Properties of Thin Epitaxial Aerosol MOCVD Ce02 Films Grown on ...

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Abstract. We have exanlined the properties of thin epitaxial C e Q films prepared by aerosol MOCVD. The films were deposited on (li02) sapphire at deposition ...
JOURNAL DE PHYSIQUE IV Colloque C5, s u p p l C m e n t au Journal de Physique 11, Volume 5, juin 1995

Properties of Thin Epitaxial Aerosol MOCVD C e 0 2 Films Grown on (1102) Sapphire K. Frijhlich, J.

hut, D. Machajdik, A.P. Kobzev*, F. Weiss**, J.P. Senatem** and K.H. Dahrnen***

Institute of Electrical Engineering, SAS, Dlibravski cesta 9, 842 39 Bratisiava, Slovakia *Frank Laboratory of Neutron Physics, JINR Dubna, Head P.O. Box 79, Moscow, Russia ** Laboratoire des Matkriaux et du Gknie Physique, ENSPG, URA 1109 du CNRS, BP. 46, 38402 Saint Martin dHkres, France *** Laboratorium fur Anorganische Chemie, ETH Zentrum, 8092 Zurich, Switzerland

Abstract. We have exanlined the properties of thin epitaxial C e Q films prepared by aerosol MOCVD. The films were deposited on (li02) sapphire at deposition temperatures between 500 "C and 900 "C. The best properties were observed for the film grown at high deposition temperatures. The films have thickness - 0.2 pm and the full width at half maximum of the rocking curve 0.3 - 0.4 ". The milumal yield measured by backscattering spectrometry in the channeling mode mas 5.5 %, confirming high preferred orientation of the depos~tedfilms. The epitaxial character of the CeQ films was revealed by the measurement of the p s c a n . The aerosol MOCVD CeOL films were found to be suitable as a buffer layer for a preparation of superconducting high-T, films YBa2Cu,0superconducting films deposited on the CeOz l(ll02)sapphire exhibit superconducting transition temperature = 86 K.

Ta4,

1. INTRODUCTION

Sapphire with b u f f e r C e 0 2 layer becomes rather a u~liversaland attractive substrate for the gowtli of superconducting high-T,films [I-41. Sapphire exhibits excellent mechanical properties cornbi~iedwith very good dielectric behaviour. The R-plane sapphire (sapphire cut along (1i02) plane) is commo~llyused in s e m i c o n d u c t o r t e c h n o l o g y a n d is available in d i a m e t e r s u p to 100 mm. Mean value of the thermal expansion coefficient for sapphire is 8 x 10" C", which is rather close to those for high-T, compounds (911 x C-' for YBa2Cu307). However, bare sapphire chelnically reacts at elevated temperatures with high-T, oxides and therefore a buffer layer is necessary for the growth of the high-T, films. A buffer layer of CeO? seems to be an appropriate choice. Ce02 is chemically and thermally stable and has cubic structure with a lattice parameter a = 0.541 1 nm. Its lattice is directly (or rotated in basal plane by 45 ") perfectly matched to most lattices of the high-T, oxides (see Tab. I), thereby permitting their epitaxial growth. Tab. I Lattice constants of 111gl1-T,ox~desand lattice mismatch \nth C e Q

Compound

YBaCuO (123)

Lattice parameter in the basal plane nm 0.3886, 0.3818

Lattice mismatcli with Ce02

[%I 0.9

0.2, 0.7

BiSrCaCuO (22 12), (2223)

0.54.

TIUaCaCuO (1113). (1113)

0.38;

0.75

HgBaCaCuO ( 17 11).( 1123)

0.385

0.75

0.38

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1995562

JOURNAL DE PHYSIQUE IV As ligh quality high-T, oxide films grow epitaxially, the preferred orientation of deposited superconducting film is strongly determined by the crystallographic structure and degree of the preferred orientation of the bottom substrate layer. For the growth of c-axis oriented high-Tc films on CeO, buffer -

