Electronic properties of diamond surfaces - blessing or curse for devices?

For the potential electronic applications of diamond as a wide band gap semiconductor, the properties of its surfaces are of fundamental importance. These properties in general depend on the kind of passivation of the surface dangling bonds, either by chemisorbed adsorbates or by the formation of mutual chemical bonds as a consequence of surface reconstruction. The principal crystallographic surfaces, (100), (111) and (110), are essentially understood in their adsorbate free form in terms of surface states and reconstructions. The corresponding results are briefly summarized in Section 2. The consequences of adsorbate passivation are then discussed for the case of hydrogen as the most important radical for surface termination with emphasis on different aspects. Hydrogen passivation leads to a negative electron affinity due to a dipole layer which is induced by the heteropolar carbon-hydrogen bonds of the surface atoms. This aspect is discussed quantitatively in Section 3. Hydrogenation also reduces the amount of sp(2) bonded carbon at the surface, which turns out to be the crucial parameter determining the position of the surface Fermi level and thus the surface band bending in diamond. This aspect is covered in Section 4. Finally, the combination of the electron affinity reduction by hydrogen and the supply of electrons at the Fermi level by sp(2) bonded carbon phases has important consequences for the low threshold electron emission into vacuum. This is dealt with in the framework of an inhomogeneous emission model in Section 5. The model can be expected to be applicable for all system containing sp(2) bonded or graphitic phases together with an sp(3) bonded diamond matrix, especially for CVD diamond, and it should have important implications also for field emission from diamond films. (C) 2000 Elsevier Science S.A. All rights reserved.