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Passivation of Extended Defects in Silicon by Catalytically Dissociated Molecular Hydrogen
S. Binetti, S. Basu, M. Acciarri, S. Pizzini
To cite this version:
S. Binetti, S. Basu, M. Acciarri, S. Pizzini. Passivation of Extended Defects in Silicon by Catalytically Dissociated Molecular Hydrogen. Journal de Physique III, EDP Sciences, 1997, 7 (7), pp.1487-1493.
�10.1051/jp3:1997201�. �jpa-00249659�
Passivation of Extended Defects in Silicon by Catalytically
Dissociated Molecular Hydrogen
S. Binetti (~,*), S. Basu (~,**), M. Acciarri (~) and S. Pizzini (~)
(~) INFM Istituto Nazionale di Fisica della Materia, Department of Materials Science, Via Emanueli 15, 20126 Milano, Italy
(~) Consorzio Milano Ricerche, Via Cicognara 7, 20133 Milano, Italy
(Received 3 October1996, accepted 14 March 1997)
PACS.66.30.Jt Diffusion, migration and displacement of impurities PACS.61.72 -y Defects and impurities in crystals; microstructure
Abstract. This paper reports the results of
a newhydrogenation process, which applies the properties of noble metals
aschemisorptive dissociation catalysts for molecular hydrogen. Used to passivate deep states in several kinds of polycrystalhne materials, H has been shown to be
particularly effective for samples grown by the EFG (Edge Film Grown) technique. These results
are
compared with former
onesobtained
ondislocated single crystals, which
werepassivated under
anhydrogen plasma, to speculate about the role of dislocations
onthe yield of
ahydrogen passivation process
1. Introduction
Atomic hydrogen from various sources is known to passivate surface states as well as shallow and
deep levels arising from dopants, impurities and defects in silicon or other semiconductors, thus
inducing important changes in their electronic properties, like the surface recombination rate, the ionized dopant profile, the minority carriers lifetime and the majority carrier concentration.
Albeit HF can be an effective but temporary surface passivating agent, permanent passivation is usually obtained using ion implantation or Radio-Frequency (RF) and microwave plasma
hydrogenation. Both these techniques present, however, an intrinsic disadvantage. In fact,
in both cases hydrogen atoms are physisorbed and chemisorbed at the semiconductor surface,
which is thus passivated and which behaves as an hydrogen reservoir continuously replenished
as long as the hydrogenation process continues, and from which H diffuses into the bulk until is trapped at a suitable point or extended defect. The effective passivation depth comes from
a delicate balance of inward and outward diffusion and of trapping and de-trapping processes, all having a strong temperature dependence. In addition, the diffusion of H in silicon itself is
known to be a complex process, influenced by quantum mechanical effects at low temperatures and by interactions of H with H itself or with other impurities at medium temperatures Iii. We expect therefore that the passivation depth and yield would strongly depend on temperature. if
we would be able to use extended defects like Grain Boundaries (GB) or segments of emerging (*) Author for correspondence
(**)Now at India Institute of Technology, 721302 Kharagpur, W. Bental, India
Q Les #ditions de Physique 1997
1488 JOURNAL DE PHYSIQUE III N°7
Table I. Imp~rity concentration of polycrystalline samples.
Samples [Cl (ppma) [O] (ppma)
EFG 15.6 ~ 0.5 2.2 ~ 0.5
EU 15.8 ~ 0.5 1.5 ~ 0.5
dislocations as preferential hydrogen chemisorption centers and as pathways for fast H-diffusion,
the further diffusion process to bulk H-traps would use as the atomic hydrogen source the extended defects themselves, thus strongly enhancing the kinetics of the overall hydrogenation
process and strongly increasing the in-depth passivated layer. Eventually, if we would be able to decorate selectively emerging dislocations and GB with a catalyst capable to favor the chemisorptive dissociation of molecular hydrogen, we would be able not only to use molecular
hydrogen instead of atomic hydrogen, but, possibly, to discriminate between different kinds of dislocations and GB, in view of their ability to be decorated with metals.
We will show that this process can be carried out using Pt as the catalyst on different types of polycrystalline and single crystal silicon substrates.
2. Experimental Details
2.I. SAMPLE SELECTION. Two kinds of polycrystalline samples, used as substrates for solar cells, were considered in detail, both exhibiting very low diffusion length (LD) values in
as-grown conditions and thus requiring passivation to improve the photovoltaic yield.
The former were grown by the "Edge Film Growth Technique" (EFG) [2] and chosen in view of their ability to show a large LD increase after plasma hydrogen passivation [3].
The latter, supplied by Eurosolare SpA (Nettuno, Roma), were grown by the directional solidification method [4] and cut from the lateral top region of a 55 x 55 x 21 cm3 multicrystalline
silicon ingot.
