STRUCTURAL AND ELECTRONIC PROPERTIES OF CVD SILICON FILMS NEAR THE CRYSTALLIZATION TEMPERATURE

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STRUCTURAL AND ELECTRONIC PROPERTIES OF CVD SILICON FILMS NEAR THE

CRYSTALLIZATION TEMPERATURE

J. Magariño, D. Kaplan, R. Bisaro, J. Morhange, K. Zellama

To cite this version:

J. Magariño, D. Kaplan, R. Bisaro, J. Morhange, K. Zellama. STRUCTURAL AND ELECTRONIC PROPERTIES OF CVD SILICON FILMS NEAR THE CRYSTALLIZATION TEMPERATURE.

Journal de Physique Colloques, 1982, 43 (C1), pp.C1-271-C1-276. �10.1051/jphyscol:1982137�. �jpa-

00221794�

JOURNAL DE PHYSIQUE

CoZZoque C1, suppZ&ment au nOIO, Tome 43, octobre 1982 page C1-271

STRUCTURAL AND ELECTRONIC PROPERTIES OF CVD SILICON FILMS NEAR THE CRYSTALLIZATION TEMPERATURE"

J. ~ a g a r i h , D. Kaplan, R. Bisaro, J . F . Morhange* and K . Zellama**

THOMSON-CSF, Laboratoire Central de Recherches, Domaine de CorbeviZZe, 91401 Orsay Cedex, France

"Laboratoire de Physique des SoZides de ZrUniversit& Pierre e t Marie Curie, 4, Place Jussieu, 75230 Paris Cedex 05,France

**Groupe de Physique des Sobides de ZrEcoZe Normale Supgrieure, Universitg Paris V I I , 2, Place Jussieu, 75221 Paris Cedex 05, France

RBsumg. - Nous avons examin6 les proprigtgs structurales ae films dgposgs par C.V.D. dans la gamme de tempgrature 600-750°C avec une attention particuliere sur la repartition en volume des cristaux pour les films dgposgs autour de la tempgrature de cristallisation. Nous dEmontrons que le mgcanisme ggnbral de cristallisation dgterminant la structure est celui de d6pbt de matgriau amorphe suivi de cristallisation dans la phase solide pendant la durge du dgp6t. Ce processus dans la phase solide apparazt par une nuclgation prgdominante a l'interface film-substrat. Les implications de ceci sur la morphologie et les propri6tGs physiques des films seront discutges.

Abstract. - We have examined the structural properties of films grown by CVD in the range of temperatures 600-750°C.

with particular emphasis on the volume repartition of crystals for films deposited around the crystallization temperature. We find that the general crystallization

mechanism which determines the structure is one of deposition of an amorphous material followed by subsequent solid phase crystallization during the deposition time. This solid phase process occurs through a dominant nucleation at the film-substrate interface. The implications of this on the physical properties and morphology of these films will be discussed.

We present a study of the crystallization process for silicon films prepared by chemical vapor deposition (CVD) from silane.(SiH4) at atmospheric pressure on fused silica substrates. The study concerns crystallization during deposition and by subsequent annealing, which has been the subject of a number of previous reports [l-31 describing X-ray texture, crystallization rates, conductivity changes, etc...

Relatively minor attention has been paid to volume repartition of crystals within the film [ 4 1 . In this study, we have concentrated on this question and found that partially crystallized films are, under many conditions, inhomogeneous in depth with a repartition indicative of the dominance of an heterogeneous nucleation mechanism at the film- substrate interface. We discuss the implications of these findings as regards measurements of crystallization rates and morphology of polycrystalline films.

The films have been prepared in a CVD reactor (Applied Materials A M V 800) industrially used for classical Si/Si epitaxy at atmospheric

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982137

C 1 - 2 7 2 JOURNAL DE PHYSIQUE

pressure. Pure silane (1 1 min-1) was diluted in hydrogen (32 1 min-l) into a vertical open-flow reactor with a silicon carbide susceptor heated by induction. n-type doping has been achieved by mixing phos- phine (pH3) with silane. Films thicknesses are of the order of 1 pm.

