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A comprehensive review of zno materials and devices

30 Aug 2005-Journal of Applied Physics (AIP Publishing)-Vol. 98, Iss: 4, pp 041301

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of ZnO materials and
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Ü. Özgür
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Ya. I. Alivov
Virginia Commonwealth University
C. Liu
Virginia Commonwealth University
See next page for additional authors
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Authors
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APPLIED PHYSICS REVIEWS
A comprehensive review of ZnO materials and devices
Ü. Özgür,
a
Ya. I. Alivov, C. Liu, A. Teke,
b
M. A. Reshchikov, S. Doğan,
c
V. Avrutin,
S.-J. Cho, and H. Morkoç
d
Department of Electrical Engineering and Physics Department, Virginia Commonwealth University,
Richmond, Virginia 23284-3072
Received 2 February 2005; accepted 13 June 2005; published online 30 August 2005
The semiconductor ZnO has gained substantial interest in the research community in part because
of its large exciton binding energy 60 meV which could lead to lasing action based on exciton
recombination even above room temperature. Even though research focusing on ZnO goes back
many decades, the renewed interest is fueled by availability of high-quality substrates and reports of
p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which
remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As
mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter
dating back to 1935 by Bunn Proc. Phys. Soc. London 47, 836 1935兲兴, studies of its vibrational
properties with Raman scattering in 1966 by Damen et al. Phys. Rev. 142, 570 1966兲兴, detailed
optical studies in 1954 by Mollwo Z. Angew. Phys. 6, 257 1954兲兴, and its growth by
chemical-vapor transport in 1970 by Galli and Coker Appl. Phys. Lett. 16, 439 1970兲兴. In terms
of devices, Au Schottky barriers in 1965 by Mead Phys. Lett. 18, 218 1965兲兴, demonstration of
light-emitting diodes 1967 by Drapak Semiconductors 2, 624 1968兲兴, in which Cu
2
O was used
as the p-type material, metal-insulator-semiconductor structures 1974 by Minami et al. Jpn. J.
Appl. Phys. 13, 1475 1974兲兴, ZnO/ZnSe n-p junctions 1975 by Tsurkan et al. Semiconductors
6, 1183 1975兲兴, and Al/Au Ohmic contacts by Brillson J. Vac. Sci. Technol. 15, 1378 1978兲兴
were attained. The main obstacle to the development of ZnO has been the lack of reproducible and
low-resistivity p-type ZnO, as recently discussed by Look and Claflin Phys. Status Solidi B 241,
624 2004兲兴. While ZnO already has many industrial applications owing to its piezoelectric
properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet
materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be
called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have
been prepared early on and used to deduce much of the principal properties of this material,
particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the
last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for
optoelectronic applications. The attraction can simply be attributed to the large exciton binding
energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based
emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also
fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room
temperature for potential spintronics applications. This review gives an in-depth discussion of the
mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological
issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
© 2005 American Institute of Physics. DOI: 10.1063/1.1992666
TABLE OF CONTENTS
I. INTRODUCTION............................ 2
II. PROPERTIES OF ZnO....................... 3
A. Crystal structures....................... 3
B. Lattice parameters....................... 6
C. Electronic band structure................. 7
D. Mechanical properties.................... 12
E. Lattice dynamics........................ 15
F. Thermal properties...................... 18
1. Thermal-expansion coefficients.......... 18
2. Thermal conductivity.................. 19
3. Specific heat......................... 21
G. Electrical properties of undoped ZnO....... 22
1. Low-field transport................... 23
a
Electronic mail: uozgur@vcu.edu
b
Present address: Balikesir University, Faculty of Art and Science, Depart-
ment of Physics, 10100 Balikesir, Turkey.
c
Present address: Atatürk University, Faculty of Art and Science, Depart-
ment of Physics, 25240 Erzurum, Turkey.
d
Electronic mail: hmorkoc@vcu.edu
JOURNAL OF APPLIED PHYSICS 98, 041301 2005
0021-8979/2005/984/041301/103/$22.50 © 2005 American Institute of Physics98, 041301-1
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2. High-field transport................... 25
III. ZnO GROWTH............................. 26
A. Bulk growth........................... 26
B. Substrates............................. 28
C. rf magnetron sputtering.................. 29
D. Molecular-beam epitaxy.................. 30
E. Pulsed-laser deposition................... 34
F. Chemical-vapor deposition................ 36
IV. OPTICAL PROPERTIES OF ZnO.............. 39
A. Prelude............................... 39
B. Optical transitions in ZnO................ 39
1. Free excitons and polaritons............ 39
2. Bound excitons...................... 42
3. Two-electron satellites in PL............ 44
4. DAP and LO-phonon replicas in PL...... 45
5. Temperature-dependent PL measurements.. 45
C. Time-resolved PL on ZnO................ 47
D. Refractive index of ZnO................. 48
E. Stimulated emission in ZnO............... 51
1. Thin films........................... 51
2. Polycrystalline ZnO films and “random
