Nov., 1953
TRAPSAND TRAPPIKG ~’ROCESSES
785
TRAPS AND TRAPPING PROCESSES BY RICHARD H. BUBE Radio Corporation of America, RCA Laboratories Division, Princeton, New Jersey Received March I%, 1969
Both emission centers and trapping centers are associated with localized imperfections in crystals. There are many case8 in which these localized imperfections are caused by the presence of added impurities, but there are other examples of emission centers and trapping centers being associated with imperfections of the crystal defect type. Crystal defects thus may be considered as a kind of “intrinsic impurity” which plays an important role in the formation of host-crystal emission centers and host-crystal trapping centers. The results reported in this paper give some recently suggested answers to the following questions. (1) What is the nature of the trapping centers? (2) Is trapping dependent on the motion of the excited electron through the conduction band? (3) Is retrapping an important phenomenon? (4) What is the transition involved in optical trap emptying? Studies of trapping have yielded the following answers to these questions. (1) Trapping centers in sulfide (and silicate) phosphors are associated with crystal defects located at substitutional sites. ThlR conclusion is based on the evidence that (a) traps occur in phosphor without added impurity, (b) trap depths are independent of added impurity, (c) trap depths are independent of crystal form, (d) the number of traps increases exponentially with the reciprocal preparation temperature, ( e ) intensification effects are found in which an added impurity increases the number of host-crystal emission centers and host-crystal trapping centers, (f) the excitation of photoconductivity varies in the same way with excitation energy as the production of trapped electrons, indicating a separate identity for emission centers and trapping centers. (2) At least in sulfide phosphors, trapping is always accompanied by photoconductivity, indicating that electrons move in the conduction band to and from traps. (3). Retrapping occurs whenever the number of empty traps becomes appreciable. (4) If optical trap emptying produces stimulation, trapped electrons are raised into the conduction band, whereupon they may be retrapped or return to emission centers.
Introduction Nature of Trapping.-Trapping is a fundamental process for energy storage in electronically active solids. This energy storage is accomplished by the spatial localization of an excited electron or hole, in such a way that the electron or hole is prohibited from moving freely through the crystal unless supplied with thermal or optical energy. When the trapped electron or hole is released, it is free to move until captured by a center, thereupon making a return transition to a ground-state level by either a radiative or non-radiative process, thus releasing the stored energy as light or heat. Those regions of the crystal which are able to capture electrons or holes and detain them in such a restricted locality are called traps. The possible transitions involving electron and hole traps are summarized in the simplified energylevel diagram of Fig. 1. The arrows indicate electron transitions.
Fig. 1.-Possible transitions involving electron traps, E, and hole traps, H. The arrows indicate elect,ron transitions.
For an electron trap, E, the possible transitions for electrons are (a) filling of trap by capture from
conduction band-the iiormal transition in electron trapping; (b) emptying of trap by transition to conduction band-the normal transition in trap emptying resulting from thermal or optical stimulation; (c) filling of trap by excitation from the filled band or from low-lying energy levels-an unobserved transition to date; and (d) emptying of traps by transition to filled band or to low-lying energy levels-a transition recently given particular attention by Broser and Warminsky,’ and by Kroeger and Dikhoff .z For a hole trap, H, the possible transitions for holes are (a’) filling of trap by capture from filled band-an important transition in the transfer of energy by hole migration as developed by Klasens, et aL3; (b’) emptying of trap by transition t o filled band-a possible transition involved in the temperature-quenching of luminescence; (c‘) filling of trap by excitation from the conduction band or from high-lying energy levels--the excitation transition i n luminescence; (d’) emptying of trap by transition to the conduction band or to high-lying energy levels-the emission trailsition in luminescence if the transition occurs by a radiative process. The concept of hole traps is not generally used in the discussion of luminescence, because the transitions involving hole traps may be more simply described in terms of the concept of luminescence centers. In addition, the phenomena of energy storage which are commonly observed in luminescence are dependent on electron trapping rather than on hole trapping, for the freeing of holes is usually associated with the process of temperature-quenching of luminescence. In the following discussion, therefore, it is assumed that electron trapping is under consideration unless hole trapping is specifically mentioned. Trapping Effects.-Trapping is an important factor in determining the magnitude of the time constant for many processes in solids, and hence in (1) I. Broser and R. Warminsky, Ann. Physik, 7 , 288 (1950). (2) F. A. Kroeger and J. Dikhoff, Physica, 16, 297 (1950). (3) H. A. Klasens, W. Ramsden and Chow Quantie. J. O p t . S O C . A m , 38, 60 (1948).
