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Regular arrays of LaB6 whiskers grown on singlecrystal substrates by the vapour-liquid-solid method
Journat of the Less-Common
REGULAR CRYSTAL
Metals, 117 (1986)
97
97 - 103
ARRAYS OF LaB6 WHISKERS GROWN ON SINGLESUBSTRATES BY THE V~OUR-LIQUI~SOLID METHOD*
E. I. GIVARGIZOV and L. N. OBOLENSKAYA Insfifufe of ~rys~a~~o~ap~y, U.S.S.R. Academy of Sciences, Moscow If 7333 (U.S.S.R.)
Summary Experiments on the controlled growth of LaB6 whiskers from the vapour phase by the vapour-liquid-solid (VLS) method are described. Single-crystal LaB, wafers were used as substrates, and gold and platinum served as liquid-forming agents to the VLS growth. Chemical transport of the solid LaB6 source by an Hi, + BBr, gas mixture at high temperatures was used to deliver the material for the c~st~lization, the transport proceeding from a cold zone to a hot zone. Regular arrays of LaB, whiskers were grown on [ 1001 substrates.
1. Introduction Lanthanum hexaboride is a metal-like compound with high electronic conductivity, and is of interest as an electron the~oemissive material El, 21. Many studies have been performed to grow single crystals of the compound. Melt-growth techniques are not quite suitable because of the high melting point of LaB6 (about 2530 “C) as well as its high evaporation rate and high reactivity near the melting point. In addition, the melt-grown crystals are rather imperfect and contain low-angle boundaries, microtwins etc. Vapour-phase crystallization is more suitable for preparation because of a substantial lowering of the growth temperature. Niemyski and KierzekPecold [3] grew, for the first time, LaB6 single crystals from the vapour phase by chemical transport and synthesis in the LazO, + BCla + HZ system, the transport proceeding from a cold zone to a hot zone. Results obtained in whisker growth of LaB, are of interest. Motojima et al. [4] have grown LaB, whiskers from a gas mixture of LaCl, + BCl, + Hz + Ar on a graphite substrate at between 1100 and 1300 “C. The whisker growth was, however, difficult to control, although the authors assumed a VLS growth mechanism. Oriented arrays of LaB, whiskers were grown on LaB, substrates by the VLS mechanism with gold as a liquid-forming agent E53. *Paper presented at the 8th International Symposium on Boron, Borides, Carbides, Nitrides and Related Compounds, Tbilisi, October 8 - 12,1984. Elsevier Sequoia/Printed
in The Netherlands
98
In this paper, further investigations on the controlled growth of LaB, whiskers are described. In addition to gold, platinum was also used as the liquid-forming agent, the temperature interval for the LaB, whisker growth being substantially expanded. Regular arrays of LaB, whiskers on singlecrystal LaB, substrates were grown with gold as the agent. New morphological data on the whisker growth are given.
2. Experimental
details
The whisker arrays were grown by a technique, the principle of which was described in a previous publication [5]. Single-crystal LaBI, wafers, 5 to 8 mm in diameter and about 1 mm thick, were cut from an ingot grown by zone melting. The wafers were mechanically ground and polished and, finally, etched chemically in an HNOs solution. Gold or platinum as liquidforming agents were evaporated onto the substrates either as a continuous film 0.05 to 0.4 E.crnthick, or as regularly arranged separate islands of the same thickness using a mask. Chemical transport in the system LaB, + BBr, + H, was used to deliver LaB, for crystallization, the material being transported from a cold zone to a hot zone. The whiskers grown were studied by scanning electron microscopy.
