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J Mater Sci: Mater Electron (2006) 17: 483488

DOI 10.1007/s10854-006-8222-4

Growth and morphology of c-axis oriented Nd1.85Ce0.15CuO4ythin films prepared by pulsed laser deposition technique

B. Prijamboedi S. Kashiwaya

Received: 12 August 2005 / Accepted: 24 February 2006C Springer Science+Business Media, LLC 2006

Abstract On the electron-doped superconductor of

Nd2xCexCuO4y (NCCO) thin film, the presence of

non-c-axis oriented grains, which are identified as (110)

reflection peak at 2= 32.5 in the x-ray diffraction (XRD)

spectrum is always observed. Meanwhile, high quality thin

films without having impurities are necessary for device

applications. We study the growth of NCCO thin film

prepared by pulsed laser deposition technique and found

that the volume fraction of (110) oriented grains depends

on the laser fluence. With the laser fluence of around

2.2 J/cm2, NCCO thin film, which is free from the presence

of non-c-axis oriented grains, could be obtained. The atomic

force microscope images show that with the absence of

(110) oriented grain the c-axis oriented grains grow into

rectangular shape with a spiral growth mode. The rocking

curve measurement for (004) peak give a full width at

half maximum value of 0.12, which confirms the superior

quality of the film and this film has superconducting critical

temperature (Tc) at 21 K with a transition width (Tc)

of 1 K.

1. Introduction

The discovery of superconductivity in Nd2xCexCuO4y(NCCO) has received much attention since this material has

unique properties compared with the hole-doped cuprates su-

perconductor [1]. The charge carrier type in this system as a

B. Prijamboedi () S. Kashiwaya

Nanoelectronics Research Institute of AIST Tsukuba Central 2,

Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, JAPAN

e-mail: [email protected]

result of Ce4+ substitution is electron, which is confirmed by

Hall measurement [2]. The other unique properties are the

absent of apical oxygen atoms outside the CuO2 layer and

T2-like temperature dependence of resistivity in the normal

state. It is believed that this material could provide useful

information to elucidate the mechanism of superconductiv-

ity in the cuprates. Meanwhile, the NCCO has number of

difficulties and disadvantages for application such as a rela-

tively low superconducting critical temperature (Tc = 23 K

with x = 0.15), the presence of superconductivity within a

narrow Ce concentration range [1] and the necessity to re-

move oxygen atom in order to obtain high Tc value. Despite

these problems, NCCO has some advantages such as better

surface stability compared to YBa2Cu3O7y (YBCO) mate-

rial [3] and longer in-plane coherence length of 7080 A [4],

which make this material a potential candidate to be used

as device. The superconductor material could open new pos-

sibility in the development of new electronic devices such

as superconductor-semiconductor hybrid junction [5, 6],

hybrid Josepshon field effect transistor [7] and for

NCCO; it is a potential candidate for the fabrication of p-n

junction device from the hole- and electron-doped supercon-

ductors [8].

Thin film is the most suitable form for device applica-

tion. The common method to fabricate NCCO thin films

is by pulsed laser deposition (PLD) technique. This tech-

nique is simple, versatile and can reproduce the stoichiomet-

riccompositionof depositedthin filmfrom thetarget material

[9]. Meanwhile, the PLD technique also has some disadvan-

tages, such as medium quality of the thin film surface and

many interdependent deposition parameters. Some NCCO

thin films with high critical superconducting transition tem-

perature have been successfully fabricated by PLD methods

[1012]. However, the presence of peaks at 2= 32.5 and

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484 J Mater Sci: Mater Electron (2006) 17: 483488

68 in the X-ray diffraction (XRD) spectra are always re-

ported even in the sample prepared by RF-sputtering method

[13]. Those peaks indicate the presence of non-c-axis ori-

ented grains and it could induce defects on the film. There

were reports that discussed the origin of these peaks. Some

reported that these peaks come from the reflection of (110)

and (220) planes of NCCO crystal [14] and the other sug-

gested that it comes from the reflection of (400) and (800)plane of NdCeO3.5 (NCO) compound [15]. The same peaks

were also observed in the Ce-free Pr2CuO4y thin films [16]

and it raised a doubt that NCO compound is responsible for

the origin of those peaks.

