PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 139 1965

ON THE EFFECT OF IMPURITIES ON THE CANDOLUMINESCENCE OF PHOSPHORS

O. G. BRIK, V. K. IVANOV

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

Candoluminescence arises due to the energy released during the recombination of free atoms and radicals present in the flame on the surface of the phosphor [1—5].

The first stage of candoluminescence is the adsorption of radicals, which depends on the state of the surface [6].

The state of the surface can be affected both by changing the temperature of the crystal and by introducing various impurities. The purpose of the present work is to study the behavior of candoluminescence when various impurities are introduced and when the temperature is changed.

Experimental Method

In the work an apparatus was used (Fig. 1) that made it possible to obtain a flame at reduced pressure (~ 90 mm Hg). The mixture, sup—

Fig. 1. Experimental apparatus:
1 — reaction vessel, 2 — support with phosphor, 3 — incandescent filament, 4 — salt, 5 — inlet of the gas-and-air mixture, 6 — inlet of impurity, 7 — to the fore-vacuum pump, 8 — to the manometer, 9 — PSU, 10 — to the microammeter, 11 — thermocouple, 12 — tube for fastening the thermocouple and heater leads, 13 — heater lead.

entering the reaction vessel was ignited by a heated filament. The resulting flame was passed through two bends of a tube, in one of which the phosphor was located under constant conditions; in the other bend it was possible to introduce the impurity and to vary the temperature of the phosphor. The phosphor was applied to a substrate. The luminescence was recorded by a photoelectric method (a photomultiplier with an M-193 instrument).

The apparatus made it possible to measure directly the change in the brightness of candoluminescence (in the region of the visible spectrum \(\lambda = 520\text{—}580\ \mathrm{m}\mu\)).

The temperature of the phosphor was measured with a chromel–alumel thermocouple 0.2 mm in diameter. The concentrations of combustible gas, air, and impurity were regulated by flowmeters.

The phosphor was applied to a brass substrate. Before each experiment the phosphor was heated in vacuum for 10 minutes at \(300^\circ\mathrm{C}\). It was then slowly cooled.

The influence of the following gases was investigated: nitrogen, carbon dioxide, oxygen, and hydrogen.

Results of the Experiments

Dependences were obtained for the brightness of candoluminescence of the phosphors ZnS, CdS—Cu, ZnS, ZnSe—Cu on the concentration of the gaseous impurities \(N_2\), \(CO_2\), \(O_2\), and \(H_2\) at various temperatures. The graphs are presented as functions \(J/J_0 = f(\eta, T)\), where \(J_0\) is the maximum intensity of the phosphor without the influence of impurities.

1. The influence of nitrogen impurity is presented in Fig. 2. With increasing nitrogen impurity, the brightness of candoluminescence decreases. As the temperature changes, the course of the dependence is preserved.

Fig. 2. Dependence of relative intensity on the concentration of nitrogen impurity.
1 — at \(25^\circ\mathrm{C}\), 2 — at \(100^\circ\mathrm{C}\), 3 — at \(200^\circ\mathrm{C}\).

1. Carbon dioxide at low temperatures acts analogously to nitrogen impurity, while at a temperature of \(200^\circ\mathrm{C}\) the quenching action is manifested more strongly (Fig. 3, 3).

2. Influence of oxygen. Because of strong fluctuations of the flame, only the dependence on the oxygen concentration at which there occurs

complete quenching of candoluminescence on temperature. With increasing temperature, the quenching concentration of oxygen increases (Fig. 4).

Fig. 3. Dependence of the relative intensity on the concentration of the \(CO_2\) impurity: 1 — at 25°C, 2 — at 110°C, 3 — at 200°C.

1. Hydrogen was introduced directly above the phosphor. The brightness of the candoluminescence increased up to a certain value, after which it remained constant (Fig. 5).

Fig. 4. Dependence of the quenching concentration of oxygen on the temperature of the phosphor.

1. The temperature dependence of the candoluminescence of the phosphor was obtained (Fig. 6). On the curve a maximum is clearly manifested at a temperature of 80–140°C. For comparison, a plot of the temperature dependence of the photoluminescence of the same phosphor is given.

Fig. 5. Dependence of the relative intensity on the concentration of the hydrogen impurity.
1 — at 25°C, 2 — at 110°C, 3 — at 200°C.

Fig. 6. Dependence of the relative intensity of candoluminescence (1) and photoluminescence (2) for the phosphor on temperature.

