Characteristics of atomic-layer-deposited Al2O3 high- k dielectric films grown on Ge substrates

Article abstract of DOI:10.1149/1.2965495

This paper describes the structural and electrical properties of Al2 O3 thin films grown through atomic layer deposition onto Ge substrates over a wide deposition temperature range (50-300°C). From grazing-incidence X-ray reflectivity and X-ray photoelectron spectroscopy, we found that increasing the deposition temperature improved the Al2 O3 film density and its dielectric stoichiometry; nevertheless, dielectric intermixing between main Al2 O3 and interfacial Ge O2 appeared at temperatures above 200°C, along with degradation of the Ge O2 Ge interface. Accordingly, a relatively large gate leakage current (Jg) and a high density of interfacial states Dit (> 1013 cm-2 eV-1) were observed as a result of deterioration of the entire Al2 O3 Ge structure at higher deposition temperatures. In addition, although subsequent high-temperature processing at 600°C in a N2 ambient could relieve the oxygen-excessive behavior further, i.e., to provide a more stoichiometric film, the accompanying Ge Ox volatilization close to the dielectric interface caused greater damage to the electrical performance. Only forming gas annealing (H2 N2, 1:10) at low temperature (300°C) improved the capacitance-voltage characteristics of the Pt Al2 O3 Ge structure, in terms of providing a lower value of Dit (ca. 6× 1011 cm-2 eV-1), a lower value of Jg, and a reduced hysteresis width.

Full text of DOI:10.1149/1.2965495

Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
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0013-4651/2008/155͑10͒/G203/6/$23.00 © The Electrochemical Society
Characteristics of Atomic-Layer-Deposited Al2O3 High-k
Dielectric Films Grown on Ge Substrates
Chao-Ching Cheng,a Chao-Hsin Chien,a,b,z Guang-Li Luo,b Jun-Cheng Liu,a
Chi-Chung Kei,c Da-Ren Liu,c Chien-Nan Hsiao,c Chun-Hui Yang,b and
Chun-Yen Changa
aInstitute of Electronics, National Chiao-Tung University, Hsinchu, 300 Taiwan
bNational Nano Device Laboratory, Hsinchu, 300 Taiwan
cInstrument Technology Research Center, National Applied Research Laboratories, Hsinchu, 300 Taiwan
This paper describes the structural and electrical properties of Al2O3 thin films grown through atomic layer deposition onto Ge
substrates over a wide deposition temperature range ͑50–300°C͒. From grazing-incidence X-ray reflectivity and X-ray photoelec-
tron spectroscopy, we found that increasing the deposition temperature improved the Al2O3 film density and its dielectric stoichi-
ometry; nevertheless, dielectric intermixing between main Al2O3 and interfacial GeO2 appeared at temperatures above 200°C,
along with degradation of the GeO2/Ge interface. Accordingly, a relatively large gate leakage current ͑Jg͒ and a high density of
interfacial states Dit ͑Ͼ1013 cm−2 eV−1͒ were observed as a result of deterioration of the entire Al2O3/Ge structure at higher
deposition temperatures. In addition, although subsequent high-temperature processing at 600°C in a N2 ambient could relieve the
oxygen-excessive behavior further, i.e., to provide a more stoichiometric film, the accompanying GeOx volatilization close to the
dielectric interface caused greater damage to the electrical performance. Only forming gas annealing ͑H2/N2, 1:10͒ at low
temperature ͑300°C͒ improved the capacitance–voltage characteristics of the Pt/Al2O3/Ge structure, in terms of providing a lower
value of Dit ͑ca. 6 ϫ 1011 cm−2 eV−1͒, a lower value of Jg, and a reduced hysteresis width.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2965495͔ All rights reserved.
