The effects of boron and phosphorus doping on the electrical properties of a-SiNx:H films are studied. The material is obtained by the glow discharge of SiH4 and N2 mixtures. It is found that for Si-rich materials boron doping produces large conductivity variations while phosphorus appears to be a much less efficient dopant. A phenomenological explanation is given assuming that phosphorus prefers to form bonds in accordance with its own valence configuration; i.e., it will go substitutionally to threefold coordinated N sites. Experimental evidence of such behavior obtained from published IR transmission measurements confirms this assumption.441116118IEEE Italy Sectio

Alvarez F.

Chambouleyron I.

Cisneros J.I.

Constantino C.

English

Repositorio da Producao Cientifica e Intelectual da Unicamp

Doping effects in off‐stoichiometric glow discharge amorphous silicon nitride
F. Alvarez, I. Chambouleyron, C. Constantino, and J. I. Cisneros 
 
Citation: Applied Physics Letters 44, 116 (1984); doi: 10.1063/1.94569 
View online: http://dx.doi.org/10.1063/1.94569 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/44/1?ver=pdfcov 
Published by the AIP Publishing 
 
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Doping effects in off-stoichiometric glow discharge amorphous silicon 
nitride 
F. Alvarez, I. Chambouleyron, C. Constantino, and J. I. Cisneros 
Institute of Physics, UNICAMP, P. O. Box 6165, Campinas, SF, Brazil 
(Received 29 June 1983; accepted for publication 13 October 1983) 
The effects of boron and phosphorus doping on the electrical properties of a-SiNx:H films are 
studied. The material is obtained by the glow discharge of SiH4 and N 2 mixtures. It is found that 
for Si-rich materials boron doping produces large conductivity variations while phosphorus 
appears to be a much less efficient dopant. A phenomenological explanation is given assuming 
that phosphorus prefers to form bonds in accordance with its own valence configuration; i.e., it 
will go substitutionally to threefold coordinated N sites. Experimental evidence of such behavior 
obtained from published IR transmission measurements confirms this assumption. 
PACS numbers: 73.60.Fw, 78.65.Jd, 81.40.Rs, 61.70.Tm 
In 1976, Spear and Le Comber demonstrated that glow 
discharge (GD) hydrogenated amorphous silicon (a-Si:H) 
can be effectively doped.! Good transport properties com-
bined with a high absorption coefficient in the visible part of 
the solar spectrum made this material a potentially useful 
semiconductor for photovoltaic applications and 5% con-
version efficiency solar cells were soon produced. Recent 
work shows that higher efficiencies can be obtained with 
heterostructures using a-Si:H as the active absorber layer 
and a wide gap material for the window side junction of a pin 
solar cell. Amorphous silicon carbon alloy (B doped) allowed 
the Osaka group to attain 8% conversion efficiencies.2 At-
tempts to develop new window materials have been made 
with different amorphous alloys such as a-SiNx:H and a-
SiOx :H. Knights et al. found optical gap changes for differ-
ent oxygen contents in the a-SiOx:H network.3 Anderson 
and Spear were able to produce off-stoichiometric amor-
phous silicon nitride with an optical gap varying between 2.5 
and 5 eV.4 However, their material did not possess good 
semiconducting properties. Recently Kurata et al. prepared 
variable band-gap a-SiNx:H by plasma decomposition of 
SiH4 and NH3 mixtures. 5 As SiH4 and NH3 have similar 
dissociation energies the nitrogen content in the sample is 
closely related to the NH3/SiH4 gaseous ratio used in the 
reactor chamber. Doping experiments using B2H6 and PH3 
were also performed. Large conductivity variations were 
measured with the former dopant while phosphorus ap-
peared to produce a much less important effect. 
In this letter we present results on the optical and elec-
trical properties of off-stoichiometric silicon nitride com-
pounds. Samples were prepared by the simultaneous plasma 
decomposion ofSiH4/N2 gaseous mixtrues in a capacitively 
coupled glow discharge system, similar to the one described 
by Knights.6.7 Combined SiH4 and N2 flow varied between 
280 and 360 sccm. In all experiments the substrate tempera-
ture was kept constant at 280°C. Deposition pressure was 