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RICHARD H. BUBE
setting limits to the frequency-response of. these solids. In phosphors, trapping effects may be detected and studied by measurements of the following phenomena. (1) Growth of Luminescence Emission Intensity.-The presence of traps decreases the rate of growth of emission intensity a t the beginning of excitation. The area between the growth curve and the equilibrium value of emission intensity is proportional t o the number of trapped electrons. Deep traps, which would produce electrons that would make unobservable non-radiative transitions if emptied thermally, may be detected by measurements of the growth of emission intensity. (2) Decay of Luminescence Emission Intensity.-The presence of traps of various depths causes the decay of phosphorescence emission to have a power-law variation with time ( I = Only traps for which freed electrons undergo radiative transitions may be detected by decay measurements. The actual power-law decay curves found may be considered to be the result of a superposition of exponential decay curves, each exponential decay constant being associated with a single trap depth. The analytical calculations based on decay measurements have as one of their assumptions that the probability P of an electron escaping from a trap of depth E , at temperature T, is given by the equation of Randall and Wilkins4 P = f exp ( - E / k T )
(1)
where f is the so-called frequency-factor, the product of the number of times per second the trapped electron may receive energy from the crystal due to crystal vibrations, and the transition probability to the conduction band. (3) Thermally Stimulated Luminescence Emission Intensity.-If traps are filled by excitation a t a low temperature, they may be emptied by raising the temperature. Only traps from which freed electrons undergo radiative transitions may be detected by measurements of thermally stimulated emission. For traps of a single depth, the following relation may be derived from equation (1) between the trap depth and the temperhture, TG,for which a maximum emission intensity is found when the phosphor is heated a t a linear rate, assuming no retra~ping.~ E
=
l c T ~(1 + a ) Inf
(2)
where a is a constant with a value of about 0.1, depending on the particular values of the heating rate, E , f and TG. (4) Optically Stimulated Luminescence Emission Intensity.-Filled traps may be emptied by the absorption of optical energy as well as by the absorption of thermal energy. Information about trapping processes may be obtained from (a) a comparison of optical and thermal light sums, and (b) a study of optical trap emptying as a function of optical energy. Only traps from which freed electrons undergo radiative transitions may be detected by measurements of optically stimulated emission. (4) J. T.Randall and M. H. F. Wilkins, Proo. Roy. Soc. (London), 1848,3G6, 390 (1945).
Vol. 57
( 5 ) Luminescence Emission Intensity during Excitation as a Function of Temperature.-If the number of luminescence centers is of the same order of magnitude as the number of traps, the emission intensity will be lower a t temperatures such that most of the traps remain filled, than a t higher temperatures where the majority of the traps will be filled for only very short times. The presence of traps thus affects the number of centers available for repetitive excitation. Any traps with appropriate depths for the temperature range of high luminescence efficiency may be detected by this method. (6) Photoconductivity.-Traps from which freed electrons enter t,he conduction band may be detected by (a) effects on the time constant of photoconductivity, and (b) the thermally stimulated current measured in a manner similar to that used for thermally stimulated emission. (7) Dielectric Constant.-Changes in dielectric constant and dielectric loss are found when a phosphor is excitede6 Although the effects have not yet received a generally accepted explanation, there is evidence that the changes in dielectric properties are associated a t least in part with trapped electrons, and that thermally stimulated dielectric changes accompany trap emptying in much the same way as thermally stimulated emission and current changes. (8) Paramagnetic Susceptibility.-With sufficient sensitivity in the experimental arrangement, trapped electrons should be detectable in situ because of the unpaired spin of a trapped electron. In principle, measurements of paramagnetic susceptibility should indicate the total number of trapped electrons at the temperature of the measurement, if susceptibility changes due to activator excitation can be separated from changes due to electron trapping. Types of Traps.-Traps may be divided into two principal types: (1) traps associated with particular impurities, and (2) traps associated with defects in the host crystal. For trapping associated with an impurity, the trapping process involves a change in the effective valence of the impurity ion. One of the best known of such impurities is Sm, which is used as a trapping activator in infrared stimulable phosphors t o produce deep traps. Presumably the trapping process involves a change from Sm+3to Sm+2. Other impurities which act in a similar manner are Sn and AS.^ Trapping associated with crystal defects is the result of the formation of regions in the crystal surrounding the defects with an effective excess of positive charge (electron trap) or negative charge (hole trap). A simple example of such a defect is an anion omission defect, which would be effective as an electron trap. Assemblages of omission defects and other types of defects to form more complex dislocations may have a similar ability to trap electrons or holes. Considerable evidence has been reported to indicate that a t least some of the traps ( 5 ) G. F. J. Garlick and A. F. Gibson, ibid., 188, 485 (1947). (6) H.W. Leverenz, “An Introduction to Luminescence of Solids,” John Wiley and Son, Inc., New York, N. Y.,1950,P. 286.
