AP08_1.vp 1 Introduction Carbon thin films have attracted the attention of various investigators due to their unique optical, electrical and me- chanical properties [1]. Various deposition techniques and processes such as Plasma Enhanced Chemical Vapor Deposi- tion (PECVD), sputter deposition etc., have been employed to deposit carbon films [2, 3]. Research on the deposition of carbon layers is mainly focused on the fabrication of dia- mond-like carbon (DLC) or carbon nanotubes. Our study deals with the development of carbon layers, which should have good optical properties (low optical losses, suitable re- fractive indices, high photoluminescence efficiency) for potential application in integrated optics. [4] and [5] have shown that it is possible to prepare carbon planar waveguides with attenuation lower than 1 dB�cm�1. Low optical losses of the optical materials are one of the most important precondi- tions for using this material in integrated optics and for dop- ing it with active ions such as erbium. Er3�-doped optical materials are candidates for fabri- cating optical amplifiers or lasers operating at 1530 nm [6, 7, 8], due to Er3� intra-4f emission, which corresponds to 4I13/2 � 4I15/2 transition. This wavelength is commonly used in telecommunication system, due to the fact that it corre- sponds to the low loss window of silica-based optical fibers. The only papers analyzing carbon layers containing er- bium ions are by Hüttel et al. [9, 10, 11], by Baranov [12] and by Speranza [13]. The articles report on fabricating erbium doped carbon layers by using magnetron sputtering on a sili- con substrate. Baranov and Speranza used a graphite target partially covered with pieces of metallic erbium, and deposi- tion was performed in a gas mixture of argon and C6H12 at a pressure of about 10�3 Pa. The carbon layers had an amorphous structure with sp2 arrangement in which the Er concentration varied from 0.15 % to 0.8 %. They observed photoluminescence due to the Er3� transition 2H11/2 � 4I15/2 only in the sample with the highest amount of erbium. In this paper we will report on our attempts to fabricate carbon layers with erbium content higher than those in the papers mentioned above. 2 Measurements The structure of the deposited carbon layers was studied by XRD (X-ray diffraction). The compositions of the fabri- cated samples were determined using nuclear chemical analy- ses such as Rutherford Backscattering Spectroscopy (RBS) and Elastic Recoil Detection Analysis (ERDA). The amounts of erbium ions were checked by RBS using both 2.4 MeV protons and 2.2 MeV alpha particles. The RBS and ERDA spectra were evaluated by GISA3 [18] and SIMNRA [19] code, respectively. The absorbance spectra of the samples were taken in the spectral region from 400 nm to 600 nm at room temperature. A tungsten lamp and MDR 23 monochromator were used as a light source, and the light transmitted through the samples was detected by the pyro-detector. The photoluminescence measurement was carried out at three excitation wavelengths: � An ILA-120 Ar laser operating at �ex � 488 nm, Eex � 100 mW, (300 K). � Ar lasers operating at �ex � 514.5 nm, Eex � 300 mW, (4 K). � A semiconductor laser P4300 operating at �ex � 980 nm, E ex � 500 mW, (300 K). The FEU62 photocell was used for detecting the wave- length from 500 to 1000 nm, while a Ge detector was used for the wavelength from 1000 nm to 1600 nm. 3 Sample fabrication and results Erbium containing carbon thin films were deposited using three different procedures: � sputtering from carbon targets with built-in metallic er- bium, 36 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 1/2008 Properties of Erbium Doped Hydrogenated Amorphous Carbon Layers Fabricated by Sputtering and Plasma Assisted Chemical Vapor Deposition V. Prajzler, Z. Burian, V. Jeřábek, I. Hüttel, J. Špirková, J. Gurovič, J. Oswald, J. Zavadil, V. Peřina We report about properties of carbon layers doped with Er3� ions fabricated by Plasma Assisted Chemical Vapor Deposition (PACVD) and by sputtering on silicon or glass substrates. The structure of the samples was characterized by X-ray diffraction and their composition was determined by Rutherford Backscattering