TEXTUREOF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER

Transcription

1 R 595 Philips Res. Repts 21, , 1966 TEXTUREOF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER Abstract 1. Introduetion by A. van der DRIFT The texture of a vapour-deposited PbO layer, similar to the layer in the "Plumbicon" television pick-up tube, is investigated with the aid of several X-ray-diffraction methods, electron-microscopic photographs of the surface as well as of cross-sections and measurements of the total area by means of gas adsorption. It appears that under certain conditions the greater part of the layer is of the red tetragonal form, crystallized in plates. They measure about 2 (.L x 0 5 (.L and have a thickness of about 50 Á. The plates are upright and the (llo)-direction shows a marked orientation determined by the angle of incidence of the PbO molecules. This orientation is caused by evolutionary selection during the deposition of the layer. The essential part ofthe "Plumbicon", a television pick-up tube, is a polycrystalline layer of PbO. A theoretical explanation of how this photoconductive layerworkshas beengivenin previous papers 1.2). This hypothesis explains most of the properties of the layer and appears to be a very useful approach. However, when going into detail one is compelled to admit the necessity of taking into account the polycrystallinity already demonstrated by Heijne 2). The surfaces of the crystals may be important. Some of the surface properties have been examined by Van den Broek 3). Attention will be paid in this paper to the texture of vapour-deposited PbO layers. Measurements are made to find out whether it is the tetragonal red form of PbO, stable below 488 oe, or the orthorhombic yellow form, stable above 488 oe 4), that is present. The shape, dimensions and orientation of the crystals as well as the crystallographic axes are investigated. To some extent the changes across the layer and the effect of evaporation parameters on its texture have been examined. 2. Sample preparation A certain amount of lead monoxide contained in a small platinum crucible is evaporated by inductive heating and condenses on the glass substrate (fig. I), which is maintained, within close limits, at a chosen température. The crucible is kept at a temperature somewhat above the melting point of PbO (888 C) where the equilibrium vapour pressure is about 0 5 torr, thus causing rather fast evaporation. Thè vapour passes through an atmosphere of fixed cornposition. Under these circumstances layers are obtained with a porosity of about 50 %. To get an angle of incidence different from zero (relative to the normal

2 290 A. van der DRIFT a e..e h g + b -~ 0 10mm f Fig. 1. Sample-preparation apparatus. (a) platinum crucible with PbO, (b) thermocouple wires, (c) bath to maintain the substrate temperature, (d).substrateconnected with the tube via a neoprene ring(i), (e) coil for inductive heating, (f) capillary tube for gas supply, (g) direction to the pump, (h) tube for measuring the pressure. on the substrate) we made use of arrangements in which the glass substrate is oblique. In addition we also prepared "stepwise-evaporated" samples as follows: from the substrate, which is completely exposed to the vapour at the start of the evaporation, part after part is screened off by a mask at certain times during the evaporation. In this way we obtain severallayer thicknesses side by side and we hope the properties of the surface of the thin part of the sample are comparable with those of the interior of the thicker parts of the sample at the same distance from the substrate. To prevent any chemical reaction of the PbO (in moist air Pb3(OH)iC0 2 )2 is easily formed) we also used arrangements in which the glass substrate was part of the tube which was sealed off afterwards. This technique was important specially for adsorption measurements. For X-ray measurements it was sufficient to cover the layer with a very thin film of an adhesive by impregnating the sample with a dilute solution of this adhesive in acetone. 3. Measuring techniques (a) Electron microscopy of vapour-deposited layers via direct carbon replicas, preshadowed with platinum. The photographs are always made in stereoscopie pairs. This gives more information than the single micrographs shown in this paper. (b) Electron microscopy of cross-sections of the layer via direct carbon replicas

