RU2750295C1 - Method for producing heteroepitaxial layers of iii-n compounds on monocrystalline silicon with 3c-sic layer - Google Patents

Abstract

FIELD: semiconductor technology.

SUBSTANCE: invention relates to semiconductor technology. The method is intended for producing heteroepitaxial layers of metal nitride compounds of group III (III-N), such as AlN, GaN, AlGaN and others, on monocrystalline silicon substrates. The III-N connections are used to create semiconductor power and microwave electronics devices. The method consists in the initial formation of a solid monocrystalline layer of silicon carbide of a cubic polytype (3C-SiC) with a thickness not exceeding the critical thickness thereof on the surface of a silicon substrate. On the surface of 3C-SiC, the first layer of aluminium nitride is grown at a temperature of not more than 1000°C and the second layer of aluminium nitride at a temperature of not more than 1250°C. The supergrille of the composition AlN/Al_xGa_1-xN (0,5≥Х≥0,3) with the thickness of alternating layers of 10-15 nm with a total thickness of at least 200 nm. A multilayer buffer composition of the Al_xGa_1-xN composition is produced, where X decreases from 0.3 to 0 as the structure grows. Finally, the monocrystalline layer of gallium nitride (GaN) of hexagonal orientation is grown.

EFFECT: invention enables increasing the structural quality and reducing the mechanical stresses of heteroepitaxial layers of III-N compounds on monocrystalline silicon.

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Description

The invention relates to semiconductor technology, more specifically to the field of forming heteroepitaxial layers of III-N compounds, such as AlN, GaN, AlGaN, etc., on monocrystalline silicon substrates. Compounds of nitrides of Group III metals of the Periodic Table of Chemical Elements (III-N compounds) were selected as materials for a new generation of power and microwave electronics. This choice is due to the unique characteristics of electronics based on wide-gap III-N connections, such as high switching speed of transistors, reduced power dissipation, high breakdown voltages, and others.

Currently, there are no available substrates of III-N compounds on the world market, which necessitates the formation of device layers of these materials using heteroepitaxy. For this reason, the attractiveness of using affordable and high-quality monocrystalline silicon (Si) substrates for heteroepitaxy of III-N compounds is difficult to overestimate. However, obtaining perfect heteroepitaxial III-N layers on silicon substrates is an extremely difficult technical problem due to the high degree of lattice mismatch of these materials. It is much more difficult to carry out III-N gas-phase epitaxy on Si than on other substrates, such as silicon carbide or sapphire, due to chemical damage to the silicon substrate by gaseous reagents of the vapor-gas mixtures used. Similar problems accompany the process of molecular beam epitaxy, since the silicon surface is subject to destruction under the direct action of atomic fluxes of Group III metals. In order to prevent the above problems, developers use complex buffer compositions.

A known method of forming buffer multilayer structures for III-N compounds on foreign substrates [1]. Silicon, sapphire, silicon carbide and other materials can be used as substrates. The method consists in the formation of a first aluminum-containing III-N buffer layer on the surface of a foreign substrate, then a second layer with a thickness of more than 15, 30 or 100 nanometers is formed, after which a third layer is formed, and so on. In this case, the Al content in the first buffer layer can be either constant or decrease with the growth of this layer. The Al content in the second layer must be higher than in the first and in the third, it can be constant in thickness or vary. The third buffer layer may contain no or less aluminum than the first and second layers. Each buffer layer or only one of them can be doped with elements from the series: iron (Fe), magnesium (Mg) and carbon (C). Initially, a seed III-N layer may be formed on the foreign substrate. The total thickness of the resulting buffer structure can be greater than 3, 5 or 7 microns. Subsequently, an active device layer is formed on this buffer structure. The formation of this buffer structure makes it possible to compensate for mechanical stresses in the finished heterostructure, which were formed due to the difference in the coefficients of thermal expansion of the foreign substrate and III-N layers. Thus, this makes it possible to avoid the formation of defects in the III-N layers due to relaxation, as well as mechanical destruction of the heterostructure.

