Optical, luminescence, and scintillation properties of ZnO and ZnO:Ga ceramics
E. I. Gorokhova, G. V. Anan'eva, V. A. Demidenko, P. A. Rodny, I. V. Khodyuk, E. D. Bourret-Courchesne
OOptical, luminescence, and scintillation properties of ZnO and ZnO:Ga ceramics
E. I. Gorokhova, a (cid:1) G. V. Anan’eva, and V. A. Demidenko
Scientific Research and Technological Institute of Optical Material Science, S. I. Vavilov State OpticalInstitute All-Russia Science Center, St. Petersburg
P. A. Rodny and I. V. Khodyuk
St. Petersburg State Polytechnic University, St. Petersburg
E. D. Bourret-Courchesne
Lawrence Berkeley National Laboratory, University of California, Berkeley, USA (cid:1)
Submitted May 20, 2008 (cid:2)
Opticheski (cid:2)
Zhurnal , 66–72 (cid:1) November 2008 (cid:2)
Uniaxial hot pressing has been used to obtain ceramics based on zinc oxide, and their optical,x-ray-structure, luminescence, and scintillation characteristics have been studied. It is shown that,by changing the concentration of the dopant (cid:1) Ga (cid:2) and the codopant (cid:1) N (cid:2) , it is possible to changethe intensities of the edge band (cid:1) (cid:2) and the intraband luminescence (cid:1)
510 nm (cid:2) of the ZnOluminescence, as well as their ratio. Undoped ZnO ceramic has good transparency in the visibleregion and fairly high luminous yield: 9050 photons per MeV. Ceramic ZnO: Ga possesses in-tense edge luminescence with a falloff time of about 1 ns. © . INTRODUCTION
Powdered zinc oxide doped with gallium possesses ashort de-excitation time of 0.7 ns and a high luminous yieldof 15 000 photon / MeV. As a result, ZnO: Ga has the high-est quality (cid:1) the ratio of the luminous yield to the falloff time (cid:2) among known phosphors. Good scintillation characteristicsare also shown by the phosphors ZnO: In (cid:1)
Refs. 1 and 2 (cid:2) andZnO: Zn.
However, powdered and thin-film materials areused only for detecting neutrons and alpha particles, whereas scintillators that possess a large volume (cid:1) single crys-tals or optical ceramics (cid:2) and high transparency in the spectralemission region of the material are needed for recordinggamma and x-ray quanta. The production of bulk, single-crystal ZnO is a difficult, time-consuming, and expensivetechnological process. There are at present only isolated re-ports of the growth of ZnO-based single-crystal scintillators,in particular ZnO: In, of millimeter dimensions. A scintillation ceramic, which is a pressed polycrystal-line material whose grain size can vary within wide limits, isa promising alternative to traditional scintillation single crys-tals. Crystals of cubic syngony are most acceptable for pro-ducing optical ceramics (cid:1) ceramics that are transparent totheir own radiation (cid:2) . It is much harder to obtain a transpar-ent ceramic in the visible region in the case of crystals withlow symmetry (cid:1) such as ZnO (cid:2) , since microstructure is formedfrom the disoriented grains during the hot pressing of anoptical ceramic, and this causes significant light scatteringeven when the refractive-index difference (cid:1) (cid:1) n (cid:2) is insignifi-cant. The negative influence of (cid:1) n can be minimized bymaking the predominant orientation of the grains coincidewith the direction of the optic axis. It is well known that thegrain boundaries have a significant effect on the other prop-erties of ceramics, and therefore the characteristics of ce-ramics and single crystals usually differ. Zinc oxide manifests typical semiconductor properties,possessing in this case a large fraction of ionic bonding. Thecharacteristics of zinc oxide are most fully presented in arecent review. Under ordinary circumstances, ZnO pos-sesses the hexagonal wurtzite structure, in which each O ion is surrounded by a tetrahedron composed of four Zn ions. The crystal-structure constants have the following val-ues: a = 3.2497 Å and c = 5.2069 Å, and their ratio of c / a = 1.602 is close to ideal. The refractive-index difference ofzinc oxide oscillates within the limits 0.016–0.018 in thevisible region and equals 0.009 at 405 nm. There are numer-ous forms of ZnO: single crystals, thin films and filaments,nanocrystals, needles, etc., and, as a rule, two emission bandsare recorded: a short-wavelength one close to the absorptionedge of the crystal—i.e., edge luminescence—and a long-wavelength (cid:1) green (cid:2) band, which we call intraband lumines-cence. Edge luminescence has an exciton nature, while theintraband luminescence is associated with the presence ofoxygen or zinc vacancies or with residual impurities. The prospects of using ZnO and ZnO: Ga in short-wavelength optoelectronics and laser and scintillation engi-neering have been considered.
