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Effects of co-depositing oxygen on the growth morphology of (111) and (100) Al single crystal faces in thin films
Vacuum/volume 33/number 1/2/pages 25 to 30/1983 Primed in Great Britain
0042-207X/83/010025-06503.00/0 Pergamon Press Ltd
Effects of co-depositing oxygen on the growth morphology of (111) and (100) AI single crystal faces in thin films P B Barna, F M Reicha, G Barcza, L G o s z t o l a and F K o l t a i , Research Institute for Technical Physics of the Hungarian Academy of Sciences, H- 1325 Budapest, PO Box 76, Hungary
Correlation between the growth surface structure and the oxygen-to-aluminium flux ratio was found in vapour deposited AI films. Studies of the effect of co-depositing oxygen species on the growth surface structure were carried out in the case of epitaxially grown AI films (on NaCl, mica and GaAs substrates) containing crystals of ( 1 11 ), (100) and ( 1 10) orientations. On ( 11 1) crystal faces bunches of growth steps decorated by pinning sites, dents, macro steps correlating to grain boundaries and hillocks were developed. However, on (100) and (1 10) crystal faces only dents have been found. These results help to explain the correlation between the crystal face anisotropy in the surface micro-chemistry and anisotropy in the participation of oxygen species in the growth mechanism on the various crystal faces. Whiskers and "mushroom whiskers" were detected in AI films deposited at high level of oxygen contamination on amorphous substrates at first. Their formation can be ascribed also to these phenomena.
Introduction The growth of crystals (single crystal growth and coalescence) in vapour deposited thin films takes place usually under the influence of the environment ~-3. The surface chemical interactions between the impinging species of environmental impurities and various faces of growing crystals control the codeposition of impurity species and their participation in the atomby-atom building process of the crystal system of films 4. The very high sensitivity of structure, surface morphology and properties of polycrystalline films to impurities 4-8 can be ascribed also to the same phenomena. The surface studies carried out in the last decade presented much information, both, on the mechanism of surface chemical interactions, kind of adsorbed species and also their sites on static crystal faces. These results can be used to approach and understand phenomena of thin film formation under the influence of environmental impurities. We are going now to discuss the aluminium-oxygen system as an example. The detailed studies on the oxygen uptake properties of the crystallographic faces orAl single crystals revealed also the sites of adsorbed species TM. These results proved a considerable difference in the surface micro-chemistry of the various crystallographic faces, i.e. the existence of crystalf~a~e anisotropy in the surface micro-chemistry. On the other hand, the systematic investigations carried out on the formation of A1 films showed an unambiguous correlation between the surface growth morphology of the film and the oxygen-to-aluminium flux ratio, Ko.yg~, (Figure 1)~2. In oxygen contaminated films very characteristic growth features have been found too, e.g. bunches of growth steps decorated by pinning sites, protruded crystals (hillocks) exhibi-
ting points of truncated octahedrons; preferential truncation of crystal shapes in epitaxially grown films 12-17. These growth features raised the idea that a correlation between crystal face anisotropy in the surface micro-chemistry and the anisotropy in the crystal growth could exist ~5. A mechanism has been proposed for the description of the formation of growth peculiarities mentioned above ~2'~6. The main ideas of this mechanism are based on the accumulation of oxygen species to the surface of the growing crystals 3, as well as on the possible difference in the participation of oxygen species in the monolayer growth processes on the various crystal faces according to the crystal face anisotropy in the surface microchemistry. To prove the reality of this latter idea the effect of admitted oxygen on the growth morphologies of (100), (111) and (110) faces have been studied in AI films. These films were prepared on various single crystal substrates (NaC1, mica and GaAs) in the same deposition process under the same conditions. Some of the preliminary results of these experiments and growth peculiarities developed in films prepared on amorphous substrates at high oxygen-to-aluminium flux ratio are presented in this paper.
