Microstructure and physical properties of Ge/Si and InAs/GaAs quantum dots

• Main Authors: Hung-Chin Chung, 鍾鴻欽
• Other Authors: Chuan-Pu Liu
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/24030155364497721047

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 96 === This dissertation explores the microstructure and physical properties of Ge/Si and InAs/GaAs quantum dots. We have investigated the microstructures and growth mechanism of the self-assembled Ge quantum dots by high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The electrical properties of the samples were investigated by conductive-AFM. In addition, we have investigated the chemical composition and optical properties of InAs/GaAs quantum dots. The main focus of this dissertation can be divided into four parts. First, we report on the formation of self-assembled Ge coherent islands on silicon (001) substrates using higher ion energy with ultra-high vacuum ion beam sputter deposition. At 500�aC, the shape of the islands is transformed from a small dome of low aspect ratio, to a large dome of high aspect ratio. The growth evolution appears to differ from conventional pyramid-to-dome transition. Lower-aspect-ratio islands surrounded by {310} and {510} facets are found without trench formation and alloying. The largest shallow dome islands can only relax about 4% of the lattice mismatch from transmission electron microscopy strain measurements. By comparing these results with theoretical calculations, the shallow domes correspond to a larger characteristic length, and the mechanisms involved are subsequently discussed Then, Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here we report on the fabrication of Ge self-assembled quantum dots and the electrical properties of individual quantum dots. Ge quantum dots were deposited onto silicon (100) substrates by ultra-high-vacuum ion beam sputtering. AFM results show that the Ge island shape is semi-spherical with a round base. Dark-field TEM images show that samples with incoherent or coherent islands can be produced under different ion energies. Subsequently the current-voltage (I-V) characteristics from individual islands were directly measured with conductive-AFM at room temperature. Whereas the I-V curves from coherent islands exhibit linear behavior at low bias and non-linear behavior at large bias, the staircase structures are clearly observed in the I-V curves from incoherent islands which are attributed to electron tunneling through the quantized energy levels of a single Ge quantum dot. In the third part, we have investigated the microstructure and the optical properties of InAs/GaAs quantum dots (QDs) by using TEM and PL measurements. A 30nm thick AlAs insertion layer was employed to change the shape and size of the InAs QDs. The InAs QDs composed of an AlAs layer exhibit a larger dot size of 20 nm and a higher aspect ratio of 0.4. The temperature-dependent PL spectra showed that the main emission originated from the band-tail state transition at low temperature and the ground state transition at high temperature. The PL intensity of the InAs dots was significantly enhanced by the GaAs/AlAs/GaAs capped layers, and the thermal activation energy of the InAs dots was decreased. Furthermore, the In-Ga intermixing behavior for InAs QDs is also discussed based on the TEM and PL measurements. In the last, we have investigated the shape and composition profiles of buried and surface InAs/GaAs Stranski-Krastanow quantum dots (QDs) by the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075 eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In-Ga intermixing in the vicinity of the buried QDs.