Manuscript submitted March 21, 2008; revised manuscript received June 16, 2008. Published August 19, 2008.
method, involving cyclical rinsing with deionized ͑DI͒ water and
dilute HF solution ͑HF/DI water, 1:30͒ for 5 and 1 min, respectively,
was performed prior to dielectric film deposition. The Al2O3 thin
films were subsequently grown on the bulk Ge at substrate tempera-
tures of 50, 100, 200, and 300°C, respectively, while maintaining
the chamber pressure at 10 Torr in the ALD system. Al͑CH3͒3 and
H2O were chosen as the Al metal source and oxidant, respectively;
they were pulsed alternately into the reactor for 1 s per pulse, sepa-
rated by a N2 purge of 10 s to remove redundant reactants. Next, a
700 Å thick layer of Pt was deposited using a sputtering system to
pattern the capacitor electrode through the shadow mask. Two post-
thermal processes were tested: ͑i͒ rapid thermal annealing ͑RTA͒ at
400 or 600°C in a N2 ambient for 30 s and ͑ii͒ forming gas anneal-
ing ͑FGA͒ at 300°C in a H2/N2 ͑10%͒ mixed ambient for 30 min.
Finally, a thermal coater was used to deposit Al as the back-side
contact. Optical microscopy revealed that the gate capacitance area
was ca. 4 ϫ 10−4 cm2.
High-k Al2O3 is considered a potential alternative gate dielectric
material on Si substrates for application to metal oxide semiconduc-
tor field-effect transistors ͑MOSFETs͒ because of its wide bandgap
energy ͑ca. 8.8 eV͒, large conduction/valance band offsets, and high
thermodynamic stability. Nevertheless, there are several drawbacks
affecting the deposition of Al2O3 on Si, relative to SiO2, including
the low crystallization temperature, the high thermal expansion co-
efficient, and the high densities of negative fixed charges and inter-
face states ͑Dit͒. Recent progress in the deposition of high-k mate-
rials, e.g., the use of deposition techniques such as atomic layer
deposition ͑ALD͒, has meant that some of these problems can be
overcome.1 Several reports have described the characteristics of
ALD-Al2O3 dielectric films grown on Si using trimethylaluminum
͓TMA, Al͑CH3͒3͔ as the precursor and H2O as the oxidant because
of the excellent ALD mechanism and broad process window.2,3 In
recent years, many investigators have studied the deposition of
ALD-HfO2 high-k layers on high-mobility Ge substrates4 using a
variety of oxidants5 in efforts aimed at enhancing the driving current
in MOSFETs. Meanwhile, various surface preparation techniques,
e.g., the incorporation of ultrathin Si,6,7 AlNx, and GeON dielectric
interlayers,8,9 plasma treatment with NH3 or PH3,10,11 and chemical
passivation12 with ͑NH4͒2S, have been developed to enhance the
electrical performance of Ge capacitors and transistors13 because the
primary obstacle affecting the fabrication of high-k/Ge structures is
the presence of GeOx native oxides and their thermal desorption,
which debases the Ge device characteristics, e.g., to provide a high
value of Dit, a large gate leakage current, and severe charge trap-
ping. In this study, we analyzed the composition and interfacial
chemistry of an ALD-Al2O3 dielectric deposited on a bulk Ge sub-
strate at temperatures in the range of 50–300°C; subsequent thermal
processing strongly influenced the structural stability and electrical
properties of the Al2O3/Ge system.
Physical analyses: high-resolution transmission electron micros-
copy ͑TEM͒, secondary-ion mass spectroscopy ͑SIMS͒, grazing-
Experimental
The p-Ge substrate had a doping concentration of ca. 2
ϫ 1015 cm−3 ͑resistivity ca. 2 ⍀ cm͒. The split samples were etched
in HF solutions of various concentrations; the surface roughness was
measured using atomic force microscopy ͑AFM͒ over a scanning
area of 1 ϫ 1 ␮m. The optimal wet-chemical cleaning ͑Fig. 1͒
Figure 1. ͑Color online͒ AFM images of the etched Ge substrate subjected
to HF solutions of three different concentrations.
