Optical properties and electronic structure of rock-salt ZnO under pressure

Abstract

This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45+/-0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40+/-3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.

Full text

Optical properties and electronic structure of rock-salt ZnO under pressure
A. Segura
a)
and J. A. Sans
Institut de Cie
`
ncia dels Materials-Dpto. de Fı
´
sica Aplicada, Universitat de Vale
`
ncia, Ed. Investigacio
´
,
E-46100 Burjassot (Vale
`
ncia), Spain
F. J. Manjo
´
n
Departimento de Fı
´
sica Aplicada, Universitat Polite
`
cnica de Vale
`
ncia, 03801 Alcoy (Alicante), Spain
A. Mun
˜
oz and M. J. Herrera-Cabrera
Departimento De Fı
´
sica Fundamental II, Universidad de la Laguna, 38204 La Laguna (Tenerife), Spain
共Received 10 February 2003; accepted 14 May 2003兲
This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt
phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica
have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of
2.45⫾ 0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct
transition is observed 共4.6 eV at 10 GPa兲, with a positive pressure coefficient 共around 40
⫾ 3 meV/GPa between 5 and 19 GPa兲. These results are interpreted on the basis of first-principles
electronic band structure calculations. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1591995兴
Zinc oxide 共ZnO兲 is attracting a renewed attention owing
to its potential applications in ultraviolet optoelectronic
devices.
1
At room pressure, ZnO crystallizes in the wurtzite
structure 共W-ZnO兲 and transits to the rock-salt structure 共RS-
ZnO兲 at about 9 GPa.
2
Upon decompression, the reverse
transition in bulk crystals has a large hysteresis and occurs
below 4 GPa.
3,4
Several authors have claimed to be able to
obtain metastable RS-ZnO at ambient conditions.
5,6
This
possibility has recently been proved for nanocrystalline
samples in which the RS phase is maintained at ambient
conditions after a pressure cycle up to 16 GPa.
7
It has also
been proved that thin films of Mg
x
Zn
1⫺x
O grown by pulsed
laser deposition 共PLD兲 have the RS structure for x⬎ 0.5 with
band-gap energies larger than 5 eV and increasing with the
Mg content.
8
Therefore, the knowledge of the electronic
structure of RS-ZnO and, more specifically, of the nature and
value of its band-gap, is a subject of interest for future tech-
nological applications of these nanocrystallites or thin films.
Apart from being a powerful material preparation tech-
nique, high pressure is also a very efficient tool for under-
standing the electronic structure of semiconductors.
9
How-
ever, relatively few experimental results have been reported
on the electronic structure of W-ZnO under pressure,
10–12
as
compared to other II-VI compounds. The same holds for
RS-ZnO, whose electronic properties have been investigated
only in theoretical papers,
13–15
which predict an indirect
semiconductor character.
In this letter, we report on the pressure dependence of
the absorption edge of RS-ZnO measured at room tempera-
ture 共RT兲 and discuss the results on the basis of first-
principles density-functional-theory calculations.
For the optical absorption measurements, both W-ZnO
bulk single crystals and thin films were used. Large bulk
single crystals were grown by the vapor-phase method
16
and
broken into small splints with parallel faces and 15-
␮
m thick
for measurements in the diamond anvil cell 共DAC兲. Thin
films were prepared by PLD on mica monocrystalline sub-
strates.
The target for the thin-film preparation was a com-
pressed pellet of 5N ZnO powder annealed at 950°C for 6 h
in air atmosphere. Samples were prepared at relatively low
temperature (400 °C) with a dynamically controlled atmo-
sphere of 5N oxygen at 2⫻10
⫺ 4
mbar and then subjected to
6 h annealing in air at higher temperatures 共up to 600°C).
For optical absorption measurements in the UV-Vis-NIR
range under pressure, a sample was placed together with a
ruby chip into a 200-
␮
m-diameter hole drilled on a 50-
␮
m-
thick Inconel™ gasket and inserted between the diamonds of
a membrane-type DAC.
17
Methanol–ethanol–water 共16:3:1兲
was used as a pressure transmitting medium, and the pressure
was determined through the ruby luminescence linear
scale.
