843
35. Doping Aspects
of Zn-Based Wide-Band-Gap Semiconductors
Doping Aspect
The present Chapter treats the wide-band-gap (defined
here as greater than 2 eV) Zn chalcogenides (as well
as ZnBeSe), i. e., ZnSe, ZnS, and ZnO, with roomtemperature band gaps of 2.7 eV, 3.7 eV, and 3.4 eV,
respectively. We shall here concentrate mainly on bulk
properties, since quantum dots and quantum wells are
treated elsewhere in this Handbook except when these
(or other nanostructures) are involved in bulk doping
(Sect. 35.1.2). The primary emphasis will be on literature from 2000 to 2004. Moreover, since there have been
few publications on ZnS in the last four years (our litarture search showed only seven publications) [35.1–7],
the present review will effectively cover ZnSe, ZnBeSe,
and ZnO.
It is well known that the primary interest in these
materials is their ability to provide light emission and/or
detection in the green and higher spectral ranges. One
of the major problems for these materials is obtaining
good bipolar doping, in particular good p-type doping
35.1
ZnSe ...................................................
35.1.1 Doping – Overview ....................
35.1.2 Results on p-Type Material
with N as the Primary Dopant .....
35.2 ZnBeSe ...............................................
35.3 ZnO ....................................................
35.3.1 Doping.....................................
35.3.2 Optical Properties ......................
References ..................................................
843
843
845
848
849
849
850
851
in ZnO is not yet well established, so this aspect is
also briefly covered.
for ZnO, ZnSe, and ZnBeSe with low fractions of Be;
this problem has for instance been reviewed for ZnO
by Pearton et al. [35.8] and by Look and Claflin [35.9]
and for ZnSe by Neumark [35.10]. A second problem,
especially for ZnSe-based devices, is that of degradation under photon irradiation, including those generated
during light emission [35.11–14]. It is for this reason
that ZnBeSe is of high interest, since Be is expected to
harden ZnSe, i. e. to reduce defect formation and thus
degradation [35.15–17].
ZnO is one of the most studied materials in
the group of II–VI semiconductors because of its
wide band gap (3.36 eV at room temperature) and
its bulk exciton-binding energy (60 meV), which is
larger than the room-temperature thermal energy. In
addition to room-temperature ultraviolet (UV) optoelectronic devices, it can be used for magnetic [35.18]
and biomedical applications [35.19] and references
therein.
35.1 ZnSe
35.1.1 Doping – Overview
Despite many years of effort, p-type doping of ZnSe
is still a problem. The main success to date has been
achieved with nitrogen as the primary dopant. Of
other dopants, Li diffuses extremely fast [35.20] and
also self-compensates via interstitial Li [35.20, 21], Na
has a predicted maximum equilibrium solubility of
5 × 1017 cm−3 [35.22] and also self-compensates (via interstitial Na), as shown by Neumark et al. [35.23], P and
As give DX centers and thus give deep levels (as summarized for instance by Neumark [35.10]), and Sb to
date has given net acceptor concentrations of only about
1016 cm−3 (see Table 35.1). Regarding, N doping, Table 35.2 lists recent results on concentrations of holes (p)
or net acceptors ([n a − n d ]), where n a (n d ) is the accep-
Part D 35
The present Chapter deals with the wide-bandgap (defined here as greater than 2 eV) Zn
chalcogenides, i. e. ZnSe, ZnS, and ZnO (mainly in
bulk form). However, since recent literature on ZnS
is minimal, the main coverage is of ZnSe and ZnO.