Spectroscopy and Elastic Recoil Detection Analysis. The Absorbance spectrum was taken in the spectral range from 400 nm to 600 nm. Photoluminescence spectra were obtained using two types of Ar laser (�ex � 514.5 nm, �ex � 488 nm) and also using a semiconductor laser (�ex � 980 nm). Samples fabricated by magnetron sputtering exhibited typical emission at 1530 nm when pumped at 514.5 nm. Keywords: carbon layers, Erbium, PACVD, sputtering, photoluminescence � PACVD growth of carbon thin films from a gas mixture ( H 2� Ar � erbium isopropoxide or CH4� Erbium tris), � magnetron sputtering placing metallic erbium on the car- bon target. The structures of the deposited carbon layers were then investigated by XRD (X-ray diffraction) and it was found that all of them were amorphous. 3.1 Growth of erbium doped carbon thin films using sputtering The first procedure for depositing erbium doped car- bon thin films was sputtering using a carbon (99.8 %) target that contained pieces of metallic erbium. The target was made by pressing milled graphite and erbium metal (7 g C � 0.8 g Er) applying different pressures (100 MPa�cm�2 and 200 MPa�cm�2). The final targets were 5 cm in diameter. De- position was performed in an Ar atmosphere at a pressure of 12 Pa for 20 min. Voltage on the grounded electrodes was �75 V and the distance of the electrodes was 3 cm. A schema of the experimental set-up is shown in Fig. 1. The composition of the carbon layers determined by a nu- clear chemical analysis as RBS and ERDA is given in Table 1. The actual content of erbium ions depends on the fabrication parameters, and the maximum amount obtained after opti- mization was 0.02 at %. However, this content was still too low for photoluminescence at 1530 nm. 3.2 Growth of erbium doped carbon thin films using PACVD Here “Erbium isopropoxide” (C9H21ErO3) was used for the first time as the erbium source. The fabrication process was based on modifying the reactor, as shown in Fig. 2 (com- pare with Fig. 1). The deposition of erbium doped carbon layers was carried out using methane (flow of 50 cm3�min�1) and argon (flow of 30 cm3�min�1). The erbium isopropoxide (Sigma-Aldrich) vapor was added into this gas mixture at a temperature from 240 °C to 260 °C and injected into the reac- tor by flow of methane and argon. The bias applied at one of the electrodes was �100 V and the power density was from 0.31 W�cm�2 to 0.35 W�cm�2. The deposition rate was in the © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 37 Acta Polytechnica Vol. 48 No. 1/2008 Vacuum gauge Substrate Pump Ar Carbon target Vacuum chamber Matching unit RF power 13.56 MHz (7 g C + 0.8 g Er) Fig. 1: Schema of the sputtering set-up (see above) Methods used for fabrication of carbon layers C H O Er (at %) (at %) (at %) (at %) Growth of erbium doped carbon thin films using sputtering (7g C � 0.8 g Er) 52 45 3 0.02 Growth of erbium doped carbon thin films using PACVD (CH4� Ar � “erbium isopropoxide”) 65 32 3 0 Growth of erbium doped carbon thin films using PACVD (CH4� “erbium tris”) – position 1* 51 27 15 8.7 Growth of erbium doped carbon thin films using PACVD (CH4� “erbium tris”) - position 2* 61 40 5 0.012 Growth of erbium and ytterbium doped carbon thin films using magnetron sputtering 73 23 4 0.2–20** * For more details see Fig. 3 ** Depending on Er or Er � Yb powder laid onto the top of the carbon target (see Table 2) Table 1: Composition of erbium-doped carbon samples as determined by Rutherford Backscattering Analysis and Elastic Recoil Detec- tion Analysis range from 20 nm�min�1 to 22 nm�min�1. The temperature of the silicon substrates varied from 50 °C to 100 °C. Because the deposited samples did not contain erbium even after optimization (for more details see Table 1), another erbium source had to be searched for. Chryssou and Pitt [16] reported the fabrication of erbium doped Al2O3 layers using PACVD. They used “Erbium tris” (Er(thd)3) as an erbium source and the fabricated samples had photoluminescence at 1530 nm even at room temperature. Therefore it was decided to fabricate carbon layers doped with erbium us- ing “Erbium tris” ([Tris (2,2,6,6-tetramethyl-3,5-heptanedionate)] Er(+III)), (Sigma-Aldrich) as