Interfacial oxide re-growth in thin film metal oxide III-V semiconductor systems

Article abstract of DOI:10.1063/1.3700863

The Al₂O₃/GaAs and HfO₂/GaAs interfaces after atomic layer deposition are studied using in situ monochromatic x-ray photoelectron spectroscopy. Samples are deliberately exposed to atmospheric conditions and interfacial oxi

Full text of DOI:10.1063/1.3700863

Interfacial oxide re-growth in thin film metal oxide III-V semiconductor systems
S. McDonnell, H. Dong, J. M. Hawkins, B. Brennan, M. Milojevic, F. S. Aguirre-Tostado, D. M. Zhernokletov, C.
L. Hinkle, J. Kim, and R. M. Wallace
Citation: Applied Physics Letters 100, 141606 (2012); doi: 10.1063/1.3700863
View online: http://dx.doi.org/10.1063/1.3700863
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APPLIED PHYSICS LETTERS 100, 141606 (2012)
Interfacial oxide re-growth in thin film metal oxide III-V semiconductor
systems
a)
S. McDonnell, H. Dong, J. M. Hawkins, B. Brennan, M. Milojevic, F. S. Aguirre-Tostado,
b)
D. M. Zhernokletov, C. L. Hinkle, J. Kim, and R. M. Wallace
Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080,
USA
(Received 20 February 2012; accepted 17 March 2012; published online 4 April 2012)
The Al O /GaAs and HfO /GaAs interfaces after atomic layer deposition are studied using in situ
2
3
2
monochromatic x-ray photoelectron spectroscopy. Samples are deliberately exposed to
atmospheric conditions and interfacial oxide re-growth is observed. The extent of this re-growth is
found to depend on the dielectric material and the exposure temperature. Comparisons with
previous studies show that ex situ characterization can result in misleading conclusions about the
interface reactions occurring during the metal oxide deposition process. VC 2012 American Institute
of Physics. [http://dx.doi.org/10.1063/1.3700863]
There is currently much interest in a range of III-V
semiconductor substrates which, due to their high carrier
mobilities, could potentially be implemented as high mobil-
ity of this self-cleaning effect, the variety of III-V substrates
9
and metal oxides being studied has increased.
–12
TEM and XPS appear to be two of the most utilized
techniques in studying this effect with TEM allowing for
both the detection of interfacial oxides and an accurate deter-
mination of thickness, while XPS provides positive identifi-
cation of specific oxidation states. Due to the sample
preparation required for TEM, the characterization is always
carried out ex situ. However, some systems allow for in situ
deposition and XPS characterization of thin films. It is clear
from the early work on this topic that for thicker films
(>5 nm) analyzed ex situ with TEM and ex situ with sputter
depth profiling þ XPS that the interfacial self-cleaning effect
is observed for the TMA/III-V systems of GaAs and
ity channel materials in improved metal oxide semiconductor
(
1
MOS) devices. Controlling interfacial oxidation at metal
oxide III-V semiconductor interfaces will be required for
their implementation as alternative channel materials. The
detection and identification of these oxides is critical in any
attempts to correlate interface chemistry with device charac-
teristics, with a number of post deposition characterization
techniques being used for this purpose.
The interfacial chemistry resulting from the deposition
of high-k metal oxides on a range of semiconductor surfaces
by atomic layer deposition (ALD) has been studied exten-
sively. However, some reports relating to the precursor de-
pendent self-cleaning effect observed on many III-V
surfaces appear to conflict. Early reports by Ye et al. and
Frank et al. observed, using transmission electron micros-
copy (TEM) and thick (>4 nm) films, a clean up of arsenic
oxides when Al O was deposited by trimethylaluminum
2
–5
InGaAs. In contrast, when thin films (<2 nm) are exposed
6
to atmosphere prior to analysis the effect is not observed.
This study intends to provide an explanation for this contra-
diction by comparing identical Al O /GaAs and HfO /GaAs
samples with both in situ and ex situ XPS.
