Electroluminescence of copper and chlorine doped zinc ...trnres.com/ebook/uploads/onbaslicontent/T_13507230271 Onbasli.pdf · of E.L. on copper and clorine doped zinc sulfide, ZnS(Cu,Cl)

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Lifetime of the Waves from Nano to Solitons in My Life, 2012: 1-36 ISBN: 978-81-7895-556-8 Editor: Ülker Onbaşlı

Chapter 1

Electroluminescence of copper and chlorine doped zinc sulphide (ZnS:Cu, Cl) phosphor

Ülker Onbaşlı

Marmara University, Faculty of Science & Letts., Physics Department (Retired Faculty & Senior Researcher)

The literature on electroluminescence (E.L.) phenomenon of both its fundamental investigation and technological development is overwhelmingly extensive and no attempt at a comprehensive review will be made here, only a brief description is intended with emphasis on experimental methods to be significant in supporting one or more of the current models that have been applied to this complex and diverse phenomenon. For many decades, the physicists have been active in applying the investigative methods and concept of solid state physics to the analysis of luminescence phenomena in II-VI compounds. Luminescence of the II-VI compounds has been and still is an extremely active area for scientific and technological studies. The high quantum efficiency and durability of E.L. devices made by II-VI compounds have been important factors in the development and growth of this field. Some commercial applications of E.L are in display panel of television with sharper, clear image and wide viewing angle, car instrument panel backlighting, radars, high efficient electroluminescent safety lamps, architectural needs etc. Correspondence/Reprint request: Prof. Dr. Ülker Onbaşlı, Rıdvanpaşa cad., 3.Sok., 85/12, Göztepe 34730 Istanbul, Turkey. E-mail: [email protected]

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Particularly, electroluminescence is a fascinating phenomenon at which the light emission occurs due to the absorption of energy from applied electric field. One of the most important features of E.L. is that a direct conversion of electric energy into visible light takes place without generation of heat in about 10-9 seconds. E.L. materials are usually called as phosphors that contain appropriate amount of donor centers, can be excited by the application of an electric field. Two distinct mechanisms are relevant for the production of E.L. light, one of which is called as pure (intrinsic) E.L. and injection E.L.. This chapter is devoted to the investigation of the intrinsic type of E.L. by means of copper and chlorine doped zinc sulphide phosphors. In 1924, the first example of E.L. had been observed on silicium carbide, SiC phosphors. The SiC n-type junction is also the first example for direct current, d.c. electroluminescence that has been recognized with the yellow emission of E.L. light. Thereafter, Destriau invented the green light emission of E.L. on copper and clorine doped zinc sulfide, ZnS(Cu,Cl) II-VI compounds. ZnS was the first material in which alternating current, a.c. electroluminescence was observed in 1937 and it has been maintained its position of having the highest rank to the present. The ZnS phosphor is usually dispersed in a dielectric material such as dinitroceluloz used in this work. To construct an E.L. device, the mixture is placed in between the electrically conducting by means of opaque and transparent electrodes. In order to detect the emission of light at least one of the electrodes is preferred to be transparent. The wave length of the emitted light depends on the type of dopants used during the preparation process of ZnS phosphors. For example, the green emission of ZnS:Cu may be shifted from 2.4 eV to 1.2 eV (green to infrared) by the substitution of Cd for Zn, or from 2.4 to 2.0 eV (green to orange) by the substitution of selenium for sulphur [1]. Under the light of information given above, this chapter is devoted to the electroluminescence of ZnS:Cu, Cl phosphor which is mainly based on the master thesis of the author. (Master of science thesis was defended by Ülker Onbaşlı and accepted by Istanbul University, Institute of Science,1974). I.1. Theoretical work As it has been mentioned above that electroluminescence is a type of luminescence excited by an electric field. Such luminescence is observed in some solids whose atoms or molecules undergo a transition to an excited state when some type of electric discharge occurs. In pure E.L., the electrons from donor levels are set free by the electric field and then accelerated by the

Electroluminescence of copper and chlorine doped zinc sulphide (ZnS:Cu, Cl) phosphor 3

field until they collide with luminescent centers and ionizing them. Light is emitted in the normal way as soon as an electron recombines with an ionized atom of the centre. Life time of the electrons is generally of the order of nanoseconds. Hence, an alternating voltage is used to establish a sustained light emission. The quantum efficiency can be very high in direct band gap semiconductors such as CdS, CdSe, CdTe, ZnS and ZnO with low density of crystal defects: on average, nearly one photon is obtained for each electron-hole pair or for each electron sent into device. The photon energy and thus the color of the emitted light are essentially determined by the band gap energy of the material and the dopants used. ZnS semiconductor investigated in this chapter is one of the II-VI compounds in the periodic table that has both cubic and hexagonal crystallographic structure. The forbidden gap of ZnS, which is one of the direct band gap semiconductors, is 3.7 eV at room temperature. In ZnS phosphor, copper works as an activator while chlorine acts as a co-activator. I.1.1.Band structure models of ZnS type phosphor Band structure models of ZnS phosphor may coincides with one of models given below according to activator and co-activator centers being deep or shallow [2,3]. a-Riehl-Schön-Klasens Model b-Lambe-Klick Model c-Prener-Williams Model a-Riehl-Schön-Klasens Model: The phosphors, which contain shallow co-activator and deep activator levels, match to this model. When the phosphor absorbs energy lower than the forbidden gap energy, an electron on the activator level A rises to the conducting band.

