Cubic Phase HfO2 Epitaxial Thin Films Grown by MBE
Z. K.Yang (楊智凱)1, W. C. Lee (李威縉)1, Y. J. Lee (李毅君)1,
P. Chang (張翔筆)1, Y. L. Huang (黃怡霖)1, K. L. Yu (于冠禮)2,
C. H. Hsu (徐嘉鴻)2, M. Hong (洪銘輝)1, and J. Kwo (郭瑞年)3
1
Department of Materials Science and Eng., National Tsing Hua University, Hsinchu, Taiwan
2
National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3
Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
HfO2, due to its high  value, is the leading candidate as a gate dielectric for SiO2 in the Si
technology, which with the rapid shrinkage of its transistor feature size has forced the SiO2 gate
oxide thickness to reduce to 1.0nm, the quantum tunneling limit. There are three known structures
of HfO2: monoclinic, cubic, and tetragonal with the dielectric constants being ~20, ~30, and ~70,
respectively, as obtained from the first-principles calculation.[1] The common structure of HfO2
thin films from sputtering, atomic layer deposition, and MBE is monoclinic phase. Moreover, the
MBE oxide films are epitaxially grown on GaAs with four domains and atomically sharp interface
without any interfacial layer. [2]
In this work, HfO2 doped with Y2O3 have been grown on GaAs (100) and Si (111) in a
multi-chamber MBE system with electron beam evaporation from two separate charges of HfO2 and
Y2O3. GaAs epi layers were grown in a solid source GaAs-based III-V chamber. The wafers were
then in-situ transferred to the oxide chamber. Si wafers were RCA cleaned, and HF-dipped before
being put into the MBE chamber. A reconstructed (7x7) RHEED was obtained prior to the oxide
growth. The oxide films were found to grow epitaxially on the semiconductors, again with very
sharp interfaces. Furthermore, the structure of the oxide films was found to change from the
monoclinic phase with a low dielectric constant to the cubic phase with a dielectric constant above
30. The amount of doping is about 15% as measured by x-ray photoelectron spectroscopy (XPS).
It was found that the cubic phase was not obtained with a lesser Y2O3 doping. The structural
characteristics of these thin oxide films were studied using high-resolution x-ray diffraction
conducted at BL17B1 wiggler beamline in Taiwan’s National Synchrotron Radiation Research
Center.
HfO2+Y2O3 system is subsitutional cubic solid solution [3], therefore, if HfO2 with cubic
fluorite structure was stabilized by a solution of Y2O3, the yttrium atoms should occupy the hafnium
atomic sites of cubic structure lattice to substitute the hafnium atoms. We conducted an AXS
experiment at Taiwan beamline 12B2 at SPring-8 in Japan to further verify the yttrium composition
was incorporated in the cubic crystalline lattice of yttrium doped in HfO2 (YDH) films.
[1] X. Zhao and D. Vanderbrit, PHYSICAL REVIEW B, V65, 233106.
[2] C.H. Hsu, et al, APL (2006) accepted.
ZhiKai Yang, u890105@alumni.nthu.edu.tw
Department of Material Science and Eng., National Tsing Hua University, Hsinchu, Taiwan
Fig1. L scans along 200 and 220 direction of monoclinic
phase HfO2
Fig 2. L scans along 200 and 220 direction of cubic
phase HfO2