2
3
2
For this study, TMA and TDMA-Hf were used as metal
precursors. GaAs(100) wafers were degreased and NH OH
2
3
2
,3
(
TMA) but not for HfO using HfCl . Haung et al. and
2
4
4
8
treated and loaded in a UHV multi deposition/characteriza-
Chang et al. reported self-cleaning on InGaAs substrates
using TMA (Ref. 4) and tetrakis(ethylmethylamino) hafnium
ꢁ
11
tion system (base pressure <2 ꢀ 10
mbar) described else-
Depositions were carried out in an attached
Picosun ALD reactor. For Al O (HfO ) depositions, TMA
5
TEMA-Hf), which supplied evidence that the clean-up was
13,14
(
where.
precursor rather than metal dependent. Both of these studies
utilized in situ Ar-ion sputtering of thick (>5 nm) films to
allow interfacial analysis with x-ray photoelectron spectros-
copy (XPS). In contrast to the work by Frank et al. with
2
3
2
(TDMA-Hf) and H O were used as the metal and oxidant
2
precursors, respectively, while the substrate was held at
ꢂ
ꢂ
300 C (200 C) during deposition. ALD reactor tempera-
tures were chosen to provide the highest film quality without
precursor decomposition. Samples were transferred in UHV,
at various stages in the deposition process, to an attached
XPS analysis chamber where they were analyzed using a
TMA and HfCl precursors, Dalapati et al. showed via ex
4
situ XPS that both Al O and HfO films had detectable ar-
2
3
2
senic oxides present at the oxide/semiconductor interface, as
6
observed by XPS after the deposition of thin 1 nm layers.
15
Also, Suri et al. showed that the clean-up effect for the tetra-
kis(dimethylamino) hafnium (TDMA-Hf) precursor was sub-
monochromatic Al Ka x-ray source described previously.
For spectral analysis, the curve fitting software AANALYZER
7
8
16
was used. Spectra were fitted with a Voigt line shape and a
strate temperature dependant. Since an in situ XPS study
of 1 nm Al O and HfO films on GaAs substrates using
TMA and TEMA-Hf highlighted the oxidation state selectiv-
Shirley background subtraction was used. The Lorentzian
component of the peak widths and core-level chemical shifts
with respect to the bulk components were kept at constant
2
3
2
17,18
values consistent with previous studies on InGaAs.
In Figures 1(a) and 1(b), the evolution of the As 2p3/2
a)
Electronic mail: stephenmcd@utdallas.edu.
b)
Electronic mail: rmwallace@utdallas.edu.
and Ga 2p3/2 core-levels through an Al O deposition and
2
3
0003-6951/2012/100(14)/141606/4/$30.00
100, 141606-1
VC 2012 American Institute of Physics
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1

141606-2
McDonnell et al.
Appl. Phys. Lett. 100, 141606 (2012)
arsenic oxides. The Ga-O states are also seen to be reduced
by the first TMA pulse, with continued reduction noted after
the final 9 cycles. The lack of change after the deionized
water (DIW) pulse suggests that the TMA is solely responsi-
25
ble for this clean-up.
In order to simulate an experiment that may include
both in situ and ex situ complementary techniques, the sam-
ple was then exposed to air for ꢃ10 s and for increasing
2
3
4
5
durations of time (e.g., 10 s, 10 s, 10 s, 10 s), with XPS
acquired after each exposure. Small concentrations of As-O
are seen at the limit of detection after the ꢃ10 s but steadily
increase with each exposure thereafter. For longer exposures
2
(
Dt ꢄ 10 s), a higher binding energy feature separated
5.1 eV (labeled As*) from the bulk begins to emerge and
increase. This feature has been attributed to an amorphous
18,26
blend of GaAsO in previous studies. A similar trend is
observed for the gallium with a concurrent emergence and
4
increase in a higher binding energy GaAsO like feature sep-
4
arated from the bulk feature by 1.6 eV (Ga*). The assign-
ment of these features to GaAsO is tentative as hydroxides
could be another possible explanation.
4
Given the small increases in interfacial oxide up to
ꢃ100 s as seen in Figure 1(c), it could be inferred that ex situ
interface analysis of thin films on III-V surfaces may be pos-
sible if the air exposure is minimized. However, this may
only be true for the process conditions used in this experi-
ment. To test this, a similar experiment was carried out using
TDMA-Hf instead of TMA to deposit HfO2 rather than
Al O . In Figure 2, the evolution of the As 2p and Ga 2p
2
3
core-level spectra during this experiment on the HfO /GaAs
2
system is presented. Here the initial surface was characterized
and ꢃ1 nm of HfO was deposited at a substrate temperature
2
ꢂ
of 200 C in a single deposition. The expected reduction in
interfacial oxides is observed as is the small residual signal
FIG. 1. Photoelectron spectra after (i) NH
(
4
OH treated, (ii) 1 TMA pulse,
5
iii) 10 ALD cycles of TMA/H
2
O, (iv) 1.1 ꢀ 10 s air exposure, for the (a)
3þ
As 2p and (b) Ga 2p core-level features. (c) The ratio of the As-O/As-Ga
and Ga-O to Ga-As signals for the As 2p and Ga 2p core-levels each deposi-
tion/exposure step. Oxygen bond conversion from As-O to Ga-O is noted af-
due to As which indicates that the TDMA-Hf clean-up is
8
similar to that of the TEMA-Hf reported previously. This is
ꢂ
contrary to the ex situ XPS study by Suri et al. which showed
ꢂ
ter 300 C. Both As-O and Ga-O are reduced by the first TMA pulse. As-O
and Ga-O signals gradually increase after exposure to the air.
