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Thin Solid Films 501

Characterization of grain growth, nature and role of grain boundaries

in microcrystalline silicon—review of typical features

J. Ko*ka *, T. Mates, H. Stuchlıkova, J. Stuchlık, A. Fejfar

Institute of Physics, AS-CR, Cukrovarnicka 10, 16253 Prague 6, Czech Republic

Available online 11 August 2005

Abstract

A boundary between amorphous and microcrystalline growth of silicon thin films was explored for the study of grain growth by changing

parameters of plasma enhanced deposition (substrate temperature, silane dilution or deposition duration and thus the film thickness).

Resulting series of a-Si:H/Ac-Si:H samples were characterized by values of dark conductivity and corresponding activation energy and

prefactor, diffusion length, hydrogen content and morphology. The abrupt change of conductivity at the boundary is accompanied by a peak

in surface roughness and diffusion length, which were connected to the formation of a percolating network of microcrystalline large grains.

Results are discussed using the idea of barriers for electronic transport forming at the large grain boundaries. Comparison of transport

properties with the hydrogen content pointed out the fundamental role of hydrogen present at the grain boundaries.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Microcrystalline silicon; Growth; Transport; Hydrogen

1. Introduction

As concerns the form of (solid state) matter, there are two

limiting cases: mono-crystalline and amorphous. In between

there are many intermediate cases like poly-, micro-, nano-

and also proto-crystalline forms. This situation is typical not

only for silicon but also for carbon (amorphous C,

crystalline diamond, nano-diamond and even ultranano-

diamond. . .).Amorphous hydrogenated silicon (a-Si:H) and, during

the last 10 years [1], hydrogenated microcrystalline silicon

(Ac-Si:H) are studied as important thin film materials for

photovoltaic (PV) solar cells and many other applications.

The possibility to use a wide range of substrates, including

flexible and cheap polymers, led us recently to study the Ac-Si:H sample series prepared at low substrate temperatures

TS�100 -C [2–4].

Here we concentrate on a-Si:H/Ac-Si:H samples, pre-

pared by plasma enhanced chemical vapour deposition (PE

0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2005.07.212

* Corresponding author.

E-mail address: kocka@fzu.cz (J. Ko*ka).

CVD) and compare them with the hot wire (HW) CVD

ones. These series of samples represent the ideal model

material because the a-Si:H/Ac-Si:H transition can be

achieved just by change of one technological parameter

like substrate temperature (TS), discharge frequency, hydro-

gen dilution (rH=[H2] / [SiH4]) or thickness.

The aim of this work is to summarize the characteristic

features of the a-Si:H/Ac-Si:H transition in topography,

crystallinity, H content and the electronic transport proper-

ties. Substantial changes observed outside the transition

region are also discussed.

2. Experimental details

2.1. Samples

We have evaluated three thickness series, two dilution

and two temperature series of a-Si:H/Ac-Si:H samples. First

thickness series was prepared by Toppan Printing Co. [2] at

TS=75 -C and 54 MHz with the H dilution rH=[H2] /

[SiH4]=29, the other two were prepared at our institute at

TS=250 -C; 13.56 MHz; with rH=21 and rH=32.

(2006) 107 – 112

J. Ko*ka et al. / Thin Solid Films 501 (2006) 107–112108

Both dilution series were prepared by Toppan Printing

Co. [2–4] (first at TS=60 -C; 54 MHz; with rH=26–168

and the second series at TS=80 -C; 54 MHz; with rH=34–

168). First of the temperature series was prepared by Toppan

Printing Co. [3,4] (TS=35–200 -C; rH=133; 54 MHz) and

the second temperature series was prepared in our institute

(TS=40–250 -C; rH=32; 13.56 MHz).

2.2. Experimental techniques

As the simplest characterization, the measurement of DC

dark conductivity (rD) with coplanar contacts at room

temperature has been used, often complemented with the

evaluation of the activation energy (Ea) and the prefactor

(r0) from the temperature dependencies of rd. Steady-State

Photocarrier Grating (SSPG) method was used for evalua-

tion of the ambipolar diffusion length (Ldiff) [2]. The

topography and RMS roughness have been studied by the

modified atomic force microscope (AFM) [5]. The total H

content has been evaluated by elastic recoil detection

analysis (ERDA). The Raman spectroscopy has been used

for evaluation of crystallinity XC, see [6].

Fig. 1. Schematic picture of the electric field and optical concentration (a) due to

thickness series prepared at TS=75 -C.

