Journal of Non-Crystalline Solids 227–230 Ž1998. 332–337

Section 5. Defects, doparts, impurities

Microscopic nature of localized states in a-Si:H and their role inmetastability
S. Yamasaki a,) , T. Umeda a , J. Isoya a , J.H. Zhou b, K. Tanaka aaJoint-Research Center for Atom Technology, National Institute for AdÕanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba-City,
Ibaraki 305, Japanb
Joint-Research Center for Atom Technology, Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba-City, Ibaraki 305, Japan

Abstract
Microscopic properties of light-induced electron spin resonance ŽLESR. centers of g s 2.004 and that of metastable
dangling bonds Ž g s 2.0055. in undoped a-Si:H have been investigated by means of pulsed electron spin resonance ŽESR.
techniques. The 29 Si hyperfine structure of the g s 2.004 signal shows that the wave function of this center spreads mainly
over two Si atoms, which suggests that the origin of g s 2.004 is electrons trapped at anti-bonding states of weak Si–Si
bonds. The microscopic creation mechanism of metastable dangling bonds is discussed on the basis of the frameworks
obtained by pulsed ESR. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Electron spin resonance; a-Si:H; Wave function

1. Introduction
Hydrogenated amorphous silicon Ža-Si. has been
widely utilized in the fields of thin film transistors
and solar cells. The understanding of localized statesin a-Si:H is one of the key issues to improve thedevice performances. For this purpose, numerous
number of studies has been done so far. Electron
spin resonance ŽESR. is a powerful tool to detect the
localized states.

Undoped a-Si:H shows a notorious dangling-bondŽDB. ESR signal of g s 2.0055. 1 The light-induced
metastability is a current topic in a-Si:H and has
been attributed to a change in the concentration of
the DB ESR signal w1,2x. The understanding of the
microscopic mechanism is required from the viewpoint of solar cell applications and is of scientificinterest as well. Undoped a-Si:H also shows light-induced ESR ŽLESR. signals of g s 2.01 and g s
2.004 under illumination at temperatures less than
100 K, which are ascribed to conduction-band-tailelectrons and valence-band-tail holes, respectively
w3x. We have detected hyperfine Žhf. structures of)Corresponding author. Present address: Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba, 305, Japan. Tel.: q81-298
54 2732; Telefax: q81-298 54 2786; e-mail: satoshi@jrcat.or.jp.

1
g s hn r b Hn , where h, n , and b have their customary
meanings and Hn is the field at which a singularity of a spectral
component is observed.

0022-3093r98r$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 3 3 3 - 0

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 332–337
29

Si Žnuclear spin of I s 1r2, natural abundance
4.7%. of the LESR spectrum using a pulsed ESR
technique w4x. However, due to the overlapping of
two ESR signals of g s 2.004 and 2.01 which arebroadened by the random orientation and site-to-site
variation of structures, the 29 Si hf structures could
not be deconvoluted into the two independent hf
structures, which is necessary for more detailed arguments.
In P-doped a-Si:H, in addition to the tail statesignals of g s 2.004, a signal with 31 P Ž100% natural abundance, I s 1r2. hf structure with 28 mT
splitting, which was first reported by Stutzmann andStreet w5x, appears. They proposed that this component should be ascribed to the electron bound to thedonor states. However, from the ESR measurementsof P-doped samples with mid-gap Fermi energy, it
was found that the location of this state extends
down to near the midgap w6,7x. B-doped a-Si:H
shows a valence-band-tail signal of g s 2.01 w8x.
Different from P-doped samples, the 2.01 signal does
not show any discernible hf structures related to 10 B
Ž19.9% natural abundance, I s 3. and 11 BŽ80.1%
natural abundance, I s 3r2. w9x.
In the first part of this paper, we report thedeconvoluted LESR spectra of g s 2.004 and 2.01
signals in undoped a-Si:H, and analyze the 29 Si hf
structures of the g s 2.004 signal, which shows theexperimental evidence of the existence of weak bonds
in a-Si:H w10x. In the second part, on the basis ofexperimental results obtained using a pulsed ESR
technique, we discuss the microscopic creation
mechanism of metastable DBs w11,12x.

333

reduce the contribution of the g s 2.0055 signal,
LESR spectra of g s 2.004 and 2.01 were measured
at 30 K illuminated by a Ti-sapphire laser Ž hn s 1.7
eV, output powers 30 mWrcm2 .. Under those conditions, the peak height of the LESR spectrum wasmore than 20 times larger than that of the dark ESR
spectrum ŽDB. in the range of 10 15 cmy3 in thesesamples.