3 TEXTURE OF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER 291 Parts of the layer are roughly removed by scraping with a pin, After the carbon has been evaporated and the Pbo' has been dissolved replicas of the crosssections are examined. Stereoscopie pairs are made in this case too, With this technique the cross-section is not necessarily perpendicular to the plane of the surface. In fact one obtains fracture edges along certain crystal planes. The problems involved in this method were similar to that of the method described by Nieuwenhuizen and Haanstra 5). Techniques to obtain electron micrographs of fracture edges of vapour-deposited layers have been mentioned before by Berlaga and Rudenok 6). \ (c) X-ray diffraction of crystal planes parallel to the substrate surface This is done with a normal diffractometer. The spectrum obtained is characteristic of a chemical compound in a certain crystalline form and gives some information on the orientation as well. We shall refer to this method as X-raydiffraction measurement. (d) X-ray diffraction of a chosen crystallographic plane in different directions In order to perform measurements more accurately, more quickly and from a smaller part of the layer, the results given in this paper were not obtained with an automatic texture goniometer but with an arrangement specially developed for the purpose by Langereis 7). In this arrangement the sample is turned around an axis perpendicular to the plane through the X-ray source, sample and Geiger tube. This is done in such a way that the normal to the substrate surface and the direction of the vapour source are also in the plane as defined, A diagram is obtained giving the frequency of the crystallographic plane having a given angle cp between the normal to the plane and the normal to the substrate surface. After correction 7) the curves obtained are characterized by two parameters: CPo is the value of cp at the top of the curve and will be termed the main orientation and Llcp is the width of the curve at half maximum intensity. We will refer to this method as X-ray-orientation measurement. (e) Determination of the surface area of the crystals This is done by means of the adsorption of a monomolecular layer of a noble gas as described by Brunauer, Emmett and Teller (BET) 8). The measurements were carried out at 77 OK using krypton. 4. Results and discussion 4.1. Crystallineform The X-ray-orientation measurements give us information on the crystalline form. The depth of penetration of the X-rays, however, is only a few microns., Stepwise evaporation as described in sec. 2 (0 2 at 0 01 torr, 100 C) enables

4 292 A. van der DRIFT an idea of the composition across the layer to be obtained; the greater part of the layer consists ofthe red form of PbO. Some yellow PbO is present, especially in the first deposited part of the layer. We shall deal primarily with red PbO. A crystal model is shown in fig. 2. The structure is talc-like and consists of sandwiches containing a plane with oxygen atoms flanked by two planes each containing halfthe number of lead atoms 9.10). Fig. 2. Crystal model of tetragonal PbO. Thewhite balls represent oxygen atoms, the black balls the lead atoms. Two sandwiches can be seen. The crystallographic axes and the < j 10)-direction, as they are orientated at perpendicular incidence of the molecules, are indicated. The bonds between the sandwiches are very weak compared with the bonds between atoms of the same sandwich 11). Red PbO would therefore be expected to crystallize mainly in the form of plates parallel to the sandwich plane Crystal shape and orientation A look at the electron micrographs in fig. 3a of a layer prepared by evaporating PbO through O 2 at 0 01 torr would seem to show the crystals to be needles rather than plates. This is a photograph of a PbO layer deposited on a substrate that was kept at 100 oe during the evaporation. By deposition of PbO on a substrate that was maintained at 150 oe we obtained the electron micrograph in

5 TEXTURE OF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER Electron micrographs ofthe surface of a PbO layer. During the deposition of th is layer ne half ofthe substrate was kept at 100 oe, giving the structure of fig. a, the other half at 50 oe (fig. b). The atmosphere was O 2 at 0 01 torr.

6 294 A. van der DRIFT Fig. 4a. Cross-section electron micrograph of a vapour-deposited PbO layer. As explained in fig. 4c part of the surface can also be seen.

7 TEXTURE OF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER 295 Fig. 4b. Cross-section electron micrograph of a vapour-deposited PbO layer. As shown in fig. 4c part of the PbO surface at the glass side can also be seen.

8 296 A. van der DRIFT _".. Fig. 4c. Artist's sketch of PbO layer, showing the parts to which the electron micrographs in figs 4a and 4b relate: fig: 3b which shows plates (in effect the two layers were prepared 'at the same time; see legend to fig. 3). We now suppose the crystals of fig.; 3a to be plates as well, however, out standing perpendicular to the substrate surface. Cross-section electron micrographs will have to be taken t? confirm this. Figures 4a and 4b show cross-sections of PbO layers (atmosphere O 2 at 0 01 torr, substrate temperature 100 -c,.nearly perpendicular incidence ~f the PbO molecules), the first with part of the vapour-source side of the layer, the latter with part of the substrate.side of the layer. Figure 4e shows how the photographs in figs 4a and 4b were taken. The vertical structure can clearly be seen and proves the assumption that the crystals are, indeed, plates and that they are standing at least nearly perpendicular to the edge line marked with an arrow in fig. 4e.. The dimensionsof the.plates we obtain when depositing through O 2 at 0 01 torr and on a substrate at.a temperature of 100 C are estimated to be o 5!L X 2 Il. But this is at the boundary of the layer on the vapour-source side. To obtain information about changes 'across the layer, the electron micrographs in figs 5a, b and e were made from several 'parts of a stepwise-evaporated sample (see sec. 2). Tlle dimensions clearly increase' during the depositon. As a consequence of the 'method of preparatien of the sample the layer in fig. 5e is formed with an oblique incidence 'of the PbOmolecules giving rise to an extra orientation, not discussed' in this paper. ',,, As far as the thickness of the plates is concerned we can make use ofthe results of the BET-adsorption measurements. From the measured total area of about 50 m 2 fg (0 2 at 0 01 torr, 100 0C), supposing a roughness ofthe crystal surfaces of unity, we calculate an average thickness of the crystals of about 50 A. Possible changes ofthe thickness across the layer have not been investigated up to now.

