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Correlation of thermoluminescence in MgO and valence changes of iron and chromium impurities detected by EPR
J. Phys. Chem. Solids
Pergamon
CORRELATION AND
Press 1960. Vol. 13. pp. 124-131.
OF
Printed in Great Britain.
THERMOLUMINESCENCE
IN
MgO
VALENCE CHANGES OF IRON AND CHROMIUM IMPURITIES DETECTED BY EPR R. L. HANSLER Lamp Research Laboratory,
and W. G. SEGELKEN
General Electric
Company,
Nela Park, Cleveland 12, Ohio
(Received 9 October 19.59)
Abstract-EPR measurements show that during 2537 A irradiation the valence of chromium and iron impurities (~10 p.p.m.) in MgO single crystals changes from 2 to 3 or vice versa dependent on treatment. The reverse valence changes as well as thermoluminescence are observed during subsequent heating. It was shown by optical absorption that the energy levels of Fe+s and Cr+slie about 5.8 and 6.0 eV respectively above the valence band while the levels of Fe+2 and Cr+2 are respectively 6.9 and 5.1 below the conduction band. The possible luminescent processes are: Fe+3 Fe+2 Cr+3
Cr+s
+ free f free + free + free
electron hole electron hole
--f + --f +
Fe+2 Fe+s Crfs Cr+3
+ 2.9 eV; + 2.0eV; + 2.7 eV; + 3.6 eV.
Four crystals were studied each of which showed preponderantly only one of the above luminescent processes. By assuming the band gap is 8.7 eV, good agreement between the color of the observed thermoluminescence and that predicted by the model was obtained.
1. INTRODUCTION THE luminescence of MgO when excited in various ways has been the subject of a number of studies.(l-6) Irradiation with X-rays or ultraviolet or heating in oxygen or magnesium vapor will produce optical absorption bands in MgO crystals which contain no intentionally added foreign ions. Until recently it was assumed that both the luminescence and the optical absorption were associated with defects in the host lattice other than foreign ions. HABY and SOSHEA et aZ.(s) have recently presented evidence that certain of the ultraviolet absorption bands are due to Fe+a. We support this hypothesis. WERTZ et aZ.@,10) have evidence for changes in valence of various foreign ions in MgO during X-irradiation and other treatments. We wish to present evidence that certain luminescent effects in relatively high purity MgO are due to the transfer of electrons (and holes) between the iron and chromium impurities and depend on 124
defects such as ion vacancies manner. 2. EXPERIMENTAL
only in an indirect
PROCEDURE
Crystals studied Most of the single crystals used in this investigawere grown in this laboratory by L. J. SCHUPP by the arc melt method. In our highest purity crystals (designated R-58 and R-72) the chief impurity is calcium (<0*05 per cent)* which probably has little effect because of its chemical similarity to magnesium. The following were spectroscopically detected in amounts less than 0.01 per cent: Si, Al, Fe, Ti, Ni, Zn, Sr, Na, K. Using other methods, R-58 crystals were found to contain approximately 5 p.p.m. Sr, 8 p.p.m. Fe, 3 p.p.m. Li and 5 p.p.m. Cr. * The concentrations of impurities are given either as wt. ‘/, or p.p.m. by weight.
CORRELATION
OF THERMOLUMINESCENCE
measure?nents Mn+s, V+s, Fe+3 and Cr+s ions in MgO give readily observable peaks in the EPR absorption spectrum. We have observed the changes in concentration of these ions in a number of crystals subjected to various treatments. Quantitative data were obtained for the concentrations of Cr+s and Fe+3 in some crystals subjected to certain treatments. The EPR spectra were taken with a Varian spectrometer and &in magnet. The crystals were cemented to a quartz rod and oriented so that I& was parallel to the [loo] axis and &I was parallel to the [OlO] axis. A CuSO4 * 5HsO sample was placed in the cavity and the Cu+s absorption line used as a relative intensity standard. Only one line of each of the complex spectra of Cr+s and Fe+3 was used for the quantitative intensity measurements. For Cr+s the “cubic” line was used and for Fe+s the central line of the five line spectrum was used.@@
IN MgO AND
VALENCE
CHANGES
125
TATION
Preliminary results of EPR measurements on a number of samples suggested that neither the manganese nor vanadium were directly involved in the observed thermoluminescence. The amount of Mn+a remained remarkably constant under quite a variety of treatments. No correlation seemed to exist either between the amount of V+2 present and the luminescence or between the valence change of the vanadium and the luminescence.
Optical absorption measurements A Beckman model DU spectrophotometer was used for most of the me~urements. After excitation with ultraviolet it was sometimes necessary to check to make sure that long-lived phosphorescence did not interfere with the measurement. The measurements in the vacuum ultraviolet were made by Dr. P. D. JOHNSON at the General Electric Research Laboratory, Schenectady, New York.
Most of the measurements were of a qualitative type in which the color of the thermoluminescence was noted when the crystal was placed on a hot plate at about 300-400°C after being irradiated with light from a 4-W germicidal lamp.* A small quartz spectrograph was used to measure the emission in a few cases. G&u curves Thermoluminescent glow curves were obtained for a few crystals. The crystals were heated at a uniform rate and the light output measured with a lP28 photomultiplier. * This low pressure mercury discharge lamp produces principally 2537 A (4.9 eV) radiation.
