CN113684536A - Preparation of Al by physical vapor transport method1-xScxMethod for producing N crystal - Google Patents

Abstract

The invention discloses a physical vapor transport method for preparing Al_1‑xSc_xA method of N crystals comprising the steps of: 1) preparing a substrate and raw materials; 2) putting a substrate and a raw material into a crucible, putting the crucible into a closed high-temperature furnace, putting the raw material into a high-temperature area, and putting the substrate into a low-temperature area; 3) decomposing and synthesizing raw materials: introducing high-purity nitrogen into the furnace body, and simultaneously heating and insulating the bottom of the crucible for a certain time; 4) crystal growth: increasing the pressure and heating temperature, and keeping the temperature to perform Al_1‑xSc_xGrowing an N crystal; 5) and cooling to room temperature. The preparation method comprises the steps of forming a convex temperature field in a raw material area and forming a concave temperature field in a substrate area by adjusting a thermal field; the distribution of low supersaturation degree of Al and Sc is formed on the crystal growth surface, so that Al_1‑xSc_xN is slowly deposited on the growth surface, thereby realizing high-concentration Sc-doped Al_1‑xSc_xN crystal, thereby promoting material piezoelectric property and electromechanical coupling coefficient, and promoting the applicability of bulk acoustic wave and surface acoustic wave devices.

Description

Preparation of Al by physical vapor transport method1-xScxMethod for producing N crystal

Technical Field

The invention relates to the technical field of semiconductor material preparation, in particular to a method for preparing a doped aluminum nitride crystal by a physical vapor transport method.

Background

The filters mainly include a film bulk acoustic resonator Filter (FBAR), a surface acoustic wave filter (SAW), and a bulk acoustic wave filter (BAW). FBARs are used more in future single-chip multi-band wireless communication RF front-end systems and have a size advantage compared to SAW filters. The filters are typically in LiNbO₃In recent years, attention has been paid to bulk piezoelectric materials that are suitable for integration with other devices on the same substrate. The SAW propagation speed of the AlN thin film is the fastest, and the AlN SAW device has good chemical and thermal stability, has extremely high sensitivity to external environments such as pressure, temperature, stress, gas and the like, is compatible with the conventional traditional Si CMOS technology, thereby becoming a key part for passive sensing, wireless sensing and mobile signal processing, and the AlN is mature in the SAW/BAW filter and has been commercialized.

However, AlN for surface acoustic wave applications inherently suffers from its low electromechanical coupling coefficient (K) typically in the range of less than 1%²) This limits surface acoustic wave based applications (e.g., sensors, drivers, and surface acoustic wave based microfluidics). The lattice parameter can be changed by doping Sc into AlN, as the concentration of Sc in a wurtzite phase increases, an ion potential well becomes smaller, the displacement of ions in an electric field becomes larger, the dielectric and piezoelectric response becomes larger, an inherent alloying effect is formed, the topological structure of the surface is greatly influenced, the elastic softening along the lattice parameter c is intensified, and the inherent sensitivity of axial strain is remarkably improved, so that the piezoelectric constant and the electromechanical coupling coefficient are increased.

Compared with the conventional thin film growth process (PVD, MOCVD, HVPE and the like), the physical vapor transport method has low production cost and high crystal quality, is the method proved to be the most effective method for growing the AlN at present, but the physical vapor transport method is used for growing the Sc-doped AlN, namely AlScN, and the doping concentration which can be reached by the Sc-doped AlN is very low, so that high-concentration doping cannot be formed; mainly because the Sc doping can be continuously precipitated to the growth surface in the technological process of the physical vapor transport method of the AlN doped with Sc. Particularly, the preparation of the Sc-doped AlN crystal has the defects that the Sc doping precipitation is more obvious to the growth surface of the crystal because the crystal growth rate is high relative to the film growth, so that the physical vapor transport growth of the AlScN crystal with high concentration is more difficult than the film form. Therefore, in the prior art, thin film-shaped AlScN is generally prepared, and a sputtering method with high cost is generally adopted.

Disclosure of Invention

Based on the problems in the prior art, the invention aims to provide a physical vapor transport method for preparing an AlScN crystal, and high-concentration Sc doping is achieved, so that an AlScN material suitable for being applied to devices based on bulk acoustic waves and surface acoustic waves is provided.

In order to achieve the above object, the present invention provides the following technical solutions.

