Developments in the Lithographic Engineering of Bi₂Sr₂CaCu₂O_8+δ mesa terahertz-emitting devices

Mesas of stacked intrinsic Josephson junctions, lithographically patterned from the high-Tc superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) are one of the most promising technologies for filling the ‘terahertz gap’. Spanning from approximately 0.3 to 2.0 THz, this range of frequencies is not well served by existing coherent sources. One longstanding aim in making these devices practicable terahertz sources is to maximize the power output achieved at an operation temperature of 77.4 K or more. Due to the low thermal conductivity of Bi-2212, the performance of such devices is significantly affected by the thickness of the base crystal on which these mesas stand. Heat-sinking, phase synchronization of the stacked Josephson junctions, and device reproducibility can all be dramatically enhanced if the base crystal can be microfabricated to consistently have the minimum possible thickness. However, imprecise crystal thickness measurements due to topographic surface variation, combined with inherent imprecision in cleaving techniques makes this a nontrivial challenge. Additionally, argon ion-milling of Bi-2212 crystals to a precise thickness induces surface deoxygenation, compromising electrical contact with the underlying crystal.

To address these issues, we demonstrate that Bi-2212 crystal thicknesses can be readily and precisely determined employing energy-dispersive X-ray spectroscopy (EDAX). We also examine the effects of ion-milling at specific ion energies and the use of thermal annealing in promoting uniform oxygen diffusion from the base crystal.