Optimal oxygenation of YSr Cu Mo O thin filmsbuhrman.research.engineering.cornell.edu/pubs/physicaC299_147.pdf · YSr Cu W O at 220 atm of O and 70022.800.20z 2 8Cto achieve a maximal

Ž .Physica C 299 1998 147–167

Optimal oxygenation of YSr Cu Mo O thin films2 2.75 0.25 z

J.P. Sydow ), D. Chamberlain, F. Ronnig, Y. Xu, R.A. BuhrmanSchool of Applied and Engineering Physics, Cornell UniÕersity, Ithaca NY 14853-2501, USA

Received 20 October 1997; revised 15 December 1997; accepted 18 December 1997

Abstract

Ž .Results are presented on the optimization of oxygen content and order in YSr Cu Mo O YSCMO thin films. It is2 2.75 0.25 z

shown that due to the high vapor pressure of the chain oxygen of this doped cuprate, optimal oxygenation cannot beachieved through annealing in O at 1 atm. Optimization of oxygen content can be accomplished globally by ozone anneals2

at 1 atm, and locally, in thin film microbridges, by electromigration of chain oxygen vacancies out of the microbridges. Bothtechniques lead to a maximal T of 75 K, substantially higher than has been previously achieved in this material. The strongc

correlation between the enhancement in superconducting and normal state properties, and improvements in oxygen contenthas been verified by micro-Raman spectroscopy. These results suggest that YSr Cu M O materials in general may be2 3yx x z

substantially under-oxidized if annealed in O at less than extremely high pressure, or without some other means that can2

overcome a comparatively high chain-oxygen vapor pressure at typical annealing temperatures. This could affect theinterpretation of previous experiments with doped cuprate superconductors. q 1998 Elsevier Science B.V.

Keywords: Synthesis of YSr Cu Mo O ; Raman scattering; Electromigration; Oxygen stoichiometry2 2.75 0.25 z

1. Introduction

Ž .The discovery of YBa Cu O YBCO as a2 3 7yd

high temperature superconductor prompted, and con-tinues to drive, many investigations into the effectsof cation doping in this class of materials. Thesestudies are often conducted in an attempt to improvematerials properties, and increase our understandingof the physical processes involved in high tempera-ture superconducting oxides. Early work determinedthat the Y cation can be substituted with a variety ofother rare earth elements with little or no effect on Tc

) Corresponding author.

w x1 . On the other hand, despite the chemical similar-ity between Ba and Sr, YBa Sr Cu O exhibits2yx x 3 z

interesting changes in transport properties and crys-tallographic structure as the Sr content is increased:T decreases monotonically, and the unit cell con-c

w xtracts 2 . There is also a possible ordering of Ba andw xSr at xs1 2 . With typical preparations in one

atmosphere of oxygen, second phases begin to formŽ .at xs1.25, and pure YSr Cu O YSCO is2 3 7yd

w ximpossible to synthesize at 1 atm of O 2,3 . This is2

attributable to the contraction of the unit cell, and thestructural stress caused by the smaller ionic radii of

qq ˚ qq ˚Ž . Ž .Sr 1.13 A as compared to Ba 1.35 A .This stress can be relieved via two routes. The

Žfirst is to synthesize YSCO at high O pressure 6–72. Ž .GPa and high temperatures 13808C . Under these

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0921-4534 98 00014-8

( )J.P. Sydow et al.rPhysica C 299 1998 147–167148

preparation conditions, it is possible to synthesizew xpure YSCO with a T onset of 60 K 4 . This reducedc

T is obtained with material that has a crystal struc-c

ture that is still contracted compared to YBCO. Thetetragonal structure of YSCO has as3.79 and cs

˚ w x11.41 A 4 , while the orthorhombic structure of˚ w xYBCO has as3.82, bs3.89 and cs11.71 A 5 .

The second route is to replace some of the Cucations in the structure with smaller ions to form

w xYSr Cu M O 6 . These dopant atoms relieve2 3yx x z

the crystal stress by three means. First, their smallersize allows shorter M–O bonds which can relievestress on the compressed Cu–O bonds. Second,dopant atoms with a higher oxygen coordination;such as Moq6 , Wq6 , or Req7; will encourage oxy-genation of the crystal structure. X-ray and neutrondiffraction studies have determined that dopants such

Ž .as Mo, W, and Re substitute for the Cu 1 site on thew x q6 Ž .Cu–O chains 7 . M dopants on the Cu 1 sites

will increase the oxygenation of the basal plane, andŽ . q2thus, increase the valence of Cu 1 from Cu to-

wards Cuq3. This in turn decreases the ionic radii ofŽ .the Cu 1 cation and alleviates more of the crystalw x q6stress 7 . Finally, the M dopant atoms on the

Ž .Cu 1 site alter the structure of the Cu–O chains inŽ . Ž . Ž .the basal planes. The O 3 –M 1 –O 3 bond angle,

which is typically 1808 for Cu–O chains in YBCO,w xis decreased to 130–1408 7 . This corrugation of the

M–O chains accommodates the smaller unit cellrequired by the Sr atom, and still allows oxygenationup to, and in excess of O .7

Ž .Doping for Cu 1 , combined with synthesis in O2

at 1 atm, has produced materials which range frominsulators, e.g., YSr Cu Cr O , to superconduc-2 2.8 0.2 z

tors with T ’s up to 53 K, e.g., YSr Cu Re Oc 2 2.85 0.15 zw x6 . In general, the highest T s are observed at thec

minimum doping concentration necessary to formstable single phase material. Routes one and twohave recently been combined by annealingYSr Cu W O at 220 atm of O and 7008C to2 2.80 0.20 z 2

w xachieve a maximal T of 75 K 8 . This material stillc

exhibits a contracted tetragonal unit cell with as3.80 and cs11.50, but a major portion of the 18%decrease in T from that of bulk YBCO can bec

accounted for by the reduction in carrier density dueŽ .to the 20% decrease of the Cu 1 in the basal plane.

This result raises the question, were specimens inprevious experiments with doped YSCO materials

optimally oxygenated by standard 5008C O anneals2

at 1 atm or even high pressure O anneals?2

To examine this issue in detail, we have producedthin films of one particular doped YSCO material,

Ž .YSr Cu Mo O YSCMO and have studied the2 2.75 0.25 z

transport properties of these films as a function oftheir oxygenation state. We have also employedmicro-Raman spectroscopy to correlate the improve-ment in superconducting transport properties withthe improvement in basal plane oxygen content andorder. In this study, we utilized two alternative meth-ods of oxygenation, ozone annealing and electromi-gration of oxygen vacancies. We have found both ofthese to be much more effective in optimally popu-lating the basal plane chain oxygen sites than anneal-ing in O at a pressure of 1 atm.2

Ozone decomposes quickly when heated to mod-erate temperatures, and thus, can provide a muchhigher partial pressure of atomic oxygen at the de-composition site than O at the same temperature.2

For this reason, ozone has often been used in loww xpressure deposition 9,10 of cuprate materials in thin

film evaporation systems where the ozone is directedonto the heated substrate. In our case, we annealedpreviously grown YSCMO films in flowing O rO2 3

mixtures at 1 atm where the O concentration was as3

high as ;2%.In the electromigration approach, a strong DC

current bias drives the diffusion of chain oxygenvacancies out of a thin film microbridge into theadjacent, negatively biased electrode. This has provento be a powerful technique for the local delivery ofoxygen ions to vacant chain sites and for the subse-quent refinement of oxygen order and content in

w xYBCO 11 .Previous studies of YSr Cu Mo O have2 2.75 0.25 z

achieved T ’s of 31 K after 1 atm O anneal atc 2

10508C, and a maximum of 55 K after 400 atm O2w xanneal at 10008C 6 . It should also be noted that a Tc

of 71 K has been reported for YSr Cu Mo O ,2 2.8 0.2 z

after the same 400 atm O anneal, and that this T2 c

can be increased to a maximal value of 73 K throughw xthe addition of 10% Ca for Y 8 . In comparison, we

find that with ozone anneals at 1 atm and 5008C, orwith electromigration, the T of our thin film sam-c

ples can be increased to 75 K. This equals themaximum T that has been reported to date for thec

entire class of doped YSCO. Our results for YSCMO

( )J.P. Sydow et al.rPhysica C 299 1998 147–167 149

meet this limit, without the necessity of a highpressure O anneal or calcium doping, and with a2

Ž .different dopant element Mo vs. W at a higherŽ Ž . .concentration 25% substitution for Cu 1 vs. 20% .

