Effective electrical passivation of Ge(100) for high- k gate dielectric layers using germanium oxide

Article abstract of DOI:10.1063/1.2773759

In search of a proper passivation for high- k Ge metal-oxide-semiconductor devices, the authors have deposited high- k dielectric layers on Ge O2, grown at 350-450 °C in O2. Zr O2, Hf O2, and Al2 O3 were deposited by atomic layer deposition (ALD). Ge O2 and Zr O2 or Hf O2 intermix during ALD, together with partial reduction of Ge4+. Almost no intermixing or reduction occurs during Al2 O3 ALD. Capacitors show well-behaved capacitance-voltage characteristics on both n - and p-Ge, indicating efficient passivation of the GeGe Ox interface. The density of interface states is typically in the low to mid- 1011 cm-2 eV-1 range, approaching state-of-the-art SiHf O2 matal gate devices.

Full text of DOI:10.1063/1.2773759

Effective electrical passivation of Ge(100) for high- k gate dielectric layers using
germanium oxide
Annelies Delabie, Florence Bellenger, Michel Houssa, Thierry Conard, Sven Van Elshocht, Matty Caymax, Marc
Heyns, and Marc Meuris
Citation: Applied Physics Letters 91, 082904 (2007); doi: 10.1063/1.2773759
View online: http://dx.doi.org/10.1063/1.2773759
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/8?ver=pdfcov
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APPLIED PHYSICS LETTERS 91, 082904 ͑2007͒
Effective electrical passivation of Ge„100… for high-k gate dielectric layers
using germanium oxide
a͒
b͒
b͒
Annelies Delabie, Florence Bellenger, Michel Houssa, Thierry Conard,
Sven Van Elshocht, Matty Caymax, Marc Heyns, and Marc Meuris
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
b͒
͑
Received 30 March 2007; accepted 29 July 2007; published online 22 August 2007͒
In search of a proper passivation for high-k Ge metal-oxide-semiconductor devices, the authors have
deposited high-k dielectric layers on GeO , grown at 350–450 °C in O . ZrO , HfO , and Al O
2
2
2
2
2
3
were deposited by atomic layer deposition ͑ALD͒. GeO and ZrO or HfO intermix during ALD,
2
2
2
4
+
together with partial reduction of Ge . Almost no intermixing or reduction occurs during Al O
2
3
ALD. Capacitors show well-behaved capacitance-voltage characteristics on both n- and p-Ge,
indicating efficient passivation of the Ge/GeO interface. The density of interface states is typically
x
in the low to mid-1011 cm eV
−2
−1
range, approaching state-of-the-art Si/HfO /matal gate
2
devices. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2773759͔
Si/SiO based transistors are unable to accommodate the
deposited at 300 °C in the ALD reactor from ZrCl , HfCl ,
4 4
2
severe scaling requirements for sub-45 nm complementary
metal oxide semiconductor ͑MOS͒ devices. To further im-
prove performance, alternative materials are considered. Ger-
manium is introduced as a channel material due to its higher
electron and hole mobility as compared to silicon. As alter-
native gate dielectric layers, high-k materials show reduced
or Al͑CH ͒ with H O. Some samples were annealed in N
3 3 2 2
at 550 °C for 5 min to simulate the thermal budget of device
processing. X-ray photoelectron spectroscopy ͑XPS͒ was
performed on a Theta300 system ͑Thermo Instruments͒ in
parallel angle-resolved mode with monochromatized Al Ka
radiation. Analysis was based on the Ge 3d or Ge 3p peaks.
Thicknesses were calculated assuming a two- or three-layer
tunneling currents at an equivalent SiO thickness. About
2
model ͑Ge/GeO /high-k͒. Air exposure between sample
five years ago, the successful combination of a high-k dielec-
2
1
preparation and XPS analysis was limited to 15 min or less.
tric film in Ge-based transistors was demonstrated. Today, it
2
The areal density of Hf ͑Hf/nm ͒ on GeO was measured by
is, however, still not straightforward to get a sufficiently low
2
2
,3
rutherford back scattering ͑RBS͒ in a RBS400 end station
with a 1 MeV He beam. Electrical measurements were per-
interface state density at the Ge/high-k interface. Also, the
+
Ge/GeO interface is generally considered to be more defec-
2
formed on MOS capacitors with TiN gates. The TiN gate was
deposited ex situ by physical vapor deposition. Gate pattern-
ing was done by lithography, followed by resist strip, TiN
etching, and dielectric removal. The devices were annealed
tive than the Si/SiO interface, although reports are scarce.
2
Currently, more effort is being made to investigate and re-
4
,5
duce the interface state density of the Ge/GeO interface.
2
In this work, we have investigated thermally grown
at 550 °C in N to activate dopants. The electrical character-
GeO as passivation layer for high-k layers deposited by
2
2
istics were measured with a Keithley K4200 semiconductor
parameter analyzer and an HP4284 LCR meter.
atomic layer deposition ͑ALD͒. The electrical quality of
GeO can be affected by air exposure and thermal process
2
GeO is grown when Ge͑100͒ is oxidized at low tem-
2
steps because GeO is hygroscopic and thermally unstable
2
perature, as evidenced by the XPS Ge 3d spectra ͓Fig. 1͑a͔͒.
͑
GeO desorbs at 420 °C in N or vacuum͒. Therefore, we
2
have avoided air exposure between surface preparation, oxi-
dation, and ALD. Germanium oxidation and high-k deposi-
tion were performed at low temperature to avoid decompo-
sition of GeO and desorption of GeO. Efficient electrical
2
passivation of the Ge/GeO interface will be demonstrated.
x
Ge͑100͒ substrates were cleaned in HF solution. The
subsequent surface preparation, germanium oxidation, and
high-k deposition were performed in a Polygon® 8200
6
cluster without air breaks between these process steps ͑un-
less where mentioned otherwise͒. The Polygon cluster is
equipped with an EPSILON™ reactor for thermal treatments
and an ALCVD™ PULSAR® 2000 reactor. The samples
were first annealed in H at 650 °C for 10 min to desorb O
2
7
or C that can be present after HF cleaning. Oxidations were
subsequently performed in the same reactor in O2 at
7
00 Torr at 350 or 450 °C. ZrO , HfO , and Al O were
2 2 2 3
a͒
Electronic mail: annelies.delabie@imec.be
Also at Department of Electrical Engineering, K.U. Leuven, Celestijnen-
laan 200F, B-3001 Leuven, Belgium.
FIG. 1. XPS Ge 3d spectra for ͑a͒ germanium oxide grown at different
b͒
temperatures and times and ͑b͒ 1.3 nm GeO ͑450 °C, 5 min͒ covered with
2
2 nm Al O with and without post deposition anneal ͑PDA͒ ͑N 550 °C͒.
2
3
2
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003-6951/2007/91͑8͒/082904/3/$23.00 91, 082904-1 © 2007 American Institute of Physics
31.111.164.128 On: Sun, 21 Dec 2014 02:48:26
0
1