The main disadvantage of this method is the lack of protection of the surface of the silicon substrate from destruction at the first stages of heteroepitaxial growth and the bulk of silicon from auto-doping with an aluminum impurity. As a result, the formation of a structurally perfect III-N layer is difficult, and it is also possible for a parasitic capacitance to appear, which has a detrimental effect on the microwave characteristics of the active device layer, even in the case of using high-resistance Si substrates. In addition, it remains unclear due to what relaxation mechanisms the mechanical stresses and deflection of the finished III-N heterostructures decrease.

A known method of forming an epitaxial GaN layer on a silicon single-crystal substrate with growth seeds from 3C-SiC [2]. The method consists in the initial formation of a layer of SiO ₂ on the surface of the silicon substrate. Then a mask is formed on this layer, etched, leaving SiO ₂ strips. On the Si surface open after etching, seed regions are formed for the growth of SiC by carbonization. Thereafter, the remaining SiO _{2 is} thermally decomposed and monocrystalline SiC is grown on the seed regions, as well as polycrystalline SiC on the damaged Si surface. Thereafter, single crystal GaN is formed on the surface of single crystal SiC and polycrystalline GaN on the surface of polycrystalline SiC.

Due to the fact that a polycrystalline layer is formed between the strips of single-crystal GaN, mechanical stresses are compensated, and the final deflection of the finished III-N heterostructure is minimized. As is known, in a polycrystalline layer, mechanical stresses of the crystal lattice are almost completely absent due to the high density of defects and disordered crystal structure of the layer.

The first disadvantage of this method is the presence of a polycrystalline phase in the bulk of the material, which leads to the impossibility of using these heterostructures in the field of power electronics due to the appearance of extended regions of space charge at the interphase boundaries. In addition, selective epitaxy of single-crystal and polycrystalline regions on a single substrate with preliminary masking is an extremely complex and multistage process that significantly increases the cost of the final product. In the case of the formation of a hexagonal GaN phase on the silicon (111) plane, this approach is unable to lead to stress compensation, since the difference in the thermal expansion coefficients between the substrate and heteroepitaxial layers in this case is much higher.

) растет на подложке с малым рассогласованием за счет слоя 3С-SiC (а=4,3596

, 3,4% несоответствие, сжимающее напряжение). В то же время рост GaN на подложке Si (а=5,431

, -17% несоответствие, растягивающее напряжение) сопровождается формированием дефектной мозаичной структуры слоя за счет большего рассогласования параметров решетки слоя и подложки.The closest in essence to the proposed technical solution is a method of forming layers of III-N materials oriented in the cubic plane (002) and oriented in the hexagonal plane (10-11) [3]. The III-N heteroepitaxial layer is formed on a silicon substrate coated with a cubic silicon carbide (3C-SiC) layer. An initial seed layer is formed on this substrate by nitridation at 500-700 ° C, which is subsequently recrystallized. Next, a cubic III-N layer (for example, a GaN layer) is formed using gas-phase epitaxy at 750-1000 ° C to a thickness of at least 0.3 μm. The N / III ratio is set in the range of 76-300. Thus, perfect heteroepitaxial layers III-N of cubic and hexagonal syngony are obtained, which are used in the production of green LEDs and other devices. The formation of quantum dots in the bulk of cubic GaN makes it possible to significantly increase the power of green LEDs, since the internal polarization field is not limited to a single crystallographic direction, due to which radiative recombination increases. Coating a silicon substrate with a 3C-SiC layer avoids the formation of amorphous silicon nitride (Si ₃ N ₄ ) at the stage of nitridation. The ratio of the intensity of diffraction reflections of the hexagonal (wurtzite) crystalline phase to the cubic (zinc blende) crystalline phase in the X-ray phase analysis should be less than or equal to 0.05. Thus, the epitaxial layer of cubic GaN (GaN lattice constant: a = 4.5059

) grows on the substrate with a small mismatch due to the 3C-SiC layer (a = 4.3596

, 3.4% mismatch, compressive stress). At the same time, the growth of GaN on a Si substrate (a = 5.431

, -17% mismatch, tensile stress) is accompanied by the formation of a defective mosaic structure of the layer due to a greater mismatch between the lattice parameters of the layer and the substrate.