The growth rates of undoped and lightly doped films

([PH31/CSiH41 = 20 ppm) as a function of the deposition temperature are shown in figure 1. An activation energy of 2.2 eV is obtained for the growth rate. This value is higher than that reported in experiments at higher deposition temperature C51 or using nitrogen or helium as carrier gas C2, 31 but is very near of the value recently reported by Beers and Bloem C61 under similar experimental conditions.

I , I I I I I I

- -

DEPOSITION

- ^-

- -

CRlSTALLlZATlON

- -

GROWTH RATE

- -

I I I I

Fig. 1 : Comparison of deposition rates

un- doped, lightly doped films) and crystal growth rates ( undoped,

TD=6000C

lightly do- ped, TD=6400C).

To investigate the volume repartition of crystals we use Raman scat- tering in conjunction with X-ray diffraction : for the typical thick- ness of our films, the light of the Ar ion laser (488 nm) used in the Raman scattering probes only a thin surface layer due to the strong optical absorption (between 40 and 200 nm respectively for amorphous and polycrystalline silicon). By contrast, X-ray diffraction using copper K , radiation probes the total depth of the sample [ 7 1 . We find three kinds of films :

a) films deposited at 600°C are amorphous in structure. In this case, incident radiation at glancing angle has been used to detect the weak

"halos" corresponding to the amorphous structure. A first "halo" is

observed near the position of the (111) crystalline peak and a single

second "halo" at an angle between the (220) and (311) crystalline

peaks. This could not originate as a superposition of broadened

crystalline (220) and (311) peaks, because the width of the second

halo is less than the separation of these two crystalline peaks. The diffraction spectra are indistinguishable from those obtained in films deposited by glow discharge at 250°C. The Raman spectra show a broad peak near 480 cm-1 characteristic of the optical phonons in. an amor- phous silicon structure C81.

b) Films deposited at TD = 700°C or higher temperatures are

all polycrystalline. The crystalline peaks (ill), (220) and (311) are observed by X-ray diffraction using a standard 8-28 geometry. A pre- ferred orientation (111) for films deposited at 700°C becomes stron er

(220) at 750°C. The Raman spectrum shows a narrow peak near 520 cm- as in single crystal silicon.

?

C)

Films deposited in the intermediary range of temperatures

(620 < T < 680°C) are generally partially crystallized. X-ray crystal- line peaRs are observed, but the striking result is that in Raman scattering, for radiation incident on the free surface side, the spectrum shows the broad amorphous peak, whereas for radiation inci- dent through the transparent substrate, probing only the substrate- film interface, the narrow crystalline peak is observed (figure 2).

Fig. 2 : Raman scattering intensity as a function of frequency. A Light incident on the free surface side. B Light incident through the substrate.

h CI U)

.-

C 3 h L CI

F

.- 13 V

k

>

!z

V)

z

W

I-

Z

Such inhomogeneous structure can be understood by assuming : i) that the material is amorphous when deposited and crystallizes by subse- quent solid phase crystallization during the deposition time

ii) A fast nucleation mechanism at the film-substrate interface is dominant for the solid phase crystallization. Under these conditions the films will have a polycrystalline layer near the film-substrate interface, the thickness of which will be dependent on the relative magnitude of the deposition and crystal growth rates.

I I

A

I I

I I I

B

I I I

500 450

,s-. F

400 520 510

FREQUENCY (cm '1

Cl-274 JOURNAL DE PHYSIQUE

This is a favorable situation to study the solid phase crystallization by measuring the dependence with time of the conductivity in isother- mal annealing treatments E91. In this case the measured conductivity is that of two layers in parallel, the first one being the polycrystal- line layer of thickness Vgt, where V is the crystal growth rate, and the second one the amorphous layer 03 thickness e-Vgt, where e is the thickness of the total layer.

