Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications
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284 Room Temperature Stimulated Emission and ZnO-Based Lasers
11
ZnO-Based Ultraviolet Detectors
Jian Zhong and Yicheng Lu
Department of Electrical and Computer Engineering,
Rutgers University, Piscataway, NJ, USA
(
Email address: ylu@rci.rutgers.edu)
11.1 Introduction
The ultraviolet (UV) region covers the spectral wavelength of 400–10 nm. The UV
radiation is commonly divided into three regions:
[1,2]
.
UV-A (400–320 nm);
.
UV-B (320–280 nm);
.
UV-C (280–10 nm).
Another widely used division of the UV wavelengths is:
.
near-UV (400–300 nm);
.
middle-UV (300–200 nm);
.
far-UV (200–100 nm);
.
extreme-UV (100–10 nm).
In UV semiconductor photodetectors, electron–hole pairs are generated when incident
photons at UV wavelengths are absorbed in the semiconductor. The photogenerated
electrons and holes are separated by the electric field due to the presence of built-in
potential or external applied voltage. A photocurrent is then produced and collected
proportional to the incident UV radiation power.
ZnO is a wide-band-gap semiconduct or that is currently undergoing a renaissance
because of its many promising properties. The earliest work on ZnO can be dated back to
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
the 1930s, particula rly optical characterizations in the 1950s and 1960s.
[3–5]
ZnO has
the following advantages:
[6]
(i) a 60-meV exciton binding energy for more efficient
optical emission and detection (much higher than the 25 meV of GaN and 4 meV of
GaAs), which is 2.4 times the thermal energy at room temperature; (ii) a commercially
available native substrate; (iii) radiation resistanc e (one order higher than that of
GaN), which is particularly useful for detectors operating in hostile envir onments;
[7]
(iv) low cost for the basic materials; (v) wet-chemical processi ng is feas ible, which eases
the fabrication process and cuts the production cost; and (vi) high-quality epitaxial films
and nanostructures can be grown at a low temperature (400
C), hundreds of degrees
lower than GaN. A high saturation velocity has been predicted to be 3 10
7
cm s
1
,
[8]
potentially leading to fast electronic and optoelectronic devices.
The UV photoreponses in ZnO films were first reported by Mollwo
[9]
and Miller
[10]
in
the early 1950s. Since then, photoconduction from various types of ZnO materials has
been extensively studied, including single crystal,
[11,12]
polycrystalline films,
[13–18]
epi-
taxial films
[19–21]
and nanostructures.
[22–26]
Recently, in the research and development of
p-type doping,
[27,28]
ZnO has demonstrated potentially superior properties for optoelec-
tronic devices, including light-emitting diodes (LEDs) and UV photodetectors.
By alloying ZnO with MgO to form the ternary compound Mg
x
Zn
1x
O, the energy band
gap of ZnO can be increased from 3.3 to 7.8 eV.
[29–31]
Ryu et al. proposed Be
x
Zn
1x
Oasa
good candidate for modulating the band gap to the deep UV region.
[32]
Since BeO has the
same wurtzite structure of ZnO, the band gap of Be
x
Zn
1x
O can be tuned from 3.37 to
10.6 eV without phase segregation. The quaternary alloy, Zn
1xy
Be
x
Mg
y
O, continuously
tunes the band gap from 3.7 to 4.9 eV.
[33]
The band gap engineering for ZnO opens up for
the broad application areas, such as solar blind and UV-sensitive space detection, in situ
combustion monitoring, and biodetection.
Several good reviews on semiconductor UV photodetectors have been published,
[1,2]
which cover the basic theory of photoconductive and photovoltaic detectors built on other
wide-band-gap semiconductors. Table 11.1 summarizes the basic mater ial parameters of
semiconductors used for UV detection.
