Browsing by Author "Kocaay, Deniz"

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Open Access
High performance floating gate memories using graphene as charge storage medium and atomic layer deposited high-k dielectric layers as tunnel barrier
(2013) Kocaay, Deniz
With the ongoing development in portable electronic devices, low power consumption, improved data retention rate and higher operation speed are the merits demanded by modern non-volatile memory technology. Flash memory devices with discrete charge-trapping media are regarded as an alternative solution to conventional floating gate technology. Flash memories utilizing Sinitride as charge storage media dominate due to enhanced endurance, better scaling capability and simple fabrication. The use of high-k dielectrics as tunnel layer and control layer is also crucial in charge-trap flash memory devices since they allow further scaling and enhanced charge injection without data retention degradation. Atomic layer deposition (ALD) is a powerful technique for the growth of pinhole-free high-k dielectrics with precisely controlled thickness and high conformality. The application of graphene as charge trapping medium in flash memory devices is promising to obtain improved charge storage capability with miniaturization. Graphene acts as an effective charge storage medium due to high density of states in deep energy levels. In this thesis, we fabricate graphene flash memory devices with ALD-grown HfO2/AlN as tunnel layer and Al2O3 as control layer. Graphene oxide nanosheets are derived from the acid exfoliation of natural graphite by Hummers Method. The graphene layer is obtained by spin-coating of water soluble graphene oxide suspension followed by a thermal annealing process. Memory performance including hysteresis window, data retention rate and program transient characteristics for both electron and hole storage mechanisms are determined by performing high frequency capacitance-voltage measurements. For comparing the memory effect of graphene on device performance, we also fabricate and characterize identical flash capacitors with Si-rich SiN layer as charge storage medium and HfO2 as tunnel oxide layer. The Si-nitride films are deposited with high SiH4/NH3 gas flow ratio by plasma-enhanced chemical vapor deposition system. Graphene flash memory devices exhibit superior memory performance. Compared with Si-nitride based cells, hysteresis window, retention performance and programming speed are both significantly enhanced with the use of graphene. For electron storage, graphene flash memory provides a saturated flat band shift of 1.2 V at a write-pulse duration of 100 ns with a voltage bias of 5 V. The high density of states and high work function of graphene improve the memory performance, leading to increased charge storage capability, enhanced retention rate and faster programming operation at low voltages. The use of graphene as charge storage medium and ALD-grown high-k dielectrics as tunnel and control layers improves the existing flash technology and satisfies the requirements including scalability, at least 10-year retention, low voltage operation, faster write performance and CMOS-compatible fabrication.
Open Access
Label-free nanometer-resolution imaging of biological architectures through surface enhanced raman scattering
(Nature Publishing Group, 2013) Ayas, Sencer; Çınar, Göksu; Özkan, Alper Devrim; Soran, Zeliha; Ekiz, Oner; Kocaay, Deniz; Tomak, A.; Toren, Pelin; Kaya, Yasin; Tunc, I.; Zareie, H.; Tekinay, T.; Tekinay, Ayse Begum; Güler, Mustafa O.; Dana, Aykutlu
Label free imaging of the chemical environment of biological specimens would readily bridge the supramolecular and the cellular scales, if a chemical fingerprint technique such as Raman scattering can be coupled with super resolution imaging. We demonstrate the possibility of label-free super-resolution Raman imaging, by applying stochastic reconstruction to temporal fluctuations of the surface enhanced Raman scattering (SERS) signal which originate from biomolecular layers on large-area plasmonic surfaces with a high and uniform hot-spot density (> 10(11)/cm(2), 20 to 35 nm spacing). A resolution of 20 nm is demonstrated in reconstructed images of self-assembled peptide network and fibrilated lamellipodia of cardiomyocytes. Blink rate density is observed to be proportional to the excitation intensity and at high excitation densities (> 10 kW/cm(2)) blinking is accompanied by molecular breakdown. However, at low powers, simultaneous Raman measurements show that SERS can provide sufficient blink rates required for image reconstruction without completely damaging the chemical structure.