Dissertation in the field of micro- and nanosciences, Jannatul Susoma
The title of thesis is ”Low-dimensional semiconducting materials for next-generation nanoelectronics”.
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2D materials have recently attracted significant interest among researchers pursuing applications in next-generation electronics. Among 2D materials, GaTe is a layered semiconductor with a direct band-gap of 1.65 eV. Thus, nonlinear optical properties of GaTe in the “2D material regime” are of high interest to the 2D mate-rial community. To the best of our knowledge, this is the first time that both the second-order nonlinear suscep-tibility, χ(2) and third-order nonlinear susceptibility, χ(3) have been determined for few-layer GaTe. Experi-ments have been carried out using a unique multi-photon microscopy at the telecommunication wavelength of 1.56 μm that has huge technological interest. Strong second-harmonic generation was measured from few-layer GaTe and χ(2) was determined to be 1.15 pm/V. The value of χ(2) is comparable to that of monolayer MoS2 at the same excitation wavelength. This indicates that few-layer GaTe is a promising material for sec-ond-order nonlinear optical applications such as sum and difference frequency generation and electro-optic modulation. Characterization of third-order nonlinearities for all-optical signal processing applications: We also observed strong third-harmonic generation and estimated χ(3) to have a value of 1.4×10-8 esu. This is one order of magnitude smaller than for graphene - however, GaTe does not suffer from the band-to-band absorp-tion as graphene does. Furthermore, it is still about three orders of magnitude higher than that of silicon at 1.56 μm. We demonstrate that the SHG of GaTe is enhanced with increasing number of layers until the coher-ence length of the material is reached. This is in contrast to MoS2 where SHG is observed only in odd number of layers N and the signal of MoS2 is significantly reduced with increasing N. These results show the potential of GaTe in the development of new optoelectronic devices and will surely encourage more research groups to pursue further studies on this field. Similar to GaTe, GaSe is also layered semiconductor with a direct band-gap of 2 eV. Crystal quality of GaTe and GaSe were investigated using Raman fingerprint. Graphene-GaSe heterostructure in device application is also investigated, where few-layer GaSe is used as a channel material and monolayer graphene is used as a contact material. The device concept we presented is expected to be useful for the progress of 2D material-based electronics making device processing compatible with conven-tional semiconductor technology.
Silicon nanocrystals (SiNCs) also have attracted much attention from researchers because of their unique properties and many promising applications. However, SiNC-based devices suffer from very low electrical conductivity owing to the high surface reactivity of SiNCs and the large number of tunnelling barriers provided by each of the SiNCs along conduction paths. By scaling down the channel length, the number of tunnel barri-ers can be reduced in order to increase conductivity. The surface nitridation of SiNCs was also carried out in order to keep the conductivity high.
Opponent: Professor Ilari Maasilta, University of Jyväskylä, Finland
Supervisor: Professor Harri Lipsanen, Aalto University School of Electrical Engineering, Department of Micro- and Nanosciences
Contact information:
Jannatul Susoma
tel +358 50 431 5459
jannatul.susoma@aalto.fi