Figure 1. Riehl-Schön-Klasens Model.

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Luminescence occurs by means of recombination of the free electron in the conduction band with the electron hole in the level A shown in Figure 1. One of the examples of II-VI compounds is CdS that obeys to this model. b- Lambe-Klick Model: In this model, the co-activator levels are deep, whereas the activators’ are shallow. When the crystal absorbs an energy being approximately equal to the forbidden gap energy, the luminescence occurs by the recombination of the bound electron with the free hole by means of emission of the equivalent energy as light. For example, the luminescence of ZnTe phosphor is usually explained by this model given in Figure 2.

Figure 2. Lambe-Klick Model. c-Prener-Williams Model: According to this model, both of the activator and co-activator levels are deep. The luminescence light occurs by an annihilation of the bound electron in state D with the bound hole in state A shown as Figure 3. The most important difference of this model is that the system does not display a photoconduction due to the fact that the electron bounded to the donor-acceptor pairs during emission. The luminescence of III-V compounds such as GaAs, GaP, GaSb obeys to this model.

Figure 3. Prener-Williams Model.


According to the preparation of phosphors containing activator and co-activator concentrations and the dependence on the temperature, specimens obey to one of the models mentioned above. The determination of a convenient model for II-VI compound phosphors, which is one of the II-VI compounds,- has additional difficulty, due to the fact that they have both cubic and hexagonal phases that exist in the specimen at the same time. Due to these reasons, the luminescence of ZnS phosphor giving green light E.L. can be explained by donor-acceptor bound pair model. I.1.2.Crystal lattice defects The fundamental reason of luminescence phenomenon in the condensed matter is based on the lattice defects. As is known that defects increase the entropy of the crystal and they are classified in three groups. 1-Temporary defects 2-Elemanter excitations 3-Static defects 1-Temporary defects: This kind of defects temporarily occurs due to photon, neutron or electrons coming to crystal from outside. 2-Elemanter excitations: The conducting electrons and holes, the elementary excitations such as phonon, magnon, exciton, polaron, bogolon and plasmon etc. increase the entropy of the crystal, so they can be considered as a defect. 3-Static defects: a-Schottky defects: When an atom or ion at the lattice point of the crystal leave their position, a defect occurs due to the hole formation. For example, as shown in Figure 4, when S- - or Zn++ ions leave their place at the lattice in ZnS phosphor, the defect is formed.

Figure 4. An illustration of Schottky defects.

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b-Frenkel defects: This type of defects is obtained when the same type or a foreign atom is placed among the lattice points interstitially (Figure 5).

Figure 5. An illustration of Frenkel defects. c-The bound electron-the bound hole: Presence of extra electrons or holes in crystal than the natural composition of the solid probably forms defects at lattice. d-Dislocations: During the preparation process of the crystal, due to the some factors such as sudden cooling or some other factors, the translation vectors of the same part of the crystal would change, so dislocation type of defects occurs. In addition to the preparation procedure, this defect is produced by introducing foreign materials called as activator. In order to produce luminescence, the rare amount of convenient materials added to the crystal is called as activator. In order to balance the charge distribution modified by activators, another atoms called as co-activator are added to the crystal. For example, in this study, ZnS phosphor having a non-completed transition from cubic to hexagonal phase has been activated by a two valued Cu-salt, so the dislocation defects have been created. Hence, Cu-activators and Cl co-activator have been provided to the luminescent phosphor. I.1.3.Elements acting as activators in ZnS phosphor There are two main groups of activators for ZnS phosphor. 1-Characteristic type of activators 2-Fundamental type of activators 1-Characteristic type of activators: The rare earth elements such as Tl, Mn, Fe are in this group. For the luminescence of characteristic type of activators, the electronic transition occurs between levels of activator atoms.


2-Fundamental type of activators: One valued elements such as copper, silver, gold and indium are in this group. According to Kröger et al. [4], the activator ions of Cu+, Ag+, Au+, Li+ can take the place of zinc atoms by only making charge compensation. If a one valued positive ion is substituted into the two valued zinc atom (Zn+2), the deficiency of the positive charge can be compensated by means of the ways as follows [4,5]. a) In order to understand the role of the halogenour ions such as Cl-, Br-, which are an equivalent amount of activators, they take the place of S-2 ions (Figure 6). Hence the compensation occurs by using the lowest energy.

Figure 6. Fundamental type of activators. b) Three valued cation such as Al+3, Ga+3 take the place of Zn+2 ions (Figure 7).

Figure 7. Defects created by cation by means of aluminum. c) The anion hole is created by removing the sulphur atoms (Figure 8).

Figure 8. Defects created by anions by means of sulphur.

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d) The energy band interaction, which occurs due to the periodic structure of the main crystal lattice, is not observed when foreign atoms are added to the crystal (Figure 9). In other words, foreign atoms place into discrete energy levels of the forbidden gap. Both the depths of activator and co-activators have been calculated by Fischer and determined that while Cu-centers are 1.2 eV above valance band, co-activators locate at between 0.1-0.6 eV below conduction band [6].

Figure 9. Defects created by copper atoms.

