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Cathodoluminescence spectra of pure LiF
Nuclear
Instruments
and Methods
in Physics Research
Nuclear Instruments & Methods in Physics Research Sk,1 IIWR
B6S (1092) 407-501
North-Holland
Cathodoluminescence B. Yang
of pure LiF
‘, B.J. Luff and P.D. Townsend
(CL) of pure LiF and H + ion-implanted
Cathodoluminescence
K.
spectra
Emission intensities vary with temperature
broad band consisting of
and the electron
several peaks at X%500
nm were observed
samples. The present studies support the identification recombination recombining
of the with the
FT F,
that the h70
centres, and the 300-500
address: Department Beijing
016X-S1;3X/Y2/$OS.O0
range SO to 300
the emission hands at 670 nm. 530 nm and a
at all temperatures
nm band is the emision
in both implanted
band of F, centres
and unimplanted produced
by the
ccntres resulting from the holes
nm band is associated with the intrinsic emission of LiF.
It has been rcportcd in the past few dccadcs that many of the luminescence processes observed in alkali halides over a wide temperature range arc related to the recombination of radiation-induced lattice defects [ 1-31. The most studied material is probably the variety of LiF manufactured by Harshaw for radiation dosimctry. Typical of the reported rcscarch is an attempt to correlate the absorption of point defects (colour centres) and the thermoluminescence (TL) peaks by Mayhugh [2]. comparison of growth rate with y-ray dose of the F-center absorption and of the TL intensity by Ehrlich [3] and mcasurcmcnts of the TL spectrum in crystals produced under a variety of different growth conditions by Townsend [4], but very little research has been done on pure LiF. Models of luminescent proccsscs given in thcsc reports are mostly associated with the doped impurities in LiF crystals. although intrinsic luminescence has been reported as well, c.g. the study of V, + c emissions in LiF by Cooke [S] and Jain [h] and the investigation of the exciton emissions in NaCl by Aguilar et al. [7]. It has been reported that the number of emission bands is reduced and the intensities arc wcakcr in pure LiF [l]. The emission bands at 320, 340 and 415 nm and a strong band at 663 nm found in heavily exposed (IO’ Gy) samples were reported by Sagastibclza et al. [X] in their TL study of pure LiF crystals. It was suggested that this complex emission spectrum of pure LiF might bc due to the existence of a wide variety of background impurities.
University.
However,
centres with the electrons; the 530 nm band is the emission hand of F:
1. Introduction
’ Permanent
pure LiF has been studied over the temperature
beam dose.
Cathodolumincsccncc (CL) spectra of pure LiF and H ’ ion-implanted pure LiF are studied in this work. Both intrinsic and nonintrinsic CL emissions have been observed, and models of the luminescent proccsscs producing thcsc emission bands arc proposed. An important role for F-aggrcgatc ccntrcs in nonintrinsic CL emissions in pure LiF is suggested.
2. Experimental Samples of pure and H ’ ion-implanted pure LiF were examined over the tcmpcrature range SO-300 K. The pure samples wcrc obtained from the University of Utah, USA, and from the Shanghai Institute of Optical Instruments. China: H + implantation was carried out on the pure samples at an energy of 3 McV to
10000
1
,
I
9000
i
‘(
of Physics, Beijing Normal
100875, China.
i‘ 1092
Elsevier
Science Publkhers
B.V. All rights reserved
VIII.
ALKALI
HALIDES
Fig. 2. Cl intensity of H
’ ion-implanted pure LiF vs wavelength and temperature whilst cooling from
;I final dose of 10” ions/cm’. C~rthodoluminescence was cxcitcd using a 2 mm diamctcr clcctron beam of cncrgy 2-20 keV with beam currents of 0.01 to 3 PA. Spectral information was ohtaincd using an f/4 scanred-scnsitivc photomultiplicr ning monochromator, tube, and lock-in amplifier: modulation of the exciting electron beam over a range of frequencies was carried out by using a parallel-plate capacitor inserted into the beam column. Measuremcnts at low tcmpcrature were made by mounting the sample on ;I cold stage of a closed-cycle cryostat. All spectra presented hcrc XC corrcctcd for the response of the whole detection systcm. Detailed spectra dcconvolution was not attempted but as seen in figs. I and 2 the major bands arc clearly resolved. Further clarity is provided by observations of temperature. dose and lifctimc depcndenciea.
