A method for preparing vanadium dioxide thin film

Abstract

The present invention discloses a method for preparing a vanadium dioxide film, comprising the following steps: Step 1: Substrate cleaning: After the substrate is sequentially cleaned with deionized water, acetone, ethanol, and ultrasonically cleaned with deionized water, a nitrogen gun is used to blow the substrate dry for standby use; Step 2: A first vanadium metal layer is plated on the substrate using a magnetron sputtering method; Step 3: The first vanadium metal layer is oxidized to obtain a vanadium oxide layer; Step 4: A second vanadium metal layer is plated on the vanadium oxide layer using a magnetron sputtering method; Step 5: The substrate plated with the material is subjected to an annealing step. The preparation method of the present invention adjusts the vanadium / oxygen molar ratio in the overall film layer by re-plating a metal vanadium layer of different thicknesses according to the oxidation degree of the metal vanadium. The method has low requirements for film deposition and annealing, and the prepared VO2 film has excellent electrical properties. Before and after the phase change, the change amplitude of the square resistance can reach 4 orders of magnitude.

Description

Technical Field

[0001] The invention relates to the technical field of functional materials and thin films, and in particular to a method for preparing vanadium dioxide (VO2). Background Art

[0002] Vanadium dioxide (VO2) is a functional material with thermoinduced phase change properties. It can undergo a reversible transition from a low-temperature semiconductor phase to a high-temperature metal phase at about 68°C, accompanied by a change in the crystal structure from a tetragonal to a monoclinic structure. In particular, when a well-crystalline VO2 film is grown on an α-Al2O3 substrate, its sheet resistance before and after the phase change can reach a mutation of 4 to 5 orders of magnitude, making VO2 film an ideal material for the next generation of transistor switching devices, optical switching devices, infrared sensors and other devices.

[0003] At present, the methods for preparing VO2 thin films mainly include sol-gel method, pulsed laser deposition, magnetron sputtering and other methods.

[0004] Although the cost of preparing VO2 thin films by the sol-gel method is low, the uniformity of the prepared films is poor and the thickness is difficult to control.

[0005] The pulsed laser deposition method has high requirements on equipment precision and is expensive, making it unsuitable for large-scale industrial production.

[0006] The preparation of VO2 thin films by magnetron sputtering usually requires heating the substrate to a relatively high temperature (above 400°C) during the sputtering process, and the ratio of argon to oxygen in the introduced gas needs to be strictly controlled. As a result, other vanadium oxide impurities besides VO2 are easily present, increasing the difficulty of film preparation. Summary of the invention

[0007] The technical problem to be solved by the present invention is to overcome the defects of the existing vanadium dioxide thin film preparation method such as harsh conditions and high cost, and propose a method for preparing VO2 thin film by self-adjustment. The advantage of self-adjustment is that the vanadium / oxygen molar ratio in the overall film layer is adjusted by re-plating a metal vanadium layer of different thicknesses according to the oxidation degree of the metal vanadium. The method has low requirements for film deposition and annealing, and the prepared VO2 thin film has excellent electrical properties. The change in square resistance before and after the phase change can reach 4 orders of magnitude.

[0008] In order to solve the above technical problems, the present invention proposes the following technical solutions: A method for preparing a vanadium dioxide film comprises the following steps:

[0009] Step 1: Substrate cleaning: The substrate is cleaned with deionized water, acetone, ethanol, and deionized water in sequence, and then the substrate is blown dry with a nitrogen gun for later use;

[0010] Step 2: Plating a first vanadium metal layer on the substrate by magnetron sputtering;

[0011] Step 3: oxidizing the first vanadium metal layer to obtain a vanadium oxide layer;

[0012] Step 4: Plating a second vanadium metal layer on the vanadium oxide layer by magnetron sputtering;

[0013] Step 5: Perform an annealing step on the substrate coated with the material.

[0014] The above technical solution is further limited in that, in step 2: the substrate includes silicon / silicon dioxide, sapphire, mica or quartz.

[0015] The above technical solution is further limited in that, in step 3: the oxidation conditions are that the gas introduced into the oxidation equipment is oxygen and inert protective gas, the oxidation temperature is 300-500° C., the oxidation time is 0.5h-2h, and the oxygen partial pressure is 5-1000Pa;

[0016] The above technical solution is further limited in that, in step 2 and step 4: the sputtering target is a metal vanadium target with a purity greater than 99%; and the sputtering speed is less than 5 nm / second.

