Note: This page will be reworked in the near future as several new projects have recently been added.
Cross-sectional Scanning Tunneling Microscopy inside semiconductor nanostructures
Cross-sectional STM (XSTM) is used for atomically resolved studies of a number of novel III-V semiconductor nanostructures such as nanowires, quantum dots, superlattices and functional materials[1,2,3,4]. In XSTM the III-V semiconductor is cleaved perpendicular to the (100) growth direction exposing the (110) surface. This procedure enables a sharp STM tip to probe the atomic scale details of a wide variety of buried nanostructures. We have recently combined XSTM with a novel embedding scheme allowing us to directly image the interior of semiconductor nanowires with atomic resolution . Defects structures such as planar twin segments and single atom impurities have been imaged inside GaAs nanowires. Further we can image nanowires cleaved through the face of the wire and the wire/substrate interface. With XSTM it is possible to study the atomic scale structure of dopants and defects inside a nanowire, and quantitatively determine distribution and abundance of dopants in for example delta-doped coaxial layers. Using STM spectroscopy and luminescence STM, the electronic and photonic properties of a nanowire can be studied on the atomic length scale and directly related to structural properties such as twin defects. Finally, this method enables the easy access to the study of heterostructures and heterostructure interfaces grown inside the nanowire.
Left: 300x300nm STM image revealing the part of a GaAs nanowire near the GaAs(001) substrate. The wire grows in the [1-11] direction, thus with a 35 degree angle to the substrate. The cleaved surface is completely flat except for occasional monoatomic steps resulting from the cleavage of the crystal, such steps are seen going through the middle of the wire perpendicular to the wire growth direction. This and subsequent images were obtained with bias voltages around –2V (filled state imaging) and feed back currents around 0.1nA The inset shows an extreme zoom-in on the interior of the nanowire revealing the structural arrangement of the As atoms in the GaAs nanowire. A twin segment is seen going through the image indicated by the directional change of the As rows.
Right: 90x80nm STM image revealing the cross-section through a nanowire. Notice that the wire consists of two distinct almost equally large nanocrystallites with completely different orientations of the crystal lattice. Inset shows a 10x10nm zoom in on the boundary between the two nanocrystalites of the wire. The two nanocrystalites can be modelled as the two twin segments of the same wire.
1. ”Direct imaging of the atomic structure inside a nanowire by Scanning Tunneling Microscopy”, A. Mikkelsen, N. Sköld, L. Ouattara, M. Borgström, J. Andersen, L. Samuelsson, W. Seifert, E. Lundgren,
Nature Materials, 3 (2004) 519
2. “GaAs / AlGaAs nanowire heterostructures studied by Scanning Tunneling Microscopy”, L. Ouattara, A.Mikkelsen, N. Sköld, J. Eriksson, T. Knaapen, E. Cavar, W. Seifert, L. Samuelson, and E. Lundgren,
Nano Lett. (2007) In Press
3. “Mn diffusion in Ga1-xMnxAs / GaAs superlattices.”, A. Mikkelsen, , L. Ouattara, H. Davidsson, and E. Lundgren, J. Sadowski, O. Pacherova,
Appl. Phys. Lett 85 (2004) 4660
4. ”Spontaneous InAs quantum dot nucleation at strained InP/GaInAs interfaces”, M. Borgstrom, L. Samuelson, W. Seifert, A. Mikkelsen, L. Outtara, E. Lundgren,
Appl. Phys. Lett. Vol. 83 (2003) 4830
Metal nanoparticles / substrate interaction: Structure, reactivity and growth
Metal nanoparticles adsorbed on semiconductor or oxide substrates are prerequisites for a wide range of nanoscale processes such as the growth of 1D nanowires and nanotubes and many catalytic processes. Very often the exact composition and structure of the nanoparticles plays a crucial role in the determining the resulting nanostructure growth or catalytic properties.
synchrotron based photo electron spectroscopy (XPS) and scanning tunneling
microscopy (STM) we aim for a direct atomic scale understanding of the
structure and substrate modification of metal nanoparticles
on semiconductor and oxide surfaces. We are currently especially focused on the
understanding of the metal-substrate interactions responsible for III-V nanowire growth. Further we investigate how addition of
small amount of organic compounds can strongly influence nanowire
growth. The investigations will be further
strengthened by our access to a new world-class Photo Emission Electron/ Low
Energy Electron Microscope (PEEM/LEEM) at the
(a) 40x40nm STM image of GaAs(111)B surface with 0.5 Au particles/μm2 after annealing to 600C. The regions framed by the yellow/red broken lines are regions of clean GaAs(111) and a structure containing 1/3 monolayer of Au respectively. Upon annealing, the small amounts of Au in the particles will spread over the surface and form the patches with a 1/3 monolayer Au. This Au wetting layer will cover the whole surface for higher densities of Au particles. (b) 50x25 μm Photo Emission Electron Microscopy image of the Au particles. Depending on the energy of the incoming photons chemically specific or electronic contrast can be achieved. In this case we use very low energy photons (5eV) and thus image workfunction differences. (c) 3D STM image of a Au particle after annealing to 600C. It is possible both to image the Au particle and the surface with atomic resolution revealing the structure of both particle and substrate surfaces.
1. “Lysine influence on InP(001) surface ordering and nanowire growth”, A. Mikkelsen,J. Eriksson, E Lundgren, J. N. Andersen, J. Weissenreider and W. Seifert, Nanotechnology, 16 (2005) 2354
2. “Spectroscopic PhotoEmission and Low Energy Electron Microscopy (SPELEEM) at MAX-lab”, A.A. Zakharov, R. Nyholm, U. Johansson, I Maximov, J. N. Andersen and A. Mikkelsen,
“MAX-lab Activity Report 2004”, p 166
3. “Au wetting and nanoparticle stability on GaAs(111)B”, E. Hilner, A. Mikkelsen, J. Eriksson, J. N. Andersen, E. Lundgren, A. Zakharov, H. Yi and P. Kratzer,
Appl. Phys. Lett., 89 (2006) 251912
4. “Epitaxial Growth of Indium Arsenide Nanowires on Silicon Using Nucleation Templates Formed by Self-Assembled Organic Coatings”, T. Mårtensson, J. B. Wagner, E. Hilner, A. Mikkelsen, C. Thelander, J. Stangl, B. J. Ohlsson, A. Gustafsson, E. Lundgren, L. Samuelson, and W. Seifert,
Adv. Mater., (2007) In Press (Cover story)