It is important in the lattercase that ZnO possesses a relatively small (cid:1) for scintillators (cid:2) band gap (cid:1) E g = 3.37 eV (cid:2) , since the conversion efficiency ofscintillators increases as the band gap decreases. Moreover,the exciton binding energy (cid:1)
60 meV (cid:2) in ZnO is greater by afactor of 2.4 than the thermal energy kT (cid:1) for T (cid:3)
290 K (cid:2) . Asa result, the edge luminescence has high intensity at roomtemperature. Edge luminescence is the most important in thecase of rapid scintillators, since it is characterized by nano-second and subnanosecond de-excitation times. The follow-ing characteristics of ZnO are also important for scintillationdetectors: transparency in the visible region, good thermaland mechanical properties, fairly high density (cid:1) g / cm (cid:2) and high radiation stability.
741 741J. Opt. Technol. (cid:2) (cid:1) , November 2008 1070-9762/2008/110741-06$15.00 © 2008 Optical Society of America his paper presents the first results on the synthesis andstudy of ZnO and ZnO: Ga scintillation ceramics. Uniaxialhot pressing was used to obtain the ceramics. This methodwas used earlier when developing the scintillation ceramicsGd O S: Pr, Ce (cid:1)
Ref. 11 (cid:2) and ZnO: Zn. The optical, lumi-nescence, scintillation, and other properties of the resultingZnO and ZnO: Ga ceramics are studied here.
EXPERIMENTAL TECHNIQUE
All the ceramics were obtained in the form of disks24 mm in diameter and 1.5 mm thick (cid:1) after polishing (cid:2) . Boththe pure and the doped ceramics were synthesized from twoforms of the starting zinc oxide: commercial ZnO powders ofVHP grade (cid:1) designated ZnO I (cid:2) produced by ZAO NPF Ly-uminofor (cid:1) Stavropol (cid:2) and a specially purified product of thefirm Alfa Aesar (cid:1) designated ZnO II (cid:2) . Powdered Ga O andGa (cid:1) NO (cid:2) , also produced by Alfa Aesar, were used for dop-ing. Based on an earlier test of the synthesis of ceramics anddata on powdered ZnO: Ga, the gallium concentration inZnO was chosen within the limits from 0.05 to 0.1 wt% (cid:1) inwhat follows, the impurity concentration is indicated every-where in weight percent (cid:2) . Gallium was introduced in theform of Ga O and Ga (cid:1) NO (cid:2) , corresponding to the samplesdesignated as ZnO: Ga or ZnO: Ga,N. Unlike gallium, whichforms donor levels in ZnO, nitrogen forms shallow acceptorlevels. It is assumed that, because of donor-acceptor recom-bination, the edge band in the samples doped with galliumand nitrogen must be shifted toward longer wavelengths bycomparison with those in ZnO: Ga.The morphology and mean size of a powder grain and ofthe ceramic samples was studied by means of optical micros-copy. The lattice parameters and the degree of texturing ofthe ceramic were studied on a DRON-2 x-ray diffractometerwith a copper anode and a nickel filter when the x-ray reflec-tions were recorded on a recorder chart. To determine theparameters of the crystal cell, a system of crystallographicplanes (cid:1) (cid:2) and (cid:1) (cid:2) with large reflection angles 2 (cid:2) (cid:1) (cid:2) was chosen. A technique proposed inRef. 13 for calculating the texture factor was used to monitorthe texture—i.e., the predominant orientation of the crystal-lographic planes in the ceramic samples. The texture factorwas calculated from unambiguously indexed reflection linesfrom the (cid:1) (cid:2) , (cid:1) (cid:2) , (cid:1) (cid:2) , (cid:1) (cid:2) , (cid:1) (cid:2) , and (cid:1) (cid:2) planesby comparing the intensities of these lines from the textur-ized test sample with standard data for this material, takenfrom the I.C.P.D.S. card file. The total spectral transmittanceof the samples was determined on a Hitachi-330 spectropho-tometer