Patent Application
20240376635
The present invention relates to an AlN single-crystal substrate and a device comprising the AlN single-crystal substrate.
Aluminum nitride (AlN) single crystals have recently attracted attention as base substrates for deep ultraviolet light emitting elements using AlN-based semiconductors. For example, AlN, AlGaN, and the like are used as AlN-based semiconductors. These AlN-based semiconductors have a direct band gap structure, which makes them suitable for light-emitting devices and applicable to light emitting diodes (LEDs) and laser diodes (LDs) in the deep ultraviolet region.
For example, Patent Literature 1 (WO2015/108089A1) discloses an ultraviolet light-emitting diode having a stacked structure in which a substrate with a light-emitting main surface from which light is emitted, an n-type layer, an active layer, and a p-type layer are stacked in this order. This literature discloses that when fabricating an ultraviolet light source such as an ultraviolet light-emitting diode, an AlN single crystal used as the substrate with a light-emitting main surface is required to have high ultraviolet transmittance. Patent Literature 2 (JP2009-190965A) discloses a method for growing an AlN crystal comprising the steps of providing a stacked substrate having a layer structure of a base substrate/a first layer/a second layer; and growing an AlN crystal on a main surface of the base substrate using a vapor phase growth method, wherein the first layer is made of a material that is less likely to sublimate than the base substrate at a growth temperature of the AlN crystal, and wherein the second layer is made of a material with a thermal conductivity higher than a thermal conductivity of the first layer. This literature discloses that AlN crystals have attracted attention as substrate materials for semiconductor devices, such as optical and electronic devices, because of their high thermal conductivity and high electrical resistance.
As described above, AlN single crystals have attracted attention for various applications. However, AlN single-crystal substrates as disclosed in Patent Literatures 1 and 2 have the drawback of being likely to crack when processed (such as ground, polished, or cut), resulting in a reduced yield. Therefore, it is desirable to reduce cracking of AlN single-crystal substrates during processing of the AlN single-crystal substrates.
The prevent inventors have now found that when an AlN single-crystal substrate satisfies a specific relation concerning (ultraviolet and visible) transmittances, thermal conductivities, and electrical resistivity, the AlN single-crystal substrate is less likely to crack when processed (such as ground, polished, or cut).
Therefore, it is an object of the present invention to provide an AlN single-crystal substrate that is less likely to crack when processed (such as ground, polished, or cut).
According to an aspect of the present invention, there is provided an AlN single-crystal substrate, which satisfies a relation:
5≤[(λ25−λ200)×log10 ρ]/(T640-660−T260-280)≤50
wherein λ25 is a thermal conductivity (W/m·K) at 25° C. of the AlN single-crystal substrate; λ200 is a thermal conductivity (W/m·K) at 200° C. of the AlN single-crystal substrate; ρ is an electrical resistivity (Ω·cm) at 25° C. of the AlN single-crystal substrate; T640-660 is an average value of transmittance (%) at 640 to 660 nm in a transmission spectrum of the AlN single-crystal substrate; and T260-280 is an average value of transmittance (%) at 260 to 280 nm in the transmission spectrum.
According to another aspect of the present invention, there is provided a device comprising the AlN single-crystal substrate.
An AlN single-crystal substrate according to the present invention satisfies a relation concerning transmittances, thermal conductivities, and electrical resistivity: 5≤[(λ25−λ200)×log10 ρ]/(T640-660−T260-280)≤50. In the inequation, λ25 is a thermal conductivity (W/m·K) at 25° C. of the AlN single-crystal substrate; and λ200 is a thermal conductivity (W/m·K) at 200° C. of the AlN single-crystal substrate. ρ is an electrical resistivity (Ω·cm) at 25° C. of the AlN single-crystal substrate. T640-660 is an average value of transmittance (%) at 640 to 660 nm in a transmission spectrum of the AlN single-crystal substrate; and T260-280 is an average value of transmittance (%) at 260 to 280 nm in the transmission spectrum. When the AlN single-crystal substrate thus satisfies the predetermined relation concerning (ultraviolet and visible) transmittances, thermal conductivities, and electrical resistivity, the AlN single-crystal substrate is less likely to crack when processed (such as ground, polished, or cut). Therefore, by subjecting such an AlN single-crystal substrate to processing, the AlN single-crystal substrate can be manufactured at a high yield. That is, as described above, the conventional AlN single-crystal substrates have the drawback of being likely to crack when processed (such as ground, polished, or cut), resulting in a reduced yield. In this respect, the AlN single-crystal substrate of the present invention can advantageously overcome the aforementioned drawback.