Indeed, in the case of this YSCMO result, the 20%depression of T below 92 K can possibly be ex-c

plained in large part by the lower hole carrier con-Ž . Ž . Ž . Žcentration in the Cu 2 –O 2 , O 3 planes i.e., the

.CuO planes that is caused by the 25% cation2Ž .doping of Mo for Cu 1 on the Cu–O chains.

The elevated T of YSCMO that can be obtainedc

by ozone annealing and electromigration, and therelated evidence of the high vapor pressure of thechain oxygen that we discuss below, clearly demon-strate that optimal oxygen content and order cannotbe achieved in this material through standard 5008CO anneals at 1 atm, or perhaps, even by high2

pressure O anneals. In light of these results, it2

appears necessary to re-examine experimental resultspreviously obtained for the entire class of dopedYSCO systems. Indeed, some of the effects andtrends which have been attributed to the types andconcentrations of dopants used in YSCO materials,may be caused in part by reduced oxygen contentand increased oxygen disorder which cannot be opti-

mized through typical atmospheric or even highpressure oxidation.

2. Film deposition

Thin YSCMO films were deposited on un-bufferedLaAlO substrates by laser ablation of a3

w xYSr Cu Mo O target 12 . The laser pulse en-2 2.75 0.25 z

ergy was 100 mJ at a repetition rate of 5 Hz. Unlessnoted, all samples discussed in the work were de-posited in 1200 mTorr of O and 8408C, cooled to2

and held at 5008C for a period of time during thebeginning of which the O pressure in the deposition2

chamber reached 1 atm, and then cooled to roomtemperature in 30 min. Films were analyzed by

Ž .X-ray diffraction XRD , and scanning electron mi-Ž .croscopy SEM . XRD analysis shows that the as-de-

posited films are predominantly single phase, tetrag-onal, and c-axis normal with a c-axis length of

˚Ž .11.4 7 A. SEM images, however, do show squareand rectangular inclusions which may be a-axisgrains. This hypothesis is supported by the fact thatthe density of these inclusions increased with filmthickness, an effect which is known to occur for

Ž .Fig. 1. Optical micrograph of a YSr Cu Mo O YSCMO thin film microbridge.2 2.75 0.25 z


a-axis grains in YBCO films on un-buffered LaAlO3w xsubstrates 13,14 .

In general, film quality with respect to c-axisalignment and roughness was poor in comparison tostate-of-the-art PLD YBCO films. On the other hand,

Ž .the zero resistance T ;20 K of the as-grownc

YSCMO films were similar to bulk sample resultsreported previously for the same material and com-position, and as we discuss below, major improve-ments in T and normal state resistivity are possiblec

through refinement of the oxygen order and content.Thin films were patterned into microstructures byoptical lithography and argon ion beam milling. Anoptical micrograph of a 3=40 mm bridge is pic-tured in Fig. 1. AgrAu contact pads were thermallydeposited after an in situ 90% argon and 10% oxy-gen ion beam clean of the contact pad area definedby photoresist.

3. Raman spectroscopy

Throughout this study, Raman spectroscopy wasused to evaluate oxygen order and content in bothbulk YSCMO films and patterned microstructures. Inthis section, we will first briefly review the majorRaman modes of YBCO and the effect that varyingthe oxygen content and order has on these modes.The analogous modes and effects of oxygen orderand content will then be presented for YSCMO.Raman spectra from electromigrated microstructuresand ozone annealed samples will provide informa-tion about the effects of these processes on oxygencontent and order.

Raman spectra were obtained with a Dilor micro-Raman spectrometer focused to a 2 mm spot sizeoperating at the 514.5 nm argon line. Spectra werecollected with a Thomson CCD detector. The excita-tion laser is intrinsically linearly polarized, and theorientation of the multiple spectrometer gratings en-sures that the detected polarization is also linearlypolarized. All measurements were made on c-axisnormal films, and the excitation light was incidentfrom the c-axis, or z direction. Generally, the excita-tion and detection polarization were along the arb,or xry direction, however, in some cases, co- andcross-polarized measurements were made with the

incident and reflected light polarized at 458 from thecrystal axis.

3.1. Raman spectra of YBCO

ŽEarly Raman work on YBCO for a review seew x.e.g., Refs. 15–17 has shown that the spectral

features for light incident along the c-axis consists offour major peaks; the first centered at 117 cmy1 andattributed to the axial stretching of the Ba atoms; oneat 150 cmy1, attributed to the axial stretching of

Ž . y1Cu 2 atoms, one at 338 cm due to the out-of-phaseŽ . Ž .motion of the O 2 and O 3 atoms, and one at

;500 cmy1, attributable to the axial stretching ofŽ .the apical O 4 oxygen atom. A small peak at ;450

cmy1 is also often seen in somewhat oxygen defi-cient material and is ascribed to the in-phase motion

Ž . Ž . w xof the O 2 and O 3 atoms 15–18 .In addition to the expected Raman modes, an

additional Raman peak is often observed at ;600cmy1. This has been associated with the presence ofoxygen defects in the basal plane and thus has beenconcluded to arise from the out-of-phase vibrationsof the Cu–O chain atoms. This mode can becomeRaman active upon the introduction of defects thatlocally break the crystal symmetry of the Cu–O

w xchains 15–18 .All of the YBCO Raman peaks are sensitive, in

varying degrees and manner, to the oxygen concen-tration. As the chain oxygen content decreases themost notable changes are; a shift down in frequencyand increase in intensity of the 150 cmy1 peak, anincrease in the intensity of the 338 cmy1 mode and

Ž .change in its line shape from an asymmetric Fanoshape to a Lorentzian, an increase in the 435 cmy1

mode, an apparent shift down in frequency of the500 cmy1 apical oxygen mode, and an increase in

y1 w xthe intensity of the 600 cm mode 19 .