maintained between 0.6 and 0.8 Torr. Peak to peak voltage 
was continuously monitored and a dc bias of - 100 V was 
applied to the substrate, all the samples being of the anodic 
type. The power delivered by the lO-MHz generator was 
measured by a rf power meter set as close as possible to the 
input electrode. The growth rate of 1.5 J..../s for the reported 
samples. A decreasing growth rate with rf power was ob-
served indicating a competitive phenomenon between film 
formation and film etching. Argon diluted phosphine and 
diborane, as well as pure silane and nitrogen of electronic 
grade, were used. 7059 Corning glass substrates were used in 
all cases. Optical measurements were made with a Zeiss 
DMC 25 spectrophotometer. The film thickness of the sam-
ples used to obtain the conductivity data was 1 J.Lm. Sample 
thicknesses were deduced from optical measurements and 
confirmed by mechanical profilometer techniques. Dark 
conductivity as a function of temperature, for intrinsic and 
doped material, was measured in an inert chamber. Co-
planar lO-mm-Iong aluminum strips 0.5 mm apart were eva-
porated on the samples. Excellent ohmic behavior was ob-
served in the 10-50-V polarization range. Conductivity 
measurements were performed within this bias range. 
Brenzikofer and Chambouleyron have shown that the 
optical gap of glow discharge a-SiNx:H samples prepared 
from SiH4 and N2 mixtures shifts with increasing rf power 
densities and depends on silane to nitrogen gaseous ratio. H 
Optical band gap is here defined as the extrapolated energy 
value of the high absorption portion of the (ahv)1I2 vs hv 
plot. Highly nitrogen diluted silane allows scanning an opti-
cal band gap that goes from 1. 8 to more than 5 e V. Why does 
an increasing rf power change the NISi ratio in the sample? 
A phenomenological explanation follows. 8 The dissociation 
energy of the nitrogen is 2 to 3 times larger than that of the 
silane molecule. At low rf power densities the plasma will 
essentially break silane molecules and the material will not 
differ much from a-Si:H. Higher power densities produce 
more and more nitrogen radicals in the plasma increasing 
the number of nitrogen atoms incorporated into the 
network. The concentration of active nitrogen in the plasma 
is then related to rf power density. Thus the composition of 
the sample depends on both rf power density and SiH4/N 2 
gaseous ratio. This phenomenological picture was con-
firmed by recent IR measurements.9 
Figure 1 shows the (ahv)l12 vs hv curves of a series of 
samples prepared using identical conditions except rf power 
density. It can be seen that for a fixed silane dilution (SiH41 
N 2 - 33 %) the absorption edge shifts towards higher ener-
gies with increasing rf power density. 
116 Appl. Phys. Lett. 44 (1 ). 1 January 1984 0003-6951/84/010116-03$01.00 © 1983 American Institute of Physics 116 
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power [~cm21 density 
A - 0.6 
B - 1.0 
.00 c- 1.5 
0- 5.0 
0 
• 
~ 300 
'E 
u 
:> 
1/ 
;::::~ 200 
:::. 
.r. 
~ 
~
100 
• 
O~ __ ~L-~L-~ ____ ~ ______ ~ ____ ~ 
1.5 Z.O Z.5 3.0 
PHOTON ENERGY [eV) 
FIG. I. (ahv)1/2 vs photon energy for different a-Six N'_x:H layers obtained 
from the GD of a SiH4 and N2 mixture of fixed composition (SiH41 
(N2) = 0.33. All samples are non intentionally doped and rfpower density is 
indicated on each curve. 
The particular growth conditions under which we have 
chosen to investigate doping effects produce an alloy having 
an extrapolated optical gap of -2.0 eV (curve C). 
The extrapolated room-temperature conductivity and 
the correspoding activation energies versus B2H~SiH4 and 
PH3/SiH4 gaseous ratio are shown in Figs. 2(a) and 2(b). The 
former shows that the material is slightly n-type when defect 
controlled. Minute amounts of diborane decrease the con-
ductivity and increase the activation energy indicating that 
Fermi level is pushed downwards towards midgap. Dibor-
ane dilutions higher than B2H~SiH4 - 10-3 lead to a con-
tinuous conductivity increase of several orders of magni-
tude. For a dilution ratio of B2H~SiH4 - 1.2% the 
activation energy drops to 0.53 eV [Fig. 2(b)]. On the right 
side of the figure results corresponding to phosphine doping 
are plotted. Conductivity and activation energy variations 
are much less impressive indicating that, concerning trans-