As a consequence of the different growth techniques used, the morphology, the microstructure and the impurity content of EFG and Eurosolare (EU) samples are very different. While their composition is reported in Table I, the dislocation density determined using the etch pit counting procedure is almost constant in EFG samples (around 10~ cm~2) and very variable from region to region in EU samples.
Moreover, as already shown in a previous paper [5], the EU samples are characterized by
the presence of silicon nitride and iron silicide submicrometric particles. The density of these
particles is very large in the edge region of EU ingots, where the minority carriers diffusion
length is remarkably lower than the ingot average value.
As reference, we used single crystalline samples, cut from high-oxygen ([Oi]= 22 ppma)
boron doped (p
=1-2 Q cm) CZ-grown 4" diameter silicon wafers. As obtained by the
supplier, the wafers are dislocation free. These samples were then submitted to a dislocation
generation procedure which follows a practice established at the Laboratoire MATOP-CNRS
(Marseille). By this procedure [6] dislocation sources are nucleated by scratching the surface of the samples along the loll) direction with a diamond tip loaded with a 0.3 N weight (other
details are reported in [7]). The dislocation density, determined ma etch pit counting and
X-ray topography, is shown to vary from 10~-10~ cm~2 near the high stressed end of the scratch (maximum resolved shear stress 35 MPa) to 10~-10~ cm~2 near the low stressed end
(maximum resolved shear stress 4 MPa).
2.2. HYDROGENATION PROCEDURES. Many noble metals show catalytic properties for molecular hydrogen dissociation, including gold, platinum and palladium. In order to satisfy
both the chemistry and the physics of the hydrogen passivation process, however, the suitable catalyst should have a low diffusion coefficient in silicon, in order to avoid any electrical degra-
dation of the substrate associated to deep traps or recombination centers. For this reason, gold
must be rejected.
Moreover, as the dissociation of the dimer to atomic hydrogen can occur everywhere at the surface of the catalyst, H dissociatively chemisorbed at the catalyst surface should diffuse
quickly to the silicon-catalyst interface. Dissolution or retainment of hydrogen in the catalyst
could influence adversely the overall reaction kinetics. For these reasons, Pd must be rejected
in favor of Pt which presents the most favorable properties, including a very low diffusion depth
in silicon [8], at the hydrogenation process temperatures (T
=
300 °C, see below).
Pt films of various thickness were deposited on the silicon surface both by DC sputtering and by pyrolysing an aqueous solution of 7.72 x 10~3 M in (NH3)4Pt(N03)2 (9] at 250 °C, this last procedure being more suitable as creates a greater surface area for hydrogen dissociation.
Before the metal deposition the samples were chemically polished using a CP4 solution (HN03
HF
:CH3COOH) to remove the cutting damage. The duration of the chemical treatment was
long enough to remove a 10 pm thick layer.
The catalytic hydrogen dissociation process was then carried out at 300
°C, in a quartz tube, using a 5%H2/Ar mixture as the hydrogen carrier gas. The hydrogenation process lasts 2 h.
The furnace temperature and process time were chosen in order to have a temperature high enough to allow a fast diffusion of hydrogen in silicon, while keeping the hydrogen desorption
rate as low as possible, to minimize the diffusion of platinum in the bulk of the samples and to avoid the formation of the boron-hydrogen complexes which are stable up to 200
°C [10].
In order to test possible lifetime degradation effects associated with Pt-diffusion during
the heat treatment, some preliminary experiments were carried out at 300
°C, in a pure argon
atmosphere. The results of these experiments show that the electrical properties of the material
are practically unaffected by such treatment.
In order to compare the effect of catalytic hydrogenation with that obtained by using a plasma source, the same kinds of samples were exposed for 2 hours to a Radio-Frequency (13.56 MHz) driven hydrogen or deuterium plasma, using a Plasmalab system ii ii.
Before and after the hydrogenation process, all the samples were submitted to diffusion
length measurements using the SPV method, with a light spot of about IA x 4 mm2, in at least three different positions on each sample.
Before the SPV measurements and after the catalytic hydrogenation the Pt or land Pt sili- cides deposits eventually formed at the sample surface were removed by purple etch (10$lo HF: 48$loHN03~42%CH3COOH saturated with 12) at room temperature for 15 s [12].
As it has been demonstrated [13] that the effect of plasma hydrogenation is significantly
enhanced by a post-hydrogenation annealing at 450
°C, which induces the'diifusion of hydrogen,
accumulated in shallow subsurface region (about 0.4 pm) towards the entire volume of the
material, we applied systematically this post-hydrogenation step both to samples which were
hydrogenated by RF plasma or catalytically.
Eventually, for microstructural studies before and after the hydrogenation procedures, Scan-
ning Electron Microscopy (SEM) was used on samples anisotropically etched in a Yang solution
(IHF:I(1.5MCr03)) which delineates defects on (iii), (100) and (l10) silicon planes.
