Electroluminescence of ZnSe:ErF3 thin films

Journal of Luminescence 23 (198!) 315—324 North-Holland Publishing Company

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ELECTROLUMINESCENCE OF ZnSe:ErF3 THIN FILMS I. SZCZUREK UNITRA CEM!, 87-100 Toruñ, Poland

H.J. LOZYKOWSKI and T. SZCZUREK Institute of Physics, N. Copernicus University,,87-1 00 Toruñ, Poland Received 15 September 1980

The result of electrical, dynamical and spectral investigations concerning the electroluminescence of ZnSe: ErF3 thin films as well as some theoretical considerations are presented. They lead to the conclusion that in the system studied a wide variety of electroluminescent centers occur and that the direct impact excitation mechanism dominates.

1. Introduction In our previous paper [1] we have reported on the electroluminescence spectra of zinc selenide thin films doped with various rare-earth fluorides. Since zinc selenide was not used before as a host in electroluminescent thin-film devices, more complex investigations have been performed. Below the results of these investigations concerning ZnSe: ErF3 are presented. The electrical, dynamical and spectral investigations as well as some theoretical considerations are in the scope of the present paper. 2. Experimenta’ The samples have been prepared in the form of sandwiches consisting of SriO2, ZnSe: ErF3, Si02 and Al layers deposited on a glass substrate. A method of their preparation has been described elsewhere [1]. In order to determine the driving conditions the voltage across the active ZnSe: ErF3 layer as well as other electrical quantities were measured using the methods proposed by Chen and Krupka [2]. For observing the photogenerated current in the ZnSe: ErF3 layer a mercury lamp HBO 200 and photoelastic modulator Morvue PEM 3 working at 50 kHz were used. Luminescence decay measurements have been performed by a single-photon 0022-2313/81 /0000—0000/$02.50 © 1981 North-Holland

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Electroluminescence of ZnSe: ErF~

gated counting method. In order to simulate an AC operation in pulse driving mode, the electroluminescence was excited by pairs of voltage pulses of opposite polarity. The pulses were supplied by two synchronized pulse generators. The pulses were of the same duration (10 its), amplitude and polarity, but being applied to different electrodes of an electroluminescent device, they were equivalent to the pulses of opposite polarity. The emission spectra were measured using Zeiss SPM 2 and GDM 1000 scanning monochromators. For spectral investigations the electroluminescent devices were immersed in liquid helium and excited by a 1 kHz sine voltage generator.

3. Experimental results and discussion The ZnSe thin films doped with ErF3 display rather bright visible electroluminescence in comparison with those doped with the other rare-earth fluorides. The scheme of energy levels involved in this electroluminescence is shown in fig. 1. Three groups of emission lines 550 and 660 2H centered 4S at 525, 4F nm originate from the transitions from the 1 1/2’ 3/2 and 9/2 excited manifolds to the ~~I5/2 ground state respectively. Fig. 2 shows the waveform of total emission excited by a 1 kHz trapezoidal voltage as well as conduction current flowing through the ZnSe: ErF3 layer measured at room temperature. The sample is slightly asymmetrical. The light output is higher when the Al electrode is charged negatively. If current pulses are concerned the asymmetry is reversed. In order to determine the instantaneous voltage direction at a ZnSe: ErF3 layer, the sample was illuminated by modulated UV light and additional photogenerated current peaks shown in fig. 3 were observed. The polarity of these peaks indicates the direction of the instantaneous voltage. From fig. 3 it is evident that the zero voltage at the ZnSe: ErF3 layer and the one at the sample are not exactly in phase. Integrating the current peaks of fig. 2 the change in the peak-to-peak surface-charge density i~Qon the insulator layer may be determined. The ~Q depends on the applied voltage, as shown in fig. 4. The slope of the curve at a high voltage evaluates2.theInstead, capacitance unitat area of the Si02 insulator the per slope a low voltage evaluateslayer the of about 30 nF/cm capacitance of the insulator in series with that of the ZnSe: ErF 3 layer, hence the capacitance of the ZnSe: ErF3 layer itself can also be derived (about 4.6

In our previous paper [I] the assignment of some 4S emission 41 lines should be corrected as follows: emission at about 800 nm corresponds to 312— 1312, and the one at 1000 nm to the 111/2 ‘15/2 transition.