The invention provides a physical vapor transport method for preparing Al_1-xSc_xThe method for preparing the N crystal specifically comprises the following steps:

a method for preparing aluminum-scandium-nitrogen crystal by physical vapor transport method, wherein the chemical expression of aluminum-scandium-nitrogen is Al_{1- x}Sc_xN, comprising the following steps:

1) preparing a substrate and raw materials;

2) putting a substrate and a raw material into a crucible, putting the crucible into a closed high-temperature furnace, putting the raw material into a high-temperature area, and putting the substrate into a low-temperature area;

3) introducing high-purity nitrogen into the furnace body to 0.1-10Kpa, simultaneously heating the bottom of the crucible to 1000-1200 ℃, and preserving heat for 1-20 hours;

4) raising the air pressure in the furnace body to 30-100Kpa, simultaneously heating the bottom of the crucible to 1900-2200 ℃, and preserving the heat for a period of time until the aluminum-scandium-nitrogen crystal grows to a certain thickness; in the heat preservation process, the saturation degree distribution of the vapor Al and Sc in the growth chamber is adjusted through the regulation and control of the distribution of the thermal field, so that the low saturation degree of the vapor Al and Sc is formed on the growth surface of the substrate, a two-dimensional laminar flow growth mode is formed, and the precipitation of Sc is avoided.

5) And after the heat preservation is finished, cooling to room temperature, and taking out the prepared aluminum scandium nitrogen crystal from the crucible.

Optionally, in step 1), the substrate is made of silicon, germanium, or tungsten, or a compound of zinc oxide, sapphire, silicon carbide, gallium nitride, aluminum nitride, gallium arsenide, indium phosphide, lithium niobate, or lithium tantalate.

Further, the substrate material includes a polycrystal including a crystal composition of an arbitrary orientation, or a single crystal of a fixed orientation.

Optionally, in step 1), the raw material includes one or more of a material combination of AlN powder and ScN powder or Sc metal, a material combination of AlN sintered body and ScN powder or Sc metal, and a material combination of AlN polycrystal or Al metal and AlSc alloy.

Optionally, in step 2), the crucible is made of tungsten, tantalum carbide, tungsten carbide, tantalum nitride, or zirconium carbide.

Optionally, Al grown in step 5)_1-xSc_xThe content of Sc in the N crystal is 0.1-50 at%, more preferably 0.1-43at%, and the content of Sc in the material is controlled by the Sc concentration in the raw material and the crystal growth conditions.

The invention has the following beneficial effects:

by adopting the preparation method of the invention, high-purity and high-quality Al can be prepared_1-xSc_xAnd (4) N crystals. With the increase of the concentration of the wurtzite phase Sc in AlN, an ion potential well is reduced, the displacement of ions in an electric field is increased, the dielectric and piezoelectric response is increased, an intrinsic alloying effect is formed, the surface topological structure is greatly influenced, the elastic softening along the lattice parameter c is increased, the inherent sensitivity of axial strain is remarkably improved, and the piezoelectric constant and the electromechanical coupling coefficient are increased. The problem that the high scandium doped Al cannot be prepared by a physical vapor transport method is solved through the regulation and control of a growth mode_1-xSc_xN crystal. Highly crystalline Al_1-xSc_xN crystal as substrate material, (0002) rocking curve full width at half maximumThe piezoelectric coefficient is less than 1 degree, the piezoelectric coefficient is more than 7.6 pC/N after detection, the electromechanical coupling coefficient of a surface acoustic wave device (SAW) is 2-10% (different sc concentrations correspond to different detection results), and the optimal value is 4-5 times of the corresponding performance compared with a pure AlN film template, so that the AlN film template can be better suitable for bulk acoustic waves and surface acoustic wave device applications (such as sensors, drivers, microfluid based on surface acoustic waves and the like) and realizes more excellent device performance; in addition, Al prepared by physical vapor transport method_1-xSc_xThe N crystal has extremely low oxygen content, the total content of main impurities detected by GDMS/EGA is less than 100ppm, the purity is more than 99.95 percent, and the crystal grains are uniformly distributed, thereby being capable of providing large-size high-purity Al for preparing the AlScN film_1-xSc_xThe N target solves the problems of small size and high oxygen content of the existing AlScN target.

Drawings

FIG. 1 shows Al production by the production method of the present invention_1-xSc_xThe crucible structure of the N crystal is shown schematically.

In the figure, 1 is a substrate material, and 2 is prepared Al_1-xSc_xN crystal, 3 as raw material and 4 as crucible.

