Near-infrared photoluminescence from ZnOMingsong Wang, Yajun Zhou, Yiping Zhang, Eui Jung Kim, Sung Hong Hahn et al. Citation: Appl. Phys. Lett. 100, 101906 (2012); doi: 10.1063/1.3692584 View online: http://dx.doi.org/10.1063/1.3692584 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i10 Published by the American Institute of Physics. Related ArticlesPhotoluminescence associated with basal stacking faults in c-plane ZnO epitaxial film grown by atomic layerdeposition Appl. Phys. Lett. 100, 101907 (2012) Near-resonant two-photon absorption in luminescent CdTe quantum dots Appl. Phys. Lett. 100, 081901 (2012) Oxygen vacancy–induced ferromagnetism in un-doped ZnO thin films J. Appl. Phys. 111, 033501 (2012) Direct evidence of type II band alignment in ZnO nanorods/poly(3-hexylthiophene) heterostructures Appl. Phys. Lett. 100, 021912 (2012) X-ray luminescence of CdTe quantum dots in LaF3:Ce/CdTe nanocomposites Appl. Phys. Lett. 100, 013109 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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Near-infrared photoluminescence from ZnO

Mingsong Wang,1,a) Yajun Zhou,1 Yiping Zhang,1 Eui Jung Kim,2,b) Sung Hong Hahn,3,c)

and Seung Gie Seong4

1School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China2Department of Chemical Engineering, University of Ulsan, Ulsan 680-749, South Korea3Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749,South Korea4Division of General Studies, Ulsan National Institute of Science and Technology, Ulsan 689-805, South Korea

(Received 1 January 2012; accepted 20 February 2012; published online 8 March 2012)

Understanding the defect physics of ZnO is crucial in controlling its properties for various

applications. We report the observation of an interesting 1.64 eV near-infrared (NIR)

photoluminescence from ZnO and its evolution with annealing temperature. Based on a recent

calculation on the transition levels of native point defects of ZnO [A. Janotti and C. G. Van de

Walle, Phys. Rev. B 76, 165202 (2007)], the NIR emission can be successfully explained by the

donor-acceptor transition between VO and VZn and/or the radiative recombination of shallowly

trapped electrons with deeply trapped holes at Oi. VC 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.3692584]

Zinc oxide (ZnO) is a II–VI semiconductor with a direct

wide band gap of �3.3 eV at room temperature.1 Great inter-

est in ZnO is triggered by its prospects for use in optoelec-

tronic devices owing to its versatile combination of

interesting optical, electrical, and magnetic properties.2

Understanding the defect physics of ZnO is necessary to

pave the way for control over its electrical conductivity and

luminescence.3 For example, the presence of a variety of

native point defects, even in high-quality ZnO single crys-

tals, gives rise to broad visible emission bands such as green,

yellow, orange, and red ones observed at room temperature.4

However, the assignments of point defects to specific emis-

sion bands remain speculative and highly disputable. Particu-

larly, the ubiquitous green luminescence was suggested to be

associated with oxygen vacancies,5–7 zinc vacancies,8–11

zinc interstitials,12,13 or donor-acceptor transition between

defect complex.14,15 Furthermore, the localized emission at

the surface makes the interpretation of this green band even

more complicated.16–19

In comparison to the near-band-edge (NBE) UV emis-

sion and the broad defect-related visible emissions that are

commonly observed, the near-infrared (NIR) luminescence

from ZnO has received far less attention. Lauer20 recorded a

1.70 eV NIR emission for high temperature (900–1000 �C)

oxygen-annealed ZnO, which appeared only as a weak side

band accompanying the broad visible emission. In addition,

a broad emission band covering the NIR and visible spec-

trum was found to center at 1.55, 1.66, and 1.9 eV for NH3-

annealed,21 electron-irradiated,22 and N-implanted ZnO,23

respectively. The above studies indicated that the NIR emis-

sion was normally a concomitant of the broad visible emis-

sion. However, none has monitored an independent NIR

band. Extreme care should be taken to avoid misinterpreting

the second order of the UV emission at �750 nm as a NIR

emission.24–26 In this work, we report the observation of a

1.64 eV NIR emission and its evolution with annealing tem-

perature. By combining our experimental observations with

recent theoretical calculations, possible mechanisms for the

NIR luminescence from ZnO have been proposed. The ob-

servation of this unambiguous NIR band helps one better

understand the defect physics of ZnO.