3. Results
Fig. 1 shows the LESR spectra of the samples
with different contents of 29 Si Žw29 Six s 9.1, and 4.7
at.%., which were recorded using a two-pulse
Hahn-echo detected ESR technique Ž908 pulse– t –1808 pulse– t-echo, t s 240 ns, repetition time of apulse sequence, RT s 10 ms. w14x. The linewidth isbroadened by the increase of 29 Si content, whichmeans that the weak hf interactions with distant 29 Si
contributes to the linewidth of the g s 2.004 signal.
In the magnetic-field range ) 392.5 mT, a long tailis noted, and its intensity increases with increase of
29
Si. The ratio of the integrated area of the tailstructure to the total area was found to be proportional to the 29 Si content ŽFig. 2., clearly indicatingthat the tail structure originates from the higher-fieldside peak of the 29 Si hf doublet. This identification
has confirmed that of our previous report w4x.

2. Experimentals
For detailed study of the LESR spectrum, we used
Ž1. a pulsed ESR technique to obtain a wide-dynamic-range ESR spectrum, Ž2. an ESR microwave
frequency of 11 GHz so that the overlapping of the
2.004 and 2.01 signals should be reduced comparedto the case of standard X-band Ž; 9 GHz. measurements, and Ž3. a sample with increased 29 Si content
to increase the intensity of 29 Si hf structure w13x. In
an echo-detected ESR technique, the spectrum isobtained by recording the amplitude of a spin echo
signal as a function of magnetic-field strength. To

Fig. 1. LESR spectra at 30 K of undoped a-Si:H with w29 Six s9.1,
4.7 at.%, which were recorded by two-pulse Hahn echo-detected
ESR technique Žt s 240 ns, RT s10 ms, the microwave frequency 10.98 GHz.. The spectra were normalized to their peakheights.

334

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 332–337

Fig. 2. The intensity of the tail structure on the high-field side as afunction of w29 Six. The ratio fraction relative to the total area of thewhole ESR spectrum is given.

We have studied a variety of spin-relaxation behaviors of both signals using pulsed ESR w10x. Eventually, we have found that the spin-lattice relaxation
time of the components ŽT1 . differs. The T1 forg s 2.004 and 2.01 were determined to be 1.3 ms
and 0.47 ms at 30 K, respectively. Thus, the T1 ofg s 2.01 is about one third of that of g s 2.004.
Deconvolution of the two overlapping LESR signals of g s 2.004 and 2.01 has been achieved byusing the difference in T1 between the two signals.
When we use a short RT of 0.5 ms for two-pulse
Hahn-echo detected ESR, the 2.01 signal becomes
relatively stronger than the 2.004 signal because thereduction of the echo intensity due to partial recovery is larger in the 2.004 signal than in the 2.01

Fig. 3. Deconvoluted spectra of the LESR Ž g s 2.004, 2.01. of the
Si-enriched Ž9.1 at.%. undoped a-Si:H. Solid lines indicate thedeconvoluted 2.004 and 2.01 spectra with a high reliability in data
analyses, while dashed lines include some ambiguity.
29

signal. On the other hand, when we adopt a threepulse stimulated echo Ž908 pulse– t –908 pulse– t X –908
pulse– t-echo, t s 240 ns, RT s 10 ms. for measuring the echo-detected ESR, the 2.01 signal is relatively suppressed by using a long t X of 1 ms,
because the decay time of the stimulated echo signal
with respect to t X corresponds to T1. In this way, byusing two different pulse sequences, we obtained two
echo-detected ESR spectra with different relative
intensities between 2.01 and 2.004 signals. The LESR
spectrum of the 29 Si-enriched Ž9.1 at.%. sample wasdeconvoluted into two independent spectra by subtracting one from the other to cancel out either the
2.004 or the 2.01 signal alternatively, which subtraction is shown in Fig. 3.

