WO2012093925A1

WO2012093925A1 - Lanthanum cerium oxide thin film and its preparation thereof

Info

Publication number: WO2012093925A1
Application number: PCT/MY2011/000179
Authority: WO
Inventors: Kuan Yew Cheong; Zainovia Lockman; Way Foong LIM
Original assignee: Universiti Sains Malaysia
Priority date: 2011-01-04
Filing date: 2011-07-22
Publication date: 2012-07-12
Also published as: MY153033A

Abstract

The present invention relates to a method of preparing lanthanum cerium oxide (La₂Ce₂O₇) precursor using metal-organic decomposition method and transforming the precursor to a ternary lanthanum cerium oxide solid thin film with thickness range of 50 to 100 nm via post-deposition annealing in Argon ambient.

Description

LANTHANUM CERIUM OXIDE THIN FILM AND ITS PREPARATION

THEREOF

FIELD OF INVENTION

The present invention relates to a method of preparing ternary oxide thin film from lanthanum cerium oxide precursor.

BACKGROUND OF THE INVENTION

Lanthanum cerium oxide (La₂Ce₂0₇) is commonly utilized as a thermal barrier coating material for high temperature applications. This is due to its fascinating properties, such as low thermal conductivity and large thermal expansion coefficient close to that of the bond coat. It is found that La₂Ce₂0₇ has high stability. It remains stable without phase transformation after long-term annealing at 1400°C. Ternary La₂Ce₂0₇ has been produced by means of solid-state reaction, using Ce02 and La₂0₃ powders as starting materials. These powders are wet ball-milled for 16 hours, using a milling pot made of resin and resin-coated balls, and ethanol as dispersion reagent. The powder mixtures are calcined at 1100 °C for 5 hours in air after drying. Subsequently, the powders are sieved under 53 um in mesh size and molded uniaxially under pressure of 50 MPa and subjected to rubber press at 200 MPa. The obtained compacts are then sintered at 1600 °C for 10 hours in air. Other invention utilizes Ce0₂ and La₂0₃ as starting powders by firing a mixture of corresponding oxides, followed by spray-drying. Another method forms La₂Ce₂0₇ by cold pressing the powders, followed by sintering at 1873 K for 6 hours with heating and cooling rates of 180 K hour.

Besides solid-state reaction method, lanthanum cerium oxide has also been prepared based on conventional sol-gel process, which utilizes La₂0₃ and Ce(N0₃)3.6H20 as starting materials. In order to achieve a stoichiometric lanthanum cerium oxide, 1:1 molar ratio of La/Ce is maintained. The La₂0₃ is first converted to lanthanum nitrate by dissolving in concentrated nitric acid. Ce(N0₃)3.6H₂0 is dissolved into de-ionized water and then added to the lanthanum nitrate solution. This is followed by addition of ethanol containing poly(vinyl pyrrolidone) (PVP). Subsequently, the mixture is magnetically stirred at room temperature for 2 hours. Although sol-gel method appears to be more convenient compared to the solid-state reaction, the reactivity is high and it needs to undergo hydrolysis and polycondensation processes. Existing technology utilizes either solid-state reaction or conventional sol-gel method to produce lanthanum cerium oxide powder. Solid-state reaction method requires processes, such as ball milling, drying, calcination, pressing, and sintering. These processes will reduce efficiency of producing lanthanum cerium oxide (La₂Ce₂07) in a short period of time and increases manufacturing cost. Conventional sol-gel method, which is a chemical solution deposition method, is better than solid-state reaction method, because such processes do not exist. Sol-gel method involves hydrolysis and pyrolysis and it utilizes alkoxide precursor in order to transform metal alkoxides to metal oxides. Nevertheless, reactivity of sol-gel method is high. Therefore there is a need for a method to reduce the reactivity and chemical interactions between different precursor compounds.