The measured conductivity is linear with time and the crystal growth rate can be directly obtained at different annealing temperatures, as shown in figure 1. By comparing in this figure deposition and crystal growth rates at a given temperature, we note that at 700°C both rates are equal. This is in agreement with the'experimental fact that the deposited films are totally polycrystalline at this temperature. At lower temperatures, the crystal growth rate is less than the deposi- tion rate and the relative thickness of the polycrystalline layer decrease with decreasing deposition temperature. However for films deposited at 600°C, where the crystal growth rate is about 1/4 of the deposition rate, we cannot observe any evidence of a polycrystalline layer. We think that the nucleation rate at the interface is tempera- ture dependent C41 and is too low at this temperature to produce a significant fraction of polycrystalline material.

The measured values of the crystal growth rates can be compared with those obtained on amorphous silicon prepared by other deposition techniques. For example, the values of Vg obtained here are very close to that obtained in similar experiments on evaporated amorphous

silicon films C91. Drosd and Washburn ClOl have measured crystal growth rates in the crystal directions C1001, CllOl and C1111 on implanted silicon. Our results agree within a factor of two in abso- lute magnitude and a difference of 0.1 eV in activation energy with the results reported for the Cllll growth C101. On the contrary, crystal growth rates in C1001 and CllOl directions are more than one order of magnitude faster than in our case. This is to be related with

the observed [llll preferred orientation in films deposited at 700°C or in the inhomogeneous films produced during deposition or annealing at lower temperatures.

The inhomogeneous distribution can give erroneous results in the interpretation of the physical measurements in films deposited at the intermediary range of temperatures 620-680°C. For example, for a film lightly doped (20 ppm PH3) with a relative thinner polycrystalline layer and a thicker amorphous layer, the apparent conductivity is near that of polycrystalline silicon because at this doping

a,

10-7 (Qcm) -1 << ap

10-3 (Qcm)-l. On the contrary, the apparent optical absorption coefficient is near that of the amorphous silicon because of their higher optical absorption in the visible range and the relative thickness of the amorphous layer. A particular interes- ting case is the measurement of the density of paramagnetic defects by EPR. In undoped amorphous CVD silicon the density of dangling bond defects at g = 2.0055 is ~ l O 1 ~ c m - ~ ~ l l l . The spin density in undoped films, decrease with increasing deposition temperature, corresponding to the diminution of the relative thickness of the amorphous layer.

At 700°C a density of 10l8cm-~ spins remains, corresponding probably to amorphous regions between the crystals.

The dependence of the spin density with temperature for lightly doped films is shown on figure 3. At this doping, the density of donors C123 is too low to compensate the number of defects in an amorphous

structure, so the same density of paramagnetic defects is observed

for films deposited at 600'~. On the contrary, for films deposited at

700°C, the density of donors is enough to supress the dangling bond

spin signal. This resonance is replaced by a new signal with the same g-factor of 1.998 as that of conduction electrons in doped crystalline silicon r131.

We believe that films deposited at 750°c cannot be described by the same mechanism of crystallization because : i) A spin resonance is observed at g

2.0050 which differs little from that of dan lin bonds in amorphous silicon, with a high density of 5 x 1 0 1 % ~ m - ~

Fig. 3 : Spin density as.

a function of the depo- sition temperature obtained by EPR experi- ments at 90K.

TEMPERATURE

C 1

ii) The conductivity of these films at light doping is comparable to that of amorphous silicon ; iii) The grain sizes measured in these films (10-20 nm) are much lower that for films deposited at 700°C

(50-80 nm)

iv) The morphology of these films is different: they have a dominant texture (220) and a surface roughness observed by scanning electron microscopy with images caracteristic of grain clus- ters, whereas deposited films at 700°C show no featureless surfaces, though some grains can be revealed after a chemical etching.