I.1.4. E.L. Mechanism of ZnS phosphor The probability of impact ionization of electrons in luminescence centers of the crystal lattice and the electrons at the conduction band speeding in the electric field, which are initially in the thermal equilibrium, is the most convenient mechanism explaining the electroluminescence of ZnS phosphor. The schematic representation of the related mechanism is displayed in Figure 10. Free electrons make a recombination with the hole in the activator center at the end of its living time, so the green luminescence light, which coincides to this energy, is observed. For further understanding of the E.L. mechanism of powder ZnS phosphor, a thin film is to be considered. For this reason, the ZnS phosphor is placed into electrodes with the dielectric adhesive. Hence some microscopic observations have been done. In the situation of the growth of ZnS monocrystals in the vapor phase, some adjacent cubic and hexagonal regions perpendicular to the c-axis occur due to the increasing temperature which is close to the transition temperature of zinc blend to Wurtzit phases of ZnS (Figure 11). The E.L. light of ZnS crystal prepared as explained above has only been observed as light lines by means of comets that have the net direction in the electric field applied [7-10].


Figure 10. The schematic representation of the “speeding-impact ionization” E.L. mechanism. 1) Electrons in co-activator centers of the crystal and one of electrons in the conduction band at the thermal equilibrium. 2) The production of the hot electrons in the conduction band by the high electric field applied to the crystal. 3) The ionization of Cu-centers by impact ionization. 4) The occurrence of two slow electrons and a hole.

Figure 11. The band schema proposed for ZnS crystal having both cubic and hexagonal phases.

According to Haupt and Nelhowski, the light lines observed occur due to conducting Cu or CuxS (decoration) which place into sliding lines of crystal (1≤ x ≤ 2). The potential barrier, which has been explained by Merz, occurs due to the ZnS crystal structure that is shown in Figure 12. Cubic and hexagonal structures have both same tetrahedral groups as shown in Figure 12. For a transition from one structure to another during the

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Figure 12. The crystallographic structure of cubic and hexagonal ZnS phosphor.

preparation of ZnS phosphor, the rotation of 60o with respect to the [111] direction of tetrahedral at cubic structure or c-axis at hexagonal structure is suffice. Due to the symmetry, rotations at the clockwise or anti-clockwise are identical. Comets illustrated in Figure 13, have been observed on ZnS single crystal via the oil immersion microscopy by Fischer [11]. Gillson and Dernell took photographs of light lines [12]. Due to the high refractive index of ZnS crystal and both of the refractive indexes of the dielectric adhesive and the conducting glass electrode, the microscopic observations on the E.L light of ZnS phosphor have some uncertainties and difficulties. As is shown in Figure 14, according to microscopic observations, a model is relevant for E.L. of the ZnS phosphor. By means of the preparation method of ZnS phosphor, the electric field applied to the phosphor becomes a few hundred times intensive than its real value at the conducting Cu or CuxS needles that form in the semi-insulator ZnS. On the other hand, by means of the hot electrons produced in the electric field and the mechanism of impact ionization, the E.L. of the powder ZnS phosphor is schematically explained in Figure 14.


Figure 13. The comet observed on conducting lines decorated by CuxS in ZnS:Cu,Cl phosphor.

Figure 14. The schematic representation of an idealised ZnS grain. The dispersion of force lines a)Before the application of electric field b) After the application of electric field [13].

The idea used in explaining the E.L. of ZnS phosphor, in which the electric field concentrates on some specific points of crystal, had been proposed by Piper and Williams for the first time in 1955 [14]. In addition to the microscopic observations, Levialdi [15] and Ince [16] have been observed that the intensified magnetic field (~105 Oerstet), which is perpendicular to the electric field applied to the E.L. sandwich, does not reduce the brightness of E.L.. Hence, the idea mentioned above that the electric field applied concentrates on some particular points of the E.L. specimen, has been accepted.

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I.2.Experimental study I.2.1.Preparation of copper and chlorine doped ZnS powder phosphor (ZnS:Cu,Cl) The pure ZnS crystal has Cu activators and Cl co-activators when it is activated by the process summarized below [17,18].

100 g ZnS + 5 g NaCl + 5 g MgF2 The pure materials, which have four nine purity, have been mixed in a quartz crucible and then the 10 cc activator solution has been added. After the mixture has been dried, it has been annealed in a furnace for half an hour at 1000 oC and then it has been cooled suddenly. Hence, 1 g ZnS containing 10-4 g Cu, which is called as phosphor giving green electroluminescence (E.L.), has been obtained according to the recipe mentioned above. I.2.2.Preparation of Destriau type of E.L. specimen A Destriau type of E.L. specimen has two electrodes. While one of the electrodes is metal, the other is transparent and conductor. ZnS:Cu,Cl phosphor has been mixed with dinitroseluloz adhesive (collodium) having the

Figure 15. A Destriau type of E.L. specimen a. Aluminum coated glass electrode. b. Aluminum metallic layer. c & e. The thin colodium (dinitroseluloz) film d. The ZnS: Cu, Cl layer dissolved in collodium f. Transparent and conducting glass electrode coated by SnO2.


refractive index of 1.5 and dielectric constants of 7 (for 50 Hz) and 6 (for 1 MHz) [19]. Afterwards, it has been placed in between the two electrodes as illustrated in Figure 15. Hence, the various E.L. specimens excited by sinusoidal or square pulse potentials, which gives the green light at different potentials in the range of 50-1000V, have been prepared. Moreover, in order to investigate E.L. brightness by the adjustable potential applied, the durable specimens, which emit the green light at different potentials varying from 100V to 1000V, have been obtained. I.2.3. Preparation of the conducting and transparent glass electrodes In order to observe the luminescence light of Destriau type of E.L. specimen, one of the electrodes has been chosen as transparent and conductor. Due to this reason, the conducting SnO2 film, whose refractive index approximately equals to ordinary glass’ refractive index, has been coated on the glass electrode as explained below [20]. 10 cc SnCl4 and 90 cc isopropyl alcohol have been sprayed by a capillary apparatus specially prepared for this purpose to glass electrodes gradually heated at 600oC shown in Figure 16. Hence, the electrodes, which posses the resistance of the order of kΩ, have been obtained. Furthermore, an additionally process has been applied by spraying solution together with oxygen flow provided by an oxygen bomb. So, at the end of the twice applications, transparent and conducting electrodes of 50-200 Ω⁄cm2, have been prepared.

