Litton C.W., . Reynolds D.C., Collins T.C. Zinc Oxide Materials for Electronic and Optoelectronic Device Applications

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up to 750 times higher UV photoresponse (l ¼365 nm) than that of a bare ZnO

nanobelt.

[114]

The mechanism for this giant photoconductivity is attributed to the efﬁcient

exciton dissociation under UV excitation, which is caused by enhanced electron hopping

transfer from the ZnO VB to the PP-AN then transit to the ZnO CB. In addition, various

schemes of surface passivation were conducted with Si

/SiO

[111]

polymide,

[112]

poly

(methyl methacrylate) (PMMA)

[113]

and polyacrylonitrile coating.

[114]

To remove surface

contamination, oxygen plasma cleaning is typically applied to the sample prior to the spin

coating of polymers or CVD deposition of SiO

11.4.2 Noise Characteristics of ZnO NW UV Photodetector

Noise equivalent power (NEP) and normalized detectivity (D



) are the main parameters to

characterize the signal-to-noise performance of a photodetector. NEP is the optical input

power for which the signal-to-noise ratio is equal to one. D



is the reciprocal of the NEP,

which is normalized for bandwidth and the photodetector area.

Noise characteristics of vertically well -aligned ZnO NW UV photodetectors were

reported.

[116]

To fabricate the vertical-structured device, undoped ZnO NWs were ﬁrst

grown on a ZnO:Ga/glass template by a self-catalyzed vapor–liquid–solid method at

600



C. To prevent the metal particles from falling onto the bottom and shorting the device,

a thin spin-on-glass (SOG) ﬁlm was spin coated onto the top of ZnO NWs followed by

thermal annealing. Au was used as the top electrode, with a diameter of 200 mm.

Figure 11.35(a) shows the FESEM image of as-grown ZnO NWs on a ZnO:Ga/glass

template, and Figure 11.35(b) is a top view of ZnO NWs coated with SOG.

Figure 11.36(a) shows the spectral response of a vertical ZnO NW photodetector. The

peak photoresponse occurs at 365 nm, with a UV to visible rejection ratio of more than

three orders of magnitude (R 365 nm/R 425 nm). The ZnO NW photodetector has a

maximum quantum efﬁciency of 12.6%. Figure 11.36(b) shows the measured low-

frequency noise power spectra under various applied biases. In a ZnO NW photodetector,

Figure 11.34 (a) Spectral response of an array of ZnO NWs in air (triangles) and in vacuum

(10

4

Torr) (circles), at an applied bias of 0.3V and a modulation frequency of 160 Hz. (b) Plot

of photoconductive gain vs photon absorption rate. Reprinted from C. Soci, et al., ZnO

Nanowire UV Photodetectors with High Internal Gain, Nano Lett 7, 1003. Copyright (2007)

with permission from American Chemical Society

ZnO NW UV Photodetectors 323

at low frequency (100 Hz), the noise power spectra are dominated by 1/f noise. Above

100 Hz, the noise decreases as 1/ f

, possibly due to interfacial traps in the SOG ﬁlm.

The total noise current power is estimated by:

ðf Þdf ¼

ð1Þdf þ

ðf Þ¼S

lnðBÞþ1½ð11:36Þ

where S

(f) is the spectral density of the noise power and B is the photodetector bandw idth.

We assume that S

(f) ¼S

(1 Hz) when f G 1 Hz. NEP and D



are then given by:

NEP ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃ

ð11:37Þ

Figure 11.36 (a) Spectral response of ZnO NW photodetector at an applied bias of 1 V. (b)

Measured low-frequency noise power spectra under various applied biases. Reprinted from C.

Lu, S. Chang, S. Chang, Y. Chiou, C. Kuo, H. Chang, C. Hsu, and I. Chen, IEEE sensors J. 7,

1020. Copyright (2007) IEEE

Figure 11.35 FESEM images of (a) the cross-sectional view of as-grown ZnO NWs on a ZnO:

Ga/glass template and (b) the top view of ZnO NWs coated with SOG. Reprinted from C. Lu, S.

Chang, S. Chang, Y. Chiou, C. Kuo, H. Chang, C. Hsu, and I. Chen, IEEE sensors J. 7, 1020.

324 ZnO-Based Ultraviolet Detectors

ﬃﬃﬃ

NEP

ð11:38Þ

where R is the responsivity of the photodetector, A is the area of the photodetector and B is

the bandwidth. For a given bandwidth of 100 Hz at 1 V bias, the NEP and D



of the NW

photodetector are 5.73 10

11

W and 6.17 10

cm Hz

1/2

1

, respectively. The NEP of

the NW photodetector is more than one order of mag nitude larger than that of the planar-

structured ZnO MSM UV photodetectors (see Table 11.2). This is presumably attributed to

the much larger surface area and larger carrier trapping and detrapping effects of the NWs.

