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AC electroluminescence of ZnS: Cu, Cl, Mn thin films in the structure In2O3(Sn)-ZnS: Cu, Cl, Mn-SiOx-Al
Journal of Luminescence 21(1980)129—135 © North-Holland Publishing Company
AC ELECTROLUMINESCENCE OF ZnS: Cu, Cl, Mn THIN FILMS IN THE STRUCTURE 1n203(Sn) ZnS : Cu, Cl, Mn SiO, Al —
—
—
Z. PORADA Institute of Electrical Engineering and Electronics, Technical University, Cnjcow, Poland
and E. SCHABOWSKA Department of Solid State Physics, Institute ofMetallurgy, Academy ofMiningand Metallurgy, 0-acow, Poland Received 7 September 1979
AC electroluminescence of ZnS: Cu, Cl, Mn thin films in the structure 1n203(Sn) — ZnS: Cu, Cl, Mn — SiO~— Al was studied. Vacuum-evaporated films 0.5 to 2.0 ~m thick, excited with sinusoidal voltage of 80—200 V and up to 2 kHz gave the luminance response fulfilling Aifrey—Taylor’s relation. Thus the electroluminescence model, suggested by these authors for a ZnS monocrystal, can be applied also for ZnS thin films.
1. Introduction Since Thornton [1] studied electroluminescence in evaporated thin films of ZnS: Cu, Cl, Mn, many reports in this field have been published. Most of them describe the basic preparation technique and the properties of electroluminescent layers, but they do not answer a great number of unsolved problems [2—8].Furthermore, the results obtained by various authors are not consistent and do not allow to assume any adequate model of ZnS thin films electroluminescence. This creates the necessity for further investigation of this phenomenon. This paper presents a study on electroluminescence of ZnS: Cu, Cl, Mn thin films under excitation with ac voltage. The results are in good agreement with the model suggested by Alfrey and Taylor [9]. This model assumes the emission from luminescent centers excited by high-energy electrons in the strong electric field of the metal—semiconductor junction. 129
130 2.
Z Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
Theoretical considerations
Although a uniform model of the mechanism of electroluminescence has not been yet worked out, attempts have been made to explain the phenomena occurring in the luminescent cell. According to Piper and Williams [10], the process of excitation of the luminescent centers is as follows: Electrons released by the electric field from the deep donor levels are supplied to the conduction band and are accelerated in the region of the potential barrier of the electron-depleted layer. This latter is formed in the close vicinity of the negative electrode. Those electrons which have gained a sufficient amount of energy, may cause ionization of the centers resulting from the impacts and subsequently yield radiative or non-radiative recombination of the electron with the ionized center. If sinusoidal voltage is applied to the electrodes, then the space of electron depletion will contact and extend cyclically, in compliance with the periodic changes of the voltage applied (fig. 1). Thus, the potential barrier near each electrode will rise and fall accordingly during each cycle. As soon as this barrier begins to rise, all electrons which are thermally excited in the conduction band will be accelerated by the field in the barrier and will cause luminescence. According to Aifrey and Taylor [9], after time t has elapsed, the number of
c ectr
~
I eLee*ro~e—
A°
Fig. 1. Potential distribution at various times during a cycle of ac sinusoidal voltage [10].
Z Porada, E. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
131
electrons originating from the traps, whose depth is e, will be ~y exp(—yt~,
(1) 1), where ~y= s exp(—e/kT), (s is the Alfrey and Taylor constant equal to 5 X 106 s i~is a function of the photon production yield equal to 77
~
(2)
exp(—a/E~),
where a = constant for given material, E~ critical value of the intensity of electrical field causing ionization of the center. As those electrons are affected by the field proportional to .,,/U 0 sin ~t, an instantaneous value of the light flux may be described by B~~-‘
‘y exp(—7t) exp [—a/(’..,/U0 sin wt)]
,
(3)
and hence the total light flux per cycle will be given by equation In
B
f
______
LI
7
exp(—’yt) exp [—a/(s./U0sin
wt)] dt.