In addition Zn1−x Bex Se (x ≤ 0.5) is included, since
Be is expected to reduce degradation (from light
irradiation/emission) in ZnSe. The main emphasis
for all these materials is on doping, in particular
p-type doping, which has been a problem in all
cases. In addition, the origin of light emission
844
Part D
Materials for Optoelectronics and Photonics
Table 35.1 p-type doping of ZnSe with dopants other than N
p (cm−3 )
Dopant
1.5 × 1016
Sb
Sb
Sb
Sb
K
Na
Co-doping Li, I
Co-doping Li, Cl
GaAs:Zn nano-cluster
Part D 35.1
a metalorganic
na − nd (cm−3 )
Ea (meV)
Method
Reference
69
MOVPE
MOVPE
MOVPE
PVT
Eximer laser
Eximer laser
MOVPE
MBE
MOMBEa
[35.25]
[35.26]
[35.27]
[35.28]
[35.29]
[35.29]
[35.30]
[35.31]
[35.32]
≈ 1016
55±5
(7±3) × 1016
9 × 1017
5 × 1019
2 × 1016
3.8 × 1016
1 × 1017
molecular beam epitaxy
Table 35.2 Doped ZnSe with p or (n a − n d ) above 1018 cm−3
δ3 -doped
Sub-monolayer (N + Te)
Li3 N diffusion
MOVPE-grown N-doped
ZnSe/ZnTe:N δ-doped superlattice
Best p or na − nd (cm−3 )
Ea (meV)
Comments on degradation
Reference
6 × 1018
8 × 1018
1 × 1018
7 × 1018
38– 87
Expected to be minimal
30
Expected to be high
[35.24, 33]
[35.34]
[35.35]
[35.36]
Table 35.3 n-type doping of ZnSe
Dopant
n (cm−3 )
Method
Cl
Cl
MBE
Cl
MBE
Al
4.2 × 1018 − 1.2 × 1019
MBE
Br
Br
1.4–4.1 × 1017
4.0 × 1016
Vertical sublimation
PVT
In
a donor–acceptor
Dopant diffusion
Comments
Reference
The PL is dominated by the Cl0 X line at 2.797 eV
(10 K). Above 200 K, the intensity of the Cl0 X
[35.37]
line decreases rapidly due to the presence of a
nonradiative center with a thermal activation energy of ≈ 90 meV. The decrease of the Cl0 X line
over the temperature range 10 –200 K is due to
the thermal activation of the Cl0 X line bound exciton to a free exciton with abactivation energy
of ≈ 9.0 meV
At high ZnCl2 beam intensity, crystallinity deteriorates due to excess Cl atom
At low T , the dominate PL is due to neutral donorbound excitons; at high T , the dominate PL is due
to free-to-bound recombination.
At low T , two additional lines on the high-energy
side are observed (light- and heavy-hole freeexciton transitions); one additional peak at the
low-energy side (DAPa transition)
Three deep levels are reported: an acceptor-like
state at 0.55 eV above VBMb and two donor states
at 0.16 eV and 0.80 eV below CBMc
Two deep electron traps with thermal activation
energy 0.20 eV and 0.31 eV are reported
A temperature range can be found where electron concentration decreases with an increase in
temperature
pair; b valence band maximum; c conduction band maximum
[35.38]
[35.39]
[35.40, 41]
[35.42]
[35.43]
[35.44]
Doping Aspects of Zn-Based Wide-Band-Gap Semiconductors
For completeness, we also list in Table 35.3 recent
results on n-type doping.
35.1.2 Results on p-Type Material
with N as the Primary Dopant
Recent methods for p-type doping with p or [n a − n d ] exceeding 1018 cm−3 have been listed in Table 35.2. Note
that all of these use N as the primary dopant. As additional comments we note that growth by metalorganic
vapor-phase epitaxy (MOVPE) is now relatively standard, and that a quite comprehensive discussion of this
method has recently been given [35.49] (although it must
be noted that the “hole concentration” of 3 × 1018 cm−3 ,
given in Table 1 of Prete et al. [35.49] from data given
in Fujita et al. [35.50], is in fact the N concentration,
with Fujita et al. [35.50] giving p as 8 × 1017 cm−3 ); in
view of this extensive recent paper, we do not discuss
MOVPE here, but merely give in Table 35.4 some recent references (not in [35.49]). We further note that the
use of a δ-ZnSe/ZnTe superlattice (SL) resulted in average Te concentrations of around 9%, which in turn
increases the lattice mismatch between the GaAs substrate and the film, since the ZnTe lattice constant is
larger than that of ZnSe. This is expected to lead to
degradation problems [35.36].