11.5 Conclusions

This chapter reviews the progr ess of ZnO UV photodetectors. ZnO-based materials and

nanostructures possess great promise for UV detection technologies. Various types of

ZnO-based UV photodetectors have been demonstrated. The ZnO photoconductive MSM

photodetectors show a large photoresponsivity and fast photoresponse time due to the

excellent material quality of the ZnO epitaxial ﬁlms. The ZnO epitaxial ﬁlm-based

Schottky barrier photodetector has been demonstrated, showing a faster photoresponse

component of tens of nanoseconds. Through band gap engineering, Mg

1x

O photo-

detectors extend the cut-off wavelength to the deep UV region, which is important for

solar-blind applications. ZnO can be made multifunctional through proper doping. The

semiconducting ZnO has been integrated with piezoelectric ZnO to form a multilayer

structure where the acousto-optic interaction is used for the novel UV detection technolo-

gy, such as UV-SAW detectors. Unlike the traditional photodetectors which have the

output in current or voltage, the ZnO UV-SAW photodetector has its output in the

frequency doma in, suitable for a wireless sensor and low power operation. Recently, ZnO

nanostructures have been emerging as one of most widely used functional nanomaterials.

The single crystalline 1D ZnO nanostructures can be grown on various substrates,

including glass, GaN, Si, metals, and ﬂexible substrates at low temperature. ZnO NW-

based photodetectors demonstrate the very large internal gain, which is promising for

unique applications such as single-photon detection.

Currently, the development of ZnO p-n junction- based UV detectors is still in the early

stage due to the constraints of lack of high quality and reliable p-doping technology. With

successful development of ZnO p-type materials and multiple quantum well technology,

ZnO-based devices will become one of the major UV detection technologies and ﬁnd

broad applications.

Acknowledgements

This work was supported by NSF, AFOSR, and NJ Commission of Science and

Technology. One of the authors (YL) acknowledges the technical assistance from his

PhD students and postdoctors, S. Liang, Y. Liu, H. Sheng, N. W. Emanetoglu, Y. M. Chen,

J. Zhu, Y. Chen, C. R. Gorla and S. Muthukumar.

Acknowledgements 325

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Room-Temperature Stimulated

Emission from ZnO Multiple Quantum

Wells Grown on Lattice-Matched

Substrates

Takayuki Makino

, Yusaburo Segawa

, Masashi Kawasaki

and Hideomi Koinuma

WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan

Cross-Correlated Materials Research Group, Advanced Science Institute,

RIKEN, Wako, Japan

WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan

Cross-Correlated Materials Research Group, Advanced Science Institute, RIKEN, Wako,

Japan, and CREST, Japan Science and Technology Agency, Tokyo, Japan

Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba, Japan

12.1 Introduction

Recent demonstration of zinc oxide (ZnO) heterostructures opens up signiﬁcant opportu-

nities for fabrication of electronic and photonic nanodevices. In particular, semiconductor

quantum structures with well-deﬁned interfaces are the main components for nanoscale

resonant tunneling devices, ﬁeld effect transistors, and light-emitting devices (LEDs). There

have been many studies recently on the properties of wide-gap semiconductors due to the

industrial demand for short-wavelength LEDs or laser diodes. ZnO is a wide-gap semicon-

ductor with a direct gap around 3.4 eV (i.e. in the near UV in the electromagnetic spectrum)

and crystallizes preferentially in the hexagonal wurtzite-type structures.

Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, First Edition.

Edited by Cole W. Litton, Donald C. Reynolds and Thomas C. Collins.

Howe v er, ZnO has some adv antages over gallium nitrides

[1,2]

among which are the

av ailability of fairly high-quality bulk single crystals and a large exciton binding energy

(60 meV). It has also much simpler crystal growth technology, resulting in a potentially lower

cost for the devices. High quality ZnO ﬁlms can be grown at relatively low temperatures. The

large exciton binding energy of 60 meV allows us to observe an intense near-band-edge

emission at room and higher temperatures, because this value is 2.4 times greater than that of

the room temperature (RT) thermal energy (approximately 25 meV). One crucial problem

should be ov ercome for the real application of devices, that is, the gro wth of p-type conductivity

ZnO layers. Despite all the progress that has been made, it remains the most extensive topic in

current research, and most of the efforts are directed just to solving this problem.

In all commercially available semiconductor lasers including GaN lasers, a recombi-

nation of electrons and holes is used as the mechanism of laser action. In such cases, the

threshold carrier density required to accomplish the inversion distribution of a population

for an electron–hole system is one or two orders of magnitude higher than the Mott

transition density. If an exciton-related recombination is used as the mechanism of laser

action, the resultant threshold value is expected to be two or three orders of magnitude

lower with higher quantum efﬁciency. Thus, the advantage of ZnO is that excitons can

exist stably even at RT and even under a high-density condition.

In thi s chapter, we present a review of optical properties of ZnO multiple quantum

well (MQW) structures grown on lattice -matched substrates based on the studies of

the luminescence of quantum wells (QWs) both under weak and strong excitation. We

also compare the properties of the QW deposited on sapphire. Studies on sapphire

substrates revealed that the ZnO epilayers are a dapted for optoelectronic applications

in several aspects includi ng an optically pumped laser oscillation of excitonic origin

even at RT

[3,4]

and a band gap engineering tunable by preparing the (Zn,Mg)O and

(Zn,Cd)O solid solutions.