The first disadvantage of this method is the absence of any transitional or buffer compositions aimed at compensating mechanical stresses in the finished III-N heterostructure on a silicon substrate coated with a 3C-SiC layer formed due to the difference in the values of thermal linear expansion coefficients (TCLE) and parameter mismatch gratings of the growing layers III-N and the substrate. The absence of such compositions in the manufacture of heterostructures with a large diameter (from 100 mm and more) will lead to the destruction of finished III-N heterostructures during growth or upon cooling due to high mechanical stresses, as well as to the appearance of large deflection (bending) values of heterostructures, which significantly reduces the productivity of the manufacturing process according to this method. The formation of a cubic phase of the III-N layer on silicon also narrows the field of application, since for most applications (LEDs of the visible spectrum of radiation, microwave and power transistors) a monocrystalline hexagonal phase (without inclusions of foreign crystalline phases) is required, for example, the crystalline GaN phase oriented in the plane ( 0002).

The objective of the present invention is to improve the structural quality and reduce the mechanical stress of heteroepitaxial layers of III-N compounds on monocrystalline silicon.

This is achieved by the fact that in the method for manufacturing heteroepitaxial layers of III-N compounds on the surface of monocrystalline silicon, including the formation of layers of III-N compounds on a silicon substrate coated with a layer of silicon carbide of cubic polytype (3C-SiC), a continuous monocrystalline layer is initially formed on the silicon surface 3C-SiC with a thickness of no more than its critical thickness, then on the surface of 3C-SiC the first layer of aluminum nitride is grown at a temperature of no more than 1000 ° C and the second layer of aluminum nitride at a temperature of no more than 1250 ° C, after which a superlattice of composition AlN / Al _{x is formed} Ga _1-x N (0.5≥X≥0.3) with a thickness of alternating layers of 10-15 nm with a total thickness of at least 200 nm and create a multilayer buffer composition of the composition Al _x Ga _1-x N, where X decreases from 0 , 3 to 0 as the structure grows, after which a single crystal layer of gallium nitride (GaN) of hexagonal orientation is grown.

As a result of the formation of a continuous monocrystalline 3C-SiC layer with a thickness not exceeding its critical thickness, the silicon surface is completely protected from the action of organometallic compounds and ammonia with the subsequent growth of III-N layers from the gas phase. Organometallic compounds of Group III metals, for example, such as trimethylgallium (TMG), destroy the silicon surface at the standard process temperature used (above 900 ° C). The presence of ammonia vapors in the reactor chamber at temperatures above 900 ° C leads to nitridation of the silicon surface, forming an amorphous layer of Si ₃ N ₄ , on which it is impossible to carry out epitaxial growth of nitrides. A layer of monocrystalline cubic silicon carbide plays the role of a chemical barrier and protects the silicon surface from the chemically active medium of VOC from organometallic compounds (MOVPE), thereby improving the structural perfection of the formed III-N layers. In addition, the 3C-SiC layer acts as a buffer, compensating (decreasing) the stresses in the III-N heteroepitaxial layers. As an example, the difference in the LTEC values between GaN (5.6 × 10 ^-6 K ^-1 ) and Si (2.6 × 10 ^-6 K ^-1 ) leads to cracking of the formed epitaxial layers after cooling the heterostructure to room temperature. At the same time, 3C-SiC has an LTEC of about 4.5 × 10 ^-6 K ^-1 and a lattice constant of 0.329 nm closer to III-N compounds (the mismatch is no more than 4% with GaN and 1% with AlN). The use of a 3C-SiC layer provides a reduction in mechanical stresses in the III-N / Si crystal structure. Exceeding the critical thickness during the formation of the 3C-SiC layer will lead to relaxation of elastic stresses in the crystal lattice of silicon carbide and the formation of various structural defects.

As a result of the formation of the first layer of aluminum nitride at a temperature of no more than 1000 ° C, a seed layer is formed, designed to reduce the effect of the cubic structure on defectiveness and stresses in the next III-N layers. The formation of the first layer at temperatures above 1000 ° C will lead to an increase in the surface roughness of the AlN layer due to the appearance of a large number of growth defects, such as edge and threading dislocations.

