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EFFECT OF HYDROGENATION ON DOPED a-Si PREPARED BY CVD
J. Magarino, A. Friederich, D. Kaplan, A. Deneuville
To cite this version:
J. Magarino, A. Friederich, D. Kaplan, A. Deneuville. EFFECT OF HYDROGENATION ON DOPED a-Si PREPARED BY CVD. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-737-C4-740.
�10.1051/jphyscol:19814161�. �jpa-00220785�
CoZZoque C4, suppldment au nO1O, Tome 42, octobre 1981 page C 4 - 7 3 7
EFFECT OF HYDROGENATION ON D O P E D a-Si PREPARED B Y C V D
J. Magarino, A. Friederich, D. Kaplan and A. ~eneuville*
Laboratoirc CenCraZ de Recherches, Thomson-C. S . F., B. P. N 0 l O , 91401 Orsay, France
*troupe
de T m n s i t i o n s de Phases, C.IJ.R. S . , B . P. I ~ ' ~ 3 6 6 , 38042 CrenobZe, France Abstract.- The effect of post-hydrogenation of phosphorus and boron doped a-Si films dewosited by CVD has been studied by electrical conductivity and ESR. It is found that, contrary to the phos~horus case, hydrogenation decreases the conductivity of stronglv boron doped swecimens and reduces the intensity of the doping induced ESR g = 2.013. The results are interpreted in terms of a reduction of the number of electrically active boron atoms.Introduction.- There is widespread interest, both fundamental and practical, in understanding properties of doped hydrogenated
amorphous silicon. A number of studies in recent years have examined how physical properties (e.g. electrical conductivity (l), owtical absorption (2)
,
Electron Spin Resonance (3),
etc.. .) are modified by introducing phoswhorus and boron in the films. In analysing the results, an inherent complexity arises from the fact that one is dealing with a three component system ; silicon, hvdroqen, and dopant. Observed variations in properties with dopant introduction in a glow discharge svstem, for instance, may be interpreted as due to substitution of dopant in a fixed silicon-hvdroqen matrix or to more complex structural variations involving dopant-hydrogen interactions. In this paper, we try to shed some light on the problem by reporting results obtained upon post-hydrogenation of CVD deposited films which contain as-deposited only small amounts of hydrogen. The idea is that by separating the film prenaration in two steps, one may extract specific information on hydrogen- dopant interactions.Samule prewaration.- Films have been deposited at 600C in a conven- tional atmospheric-pressure CVD system described wreviouslv (4).
Post-hydrogenation was nerformed by plasma treatment. We wished to obtain different levels of hydrogenation by varying the plasma intensity, i.e. the density of atomic hydrogen at the sample's surface. This is however not easy to achieve with a single plasma system since the range of o ~ e r a t i n g conditions is rather narrow.
We shall compare here results obtained with a mild wlasma, generated by DC excitation (4) (hvdrogenation 1) and a stronqer plasma,
generated by microwave excitation at 2450 MHz (hydroqenation 2).
SIMS analysis on undoped samples treated at 400C in a deuterium plasma give deuterium concentrations at the sample's surface of order
1% for hydrogenation 1, and 10% for hydrogenation 2. Analysis of the depth profile yields a diffusion coefficient ~ = 8 f 4 x 1 0 - ' ~ c m ~ s - ' at T = 400C. The hydroqen diffusion ropert ties are dependent upon doping and this will be reported elsewhere. Because of complications associa- ted with non-flat hydroqen depth profiles, we limit ourselves to the consideration of general trends.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814161
C4-738 JOURNAL DE PHYSIQUE
The doped samoles are characterized bv the concentration ratios CPH33/[SiH,,l and CB,H,I/KSiH,I in the gas phase during deuosition.
SIMS measurements have been ~erformed o n some of the samples, usinq implantations in crystalline Si as calibration references. For exam- ple samples prepared wlth CPH31/rSiH,1 = yielded rPl/CSil=7 and those with [B,H,l/CSiH,] = lo-', rBl/rSil = 3 x 1 0 ~ ~ . Experimental results.- Room temperature electrical conductivity results are presented in fig. 1 for phosphorus doping and fig.2 for boron doping. In the case of as-deoosited films these data are similar to those reported by other authors. Part of the observed features can be understood in terms of the common picture :
a) Doping induced shifts of the Fermi level ~ o s i t i o n , which are controlled by the gap state density.
b) Removal of gap states by hydrogen.
