Journal of Non-Crystalline Solids 227–230 Ž1998. 353–357

Energy location of light-induced ESR centers in undoped a-Si:H
T. Umeda a,b,) , S. Yamasaki a , J. Isoya a,c , A. Matsuda d , K. Tanaka a,baJoint Research Center for Atom Technology (JRCAT)-National Institute for AdÕanced Interdisciplinary Research (NAIR), 1-1-4, Higashi,
Tsukuba 305, Japanb
Institute of Materials Science, UniÕersity of Tsukuba, Tsukuba 305, JapancUniÕersity of Library and Information Science, 1-2, Kasuga, Tsukuba 305, JapandElectrotechnical Laboratory, 1-1-4, Umezono, Tsukuba 305, Japan

Abstract
We report the light intensity dependence of 29 Si hyperfine Žhf. structures of light-induced electron spin resonance ŽLESR.
in undoped a-Si:H in order to detect changes of electronic structures of the g s 2.004 center against the shift of energy
locations of g s 2.004 electrons. Within the light intensity range from 0.004 to 450 mWrcm2 , 29 Si hf structures ofg s 2.004 were almost unchanged, which indicates that a magnitude of the spatial localization of g s 2.004 electrons isalmost constant. Based on the experimental results, we discuss the energetic distribution of g s 2.004 electrons. q 1998
Elsevier Science B.V. All rights reserved.
Keywords: Hydrogenated amorphous silicon; Light-induced electron-spin resonance; Energy shift; 29 Si hyperfine structure

1. Introduction
In hydrogenated amorphous silicon Ža-Si:H.,
light-induced electron-spin-resonance ŽLESR. signals of g 1 s 2.004 and 2.01 are observed underillumination at low temperatures, in addition to thedangling bond signal of g s 2.0055. On the basis of
doping and photoluminescence experiments, the two
LESR signals were tentatively attributed to conduction-band-tail electrons Ž g s 2.004. and valenceband-tail holes Ž g s 2.01. w1x. Therefore, the 2.004)
Corresponding author. Tel.: q81-298 54 2734; fax: q81-298
54 2786; e-mail: umeda@jrcat.or.jp.
1
g s hn r b H, where h, n , and b have their customary
definitions and H is the magnetic field at which the maximum
amplitude of a spectral component is observed.

and 2.01 centers directly correlate with band-tailstates in the gap which play an important role in thetransport properties.
In our recent paper w2x, overlapped LESR signalsof g s 2.004 and 2.01 were experimentally deconvoluted into two signals, by making use of the difference in their spin-lattice relaxation times. The deconvolution clearly showed that the high field side
hyperfine Žhf. structure of 29 Si Žnatural abundance of
4.7 at.%, nuclear spin of 1r2. belongs mainly to theg s 2.004 signal. Furthermore, the fraction of thedeconvoluted hf structure of g s 2.004 in the total
2.004 signal was found to be about twice the 29 Si
concentration in the film. It was therefore concluded
that the wave function of the 2.004 center extends
mainly over two Si atoms with an approximately
uniform distribution. Accordingly, we suggested thatthe origin of the 2.004 center is electrons trapped at0022-3093r98r$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 7 5 - 1

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T. Umeda et al.r Journal of Non-Crystalline Solids 227–230 (1998) 353–357

anti-bonding states of weak Si–Si bonds rather thanthose trapped at positively charged dangling bonds.
On the basis of the above results, an interestingquestion arises; where are these electrons energetically distributed in the band-tail states? In general,
the extent of the wave function of localized electrons
Žor holes. depends on their energy positions in theenergy gap, which should be directly reflected in29
Si hf interactions such as splittings andror intensities of hf signals. In this paper, we present experimental results of the light-intensity dependence of
LESR spectra, especially focusing on changes of the29
Si hf structures in the LESR spectrum, which is
obtained through echo-detected ESR measurementsof pulsed ESR on a 29 Si enriched sample. Based on
the results, we discuss the energy position of g s
2.004 center and electron trapping process.

