Paper NS-MoM4
Template Guided Self-Assembly of Metal Oxide Quantum Dots

Monday, October 15, 2007, 9:00 am, Room 616

This talk reports the use of focused ion-beams (FIB) to direct the self-assembly of Cu₂O quantum dots (QDs) on SrTiO₃ (100) substrates via point implants of Ga⁺. Oxygen plasma-assisted molecular beam-epitaxy (OPA-MBE) is used to grow Cu₂O QDs. It is found that, for high FIB implant dose (>10¹⁸ ions/cm²) and large interdot spacing (~1000 nm), multiple QDs can be formed preferentially on the edges of FIB modified pits. For lower doses (<10¹⁵ ions/cm²) and/or smaller interdot spacings (~100 nm), individual QDs nucleate first within the pits. Under carefully controlled conditions, the separation and arrangement of the Cu₂O QDs follows the FIB patterned template. This study finds that the FIB directed self-assembly technique works for different FIB doses, interdot spacings and deposition thicknesses, suggesting that this method is robust and flexible. Examination of QD growth on low-dose implant surfaces revealed a multi-step growth process. To further interpret the low dose implant results, calculations of total free-energy changes have been performed to study the differences between nucleation on a flat substrate surface and nucleation within a surface pit. This analysis shows that nucleation within a pit is almost always energetically favorable. In some special cases, assuming the pits have an inverted pyramidal shape, calculations show that island formation within the pits lowers the system total free-energy from the beginning of growth, i.e. there is no critical radius or energy barrier before a stable nucleus can be formed. Continued study of directed self-assembly of metal oxide quantum dots should lead to better understanding of the creation of well ordered, precisely controlled, high density QD arrays, ultimately contributing to the development of next generation nanoelectronic, magnetic, and optical devices.