Journal of Non-Crystalline Solids 227–230 Ž1998. 83–87

Existence of surface region with high dangling bond density
during a-Si:H film growth
S. Yamasaki ) , T. Umeda, J. Isoya, K. Tanaka
Joint Research Center for Atom Technology, National Institute for AdÕanced Interdisciplinary Research, 1-1-4, Higashi, Tsukuba,Ibaraki 305-8562, Japan

Abstract
In-situ electron-spin-resonance measurements have been made on the growth of hydrogenated amorphous silicon Ža-Si:H.
film using a remote hydrogen plasma technique. Dynamic changes of the Si dangling-bond signal intensity were observed
during and after the deposition of the film, which confirms the existence of a surface region with a large spin density only
during the deposition process due to the interaction with reactive species of SiH 3 and H in the gas phase. q 1998 Elsevier
Science B.V. All rights reserved.
Keywords: Dangling bond; a-Si:H; Surface region

1. Introduction
Electron-spin-resonance ŽESR. technique has been
applied to the study of a variety of hydrogenatedamorphous silicon Ža-Si:H. films, and provided uswith important information on the defect structure inthe bulk and on its surface. On the other hand, forthe growth of a-Si:H, dangling bonds in the surface
region would play a crucial role as reacting sites forprecursors to make bonding to the surface. It is
widely accepted from the measurement of film thickness dependence of the dangling bond signal that
there exists a larger density region of dangling bonds)

Corresponding author: Permanent address: ElectrotechnicalLaboratory, 1-1-4, Umezono, Tsukuba, Ibaraki 305, Japan. Tel.:
q81-298 54 2732; fax: q81-298 54 2786; e-mail:satoshi@jrcat.or.jp

close to the surface w1,2x. This density might be theindication of the surface reaction during film growth.
However, there had been no information on dangling
bonds during deposition until our recent report of thefirst success of in situ ESR study of a-Si:H film
growth w3x.
Several groups have reported on the surface morphology and bonding configuration during a-Si:H
film growth. Collins et al. w4x studied the nucleationand initial growth process of a-Si:H on different
substrates by means of real-time spectroscopic ellipsometry, and discussed its microstructural evolution
using a two-layer model. Using ultrahigh vacuum
scanning tunneling microscopy, Ikuta et al. w5x reported on a direct subnanometer scale observation of
the initial growth of a-Si:H. Using infrared absorption reflection spectroscopy, Toyoshima et al. w6x
reported that a hydrogen-rich surface exists during0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 0 2 4 - 6

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S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 83–87

the film growth. In situ ESR measurements during
deposition should give us the dynamic chemical
reactions governing the film growth.
We have reported the first successful in situ ESR
measurement during film growth using a remotehydrogen plasma and showed that it is possible toobserve the dangling bond ESR signal during film
growth in addition to the gas-phase ESR signals w3x.
In this paper, we present the dynamic changes of thedangling bond signal intensity during and after
growth, and discuss the nature of the film surface
during deposition.

2. Experimental procedure
For in-situ ESR measurements of a-Si:H filmgrowth, a remote hydrogen plasma system was used,which is schematically shown in Fig. 1. The hydrogen plasma discharge was sustained within a microwave cavity Ž2.45 GHz., which was located at theupstream position with respect to the X-band ESR
cavity w7x. Hydrogen molecules flowing through asilica tube, with an outer diameter of 6 mm, weredissociated into hydrogen atoms when passing
through the discharge cavity. A T-shaped discharge
tube was used to avoid incidence of hydrogen plasma
light into the deposition space in the ESR cavity. Thesilane gas was mixed with the atomic hydrogen in

Fig. 1. Sketch of experimental setup. The T-shaped discharge tube
was used to avoid incidence of hydrogen plasma light into thedeposition space in the ESR cavity.

Fig. 2. The dangling bond ESR signal as a function of time. The
vertical scale is calculated the number of spins per square centimeters, Ns .

the ESR cavity and the primary reaction taking place
was SiH 4 q H ™ SiH 3 q H 2 w8x. As a result, ana-Si:H film was deposited on the inner wall of theouter silica tube having an inner diameter of 8 mm.
The substrate temperature of deposition shown inthis paper was room temperature. The ESR measurements were made using an X-band Cavity ŽBRUKERESP 300E.. To get time-resolved change of dangling
bond ESR intensity, the magnetic field was set at thepeak position on the lower field side of the spectrum
before deposition, the peak position being calculated
from the ESR microwave frequency. By using theT-shaped tube, we could suppress the noise levelarising from the hydrogen plasma system. As aresult, the noise level in the plasma-on state becamealmost the same as that in the plasma-off state. Thechange of thickness was monitored by measuring aninterference fringe of transmitted He–Ne laser light.
The number of hydrogen atoms in the gas phase
can also be measured by the ESR technique w9x,
which is one of the advantages of this in-situ ESR
system. For the deposition conditions used here, an
atomic hydrogen signal of ; 2 = 10 12 cmy3 wasdetected. This number is considered as the upper
limit in the reaction region, because hydrogen atoms
inside the inner tube are also included in the total
atomic hydrogen signal.
When we used the experimental setup having astraight inner tube, the hydrogen plasma light illuminates the deposition space in the ESR cavity, and thesignal originating from photo-excited SiH 4 related

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 83–87

species was observed w10x. This signal is thought tobe caused by chemically-induced dynamic electron
polarization ŽCIDEP. w11x, which arises from species
that are formed with non-equilibrium spin state populations. However, for the present setup, this signal
was not observable due to no incident light from thehydrogen plasma.
The deposition conditions were a gas pressure of5 Torr, a silane flow rate of 10 SCCM Žstandard
cubic centimeters per minute., a hydrogen flow rateof 100 SCCM, and a discharge microwave power of20 W. The deposition time was 5640 s and the film
thickness was 510 nm. Due to the spatial distributionof film thickness and of the sensitivity in the ESR
cavity, the dangling-bond density is estimated. The
deposition area was 2.5 cm2 .

