Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications
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(zinc-rich), C
(oxygen-rich), and nonpolar ZnO faces.
[84]
Sakagami et al. measured
current–voltage and capacitance–voltage characteristics on the crystallographic C
þ
,C
,
and m faces.
[85]
Along all crystallographic axes studied, they observed nearly ohmic
behavior when surfaces were zinc-rich and rectifyi ng behavior when surfaces were
oxygen-rich. The authors’ judge ment that m-sectors are more suitable than C
þ
and C
sectors for making electrical contacts, if confirmed, has potential significance for device
fabrication that could stimulate interest in ZnO crystal growth on nonbasal-plane seeds.
8.6.4 Optical Properties
To date, the overwhelming majority of optical measurements of hydrothermal ZnO have
been at room temperature or liquid nitrogen temperature. Such measurements may
usefully compare the luminescence efficiencies of samples prepared under conditions in
which a researcher varies a growth parameter, and they provide qualitative indications of
crystal quality or the presence of impurities. However, the most sensitive indications of
crystal quality often come from optical measurements at near-liquid-helium temperatures,
where excitonic and other bands indicate the underlying quality (or lack thereof) of the
material. Broadband spectra at both high and low temperatures can provide useful
information about impurities and native defects.
Near-band-edge PL spectra, obtained at 4.2 K, are shown in Figure 8.19, which
compares hydrothermally grown and vapor-phase-grown ZnO (from AFRL-Hanscom
AFB and Eagle Picher Technologies, respectively).
[12]
The narrow-line spectra – emis-
sions from decays of excitons bound to a neutral donor and to some unidentified impurity
or defect – demonstrate that carriers in both samples have the long lifetimes characteristic
8
6
4
2
0
3.355 3.360 3.365 3.370
Energy (eV)
PL Intensity (arb. units)
ZnO, Zinc Face, Near-Band-Edge Region
Before Polishing (times 1.1x10
3
)
After (times 1)
Eagle-Picher Material
4.2K
Figure 8.19 Zinc-face PL spectra of AFRL-Hanscom AFB hydrothermal ZnO and Eagle Picher
Technologies vapor-grown ZnO. The two narrow-line spectra are for samples polished by Eagle
PicherTechnologies; the broad weak band is for the AFRL-Hanscom AFB sample after
conventional polishing. With kind permission of Dr. David Look
Properties of Bulk Hydrothermal ZnO 213
of excellent crystallinity. The weak, broad band exhibited in Figure 8.19, caused by
damage introduced by a mechanical polishing, contrasts starkly with the intense narrow
bands that characterize undamaged surfaces (which were prepared by a proprietary Eagle
Picher Technologies chemical–mechanical polish). Near-helium-temperature PL is thus
seen to be a sensitive indicator of both crystal and surface quality for ZnO.
Higher temperature PL spectra tend to be less sensitive to surface quality. Room
temperature PL from the polish-damaged C
face was approximately one-third as intense
as PL from the undamaged surface.
[12]
Thus an investigator who does not know a priori
what PL intensity should be expected from an undamaged surface might conclude,
erroneously, that ZnO sample surfaces were well-prepared. (PL on the C
þ
surface, on
the other hand, might have raised a red flag.)
[12]
The broadband low temperature PL spectrum of AFRL-Hanscom AFB sample
ZnO34A2 is shown in Figure 8.20. In addition to the excitonic and donor–acceptor bands
at energies above 3 eV, there is a broad band, centered at 2.3 eV (it is probably not a
composite of sub-bands, since its shape remains essentially unchanged over three orders of
magnitude of excitation intensity). The intensity of the broad band (measured at both 10 K
and 300 K) increases from sample ZnO34A1 to ZnO34A3, i.e., as the material becomes
more heavily colored. Similar broad extrinsic “orange” and “green” bands have been
reported in several PL and cathodoluminescence (CL) measurements of hydrothermal and
vapor-grown ZnO.
Conflicting mechanisms have been advanced for the origin of the green band. Reynolds
et al. compared the ZnO green band to the “yellow band” in GaN (a wurtzitic crystal
having a band gap similar to that of ZnO), whose origin remains a matter of debate, and
concluded that both bands arise from a transition between a shallow donor and a deep
level.
