Luminescence and nonradiative deactivation of excited states involving oxygen defect centers in polycrystalline ZnO

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Journal of Crystal Growth 161 (1996) 190-194

Luminescence and nonradiative deactivation of excited states involving oxygen defect centers in polycrystalline ZnO H.-J. Egelhaaf *, D. Oelkrug Institute of Physical Chemistry, University of T'~bingen, Auf der Morgenstelle 8, D-72076 T'~bingen, Germany

Abstract The green luminescence of polycrystalline ZnO is investigated by diffuse reflection, steady state and time-resolved photoluminescence as well as photoconductivity and is assigned to a donor-acceptor-type transition. Due to efficient quenching by surface states it is most effectively excited upon excitation into oxygen defect states. This "dead-layer effect" is used to determine the donor density (N D = 2.1 × 1018 cm 3) and the flatband potential (Ufb = --530 mV).

1. Introduction Polycrystalline ZnO has received ample attention as starting material for electroluminescent devices. A major drawback of these cells is their low luminescence quantum yield. A thorough understanding of the deactivation processes is essential in order to enhance the radiative recombination channel. It has been shown [1] that the final elementary steps of electro- and photoluminescence are identical. We therefore apply photoluminescence, both steady state and time resolved, in combination with diffuse reflectance and photoconductivity measurements to distinguish between luminescent band states, luminescent states associated with defect centers in the bandgap, and luminescence quenching surface states. It is shown by photoluminescence excitation spectra that the density of the luminescent defect centers, and hence the luminescence quantum yield, can be manipulated by the sintering conditions. The quan-

* Corresponding author.Fax: +49 7071 ctioe01 @mailserv.zdv.uni-tuebingen.de.

296910;

E-mail:

tum yield can also be influenced by applying an electric field across the semiconductor-liquid-interface. This so-called "dead-layer-effect" [2,3] is made use of to determine the flatband potential Ufb and the donor density N D in the ZnO pellets.

2. Experimental procedure Samples are prepared by forming ZnO powder (Aldrich, 99.99%) into pellets under a pressure of 2 × 107 N / m 2 and by subsequent sintering at a temperature of 1300°C for 6 h under air, argon and oxygen at atmospheric pressure. Diffuse reflectance spectra were recorded on a Cary 14 spectrometer using BaSO 4 as a standard. Fluorescence and fluorescence excitation spectra were obtained on a Zeiss fluorometer with MM12 double monochromators. Time-resolved luminescence spectra were measured with a sensitivity-enhanced SI-OSMA-system, gated by a PG 10 pulse generator, triggered by a photodiode. Luminescence was excited by an N 2 laser. For low-temperature

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H.-J. EgelhaafD. Oelkrug/Journalof CrystalGrowth161 (1996)190-194 measurements the pellets were sealed in evacuated fused silica tubes and immersed into liquid nitrogen. For electrical measurements the pellets were connected to a copper wire by wetting the surface with Ga-In alloy at 300°C. The contact was sealed to a glass pipe and the pellets were immersed into the electrolyte. Bias potential was controlled by a Wenking STP 84 potentiostat using a standard three electrode configuration. Photoluminescence cyclovoltagrams [2] were obtained by recording photoluminescence intensity as a function of the applied bias potential, which was scanned with a Wenking VSG 72 scan generator. Photoconductivity was measured by applying a bias potential and exciting with chopped light (u = 40 Hz). The resulting photocurrent was recorded with the lock-in technique to eliminate dark current. Photoaction spectra and photocurrent cyclovoltammograms were recorded by scanning the wavelength of the exciting light or by scanning the applied bias potential, respectively.

3. Results and discussion

3.1. Assignment of transitions Unsintered ZnO pellets possess a diffuse reflectance spectrum, which shows the typical absorption edge connected with the interband transition. A weak green photoluminescence is observed, whose excitation spectrum almost coincides with the diffuse reflectance spectrum. After sintering the pellets at temperatures T = 1600 K, the diffuse reflectance spectrum shows strong absorptions in the bandgap region (Fig. 1). Concomitantly the quantum efficiency of the green luminescence upon band-to-band excitation rises fivefold. In contrast to untreated samples the luminescence is now excited with highest efficiency at energies of 160 meV below the bandgap (peak I of the fluorescence excitation spectrum in Fig. 1) and, depending on sintering conditions, also of 50 meV (peak II in Fig. 1) below the bandgap. However, the photoaction spectrum of a sintered ZnO pellet in 1 n NaOH (Fig. 1) shows a steep rise with an onset at ~ = 26 500 cm -~ , corresponding to the direct bandgap of ZnO, as revealed .2 versus P plot. Obviously, the states by a linear tph corresponding to peaks I and II in the photolumines-

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U = + 1 V) (1) of a ZnO pellet sinteredat 1300°Cunder argon. L, D and S denote localized,delocalizedand surface states, respectively. For peaks I and If, see text. cence excitation spectra are associated with localized defect centers in the bandgap (region L in Fig. 1). The high fluorescence yield observed upon irradiation into the L-states leads to their assignment as bulk states, because fluorescence is most effectively quenched at the surface, as will be shown below. The states responsible for the absorption on the low energy side of peak I (region S in Fig. 1) lead to fluorescence quenching, as obvious from a comparison of absorption and fluorescence excitation spectra. Consequently, they are identified as surface states. Also, the S-absorption, responsible for the yellow color of sintered ZnO, can be removed by etching [4], which supports its assignment to surface states [5]. Sintering under oxygen only produces peak I together with inefficient band-to-band excitation of the green luminescence, whereas sintering under argon produces both peak I and II (Fig. 2). Also the photoluminescence quantum yield of the band-toband excitation is four times higher in argon-treated pellets than it is in samples sintered under oxygen. Thus, the photoluminescence quantum yield in argon-treated ZnO is approximately a factor of twenty higher than in untreated samples. However, after four weeks under air at room-temperature, peak II in the excitation spectrum of the argon-treated sample has vanished. Concomitantly the quantum yield is reduced to the value of the oxygen sample. This

H.-J. Egelhaaf D. Oelkrug /Journal of Crystal Growth 161 (1996) 190-194

192

should undergo a red-shift with increasing time after excitation (Eq. (2)).

