Pulsed Laser Ablation and Pulsed Laser Depostion
The technique is based on a small turbomolecular pumped vacuum vessel with access for the laser beam (1064, 532, 355 and 266 nm from a Q-switched Quantel Brilliant b laser), an optical port for stereomicroscopic visual inspection of the ablation sample, an electrostatic deflection system to discriminate between ablated neutrals and ions, a manually microtranslated (XYZ) mask holder, and a liquid nitrogen cooled cold finger cryostat. The available setup is reported in Figure 1.
Figure 1 – The Pulsed Laser Depostion (PLD) equipment has the possibility to micrometrically translate the sample or the shadow mask in order to achieve micropatterned depositions, and to ablate biological samples from frozen solutions. Other micro or nanopatterned deposits can be obtained by e-beam lithography (A. Gerardino, CNR-IFN).
The possibility to steer ionized atoms and molecules out of the ablated neutral beam and address ions on a surface along a trajectory at right angle with respect to the ablation direction allows accomplishment of extremely uniform growth at very low growth rates. This has produced recently very uniform and thin metal layers. Figure 2 reports a lithographically fabricated pattern of platinum squares as imaged by SEM (left) and by AFM (right)
Figure 2. SEM image on the left, AFM detail on the right
The Pt layers we obtain by PLD are extremely thin and uniform. In this AFM scan we have an atomically uniform, 9 Angstrom thick layer, while Figure 3 reports a detail of one such square showing the step height and thickness.
The associated histogram reports the height distributions of the topographic image for both the uncoated native silicon oxide on silicon and for the platinum coated part.
Figure 3
We can see that the step height is 9 Angstrom, while the distribution width of the substrate and of the deposition is identical, showing that the Pt coat does not contribute to the substrate roughness. The same apparatus is used for micropatterned metal deposition (Figure 4) and “soft” biomolecule deposition (Figure 5) using microtranslated shadow masks. Furthermore, visual microscopic control of the laser spot on the target allows quick and easy fabrication of micrometric shadow masks (Figure 6). Platinum micropatterned PLDs on Si as imaged by SEM on the left. On the right, selectively coloured luminescence from chromophore tagged antigens reacting with Pulsed Laser Depositions of their specific antibodies (confocal microscopy). Pulsed Nd-YAG laser drilled shadow mask on 100 micron thick stainless steel blade.
Figure 4 Figure 5 
Figure 6
STM under SEM observation with laser excitation of the specimen
A home made piezoscanner, designed for xy scanning of the sample, is hosted on the sample stage of a modified Cambridge S200 scanning electron microscope, while the STM tip is positioned on the desired feature under study by a ρ,θ,φ nanomanipulator. This configuration is the most suitable to minimize mechanical vibration of the STM; alternatively, a particularly low weight scanner is mounted on the nanomanipulator while the sample is fixed. Laser beams can be directed to the interaction zone through an optical port at the back of the SEM chamber, which also serves for microscopic observation of the laser beam alignment on the STM junction. Directing the Laser beam onto the tip allows tip enhanced localised laser nanostructuring (ablation or multiphoton deposition) and localised spectroscopy.
A scheme of the STM-SEM facility is reported below (left). The images on the right show an optical microscope view of the laser irradiated STM tip on the sample, and an SEM scan of the STM tip operating to deposit material on the tips of a couple of lithographic nanoelectrodes fabricated by A. Gerardino at the CNR-IFN electron beam facility. 
This facility is particularly useful for precise positioning of the tip on the specific features or devices to be analyzed or operated upon. It allows STM electronic spectroscopy to be effected on features previously easily located on the basis of the difference in secondary electron emission. It allows a prompt characterization of both tip and sample surface damage. It supplies a first easy characterization of surface modifications, like nanometric pit formation or inorganic material depositions, when the sample is irradiated by laser beams. Finally it gives us the possibility to effect much shorter, and as a consequence slower and less damaging, STM scans for a complete characterization of the sample.