Photon Energy fe Vi
FIG. 1. The absorption coefficient vs. photon energy of an MgO :3 %FesOs crystal which was tired 6 hr at 1100°C in hydrogen in order to reduce the iron to the divalent state. The sharp rise in absorption at about 6.7 eV we attribute to the Fef2.
Depending on the previous treatment the amount of Fe+s could either be increased or decreased during ultraviolet irradiation and showed the reverse change during thermoluminescence. The same was observed with Cr+3. These preliminary results suggested the luminescence might be correlated with these changes in valence. The positions of the energy levels of Fe*s, Fe+s, Cr+s and Cr+s in the MgO lattice were determined by optical absorption. HAXBY@) showed the correlation between the amount of Fe-@ in the lattice and the 4.3 and 5-7 eV absorption peaks. We found that by hydrogen firing an MgO :3OkF15&~ t crystal for six hr at 1100°C all of iThis notation means, “grown from a melt to which 3 per cent by weight FesOs was added”. The amount of foreign ion in the crystals usually is about one-tenth of rhis amount.
126
R.
L.
HANSLER
and
the 4.3 and 5.7 eV absorption could be removed. Fig. 1 shows the absorption edge at approximately 6.7 eV which we attribute to Fe+s. That the iron was not removed by the hydrogen firing but merely reduced was shown by observing the return of the 4.3 and 5.7 eV absorption after heating the crystal briefly in air. Fig. 2 shows the absorption of an MgO: 0*10j,CrsOs crystal after hydrogen firing 5 hr at
W.
G.
SEGELKEN
of metallic iron is inferred from valence studies carried out by R. P. TAYLOR of this laboratory using wet chemical methods. It is also suggested by the fact that crystals containing more than 100 p.p.m. of iron are attracted by a magnet after firing in Mg vapor. We have been able to produce Cr+s in high purity MgO crystals (total chromium about 3 p.p.m.) by passing direct current through the crystals at high temperature. Fig. 3 shows the
Photon
Energy
(ev)
FIG. 3. The absorption coefficient vs. photon energy for a high purity MgO crystal which has been treated by the passage of direct current at high temperature for a few minutes. We attribute the band at 5 eV to Cr+a. Photon
Energy
Ce VI
FIG.2. The absorption coefficient vs. photon energy of an MgO :O.l o/OCrsOs crystal which was fired 5 hr at 1200°C in hydrogen to reduce the iron present. This treatment does not reduce the chromium. The 5.9 eV peak is due to Cr+s.
1200°C. A strong peak at 5.9 eV is observed. This crystal contains some iron although none was intentionally added. Our EPR results show that the iron is reduced to Fe+2 but that the amount of Cr+s present is a maximum after hydrogen firing. The absence of any 4.3 eV absorption confirms that no Fefs is present. The occurrence of this absorption at 5.9 eV which we attribute to Cr+s is close to the 5.7 eV Fe+3 absorption maximum. It may explain the shifts in the peak observed in this region which were reported by SOSHEA etal@) The chromium can be reduced to Cr+s only under unusually strong reducing conditions. EPR measurements show that even firing in magnesium vapor does not reduce an appreciable fraction although such firing will reduce the iron present in the crystal to the metallic state. The presence
absorption spectrum after this treatment. The band at 5 eV which we think is due to Cr+s has not been observed previously by optical absorption. PERIA(~~. 12)reports a peak in the photoconductivity spectrum at 5.05 eV. He states that the charge carriers arising from this center are electrons. This is consistent with the change in valence from Crt2 to Crf3. The positions of the absorption bands associated with these ions are tabulated in Table 1. Table 1 ..___.. Ion Fe+2
Fe+3 Cr+a Cr+3
Optical absorption
!
6.7 eV 4.3 & 5.7 eV 5 eV 5.9 eV
___~
In order to explain the color of the luminescence which is associated with transitions at these four
CORRELATION
OF THERMOLUMINESCENCE
ions we propose the band model shown in Fig. 4. The large Stokes shift between the occupied (Fe+2 and Crfa) and the unoccupied (Fe+3 and Cr+s) levels is the result of the large polarizability of MgO. The excited states of the centers are sufficiently close to the valence and conduction bands that the carriers can be thermally excited into the bands even at room temperature. If we accept the value for the band gap of 8.7 eV given by REILING and HENSLEY(I~) the energy of the photons produced during luminescence is roughly the difference between 8.7 eV and the optical absorption energy. The solid arrows in Fig. 4 indicate the optical absorption energies. The dashed arrows show the electron and hole transitions giving rise to luminescence. With our proposed model in mind we sought out crystals in which one of four reactions dominated the others. The processes which produce the luminescence, and the energy and color of the
IN MgO AND
Fe”
VALENCE
Cr” Cr+t
Fe+’
CHANGES
127
COdl&iO~
FIG. 4. An energy level scheme showing the energy levels introduced in magnesium oxide by di- and trivalent iron and chromium. The solid arrows show the transitions causing optical absorption. The dashed arrows show the transitions which produce luminescence. The trapping states for electrons and holes which can be emptied by heating the crystal are not shown.
Table 2. The concentration of Fe+s and Cr+s in four MgO crystals after various treatments. The arrow shows which change in concentration produced the luminescence indicated in the reaction. Concentration Sample no.
Reaction
Fe+3
--
1.
Fe+3+free electron+ Fef2+2.9 eV (blue)
2.
Fe+“+free hole+ Fe+3+2.0 eV (red)
of
Treatment Cr+3 12 X 1015 cm-3 5 6
Ultraviolet Partial thermoluminescence Complete thermoluminescence
78 x 106 48 J
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak
290 4404
199 194
570 4
206
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak Complete thermoluminescence
78 109
85 74 &
116
69
1015
cm-3
-N_
I 3.