port properties, P is a less efficient dopant than B. In fact, 
these results are similar to those obtained by Kurata et al. on 
off-stoichiometic silicon nitride produced by plasma decom-
position of SiH4/NH3 gaseous mixtrues. 5 We believe that 
large differences in doping efficiencies are related to the 
chemical structure of the material and not to the particular 
method used to prepare the samples. 
We have also performed IR absorption experiments on 
intrinsic samples and strong Si-H and Si-N absorption 
bonds were observed. The integrated areas of the former 
band indicate a hydrogen content of - 17 at. %. 9 Due to the 
uncertain absorption strength constant for Si-N band we 
have not yet a reliable experimental value for nitrogen con-
centration. However, from theoretical plots of N/Si struc-
tures versus refractive index we estimate the nitrogen con-
117 Appl. Phys. Lett., Vol. 44, No.1, 1 January 1984 
-4 I 
.- P -1- N E 
0 
-6 2.3;; c 
...; Q) ........., 
'" ~ -8 2.1 .... Q. 0 
en en 
0 
-10 I 1.9 W 
.. ' 1.7 
1.0 t 0.9 • ., ,.--, > 0.8 Q) • \...-..J 0.7 
to 0.6 
, 
~ , 0.5 • 
o -2 -4 -6 -6 -4 -2 0 
log gas. imp. ratio 
FIG. 2. (a) Room-temperature conductivity vs gaseous doping mixture for 
samples grown under conditions identical of those of curve C;, Fig. I. The 
effect of optical gap shrinking due to boron doping is also indicated. (b) Dark 
conductivity activation energies vs gaseous doping mixture. It is found ex-
perimentally that the conductivity varies with temperature according to 
a = ao exp(-Ea1kT) in the 300-500-K temperature range. 
tent of our samples to be - 20-30 at. %.10 
What follows is an attempt to explain the experimental 
results. Off-stoichiometric amorphous silicon nitrides are 
midway materials between a-Si:H and a-Si3N4 :H. This can 
be interpreted essentially in two ways: (1) a microcluster 
model where small a-Si3N4 units are randomly distributed in 
the network; (2) a more uniformly perturbed network where 
nitrogen is included in a way that clustering is not preferred. 
Presently we cannot conclusively determine which of 
the above models is more appropriate and thus more experi-
ments are needed. However, one can say that the process 
which decreases the network energy will dominate the 
growth mechanism giving either a homogeneouslike matrix 
or a two-phase material. Furthermore, in GD off-stoichio-
metric silicon nitride the presence oflarge amounts of hydro-
gen contributes to network relaxation. 
It is by now well accepted that amorphous semiconduc-
tors might not be homogeneous random networks but in-
stead might possess structural and/or compositional mi-
croscopical inhomogeneities. If these inhomogeneities can 
grow, formation of clusters will depend on how nitrogen 
atoms interact with the network. They might prefer to ac-
commodate, in a strain-free region instead of in the vicinity 
Alvarez sf al. 117 
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143.106.108.136 On: Thu, 25 Jun 2015 17:36:57
of an already existing N-Si bond giving a more uniformly 
perturbed network. 
Chemical vapor and glow discharge deposited silicon 
nitrides are amorphous. Radial distribution function experi-
ments show that local configurations are very similar to 
those in stoichiometric polytype,8-Si3N4 with planar bonded 
N and tetrahedral bonded Si. II This structure is consistent 
with Sp3 silicon hybrid orbitals while N bonding has been 
explained in terms of a linear combination of p orbitals, the 
planar geometry being given by a strong repulsion of non-
bonded Si atoms. According to these calculations nitrogen 
electrons do not participate in bonding and the top of the 
valence band is given by nonbonding N-p; electrons. 12 
An alternative explanation could be a nitrogen Sp2 hy-
bridization and a N p; nonbonding orbital, the repulsion 
between lone pairs being responsible for the hexagonal close 
packed structure. \3 Both explanations lead to the interpreta-
tion of silicon nitride as a lone pair semiconductor. 
We now consider the differences in boron and phos-
phorus doping effects. It was once believed that amorphous 
semiconductors could not be doped. Not being constrained 
by a crystalline matrix the impurity atom would form bonds 
in accordance with its own valency. It is now well known 