Figure 16. The hand-made capillary apparatus for transparent conducting electrode production.

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I.2.4. Observation method of the E.L. light E.L. sandwiches prepared give the green light that can be observed by naked eye in a dim medium. For quantitative analysis, E.L. light has been detected by EM 6097 S photomultiplier (P.M.) tube being sensitive to the wave length of 500 nm. Due to the E.L. light, a potential variation has been induced at the anode of the P.M. tube working at 1.5 kV. Subsequently, the potential variation has been taken from anode on the charge resistance of RL= 200 kΩ at the exterior circuit as a negative pulse or eleventh dynode as a positive pulse. The related P.M. tube and the block diagram of the circuit are given in Figure 17.

Figure 17. P.M. tube potential divider circuit with EMI 6097 S Cesiumantimony cathode.

I.2.5. Circuits and devices used in experiments in order to observe E.L.brightness waves The E.L. specimens have been excited by the Heathkit audio oscillator that the output of the oscillator has been intensified to 800V by a special transformer (Figure 18). In order to protect from various magnetic fields, the P.M. tube has been put into a container covered by μ-metal. Afterwards, the container has been placed in a special box made of copper with the E.L. sample in order to eliminate the undesirable light contribution. The regulator of the high voltage source, which requires for E.L. tube, has been prepared in a sophisticated design of the circuit proposed by Seely [21].


Figure 18. The electronic circuit utilized for the excitation for E.L. brightness waves by sinusoidal potential adjustable between 50-800V and the circuit with the P.M tube for its detection.

Figure 19. E.L. brightness waves (above). E.L. light pulse is about 2V. The potential applied to E.L. specimen (below), 400V, f=400Hz. In the photographs, taken from double trace and double beam Tektronix 546 oscilloscope, the shape of the E.L. light pulse of the sample (brightness waves) excited by alternative potential is shown (Figure 19). Every one period of the potential applied corresponds to two maximum of E.L. light which is one of the most fundamental property of E.L. sample. I.2.6. Measuring circuit In order to observe the retardation of the E.L. light, the electronic switching circuit, which opens and closes the potential applied to the E.L. sample, has been set up (Figure 20). In order to obtain the potential generated by the audio oscillator that requires for the divider circuits of the electronic switching, the circuit I, which transforms the sinusoidal pulse into square pulse, has been setup. The schematic representation of the circuits I,II, III and IV is given in Figure 20.

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Figure 20. The schematic representation of the circuit, which is used for opening and closing the alternative potential applied to the E.L. specimen for some specific time periods.

Figure 21. The schematic representation of the electronic switching circuit.

The integrated circuits (TL 7490) in Figure 21, divide the frequency applied into 20 times in total. (The detailed information is in the Telefunken integrated circuit catalogue 1973-74).


While the first mono stable multi-vibrator retarder (TL 74122) controls the beginning of the applied alternative potential, the second retarder varies the duration of the pulse in Figure 21. By means of the switching circuit, the potential applied to E.L. specimen, which increases up to 800V, has been switched in milliseconds of periods via retarders. Moreover, the expected delay of the switching, which occurs due to the inductive charge in the circuit, has been eliminated by Triac connected to the circuit in Figure 20. Photographs of the shape of the E.L. light pulses taken from oscilloscope via circuit illustrated in Figure 20 have been given in Figure 22. A weak E.L. light pulse occurs with a specific delay as soon as the alternative potential is applied to the E.L. specimen. At the end of the

Figure 22. The retardation of the E.L. light and the alternative and constant components of the E.L. brightness waves at the upper part of the photograph. The lower part corresponds to the applied potential of 200V/cm. The E.L. light: 1V/cm, f= 1.25kHz, time:1msec/cm.

Figure 23. Calculation of the retardation of the phase of E.L. light pulse. Voltage= 200V/cm, The E.L. light: 1V/cm, f =1.1kHz, time=0.5msec/cm.

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Figure 24. The saturation of the E.L. light pulse. Voltage= 200V/cm, The E.L. light: 1V/cm, f = 1kHz, Time=2msec/cm. 7-8 periods of the potential, the specimen gives the light that corresponds to the equilibrium state. The delay of the E.L. light has been calculated by Figure 23. After the application of potential, the E.L. light pulse has been observed within 150μs. By means of retarders, the circuit has been closed for about 16 ms, hence the saturation of E.L. light has been observed in Figure 24.

Figure 25. The variation of brightness waves with applied voltage. Voltage= 200V/cm, The E.L. light:1V/cm, f = 1kHz, time=2.5msec/cm.

Figure 26. The variation of brightness waves with applied voltage Voltage= 200V/cm, The E.L. light:1V/cm, f = 1 kHz, time=2.5msec/cm.


Figure 27. The variation of brightness waves with applied voltage f = 1kHz. Voltage = 200V, The E.L. light:1V/cm, time=2.5msec/cm.

