Correlation between photoluminescence and oxygen vacancies in ...

Correlation between photoluminescence and oxygen vacancies in ZnO phosphors K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt Citation: Appl. Phys. Lett. 68, 403 (1996); doi: 10.1063/1.116699 View online: http://dx.doi.org/10.1063/1.116699 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v68/i3 Published by the American Institute of Physics.

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Correlation between photoluminescence and oxygen vacancies in ZnO phosphors K. Vanheusden,a) C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt Sandia National Laboratories, Albuquerque, New Mexico 87185-1349

~Received 20 September 1995; accepted for publication 9 November 1995! By combining electron paramagnetic resonance ~EPR!, optical absorption, and photoluminescence ~PL! spectroscopy, a strong correlation is observed between the green 510 nm emission, the free-carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free-carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low-voltage field emission displays is discussed. © 1996 American Institute of Physics. @S0003-6951~96!05003-6#

Luminescence of ZnO phosphors has recently regained much interest because of its potential use in new low-voltage fluorescence applications, such as the field emissive display technology.1 Although ZnO luminescence has been the subject of studies for several decades, the centers and mechanisms responsible for many of its luminescence properties are still a matter of controversy. To explain the green emission, which has been known at least since the turn of the century, various models have been proposed, including the involvement of O vacancies,2– 4 interstitial O,5,6 Zn vacancies, and Zn interstitials,7 or even substitutional Cu.8 In the present work, electron paramagnetic resonance ~EPR!, optical absorption, and photoluminescence ~PL! spectroscopy were combined to study commercial ZnO powders from different suppliers. The powders were further adapted by oxidation in pure oxygen or reduction in forming gas at various temperatures, enabling us to vary the intensity of the green emission in a controlled and systematic manner. Our observations show a correlation between the green luminescence, the oxygen vacancy density, and free-carrier concentration, thus providing new insights in the green photoluminescence mechanism in ZnO. Two different powders: ZnO ~powder A! and ZnO:Zn ~powder B! were obtained from commercial suppliers. Electron microscopy images showed powder particle sizes of 50– 400 nm for powder A and 0.5–1.5 mm for powder B. For both powders, the powder particles were observed to be single-crystal grains, rather than multiple-grain colloids. Reduction and oxidation treatments were performed using a flow of forming gas @ N2 :H2; 95:5 ~by volume, 99.999% pure!# or pure O2 ~99.9999% pure!, respectively, through a quartz tube inserted into a tube furnace, at a constant temperature between 500 and 1050 °C. The reduction treatments in the forming gas ~FG! ambient were observed to cause a significant release of Zn vapor, which increased with temperature. This was not observed when annealing in pure N2. After the 1-h oxidation, or the 20-min reduction process, the powders, loaded on a quartz boat, were pulled out and quenched in air to room temperature. a!

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Appl. Phys. Lett. 68 (3), 15 January 1996

To obtain optical absorption data for powders, normal transmission techniques are not appropriate because of scattering. We have overcome this effect by use of a calorimetric technique known as photothermal deflection spectroscopy ~PDS!, which monitors the optical energy absorbed in the sample by detecting the temperature rise in an adjacent liquid medium ~Fluorinert! by a laser deflection method.9,10 Extraction of the absorption coefficient ~a! from the data has been calibrated with In-doped ZnO single crystals which were ground into powder after initial optical characterization. The free-carrier concentration of the powders can be derived directly from their a spectra.11 Photoluminescence ~PL! excitation and emission spectra were obtained using a spectrometer equipped with a charge-coupled detector linked to excitation and emission monochromators. The green 510-nm PL emission peak was generated using 370-nm excitation. In addition, a 392-nm emission peak could be generated using pulsed 232-nm excitation. EPR measurements were performed with an X-band ~'9.430 GHz! Bruker ESP-300E spectrometer between 4.3 and 294 K. Cylindrical quartz tube containers were used to insert the powders into the microwave cavity. A Hg arc lamp in line with a monochromator and an optical access microwave cavity enabled in situ illumination both with broadband and narrow-band ultraviolet ~UV! and visible light during EPR measurements. The singly ionized oxygen vacancy (V •O) with g51.96 ~Ref. 2! was the predominant paramagnetic defect observed in all the samples studied here. Figure 1~a! shows the density ~in the dark! of paramagnetic oxygen vacancies (V •O), together with the intensity of the 2.42 eV ~510 nm! green emission peak for the ZnO powder ~type A! as a function of reduction anneal temperature, both measured at 294 K. These data reveal a good correlation between the intensity of the green emission and the dark density of V •O, i.e., both peak at about 800 °C and the variations roughly occur within one order of magnitude. No correlation with the intensity of the 3.14 eV ~392 nm! UV emission was observed. In the type A powders the V •O signal was observed to be photosensitive for photon energies down to as low as 2.3 eV, as has been previously observed.12,13 Upon illumination, the V •O density was observed to grow to 2.531015 cm23 in all samples studied, i.e., the light-on V •O density in the ZnO

