Effects of preferred orientation on electrical properties ofMn1.56Co0.96Ni0.48O47δ spinel films

Wenwen Kong a,b,c, Haijun Bu c, Bo Gao a, Long Chen a,b,c, Fei Cheng a,b, Pengjun Zhao a,b,Guang Ji c, Aimin Chang a,n, Chunping Jiang a,c,n

a Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials andDevices, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, Chinab University of Chinese Academy of Sciences, Beijing 100049, Chinac Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Science, Suzhou 215123, China

a r t i c l e i n f o

Article history:Received 4 July 2014Accepted 23 August 2014Available online 30 August 2014

Keywords:Mn1.56Co0.96Ni0.48O47δThin filmCrystal structureElectrical properties

a b s t r a c t

Mn1.56Co0.96Ni0.48O47δ thin films are fabricated by Pulsed Laser Deposition process at different oxygenpartial pressure. The X-ray diffraction shows that the preferential orientation of the films changes from(4 0 0) plane to (1 1 3) as the oxygen pressure increases. The resistance of the (1 1 3)-oriented films isfound to decrease compared to those of the (4 0 0)-oriented ones. Meanwhile, the activation energyEa increases dramatically. A detailed X-ray photoemission spectroscopy study of the lattice oxygencontent and Mn cation distribution is performed. The results reveal that higher oxygen partial pressureduring deposition should be responsible for the preferred (1 1 3) plane growth and the greatly elevatedamount of Mn3þ and Mn4þ , which can result in significantly different electrical properties ofMn1.56Co0.96Ni0.48O47δ spinel films.

& 2014 Published by Elsevier B.V.

1. Introduction

The Mn–Co–Ni–O (MCN) spinel oxides exhibit excellent nega-tive temperature coefficient (NTC) characters and high stability,which makes them well suited for use as precise measurements inthermometry and uncooled infrared detections [1,2]. Though theelectrical, magnetic, thermal and mechanical properties of theMn1.56Co0.96Ni0.48O47δ (MCN) have been extensively studied inrecent decades [2–5], the effects of oxygen partial pressure used infabricating MCN films, which is closely related to the chargecarriers density and cation vacancies concentration and thuselectrical properties, still attracts much considerations [2,4,6,7].Until now, little effort has been made to study on the differentpreferred orientation on electrical properties of MCN thin filmsbased on oxygen stoichiometry.

In this paper, the electrical properties of MCN films fabricatingby PLD process at various oxygen pressures were reported. It iscurious that the oxygen pressure has a significant influence on thepreferential orientation and manganese cations distribution,which further affects the electrical properties of MCN films.

2. Experimental details

Mn1.56Co0.96Ni0.48O47δ (MCN) thin films were deposited byPLD with KrF (248 nm) excimer laser on thermal-grown silicacapped silicon. Stoichiometric MCN target with 25 mm diameterwas installed on a rotator. The target-to-substrate distance was71 mm. The repetition rate of the laser pulse was 2 Hz with energyof 250 mJ. The substrate temperature was kept at approximately250 1C during deposition. The oxygen partial pressure (PO2 ) variedin the range of 2�10�4–2�10�1 Pa. All films were deposited for120 min which amounts to a thickness of about 50 nm. Afterdeposition, the films were in situ rapid annealed under the growthatmosphere at 700 1C for 10 min. Finally, the as-prepared filmswere all characterized by AFM, XRD, XPS and electrical propertiesmeasurements.

3. Results and discussion

Fig. 1 gives the AFM pictures and the normalized XRD patternsof MCN thin films deposited at various oxygen partial pressures.All the obtained MCN films are atomically smooth and compact,and no obvious cracks are observed. The root-mean-square rough-ness are 1.0, 0.8, 0.7 and 0.5 nm for films deposited at 2�10�4,2�10�3, 2�10�2 and 2�10�1 Pa, respectively. All the films

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Materials Letters

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n Corresponding authors.