z E-mail: chchien@faculty.nctu.edu.tw
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Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
incidence X-ray reflectivity ͑GI-XRR͒, and X-ray photoelectron
spectroscopy ͑XPS͒, were performed to investigate the as-deposited
Al2O3/Ge structure and its compositional variation after thermal an-
nealing. The GI-XRR system employed parallel collimated and
monochromatic beams ͑Cu K␣ radiation; ␭ = 1.5406 Å͒ and a
scanned angle ranging from 0 to 7600 arc. The reflected X-ray sig-
nal was analyzed using a commercial Bede REFS Mercury software
package based on the four-layered stack model; the available data in
experiments were applied to minimize the cost function ␹2 and more
accurately extract all the parameters during curve fitting. For XPS
analysis, an Al K␣ excitation source operating at 1486.6 eV was
employed to collect the photoelectrons at a take-off angle of 60°
with respect to the surface horizontal. These measurements allowed
us to examine not only the density and stoichiometry of the Al2O3
dielectric films but also the interfacial GeOx chemistry and thermal
diffusion behavior within the high-k dielectric. For electrical char-
acterization, the parallel capacitance–voltage ͑C-V͒, parallel
conductance–voltage, and gate leakage impurity vacancy ͑I-V͒ be-
havior of Pt/ALD-Al2O3/p-Ge metal-oxide semiconductor ͑MOS͒
structures were measured. Nicollian and Goetzberger’s conductance
method considering the fluctuation of surface band bending was
used to determine the value of Dit.14
Results and Discussion
Physical properties of ALD-Al2O3/Ge structures.— Figures 1a-d
display AFM images of the Ge surface before and after cyclical HF
wet-chemical cleaning; the surface morphology improved upon de-
creasing the HF concentration. The root-mean-square roughness de-
creased from 0.241 to 0.113 nm after dilute HF etching ͑HF/DI wa-
ter, 1:30͒, i.e., to a value comparable to the common value of ca.
0.1 nm obtained for a HF-last Si wafer. In contrast to Si, however,
the hydrophobic behavior of Ge gradually diminishes as either the
acid concentration decreases or the exposure time in air increases,
and it disappears after rinsing in DI water again. Two possible
mechanisms might be responsible for this behavior: ͑i͒ Ge–H bonds
are weaker than Ge–OH bonds, as evidenced by their respective
binding energies of 4.6–4.9 and 5.6–5.8 eV;15 the reversibility of
the chemical transformation of GeH into GeOH has also been
characterized.16 ͑ii͒ The contamination of GeC-based carbide spe-
cies, rather than a pure GeH surface coverage, can occur within a
few minutes in air, thereby resulting in a hydrophilic Ge surface.17
The TEM images in Fig. 2a-d reveal that the thicknesses of as-
deposited Al2O3 thin films on Ge substrate were dependent on the
deposition temperature ͑Tdep͒; they were ca. 53, 62, 70, and 48 Å
after 60 deposition cycles at values of Tdep of 50, 100, 200, and
300°C, respectively. The Al2O3 exhibited an amorphous phase in
these samples, but we did not observe the existence of an obvious
interfacial layer; nevertheless, the oxygen atoms are likely to react
with the Ge substrate surface during deposition, probably in the
form of GeOx species, because Ge is readily oxidized. A blurred
oxide–substrate interface became apparent upon increasing the value
of Tdep to above 200°C, presumably correlating with the interfacial
GeOx desorption; the detailed surface chemistry is discussed in the
following sections describing the XRR and XPS analyses. Figure 3
displays a plot of the corresponding ALD-Al2O3 deposition rate on
Ge determined in this study, together with those on Si as reported by
Ott et al.2 and Groner et al.;3 the results indicate a similar ALD
growth behavior with respect to the value of Tdep. Meanwhile, these
experimental data were simulated well using our proposed ALD
model,18 suggesting that the growth mechanism was mainly deter-
mined by the activation energy in Al2O3 chemical reactions at low
Figure 2. TEM images of the as-deposited Pt/ALD-Al2O3/Ge structure after
60 deposition cycles under different values of Tdep
.
