Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications
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where E
D
is the screened donor binding energy, E
D0
is the unscreened donor binding
energy, and the suggested value of a for n-type GaN is a ’2.1 10
5
meV cm.
[69]
For
ZnO we c an a pprox imate a as [(2
ZnO
/2
GaN
)(m
GaN
/m
ZnO
)]a
GaN
giving a value a
’1.2 10
5
meV cm. One sees that the screening in ZnO is somewhat weaker than
that in GaN. Assuming 1 10
16
cm
-3
free electrons, one calculates from E quation (2.5)
a reduction in the donor binding energy in ZnO of ’2.7 me V due to screenin g.
From screening alone, this should result in a blue shift of the donor-bound-exciton
(D
,X) transition.
The transition energy of D
,X is the free exciton energy minus the energy with
which the exciton is bound to the donor. By Haynes’s rule
[70]
the exciton is bound to
the donor by some fraction of the donor binding energy. It would be expected that the
free exciton binding energy will be screened very similar to that of the donor binding
energy. Therefore, in the screened case, the D
,X transition energy will be that of the
screened free exciton energy minus the energy with which the exciton is bound to
the screened donor. A reasonable est imate of the blue energy shift of D
,X with
screening would then be obtained from Equation (2.5). The D
,X PL s pectr a as a
function of Ar
þ
ion laser intensity are shown in Figure 2.22. The D
,X intensity
decreases as the Ar
þ
ion intensity increases, as expected. As the excitons are screened
from the donors, the D
,X decay route is eliminated as one of the processes by which
excitons are dissipa ted. Therefore, as more excitons are screened from the donors, the
D
,X intensity will decrease and the free exciton intensity will increase. This is
verified in Figure 2.23 where the free exciton intensity is plotted as a function of t he
Ar
þ
ion laser intensity. Here the G
5
and G
6
free excitons are clearly identified. O ne
other feature is observed in both the D
,X spectra and the free exciton spectra and that
is the energy red shift.
Figure 2.22 Intensity and energy position of the D,
X transitions in ZnO as a function of the
Ar
þ
ion laser intensity. Reprinted with permission from Donald Reynolds. Copyright American
Institute of Physics
52 Optical Properties of ZnO
The GaN sample was excited in exactly th e same way as the ZnO sample. Making the
assumption of 1 10
16
cm
-3
free electrons for GaN, then from Equation (2.5) a reduction
in the donor binding energy of 4.5 meV due to screening is obtained. The PL emission
spectra from GaN as a function of Ar
þ
ionlaserintensityareshowninFigure2.24.
Figure 2.23 Intensity and energy of the free-exciton transitions in ZnO as a function of the Ar
þ
ion laser intensity. Reprinted with permission from Donald Reynolds. Copyright American
Institute of Physics
Figure 2.24 The PL emission spectra from GaN as a function of the Ar
þ
ion laser intensity.
Reprinted with permission from Donald Reynolds. Copyright American Institute of Physics
The Combined Effects of Screening and Band Gap Renormalization 53
The D
,X emission peak (3.4809 eV) as well as the ionized-donor-bound-exciton
(D
þ
,X) p eak (3.4762 eV) are dominant when the Ar
þ
ion laser intensity is zero. As
the Ar
þ
intensity is increased, more free electrons are created which neutralize the
ionized donors, and at the highest Ar
þ
intensity the ionized donor-bound excitons are
essentially all neutralized. The D
,X ce nters experience a n increase in numbers as the
ionized dono rs are neut rali zed, but they also experience a decrease due to screening. At
the highest intensity of the Ar
þ
ion las er the free exciton associ ated with t he A valence
band (3.4875 eV) is the dominant transi tion and the free exciton associated with the B
valence band (3.4951 eV) is clearly evident. In GaN, as in ZnO, t he bound excitons
decrease in intensity while the free excitons increase in intensity with an increase in the
Ar
þ
ion laser intensi ty. The top solid line i n Figure 2.24 shows the reflection minima,
which coincide with the A- and B-band free excitons. In the case of GaN, the bound and
free excitons do not s how an obvious blue or red shift as the Ar
þ
ion laser intensity
is increased.
The addition al ele ctrons excited by the Ar
þ
ion laser may also result in many-body
effects in an intrinsic semiconductor, which will lead to a renormalization of the
fundamental energy gap. The renormalization reduces the energy gap as shown in
Figure 2.25. In this figure, the energy shift is estimated from density functional theory
within the local density appr oximation using the Hedin– Lundquist parameteriza-
tion.
[71]
Other parameterized forms exist and yield similar results.