Materials Letters 137 (2014) 36–40

crystallize in single spinel structure. However, the preferentialorientation shows obvious dependence on PO2 . At low oxygenpartial pressure PO2 ¼2�10�4 Pa, only the (4 0 0) diffraction peakappears. As PO2 increases, the (4 0 0) peak intensity graduallydiminishes while the (1 1 3) diffraction peak starts to dominate.In addition, the increase of PO2 leads to further increase of the(1 1 3) peak intensity, which generally means the improvement ofcrystallinity of the films [8].

In the deposition of MCN oxide films, the films initially nucl-eate without preferential orientation. When the film grows from theinitial nuclei, the crystal plane of the nuclei with minimum surface freeenergy may remain parallel to the film surface, because the growthrate of the crystal plane with minimum surface free energy is lowerthan that of the other crystal planes [9]. Thus, the crystal orientation ofthe MCN film is controlled by the nucleation and growth of the grains,which is determined by the variation of the oxygen pressure.

Fig. 1. AFM pictures and the normalized XRD patterns of MCN films deposited at various PO2 from 2�10�4 to 2�10�1 Pa.

W. Kong et al. / Materials Letters 137 (2014) 36–40 37

In order to demonstrate that more oxygen is incorporatedinto the lattice under higher oxygen partial pressure, O1s X-rayphotoemission spectroscopy (XPS) was performed (as shown in

Fig. 2(a)). There are two peaks in the O1s XPS spectra. The peakaround 529 eV can be assigned to the lattice oxygen and anotherpeak around 531 eV is due to the absorbed oxygen [10]. From

Fig. 2. (a) O1s XPS spectra and (b) Mn2p3/2 XPS spectra of the MCN thin films.

W. Kong et al. / Materials Letters 137 (2014) 36–4038

Fig. 2(a), the O1s XPS peak shape of the films deposited at2�10�4 Pa is clearly distinguished from other ones. It suggeststhat the lattice oxygen content of these two kinds of films isdifferent. We estimate the percentage of lattice and absorbtionoxygen by applying a peak synthesis procedure [8], and the resultis summarized in Table 1. Obviously, the lattice oxygen contentincreases with PO2 , and it increases dramatically in (1 1 3)-orientedfilms compared to that in (4 0 0)-oriented ones.

In MCN, carrier hopping between neighboring Mn3þ and Mn4þ

in octahedral sites is responsible for the electrical conductivitywhile Mn2þ doesn’t participate in this process for the large on-sitecoulomb repulsion energy [8]. And also, the carrier type in MCN isclosely related to Mn3þ and Mn4þ through the thermopower

Q defined as Q ¼ kB=e� �

lnð1β½Mn3þ �½Mn4þ �Þ, where kB is Boltzmann constant,

e is the unit charge and β¼5/4 is the spin degeneracy factor [7].Clearly, Q is temperature independent but depends on Mn3þ/Mn4þ ratio thus the lattice oxygen content. The MCN is p-typesemiconductor for Q40 or n-type for Qo0 [7]. In order to obtainthe value of Q, we analysis the Mn2p3/2 XPS result of MCN filmspecimens in Fig. 2(b) and summarize the relative concentration ofMn2þ (641.9 eV), Mn3þ (642.8 eV) and Mn4þ (643.8 eV) [8]cations also in Table 1. The data illustrates that the Mn3þ andMn4þ pairs gradually increase with the increase of oxygen con-tent, especially from the (4 0 0)-oriented films to the (1 1 3)-oriented films. The value of Mn3þ/Mn4þ ratios of films is 1, 0.31,0.53, and 0.62 for films deposited at 2�10�4, 2�10�3, 2�10�2

and 2�10�1 Pa, respectively. Then, the values of Q can be alsocalculated, and summarized in Table 1 as well. All the Q values arenegative for MCN films, which means that films deposited from2�10�4 to 2�10�1 Pa are all n-type semiconductor materials.This is very different from many previous reports where MCN isconsidered as p-type semiconductor [7].