As used herein, “average value of transmittance” in a specific wavelength range in the transmission spectrum is determined by dividing a sum of transmittances (%) at the wavelengths (nm) measured in the specific wavelength range (for example, from 260 to 280 nm or from 640 to 660 nm), by the number of measurement points. For example, when a sum of transmittances measured in 1 nm increments in the region from 640 to 660 nm is 1050, the average value of transmittance can be obtained by dividing the sum of transmittances by the number of measurement points, i.e., 21 (for example, 1050/21=50%). The transmittance in this case is preferably the transmittance T100 μm with the thickness of the AlN single-crystal substrate converted to 100 μm. This is because if there is a variation in the thickness of the AlN single-crystal substrate to be measured, the transmittance also changes accordingly. For example, the transmittance decreases with increasing thickness of the AlN single-crystal substrate and increases with decreasing thickness of the AlN single-crystal substrate.
The transmittance in the transmission spectrum can be determined by, for example, the below-described calculation method. The total light transmittance Ta of the AlN single-crystal substrate is measured using a spectrophotometer. The absorption coefficient α of the AlN single-crystal substrate is determined using the measured value of Ta and theoretical transmittance Tt of the AlN single-crystal substrate. Then, the transmittance T100 μm with the thickness of the AlN single-crystal substrate converted to 100 μm is calculated. Here, α and T100 μm can be determined according to the following equations:
wherein t represents an actual thickness (cm) of an AlN single-crystal sample. For an AlN single-crystal sample that has low transmittance and for which the absorption coefficient α is difficult to calculate, the actual thickness may be reduced, and the total light transmittance Ta may be measured. In this manner, a transmission spectrum based on the transmittance T100 μm with the thickness converted to 100 μm is obtained.
In the transmission spectrum of the AlN single-crystal substrate, a difference between the average value of transmittance at 640 to 660 nm (T640-660) and the average value of transmittance at 260 to 280 nm (T260-280) (T640-660−T260-280) is preferably 10 to 80 percent points (% pt), more preferably 20 to 75% pt, and still more preferably 30 to 70% pt.
In the AlN single-crystal substrate, a difference between the thermal conductivity (λ25) at 25° C. and the thermal conductivity (λ200) at 200° C. (λ25−λ200) is preferably 60 to 90 W/m·K, more preferably 65 to 85 W/m·K, and still more preferably 70 to 80 W/m·K.
The AlN single-crystal substrate preferably has an electrical resistivity ρ at 25° C. of 1×103 to 1×1017 Ω·cm, more preferably 5×103 to 1×1011 Ω·cm, and still more preferably 1×104 to 1×106 Ω·cm.
As described above, the AlN single-crystal substrate satisfies the relation: 5≤[(λ25−λ200)×log10 ρ]/(T640-660−T260-280)≤50; preferably 5≤[(λ25−λ200)×log10 ρ]/(T640-660−T260-280)≤35; and more preferably 5≤[(λ25−λ200)×log10 ρ]/(T640-660−T260-280)≤50. When the AlN single-crystal substrate satisfies such relations, it is less likely to crack when processed (such as ground, polished, or cut). Moreover, by subjecting such an AlN single-crystal substrate to processing, the AlN single-crystal substrate can be manufactured at a higher yield.
The AlN single-crystal substrate according to the present invention is preferably an oriented layer that is oriented in both the c- and a-axis directions, and may contain a mosaic crystal. The mosaic crystal refers to a crystal that does not have distinct grain boundaries but is an aggregation of crystals whose crystal orientation slightly deviates from one or both of the c- and a-axes. Such an oriented layer has a structure in which the crystal orientation is substantially aligned with a substantially normal direction (c-axis direction) and an in-plane direction (a-axis direction). Such a structure allows a semiconductor layer with an excellent quality, particularly an excellent orientation, to be formed on the oriented layer. That is, when forming a semiconductor layer on the oriented layer, the crystal orientation of the semiconductor layer substantially matches the crystal orientation of the oriented layer. Therefore, a semiconductor film formed on the AlN single-crystal substrate tends to be an oriented film.
