- Email: [email protected]
The columnar microstructure and nodular growth of a-As2Se3 films
Thin Solid Films, 78 (1981) 15-23 METALLURGICAL AND PROTECTIVE COATINGS
15
THE COLUMNAR MICROSTRUCTURE AND NODULAR GROWTH OF a-As2Se a FILMS FRANK JANSEN Xerox Corporation, Webster Research Center, Rochester, N Y 14580 (U.S.A.) (Received July 2, 1980; accepted September 26, 1980)
The occurrence and direction of a columnar microstructure in a-As2Se a films were investigated. As2Se3 films deposited at room temperature show a microscopically observable columnar structure only when the angle of incidence of the vapor, measured from the film normal, exceeds 40-50 °. Such a structure affects the surface morphology of these films and decreases the density. At smaller angles of incidence a relatively dense and smooth film is formed. The direction of the columnar microstructure which forms at larger angles of incidence is considerably closer to the direction of the incident vapor than is predicted by the tangent rule. The applicability of this rule to materials with a non-unity sticking coefficient and the effect of the columnar microstructure on macroscopic growth defects, such as the observed nodular structures, are discussed. 1. INTRODUCTION It is well known 1-3 that vapor-deposited amorphous films commonly exhibit a structural anisotropy in the sense that the material condenses in a columnar fashion. The needle-like or rod-like structure is reminiscent of the grain structure in some polycrystalline metal films since in both types of films the columns originate at the substrate and extend throughout the film. Both crystalline and amorphous columnar formations often grow more or less in the direction of the vapor source. The study of columnar structures is of importance as these structures can lead to remarkable anisotropies in many of the physical properties of the film. Furthermore, the relatively porous columnar structures can be easily densified under the influence of light or heat (or both) so that the columnar structure is of potential technological importance for purposes of permanent optical image recording4. We observed the occurrence of columnar growth in As2Se3 films deposited under certain conditions of vapor incidence and at substrate temperatures well below the glass transition temperature of about 180°C. The columnar growth structure is readily evident from visual observation of the surface of the films as this structure affects the surface topography. The blue-green hue which shows at certain angles of incidence of white light indicates that the surface roughness has a dominant correlation length related to the diameter of the columns and their inclination towards the substrate. Elsevier Sequoia/Printed in The Netherlands
16
r. JANSEN
As2Se3 is an important technological material and is commonly used as a base material for the production of Semiconducting films Of large area to be used, for example, as photoreceptors. The ideal surface structure of such a film is One with a minimum number of surface undulations due to columnar growth or nodular structures. The latter type of defect is quite common s, 6 in single-layer and multilayer films and is often nucleated by substrate defects and irregularities. The conical shape with which these nodules grow has not been fully explained and it gave these defects the name "cone defects". The cone defect often does not form an integral part of the rest of the film. It can easily be removed from the film by external forces; upon removal it leaves a cavity behind. The purpose of this paper is to discuss the conditions under which columnar growth takes place in vapor,deposited As2Se s filmS and to consider what some of the characteristics and effects of this observed growth pattern are. We will subsequently discuss the occurrence of the cone defect and its relation to columnar growth. 2. EXPERIMENTAL DETAILS
S e A s films were prepared by the thermal evaporation of amorphous As2Sea onto glass substrates, The amorphous starting material was in bead form. Itwas found by X-ray fluorescence spectroscopy to contain 38.7 -+0.1 wt,~ As and its nonvolatile residue was less than 15 wt. ppm. The evaporation source is a closed stainless steel container with a hole 12 mm in diameter. This source is resistively heated and the radiative coupling between it and the substrates is minimized by the use of a water-cooled heat shield. The temperature of the evaporation source is kept constant during the deposition process at 460-+3°C, unless otherwise stated. Operation at this temperature yields effusion rates of about 0.02 g c m - 2 s- 1. The crucible is loaded with 100 g of As2Sea per run and this amount is evaporated to completion, The glass substrates (Coming 7084) are situated with respect to the evaporation source as indicated in Fig. 1. The temperature increase of the substrates during the course of the evaporation at 460 °C is less than 25 °C; it is primarily due to i . 50"
~
"roe
80e
90e
' RE%'1
i
i
I
i
I
t
4 t0 1 % I 30
t, ~mm
20°
4G 30 --~ 20
ioo
10 t
0 Fig. 1. Schematic diagram of the experimental configuration. Fig. 2. The specular reflectance of a-As2Sez films as a function of the angle of incidence of the vapor.
