Characteristics of La 0 . 7 Sr 0 . 3 MnO 3 Films Modified by Aluminum Ions Implantation and Post-Implantation Annealing

The magnetron sputtered La0.7Sr0.3MnO3 films were implanted with different doses (5 × 1015 ions⋅cm−2 and 5 × 1016 ions⋅cm−2) of Al ions at different negative pulsed voltages (30 kV and 50 kV) by plasma based ion implantation and then annealed at 973 K for 1 h in air. The microstructure, surface morphologies, surface roughness, metal-insulator transition and room temperature emittance properties of the post-implantation annealed films were investigated and compared with those of the La0.7Sr0.3MnO3 film annealed at 973 K for 1 h in air. The results indicate that the postimplantation annealed films show single perovskite phase and obvious (100) preferred orientation growth. The Mn-O bond length, surface roughness and metal-insulator transition temperature (TMI) of the films can be effectively adjusted by changing implantation voltage or implantation dose of Al ions. However, the change of implantation parameters just has a small effect on room temperature emittance of the films. Compared with the annealed film, the post-implantation annealed films have shorter Mn-O bond length and lower room temperature emittance. The TMI of the films implanted at low voltage is lower than that of the annealed film, which mainly results from the degradation of oxidization during annealing process and the part displacement of Mn3+-O2−Mn4+ double exchange channels by Al3+-O2−-Mn4+. The post-implanted annealed film implanted at 50 kV/5 × 1016 ions⋅cm−2 has a higher TMI than the annealed film, which is 247 K. The increase of TMI of the film implanted with high dose of Al ions at high voltage can be attributed to the improvement of microstructure.


Introduction
ABO 3 type perovskite manganites La 1−x M x MnO 3 (M = Ca, Sr or Ba) with appropriate doping concentration (x) have a variable emittance property based on the metal-insulator transition [1]- [3].With the increase of temperature, they change from low emissive metal state to high emissive insulator state.Moreover, the emissivity exhibits drastic change in the vicinity of the metal-insulator transition temperature (T MI ) [4] [5].Due to the unique regulation capability of heat dissipation, these compounds have potential application for a smart cool radiator, which can automatically self-adjust its emissivity in response to the variations of environmental temperature and thermal load [6] [7].In order to get good thermal control effect, the emittance property of La 1−x M x MnO 3 should be adjusted to meet different thermal control applications because of the difference of thermal control demands.Generally, the emissivity of La 1−x M x MnO 3 is mainly affected by the composition, structure, metal-insulator transition temperature and surface state, etc.The T MI is especially important, which determines the thermal control temperature range of La 1−x M x MnO 3 [8].The relative researches indicated that the changes of doping metal element and doping concentration could result in different structure and T MI of La 1−x M x MnO 3 [9]- [11].Therefore, the researchers adjusted the composition, microstructure and T MI of La 1−x M x MnO 3 mainly by changing doping element and doping concentration at A-site, so as to gain a series of La 1−x M x MnO 3 with different emittance property [3] [12] [13].From a technical feasibility viewpoint, the emissivity of La 1−x M x MnO 3 can also be adjusted by modification methods.Ion implantation is an effective surface modification technology which can lead to a controlled introduction of defects and dopants in a material system and generate different surface properties [14] [15].So it is worthwhile to try to adjust the emissivity of La 1−x M x MnO 3 by ion implantation.
The modification effect of ion implantation strongly depends on the kind of implanted ions.Aluminum has a low density and is easy to be oxidized.Thus implanting with aluminum cannot obviously increase the volume density but potentially improves oxygen content of La 1−x M x MnO 3 .Meanwhile, this treatment also affects the double exchange interaction.It means that the structure, composition and T MI of La 1−x M x MnO 3 will change, which will result in different emittance properties.Up to now, the investigation on microstructure and properties especially the emittance property of La 1−x M x MnO 3 implanted with aluminum is very little.Therefore, In this work, the La 0.7 Sr 0.3 MnO 3 films were implanted with aluminum at various negative pulsed voltages and implantation doses by plasma based ion implantation and post-implantation annealed in air.The structure, surface state, metal-insulator transition and room temperature emittance property of the modified films have been studied.