(4)
With the assumption that y’.JU~/ac.~,> 1, this formula may be replaced by the approximation B ~ y exp(—yir/2w) exp(—a/\/U~,
(5)
that may be written in a more convenient form for further calculations, in the following way B
B0 exp(—A/f) exp(—a/~JU~),
(6)
where B0 = constant dependent on the units in which luminance B is expressed, A = parameter dependent on the technological conditions under which the samples were prepared and on the temperature,f= frequency, a = parameter dependent on the value of critical intensity of the electric field causing ionization of the luminescent center, U0 = voltage amplitude.
3. Experimental results ZnS thin films were produced by vacuum evaporation on a Corning 7059 glass substrate. Before that, a photoconducting layer of 1n203(Sn) (degenerate semiconductor) was produced 2. on the glass surface by cathode sputtering. Its resistance was about &~/cm The 100 material for evaporation was powdered ZnS with Cu, with Cl and Mn admixtures added. The contents of the admixtures were as follows: Cu = 3.2 X 10—2 g/g ZnS, Cl = 3.6 X 10_2 g/g ZnS and Mn = 2.5 X 10_2 g/g ZnS. The material produced in such a way was then evaporated under vacuum 5 X 1 ~_6 Torr from the alundum
Z. Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
132
AL
ZnS (CU,CL
/
3n,0j(Sn)
I —. Fig. 2. Thin film electroluminescence capacitor.
mB
500 200
c1~
100 .
c/s
I
~ [v_I/’] 0.07
0.08
0.09
0.10
Fig. 3. Luminance characteristics for ZnS : Cu, Cl, Mn thin films at various frequency.
Z. Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
133
crucible heated to about 1100°Con the substrate, which was placed 8 cm apart from the source and heated to 200°C.Processing time was varied from 50 to 120 mm, which resulted in the layer thicknesses varying from 0.5 to 2.0 pm. The ZnS film so obtained was then recrystallized under vacuum for 30 mm at about 3 50°C [11]. Silicon oxide SiO~film about 500 A thick was evaporated thereafter on ZnS film. Finally, the aluminium electrode was evaporated onto the SiO~film (fig. 2). The active surface of such an arrangement was 0.28 cm2. In the measurement of light emission, the photomultiplier with spectrum range of sensibility from 3500 to 6500 Awas used. The luminescent characteristics were obtained for the 0.8 pm thick film excited with sinusoidal voltages of 100, 200, 500 and 2000 Hz frequency (fig. 3). It can be seen from the plot, that the function lnB(1/~/U~) is linear, irrespective of the applied frequency. Thus the Alfrey—Taylor formula, eq. (6), is fulfilled for all voltages used. Luminance characteristics were obtained at room temperature, so the parameter A was constant for a given sample. On the basis of performed measurements, the mean values of A and a were calculated. For a 0.8 pm thick sample they equal 240 51 and 16.5 V”2, respectively.
LnB 5
0
;~
*1m5&
Fig. 4. Plot of mB vs. 1/f for 0.8 pm thick ZnS: Cu, Cl, Mn film.
134
Z. Porada, E. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
Fig. 5. Luminance intensity vs. time over a voltage cycle. Frequency 100 Hz, amplitude 180 V, experimental results, (———) theoretical values.
When exciting the sample with a sinusoidal voltage of constant amplitude, the characteristic in B(l/f) should be — according to eq. (6) — a straight line with slope independent of the applied voltage and depending only on the value ofA. The plots presented in fig. 4 indicate, that all points of measurement are situated on the straight lines, whose slopes are the same and independent of the applied voltage. The Aifrey—Taylor relation is then fulfilled. The luminance vs time for single cycle of applied voltage at 100 Hz frequency was observed for the film studied (fig. 5). The B(t) function was estimated from formula (3) for the earlier calculated parameter A and for the amplitude of applied voltage U0 = 180 V. Pulses obtained from the oscillograph and those numerically calculated were similar in shape and the maximum of luminance pulse appeared after relaxation time r 1.05 ms from the beginning of the pulse. It complies with the numerically calculated maximum value, thus confirming the validity of formula (3) applied by Alfrey and Taylor for the model describing electroluminescence.