2. Experimental
We prepared a powdered a-Si:H sample Ž30 mg.
with a 29 Si concentration of 9.1 at.% by using thestandard rf-glow discharge technique at a substratetemperature of 2508C. The flakes with more than 1
m m in thickness were collected into a high-purity
vitreous-silica tube Ž2.5 mm in inner diameter. for
ESR measurements. The ESR spectra were measured
using the two-pulse Hahn echo-detected ESR technique Ž908 pulse-t-1808 pulse-t-echo, t s 240 ns..
Details of our pulsed ESR spectrometer have been
described previously w3x. LESR measurements werecarried out at 30 K using a laser diode Ž685 nm.. The
sample temperature was monitored by a thermocouple placed in the sample during ESR measurements.
With increasing light intensity, the measurement time
was shortened to prevent the effect of photo-createddangling bonds. The repetition time for accumulating
the echo signals was 150 ms to ensure complete
recovery of the spin system.

Fig. 1. Light-intensity dependence of LESR spin densities at 30 K.

respectively. The dangling bond density in the dark
is also shown by an open triangle. It is noted that theabove LESR spin densities are nominal, because in
our experiments there are small unilluminated regions in the powder sample. The solid line in the
figure represents the dependence of Žlightintensity. 0.2 .
LESR spectra at various light intensities and thedangling bond signal are shown in Fig. 2. Light
intensities of 0.004 and 450 mWrcm2 were thesmallest and largest, respectively, in our LESR measurements. The highest light intensity was limited by

3. Results
Fig. 1 shows the light-intensity dependence of the
LESR intensity at 30 K. Solid and open circles
represent the LESR spin density before and after
subtracting the dangling bond density in the dark,

Fig. 2. LESR spectra of the 29 Si-enriched Ž9.1 at.%. sample atlight intensities of 0.004, 0.20, 19, and 450 mWrcm2 , which wereobtained by the echo-detected ESR technique. A dangling bondspectrum is also shown.

T. Umeda et al.r Journal of Non-Crystalline Solids 227–230 (1998) 353–357

a non-negligible increase of the sample temperature
above 30 K. As shown in the figure, even for 0.004
mWrcm2 , the high-field-side 29 Si hf structure could
be clearly detected, owing to the very flat baseline of
the echo-detected ESR spectrum and large 29 Si concentration.

4. Discussion
According to the exponential band-tails model w4x,
the amount of shift of the quasi Fermi energy forelectrons was roughly estimated to be about 50 meV
for a change in the LESR spin density by a factor of
10–20, as is the case of Fig. 1. The estimation wasmade on the assumption that the Urbach energy for
conduction band-tail is 20 meV w4,5x and the occupation of the tail states is given by a step function.
Under this assumption, the maximum energy ofband-tail electrons with a density of 7 = 10 16 cmy3 ,which corresponds to the maximum density of g s
2.004 signal of Fig. 1, was estimated to be 160 meV
below the mobility edge, assuming 10 19 cmy3 w6x forthe effective density of states at the mobility edge.
The estimated 160 meV agrees with the thermal
activation energy of the dark conductivity of theheavily phosphorous doped a-Si:H having a comparable density of g s 2.004 spins w7x.
Generally speaking, with decreasing energy of atrapped electron below the conduction band edge, theextent of its wave function should monotonicallydecrease. By applying a simple bounded-electron
picture w7x to electrons at the shallowest and deepest
levels in our experiments, which has been calculated
to be 160 and 210 meV below the mobility edge,
respectively, the ratio of their effective Bohr radii,
a 210ra160 , was calculated to be 0.87. This reductionin the extent of electron wave function should resultin an appreciable increase in the isotropic hf interactions which is determined by electron densities juston Si atoms. In the present case, an increase of theisotropic hf splitting of g s 2.004 can be roughly
estimated to be 13% of itself, i.e., 7 mT Žfrom Ref.
w2x. =13%s 0.9 mT.
To detect the changes of 29 Si hf structures in the
LESR spectrum against the energy shift of electrons,
LESR spectra at various light intensities were normalized to their peak heights, which are shown in

355

Fig. 3. In the figure, the heights of the high-field-side
hf structures gradually decrease with decreasing lightintensity, i.e., decreasing LESR intensity. This decrease is assumed, first of all, to originate fromsuperposition of 29 Si hf structures of the dangling
bond whose area fraction in the g s 2.0055 signal isapproximately equivalent to the content of 29 Si, whichis around one half of the case of g s 2.004 signal
w2x.
The number of dangling bonds under illumination
has not been well estimated so far. However, as isseen in Fig. 2, the LESR spectra for light intensitiesof 450 and 19 mWrcm2 are almost identical, whichindicates no apparent contribution of the dangling
bond signal to the LESR for high light intensities.
Thus, we have to consider the contribution of thedangling bond signal on the LESR spectra only forthe cases of illumination with low light intensities.
To clarify the contribution of the dangling bondsignal to the whole LESR for the lowest light intensity, we carried out a fitting analysis. We subtracted
the dangling bond spectrum from the LESR spectrum for the lowest light intensity so that the resultant spectrum can fit to the LESR spectrum for thehighest light intensity Ž450 mWrcm2 .. The dangling
bond signal intensity in the LESR for the lowest
light intensity was estimated to be about 40% of theintensity in the dark.
Fig. 4 shows the original LESR spectrum for thehighest light intensity Žsolid line. and the best fitted
spectrum for the lowest light intensity Ždashed line..