Hydrothermally grown ZnO microspheres were

employed in the present study. A precursor solution was pre-

pared by sequentially dissolving zinc nitrate, ammonium fluo-

ride, and sodium hydroxide with a mole ratio of 1:6:8 to

deionized water. The precursor solution was hydrothermally

treated at 75 �C for 10 h. The as-prepared ZnO was further

annealed in air for 1 h at temperatures ranging from 700 to

1000 �C. Structural characterization was carried out using

x-ray diffractometer (XRD, Bruker Axs D8 Advance) with Cu

Ka radiation. Sample morphology was examined by a JEOL

JSM-7001F field emission scanning electron microscope

(FESEM) operating at 15 kV and a JEOL JEM-2100F high-re-

solution transmission electron microscope (HRTEM) operated

at 200 kV. Photoluminescence (PL) spectra were recorded at

room temperature by exciting the samples with a 325 nm

He–Cd laser at an output power of 30 mW. Raman spectra

were taken on a Laser Raman Spectrometer SPEX 1403 with

a He–Ne laser at an excitation wavelength of 632.8 nm.

Figure 1 shows the SEM image and XRD pattern of the

as-prepared ZnO microspheres. As seen in Fig. 1(a), the

microspheres are constructed of uniform thin sheets of

�20 nm in thickness. A fine arrangement of the nanosheets

in the microsphere indicates the self-assembly of nanosheets

by epitaxy.27 The HRTEM image of a single sheet and its

corresponding fast Fourier transform (FFT) pattern shown in

the insets of Fig. 1(b) reveal the single crystalline nature of

the nanosheet. Energy-dispersive x-ray spectroscopy analysis

(coupled with SEM) reveals that there is no impurity element

(not shown). All diffraction peaks in Fig. 1(b) are identified

as the hexagonal wurtzite ZnO; no crystalline impurity is

detected.

a)Electronic mail: wangms@ujs.edu.cn.b)Electronic mail: ejkim@ulsan.ac.kr.c)Electronic mail: shhahn@ulsan.ac.kr.

0003-6951/2012/100(10)/101906/4/$30.00 VC 2012 American Institute of Physics100, 101906-1

APPLIED PHYSICS LETTERS 100, 101906 (2012)

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Raman spectroscopy is employed to investigate the

phase purity and crystallinity of ZnO. Figure 2 shows the

Raman spectra of the as-prepared and annealed ZnO micro-

spheres. All bands in the Raman spectra can be assigned to

wurtzite ZnO,28 i.e., E2 modes at 98 and 437 cm�1, transver-

sal optical TO modes with E1 symmetry at 409 cm�1 and

with A1 symmetry at 380 cm�1, longitudinal optical LO

modes with E1 symmetry at 583 cm�1 and with A1 symmetry

at 573 cm�1, and the second order vibrations at 202, 331,

538, and 1050–1200 cm�1. An enhanced Raman intensity for

the annealed ZnO indicates its improved crystallinity. No

Raman peaks related to impurity phases are observed con-

firming the pure wurtzite phase of the investigated ZnO.

Figure 3 illustrates the room temperature PL spectra of

the as-prepared and annealed ZnO microspheres. In addition

to a weak NBE UV emission at 385 nm, the as-prepared ZnO

exhibits a broad intense deep-level (DL) emission that

appears in the range of 440–860 nm, which can be readily

Gaussian-resolved into three bands located at 566, 637, and

759 nm, respectively (Fig. 3(a)). Upon annealing at 700 �C,

the DL emission maintains its broad profile while the band

intensity is weakened (Fig. 3(b)). This broad DL emission,

however, changes to separate green and NIR emissions peak-

ing at 537 (2.31) and 758 nm (1.64 eV), respectively, for

800 �C-annealed ZnO. A further increase in annealing tem-

perature leads to a monotonic increase (decrease) of the NIR

(visible) emission, as seen in Figs. 3(b) and 4. The NIR band

does not shift with annealing temperature and finally domi-

nates the PL spectrum at 1000 �C. Note that the 1.64 eV NIR

luminescence is not a second order of the UV emission: first,

the absence of a UV emission can not create a second order

NIR band; second, the NIR band due to the second order of

the UV emission is much weaker and narrower than the NIR

emission shown in Fig. 3(b).29 Therefore, we have demon-

strated unambiguously a 1.64 eV NIR luminescence from

ZnO and its evolution with annealing temperature.

FIG. 1. Typical SEM image (a) and XRD pattern (b) of the as-prepared

sheet-constructed ZnO microspheres. The insets in (b) show the HRTEM

image of a single sheet and the corresponding FFT pattern.

FIG. 2. (Color online) Raman spectra of the as-prepared and annealed ZnO

microspheres.