3.2. Raman spectra of YSCMO

Fig. 2 represents a series of Raman spectra col-lected from a YSCMO microbridge as the sample

Ž .was first annealed in O at 5008C, spectrum b , then2

annealed in N at successively higher temperatures2Ž . Ž .from 2008C to 5008C in 508C steps, spectra c – i ,

Ž .and finally re-annealed in O at 5008C, spectrum a .2


Ž . Ž .Fig. 2. Raman spectra of a YSCMO bulk film after various 1 atm N and O anneals: a after 2nd 5008C O anneal after all N anneals, b2 2 2 2Ž . Ž . Ž . Ž . Ž .after the initial 5008C O anneal, c after a 2008C N anneal, d 2508C N anneal, e 3008C N anneal, f 3508C N anneal, g 4008C N2 2 2 2 2 2

Ž . Ž . Ž . Ž . Ž .anneal, h 4508C N anneal, i 5008C N anneal. Each anneal was for 1 h, and the annealing sequence was b through i , followed by a .2 2

Each anneal was 1 h long. Clear, although generallybroad peaks are observed at 148, 310, 326, 450, 540,and 601 cmy1. The additional sharp peak at 117cmy1 is a plasma line of the excitation laser. It is

straight forward to associate the 148 cmy1 peak withŽ . y1the Cu 2 axial stretching mode, the 326 cm peak

and the small 450 cmy1 peak with the out-of-phaseŽ .and in-phase motion, respectively of the O 2 and

Ž . 2Fig. 3. Raman spectra of a YSCMO microbridge: a co-polarized measurement at 458 to the principal arb axes after EM at 1.67 MArcm ,Ž . Ž . Ž .b 458 co-polarized measurement before EM, c 458 cross-polarized measurement after EM, d 458 cross-polarized measurement beforeEM.


Ž . y1O 3 atoms, and the peak centered at 540 cm withthe apical oxygen mode, shifted upwards due to theSr replacement for Ba.

Depending on the oxygen state of the film, theRaman spectra in the 300 cmy1 region show eithertwo resolvable peaks or a single, broad and asym-metric peak, with the latter being the case for the O2

Ž . Ž .annealed films. In Fig. 3, spectra b and d showco- and cross-polarized Raman measurements madewith the incident and reflected light polarized at 458

to the arb axes of an oxygen annealed YSCMOfilm. These spectra demonstrate that this broad peakactually consists of two modes. The 326 cmy1 modeis present for cross-polarized incident and detected

Ž .light, spectrum d , but not for the co-polarized case,Ž .spectrum b . This is the expected polarization de-

pendence for the Raman mode due to the out ofŽ . Ž .phase motion of the O 2 and O 3 atoms. The 310

cmy1 mode, on the other hand, is clearly present forŽ .the co-polarized spectrum b , and may also be

present in what might otherwise be interpreted as theFano-like line shape of the 321 cmy1 mode in the

Ž .cross-polarized spectrum d . The origin of this 310cmy1 mode is not known, but presumably is due to

Ž .the Mo substitution or the high concentration of O 5defects that they induce.

Returning to Fig. 2, as the N anneal temperature2

is increased, the effects of oxygen loss becomeevident. The 148, 326, and 450 cmy1 modes in-crease in intensity and the 148 cmy1 mode decreasesin wave number. These changes are analogous to thechanges that occur in YBCO. In addition, the inten-sity of the anomalous 310 cmy1 mode increases,becoming greater than that of the 326 cmy1 modeafter the 2508C and 3008C anneals. Other changes asthe oxygen content decreases are a sharpening of the540 and 600 cmy1 modes, and finally a decrease intheir intensity at the highest anneal temperatures.This sharpening may be due to an increase in theordering of remaining chain oxygen atoms as thevacancy concentration increases. The effect of oxy-gen concentration on the Raman modes in YSCMO,and the equivalent changes in YBCO, are summa-

Ž .rized in Table 1. Spectrum a in Fig. 2 demonstratesthat re-annealing the sample in 1 atm of O at 5008C2

returns the Raman spectra to the initial state, demon-strating that the de-oxygenation induced by the N2

anneals is reversible. Tab

le1

Ž.

Ž.

The

effe

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299

1998

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1Ž.

150r

148

cmax

ialm

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Cu

2in

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ityin

crea

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inte

nsity

incr

ease

sin

Cu–

Opl

anes

wav

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crea

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wav

enu

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crea

ses

2y

1y

r31

0cm

unkn

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133

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6cm

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544

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4. Electromigration

In previous work, it has been shown that electro-Ž .migration EM can be used to significantly improve

the basal plane oxygen content and order of well-an-w xnealed YBCO microstructures 11,19 . Electromigra-

tion consists of the application of an electrical biaswith a current density of 1–5 MArcm2 to a thinfilm microstructure. This process is generally con-ducted in 1 atm of He and at room temperature,although there is some ohmic heating of the mi-crostructure. The current bias induces long rangemigration of the positively charged oxygen vacanciestowards the negative electrode. This is possible be-cause the chain oxygen is relatively mobile, presum-

Ž .ably due to the normally vacant off-chain or O 5site in the a-axis direction of the crystal lattice. Once

Ž .an oxygen atom is thermally excited to the O 5 site,the local electric field will bias it to diffuse towardsa vacancy closer to the positive electrode. Whensuch a bias is applied to a cuprate superconductormicrobridge for a sufficient period of time, the endeffect is illustrated in Fig. 1. The resultant build upof oxygen vacancies is demonstrated by the morereflective, or whiter material which has the visualappearance of a plume shape at the end of the

microbridge. The electric field and current densitiesdecrease in the plume, resulting in the re-depositionof vacancies, similar to the re-deposition of silt in ariver delta. In practice, EM experiments of this typeare generally performed under constant current, whilethe microbridge resistance is monitored as a functionof time.

The history of the EM experiment portrayed inFig. 4 illustrates the key aspects of this process. Thisfigure shows the response of the microbridge resis-tivity to a constant current bias applied through themicrobridge for a certain time interval, and thenreduced briefly to a low level sensing bias current of16.7 kArcm2 so that any permanent resistivitychange can be determined before applying a step-wisehigher EM bias. The sensing bias current is neces-sary since ohmic heating of the microbridge tem-porarily increases the resistivity during the applica-tion of the EM bias. The temperature dependence ofthe resistivity must be deconvolved from EM in-duced materials changes. These thermal effects areused to determine the microbridge temperature dur-ing the EM process. As can be seen, the first threebiases of 0.17, 0.33 and 0.5 MArcm2 have nodiscernible effect on the microbridge room tempera-ture resistivity, r .n

Fig. 4. Resistivity vs. time portrayal of an electromigration experiment with step-wise increasing bias current. See text for details of theexperiment.


A markedly different result is obtained once apower threshold region is reached at 0.67 MArcm2

for the sample in question. At this threshold, thetemperature of the microbridge increased to a pointwhere the resistivity begins to increase dramaticallyover time. The response time for this increase isslower than that required for the sample to reach asteady state temperature due to the ohmic heating.The effect of this linear increase in resistivity duringthe bias remains after the bias current is reduced to16.7 kArcm2 and thus, demonstrates a permanentchange rather than simple heating effects. Thisthreshold behavior is repeatable from sample to sam-ple, but comparison between samples indicates thatthe key parameter is the ohmic power dissipation perunit surface area of the sample, and not the biascurrent, with the power threshold being ;14 mW,or 47 kWrcm2. The universality of the powerthreshold indicates that the increase in resistance

Žbegins at a characteristic temperature region ;.2008C , and is due to the thermally activated loss of

oxygen from the microbridge into the surroundingHe gas ambient, presumably by out-diffusion throughsmall angle grain boundaries and twin boundaries.This conclusion regarding thermally activated oxy-gen loss is supported by resistivity vs. temperatureand Raman spectroscopy data following various O2

and O rO anneals, as we discuss below.2 3

When the EM bias is raised significantly beyondthe power threshold, to 0.83 MArcm2 in the case ofFig. 4, the resistivity immediately undergoes a dra-matic decrease, and then decays more slowly in astretched exponential fashion. At this point, electro-migration of oxygen vacancies out of the micro-bridge, into the negative electrode, has begun. Thebasal plane oxygen content and order are beingimproved within the microbridge. If the bias is stillincreased further, an even more rapid and morepronounced reduction in resistivity results, again fol-lowed by a stretched exponential response.