Fig. 3. LESR spectra of the 29 Si-enriched Ž9.1 at.%. sample atvarious light intensities ŽFig. 2., which are normalized to theirpeak values.

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T. Umeda et al.r Journal of Non-Crystalline Solids 227–230 (1998) 353–357

Fig. 4. The LESR spectra for the highest Ž450 mWrcm2 . andlowest Ž0.004 mWrcm2 . light intensities, which are indicated bysolid and dashed lines, respectively. The LESR spectrum for thelowest light intensity was calculated by subtracting 40% dangling
bond signal in the dark from the original spectrum. Thin lines
show estimated high-field-side 29 Si hf structures which wereobtained by a procedure described in the text ŽSee Ref. 9 of Ref.
w2x..

an ab initio theoretical calculation of charged weak
Si–Si bonds w8x. Although the energy location ofg s 2.004 electrons has not been determined quantitatively, we speculate that it is relatively deep by anenergy gain of the structural relaxation compared tothe empty states.
Another possibility is given by weaker dependence of the spatial localization of g s 2.004 electrons on their energy positions. In the above discussion, we expected about 0.9 mT change in theisotropic hf splittings on the basis of the simple
model for electrons bound by a Coulomb force.
However, if the extent of the wave function ofg s 2.004 had much weaker dependence on a binding energy than that we have expected, it would be
difficult to detect the changes in the isotropic hf
interactions due to the 50 meV shift in the energy
positions of g s 2.004.

5. SummaryThin lines show the high-field-side hf structures ofboth spectra which were obtained on the assumptionthat the tail region of the main peak of g s 2.004
decays exponentially ŽSee Ref. 9 of Ref. w2x.. The
high-field-side hf structures of both spectra are almost identical, although the height of the high-fieldside hf structure in the LESR for the lowest lightintensity appeared to be still a little bit less than thatfor the highest light intensity. As is clearly seen on alogarithmic plot in the inset of Fig. 4, the isotropic hf
splitting was found to remain unchanged within errors of "0.2 mT.
Our experimental result indicates that densities ofg s 2.004 electrons just on two Si atoms are almost
constant even for an increase in the spin density bymore than a factor of 10, i.e., a shift of their energy
position by about 50 meV. We speculate the reason
why we could not detect the changes of electronic
structures of the 2.004 electrons in two ways.
The first one is that g s 2.004 electrons distributein a relatively narrow energy range compared to their
original empty states, resulting in almost constantlocalization of every g s 2.004 electrons. We suggest that such distribution will be generated by somestructural relaxation just after excited electrons aretrapped at initial empty states. The structural relaxation of the weak Si–Si bonds has been speculated in

We have measured the 29 Si hf structures of LESR
spectra in the light intensity range from 0.004 and
450 mWrcm2 . However, no discernible change inthe high-field-side 29 Si hf structure of g s 2.004 wasobserved. Based on the results, we suggest two
possibilities. One is that the energy position of g s
2.004 electrons will be lowered towards the mid-gap
and located in a relatively narrow range of energy,
which may be caused by some structural relaxation
just after excited electrons are captured in the bandtail states. The other proposal is that g s 2.004
electrons have weaker dependence of their spatial
localization on their energy positions than that expected from the simple model for such as electrons
bound by the Coulomb force.

Acknowledgements
The authors are thankful to H. Okushi for helpful
discussions. This work, partly supported by NEDO,
was performed in the Joint Research Center for
Atom Technology ŽJRCAT. under the joint research
agreement between the National Institute for Advanced Interdisciplinary Research ŽNAIR. and theAngstrom Technology Partnership ŽATP..

T. Umeda et al.r Journal of Non-Crystalline Solids 227–230 (1998) 353–357

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