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1
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TRAPSAND TRAPPING PROCESSES
Nov., 1953
may be conveniently separated into a low temperature glow peak a t -110", associated with shallow traps, and a high temperature glow peak a t about -20", associated with deeper traps. There is no indication of a sudden change i n the glow curves a t Dependence of Trapping on Preparation the crystal transition from cubic to hexagonal, thus The production of traps in zinc sulfide phosphors indicating that trapping centers are located at subsites in the crystal. has been studied as a function of (1) the preparation stitutional It is found that the number of traps, as detertemperature,' ( 2 ) the preparation a t m ~ s p h e r e , ~mined from glow curve areas, varies exponentially In and (3) the presence of added impuritie~.~~~O addition, the production of traps in silicate phos- with the reciprocal preparation temperature, and phors has been studied as a function of added im- therefore activation energies for trap formation may be calculated for the cubic and hexagonal crystal purities. * of zinc sulfide (see Table I). The ratio of forms Preparation Temperature.-The number and activation energies for the shallow traps in the cubic distribution of traps in zinc sulfide phosphor, pre- and hexagonal phosphors is found to be the same pared without the addition of impurity, were measfor the deep traps in the two crystal ured by means of glow curves. Ten phosphors as the ratio were measured for preparation temperatures be- forms. It is also found that the ratio of activation tween 900 and 1350". X-Ray analysis of the phos- energy for defect formation to the average trap phors showed that phosphors prepared below 1050' depth corresponding to the defect, is a constant for were cubic, and that phosphors prepared above shallow and deep traps. 1100" were hexagonal; those prepared a t 1050 TABLEI and 1100" were essentially cubic with small fracACTIVATION ENERGIES,E*, FOR DEFECT FORMATION tions of hexagonal. Figure 2 shows the glow curves Auproxiobtained with this series of phosphors. The curves inate trap E*oub/
in zinc sulfide phosphors are associated with crystal defects,' and also that crystal defects are responsible for the trapping found in silicate and germanate phosphors.8
E*hex,
E*oub,
e.v.
depth, E , e.v.
1.6
4.5
2.6
7.4
e.v.
I00
50
200
I00
1250%
200 I00
300 13OO0C
200 I00
l
a
TEMPERATURE.'C.
Fig. 2.-Glow curves for a series of ZnS phosphors prepared a t various temperatures between 900 and 1350' without any additives in the preparation. Ultraviolet excitation was used. (7) R. H. Bube, J. Chem. Phys., SO, 708 (1952). (8) R. H. Bube and 8. Larach, .ibdd., 21, 5 (1953). (9) F. A. Kroeger and J. Dikhoff, J . Electrochem. Soc., 99, 144 (1952). (10) R. H. Bube, Phys. Rev., 90, 70 (1953).
Traps with glow peak a t -110" Traps with glow peak a t -20'
E'hex,
E*hex/
e.v.
E , e.v.