Experimental AI films have been prepared in a high vacuum system using a hairpin-type evaporation source of a double spiral of tungsten wires. The purity of evaporated A1 was 99.999%. The pressure in the system was measured by a nude Bayard-Alpert gauge situated near the substrate, but shadowed from the direct vapour beam of the evaporation source. The baseline pressure of the system using 25
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal races
Figure 1. Crystal growth features in AI films deposited onto glass substrates at 300°C. Film thickness I /Jm Ko,yge,:(a) and (b) 10-3; (C) |O -2 and (d) about 1.
a Meissner trap was 5 x 10- 5 Pa. Oxygen was introduced into the chamber through a needle valve at the bottom plate and directed both, to the substrate and vacuum gauge. To evaluate the oxygenspecies-to-aluminium-atom flux ratio (Ko,rg~,) the oxygen fill pressure was measured before the evaporation of aluminium in the unthrottled system. A quartz crystal monitor was used to control the aluminium-atom flux. Films were deposited onto cleaved NaC1 and mica, chemically cleaned GaAs single crystal substrates as well as onto amorphous substrates (carbon films supported by micro grids and glass plates). By using a tube oven the substrates were degassed at 550°C for 30 rain before deposition. Films were deposited at 300°C substrate temperature and the Koxygen varied for the different depositions. The surface morphology of films was studied by SEM and by single stage Pt-C replica transmission electron microscopy, while the orientation and structure of crystals was analysed by TEM (selected area diffraction) in samples floated off the substrate and by channelling in samples supported on GaAs. For studying the cross-section thick films have been torn from the substrate. Results Surface growth structures on (111), (100) and (I 10) faces. Cleaved surfaces of NaCI ((100) face) and mica (111 ) as well as chemically polished surface of GaAs ((100) face) were used to grow epitaxial films. AI crystals of (111) orientation grow both on NaCI and mica while (110) oriented crystals develop on NaCI surfaces contaminated by a very thin deposit (a few monolayers) of carbon before evaporating the AI. Sometimes both, the (111) and (100) orientations coexist on the contaminated (100) face of NaCI. The film formation starts by nucleation, growth and coalescence of three-dimensional crystals on these single crystal surfaces at 26
10-4Pa. K,,xyg~n was l0 -3 calculated from the oxygen fill pressure. The growth morphology was studied mainly in the continuous films or in films of island structure by sampling. As a consequence of this, the surface growth structures of the film at this growth stage result both from the coalescence and from the crystals developed during the coalescence. However, the coalescence of crystals is controlled by the surface conditions of participating crystals developed under the influence of K,,xygen before coalescence t s,~6. The coalescence reorganizes usually both, the surface growth structure of coalescing crystals as well as the occasional contamination (oxide) layers formed on their surfaces. This newly developed surface structure will influence or even determine the surface growth phenomena of crystals composing the new islands and/or continuous film. Surface growth structures presented here should be regarded as a result of phenomena mentioned abovc.
(111) growth faces. Bunches of growth steps decorated by pinning sites and hillocks inside the single crystals (hillock type I) as well as macro steps at grain boundaries are the characteristic elements of surface growth structures in the regions of (11 I) oriented crystals in epitaxial films grown on contaminated surfaces of NaC1 (area marked by (A) in Figures 2 and 4) and in films grown epitaxiaUy on mica. These surface growth structures are the same as those detected on the surface of (111) oriented crystals in polycrystalline films deposited onto glass substrates under the same process (Figure 3 and in refs 12, 15, 16). Surface macro steps belonging to grain boundaries are clearly detectable in TEM images of shadowed and floated-off films too (Figure 4). The development of macro steps indicates that the height difference of the coalescing crystals is preserved at coalescence probably
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai. Growth morphology of (111 ) and (100) AI single crystal faces
Figure 2. Epitaxial AI film deposited onto carbon contaminated surface of cleaved NaC1 at 300°C and orientation area (A) (111); area (B) (100).
Figure 3. Growth surface structure of AI film deposited onto glass :,ubstrate in the same process as film shown in Figure 2.