[86]
However, Garces et al . concluded, from electron paramagnetic resonance (EPR)
and PL spectra of unanne aled and annea led vapor-phase-grown ZnO, that ZnO green
bands are caused by emission from Cu
þ
and Cu
2þ
ions, respectively.
[69]
10
5
10
4
10
3
10
2
10
1
10
0
32.52
PL Intensity (a.u.)
Photon Energy (eV)
03/11/00 T = 10 K
Bulk ZnO # ZNO34a2
Max at 3.3597 eV
FWHM=0.55 meV
Q.E.= 26% (20% from Exciton)
Unfocused (0.1 W/cm
2
)
Normalized PL intensity
Focused (100 W/cm
2
)
Figure 8.20 Low temperature broadband PL spectrum of AFRL-Hanscom AFB sample
ZnO34A2. Courtesy of M. Reshchikov. With Kind Permission of M. Reshchikov
214 Growth Mechanisms and Properties of Hydrothermal ZnO
Orange and green CL have been observed in the m, C, and p sectors of hydrothermal
ZnO crystals. Strong green emission (in addition to near-band-edge emission, which we
ignore for the purposes of this discussion) was observed from C
þ
(Zn-terminated)
surfaces, and weak orange emission from C
(oxygen-terminated) surfaces; green
emission was observed from P
surfaces (Zn-terminated) and orange emission from P
þ
(oxygen-terminated) surfaces; and there was weak orange emission from the (nonpolar)
M face.
[53]
Combining these and related observations with the growth kinetic models
presented in Section 8.5.2, Sekiguchi et al.
[53]
and Urbieta et al.
[87,88]
associated the
occurrence of ZnO green and orange CL bands with impurity incorporation efficiencies
(during crystal growth) that the growth model attributes to the polarization state of ZnO
sectors (C
þ
,C
, M, etc.); similarly, UV and broadband luminescence efficiencies were
associated with presumed incorporation rates of nonradiative recombination centers. In
this vein, Sekiguchi et al. noted that orange emission was strongest in their sample that
had the highest Li concentration and lower in a flux-grown crystal that contained
virtually no Li; they also noted that use of H
2
O
2
in the hydrothermal growth solution,
which presumably lowered the concentration of oxygen vacancies, significantly reduced
the orange emission.
[53]
The broad emission band from an industrial ZnO wafer was measured. The band went
from 1.7 to 2.8 eV with a maxium intensity at approximately 2.3 eV. The nature of this
broad emission involves donor–acceptor pair recombination due to Li.
[89,90]
As already
above reported, V
Zn
as neutral defect complexes and V
O
as neutral oxygen are typically
observed in hydrothermal ZnO.
[76]
A recent overview of visible luminescence in ZnO has
more comprehensive information on the possible mechanisms of broadband emission in
ZnO bulk crystals and thin films.
[91]
We complete our summary of the optical properties of hydrothermal ZnO with
transmission spectra (Figure 8.21).
[92]
Insulating ZnO is transparent from the near-
ultraviolet almost to 10 mm (spectrum in Figure 8.21 labeled “Clear Cþ Sector”).
Electrically conducting sam ples (e.g., spectrum labeled “Dark C- sector”) exhibit a long-
wavelength free-carrier absorption tail. The optical tr anspa rency shown in Figure 8.21,
together with the high laser breakdown strength of ZnO,
[44]
have m ade ZnO a leading
candidate for transparent conducting electrodes for high-power near-infrared laser
beam-steering devices.
[93]
8.6.5 Etching and Polishing
Because the literature
[52,70,94–96]
provides in-depth discussions of the etching behavior of
ZnO, we offer a very brief summary. Oxygen-rich surfaces etch faster (e.g., in phosphoric
acid) than zinc-rich surfaces; etch pyramids and hillocks develop. The Zn surface becomes
smooth, but small pits develop where dislocations intersect the surface . The prism face
exhibits triangular etch pits that look like arrowheads pointing in the h0001i direction.
Thus, etching can be a quick and simple method for obta ining information about crystal
orientation and the polarity of ZnO facets.
