electro-optical studies of kbr and kcl...
TRANSCRIPT
155
Chapter V
Electro-Optical Studies of KBr and KCl Crystals
V.1 Introduction
The subject of crystal growth has advanced greatly in the last few
decades due to the practical applications of the crystals. The most common
methods of crystal growth are solution growth [1] and melt growth [2]. In
practice, all materials can be grown in single crystal from the melt; provided
they melt congruently, they do not decompose before melting and they do
not undergo a phase transition between the melting point and the room
temperature. Among the normal freezing methods, Bridgeman technique is
one of the oldest methods for growing crystals. This technique produces
nucleation on a single solid-liquid interface by carrying out the crystallization
in a temperature gradient. Many of the technically important crystals are
obtained by this method. The principle is that a melt of the correct
composition of the substance is slowly cooled from above the equilibrium
melting point to produce the desired crystal. Melt crystallization is often
considered to be commercially attractive, since it offers the potential for low
energy separation compared to distillation, because latent heats of fusion are
generally much lower than latent heats of vaporization [3]. Due to their
simple cubic structure, the alkali halides have played a very important role in
the development of Solid State Physics. Europium doped alkali halide single
crystal has been the subject of very intensive investigation due to its
156
applications in digital medical radiography, optical memories and
environmental dosimetry etc [4].
The present chapter proposes the electro-optical studies of the
undoped and Mn doped KBr and KCl crystals prepared by melt growth. The
lattice of KBr/KCl is face centered cubic with lattice parameters a = b = c =
6.60050Å for KBr (JCPDS – CAS : 7758-02-3) and a = b = c = 6.29170Å for
KCl [5]. The basis consists of one K atom and one Br/Cl atom separated by
one half the body diagonal of a unit cube. There are four units of KBr/KCl in
each unit cube. The inter ionic separation of pure KBr crystal is 3.300 Å and
that of pure KCl crystal is 3.147 Å [5].
V.2 Results and Discussion V.2.1 Structural Properties
Structures of the KBr and KCl crystals in the present study is
analyzed by taking XRD using Cu Kα (λ = 0.154056nm) radiation and are
compared with JCPDS data card. The XRD patterns of KBr and KCl
crystals (precursor, undoped and Mn doped) with d and (hkl) values are
shown in Fig. 5.1 and Fig.5.2 respectively. The XRD pattern reveals a
crystalline nature for the crystals. The orientation of higher intensity peak
is found to be along (200) and (220) planes at 2θ =27.073o and 38.600o for
KBr crystal and (200) and (220) planes at 2θ =28.612 o and 40.82 o for
KCl crystal. The presence of other orientations such as (111), (311),
(222), (400), (420) and (422) is also detected at 2θ = 23.311o, 45.525o,
47.723o, 55.692o, 62.930o and 69.857o respectively for KBr. Similarly
orientations such as (222), (400), (420) and (422) is also detected at
2θ = 50.45o, 59 o, 66.81oand 74.15 o respectively for KCl crystals with
lower intensities.
157
0 20 40 60 80 100
4 wt %3 wt %2 wt %1 wt %
crystalundoped
precursor
d =
1.34
538
(422
)d
= 1.
4757
(420
)
d =
1.64
9(4
00)d
= 1.
9042
(222
)d
= 1.
9909
(311
)d
= 2.
33(2
20)
d =
3.29
1(2
00)
d =
3.81
(111
)
2θo
L in
tens
ity (A
.U)
Fig.5.1. XRD pattern of precursor, undoped and Mn doped KBr crystal.
Extra peaks corresponding to the dopant or their compounds are not
detected, for any crystals but the intensity of the prominent peaks in the host
sample is decreased in the crystals on Mn doping. The intensities of the peaks
are further decreased on increasing the concentration (in wt %) of the dopant.
It is due to the decrease in the host atomic density in these planes. Increase in
dopant concentration leads to the movement of Mn2+ ions to the interstitial
sites and also increases the state of amorphous nature and disorders.
158
0 20 40 60 80 100
(422
)1.
284
d =
1.40
7
d =
1.57
3
d =
1.81
7
d =
2.22
5
(420
)
(400
)
(222
)
(220
)
d =
3.14
6(2
00)
4 wt %3 wt %2 wt %1 wt %
crystalundopedprecursor
L in
tens
ity (A
.U)
2θo
Fig.5.2. XRD pattern of precursor, undoped and Mn doped KCl crystal.