SUMMARY OF INVENTION

The present invention relates to a method of preparing lanthanum cerium oxide precursor. The method includes the steps of (i) preparing a cerium containing precursor by dissolving carboxylate or β-diketonate [acetylacetone (acac)-type] compound into acid and alcohol mixture with a mole ratio of 1:42:30; (ii) preparing a lanthanum containing precursor by dissolving lanthanum nitrate salt into a solution containing carboxylate or β- diketonate [acetylacetone (acac)-type] compound or acetylacetone; (iii) mixing the cerium containing precursor and the lanthanum-containing precursor to produce a 1:1 molar mixture of cerium and lanthanum containing precursors; and (iv) subjecting the mixture obtained from step (iii) under reflux condition for two hours at 100-110°C. The β- diketonate [acetylacetone (acac)-type] compound used in step (i) is cerium (ΠΙ) acetylacetonate hydrate powder. The alcohol used in step (i) is methanol. The acid used in step (i) is acetic acid. The lanthanum nitrate salt and acetylacetone solution used in step (ii) having a mole ratio of 1:4005. The present invention also relates to a method of preparing lanthanum cerium oxide thin film. The method includes the steps of (i) preparing a cerium-containing precursor by dissolving carboxylate or β-diketonate [acetylacetone (acac)-type]compound into acid and alcohol mixture with a mole ratio of 1:42:30; (ii) preparing a lanthanum- containing precursor by dissolving lanthanum nitrate salt into a solution containing carboxylate or β-diketonate [acetylacetone (acac)-type] compound or acetylacetone; (iii) mixing the cerium containing precursor and the lanthanum-containing precursor to produce a 1 : 1 molar mixture of cerium and lanthanum-containing precursors; (iv) subjecting the mixture obtained from step (iii) under reflux condition for 2 hours at 100- 110°C; (v) coating of the mixture obtained from step (iv) on a substrate to produce a wet film; and (vi) post-deposition annealing is performed on the wet film obtained from step

(v) under argon ambient, forming gas ambient or oxidizing ambient. The β-diketonate [acetylacetone (acac)-type] compound used in step (i) is cerium (ΠΙ) acetylacetonate hydrate powder. The alcohol used in step (i) is methanol. The acid used in step (i) is acetic acid. The lanthanum nitrate salt and acetylacetone solution used in step (ii) having a mole ratio of 1:4005. The coating of the mixture obtained from step (iv) on a substrate is achieved by spin-coating at 3000-6000rpm and spinning time of 15-45 seconds. The substrate is silicon, silicon carbide or gallium nitride. The post-deposition annealing in step

(vi) is performed for at least 15 minutes each at a temperature range 400-1100°C. Lanthanum cerium oxide thin film produced having thickness of 50-100nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

Figure 1 shows a flow diagram of for a method of preparing a lanthanum cerium oxide (La₂Ce₂0₇) thin film; and Figure 2 shows X-Ray-Diffraction (XRD) patterns of lanthanum cerium oxide (La₂Ce20₇₎ thin film post-deposition annealed at different annealing temperatures (400-1100 °C). DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of preparing lanthanum cerium oxide precursor and transforming the precursor to a ternary lanthanum cerium oxide solid thin film via post-deposition annealing in Argon ambient. It should be understood, however, that the disclosed preferred embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limiting, but merely as the basis for the claims and for teaching one skilled in the art of the invention. Referring to Figure 1 , in a preferred embodiment, a method is provided for formulating and preparing lanthanum cerium oxide precursor and transforming the lanthanum cerium oxide precursor to lanthanum cerium oxide thin film. The method includes the steps of preparing a cerium-containing precursor, preparing a lanthanum- containing precursor, mixing both cerium and lanthanum-containing precursors, pouring said mixture into a two-neck flask equipped with a reflux condenser and a thermometer, leaving the mixture in the reflux condition, i.e. two hours, 100°C-110°C, to allow complete reaction between the cerium and lanthanum-containing precursors, spin-coating the mixture solution on a substrate at a spinning rate of 3000-6000rpm and spinning time of 15-45 seconds and post-deposition annealing to transform wet film obtained from the spin coating to a ternary oxide thin film. The ternary oxide thin film herein is the lanthanum cerium oxide thin film. The reflux hours can be varied, i.e. 30 minutes, 1 hour, 3 hours, etc.

Metal-organic decomposition (MOD) method is used to produce the precursor of ternary lanthanum cerium oxide. The MOD method minimizes reactivity and chemical interactions between different precursors. The MOD method is a straight forward process that uses large carboxylate or β-diketonate [acetylacetone (acac)-type] compounds. It also does not produce any aligomerization behavior. The precursor of lanthanum cerium oxide is produced by metal-organic decomposition method, using a mixture of two types of precursors. The first precursor is cerium-containing precursor, while the second precursor is lanthanum-containing precursor. Cerium-containing precursor is derived from a mixture containing cerium (ΠΙ) acetylacetonate hydrate powder, methanol and acetic acid. Lanthanum-containing precursor is prepared by dissolving La(N0₃)3.6H₂0 powder into acetylacetone solution to form lanthanum acetylacetonate hydrate solution. Both the precursors are mixed to produce 1 :1 molar ratio of Ce La mixture, which is to obtain a stoichiometric solution. The solution is left in a reflux condition to allow complete reaction between the cerium and lanthanum-containing precursors. The solution is then deposited by any of the optional masking techniques preferably, spin-coating onto a substrate to produce a wet film of lanthanum cerium oxide film and post-deposition annealing is performed at different annealing temperatures, preferably at 400, 600, 800, 1000, and 1100 °C in an inert atmosphere, preferably argon ambient for at least 15 minutes, preferably 15 minutes, in a horizontal tube furnace to transform the wet film to a ternary lanthanum cerium oxide thin film with thickness of 50 - 100 nm. The post deposition annealing can also be performed in other ambient such as forming gas ambient and oxidizing ambient. The forming gas ambient includes 95% N₂ gas and 5% H₂ gas while the oxidizing ambient includes O₂ gas.