Nanomanipulated Piezoelectric Dynamometers (AFM and Piezoresistive Cantilevers)
Within the general issue of nanofabrication in our laboratory we are interested in controlling the applied forces during sharp probe interaction with the samples. We have purchased commercial piezoresistive cantilever based dynomometer probes and are planning to build more sensitive ones. Attaching this kind of piezoresistive or mechanical probes to a piezoelectric inertial nanomanipulator allows measurement of the stiffness and compliance of a wide range of materials. If the interaction is made under electron microscopy observation, we can easily gain a practical experience on the interaction of sharp tips with materials. We show here how a commercial AFM probe can be stressed and used to dig pits on a gold surface (movie), how we can measure the sensitivity of insects’ tactile hair sensors (movie), how we can measure the compliance of carbon nanotubes wool (movie), and even how we can accomplish a micrometric electrical switch based on a carbon nanotubes helical spring (movie).
Atomic Force Microscope (AFM) & Scanning Tunnelling Microscopy (STM)
The Variable Temperature STM/AFM
Characteristics
Stainless steel UHV system (Standard pressure 6 10-11mbar).Turbo molecular pump for roughing: 240 l/sec – Ionic pump: 500 l/sec – Titanium sublimator.X-Y-Z manipulator with direct current or resistive heating. Tmax=1500 K.STM/AFM (piezoresistive).Heating at the STM position: up to 1500 K.Cooling at the STM position: down to 25 K.E-beam evaporators at the STM position: Ge and Si.Reverse view LEED-Auger with LaB6 filament.
Some Results
Ge/Si(111) – experiments performed were related to the visualization of the growth by Physical Vapour Deposition of Ge nanostructures on 7×7 Si(111) reconstructed surfaces. By evaporating Ge on Si(111) at T=500°C the formation ofwetting layers have been studied by Scanning Tunneling Microscopy in situ. The evolution of the Ge islands appearing after the wetting layer (completed at 3 ML). The 3D islands appear as truncated tetrahedrons (7×7 reconstructed on the top) and evolves into rounded shape, flat islands with a central hole. An erosion of the substrate around the islands has been also evidenced and measured. The island have lateral dimensions in the range 200 – 500 nm. The statistical distribution of the islands shapes and contact angles has been analysed. On Si step bunched surfaces (obtained by flashing at 1200°C in proper condition the Si substrate) the self aggregation and ordering of the islands has been evidenced. These results have been published in several papers and conference or school proceedings [1-6].
Fig 1. Ge islands grown on Si(111) observed by STM and AFM at various stages of evolution. Top left, a) an island after the nucleation (STM: 236×236x8 nm), top rigth, b) new facets (100 and 117) appear (STM: 230×230x38 nm); bottom left, c). First stage of ripening. (STM: 527×527x12 nm) bottom rigth, d) final stage of ripening (AFM image: 527×527x10 nm). Ge flux was 1 Å/min. The substrate temperature was 530 °C for a), 450 °C for b) and 500 °C for c) and d).