A novel, interesting approach, which has given
net acceptor concentrations up to 6 × 1018 cm−3 , is
that of incorporating both N as a dopant and Te as
a co-dopant into the δ-layer(s) with fractional ZnTe
coverage, via molecular-beam epitaxy (MBE) [35.24];
as previously mentioned (Sect. 35.1.1), co-dopant here
means a material which aids in the incorporation of
the dopant, and it is well known that it is easy
to obtain p-type ZnTe [35.51, 52]. Electrochemical
capacitance–voltage (E–CV) profiling results for various samples are shown in Fig. 35.1 (Fig. 3 of [35.24]);
it can be seen that good doping was obtained when
three contiguous layers of N and Te were incor-
Table 35.4 Nitrogen-doped ZnSe grown by metalorganic chemical vapor deposition (MOCVD) or MOVPE
na − nd (cm−3 )
Ea (meV)
Comments
Reference
6.7 × 1017
109
ZnSe:N epilayers were grown on ZnSe substrates by low-pressure MOCVD
at 830 K and annealed in Zn saturated vapor. The net acceptor concentration
is enhanced
ZnSe:N grown on GaAs. A radio-frequency (RF) plasma nitrogen source
was used for doping
ZnSe:N grown by MOVPE with hydrazines as dopants. The acceptor concentration is limited by the residual impurities in the sources
ZnSe:N was grown by photo-assisted MOVPE. Post-growth annealing is
critical to reducing the hydrogen concentration (by a factor of 10)
[35.45]
1.2 × 1018
845
[35.35]
[35.46]
[35.47]
Part D 35.1
tor (donor) concentration, in various approaches, where
these are greater than 1018 cm−3 . We note in connection
with Table 35.2 that degradation associated with N can
be a severe problem [35.11, 12], and we also give some
comments on degradation in the table. We shall discuss
two N-doped systems in more detail below Sect. 35.1.2.
One uses delta-doping with Te as co-dopant (for this system, a material used to help in incorporating the dopant);
this system has given net acceptor concentrations up to
6 × 1018 cm−3 [35.24] with very low Te concentrations,
so that minimal degradation is expected. The second
system is that of Li3 N doping, with a report of carrier
concentrations close to 1019 cm−3 . We list recent work
on p-type dopants other than N in Table 35.1.
Interestingly, there are two reports that Sb gives quite
low activation energies, one being 69 meV [35.25] and
the other being 55 meV [35.27] (note that the activation
energy for N is 111 meV [35.48]), with the former paper
giving a net acceptor concentration of about 1016 cm−3 ;
in this connection it should still be noted that, as mentioned, As and P are generally believed to form DX
centers and give deep levels (for a summary [35.10]).
Other dopants used were K and Na, with doping carried
out via excimer laser annealing; high doping levels were
reported, but the excimer procedure would be expected
to introduce high defect densities and resultant strong
degradation (note that the maximum equilibrium solubility for Na was predicted to be about 5 × 1017 cm−3
by Van de Walle et al. [35.22]). A further approach was
that of co-doping, where the term in this case means
incorporation of both donors and acceptors; here, experimental tests were reported for Li with I in one case,
and with Cl in another, but in both cases net acceptor
concentrations were only in the 1016 cm−3 range. An
additional method was to use planes of p-type GaAs
(doped with Zn) to inject holes into ZnSe; net acceptor
concentrations of 1017 cm−3 were reported in [35.32],
where metalorganic molecular beam epitaxy (MOMBE)
was used.