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© 1996 American Institute of Physics


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FIG. 1. Intensity of the ~a! 2.42 eV ~510 nm! green emission peak and ~b! free-carrier electron density for powder A ~ZnO! as a function of reduction anneal temperature. Anneals were performed in a flow of forming gas @ N2 :H2 95:5 ~by volume!# for 20 min. The densities of paramagnetic singly ionized oxygen vacancies (V •O) as measured by EPR ~dark! have been added for comparison. All data were recorded at 294 K.

powder was not affected by the FG anneal treatments. Figure 1~b! shows the V •O density together with the freecarrier electron density (n e ) as a function of reduction anneal temperature. Again, there exists a good correlation between both entities. However, notice the difference in absolute scale between the V •O density and n e ; n e is much larger than the V •O density. Oxidation at 900 °C following the FG anneal reversed the effects induced by the reduction anneal. Figure 2~a! shows the density V •O ~dark! recorded at 20 and 294 K, together with the intensity of the 2.42 eV ~510 nm! 294-K green emission peak for the ZnO:Zn powder ~type B!, as a function of oxidation temperature. The ZnO:Zn powder starts with a high green PL as compared to the type A ZnO powder in Fig. 1 ~about 50 times!. Figure 2~b! shows the same V •O densities as in Fig. 2~a! together with the 294-K n e density, again versus oxidation temperature. In general, and especially in the 500–700 °C oxidation temperature region, the V •O density is higher at 20 K as compared to 294 K. Both plots reveal a reasonable correlation with the density of V •O, i.e., the three parameters drop over roughly one order of magnitude between 700 and 1050 °C. In the type B ZnO:Zn powders ~as-prepared or after oxidation!, the V •O signal did not display any significant photosensitivity. Oxygen vacancies in ZnO can occur in three different • charge states: V O , V •O, and V •• O . Only V O is paramagnetic, and consequently observable by EPR. Because V O is a very shallow donor,14 it is expected that most oxygen vacancies will be in their paramagnetic V •O state under flatband condi404

Appl. Phys. Lett., Vol. 68, No. 3, 15 January 1996

FIG. 2. Intensity of the ~a! 2.42 eV ~510 nm! green-emission peak and ~b! free-carrier electron density for powder B ~ZnO:Zn!, both measured at 294 K, vs oxidation anneal temperature. Oxidation treatments were done in a flow of pure oxygen for 1 h. The densities of paramagnetic singly ionized oxygen vacancies (V •O) detected with EPR ~dark! at 20 and 294 K have been added for comparison.