--
Cr+3 +free electron+ Cr+2+2*7 eV (blue)
-1
96
47 I
65 96
256 269 &
92
228
--
Crf2 +free hole+ Cr+s+3.6 eV (blue)
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak 3rd Thermoluminescence peak
129
276 I
128
R.
L.
HANSLER
and
light are shown in Table 2. The concentrations given in the last column of Table 2 are the EPR intensities multiplied by a calibration factor. The calibration factor was obtained by comparing the maximum Fe+a or Cr+s EPR intensity from a given crystal (highly oxidized) with the total amount of Fe or Cr in that crystal as determined by chemical means. Therefore, the numbers in the table are really the upper limits on the concentrations instead of the concentrations. The relative amounts of Fe+s (or of Cr+a), however, are in error by no more than 10 per cent (due to “noise” in the EPR spectrum). The first sample mentioned in Table 2 was of high purity (from melt R-72) and had been treated by passing d.c. through it at a high temperature resulting in a reduction of most of the iron and chromium. During ultraviolet excitation evidently both holes and electrons are freed from iron and chromium and become trapped elsewhere. The glow curves for this crystal are shown in Fig. 5.
FIG. 5. The light as measured by a lP28 photomultiplier as a function of crystal temperature for a uniform heating rate of 0.23”/sec. The (R-72) crystal had been treated by the passage of direct current at high temperature. It was irradiated with 2537 d light before being heated to produce the glow curve A. B was obtained by heating the crystal only to 115” after the irradiation, cooling to room temperature and then heating while recording the light output.
The complete curve (A) with a strong peak at 160°C and a very weak one at 90°C was made by continuous heating at a rate of 0.23”/sec. The crystal was then irradiated again and heated to
W.
G.
SEGELKEN
ll.S”C, cooled to room temperature and the EPR spectrum measured. In Table 2 the resulting changes are recorded after “Partial Thermoluminescence”. Without further irradiation it was then heated to 255°C (curve B), cooled to room temperature and the EPR spectrum measured again. The results are tabulated after “complete thermoluminescence” in Table 2. The Cr+s decreases during the first part of the thermoluminescence while the Fefs increases. This implies that the Cr+a must have a larger capture cross section for electrons and the Fe+2 for holes. During the complete thermoluminescence the Fe+3 picks up most of the electrons liberated at the higher temperature. Evidently the majority of the holes are liberated during the first step. The 90°C peak may be associated with the untrapping of holes. The luminescence would be red corresponding to hole capture by Fe+z. No red luminescence was visible. The relatively intense blue luminescence probably masks it. During the second step the emission consists of a blue band between 2.7 and 3.4 eV, in good agreement with the predicted 2.9 eV. It should be pointed out that the position (temperature) of the glow peaks is only dependent on the nature of the hole and electron traps and the rate of heating. The position is not dependent on the nature of the luminescent center. The second sample, also of high purity but from
0
FIG. 6. The glow curve of a high purity (R-58) untreated MgO crystal. Curve A was made during continuous heating folIowing 2537 A irradiation. Curve B was obtained after a preliminary heating to 110°C to remove the carriers causing the lower temperature peak.
CORRELATION
OF THERMOLUMINESCENCE
a different melt (R-58), was not treated other than to cleave it from a larger piece. The concentrations of both Fe+3 and Cr+s are much higher. Fig. 6 shows the glow curve made in one continuous heating (curve A) and that made to determine the valence changes which occurred during the second peak (curve B). As shown in Table 2, only holes freed from the Fe+3 during ultraviolet excitation and returned during the thermoluminescence appear to be involved. The two peaks in the glow curve suggest the presence of two different types of hole traps. The luminescence consists only of red light in a rather narrow band at 2.0 eV. This is consistent with our model. The third sample was from the same melt as the second (R-58) but was treated by passage of direct
IN MgO AND
VALENCE
CHANGES
129
Only blue luminescence was observed. The slight amount of red which our model predicts we might see during the first peak was probably masked by the relatively intense blue luminescence. The Cr+a concentration dropped during each step. The light emitted during the first peak consists of a broad band from 2.75 to 4.3 eV. The light produced during the highest temperature peak is weaker and extends approximately from 2.75 to 3.45 eV. The fact that the Fe+3 concentration increased during the first peak is evidence for the presence of free holes. If some of these holes are trapped by the Cr+2, we should expect some 3.6 eV photons. The emission during the first peak extending to 4.3 eV might be explained in this way. During the highest temperature peak and at higher temperatures (the spectrum was taken by heating to about 400°C following the heating and
r
T (‘C) FIG. 7. The glow curve of a high purity (R-58)
MgO crystal which had been treated by passage of direct current at high temperature. Curve A was obtained by continuous heating following 2537 A irradiation. Curve B was obtained after a preliminary heating to remove the carriers causing the lower temperature peak.
glow curves for a moderate purity MgO crystal which was treated by oxygen firing one hour at 1200°C. Three curves taken to separate the three types of traps are shown.
current at high temperature. Fig. 7 shows the glow curve obtained with continuous heating (curve A) and that made to check on the changes occurring during the highest temperature peak (curve B). The shoulder on the first peak is evidence that we are not separating all the effects. With the glow curve apparatus it was not possible to obtain temperatures high enough to untrap all the carriers. The last entry in Table 2 under sample 3 gives the concentrations of Fe+3 and Cr+s after heating the crystal on a hot plate at 300-4OO”C for 15 min.
cooling carried out to obtain the spectrum of the first peak) there is little evidence for the presence of holes. The observed spectrum extending from 2.75 to 3.45 eV is thus in fair agreement with the 2.7 eV peak predicted by the model for electron capture by Cr+s. The fourth sample is from a different melt (P-5) and contains a relatively larger amount of chromium. It was heated 1 hr in oxygen at about 1200°C. The glow curves in Fig. 8 are shown for continuous heating (curve A) and for heating
T (“Cl FIG. 8. The
130
R.
