Preparation and cathodoluminescence of ZnO phosphor

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Preparation and cathodoluminescence of ZnO phosphor

C.H. Lin

a

, Bi-Shiou Chiou

a,∗

, C.H. Chang

a

, J.D. Lin

b

a_{Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan} b_{Electronic Research Service Organization, Industrial Technology Research Institute, Hsinchu, Taiwan}

Received 27 July 2001; received in revised form 2 January 2002; accepted 15 January 2002

Abstract

Zinc oxide phosphors (ZnO:Zn) were prepared with solid state sintering of ZnO powders with ZnS and screen printed onto ITO-coated glass substrates to form a thin layer. Structural characterization carried out by X-ray diffraction (XRD) analysis suggests that the Zn atoms originated from the decomposition of ZnS, and diffuse to occupy the oxygen vacancies in the host lattice. Both lattice parameters a and

c decrease slightly with the increase of ZnS content and/or firing temperature. The cathodoluminescent (CL) peak band varies with ZnS

content and ranges from 494 to 508 nm, which is close to the green band of ZnO phosphor. Various mechanisms have been proposed in literatures to explain the origin of the green emission band. Among them, the presence of oxygen vacancies and trace ZnS residue are possible causes for the green emission in this study. CL spectra and CIE color loci suggest that ZnO:Zn phosphor with 5 wt.% ZnS can be used as a green primary for color picture tubes.

1. Introduction

Phosphors are materials that emit photons with a high efficiency. They are widely used as radiation detectors and in visual displays such as computer monitors and television screen. Given the recent thrust of activity in the flat panel display area, it is imperative to optimize phosphor properties to reduce power consumption and increase brightness[1–4]. As a wide band-gap material, ZnO has an electronic structure which makes it a useful candidate for luminescent devices. Recently, ZnO has regained considerable interest because of its potential use in new low voltage lumines-cence applications such as field emission displays (FEDs)

[5]. Pure ZnO emits a narrow luminescence band in the

blue–violet spectral region [6]. However, upon

incorpo-ration of various dopants, a variety of emissions, ranging from blue to orange spectral region occur[7].

In this research, ZnO doped with Zn (ZnO:Zn) phosphors were prepared by solid state sintering of ZnO powders with various amounts of zinc sulfide (ZnS) at various tempera-tures in nitrogen atmosphere. The effects of sintering tem-perature and ZnS content on the phosphor powder shape, powder size distribution, lattice parameters, and luminescent characteristics of phosphors were investigated. Mechanisms for the green emission of ZnO:Zn phosphor are discussed.

∗_{Corresponding author.}

E-mail address: [email protected] (B.-S. Chiou).

2. Experimental

Zinc-doped zinc oxide was prepared by solid state

sinter-ing of mixtures of ZnO and ZnS powders in N2atmosphere

at temperatures ranging from 800 to 1200◦C for 1 h. N2

atmosphere was employed because previous study on the effect of sintering ambient suggested that specimens

sin-tered in N2 had higher brightness and better CIE color.

The as-sintered powders were examined with a scanning electron microscope (SEM, Leica S440, Japan) to investi-gate the particle size, shape and surface morphology. The phosphor powder was further examined with a laser par-ticle size analyzer (Analysette 22, Fritsch) to evaluate the particle size distribution. The phase and crystal structure of the as-fabricated phosphor were identified with an X-ray diffractometer (Rigaku Ru-200, Japan) with a wavelength of Cu K␣ (λ = 1.5406 Å). The scanning rate was 4◦min−1. Screen printing method was employed to deposit the phos-phor powders onto indium–tin-oxide (ITO) coated glass. Paste consisting of polyvinyl alcohol (PVA, as a binder) and as-sintered phosphor was printed onto ITO glass and baked at 450◦C for 1 h. The film thickness was about 100␮m.

The cathodoluminescence (CL) spectra were measured with an electron gun in a vacuum chamber. The chamber

was pumped down to 5×10−2Torr with a mechanical pump

and to 5× 10−7Torr with a turbo pump. Samples were

ex-cited by electron beam with an accelerating voltage of 1 kV

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of the hexagonal phase of ZnO, peaks of unreacted ZnS are also found for compositions with a ZnS content of 7 wt.%

Fig. 1. The XRD patterns for ZnO with various amounts of ZnS sintered in N2 at 1100◦C for 1 h.