MICROSTRUCTURE AND G R O W T H OF
a-As2Se 3 FILMS
17
heat of condensation effects and some thermal radiation from the evaporation source assembly. The experimental assembly is contained in a conventional oilpumped vacuum system capable of background pressures better than 10-s Pa. Samples for microscopic surface and cross-sectional analyses are easily obtained when the film thickness is of the order of tens of microns. The densification of the film which takes place after the material has condensed on the substrate induces tensile film stresses which arc relieved by adhesion failure when the film is thick enough. Film fragments of which at least one edge coincides with the plane of incidence of the vapor were selected. The plane of the cross section is thus the same as the plane of incidence of the vapor. Samples were always taken from the same place on the substrates (the top) so that the angles of incidence were accurately known. 3. RESULTS
3.1. The surface topography and the columnar structure as functions of the angle of incidence of the vapor The specular sample reflectivity for incandescent light is shown in Fig. 2 as a function of the angle of incidence of the vapor. The quarter-plane of incidence of the vapor is the same as the quarter-plane of incidence of the light. The scanning electron micrograph in Fig. 3 shows the surface morphology of a sample for which 0 = 60 °, where 0 is the angle of incidence of the vapor measured from the surface normal. Surfaces of samples with angles of incidence smaller than 50 ° are featureless or nearly featureless. The hardness of the films, as measured by the size of the impression of a diamond stylus under a 25 g load, also decreases when 0 > 50° (Fig. 4). Optical and scanning electron micrographs of cross sections of films at positions corresponding to 0 > 50 ° are shown in Fig. 5. Cross sections of films at angles less than 50 ° are not shown since these do not show a columnar structure observable either by optical microscopy or by scanning electron microscopy. Measurements of the angle 0 between the substrate normal and the columnar microstructure are given in Table I as a function of the angle of incidence of the vapor. In Fig. 6 the film cross i
,
I
,
~
i
i
I
"
\ P
Fig. 3. The surface morphology of an a-AszS% film deposited at a vapor angle of incidence of 60 °. Fig. 4. The microhardness ofa-AszSe 3 films a s a function of the angle of incidence of the vapor.
i
18
F. JANSEN
I
I
2.s,uM go °
'
~'
I
I
I
I
"
;
2s uM
2s uM
25)uM
2s)uM
eo °
70 °
80 °
5 o°
Fig. 5. Micrographs of cross sections of a-As2Sea deposited at oblique angles of incidence. TABLE I 0 (deg) a
~o(deg) b
~0tan (deg) ¢
0-40 50 60 70 80 90
No columnar structure 45-t-4 56 ---2 66 + 2 75+2 < 80
0-23 31 41 54 71 90
a 0 is the angle of incidence of the vapor measured from the surface normal. b ~ois the angle between the direction of the columnar structure and the surface normal. ° ~ot~n is the angle of columnar structure calculated from the tangent rule. section from a s a m p l e t a k e n from the b o t t o m of o n e o f the s u b s t r a t e s (70 °) is shown, illustrating the difference in the m i c r o s t r u c t u r e of films d e p o s i t e d at different angles of incidence. F i n a l l y the sticking coefficient was m e a s u r e d as a function of the rate o f a r r i v a l of the v a p o r species b y v a r y i n g the distance (20, 40 a n d 60 cm) of p r e n u c l e a t e d s u b s t r a t e s f r o m the e v a p o r a t i o n source. T h e m e a s u r e m e n t s were r e p e a t e d at different t e m p e r a t u r e s of the e v a p o r a t i o n source (440, 460 a n d 480°C). T h e m a x i m u m s u b s t r a t e t e m p e r a t u r e r e a c h e d d u r i n g the c o u r s e o f these d e p o s i t i o n s v a r i e d between 40 a n d 60 °C. T h e d a t a are s h o w n in Fig. 7. T h e highest sticking coefficient of 0.90 was e s t i m a t e d b y e x t r a p o l a t i n g the d a t a to zero e v a p o r a t i o n times (infinite e v a p o r a t i o n rates), where the r e - e v a p o r a t i o n rate was a s s u m e d to be constant.
3.2. The formation of nodular defects (cone defects) in As2Se3films T h e t e r m " n o d u l a r defect" is used for a variety of g r o w t h p a t t e r n s which m a y
MICROSTRUCTURE AND GROWTH OF
P
a-As2Se 3 FILMS
19
STICKINGCOEFFICIENT L¢
.8
.6
'1 .2
RATEOFINCIDENCE [lO'Sgcml:s-I] 4..