L.
HANSLER
and
schedules used to separate the changes occurring during the three peaks (curves B and C). The first and third peaks indicate that hole capture by Cr+a is occurring while the second peak seems to involve electron capture by Cr+s. The thermoluminescence of sample 4 was too weak to be detected by our spectrograph. During thermoluminescence the crystal first appears purple and then red. The purple color is the result of both red and blue emission occurring simultaneously. The red should arise from hole capture by Fe+2 while the blue is the result of hole capture by Cr+a. This is the least convincing of the four experiments, but thus far we have been unable to find any crystal in which the only significant change was hole capture by Crf2. The results of the measurements on these four crystals offer support for the model we are proposing. Measurements on many other crystals have failed to produce any effect which is not satisfactorily explained in terms of this model. We have thus far said nothing about the exact
W.
G.
SEGELKEN
nature of the hole and electron traps. At least two optical absorption bands have been observed (2.3 eV and 3.6 eV) which we may attribute to trapped holes and electrons. Fig. 9 shows the optical absorption spectrum of a moderately high purity (P-5) neutron-irradiated sample. Peaks at 1.3, 2.2, 3.0 and 3.5 eV are seen. An analysis of the glow peaks of the type described by RANDALL and WILKINS in which we assume no retrapping, gives values for the activation energy of 0.2-l eV and frequency factors of 105 to 1012 set-1. Such calculations are probably of little significance, however, for there is no reason to believe that retrapping does not occur. The complexity of the glow curves indicates that at least four types of traps exist. Magnesium vacancies should trap either one or two holes and thus give rise to two types of trapping centers. The observation by oRTON of Fe+1 in crystals which have been ultraviolet irradiated makes Fe+2 a good possibility for an electron trap. The identification of the traps with the optical absorption bands is one of the attractive problems remaining in the study of magnesium oxide. 4. SUMMARY 1. Optical
I-
% ,-
7
6
5
4
3
Photon
Energy
(ev)
2
I
FIG. 9. The absorption coefficient vs. photon energy for a neutron irradiated moderate purity (P-5) MgO crystal. The integrated neutron dose was 1.2 f 0.2 x 10s” nvt.
absorption studies have shown the position of the energy levels due to Fe+a, Fe+s, Crfa and Cr+s. 2. By assuming the band gap in MgO is 8.7 eV (after bILING(13)) the energy of the luminescence due to recombination of free holes with Fe+2 and Cr+a and free electrons with Fe+s and Cr+s is predicted. 3. EPR studies have made it possible to select crystals in which only one of the four processes takes place during some part of the thermoluminescence which is carried on after excitation with 4.9 eV (2537 A) photons. 4. The spectrum of the light emitted during this one process has been measured for each of the four cases and compared with that predicted by the model. The agreement is good. 5. The thermoluminescent glow curves offer information about the hole and electron traps. No identification of the traps has been made. Acknowledgements-It is our pleasure to thank our colleagues in the Lamp Research Laboratory for their contributions to this paper. We also wish to express
CORRELATION
OF THERMOLUMINESCENCE
our appreciation to the groups at the University of Minnesota and the University of Missouri who have contributed so much to the knowledge of magnesium oxide. REFERENCES 1. BUBE H. and STIPP K. F., J. Chem. Phys. 20, 193 (1952). 2. PR&ER J. S., J. Chem. Phys. 21, 160 (1953). 3. YtwAuE.,Phvs.Rev.%.293(1954). 4. SAKSENAB: D: and PAN; L. &I., &c. Phys. Sac., Land. 67,811 (1954). 5. SAKSENAB. D. and PANT L. M., Curr. Sci. 23, 393 (1954). 6. SAKSENAB. D. and PANT L. M., J. Chem. Phys. 23, 989 (1955).
IN MgO AND
VALENCE
CHANGES
131
7. HAXBY B. V., A Study of Color Centers in Magnesium Oxide, Ph.D. Thesis, Univ. of Minn. (1957). 8. SOSHEAR. W., DEKKER A. J. and STURTZ J. P., J. Phys. Chem. Solids 5, 23 (1958). 9. WERTZ J. E. and AUZINS P., Phys. Rev. 106, 484 (1957). 10. WERTZ J. E., AUZINS P., GRIFFITH~ J. H. E. and ORTON J. W., Disc. Faraday Sot. 26, 66 (1958). 11. PERIA W. T., Optical Absorption and Photoconductivity in M&g&urn Oxide Crystals, Ph.D. Thesis, Univ. of British Columbia (1957). 12. PERIA W. T., Phys. Rev. 112,4i3 (1458). 13. REILING G. H. and HENSLEY E. B., Phys. Rev. 112, 1106 (1958). 14. RANDALLJ. T. and WILKINS M. H. F., Proc. Roy. Sot. A184, 366 (1945). 15. ORTONJ., private communication.
