ELSEVIER

Journal of Non-Crystalline Solids 198-200(1996) 330-333

Spatial distribution of phosphorus atoms surrounding spin centersof P-doped hydrogenated amorphous silicon elucidated by pulsed
ESR
Satoshi Yamasaki a7*, Jung-kyu Lee b, Takahide Umeda a, Junichi Isoya a,c,
Kazunobu Tanaka a’ Joint Research Center for Atom Technology (JRCAT) - National Institute for Advanced Interdisciplinary Research (NAIR), l-l-4,Higashi, Tsukuba-city, Ibaraki 305, Japanb JRCAT-Angstrom Technology Partnership (ATPI, I-1-4, Higashi, Tsukuba-city, Ibaraki 305, Japan’ Unioersity of Library and Information Science, l-2, Kasuga, Tsukuba-city, Ibaraki 305, Japan

Abstract
Local concentration of phosphorus (Cr.,,,) in the vicinity of spin centers in P-doped a-Si:H has been determined forelectron spin resonance components g = 2.0043, for g = 2.0055, and for a hyperfine structure (hf) with a splitting N 24 mT
It has been observed, for both g = 2.0043 and 2.0055, that Cp_,Oc
as a function of gaseous ratio of CP_sas = [PH,]/[SiH,].
increases with the increase of CP_sas, however, that the ratio CP_t,JCP_saq decreases with the increase of CP_sas. In the films
with small CP_aas, Cp_,ac of g = 2.0043 and that of 2.0055 are significantly higher than the average P concentration in films.

1. Introduction
In phosphorus
icon(P-dopeddopeda-Si:H),hydrogenatedamorphous

threeof spinkinds

sil-centers

have been detected by electron spin resonance (ESR);g = 2.0043 in dark [l], g = 2.0055 under illumination [2], and a hyperfine (hf) structure of 3’P nucleus
with splitting
N 24 mT [3]. Their origins are crucially related to the doping mechanism in P-dopeda-Si:H. Although these are assigned to be conduction
band tail electrons, dangling bonds, and donor states,
no clear experimental evidence to confirm their origins and microscopic structure has been reported fora long time [4].

In ESR spectra of amorphous semiconductors,
weak hf interactions that give information of distribution of magnetic nuclei (’H, 29Si, 3’P, etc.) nearby
paramagnetic spin centers are hidden underneath theline broadening
due to random orientations
andstructural fluctuation. By applying the electron spinecho envelope
modulation(ESEEM)
method ofpulsed ESR, the weak hf interaction can be extracted[5,6]. In the present work, the local P concentrationsin the vicinity of the three spin centers in P-dopeda-Si:H have been determined as a function of gaseous
ratio of [PH,]/[SiH,],
CP_gas.

2. Experimental
X Corresponding author. Tel.: + 8 l-298-542732;
542786; e-mail: satoshi@jrcat.or.jp.
0022.3093/96/$15.00
Copyright
SSDI 0022-3093(95)00696-h

fax: + 81-298-

The samplesdeposited from0 1996 Elsevier Science B.V. All rights reservedof P-doped a-Si:H used here wereSiH, and PH, mixtures with theS. Yamasaki et al. /Journalof Non-Crystalline Solids 198-200 (19961330-333