ZnO UV photodetectors are categorized in the following device configurations:
1. photoconductive detector;
2. Schottky barrier photodetector;
3. metal–insulator–metal (MIS) structure;
4. p-n junction and p-i-n photodiode;
5. field-effect phototransistor, including thin-film transistor (TFT);
6. integrated surface acoustic wave (SAW) and photoconductive wireless UV detectors
(UV-SAW);
[35]
Among these, p-n photodiodes have the advantages of fast photoresponse speed and
low dark current. However, because of the difficulties in obtaining high quality and reliable
p-type materials, ZnO homojunction-based photodetectors show poor performances at the
current stage. In this chapter, we will first review photoconductivity mechanism of ZnO for
UV detection. Various types of ZnO-based UV photodetectors will be discussed, including
photoconductive-type, Schottky-type, UV-SAW, and Mg
x
Zn
1-x
O photodetectors, etc.
Finally, ZnO one-dimensional (1D) nanostructure-based UV detectors are reviewed and
compared with the thin-film devices.
286 ZnO-Based Ultraviolet Detectors
Table 11.1 Basic material parameters of semiconductors used for UV detection
[2,34]
Si GaAs GaP 3C-SiC 4H-SiC 6H-SiC Diamond GaN AIN ZnS ZnSe CdS ZnO
E
g
(eV) 1.12 1.43 2.27 2.39 3.2 2.86 5.5 3.39 6.2 3.6 2.82 2.50 3.37
direct direct direct direct direct direct direct
Thermal conductivity
(W cm
1
K
1
)
1.5 0.5 1.1 3.2 4.9 4.9 20 1.3 3.19 (0.18)
Melting point (K) 1683 1513 1740 2830 2830 3773 1100 1750 1975
Electron saturation velocity
(10
7
cm s
1
)
1 2 1.25 2 2 2.7 2.5 1.4 3
Mobility (cm
2
V
1
s
1
)
Electrons 1400 8500 350 1000 950 400 2200 1000 135 165 340 205
Holes 600 400 100 50 120 75 1600 30 14 5 340 180
Dielectric constant 11.8 12.5 11.1 9.7 9.7 9.7 5.5 8.9 8.5 9.6 9.1
Breakdown field (10
5
Vcm
1
) 3 6 10 20 20 24 100 26 20
Introduction 287
11.2 Photoconductivity in ZnO
It has been found that the photoresponse of ZnO consists of two components: a fast
response and a slow response. The fast response results from the reversible solid-state
process, such as intrinsic interband or excitonic transition, while the slow one is governed
by the surface-related oxygen adsorption and photodesorption process, or by the bulk
defect related recombination process.
[11,15,36–39]
In polycrystalline ZnO films, the slow photoreponse component is found to be the
dominant process.
[15,38,40]
The slow process in the photoresponse of polycrystalline ZnO
has been mainly attributed to a two-step process, in which free carriers interact with
surface states and grain boundaries:
i. Under the dark condition, oxygen adsorbs at the surface states (e.g. oxygen
deficiency sites) to create a negatively charged ion by capturing a free electron
from n-type ZnO:
O
2
ðgÞþe
!O
2
ðadÞð11:1Þ
Thus, a depletion region and band bending are created near the surface, resulting in a
decrease of the film conductivity.
ii. Under UV irradiation, forced by the electrical field in the surface depletion region, the
photogenerated holes move toward the surface to neutralize the negative oxygen ion.
This process is called the photodesorption of O
2
:
h
þ
þO
2
ðadÞ!O
2
ðgÞð11:2Þ
When the free carriers generated by optical absorption are spatially separated, the film
conductivity is increased.
This model has been supported by photoconductivity decay measurements of ZnO,
where the decay is slower in vacuum but faster in oxygen.
[15,36,37,41]
A hole-capture model
in polycrystalline ZnO films was proposed by using photo-Hall and transient photocon-
ductivity measurements.