Fig. 6. Scanning electron micrograph of an angled a-As2Se 3 film structure illustrating a difference in the microstructure at the different angles of incidence of the vapor. Fig. 7. The sticking coefficient of a-As2Se 3 as a function of the mass density rate: I-q, 480 °C; O, 460 °C; x , 440 °C.
(a)
10~uM
(b)
1o,uM
Fig. 8. Two different types of nodular defects in a-As2Se3 films: (a) the pointed type show an angle of incidence of the vapor at which columnar growth is favored; (b) the rounded type show observable columnar growth only on their sides where the angle of incidence is large. morphologically be very different, as shown in Fig. 8. What the cone defects of different shapes have in c o m m o n is that they protrude from the surface and that their origin can often be traced to macroscopic substrate undulations such as those caused by dust, scratches etc. Cone defects also form under normal incidence although they are more easily observable when the vapor is incident at oblique angles. This is illustrated in Fig. 9 for cone defects grown on rough aluminum substrates suspended at different angles. The size of the cone defects dearly depends
20
F. JANSEN
on the angle of incidence and their areal density is determined by the density of macroscopic irregularities. Optical microscopy of cross sections of films and deposits on coater walls and fixtures showed that the defects grow in the direction of the vapor source (within -+ 1~).
(a)
15"~M
(b)
I~M
Fig. 9. The morphology of the cone defects as a function of the angle of incidence: (a) 80°; (b) 50 °.
4. DISCUSSION The abrupt decrease in the specular reflectance of a film when the angle of incidence of the vapor exceeds 50 ° (Fig. 2) is caused by a change in the surface morphology of the sample. The surfaces of films deposited at oblique angles of incidence have a rough texture (Fig. 3) which does not persist at smaller angles of incidence. The "scales" which can be observed on the surface are the exposed ends of columns whose areal density is approximately l0 s cm-2. That the change in the surface texture of the film is indeed correlated with a change in the bulk structure of the film can be seenfrom both the film hardness data in Fig. 4 and more d i r e ~ ' f r 0 m the micrographs in Fig. 5. The film hardness starts to decrease at about the same angle of incidence at which the film reflectance decreases. Computer simulations of the deposition process, performed by Henderson et al. 7 and by Kim et al. a, predict an increase in the film density and a Columnar structure that is increasingly hard to observe when the angle of incidence of the vapor decreases. This is in agreement with our observations. These computer simulations have also confirmed the validity of the following relationship between the angle of incidence 0 of the vapor and the direction goof the columnar structure: go = arctan(½ tan 0)
(1)
This empirical relationship is known as the tangent rule 1'2 and it predicts the direction of columnar growth rather well for many materials. Our observations do not confirm the validity of such a relationship for As2Sea (Table I). We refer to Fig. 10 for a discussion of the tangent rule in an attempt to understand why such large deviations from the behavior predicted by eqn. (1) are
MICROSTRUCTURE AND GROWTH OF a - A s 2 Se 3 FILMS
21
found. Figure 10 shows a two-dimensional rectangular obstacle on an otherwise flat surface which is placed in a unidirectional vapor stream. The condensation rate per unit area on an imaginary substrate placed perpendicular to the direction of the vapor is taken to be equal to unity. The condensation rate on the leading planes of the obstacle is then equal to the cosine of the angle of incidence. The edges L 1 and L 2 will therefore propagate into the direction of the vapor stream, thus replicating the shape of the obstacle. The situation at the trailing edge T is more interesting. In addition to a net displacement of the edge into the direction TT' of the vapor stream, the mechanism underlying the tangent rule 1 causes the edge to have an additional component of growth parallel to the surface in the direction T'T".
%
Fig. 10. Deposition on and shadowing by a two-dimensional rectangular obstacle placed in a unidirectional vapor stream.
