Scanning tunneling microscopy of compound semiconductor heterostructures: From alloy ordering to composition determination.

摘要

Cross-sectional scanning tunneling microscopy (XSTM) has been employed to explore various material properties of III-V compound semiconductor heterostructures. Regarding GaInP alloys, the (111)-type alloy ordering is observed in organometallic vapor phase epitaxy (OMVPE) grown alloy region, while the molecular beam epitaxy (MBE) grown region shows a very small degree of ordering. Two types of ordering, namely (InP)1(GaP)1 and (InP)2(GaP) 1, have been found in OMVPE grown GaInP alloys. Then, the focus is shifted to the structural properties of self-assembled InGaAs quantum dots (QDs) grown by migration enhanced epitaxy (MEE) and heterogeneous droplet epitaxy (HDE) respectively. Size, shape, orientation, spatial distribution, strain, and composition have been examined. Regarding MEE grown InGaAs QDs, the composition appears highly non-uniform, with an Indium-rich core having an inverted-triangle shape. With atomic resolution, compositional analysis has been done for both the QDs and wetting layers quantitatively. Depletion of the wetting layer, due to the formation of the QDs, is also demonstrated. For HDE grown InGaAs QDs, the size, shape, composition, and strain, etc., are quite different from that of MEE or MBE grown InGaAs QDs. Indium distribution inside the QDs is rather uniform compared with MEE grown InGaAs QDs. More importantly, it is found that the self-compensation between the size and indium concentration of the QDs appears to be the key factor that controls the sharpness of the photoluminescence (PL) linewidths in the investigated samples.