novel hypostasis of «old» materials in oxide electronics: metal oxides for resistive random...

8/21/2019 Novel hypostasis of old materials in oxide electronics: Metal oxides for resistive random access memory (ReRA

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Novel hypostasis of old materials in oxide electronics:

Metal oxides for resistive random access memory (ReRAM) applications

A. Pergament !. "tefanovich A. #elich$o

#. P%trolainen &. '%ndoerova . "tefanovich

Physics and Technology Department, Petrozavodsk State University,

185910, Petrozavodsk, !ssia

e"mail# aperg$ps!%karelia%r!

ABSTRACT

&ransition*metal oxide films demonstrating the effects of +oth threshold and nonvolatile memory

resistive s,itching have +een recently proposed as candidate materials for storage*class memory.

-n this ,or$ ,e descri+e some experimental res%lts on threshold s,itching in a n%m+er of

vario%s transition metal (# &i e N+ Mo / 0f 1r Mn 2 and &a) oxide films o+tained +y

anodic oxidation. &hen the res%lts concerning the effects of +ista+le resistive s,itching in M3M

and M3" str%ct%res on the +asis of s%ch oxides as #435 N+435 and Ni3 are presented. -t issho,n that sand,ich str%ct%res on the +asis of the A%6#4356"i346"i N+6N+4356A% and Pt6Ni36Pt

can +e %sed as memory elements for ReRAM applications. inally model approximations are

developed in order to descri+e theoretically the effect of nonvolatile %nipolar s,itching in

Pt*Ni3*Pt str%ct%res.

1. INTRODUCTION

1.1. Contemporary trends in oxide electronics

Metal oxides (transition metal oxides &M3 incl%ded) have +een $no,n for a long time

and their properties ,o%ld seem to have +een st%died in detail and 7%ite comprehensively. /hile

the %tility of metal oxide films e.g. as good ins%lators has +een ,ell $no,n their active

properties have +een so to say in the shado, 8%st till recently. 3ne of the most prono%nced

among those active properties is the effect of electronic s,itching 9;.
mailto:[email protected]:[email protected]


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At present the pro+lem of standard "i*+ased


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1.2. Electrical sitc!in" in metal oxides

?lectrical s,itching of a $ind can +e o+served in a great variety of materials in many

different forms and str%ct%res 9;.


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#i".2. "chematic c%rrent*voltage characteristic (3* a) and HthresholdI

(ig.> +) -*# c%rves. &herefore ,e ,ill not differentiate -*# characteristics in this sense and theterms H"*type s,itchingI H"*NFRI etc ,ill +e %sed only to indicate the type of NR i.e. to

disting%ish it from the N*type NFR (see ig. a and +).

#i".$. "*shaped (a) threshold (+) and memory (c d) -*# c%rvesB +ista+le (c) and Hanalog%eI (d)

memory s,itching 9J;.

=


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3riginally the s,itching effect ,ith c%rrent*controlled negative resistance ,as fo%nd and

investigated in chalcogenide*glass semicond%ctors (

electroforming ,ill +e disc%ssed later) are "*shaped. "imilar phenomena have also +een

o+served in oxides (mainly those of transition metals) and oxide glasses amorpho%s and

polycrystalline "i and other semicond%ctors halides nitrides s%lfides of metals car+on*

containing materials partic%larly nano*t%+es organic compo%nds s%ch as cond%cting polymers

and self*assem+led monolayers and many other materials 9 4 =*C;.

As*fa+ricated devices rarely sho, threshold or memory s,itching effects ,itho%t an

initial modification of their str%ct%re a process ,hich is %s%ally called HformingI or

electroforming (?). &here is a stri$ing similarity in the electroforming +ehavior o+served in a

,ide range of oxide halide s%lfide polymer and

permanent filament formation is a conse7%ence of temporary filamentary +rea$do,n often

o+served in M-M or M"M sand,ich str%ct%res 95;. 0o,ever electroforming differs from

dielectric +rea$do,n in the sense that it is a non*destr%ctive process (or in other ,ords it is an

irreversi+le metal*ins%lator transition 9;) i.e. it rather resem+les a self*healing type of

+rea$do,n 94D;.

-t sho%ld +e emphasied that the ? characteristics of different materials are generally ill

defined and exhi+it large variations even for devices fa+ricated %nder the same conditions.

Nevertheless since this process is o+served in a ,ide variety of ins%lating films it is %nli$ely to

res%lt from a pec%liarity of any one system. -n the case of amorpho%s thin film str%ct%res

s,itching channel formation can occ%r via crystalliation of an amorpho%s film stoichiometric

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changes diff%sion of the electrode material into the film or ioniation of deep traps 95 J;. All

of these changes %s%ally refer to a localied modification of the str%ct%re in the area of the

filament. &he modification of the initial str%ct%re d%ring the forming process is the most

important factor determining the s%+se7%ent s,itching operation. &his is +eca%se in most cases

forming creates a Hne, deviceI ,ithin the original str%ct%re the characteristics of ,hich

determine the ens%ing threshold or memory s,itching operation.

Note that in +oth threshold*type devices and those of the memory type the initial

s,itching mechanism appears to +e the same 95;. -t is initiated +y field*dependent non*ohmic

cond%ctivity and a conse7%ent insta+ility. /hether ,hat follo,s is threshold s,itching memory

s,itching or in some cases destr%ctive electrical +rea$do,n depends %pon the properties of the

material and on the presence or a+sence of s%ita+le feed+ac$ in the system. ormemory

s)itchingthe active material m%st +e capa+le of changing in some ,ay (e.g. an overall or

localied change in the electronic atomic or microscopic str%ct%re) into a permanent cond%cting

state +%t one that can +e reversed to the 3 state +y a s%ita+le c%rrent (energy) p%lse.

3+vio%sly the system m%st also +e a+le to a+sor+ the reversing p%lse ,itho%t destr%ctive

+rea$do,n. &he s%ggested models for memory s,itching can therefore +e divided into t,o +road

categories: electronic or str%ct%ral 95;. &he former is +ased on the long term storage of charge

(i.e. ,itho%t any ma8or str%ct%ral modification) to acco%nt for the non*volatile nat%re of the

s,itch. 3ne of the most commonly proposed charge storage sites are traps either in the +%l$ or at

an interface +et,een t,o dissimilar materials. &he necessary characteristic of s%ch traps is that

they have a release time compara+le to the retention time of the memory ,hich may range from

fe, ho%rs to many years 94 5*J;.