Ko, rg,, ~
10-3. Film thickness 200 nm. Crystal
Figure 4. TEM image of the same film as in Figure 2, floated off. Selected area diffraction is taken from the encircled area. 27
P B Bama, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) A[ single crystal faces
because of the contamination of contacting surfaces iS. Pits situated at grain boundaries can be found too. (100) growth faces. There are regions of (100) oriented crystals in films prepared on contaminated surfaces of NaC1 (region marked by (B) in Figures 2 and 4). Selected area diffraction taken from the area marked by a circle proves the (100) orientation in this region of the shadowed sample. This area is identified by the region (B) in Figure 2. The replica images in Figures 2 and 4 prove that neither bunches of growth steps nor macro steps are developed on (100) faces. However, one can find and distinguish two kinds of dents. One is pit-like while the other is more or less cup-shaped. The origin of these dents and their correlation to the bulk structure needs further investigations.
truncated octahedrons and develop both, by 'orientation' and 'coalescence selection q 5.~6. Both of the selection mechanisms are assumed to correlate to the crystal face anisotropy in the participation of oxygen species in the building of crystal structure. The formation of a st~rface covering contamination layer on the preferential regions of crystal surfaces hinders or even interrupts the growth of crystals in the direction perpendicular to the covered surface areas. The above-mentioned selection mechanisms become more active at Ko~y,~. > 10 -2 and control the development of a very inhomogeneous structure from the very early stages of film formation 12. This results in the formation of whiskers in films of 1 ltm thickness (Figures 6 and l(b)). They have different length but the longest one seems to have a (100) axis according to the
(110) growth face. Epitaxial films of this orientation are developed on chemically cleaned (100) faces of GaAs. The orientation of films determined by a SEM Selected area channelling pattern was (1 I0)AI//(100)GaAs and [110] AI//[100] GaAs. On (110)growth faces neither bunches of steps nor macro steps can be detected (Figure 5). However a high density of small dents can be found
Figure 6. Whiskers grown in AI film deposited onto glass substrate at !
Figure 5. Surface growth structure of(110) crystal face in AI film deposited onto GaAs single crystal substrate in the same process as film shown in Figure 2. both, on the surface of a continuous film and islands. Samples taken from the different stages of film formation prove that these dents are present from the early stages of growth. Dents developing on (110) faces are less regular than those on (100) faces. It is difficult to explain fully their origin at that moment, but they may result from the coalescence of crystals.
Crystal growth peculiarities in AI films deposited at high value of oxygen-species-to-almninium-atom flux ratio on amorphous substrates The phenomenon of coalescence plays an important role in the development of the texture in films condensed on amorphous substrates 3,1~ Increasing the level of contamination (Koxyge,),i.e. the contamination content of films, the degree of texture decreases while the growth peculiarities start to develop 3'~. At a value of Ko,yg,, ~ 10- 2, single crystals are protruding over the film surface termed 'hillocks type II'. These configurations exhibit points of 28
300°C and
Ko,~,go,, ~ 1. Film thickness is 1 /zm.
SEM image (Figure 7) ~s. By increasing the thickness or the contamination level, the points of the whiskers become contaminated and the impinging Ai vapour flux initiates the formation of new nuclei on the tip surfaces 3 (Figure 8). These growing nuclei can develop a polycrystalline head the size of which increases during deposition. Very characteristic crystal configurations can develop in this way, the so-called 'mushroom whiskers' (Figures 9 and 10). These have conically shaped dents around their 'stem' as a result of the shadowing effect of the growing head. These configurations appear also in the cross section of films. The conical dents will be closed when the surface of surrounding areas gets to the periphery of.the heads during film growth. The heads of these 'mushroom whiskers' will be detected as cup-shaped hills beginning with that stage of growth.
Conclusions The studies of growth surface structures of (111), (100) and (110) faces of epitaxial Al films indicate a correlation between the crystal face anisotropy in the surface micro-chemistry of oxygen
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal faces
Figure 7. The longest whisker in the film shown in Figure 6.
Figure 9(a).
Figure 8. Whisker with contaminated tip, developed in a 1.5 ttm thick film, deposited under the same conditions as the film in Figure 7.
