@article{porto_boyanov_sayers_nemanich_1999, title={Cobalt suicide formation on 6H silicon carbide}, volume={6}, ISSN={["0909-0495"]}, DOI={10.1107/S0909049599001326}, abstractNote={Cobalt films (1, 25 and 100 A) have been directly deposited on top 6H-SiC(0001) wafers by molecular beam epitaxy and annealed at 500-800°C in UHV. The structure of the metal-semiconductor interface was investigated by XAFS. The results show that Co-Si bonds were preferentially formed in the 1 A Co films. In the 25 and 100 A Co films only Co-Co bonds were identified. The XRD pattern of the 100 A Co film exhibits a Co (200) peak confirming the presence of unreacted metal even after annealing at 800°C.}, journal={JOURNAL OF SYNCHROTRON RADIATION}, author={Porto, AO and Boyanov, BI and Sayers, DE and Nemanich, RJ}, year={1999}, month={May}, pages={188–189} }
 @article{newville_boyanov_sayers_1999, title={Estimation of uncertainties in XAFS data}, volume={6}, ISSN={["0909-0495"]}, DOI={10.1107/S0909049598018147}, abstractNote={The estimate of measurement uncertainties in XAFS spectra is one of the most important and difficult aspects of error analysis for XAFS data.Since several different analysis methods are used to determine structural parameters and their uncertainties from XAFS data, and no universally agreed-upon standard exists, the ability to compare estimates of uncertainties from different methods is vital.We discuss numerical aspects of the propagation of measurement uncertainties from raw absorption coefficient to analyzed signal in k-and R-space.We focus our attention on the conversion of measurement uncertainties between k-and R-space representations of XAFS data, and on the testing of this conversion procedure for data dominated by random fluctuations.}, journal={JOURNAL OF SYNCHROTRON RADIATION}, author={Newville, M and Boyanov, BI and Sayers, DE}, year={1999}, month={May}, pages={264–265} }
 @article{goeller_boyanov_sayers_nemanich_myers_steel_1999, title={Germanium segregation in the Co/SiGe/Si(001) thin film system}, volume={14}, ISSN={["0884-2914"]}, DOI={10.1557/JMR.1999.0592}, abstractNote={Cobalt disilicide contacts to silicon–germanium alloys were formed by direct deposition of pure cobalt metal onto silicon–germanium films on Si(001) substrates. Segregation of germanium was observed during the reaction of the cobalt with the silicon–germanium alloy. The nature of the Ge segregation was studied by transmission electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. In the case of cobalt films deposited onto strained silicon–germanium films, the Ge segregation was discovered to be in the form of Ge-enriched Si1−xGex regions found at the surface of the film surrounding CoSi and CoSi2 grains. In the case of cobalt films deposited onto relaxed silicon–germanium films, the Ge segregation was dependent on formation of CoSi2. In samples annealed below 800 °C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the strained Si1−xGex case. In samples annealed above 800 °C, where CoSi2 was the dominant silicide phase, the Ge segregation was also in the form of tetrahedron-shaped, Ge-enriched, silicon–germanium precipitates, which formed at the substrate/silicon– germanium film interface and grew into the Si substrate. A possible mechanism for the formation of these precipitates is presented based on vacancy generation during the silicidation reaction coupled with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.}, number={11}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Goeller, PT and Boyanov, BI and Sayers, DE and Nemanich, RJ and Myers, AF and Steel, EB}, year={1999}, month={Nov}, pages={4372–4384} }
