New optics for X’Pert PRO The hybrid monochromator for high-resolution measurements In this presentation, the use of the 4-bounce hybrid monochromator for high-resolution measurements is explained. It is shown, by measurements on a superlattice, that the hybrid monochromator can best be used when fast collection of data with a good resolution is required. For ultra-high resolution, the Ge(220) and Ge(440) 4-crystal monochromators are better, but measurement times are much longer.
Contents What is a hybrid monochromator? For which applications can it be used? What is the advantage in the analysis of epitaxial layers? Contents of the talk: it will be shown what a hybrid monochromator is, for which applications it can be used and what the advantage is when the hybrid monochromator is used for high-resolution measurements of epitaxial layers.
What is a hybrid monochromator? A combination of an X-ray mirror and a channel-cut Germanium crystal Only Cu K1 is transmitted The combination gives a high-intensity, parallel, monochromatic X-ray beam Two varieties: two bounces in the monochromator four bounces in the monochromator The hybrid monochromator is a combination of an X-ray mirror and a channel cut Germanium crystal. The X-ray mirror transfers the diverging beam coming from the line focus of the X-ray tube into a high-intensity parallel beam. In the Germanium crystal, only a part of the Copper K-Alpha1 line is selected.
For which applications? Two-bounce type: Phase analysis on rough samples with complex patterns Samples in glass capillaries Four-bounce type: Measurement of rocking curves and reciprocal space maps How are these two types positioned with respect to each other? One is meant for phase analysis of powder samples for which a monochromatic parallel beam is required. In this monochromator, two bounces (reflections) of the X-ray beam in the Germanium crystal yield a strong signal with a divergence of about 25 arcseconds (about 0.007°). Optionally, it can be used for measurement of rocking curves and reciprocal space maps when intensity is of higher importance than resolution. The other one has four reflections in the Germanium crystal and is meant for fast measurement of rocking curves and reciprocal space maps. It has a divergence of 18 arcseconds (about 0.005°). In the remainder of this talk we will discuss the four-bounce hybrid monochromator and demonstrate its capabilities when compared with other monochromators.
Example: measurements on a superlattice What is a superlattice? What is the layer thickness? GaAs substrate GaAs AlAs 8 x } In order to demonstrate the power of the hybrid monochromator, we consider an example, measurements on a superlattice. A superlattice is a periodic repetition of several layers with different composition. Superlattices are used for many applications, such as Bragg reflectors, quantum wells and wave guides. We use this sample to demonstrate the versatility of the X’Pert PRO MRD for analyzing such a superlattice with different optical components.
Which instrument? X’Pert PRO MRD: Standard MRD Extended MRD In the next slides we will show the usefulness of the four-bounce hybrid monochromator on a standard MRD and on an extended MRD.
Standard X’Pert PRO MRD Tube focus: point Ge(440) monochromator Ge(220) monochromator Tube focus: line four-bounce hybrid monochromator For the standard MRD, we have four possibilities. Depending on the demands on resolution and intensity, one can choose different incident beam monochromators. With the tube rotated to the point focus position one can choose between the Ge(440) 4-crystal monochromator, the Ge(220) 4-crystal monochromator and the Ge(220) asymmetrical 4-crystal monochromator. In this example we will take a look at the first two only. Once the tube is rotated to the line focus position, it is possible to use the four-bounce hybrid monochromator. On the next slide we will compare the different measurements in one graph, created with the Philips X’Pert Epitaxy software package.
This graph shows the difference in intensity between the three monochromators. In each experiment, the illuminated area on the sample was taken as large as possible. The size of the illuminated area thus depended on the perfection of the sample. Because the measurement with the Ge(440) high-resolution 4-crystal monochromator was most sensitive to sample imperfections, the smallest beam size was used in that experiment. For the other measurements it was found that the beam size could be increased without decreasing the resolution. On the next slide we will show the trade-off between resolution and intensity in more detail by zooming in on the central peaks.
This graph shows the trade-off between intensity and resolution in more detail. In the high-resolution measurement with the Ge(440) monochromator, fine fringes between the main peaks are visible, resulting from the interference between the diffracting X-rays. The presence of these fringes illustrates the perfectness of the superlattice sample. With the hybrid monochromator however, the superlattice peaks are clearly resolved with an intensity increase of more than 1000 with the hybrid monochromator. An intensity increase of about 40 is possible with respect to the Ge(220) point focus 4-crystal monochromator. Imagine the increase in efficiency you can reach by decreasing your measurement time for each sample!
Normal MRD: conclusions Features of the hybrid monochromator Intensity increase: > 30 compared with Ge(220) point focus > 10 compared with Ge(220) asymm. Fast, compact tool for reciprocal space maps and rocking curves How can this be compared with Extended MRD? The plots in the previous slide show that the 4-bounce hybrid monochromator can well be compared with the Ge(220) 4-crystal monochromator. The resolution of the hybrid monochromator is slightly lower, this can be seen in the less perfect separation between the two largest peaks in the spectrum. In terms of intensity, however, a increase of around 30 can be reached, depending on the application. In other words: the same information is gathered in 4% of the time! When compared to the Ge(220) asymmetrically cut 4-crystal monochromator (not shown in the previous slides), the resolution of the 4-bounce hybrid monochromator is comparable, the typical intensity gain will be about 10. Let us compare the 4-bounce hybrid monochromator with a diffraction system in which the 4-crystal monochromators can be used together with a separate X-ray mirror; the so-called Extended MRD.
Extended MRD Mirror with Ge(220) and Ge(220) asymmetrically channel-cut monochromator Compare with four-bounce hybrid monochromator and with Ge(220) point focus In this case, we will compare the following configurations: tube focus: line X-ray mirror with 4-crystal Ge(220) monochromator X-ray mirror with asymmetrically cut 4-crystal Ge(220) monochromator 4-bounce hybrid monochromator tube focus: point 4-crystal Ge(220) monochromator
It is clear that in this case the difference between the hybrid monochromator and the mirror + 4-crystal monochromators is less pronounced.
Especially in this picture it can be seen that the resolution of the 4-bounce hybrid monochromator is comparable to the asymmetrically cut 4-crystal Ge(220) monochromator in combination with the separate X-ray mirror.
Conclusions (1) Let us compare the intensities of the possible incident beam optical modules for the analysis of epitaxial layers.
Conclusions (2) The four-bounce hybrid monochromator is an excellent tool for fast measurements of rocking curves and reciprocal space maps It can best be used when high intensity is required in combination with moderate resolution When ultra-high resolution is required, it is easy to change to a four-crystal monochromator In this talk I have introduced the hybrid monochromator to you. Two types of hybrid monochromator exist, the 2-bounce and the 4-bounce hybrid monochromator. The 4-bounce types is mainly designed for measuring rocking curves and reciprocal space maps. It can best be used when a high intensity is required at moderate resolution. When ultra-high resolution is required, the hybrid monochromator can easily be exchanged for a four-crystal monochromator. The ultimate flexibility is possible with an extended-MRD, in which the mirror and four-crystal monochromators can be used together. The hybrid monochromator, however gives the highest intensity and can also be used on standard MRD systems.