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Growth mechanism of CdS platelets
Journal of Crystal Growth 16 (1972) 99—109 ~ North-Holland Publishing Co.
GROWTH MECHANISM OF CdS PLATELETS G. H. DEERSSEN and T. GABOR Central Research Laboratories, 3M Co., St. Paul, Minnesota 55133,
U.S.A.
Received 28 April 1972; revised manuscript received 29 July 1972 Needles were observed to grow at 1.0 to 2.5 Jim/sec during the transport of CdS from 1100 °Cto 900 °C in a stream of argon. In many instances an acicular crystal, usually ofmuch smaller crossectional area (i.e. a side-whisker), did form on a needle thus defining a plane. A platelet developed in this plane by favored nucleation at the corners. From the growing platelet arrays of whiskers sprouted in both directions parallel to the needle. The additional corners thus formed served as nucleation sites for filling-in between the whiskers. The axes of all whiskers and needles were found to be <0001>. Recrystallization of the side-whisker eliminated the lattice mismatch. The morphology of the whiskers, needles and platelets was much varied. There was a tendency for the formation of crystals with large surface to volume ratios. Epitaxy was found to have played an important role in the elimination of vicinal and high index planes.
I. Introduction
specimen plane from the camera and also to the large numbers of crystals growing in close proximity of the tube-wall, only those crystals which were growing relatively fast and in directions approximately perpendicular to the tube wall could be resolved. For this reason little detail could be observed for about the first 2 hr after finite supersaturation was established. The growth rate of crystals could be measured during the time interval of about 170 to 390 mm, after which the image became blurred because of the overlap of crystals even close to the center of the tube. To obtain information on the early stages of the growth process, and to avoid changes taking place at late stages, occasionally the growth tube was removed from the furnace at the growth temperature and was transferred into a nitrogen-purged ceramic tube. Despite fast purging with the inert gas, some oxidation usually occurred during the transfer. After cooling to room temperature, the crystals could readily be observed at magnifications up to about 30 x while still attached to the tube-wall. The crystals were then removed from the tube for examination with the optical microscope, with the scanning electron microscope, by X-ray diffraction, and by etching studies.
Many investigators have studied the growth of hexagonal CdS crystals from the vapor phase. The original work was reported by Frerichst), and one early and extensive study was that of Ibuki2). The latter described various crystal morphologies found when varying growth parameters like temperature and supersaturation. More recently, a detailed theory on the growth mechanism of CdS platelets was advanced by Chikawa and Nakayama3) (C & N). They claim that platelet growth commences by the growth of a whisker with its axis in a <1010> direction. The platelet is subsequently formed by growth on the whisker in one of the <0001> directions. Using a technique similar to the one used by C & N, crystals of CdS were also grown in these laboratories. Some of the observations and interpretations made during the course of this work are presented in this communication,
2. Experimental The CdS crystals were grown on the inside wall of a longitudinally split fused silica tube (7 cm diameter) in the cooler zone of a two-zone furnace. A stream of argon (1 cm/sec) at atmospheric pressure was used 3. Results to transport the vapors from the CdS source (1100 °C) to the deposition zone (900 CC). The growth was ob- ~ j~ OBSERVATIONS DURING GROWTH served and photographed through a window across the A few examples illustrating a characteristic growth end of the furnace tube. Owing to the distance of the run are shown in fig. I. In accordance with the observa99
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tions of C & N, the growth of crystals commenced with the growth of needles. However, owing to poor resolulion, it could not be ascertained whether or not the needles were formed by the increase in thickness of whiskers. In the present communication a whisker is defined as a needle observable only under the optical microscope. The micrographs show that some of the needles widened to platelets, the growth perpendicular to the axis often extending to both sides of the needle. In most cases, this growth started close to the tip of the needle. In the micrograph taken at 328 mm three examples caii be seeti in the bottom left part of the picture. The platelets thus formed also grew in the two directions parallel to the axis. The rate of growth of the needles and of the platelets (both perpendicular and parallel to the needle) was about the same: 1.0 to
2.5 pm/sec. It appears that most ot the needles grew during the whole period during which measurements could be made, while the growth of platelets perpendicular to the needle direction frequently came to a halt.
3.2.
MORPHOLOGY OF CRYSTALS AS FXAMINEI) AFFER GROWTH
The crystals were randomly oriented in all directions on the inner wall. Similarly to C&N, it was found practical to categorize the crystals as whiskers, needles. ribbons and platelets, even though the distinction hetween theni was in many cases difficult. Needles and whiskers differ in cross-sectional area (see above) and a ribbon may be deflned as a narrow platelet. As ohserved by the naked eye, the most prominent growth form was the needle. Those needles which served as
GROWTH MECHANISM OF
seeds for platelet growth will be defined as seed-needles. The position of these was evident in most platelets by visual observation, Microscopic examination of needles and of platelets revealed that whiskers played an important role in their growth. Examination of many whiskers and needles by X-ray diffraction and in polarized light showed that they all had their axes oriented in the <0001> directions. On about 20~of the platelets, arrays of whiskers parallel to the seed-needle and in many cases growing in both <0001> directions were present at the edges. In some platelets, especially when the growth run was prematurely terminated and the tube cooled fast, a whisker which was not parallel to the seed needle was also present. Such whiskers will be called a side-whisker. The morphology of these various crystals will be described in the following, 3 .2. 1. Needles and seed-needles Both the needles and seed-needles showed a varied morphology. Some had regular hexagonal cross-seetions, while the cross-section of others was very irreg-
CdS
101
PLATELETS
ular. Some were hollow, others not. Some tapered off to a whisker or, in case of hollow ones, to a number of parallel whiskers, while others were bounded by prismatic planes and by a smooth or rough {000l } end plane. There appeared to be a tendency for the formation of crystals having large surface to volume ratios. For example, in some cases the seed-needle terminated in a large number of whiskers (fig. 2a), while in other cases the void in hollow needles became filled by the propagation and the winding-around of several lamellae (fig. 3). The polished {000l } section of a hollow seed-needle with a platelet growing from it is shown in fig. 4. 3.2.2. Whiskers growing parallel to a seed-needle Whiskers grew individually and as parts of needles and platelets. Only the latter will be considered here. Arrays of whiskers growing in both <0001> directions and on both sides of a seed-needle were shown in figs. 2b and 2c. These whiskers were straight and parallel. In crystals of poorer quality some of the whiskers were irregular and bent, as is illustrated in fig. 5. This
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Fig. 2. Platelet ~ ith long seed-needle. Corresponding areas in photomicrographs. (a) to Ic). and in enlarged contact print (d) are marked by letters.
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micrograph Fig. ~. Part of rough 0001 end of holloss needle (1.2 mm wide) of irregular circumference. Void is in the process of fillingstd~ssisu.prop t~ation md ssmnding around of several
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micrographs shows that growth of the platelet can take place by the filling up of space between the whiskers. As the whisker axts always coincided with the <0001> direction, it is easy to see how bending of whiskers introduced defects in the platelets. Actually, one can see the lines tn the platelet where the whiskers were Incorporated. To illustrate an edge which was not perpendicular to <0001> but which still strived to maintain the .~000l} orientation, fig. 6 is shown. One notes that the edge consists of an array of {000l } steps, and that each step terminates in a whisker.
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Whiskers growing at an edge parallel to the seedneedle (see fig. 7) are of special interest. One notes Iliat outward-bending of such whiskers and filling-in of space between whisker and platelet can lead to an increase in width of the platelet.
GROWTH MECHANISM OF
5 0P
CdS
103
PLATELETS
cally unchanged, while in other cases the whiskers were strongly tapered. In many cases the diameter of the whiskers was identical to the thickness of the platelet (fig. 8), while in other cases it was smaller. Many of the whiskers were bounded by 12 (fig. 8) or 6 prismatic planes, the latter tending to form a whisker having a regular hexagonal cross-section. In other cases the whiskers were bounded also by vicinal planes and even by curved surfaces. It was attempted to establish whether screw dislocations were present in the whiskers. Encapsulated polished {000l } sections of arrays of whiskers were etched by the “HPC-etch5). As no pits characteristic of dislocations were detected, it was concluded that at least after the growth process, dislocations were not present in the whiskers. The observation that whiskers can grow in both
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Fig. 7. l’io omicrographs shosu ing Iwo platelets ssliich appear to grosv perpendicular to the seed-needle by the bending of whiskers which are out the edges parallel to <0001
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Fig. 8. Scanning electron micrograph of 12—sided ss hisker and platelet growing behind it. The diameter of the whisker is identical with the thickness of the platelet.
Analogously to the needles, the morphology of the whiskers was also varied. However, in no case was the bulb often present in the VLS growth4) observed at the tip not even at 1000 x magnification. Some of the whiskers terminated with cross-sectional areas practi-
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Fig. 9. perpendicular l’latelcts ssithtosshiskcrs at all apices sshich spearhead growth the needle. Note whisker even on apex of side-growth on whisker in (b). Photographed on wall of silica tube. The change in contrast between left and right side of the
platelet (b) is duepictures to changed and/or focusing when theinindividual of theillumination composite micrograph were taken.
104
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II. DIERSSEN AND T. GABOR
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Fig. II
I— ig. 10. Side—us hiskers grossing perpendicular to needle. I lie 0001> direction in the platelet is indicated by the striations.
shown by optical inhomogencit~ us here rcer~st,ill i/at ~Oit h.s not yet been completed. (a) Striation’. shosu 0001 dircetiot one notes that the recrystallization is comi,leted about 0.4 mnt from apex. (b) Selected area of(a) better illustrattnn in.ornplete recrss-
Side-whisker shown was found by X-ray diffraction to hase 0001 axis. (b) Side-whisker thickened into spike; columnar crystal, evidently formed by filling-in between whiskers and the individual whisker indicate how growth proceeds perpendicular to side-whisker. )tt)
Apex of platelet ssitli position of original side—suhisker
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of fig. 9. in most cases the original needle could he clearly differentiated froni the side-whisker by its larger .
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directions parallel to the seed-needle (see figs. 2 and 7a) raised the question whether reversal of polarity did take place within the crystals and thus the growth in reality occurred only in one of the <0001> directions. Therefore {000l} sections were cut close to the arrays of whiskers sprouting in opposite directions on a platelet, After polishing, the sections were etched in “HPC”,
dtmension, and by It being parallel to the striations which always run in the <0001> direction. However, in rare cases, the needle and side-whisker were of comparable dimensions, as is shown at the top right corner of the platelet of fig. 9b. In this case the side-growth of the platelet was blocked by the original side-whisker having developed its own side-whisker, which led to the formation of a new platelet epitaxial to the first side-whisker.
which etchant was shown to differentiate between the cadmium and sulfur faces. It was thus established that reversal of polarity perpendicular to <0001> did not take place, i.e., the whiskers can grow in both <0001> directions.
Numerouscrystals having side-whiskers of significant cross-sectional area were studied in transmitted polarized light. These studies, combined with the X-ray examination in a Weissenberg camera of two sidewhiskers, one perpendicitlar (11g. lOa) and the other
3.2.3. Side-whiskers Generally the side-whiskers were apparent only on the edges of some platelets and no trace of them was observed in the platelets, as is shown in the examples
not perpendicular to the seed-needle proved that the axes of the side-whiskers are <0001>. In a specimen (fig. lOb) which was somewhat similar in appearance to the one shown in fig. lOa, the sidewhisker must have recrystallized and it thickened into
GROWTH MECHANISM OF
a spike. One notes the presence of a whisker and of a column growing parallel to the seed-needle. in some specimens, even though a side-whisker was not present (probably it broke off either during the growth or when the specimen was removed from the tube wall) evidence was seen in the platelet of the previous presence of a side-whisker. Thus, for example an inhomogeneity was observed along a line from the
CdS
PLATELETS
105
the material filling-in the gap had the same orientation as the rest of the platelet. 3.2.4. Platelets The great variety in morphology of needles and whiskers also extended to the morphology of platelets, their size and shape varying greatly. Most of them grew only from one prismatic plane of a seed-needle. However, a significant number extended in opposite directions perpendicular to a seed-needle, as was illustrated in fig. 2. This type of platelet grew parallel on a prismatic plane of a seed-needle (fig. 13) and was not nucleated on opposite prismatic planes of the needle. Many of the platelets had edges parallel to the seedneedle (e.g., figs. 2, 6 and 7) while a smaller number
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Fig. 12. Platelet ssi(h side-’.uhiskcr fallen out. (a) Optical micrograph showing that at left edge regrowth is completed and all optical inhomogeneity has disappeared; a void exists at the apex, while in between these extremes, varying stages of the filling-in process can be seen. (b) Scanning electron micrograph of area where whiskers are in the process ofjoining up and thus filling-in the gap. (c) Optical micrograph of apex.
apex (fig. 11). In still other cases, there was a gap at the apex and a line was seen right through the platelet (fig. l2a). An area along the line where the filling-in process took place by the intergrowth of whiskers growing parallel to the seed-needle and originating at both edges of the gap is shown at a higher magnification in fig. 12b. it was shown by microprobe that these whiskers consisted of CdS. The gap at a magnification intermediate between that of figs. 12a and l2b is shown in fig. 12c. Examination in polarized light showed that
Fig. 13. Photomicrograph shossing part of a platelet us hich grew on a prismatic plane of a seed-needle.
of platelets terminated in an apex furthest away from the seed-needle (e.g., figs. 9 to 12). The shape of all but a few platelets deviated from the ideal plane-parallel configuration, and most of the platelets contained defects. It was already mentioned (see e.g., figs. 2, 4, 9 and 13) that the position of the seed-needle was clearly visible in most platelets. In a few thin platelets though the seed-needle blended completely with the rest of the platelet. In the majority of platelets, as was illustrated in figs. 9 to 12, striations were found running parallel to the seed-needle. Etching studies of polished cross-seetions revealed that the striations in most eases corresponded with planar defects perpendicular to the
106
G. I-!. I)IERSSEN AN!) T. GABOR
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Fig. 14.
Polished and ‘~HPC’~-etched{000l}-section shosving
arrays of pits in traces of loTO} planes.
principal planes. In other cases though, they were ridges or “rooftop-type” structures6). In case of one platelet with I OIO}-type principal planes, planar defects in {IOTO}-type planes at 600 angle to the principal planes were found (fig. 14). Many of the platelets, especially the ones grown in interrupted runs, were wedge-shaped. Another irregularity observed in case of a polished cross-section is shown in fig. 1 5. One notes that the platelet narrowed down, forming two small “channels”. On removing a layer of about 10 to 15 pm by etching, the same area appeared straight and much wider. Some of the platelets were found to be composites as is illustrated in fig. 16. Here an epitaxially growing
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Fig. 15. Polished 0001 }-section close to edge. showing irregular nature of initially grossn crystal.
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platelet was sprouting its arrays of whiskers in <0001> on its substrate-platelet. In some composite platelets the arrays of whiskers of both Iamellae were at a cornmon edge. When viewed with the axis of the optical microscope perpendicular to the principal planes, the two arrays came into focus at different levels, thus confirming that the growth of each lamella was led by its own array of whiskers. 3.3. ANALYTICAL By emission spectrographic analysis, elements other than sulfur and cadmium were detected only in the parts per million range, the highest values being that for silicon (1—5 ppm). The surfaces of various crystals and especially the tips of the whiskers were frequently subjected to electron microprobe analysis. No elements other than sulfur and cadmium were detected. 4. Discussion In this presentation only the growth mechanism of the platelets will be discussed. The growth mechanism of needles and of seed-needles will form the subject of a later communication. 4.1. FORMATION OF PLATELETS As observed during the growth process (fig. I) most of the platelets originated as small triangular “flags” on the shafts of the needles. The random orientations of the platelets relative to the growth chamber indicated that local supersaturation gradients did not play a significant role in their growth. This was confirmed by the evidence shown in figs. 2 and 13 where growth occurring in both directions perpendicular to the seed needle was presented. Clearly a supersaturation gradient perpendtcular to the needle tn one dtrection would not have favored growth in the direction l80~opposite to it. Microscopic examination of platelets from early stages of the growth process revealed the presence of sidewhiskers on the small platelets (e.g.. fig. 9). As junctions of whiskers are known to he favored growth Sites8-9) and the growth of platelets at the intersection of whiskers has been observed in the case of Zn ZnO9) and tellurium and as the presence of a sidewhisker was shown on a platelet of CdSt2). it is now .
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proposed that most of the platelets of the present work grew by the following mechanism: (I) A side-whisker having an <0001> axis nucleates
GROWTH MECHANISM
OF
CdS
PLATELETS
107
[in many cases in twin orientation7)] on a prismatic plane of a needle. (2) Growth layers nucleate at and grow from the corners formed by the needle and side-whisker. The growing platelet is likely to assume the lattice orientation of the larger of the two acicular crystals. (3) Due to <0001> being the favored whisker growth direction, whiskers parallel to the needle sprout out of the growing layer. (4) The filling-in of space between the seed-needle and whisker and between neighboring whiskers proceeds by the propagation of layers nucleated at the numerous corners (figs. 2, 5 and 6).
relative to the platelet and thus some epitaxial relationship existed between the two crystals, the formation of a homogeneous single crystal still involved some decrease in free energy. Evidently at the temperature of the growth process, the activation energy for such recrystallization was available, and thus in most platelets no sign of the side-whisker was seen. However, the presence of the vague line, as shown in fig. II, indicated that in such crystals the recrystallization has not yet been completed. In the preceeding it was assumed that platelets grew in epitaxy matching the orientation of the needles. However, one may also argue that once the plane of a platelet was defined by two acicular crystals, except for 4.2. ORIENTATION OF CRYSTALS size of these crystals, it was immaterial whether it was The principal planes of the platelets were defined by the first or the second-formed acicular crystal which the relative orientations of the seed-needles and the imposed its orientation upon the platelet. The sequence side-whiskers. As the most prevalent planes of the of events that took place cannot be ascertained after needles, side-whiskers and platelets were of the types the growth experiment, as in the case of the second{lOTO} and {l I~0},it must be concluded that most of grown acicular crystal having “won-out”, it may well the side-whiskers nucleated in a plane defined by the have extended back to the growth tube, thus giving the axis of the seed-needle and the perpendicular or parallel impression that it formed first. The “first” and “second” to one of its prismatic planes. In the parallel case, the sequence need not even have been present in all cases, seed needle need not be at the edge of the platelet. as one may even postulate that some of the platelets It was mentioned earlier that the axes of all whiskers developed through random junction formation beand side-whiskers were always in the <0001> directions. tween two independently nucleated whiskers or neeAs the source material was CdS, the Cd: S ratio in the dlesi One need not even postulate that in all cases the gas phase must have been close to I and thus according planar crystal grew from two individual acicular crysto Bulakht3), Cd 6—S6 or Cd3—S3 complexes may have tals. The plane may also have been defined by the played a role in favoring the <0001> growth direction. bending and/or kinking of one needle. One must conclude that contrary to the findings of In case of specially unfavorable orientation relationC&N, the side-whiskers do not form a single crystal ships between side-whisker and platelet, the former, with the seed-needle. Angles between seed-needle and of recrystallizing, fell out (fig.between 12), indicating 7) instead the presence of especially weak bonds the two side-whisker which correspond to twin orientations may be energetically favored. For example, the ap- orientations. The filling-in of the gap showed that the parently perpendicular orientation (fig. 10) may ac- falling-out must have occurred during the growth run tually be 92°,thus corresponding to the 46°twin7), and while there was still time left for further growth (having giving an epitaxial relationship, i.e., {000l} of the the same orientation as the platelet) to take place. needle being parallel to {l 120} of the side-whisker. 4.4. PREVALENCE OF THE SIDE-WHISKER GROWTH ME4.3. RECRYSTALLIZATION OF SIDE-WHISKERS CHANISM 0)
When no simple matching pattern existed between the lattices of the side-whisker and the platelet, the recrystallization of the side-whisker into the crystallographic orientation of the platelet must have involved a considerable decrease in free energy. it is expected that even when the side-whisker was in twin orientation
The prevalence of the proposed side-growth mechanism could not be ascertained as it is conceivable that in many platelets the filling-in process proceeded just as fast as did the growth of the side-whisker, thus leading to the formation of edges without any detectable protruding side-whisker being present. Fast re-
108
G. H. DIERSSEN
crystallization of the embedded side-whisker then made the detection of the mechanism impossible. Also, the absence of side-whiskers after the growth experiment may only be apparent. They may be present but too small to be detected at the low magnifications which were attained when examining the crystals while still attached to the tube wall, while during the removal of the crystals from the tube, most of the side-whiskers could have broken off. This is to be expected when one considers that recrystallization of the embedded parts of the side-whiskers caused grain boundaries to form at the apices or the edges of the platelets where the protruding parts of the side-whiskers were attached. The breaking off of protruding side-whiskers at the grain boundary is also expected to have occurred in many cases during the growth run. It should certainly have proceeded more readily than the falling out of an embedded whisker, which involved not only a break at a grain boundary perpendicular to the whisker axis, but also the separation of the bonds between platelet and whisker along the shaft of the latter. The growth perpendicular to the seed needle by the above mechanism should have come to a halt when a whisker broke off during the growth run. However, there are indications that growth perpendicular to the needle also proceeded. though at a much slower rate, by conventional twodimensional growth. The simultaneous presence of a side-whisker and an array of whiskers was noted on some platelets. These specimens though happened to be less illustrative than the examples selected. The closest example illustrating growth both perpendicular and parallel to a seed-needle was given in fig. lob, where a whisker and a column (evidently formed by filling in between whiskers) was shown on a platelet which terminated in a spike perpendicular to the seed-needle. The widening of platelets by the filling-in of space between an outward-bending whisker and the edge of the platelet (fig. 7) was not frequently observed. The phenomenon itself though is not novel as Ibuki has suggested, it is an explanation for the shape of rod-type crystals of CdS2). 4.5. CRYSTALLOGRAPHIC PERFECTION Several examples were shown indicating that the original growth was imperfect. For example the dark band parallel to the edge in fig. 6a was caused by a rough
ANI) T. GABOR
surface. Apparently this roughness annealed out continuously as the growth process proceeded. Wedge-shaped crystals which by definition have vicinal or high-index planes were prevalent (see, e.g., fig.4).Aplatelethavingtwosmall”channels”atitsedge was shown in fig. I 5. In the latter case, the curved surfaces may be considered as a series of microscopic highindex planes. It is probable that at the center of the convex area a whisker was leading the growth in the <0001> direction, and the section happened to he at a level where the gap between this whisker and its two neighbors had just been bridged. There is a driving force for the elimination of high index (i.e., high energy) surfaces, as is shown by the fact that 10 to 15 pm below the level shown, the cross-section appeared much thicker and was bounded by straight parallel lines. Evidently epitaxial growth led to the formation of low index surfaces. The preferential filling-in of channels is not surprising as it was already observed to take place in the heteroepitaxial growth of GaAs on germanium by Gabort4), and the phenomenon was later studied in more detail by Shaw’ It is expected that epitaxial growth also led to the decrease in the wedge character, as the steps on vicinal planes must have served as favored nucleation sites. The planar defects perpendicular to the principal planes and which were revealed by etching of polished cross-sections were thought to he low angle grain houndaries. In case of fig. 14, by assuming that each pit corresponded to the presence of one terminating plane (b = I), the angle of tilt around each dislocation line was calculated to he about I minute. Reversal of polarityt ~ 7) was revealed in planes parallel and also perpendicular to the principal planes of some platelets. 5),
4.6. SURFACE TO VOLUME RATIO All throughout this work, the formation of crystals with large surface to volume ratios was favored. Not only did whiskers, platelets, needles and hollow needles form, hut even the filling-in of the latter involved the propagation of two-dimensional crystals (Pg. 3). A further proof of the driving force for the formation of large surface areas is shown in fig. 17. This micrograph was taken of a platelet which was sublimation-etched under conditions close to equilibrium, leading to the formation of large-area skeleton consisting of partly
GROWTH MECHANISM OF
CdS
PLATELETS
109
axes of both of which had <0001> orientation. The lattice orientation of the platelet was identical with the orientation of one of the acicular crystals (probably with the large one) while the other one recrystallized to give a homogeneous single crystal. Acknowledgement We thank D. M. Harshbarger, E. 0. Odegard and D. W. Johnson for their help in growing the crystals, P. A. Christensen and W. E. Thatcher for the X-ray examinations and J. A. Leys for the scanning electron rnicrographs, and R. L. Weiher and R. A. Hatch for constructive criticism of the manuscript. References Fig. 17. Platelet after sublimation-etching under conditions close to equilibrium. The background is gray. The black whiskers and the bright interconnecting areas are what remains of the original platelet.
interconnected whiskers. As the formation of surfaces
I) R. Frerichs, Phys. Rev. 72 (1947) 594. 2) S. Ibuki, J. Phys. Soc. Japan 14(1959)1181.
3) J. Chikawa and T. Nakayama, J. AppI. Phys. 35 (1964) 2493. 4) R. S. Wagner and W. C. Ellis, AppI. Phys. Letters 4 (1964) 89; Trans. AIME 233 (1965) 1053. 5) N. Hemmat and M. Weinstein, J. Electrochem. Soc. 114 (1967) 851. 6) R. J. Caveney, Phil. Mag. 12(1965) 423. 7) K. H. Jost, Z. Naturforsch. 9a (1954) 435. 8) T. Gabor and J. M. Blocher, J. AppI. Phys. 40 (1969) 2696. 9) R. B. Sharma, Indian J. Pure AppI. Phys. 7 (1969) 736.
in general leads to an increase in free energy, it is now postulated that such increase may be partially cornpensated by the lower free energy of formation of offstoichiometric CdS, stable at the surface while in the bulk at 900°Cthe off-stoichiometry must be less than 1.3 x I0~atom fractions’8).
10) G. W. Sears and R. V. Coleman, J. Chem. Phys. 25 (1956) 635. II) N. Furuta, H. Itinose and Y. Takeda, Japan. J. AppI. Phys.
5. Conclusions
13) B. M. Bulakh, J. Crystal Growth 5 (1969) 243.
CdS platelets were found to grow by a mechanism different from the one advanced by Chikawa and Nakayama3) for the growth of their platelets. in the present work, many of the platelets did not commence their growth as homogeneous single crystals. They developed in a plane defined by two acicular crystals, the
15) D. W. Shaw, J. Electrochem. Soc. 113 (1966) 904; 115 (1968) 777.
10(1971)811.
12) S. Balabanov, E. Kantardieva, E. Nikolova, N. Pashov and S. Simov, Phys. Status Solidi (a) 6 (1971) K73.
14) T. Gabor, J. Electrochem. Soc. 111 (1964) 825. 16) R. B. Wilson, J. AppI. Phys. 37 (1966) 1932.
17) S. B. Austerman and W. G. Gehman, J. Mater. Sci. 1(1966) 249. 18) L. R. Shiozawa and J. M. Jost, Research on Improved
Il—VI Crystals, Contract No. F33615-68-C-1601-P002, ARL
71-0017, Jan., 1971, p. 83.
GROWTH MECHANISM OF CdS PLATELETS G. H. DEERSSEN and T. GABOR Central Research Laboratories, 3M Co., St. Paul, Minnesota 55133,
U.S.A.
Received 28 April 1972; revised manuscript received 29 July 1972 Needles were observed to grow at 1.0 to 2.5 Jim/sec during the transport of CdS from 1100 °Cto 900 °C in a stream of argon. In many instances an acicular crystal, usually ofmuch smaller crossectional area (i.e. a side-whisker), did form on a needle thus defining a plane. A platelet developed in this plane by favored nucleation at the corners. From the growing platelet arrays of whiskers sprouted in both directions parallel to the needle. The additional corners thus formed served as nucleation sites for filling-in between the whiskers. The axes of all whiskers and needles were found to be <0001>. Recrystallization of the side-whisker eliminated the lattice mismatch. The morphology of the whiskers, needles and platelets was much varied. There was a tendency for the formation of crystals with large surface to volume ratios. Epitaxy was found to have played an important role in the elimination of vicinal and high index planes.
I. Introduction
specimen plane from the camera and also to the large numbers of crystals growing in close proximity of the tube-wall, only those crystals which were growing relatively fast and in directions approximately perpendicular to the tube wall could be resolved. For this reason little detail could be observed for about the first 2 hr after finite supersaturation was established. The growth rate of crystals could be measured during the time interval of about 170 to 390 mm, after which the image became blurred because of the overlap of crystals even close to the center of the tube. To obtain information on the early stages of the growth process, and to avoid changes taking place at late stages, occasionally the growth tube was removed from the furnace at the growth temperature and was transferred into a nitrogen-purged ceramic tube. Despite fast purging with the inert gas, some oxidation usually occurred during the transfer. After cooling to room temperature, the crystals could readily be observed at magnifications up to about 30 x while still attached to the tube-wall. The crystals were then removed from the tube for examination with the optical microscope, with the scanning electron microscope, by X-ray diffraction, and by etching studies.
Many investigators have studied the growth of hexagonal CdS crystals from the vapor phase. The original work was reported by Frerichst), and one early and extensive study was that of Ibuki2). The latter described various crystal morphologies found when varying growth parameters like temperature and supersaturation. More recently, a detailed theory on the growth mechanism of CdS platelets was advanced by Chikawa and Nakayama3) (C & N). They claim that platelet growth commences by the growth of a whisker with its axis in a <1010> direction. The platelet is subsequently formed by growth on the whisker in one of the <0001> directions. Using a technique similar to the one used by C & N, crystals of CdS were also grown in these laboratories. Some of the observations and interpretations made during the course of this work are presented in this communication,
2. Experimental The CdS crystals were grown on the inside wall of a longitudinally split fused silica tube (7 cm diameter) in the cooler zone of a two-zone furnace. A stream of argon (1 cm/sec) at atmospheric pressure was used 3. Results to transport the vapors from the CdS source (1100 °C) to the deposition zone (900 CC). The growth was ob- ~ j~ OBSERVATIONS DURING GROWTH served and photographed through a window across the A few examples illustrating a characteristic growth end of the furnace tube. Owing to the distance of the run are shown in fig. I. In accordance with the observa99
100
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I ig I ( dS needles and pIateIet~ gro\~ing h~ ~uhlimation in ~i stream of argon. 1 he numheN denote minuIe~alter the lo~~ering the temperature of the gur~111 lOFiC from I I 0(t C to 900 C oa~begun.
tions of C & N, the growth of crystals commenced with the growth of needles. However, owing to poor resolulion, it could not be ascertained whether or not the needles were formed by the increase in thickness of whiskers. In the present communication a whisker is defined as a needle observable only under the optical microscope. The micrographs show that some of the needles widened to platelets, the growth perpendicular to the axis often extending to both sides of the needle. In most cases, this growth started close to the tip of the needle. In the micrograph taken at 328 mm three examples caii be seeti in the bottom left part of the picture. The platelets thus formed also grew in the two directions parallel to the axis. The rate of growth of the needles and of the platelets (both perpendicular and parallel to the needle) was about the same: 1.0 to
2.5 pm/sec. It appears that most ot the needles grew during the whole period during which measurements could be made, while the growth of platelets perpendicular to the needle direction frequently came to a halt.
3.2.
MORPHOLOGY OF CRYSTALS AS FXAMINEI) AFFER GROWTH
The crystals were randomly oriented in all directions on the inner wall. Similarly to C&N, it was found practical to categorize the crystals as whiskers, needles. ribbons and platelets, even though the distinction hetween theni was in many cases difficult. Needles and whiskers differ in cross-sectional area (see above) and a ribbon may be deflned as a narrow platelet. As ohserved by the naked eye, the most prominent growth form was the needle. Those needles which served as
GROWTH MECHANISM OF
seeds for platelet growth will be defined as seed-needles. The position of these was evident in most platelets by visual observation, Microscopic examination of needles and of platelets revealed that whiskers played an important role in their growth. Examination of many whiskers and needles by X-ray diffraction and in polarized light showed that they all had their axes oriented in the <0001> directions. On about 20~of the platelets, arrays of whiskers parallel to the seed-needle and in many cases growing in both <0001> directions were present at the edges. In some platelets, especially when the growth run was prematurely terminated and the tube cooled fast, a whisker which was not parallel to the seed needle was also present. Such whiskers will be called a side-whisker. The morphology of these various crystals will be described in the following, 3 .2. 1. Needles and seed-needles Both the needles and seed-needles showed a varied morphology. Some had regular hexagonal cross-seetions, while the cross-section of others was very irreg-
CdS
101
PLATELETS
ular. Some were hollow, others not. Some tapered off to a whisker or, in case of hollow ones, to a number of parallel whiskers, while others were bounded by prismatic planes and by a smooth or rough {000l } end plane. There appeared to be a tendency for the formation of crystals having large surface to volume ratios. For example, in some cases the seed-needle terminated in a large number of whiskers (fig. 2a), while in other cases the void in hollow needles became filled by the propagation and the winding-around of several lamellae (fig. 3). The polished {000l } section of a hollow seed-needle with a platelet growing from it is shown in fig. 4. 3.2.2. Whiskers growing parallel to a seed-needle Whiskers grew individually and as parts of needles and platelets. Only the latter will be considered here. Arrays of whiskers growing in both <0001> directions and on both sides of a seed-needle were shown in figs. 2b and 2c. These whiskers were straight and parallel. In crystals of poorer quality some of the whiskers were irregular and bent, as is illustrated in fig. 5. This
d
____
___
2mm
Fig. 2. Platelet ~ ith long seed-needle. Corresponding areas in photomicrographs. (a) to Ic). and in enlarged contact print (d) are marked by letters.
102
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V~Itiskers. mait~ot them bent, on edge ol platelet. P1 oto— 55,15 Liken ss ith phase contrast.
micrograph Fig. ~. Part of rough 0001 end of holloss needle (1.2 mm wide) of irregular circumference. Void is in the process of fillingstd~ssisu.prop t~ation md ssmnding around of several
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micrographs shows that growth of the platelet can take place by the filling up of space between the whiskers. As the whisker axts always coincided with the <0001> direction, it is easy to see how bending of whiskers introduced defects in the platelets. Actually, one can see the lines tn the platelet where the whiskers were Incorporated. To illustrate an edge which was not perpendicular to <0001> but which still strived to maintain the .~000l} orientation, fig. 6 is shown. One notes that the edge consists of an array of {000l } steps, and that each step terminates in a whisker.
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contr,tet print (b) : the latter is rotated and inserted relatisc to tal
Whiskers growing at an edge parallel to the seedneedle (see fig. 7) are of special interest. One notes Iliat outward-bending of such whiskers and filling-in of space between whisker and platelet can lead to an increase in width of the platelet.
GROWTH MECHANISM OF
5 0P
CdS
103
PLATELETS
cally unchanged, while in other cases the whiskers were strongly tapered. In many cases the diameter of the whiskers was identical to the thickness of the platelet (fig. 8), while in other cases it was smaller. Many of the whiskers were bounded by 12 (fig. 8) or 6 prismatic planes, the latter tending to form a whisker having a regular hexagonal cross-section. In other cases the whiskers were bounded also by vicinal planes and even by curved surfaces. It was attempted to establish whether screw dislocations were present in the whiskers. Encapsulated polished {000l } sections of arrays of whiskers were etched by the “HPC-etch5). As no pits characteristic of dislocations were detected, it was concluded that at least after the growth process, dislocations were not present in the whiskers. The observation that whiskers can grow in both
I ~
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a
Fig. 7. l’io omicrographs shosu ing Iwo platelets ssliich appear to grosv perpendicular to the seed-needle by the bending of whiskers which are out the edges parallel to <0001
-
-
______________
Fig. 8. Scanning electron micrograph of 12—sided ss hisker and platelet growing behind it. The diameter of the whisker is identical with the thickness of the platelet.
Analogously to the needles, the morphology of the whiskers was also varied. However, in no case was the bulb often present in the VLS growth4) observed at the tip not even at 1000 x magnification. Some of the whiskers terminated with cross-sectional areas practi-
0.5mm
—-
.
-
Fig. 9. perpendicular l’latelcts ssithtosshiskcrs at all apices sshich spearhead growth the needle. Note whisker even on apex of side-growth on whisker in (b). Photographed on wall of silica tube. The change in contrast between left and right side of the
platelet (b) is duepictures to changed and/or focusing when theinindividual of theillumination composite micrograph were taken.
104
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II. DIERSSEN AND T. GABOR
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Fig. II
I— ig. 10. Side—us hiskers grossing perpendicular to needle. I lie 0001> direction in the platelet is indicated by the striations.
shown by optical inhomogencit~ us here rcer~st,ill i/at ~Oit h.s not yet been completed. (a) Striation’. shosu 0001 dircetiot one notes that the recrystallization is comi,leted about 0.4 mnt from apex. (b) Selected area of(a) better illustrattnn in.ornplete recrss-
Side-whisker shown was found by X-ray diffraction to hase 0001 axis. (b) Side-whisker thickened into spike; columnar crystal, evidently formed by filling-in between whiskers and the individual whisker indicate how growth proceeds perpendicular to side-whisker. )tt)
Apex of platelet ssitli position of original side—suhisker
t,mlli,’.ation.
of fig. 9. in most cases the original needle could he clearly differentiated froni the side-whisker by its larger .
.
.
.
directions parallel to the seed-needle (see figs. 2 and 7a) raised the question whether reversal of polarity did take place within the crystals and thus the growth in reality occurred only in one of the <0001> directions. Therefore {000l} sections were cut close to the arrays of whiskers sprouting in opposite directions on a platelet, After polishing, the sections were etched in “HPC”,
dtmension, and by It being parallel to the striations which always run in the <0001> direction. However, in rare cases, the needle and side-whisker were of comparable dimensions, as is shown at the top right corner of the platelet of fig. 9b. In this case the side-growth of the platelet was blocked by the original side-whisker having developed its own side-whisker, which led to the formation of a new platelet epitaxial to the first side-whisker.
which etchant was shown to differentiate between the cadmium and sulfur faces. It was thus established that reversal of polarity perpendicular to <0001> did not take place, i.e., the whiskers can grow in both <0001> directions.
Numerouscrystals having side-whiskers of significant cross-sectional area were studied in transmitted polarized light. These studies, combined with the X-ray examination in a Weissenberg camera of two sidewhiskers, one perpendicitlar (11g. lOa) and the other
3.2.3. Side-whiskers Generally the side-whiskers were apparent only on the edges of some platelets and no trace of them was observed in the platelets, as is shown in the examples
not perpendicular to the seed-needle proved that the axes of the side-whiskers are <0001>. In a specimen (fig. lOb) which was somewhat similar in appearance to the one shown in fig. lOa, the sidewhisker must have recrystallized and it thickened into
GROWTH MECHANISM OF
a spike. One notes the presence of a whisker and of a column growing parallel to the seed-needle. in some specimens, even though a side-whisker was not present (probably it broke off either during the growth or when the specimen was removed from the tube wall) evidence was seen in the platelet of the previous presence of a side-whisker. Thus, for example an inhomogeneity was observed along a line from the
CdS
PLATELETS
105
the material filling-in the gap had the same orientation as the rest of the platelet. 3.2.4. Platelets The great variety in morphology of needles and whiskers also extended to the morphology of platelets, their size and shape varying greatly. Most of them grew only from one prismatic plane of a seed-needle. However, a significant number extended in opposite directions perpendicular to a seed-needle, as was illustrated in fig. 2. This type of platelet grew parallel on a prismatic plane of a seed-needle (fig. 13) and was not nucleated on opposite prismatic planes of the needle. Many of the platelets had edges parallel to the seedneedle (e.g., figs. 2, 6 and 7) while a smaller number
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Fig. 12. Platelet ssi(h side-’.uhiskcr fallen out. (a) Optical micrograph showing that at left edge regrowth is completed and all optical inhomogeneity has disappeared; a void exists at the apex, while in between these extremes, varying stages of the filling-in process can be seen. (b) Scanning electron micrograph of area where whiskers are in the process ofjoining up and thus filling-in the gap. (c) Optical micrograph of apex.
apex (fig. 11). In still other cases, there was a gap at the apex and a line was seen right through the platelet (fig. l2a). An area along the line where the filling-in process took place by the intergrowth of whiskers growing parallel to the seed-needle and originating at both edges of the gap is shown at a higher magnification in fig. 12b. it was shown by microprobe that these whiskers consisted of CdS. The gap at a magnification intermediate between that of figs. 12a and l2b is shown in fig. 12c. Examination in polarized light showed that
Fig. 13. Photomicrograph shossing part of a platelet us hich grew on a prismatic plane of a seed-needle.
of platelets terminated in an apex furthest away from the seed-needle (e.g., figs. 9 to 12). The shape of all but a few platelets deviated from the ideal plane-parallel configuration, and most of the platelets contained defects. It was already mentioned (see e.g., figs. 2, 4, 9 and 13) that the position of the seed-needle was clearly visible in most platelets. In a few thin platelets though the seed-needle blended completely with the rest of the platelet. In the majority of platelets, as was illustrated in figs. 9 to 12, striations were found running parallel to the seed-needle. Etching studies of polished cross-seetions revealed that the striations in most eases corresponded with planar defects perpendicular to the
106
G. I-!. I)IERSSEN AN!) T. GABOR
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Fig. 14.
Polished and ‘~HPC’~-etched{000l}-section shosving
arrays of pits in traces of loTO} planes.
principal planes. In other cases though, they were ridges or “rooftop-type” structures6). In case of one platelet with I OIO}-type principal planes, planar defects in {IOTO}-type planes at 600 angle to the principal planes were found (fig. 14). Many of the platelets, especially the ones grown in interrupted runs, were wedge-shaped. Another irregularity observed in case of a polished cross-section is shown in fig. 1 5. One notes that the platelet narrowed down, forming two small “channels”. On removing a layer of about 10 to 15 pm by etching, the same area appeared straight and much wider. Some of the platelets were found to be composites as is illustrated in fig. 16. Here an epitaxially growing
~
Fig. 15. Polished 0001 }-section close to edge. showing irregular nature of initially grossn crystal.
0.2 mm
platelet was sprouting its arrays of whiskers in <0001> on its substrate-platelet. In some composite platelets the arrays of whiskers of both Iamellae were at a cornmon edge. When viewed with the axis of the optical microscope perpendicular to the principal planes, the two arrays came into focus at different levels, thus confirming that the growth of each lamella was led by its own array of whiskers. 3.3. ANALYTICAL By emission spectrographic analysis, elements other than sulfur and cadmium were detected only in the parts per million range, the highest values being that for silicon (1—5 ppm). The surfaces of various crystals and especially the tips of the whiskers were frequently subjected to electron microprobe analysis. No elements other than sulfur and cadmium were detected. 4. Discussion In this presentation only the growth mechanism of the platelets will be discussed. The growth mechanism of needles and of seed-needles will form the subject of a later communication. 4.1. FORMATION OF PLATELETS As observed during the growth process (fig. I) most of the platelets originated as small triangular “flags” on the shafts of the needles. The random orientations of the platelets relative to the growth chamber indicated that local supersaturation gradients did not play a significant role in their growth. This was confirmed by the evidence shown in figs. 2 and 13 where growth occurring in both directions perpendicular to the seed needle was presented. Clearly a supersaturation gradient perpendtcular to the needle tn one dtrection would not have favored growth in the direction l80~opposite to it. Microscopic examination of platelets from early stages of the growth process revealed the presence of sidewhiskers on the small platelets (e.g.. fig. 9). As junctions of whiskers are known to he favored growth Sites8-9) and the growth of platelets at the intersection of whiskers has been observed in the case of Zn ZnO9) and tellurium and as the presence of a sidewhisker was shown on a platelet of CdSt2). it is now .
.
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.
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______
I itt. 16. Arras of us hiskcrs of a platelet gross inc e~iitax a Is on the principal plane of another platelet. -
Ii)
proposed that most of the platelets of the present work grew by the following mechanism: (I) A side-whisker having an <0001> axis nucleates
GROWTH MECHANISM
OF
CdS
PLATELETS
107
[in many cases in twin orientation7)] on a prismatic plane of a needle. (2) Growth layers nucleate at and grow from the corners formed by the needle and side-whisker. The growing platelet is likely to assume the lattice orientation of the larger of the two acicular crystals. (3) Due to <0001> being the favored whisker growth direction, whiskers parallel to the needle sprout out of the growing layer. (4) The filling-in of space between the seed-needle and whisker and between neighboring whiskers proceeds by the propagation of layers nucleated at the numerous corners (figs. 2, 5 and 6).
relative to the platelet and thus some epitaxial relationship existed between the two crystals, the formation of a homogeneous single crystal still involved some decrease in free energy. Evidently at the temperature of the growth process, the activation energy for such recrystallization was available, and thus in most platelets no sign of the side-whisker was seen. However, the presence of the vague line, as shown in fig. II, indicated that in such crystals the recrystallization has not yet been completed. In the preceeding it was assumed that platelets grew in epitaxy matching the orientation of the needles. However, one may also argue that once the plane of a platelet was defined by two acicular crystals, except for 4.2. ORIENTATION OF CRYSTALS size of these crystals, it was immaterial whether it was The principal planes of the platelets were defined by the first or the second-formed acicular crystal which the relative orientations of the seed-needles and the imposed its orientation upon the platelet. The sequence side-whiskers. As the most prevalent planes of the of events that took place cannot be ascertained after needles, side-whiskers and platelets were of the types the growth experiment, as in the case of the second{lOTO} and {l I~0},it must be concluded that most of grown acicular crystal having “won-out”, it may well the side-whiskers nucleated in a plane defined by the have extended back to the growth tube, thus giving the axis of the seed-needle and the perpendicular or parallel impression that it formed first. The “first” and “second” to one of its prismatic planes. In the parallel case, the sequence need not even have been present in all cases, seed needle need not be at the edge of the platelet. as one may even postulate that some of the platelets It was mentioned earlier that the axes of all whiskers developed through random junction formation beand side-whiskers were always in the <0001> directions. tween two independently nucleated whiskers or neeAs the source material was CdS, the Cd: S ratio in the dlesi One need not even postulate that in all cases the gas phase must have been close to I and thus according planar crystal grew from two individual acicular crysto Bulakht3), Cd 6—S6 or Cd3—S3 complexes may have tals. The plane may also have been defined by the played a role in favoring the <0001> growth direction. bending and/or kinking of one needle. One must conclude that contrary to the findings of In case of specially unfavorable orientation relationC&N, the side-whiskers do not form a single crystal ships between side-whisker and platelet, the former, with the seed-needle. Angles between seed-needle and of recrystallizing, fell out (fig.between 12), indicating 7) instead the presence of especially weak bonds the two side-whisker which correspond to twin orientations may be energetically favored. For example, the ap- orientations. The filling-in of the gap showed that the parently perpendicular orientation (fig. 10) may ac- falling-out must have occurred during the growth run tually be 92°,thus corresponding to the 46°twin7), and while there was still time left for further growth (having giving an epitaxial relationship, i.e., {000l} of the the same orientation as the platelet) to take place. needle being parallel to {l 120} of the side-whisker. 4.4. PREVALENCE OF THE SIDE-WHISKER GROWTH ME4.3. RECRYSTALLIZATION OF SIDE-WHISKERS CHANISM 0)
When no simple matching pattern existed between the lattices of the side-whisker and the platelet, the recrystallization of the side-whisker into the crystallographic orientation of the platelet must have involved a considerable decrease in free energy. it is expected that even when the side-whisker was in twin orientation
The prevalence of the proposed side-growth mechanism could not be ascertained as it is conceivable that in many platelets the filling-in process proceeded just as fast as did the growth of the side-whisker, thus leading to the formation of edges without any detectable protruding side-whisker being present. Fast re-
108
G. H. DIERSSEN
crystallization of the embedded side-whisker then made the detection of the mechanism impossible. Also, the absence of side-whiskers after the growth experiment may only be apparent. They may be present but too small to be detected at the low magnifications which were attained when examining the crystals while still attached to the tube wall, while during the removal of the crystals from the tube, most of the side-whiskers could have broken off. This is to be expected when one considers that recrystallization of the embedded parts of the side-whiskers caused grain boundaries to form at the apices or the edges of the platelets where the protruding parts of the side-whiskers were attached. The breaking off of protruding side-whiskers at the grain boundary is also expected to have occurred in many cases during the growth run. It should certainly have proceeded more readily than the falling out of an embedded whisker, which involved not only a break at a grain boundary perpendicular to the whisker axis, but also the separation of the bonds between platelet and whisker along the shaft of the latter. The growth perpendicular to the seed needle by the above mechanism should have come to a halt when a whisker broke off during the growth run. However, there are indications that growth perpendicular to the needle also proceeded. though at a much slower rate, by conventional twodimensional growth. The simultaneous presence of a side-whisker and an array of whiskers was noted on some platelets. These specimens though happened to be less illustrative than the examples selected. The closest example illustrating growth both perpendicular and parallel to a seed-needle was given in fig. lob, where a whisker and a column (evidently formed by filling in between whiskers) was shown on a platelet which terminated in a spike perpendicular to the seed-needle. The widening of platelets by the filling-in of space between an outward-bending whisker and the edge of the platelet (fig. 7) was not frequently observed. The phenomenon itself though is not novel as Ibuki has suggested, it is an explanation for the shape of rod-type crystals of CdS2). 4.5. CRYSTALLOGRAPHIC PERFECTION Several examples were shown indicating that the original growth was imperfect. For example the dark band parallel to the edge in fig. 6a was caused by a rough
ANI) T. GABOR
surface. Apparently this roughness annealed out continuously as the growth process proceeded. Wedge-shaped crystals which by definition have vicinal or high-index planes were prevalent (see, e.g., fig.4).Aplatelethavingtwosmall”channels”atitsedge was shown in fig. I 5. In the latter case, the curved surfaces may be considered as a series of microscopic highindex planes. It is probable that at the center of the convex area a whisker was leading the growth in the <0001> direction, and the section happened to he at a level where the gap between this whisker and its two neighbors had just been bridged. There is a driving force for the elimination of high index (i.e., high energy) surfaces, as is shown by the fact that 10 to 15 pm below the level shown, the cross-section appeared much thicker and was bounded by straight parallel lines. Evidently epitaxial growth led to the formation of low index surfaces. The preferential filling-in of channels is not surprising as it was already observed to take place in the heteroepitaxial growth of GaAs on germanium by Gabort4), and the phenomenon was later studied in more detail by Shaw’ It is expected that epitaxial growth also led to the decrease in the wedge character, as the steps on vicinal planes must have served as favored nucleation sites. The planar defects perpendicular to the principal planes and which were revealed by etching of polished cross-sections were thought to he low angle grain houndaries. In case of fig. 14, by assuming that each pit corresponded to the presence of one terminating plane (b = I), the angle of tilt around each dislocation line was calculated to he about I minute. Reversal of polarityt ~ 7) was revealed in planes parallel and also perpendicular to the principal planes of some platelets. 5),
4.6. SURFACE TO VOLUME RATIO All throughout this work, the formation of crystals with large surface to volume ratios was favored. Not only did whiskers, platelets, needles and hollow needles form, hut even the filling-in of the latter involved the propagation of two-dimensional crystals (Pg. 3). A further proof of the driving force for the formation of large surface areas is shown in fig. 17. This micrograph was taken of a platelet which was sublimation-etched under conditions close to equilibrium, leading to the formation of large-area skeleton consisting of partly
GROWTH MECHANISM OF
CdS
PLATELETS
109
axes of both of which had <0001> orientation. The lattice orientation of the platelet was identical with the orientation of one of the acicular crystals (probably with the large one) while the other one recrystallized to give a homogeneous single crystal. Acknowledgement We thank D. M. Harshbarger, E. 0. Odegard and D. W. Johnson for their help in growing the crystals, P. A. Christensen and W. E. Thatcher for the X-ray examinations and J. A. Leys for the scanning electron rnicrographs, and R. L. Weiher and R. A. Hatch for constructive criticism of the manuscript. References Fig. 17. Platelet after sublimation-etching under conditions close to equilibrium. The background is gray. The black whiskers and the bright interconnecting areas are what remains of the original platelet.
interconnected whiskers. As the formation of surfaces
I) R. Frerichs, Phys. Rev. 72 (1947) 594. 2) S. Ibuki, J. Phys. Soc. Japan 14(1959)1181.
3) J. Chikawa and T. Nakayama, J. AppI. Phys. 35 (1964) 2493. 4) R. S. Wagner and W. C. Ellis, AppI. Phys. Letters 4 (1964) 89; Trans. AIME 233 (1965) 1053. 5) N. Hemmat and M. Weinstein, J. Electrochem. Soc. 114 (1967) 851. 6) R. J. Caveney, Phil. Mag. 12(1965) 423. 7) K. H. Jost, Z. Naturforsch. 9a (1954) 435. 8) T. Gabor and J. M. Blocher, J. AppI. Phys. 40 (1969) 2696. 9) R. B. Sharma, Indian J. Pure AppI. Phys. 7 (1969) 736.
in general leads to an increase in free energy, it is now postulated that such increase may be partially cornpensated by the lower free energy of formation of offstoichiometric CdS, stable at the surface while in the bulk at 900°Cthe off-stoichiometry must be less than 1.3 x I0~atom fractions’8).
10) G. W. Sears and R. V. Coleman, J. Chem. Phys. 25 (1956) 635. II) N. Furuta, H. Itinose and Y. Takeda, Japan. J. AppI. Phys.
5. Conclusions
13) B. M. Bulakh, J. Crystal Growth 5 (1969) 243.
CdS platelets were found to grow by a mechanism different from the one advanced by Chikawa and Nakayama3) for the growth of their platelets. in the present work, many of the platelets did not commence their growth as homogeneous single crystals. They developed in a plane defined by two acicular crystals, the
15) D. W. Shaw, J. Electrochem. Soc. 113 (1966) 904; 115 (1968) 777.
10(1971)811.
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14) T. Gabor, J. Electrochem. Soc. 111 (1964) 825. 16) R. B. Wilson, J. AppI. Phys. 37 (1966) 1932.
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