FIG. 2 shows Al production in example 1 of the present invention_1-xSc_xThe process curve chart adopted by the N crystal.

FIG. 3 shows Al production in example 1 of the present invention_1-xSc_xAnd (4) distributing the thermal field of the N crystal.

FIG. 4 shows Al production in example 1 of the present invention_1-xSc_xDistribution pattern of supersaturation degree of vapor phase Al and Sc in N crystal

FIG. 5 shows Al prepared by the preparation method of the present invention_1-xSc_xRaw material proportion of N crystal and Al prepared_1-xSc_xThe change trend of the Sc content in the N crystal is shown in the figure.

FIG. 6 shows Al prepared by the preparation method of the present invention_1-xSc_xAnd the Sc concentration in the N crystal is plotted against the piezoelectric coefficient.

Detailed Description

For a further understanding of the invention, reference will now be made to the preferred embodiments of the invention by way of example, and it is to be understood that the description is intended to further illustrate features and advantages of the invention, and not to limit the scope of the claims.

Example 1

As shown in FIGS. 1 and 2, the preparation of aluminum scandium nitrogen (Al) in this example is performed respectively_1-xSc_xN) crucible structure schematic diagram and process curve diagram of the crystal. Next, the high piezoelectric performance material Al of this embodiment is described with reference to the drawings_1-xSc_xThe method for producing the N crystal is explained in detail.

1) A substrate material is prepared. In the embodiment, a (0002) crystal orientation aluminum nitride single crystal substrate is used as a substrate material, the used aluminum nitride single crystal substrate is a standard specification polishing substrate sheet, the surface is a polishing surface, and the roughness is less than 0.3 nm through an RCA (rolling circle reactor) cleaning epitaxial preparation process; the back is grinding grade, and the roughness is 1 +/-0.2 mu m;

2) preparing Al_1-xSc_xN is a raw material for crystal growth. In the present example, a mixed material of high-purity AlN powder (purity 3N) and high-purity Sc metal (purity 3N) was used as Al_1-xSc_xThe N material is used as a raw material for growth, wherein the AlN powder accounts for 13 wt% of the total mass of the raw material. The ratio of AlN in the raw material to Al finally prepared_1-xSc_xThe variation trend of the Sc content in the N crystal is shown in FIG. 5. In this example, AlN powder accounting for 13 wt% of the total mass of the raw materials was used, and Al was finally obtained_1-xSc_xThe test result of the N crystal shows that Sc is in Al_1-xSc_xThe content of N crystals was 30at% (as a result of the test in step 6 of this example). In addition to this example, the inventors found the ratio of AlN in the raw material and Al finally obtained as shown in FIG. 5 after many times of experimental searches_1-xSc_xA Sc content variation trend graph in the N crystal; in repeated research experiments of the inventor, the preparation method of the invention realizes Al_1-xSc_xThe Sc content in the N crystal can reach about 50at percent at most, and the preparation method is reliable in process control when used in the range.

3) Mixing Al_1-xSc_xThe method comprises the following steps that N raw materials for crystal growth are placed at the bottom of a crucible (high-temperature area), a substrate material is placed at the top of the crucible (low-temperature area), the crucible is placed in a closed furnace body, and a heating mechanism for heating the crucible, a vacuumizing mechanism for vacuumizing the furnace body, a nitrogen input mechanism for introducing high-purity nitrogen into the furnace body and a temperature measuring mechanism for measuring the temperature of the crucible are arranged in the furnace body; the purity of the selected high-purity nitrogen is at least 99.999 percent; the crucible material selected is tungsten (the purity is 3N);

4) heating the bottom of the crucible to 1100 ℃ (T1) within 2 hours (T1) by a heating mechanism, introducing high-purity nitrogen to 5Kpa, and preserving heat for 10 hours (T2), wherein the AlN and ScN are decomposed and synthesized by using low-pressure conditions to achieve a mutual melting state of the AlN and the ScN;

5) the gas pressure in the furnace body is adjusted by the vacuum adjusting mechanism, so that the gas pressure in the furnace body is increased to 60Kpa, in the embodiment, the bottom of the crucible is heated to 2100 ℃ (T2) within 3 hours (T3) by the heating mechanism, the synthesized raw materials start to sublimate, and gas-phase substances (Al, Sc and N)₂) The Al is transferred to the surface of the substrate material under the drive of temperature gradient to start growing Al_1-xSc_xN crystal, the process is kept warm for 60 hours (t 4), the average growth rate is about 20um/h, and the thickness of the crystal reaches 1-1.2 mm. The holding time of the process is adjusted according to the thickness required for the crystal to be produced.