These arguments suggest the non-validity of one of the hypothesis

given to explain the mechanism of crystallization at lower tempe-

ratures: either the material is polycrystalline when deposited, or

volume nucleation is comparable or higher than surface nucleation. The

first process is a possibility if one considers that, in the case of

ultrahigh vacuum evaporated films C14, 151, polycrystalline silicon

is deposited at temperatures lower than 500°C, where nucleation and

crystal growth rates are certainly negligible. The principal diffe-

rence between ultrahigh vacuum evaporated and CVD silicon films is the

hydrogen coverage under the surface during deposition which modifies

the surface mobility of the silicon atoms at this surface. Evidence

of crystallization at deposition is given by the work of Kamins and

Cass C11 which shown a definite island-type nucleation in the initial

stage of deposition in CVD films at 850°C.

C1-276 JOURNAL DE PHYSIQUE

In summary, we have shown that : i) amorphous films are obtained at deposition temperatures of 600°C or lower. ii) In the range of tempe- ratures 620-680°C, two layered films are produced by nucleation at the substrate-film interface and subsequent crystal growth. The relative thickness of the two layers, amorphous and polycrystalline, depends on the relationship between deposition and crystal growth rates.

iii) At 700°C, where the crystal growth rate equals the deposition rate, polycrystalline films with a small density of defects and a correlated maximum in conductivity for lightly doped films are

obtained. iv) A new mechanism of crystallization corresponding to nucleation of crystallites at deposition appears at temperatures higher than 750°C, giving a different morphology of the polycrystal- line material with an increased density of defects and lower grain sizes.

References

C11 Kamins T.I. and Cass T.R., Thin Solid Films 16 (1973)147.

[2] Janai M., Allred D.D., Booth D.C. and Seraphin B.O., Solar Energy Materials 1 (1979) 11.

C 3 1 H i r o s e M., T a n i g u c h i M. and Osaka Y., J. Appl. Phys. 50

(1979) 377.

Taniguchi M., Hirose M., Osaka Y. Hasegawa S. and Shimizu T., Japanese J. Appl. Phys. 19 (1980) 665.

C41 Barna A., Barna P.B. and Pocza J.F.. J. Non-Crvst. Solids.

8-10 (19j2) 36.

C51 Bloem J. and Giling L.J , Current Topics in Materials Science 1, Edited by E. Kaldis, North Holland, Amsterdam (1978) 147.

[61 geers A.M. and Bloem J., Appl. Phys. Lett., 41 (1982) 153.

[71 Cullity B.D., Elements of X-Ray Diffraction, Addison-Wesley Publishing Co., Reading, Massachussets (1956) 270.

C81 Brodsky M.H., Cardona M. and Cuomo J.J., Phys. Rev. B16

(1977) 3556.

C91 Zellama K., Germain P., Squelard S., Bourgoin J.C. and Thomas P.A., J. Appl. Phys. 50 (1979) 6995.

C101 Drosd R. and Washburn J.,J. Gp+. ^Phys. 53 (1982) 397.

[lll ~agariiio J., Kaplan D., Friederlch A. and Deneuville A,, Phil. Mag. B45 (1982) 285.

C121 Szydlo N., G a r i z o J. and Kaplan D., J. Appl. Phys. 12 (1982) 5044.

El31 Maekawa S. and Kinoshita N., J. Phys. Soc. Japan, 2 (1965) 1447.

C141 Thomas P.A., Brodsky M.H., Kaplan D. and LBpine D., Phys. Rev.

B18 (1978) 3059.

C151 G s u i M., Shiraki Y., Katayama Y., Kobayashi K.L.I., Shintani A.

and Maruyama E., Appl. Phys. Lett. 21 (1980) 936.

zk

This work has been partially supported by Deldgation GBndrale 2- la

Recherche Scientifique et Technique on the contract no 80 A 0.581