Lifetime and leakage current considerations in metal-dopedgermanium
E. Simoen Æ C. Claeys Æ S. Sioncke ÆJ. Van Steenbergen Æ M. Meuris Æ S. Forment ÆJ. Vanhellemont Æ P. Clauws Æ A. Theuwis
Received: 6 September 2006 / Accepted: 26 December 2006 / Published online: 30 January 2007� Springer Science+Business Media, LLC 2007
Abstract This paper discusses the impact of transi-
tion metal (TM) impurities in germanium on the
electrical material and device characteristics, i.e., the
recombination and generation lifetime and the leak-
age current in a p–n junction. As will be shown, most
relevant data have been obtained in the fifties and
sixties from Hall effect and lifetime measurements,
while there is a lack of accurate Deep Level Transient
Spectroscopy (DLTS) results on metal-doped Ge. It
will be shown that the most efficient lifetime killers
are Fe, Ni and most likely Co, while Cu, which is the
most notorious contaminant, has little impact on the
minority carrier lifetime. Owing to the asymmetry in
the capture cross sections, n-type material will gener-
ally have a lower lifetime than p-Ge for the same
concentration of substitutional metal centers. Due to
the presence of near mid-gap acceptor levels, most
TMs will be efficient leakage current generators at
room temperature. However, the expected maximum
values for the leakage current density are one to two
decades smaller than what is typically found in
state-of-the-art ion-implanted shallow p–n junctions
in Ge.
1 Introduction
Since the early days of semiconductor processing, it is
clear that great care has to be taken during high-
temperature treatments in order to avoid inadvertent
contamination by fast diffusing metals. In the case of
germanium, copper is the most notorious contaminant
and also the most prominent residual impurity in high-
purity Czochralski-grown material [1–5]. Currently,
there is a strong revival in the materials’ study of
germanium, triggered by the interest of the microelec-
tronics community and based on its high low-field
mobility. One of the advantages of employing Ge for
nanoelectronic devices is the reduced thermal budget,
typically below 600 �C, required for the activation of
dopants [6]. At the same time, in modern CMOS
technologies several fast diffusing metals are being
implemented, either as a constituent of the high-jmetal oxides used as gate dielectric, or for the
formation of self-aligned, low-resistance germanides
on the source, drain and gate electrodes or in the back-
end metallization (copper...) Therefore, it is relevant to
estimate the effect of certain metals on the electrical
parameters, namely the minority carrier lifetime, s, and
the reverse current in the depletion region of a p–n
junction.
Paper to be published in the Proceedings of the 2nd CADRESWorkshop, Crete, 8–11 September, 2006, J. Materials Sci:Materials in Electron.
E. Simoen (&) � C. Claeys � S. Sioncke �J. Van Steenbergen � M. MeurisIMEC, Kapeldreef 75, Leuven 3001, Belgiume-mail: [email protected]
C. ClaeysESAT-INSYS, KU Leuven, Kasteelpark Arenberg 10,Heverlee 3001, Belgium
S. Forment � J. Vanhellemont � P. ClauwsDepartment of Solid State Sciences, Ghent University,Krijgslaan 281-S1, Gent 9000, Belgium
A. TheuwisUmicore, Watertorenstraat 33, Olen 2250, Belgium
J Mater Sci: Mater Electron (2007) 18:799–804
DOI 10.1007/s10854-006-9110-7
123
The minority carrier lifetime of a deep-level point
defect with concentration NT is defined by
sn = [NTrnvthn]�1(p � Ge) ð1Þ
sp = [NTrpvthp]�1(n � Ge) ð2Þ
and is essential for the operation of Ge solar cells for
example, as it sets the diffusion length of minority
carriers. In Eqs. (1), (2) rn, rp is the capture cross
section for electrons and holes of the recombination
center with energy level ET in the band gap and vthn,
vthp is the thermal velocity for electrons and holes,
respectively. It is clear from Eqs. (1) and (2) that at low
injection level, the carrier lifetime in n- or p-type
material is determined by the deep-level concentration
and the capture cross section for the respective
minority carriers. The latter parameters determine
the capture rates given by:
cn = rn vthn ð3Þ
cp = rp vthp ð4Þ
While the precise energy position of a recombina-
tion center is generally not so crucial, it is when it
comes down to carrier generation. From the expression
of the generation lifetime [7]:
sgen = 2srec cosh(ET � Ei/kT) ð5Þ
one can clearly derive that deep levels with activa-
tion energy close to the intrinsic level Ei (�EG/2 with
EG the band gap energy) are very efficient generation
centers. In Eq. (5) srec is the recombination lifetime
given by [1/sn + 1/sp]–1. It is also obvious that in
general sgen „ 2srec, unless ET = Ei. We have further
that k is Boltzmann’s constant and T the absolute
temperature.