[0017] The above technical solution is further limited in that, in step five, the annealing temperature is 300-600° C., and the oxygen content in the annealing atmosphere is less than 1%.

[0018] The above technical solution is further limited in that, in step 3: the vanadium oxide layer is VO2, V6O 13 、V4O9、V7O 16 , V3O7, V2O5, one pure phase or several mixed phases.

[0019] The above technical solution is further limited in that the thickness Y of the second vanadium metal layer depends on the thickness X of the first vanadium metal layer and the oxygen / vanadium molar ratio A in the vanadium oxide layer, wherein the relationship between the three is Y=X(A-2) / 2.

[0020] A further limitation of the above technical solution is that the thickness H of the VO2 film finally obtained in step five is linearly related to the sum of the thickness X of the first vanadium metal layer and the thickness Y of the second vanadium metal layer, and the relationship between the three is H≈2.29X+2.29Y.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The method for preparing the vanadium dioxide film provided by the present invention is simple and easy to implement, with high repeatability and strong universality. According to the different oxidation degrees of the first vanadium metal layer, the thickness of the second vanadium metal layer is adjusted to achieve the purpose of loose experimental conditions;

[0023] 2. The preparation method of the vanadium dioxide film of the present invention does not involve dangerous gases such as hydrogen. For example, hydrogen is used in (Shigesato Y, Enomoto M, Odaka H. Thermochromic VO2 films deposited by RF magnetronsputtering using V2O3 or V2O5 targets [J]. Japanese Journal of Applied Physics, 2000, 39 (10R): 6016.). The preparation process of the present invention has loose conditions and is relatively safe.

[0024] 3. The thickness of the prepared VO2 film is controllable;

[0025] 4. The preparation method of the present invention adjusts the vanadium / oxygen molar ratio in the overall film layer by re-plating a metal vanadium layer of different thicknesses according to the oxidation degree of the metal vanadium. The method has low requirements for film deposition and annealing, and the prepared VO2 film has excellent electrical properties. The change range of the square resistance before and after the phase change can reach more than 4 orders of magnitude. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the steps and structure of the method for preparing the vanadium dioxide film of the present invention.

[0027] Figure 2 This is a Raman spectrum data diagram of the vanadium oxide layer obtained in Preparation Example 1 of the present invention.

[0028] Figure 3 This is a graph of the Raman spectrum data of VO2 after annealing obtained in Preparation Example 1 of the present invention.

[0029] Figure 4 It is the final VO2 film thickness of Preparation Example 1 of the present invention.

[0030] Figure 5 This is the square resistance-temperature curve of the final VO2 thin film prepared in Example 1 of the present invention (the lower left small figure is the corresponding first-order differential Gaussian fitting curve).

[0031] Figure 6 This is a Raman spectrum data diagram of the vanadium oxide layer obtained in Preparation Example 2 of the present invention.

[0032] Figure 7 This is a graph of the Raman spectrum data of VO2 after annealing obtained in Preparation Example 2 of the present invention.

[0033] Figure 8 It is the final VO2 film thickness of Preparation Example 2 of the present invention.

[0034] Fig. 9This is the square resistance-temperature curve of the final VO2 thin film prepared in Example 2 of the present invention (the lower left small figure is the corresponding first-order differential Gaussian fitting curve).

[0035] Fig.10 This is a Raman spectrum data diagram of the vanadium oxide layer obtained in Preparation Example 3 of the present invention.

[0036] Fig.11 This is a graph of the Raman spectrum data of VO2 after annealing obtained in Preparation Example 3 of the present invention.

[0037] Fig.12 It is the final VO2 film thickness of Preparation Example 3 of the present invention.

[0038] Fig.13 This is the square resistance-temperature curve of the final VO2 thin film prepared in Example 3 of the present invention (the lower left small figure is the corresponding first-order differential Gaussian fitting curve). DETAILED DESCRIPTION

[0039] See also Figure 1 The present invention provides a method for preparing a vanadium dioxide film, comprising the following steps:

[0040] Step 1: Substrate cleaning: The substrate is cleaned with deionized water, acetone, ethanol, and deionized water in sequence, and then the substrate is blown dry with a nitrogen gun for later use;

[0041] The substrate in step one includes silicon / silicon dioxide, sapphire, mica or quartz.

[0042] Step 2: Plating a first vanadium metal layer on the substrate by magnetron sputtering;

[0043] In step 2, the sputtering target is a metal vanadium target with a purity greater than 99%, and the sputtering speed is less than 5 nm / second.