18
The optical setup was similar to the one described in
Ref. 17. It consists of a Xe lamp, fused silica lenses, reflect-
ing optics objectives, and an UV-Vis spectrometer, which
allows for transmission measurements up to the absorption
edge of IIA diamonds 共about 5.5 eV兲.
The electronic structure at different pressures of RS-ZnO
has been calculated through the ab initio total energy
pseudopotential plane wave method using the density func-
tional theory 共DFT兲 in the framework of the local density
approximation 共LDA兲. The semi-core 3d electrons of Zn are
treated as forming part of the valence states. Plane waves up
to 130 Ry energy cutoff were used in order to have highly
convergent results.
As regards the wurtzite phase, in both the bulk and thin
film samples, a monotonous blueshift of the absorption edge
is observed as pressure increases. In the experiment with
bulk ZnO samples, the use of a 15-
␮
m-thick sample allows
only for the observation of the low-energy tail of the absorp-
tion edge in W-ZnO. The transition from the W-ZnO bulk
sample to the NaCl phase occurs at about 9.5⫾ 0.2 GPa, and
is observed as a neat change in the shape of the absorption
coefficient 关see Fig. 1共b兲兴. The absorption edge exhibits a
structure related to the wurtzite phase up to 16 GPa. Above
a兲
Electronic mail: alfredo.segura@uv.es
APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 2 14 JULY 2003
2780003-6951/2003/83(2)/278/3/$20.00 © 2003 American Institute of Physics
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that pressure, and contrary to what happens for most high-
pressure phases of II-VI semiconductors, the sample does not
exhibit any trace of light scattering and the absorption edge
can be accurately measured. Figure 2 shows the absorption
edge in RS-ZnO as measured in the down-stroke from 19 to
4 GPa with the bulk sample.
The low-energy tail of the absorption edge has a qua-
dratic dependence on the photon energy at all pressures, as
illustrated in the inset of Fig. 2 for P⫽14.5 GPa. Then this
tail can be assigned to an indirect transition with a band-gap
of 2.47⫾ 0.02 eV 共at 14.5 GPa兲. At other pressures 共from 4.7
to 19.9 GPa兲 the band-gap values vary between 2.33 and
2.61 eV, but do not show any clear trend of pressure depen-
dence. The average value would be 2.45⫾ 0.15 eV. This is
about half the theoretical values predicted by correlated
Hartree–Fock calculations 共5.54 eV兲
13
or by quasiparticle
methods 共4.51 eV兲.
15
Our result is also to be compared to the extrapolated
value of the band gap of RS-Mg
x
Zn
1⫺ x
O thin films for x
⫽ 0, that is slightly higher than 3 eV.
8
This is a normal dis-
agreement, as transmission measurements in thin films are
not sensitive to the low values of the absorption coefficient
typical of indirect transitions and, consequently, yield over-
estimated band-gap values.
Contrary to the low-energy tail, the absorption spectrum
at higher photon energies exhibits a clear pressure depen-
dence. Above 3.5 eV 共see Fig. 2兲, the absorption edge be-
comes steeper and blueshifts with a increasing pressure. The
pressure coefficient, as determined from the shift of the pho-
ton energy at a constant absorption coefficient, is about 34
⫾ 2 meV/GPa between 5 and 19 GPa.
In ZnO/mica thin films the phase transition to the NaCl
phase is observed also at about 9.5⫾ 0.2 GPa, as a sudden
change in the absorption edge, from the step-like shape typi-
cal of the wurtzite phase, with its exciton-related maximum,
to a structureless absorption tail 关see Fig. 1共a兲兴. As discussed
earlier, the small thickness of the films prevents the observa-
tion of the indirect transition at 2.47 eV. Instead, an absorp-
tion edge at photon energies higher than 4.5 eV is observed,
which is likely related to an allowed direct transition. Figure
3 shows this absorption edge at several pressures. The inset
of Fig. 3 shows the pressure dependence of the direct gap, as
determined from extrapolating the linear part of the
␣
2
ver-
sus h
v
plot at each pressure. The pressure coefficient of this
absorption edge is 40⫾ 3 meV/GPa.