4. Summary It follows from this study that for vacuum-evaporated ZnS: Cu, Cl, Mn thin films 0.5 to 2.0 pm thick, excited with sinusoidal alternating voltage of amplitude
Z. Porada, E. Schabowska
/ AC electroluminescence of ZnS: Cu,
Cl,
Mn
135
80—200 V at frequency up to 2 kHz, the Alfrey—Taylor relation is in good agreement with experimental results. Accordingly, the theoretical model of electroluminescence suggested by Aifrey and Taylor for ZnS monocrystals can be applied also for ZnS thin films.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W.A. Thornton, J. App!. Phys. 30(1959)123. P. Goldberg and J.W. Nickerson, J. App!. Phys. 34(1963)1601. K. Sasaki and S. Kurita, Jap. J. Appl. Phys. 7 (1968) 1039. T. Mijata, S. Nakagawa, H. Mirayama, H. Hasegawa and M. Korenaga, Jap. J. AppL Phys. 9(1970)615. W. Uchida, Jap. J. Appi. Phys. 14 (1973) 670. E. Walentynowicz, J. Luminescence 18/19 (1979) 362. W. Ruffle, V. Morreilo and A. Onton, J. Luminescence 18/19 (1979) 729. J. Benoit, P. Benailoul, R. Parrot and J. Mattler, J. Luminescence 18/19 (1979) 739. C.F. Alfrey and J.B. Taylor, But. J. AppL Phys., SuppL 4 (1955) 44. W.W. Piper and F.E. Williams, Brit. J. AppL Phys., Suppl. 4 (1955) 39. Z. Porada, Thesis, Academy of Mining and Metallurgy, Cracow (1978).
[io] [111
AC ELECTROLUMINESCENCE OF ZnS: Cu, Cl, Mn THIN FILMS IN THE STRUCTURE 1n203(Sn) ZnS : Cu, Cl, Mn SiO, Al —
—
—
Z. PORADA Institute of Electrical Engineering and Electronics, Technical University, Cnjcow, Poland
and E. SCHABOWSKA Department of Solid State Physics, Institute ofMetallurgy, Academy ofMiningand Metallurgy, 0-acow, Poland Received 7 September 1979
AC electroluminescence of ZnS: Cu, Cl, Mn thin films in the structure 1n203(Sn) — ZnS: Cu, Cl, Mn — SiO~— Al was studied. Vacuum-evaporated films 0.5 to 2.0 ~m thick, excited with sinusoidal voltage of 80—200 V and up to 2 kHz gave the luminance response fulfilling Aifrey—Taylor’s relation. Thus the electroluminescence model, suggested by these authors for a ZnS monocrystal, can be applied also for ZnS thin films.
1. Introduction Since Thornton [1] studied electroluminescence in evaporated thin films of ZnS: Cu, Cl, Mn, many reports in this field have been published. Most of them describe the basic preparation technique and the properties of electroluminescent layers, but they do not answer a great number of unsolved problems [2—8].Furthermore, the results obtained by various authors are not consistent and do not allow to assume any adequate model of ZnS thin films electroluminescence. This creates the necessity for further investigation of this phenomenon. This paper presents a study on electroluminescence of ZnS: Cu, Cl, Mn thin films under excitation with ac voltage. The results are in good agreement with the model suggested by Alfrey and Taylor [9]. This model assumes the emission from luminescent centers excited by high-energy electrons in the strong electric field of the metal—semiconductor junction. 129
130 2.
