JP2006222361A - Nitride semiconductor crystal and its production process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress deterioration in crystal quality (surface planarity, orientation, dislocation density) of a thin film due to difference in lattice constant as much as possible in an AlN thin film as a buffer layer when III-V nitride semiconductor crystal is grown on an Si substrate. <P>SOLUTION: In the process for producing nitride semiconductor crystal where an AlN layer 2 is formed as a buffer layer on an Si substrate 1 and a thin film of III-V nitride semiconductor crystal is formed thereon, a mixed crystal layer of AlN and Si is formed by growing the AlN layer 2 while supplying Si material together with AlN material in the initial stage of growth thereby adding Si element to have a concentration of 1×10<SP>20</SP>cm<SP>-3</SP>or above. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a nitride semiconductor crystal and a method of manufacturing the same, and more particularly, a high-quality III-V nitride semiconductor thin film crystal is grown on a Si substrate, and has excellent characteristics compared to conventional products. The present invention relates to a technique for realizing wafer supply.

  III-V nitride semiconductor epitaxial wafers in which GaN, AlN, InN, and mixed crystals thereof are laminated and grown in an optimum structure have already been put on the market as epitaxial wafers for blue light emitting diodes (LEDs). Blue laser diode (LD) epitaxial wafers and ultraviolet LED epitaxial wafers are also being developed. Further, with the recent demand for high-power transistors, the use of nitride semiconductor crystals is not limited to optical devices, but has been developed as epitaxial wafers for GaN-HEMT. As substrates used for these epitaxial wafers for optical / electronic devices, there are only sapphire substrates and SiC substrates on the market that are in practical use.

  However, in recent years, research on crystal growth of III-V nitride semiconductor thin films on Si substrates has become active. This is because the Si substrate is less expensive than the sapphire substrate or the SiC substrate, so that the cost of the epitaxial wafer can be further reduced. Furthermore, since the diameter can be easily increased and the existing process line for Si devices can be applied, it is possible to reduce the cost of the device.

However, it is difficult to grow a high-quality group III-V nitride semiconductor thin film on a Si substrate. The main cause is
(i) Wafer warpage and cracks due to differences in thermal expansion coefficient,
(ii) Deterioration of the surface state and orientation of the thin film crystal due to the difference in lattice constant from the substrate, and increase of dislocations,
There are two.

  In order to suppress the warpage and cracking of the wafer, the nitride semiconductor crystal must be made as thin as possible. However, in the case of a thin thin film crystal, a difference in lattice constant from the substrate is remarkably reflected, so that it is difficult to realize a high quality crystal.

  In order to solve this problem, various strain relaxation buffer layer structures have been proposed for a long time, and hetero buffers such as metal Ti and ZnO have been reported.

For example, since an AlN (aluminum nitride) thin film has a lattice mismatch rate of 1% with a SiC substrate and a lattice mismatch with a Ga thin film of 2.5%, this AlN thin film or GaN layer is used as a SiC substrate. Research has been conducted to use the GaN-based thin film as a buffer layer (see, for example, Patent Document 1).
JP 2001-176804 A

  By the way, the reason why the Si substrate is used as the substrate is to realize further cost reduction as compared with the existing products, and it is desired to collectively perform buffer layer formation → nitride thin film growth in one apparatus. For this purpose, it is desirable to use an AlN buffer layer and make the nitride semiconductor thin film crystal grown thereon as flat and oriented as possible, and to have low dislocations. For this purpose, the AlN buffer layer itself must realize good surface flatness, orientation, and dislocation density.

  However, the difference in lattice constant between AlN and Si is large as shown in Table 1 below. Therefore, AlN on the Si substrate in the initial stage of growth grows as a collection of granular lumps. Even if AlN growth is continued as it is, even if the thickness is increased to a certain extent, surface flatness, orientation, and low dislocation cannot be expected.

  Accordingly, an object of the present invention is to solve the above-mentioned problems, and when growing a group III-V nitride semiconductor crystal on a Si substrate, in the AlN thin film as the buffer layer, the crystal quality of the thin film due to the difference in lattice constant (surface flatness) Property, orientation, dislocation density).

