METHOD

Abstract

The invention relates to methods for the formation of rare earth nickelate thin films and “doped” (i.e. cation-substituted) variants thereof on a substrate using atomic layer deposition (ALD). The films can be deposited at low temperature (e.g. at temperatures as low as 225° C.) and have a range of useful properties including good crystallinity and high electrical conductivity, as well as interesting magnetic, optic and catalytic properties. These properties make the materials suitable for use in microelectronic applications, in the production of electrodes and as catalytic surfaces.

Claims

1. A method for the formation of a rare earth nickelate-containing film, or a doped variant thereof, on a substrate by atomic layer deposition, said method comprising the following steps: a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions; b) depositing a rare earth nickelate, or doped variant thereof, on at least a portion of said substrate by means of a deposition cycle which comprises sequential pulsing of a rare earth precursor, an oxygen precursor, a nickel precursor and, optionally, one or more dopant precursors, through said reaction chamber whereby to cause each precursor to deposit on and/or react with at least one surface of said substrate; and c) repeating step b), if desired, until the required film thickness is obtained; wherein the deposition cycle in step b) comprises the following steps i) and ii) carried out sequentially: i) sequential pulsing of said rare earth precursor and said oxygen precursor, repeated “A” times; ii) sequential pulsing of said nickel precursor and said oxygen precursor, repeated “B” times, in which at least one pulse of said nickel precursor is optionally substituted by a pulse of a dopant precursor; wherein: A is 5 or 10; B is 2 or 4; and the ratio of A:B is 2.5:1; and further wherein the nickel precursor is [Ni(acac).sub.2].sub.3.

2. A method as claimed in claim 1, wherein said rare earth precursor comprises La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, preferably La, Pr, Nd, Sm, Eu, Gd, or Tb, more preferably La, Pr, Nd, or Sm.

3. A method as claimed in claim 2, wherein said rare earth precursor is RE(thd).sub.3, wherein RE=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

4. A method as claimed in claim 3, wherein said rare earth precursor is La(thd).sub.3.

5. A method as claimed in any one of the preceding claims, wherein said nickel precursor is formed by sublimation of Ni(acac).sub.2 prior to introduction into the reaction chamber.

6. A method as claimed in any one of the preceding claims, wherein at least one, but not all, of the pulses of said nickel precursor are replaced by a pulse of a copper precursor.

7. A method as claimed in claim 6, wherein the copper precursor is Cu(acac).sub.2.

8. A method as claimed in any one of the preceding claims, wherein the resulting rare earth nickelate-containing film, or doped variant thereof, is not subjected to any post-annealing treatment.

9. A method as claimed in any one of the preceding claims, wherein said oxygen precursor is H.sub.2O, O.sub.3, or a combination thereof, preferably O.sub.3.

10. A method as claimed in any one of the preceding claims, wherein said substrate is Si, SiO.sub.2, Si/SiO.sub.2, Si/Pt, Pt, Ti, Al.sub.2O.sub.3, glass, LaAlO.sub.3, MgO, SrLaAlO.sub.4, SrTiO.sub.3, KNbO.sub.3, K(Nb,Ta)O.sub.3, NdGaO.sub.3, TbScO.sub.3, or YAlO.sub.3.

11. A method as claimed in any one of the preceding claims, wherein said method is performed at a temperature of less than 300° C., preferably less than 250° C.

12. A method for the formation of a rare earth nickelate-containing film, or a doped variant thereof, on a substrate by atomic layer deposition, said method comprising the following steps: a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions; b) depositing a rare earth nickelate, or doped variant thereof, on at least a portion of said substrate by means of a deposition cycle which comprises sequential pulsing of a rare earth precursor, an oxygen precursor, a nickel precursor and, optionally, one or more dopant precursors, through said reaction chamber whereby to cause each precursor to deposit on and/or react with at least one surface of said substrate; and c) repeating step b), if desired, until the required film thickness is obtained; wherein the nickel precursor is [Ni(acac).sub.2].sub.3; and wherein the rare earth precursor is other than La(thd).sub.3.

13. A method as claimed in claim 12, wherein the rare earth precursor does not contain lanthanum.

14. A method as claimed in claim 12, wherein the rare earth precursor comprises Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, or any combination thereof.

15. A method as claimed in claim 12, wherein the rare earth precursor comprises lanthanum in combination with a different rare earth element.