layer it is necessary to have (001) buffer orieutatiou. The atomic st~uctureon the surface of (1102)plane of sapphire is pseudo-rectangular with the atomic distance along [1210] and [loill directions 0.476 nm and 0.5 12 nm, respectively. Tlis results in rather high value of the lattice mismatch, 13.7 % and 5.7 %, in these two directions. Consequently, the conditions for the CeO, growth sliould be carehlly adjusted in order to prepare hi& quality epitaxial film. We have recently developed a new modification of MOCVD process for oxide f i s growth called aerosol MOCVD 151. The process is based on die tra~lsportof metalorganic precursors, dissolved in appropriate sohent, in the form of aerosol. After the evaporation of the aerosol, the process proceeds as in a conventional MOCVD method. Using the aerosol MOCVD we have prepared highly oriented thin films of CeO, [ 5 ] , SrTiO, [ 6 ] , and YBa,Cu,O, [5,7]. CeO, & n s were prepared on (001) MgO and (li02)saplire single crystal substrates. While CeO, films on MgO showed a small amount of in-plane ~nisoriented crystallites, CeO, films on sapphire exhibited higli degree of in-plane epitaxy. However, a small quantity about 1 % of (1 1 1) oriented grains was always present in these films. In this paper we describe in more detail the growth and properties of CeO, films on (1102) sapphire. The conditions for the growth of epitaxial CeO, buffer layers are examined and optimized. It is shown, that using aerosol MOCVD, it is possible to prepare high quality epitaxial buffer layers for the gowtli of high-Tc iilms.

2. EXPERIMENTAL

Thin CeO, films were prepared in a low-pressure aerosol MOCVD apparatus depicted in Fig. 1. The apparatus consists of a system for gas flow control, ultrasonic injector (I), liquid solution delivery (7,3). hot wall type reactor, liquid nitrogen trap (6) and vacuum pump (7). Reactor is a quartz tube with 30 mm outer diameter in the deposition part and 50 mm in the evaporation part. The reaction space is heated externally by resistive furnaces. The liquid solution delivery is controlled using a needle valve (2) at the input of t l ~ eultrasonic injector. The amount of generated aerosol depends on the solution delivery and on tlie amplitude of ultrasonic vibrations. Iu order to produce a homogeneous spray pattern by the ultraso~iic injector, there exists an optimal delively rate of the liquid solution. III our case this optimal delivery rate corresponds to -0.4 mllmin. The injected aerosol is transported using Ar to the evaporation part bf the reactor kept at 170 Tab. 2 Typical growth conditions for Ce02 films "C. After the evaporation and mixing with oxygen, the precursor vapours enter the Precursor concentration 0.01 molefl deposition chamber, heated at the temperatures TI, between 500 "C and 900 VoIume of the liquid solution 5 ml "C. The substrates (4) are placed on a Flow rate of Ar 36 141 sampler holder. tilted in an angle of 22 ". We have observed that tlie deposition occurs Flow rate of O2 30 IAi ~nostlyon the samples on the holder and the Total pressure 10 torr depositio~ion the reactor walls is negligible. Nonconsumed reaction products condense in Deposition time 15 mi11 a liquid nitrogen trap placed between the reactor and vacuum pulnp. The wl~ole apparatus is kept dal-ing the deposition at 11 10 t o n by tlie pressure regulating valve (5). -

1 ultrasonic injector

2 needle valve 6

3 4 5 6 7

reservoir with the liquid solution sample regulating valve LNZtrap vacuum pump

Fig. 1 Scheme of the aerosol MOCVD apparatus

We have used two types of Ce precursors, heptafluorodimethyl octanodionate, Ce(fodl3, and ether tetramethyl heptanodionate, Ce(tmhd)4. The precursors were dissolved in diethyleneglycol di~netl~yl (diglyme) in a concentration of 0.01 molil. Ce(tmhd)J was fouud to be less soluble hi diglyrne and less stable. The solution of Ce(tmhd)4 ui diglyme became cloudy after several hours, indicating reaction of tlie precursor, presumably with the water in diglyme. Therefore we have used in all further experiments Ce(fodh as a precursor. For one deposition run we used 5 ml of the solution. A typical deposition time was I5 ~nin.Slides of polished (li02)sapphire having dimensions 5 x 5 Iwere used as a substrate. Tlie substrates were cleaned before the deposition by acetone. Together with the sapphire. (001) Si sl~deswere placed on tlie substrate holder. The films deposited on Si substrates were polycrystalline and it was possible to etch them using dilute W. After photolithographic pattenling we liave used the films on Si substrate for the determination of the thickuess using Talystep p~.ofilometcr. The prepared films were characterized by X-ray diEaction and Rutherford backscatte~i~~g spectrometry (REIS). Standard 0 - 3 0 technique was employed for the detection of ( I I I ) oriented crystallites present in the film. The degree of the preferred orientation perpendicular to the fi11~1siirface was deduced from the value of the full width at half maximum (FWHM) of the rocking curve of tlie 003 CeO? diffraction. The quality of the preferred orielitation was for some samples examinated also by RBS 111 channeling mode using 3.05 MeV He' ion beam. The epitaxial character of tlie films was studied by performing azi~nutiial@-scan of tlie 53 1 d i f i a c t i o ~ ~ . h~ order to examine the suitability of the prepared buffer substrates, we liave deposited a YBa3Cu307 film on the top of CeOy'sapphire substrate. Tetramethyl heptanodionates of Y, Ba. and Cu were used as precursors. Tlie deposition was canied out at the substrate temperature TI, = 800 "C. The film has thickuess of 0.3 pm. Supercoliducting trausitioli was measured using standard four probe uietl~od.