quired for Al2O3 deposition as well as the desorption rate of formed
surface species; further details of this behavior are described
elsewhere.18
Figure 4a compares the XRR curves of Al2O3/Ge samples de-
posited at 50 and 300°C. The four-layer model of the
Al2O3/GexAl1−xO/GeO2/Ge gate stack fit the measured curve nearly
perfectly; note that the presence of a GexAl1−xO interlayer, arising
from the intermixing of Al2O3 and GeO2, had to be considered in
the model to accomplish an excellent fit to the curve. The oscillation
frequency is determined primarily by the thickness of the overlying
Al2O3 film, and the oscillation amplitude is very sensitive to the
variation in roughness between the top surface and the deposited
interface. The 300°C Al2O3 on the Ge substrate exhibited a slower
oscillation frequency with a higher amplitude variation, reflecting its
thinner Al2O3 film and rougher Ge interface, with respect to those of
the substrate prepared at 50°C. Note that the top surface roughness
͑Ͻ3 Å͒ was similar in all samples. We also observed that the peak
*
*
temperatures and by the adsorption coverage of AlOH or Al͑CH3͒
reactive species at high temperatures. These two effects ensured that
the growth rate achieved a maximum at a critical temperature
͑Tmax͒; in our case, the value of Tmax was ca. 170°C, close to the
value ͑ca. 175°C͒ reported by Ott et al., but higher than that ͑ca.
125°C͒ observed by Groner et al. The resultant shift of the value of
Tmax was correlated to differences in the chemisorption energy re-
Figure 3. Growth rates of ALD-Al2O3 films plotted against the value of Tdep
for both this and previous studies ͑by Ott et al. and Groner et al.͒. Solid lines
represent the simulation of our proposed ALD model ͑Ref. 18͒.
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Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
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Figure 5. SIMS depth profiles of carbon second-ion intensity within
ALD-Al2O3 ͑60 cycles͒ on Ge substrate deposited at various values of Tdep
.
The Al and O second-ion intensities of the Al2O3 ͑Tdep = 300°C͒/Ge sample
are labeled as a reference of the relative depth.
process, which caused an increase in the levels of H, OH, and C
impurities in films. Using forward recoil spectrometry, the hydrogen
concentration in ALD-Al2O3 has been observed to increase upon
decreasing the value of Tdep from 175 to 35°C.3 SIMS analyses
͑Fig. 5͒ provided further evidence for a relatively large carbon con-
centration within the main Al2O3 prepared at temperatures below
Tmax, i.e., Ͻ200°C. When excess O atoms or the C/H contaminants
were present within the Al2O3, probably in the form of aluminum
hydroxide ͓Al͑OH͒ ͔ and aluminum oxyhydroxide, the densities of
3
which are 2.42 and 3.44 g cm−3, respectively,21 the film density was
lowered accordingly.
Figure 4c depicts the respective thicknesses of main Al2O3,
mixed Al2O3–GeO2, and interfacial GeO2 layers after 60 deposition
cycles as a function of Tdep; at first, the overall XRR thicknesses
were close to the TEM observations. Interestingly, as the value of
Tdep increased from 50 to 300°C, the thickness of the underlying
GeO2 layer decreased gradually from ca. 5 Å to below 1 Å, whereas
that of the intermediate GexAl1−xO layer increased dramatically
from ca. 1 to ca. 14 Å, accompanied by an increased roughness
between the GeO2 layer and Ge substrate. These structural degrada-
tions became severe at temperatures above 200°C, implying that,
during ALD deposition, the poorly oxidized GeOx species at the
surface were less stable and probably diffused into the top Al2O3 at
higher values of Tdep, leading to a more severe dielectric intermixing
and degraded interfacial roughness. Seo et al. speculated that a simi-
lar behavior occurred during the growth of HfO2 thin films on Ge
substrates using molecular beam epitaxy;22 they observed that the
surface GeOx species possibly dissolve in HfO2 at a growth tem-
perature of 360°C.
Next, we examined the relevance of the Al 2p and O 1s photo-
emission spectra, in particular their peak energy spacing; Fig. 6a
provides an example of the as-deposited Al2O3/Ge. The value of the
energy spacing was ca. 456.95 eV, close to the value of 457 eV for
sapphire, but we did not observe a correlation between the energy
spacing and the deposition temperature because the chemical shift
was within 1 eV. Therefore, we further investigated the stoichiom-
etry of the temperature-dependent grown Al2O3 before and after
rapid thermal processing ͑Fig. 6b͒. Two noteworthy features are that
͑i͒ the O/Al composition ratio gradually achieved the ideal value of
1.5 upon increasing the value of Tdep to 200°C, but it increased to
1.65 at 300°C, and ͑ii͒ a more stoichiometric film was formed after
subsequent RTA. We can explain the variation of the O/Al compo-
sition ratio reasonably in terms of the controlled growth mechanism.