  In order to achieve the above object, the present invention is configured as follows.

The nitride semiconductor crystal according to the invention of claim 1 is a nitride semiconductor crystal in which an AlN layer is formed as a buffer layer on a Si substrate, and a group III-V nitride semiconductor crystal is stacked thereon as a thin film. A feature is that a mixed crystal layer of AlN and Si is formed by adding Si element so as to have a concentration of 1 × 10 ²⁰ cm ⁻³ or more in a part of the vicinity of the substrate of the layer.

  According to a second aspect of the present invention, in the nitride semiconductor crystal according to the first aspect, the thickness of the Si-added portion of the AlN layer is less than half that of the AlN layer (for example, 100 nm or less). .

  The invention of claim 3 is characterized in that, in the nitride semiconductor crystal of claim 1 or 2, the additive concentration of Si element in the Si addition part of the AlN layer is continuously reduced from the substrate toward the surface. To do.

According to a fourth aspect of the present invention, there is provided a nitride semiconductor crystal manufacturing method comprising: forming an AlN layer as a buffer layer on a Si substrate; and laminating a group III-V nitride semiconductor crystal as a thin film thereon. In the manufacturing method, at the initial stage of growth of the AlN layer, the Si element is added to a concentration of 1 × 10 ²⁰ cm ⁻³ or more by growing while flowing the Si raw material together with the AlN raw material. A mixed crystal layer is formed.

  According to a fifth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the fourth aspect, the thickness of the Si-added portion of the AlN layer is less than half the thickness of the AlN layer (for example, 100 nm or less). A method for producing a nitride semiconductor crystal.

  The invention of claim 6 is characterized in that, in the method of manufacturing a nitride semiconductor crystal according to claim 5, the additive concentration of Si element in the Si addition part of the AlN layer is continuously reduced from the substrate toward the surface. To do.

  The invention according to claim 7 is the method for producing a nitride semiconductor crystal according to claim 5 or 6, wherein the AlN layer as the buffer layer is grown by the MOVPE method and monosilane, disilane, dichlorosilane, Either trimethyl silicon or triethyl silicon is used.

  The invention according to claim 8 is the method for producing a nitride semiconductor crystal according to any one of claims 4 to 7, wherein a temperature during growth of the AlN layer as the buffer layer is set to 1000 ° C. or more. .

  According to a ninth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the eighth aspect, a growth pressure during the growth of the AlN layer as the buffer layer is set to 200 Torr (approximately 267 hPa) or less.

<Key points of the invention>
In the present invention, an AlN and Si mixed crystal layer is formed by growing an initial AlN layer while flowing Si raw material. Further, by continuously changing the composition of the mixed crystal layer, the lattice constant is continuously changed from the Si substrate to the AlN layer.

  In the early stage of AlN growth, Al supplied from the gas phase diffuses on the surface of the substrate for a certain period of time, eventually reacts with nitrogen radicals in the gas phase, and is fixed as an AlN crystal nucleus in an arbitrary place. After AlN nuclei are randomly formed in the substrate surface in this way, the nuclei expand and become islands with growth. Normally, this island expands two-dimensionally, and the islands merge to form a single film. However, if the lattice constant difference is large, the island cannot be expanded two-dimensionally but expanded in three dimensions. If it becomes so, it will become a thin film that a granular lump gathered.

  Si also serves as an anti-surfactant. Atoms on the surface of AlN are bonded to atoms in the lower layer, but since there is no upper layer, the bonding is more difficult and the surface free energy is higher. If Si of group IV element is supplied there, it will combine with surplus hands and the surface free energy will be lowered. The AlN reactive species on the surface causes surface diffusion so as to avoid the group IV element Si, and consequently promotes the lateral growth of AlN. As a result, the two-dimensional expansion of the island is promoted, and as a result, the deterioration of the crystal quality is suppressed.