[36–38]
The transi ent photoconductivity study shows that the hole-
capture process is of nonthermal nature; the excitation rate of photoconductivity does not
depend on temperature but depends on ambient oxygen partial pressure. The photocon-
ductivity relaxation is attributed to electron tunneling to the surface oxygen states. The
slow photoconductivity process associated with the surface trap states has been also
observed in the epitaxial ZnO films.
In general, the fast photoresponse process in ZnO is related to the solid-state process
such as intrinsic interband photoconductivity, which is a general photoconduction process
in semiconductor materials. When the energy of incident photons is higher than the energy
band gap, electrons are excited from the valence band to the conduction band. Free
electrons and holes are generated:
hn !h
þ
þe
ð11:3Þ
Thus, the film conductivity is increased. This process is characterized by the cut-off
wavelength of the photoresponse corresponding to the energy band gap.
288 ZnO-Based Ultraviolet Detectors
Other photoconduction mechanisms, including exciton photoconductivity and phonon-
assisted photoconductivity, have been also connected with the fast photoresponse process
in ZnO. An exciton is a hydrogen-like two-particle system composed of an electron and a
hole formed by coulomb interaction. Since the exciton is typically quasi-charge-neutral, it
does not directly contribute to the photoconductivity. However, exciton photoconductivity
has been observed in II–VI semiconductors including ZnO,
[42,43]
which may be due to
ionization of the excitons. The mechanisms of the exciton ionization could be: (i)
ionization under the electrical field; (ii) exciton–phonon or exciton–impurity
interaction; and (iii) the Auger process. A phonon-assisted photoconductivity is caused
by interaction between photogenerated carriers and phonons. Oscillations in
photoconductivity of single crystal ZnO have been observed in the infrar ed (IR)
region. By studying the oscillation energies in the surface photoconductivity spectra,
L
€
uth attributed the oscillations to the bulk longitudinal optical (LO) phonons and surface
phonons.
[44]
In order to make a high speed UV photodetector, it is critical to use the high quality ZnO
films in which the slow photoresponse process is suppressed by reducing the trap density
and background carrier concentration, and the fast process is dominant.
Effects of ambient gas. Zinc oxide shows a strong chemisorption behavior at the surface,
which has been widely used for various chemical/gas sensing. The conductivity and
photoconductivity of ZnO films are strongly dependent on ambient gas conditions.
[15,45–47]
An oxidizing atmosphere, such as CO
2
and water vapor, effectively accelerates the decay
of photoconductivity in ZnO films.
[15]
It is suggested that oxygen in water vapor may
diffuse into the interior part of ZnO films from the surface through pores inside. The
oxygen adsorption modifies the electrical conductivity and photoresponse time of the ZnO
films.
Effects of oxygen plasma treatment and surface passivation. Enhancement of UV
photoresponse properties of a ZnO photoconductor is obtained by use of an oxygen plasma
treatment. Liu and Kim reported that an oxygen plasma treatment dramatically improved
the UV detection properties of ZnO metal–semiconductor–metal (MSM) photoconductors
grown by radio frequency (RF)-magnetron sputtering,
[48]
through reducing the decay time
constant and increasing the on/off ratio of photocurrent. The as-grown ultrathin (20 nm)
ZnO film was exposed to the oxygen plasma for 30 min using an induc tively coupled
plasma reactive ion etching (ICP-RIE) system with ICP power of 125 W and RIE power of
5 W, oxygen pressure of 1.5 10
2
Torr and a flow rate of 30 sccm. Figure 11.1 shows the
current–voltage (I–V) characteristics of (a) an as-grown ZnO MSM photoconductor and
(b) an oxygen-plasma-treated ZnO MSM photoconductor. The dark leakage current of
ZnO photoconductor is reduced from 5 mA to 400 pA at 3 V after the oxygen plasma
treatment. The normalized photocurrent transient is measured in air and in vacuum
(10
6
Torr) ambient and compared for the as-grown and oxygen-plasma-treated ZnO
MSM photoconductor, respectively, shown in Figure 11.2 (a) and (b).