The cerium-containing precursor is prepared by dissolving carboxylate or β- diketonate [acetylacetone (acac)-type] compound into acid and alcohol mixture with a mole ratio of 1:42:30 respectively. In the present invention, the β-diketonate [acetylacetone (acac)-type] compound is cerium (III) acetylacetonate hydrate powder. The acid is acetic acid and the alcohol is methanol.

Lanthanum-containing precursor is prepared by adding La(N0₃)3.6H₂0 salt into a solution containing carboxylate or β-diketonate [acetylacetone (acac)-type] compound. The lanthanum-containing precursor may also be prepared by adding La(NC>3)3.6H20 powder into acetylacetone solution to form lanthanum acetylacetonate hydrate solution. The mole ratio of La(N0₃)3.6H₂0 powder: acetylacetone solution is 1:4005.

Equimolar amounts of cerium-containing precursor and lanthanum-containing precursor are mixed to produce precursor of lanthanum cerium oxide with a 1 : 1 molar ratio of Ce/La. The stoichiometric lanthanum cerium oxide is left in a reflux system for two hours at 100°C-110°C. In another embodiment, the refluxed solution containing stoichiometric lanthanum cerium oxide is spin-coated onto a substrate. The substrate is such as silicon, silicon carbide or gallium nitride. Post deposition annealing is performed on the spin coated lanthanum cerium oxide to obtain ternary phase of lanthanum cerium oxide. The post- deposition annealing is performed at different temperatures, preferably 400, 600, 800, 1000, and 1 100 °C in a horizontal tube furnace. Post-deposition annealing time used is at least 15 minutes, preferably 15 minutes, for each annealing temperature. Post-deposition annealing is accomplished in an inert atmosphere. The inert atmosphere can be Argon ambient. The post deposition annealing can also be performed in other ambient such as forming gas ambient and oxidizing ambient. The forming gas ambient includes 95% N₂ gas and 5% ¾ gas while the oxidizing ambient includes O₂ gas. Post-deposition annealing temperature, time, and ambient are sufficient to transform the wet film to a ternary lanthanum cerium oxide thin film. Formation of ternary lanthanum cerium oxide is shown by X-Ray Diffraction

(XRD) pattern as shown in Figure 2. The XRD results indicate that four diffraction peaks, ascribed to (200), (220), (311), and (222) planes. These peaks are detected in the precursor of lanthanum cerium oxide annealed from 600°C. The precursor annealed at 400° C demonstrates amorphous characteristics, which means that no crystalline phase of La₂Ce₂0₇ is formed. The peaks mentioned above correspond to La₂Ce20₇, which is a ternary phase of lanthanum cerium oxide. This phase is determined by comparing the XRD patterns with previous literature [H. Yamamura, H Nishino, K. Kakinuma, and K. Nomura, J. Cer. Soc. Jpn 111 [12] (2003) 902-906; X. Q. Cao, R. Vassen, W. Fischer, F. Tietz, W. G. Jungen, and D. Stover, Adv. Mater. 15 (2003) 1438-1442; W. Ma, S. . Gong, H. B. Xu, and X. Q. Cao, Sur. Coat. Tech. 200 (2006) 5113-5118; J. Y. Li, H. Dai, X. H. Zhong, Y. F. Zhang, and X. Q. Cao, Adv. Eng. Mater. 9 (2007) 205-207] since it does not exist in ICDD database. The patterns of this phase should be similar to pure CeC^ as La2Ce20₇ is a solid solution of La₂0₃ in Ce0₂ [X. Q. Cao, R. Vassen, W. Fischer, F. Tietz, W. G. Jungen, and D. Stover, Adv. Mater. 15 (2003) 1438-1442; W. Ma, S. K. Gong, H. B. Xu, and X. Q. Cao, Sur. Coat. Tech. 200 (2006) 5113-5118]. Besides, these XRD patterns can be compared with the standard stick pattern of Ceo.56Lao.44O1.-78 _wj_m ICDD file no. 98-006- 2223, which is also a ternary phase of lanthanum cerium oxide. A minute deviation in the diffraction angles of Ι^¾θ7 and Ceo.56Lao.4401.78 is observed. This deviation is due to the 1:1 molar ratio of La/Ce used in present work if compared to the one in

Ceo.S6Lao.44O1.78^' The various embodiments will now be illustrated with a working example, which is intended to illustrate the working of invention and not intended to take restrictively to imply any limitations on the scope of the present invention.