-n


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phase*change memory (PD;.

D;.

&a$ing the aforementioned into acco%nt for vanadi%m anodiation the electrolyte +ased

on either acetone or acetic acid ,as %sed. 3ther metals (&i e N+ Mo / 0f 1r Mn 2 and

&a) ,ere oxidied in standard condition 9 4J 4C >;.


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from 4 to 5DD A. -n some cases a standard and simple to operate oscilloscopic techni7%e 9>;

,as also %sed for mass meas%rements.

2.2. Electro-ormin" and Sitc!in"

-nitial str%ct%res demonstrate the nonlinear and slightly asymmetric c%rrent*voltage

characteristics. &he resistance at ero +ias meas%red ,ith the point contact is in the range DJ*

DCfor the vanadi%m anodic oxide ,hich is the most highly cond%ctive among all the

materials st%died. /hen the amplit%de of the applied voltage reaches the forming voltage #f a

sharp and irreversi+le increase in cond%ctivity is o+served and the -(#) c%rve +ecomes "*

shaped. /ith increasing c%rrent the -*# characteristic may change %ntil the parameters of the

s,itching str%ct%re are finally sta+ilied. &he process o%tlined a+ove is 7%alitatively similar to

the electroforming of the s,itching devices +ased on amorpho%s semicond%ctors 9@ J;.

&h%s the first stage of the forming does not differ from the conventional electrical

+rea$do,n of oxide films. 0o,ever if the post*+rea$do,n c%rrent is limited this res%lts in

#i".. (a) &he sand,ich s,itching str%ct%re (schematic): E vanadi%m 4 E anodic oxide film > E

A% electrical contact = E s,itching channel (#34 as ,ill +e sho,n +elo,)B (+) the -*#

characteristic for one of the samples at room temperat%re after ?.

formation of a s,itching channel rather than a +rea$do,n one. &he latter ,o%ld +e expected to

have metallic*li$e cond%ctivity and no negative resistance in the -*# characteristic. -t is 7%ite

evident that the phase composition of this s,itching channel m%st differ from the material of the

initial oxide film +eca%se the channel cond%ctivity exceeds that of an %nformed str%ct%re +y

C


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several orders of magnit%de. orming does not al,ays res%lt in an "*type characteristic. -n some

instances a transition of the str%ct%re to a high cond%ctivity state (RQDD ) ,ith ohmic +ehavior

ta$es placeB i.e. in this case +rea$do,n rather than forming occ%rs.

&he voltage*c%rrent characteristic for electroformed vanadi%m*A3*metal str%ct%re is

sho,n in ig.=. &he parameters of the str%ct%res (threshold voltage #thand c%rrent -th resistances of

3 and 3N states) may vary +y %p to an order of magnit%de from point to point for the same

specimen. "%ch a ,ide range of variation of the #thand Roffval%es as ,ell as the a+sence of a

correlation +et,een these s,itching parameters and the parameters of the sand,ich str%ct%res (the

electrode material and area the film thic$ness) leads to the concl%sion that resistance and

threshold parameters are mainly determined +y the forming process.


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#i"./. &hreshold voltage (in ar+itrary %nits ##th(&)6#th(&S)) for the M3M str%ct%res on the

+asis of different oxides as a f%nction of am+ient temperat%re. E e 4 E / > E # = E &i

and 5 E N+ anodic oxide. Fata for different samples have +een averaged and normalied

to a certain temperat%re &S (different for different materials).

temperat%re range (ig.@ a) it has +een sho,n that as the field strength increases the transition

temperat%re (i.e. the temperat%re at ,hich the s,itching event inside the channel occ%rs)

decreases (ig. @ *). -n addition n%merical sim%lation of the free charge carrier density has +een

made (ig. J) %sing the relation + ne ,here +is the channel cond%ctivity meas%red

experimentally and D.5 cm4 #* s*9>4; is the mo+ility of electrons in vanadi%m dioxide. 3ne

can see that at -D T 4DD ' (.U D5# cm*) the val%e of n+ecomes less than the critical Mott

density ncin e7%ation for the Mott criterion 9>4; ,hich for vanadi%m dioxide has +een estimated

to +e ncQ >VDCcm*>9 4=;. Note that this +ehavior of n(T) in the lo,*temperat%re (W&) region

can not +e explained +y a dependence of on temperat%re E see c%rve 4 in ig. J.

D


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a

b

#i".0. (a) ?xperimental c%rrentEvoltage characteristics of the #34+ased s,itch at vario%s

am+ient temperat%res TD('): () 4>B (4) 4=B (>) 4B (=) ==B (5) B (@) JDB (J) 5.


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const 'curve () and&'T) 'curve "), where'T) * exp'-W/kT) is a typical

temperature dependence for the mobility in strongly correlated systems !(, 4=

+"$.

2.3. Switching Mechanism

s was discussed above 'in ection (.") the switching mechanism has

for a long time been a matter of especial concern. Many a one is liely to

remember heated discussions of the (/01s about the mechanism of electrical

switching in dierent systems, 23 included, and the principal issue ,as:

whether the mechanism is primarily thermal or electronic !($. %here have

also been a lot of wors treating the switching eect as a metal-insulator

transition occurring in electric 4eld !(, +(, ++-#($. %he 4eld eect upon the

MI% in V5" has been studied previously, both theoretically and

experimentally, in a number of wors !(, #, "#, ++-+$. 6articularly, a

thermodynamic analysis based on the standard phenomenological approach

!#, +#$, using the e7uation for the free energy, shows that the shift of the

transition temperature in electric 4eld is

Tt~ Tt0E"/q,

'()

,here q& 45D Y cm*> is the latent heat of the transition 9>=;. &he change of Ttis negligi+le in

this case (Q ' for EQ D5# cm*). Also since the entropy of #34increases at the transition into

metallic phase the val%e of Ttincreases ,ith increasing E i.e. ZTtU D in e7%ation () 9=;. A

decrease of Ttin an electric field (ig. @ *) and finally its fall do,n to ero at a certain critical

field/[ can +e o+tained %sing a microscopic not thermodynamic approach +ased on the detailed

M-& mechanism. Onfort%nately as noted in 9=; ,e have no a 7%antitative theory to descri+e s%ch

a transition.