lasers”.............................. 54
3. Multiple-quantum wells................ 56
4. Stimulated-emission dynamics.......... 56
V. DEFECTS IN ZnO........................... 57
A. Predictions from first principles............ 57
B. Experimental studies of native and
unintentionally introduced defects.......... 58
1. Shallow acceptor in ZnO............... 58
2. Green luminescence band.............. 59
3. Yellow luminescence band............. 61
4. Red luminescence band................ 62
VI. DOPING OF ZnO.......................... 62
A. n-type doping.......................... 62
B. p-type doping..........................
63
1. Nitrogen doping...................... 63
2. Codoping method: Nitrogen+group III... 65
3. Other dopants in group V.............. 67
VII. ZnO-BASED DILUTE MAGNETIC
SEMICONDUCTORS....................... 68
A. Theory of ZnO-based magnetic
semiconductors......................... 69
B. Experimental results on ZnO-based
magnetic semiconductors................. 72
VIII. BAND-GAP ENGINEERING................ 76
A. Mg
x
Zn
1−x
O alloy........................ 77
B. Cd
y
Zn
1−y
O alloy........................ 78
IX. PROCESSING, DEVICES, AND
HETEROSTRUCTURES..................... 79
A. Ohmic contacts to ZnO.................. 79
B. Schottky contacts to ZnO................. 80
C. Heterostructure devices.................. 82
1. Light-emitting devices................. 83
2. Photodiodes......................... 85
D. Metal-insulator-semiconductor diodes....... 86
E. Transparent thin-film transistors............ 87
X. ZnO NANOSTRUCTURES. .................. 88
XI. SUMMARY............................... 95
I. INTRODUCTION
There has been a great deal of interest in zinc oxide
ZnO semiconductor materials lately, as seen from a surge
of a relevant number of publications. The interest in ZnO is
fueled and fanned by its prospects in optoelectronics appli-
cations owing to its direct wide band gap E
g
3.3 eV at
300 K. Some optoelectronic applications of ZnO overlap
with that of GaN, another wide-gap semiconductor E
g
3.4 eV at 300 K which is widely used for production of
green, blue-ultraviolet, and white light-emitting devices.
However, ZnO has some advantages over GaN among which
are the availability of fairly high-quality ZnO bulk single
crystals and a large exciton binding energy 共⬃60 meV. ZnO
also has much simpler crystal-growth technology, resulting
in a potentially lower cost for ZnO-based devices.
As indicated in the abstract, ZnO is not really a newly
discovered material. Research on ZnO has continued for
many decades with interest following a roller-coaster pattern.
Interest in this material at the time of this writing is again at
a high point. In terms of its characterization, reports go back
to 1935 or even earlier. For example, lattice parameters of
ZnO were investigated for many decades.
1–9
Similarly, opti-
cal properties and processes in ZnO as well as its refractive
index were extensively studied many decades ago.
10–25
Vi-
brational properties by techniques such as Raman scattering
were also determined early on.
26–32
Investigations of ZnO
properties presumes that ZnO samples were available.
Growth methods not much different from what is employed
lately have been explored, among which are chemical-vapor
transport,
33
vapor-phase growth,
34
hydrothermal growth
35
which also had the additional motivation of doping with Li
in an effort to obtain p-type material, high-quality platelets,
36
and so on.
37
The ZnO bulk crystals have been grown by a number of
methods, as has been reviewed recently,
38
and large-size
ZnO substrates are available.
39–41
High-quality ZnO films
can be grown at relatively low temperatures less than
700 °C. The large exciton binding energy of 60 meV
paves the way for an intense near-band-edge excitonic emis-
sion at room and higher temperatures, because this value is
2.4 times that of the room-temperature RT thermal energy
k
B
T=25 meV. There have also been a number of reports on
laser emission from ZnO-based structures at RT and beyond.
It should be noted that besides the above-mentioned proper-
ties of ZnO, there are additional properties which make it
preferable over other wide-band-gap materials: its high-
energy radiation stability and amenability to wet chemical
etching.
38
Several experiments confirmed that ZnO is very
resistive to high-energy radiation,
42–44
making it a very suit-
able candidate for space applications. ZnO is easily etched in
all acids and alkalis, and this provides an opportunity for
fabrication of small-size devices. In addition, ZnO has the
same crystal structure and close lattice parameters to that of
GaN and as a result can be used as a substrate for epitaxial
growth of high-quality GaN films.
45,46
ZnO has recently found other niche applications as well,
such as fabrication of transparent thin-film transistors, where
the protective covering preventing light exposure is elimi-
041301-2 Ozgur et al. J. Appl. Phys. 98, 041301 2005
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nated since ZnO-based transistors are insensitive to visible
light. Also up to 2 10
21
cm
−3
charge carriers can be intro-
duced by heavy substitutional doping into ZnO. By control-
ling the doping level electrical properties can be changed
from insulator through n-type semiconductor to metal while
maintaining optical transparency that makes it useful for
transparent electrodes in flat-panel displays and solar cells.
ZnO is also a promising candidate for spintronics applica-
tions. Dietl et al.
47
predicted a Curie temperature of 300 K
for Mn-doped ZnO. n-type doping in Fe-, Co-, or Ni-alloyed
ZnO was predicted to stabilize high-Curie-temperature ferro-