We studied the growth of high quality c-axis oriented

NCCO thin films prepared by pulsed laser deposition tech-

nique, which do not show foreign peaks at 2= 34.5 and

68 in the XRD spectrum. High quality NCCO thin film with

less defect and impurity is necessary for the device applica-

tion since the cuprate based superconductor device is very

sensitive to the disorder [17]. We focused the study on the

effect of laser fluence on the target surface to the growth

and morphology of the NCCO thin film. We used a method

that has been reported before that the laser fluence could be

simply adjusted by shifting the distance between the waist

of the focusing laser lens and the target surface, d wt [18].

We found that the intensity of the foreign peaks decreases

with the decreasing of the laser fluence. We could prepare

high quality NCCO film without the presence of (110) ori-

ented grains as it was confirmed by XRD and atomic force

measurement (AFM) and found different morphology of the

NCCO thin film affected by the presence of (110) oriented

grains.

2. Experimental

Polished single crystal SrTiO3 (001) with thickness of

0.5 mm was used as substrate and it was glued on a stainless

steel block heater with silver paste. The deposition chamber

then was evacuated into a typical pressure of 1.33 104 Pa

and the heater temperature was raised to 820C. The tem-

perature was measured by a sheathed thermocouple inserted

into the block heater.During deposition,oxygen pressure was

maintained at 93.3 Pa by flooding the oxygen gas with flow

rate of 17.5 sccm into the chamber. An ArF excimer laser

(Compex 205 from LambdaPhysik, = 193 nm) with a rep-

etition rate of 2 Hz was used as excitation source. A mask with

an opening with size of 2 1 cm2 was put on the laser beam

output window. The laser energy measured after it passed the

mask is about 100 mJ. The laser beam is focused by a lens

with focal length of 50 cm onto the surface of NCCO ceramic

target prepared by a conventionalsolid-state reaction method.

Thepurity of theNCCO target(Dowa Co.Ltd.) is 99.9% (3N)

and with density of 90%. During deposition, target is rotated

at 10 rpm so that the laser beam does not impinge on the same

spot. The target-substrate distance was set at 32 mm. After

deposition, the NCCO thin film sample was subsequently

annealed at 700C in vacuum of 1.33 103 Pa for about

15 minutes.

To study the effect of the laser fluence on the growth of

NCCO thin films, some samples were prepared in different

distance between the waist of the lens and the target surface,dwt; sample A: 50 cm, sample B: 51.5 cm and sample C:

53 cm. The laser fluence was estimated by measuring the size

of the laser spot on the target for each different dwt value.

With constant deposition time (45 minutes), the thickness of

the films are 3300 A for sample A, 2200 A for sample B and

1600 A for sample C. Thin films sample then were charac-

terized by an atomic force microscope (AFM) to analyze its

surface morphology and it was performed in the air at room

temperature. The x-ray spectrometer was used to study the

NCCO thin film crystal structure and resistivity and super-

conducting critical temperature were measured by a standard

four-probe technique in the temperature range between 2 K

and 300 K.

3. Results and discussions

Figure 1 shows the x-ray diffraction spectra of NCCO thin

film sample prepared with different waist of the lens to tar-

get surface distance, dwt. The spectra show that it mainly

consists of the peaks from c-axis oriented grains of (001)

reflection planes and SrTiO3 substrate. In sample A and B,

peaks at 2= 32.5 and 68 are clearly observed, indicating

the presence of non-c-axis oriented grains. The same peaks

are not observed in sample C. The ratio between the intensity

of peak at 2= 32.5(I110) and the intensity of peak from the

(002) reflection (I002) is shown in Table 1 as well as the de-

position rate and the laser fluence on the target. It shows that

by increasing the waist of lens to the target surface distance

or decreasing the laser fluence, the deposition rate decreases

and it will reduce the volume fraction of non-c-axis oriented

grain on the NCCO film.