It is the aim of the paper to discuss the deep level
parameters for a number of transition metals (TMs) in
germanium, from Mn up to Cu as they are commonly
present in an industrial processing environment. First,
we will have a look on what information is available on
the energy levels of the respective substitutional metal
centers, followed by an inventory of the capture cross
sections and related minority carrier lifetime. Finally,
an attempt is made to estimate the impact of substi-
tutional TMs on the leakage current of a Ge p–n
junction.
2 Activation energy of TMs in Ge
Early investigations of metal-doped Ge were relying
on Hall effect measurements versus temperature T to
derive the nature (i.e., donor or acceptor) and position
of the deep levels associated with impurity atoms of
transition metals in Ge. A summary of the old results
can be found in Refs. [8–12]. It turns out that most TMs
in Ge occupy a lattice site (they are substitutional)
giving rise to one or more deep acceptor states in the
band gap. The number of levels corresponds with the
charge configuration in the outer shells of the respec-
tive species, whereby the tendency exists to satisfy all
the bonds with the four nearest neighbor Ge atoms. A
summary of the levels found by Hall effect is given in
Table 1.
Since the introduction of deep-level transient spec-
troscopy (DLTS), the technique has also been applied
to the study of metal-related impurities in germanium
[1–3]. The resulting value of the activation energy
(EA), derived from an Arrhenius plot for the selected
TMs is also given in Table 1. From Table 1, a few clear
conclusions can be drawn. In contrast to the case of
silicon, the deep levels of most of the TMs in Ge have
not been well characterized by DLTS. This is partly
due to the difficulty in preparing samples with a
sufficiently high concentration. However, recent exper-
iments with TM ion-implanted germanium show prom-
ising results: a short anneal at 500 �C seems to be
sufficient to remove most of the implantation damage
and to activate only the metal in question in detectable
concentrations, without cross contamination with other
metals like Cu or Ni [20].
Table 1 Energy levels derived from Hall effect and DLTS forsome of the technologically relevant TMs in Ge. Also included isthe activation energy of the majority capture cross section, i.e., rn
for electron traps (negative energy, defined with respect to thebottom of the conduction band EC) and rp for hole traps (posi-tive energy, defined with respect to the top of the valence bandEV)
Charge state EA Hall (eV) EA DLTS (eV) Er DLTS (eV)
Cus2–/3– –0.26 [13] –0.324 [14] 0.065[14]
Cus–/2– +0.32 [13] +0.318[15] –0.015 [15]
Cus0/– +0.04 [13]* +0.038[15] –0.0033 [15]
Nis–/2– –0.30 [16] –0.30 [17] –
Nis0/– +0.22 [16] +0.217 [17] –
+0.23 [18]Cos
–2– –0.31 [19] –0.33 [20] –Cos
0/– +0.25 [19] – –Fes
–/2– –0.27 [21] –0.36 [20] –Fes
0/– +0.34 [21] – –
* a value of 0.043 eV has been derived from piezospectroscopicmeasurements
123
800 J Mater Sci: Mater Electron (2007) 18:799–804
Another promising route for doping with certain
metals is by a low-temperature germanidation treat-
ment [22, 23]. An example is given in Fig. 1, repre-
senting the DLT-spectrum of a Co-germanide Schottky
barrier on n-type Ge. Germanidation was achieved by
sputtering of 30 nm of Co, followed by a Rapid
Thermal Anneal (RTA) step at 650 �C. Two electron
traps are observed, whereby the EC–0.33 eV peak can
be identified as the second acceptor state of substitu-
tional Co (Cos). EC is the bottom of the conduction
band. The deeper peak at EC–0.35 eV has been
identified as belonging to the third acceptor state of
Cus. As shown in Fig. 1, it is the only peak found in a
Cu-germanided n-type sample, for a RTA temperature
of 600 �C. This result demonstrates that metals can be
in-diffused during a low thermal budget heat treat-
ment, but at the same time, cross-contamination with
fast diffusers like Cu and Ni is hard to avoid for
treatments above ~600 �C.