tions between 294 and 4.3 K. The increase in the V •O density when cooling down to 20 K, as can be seen in Fig. 2, may be • due to V •• O →V O transitions caused by partial free-carrier freezeout. The oxygen vacancy is an intrinsic donor in ZnO.15 However, the coexistence of another donor ~possibly interstitial Zn! with a much higher density must be inferred to explain the large difference between the V •O and the freecarrier densities at 294 K as shown in Figs. 1~b! and 2~b!. Band bending at the particle boundaries must be taken into account,13,16 not only to explain changes in the ionization state of the oxygen vacancy but also to understand some of the observed variations in green PL intensity and n e . Band bending will create an electron depletion region at the ZnO particle surface of width W. In this region all oxygen vacancies will be in the diamagnetic V •• O state. Under illumination, these V •• O centers can trap a photoactivated electron and change to their paramagnetic V •O state. When the light was switched off, the V •O signal in type A powders was observed to decay. At 294 K this decay was instantaneous, but at 150 K it required several minutes. The decay kinetics did not follow a pure exponential as would be expected for simple thermal activation of the unpaired V •O electron into the conduction band.12 This suggests that a more complex, depletion region related, mechanism is involved; for example electron emission into surface states causing an increasing surface barrier.17 Vanheusden et al.


The width of the depletion region is given by W5

Ae 2

ZnOV bi

eN D

,

where V bi is the potential at the boundary, e is the electronic charge, N D is the donor density, and e ZnO is the static dielectric constant of ZnO. Assuming that the measured freecarrier concentration is a good measure for the donor density, the depletion layer width can be calculated to be '100 nm for type A and '30 nm for type B particles. Due to the difference in particle size and n e , the depletion region will cover a significant fraction of the particles in the type A ZnO powder, whereas it only represents a few percent of the total volume in the type B ZnO:Zn powder. The fact that the V •O signal is not photosensitive in the type B powders, but highly photosensitive in the type A powders ~smaller particles!, supports this surface depletion model. This model also explains the correlation between the dark V •O concentration and n e in the ZnO powders @Fig. 1~b!#: the width of the depletion region, and therefore, the density of V •• O , is inversely proportional to the square root of the free-carrier concentration in the particles. Additional evidence for significant changes in the depleted fraction of the ZnO particles is provided by looking at the effect of band-gap optical excitation on the free-carrier density. As expected, the photosensitivity of the free-carrier concentration was found to be proportional to the predicted depletion layer widths ~not shown!. The fact that the light-on V •O density was observed to be constant for all the type A ZnO powders implies that the reduction anneals only affects the free-carrier concentration (n e ) and not the total oxygen vacancy density ~as detected by EPR under illumination!. This observation is quite surprising and may be linked to the observed strong release of Zn during the FG anneals, especially at higher temperatures, indicating that this treatment merely etches the particle surface. The Zn atoms thermally released in this process are likely to diffuse into the particles in interstitial sites where they act as shallow donors.16 The oxidation of the ZnO:Zn powders on the other hand, appears to reduce the oxygen vacancy and the free-carrier concentration. However, the key observation is that in both powders ~type A and B! there seems to be a direct correlation between the ~dark! number of paramagnetic V •O center and the PL intensity. One obvious, although still tentative, mechanism responsible for the green emission is the recombination of

Appl. Phys. Lett., Vol. 68, No. 3, 15 January 1996

V •O electrons with photoexcited holes in the valence band. But clearly, more experimental evidence is required to confirm such an assignment. In summary, we provide new insights in the PL mechanisms responsible for green emission in ZnO. We observe a good correlation between the green emission, the free-carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO powders with different particle sizes. The impact of particle-surface depletion on the green emission intensity, the density of singly ionized oxygen vacancies, and the photosensitivity of the charge state of this defect has been revealed. Electronic particle-boundary effects will gain technological relevance because they are bound to play an even more crucial role in new low-voltage cathodoluminescence applications such as field emissive displays, where the penetration depth of the incident low-energy electrons is of the order of, or less than, the depletion layer width. Last, our results suggest that intrinsic defects, more specifically oxygen vacancies, play a key role in the green PL in ZnO. We would like to thank B. McKenzie for her contribution in the electron microscopy imaging, and D. Dimos and B. E. Gnade for many useful discussions. This work performed at Sandia National Laboratories was supported by the U.S. Department of Energy under Contract DE-AC0494AL85000.

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