Pergamon
CORRELATION AND
Press 1960. Vol. 13. pp. 124-131.
OF
Printed in Great Britain.
THERMOLUMINESCENCE
IN
MgO
VALENCE CHANGES OF IRON AND CHROMIUM IMPURITIES DETECTED BY EPR R. L. HANSLER Lamp Research Laboratory,
and W. G. SEGELKEN
General Electric
Company,
Nela Park, Cleveland 12, Ohio
(Received 9 October 19.59)
Abstract-EPR measurements show that during 2537 A irradiation the valence of chromium and iron impurities (~10 p.p.m.) in MgO single crystals changes from 2 to 3 or vice versa dependent on treatment. The reverse valence changes as well as thermoluminescence are observed during subsequent heating. It was shown by optical absorption that the energy levels of Fe+s and Cr+slie about 5.8 and 6.0 eV respectively above the valence band while the levels of Fe+2 and Cr+2 are respectively 6.9 and 5.1 below the conduction band. The possible luminescent processes are: Fe+3 Fe+2 Cr+3
Cr+s
+ free f free + free + free
electron hole electron hole
--f + --f +
Fe+2 Fe+s Crfs Cr+3
+ 2.9 eV; + 2.0eV; + 2.7 eV; + 3.6 eV.
Four crystals were studied each of which showed preponderantly only one of the above luminescent processes. By assuming the band gap is 8.7 eV, good agreement between the color of the observed thermoluminescence and that predicted by the model was obtained.
1. INTRODUCTION THE luminescence of MgO when excited in various ways has been the subject of a number of studies.(l-6) Irradiation with X-rays or ultraviolet or heating in oxygen or magnesium vapor will produce optical absorption bands in MgO crystals which contain no intentionally added foreign ions. Until recently it was assumed that both the luminescence and the optical absorption were associated with defects in the host lattice other than foreign ions. HABY and SOSHEA et aZ.(s) have recently presented evidence that certain of the ultraviolet absorption bands are due to Fe+a. We support this hypothesis. WERTZ et aZ.@,10) have evidence for changes in valence of various foreign ions in MgO during X-irradiation and other treatments. We wish to present evidence that certain luminescent effects in relatively high purity MgO are due to the transfer of electrons (and holes) between the iron and chromium impurities and depend on 124
defects such as ion vacancies manner. 2. EXPERIMENTAL
only in an indirect
PROCEDURE
Crystals studied Most of the single crystals used in this investigawere grown in this laboratory by L. J. SCHUPP by the arc melt method. In our highest purity crystals (designated R-58 and R-72) the chief impurity is calcium (<0*05 per cent)* which probably has little effect because of its chemical similarity to magnesium. The following were spectroscopically detected in amounts less than 0.01 per cent: Si, Al, Fe, Ti, Ni, Zn, Sr, Na, K. Using other methods, R-58 crystals were found to contain approximately 5 p.p.m. Sr, 8 p.p.m. Fe, 3 p.p.m. Li and 5 p.p.m. Cr. * The concentrations of impurities are given either as wt. ‘/, or p.p.m. by weight.
CORRELATION
OF THERMOLUMINESCENCE
measure?nents Mn+s, V+s, Fe+3 and Cr+s ions in MgO give readily observable peaks in the EPR absorption spectrum. We have observed the changes in concentration of these ions in a number of crystals subjected to various treatments. Quantitative data were obtained for the concentrations of Cr+s and Fe+3 in some crystals subjected to certain treatments. The EPR spectra were taken with a Varian spectrometer and &in magnet. The crystals were cemented to a quartz rod and oriented so that I& was parallel to the [loo] axis and &I was parallel to the [OlO] axis. A CuSO4 * 5HsO sample was placed in the cavity and the Cu+s absorption line used as a relative intensity standard. Only one line of each of the complex spectra of Cr+s and Fe+3 was used for the quantitative intensity measurements. For Cr+s the “cubic” line was used and for Fe+s the central line of the five line spectrum was used.@@
IN MgO AND
VALENCE
CHANGES
125
TATION
Preliminary results of EPR measurements on a number of samples suggested that neither the manganese nor vanadium were directly involved in the observed thermoluminescence. The amount of Mn+a remained remarkably constant under quite a variety of treatments. No correlation seemed to exist either between the amount of V+2 present and the luminescence or between the valence change of the vanadium and the luminescence.
Optical absorption measurements A Beckman model DU spectrophotometer was used for most of the me~urements. After excitation with ultraviolet it was sometimes necessary to check to make sure that long-lived phosphorescence did not interfere with the measurement. The measurements in the vacuum ultraviolet were made by Dr. P. D. JOHNSON at the General Electric Research Laboratory, Schenectady, New York.
Most of the measurements were of a qualitative type in which the color of the thermoluminescence was noted when the crystal was placed on a hot plate at about 300-400°C after being irradiated with light from a 4-W germicidal lamp.* A small quartz spectrograph was used to measure the emission in a few cases. G&u curves Thermoluminescent glow curves were obtained for a few crystals. The crystals were heated at a uniform rate and the light output measured with a lP28 photomultiplier. * This low pressure mercury discharge lamp produces principally 2537 A (4.9 eV) radiation.
Photon Energy fe Vi
FIG. 1. The absorption coefficient vs. photon energy of an MgO :3 %FesOs crystal which was tired 6 hr at 1100°C in hydrogen in order to reduce the iron to the divalent state. The sharp rise in absorption at about 6.7 eV we attribute to the Fef2.