Another trend, which becomes clear from Table 1 is
that the Hall effect measurements not necessarily
match the DLTS results. As pointed out by Kotina
et al. [18] this difference can be ascribed to two factors:
one, DLTS is generally applied at a non-negligible
electric field in the depletion region, which may
influence the activation energy and, second, the acti-
vation energy EA derived from an Arrhenius plot in
DLTS is affected by the temperature dependence of
the capture cross section. In many instances, this last
factor will dominate, as also illustrated by the results of
Table 1 for the three Cus acceptor levels [14, 15].
Subtracting the activation energy for the relevant
capture cross section (Er) from EA yields a satisfactory
agreement with the Hall effect data. Er is defined as
follows:
rn;p = rn;p0 exp(� Er/kT) ð6Þ
with rn,p0 the capture cross section extrapolated to
infinite temperature. Note that Er is positive for the
third Cus level, meaning that the electron capture cross
section for the repulsive Cus2– center increases with
temperature. The opposite holds for the two acceptor
levels in p-type Ge: a negative Er is obtained,
indicating that the hole capture cross section for the
attractive charge states lowers with increasing T.
It is clear from Table 1 that for most metals even the
trap level positions are not yet clearly identified in
DLTS. Different values for the ionization energies can
be found in the literature. However, assuming that the
Hall data provide a reliable estimate of the true
activation energy for the double acceptor levels in the
upper half of the band gap, one can anticipate for Er a
negligible value for Nis, while Er is estimated ~20 meV
for Cos and ~90 meV for Fes. This points to an
increasingly repulsive character of the singly negative
charge state going from Ni to Fe (lower atomic mass).
The rather high value for the third Cus level supports
this idea, since the center is in a doubly negative charge
state, strongly repulsive for the capture of a third
electron.
For a more extensive recent update of the properties
of metal impurities in Ge, the Reader is referred to
[5, 20, 24].
3 Minority carrier lifetime
Filling in a thermal velocity vth~2–3 � 107 cm/s in Eqs.
(3), (4) results in the room temperature values of Fig. 2
(p-Ge) and 3 (n-Ge). It is obvious from Figs. 2 and 3
that there are several capture rates for the different
TMs owing to the fact that they can have different
charge states—they act as multiple acceptors. Accord-
ing to Eqs. (1) and (2), a higher capture rate means a
lower minority carrier lifetime.
A few conclusions can be drawn: in both cases, Fe
and Ni (in p-Ge also Co) are the most efficient lifetime
killers and higher lifetimes may be achieved in p- than
in n-type Ge at 300 K, for the same TM concentration.
For example, for a Ni concentration of 1012 cm–3, one
achieves a low-injection lifetime of 16 ls in p-Ge and
of 2.5 ls in n-Ge. This is related to the difference in
electron and hole capture cross section. Higher hole
capture cross sections are generally observed since
-4 10-2
0 100
4 10-2
8 10-2
1.2 10-1
50 100 150 200 250
30 nm Co or Cu on n-type Ge
DL
TS
b1
(pF
)
Temperature (K)
tw=51.2 ms
tp=5/10 ms
-4-->0 V
Co 650oC
Cu 600oC
EC-0.33 eV E
C-0.35 eV
Fig. 1 DLT-spectra for a 650 �C annealed Co- and a 600 �Cannealed Cu-germanide Schottky barrier on n-type Cz Ge from–4 fi 0 V. The pulse duration tp = 5 (Co) or 10 (Cu) ms andthe period is tw = 51.2 ms
123
J Mater Sci: Mater Electron (2007) 18:799–804 801
most of the TMs give only rise to deep (multiple)
acceptor levels, which are attractive for holes and
repulsive for electrons. This is generally in good
agreement with lifetime measurements on nickel-
doped n- and p-type Ge [25–32].
Some additional comments regard the assignment of
the high capture rate of Ni in p-Ge to the singly
negative charge state (cn- ). This seems in contradiction
to the overall trend and requires a further in-depth
analysis of the published data. It is more likely
belonging to the neutral state (cn0). No data for hole
capture by substitutional Co in n-Ge is available in the
literature (Fig. 3) although it could correspond to an
efficient recombination center as well.