9 TEXTURE OF A VAPOUR DEPOSITED LEAD MONOXIDE LAYER 297 a Fig. 5. Electron micrographs of the surface of PbO layers at several stages of a stepwise evaporation through O 2 at 0 01 torr. The substrate temperature was 100 C. The thicknesses of the layers are 0 7 [L Ca), 2 [L Cb) and 16 [L Cc), respectively.

10 298 A. van der DRIFT 4.3. Orientation of the crystal axes Figure 6 shows some results ofx-ray-orientation measurements ofthe (110)- direction made at PbO layers deposited through O 2 at 0 01torr on a substrate at a temperature of 109 C; y is the angle between the normal to the substrate and the direction to the vapour source. The samples were made in the arrangement as shown in fig. 1, when necessary with a sloping glass substrate. The measurements have been carried out ih the centre of the substrate in order to avoid effects of asymmetry. As a consequence of the special way of tilting the sample a two-dimensional curve obtained is only a cross-section of a three-dimensional orientation. By making some complete measurements with an automatic texture goniometer in Relative intensity Ba ]'=0 :1=10" :1=21 :1=30" l!...:ti t\ f \ Ir+, 1" I \ I \ 17" iy - Î \. } \+ '\ < V 1 'i \ I ~ f J \. t j f \ J 1\ I \ \ / / \ / V \ I1,,+ x I /\ \ 1\ / v j 'x, \... v I----' V I,.( V..." I~ -, a a degrees -cp Fig.6. Relative intensities pf the (llo)-direction as a function of rp, the angle between the normal to the (110)-plane jmd the normal to the substrate, at several values of y, the angle between the direction to $he vapour source and the normal to the substrate. addition (we hope to report on the results later on) we showed that the threedimensional diagram is more or less rotation-symmetric around the maximum we found in the two-dimensional curve, which consequently provides us with sufficient information. ' Figure 7 shows that there is a relation between CPo and y, whicli means that 1. the direction of the vapour source plays an important part in the orientation of the (110)-plane~, If,in the case of y = 0, we combine the orientation ofthe (110)-plane (shown in fig. 2) with the orientation of the crystal plates (see sec. 4.2) we are sure that the crystals grow in the (110)-direction, as was already expected (see sec, 4.1).

11 TEXTURE OF A VAPOUR DEPOSITED LEAD MONOXIDE LAYER 299 degrees <Po 30', , r-----r------, Î degrees ----3' Fig. 7. Relation between CPa, the main orientation of the (110) direction, and y, the direction of the vapour source as determined from the measurements of fig. 6. The orientation width LI(/J is about 8 0, measured at the boundary of the layer at the vapour-source side. We should like to know what the orientation is like within the layer. Therefore we made X-ray-orientation measurements (fig. 8) of several.parts of a stepwise-evaporated sample (see sec. 2). The value of (/Jo was not precise enough to enable us to draw conclusions concerning possible changes of (/Jo across the layer. For easier comparison (/J - (/Jo has been chosen as the abscissa. The intensity is standardized at a maximum referred to as 100 %. The accuracy of the measurements is determined by the number of counts of the Geiger tube. The standard deviation was calculated at the 100% level ofthe curves. Table I gives the results. We see that the orientation becomes sharper as evaporation continues. There evidently is an evolutionary selection of favourable orientations. Less favourable orientations die out making room for other crystals. This is in accordance with the increasing dimensions (see fig. 5). TABLEI thickness of the layer orientation width standard deviation (urn) (degrees) at top (%) 1'5 > ~

