(Negative) Electron Affinity of AlN and AlGaN Alloys
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INTRODUCTION Wide bandgap semiconductors have the potential of exhibiting a negative electron affinity (NEA). These materials could be key elements of cold cathode electron emitters which could be used in applications that include flat panel displays, high frequency amplifiers, and vacuum microelectronics. The surface conditions have been shown to be of critical importance in obtaining a negative electron affinity on diamond surfaces.[1,2,3,4] In this paper, angle resolved UVphotoemission spectroscopy (ARUPS) is used to explore this effect on AIN,[5] GaN and AlGaN alloy surfaces. The value of UV photoemission in characterizing electron emission is that the technique emphasizes effects of the emission process. To fully characterize electron emission properties it is necessary to also employ additional measurements such as field emission, and secondary electron emission. The measurements are interpreted with the help of theoretical calculations. Measurements of field emission from AN on 6H-SiC are presented to demonstrate the device potential of the materials. The electron affinity of a semiconductor is defined as the energy required to remove an electron from the conduction band minimum to a distance macroscopically far from the semiconductor (i.e. away from image charge effects.). At the surface this energy can be shown schematically as the difference between the vacuum level and the conduction band minimum. The electron affinity is not, in general, dependent on the Fermi level of the semiconductor. Thus while doping can change the Fermi level in the semiconductor and the work function will change accordingly, the electron affinity is unaffected by these changes. An alternative view is that the electron affinity is a measure of the heterojunction band offset between the vacuum and a semiconductor of interest. For most semiconductors, the conduction band minimum is below the vacuum level and electrons in the conduction band are bound to the semiconductor by an energy equal to the electron affinity. In some cases, surface conditions can be obtained in which the conduction band minimum is above the vacuum level. In that case, the first conduction electron 777
Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
would not be bound to the sample but could escape with a kinetic energy equal to the difference in energy of the conduction band minimum and the vacuum level. This situation is termed a negative electron affinity. (Note that the electron is still bound to the vicinity of the sample by coulomb forces.) The electron affinity or work function of a material is usually ascribed to two aspects of the material: (1) the origin of the atomic levels, and (2) the surface dipole due to the surface termination.[6] These effects are shown schematically in Fig. 1. The atomic levels are more or less intrinsic to a material and cannot be changed. This is not the case for the surface dipole. The surface dipole can be substantially affected by surface reconstructions and surface adsorbates. Recent results on di
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