FIG. 3. (Color online) PL spectra of the as-prepared (a) and annealed (b)

ZnO microspheres. The broad visible-NIR emission in (a) is Gaussian-

resolved into three bands located at 566, 637, and 759 nm, respectively.

FIG. 4. (Color online) PL intensity for the visible and NIR emission bands

as a function of annealing temperature.

101906-2 Wang et al. Appl. Phys. Lett. 100, 101906 (2012)

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Although the as-prepared sample has a large surface

area, the DL emissions observed herein for the annealed

ZnO are believed to be a bulk effect rather than a surface

effect as the nanosheets become much thicker (of over

200 nm after 900 �C annealing, not shown). Before interpret-

ing the NIR luminescence, it is necessary to review recent

calculations on the defect energetics and electronic structure

in ZnO. The calculations consistently indicate that oxygen

vacancies (VO), zinc vacancies (VZn), and oxygen interstitials

(Oi) create deep energy levels in the band gap, and among

them VO has the lowest formation energy.30 Furthermore,

reliable results concerning the charge transition levels can be

obtained by aligning the electronic band structure through an

external potential, which yields the (þ2/0) charge transition

level of the VO in ZnO to be 2.2–2.4 eV from the valence

band maximum (VBM).31 Therefore, one can justify the DL

emissions of ZnO provided the accurate defect levels are pre-

dicted. Recently, Janotti and Van de Walle10 performed a

detailed study on the native point defects in ZnO. Accord-

ingly, the observed DL emissions herein are explained based

on their calculated transition levels of VO, VZn, and Oi. We

exclude zinc and oxygen antisites as they show a large off-

site displacement and induce a large local lattice

relaxation.10

As illustrated in Fig. 5, exciting ZnO with photons of

energy higher than the band gap leads to the band-to-band

excitation (1) and the formation of excitons (2). Excited free

charge carriers are then trapped by VO, VZn, and Oi occupy-

ing the deep energy levels in the band gap (corresponding to

processes 4, 7, and 9, respectively). De-excitation and radia-

tive recombination processes 3, 5, and 6 give rise to the

emission bands at 3.23, 2.51, and 2.36 eV, respectively. The

energy level of the shallowly trapped electrons to be com-

bined with deep acceptors (VZn and Oi) is thought to be the

same as that of the excitons since the DL emissions compete

with the exciton-related UV emission.16 It is found that both

VO and VZn contribute to the green emission, but with

slightly different peak energy, which may explain the

observed high- and low-energy green luminescence from

annealed ZnO in different atmospheres.7 Although the green

emission ascribed to VO was in question,10 we have the fol-

lowing positive evidences for it. (i) VO has the lowest forma-

tion energy and, thus, is most abundant in a Zn-rich

condition,10 e.g., reduced ZnO, and a high-energy green

emission do appear in reduced ZnO.7 (ii) Thermodynami-

cally unstable VþO is experimentally detected in the optically

excited ZnO by electron paramagnetic resonance, revealing

the participation of VO in the green emission process.32 (iii)

Subband excitation of ZnO generates DL emissions,32 sug-

gesting that VO as a deep donor is a source of green emission

(see Fig. 5, process 11); in contrast, one can never expect

any DL emission if VZn is involved.

Finally, it is seen from Fig. 5 that a 1.64 eV NIR emis-

sion is generated as a result of the radiative recombination of

shallowly trapped electrons with deeply trapped holes at Oi,

which is amazingly consistent with the observed NIR band

in Fig. 3. Alternatively, the same NIR emission is obtained

in case a donor-acceptor transition takes place between VO

and VZn. A dominant process for the NIR emission may

depend on the concentration of these native defects that are

present. The proposed recombination processes in Fig. 5 can

successfully explain the green and NIR luminescence from

ZnO. However, further investigation is required to interpret

other emission bands such as yellow and red ones.

In conclusion, ZnO microspheres constructed of nano-

sheets were prepared by a facile hydrothermal method. We

demonstrate clearly a 1.64 eV NIR luminescence from ZnO

and its evolution with annealing temperature. Combined

with the calculated defect energy levels of native point

defects in ZnO by Janotti and Van de Walle,10 we success-

fully explain the green and NIR luminescence from ZnO.

Both VO and VZn are responsible for the ubiquitous green

emission, while donor-acceptor transition between them and/

or the radiative recombination of shallowly trapped electrons

with deeply trapped holes at Oi give rise to the 1.64 eV NIR

luminescence.

This work was supported by the Natural Science Foun-

dation of China (Grant No. 51002066), the Research Founda-

tion of Jiangsu University (Grant No. 09JDG004), and the

Research Fund of the University of Ulsan.

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