While the ‘healing’ effects of EM that are seen atthe highest bias levels in these YSCMO experimentsare qualitatively similar to, although quantitativelystronger than those seen earlier in EM experimentson YBCO, a significant oxygen outgassing effect at abias power threshold was not observed in the YBCO

w xcase 19 . This is due to the fact that the microbridgetemperatures are elevated to, at most, 2508C by

ohmic heating. Such moderate temperatures are notenough to induce substantial oxygen loss in YBCO,but as we have found, are more than enough to resultin substantial oxygen loss from YSCMO microstruc-tures. As we discuss below, this is because theeffective vapor pressure of the basal plane oxygen inthe YSCMO material is sufficiently high at ;2008Cthat even before the electromigration force is greatenough to directionally bias diffusion, oxygen beginsto leave the sample unless there is a substantialover-pressure of atomic oxygen supplied to the sam-ple’s surface.

As suggested by the overall resistivity changeseen in Fig. 4, a decrease in room temperatureresistivity from 4.5 to 2.5 mV cm, the results of EMbiases applied to YSCMO microbridges can be rela-tively spectacular. The effects on the low tempera-ture transport properties of a YSCMO microbridgeare demonstrated by Fig. 5 which shows the resistiv-

Ž .ity vs. temperature RT behavior of a bulk film andthat of a microbridge patterned from the film, beforeand after two EM biases. The zero resistance T ofc

the bulk film at 23 K is slightly lower than thatachieved by bulk synthesis in flowing oxygen at

w x10008C 6 . After patterning, there is an increase inresistivity and a transition to a semiconducting be-

Ž .havior r increasing with decreasing T at lowtemperature which indicates oxygen loss or vacancyaggregation during the patterning process. This isoften seen in lithographic processing of pure anddoped YBCO thin films. An anneal at 5008C in 1atm of O can substantially, but not fully, restore the2

properties of the microbridge back to those of thebulk film before processing, i.e., T ;23 K. How-c

ever, such an anneal was not applied in this case.Fig. 5 also portrays RT data for the microbridge

after the application of two EM biases, the first at1.17 MArcm2 applied for 16 h followed by the

2 Žsecond at 1.50 MArcm applied for 7.4 min there2 Ž .were previous 1.33 MArcm 18 min and 1.42

2 Ž ..Marcm 12 min EMs. After the final EM, theresistivity of the microbridge at 300 K was reducedfrom 4.57 to 1.22 mV cm, and T was increased toc

62 K. This elevated T is nearly three times thec

original value for the un-patterned film, even afterextended oxygen annealing. A T of 62 K is alsoc

higher than has been reported for any bulk sample ofŽ .this material composition YSr Cu Mo O ,2 2.75 0.25 z


Fig. 5. Resistivity vs. temperature for a YSCMO film and microbridge as-deposited, after patterning into a microbridge, and afterelectromigration of the microbridge at 1.17 and 1.50 MArcm2.

even after high pressure O annealing. This dramatic2

increase in T is caused by the increase in basalc

plane oxygen content and order that EM induces, andsuggests that YSCMO films, and, by inference, bulkYSCMO material, even when carefully and exten-sively annealed in 1 atm O can still be substantially2

oxygen deficient. A T of 62 K for YSCMO micro-c

bridges is, however, not the limit for this material.Higher bias electromigration of this and similar

samples was hindered by the aggregation of oxygenvacancies in the negative electrode. At the highestEM bias levels, the resistivity of the vacancy-richplume region increases until catastrophic thermalfailure occurs in the negative electrode. As we showbelow, with another means of oxygenating YSCMOfilms, it is possible to achieve T ’s for YSCMO filmsc

and microbridges as high as 75 K.

4.1. Raman spectra from electromigrated structures

Raman spectroscopy was employed to directlyexamine the effect of electromigration on the oxygencontent and order of YSCMO microbridges. Fig. 6shows Raman spectra collected from a series ofpoints in a microbridge after a long EM at 1.5

2 Ž .MArcm . Spectrum a was taken in the positive

electrode near the entrance of the microbridge andindicates relatively well-oxygenated material there.

Ž . Ž .Spectra b – g were taken along the bridge region,and demonstrate the effect of the maximal electromi-

Ž .gration current density. Spectrum h was taken inthe negative electrode near the exit of the micro-bridge and also indicates relatively well-oxygenated

Ž .material. Spectrum i was taken from the plume areawhere oxygen vacancies have been deposited by the

Ž .action of electromigration. Spectrum j was obtainedfrom a section of the microbridge in the voltageelectrode which was not subjected to large currentdensities.

The Raman spectra collected from the plumeŽ Ž ..region spectrum i demonstrate a substantial in-

crease in the intensity of the 148, 310, 326, and 450cmy1 modes. Just as in the Raman spectra collectedafter a 5008C N anneal, an increase in the intensity2

of these modes demonstrates a high concentration ofŽ .oxygen vacancies in the plume region. Spectrum j

does not demonstrate the degree of de-oxygenationŽ .seen in spectrum i , and, as expected, is similar to

spectra taken from the bulk film after a 5008C annealin O .2

Ž . Ž .Returning to spectra b – h in Fig. 6, which werecollected from the electromigrated bridge region, we


2 Ž .Fig. 6. Raman spectra collected from 10 separate points along a YSCMO microbridge after electromigration at 1.5 MArcm : a taken atŽ . Ž . Ž .the entrance of the microbridge in the positive electrode, b through g along the length of the microbridge, h at the exit of the

Ž . Ž .microbridge in the negative electrode, i in the plume region, and j in a bulk region of a voltage tap.

note that most of the resultant changes in the Ramanmodes seen there are simply a continuation, in re-verse, of the effects of de-oxygenation observed in

Ž .the plume and N annealed spectra Fig. 2 . The 148,2

310, and 450 cmy1 modes have decreased to non-ex-istence, and the 328 cmy1 mode has decreased inintensity while the line shape has changed fromLorentzian to Fano. The behavior of these modes,apart from the anomalous 310 cmy1 mode, followsthe expected extrapolation from the behavior ofYBCO as oxygen content and order is increased tonear optimum levels. This information verifies thatthe oxygen content in the bridge region has beenincreased by EM, as indicated by the dramatic im-provements in electrical transport properties.

Ž . Ž .Spectra b – g in Fig. 6 also demonstrate a highdegree of point to point uniformity throughout theelectromigrated structure. The disappearance of the310 cmy1 mode, which is demonstrated by polarizedRaman measurements shown in Fig. 3, made on adifferent sample before and after undergoing a simi-lar EM, confirms that this mode is associated withthe presence of basal plane oxygen vacancies anddisorder.

The effect of electromigration on the broad peaks

at 540 and 600 cmy1 is somewhat different forŽ . Ž .YSCMO than YBCO. Spectra b – g in Fig. 6

demonstrate that the peak at ;540 cmy1 decreasesin wave number to ;520 cmy1 and sharpens signif-icantly when the oxygen content is maximized. Sucha downward shift is the reverse of the observedeffect in YBCO. In the case of YBCO, it has beenshown that the apparent upward frequency shift of

y1 Ž y1 .the 501 cm mode 540 cm in YSCMO withincreasing oxygen content is actually the result of achanging ratio within the measurement volume ofthe amount of Ortho I phase material relative to thatof Ortho II and tetragonal material, each of whichhave different apical oxygen vibration frequenciesw x20 . Thus, we take the apparent downward fre-quency shift of the YSCMO apical oxygen modewith increasing oxygen content as a result of thegrowth of the strength of the apical oxygen mode offully oxygenated material at the expense of thestrength of the higher frequency apical mode ormodes of oxygen deficient YSCMO material.

