Photon Factory Activity Report 2005 #23 Part B (2006)
Electronic Structure of Condensed Matter

2C/2005S2-002

Composition dependence of band offsets for (LaAlO3)1-x(Al2O3)x gate dielectrics
determined by photoelectron spectroscopy and x-ray absorption spectroscopy
Ryutaro YASUHARA1, Makoto KOMATSU1, Haruhiko TAKAHASHI1, Satoshi TOYODA1,Jun OKABAYASHI1, Hiroshi KUMIGASHIRA1, Masaharu OSHIMA*1, Dmitry KUKURZNYAK2
and Toyohiro CHIKYOW2
1
The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan
Introduction
LaAlO3 is regarded as a promising candidate for highdielectric constant (high-k) gate dielectrics dielectrics inmetal-oxide-semiconductor
field-effecttransistors(MOSFETs). In order to search for the optimum
composition for the mixture of La2O3 and Al2O3, Fujitsuka
et al. investigated the thermal stability and electricalproperties for (La2O3)1-x(Al2O3)x composite films (x=0 ~0.5)[1]. On the other hand, several results have beenreported for Al2O3 rich side, namely (LaAlO3)1-x(Al2O3)x[2,3]. However, studies on band offsets have not beenreported yet. In this study, we performed systematicinvestigation of band offsets in the composition range
from LaAlO3 to Al2O3 in order to propose the optimum
composition for gate dielectrics.

that the CBO becomes smaller with increasing the ratio ofAl2O3, while the VBO becomes larger. Since valence bandof oxides mainly consists of O 2p states, the shift of theVBO can be related to the chemical shift of O 1s states.
On the other hand, conduction bands of La2O3, LaAlO3,and Al2O3 mainly consist of La 5d* and Al 3s*/3p* states.
Thus, the structure of the conduction band is significantly
changed as a function of the ratio of La to Al. The shift of
the CBO is, therefore, caused by both the effect of thechemical shift for core levels and the change in
conduction-band structure. Since the values of VBO aresufficiently large in all compositions, the optimum
composition of (LaAlO3)1-x(Al2O3)x in terms of band
offsets are dominated by the value of CBO. Thus, LaAlO3(x=0) was determined to be the optimum composition in(LaAlO3)1-x(Al2O3)x.

Experimental(LaAlO3)1-x(Al2O3)x composite films (x=0, 0.2, 0.33, 0.5,and 1) were prepared by the combinatorial pulsed laser
deposition (PLD) method on clean p-type Si (100)osubstrates at the growth temperature of 300 C using a
KrF excimer laser. Ambient oxygen pressure during-6
deposition was 10 Torr. The total thickness of eachsample was set at 5 nm. Synchrotron radiation
photoelectron spectroscopy and x-ray absorption
spectroscopy (XAS) measurements were carried out at an
undulator beamline BL-2C of the Photon Factory inHigh-Energy Accelerator Organization (KEK).
Results and discussion
Figure1 shows the values of bandgap (Eg),valence-band offset (VBO), and conduction-band offset(CBO) for (LaAlO3)1-x(Al2O3)x as a function ofLaAlO3-Al2O3 composition (x). The values of VBO andCBO for LaAlO3 were determined to be 3.4 eV and 0.9 eV,respectively. These values are in good agreement with theprevious ones theoretically obtained by Peacock et al.[4].
The values of VBO and CBO for Al2O3 are 4.7 eV and 2.0eV, respectively. Although the value of VBO is in good
agreement with the theoretical value, the value of CBO israther smaller than the theoretical one. This is probably
because the theoretical value of CBO was determined forAl2O3 bulk crystal. In fact, the reported experimental
values of CBO for Al2O3 thin films were smaller than thatof bulk Al2O3[5, 6]. The reduction of CBO in the Al2O3
thin film is consistent with our results. It should be noted

Fig. 1. Plot of bandgap (Eg), valence-band offset(VBO), and conduction-band offset (CBO) for(LaAlO3)1-x(Al2O3)x as a function of LaAlO3-Al2O3
composition.
Reference[1] R. Fujitsuka et al., Jpn. J. Appl. Phys. 44, 2428
(2005).
[2] B. S. Lim et al., Appl. Phys. Lett. 84,3957 (2004).
[3] A. D. Li et al., Appl. Phys. Lett. 83, 3540 (2003).
[4] P. W. Peacock et al., J. Appl. Phys. 92, 4712 (2002).
[5] S. Miyazaki, J. Vac. Sci. Technol. B 19, 2212 (2001).
[6] V. V. Afanas’ev et al., J. Appl. Phys. 91, 3079 (2002).
*oshima@sr.t.u-tokyo.ac.jp

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