Atomic scale study of impurities and nanostructures in compound semiconductors

This thesis deals with cross-sectional scanning tunneling microscopy analyses on III/V and II/VI semiconductor nanostructures and single dopants in various III/V materials. A detailed atomic scale study of the structural properties of capped InAs quantum dots and ZnSeTe/ZnTe based quantum wells and the local charge distribution around acceptor type dopants in GaAs and GaP are presented. Self-assembled InAs quantum dots are interesting objects from fundamental and technological points of view because they form nearly ideal zerodimensional systems in which quantum confinement effects become important. For example, InAs quantum dots are employed in quantum dot lasers, single electron transistors, midinfrared detectors, single-photon sources, etc. For any device application, the quantum dots are capped after growth to prevent unwanted effects occurring at the surface of uncapped dots. However, the use of capping materials such as InP, GaAs and InGaAs strongly affects the structural properties of quantum dot size, shape and composition. In chapter 3, critical issues occurring during the capping process like dot decomposition, intermixing, segregation, As/P exchange, and compositional modulation in the dot/capping layer interface have been identified on the atomic scale. In chapter 4, II/VI semiconductor heterostructures grown by MBE are investigated on the atomic level. Some peculiar effects are found which are not observed in III/V semiconductors. For ZnTe, mono-atomic vacancy chains are observed on the Te sublattice. These chains are found to be created during the cleavage process, pulling straight atomic rows out of the surface. The measured distances between these missing rows were found to vary between a few nanometers and tens of nanometers whereas their lengths go up to several hundreds of nanometers. It can be an interesting breakthrough to use these vacancy chains as templates to create 1D magnetic quantum wires with a cross section of only one atom by incorporating magnetic ions (e.g. Fe, Mn or Co) into these vacancy chains. Atom manipulation as a result of applying positive bias voltages to the sample is observed on ZnTe. In this process, atoms are randomly pulled out of the surface by STM tip. Moreover, the quantitative extent of the atom manipulation appears to increase with decreasing bias voltage while maintaining the constant tunnel current. Atomic vacancy chains in particular are determined to be highly sensitive to the change of the bias voltage. The most important result concerns with the observation of ZnSeTe/ZnTe quantum well structures. The compositional profile of these quantum wells is determined directly by atom counting and by numerical fitting of the cleavage induced outward relaxation of the ZnSeTe/ZnTe layered structure. In chapter 5, the identification of the spatial position of Mn acceptors in GaAs is studied with atomic layer precision at T = 5 K. In the experiments, the STM tip is used not only to probe the hole distribution around the Mn acceptors, but also to manipulate the transition metal atoms and their adsorbate related complexes on the GaAs surface. The symmetry of Mn is studied experimentally both at GaAs(110) and GaAs(1¯10) cleavage surface. In order to examine the electronic structure of the Mn state, we compared our measurements with a multi-band tight-binding model. The model considers the non-spherical symmetry of the GaAs top most valence band structure and takes into account the spin-orbit interaction. The calculations were performed for a bulk-like (Mn2+3d5 + hole) complex and excluded the possible effects caused by the presence of the surface and a vacuum-half sphere. The results of the bulk calculations made it possible to distinguish the experimentally observed actual surface induced effects from the bulk properties of the Mn acceptor state located near the vacuum interface. In chapter 6, the wave functions of single Mn, and Cd acceptors at 10atomic layers below the (110) cleavage surface of GaAs and GaP are spatially mapped both at room and low temperature (T = 5 K). In particular, the effect of the spin-orbit interaction on the acceptor wave functions is investigated by comparing a set of measurements on Mn doped GaAs and Cd doped GaP samples. Although the binding energy of Cd (102 meV) in GaP is nearly identical to that of Mn (113 meV) in GaAs, the spin-orbit interaction differs by a factor of about 4 between the two materials. Similar to the Mn hole wave function in GaAs, we found a highly anisotropic cross-like shape of the hole bound to Cd acceptor at GaP(110) surface. The observed similarity of the symmetry properties of Mn:GaAs and Cd:GaP clearly showed that the anisotropic structure of the acceptor states in III/V semiconductors is found to be determined by the cubic symmetry of the host crystal. The role of the spinorbit interaction on the overall shape of acceptor wave functions is identified to be negligible. Nevertheless, the weak spin-orbit coupling in GaP gives rise to additional components in the Cd acceptor wave function as compared to Mn:GaAs. Our experimental results are confirmed by two independent models based on tight-binding and effective mass approaches. In chapter 7, a detailed depth dependence of the Mn acceptor wave function symmetry is characterized and quantified experimentally at T = 5 K as a function of the depth of the Mn atom up to several layers below the surface. The experimental results are compared with the results of theoretical tightbinding calculations in the presence of an internal homogeneous strain and the strain induced by the surface relaxation, is identified to be the dominant cause of the symmetry breaking of both shallow and deep acceptors in III/V materials. To approximate the case of a relaxed GaAs surface within the tightbinding model, the Ga sub-lattice of the GaAs crystal is displaced vertically relative to the As sub-lattice. Such a shift of the Ga sub-lattice created an internal strain which was found to break the Mn wave function symmetry along a particular direction as observed experimentally. Finally, the effect of the surface related strain on different acceptor species with different binding energies is investigated with an emphasis on their (001)-reflection asymmetry. As the strain driven effect is found to be stronger for weakly-bound acceptors, this work explains within a unified approach the long-standing problem of the commonly observed triangular-shaped STM images for all shallow acceptors and the cross-like features of deeply-bound acceptor wave functions in III/V semiconductors. The results are of general importance for the dopant atoms in a stain field as well as near a strained interface. Further analysis on the acceptor wave functions, as presented in this thesis, suggests that the deep acceptor wave functions can be used as atomic scale sensors to trace the strain profile in a strained semiconductors.