A non-zero growth component of the edge T in the direction T'T" will give rise to shadowing effects and a porous structure with the orientation of the open space along TT". The tangent rule (eqn. (1)), based on shadowing of the vapor beam by atoms within the growing film 1, predicts a component of ½sin 0 for T'T" which is just half the film thickness on the vertical wall L 1L2. Film samples from the bottom of the suspended substrates (Fig. 1) were microscopically investigated but the results fail to indicate any appreciable growth component in the direction T'T". The edges TT" are always within 2 ° from the direction of the vapor stream. The displacement component T'T" is therefore much smaller than ½sin 0 in the case of As2Se3. A possible explanation lies in the fact that the sticking coefficient of As2Se3 drops very steeply when the rate of incidence of the vapor species is small (Fig. 7). Condensation of vapor species in low flux ("shadowed") regions then becomes rather unlikely and this condition may prevent an appreciable build-up of material in the region TT'T". This causes the directions of both the trailing edges and the microstructure to be relatively close to the direction of the incident vapor. The closer the direction of the mierostructure becomes to that of the incident vapor, the smaller
22
F. JANSEN
the open regions between the columns and the higher the film density will be. A decrease in the angle of incidence also leads to a denser film, since the width of the open areas decreases 1. The combination of these two effects is thought to lead to the disappearance of an observable microstructure in As2Sea at angles of incidence of less than 50 °. The abrupt disappearance of the columnar microstructure at smaller angles of incidence has an effect on the cone morphology. The cone shapes which were observed are the pointed structure shown in Fig. 8(a) and the more rounded version as in Fig. 8(b). The pointed structure presents surfaces to the vapor at angles of incidence which exceed the critical angle above which columnar growth was observed to occur. When columns at the tip conglomerate a rounded surface is formed and the shape which develops will be more like the one shown in Fig. 8(b). The tops of these cones are relatively smooth owing to the small angle of incidence of the vapor at which the formation of a columnar structure is not favored. When the angle of incidence becomes larger at the side of the cone, the columnar structure develops again. The size and the type of cone which develops will depend on the size of the defect which gave rise to the cone growth. Very small defects, say of the order of a few column diameters, are expected to favor the initial development of the pointed structure. This type is likely to develop, in a later stage of growth, the rounded tip. Larger substrate imperfections will not give rise to the growth of pointed cones but will favor the development of rounded types. The conical shape of the nodular defect can be understood since the exposure of a surface to the vapor stream at an angle favors the development of a relatively low density film. Defects with surfaces on which vapor is obliquely incident will therefore show a disproportionate growth perpendicular to these sides. If the top of the starting defect is close enough to a hemisphere, as is the case with a cylindrical piece of dust, then the developing cone will partially shadow the substrate on which it grows. The cone will then not be an integral part of the film and may be easy to remove, leaving a cavity behind. An oblique angle of incidence of the vapor onto a rough substrate favors the development of these conical growth structures as the probability is then large that the angle of vapor incidence on defects is greater than the critical angle. At smaller angles of incidence onto a rough substrate, the cone structures are surrounded by film that appears smooth; this indicates the absence of a columnar microstructure (Fig. 9). 5. CONCLUSIONS
The direction of both the columnar microstructure in As2Se 3 as well as the orientation of trailing film edges was found to be much closer tO the direction of the incident vapor than might be expected from the tangent rule. This is thought to be because of re-evaporation of material preferentially from the shadowed regions of the film. The columnar microstructure cannot be observed in films for which the angle of incidence is smaller than 50°. Both the film density and the reflectivity are increased for these films. The shape and the size of conical growth structures depend on the size of the substrate irregularities from which they originate. Two types of nodular defects were
MICROSTRUCTUR.E AND GROWTH OF a - A s 2 S e 3 FILMS
23
observed and are distinguished by the shape of the tip, which can be pointed or rounded. The pointed type presents surfaces to the vapor at angles of incidence at which columnar growth is favored. The rounded type develops the low density columnar structure only at its sides, giving rise to its observed conical shape. REFERENCES
1 2 3 4 5 6 7 8
A . G . DirksandH. J. Leamy, ThinSolidFilms, 47(1977)219. J.M. Nieuwenhuizen and H. B. Haanstra, Philips Tech. Rev., 27 (1966) 87. N.G. Nakhodkin and A. I. Shaldervan, Thin Solid Films, 10 (1972) 109. B. Singh, S. Rajagopalan, P. K. Bhat, D. K. Pandya and K. L. Chopra, Solid State Commun., 29 (1979) 167. D.H. Boone, T. E. Strangman and L. W. Wilson, J. Vac. Sci. Technol., 11 (1974) 641. T. Spalvins, Thin Solid Films, 64 (1979) 143. D. Henderson, M. H. Brodsky and P. Chaudari, Appl. Phys. Lett., 25 (1974) 641, S. Kim, D.J. HendersonandP. Chaudari, ThinSolidFilms, 47(1977) 155.