3. Results and discussion Fig. 1 shows the CL spectrum of pure LiF at 300. I90 and 60 K. A strong red band pcakcd at 670 nm and a wcakcr band around 530 nm arc obscrvcd. A broad band at 300-500 nm which probably consists of scvcral overlapping peaks. e.g., at 325. 350, 370, 410 and 450 nm is also observed. The intensities of these peaks arc strongly dependent upon the irradiation history and temperature, c.g. those at 325 and 350 nm arc atrongcr at higher temperature. but the 450 nm band can only
bc dctcctcd at low tcmpcraturcs. Another broad band in the near-infrared region, which partially overlaps the 670 nm band, is obscrvcd at room temperature, but the peak position of this band cannot bc dctcrmincd by our instrument. CL spectra from H + ion-implanted pure LiF wcrc also rccordcd. The number and positions of the CL peaks arc the same as those in the unimplanted samplcs. The Cl intensities of thcsc two kind of samples arc nearly cqual at room tempcraturc, but at low tcmpcraturc the intensity of the 070 nm band in the implanted sample is obviously much stronger. A three dimensional diagram of the CL intensity of H ’ ion-implanted LiF vb wavclcngth and temperature whilst cooling from 300 to 50 K is shown in fig. 2. The relation bctwccn emission intensity and irradiation dose is investigated at diffcrcnt tcmpcraturcs with various incident clcctron beam currents bctwccn 0.01 and 3 FA and various irradiation times of up to one hour. Complex bchaviour occurs for all the CL bands in both implanted and unimplanted samples. In gcneral, the intensity of cvcry emission band increases with increasing beam current and dccrcases with the irradiation time. But sometimes the intensity shows 21 small increase in the first 5 or IO min if the sample is irradiated with a low beam current (< 0.4 FA). Examplcs of thcsc results arc given in fig. 3. A graph of CL intensity vs temperature is shown in fig. 4. Mcnsurcmcnts have been performed during a
Fig. 3. Cl intensitie
time:
of 370. 530 and 670 nm bands in LiF aa a function of irradiation
the
incident electron energy is IO keV.
the beam current is 0.3 uA. (a) T = 300 K, (b) T = 50 K.
cooling cycle from RT to SO K (a), and a warming cycle from SO K to RT 0~). At the start of each cycle a position on the crystal which had not been previously irradiated was selected to rcducc the influence of irradiation history. The mcasurcd temperature is the tcmperature of the sample holder block. It is likely that thcrc is a temperature difference bctwccn the emission surface of the sample and the sample block as LiF is not a good thermal conductor and fairly thick (2 mm) samples were used. Therefore the measured temperatures during heating and cooling cycles may differ. For example, in fig. 4a they may be lower than the temperature of the emission surface of the sample, but in fig. 4b we would cxpcct the latter to be lower. Note. howcvcr, that the apparent differcnccs can also disguise dose dependent fcaturcs and differences in dc-
feet structures formed during the temperature cycle. The following observations are made from these measurements: ti) Below 240 K, the 670 nm emission increases as the temperature decreases: there is a slow increase from 240 to I10 K and a rapid increase below I IO K. (ii) The 530 nm emission bccomcs more intcnsc as the sample is cooled from 240 to 200 K; it rcachcs its highest intensity at about 200 K, and then decreases. (iii) In fig. 4a, the 670 nm and 530 nm emission bands decrease in intensity during a cooling cycle from RT to 240 K, but opposite results arc shown in fig. 4b for a warming cycle. This diffcrcncc may be caused by the cffcct of irradiation dose. (iv) Because of the considerable overlapping of emission bands, it is very difficult to make a detailed
15r
15
12r
LJ’
2 3r
0
~~
50
I
~__J 100
Fig. 4. Cl intensities
150 'Sempcrature
200 (K
)
250
0
300
L_
50
IO0
\
'-. _._ __ 150
200
Temperature
of 530 and 670 nm emission hands in
LiF vs temperature
250
300
350
(K )
during a cooling cycle from
RT to SO K (a), and a
warming cycle from SO K to RT (b).
VIII. ALKALI HALIDES
study of emission bands in the range 300-500 nm. The main observation is that the total intensity of the whole broad band reachcs its maximum value at about 200 K and becomes relatively lower at both RT and low tcmpcraturcs (below 100 K). Similar investigations wcrc made on the H ’ ion-implanted LiF sample. The only diffcrcncc bctwccn this sample and the previous sample is that the rate of change of the intensity of the 670 nm band with tcmpcraturc is grcatcr in the implanted sample. The optical absorption spectrum of unimplantcd LiF which has been irradiated by the electron beam for two hours was recorded in the ultraviolet and visible ranges at room tcmpcraturc. The strong absorption bands of F and F, ccntrcs, as well as the wjcak F, and F4 bands wcrc detected.
