Emission and Excitation Spectra of ZnO:Ga and ZnO:Ga,N Ceramics
P. A. Rodnyi, I. V. Khodyuk, E. I. Gorokhova, S. B. Mikhrin, P. Dorenbos
ISSN 0030-400X, Optics and Spectroscopy, 2008, Vol. 105, No. 6, pp. 908–912. © Pleiades Publishing, Ltd., 2008.Original Russian Text © P.A. Rodny ˇ , I.V. Khodyuk, E.I. Gorokhova, S.B. Mikhrin, P. Dorenbos, 2008, published in Optika i Spektroskopiya, 2008, Vol. 105, No. 6, pp. 988–993. INTRODUCTIONZinc oxide is of great interest for researchers due toits unique properties. When the degree of ionic bondingis significant, ZnO is a semiconductor material, inwhich Wannier–Mott excitons manifest themselves as anarrow emission band near the fundamental absorptionedge. Concerning the conductivity, ZnO can be trans-formed from an insulator (in the inactivated state) to atypical semiconductor by introducing IIIa-group impu-rity elements: Al, Ga, and In [1]. Prospects of applyingzinc oxide in optical contacts, solar cells, and spintron-ics are considered. The characteristics of zinc oxidehave been most completely described in reviews [1, 2].Under standard conditions, ZnO has a hexagonalwurtzite structure, in which each O ion is coordinatedby a tetrahedron composed of four Zn ions. The lat-tice constants are a = 3.2497 Å and c = 5.2069 Å; theirratio c / a = 1.602 is close to ideal. In view of the largedegree of ionic bonding, 72-meV optical phonons playa key role in ZnO. Two emission bands are generallydetected in all diverse forms of ZnO (single crystals,thin films, whiskers, nanocrystals, needles, etc.), i.e., ashort-wavelength band near the absorption edge (theluminescence edge) and a long-wavelength (green)band, which will be referred to as the intraband lumi-nescence. The edge luminescence has the excitonnature, while the intraband luminescence is due to thepresence of oxygen or zinc vacancies [1] or residualimpurities [3]. The edge deexcitation time lies in thenanosecond or subnanosecond range; therefore, it is most important for high-speed devices (lasers, scintilla-tors, phosphors).Gallium-doped zinc oxide is used in the form ofthin-film coatings in scintillation detectors to detect a particles [4]. To measure X rays and g rays, ceramicsbased on zinc oxide have been developed [5, 6]. It isimportant that ZnO has a relatively narrow (for scintil-lators) band gap ( E g = 3.37 eV) because the conversionefficiency of scintillators increases with a decrease inthe band gap. In addition, the exciton binding energy inZnO (60 meV) in higher by a factor of 2.4 than the ther-mal energy kT (for T » K), as a result of which theedge luminescence is strong at room temperature. Forscintillation detectors, the following ZnO parametersare important: transparency in the visible spectralrange, good thermal and mechanical properties, suffi-ciently high density (5.61 g/cm ), and high radiationresistance.In this paper, we report the results of studying theluminescence characteristics of the ZnO:Ga andZnO:Ga,N ceramics obtained by uniaxial hot pressing.The optical, X-ray diffraction, and scintillation charac-teristics of the samples were described in [7].EXPERIMENTALThe ceramics were prepared by uniaxial hot press-ing [6, 7] from specially purified initial powders (AlfaAesar production) in the form of 24-mm disks with athickness of 1.5 mm (after polishing). A mixture ofZnO and Ga O powders was used to prepare ZnO:Ga CONDENSED-MATTERSPECTROSCOPY
Emission and Excitation Spectra of ZnO:Ga and ZnO:Ga,N Ceramics
P. A. Rodny ˇ a , I. V. Khodyuk a , E. I. Gorokhova b , S. B. Mikhrin a , and P. Dorenbos c a St. Petersburg State Technical University, St. Petersburg, 195251 Russia b All-Russia Research Center Vavilov State Optical Institute, St. Petersburg, 192171 Russia c Faculty of Applied Sciences, Delft University of Technology, 2629 JB Delft, The Netherlandse-mail: [email protected]
Received June 17, 2008
Abstract —The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressinghave been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescencespectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, isobserved in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and thebandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E m (0) =3.367 – eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectrain the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescencecenters is considered.PACS numbers: 78.55.-m DOI:
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EMISSION AND EXCITATION SPECTRA 909 ceramics. In addition, we attempted to introduce nitro-gen coactivator into ZnO:Ga. In contrast to gallium,which forms donor levels in ZnO, nitrogen generatesshallow acceptor levels [1, 8]. It is believed that, due tothe donor–acceptor recombination, the edge band in thesamples activated with gallium and nitrogen should beshifted to longer wavelengths in comparison with thatin ZnO:Ga. ZnO:Ga,N ceramics were prepared from amixture of ZnO and
Ga(NO ) powders. On the basis ofthe previous experiments on ceramics synthesis [6] andthe data on the powder [8, 9] and single-crystal [10]ZnO:Ga, the gallium concentration in the ZnO:Ga andZnO:Ga,N samples was chosen to be 0.075 wt %,which corresponds to ~1.5 · cm —3 .The X-ray luminescence spectra were measuredusing an X-ray tube with a copper anode, operating at55 kV and 40 mA. To cut off the X-ray soft component,we applied a 3-mm thick Al filter. The emission spectrawere measured on a VM504 monochromator (ActonResearch) with a resolution of 1 nm (diffraction grating1200 lines/mm). An R934-04 photoelectron multiplier(Hamamatsu) was used as a photodetector. All mea-sured spectral curves were corrected taking intoaccount the multiplier sensitivity and the transmissioncapacity of the monochromator at different wave-lengths.The excitation spectra were measured in the UVspectral region; the UV source was an E7536 CW Xelamp with a power of 150 W (Hamamatsu). The excita-tion scheme contained an ARC monochromator (modelVM502, 1200 lines/mm (slit 0.3 mm), with an Al/MgF -coated mirror), MgF window, and focusingelements. The detection scheme included a Macan 910monochromator (resolution 8 nm, 1200 lines/mm) andan XP2254/B photoelectron multiplier (Philips). Allrecorded spectra were corrected taking into account theemitter intensities at different wavelengths.RESULTSFigure 1 shows how the shape and position of theceramics luminescence spectra depend on temperature.The integrated intensity (specific light yield) of thesamples turned out to be the same; however, the emis-sion in ZnO:Ga ceramics was concentrated predomi-nantly in the short-wavelength region, and ZnO:Ga,Nexhibits dominant intraband luminescence. In ZnO:Ga,the intraband band peak shifts from 2.05 eV at 78 K to2.3 eV at 300 K (Fig. 1a). The wide intraband band inZnO:Ga,N peaks at 2.37 eV; the peak position is tem-perature-independent (Fig. 1b). The peak positions ofthe edge-luminescence bands in ZnO:Ga andZnO:Ga,N were found to be the same, i.e., 3.12 eV at300 K, which indicates that the expected long-wave-length shift due the acceptor (nitrogen) doping was notobserved. Moreover, the edge-band peak in the dopedsamples is shifted with respect to that for undoped ZnOceramics [6] by only ~10 meV. The integrated edge- luminescence intensity for ZnO:Ga at 78 K is higherthan at 300 K by a factor of 3; at T > 300 K, it exceedsthe intraband luminescence intensity.The edge-luminescence spectra of the ZnO:Ga andZnO:Ga,N ceramics at different temperatures areshown in Fig. 2 (the spectra were measured at temper-atures from 78 to 600 K with a step of 25 K; to avoidcumbersomeness, only part of them are shown). It canbe seen that, with an increase in temperature, the peakof the edge-luminescence band shifts to lower energies(undergoes a red shift), and the bandwidth increases.The arrow in Fig. 2 indicates a feature in the spectrumat 78 K, which is due to the contribution of longitudinaloptical (LO) phonons to the luminescence.The temperature dependence of the position of theedge-luminescence peak for the ZnO:Ga ceramics isshown in Fig. 3 (a similar dependence was obtained for T , (cid:138) E , e V I n t e n s i t y , r e l . u n i t s
10 2.0 4.0 400 T , (cid:138) E , e V
50 2.5 3.0 3.5 300 200100(b)((cid:224))
Fig. 1.
Dependences of the luminescence intensities of(a) ZnO:Ga and (b) ZnO:Ga,N ceramics in the energy–tem-perature coordinates.