As we mentioned earlier, raising the value of Tdep led to redundant
oxygen-based radicals being expelled from the deposited films,
Figure 4. ͑a͒ Measured and simulated GI-XRR curves of the as-deposited
Al2O3 ͑60 cycles͒/Ge structures at values of Tdep of 50 and 300°C. ͑b͒ Ex-
tracted Al2O3 film densities plotted with respect to the value of Tdep. ͑c͒
Extracted thicknesses of each layer and GeO2/Ge interface roughness plotted
against the value of Tdep. Note that the modeled XRR structure is the
Al2O3/GexAl1−xO/GeO2/Ge gate stack.
positions of the maxima shifted with the incident angle in these two
samples, a typical effect of the Al2O3 density changing with the
value Tdep. Indeed, the film density increased from ca. 3.3 to ca.
3.7 g cm−3 ͑Fig. 4b͒. Compared with the high density ͑3.97 g cm−3
͒
of crystalline ␣-Al2O3,19 the density of each as-deposited Al2O3
film was indeed closer to those reported for ␥-Al2O3 or amorphous
alumina ͑3.5–3.7 g cm−3͒.19,20 The lower densities of the Al2O3
films observed upon decreasing the value of Tdep arose mainly from
the thermally activated reaction kinetics dominating at lower tem-
peratures, especially because we used TMA and H2O in the ALD
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Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
Figure 6. ͑a͒ Binding energy difference between the peak positions of Al 2p
and O 1 s core levels for the as-deposited Al2O3 ͑Tdep = 100°C͒/Ge struc-
ture. ͑b͒ Oxygen-to-aluminum elemental ratios estimated from the respective
photoemissions of the Al2O3 ͑Tdep = 50–300°C͒/Ge samples subjected to
RTA at 400 and 600°C, respectively.
Figure 7. ͑a͒ Ge 2p3/2 core level spectra of the Al2O3 ͑Tdep = 100°C͒/Ge
structure before and after RTA. Inset: Emission ratio of Ge oxides to sub-
strate extracted from the Ge 3d core level. ͑b͒ Calculated GeO2 atomic con-
centration within Al2O3 high-k dielectric ͑Tdep = 50–300°C͒ grown on Ge
substrate before and after RTA.
which in turn suppressed the oxygen content and, thus, improved the
Al2O3 stoichiometry. Above Tmax ͑ca. 180°C͒, the deposition pro-
*
*
cess relies strongly on the presence of AlOH or Al͑CH3͒ reactive
sites because of the lower adsorption rate. In other words, a rela-
tively higher H2O concentration exists close to the surface at higher
Tdep, presumably leading to the oxygen-excessive Al2O3 film ob-
served when Tdep was 300°C. A similar tendency was also charac-
terized in the electrical permittivity, ca. 5.1, 6.2, 7.9, and 5.8 for
as-deposited Al2O3 films grown at Tdep of 50, 100, 200, and 300°C,
respectively; a relatively higher value was obtained at 200°C. Fur-
thermore, we suggest that high-temperature annealing provides ad-
ditional thermal energy, acting as an external driving force, to return
these as-deposited Al2O3 samples to their stable, stoichiometric
phase.
gas, oxide desorption ͑rather than oxide growth͒ should dominate
the thermal reaction mechanism. Together with the finding that the
critical temperature of GeO desorption was in the range of
360–425°C,23,24 it is probable that GeO2 continued to grow at ca.