Fig. 2. The XRD patterns for ZnO with 5 wt.% ZnS sintered in N2 at various temperatures for 1 h.

or larger.Fig. 2shows XRD spectra of samples with 5 wt.% ZnS sintered in N2at various temperatures for 1 h. Samples

fired at either 800 or 900◦C exhibit peaks corresponding to the ZnS phase, which indicates that the firing temperature should be above 1000◦C for 5 wt.% ZnS to react completely.

The average particle size of as-sintered (ZnO + ZnS)

phosphor increases with the increase of firing temperature as shown inFig. 3, while no apparent difference in particle size is observed for ZnO:Zn powders with various amounts of ZnS (Fig. 4).

On the basis of the XRD patterns shown inFigs. 1 and 2, the ZnS peaks disappear for samples sintered at a

tempera-ture900◦C and with a ZnS content of 5 wt.%. The Zn

atoms from the decomposition of ZnS may diffuse either to the interstitial sites or to the host lattices. For the former the reaction is as follows[7]:

ZnO+ xZnS → Zn(O1−X· xVo) + xSO

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Fig. 3. Particle size distribution for ZnO:Zn sintered in N2 at various temperatures for 1 h. Starting composition: 95 wt.% ZnO–5 wt.% ZnS.

Fig. 5. Lattice parameters a and c of ZnO:Zn phosphors prepared with ZnO and various amounts of ZnS sintered in N2at 1100◦C for 1 h. Fig. 4. Particle size distribution for ZnO with various amounts of ZnS sintered in N2 at 1100◦C for 1 h.

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Fig. 6. Lattice parameters a and c of ZnO:Zn phosphors as a function of firing temperatures.

There are oxygen vacancies created in ZnO lattices and excess Zn atoms in the interstitial sites. However, as the radius of Zn is 0.75 Å and that of the oxygen vacancy is 1.4 Å[8], it is argued that Zn atom is too large to remain as an interstitial and possibly occupies the oxygen vacancy already created as shown in the following[9]:

ZnO+ xZnS → Zn(O1−X· xVo) + xSO

+ xZni2++ 2xe (1)

xZni2++ 2xe → xZn0 (2)

xZn0_{+ xV}

o→ xZno0 (3)

ZnO1−X+ xZno0→ ZnO1−X:xZno0 (4)

The defect formation behavior is one where the sulfide de-composes, reduces the host lattice and forms an oxygen va-cancy plus two free electrons. These electrons then reduce the interstitial Zn2+cations to the metal valance state Zn0.

Figs. 5 and 6are the lattice parameters of ZnO:Zn

phos-phor calculated on the basis of XRD data shown inFigs. 1

and 2, respectively. Both lattice parameters a and c de-crease slightly (<0.2%) with the increase of ZnS content

Fig. 7. CL spectra of ZnO:Zn phosphors prepared with ZnO and various amounts of ZnS sintered in N2at 1100◦C for 1 h. Samples measured at l kV and 10␮A. Number in parenthesis is the peak wavelength in nm.

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and/or firing temperature. As stated previously, both zinc ion and oxygen vacancies are generated during firing of

(ZnO+ ZnS). The presence of Zn atoms in the interstitial

tends to enhance the lattice parameters. As the lattice param-eters decrease when more ZnS is consumed, it suggests that zinc atoms diffuses to occupy the oxygen vacancies, since the presence of Zn in the interstitial site would enhance the lattice parameters. Besides, the radius of Zn atom is smaller than that of oxygen vacancies, the more ZnS consumed (i.e., higher firing temperature or higher ZnS content), the smaller the lattice parameter, as observed. However, the above discussion must be regarded as speculative and not

Fig. 8. CIE color loci of ZnO:Zn phosphors prepared with ZnO and various amounts of ZnS sintered in N2 at 1100◦C for 1 h. Inset is the exploded view of the X- and Y-coordinates of specimens studied. The number in parenthesis is wt.% ZnS. CIE measured at 1␮A, 800 V.

explicitly proven, even though it is consistent with the XRD results. Further investigation is needed to verify this defect formation mechanism.

Fig. 7 exhibits CL spectra of ZnO:Zn with various amounts of ZnS. The wavelength of emission ranges from 420 to 620 nm (corresponding to an energy 2.95–2 eV), and a shift in the peak luminescent band is observed among

samples with various ZnS content as indicated in Fig. 7.