 @article{boyanov_goeller_sayers_nemanich_1999, title={Growth of epitaxial CoSi2 on SiGe(001)}, volume={86}, ISSN={["0021-8979"]}, DOI={10.1063/1.370894}, abstractNote={A technique for achieving epitaxial growth of (001)-oriented CoSi2 on strained epitaxial layers of Si1−xGex(001) is described. The technique is based on a variation of the template method, and is designed to control the local environment of Co atoms at the CoSi2/SiGe interface. The effects of the Co–Ge interactions on the interfacial reaction and the epitaxial orientation and the morphology of the silicide film were investigated. This reaction was found to cause pitting in (001)-oriented CoSi2 films, and to stabilize the (221¯) orientation for films codeposited under conditions where CoSi2(001) growth is achieved on Si(001) substrates. The (221¯)-oriented CoSi2 films were islanded after annealing at 700 °C. The islands were terminated by (1¯11) and (110) facets inclined at 15.8° and 19.5°, respectively, from CoSi2 [221¯] towards CoSi2 [114]. These results were interpreted in terms of reduction of interfacial and surface energies, and geometric effects. Silicide films up to 730-Å-thick were deposited and annealed up to 900 °C. The films were stable against agglomeration, and retained tensile stress in the CoSi2 layer after annealing at 700 °C. The rms roughness of the CoSi2 films was comparable to that of the Si(001) substrate—less than 15 Å over areas as large as 20×20 μm2. Films annealed at 900°C were severely agglomerated.}, number={3}, journal={JOURNAL OF APPLIED PHYSICS}, author={Boyanov, BI and Goeller, PT and Sayers, DE and Nemanich, RJ}, year={1999}, month={Aug}, pages={1355–1362} }
 @article{maillard-schaller_boyanov_english_nemanich_1999, title={Role of the substrate strain in the sheet resistance stability of NiSi deposited on Si (100)}, volume={85}, ISSN={["1089-7550"]}, DOI={10.1063/1.369774}, abstractNote={In order to study the influence of strain on the formation and stability of NiSi, Ni has been deposited on strained and relaxed Si(100) n-type substrates. Strained Si substrates have been produced by depositing a pseudomorphic silicon film onto a 3000 Å thick relaxed Si0.8Ge0.2 film. Raman spectroscopy has established that the silicon film is strained. The presence of a characteristic cross-hatch pattern has been identified by atomic force microscopy. Measurements show that the sheet resistance (Rs) of the silicide formed on strained silicon remains stable up to 700 °C while the Rs of the silicide formed on bulk silicon (100) shows a significant increase at 600 °C . X-ray photoelectron spectroscopy shows that the NiSi–NiSi2 phase transition occurs at a higher temperature and is, therefore, not responsible for the Rs instability. Scanning electron microscopy measurements indicate that islanding occurs in the temperature region of the resistivity increase. Photoelectron emission microscopy has been employed to observe the surface morphology during annealing, and islanding is not observed until a higher annealing temperature for the NiSi on strained Si. The increase in Rs is apparently correlated to the islanding of NiSi which appears at lower temperature on the bulk silicon substrate than on the strained silicon substrate. The stability of the NiSi film on the strained Si substrate is related to the strain induced by thermal expansion and the increased lattice constant of the strained Si.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Maillard-Schaller, E and Boyanov, BI and English, S and Nemanich, RJ}, year={1999}, month={Apr}, pages={3614–3618} }
 @article{boyanov_goeller_sayers_nemanich_1999, title={The effect of germanium on the Co-SiGe thin-film reaction}, volume={6}, ISSN={["0909-0495"]}, DOI={10.1107/S0909049599000060}, abstractNote={Ge Co Germanium was found to have a strong influence on the path and products of the Co-SiGe reaction, and on the interfacial stability and crystallographic orientation of the silicide film. The segregation of Ge that occurs during the reaction of blanket Co films with SiGe results in thickness effects not present in the reaction of Co with Si. The thickness effect was modelled in terms of the energy cost of Ge segregation, and good agreement with experimental results was obtained. In s i tu EXAFS experiments on sub-monolayer Co films annealed on SiGe substrates indicate a strong preference for the formation of Co-Si bonds at the silicide-SiGe interface. The implications of these results for the stability of the interface and the epitaxial orientation of co-deposited cobalt disilicide (CoSi2) films will be discussed.}, journal={JOURNAL OF SYNCHROTRON RADIATION}, author={Boyanov, BI and Goeller, PT and Sayers, DE and Nemanich, RJ}, year={1999}, month={May}, pages={521–523} }
