Field emitter arrays (FEAs) are a promising class of cold cathode electron sources with applications in RF amplifiers, terahertz sources, lithography, imaging, and displays. FEAs are yet to achieve commercial success because of serious challenges which have limited their viability in systems that require advanced electron sources. We identified four major challenges that posed significant barriers to the application of previous field emitter arrays in systems. These challenges are (1) charge injection and breakdown of the insulator between the emitter and the extraction gate, (2) Joule heating and thermal runaway at the emitter tip, (3) back-ion bombardment resulting in emitter tip damage (4) large capacitance and stored energy between the gate and the substate that limits switching performance.
In this thesis, we address these challenges with a new device architecture that consists of a sharp silicon emitter atop a silicon nanowire embedded in a dielectric matrix of SiO2 and SiNx. The 10-μm tall, 200-nm diameter silicon nanowire limits current and improves reliability through velocity saturation and the pinch-off of majority carriers. The 2-μm thick SiO2 insulator between the gate and the substrate and the conformal dielectric matrix that embeds the nanowire current limiters prevents charge injection and minimizes the capacitance between the gate and the substrate. Since the nanowire current limiter is fabricated directly underneath each field emitter, we maintain an emitter density of 10^8 emitters/cm2, enabling high current density. The design of the anode prevents tip erosion from back-streaming ions.
These arrays demonstrate consistent current scaling of array sizes from a single emitter to 25,000 emitters, low voltage (V_GE < 60 V), high current density (J > 100 A/cm2), and long lifetime (t > 100 hours @ 100 A/cm2, > 300 hours at 100 mA/cm2). The current density enabled by our device structure is an improvement of > 10x over state-of-the art (~ 1-10 A/cm2) for Si field emission cathodes operated in a direct current mode. Device demonstrated a turn-on voltage as low as 8.5 V. This low-voltage enables operation in a 500 Torr He ambient with an anode-emitter voltage below the first ionization potential of He (~ 19 V). These high current, high current density, long lifetime cold cathodes could enable new approaches to x-ray imagers, RF amplifiers, THz sources, and deep UV sources.
Prof. Akintunde I. (Tayo) Akinwande (Thesis supervisor)
Provost Martin A. Schmidt
Prof. Karl. K. Berggren