All entries for michael swanwick

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High-Current Cold Cathodes with Temporal and Spatial Emission Uniformity

• Authors: Michael E. Swanwick, Luis F. Velásquez-García
• Sponsorship: DARPA

Field emission arrays (FEAs) are an attractive alternative to mainstream thermionic cathodes, which are power hungry and require high vacuum and high temperature to operate. Field emission of electrons consists of the following two processes: 1) tunneling of electrons through the potential barrier that holds electrons within the material (workfunction φ) when the barrier is deformed by the application of a high electrostatic field^[1] and 2) supply of electrons from the bulk of the material to the emitting surface. Either the transmission process or the supply process could be the limiting step that determines the emission current of the field emitter. Control of the transmission process (tunneling) to produce high uniform current from FEAs is very challenging due to the physics of the field emission process. Due to the exponential dependence on the field factor and, hence, the tip radius, emission currents are extremely sensitive to tip radii variation; unfortunately, nanometer-sized tip radii in FEAs have a distribution with long tails that causes severe FEA underutilization. A better approach for achieving uniform emission from nanosharp FEAs is controlling the supply of electrons to the emitting surface. In a metal, the supply of electrons is very high, making the control of the supply challenging. However, in a semiconductor, where the local doping level and the local potential determine the concentration of electrons, it is possible to configure the emitter such that either the supply process determines the emission current.

We are developing high current FEAs where each field emitter is individually ballasted using a vertical ungated field effect transistor (FET) made from a high aspect ratio (40:1) n-type silicon pillar. Each emitter has a proximal extractor gate that is self-aligned for maximum electron transmission to the anode (collector). Our modeling suggests that these cathodes can emit as much as 30 A.cm^-2 uniformly with no degradation of the emitters due to Joule heating; also, these cathodes can be switched at microsecond-level speeds. The design process flow and mask set have been completed (Figure 1). An ultra-high vacuum chamber has been built to test the devices (Figure 2). The chamber can test full 150mm wafers with four high voltage probes at 10^-10 torr pressure.



Large array of high aspect ratio pillars with 10nm radius tips with 5 µm hexagonal packing for individually ballasted field emission arrays.



UHV chamber designed and built to conduct electrical characterization of FEAs




1. R. H. Fowler and L. W. Nordheim, “Electron emission in intense electric fields,” Proc. R. Soc. Lond. A, vol. 119, no. 781, pp. 173–181, May 1928. [↩]

Next-Generation Ultrafast Photo-Triggered Cathodes

• Authors: Michael E. Swanwick, Phillip D. Keathley, Franz X. Kärtner, Luis F. Velásquez-García
• Sponsorship: DARPA

State-of-the-art ultrafast cathodes are based on the photoelectric effect, where electrons are emitted from a flat surface using ultraviolet (UV) pulses; however, these cathodes have a number of shortcomings including difficult manufacture, need for ultra-high vacuum to operate, and short lifetime^[1].  Photon-triggered field emission cathodes are an attractive alternative to circumvent these issues.  Strong-field electron tunneling from solids without damage occurs when the electric field of high-intensity optical pulses interacts with field enhancing structures to lower the incident flux necessary for barrier suppression. In this project we are using wafer-level semiconductor batch fabrication techniques to create massively multiplexed arrays of nano-sharp high-aspect-ratio silicon pillars with high uniformity (4.6-million tips.cm^-2), resulting in greatly enhanced array electron emission.  A high-aspect-ratio silicon column topped by a nano-sharp tip achieves electron emission at low power by greatly enhancing the incident electric field, and the massive multiplexing of the pillars drastically increases the total current emission. We developed a fabrication process that attains small tip variation across the array due to the diffusion-limited oxidation step that sharpens the tips, resulting in large array utilization.  As shown in Fig. 1, the high field of the ultra-short laser pulses combined with the field enhancement of the nano-sharp high-aspect-ratio silicon tip array resulted in large current emission using small laser energies. Between 25 nJ and 0.1 µJ laser pulse energy, the data follow a multi-photon absorption process; the high power dependence of the emitted current on the laser pulse energy comes from the electrons oscillating back into the tip.  For larger laser pulse energies the curve bends over, evidencing operation of the cathode in the tunneling regime^[2]; this transition occurs because the electric field becomes so strong that the electrons tunnel faster than they can oscillate back into the tips. We have also demonstrated long-term operation without degradation for 3.6 nA average current from an array of about 2,220 tips^[3].



Log-log plot of emitted current versus incident laser pulse energy for 10 V anode voltage^[4]



Photo-emitted current versus time. Top green line is 9.5 µJ with 1000 V anode bias, middle red line is 9.5 µJ with 500 V anode bias and the bottom blue line is 3.0 µJ with 500 V anode bias^[5]




1. Q. Guo,  K. Takahashi, K. Saito, H. Akiyama, T. Tanaka, and M. Nishio, “Band alignment of ZnTe/GaAs heterointerface investigated by synchrotron radiation photoemission spectroscopy,” Applied Physics Letters, vol. 102, no. 9, pp. 092107–092107–4, Mar. 2013. [↩]
2. M. Swanwick, P. D. Keathley, F. X. Kärtner, and L. F. Velásquez-García, “Ultrafast Photo-Triggered Field Emission Cathodes Using Massive, Uniform Arrays Of Nano-Sharp High-Aspect-Ratio Silicon Structures”, Technical Digest of the 17^th International Conference on Solid-State Sensors, Actuators, and Microsystems, Barcelona, Spain, pp. 2680 – 2683, June 16 – 20 2013. [↩]
3. M. Swanwick, P. D. Keathley, F. X. Kärtner, and L. F. Velásquez-García, “Nanostructured Silicon Photo-cathodes for X-ray Generation”, Technical Digest of the 26^th International Vacuum Nanoelectronics Conference, Roanoke VA, July 8 – 12 2013. [↩]
4. [M. Swanwick, P. D. Keathley, F. X. Kärtner, and L. F. Velásquez-García, “Ultrafast Photo-Triggered Field Emission Cathodes Using Massive, Uniform Arrays Of Nano-Sharp High-Aspect-Ratio Silicon Structures”, Technical Digest of the 17th International Conference on Solid-State Sensors, Actuators, and Microsystems, Barcelona, Spain, pp. 2680 – 2683, June 16 – 20 2013. [↩]
5. M. Swanwick, P. D. Keathley, F. X. Kärtner, and L. F. Velásquez-García, “Nanostructured Silicon Photo-cathodes for X-ray Generation”, Technical Digest of the 26th International Vacuum Nanoelectronics Conference, Roanoke VA, July 8 – 12 2013. [↩]