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Chemical etching of (100) and (110) faces of flux-grown LaBO3 crystals
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applied surface science ELSEVIER
Applied SurfaceScience 84 (1995) 65-73
Chemical etching of ( 100) and ( 110) faces of flux-grown LaBO, crystals Anima Jain a, Ashok K. Razdan a, P.N. Kotru aY*,B.M. Wanklyn b aDepartment of Physics, Universiv of Jammy Jammu 180001, India b Department of Physics, Clarendon Laboratory, Uniuersity of Oxford, Oxford, UK Received21 May 1992;acceptedfor publication 14 August 1994
Abstract Experiments on the etching of (100) and (110) faces of LaBO, crystal surfaces are offered. The results reveal HNO, to be a dislocation etchant for both LaBO, crystal faces. It is shown that the shape of the etch pits due to HNO, is different for different habit faces. The dependence of the etch rates for (100) and (110) faces (lateral as well as vertical) on the concentrations and temperature of the etchant, are described and discussed. It is shown that the faces resist attack of the etchant in the direction of the normal to the surface after 2 h of etching irrespective of the concentration of the etchant used at different temperatures. It is further shown that till the time the passivity sets in, the variation of depth with etching time is linear in all the cases.
1. Introduction
the point of view of studies on magnetic interactions
The rare-earth borates, RBO,, are in general of particular interest for the following reasons: (i> the concentration of rate-earth ions is sufficient for magnetic interactions at low temperatures to be expected; (ii) they undergo phase and structural transformations as for example LaBO, and NdBO, undergo a phase transformation at 1490 and 109O”C, respectively [l]. PrBO, is reported to undergo a crystallographic transition near 1220°C. If R = Sm to Yb, it appears that the high-temperature vaterite structure transforms to a room-temperature structure of lower symmetry. Thus RBO, crystals are of interest from
* Corresponding
author.
0169-4332/95/.$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00289-4
at low temperatures and phase/ structural transformation studies. Flux-grown experiments need to be designed so as to yield good-quality single crystals of LaBO, below their transition temperature. LaBO, crystals were thus grown by the flux technique using the system La,O,-PbO-B,O, as reported by Kotru and Wanklyn [2]. The starting composition consisting of La203, B,O, and PbO in stoichiometric proportions pressed into a Pt crucible was first soaked at the maximum temperature of 1250°C for 15 h and then cooled at a rate of 3°C h-’ up to 700°C. The crystals were separated from the flux by leaching them in 10% HNO, under an infrared lamp. The crystals were obtained as platelets, tabular and equidimensional types.
66
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
Lanthanum borate has the orthorhombic structure of aragonite (space group) Vi6 (Pbnm) and is pseudo-hexagonal with the pseudo-hexagonal c-axis parallel to the orthortombic b-axis;Othe cell parameters being a, = 8.25 A, b, = 5.872 A and ca = 5.104 A [3]. The flux technique as such is a multi-componentbased system where the possibility of obtaining defect-free crystals is to be carefully attended to. Firsthand information could be got from defect studies of flux-grown LaBO, crystals. Etching is a powerful, simple and economical tool which provides considerable information regarding defects in crystals. Search for a good dislocation etchant for LaBO, crystals is significant especially since no such attempt has been reported to date. The authors have performed experiments to establish a good defect etchant for LaBO, crystals. The results of etching kinetic studies are presented and discussed here.
Fig. 1. (a) Etch patterns on the (100) face due to 80% HNO, etching of 3.5 h ( X 400).
2. Results and discussion 2.1. Etching of (100) and (110) faces at 35°C Etching of (100) faces of LaBO, crystals in HNO, results in square or rectangular etch pits oriented with one of their sides parallel to the edges of intersection between (100) and (110) faces. Fig. la shows etch patterns on the (100) face due to 80% HNO, at 35°C for 1 h. Successive etching for longer duration indicates that the shape of the etch pits is only slightly altered. Fig. lb depicts the region of Fig. la after it had been etched for a total of 3.5 h. The following points emerge: (i) The point-bottomed pits of Fig. la persist at their own positions even after 3.5 h of etching. No new pit (point-bottomed) has developed in Fig. lb but for some flat-bottomed pits. (ii) The point-bottomed pits have increased in their
at 35°C for etching time of 1 h (X400).