[0044] Step 3: oxidizing the first vanadium metal layer to obtain a vanadium oxide layer;

[0045] The oxidation conditions in step 3 are: the gases introduced into the oxidation equipment are oxygen and inert protective gas, the oxidation temperature is 300-500° C., the oxidation time is 0.5 h-2 h, and the oxygen partial pressure is 5-1000 Pa;

[0046] The oxide layer of vanadium in step 3 is VO2, V6O 13 、V4O9、V7O 16 , V3O7, V2O5, one of the pure phases or several mixed phases, marked as VO A , A is the oxygen / vanadium molar ratio in the vanadium oxide layer (where 2≤A≤2.5).

[0047] Step 4: Plating a second vanadium metal layer on the vanadium oxide layer by magnetron sputtering;

[0048] In step 4, the sputtering target is a metal vanadium target with a purity greater than 99%, and the sputtering speed is less than 5 nm / second.

[0049] Step 5: performing an annealing step on the plated substrate;

[0050] In step five, the annealing temperature is 300-600° C., and the oxygen content in the annealing atmosphere is less than 1%.

[0051] The thickness Y of the second vanadium metal layer depends on the thickness X of the first vanadium metal layer and the oxygen / vanadium molar ratio A in the vanadium oxide layer, wherein the three are related by Y=X(A-2) / 2. The relationship is derived as follows:

[0052] (1) V+(A / 2)O2→VO A

[0053] (2) VO A +((A-2) / 2)V→(A / 2)VO2

[0054] in,

[0055] (1) corresponds to step 2 vanadium plating and step 3 oxidation;

[0056] (2) Corresponding to the vanadium plating in step 4 and the annealing in step 5

[0057] In step five, the thickness H of the VO2 film finally obtained is linearly related to the sum of the thickness X of the first vanadium metal layer and the thickness Y of the second vanadium metal layer. The relationship between the three is H≈2.29X+2.29Y.

[0058] The theoretical derivation is as follows:

[0059]

[0060] H VO2 , H V Represent the thickness of VO2 and V films respectively;

[0061] V VO2 、V V Represent the volumes of VO2 and V membrane respectively;

[0062] S VO2 , S V They represent the bottom areas of VO2 and V membranes respectively, which are equal;

[0063] m VO2 、m V Respectively represent the quality of VO2 and V membrane;

[0064] n VO2 、n VThey represent the amount of substances VO2 and V respectively, which are equal;

[0065] ρ VO2 , V They represent the densities of VO2 and V respectively;

[0066] M VO2 、M V represent the molar masses of VO2 and V respectively;

[0067] Preparation Example 1

[0068] Step 1: Take a 1cmx1cm double-sided polished sapphire substrate, and clean it with deionized water, acetone, ethanol, and deionized water ultrasonically for 10 minutes each, and then use a nitrogen gun to blow dry the substrate for later use;

[0069] Step 2: Use magnetron sputtering to deposit the first vanadium metal layer on the substrate, using a 99.99% vanadium target. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3 After 300 Pa, argon gas was introduced to deposit a first vanadium metal layer of 45 nm on the substrate at a rate of 1 nm / s;

[0070] Step 3: Oxidize the first vanadium metal layer in an annealing device to obtain a vanadium oxide layer. The oxidation conditions are: oxygen and argon are introduced, the oxygen partial pressure is 300Pa, the oxidation temperature is 400°C, and the oxidation time is 0.5h. The Raman spectrum of the vanadium oxide layer is tested to determine that the oxide layer composition is single V7O 16 Phase, such as Figure 2 As shown;

[0071] Step 4: Use magnetron sputtering to deposit the second vanadium metal layer on the vanadium oxide layer, using a 99.99% vanadium target. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3 After the argon gas is introduced into the reactor at a rate of 0.6 nm / s, a second vanadium metal layer is deposited. The thickness of the second vanadium metal layer is calculated to be 45x(16 / 7-2) / 2≈6.7 nm using the above relationship Y=X(A-2) / 2.

[0072] Step 5: Place the plated substrate into a tube furnace for annealing. The protective gas is argon. The annealing temperature is 430°C and the annealing time is 2 hours. After annealing, the Raman spectrum is as follows: Figure 3 As shown, it is VO2 phase. The final VO2 thickness is Figure 4 The thickness is 120nm, which is close to the theoretical calculated value 2.29x(45+6.7)=118nm (calculated according to the relationship H≈2.29X+2.29Y).

