Supplemental Material for “Impact of Mg Content on Native Point Defects in MgxZn1-xO (0
≤x≤0.56) by J. Perkins et al.
(Mg,Zn)O Growth - A series of 1 µm thick (Mg,Zn)O samples were grown on r-plane sapphire
in a Riber Epineat MBE system equipped with effusion cells for elemental Zn and Mg. Atomic
oxygen was supplied via an Addon radio-frequency (rf) plasma cell using an rf power operating
at 320W under a flux of 0.5sccm. The growth temperature was controlled by a thermocouple
located behind the substrate and by an infrared pyrometer. The epilayers were grown at 400 °C at
a growth rate of 0.2 mm/h. The Mg concentration was controlled by changing the temperature of
the Mg cell and, thus the flux ratio between the Zn and the Mg elements from 40 to 25.
Additional details of the growth conditions for a-MgZnO/r-sapphire can be found in ref.. J.-M.
Chauveau, J. Vives, J. Zuniga-Perez, M. Laugt, M. Teisseire, C. Deparis, C. Morhain and B.
Vinter, Appl. Phys. Lett., 2008, 93, 231911. J.-M. Chauveau, M. Laugt, P. Vennéguès, M.
Teisseire, B. Lo, C. Deparis, C. Morhain and B. Vinter, Semicond. Sci. Technol., 2008, 23,
035005. Under these conditions, the growth is slightly Zn-rich but close to the stoichiometry.
Defect Thermodynamic Stability - According to ref. 12 (Rep. Prog. Phys. 72 (2009) 126501),
oxygen vacancies and zinc vacancies are the lowest energy defects, followed by the Zn
interstitial and the ZnO antisite. O vacancies and OZn antisites are found to be higher in energy.
Under O-rich conditions, oxygen interstitials are more likely than O vacancies but still higher in
energy than Zn vacancies. If present, Zn interstitials would be shallow donors, e.g., 30-50 meV,
which is not resolvable in the near band edge region. The formation energy of oxygen
interstitials is “very high except under extreme O-rich conditions,” which is opposite to our Znrich MBE growth conditions, and would appear as a deep acceptor in n-type material at ~ 2.8 eV,
which our spectra do not exhibit.
Sample Growth and Reproducibility – Samples were measured after growth by
photoluminescence spectroscopy (PL),RBS and energy dispersive X-ray (EDX). The measured
compositions are close to those of the nominal value with a deviation of +/-2%. The control of
the composition is reproducible because it only depends on the growth rate and the temperature
of the Mg Cell, which is the only modified parameter in this series.
Experimental Conditions: Depth-Resolved Cathodoluminescence Spectroscopy (DRCLS) Compared to photoluminescence spectroscopy (PL), DRCLS yields very similar results for bulk
1
excitation of thick samples. In this paper, we emphasize the importance of probing away from
the surface to avoid strong surface segregation effects. PL excitation is maximum at the surface,
decaying exponentially away with increasing depth whereas the 2 keV beam excitation is
primarily below this segregated region. 2 keV excitation provides representative bulk spectra
without possibly exciting the film near its interface with the growth substrate. Zn vacancy defect
densities were determined by direct comparison between DRCL spectra and positron annihilation
spectroscopy (PAS) spectra. Positron annihilation spectroscopy is a well-established and
quantitative tool to calibrate defect densities.
Integrating peak areas with two types of peak fitting to the spectra yielded nearly the same
variation (orders-of-magnitude decrease and minimum) of I(defect)/I(NBE) as the peak intensity
ratios plotted in Figure 4, which required no peak fitting. Peak energies are extracted from
DRCL spectra on linear scales.
Surface Photovoltage Spectroscopy (SPS) - SPS was performed with a gold tip, in single-pass
mode, and a frequency of 0.1 Hz. Samples were allowed to reach a steady state in the dark for 20
minutes before scanning to allow charge in deep levels to reach equilibrium. Multiple scans were
taken at different spots on the surface to insure representative SPS spectra. Transition energies
are obtained from the clearly pronounced slope changes in SPS. Transient SPS provided
densities of 0.097, 0.235, 2.03, and 1.28 x 1016 cm-3 for x = 0.31, 0.44, 0.52, and 0.56,
respectively, in good agreement with deep level optical spectroscopy (DLOS) E3 densities5 and
consistent with the pronounced defect decreases with Mg addition. Similar agreement between
DLOS, DLTS and t-SPS using the same XE-70 KPFM was reported for etched ZnO6 and
neutron-irradiated GaN.7
Band Gap Variation - Figure S1 displays the band gap variation with Mg alloy composition
over the widest range measured in high quality (Mg,Zn)O. For 0 < x < 0.52, the band gap EG
appears to increase linearly according to EG(x) = 3.309 +1.69*x, above which it increases at a
higher rate toward the 7.8 eV gap of MgO, reflecting the crystal strain present near the wurtziterocksalt crossover. These band gap variations are consistent with theory1 and other reports.2,3,4
2
Figure S1. Comparison of NBE energies versus Mg content for DRCLS, SPS, and SSPC.
Also shown is a comparison of band gap measurements of the (Mg,Zn)O alloys studied in this
paper by steady state photocapacitance (SSPC)5, DRCLS, and SPS. All three methods measure a
similar band gap increase with Mg content that deviate by ≤ 5%. Compared with the SSPC
threshold, the primary DRCLS peak is several percent lower while the SPS onset is slightly
higher. All three are consistent with the nearly linear increase of (Mg,Zn)O band gap with Mg
content up to 52% Mg. An optical absorption threshold of 4.5 eV for 56% Mg agrees with SSPC
and DRCLS values, consistent with the single phase wurtzite crystal structure of the entire alloy
series.
1. R. Schmidt-Grund, D. Fritsch, M. Schubert, B. Rheinländer, H. Schmidt, H. Hochmuth, M. Lorenz,
C.M. Herzinger, M. Grundmann, AIP Conf. Pro. 772, 201 (2005).
2. M. W. Williams and E. T. Arakawa, J. Appl. Phys. 38, 5272 (1967).
3. A.S. Rao and R.J. Keamey, Phys. Status Solidi (b) 95, 243 (1979).
4. T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, and Y. Hamakawa, Thin Solid Films 372, 173
(2000).
5. E. Gür, G. Tabares, A. Arehart, J. M. Chauveau, A. Hierro and S. A. Ringel, J. Appl. Phys. 112,
123709 (2012).
3
6. Z. Zhang, V. Quemener, C.-H. Lin, B.G. Svensson, and L.J. Brillson, Appl. Phys. Lett. 103,
072107 (2013).
7. C.- H. Lin, E. J. Katz, J.Qui, Z. Zhang, U. K. Mishra, L. Cao, and L. J. Brillson, Appl. Phys.
Lett. 103, 162106 (2013).
4