(b) Region of (a) after prolonged
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
dimensions and the shape of the etch pits has changed. The two comers of the etch pits (of Fig. la) have been truncated giving rise to Dshaped pits (in Fig. lb). The same type of experiments when repeated at 100% concentration of HNO, gave similar results. HNO, (50%100% concentration) at its boiling point attacks the crystal vigorously, leading to uncontrolled dissolution and hence defacing of the surface. Fig. 2a represents an etch pattern on a (110) face after etching in 80% HNO, for 1 h. Prolonged etching of the surface of Fig. 2a resulted in an increase in dimensions of point-bottomed pits. Examination of the surface at different stages of etching (after every 0.5 h of etching) revealed that the pointbottomed pits were getting larger at every stage of etching, whereas flat-bottomed pits got washed off. Fig. 2b shows the etch pattern on the (110) face of Fig. 2a after the last stage of etching (after 3.5 h) in 80% HNO,. From the experiments performed on etching of (110) faces in different concentrations of the etchant
67
at 35°C as described above, the following major points come forth: ‘(3 Although the shape of the etch pits is not very much altered on changing the concentration of the etchant, their definition is very much affected. However, lower concentrations produce better defined pits as compared to the higher concentrations, i.e. 90% and 100%. (ii) Continuation of point-bottomed etch pits at successive stages of prolonged etching for all concentrations (70%, 80% and 100%) suggests that HNO, is a dislocation etchant for (110) planes of LaBO, crystals also. 33 Etching of (100) and (110) faces at different
I.“.
temperatures in 90% HNO,
Etching experiments on LaBO, crystals were performed using 90% HNO, at different room temperatures, viz. 20,27,35 and 44°C. The shape of the etch pits on (100) faces at these temperatures remains nearly the same (after a total of 3.5 h of etching) as that of the pits produced in 80% or 100% HNO, at
Fig. 2. (a) Etch patterns on the (110) face after etching in 80% HNO, at 35°C for 1 h (X 400). (b) Region of (a) after a total etching period of 3.5 h (x400).
68
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
35°C. Figs. 3a and 3b show etch patterns on (100) faces due to 90% HNO, at 20 and 44°C respectively, after an etching of 1 h. The deep etch pits persisted at their positions even on prolonged etching to a total period of 3.5 h. Figs. 4a and 4b are optical micrographs of the same region showing convex-type etch pits produced on a (110) face after 1 and 3.5 h of etching, respectively, in 90% HNO, at 20°C. The pits persist on successive etching with increased dimensions. For (110) faces also the shape of the etch pits remained almost the same for various temperatures like 27, 35 and 44°C when 90% HNO, is used. However, the size of the etch pits increases manifold for the same period of etching at temperatures above 35°C. In the temperature range 95-121°C the etching gets so vigorous that the faces get mutilated. 2.3. Etching kinetics of (100) and (110) faces In order to study the dependence of etch rates on the concentration of the etchant, measurements of
lateral dimensions (length and breadth) and depth of etch pits for different concentrations (80%-100%) of the etchant were recorded. For the (110) face, the longer diagonal of the etch pit was taken as its length. The results obtained are compiled in the form of graphs shown in Figs. 5 and 7. From Fig. 5, it is evident that the variation of length of etch pits on the (100) surface with time is non-linear for 80% HNO, (curve 1 of Fig. 5) and in the case of 100% HNO, (curve 21, the observed variation is linear but having two different slopes. Figs. 6a and 6b show the variation of length and depth, respectively, of etch pits with time at different temperature when 90% HNO, is used for (100) surfaces. The variation is linear at the temperatures of 27 and 35°C but not at 20 and 44°C. The rates of linear etching being independent of time in the former cases. Fig. 6b illustrates the variation of depth of etch pits with time at different temperatures of the etchant for (100) surfaces; the temperatures taken at 20, 27,
Fig. 3. (a) Etch patterns on the (100) face after 1 h of etching in 90% HNO, at 20°C (X 400). (b) Etch patterns on the (100) face formed due to etching in 90% HNO, at 44°C for 1 h ( X 400).
69
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
Fig. 4. (a) Etch patterns on the (110) face formed after 1 h of etching in 90% HNO, period of 3.5 h (x400).
35 and 44°C. From Fig. 6b, it is interesting to note the following: (i) The variation of depth with etching period is linear till a total etching period of 3.5 h at all the temperatures of the etchant considered here; the linear dependence ceases at 2 h of etching in all the four cases. After 2 h of etching, saturation sets in and the depth remains the same at all the temperatures on further etching. (ii) The vertical etch rates dD/dt (compiled in Table 1) are almost the same at different temperatures of the etchant. The dependence of length on the time of etching for (110) faces at each concentration of the etchant (Fig. 7a) is different when compared with that of the (100) face (Fig. 5). The variation for (110) faces is nearly linear for at least some part of the etching time in case of 100% and 70% concentrations of the etchants. Measurements of depth of etch pits at different intervals of etching time for (110) faces lead to interesting results. The results obtained are compiled
at 20°C (X400).