082904-2
Delabie et al.
Appl. Phys. Lett. 91, 082904 ͑2007͒
FIG. 2. XPS Ge 3p spectra of 1.3 nm GeO2 ͑450 °C, 5 min͒ before and
after 2 nm HfO and ZrO ALD. ͑b͒ Growth per cycle ͑GPC͒ as a function
2
2
of the number of cycles for HfO and ZrO ALD on 1.3 nm GeO ͑450 °C,
2
2
2
5
min͒ and 0.3 nm GeO ͑HF cleaned Ge͒ and O-free Ge.
x
The difference in binding energy of the oxide and substrate
4
+ 8
peaks is 3.3 eV, indicating Ge . No significant components
of lower oxidation states are detected. Oxidation at 450 °C
FIG. 3. Capacitance-voltage characteristics of ͑a͒ p-Ge/GeO /HfO and ͑b͒
x
2
p-Ge/GeO /Al O gate stacks ͑1.3 nm GeO was grown at 450 °C during
x
2
3
2
yields 1.3 and 2.3 nm of GeO after 5 and 18 min, respec-
5 min͒ measured at 100 Hz, 10 kHz, and 1 MHz. The inset in ͑a͒ shows the
2
normalized conductance as a function of frequency at V =0.65 V.
tively, in agreement with oxidation studies in similar condi-
tions reported previously. At 350 °C, 1.1 nm of GeO was
G
9
2
grown in 60 min.
surface. Intermixing could occur by this mechanism if Ge is
included in the volatile hafnium hydroxychloride intermedi-
ate that readsorbs on the surface. Very little or no intermixing
During Al O ALD on GeO , the main oxidation state of
2
3
2
4
+
Ge remains Ge , as indicated by the Ge 3d spectrum ͓Fig.
1͑b͔͒. The binding energy shift of the oxidized Ge 3d peak
with Ge was observed when HfO is deposited directly on
2
remains 3 eV, and the contributions of Ge in oxidation states
lower than 4+ are small. The reconstructed composition
depth profile from angle-resolved XPS intensities indicates
Ge͑100͒ but intermixing occurred during O annealing at
2
1
4
5
00 °C.
As germanium oxide is hygroscopic, we have investi-
gated the effect of air exposure between Ge oxidation and
sharp interfaces between Al O and GeO ͑not shown͒. Post-
deposition annealing in N at 550 °C results in a slight in-
crease of the GeO component in the Ge 3d spectrum. The
main oxidation state of Ge remains 4+, although a small shift
2
3
2
2
ALD. When GeO is exposed to air, the intermixing between
2
2
GeO and HfO or ZrO also occurs, while the interface with
2
2
2
Al O remains sharp.
2
3
of the oxide component is observed ͓Fig. 1͑b͔͒.
The frequency dependent capacitance-voltage ͑C-V͒
characteristics of p-Ge/GeO /HfO and p-Ge/GeO /Al O
3
During ALD of HfO or ZrO on GeO , the GeO and
2
2
2
2
x
2
x
2
HfO2 or ZrO2 layers partly intermix. Unfortunately, the
Ge 3d peak is located at a binding energy very close to O 2s
gate stacks are shown in Figs. 3͑a͒ and 3͑b͒, respectively.
Well-behaved C-V characteristics are observed for both gate
stacks with minimal frequency dispersion, minimal stretch
out, and the absence of bumps near the flatband voltage. This
indicates the efficient electrical passivation of the Ge/GeOx
interface obtained for high-k gate dielectric ALD on ther-
mally grown GeO2 ͑air exposure between oxidation and
ALD was avoided͒. Similar results were observed on n-type
Ge substrates ͑not shown͒. The density of interface states,
estimated from conductance measurements as a function of
͑
from HfO , 32.5 eV͒ and Zr 4p ͑27.1–28.5 eV͒ and was,
2
1
0
therefore, not analyzed. Ge 3p has no overlap with HfO or
2