16. A method for the formation of a doped rare earth nickelate-containing film on a substrate by atomic layer deposition, said method comprising the following steps: a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions; b) depositing a doped rare earth nickelate on at least a portion of said substrate by means of a deposition cycle which comprises sequential pulsing of a rare earth precursor, an oxygen precursor, a nickel precursor, and one or more dopant precursors, through said reaction chamber whereby to cause each precursor to deposit on and/or react with at least one surface of said substrate; and c) repeating step b), if desired, until the required film thickness is obtained; wherein the nickel precursor is [Ni(acac).sub.2].sub.3.

17. A substrate carrying a rare earth nickelate-containing film, or doped variant thereof, obtained or obtainable by a method as claimed in any one of claims 1 to 16.

18. A substrate carrying a doped variant of a rare earth nickelate-containing film as claimed in claim 17, wherein said film comprises copper.

19. A substrate carrying a rare earth nickelate-containing film, or doped variant thereof, as claimed in claim 17 or claim 18, wherein said film has a thickness in the range of 1.5 to 200 nm.

20. A substrate carrying a rare-earth nickelate-containing film, or doped variant thereof, as claimed in any one of claims 17 to 19, wherein said film has a resistivity in the range of 1×10.sup.−5 to 100 Ωcm, preferably 3×10.sup.−4 to 100 Ωcm, e.g. 3×10.sup.−4 to 2×10.sup.−3 Ωcm.

21. A rare-earth nickelate-containing film as characterised by one or more of the following: (i) an X-ray diffraction pattern according to that labelled 5:2 in FIG. 2; (ii) an X-ray diffraction pattern according to that labelled 10:4 in FIG. 2; (iii) an X-ray diffraction pattern according to that labelled 5:2 in FIG. 3; or (iv) an X-ray diffraction pattern according to that labelled 10:4 in FIG. 3.

22. Use of a substrate carrying a rare-earth nickelate-containing film, or doped variant thereof, as claimed in any one of claims 17 to 21 in the production of a battery, a supercapacitor, a surface acoustic wave device, a ferroelectric random-access memory, a transducer, an ion conductor, an optoelectronic device, a Mott transistor, an actuator, a sensor, a magnetoelectric device, an electrode, or a catalytic surface.

Description

[0074] The invention is illustrated further in the following non-limiting Examples and in the attached Figures, in which:

[0075] FIG. 1: Electrical resistivity of thin films of LaNiO.sub.3 deposited on LaAlO.sub.3 as a function of the pulsing ratio. 3:1:2:1 denotes a repeated pulsing sequence of La:La:La:Ni:La:La:Ni, whereas 5:2 denotes La:La:La:La:La:Ni:Ni, and so on. Squares denote as deposited films, whereas circles denote post-annealed films.

[0076] FIG. 2: X-ray diffractograms of as deposited LaNiO.sub.3 thin films, deposited with 5 different pulsing sequences.

[0077] FIG. 3: X-ray diffractograms of post annealed LaNiO.sub.3 thin films, deposited with 5 different pulsing sequences.

[0078] FIG. 4: X-ray diffractogram of LaNiO.sub.3 on SrTiO.sub.3 as deposited at 225° C. The material is highly oriented and phase pure.

[0079] FIG. 5: Electrical resistivities of RENiO.sub.3 thin films under ambient conditions (i.e. at room temperature), with RE-element shown on x-axis. All resistivities are from films as deposited at 225° C., with no post-treatment. For comparison, an uncoated substrate exhibits a resistivity in the 10.sup.5 Ωcm range.

[0080] FIG. 6: X-ray diffractograms of RENiO.sub.3 (RE=Pr, Nd, Sm), showing the (200)-reflection of oriented thin films as-deposited at 225° C. The decreasing crystallinity is believed to be attributed to a slight alteration of the optimal pulsing ratio between RE and Ni.

[0081] FIG. 7: Resistivity as a function of Ni:Cu-ratio in the LaNi.sub.1−xCu.sub.xO.sub.3-system.

[0082] FIG. 8: Resistivity as a function of temperature for the different samples in the copper-substituted (i.e. doped) series.