We note the persistent presence of the 600 cmy1

defect mode in the well-oxygenated bridge region,and the fact that this peak is sharper in both the

Ž Ž . Ž ..oxygen rich bridge region spectra b – g and the


Ž Ž ..oxygen poor plume region spectrum i , than in thematerial that has not been subjected to EM, i.e.,

Ž . y1spectrum j . The persistence of the 600 cm defectmode, even when the material is well-oxidized byprolonged EM, is explained by the incorporation ofMo dopant atoms which permanently break the sym-

Ž .metry of the Cu Mo –O chains. The sharpness of thedefect mode in both the well-oxygenated bridge re-gion and the poorly oxygenated plume region maybe explained by an increase in vacancy or oxygenorder at both high or low oxygen concentrations,respectively.

Finally, we note one anomalous change in theRaman spectra after EM. The 148 cmy1 mode hasbeen replaced by a broader structure centered about

y1 Ž . Ž .165 cm . Spectra a and c shown in Fig. 3demonstrate that this 165 cmy1 mode is present inboth the 458 co and cross-polarized measurementscontrary to the polarization dependence expected for

Ž . y1the mode due to the Cu 2 atoms. The 148 cmfeature, on the other hand, is not observable in thecross polarized case, as expected for this mode. Theorigin of the anomalous Raman feature at 165 cmy1

is not known, but is presumably due to the Modopants. It is only seen in microbridge YSCMO

samples after they have been subjected to a strongEM bias. A similar, but broader anomalous featurehas been seen at ;180 cmy1 in Co-doped YBCO

wmicrobridges after EM D. Chamberlain, B.D.xMoeckly, K. Char, R.A. Buhrman, unpublished. .

In Fig. 7, we show Raman spectra collected froma number of microbridges fabricated on the samechip, each of which is subject to an EM bias rangingin magnitude from 0–1.67 MArcm2. There is no

Ž . Žsubstantial difference between spectrum a no EM. Ž . Ž 2 .bias to spectrum b EM bias of 0.5 MArcm .

Ž y1The variation in the structure at 500–600 cm istypical of variations seen across a film prior to EM

. Ž . Žor annealing experiments. Spectrum c 0.672 . Ž . Ž 2 .MArcm , spectrum d 0.83 MArcm , and spec-

Ž . Ž 2 .trum e 0.92 MArcm begin to demonstrate theeffect of the temperature excursion due to the ohmicheating during the EM. In particular, the dual peakstructure at 310 and 328 cmy1 becomes pronouncedafter biasing at these current levels. In Fig. 2, thisspectral structure first appeared at N anneal temper-2

atures of ;2508C. Current levels of 0.67 to 0.92MArcm2 correspond to the power threshold seen inthe electromigration experiment portrayed in Fig. 4which caused a linear increase in sample resistivity.

Ž . Ž .Fig. 7. Raman spectra from YSCMO microbridges after various electromigration experiments at increasing bias: a No EM, b after a 0.502 Ž . 2 Ž . 2 Ž . 2 Ž . 2 Ž . 2MArcm bias, c after 0.67 MArcm , d after 0.83 MArcm , e after 0.92 MArcm , f after 1.17 MArcm , g after 1.50 MArcm ,

Ž . 2 Ž . 2h after 1.58 MArcm , i after 1.67 MArcm .


Fig. 8. Resistivity vs. temperature measurements for the same YSCMO microbridge: as measured after electromigration at 1.50 MArcm2,after a subsequent 19 h anneal in O at 5008C, and after a re-electromigration at 1.67 MArcm2.2

The similarity of the Raman spectra from micro-bridges subjected to electromigration power levelswhich cause an increase in resistiÕity, to the Ramanspectra of samples annealed in an N ambient at 2002

to 3008C confirms that the first stage of the electro-

migration in YSCMO is to resistively heat the sam-ple to temperatures of 200–3008C, which in thismaterial results in the onset of oxygen loss.

As the electromigration bias is increased furtherto 1.17, 1.50, 1.58, and finally 1.67 MArcm2, the

Fig. 9. Raman spectra collected for the same YSCMO microbridge as in Fig. 8 as taken after electromigration at 1.50 MArcm2, after asubsequent 19 h anneal in O at 5008C, and after re-electromigration at 1.67 MArcm2.2


resultant Raman spectra in Fig. 7 indicate that theoxygen content and order are increasing substantiallyrather than decreasing. This is demonstrated by thedecrease in intensity of the 450, 326, 310 and 148cmy1 modes in addition to the sharpening of thestructure encompassing the 544 and 601 cmy1

modes. At these bias levels the rate of oxygen va-cancy aggregation under the EM bias, which leads toenhanced vacancy diffusion, has become such thatthe EM bias is able to sweep vacancies out of themicrobridge faster than new vacancies are generated

w xby thermal activation 11,21,22 . Thus, the oxygencontent in the microbridges increases dramatically asindicated by the Raman spectra, the decrease inresistivity, and low temperature transport properties.

These dramatic effects are, however, reversible.Fig. 8 portrays RTs for a microbridge sample afterelectromigration at 1.50 MArcm2, after a subse-quent 19 h anneal at 5008C in 1 atm of O , and2

finally after a re-electromigration at 1.67 MArcm2.Fig. 9 portrays Raman spectra from the bridge afterthe first and second EM. These figures demonstratethat the increase in oxygen content and order can beachieved, eliminated, and re-achieved with littlechange in the end result as demonstrated by thenearly identical RTs and Raman spectra of the well-oxygenated, electromigrated bridge. Figs. 8 and 9

also clearly demonstrate that a canonical anneal in5008C at 1 atm O does not achieve optimum oxy-2

genation for this material, and actually destroys mostof the benefits of electromigration.

5. Ozone annealing studies

To qualitatively examine the issue of the equilib-rium oxygen vapor pressure of YSCMO, we haveperformed a series of ozone and oxygen annealingstudies on un-patterned thin films and microbridges.Ozone is often used in molecular beam epitaxyŽ .MBE and co-evaporation of cuprate superconduc-tors to provide a high flux of atomic oxygen to thefilm’s surface during deposition, while keeping thebackground pressure low enough to allow these tech-

w xniques to be effective 9,10 . In our annealing experi-ments, we have used an industrial ozone generator toflow ozone at atmospheric pressure through a cold-wall annealing chamber while the film to be an-nealed is mounted on a temperature controlled heaterblock.

Fig. 10 shows the effect of increasing O rO2 3

anneal temperature on the RT behavior of four dif-ferent YSCMO films. All anneals were performed inflowing O with ;2% O at 1 atm. Once the heater2 3

Fig. 10. Resistivity vs. temperature data for four different YSCMO bulk films before and after O rO annealing.2 3


was turned on, samples would ramp up to, but notexceed, the anneal temperature in less than 10 min.The heater was left on for 1.5 h. Once turned off, thesamples would cool to room temperature in ;30min. Sample A was initially a comparatively poorfilm with a very low T due to its less than optimumc

growth conditions, but a 2008C anneal increased Tc

from 3 to 14 K. Thus, even at temperatures as low as2008C, the large partial pressure of atomic oxygenprovided by ozone decomposition can drive addi-tional oxygen into the crystal structure, increasing Tc

and decreasing r .n

By annealing sample B at 3008C, we were able toincrease T from 19 to 72 K. This value is 10 Kc

greater than the best result obtained from electromi-gration of microbridges grown in O . Sample C was2

annealed at 4008C, increasing the T from 24 to 68c

K, while sample D was annealed at 5008C increasingT from 24 to 75 K. Variation in initial film qualityc

may account for the relatively small differences inthe final T for the 300, 400, and 5008C anneals.c

While electromigration has been shown to locallyincrease the concentration of atomic oxygen in amicrobridge, ozone anneals can globally increase theconcentration of atomic oxygen at the surface of theentire film, allowing a much higher concentration ofoxygen to diffuse into the film.