ratio of CP_gas= 0.1, 0.3, and 1% by a rf glow
discharge method at a substrate temperature of 250°C.
ESEEM spectra were recorded at 50 K using atwo-pulse sequence. The detailed experimental conditions are similar to those described in Ref. [5].
ESEEM is a periodic variation of the echo intensity due to weak hf interaction between electron andnuclear spins which appears superimposedon theslow echo decays produced by a pulse sequence of a
two-pulse Hahn echo (90”-7-180°-recho, T isscanned) or three-pulse stimulated echo (90”-r-90”T-90”-7 echo, T is scanned and r is fixed) [5,7]. In
two pulse ESEEM employed in this study, the modulation contains ENDOR frequenciesv,( V~) = vn +aswellasthesumfrequency(v, + r+‘P)(-)1/2V*,= 2 V, and the difference frequency ( V, - VJ) = vA,
where uA is the hf splitting, and V, the NMRfrequency. In amorphous materials, an ESEEM spectrum in the frequency domain usually shows featureless peaks centered at vn and 2v,, since an ESEEM
spectrum is a superposition of spectra at many different orientations. The detection of V, c3’P> was rather
difficult in this experiment, since the modulation ofV, (“‘P) in time domain damps out rapidly due to a
distribution of V, in the frequency domain which islikely to be caused by the small extent of the wave
function of the unpaired electron onto the P atoms.
Therefore, we have utilized, in this study, the sum
frequency 2~” (3’P) peak which is, at the first order,
free from broadening due to the distribution of v*.
The local concentration of P in the vicinity of spin
centers (C,,,,)
is estimated, at first approximation,from the intensity ratio of the 2~” peak of 3’P(I = l/2, natural abundance: 100%) to that of 29Si
(l/2,4.7%) in the frequency domain spectra. The
29Si at the first-nearest neighbors, i.e., back-bonded
silicon in the case of dangling bond, does not contribute to ESEEM since the hf interaction is large[5,8]. The 29Si modulation arises from 29Si nuclei
occupying the silicon sites which are the second
nearest neighbors and furthest neighbors. Since themodulation depth depends on the distance r between
electron spin and nucleus (proportionalto rm6 fornuclei with r > 0.36 nm), main contributions
to the29Si modulation are from the second and the third
nearest neighbors. It is likely that the 3’P occupying
these silicon sites contributes
mainly to the 3’P
modulation. The 3’P modulation arises from P atoms

311

which are located nearby spin centers and which do
not have unpaired electrons such as P: and Pt.

3. Results
The frequency domain spectrum of two-pulse ESEEM at the center position of g = 2.0043 in dark
and g = 2.0055 under illumination
for CP+ = 1%
is shown in Fig. 13(a) and (b), respectively. ‘One cansee the V” and 2~” peaks from 29Si (u, = 2.478 MHz
at 293 mT), the 2~” peak from 3’P (v, = 5.050 MHz
at 293 mT), as well as the V, peak from ’H (I = l/2,lOO%, V, = 12.46 MHz at 293 mT). Table 1 showsC p_,Ocfor different gaseous ratios, Cp_E,\. As seenfrom Table 1, in the sample of CP.gas = l%, CI,_,oc indark (g = 2.0043) and that under illumination (i: =2.0055) are higher than Cp_g,s. However, the average

P-doped a-Si:Hin dark
H=293mT

010515rd20

frequency (MHz)7‘I’2vn(29Si)

x.Z

P-doped a-Si:HCP_gas = 1%
under ill.
H=293mT

J0510frequency (MHz)

1520Fig. 1. The frequency domain spectra of 2-pulse echo ESEEM indark (a) and under illumination (b) at the center position ofg = 2.0043 and 2.0055 for CP.pilj = l%, respectively.

332
Table 1Local concentration

5’. Yamasaki et al. / Journal of Non-Crystalline Solids 198-200 (1996) 330-333

of P atom surrounding

spin centers (C,,,,)(%I[PH,l/[SiH,l

1.0

0.3

0.1

g = 2.0043
g = 2.0055
hf

2.2
2.3
< 1.6

1.1
1.00.5 I
0.44

P concentrationin the film (C,,,,)
measured byEPMA is almost twice of CP_aas and proportional toc r__. Therefore, in the sample of CP_aas= I%, Cp_,oc
is almost equal to the CP_rilm. When P atoms are
spatially distributed randomly like 29Si, Cp_,Ocshould
vary in proportion to CP_aas. However, Cp_,Oc for
C P_gas= 0.1% is almost twice (2.3 for g = 2.0043
and 1.9 for g = 2.0043) of that expected from thevalue of Cp_lOc for CP_gas= 1%. In the films with
low CP-lot is significantly higher than CP_rilm.