3. Experimental results and discussion

3.1. Growth of microcrystalline grains

Besides the proper identification of grains, the important

input data for the characterization of the growth of grains

are their shape and size. For the grain identification and

size detection, we have used the modified AFM which

measures simultaneously the topography and local con-

ductivity [5]. The shape of grains is often studied by cross-

section TEM (Transmission Electron Microscope) [7]. We

have shown [8] that the grain shape can be also deduced

from the topography of samples from the thickness series,

see Fig. 1. The specific conical shape of microcrystalline

grains could lead to the optical and electric field concen-

tration and influence the function of PV cells, as discussed

elsewhere [8].

Fig. 2 serves to recall the basic idea of our model of

electronic transport [9], i.e., that the formation of so-called

‘‘large grain boundaries’’ (LGB) leads to the potential

barriers at LGB, which results in the drop of prefactor ro

and activation energy Ea of dark conductivity.

specific shape of grains (b), see text, based on topography (c) of the VHF

Fig. 2. Schematic illustration of the potential barriers, related to the ‘‘large

grain boundaries’’ (a) and the conductivity prefactor r0 and activation

energy Ea (b) of two RF thickness series, used for the detection of the

formation of these boundaries.

Fig. 3. Dark conductivity and crystallinity (a), SSPG diffusion length and

RMS roughness (b) together with the conductivity prefactor r0 and

activation energy Ea (c) of the TS=80 -C dilution series, displaying typical

features. The transition zone is highlighted by the grey background.

J. Ko*ka et al. / Thin Solid Films 501 (2006) 107–112 109

3.2. Characteristic features of a-Si:H/lc-Si:H transition

From measurements on many series of Ac-Si:H samples,

prepared by PE CVD (see for example [9]), we have learned

that the typical features (see Fig. 3) of the a-Si:H/Ac-Si:Htransition are:

& a clear peak in diffusion length (Ldiff) and surface

roughness,

& the drop of conductivity to below rD<10� 7 V� 1 cm� 1

and crystallinity below XC<70%,

& and a percolation threshold accompanied by LGB

formation.

Very similar features, both in structure and transport

properties, have been observed also on HW CVD

samples prepared by Niikura et al. [10,11] at substrate

temperature TS¨240 -C, filament temperature Tfil =1500

-C (see Fig. 4). Although this dilution series has been

prepared by a different deposition technique, i.e., HW

CVD, the a-Si:H/Ac-Si:H transition was crossed by

variable dilution of rH=7.3–49, keeping similar film

thicknesses around 1000 nm. However, for a detailed

review of the electronic properties of HW CVD samples

in Fig. 4, the data of the activation energy (Ea) and

conductivity prefactor (r0), as shown in Fig. 3(c) for PE

CVD series, would be needed.

Kroll et al. [12] have studied H content in PE CVD

samples (TS¨225 -C) and clearly illustrated another

feature typical for the a-Si:H/Ac-Si:H transition region,

i.e., a peak in H content (see Fig. 5) and a systematic

difference between results from ERDA (which probes all

hydrogen present in the material) and FT-IR (which detects

bonded hydrogen only).

While the common feature of HW CVD and PE CVD

samples is that the H content on the amorphous side of the

transition is about twice as high as compared to the Ac-Si:H

Fig. 6. The crystallinity and dark conductivity (a) together with the

conductivity prefactor r0 and activation energy Ea (b) of the samples of

VHF temperature series with rH=133. The transition zone is highlighted by

the grey background.

Fig. 4. SSPG diffusion length and RMS roughness together with

crystallinity of the TS¨240 -C HW CVD dilution series from [10,11],

displaying the same features as the PE CVD dilution series in Fig. 3. The

transition zone is highlighted by the grey background and the top rH axis

was added for comparison purposes.

J. Ko*ka et al. / Thin Solid Films 501 (2006) 107–112110

side, there are some evident differences in the bonding of H

in HW CVD and PE CVD samples. For the HW CVD

dilution series by Niikura et al. [10,11], there is no peak in H

at the transition region and both techniques (ERDA and FT-

IR) agree quite well, with a small difference just in the

transition region.

3.3. Nature of grain boundaries and role of hydrogen in

lc-Si:H

For the first (VHF) temperature series, see Fig. 6(a), the

drop of the conductivity and crystallinity, expected for the

transition region, is observed for TS�60 -C. In agreement

with Kroll et al. [12] also in this VHF series, see Fig. 7(a),

Fig. 5. Hydrogen content detected by ERDA and FT-IR of the PE CVD

dilution series from [12] prepared by Kroll et al. and HW CVD dilution

series from [10,11] prepared by Niikura et al. The transition zone is

highlighted by the grey background. Please note the reversed orientation of

the top rH axis added for comparison purposes.