35.1 ZnSe
846
Part D
Materials for Optoelectronics and Photonics
na–nd(cm3)
1015
Concentration (atoms/cm3)
1021 ≈10 nm
a)
1020
1021
1019
1016
ZnSe: N
ZnSe: δ N
1018
1020
0.06 0.09 0.12 0.15
1017
b)
1019
1019
ZnSe: (N + Te)δ
Part D 35.1
1018
ZnSe: N
1018
40 s
1017
c)
1019
0
1018
ZnSe: N
ZnSe: (N + Te)δ3
1017
0.0
0.1
0.2
0.3
0.4
0.5
Depth (µm)
Fig. 35.1 (a) Depth-dependent (n a − n d ) of a conventional
δ-doped sample with 5-ML spacer (nominal undoped
ZnSe); (b) a (N + Te) δ-doped sample with 4-ML spacer;
(c) a (N + Te) δ3 -doped sample with 7-ML. After [35.24]
porated (δ3 -doped). A very important aspect of this
system, established by subsequent work [35.33], is that
the N is preferentially located within ZnTe, which
was shown conclusively [35.33] to be present in submonolayer quantities, without formation of a standard
SL (considering a standard SL to require full monolayers). This result was established by transmission
electron microscopy (TEM), secondary-ion emission
spectroscopy (SIMS), and high-resolution X-ray diffraction (HRXRD). SIMS data were taken on a specially
prepared sample, in which the spacer regions (undoped
ZnSe separating the δ-layers) were thick enough (in
view of the SIMS resolution) that the SIMS measurements effectively gave the N and Te concentrations in
the delta region. The results for a triple-doped sample are shown in Fig. 35.2, with a Te concentration of
about 5 × 1020 cm−3 and an N concentration of about
5 × 1019 cm−3 for a “standard” 5 s Te + N deposition
0.1
20 s
0.2
10 s
0.3
0.4
N
Te
5s
0.5
0.6
0.7 0.8
Depth (µm)
Fig. 35.2 SIMS results on a δ3 -doped ZnSe:(Te, N) sample.
The upper line represents the [Te] concentration, and the
lower line represents the [N] concentration
time [35.24]. Thus, both are present at far less than
monolayer quantities.
Results from HRXRD are shown in Fig. 35.3, which
gives (004) θ − 2θ (solid black line) of a triple delta
ZnSe:(Te,N) sample; this sample was grown in the [001]
direction with a 10 nm ZnSe buffer layer, spacers of 10
monolayer (ML, where we here assume 1 ML, in the
[001] growth direction, to be half of the lattice constant), and 200 spacer/δ-region periods and was grown
using a standard Te deposition time of 5 s [35.24]. The
strongest peak, at 2θ ≈ 66.01 ◦ , is from the GaAs substrate. In addition, satellite peaks associated with the
periodic structure along the growth direction are observed. The result of a simulation using dynamical
diffraction theory [35.53–55] is shown by the dashed
line. This fit is obtained with a δ-layer and spacer thicknesses of 0.25 ML and 10.4 ML, respectively. These
values are in excellent agreement with the nominal
growth conditions. The average Te concentrations are
≈ 37% and 2.2% in the δ-layers and the spacers, respectively. The low average Te coverage and its relatively
high concentration within the δ3 -layers indicate that Te
is not uniformly distributed within these layers, and,
thus, forms ZnTe-rich nano-islands (such nano-islands
have been observed by Gu et al. [35.56], optically, in
similar samples grown without nitrogen). The relatively
Doping Aspects of Zn-Based Wide-Band-Gap Semiconductors
GaAs
SL(0)
SL(–1)
SL(+1)
SL(–2)
50
55
60
65
70
75
80
85
2Θ (deg)
Fig. 35.3 Symmetric θ − 2θ scan of a δ3 -doped ZnSe:
(Te,N). The solid black line is the experimental result, and
the dashed line is the result of simulation. For clarity, the
curves are shifted vertically
high Te concentration in the δ-layers is consistent with
doping results obtained for ZnSeTe alloys, where it was
shown that high acceptor concentrations are observed
only for Te concentrations exceeding 15% [35.57].
