Interface of atomic layer deposited Al2O3 on H-terminated silicon

Article abstract of DOI:10.1002/pssa.200566014

Al₂O₃ films 1 to 20 nm thick were deposited as alternative high-κ gate dielectric on hydrogen-terminated silicon by Atomic Layer Deposition (ALD) and characterized by Synchrotron X-ray Photoelectron Spectroscopy (SXPS), Fourier Transform Infrared (FTIR) absorption spectroscopy and admittance measurements. The SXPS results indicate that about 60% of the original Si-H surface bonds are preserved at the Al₂O ₃/Si interface and this is confirmed by monitoring the Si-H stretching modes by FTIR spectroscopy in the Attenuated Total Reflection (ATR) mode both before and after ALD of Al₂O₃. The remaining 40% of Si-H bonds are replaced by Si-O bonds as verified by SXPS. In addition, a fraction of a monolayer of SiO₂ forms on top of the Al ₂O₃ dielectric during deposition. The presence of OH-groups at a level of 3% of the total oxygen content was detected throughout the Al₂O₃ layer through a chemically shifted O Is component in SXPS. Admittance measurements give a dielectric constant of 9.12, but a relatively high density of interface traps between 10¹¹ and 10¹² cm^-2 eV^-1.

Full text of DOI:10.1002/pssa.200566014

phys. stat. sol. (a) 203, No. 9, 2194–2199 (2006) / DOI 10.1002/pssa.200566014
Interface of atomic layer deposited Al O
2
3
on H-terminated silicon
*
, 1
1
1
1
1
2
K. Y. Gao , F. Speck , K. Emtsev , Th. Seyller , L. Ley , M. Oswald , and W. Hansch
2
1
Lehrstuhl für Technische Physik, Institut für Physik der Kondensierten Materie, Universität
Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
2
Lehrstuhl für Technische Electronik, Technische Universität München, Arcisstraße 21,
80333 München, Germany
Received 5 September 2005, revised 23 May 2006, accepted 23 May 2006
Published online 27 June 2006
PACS 68.35.–p, 73.20.At, 78.30.–j, 78.67.Pt, 79.60.–i, 81.15.Gh
Al O films 1 to 20 nm thick were deposited as alternative high-κ gate dielectric on hydrogen-terminated
2
3
silicon by Atomic Layer Deposition (ALD) and characterized by Synchrotron X-ray Photoelectron Spec-
troscopy (SXPS), Fourier Transform Infrared (FTIR) absorption spectroscopy and admittance measure-
ments. The SXPS results indicate that about 60% of the original Si–H surface bonds are preserved at the
Al O /Si interface and this is confirmed by monitoring the Si–H stretching modes by FTIR spectroscopy
2
3
in the Attenuated Total Reflection (ATR) mode both before and after ALD of Al O . The remaining 40%
3
2
of Si–H bonds are replaced by Si–O bonds as verified by SXPS. In addition, a fraction of a monolayer of
SiO forms on top of the Al O dielectric during deposition. The presence of OH-groups at a level of 3%
2
2
3
of the total oxygen content was detected throughout the Al O layer through a chemically shifted O 1s
2
3
component in SXPS. Admittance measurements give a dielectric constant of 9.12, but a relatively high
11
12
–2
–1
density of interface traps between 10 and 10 cm eV .
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
The continuous downscaling of the basic field effect transistor device in ULSI requires a concomitant
reduction in the thickness of the gate oxide SiO . At the 80 nm node this thickness passes the critical
2
value of 1.2 nm where the gate leakage current due to tunnelling across the gate oxide exceeds 10 A/cm2
[2]. A reduction of this unacceptably high leakage current is possible by replacing SiO with an insulator
2
of higher dielectric constant (high κ dielectric) such that the thickness increases by a factor of κ/κ
for
SiO
2
the same gate capacitance with a concomitant reduction in tunnelling current [2]. Among the candidates
is Al O with κ = 9 compared to κ
2 3
= 3.9 and a band gap of 7 eV (SiO :9 eV) [3, 4]. Al O is further-
2
2
3
SiO
2
more thermodynamically stable on Si [5].
Al O is commonly deposited on Si by atomic layer deposition (ALD). ALD is a low temperature
2
3
(~300 °C) CVD process whereby Al O is synthesized atomic layer by atomic layer from trimethylalu-
2 3
minum (TMA) and water (H O) by repeated cycles of self limiting surface reactions. In this way an op-
2
timal thickness control is possible and the resulting films grow highly conformal [6, 7].
