Diagnosis of the optical properties and structure of lanthanum hexaboride thin films

Abstract

Growth and surface properties of lanthanum hexaboride crystals J. Vac. Sci. Technol. 21, 952 (1982); 10.1116/1.571872 Structure of lanthanumhexaboridecoated rhenium filaments

Borides in Thin Film Technology

“…The application of triode sputtering (30,32,40), dc (4,5,22,33,38,(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61) and rf magnetron sputtering (31,(34)(35)(36)(37)(38)(39)(40)(41)(42)(43), electron-beam evaporation (20)(21)(22)(23)(24)(25)(26)(27), and arc evaporation (28,29) for the deposition of boride coatings is reported in the literature. Because of the low electrical and thermal conductivity of boron (22) and because of the high reactivity of certain rare-earth metals with oxygen (e.g., lanthanum and cerium), most PVD processes make use of the corresponding boride as the vapor source in...…”

“…However, since stress relaxation is not possible, the highest compressive stresses with values up to 2.7 GPa are reported for this film type (42). Consequently, these high stress levels impose strong requirements on the adhesion strength of the films, which may not be satisfied in every case (6,21,27,28,34,50,51,(55)(56)(57).…”

“…1a and 1c). Typical grain sizes for this structure type revealed from X-ray diffraction (XRD) measurements on ZrB -and LaB -based films were found to be below 40 nm for electron-beam evaporation (21) and below 20 nm (21,30,48) for sputtering. In the complementary growth region, i.e., extremely low or high energetic contribution (e.g., high argon pressure, low substrate temperature or high bias voltage, respectively) and due to the addition of nitrogen using reactive PVD processes, the structure of the coatings becomes extremely fine-grained to amorphous (see Figs.…”

“…2), other rare-earth boride phases could not be detected. For crystalline films, the preferred orientation of the hexaboride phase is (100) (6,21,22,26,34,41,42,50,51,(55)(56)(57)(58)(59). With increasing kinetic energy of the condensing target atoms, a change in the texture from (100) to (110) and (111) is observed (6,41,45,55,(57)(58)(59), which is related to different contributions of the strain and surface energy of the films.…”

“…Figure 7 shows the spectral reflectivity calculated from ellipsometric data for boride coatings and for the corresponding bulk hexaborides. Due to the highly disordered microstructure of these coatings, their electronic and, consequently, their optical properties differ from those of the bulk values (21,30,(86)(87)(88)(89). Structural defects like interstitials, vacancies, and impurities which are typical for coatings grown by low-temperature PVD processes are expected to affect the shape of the plasma oscillation edge in rare-earth hexaborides (90).…”