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Electroluminescence of ZnSe: ErF~

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nF/cm2). The Si0 2 and ZnSe:ErF3 layers are 0.2 and 1.6 nm thick respectively. Fig. 4 shows the plot of t~Qversus applied voltage. From the linear relationship between z~Qand applied voltage exceeding about 100 V it is evident that the electric field in a ZnSe: ErF3 layer is clamped. The same conclusion can be inferred from the linear relationship between theinelectro2 shown fig. 5. luminescence intensity and z~Q 0.2 zC/cm The electric field is clamped at exceeding about 0.5 Xabout 106 V/cm. This value is about 6 times smaller than that reported by Chen and Krupka [2] for their electroluminescence structures composed of a ZnS layer, but very close to the magnitude of the electric field threshold for intrinsic impact ionization processes in ZnSe [3]. Attainable 1~Qand, then, the brightness of electroluminescent devices are

Fig. 2. Oscilloscope photograph of electroluminescence waveform (upper trace) and conduction current flowing through a ZnSe: ErF 3 layer (medium trace) following a trapezoidal voltage excitation (lower trace).

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Electroluminescence of ZnSe: ErF~

Fig. 3. Oscilloscope photograph of photogenerated current (upper trace) showing the direction of electric field at a ZnSe: ErF 3 layer during the increase of exciting voltage (lower trace).

limited by the peak-to-peak displacement which the insulator can sustain. 2.TFor his our Si02 insulator we order have attained the value of about value is more thanlayer by an of magnitude smaller than 0.6 thatp~C/cm reported by Chen and Krupka [2] for their structures composed with a Ta 205 insulator layer. Therefore, it may be expected that replacing Si02 by a better insulator would bring a substantial improvement in brightness of our electroluminescent devices.

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Fig. 4. Surface-charge density on the insulator as a function of applied voltage.

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electroluminescence spectra: (a) for ZnSe : ErF 2~ 3 thin film as it was deposited doped and heat ZnSe treated singlein crystal vacuumrecorded at 500°C, at 77 recorded K [4]. at 4.2 K, (b) for Er doped ZnSe and (c) for Er

The results of electroluminescence decay measurements are presented in fig. 6 as a semilogarithmic plot. The slopes of the curves (a) and (b) are identical reflecting the fact that at room temperature the populations of the 2H 4H 1 1/2 and 3/2 manifolds are thermally equalized. Following immediately the exciting voltage pulse, the decays are characterized by the time constants T 10 ~ts (curves (a) and (b)) and i~= 20 ~ss(curve (c)). The decay curves are far from being exponential. This can be interpreted as evidence of a wide variety of electroluminescent centers occurring in ZnSe: ErF3, each of them being characterized by its own lifetime. There e~iistsalso the possibility, however, that the sample is not completely electrically relaxed immediately after the exciting voltage pulse has been finished. If so, a remnant excitation may be prolonged beyond the exciting voltage pulse. The occurrence of a quite large number of different sites is confirmed by 4S spectroscopic investigations. The spectra of the 3/2 —~I15/2 transition shown in fig. 7 revealed dozens of sharp lines. Relative intensities of these lines change slightly from sample to sample. They depend also upon the heat treatment of the sample (compare the curves (a) and (b)) as well as upon the applied voltage. Some of thecrystal. lines coincide withhad those observed by Brown et al. 3 + single These lines been classified by them as [4] for a ZnSe: belonging to theErsites G and H. ZnSe: ErF 3 thin films displaying rather strong electroluminescence are wealdy active in photoluminescence. This observation suggests that in this system a direct impact3+, excitation by hot electrons dominates. similar the impact excitation mechanism has In been well systems, e.g. in ZnS Er

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Electroluminescence of ZnSe: ErF

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Fig. 8. Experimental intensity ratio versus applied voltage.

established [5]. In order to support the above suggestion we have investigated the brightness versus the applied voltage characteristics. Experimental results of these investigations are plotted in figs. 8 and 9 as the ratios of the electroluminescence intensities originated from three excited manifolds to the ground state. For simplicity, the intensities are labelled by the consecutive numbers of the levels appearing in fig. 1, instead of a full spectroscopic notation. The results differ slightly from sample to sample. Those shown in figs. 8 and 9 were obtained for two representative samples. The intensity ratios

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Fig. 9. The same as in fig. 8, except for another sample.