8evertheless, taing into account the fact that the MI% in V5 " is an

electronically-induced transition, we next consider the following model. 9et us

imagine an array, e.g. three-dimensional latticeof partly ioni:ed one-electron

sites 'positively-charged centres) with the locali:ation radius a; and the

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number of free electrons n 1


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Tm2Tt )6ln()6(:

)6:(:

D

46

ct

c

nn9kT

nn

(=)

,here TtD is the e7%ili+ri%m transition temperat%re and is the cond%ctivity activation energy.

&he graphs of these dependences are presented in ig. C ,here and (kTtD2) are considered as

varia+le parameters. 3ne can see that the forms and positions of the c%rves in ig.C are almost

independent of the variation of the parameter (kTtD2) in the range D.D5 to D.>D ,hich

corresponds to the activation energy Q D.5ED. e# for TtD >=D ' (for vanadi%m dioxide.g Q

e# and Q D.5 e# at T : Tt). As to the exponent the +est accordance ,ith the experimental

data is achieved for ;D (c(. c%rves 4 and din ig. C). &his val%e of is in a good

Fig.8. Maxim%m s,itching channel temperat%re as a f%nction of electron density at/ /[

(e7%ation ()) ,ith (a) (b) > (c) and D (d) and (kTt6W) D.D5 (solid lines) and D.>

(dotted lines). ?xperimental data (c%rves and 4) correspond to those in fig%re J.

agreement ,ith experimental data for materials ,ith M-& 9 4=;. -t sho%ld +e noted that tho%gh

the coincidence of the experimental data (c%rves and 4 in ig. J) and theoretical res%lts (c%rves

din ig.C) is only approximate nevertheless it is 7%ite satisfactory.

=


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-n concl%sion the proposed model not only allo,s the 7%alitative description of the

s,itching mechanism +%t it is in 7%antitative agreement ,ith the experimental res%lts in

partic%lar ,ith those concerning the critical concentration (igSs J and C). &h%s the main

feat%res of s,itching +oth in transition metal compo%nds and in other strongly correlated systems

can +e nat%rally explained ,ithin the frame,or$s of a %niversal s,itching mechanism +ased on

the electronically*ind%ced metal*ins%lator transition. -n some cases a preliminary electroforming

is re7%ired. &he s,itching channel forms in the initial str%ct%re d%ring s%ch electroforming

res%lted from the electrothermal and electrochemical processes %nder the action of the applied

forming voltage 9 4= >;. &his channel consists (partly or completely) of a material 9>; ,hich

can %ndergo a M-& from one sta+le state into another at a certain critical temperat%re Tt or

electron density nc. &he "* or N*shaped I-Vc%rve is conditioned +y the development of an

electrothermal insta+ility in the s,itching channel. F%e to the effect of Yo%le heating ,hen the

voltage reaches a critical val%e V & Vth the channel is heated %p to T & Ttand the str%ct%re

%ndergoes a transition from the ins%lating 3 state to the metallic 3N state (for the case of "*

s,itching). &his is the model of Hcritical temperat%reI (i.e. a simple electrothermal mechanism

al+eit ta$ing into acco%nt the specific +'T)dependence of the material at the M-&) tho%gh the

mechanism of the M-& itself is of co%rse essentially electronic. /hen the am+ient temperat%re

T1is m%ch less than the transition temperat%re and the val%e of Ethis high eno%gh the effect of

electronic correlations %pon the M-& is feasi+le. -n high electric fields electronic effects infl%encethe M-& so that a field*ind%ced increase in the charge carrier concentration (d%e to either

in8ection from contacts or impact ioniation or d%e to field*stim%lated donor ioniation E i.e. the

Poole*ren$el effect 94=;) leads to the elimination of the Mott*0%++ard energy gap at T < Tt!(,

+/$. &his effect may +e treated also as a lo,ering of Ttd%e to an excess negative charge

(electrons) and the dependence of Vthon Tdeviates from the +ehavior descri+ed +y the critical

temperat%re model. -n this case s,itching commences ,hen a certain critical electron density nc

is achieved. -n the e7%ili+ri%m conditions the val%e of ncis achieved merely d%e to the thermal

generation of carriers at Yo%le heating %p to T ~ Tt. Also in even higher fields s,itching occ%rs

at E& Ec ,hen Tm < Ttandn < nc.

&o s%mmarie the res%lts presented a+ove as ,ell as the other data on the s,itching in

transition metal oxides 9>C*==; indicate that c%rrent insta+ilities ,ith the "*type NR exhi+it

several common feat%res. -n partic%lar for each of the investigated materials there is a certain

fixed temperat%re To a+ove ,hich s,itching disappears. At T T To the threshold voltage

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decreases as the temperat%re rises tending to ero at T To. *x

>=D ' for #34 Q 5DD ' for &i43> and DJD \ for N+34) E see ig.5 E sho,s that the s,itching

effect is associated ,ith the ins%lator*metal transition in an electric field. &he channels

consisting of these lo,er oxides are formed in initial anodic films d%ring preliminary

electroforming.

$. *E*OR S'ITC%IN( IN %ETEROSTRUCTURES

BASED ON TRANSITION *ETA& O)IDES

$.1. Bistale resisti3e sitc!in" in A,452O/4SiO24Si 6,nctions

&,o*terminal sand,iched str%ct%res ,ith a memory s,itching are conditionally classified

into t,o types according to the mechanism involved: those ,ith an electrical capacity change

(capacitance memory) 9=5; and ,ith a cond%ctivity change (resistance memory) 9=@;. &he latter

can contain several layers incl%ding dielectric ones in order to improve the characteristics of

reversi+le electrical s,itching. &hese str%ct%res may also change the capacity 9=J; tho%gh this

feat%re is not critical for their operation. Recently +ista+le resistance s,itching has +een fo%nd in

transition metal oxides revived the interest to the phenomenon of reversi+le electrical s,itching

first o+served in vario%s types of disordered chalcogenide*+ased semicond%ctors +y 3vshins$y

9=C;. "ince then the effect ,as o+served in N+435 &i34 Ni3 Al43> ;. Also the effect of non*volatile memory

has +een fo%nd in "i6#346A% str%ct%res 95=; and the s,itching mechanism has +een attri+%ted to

the phenomena in the "i*#34 hetero8%nction.