In order to determine the alternative potential dependence of the E.L. light pulse, three different voltages have been applied to the E.L. specimen to get an opinion about the variation of the specimen light with the different potentials. Hence, photographs, which show the shapes of pulse with respect to various potential values, are given in Figures 25, 26 and 27. I.2.7. The variation of the alternative component of the brightness waves, Bω with the potential applied In this section, the relation between the alternative component, Bω of the shape of the E.L. light pulse and the potential applied has been investigated. In order to observe the relation, the output of the P.M. tube in Figure 17 has been connected to the oscilloscope by means of circuit given in Figure 28.

Figure 28. The investigation of the variation of the alternative component of the brightness waves by the potential applied.

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Table 1. The variation of the brightness waves of the E.L. sample with the potential applied at the constant frequency of 1 kHz.

V(V) (V-1/2)

450 2.5 205 0.39

500 5 193 0.69

550 13 185 1.11

600 29 175 1.46

650 56 167 1.75

700 98 163 1.99

725 120 159 2.08

750 140 156 2.14

The alternative potential on the eleventh dynode of the P.M. tube has been connected by diode, so the data have been read by the double trace and double beam oscilloscope. In order to keep all of the system as well as the A output of the diode to be kept in the same level with respect to the ground, the resistance of 100kΩ has been added to the circuit. Another resistance of 100kΩ has been linked to the circuit to prevent the charge accumulation of the capacitance. The related curves obtained from Table 1 are shown in Figure 29.

(a) (b) Figure 29. (a) Brightness waves (b) The logarithmic values of the brightness waves of E.L. sample versus potential curve at the constant frequency of 1 kHz.


I.2.8. The variation of the alternative component of the brightness waves, Bω with the frequency applied The variation of the alternative component, Bω of the E.L. pulse shape with the frequency has been given in Table 2. Table 2. The variation of the brightness waves of the E.L. sample with the frequency applied at the constant potential of 600 volt.

200 70

600 30

1000 24

1400 23

1800 21

2200 20

2600 20

Figure 30. The brightness waves versus frequency curve of green light emitting E.L. sandwiches.

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The curve obtained from data in Table 2 is seen in Figure 30. The curve obeys the equation following as y =13414.4112 + (19.06229 − 13414.4112) × x 1.44867⁄ (4.245991.44687− x1.44687). I.2.9. Electroluminescence of the Destriau type of E.L. samples under squared wave excitations In order to calculate the build up and the decay time of E.L. brightness waves of green light emitting specimens, the electronic circuit shown in Figure 31 has been setup. In order to change the width of the square pulses applied to the E.L. specimen up to 180 μs, the necessary alteration has been done at the exterior circuit of the retarders. In the Figure 32, the B1 light peak which occurs when the square potential pulse is applied to the E.L. sample and the B2 light peak appears when the potential is switched off. The raising time (build-up time) of the E.L. light pulse has been determined as 6msec by B1 light peak.

Figure 31. The electronic circuit block diagram used for the observation of the response of the E.L. samples to the square potential pulses.


Figure 32. The shape of the E.L. light pulse of E.L. specimen excited by square pulse. The pulse duration = 80msec, Time=20msec/cm, Potential=100V/cm, The E.L. light=2V/cm.

I.2.10.Variation of the shape of E.L. light output with the pulse duration applied It has been determined that when the square pulses have been applied to the same E.L. specimen in different time periods, the shape and the intensity of the E.L. light pulses changed as shown in Figure 33.

Figure 33. The shape of E.L. light output during the excitation of the E.L. specimen by 50 msec pulse duration. Time=20msec/cm, Potential=100V/cm, The E.L. light=2V/cm.

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Figure 34. The shape of E.L. light output during the excitation of the E.L. specimen by 12msec pulse duration. Time=4msec/cm, Potential=100V/cm, The E.L. light=2V/cm.

When the pulse duration decreases, two light peaks of E.L. light become one Peak, so that it appears as if the E.L. brightness reduces (Figure 34). I.2.11. Determination of the variation of first brightness waves with the potential applied By means of circuit in Figure 31, the variation of the first light peak with square potential pulse is listed in Table 3. Graphics obtained from data in Table 3 are illustared in Figure 35. The function obtained from Figure 37 which gives the dependence of the brightness to the voltage has been determined as B1~eV. Table 3. The variation of the first light peak of the E.L. samples with square potential pulse.

V(V) B1(V) logB1(V)

150 0.12 -0.92

200 0.48 -0.32

250 1.2 0.08

300 2.8 0.45

350 5.8 0.76


(a) (b) Figure 35. (a) The first light peak (b) the logarithmic values of the first peak versus the square potential pulse curve.

I.2.12.The decay time calculation of the electroluminescence As is seen in the subsection I.2.10, when the appropriate square pulse duration is applied to the E.L specimen, the decay time of the E.L light can be observed exactly. Due to this reason, when the pulse duration applied to the E.L. specimen equals to 80msec, the decay time of B1 of the E.L. light maximum has been determined as 23msec from Figure 36. As is seen from Figure 37, the time required for the emission of all of the centers that give luminescence is obtained in the order of milliseconds.

Figure 36. The determination of E.L. decay time with 80 msec square pulse excitation. V=100V, The E.L. light = 1V/cm, Time: 10msec/cm.