Figure 4 shows the pressure dependence of the ab initio
calculated electronic band structure, on the basis of which we
will discuss the experimental results. RS-ZnO turns out to be
an indirect semiconductor. Its conduction band minimum
共CBM兲, located at the ⌫ point of the Brillouin zone 共BZ兲,is
about 1.1 eV above the valence band maxima 共VBM兲, lo-
cated at the L point and midway in the ⌫–K 共or ⌺兲 direction.
The fundamental transition is correctly predicted to be indi-
rect, even if the band gap is underestimated 共1.1 eV versus
the experimental value of 2.45 eV兲.Inthe⌫ point of the BZ
in the RS structure (O
h
point group兲 p and d states belong to
different representations and do not mix. Away from the ⌫
point, p and d states mix and the resulting p–d repulsion
results in the upwards dispersion of p bands in directions
⌫–K 共⌺兲 and ⌫–L 共⌳兲 of the BZ Consequently, the VBM
FIG. 1. Change of the absorption coefficient at the wurtzite-to-rock-salt
transition in ZnO thin films on mica 共a兲 and bulk samples 共b兲.
FIG. 2. RT absorption edge of rock-salt ZnO at different pressures, as mea-
sured in a bulk sample. Inset: square root of the absorption coefficient as a
function of photon energy at 14.5 GPa.
FIG. 3. RT absorption edge of rock-salt ZnO at different pressures, as mea-
sured in a thin film deposited on mica. Inset: pressure dependence of the
photon energy at a constant absorption coefficient (
␣
⫽ 10
5
cm
⫺ 1
).
279Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003 Segura
et al.
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occurs away from the ⌫ point and the material has indirect
character.
The calculated pressure coefficient of the indirect band
gaps 共L–⌫ and ⌺–⌫兲 are both positive (⬇25 meV/GPa).
This result contrasts with the pressure insensitivity of the
observed indirect transition and prevents an unambiguous
assignment. On the opposite, the direct transition at higher
photon energy can be reasonably assigned. Direct transitions
at X and L points have very large energies and pressure
coefficients 共see Fig. 4兲, then the most reasonable assignment
seems to be the ⌫–⌫ direct transition. This assignment is
supported by the comparison of the theoretical pressure co-
efficient for this direct transition 共45.4 meV/GPa兲 and the
experimental one (40⫾ 3 meV/GPa).
Table I compares the pressure coefficients and deforma-
tion potentials of the ⌫–⌫ direct transition in ZnO wurtzite
and RS phases. The fact that both parameters are much larger
in the RS phase is a consequence of role played by p–d
interaction in each phase. P–d repulsion in the valence band
has been proposed to be responsible for the so-called band-
gap anomaly in wurtzite II-VI
20
and in I-III-VI
semiconductors.
21
It has also been invoked as responsible for
the low deformation potential of the band gap in wurtzite and
zinc-blende II-VI materials.
22
According to that model, when
p–d mixing is symmetry forbidden, one should expect de-
formation potentials for direct transitions at the ⌫ point to be
close to those in light III-V zinc-blende compounds,
22
as it
actually happens in RS-ZnO. In the same direction, it is also
relevant to notice that the ‘‘band-gap anomaly,’’
20,21
illus-
trated in the II-VI wurtzite semiconductors by the larger
band gap of ZnS 共3.7 eV兲 with respect to ZnO 共3.35 eV兲,
does not occur in the RS phase: the band gap of RS-ZnO
共2.45 eV兲 is larger than the one of RS-ZnS 共2.0 eV兲.
23
In summary, we have investigated the electronic band
structure of rock-salt ZnO in bulk crystals and thin films
observing the same wurtzite-to-rock-salt phase transition
pressure (9.5⫾ 0.2 GPa) in both types of samples. RS-ZnO is
an indirect gap semiconductor with a band gap of 2.45
⫾ 0.15 eV 共at 13.5 GPa兲. An intense direct transition at
higher energy 共about 4.5 eV at 10 GPa兲, with a large positive
pressure coefficient (40⫾ 3 meV/GPa), has also been ob-
served and assigned to the lowest direct transition at the ⌫
point of the BZ. Further investigations are needed to solve
the discrepancy between the measured and calculated ener-
gies and pressure coefficients of the indirect band gaps.
The authors thank R. Lauck for kindly providing the
bulk single crystals. This work was supported through Span-
ish Government MCYT grants MAT2002-04539-C02-01共02兲
and BFM2001-3309-C02-01 共02兲. One of the authors
共M.J.H.C.兲 wish to thank the Spanish MCYT FPI fellowship
program.