0.37
2.8
4.3
.BO
2.8
4.3
The fact that the number of traps with glow peak at -25" in Fig. 2 varies approximately as the square of the number of traps with glow peak a t - 120", when the total number of traps is increased by raising the preparation temperature, indicates a possible correlation between the defects associated with these two glow peaks. The functional relationship is the same as that between pairs of defects and isolated defects. Reparation Atmosphere.-The phosphors described in the previous section were prepared in air. Kroeger and his associates have reported that the occurrence of traps with depths greater than the shallowest mentioned above, with glow peak a t about - l l O o , is dependent on the presence of oxygen in the preparation atmosphere. ZnS phosphors prepared (with halide present) in H2S or N? are reported to show only a single low-temperature glow peak, and a direct correlation is reported between the intensity of higher temperature glow peaks and the proportion of ZnO added in the preparation. That the effect of oxygen may be a secondary rather than a primary one in the production of traps, ie., that oxygen may aid the formation of trapping defects under ordinary preparation conditions, rather than be directly associated with traps themselves, is indicated by recent experiments. ZnS crystals were prepared by Thomseri a t a temperature between 1000 and 1100" in a reaction between Zn vapor and H2S. Two types of crystals were found: (1) with an intense blue emission and moderate afterglow a t room temperature, growing near the location where the Zn vapor and H2S first interact, and (2) with an intense green emis-
788
RICHARD H. BUBE
Vol. 57
I
7 -200
I
-100
I
I
0
IO0
TEMPERATURE, 'C. Fig. 3.-Glow
curves for a series of ZnS phosphors prepared at 1200' wi?n various proportions, between 0 and I%, of "IC1 added in the preparation. Ultraviolet excitation was used.
sion and long afterglow a t room temperature, growing further down the reaction tube, and hence probably contaminated with Cu impurity from the silica reaction tube. The green-emitting crystals show the rapid quenching characteristic under infrared irradiation which is common t o ZnS:Cu crystals prepared in the absence of oxygen or halide, but which is totally absent in ZnS:Cu phosphors prepared in air with halide present. Impurities.-For ZnS phosphors prepared in air, studies have been made on the effects of halide impurity and of Ag, Cu and Mn impurity. Phosphors prepared with halide impurity have an increased emission intensity and an increased glow curve intensity, The same trap depths found for the phosphors prepared without halide are also found for the phosphors prepared with halide, and only second-order differences are found between phosphors prepared with chloride, bromide, or iodide. Figure 3 shows the effect of increasing proportions of NH&l added in the preparation of ZnS phosphor a t 1200O. The halide produces a change in the distribution of the glow curve wherein the high-temperature peak becomes more prominent relative to the low-temperature peak, exactly the same effect as was produced by raising the preparation temperature without adding halide, as shown in Fig. 2. It may be concluded that the halide produces defects in the ZnS crystal of the same type as may alternatively be produced by thermal effects. Figure 4 shows the glow curves obtained for a series of ZnS phosphors, without added impurity, and with Ag, Cu and Mn impurity. Measurements obtained down t o very low glow intensities show
that five trap depths are common t o all of the ZnS phosphors regardless of the added impurity. The five glow peaks are located a t about -120, -60, -25, 15 and 90". The principal reason for the deep traps being more prominent in the Cu and Rln
TEMPERATURE.%.
Fig, 4.-Glow curves for ZnS phosphors after excitation at -160" with ultraviolet: 1, ZnS phosphor without any additives; 2, ZnS, [NaC1(2)]; 3, ZnS:Ag(O.Ol), [NaC1(2)] ; 4, ZnS:Cu(O.Ol), [NaC1(2)]; 5 , orange band of ZnS:Mn (0.01),[NaC1(2)]. All are hexagonal phosphors.