Discussion of the Results

Taking into account the electron–hole mechanism of candoluminescence proposed in works [3, 4, 5], the process of excitation of candoluminescence can be written as follows:

1. Formation of a “weak” bond

\[ \mathrm{C} + \mathrm{L} \to \mathrm{CL}. \]

1. “Strengthening” of the bond

\[ \mathrm{CL} \to \mathrm{CpL} + \mathrm{eL}, \tag{a} \]

\[ \mathrm{CL} + \mathrm{pL} \to \mathrm{CpL}. \tag{b} \]

1. Recombination of an adsorbed atom with an atom from the gas phase, causing the release of a hole

\[ \mathrm{CpL} + \mathrm{C} \to \mathrm{C}_2 + \mathrm{pL}. \]

1. Ionization of the luminescence center

\[ \mathrm{AL}+pL \to \mathrm{A}pL, \]

1. Neutralization of the luminescence center—the act of emission:

\[ \mathrm{A}pL+eL \to \mathrm{AL}+h\nu . \]

Thus, a necessary condition for the excitation of candoluminescence is the presence of free hydrogen atoms [7], as well as the presence of an active surface and free electrons.

By introducing various gaseous impurities, one can influence the conditions listed above.

The data obtained (Fig. 2) indicate that nitrogen does not change the properties of the surface, but only affects the concentration of free hydrogen atoms, causing their destruction in the volume by the reaction

\[ \mathrm{H}+\mathrm{H}+\mathrm{M}\to \mathrm{H}_2+\mathrm{M}. \]

This is indicated by the fact that an increase in the temperature of the phosphor does not affect the course of the dependence of the decrease in brightness on concentration.

At low temperatures carbon dioxide affects candoluminescence in a manner similar to nitrogen. At a temperature of \(200^\circ\mathrm{C}\), along with the destruction of atoms in the volume, adsorption of \(\mathrm{CO}_2\) on the surface of the phosphor takes place, since at this temperature the character of the dependence of brightness on impurity concentration changes.

We believe that, in the case where the impurity consists of oxygen molecules, which have a high electron affinity, quenching of candoluminescence occurs mainly as a result of adsorption of the oxygen molecule on the surface of the phosphor. This phenomenon is reversible. After heating, the phosphor again candoluminesces (as a result of heating, desorption of \(\mathrm{O}_2\) occurs). According to Vol’kenshtein [6], the transition of a “weak” bond into a “strong” one occurs as a result of capture of a free electron of the surface, the supplier of which may be act (2a). Thus, the formation of the ion-radical \(\mathrm{O}_{2\text{ads}}^{-}\) entails a decrease in the probability of reaction (5), and consequently also a decrease in the intensity of candoluminescence.

In the case where an oxygen molecule is adsorbed on an activator atom, the number of centers capable of being ionized decreases, which is an additional cause of the decrease in luminescence.

Quenching is also possible as a result of the reaction on the surface

\[ \mathrm{H}_{\text{ads}}+\mathrm{O}_2 \to \mathrm{HO}_2, \]

i.e., as a result of a decrease in the probability of reaction (3).

The experimental data show that, with an increase in the temperature of the phosphor, the concentration of oxygen at which complete quenching of the phosphor occurs increases.

In this case there is probably a change in the ratio between the “weak” and “strong” bonds. In accordance with the assumptions of Vol’kenshtein [8], an acceptor impurity decreases the concentration of free electrons, and in this case the “strong” bond decreases. A donor impurity, on the contrary, increases the concentration of electrons, and the luminescence should thereby increase. Such a dependence was observed by us in the case of a hydrogen impurity. (In the case of hydrogen, an increase in the concentration of free H atoms is also possible.) With increasing temperature the concentration of electrons increases, which promotes adsorption of impurity molecules. As a result, in order to quench candoluminescence at elevated temperature, a larger amount of oxygen is necessary.

With an increase in the temperature of the phosphor, the concentration of free electrons increases, and together with this the catalytic

activity and adsorption capacity of the crystal. The latter causes an increase in the intensity of candoluminescence, which agrees well with the experimental data (Fig. 6, 1). The decrease in the intensity of candoluminescence with a further increase in temperature is associated mainly with thermal quenching of luminescence. It is interesting to note that analogous results were obtained in [9] in a study of radicoluminescence under the action of active nitrogen.

As a result of adsorption, adsorbed particles may create a surface charge causing a shift of the Fermi level, on whose position at the crystal surface the intensity of candoluminescence depends [10]. In this case there must be an opposite action of acceptor and donor impurities, which we have recently found in the case of \(O_2\) and CO.

REFERENCES

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4. A. N. Gorban, V. A. Sokolov. Opt. i spektr. 7, 569, 1959.

5. A. N. Gorban, F. F. Vol'kenshtein, V. A. Sokolov. In: Surface Properties of Semiconductors, p. 179, USSR Academy of Sciences Press, 1962.

6. F. F. Vol'kenshtein. Electronic Theory of Catalysis on Semiconductors. USSR Academy of Sciences Press, 1960.

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