This is the emission band of the F, centrc in LiF. The F, centrc is also charactcrizcd by absorption at 450 nm [Y]. When the LiF sample is irracliatcd by the high energy clcctron beam, irradiation dcfccts would be produced. Some of thcsc defects. such as F, F,. F,. F4 ccntrcs can easily bc detected by room tcmpcraturc optical absorption mcasurcmcnts. whcrcas some other dcfccts; e.g., Ft. F, , F; . Fj- and various V ccntres arc difficult to dctcct by this method, bccausc they are unstable at room tcmpcrnture or exhibit obviously overlapping absorption bands. In some casts the peak position is beyond the detection range of the instruments [Y]. Such colour ccntrcs undoubtedly play complex and important roles in the CL proccsscs. Thcrc arc several possible lumincsccnt mechanisms that could give rise to the FI emission: (I ) the rccombination of a rclcascd electron with an Fi ccntrc: (2) F absorption band excitation: (3) recombination of a rcleased hole with an F, ccntrc; (3) F, absorption band excitation. (I) F; ccntrcs in LiF arc thermally stable at tcmperatures below 230 K. From fig. 3a, the intensity of 670 nm emission starts increasing near 220 K. From this tempcraturc the concentration of Fi ccntrcs gradually incrcascs with decreasing tempcraturc and increasing irradiation dose. Below I30 K the stability of the V, ccntrc increases the probability of a released clcctron recombining with an Fi ccntrc, as does the impurity stabilized H center when it is below I IO K. Therefore. with this first model, the increasing hole stability would lcad to a rapid increase of 670 nm emission. Howcvcr, the change in signal intensity near I IO K can arise from a variety of factors. It is not only the tcmpcrature region where the impurity stabilized H ccntrcs anncal but also there is cvidcnce that an electron trap empties in this tcmperaturc range since in an early paper on TL of LiF [4] it was found that ;I TL peak near I I5 K could bc rcgencratcd by illumina-
tion in the F band region. Electron relcasc was implied via this TL cxpcrimcnt whcrcas hole release is associatcd with H ccntre annealing. By cithcr charge mechanism it is expected that CL efficiency will bc altered. probably with enhanced F, emission. For cxamplc, holes incrcasc the conversion of F, to F; , and clcctron capture produces the rcvcrsc rcacfion and hcncc F, luminesccncc. (2) It has been suggcstcd previously [IO] that F band excitation may cause F, band emission. Cooke [5] and Wayne [I I] rcportcd in their TL study of LiF that the direct recombination of a hole trapped at a V, centrc with a thermally relcascd electron would produce the 270 nm emission at IX- I60 K. Presumably, this cmission occurs at any tcmpcraturc below the V, ccntrc’s decay temperature in the cast of CL. Jain [h] rcportcd that from 200 to 350 nm many TL glow peaks wcrc obscrvcd at the temperatures below I40 K. The F ccntrc absorption band in LiF is at 250 nm. This implies that the emissions dcscribcd above will cxcitc the F band. This is therefore one possible explanation for the very sharp increase of the 670 nm CL emission band with decreasing tcmpcraturc below I IO K. (3) A totally different viewpoint may bc that the cxpccted high CIA intensity is dcprcsscd from 100-250 K because of the hole rclcasc from H and V type centres in the tcmpcraturc range. Furthermore WC can conclude that a rclcascd hole recombining with an F2 ccntre is not involved in the 670 nm emission ob\crvcd in the prcscnt work. A strong 663 nm TL band in pure LiF was obscrvcd by Sagastibclza et al. [Xl. but they suggested that this is due to the background impurity in the samples. They could only obscrvc this band in heavily irradiated (10’ Gy) samples and its maximum intensity was obtained at about 470 K. We presume this is the same band as the 670 nm F, emission band obscrvcd in the CL spectrum. One possible lumincscent process is the following: a thermally rclcascd hole an cxcitcd recombining with an F, ccntrc produces state F2 ccntrc and then :I 1.X4 cV photon is cmittcd with the transition from the cxcitcd state to the ground state. This suggestion is supported by the charactcristics of the 663 nm TL band; that is the fact that it could only be observed in the heavily exposed samples indicates it is associated with F-aggrcgatc ccntrcs and 370 K is near the decay temperature of the F, centrc in LiF. (4) Only very weak emission around 350 nm is dctcctcd as shown in fig. I, thcrcforc it is not possible to say that the 670 nm CL emission in the present work is induced also by a direct F, absorption band cxcitation.