In Patent Document 1, an AlN thin film is formed as a buffer layer on a 6H—SiC (0001) substrate (FIG. 3A of Patent Document 1), and a GaN buffer layer is formed on the AlN thin film. Is supplied with tetraethylsilane (TESi) as an n-type impurity source (FIG. 3B of Patent Document 1), and then trimethylgallium (TMG) and ammonia (NH ₃ ) are supplied to form a GaN layer as a semiconductor layer (Patent Document 1, paragraph numbers 0049 to 0051). Patent Document 1 also discloses a method of supplying TESi and TMG and NH ₃ for forming a GaN layer at different timings and a method of performing them at the same timing (Patent Document 1, paragraph number 0106). ).

  However, Patent Document 1 is directed to a SiC substrate. Patent Document 1 describes a GaN buffer layer, and does not describe an AlN buffer layer. That is, there is no description of the mixed crystal layer of AlN and Si, and there is no mention of the numerical value or thickness of the Si concentration in the Si addition portion, or the point of continuously decreasing the Si addition concentration. Patent Document 1 describes not an AlN buffer layer but an example of a Si-doped GaN buffer layer thereon. Therefore, the present invention is different from the technique of devising the AlN buffer layer itself on the Si substrate as in the present invention, particularly the technique of inserting a mixed crystal layer of Si and AlN at the interface in the AlN buffer layer on the Si substrate.

According to the present invention, when an AlN layer is formed as a buffer layer on a Si substrate and a group III-V nitride semiconductor crystal is laminated thereon as a thin film, an SiN raw material together with an AlN raw material is formed at the initial stage of the growth of the AlN layer. The SiN element is added to a concentration of 1 × 10 ²⁰ cm ⁻³ or more by growing the substrate while flowing, thereby forming a mixed crystal layer of AlN and Si in a part of the AlN layer near the substrate. For this reason, Si plays a role as an anti-surfactant when the nuclei of AlN expand, and the lateral growth of AlN is promoted to obtain a high-quality crystal. Therefore, according to the present invention, the III-V nitride semiconductor thin film crystal such as GaN formed on the AlN buffer layer is also of a high quality.

  Therefore, according to the group III-V nitride semiconductor thin film crystal on the Si substrate according to the present invention, a blue LED, a white LED, an ultraviolet LED, or a HEMT device can be realized at a lower price than before.

  Hereinafter, the present invention will be described based on the illustrated embodiments.

  As shown in FIG. 1, a high-purity AlN layer 2 is formed as a buffer layer on a single crystal Si substrate 1, and a GaN layer (not shown) is grown thereon by the MOVPE method. A sequence for forming the AlN layer 2 on the Si substrate 1 is shown in FIG.

  As shown in FIG. 2, only the hydrogen atmosphere is used in the temperature rising process until the substrate is heated to just reach the first set temperature (homoepitaxial start temperature) T <b> 1 at which Si homoepitaxy is possible from room temperature. The set value of the homoepitaxy start temperature T1 is preferably set to a temperature of 800 ° C. or higher at which Si homoepitaxy can be reliably performed. In the case of FIG. 2, the temperature is set to 800 ° C. (point “a” in FIG. 2), and only the hydrogen atmosphere is used until immediately before the temperature is raised to 800 ° C. (point “a” in FIG. 2).

In the temperature rising process from the homoepitaxy start temperature T1 of 800 ° C. to the second set temperature (growth temperature) T2 of 1100 ° C. or higher, the Si substrate is heated while supplying the source gas containing the Si element. I do. In FIG. 2, the growth temperature T2 is set to 1100 ° C., and monosilane (SiH ₄ ) is flown in the temperature rising process (a section between points a and b in FIG. 2) until the growth temperature reaches 1100 ° C. The temperature is raised in a mixed atmosphere of silane and monosilane. Thereby, Si grows on the surface of the Si substrate. By raising the temperature while performing homoepitaxy in this way, the surface of the Si substrate is always exposed in an unreacted state.

