Browsing by Author "Polat, Mustafa"

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Characterization of GaAs (211) surface for epitaxial buffer growth
(Izmir Institute of Technology, 2014) Polat, Mustafa; Selamet, Yusuf
GaAs (211)B wafer can be used for the growth of CdTe buffer layer by MBE after thermal desorption of oxide presents on its surface. Then, CdTe buffered GaAs (211)B called as composite substrate can be used as a template for the growth of HgCdTe. Thermal desorption can be detrimental to surface in some cases if the structure and constituents of this oxide are not fully understood. In this thesis, HF:H2O and H2SO4:H2O2:H2O chemical treatments were applied to epiready GaAs (211)B samples for the determination of suitability of their usage for CdTe buffer layer growth. Effects of these wet chemical etching processes on the surface of samples are characterized and determined by various kinds of characterization techniques including XRD, XPS, SEM, EDX, AFM, and optical microscope. We also analyzed samples cut from 3" epiready GaAs (211)B wafers to determine their oxide structures, surface roughnesses, crystal qualities, and surface morphologies. Crystal quality of as-received samples measured by RC were about 18-21 arcsec. Amounts of arsenic and gallium oxides were decreased after HF treatment according to XPS results. Gallium rich surface was obtained for samples treated with piranha solution. Surface roughnesses of samples increased after piranha treatment. However, it was determined that others treated with HF had smaller surface roughnesses than asreceived samples.
Electronic, magnetic and transport properties of graphene quantum dots with charged impurities
(Izmir Institute of Technology, 2020-12) Polat, Mustafa; Güçlü, Alev Devrim; Izmir Institute of Technology
In this thesis, electronic, magnetic, and transport properties of armchair edged hexagonal and zigzag edged triangular graphene quantum dots (GQDs) are investigated in the presence of charged impurities. In this manner, a special attention has been paid to the Coulomb impurity problem in these structures. The collapse of the wave functions starting from the 1S$_{1/2}$ state is studied in the presence of not only the Coulomb impurity but also in the presence of a Coulomb charged vacancy with the help of tight-binding and extended mean-field Hubbard (MFH) models. Here, we report an interaction induced renormalization of the critical coupling constant ($\beta_{c}$). In addition, our results suggest that the induced charge for the interacting fermions is smaller than that of the non-interacting fermions. Furthermore, the transport coefficients reveal two different characteristics of the subcritical ($\beta$ $<$ $\beta_{c}$) and supercritical ($\beta$ $>$ $\beta_{c}$) regimes. As for the charged vacancy, the bare carbon vacancy induces a local magnetic moment in the hexagonal GQDs, but it is suppressed when the vacancy is charged with the subcritical Coulomb potential. Except the pristine cases of the GQDs, we numerically study a Coulomb impurity problem for the interacting fermions restricted in disordered hexagonal GQDs. In the presence of randomly distributed lattice defects and spatial potential fluctuations induced by Gaussian impurities, the response of $\beta_{c}$ for atomic collapse is mainly investigated by local density of states (LDOS) calculations within the MFH model. We find that both types of disorder cause an amplification of the critical threshold. As for the zigzag edged triangular GQDs, in the presence of the bare vacancy, we exactly obtain the spin splitting with the help of LDOS calculations in the energy spectrums, which are dominated by the edge states around the Fermi level. Similar to the hexagonal GQDs, if the vacancy is charged, the local magnetic moment disappears in these GQDs.