GaN quantum dots as charge storage elements for memory devices
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GaN quantum dots as charge storage elements for memory devices P. Dimitrakis1, P. Normand1, C. Bonafos2, E. Papadomanolaki3, E. Iliopoulos3,4 1
IMEL – NCSR “Demokritos”, P.O.Box 60228, 153 10 Ag. Paraskevi, Greece CEMES – CNRS, 29 Rue Jeanne Marvig, BP 94347, 31055 Toulouse cedex 4, France 3 Physics Department, University of Crete, P.O.Box 2208, 71003 Heraklion, Greece 4 Microelectronics Research Group, IESL-FORTH, P.O.Box 1527, 71110 Heraklion, Greece 2
ABSTRACT We investigated the fabrication and the memory characteristics of metal-oxidesemiconductor (MOS) capacitors with GaN quantum-dots (QDs) embedded in the gate insulator. The GaN-QDs, which act as discrete charge storage nodes, were deposited by radio-frequency molecular-beam-deposition (RF-MBD). The influence of the deposition dose on the QDs size and density was investigated by TEM studies. Subsequent electrical characterization measurements on memory capacitors revealed enhanced electron charge trapping leading to significant memory windows. Charge retention measurements at room temperature showed that the sample with the lowest concentration of QDs exhibits a significant programming window after ten-years. INTRODUCTION Despite tremendous technological advances over the past three decades, the conventional flash memory faces physical limitations and challenging reliability issues which hinder its further scaling beyond the 16 nm technology node. The last decade, a number of memory alternatives based on different concepts and/or technologies have been proposed as potential solutions to flash downscaling limitations. Among them, the nanocrystal or quantum dot (QD) nonvolatile memories (NC/QD-NVM) have retained considerable attention and have been intensively explored for CMOS and flexible electronic devices [1]. There, the QDs act as discrete charge storage nodes and due to quantum effects, the trapped charge is retained for a longer time than in conventional floating-gate flash memories. Furthermore, QDs NVMs have the potential for faster operation at low voltage [2]. However, despite the various approaches that have been proposed such as the tunnel barrier engineering approach [3] and the use of QDs of metals with high work function [4], the requirements of both long charge retention and fast operation at low voltages have not been met yet. The work function engineering of QDs is of particular interest and may fulfill the above requirements for semiconductor QDs which exhibit a negative conduction band offset of about 0.7 eV with respect to the substrate [5]. In such case, the injected electrons can be trapped into the QDs at low voltage, while the retention time can be significantly improved due to the higher energy barrier the carriers have to overcome in order to tunnel back to the Si substrate. Assuming a Si substrate with a thin SiO2 tunneling layer, gallium nitride (GaN) QDs fulfill these requirements. The present communication reports on the fabrication and characterization of SiO2 gate dielectrics with embedded GaN-QDs which are formed under diffe
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