A further important result for understanding the doping
mechanism in the present system was that the photoluminescence (PL) quenched, with increasing temperature,
with quite a low activation energy [35.33]; results are
shown in Table 35.5, where it can be seen that the
activation energies are far lower than for N in ZnSe
(111 meV [35.48]) and decrease with increasing Te concentration, down to 38 meV, which is within the range
30–65 meV reported for ZnTe [35.36,58]. Thus, the N is
associated primarily with ZnTe, i. e., the N is embedded
primarily in Te-rich regions. It can also be noted from
the SIMS results (Fig. 35.2) that N and Te are located
in the same spatial region, and this is of course totally
consistent with the view that N is embedded in Te-rich
nano-islands.
We next consider the case of doping by diffusion of
Li3 N into MOVPE-grown material [35.34]. The view
has been expressed [35.59] that the resultant good doping was due to the incorporation of Li into Zn sites,
and N into Se sites, with both such species being acceptors. In our view, this conjecture is unlikely. Thus,
we note that in hard-to-dope wide-band-gap materials,
strong compensation is expected [35.60]; since interstitial Li is a donor, it seems very likely that considerable
Li is incorporated into the interstitial site after diffusion. Moreover, it is known [35.20] that Li diffuses very
quickly. Thus we suggest that, during contact formation, even with minimal heating, a good fraction of the
interstitial Li diffuses into precipitates, leaving the material p-type. We note that this view is reinforced by
the work of Strassburg et al. [35.61], who show that
this method works very well if doping is carried out by
ion implantation; such implantation is expected to cause
a high density of lattice defects, where defects would be
expected to act as nucleation sites for precipitation of
interstitial Li.
Last, but not least, no discussion of N doping would
be complete without pointing out that it is now realized that N, to a greater or lesser extent (depending
on conditions), does self-compensate, i. e. it does introduce donors. The nature of the donors will depend
on the Fermi level and on the Zn (Se) and N chemical potentials, as shown in theoretical analyses by
Van de Walle et al. [35.22], Kwak et al. [35.62] and
Cheong et al. [35.63]. A discussion and comparison of
these papers, as well as of the minimum requirements
for reliable first-principles calculations, has been given
by Neumark [35.10]. Additional work by Faschinger
et al. [35.64] and Gundel and Faschinger [35.65] suggested, based on first-principles calculations, a complex
between interstitial N (Ni ) and a Se vacancy (VSe ), but
no dependence on the Fermi level or chemical potentials
was given.
Moreover, experimentally, Kuskovsky et al. [35.66]
have reported a double N interstitial donor at high N
doping, and Desgardin et al. [35.67] have reported a
[VSe NSe ] complex and a VZn point defect. Furthermore,
it has also been shown that the Ni species (and probably
complexes) contribute to degradation [35.11,12,68], but
details of this process do not yet appear well understood,
and we thus merely mention its existence. However,
Table 35.5 Sample parameters and photoluminescence properties of δ3 (Te, N)-doped ZnSe
Te concentration (%)
na − nd (cm−3 )
PL quenching activation energy (meV)
<3
1.3
<1
6.0 × 1018
38
72
87
4.0 × 1018
3.0 × 1018
847
Part D 35.1
45
SL(+2)
35.1 ZnSe
848
Part D
Materials for Optoelectronics and Photonics
a point we do want to emphasize in this regard is that it
might be highly advantageous to be able to use a dopant
other than N. We thus note that, with the approach of
Lin et al. [35.24], one can envision the use of a dopant
other than N, since the acceptors are not located within
ZnSe, but rather in a favorable ZnTe-rich environment.
We thus point out that P and As are excellent p-dopants
in ZnTe [35.69, 70].
35.2 ZnBeSe
Part D 35.2
As mentioned in Sect. 35.1, the best p-type dopant
developed to date for ZnSe is nitrogen, and such
N-doped material suffers from degradation problems.
To alleviate this problem, the use of ZnBeSe has
been suggested [35.15, 16]. BeSe is harder than
ZnSe (Fig. 35.4 [35.71]) and, since it is expected that
harder materials are less susceptible to defect formation (dislocations etc.), it is expected to be less
susceptible to degradation [35.15, 16]. It has been
shown that the hardness of ZnBeSe increases with
increasing Be content, at least up to 60% Be, as
shown in Fig. 35.4 [35.71], where it should be noted
that the experimental error at the higher Be concentrations is quite large (moreover, the main interest
in ZnBeSe is in the direct-band range, i. e. below
46% [35.72]).