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Figure 37. The decay time calculation of the electroluminescence centers. Moreover, as is seen from the same curve, the decay time of the first light peak obeys the exponential relation as B = constant .exp (− t⁄τ) where τ is the decay time and t is time. I.2.13. Results and discussion 1-The most known property of the ZnS phosphor giving green E.L. light, which had first been observed in 1936, possesses an E.L. light maximum at every half period of sinusoidal excitation potential as shown in Figure 38.

Figure 38. The light maximums of E.L. so called in-phase and out of-phase.


When the positive potential is applied to the sample, the light maximum called as in phase occurs. The second light maximum observed which refers to the second alternate is named as out of phase. This property observed and shown in Figure 19 which is the result of the impact ionization that has been explained by the intensification of the electric field on the stacking faults in ZnS phosphor. At the first half period of the potential applied to the Destriau type of E.L. specimen, the electric field causes ionization of copper-activator centers. Some electrons, which reach to the conducting band, are trapped at the electrons traps. Due to the thermal equilibrium, the untrapped electrons, which come together with electron holes, cause the light maximum called as in phase brightness wave. When the direction of the field changes, the trapped electrons make recombination with ionized activators. Hence, the light emission that corresponds to the second light maximum called as out of phase brightness wave. 2-The building up procedure of the electroluminescence: Due to the fact that ZnS:Cu,Cl phosphor being dispersed in the dielectric media, after the application of potential, the E.L. light is observed with a retardation. The retardation for the Destraiu type E.L. specimen shown in Figure 23 has been determined as 150μs. The time duration required for the formation of the polarization field in the E.L. specimens had been observed as 20μs by Marenkov-Bonch [22], 700μs by Orlov [23], 7μs by Johnson [24] and 100μs by Thornton [25]. One of the most known properties of E.L. [26] is that the most appropriate observation of the E.L. light can be achieved after 5-10 periods of the potential applied. This event is called as the building up procedure of electroluminescence. All center of a Destriau type of E.L. specimen having phosphor being heterogeneously dispersed in the dielectric media, which give the luminescence, are not excited at the same time. Hence, E.L. light reaches to the saturation value after a while. As is seen in Figure 22, the intensification of every two maximums at the beginning periods is small in the frequency value at 1.25kHz and the light reaches to the saturation after eighth periods. Moreover, it has been determined from the Figure 22 that the constant component of the E.L. increases (Figure 39). The effect of frequency on the brightness waves has been studied in this thesis and it has been realized that the results obtained has the same

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Figure 39. The brightness waves of the E.L. light (Bw=The alternative component of E.L., Bc=The constant component of E.L.).

relationship that had been investigated by Destriau [26], Haake [27], Thornton[28], Zalm [29], Tanak[30] and other researchers for the first time. The constant (Bc) and alternative (Bw) components of the E.L. light had been observed for the first time by Destriau [31] and it has been determined that while the constant component increases with the increasing frequency of the alternative field applied, the alternative component decreases. 3-It has been determined that the alternative component of the E.L. specimen excited by the sinusoidal potential, Bw exponentially varies with the potential applied as following,

This result is the characteristic behavior for Destriau type of E.L. specimen which contains various kind of light emission centers and given the relation below [32-35].

where V0 and B0 appear due to the integration constants. By taking integral of the formula given above along the all emission centers points of specimen, the brightness relation has been found as,


4-As is seen in Figure 38, two light maximums have been obtained for both of the cases at while the sinusoidal potential is applied and removed. The raising time of the first light peak has been calculated as 6msec. This time is equal to the saturation time of the E.L. light when the specimen is excited by sinusoidal potential. One of the important results is that in order to determine the real shape of the brightness waves and the decay time of E.L., excitation of the specimen with the particular duration of pulses are crucial. When the pulse duration time decreases, the trapped carriers speed up in the opposite direction without making recombination with all of the luminescence centers, so the second light maximum occurs. In this work, it has been determined that the most convenient exciting pulse duration is 80msec. Previously, some researchers had observed the luminescence decay time as in order of microseconds when the specimen was excited by relatively short time pulses [36,37]. On the other hand, the luminescence decaying of ZnS phosphor with long persistence had been determined in the order of milliseconds by Haake [38] and Zallen [39] and co-workers. The dependence of the brightness waves on the potential applied has been explained in the subsection I.3 [40]. 5- The variation of the first light maximum (B1) with the potential applied has been investigated and B1 is proportional to exponential function of potential (B1~eV). This result obeys the relation given below

which is valid for the square pulse potential excitations. 6- Consequently, in order to calculate the decay time of the B1 light maximum and to find the corresponding law, lnB1= f(t) curve has been obtained (Figure 37). Hence, the curve has been determined that the law is exponential.

This result obeys to the calculation made by Antonov and Romanovskii [41] which has been derived via the initial dispersion of the free carriers. According to the calculation, the decay of initial concentration of centers, n(0) possessing localize carriers, obeys the law given below