15
THE COLUMNAR MICROSTRUCTURE AND NODULAR GROWTH OF a-As2Se a FILMS FRANK JANSEN Xerox Corporation, Webster Research Center, Rochester, N Y 14580 (U.S.A.) (Received July 2, 1980; accepted September 26, 1980)
The occurrence and direction of a columnar microstructure in a-As2Se a films were investigated. As2Se3 films deposited at room temperature show a microscopically observable columnar structure only when the angle of incidence of the vapor, measured from the film normal, exceeds 40-50 °. Such a structure affects the surface morphology of these films and decreases the density. At smaller angles of incidence a relatively dense and smooth film is formed. The direction of the columnar microstructure which forms at larger angles of incidence is considerably closer to the direction of the incident vapor than is predicted by the tangent rule. The applicability of this rule to materials with a non-unity sticking coefficient and the effect of the columnar microstructure on macroscopic growth defects, such as the observed nodular structures, are discussed. 1. INTRODUCTION It is well known 1-3 that vapor-deposited amorphous films commonly exhibit a structural anisotropy in the sense that the material condenses in a columnar fashion. The needle-like or rod-like structure is reminiscent of the grain structure in some polycrystalline metal films since in both types of films the columns originate at the substrate and extend throughout the film. Both crystalline and amorphous columnar formations often grow more or less in the direction of the vapor source. The study of columnar structures is of importance as these structures can lead to remarkable anisotropies in many of the physical properties of the film. Furthermore, the relatively porous columnar structures can be easily densified under the influence of light or heat (or both) so that the columnar structure is of potential technological importance for purposes of permanent optical image recording4. We observed the occurrence of columnar growth in As2Se3 films deposited under certain conditions of vapor incidence and at substrate temperatures well below the glass transition temperature of about 180°C. The columnar growth structure is readily evident from visual observation of the surface of the films as this structure affects the surface topography. The blue-green hue which shows at certain angles of incidence of white light indicates that the surface roughness has a dominant correlation length related to the diameter of the columns and their inclination towards the substrate. Elsevier Sequoia/Printed in The Netherlands
16
r. JANSEN
As2Se3 is an important technological material and is commonly used as a base material for the production of Semiconducting films Of large area to be used, for example, as photoreceptors. The ideal surface structure of such a film is One with a minimum number of surface undulations due to columnar growth or nodular structures. The latter type of defect is quite common s, 6 in single-layer and multilayer films and is often nucleated by substrate defects and irregularities. The conical shape with which these nodules grow has not been fully explained and it gave these defects the name "cone defects". The cone defect often does not form an integral part of the rest of the film. It can easily be removed from the film by external forces; upon removal it leaves a cavity behind. The purpose of this paper is to discuss the conditions under which columnar growth takes place in vapor,deposited As2Se s filmS and to consider what some of the characteristics and effects of this observed growth pattern are. We will subsequently discuss the occurrence of the cone defect and its relation to columnar growth. 2. EXPERIMENTAL DETAILS
S e A s films were prepared by the thermal evaporation of amorphous As2Sea onto glass substrates, The amorphous starting material was in bead form. Itwas found by X-ray fluorescence spectroscopy to contain 38.7 -+0.1 wt,~ As and its nonvolatile residue was less than 15 wt. ppm. The evaporation source is a closed stainless steel container with a hole 12 mm in diameter. This source is resistively heated and the radiative coupling between it and the substrates is minimized by the use of a water-cooled heat shield. The temperature of the evaporation source is kept constant during the deposition process at 460-+3°C, unless otherwise stated. Operation at this temperature yields effusion rates of about 0.02 g c m - 2 s- 1. The crucible is loaded with 100 g of As2Sea per run and this amount is evaporated to completion, The glass substrates (Coming 7084) are situated with respect to the evaporation source as indicated in Fig. 1. The temperature increase of the substrates during the course of the evaporation at 460 °C is less than 25 °C; it is primarily due to i . 50"
~
"roe
80e
90e
' RE%'1
i
i
I
i
I
t
4 t0 1 % I 30
t, ~mm
20°
4G 30 --~ 20
ioo
10 t
0 Fig. 1. Schematic diagram of the experimental configuration. Fig. 2. The specular reflectance of a-As2Sez films as a function of the angle of incidence of the vapor.
