From where we know that the CB edge of CdS is upper than the CB of ZnO. Where will be the zero level? What factor decides this band edges? What is the fermi level and vacuum level of these?
There are several methods for drawing the band diagrams of hetero-junctions. Most popular is the Anderson model. In this model, to draw a band diagram of a hetero-junction between two materials, you should know the work function and electron affinity of the materials. The work function gives you the difference between the Fermi level and the vaccum level while electron affinity gives you the difference between the vacuum level and the edge of the conduction band. So these two parameters will setup the conduction band edge and Fermi levels of the materials w.r.t vacuum level. Once you get this, you can further locate the edge of valance band as the difference between the edges of conduction band and valence band is nothing but the band gap of the material.
If you want to evaluate band bending at the interface, then you will have to know the charge carrier densities on either side.
These methods are just rough tools. The success of these methods vary from one hetero junction to the other. The actual band structure can only be determined experimentally.
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