EXAMPLES

[0083] General Procedure: ALD of RE(Ni,Cu)O.sub.3 where “RE”=Rare Earth Element

[0084] Depositions of LaNiO.sub.3 were carried out using an F-120 Sat ALD reactor (ASM Microchemistry) with La(thd).sub.3 (thd=2,2,6,6-tetramethyl-3,5-heptadionate), Ni(acac).sub.2 (acac=acetyl acetonate) and O.sub.3 as precursors. O.sub.3 was supplied externally by an AC-2505 ozone generator, yielding 15 mass % O.sub.3 in O.sub.2, and fed to the ALD reactor in inert tubes. For copper substituted (i.e. doped) variants, a fraction of Ni(acac).sub.2 pulses were exchanged by Cu(acac).sub.2 pulses. For other RENiO.sub.3 systems, La(thd).sub.3 was exchanged with the appropriate RE(thd).sub.3 compound. The metal precursors were used in powder form and were held in open boats internally in the reactor for the duration of deposition. The reactor system was constructed with eight separate heat zones in which precursors could be supplied from four.

[0085] For the deposition of LaNiO.sub.3, La(thd).sub.3 and Ni(acac).sub.2 were held at 185° C. throughout the deposition, ensuring sufficient vapour pressure for surface saturation during growth. Metal precursors were pulsed into the chamber by internal inert gas valves, while 03 was pulsed by an external pneumatic valve. The reactor was maintained at 2.6 mbar throughout the deposition with a primary and secondary N.sub.2-flow of 300 sccm and 200 sccm, respectively, used as purging gas. The temperature in the reaction chamber was maintained at 225° C. throughout deposition.

[0086] For copper substitution, Cu(acac).sub.2 was held at 140° C. throughout the deposition.

[0087] Ni(acac).sub.2 was resublimated before use as a precursor. This was carried out in vacuo at 175° C. with a cold finger at which the precursor was back-deposited upon sublimation. This changed the colour of the precursor from light green to dark emerald green, ensuring a transformation from Ni(acac).sub.2(H.sub.2O).sub.2 to [Ni(acac).sub.2].sub.3. We have found that this transformation, which has been confirmed by X-ray diffraction, is required for the epitaxial growth of the films. Without wishing to be bound by any theory, it is this thought that the Ni—O—Ni bond distances in [Ni(acac).sub.2].sub.3 are similar to those in LaNiO.sub.3, and that this aids in forming an epitaxial film as deposited.

[0088] Crystallinity and orientation were determined using a Bruker AXS D8 Discover diffractometer, with a Cu-Kai source and equipped with a LynxEye detector. Film thickness was determined by ellipsometry (J.A. Woolam α-SE) and X-ray reflectivity (PANalytical Empyrean). Resistivity at room temperatures was measured using a Jandel Cylindrical four point probe head connected to a Keithley 2400 Sourcemeter. Temperature dependent resistivity was measured in a Model 4000 Physical Property Measurement System (PPMS, Quantum Design), cooled with liquid helium.

Example 1: Deposition of LaNiO.SUB.3 .on LaAlO.SUB.3

[0089] LaNiO.sub.3 was deposited by sequentially pulsing the precursors, allowing substrate surface saturation for every pulse. The deposition was carried out in cycles, where one deposition cycle may be represented as:

[{La(thd).sub.3+O.sub.3}×A+{[Ni(acac).sub.2].sub.3+O.sub.3}×B]×n

This should be interpreted as:
2 s pulse of La(thd).sub.3, 2 s purge, 4 s pulse of O.sub.3, 2 s purge, repeated A times;
followed by:
2 s pulse of [Ni(acac).sub.2].sub.3, 2 s purge, 4 s pulse of O.sub.3, 2 s purge, repeated B times.

[0090] This is one deposition cycle which may be repeated as many times (n) as needed to achieve the desired film thickness.

[0091] The growth per cycle of the process was ˜0.3 Å depending on the substrate, and so this is also the resolution for thickness control. Films were deposited ranging from 1.5 to 40 nm by varying n.

[0092] Thin films were deposited using the following pulse sequences: 5:2, 10:4, 20:8 and 40:16. To test the conventional principle of maximum mixing, a thin film was deposited using a pulsing sequence of 3:1:2:1, i.e. three pulses of La(thd).sub.3, followed by one pulse of [Ni(acac).sub.2].sub.3, then two pulses of La(thd).sub.3, then one pulse of [Ni(acac).sub.2].sub.3, each interspersed with pulses of O.sub.3 and purging. All films were annealed at 650° C.