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suggests that at least peak II is associated with an oxygen defect center or oxygen defect clusters [6]. The assignment of the observed transitions in the literature is controversial [7-9], which is partly due to the fact that the nature of the observed transitions depends sensitively on the preparation conditions. There seems to be agreement only on the luminescence peak at 26 600 cm-1 (Fig. 3 and 5), which is assigned to the decay of an exciton, lying ca. 40 meV below the conduction band edge [10,11]. For the green luminescence a transition from a donor- to an acceptor-center (DA mechanism) has been proposed (Eq. (1)) [1,7]. Due to the distance dependence of the transition energy, the luminescence spectrum i

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where E D and E A are the energies of the donor and acceptor states, respectively, and r ( D - A) is the distance between acceptor and donor. We actually find a red-shift of A ~ = 2000 c m - i within A t = 600 ns after laser excitation in the time-resolved luminescence spectra of our samples at T = 77 K (Fig. 3). At room temperature, almost no red-shift is observed, probably due to detrapping-retrapping processes. These observations are strong evidence that the green luminescence is actually caused by a DA-type transition. The donor is probably one of the oxygen defect centers V° and V~, which are located at 50 and 190 meV below the band edge [12], and which can be assigned to peak I and II in the photoluminescence spectra, respectively. The participation of Vo° and V~ in the DA luminescence is supported by the observation that the intensity of the green luminescence is proportional to the intensity of the ESR signal at g = 1.96 [12]. Also, the green luminescence can be quenched with an activation energy of ca. 160 meV [4]. However, at the moment it can not be ruled out that these oxygen vacancies only act as traps and that the excitation is transferred to still another donor center before the actual DA transitions occurs. The acceptor is assumed to be a Zn defect Vz~ center, lying 2.5 eV below the conduction band edge [12]. This is in accord with the observation [4] that the green luminescence requires the existence of Zn + sites. The proposed deactivation mechanism is summarized in Fig. 4.

3.2. Polarization effects on the photoluminescence quantum yield Application of a negative bias potential leads to a strongly enhanced quantum yield, up to a factor of five for the green photoluminescence and an additional factor of two for the UV luminescence, when • e luminescence is excited in the band-to-band region, whereas there is almost no effect when exciting

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Fig. 4. Proposed deactivation mechanisms in sintered ZnO pellets. (1) Band-to-band excitation; (2) trapping of charge carriers by zinc and oxygen vacancies; (3) transfer of trapped electrons to localized donor centers D; (4) radiative recombination of trapped electrons and holes from the valence band; (5) donor-acceptor transition; (6) and (7) absorption and emission by excitons E.

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layer" effect [2,3] is caused by the fact that due to "upward"-band bending under negative bias, photogenerated mobile minority carriers, i.e. holes, do not reach the defect centers at the surface observed in the low energy region of the diffuse reflectance spectra, which act as the main quenching sites. In a very similar way to external electric fields, the quantum yield of band-to-band excitation can also be influenced by varying the pH of the solution (Fig. 5b). In this case the band bending is caused by specific adsorption of H ÷ and O H - ions at the ZnO surface. On going from 1 n NaOH to 1 n HCI, the band-to-band excitation efficiency increases by a factor of two, accompanied by a shift of the flat band

H.-J. Egelhaaf. D. Oelkrug /Journal of Crystal Growth 161 (1996) 190-194

194

potential, indicated by the onset of the photocurrent, of AU = 760 mV, which is close to the Nernstian value of AU = 780 mV. A negative surface charge, probably O~- [4], is also the reason for the higher luminescence quantum yield of band-to-band excitation in argon-treated pellets. The dead-layer effect can be exploited to determine the donor density and the flat band potential [2,3] in the sintered ZnO pellets from

is partly due to strong frequency dispersion [13]. Also, the effective surface area Aef f of the sintered electrodes exceeds their geometric one, Ageom, tO an extent not exactly known. As the square of the geometric area is inserted into the Mott-Schottky equation this leads to the overestimation of the donor density N D by a factor (Aeff/mgeom)2.

References

~o 2

2~2EE° ( U - ufo)

[In( IpL/IpL )] = eN-----~

where U is the applied bias, N D the donor density, a the absorption coefficient, and I ~ the intensity of the photoluminescence at the flatband potential. With a ( ~ = 27100 c m - J ) = 2.8 x 105 cm -j and e = 10, we obtain from the plot in Fig. 6b a donor density N o = 2.1 X 1018 cm -3 and a flatband potential of Ufb = - 5 3 0 mV for a sintered ZnO electrode in 1 n NaOH. The depth of the depletion layer is calculated from W=ct-lln(l~/IpL ) to be W = ( 3 . 5 - 5 . 4 ) X 10 -6 cm. In the case of the sintered ZnO pellets, the donor densities determined with the dead-layer model seem to be more reliable than those obtained from a Mott-Schottky plot, which yields unrealistically high values for the donor density (N D = 1.7 X 1021 cm -3, Uro = - 3 5 0 mV). The breakdown of the simple Mott-Schottky model, a phenomenon often encountered with polycrystalline semiconductor electrodes,

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