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Project

Alternative semiconducting oxides for future DRAM applications

Dynamic random-access memory (DRAM) is a type of random-access semiconductor memory that stores each bit of data, represented by an electric charge, in a cell which typically consists of a capacitor and a semiconductor transistor. Since the charge that is stored in each cell leaks away slowly, DRAM requires a periodic refresh that rewrites the information. To reduce the refresh rate and to enable hybrid technologies, like integration of DRAM with CMOS logic, semiconducting oxides (SCO’s) are potential alternatives for silicon as a channel material for the DRAM transistor. Indium gallium zinc oxide (IGZO) is an exclusively n-type SCO that is characterized by a high bandgap, and therefore enables the formation of low leakage, junction-less thin film transistors for future DRAM applications. The main challenges related to the use of IGZO as a channel material are its stability and reliability under the presence of hydrogen. Hydrogen is a n-type dopant in IGZO that contributes to the formation of oxygen vacancies which are also n-type doping defects. Moreover, electrical performance characteristics, like its electron mobility, need to be improved. The subject of this doctoral project is the experimental exploration of different IGZO compositions that have the potential of higher electron mobility and better chemical (stoichiometry) and electrical stability. Furthermore, addition of alternative metals to the SCO’s will be explored, since they could be less sensitive to hydrogen induced reduction. The experimental work of this project involves optimization of stoichiometries, morphologies and doping with other elements, like lanthanides and silicon, that are known in literature for suppressing oxygen vacancies. From the different IGZO morphologies, spinel is identified as promising, due to its better structural and electrical properties, and will be used as a prototype crystal phase to further investigate the impact of incorporating alternative elements. Initially, the new materials will be obtained by physical vapor deposition (PVD) and will be characterized by techniques like X-ray diffraction, X-ray fluorescence, ellipsometry and others. Atomic layer deposition (ALD) is another deposition method that can follow learnings from PVD, since its conformality is very beneficial in complex structures, and therefore enables 3D DRAM designs. Both deposition processes allow the control of the composition, the crystallinity and as such the properties of the SCO’s. Therefore, a good understanding of the deposition process is necessary. Finally, lab devices will be fabricated and characterized electrically in order to examine the dependence of their electrical properties on the deposition parameters and the composition of the material.

Date:25 May 2022 →  Today
Keywords:Indium gallium zinc oxide (IGZO), semiconducting oxides, Physical Vapor Deposition (PVD), material characterization
Disciplines:Nanomaterials
Project type:PhD project