Luminescence of a ZnO:Ga Crystal upon Excitation in Vacuum UV Region
IISSN 0030-400X, Optics and Spectroscopy, 2008, Vol. 104, No. 2, pp. 210–212. © Pleiades Publishing, Ltd., 2008.Original Russian Text © P.A. Rodnyi, G.B. Stryganyuk, I.V. Khodyuk, 2008, published in Optika i Spektroskopiya, 2008, Vol. 104, No. 2, pp. 245–247.
Zinc oxide has been actively investigated since it hasa number of interesting physical properties as a wide-gap ( E g = 3.4376 eV at 4.2 K) semiconductor with a sig-nificant ionic-bonding component [1]. The prospects ofapplication of ZnO and ZnO:Ga in short-wavelengthoptoelectronics and laser and scintillation techniquewere considered in [2–4]. Photoluminescence investi-gations, performed on thin films [2, 3], ceramics [4],nanopowders [5], and single crystals [6–8] showed thatzinc oxide exhibits a narrow UV line peaking near 3.3eV and a wide long-wavelength ( ~2.2 eV) emissionband. The narrow UV line has an exciton nature, whilethe long-wavelength band is attributed to recombina-tion of electrons with oxygen vacancies [5]. Mostinvestigations of the ZnO photoluminescence were per-formed upon excitation by a He–Cd laser (325 nm) [5–8]. In this study, the spectral–kinetic characteristics ofa ZnO:Ga single crystal excited by vacuum UV (VUV)radiation were investigated.Measurements were performed on a small (approx-imately × × mm) ZnO:Ga single crystal with agallium concentration of cm –3 . The excitation andemission spectra of ZnO:Ga were measured at temper-atures of 8 and 300 K at the Synchrotron RadiationLaboratory HASYLAB (DESY, Hamburg), using theexperimental equipment of the SUPERLUMI station.The luminescence spectra were recorded using a mono-chromator with the best resolution (for low-tempera-ture measurements): 2 meV. The luminescence excita-tion spectra were measured in the range from 4 to12.5 eV (resolution Å) using an R6358P photomul-tiplier (Hamamatsu) on a secondary monochromator. Inmeasurements of kinetic curves, the excitation pulsewidth was 290 ps. Figure 1 shows the crystal luminescence spectrameasured upon excitation by 6.89-eV photons at tem-peratures of 8 and 300 K (Figs. 1a and 1b, respectively).The low-temperature spectrum contains a very narrow(width at half-maximum 7.2 meV) line peaking at3.356 eV (Fig. 1‡, inset) and a wide band peaking near2.0 eV. The narrow 3.356-eV UV line is due to freeexcitons in zinc oxide. The weak peak at 3.214 eVshould be attributed to the superposition of a free exci-ton and two longitudinal optical (LO) phonons, sincethe energy of LO phonons in ZnO is 72 meV [7]. Thewide band is red-shifted in comparison with the similarband in ZnO nanocrystals. It is believed that this bandis due to several types of electron recombination: atoxygen and zinc vacancies and at interstitial zinc [8].At room temperature, the UV line is red-shifted( h ν max = 3.3 eV) and broadened (Fig. 1b). Thesechanges are due to the change in the luminescencemechanism: the exciton luminescence is suppressed atroom temperature and emission occurs between donor(gallium) levels and valence-band holes [3, 9]. Thewide band in the visible spectral range (Fig. 1b) peaksat 2.1 eV and has a feature near 2.3 eV.The reflection and luminescence excitation spectraof a ZnO:Ga crystal are shown in Fig. 2. The excitationspectra exhibit peaks near 6.0, 7.8, and 11.4 eV. Gener-ally, peaks in this region are due to the formation ofhigh-energy excitons. UV and visible (Fig. 2, curves and , respectively) luminescence excitation bandshave a similar shape; this fact is indicative of similarmechanisms of energy transfer to the correspondingluminescence centers. In the crystal reflection spectrum(Fig. 2, ), the high-energy peaks at 11.2 and 12.0 eVshould be attributed to the core zinc states d and d , respectively. Luminescence of a ZnO:Ga Crystal upon Excitationin Vacuum UV Region
P. A. Rodnyi a , G. B. Stryganyuk b , and I. V. Khodyuk a a St. Petersburg State Technical University, St. Petersburg, 195251 Russia b Franko Lviv National University, Lviv, 79005 Ukraine
Received March 29, 2007
Abstract —The spectral–kinetic characteristics of a ZnO:Ga single crystal upon excitation in the vacuum UVregion have been studied. At a temperature of 8 K, the exciton luminescence line peaking at 3.356 eV has anextremely small half-width (7.2 meV) and a short decay time (360 ps). In the visible range, a wide luminescenceband peaking at ~2.1 eV with a long luminescence time at 8 K and a decay time in the nanosecond range at300 K is observed. The luminescence excitation spectra of ZnO:Ga have been measured in the range from 4 to12.5 eV.PACS numbers: 78.55.-m
DOI:
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LUMINESCENCE OF A ZnO:Ga CRYSTAL 211
The kinetics of UV and visible luminescence uponVUV excitation is shown in Fig. 3. At 8 K, the main UVluminescence decay constant is 360 ps (Fig. 3a); thisvalue is characteristic of excitons in ZnO. The decaytime of the band at 2.0 eV is difficult to determine in ourcase, since it is in the microsecond range (Fig. 3b, ).At room temperature, the visible luminescence kineticshas a complex character; in this case, the luminescenceintensity decreases by two orders of magnitude over ~20 ns (Fig. 3b, ). The significant decrease in thedecay time of the visible luminescence at 300 K incomparison with that at 8 K indicates the presence ofshallow traps in the crystal. At low temperatures, therelease rate of carriers from traps is low, as a result ofwhich the luminescence decay constant is large. Withan increase in temperature, the carrier release rateincreases, and, accordingly, the deexcitation timedecreases. The possibility of forming shallow electrontraps due to the introduction of Ga into ZnO was notedin [8].Thus, the VUV-excitation luminescence spectra ofZnO:Ga crystals have a number of features, amongwhich the most important ones are the extremely small Intensity, rel. units
Energy, eV (‡) 431 2
Energy, eV
Fig. 1.
Luminescence spectra of a ZnO:Ga crystal excitedby 6.89-eV photons at (a) 8 and (b) 300 K. The inset showsthe luminescence spectrum at 8 K on the enlarged energyscale.
Energy, eV
Quantum efficiency, rel. units
Fig. 2.
Excitation spectra of ( ) UV and ( ) visible lumines-cence of a ZnO:Ga crystal at 8 K and (3) its reflection spec-trum at 300 K. Number of centers
Time, ns (b)100010010 Fig. 3.
Luminescence kinetics of a ZnO:Ga crystal: (a) theUV line at 8 K and (b) the visible band at temperatures of( ) 8 and ( ) 300 K. OPTICS AND SPECTROSCOPY
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RODNYI et al. width of the exciton luminescence line, a short decaytime of this luminescence, and a large difference in thedecay times of the visible luminescence band at lowand high temperatures.REFERENCES