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Electroluminescence of ZnSe: ErF

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eE~leVI Fig. 10. Theoretical intensity ratio versus electron mean energy: curves (a) and (c) represent the ratios 14/12, while (b) and (d) represent the ratios I3/’2. Curves (a) and (b) were obtained using the temperature corrected average energy loss ~E0), while (c) and (d) used the optical phonon energy E0 in electron energy distribution function.

decrease, eventually pass through the maximums, with increasing applied voltage. No case was observed where the intensity ratio increases with increas3~. ing applied voltage, as related was reported by Kobayashi et al. [5] ZnS: Er This difference may be to the difference in numbers of for electroluminescence levels of Er3~located within energy gaps of ZnSe and ZnS. In the following section an attempt is made to reproduce theoretically the curves of the experimental intensity ratios versus applied voltage on the basis of the direct impact excitation mechanism.

4. Theoretical considerations The theory is based on a four-level model (fig. 1). The intensity ratio originating from the transitions from two arbitrary levels, each of them populated by n, Er3~ions may be derived from the rate equations

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provided that all are known. The radiative plus nonradiative relaxation rates a 11 can be roughly estimated, some of them from observed radiative

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decays (a14 =a13 = l.4X iO~s”t and a12 =2.8 X iO~s’), and others from the general laws ruling the forbidden and phonon-assisted transitions (a24 = X l0~Is’ and a23 = 5>< l0~s~).At room temperature the levels 3 and 4 are thermally equalized, therefore = 0.resulting The expression for collisions the rate ofwith produc3~ ions in the 6, a34 state, from the hot tion of Er electrons, is an integral over the energy 6, therefore ~

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=

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1V~In 2 can be fitted for each excited level by choosing the value of pt,. The curves shown in fig. 10 were obtained for the case when the half-widths are: ~ = I eV, ~ = 2 eV and = 4 eV. These curves are in qualitative agreement with the experimental ones shown in figs. 8 and 9. A better quantitative agreement is achieved when instead of E0, in the electron energy distribution function, is used (compare the curves (a) and (b) and the curves (c) and (d) respectively). The theory is based on a very simple model which does not take into account the variety of electroluminescent centers occurring in ZnSe: ErF3 thin

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films, and interactions between them. Therefore, one should not expect a better agreement of theory and experiment. Nevertheless, qualitative agreement seems to support the suggestion that in this system, like in ZnS: Er3~,the direct impact excitation dominates.

Acknowledgement The authors are grateful to Prof. W. Wardzyñski, Institute of Physics, Polish Academy of Science, Warsaw, for his personal, and instrumental, contribution to the spectral measurements at liquid helium temperature.

References [I] [2] [3] [4] [5] [6] [7] [8]

I. Szczurek and H.J. Lo~ykowski,1. Lumin. 14 (1976) 389. Y.S. Chen and D.C. Krupka, J. Appl. Phys. 43 (1972) 4089. R. Mach and W. Ludwig, Phys. Stat. Sol. (a) 23(1974) 507. MR. Brown, A.F.J. Cox, WA. Shand and J.M. Williams, J. Phys. C4 (1971) 1049. H. Kobayashi, S. Tanaka and H. Sasakura, Japan. J. Appi. Phys. 12 (1973) 1637. CR. Crowell and SM. Sze, Appl. Phys. Lett. 9 (1966) 242. D.C. Krupka, J. Appi. Phys. 43 (1972) 476. H.M. Jongerius, Philips Res. Repts., Suppl. No. 2 (1962).