Surface characterization of amorphous hydrogenated carbon thin films containing nanoclusters of noble metals (2003)

In this work nanocomposite thin films of amorphous hydrogenated carbon\n(a-C:H) doped with noble transition metals of 1B group (gold, silver, and copper) are\nstudied. The composite materials are obtained by combined magnetron sputtering\n(MS) of a metal target by argon, and plasma-assisted chemical vapor deposition\n(PACVD) of methane under vacuum conditions. Particular attention is devoted to the\nlow metal-content a-C:H samples, in which metallic inclusions have a form of isolated\nnanoclusters. Our aim was to reveal surface cluster arrangement, i.e. to figure out\nwhether topmost metallic nanoclusters are covered with a layer of a-C:H or are bald\non the surface and hence exposed to the surrounding environment. We deposited\nour samples onto substrates kept on the ground potential and on �150 V dc bias\nvoltage. The differences encountered in the surface structure and nanocluster arrangement\nbetween samples, which differed in the deposition process in this parameter\nonly, provided an answer to the question of surface clusters coverage.\nThe experimental techniques used to reach this goal comprised in vacuo and\nex vacuo photoelectron spectroscopy (PES), direct imaging by atomic force microscopy\n(AFM) and scanning electron microscopy (SEM), and grazing incidence smallangle\nx-ray scattering (GISAXS). The majority of work is made in several photoemission\nexperiments, using both x-ray and UV-excited photoelectron spectroscopy (XPS\nand UPS, respectively). In the course of the work, series of (grounded only) a-C:H/Au, a-C:H/Ag, and\na-C:H/Cu samples, with metallic content varying from zero to 100 at.%, have been\ndeposited and studied in vacuo by XPS and UPS (Section 3.1). The XPS results of\nthese series show that, with decreasing metal content below the percolation threshold\n(at about 40-50 at.% of metal concentration), a shift in binding energy (BE) of\nmetal core levels towards higher BEs is observed. With non-carbidic metals like the\nones we used in our work, these shifts can be related to their isolated cluster structure\nin the host matrix. Decrease of the total metal content in the sample is followed\nby the decrease in the cluster size, which is reflected in increased binding energy of\nelectrons escaping from them (Paragraph 2.2.2 and Section 3.1). At the same time,\ncarbon C 1s core level is shifted in the opposite direction, towards lower binding energies,\nand this shift rises as metal content increases in the sample. Negative shift in\nC 1s binding energy reveals increased relative content of sp2-coordinated carbon in\nthe a-C:H matrix, which, we believe, should be attributed to the compressive stress\nthat metallic inclusions introduce in the host a-C:H, being higher with higher metal\ncontent in the sample. Another possible reason is catalytic reduction of hydrogen in\nthe a-C:H matrix with increasing metallic content. UPS of the same series (Fig. 3.2)\nshowed increased relative contribution of metal valence band features with increased\nmetal content. The Fermi edge evolution from monocrystalline metal reference samples\nto low metal-content a-C:H showed both decrease in the density of states near the Fermi level and its shift to the higher BEs, related to the cluster structure of metallic\ninclusions and to the same direction-shifts in metal core levels.\nOur further attention was focused on low metal-content a-C:H that is, typically\nbelow 10 at.%, and to the differences between grounded and biased samples. The\nreprint of the communication on these effects encountered in a-C:H/Au and published\nin the Applied Physics Letters (Vol. 80, 2002, p.2863) is given in Section 3.2.\nDirect imaging techniques (Sections 3.2 and 3.3) reveal that even small\namount of metal included in the a-C:H significantly changes the surface morphology\nand increases roughness. For all three nanocomposite materials, biased samples\nshow similar surface morphology, characterized by relatively flat basis and isolated\nbump structures, of about 30 nm in diameter and up to 15 nm in height. These\nstructures are attributed to the altered morphology of a-C:H component of a nanocomposite,\nwith metallic clusters concentrated on them. Grounded samples characterize\nlow roughness in a-C:H/Au and increased in a-C:H/Ag and a-C:H/Cu. The increase\nof roughness in the latter two materials is explained by enhanced surface diffusion\nof metal atoms and clusters coalescence into bigger islands.