Ülker Onbaşlı 30

From the equations given above, the final equation has been obtained as follows

where B(t) is the intensification of the luminescence, n(t) is the average concentration of the one type carrier (two carriers have the same concentration), t is time and τ is the decay time of the luminescence. References 1. F.Morehead, J.Phys. Chem.Solids, 245, 37, 1963. 2. S.Akpınar, Selected Topics in Solid Sate Physics, Vol2, p35, 1971. 3. H.P.Kallman, Org.and Inorg. Materials, p418, 1962. 4. a) F.A. Kröger, J.Dikhoff, Physica 16, 297-316, 195.0 b) F.A. Kröger, J.E. Hellingmann, Physica 15, 990-1018, 1949. 5. H.P.Kallman, G.M. Spruch, Lum. of Org.and Inorg. Materials, p344, 1962. 6. P. Goldberg, A.G. Fischer, Lum.of Inorg. Solids, p554, 1966. 7. S.Akpınar, Selected Topics in Solid State Physics, Vol4, p46, 1973. 8. P.Zalm, Phil. Res. Repts, 11, 417, 1956. 9. G.Diemer, Phil. Res. Repts, 10, 194, 1955. 10. L.Strock, Acta.Cryst, 10, 840, 1957. 11. a)A.Fischer, J.Elect.Soc. 109,1043, 1962. b) A.Fischer, Luminescence of Inorg. Sol., (Ed.P.Goldberg), New York, 1966. 12. J.L. Gillson, J.Darnell, Phys. Rev.125, 149, 1966. 13. R.Thornton, The Physics of E.L. Devices, p120, 1967. 14. W.Piper, F.Williams, Brit.J.Appl.Phys. Supp, 4, 39, 1955. 15. A. Levialdi, E.Guercigli, Comp. Rend., 257, 852, 1963. 16. N.İnce, Proc.Phys. soc., (London), B67, 870, 1954. 17. S.Akpınar, Selected Topics in Solid State Physics, Vol3, issue6, 1971. 18. W.Espe, Werkstoffkunde der Hochvakuumtechnik, 3, 44, 1961. 19. W.J. Roff, J.R. Scott, Fibres, Films, Plastics and Rubbers Handbook of Common

Polymers, pp 161-162, 1971.


20. L.Holland, High Vacuum Deposition of Thin Films, 1956. 21. S.Seely, Electron Tube Circuits, Mc.Graw Hill, p244, 1958. 22. M.Bonch, S.Marenkov, Optics Spect., 8, 449, 1960. 23. I.Orlov, Inst. Min.Tadiotech. Prom., 9, 3, 1957. 24. E.Johnson, J.Elect. Soc. 108, No9, 852, 1961. 25. R.Thornton, The Physics of E.L. Devices, p125, 1967. 26. G. Destriau, Brit. J. Appl. Phys.Supp., 4, 49, 1955. 27. W. Haake, J.Appl.Phys., 28, 245, 1957. 28. R.Thornton, The Physics of E.L. Devices, III, 1967. 29. P.Zalm, Phil.Res.Repts., II, 353, 417, 1956. 30. S.Tanaka, J.Phys.Soc., Japan, 14, 1123, 1959. 31. G. Destriau, F.Ivey, IRE, 43, (12), 1911, 1955. 32. S.Akpınar, Selected Topics in Solid State Physics, vol.4, p.48, 1973. 33. R.Thornton, The Physics of E.L. Devices, p128, 1967. 34. G.Fischer, J.Elect. Soc., 110, 734, 1963. 35. Yu. P. Chukova, Elect. and Opt. Properties of Semicond., Ed.D.V. Skobel’tsyn,

New York, p160, 1968. 36. R. Briggs, Bull. Ann. Phys. Soc., II, 2, 155, 1957. 37. A.Rajchman, R.Briggs, Proc. I.R.E., 46, 1808, 1958. 38. H.Haake, J.Appl.Phys., 28, 245, 1957. 39. R.Zallen, W.Eriksen, J.Elect. Soc., 107, 9, 288, 1960. 40. Destriau’s Electrolumınescence Of Zns:Cu,Cl Phosphors, Ülker Onbaşlı (Dalay),

2th Congress of Turkish Association of Physics, 1978. 41. V.Antonov-Ramonovskii, Journal of Luminescence, 3, 456-466, 1971.

Ülker Onbaşlı 32

I.3. Publication related to the master of science thesis 2th Congress of Turkish Association of Physics, 1978 I.3.1. Destriau’s1 electroluminescence of ZnS:Cu, Cl phosphors

Ülker Onbaşlı

University of İstanbul, Experimental Physics Dept.

Electroluminescence (E.L) brightness waves of green light emitting copper activated Zn phosphors (ZnS:Cu,Cl), have been investigated by both the excitation of sinusoidal and square potential pulses and some specific values of build up and decay time calculations have been made to give 6ms and 23ms, respectively. Moreover, the phase retardation of E.L. has been determined to be equal to 150μs for the specimens prepared. It has been determined that E.L. brightness waves (B) of the samples under the sinusoidal pulse and the squared wave excitations obey to the relations of and respectively 2,3.

Moreover, the decay time of E.L. has been determined to be in the form of that is in accord with the theoretical result obtained by Antonov and Romanovskii from the initial distribution of the free carriers4. The decay time calculations of Destriau’s type of E.L. have been made by several researchers that lead to different results varying from microseconds to milliseconds for the samples that have been doped with various dopants5. The phosphor that contains 10-4g Cu/1g ZnS have been prepared in our laboratories. The most suitable pulse duration of E.L. samples has been determined to be equal to 80 ms. 1G. Destriau, J.Chem. Phys. 33, 587 (1936) 2S.Akpınar, Selected Topics in Solid State Physics, c.4, 48 (1973) 3U.Onbaşlı (Dalay), Master of Science Thesis, Destriau’s Type of Electroluminescence of ZnS: Cu,Cl Phosphorus (1974) 4V.Antonov-Romanovskii, Journal of Luminscence 3, 459-466 (1971) 5P.R.Thorton, The Physics of Electroluminescence Devices p.125 (1967)


