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X-ray diffractometry situ of evaporated thin films: application to amorphous tellurium
Thin Solid
Films,
@ Elsevier
Sequoia
36 (1976)
Ll - L4 S.A., Lausanne - Printed
Ll in Switzerland
Letter
X-ray diffractometry phous tellurium* J. C. MALAURENT Laboratoire
(Received
in situ of evaporated
to amor-
and J. DIXMIER
de Physique
August
thin films: application
des Solides,
25, 1975; accepted
Universitk
Paris&d,
91405
Orsay (France)
April 26, 1976)
Recently, energy scanning X-ray diffraction has been introduced in radiocrystallography as a new method for structure determination’,’ The most recent paper on the subject is that of Prober and Schultz3. In their work, the technique is applied to the study of the liquid structure of mercury. The interference function given is quite similar to that obtained by other authors in the past using the conventional X-ray angle scanning technique. The principle of the energy scanning X-ray diffraction technique is derived from the Bragg formula 2d sin 0 = X which is the basis of every diffraction study. This relation can be used in two ways: firstly by 0 scanning with a conventional goniometer and alternatively by keeping 0 constant and using a polychromatic or “white” X-ray beam. The incident X-ray beam and the axis of the detector are fixed at a given 28 value with respect to the sample surface. A solid state detector connected to a multichannel analyser records the intensity of the diffracted beams as a function of their energy. The diagram is modulated by the shape of the characteristic continuous spectrum of the X-ray tube. The first advantage of either method is an important increase of the incident beam intensity compared with that reflected from a monochromator. Since the whole spectrum is directly utilized, one can evaluate the time saving to one order of magnitude. As the record is made simultaneously for all radiations, the instability of the incident beam does not affect the resulting curve, and this is a second advantage. Our present work is concerned with another application of this method. Since the record of a diagram can be made without moving the detector, one can study the structure of evaporated thin films in situ, i.e. the sample can be observed directly on its substrate in the vacuum system.
*Paper presented at the Third International Conference on Thin Films, “Basic Problems, Applications and Trends”, Budapest, Hungary, August 25 -29, 1975, but appearing in the Conference Proceedings only in abstract form.
L2
DIFFRACTOM6TRE I”
-
51tu
Fig. 1. (1) Schematic drawing of the energy scanning X-ray apparatus; (3) Be windows; (4) cooling system of the sample substrate; (5) (6) moving arms for angle setting; (7) solid state detector; (9) X-ray tube; (10) vacuum system. Scale l/6.
As shown in Fig. 1, the apparatus is divided into three parts: the X-ray tube, the vacuum system and the solid state detector. Both tube and detector can rotate about the same axis, which is situated in the plane of symmetry of the vacuum cylinder. Two Be windows permit the entry and exit of the incident and reflected beams. The angular 20 setting can be chosen and read with good accuracy (- 0.1”). In this apparatus the available 20 range is from 12” to 60”. The sample can be a thin film evaporated at the top of the vacuum chamber or, by translating the axis of rotation along a vertical slit, a liquid at its bottom. The solid state detector is a Si( Li) crystal which must be kept at liquid nitrogen temperature when in use. The energy resolution of such a detector is 165 eV at 5 keV and 400 eV at 30 keV, which is better than the resolving power of a classical detector (1 keV) The various peaks given by the detector are classified by a 1024 channel analyser, and the data are recorded on a Teletype ribbon. The amorphous Te interference function has already been obtained by Ichikawa4 by electron diffraction, but we chose amorphous tellurium for testing the apparatus because this material is unstable at room temperature and no X-ray experiments have been performed on it up to now. A layer of amorphous Te 2 pm thick was evaporated onto a copper substrate. Copper was used for two reasons. Firstly it has a good thermal
L3
Fig. 2. Interference functions of amorphous tellurium dotted line, Ichigawa; full line, present work. Fig. 3. Radial distribution
functions:
dotted
(I(k)
line, Ichigawa;
-
-_-
T. lchikawa
_
Present
work
1) with K = (47r sinO)/n:
full line, present
work.