Fig. 11 demonstrates the effects of subsequentO rO anneals on sample A. An anneal in flowing2 3

O with ;1% O at 2508C for 2 h increased the T2 3 c

to 37 and an additional 2 h at 2508C with ;1% O3

increased T to 45 K. Thus, flowing O rO annealsc 2 3

at temperatures as low as 2508C can surpass the levelof oxygenation achieved by canonical 450–5008C 1atm O anneals used to oxygenate YBCO. In addi-2

tion, the results from sample A demonstrate that bulkYSCMO thin films do not reach an equilibriumoxygenation state when exposed to O rO at 2508C2 3

for only 2 h.Once established, this increase in oxygen concen-

tration is unstable upon subsequent annealing in O .2

The effect of O annealing on the RT behavior of a2

YSCMO film that was deposited and cooled inO rO instead of O is shown in Fig. 12. After2 3 2

growth, the sample was subjected to O anneals at 12

atm, and at increasing temperatures of 200, 250, and3008C for 3 h each. The moderate 2008C anneal wasenough to decrease the T of the film from 71 to 48c

K suggesting a partial loss or disordering of thechain oxygen. The 2508C anneal continued this trendby decreasing T to 19 K. At this point, the resistiv-c

ity had increased by nearly a factor of three and theRT curve began to show an upturn in resistancebefore T is achieved. The 3008C anneal caused ac

Fig. 11. Resistivity vs. temperature data for a YSCMO bulk film before and after sequential O rO anneals.2 3


Fig. 12. Resistivity vs. temperature data for a YSCMO bulk film deposited in an O rO ambient after various anneals.2 3

dramatic quantitative and qualitative change in thematerials properties of the film. The film now ex-hibits semiconducting transport properties, i.e., in-creasing resistivity with decreasing temperature.

Increasing the O anneal temperature to 5008C,2

did not re-establish superconductivity. As Fig. 12demonstrates, the first, 1 h, 5008C anneal decreasedthe room temperature resistivity to 4900 mV cm, butthe film was still semiconducting in nature. Anadditional 3 h anneal at 5008C in O further de-2

creased the room temperature resistivity to 4560 mV

cm, indicating that equilibrium had not been reachedafter 1 h at 5008C. The semiconducting nature of theRT curve, however, remained unchanged. The effectof these O anneals was reversed by a 5008C 1.5 h2

anneal in ;2% flowing O , which returned T to 723 c

K and the resistivity to 1220 mV cm.In addition to the resistance vs. temperature data

described above and portrayed in Fig. 11, Ramanspectra were also collected from this sample, firstfollowing its growth in O rO and then after each2 3

annealing step. The results are shown in Fig. 13. Thelow intensity of the 148, 310, and 328 cmy1 modes,and the sharpness of the 540 and 600 cmy1 modes in

Ž .spectrum a indicates that, as expected, the as-grownfilm is relatively well-oxygenated. Following the 3 h

Ž .2008C O anneal, the Raman response, spectrum b ,2

did not demonstrate any pronounced changes, al-though T of the film had decreased considerably.c

This result is attributed to a small degree of ther-mally activated oxygen loss localized near smallangle grain boundaries, etc., accompanied by local-ized disordering of the remaining chain oxygen.Such a non-uniform loss can impact the transportproperties of the cuprate more strongly than theRaman spectra which is more sensitive to changes inthe average oxygen content under the 2-mm mea-surement spot than to localized changes in oxygenorder which are represented in a potentially smallvolume fraction.

After the 2508C O anneal, the more substantial2

quantitative and qualitative change in the nature ofthe normal metal conduction in the film is accompa-nied by discernible changes in the Raman signal.

Ž . y1Spectrum c in Fig. 13 shows that the 148 cmmode has sharpened and increased in intensity. The310 and 328 cmy1 modes have increased while thestructure at 540 to 600 cmy1 has softened, indicatinga decrease in oxygen order as well as content. Fol-

Ž .lowing the 3008C 3 h, 1 atm O anneal, spectrum d2

indicates that the film was highly oxygen deficient asdemonstrated by the increase in the strengths of 148,


Ž . Ž . Ž .Fig. 13. Raman spectra collected from a YSCMO bulk film: a as-deposited in O rO , b post 2008C O 3 h anneal, c post 2508C O 32 3 2 2Ž . Ž . Ž .h anneal, d post 3008C O 3 h anneal, e post 5008C O 1 h anneal, f post 5008C O rO 1.5 h anneal.2 2 2 3

310, and 328 cmy1 modes and by the downwardshift of 148 cmy1 mode. Interestingly, the sharpnessof the 540 cmy1 and 600 cmy1 modes has recoveredslightly. This was also seen in the severely de-oxygenated plume regions of electromigrated sam-ples, and indicates that with low oxygen content, thevacancies become more ordered. After the 1 h, 5008C,

Ž .O anneal; spectrum e shows that the intensity of2

the 148, 310, and 328 cmy1 modes have decreased,and the wave number of 148 cmy1 mode has movedupwards, confirming that the oxygen content hadincreased, but was still below the well-oxygenatedlevel. Finally, after the 5008C O rO anneal, spec-2 3

Ž .trum f in Fig. 13 shows that the film is nowcomparatively well-oxygenated, with the slight de-crease in the intensity of the 310 and 328 cmy1

Ž .peaks compared to that of spectrum a indicating asmall improvement in overall oxygen content andorder compared to that of the initial film. This isconsistent with the slight increase in T observed inc

Fig. 12.

5.1. Electromigration of YSCMO microbridges an-nealed in ozone

Since the degree of T enhancement achieved byc

electromigration of O annealed films was limited by2

the creation of the defect rich plume region in serieswith the microbridge, we pursued EM experimentswith microbridges that had been annealed in ozone.By annealing in ozone, the films began with a lowervacancy density, and electromigration could achievean even higher level of oxygenation and T enhance-c

ment in the bridge region. The improvementsachieved were equivalent to the best results obtainedby ozone annealing.

Fig. 14 portrays the RT for a sample before andafter electromigration. They are nearly identical witha T of 75 K in each case. Essentially, we havec

found that electromigration of O rO annealed2 3

structures only acts to create oxygen vacancies in thebridge region due to heating, which at higher biaslevels are removed by the electromigration process,more or less returning the microbridge to its originalstate before the imposition of the electromigrationbias.

Raman spectroscopy measurements as shown inFig. 15 do, however, indicate some small differencesin ozone annealed microbridges before and afterelectromigration. These spectra were taken after thefilm had been deposited in O rO , patterned and2 3

then re-annealed in ozone after patterning. SpectraŽ . Ž .a and b are from the microbridge taken after theelectromigration whose minimal effect on the RT


Fig. 14. Resistivity vs. temperature data for a YSCMO microbridge before and after the electromigration at 1.75 MArcm2.

