CN111472045A - Aluminum nitride single crystal preparation method based on large-size seed crystals - Google Patents

Abstract

The invention discloses a preparation method of aluminum nitride single crystal based on large-size seed crystals. Then, adjust the temperature field to return to the normal state of long-term growth and continue to grow AlN to obtain a large-sized AlN single crystal. In the present invention, the surface of the seed crystal is dense and flat, without primary cracks, which is beneficial for subsequent growth to obtain crack-free crystals; the size of the seed crystal can reach more than 2 inches in the in-plane size, and different sizes can be customized according to needs to meet the requirements of 2 inches and more. The growth of AlN bulk single crystal; sputtering AlN effectively reduces the cost and improves the efficiency.

Description

A kind of preparation method of aluminum nitride single crystal based on large-size seed crystal

technical field

The invention relates to the technical field of crystal growth, in particular to a method for preparing a large-sized aluminum nitride (AlN) single crystal by a physical vapor transport (Physical VaporTransport, PVT) method.

Background technique

Aluminum nitride (AlN) crystal, with wider band gap, high thermal conductivity and small mismatched lattice constant, has become indispensable for the development of high-performance deep ultraviolet optoelectronic devices and ultra-high voltage high-power power electronic devices. base substrate. Due to the implementation of the "Minamata Convention on Mercury" in 2020, there has been a wide and urgent need for deep ultraviolet LEDs to replace traditional mercury-containing light sources, which can be used for medical sterilization and disinfection. With the further expansion of the market scale and the expectation for the performance of power devices and radio frequency electronic devices on AlN substrates, AlN substrates have become a research hotspot in the field of semiconductor optoelectronics, and are also an urgent strategic demand of the country.

At present, only HexaTech in the world has achieved the world's largest 2-inch AlN single crystal substrate without macroscopic defects in 2019, which is the highest level of AlN single crystal preparation, but the 2-inch AlN substrate wafer has still not been achieved. Commercialization, small quantity and high price, and due to the strict embargo on my country, there is no AlN single crystal substrate available in the domestic market. Large-sized AlN seed crystals or alternative seed crystal substrates are one of the keys to technological breakthroughs in AlN single crystal substrates larger than 2 inches.

There are two commonly used seed crystal schemes: one is the diameter expansion technology of spontaneous nucleation, that is, the spontaneous nucleation process obtains AlN crystals of good quality in millimeters, and then the spontaneous nucleated AlN small crystals are used as seed crystals, through a specially designed crucible. The diameter-expanding growth is carried out, and large-sized AlN crystals are obtained by multiple iterations. The biggest difficulty of this method is the large size. Both theoretical calculation and practice have found that in the expansion technology, the inclination angle of the sidewall of the seed crystal must be less than 22-27o. Using this expansion technology has low efficiency and high cost of seed crystals. More than 2 inches AlN crystal growth is difficult; the second is the PVT iterative technology of SiC seed crystal, that is, the AlN layer is grown by heterogeneous PVT on the SiC substrate, and then the AlN layer is used as the seed crystal for multiple iterative growth to obtain AlN single crystal with optimized quality substrate. This method can easily enlarge the size, but the biggest problem is low single crystallinity, poor crystal quality, and accompanied by serious cracking and low light transmittance. Obviously, the current conventional technology has problems of low efficiency and high cost, and it is difficult to solve many problems existing in the growth of 2-inch seed crystals.

The sputtering AlN thin layer technology has a wide range of applications in the MOCVD growth of nitride-based optoelectronic device structures. The study found that the magnetron sputtering AlN film has the advantages of high density, c-direction preferential growth, arbitrary size and low cost, and has a good seed layer effect. The seed effect of the AlN layer depends more on its own structure. , and has little to do with the substrate orientation. Based on the above characteristics, we propose a large-size seed crystal technology of sputtering AlN layer directly on the metal cover of the crucible and a special PVT growth technology of two-stage positive temperature field to obtain large-size AlN crystals. Sputtering AlN/W or sputtering AlN/(Ta or TaC or Ta/TaC) composite seed crystal technology can reduce the difficulty of AlN seed crystal technology, arbitrarily enlarge the size, and avoid the technical difficulty of high temperature seed crystal bonding. It has high practicability and industrialization value.