V.2.2 Diffused Reflectance Spectroscopy Band gap (Eg) of melt grown KBr and KCl crystals is measured
from Diffused Reflectance Spectroscopy as described in [6]. Diffused
Reflectance Spectrogram for these crystals with percentage of reflectivity
R versus wavelength λ is incorporated in Fig. 5. 3(a) and Fig. 5.4 (a). Eg is
found by extrapolating the straight line graph of {(k/s)hυ}2 versus hυ
(Fig. 5.3 (b) and Fig. 5.4(b)) at k = 0. Where k is the absorption
coefficient and s is the scattering coefficient and Eg is found to be 5.05 eV
for KBr and 4.94 eV for KCl crystal. Wide band gap compounds are
especially promising for light emitting devices in the short wavelength
region of visible light.
159
1 2 3 4 5 6
0
500
1000
1500
2000
2500
3000
(b)
(a)
200 300 400 500 600 700 800 9000
5
10
15
20
25
30
35
40
% R
λ (nm)
hν (eV)
[(k/s
) hν]
2
Fig.5.3. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ}2 versus
hυ of KBr crystal.
1 2 3 4 5 6
0
500
1000
1500
2000
2500
(b)
(a)
200 300 400 500 600 700 800 9000
5
10
15
20
25
30
35
40
45
λ (nm)
% R
[(k/s
) hν]
2
hν (eV) Fig.5.4. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ}2 versus
hυ of KCl crystal.
160
V.2.3 Electro-Optical Studies V.2.3.1 Photoconductivity Studies The basic principle of photoconductivity (PC) is the production
of ‘free’ charge carriers in a material by optical excitation. Saturated
photocurrent is reached after some time as the PC cell is exposed to
excitation source. The samples are annealed to various temperatures such
as 50 oC, 100oC, 150 oC, 200oC, and 250oC. A plot of the time dependence
of Photocurrent (primary photocurrent) of KBr and KCl samples at 100 oC
annealing temperature is shown in Fig.5.5 (Saturated value of
photocurrent of the crystal is measured for each case separately).
Saturated photocurrent is different at different annealing temperature and
it increases as annealing temperature increases and reaches the maximum
for the samples annealed at 100oC (Fig.5.6). Annealing increases the
crystallanity of the KBr and KCl crystals, which produces an increment in
photocurrent [7].
-2 0 2 4 6 8 10 12 14 16
0
2
4
6
8
10
12
14
Time (mins.)
Phot
o C
urre
nt (µA
)
KBr
KCl
Fig.5.5. Variation of photocurrent versus time of KBr and KCl crystals annealed
at 100 oC.
161
Saturated photocurrent decreases as the annealing temperature is
increased from 100oC to 250oC. The observed decrease in photocurrent
when the sample is annealed above 100oC can be explained on the basis
of defects in the material [8] as explained in section IV.2.3.1. It is found
that saturated value of photocurrent increases with increasing intensity of
excitation (Fig.5.7) and also with increase in applied voltage (Fig. 5.8).
More and more charge carriers reach at the respective electrodes and the
photocurrent increases with the increase in intensity of light and applied
voltage. The non-linearity in Fig.5.7 and Fig.5.8 for these crystals
represents the dependence of saturated value of photocurrent (secondary
photocurrent) on the intensity of excitation and applied voltage [8].
0 50 100 150 200 2500
1
2
3
4
5
6
7
8
9
10
11
Phot
o C
urre
nt (µA
)
Temperature (oC)
KBr
KCl
Fig.5.6. Variation of saturated photocurrent of KBr and KCl crystals with
annealing temperature.
162
40 60 80 100 120 140 1600.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
KBr
KCl
Phot
o C
urre
nt (µA
)
Intensity (mW/cm2)
Fig.5.7. Variation of saturated photocurrent of KBr and KCl crystals at
different intensities of light.
0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
KBr
KClPhot
o C
urre
nt (µA
)
V (volts)
Fig.5.8. Variation of saturated photo current of KBr and KCl crystals
with applied voltage.
163
0 1 2 3 40
5
10
15
20
25
30
KBr
KClPhot
o C
urre
nt (µA
)
Concentration (Arb. Units)
Fig.5.9. Variation of saturated photocurrent of KBr and KCl crystals at
different concentrations of the dopant.