Example 1

A 0.0025 moles of cerium containing precursor is produced by dissolving 1.0936 g Ce(C5H702)3.xH₂0 powder into 6 ml of acetic acid. 3 ml of methanol is then added into the solution to make up a 0.25 M solution. 1.0825 g of La(N0₃)3.6¾0 powder is dissolved into 10 ml of acetylacetone solution to form lanthanum acetylacetonate hydrate solution to yield 0.0025 mole of solution. A 0.25 M lanthanum-containing precursor is produced. 0.25 M of cerium-containing precursor and 0.25 M of lanthanum-containing precursor are mixed together to produce precursor of lanthanum cerium oxide with a 1:1 molar ratio of Ce/La. The mixture is poured into a two-necked flask equipped with a reflux condenser and a thermometer. The mixture is refluxed for two hours at 100-110 °C to drive the reaction to completion. The refluxed solution is spin-coated onto a substrate at a spinning rate of 4000 rpm and a spinning time of 30 seconds. The resulting wet film is transformed to a ternary phase of lanthanum cerium oxide thin film by performing post- deposition annealing onto the wet film resulting in solid ternary lanthanum cerium oxide thin film. Post-deposition annealing is performed at 400, 600, 800, 1000, and 1100 °C in a horizontal tube furnace for 15 minutes for each annealing temperature in Argon ambient. The ternary lanthanum cerium oxide will act as a thin film with thickness ranging from 50 to 100 nm.

The lanthanum cerium oxide thin film obtained in the present invention is a high dielectric constant film which can be utilized as a gate oxide material for metal-oxide- semiconductor based high power devices in future.

Claims

1. A method of preparing lanthanum cerium oxide precursor, the method includes the steps of:

i) preparing a cerium containing precursor by dissolving carboxylate or β-diketonate [acetylacetone (acac)-type] compound into acid and alcohol mixture with a mole ratio of 1:42:30;

ii) preparing a lanthanum containing precursor by dissolving lanthanum nitrate salt into a solution containing carboxylate or β-diketonate [acetylacetone (acac)-type] compound or acetylacetone;

iii) mixing the cerium containing precursor and the lanthanum- containing precursor to produce a 1 : 1 molar mixture of cerium and lanthanum containing precursors; and

iv) subjecting the mixture obtained from step (iii) under reflux condition for two hours at 100-110°C.

The method of preparing lanthanum cerium oxide precursor as claimed in claim 1 wherein the β-diketonate [acetylacetone (acac)-type] compound in step (i) is cerium (ΠΙ) acetylacetonate hydrate powder.

The method of preparing lanthanum cerium oxide precursor as claimed in claim 1 wherein the alcohol in step (i) is methanol.

The method of preparing lanthanum cerium oxide precursor as claimed in claim 1 wherein the acid in step (i) is acetic acid.

The method of preparing lanthanum cerium oxide precursor as claimed in claim 1 wherein the lanthanum nitrate salt and acetylacetone solution in step (ii) having a mole ratio of 1:4005.

A method of preparing lanthanum cerium oxide thin film, the method includes the steps of:

i) preparing a cerium-containing precursor by dissolving carboxylate or β-diketonate [acetylacetone (acac)- type]compound into acid and alcohol mixture with a mole ratio of 1:42:30;

ii) preparing a lanthanum-containing precursor by dissolving lanthanum nitrate salt into a solution containing carboxylate or β-diketonate [acetylacetone (acac)-type] compound or acetylacetone;

iii) mixing the cerium containing precursor and the lanthanum- containing precursor to produce a 1 : 1 molar mixture of cerium and lanthanum-containing precursors;

iv) subjecting the mixture obtained from step (iii) under reflux condition for two hours at 100-110°C;

v) coating of the mixture obtained from step (iv) on a substrate to produce a wet film; and

vi) post-deposition annealing is performed on the wet film obtained from step (v) under argon ambient, forming gas ambient or oxidizing ambient.

The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the carboxylate or β-diketonate [acetylacetone (acac)-type] compound in step (i) is cerium (III) acetylacetonate hydrate powder.

8. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the alcohol in step (i) is methanol.

9. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the acid in step (i) is acetic acid.

10. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the lanthanum nitrate salt and acetylacetone solution in step (ii) having a mole ratio of 1:4005.

11. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the substrate is silicon, silicon carbide or gallium nitride.

12. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the coating of the mixture obtained from step (iv) on a substrate is achieved by spin-coating at 3000-6000rpm and spinning time of 15-45 seconds.

13. The method of preparing lanthanum cerium oxide thin film as claimed in claim 6 wherein the post-deposition annealing in step (vi) is performed for at least 15 minutes each at a temperature range 400-1100°C.

14. Lanthanum cerium oxide thin film produced according to claims 6 to 13 having thickness of 50-100nm.