400°C due to the presence of some residual oxygen in the N2 am-
bient, but it most likely desorbed at the higher temperature of
600°C. In Fig. 7b, the Al2O3/Ge deposited at a lower temperature of
50°C exhibited results similar to that deposited at 100°C after RTA;
moreover, the higher-temperature Al2O3 samples appeared to dis-
play a lower resistivity in GeOx incorporation. Note that the calcu-
lated value of the GeO2 atomic concentration in Al2O3, based on a
standard sampling depth of ca. 30 ͑Ϯ4͒ Å for the Ge 2 p3/2 spec-
trum, might be affected by different Al2O3 overlaying thicknesses in
these samples. Here, the GeO2 concentration remained low ͑Ͻ1.0
atom %͒ in all cases. In view of the earlier XRR examination, the
GeOx out-diffusion after thermal annealing is a critical issue in
the preparation of high-k/Ge structures. Surface-sensitive Ge 2p3/2
spectroscopy ͑Fig. 7a͒ revealed that no Ge oxides diffused into the
top of the Al2O3 dielectric after 400°C RTA, but the bulk-sensitive
Ge 3d spectrum ͑inset͒ revealed an increased GeOx/Ge intensity
ratio, indicative of GeO2 growth near the lower interface. When we
increased the RTA temperature to 600°C, we detected a small peak
corresponding to GeO2 in the Ge 2p3/2 spectrum along with a re-
duced GeOx/Ge ratio in the Ge 3d spectrum. We speculate that the
balance between two competing processes, oxide growth and oxide
desorption, which are strongly dependent on both the temperature
and oxygen concentration, determines the amount of residual Ge
oxide and its distribution. Because we used a N2 ambient as the feed
as-deposited Al2O3/Ge structures displayed an increased GexAl1−x
O
intermixing phenomenon, especially at temperatures up to 200°C.
Therefore, we conclude that high-temperature annealing is likely to
cause interfacial GeOx volatilization on the bottom of the Al2O3
dielectric with a small degree of Ge incorporation. It was suspected
that Al2O3/Ge capacitors undergoing RTA at 600°C might suffer
from resultant dielectric intermixing, as in the case of the high-Tdep
systems ͑ജ200°C͒; indeed, both the interface quality and the leak-
age current characteristics deteriorated ͑data not shown͒. These ex-
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Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
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hellemont and Simoen reported that metal impurities, in particular
Fe and Ni, are responsible for these deep levels, with a trap energy
of ca. 0.3 eV within the bandgap.27 From electrical characterization,
the trap density was estimated to be ca. 1015–1016 cm−3 in the bulk
Ge substrate.28 Figure 8b provides a comparison of the I-V charac-
teristics; the inset presents the bidirectional sweep of the C-V
curves. At a similar value of the capacitance equivalent thickness,
the value of Jg and the degree of charge trapping were both reduced
after FGA; we attribute these improvements to the resultant passi-
vation of the interfacial traps.
The common approaches toward extracting the value of Dit for Si
MOS capacitors, namely the Terman,29 high-low frequency
capacitance,30 and conductance12 methods, cannot be fully applied
to Ge devices because carrier exchange between the interface states
and the minority-carrier bandedge influences the Dit extraction in the
weak inversion regime.28 Here, the statistical conductance model
was utilized to evaluate the energy distribution of Dit only within the
limited gate biases. Figure 9a provides a plot of the conductance
͑Gp/␻͒ as a function of the measured frequency ͑fm͒ for the as-
deposited Al2O3/Ge capacitor under gate biases ranging from
0.2 to 0.7 V, i.e., where the device is biased from depletion to weak
inversion. During the Dit extraction, only the conductance peak hav-
ing the shoulder near the higher-frequency side was fitted well. Most
importantly, two interesting features appeared upon increasing the
positive gate bias: ͑i͒ the Gp/␻ peak increased in intensity gradually
and moved to lower frequency and ͑ii͒ the plateau in the range
1–10 kHz had a higher magnitude. As it is well known, two main
physical mechanisms are responsible for the onset of the ac conduc-
tance loss peak: ͑i͒ the carrier transition between the interface states
near the Fermi level and the majority-carrier bandedge, which cer-
tainly dominates at gate bias in depletion, and ͑ii͒ the dark current of
the minority carriers that charge the inversion layer; they arise not
only from the bulk traps within the depletion layer but also from the
diffusion current from the bulk semiconductor. Based on the obser-
vation that the generation and recombination of the bulk traps over
the diffusion mechanism contributed to the main minority-carrier
current at temperatures below ca. 45°C,26 we suppose that in the
weak inversion region the competition between bulk trap loss and
interface state loss is intense. The resultant shift of the gate bias-
dependent Gp/␻ peak reflected the fact that charging and discharg-
ing of the interface traps should dominate these energy losses. As for
the conductance plateau observed on the lower-frequency side
͑1–10 kHz͒, Evangelou et al. successively simulated this anomalous
behavior using the continuum Dit model accompanying the tunnel-
ing effect of the majority carriers from the semiconductor surface to
the bulk oxide defects.30 We suspect, however, that the insensitive
conductance loss should be strongly related to the intrinsic proper-
ties of Ge; studies of temperature- and doping concentration depen-
dences are currently under investigation.
