Nature of millimeter wave losses in low loss CVD diamonds

The CVD diamonds dielectric loss mechanisms at the millimeter range of wavelengths are researched. The loss tangent tan 8 at frequencies fcongruent to50-200 GHz and temperatures tcongruent to20-500 degreesC was measured. The loss tangent frequency dependence (at room temperature) is similar to1/f at frequencies <140 GHz, due to electric conductance in non-diamond inclusions, and deviates upwards at f>140 GHz. From the comparison to the theory it follows that this deviation can be interpreted as exhibition of the nonintrinsic lattice loss connected with the lattice disorder induced one-phonon excitation of acoustic vibrations. The loss increase at temperatures >400 degreesC is sharper compared to the theoretical dependence for two-phonon intrinsic lattice loss (ILL). It can be connected with the loss due to dipole relaxation (of Debye type) with activation energy similar to0.56 eV. It was confirmed that the theoretical lower loss limit (determined by the ILL) in diamond is extremely low (particularly, at ambient temperatures, at least one order of magnitude lower than the lowest observed loss). (C) 2003 Elsevier Science B.V. All rights reserved.