AlN and GaN epitaxial heterojunctions on 6H-SiC(0001): Valence band offsets and polarization fields

From a series of in situ photoemission experiments macroscopic electric fie lds are clearly demonstrated in SiC/AlN, SiC/GaN, and GaN/AlN heterostructu res grown by molecular beam epitaxy on 6H-SiC(0001). A significant contribu tion is due to the spontaneous polarization; the piezoelectric term alone w ould not explain the sign of the field measured in SiC/AlN. The experimenta l field has lower intensity as compared to theory: the role of electronic g ap slates at the surface is pointed out. A self-consistent tight-binding ap proach which is able to describe polarization fields, dielectric screening, and free carrier screening is applied for a more consistent theoretical di scussion of the experimental data. The valence band offset (VBO) has been d etermined for all heterojunctions under study and the apparent dependence o n the overlayer thickness, due to the presence of the strong polarization f ields, has been pointed out in view of a correct determination of the VBO. The VBOs at the heterojunctions obtained by extrapolation to zero overlayer thickness are (substrate/overlayer) : [(1.5-1.7)+/-0.1] eV for SiC/AlN, [( 0.7-0.9)+/-0.1] eV for SiC/GaN, (-0.3+/-0.1) eV for AlN/GaN, and [(0.15-0.4 )+/-0.1] eV for GaN/AlN. (C) 1999 American Vacuum Society. [S0734-211X(99)0 2704-3].