Local epitaxial growth of ZrO2 on Ge (100) substrates by atomic layer epitaxy

Article abstract of DOI:10.1063/1.1613031

A study was performed on local epitaxial growth of ZrO₂ on Ge (100) substrates by atomic layer epitaxy. Due to the high density of interface states, large hysteresis and high frequency dispersion were observed in capacitance-voltage measurements. A low leakage current density, comparable to values obtained on Si substrates, was observed with the same capacitance density.

Full text of DOI:10.1063/1.1613031

Local epitaxial growth of ZrO 2 on Ge (100) substrates by atomic layer epitaxy
Hyoungsub Kim, Chi On Chui, Krishna C. Saraswat, and Paul C. McIntyre
Citation: Applied Physics Letters 83, 2647 (2003); doi: 10.1063/1.1613031
View online: http://dx.doi.org/10.1063/1.1613031
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 13
29 SEPTEMBER 2003
Local epitaxial growth of ZrO on Ge „100… substrates
2
by atomic layer epitaxy
Hyoungsub Kima)
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Chi On Chui and Krishna C. Saraswat
Department of Electrical Engineering, Stanford University, Stanford, California 94305
Paul C. McIntyre
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
͑
Received 9 June 2003; accepted 28 July 2003͒
High-k dielectric deposition processes for gate dielectric preparation on Si surfaces usually result in
the unavoidable and uncontrolled formation of a thin interfacial oxide layer. Atomic layer deposition
of ϳ55-Å ZrO film on a Ge ͑100͒ substrate using ZrCl and H O at 300 °C was found to produce
2
4
2
local epitaxial growth ͓͑001͒ Ge//͑001͒ ZrO2 and ͓100͔ Ge//͓100͔ ZrO ] without a distinct
2
interfacial layer, unlike the situation observed when ZrO is deposited using the same method on Si.
2
Relatively large lattice mismatch ͑ϳ10%͒ between ZrO and Ge produced a high areal density of
2
interfacial misfit dislocations. Large hysteresis ͑Ͼ200 mV͒ and high frequency dispersion were
observed in capacitance–voltage measurements due to the high density of interface states. However,
a low leakage current density, comparable to values obtained on Si substrates, was observed with the
same capacitance density regardless of the high defect density. © 2003 American Institute of
Physics. ͓DOI: 10.1063/1.1613031͔
As the continued scaling of Si complementary metal-
oxide-semiconductor ͑MOS͒ devices approaches its funda-
mental limits, various methods are being investigated to in-
crease the saturation current by improving carrier mobility in
the channel region. Improvements of carrier mobility have
been obtained by replacing Si channel with strained Si;1
however, a major breakthrough may be achieved if the con-
ventional Si substrate is replaced by alternative semiconduc-
tor materials, such as Ge, which have high intrinsic carrier
mobilities. Because of its higher low-field carrier mobility
and smaller mobility band gap for supply voltage scaling,
there have been many attempts to use Ge as a channel ma-
produces a low-k SiO -containing layer. Microstructural and
2
electrical properties of ALD ZrO on Ge were investigated
2
using transmission electron microscopy ͑TEM͒, x-ray photo-
electron spectroscopy ͑XPS͒, and basic MOS electrical
characterizations.
As a substrate, 4-in.-diameter n-type ͗100͘ Ge wafers
having 0.25 ͑⍀ cm͒ resistivity were used and the native ox-
ide (GeO ) was removed by exposing the wafers to HF va-
x
por before ZrO ALD. After cleaning, the Ge wafers were
2
immediately transferred to a cold wall-type high vacuum
ALD system within an hour, and deposition of ϳ55-Å ZrO2
was performed at 300 °C using alternating surface-saturating
terial in high-speed field-effect transistors.2 Although, dur-
ing device manufacturing, the lack of a sufficiently stable
native oxide poses problems in obtaining a high quality sur-
face passivation, the possibility of Ge-based MOS capacitors
,3
reactions of ZrCl and H O. Each precursor was pulsed for 2
4 2
s and N purging was followed for 30 and 60 s after each
2
H O and ZrCl pulse, respectively. The base pressure of the
2
4
Ϫ8
system was around 5ϫ10 Torr and the process pressure
was maintained at 0.5 Torr during ALD. Various sizes of Pt
gate electrodes for electrical measurements were deposited
by the room temperature e-beam evaporation through
shadow mask and, subsequently, Al was deposited on the
backside of the Ge wafers to reduce the contact and series
resistance of the samples. C–V measurements were per-
formed after forming gas anneal ͑4% H /N , 400 °C, 30
and transistors showing superior electrical properties using
high-k dielectrics was demonstrated recently.2,4
Among many possible deposition techniques for high-k
gate dielectrics, atomic layer deposition ͑ALD͒ has drawn
attention as a method for preparing ultrathin metal-oxide lay-
ers with excellent electrical characteristics and near-perfect
film conformality because of the layer-by-layer nature of the
2
2
min͒ using an HP4284A precision LCR meter. I–V measure-
ments were performed using a Keithley 230 programmable
voltage source and 6512 programmable electrometer. The
thickness and film microstructures of selected samples were
analyzed by both cross-sectional and plan-view TEM ͑JEOL
5
deposition kinetics. This technique is promising from the
perspective of deposition rate control and conformality com-
pared to other techniques, especially for proposed future de-
vices with vertical transistor channels.6
In this letter we demonstrate ZrO2 high-k dielectric
deposition onto a pure Ge ͑100͒ substrate by ALD. We ob-
3
010 and Philips CM20 FEG-TEM͒. The compositional
characterization of a ZrO /Ge structure was carried out by
2
served local epitaxial growth of ZrO films without any dis-
2
angle resolved-XPS using a Surface Science Instruments
tinct amorphous interfacial layer. This behavior stands in
S-Probe (Al K x-ray source͒.