3. Results and discussion Using gold and platinum as stimulating impurities, epitaxially oriented LaB, whiskers were grown, and direct evidence of the VLS mechanism (namely, frozen globules on whisker tips) were obtained. Earlier, it was shown by X-ray analysis that gold is present in similar globules in remarkable amounts, i.e. the metal plays an important role in whisker growth by the VLS mechanism [ 51. The results obtained in this paper were rather different for gold- and platinum-stimulated growth, the differences concerning both temperature intervals for the whisker growth and the morphology of the whiskers. For gold, the interval was rather narrow, from about 1050 to 1150 ‘C, while it spread from about 1100 up to 1600 “C using platinum. Lower limits of the intervals seem to be caused by two factors: by the low solubility of LaB, in the metals and/or by the oxidation of substrates owing to residual oxygen in the gas phase. As is known, boron is practically insoluble in gold, whereas the solubility of lanthanum in gold is rather high, more than about 10 at.% at temperatures in the range 1100 - 1300 “C [6]. Platinum forms a low-melting eutectic (about 880 “C) with boron, while nothing is known about its interaction with lanthanum. The low solubility means that high supersaturations are developed in the droplets and, as a result, instabilities can take place during whisker growth. ‘The oxidation of substrates acts in the same direction, i.e. causes instabilities. Indeed, at relatively low tem-
Fig. 1. LaB6 whiskers grown by means of gold at 1070 “C. Fig. 2. LaB6 whiskers grown by means of platinum at 1350 “C. peratures
instabilities, such as branching, kinking etc., were observed in the LaB, whisker growth. Upper limits of the whisker growth intervals are different for different agents. In the case of gold, it is due to evaporation of the metal, as demonstrated by the conical shape of the whiskers (see below). In the case of platinum, the limit is most probably determined by an increase in the solubility of LaB, in the metal, and hence, by a decrease in the supersaturation in the droplets [7]. Typical LaB, whiskers grown by means of gold and platinum are shown in Figs. 1 and 2 respectively. As can be seen, gold-stimulated whiskers grown even at relatively low temperatures have a slightly conical shape indicating gold evaporation during the growth process. However, in the case of platinum, whiskers grown at 1300 - 1400 “C (this is approximately the middle of the platinum-stimulated growth interval) have cylindrical or prismatic shape, i.e. neither metal evaporation nor overgrowing of whiskers by the vapour-solid mechanism takes place. Growth rates are here as high as 10 pm s-’ indicating relatively high supersaturations in the vapour phase. At such supersaturations the minimum diameter of a whisker, caused by the Gibbs-Thomson effect [ 71, is as low as about 500 a (Fig. 3). Although each whisker carries a frozen droplet on its tip, far from every droplet formed on the LaB, substrates initiates whisker growth. The ratio between the number of droplets and number of whiskers grown can be obtained from Fig. 4. As can be seen, the ratio lies between 2O:l and 50:1, values close to those obtained earlier for TiP whiskers grown by gold in the VLS process [ 81. At relatively high temperatures (Fig. 4(b)), crystal growth proceeds under practically every droplet causing formation of at least a hillock. However, only some droplets leave the substrate giving rise to whiskers. Such a situation is typical for small supersaturations characteristic
Fig. 3. The thinnest LaB6 whiskers grown by means of platinum (indicated by arrow).
(a)
(b)
Fig. 4. Relationship between whiskers grown and droplets of LaB6 + F’t on the substrate: growth temperature (a) 1150 “C; (b) 1550 “C.
of high temperatures. In contrast, at relatively low temperatures, no hillocks are observed under those droplets which do not stimulate whisker growth, while fairly long whiskers have grown on “active” droplets (Fig. 4(a)). In this case, poisoning of the liquid-solid interface by impurities (e.g. by oxides) is the most probable reason for the inactivity of the droplets. Growth directions and orientations of the whiskers are fairly characteristic for this material. It has been found previously [ 51 that LaB6 whiskers grow preferentially in (100) directions perpendicular to cleavage planes of the material. All three directions of the form (100) are seen in Fig. 5. In some cases, the whiskers develop directly from the substrates; in other cases, the various directions appear as a result of branching. As is seen in Fig. 5(b), the contact angle of LaB6 + Pt + droplet is rather large, estimated
(4
(b)
Fig. 5. A family of LaB6 whiskers growing in (100) directions
Fig. 6. Side faceting of LaB6 whiskers. Fig. 7, Differences in growth rates of LaB6 whiskers having different diameters.
120” to 130” when measured from the plane perpendicular to the growth direction of a given whisker. Side faces of the whiskers correspond to the growth directions, namely the four prismatic (100) faces, see Fig. 6. A further characteristic is the dependence of the growth rate of the whiskers on their diameters. In Fig. 7, a family of whiskers grown from the same platinum “spot” on the substrate is shown. When heated, the spot broke into lots of droplets having different diameters. Two relatively large droplets gave rise to the growth of “thick” central whiskers and others initiated a lot of tiny satellites. As is seen from Fig. 7 (as well as from Fig. 5), the smaller the whisker diameter, the less, in general, is the growth rate,
102
as was also repeatedly
observed
for whiskers
of silicon and other materials
r71. Finally, under certain conditions, it was possible to grow regular arrays of LaB, whiskers (Figs. 8 and 9). To obtain this result, it is important to have a certain ratio between the thickness of the evaporated metal and the diameter of the spot: the ratio, generally, should be not less than about l/20, although an optimal value depends on the growth temperature, kind of metal, supersaturation in the vapour phase, purity of the medium etc. As is seen in Figs. 8 and 9, formation of satellites is difficult to avoid. In addition, comparison of these figures shows that, at sufficiently high growth temperatures, whiskers are sharpened owing to evaporation of the liquidforming metal (gold in this case).