[72]
The reduction of
the renormal ized energy gap, as a function of electron density, is shown for both GaN
and ZnO in Figure 2.25. Here it is seen that renormalization reduces the Z nO band gap
more than the GaN band gap for the same electron density. The PL energy positions are
determined by a combination o f renormaliza tion and screening, the former giving a red
Figure 2.25 The reduction of the renormalized energy gap as a function of electron density for
both ZnO and GaN. Reprinted with permission from Donald Reynolds. Copyright American
Institute of Physics
54 Optical Properties of ZnO
shift and the latter an offsetting blue shift. For a g iven electron density, the blue shift can
be obta ined from Eq uation (2.5) and the red shift fro m Figure 2 .25. Taking ele ctron
densities within an order of magnitude of those calculated to give substantial screening,
the blue and red shifts for both ZnO a nd GaN are given in Table 2.2. From the table, one
would expect to observe an increasing red shift of the exciton transitions from ZnO as
the Ar
þ
ion laser intensity is increased. In the case of GaN, essent ially no shift in the
energy positi ons of the exciton transitions woul d b e expe cted. These predic tions are
verified in Figures 2.22–2.24
These resu lts show that the energy positions of optical transitions are modified when
additional free electrons are introduced into the system. The energy positions are
determined by the combined effects of screening and band gap renormalization resulting
from the additional free electrons. This study establishes trends for the optical transition
energies, but is not intended to give the exact energy positions.
2.8 Closely Spaced Donor–Acceptor Pairs in ZnO
Closely spaced donor–acceptor pair spectra have been observed in the PL radiative
recombination of selected donor bound exciton complexes in ZnO single crystals, that
have been ion implantation-doped with H and Zn atoms and subsequently annealed in a
nitrogen (N
2
) atmosphere at temperatures in the range of 700–800
C.
[73]
Observations of
the donor–acceptor pair spectra are explained as the bound states of Hopfield’s bound
exciton complex model,
[74]
in terms of the effective mass approximation of the wurtzitic
ZnO energy band structure.
The closely spaced donor–acceptor pair spectra for a H-implanted ZnO sample are
shown in Figure 2.26; sample A (solid lin e), was implanted into the O fac e, while
sample B (dashed line), was implante d into the Zn face. Both s amples were then
annealed in N
2
vapor at 700
C for 1 h. The sample implanted into its Zn face shows an
emission line at 3.2888 eV, and an LO phonon replica at 3.2155 eV, the same energies
as observed for the implanted sample prior to annealing. The sample implanted into
the O face shows an emission line at 3 .2845 eV, and an LO phonon replica a t
3.2118 eV. It is noted that the higher energy emission lines at 3.3223 and 3 .3356 eV
do not show an energy shift with N
2
annealing. It was reported by Hopfield,
[74]
that
when donor–acceptor pairs are close enough together, they can bind neither an
Table 2.2 The blue and red energy shifts of the optical transitions in both ZnO and GaN as the
electron density is increased
Sample Electron
density (cm
3
)
Renormalization
red shift (meV)
Screening blue
shift (meV)
ZnO 5 10
15
4.5 2.1
GaN 5 10
15
3.5 3.6
ZnO 1 10
16
5.7 2.7
GaN 1 10
16
4.5 4.5
Closely Spaced Donor–Acceptor Pairs in ZnO 55
electron nor a hole but they c an bind an exciton. The exciton has an interaction energy
with the donor and acceptor ions and this interaction energy is negative. This will
move the transit ion to lower energy, as is observed. It is believed that the emission
lines at 3.2888 and 3.2155 eV are associated with more distant spaced pairs that bind
an electron and then an exciton. These lines a re observed from the O face. The
emission lines at 3.2845 and 3.2118 eV, from the Zn face, result from more closely
spaced pairs that bind only the exciton. The energies are shifted due to the total
interaction energy of the exciton with the donor and acceptor ions.
In the same samples, emission lines are observed at higher energies of 3.3606 and
3.3567 eV, as shown in Figure 2.26 (inset). Here again, from the O face (solid line), the
higher energy line is much more intense; however, when the Zn face (dashed line) is
examined, the line intensities are reversed, showing a shift to lower energy with annealing
in N
2
. As was observed with the lower energy lines, it is believed that the line at 3.3606 eV
results from more distant spaced pairs that bind an electron and then the exciton, whereas
the line at 3.3567 eV results from closely spaced pairs that bind only the exciton. The
energy shift is comparable with that of the lower energy lines at 3.2888 and 3.2845 eV,
suggesting that we are observing the interaction energy of the exciton with the closely
spaced donor–acceptor pair ions.
Similar spectra from Zn-implanted ZnO samples are shown in Figure 2.27. Here again,
sample A (solid line) is implanted into the O face and sample B (dashed line) into the Zn
face. The samples were then annealed, in this case at 800
C, for 1 h in N
2
vapor. Here we
note that there are lines at 3.2903 and 3.2173 eV that are observed from the Zn face
(dashed curve), whereas there are lower energy lines at 3.2836 and 3.2123 eV that are
observed from the O face (solid curve). The energy shift to a lower energy is observed on
the opposite face from that which was observed for the H-implanted samples.