(b) Region of (a) after total etching
graphically as shown in Fig. 7b. Ignoring tion period of etching (shown by dotted curves obtained are linear till 7.2 X lo3 s at all the etchant concentrations (70%,
the induclines) the of etching 80% and
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TIMECTXlCkEC) Fig. 5. Graph showing the lateral extension and depth of etch pits against time for the (100) face. Curves (1) and (2) show the variation of lateral extension while curves (3) and (4) show the variation of depth for 80% and lOO%, respectively.
70
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
100%). After 7.2 X lo3 s of etching there is no further increase in depth with time of etching. The variation of depth of etch pits with time of etching on using 10% HNO, at 95°C is shown in Fig. 7c. The variation is similar to that of other concentrations of the etchant (70%, 80% and 100%) at 35°C except that the variation is linear till 1.5 X lo3 s and thereafter V,, = d D/dt suddenly drops down to zero. It may be noted here that the depth at which the saturation is reached is different in all the cases. In other words, the onset of passivity is in some way related to the time of etching and not the depth. Figs. 8a and 8b illustrate the curves recorded on the basis of measurements of length and depth, respectively, of etch pits on (110) surfaces due to etching by 90% HNO, at temperatures of 20, 27, 35 and 44°C. From Fig. 8a, it is clear that the lateral etch rates increase with increase in the temperature of the etchant; the etch rate being highest at 44°C in the temperature range of 20-44°C. The dependence of depth on time of etching of (110) faces by 90% HNO, at different temperature (20~44°C) as illustrated by Fig. 8b reveals a linear variation of depth with time till 7.2 X lo3 s of etching time. After 7.2 X lo3 s of etching, saturation sets in and the depth no longer increases with increase in the time of etching at all the temperatures considered here.
After 7.2 X lo3 s of etching of the (110) surfaces, the vertical etch rate drops down to zero for all the temperatures of the etchant. The saturation time (i.e., the time at which the pit stops deepening further) is the same irrespective of the temperature of the etchant. The data on lateral and vertical etch rates (V, = dL/dt and V,, = dD/dt) as estimated from the linear portions of the curves for (100) and (110) surfaces are compiled in Table 1. From experiments on etching kinetics of the HNO,-(110) surface system, the following points are noteworthy: (8 The variation of length of etch pits with time of etching is not linear for any of the concentrations (70%, 80% and 100%) of the etchant at 35°C. (ii) The variation of depth of an etch pit with time of etching is linear till saturation is reached after 120 min of etching in case of any of the concentrations (70%, 80% and 100%) of HNO,. The depth of an etch pit at which the saturation is reached is different for different concentrations of an etchant. After 120 min of etching in any of these three different concentrations of the etchant, the velocity of dissolution perpendicular to the surface suddenly drops down to zero.
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TIME(TXlOkC) Fig. 6. (a) Graph showing the variation of length with time of etching for 90% HNO, at 20, 27, 35 and 44°C for the (100) face. (b) Graph showing the variation of depth with time of etching for 90% HNO, at 20, 27, 35 and 44°C for the (100) face.
A. Jain et al./Applied
I
Surface Science 84 (1995) 65-73
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Fig. 7. (a) Variation of length with time of etching for different concentrations (70%, 80% and 100%) of etchant for the (110) face. (b) Variation of depth of etch pits with time of etching for different concentrations (70%, 80% and 100%) of etchant for the (110) face. (c) Variation of depth with time of etching in 10% HNO, at 95°C for the (110) face.
71
(iii) The variation of depth with time of etching in 10% HNO, at 95°C (see Fig. 7c) has the same form as that with different etchant concentrations (70%, 80%, 90% and 100%) at 35°C. The difference, however, being that in the former case, the saturation is reached after 25 min, whereas in the latter cases, it is reached after 120 min of etching. Here also the depth of etch pits at which the velocity of dissolution perpendicular to the surface drops down to zero is different for any of the etchant concentrations at 35°C. From what is described above, it is clear that prolonged etching leads to cessation of any further increase in depth at some stage, whereas it does not happen so in the case of lateral extension. This phenomenon is true not only at different concentrations but also at different temperatures of the etchant. The explanation for the occurrence of this type lies in the following: (9 The dislocations emerging on (100) and (110) planes do not go too far. They may bend and emerge on the other surface. If so, the dislocation etch pits may get laterally extended but not deepen after some stage of progressive etching. (ii) Undersaturation of the etchant decreases and so the in-depth etching stops abruptly, thus suggesting the occurrence of a critical Ap, as has been reported in the case of alum and Si [4] supporting the theory of Cabrera and Levine [5]. (iii) This could as well be attributed to the phenomenon of passivity as reported by Kotru et al. [6] for the KNiF,-HNO, system and by Kotru et al. [7] for the LaAlO,-HNO, system. The phenomenon of passivity of the surface is discussed by Milazzo [S] and Baker et al. [9]. The first explanation is ruled out because it is unlikely that all dislocations will bend suddenly on reaching the same depth from the exposed surface. The second explanation could have been applicable in this case too if the above given results obtained on using fresh HNO, at every successive stage were not in agreement with the results of experiments with the same HNO, at each successive stage of etching. The authors repeatedly performed experiments to verify and confirm if the etching kinetic results on using the same old HNO, and each
72
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
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Fig. 8. (a) Variation of length with time of etching for 90% HNO, at different temperatures (20, 27, 35 and 44°C) for the (110) face. (b) Variation of depth with time of etching for 90% HNO, at different temperatures (20, 27, 35 and 44°C) for the (110) face.