␣
contrast to that observed in ZrO deposition onto Si, which
2
Figure 1 shows the cross-sectional and plan-view image
of ALD ZrO ͑ϳ55 Å͒ grown on a HF-vapor-cleaned Ge
substrate. In contrast to the deposition characteristics typi-
2
a͒Electronic mail: hsubkim@stanford.edu
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003-6951/2003/83(13)/2647/3/$20.00 2647 © 2003 American Institute of Physics
29.120.242.61 On: Sat, 29 Nov 2014 08:30:58
0
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Appl. Phys. Lett., Vol. 83, No. 13, 29 September 2003
Kim et al.
FIG. 2. X-ray photoelectron spectrum and peak fitting results for the Ge 3p
feature of a 55-Å ZrO /Ge specimen for several sputter etch times and a 90°
2
detection take-off angle.
possible that any interfacial native oxide which forms in one
part of the deposition process may be dissociated and re-
moved in a subsequent step.
Due to the large lattice mismatch ͑ϳ10%͒ between ZrO2
and Ge, a significant areal density of interfacial dislocations
can be seen in the cross-sectional image in Fig. 1͑a͒. The
epitaxial relationship between film and substrate was verified
using plan-view imaging and electron diffraction ͑ED͒ analy-
sis, as shown in Fig. 1͑b͒. Indexing of ED patterns obtained
during electron microscopy indicates that the ALD-grown
FIG. 1. ͑a͒ Cross-sectional high-resolution-TEM micrograph of Pt/55-Å
ZrO /Ge ͑100͒ along the ͗110͘ zone-axis, and ͑b͒ bright-field plan-view
2
image of 55-Å ZrO /Ge ͑100͒. Inset of the figure shows selected area elec-
tron diffraction pattern and corresponding epitaxial relationship.
ZrO film may be in either the tetragonal or cubic phase. It is
2
2
difficult to distinguish a- or b-axis oriented tetragonal grains
for the epitaxial orientation present in these films. However,
careful investigation of electron diffraction patterns has veri-
fied that ZrO films grown by ALD on to SiO /Si substrates
cally observed on Si substrates, ALD deposition of ZrO2
films on a Ge ͑100͒ showed local epitaxial growth without a
distinct interfacial layer, as shown in Fig. 1͑a͒. High-k di-
electric deposition by ALD occurs readily on hydroxylated
2
2
1
0
are in the tetragonal phase. Therefore, it is quite likely that
ALD ZrO on the Ge ͑100͒ substrate may also be tetragonal.
2
Due to the large lattice mismatch, local epitaxial growth gen-
erated numerous distorted Moir e´ fringes, as shown in the
plan-view image in Fig. 1͑b͒. The mosaic spread of the epi-
taxial film orientation also manifests itself in a distortion of
diffraction seen in the ED pattern that is inset in the same
surfaces, such as chemical SiO , or a hydroxylated oxyni-
2
tride passivation. Moreover, uncontrolled formation of a thin
interfacial oxide layer is observed if the Si is not already
7
passivated by such a layer prior to deposition. High-k ma-
terials, such as ZrO and HfO , are stable with respect to
2
2
2
figure. The ͑001͒ Ge//͑001͒ ZrO and ͓100͔ Ge//͓100͔ ZrO2
solid-state reactions with Si; however, because metal-oxide
films are often deposited in an oxidizer-rich environment,
some oxidation of the Si surface during high-k film deposi-
tion is almost unavoidable, and the growth of this low-k
interface layer tends to reduce the possibility of scaling of
equivalent oxide thickness ͑EOT͒. Considering the Gibbs
epitaxial relationship is observed, as expected for this sys-
tem, and the existence of one extra ZrO ͑111͒ atomic plane
2
per every 10 planes in the cross-sectional image indicates
that the compressive misfit strain of the ZrO film is almost
2
fully relieved by misfit dislocations.
