MEASUREMENTS OF VACUUM FIELD-EMISSION FROM BIO-MOLECULAR AND SEMICONDUCTOR-METAL EUTECTIC COMPOSITE MICROSTRUCTURES

We present designs, calculations, and initial data from a program investigating the use of bio-molecular and eutectic composite microstructures as vacuum field emission cathodes. Calculations, supported by the initial data, indicate that these electron sources should be capable of macroscopic beam current densities J greater-than-or-equal-to 100 A/cm2 without the formation of a surface plasma. The absence of a plasma allows these cathodes to operate as a long-pulse to dc electron sources. These composite materials are processed to produce surface structures which consist of rod-like tips protruding from a uniform base, where the protruding rods are approximately 0.5-mu-m in diameter by approximately 10-mu-m tall, with an average surface density of > 10(6) tips/cm2. Such structures are essentially microanalogs of the velvet cathodes which have found widespread use in the exploding field-emission cathode domain. However, unlike the velvet cathodes, the aim of the present research is the controlling of the surface emission and preventing the formation of surface plasma. The present designs seek to avoid the formation of surface plasma by forcing the current emitted from the tips to transit a thin layer of silicon. The natural high-field current-limiting nature of the silicon serves to prevent the ablation of the tip structure and the subsequent formation of cathode plasma. Initial data obtained with these microstructures has demonstrated Fowler-Nordheim characteristic emission. With a final surface coating of amorphous, p-doped silicon, the bio-molecular composite cathodes have demonstrated stable dc (10 s, +/- 10% p-p) current densities of approximately 100 mA/cm2, and the eutectic composites cathodes have demonstrated similarly stable dc current densities of approximately 1 A/cm2. It is anticipated that using crystalline or polycrystalline n-type silicon coatings can increase these values to greater than 100 A/cm2.