Ž . Ž .was shown in Fig. 14. Spectra c and d are froman adjacent microbridge which was not electromi-

Ž . Ž .grated, and spectra e and f are from a bulk section

of the film. Note that the 148, 310, and 328 cmy1

Ž . Ž .modes in spectra c and d have a slightly lowerŽ . Ž .intensity than those of spectra e and f , indicating

Ž . Ž .Fig. 15. Raman spectra for a YSCMO film deposited in an O rO ambient and then annealed in O rO after patterning: spectra a and b2 3 2 32 Ž . Ž . Ž . Ž .were taken in a bridge after EM at 1.75 MArcm , spectra c and d were taken in a bridge with no EM, spectra e and f were taken in

an unpatterned portion of the thin film.


a greater oxygen content in the microbridge evenbefore electromigration. This can be attributed to theincreased ability of oxygen to diffuse into a pat-terned microbridge due to the exposed sidewalls.

The only notable changes in Fig. 15 betweenŽ . Ž .microbridge spectra with, spectra a and b , and

Ž . Ž .without, spectra c and d , electromigration is thatthe 148 cmy1 mode is replaced by the more roundedmode centered at 165 cmy1 and the 310 cmy1 modehas disappeared leaving behind only the small Fanoshaped 328 cmy1 mode. These changes indicate agreater degree of oxygen ordering in the microbridgethat has been electromigrated even though there islittle or no effect on T . The equivalence of thec

transport measurements and the near equality ofRaman spectra achieved by electromigration andozone annealing of a microbridge supports the asser-tion that each of these techniques has achieved anear optimum level of oxidation.

6. Discussion

The electromigration and ozone annealing resultshave shown that a high level of oxygenation can beobtained in YSCMO thin films at 3008CFTF5008C

Ž .if an adequate delivery of oxygen ions to the Cu 1basal plane sites can be effected. The degree ofoxygenation is verified by the Raman spectra ofmaterial that has undergone such treatment. Asdemonstrated by Figs. 6, 7, 13 and 15; the strengthand position of the principal YSCMO Raman modesafter O rO annealing or electromigration are fully2 3

consistent with what would be expected on the basisof Raman spectra of fully oxidized YBCO, apartfrom the high frequency defect mode attributable tothe symmetry breaking presence of the Mo in thebasal plane. In both cases, the highest T , 75 K, thatc

has been obtained for YSr Cu Mo O thin films2 2.75 0.25 z

is substantially higher than has been previously ob-tained with YSCMO bulk material of the same Mocontent, even with the use of a high pressure, 400

w xatm, post-growth O annealing protocol 6 . This2

suggests that in earlier efforts, the material wasunder-oxidized and oxygen disordered despite highpressure anneals. Certainly, our Raman and T mea-c

surements indicate a high degree of oxygen defi-ciency and disorder in films annealed at 5008C in 1

atm O as compared to the conditions indicated by2

Raman spectroscopy and T for films annealed in 1c

atm O rO .2 3

The basal plane oxygenation of a cuprate super-conductor upon exposure to O can be considered as2

a three-step process; the adsorption of a O molecule2

upon the cuprate surface, the dissociation of thismolecule into atomic oxygen, and the diffusion ofthe oxygen atom or ion into the bulk of the materialwhere it eventually takes occupancy in a vacant

Ž .basal plane O 1 site. The equilibrium oxygen con-tent in the cuprate, and the material’s oxygen vaporpressure are then determined by the equilibrationbetween the chemical potential of the basal planeoxygen, which depends on the activation energy forthe generation of chain oxygen vacancies, and thechemical potential of the two-dimensional atomicoxygen gas at the cuprate surface, which is generatedby the thermal decomposition of adsorbed O2

molecules.Equilibrium partial pressure studies of well-

oxygenated YBCO have indicated that it begins tow xlose oxygen in 1 atm of O at ;4758C 23 . In2

contrast, we find that the onset of substantial loss ofbasal plane oxygen from electromigrated or ozoneannealed YSCMO samples begins at even moremodest temperatures, ;200–2508C in 1 atm O . On2

the other hand, annealing of highly oxygen deficientYSCMO films in 1 atm O at 5008C and then2

cooling the film to 208C can partially re-oxygenatethe material. Thus, we conclude that the effectivevapor pressure of the basal plane oxygen in YSCMOis strongly concentration dependent, particularly so

w xin the oxygen rich regime, as it is for YBCO 23 .However, for fully or nearly fully oxygenatedYSCMO material this oxygen vapor pressure is muchhigher for TF5008C than is the case for equallywell-oxygenated YBCO. Because of the apparentlystrong concentration dependence of equilibrium oxy-gen vapor pressure of YSCMO, it is possible topartially oxygenate the material in 1 atm O at2

5008C for example. Yet judging from previous bulkstudies, annealing in 200 to 400 atm O may not be2

effective in completely or even nearly completelyoxygenating YSCMO.

The decomposition of ozone near and at the film’ssurface results in a much higher chemical potentialfor the surface gas of atomic oxygen. Thus, exposing


a YSCMO film to ozone at 250 to 5008C is sufficientto drive a complete or nearly complete oxygenationof the material. Equivalently, electromigration is ableto sweep pre-existing and thermally generated oxy-gen vacancies out of a microbridge in a drivendiffusion process faster than new vacancies can bethermally generated, again resulting in a complete ornearly complete local oxygenation of the material.

While the effective oxygen vapor pressure ofYSCMO is much higher, and perhaps more stronglydependent on basal plane oxygen concentration, thanthat of YBCO, the diffusion of oxygen in YSCMOappears to be substantially slower than in YBCO.We have found that, as shown in Fig. 12, a 1 h ofexposure to 1 atm O at 5008C is insufficient to2

establish an equilibrium level of oxygenation, eventhough we find oxygen can be lost from this materialthrough exposure to inert gas ambient at tempera-tures as low as 2008C. Thus, the effective oxygendiffusion rate for YSCMO, at least in thin films,appears to be much slower than in YBCO, eventhough the equilibrium vapor pressure is much higher.Presumably, this difference in diffusion rates is dueto the presence of the Moq6 dopant atoms in thebasal plane, which would retard oxygen diffusiondue to the 6-fold oxygen coordination in the fullyoxidized state, and would include bonds to oxygen

Ž .atoms in the otherwise normally vacant O 5 sitesbetween Cu–O chains.

This low oxygen diffusion rate in YSCMO, if italso occurs in bulk material and other doped cupratesof this type, could be important in thermogravimetricstudies because temperature changes as slow as 0.68

per minute may not be gradual enough to maintainquasi-equilibrium with respect to oxygen content inthe 5008C and lower temperature range. In a recentextensive study of the bulk synthesis and characteri-zation of transition element doped YSr Cu O2 3 7yd

material, it was found that the optimized synthesisand high pressure, 220 atm, O annealing of such2

systems could result in single phase material with thehighest bulk T ’s yet reported for this class of cupratec

w xsuperconductors 8 . For example, T ’s of 75 K werec

obtained with bulk Y Ca Ba Cu W O which0.9 0.1 2 2.8 0.2 z

was synthesized at 9908C and then extensively an-nealed in 220 atm O at 7008C. While no results2

were reported for YBa Cu Mo O , similar tech-2 2.75 0.25 z

niques were used to produce YBa Cu Mo O2 2.8 0.2 z

superconductors with a maximum T of ;66 K.c

This is a substantial enhancement over the previousw xbest result for this material 6 . However, if every-

thing else is held constant, an increased dopantconcentration will lead to a lower T due to thec

lower carrier concentration. Thus, a comparison withthe 75 K T of our best YBa Cu Mo O thinc 2 2.75 0.25 z

films does suggest that, despite the good supercon-ducting properties of the bulk YBa Cu Mo O2 2.8 0.2 z

material, it still may be somewhat under oxidized,although considerably less than our films which areannealed in 1 atm O at 5008C and below.2

In the case of the bulk W-doped material, thermo-Ž .gravimetric analysis TGA was used by Dabrowski

et al. to determine the oxygen composition z to be7.14 when the material was annealed in 220 atm O ,2

but TGA also showed that slowly cooling this mate-rial, at 0.68 per minute, in 1 atm O only reduced z2

to 7.12. Neutron scattering results for the same sam-ple indicated an oxygen content of zs7.3 with thedifference with the TGA result being tentatively

w xattributed by Dabrowski et al. 8 to a possiblenon-stoichiometric character of the tungsten oxideupon hydrogen reduction. This, however, would notaffect the TGA measurement of the relatiÕe changein z upon cooling.

