Patent Application
20170278754
The technical field relates to a method for producing a Group III nitride crystal, and an RAMO4 substrate.
An ScAlMgO4 substrate has been known as a substrate represented by the general formula RAMO4 (wherein R represents one or a plurality of trivalent element selected from the group consisting of Sc, In, Y, and a lanthanoid element, A represents one or a plurality of trivalent element selected from the group consisting of Fe(III), Ga, and Al, and M represents one or plurality of divalent elements selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd). The ScAlMgO4 substrate is used as a growth substrate for a nitride semiconductor, such as GaN (see, for example, Patent Literature 1).
In Patent Literature 1 as described above, in the production of a GaN substrate, the RAMO4 substrate is removed by etching or polishing in the step S204. In the case where a part of the RAMO4 substrate is removed by cleaving, the RAMO4 substrate thus removed is once dissolved and then reused by forming again into a single crystal. However, in consideration of the cost, the production efficiency and the like of the production of a GaN substrate in recent years, there is a demand of easy reuse of the removed RAMO4 substrate.
An object herein is to provide a method for enhancing the use efficiency of a RAMO4 substrate in the production of a Group III nitride.
For achieving the aforementioned and other objects, there is provided, as one aspect, a method for producing a Group III nitride crystal, containing: preparing an RAMO4 substrate containing a single crystal represented by the general formula RAMO4 (wherein R represents one or a plurality of trivalent elements selected from the group consisting of Sc, In, Y, and a lanthanoid element, A represents one or a plurality of trivalent elements selected from the group consisting of Fe (III) , Ga, and Al, and M represents one or a plurality of divalent elements selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd) having a notch on a side portion thereof; growing a Group III nitride crystal on the RAMO4 substrate; and cleaving the RAMO4 substrate from the notch as an origin.
There is also provided, as another aspect, a RAMO4 substrate containing a single crystal represented by the general formula RAMO4 (wherein R represents one or a plurality of trivalent elements selected from the group consisting of Sc, In, Y, and a lanthanoid element, A represents one or a plurality of trivalent elements selected from the group consisting of Fe(III), Ga, and Al, and M represents one or a plurality of divalent elements selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd), the RAMO4 substrate having a notch on a side portion thereof.
According to the aforementioned and other aspects, the method produces a Group III nitride and a RAMO4 substrate that is capable of easy reuse.
An embodiment will be described with reference to
The upper figure of
A method for producing a Group III nitride crystal including a method for producing the ScAlMgO4 substrate 10 will be described in more detail. The method will be described with reference to GaN (gallium nitride) for the Group III nitride.
As shown in
In the ScAlMgO4 ingot preparing step, a single crystal ScAlMgO4 ingot produced, for example, with a high-frequency induction heating type Czochralski furnace is prepared. As an example of the production method of the ingot, a method for producing an ingot having a diameter of 50 mm will be described. As a starting material, Sc2O3, Al2O3, and MgO each having a purity of 4N (99.99%) are mixed in the prescribed molar ratio. 3,400 g of the starting material is placed in a crucible formed of iridium having a diameter of 100 mm. The crucible having the starting material placed therein is then placed in a high-frequency induction heating type Czochralski furnace (growing furnace), and the interior of the furnace is vacuumed. Thereafter, nitrogen is introduced into the furnace, and at the time when the interior of the furnace is at atmospheric pressure, the crucible is heated. The starting material is melted by gradually heating the material over 12 hours to the melting point of ScAlMgO4. An ScAlMgO4 single crystal having been cut into the (0001) azimuth is used as a seed crystal, and the seed crystal is descended to the vicinity of the molten liquid in the crucible. Thereafter, while rotating the seed crystal, the seed crystal is gradually descended to make the tip end of the seed crystal contact the molten liquid. Then, the seed crystal is raised at a raising speed of 0.5 mm/h (i.e., withdrawing in the (0001) azimuthal direction) while gradually decreasing the temperature to grow a crystal. According to the procedure, a single crystal ingot having a diameter of 50 mm and a length of the straight body portion of 50 mm is obtained.