In order to verify the absence of an interfacial germa-
nium oxide, XPS analysis using sputter depth profiling tech-
nique was performed, as shown in Fig. 2. A 90° take-off
angle was used to maximize the information depth and
Ge 3p feature was monitored because the Ge 3d signal over-
laps with the Zr 4p signal. As the sputtering etch time in-
creases, a significant increase of the peak associated with the
Ge substrate was seen. Simultaneously, the Zr and O peaks
from ZrO2 decreased, and the metallic Zr ͑silicide͒ peak hav-
ing lower binding energy appeared due to the sputtering ar-
tifact ͑not shown here͒. However, no higher binding energy
free energy of formation for ZrO ͑Ϫ1135 kJ/mol at 600 K͒
2
and GeO ͑Ϫ610 kJ/mol at 600 K͒, it is plausible that ZrO2
2
should be thermodynamically stable with respect to solid-
state reactions with the Ge substrate, similar to the Si case.8
On the contrary to the ZrO /Si case, ZrO deposited by ALD
2
2
onto Ge ͑100͒ consistently resulted in an interfacial layer-
free interface structure. This was the case in spite of the fact
that the Ge substrates were exposed to air after HF treatment
to remove the native oxide, possibly resulting in some refor-
mation of native oxide prior to ALD. However, it is well
known that Ge oxides ͑GeO and GeO ) are very unstable at
2
moderately high temperatures ͑Ͼ400 °C͒ in vacuum and also
peaks, which might correspond to Ge suboxide or ZrGeO
x
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9
are readily dissolved in H O. During ALD deposition, it is
͑zirconum germanate͒, were detected.
2
1
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Appl. Phys. Lett., Vol. 83, No. 13, 29 September 2003
Kim et al.
2649
FIG. 3. C–V characteristics of a Pt/55-Å ZrO /n-type Ge ͑100͒ structure
2
FIG. 4. Leakage current characteristics of a Pt/55-Å ZrO /n-type Ge ͑100͒
stack for both bias polarities. Applied voltage is defined as positive when the
top Pt electrode is positively biased.
2
measured at three different frequencies after forming gas anneal ͑4%
H /N , 400 °C, 30 min͒.
2
2
High-frequency C–V measurements were performed af-
the interfacial dislocation density is expected to improve the
electrical properties of these high-k/Ge gate stacks.
In this letter, the microstructural and electrical character-
ter the forming gas annealing ͑400 °C, 30 min͒ using 7 090
2
␮
m circular capacitor patterns. The capacitance was mea-
sured at various frequencies as a function of gate voltage and
the capacitor was swept from inversion to accumulation and
back to check the amount of hysteresis. As shown in Fig. 3,
a significant amount of frequency dispersion was observed
even after series resistance correction using the two-
istics of ALD-grown ZrO dielectric layers on Ge ͑100͒ sub-
2
strates were reported. Locally epitaxial growth of the ZrO2
films was observed. Failure to obtain high-quality epitaxy
over the entire film is attributed to the very large lattice mis-
match in this system. No amorphous interface layer between
the Ge substrate and crystalline high-k film was observed
through TEM and XPS analyses, indicating that native ger-
manium oxide and suboxides may be unstable under typical
ALD growth conditions or they may be removed by HF-
vapor treatment producing a chemical termination that is
stable during subsequent ALD processing. Although large
hysteresis and frequency dispersion were observed, very low
leakage current densities are promising for the future poten-
tial of epitaxial metal-oxide/Ge gate stacks.
11
frequencies correction method, and a very large hysteresis
Ͼ200 mV͒ was observed across the entire frequency range.
͑
After scanning from inversion to the accumulation condition,
the C–V curve shifted along the positive axis during reverse
scan as a consequence of electron trapping from the n-type
Ge substrate injection. This significant electron trapping ͓3
1
2
12
2
ϫ10 ϳ3.5ϫ10 (/cm )͔ and frequency dispersion are be-
lieved to originate either from the large areal density of in-
terfacial dislocations ͓ϳ7ϫ1012 (/cm )͔ due to the rela-
tively large lattice mismatch or because of a very high
density of interface states due to intrinsic differences in
2
The authors thank Dr. Tamara Radetic of the National
Center for Electron Microscopy at Lawrence Berkeley Na-
tional Laboratory for assistance with TEM. This work was
supported in part by the NSF/SRC Center for Environmen-
tally Benign Semiconductor Manufacturing, award no.
Q423740, DARPA HGI Program, and the Mayfield Stanford
Graduate Fellowship.
bonding corrdination across the chemically-abrupt ZrO /Ge
2
interface. With decreasing measurement frequency, the inver-
sion capacitance was observed to increase significantly. This
may be attributed either to an increase in minority carrier
generation due to the diffusion of impurities from the gate
dielectric into the substrate, or, perhaps, to the interaction of
interface slow states. Although the exact evaluation of the
EOT is impossible due to the significant frequency disper-
sion, an EOT of ϳ13 Å can be extracted from the 10-kHz
C–V data, without quantum-mechanical corrections. For an
1
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2
3
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5
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2
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using SiO gate stacks with similar EOT was measured.
9
0
2
1
These excellent leakage current characteristics suggest that
other crystalline high-k metal oxides with closer lattice
match to Ge may be good candidates for epitaxial in
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11
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13
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