Fig. 3 shows the resistance (R)–temperature (T) dependences ofall the MCN films deposited at different oxygen pressure. Note thatthe films deposited at higher PO2 have lower resistance, the

resistance of (1 1 3)-oriented films (PO2 42�10�4 Pa) decreasesdramatically compared to that of the (4 0 0)-oriented ones (PO2 ¼2�10�4 Pa). Usually, electronic transport in MCN is described bythe hopping of small polarons, and the R–T relationship is given byR¼ CTexp Ea=kBT

� �, where C is a constant, Ea is the activation

energy [11]. By plotting ln R=T� �� 1=T and linear-fitting the data,

Ea can be subtracted from the slope. B constant defined asB¼ ln R1=R2

� �= 1=T1�1=T2� �

is used to characterize the sensitivityof thermistors [8]. For the potential temperature operational rangeof our films, we choose T1 ¼ 50 K and T2 ¼ 80 K as the twostandard temperatures to calculate B50/80. Table 1 also presentsthe B50/80 and Ea constants of MCN films. Both the B50/80 constantsand Ea of (4 0 0)-oriented films are far less than those of the(1 1 3)-oriented films, while these values of the (1 1 3)-orientedfilms only slightly change with increasing PO2 . Thus, the (1 1 3)-oriented films are more sensitive to the temperature variationthan the (4 0 0)-oriented films.

In (4 0 0)-oriented films, the content of Mn3þ and Mn4þ is verylow, which induces very low mobile electron density. Meanwhile,a large fraction of octahedral sites are occupied by cations otherthan Mn3þ and Mn4þ , the electronic transport from Mn3þ toMn4þ is interrupted and the carriers are strongly localized withina very short range. As PO2 increases, the content of Mn3þ andMn4þ increases dramatically, the electron density is also greatlyelevated. Moreover, the conduction path of the Mn3þ–O2�–Mn4þ

increases, the localization effect is strongly depressed. So, theresistivity of (1 1 3)-oriented films is considerably decreasedcompared to that of the (4 0 0)-oriented ones.

4. Conclusion

We have prepared MCN films at different oxygen pressures andinvestigated the relationship between the preferred orientationand electrical properties. With the increase of oxygen pressure, thepreferential orientation in films changes from (4 0 0) to (1 1 3).The electrical measurement shows that the resistance shows aconsiderable decrease from the (4 0 0)-oriented films to the(1 1 3)-oriented films. In the (1 1 3)-oriented films group, the Band Ea decrease as oxygen partial pressure increases from 2�10�3

to 2�10�1 Pa, which is due to the increase in Mn3þ and Mn4þ

pairs in the octahedral sites.

Acknowledgement

The authors acknowledge the financial support of the “OneHundred Talents Project Foundation Program” of Chinese Academyof Sciences, West Light Foundation of the Chinese Academy of Sciences(no. XBBS201111), the Xinjiang Program of Cultivation of YoungInnovative Technical Talents (no. 2013731022) and National NaturalScience Foundation of China (no. 21103225).

Table 1The analysis of the XPS spectra of O1s and Mn2p3/2, as well as the B50/80, Ea and Q.

PO2 (Pa) O Mn B50/80 (K) Ea (eV) Q

[Lattice oxygen] (%) [Absorbtion oxygen] (%) [Mn2þ] (%) [Mn3þ] (%) [Mn4þ] (%) [Mn3þ]/[Mn4þ]

2�10�4 28.85 71.15 58 21 21 1 1557 0.13 �0.2232�10�3 57.12 42.88 21 19 60 0.31 3807 0.32 �1.3942�10�2 58.20 41.80 20 28 52 0.53 3413 0.29 �0.8582�10�1 65.22 34.78 19 31 50 0.62 3263 0.28 �0.701

Fig. 3. The ln(R/T) versus 1/T plots of the MCN thin films.

W. Kong et al. / Materials Letters 137 (2014) 36–40 39

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