MICROSTRUCTURE AND G R O W T H OF
a-As2Se 3 FILMS
17
heat of condensation effects and some thermal radiation from the evaporation source assembly. The experimental assembly is contained in a conventional oilpumped vacuum system capable of background pressures better than 10-s Pa. Samples for microscopic surface and cross-sectional analyses are easily obtained when the film thickness is of the order of tens of microns. The densification of the film which takes place after the material has condensed on the substrate induces tensile film stresses which arc relieved by adhesion failure when the film is thick enough. Film fragments of which at least one edge coincides with the plane of incidence of the vapor were selected. The plane of the cross section is thus the same as the plane of incidence of the vapor. Samples were always taken from the same place on the substrates (the top) so that the angles of incidence were accurately known. 3. RESULTS
3.1. The surface topography and the columnar structure as functions of the angle of incidence of the vapor The specular sample reflectivity for incandescent light is shown in Fig. 2 as a function of the angle of incidence of the vapor. The quarter-plane of incidence of the vapor is the same as the quarter-plane of incidence of the light. The scanning electron micrograph in Fig. 3 shows the surface morphology of a sample for which 0 = 60 °, where 0 is the angle of incidence of the vapor measured from the surface normal. Surfaces of samples with angles of incidence smaller than 50 ° are featureless or nearly featureless. The hardness of the films, as measured by the size of the impression of a diamond stylus under a 25 g load, also decreases when 0 > 50° (Fig. 4). Optical and scanning electron micrographs of cross sections of films at positions corresponding to 0 > 50 ° are shown in Fig. 5. Cross sections of films at angles less than 50 ° are not shown since these do not show a columnar structure observable either by optical microscopy or by scanning electron microscopy. Measurements of the angle 0 between the substrate normal and the columnar microstructure are given in Table I as a function of the angle of incidence of the vapor. In Fig. 6 the film cross i
,
I
,
~
i
i
I
"
\ P
Fig. 3. The surface morphology of an a-AszS% film deposited at a vapor angle of incidence of 60 °. Fig. 4. The microhardness ofa-AszSe 3 films a s a function of the angle of incidence of the vapor.
i
18
F. JANSEN
I
I
2.s,uM go °
'
~'
I
I
I
I
"
;
2s uM
2s uM
25)uM
2s)uM
eo °
70 °
80 °
5 o°
Fig. 5. Micrographs of cross sections of a-As2Sea deposited at oblique angles of incidence. TABLE I 0 (deg) a
~o(deg) b
~0tan (deg) ¢
0-40 50 60 70 80 90
No columnar structure 45-t-4 56 ---2 66 + 2 75+2 < 80
0-23 31 41 54 71 90
a 0 is the angle of incidence of the vapor measured from the surface normal. b ~ois the angle between the direction of the columnar structure and the surface normal. ° ~ot~n is the angle of columnar structure calculated from the tangent rule. section from a s a m p l e t a k e n from the b o t t o m of o n e o f the s u b s t r a t e s (70 °) is shown, illustrating the difference in the m i c r o s t r u c t u r e of films d e p o s i t e d at different angles of incidence. F i n a l l y the sticking coefficient was m e a s u r e d as a function of the rate o f a r r i v a l of the v a p o r species b y v a r y i n g the distance (20, 40 a n d 60 cm) of p r e n u c l e a t e d s u b s t r a t e s f r o m the e v a p o r a t i o n source. T h e m e a s u r e m e n t s were r e p e a t e d at different t e m p e r a t u r e s of the e v a p o r a t i o n source (440, 460 a n d 480°C). T h e m a x i m u m s u b s t r a t e t e m p e r a t u r e r e a c h e d d u r i n g the c o u r s e o f these d e p o s i t i o n s v a r i e d between 40 a n d 60 °C. T h e d a t a are s h o w n in Fig. 7. T h e highest sticking coefficient of 0.90 was e s t i m a t e d b y e x t r a p o l a t i n g the d a t a to zero e v a p o r a t i o n times (infinite e v a p o r a t i o n rates), where the r e - e v a p o r a t i o n rate was a s s u m e d to be constant.
3.2. The formation of nodular defects (cone defects) in As2Se3films T h e t e r m " n o d u l a r defect" is used for a variety of g r o w t h p a t t e r n s which m a y
MICROSTRUCTURE AND GROWTH OF
P
a-As2Se 3 FILMS
19
STICKINGCOEFFICIENT L¢
.8
.6
'1 .2
RATEOFINCIDENCE [lO'Sgcml:s-I] 4..