that this is not necessarily so. In the a-Si:H network impuri-
ties like Band P are partially incorporated in substitutional 
sites. Nevertheless, the problem concerning doping efficien-
cies has not received a definitive answer and is probably re-
lated to network topology and partial microcrystallization. 
Additional information to our problem is given by the IR 
transmission measurements made by Kurata et al.5 These 
researchers studied Band P-doped off-stoichiometric a-
SiNx:H samples. In B-doped materials they found an in-
creasing amount of B-N bonds with increasing boron dop-
ing in detriment of N-Si bonds. On the other hand P-doped 
films do not exhibit a P-N absorption band, as one might 
expect if P replaced Si atoms in the network. 
Electrical and optical measurements and the above-
mentioned results lead us to conclude that (I) boron is largely 
incorporated in a fourfold bonded configuration and acts as 
an acceptor impurity in the silicon structure, (2) phosphorus 
prefers to form bonds in accordance with its own valence 
configuration. It will prefer N sites and will not produce an 
extra electron in the conduction band. However, small con-
ductivity changes are measured indicating that some phos-
phorus atoms are fourfold bonded. 
We believe that the behavior of boron and phosphorus 
118 Appl. Phys. Lett., Vol. 44, No.1, 1 January 1984 
is strongly related to material composition. In silicon-rich 
samples the conductivity variations would be larger for both 
dopants. As the composition approaches the silicon nitride 
stoichiometry, doping becomes less and less effective. A 
greater availability of threefold coordinated sites increases 
phosphorus probability of coordination according to its va-
lence configuration. On the other hand, fourfold bonded bo-
ron will have less opportunities of being surrounded by Si 
atoms. 
The low phosphorus doping efficiency could also be re-
lated to a high density of gap states coming from structural 
defects. However, Fig. 1 shows steep straight lines on the 
high absorption part of the curves. By extrapolation a well 
defined optical gap is obtained indicating low density of 
states in the gap. Furthemrore, absorption coefficients de-
duced from photoconductivity measurements indicate an 
exponential shape down to a few cm - I. 
Finally, although samples prepared from SiH4/NH3 
and SiH4/N2 processes might have a different H content we 
believe that the H incorporation produces a minor effect as 
far as gap widening is concerned. 
The authors are indebted to Professor C. Gon~alves da 
Silva (UNICAMP) for fruitful discussions. They also ac-
knowledge clarifying comments from Professor M. Cardona 
(Max Planck Institute, Stuttgart), Dr. S. Pantelides (IBM, 
Yorktown Heights), and Professor W. Paul (Harvard Uni-
versity, Cambridge). 
'w. E. Spear and P. G. Le Comber, Philos. Mag. 33, 935 (1976). 
2y. Tawada, K. Tsuge, M. Kondo, H. Okamoto, and Y. Hamakawa, J. 
Appl. Phys. 53,5273 (1982). 
3J. C. Knights, R. A. Street, and G. Lucovsky, J. Non-Cryst. Solids 35 & 
36,279 (1980). 
4D. A. Anderson and W. E. Spear, Philos. Mag. 35, 935 (1976). 
5H. Kurata, H. Hiyamoto, M. Hirose, and Y. Osaka, 3rd Photovoltaic 
Science and Engineering Conference Japan, Kyoto, 1982. 
6J. C. Knights, in Structure and Excitation of Amorphous Solids, edited by 
G. Lucovsky and F. L. Galeener (AlP, New York, 1976), p. 296. 
7F. Alvarez, Rev. Bras. Fiz. 12, 832 (1982). 
SR. Brenzikofer and I. Charnbouleyron, 7th Latin American Symposium 
on Solid State Physics, Gramada, Brasil, 1981 (unpublished). 
91. Chambouleyron, F. Alvarez, C. Constantino, and J. I. Cisneros, 5th E. 
C. Photovoltaic Solar Energy Conference, Athens, Greece, 1983 (to be 
published). 
lOA. R. Reimberg, J. Electron. Mater. 8, 345 (1979). 
liT. Aiyama, T. Fukunaga, K. Nihara, T. Hirai, and K. Suzuki, J. Non-
Cryst. Solids 33, 31 (1979). 
12J. Robertson, Philos. Mag. B 44, 215 (1981). 
13C. E. Gon<;alves da Silva (private communication). 
Alvarez at al. 118 
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Doping Effects In Off-stoichiometric Glow Discharge Amorphous Silicon Nitride

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Doping Effects In Off-stoichiometric Glow Discharge Amorphous Silicon Nitride

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