-n the ,or$ 955;p]*"i*"i34*"n34*metal str%ct%res ,ith a t%nnel "i34 layer have +een

st%died. &he physical mechanism of memory and s,itching is attri+%ted to redistri+%tion ofionied imp%rity in "n34+eca%se of electromigration occ%rring d%e to the Yo%le heating of the

s,itching channel. After the voltage is s,itched off ne, imp%rity distri+%tion +ecomes froen

and ens%res a decrease of the c%rrent activation energy and there+y an increase of the density of

states participating in the t%nneling transitions.

@


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-n this section ,e report on the resistance s,itching characteristics in vanadi%m oxide

+ased str%ct%res (A%6#4356"i346"i) feasi+le for the application as nonvolatile memory cells ,ith

nondestr%ctive reado%t operation.

F%e to the existence of %nfilled d*shells transition metals possess a set of valence states

and in compo%nds ,ith oxygen form a n%m+er of oxides. -n the vanadi%m*oxygen system there

have +een fo%nd the follo,ing phases: s%+oxides #3=,ith=T vanadi%m monoxide #3 #43>

the Magneli phases #n34n*(n >*) #34 #@3> and #4359 >4 5@ 5J;. Wo,er oxides and

#J3>exhi+it metallic properties vanadi%m pentoxide is an ins%lator ,ith the energy gap .g 4.5

e# and the other oxides %ndergo the metal*ins%lator transitions at different temperat%res. -n

vanadi%m dioxide for example the M-& ta$es place at Tt >=D ' 9>4;. &he a+ility of these

materials to s,itch their cond%ctance +et,een t,o sta+le states provides the +asis for many

electronic devices 9J; incl%ding memory devices.

ig%re (inset) sho,s the device str%ct%re %sed in this st%dy. &,o*terminal devices have

+een fa+ricated on "i ,afers and the parameters of the samples are indicated in &a+le -.

irst a O# resist ,as deposited onto the "i34s%+strate and the DDDD m4regions

,ere developed after expos%re. &he regimes of exposing and developing ens%red almost vertical

,alls after etching. Next DD nm thic$ vanadi%m oxide film ,as deposited at room temperat%re

+y thermal evaporation. #ac%%m thermal evaporation ,as performed in a #OP*5 vac%%m system

at the +ac$gro%nd press%re Q 4VD

*@

&orr %sing a li7%id nitrogen trap. ?vaporation of #435po,der,as carried o%t +y a flash method %sing a standard #OP*5 +atcher. Al43>+oats ,ith a +%ilt*in

t%ngsten heater ,ere %sed to evaporate #435po,der. &he distance +et,een the +oat and

&a+le -.

"ample

^

"i

type"i

Vcm

"i34thic$ness nm6 fa+rication

method

#435thic$ness nm

p =D JD D thermal DD> n D.> >D thermal 4DD

s%+strate ,as D to 4 cm and the +oat temperat%re ,as maintained at a+o%t CDD_


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room temperat%re %sing thermal evaporation. inally an array of A%6#435DDDD m4

8%nctions ,as fa+ricated on "i346"i s%+strate %sing a lift*off techni7%e +y etching in acetone.

`*ray str%ct%ral analysis has revealed the deposited #435 films to +e amorpho%s E the `*

ray patterns do not sho, any diffraction pea$s 95C;.

-n the virgin state the 8%nctions for all the samples demonstrate ins%lating properties. &he

electrical forming is performed +y increasing the +ias voltage a+ove 5DEDD # ,ith positive or

negative polarity on A% for the cases of p* and n*"i respectivelyB reverse polarity does not lead to

formation of str%ct%res ,ith the memory effect. &his happens as a Hsoft +rea$do,nI of #4356"i34

+ilayer accompanied +y the formation of a s,itching channel ,ith the memory effect.

&he -E#characteristic of sample is sho,n in fig%re B one can see that s,itching of the

8%nction resistance occ%rs to the WR" (@*J*C) ,ithQ D.E M and the resistance ratio is as

high as D5ED@. &he resistance of the 0R" is Q DD !(>*=**5). &oo high c%rrent compliance

&comsettled at the process of electrical forming may lead to a HhardI +rea$do,n of the #4356"i34

+ilayer ,ith nonreversi+le 0R"*to*WR" transition.

WR" conserves after s,itching the +ias voltage off and can +e reado%t nondestr%ctively

applying a lo, pro+ing voltage. -n properly electrically formed 8%nctions reprod%ci+le s,itching

+et,een WR" and 0R" can +e performed +y contin%o%s s,eeping or p%lsing the +ias voltage of

opposite polarity (*4). WR" is s,itched +ac$ to 0R" (4*>) %nder negative voltage aro%nd D #

(the c%rrent of s,itching off is&o((). Positive voltage of a+o%t 5 # ca%ses s,itching (5*@) of 0R"

again to WR" (the c%rrent right after s,itching on is&on). WR" conserves after s,itching the +ias

voltage off and can +e reado%t nondestr%ctively applying a lo, pro+ing voltage. -n properly

electrically formed 8%nctions reprod%ci+le s,itching +et,een WR" and 0R" can +e performed +y

C


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#i". 7.-*# characteristic of A%6#4356"i346"i (sample ) 8%nctions at room temperat%re ,ith

arro,s indicating the direction of the voltage s,eep.

&he str%ct%re %nder st%dy is also depicted schematically.

contin%o%s s,eeping or p%lsing the +ias voltage of opposite polarity (*4). WR" is s,itched +ac$

to 0R" (4*>) %nder negative voltage aro%nd D # (the c%rrent of s,itching off is&o((). Positive

voltage of a+o%t 5 # ca%ses s,itching (5*@) of 0R" again to WR" (the c%rrent right after

s,itching on is&on).