Depending on the previous treatment the amount of Fe+s could either be increased or decreased during ultraviolet irradiation and showed the reverse change during thermoluminescence. The same was observed with Cr+3. These preliminary results suggested the luminescence might be correlated with these changes in valence. The positions of the energy levels of Fe*s, Fe+s, Cr+s and Cr+s in the MgO lattice were determined by optical absorption. HAXBY@) showed the correlation between the amount of Fe-@ in the lattice and the 4.3 and 5-7 eV absorption peaks. We found that by hydrogen firing an MgO :3OkF15&~ t crystal for six hr at 1100°C all of iThis notation means, “grown from a melt to which 3 per cent by weight FesOs was added”. The amount of foreign ion in the crystals usually is about one-tenth of rhis amount.
126
R.
L.
HANSLER
and
the 4.3 and 5.7 eV absorption could be removed. Fig. 1 shows the absorption edge at approximately 6.7 eV which we attribute to Fe+s. That the iron was not removed by the hydrogen firing but merely reduced was shown by observing the return of the 4.3 and 5.7 eV absorption after heating the crystal briefly in air. Fig. 2 shows the absorption of an MgO: 0*10j,CrsOs crystal after hydrogen firing 5 hr at
W.
G.
SEGELKEN
of metallic iron is inferred from valence studies carried out by R. P. TAYLOR of this laboratory using wet chemical methods. It is also suggested by the fact that crystals containing more than 100 p.p.m. of iron are attracted by a magnet after firing in Mg vapor. We have been able to produce Cr+s in high purity MgO crystals (total chromium about 3 p.p.m.) by passing direct current through the crystals at high temperature. Fig. 3 shows the
Photon
Energy
(ev)
FIG. 3. The absorption coefficient vs. photon energy for a high purity MgO crystal which has been treated by the passage of direct current at high temperature for a few minutes. We attribute the band at 5 eV to Cr+a. Photon
Energy
Ce VI
FIG.2. The absorption coefficient vs. photon energy of an MgO :O.l o/OCrsOs crystal which was fired 5 hr at 1200°C in hydrogen to reduce the iron present. This treatment does not reduce the chromium. The 5.9 eV peak is due to Cr+s.
1200°C. A strong peak at 5.9 eV is observed. This crystal contains some iron although none was intentionally added. Our EPR results show that the iron is reduced to Fe+2 but that the amount of Cr+s present is a maximum after hydrogen firing. The absence of any 4.3 eV absorption confirms that no Fefs is present. The occurrence of this absorption at 5.9 eV which we attribute to Cr+s is close to the 5.7 eV Fe+3 absorption maximum. It may explain the shifts in the peak observed in this region which were reported by SOSHEA etal@) The chromium can be reduced to Cr+s only under unusually strong reducing conditions. EPR measurements show that even firing in magnesium vapor does not reduce an appreciable fraction although such firing will reduce the iron present in the crystal to the metallic state. The presence
absorption spectrum after this treatment. The band at 5 eV which we think is due to Cr+s has not been observed previously by optical absorption. PERIA(~~. 12)reports a peak in the photoconductivity spectrum at 5.05 eV. He states that the charge carriers arising from this center are electrons. This is consistent with the change in valence from Crt2 to Crf3. The positions of the absorption bands associated with these ions are tabulated in Table 1. Table 1 ..___.. Ion Fe+2
Fe+3 Cr+a Cr+3
Optical absorption
!
6.7 eV 4.3 & 5.7 eV 5 eV 5.9 eV
___~
In order to explain the color of the luminescence which is associated with transitions at these four
CORRELATION
OF THERMOLUMINESCENCE
ions we propose the band model shown in Fig. 4. The large Stokes shift between the occupied (Fe+2 and Crfa) and the unoccupied (Fe+3 and Cr+s) levels is the result of the large polarizability of MgO. The excited states of the centers are sufficiently close to the valence and conduction bands that the carriers can be thermally excited into the bands even at room temperature. If we accept the value for the band gap of 8.7 eV given by REILING and HENSLEY(I~) the energy of the photons produced during luminescence is roughly the difference between 8.7 eV and the optical absorption energy. The solid arrows in Fig. 4 indicate the optical absorption energies. The dashed arrows show the electron and hole transitions giving rise to luminescence. With our proposed model in mind we sought out crystals in which one of four reactions dominated the others. The processes which produce the luminescence, and the energy and color of the
IN MgO AND
Fe”
VALENCE
Cr” Cr+t
Fe+’
CHANGES
127
COdl&iO~
FIG. 4. An energy level scheme showing the energy levels introduced in magnesium oxide by di- and trivalent iron and chromium. The solid arrows show the transitions causing optical absorption. The dashed arrows show the transitions which produce luminescence. The trapping states for electrons and holes which can be emptied by heating the crystal are not shown.
Table 2. The concentration of Fe+s and Cr+s in four MgO crystals after various treatments. The arrow shows which change in concentration produced the luminescence indicated in the reaction. Concentration Sample no.
Reaction
Fe+3
--
1.
Fe+3+free electron+ Fef2+2.9 eV (blue)
2.
Fe+“+free hole+ Fe+3+2.0 eV (red)
of
Treatment Cr+3 12 X 1015 cm-3 5 6
Ultraviolet Partial thermoluminescence Complete thermoluminescence
78 x 106 48 J
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak
290 4404
199 194
570 4
206
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak Complete thermoluminescence
78 109
85 74 &
116
69
1015
cm-3
-N_
I 3.