In terms of the frequency-dependent conductance peaks, Fig. 9b
reveals the change in Dit after thermal processing. The value of Dit
of the as-deposited sample was ca. 4 ϫ 1012 cm−2 eV−1; it increased
to ca. 2 ϫ 1013 cm−2 eV−1 after RTA at 600°C, but decreased to ca.
5 ϫ 1011 cm−2 eV−1 after FGA at 300°C, indicating that the low-
temperature FGA did help to repair the high-k–Ge interface.31 The
in-diffused hydrogen possibly filled the existed defects inside poor-
quality GeOx between the high-k/Ge interface.32 In addition, as is
evident in Fig. 9c, the employed value of Tdep was also a determin-
ing factor affecting the quality of the interface between the grown
Al2O3 and Ge. Apparently, the value of Dit increased to greater than
1013 cm−2 eV−1 when Tdep was 200 and 300°C, showing consis-
tency with the results of appreciable interfacial dielectric intermix-
ing. In other words, it is essential to protect the Ge devices from
GeOx formation and volatilization under either higher-temperature
deposition or higher-temperature post-thermal processing to achieve
the optimal electrical performance.
Figure 8. ͑a͒ Multifrequency C-V and ͑b͒ I-V characteristics of the Pt/Al2O3
͑Tdep = 100°C͒/p-Ge MOS capacitors before and after FGA at 300°C. Inset:
Bidirectional C-V curves measured at 100 kHz.
perimental findings agree with those of previous studies suggesting
that the volatilization of gaseous GeO into high-k layers degrades
the insulator properties.6,25
Electrical characteristics of ALD-Al2O3/Ge structure.— The
physical analyses described above suggested that Al2O3 ALD films
grown on p-Ge substrates at the Tdep range of 100–200°C should
exhibit improved dielectric and interfacial qualities. Thus, we stud-
ied the electrical properties of Ge MOS capacitors incorporating
Al2O3 deposited at 100°C; Fig. 8a depicts the multifrequency C-V
curves recorded before and after FGA at 300°C. Note that the C-V
sweep direction presented here was from strong inversion to strong
accumulation, and that the as-deposited Al2O3 thickness increased
to ca. 80 Å in this case. For the as-deposited sample, we observed
C-V stretch-out behavior along with humps that emerged in the
depletion and weak inversion regimes, arising from the onset of
surface slow states. Subsequent FGA processing improved the qual-
ity of the Al2O3-Ge interface, such that the C-V curves reached full
accumulation saturation with a deeper “dip” in depletion, i.e., the
deviation of the 1 kHz curve was diminished with respect to that of
the 1 MHz curve. Another noticeable phenomenon is the significant
frequency dispersion in the strong inversion region, which is consis-
tent with a high intrinsic carrier concentration and a large density of
bulk traps close to the midgap in Ge; consequently, a fast accumu-
lation of minority carriers could occur at the Ge surface.26 Van-
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G208
Journal of The Electrochemical Society, 155 ͑10͒ G203-G208 ͑2008͒
GeO2–Ge interface. Therefore, a relatively large value of Jg and an
increased value of Dit ͑Ͼ1013 cm−2 eV−1͒ arose from structural de-
terioration in Al2O3/Ge at higher values of Tdep. Although N2 RTA
at 600°C abated the resultant oxygen-excessive behavior, i.e., pro-
viding a more stoichiometric film, interfacial GeOx volatilization
resulted in the greater electrical degradation. In contrast, FGA at
300°C improved the C-V curves of Pt/Al2O3/Ge MOS capacitors,
including lower values of Dit ͑ca. 6 ϫ 1011 cm−2 eV−1͒ and Jg and
reduced hysteresis width.
Acknowledgment
This study was sponsored primarily by the National Science
Council of Taiwan ͑contract no. NSC94-2215-E009-066͒.
National Chiao-Tung University assisted in meeting the publication costs
of this article.
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