equipped with an attachment having an integratingsphere 60 mm in diameter.The x-ray-luminescence spectra were measured using anx-ray tube with a copper anode, operating in the 55-kV,40-mA regime. An Al filter 3 mm thick was used to cut offthe soft component of the x-ray emission. The emission spec-tra were measured by means of an Acton Research Co. VM-504 monochromator (cid:1) diffraction grating 1200 line / mm (cid:2) . AHamamatsu R934-04 photomultiplier was used as a photode-tector. All the measured spectral curves were corrected tak- ing into account the photomultiplier sensitivity and themonochromator transmissivity for various wavelengths.The absolute luminous yield of single-crystal BaF wasmeasured using a Hamamatsu R1791 photomultiplier bycomparing the position of the maximum of the photopeak forthe Cs spectrum (cid:1)
662 keV (cid:2) with the position of the centerof gravity of the single-electron spectrum. The luminousyield of standard single-crystal BaF was equal to 8880 pho-tons per MeV. For the ceramic samples of ZnO and ZnO: Ga,it was not possible to reliably resolve the position of thephotopeak using excitation with Cs gamma quanta. Theabsolute luminous yield of the resulting ceramic was deter-mined by the ratio of its overall intensity to the x-ray-luminescence intensity of BaF , multiplied by the absoluteluminous yield of barium fluoride. L = (cid:4) (cid:5) I ZnO (cid:1) (cid:3) (cid:2) d (cid:3) (cid:6) (cid:5) I BaF (cid:1) (cid:3) (cid:2) d (cid:3) (cid:7) / MeVThe luminescence kinetics were measured using anoriginal experimental apparatus. An x-ray source withpulse width (cid:4)
OPTICAL AND X-RAY CHARACTERISTICS OF THESAMPLES
The original ZnO powders were characterized by a fairlyhomogeneous grain composition both in morphology and insize. The grains were predominantly round and close to iso-metric in shape, with a size of about 1 – 3 (cid:5) m. The results ofthe x-ray structural analysis showed that the lattice param-eters of the original powders correspond to those for singlecrystals: a = 3.2497 Å and c = 5.2069 Å. The hot pressingprocess has a significant effect on the lattice parameters: Adecrease of the c parameter to 5.2034– 5.2043 Å was ob-served for the undoped ZnO ceramics. The introduction of adopant impurity with smaller ionic radius (cid:1) r (cid:8) Ga (cid:9) = 0.47 Å,and r (cid:8) Zn (cid:9) = 0.6 Å (cid:2) into the ZnO lattice did not appreciablychange the parameters of the ZnO: Ga ceramic. When gal-lium nitrate was introduced into the ZnO, parameter c de-creased. A large decrease of parameter a was also noted,which is observed in ceramic samples when there is a mini-mum dopant concentration, regardless of the type of com-pound in the form of which it was introduced.Studies of the predominant orientation of the principalcrystallographic planes showed that all the ceramics are char-acterized by texture along the (cid:1) (cid:2) and (cid:1) (cid:2) planes of theprism and the (cid:1) (cid:2) planes of the rhombohedron. The mani-festation of texture over the three planes is a consequence ofthe fact that it spread (cid:1) the angle between the planes does notexceed 30° (cid:2) . The predominant orientation is observed alongthe (cid:1) (cid:2) plane of the rhombohedron for the undoped ZnO
