In order to make a diamond device for nuclear applications we graphitized in front and back an high quality Chemical Vapor Deposition (CVD) diamond.
The sample was an undoped synthetic detector grade polycrystalline dia-mond with a thickness of 300 µm after chemical polishing and laser cutted to a size of 0.5⇥ 0.5 cm2. Chemical polishing was given by 15 minutes boiling diamond sample in a chromic acid powder dissolved in H2SO4 followed by 5
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Fig. 4.7. SEM micrographs from C area at increasing magnification from (a) to (c)
minutes boiling in piranha solution (50:50 H2SO4 and H2O2). The next step was the immersion of the sample in a boiling solution: 4:1:1 deionized (DI) Water + HCl + H2O2 for 1 minute. Finally the sample was blowed dry in ultra dry N2 gas for 7 minutes ⇡ 40 watt plasma in 50 cubic in chamber.
In each case when we changed the boiling solution, prior to immerge the sample, we made a DI water rinse by ultrasonic bath. After chemical polish-ing diamond device was prepared with an experimental procedure analogous to that described in section 4.1.1, i.e. we irradiated in front (grow side) and back (seed side) diamond surfaces with the ArF excimer laser (Lambda Physik LPX305i) at 193 nm. In this way graphite electrodes with an area of about 3⇥3 mm2 were made on both sides of diamond by using the automatic handling to scan the surfaces with a velocity of about 0.3 mm/s [see Fig.
4.8 (a)]. In first analysis we verified the strong mechanical resistance about graphitic layers, because we couldn’t remove any graphite piece by mechan-ical instruments, such as cutters or knives. In second analysis micro-Raman
Fig. 4.8. (a) Optical microscopy image of the polycrystalline CVD diamond detector grade after laser treatment. The black pad at the center of the device is the photo-generated graphite electrical contact. The coloured circles represent a scheme of points in which we performed micro-Raman and SEM characterizations. The graphitised sample is turned by the grow side surface as explained in the text; (b) Micro-Raman spectrum of unirradiated CVD diamond detector grade.
and photoluminescence (PL) measurements, SEM images, were taken using the same experimental setup described in section 4.1.1 to characterize the ir-radiated area. Micro-Raman and PL spectroscopy was collected in front and back at di↵erent points, i.e. at the vertexes of a triangle and at the center in order to warrant the reproducibility of the measures in refer to Fig. 4.8 (a), while SEM images approximatively at the center of the irradiated pad for both surfaces. A preliminary visual and optical microscopy observation indicates that throughout them thin surface layers of carbonaceous material were photon modified.
A representative micro-Raman spectrum from our unirradiated diamond [Fig. 4.8 (b)] displays a narrow peak centered at 1335 cm 1 (FWHM, 8 cm 1), and a broad G band extending from about 1450 to 1640 cm 1, with a maximum around 1565 cm 1; both these features grow up from a lumines-cence background. This spectrum was taken from the front face of the sample and it is identical to a spectrum taken from the back face of the sample (not reported). The same analysis made on irradiated area is reported and we can see in Fig. 4.9 micro-Raman spectra. For front side surface [Fig. 4.9 (a)] the general information is that laser irradiation brings about diamond
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(a) (b)
Fig. 4.9. Micro-Raman spectra taken at four di↵erent positions on graphitised pad for detector grade CVD diamond in refer to Fig. 4.8, (a) is for grow side surface (front) and (b) for seed side (back).
graphitization, as indicated by the presence of the G and D bands; yet the G band extends up to about 1650 cm 1, peaking between about 1590 and 1600 cm 1: this important blue-shift is likely to be associated to the presence of a considerable fraction of disordered diamond, sp3 coordinated [58]. When we look at the details of the di↵erent micro-Raman spectra we see that the spectrum taken at vertex 3 has a shoulder at about 1540 cm 1 that appears in the other spectra as a weak feature around 1532 cm 1 whose intensity is scales inversely with the D/G ratio of the considered spectrum. As to the D band, in all spectra it is centered at about 1350 cm 1: this is expected in micro-Raman spectrum of a disordered graphitic carbon when green excita-tion light is used, as in our measurements. From Fig. 4.9 (b) Raman G and D bands are evident; G bands are very broad, peaked at about 1590 cm 1, with a shoulder at about 1630 cm 1, while D bands are centered around 1360 cm 1. The overall information from Raman spectroscopy of the back face of the sample agrees with that from the front face, namely that under this type of laser irradiation detector grade CVD diamond is converted to graphitic, strongly disordered.
We discuss now the spectral features of the front and the back irradiated sample faces about photoluminescence (PL). In Fig. 4.10 (a) is displayed the collection of PL spectra taken at the four selected points on the front side
Fig. 4.10. PL spectra taken at four di↵erent positions in refer to Fig. 4.8, (a) is for grow side surface (front) and (b) for seed side (back).
of the sample in refer to Fig. 4.8 (a). We observe that the irradiation e↵ect was not uniform cross the irradiated area: although PL intensity is low in all spectra, it is even lower at the center and at vertex 2 with respect to vertices 1 and 3. The second order Raman G’ band, centered at about 2720 cm 1, is weak. While about back irradiated side of the sample, PL spectra in Fig.
4.10 (b) again show that the e↵ect of laser irradiation was not uniform: at vertices 2 and 3 the intensity of PL spectra is high and comparable to what is expected from diamond, while it is strongly reduced at vertex 1 and at the center of the sample. Also in this set of spectra second order Raman features at about 2720 cm 1, are weak.
In Fig. 4.11 are displayed SEM micrographs taken on the front and back faces of the sample, approximately at the center position. Low magnification pictures show that the surface morphology of the sample is uniform across the whole irradiated surface on both faces. From both Fig. 4.11 (a) and (c), taken at the same intermediate magnification (20000 x) the surfaces appear stepped, with an irregular, dense distribution of nanoparticles, about 200 to 400 nm in size, partly protruding outwards. The bigger ones are located mostly in the step regions. In Fig. 4.11 (b), that was taken from a portion of the area imaged in Fig. 4.11 (a) for a magnification of 50000⇥, the spongy nanoparticle morphology is more evident.
For unirradiated area an optical profile image was given by Diamond
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(a) (b)
(c)
Fig. 4.11. SEM micrographs taken at the center of the irradiated area in refer to Fig.
4.8 (a).
Detectors Ltd company (Fig. 4.12). This image was taken from the front face of the sample and it is identical to the image taken from the back face of the sample (not reported). They used a Wyko NT9100 Optical Profiling System that employed coherence scanning interferometry, also known as white-light interferometry, white-light confocal, or vertical scanning interferometry to produce high quality three-dimensional surface maps of the object under test with a sub-nanometer vertical resolution at all magnifications. From a confront of the images about irradiated and unirradiated diamond we can
Fig. 4.12. Optical profile image for CVD diamond detector grade on unirradiated surface.
see that laser irradiation produce changes about the structure of the sample surfaces.