The as-grown ZnO photoconductor exhibits a slow photocurrent decay profile with a
time constant in an order of tens of minutes, which is strongly dependent on the oxygen
ambient pressure [Figure 11.2(a)]. The slow photocurrent decay process indicates that a
slow charge-transfer process due to the oxygen chemisorption occurs, which is slower in
vacuum than in air. After the oxygen plasma treatment, the ZnO photoconductor becomes
insensitive to the ambient with a dramatically reduced decay time constant (below 10 ms)
Photoconductivity in ZnO 289
[Figure 11.2(b)]. It is suggested that the plasma treatment suppresses the chemisorption
sites (primarily the oxygen deficiency sites) on the ZnO surface and the oxygen vacancies
in bulk ZnO, leading to reduction of the chemisorption of oxygen through the surface and
decrease of free carrier concentration in the bulk for the dark current. In the plasma
treatment process, highly diffusive atomic oxygen may effectively fill the chemisorption
sites on the ZnO surface, while positive oxygen ions in the plasma may neutralize the
negatively charged oxygen molecular ions adsorbed on the ZnO surface.
Surface passivation effectively suppresses the slow photoresponse decay in the ZnO
photoconductor. One of the surface passivation schemes is to deposit a SiO
2
layer after wet
chemical etching of bare ZnO surface in 30% H
2
O
2
for 15 min.
[49]
However, the quantum
efficiency is also reduced.
Effects of trap levels. The slow UV photoresponse process of ZnO photodetector is
controlled by the trap levels on the surface and in the bulk ZnO. Sh arma et al. reported a
very slow decay and rise time of photocurrent (800 ms) in polycrystalline ZnO films
Figure 11.2 Photocurrent transients of ZnO photoconductors with (a) as-grown ZnO film
measured in air (dashed curve) and in vacuum (10
6
Torr) (solid curve) ambient and (b)
oxygen-plasma-treated ZnO film measured in air (solid curve) and in vacuum (10
6
Torr)
(dashed curve) ambient. Reprinted from M. Liu and H. K. Kim, Ultraviolet detection with
ultrathin ZnO epitaxial films treated with oxygen plasma, Appl. Phys. Lett. 84, 173. Copyright
(2004) with permission from American Institute of Physics
Figure 11.1 I–V characteristics of ZnO photoconductors with (a) as-grown ZnO film and (b)
oxygen-plasma-treated ZnO film. Reprinted from M. Liu and H. K. Kim, Ultraviolet detection
with ultrathin ZnO epitaxial films treated with oxygen plasma, Appl. Phys. Lett. 84, 173.
Copyright (2004) with permission from American Institute of Physics
290 ZnO-Based Ultraviolet Detectors
grown by unbalanced RF-magnetron sputtering. They attributed the observed slow time
response to the trap levels in the band gap.
[50]
Oh et al. found the trap centers in ZnO layers with different Zn/O flux ratios by
admittance spectroscopy.
[51]
The ZnO layers exhibited three electron-trap centers ET1
(33 meV), ET2 (0.14 eV) and ET3 (65 meV), near the conduction-band edge, where ET1
and ET2 are ascribed to singly and doubly ionized Zn interstitials, respectively. The trap
densities of ET1, ET2, and ET3 were estimated to be 3 10
15
cm
3
,210
16
cm
3
and
3 10
15
cm
3
, respectively. Since the ZnO layers used were grown under Zn-rich growth
conditions, it is suggested that the large photoresponse time observed in ZnO layers can be
ascribed to Zn-interstitial type defects.
[52]
Epitaxial ZnO thin films grown by pulsed laser deposition (PLD) were investigated by
deep level transient spectroscopy (DLTS) and by Fourier transform infrared photocurrent
(FTIR-PC) spectroscopy.