It can be pointed out that two additional advantages
of ZnBeSe over ZnSe are that one can adjust the lattice
constant for better lattice-matching to various materials
of interest (GaAs – the most frequently used substrate),
and that one can obtain a wider band gap. For instance,
Zn0.028 Be0.972 Se is lattice-matched to GaAs [35.73] and
Zn0.55 Be0.45 Se [35.74] is lattice-matched to Si (assuming a BeSe lattice constant of ≈ 5.138 Å). The variation
of the band gap has been studied in a number of recent
papers [35.72, 75]. One result, over the entire concentration range, is shown in Fig. 35.5 [35.72]. It can be
seen that the band gap becomes indirect for Be concentrations above 46%; thus, very high Be concentrations
are not as interesting, since they cannot be used to give
diode lasers.
Energy (eV)
Young’s modules (GPa)
160
5.6
Γ
x in Zn(1–x)BexSe alloys
5.4
10 K
5.2
140
5.0
L
4.8
120
4.6
X
4.4
100
4.2
4.0
80
L
3.8
0
0.2
0.4
0.6
0.8
x in Zn(1–x)BexSe
Fig. 35.4 Variation of elastic modulus E as a function of
alloy composition x in ZnBex Se1−x . The data points represent the average value. The squares – joined by the full line
– show results obtained under peak loads of 1 mN for alloys grown onto GaAs. Crosses are for data obtained under
10 mN for alloys grown onto GaAs and open circles show
data obtained under 1 mN for alloys grown onto GaP. We
note that in general Young’s modulus is related to material
hardness [35.17]. After [35.71]
X
3.6
3.4
3.2
3.0
Γ
2.8
0.0
0.2
0.4
0.6
0.8
1.0
Be content
Fig. 35.5 Evolution of the direct band gap ( ) and of the
main PL peak ( ) as a function of the Be content in
Zn1−x Bex Se alloys. After [35.72]
Doping Aspects of Zn-Based Wide-Band-Gap Semiconductors
35.3 ZnO
849
Table 35.6 XRD, EPD, and C–V results for undoped, N-doped, and (N + Te) δ-doped ZnBeSe epilayers [35.76]
FWHM (arcs)
Be content (%)
Te content (%)
EPD (cm−2 )
n a − n d (cm−3 )
ZnBeSe
ZnBeSe : N
ZnBeSe : (N + Te)δ
ZnBeSe : (N + Te)δ3
23
3.1
0
4 × 104
–
30
2.6
0
1 × 105
2 × 1017
45
2.6
0.3
6 × 105
3 × 1017
51
2.5
0.5
5 × 105
1.5 × 1018
A problem for ZnBeSe, as for ZnSe, is that of p-type
doping. The highest bulk net acceptor concentration in ptype ZnBeSe does not exceed ≈ 2 × 1017 cm−3 [35.76].
The best p-type results were again obtained via delta-
doping [35.76], using the same method that Lin
et al. [35.24] used for ZnSe. The results from Guo
et al. [35.76] are shown in Table 35.6 (Table I
from [35.76]).
Part D 35.3
35.3 ZnO
ZnO is a wide-band-gap (3.36 eV at room temperature) semiconductor with a bulk exciton-binding
energy (60 meV), larger than the room-temperature thermal energy, which makes this material very suitable
for a variety of applications (see recent reviews by
Pearton et al. [35.18], Heo et al. [35.87] and Djurišić
et al. [35.88]) in the UV spectral range. However, as for
ZnSe and ZnBeSe, one of the major problems for ZnO
is p-type doping, and we shall therefore emphasize this
aspect.