Fig. 8. Regular array of LaBs whiskers grown by means of gold at relatively low temperatures (less than about 1080 “C). Fig. 9. The same as Fig. 8 but grown at higher temperatures (more than about 1100 “C).
4. Conclusions 1. Whiskers of LaB, can be grown as oriented arrays by the VLS mechanism with gold or platinum as liquid-forming agent. 2. The temperature interval of whisker growth is limited at the lower end by the low solubility of LaB, in the metals and/or by the oxidation of the substrate and at the higher end by the evaporation as a result of enhanced solubility of LaB, in the metal (in the case of platinum). 3. The LaB, whiskers grow in (100) directions and are faceted by (100) planes. 4. The growth rate of the whiskers depends on their diameters in accordance with the Gibbs-Thomson effect (i.e. lowering of supersaturations
103
owing to enhanced equilibrium vapour pressure and/or enhanced solubility of the material in the droplets). 5. Regular arrays of LaB, whiskers can be grown on single-crystal substrates.
References 1 2 3 4 5 6
J. M. Lafferty, J. Appt. Phys., 22 (1951) 299. G. A. Kudintseva and M. I. Elinson, Radio Eng. Electron. Phys., 12 (1968) 798. T. Niemyski and E. Kierzek-Pecold, J. Cryst. Growth, 3/4 (1968) 162. S. Motojima, Y. Takahashi and K. Sugiyama, J. Cryst. Growth, 44 (1978) 106. E. I. Givargizov and L. N. Oholenskaya, J. Cryst. Growth, 51 (1981) 190. M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 7 E. I. Givargizov, Growth of whiskers by the vapor-liquid-solid mechanism. In E. Kaldis (ed.), Current Topics in Material Science, Vol. 1, Amsterdam/North-Holland, 1978, pp. 79 - 145. 8 K. Sugiyama, M. Takigawa, S. Motojima and Y. Takahashi, J. Cryst. Growth, 44 (1978) 499.
REGULAR CRYSTAL
Metals, 117 (1986)
97
97 - 103
ARRAYS OF LaB6 WHISKERS GROWN ON SINGLESUBSTRATES BY THE V~OUR-LIQUI~SOLID METHOD*
E. I. GIVARGIZOV and L. N. OBOLENSKAYA Insfifufe of ~rys~a~~o~ap~y, U.S.S.R. Academy of Sciences, Moscow If 7333 (U.S.S.R.)
Summary Experiments on the controlled growth of LaB6 whiskers from the vapour phase by the vapour-liquid-solid (VLS) method are described. Single-crystal LaB, wafers were used as substrates, and gold and platinum served as liquid-forming agents to the VLS growth. Chemical transport of the solid LaB6 source by an Hi, + BBr, gas mixture at high temperatures was used to deliver the material for the c~st~lization, the transport proceeding from a cold zone to a hot zone. Regular arrays of LaB, whiskers were grown on [ 1001 substrates.
1. Introduction Lanthanum hexaboride is a metal-like compound with high electronic conductivity, and is of interest as an electron the~oemissive material El, 21. Many studies have been performed to grow single crystals of the compound. Melt-growth techniques are not quite suitable because of the high melting point of LaB6 (about 2530 “C) as well as its high evaporation rate and high reactivity near the melting point. In addition, the melt-grown crystals are rather imperfect and contain low-angle boundaries, microtwins etc. Vapour-phase crystallization is more suitable for preparation because of a substantial lowering of the growth temperature. Niemyski and KierzekPecold [3] grew, for the first time, LaB6 single crystals from the vapour phase by chemical transport and synthesis in the LazO, + BCla + HZ system, the transport proceeding from a cold zone to a hot zone. Results obtained in whisker growth of LaB, are of interest. Motojima et al. [4] have grown LaB, whiskers from a gas mixture of LaCl, + BCl, + Hz + Ar on a graphite substrate at between 1100 and 1300 “C. The whisker growth was, however, difficult to control, although the authors assumed a VLS growth mechanism. Oriented arrays of LaB, whiskers were grown on LaB, substrates by the VLS mechanism with gold as a liquid-forming agent E53. *Paper presented at the 8th International Symposium on Boron, Borides, Carbides, Nitrides and Related Compounds, Tbilisi, October 8 - 12,1984. Elsevier Sequoia/Printed
in The Netherlands
98
In this paper, further investigations on the controlled growth of LaB, whiskers are described. In addition to gold, platinum was also used as the liquid-forming agent, the temperature interval for the LaB, whisker growth being substantially expanded. Regular arrays of LaB, whiskers on singlecrystal LaB, substrates were grown with gold as the agent. New morphological data on the whisker growth are given.