In this framework :
1) The conductivity ~ o - ~ R - ' cm-
'
in undoped and non-hydrogenated films involves transport near the Fermi level.2) The conductivity rises rapidly with doping when all states near the center of the gap have been filled. The threshold concen- tration for this rise in non-hydrogenated films is x = 3x10-' for phosphine and x = lo-' for diborane.
3) In undoped films, hydrogenation reduces the conductivity by an order of magnitude to a level of 1 0 - ~ ~ - ' c m - ' by e,liminatinq gap states and transport near the Fermi level. The conductivity is then slightly n-type as evidenced from the conductivity decreases induced by slight boron doping and conductivity increases induced slight phosphorus doping.
1 I I 7 -
lo-' -
-
Fig. 1 Room temperature conductivity7
6 for phosphorus dooed films
as-desposited (no H) and hy-
c
-'.01 drogenated. The value of>-
t conductivity for hydrogenated
2 lop-
,- u n d o ~ e d specimens is ~ n d i c a -
"
ZY ted by the line labeled
2 10-5 - "intrinsic".
8 lo-& - 1r7 -
10-~ '
lo-' lo-5 10.' TO-' x = [Pn,l/lslH,l
However, although the behaviour of phosphorus doped soecimens can be accounted for by the above picture in the complete d o ~ i n g range, this not the case for boron doping at high doping levels.
The most striking difference is that the stronger hydrogenation (hydrogenation 2) in fig. '2 decreases the conductivity instead of increasing it, as is the case for phosphorus in fig. 1. It should be noted that the conductivities at hiqh boron doping after
hydrogenation 2 are comparable to that of highly dowed glow discharge films ( I ) , whereas they are more than an order of maqnitude higher
for boron doped films.
for hydrogenation 1 or no hydroqenation.
ESR measurements confirm the specifity of hydrogen interaction with boron doped samples. Fig. 3 shows a compilation of spin density measurements. The spins observed at high boron doping are characte- rized by a g-value of 2.013. This resonance has been ascribed to holes trapped in states above the valence band (3). The data of fig.3 indicates that the density of such holes decreases with increasing hydrogenation (i.e. going from DC to microwave plasma and 400C to 300C treatment temperatures T p ) . Annealing without the plasma resto- res values characteristic of weakly hydrogenated specimens. In highly phosphorus doped samples the effect hydrogenation is in general a small increase of the ESR.
0.C T,,=&ooc/'i plasma L
1
microwave plosma
Tp = 300 C
lo-' lo-' lo-' l o 3 M-'
X = [ ~ , ~ d / b H , l
Fig. 3 ESR spin density for the q = 2.013 line. T refers to the measure- ment temperature, T, to the temperature of the plasma treatment. The point designated by a star refers to a sample hydrogenated in the mi- crowave plasma at Tp = 400C subsequently annealed at 300C under vacuum ( 1 hour)
.
Discussion.- In trying to explain the present data, we cannot a-priori exclude some effect of hydrogen on the density of states.
On the other hand, all the above observations might be simply related to a decrease of the number of electricallv active boron atoms with increasing hydroqen concentration : it is clear that this would decrease the conductivity and trapped hole ESR, without giving rise to any extra ESR signal. If this is the main effect involved, one may expect a one to-one-correspondence between conductivity and ESR
C4-740 JOURNAL DE PHYSIQUE
intensity, both being related to the Fermi level position, irrespec- tive of whether this is obtained by changing boron or hydrogen concen- tration. Fig. 4 shows a plot of ESR versus conductivity for a varie- ty of samples and sample treatments. The observed correlation
supports the pro~osed explanation.
r A r m - + m . o c A PbmnNu
*
Anmat-
w' lu' IC'
IUX*( lE)9ERITUIL CMmUCTMlY 19-' cm-' 1
Fig. 4 Correlation between conductivity and spin density at g = 2.013. The sample treatments are indicated. Plasma treat- ments are performed at 400C and vacuum annealing at 350C.
A microscopic model that could account for this hydrogen effect might involve fourfold coordinated boron atoms (i.e. electrically active) becoming threefold coordinated (i.e. inactive) with a hydro- gen atom tying the remaining silicon bond. It is however temptinq to relate our observations to recent reports by Shen and Cardona (7) of a strong IR absorption due to boron-hydrogen bonds in very
highly doped specimens. Although we have not yet been able to detect this absorption in our samples, the question deserves further
investigation.
Acknowledgments.- The authors thank A. Huber, N. Proust, P. Landouar, G. Morillot, E. Criton for their cooperation in this work.
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