a source. Deposition was carried out using methane (flow of 50 cm3�min�1). “Erbium tris” vapor was produced at tempera- tures from 130 °C to 160 °C, and it was always mixed with methane. There were two ways to insert the “Erbium tris” into the vacuum chamber (Fig. 3). When the “Erbium tris” was in position (1) the concentration of erbium ions in the deposited carbon layers was as high as 9 at %. When the “Erbium tris” was in position (2) the erbium concentration was always lower, reaching a maximum of 0.012 at % (see Table 1). Fig. 4 shows the photoluminescence spectrum of the car- bon layer fabricated by PACVD. The bands at wavelength 582 nm and 645 nm come from the carbon layers themselves 38 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 1/2008 Vacuum gauge Substrate Pump Ar Vacuum chamber Matching unit RF power 13.56 MHz Erbium isopropoxide Heating CH4 Fig. 2: Schema of the PACVD set-up used for depositing of carbon layers from methane and “erbium isopropoxide” Vacuum gauge Substrate Pump Vacuum chamber Matching unit RF power 13.56 MHz 1. Erbium tris Heating Ar CH4 2. Erbium tris Heating Ar Fig. 3: Schema of the PACVD set-up used for deposition of carbon layers from methane and “erbium tris”. For the erbium precursor two positions (1. and 2.) were experimentally evaluated. (similar photoluminescence had already been reported in [17]). Obviously, photoluminescence at 1530 nm (PL band 4I15/2� 4I11/2) did not occur. The probable reason is that the samples contained OH groups (see Table 1) due to the gas source applied for the growing process (methane as the hydrogen source, oxygen residual contamination). OH impu- rities are known to quench Er luminescence. 3.3 Growth of erbium doped carbon thin films using magnetron sputtering The carbon layers were deposited by RF magnetron sput- tering (Balzers Pfeiffer PLS 160) using a carbon target of 99.9999 % purity. The RF magnetron sputtering set-up is depicted in Fig. 5. The typical growth parameters were: deposition temperature 300 K, Ar or Ar � CH4 gas mixture, total gas pressure from 1 Pa to 4 Pa, and the growing time ranged from 30 min to 4 hrs, with power from 50 W to 150 W (13.56 MHz). For Er3� doping into the carbon samples, pow- der of Er (99.9 % purity) was placed on top of the carbon target. The RBS analyses and also the ERDA analyses of the de- posited layers proved that the samples contained carbon, ar- gon, oxygen, hydrogen and erbium (see Tab. 1). The list of carbon samples and the relevant content of erbium or erbium ions is given in Table 2. The amount of incorporated erbium ions differed depending on the area of the target covered by erbium powder placed on the target, and also depending on the erosion area represented by the part of the surface cov- ered by the erbium ions. The samples had a total erbium con- tent ranging from 1 at % to 17.1 at %. The highest concentra- tion (17.1 at %) of erbium was found in the sample doped using 1.0004 g of erbium powder. © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 39 Acta Polytechnica Vol. 48 No. 1/2008 500 550 600 650 700 750 800 0 200 400 600 800 1000 1200 1400 1600 582 nm P L In te n s it y (a .u .) Wavelength (nm) 645 nm � ex = 488 nm T= 300 K Fig. 4: Photoluminescence spectrum of a carbon layer fabricated by PACVD Window Substrate Pump Ar Cooling water SS N C-target Vacuum gauge Vacuum chamber Er powder Fig. 5: The schema of magnetron sputtering set-up The absorbance spectrum of erbium doped carbon layers fabricated by magnetron sputtering is given in Fig. 6. This fig- ure shows typical strong 2H11/2 (523 nm) transitions, which correspond to optical materials doped with erbium ions. Other Er3� transitions 4F11/2 (451 nm) and 4F7/2 (490 nm) are not so strong, and transitions 2H9/2 (405 nm), 4F3/2 (441 nm) and 4S3/2 (539 nm) are almost hidden in the background. Photoluminescence spectra of erbium doped carbon lay- ers fabricated by magnetron sputtering are shown in Fig. 7 and Fig. 8. The spectrum excited by an Ar laser at very low temperature (�ex � 514.5 nm, temperature 4 K) is given in Fig. 7 and shows quite well the typical photoluminescence bands attributed to the erbium transition 4I13/2� 4I15/2. Fig. 8 shows the