In summary, the most efficient lifetime killers in Ge
are expected to be Fe, Co and Ni, while Cu has only a
modest effect on the lifetime. This trend is confirmed
by microwave absorption lifetime measurements on
heat treated Ge, suffering from contamination in the
furnace [32]. This is illustrated by Fig. 4. The micro-
wave photoconductivity method (Semilab WT-85 Life-
time Scanner) is used to determine carrier lifetimes in
the Ge bulk. The wafer pieces were passivated with a
polymer on both sides in order to suppress surface
recombination. Measurements have been performed
on Ni and Ge implanted n-type Ge samples before
(labeled no-) and after (labeled a-) a 700 �C 1 h anneal
under N2 atmosphere. Both a low (low) and a 10 times
higher dose (high) have been employed. Further
details are published elsewhere [32]. The Minority
Carrier LifeTime (MCLT) of the as-implanted samples
is in all cases dominated by the amorphization damage
close to the surface, yielding a typical value of ~50 ls.
A 1 h anneal at 700 �C completely removes the
implantation damage in the case of the Ge-implanted
material, giving rise to a carrier lifetime close to the
value of the starting material. Nevertheless, DLTS
reveals the presence of a significant Cus2–/3– contami-
nation peak [32]. The annealed Ni-implanted samples
exhibit only nickel-related electron traps [32], produc-
ing a strong reduction in the lifetime from 50 to 1 ls
and corresponding with a DLTS concentration of a few
1012 cm–3. This underlines the difference in lifetime
killing ability between Cus and Nis in n-type Ge.
4 Leakage current
In order to estimate the impact of TMs in Ge on the
carrier generation lifetime sgen, the parameters of
Table 2 have been used. As a further simplification, for
each metal only the level with activation energy ET
10-10
10-9
10-8
10-7
24 25 26 27 28 29 30
p-type Ge
cn0
cn-cn2-
Ele
ctro
n C
aptu
re R
ate
(cm
3 s-1
)
Atomic Number
vthn
=3.1x107 cm/s
T=300 K
Mn Fe Co
Ni
Cu
Fig. 2 Electron capture rate at 300 K for transition metals inp-type germanium. The charge state of the empty substitutionalmetal centers is either neutral (s), single (¤) or double negative(x)
10-9
10-8
10-7
10-6
24 25 26 27 28 29 30
n-type Ge
cp-cp2-cp3-
Ho
le C
aptu
re R
ate
(cm
3 s-1
)
Atomic Number
vthp
=1.9x107 cm/s
T=300 K
Mn CuNiCoFe
Fig. 3 Hole capture rate at 300 K for transition metals in n-typegermanium. The charge state of the empty substitutional metalcenters is either single (s), double (¤) or triple negative (x)
100
101
102
103
no-low no-high a-low a-high no-h a-low a-high
n-type Ge
MC
LT
( s
)
Sample
Ni implanted
Ge implanted
µ
Fig. 4 Minority carrier lifetime for n-type Cz Ge samplesimplanted with Ni or Ge at two different doses indicated bylow and high. As-implanted (no) data are compared with thelifetime after a 700 �C 1 h anneal under N2 atmosphere (a)
123
802 J Mater Sci: Mater Electron (2007) 18:799–804
closest to mid gap is considered, as this is the most
efficient for carrier generation. This follows from the
cosh(ET – Ei) term in Table 2 and Eq. (5). Moreover,
the leakage current per TM atom is calculated starting
from the Shockley-Read-Hall expression for the
reverse current density of a p–n junction.
Jgen = qni W/sgen ð7Þ
with q the electron charge, ni is the intrinsic carrier
concentration (~2 � 1013 cm–3 at 300 K for Ge) and W
the depletion width. In Eq. (7) no electric field
dependence is taken into account. This could arise
from:
– Poole Frenkel lowering of the emission barrier for
attractive generation centers
– trap-assisted tunneling
– a combination of these effects
This will become important for higher substrate
doping levels (p–n junction in a well or a highly doped
substrate) for fields starting from ~105 V/cm approxi-
mately. In practice, this will lead to a stronger bias
dependence of the generation current density than
predicted by Eq. (7) (~V1/2). In the case of junctions in
a silicon p-well, one regularly finds either a power law
(Vn with 3 < n < 5) or even an exponential depen-
dence (I ~ exp(dV)). For the rest of the analysis, we
assume no field-enhancement, which could be valid for
lightly doped Ge substrates (~1014 cm–3).