35.3.1 Doping
ZnO can be grown by a wide range of techniques (some
of which are listed in Table 35.7). As-grown ZnO is
usually n-type, and heavily n-type ZnO is easily obtained by using group III elements. It is assumed that
nominally undoped ZnO is n-type due to shallow native defects such as interstitial zinc (Zni ) [35.89, 90] or,
alternatively, due to the presence of hydrogen [35.91].
Experimentally, hydrogen in ZnO has been observed
Table 35.7 p-type doping of ZnO
Dopant
Growth method
Resistivity
(cm)
As
Evaporation followed
by sputtering
Hybrid beam
deposition
RF sputtering
followed by RTA
Ultrasonic spray
pyrolysis
Thermal oxidization
of Zn3 N2 thin films
Implantation
MOCVD
CVD
Direct-current (DC)
reactive
magnetron sputtering
Ultrasonic spray
pyrolysis
As
P
N
N
N
N
N
N + Al
N + In
Carrier
concentration
(cm−3 )
4 × 1018
Mobility
(cm2 /Vs)
Reference
0.4
Dopant
concentration
(cm−3 )
Mid 1019
4
[35.77]
2
3 × 1018
4 × 1017
35
[35.78]
1.0 × 1017
−1.7 × 1019
8.59 × 1018
0.53–3.51
[35.79]
24.1
[35.80]
0.59–4.4
≈ 0.03
Up to 1021
10.11–15.3
3.02
17.3
57.3
0.017
Up to 3 × 1020
4.16 × 1017
[35.81]
Up to 7.3 × 1017
1.97 × 1018
1.06 × 1018
2.25 × 1017
2.51–6.02
1
0.34
0.43
[35.82]
[35.83]
[35.84]
[35.85]
2.44 × 1018
155
[35.86]
850
Part D
Materials for Optoelectronics and Photonics
Part D 35.3
via electron paramagnetic resonance (EPR), electron
nuclear double resonance (ENDOR), optical, and IR
absorption measurements [35.92–95]. The activation energy of the hydrogen donor is 35–46 meV [35.92, 96].
We note that sometimes oxygen vacancies (VO ) are cited
as shallow donors [35.97]; however, Zhang et al. [35.90]
estimated this species to be a relatively deep level. Also,
Vanheusden et al. [35.98, 99] suggested that charged
oxygen vacancies are responsible for the deep green
luminescence in ZnO (see below).
Obtaining good p-type ZnO has however proven
difficult. There is a good discussion and summary of
growth methods as well as achieved resistivities in ptype ZnO up to 2003 in Look and Claflin [35.9] and
Look et al. [35.102]. The latter publication also discusses
background impurities in ZnO. We therefore present
only some later results and give a short discussion of
models proposed for p-type doping of ZnO.
Group V acceptors, based on theoretical arguments,
are expected to form very deep substitutional acceptors;
for instance, Park et al. [35.103] have calculated that the
ionization energies of N, P, and As are 0.40 eV, 0.93 eV,
and 1.15 eV, respectively. So successes (Table 35.7) in
obtaining p-type ZnO with N, P, and As are surprising.
Also group I (Li, Na, and K) impurities [35.103] have, in
general, lower ionization energies, but these impurities
are amphoteric and thus self-compensate. Experimentally, interstitial Li and Na donors were observed by
Orlinskii et al. [35.104], and recent attempt to use Li3 N
to dope ZnO to be p-type produced n-type conductivity
instead [35.105].
To achieve p-type doping, Wang and Zunger [35.106]
have proposed a cluster co-doping method using Ga or
Al as co-dopants along with group V dopants; experimentally, p-type ZnO has been obtained using co-doping
with Al and In (Table 35.7); N−Ga co-doping has been
attempted [35.107] but no p-type conductivity has been
observed via the Hall effect.
Recently, to explain p-type ZnO obtained via
group V doping, Limpijumnong et al. [35.108] proposed, using first-principles calculations, that group V
elements give shallow acceptors by forming complexes
with native defects. Specifically, these authors proposed
that ZnO:As and ZnO:Sb are p-type due to AsZn −2VZn
and SbZn −2VZn complexes, which behave as shallow
acceptors. These complexes have low formation energies (1.59 eV and 2.00 eV, respectively) as well as low
ionization energies (0.15 eV and 0.16 eV, respectively).