742 742J. Opt. Technol. (cid:2) (cid:1) , November 2008 Gorokhova et al. eramic. The texture over the planes of the prism predomi-nates in the gallium-doped samples. As a whole, the value ofthe texture does not exceed 0.3.Figure 1 shows photographs of typical microstructuresof the ZnO II and ZnO: Ga II ceramics. The character of therepresented microstructures reflects, first, the intenseprogress of recrystallization processes during the formationof the ZnO ceramic, as a result of which the grain size in-creases by 1.5-2 orders of magnitude by comparison with thesize of the original particles, and, second, the inhibiting roleof the dopant, a consequence of which is that the recrystal-lization processes ceases. An analysis of the data in Fig. 1showed that the grain size is 30– 90 (cid:5) m in the undopedsample and 15– 35 (cid:5) m in ZnO: Ga II .The density of all the ceramic samples was more than0.99 relative to the x-ray-structural density of ZnO. The re-sulting samples had a characteristic color. The ZnO I sampleswere an intense red, but the ZnO II had a yellow-red tint. Thedoped ceramic was in between light blue and dark blue,whose intensity increased with increasing gallium concentra-tion. The highest transparency characterizes the ZnO II ce-ramic, whose total transmission spectrum is shown in Fig. 2.Absorption in the short-wavelength region of transparencyreduces the transmission level of the ZnO II ceramic in thisregion because of the presence of color (cid:1) the cause of whichwill be explained (cid:2) . A similar phenomenon for single-crystalZnO (cid:1) the presence of red coloration and the shift of the short-wavelength transmission limit toward longer wave-lengths (cid:2) was noted in Ref. 16 and was explained by the pres-ence of a high concentration of oxide vacancies. High-temperature annealing in an oxygen atmosphere allowed theauthors to decolor the ZnO single crystals, and this was ac-companied by an increase of transparency in the entire vis-ible region and a corresponding shift of the transmissionlimit toward short wavelengths.It should be pointed out that the resulting transparencylevel of the undoped ceramic in the long-wavelength region (cid:1) (cid:3) (cid:6)
600 nm (cid:2) approaches that for single-crystal ZnO. It isessential in this case that such a result is achieved for aceramic in which the predominant orientation of the grains isnot optimal for a hexagonal structure, since it does not coin-cide with the direction of the optic axis.
THE LUMINESCENCE AND SCINTILLATIONCHARACTERISTICS OF THE CERAMICS
Figure 3a shows the x-ray-luminescence spectra of ZnO I and ZnO II ceramics in comparison with the spectrum of thetraditional scintillator BaF . It can be seen that intrabandluminescence, i.e., a wide band with a maximum at 520 nm,predominates in the undoped ceramics. The ZnO I ceramicmanifests a weak edge luminescence (cid:1) Fig. 3b (cid:2) , whereas theedge luminescence in the original powders and single crys-tals is fairly intense.
The luminous yield of ZnO II wasgreater than that of BaF and substantially greater than thatof ZnO I .The luminescence spectra of the doped ceramics areshown in Fig. 4. The intensities of both luminescence bandsincrease in the transition I → II (cid:1) Figs. 4a and 4b (cid:2) , and this isvalid both for both the ZnO: Ga and the ZnO: Ga,N samples.When the gallium concentration is increased from 0.1% (cid:1)
Fig.4c (cid:2) , the luminous yield decreases by comparison with thatfor ZnO: Ga,N II (cid:1) (cid:2) (cid:1) Fig. 4b (cid:2) . The radiation in theZnO: Ga ceramics is concentrated predominantly in theshort-wavelength region (cid:1)
Fig. 4d (cid:2) , whereas the intraband lu-minescence with a maximum at 510 nm predominates inZnO: Ga,N (cid:1)
Figs. 4a–4c (cid:2) . It should be pointed out that theluminous yield of the ZnO: Ga II and ZnO: Ga,N II ceramicsis identical, but the presence of nitrogen in the samplestrengthens the intraband luminescence and weakens theedge luminescence. The position of the maximum of theedge-luminescence band was identical in ZnO: Ga and ZnO: FIG. 1. Microstructure of ZnO II (cid:1) a (cid:2) and ZnO:Ga II ceramics (cid:1) b (cid:2) . FIG. 2. Total transmission spectrum of ZnO II ceramic 1.5 mm thick.