[0073] like Figures 2 to 4As shown in the Raman spectrum of the film, it can be confirmed that the first vanadium metal layer is oxidized to obtain pure V7O 16 The thickness of the second vanadium metal layer is then approximately calculated based on the relationship between the thickness of the second vanadium metal layer, the thickness of the first vanadium metal layer, and the oxygen / vanadium molar ratio in the vanadium oxide layer. The AFM atomic force microscope morphology image shows that the surface of the film is uniform, and the thickness data image shows that the thickness of the synthesized VO2 film is controllable and meets the thickness expectations. Figure 5 The square resistance-temperature curve of the VO2 film after annealing (the lower left small figure is the corresponding first-order differential Gaussian fitting curve) shows that the phase change amplitude of the obtained VO2 film is greater than 10 4 The heating phase transition temperature is 67.8℃, the cooling phase transition temperature is 62.8℃, and the loop width is 5℃, which shows that the VO2 film has excellent thermoinduced phase transition properties and electrical properties.

[0074] Preparation Example 2

[0075] Step 1: Take a 1cmx1cm double-sided polished sapphire substrate, and clean it with deionized water, acetone, ethanol, and deionized water ultrasonically for 10 minutes each, and then use a nitrogen gun to blow dry the substrate for later use;

[0076] Step 2: Use magnetron sputtering to deposit the first vanadium metal layer on the substrate, using a 99.99% vanadium target. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3 After 300 Pa, argon gas was introduced to deposit a first vanadium metal layer of 160 nm on the substrate at a rate of 1 nm / s;

[0077] Step 3: Oxidize the first vanadium metal layer in an annealing device to obtain a vanadium oxide layer. The oxidation conditions are: oxygen and argon are introduced into the gas, the oxygen partial pressure is 1000Pa, the oxidation temperature is 450°C, and the oxidation time is 1.0h. The Raman spectrum of the vanadium oxide layer is tested to determine that the oxide layer composition is a single V2O5 phase, such as Figure 6 As shown;

[0078] Step 4: Use magnetron sputtering to deposit the second vanadium metal layer on the vanadium oxide layer, using a 99.99% vanadium target. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3 After the argon gas is introduced into the reactor at a rate of 1 nm / s, a second vanadium metal layer is deposited. The thickness of the second vanadium metal layer is calculated to be 160x(5 / 2-2) / 2≈40 nm using the above relationship Y=X(A-2) / 2.

[0079] Step 5: Place the plated substrate into a tube furnace for annealing. The protective gas is argon. The annealing temperature is 430°C and the annealing time is 2 hours. After annealing, the Raman spectrum is as follows: Figure 7As shown, it is VO2 phase. The final VO2 thickness is Figure 8 The thickness is 450nm, which is close to the theoretical calculated value 2.29x(160+40)=458nm (calculated according to the relationship H≈2.29X+2.29Y).

[0080] like Figures 6 to 8 As shown, the Raman spectrum of the film can confirm that a pure V2O5 film is obtained after the first vanadium metal layer is oxidized. Then, the thickness of the second vanadium metal layer is approximately calculated based on the relationship between the thickness of the second vanadium metal layer, the thickness of the first vanadium metal layer, and the oxygen / vanadium molar ratio in the vanadium oxide layer. The AFM atomic force microscope morphology image shows that the film surface is uniform, and the thickness data graph shows that the thickness of the synthesized VO2 film is controllable and meets the thickness expectations. Fig. 9 The square resistance-temperature curve of the VO2 film after annealing (the lower left small figure is the corresponding first-order differential Gaussian fitting curve) shows that the phase change amplitude of the obtained VO2 film is greater than 10 4 The heating phase transition temperature is 67.8℃, the cooling phase transition temperature is 62.8℃, and the loop width is 5℃, which shows that the VO2 film has excellent thermoinduced phase transition properties and electrical properties.

[0081] Preparation Example 3

[0082] Step 1: Take a 1cmx1cm double-sided polished sapphire substrate, and clean it with deionized water, acetone, ethanol, and deionized water ultrasonically for 10 minutes each, and then use a nitrogen gun to blow dry the substrate for later use;

[0083] Step 2: Use magnetron sputtering to deposit the first vanadium metal layer on the substrate, using a 99.99% vanadium target. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3 After 300 Pa, argon gas was introduced to deposit a first vanadium metal layer of 50 nm on the substrate at a rate of 1 nm / s;

