Crystallization of manganese cobalt nickelate films ...

Crystallization of manganese cobalt nickelate films prepared by chemical deposition Yujian Ge*, Zhiming Huang, Yun Hou, Tianxin Li and Junhao Chu National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yu Tian Road, Shanghai, China 200083 ABSTRACT Manganese cobalt nickelate films (Mn x Co y Ni 3-x-y )O 4 (MCN) are successfully prepared by chemical deposition method at a crystallization temperature of 600 °C , which is greatly reduced from the traditional sintered temperature of ~10501200 °C . From the XRD and AFM, we find the grain size of the MCN films increases from 20 to 60 nm with the annealing temperature increase from 600 °C to 900 °C . Keywords: diffraction, Atomic force microscopy, manganese cobalt nickelate film

1.

INTRODUCTION

Manganese cobalt nickelate (Mn x Co y Ni 3-x-y )O 4 [1] has been widely utilized in commerce and space as a material for negative temperature coefficient (NTC) thermistor devices [2, 3], such as temperature compensation devices [4, 5], temperature sensor devices [6], surge protection devices [7], room-temperature thermometer bolometer infrared detectors. However, manganese cobalt nickelate (MCN) is commonly prepared from a mixture of elemental oxides by solid state reactions at high temperatures (~1050-1200 °C ), which cannot be explored for modern devices with silicon microfabrication technology. In this paper, we report the successful preparation of MCN film Mn1.56 Co 0.96 Ni 0.48 O 4 with semiconducting behavior by chemical deposition method at a low crystallization temperature of 600 °C , which is much lower temperature than those previously reported ones [8-10].

2.

EXPERIMENTAL PROCEDURE

The starting materials were the tetra-hydrated acetates Ni(CH 3 CO 2 ) 2 ⋅ 4(H 2 O) , Mn(CH 3 CO 2 ) 2 ⋅ 4(H 2 O) and

Co(CH 3CO 2 ) 2 ⋅ 4(H 2 O) (quoted purity >99%.). The weighed acetates were mixed in a ratio of Mn:Co:Ni=52:32:16, and then dissolved in glacial acetic acid. The mixture was then filtrated as the stock solution. The Mn 1.56 Co 0.96 Ni 0.48 O 4 films were deposited on Si and Pt/Ti/SiO 2 /Si substrates, respectively, by spin coating of the solution at 4500 rpm for 20 s. After coating each layer, the films were given by a heat treatment at 450 °C for 5 min using a rapid thermal annealing method in the air atmosphere to remove residual organics. Then films were annealed at 600, 700, 800, 900 °C for 8 min, respectively. The average thickness of a single coated as-grown layer, measured by ellipsometry, was about 20 nm. The desired film thicknesses of ∼600 nm were achieved by repeating the spin-coating and heat-treating cycles. The crystallization of the MCN films were studied by x-ray diffraction (XRD) using a Rigaku D/MAX-550 X-ray diffractometer. The surface and grain size of the films were observed using atomic force microscopy (AFM). *[email protected]; phone 86-21-65420850 ext. 24205; fax 86-21-65830734

Sixth International Conference on Thin Film Physics and Applications, edited by Wenzhong Shen, Junhao Chu, Proceedings of SPIE Vol. 6984, 69842G, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.792120

Proc. of SPIE Vol. 6984 69842G-1 2008 SPIE Digital Library -- Subscriber Archive Copy

3.

RESULTS AND DISCUSSION

Si(222) (511)

(440)

(111)

(422)

(220)

(400)

(311)

Si(111)

Figure 1 shows the XRD patterns of the crystallographic structure and orientation of the MCN films on Si(111) annealed at 600 °C , 700 °C , 800 °C , 900 °C . We can see that multi-orientation peaks appear at a low temperature of 600 °C . The temperature is reduced nearly one half from the traditional sintered temperature of ~1050-1200 °C for the MCN material. It demonstrates that the crystallization of the MCN films has been realized at a very low temperature compatible with that of modern silicon semiconductor integration technology.

o

900 C o

800 C o

700 C o

600 C 20

30

40

2θ (deg)

50

60

Fig. 1 XRD patterns of the MCN films on Si substrate annealed at 600 °C , 700 °C , 800 °C , 900 °C .

The average grain size is calculated using Scherrer’s formula from the XRD patterns,

d = 0.9λ B cos θ

(1)

in which d is grain size, λ is the x-ray wavelength, B is the full width at half maximum, and θ is the diffraction angle.The calculated mean size of crystal grains is about 20-50 nm for the MCN films on Si substrate annealed at 600, 700 ,800 and 900 °C , respectively, and the grain size increases with the annealing temperature from 600 °C to 900 °C . Table 1. Grain size versus annealing temeprature Temperature

600 °C

700 °C

800 °C

900 °C

Grain size

17nm

28.9nm

39.9nm

46.9nm

Proc. of SPIE Vol. 6984 69842G-2

2.00

60.0 nm

10.0 flm

1.00

0.0 flm

1.

(a) (b) Fig. 2 AFM surface morphology of MCN films grown on Si substrate: (a) annealed at 600 °C ; (b) annealed at 900 °C .

The grain size is more clearly observed by the AFM image, as shown in Fig. 2. The size of the grains grows from 20 to 50 nm when the annealing temperature changes from 600 to 900 °C . The grain size is in good agreement with the XRD results. Meanwhile, we can see that the rectangular-shaped crystal grains are stacked one by one in multiple orientations. The rectangular boundaries of the grains are not parallel each other. Fig. 2 further indicates that the films are composed of the nano-size grains with random orientation. So the MCN films we prepared are multiple crystalline films.

4.

CONCLUSION

In conclusion, we have successfully prepared stable and uniform semiconducting MCN thin films by chemical deposition method at a crystallization temperature of as low as 600 °C . The grain size of the MCN film increases from 20 to 60 nm with the annealing temperature from increase 600 °C to 900 °C .

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation (No. 60407014 and 60527005), and Shanghai Grants (06QH14018 and 06QA14056).

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