Fig. 6. Scanning electron micrograph of an angled a-As2Se 3 film structure illustrating a difference in the microstructure at the different angles of incidence of the vapor. Fig. 7. The sticking coefficient of a-As2Se 3 as a function of the mass density rate: I-q, 480 °C; O, 460 °C; x , 440 °C.
(a)
10~uM
(b)
1o,uM
Fig. 8. Two different types of nodular defects in a-As2Se3 films: (a) the pointed type show an angle of incidence of the vapor at which columnar growth is favored; (b) the rounded type show observable columnar growth only on their sides where the angle of incidence is large. morphologically be very different, as shown in Fig. 8. What the cone defects of different shapes have in c o m m o n is that they protrude from the surface and that their origin can often be traced to macroscopic substrate undulations such as those caused by dust, scratches etc. Cone defects also form under normal incidence although they are more easily observable when the vapor is incident at oblique angles. This is illustrated in Fig. 9 for cone defects grown on rough aluminum substrates suspended at different angles. The size of the cone defects dearly depends
20
F. JANSEN
on the angle of incidence and their areal density is determined by the density of macroscopic irregularities. Optical microscopy of cross sections of films and deposits on coater walls and fixtures showed that the defects grow in the direction of the vapor source (within -+ 1~).
(a)
15"~M
(b)
I~M
Fig. 9. The morphology of the cone defects as a function of the angle of incidence: (a) 80°; (b) 50 °.
4. DISCUSSION The abrupt decrease in the specular reflectance of a film when the angle of incidence of the vapor exceeds 50 ° (Fig. 2) is caused by a change in the surface morphology of the sample. The surfaces of films deposited at oblique angles of incidence have a rough texture (Fig. 3) which does not persist at smaller angles of incidence. The "scales" which can be observed on the surface are the exposed ends of columns whose areal density is approximately l0 s cm-2. That the change in the surface texture of the film is indeed correlated with a change in the bulk structure of the film can be seenfrom both the film hardness data in Fig. 4 and more d i r e ~ ' f r 0 m the micrographs in Fig. 5. The film hardness starts to decrease at about the same angle of incidence at which the film reflectance decreases. Computer simulations of the deposition process, performed by Henderson et al. 7 and by Kim et al. a, predict an increase in the film density and a Columnar structure that is increasingly hard to observe when the angle of incidence of the vapor decreases. This is in agreement with our observations. These computer simulations have also confirmed the validity of the following relationship between the angle of incidence 0 of the vapor and the direction goof the columnar structure: go = arctan(½ tan 0)
(1)
This empirical relationship is known as the tangent rule 1'2 and it predicts the direction of columnar growth rather well for many materials. Our observations do not confirm the validity of such a relationship for As2Sea (Table I). We refer to Fig. 10 for a discussion of the tangent rule in an attempt to understand why such large deviations from the behavior predicted by eqn. (1) are
MICROSTRUCTURE AND GROWTH OF a - A s 2 Se 3 FILMS
21
found. Figure 10 shows a two-dimensional rectangular obstacle on an otherwise flat surface which is placed in a unidirectional vapor stream. The condensation rate per unit area on an imaginary substrate placed perpendicular to the direction of the vapor is taken to be equal to unity. The condensation rate on the leading planes of the obstacle is then equal to the cosine of the angle of incidence. The edges L 1 and L 2 will therefore propagate into the direction of the vapor stream, thus replicating the shape of the obstacle. The situation at the trailing edge T is more interesting. In addition to a net displacement of the edge into the direction TT' of the vapor stream, the mechanism underlying the tangent rule 1 causes the edge to have an additional component of growth parallel to the surface in the direction T'T".
%
Fig. 10. Deposition on and shadowing by a two-dimensional rectangular obstacle placed in a unidirectional vapor stream.