Figure 9. Mushroom whiskers in A1filmsdeposited onto glass substrate at 300°C and Ko,ygcn~ 1. Film thickness is 1,5/am.
species 9-~1 and their participation in the building processes on various crystal faces. The presence of oxygen develops bunches of growth steps decorated by pinning sites, dents, macro steps and hillocks on (111) faces. Under the same conditions only dents develop on (100) and (110) faces. Contamination layers partially or completely covering the surface of crystals can develop mainly by the processes accumulating the oxygen species on (111) faces 1s,16. This phenomenon
leads to the separation of crystals during their growth, according to their orientation. Whiskers and 'mushroom whiskers' develop at a high level of oxygen contamination on amorphous substrates. Their formation can be ascribed to the orientation and coalescence selection of crystals controlled by the anisotropic incorporation of oxygen species and to the development of contamination layers on specific surface areas of growing crystals. 29
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal faces
References
Figure 10. Cross section of film shown in Figure 9.
Acknowledgements A u t h o r s ' t h a n k s are due to D r G Gergely, D r ,/~ Barna, D r M Malicsk6 for their valuable discussions, to D r I Pozsgai for s u b m i t t i n g S E M m i c r o g r a p h s (Figures 6 a n d 7), M r s 1~ H a j m ~ y , M r s G y Ghizer a n d M r P Lhnyi for their help in p r e p a r i n g samples.
30
1 1 E Bauer, Single CrystalFilms. (Edited by M H Francombe and H Sato), p 43. Pergamon Press, Oxford (1964). 2 j A Venables and G L Price, in Epitaxial Growth (Edited by J W Matthews), Part B, p 381: Academic Press, New York (1975). 3 j F Pbcza, A. Barna, P B Bama, I Pozsgai and G Radnbczi, Jpn J Appl Phys, Suppl 2, Part I, p 525 (1974). '* P B Barna, ,~ Barna and Z Pahl, Acta Phys Hung, 49, 77 (1980). 5 L E Murr and M C Inman, Phil Mag, 8, 135 (1966). 6 G Hass, J Fac Sci Technol, 16, 113 (1979). 7 M Laugier, Thin Solid Films, 79, 15 (1981). 8 T J Faith, R S lrven, J J O'Neill, Jr and F J Tams, J Vac Sci Technol, 19, 709 (1981). 9 C W B Martinson, S A Flodstr6m, J Rundgren and P Westrin, Surface Sci, 89, 102 (1979). 10 R Michel, J Gastaldi, C Allasis, C J Jourdon and J Derrien, Surface Sci, 95, 309 (1980). 11 R Z Bachrach, G V Hansson and R S Bauer, Surface Sci, 109, L 560 (1981). 12 ,~ Barna, P B Barna, G Radn6czi, F M Reicha and L T6th, Phys St Solidi 55, 427 (1979). 13 F M d'Heurle, L Berenbau and R Rosenberg, Trans MS AIME, 242, 502 (1968). 14 N G Dbere, Th P Arsenio and B K Patnaik, Thin Solid Films, 30, 267 (1975); 44, 29 (1977). 15 p B Barna and F M Reicha, Proc 8 Int Vacuum Congress, 22-26 September 1980, Cannes, France, Vol 1, p 165. Suppl le Vide, les Couches Minces No 201 (1980). 16 F M Reicha, P B Barna, Acta Phys Hung, 49, 237 (1980). 17 j F P6cza, ,,~ Barna, P B Bama, Kristall Technik, 5, 315 (1970). 18 p B Barna, F M Reicha and G Barcza (to be published).
0042-207X/83/010025-06503.00/0 Pergamon Press Ltd
Effects of co-depositing oxygen on the growth morphology of (111) and (100) AI single crystal faces in thin films P B Barna, F M Reicha, G Barcza, L G o s z t o l a and F K o l t a i , Research Institute for Technical Physics of the Hungarian Academy of Sciences, H- 1325 Budapest, PO Box 76, Hungary
Correlation between the growth surface structure and the oxygen-to-aluminium flux ratio was found in vapour deposited AI films. Studies of the effect of co-depositing oxygen species on the growth surface structure were carried out in the case of epitaxially grown AI films (on NaCl, mica and GaAs substrates) containing crystals of ( 1 11 ), (100) and ( 1 10) orientations. On ( 11 1) crystal faces bunches of growth steps decorated by pinning sites, dents, macro steps correlating to grain boundaries and hillocks were developed. However, on (100) and (1 10) crystal faces only dents have been found. These results help to explain the correlation between the crystal face anisotropy in the surface micro-chemistry and anisotropy in the participation of oxygen species in the growth mechanism on the various crystal faces. Whiskers and "mushroom whiskers" were detected in AI films deposited at high level of oxygen contamination on amorphous substrates at first. Their formation can be ascribed also to these phenomena.