ZrO2 peaks and indicates a binding energy shift of the
Ge 3p1/2 oxide component of 0.4 eV with respect to samples
without high-k dielectric. The reconstructed composition
depth profiles from angle-resolved XPS indicate partial inter-
mixing of GeO and HfO or ZrO ͑not shown͒.
2
2
2
Intermixing has been observed previously for GeON and
HfO deposited by ALD, either from HfCl /H O at 350 °C
1
1
−2
−1
2
4
2
frequency ͓inset of Fig. 3͑a͔͒, is about 3ϫ10 cm eV ,
1
1,12
or from hafnium-alkylamide/H O at 150 °C.
approaching state-of-the-art Si/SiO /HfO /metal gate stacks
2
x
2
1
0
−2
−1
It has been reported previously that GeO surfaces en-
͑typically mid to high 10 cm eV ͒.
x
hance the growth per cycle in the first reaction cycle of the
The equivalent oxide thickness ͑EOT͒, extracted from
the fits of the C-V characteristics with a Ge-based simulator,
is about 1.5 and 2.4 nm for the GeO /HfO and GeO /Al O
1
3
HfO ALD. The growth enhancement also occurs for HfO2
2
and ZrO ALD on thermally grown GeO ͓Fig. 2͑b͔͒. In the
2
2
x
2
x
2
3
2
2
first reaction cycle, 6.5 Hf/nm and 12.8 Zr/nm are depos-
gate stacks, respectively. This leads to an EOT contribution
ited on GeO . The reaction mechanism proposed to account
of about 0.7–0.8 nm for 1.3 nm GeO , assuming EOT con-
2
x
for the growth enhancement was an agglomeration mecha-
tributions of 0.8 nm for 4 nm HfO and 1.6 nm for 4 nm
Al O . The relative dielectric constant of GeO is thus 6–7,
2 3 x
2
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nism in which HfCl exchanges Cl with OH from the starting
4
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082904-3
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Appl. Phys. Lett. 91, 082904 ͑2007͒
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FB
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͑ϳ200 mV͒ and GeO /HfO2 stacks
x
2
3
x
͑
ϳ900 mV͒ is observed when the gate voltage is swept from
1
inversion to accumulation and back to inversion. We point
out that the maximum gate voltage reached in accumulation
during the C-V measurement corresponds to the same elec-
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C-V hysteresis observed for these devices, more specifically
for GeO /HfO stacks, is caused by bulk oxide traps and is
not related to the passivation of the Ge interface. Partial in-
termixing of GeO and HfO , as shown by XPS, could create
more bulk oxide traps ͑slow states͒, leading to a larger C-V
hysteresis for HfO as compared to Al O gate stacks. The
choice of the high-k material, its deposition process, and/or
device processing have to be investigated further to reduce
the hysteresis.
To conclude, many studies have focused on the passiva-
tion of Ge with elements other than oxygen ͑N, Si, S͒,
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6–18
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as
2
In this work, we have shown that thermally grown GeO can
2
1
1
5
6
act as a good electrical passivation layer for high-k dielec-
trics deposited by means of ALD. Downscaling the GeO2
thickness while maintaining good passivation properties will
be the subject of further investigation.
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