some initial etching. At this stage only the side faces of the etch pits move, flattening the etch pits more and more. It is reported in the literature that metals do become passive to chemical attack by strong acids such as HNO, [8,9]. LaBO, on reaction with HNO, may suddenly stop being attacked after some time on account of passivity which sets in after the local formation of an oxide film at the preferential
time fresh HNO, at every stage of etching were in any way different. It was found that the etching kinetic results in the two cases were in agreement within the acceptable limits, thus excluding the application of this explanation. It then leaves one to think that the passivity of the LaBO,-HNO, surface system may be responsible for the abrupt cessation of “in-depth” etching after
Table 1 Estimation
of etch rates on (100) and (110) faces of LaBOt
Face
Etchant
crystals Lateral etch rate
Vertical etch rate
Ratio of vertical and lateral etch rates
35 35 20 27 35 44
2.177 1.878 0.806 0.975 1.896 1.899
0.281 0.276 0.275 0.271 0.272 0.270
0.129 0.146 0.341 0.277 0.143 0.142
35 35 35 20 27 35 44 95
4.948 3.448 3.991 0.925 2.033 2.649 3.045
0.548 0.535 0.958 0.274 0.270 0.555 0.283 1.793
0.110 0.155 0.240 0.296 0.132 0.207 0.092
Chemical formula
Cont. (%)
Temp. PC)
(100)
HNO,
80 100 90 90 90 90
(110)
HNO,
70 80 100 90 90 90 90 10
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
sites of attack after initial reaction for some time, thus protecting the surface from further in-depth etching (i.e., rendering it passive to further in-depth chemical attack). Higher temperatures rather than higher concentrations of HNO, appear to be more significant in bringing the early on-set of passivity. It is clear from Fig. 7b that the etch rate (till the variation is linear and the so-called passivity is not called into play) is different for different concentrations of the etchant.
13
in, the variation of depth with etching time is linear in all the cases (70%, 80% and 100% HNO, at 35°C and 10% HNO, at 95°C).
Acknowledgement One of the authors (A.J.) wishes to thank the CSIR for the award of a Senior Research Fellowship.
References 3. Conclusions
(1) HNO, is a dislocation etchant for (100) and (110) surfaces of LaBO, crystals.
(2) The shape of each pits due to HNO, being different for different habit faces [(loo) and (llO)] of LaBO, crystals, the former can, therefore, be used for the identification of the latter. (3) Suitable conditions of etching for (100) and (110) faces are 80% HNO, at 35°C for an etching period of 1 h. (4) The data on etching kinetics indicate that both (100) and (110) faces resist attack of the etchant in the direction of the normal to the surface after some period of etching (120 min for different etchant concentrations at 35°C and 25 min for 10% HNO, at 95°C). Till the time passivity sets
[l] E.M. Levin, R.S. Roth and J.B. Martin, Am. Mineral. 46 (1961) 1030. [2] P.N. Kotru and B.M. Wanklyn, J. Mater. Sci. L&t. 14 (1979) 755. [3] R.W.G. Wyckoff, Crystal Structures, 2nd ed., Vol. 2 (Interscience, New York, 1964). [4] P. Bennema and W.J.P. van Enckevort, Ann. Chim. Fr. 4 (1979) 451. [5] N. Cabrera and M.M. Levine, Phil. Mag. 1 (1956) 450. [6] P.N. Kotru, S. Gupta, S.K. Kachroo and B.M. Wanklyn, J. Mater. Sci. 20 (1985) 3949. [7] P.N. Kotru, A.K. Razdan, K.K. Raina and B.M. Wanklyn, J. Mater. Sci. 20 (1985) 3365. [8] G. Milazo, Electra-Chemistry (Eisevier, New York, 1963) p. 496. [9] D. Baker, D.C. Koehler, Wofeckenstein, C.E. Roden and R. Sabia, Material Technology: Physical Design of Electronic Systems, Vol. II (Prentice-Hall, Englewood Cliffs, NJ, 1970) p. 217.