2. Experimental
details
The whisker arrays were grown by a technique, the principle of which was described in a previous publication [5]. Single-crystal LaBI, wafers, 5 to 8 mm in diameter and about 1 mm thick, were cut from an ingot grown by zone melting. The wafers were mechanically ground and polished and, finally, etched chemically in an HNOs solution. Gold or platinum as liquidforming agents were evaporated onto the substrates either as a continuous film 0.05 to 0.4 E.crnthick, or as regularly arranged separate islands of the same thickness using a mask. Chemical transport in the system LaB, + BBr, + H, was used to deliver LaB, for crystallization, the material being transported from a cold zone to a hot zone. The whiskers grown were studied by scanning electron microscopy.
3. Results and discussion Using gold and platinum as stimulating impurities, epitaxially oriented LaB, whiskers were grown, and direct evidence of the VLS mechanism (namely, frozen globules on whisker tips) were obtained. Earlier, it was shown by X-ray analysis that gold is present in similar globules in remarkable amounts, i.e. the metal plays an important role in whisker growth by the VLS mechanism [ 51. The results obtained in this paper were rather different for gold- and platinum-stimulated growth, the differences concerning both temperature intervals for the whisker growth and the morphology of the whiskers. For gold, the interval was rather narrow, from about 1050 to 1150 ‘C, while it spread from about 1100 up to 1600 “C using platinum. Lower limits of the intervals seem to be caused by two factors: by the low solubility of LaB, in the metals and/or by the oxidation of substrates owing to residual oxygen in the gas phase. As is known, boron is practically insoluble in gold, whereas the solubility of lanthanum in gold is rather high, more than about 10 at.% at temperatures in the range 1100 - 1300 “C [6]. Platinum forms a low-melting eutectic (about 880 “C) with boron, while nothing is known about its interaction with lanthanum. The low solubility means that high supersaturations are developed in the droplets and, as a result, instabilities can take place during whisker growth. ‘The oxidation of substrates acts in the same direction, i.e. causes instabilities. Indeed, at relatively low tem-
Fig. 1. LaB6 whiskers grown by means of gold at 1070 “C. Fig. 2. LaB6 whiskers grown by means of platinum at 1350 “C. peratures
instabilities, such as branching, kinking etc., were observed in the LaB, whisker growth. Upper limits of the whisker growth intervals are different for different agents. In the case of gold, it is due to evaporation of the metal, as demonstrated by the conical shape of the whiskers (see below). In the case of platinum, the limit is most probably determined by an increase in the solubility of LaB, in the metal, and hence, by a decrease in the supersaturation in the droplets [7]. Typical LaB, whiskers grown by means of gold and platinum are shown in Figs. 1 and 2 respectively. As can be seen, gold-stimulated whiskers grown even at relatively low temperatures have a slightly conical shape indicating gold evaporation during the growth process. However, in the case of platinum, whiskers grown at 1300 - 1400 “C (this is approximately the middle of the platinum-stimulated growth interval) have cylindrical or prismatic shape, i.e. neither metal evaporation nor overgrowing of whiskers by the vapour-solid mechanism takes place. Growth rates are here as high as 10 pm s-’ indicating relatively high supersaturations in the vapour phase. At such supersaturations the minimum diameter of a whisker, caused by the Gibbs-Thomson effect [ 71, is as low as about 500 a (Fig. 3). Although each whisker carries a frozen droplet on its tip, far from every droplet formed on the LaB, substrates initiates whisker growth. The ratio between the number of droplets and number of whiskers grown can be obtained from Fig. 4. As can be seen, the ratio lies between 2O:l and 50:1, values close to those obtained earlier for TiP whiskers grown by gold in the VLS process [ 81. At relatively high temperatures (Fig. 4(b)), crystal growth proceeds under practically every droplet causing formation of at least a hillock. However, only some droplets leave the substrate giving rise to whiskers. Such a situation is typical for small supersaturations characteristic
Fig. 3. The thinnest LaB6 whiskers grown by means of platinum (indicated by arrow).
