Browsing by Subject "Zinc Oxide"

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    Enhanced memory effect with embedded graphene nanoplatelets in ZnO charge trapping layer
    (AIP Publishing, 2014) El Atab, B.; Cimen, F.; Alkis, S.; Okyay, Ali Kemal; Nayfeh, A.
    A charge trapping memory with graphene nanoplatelets embedded in atomic layer deposited ZnO (GNIZ) is demonstrated. The memory shows a large threshold voltage Vt shift (4 V) at low operating voltage (6/-6 V), good retention (>10 yr), and good endurance characteristic (>104 cycles). This memory performance is compared to control devices with graphene nanoplatelets (or ZnO) and a thicker tunnel oxide. These structures showed a reduced Vt shift and retention characteristic. The GNIZ structure allows for scaling down the tunnel oxide thickness along with improving the memory window and retention of data. The larger Vt shift indicates that the ZnO adds available trap states and enhances the emission and retention of charges. The charge emission mechanism in the memory structures with graphene nanoplatelets at an electric field E ¥ 5.57 MV/cm is found to be based on Fowler-Nordheim tunneling. The fabrication of this memory device is compatible with current semiconductor processing, therefore, has great potential in low-cost nano-memory applications. © 2014 AIP Publishing LLC.
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    Surface engineered angstrom thick ZnO-sheathed TiO2 nanowires as photoanodes for performance enhanced dye-sensitized solar cells
    (Royal Society of Chemistry, 2014) Ulusoy, T. G.; Ghobadi, A.; Okyay, Ali Kemal
    This paper presents a systematic study on the effects of angstrom-thick atomic layer deposited (ALD) ZnO sheaths on hydrothermally-grown TiO2 nanowires (NWs) used as photoanodes in dye-sensitized solar cells (DSSCs). We designed, synthesized and characterized the samples prepared using different numbers of ZnO cycles and compared their photovoltaic (PV) performances. The device consisting of TiO2 NWs coated with the optimum thickness (two cycles) of ZnO shell exhibits a three-fold increase in efficiency compared to a control reference device. This paper reports results and features that demonstrate the passivation of surface state traps upon deposition of ZnO shells. While this passivation of surface traps provides a reduction in the back-reactions of the surface state mediated electrons (KET trap), it is speculated that ZnO-induced surface band bending (SBB) substantially reduces the recombination rate of the device by reducing the recombination rate of the conduction band (CB) electrons (KET CB). Moreover, an enhancement in the amount of dye uptake for ZnO-coated TiO2 samples is observed and explained with the isoelectric point (IEP) concept. In spite of the excellent PV power conversion efficiencies achieved by the first ZnO cycles, thicker layers impede the electron injection rate, reducing the efficiency of the device by capturing the photogenerated dye electrons in ZnO quantum wells. Here, we investigate the mechanisms contributing to this unprecedented change and correlate them with the enhancement in device efficiency.