Next, when the growth temperature T2 reaches 1100 ° C. and the temperature is stabilized (point b in FIG. 2), trimethylaluminum (TMA) and ammonia (NH ₃ ) are simultaneously used as raw materials for AlN in a hydrogen atmosphere. Then, the growth of the AlN layer 2 is started. In the early stage of the growth of the AlN layer 2, the Si element is added to a concentration of 1 × 10 ²⁰ cm ⁻³ or more by growing while continuously flowing the monosilane (SiH ₄ ) as the Si raw material together with the AlN raw material. Thus, a mixed crystal layer of AlN and Si is formed.

The thickness of the Si-added portion of the AlN layer is a layer having a thickness that is half or less (eg, 100 nm or less) of the thickness (eg, 200 nm) of the AlN layer 2. In addition, in the Si-added portion of the AlN layer, the additive concentration of Si element is continuously reduced from the substrate toward the surface. In this example, at the same time as the growth of the AlN layer 2 is started, the supply amount of monosilane (SiH ₄ ) is reduced, and the Si-added portion has a thickness of 50 nm, which is ¼ of the thickness of the AlN layer 2. And the flow rate of monosilane (SiH ₄ ) is made zero. As a result, a mixed crystal layer of Si and AlN is formed in a part of the AlN layer 2 near the substrate, that is, at the interface between the Si substrate and the AlN buffer layer thereon.

Thereafter, when the AlN layer 2 is grown to a predetermined thickness, for example, 200 nm (point c in FIG. 2), the supply of TMA is stopped, the growth of the AlN layer 2 is stopped, and the temperature of the furnace is started. Then, the temperature is lowered in an NH ₃ atmosphere, and the supply of NH ₃ is stopped when the temperature of the substrate is lowered to a predetermined temperature T3, for example, 600 ° C. (point d in FIG. 2).

  As described above, when Si element is added in the early stage of growth of the AlN layer, Si plays a role as an anti-surfactant when the nuclei of AlN expand, so that the lateral growth of AlN is promoted, and a good quality AlN film. Is obtained. Therefore, a III-V nitride semiconductor thin film crystal such as GaN formed on the AlN buffer layer is also good quality.

  In order to confirm the effect of the present invention, as a prototype example (sample), as shown in FIG. 1, a high-purity AlN layer 2 is formed on a single-crystal Si substrate 1, and the characteristics of the AlN layer 2 change. , And the characteristics of the GaN layer grown thereon.

  The Si substrate used on-axis specifications in the <111> direction. Also, RCA cleaning of the surface was performed before thin film growth to remove organic and inorganic impurities and oxide films.

  FIG. 2 shows a sequence (Example) when the sample of FIG. 1 is grown by applying the present invention, and FIG. 3 shows a sequence (Comparative Example) when the sample of FIG. 1 is grown by the conventional method. The former is a growth sequence of a thin film (product applied to the present invention) grown while flowing an Si raw material together with an AlN raw material in the early stage of growth of the AlN layer, and the latter is a thin film grown without flowing an Si raw material together with the AlN raw material (conventional). (Growth product) growth sequence.

As shown in FIG. 1, the measured sample has a simple structure in which an AlN layer is grown to 200 nm on a Si substrate. For both the conventionally grown product and the product to which the present invention was applied, an AlN layer was grown by simultaneously flowing trimethylaluminum (TMA) and ammonia NH ₃ in a hydrogen atmosphere. At this time, the growth temperature was 1100 ° C., the furnace pressure was 135 Torr, and the flow rates of TMA and NH ₃ were 2.00 × 10 ⁻⁵ mol / min and 4.46 × 10 ⁻³ mol / min, respectively.

As described above, when the sample in which the monosilane was flown at the initial stage of the growth of the AlN layer is the product of the present invention and the sample not to be flowed is the conventional product, the flow rate of monosilane flowed by the product of the present invention is 5.00 × 10 ^-5 mol / min, and then continuously reduced to zero when 50 nm is grown.

  FIG. 4 shows a comparison between conventional growth and AFM surface observation of AlN applied to the present invention, and FIG. 5 shows a comparison of rocking curves of (002) plane diffraction of X-rays.