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ZnO:Ga,N). This dependence has two specific features:a large peak shift (by 0.4 eV) in the temperature rangeof 100–600 K and almost linear temperature depen-dence of the peak position.The temperature dependence of the half-width athalf maximum ( G ) for the edge-luminescence band ofZnO:Ga is shown in Fig. 4. The data obtained have alarge spread because they were derived directly fromthe experimental curves (Fig. 2). These spectral curvesinclude (i) phonon replicas, which expand the low-energy band edge, and (ii) the fundamental absorptioncomponent, which cuts off the high-energy edge of theluminescence band. In addition, the temperature decayof the luminescence intensity is imposed on theseeffects, as a result of which the true value is difficult todetermine.Figure 5 shows the dependences of the quantumyields of the edge and intraband luminescence on theincident photon energy h n exc for the ZnO:Ga andZnO:Ga,N ceramics at 290 K. The intraband lumines- cence is effectively excited in the range of exciton gen-eration (the band peaking at 3.17 eV) and is not excitednear the interband transition, i.e., at h n exc > E g = 3.37 eV(Fig. 5, curves , ). The luminescence edge is excitedat incident photon energies slightly exceeding the zincoxide band gap (the inset in Fig. 5 shows the depen-dence for the ZnO:Ga ceramics; for ZnO:Ga,N, thedependence is the same, and the curves coincide). Boththe edge and intraband bands are barely excited in therange of 3.5–10 eV. DISCUSSIONThe intraband luminescence in ZnO may be due tozinc vacancies V Zn [1], oxygen vacancies V O [11], anti-site zinc O Zn [12], and other centers. A recent investiga-tion [13] showed that the luminescence band peaking at2.35 eV is due to V Zn centers, while oxygen vacanciesare responsible for the shorter-wavelength radiation(2.53 eV). The position of the wide long-wavelengthluminescence band (Fig. 1) in the spectra of the ceram-ics studied suggests that this band is due to V Zn centers.Obviously, in our case, the introduction of Ga O intoZnO decreases the content of zinc vacancies in the sam-ple and enhances the edge luminescence.The red shift of the edge luminescence in ZnO isgenerally attributed to the decrease in the crystal bandgap upon cooling. The ZnO band gap decreases by65 meV in the temperature range from 100 to 300 K[1], the red shift for single crystals [14] and films [15]is approximately the same. For the ceramics, the corre-sponding value is D E m = 0.18 eV. The shift of the high-energy band edge in the range 100–300 K is also large,i.e., 0.11 eV (Fig. 3). Even taking into account theUrbach absorption tails, one cannot explain such a largered (Stokes) shift in the spectra of ZnO:Ga and Intensity, rel. units E , eV Fig. 2.
Edge-luminescence spectra of the (a) ZnO:Ga and(b) ZnO:Ga,N ceramics at temperatures T = ( ) 78, ( ) 100,( ) 150, ( ) 200, ( ) 250, ( ) 300, ( ) 350, and ( ) 400 K. S , eV Peak position, eV T , (cid:138) Fig. 3.
Temperature dependence of the position of the edge-luminescence peak for the ZnO:Ga ceramics. The lumines-cence Stokes shift S is plotted on the right ordinate axis.OPTICS AND SPECTROSCOPY Vol. 105
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EMISSION AND EXCITATION SPECTRA 911
ZnO:Ga,N ceramics. In addition, the edge-lumines-cence bands do not have a sharp high-energy wing at T ‡ K; i.e., they are less affected by the intrinsicabsorption in ZnO. The intrinsic absorption is signifi-cant for temperatures T < 250 K, at which the edge-luminescence band is asymmetric (Fig. 2). At 78 K, theluminescence band of ZnO:Ga (Fig. 2b, upper curve)can be approximated by two Gaussians peaking at E m = 3.302 eV and E m = 3.230 eV. The former and lat-ter bands should be attributed to the excitons localizedon donors ( D , X centers) and excitons bound with opti-cal phonons ( D , X –1LO) because E m — E m = 72 meV.The specific features of the temperature changes inthe edge-band parameters—large Stokes shift and lin-ear dependence of the peak position—should be attrib-uted to the presence of several gallium levels near thetop of the conduction band. It is known that gallium inlow concentrations ( ~10 cm —3 ) forms single levelsnear the bottom of the conduction band in ZnO,whereas a system of levels is formed at high concentra-tions ( ‡ cm —3 ) [9]. The shift of the edge-band peak(Fig. 3) can be caused by to the successive deoccupa-tion of gallium levels in ZnO:Ga. Upon heating, theelectrons from high-lying levels are thermally excitedto the conduction band, and low-lying levels areinvolved in emission. This mechanism is confirmed bythe exponential decrease in the edge-luminescenceintensity with an increase in temperature (Fig. 1). Nat-urally, the temperature change in