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applied surface science ELSEVIER
Applied SurfaceScience 84 (1995) 65-73
Chemical etching of ( 100) and ( 110) faces of flux-grown LaBO, crystals Anima Jain a, Ashok K. Razdan a, P.N. Kotru aY*,B.M. Wanklyn b aDepartment of Physics, Universiv of Jammy Jammu 180001, India b Department of Physics, Clarendon Laboratory, Uniuersity of Oxford, Oxford, UK Received21 May 1992;acceptedfor publication 14 August 1994
Abstract Experiments on the etching of (100) and (110) faces of LaBO, crystal surfaces are offered. The results reveal HNO, to be a dislocation etchant for both LaBO, crystal faces. It is shown that the shape of the etch pits due to HNO, is different for different habit faces. The dependence of the etch rates for (100) and (110) faces (lateral as well as vertical) on the concentrations and temperature of the etchant, are described and discussed. It is shown that the faces resist attack of the etchant in the direction of the normal to the surface after 2 h of etching irrespective of the concentration of the etchant used at different temperatures. It is further shown that till the time the passivity sets in, the variation of depth with etching time is linear in all the cases.
1. Introduction
the point of view of studies on magnetic interactions
The rare-earth borates, RBO,, are in general of particular interest for the following reasons: (i> the concentration of rate-earth ions is sufficient for magnetic interactions at low temperatures to be expected; (ii) they undergo phase and structural transformations as for example LaBO, and NdBO, undergo a phase transformation at 1490 and 109O”C, respectively [l]. PrBO, is reported to undergo a crystallographic transition near 1220°C. If R = Sm to Yb, it appears that the high-temperature vaterite structure transforms to a room-temperature structure of lower symmetry. Thus RBO, crystals are of interest from
* Corresponding
author.
0169-4332/95/.$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00289-4
at low temperatures and phase/ structural transformation studies. Flux-grown experiments need to be designed so as to yield good-quality single crystals of LaBO, below their transition temperature. LaBO, crystals were thus grown by the flux technique using the system La,O,-PbO-B,O, as reported by Kotru and Wanklyn [2]. The starting composition consisting of La203, B,O, and PbO in stoichiometric proportions pressed into a Pt crucible was first soaked at the maximum temperature of 1250°C for 15 h and then cooled at a rate of 3°C h-’ up to 700°C. The crystals were separated from the flux by leaching them in 10% HNO, under an infrared lamp. The crystals were obtained as platelets, tabular and equidimensional types.
66
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
Lanthanum borate has the orthorhombic structure of aragonite (space group) Vi6 (Pbnm) and is pseudo-hexagonal with the pseudo-hexagonal c-axis parallel to the orthortombic b-axis;Othe cell parameters being a, = 8.25 A, b, = 5.872 A and ca = 5.104 A [3]. The flux technique as such is a multi-componentbased system where the possibility of obtaining defect-free crystals is to be carefully attended to. Firsthand information could be got from defect studies of flux-grown LaBO, crystals. Etching is a powerful, simple and economical tool which provides considerable information regarding defects in crystals. Search for a good dislocation etchant for LaBO, crystals is significant especially since no such attempt has been reported to date. The authors have performed experiments to establish a good defect etchant for LaBO, crystals. The results of etching kinetic studies are presented and discussed here.
Fig. 1. (a) Etch patterns on the (100) face due to 80% HNO, etching of 3.5 h ( X 400).
2. Results and discussion 2.1. Etching of (100) and (110) faces at 35°C Etching of (100) faces of LaBO, crystals in HNO, results in square or rectangular etch pits oriented with one of their sides parallel to the edges of intersection between (100) and (110) faces. Fig. la shows etch patterns on the (100) face due to 80% HNO, at 35°C for 1 h. Successive etching for longer duration indicates that the shape of the etch pits is only slightly altered. Fig. lb depicts the region of Fig. la after it had been etched for a total of 3.5 h. The following points emerge: (i) The point-bottomed pits of Fig. la persist at their own positions even after 3.5 h of etching. No new pit (point-bottomed) has developed in Fig. lb but for some flat-bottomed pits. (ii) The point-bottomed pits have increased in their
at 35°C for etching time of 1 h (X400).