&here is no s,itching if the threshold voltage is not overcome th%s the -*# characteristics

have reversi+le character. Note also that s,itching from the 0R" to the WR" only occ!rs ,hen

positive +ias is applied to the top electrode. "imilarly s,itching from the WR" to the 0R" only

occ%rs ,hen negative +ias is applied to the top electrode. &his means that the -*# c%rve sho,n in

ig. cannot +e reprod%ced ,hen s,eeping the voltage in the opposite direction of the arro,s.

Kased on these o+servations electrical s,itching in A%6 #4356"i346"i 8%nctions m%st +e classified

as a reversi*le polar e((ect.

&he -E#characteristic of sample 4 is 7%alitatively similar to the res%lts for sample . &he

0R"*to*WR" resistance ratio for this sample is Q D4 i.e. m%ch less than that for sample

+eca%se of lo, maxim%m c%rrent in WR"&on ; D*JA. &he val%e of&onseems to +e conditioned +y

the effect of c%rrent selfcompliance in WR" (@*J) allo,ing reversi+le s,itching of the str%ct%re


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,itho%t the c%rrent compliance +y the so%rcemeter. As sho,n after the st%dy of the "i34film

+rea$do,n the selfcompliance effect is not lin$ed to the #435layer and is associated ,ith the

effect on the "i6"i34interface ,hich is in the depletion regime. &he maxim%m val%e of the&on

c%rrent depends on the ill%mination of A%6#4356"i346"i8%nction (the A% layer is semi*

transparent). At a external ill%mination of D> Wx the 0R"6WR" resistance ratio reaches the val%e

of Q D=. or sample the effect of ill%mination is ,ea$er and&onincreases +y 4*> times.

&he difference +et,een samples and 4 is evidently connected ,ith the different

techni7%es of "i34fa+rication (thermal or I in &a+le - does not demonstrate any memory

effects. Also the 0R" is act%ally a+sent and the str%ct%re is in the WR" right after

electroforming. &he same +ehavior ,as fo%nd for the A%6#4356"i str%ct%res. &h%s the processes

on the #4356"i34 interface seem to +e responsi+le for the o+served nonvolatile memory

phenomena.

%alitatively the mechanism of reversi+le resistance s,itching seem to +e associated

,ith the electric field promoted n%cleation of cond%cting #3=and "i3xchannels in isolating

#4356"i34 matrix (ig. ). &hin "i34layer plays the role of the ins%lating +arrier ena+ling

appearance of high electric field strength applied across #435 layer every time incl%ding the

initial electrical forming process ,hen 0R" is s,itched to WR". At positive +ias (*5) the

transition of #435 matrix into a metallic*li$e state occ%rs thro%gh the electric field driven

redistri+%tion of oxygen ions and conse7%ent red%ction of #435 to #3=(and "i34to "i3=) in a

filament perforating #435 and "i34 ins%lator the oxygen depletion of the #3x6"i3xinterface

occ%rs. /hen the str%ct%re is in the WR" at positive +ias (*4) the motion of the oxygen ions

to,ard the +o%ndary of the #3x6"i3x channel ta$es placeB as a res%lt the cond%cting channel

collapses. A sharp transition (4*>) from WR" to 0R" can +e conditioned +y a positive feed+ac$ E

the field in the #3x6"i3x interface increases sharply at the moment of the channel collapse.

Also ,e have o+served the -E#characteristic of sample and 4 in dynamical regime. -n

this case the sta+le s,itching has +een o+served at fre7%encies %p to $0. At higher

fre7%encies the effect of memory s,itching ,as %nsta+le. &his fact also s%pports a slo, ionic

mechanism of s,itching. &he a+sence of the memory effect in sample > (,ith the n*type "i)

might +e associated ,ith either high cond%ctivity of silicon or high for,ard +ias c%rrent&onof the

"i346"i +o%ndary in this case.

4D


21/37

&h%s in the present section the res%lts of st%dy of a ne, memory element on the +asis of

A%6#4356"i346"i str%ct%res have +een presented. &he memory effect is associated ,ith +ista+le

s,itching from a high*resistance state to a lo,*resistance state d%e to the reversi+le local changes

of the oxygen content at the #4356"i34interface %nder the action of electric field. At present the

only advantage of the str%ct%re is its high 0R"*to*WR" resistance ratio. &he st%dy of its time

characteristics is going on. inally the res%lts presented may +e %sef%l for potential applications

in memory devices.

$.2. Unipolar resisti3e memory sitc!in" in nioi,m and nic8el oxides

-n this section ,e present the res%lts on the s,itching effect in thin*film M3M (metal*

oxide*metal) str%ct%res +ased on nio+i%m oxide. &he str%ct%res %nder st%dy ,ere o+tained +y

oxidation of the metal s%rface (either N+ foils or layers deposited on glass*ceramic or silicon

s%+strate) ,ith s%+se7%ent deposition onto the s%rface of the oxide film of Al* or A%*electrodes.

3xide films ,ere prepared +y anodic oxidation of nio+i%m in D. N a7%eo%s sol%tion of

phosphoric acid (0>P3=) at room temperat%re in the galvanostatic regime. or different samples

the oxide film thic$ness ,as in the range from DD to >DD nm.

or the meas%rements of the electrical properties ,e %sed a t,o*pro+e system on the

+asis of a 'eithley Model 4=D "o%rceMeter. &he system ,as designed to meas%re c%rrent*

voltage characteristics of micro* and nanostr%ct%res +oth in galvanostatic and in potentiostatic

modes. A ramp voltage %p to 5 # of a certain polarity ,as applied to the M3M str%ct%re and

the c%rrent ,as limited in order to prevent an irreversi+le +rea$do,n of the str%ct%re.

ig.D sho,s a typical c%rrent*voltage dependence of the N+*oxide +ased str%ct%res after

electrical forming. &he o+tained -*# characteristics demonstrate !nipolarresistive s,itching.

-n case of %nipolar s,itching the cond%ctivity change does depend on the amplit%de of

the applied voltage and the polarity is not important. /hen initial str%ct%re %nder an electric field

transits into the WR" this process also is referred to as the process of ? li$e,ise this ta$es place

in

process) is achieved +y %sing a larger threshold voltage than the voltage reset. Resistance ratio

R0P"6 RWR"is as high as D4*D>. -n the process of recording the c%rrent sho%ld +e limited.

