Light-induced point defect reactions of residual iron in crystalline silicon
after aluminum gettering
D. Abdelbarey, V. Kveder, W. Schröter, and M. Seibt
a͒
IV. Physikalisches Institut der Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077
Göttingen, Germany
͑Received 25 June 2010; accepted 7 July 2010; published online 25 August 2010͒
Deep level transient spectroscopy is used to study light-induced reactions of residual iron impurities
after aluminum gettering ͑AlG͒ in crystalline silicon. White-light illumination at room temperature
leads to the formation of a defect which is associated with a donor level at 0.33 eV above the
valence band. This defect is stable up to about 175 ° C where it dissociates reversibly in case of
small iron concentrations and irreversibly for high iron concentrations. Since marker experiments
using gold and platinum diffusion show a high vacancy concentration after AlG a tentative
identification of the new defect as the metastable iron-vacancy pair is proposed. © 2010 American
Institute of Physics. ͓doi:10.1063/1.3474658͔
I. INTRODUCTION
Iron is one of the most frequently studied metal impuri-
ties in crystalline silicon due to its detrimental effects on the
material quality and the plethora of device failures associated
with various iron-related species.
1
Most prominent examples
of iron-related complexes are pairs with shallow acceptors
like B, Al, Ga, and In in p-type silicon materials.
2
Recently,
research activities in the silicon photovoltaics area have re-
newed the interest in defect reactions of iron in B-doped
crystalline silicon.
3
Further developments of injection-
dependent lifetime measurement techniques are now rou-
tinely used to measure concentrations of interstitial iron ͑Fe
i
͒
and photoluminescence mapping is utilized to image Fe
i
dis-
tributions in silicon wafers.
4,5
Various pairs with other point
defects have been reported depending on the defect popula-
tion of the material under investigation, as is exemplified by
FeAu pairs after Au–Fe codiffusion silicon
6
or FeH com-
plexes if H is introduced into the silicon by chemical
etching.
7
Due to detrimental effects of transition metal impurities,
like iron, on solar cells efficiency some gettering steps are
routinely included into silicon solar cells manufacturing.
Standard solar cell processing include two potential gettering
steps, i.e., the emitter diffusion ͑phosphorus-diffusion getter-
ing, PDG͒ and the backsurface field formation ͑aluminum
gettering, AlG͒ although the gettering capabilities are not
fully exploited in present-day solar cell processing schemes.
Both techniques base on an increased solubility of metal im-
purities in a thin surface-near gettering layer, i.e., in the
highly P-doped region for PDG and in the Al–Si melt for
AlG. An additional long-range beneficial effect of AlG in
connection with subsequent H passivation has been attrib-
uted to the injection of vacancies ͑V͒͑Refs. 8–11͒ which
enhances H diffusion
9
and promotes dissociation of molecu-
lar hydrogen.
12
Such synergetic effects have been concluded
to contribute to material improvement after AlG in various
studies.
13–16
From the behavior of intrinsic point defects dur-
ing silicon crystal growth
17
it can be concluded that vacan-
cies survive in complexes like V0, V0
2
,V
2
, and VH
n
after
AlG.
The reaction of vacancies with a typical minority carrier
life-time controlling impurity like Fe
i
or FeB pair in Si has
been studied by electron paramagnetic resonance ͑EPR͒ and
theoretical modeling.
18–20
For these experiments 1–3 MeV
electron irradiation has been used to generate vacancies,
which survive in a great variety of complexes, among
these—if the specimen contained Fe
i
or ͑FeB͒ before
irradiation—͑Fe
i
V͒ and ͑Fe
i
V
2
͒-pairs and possibly Fe
s
. Their
atomic structure, spin states, binding, and formation energies
have been determined experimentally or theoretically but
their levels within the silicon band gap could not be studied
experimentally because of the large variety of irradiation-
induced defects.
In this paper we report a detailed deep level transient
spectroscopy ͑DLTS͒ study of point defect reactions in iron
containing float-zone silicon after AlG. In iron contaminated
samples before and after AlG iron can be found exclusively
in pairs with boron ͑FeB pairs͒ as is expected in agreement
with previous data.
21,22
However, after AlG a white-light il-
lumination leads to the formation of an iron-related defect
associated with a deep state at E
V
+0.33 eV and subse-
quently referred to as the FeD defect. It is a donor as evi-
denced by the absence of the Poole–Frenkel effect which is
expected in case of an acceptor level. In addition, a small
DLTS line associated with the emission characteristics of the
divacancy ͑V
2
͒ appears.
The iron-related FeD defect is stable at room tempera-
ture and can be destroyed during annealing at about 175 °C
leaving the V
2
-defects unaffected in accord with literature
data.
23
Hence, our results provide evidence that AlG drasti-
cally changes the point defect population of crystalline sili-
con.
In an additional set of experiments, indiffusion of Pt or
Au into silicon after AlG has been used to monitor vacancy
concentrations. They consistently show strongly enhanced
concentrations of substitutional Pt and Au indicating vacancy
a͒
Electronic mail: seibt@ph4.physik.uni-goettingen.de.
JOURNAL OF APPLIED PHYSICS 108, 043519 ͑2010͒
0021-8979/2010/108͑4͒/043519/6/$30.00 © 2010 American Institute of Physics108, 043519-1