A change of z of 0.02 upon cooling in 1 atm O2

suggests that the bulk YBa Cu W O material is2 2.8 0.2 z

relatively stable, with a much lower oxygen vaporpressure at 5008C and below, than is the case for ourYBa Cu Mo O thin films. However, given our2 2.75 0.25 z

qualitative observation of a very slow oxygen diffu-sion rate for TF5008C in YBa Cu Mo O thin2 2.75 0.25 z

films, it may be possible that the bulk powder TGAmeasurements were not gradual enough to establishcomplete equilibrium with the 1 atm O ambient for2

TF5008C. Unfortunately, the effect of the TGAprocess on the superconducting properties of thebulk YBa Cu W O material was not reported2 2.8 0.2 z

since that could be helpful in establishing whetherthe material was still uniform after the 1 atm O2

slow cooling.

7. Conclusions and summary

We have grown thin films of YSr Cu Mo O2 2.75 0.25 zŽ .YSCMO by pulsed-laser deposition and have used


Raman micro-spectroscopy to extensively character-ize the material in various degrees of oxygenation.Compared to what can be achieved with a 1 atm O2

anneal, we have found that annealing in 1 atmO rO mixtures at TF5008C can greatly increase2 3

the oxygen content and order, as determined by theresulting transport properties and Raman spectra.Ozone annealing can bring the material to the pointwhere the peak positions of the Raman spectra arequite close to what would be expected for optimallyoxygenated material based on an extrapolation fromRaman results on well-oxygenated YBCO. With suchannealing, we have been able to increase the T ofc

our films to a value, 75 K, which is substantiallyhigher than has been previously achieved with bulkmaterial of the same composition. At the same time,the increased sharpness of the 450, 544, and 601cmy1 Raman peaks modes provide qualitative evi-dence for an improvement in oxygen order.

The application of electrical biases to YSCMOmicrobridges, which are nominally at room tempera-tures, results in the long range electromigration ofbasal plane oxygen vacancies out of the microbridge.This process can be equally or, as indicated by theRaman spectra, slightly more effective than ozoneannealing in locally oxygenating the microbridgematerial and in enhancing the oxygen order. How-ever, in either case, subsequent anneals in 1 atm O2

at TF5008C results in substantial oxygen loss andmuch lower T ’s.c

We conclude that the dopant oxygen in YSCMOthin films is not more stable than in YBCO, at leastfor TF5008C. Instead, the oxygen vapor pressure ofwell-oxygenated YSCMO is sufficiently high, that atsuch temperatures, the dissociation of molecular oxy-gen on the surface of the material is too low for 1atm O to provide an adequate over-pressure of2

atomic oxygen to maintain the fully oxygenated state.However, annealing studies of our thin films havequalitatively indicated that the low temperature oxy-gen diffusion rate is substantially lower in YSCMOfilms than in YBCO. Thus, while oxygen is lessstable in YSCMO films than in YBCO, it also takesmuch more time to establish the equilibrium state inYSCMO than in YBCO.

These YSCMO results raise the question as towhether other cation-doped or fully substitutedYBCO materials are also substantially grossly oxy-

gen deficient in comparison to what can be achievedwith YBCO when both are carefully annealed in 1atm of molecular oxygen. Co-doped YBCO thinfilms have been widely examined and employed asthe normal metal layer in YBCO superconductor-normal metal-superconductor Josephson junctions.Early experiments have shown that Co-doping onto

Ž .the Cu 1 site distorts the crystal lattice of the YBCO,but to a lesser degree and certainly in a differentmanner than does the combined complete substitu-tion of Sr for Ba and partial substitution of Mo for

Ž .Cu 1 . Bulk studies of Co-doped YBCO typicallyemploy O anneals sufficient for the complete oxida-2

w xtion of YBCO 24,25 . However, recent experimentson Co-doped YBCO microbridges have shown thatthe oxygen content of O annealed material can be2

substantially enhanced and T generally increased bycw xoxygen vacancy electromigration 26 .

In addition to their possible usage as normal metallayers in high T Josephson junctions, doped cupratesc

have been of considerable interest due to the insightstheir study may yield into the details of cupratesuperconductivity. Our observations regarding oxy-gen stability and order in YSCMO and Co-dopedYBCO might raise some questions regarding theproper interpretation of some of these studies. Forinstance, recent work with RSr Cu Re O has2 2.85 0.15 z

reported a strong correlation between the ionic radiiw xof R, T , and orthorhombicity 27 . These samplesc

were annealed at 5008C in 1 atm O for 10 h and2

then slow cooled at 18Crmin to room temperature.In light of the results presented here, it is likely thatthose samples were not fully oxygenated or ordered.Thus, the maximal T may not have been achieved.c

If the samples were oxygenated in O rO , the corre-2 3

lation between the variation of T , orthorhombicity,c

and ionic radii might have been affected.In general, the crystal lattice of doped cuprate

superconductors of the YBCO type are stressed incomparison to the situation in undoped YBCO mate-rial. This lattice stress is affected by the dopantspecies, concentration and distribution, i.e., whetherthe dopants are uniformly dispersed or microscopi-cally clumped. This in turn affects the bond anglesand bond lengths which can strongly affect the sta-bility of the chain oxygen, and thus, the oxygenvapor pressure of the material. We have demon-strated this in YSCMO films when in the well-


oxygenated state. At the same time, the presence ofŽ .transition metal dopants on the Cu 1 chain sites can

lower the oxygen diffusion rate. Thus, there can be asubstantial effect on the oxygen content and orderthat can be achieved or maintained under variousoxygen annealing protocols. As a result, oxygenannealing protocols which are effective for YBCOthin films are not fully effective for YSCMO thinfilms. A substantially higher partial pressure ofatomic oxygen, such as can be provided by ozoneannealing, is required to achieve optimum oxygena-tion, and a longer time is required to reach thiscondition. Whether a similar situation occurs moregenerally in the YSr Cu M O class of materials,2 3yx x z

and perhaps also in doped YBCO superconductors,as the Co-doped YBCO thin film studies suggest, isan intriguing issue that warrants further investiga-tion.

Acknowledgements

This research was supported by the Office ofNaval Research, Grant No. N00014-97-I-0142, andby the National Science Foundation through the useof the Cornell Nanofabrication Facility which is partof the National Nanofabrication User Network, andthrough the use of the central research facilities ofthe Cornell Materials Science Center.

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Documents

Optimal oxygenation of YSr Cu Mo O thin filmsbuhrman.research.engineering.cornell.edu/pubs/physicaC299_147.pdf · YSr Cu W O at 220 atm of O and 70022.800.20z 2 8Cto achieve a maximal