Fig. 7. Hydrogen content by ERDA and optical gap (a) together with SSPG

diffusion length and RMS roughness (b) of the samples of a VHF

temperature series with rH=133. The transition zone is highlighted by the

grey background.

Fig. 8. Dark conductivity and crystallinity (a) together with SSPG diffusion

length and RMS roughness (b) and hydrogen content by ERDA (c) of the

RF samples of the temperature series with rH=32. The transition zone is

highlighted by the grey background.

J. Ko*ka et al. / Thin Solid Films 501 (2006) 107–112 111

for TS�60 -C, the H content increases, giving further

confirmation that, for TS�60 -C, there is the transition

region. As expected for the transition region, see Fig. 7(b),

both the roughness and Ldiff increase for TS�60 -C (their

maximum is not yet reached).

However, what is the reason for the surprising minimum

in conductivity, see Fig. 6(a), around TS=100 -C? While

roughness is flat at about TS=100 -C, see Fig. 7(b), dramatic

changes appear in Ldiff as well as in conductivity. Fig. 6(b)

indicates that not only for TS�60 -C but also around

TS=100 -C the GB nature changes. The observed (see Fig.

7(a)) smooth increase of the optical gap for TS<120 -C,usually related to the increase of H content, indicates that H

can play a role.

Both the roughness and the grain size are constant at

about TS=100 -C, however, H content dramatically changes

also outside the a-Si:H/Ac-Si:H transition region, see Fig.

7(a), and hence, the most probable explanation of the

change of rD and Ldiff around TS=100 -C is the change of

the GB nature induced by H.

We have observed as the typical feature of Ac-Si:H [5,9]

that there are two different grain sizes, the ‘‘small grains’’

(15–30 nm) and so-called ‘‘large grains’’ (about 300–500

nm). We have suggested that LGB and the related potential

barriers play a detrimental role in the transport properties

[9,13].

The role of H in the change of the grain boundaries has

been suggested as the explanation of the annealing induced

change of transport properties by Yoon [14] and in agree-

ment with our analysis [9], our point of view is as follows.

Formation of LGBs is related to the increase of the H

content, which concentrates at the tissue forming LGBs, and

as a result, potential barriers are formed. On the other hand,

when H content decreases (as it occurs for TS�90 -C in our

VHF temperature series), possibly supported by a few

percent increase of crystallinity, the LGBs are destroyed and

the transport properties improve, see the sharp increase of

Ldiff for TS�90 -C in Fig. 7(a).

We can test these ideas on the second temperature series

of samples—the RF series of Ac-Si:H samples, prepared at

40 -C�TS�250 -C. In Fig. 8(a) and (b), it is nicely

illustrated by the drop of rD and XC, as well as by the peak

in Ldiff and RMS roughness that for this series, the a-Si:H/

Ac-Si:H transition is found at TS¨80 -C.Also in this series, there are changes of transport

properties outside the transition region, namely, see Fig. 8,

substantial increase of Ldiff for TS�120 -C. Again for 120

-C�TS�250 -C, the roughness and also grain size are

practically constant, while for TS�120 -C, the crystallinity,slightly but systematically increases (from 80 to 86%). At

the same time, the total H content (by ERDA) clearly

decreases for TS�120 -C.So for both VHF and RF temperature series, the change

of transport properties (increase of Ldiff) tightly follows the

change (decrease) of H content, accompanied by a slight

change (increase) of the crystallinity.

Here it is important to note that the role of H in Ac-Si:His quite distinct from the role of H in poly-Si, see for

example [15]. The main difference is probably in different H

content in both structures—while in Ac-Si:H, there is 5–

30% of H, poly-Si usually contains less then 1% of H.

4. Conclusions

Hydrogen plays a key role during the Ac-Si:H growth

which is even more pronounced in the samples grown close

to the a-Si:H/Ac-Si:H transition at the low substrate

temperatures. It may also influence the properties away

from the transition by modifying the large grain boundaries.

Acknowledgements

The authors would like to thank, most of all, Manabu Ito,

Toppan Printing Research Institute, for providing samples

J. Ko*ka et al. / Thin Solid Films 501 (2006) 107–112112

and supporting this research. Support by projects AVOZ

1010914, Kontakt ME537, VaV/300/01/03, GAAV

IAA1010316, IAA1010413 and GA CR 202/03/0789 is

gratefully acknowledged.

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