(b) Region of (a) after prolonged
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
dimensions and the shape of the etch pits has changed. The two comers of the etch pits (of Fig. la) have been truncated giving rise to Dshaped pits (in Fig. lb). The same type of experiments when repeated at 100% concentration of HNO, gave similar results. HNO, (50%100% concentration) at its boiling point attacks the crystal vigorously, leading to uncontrolled dissolution and hence defacing of the surface. Fig. 2a represents an etch pattern on a (110) face after etching in 80% HNO, for 1 h. Prolonged etching of the surface of Fig. 2a resulted in an increase in dimensions of point-bottomed pits. Examination of the surface at different stages of etching (after every 0.5 h of etching) revealed that the pointbottomed pits were getting larger at every stage of etching, whereas flat-bottomed pits got washed off. Fig. 2b shows the etch pattern on the (110) face of Fig. 2a after the last stage of etching (after 3.5 h) in 80% HNO,. From the experiments performed on etching of (110) faces in different concentrations of the etchant
67
at 35°C as described above, the following major points come forth: ‘(3 Although the shape of the etch pits is not very much altered on changing the concentration of the etchant, their definition is very much affected. However, lower concentrations produce better defined pits as compared to the higher concentrations, i.e. 90% and 100%. (ii) Continuation of point-bottomed etch pits at successive stages of prolonged etching for all concentrations (70%, 80% and 100%) suggests that HNO, is a dislocation etchant for (110) planes of LaBO, crystals also. 33 Etching of (100) and (110) faces at different
I.“.
temperatures in 90% HNO,
Etching experiments on LaBO, crystals were performed using 90% HNO, at different room temperatures, viz. 20,27,35 and 44°C. The shape of the etch pits on (100) faces at these temperatures remains nearly the same (after a total of 3.5 h of etching) as that of the pits produced in 80% or 100% HNO, at
Fig. 2. (a) Etch patterns on the (110) face after etching in 80% HNO, at 35°C for 1 h (X 400). (b) Region of (a) after a total etching period of 3.5 h (x400).
68
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
35°C. Figs. 3a and 3b show etch patterns on (100) faces due to 90% HNO, at 20 and 44°C respectively, after an etching of 1 h. The deep etch pits persisted at their positions even on prolonged etching to a total period of 3.5 h. Figs. 4a and 4b are optical micrographs of the same region showing convex-type etch pits produced on a (110) face after 1 and 3.5 h of etching, respectively, in 90% HNO, at 20°C. The pits persist on successive etching with increased dimensions. For (110) faces also the shape of the etch pits remained almost the same for various temperatures like 27, 35 and 44°C when 90% HNO, is used. However, the size of the etch pits increases manifold for the same period of etching at temperatures above 35°C. In the temperature range 95-121°C the etching gets so vigorous that the faces get mutilated. 2.3. Etching kinetics of (100) and (110) faces In order to study the dependence of etch rates on the concentration of the etchant, measurements of
lateral dimensions (length and breadth) and depth of etch pits for different concentrations (80%-100%) of the etchant were recorded. For the (110) face, the longer diagonal of the etch pit was taken as its length. The results obtained are compiled in the form of graphs shown in Figs. 5 and 7. From Fig. 5, it is evident that the variation of length of etch pits on the (100) surface with time is non-linear for 80% HNO, (curve 1 of Fig. 5) and in the case of 100% HNO, (curve 21, the observed variation is linear but having two different slopes. Figs. 6a and 6b show the variation of length and depth, respectively, of etch pits with time at different temperature when 90% HNO, is used for (100) surfaces. The variation is linear at the temperatures of 27 and 35°C but not at 20 and 44°C. The rates of linear etching being independent of time in the former cases. Fig. 6b illustrates the variation of depth of etch pits with time at different temperatures of the etchant for (100) surfaces; the temperatures taken at 20, 27,
Fig. 3. (a) Etch patterns on the (100) face after 1 h of etching in 90% HNO, at 20°C (X 400). (b) Etch patterns on the (100) face formed due to etching in 90% HNO, at 44°C for 1 h ( X 400).
69
A. Jain et al. /Applied Surface Science 84 (1995) 65-73
Fig. 4. (a) Etch patterns on the (110) face formed after 1 h of etching in 90% HNO, period of 3.5 h (x400).
35 and 44°C. From Fig. 6b, it is interesting to note the following: (i) The variation of depth with etching period is linear till a total etching period of 3.5 h at all the temperatures of the etchant considered here; the linear dependence ceases at 2 h of etching in all the four cases. After 2 h of etching, saturation sets in and the depth remains the same at all the temperatures on further etching. (ii) The vertical etch rates dD/dt (compiled in Table 1) are almost the same at different temperatures of the etchant. The dependence of length on the time of etching for (110) faces at each concentration of the etchant (Fig. 7a) is different when compared with that of the (100) face (Fig. 5). The variation for (110) faces is nearly linear for at least some part of the etching time in case of 100% and 70% concentrations of the etchants. Measurements of depth of etch pits at different intervals of etching time for (110) faces lead to interesting results. The results obtained are compiled
at 20°C (X400).
(b) Region of (a) after total etching
graphically as shown in Fig. 7b. Ignoring tion period of etching (shown by dotted curves obtained are linear till 7.2 X lo3 s at all the etchant concentrations (70%,
the induclines) the of etching 80% and
I
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I
4
6
9
I
IO
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TIMECTXlCkEC) Fig. 5. Graph showing the lateral extension and depth of etch pits against time for the (100) face. Curves (1) and (2) show the variation of lateral extension while curves (3) and (4) show the variation of depth for 80% and lOO%, respectively.