This band is suggcstcd to be the FT emission by the recombination of B rclcascd
[ 121 induced
band hole
with an F, centrc produced by the electron beam irradiation. Irradiation at low temperature produces hole-trap ccntres, e.g., the H type and V type centre. as well as F-aggregate ccntrcs. When the temperature is lower than 110 K, both the impurity associated H ccntre and the V, centrc arc thermally stable and the holes that recombine with the F, ccntrcs arc mainly freed by the continuous irradiation. When the sample is hcatcd to about 110 K, as described in the above section, the impurity stabiliLcd H centre decays by releasing a hole and then when the temperature is higher than 130 K, the V, centrc becomes mobile leading to the release of the trapped hole or the generation of another V centre - a V, or V, centrc. which will release the trapped hole at 180 or 200 K, respectively. This is a good explanation for the behaviour shown by the solid lint in fig. 4b. The intensity of the 530 nm emission indicated by this line incrcascs from 120 K and reaches a maximum at 200-220 K. The absorption band of the F: centrc overlaps that of the F2 ccntrc, [I21 therefore the Fj’ band cannot bc found in the absorption spectrum of the irradiated LiF samples.
of the cxciton and observed at 320-500
the overlapping bands nm are -rr-polarized.
WC have
3.3. In trirlsic emission
Acknowledgements
The overlapping emission bands at 320-500 nm in fig. 1 arc associated with the intrinsic emission of LiF. Many fret electrons are produced by electron beam irradiation. The intrinsic emission is due to the recombination of an electron with a V, ccntre, or self-trapped hole. This so-called exciton emission can be (r-polarized or n-polarized. Aguilar et al. [7] studied the electron beam excited luminescence spectra of NaCI. Similar emission bands wcrc observed at 330-600 nm. These bands overlapped with each other and the relative intensities varied with the crystal history, temperature and the irradiation history. They suggested that these variations resulted from perturbations by lattice imperfections, i.e. excitons or (V, + e )-like states wcrc mobile and diffused through the lattice; once trapped or perturbed by a lattice imperfection decay occurs to give a modified luminescence spectrum. This is also a suitable model for the CL bands of LiF at 320-500 nm. Ultra pure LiF samples are used in the present work, so one assumes that the main lattice imperfections which cause the variations of the spcctrum result from irradiation damage rather than impurities. Previous reports [5,6,11] suggested that the V, + e emission in LiF was positioned at 270 or 305 nm. This is beyond the range of our instrument. We propose that the 270 or 305 nm band is possibly the a-emission
We are grateful to the SERC and the State Education Commission of the Pcoplc’s Republic of China for financial assistance, and thank the Analysing and Tcsting Ccntrc of Beijing Normal University for carrying out the implants.
4. Conclusions (I) The same emission bands wcrc obscrvcd at all tcmpcratures in both implanted and unimplantcd samples: a strong band at 670 nm, a weaker band at 530 nm and a broad band consisting of several peaks at 300~500 nm. (2) The emission intcnsitics of all CL bands arc strongly dependent on the electron energy, beam current. irradiation time and temperature. The relations arc complex. (3) It is identified that the 670 nm band is the emission band of F, ccntres produced by the recombination of the F: centres with the electrons; the 530 nm band is the emission band of F: centres resulting from the holes recombining with the F, centres and the 300-500 nm band is associated with the intrinsic cmission ~ the T-polarized emission from electrons recombining with the V, ccntrcs.
References [I] T.G. Stoehe and S. Watanabe. Phys. Status. Solidi A20 (1973) Il. [2] M.R. Mayhugh. R.W. Christy and N.M. Johnson, J. Appl. Phys. 40 (1970) 296X. [3] M. Ehrlich. .I. Appl. Phys. 40 (196’)) XYI. [4] P.D. Townsend, C.D. Clark and P.W. Levy. Phys. Rev. IS5 (lYh7) YOX. [S] D.W. Cooke. J. Appl. Phys. 4Y (lY78) 4206. [6] V.K. Jain, J. Phys. DIY (19Xh) 1791. [7] M. Aguilar, P.J. Chandler and P.D. Townsend, Radiat. Eff. 40 (lY7Y) I. [Xl F. Sagastihelza and J.L. Alvarez Rivas. J. Phys. Cl4 (19X1) 1873. [Y] W.B. Fowler. H. Seidel and H.C. Wolf, in: Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York/London, lY68) pp. 110-17, 121-122. SY4-506. [Ill] J. Nahum. Phys. Rev. IS8 (1967) 814. [II] D. Wayne. D.W. Cooke and J.F. Rhodes, J. Appl. Phys. 52 (lYX1) 4244. [12] Y. Farge. G. Toulouse and M. Lamhert. J. Phys. 27 (1966) 2X7.