Experimentally, the activation energy for ZnO:As was
reported to be between 0.12 [35.78] and 0.18 [35.109].
As for nitrogen, the most often used p-type dopant,
Look et al. [35.77] reported that the ionization energy
was as low as 0.090 eV for heavily doped material (see
also [35.110] and references therein).
Regarding doping using phosphorous, we note that
Kim et al. [35.79] obtained p-type ZnO only after annealing at high temperatures using rapid thermal annealing
(RTA), while as-grown material was n-type. The authors
suggested that the annealing removes the compensating
donors; however, we suggest that the formation of shallow acceptor complexes cannot be ruled out, especially
in view of enhanced n-type behavior with increased P
concentration [35.87].
Finally, we note that Lee and Chang [35.111] have
proposed, theoretically, ways to use [group I – Hydrogen] complexes for p-type doping. These authors have
found that an intentional co-doping with H impurities
suppresses the formation of compensating interstitials
and greatly enhances the solubility of Li and Na acceptors. This type of effect, in general, was clearly predicted
by Neumark [35.112]. H atoms can be easily removed
from ZnO by post-growth annealing at relatively low
temperatures. Apparently, this method is similar to that
used to obtain p-type GaN. These authors [35.111] also
found, as did Park et al. [35.103], that Li and Na have
lower ionization energies than substitutional group V
dopants such as nitrogen.
35.3.2 Optical Properties
Finally, we shall briefly discuss some optical properties of ZnO. Low-temperature PL of undoped ZnO
is dominated by near-band, edge emission, with
up to 20 lines observed within the spectral range
3.34–3.38 eV [35.96]. Detailed studies of bound excitons (BX) and donor–acceptor pair luminescence have
recently been published by Meyer et al. [35.96], so here
Table 35.8 Low-temperature bound-excitonic position and
assignments
BX line
energy
(eV)
≈ 3.3567
Assignment
In
Donor
binding
energy (meV)
63.2
≈ 3.3598
Ga
54.6
≈ 3.3628
H
≈ 3.3608
Al
37
35
46.1
54.8
51.55
References
[35.96,
100]
[35.96,
101]
[35.92]
[35.95]
[35.96]
[35.96]
[35.100]
Doping Aspects of Zn-Based Wide-Band-Gap Semiconductors
usually observed at (2.38 ± 0.04) eV [35.118–121]. The
origin of this band, however, remains controversial: transitions associated with OZn antisites [35.119], oxygen
vacancies [35.97–99, 122], zinc interstitials [35.123],
ZnO antisites [35.124], donor–acceptor pairs [35.125],
and Cu2+ ions [35.126] have all been suggested. It must
be noted that the origin of the green luminescence could
be different in ZnO prepared via different methods, since
various defects and/or impurities can contribute to the
emission [35.127]. We note that oxygen vacancies are
the species most often suggested as the defect associated with the green luminescence. Oxygen vacancies can
have three states – neutral, singly and doubly positively
charged. The transition thus depends on the type of free
carrier that is participating in recombination. Vanheusden et al. [35.98, 99] suggested that holes participate in
this recombination while Djurišić et al. [35.88] (also references therein) suggested the involvement of electrons.
We have recently shown that the green luminescence
most likely involves holes rather than electrons [35.128],
via studies of quantum ZnO wires. A similar conclusion
was also recently obtained by Kang et al. [35.129] who
investigated PLD-grown ZnO. Their conclusion was that
the most likely channel for the green PL is through a deep
donor (attributed to oxygen vacancies) and the holes
in the valence bands. Lastly, a further suggestion by
Harada and Hashimoto [35.130] is that surface states
associated with oxygen vacancies could play a significant role in the emission within the visible spectral
region in bulk ZnO.
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Part D 35
we merely summarize some of their results and compare
these with other reports.
An important conclusion was that all boundexcitonic lines are due to neutral donor-bound excitons,
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