4


22/37

&he physical mechanism of the effect of +ista+le s,itching in %nipolar str%ct%res +ased on N+

anodic oxide re7%ires f%rther st%dy +ased on a more experimental data and analysis of different

models offered in the literat%re 95*@;. Note that in the amorpho%s oxide N+ 435 there is also

monosta+le (threshold) 9@; and +ista+le (memory effect) ,ith *ipolars,itching 9@D;. /hat type of

s,itching (threshold or memory +i* or mono*polar) is implemented in a real M3M str%ct%re E it

depends on the conditions of the ? process 95; and c%rrent compliance d%ring the meas%rements.

#i".

19.


23/37

#i". 11.&ypical c%rrent*voltage characteristic for Pt*Ni3*Pt str%ct%re ,ith nonvolatile %nipolar

s,itching in voltage controlled regime of the meas%rement. &? and K? are Pt top and +ottom

electrodes.

#i". 12.


24/37

-n addition to the considered here oxides of # and N+ as ,as already said a+ove a

n%m+er of other &M3s demonstrate the memory s,itching effects 94*C 4@ >C*=4 =5 @D*@>;.

&herefore finally ,e present some experimental res%lts (see igSs and 4) on +ista+le

%nipolar s,itching in Ni3 94@ @=*@@;. Note that nic$el oxide seems to +e the most investigated

material for ReRAM applicatioins at least experimentally 94 4@ @J*J;.

&he typical c%rrent*voltage characteristics of the M3M str%ct%re on the +asis of

magnetron sp%ttered Ni3 ,ith Pt top and +ottom electrodes ,hich ,ere %sed for modeling

presented in the next "ection are sho,n in igSs and 4. Also ig. demonstrates also a

cross section of the str%ct%re giving geometry and some indications.

. *ODE& O# CONDUCTIN( C%ANNE& #OR*ATION INSIDE

T%E DIE&ECTRIC O)IDE *ATRI)

-n this section ,e present a model of the electrically act%ated formation of the nanosied

metal filament inside an oxide matrix. &he dielectric +rea$do,n of the oxide ,ith conditions of

the ade7%ate c%rrent compliance and s%+se7%ent capacitance discharge of the energy ,hich have

+een stored in thin film oxide capacitor str%ct%re +efore +rea$do,n res%lts in sharp local

temperat%re gro,th and as a res%lt in fast local oxide red%ction. &he so*called "oret state ,ith

metal segregation on the center of the high temperat%re region is esta+lished +y temperat%re

gradient*driven diff%sion. &he nanosied metal filament is 7%enched +y fast temperat%re drop

after capacitance discharge ending. At the next +iasing the local domain ,ith high resistance and

high electric field is created near the cathode end of a filament +y metal electromigration d%e to

the electron ,ind ind%ced +y high density electron c%rrent. A part of the metal filament is

transformed in oxide +y the s%+se7%ent fast electric field enhanced thermal oxidation.

&he st%dy ,as initiated +y the experimental o+servation of the electrically act%ated

nonvolatile s,itching +et,een t,o resistance states in thin films oxide str%ct%res 9@=;. &o date

this phenomenon is considered as promising candidate for development of the high density

stac$a+le nonvolatile memory. &he nonvolatile s,itching %s%ally can +e divided into t,o

different gro%ps * polarity dependent s,itching and practically symmetrical %nipolar s,itching

(see ig.>). &here exists a common %nderstanding of the +ipolar s,itching ,ith memory. &he

ma8ority of the researches sho, that +ipolar s,itching is interface phenomenon in ,hich the

interface properties (interface transition layer resistance 9@5; or "hott$y +arrier height and form

4=


25/37

ig.>. (a) -*# characteristics of %nipolar s,itching in Pt6Ni36Pt str%ct%re and (+) +ipolar

s,itching in &i6Wa4


26/37

-*# c%rve and a thin cond%ctive channel is formed +et,een electrodes. F%ring second stage of

the +rea$do,n the permanent cond%ctive filament ,hose str%ct%re and chemical composition

differs from native oxide is formed inside the ins%lator 9J4;. &a$ing into attention this %niversal

phenomenological +ehavior of the thin film ins%lators the first +rea$do,n stage is not so

important for presented model. /hen any electronic or electrothermal insta+ility has +een

initiated and as a res%lt the cond%ctive channel has +een formed the temperat%re increases d%e

to the Yo%le heating of this local region ,hich co%ld res%lt in a local thermochemical modification

of the oxide.

&here are t,o approaches to the estimation of the energy dissipation region sie. &he

%niversal thermodynamic consideration 9J>; sho,s that in system ,ith initial %niform c%rrent

distri+%tion the trend of the c%rrent to collect in local domain is governed +y the principal of least

entropy prod%ction. Osing approximation ,hich have +een developed in 9J>; ,e calc%late that

radi%s aof cross*sectional area of the cylinder cond%ctive channel ,hich have +een formed after

first +rea$do,n stage is 5 nm.

Another approach is +ased on a strong non%niform distri+%tion of the c%rrent in pre*

+rea$do,n state of the defect ins%lator 9J= J5;. &he statistical model of the electric field

enhancement +y local geometric thinning of the oxide thic$ness ass%mes that the sie of the high

cond%ctive path after first stage of the +rea$do,n is the same as interface irreg%larities sie E 5

nm 9J=;. Note also that the other high cond%ctive defect in polycrystalline Ni3 is the grain

+o%ndaries ,hich sie have +een meas%red as 5*D nm 9J= J@;. &herefore the 5 nm as the

dimension scale for ais a reasona+le estimation.

-t is o+vio%s that there are t,o energy so%rces for Yo%le heating of the cond%ctive domain.

At first it is necessary to ta$e into acco%nt the action of the direct c%rrent tro%gh cond%ctive

channel. &he density of the dissipated po,er can +e calc%lated asPD>&c'c 6v ,here&cis the

c%rrent compliance 'cis the voltage ,hich corresponds to&c, and vE the vol%me of the

cond%ctive domain. 3+vio%sly that 'c '? ,here '?is forming voltage +eca%se right after the

first +rea$do,n stage the str%ct%re has -*# charactristic ,ith c%rrent*controlled NFR. Accepting

a high c%rrent domain as cylindrical +ody ,ith +asis radi%s a 5 nm and oxide thic$ness as

height 5D nm ,e o+tainPD> 5D>/cm*>.