In the ScAlMgO4 ingot outline machining step, the ingot thus withdrawn is machined into a cylinder shape, and a notch is formed on the side portion of the cylinder. The both ends (i.e., the top and the tail) of the ingot are cut out with a band saw, a slicer having an inner blade or an outer blade, a single wire saw, or the like. The ScAlMgO4 ingot withdrawn does not have a regular circular shape, and thus formed into a cylinder by outline grinding with a diamond wheel or the like or by polishing with a polishing cloth.
The ScAlMgO4 single crystal will be described. The ScAlMgO4 single crystal has a structure containing an ScO2 layer like the (111) plane of the rock salt structure and an AlMgO2 layer like the (0001) plane of the hexagonal structure, which are laminated alternately. The two layers like the (0001) plane of the hexagonal structure are of a planar structure as compared to the wurtzite structure, and the bond between the upper and lower layers is longer than the bond in the plane by approximately 0.03 nm, and has a weak bond strength. Accordingly, the ScAlMgO4 single crystal can be cleaved at the (0001) plane. By utilizing the characteristics, in the ScAlMgO4 substrate preparing step, the ScAlMgO4 ingot 1 having a cylinder shape is formed into a substrate having a prescribed thickness. At this time, an origin for cleavage is necessary, and notches 2a and 2b are formed on the side portion in the ScAlMgO4 ingot outline machining step. The method for forming the origin will be described with reference to
As shown in
The method for forming the notch will be described.
In the case where the ScAlMgO4 ingot 1 is actually cleaved, the blade 3 having the shape shown in
It may be considered to provide plural notches 2b on the same plane, but when the ingot is cleaved from plural positions without sufficient positioning, steps may be formed on the cleaved surface. Accordingly, in the case where plural notches are provided on the same plane, it is necessary to perform atomic level positioning, which is practically difficult, and therefore it suffices that only one notch may be provided on the same plane.
The ScAlMgO4 substrate preparing step will be described. In this step, for providing an ScAlMgO4 substrate 10 having a notch, the blade 3 shown in
It is not easy to remove the irregularities of 500 nm or more formed through cleavage. In particular, the processing of the cleaved surface of the ScAlMgO4 substrate encounters the following difficulty. In the case where the irregularities formed through cleavage are tried to be removed, when the proportion of the flat portion in the entire surface is large, the processing load tends to be concentrated to a partial area (irregularity) on processing the flat portion, and cracks occur due to cleavage in the deeper interior from the surface, but not on the surface. Accordingly, it is considered that irregularities are newly formed due to removal of the cracked portions. When the proportion of the flat portion is large, the application of a load that does not cause cleavage in the interior substantially cannot remove the irregularities formed in the cleaving step.
In view of the properties of the ScAlMgO4 material, the processing method described in detail below (i.e., the coarse irregularity forming step and the minute irregularity forming step) has been found, and designated as the irregularity removing step of the embodiment. Specifically, an irregularity shape having a uniform height is formed over the entire surface of the region to be an epitaxial growth surface of the ScAlMgO4 substrate (i.e., the coarse irregularity forming step). Subsequently, the irregularity shape having a uniform height having been formed on the entire surface is gradually reduced, while the pressing force is reduced stepwise to decrease the absolute value of the fluctuation of the pressing force, thereby preventing the cleavage in the interior (i.e., the minute irregularity forming step). More specifically, the ScAlMgO4 single crystal is cleaved at the cleavage surface from the notch 2a as an origin to prepare the ScAlMgO4 substrate 10 shown in the upper figure of
In the coarse irregularity forming step, the irregularity shape is distributed over the entire surface of the region to be an epitaxial growth surface in such a manner that the areas of the regions each having continuously a height of the irregularities of 500 nm or less (which may be hereinafter referred to as a “flat portion”) are 1 mm2 or less. This is because when the flat portion having an area exceeding 1 mm2 is formed in the coarse irregularity forming step, the cleavage in the interior occurs in the minute irregularity forming step due to the processing load concentration, forming irregularities having a height exceeding 500 nm. The difference among the heights of the plural protruded parts of the irregularities formed in the coarse irregularity forming step is preferably within a range of ±0.5 μm. When the irregularities having a uniform height, which have the fluctuation in height within the range, are formed over the entire surface, the height of the irregularities can be gradually reduced in the minute irregularity forming step, and thereby a uniform flat portion can be formed on the surface.