Lehman reported that ZnO phosphor has four different lu-minescence bands, two intrinsic bands, 3.19 eV (388 nm, UV band) and 2.43 eV (510 nm, green band), and two extrinsic bands, 1.97 eV (629 nm, red band) and 1.35 eV

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Fig. 9. CIE color loci of ZnO:Zn phosphor sintered in N2 at various temperatures for 1 h. Inset is the exploded view of the X- and Y-coordinates of specimens studied. The number in parenthesis is the sintering temperature in◦C. Starting composition: 95 wt.% ZnO–5 wt.% ZnS, CIE measured at 1␮A, 800 V.

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Fig. 10. CL spectra of ZnO:Zn phosphors with CL peak deconvoluted. Samples with (a) 0 wt.% ZnS and (b) 15 wt.% ZnS. (—) CL spectrum, (- - -) deconvoluted green emission (500 nm), (– – –) deconvoluted yellow emission (573 nm).

Table 1

CL peaks for ZnO:Zn phosphors prepared with ZnO:Zn and various amounts of ZnS

ZnS content (wt.%) Position (nm) Relative strengths (%) Peak ratio (G/Y)

G (green) Y (yellow) G Y 0 503 576 75.7 24.3 3.12 1 494 562 81.9 18.1 4.52 3 497 572 85.3 14.7 5.80 5 496 577 86.8 13.2 6.58 7 508 587 84.8 15.2 5.58 10 503 576 75.8 24.2 3.13 15 494 562 81.9 18.1 4.52

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main peak for the CL spectra in all samples (Fig. 7). The deconvoluted CL spectra allow one to distinguish between a peak located at around 500 nm (referred to as G, green) and a peak around 573 nm (referred to as Y, yellow). The corre-sponding deconvoluted peaks are indicated by full curves as shown inFig. 10. The resolution procedure for the CL peak uses two Gaussian functions with variable positions, width

and intensities.Table 1 summarizes the corresponding

re-sults. The relative strengths are obtained by comparing the integrated area of the G and Y peaks. As can be seen in

Table 1, the peak ratio G/Y of the 5 wt.% ZnS specimen is the largest among all compositions studied. This also sug-gests that ZnO:Zn phosphor with 5 wt.% ZnS can be used as a green primary for color picture tubes.

4. Conclusion

Zinc-activated zinc oxide (ZnO:Zn) films have been pre-pared by screen printing of solid state sintered (100−x) wt.%

ZnO−x wt.% ZnS (x: 0–15) onto ITO-coated glass

sub-strates. Polycrystalline ZnO:Zn exhibits a hexagonal crystal structure with lattice parameters a and c decreasing slightly as the ZnS content increases. This suggests that, instead of remaining as an interstitial, the excess Zn atom occupies the oxygen vacancy of the host lattice. The CL spectra show the

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References

[1] P.H. Holloway, S. Jones, P. Pack, J. Sebastian, T. Trottier, in: Proceedings of the 10th IEEE International Symposium on Applications, Vol. 1, 1996, p. 127.

[2] A. Vecht, Display Phosphors, Vol. M-5, Society for Information Display (SID), 1999, pp. 3–48.

[3] Y.H. Tseng, B.-S. Chiou, C.C. Peng, L. Ozawa, Thin Solid Films 330 (1998) 173.

[4] C.L. Lo, J.G. Duh, B.-S. Chiou, C.C. Peng, L. Ozawa, Mater. Chem. Phys. 71 (2001) 179.

[5] B. Allieri, L.E. Depero, L. Sangaletti, L. Antonini, M. Bettinelli, Mater. Res. Soc. Symp. Proc. 508 (1998) 275.

[6] L. Ozawa, Cathodoluminescence Theory and Applications, Kodansha Ltd., Tokyo, Japan, 1990, pp. 255–261.

[7] A. Pfahnl, J. Electrochem. Soc. 109 (1962) 502.

[8] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd Edition, Wiley, USA, 1976, p. 58.

[9] R.C. Ropp, Luminescence and the Solid State, Elsevier, USA, 1991, p. 295.

[10] W. Lehman, J. Electrochem. Soc. 115 (1968) 538.

[11] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983.

[12] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, Mater. Res. Soc. Symp. Proc. 424 (1997) 433.

[13] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403.