(NEGATIVE) ELECTRON AFFINITY OF AIN AND AIGaN ALLOYS R.J. Nemanich, M.C. Benjamin, S.P. Bozeman, M.D. Bremser, S.W. King, B.L. Ward, R.F. Davis, B. Chen, Z. Zhang, and J. Bernholc Department of Physics and Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-8202 ABSTRACT The electron affinity of a semiconductor defines the relationship of the vacuum level and the semiconductor band structure. It is dependent on the atomic orbitals of the material and the surface termination. We report experimental and theoretical results that support the presence of a negative electron affinity on AIN and the Al rich AlGaN alloys. The GaN surface is found to exhibit a (positive) electron affinity of 3.3eV. The experimental measurements employ UV-photoemission spectroscopy on in situ gas-source MBE samples and on CVD samples. Theoretical results indicate that the (negative) electron affinity of AIN depends sensitively on the surface reconstruction and adatom termination. The experimental dependence of the electron affinity on alloy concentration is presented. The results indicate that AlGaN alloys with band gap similar or greater than that of diamond will exhibit a negative electron affinity. Field emission results are reported, and the characteristics are similar to those obtained from a diamond film. Issues related to cold cathode electronic devices based on NEA surfaces are noted. 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 UV- photoemission 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