Optical Transmission And Reflection Of Freestanding Filament Assisted CVD Diamond Films

Abstract

Results of room-temperature optical studies on A¢â‚¬â€?10 micron free-standing diamond films are reported. The films were grown on Si(100) substrates by hot filament-assisted chemical vapor deposition (CVD) from a methane/hydrogen mixture. The as-grown, free surface of the film exhibits a surface roughness of scale 0.2-5 microns, depending on the methane/hydrogen ratio of the growth gas mixture, which introduces significant optical scattering losses for frequencies greater than 0.5 eV. Reflection and transmission spectra in the range 0.01-10 eV were collected for films grown in different methane/hydrogen mixtures. Below the threshold for interband adsorption, the film behaves approximately as a thin parallel plate of refractive index 2.4, with the rough free surface leading to increasingly larger loss of specular transmission/reflection with decreasing wavelength (λ). For λ>s, where s is the average scale of the surface roughness, distinct interference maxima are observed, and the data in this region can be analyzed to determine the refractive index and film thickness. Structure associated with absorption from one-, and multi-phonon processes and chemisorbed hydrogen are also observed. Near 5.3 eV the onset of interband adsorption is observed, in good agreement with the value of the indirect bandgap in crystalline type IIa diamond. The films are found to exhibit optical properties similar to that of bulk diamond. However, the surface roughness must be better controlled by the deposition process, if these CVD films are to be used in applications as protective, high refractive index optical coatings.