--
Cr+3 +free electron+ Cr+2+2*7 eV (blue)
-1
96
47 I
65 96
256 269 &
92
228
--
Crf2 +free hole+ Cr+s+3.6 eV (blue)
Ultraviolet 1st Thermoluminescence peak 2nd Thermoluminescence peak 3rd Thermoluminescence peak
129
276 I
128
R.
L.
HANSLER
and
light are shown in Table 2. The concentrations given in the last column of Table 2 are the EPR intensities multiplied by a calibration factor. The calibration factor was obtained by comparing the maximum Fe+a or Cr+s EPR intensity from a given crystal (highly oxidized) with the total amount of Fe or Cr in that crystal as determined by chemical means. Therefore, the numbers in the table are really the upper limits on the concentrations instead of the concentrations. The relative amounts of Fe+s (or of Cr+a), however, are in error by no more than 10 per cent (due to “noise” in the EPR spectrum). The first sample mentioned in Table 2 was of high purity (from melt R-72) and had been treated by passing d.c. through it at a high temperature resulting in a reduction of most of the iron and chromium. During ultraviolet excitation evidently both holes and electrons are freed from iron and chromium and become trapped elsewhere. The glow curves for this crystal are shown in Fig. 5.
FIG. 5. The light as measured by a lP28 photomultiplier as a function of crystal temperature for a uniform heating rate of 0.23”/sec. The (R-72) crystal had been treated by the passage of direct current at high temperature. It was irradiated with 2537 d light before being heated to produce the glow curve A. B was obtained by heating the crystal only to 115” after the irradiation, cooling to room temperature and then heating while recording the light output.
The complete curve (A) with a strong peak at 160°C and a very weak one at 90°C was made by continuous heating at a rate of 0.23”/sec. The crystal was then irradiated again and heated to
W.
G.
SEGELKEN
ll.S”C, cooled to room temperature and the EPR spectrum measured. In Table 2 the resulting changes are recorded after “Partial Thermoluminescence”. Without further irradiation it was then heated to 255°C (curve B), cooled to room temperature and the EPR spectrum measured again. The results are tabulated after “complete thermoluminescence” in Table 2. The Cr+s decreases during the first part of the thermoluminescence while the Fefs increases. This implies that the Cr+a must have a larger capture cross section for electrons and the Fe+2 for holes. During the complete thermoluminescence the Fe+3 picks up most of the electrons liberated at the higher temperature. Evidently the majority of the holes are liberated during the first step. The 90°C peak may be associated with the untrapping of holes. The luminescence would be red corresponding to hole capture by Fe+z. No red luminescence was visible. The relatively intense blue luminescence probably masks it. During the second step the emission consists of a blue band between 2.7 and 3.4 eV, in good agreement with the predicted 2.9 eV. It should be pointed out that the position (temperature) of the glow peaks is only dependent on the nature of the hole and electron traps and the rate of heating. The position is not dependent on the nature of the luminescent center. The second sample, also of high purity but from
0
FIG. 6. The glow curve of a high purity (R-58) untreated MgO crystal. Curve A was made during continuous heating folIowing 2537 A irradiation. Curve B was obtained after a preliminary heating to 110°C to remove the carriers causing the lower temperature peak.
CORRELATION
OF THERMOLUMINESCENCE
a different melt (R-58), was not treated other than to cleave it from a larger piece. The concentrations of both Fe+3 and Cr+s are much higher. Fig. 6 shows the glow curve made in one continuous heating (curve A) and that made to determine the valence changes which occurred during the second peak (curve B). As shown in Table 2, only holes freed from the Fe+3 during ultraviolet excitation and returned during the thermoluminescence appear to be involved. The two peaks in the glow curve suggest the presence of two different types of hole traps. The luminescence consists only of red light in a rather narrow band at 2.0 eV. This is consistent with our model. The third sample was from the same melt as the second (R-58) but was treated by passage of direct
IN MgO AND
VALENCE
CHANGES
129
Only blue luminescence was observed. The slight amount of red which our model predicts we might see during the first peak was probably masked by the relatively intense blue luminescence. The Cr+a concentration dropped during each step. The light emitted during the first peak consists of a broad band from 2.75 to 4.3 eV. The light produced during the highest temperature peak is weaker and extends approximately from 2.75 to 3.45 eV. The fact that the Fe+3 concentration increased during the first peak is evidence for the presence of free holes. If some of these holes are trapped by the Cr+2, we should expect some 3.6 eV photons. The emission during the first peak extending to 4.3 eV might be explained in this way. During the highest temperature peak and at higher temperatures (the spectrum was taken by heating to about 400°C following the heating and
r
T (‘C) FIG. 7. The glow curve of a high purity (R-58)
MgO crystal which had been treated by passage of direct current at high temperature. Curve A was obtained by continuous heating following 2537 A irradiation. Curve B was obtained after a preliminary heating to remove the carriers causing the lower temperature peak.
glow curves for a moderate purity MgO crystal which was treated by oxygen firing one hour at 1200°C. Three curves taken to separate the three types of traps are shown.
current at high temperature. Fig. 7 shows the glow curve obtained with continuous heating (curve A) and that made to check on the changes occurring during the highest temperature peak (curve B). The shoulder on the first peak is evidence that we are not separating all the effects. With the glow curve apparatus it was not possible to obtain temperatures high enough to untrap all the carriers. The last entry in Table 2 under sample 3 gives the concentrations of Fe+3 and Cr+s after heating the crystal on a hot plate at 300-4OO”C for 15 min.
cooling carried out to obtain the spectrum of the first peak) there is little evidence for the presence of holes. The observed spectrum extending from 2.75 to 3.45 eV is thus in fair agreement with the 2.7 eV peak predicted by the model for electron capture by Cr+s. The fourth sample is from a different melt (P-5) and contains a relatively larger amount of chromium. It was heated 1 hr in oxygen at about 1200°C. The glow curves in Fig. 8 are shown for continuous heating (curve A) and for heating
T (“Cl FIG. 8. The
130
R.