Fig. 5.9 shows the variation of the saturated value of photocurrent
with concentration of the dopant Mn for KBr and KCl crystals. As the
concentration increases, the charge carriers also increase and it is
observed that photocurrent increases and reaches the maximum at 2 wt %
concentration of Mn for both the samples. At this concentration, the
charge concentration seems to be optimum for better PC [8]. PC is found
to be decreased on further increase in the concentration of Mn. The
increase in concentration of Mn causes increases the charge carrier
concentration and photocurrent. But, when it goes beyond an optimum
value the carrier collision probability increases which results in reduction
of the photocurrent.
164
V.2.3.2 Photovoltaic Studies Photovoltaic (PV) effect is involved, when absorption of radiation
by a material causes the formation of a p.d. between the two portions of
the material. Fig.5.10 shows the variation of photovoltage of KBr and KCl
crystals annealed at 100 oC respectively with respect to time as the PV cell
is exposed to light source. As annealing temperature increases, it is
observed that the saturated photovoltage increases and becomes the
maximum at 100oC for the crystals (Fig.5.11). (Saturated value of
photovoltage of crystals is measured for each case separately). When the
annealing temperature is increased further, saturated photovoltage
decreases which is due to the similar cause as given in section IV.2.3.2.
The maximum value of the photovoltage also increases with increasing
intensity of excitation (Fig.5.12). With the increase in intensity, more and
more charge carriers are separated at the respective electrodes and the
photovoltage is increased [8].
0 2 4 6 8 10 12
0
20
40
60
80
100
120
KBr
KCl
Time(mins.)
Phot
o Vo
ltage
(m
V)
Fig.5.10. Variation of photovoltage of KBr and KCl crystals annealed at 50 oC with respect to time.
165
0 50 100 150 200 250
30
40
50
60
70
80
90
100
110
Temperature (oC)
Phot
o Vo
ltage
(m
V) KBr
KCl
Fig.5.11. Variation of saturated photovoltage of KBr and KCl crystals with temperature.
40 60 80 100 120 140 16020
30
40
50
60
70
80
90
Intensity (mW/cm2)
Phot
o Vo
ltage
(m
V)
KBr
KCl
Fig.5.12. Variation of saturated photovoltage of KBr and KCl crystals at different intensities of light.
166
0 1 2 3 430
60
90
120
150
180
KBr
KCl
Phot
o Vo
ltage
(m
V)
Concentration (wt %)
Fig.5.13. Variation of saturated photovoltage of KBr and KCl crystals at different concentrations of the dopant.
Fig. 5. 13 depicts the variation of the saturated photovoltage of KBr and
KCl crystals with concentration of the dopant Mn and it is observed that PV is the
maximum at 2 wt % concentration of Mn. At this concentration, the charge
separation may be more in number compared to other cases for better PV [8].
Photo-electronic properties of the prepared crystals are very much influenced by
the presence of defects in the original crystal lattice [8]. PV increases on increasing
the concentration of Mn and reached the maximum at 2 wt % concentration and
decreases on increasing the dopant concentration. Decrease of PV on increasing
dopant concentration, may be attributed to increase in amorphous phase and
concentration quenching, consequent to doping.
V.2.3.3 Electroluminescence Studies Electroluminescence (EL) is the phenomenon in which electrical
energy is converted directly into electromagnetic energy in the visible
167
region of the spectrum. In this process heating does not play an essential
part and an electroluminescent device is not designed to operate at an
incandescent temperature. The observation of EL was first reported by
Round [9], but the phenomenon was originally discovered by Lossew [10-
12]. This effect in inorganic phosphors was first observed by Destriau [13-
15]. A great deal of progress has been made recently in improving the
performances of various classes of EL devices. The light emitted from the
AC/DC EL cell, on application of suitable voltage, was detected by the
PMT and the corresponding photocurrent is measured with the help of a
digital nanoammeter.
V.2.3.3.1 AC Electroluminescence The mechanism of AC EL is as explained in section IV.2.3.3.1. The
integrated light intensity is accurately given by the expression [16-19],
B= A exp (-C/V1/2) → (5.1)
0.05 0.06 0.07
-1.0
-0.8
-0.6
-0.4
-0.2
Lo
g (B
) (A
rb. U
nits
)
1/V1/2 (volts-1)
Fig.5.14. Plot of Log (B) Vs. V-1/2 of KBr crystal.
where A and C are constants independent of the voltage. This expresses
that the mechanism of excitation is an acceleration- collision one. Log (B)
168
(Brightness) versus V-1/2 (applied AC voltage) graph of KBr and KCl
electroluminors is given in Fig. 5.14 and Fig. 5.15 respectively. Linearity of
these plots holds the above relation and proves the mechanism of excitation
is acceleration- collision one. The EL phenomenon is very much influenced
by presence of defects in the original lattice [8].
