Emission spectra in Ca1-xCdxS solid solution at 77 and 300 K

644

Journal of Crystal Growth 86(1988)644—649 North—Holland. Amsterdam

EMISSION SPECTRA IN Ca1 ~Cd~S SOLID SOLUTIONS AT 77 AND 300 K B. RAY, J.W. BRIGHTWELL, D. ALLSOP and A.G.J. GREEN Department of Applied Physical Sciences. Coi’enrrr Polytechnic. Priory Street, Coi’entry

CVJ

SF8. UK

Solid solutions of Cai — ~Cd~~Shave been prepared for 0.12 x 0 from mixed suiphates in flowing H~/H~Sat 1050°C. Compositions have been determined by atomic absorption spectrophotometry recognising the difficulties in retaining cadmium. A monotonic linear variation of lattice parameter with composition is observed with the rock salt structure being sustained throughout. Both emission and excitation spectra have been determined at 77 and 300 K throughout the composition range. Excitation at wavelengths shorter than the band edge gave rise at 300 K to increased emission in the ultraviolet-violet region between 320 and 420 nm for x between 0.008 and 0.04, whilst at 77 K the emission was restricted to a 390 nm peak with excitation by longer wavelengths at 280 to 285 nm. Cooling to liquid nitrogen temperatures generally sharpened the emission peaks and increased intensities by up to two orders of magnitude. The absorption edge is observed to decrease linearly with increasing cadmium content. An explanation for the excitation and emission behaviour with change in temperature and composition is discussed.

1. Introduction

(t)

The rock salt structure is reported to be retamed in Ca1 ~Cd~~S solutions up to x = 0.55 [1,2]. The substitution of cadmium on the calcium sub-lattice is accompanied by a linear vartation of lattice parameter with composition [2]. The apparent optical absorption edge energy ts observed [31to decrease monotonically with composition from a value of 4.8 eV for CaS to 2.8 eV for Ca045Cd055S. Of particular interest is the optical emission spectrum found in Ca099Cd001S prepared at 1200°C in a sulphurising atmosphere and this extends from 4.0 to 1.5 eV (310 to 825 nm). Increasing the cadmium content is reported to concentrate the emission more and more into a single energy band at 2.2 eV (560 nm). The origin of this emission is not certain but it has been suggested that it may be derived 2t from clusters of tetrahedrally coordinated Cd reported here has The objective of the study been to reexamine the excitation and emission spectra of Ca 1 - ,Cd,~S for 0 x 0.12 both at ambient temperature and 77 K (not prevtously studied) on a range of carefully characterised samples. CaS and CaS solid solutions with CdS may be prepared by a number of techniques which inelude:

ncr; (iii) reduction of CaSO4 in fluxes, such as Na2SO4 or Na2CO5, with either sulphur or carbon; (iv) reaction of CaS and CdS in a flux such as KCI; (v) gas phase reduction of CaSO4 and CdSO4 with H2 or H2/H2S; (vi) reduction of CaO and CdO with CS~. Each technique has its difficulties and limitations; method (i) demands considerable technology, (ii), (iii) and (iv) yield products which may contain unacceptable levels of foreign ions from the carrier or the flux system, and products of (v) and (vi) have highly active surfaces susceptible to rapid reactions with atmospheric components such as oxygen, carbon dioxide and water vapour. growntosamples, using carbon reduction, areFlux reported have decreased phosphor effi-

0022-0248/88/$03.50

crystal growth from the melt; (ii) vapour phase transport using a halogen car-

ciency compared to gas phase reduced samples [1], although those produced in sulphur containing fluxes are comparable [4,5]. The method selected was (v), the reduction of mixed sulphates using H2/H2S at 1050°C, the purpose of the H,S being to eliminate trace levels of oxygen, which are known to influence the emission characteristics of CaS.

© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

B. Ray et al.

/

Emission spectra in Ca,

-

~Cd~ S solid solutions at 77 and 300 K

2. Experimental methods

3. Results

The calcium and cadmium sulphates used were of Analar grade. The CaSO4 2H20 was dried at 180°C to generate. CaSO4 0.5H20, which was wetted with a quantity of aqueous CdSO4 solulion, allowed to set and then the product was finely ground. Approximately 10 g samples were heated to 1050°Cin a tube furnace in a flowing H2/H2S atmosphere for periods of 1 to 4 h. The samples were thermally quenched by withdrawing the tube from the furnace with a H2 flow maintained until cool. The samples were analysed for phase purity with a Philips PW 1710- 1740 X-ray diffractometer using Cu Ka radiation; if traces of phase impurity, e.g. oxide, were detected, elimination was effected by repeat treatment with H2S/H2. The samples were then analysed by atomic absorption spectrophotometry (Varian AA475) for calcium and cadmium content. In all samples, substantial loss of cadmium occurred, due to the reaction, CdS + H2 ~=s Cd + H2S,

3.1. Structural determinations

since equilibrium for this reaction is to the right in the equation. The cadmium content is related to the treatment time and the gas flow rates. The compositions of the samples used in the study had values of x in Ca1 ~Cd~S of 0, 0.008, 0.02, 0.04. 0.06, 0.90, 0.10 and 0.12. Emission and excitation spectra were determined for freshly prepared samples using a Perkin-Elmer SP 3000 Fluorescence Spectrophotometer with a cold stage attachment. For each sample, the excitation spectra were recorded for each of the principal emission peaks at both 77 and300K. The emission characteristics of samples were determined with an excitation wavelength of 230 nm, which is of an energy greater than that of the band edge in CaS (A 255 nm). The measurements were made with and without a 330 nm long pass filter to allow high sensitivities at shorter -

Sharp X-ray diffraction traces were observed in all the samples subjected to optical investigation and only lines related to the sodium chloride structure were observed. The lattice parameter was found to vary linearly with composition with values of 569.5 and 564.2 pm in CaS and Ca0 58Cd0 12S respectively. 3.2. Emission spectra 3.2.1. At 300K

Ca1 ~Cd~S samples excited at a wavelength of 230 nm showed a progressive increase in the overall emission intensity of greater than an order of magnitude as x increased from 0 to 0.04, as shown in table 1. At higher x-values the overall intensity declined to little greater than that of CaS for x 0.12. Fig. la indicates the nature of the dependence of the integrated emission on composition at 300 K within the limits of sample by sample compartson. The emission intensity as a function of wavelength varies markedly between the different seetions of the composition range investigated and fig. 2 illustrates the point with spectra for x 0, 0.04, 0.06 and 0.10. The dominant emission peak =

=

10

I

I

I

—

-

300 K

RelQtIv0e6

-

Intensity 01

L.

-

x 50

I

02

“~

0

wavelengths and to remove the influence of see-

it .

.

—

\77 K I -..~L.~ L 004 008

=

ond order diffraction effects from the monochromator in the emission in the region 400 to 500 nm.

645

fl .

.

(a) b) 012

—

.

CQi_~Cdx S .

.

Fig. 1. Relative peak emission intensity excited by radiation of wavelength 230 nm as a function of cadmium concentration in Ca~ ~Cd~S at (a) 300 K and (b) 77 K.

646

B. Ray et at.

/

Emission spectra in Ca

1

,Cd, S solid solutions at 77 and 300 K

Table Relative intensities of peaks in the emission spectra for Ca1 - ~Cd ~ excited by 230 nm radiation at 300 K v

Relative intensity I~at A. in nm 290—310

=

0 1)008 0.02

0.04 0.06 0.09 0.10 0.12

52

=

1i

‘2

0.35 3.4 2.7 1.8 2.0

0.45 3.1 3.8 3.8 3.5 1.7 1.3 0.7

2.0

1.3 0.6

330—350

360—375 ‘~ 0.6 2.9 4.4 5.8 2.4

54

=

410—430

0.3 1.7

1.2 2.0

4.6 6.9 1.6

3.0 1.2

2.5 1.4

2.7

1.7

1.9

1.1

0.8

0.7

0.5

in CaS. which has low overall emission intensity, is at 360 nm with well defined satellite peaks at longer wavelengths of 420 and 480 nm whilst at shorter wavelengths there are two broad shoulders centred about 335 and 310 nm. For x 0.008. the overall emission intensity is enhanced by a factor of five with sharpening and intensification of peaks at 335 and 290 nm to the extent that the 360 nm emission peak observed in CaS becomes a well distinguished shoulder as the emission intensity declines towards longer wavelengths: the 420 and 480 nm satellite peaks remain well defined in the long wavelength tail. Increasing x to 0.02 and 0.04 =

480—495

=

‘~4

progressively enhance the emission intensity with a series of peaks between 370 and 420 nm and rapidly declining emission levels at shorter and longer wavelengths. At x 0.06 and 0.09. the emission level has declined and is predominantly centred between 330 and 340 nm with low level shoulders at 300 and 395 nm. whilst at 0.10 and 0.12. sharper but even less intense emission peaks are observed at 280. 305. 375 and 405 nm. =

3.2.2. At 77 K The impact of liquid nitrogen temperatures on emission intensity levels excited at a wavelength of 230 nm was significant for x 0 to 0.04 giving up to 100 fold increases on the values at room ternperature; table 2 summartses the relative emission intensities at different wavelengths for .v from 0 to 0.12 at 77 K. The dominant emission peak at these compositions was between 380 and 390 nm and =

_______________________

1~0

I

075j~

.

i.~

10

other emissions at shorter and longer wavelengths

were very much part of the tail of that peak. For .v = 0.06 the effect of temperature on emission was

X

Relative

—

—

virtually nil and represented the composition at

Intensity xlO

b) “it

~-©

r

025

9

dl

presents the relative emission intensity as a func-

x20 0

————

440

380

which the emission spectrum changed from a strong single dominant peak at 380 to 390 nm to a generally lower intensity level with a more structured emission at shorter wavelengths. Fig. lb

320

260

?~/ nm Fig. 2. Emission spectra excited by radiation of wavelength 230 nm at 300 K in Cai,Cd,S for x values of(a) 0, (b) 0.04. (c) 0.06 and (d) 0.10.

tion of composition at 77 K in which it can be seen that the greatest emission intensity is ohserved at x 0.008 compared to x 0.04 at 300 K. Fig. 3 shows emission intensity as a function of wavelength when excited by radiation of 230 nm wavelengths for x 0. 0.02. 0.06 and 0.10. =

=

.

=

.

a a a

ii ii ii

a B. Ray et a!

/

Emission spectra in Ca, - ~Cd~ S so/id solutions at 77 and 300 K

Table 2 Relative intensities of peaks in the emission spectra for Ca

1

Cd~Sexcited by 230 nm radiation at 77 K

x

Relative intensity 1~at A~in nm 290—315 A,=335—340 Xi= 12

0 0.008

1.4 16

2.1 49

0.02

20

39

0.04 0.06

19 2.4

48 2.9

96 3.2

0.09

4.5

5.2

0.10

1.3

1.1

0.12

1.0

0.9

53=380—390 13

The excitation spectra for near band edge emission, 290—310 nm, observed in all samples at various relative intensity levels, are characterised by two narrow peaks (half width 5 nm) at 235 and 255 nm. The emission peaks at 360—390 nm and 400—420 nm found in samples with x 0.008, 0.02 and 0.04 were excited by a broad band of half width 35 nm and centred at 300 nm, whilst satellite excitation peaks existed at 235 and 255 nm for =

I

I

A~=485—495 15

2.0 96

24

65

13

91 2.4

45 1.8

6.2

6.7

4.3

1.3

1.9

1.0

1.1

1.2

0.7

3.6

3.3.1. At 300 K

\x1~P

54=410—425 14

300 178

3.3. Excitation spectra

~•°

—

the same emissions but only about one fifth the level of the 300 nm excitation peak. The 360 nm emission peak in CaS, however, has a single, skewed, broad excitation peak at 245 nm (half width 45 nm). For x 0.06, 0.09 and 0.10, the emission peak at 330—340 nm is excited by 230, 255 and 270 nm to similar but relatively low levels, as is the 420 nm emission for x 0.10; however, for x 0.06 and 0.09, the 420 nm peak is most strongly excited by radiation at wavelengths between 290 and 305 nm. Fig. 4 shows a number of characteristic excitation spectra. =

=

=

~•o

I

I

I I

Ic)

__ 430

370 A /

310 nm

250

Fig. 3. Emission spectra excited by radiation of wavelength 230 nm at 77 K in Cai

647

Cd,~Sfor x values of (a) 0. (b) 0.02, (c) 0.06 and (d) 0.10.

0

I

II (a)

Ib)

]~ A ~‘ _

as r,3

I

I

0’ (“.1

I I I

~.o

rvs c’.J

A

/

n5

C’.J

.LL

0’ i”J

rn

C’~1

nm

Fig. 4. Excitation spectra at 300 K for emission at (a) 320 nm, (b) 365 nm and (c) 400 nm in Ca 0 98Cd002S.

648

B. Ray et al.

10

/

Emission spectra in Ca,

sulphate. As the cadmium content was increased the weighted scattering factor of that the cations came increasingly different from of the bean-

~/ ..~.cl

ions making increasingly obvious the sodium chlo-

—

ride structure of the solid solutions rather than a

\~

(dl

Relative X20

oso’~\

Intensity

~

“~

025

“.

I 350

~

/2

/

/

~L

~

.~

~,

Xl

‘

‘~.

\.~.

..~

/ X10~’

380

/

/

—

! X11

‘L~1L I 320 290 260 t / nm

-

23C

Fig. 5. Excitation spectra at 77 K for emission at 380 br emission at 380—420 nm in Cai ,Cd~S for r values of (a) 0 (380 nm). (b) 0.02 (390 nm). (c) 0.06 (420 nm) and (d) 0.09 (420 nm).

3.3.2. At 77 K

The most striking feature of the excitation spectra at 77 K is that the already very bright violet emission observed at 390 nm under 230 nm excitation for x 0.008, 0.02 and 0.04 is enhanced by more than an order of magnitude at an excitation wavelength of 285 nm. On the other hand, the intensity of the 485 nm emission in the same composition range is only marginally improved by moving to longer wavelength excitation at 300 nm. For x 0.06, the 335--340 nm peak is excited almost equally by 240. 255 and 270 nm radiation but the intensity is very low compared to that of the phosphors with lower CdS concentrations. The 420 emission peak is excited to similar intensity levels by 240, 260 and 300 nm radiation. Fig. 5 illustrates some of the excitation spectra observed at 77 K over the composition range x 0 to 0.10. =

=

4.

,Cd~Ssolid solutions at 77 asid 300 A

tamed with no traces of CaO, CdS or unreacted

Ib)

(I/I~l 075

-

Discussion

The structural determinations indicated that for all compositions a single phase condition per-

superficially primitive cubic type of half the cell parameter caused the innear 2 andbyS~ CaS.identical scattering factors of Caabsorption Atomic analyses showed that the Ca50 4’ 2H20 starting material contained in the region of 40 ppm of Mn as the major impurity. However, at that concentration level it would not be expected to show any significant effect, at-

though Pot and Sinha [7] have reported substantial emission in the region 540—600 nm in Ca5, 9Cd, S containing 0.02 mol Mn. The wavelenoth emissions, reported by Leh0 mann [3] and peaking at approximately 550 nm, were not observed in any of the samples; the