Destriau’s type electroluminescence of copper activated powder ZnS The possibility of transforming of electric energy into light energy by applying electric field to solid matters has been first observed in SiC crystal by Lossew in 1924. In this study, the ZnS powder crystals investigated has been dissolved in a liquid dielectric media that gives luminescence by a strong alternative field which has been first observed in 1936 by Destriau, so this event is called as Destriau’s type of electroluminescence. In order to prepare the E.L. sandwich giving such type electroluminescence, at first pure ZnS powder has been doped with 10-4g Cu-activator in 1g of the sample with a special process. Then, the obtained Zn phosphors have been mixed with the insulating dinitroseluloz adhesive (collodium) and it has been placed in between two electrodes, conveniently. While one of the electrodes is metal, the other is transparent and conductor. Hence, the E.L. samples have been obtained by the procedures mentioned is shown in Figure 15 in section I.2.2. The E.L. sandwiches prepared have been excited by adjustable sinusoidal potential between 50-800V by means of the circuit illustrated in Figure 18 in section I.2.5. Hence, E.L. brightness waves have been obtained and the related photographs have been taken from double beam and double traces oscilloscope (Figure 19 in section I.2.5). As is seen in the picture, in response to every half cycle of the exciting sinusoidal potential, one light peak occurs. In order to observe the delay in the beginning of the event, the electronic switching circuit has been set up (Figure 20 in section I.2.6), to supply the necessary potential from the square pulse generator from the same audio oscillator, the electronic switching circuit has been established (Figure 21 in section I.2.6). By using frequency dividers, the frequency applied to the E.L. specimen has been divided 20 times in total. While the first retarder controls the applied sinusoidal potential in the beginning, the second retarder changes the duration of the pulse applied to the E.L. specimen. The photographs taken by means of this circuit are given in Figures 22, 23, 25 and 27 in section I.2.6. In Figure 22 in section I.2.6, both alternative and linear components of E.L. brightness waves are seen together. The delay of E.L. brightness has been found as 150 microseconds for sandwiches prepared (Figure 23 in section I.2.6). Hence, its variation by the amplitude of the applied potential has been determined.

Ülker Onbaşlı 34

In order to exclude the linear component of the brightness waves and to investigate the response of the alternative component of the E.L. brightness waves versus the applied potential, the electronic circuit giving in the Figure 28 in section I.2.7. has been set up. Hence the relation,

has been obtained from Figure 29 in section I.2.7. Moreover, E.L. samples prepared have been excited by square potential by means of the circuit which is shown in Figure 31 in section I.2.9. The variation function of the brightness waves with the voltage applied has been found as . In order to determine the decay time of E.L., the exciting pulse duration required has been sought by several researchers by using different methods and various doping rates and it has been determined as varying from microseconds to milliseconds for ZnS. The most convenient excitation pulse duration for E.L. samples, which has been prepared by ZnS powder having Cu-activator in the ratio of 1/10 000 in our laboratory, has been determined as 80ms (Figures 32, 33 and 34 in sections I.2.9 and I.2.10). Besides, the raising and decaying times of E.L. have been found as 6ms and 23ms from Figures 32 and 36 in subsections I.2.9 and I.2.12, respectively. The decay of E.L. has been determined to be in the form of

, where t is time, τ is the decay time. In this study, the approximate calculation for the E.L. decay time, which had been made in theoretical way by Antonov and Romanovskii, has been determined in accord with the experiment. Conclusion and discussion The main issues related to the study are summarized as follows; I-One of the observed properties of ZnS phosphors giving green electroluminescence light has a maximum that occurs in every half cycle of the excitation potential. II- The building up procedure of electroluminescence displays the particular amount of delay. After the potential is applied, the E.L. light is observed with retardation and reached to the saturation after a while. The retardation has been found as 150µs for samples prepared. Also, the reaching


time to the saturation of the E.L. light has been determined as 6ms (Figure 32 in section I.2.9). III- In the excitation process with square potential (as is seen Figures 33 and 34 in section I.2.10), when the potential has been applied, the amplitude of the first light peak is higher than that of the light peak appearing in the absence of the potential. IV- Ultimately, the most convenient exciting pulse time of the E.L samples mentioned has been determined to be equal to 80ms. Moreover, the decay and build up time of the E.L. samples prepared have been found as 23ms and 6ms, respectively. I.4. Concluding remarks of the section However, the E.L. of solids had been first described in the scientific literature by G.Destriau in 1936, the contemporary and prospective applications of E.L. devices have been still widely utilized in technology. The most common applications of E.L. devices are laser diodes and light emitting diodes (LEDs). As is known, LEDs devices form the basis of the solid-state lighting industry. Due to the low radiative efficiency of indirect gap material, modern commercial E.L. devices are made from direct gap compounds such as ZnS phosphor investigated in this chapter. Direct gap II-VI compounds should make good LEDs for the blue/green/yellow spectral region. Semiconductor lasers are another application of the E.L. processes, usually very much the same as in corresponding LEDs that have been industrially produced over the last four decades. The overall consumption per year has been about 50 LEDs per human6. Due to the fact that electroluminescent devices are indispensable for daily life from building lightening, display panel of television, radars or safety lamps to automotive instrument panel backlighting, it is concluded that the electroluminescence of the II-VI compounds has been and still is an active and promising area for scientific and technological studies.

6 [a]Electroluminescence I, Volume editor Gerd Mueller, 2000, Academic Press, USA [b]Optical Properties of Solids, Mark Fox, Second Ed., 2010, Oxford University Press Inc., NewYork

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