In conclusion, ZrO2is an attractive candidate with enormous poten-tial to substitute for conventional SiO2in gate dielectric applications in downsized ULSI technology due to high dielectric constant, large band offset, good thermal stability in contact with Si, and process
0 1 2 3 4 5 6 7
10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
Pt/ZrO2/Pt Pt/ZrO2/Au:Pt reduced forming voltage
Current (A)
Voltage (V)
forming
Voff Von
a b
Fig. 23. (a) SEM image of Au nanospheres prepared by a lithography process. The inset shows the arrangement of the NSs on Pt/Ti/SiO2/Si substrate. (b) Typical I–V switching curves for Pt/ZrO2/Pt and Pt/ZrO2/Au:Pt devices[173].
Fig. 24. Unipolar resistive switching characteristics for a Pt/ZrO2/HfO2/TiN device in a 1D1R architecture. ON and OFF states are shown after 10, 50, and 100 cycles[175].
compatibility. Numerous deposition methods including evaporation, sputtering, PLD, ALD, MOCVD, and chemical sol–gel are commonly used to deposit ZrO2 thin films on Si, metalized Si, strained-Si-based substrates, and III–V substrates. The recent progress of ZrO2 di-electric layers for volatile DRAM applications, in terms of material and electrical properties produced by various deposition methods and different structures such as MIS and MIM has been briefly reviewed. The influence of different electrodes including highly-doped Si, and the effects of different metal and semiconductor dop-ing on the dielectric properties has been investigated. Dielectric properties of stacked ZrO2and Al2O3MIM capacitors were briefly reviewed. ZrO2-based device structures embedded with Ge and SiGe nanocrystals are applicable for nonvolatileflash memory applica-tion. Sputter deposition of ZrO2is primarily used for nonvolatile mem-ory applications due to several advantages. ZrO2films can be used as a charge trapping layer in SOZOS (poly-Si/SiO2/ZrO2/SiO2/Si) and TAZOS (TaN/Al2O3/ZrO2/SiO2/Si) based nonvolatileflash memory stacks. Bipo-lar, unipoBipo-lar, and nonpolar resistive switching properties are also observed in ZrO2-based RRAM devices, depending on the device struc-ture. Choice of electrodes plays a significant role in determining the na-ture of resistive switching properties by controlling the oxygen vacancy concentration in ZrO2-based RRAMs. Metal nanocrystals, metal implan-tation, metal doping, and embedded metal layers in ZrO2films improve resistive switching behaviors. Oxygen ion conducting buffer layer can also be used to modify the oxygen vacancy concentration in the active layer. This review provides a roadmap of ZrO2thinfilms in future low power, nanoscale microelectronic device applications.
Acknowledgment
This work is supported by the National Science Council, Taiwan under grant nos. NSC-100-2811-E-009-026 and NSC-99-2221-E-009-166-MY3.
The authors are grateful to Dr. M-C. Wu and Dr. D-Y. Lee of NCTU for providing some experimental results discussed in this review.
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Comparison of resistive switching parameters of ZrO2-based RRAM devices; VF= forming voltage, IC= compliance current for forming, Roff/on= off and on state resistance ratio, ND = no data available.
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