Periodic nanowires are observed from (001) orientation of Si and GaAs when the surfaces are irradiated interferentially by high power laser pulses. These nanowires are self-assembled and can be strain-free while their period is consistent with interference period. The nanowire morphologies are studied by atomic force microscopy. The observed period between nanowires depends on the wavelengths used and interference angle. The nanowire width increases with laser intensity. The narrowest nanowires observed have the width smaller than 20 nm, which is more than 10 times smaller than the interference period.
We report a direct method of fabricating high density nanodots on the GaAs(001) surfaces using laser irradiations on the surface. Surface images indicate that the large clumps are not accompanied with the formation of nanodots even though its density is higher than the critical density above which detrimental large clumps begin to show up in the conventional Stranski-Krastanov growth technique. Atomic force microscopy is used to image the GaAs(001) surfaces that are irradiated by high power laser pulses interferentially. The analysis suggests that high density quantum dots be fabricated directly on semiconductor surfaces.
Interferential irradiation of high power laser pulses can produce arrays of periodic nanostructures on surfaces. Patterning Si wafers directly by high power laser pulses indicates that the trench depth is limited to the laser pulse intensity. We present our recent studies on direct laser patterning of polystyrene coated Si wafers, which are irradiated interferentially by high power laser pulses. Polystyrene films were formed on silicon wafers with thickness controlled based on a previously developed method. Interferential irradiations of laser pulses are applied on the polystyrene coated Si wafer. The laser pulse intensities are varied along with other interferential parameters such as interference angle and laser wavelengths of 532, 355, and 266nm. The polystyrene film is dissolved to expose the patterned Si surfaces. Atomic force microscopy (AFM) images from the patterned Si surfaces indicate that the area covered with the films has trenches deeper than those on bare Si wafers patterned at the same laser intensity. Furthermore, studies of AFM images indicate that the thicker the polystyrene coating, the deeper the trenches that are produced by direct laser patterning Si surfaces. The enhancement and modification due to polymer films may enhance the security features by improving the quality of holograms.
The so-called Stranski-Krastanov (S-K) growth technique is useful to fabricate quantum dots in large quantity. However, it is limited to hetero-epitaxial systems because the S-K growth method requires a lattice mismatch generally larger than 2% such as in InGaAs quantum nanostructures. We present a study on direct laser fabrication of a strain-free selfassembled GaAs nanostructures on GaAs(001) surfaces in a molecular beam epitaxy (MBE) growth reactor in-situ. This self-assembly is due to the rapid thermal relaxation of materials heated at the interference maxima lines that are created by overlapping two laser pulses interferentially on the epitaxial growth front inside an MBE growth reactor. The morphologies of the GaAs nanostructures are characterized by atomic force microscopy and field emission scanning electron microscopy (FESEM) while their stoichiometry has been characterized by low voltage energy dispersive X-ray spectroscopy that is coupled with FESEM. The morphological study indicates that the width and length of nanodots are a few tens of nanometers while their height is around ten nanometers. The nanodot dimensions are much smaller than the interferential period and the wavelength of laser used but comparable to findings in our recent reports of quantum dots produced by direct laser annealing. For the stoichiometry study of the nanostructures, low electron voltages less than 5 kilovolts have been used in order to enhance the surface sensitivity of the resulting X-ray fluorescence due to the small inelastic mean free path of electron (~ 4 nm at 3 kV) in GaAs. The stoichiometric analysis indicates that the relative gallium content increases with size. However, the nanodots’ arsenic content as well as the relative Ga composition reaches to those of GaAs substrate when the dot size becomes smaller than 100 nm. The chemical analysis suggests a novel route of strainfree semiconductor nanodots.
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