Upon annealing in air, the C
þ
surface becomes smoother, and dimples gradually
disappear. On the other hand, the C
surface roughens, but the size of bumps decreases.
ZnO is a very soft material whose surface is easily damaged by polishing. In
Section 8.6.4 we showed that near-band-gap PL spectra measured at temperatures 15 K
Properties of Bulk Hydrothermal ZnO 215
are sensitive to surface damage. This sensitivity arises from the fact that the above-band-
gap excitation employed in such measurements penetra tes only a few tens of nanometers
into the material. Near-band-gap PL at higher temperatures tends to be less sensitive to
surface damage because the emission is due to transitions of thermally dissociated
excitons and of carriers that have been thermally excited into the conduction band. In
the absence of information about emission from undamaged surfaces, higher temperature
PL from damaged surfaces might erroneously be interpreted as “good enough”. The same
holds true for measurements of the widths of X-ray rocking curves.
Figure 8.21 Infrared transmission spectra of ZnO slices cut from C
þ
and C
growth sectors of
crystal ZnO37A. Reprinted from D. F. Bliss, Encyclopedia of Materials: Science and Technolo-
gy. Copyright (2008) with permission from Elsevier
216 Growth Mechanisms and Properties of Hydrothermal ZnO
Most X-ray diffraction measurements sample to depths of 1–2 mm, whi ch may include
comparatively undamaged material. In the data of Table 8.1, for example, rocking curve
widths of 50–100 arc-sec might be mistakenly attributed to mediocre crystal quality rather
than to damaged surfaces in an otherwise excellent crystal.
8.7 Conclusion
Hydrothermal growth can produce very high quality ZnO crystals. Nearly dislocation-free
growth can be produced in zinc-terminated growth sectors and on nonpolar prism faces.
Semi-insulating crystals have been grown, as well as crystals exhibiting superior low
temperature PL characteristics.
The various crystallographic faces of hydrothermal ZnO exhibit pronounced differences
not only in growth rates, but also in structural, electrical, and optical properties. The
growth mechanisms, the incorporation of impurities, and the generation of native defects
are (at least in part) connected to the polarity of the growth surfaces; however, the precise
mechanisms – and especially the properties of impurities and native defects – have not
been fully elucidated. Ex ploiting faceted morphology of hydrothermal crystals may
facilitate studies of these phenomena, many of which surely have counterparts in ZnO
thin film growth and ZnO bulk growth by other methods. The hydrothermal technique has
been used for the economical growth of large, low-defect-density ZnO crystals. Advances
in homoepitaxy P-doping, and device design and fabrication will drive demand for
hydrothermal ZnO substrates in the future.
Acknowledgements
We give special thanks to collaborators whose work either influenced or was explicitly
incorporated into this article. Finally, we thank the Air Force Office of Scientific Research
for its support of our research on ZnO and other wide-band-gap semiconductors.
Table 8.1 Comparison of X-ray rocking curves of AFRL-Hanscom AFB ZnO after application of
different polishing techniques
[12]
Sample number and
surface termination
v Scan after abrasive
polish, in arc-sec (002)
v Scan after chemical–mechanical
polish, in arc-sec (002)
34A1 C 52 17.5
34A1 Cþ 78 23.2
34A2 C 143 18.6
34A2 Cþ 214 20.3
34A3 C 62 13.2
34A3 Cþ 92 18.0
Acknowledgements 217
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220 Growth Mechanisms and Properties of Hydrothermal ZnO
9
Growth and Characterization of GaN/
ZnO Heteroepitaxy and ZnO-Based
Hybrid Devices
Ryoko Shimada and Hadis Morko
¸c
Department of Electrical and Computer Engineering, Virginia Commonwealth University,
Richmond, VA, USA
9.1 Introduction
It is a well known fact that GaN-based heterostructures have been playing a major role in
optoelectronic devices such as blue-green light-emitting diodes (LEDs), laser diodes
(LDs), color displays, moving signals , low level lighting, traffic signals, data storage, UV
and solar blind detectors, and high temperature high-power electroni cs such as field effect
transistors and Schottky devices.
