IV Characteristics of 1D1R Memory Cell Operation of 1D-1R cell with select transistor biased to off, and on in both saturation and in linear regime.1D1R cells with GIZO TFTsSchematic diagramAdv. Funct. Mater. (2008)

Dept. of Applied PhysicsMaterial & Device Physics Lab.

OutlineResistance switching in nanometer scaleReduction of reset currentOxide Diode switch element to embody RRAM arrayReview of next-generation nonvolatile memory candidatesSummaryRRAM

Dept. of Applied PhysicsMaterial & Device Physics Lab.

High reset current problem in RRAMPresent status of RRAMFinal goal of reset currentTwo important problems

Reducing reset currentlow power consumption and reliable operation 1D-1R or 1T-1R devices structure

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Forming processUnderstanding of conducting filament generation in oxideIn Unipolar resistance switching, Conducting filament generation decides reset current.

How can we block excessive conducting filament generation?

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Control parameters of reset currentPt/Ti:NiO/PtS.-E. Ahn, Adv. Mater. 20, 924 (2008)!! Compliance current !! We restrict current level to avoid electric breakdown during forming & set processes.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

1 A2 A3 ACurrent compliance Physics: Compliance current controls percolating growth of conducting filaments.Physical meaning of compliance currentRCB network modelIdea: How can we reduce compliance current to solve high reset current problem?Random circuit breaker (RCB) network model

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Unipolar resistance switching of Pt/SrTiO3-x/PtSrTiO3 is very famous material for high-k applications. Therefore, many methods for reducing leakage current have been developed.Pt/SrTiO3-x/Pt capacitors

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Reduction of leakage current of SrTiO3-xCo dopingMnAlNbPureAcceptor doping : Co, Mn decrease of currentDonor doping : Nb increase of currentS. Y. Wang, APL 84, 4116 (2004) and many referencespure0.2 % Co1.0 % Co

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Reduction of leakage current of SrTiO3-xLeakage currentNb:STO > pure STO > Co:STO > Mn:STOAll acceptor doped SrTiO3-x showgood unipolar RS.Note: we made all materials in same PLD conditions.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

I-V curves of undoped/doped STO for various Icomp value

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Reset current reduction using compliance currentSmall compliance current reduce reset current by 1/20.We can reduce compliance current using acceptor (Mn or Co) doping in SrTiO3-x.If we reduce electrode sizes, we can more reduce compliance current. Reducing reset current. Note: we control forming & set processes with same compliance current.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

The relationship of IR Icomp was maintained down to Icomp = 0.7 mA using external transistor.Very important problem to reduce the reset current Reset current IR is not scaled with Icomp.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Unipolar RS of acceptor doped SrTiO3-xWe can reduce compliance current in Co and Mn. Smaller than pure STO by the order of 1~2.Current levels near forming process

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Summary A room-temperature-grown oxide thin film diode with the highest current density could be achieved by employing p-type CuO with small bandgap and n-type InZnOx with low resistivity. Difficulties in further (1) reduction of reset current in NiO, (2) increase of current density of oxide diode, and (3) lowering of process temperature of poly-Si diode must be solved to realize 3D ReRAM array. Pt/IZO/Pt Schottky diode showed an asymmetric J-V characteristics, possibly due to the tunneling contact at the top interface. Aging caused the increase of donor-like defects such as oxygen vacancies and zinc interstitials. Consequently, serious degradation in rectification property occurred. Control of compliance current can be one of the ways to reduce the high reset current of unipolar resistance switching.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

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Dept. of Applied PhysicsMaterial & Device Physics Lab.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Bipolar RRAM and Two-way switchstoragesteeringsetreadoffresetread

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Vs/2Set-Vs/200000000An example of operation schemeVsVs/20Vr/2Read-Vr/200000000-Vr/2ResetVr/200000000

Dept. of Applied PhysicsMaterial & Device Physics Lab.

Do we have such a switch device?Threshold device ?Tunnel device ?Zener diode ?

Dept. of Applied PhysicsMaterial & Device Physics Lab.

The relationship of IR Icomp was maintained down to Icomp = 7.5 mAReset current reductionVery important problem to reduce the reset current Reset current IR is not scaled with Icomp.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

K. Kinoshita, APL 93, 033506 (2008)Importance of current limiterWe should reduce RC time between RRAM and current limiter.

Dept. of Applied PhysicsMaterial & Device Physics Lab.

BJT 2N2369: rising time 6 ns cf. set time: 10 nsRRAMBCEDC powersupplierHP 4155C10 kW2N2369CC = 100 mAWe control Icomp using a bipolar junction transistor.S.-E. Ahn (SAIT), Adv. Mater. 20, 924 (2008)B. J. Choi (), APL 89, 012906 (2006)

Dept. of Applied PhysicsMaterial & Device Physics Lab.

ITRS for Emerging Research Devices2007 ITRS

Dept. of Applied PhysicsMaterial & Device Physics Lab.

NVNC/NA exp(-Eg_p/kT)J=Jn (electron contribution) +Jp (hole contribution) =q(Dn/Ln)np0 [exp(qV/kT)-1]+q(Dp/Lp)pn0 [exp(qV/kT)-1]NVNC/ND exp(-Eg_n/kT)diffusioncarrier concentrationexternal biasSmall bandgap materials favorable.ln JV?Shockley equation for ideal p-n diodeActual diodeSeries resistance dominates at high injection region and finally permanently damaged.Series resistance

Dept. of Applied PhysicsMaterial & Device Physics Lab.

How to increase forward currentln JVln JVln JVln JVUse small bandgap materialsRemove other unintended junctionsReduce the series resistancePrevent the damage from high current