VIII. ALKALI HALIDES
Instruments
and Methods
in Physics Research
Nuclear Instruments & Methods in Physics Research Sk,1 IIWR
B6S (1092) 407-501
North-Holland
Cathodoluminescence B. Yang
of pure LiF
‘, B.J. Luff and P.D. Townsend
(CL) of pure LiF and H + ion-implanted
Cathodoluminescence
K.
spectra
Emission intensities vary with temperature
broad band consisting of
and the electron
several peaks at X%500
nm were observed
samples. The present studies support the identification recombination recombining
of the with the
FT F,
that the h70
centres, and the 300-500
address: Department Beijing
016X-S1;3X/Y2/$OS.O0
range SO to 300
the emission hands at 670 nm. 530 nm and a
at all temperatures
nm band is the emision
in both implanted
band of F, centres
and unimplanted produced
by the
ccntres resulting from the holes
nm band is associated with the intrinsic emission of LiF.
It has been rcportcd in the past few dccadcs that many of the luminescence processes observed in alkali halides over a wide temperature range arc related to the recombination of radiation-induced lattice defects [ 1-31. The most studied material is probably the variety of LiF manufactured by Harshaw for radiation dosimctry. Typical of the reported rcscarch is an attempt to correlate the absorption of point defects (colour centres) and the thermoluminescence (TL) peaks by Mayhugh [2]. comparison of growth rate with y-ray dose of the F-center absorption and of the TL intensity by Ehrlich [3] and mcasurcmcnts of the TL spectrum in crystals produced under a variety of different growth conditions by Townsend [4], but very little research has been done on pure LiF. Models of luminescent proccsscs given in thcsc reports are mostly associated with the doped impurities in LiF crystals. although intrinsic luminescence has been reported as well, c.g. the study of V, + c emissions in LiF by Cooke [S] and Jain [h] and the investigation of the exciton emissions in NaCl by Aguilar et al. [7]. It has been reported that the number of emission bands is reduced and the intensities arc wcakcr in pure LiF [l]. The emission bands at 320, 340 and 415 nm and a strong band at 663 nm found in heavily exposed (IO’ Gy) samples were reported by Sagastibclza et al. [X] in their TL study of pure LiF crystals. It was suggested that this complex emission spectrum of pure LiF might bc due to the existence of a wide variety of background impurities.
University.
However,
centres with the electrons; the 530 nm band is the emission hand of F:
1. Introduction
’ Permanent
pure LiF has been studied over the temperature
beam dose.
Cathodolumincsccncc (CL) spectra of pure LiF and H ’ ion-implanted pure LiF are studied in this work. Both intrinsic and nonintrinsic CL emissions have been observed, and models of the luminescent proccsscs producing thcsc emission bands arc proposed. An important role for F-aggrcgatc ccntrcs in nonintrinsic CL emissions in pure LiF is suggested.
2. Experimental Samples of pure and H ’ ion-implanted pure LiF were examined over the tcmpcrature range SO-300 K. The pure samples wcrc obtained from the University of Utah, USA, and from the Shanghai Institute of Optical Instruments. China: H + implantation was carried out on the pure samples at an energy of 3 McV to
10000
1
,
I
9000
i
‘(
of Physics, Beijing Normal
100875, China.
i‘ 1092
Elsevier
Science Publkhers
B.V. All rights reserved
VIII.
ALKALI
HALIDES
Fig. 2. Cl intensity of H
’ ion-implanted pure LiF vs wavelength and temperature whilst cooling from
;I final dose of 10” ions/cm’. C~rthodoluminescence was cxcitcd using a 2 mm diamctcr clcctron beam of cncrgy 2-20 keV with beam currents of 0.01 to 3 PA. Spectral information was ohtaincd using an f/4 scanred-scnsitivc photomultiplicr ning monochromator, tube, and lock-in amplifier: modulation of the exciting electron beam over a range of frequencies was carried out by using a parallel-plate capacitor inserted into the beam column. Measuremcnts at low tcmpcrature were made by mounting the sample on ;I cold stage of a closed-cycle cryostat. All spectra presented hcrc XC corrcctcd for the response of the whole detection systcm. Detailed spectra dcconvolution was not attempted but as seen in figs. I and 2 the major bands arc clearly resolved. Further clarity is provided by observations of temperature. dose and lifctimc depcndenciea.