magnetism. There have been a number of publications which
appear to confirm these predictions, as has been reviewed
recently,
48,49
albeit with a good deal of controversy, as will
be discussed in Sec. VII.
However, one important problem should be overcome
before ZnO could potentially make inroads into the world of
optoelectronics devices: the growth of p-type-conductivity
ZnO crystals. Despite all the progress that has been made
and the reports of p-type conductivity in ZnO films using
various growth methods and various group-V dopant ele-
ments N, P, As, and Sb, a reliable and reproducible high-
quality p-type conductivity has not yet been achieved for
ZnO. Therefore, it remains to be the most pivotal topic in
ZnO research today, and congruently most of the research
efforts are directed just to solving this problem. In order to
overcome this bottleneck and to control the material’s prop-
erties, a clear understanding of physical processes in ZnO is
necessary in addition to obtaining low n-type background. In
spite of many decades of investigations, some of the basic
properties of ZnO still remain unclear. For example, the na-
ture of the residual n-type conductivity in undoped ZnO
films, whether being due to impurities of some native defect
or defects, is still under some degree of debate. Some authors
ascribe the residual background to intrinsic defects oxygen
vacancies V
O
and interstitial zinc atoms Zn
i
兲兴, and others
to noncontrollable hydrogen impurities introduced during
growth. The well-known green band in ZnO luminescence
spectra manifesting itself as a broad peak around
500530 nm, observed nearly in all samples regardless of
growth conditions, is related to singly ionized oxygen vacan-
cies by some and to residual copper impurities by others.
Simply, a requisite consensus is lacking.
While p-type ZnO is difficult to attain, the advantages of
ZnO are being explored and exploited by alternative methods
such as heteroepitaxy in which p-n heterostructures can be
obtained by depositing n-type ZnO films on other p-type
materials while still utilizing ZnO as the active layer.
Progress has been made in this arena with a number of het-
erostructures fabricated wherein one of the following, Si,
NiO, GaN, AlGaN, SiC, ZnTe, CuO, CdTe, etc., plays the
role of p-type layer. In particular, high-intensity UV emission
has been observed from the n-ZnO/p-AlGaN heterojunction
in which ZnO served as the active layer. These results are
just harbingers of what can be expected of ZnO in an effort
to position it for future device applications. As in the early
developments of GaN predating the demonstration of p-type
conductivity, metal-insulator-semiconductor device struc-
tures not requiring p-type ZnO have been introduced but lack
the high efficiency.
In this paper we collate the properties of ZnO as well as
review the recent progress in ZnO research. This present
review is distinguishable from the other reviews
50–55
in that
the previous ones focused mainly on material processing,
doping, and transport properties, while the present one treats
those topics in greater depth in addition to an in-depth dis-
cussion of the growth, optical properties, p-type doping, and
device fabrication aspects. The organization of this review is
as follows: First, structural, chemical, and electrical proper-
ties of undoped ZnO are discussed in Sec. II. This is fol-
lowed by ZnO crystal growth, both bulk and film Sec. III,
and optical properties of ZnO Sec. IV. Sections V–VII are
devoted, respectively, to defects in ZnO, doping, and mag-
netic properties. Alloys of ZnO band-gap engineering are
discussed in Sec. VIII, and this is followed by ZnO-based
devices and their applications Sec. IX. Finally, ZnO nano-
structures are reviewed in Sec. X.
II. PROPERTIES OF ZnO
In this section crystal structures, inclusive of lattice pa-
rameters, electronic band structures, mechanical properties,
inclusive of elastic contants and piezoelectric constants, lat-
tice dynamics and vibrational processes, thermal properties,
electrical properties, and low-field and high-field carrier
transports are treated.
A. Crystal structures
Most of the group-II-VI binary compound semiconduc-
tors crystallize in either cubic zinc-blende or hexagonal
wurtzite structure where each anion is surrounded by four
cations at the corners of a tetrahedron, and vice versa. This
tetrahedral coordination is typical of sp
3
covalent bonding,
but these materials also have a substantial ionic character.
ZnO is a II-VI compound semiconductor whose ionicity re-
sides at the borderline between covalent and ionic semicon-
ductor. The crystal structures shared by ZnO are wurtzite
B4 , zinc blende B3, and rocksalt B1, as schematically
shown in Fig. 1. At ambient conditions, the thermodynami-
cally stable phase is wurtzite. The zinc-blende ZnO structure
can be stabilized only by growth on cubic substrates, and the
rocksalt NaCl structure may be obtained at relatively high
pressures.
The ground-state total energy of ZnO in wurtzite, zinc-
blende, and rocksalt structures has been calculated as a func-
tion of unit-cell volume using a first-principles periodic
Hartree-Fock HF linear combination of atomic orbitals
LCAO theory by Jaffee and Hess.
56
The total-energy data
versus volume for the three phases are shown in Fig. 2 along
with the fits to the empirical functional form of the third-
order Murnaghan equation, which is used to calculate the
derived structural properties,
041301-3 Ozgur et al. J. Appl. Phys. 98, 041301 2005
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Journal ArticleDOI
Abstract: In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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23,109 citations