Specifically, in the coarse irregularity forming step, irregularities having a height of 500 nm or more are formed with first abrasive grains, and in the minute irregularity forming step, irregularities having a height of less than 500 nm are formed with second abrasive grains having a hardness that is smaller than that of the first abrasive grains.
More specifically, in the irregularity forming step of processing an irregularity shape having a uniform height, a grinding process is performed with a diamond fixed whetstone having a large abrasive grain size. As for the abrasive grain size, diamond abrasive grains of #300 or more and #20000 or less (preferably #600) may be used. By the process using the diamond abrasive grains having a size within the range, the difference in height of the irregularities on the processed surface can be made within a range of ±5 μm. The processing conditions in the coarse irregularity forming step may be a rotation number of the whetstone of 500 min−1 or more and 50,000 min−1 or less (preferably 1,800 min −1) , a rotation number of the ScAlMgO4 substrate of 10 min−1 or more and 300 min−1 or less (preferably 100 min−1), a processing speed of 0.01 μm/sec or more and 1 μm/sec or less (preferably 0.3 μm/sec), and a processing elimination amount of 1 μm or more and 300 μm or less (preferably 20 μm).
The minute irregularity forming step of gradually removing the irregularities formed in the coarse irregularity forming step will be then described. In the minute irregularity forming step, while the irregularities having a height of 500 nm are removed, irregularities having a height of less than 500 nm are formed by polishing with a pressing force that is reduced stepwise. In the minute irregularity forming step, a slurry containing colloidal silica as a major component is preferably used as abrasive grains, and polishing is preferably performed with a nonwoven cloth pad as a polishing pad at a rotation number of 10 min−1 or more and 1,000 min−1 or less (preferably 60 min−1) and a slurry supplying amount of 0.02 mL/min or more and 2 mL/min or less (preferably 0.5 mL/min). The slurry supplying amount may be changed depending on the area of the substrate. Specifically, the slurry supplying amount is preferably increased when the area of the substrate is larger. In the case where a large amount of irregularities are formed, the processing force tends to be concentrated to the protruded parts thereof. Accordingly, the pressing force is preferably in a range of 10,000 Pa or more and 20,000 Pa or less in the initial stage of the minute irregularity forming step, then in a range of 5,000 Pa or more and less than 10,000 Pa when the protruded parts are being flattened, and finally in a range of 1,000 Pa or more and 5,000 Pa or less. By decreasing the pressing force stepwise in this manner, the irregularities having a height of 500 nm or more can be removed from the region to be an epitaxial growth surface while preventing the cleavage in the interior from occurring.
The GaN crystal growing step will be then described. Examples of the method for growing a GaN single crystal include a vapor phase epitaxial growth method, in which Group V and Group III raw material gases are reacted with each other to synthesize the crystal, and a liquid phase epitaxial growth method using a solution or a molten liquid. Examples of the vapor phase epitaxial growth method used include an HVPE (hydride vapor phase epitaxial) method and an OVPE (oxide vapor phase epitaxial) method. Examples of the liquid phase epitaxial growth method include an Na flux (sodium flux) method.
