Basic Physics of NEA Operation

NEA photocathodes are photoconductive semiconductors whose surface has been treated to obtain a state of negative electron affinity . This state is reached when the energy level of an electron at the bottom of the conduction band in the bulk of the semiconductor is greater than the zero energy level of an electron in the vacuum. Hence an electron excited to the conduction band within the bulk can, if it travels to the activated surface without first recombining, energetically fall out of the photocathode into free space. 

NEA surfaces differ from classical photoemissive surfaces in that conduction band electrons require no excess thermal energy above to escape. It is a cold electron emission device, while in classical positive electron affinity surfaces a small barrier is present at the surface and either hot electron escape or tunneling of thermalized electrons is required for emission. NEA and classical electron emission are contrasted in Fig. 5.8 [5.67]. Part (a) is a classical emitter, (b) is a p-type semiconductor treated to obtain NEA, and (c) is an n-type semiconductor similarly treated but without attaining NEA. (Many details of this figure are clarified in later sections.) 

As shown for the p-type NEA surface (Fig. 5.8b), electrons at the bottom of the conduction band at the surface do not have energy greater than or that required to escape into the vacuum the very surface is similar to that of a classical, positive electron affinity photoemitter (Fig. 5.8a). But there is a region in the bulk beyond x = Xgg where the energy of even a thermalized electron at the bottom of the conduction band exceeds the vacuum potential. This is the condition of NEA and it does not occur in classical emitters. 

A full understanding of the NEA figure involves at least the following three facts relating to cesiated surfaces 1) most NEA surfaces do exhibit a finite upward discontinuity in the conduction band (a heterojunction energy barrier) at the surface [5.65,69] 2) the real value of Xs is independent of the bulk doping 

The attainment of NEA requires selection of a surface coating which has both small work function and a small barrier introduced at its interface with the semiconductor active layer. Several coatings have been investigated to date [5.49,71-74], but thus far the best surface coating in use is CS2O [5.75], normally of monolayer dimension and deposited on a separate initial monolayer of Cs. A Cs coating by itself can, however, activate GaAs nearly to full NEA, as discussed below. 

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