 @article{goeller_boyanov_sayers_nemanich_1998, title={Co-deposition of cobalt disilicide on silicon-germanium thin films}, volume={320}, ISSN={["0040-6090"]}, DOI={10.1016/S0040-6090(97)00941-3}, abstractNote={The formation of CoSi2 on strained epitaxial Si0.8Ge0.2/Si(100) films has been studied as a function of the deposition method and annealing temperature. Two types of deposition processes were used: a direct method, where 5 nm of pure Co metal were deposited at room temperature onto a strained 80 nm thick Si0.8Ge0.2 layer; and a co-deposition method, where 5 nm Co and 18.2 nm Si were simultaneously deposited in a 1:2 ratio onto a strained Si0.8Ge0.2 layer at 450°C. Samples were then annealed at temperatures ranging from 500 to 800°C. Extended X-ray absorbance fine structure spectroscopy (EXAFS) and X-ray diffraction (XRD) were used to characterize the structure of the resulting films. It was found that the samples prepared via the direct deposition method did not convert to CoSi2 at any annealing temperature up to 800°C, while the co-deposited samples formed epitaxial CoSi2 at even the lowest annealing temperature of 500°C. These results are discussed in terms of proposed reaction mechanisms of the different deposition methods, based on consideration of the Co–Si–Ge ternary phase diagram.}, number={2}, journal={THIN SOLID FILMS}, author={Goeller, PT and Boyanov, BI and Sayers, DE and Nemanich, RJ}, year={1998}, month={May}, pages={206–210} }
 @article{boyanov_goeller_sayers_nemanich_1998, title={Film thickness effects in the Co-Si1-xGex solid phase reaction}, volume={84}, ISSN={["0021-8979"]}, DOI={10.1063/1.368872}, abstractNote={The thickness dependence of the reaction of cobalt with epitaxial silicon–germanium alloys (Si1−xGex) has been studied. The reaction products of Co with (100)-oriented Si0.79Ge0.21 after annealing at 800 °C depended on the thickness of the Co film. Complete conversion to CoSi2 occurred only when the thickness of the Co layer exceeded 350 Å. Interface reactions with Co layers thinner than 50 Å resulted in CoSi formation, while a mixture of CoSi and CoSi2 was formed at intermediate thicknesses. X-ray diffraction and extended x-ray absorption fine structure measurements indicated no measurable incorporation of Ge had occurred in either the CoSi or CoSi2. The threshold thickness for nucleation of CoSi2 on (100)-oriented Si1−xGex was determined in the range 0⩽x⩽0.25. The threshold thickness increased superlinearly with the Ge concentration x, and did not depend on the doping of the Si(100) substrate or the strain state of the Si1−xGex film. The observed thickness effect was attributed to preferential Co–Si bonding in the reaction zone and the energy cost of Ge segregation, which accompanies the formation of CoSi and CoSi2 during the reaction of Co with Si1−xGex.}, number={8}, journal={JOURNAL OF APPLIED PHYSICS}, author={Boyanov, BI and Goeller, PT and Sayers, DE and Nemanich, RJ}, year={1998}, month={Oct}, pages={4285–4291} }
 @article{sayers_goeller_boyanov_nemanich_1998, title={In situ studies of metal-semiconductor interactions with synchrotron radiation}, volume={5}, ISSN={["0909-0495"]}, DOI={10.1107/S0909049597015240}, abstractNote={The capabilities and performance of a UHV system for in situ studies of metal–semiconductor interactions are described. The UHV system consists of interconnected deposition and analysis chambers, each of which is capable of maintaining a base pressure of approximately 1 × 10−10 torr. The deposited materials and their reaction products can be studied in situ with RHEED, XAFS, AES, XPS, UPS and ARUPS. Results from a study of the reaction of 0.7- and 1.7-monolayer-thick films of cobalt with strained silicon–germanium alloys are presented. The signal-to-noise ratio obtained in these experiments indicates that the apparatus is capable of supporting in situ EXAFS studies of ∼0.1-monolayer-thick films.}, journal={JOURNAL OF SYNCHROTRON RADIATION}, author={Sayers, DE and Goeller, PT and Boyanov, BI and Nemanich, RJ}, year={1998}, month={May}, pages={1050–1051} }