L.
HANSLER
and
schedules used to separate the changes occurring during the three peaks (curves B and C). The first and third peaks indicate that hole capture by Cr+a is occurring while the second peak seems to involve electron capture by Cr+s. The thermoluminescence of sample 4 was too weak to be detected by our spectrograph. During thermoluminescence the crystal first appears purple and then red. The purple color is the result of both red and blue emission occurring simultaneously. The red should arise from hole capture by Fe+2 while the blue is the result of hole capture by Cr+a. This is the least convincing of the four experiments, but thus far we have been unable to find any crystal in which the only significant change was hole capture by Crf2. The results of the measurements on these four crystals offer support for the model we are proposing. Measurements on many other crystals have failed to produce any effect which is not satisfactorily explained in terms of this model. We have thus far said nothing about the exact
W.
G.
SEGELKEN
nature of the hole and electron traps. At least two optical absorption bands have been observed (2.3 eV and 3.6 eV) which we may attribute to trapped holes and electrons. Fig. 9 shows the optical absorption spectrum of a moderately high purity (P-5) neutron-irradiated sample. Peaks at 1.3, 2.2, 3.0 and 3.5 eV are seen. An analysis of the glow peaks of the type described by RANDALL and WILKINS in which we assume no retrapping, gives values for the activation energy of 0.2-l eV and frequency factors of 105 to 1012 set-1. Such calculations are probably of little significance, however, for there is no reason to believe that retrapping does not occur. The complexity of the glow curves indicates that at least four types of traps exist. Magnesium vacancies should trap either one or two holes and thus give rise to two types of trapping centers. The observation by oRTON of Fe+1 in crystals which have been ultraviolet irradiated makes Fe+2 a good possibility for an electron trap. The identification of the traps with the optical absorption bands is one of the attractive problems remaining in the study of magnesium oxide. 4. SUMMARY 1. Optical
I-
% ,-
7
6
5
4
3
Photon
Energy
(ev)
2
I
FIG. 9. The absorption coefficient vs. photon energy for a neutron irradiated moderate purity (P-5) MgO crystal. The integrated neutron dose was 1.2 f 0.2 x 10s” nvt.
absorption studies have shown the position of the energy levels due to Fe+a, Fe+s, Crfa and Cr+s. 2. By assuming the band gap in MgO is 8.7 eV (after bILING(13)) the energy of the luminescence due to recombination of free holes with Fe+2 and Cr+a and free electrons with Fe+s and Cr+s is predicted. 3. EPR studies have made it possible to select crystals in which only one of the four processes takes place during some part of the thermoluminescence which is carried on after excitation with 4.9 eV (2537 A) photons. 4. The spectrum of the light emitted during this one process has been measured for each of the four cases and compared with that predicted by the model. The agreement is good. 5. The thermoluminescent glow curves offer information about the hole and electron traps. No identification of the traps has been made. Acknowledgements-It is our pleasure to thank our colleagues in the Lamp Research Laboratory for their contributions to this paper. We also wish to express
CORRELATION
OF THERMOLUMINESCENCE
our appreciation to the groups at the University of Minnesota and the University of Missouri who have contributed so much to the knowledge of magnesium oxide. REFERENCES 1. BUBE H. and STIPP K. F., J. Chem. Phys. 20, 193 (1952). 2. PR&ER J. S., J. Chem. Phys. 21, 160 (1953). 3. YtwAuE.,Phvs.Rev.%.293(1954). 4. SAKSENAB: D: and PAN; L. &I., &c. Phys. Sac., Land. 67,811 (1954). 5. SAKSENAB. D. and PANT L. M., Curr. Sci. 23, 393 (1954). 6. SAKSENAB. D. and PANT L. M., J. Chem. Phys. 23, 989 (1955).
IN MgO AND
VALENCE
CHANGES
131
7. HAXBY B. V., A Study of Color Centers in Magnesium Oxide, Ph.D. Thesis, Univ. of Minn. (1957). 8. SOSHEAR. W., DEKKER A. J. and STURTZ J. P., J. Phys. Chem. Solids 5, 23 (1958). 9. WERTZ J. E. and AUZINS P., Phys. Rev. 106, 484 (1957). 10. WERTZ J. E., AUZINS P., GRIFFITH~ J. H. E. and ORTON J. W., Disc. Faraday Sot. 26, 66 (1958). 11. PERIA W. T., Optical Absorption and Photoconductivity in M&g&urn Oxide Crystals, Ph.D. Thesis, Univ. of British Columbia (1957). 12. PERIA W. T., Phys. Rev. 112,4i3 (1458). 13. REILING G. H. and HENSLEY E. B., Phys. Rev. 112, 1106 (1958). 14. RANDALLJ. T. and WILKINS M. H. F., Proc. Roy. Sot. A184, 366 (1945). 15. ORTONJ., private communication.