Kefore +rea$do,n the str%ct%re is the capacitor ,ith capacitance of >,hich is charged

%p to the voltage of '?. At the second +rea$do,n stage this energy is li+erated +y electrical

4@


27/37

discharge thro%gh cond%ctive channel. &he storage energy can +e ,ritten as.> 9>@'?*'>A4;64

and it is e7%al to D*>Y for the analyed sample. &he capacitance discharge po,er densityP>

changes d%ring energy li+eration process +%t ,e ass%me that capacitance discharges occ%rs ,ith

constant rate at characteristic time 0 >'c2&cD*s and %nder s%ch an ass%mption the po,er

densityP>.>6@v0A 101B/cm*> thereforeP>>>PD>. Note that D*s and it is typical

transient time of second +rea$do,n stage for many thin ins%lator films 9J4 J5;.

or estimations of the temperat%re space*time distri+%tions ,e ,ill ass%me that the heat

prod%ction is confined to a cond%ctive cylinder ,ith height and radi%s a. &he temperat%re ,ill

+e determined +y oxide thermal cond%ction in +oth axial and radial direction and spreading

thermal resistance in electrodes. /hen ais eno%gh small as compared to other dimensions of the

str%ct%re (thic$ness of oxide thic$ness and sie of metal electrodes) ,e can ass%me that the

temperat%res of electrodes and oxide matrix vol%me are e7%al am+ient. Also for thin cond%ctive

path the cylinder lateral s%rface is m%ch greater than the +asis s%rface and ,e can ass%me that

the radial heat flo, ,ill dominate.

&he steady*state sol%tion of the heat e7%ation in cylindrical coordinates ,ith heating +y a

line so%rce of strength C> P>2along cylinder axis and ,ith T D at r d,anddT2dr D at

r D is 9JJ;

ln4

=r

d

DiEF

>C

>T

(5)

-n practice d,ill not +e the sample or electrode sie and in order to o+tain a realistic

estimation ,e therefore replace d,iththe oxide thic$ness. &a$ing nic$el oxide thermal

cond%ctivityFiE D.J /6(cmD=DDDD< that is more than the oxide melting point TmiE DD,ithP>

,hich yields TD>DDD


28/37

&he melting time tm can +e eval%ated from 7%asistationary approximation of the "tefan

pro+lem of cylindrical +ody melting d%e to a line heat so%rce of strength C> at r D. &he

appropriate sol%tion is given +y the e7%ation 9JC; : tma7iEG(iE2C>, ,hereG(iEis the oxide

latent heat of f%sion. &a$ingG( D.JC $Y6g tm D*>s. /e have th%s arrive to a concl%sion that

d%ring high temperat%re forming stage the Ni3 cond%ctive domain and some region aro%nd it

sho%ld +e transformed to melting state.

"econd process ,hich sho%ld +e considered at high temperat%re forming stage is oxide

red%ction. ?xtensive st%dies of the Ni3 red%ction have appeared in literat%re and the important

res%lt in frames of o%r consideration is that the red%ction of Ni3 is irreversi+le since the

e7%ili+ri%m constantFeHof the red%ction reaction reaches D>in high temperat%re limit 9J;. Note

that oxide red%ction d%e to direct thermal decomposition is reaction*limited process and ,e can

neglect diff%sion of the reaction prod%cts for estimation of the red%ction time scale. i2>iE 9"e=p@"ktA; ,here >iis the Ni concentration >0iEis the initial

Ni3 concentration andk k0 e=p@".rmol2TA ,here kis the red%ction reaction rate constant.rmol

is the molar activation energy andis the gas constant. Osing experimental val%es:.rmol D

$Y6mol and k0 @D>s*9CD; ,e can estimate the characteristic time constant of the Ni3

red%ction as R 6$


29/37

direction of the diff%sion is determined primarily +y the mass differences: the lighter component

migrates to the ,armer end and the heavy component to the cold one. &a$ing

#i". 1. "-M" images of the Ni and 3 distri+%tions near Ni3*Pt interfaces in initial state and

after forming

this fact into acco%nt ,e can ass%me that Ni ions migrate to,ards the hot region ,hereas the 3

ions diff%se to peripheryof the melt region. As a conse7%ence a temperat%re gradient drives the

esta+lishment of concentration gradients. -n the stationary state this concentration gradient

depends on the +o%ndary conditions.Asmelt region are closed for the exchange of oxygen ,ith

the s%rro%nding gas phase the process ends %p ,ith ero atom fl%xes defining the so*called "oret

state ,ith Ni rich region in the center of the melt.

&he data given in ig.= confirm an opport%nity of an esta+lishment of the "oret state at

high temperat%re stage of the forming. &he presented res%lts are the "-M" images of the 3 and

Ni distri+%tion near Ni3*Pt interfaces for initial oxide str%ct%re and after forming. /e can see

that only 3 diff%ses a,ay from local nonhomogeneo%s regions of the Ni3 d%ring forming.

Ass%ming that these local regions have highest cond%ctivity and as conse7%ence high

temperat%re d%e to Yo%le heating the atoms redistri+%tion can +e defined +y thermomigration and

"oret state esta+lishment.

4


30/37

At the last forming stage ,hen li+eration of the capacitance energy ,ill +e finished the

temperat%re drops do,n to a+ove estimated lo, val%es d%e to the thermal cond%ctivity and the

solidification of the melting region sho%ld occ%r in time ts. &his timecan +e estimated from time

dependence of the solidification front position@tA%-n o%r case temperat%re difference +et,een

solid and li7%id phases near the interface is not so +ig and ,e can ass%me that li7%id has melting

temperat%re and temperat%re profile in the solid is linear. &he sol%tion of the appropriate "tefan

pro+lem can +e ,ritten as 9JJ JC;:ts a7iG(i27FiTm% &he val%e of tsis less than D*s and fast

solidification sho%ld 7%ench the Ni filament inside the oxide matrix.

