Self-assembled Growth And Characterization Of Ordered Germanium Quantum Dots On Silicon

Silicon-based self-assembled Ge quantum dots(QDs)have been intensively investigated for their light emission in the telecommunication band and compatibility with complementary metal-oxide-semiconductor(CMOS)fabrication processes.At present,Ge QDs have demonstrated their application potential in the fields of telecommunication band lasers,telecommunication band photodetectors,mid-to-far infrared detectors,single-hole transistors,and quantum computers.Ge QDs are very suitable for studying the light-matter interaction and the growth mechanism of semiconductor materials.However,Ge QDs have many shortcomings,including much larger size than the Bohr radius of Ge,size inhomogeneity,random distribution,the Ge composition heterogeneity,and the low photoluminescence efficiency of the type ? band structure,which limit its application prospect.There is an urgent need to prepare controllable Ge QDs.In this research,the growth principles of ordered Ge QDs are fully explained.A series of patterned substrates have been designed and fabricated.The ordered growth of Ge QDs on patterned substrates is fulfilled by molecular beam epitaxy(MBE).The PL enhancement of the ordered Ge QDs from microcavities is also observed.The specific research results are as follows.(1)The basic principles of the growth of ordered Ge QDs and the factors affecting the growth results are summarized.By combining the simulation of the surface morphology and surface chemical potential(SCP)distribution of the pattern substrates and the principles of crystallography,the growth of ordered Ge QDs can be predicted and guided.(2)The growth of ordered Ge QDs on Si nanopillar-patterned substrates is realized.Four Ge QDs of the same size and morphology are regularly arranged around the Si nanopillars,which formed a special QD molecular structure.The growth results fully confirmed the great influence of the degassing temperature on the growth results,indicating that the higher the degassing temperature,the larger the interval between the four Ge QDs,which can be used to artificially adjust the QD spacing.It is also proved that Si atoms have the ability of rapid migration when depositing Ge.(3)The growth of ordered Ge QDs on Si hollow nanopillar(HNP)patterned substrates is realized.Ordered Ge QDs preferentially grow at the central hole of the HNPs.There are also Ge clusters formed at the bottom of the HNPs.The simulation of the morphology and SCP distribution of the HNPs fully proved the structure's ability to restrict the expansion of the central hole size and the lateral growth of Ge QDs,which can make the size of Ge QDs smaller,and the growth results have been confirmed.HNPs have a broader application potential than pits,which will greatly help the research on the growth of ordered QDs.(4)Ordered Ge QDs are grown on nanowires with a diameter on the order of ?100 nm.Due to the symmetry of the structure,the Ge QDs are elliptical.Ge QDs were grown on controllable suspended Si nanowires,which avoided the shortcomings of the growth of QDs on nanowires prepared by vapor-liquid-solid methods.(5)The growth of two ‘necklaces' of ordered Ge QDs on microring resonators(MRRs)and one ‘necklace' of QDs on microdisk by MBE is demonstrated.Sharp resonant peaks with the quality factors in the range of 450–850 in ?PL spectra from ordered Ge QDs embedded in groove-patterned MRRs(GPMRRs)are observed at 295 K.The growth results confirm the effect of SCP on the position of ordered Ge QDs.The growth of Ge QDs at the surface edges is generally separated into two phases: the early accumulation stage and the growing up stage.Sufficient Ge adatoms should be accumulated at the surface edges to reduce the SCP during the accumulation stage.Reducing the surface diffusion length of Ge adatoms can achieve this goal,which means a high Ge growth rate and low growth temperature during the accumulation stage.During the growing up stage,however,the longer surface diffusion length of Ge adatoms is appreciated,which means higher temperature or a slower growth rate.It is found that the buried oxide has a strong adsorption effect on Ge atoms,so the top silicon layer of silicon-on-insulator(SOI)cannot be etched through.