the band gap and theshift of the fundamental absorption edge of ZnO con-tribute to the shift of the edge-band peak.The dependence E m = E m ( T ) for ZnO crystals [14]and films [15, 16] differs from that obtained by us forthe ZnO:Ga ceramics (Fig. 3). In the case under consid- eration, the following linear dependence is observed forthe red shift :Using the dependence in Fig. 3, we obtain E m (0) =3.367 – eV; a = 0.774 meV/K. The position ofthe edge-band peak at zero temperature, E m (0) , is con-sistent with the energy of the excitons localized atdonors ( D , X ) in single crystals (3.360 eV) [14] andfilms (3.3651 eV) [15]. The coefficient a for ceramicsis between those for D and X excitons in single crystals( a = 0.67 meV/K) and nanocrystals ( a = 1.14 meV/K)[14].The Stokes shift of the edge luminescence at roomtemperature is 0.24 eV (Fig. 3). For comparison, theStokes shift in thin ZnO:Ga films with a gallium con-centration of 10 cm –3 is ~0.2 eV at room temperature[9]. The change in the edge-band half-width at half-maximum cannot be described by the generallyaccepted dependence [14] due to the spread in G andabsence of low-temperature ( T < 78 K) experimentaldata. We can only state that G for ceramics changesfrom 0.25 to 0.4 eV in the temperature range of 100–300 K. For comparison, in ZnO:Ga films with a galliumconcentration of · cm —3 , G = 195 meV at293 K [9].Intraband luminescence is effectively excited in therange of exciton generation (the band peaking at3.17 eV in Fig. 5). This fact indicates that intrabandluminescence is due to the generation of excitons,which then radiatively annihilate at luminescence ( V Zn ) Generally, the formula [15] E m ( T ) = E m (0) — [ a T /(1 + b / T )] isused for the red shift. It is likely that the ratio b / T does not differmuch from unity in our case. E m T ( ) E m ( ) a T .–= G , eV T , (cid:138) Fig. 4.
Temperature dependence of the half-width at half-maximum, G , of the edge-luminescence band of ZnO:Ga.The errors are shown by vertical lines. Energy, eVQuantum yield, rel. units
210 543 g E g Energy, eV
Fig. 5.
Intraband-luminescence excitation spectra of the( ) ZnO:Ga,N and ( ) ZnO:Ga (2) ceramics at T = 290 K.The inset shows the edge-luminescence excitation spectrumof ZnO:Ga. OPTICS AND SPECTROSCOPY
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RODNY˛ et al. centers. Note that the conventional recombinationmechanism of intraband luminescence was accepted ina number of studies; i.e., the recombination of conduc-tion-band electrons with V Zn or V O centers was consid-ered [11, 16]. The sharp increase in the luminescencequantum yield at h n exc < 3 eV (Fig. 5, curves , ) isobviously caused by direct excitation of luminescence( V Zn ) centers.The edge-luminescence excitation spectrum (Fig. 5,inset) shows that excitons in ZnO:Ga are formed upongeneration of the so-called genetic electron–hole pairsand under direct excitation of exciton states. If theenergy of generated electrons exceeds ~0.15 eV (withrespect to the bottom of the conduction band), they can-not be excited to gallium levels and are apparently cap-tured by traps.Note that an increase in the quantum yield of theedge and intraband luminescence was observed at ener-gies h n exc > 10.2 eV (the corresponding curves are notreported because the deuterium lamp used by us has alow intensity at h n exc > 10.5 eV, and the data are not reli-able). A similar increase in the quantum yield wasobserved in a ZnO:Ga single crystal [10]; it was attrib-uted to the electron transitions from the Zn d levels tothe conduction band. An increase in the edge-lumines-cence quantum yield at h n exc > 10 eV was detected inundoped ZnO ceramics [17]. Therefore, this increase isa fundamental feature of the base (zinc oxide).CONCLUSIONSAt room temperature, the edge- (exciton-) lumines-cence band peaking at 3.12 eV dominates in ZnO: G . InZnO:Ga,N, an intraband-luminescence band at 2.37 eVis detected, which is apparently due to zinc vacancies.Upon heating, the edge-band peak shifts to lower ener-gies and the bandwidth increases. For ceramics (in con-trast to crystals), a linear temperature dependence (withthe slope a = 0.774 meV/K) of the edge-band peakposition was recorded. The linear dependence E m ( T ) isthe result of superposition of several processes. Theposition of the edge-band peak at zero temperature, E m (0) = 3.367 – eV, obtained by extrapolation, isin agreement with the data for thin films and crystals.The luminescence excitation spectra suggest thatintraband luminescence is excited through exciton states. The edge luminescence arises upon direct exci-ton generation and formation of electron–hole pairswith energies slightly exceeding the ZnO band gap.REFERENCES
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