[1]
The semiconductors, which show a higher electron
drift velocity, high mobility and large dielectric breakdown field, are essential for high
frequency power devices in communication and other systems. The nitride semiconductor
family satisfies many of these requirements. A mobility of 160 000 cm
2
V
1
s
1
and sheet
density of 1.5 10
12
cm
2
at 4.2 K for AlGaN/GaN have been recently reported.
[2]
Almost
on a daily basis new milestones are attained. Sapphire has been used as a substrate for GaN
growth, owing to demonstration of reasonable quantum efficiency from the LEDs and
pulsed laser devices grown on sapphire. For example, Nichia Company, Japan and others
have produced high efficiency blue LEDs with lifetimes in excess of 60 000 h, output
powers near 300 mW with efficiencies near 200 lumens per watt (lm W
1
),
[3]
despite of
high density of defects occurring in the layers. The origin of this high density of defects is
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
due to poor lattice match and thermal match of substrates, typically sapphire, to GaN. They
affect both the electrical and optical properties of the devices. The origin of the defects
has been the topic of many investigations, as detailed in reference [4]. There is still a need
to achieve more efficient violet lasers, green lasers and LEDs, which could be operated
continuously at room temperature for long lifetimes.
The role of the buffer layer is predominant for better wetting of the growing layer with
the substrate, i.e. from three-dimensional growth to layered growth. A suitable buffer
layer, either homo or hetero, is essential to grow nitride compounds to promote nucleation
centers and to reduce defect level concentration, hence, to increase the quality of the
growth. Various buffer layers have been explored to grow high quality GaN such as AlN,
GaN and ZnO. Early on AlN was identified as a good buffer layer for GaN and AlGaN on
sapphire substrates, which led to improved optical and electrical properties. As further
development took place GaN buffer layers also began to be used successfully.
[5]
It is preferable to employ a nearly lattice-matched and also atomic stacking match buffer
layers for follow up epitaxial layers, in order to avoid defect formation and to yield quality
epitaxial growth. Among these types of strategies, ZnO has been reported to be a suitable
buffer layer for GaN epitaxial layers and templates, owing to low lattice mismatch,
about less than 2% between ZnO and GaN. An additional advantage of ZnO is that it has
a common stacking order with GaN.
Pulsed laser vapor deposition (PLD), molecular beam epitaxy (MBE) , hydride vapor
phase epitaxy (HVPE), and metal organic vapor phase epitaxy (MOVPE) techniques have
been employed to grow GaN using ZnO buffer on c-plane (0001) sapphire. It was found
that the growth of ZnO is in the c-direction, which is perpendicular to the sapphire
substrate. ZnO perfectly matches with In
0.22
Ga
0.78
N. In many respects, ZnO may also be
considered as a suitable buffer for GaN because of the similarity in its physical properties
to those of GaN, such as its wide band gap leading to transparency in the visible region.
Moreover, ZnO can be grown on c-, a-, and r-plane sapphi re.
9.2 Growth of GaN/ZnO
Several techniques have been employed and are being developed to grow nitride-based
compounds on different substrates. However, we focus our attention on reviewing growth
of GaN on ZnO by these techniques and the role of ZnO as either a buffer layer or a
substrate. The PLD technique has been extensively used for deposition of oxide com-
pounds such as superconducting, ferrite, and magnetic materials and has been successfully
extended to grow nitride compounds. Vispute et al.
[6,7]
prepared GaN layers using ZnO as
a buffer layer by the PLD technique. A KrF excimer laser with a wavelength of 248 n m,
and pulse duration of 25 ns was used for ablation of a ZnO (99.99 %) target. The focused
beam energy and pulse repetition rate were 2–3 J cm
2
and 10–15 Hz, respectively
(Figure 9.1). The substrate temperature for the deposition of ZnO was in the range of
300–800
C. The background oxygen pressure was in the range of 10
5
–10
2
Torr. In
the same fashion, GaN films were deposited on ZnO/sapphire templates at 850
C under
a background NH
3
pressure of 10
6
–10
5
Torr.
Note that the growth temperature is very important. If it is too low, the obtained smaller
grains or number of grain boundaries and defect levels lead to poor quality of the growth.
222 Growth and Characterization