[0093] The resistivity and crystallinity of the thin films was studied before and after the annealing step. FIG. 1 shows the electrical resistivity, which correlates to conductivity, of the thin films before and after annealing. As can be seen, thin films deposited using the 5:2 and 10:4 pulsing sequences exhibit lower resistivity than that produced with the 3:1:2:1 pulse sequence. This resistivity was reduced further by annealing. Thin films deposited using the 20:8 and 40:16 pulse sequences had high resistivity as deposited. However, annealing reduced the resistivity of these films to levels similar to that of the annealed thin film deposited using the 3:1:2:1 pulse sequence.

[0094] The resistivity is closely related to crystallinity and orientation of the films. This can be seen from X-ray diffractograms for as deposited (FIG. 2) and post-annealed films (FIG. 3), respectively. Sharper reflections represent a greater degree of crystallinity. It is clear that the reflections for the thin films deposited using the 5:2 and 10:4 pulsing sequences show a greater degree of crystallinity both before and after annealing than those deposited using other pulse sequences.

[0095] The pulsing sequence (i.e. the order of pulsing the different precursors) was thus found to be of high importance in achieving the optimal crystallinity and electrical properties. This was most pronounced for the as deposited films, i.e. films that had not undergone any post annealing. A pulse ratio of 5 La(thd).sub.3: 2 [Ni(acac).sub.2].sub.3 in a given cycle was found to be optimal for the deposition of layers having a 1:1 ratio of lanthanum and nickel.

Example 2: Deposition of LaNiO.SUB.3 .on Other Substrates

[0096] LaNiO.sub.3 thin films were deposited on other substrates using the same reaction conditions and pulsing sequences as in Example 1.

[0097] Highly crystalline thin films were obtained as deposited on silicon (HF-etched Si), LaAlO.sub.3, SrTiO.sub.3, MgO, Al.sub.2O.sub.3 and SrLaAlO.sub.4 (FIG. 4 exemplifies SrTiO.sub.3) and found to conduct electricity with resistivities ranging from 3×10.sup.−4 Ωcm to 2×10.sup.−3 Ωcm, depending on the substrate (lowest on LaAlO.sub.3). This is comparable to bulk values of resistivity for LaNiO.sub.3, reported at 1×10.sup.−4 Ωcm. After annealing at 650° C., the resistivity reduced to 8×10.sup.−5 Ωcm, which is lower than bulk values for resistivity in LaNiO.sub.3. When silicon was used as the substrate, the LaNiO.sub.3 film exhibited a resistivity of 2×10.sup.−3 Ωcm as deposited, which decreased to 5×10.sup.−4 Ωcm after annealing. The lowest resistivity reported so far for LaNiO.sub.3 thin films by ALD is 5×10.sup.−4 Ωcm, but this is for films post-annealed at 700° C. The methods herein described are able to produce thin films that have a much higher conductivity than this without undergoing any high temperature post-annealing process.

Example 3: Deposition of Other Rare Earth Nickelates—RENiO.SUB.3

[0098] A series of other rare earth nickelates were deposited using the same method as in the General Procedure, but replacing the La(thd).sub.3 precursor with the analogous rare earth element precursor. The films were deposited on LaAlO.sub.3 using a pulsing sequence of 5:2.

[0099] The resistivities of the as deposited thin films obtained in this way were measured and are shown in FIG. 5. As can be seen, thin films containing La, Pr and Nd have low resistivities as deposited. The resistivity of thin films containing Sm, Eu, Gd and Tb was somewhat higher; this is as expected due to their intrinsic material properties. Crystallinity of as deposited films of PrNiO.sub.3, NdNiO.sub.3 and SmNiO.sub.3 was confirmed by X-ray diffraction, as shown in FIG. 6.

Example 4: Doping of LaNiO.SUB.3 .with Cu

[0100] The LaNiO.sub.3 matrix can be doped with Cu to alter its electrical properties. This was done by exchanging some pulses of nickel precursor in the method of Example 1 with pulses of Cu(acac).sub.2. Crystalline films were obtained as deposited on HF-etched silicon substrates. No special preparation of the silicon substrates was required. For copper substitution, Cu(acac).sub.2 was held at 140° C. throughout the deposition. The copper precursor does not exhibit a trimeric structure upon sublimation, and the crystallinity of the films decreased as the amount of the copper in the films was increased. It was found that the electrical conductivity of the films could be tuned smoothly from 3×10.sup.−4 Ωcm to approximately 100 Ωcm, covering more than 6 orders of magnitude (see FIG. 7). The samples in this composition series exhibited completely different resistivity variation upon changes in temperature (see FIG. 8).