4. Discussion
The above experimental result can be tentatively
explained by an inhomogeneous
distributionof Patoms, although the additional measurements,
especially, for samples with CP_gas lower than 0.1% areneeded for confirmation.
One possibility is that, inthe films with low CP_aas, the spin centers might bepreferentiallyformed in the region with higher P
concentration. The other possibility for Cp_,Ochigher
than CP_fi,m is that the spin centers in P-doped a-Si:H
might accompany specific P atom(s) nearby in addition to P neighbors which are randomly distributedin the P-doped a-Si:H network. The distance to thespecific P atom(s) is not closer than the second
nearest neighbors since the intensity of 2 v”(~’P) peakis comparable to the 2~,,(*~Si) peak coming from 29Si
nuclei statistically distributed in the silicon sites of
the network in its natural abundance of 4.7%. The
contribution from the 31P nuclei randomly distributedin the network is dominant for higher CP_f,,m, while
the modulationarises mainly from the specific P
atoms for low CP_rilm. The existence of P atoms
nearby dangling bonds with g = 2.0055 in P-dopeda-Si:H was suggested by Okushi et al. on the basis of

their experimental
results using isothermal capacitance transient spectroscopy [9]. They proposed aP-related charged dangling bond (g = 2.0055) in
P-doped a-Si:H, in which P4+ is located proximate toa negatively charged dangling bond (D-) due to thereduction of Coulombic energy. The ESR signals
with g = 2.0043 which is observed in dark in Pdoped a-Si:H is observed under illuminationin undoped a-Si:H. The role of P atoms in P-doped a-Si:H
might be assumed by impurities or charged defects
in the case of undoped a-Si:H.
The values of Cp_,Oc are listed (Table 1) for
different spin centers of g = 2.0043 in dark, g =2.0055 under illumination,
and a hf structure with a(measured atsplitting - 24 mT under illumination
the low magnetic field component). As seen in Table
1, the spin centers of g = 2.0043 show a similar
trend of Cp_,Octo that of g = 2.0055. This similarity
might suggest that the origin of the g = 2.0043
signal is close to dangling bonds with g = 2.0055,
and not like the band tail electrons with spreading
wave functions. For g = 2.0043 in undoped a-Si:H,
we reported that the wave function is localized likedangling bonds [4].
For spin center with a hf splitting- 24 mT,
Cp_,Ocwas measured only for CP_aas= l%, since theESR signal intensity was weak for the films withlower CP_aas. The 3’P nucleus with such a large hf
splitting does not give rise modulation [5,8]. Therefore, Cp_ioc for the spin center with a large hfsplitting does not count a P atom which is among theatoms constituting the spin centers and contains only
the contribution
from the outside P atoms in thenetwork. As seen from the table, the Cp_,oc of the hf
structure is smaller than that of other spin centers.
This might suggests that, if we include the P atom
exhibiting hf splitting - 24 mT into Cp_,Oc,the local
concentration of P for the spin center with a large hf
splitting might be close to those of other centers.
In this paper we did not touch upon the coordination number of P atoms, which is assumed to be
three-fold and four-fold coordinated. This assumption is crucially related with the doping as well as
defect creation mechanism and will be clarified by
the ESEEM measurements using As (I = 3/2, natural abundance: 100%) doped a-Si:H through the information on the quadrapole coupling constant withAs atoms surrounding electrons spins.

S. Yamusaki et al./JournalofNon-Crystalline

5. Summary
Local P concentrationsin the vicinity of the spin
centers in P-doped a-Si:H have been estimated by
the ESEEM technique of pulsed ESR. The followingexperimental results have been obtained: (1) There
was no linear relationship between Cp_,Ocand CP_Eaq,and Cp_,,~cfor CP_ga5= 0.1% was almost twice that
expected from the value of Cp_,Ocfor CP_gas= 1% in
the spin centers with g = 2.0043 and 2.0055. (2)Local P concentrationsin the vicinity of the spin
centers for g = 2.0043 and 2.0055 were higher than
the average P concentrationof the films for low
CP_ga\ films. (3) The value of Cp_,oc of the hf structure was smaller than that of other spin centers. To
explain these experimental results the tentative model
related with the spatial inhomogeneityof P atoms
has been suggested.

Acknowledgements
We thank A. Matsuda and H. Okushi for many
stimulating discussions. We also thank Mitsui Toatsu

Solids 15X-200~19%) 330-333

333

Chemicals Inc. for providing P-doped samples. This
work was performed under the management of Joint
Research Center for Atom Technology(JRCAT)
partly supported by New Energy and Industrial
Technology Development Organization (NEDO).

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