743 743J. Opt. Technol. (cid:2) (cid:1) , November 2008 Gorokhova et al. a,N: 397.5 nm at 300 K; i.e., the expected long-wavelengthshift when an acceptor (cid:1) N (cid:2) was introduced was not observed.Moreover, the maximum of the edge band in the dopedsamples (cid:1) Fig. 4 (cid:2) is shifted relative to the maximum in ZnO I (cid:1) Fig. 3b (cid:2) by only about 10 meV.The intraband luminescence of ZnO can be associatedwith zinc vacancies V Zn , oxygen vacancies V O , antinodezinc O Zn , and other centers. A recent special study showed that V Zn centers are responsible for the luminescenceband with the maximum at 2.35 eV that is recorded in ourceramics, while oxygen vacancies result in shorter-wavelength radiation (cid:1) (cid:2) . Obviously, the introductionof Ga O and ZnO in our case reduces the number of zincvacancies in the sample and strengthens the edge lumines-cence.Figure 5 shows how the luminescence intensity dependson temperature for a sample of ZnO: Ga II . The intensity ofthe edge luminescence at 78 K is a factor of 3 greater than at300 K. For T (cid:7)
300 K, the intensity of the edge band be-comes greater than that of the intraband luminescence.Examples of the kinetic curves for samples of ZnO: Gaand ZnO: Ga,N are shown in Fig. 6. A rapid falloff of thex-ray luminescence predominates n the samples. The rapidcomponent in ZnO: Ga with a falloff constant of about 1 nsoccupies two decimal places in intensity. The ZnO: Ga,Nceramic possesses a hyperbolic luminescence falloff, and asection with a falloff constant of 3.3 (cid:8) (cid:1)
Fig. 6b (cid:2) . The ZnO: Gaceramic is thus preferable to ZnO: Ga,N in response rate.The main characteristics of the ceramics are shown inTable I. The luminous yield of the pure sample of ZnO II isfairly high: 9050 photon / MeV. The doped ceramics have alow luminous yield: 420 photon / MeV. However, this is theso-called technical output, associated with the low transpar-ency of the ceramics so far obtained. The physical luminousyield must be greater, since it is very high in the initial ZnO:Ga. Moreover, it should again be emphasized that these datarelate to samples that have not undergone heat treatment.These data need to be regarded as preliminary, since heattreatment in a special atmosphere is an integral procedure forforming scintillators not only on the basis of ZnO.
Ourexploratory experiments on the heat treatment of ceramic
FIG. 3. (cid:1) a (cid:2) Luminescence spectra of undoped ceramics ZnO I — , ZnO II — and single-crystal BaF — at room temperature. (cid:1) b (cid:2) Segment of spectra atan enlarged intensity scale.FIG. 4. Luminescence spectra of ZnO: Ga ceramics at room temperature. (cid:1) a (cid:2) ZnO: Ga,N I (cid:1) (cid:2) , (cid:1) b (cid:2) ZnO: Ga,N II (cid:1) (cid:2) , (cid:1) c (cid:2) ZnO: Ga,N II (cid:1) (cid:2) , (cid:1) d (cid:2) ZnO: Ga II (cid:1) (cid:2) . FIG. 5. Temperature dependences of the intensities of —edge lumines-cence (cid:1) (cid:2) , —intraband luminescence (cid:1)
510 nm (cid:2) , and —total lumi-nescence of ZnO: Ga II ceramic.
744 744J. Opt. Technol. (cid:2) (cid:1) , November 2008 Gorokhova et al. amples of ZnO: Ga showed that the intensity of the edgeluminescence becomes somewhat greater in an annealed ce-ramic than in the original sample, with the transparency ofthe ceramic also increasing slightly.From the viewpoint of luminous yield, the optimum gal-lium concentration (cid:1) when it is introduced in the form ofGa O (cid:2) in ZnO was 0.075%, and the result is found to be inagreement with the data for powdered and thin-film ZnO:Ga. An increase of the gallium concentration by 30% re-sulted in a decrease of the luminous yield by almost a factorof 2. Concentration quenching of the edge luminescence inZnO: Ga thus begins at a gallium concentration that exceeds 0.075%. Similar values are characteristic of the ceramic scin-tillators Gd O S: Pr,Ce. This is a fairly low value, since theoptimum dopant concentration is 0.1–0.2% in traditionalscintillators (cid:1)
NaI: Tl, CsI: Tl (cid:2) . It should be emphasized that adecrease of the optimum dopant concentration in a ceramicby comparison with the powdered analogs was observed bythe authors using such luminescent ceramics as ZnS:Cu asexamples. This feature of the ceramics is probably caused bythe higher solubility of dopant impurities in the matrix latticeduring hot pressing.