The rocking curve measurement ( scan) was carried out

for the (004) reflection plane of the NCCO thin films. In

Figure 2, the results of the rocking curve scan for (004) peak

from all samples are shown. The rocking curve measurement

gives full width at half maximum (FWHM) values of 0.9 for

sample A, 0.6 for sample B and 0.12 for sample C. The

small FWHM value obtained for sample C indicates a highly

oriented epitaxial growth of NCCO thin films characterized

by a narrow mosaic spread distribution.

In Figure 3(a) and 3(b), AFM images from sample A,

scanned from a 5 5 m2 and 0.5 0.5 m2 areas are

shown. On this film, we can clearly observed two different

types of grain. We notice some large and flat grains with size

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J Mater Sci: Mater Electron (2006) 17: 483488 485

Fig. 1 Plots of intensity vs. 2 of x-ray diffraction spectra for

Nd1.85Ce0.15CuO4y thin film grown on SrTiO3 with different distance

between the waist of lens and target surface (dwt): a). Sample A;

dwt = 50 cm. b). Sample B; dwt = 51.5 cm. c). Sample C; dwt =

53 cm

around 80 200 nm2. The other grains have smaller size and

round shape with diameter around 60 nm2 and these grains

are higher than the previous mentioned one. The size of the

small grains is very homogenous and well distributed on the

thin film surface. The small grains observed on sample A

seem to be different with those of the spherical particulate or

droplet, which are commonly observed on the film prepared

by laser deposition technique since the particulates has in-

homogenous size. In sample B (Figure 3(c)), larger grains or

islands were observed compared to the previous film. These

Fig. 2 Graph of rocking curve measurement result for the (004) re-

flection peak for Nd1.85Ce0.15CuO4y thin films prepared with different

waist lens to target surface distance

grains are not regularly shaped. Detail scan on sample B

(Figure 3(d)) reveals the presence of nano-sized grains grow

between the large grains as well as on the top ones. TheAFMmicrographs of sample C are shown in Figure 3(e) and 3(f).

On the sample C, we can clearly see grains grow with rect-

angular shape. The grain is characterized by hillock growth

with a clearly seen terracing structure. Some screw disloca-

tions can be observed and it indicates the spiral growth mode

of the NCCO grains. The step height of the terrace is around

10 A, which was determined from a cross section analysis

and it is close to one unit cell c-axis length of NCCOof 12 A.

We can mention here that few nano-sized grains or particles

can still be observed on some of NCCO grains.

By comparing the AFM micrographs from these samples,

we realize that the population and size of small and sphericalgrains decreases as the intensity of (110) peak in the XRD

spectra decreases. It would indicate that these grains are re-

sponsible for the (110) and (220) peaks observed in the XRD

spectra. In sample A, the (110) oriented grains are dominant

compared with the c-axis oriented ones, where the c-axis

oriented grains are seen as flat grains and it is confirm with

the high value of I(110)/I(002). Meanwhile, in the sample B,

we should expect that the volume fraction of c-axis oriented

grains increases and it grows as large irregular shape islands.

As we saw on the AFM micrograph of sample A and B, the

non-c-axis grains hinder the growth of the large flat c-axis

Table 1 Values of some deposition parameters (waist of lens to target distance and laser fluence), NCCO growth

rate and ratio between the intensity of (110) and (002) reflection peaks of NCCO.

The waist of lens

to target Laser fluence NCCO growth

Sample Name distance, dwt (cm) (J/cm2) rate (A/pulse) I110/I002

A 50 4.2 0.6 4.75

B 51.5 2.8 0.4 1

C 53 2.2 0.3 0

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(a) (b)

(d)

(f)

(c)

(e)

Fig.3 AFMimages of Nd1.85Ce0.15CuO4y thin filmgrown on SrTiO3.

a). Sample A, scanned on 5 5 m2 surface area. b). Sample A,

scanned on 0.5 0.5 m2 surface area. c). Sample B, scanned on

5 5 m2 surface area. d). Sample B, scanned on 1 1 m2 surface

area. d). Sample C, scanned on 5 5 m2 surface area. e). Sample C,

scanned on 1 1 m2 surface area

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J Mater Sci: Mater Electron (2006) 17: 483488 487

oriented grains and as the result, we have very small c-axis

oriented grains in sample A and irregular shape of c-axis ori-

ented grains in sample B. The AFM image of sample C give

a detail image of defects caused by the small (110) oriented

grains on the c-axis oriented ones (Figure 3(f)) but since the

number of (110) oriented grains is so small, we could not see

the peaks at the XRD spectrum. The AFM analysis shows

that by reducing the laser fluence on the target surface wouldgive a favorable condition for c-axis oriented grains to grow

as large rectangular grains.