70
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
100%). After 7.2 X lo3 s of etching there is no further increase in depth with time of etching. The variation of depth of etch pits with time of etching on using 10% HNO, at 95°C is shown in Fig. 7c. The variation is similar to that of other concentrations of the etchant (70%, 80% and 100%) at 35°C except that the variation is linear till 1.5 X lo3 s and thereafter V,, = d D/dt suddenly drops down to zero. It may be noted here that the depth at which the saturation is reached is different in all the cases. In other words, the onset of passivity is in some way related to the time of etching and not the depth. Figs. 8a and 8b illustrate the curves recorded on the basis of measurements of length and depth, respectively, of etch pits on (110) surfaces due to etching by 90% HNO, at temperatures of 20, 27, 35 and 44°C. From Fig. 8a, it is clear that the lateral etch rates increase with increase in the temperature of the etchant; the etch rate being highest at 44°C in the temperature range of 20-44°C. The dependence of depth on time of etching of (110) faces by 90% HNO, at different temperature (20~44°C) as illustrated by Fig. 8b reveals a linear variation of depth with time till 7.2 X lo3 s of etching time. After 7.2 X lo3 s of etching, saturation sets in and the depth no longer increases with increase in the time of etching at all the temperatures considered here.
After 7.2 X lo3 s of etching of the (110) surfaces, the vertical etch rate drops down to zero for all the temperatures of the etchant. The saturation time (i.e., the time at which the pit stops deepening further) is the same irrespective of the temperature of the etchant. The data on lateral and vertical etch rates (V, = dL/dt and V,, = dD/dt) as estimated from the linear portions of the curves for (100) and (110) surfaces are compiled in Table 1. From experiments on etching kinetics of the HNO,-(110) surface system, the following points are noteworthy: (8 The variation of length of etch pits with time of etching is not linear for any of the concentrations (70%, 80% and 100%) of the etchant at 35°C. (ii) The variation of depth of an etch pit with time of etching is linear till saturation is reached after 120 min of etching in case of any of the concentrations (70%, 80% and 100%) of HNO,. The depth of an etch pit at which the saturation is reached is different for different concentrations of an etchant. After 120 min of etching in any of these three different concentrations of the etchant, the velocity of dissolution perpendicular to the surface suddenly drops down to zero.
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TIME(TXlOkC) Fig. 6. (a) Graph showing the variation of length with time of etching for 90% HNO, at 20, 27, 35 and 44°C for the (100) face. (b) Graph showing the variation of depth with time of etching for 90% HNO, at 20, 27, 35 and 44°C for the (100) face.
A. Jain et al./Applied
I
Surface Science 84 (1995) 65-73
I
nME(TXldSEC)
TIME(TXdSEC)
Fig. 7. (a) Variation of length with time of etching for different concentrations (70%, 80% and 100%) of etchant for the (110) face. (b) Variation of depth of etch pits with time of etching for different concentrations (70%, 80% and 100%) of etchant for the (110) face. (c) Variation of depth with time of etching in 10% HNO, at 95°C for the (110) face.
71
(iii) The variation of depth with time of etching in 10% HNO, at 95°C (see Fig. 7c) has the same form as that with different etchant concentrations (70%, 80%, 90% and 100%) at 35°C. The difference, however, being that in the former case, the saturation is reached after 25 min, whereas in the latter cases, it is reached after 120 min of etching. Here also the depth of etch pits at which the velocity of dissolution perpendicular to the surface drops down to zero is different for any of the etchant concentrations at 35°C. From what is described above, it is clear that prolonged etching leads to cessation of any further increase in depth at some stage, whereas it does not happen so in the case of lateral extension. This phenomenon is true not only at different concentrations but also at different temperatures of the etchant. The explanation for the occurrence of this type lies in the following: (9 The dislocations emerging on (100) and (110) planes do not go too far. They may bend and emerge on the other surface. If so, the dislocation etch pits may get laterally extended but not deepen after some stage of progressive etching. (ii) Undersaturation of the etchant decreases and so the in-depth etching stops abruptly, thus suggesting the occurrence of a critical Ap, as has been reported in the case of alum and Si [4] supporting the theory of Cabrera and Levine [5]. (iii) This could as well be attributed to the phenomenon of passivity as reported by Kotru et al. [6] for the KNiF,-HNO, system and by Kotru et al. [7] for the LaAlO,-HNO, system. The phenomenon of passivity of the surface is discussed by Milazzo [S] and Baker et al. [9]. The first explanation is ruled out because it is unlikely that all dislocations will bend suddenly on reaching the same depth from the exposed surface. The second explanation could have been applicable in this case too if the above given results obtained on using fresh HNO, at every successive stage were not in agreement with the results of experiments with the same HNO, at each successive stage of etching. The authors repeatedly performed experiments to verify and confirm if the etching kinetic results on using the same old HNO, and each
72
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
(b)
Ia) 0
2
4
6
8
ti
iz
14
:!
