Supporting Information
The role of beryllium in the band structure of MgZnO:
lifting the valence band maximum
S.S. Chen, X.H. Pan*, W. Chen, H.H. Zhang, W. Dai, P. Ding, J.Y. Huang, B. Lu, Z.Z.
Ye*
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials
and Applications, Department of Materials Science and Engineering, Zhejiang
University, Hangzhou 310027, People’s Republic of China
* Corresponding authors. Tel.: + 86 571 87952187; fax: + 86 571 87952124. E-mail
addresses: panxinhua@zju.edu.cn (X.H. Pan), yezz@zju.edu.cn (Z.Z. Ye).
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I.
XPS and SIMS evidence for the incorporation of Be.
Fig. S1 XPS spectrum of the Be element of the BexMgyZn1-x-yO film.
As shown in Fig. S1, the Be component corresponding to the Be 1s core level is
distinguished by Gaussian fitting. The Be 1s core level located at 113.3 eV is
attributed to the chemical bonding of Be-O. This weak peak indicates little content of
Be in the BexMgyZn1-x-yO film.
Fig. S2 SIMS depth profile of
Fig. S3 Cross-section view of SEM image
MgZnO/BeMgZnO heterojunction grown
of MgZnO/BeMgZnO heterojunction grown
on c-sapphire.
on c-sapphire.
A film with two-layer-structure containing 350 nm MgZnO layer and 375 nm
BeMgZnO layer was deposited on c-plane sapphire substrate under the same growth
parameters with the MgZnO and BeMgZnO films. Fig. S2 shows the SIMS depth
profile of the film with two-layer-structure. The element depth profiles are consistent
with the cross section view of SEM image in Fig. S3. It is evident that Be signal is
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mainly detected on the surface of the samples. Moreover, a very weak Be signal is
also detected on the bottom layer due to the thermal diffusion process. SIMS depth
profile offers a strong evidence of the existence of Be.
II. Calculation of the Be content.
Fig. S4 θ-2θ scanned XRD patterns of BexMgyZn1-x-yO films grown on c-sapphire.
We use the lattice parameters of the BexMgyZn1-x-yO film to estimate the
approximate content of Be. This method has been widely accepted to calculate the
content of Be.1,2 Both of the ionic radii of Be2+ and Mg2+ are less than that of Zn2+. As
a result, the c-axis lattice of BexMgyZn1-x-yO decreases. The crystalline structure of the
thin films is analyzed by X-Ray diffraction (XRD) with a Cu Kα radiation source
(λ=1.54056 Å). Fig. S4 shows typical θ–2θ XRD patterns of the BexMgyZn1-x-yO thin
films grown on c-plane sapphire substrates. The related c-axis lattice of
BexMgyZn1-x-yO can be obtained based on Bragg diffraction formula. The previous
researches claimed that the lattice constant of the BexMgyZn1-x-yO alloy follows
Vegard’s law.3-6 So, the c-axis lattice constant of the BexMgyZn1-x-yO alloy is derived
from:
c(BexMgyZn1-x-yO) = (1-x-y)c(ZnO) +xc(BeO)+yc(MgO),
where c(ZnO), c(BeO) and c(MgO) are 5.207 Å, 4.408 Å and 5.174 Å, respectively.4-6
The Mg compositions of BexMgyZn1-x-yO films are determined to be 19% and 12% by
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XPS measurements, respectively. Therefore, the corresponding Be content can be
estimated to be 0.5% and 0.7%, respectively.
III. Multiple measurements.
Fig. S5 XPS energy spectra of the Mg0.15Zn0.85O and Be0.005Mg0.19Zn0.805O films.
Table S1 Values of band offsets of the Mg0.15Zn0.85O/Be0.005Mg0.19Zn0.805O heterojunction.
Sample Zn2p3/2
VBM Zn2p3/2 - VBM ΔEV
A1
B1
1021.19 2.33
1021.00 2.12
1018.86
1018.88
-0.02
A2
B2
1021.00 2.10
1021.00 2.08
1018.90
1018.92
-0.02
A3
B3
1020.68 1.84
1020.49 1.64
1018.84
1018.85
-0.01
To demonstrate the reliability of our results, multiple measurements are required.
For each heterojunction, we have done additional three times tests. Fig. S5 shows
additional three sets of XPS energy spectra. A and B represent for the Mg0.15Zn0.85O
and Be0.005Mg0.19Zn0.805O, respectively. According to the XPS energy spectra, we
calculate the values of band offsets in Table S1. The calculated ΔEV values above are
within the error range. Although these values show deviation with the result (0.01 eV)
in the manuscript, all values are smaller than 0.03 eV. Therefore, the conclusion that
the little amount of Be lifts the VBM of MgxZn1-xO still can be drawn.
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Fig. S6 XPS energy spectra of the Mg0.11Zn0.89O and Be0.007Mg0.12Zn0.873O films.
Table S2 Values of band offsets of the Mg0.11Zn0.89O/Be0.007Mg0.12Zn0.873O heterojunction.
Sample Zn2p3/2 VBM Zn2p3/2 - VBM ΔEV
C1
1020.46
1.84
1018.62
-0.05
D1
1020.56
1.89
1018.67
C2
D2
1020.70
1020.82
1.95
2.05
1018.75
1018.77
-0.02
C3
D3
1020.84
1020.76
2.13
2.02
1018.71
1018.74
-0.03
Fig. S6 shows additional three sets of XPS energy spectra of the Mg0.11Zn0.89O
and Be0.007Mg0.12Zn0.873O films. C and D represent for the Mg0.11Zn0.89O and
Be0.007Mg0.12Zn0.873O, respectively. Table S2 shows the calculated values of band
offsets. Note that all the calculated ΔEV values are within the error range. Considering
the values of band offsets, it is, therefore, believed that the VBM of MgxZn1-xO shows
rising trends with a little amount of Be. This result agrees well with the conclusion in
our manuscript.
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References
1
Chen, M. et al. Stabilization of p-type dopant nitrogen in BeZnO ternary alloy
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epitaxial thin films. Journal of Physics D: Applied Physics 45, 455101 (2012).
Yu, J. H. et al. A study on structural formation and optical property of wide
band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system. J
3
Cryst Growth 312, 1683-1686 (2010).
Fan, X. F. et al. A direct first principles study on the structure and electronic
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properties of BexZn1−xO. Appl Phys Lett 91, 121121 (2007).
Kim, W. J. et al. Crystalline properties of wide band gap BeZnO films. J Appl
5
Phys 99, 096104 (2006).
Ryu, Y. R. et al. Wide-band gap oxide alloy: BeZnO. Appl Phys Lett 88,
052103 (2006).
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Vashaei, Z. et al. Structural variation of cubic and hexagonal MgxZn1−xO
layers grown on MgO(111)/c-sapphire. J Appl Phys 98, 054911 (2005).
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