Xov., 1053
TRAPSAND TRAPPING PROCESSES
phosphors than in the others is probably simply the higher luminescence efficiency of the Cu and M n phosphors at these temperatures. The glow peak a t -60" was present also in the glow curves of Fig. 2, but was not clearly resolved; the peaks at 15 and 90" were of too low an intensity to be shown in Fig. 2. These results show that the trap depths are independent of any added impurity, but are characteristic of defects in the host crystal. For ZnS phosphors prepared in HzS, it has been reported by Kroeger and his associates that single glow peaks are obtained when Al, Ga, or In is incorporated, the location of the glow peaks in these cases being at temperatures characteristic of the particular impurity. These traps are associated, it is suggested, with perturbed Zn ions. The action of halide in ZnS phosphors, causing both an intensification of the host crystal emission and an increase in the number of trapping defects, is duplicated to a considerable extent by gallium in zinc silicate phosphor. Gallium acts as an intensifier for the blue host crystal emission of zinc silicate and also greatly increases the glow intensity of one of the two principal glow peaks found for zinc silicate phosphor above room temperature. Both of these effects can be explained in the same way as the action of halide on ZnS; the gallium produces defects of the same nature as are formed to a much less extent by thermal disorder in the preparation of the phosphor. These defects play the role both of perturbing certain SiOr groups in ZnzSi04, thus producing luminescence centers, and of creating trapping sites. Evidence for the identification of traps in silicate phosphors with defects in the host crystal is based on measurements of glow curves for ZnzSiOc, ZnO.SiOz, MgzSi04 and Be2Si04,without added impurity, and with Mn and Ga impurity. From all
789
the glow curves of these various phosphors, about forty-nine definite glow peaks or clear indications of glow peaks can be identified. If two of these glow peaks are neglected, since they appear only once each (in the magnesium silicate phosphors), each of the other forty-seven glow peaks may be identified with one of only six different glow peaks at 57, 75, 100, 125, 210 and 250'. Each of these six different glow peaks occurs at least once in a host crystal without added impurity; it may therefore be concluded that all glow peaks are associated with host-crystal defects. Trapping and Photoconductivity Direct measurements on the traps involved in luminescence and photoconductivity have shown a close correlation between the two processes in powder phosphors,'O but some quantitative differences have been found in measurements with a ZnS crystal. l 1 The total number of electrons trapped and the photoconductivity were measured for ZnS, ZnS :Ag, ZnS:Cu and ZnS:Mn phosphors as a function of excitation energy. Figure 5 shows that trapping and photoconductivity have exactly the same variation with excitation energy. Both trapping and photoconductivity occur when the excitation is absorbed by the host crystal (excitation energy greater than about 3.2 e.v.) and when the excitation is absorbed by the Ag (3.0-3.5 e.v.) or Cu (2.6-3.5 e.v.) centers, but neither trapping nor photoconductivity occurs when the excitation is absorbed by the Mn centers (2.2-3.5 e.v.). I n ZnS phosphors, therefore, excited electrons must enter the conduction band in order to be trapped, and trapping centers are separate from emission centers. Measurements of thermostimulated current in a ZnS crystal compared with glow curve measurements indicated that the same trap depths were involved in both the luminescence and photoconductivity processes, but that shallow traps were more prominent in contributing to the glow curve than deep traps, whereas deep traps were more prominent in contributing t o the thennostimulated current than shallow traps. The difference cannot be explained by a simple variation of electron mobility with temperature, since this variation would produce an opposite effect to that observed. A possible explanation is that electrons freed from shallow traps have a higher probability of finding an excited center per unit time than do electrons freed from deep traps, simply because the number of excited centers is considerably smaller when deep traps are being emptied.
Retrapping Since electrons freed from traps enter the conduction band in the process of returning to emisFig. 5.-(a) Light sums, measured with thermal stimula- sion centers, the retrapping of electrons by other tion, for ZnS phosphors, as a function of excitation energy: traps would be expected. Detailed glow curve 1, ZnS Phosphor without any additives; 2, ZnS, [NaC1(2)] : measurements have been made, both with ZnS :CuI2 3, ZnS:Ag(O.Ol), [NaC1(2)]; 4, ZnS:Cu(O.Ol), [NaCI(2)]; and ZnS :Mn'O phosphors, which show that retrap5, ZnS:Mn(O.Ol), [NaC1(2)]; 6, ZnS:Mn(l.O), [NaC1(2)]. ping, by deep traps, of electrons freed from shallow All are hexagonal phosphors. (b) Photocurrent for ZnS phosphors as a function of excitation energy. numbered the mme as in (a).
Curves are
(11) (12)
R. H.Bube, Phys. Rev., 83, 393 (1951). R,H. Bube, (bid., 80, 666 (lQ50).
RICHARD H. BUBE
790
traps in the course of the glow curve measurement is a common occurrence whenever there is an appreciable number of empty deep traps at the beginning of the glow curve measurement. The effect of retrapping is to make high-temperature glow peaks more prominent relative to glow peaks at lower temperatures, as the total number of electrons initially trapped decreases. These effects are demonstrated in Fig. 6, in which the relative intensities of the glow peaks a t -60, -25 and 15" in a ZnS :Mn (0.01) phosphor are plotted, compared to the intensity of the glow peak a t - 120°, as a function of the per cent. of total traps filled a t the beginning of the glow curve measurement. Approximate calculations based on retrapping measurements in ZnS :Cu phosphors indicated that one electron was retrapped for about every four empty deep traps, capable of retrapping.
