and an activation energy of 2.7 eV was determined. The amorphous to polycrystalline transformation was investigated in vacuum-deposited4 and in chemical vapor deposited' (CVD) material, and the acti­vation energy for grain growth was determined to be 2,4-3.3 eV. The CVD samples were ion implanted with Si ions prior to the thermal treatment. 6 The results indicated that for high-dose implants, the polycrystalline transition can be de­scribed by classical nucleation and growth mechanisms. For low implantation doses, the presence of nucleation seeds was argued by the crystallization kinetics.

Recently the amorphous to single-crystal transition has been studied in the presence of an energetic ion beam. 7-9 The measured crystal growth rate is characterized by an activa­tion energy of -0.3 eV and the growth process occurs at temperatures as low as 250°C. The ion beam induced epitax­ial crystallization is capable of regrowing even a CVD layer deposited onto an uncleaned surface, 10 In this case the re­quirement to obtain a single crystal is the preamorphization of the deposited layer by a suitable As ion dose, II Without such a preamorphization procedure, a polycrystaHine layer was obtained. In this letter we report some results obtained on the amorphous to polycrystalline transition induced at 450°C by ion irradiation in CVD material.

Silicon substrates were covered by a thick silicon oxide layer and a 90-nm-thick CVD silicon was then deposited in a conventional reactor at a substrate temperature of 540°C. Some samples were subsequently preamorphized by implan­tation at room temperature with 130 keY Ge+ ions to doses between 8 X 1013 and 2X 10 15 ions/cm:?, Kr+ ions at 600 keY were then used to irradiate the samples held at 450'C. Ge and Kr ions were chosen to avoid chemical effects due to the introduced impurities. The energy was chosen so that the projected range of Kr is larger than the CVD silicon layer thickness, thus avoiding problems related to the end of range damage; the Kr f

- dose ranged from I X 10 15 to 1.1 X 10 16

ions/cm2 and used a beam current density of about 0.4 f-lAI cm2

• After preamorphization, some samples were thermally annealed in the temperature range 600-700 °C in a rapid thermal annealing (RT A) apparatus for time in the 30-150 s range. The CVD layers, either ion irradiated or thermally treated, were examined by transmission electron microscopy

(TEM) after a Hft-oifprocess. By the TEM method we have measured the grain size distribution and the fraction of crys­tallized materiaL

We have observed that after either thermal treatment or ion beam irradiation, a portion of the material transforms into polycrystals. The grain density is a function of the prea­morphization dose and, in order to have a controlled density of crystal grains, we will report results obtained after a prea­morphization dose of 1 X 1014 Ge+ /cm2

• For this dose a suit­able grain density is obtained.

The TEM micrographs reported in Fig. 1 refer to ion­irradiated and thermally annealed samples. Irradiation con­ditions are 600 keY Kr f ions at 450 oe, for doses of (a) 4 X 1015

, (b) 6 X 10 15, and (c) 8 X 1015 at/cm2. The thermal­

ly annealed sample has been treated for 30 s at 675 °C.ln ion­irradiated materials we note the presence of spherical crystal grains imbedded in the amorphous matrix; the average grain size of these crystal spheres increases with the irradiation dose. For grain diameter larger than the film thickness, the shape of these grains becomes cylindrical. Some samples were placed on the same holder but shadowed from the ions

FIG. 1. Tran;mis;;ion electron micrographs of CVD silicon irradiated at 450"C with 600 keY K1'+ ions at doses of (al 4X 1015/cm', (b) 6X 10"; em', and (el 8x 1O"/cm', and (d) after thermal treatment for 30 s at 675 "C.

109 Appl. Phys. Lett. 55 (2), 10 July 1989 0003-6951/89/2801 09-03$01.00 (c) 1989 American Institute of Physics 109

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so that they could experience the same thermal history of the ion-irradiated samples. No sign of crystallinity was observed in these reference samples even after the about 4 h time rc­quired to reach the lO'6/cm2 Kr dose.

The grain size in irradiated material has been measured and results arc reported in Fig. 2. The grain diameter in­creases linearly with the ion dose and the experimental trend extrapolates wen to the zero value at null dose, without showing an initial delay, as might be anticipated by the clas­sic theory of nucleation and growth. 12 In the as-deposited material, no sign of crystallinity has been observed by TEM even under high-resolution observations. We have also mea­sured the grain density under different irradiation doses and found that the average number of grains shows very little dependence on the Kr-+ dose. TEM observations of cross­sectioned samples have revealed that the grain density at the interface is larger than in the near surface region. This may be due either to the profile of energy deposited in elastic collisions or to a thickness dependence of the crystalline seeds. The hypothesis that the CVD material deposited at 540°C contains a fixed density of seeds (depending on the preamorphization dose only) that can grow under the ion beam irradiation at elevated temperatures has been further tested by increasing the Gc /- preamorphization dose to a value of 8 X 10 14/cm2. In this case, an amorphous layer should be obtained and thus no growth should be observed. Irradiation with Kr t at 450°C up to a dose of 8 >< 10 15 ions/ em} did not show crystalline grain in any of the samples analyzed. This result explains the epitaxial crystallization of CVD amorphous layers on a crystalline Si substrate. 10.11 In fact, if the preamorphization dose is high enough to produce a good amorphous layer, no polynucleation induced by Kr ion beam is observed and the deposited layer can thus regrow epitaxially onto the underlying single crystal.