SUMMARY OF THE INVENTION

The invention provides a method for preparing an aluminum nitride single crystal based on a large-size seed crystal. After growing an AlN crystal with a certain thickness on the AlN seed layer, adjust the temperature field to return to the normal state of long-term growth and continue to grow to obtain a large-sized AlN single crystal, which effectively reduces costs and improves efficiency.

The core of the preparation method that is different from the prior art is the large-size seed layer of sputtering AlN and the special PVT growth method matched with it, specifically:

On the surface of the polished crucible lid (made of one or more of the materials used for PVT growth of AlN, such as tungsten W, tantalum Ta, tantalum carbide TaC, etc.), the large size of AlN is deposited by magnetron sputtering A seed layer is formed to form an AlN composite substrate. On the surface of this composite substrate, a two-stage positive temperature field continuous growth technology is used: moving the crucible in the thermal field or changing the power ratio of the multi-temperature zone, to achieve a lower temperature on the surface of the sputtering seed crystal during the heating stage and the formation of the inside of the crucible. the purpose of a larger temperature difference. In this way, the decomposition of the sputtering seed layer can be suppressed, and the temperature of the source powder can be ensured to be sufficiently high to have a certain degree of sublimation. After growing a certain thickness of AlN crystal on the sputtered AlN seed layer, adjust the temperature field to return to the normal long-term growth state (achieved by moving the crucible or changing the power ratio), and continue to grow to obtain a large-sized AlN single crystal.

The technical scheme provided by the present invention is:

A method for preparing aluminum nitride single crystal based on large-size seed crystals: first, select the crucible cover of the crucible (the diameter of the crucible cover is > 2 inches, which is made of tungsten W, tantalum Ta, tantalum carbide TaC and other materials used for PVT growth of AlN. One or more of these are made, the crucible cover is used as the substrate) for pretreatment such as mechanical polishing and chemical cleaning, and the surface roughness Ra<1.6 μm is required; The sputtering method controls the thickness of the AlN seed layer >600nm to form a composite substrate; finally, the composite substrate is placed on the crucible, an appropriate amount of pre-sintered AlN powder is placed in the crucible, and the whole crucible is placed in the PVT equipment for subsequent follow-up. grow. The two-stage positive temperature field technology is used in the continuous growth: in the heating stage and the initial stage of growth, by moving the position of the crucible in the thermal field or changing the power ratio of the multi-temperature zone, the surface temperature of the composite substrate is guaranteed to be less than 1900 ℃, and grow at this lower temperature for 10-60min; then increase the temperature (by moving the position of the crucible in the thermal field or changing the power ratio of the multi-temperature zone), so that the surface temperature of the composite substrate is > 2100 ℃, The heating time is t<10min; finally, AlN single crystal is grown for a long time at a higher temperature of >2100℃.

The method of the present invention specifically comprises the following steps:

1) Substrate (crucible cover) selection:

The AlN single crystal is grown by PVT method, and the crucible cover made of W, Ta, TaC and other materials is selected as the substrate, the diameter is more than 50mm, the thickness is more than 1mm, highly polished, and the surface roughness Ra<1.6μm;

2) Substrate pretreatment:

First, soak the substrate material in dilute hydrochloric acid for 5-10min (or other methods to remove contaminants and oxide layers), then wash with deionized water, and then ultrasonically clean in acetone and alcohol for 10-30min. It must be stored in a vacuum before sputtering;

3) Sputtering AlN film:

Using magnetron sputtering technology, using high-purity Al source as the target, first pre-sputtering in an argon atmosphere to remove the surface oxide layer and contaminants, and then sputtering in a mixed atmosphere of nitrogen and argon at a temperature of 100 -300℃, after sputtering, the temperature is cooled in an argon atmosphere, a c-plane AlN film with a thickness of 200-300nm can be obtained at one time, and an AlN seed layer larger than 600nm can be obtained by multiple sputtering, and then a composite substrate can be obtained;

4) Crystal growth:

The pre-sintered AlN powder is loaded into the crucible, and the composite substrate and the crucible are placed in a metal PVT growth furnace, pumped and ventilated, and heated in a vacuum or nitrogen atmosphere. The two-stage positive temperature field technology is used in the continuous growth: in the heating stage and the initial stage of growth, by moving the position of the crucible in the thermal field or changing the power ratio of the multi-temperature zone, the surface temperature of the composite substrate is guaranteed to be less than 1900 ℃, and grow at this lower temperature for 10-60min; then by moving the position of the crucible in the thermal field or changing the power ratio of the multi-temperature zone, the surface temperature of the composite substrate is >2100℃, and the heating time t<10min ; Finally, AlN single crystal was grown for a long time at a higher temperature > 2100 °C, and the growth time was 4-96 hours.

Through the above steps, large-sized AlN crystals are prepared.

Compared with the prior art, the beneficial effects of the present invention are:

The invention provides a preparation method of aluminum nitride single crystal based on large-size seed crystal. First, the surface of the seed crystal is dense and flat without primary cracks, which is beneficial for subsequent growth to obtain a crack-free crystal; second, the size of the seed crystal can reach more than 2 inches in the in-plane size, and can be customized to be larger or other as needed. Different sizes, so as to meet the growth of AlN bulk single crystals of 2 inches and above; third, compared with traditional AlN seed crystals (spontaneous nucleation or SiC heteroepitaxy), sputtered AlN effectively reduces costs and improves efficiency .

Description of drawings

Fig. 1 is the basic schematic diagram of the furnace body used for growing AlN bulk single crystal by PVT method in the embodiment of the present invention:

1 is the furnace body and nitrogen environment; 2 is the air inlet; 3 is the air outlet; 4 is the crucible; 5 is the composite substrate; 6 is the high temperature growth chamber; 7 is the high-purity AlN source powder; 8 is the support base , the height is adjustable; 9 is a double heater (heating body), the power is adjustable.

Detailed ways

Below in conjunction with the accompanying drawings, the present invention is further described by means of embodiments, but the scope of the present invention is not limited in any way.

FIG. 1 is a basic schematic diagram of a furnace body used for growing AlN bulk single crystal by PVT method in an embodiment of the present invention. Inside the crucible 4 is the reaction zone. The double heaters 9 can provide the crucible with a high temperature of over 2000°C. The high temperature zone (high temperature growth chamber 6) has a certain temperature gradient distribution and can be maintained to meet the requirements of the temperature field distribution for crystal growth. The heater can adjust the power ratio, and the support base 8 can be raised and lowered, so that the crucible 4 can switch between high temperature difference and low temperature difference. The crucible 4 and the outer heating and holding area are not completely sealed, and the air flow can pass through the gap. Dry pumps and molecular pumps can pump and maintain a certain degree of vacuum. When nitrogen is continuously fed during the growth, the vacuum pump works, so that the gas in the furnace can be replaced and the pressure is stable. Inside the high temperature growth chamber 6, the AlN source powder is sublimated and AlN deposition is performed at the top composite substrate.

Example 1 W plate - sputtering AlN seed layer composite substrate for PVT growth

1) Substrate pretreatment:

First, soak the highly polished W plate in dilute hydrochloric acid for 5-10 minutes, then wash it with deionized water, and then ultrasonically clean it in acetone and alcohol successively for 10-30 minutes. It must be stored in vacuum before sputtering;

2) Sputtering AlN thin films:

Using magnetron sputtering technology, using high-purity Al source as the target, first pre-sputtering in an argon atmosphere to remove the surface oxide layer and contaminants, and then sputtering in a mixed atmosphere of nitrogen and argon at a temperature of 100 -300℃, after sputtering, the temperature is cooled in an argon atmosphere, a c-plane AlN film with a thickness of 200-300nm can be obtained at one time, and an AlN seed layer larger than 600nm can be obtained by multiple sputtering, and then a composite substrate can be obtained;

3) Place the crucible:

The tungsten crucible containing the sintered AlN source powder is sealed with a composite substrate, the crucible is placed in a metal furnace, and the furnace body is closed;

4) Holding pressure and heating up:

Evacuate the furnace environment to a vacuum (10 ^-4 Pa), keep the vacuum or inject a certain amount of nitrogen to maintain the pressure, start to heat up, and ensure that the crucible is at the highest position during the heating process, so that the composite substrate is always lower than 1900°C;

5) Low temperature growth:

After the heating is completed, the pressure is maintained at 200-1000 Torr, the crucible is moved down, and the power ratio is adjusted to ensure that the temperature of the composite substrate is below 1900 ℃, and the temperature of the source powder is high enough, and the growth time is 10-60min;

6) High temperature growth:

Adjust the power ratio so that the surface temperature of the composite substrate is >2100℃, and the heating time is t<10min; finally, AlN single crystal is grown for a long time at a higher temperature >2100℃, and the growth time is 4-96 hours.

Example 2 TaC/Ta plate - sputtered AlN seed layer composite substrate for PVT growth

1) Carbonization of Ta plate:

The Ta plate is washed and carbonized at high temperature above 2000℃ in a graphite furnace to form a dense TaC layer on the surface;

2) Substrate pretreatment:

Soak the TaC/Ta plate in dilute hydrochloric acid for 5-10min, then wash it with deionized water, and then ultrasonically clean it in acetone and alcohol successively for 10-30min. It must be stored in vacuum before sputtering;

3) Sputtering AlN film:

Using magnetron sputtering technology, using high-purity Al source as the target, first pre-sputtering in an argon atmosphere to remove the surface oxide layer and contaminants, and then sputtering in a mixed atmosphere of nitrogen and argon at a temperature of 100 -300℃, after sputtering, the temperature is cooled in an argon atmosphere, a c-plane AlN film with a thickness of 200-300nm can be obtained at one time, and an AlN seed layer larger than 600nm can be obtained by multiple sputtering, and then a composite substrate can be obtained;

4) Place the crucible:

The tungsten crucible containing the sintered AlN source powder is sealed with a composite substrate, the crucible is placed in a metal furnace, and the furnace body is closed;

5) Holding pressure and heating up:

Evacuate the furnace environment to a vacuum (10 ^-4 Pa), keep the vacuum or inject a certain amount of nitrogen to maintain the pressure, start to heat up, and ensure that the crucible is at the highest position during the heating process, so that the composite substrate is always lower than 1900°C;

6) Low temperature growth:

After the heating is completed, the pressure is maintained at 200-1000 Torr, the crucible is moved down, and the power ratio is adjusted to ensure that the temperature of the composite substrate is below 1900 ℃, and the temperature of the source powder is high enough, and the growth time is 10-60min;

7) High temperature growth:

Adjust the power ratio so that the surface temperature of the composite substrate is >2100℃, and the heating time is t<10min; finally, AlN single crystal is grown for a long time at a higher temperature >2100℃, and the growth time is 4-96 hours.

It should be noted that the purpose of the published embodiments is to help further understanding of the present invention, but those skilled in the art can understand that various replacements and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. of. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection of the present invention shall be subject to the scope defined by the claims.