 @article{strashilov_djordjev_boyanov_avramov_1997, title={A coupling-of-modes approach to the analysis of STW devices}, volume={44}, ISSN={["0885-3010"]}, DOI={10.1109/58.658322}, abstractNote={The dispersion characteristics of surface transverse waves (STW) on 36/spl deg/ rotated Y-cut quartz have been analyzed numerically. A closed form dispersion relation has been derived and introduced in the equations of the coupling-of-modes (COM) theory to account for the variation of energy trapping with frequency. The transmission and reflection coefficients at the finger edges have been determined from the stopband characteristics. Through the introduction of an electromechanical coupling which varies linearly with the electrode mass loading, quantitative agreement with experimental results on two different types of two-port STW resonators has been achieved, and characteristic features have been explained.}, number={3}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, author={Strashilov, VL and Djordjev, KD and Boyanov, BI and Avramov, ID}, year={1997}, month={May}, pages={652–657} }
 @article{boyanov_goeller_sayers_nemanich_1997, title={Preferential Co-SI bonding at the Co/SiGe(100) interface}, volume={71}, DOI={10.1063/1.119436}, abstractNote={The initial stages of the reaction of Co with Si0.79Ge0.21(100) were studied in situ with extended x-ray absorption fine structure spectroscopy and reflection high energy electron diffraction. The Si:Ge ratio in the first coordination shell of Co in sub-monolayer Co films was found to increase with film thickness and annealing temperature, indicating preferential formation of Co–Si bonds. The impact of the observed preference for Co–Si bonding on the morphology of epitaxial CoSi2/Si1−xGex heterostructures is discussed.}, number={21}, journal={Applied Physics Letters}, author={Boyanov, B. I. and Goeller, P. T. and Sayers, D. E. and Nemanich, R. J.}, year={1997}, pages={3060–3062} }
 @article{goeller_boyanov_sayers_nemanich_1997, title={Structure and stability of cobalt-silicon-germanium thin films}, volume={133}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(97)00458-8}, abstractNote={The phase formation and stability of CoSi2 on strained epitaxial Si0.80Ge0.20Si (0 0 1) thin films has been investigated. Silicide films prepared via direct deposition of cobalt (CoSiGe), and via co-deposition of silicon and cobalt (Co+2SiSiGe), were compared. EXAFS, XRD, and sheet-resistance measurements indicated that co-deposited Co+2Si films annealed at 400–700°C exhibit the expected low-resistivity CoSi2 structure but were susceptible to roughening, pinhole formation, and agglomeration. In contrast, the CoSiGe structure formed CoSi2 only after annealing at 700°C and silicide formation was accompanied by Ge segregation in the contact region. In situ RHEED experiments indicated that growth of CoSi2 co-deposited on SiGe at 400–500°C results in immediate island formation. Template methods, which are often used to enhance the quality of co-deposited Co+2SiSi structures, did not lead to two-dimensional growth in the Co+2SiSiGe system. In situ EXAFS measurements of 2 Å Co films deposited on SiGe substrates and annealed at 450°C suggested that the failure to achieve two-dimensional growth may be due to preferential bonding of Co to Si atoms at the interface, which prevents the formation of a continuous CoSi2 template.}, number={1-4}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Goeller, PT and Boyanov, BI and Sayers, DE and Nemanich, RJ}, year={1997}, month={Dec}, pages={84–89} }