From the data reported in Fig. 2 we determine that the growth rate of the grain radius is about 10 nm/( 10 15 at! em2

), a factor 2.5 lower than the rate determined" during epitaxial crystallization of the deposited layer. During ther­mal treatment the crystal radius growth rateS is a factor of 10-50 smaller than the planar interface velocity during solid phase epitaxial regrowth. This difference thus persists dur­ing ion beam regrowth, although attenuated.

300

[VO Silicon

E 600 keV Kr' c 200 '-

-: =450·C 2 0 OJ E ~

1

.~

't/ 0

c: []

ro '-

l::J

I 1 0 4 8 12

Dose (1015 at/cm 2)

FIG. 1. Crystal grain diameter vs ion fluencc for CVD silicon irradiated with 600 keV Kr· :II a substrate temp<?rawre of 450 T.

110 Appl. Phys. lett., Vol. 55, No.2. 10 July 1989

The grain shape in the thermally annealed sample, also reported in Fig. 1, is quite different from that observed after ion-assisted growth. The grains are no more spherical but elongated, due to an orientation-dependent transformation velocity. The thermal growth rate ratio between < I 11) and (100) directions is about a factor 1:20, while during ion beam assisted growth the ratio falls to a value 1:3.9 More­over, during thermal growth the grain density is a function of the annealing time and the results can be described as al­ready reported in the literature.6

A possible explanation of the differences between ther­mally and ion beam grown grains is that in a 540°C deposit­ed CVD material there are some nuclei that can grow under the ion beam irradiation without showing the incubation be­havior required by the nucleation mechanism. In contrast, during thermal treatment a transient in the transformation rate is observed due to the requirements for subcritical grains to reach a critical size.

As a further test we have measured the grain size distri­bution. The results are reported in Fig. 3 for a Kr+ dose of 8 >~ lOIS ions/cm1

. As it appears the grain size distribution in ion-irradiated material (shown for an irradiation dose of 8 X IOI5/em2) is very narrow and centered around the values aiready reported in Fig. 2. The grain size distribution in ther­mally grown grains is instead much broader, although the average grain size of the two distributions reponed in Fig. 3 is almost the same. During thermal treatment the grains first appearing have a time interval allotted for growth much longer than those appearing in the later stages of the anneal­ing process. The final result for a pure nucleation and growth process is a lognormal distribution of grain size. 13 For the present case of CVD material, the presence of an initial dis­tribution of very small grains gives 11 final distribution differ­ent from a pure lognormal one, but, in any case, much broader than that observed after ion beam crystallization. It is interesting to note that even at the highest temperature used in the present expetiment (700 °C) no crystal grains should be observed using a truly amorphous layer as starting material. In this case in fact, the time required to get nuclei of critical size is of the order of 103

S,4 much longer than the

30

1 '" C

t'O 20 '-

l:J -0

<-OJ .0 E 10 :::J :z:

I eVD Silicon I 1 I 600 keV Kt S,1dYcm2 ~

/"1 at T = 450 ·c /

I

T/i -30,.1675"C l I

200 400 600 800

Grain Diameter [nm)

FIG. 3. Comparison of grain diameter distriblltion for thermally and iOIl

beam recrystallized CVD silicon. The comparison is performed for almost equal average gmin size.

Spinella, lombardo, and Campisano 110

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processing time we adopt. In conclusion, we have reported experimental evidence

that in CVD-deposited layers the amorphous to polycrystal­line transformation occurs at temperatures as low as 450°C in the presence of an energetic ion beam. The shape and the distribution of these grains are substantially different from those growing by a pure thermal process at T - 675 'c The results are consistent with a description where the crystalline nuclei present in the CVD material can grow at the same time, independently of their initial size. For layers complete­ly preamorphized no beam induced nucleation is observed.

We are indebted to W. L Brown for comments and sug­gestions. Thanks are due to F. Baroetto for the CVD deposi­tions and to Professor S, Sanfilippo for the electron micros­copy facilities, Work supported in part by Progetto Finalizzato Materiali e Dispositivi per l'Elettronica a Stato Solido-Co N. R

111 AppL Phys. Lett, Vol. 55, No.2, 10 July 1 SS9

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Spinella, Lombardo, and Campisano 111

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