For any alternative dielectric, a low density of interface states is of the essence and the state of the Si
surface before Al O deposition is of crucial importance in this respect. H-termination is one way to
2
3
prepare a chemically and electronically well passivated Si surface [8, 9]. A previous study of the interac-
*
Corresponding author: e-mail: gao@physik.uni-erlangen.de
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 203, No. 9 (2006)
2195
Fig. 1 High resolution Si 2p spectra of 10 cycles Al O on
2
3
ν
h = 140 eV, λeff = 3.6 Å
Si2p
H-terminated Si taken at photon energies of 140 eV, 170 eV, and
510 eV, which correspond to electron mean free paths λ of 3.6,
eff
4, and 12 Å, respectively.
h
ν
= 170 eV, λeff = 4 Å
o
_
Exp. data
Fit
Bulk
Si-H
Si4+
Si3+
Si+
Si2+
h = 510 eV, eff = 12 Å
ν λ
105 104 103 102 101 100
99
98
Binding Energy (eV)
tion of H O with H-passivated silicon suggests that the hydroxylation step (exposure to H O) as a neces-
2 2
sary first step in Al O ALD leads to a partial oxidation of the Si surface [10].
2 3
Here we present a detailed study of the chemical, structural and electronic properties of Al O depos-
2 3
ited by ALD on H-terminated Si(100) and Si(111) using a variety of surface and interface sensitive tech-
niques.
2 Experimental details
N-type Cz-Si(111) and Si(100) wafers with resistivities of 4–6 Ω cm and 4–7.2 Ω cm, respectively,
were used as substrates. Samples were cleaned by the standard wet chemical procedure until a thin layer
of oxide was formed. For Si(100), this was followed by a dip in 1% HF solution for 2 minutes and rins-
ing in deionised water. This removed the surface oxide and left the surface terminated with silicon mono-
hydride and dihydride species [8]. For Si(111), the last step was etching in a 40%, N -bubbled NH F
2
4
solution for 5 minutes followed by a rinsing in deionised water which yields an atomically flat surface
terminated by silicon monohydride [9]. Next, Al O was deposited by ALD at a substrate temperature of
2
3
300 °C using a F120 SAT reactor from ASM Microchemistry Ltd.. The hydrogen-terminated samples
were first exposed to water for 10 seconds followed by a N purge for 8 seconds. Then the two precur-
2
sors, trimethylaluminium (TMA) and water, were sequentially introduced into the chamber for 1 second
each, separated by purging cycles with nitrogen for 4 seconds. A growth rate of 0.1 nm per cycle was
determined by spectroscopic ellipsometry for a thick film (100 nm) in good agreement with previous
reports [6, 7, 11].
The density of interface states and the dielectric constant of Al O were measured via admittance spec-
2 3
troscopy on MOS capacitors. The chemical, structural, and electronic properties of Al O /Si were inves-
2 3
tigated by soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation from the storage
ring BESSY II. In order to detect hydrogen at the interface and/or surface, the samples were additionally
www.pss-a.com
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2196
K. Y. Gao et al.: Interface of atomic layer deposited Al2O3
5.0
model 1
model 2
16
14
12
10
8
(a)
(b)
4.5
SiO2
Al2O3
SiO2
Si
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Al2O3
Si
exp. data from ref. [12]
exp. data from the present work
exp. data from ref. [12]
exp. data from the present work
6
4
2
4
6
8
10
12
2
4
6
8
10
12
14
Electron mean free path (Å)
Electron mean free path (Å)
Fig. 2 Thickness of SiO calculated from the relative intensity data deconvoluted from Si 2p spectra using (a) the
2
three-layer interface model (model 1) and (b) three-layer surface model (model 2) shown in insets. The lines are
meant as guides to the eyes.
investigated by Fourier transform infrared (FTIR) spectroscopy with a resolution of 0.5 cm–1 using the
attenuated total reflection mode (ATR).