The HVPE method uses GaCl as the Group III source. Specifically, metallic Ga and HCl gas are reacted to form GaCl gas in a raw material gas forming part of a quartz tube furnace. By increasing the reaction efficiency herein, substantially 100% of HCl is reacted to form GaCl gas. The GaCl gas is transported to a growth part having a seed substrate (ScAlMgO4 substrate 10) disposed therein. The GaCl gas is then reacted with NH3 gas at a temperature of approximately from 1,000 to 1,100° C. to form GaN. A characteristic feature of the HVPE method is that GaN can be grown at a high speed, and a GaN independent substrate of several hundred nanometers to several millimeters can be formed. In this embodiment, as shown in
As another method for forming the GaN layer 4, the OVPE method will be described. In the OVPE method, Ga2O is used as a Ga source. In this method, Ga2O gas is formed, and the Ga2O gas and NH3 gas are reacted with each other in the growing part to grow a GaN crystal. In this method, the by-products are only hydrogen and water vapor, which do not clog the exhaust system, and thus continuous growth for a prolonged period of time can be performed in principle.
In the case where the equipment for the HVPE method or the OVPE method is such an equipment that the gas is made to flow in the lateral direction, the ScAlMgO4 substrate 10 is preferably disposed to make the notch 2b directed to the downstream side of the gas flow.
As still another method for forming the GaN layer 4, the Na flux method may be used. In the Na flux method, nitrogen is dissolved in a Ga-Na molten liquid at a high temperature to create a supersaturated state, in which GaN is grown. Specifically, a seed substrate (ScAlMgO4 substrate 10), Ga, and Na are placed in a crucible, which is then placed in a stainless steel tube, and heated to a temperature of from 800 to 900° C. with a heater under application of a nitrogen pressure of from 3 to 4 MPa. Nitrogen is dissolved in the heated Ga-Na molten liquid, and thereby a GaN crystal is grown on the ScAlMgO4 substrate 10. A characteristic feature of the Na flux method is that although the growing speed is lower than the HVPE method, the dislocation density is small, and a high quality GaN crystal with less defects can be obtained.
The ScAlMgO4 removing step will be then described. In the case where a sapphire substrate is used as a seed substrate in an ordinary method having been practiced, the sapphire substrate and the GaN substrate are separated from each other in the temperature decreasing step of the GaN crystal growing process, due to the difference in expansion and contraction caused by the difference in thermal expansion of the sapphire substrate and the GaN layer. However, the thermal expansion coefficients of ScAlMgO4 and GaN are close to each other, and in this embodiment, the ScAlMgO4 substrate 10 and the GaN layer 4 are not separated from each other even after completion of the GaN crystal growth process. Accordingly, a separation process is necessarily performed.
The GaN layer 4 formed in the GaN crystal growing step has a distorted outer shape, and thus is processed to make the outer shape circular. For example, grinding with a rotating whetstone is performed. Subsequently, for removing the part of the ScAlMgO4 substrate (corresponding to the part 10a in
The GaN substrate machining step will be then described. In this step, a Ga surface 20, which is the epitaxial growth surface of the GaN layer 4, and an N surface 21 having ScAlMgO4 present thereon are ground and polished to finalize a GaN independent substrate 4a capable of growing epitaxially. The grinding may be performed by grinding with fixed abrasive grains or lapping with free abrasive grains. The polishing of the Ga surface 20 may be a method including lapping with small free abrasive grains and then removing a process affected layer, which is damaged crystals, by CMP (chemical mechanical polishing) . For the N surface 21, on the other hand, the ScAlMgO4 substrate 10b is removed by grinding, and the N surface 21 is finished by polishing. According to the procedures, the GaN independent substrate 4a capable of growing epitaxially is formed as shown in
In this embodiment, the ScAlMgO4 substrate 10a separated in the ScAlMgO4 removing step is subjected again to the irregularity removing step, and thereby the ScAlMgO4 substrate 10a can be used as a seed substrate. Accordingly, a GaN independent substrate can be formed by using again the regenerated ScAlMgO4 substrate 10a. Therefore, ScAlMgO4 can be efficiently used, and the material yield can be enhanced. The number of the notches 2b of the ScAlMgO4 substrate 10 may not be necessarily one, and plural notches may be formed in the thickness direction of the substrate in the preparing step of an ScAlMgO4 substrate having a notch, depending on the number of times of regeneration of the seed substrate.