CONCLUSION
Uniaxial hot pressing has been used to obtain ceramicsbased on zinc oxide, and their optical, x-ray-structural, lumi-nescence, and scintillation characteristics have been studied.It has been shown that, by changing the concentration of thedopant (cid:1) Ga (cid:2) and the codopant (cid:1) N (cid:2) , it is possible to changethe intensity of the edge band and the intraband lumines-cence of ZnO, as well as their ratio. The resulting ceramicspossess short luminescence-falloff times.Undoped ZnO II ceramic has good transparency in thevisible region and a fairly high luminous yield, unlike ZnO:Ga ceramic. Based on the first data obtained by the authors,they have developed a strategy for improving the quality ofthe ZnO: Ga ceramics (cid:1) producing, most importantly, in-creased transparency of the samples (cid:2) , which should improvethe scintillation characteristics of ceramics based on zinc ox-ide. The authors consider it their pleasant duty to expressgratitude to Professor S. E. Derenzo and to Drs. W. W.Moses and P. Dorenbos for collaboration in carrying out thiswork. We also express appreciation to E. A. Oreshchenko forhelp in carrying out the experimental part of this work. a (cid:2) Email: [email protected] S. E. Derenzo, M. J. Weber, and M. K. Klintenberg, “Temperature depen-dence of the fast, near-band-edge scintillation from CuI, HgI , PbI , ZnO:Ga and CdS: In,” Nucl. Instrum. Methods Phys. Res. A , 214 (cid:1) (cid:2) . P. J. Simpson, R. Tjossem, A. W. Hunt, K. G. Lynn, and V. Munne,“Superfast timing performance from ZnO scintillators,” Nucl. Instrum.Methods Phys. Res. A , 82 (cid:1) (cid:2) . M. Katagiri, K. Sakasai, M. Matsubayashi, T. Nakamura, Y. Kondo, Y.Chujo, H. Nanto, and T. Kojima, “Scintillation materials for neutron-imaging detectors,” Nucl. Instrum. Methods Phys. Res. A , 274 (cid:1) (cid:2) . N. Kubota, M. Katagiri, K. Kamijo, and H. Nanto, “Evolution of ZnS-family phosphors for neutron detectors using counting method,” Nucl.Instrum. Methods Phys. Res. A , 321 (cid:1) (cid:2) . E. D. Bourret-Courchesne, S. E. Derenzo, and M. J. Weber, “Semiconduc-tor scintillators ZnO and PbI : co-doping studies,” Nucl. Instrum. MethodsPhys. Res. A , 1 (cid:1) (cid:2) . S. J. Duclos, C. D. Greskovich, R. J. Lyons, J. S. Vartuli, D. M. Hoffman,R. J. Riener, and M. J. Lynch, “Development of the HiLight scintillator forcomputed tomography medical imaging,” Nucl. Instrum. Methods Phys.Res. A , 68 (cid:1) (cid:2) . Y. Sato, T. Yamamoto, and Y. Ikuhara, “Atomic structures and electricalproperties of ZnO grain boundaries,” J. Am. Ceram. Soc. , 337 (cid:1) (cid:2) . U. Orgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshnikov, S. Dogan, V.Avrutin, S.-J. Cho, and H. Morkoc, “A comprehensive review of ZnOmaterials and devices,” J. Appl. Phys. , 041301 (cid:1) (cid:2) .FIG. 6. Luminescence kinetics of ZnO: Ga II (cid:1) a (cid:2) and ZnO: Ga,N II (cid:1) b (cid:2) ceramics.TABLE I. Comparison of the characteristics of ceramics based on zincoxide and a BaF standard scintillator. a The luminous yield is obtained for the available BaF crystal, having thesame size as the test ceramic. The best samples of BaF scintillators possessa luminous yield of up to 11 000 photons per MeV.
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