12 300 A. van der DRIFT f({r' ;orenstty ~\l-.v/ 'ril 1\'\'---'.o.~~.;/ P vt ~... /' I ~ ~... rr~.. n,~ '4/,f "/.\ ~/.. '\ /..Jl 11 \ V ft I/~ 1\ \..,, -r!f l./ :J/ \....+ I! \ \ I / / I ~,,>('.j."+ o / "ro e degrees' -9>-<Po -Fig, 8.'Relative intensities of the (110 )-direction as a function of ffj- ffjo, measured at s'everal stages of a stepwise evaporation through O 2 at 0'01 torr., The substrate température was 100 oe. The ffjo-value ofthe layer of 1 5 [.I. thickness used is assumed to follow the relation of fig. 7. The indefinite parts of the curves are dashed Effect of the evaporation parameters on the texture of the layer. We have: been investigating the effect of the substrate temperature and the. kind of gas on the texture of the layer. Figure 3 already shows the increased size of the crystals at a more elevated substrate temperature. The samples of which these photographs were taken have been prepared at the same time in the same arrangement (see iegènd to fig. 3). It is assumed that, as a result of the larger size, the evolutionary selection proceeds more slowly. That the. orientation is therefore reduced may be seen in fig. 3. It was also determined by means of the X-ray-orientation measurements. We measured zl (p = 14 at 150 oe and LlqJ= 8 at 100 oe substrate temperature.. The effect of the kind of gas (see also ref. 2) can be seen in the electron micrographs in ûg, 9 where a layer evaporated through O 2 is compared with one evaporated through H 2 0. Both eyaporations were carried out at 0 01 torr. The substrate temperaturé was 100 oe. The amount of PbO is chosen so that in both samples. the same amount of PbO was deposited. Water apparently gives smaller crystals than does O 2, The orientation widths of the Xeray-orientation curves also differed.: We measured LtqJ= 16 in thecaseof HçO and LlqJ= 8 inthe case of O 2,. The texture of the layer on different substrates (glass and SnOz-coated glass) appeared to ge identical. Although we only discussed properties of layers evaporated through O 2 or H 2 0 at 0 01 torr at a substrate with a temperature of 100 oe or 150 oe we

13 TEXTURE OF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER 301 a Fig. 9. Electron micrographs of the surfaces of PbO layers deposited on a substrate temperature of 100 C. (a) Evaporation through O 2 at 0 01 torr (the same micrograph as in fig. 3a). (b) Evaporation through H 2 0 at 0 01 torr. at a

14 302 A. van der DRfFT Fig. 10. Electron micrograph made from the surface of a "Plurnbicon" PbO layer. measured that layers prepared under conditions somewhere between these extremes showed properties between the discussed properties ofthe extreme layers. As an example an electron micrograph of a layer as present in a "Plurnbicon " suitable for television-studio use is shown in fig Conclusions The PbO layer produced by evaporating PbO through oxygen on a substrate of glass at 100 oe consists mainly of the red modification. Within the layer, the crystal plates, with the (OOI)-planes as their boundaries, become orientated to a high degree due to an evolutionary selection. The direction of the orientation depends on the direction of the vapour source. At perpendicular incidence of the molecules the crystal plates are also perpendicular to the substrate surface. The dimensions of the crystals are about X O'5 11- X 50 A. The dimensions of crystals produced by evaporating PbO through water were smaller, those of crystals produced at a higher substrate temperature were larger. The "Plumbicon" layer has properties somewhere between the properties of the above-mentioned layers. The thickness of the crystal plates may be of particular importance in more extended theories of the "Plumbicon".

15 , TEXTURE OF A VAPOUR-DEPOSITED LEAD-MONOXIDE LAYER 303 Acknowledgement The Jxperiments described in this paper would not have been possible without the preparation of the samples by Miss T_ E. Horsman. The author is indebted to her and also wishes to thank Messrs C. Langereis for the X-ray measurements, H. B. Haanstra and J. M. Nieuwenhuizenfortheelectron-microscopic work and A. H. Boonstra for the adsorption measurements.-finally he'would like to acknowledge with gratitude the useful discussions with his colleagues. Eindhoven, November 1965 REFERENCES 1) E. F. de Haan, A. van der Drift and P. P. M. Schampers, Philips tech. Rev. 25, , ) L. Heijne, Philips Res. Repts Suppl. 1961no, 4. 3) J. van den Bro ek, Philips Res. Repts 20, , ) E. Cohen and N. W. H. Addink, Z. phys, Chem. A168, , S) J. M. Nîe uw en hu izen and H. B. Haanstra, Philips tech. Rev: 27, ,1965/66. 6) R. Ya. Berlaga and M. 1. Rudenok, Soviet Phys, solid State3, , ) C. Langereis, to be published in Philips Res. Repts 21 (1966). 8) S. Brun auer, P. H. Emmett ande. Teller, J. Am. chem. Soc. 60, ,'1938" ' 9) R. W. G. Wyckoff, Crystal structures I, Interscience Publishers, Inc., New York and London, 1963, p ) J. Leciejewicz, Acta crystallogr. 14, 1304, ) B. Dickens, J. inorg. nuel. Chem. 27, , 1965.