  According to the results of AFM, the surface of the conventionally grown product has a shape in which spherical particles are gathered, whereas the product to which the present invention is applied has a nanoscale hole, but it is a single film. Recognize. Further, as can be seen from FIG. 5, the product to which the present invention is applied has a small half-width of the diffraction peak. From this, it was confirmed that the orientation of the thin film was overwhelmingly improved by applying the present invention.

  In the present invention, the composition, thickness, and compositional modulation in the thickness direction of the mixed crystal layer of AlN and Si vary depending on the target thin film crystal stack structure or the final device specifications, so it is optimal for the application. It must be converted. Further, since the Si raw material concentration, growth temperature, pressure, V / III ratio, atmosphere, film forming speed, and growth pressure for growing this vary depending on the apparatus used, trial and error must be carried out.

  In the above embodiment, monosilane is used as the Si raw material, but instead, disilane having a low decomposition temperature, tetramethyl silicon or tetraethyl silicon as organic raw materials can be used, and similar effects can be obtained.

It is a figure which shows a sample structure when the change of the AlN layer on the Si substrate used as the foundation | substrate of the III-V group nitride type compound semiconductor crystal of this invention is investigated. It is a figure which shows a sequence when the prototype example (sample) of FIG. 1 is grown by applying this invention. It is a figure which shows a sequence when the trial manufacture example (sample) of FIG. 1 is grown by the conventional method. The AFM observation result on the surface of the AlN layer of the prototype is shown, (a) is a conventionally grown product, and (b) is a drawing substitute photograph of the product to which the present invention is applied. It is the figure which showed the X-ray-diffraction peak of the AlN layer (002) surface of this invention application product compared with the conventionally grown product.

Explanation of symbols

1 Si substrate 2 AlN layer

Claims (9)

In a nitride semiconductor crystal in which an AlN layer is formed as a buffer layer on a Si substrate and a group III-V nitride semiconductor crystal is laminated thereon as a thin film, 1 element of Si is added to a part of the AlN layer near the substrate. A nitride semiconductor crystal comprising a mixed crystal layer of AlN and Si added so as to have a concentration of × 10 ²⁰ cm ⁻³ or more.   2. The nitride semiconductor crystal according to claim 1, wherein the thickness of the Si-added portion of the AlN layer is not more than half that of the AlN layer. 3.   3. The nitride semiconductor crystal according to claim 1, wherein the Si element addition concentration in the Si addition portion of the AlN layer is continuously reduced from the substrate toward the surface. 4. In a method for manufacturing a nitride semiconductor crystal in which an AlN layer is formed as a buffer layer on a Si substrate and a group III-V nitride semiconductor crystal is stacked as a thin film on the SiN substrate, in the initial growth stage of the AlN layer, together with the AlN raw material A nitride semiconductor crystal characterized in that by growing while flowing Si raw material, Si element is added to a concentration of 1 × 10 ²⁰ cm ⁻³ or more to form a mixed crystal layer of AlN and Si. Production method.   5. The method of manufacturing a nitride semiconductor crystal according to claim 4, wherein the thickness of the Si-added portion of the AlN layer is less than half that of the AlN layer.   6. The method of manufacturing a nitride semiconductor crystal according to claim 5, wherein the Si element addition concentration in the Si addition portion of the AlN layer is continuously reduced from the substrate toward the surface. Method.   7. The method for producing a nitride semiconductor crystal according to claim 5, wherein the AlN layer as the buffer layer is grown by MOVPE, and any of monosilane, disilane, dichlorosilane, trimethylsilicon, and triethylsilicon is added as a Si raw material to be added. A method for producing a nitride semiconductor crystal characterized by using the above.   8. The method for producing a nitride semiconductor crystal according to claim 4, wherein a temperature during growth of the AlN layer as the buffer layer is set to 1000 ° C. or higher. .   9. The method for manufacturing a nitride semiconductor crystal according to claim 8, wherein a growth pressure during the growth of the AlN layer as the buffer layer is 267 hPa or less.