4. Discussion
4.1. Electronic structure of band-tail electrons in
a-Si:H
Although the center part of the spectrum could
not be deconvoluted due to the overlap of the two
signals, the hf structure observed on the high-fieldside belongs mainly to the g s 2.004 signal, as isseen in the figure. The area of the high-field-side hf
structure, which should correspond to one-half of thearea of the entire hf structure Ž I s 1r2., was estimated to be 20 " 3% of one-half of the total area of
the g s 2.004 signal, being almost twice the 29 Si
abundance Ž9% for the sample used here.. Thismagnitude is quite different from the case of a DB
signal Ž g s 2.0055., in which the ratio is almost thesame as the 29 Si abundance ratio. The area fractionof the hf structure depends on the number of Si
atoms, N, on which an electron spin is mainly
located. By the quantitative discussion, we conclude
that the wave function of the 2.004 center spreads
mainly over two Si atoms w10x.
In an amorphous network, it is quite possible thatlargely elongated covalent bonds Žweak bonds. arepresent and give rise to energy levels in the tailregions of the conduction and valence bands. In thiscase, when an electron is trapped in an anti-bonding
state, its wave function is expected to extend over
the two Si atoms associated with the weak Si–Si

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 332–337

bond. Consequently, the present results suggest thatthe origin of the LESR signal of g s 2.004 is electrons trapped at weak Si–Si bonds whose anti-bonding states are located at the conduction-band tail.
Recently, the LESR signal has been measured as aparameter of light intensity, which shows that the hf
coupling is almost unchanged in spite of the changeof quasi-Fermi energy due to the change of lightintensity. This suggests that the electronic state ofg s 2.004 signal is decreased and located at a uniqueenergy compared to that of an empty state. The
details are described in the another paper in thisvolume w13x.
4.2. Metastable dangling bonds
4.2.1. Experimental eÕidence of metastable DB
To understand the microscopic creation mechanism of the metastable DBs, many works have been
done using continuous-wave ŽCW. ESR technique.
Although a change of DB densities by light illumination was easily observed by CW-ESR, no spectralchange between after and before light illumination
was detected by CW-ESR. Although CW-ESR mightseem to be incapable of elucidating microscopic
structure of the metastable DBs, it supplies us some
important insights.
Hydrogen nearby an electron spin should be evidenced from the change in an ESR spectrum whichis expected from the difference of hf interaction byreplacing hydrogen atoms Ž I s 1r2, g n s
5.5856912. by deuterium atoms Ž I s 1, g n s
0.8574376.. However, the effect of deuteration on
the linewidth was not detected for the case ofmetastable DBs of a-Si:H in the X-band ESR spectrum w14,15x.
An electron spin experiences a dipolar field froma nearby nuclear spin. The hf interaction due todipole–dipole interaction Žin mT. is given by:
A diprg be s g n bn Ž 3 cos 2u y 1 . rr 3 ,335

tively. Therefore, if DBs accompany hydrogen at a
˚ the CW-ESR linewidth
distance smaller than 3 A,
should be affected by deuteration, because thelinewidth change by 0.1 mT is detectable for a DB
spectrum with the linewidth of 0.7 mT. It is concluded that the metastable DB does not form acomplex accompanied by a hydrogen atom within 3Å.
For more detailed check, we performed the L-band
Ž1.06 GHz. ESR experiments, since, by changing
from conventional X-band Ž9 GHz. to L-band, theline broadening originating from g-value distribution
should be reduced by a factor of nine while the line
broadening due to hf interaction is unaffected. L-band
ESR spectra of a-Si:H deposited at room temperature
were measured as a function of post-annealing temperatures, in which the spin density of the sample
was varied from 2 = 10 18 down to 1 = 10 17 cmy3
ŽFig. 4.. Every measured spectrum shows a single
sharp peak and no characteristic feature due to a hf
structure originating from close hydrogen was observed, which suggests that DBs with a strong 1 H hfinteraction are very few if exists and that major DBs
are spatially located in a hydrogen-depleted region.
This result is quantitatively consistent with the results obtained from electron-spin-echo envelopemodulation of pulsed ESR w11,12x. Quite recently,
Stutzmann reported using the high resolution spinresonance spectra obtained by low-field spin-dependent transport that no close hydrogen atoms weredetected around the metastable DB w16x. Our resultof L-band ESR is consistent with his report.

Ž 1.

where be and bn are the Bohr magneton and nuclear
magneton, respectively, u the angle between theexternal field and the vector from an electron spin toa nuclear spin, r the distance between electron and˚ g n bnrr 3 is 0.1045 mT
nuclear spins. At r s 3 A,
and 0.0160 mT for hydrogen and deuterium, respec-

Fig. 4. L-band CW-ESR spectrum of the DB Ž g s 2.0055. ofundoped a-Si:H.