1
H. Ohta, K. Kawamura, M. Orita, N. Sarukura, M. Hirano, and H.
Hosono, Electron. Lett. 36,984共2000兲.
2
C. H. Bates, W. B. White, and R. Roy, Science 137,993共1962兲.
3
F. J. Manjo
´
n, K. Syassen, and R. Lauck, High Press. Res. 22,299共2002兲.
4
H. Karzel, W. Potzel, M. Ko
¨
fferlein, W. Scchiessel, M. Steiner, U. Hiller,
G. M. Kalvius, D. W. Mitchel, T. P. Das, P. Blaha, K. Schwarz, and M. P.
Pasternak, Phys. Rev. B 53, 11425 共1996兲.
5
L. Gerward and J. Staun Olsen, J. Synchrotron Radiat. 2,233共1995兲.
6
J. M. Recio, M. A. Blanco, V. Luan
˜
a, R. Pandey, L. Gerward, and J. Staun
Olsen, Phys. Rev. B 58, 8949 共1998兲.
7
F. Decremps, J. Pellicer-Porres, F. Datchi, J. P. Itie
´
, A. Polian, F. Baudelet,
and J. Z. Jiang, Appl. Phys. Lett. 81,4820共2002兲, and references therein.
8
S. Choopun, R. D. Vispute, W. Yang, R. P. Sharma, T. Venkatesan, and H.
Shen, Appl. Phys. Lett. 80, 1529 共2002兲.
9
W. Paul, High Pressure in Semiconductor Physics I, edited by T. Suski and
W. Paul, Semiconductors and Semimetals Vol. 54 共Academic, San Diego,
1998兲,p.1.
10
R. L. Knell and D. W. Langer, Phys. Lett. 21,270共1966兲.
11
D. R. Huffman, L. A. Schwalbe, and D. Schiferl, Solid State Commun. 44,
521 共1982兲.
12
A. Mang, K. Reimann, and St. Ru
¨
benache, Solid State Commun. 94,251
共1995兲.
13
J. E. Jaffe, R. Pandey, and A. B. Kunz, Phys. Rev. B 43, 14030 共1991兲.
14
J. E. Jaffe, J. A. Snyder, Z. Lin, and A. C. Hess, Phys. Rev. B 62,1660
共2000兲.
15
H. Q. Ni, Y. F. Lu, and Z. M. Ren, J. Appl. Phys. 91, 1339 共2002兲.
16
R. Helbig, J. Cryst. Growth 15,25共1972兲.
17
R. Le Toullec, J. P. Pinceaux, and P. Loubeyre, High Press. Res. 1,77
共1988兲.
18
G. J. Piermarini, S. Block, J. D. Barnett, and R. A. Forman, J. Appl. Phys.
46, 2774 共1975兲.
19
S. Desgreniers, Phys. Rev. B 58, 14102 共1998兲.
20
S. H. Wei and A. Zunger, Phys. Rev. B 37, 8958 共1988兲.
21
J. E. Jaffe and A. Zunger, Phys. Rev. B 29, 1882 共1984兲.
22
S. H. Wei and A. Zunger, Phys. Rev. B 60, 5404 共1999兲.
23
S. Ves, U. Schwarz, N. E. Christensen, K. Syassen, and M. Cardona, Phys.
Rev. B 42,9113共1990兲.
FIG. 4. Electronic band structure of rock-salt ZnO along high-symmetry
directions of the BZ at several pressures, as calculated through ab initio
DFT-LDA pseudopotential method.
TABLE I. Pressure coefficient and deformation potential of the ⌫–⌫ direct
transition in ZnO crystalline phases.
ZnO
phase
dE
gd
⌫⌫
/dP
共meV/GPa兲
B
0
共GPa兲
dE
gd
⌫⌫
/d ln V
共eV兲
Wurtzite 24.5⫾ 2
a
142.6⫾ 2
b
⫺ 3.5⫾0.4
RS 40⫾ 3
a
202.5⫾ 2
b
⫺ 8.1⫾0.5
a
This work.
b
Reference 19.
280 Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003 Segura
et al.
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