Fig 2. Organization of the Ge islands on a Si(111) step-bunched substrate a) STM image of 2.5 nm Ge deposition on Si(111) at T=450 °C 2.7×3.7 μm2. The total height of the image is 56 nm. b) STM image of 6 nm Ge deposition on Si(111) at T=450 °C 10×10 μm2. The total height of the image is 82 nm.InAs/GaAs quantum dotsThe InAs quantum dots on GaAs were prepared ex-situ by MBE as follows: the GaAs(001) wafer was initially deoxidized in As flux at 640 °C until a weak 2×4 RHEED pattern appeared. Afterwards, the substrate temperature was lowered to 590 °C and an epitaxial GaAs buffer layer of approximately 0.75 mm was grown at a rate of 1 μm/h. After 10 min of annealing, the temperature was further lowered to 500 °C for the InAs growth. The deposition rate was 0.028 ML/s with an In/As flux ratio of 1/15. The InAs coverage was 3 ML (the 2D-3D transition occurs at 1.6 ML). A 50 nm As capping layer is grown on top of the quantum dots, in order to protect the surface during the transfer to the AFM/STM chamber if atomic resolution and reconstruction have to be obtained by STM. The other samples are grown on intrinsic GaAs exhibiting a resistance too high to be imaged by STM, so the measurements are normally performed by AFM. Many samples, with different InAs coverages and growth modes have been prepared and measured. The size and distribution of the dots obtained in various growth conditions have been measured. Two main growth modes have been analysed: Continuous (the In flux was never interrupted) and Migration Enhanced (the In flux was interrupted at regular intervals, in order to increase the atomic movements ad aggregation on the surface). The island size increases in the Migration Enhanced mode. The onset of the quantum dot formation has been evidenced by also by STM, by imaging 2D islands after which act as precursors. The results have been published in two papers [7-8]
Fig 3. STM images at 1.3 ML of InAs on GaAs(001); a) 2D-islands of average height 0.4 nm (precursors); b) high resolution image of the wetting layer. The RHEED image has been acquired in the MBE system at the end of the growth.
Fig 4. Needle-Sensor AFM of self-assembled quantum dots of InAs on GaAs(001) imaged using the VT SPM; dimensions: 300×300 nm. Height: 10 nm. The typical dot size is around 25 nm, and the height is 7 nm.
References on Ge/Si
[1] F. Boscherini, G. Capellini, L. Di Gaspare, F. Rosei, N. Motta and S. Mobilio, “Ge-Si intermixing in Ge quantum dots on Si(001) and Si(111)”, Appl. Phys. Lett. 76, 682 (2000).[2] F. Rosei, N. Motta, A. Sgarlata, G. Capellini and F. Boscherini, “Formation of the Wetting Layer in Ge/Si(111) studied by STM and XAFS”, Thin Solid Films 369 p29 (2000).[3] F. Boscherini, G. Capellini, L. Di Gaspare, F. Rosei, N. Motta and S. Mobilio, “Atomic intermixing in Ge Quantum Dots”,“ESRF Highlights” 1999, Surfaces and Interfaces (may be visualized on the web site www.esrf.fr).[4] F. Boscherini, G. Capellini, L. Di Gaspare, M. De Seta, F. Rosei, N. Motta A. Sgarlata and S. Mobilio, “Ge-Si intermixing in Ge quantum dots on Si”, Thin Solid Films 380, 173 (2000).[5] A. Sgarlata, F. Rosei, M. Fanfoni, N. Motta and A. Balzarotti, “STM/AFM study of Ge Quantum Dots grown on Si(111)”, IEEE Proceedings of the 11th International Semiconducting and Insulating Materials Conference, February 2000.[6] F. Rosei, N. Motta, A. Sgarlata and A. Balzarotti, “Growth and characterization of Ge nanostructures on Si(111)”, submitted for publication to Lecture Notes in Physics. (February 2001).[7] Nunzio Motta Self-assembled Quantum Dots studied by scanning probes and other structural techniques. Proc. Workshop on Nanotubes and Nanostructures 2000, S.M.Pula (CA), Ed. S.Bellucci, (Editrice Compositori 2001)[8] N. Motta, F. Rosei, A. Sgarlata, G. Capellini, S. Mobilio, F. Boscherini, Evolution of the intermixing process in Ge/Si(111) Self-assembled islands, submitted to Materials Science B
References on InAs/GaAs
[1] F. Patella, M. Fanfoni, F. Arciprete, S. Nufris, E. Placidi, and A. Balzarotti; Kinetic aspects of the morphology of self-assembled InAs quantum dots on GaAs(001) Applied Physics Letters 76 (2001) 320.[2] Arciprete, A. Balzarotti, M. Fanfoni, N. Motta, F. Patella, A. Sgarlata; Morphology of self-assembled quantum dots of InAs on GaAs(001) and Ge on Si(111) in Recent Research developments in Vacuum Science & Technology. Publisher: Transworld Research Network (2001).