336

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 332–337

The distance between metastable DBs is also
important information required for understanding its
creation mechanism, because the bond breaking,which is a simple way to create DBs in an a-Si:H
network, makes a pair of DBs simultaneously. For
the closest pair case, where there is an overlappingof wave functions between a pair of DBs, an exchange narrowing effect is usually expected. However, we confirmed there is no exchange narrowing
effect in metastable DBs using 1 H-ENDOR detected
ESR spectrum as well as using pulsed ESR w17x. The
other interaction between two electron spins of a pairis a dipole–dipole interaction. When an electron spinof A is located in the vicinity of another electron
spin of B, the dipolar field at the position of A due tothe B spin is expressed as:
Bdiprg be s Ms g be Ž 1–3 cos 2u . rR 3 ,4.2.2. Microscopic creation mechanism of metastableDB
A simple way to create DBs in an a-Si:H network
is breaking of weak Si–Si bonds. Many models
involving Si–Si bond breaking have been proposed.
In order for two DBs to be stabilized in the networkand to avoid restoring to the original Si–Si bonding,bond switching mechanisms have been proposed andare shown in Fig. 5a–e w20–24x. For these cases,
except Žd. involving floating bonds and Že. involving
a volume rearrangement caused by the change of thelocalized state independent of hydrogen atoms,

Ž 2.

where u is the angle between the external field andthe vector from B spin to A spin, R the distance
between B spin and A spin. The dipolar field around˚ spin–spin
1 mT is roughly equivalent to a 10 A˚ g berR 3 is 1.86 mT.,separation ŽAt r s 10 A,
which must clearly be detected in the CW-ESR
spectra of a-Si:H w18x, since the peak-to-peak widthof a DB signal in a-Si:H is around 0.7 mT. This
points out that the metastable DBs are separated each˚other at least by 10 A.
In order to obtain more detailed microscopic information than that obtained from a CW-ESR spectrum, we have used a pulsed ESR technique which
can extract a small hf interaction with surrounding
nuclei as well as a small dipole–dipole interaction
with surrounding electron spins. The experimental
results have pointed out the frameworks for understanding the microscopic mechanism of metastable
DBs in a-Si:H. Although there is no space to describe details, the important experimental results are
just summarized as:
1. No DB–H complex in which DB accompanieshydrogen at specific close distance has been detected.
2. No DB pair has been detected Žmetastable DBs
are distributed randomly with the average separa˚ ..
tion between DBs larger than 400 A
The details were described in the references ofRefs. w11,12,19x.

Fig. 5. Models for the creation mechanism of the light-induced
metastable DBs. The references are Ža. Ref. w20x, Žb. Ref. w21x, Žc.
Ref. w22x, Žd. Ref. w23x, Že. Ref. w24x, Žf. Ref. w25x, Žg. Ref. w26x,
Žh. Ref. w27x, Ži. Ref. w34x, and Žj. Ref. w33x.

S. Yamasaki et al.r Journal of Non-Crystalline Solids 227–230 (1998) 332–337

metastable DBs Žat lease one of two DBs. are forming DB–H complexes. However, as the experimental
results of 1, these DB–H complexes have been
excluded by the pulsed ESR study.
For bond breaking models of Ža. – Že., two DBs
are simultaneously created. In the models of Žd., two
DBs spatially separate each other through high mobility of the floating bonds after bond breaking of
weak Si–Si bonds. The models of Ža. – Žc. and Že.
form intimate DB pairs. However, from the experimental result of 2, these models are unlikely.
On the other hand, the models related with impurities such as oxygen, nitrogen, carbon, and phosphorus atoms have been proposed w25–29x. The pulsed
ESR technique can provide the detailed informationof the spatial relationship between an electron spin
and its adjacent nuclear spins. However, we could
not detect any relationship with impurities of 14 N,
17O, and 13 C using 17O and 13 C enriched samples
w30x. The CW-ESR measurements using samples withquite low oxygen and carbon content also pointed
out that there is no relationship between metastable
DB and impurities w11,31,32x.
The models involving floating bond of Že. and Žj.
w33x, and charged DBs with negative effective-correlation-energy of Ži. w34x do not conflict with theframeworks of 1 and 2. However, the existence offloating bonds and charged DBs themselves has notbeen experimentally confirmed so far. Although themicroscopic structure has been understood step bystep, we need new experimental evidence to obtain
the perfect understanding of the microscopic originof metastable DBs in a-Si:H.

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