Atomic-scale study of Mn- and Sb-containing III-V semiconductor nanostructures

Cross-sectional Scanning Tunneling Microscopy (X-STM) is used to study Mn and Sb containing III-V semiconductor materials. In this technique the cleavage plane of a nanostructured semiconductor material is imaged with an STM tip at the atomic scale revealing the structural and the electronic properties of the embedded nanostructures. Mn doped III-V semiconductors have been extensively explored in the last decade because of their ferromagnetic properties. Sb is often used as a surfactant to assist in the formation of nanostructured materials or to open special wavelength areas in photonic applications. Both Mn and Sb show a complex behavior and often ill understood behavior during growth. In this study we have used XSTM to explore a number of issues related to the incorporation and behavior Mn and Sb during the growth of nanostructured materials. Thin GaMnAs/(Al,Ga)As superlattices have been predicted to have an enhanced Curie temperature compared to bulk GaMnAs. However X-STM measurements on thin GaMnAs/(Al,Ga)As multilayers structures have shown that even if the nanostructures are grown at low-temperature (250ºC), about 20% of Mn atoms segregate into the (Al,Ga)As barriers putting a serious constraint in the realization of an enhanced Curie Temperature. Recently InMnSb has been explored as a new semiconductor material with ferromagnetic properties at room temperature. This material exhibits ferromagnetism at room temperature and has a Curie temperature exceeding 500ºC but several material related questions such as phase purity have been raised. The X-STM study of InMnSb revealed that the crystal phase is purely zinc-blend with no evidence of a second phase or cluster formation. The Mn distribution inside the InSb is fully random beyond the X-STM resolution for observing individual impurities, which is about 3 nm. Well-resolved percolation pathways due to the random Mn distribution in InSb were revealed for the first time. The percolative pathways are suggested to be responsible for the observed ferromagnetism at room temperature. In order to create spintronic semiconductor materials that show magnetic behavior at room temperature, MnAs nanoclusters have been embedded in a semiconductor host. X-STM measurements have shown that the MnAs clusters in GaAs have a hexagonal shape and that the Mn content has a strong influence on the size of the nanoclusters. The GaAs matrix exhibits a perfect zincblende crystal phase around the clusters while Mn concentration around the clusters is much smaller than the intended concentration indicating that Mn efficiently migrates during the last annealing step to form the clusters. The influence of Sb on the growth AlAsSb/InGaAs quantum wells has been investigated. The effect of surface termination of the interface layers has been studied. It was proven that the sharpness of the interface is uncorrelated with the conditions of the surface termination. Every AlAsSb barrier shows a steep onset but a gradual Sb profile into the capping layer. Furthermore a comparison has been made between InGaAs barriers grown by the so-called digital alloy method or by a conventional MBE method. The samples grown by the so-called digital alloy method show degraded structural properties but a more intense PL emission than the samples where the InGaAs barriers were grown with a conventional MBE method. X-STM measurements have shown that digital InGaAs alloy exhibit periodic composition modulation in the direction lateral to the growth with a periodicity of about 10 nm. This lateral modulation leads to a better confinement of the carriers and this results in a higher recombination rate due to a better overlap of the electron-hole wavefunctions. Nominally symmetric GaAsSb/InGaAs quantum cascade lasers were also investigated by X-STM. PL emission of these lasers showed different spectra depending on the direction of the current flow. X-STM measurements have shown that the profile of the Sb distribution is steep on the onset of the GaAsSb layers but less sharp at the interface with the capping layer. Also an asymmetric strain distribution is observed in the GaAsSb layers, which is due to segregation of Sb into the InGaAs layers that lowers the Sb concentration in the GaAsSb layers. Both effects break the symmetry of the structure of the layer structure and results in the observed current polarity effects in the PL spectra. The effect of the substrate orientation on the growth of InAs in GaAsSb was explored. Quantum dot formation was observed when InAs was deposited on a 311B GaAsSb surface However, the deposition of InAs on a 100 GaAsSb surface leads only to the formation of a wetting layer. This is due to the curious role of Sb in semiconductor growth