0.045 0.050 0.055 0.060 0.065 0.070 0.075
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Lo
g (B
) (A
rb. U
nits
)
1/V1/2 (volts-1) Fig.5.15. Plot of Log (B) Vs. V-1/2 of KCl crystal.
AC EL Brightness as a function of applied a.c voltage for the two
electroluminors at different concentrations (up to 4 wt %) of the dopant
(Mn) is given in Fig. 5.16 and Fig.5.17 respectively. The EL intensity is
almost same as that of the undoped sample below 1 wt % concentration of
the dopant. Hence the corresponding curves are not included in the graph.
EL intensity increases with increasing applied voltage. EL intensity
increases as the concentration of the dopant increases and becomes the
maximum at 1wt % for KBr and KCl crystals. EL intensity then decreases
169
100 200 300 400 500 600
0
10
20
30
AC Voltage
EL
Inte
nsity
(Arb
.uni
ts)
1wt%
2wt%
undoped3wt%4wt%
Fig.5.16. AC EL of KBr crystal at different concentrations of the dopant
(Mn).
200 250 300 350 400 450 500
0
10
20
30
401wt%
2wt%
3wt%
undoped
4wt%
EL
Inte
nsity
(Arb
.uni
ts)
AC Voltage Fig.5.17. AC EL of KCl crystal at different concentrations of the dopant
(Mn).
on increasing the concentration above 1wt %. This decrease in intensity is
due to concentration quenching. AC EL intensity of the electroluminors at
different annealing temperatures is given in Fig. 5.18 and Fig.5.19
respectively. EL intensity increases with increasing annealing temperature
from room temperature (crystallanity increases) and reaches the
170
100 200 300 400 500 600
0
10
20
30
40
50
EL
Inte
nsity
(Arb
.uni
ts)
AC Voltage
500C1000C1500C2000C2500C
Room Temperature
Fig.5.18. AC EL of KBr crystal at different annealing temperature.
150 200 250 300 350 400 450 500 550
0
10
20
30
40
50500C
1000C
1500C
Room Temperature2000C2500C
AC Voltage
EL In
tens
ity (A
rb.u
nits
)
Fig.5.19. AC EL of KCl crystal at different annealing temperature.
171
Fig.5.20. AC EL emissionphotograph of KBr crystal.
Fig.5.21. AC EL emissionphotograph of KCl crystal.
maximum at 500C for KBr and KCl crystals. EL intensity then decreases
on increasing annealing temperature due to the increase in the amorphous
phase and disorders. The AC EL emission of both the electroluminors is
bluish. AC EL emission photographs of the electroluminors taken with a
digital camera are given in the figures (Fig. 5.20 for KBr and Fig .5.21 for
KCl crystals).
V.2.3.3.2 DC Electroluminescence
DC EL powder panels have become reasonably successful as
displays. An efficient DC powder EL device was first reported by A.
Vecht [20]. Two essential features of any DC EL panel are that the
phosphor particles are in contact with each other and with the electrodes.
The mechanism of DC EL is as explained in section IV.2.3.3.2.
172
100 200 300 400 500 600
0
10
20
30
40
1wt%2wt%
undoped3wt%
4wt%EL
Inte
nsity
(Arb
.uni
ts)
DC Voltage
Fig.5.22. DC EL of KBr crystal at different concentrations of the
dopant (Mn).
200 250 300 350 400 450
0
10
20
30
40
1wt%
2wt%
3wt%
undoped4wt%
DC Voltage
EL In
tens
ity (A
rb.u
nits
)
Fig.5.23. DC EL of KCl crystal at different concentrations of the
dopant (Mn).
173
100 150 200 250 300 350 400 450 500 550 600
0
10
20
30
40
50500C
1000C1500C2000C2500C
Room Temperature
EL In
tens
ity (A
rb.u
nits
)
DC Voltage
Fig.5.24. DC EL of KBr crystal at different annealing temperature.
DC EL Brightness as a function of applied DC voltage of KBr and
KCl electroluminor at different concentrations of the dopant (Mn) is given
in Fig. 5.22 and Fig.5.23 respectively. The EL intensity is almost same as
that of the undoped sample below 1 wt % concentration of the dopant.