3. Results and discussion Fig. 1 shows the CL spectrum of pure LiF at 300. I90 and 60 K. A strong red band pcakcd at 670 nm and a wcakcr band around 530 nm arc obscrvcd. A broad band at 300-500 nm which probably consists of scvcral overlapping peaks. e.g., at 325. 350, 370, 410 and 450 nm is also observed. The intensities of these peaks arc strongly dependent upon the irradiation history and temperature, c.g. those at 325 and 350 nm arc atrongcr at higher temperature. but the 450 nm band can only
bc dctcctcd at low tcmpcraturcs. Another broad band in the near-infrared region, which partially overlaps the 670 nm band, is obscrvcd at room temperature, but the peak position of this band cannot bc dctcrmincd by our instrument. CL spectra from H + ion-implanted pure LiF wcrc also rccordcd. The number and positions of the CL peaks arc the same as those in the unimplanted samplcs. The Cl intensities of thcsc two kind of samples arc nearly cqual at room tempcraturc, but at low tcmpcraturc the intensity of the 070 nm band in the implanted sample is obviously much stronger. A three dimensional diagram of the CL intensity of H ’ ion-implanted LiF vb wavclcngth and temperature whilst cooling from 300 to 50 K is shown in fig. 2. The relation bctwccn emission intensity and irradiation dose is investigated at diffcrcnt tcmpcraturcs with various incident clcctron beam currents bctwccn 0.01 and 3 FA and various irradiation times of up to one hour. Complex bchaviour occurs for all the CL bands in both implanted and unimplanted samples. In gcneral, the intensity of cvcry emission band increases with increasing beam current and dccrcases with the irradiation time. But sometimes the intensity shows 21 small increase in the first 5 or IO min if the sample is irradiated with a low beam current (< 0.4 FA). Examplcs of thcsc results arc given in fig. 3. A graph of CL intensity vs temperature is shown in fig. 4. Mcnsurcmcnts have been performed during a
Fig. 3. Cl intensitie
time:
of 370. 530 and 670 nm bands in LiF aa a function of irradiation
the
incident electron energy is IO keV.
the beam current is 0.3 uA. (a) T = 300 K, (b) T = 50 K.
cooling cycle from RT to SO K (a), and a warming cycle from SO K to RT 0~). At the start of each cycle a position on the crystal which had not been previously irradiated was selected to rcducc the influence of irradiation history. The mcasurcd temperature is the tcmperature of the sample holder block. It is likely that thcrc is a temperature difference bctwccn the emission surface of the sample and the sample block as LiF is not a good thermal conductor and fairly thick (2 mm) samples were used. Therefore the measured temperatures during heating and cooling cycles may differ. For example, in fig. 4a they may be lower than the temperature of the emission surface of the sample, but in fig. 4b we would cxpcct the latter to be lower. Note. howcvcr, that the apparent differcnccs can also disguise dose dependent fcaturcs and differences in dc-
feet structures formed during the temperature cycle. The following observations are made from these measurements: ti) Below 240 K, the 670 nm emission increases as the temperature decreases: there is a slow increase from 240 to I10 K and a rapid increase below I IO K. (ii) The 530 nm emission bccomcs more intcnsc as the sample is cooled from 240 to 200 K; it rcachcs its highest intensity at about 200 K, and then decreases. (iii) In fig. 4a, the 670 nm and 530 nm emission bands decrease in intensity during a cooling cycle from RT to 240 K, but opposite results arc shown in fig. 4b for a warming cycle. This diffcrcncc may be caused by the cffcct of irradiation dose. (iv) Because of the considerable overlapping of emission bands, it is very difficult to make a detailed
15r
15
12r
LJ’
2 3r
0
~~
50
I
~__J 100
Fig. 4. Cl intensities
150 'Sempcrature
200 (K
)
250
0
300
L_
50
IO0
\
'-. _._ __ 150
200
Temperature
of 530 and 670 nm emission hands in
LiF vs temperature
250
300
350
(K )
during a cooling cycle from
RT to SO K (a), and a
warming cycle from SO K to RT (b).
VIII. ALKALI HALIDES
study of emission bands in the range 300-500 nm. The main observation is that the total intensity of the whole broad band reachcs its maximum value at about 200 K and becomes relatively lower at both RT and low tcmpcraturcs (below 100 K). Similar investigations wcrc made on the H ’ ion-implanted LiF sample. The only diffcrcncc bctwccn this sample and the previous sample is that the rate of change of the intensity of the 670 nm band with tcmpcraturc is grcatcr in the implanted sample. The optical absorption spectrum of unimplantcd LiF which has been irradiated by the electron beam for two hours was recorded in the ultraviolet and visible ranges at room tcmpcraturc. The strong absorption bands of F and F, ccntrcs, as well as the wjcak F, and F4 bands wcrc detected.