3 Results and discussion
Figure 1 shows high resolution Si 2p core level spectra for 10 cycles ALD-deposited Al O on hydrogen-
2 3
terminated Si(100). Taking the incubation effects into account, the thickness of Al O is about 8 Å. The
2 3
electron mean free path, i.e., sampling depth of the SXPS measurements was varied between 3.6 and
12 Å by using photon energies of 140 eV, 170 eV, and 510 eV, respectively. All spectra show the Si 2p
spin–orbit doublet from the Si substrate and a broad feature at ~103 eV binding energy that corresponds
to SiO , i.e. to Si surrounded by four oxygen atoms (formal oxidation state Si4+). From Fig. 1 it is obvi-
2
ous that this peak is strongest for the most surface sensitive spectrum (hν = 140 eV). Lee and Oh con-
cluded from similar spectra that the SiO was formed at the Si/Al O interface [12]. However, a detailed
2 2 3
analysis of their data as well as ours proves that this is not the case. We have calculated the nominal SiO
2
layer thickness from the I /I intensities ratio as a function of electron mean free path for the interface
SiO Si
2
model 1 as sketched in inset of Fig. 2a using well known formula [13]. Both sets of data yield thick-
nesses for the SiO layer that increase with decreasing mean free path and are unrealistically high (up to
2
14 Å) for the data of Ref. [12]. This is a clear indication that most of the SiO is not situated at the inter-
2
face but at the surface of Al O . Indeed, using model 2 (see inset of Fig. 2b), we obtain reasonable values
2 3
for the SiO layer thickness for both data sets. The data of Lee and Oh [12] yield a constant SiO thick-
2 2
ness which implies that in their case all SiO is indeed on the surface of Al O . In our case, d increases
2 2 3 SiO
2
slightly with electron mean free path which of course means that not all SiO can be at the surface but
2
some must reside at the interface as well. A complete analysis of our data including the suboxides (Si+,
Si2+ and Si3+) and a Si–H contribution at +0.38 eV (see Fig. 1 and below) yields the structure model 3
shown in Fig. 3. There is an interfacial layer between Al O and Si that is 3.5 Å thick, about one
2
3
monolayer. About 40% consists of Si-suboxides which provide the connection between Si and Al O and
2 3
60% comprises Si–H units. And there is a thin SiO layer at the surface with a thickness of 0.4 Å, corre-
2
sponding to only 0.1 monolayer.
Originally, the fit of the spectra shown in Fig. 1 was performed with a total of five equally spaced
components corresponding to Si in the formal oxidation states 0 to 4+. This always left a residuum at a
binding energy around 0.4 eV higher than the bulk component (Si0). The best fit shown in Fig. 1 does
therefore include a sixth component with a chemical shift of 0.38 0.05 eV. We ascribed this component
to Si–H bonds that are situated at the Al O /Si interface. This assignment is based for one on the chemi-
2
3
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 203, No. 9 (2006)
2197
0.4 Å SiO2
Fig. 3 Schematic picture of the structure model 3
of the sample deposited with 10 cycles Al O on Si.
2
3
8 Å Al2O3
3.5 Å
SiOx(0.4)+Si-H(0.6)
Si
cal shift which falls between the +0.24 and +0.48 eV reported for Si–H and Si–H species at the silicon
2
surface, respectively [14].
More important, the presence of Si–H at the Si/Al O interface is confirmed by FTIR spectra where
2 3
the necessary surface sensitivity is achieved via attenuated total reflection (ATR) [8, 9]. Figure 4 shows
the FTIR-ATR spectra of the hydrogen terminated Si(111):H surface before and after deposition of
15 cycles Al O (about 12 Å). The spectrum of Si(111):H before Al O deposition shows a sharp and
2
3
2
3
prominent absorption peak at 2083.6 cm–1 which is due to Si–H stretching vibrations at the surface [9].
The smaller satellite at 2087.1 cm–1 is probably related to Si–H bonds at steps [9]. Both peaks are ob-
served in p-polarisation only, attesting to the high quality of the H-terminated surface. After Al O depo-
2
3
sition, a clear (see insert of Fig. 4) albeit considerably broadened (64 cm–1 FWHM) adsorption feature is
seen in the same energy region with the maximum shifted by 23.6 cm–1 down to 2060 cm–1 compared to
the Si–H stretching mode on the uncovered surface. We ascribe this broad band to stretching vibrations
of Si–H bonds at the Si/Al O interface. Considerable damping, a certain degree of inhomogeneity and
2
3
possibly contributions from higher silicon hydrides are likely to contribute to the width of the peak. The
down-shift of the stretching frequency is ascribed to the tendency to form Si–H···Al–O hydrogen
bridges between partially negative hydrogen and partially positive aluminium atoms.
SXPS spectra of the Al 2p and O 1s core levels (not shown) were also recorded with different photon
energies and thus varying sampling depths. For the Al 2p spectra no differences in spectral shape could
be observed; Al is obviously present only in the form of Al O . The O1s spectra, by contrast, can be
2
3
Al2O3 / H-Si(111)
2000 2020 2040 2060 2080 2100 2120
Al2O3 / H-Si(111)
o
H-Si(111)
~2060 cm-1 2083.6 cm-1
2000
2020 2040
2060
2080
2100
2120
Wavenumber (cm-1)
Fig. 4 FTIR spectra measured in the ATR mode on H-terminated Si(111) before and after deposition of
15 cycles Al O .
2
3
www.pss-a.com
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2198
K. Y. Gao et al.: Interface of atomic layer deposited Al2O3
10
Fig. 5 Histogram of the density of interface states
1. run (neg. -> pos.)