[53]
The IR photo current of undoped ZnO films shows transitions
between 100 meV and 500 meV from deep defect states to the band edges, including
intrinsic deep d efects E1 at 110 meV and E3 at 320 meV. Cordaro and Shim
reported trap levels T1 at 170 meV and T2 at 330 meV below the conduction band
edges, and related them to the second ionization of zinc interstitial and the oxygen
vacancy, respectively.
[54]
There are reports of trap levels L1 (close to T1) and L2 at 170
and 260 meV and E1, E3 (close to T2) and E4 at 120, 290 and 570 meV,
respectively.
[55,56]
However, the chemical origins of many trap states related to the
photoconduction process in ZnO are still unclear.
Doping effects. Doping and co-doping with multiple elements (N, Te, P, Cu, etc.) are
used to improve the photoresponse of n-type ZnO films.
[19,57–59]
Photosensitivity is
defined as a ratio of the photocurrent to the dark current:
Photosensitivity ¼
I
photo
I
dark
1 þ
Dn
n
0
ð11:4Þ
where n
0
is the background carrier concentration and Dn is the photogenerated excess
carrier density. When the equilibrium carrier concentration is suppressed, the photosensi-
tivity is increased. The approach to use the doping and co-doping methods is to reduce the
background carrier concentration of n-type ZnO films by incorporating compensation
dopants of group V elements.
Ultraviolet photoresponse of ZnO film deposited by PLD was enhanced by phosphorous
doping and post-annealing.
[57]
Phosphorous introduced a deep level in the energy band
gap, heavily compensating the defect-related shallow donors in the intri nsically n-type
ZnO films. On the other hand, the incorporated acceptor atoms such as N and P may
compensate the oxygen vacancies at the ZnO surface, reducing the photoresponse time.
Porter et al. proposed to co-dope Zn O films with nitrogen and tellurium during PLD to
improve the photoconductivity.
[58]
The isoelectronic impurity Te is chosen as a co-dopant
due to the anion-rich nature of ZnTe, in contrast to the cation-rich nature of ZnO. After N
and Te co-doping, the sample’s dark resistivity increases from 10
2
–10
1
to 10
3
–10
5
W cm.
A peak photoconductivity was achieved at a Te concentration of 10
20
cm
3
, presumably
limited by the solid solubility of Te in ZnO.
Effects of surface polarity . Surface polarity is known to strongly affect the photocon-
ductivity of ZnO. In this study, ZnO single crystal samples were prepared by a
hydrothermal method.
[60]
After chemical–mechanical polishing, the ZnO substrates were
annealed at 850
C for 2 h. One side of the c-plane ZnO substrates has an O-face, and the
Photoconductivity in ZnO 291
other side has a Zn-face. The spectral responses of photocurrent for different faces of the
c-plane ZnO single crystals were measured under UV irradiation of 10 mW cm
2
.
Figure 11.3 shows the spectral response of the O-face and the Zn-face of a c-plane ZnO
single crystal. Photocurrent of the O-face is about two orders of magnitude larger than that
of the Zn-face. It is also noted that the photoresponse of the O-face ZnO in the shorter
wavelength of 250–200 nm is stronger than that of the Zn-face ZnO.
The different behavior for the photoresponsivity of O- and Zn-face ZnO single crystals
is explained by an energy band diagram model illustrated in Figure 11.4. On the O-face,
the surface-terminated oxygen atoms act as the acceptor-like surface states. They are deep
Figure 11.4 Energy band diagram model for (a) O-face and (b) Zn-face of a c-plane ZnO
single crystal. Reprinted from F. Masuoka, et al. Phys. Stat. Sol. (c) 3, 1238 (2006). Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission
Figure 11.3 Spectral response of the photocurrent of c-plane ZnO single crystals: (a) O-face;
and (b) Zn-face. Reprinted from F. Masuoka, et al. Phys. Stat. Sol. (c) 3, 1238 (2006).
Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission
292 ZnO-Based Ultraviolet Detectors