(a)
(b)
Fig. 4. Relationship between whiskers grown and droplets of LaB6 + F’t on the substrate: growth temperature (a) 1150 “C; (b) 1550 “C.
of high temperatures. In contrast, at relatively low temperatures, no hillocks are observed under those droplets which do not stimulate whisker growth, while fairly long whiskers have grown on “active” droplets (Fig. 4(a)). In this case, poisoning of the liquid-solid interface by impurities (e.g. by oxides) is the most probable reason for the inactivity of the droplets. Growth directions and orientations of the whiskers are fairly characteristic for this material. It has been found previously [ 51 that LaB6 whiskers grow preferentially in (100) directions perpendicular to cleavage planes of the material. All three directions of the form (100) are seen in Fig. 5. In some cases, the whiskers develop directly from the substrates; in other cases, the various directions appear as a result of branching. As is seen in Fig. 5(b), the contact angle of LaB6 + Pt + droplet is rather large, estimated
(4
(b)
Fig. 5. A family of LaB6 whiskers growing in (100) directions
Fig. 6. Side faceting of LaB6 whiskers. Fig. 7, Differences in growth rates of LaB6 whiskers having different diameters.
120” to 130” when measured from the plane perpendicular to the growth direction of a given whisker. Side faces of the whiskers correspond to the growth directions, namely the four prismatic (100) faces, see Fig. 6. A further characteristic is the dependence of the growth rate of the whiskers on their diameters. In Fig. 7, a family of whiskers grown from the same platinum “spot” on the substrate is shown. When heated, the spot broke into lots of droplets having different diameters. Two relatively large droplets gave rise to the growth of “thick” central whiskers and others initiated a lot of tiny satellites. As is seen from Fig. 7 (as well as from Fig. 5), the smaller the whisker diameter, the less, in general, is the growth rate,
102
as was also repeatedly
observed
for whiskers
of silicon and other materials
r71. Finally, under certain conditions, it was possible to grow regular arrays of LaB, whiskers (Figs. 8 and 9). To obtain this result, it is important to have a certain ratio between the thickness of the evaporated metal and the diameter of the spot: the ratio, generally, should be not less than about l/20, although an optimal value depends on the growth temperature, kind of metal, supersaturation in the vapour phase, purity of the medium etc. As is seen in Figs. 8 and 9, formation of satellites is difficult to avoid. In addition, comparison of these figures shows that, at sufficiently high growth temperatures, whiskers are sharpened owing to evaporation of the liquidforming metal (gold in this case).
Fig. 8. Regular array of LaBs whiskers grown by means of gold at relatively low temperatures (less than about 1080 “C). Fig. 9. The same as Fig. 8 but grown at higher temperatures (more than about 1100 “C).
4. Conclusions 1. Whiskers of LaB, can be grown as oriented arrays by the VLS mechanism with gold or platinum as liquid-forming agent. 2. The temperature interval of whisker growth is limited at the lower end by the low solubility of LaB, in the metals and/or by the oxidation of the substrate and at the higher end by the evaporation as a result of enhanced solubility of LaB, in the metal (in the case of platinum). 3. The LaB, whiskers grow in (100) directions and are faceted by (100) planes. 4. The growth rate of the whiskers depends on their diameters in accordance with the Gibbs-Thomson effect (i.e. lowering of supersaturations
103
owing to enhanced equilibrium vapour pressure and/or enhanced solubility of the material in the droplets). 5. Regular arrays of LaB, whiskers can be grown on single-crystal substrates.
References 1 2 3 4 5 6
J. M. Lafferty, J. Appt. Phys., 22 (1951) 299. G. A. Kudintseva and M. I. Elinson, Radio Eng. Electron. Phys., 12 (1968) 798. T. Niemyski and E. Kierzek-Pecold, J. Cryst. Growth, 3/4 (1968) 162. S. Motojima, Y. Takahashi and K. Sugiyama, J. Cryst. Growth, 44 (1978) 106. E. I. Givargizov and L. N. Oholenskaya, J. Cryst. Growth, 51 (1981) 190. M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 7 E. I. Givargizov, Growth of whiskers by the vapor-liquid-solid mechanism. In E. Kaldis (ed.), Current Topics in Material Science, Vol. 1, Amsterdam/North-Holland, 1978, pp. 79 - 145. 8 K. Sugiyama, M. Takigawa, S. Motojima and Y. Takahashi, J. Cryst. Growth, 44 (1978) 499.