2. run (pos. -> neg.)
3. run (neg. -> pos.)
D
measured on MOS capacitors by admittance
it
8
6
4
2
0
spectroscopy for 200 cycles Al O on Si(100):H.
2
3
Entire: 13
Dit [cm-2eV-1]
deconvoluted into three components, one due to Al O , one due to the silicon oxide on top of Al O , and
2
3
2
3
the third one due to OH groups. OH groups are expected on top of Al O on account of the water precur-
2 3
sor [7, 16]. However, spectra recorded with different sampling depths reveal that about 3% of oxygen is
present in the form of OH throughout the Al O layer.
2
3
The electrical properties were finally measured by admittance spectra using MOS capacitors with a
thick (200 cycles, 19.5 nm) Al O layer on hydrogen terminated Si(100) as the dielectrics. The average
2
3
dielectric constant of 9.12 is comparable to what has been reported in the Ref. [2]. A histogram of re-
peated measurements of the densities of interface states, D , is given in Fig. 5. Initially, D is rather high
it
it
at 5 × 1011 cm–2 eV–1. Worse yet, it increases by a factor of two in the third run for reasons that are not
clear at present.
4 Conclusions
Al O films were deposited on H-terminated Si with the ALD technique using TMA and water as precur-
2 3
sors. High resolution core level spectra show that the Al O /Si interface is a mixture of 40% Si–O and
2 3
60% Si–H bonds. FTIR measurements are compatible with hydrogen at the interface in the form of
Si–H···Al–O bridging bonds. About 3% of the total oxygen is bound in the form of OH throughout the
Al O layer. From admittance measurements, a high dielectric constant of 9.12 is obtained, while the
2
3
density of interface traps was measured to be initially about 5 × 1011 cm–2 eV–1.
Acknowledgement This work was supported by the Bayerische Forschungsstiftung in the framework of
FORNEL.
References
[1] See The International Technology Roadmap for Semiconductors, Semiconductor Industry Association; see also
http://public.itrs.net/ for the most recent updates (2004).
[2] G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5243 (2001).
[3] R. S. Johnson, G. Lucovsky, and I. Baumvol, J. Vac. Sci. Technol. A 19, 1353 (2001).
[4] T. M. Klein, D. Niu, W. S. Epling, W. Li, D. M. Mayer, C. C. Hobbs, R. I. Hedge, I. J. R. Baumvol, and G. N.
Parsons, Appl. Phys. Lett. 75, 4001 (1999).
[5] K. J. Hubbard and D. G. Schlom, J. Mater. Res. 11, 2757 (1996).
[6] G. S. Higashi and C. G. Fleming, Appl. Phys. Lett. 55, 1963 (1989).
[7] A. W. Ott, K. C. McCarley, J. W. Klaus, J. D. Way, and S. M. George, Appl. Surf. Sci. 107, 128 (1996).
[8] V. A. Burrows, Y. J. Chabal, G. S. Hogashi, K. Raghavachari, and S. B. Christman, Appl. Phys. Lett. 53, 998
(1988).
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 203, No. 9 (2006)
2199
[9] G. S. Higashi, Y. J. Chabal, G. W. Trucks, and K. Raghavachari, Appl. Phys. Lett. 56, 656 (1990).
[10] X. Zhang, Y. J. Chabal, E. Garfunkel, and S. B. Christman, Appl. Phys. Lett. 79, 4051 (2001).
[11] L. G. Gosset, J. F. Damlencourt, O. Renault, D. Rouchon, Ph. Holliger, A. Ermolieff, I. Trimaille, J. J. Ganem,
F. Martin, and M. N. Semeria, J. Non-Cryst. Solids 303, 17 (2002).
[12] J. C. Lee and S. J. Oh, Appl. Phys. Lett. 84, 3561 (2004).
[13] M. Cardona and L. Ley, Photoemission in Solids I, Topics in Applied Physics Vol. 26 (Springer-Verlag, Berlin
1978), p. 60.
[14] J. H. Oh, H. W. Yeom, Y. Hagimoto, K. Ono, M. Oshima, N. Hirashita, M. Nywa, A. Toriumi, and A. Kakiza-
ki, Phys. Rev. B 63, 205310 (2001).
[15] C. H. Bjorkman, J. L. Alay, H. Nishimura, M. Fukuda, T. Yamazaki, and M. Hirose, Appl. Phys. Lett. 67, 2049
(1995).
[16] M. D. Groner, F. H. Fabreguette, J. W. Elam, and S. M. George, Chem. Mater. 16, 639 (2004).
www.pss-a.com
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim