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Growth and some physical properties of non-stoichiometric CdS single crystals
Journal of Crystal Growth 9 (1971) 209-216 © North-Holland Publishing Co.
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC CdS SINGLE CRYSTALS
M. HARSY, J. BALAZS, P. SVISZT, B. PODOR and E. LENDVAY Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary
CdS single crystals of various stoichiometric ratios have been grown in vacuum sealed ampoules using the vertical pulling method. Stoichiometry and physical properties of the crystals were found to be strongly dependent on the pre-treatment of the
charge. Optical transmission, electrical and microhardness measurements were used to characterize the crystals. The dependence of these properties on the growth conditions are discussed.
1. Introduction
were grown in vacuum-sealed ampoules by the gas
Large CdS single crystals of high perfection can be obtained using the gas phase pulling method1—3). Though A~B”~ compounds are of great importance and have been widely investigated, relatively little has been published which deals with the theoretical aspects of the growth process4’5). To date even the gas phase composition of the A”B~1 compounds is not known exactly. Some measurements have shown the complete absence of molecular associates6 9) while other authors have described the crystal growth as forming from the vapour’° 13). However, there still remain unsolved problems as regards the growth mechanism and the non-stoichiometry of the crystals which of course can cause enormousdifferences in theirphysical parameters. Apart from measurements of the physical parameters of CdS, including those of the basic constants, the influence of the non-stoichiometry on the crystal properties gen-
phase pulling method. The principal parameters (geometry, temperature, distribution, pulling rate, etc.) were kept constant. Only the pre-treatment of the ground material differed. The growth process, morphology, some optical and electrical parameters as well as the microhardness of the grown non-stoichiometric CdS single crystals have been investigated.
erally has not been taken into consideration. In growth systems, especially in the closed ones this deviation changes the gas phase composition and the growth process can be completely inhibited. Even devia2 in small the source tions (1—3 x 106 at Cd/mole CdS) composition considerably alter the vapour phase cornposition as well as the non-stoichiometry of the growing crystals. The object of the present paper is to investigate this phenomenon with regard to the growth mechanism and the semiconductive properties of the crystals. To this end CdS single crystals of different stoichiometry
2. The growth process All crystals examined were grown by a technique similar to that which had been first described by Kaldis and Widmer3”4 15). The furnace and the geometry of the quartz ampoule are shown in fig. 1. The ground material was luminescent grade CdS powder obtained from the Institute for Telecommunication, Budapest. Before the growth process the powder was fired in a slow-flow of dry H 2S gas (flow rate 10—15 1/h) supplied by Matheson and Co. USA. After being charged the ampoules were evacuated to about 1—6 x 10 ~ Torr 1—6 hr and heated at 200—300 °C simultaneously. Following the ampoules were sealed, taking care this that treatment neither decomposition of the CdS powder nor fixation of some crystallites in the ampoule tip occurred. A quartz rod of 3 mm diameter had been joined to the tip of the ampoule to conduct away the heat of crystallization and to pull the ampoule. During the growth process the ampoules were pulled vertically at a rate of 1 mm/h from a homogeneous temperature zone of 1180 °Cinto a part of the furnace where the
lII.A.5
210
M. HARSY, J. BALASZ, P. SVISZT, B. PODdR AND E. LENDVAY QUA/I ~‘~ROD
N T//ERTIOCOIJPLE
N
~
~ QLJARIZ WOOL
N
~
/ —Ilo/ILITE INSULAr/PIG BRICK
II
1NSULAT/fi/G
A~OULt ~.
~.
I 10 I
.~NN. _/ANTNA~,,~~
INNER Q~~Z
II I
//
~
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(convex or concave) or covered by planes. Corresponding to the observations described by Kaldis3) the crystal consisted of prismatic covering planes of first and second rank and of basal ones.
1(a) -~
boules were produced (see fig. 3a). Under these conditions the transport rate was higher than 80 mg/h. Generally a small void could be observed in the cone tip theand pointed end of the boule was co~eredby tiny and basal prismatic facets. The hexagonal c-axis formed an angle of 10—20° with the main growth direction. Depending on the pre-treatment conditions, the growing surface of the boule was either curved
I
QUARTZ TUBE
material was fired above 900 °Cfor a long period and had been heated during the evacuation of the ampoule the whole quantity of charged CdS (4 0 g) was trans ported into the conical tip of the ampoule and large
~—
I
OXIDE
properties of the crystals. The structure and the orientation of the external crystal faces was determined by goniornetry and X-ray Laue technique using Cu-Kct radiation. In limiting cases where the CdS ground
10
cm
70
17
1(b)
N
1
V
QOOLE
I
Fig. 1. (a) Schematic diagram of the furnace used for growing CdS single crystals. (b) The shape and data of ampoule used for growing. POWOEP
decreasing temperature gradient was about 5—4 °C cm_i. This is illustrated diagrammatically in fig. 2. 3. Results 3.1. GROWTH AND MORPHOLOGY Depending on the pre-treatment of the CdS powder different transport rates were observed causing changes in the morphology, the colour and the other physical
I
50
/i i ~
1000
Fig. 2.
2100
7200
7300
Temperature distribution in the furnace.
GROWTH
AND SOME PHYSICAL PROPERTIES
OF NON-STOICFIIOMETRIC
In some cases when the pre-treatment of CdS powder was carried out at a lower temperature than 900 °Cor it had not been pretreated at all, the transport rate decreased to 1.0—60.0 mg/h. At the bottom of the ampoule, on the charge surface, large and dark CdS single crystals formed (see fig. 3b). X-ray examination proved them to be perfect. Even defocussing the X-ray beam and irradiating a 4 mm diameter surface region of the basal plane of the above described crystal, the X-ray pattern showed single crystal reflections, fig. 4, confirming the high perfection of the crystals. The dominant prism planes (some of them were in the neighbourhood of 1 cm2) were (2l~0)and (ll~0) type while the others most frequently were (1010). Sometimes prismatic planes of second rank such as (1012) could also be observed. The transport rate was greatly decreased by adding 0.1—0.5 at° elementary
CdS
SINGLE CRYSTALS
211
_____ Ii
~
Fig. 4. Laue reflection from the ha~al plane of CdS single crystal grown on the source. The X-ray beam is strongly defocused.
sulphur or cadmium to the source. At concentrations higher than these values no transport took place at all (see fig. 5). After adding cadmium to the powder, a light yellow sintered CdS block was formed, while in presence of excess sulphur a block covered by small dark crystallites was produced. Table I shows some characteristic data of the experiments described. 3.2. OPTICAL PROPERTIES AND MICROHARDNESS Depending on the pre-treatment of the powder the
Fig. 3. a Large CdS boule grown in the conical tip and in the following cylindrical part. (b) Partial transport. A CdS boule
is grown in the ampoule tip while on the source large CdS crystal was formed,
colour of the crystals varied between pale yellow and light brown. All these crystals were completely transparent. In other cases, when only partial transport was observed, brown and generally non-transparent hexagonal crystals grew on the charge surface. With partial transport a characteristic difference was observed i.e. the CdS boules grown in the tips were usually lighter in colour than the large crystals grown .
.
.
212
M. HARSY, J. BALAZS, P. SVISZT, B. PODOR AND E. LENDVAY
special holder in a Beckman DU spectrophotorneter. Fig. 6 displays some characteristic curves showing that practically no change can be observed in the position of the absorption edge, however, in the 540—650 nm spectral range a strong uniform absorption occurs and its value markedly depends on the prehistory of the powder. The same samples were subjected to microhardness measurements using a Reichert Microhardness Tester attached to an Universal MeF microscope. Owing to the limited number of samples and because of the statistical character of the measured microhardnessi 6) no significant differences between the different H~ values could be observed. Control measurements on some high resistivity CdS crystals (p > 106 ohm/cm) grown by the French methods were carried out and a remarkable and well-reproducible difference could be found between the crystals prepared by the gas phase pulling method and those grown in the open-flow system. These results are shown in fig. 7.
~
Fig. 5. Sintered CdS sources containing excess elementary Cd or S after the suitable growth procedure. Left: The influence of excess Right: (b) Source with excess sulphur.
on the charge surface. Plane parallel slices were cut from the CdS boules parallel to the c-axis and the transmission of 100 ~tm thick samples was measured using a
3 .3. ELECTRICAL MEASUREMENTS Following the transmission and microhardness measurements electrical contacts were applied to the CdS samples by evaporating and annealing in on to their surfaces. For galvanomagnetic investigations the Van der Pauw technique was used. The measurements were made over a temperature range of 77—400 °K. The conductivity of the CdS samples varied between 10 ohm_i cm_i and 100 ohm_i cm_i. The charge carrier
TABLE
I
Effect of pre-treatn-ient on the CdS crystals grown by gas phase pulling method Charge No.
Temperature (°C)of firing in H 2S for 4h
15 4! 54
*
800 ‘—800 900
Temperature of heat treatment during
Transport rate
Colour
(mg/h)
evacuation No No No
24 14 28 ~—20
32 35 36
900 *9~(~at°V0S) ~900 (I at% Cd)
No No
0
47 59 72 53 51 20
800 900 *900 (0.05 at% 5) 960 1000 1050
300 300 300 300 300 300
>80 >80 50 >80 72 >80
No
0.1—1
The excess component was added to the powder after heat treatment.
Dark brown Dark brown Yellow Reddish brown
Light brown Light yellow
Yellow Dark yellow Tip: light yellow; bottom: light brown Yellow Light brown Pale yellow
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC
CdS
SINGLE CRYSTALS
213
TPANSM/TTANCE 80
I, / 1I /.1/ •/• // ~•
50
—.
~
~
i/S
~ I
21 C~N° ~51 53
/
a
‘7
30
20
‘-‘-
20
70
5~3
Fig. 6.
650
500
700
nm
Transmittance ofdifferent CdS samples.
concentration amounted to 101 5_l018 cm3. Also the Hall mobility shows a considerable change, since the it values vary between 120 and 315 cm2 V~’ sec 1, Table 2 shows the results of the experiments in detail.
A very good linear agreement was found between the donor activation energy and the cube-root of the donor concentrations1 7) according to the Pearson— Bardeen model, in the same way as the described data
H 0 (kPQ1~jND/mm°I
Ij gl
~
sjI
1/
~ s,
\\
‘~--~. sf
GAS PHASE PULLING SYSTEM 70~~<( 70 ohm cm
f
50
OPEN FLOW ~ (FREuicHS) SYSTEM ohmcm
LOAD (POUND) 0
Fig. 7.
50
100
750
Load dependence of Vickers microhardness on the (1010) prism-plane of CdS. (a) Values of CdS grown by Frerichs method. (b) Values on crystals grown by vapour phase pulling method.
214
M. HARSY, J. BALAZS, P. SVISZT, B. P~D~RAND E. LENDVAY TABLE
2
Electrical data of different CdS single crystals Charge No.
32 47 59 51 53 20 15
Conductivity (ohm cm I)
2Mobility V sec
(cm
0.34 1.0 3.4 3.0 2.95 18.5 76.0
I)
3l5 125 120 200 240 270 165
Electron concentration (cm~3) 6.96x101° 4.85x10’6 I.75x 10’~ 9.20x1016 7.45x10b6 4.ll>’l0’~ 2.9lx10’8
concerning CdS’8”9). Using our own results and those previously described in the literature2 0_23) the relationship displayed in fig. 8 can be obtained. 4. Discussion Although CdS is one of the most thoroughly studied AiiBvi compounds, the majority of its parameters which influence the growth and the non-stoichiometry are not known. In order to exclude the effects of impunities in all the above described experiments, the same pure CdS ground material was used. If we do not take into condideration the possibility of the powder being contaminated during the pre-treatment, the differences occurring between the electrical and other
Compensation degree (%)
8.8x10’6 l.lxlO’7 3.3 x l0’° I.58x10’7 3 l~
91.0 46.0 330 31.7 258 21.0 >10.0
characteristics of the crystal cannot be due to impuri ties. The only reasonable explanation for these differences is the deviation in the stoichiornetry of the crystals. Thus the prehistory of the powder proved to be one of the most important parameters during the CdS growth. According to Ballentyne et al.5) this is probably the reason why the reproducibility is so poor in the cases when industrial CdS and powders or lumps of different origin were used4’5). The fact that the different pretreatments cause a considerable change in the physical properties of the CdS crystals show that in closed systems the composition of the vapour phase is determined mainly by that of the solid phase source. There are two possible interpretations of the experimental results. Since the CdS stoichiometry is very -
sensitive to relatively small effects, e.g. precipitation conditions, heat treatment etc., and since adsorbed gases have a strong influence on the growth process, e.g. the role of heat treatment during the evacuation of the ampoule, it is a reasonable assumption that for many cases the generally accepted equilibrium.
30
o
r’ - e
~23]
LITERATURE DATA III
(.1
• NiP EXPERI.~NTS 20
Donor concentration (cm°)
._~.
(s)
r~-i
I
(g) ~ ~
S 2(g)
is fairly exact. Considering the mass-spectrolnetric and absorption data6’7’24), it is more reasonable to suppose the following equilibria:
°
CdS(S) :~± CdS(
•
IL
8),
(1)
CdS(g)
~±
Cd(g) + S(g),
(2)
S(g)
~±
+ S2(g),
(3)
which gives: a)
70~
4 function,
Fig. 8.
The Ed donor activation energy versus (Nd)
Cd(g) + ~ S2~8~, (4) where at elevated temperatures the dissociation conCdS(S) ~
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC
CdS
SINGLE CRYSTALS
215
stant of eq. (2) is very high (according to the measurements listed in ref. 5, between 800 and 1200 °Cthe dissociation is nearly 100°/a). however, it does not exclude a priori the existence of the molecular species in the vapour phase. No data are available to show how
and the vapour phase differ from each other. If inequality (7) is valid, there must be a segregation process promoted also by the weight-loss of the source. During the growth x, in eq. (1) changes continuously (but probably slowly). Since x1 determines the p, values, the
the non-stoichiometry changes the5(g) above mightequilibria, alter eq. The appearance of excess transport Cd(g) or via molecular Va(2) leading to increased pours. This variation is supported by some growth experiments where the crystal growth is supposed to occur from molecular CdS or its higher associates7”3) as well as by the theoretical predictions of Ballentyne et al. 5) which indicate that a CdS crystal of good quality is unlikely to be obtained from growth in stoichiometnc vapour, The other possibility is to treat the non-stoichio-
composition growing crystals changes. Such an effect wasofthe observed by Clark andalso Woods2 5) effectively. Some systematic correlations between the experimental conditions and the observed properties are shown in table 1. The low temperature heat-treatment has a decisive role. None of the samples was transported completely during the growth process if baking of the powder was omitted. The reason is obvious: in closed systems either the excess elementary sulphur originated by oxidizing processes during precipitation, or the adsorbed H 2S, or both are strongly inhibiting the transport process. This assumption is supported by adding elementary sulphur to the powder. This simple treatment decreases the transport rate enormously as
metric system as a two-component one. In this case the composition of the solid and vapour phases is described by the Duhem—Margules equation: (5)
seen in case of Charge 35 and 36. Even if the powder is heated, the addition of a small quantity of sulphur decreases the transport rate, as occurred for Charge 72.
where p1 is the partial pressure of the excess component of CdS in the vapour phase, x• the molecular fraction of the excess component in the solid phase (this is the measure of the non-stoichiometry of the source) and P the total pressure caused by the stoichiometric part of the CdS dissociation according to the reaction equilibria described above. This treatment is similar to those described by Ballentyne et al. 5) where the total pressure changes according to
It is, however, interesting to remark that with partial transport neglecting the low temperature heat treatment, the difference between the colour of the transported and residual CdS is not significant. When S is added to baked powders this difference becomes very distinct. Comparison of the data of tables 1 and 2 reveals interesting relationsships. The degree of compensation, i.e. the measure of non-stoichiometry shows a definite
(1—x,)
/~3ln P\
/~in
(~~—-•)
~
p.\ =
-
T
f~In P’\
0,
T
(6)
connection with the pre-treatment, except Charge 15. Rising the temperature of the pre-treatment, the corn-
y, where P’ is the total pressure above the non-stoichiometric powder and y~the molar fraction of component i in the gas phase. This model does not need the supposition of growth from stoichiometric or slightly nonstoichiometric associates the probability of which is very low at the temperatures used but explains, e.g., the observed differences at partial transport between the properties of CdS crystals grown on the charge surface and in the ampoule tip, which occurs when x. ~ ~,. (7)
pensation degree monotonously decreases. However, the donor concentrations, the free electron concentration and naturally the conductivity of the samples increase monotonously. The mobilities of carriers, however, show no connection with the measured data proving that their values depend on some other parameters. The observed correlations can be explained by assuming that the non-stoichiometry is in fact due to the S vacancies as described by Woods, and that the high temperature state can easily be quenched to room temperature. Probably the S vacancies are partly cornpensated by Cd vacancies. Since S is the most volatile
I
~ .1
—
I
I’
i.e. the compositions of the solid phase at the source
component in the system, the formation of S vacancies
216
M. HARSY, J. BALAZS, P. SVISZT, B. PODöR AND F. LEND VAY
surpasses that of the Cd vacancies during the high temperature heat-treatment. This explains the observed differences between the donor concentrations and the compensation degrees. The trend of the heat-treatments
properties and the microhardness of the different crystals were investigated. The galvanomagnetic measurements were made over a temperature range between liquid N2 and room temperature. The resistivity
agrees well with the mentioned measurements, The results reported here show that there is no definite correlation between (the the colour of the refer crystal its electrical properties same might toand the non-stoichiometrical properties). In accordance with some published data 25) crystals of a faint colour (pale yellow) can have low resistivities e.g. iO~~ ohm/cm, as well as high ones e.g. 1010 ohm/cm. Since crystals with high resistivity were not observed
of the measured samples varied from several ohm/cm to l0~~ohm/cm. The concentration the charge 3.The donorof concentration carriers was 101 5_1018 cm~ determined form these data was lObo_1019 cm~3.A considerable change was also observed in the Hallmobility where values changed from 165—330 cm2 \1 i sec~
in our work, it was not possible to decide whether the difference between the microhardness of our crystals and those prepared by the French method was caused by the non-stoichiometry of the crystals. One of the most interesting problems which remained unsolved, is the colour of the crystals. The nature of the optical absorption of these crystals is unknown and calls for further effort. 5. Summary CdS single crystals of different stoichiometry have been grown in vacuum sealed ampoules. The stoichiometry and the physical properties of the grown crystals proved to be strongly dependent on the pre-treatment of the CdS ground material and on the circumstances of the ampoule sealing. In order to clarify the problem, luminescent pure CdS ground materials were fired in pure H 2S atmosphere for ito 6 h at 600—1000 °C. During the growth process the ampoules weie pulled vertically from a homogeneous temperature zone of 1180 °Cat a rate of I mm/h to a region with a temperature gradient of 5—10 °Ccm till the tip of the ampoules reached 1130 °C.Depending on the transport properties of the ground material either the whole of the powder or only a part of it had been transported into the conical tip. In the second case, however, well developed hexagonal CdS crystals were formed on the charge, while a crystal boule was formed in the tip. The optical transmission, the galvanomagnetic
References 1) 2) 3) 4)
W. W. Piper and S. J. Polich, J. AppI. Phys. 32 (1961) 1278. L. Hildisch, J. Crystal Growth, 3,4 (1968) 131. E. Kaldis, J. Crystal Growth 5 (1969) 376. M. Toyama, Japan. J. AppI. Phys. 5 (1966) 1204.
5) D. W. G. Ballentyne, S. Wetwatana and E. A. D. White, J. Crystal Growth 7 (1970) 79. 6) P. Goldflnger and M. Jeunehomme, Trans. Faraday Soc. 59 (1963) 2851. 7) A. S. Pashinkin and B. A. Salamantin, Soviet Inorg. Mater. 5 (1969) 256 8) J. R. Marquart and J. Berkowitz, J. Chem. Phys. 89 (1963) 288. 9) G. DeMaria, P. Goldflnger, L. Malaspina and V. Piacente, Trans. Faraday Soc. 68 (1968) 2146. 10) D. C. Reynolds and L. C. Green, J. AppI. Phys. 29 (1958) 11) H. Samuelson, J. AppI. Phys. 32(1961) 309. 12) R. J. Caveney, J. Crystal Growth 7 (1970) 102. 13) B. M. Bulakh, J. Crystal Growth 5 (1969) 243. 14) E. Kaldis and R. Widmer, J. Phys. Chem. Solids 26 (1965) 1697. 15) F. Kaldis, J. Phys. Chem. Solids 26 (1965) 1701. 16) V. Fok, Phys. J. Mater. (1969)865. 747. 17) F. G. Lendvay Pearson and and J.M.Bardeen, Rev.Sci. 75 4(1949) 18) R. H. Bube, E. L. Lind and A. B. Dreeben, Phys. Rev. 128 (1962) 532. 19) W. E. 130. Spear and J. Mort, Proc. Phys. Soc. (London) 81 (1963) 20) F. A. Kroger, H. J. Vink and J. Volger, Philips Res. Rept. 10 (1955) 39. 21) W. W. Piper and R. Prague, E. Halsted, conductor Physics, 1960, in: p. Intern. 1046. Conf. on Semi. 22) M. Itakura and H. Toyoda, J. Phys. Soc. Japan 18 (1963) 150. 23) R. Crandall, Phys. Rev 169 (1968) 577. 24) Sen Gupta, Proc. Roy. Soc. (London) A 143 (1933) 438. 25) L. Clark and L. Woods, Brit. J. AppI. Phys. 17 (1966) 319.
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC CdS SINGLE CRYSTALS
M. HARSY, J. BALAZS, P. SVISZT, B. PODOR and E. LENDVAY Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary
CdS single crystals of various stoichiometric ratios have been grown in vacuum sealed ampoules using the vertical pulling method. Stoichiometry and physical properties of the crystals were found to be strongly dependent on the pre-treatment of the
charge. Optical transmission, electrical and microhardness measurements were used to characterize the crystals. The dependence of these properties on the growth conditions are discussed.
1. Introduction
were grown in vacuum-sealed ampoules by the gas
Large CdS single crystals of high perfection can be obtained using the gas phase pulling method1—3). Though A~B”~ compounds are of great importance and have been widely investigated, relatively little has been published which deals with the theoretical aspects of the growth process4’5). To date even the gas phase composition of the A”B~1 compounds is not known exactly. Some measurements have shown the complete absence of molecular associates6 9) while other authors have described the crystal growth as forming from the vapour’° 13). However, there still remain unsolved problems as regards the growth mechanism and the non-stoichiometry of the crystals which of course can cause enormousdifferences in theirphysical parameters. Apart from measurements of the physical parameters of CdS, including those of the basic constants, the influence of the non-stoichiometry on the crystal properties gen-
phase pulling method. The principal parameters (geometry, temperature, distribution, pulling rate, etc.) were kept constant. Only the pre-treatment of the ground material differed. The growth process, morphology, some optical and electrical parameters as well as the microhardness of the grown non-stoichiometric CdS single crystals have been investigated.
erally has not been taken into consideration. In growth systems, especially in the closed ones this deviation changes the gas phase composition and the growth process can be completely inhibited. Even devia2 in small the source tions (1—3 x 106 at Cd/mole CdS) composition considerably alter the vapour phase cornposition as well as the non-stoichiometry of the growing crystals. The object of the present paper is to investigate this phenomenon with regard to the growth mechanism and the semiconductive properties of the crystals. To this end CdS single crystals of different stoichiometry
2. The growth process All crystals examined were grown by a technique similar to that which had been first described by Kaldis and Widmer3”4 15). The furnace and the geometry of the quartz ampoule are shown in fig. 1. The ground material was luminescent grade CdS powder obtained from the Institute for Telecommunication, Budapest. Before the growth process the powder was fired in a slow-flow of dry H 2S gas (flow rate 10—15 1/h) supplied by Matheson and Co. USA. After being charged the ampoules were evacuated to about 1—6 x 10 ~ Torr 1—6 hr and heated at 200—300 °C simultaneously. Following the ampoules were sealed, taking care this that treatment neither decomposition of the CdS powder nor fixation of some crystallites in the ampoule tip occurred. A quartz rod of 3 mm diameter had been joined to the tip of the ampoule to conduct away the heat of crystallization and to pull the ampoule. During the growth process the ampoules were pulled vertically at a rate of 1 mm/h from a homogeneous temperature zone of 1180 °Cinto a part of the furnace where the
lII.A.5
210
M. HARSY, J. BALASZ, P. SVISZT, B. PODdR AND E. LENDVAY QUA/I ~‘~ROD
N T//ERTIOCOIJPLE
N
~
~ QLJARIZ WOOL
N
~
/ —Ilo/ILITE INSULAr/PIG BRICK
II
1NSULAT/fi/G
A~OULt ~.
~.
I 10 I
.~NN. _/ANTNA~,,~~
INNER Q~~Z
II I
//
~
/
~—
MO/I/fEE
(convex or concave) or covered by planes. Corresponding to the observations described by Kaldis3) the crystal consisted of prismatic covering planes of first and second rank and of basal ones.
1(a) -~
boules were produced (see fig. 3a). Under these conditions the transport rate was higher than 80 mg/h. Generally a small void could be observed in the cone tip theand pointed end of the boule was co~eredby tiny and basal prismatic facets. The hexagonal c-axis formed an angle of 10—20° with the main growth direction. Depending on the pre-treatment conditions, the growing surface of the boule was either curved
I
QUARTZ TUBE
material was fired above 900 °Cfor a long period and had been heated during the evacuation of the ampoule the whole quantity of charged CdS (4 0 g) was trans ported into the conical tip of the ampoule and large
~—
I
OXIDE
properties of the crystals. The structure and the orientation of the external crystal faces was determined by goniornetry and X-ray Laue technique using Cu-Kct radiation. In limiting cases where the CdS ground
10
cm
70
17
1(b)
N
1
V
QOOLE
I
Fig. 1. (a) Schematic diagram of the furnace used for growing CdS single crystals. (b) The shape and data of ampoule used for growing. POWOEP
decreasing temperature gradient was about 5—4 °C cm_i. This is illustrated diagrammatically in fig. 2. 3. Results 3.1. GROWTH AND MORPHOLOGY Depending on the pre-treatment of the CdS powder different transport rates were observed causing changes in the morphology, the colour and the other physical
I
50
/i i ~
1000
Fig. 2.
2100
7200
7300
Temperature distribution in the furnace.
GROWTH
AND SOME PHYSICAL PROPERTIES
OF NON-STOICFIIOMETRIC
In some cases when the pre-treatment of CdS powder was carried out at a lower temperature than 900 °Cor it had not been pretreated at all, the transport rate decreased to 1.0—60.0 mg/h. At the bottom of the ampoule, on the charge surface, large and dark CdS single crystals formed (see fig. 3b). X-ray examination proved them to be perfect. Even defocussing the X-ray beam and irradiating a 4 mm diameter surface region of the basal plane of the above described crystal, the X-ray pattern showed single crystal reflections, fig. 4, confirming the high perfection of the crystals. The dominant prism planes (some of them were in the neighbourhood of 1 cm2) were (2l~0)and (ll~0) type while the others most frequently were (1010). Sometimes prismatic planes of second rank such as (1012) could also be observed. The transport rate was greatly decreased by adding 0.1—0.5 at° elementary
CdS
SINGLE CRYSTALS
211
_____ Ii
~
Fig. 4. Laue reflection from the ha~al plane of CdS single crystal grown on the source. The X-ray beam is strongly defocused.
sulphur or cadmium to the source. At concentrations higher than these values no transport took place at all (see fig. 5). After adding cadmium to the powder, a light yellow sintered CdS block was formed, while in presence of excess sulphur a block covered by small dark crystallites was produced. Table I shows some characteristic data of the experiments described. 3.2. OPTICAL PROPERTIES AND MICROHARDNESS Depending on the pre-treatment of the powder the
Fig. 3. a Large CdS boule grown in the conical tip and in the following cylindrical part. (b) Partial transport. A CdS boule
is grown in the ampoule tip while on the source large CdS crystal was formed,
colour of the crystals varied between pale yellow and light brown. All these crystals were completely transparent. In other cases, when only partial transport was observed, brown and generally non-transparent hexagonal crystals grew on the charge surface. With partial transport a characteristic difference was observed i.e. the CdS boules grown in the tips were usually lighter in colour than the large crystals grown .
.
.
212
M. HARSY, J. BALAZS, P. SVISZT, B. PODOR AND E. LENDVAY
special holder in a Beckman DU spectrophotorneter. Fig. 6 displays some characteristic curves showing that practically no change can be observed in the position of the absorption edge, however, in the 540—650 nm spectral range a strong uniform absorption occurs and its value markedly depends on the prehistory of the powder. The same samples were subjected to microhardness measurements using a Reichert Microhardness Tester attached to an Universal MeF microscope. Owing to the limited number of samples and because of the statistical character of the measured microhardnessi 6) no significant differences between the different H~ values could be observed. Control measurements on some high resistivity CdS crystals (p > 106 ohm/cm) grown by the French methods were carried out and a remarkable and well-reproducible difference could be found between the crystals prepared by the gas phase pulling method and those grown in the open-flow system. These results are shown in fig. 7.
~
Fig. 5. Sintered CdS sources containing excess elementary Cd or S after the suitable growth procedure. Left: The influence of excess Right: (b) Source with excess sulphur.
on the charge surface. Plane parallel slices were cut from the CdS boules parallel to the c-axis and the transmission of 100 ~tm thick samples was measured using a
3 .3. ELECTRICAL MEASUREMENTS Following the transmission and microhardness measurements electrical contacts were applied to the CdS samples by evaporating and annealing in on to their surfaces. For galvanomagnetic investigations the Van der Pauw technique was used. The measurements were made over a temperature range of 77—400 °K. The conductivity of the CdS samples varied between 10 ohm_i cm_i and 100 ohm_i cm_i. The charge carrier
TABLE
I
Effect of pre-treatn-ient on the CdS crystals grown by gas phase pulling method Charge No.
Temperature (°C)of firing in H 2S for 4h
15 4! 54
*
800 ‘—800 900
Temperature of heat treatment during
Transport rate
Colour
(mg/h)
evacuation No No No
24 14 28 ~—20
32 35 36
900 *9~(~at°V0S) ~900 (I at% Cd)
No No
0
47 59 72 53 51 20
800 900 *900 (0.05 at% 5) 960 1000 1050
300 300 300 300 300 300
>80 >80 50 >80 72 >80
No
0.1—1
The excess component was added to the powder after heat treatment.
Dark brown Dark brown Yellow Reddish brown
Light brown Light yellow
Yellow Dark yellow Tip: light yellow; bottom: light brown Yellow Light brown Pale yellow
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC
CdS
SINGLE CRYSTALS
213
TPANSM/TTANCE 80
I, / 1I /.1/ •/• // ~•
50
—.
~
~
i/S
~ I
21 C~N° ~51 53
/
a
‘7
30
20
‘-‘-
20
70
5~3
Fig. 6.
650
500
700
nm
Transmittance ofdifferent CdS samples.
concentration amounted to 101 5_l018 cm3. Also the Hall mobility shows a considerable change, since the it values vary between 120 and 315 cm2 V~’ sec 1, Table 2 shows the results of the experiments in detail.
A very good linear agreement was found between the donor activation energy and the cube-root of the donor concentrations1 7) according to the Pearson— Bardeen model, in the same way as the described data
H 0 (kPQ1~jND/mm°I
Ij gl
~
sjI
1/
~ s,
\\
‘~--~. sf
GAS PHASE PULLING SYSTEM 70~~<( 70 ohm cm
f
50
OPEN FLOW ~ (FREuicHS) SYSTEM ohmcm
LOAD (POUND) 0
Fig. 7.
50
100
750
Load dependence of Vickers microhardness on the (1010) prism-plane of CdS. (a) Values of CdS grown by Frerichs method. (b) Values on crystals grown by vapour phase pulling method.
214
M. HARSY, J. BALAZS, P. SVISZT, B. P~D~RAND E. LENDVAY TABLE
2
Electrical data of different CdS single crystals Charge No.
32 47 59 51 53 20 15
Conductivity (ohm cm I)
2Mobility V sec
(cm
0.34 1.0 3.4 3.0 2.95 18.5 76.0
I)
3l5 125 120 200 240 270 165
Electron concentration (cm~3) 6.96x101° 4.85x10’6 I.75x 10’~ 9.20x1016 7.45x10b6 4.ll>’l0’~ 2.9lx10’8
concerning CdS’8”9). Using our own results and those previously described in the literature2 0_23) the relationship displayed in fig. 8 can be obtained. 4. Discussion Although CdS is one of the most thoroughly studied AiiBvi compounds, the majority of its parameters which influence the growth and the non-stoichiometry are not known. In order to exclude the effects of impunities in all the above described experiments, the same pure CdS ground material was used. If we do not take into condideration the possibility of the powder being contaminated during the pre-treatment, the differences occurring between the electrical and other
Compensation degree (%)
8.8x10’6 l.lxlO’7 3.3 x l0’° I.58x10’7 3 l~
91.0 46.0 330 31.7 258 21.0 >10.0
characteristics of the crystal cannot be due to impuri ties. The only reasonable explanation for these differences is the deviation in the stoichiornetry of the crystals. Thus the prehistory of the powder proved to be one of the most important parameters during the CdS growth. According to Ballentyne et al.5) this is probably the reason why the reproducibility is so poor in the cases when industrial CdS and powders or lumps of different origin were used4’5). The fact that the different pretreatments cause a considerable change in the physical properties of the CdS crystals show that in closed systems the composition of the vapour phase is determined mainly by that of the solid phase source. There are two possible interpretations of the experimental results. Since the CdS stoichiometry is very -
sensitive to relatively small effects, e.g. precipitation conditions, heat treatment etc., and since adsorbed gases have a strong influence on the growth process, e.g. the role of heat treatment during the evacuation of the ampoule, it is a reasonable assumption that for many cases the generally accepted equilibrium.
30
o
r’ - e
~23]
LITERATURE DATA III
(.1
• NiP EXPERI.~NTS 20
Donor concentration (cm°)
._~.
(s)
r~-i
I
(g) ~ ~
S 2(g)
is fairly exact. Considering the mass-spectrolnetric and absorption data6’7’24), it is more reasonable to suppose the following equilibria:
°
CdS(S) :~± CdS(
•
IL
8),
(1)
CdS(g)
~±
Cd(g) + S(g),
(2)
S(g)
~±
+ S2(g),
(3)
which gives: a)
70~
4 function,
Fig. 8.
The Ed donor activation energy versus (Nd)
Cd(g) + ~ S2~8~, (4) where at elevated temperatures the dissociation conCdS(S) ~
GROWTH AND SOME PHYSICAL PROPERTIES OF NON-STOICHIOMETRIC
CdS
SINGLE CRYSTALS
215
stant of eq. (2) is very high (according to the measurements listed in ref. 5, between 800 and 1200 °Cthe dissociation is nearly 100°/a). however, it does not exclude a priori the existence of the molecular species in the vapour phase. No data are available to show how
and the vapour phase differ from each other. If inequality (7) is valid, there must be a segregation process promoted also by the weight-loss of the source. During the growth x, in eq. (1) changes continuously (but probably slowly). Since x1 determines the p, values, the
the non-stoichiometry changes the5(g) above mightequilibria, alter eq. The appearance of excess transport Cd(g) or via molecular Va(2) leading to increased pours. This variation is supported by some growth experiments where the crystal growth is supposed to occur from molecular CdS or its higher associates7”3) as well as by the theoretical predictions of Ballentyne et al. 5) which indicate that a CdS crystal of good quality is unlikely to be obtained from growth in stoichiometnc vapour, The other possibility is to treat the non-stoichio-
composition growing crystals changes. Such an effect wasofthe observed by Clark andalso Woods2 5) effectively. Some systematic correlations between the experimental conditions and the observed properties are shown in table 1. The low temperature heat-treatment has a decisive role. None of the samples was transported completely during the growth process if baking of the powder was omitted. The reason is obvious: in closed systems either the excess elementary sulphur originated by oxidizing processes during precipitation, or the adsorbed H 2S, or both are strongly inhibiting the transport process. This assumption is supported by adding elementary sulphur to the powder. This simple treatment decreases the transport rate enormously as
metric system as a two-component one. In this case the composition of the solid and vapour phases is described by the Duhem—Margules equation: (5)
seen in case of Charge 35 and 36. Even if the powder is heated, the addition of a small quantity of sulphur decreases the transport rate, as occurred for Charge 72.
where p1 is the partial pressure of the excess component of CdS in the vapour phase, x• the molecular fraction of the excess component in the solid phase (this is the measure of the non-stoichiometry of the source) and P the total pressure caused by the stoichiometric part of the CdS dissociation according to the reaction equilibria described above. This treatment is similar to those described by Ballentyne et al. 5) where the total pressure changes according to
It is, however, interesting to remark that with partial transport neglecting the low temperature heat treatment, the difference between the colour of the transported and residual CdS is not significant. When S is added to baked powders this difference becomes very distinct. Comparison of the data of tables 1 and 2 reveals interesting relationsships. The degree of compensation, i.e. the measure of non-stoichiometry shows a definite
(1—x,)
/~3ln P\
/~in
(~~—-•)
~
p.\ =
-
T
f~In P’\
0,
T
(6)
connection with the pre-treatment, except Charge 15. Rising the temperature of the pre-treatment, the corn-
y, where P’ is the total pressure above the non-stoichiometric powder and y~the molar fraction of component i in the gas phase. This model does not need the supposition of growth from stoichiometric or slightly nonstoichiometric associates the probability of which is very low at the temperatures used but explains, e.g., the observed differences at partial transport between the properties of CdS crystals grown on the charge surface and in the ampoule tip, which occurs when x. ~ ~,. (7)
pensation degree monotonously decreases. However, the donor concentrations, the free electron concentration and naturally the conductivity of the samples increase monotonously. The mobilities of carriers, however, show no connection with the measured data proving that their values depend on some other parameters. The observed correlations can be explained by assuming that the non-stoichiometry is in fact due to the S vacancies as described by Woods, and that the high temperature state can easily be quenched to room temperature. Probably the S vacancies are partly cornpensated by Cd vacancies. Since S is the most volatile
I
~ .1
—
I
I’
i.e. the compositions of the solid phase at the source
component in the system, the formation of S vacancies
216
M. HARSY, J. BALAZS, P. SVISZT, B. PODöR AND F. LEND VAY
surpasses that of the Cd vacancies during the high temperature heat-treatment. This explains the observed differences between the donor concentrations and the compensation degrees. The trend of the heat-treatments
properties and the microhardness of the different crystals were investigated. The galvanomagnetic measurements were made over a temperature range between liquid N2 and room temperature. The resistivity
agrees well with the mentioned measurements, The results reported here show that there is no definite correlation between (the the colour of the refer crystal its electrical properties same might toand the non-stoichiometrical properties). In accordance with some published data 25) crystals of a faint colour (pale yellow) can have low resistivities e.g. iO~~ ohm/cm, as well as high ones e.g. 1010 ohm/cm. Since crystals with high resistivity were not observed
of the measured samples varied from several ohm/cm to l0~~ohm/cm. The concentration the charge 3.The donorof concentration carriers was 101 5_1018 cm~ determined form these data was lObo_1019 cm~3.A considerable change was also observed in the Hallmobility where values changed from 165—330 cm2 \1 i sec~
in our work, it was not possible to decide whether the difference between the microhardness of our crystals and those prepared by the French method was caused by the non-stoichiometry of the crystals. One of the most interesting problems which remained unsolved, is the colour of the crystals. The nature of the optical absorption of these crystals is unknown and calls for further effort. 5. Summary CdS single crystals of different stoichiometry have been grown in vacuum sealed ampoules. The stoichiometry and the physical properties of the grown crystals proved to be strongly dependent on the pre-treatment of the CdS ground material and on the circumstances of the ampoule sealing. In order to clarify the problem, luminescent pure CdS ground materials were fired in pure H 2S atmosphere for ito 6 h at 600—1000 °C. During the growth process the ampoules weie pulled vertically from a homogeneous temperature zone of 1180 °Cat a rate of I mm/h to a region with a temperature gradient of 5—10 °Ccm till the tip of the ampoules reached 1130 °C.Depending on the transport properties of the ground material either the whole of the powder or only a part of it had been transported into the conical tip. In the second case, however, well developed hexagonal CdS crystals were formed on the charge, while a crystal boule was formed in the tip. The optical transmission, the galvanomagnetic
References 1) 2) 3) 4)
W. W. Piper and S. J. Polich, J. AppI. Phys. 32 (1961) 1278. L. Hildisch, J. Crystal Growth, 3,4 (1968) 131. E. Kaldis, J. Crystal Growth 5 (1969) 376. M. Toyama, Japan. J. AppI. Phys. 5 (1966) 1204.
5) D. W. G. Ballentyne, S. Wetwatana and E. A. D. White, J. Crystal Growth 7 (1970) 79. 6) P. Goldflnger and M. Jeunehomme, Trans. Faraday Soc. 59 (1963) 2851. 7) A. S. Pashinkin and B. A. Salamantin, Soviet Inorg. Mater. 5 (1969) 256 8) J. R. Marquart and J. Berkowitz, J. Chem. Phys. 89 (1963) 288. 9) G. DeMaria, P. Goldflnger, L. Malaspina and V. Piacente, Trans. Faraday Soc. 68 (1968) 2146. 10) D. C. Reynolds and L. C. Green, J. AppI. Phys. 29 (1958) 11) H. Samuelson, J. AppI. Phys. 32(1961) 309. 12) R. J. Caveney, J. Crystal Growth 7 (1970) 102. 13) B. M. Bulakh, J. Crystal Growth 5 (1969) 243. 14) E. Kaldis and R. Widmer, J. Phys. Chem. Solids 26 (1965) 1697. 15) F. Kaldis, J. Phys. Chem. Solids 26 (1965) 1701. 16) V. Fok, Phys. J. Mater. (1969)865. 747. 17) F. G. Lendvay Pearson and and J.M.Bardeen, Rev.Sci. 75 4(1949) 18) R. H. Bube, E. L. Lind and A. B. Dreeben, Phys. Rev. 128 (1962) 532. 19) W. E. 130. Spear and J. Mort, Proc. Phys. Soc. (London) 81 (1963) 20) F. A. Kroger, H. J. Vink and J. Volger, Philips Res. Rept. 10 (1955) 39. 21) W. W. Piper and R. Prague, E. Halsted, conductor Physics, 1960, in: p. Intern. 1046. Conf. on Semi. 22) M. Itakura and H. Toyoda, J. Phys. Soc. Japan 18 (1963) 150. 23) R. Crandall, Phys. Rev 169 (1968) 577. 24) Sen Gupta, Proc. Roy. Soc. (London) A 143 (1933) 438. 25) L. Clark and L. Woods, Brit. J. AppI. Phys. 17 (1966) 319.