A non-zero growth component of the edge T in the direction T'T" will give rise to shadowing effects and a porous structure with the orientation of the open space along TT". The tangent rule (eqn. (1)), based on shadowing of the vapor beam by atoms within the growing film 1, predicts a component of ½sin 0 for T'T" which is just half the film thickness on the vertical wall L 1L2. Film samples from the bottom of the suspended substrates (Fig. 1) were microscopically investigated but the results fail to indicate any appreciable growth component in the direction T'T". The edges TT" are always within 2 ° from the direction of the vapor stream. The displacement component T'T" is therefore much smaller than ½sin 0 in the case of As2Se3. A possible explanation lies in the fact that the sticking coefficient of As2Se3 drops very steeply when the rate of incidence of the vapor species is small (Fig. 7). Condensation of vapor species in low flux ("shadowed") regions then becomes rather unlikely and this condition may prevent an appreciable build-up of material in the region TT'T". This causes the directions of both the trailing edges and the microstructure to be relatively close to the direction of the incident vapor. The closer the direction of the mierostructure becomes to that of the incident vapor, the smaller
22
F. JANSEN
the open regions between the columns and the higher the film density will be. A decrease in the angle of incidence also leads to a denser film, since the width of the open areas decreases 1. The combination of these two effects is thought to lead to the disappearance of an observable microstructure in As2Sea at angles of incidence of less than 50 °. The abrupt disappearance of the columnar microstructure at smaller angles of incidence has an effect on the cone morphology. The cone shapes which were observed are the pointed structure shown in Fig. 8(a) and the more rounded version as in Fig. 8(b). The pointed structure presents surfaces to the vapor at angles of incidence which exceed the critical angle above which columnar growth was observed to occur. When columns at the tip conglomerate a rounded surface is formed and the shape which develops will be more like the one shown in Fig. 8(b). The tops of these cones are relatively smooth owing to the small angle of incidence of the vapor at which the formation of a columnar structure is not favored. When the angle of incidence becomes larger at the side of the cone, the columnar structure develops again. The size and the type of cone which develops will depend on the size of the defect which gave rise to the cone growth. Very small defects, say of the order of a few column diameters, are expected to favor the initial development of the pointed structure. This type is likely to develop, in a later stage of growth, the rounded tip. Larger substrate imperfections will not give rise to the growth of pointed cones but will favor the development of rounded types. The conical shape of the nodular defect can be understood since the exposure of a surface to the vapor stream at an angle favors the development of a relatively low density film. Defects with surfaces on which vapor is obliquely incident will therefore show a disproportionate growth perpendicular to these sides. If the top of the starting defect is close enough to a hemisphere, as is the case with a cylindrical piece of dust, then the developing cone will partially shadow the substrate on which it grows. The cone will then not be an integral part of the film and may be easy to remove, leaving a cavity behind. An oblique angle of incidence of the vapor onto a rough substrate favors the development of these conical growth structures as the probability is then large that the angle of vapor incidence on defects is greater than the critical angle. At smaller angles of incidence onto a rough substrate, the cone structures are surrounded by film that appears smooth; this indicates the absence of a columnar microstructure (Fig. 9). 5. CONCLUSIONS
The direction of both the columnar microstructure in As2Se 3 as well as the orientation of trailing film edges was found to be much closer tO the direction of the incident vapor than might be expected from the tangent rule. This is thought to be because of re-evaporation of material preferentially from the shadowed regions of the film. The columnar microstructure cannot be observed in films for which the angle of incidence is smaller than 50°. Both the film density and the reflectivity are increased for these films. The shape and the size of conical growth structures depend on the size of the substrate irregularities from which they originate. Two types of nodular defects were
MICROSTRUCTUR.E AND GROWTH OF a - A s 2 S e 3 FILMS
23
observed and are distinguished by the shape of the tip, which can be pointed or rounded. The pointed type presents surfaces to the vapor at angles of incidence at which columnar growth is favored. The rounded type develops the low density columnar structure only at its sides, giving rise to its observed conical shape. REFERENCES
1 2 3 4 5 6 7 8
A . G . DirksandH. J. Leamy, ThinSolidFilms, 47(1977)219. J.M. Nieuwenhuizen and H. B. Haanstra, Philips Tech. Rev., 27 (1966) 87. N.G. Nakhodkin and A. I. Shaldervan, Thin Solid Films, 10 (1972) 109. B. Singh, S. Rajagopalan, P. K. Bhat, D. K. Pandya and K. L. Chopra, Solid State Commun., 29 (1979) 167. D.H. Boone, T. E. Strangman and L. W. Wilson, J. Vac. Sci. Technol., 11 (1974) 641. T. Spalvins, Thin Solid Films, 64 (1979) 143. D. Henderson, M. H. Brodsky and P. Chaudari, Appl. Phys. Lett., 25 (1974) 641, S. Kim, D.J. HendersonandP. Chaudari, ThinSolidFilms, 47(1977) 155.