The step is mainly characterized in that the thermal field is adjusted, the radial and axial temperature difference in the growth chamber is controlled simultaneously through temperature selection in the heat preservation stage, so that the axial and radial thermal fields in the growth chamber are distributed as shown in figure 3, a low-temperature region is formed in the middle of the top of the crucible, a high-temperature region is formed in the middle of the bottom of the crucible, namely, a convex thermal field is formed in a raw material region, and a concave thermal field is formed in an AlN single crystal substrate region. Under the condition of the temperature field, the supersaturation distribution of gas-phase Al and Sc in the growth chamber is shown in figure 4, and a distribution with low supersaturation of Al and Sc is formed on the crystal growth surface, the saturation degree of the distribution is far lower than that of the conventional crystal growth by one order of magnitude, and under the equilibrium state of the low supersaturation degree, Al and Sc are in equilibrium state_1-xSc_xN is slowly deposited on the growth surface to obtain a two-dimensional layered growth mode, so that Sc element is prevented from being separated out, the crystallization quality is improved, the formation of defects such as dislocation and the like is avoided, and the high scandium doped Al is realized_1-xSc_xThe preparation of N crystal solves the problem that the physical vapor phase transmission method is difficult to prepare high scandium doped Al_1-xSc_xN crystal.

6) And (5) cooling. The temperature is reduced to room temperature within 10 hours (t 5). Al prepared in example 1 by electron spectroscopy (EDS)_{1- x}Sc_xThe content of Sc in the N crystal reaches 30 at%.

Al obtained in example 1_1-xSc_xSlicing N crystal, preparing substrate, testing by Piezoelectric Force Microscope (PFM), and testing with Al_1-xSc_xThe piezoelectric coefficient of the N substrate reaches 14.6 pC/N. The electromechanical coupling coefficient after the SAW device is manufactured is as high as 6.7%.

On the basis of the embodiment 1, the inventor conducts repeated experimental exploration and properly adjusts the process parameters, such as the furnace nitrogen pressure of 0.1-10KPa, the heating temperature of 1000-1200 ℃ and the heat preservation time of 1-20 hours in the step 4; the nitrogen pressure in the step 5 is 30-100KPa, the heating temperature is 1900-2200 ℃, and the heat preservation time in the step 5) is determined according to the finally prepared Al_{1- x}Sc_xThe target thickness of the N crystal is determined. These parameters are adjusted within the above range, and the object of the present invention can be achieved. The inventors have experimented with many examples and made Al of different Sc doping concentrations by adjusting the process recipe of example 1 and within the process parameter ranges_1-xSc_xN crystal, and to these Al_1-xSc_xThe N crystal was subjected to performance tests such as high-resolution X-ray diffraction test, piezoelectric coefficient, electromechanical coupling coefficient, etc., wherein table 1 lists some data of several examples tested by the inventors, and more detailed relationship between Sc concentration and piezoelectric coefficient in the material is shown in fig. 6. As can be seen from the graph, Al was observed when the Sc concentration reached about 43at%_1-xSc_xThe piezoelectric coefficient of the N crystal reaches the highest value of about 27.5pC/N, and then the piezoelectric coefficient rapidly decreases along with the increase of Sc concentration, so that the inventor analyzes that the possibility of forming other phases such as ScN and the like is increased along with the increase of Sc concentrationAnd in addition, the lattice mismatch between the substrate AlN and the defect can also be rapidly increased, so that the piezoelectric performance of the substrate AlN is obviously reduced.

In high resolution X-ray diffraction test, to the Al produced_1-xSc_xThe N crystal is used as a substrate material, the full width at half maximum of a (0002) rocking curve is less than 1 degree, the electromechanical coupling coefficient of a surface acoustic wave device (SAW) is 2-10 percent (shown in table 1), and the optimal value of the N crystal is 4-5 times of the corresponding performance of a pure AlN thin film template, so that the N crystal can be better suitable for application of bulk acoustic waves and surface acoustic wave devices, and the more excellent device performance is realized.