that substrate temperatures of 300 C were required to
achieve significant arsenic oxide reduction during ALD using
1
9
7
the TDMA-Hf precursor. The sample was exposed to the air
subsequent air exposure is illustrated with sample spectra
at key stages in the process. Figure 1(c) plots the oxide to
bulk ratio for both As and Ga oxides from the fitted spectra.
For the starting, oxide covered, surface, chemical states
attributed to As-Ga (Bulk @ 1322.8 eV), As-As(DBE
4
for 1.1 ꢀ 10 s and compared to the Al O /GaAs system after
2
3
the same exposure. It is clear that the oxide re-growth through
the HfO is substantially greater than through the Al O . This
2
2 3
can be quantified by calculating the ratio of the oxide related
features to the bulk feature for both systems. It is found that
this ratio is twice as high for the HfO system. This result is
¼
þ0.5 eV), as well as the oxidation states corresponding to
1
þ
þ
3þ
As
(DBE ¼ þ1.3 eV), As
(DBE ¼ þ3.3 eV), and
2
5
As (DBE ¼ þ4.4 eV) are observed in the As 2p core-level
not surprising since both O and H O are known to have high
2
2
17,18
5þ
27
diffusion coefficients in HfO as shown by Ming et al. and
feature.
as has been previously reported.
attributed to Ga-As (Bulk @ 1117.3 eV) and the oxidation
It is clear that the NH OH reduces the As state
4
2
1
7,20,21
28
Chemical states
Driemeier et al., respectively. It is clear that ex situ inter-
face analysis of the HfO /III-V systems may be considerably
influenced by atmospheric exposure.
2
1þ
3þ
states corresponding to Ga (DBE ¼ þ0.5 eV) and Ga
(
DBE ¼ þ1.1 eV) are seen in the Ga 2p core-level fea-
Both the samples in the preceding studies were analyzed
by in situ XPS before exposure to air. Given that the analysis
time is approximately 2 h, this ensured that the samples were
close to room temperature before exposure to the air. Since
diffusion coefficients are explained with an Arrhenius-type
expression, higher temperatures should result in an increased
18,22
ꢂ
23
Upon annealing at 300 C, there is a transfer of
ture.
oxygen from the arsenic to the gallium (not shown). The first
TMA pulse almost completely removes the As-O signal
from the As 2p spectra which differs slightly from the ex situ
2
3
study by Lee et al. where an As-O signal was detected in
2
4
29,30
even the less surface sensitive As 3d core-level feature. Af-
ter the final 9 cycles of TMA/H O, the As-O signal is no lon-
ger detectable, indicating the removal of the remaining
interfacial oxide re-growth.
In many ALD chambers, cooling under UHV conditions
is not possible; consequently a sample will either be cooled in
2
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141606-3
McDonnell et al.
Appl. Phys. Lett. 100, 141606 (2012)
FIG. 3. Photoelectron spectra after (i) NH
4
4
OH treated, (ii) 10 ALD cycles
af-
of TMA/H
2
4
O þ 1.1 ꢀ 10 s immediate air exposure, (iii) 10 cycles Al
2 3
O
ter 1.1 ꢀ 10 s room temperature air exposure for the As 2p and Ga 2p core-
level features. Air exposure at elevated temperatures increases the interfacial
oxide re-growth.
FIG. 2. Photoelectron spectra after (i) NH
4
OH treated, (ii) 10 ALD cycles
O, (iii) 1.1 ꢀ 10 s air exposure, (iv) 10 cycles Al after
.1 ꢀ 10 s air exposure for the As 2p and Ga 2p core-level features. Interfa-
cial oxide re-growth is greater for the HfO /GaAs system in comparison to
the Al /GaAs system under these growth conditions.
4
4,5,8
of TDMA-Hf/H
1
2
2
O
3
allow interface analysis also observed the clean-up effect.
