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The bare surface barrier heights, obtained by simulation using the transient responses, are plotted against AlGaN barrier layer thickness. The solid line is a guide to the eye, to highlight the saturation of the surface barrier for higher AlGaN layer thickness. The inset shows the layer structure for the heterostructure samples used in this study.
2DEG density and BSBH for heterostructures plotted against thickness of the AlGaN barrier layer (with 35% Al alloy composition).
Schematic band diagram showing the surface donor states as the origin of the 2DEG density at the interface. As the surface barrier increases, the positive charge at the surface (this is separate from the negative polarization charges at the surface), due to ionization of the surface donors (and de-occupation of the acceptor states, if any) increases, increasing the 2DEG. and are the ionized donor charge densities, and and are the occupied acceptor charge densities.
2DEG density plotted against BSBH for AlGaN heterostructures with varying AlGaN thickness. The slope of the least-squares fit line gives an approximate value of the density of surface donor states. Here is in , and is in eV. Comparison of the surface donor density from the present work, and the surface state density from Ref. 11, as a function of energy from the conduction band, is also shown (dotted lines).
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