Experimental
The La 0.7 Sr 0.3 MnO 3 films were deposited on Si(100) by DC magnetron sputtering from a La 0.7 Sr 0.3 MnO 3 ceramic target.During deposition process, the Si substrate was kept at 853 K.The deposition was carried out in Ar (75 vol %) + O 2 (25 vol %) ambience at a total pressure of 0.5 Pa.The thickness of the films was about 700 -750 nm.After deposition, the films were implanted with aluminum at negative pulsed voltages and then were annealed in air.The treatment parameters are shown in Table 1.In order to clarify implantation effect, the corresponding results of the La 0.7 Sr 0.3 MnO 3 annealed at 973 K for 1h in air were also given.
The phase structure of the films was confirmed by Glance angle X-ray diffraction (GXRD) using Cu Kα radiation and 5˚ angle.The surface morphologies and mean roughness of the films were studied by atomic force microscope (AFM).The bond structure and IR reflectance spectra at room temperature of the films were examined using Fourier transform infrared spectroscopy (FTIR).The resistivity was measured as a function of the temperature using the standard four-probe method between 10K and 325 K in zero field.The annealed film has a phase structure similar to that of the corresponding bulk with same composition.The post-implantation annealed films show quite obvious (100) preferred orientation growth and the degree of preferred orientation depends on the implantation voltage and implantation dose.By comparison, the post-implantation annealed film implanted at 30 kV/5 × 10 15 ions⋅cm −2 has the highest degree of (100) preferred orientation.

Structure of the Films
Figure 2 shows FTIR spectra of the post-implantation annealed and annealed La 0.7 Sr 0.3 MnO 3 films in the range of 400 -1000 cm −1 .An optical phonon band is observed at about 600 cm −1 , corresponding to the Mn-O stretching vibration in the MnO 6 octahedron [14].As can be seen, the frequency of Mn-O stretching vibration mode of the post-implantation annealed films implanted at 30 kV/5 × 10 15 ions⋅cm −2 , 50 kV/5 × 10 15 ions⋅cm −2 and 50 kV/5 × 10 16 ions⋅cm −2 is 593.8 cm −1 , 583.0 cm −1 and 586.3 cm −1 , respectively.Clearly, the bond location of the post-implantation annealed films shifts towards high frequency compared with that of the annealed film.
The higher the stretching vibration frequency of Mn-O bond is, the shorter the length of Mn-O bond is.It implies that the Mn-O bond length of the post-implantation annealed films is shorter than that of the annealed film and can be adjusted by changing the implantation voltage and implantation dose.The shortening of Mn-O bond can enhance the double exchange interaction, which is propitious to the increase of metal-insulator transition temperature.

Morphologies of the Films
The AFM surface morphologies of the post-implantation annealed and annealed films are shown in Figure 3.By comparing the surface morphologies of these films with that of the as-grown film given in the reference [16], it can be found that there are plenty of new particles on the surfaces of the former.It indicates that the post-implantation annealed and annealed films regrow during annealing process.The new particles on the surface of the annealed film present in dispersive island bulges form.However, the new particles on the surfaces of the post-implantation annealed films are relatively uniform in size and close in arrangement and grow along the direction perpendicular to the surface of the films.Moreover, increasing implantation voltage or implantation dose can result in an obvious decrease in size of the new particles.The AFM results also show that the films have different surface roughness R a .The R a of the annealed film is 3.058 nm.The R a is 7.140 nm, 2.346 nm and 3.183 nm for the post-implantation annealed film implanted at 30 kV/5 × 10 15 ions⋅cm −2 , 50 kV/5 × 10 15 ions⋅cm −2 and 50 kV/5 × 10 16 ions⋅cm −2 , respectively.Clearly, the treatment of implanting with Al and post-implantation annealing can effectively adjust the surface morphology and surface roughness of La 0.7 Sr 0.3 MnO 3 films.

Metal-Insulator Transition of the Films
In order to understand the effect of the treatment of implanting with Al and post-implantation annealing on the transport property of the films, the temperature dependence of resistivity of the films was measured.But the

Room Temperature Emittance of the Films
Figure 5 gives the IR reflectance spectra from 2.5 to 25 μm at room temperature of the films.It can be found that the post-implantation annealed films have a higher reflectance than the annealed film and the difference of  reflectance is small for the post-implantation annealed films.As these films are nontransparent, the emittance of them can be evaluated based on their reflectance as per the followed formula [14] where I b (λ, T) is blackbody spectral intensity, R(λ) is reflectance, λ is wavelength, T is temperature and λ i (i = 1, 2) represents the initial wavelength and end wavelength of a certain wavelength range, respectively.It should be noted that the blackbody spectral emittance from 2.5 μm to 25 μm covers nearly 85% to the total emittance.According to the formula (1), the emittance at room temperature of the post-implantation annealed films decreases slightly compared with that of the annealed film and the changes of implantation voltage and implantation dose result in a small difference in the room temperature emittance.The change of the emittance at room temperature mainly results from the combined action of surface roughness, composition and preferred orientation of the films.According to the GXRD result, it can be found that the films with (100) preferred orientation have a relative low emittance at room temperature.

