Self-assembled IV-VI semiconductor quantum dots

Self-assembled growth of nanoscale three-dimensional islands by strained-layer heteroepitaxy has recently emerged as effective new technique for direct synthesis of semiconductor quantum dots. It is based on the fundamental morphological instability of strained layers that is driven by the elastic lattice relaxation of freestanding 3D islands on the surface. As a result, quantum dots are obtained without the interface problems and technological challenges associated with ex situ lithographic processing methods. In this research project, this technique will be applied for synthesis of novel IV-VI semiconductor quantum dot structures for mid-infrared device applications. Up to now, the optical emission of IV-VI semiconductor quantum dots based on the PbSe/PbEuTe material system has been limited by insufficient carrier confinement. The major goal of this project is therefore to overcome this limitation using two different approaches: First, the energy band gap of the quantum dots will be reduced by introducing PbSnSe alloys as dot material due to the rapid decrease of the energy band gap of this alloy with increasing Sn content. Thus, the quantum confinement will be increased both for the valence and conduction band. In the second approach, the tensile strained PbSe dots will be replaced by PbTe quantum dots under compressive strain. Due to the negative deformation potentials of the IV-VI compounds, this will lead to a strain-induced lowering of the band gap within the dots, resulting in a substantial improvement in the confinement as well. For both material systems the growth properties of the dots will be investigated in detail using various in situ as well as ex situ methods, including electron and x-ray diffraction as well as scanning tunneling and scan- ning force microscopy. In particular, we will study how differences in materials properties affect the growth and structure of the quantum dots. This will yield design rules on how the growth conditions can be tuned to achieve a high uniformity of the dots as well as and tailor their optical emission. For the new material systems we will fabricate self-organized quantum dot superlattices to obtain 3D ordered dot structures. This will allow a refinement of our theoretical models for description of the ordering processes during growth. The optical and electronic properties of the dot structures will be investigated by optical spectroscopy techniques. The final goal is to develop quantum dot structures with good confinement and efficient luminescence emission to be able to fabricate and test new prototype mid-infrared quantum dot lasers and detectors.

Self-assembled growth of nanoscale three-dimensional islands by strained-layer heteroepitaxy has recently emerged as effective new technique for direct synthesis of semiconductor quantum dots. It is based on the fundamental morphological instability of strained layers that is driven by the elastic lattice relaxation of freestanding 3D islands on the surface. As a result, quantum dots are obtained without the interface problems and technological challenges associated with ex situ lithographic processing methods. In this research project, this technique will be applied for synthesis of novel IV-VI semiconductor quantum dot structures for mid-infrared device applications. Up to now, the optical emission of IV-VI semiconductor quantum dots based on the PbSe/PbEuTe material system has been limited by insufficient carrier confinement. The major goal of this project is therefore to overcome this limitation using two different approaches: First, the energy band gap of the quantum dots will be reduced by introducing PbSnSe alloys as dot material due to the rapid decrease of the energy band gap of this alloy with increasing Sn content. Thus, the quantum confinement will be increased both for the valence and conduction band. In the second approach, the tensile strained PbSe dots will be replaced by PbTe quantum dots under compressive strain. Due to the negative deformation potentials of the IV-VI compounds, this will lead to a strain-induced lowering of the band gap within the dots, resulting in a substantial improvement in the confinement as well. For both material systems the growth properties of the dots will be investigated in detail using various in situ as well as ex situ methods, including electron and x-ray diffraction as well as scanning tunneling and scan- ning force microscopy. In particular, we will study how differences in materials properties affect the growth and structure of the quantum dots. This will yield design rules on how the growth conditions can be tuned to achieve a high uniformity of the dots as well as and tailor their optical emission. For the new material systems we will fabricate self-organized quantum dot superlattices to obtain 3D ordered dot structures. This will allow a refinement of our theoretical models for description of the ordering processes during growth. The optical and electronic properties of the dot structures will be investigated by optical spectroscopy techniques. The final goal is to develop quantum dot structures with good confinement and efficient luminescence emission to be able to fabricate and test new prototype mid-infrared quantum dot lasers and detectors.