Introduction The growth of crystals (single crystal growth and coalescence) in vapour deposited thin films takes place usually under the influence of the environment ~-3. The surface chemical interactions between the impinging species of environmental impurities and various faces of growing crystals control the codeposition of impurity species and their participation in the atomby-atom building process of the crystal system of films 4. The very high sensitivity of structure, surface morphology and properties of polycrystalline films to impurities 4-8 can be ascribed also to the same phenomena. The surface studies carried out in the last decade presented much information, both, on the mechanism of surface chemical interactions, kind of adsorbed species and also their sites on static crystal faces. These results can be used to approach and understand phenomena of thin film formation under the influence of environmental impurities. We are going now to discuss the aluminium-oxygen system as an example. The detailed studies on the oxygen uptake properties of the crystallographic faces orAl single crystals revealed also the sites of adsorbed species TM. These results proved a considerable difference in the surface micro-chemistry of the various crystallographic faces, i.e. the existence of crystalf~a~e anisotropy in the surface micro-chemistry. On the other hand, the systematic investigations carried out on the formation of A1 films showed an unambiguous correlation between the surface growth morphology of the film and the oxygen-to-aluminium flux ratio, Ko.yg~, (Figure 1)~2. In oxygen contaminated films very characteristic growth features have been found too, e.g. bunches of growth steps decorated by pinning sites, protruded crystals (hillocks) exhibi-
ting points of truncated octahedrons; preferential truncation of crystal shapes in epitaxially grown films 12-17. These growth features raised the idea that a correlation between crystal face anisotropy in the surface micro-chemistry and the anisotropy in the crystal growth could exist ~5. A mechanism has been proposed for the description of the formation of growth peculiarities mentioned above ~2'~6. The main ideas of this mechanism are based on the accumulation of oxygen species to the surface of the growing crystals 3, as well as on the possible difference in the participation of oxygen species in the monolayer growth processes on the various crystal faces according to the crystal face anisotropy in the surface microchemistry. To prove the reality of this latter idea the effect of admitted oxygen on the growth morphologies of (100), (111) and (110) faces have been studied in AI films. These films were prepared on various single crystal substrates (NaC1, mica and GaAs) in the same deposition process under the same conditions. Some of the preliminary results of these experiments and growth peculiarities developed in films prepared on amorphous substrates at high oxygen-to-aluminium flux ratio are presented in this paper.
Experimental AI films have been prepared in a high vacuum system using a hairpin-type evaporation source of a double spiral of tungsten wires. The purity of evaporated A1 was 99.999%. The pressure in the system was measured by a nude Bayard-Alpert gauge situated near the substrate, but shadowed from the direct vapour beam of the evaporation source. The baseline pressure of the system using 25
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal races
Figure 1. Crystal growth features in AI films deposited onto glass substrates at 300°C. Film thickness I /Jm Ko,yge,:(a) and (b) 10-3; (C) |O -2 and (d) about 1.
