Field emission gun
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A field emission gun consists of a sharply pointed tungsten tip held at several kilovolts negative potential relative to a nearby electrode, so that there is a very high potential gradient at the surface of the tungsten. Charge density becomes so great on the tungsten tip that electrons are stripped from the material and can be caught in phosphor, revealing part of its crystalline structure as shadows on the screen. Miller indexes can be determined by means of these shadows.
In field emission electron microscopes, a field emitter tip is used to produce an electron beam that is smaller in diameter, more coherent and with up to three orders of magnitude greater current density or brightness than can be achieved with conventional thermionic emitters such as tungsten or lanthanum hexaboride (LaB6)-tipped filaments. The result in both scanning and transmission electron microscopy is significantly improved signal-to-noise ratio and spatial resolution, and greatly increased emitter life and reliability compared with thermionic devices.
Field emitter tips for electron microscopy are either of cold-cathode type, usually made of single crystal tungsten sharpened to a tip radius of about 100 nm, or of the Schottky type, in which field emission from the tip is thermally assisted. Schottky field emitters are made by coating a tungsten tip with a layer of zirconium oxide, which has the unusual property of increasing in electrical conductivity at high temperature.