Z Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
Theoretical considerations
Although a uniform model of the mechanism of electroluminescence has not been yet worked out, attempts have been made to explain the phenomena occurring in the luminescent cell. According to Piper and Williams [10], the process of excitation of the luminescent centers is as follows: Electrons released by the electric field from the deep donor levels are supplied to the conduction band and are accelerated in the region of the potential barrier of the electron-depleted layer. This latter is formed in the close vicinity of the negative electrode. Those electrons which have gained a sufficient amount of energy, may cause ionization of the centers resulting from the impacts and subsequently yield radiative or non-radiative recombination of the electron with the ionized center. If sinusoidal voltage is applied to the electrodes, then the space of electron depletion will contact and extend cyclically, in compliance with the periodic changes of the voltage applied (fig. 1). Thus, the potential barrier near each electrode will rise and fall accordingly during each cycle. As soon as this barrier begins to rise, all electrons which are thermally excited in the conduction band will be accelerated by the field in the barrier and will cause luminescence. According to Aifrey and Taylor [9], after time t has elapsed, the number of
c ectr
~
I eLee*ro~e—
A°
Fig. 1. Potential distribution at various times during a cycle of ac sinusoidal voltage [10].
Z Porada, E. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
131
electrons originating from the traps, whose depth is e, will be ~y exp(—yt~,
(1) 1), where ~y= s exp(—e/kT), (s is the Alfrey and Taylor constant equal to 5 X 106 s i~is a function of the photon production yield equal to 77
~
(2)
exp(—a/E~),
where a = constant for given material, E~ critical value of the intensity of electrical field causing ionization of the center. As those electrons are affected by the field proportional to .,,/U 0 sin ~t, an instantaneous value of the light flux may be described by B~~-‘
‘y exp(—7t) exp [—a/(’..,/U0 sin wt)]
,
(3)
and hence the total light flux per cycle will be given by equation In
B
f
______
LI
7
exp(—’yt) exp [—a/(s./U0sin
wt)] dt.
(4)
With the assumption that y’.JU~/ac.~,> 1, this formula may be replaced by the approximation B ~ y exp(—yir/2w) exp(—a/\/U~,
(5)
that may be written in a more convenient form for further calculations, in the following way B
B0 exp(—A/f) exp(—a/~JU~),
(6)
where B0 = constant dependent on the units in which luminance B is expressed, A = parameter dependent on the technological conditions under which the samples were prepared and on the temperature,f= frequency, a = parameter dependent on the value of critical intensity of the electric field causing ionization of the luminescent center, U0 = voltage amplitude.
3. Experimental results ZnS thin films were produced by vacuum evaporation on a Corning 7059 glass substrate. Before that, a photoconducting layer of 1n203(Sn) (degenerate semiconductor) was produced 2. on the glass surface by cathode sputtering. Its resistance was about &~/cm The 100 material for evaporation was powdered ZnS with Cu, with Cl and Mn admixtures added. The contents of the admixtures were as follows: Cu = 3.2 X 10—2 g/g ZnS, Cl = 3.6 X 10_2 g/g ZnS and Mn = 2.5 X 10_2 g/g ZnS. The material produced in such a way was then evaporated under vacuum 5 X 1 ~_6 Torr from the alundum
Z. Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
132
AL
ZnS (CU,CL
/
3n,0j(Sn)
I —. Fig. 2. Thin film electroluminescence capacitor.
mB
500 200
c1~
100 .
c/s
I
~ [v_I/’] 0.07
0.08
0.09
0.10
Fig. 3. Luminance characteristics for ZnS : Cu, Cl, Mn thin films at various frequency.