a Meissner trap was 5 x 10- 5 Pa. Oxygen was introduced into the chamber through a needle valve at the bottom plate and directed both, to the substrate and vacuum gauge. To evaluate the oxygenspecies-to-aluminium-atom flux ratio (Ko,rg~,) the oxygen fill pressure was measured before the evaporation of aluminium in the unthrottled system. A quartz crystal monitor was used to control the aluminium-atom flux. Films were deposited onto cleaved NaC1 and mica, chemically cleaned GaAs single crystal substrates as well as onto amorphous substrates (carbon films supported by micro grids and glass plates). By using a tube oven the substrates were degassed at 550°C for 30 rain before deposition. Films were deposited at 300°C substrate temperature and the Koxygen varied for the different depositions. The surface morphology of films was studied by SEM and by single stage Pt-C replica transmission electron microscopy, while the orientation and structure of crystals was analysed by TEM (selected area diffraction) in samples floated off the substrate and by channelling in samples supported on GaAs. For studying the cross-section thick films have been torn from the substrate. Results Surface growth structures on (111), (100) and (I 10) faces. Cleaved surfaces of NaCI ((100) face) and mica (111 ) as well as chemically polished surface of GaAs ((100) face) were used to grow epitaxial films. AI crystals of (111) orientation grow both on NaCI and mica while (110) oriented crystals develop on NaCI surfaces contaminated by a very thin deposit (a few monolayers) of carbon before evaporating the AI. Sometimes both, the (111) and (100) orientations coexist on the contaminated (100) face of NaCI. The film formation starts by nucleation, growth and coalescence of three-dimensional crystals on these single crystal surfaces at 26
10-4Pa. K,,xyg~n was l0 -3 calculated from the oxygen fill pressure. The growth morphology was studied mainly in the continuous films or in films of island structure by sampling. As a consequence of this, the surface growth structures of the film at this growth stage result both from the coalescence and from the crystals developed during the coalescence. However, the coalescence of crystals is controlled by the surface conditions of participating crystals developed under the influence of K,,xygen before coalescence t s,~6. The coalescence reorganizes usually both, the surface growth structure of coalescing crystals as well as the occasional contamination (oxide) layers formed on their surfaces. This newly developed surface structure will influence or even determine the surface growth phenomena of crystals composing the new islands and/or continuous film. Surface growth structures presented here should be regarded as a result of phenomena mentioned abovc.
(111) growth faces. Bunches of growth steps decorated by pinning sites and hillocks inside the single crystals (hillock type I) as well as macro steps at grain boundaries are the characteristic elements of surface growth structures in the regions of (11 I) oriented crystals in epitaxial films grown on contaminated surfaces of NaC1 (area marked by (A) in Figures 2 and 4) and in films grown epitaxiaUy on mica. These surface growth structures are the same as those detected on the surface of (111) oriented crystals in polycrystalline films deposited onto glass substrates under the same process (Figure 3 and in refs 12, 15, 16). Surface macro steps belonging to grain boundaries are clearly detectable in TEM images of shadowed and floated-off films too (Figure 4). The development of macro steps indicates that the height difference of the coalescing crystals is preserved at coalescence probably
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai. Growth morphology of (111 ) and (100) AI single crystal faces
Figure 2. Epitaxial AI film deposited onto carbon contaminated surface of cleaved NaC1 at 300°C and orientation area (A) (111); area (B) (100).
Figure 3. Growth surface structure of AI film deposited onto glass :,ubstrate in the same process as film shown in Figure 2.
Ko, rg,, ~
10-3. Film thickness 200 nm. Crystal
Figure 4. TEM image of the same film as in Figure 2, floated off. Selected area diffraction is taken from the encircled area. 27
P B Bama, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) A[ single crystal faces
because of the contamination of contacting surfaces iS. Pits situated at grain boundaries can be found too. (100) growth faces. There are regions of (100) oriented crystals in films prepared on contaminated surfaces of NaC1 (region marked by (B) in Figures 2 and 4). Selected area diffraction taken from the area marked by a circle proves the (100) orientation in this region of the shadowed sample. This area is identified by the region (B) in Figure 2. The replica images in Figures 2 and 4 prove that neither bunches of growth steps nor macro steps are developed on (100) faces. However, one can find and distinguish two kinds of dents. One is pit-like while the other is more or less cup-shaped. The origin of these dents and their correlation to the bulk structure needs further investigations.
truncated octahedrons and develop both, by 'orientation' and 'coalescence selection q 5.~6. Both of the selection mechanisms are assumed to correlate to the crystal face anisotropy in the participation of oxygen species in the building of crystal structure. The formation of a st~rface covering contamination layer on the preferential regions of crystal surfaces hinders or even interrupts the growth of crystals in the direction perpendicular to the covered surface areas. The above-mentioned selection mechanisms become more active at Ko~y,~. > 10 -2 and control the development of a very inhomogeneous structure from the very early stages of film formation 12. This results in the formation of whiskers in films of 1 ltm thickness (Figures 6 and l(b)). They have different length but the longest one seems to have a (100) axis according to the
(110) growth face. Epitaxial films of this orientation are developed on chemically cleaned (100) faces of GaAs. The orientation of films determined by a SEM Selected area channelling pattern was (1 I0)AI//(100)GaAs and [110] AI//[100] GaAs. On (110)growth faces neither bunches of steps nor macro steps can be detected (Figure 5). However a high density of small dents can be found
Figure 6. Whiskers grown in AI film deposited onto glass substrate at !