This is the emission band of the F, centrc in LiF. The F, centrc is also charactcrizcd by absorption at 450 nm [Y]. When the LiF sample is irracliatcd by the high energy clcctron beam, irradiation dcfccts would be produced. Some of thcsc defects. such as F, F,. F,. F4 ccntrcs can easily bc detected by room tcmpcraturc optical absorption mcasurcmcnts. whcrcas some other dcfccts; e.g., Ft. F, , F; . Fj- and various V ccntres arc difficult to dctcct by this method, bccausc they are unstable at room tcmpcrnture or exhibit obviously overlapping absorption bands. In some casts the peak position is beyond the detection range of the instruments [Y]. Such colour ccntrcs undoubtedly play complex and important roles in the CL proccsscs. Thcrc arc several possible lumincsccnt mechanisms that could give rise to the FI emission: (I ) the rccombination of a rclcascd electron with an Fi ccntrc: (2) F absorption band excitation: (3) recombination of a rcleased hole with an F, ccntrc; (3) F, absorption band excitation. (I) F; ccntrcs in LiF arc thermally stable at tcmperatures below 230 K. From fig. 3a, the intensity of 670 nm emission starts increasing near 220 K. From this tempcraturc the concentration of Fi ccntrcs gradually incrcascs with decreasing tempcraturc and increasing irradiation dose. Below I30 K the stability of the V, ccntrc increases the probability of a released clcctron recombining with an Fi ccntrc, as does the impurity stabilized H center when it is below I IO K. Therefore. with this first model, the increasing hole stability would lcad to a rapid increase of 670 nm emission. Howcvcr, the change in signal intensity near I IO K can arise from a variety of factors. It is not only the tcmpcrature region where the impurity stabilized H ccntrcs anncal but also there is cvidcnce that an electron trap empties in this tcmperaturc range since in an early paper on TL of LiF [4] it was found that ;I TL peak near I I5 K could bc rcgencratcd by illumina-
tion in the F band region. Electron relcasc was implied via this TL cxpcrimcnt whcrcas hole release is associatcd with H ccntre annealing. By cithcr charge mechanism it is expected that CL efficiency will bc altered. probably with enhanced F, emission. For cxamplc, holes incrcasc the conversion of F, to F; , and clcctron capture produces the rcvcrsc rcacfion and hcncc F, luminesccncc. (2) It has been suggcstcd previously [IO] that F band excitation may cause F, band emission. Cooke [5] and Wayne [I I] rcportcd in their TL study of LiF that the direct recombination of a hole trapped at a V, centrc with a thermally relcascd electron would produce the 270 nm emission at IX- I60 K. Presumably, this cmission occurs at any tcmpcraturc below the V, ccntrc’s decay temperature in the cast of CL. Jain [h] rcportcd that from 200 to 350 nm many TL glow peaks wcrc obscrvcd at the temperatures below I40 K. The F ccntrc absorption band in LiF is at 250 nm. This implies that the emissions dcscribcd above will cxcitc the F band. This is therefore one possible explanation for the very sharp increase of the 670 nm CL emission band with decreasing tcmpcraturc below I IO K. (3) A totally different viewpoint may bc that the cxpccted high CIA intensity is dcprcsscd from 100-250 K because of the hole rclcasc from H and V type centres in the tcmpcraturc range. Furthermore WC can conclude that a rclcascd hole recombining with an F2 ccntre is not involved in the 670 nm emission ob\crvcd in the prcscnt work. A strong 663 nm TL band in pure LiF was obscrvcd by Sagastibclza et al. [Xl. but they suggested that this is due to the background impurity in the samples. They could only obscrvc this band in heavily irradiated (10’ Gy) samples and its maximum intensity was obtained at about 470 K. We presume this is the same band as the 670 nm F, emission band obscrvcd in the CL spectrum. One possible lumincscent process is the following: a thermally rclcascd hole an cxcitcd recombining with an F, ccntrc produces state F2 ccntrc and then :I 1.X4 cV photon is cmittcd with the transition from the cxcitcd state to the ground state. This suggestion is supported by the charactcristics of the 663 nm TL band; that is the fact that it could only be observed in the heavily exposed samples indicates it is associated with F-aggrcgatc ccntrcs and 370 K is near the decay temperature of the F, centrc in LiF. (4) Only very weak emission around 350 nm is dctcctcd as shown in fig. I, thcrcforc it is not possible to say that the 670 nm CL emission in the present work is induced also by a direct F, absorption band cxcitation.