Figure 2.26 Comparison of ZnO faces, H-implanted. Reprinted with permission from Donald
Reynolds. Copyright American Institute of Physics
56 Optical Properties of ZnO
Again, as in the H-implanted samples, higher energy lines are observed at 3.3618 and
3.3571 eV, as shown in Figure 2.27 (inset). In the Zn-implanted samples, the line at
3.3618 eV does not shift to a lower energy after annealing in N
2
, when viewed from the Zn
face (dashed line), while the more intense line at 3.3618 eV does shift to a lower energy at
3.3571 eV in the O face sample (solid line). Here again the shift to a lower energy is
believed to be caused by an exciton bound to a closely spaced pair. This shift was observed
from the O face, and it was also observed from the lower energy line at 3.2903 eV. The
energy shift from the two faces of the Zn-implanted samples is reversed from those of the
two faces of the H-implanted samples.
The Zn implant will not penetrate as deeply into a sample as will the H implant. When a
sample is annealed in N
2
, N ions will in-diffuse to occupy O sites near the surface, and pair
with donor-created sites near the surface. The more closely spaced pairs will form near the
surface, which in our case will be the O face. This accounts for the observation of the shift
to lower energy when observing the O face in Zn-implanted samples. In the H-implanted
samples, the H ion will penetrate deeper into the sample. In this case, when the sample is
annealed in N
2
vapor, the N ion must diffuse deeper into the sample to create acceptors
closer to the donors. It is believed that the acceptor results from N ions replacing O sites
and that thes e dominant acceptor sites are near the crystal surface, when viewing the O
face. Therefore, in the H-implanted samples, the closely spaced, donor–acceptor pairs
would not be created near the O surface. The N ions will diffuse deeper to occupy O sites
nearer the donors which may be H interstitials. This would result in the energy shift
occurring when observing the Zn face, as is shown, and would account for the reversal of
the faces in Zn-implanted and H-implanted samples. It is believed that the acceptor in the
donor–acceptor pair results from a N ion occupying an O site. The source of the donor is
not established and will require more study.
Figure 2.27 Comparison of ZnO faces, Zn-implanted. Reprinted with permission from Donald
Reynolds. Copyright American Institute of Physics
Closely Spaced Donor–Acceptor Pairs in ZnO 57
2.9 Summary
We have observed the G
5
and G
6
free excitons associated with the top valence band in ZnO,
as well as these same excitons associated with rotator states. The unallowed, G
6
excitons,
split in an applied magnetic field in both states, giving a g-value of 3.09, which is the sum
of the electron and hole g-values. This is the predicted splitting for the G
6
exciton from
effective mass and group theory, which is consistent with the top valence band having G
9
symmetry. We have also observed the splitting of the G
6
exciton due to combined strain
and electron–hole spin exchange, which als o reflects the G
9
symmetry of the top valence
band in ZnO.
Many bound-exciton transitions are observed in ZnO, and these include excitons bound
to neutral and ionized centers, as well as complex centers, like defect pairs. In this case,
many lines are observed which are related to the donor–acceptor spacing in the defect pair
complex. Free-to-bound and bound-to-bound transitions are observed which are not
associated with excitonic transitions. The well-known green band is associated with pair
spectra where the transition energy is associated with the donor–acceptor separation.
Similarities in some of the optical transitions in ZnO and GaN have also been observed.
This is to be expected since these materials have the same crystal structure and their band
gaps are similar.
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60 Optical Properties of ZnO
3
Electrical Transport Properties in
Zinc Oxide
B. Claflin and D. C. Look
Semiconductor Research Center, Wright State University, Dayton, OH, USA
3.1 Introduction
Zinc oxide (ZnO) is a truly remarkable and versatile material. It has long been used for a
wide variety of commercial applications ranging from phosphors, ceramics, abrasives, and
as a pigment for paints, to serving as the active (UV-absorbing) ingredient in sunscreen, a
key component in cosmetic and medical ointments, a source of Zn for mineral fortification
in foods, and as an additive for plastic and rubber processing.
[1,2]
It also possesses useful
semiconducting properties which were iden tified in the naturally occurring miner al form,
zincite, for crystal radio receivers,
[3]
and led to early electrical and optical characterization
efforts.
[4–6]
However, as-grown, unintentionally doped ZnO is almost always found to be
n-type because of high concentrations of background donors, so this early work focused
primarily on surface effects
[7–11]
for catalysis and gas sensing applications
[12]
and on the
photoresponse
[13]
of the material. The inability to identify a suitable p-type dopant for ZnO
limited progress in the development of electronic devices since p-n homojunctions could
not be formed. This doping asymmetry is fairly common in wide band gap semiconduc-
tors;
[14–19]
similar problems have limited the development of diamond films
[20]
and
initially slowed the progress of GaN-based devices.
[21]
However, demonstrations of
GaN-based blue light-emitting diodes (LEDs) in the early 1990s and diode lasers
[22–25]
in the mid-1990s generated a renewed interest in the electrical and optical properties of
ZnO. Reports of successful p-type ZnO growth
[26–33]
a few years later, while viewed with
Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, First Edition.
Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins.
© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. ISBN: 978-0-470-51971-4