The deep level information is lumped into the
generation lifetime sgen. For the latter, an approach is
followed developed by Schroder [7] and further refined
in [33]. Neglecting the effect of the majority capture
cross section one arrives at Eq. (5) [7, 33].
An estimate of the leakage current density, based on
the above formulae has been derived for the transition
metals in Ge. They can in fact be considered as typical
examples of deep levels in Ge and provide an idea
about the range of expected generation currents. In the
calculation, the parameters of Table 2 have been used
for the minority carrier capture rate cmin, based on
literature data for the respective capture cross sections.
The leakage current per defect at 300 K is calculated
from:
Idefect ¼ qnicmin=2cosh½ðET � EiÞ=kT� ðin AÞ ð8Þ
and is represented in Fig. 5. Fe and Ni stand out as
the most efficient leakage current generators. This
mainly relates to the high cmin (high electron capture
cross section).
In Fig. 6, we have tried to estimate the leakage
current density per lm depletion width for the differ-
ent transition metals. This is given by:
Jgen ¼ Idefect � NT � 1 lm ðA=cm2Þ ð9Þ
Table 2 Parameters used to calculate the leakage currentdensity per metal atom
Atomic number Element cmin
(cm3s–1)ET (eV) cosh
(ET–Ei)
25 Mn 3.1�10–9 0.37 2.0526 Fe 3.1�10–8 0.34 1.0227 Co 3.1�10–9 0.31 1.528 Ni 6.2�10–8 0.30 2.0529 Cu 3.1�10–10 0.32 1.1347 Ag 3.1�10–9 0.302 1.9279 Au 6.2�10–9 0.215 50.5
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
1011
1012
1013
1014
1015
1016
1017
Germanium at 300 K
Lea
kag
e C
urr
ent
Den
sity
per
m D
eple
tio
n (A
/cm
2 )
Defect Concentration (cm-3)
CuMn
Co
Ni; Fe
µ
Fig. 6 Leakage current per lm depletion width at 300 K fordifferent transition metals in function of the metal concentration
10-16
10-15
10-14
24 25 26 27 28 29 30
Germanium Junctions
Lea
kag
e C
urr
ent
per
Def
ect
(A)
Atomic Number
Mn
Fe
Co
Ni
Cu
Fig. 5 Leakage current generated per metal atom in germanium,calculated for the level closest to mid gap and with the capturerates of Fig. 1 or 2
123
J Mater Sci: Mater Electron (2007) 18:799–804 803
The true leakage current density then follows from
multiplying Eq. (9) by W in lm. In this way, we obtain
a Jgen that is independent of the doping density of the
substrate.
If we assume a limit of 10–3 A/cm2, which is found
typically for state-of-the-art ion-implanted p–n junc-
tions in Ge (assuming a depletion depth of 1 lm) [34],
we see that this value is reached for Ni and Fe
concentrations in the range 1015 cm–3. For the other
metals, even higher concentrations well above the
solubility limit have to be introduced. One can derive
from this that the trap densities in our ion implanted p–
n junctions should be at least of this order of
magnitude, neglecting possible electric field effects.
This should be easily observed by DLTS for example.
In practice, annealing at 500–600 �C removes most of
the implantation damage, as already mentioned above,
so that the leakage current is dominated by other than
simple point defects, e.g., extended defects or surface-
states at the Ge-dielectric interface surrounding the
p–n junction [34].
5 Conclusions
Similar as for the case of silicon, TMs in Ge can
determine the lifetime properties of the material. The
main difference is that most TMs in Ge occupy
predominantly substitutional sites, giving rise to multi-
ple deep acceptor levels. As a consequence, the
minority carrier lifetime in p-type material, governed
by electron capture, will be higher than the minority
carrier lifetime in n-type material. Overall, there is a
lack of accurate DLTS characterization of many of the
technologically relevant metals, like Fe, Cr, Hf, etc.
Acknowledgements Part of this work has been performed inthe frame of the IWT Project GEWEL (IWT030516).
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