TlhlECTX$SEC)
4
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Fig. 8. (a) Variation of length with time of etching for 90% HNO, at different temperatures (20, 27, 35 and 44°C) for the (110) face. (b) Variation of depth with time of etching for 90% HNO, at different temperatures (20, 27, 35 and 44°C) for the (110) face.
some initial etching. At this stage only the side faces of the etch pits move, flattening the etch pits more and more. It is reported in the literature that metals do become passive to chemical attack by strong acids such as HNO, [8,9]. LaBO, on reaction with HNO, may suddenly stop being attacked after some time on account of passivity which sets in after the local formation of an oxide film at the preferential
time fresh HNO, at every stage of etching were in any way different. It was found that the etching kinetic results in the two cases were in agreement within the acceptable limits, thus excluding the application of this explanation. It then leaves one to think that the passivity of the LaBO,-HNO, surface system may be responsible for the abrupt cessation of “in-depth” etching after
Table 1 Estimation
of etch rates on (100) and (110) faces of LaBOt
Face
Etchant
crystals Lateral etch rate
Vertical etch rate
Ratio of vertical and lateral etch rates
35 35 20 27 35 44
2.177 1.878 0.806 0.975 1.896 1.899
0.281 0.276 0.275 0.271 0.272 0.270
0.129 0.146 0.341 0.277 0.143 0.142
35 35 35 20 27 35 44 95
4.948 3.448 3.991 0.925 2.033 2.649 3.045
0.548 0.535 0.958 0.274 0.270 0.555 0.283 1.793
0.110 0.155 0.240 0.296 0.132 0.207 0.092
Chemical formula
Cont. (%)
Temp. PC)
(100)
HNO,
80 100 90 90 90 90
(110)
HNO,
70 80 100 90 90 90 90 10
A. Jain et al. /Applied
Surface Science 84 (1995) 65-73
sites of attack after initial reaction for some time, thus protecting the surface from further in-depth etching (i.e., rendering it passive to further in-depth chemical attack). Higher temperatures rather than higher concentrations of HNO, appear to be more significant in bringing the early on-set of passivity. It is clear from Fig. 7b that the etch rate (till the variation is linear and the so-called passivity is not called into play) is different for different concentrations of the etchant.
13
in, the variation of depth with etching time is linear in all the cases (70%, 80% and 100% HNO, at 35°C and 10% HNO, at 95°C).
Acknowledgement One of the authors (A.J.) wishes to thank the CSIR for the award of a Senior Research Fellowship.
References 3. Conclusions
(1) HNO, is a dislocation etchant for (100) and (110) surfaces of LaBO, crystals.
(2) The shape of each pits due to HNO, being different for different habit faces [(loo) and (llO)] of LaBO, crystals, the former can, therefore, be used for the identification of the latter. (3) Suitable conditions of etching for (100) and (110) faces are 80% HNO, at 35°C for an etching period of 1 h. (4) The data on etching kinetics indicate that both (100) and (110) faces resist attack of the etchant in the direction of the normal to the surface after some period of etching (120 min for different etchant concentrations at 35°C and 25 min for 10% HNO, at 95°C). Till the time passivity sets
[l] E.M. Levin, R.S. Roth and J.B. Martin, Am. Mineral. 46 (1961) 1030. [2] P.N. Kotru and B.M. Wanklyn, J. Mater. Sci. L&t. 14 (1979) 755. [3] R.W.G. Wyckoff, Crystal Structures, 2nd ed., Vol. 2 (Interscience, New York, 1964). [4] P. Bennema and W.J.P. van Enckevort, Ann. Chim. Fr. 4 (1979) 451. [5] N. Cabrera and M.M. Levine, Phil. Mag. 1 (1956) 450. [6] P.N. Kotru, S. Gupta, S.K. Kachroo and B.M. Wanklyn, J. Mater. Sci. 20 (1985) 3949. [7] P.N. Kotru, A.K. Razdan, K.K. Raina and B.M. Wanklyn, J. Mater. Sci. 20 (1985) 3365. [8] G. Milazo, Electra-Chemistry (Eisevier, New York, 1963) p. 496. [9] D. Baker, D.C. Koehler, Wofeckenstein, C.E. Roden and R. Sabia, Material Technology: Physical Design of Electronic Systems, Vol. II (Prentice-Hall, Englewood Cliffs, NJ, 1970) p. 217.