Methods of evaluating the orientation in the AlN single-crystal substrate according to the present invention include, but are not specifically limited to, known analytical techniques such as the EBSD (Electron Back Scatter Diffraction Patterns) method and X-ray pole figures. For example, when using the EBSD method, an inverse pole figure map and a crystal orientation map of a surface (plate surface) or a cross section orthogonal to the plate surface of the AlN single-crystal substrate are measured. The AlN single-crystal substrate can be defined as being oriented along the two axes in the substantially normal direction and a substantially plate-surface direction, when the following conditions are satisfied: in the obtained inverse pole figure map, (A) the crystals are oriented in a specific orientation (first axis) in the substantially normal direction with respect to the plate surface, and (B) the crystals are oriented in a specific orientation (second axis) in the substantially in-plane plate-surface direction, orthogonal to the first axis; and in the obtained crystal orientation map, (C) the inclination angle from the first axis is distributed within ±10°, and (D) the inclination angle from the second axis is distributed within ±10°. In other words, when the above-described four conditions are satisfied, the AlN single-crystal substrate can be determined as being oriented along the two axes, i.e., the c- and a-axes. For example, when the substantially normal direction with respect to the plate surface is oriented along the c-axis, the substantially in-plane plate-surface direction may be oriented in a specific orientation (for example, the a-axis) orthogonal to the c-axis. While the AlN single-crystal substrate may be oriented along the two axes in the substantially normal direction and the substantially in-plane plate-surface direction, it is preferred that the substantially normal direction is oriented along the c-axis. The smaller the inclination angle distribution in the substantially normal direction and/or the substantially in-plane plate-surface direction, the smaller the mosaicity of the AlN single-crystal substrate; and the closer the inclination angle distribution is to zero, the closer the AlN single-crystal substrate is to a perfect single crystal. Therefore, from the viewpoint of crystallinity of the AlN single-crystal substrate, the inclination angle distribution is preferably smaller in both the substantially normal direction and the substantially plate-surface direction, and is preferably within ±5° or less, and more preferably within ±3° or less, for example.
One surface of the AlN single-crystal substrate preferably has an area of 20 cm2 or more, more preferably 70 cm2 or more, and still more preferably 170 cm2 or more. By thus increasing the area of the AlN single-crystal substrate, it is possible to increase the area of the semiconductor layer to be formed thereon. Therefore, it is possible to obtain a larger number of semiconductor elements from a single semiconductor layer, and a reduction in manufacturing costs can be expected. While the upper limit of size is not specifically limited, the area of the one surface is typically 710 cm2 or less.
The AlN single-crystal substrate of the present invention can be manufactured by various methods as long as the above-described relation concerning transmittances, thermal conductivities, and electrical resistivity is satisfied. A seed substrate may be provided and then an epitaxial film may be formed thereon, or the AlN single-crystal substrate may be directly manufactured by spontaneous nucleation without using a seed substrate. The seed substrate to be used may be an AlN substrate to achieve homoepitaxial growth, or may be a substrate other than the AlN substrate to achieve heteroepitaxial growth. While any of a vapor phase deposition method, a liquid phase deposition method, and a solid phase deposition method may be used to grow a single crystal, the vapor phase deposition method is preferably used to form an AlN single crystal, and then the seed substrate portion is ground away, as required, to obtain a desired AlN single-crystal substrate. Examples of the vapor phase deposition method include various CVD (chemical vapor deposition) methods (such as thermal CVD, plasma CVD, and MOVPE), a sputtering method, hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), a sublimation method, and pulsed laser deposition (PLD), with the sublimation method or HVPE being preferred. Examples of the liquid phase deposition method include solution growth methods (such as a flux method). Alternatively, the AlN single-crystal substrate can be obtained without directly forming an AlN single crystal on a seed substrate, by the steps of forming an unoriented precursor layer, forming the unoriented precursor layer into an AlN single-crystal layer by heat treatment, and grinding the seed substrate away. Examples of methods of forming the unoriented precursor layer in this case include an aerosol deposition (AD) method and a hypersonic plasma particle deposition (HPPD) method.
While any of the solid phase deposition method, vapor phase deposition method, and liquid phase deposition method described above may employ known conditions, a technique of fabricating the AlN single-crystal substrate using the sublimation method, for example, is hereinafter described. Specifically, the AlN single-crystal substrate is fabricated by (a) heat-treating polycrystalline AlN powder, (b) forming an AlN single-crystal layer, and (c) grinding a seed substrate away and polishing the surface of the AlN single-crystal layer.
This step is the step of heat-treating polycrystalline AlN powder to obtain AlN raw material powder. As shown in
This step is the step of forming an AlN single crystal on a seed substrate in a crystal growth apparatus.
