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: simoen@imec.be

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

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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).

References

1. E.E. Haller, P.P. Li, G.S. Hubbard, W.L. Hansen, IEEETrans. Nucl. Sci. NS-26, 265 (1979)

2. A.O. Evwaraye, R.N. Hall, T.J. Soltys, IEEE Trans. Nucl.Sci. NS-26, 271 (1979)

3. E. Simoen, P. Clauws, J. Broeckx, J. Vennik, M. Van Sande,L. De Laet, IEEE Trans. Nucl. Sci. NS-29, 789 (1982)

4. E.E. Haller, W.L. Hansen, F.S. Goulding, Adv. Phys. 30, 93(1981)

5. P. Clauws, E. Simoen, Mat. Sci. Semicond. Process. 9, 546(2006)

6. A. Satta, E. Simoen, T. Janssens, T. Clarysse, B. De Jaeger,A. Benedetti, I. Hoflijk, B. Brijs, M. Meuris, W. Vandervorst,J. Electrochem. Soc. 153, G229 (2006)

7. D.K. Schroder, IEEE Trans. Electron Devices 29, 1336(1982)

8. J.A. Burton, Physica 20, 845 (1954)9. W. Dunlap Jr., Prog. Semicond. 2, 167 (1957)

10. E.M. Conwell, Proc. I.R.E. 46, 1281 (1958)11. W.W. Tyler, J. Phys. Chem. Solids 8, 59 (1959)12. S.M. Sze, J.C. Irvin, Solid-St. Electron. 11, 599 (1968)13. H.H. Woodbury, W.W. Tyler, Phys. Rev. 105, 84 (1957)14. P. Clauws, G. Huylebroeck, E. Simoen, P. Vermaercke, F.

De Smet, J. Vennik, Semicond. Sci. Technol. 4, 910 (1989)15. F.X. Zach, H. Grimmeiss, E.E. Haller, Mater. Sci. Forum 83–

87, 245 (1992)16. W.W. Tyler, R. Newman, H.H. Woodbury, Phys. Rev. 98,

461 (1955)17. G. Huylebroeck, P. Clauws, E. Simoen, J. Vennik, Solid

State Commun. 82, 367 (1992)18. I.M. Kotina, V.V. Kuryatkov, S.R. Novikov, T.I. Pirozhkova,

Sov. Phys. Semicond. 21, 635 (1987)19. W.W. Tyler, R. Newman, H.H. Woodbury, Phys. Rev. 97,

669 (1955)20. S. Forment, J. Vanhellemont, P. Clauws, J. Van Steenbergen,

S. Sioncke, M. Meuris, E. Simoen, A. Theuwis, Mat. Sci.Semicond. Process 9, 559 (2006)

21. W.W. Tyler, H.H. Woodbury, Phys. Rev. 96, 874 (1954)22. E. Simoen, K. Opsomer, C. Claeys, K. Maex, C. Detavernier,

R.L. Van Meirhaeghe, S. Forment, P. Clauws, Appl. Phys.Lett. 88, 183506 (2006)

23. K. Opsomer, E. Simoen, C. Claeys, K. Maex, C. Detavernier,R.L. Van Meirhaeghe, S. Forment, P. Clauws, Mat. Sci.Semicond. Process. 9, 554 (2006)

24. E. Simoen, C. Claeys, Germanium-Based Technologies:From Materials to Devices, Ch. 5. Elsevier Ltd, London(2007)

25. G.K. Wertheim, Phys. Rev. 115, 37 (1959)26. B.H. Schulz, Philips Res. Repts. 16, 182 (1961)27. J.A. Burton, G.W. Hull, F.J. Morin, J.C. Severiens, J. Phys.

Chem. 57, 853 (1953)28. J.F. Battey, R.M. Baum, Phys. Rev. 100, 1634 (1955)29. J. Okada, J. Phys. Soc. Jpn. 12, 741 (1957)30. J. Okada, J. Phys. Soc. Jpn. 12, 1338 (1957)31. F.D. Rosi, RCA Rev. 19, 349 (1958)32. S. Sioncke, E. Simoen, T. Janssens, M. Meuris, P. Mertens, S.

Forment, P. Clauws, A. Theuwis, ECS Trans. 3(7), 1109(2006)

33. E. Simoen, C. Claeys, IEEE Trans. Electron Devices 46,1487 (1999)

34. A. Satta, G. Nicholas, E. Simoen, B. De Jaeger, J. VanSteenbergen, Mat. Sci. Semicond. Process 9, 716 (2006)

123

804 J Mater Sci: Mater Electron (2007) 18:799–804