\nGISAXS patterns (Section 3.4) showed isotropic cluster size and intercluster\ndistance distribution in grounded a-C:H/Au and a-C:H/Cu samples. With grounded a-\nC:H/Ag sample, no spatial correlation could be revealed, probably again due to the\nsurface agglomeration of Ag clusters. These analysis of GISAXS patterns showed\nthat biased samples contain bigger clusters than grounded ones, and slightly flattened\nin the grow direction. The differences between grounded and biased samples were first detected by\nin vacuo XPS of a-C:H/Au in systematically lower, in average about 50%, Au content\nin the biased than in the grounded case. This decrease is followed by higher positive\nshift of the Au 4f7/2 core level binding energy in the biased case (Section 3.2 and\nParagraph 3.5.1). The similar situation was encountered in a-C:H/Ag samples, while\na-C:H/Cu showed higher Cu content in the biased than in the grounded case and no\ndifference in Cu 2p3/2 shifts from the reference BE. The decrease of the total metal\ncontent in biased a-C:H/Au and a-C:H/Ag samples most probably results from increased\ndistance among surface clusters and their concentration on isolated bump\nstructures. The reason for the increased BE shift of Au 4f7/2 and Ag 3d5/2 core levels\nin the biased samples should be searched in cluster baldness at the sample surface.\nThe violation of both above conclusions in a-C:H/Cu, i.e. higher Cu content in the\nbiased sample and equal shift of the Cu 2p3/2 core level in grounded and biased\nsample is probably due to high relative increase of copper cluster size upon biasing,\nwhich compensates the effect of their baldness at the surface. The topmost clusters\nof grounded nanocomposite samples are, therefore, most probably covered with a\nlayer of a-C:H. The thickness of this layer must be below escape depth of metaloriginated\nphotoelectrons, i.e. less than about 2 nm. The negative shift of the C 1s\ncore level in the biased samples is induced by increased sp2/sp3 coordinated carbon\nratio due to the sample bombardment by Ar+ ions during the deposition process. To our belief, the same effect is responsible for surface metallic clusters baldness in the\nbiased samples.\nThese conclusions are supported by several following PES experiments, other\nthan XPS of as-deposited samples. First of them was in vacuo UPS of as-deposited\nsamples. In the grounded samples, the He I spectra mostly reproduce the characteristic\nshape of a-C:H valence band, and only higher sensitivity He II UPS reveal the\npresence of metallic inclusions. Upon biasing, even when total measured metal content\nwas lower than in the grounded case (a-C:H/Au and a-C:H/Ag), all spectra\nclearly showed increased metallic features, evidencing on higher metal exposure at\nthe surface.\nXPS at off-normal take-off angle of escaping electrons also confirmed our conclusions\non the surface clusters coverage. Increasing the tilting angle of a sample,\nmeasured intensity ratio of a metal core level to appropriate C 1s showed in most\ngrounded samples monotonous decrease, and regular and steady increase in the\nbiased ones. These results support our conclusion on the surface clusters coverage\nin grounded samples and their baldness in the biased ones. The higher metalcontent\ngrounded samples of a-C:H/Ag, however, did not show the expected decrease\nin the intensity ratio, and that was the first indication that the effect of coverage\nmay be a particularity of small clusters only, i.e. low-metal content grounded\nsamples. This suspicious is confirmed in the next test experiment that we have undertaken,\nby subsequent in situ low-energy Ar+ ion etching and PES analysis of a sample.\nThe same metal to carbon core level intensity ratio curves were measured\nagainst the sputtering time. In the grounded samples, at the beginning of the sputtering,\nan increase in the intensity ratio is observed, related to the thinning of the top\na-C:H layer. In most cases, after some time of sputtering, the maximum is reached\nrelated to the total removal of the cover layer, and from that point onwards, Ar+ ion\netching erodes the metallic clusters as well. In biased samples, a monotonous decrease\nof intensity ratio curves was observed throughout the experiment and is\nclearly related to the bald surface clusters that are sputtered together with the a-C:H\nmatrix. The grounded samples intensity ratio curves showed one more important\nregularity: in higher metal-content samples the maximum is reached after shorter\ntime, i.e. these samples need less time to be fully uncovered. As a special case, a-\nC:H/Ag 32.3 at.% did not show any increase in the intensity ratio curve, but monotonous\ndecrease throughout the measurement. That encouraged the conclusion that\nthe coverage of the topmost metallic clusters of grounded samples with a-C:H is an\neffect that is characteristics of small clusters in the host matrix, i.e. low metal-content\nsamples. With