conductivity and the temperature of the target was that of liquid nitrogen. Secondly it is convenient to use a crystalline material since the thickness of the sample is not sufficiently large to be considered infinite for the shortest wavelengths. Thus one cannot avoid the substrate structure being registered, and it is more convenient to subtract lines rather than the continuous pattern of, for example, glass. The best substrate is a single crystal oriented in such a way that no reflection occurs for the 20 setting. The X-ray tube was a copper target tube using 50 kV and 25 mA. The complete continuous spectrum of the copper target was not utilized. The interesting range was situated between the characteristic radiation of copper (Cu Ka, Cu Kfl) and the first fluorescence line Ka of tellurium. This range is convenient for avoiding the anomalous dispersion of the scattering factor near the absorption edge. Settings of 20 = 18”, 26”, 40” and 53” were used. The successive rings of the amorphous Te became visible on increasing 20. For the last value (53”), no significant ripples were detected. Hence the shape of the recorded curve corresponded to the atomic scattering factor of Te modulated by the tube spectrum. This last record was utilized to derive the spectrum shape, which was the same for the four angle settings. The correction procedure for energy scanning presents specific features and has already been studied by Prober and Schultz3. Compared with the usual correction procedure for 20 scanning, the problem is complicated by the fact that instead of characteristic monochromatic radiation we are using a white beam continuously variable in frequency intensity and polarization. While the fluorescence emitted by the sample is concentrated in one peak, which is an advantage, the Compton effect gives a background more complicated to evaluate, i.e. each narrow band of the continuous beam gives its own Compton radiation. The final result is a continuous background shifted in energy with respect to the white spectrum. The major problem is in absorption correction since the incident beam is frequency variable and
L4
thus the absorption of the white beam in the sample is also variable. The correction was evaluated experimentally by checking the intensities of the crystallized Te lines compared with those obtained in a Debye-Scherrer camera. Our corrected and normalized interference function I(h) (Fig. 2) is not significantly different from the curve of Ichikawa except for the relative intensity of the first rings. This difference may be due to the difficulty of measuring the inelastic background at low angles in electron diffraction experiments. In energy scanning X-ray diffraction there is no background except that arising from the Compton effect, which is very small for heavy elements. The Fourier transforms are compared in Fig. 3. They are similar but the coordination number of the first neighbours derived from our radial distribution function is 2.2, which is higher than Ichikawa’s result (1.7). Thus energy scanning X-ray diffraction appears to be a useful technique for studying in situ amorphous structures which must be maintained at low temperature in order to remain non-crystalline. For example, the structure of amorphous pure metals could be identified by this technique to complete the electron diffraction studies which have already been made5,‘. Another field of interest is the possibility of checking the structure of a material at the same time as a physical measurement is being made. Taking into account the rapidity of the acquisition of a qualitative diagram (a few minutes), the phase transformation problem in general can be treated both structurally and physically by this technique.
1 2 3 4
G. Giessen and G. E. Gordon, Science, 159 (1968) 973-975. J. P. Lauriat and P. Perio, J. Appl. Crystallogr., 5 (1972) 177-183. J. M. Prober and J. M. Schultz, J. Appl. Crystallogr., (August 1975) T. Ichikawa,Phys. Status Solidi 6, 56 (1973) 707. 5 P. K. Leung and J. G. Wright, Phiios. Mag., (1975). 6 L. B. Davies and P. J. Grundy, J. Non-tryst. Solids, I1 (1972).
Films,
@ Elsevier
Sequoia
36 (1976)
Ll - L4 S.A., Lausanne - Printed
Ll in Switzerland
Letter
X-ray diffractometry phous tellurium* J. C. MALAURENT Laboratoire
(Received
in situ of evaporated
to amor-
and J. DIXMIER
de Physique
August
thin films: application
des Solides,
25, 1975; accepted
Universitk
Paris&d,
91405
Orsay (France)
April 26, 1976)
Recently, energy scanning X-ray diffraction has been introduced in radiocrystallography as a new method for structure determination’,’ The most recent paper on the subject is that of Prober and Schultz3. In their work, the technique is applied to the study of the liquid structure of mercury. The interference function given is quite similar to that obtained by other authors in the past using the conventional X-ray angle scanning technique. The principle of the energy scanning X-ray diffraction technique is derived from the Bragg formula 2d sin 0 = X which is the basis of every diffraction study. This relation can be used in two ways: firstly by 0 scanning with a conventional goniometer and alternatively by keeping 0 constant and using a polychromatic or “white” X-ray beam. The incident X-ray beam and the axis of the detector are fixed at a given 28 value with respect to the sample surface. A solid state detector connected to a multichannel analyser records the intensity of the diffracted beams as a function of their energy. The diagram is modulated by the shape of the characteristic continuous spectrum of the X-ray tube. The first advantage of either method is an important increase of the incident beam intensity compared with that reflected from a monochromator. Since the whole spectrum is directly utilized, one can evaluate the time saving to one order of magnitude. As the record is made simultaneously for all radiations, the instability of the incident beam does not affect the resulting curve, and this is a second advantage. Our present work is concerned with another application of this method. Since the record of a diagram can be made without moving the detector, one can study the structure of evaporated thin films in situ, i.e. the sample can be observed directly on its substrate in the vacuum system.
*Paper presented at the Third International Conference on Thin Films, “Basic Problems, Applications and Trends”, Budapest, Hungary, August 25 -29, 1975, but appearing in the Conference Proceedings only in abstract form.
L2
DIFFRACTOM6TRE I”
-
51tu
Fig. 1. (1) Schematic drawing of the energy scanning X-ray apparatus; (3) Be windows; (4) cooling system of the sample substrate; (5) (6) moving arms for angle setting; (7) solid state detector; (9) X-ray tube; (10) vacuum system. Scale l/6.