reasons for this may reside in either the preparation method (Lehmann treated his samples at 1200 rather than 1050°C)or that those emissions were due to the presence of large concentrations of Mn with or without coactivators. The optical absorption edge was observed to decrease linearly with composition over the range .v 0 to 0.10 from 259 (4.97 eV) to 278 (4.46 eV) nm at 300 K (see table 3). The excitation peaks associated with the hand edge were in general only dominant for the near hand edge emission. 290 to 310 nm, in the more strongly luminescent materials at 300 K. For x 0.008 to 0.04, the principal excitation peak wavelength. 300 nm (4.13 eV). excited luminescence at both 360—390 nm and =

=

Fable 3 Ahsorpiion wavelengths and energies asa function of composition Absorption edge A (nm) -

F (cV)

~~—_____

0 0.008 0.02 0.04

259 260 262 265

4.79 4.77 4.73 4.68

0.06

271

4.58

0.09

276

4.49

0.10

278

4.46

—.

B. Ray eta!.

/

Emission spectra in Ca, - ~Cd~S so/id solutions at 77 and 300 K

400—420 nm. Although excitation by radiation of 290—305 nm wavelength of emission at 390 and 420 nm did occur for greater x values, it was no longer the dominant exciting wavelength, Cooling the samples to 77 K, had a remarkable effect on both the excitation and emission spectra in samples with x between 0.008 and 0.04. The dominant emission peak was shifted to longer wavelengths, 390 nm, whilst the principal excited peak moved to shorter wavelengths at 280—285 nm with an ancillary peak at 310—315 nm. A possible explanation for the change in emission and excitation characteristics between 77 and 300 K for x 0.008 to 0.04 is as follows. At 300 K, electrons are excited from a complex centre’s ground state via excited states to the conduction band with the assistance of phonons; the large bandwidth of the excitation peak tends to lend further support to the notion of phonon assistance. Emission then occurs by electron transitions via higher excited states of the centre, which have a high capture cross-section for electrons in the conduction band, to the ground state. On decreasing the temperature, fewer phonons are available to assist both excitation and emission processes; thus the excitation process becomes the more highly efficient two body process of direct excitation from the ground state of the centre to conduction band at a higher energy. The emission process then occurs predominantly via the transitions from the lowest excited state to the ground state of the centre with a drop in the emitted energy. The observation that small additions, between 0.8 and 4 at% of cadmium on the calcium sub=

649

lattice, gives rise to considerably enhanced emission levels at both room and liquid nitrogen ternperatures cannot be explained simply. There does exist the possibility that, at low cadmium concentrations, aggregation of cadmium ions on adjacent octahedral sites creates local lattice distortion. Depending on the number of ions aggregating, a range of energy levels giving rise to emission will occur. At higher concentrations of cadmium, the number of defect sites of this type could be such as to have extensive interaction and result in bands of energy merging with the associated band edge, or alternatively, a random and non-localised distribution of cadmium ions exists modifying the periodic potential and thereby removing these defeet centres with strongly emitting characteristics. This explanation is not out of line with Lehmann’s hypothesis [3] which refers to clusters of cadmium ions and a tendency towards a tetrahedral configuration strong enough to create lattice disturbances.

References [1] W. Lehmann, J. Electrochem. Soc. 117 (1970) 1389. [2] Chem

and S. Taniguchi, J. Solid State

[3] W. Lehmann, J. Luminescence 5 (1972) 87 [4] F. Okamoto and K. Kato, J. Electrochem. Soc. 130 (1982) 432. [5] K. Kato and F. Okamoto, Japan. J. AppI. Phys. 22 (1983) 76. [6] J.C. Bickerton, l.V.F. Viney and B. Ray, J. Crystal Growth 72 (1985) 290.

[7] PG. Pol and A.P.G. Sinha, Indian J. Pure AppI. Phys. 2 (1973) 504.