[0084] Step 3: Oxidize the first vanadium metal layer in an annealing device to obtain a vanadium oxide layer. The oxidation conditions are: oxygen and argon are introduced, the oxygen partial pressure is 20Pa, the oxidation temperature is 380°C, and the oxidation time is 2.0h. The Raman spectrum of the vanadium oxide layer is tested to determine that the oxide layer composition is VO2 phase and V7O 16 Mixed phase, such as Fig.10 As shown;

[0085] Step 4: Use magnetron sputtering to deposit a second vanadium metal layer on the vanadium oxide layer. Use a 99.99% vanadium target material. The specific conditions are: substrate temperature 25°C, wait for the background vacuum to reach 9x10 -3After the first vanadium metal layer was oxidized, the Raman spectrum showed that the vanadium metal layer was VO2 and V7O 16 Mixed phase, quantitative calculation of the +2.29 valence state of V (i.e. V7O 16 ) accounts for about 8.2%. The thickness of the second vanadium metal layer is calculated by the relationship Y=X(A-2) / 2 to be (0.082*50)x(16 / 7-2) / 2≈0.6nm;

[0086] Step 5: Place the plated substrate into a tube furnace for annealing. The protective gas is argon. The annealing temperature is 450°C and the annealing time is 1 hour. After annealing, the Raman spectrum is as follows: Fig.11 As shown, it is VO2 phase. The final VO2 thickness is Fig.12 The thickness is 120nm, which is close to the theoretical calculated value 2.29x(50+0.6)=115.9nm (calculated according to the relationship H≈2.29X+2.29Y).

[0087] like Figures 10 to 12 As shown in the Raman spectrum of the film, it can be confirmed that VO2 and V7O are obtained after the first vanadium metal layer is oxidized. 16 The thickness of the second vanadium metal layer is approximately calculated to be 0.45nm based on the relationship between the thickness of the second vanadium metal layer, the thickness of the first vanadium metal layer, and the oxygen / vanadium molar ratio in the vanadium oxide layer. The AFM atomic force microscope morphology image shows that the film surface is uniform, and the thickness data image shows that the thickness of the synthesized VO2 film is controllable and meets the thickness expectations. Fig.13 The square resistance-temperature curve of the VO2 film after annealing (the lower left small figure is the corresponding first-order differential Gaussian fitting curve) shows that the phase change amplitude of the obtained VO2 film is greater than 10 4 The heating phase transition temperature is 67.8℃, the cooling phase transition temperature is 62.8℃, and the loop width is 5℃, which shows that the VO2 film has excellent thermoinduced phase transition properties and electrical properties.

Claims

1. A method for preparing a vanadium dioxide thin film, characterized in that: The following steps are involved: Step 1: Substrate cleaning: The substrate is cleaned with deionized water, acetone, ethanol, and deionized water in sequence, and then the substrate is blown dry with a nitrogen gun for later use; Step 2: Plating a first vanadium metal layer on the substrate by magnetron sputtering; Step 3: Oxidize the first vanadium metal layer to obtain a vanadium oxide layer, the vanadium oxide layer is VO2, V6O 13 、V4O9、V7O 16 , V3O7, V2O5, one of the pure phases or several mixed phases; Step 4: Plating a second vanadium metal layer on the vanadium oxide layer by magnetron sputtering, wherein the thickness Y of the second vanadium metal layer depends on the thickness X of the first vanadium metal layer and the oxygen / vanadium molar ratio A in the vanadium oxide layer, wherein the relationship between the three is Y=X(A-2) / 2; Step 5: subjecting the plated substrate to an annealing step, the annealing temperature being 300-600°C, and the oxygen content in the annealing atmosphere being less than 1%.

2. The method for preparing a vanadium dioxide thin film according to claim 1, characterized in that: In step 2: the substrate includes silicon / silicon dioxide, sapphire, mica or quartz.

3. The method for preparing a vanadium dioxide thin film according to claim 1, characterized in that: In step three: the oxidation conditions are that the gases introduced into the oxidation equipment are oxygen and inert protective gas, the oxidation temperature is 300-500° C., the oxidation time is 0.5 h-2 h, and the oxygen partial pressure is 5-1000 Pa.

4. The method for preparing a vanadium dioxide thin film according to claim 1, characterized in that: In step 2 and step 4: the sputtering target is a metal vanadium target with a purity greater than 99%; and the sputtering speed is less than 5 nm / second.

5. The method for preparing a vanadium dioxide thin film according to claim 1, characterized in that: In step five, the thickness H of the VO2 film finally obtained is linearly related to the sum of the thickness X of the first vanadium metal layer and the thickness Y of the second vanadium metal layer. The relationship between the three is H≈2.29X+2.29Y.

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