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'25-536 3. RESULTS AND DISCUSSION

3.1 Temperature growth rate dependence

We have observed that the deposition of CeOz in the aerosol MOCVD reactor occurs preferably on the surface of the tilted sample holder and the deposition rate on the reactor walls is low. Fig. 2 shows the thiclcness of the b s placed on the sample holder as a function of the deposition temperature. The films were prepared at To = 500 "C - 900 "C using 5 ml of the solution with the concentration c = 0.01 molell. The time of the deposition was 15 min, resulting in the growth rate of l k m h at the 500 900 800 700 600 substrate temperature TI, = 900 "C. The dependence in the Fig. 2 F indicates that tbe deposition process is F governed in the whole range by the same '3 mechanism with the activation energy E, = -- 1 C 29 kllmole. We have speculated that with regards to the observed low deposition on the reactor walls and to the rather low value of the activation energy, the process is limited by the transport in the gas phase. 0,1 This seems to be reasonable, as the precursor molecules have large dimensions, and there are also vapours of the solveut 0.0008 0.001 0 0,0012 0,0014 present in the reaction atmosphere. -

-

-

-

irr [

Fig.2

-

i ~ ]

Growth rate of the Ce(X films as a function of the substrate temperature

3.2. Competing growth of (111) oriented CeO, inclusions

The presence of (111) oriented inclusions in (001) CeO, matrix is a common problem for preparation of CeO, films by various techniques [2,3]. We have observed that the (1 11) oriented grains growth can be e h i n a t e d by lowering the deposition rate or by increasing substrate temperature T,. We have controlled t l ~ edeposition rate by changing the concentration of the precursor in the solvent. Starting from the concentration we have used in our previous experiments, c = 0.02 moleA, ref. [ 6 ] ,we have observed the intensity of the ( I 1 I ) dfiaction on 0-20 scan, corresponding to several volume percent of ( l I I) oriented grains. Using lower concentration, c = 0.01 molen or even c = 0.006 molell we have suppressed the presence of (I 11) oriented crystallites at elevated temperatures below the sensitivity limit of the X-ray diffraction analysis ( - 1 %). Competing growth of ( 1 l I) oriented blocks depends also on the substrate temperature. For the co~~centrationc = 0.01 molen of Ce(fod), in diglyme we have detected ( 1 11) grains only at the telrlpel-atul-esbelow 700 "c. Fig. 3 sllows difiaction patterns of the films prepared at two different substrate temperatures. The 1 1 1 difiaction is clearly observed in the difiaction patteni for the sample there are only 002 and 004 prepared at Ti,= 600 'c, Fig. 3a. For the sample prepared at T,, = 800 tliffi-ac~ionpcnks picscllt in t l ~ c0 - 2 0 scan. I'ig. 311. OC

Fig 3 . X-ray &£fractionpattern of the films prepared at a) TD=600 'C and b) TD = 800 "c.

3.3 Quality of the preferred orientation The prefemed orientation in the [OOl] direction was deduced from the full width at half maximum (FWHM) of the o-scan. Fig. 4 illustrates the dependence of the FWHM value on the deposition temperature. The films prepared at low deposition temperatures exhibit low preferred orientation wit11 the value of FWHM of several degrees. Highly oriented lilms (FWHM = 0.3 a - 0.4 ') were deposited at h41 deposition temperatures. T l ~ evalue of FWHM for the films deposited at high temperatures indicates high degree of epitaxial growth of (001) CeO? on (li02)sapphire. The Fig. 5 shows the o-scan for the film prepared at TI, = 900 "C.- The sharp peak in the o-scan is caused by the d a a c t i o n of 11011 monochromatic X-ray radiation on the 2204plaue of the substrate. The quality of the p r e f e ~ ~ e01-ie~~tation d in the direction along [OOI] axis of the CeO? film was esa~ninedby Rutl~erfordbackscatte~i~lg sl>ectto~netly in random and clla~ll~eling mode. Fig. 6 sllows both randon1 and channeling spectra for the film prepared at TI, = 800 "C. X-ray diffraction pattern for tllis sample exhibited only s h a ~ p002 and 004 peaks of CeO:, the 1 I I difiaction was below the X-ray detectio~l limit. The width of the ~.ockingcuwe was FWHM = 0.7 O. A cha~u~eli~lg ~nininlu~n yield of 5.5 ? ~ bwas observed for this film. indicating excellent quality of the prefened orientation. An increase of the channeling signal for Ce, corresponding to the upper part of the CeO: film is obserted in the spectrum. This supycsts tllc 111.csc11cc o f l ~ i ~ l conce~ltsatio~~ ~cs ofdcfecrs ill the ul)per 1):11tof the film

JOURNAL DE PHYSIQUE IV

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Fig.?