This step includes the grinding step of grinding the seed substrate away to expose the AlN single-crystal layer; and the polishing step of removing irregularities and defects on the surface of the AlN single crystal. The SiC single crystal remains on the AlN single-crystal layer fabricated by the steps (a) and (b) above using the SiC substrate as the seed substrate, and thus, the seed substrate is subjected to grinding to expose the surface of the AlN single-crystal layer. Additionally, to mirror-finish the surface of the formed AlN single-crystal layer, the plate surface is smoothed by lapping with diamond abrasive grains and then polished by chemical mechanical polishing (CMP) with colloidal silica or the like. In this manner, the AlN single-crystal substrate can be fabricated.
A device can also be fabricated using the AlN single-crystal substrate of the present invention. That is, preferably, a device comprising the AlN single-crystal substrate is provided. Examples of such devices include deep ultraviolet laser diodes, deep ultraviolet diodes, power electronic devices, high-frequency devices, and heat sinks. Methods for manufacturing the device using the AlN single-crystal substrate are not specifically limited, and known techniques may be employed to manufacture the device.
The present invention is described in more detail with the following examples.
As shown in
As shown in
The SiC substrate having AlN re-deposited thereon, obtained in (1b) above, was ground using a grinding wheel of a size up to #2000, until the AlN single crystal was exposed, and the plate surface was further smoothed by lapping with diamond abrasive grains. Then, the plate surface was subjected to chemical mechanical polishing (CMP) with colloidal silica to be mirror-finished. In this manner, an AlN single-crystal substrate was fabricated.
Top and bottom surfaces of the AlN single-crystal substrate were subjected to EBSD measurement. The results showed that the AlN crystals were oriented in both the c- and a-axis directions.
The total light transmittance Ta in the wavelength range of 200 to 800 nm was measured for the AlN single-crystal substrate, using a spectrophotometer (UH4150 manufactured by Hitachi High-Tech Corporation). The absorption coefficient α of the AlN single-crystal substrate was determined using the measured value of Ta and theoretical transmittance Tt of the AlN single-crystal substrate, and then the transmittance T100 μm with the thickness of the AlN single-crystal substrate converted to 100 μm was calculated. α and T100 μm were determined according to the following equations:
wherein t represents an actual thickness (cm) of an AlN single-crystal sample. In this manner, a transmission spectrum based on the transmittance T100 μm with the thickness converted to 100 μm was obtained. Based on the obtained transmission spectrum, the difference between T640-660 and T260-280 (T640-660−T260-280) (% pt) was calculated, wherein T640-660 is an average value of transmittance (%) at 640 to 660 nm, and T260-280 is an average value of transmittance (%) at 260 to 280 nm. The results are shown in Table 1.
The thermal conductivity λ25 at 25° C. and thermal conductivity λ200 at 200° C. of the AlN single-crystal substrate were determined according to the equation: (thermal conductivity)=(thermal diffusivity)×(specific heat)×(density). Here, the thermal diffusivity was measured at 25 and 200° C. for AlN single-crystal samples processed into a disc shape with a diameter of 10 mm and a thickness of 0.4 mm, using a flash analyzer thermal diffusivity measuring apparatus (LFA467HT manufactured by NETSCH). The specific heat was measured at 25 and 200° C. for AlN single-crystal samples processed into a disc shape with a diameter of 5 mm and a thickness of 0.4 mm, using a differential scanning calorimeter (DSC404 manufactured by NETSCH). The density was measured by the Archimedes' method in accordance with JIS R 1634:1998. The difference between the thermal conductivity λ25 at 25° C. and the thermal conductivity λ200 at 200° C. (λ25−λ200) (W/m·K) was calculated. The results are shown in Table 1.
Ohmic electrodes were formed on top and bottom surfaces of the AlN single-crystal substrate, and the electrical resistivity at 25° C. was measured by the two-terminal method. The results are shown in Table 1.
Based on the numerical values obtained in (2b) to (2d) above, a value obtained from the relation: [(λ25−λ200)×log10 ρ]/(T640-660−T260-280) was calculated. The results are shown in Table 1.
The surface of the AlN single-crystal substrate after grinding and polishing in (1c) above was observed with an optical microscope to examine for cracks with a maximum length of 50 μm or more. A total of 10 AlN single-crystal substrates were fabricated using the same method as described in (1) above, and it was examined how many of these AlN single-crystal substrates cracked, and then the AlN single-crystal substrates were rated according to the evaluation criteria shown below. The results are shown in Table 1.
This application is a continuation application of PCT/JP2022/013982 filed Mar. 24, 2022, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/013982 | Mar 2022 | WO |
| Child | 18779382 | US |