Conclusions
The post-implantation annealed La 0.7 Sr 0.3 MnO 3 films exhibit single perovskite phase and obvious (100) preferred orientation growth.The degree of (100) preferred orientation, Mn-O bond length, surface roughness and metal-insulator transition temperature of these films can be adjusted by changing the implantation voltage and implantation dose of Al.Compared with the annealed film, the post-implantation annealed films have shorter Mn-O bond length and lower room temperature emittance.However, the T MI change tendency of the postimplantation annealed films strongly depends on the implantation voltage and implantation dose.The postimplantation annealed films implanted with Al at low voltage or low dose have lower T MI than the annealed film due to the degradation of oxidation degree and the part displacement of Mn 3+ -O 2− -Mn 4+ by Al 3+ -O 2--Mn 4+ .The post-implantation annealed film implanted with Al at 50 kV/5 × 10 16 ions⋅cm −2 has higher T MI than the annealed film because the improvement of microstructure of the film promotes metal-insulator transition.The changes of implantation voltage and implantation dose have only a small effect on the room temperature emittance of the post-implantation annealed films.By comparison, the modified La 0.7 Sr 0.3 MnO 3 films with (100) preferred orientation have relatively lower emittance at room temperature.funded project (Project No.: 1101017C), China Scholarship Council and Outstanding innovative talents support plan of Hohai University.

Figure 1 1 .Film
Figure 1 gives the GXRD patterns of the annealed and post-implantation annealed La 0.7 Sr 0.3 MnO 3 films.All of the films exhibit single perovskite phase.No aluminum or aluminum compound phase can be detected in the Table 1.Parameters of implanting with aluminum and post-implantation annealing of La 0.7 Sr 0.3 MnO 3 films.Film Implantation voltage (V) Implantation dose (ions⋅cm −2 ) Annealing treatment 1 --

Figure 1 .
Figure 1.GXRD patterns of the annealed and post-implantation annealed La 0.7 Sr 0.3 MnO 3 films.post-implantationannealed films due to low Al concentration.However, the treatment of implanting with Al and post-implantation annealing makes the phase structure of the films quite different from that of the annealed film.The annealed film has a phase structure similar to that of the corresponding bulk with same composition.The post-implantation annealed films show quite obvious (100) preferred orientation growth and the degree of preferred orientation depends on the implantation voltage and implantation dose.By comparison, the post-implantation annealed film implanted at 30 kV/5 × 10 15 ions⋅cm −2 has the highest degree of (100) preferred orientation.Figure2shows FTIR spectra of the post-implantation annealed and annealed La 0.7 Sr 0.3 MnO 3 films in the range of 400 -1000 cm −1 .An optical phonon band is observed at about 600 cm −1 , corresponding to the Mn-O stretching vibration in the MnO 6 octahedron[14].As can be seen, the frequency of Mn-O stretching vibration mode of the post-implantation annealed films implanted at 30 kV/5 × 10 15 ions⋅cm −2 , 50 kV/5 × 10 15 ions⋅cm −2 and 50 kV/5 × 10 16 ions⋅cm −2 is 593.8 cm −1 , 583.0 cm −1 and 586.3 cm −1 , respectively.Clearly, the bond location of the post-implantation annealed films shifts towards high frequency compared with that of the annealed film.The higher the stretching vibration frequency of Mn-O bond is, the shorter the length of Mn-O bond is.It implies that the Mn-O bond length of the post-implantation annealed films is shorter than that of the annealed film and can be adjusted by changing the implantation voltage and implantation dose.The shortening of Mn-O bond can enhance the double exchange interaction, which is propitious to the increase of metal-insulator transition temperature.

Figure 4 .
Figure 4. Temperature dependence of resistivity of the postimplantation annealed and annealed La 0.7 Sr 0.3 MnO 3 films.

Figure 5 .
Figure 5. IR reflectance spectra at room temperature of the post-implantation annealed and annealed La 0.7 Sr 0.3 MnO 3 films.