Book
01 Jan 1953-
Abstract: Mathematical Introduction Acoustic Phonons Plasmons, Optical Phonons, and Polarization Waves Magnons Fermion Fields and the Hartree-Fock Approximation Many-body Techniques and the Electron Gas Polarons and the Electron-phonon Interaction Superconductivity Bloch Functions - General Properties Brillouin Zones and Crystal Symmetry Dynamics of Electrons in a Magnetic Field: de Haas-van Alphen Effect and Cyclotron Resonance Magnetoresistance Calculation of Energy Bands and Fermi Surfaces Semiconductor Crystals I: Energy Bands, Cyclotron Resonance, and Impurity States Semiconductor Crystals II: Optical Absorption and Excitons Electrodynamics of Metals Acoustic Attenuation in Metals Theory of Alloys Correlation Functions and Neutron Diffraction by Crystals Recoilless Emission Green's Functions - Application to Solid State Physics Appendix: Perturbation Theory and the Electron Gas Index.

21,934 citations


Journal ArticleDOI

13,816 citations


Journal ArticleDOI
Philip W. Anderson1Institutions (1)
01 Mar 1958-Physical Review
Abstract: This paper presents a simple model for such processes as spin diffusion or conduction in the "impurity band." These processes involve transport in a lattice which is in some sense random, and in them diffusion is expected to take place via quantum jumps between localized sites. In this simple model the essential randomness is introduced by requiring the energy to vary randomly from site to site. It is shown that at low enough densities no diffusion at all can take place, and the criteria for transport to occur are given.

8,667 citations


Network Information
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No. of citations received by the Paper in previous years
YearCitations
202221
2021529
2020552
2019610
2018645
2017684