the movpe step may be carried out in a commercial aixtron aix200 4 reactor the growth conditions may be chosen similar to standard growth conditions for movpe growth of alas gaas as known in the art other epitaxial growth techniques and their respective growth conditions are contemplated as well then a first layer 30 is deposited on the intermediate buffer layer 10 the first layer 30 may consist of a plurality of algaas sublayers for instance the first layer 30 may comprise a graded al 0 0 9 ga 1 0 1 as sublayer 31 preferred thickness between 1 nm and 100 nm e g 30 nm an al 0 9 ga 0 1 as sublayer 32 preferred thickness between 10 nm and 100 nm e g 40 nm an alas sublayer 33 preferred thickness between 1 nm and 100 nm e g 30 nm an al 0 9 ga 0 1 as sublayer 34 preferred thickness between 10 nm and 100 nm e g 40 nm and an oppositely graded al 0 9 0 ga 0 1 1 as sublayer 35 preferred thickness between 1 nm and 100 nm e g 30 nm the growth is concluded by depositing a second layer 40 which may consist of a gaas material preferred thickness between 5 nm and 200 nm e g 100 nm the resulting layer structure is shown in fig1 referring to fig2 after the epitaxy a circular mesa structure 50 of 15 25 u03bcm diameter is defined by conventional photolithography and reactive ion etching for example the mesa height is preferably chosen such that the side facets 30.