Hence the corresponding curves are not included in the graph. EL
intensity increases with increasing Mn concentration and reaches the
maximum at 1 wt % then decreases on increasing concentration due to
concentration quenching. DC EL intensities of KBr and KCl
electroluminor at different annealing temperatures are given in Fig. 5.24
and Fig.5.25 respectively. EL intensity increases with increasing
annealing temperature (crystallanity increases) and reaches the maximum
at 500C then decreases on increasing annealing temperature due to the
increase in the amorphous phase and disorders. The DC EL emission of
KBr and KCl electruluminor is also bluish.
174
150 200 250 300 350 400 450 500
0
20
40
60
500C
1000C
1500C2000C2500CRoom Temperature
DC Voltage
EL In
tens
ity (A
rb.u
nits
)
Fig.5.25. DC EL of KCl crystal at different annealing temperature.
DC EL emission photographs of the two electroluminors taken with a
digital camera are given in the following figures (Fig. 5.26 for KBr and
Fig. 5.27 for KCl crystals).
Fig.5.26. DC EL emission photograph of KBr crystal.
Fig.5.27. DC EL emission photograph of KCl crystal.
175
V.2.3.4 Photoluminescence Studies Photoluminescence (PL) is the process in which absorption of
UV/optical photons is followed by electronic transitions, associated with
the emission of photons. PL spectra are recorded by using a Flourimeter.
Excitation and emission spectra were taken by changing the excitation
wavelength (λex) under a fixed emission wavelength (λem) and vice versa. The
highest resolution used were 0.1nm for excitation and 0.3nm for the emission.
When excited with λex = 301 nm, the KBr crystal phosphors show a broad
280 300 320 340 360 380 400 420 440
6000000
9000000
12000000
15000000
18000000
undoped
1 wt%2 wt%3 wt%4 wt%
λ (nm)
PL In
tens
ity (A
rb.u
nits
)
emis
sion
exci
tatio
n
Fig.5.28. PL emission at λem = 357 nm with λexc = 301nm spectra of KBr crystal at different concentrations of the dopant (Mn).
emission band which is observed at λem = 357 nm (Fig. 5.28). The broad
peak observed around 357 nm corresponds to the 1S0→ 3P2 transition
(4p4 – 4p4) of Br [21]. The normal luminescent bands are attributed to
interaction between emission centers and the host crystal lattice [22]. The
luminescent emission is usually originated due to presence of some
defects in the host lattice, which produces certain impurity site or centers
176
during the preparation [8]. Shoulder at 334 nm is originated because of
the presence of some defects in the host lattice. When excited with λex =
274 nm, the KCl crystal phosphors show a broad emission band which is
observed at λem = 340 nm (Fig. 5.29). . The broad peak observed around
340 nm corresponds to the 5D00→ 1S0
0 transition (3s23p3(4So) 4d –
3s23p3(2Do) 3d) of Cl [21].
260 280 300 320 340 360 380 400 4200
3000000
6000000
9000000
12000000
15000000
18000000
PL
Inte
nsity
(Arb
.uni
ts)
λ (nm)
emis
sion
exci
tatio
n
undoped1 wt%2 wt%3 wt%4 wt%
Fig.5.29. PL emission at λem = 340 nm with λexc = 274 nm spectra of
KCl crystal at different concentrations of the dopant (Mn).
PL emission spectra of the photoluminors doped with Mn are given in
Fig. 5.28 and Fig.5.29. Normally the luminescent emission depends upon
the nature of the activator and its concentration in the host lattice. The
peak position is not affected because the energy levels of these additives
(Mn2+ ions) lie at the same level as that of the host materials KBr and
KCl. The emission band corresponds to Mn2+ ion found in [23] and [24] is
177
merged in the broad peak of KBr and KCl. The merged transitions of
Mn2+ ion are at 339nm, corresponding to the band assignment 6A1(S) → 4T1(P), 363nm corresponding to the band assignment (6A1(S) → 4E(D)
and 376nm corresponding to the band assignment 6A1(S) → 4T2(D). PL
intensity of the peak decreases with increasing Mn concentration for both
the crystals due to concentration quenching [25]. At higher concentration,
the activator atoms destroy the matrix, which results in quenching of
emission [26]. PL emission may be delayed due to the presence of traps
[27].