Example 5: Deposition of LaNiO.SUB.3

[0101] Thin film depositions of LaNiO.sub.3(LNO) were carried out in an F-120 Sat ALD reactor (ASM Microchemistry). The deposition temperature was 225° C. with an operating pressure of 2.4 mbar, maintained by a 300 cm.sup.3 min.sup.−1 primary flow rate of N.sub.2.

[0102] Nitrogen was supplied from gas cylinders (Praxair, 99.999%) and run through a Mykrolis purifier for removal of any oxygen or water contamination.

[0103] La(thd).sub.3 (Volatech, 99%) and Ni(acac).sub.2 (Sigma Aldrich, 97%) were used as cation precursors. Both precursors were supplied from open boats inside the reactor, and maintained at 185° C. throughout the deposition. Ni(acac).sub.2 was re-sublimated at 175° C. for purification prior to use in the reactor. The cation precursors were pulsed into the reaction chamber by means of inert gas valves. 03 was used as the oxygen source, made from O.sub.2 (Praxair, 99.5%) using an AC-2505 (In USA) ozone generator supplying 15 wt. % O.sub.3 in O.sub.2. Pulse durations were 2, 2 and 4 s for La(thd).sub.3, Ni(acac).sub.2 and O.sub.3, respectively. Purge durations were 2 s after cation precursor pulses, and 3 s after the ozone pulses. Self-limiting behavior for the employed pulse- and purge scheme was confirmed.

[0104] Thin films were deposited on 1×1 cm.sup.2 Si for routine characterization of thickness and 3×3 cm.sup.2 Si for analysis of conformality and cation stoichiometry. Selected compositions were deposited on LaAlO.sub.3 (LAO) (100) (Crystal GmbH), (110) and (111) (MTI Corp.) and SrTiO.sub.3 (STO) (100) (Crystal GmbH), (110) and (111) (MTI Corp.) single crystals for facilitation of epitaxial growth.

[0105] Room-temperature resistivity measurements were carried out using a 4-point probe and a Keithley model 2400 SourceMeter. The sheet resistivity was recorded by measuring resistance in 10 points from 1 to 10 μA. Variable temperature resistivity measurements were performed on a Model 4000 physical property measurement system (PPMS, Quantum Design). The samples were mounted on a puck and contacted with gold wires on gold pads deposited by evaporation. Resistivity was collected in a 4-point setup, while the temperature was swept from 300 to 6 K.

[0106] 4-point probe measurements (point distance 1 mm) at room temperature revealed metallic electrical resistivity. STO(100)∥LNO(100).sub.pseudocubic(pc) exhibited a resistivity of ˜100 μΩcm on average, with a lowest measured resistivity of 80 μΩcm. This is below the reported values for bulk LNO. For LA(100).sub.pc∥LNO(100).sub.pc the average resistivity measured was ˜300 μΩcm, which is slightly higher than bulk LNO.

[0107] To further investigate the electrical properties of the sample, a Hall analysis was carried out using a 4-point setup in which each corner of the 1×1 cm.sup.2 sample was contacted to a probe. The film was perturbed by a 1.02 T magnet, which allowed for measurements of induced Hall currents and deduction of carrier densities.

[0108] The resistivity was estimated to be 138 μΩcm for STO (100)∥LNO(100).sub.pc at ambient temperature, which was slightly higher than measured by the 4-point probe. This is likely because the Hall-setup uses a much larger probe distance (10× longer), which is less tolerant to any grain boundaries that may exist in the film. It should still be noted that the resistivity is on par with that of bulk LNO (˜120μΩ cm).

[0109] The induced Hall voltage was measured to be ˜2 mV A.sup.−1. Using the film conductivity, sample thickness, current and magnetic field, a Hall coefficient of 0.0549 mm.sup.3 C.sup.−1 and a charge carrier density of ˜3.6×10.sup.22 cm.sup.−3 could be deduced. This is very close to the theoretical 3.4×10.sup.22 cm.sup.−3 carriers that would be present if each NiO.sub.6-octahedron contributes with one carrier each. High carrier density is key for materials that are to be used as gates in oxide electronics.

[0110] The temperature dependent resistivity of the LNO films on STO was measured using a PPMS cooled by liquid He. The temperature was swept from room temperature to 6 K and the resistivity was collected for every 2 K. The results showing a decrease in resistivity as a function of temperature underpin that the films are metallic across the whole temperature range.