Applied Surface Science 70/71 (1993) 660-663 North-Holland applied

surface sc ience

Structural and electrical properties of highly conductive c-Si(P) layers

G . Con te , M.L. Addon iz io , G. Nobi le , A. R u b i n o and E. Terz in i ENEA, Centro Ricerche Fotovoltaiche, 80055 Portici (Na), Italy

Received 13 August 1992; accepted for publication 20 November 1992

Thin films of phosphorus doped ~c-Si have been prepared in a glow discharge reactor starting from hydrogen diluted silane using less than 5 watt R F power. Conductivities as high as 41 S / c m have been measured on samples deposited at 210°C. The discharge parameters and reactor geometry will be examined in order to correlate the really dissipated RF power to the structural and electrical properties of the material. XRD, SEM, Raman spectroscopy and electrical characterization techniques have been used. All the samples show clusters that increase with the film thickness, while at the same time the crystallite size changes in the

range 30-75 A.

1. Introduction

Phosphorus doped microcrystalline silicon (~c-Si(P)) is an interesting material for the preparat ion of amorphous silicon solar cells and related devices as back contact layer. Its main advantages are the reportedly high room temper- ature conductivity [1,2] with respect to doped amorphous material, and the low activation en- ergy, that is comparable to that of P-doped crys- talline silicon. In a device, its beneficial effect should be an increase of the built-in potential and the reduction of the series resistance.

It is commonly accepted that some conditions must be satisfied to produce microcrystalline ma- terials: the role of the H 2 dilution is well under- stood [3,4], while the effect of the RF power is not yet clear. A common trend [2-6] seems that the increase of the RF power promotes the grain growth, giving rise to higher conductivities.

For a bet ter understanding of the physical propert ies of the material, a detailed study of structural and electrical propert ies as a function of deposition conditions has been performed us- ing conductivity measurements , XRD, SEM and Raman spectroscopy. We focused on the varia-

tion of RF power, keeping the H 2 dilution and the substrate temperature constant. The temper- ature has been chosen as low as required for the deposition of other layers in the device, in order to avoid interfaces degradation.

2. Experimental

The films were prepared in a 13.56 MHz ca- pacitively-coupled PECVD reactor from PH 3, Sill 4 and H 2 mixture; the chamber has two par- allel plate electrodes (@ = 15 cm); the cathode is at 2.2 cm from the grounded anode. The films were deposited on Coming 7059, the temperature was kept constant at 210°C, while the RF power was varied between 4 and 60 watt. Silane diluted phosphine (5vo1%) was used as dopant gas, the pressure was varied from 730 to 800 mTorr, while the gas flow was adjusted to maintain the P H 3 / S i H 4 ratio to 0.01.

Thicknesses have been measured by mechani- cal profilometer with 5 A resolution in the largest magnification scale.

The conductivity measurements were per- formed by a four probe technique, while the

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G. Conte et al. / Structural and electrical properties of highly conductive ix c-Si(P) layers 661

activation energy has been calculated from an Arrhenius plot in the 25-200°C range.

Raman spectroscopy has been performed with a triple grating spectrometer equipped with an OMA-CCD photon detector. The 514.5 nm line of an Ar ÷ ion laser has been focused on the sample through a cylindrical lens. The different spectral contributions were resolved using Gauss- ian components; the convergence was provided by the "s teepest descend" method based on the Fletcher minimization routine [9].

For the X-ray analysis a 0 -20 diffractometer was used in the 10°-90 ° 20 range with the Cu K a (A = 1.54 ,~) radiation; the contribution from the substrate was subtracted and the crystallite size estimated by the War ren -Averbach analysis on the (220) and (311) peaks; no preferred orienta- tions were found.

The surface morphology and the cross section analysis has been performed using a SEM on samples overcoated with 100 ,~ of A u / P d alloy to avoid charging effects.

3. Results and discussion

A useful tool to optimize the growth condi- tions in a PECVD reactor is the Paschen-like curve [7]. We investigated two discharge pres- sures lying on the left branch of the same curve in order to produce precursors that give rise to materials with good electronic properties.

In our reactor the RF power supply is directly attached to the matching network (MN), which is connected to the cathode via a long shielded cable. This configuration has an impedance mis- match between the MN and the reactor itself, so that the net power really dissipated in the dis- charge is much lower than expected, as shown in fig. 1. In such a way we are able to deposit thin film materials with a very low power density in comparison to the values reported in literature [1-5].

Some authors report the presence of micro- crystallinity down to 95% H 2 dilution [3]; in our case 99% H 2 dilution is a critical parameter: for all the RF power values sampled here, it has been impossible to obtain a crystalline phase be-

5O

~4o

~2o

~1o 0

xlO ID'

2O

0 r~

s ~ 2:

0 10 20 30 40 50 60 70 RF Power (Watts)

Fig. 1. Dependence of the RF power effectively dissipated in the plasma on the power supplied by the generator. The full squares are the enlarged part of the low power region. Lines

are only for visual aid.

low this value. We found that the growth rate decreases from 1 to 0.1 A / s when the power changes from 60 to 4 watt for both pressures. The dependence of the conductivity on the RF power is shown in fig. 2. The material deposited at lower pressure shows bet ter electrical properties; in both cases the materials have high conductivities, with a net increase when the power is decreased to 4 -6 watt. This is the lower limit for our reac- tor: in fact the left branch of the Paschen curve increases very sharply, making the discharge un- stable for lower powers.