[5–7]

In addition, growth of ZnO/(Mg,Zn)O (MQ Ws) were

successfully pe rformed.

[8–11]

However, the abovementioned stud ies also revealed unsatisfactory properties of ZnO

epilayers and superlattices. These cannot be avoided as long as lattice-mismatched

sapphire substrates are used. In addition, the fact that neither luminescence (photolumi-

nescence) nor a stimulated emission could be observed at RT in quantum wells grown on

sapphire substrates is also a problem. These problems have been completely resolved by

using lattice-matched substrates. We have adopted hexagonal ScAlMgO

(0001) with

lattice constants of a ¼3.246 A



and c ¼25.195 A



[12]

This crystal has an in-plane lattice

mismatch as small as 0.09%. ScAlMg O

can be regarded as a natural superlattice

composed of alternating stacking layers of wurtzite-type MgAlO

and rocksalt (111)-

ScO

layers. The structure of a crystal grown by this method and a possible hetero-

interface with ZnO are schematically shown in Figure 1 of Ohtomo et al.

[13]

Although there is now a fairly good understanding of the basic properties of ZnO

epilayers, it is only recently that the basic properties of its QWs have been studied in detail.

The combination of ZnO and (Zn,Mg)O has been employed in a study by another research

group.

[14–21]

We also conducted studies on this interesting and unexplored material, part of

which are introduced in this chapter. In this chapter, we describe the opti cal properties of

ZnO/(Zn,Mg)O MQWs grown on lattice-matched ScAlMgO

(SCAM) substrates.

Not only linear properties but also nonlinear optical properties are presented. The

properties of the strongly excited electron–hole system in direct band gap material have

332 Room-Temperature Stimulated Emission from ZnO Multiple Quantum Wells

been investigated for several decades. We studied the luminescence of ZnO MQWs under

strong excitation. The bulk or thin ﬁlm forms of this material have been studied previously

by a variety of spectroscopic techniques. Compared with these, strong excitation effects

have been only recently performed for the ZnO QW systems.

This chapter is organized as follows. The experimental procedures are brieﬂy described

in Section 12.2. In Sections 12.3–12.7, quantum conﬁnement effects of excitons, the well-

width dependence of the exciton–phonon coupling constants, and possible mechanism of a

stimulated emission of ZnO MQWs are described. The summarizing remarks are given in

Section 12.8.

12.2 Experimental Details

Samples of MQWs (10 periods) were grown by the method of laser molecular beam

epitaxy (MBE). A QW is deﬁned as stacks alternately deposited by using two kinds of very

thin semiconductor layers (wells and barriers) that have different band gap energies. ZnO

was used as a well layer material and a ZnMgO solid solution, the band gap of which is

larger than that of ZnO, was used as a barrier layer. The magnesium concentration

dependence of the band gap energy is given elsewhere.

[22]

It should be noted that the in-

plane lattice mismatch between ZnO and these alloys is very small. The Mg concentration

was set to 0.12 or 0.27, because the barrier height could be changed by a change in its

concentration. Eighteen samples with different Mg concentrations and well widths (L

6.9–46.5 A



, were prepared in order to estimate the L

dependence of their optical

properties. Barrier layer thickness was ﬁxed at 50 A



. The samples were grown by the

“combinatorial” method, the concept of which has been explained elsewhere.

[23–27]

The study has been performed using time-resolved luminescence spectroscopy at 5 K.

Detection was achieved using a Hamamatsu streak camera with a time resolution of

approximately 20 ps. The third harmonics (266 nm in wavelength) from a titan–sapphire

picosecond pulsed laser was used for excitation.

12.3 Quantum Conﬁnement Effect of Excitons in QWs

As mentioned in Section 12.1, the MQWs grown on sapphire substrates have several

drawbacks due to the formation of rough interfaces caused by the use of lattice-mismatched

substrates. One of these drawbacks is that a controllability of layer thickness is not sufﬁcient

for the quantum conﬁnement effect to be elicited especially in the case of L

 15 A



Moreover, we could not observe an exciton emission at RT. In other words, the efﬁciency of

luminescence is not sufﬁciently high enough to enable observation of the exciton emission at

RT.

[8, 9]

From a viewpoint of an optoelectrical device to be operable at RT, problems must be

overcome. The use of lattice-matched substrates opens up the possibility of a solution to these

drawbacks. We descibe how the properties can be improved by using SCAM substrates.

[28]

Bragg diffraction peaks and clear intensity oscillations due to Laue patterns corresponding to

the layer thickness have been observed in the X-ray diffraction (XRD) patterns of the MQWs

studied. This suggests a high crystallinity and a high degree of thickness homogeneity.

Quantum Conﬁnement Effect of Excitons in QWs 333