After the formation of the second layer of high-temperature aluminum nitride at a temperature of no more than 1250 ° C, the crystal structure of the surface will correspond to the required value of the lattice parameter of hexagonal aluminum nitride AlN (0002) with a minimum surface roughness, as a result of which it will be possible to grow any variation of the ternary solution of Al _x Ga _{1- x} N. The formation of the second layer at a temperature above 1250 ° C will lead to an increase in the roughness of the AlN layer, as a result of which the roughness of the subsequent III-N layers, including GaN, will increase.

The formation of a superlattice with the composition AlN / Al _x Ga _1-x N at 0.5≥X≥0.3 with a thickness of alternating layers of 10-15 nm with a total thickness of at least 200 nm makes it possible to compensate for mechanical stresses formed by the difference in the lattice parameters and LTEC of the formed layers and substrates. Linear defects, such as threading dislocations, break off at AlN / Al _x Ga _1-x N heterointerfaces. As a result, the density of growth defects in the subsequent Al _x Ga _1-x N multilayer structure is significantly reduced.

To achieve the maximum structural perfection of a single-crystal GaN layer of a hexagonal orientation, a multilayer buffer composition of the composition Al _x Ga _1-x N is formed, where X decreases from 0.3 to 0 and a smooth stoichiometric transition occurs from a ternary solution of Al _x Ga _1-x N to a single crystal layer. GaN. The number of layers of the Al _x Ga _1-x N structure required for this transition to GaN is determined experimentally for each operating process and mainly depends on the chosen growth method and epitaxial equipment. The total thickness of the multilayer structure is set equal to the thickness of the further formed GaN layer in order to commensurately compensate for mechanical stresses.

Graphic images

FIG. 1 schematically shows a schematic diagram of the Ga (Al) N / 3C-SiC / Si heterostructure manufactured by the proposed method.

FIG. 2 shows an image of the surface of a 1 μm thick GaN layer on a Ga (Al) N / 3C-SiC / Si heterostructure with a multilayer buffer composition obtained by atomic force microscopy. The image demonstrates the fundamental morphology of the surface of the obtained single-crystal GaN layer, which consists of smooth growth steps of hexagonal orientation.

FIG. 3 shows the results of X-ray phase (A) and X-ray diffraction (B) analyzes of the Ga (Al) N / 3C-SiC / Si heterostructure grown on a single-crystal silicon substrate with the Si (111) orientation by the proposed method. The results obtained demonstrate the presence of clear reflections from the hexagonal single-crystal phases of III-N compounds and from the single-crystal hexagonal GaN (0002) phase. The small width of the peaks shows a high structural perfection of the obtained III-N heteroepitaxial layers and a single-crystal gallium nitride layer.

The method of manufacturing heteroepitaxial layers of III-N compounds on monocrystalline silicon with a 3C-SiC layer is implemented as follows. A single crystal silicon substrate is placed in the reactor. The reactor chamber is sealed and pumped out to a pressure of 50-60 Torr. After heating the substrate surface to a temperature of 900-1300 ° C, a silicon-containing reagent (for example, monosilane) and a carbon-containing reagent (for example, ethylene) are supplied, the 3C-SiC layer is grown to a thickness of 20-100 nm. After that, the reactor chamber is purged and the substrate surface is heated to a temperature not exceeding 1000 ° C. The supply of organometallic compounds of the III-group (for example, TMG and trimethylaluminium (TMA)) and nitrogen-containing reagents (for example, gaseous ammonia) is carried out. Next, the AlN layer is formed, necessarily using a low-temperature growth phase and a high-temperature one. Low-temperature aluminum nitride is formed at temperatures from 800 to 1000 ° C. High-temperature AlN - at temperatures from 1000 to 1250 ° C. Next, a superlattice of composition AlN / Al _x Ga _1-x N (0.5≥X≥0.3) is formed with a thickness of alternating layers of 10-15 nm with a total thickness of at least 200 nm. The growth temperature of the superlattice is selected from the standard range, taking into account the minimum roughness of the resulting surface. A multilayer structure of the composition Al _x Ga _1-x N is formed, where X decreases from 0.3 to 0 as the structure grows. Thereafter, a single-crystal GaN layer of the required thickness is grown for the subsequent formation of active device regions both in-situ and in a separate epitaxial process.