The temperature dependence of the full width at half maximu~n(FWHM) values of o-scan

0 200

400

600

800

channel Fig.6

RBS random and channeling spectra for C e 4 film prepared at T,, = 800 OC

o-scan of the 002 di&action of the CeQ film deposited at TD= 900 OC

Fig. 5

0

0

120

240

360

$["I Fig 7 0-scan measured on the 53 1/35 1 diffraction of the film prepared at T,= 900 OC

RBS method is appropriate for the detection of the diffusion in the interface between tlie film and the substrate. The diffusion of Al from tlie substrate into the film should be indicated on the RBS spectluln by broadening o f the edge co~respondingto Al. The width of the A1 edge for the film in Fig.5 is of the same value as for the surface edge of Ce. Thus. we can coliclude that tllere is no reaction between the Ce0: film and tlie AI203 substrate at the depositioti temeperature TD= 800 "C. Even for the films prepared at the temperatures up to 900 "C we didn't indicate interdiffusion at tlie interface. Consequently, CeO: film acts as all effective difksion barrier for A1303in the whole range of deyositioli temperatures. Epitaxial character of the C e 0 2 film was examined by the measurement of the @-scan on the 53 1135 1 diffraction using the tilt angle = 32.6 ". 111 tlie case of epitaxy. eight maxima are expected for tli~:, tlifli.nctio~l111 structures \\it11 cubic synietly. I'l~erefore the eight rnauim;~in the Fig. 7 clearly illdic;itcs cl)ita\-i;~lyro\\th of (00 1 ) C c 0 : o n ( 1103)snlq~l~irc.

3.4 Superconducting YBa,Cu,O, film on ~ e 0 , / ( l i 0 2 )sapphire

The ~ e 0 , / ( l i 0 2 )sapphire substrate was used for the growth of YBa,Cu,O, supercoliductil~gfilm The film was deposited at the deposition temperature TI, = 800 "C, at tlie same temperature as the preparation of the Ce02 buffer layer. X-ray dB?actiou of the deposited film revealed a typical spectrum for MOCVD grown YBa,Cu,O, superconducting film - llighly oriented 1-2-3 phase togetllel-wit11 sorrle peaks correspo~~ding to CuO. The thickness of the film was about 0.3 pm. Fis. 8 sl~owstlie depe~ldenceof the film t.esistancc as a h c t i o n of temperature. T l ~ efilm exhibits metallic behaviour \nth the resistalice ratio \ ,,,,R,,, = 3.1. The tralisitioi~to the superconducting state is sha~y.wit11 zero resistance at the temperature T,,;;,,, = 86 K Fig.8 Resistance as a function of the temperature for the YBa2Cu307 filni on CeO1_/( 1 102) sapplure si~bstrate

0

0

50

100

150

200

250

300

4. CONCLUSIONS

We have prepared high quality epitaxial CeO, films using aerosol MOCVD. The f i h s prepared at tlie deposition above 800 "C exhibited high degree of preferred orientation as iufered fiom the width of rocking curves aud from the RBS measurements in the cl~almeliugmode. In the whole range of deposition temperatures, Ce02 acts as an effective banier against the diffusion of A1 fiom the A1203substrate. The measurement of the @-scan on the 53 1/35 1 dB?action showed the epitaxial character of the prepared filin. The CeO- film prepared by aerosol MOCVD on /(li02)Al,0, substrate was found as a suitable substrate for the growth of supercouductiug high-Tc films. We have demonstrated this by tlle successive deposition of supercot~ducti~ig YBa,Cu,O, . film with T,

=

86 K on ~ e 0 , / ( l i 0 2 ) ~ 1 , 0substrate. ,

Acknowledgments The authors are s a t e f i l to A. Slani and S. ~ t e f i u i kfor their technical assistance. This work was partially supported by the Slovak Grant Agency for Science (grant No GAV 211087194) aud by the Swiss National Research Foulldatiou.

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