Figure 5. Surface growth structure of(110) crystal face in AI film deposited onto GaAs single crystal substrate in the same process as film shown in Figure 2. both, on the surface of a continuous film and islands. Samples taken from the different stages of film formation prove that these dents are present from the early stages of growth. Dents developing on (110) faces are less regular than those on (100) faces. It is difficult to explain fully their origin at that moment, but they may result from the coalescence of crystals.
Crystal growth peculiarities in AI films deposited at high value of oxygen-species-to-almninium-atom flux ratio on amorphous substrates The phenomenon of coalescence plays an important role in the development of the texture in films condensed on amorphous substrates 3,1~ Increasing the level of contamination (Koxyge,),i.e. the contamination content of films, the degree of texture decreases while the growth peculiarities start to develop 3'~. At a value of Ko,yg,, ~ 10- 2, single crystals are protruding over the film surface termed 'hillocks type II'. These configurations exhibit points of 28
300°C and
Ko,~,go,, ~ 1. Film thickness is 1 /zm.
SEM image (Figure 7) ~s. By increasing the thickness or the contamination level, the points of the whiskers become contaminated and the impinging Ai vapour flux initiates the formation of new nuclei on the tip surfaces 3 (Figure 8). These growing nuclei can develop a polycrystalline head the size of which increases during deposition. Very characteristic crystal configurations can develop in this way, the so-called 'mushroom whiskers' (Figures 9 and 10). These have conically shaped dents around their 'stem' as a result of the shadowing effect of the growing head. These configurations appear also in the cross section of films. The conical dents will be closed when the surface of surrounding areas gets to the periphery of.the heads during film growth. The heads of these 'mushroom whiskers' will be detected as cup-shaped hills beginning with that stage of growth.
Conclusions The studies of growth surface structures of (111), (100) and (110) faces of epitaxial Al films indicate a correlation between the crystal face anisotropy in the surface micro-chemistry of oxygen
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal faces
Figure 7. The longest whisker in the film shown in Figure 6.
Figure 9(a).
Figure 8. Whisker with contaminated tip, developed in a 1.5 ttm thick film, deposited under the same conditions as the film in Figure 7.
Figure 9. Mushroom whiskers in A1filmsdeposited onto glass substrate at 300°C and Ko,ygcn~ 1. Film thickness is 1,5/am.
species 9-~1 and their participation in the building processes on various crystal faces. The presence of oxygen develops bunches of growth steps decorated by pinning sites, dents, macro steps and hillocks on (111) faces. Under the same conditions only dents develop on (100) and (110) faces. Contamination layers partially or completely covering the surface of crystals can develop mainly by the processes accumulating the oxygen species on (111) faces 1s,16. This phenomenon
leads to the separation of crystals during their growth, according to their orientation. Whiskers and 'mushroom whiskers' develop at a high level of oxygen contamination on amorphous substrates. Their formation can be ascribed to the orientation and coalescence selection of crystals controlled by the anisotropic incorporation of oxygen species and to the development of contamination layers on specific surface areas of growing crystals. 29
P B Barna, F M Reicha, G Barcza, L Gosztola and F Koltai: Growth morphology of (111 ) and (100) AI single crystal faces
References
Figure 10. Cross section of film shown in Figure 9.
Acknowledgements A u t h o r s ' t h a n k s are due to D r G Gergely, D r ,/~ Barna, D r M Malicsk6 for their valuable discussions, to D r I Pozsgai for s u b m i t t i n g S E M m i c r o g r a p h s (Figures 6 a n d 7), M r s 1~ H a j m ~ y , M r s G y Ghizer a n d M r P Lhnyi for their help in p r e p a r i n g samples.
30
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