Multiple quantum wells consisting of alternating Si and SiO2 layers were studied by means of Raman scattering. The structures were fabricated by the remote plasma enhanced chemical vapor deposition of amorphous Si and SiO2 layers on quartz substrate. The structures were subjected to a rapid thermal annealing procedure for Si crystallization. The obtained results suggest that the Si layers consist of nanocrystals embedded in an amorphous Si phase. It was found that the silicon nanocrystals inside 2nm thin layers are under high residual compressive stress. Moreover, the metastable Si III phase was detected in these samples supporting the presence of large compressive stresses in the structures. The compressive stress could be relaxed upon local laser annealing.
Well-controlled fabrication of dislocation networks in Si using direct wafer bonding opens broad possibilities for nanotechnology applications. Concepts of dislocation-network-based light emitters, manipulators of biomolecules, gettering and insulating layers, and three-dimensional buried conductive channels are presented and discussed. A prototype of a Si-based light emitter working at a wavelength of about 1.5 microm with an efficiency potential estimated at 1% is demonstrated.
Amorphous Si was completely transformed to a nanocrystalline phase in nanometer thick layers of Si-SiO 2 multiple quantum wells deposited on quartz substrates employing an illumination with a continuous-wave laser. The process was controlled by micro-Raman spectroscopy. Preferential heating of amorphous Si due to selective light absorption in the employed range of laser radiation wavelengths and solid-to-solid phase transformation can explain the obtained results.Multiple quantum well ͑MQW͒ structures containing nanometer-thick alternating crystalline Si and isolating layers, individual Si-nanowires and those assembled in networks, and Si-quantum dots are examples of nanostructures to be used in microelectronics, photonics, photovoltaics, bioelectronics, etc., in the future due to expected quasidirect and controllable band-gap and advanced carrier-transport properties. 1-5 Bearing in mind the compatibility of the Si nanostructures with modern semiconductor technology, perspectives for their prompt integration in the actual devices are very good. The hitherto unresolved problem was poor crystalline quality for extended one-dimensional ͑1D͒ and two-dimensional ͑2D͒ Si nanostructures. 6 Despite the broad interest and large number of investigations in the area of phase transitions in Si ͑see Refs. 7-11, and references therein͒, the topic is still far from being clear. The formation and growth of crystalline inclusions in an amorphous matrix, so-called solid-to-solid phase transformation ͑SSPT͒, is an interesting fundamental problem. 12 Transition of a-Si to crystalline in free standing Si films at temperatures below the melting point ͑see Ref. 13, and references therein͒ represents an example of SSPT in a monatomic system. The growth of Si-nanocrystals ͑Si-nc͒ in an amorphous Si ͑a-Si͒ matrix was also considered recently in Refs. 14 and 15. Below we will present results describing light-induced SSPT of nanometer-thick a-Si layers in Si-SiO 2 MQWs deposited on quartz substrate. Differently from the broadly investigated laser annealing of a-Si ͑see Refs. 16-18, and references therein͒, the melting of the material does not occur when the proper parameters of illumination are used and complete a-Si→ Si-nc SSPT could be achieved.The preparation of a Si-SiO 2 MQW implies crystallization of a-Si in stacks of Si and SiO 2 layers deposited on a substrate. 6 Several methods, i.e., furnace annealing, rapid thermal annealing ͑RTA͒, laser annealing, and their combinations have been applied for the crystallization of the a-Si layers previously. 6,19-22 An interaction of several materials in a MQW having different thermal properties, i.e., Si-nc, a-Si, SiO 2 , and a substrate, during the heat treatments is an obvious source for problems with crystallization. Besides that, in a Si-SiO 2 MQW at high temperatures atoms from Si layers may dissolve in silicon dioxide forming a SiO x ͑x Ͻ 2͒ phase and causing the appearance of compressive stress in the system. Changes in the volumes of the materials in the MQW, i.e., variations in thickn...
The process of light-induced crystallization (LIC) of nanometer-thick amorphous silicon (a-Si) layers in Si/SiO2 multiquantum wells (MQW) was investigated using Raman spectroscopy. In the present investigations, a laser was employed as the light source. An analysis of obtained and previously published results suggests strong influence of radiation wavelength on the outcome of the process. Namely, for certain ranges of wavelengths and radiation fluxes the crystallization proceeds through the light-induced solid phase crystallization (LISPC) process. An optimal set of radiation wavelength and flux values allows formation of fully crystallized and almost strain-free layers of nanocrystalline silicon (Si-nc). The difference in the absorption coefficients between a-Si and Si-nc was considered responsible for the obtained results. A mechanism explaining the wavelength and the radiation flux dependence was proposed. Understanding of the mechanism of LISPC in MQW structures would allow improving the LIC processes for thin silicon films.
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