P E R C E N T OF SATURATION LIGHT SUM.
(30)
Fig. 6.-The li5lit sum of glow peaks a t (2) -160°, -25' and (4)15 relative to the glow peak a t (1) -120 , with the per cent. of the saturation total light sum: for hex.-ZnS: Mn(O.Ol), [NaC1(2)].
Retrapping by shallow traps of electrons freed from deep traps is observed as a result of optical trap emptying, which is discussed in the following section. In this case electrons may be optically freed from deep traps at temperatures a t which they may be stably recaptured by shallow traps, for which optical emptying is not effective or less efficient. Optical Trap Emptying Recent experiments have indicated that irradiation by almost any wave length that is able t o fill traps is able also to empty traps. Thus the maximum light sum obtained is not necessarily a measure of the total number of traps present in the phosphor but is instead a measure of the equilibrium set up between trap filling and trap emptying. There are, in addition, many other wave lengths for which irradiatioii will empty traps, although unable to fill traps. Examples have been studied where optical trap emptying results in a stimulation of luminescence emission and retrapping by shallow traps of electrons freed from deep traps, both for silicate phosphors* and for ZnS phosphors. lo A series of ZnzSiO,, ZnO.SiOz, Mg2Si04 and BezSi04 phosphors, without added impurity, and with Mn and Ga impurity, were used for measurements of trap emptying with 3650 A. ultraviolet. These phosphors may be efficiently excited by cathode
Vol. 57
rays, and the emptying of traps, filled by cathoderay excitation, by 3650 ultraviolet was studied as a function of host crystal and impurity activator. The 3650 8. ultraviolet emptied all traps t o some extent, as measured from glow curves obtained from room temperature to above 250°, but was more effective in emptying traps with glow peaks between 30 and 150" than in emptying traps between 150 and 270". The incorporation of Ga greatly in.creased the effectiveness of the ultraviolet for trap emptying. The glow peak at 210' was affected far less by ultraviolet irradiation than were the other glow peaks. Trap emptying in ZnS phosphors, without added impurity, and with Ag, Cu and Mn impurity, was measured for irradiation with energies of 3.18, 2.87, 2.43 and 2.00 e.v. It is found that the effectiveness of trap emptying in the sulfide phosphors is approximately independent of impurity or irradiating energy over this range. Traps of all depths were emptied regardless of the irradiating energy. For these phosphors it is concluded that optical stimulation of trapped electrons with visible light raises the electrons from traps into the conduction band. This conclusion is based on the fact that (1) the effectiveness of trap emptying is independent of impurity or irradiating energy; (2) a large proportion of the freed electrons return to centers with normal luminescence emission, as determined by a comparison of optical light sum, obtained with irradiation at 2.00 e.v. at a low temperature, with the difference in thermal light sums obtained without and after irradiation; and (3) electrons raised from deep traps by optical stimulation are retrapped in empty shallow traps.
s.
DISCUSSION H. J. VINK.-I would like to make a comment which is perhaps a confirmation of your ideas that defects in phosphors are important. Kroger and I did some experiments with CdS in which we incorporated a small proportion of gallium. Now the charge compensation required by the presence of gallium can be made by the crystal taking on another electron, what we call controlled valency, or by forming a cation vacancy. We found that under sufficiently sulfurizing conditions the charge compensation was not made by controlled valency, but by forming vacancies, cation vacancies which have one electron missing from the neighboring sulfurs. These vacancies cause a red coloration and a fluorescence in CdS. So we think that cadmium sulfide can be made fluorescent by cation vacancies, and furthermore we think that even the fluorescence of the self-activated zinc sulfide is not caused by monovalent zinc ion, but by a cation vacancy which has lost an electron. So there is a lot of evidence in favor of your ideas.
R. H. BuEE.-I should have mentioned that one gets the same effect in silicates by incorporating gallium as one does in sulfides by incorporating a halide; in zinc silicate the incorporation of gallium will increase the emission intensity without changing the spectral distribution, and will also intensify one of the two prominent trap depths in zinc silicate. VINK.-we find that the incorporation of gallium has the same effects as you found for increasing the preparation temperature.
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