Thickness measurement of Bi₂Sr₂CaCu₂O_8+δ single crystals via EDAX for terahertz device fabrication

Mesas of stacked ‘intrinsic’ Josephson junctions, lithographically patterned from the high-T_c superconducting cuprate Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) are a promising technology for filling the well-known ‘terahertz gap’. The range of frequencies from 0.3 to 2.0 THz is not well served by existing coherent sources. Due to the low thermal conductivity of Bi-2212, the THz emission characteristics of Bi-2212 mesas are strongly affected by the thickness of the base crystal on which they stand. Cleaving Bi-2212 crystals yields base crystal thicknesses randomly distributed over a range of 10 μm or more, often with significant thickness variation across the area of the crystal. However, if the base crystal can be ion milled to a thickness of 0.5 μm or less, then THz emission performance and reproducibility of the resulting sources can be strongly enhanced. Achieving this requires a method for probing the crystal thickness at the specific location where a device will be patterned.

 

We find that Bi-2212 crystal thicknesses can be readily determined via energy-dispersive X-ray spectroscopy (EDAX). If precise results are required, it is important to correct for electron beam current drift using a Bi-2212 reference sample, and it is also important to correct for any angular misalignment of the crystal surface.