Claims (7)

1. A method for preparing aluminum nitride single crystals based on large-size seed crystals comprises the steps of depositing aluminum nitride large-size seed crystal layers by a magnetron sputtering method, growing AlN crystals with a certain thickness on the sputtered AlN seed crystal layers, adjusting a temperature field to return to a normal growth state for continuous growth of AlN, and obtaining large-size AlN single crystals; the method comprises the following steps:

firstly, selecting a crucible with the diameter of the crucible cover larger than 2 inches, and pretreating the crucible cover to ensure that the surface roughness Ra of the crucible cover is smaller than 1.6 mu m; the crucible cover is used as a substrate; the crucible cover as a substrate is made of a material for growing an AlN single crystal by the PVT method;

then, magnetron sputtering an AlN thin film on the surface of the crucible cover by adopting a magnetron sputtering method, and controlling the thickness of the AlN seed crystal layer to be more than 600nm by adopting a multi-time sputtering method to form a composite substrate;

finally, the composite substrate is placed on a crucible, pre-sintered AlN powder is filled in the crucible, and the whole crucible is placed in PVT equipment for subsequent successive crystal growth;

the two-stage type positive temperature field technology is adopted in the successive growth of the crystal: in the temperature rising stage and the initial stage of starting growth, the surface temperature of the composite substrate is less than 1900 ℃, and the composite substrate grows for 10-60min at the lower temperature; then raising the temperature to make the surface temperature of the composite substrate be greater than 2100 ℃, and the temperature rise time t is less than 10 min; long-term growth at higher temperatures >2100 ℃;

through the steps, the large-size AlN single crystal is prepared.

2. A large-size seed crystal-based aluminum nitride single crystal preparation method as claimed in claim 1, wherein the crucible cover as the substrate is made of one or more of tungsten W, tantalum Ta and tantalum carbide TaC; the diameter of the crucible cover is more than 50mm, and the thickness of the crucible cover is more than 1 mm.

3. A large-sized seed crystal-based aluminum nitride single crystal production method as set forth in claim 1, wherein the crucible cover is subjected to pretreatment including mechanical polishing and chemical cleaning pretreatment, to remove contaminants and an oxide layer, to cleaning, and to vacuum preservation before sputtering.

4. A method for preparing a large-sized seed crystal-based aluminum nitride single crystal as claimed in claim 3, wherein the substrate material is immersed in dilute hydrochloric acid for 5-10min to remove contaminants and an oxide layer, then washed with deionized water, and then ultrasonically cleaned in acetone and alcohol for 10-30 min.

5. The method for preparing the aluminum nitride single crystal based on the large-size seed crystal as claimed in claim 1, wherein the method for magnetron sputtering the AlN thin film comprises the following specific steps:

a magnetron sputtering technology is utilized, and a high-purity Al source is used as a target material;

firstly, pre-sputtering in an argon atmosphere to remove a surface oxide layer and pollutants;

then sputtering in the mixed atmosphere of nitrogen and argon at the temperature of 100-300 ℃;

after the sputtering is finished, cooling in an argon environment to obtain a c-surface AlN thin film with the thickness of 200-300nm at one time;

and (4) carrying out multiple times of sputtering to generate an AlN seed crystal layer with the thickness of more than 600nm so as to obtain the composite substrate.

6. A method for preparing a large-size seed crystal-based aluminum nitride single crystal as claimed in claim 1, wherein the subsequent successive growth of the crystal is carried out by loading pre-sintered AlN powder into a crucible, placing the composite substrate and the crucible in a metal PVT growth furnace, evacuating and ventilating, and heating in vacuum or nitrogen atmosphere.

7. A method for preparing an aluminum nitride single crystal based on a large-size seed crystal as claimed in claim 1, wherein a two-stage type normal temperature field technology is adopted in the successive growth, and specifically:

in the heating stage and the initial stage of starting growth, the surface temperature of the composite substrate is enabled to be less than 1900 ℃ by moving the position of the crucible in a thermal field or changing the power ratio of a multi-temperature zone, and the growth time is 10-60min at the lower temperature;

then, the surface temperature of the composite substrate is enabled to be more than 2100 ℃ by moving the position of the crucible in the thermal field or changing the power ratio of the multiple temperature zones, and the temperature rise time t is enabled to be less than 10 min;

and finally growing the AlN single crystal for a long time at a high temperature of more than 2100 ℃ for 4 to 96 hours.

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