A slow growth rate as the result of low laser fluence on

target surface could be the factor for growing high qual-

ity c-axis oriented NCCO thin film. In the case of YBCO

thin film prepared by PLD method, the a-axis and c-axis ori-

ented grains both can grow on the films at a certain condition

[19]. According to the adatom migration model, the adatom

of each element would migrate in a relatively long distance

and climb steps before residing at the stable site if they are

forming c-axis oriented grains. Meanwhile, short distance

movement would be needed for a-axis oriented growth [20].

The same mechanism could be considered for the growth of

NCCO thin film and in this case, the growth of (110) oriented

grains seems to be similar with the growth of a-axis oriented

grain in YBCO. Since all three NCCO samples were pre-

pared only with different laser fluence on the target surface,

it could be assumed that the most relevant factor to be con-

sidered in the NCCO case is the density of evaporated atoms

that impinge the substrate surface. In the condition of low

evaporated atoms density, we could expect a lower probabil-

ity of inter collision and early reaction between atoms and

therefore, the mobility of the adatoms on the surface would

increase and it would give more opportunity for the adatoms

to find a stable position and to form c-axis oriented grains.

The kinetic model might explain that the observed (110) ori-

ented grains are smaller and higher than for c-axis oriented

ones since for this growth, the adatoms only need to move in

very short distance.

The result from resistivity measurement is shown in Fig-

ure 4. Sample A shows the highest resistivity compared with

other samples and has a minimum at temperature around

75 K. The onset of superconducting critical temperature

(Tc, onset) is at T = 18 K but the zero resistivity (Tc0) was

observed below 11 K. The resistivity was found to be lower

for sample B than for sample A with a minimum at tempera-

ture around 50 K. Tc0 for sample B is at 15 K. The resistivity

data of the two samples (A and B) show a characteristic of the

underdoped NCCO, which has a resistivity minimum above

Tc. However, as we have seen on the AFM micrographs, both

samples have poor inter-grain connection and dislocation de-

fect due to the presence of (110) oriented grains and it would

give high electrical resistance value. For sample C, Tc,onsetwas observed at around 22 K with Tc0 at21Kasitisshownin

the insert of Figure 4. The resistivity results again confirm the

Fig. 4 Graph of electrical resistivity vs. temperature of

Nd1.85Ce0.15CuO4y thin film grown on SrTiO3. The insert graph

shows the resistivity of sample C around its superconducting critical

temperature

quality of the NCCO thin film grown with low laser fluenceon the target surface.

4. Summary

High quality NCCO thin film has been fabricated by pulsed

laser deposition technique. The sample that did not show

(110) reflection peak in the XRD spectrum was fabricated

with low laser fluence on the target surface of 2.2 J/cm2 and

the fraction of the non-c-axis oriented grains on the NCCO

film can be controlled by changing the laser fluence on the

targetsurface.The rocking curve measurement of this NCCO

thin film gave a FHMW value of 0.12 and resistivity mea-

surement showed a Tc0 valueof 21K withTc = 1 K,which

confirm the high quality of the sample. The non-c-axis ori-

ented grains would induce some defects and change the mor-

phology on the c-axis oriented ones and the c-axis oriented

grains grew into rectangular shaped grains with a clear view

of the terracing structure.

Acknowledgements We would like to thank H. Yamasaki, Y. Naka-

gawa, and J. C. Nie for allowing us using some equipments and help

during measurement and deposition. B. P would thank M. Murugesan

for suggestion and discussion. This work was supported by the Japan

Society for the Promotion of Science (JSPS).

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Prijamboedi-B._Growth-and-morphology-of-c-axis-oriented-Nd1.85-Ce0.15CuO4-y-thin-films-prepared-by-pulsed-laser-deposition-technique_2006.pdf