Molecular beam epitaxy of laterally structured lead chalcogenides for the fabrication of buried heterostructure lasers

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Abstract
Buried heterostructure (BH) lasers with active layers of PbSe, PbEuSe, PbSrSe and PbSnSe were made using molecular beam epitaxy (MBE). After growing the active layer the growth was interrupted. By use of photolithography and Ar+ ion beam etching, mesa stripes were defined and overgrown by a second MBE step. The overgrowth was investigated employing scanning electron microscopy (SEM). A selective chemical etch was used to resolve areas of different composition. The SEM pictures indicate the crucial role of the mesa shape and surface quality for the overgrowth process. Sharp edges result in deep cracks or grooves beside a well grown ridge. The authors successfully employed a chemi-mechanical polish after the ion milling process. The obtained smooth mesa shape results in a better overgrowth. As an alternative approach for fabrication of BH lasers shadow-masked growth of the active mesa stripe was investigated. Masks with slits in the 15 mu m range and sharp edges were obtained by anisotropic etching of (100)Si BH lasers with an active stripe width of 20 mu m were made by this technique and showed a threshold current comparable to double heterostructure (DH) lasers. Lateral structuring improved BH laser far-field distributions compared with DH lasers. The observed far-fields can be explained by a waveguide model.