1490 JOURNAL DE PHYSIQUE III N°7
Table III SPV meas~rements on EFG samples. LDI, LD2 and LD3 are, respecti~ely, the average dijfwion length of the as-grown, catalytically treated and post-annealed samples. On each sample,
aminim~m of three meamrements in different position were carried o~t
Sample LDI (pm) LD2 (pm) LD3 (pm)
CS2 28 ~ 3 52 ~ 5 45 ~ 5
CG2 33~3 47~5 41~4
LS2 26~3 42~4 41~4
LG2 21~2 60~6 52~5
Table III. SPV meamrements on EFG samples. LDI, LD2 and LD3 are, respectively, the average dijfwion length of the as-grown, plasma treated and post-annealed samples. On each sample, a minim~m of three meamrements in different position were canted o~t.
Sample LDI (»m) LD2 (pm) LD3 (pm)
CSI 37 ~ 4 36 ~ 4 52 ~ 5
CGI 23~2 17~2 67~7
LSI 27~3 30~3 41~4
LGI 27~3 31~3 39~4
3. Experimental Results
The experimental results, as average values before and after the hydrogenation and post an- nealing processes are reported in Tables II to V.
It is apparent (see Tab. II) that the catalytic hydrogenation induces an increase of diffusion
length of the EFG samples which reaches values comparable with those obtained only after a 450
°C annealing of the RF plasma pre-hydrogenated samples (see Tab. III). The EFG catalyt- ically hydrogenated material does not show any improvement after a post-annealing process.
No significant improvement of the diffusion length is instead observed in the case of EU ma- terial, both in plasma and catalytically hydrogenated samples neither after the hydrogenation
nor after the post-hydrogenation annealing (see Tabs. IV and V).
In order to understand whether this different behavior would be associated to a different density of defects which can be decorated by Pt and then made preferential sites for dissociative
chemisorption, we have measured the dislocation density by the etch pit count technique on
the EFG material before and after the catalytic hydrogenation.
Table IV. SPV meas~rements on E~rosolare lateral top samples. LDI, LD2 and LD3 are, respectwely, the average dijfwion length of the as-grown, catalytically treated and post-annealed samples. On each sample, a minim~m of three meas~rements in different positions were carried
o~t.
Sample LDI (pm) LD2 (pm) LD3 (pm)
12dA 22 ~ 2 21 ~ 2 21 ~ 2
12dB 18~2 20~2 8fil
12dC 16~2 14~2 16~2
Table V. SPV meamrements on E~rosolare lateral top samples. LDI, LD2 and LD3 are, respectively, the average dijfwion length of the as-grown, plasma treated and post-annealed samples. On each sample,
aminim~m of three meas~rements in different positions were carried o~t.
Sample LDI (»m) LD2 (pm) LD3 (»m)
13dA 18 ~ 2 21 ~ 2 17 ~ 2
13dB 22 ~ 2 15 ~ 2 10 ~ l
13dC 19 ~ 2 17 ~ 2 11 ~ l
Fig. i SEM micrography of
anEFG as-grown sample (magnification 200x)
The results of these measurements, carried out on SEM (Scanning Electron Microscope) pictures (see Figs. 1, 2), after purple etch removal of the massive Pt deposits, show that after the catalytic hydrogenation the apparent number of etch pits is drastically reduced. As the hydrogenation temperature is too low to allow dislocation movement, one has to conclude that
some emerging dislocation segments are no more selective etching sites, due to Pt decoration.
Apparently, these decorated dislocations are preferential hydrogen diffusion paths, with Pt which behaves as localized dissociation center of the molecular hydrogen adsorbed on it.
4. Discussion and Conclusions
The prompt increase of the diffusion length in EFG samples after catalytic hydrogenation,
without the need of a subsequent thermal annealing, supports a preliminary conclusion that the dynamics of the catalytic process are dominated by the injection of atomic hydrogen directly in correspondence with Pt-decorated emerging dislocation segments. The need of
a post-hydrogenation anneal in the plasma hydrogenated samples, moreover, indicates that in this latter case the hydrogen is ubiquitously diffused in a thin subsurface layer during the
hydrogenation process, but that it is allowed to diffuse deep into the material by the high temperature post-hydrogenation stage. In both cases, however, dislocations should behave as
preferential pathways for hydrogen to hydrogen passivable defects.
1492 JOURNAL DE PHYSIQUE III N°7
Fig. 2. SEM micrography of an EFG sample submitted to catalytic hydrogenation (magnification
200x ).
6o
so A
~
40
E ~
~ 30 '~
~20 ,
lo Nd"10~ cm'~ Nd"10~ Nd"10~
0
o-o
Distance
from
bending