We have found that the film propert ies are dependent on the thickness; a set of films has been realized at 0.73 Torr and 6 watt with thick- nesses ranging from 230 ,~ to 0.96/xm; in such a

50

"~ 4O

~ 3 0

• ~ 20

50 . . . .

~ 2o

2 8

0 10 20 30 40 50 60 70 Power (Watt)

Fig. 2. Dependence of the RT conductivity on the RF deposi- tion power for two different pressures: • = 730 mTorr; o = 800 mTorr. The inset shows the variation of the RT conductivity with the thickness of the material deposited at 6 watt and 730

mTorr.

662 G. Conte et al. / Structural and electrical properties o f highly conductive tzc-Si(P) layers

Fig. 3. (a) Section micrograph of a microcrystalline film: the structure is clearly granular with a spherical grain shape of 1000 average size; (b) Surface micrograph of the film: the stress has caused the surface to blister out; the structure undernea th is clearly

analogous to that of the surface.

way we want to emphasize the separate effect of the thickness and growth conditions on the film properties. The conductivity saturates for thick- o nesses > 2000 A, as shown in the inset of fig. 2, therefore only samples thick enough have been analysed.

Typical SEM micrographs are shown in fig. 3: the grain size appears of the order of 1000 A. The structure is not columnar (fig. 3a), as re- ported in other papers [3]: the grains appear more or less spherical in shape. Moreover, for some films, growth stresses [8] make the surface to blister, showing clearly an analogous structure underlying (fig. 3b). The grain size ranges from 300 to 1100 A, while the average size distribution is larger for lower RF power.

The XRD analysis as well as Raman scattering have shown that the crystallite size is of the order of tens of ,~. The discrepancies between XRD and SEM analysis must be attributed to the mi- crocrystalline structure. These crystallites are ag- glomerated in clusters of about 1000 ,~, as seen at SEM.

The cluster size increases with thickness and saturates to a value of about 1100 A for thick- nesses of about 0.2/zm. For the same thicknesses the crystallite size increases from 30 ~, to a satu-

o ration value of 75 A.

All the samples show a Raman contribution of the TO-like mode at 485 + 4 cm-1 of the amor- phous phase as well as of the TO mode at 519 +_ 3 cm-1 of the crystalline phase (fig. 4). The posi- tions of these peaks are not correlated to growth conditions, but only to crystallite sizes [4]. The volume fraction of crystallinity, measured both by XRD and Raman, ranges from 45 to 60%, and

1.0

0 . 9

0 . 8

0 . 7

0 . 6

0 . 5

0 . 4

0 . 3 i

0 . 2

0.1

2 0 0 . 0 3 0 0 . 0 4 0 0 . 0 5 0 0 , 0 6 0 0 . 0 Raman Shift (I/cm)

Fig. 4. Raman spectrum for a microcrystalline film: the figure shows the experimental as well as the fitted curve together with the 6 Gaussian components needed to obtain a good correlation: the elastic line contribution ( • ) , the LO mode ( , ) , the surface mode (o), the TO modes of amorphous (o ) and crystalline (D) phases, and finally the contribution due to

the relaxation of the selection rules ( * ).

G. Conte et al. / Structural and electrical properties of highly conductive tzc-Si(P) layers 663

varies linearly with the crystallite dimension. All the fits show the presence of a peak at ~ 510 cm-1 due to the relaxation of selection rules that explain the asymmetry of the T O peak, and of another peak be tween 423 and 436 c m - 1 that has been at t r ibuted to surface modes [10]. The latter one has been found to be independen t of growth conditions; we infer that this contr ibut ion comes f rom the amorphous tissue that in terconnects the crystallites.

The most interesting proper ty of these materi- als is the high conductivity, that in the best case reaches 41 S / c m , with an activation energy of 22 meV, typical of all samples. The picture that comes out f rom the observations on the structure of the material is of microcrystals of Si(P) g rouped in clusters, and embedded in an amorphous ma- trix, lying on the grain surface. In this view, the conductivity becomes limited by the intergrain tissue. This fact is suppor ted by the evidence that the conductivity increases linearly with the crys- tallite size; this figure is typical of the t ranspor t mechanism limited by thermionic emission [11]. I f this hypothesis is real, the activation energy that we measure is correla ted to the intergrain energy barrier. Work is in progress to confirm this pic- ture.

4. Conclusions

The results of this work show how to obtain very high conductivi ty/~c-Si(P) films with low R F powers ( 4 - 6 watt) and t empera tu re (210°C). Fur-

the rmore conductivity remains high (1 S / c m ) also for films as thin as 230 ,~. These characteristics are essential for the utilization of p~c-material in realistic thin film device realization. The material does not have columnar structure, but the crystal- lites aggregate into a spherical cluster. We found that the crystalline fraction is greater then 45% for all deposi t ion condit ions tried. Some evidence has been repor ted that amorphous phase is the grain boundary region between microcrystalline grains.

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