&he lo, val%e of the diff%sion coefficient for Ni diff%sion in Ni3 and electrode materials

(d%ring the final lo,*temperat%re stage of electroforming 9C=;) allo,s ass%ming that the infl%ence

of Ni diff%sion on final Ni filament sie is negligi+le. &he oxidation process at the Ni*Ni3 interface

co%ld also +e rather slight +eca%se for this reaction at lo, temperat%res the oxidation rate is

limited +y slo, oxygen diff%sion transport to,ard the Ni3*Ni interface 9CD;.

&he strict sol%tion of the pro+lem of the Ni filament sie Rf is +ased on considering of

the energy conservation e7%ation +%t the simple estimations sho, that heating and heat transfer

terms are m%ch less in comparison ,ith melting and chemical reaction terms. Ass%ming that the

vol%me of the melt is v(7and that intensive thermal red%ction is going only in molten

region ,e can ,rite more simple integral energy conservation e7%ation for steady*state regime

mol

molDiE(DiEDiEred!ctionmelting>

I

.v


31/37

threshold s,itching ,hich then ca%ses the formation of a cond%cting filament +y the local high

c%rrent and high temperat%re conditions. A set transition time +elo, ns has +een evidenced and

the impact of parasitic capacitance has +een confirmed +y n%merical sim%lations of threshold

s,itching and Yo%le heating 94@;. Also the &i34+ased sand,ich str%ct%re st%died in 9C5; has

demonstrated +ehavior resem+ling the a+ove descri+ed processes i.e. electrored%ction and drift

process triggered +y high electric fields and enhanced +y Yo%le heating 9C5;. Also in this ,or$

the res%lts are reported revealing sta+le rectification and resistive*s,itching properties of a

&i6&i346Pt str%ct%re. &he oxygen migration and localied cond%ctive filaments play important

roles in not only the resistive*s,itching of ReRAM +%t also in the process of the rectification of

oxide diodes. &he rectification properties sta+le %p to 45_< and D>cycles %nder a+o%t > #

s,eep ,itho%t interference ,ith resistive*s,itching. &his sho,s a satisfactory relia+ility of &i34

M-M diodes for f%t%re FR (one diode E one resistor) ReRAM applications 9C5;.

&he reverse process of the Ni filament interr%ption and the transition of the str%ct%re

from WR" +ac$ to 0R" is rather more complicated for calc%lations 9C@; and ,e ,ill not develop

here all these calc%lations and estimates. -t sho%ld +e noted ho,ever that all the a+ove descri+ed

phenomena E electro* and thermo*diff%sion the "oret effect electronic ,ind and so on E play an

important role altho%gh all of them are different in their intensity and there+y in relative

contri+%tion to the mechanism of the WR" E 0R" transition. -n other ,ords several processes are

involved d%ring this WR" E 0R" transition +%t their importance (in order to interr%pt the Ni*

metal filament) ,ill +e defined +y a parity of their time scales to capacitance discharge time.

?vidently ,e sho%ld consider melting thermored%ction of oxide reoxidation diff%sion and

solidification of the components of the red%ction*oxidation reaction. Part of these processes ,ill

+e going in parallel and interdependently +%t their parity can +e defined +y separate

consideration of temporary evol%tion of each process. -n more detail all these effects have +een

considered earlier in the ,or$ 9C@;.

/. CONC&USION

&ransition*metal oxide films demonstrating the effects of +oth threshold and nonvolatile

memory resistive s,itching have +een proposed as candidate materials for storage*class memory

9 4;. -n this ,or$ ,e have descri+ed experimental res%lts on threshold s,itching in a n%m+er

of diverse transition metal oxides (# &i e N+ Mo / 0f 1r Mn 2 and &a) and the

amorpho%s oxide films of these metals have +een o+tained +y anodic oxidation.

>


32/37

-n the case of nonvolatile resistive s,itching on the +asis of &E'characteristics the

s,itching +ehaviors can +e classified into t,o types: %nipolar (nonpolar) and +ipolar.

F integrated tera+it memory ,ith a m%lti*layer stac$a+le str%ct%re 9>;.

Next ,e have st%died the effects of +ista+le resistive s,itching in M3M and M3"

str%ct%res on the +asis of transition metal oxides. 3xide films ,ere prepared +y electrochemical

oxidation of # and N+ as ,ell as +y vac%%m evaporation of vanadi%m pentoxide and Ni3 onto

"i*"i34and metal (in partic%lar Pt) s%+strates. Possi+le mechanisms of +ista+le s,itching

incl%ding a compositionally*ind%ced metal"ins!lator transition are proposed to acco%nt for the

memory properties of the H"i6vanadi%m oxide6metalI and HN+6nio+i%m oxide6metalI str%ct%res.

-n concl%sion oxide materials and physical*chemical phenomena therein termed as

Ho=ide electronicsI 9>*; is a promising direction for alternative electronics +eyond silicon along

,ith (and in addition to) s%ch %p*to*date approaches as molec%lar electronics spintronics or

s%percond%cting electronics 9=;. &hat is ,hy ,e post%late that in spite the fact that &M3s and

their properties had +eing st%died for many years this ne, +rea$thro%gh to oxide electronics

does give them a novel hypostasis. 3n the other hand "i*#3xstr%ct%res may serve as an example

demonstrating the availa+ility of hy+rid devices +ased on +oth traditional silicon technology and

ne, memory technologies. -t sho%ld +e noted that the class of oxide materials potentially s%ita+le

for ReRAM applications is not limited +y the a+ove mentioned materials: see e.g. &a+le in the

recent s%rvey 9C; representing more than =D different oxides and systems. Really ample

7%antity to selectG All the aforesaid gives hope of forthcoming oxide electronics %tiliing metal*

ins%lator transitions 9CJ; and other phenomena in &M3s ena+ling and accelerating advances in

information processing and storage +eyond conventional 5 D4.J=D..5J =.J=D..D>J

P5@ P44D and HFevelopment of "cientific Potential of 0igh "chool (4DD*4D)I

Programme pro8ect no. 4CJ.

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novel hypostasis of «old» materials in oxide electronics: metal oxides for resistive random...

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