spectrum obtained by optical pumping at 980 nm at room temperature. Obviously, the peak coming from the erbium 4I13/2� 4I15/2 transition is very weak but it is still there, as can be seen from the comparison with the PL spec- trum of the reference sample given in the same figure. The intensities of the emission bands in the 1530 nm re- gion did not show any evidence of being connected with the actual concentration of Er3� in the samples (see Table 2). The intensities are very low, so a small increase would be difficult to recognize. However, our opinion is that the higher concen- tration of Er3� resulted in the formation of clusters that, due to pair interactions, would in fact have a counterproductive concentration quenching effect. 4 Conclusion We report on carbon layers doped with erbium ions de- posited by sputtering, by PACVD and by magnetron sputter- ing onto a silicon or glass substrate. The fabricated samples had an amorphous structure. When the carbon samples were fabricated by sputtering the content of erbium ions was too low to reveal the photoluminescence at 1 530 nm. Carbon layers fabricated by PACVD using a CH4� Ar � “erbium isopropoxide” gas mixture did not even contain erbium. Erbium doped carbon samples fabricated by PACVD using “Erbium tris” as erbium source contained erbium ions but no photoluminescence at 1530 nm was observed. The main reason could be that the carbon sam- ples contained oxygen and hydrogen. OH impurities are known to be a strong quencher of Er luminescence. The most successful method for fabricating erbium doped carbon layers was magnetron sputtering. The content of erbium ions in the carbon samples was controlled by the amount of erbium powder placed on the carbon target. Un- der optical pumping at 514.5 nm (4 K) the typical photolu- minescence emission due to the transition of 4I13/2� 4I15/2 was observed. This emission, though rather weak, was ob- served even at room temperature when generated by optical pumping at 980 nm. 40 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 1/2008 Samples Doping Er (at %) #1 Reference sample * … #2 Er mEr � 0.0011g ** 1.2 #3 Er mEr � 0.0012g ** 1.7 #4 Er mEr � 0.0106g ** 4.8 #5 Er mEr � 0.0104g ** 6.0 #6 Er mEr � 0.5004g ** 8.4 #7 Er mEr � 0.5004g ** 9.0 #8 Er mEr � 0.5004g ** 12.5 #9 Er mEr � 1.0004g ** 15.7 #10 Er mEr � 1.0004g ** 17.1 * Sample without Er doping ** weight of Er powder placed on top of the carbon target Table 2: Composition of erbium-doped carbon samples fabri- cated by magnetron sputtering, showing the relations between the amounts of erbium used for doping and the actual concentrations of erbium found in the doped samples Fig. 6: Absorbance spectra of Er3� doped carbon layers Acknowledgments Our research is supported by the Grant Agency of the Czech Republic under grant number 102/06/0424 and by re- search program MSM6840770014 of the Czech Technical University in Prague. 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Thin Solid Films, Vol. 270 (1995), No. 1–2, p. 160–164. Ing. Václav Prajzler, Ph.D. e-mail: xPrajzlv@feld.cvut.cz Doc. Ing. Zdeněk Burian, CSc. e-mail: Burian@fel.cvut.cz Ing. Vitězslav Jeřábek, CSc. e-mail: jerabek@fel.cvut.cz Department of Microelectronics Czech Technical University in Prague Faculty of Electrical Engineering Technická 2 166 27 Prague, Czech Republic Doc. Ing. Ivan Hüttel, DrSc. e-mail: Ivan.Huttel@vscht.cz RNDr. Jarmila Špirková, CSc. e-mail: Jarmila.Spirkova@vscht.cz Institute of Chemical Technology Technická 5 166 27 Prague, Czech Republic Ing. Ján Gurovič e-mail: Jan.Gurovic@fs.cvut.cz Department of Physics Czech Technical University in Prague Faculty of Mechanical Engineering Technická 4 166 07 Prague, Czech Republic Ing. Jiří Oswald, CSc. e-mail: Oswald@fzu.cz Institute of Physics Czech Academy of Sciences Cukrovarnická 10 162 53 Prague, Czech Republic RNDr. Jiří Zavadil, CSc. e-mail: Zavadil@ure.cas.cz Institute of Radio Engineering and Electronics Academy of Sciences Chaberská 57 182 51 Prague, Czech Republic Vratislav Peřina e-mail: Perina@ujf.cas.cz Institute of Nuclear Physics Czech Academy of Sciences 250 68 Řež near Prague, Czech Republic 42 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 1/2008