Z. Porada, F. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
133
crucible heated to about 1100°Con the substrate, which was placed 8 cm apart from the source and heated to 200°C.Processing time was varied from 50 to 120 mm, which resulted in the layer thicknesses varying from 0.5 to 2.0 pm. The ZnS film so obtained was then recrystallized under vacuum for 30 mm at about 3 50°C [11]. Silicon oxide SiO~film about 500 A thick was evaporated thereafter on ZnS film. Finally, the aluminium electrode was evaporated onto the SiO~film (fig. 2). The active surface of such an arrangement was 0.28 cm2. In the measurement of light emission, the photomultiplier with spectrum range of sensibility from 3500 to 6500 Awas used. The luminescent characteristics were obtained for the 0.8 pm thick film excited with sinusoidal voltages of 100, 200, 500 and 2000 Hz frequency (fig. 3). It can be seen from the plot, that the function lnB(1/~/U~) is linear, irrespective of the applied frequency. Thus the Alfrey—Taylor formula, eq. (6), is fulfilled for all voltages used. Luminance characteristics were obtained at room temperature, so the parameter A was constant for a given sample. On the basis of performed measurements, the mean values of A and a were calculated. For a 0.8 pm thick sample they equal 240 51 and 16.5 V”2, respectively.
LnB 5
0
;~
*1m5&
Fig. 4. Plot of mB vs. 1/f for 0.8 pm thick ZnS: Cu, Cl, Mn film.
134
Z. Porada, E. Schabowska / AC electroluminescence of ZnS: Cu, Cl, Mn
Fig. 5. Luminance intensity vs. time over a voltage cycle. Frequency 100 Hz, amplitude 180 V, experimental results, (———) theoretical values.
When exciting the sample with a sinusoidal voltage of constant amplitude, the characteristic in B(l/f) should be — according to eq. (6) — a straight line with slope independent of the applied voltage and depending only on the value ofA. The plots presented in fig. 4 indicate, that all points of measurement are situated on the straight lines, whose slopes are the same and independent of the applied voltage. The Aifrey—Taylor relation is then fulfilled. The luminance vs time for single cycle of applied voltage at 100 Hz frequency was observed for the film studied (fig. 5). The B(t) function was estimated from formula (3) for the earlier calculated parameter A and for the amplitude of applied voltage U0 = 180 V. Pulses obtained from the oscillograph and those numerically calculated were similar in shape and the maximum of luminance pulse appeared after relaxation time r 1.05 ms from the beginning of the pulse. It complies with the numerically calculated maximum value, thus confirming the validity of formula (3) applied by Alfrey and Taylor for the model describing electroluminescence.
4. Summary It follows from this study that for vacuum-evaporated ZnS: Cu, Cl, Mn thin films 0.5 to 2.0 pm thick, excited with sinusoidal alternating voltage of amplitude
Z. Porada, E. Schabowska
/ AC electroluminescence of ZnS: Cu,
Cl,
Mn
135
80—200 V at frequency up to 2 kHz, the Alfrey—Taylor relation is in good agreement with experimental results. Accordingly, the theoretical model of electroluminescence suggested by Aifrey and Taylor for ZnS monocrystals can be applied also for ZnS thin films.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W.A. Thornton, J. App!. Phys. 30(1959)123. P. Goldberg and J.W. Nickerson, J. App!. Phys. 34(1963)1601. K. Sasaki and S. Kurita, Jap. J. Appl. Phys. 7 (1968) 1039. T. Mijata, S. Nakagawa, H. Mirayama, H. Hasegawa and M. Korenaga, Jap. J. AppL Phys. 9(1970)615. W. Uchida, Jap. J. Appi. Phys. 14 (1973) 670. E. Walentynowicz, J. Luminescence 18/19 (1979) 362. W. Ruffle, V. Morreilo and A. Onton, J. Luminescence 18/19 (1979) 729. J. Benoit, P. Benailoul, R. Parrot and J. Mattler, J. Luminescence 18/19 (1979) 739. C.F. Alfrey and J.B. Taylor, But. J. AppL Phys., SuppL 4 (1955) 44. W.W. Piper and F.E. Williams, Brit. J. AppL Phys., Suppl. 4 (1955) 39. Z. Porada, Thesis, Academy of Mining and Metallurgy, Cracow (1978).
[io] [111