As shown in Fig. 1, the apparatus is divided into three parts: the X-ray tube, the vacuum system and the solid state detector. Both tube and detector can rotate about the same axis, which is situated in the plane of symmetry of the vacuum cylinder. Two Be windows permit the entry and exit of the incident and reflected beams. The angular 20 setting can be chosen and read with good accuracy (- 0.1”). In this apparatus the available 20 range is from 12” to 60”. The sample can be a thin film evaporated at the top of the vacuum chamber or, by translating the axis of rotation along a vertical slit, a liquid at its bottom. The solid state detector is a Si( Li) crystal which must be kept at liquid nitrogen temperature when in use. The energy resolution of such a detector is 165 eV at 5 keV and 400 eV at 30 keV, which is better than the resolving power of a classical detector (1 keV) The various peaks given by the detector are classified by a 1024 channel analyser, and the data are recorded on a Teletype ribbon. The amorphous Te interference function has already been obtained by Ichikawa4 by electron diffraction, but we chose amorphous tellurium for testing the apparatus because this material is unstable at room temperature and no X-ray experiments have been performed on it up to now. A layer of amorphous Te 2 pm thick was evaporated onto a copper substrate. Copper was used for two reasons. Firstly it has a good thermal
L3
Fig. 2. Interference functions of amorphous tellurium dotted line, Ichigawa; full line, present work. Fig. 3. Radial distribution
functions:
dotted
(I(k)
line, Ichigawa;
-
-_-
T. lchikawa
_
Present
work
1) with K = (47r sinO)/n:
full line, present
work.
conductivity and the temperature of the target was that of liquid nitrogen. Secondly it is convenient to use a crystalline material since the thickness of the sample is not sufficiently large to be considered infinite for the shortest wavelengths. Thus one cannot avoid the substrate structure being registered, and it is more convenient to subtract lines rather than the continuous pattern of, for example, glass. The best substrate is a single crystal oriented in such a way that no reflection occurs for the 20 setting. The X-ray tube was a copper target tube using 50 kV and 25 mA. The complete continuous spectrum of the copper target was not utilized. The interesting range was situated between the characteristic radiation of copper (Cu Ka, Cu Kfl) and the first fluorescence line Ka of tellurium. This range is convenient for avoiding the anomalous dispersion of the scattering factor near the absorption edge. Settings of 20 = 18”, 26”, 40” and 53” were used. The successive rings of the amorphous Te became visible on increasing 20. For the last value (53”), no significant ripples were detected. Hence the shape of the recorded curve corresponded to the atomic scattering factor of Te modulated by the tube spectrum. This last record was utilized to derive the spectrum shape, which was the same for the four angle settings. The correction procedure for energy scanning presents specific features and has already been studied by Prober and Schultz3. Compared with the usual correction procedure for 20 scanning, the problem is complicated by the fact that instead of characteristic monochromatic radiation we are using a white beam continuously variable in frequency intensity and polarization. While the fluorescence emitted by the sample is concentrated in one peak, which is an advantage, the Compton effect gives a background more complicated to evaluate, i.e. each narrow band of the continuous beam gives its own Compton radiation. The final result is a continuous background shifted in energy with respect to the white spectrum. The major problem is in absorption correction since the incident beam is frequency variable and
L4
thus the absorption of the white beam in the sample is also variable. The correction was evaluated experimentally by checking the intensities of the crystallized Te lines compared with those obtained in a Debye-Scherrer camera. Our corrected and normalized interference function I(h) (Fig. 2) is not significantly different from the curve of Ichikawa except for the relative intensity of the first rings. This difference may be due to the difficulty of measuring the inelastic background at low angles in electron diffraction experiments. In energy scanning X-ray diffraction there is no background except that arising from the Compton effect, which is very small for heavy elements. The Fourier transforms are compared in Fig. 3. They are similar but the coordination number of the first neighbours derived from our radial distribution function is 2.2, which is higher than Ichikawa’s result (1.7). Thus energy scanning X-ray diffraction appears to be a useful technique for studying in situ amorphous structures which must be maintained at low temperature in order to remain non-crystalline. For example, the structure of amorphous pure metals could be identified by this technique to complete the electron diffraction studies which have already been made5,‘. Another field of interest is the possibility of checking the structure of a material at the same time as a physical measurement is being made. Taking into account the rapidity of the acquisition of a qualitative diagram (a few minutes), the phase transformation problem in general can be treated both structurally and physically by this technique.
1 2 3 4
G. Giessen and G. E. Gordon, Science, 159 (1968) 973-975. J. P. Lauriat and P. Perio, J. Appl. Crystallogr., 5 (1972) 177-183. J. M. Prober and J. M. Schultz, J. Appl. Crystallogr., (August 1975) T. Ichikawa,Phys. Status Solidi 6, 56 (1973) 707. 5 P. K. Leung and J. G. Wright, Phiios. Mag., (1975). 6 L. B. Davies and P. J. Grundy, J. Non-tryst. Solids, I1 (1972).