300 350 400 450
8000000
12000000
16000000
20000000
500CRoom Temp
1000C1500C
2000C
λ (nm)
PL In
tens
ity (A
rb.u
nits
)
Fig.5.30. PL emission spectra of KBr crystal at different annealing temperatures.
178
280 300 320 340 360 380 400 420 4400
3000000
6000000
9000000
12000000
15000000
18000000
500C
Room Temp
1000C1500C2000C
PL
Inte
nsity
(Arb
.uni
ts)
λ (nm)
Fig.5.31. PL emission spectra of KCl crystal at different annealing
temperatures.
PL emission spectra of both the photoluminors at different
annealing temperatures are given in Fig. 5.30 and Fig.5.31. PL intensity
increases with increasing annealing temperature and reaches the
maximum at 500C then decreases on increasing annealing temperature.
Shoulder at 334 nm of KBr crystal produced from the defects is
disappeared due to increase in crystallanity of the material on annealing.
Increase in PL intensity with increase in annealing temperature up to 50oC
can also be explained on the basis of impurity effect in the material.
Photosensitivity will be increased if imperfections capture more minority
carriers than majority carriers. Imperfections acting as efficient
recombination centers decrease the photosensitivity. The decrease in PL
intensity on increasing the temperature beyond 50oC is due to the increase
in the amorphous nature and disorders. This can also happen due to the
179
recombination of electrons, which are thermally freed from traps with
photo excited holes held at centers as in quenching effects reported [7].
V.3 Conclusion
Conditions for the growth of crystals of KBr and KCl crystals by
melt growth using a cost effective mini crystal growth setup have been
optimized and their crystalline nature has been confirmed by carrying out
X-ray diffraction. On Mn doping, no extra peaks corresponding to them or
their compounds were detected but the intensity of the prominent peaks
was decreased due to the decrease in the atomic density in these planes
which leads to the movement of Mn2+ ions to the interstitial sites and also
increases the amorphous phase and disorders. Band gap (Eg) is found to
be 5.05 eV for KBr and 4.94 eV for KCl crystals.
PC effects of the crystals were studied and found that both the
materials are more photosensitive at 100oC. It is found that the maximum
value of photocurrent increases with increasing intensity of excitation and
also with increase in applied voltage. Mn doping makes the photosensitive
crystals more photosensitive. As the concentration of Mn increases, it is
observed that PC increases and reaches the maximum at 2 wt %
concentration of Mn. KBr is found to be more photoconducting than KCl.
PV effects of the crystals were studied and found that both the
materials show greater photovoltage at an annealing temperature of
100oC. Maximum value of the photovoltage also increases with increasing
intensity of excitation. PV increases on increasing the concentration of
Mn in the doped crystals and becomes the maximum at 2 wt%
concentration and decreases on increasing the dopant concentration.
180
Electroluminescence Brightness increases with the applied electric
field. The brightness of a powder EL cell increases non-linearly as
excitation voltage is increased. The EL phenomenon is very much
influenced by presence of defects in the crystal lattice. AC/DC EL
Brightness as a function of applied AC/DC voltage of both crystals at
different concentrations of the dopant (Mn) shows that EL intensity
increases with increasing Mn concentration (in wt %) and reaches the
maximum at 1 wt % then decreases on increasing concentration due to
concentration quenching. AC/DC EL Brightness as a function of applied
AC/DC voltage of both electroluminors at different annealing
temperatures shows that EL intensity increases with increasing annealing
temperature and reaches the maximum at an annealing temperature of
500C then decreases on increasing annealing temperature. The AC/DC EL
emissions of KBr and KCl electruluminors are bluish in colour. The
AC/DC EL brightness intensity of plane KBr reveals that it is a better
electroluminor than plane KCl crystal. But on doping and annealing the
AC/DC EL brightness intensity of KCl is more than that of KBr crystal.
When excited with λex = 301 nm, the KBr crystal phosphors show
a broad emission band which is observed at λem = 357 nm. The broad peak
observed around 357 nm corresponds to the 1S0→ 3P2 transition of Br.
Shoulder at 334 nm is originated because of the presence of some defects
in the host lattice. When excited with λex = 274 nm, the KCl crystal
phosphors show a broad emission band which is observed at λem = 340
nm. The broad peak observed around 340 nm corresponds to the 5D00→
1S00 transition of Cl.