In addition, the inventor also finds that Al prepared by the preparation method of the invention_1-xSc_xThe N crystal has extremely low oxygen content, the total content of main impurities detected by GDMS/EGA is less than 100ppm, the purity is more than 99.95 percent, and the crystal grains are uniformly distributed, so that large-size high-purity Al can be provided for preparing the AlScN epitaxial film_1-xSc_xN target material is used and is suitable for preparing high-quality Al_1-xSc_xN thin film semiconductor materials.

TABLE 1

Sample numbering	Sc content	High resolution X-ray diffraction (0002) half-height width	Piezoelectric coefficient d33	Electromechanical coupling coefficient of SAW device
					1	9.6 at%	0.43°	7.6 pC/N	2.3%
2	20 at%	0.55°	10 pC/N	3.9%
					3	30 at%	0.72°	14.6 pC/N	6.7%
4	40 at%	0.9°	23 pC/N	8.5%

The above description of the embodiments is only intended to facilitate the understanding of the method of the invention and its core idea. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.

Claims (10)

1. The method for preparing the aluminum-scandium-nitrogen crystal by a physical vapor transport method is characterized in that the chemical expression of the aluminum-scandium-nitrogen crystal is Al_1-xSc_xN, said Al_1-xSc_xThe content of Sc in N can reach 50at%, and the method comprises the following steps:

1) preparing a substrate and raw materials;

2) putting a substrate and a raw material into a crucible, putting the crucible into a closed high-temperature furnace, putting the raw material into a high-temperature area, and putting the substrate into a low-temperature area; wherein the raw material is an AlN material or a mixture of an Al material and a material at least containing Sc;

3) introducing high-purity nitrogen into the furnace body to 0.1-10Kpa, simultaneously heating the bottom of the crucible to 1000-1200 ℃, and preserving heat for 1-20 hours;

4) continuously introducing nitrogen into the furnace body, increasing the air pressure in the furnace body to 30-100Kpa, simultaneously heating the bottom of the crucible to 1900-2200 ℃, and preserving the heat for a period of time to carry out the Al_1-xSc_xGrowth of N crystal to Al_1-xSc_xThe N crystal reaches the required thickness; in the heat preservation process, the radial and axial temperature difference in the growth chamber is controlled by adjusting the thermal field, so that the axial and radial thermal fields in the growth chamber are distributed in the raw material region to form a convex thermal field, and a concave thermal field is formed in the substrate region; and Al and Sc are distributed at the crystal growth surface with low supersaturation degree to form a two-dimensional layered growth mode, wherein Al is_1-xSc_xN is slowly deposited on the growth surface;

5) cooling to room temperature, and taking out the prepared Al from the crucible_1-xSc_xAnd (4) N crystals.

2. The method according to claim 1, wherein in step 1), the substrate material is a silicon, germanium or tungsten substrate material, or is a zinc oxide, sapphire, silicon carbide, gallium nitride, aluminum nitride, gallium arsenide, indium phosphide, lithium niobate or lithium tantalate compound.

3. The method of claim 2, wherein the substrate material comprises a polycrystal comprising an arbitrarily oriented crystal composition or a single crystal having a fixed orientation.

4. The method according to claim 1, wherein in step 1), the raw material comprises one or more of the following material combinations:

a material combination of AlN powder mixed with ScN powder or Sc metal, a material combination of AlN sintered body mixed with ScN powder or Sc metal, AlN polycrystal or a material combination of Al metal mixed with AlSc alloy.

5. The method according to claim 4, wherein the Sc content of the feedstock prepared in step 1) is dependent on the Al content_1-xSc_xAnd determining the content value of Sc in the N crystal.

6. The method of claim 1, wherein the Al is_1-xSc_xThe content of Sc in the N crystal can reach 30-43 at%.

7. The method of claim 1, wherein in step 2), the crucible is made of tungsten, tantalum carbide, tungsten carbide, tantalum nitride, or zirconium carbide material.

8. The method of claim 1, wherein the step 4) forms a distribution of low supersaturation levels of vapor phase Al and Sc at the crystal growth plane, wherein said low supersaturation levels of vapor phase Al and Sc are less than 0.06.

9. The method of claim 1, wherein: the heat preservation time in the step 5) is determined according to the Al_1-xSc_xDetermining the target thickness of the N crystal; the Al is_1-xSc_xThe thickness of the N crystal can reach more than 1 mm.

10. The method of claim 1, wherein: the Al is_1-xSc_xThe N crystal has low oxygen content, total impurity content less than 100ppm, purity greater than 99.95% and (0002) rocking curve width at half maximum less than 1 deg.

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