4
These ex situ studies all suggest that when metal oxides such
as Al O and HfO of sufficient thickness are deposited, the
2
2
3
2
2 3
O
interface is protected and accurate interface analysis is possi-
ble. In situ studies utilizing TMA on III-V surfaces have
8,10,25,37–39
repeatedly observed the interfacial self-cleaning.
flowing nitrogen or simply exposed to the air at some temper-
ature between the deposition temperature and room tempera-
ture. To test the effect of such elevated temperature exposures
on the observed interface, a second ꢃ1 nm Al O film was
We note here some examples in this context. An early
ex situ study using 1.5 nm ALD films on GaAs to examine
6
the interface reported an incomplete clean-up of arsenic
2
3
grown using 10 cycles TMA/H O. After deposition, the sam-
oxides using TMA and HfCl as aluminum and hafnium
4
2
ple was exposed to air as quickly as possible. This required a
transfer to a UHV loadlock and subsequent vent with nitrogen.
The total time between the removal of the sample from the
hot ALD stage and exposure to air was 6 min. The sample
metal precursors, respectively. Recent studies on GaAs (Ref.
40 and 41) and InP (Ref. 9) suggest that there may be a
thickness dependence of HfO on the clean-up affect utiliz-
2
ing ex situ XPS and TEM, respectively. However, interfacial
oxide regrowth at the high-k/III-V interface is a possible
explanation for the previously reported, incomplete arsenic
4
was then exposed to the air for ꢃDt ¼ 1.1 ꢀ 10 for compari-
son with the sample that saw a room temperature exposure.
The core-level spectra for these two samples are compared in
Fig. 3. The change in As-O concentration can again be quanti-
fied by examining the ratios of the oxide to bulk features and
the “greater than room temperature” exposure has an oxide to
bulk ratio 40% larger than the room temperature exposure.
The evidence that the exposure of thin films to atmos-
pheric conditions can lead to interfacial oxide re-growth
allows us to explain the apparent contradictions in the litera-
ture regarding the ALD self-cleaning effect. It is clear that
exposing thin films to atmospheric conditions prior to inter-
face analysis can lead to inaccurate conclusions about the
interface reactions during growth. Looking back at previous
reports we can see that ex situ experiments using thicker
ALD films are consistent with in situ studies using thin films.
We note that only experiments carried out ex situ using thin
6
oxide removal for thin high-k dielectric depositions and the
4
0,41
apparent thickness dependence of this clean-up.
larly, the incomplete removal of In and Sb oxides after 100
Simi-
pulses of TMA reported in an ex situ XPS study by Hou
12
et al. could also be due to interfacial re-oxidation after air
exposure. The apparent contradiction between the TDMA-Hf
induced cleaning reported here at a substrate temperature of
ꢂ
7
200 C and the ex situ work by Suri et al., which showed no
significant arsenic oxide reduction using TDMA-Hf as a haf-
ꢂ
nium precursor at 200 C, can also be explained as being due
to the atmospheric exposure in the ex situ study. Interest-
ingly, the same study by Suri et al. showed that using a sub-
strate temperature of 300 C during deposition suppresses
the arsenic oxide signal after only 5 ALD cycles. This sup-
7
ꢂ
pression was noted only in the bulk sensitive As 3d core-
24
(<2 nm) films report results that contradict in situ work.
level feature. Other studies have shown that film properties
such as thickness, density, impurity content, and leakage cur-
Ex situ HR-STEM and TEM studies of thick (>5 nm)
oxides on III-V surfaces regularly report almost no interfacial
42
rent are growth temperature dependent. As such it is possi-
ble that the diffusion coefficients of oxidizing species would
also be growth temperature dependent.
31–35
oxides.
Ex situ HR-TEM studies of a thin 1.1 nm Al O3
2
film with a thicker in situ deposited HfO layer also showed
2
3
an abrupt Al O /InGaAs interface. XPS studies of thick
6
In summary, clear changes in the oxide/semiconductor
interface are observed when ꢃ1 nm Al O and HfO films are
2
3
(>5 nm films) on III-V substrates that utilize sputtering to
2
3
2
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141606-4
McDonnell et al.
Appl. Phys. Lett. 100, 141606 (2012)
1
1
6
7
exposed to air, with interfacial oxide re-growth in the HfO2/
GaAs system being particularly large. The successful TEM
interface studies of thicker (>5 nm) oxide would suggest that
in situ capping of thin ALD grown layers could allow for accu-
rate ex situ interface analysis of thin films using TEM. The
temperature dependence of these atmosphere induced interface
reactions, which may have a significant effect on thin films
grown in stand alone ALD reactors, has been highlighted.
These studies also highlight the importance of clearly report-
ing the use of either in situ or ex situ characterization techni-
ques in the literature so that proper comparisons between the
results obtained from different experimental set-ups can be
made. Finally, this interfacial oxide re-growth may have a det-
rimental effect on aggressively scaled high-k/III-V devices
and the use of barrier layers or in situ metal capping may be
required to prevent the formation of such interfacial layers.
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