181
The peak positions of both crystals are not much affected on Mn
doping because the energy levels of these additives (Mn2+ ions) lie at the
same level as that of the host materials KBr and KCl. The emission band
corresponds to Mn2+ ion is merged in the broad peak of KBr and KCl. The
merged transitions of Mn2+ ion are at 339nm, corresponding to the band
assignment 6A1(S) → 4T1(P), 363nm corresponding to the band
assignment (6A1(S) → 4E(D) and 376nm corresponding to the band
assignment 6A1(S) → 4T2(D). PL intensity of the peak decreases with
increasing Mn concentration for both the crystals due to concentration
quenching. At higher concentration, the activator atoms destroy the
matrix, which results in quenching of emission.
The emission peak intensity reveals that the prepared materials
could be used as a scintillator phosphor. PL intensity increases as
annealing temperature increased from room temperature and reaches the
maximum at 500C then decreases on increasing annealing temperature.
Shoulder at 334 nm (originated out of defects) of KBr crystal is
disappeared due to increase in crystallanity of the material on annealing.
The PL intensity of KBr peak reveals that it is a better photoluminor than
KCl.
182
References
[1] A. Holden and P. Singer, Crystals and Crystal Growing, Vakils,
Feffer and Simons, Bombay, 1968.
[2] J. J. Gilman, The Art and Science of Growing Crystal, John
Wiley and Sons, New York, 1963.
[3] J.W. Mullin, Crystallization, IVth Edn, Butterworth-Heinemann,
A division of Reed Educational and Professional Publications Ltd,
2001.
[4] M. Pedroza-Montero, B. Castaneda, R. Melendrez, T. M. Piters
and M. Barboza-Flores, Phys. Stat. Sol(b), 220 (2000) 671.
[5] K. K. Upadhyay and K.K Sarkar, Indian J. Phys., 73A (6) (1999) 793.
[6] P. D. Fochs, “The measurement of the energy gap of semiconductors
from their Diffuse Reflection Spectra”, Research Laboratory,
Associated Electrical Industries Ltd., Aldermaston, Berks.
[7] A. T. Halperin and G. F. Garlick Proc. Phys. Soc., 6813 (1955) 758.
[8] K. E. Abraham, “Electro-optical properties of some II-VI semi
conducting compounds”, Ph. D thesis, 1990, Department of
Physics, Ravisankar University, Raipur.
[9] H. J. Round, Electrical world 19 (1907) 309.
[10] O. W. Lossew, Telegrafia i Telefonia bez Prowodow, 26 (1923) 403.
[11] O. W. Lossew, Wireless World and Radio Rev., 780 (1924) 259.
[12] O. W. Lossew, Z. Fernmelde tec., 7 (1926) 92.
[13] G. Destriau, J. Chim. Physique, 34 (1937) 117, 327, 462.
183
[14] G. Destriau, Phil. Mag., 38 (1947) 774, 880.
[15] G. Destriau and H. F. Ivey Proc. IRE, 43 (1955) 1911.
[16] P. Zalm, Philips Res. Rep., 11, 417 (1956) 353.
[17] P. Zalm, G. Diemer, and H. A. Klasens, Philips Res. Rep., 10
(1955) 205.
[18] A.G. Fischer, J. Electrochem. Soc., 109 (1962) 1043.
[19] A.G. Fischer, J. Electrochem. Soc., 110 (1963) 733.
[20] A Vecht, N. J. Werring and P.G.F Smith Brit. J. Appl. Phys. (J.
Phys. D) Ser, 134 (1968) 21.
[21] Basic Atomic Spectroscopic Data: National Institute of Standards
and Technology, Physics Laboratory, U.S.A (Internet site).
[22] C. C. Vlam, Brit. J. Appl. Phys., 5 (1954) 443.
[23] M. Moreno, F. Rodriguez and J.A. Aramburu, Physical Review B,
Vol. 28 No.10 (November, 1983) 6100.
[24] C. Marco de Lucas, F. Rodriguez and M. Moreno, Phys.Stat.Sol.
(b) 172 (1992) 719.
[25] H.W. Leverenz, An introduction to luminescence of solids, J.
Willey New York, 1950.
[26] F. A. Krojer, Physica, 14 (1948) 425.
[27] P.I.Paulose, Gijo Jose, Vinoy Thomas, N.V Unnikrishnan, James
Joseph and M.K. Rudra Warrier, Asian Journal of Physics, 2
(2001) 69.