This band is suggcstcd to be the FT emission by the recombination of B rclcascd
[ 121 induced
band hole
with an F, centrc produced by the electron beam irradiation. Irradiation at low temperature produces hole-trap ccntres, e.g., the H type and V type centre. as well as F-aggregate ccntrcs. When the temperature is lower than 110 K, both the impurity associated H ccntre and the V, centrc arc thermally stable and the holes that recombine with the F, ccntrcs arc mainly freed by the continuous irradiation. When the sample is hcatcd to about 110 K, as described in the above section, the impurity stabiliLcd H centre decays by releasing a hole and then when the temperature is higher than 130 K, the V, centrc becomes mobile leading to the release of the trapped hole or the generation of another V centre - a V, or V, centrc. which will release the trapped hole at 180 or 200 K, respectively. This is a good explanation for the behaviour shown by the solid lint in fig. 4b. The intensity of the 530 nm emission indicated by this line incrcascs from 120 K and reaches a maximum at 200-220 K. The absorption band of the F: centrc overlaps that of the F2 ccntrc, [I21 therefore the Fj’ band cannot bc found in the absorption spectrum of the irradiated LiF samples.
of the cxciton and observed at 320-500
the overlapping bands nm are -rr-polarized.
WC have
3.3. In trirlsic emission
Acknowledgements
The overlapping emission bands at 320-500 nm in fig. 1 arc associated with the intrinsic emission of LiF. Many fret electrons are produced by electron beam irradiation. The intrinsic emission is due to the recombination of an electron with a V, ccntre, or self-trapped hole. This so-called exciton emission can be (r-polarized or n-polarized. Aguilar et al. [7] studied the electron beam excited luminescence spectra of NaCI. Similar emission bands wcrc observed at 330-600 nm. These bands overlapped with each other and the relative intensities varied with the crystal history, temperature and the irradiation history. They suggested that these variations resulted from perturbations by lattice imperfections, i.e. excitons or (V, + e )-like states wcrc mobile and diffused through the lattice; once trapped or perturbed by a lattice imperfection decay occurs to give a modified luminescence spectrum. This is also a suitable model for the CL bands of LiF at 320-500 nm. Ultra pure LiF samples are used in the present work, so one assumes that the main lattice imperfections which cause the variations of the spcctrum result from irradiation damage rather than impurities. Previous reports [5,6,11] suggested that the V, + e emission in LiF was positioned at 270 or 305 nm. This is beyond the range of our instrument. We propose that the 270 or 305 nm band is possibly the a-emission
We are grateful to the SERC and the State Education Commission of the Pcoplc’s Republic of China for financial assistance, and thank the Analysing and Tcsting Ccntrc of Beijing Normal University for carrying out the implants.
4. Conclusions (I) The same emission bands wcrc obscrvcd at all tcmpcratures in both implanted and unimplantcd samples: a strong band at 670 nm, a weaker band at 530 nm and a broad band consisting of several peaks at 300~500 nm. (2) The emission intcnsitics of all CL bands arc strongly dependent on the electron energy, beam current. irradiation time and temperature. The relations arc complex. (3) It is identified that the 670 nm band is the emission band of F, ccntres produced by the recombination of the F: centres with the electrons; the 530 nm band is the emission band of F: centres resulting from the holes recombining with the F, centres and the 300-500 nm band is associated with the intrinsic cmission ~ the T-polarized emission from electrons recombining with the V, ccntrcs.
References [I] T.G. Stoehe and S. Watanabe. Phys. Status. Solidi A20 (1973) Il. [2] M.R. Mayhugh. R.W. Christy and N.M. Johnson, J. Appl. Phys. 40 (1970) 296X. [3] M. Ehrlich. .I. Appl. Phys. 40 (196’)) XYI. [4] P.D. Townsend, C.D. Clark and P.W. Levy. Phys. Rev. IS5 (lYh7) YOX. [S] D.W. Cooke. J. Appl. Phys. 4Y (lY78) 4206. [6] V.K. Jain, J. Phys. DIY (19Xh) 1791. [7] M. Aguilar, P.J. Chandler and P.D. Townsend, Radiat. Eff. 40 (lY7Y) I. [Xl F. Sagastihelza and J.L. Alvarez Rivas. J. Phys. Cl4 (19X1) 1873. [Y] W.B. Fowler. H. Seidel and H.C. Wolf, in: Physics of Color Centers, ed. W.B. Fowler (Academic Press, New York/London, lY68) pp. 110-17, 121-122. SY4-506. [Ill] J. Nahum. Phys. Rev. IS8 (1967) 814. [II] D. Wayne. D.W. Cooke and J.F. Rhodes, J. Appl. Phys. 52 (lYX1) 4244. [12] Y. Farge. G. Toulouse and M. Lamhert. J. Phys. 27 (1966) 2X7.
VIII. ALKALI HALIDES