3. Results
ESR signal intensity of the dangling bonds wasmeasured as a function of time, which is shown inFig. 2. The vertical scale represents the number of
ESR spins per square centimeters, Ns . As shown in
the figure, an increase of the signal intensity isobserved when the deposition starts, followed by arelatively slower increase of signal intensity over thewhole deposition time with increasing film thickness.
A decrease of the signal intensity occurs when thedeposition is terminated by stopping the plasma.
The initial change is shown in Fig. 3 on a shorttime scale. Up to t ; 60 s, the signal intensity in-85Fig. 4. The final stage of deposition. At t s 5640 s, deposition isstopped.

creases to around 2 = 10 13 cmy2 with a steeper
slope than that of a following increase for t ) 60 s.
This density is larger than the surface spin density
observed after deposition by around one order ofmagnitude w1,2x. If we assume a constant deposition
rate for the initial stage of deposition, a deposition
time of 60 s is equivalent to a film thickness ofaround 5 nm. The dangling bond density is calculated to be 4 = 10 19 cmy3 .
Spin density at the final stage of deposition isshown in Fig. 4, where deposition stopped at t s
5640 s. The signal intensity decays with two distinctly different time constants after stopping theplasma; a decrease just after stopping the plasma,followed by a slower decrease due to a slow structural relaxation of the deposited bulk film. The spin
density, Ns , of the film reached ; 2 = 10 14 cmy2Žequivalent to ; 4 = 10 18 cmy3 . at 10 h after thedeposition is stopped, but is still unsaturated. The
magnitude of the faster decrease in the spin density
at the end of the deposition was almost the same asthat of the initial increase. From the measurements
for different deposition times, the amount of contributing to the faster decay is almost independent of
the deposition time, namely, thickness.

4. Discussion

Fig. 3. The initial change of the dangling bond ESR signal on ashort time scale. At t ; 0 s, the deposition is started.

As the film structure is known to be affected by
the substrate during the initial film growth, one
possible candidate of a larger spin density region
might be the film–substrate interface. However, this86

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 83–87

region is not the case, because it is then not possible
to explain why the film–substrate interface keeps
being affected by reactive species in the gas phaseeven after the film is 100-nm thick. Eventually, theabove data suggest that the high spin-density region
created at the beginning of deposition is always
located near the surface-region during the film growth
independent of the film thickness, and disappears
immediately when the plasma is turned off. It isnoticeable that the larger density region is located at
the two-dimensional top-surface but also at the subsurface region with a thickness of around 5 nm inthis experimental condition.
A change of Ns was also observed when hydrogen-plasma treatments were made on a-Si:H films.
The plasma conditions were 100-SCCM H 2 flow,
5-Torr pressure, and 2-W discharge power. When thehydrogen plasma was ignited after the growth ofa-Si:H film, the surface starts to interact with reactive species in the gas phase and, as a result, Ns
increases immediately with a time constant less than
1 s, which is shown in Fig. 5. The increased dangling bond signal decreased after the H 2-plasma wasturned off. This experiment also shows that the‘active region’ with a high dangling bond density
originates from the surface region. It should be notedthat the active region with the largest Ns is formed
after the H 2 plasma being ignited.
During either deposition or hydrogen-plasma
treatment, various radicals, mostly SiH 3 and H, reachthe surface and react with it. The fact that thehydrogen plasma induces the Ns region near the top
surface suggests that H is at least one of the species
determining the Ns region. Branz et al. w12x reported
from the deuterium ŽD. tracer measurements of aSi:H that the distance D travels before retrapping isaround ; 15 nm, being determined by an exchange
process of free D with trapped H. Beyer and Zastrow
w13x reported that D–H exchange in the a-Si:H network takes place even over a distance of ; 100 nm
by D 2 plasma treatment. Although an estimated depthof the high-Ns-region is around 5 nm in the present
study, and a lower than the above two cases, some
H-mediated reaction in the surface region in thepresent study might be the origin of the high Nsstate.
An et al. w14x studied the nucleation and initial
growth process of a-Si:H by means of the real-time
spectroscopic ellipsometry and observed that a low
˚ was first formed, which wasdensity layer of ; 20 A
followed by the growth of a denser bulk layer under
the low-density surface layer. The high-spin density
surface region observed in the present study isthought to be closely related with the low density
layer observed in spectroscopic ellipsometry. Although their low density surface layer is thinner thanthe high Ns region observed in this work, this might
be understood by taking into account a difference insystems between two groups.

5. Conclusions
The experimental results clearly showed the existence of a subsurface region during the growth process, where the dangling bond density is dynamicallymuch larger than that in the bulk region.

Acknowledgements

Fig. 5. The change of Ns for hydrogen plasma treatments without
SiH 4 flow. The plasma conditions were 100-SCCM H 2 flow,
5-Torr pressure, and 2-W discharge power.

This work, partly supported by NEDO, and wasperformed in the Joint Research Center for AtomTechnology ŽJRCAT. under the joint research agreement between the National Institute for Advanced
Interdisciplinary Research ŽNAIR. and the Angstrom
Technology Partnership ŽATP..

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 83–87

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