An example of a specific implementation

A single-crystal silicon substrate with an orientation of the Si (111) working surface 100 mm in diameter with a dopant content from 1 × 10 ¹⁰ cm ^-3 to 9 × 10 ¹⁶ cm ^{-3 is} placed in a MOVPE reactor, pumped out to 100 Torr and heated to a temperature of 950 ° C. Gaseous SiH ₄ and C ₂ H ₄ are fed in equal proportions at a flow rate of 100 cm ³ / min. A layer of 3C-SiC with a thickness of 50 nm is formed. After that, the reactor chamber is purged, the substrate is heated to a temperature of 750 ° C-1000 ° C, TMA is supplied at a rate of 15 cm / min with a V / III ratio of more than 1000, and the first AlN layer is formed (composition X = 1). Then, growth continues at the same fluxes, but at a high substrate temperature of 1200 ° C, forming a second AlN layer (composition X = 1). Then, a superlattice of the composition AlN / Al _0.3 Ga _0.7 N is formed due to the time-increasing multiple opening of the supply of TMG to the reactor chamber. _{In this case, Al 0.3} Ga _0.7 N layers increasing in thickness are formed, and the thickness of the AlN interlayers decreases. The number of AlN layers and the number of Al _0.3 Ga _0.7 N layers is 20. The last discovery of TMG for the superlattice with a flow of 20 cm ³ / min begins to form a thicker layer of the composition Al _0.3 Ga _0.7 N 300 nm thick. Following the Al _0.3 Ga _0.7 N layer, an Al _0.2 Ga _0.8 N 300 nm layer grows due to an increase in the TMG flux to 40 cm ³ / min. Then, Al _0.1 Ga _0.9 N 300 nm is grown at a TMA flow of 5 cm ³ / min and a TMG ^{flow of 40 cm 3} / min. To grow the GaN layer, the TMA flow is blocked, and a single crystal GaN layer of the required thickness is grown.

The proposed method, in comparison with the prototype, provides the following advantages: protection of the silicon surface from destruction by gaseous reagents during epitaxial growth; suppression of the effect of auto-doping of silicon with Group III metals during growth (3C-SiC is a chemical barrier in which the solid-phase diffusion of these elements is negligible); reduction of mechanical stresses in the heterostructure due to close lattice parameters and LTEC of silicon carbide and III-N compounds; reducing the bending of the silicon substrate, which subsequently has a positive effect on the design of the device; improvement of the crystal structure of the final III-N layer due to a smooth stoichiometric transition from a ternary solution of AlGaN to GaN.

INFORMATION SOURCES

1. International application 2013116747, publ. 08.08.2013

2. US Patent 9515222, publ. 06.12.2016

3. International application 2018177552, publ. 10/04/2018 - prototype.

Claims (1)

A method of manufacturing heteroepitaxial layers of III-N compounds on the surface of monocrystalline silicon, including the formation of layers of III-N compounds on a silicon substrate coated with a layer of silicon carbide of a cubic polytype (3C-SiC), characterized in that initially a continuous monocrystalline layer 3C- is formed on the silicon surface. SiC with a thickness of no more than its critical thickness, then the first layer of aluminum nitride is grown on the surface of 3C-SiC at a temperature of no more than 1000 ° C and the second layer of aluminum nitride at a temperature of no more than 1250 ° C, after which a superlattice of the composition AlN / Al _x Ga _{1 is formed -x} N (0.5≥X≥0.3) with a thickness of alternating layers of 10-15 nm with a total thickness of at least 200 nm and create a multilayer buffer composition of the composition Al _x Ga _1-x N, where X decreases from 0.3 to 0 as the structure grows, after which a single-crystal layer of gallium nitride (GaN) of hexagonal orientation is grown.

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