The difference from the embodiment 1 is that a Group III nitride crystal is epitaxially grown on the ScAlMgO4 substrate, and furthermore the device is formed. The process steps until the irregularity removing step are the same as in the embodiment 1, and the Group III nitride crystal growing step and the latter will be described. However, in the irregularity removing step, the epitaxial growth surface may be formed only on one surface (front surface) of the ScAlMgO4 substrate in the embodiment 1, whereas the epitaxial growth surfaces may be formed on both the surfaces of the ScAlMgO4 substrate in the embodiment 2. In this case, epitaxial growth of a Group III nitride crystal (Group III nitride semiconductor), such as GaN, may be performed on both surfaces. For example, in the case where an LED light emitting layer is formed on the epitaxial growth surface of the substrate by utilizing the processing technique described above, the problems including the change of the composition, and light emission unevenness and reduction in luminance of the LED device caused thereby may not occur. Furthermore, the height of the irregularities on the epitaxial growth surface of the ScAlMgO4 substrate is reduced to 50 nm or less by the irregularity removing step, and thereby for example, in the case of forming an electrode after forming the LED light emitting layer on the epitaxial growth surface, the formation failure thereof (such as an etching residue at steps) due to the irregularities can be suppressed. Accordingly, the production yield of the device, such as an LED, using the substrate can be enhanced.
In the crystal growing step in the embodiment 2, vapor phase epitaxial growth of a Group III nitride is performed on the epitaxial growth surface of the ScAlMgO4 substrate 10, for example, by an MOCVD method, to grow the Group III nitride crystal layer 5 shown in
In the Group III nitride crystal layer forming step, for example, a Group III nitride crystal layer 5 containing a laminated material of an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer (which is a light emitting layer of an LED device) can be formed. The n-type nitride semiconductor layer used may be a layer formed of an n-type AluGavInwN (wherein u+v+w=1, u≧0, v≧0, and w≧0). The n-type dopant used may be silicon (Si). The n-type dopant used may also be oxygen (O) or the like in addition to Si. The active layer may be a layer (well layer) having an MQW (multiple quantum well) structure formed of GaInN/GaN containing Ga1-xInxN (wherein 0<x<1) well layers each having a thickness of approximately from 3 to 20 nm and GaN barrier layers each having a thickness of approximately from 5 to 30 nm, which are laminated alternately. The wavelength of the light that is emitted by the LED device is determined by the extent of the band gap of the nitride semiconductor constituting the active layer, and specifically determined by the composition x of In in the Ga1-xInxN semiconductor as the semiconductor composition of the well layer. The p-type nitride semiconductor layer may be, for example, a layer formed of a p-type AlsGatN (wherein s+t=1, s≧0, and t≧0) semiconductor. The p-type dopant used may be magnesium (Mg). The p-type dopant used may also be zinc (Zn), beryllium (Be), or the like in addition to Mg.
The device forming step will be then described. In the device forming step, the Group III nitride crystal layer 5 laminated in the Group III nitride crystal growing step is processed into a desired shape, for example, by a lithography method, a dry etching method, or the like. For example, a concave portion 6 is formed by removing a part of the p-type nitride semiconductor layer, the active layer, and the n-type nitride semiconductor layer, thereby forming an electrode. Subsequently, an n-electrode 8 that is electrically connected to the n-type nitride semiconductor layer and a p-electrode 7 that is electrically connected to the p-type nitride semiconductor layer are formed. The n-electrode 8 may be formed, for example, of a laminated structure (Ti/Pt) containing a titanium (Ti) layer and a platinum (Pt) layer, or the like. In addition, a laminated structure (Ti/Al) containing a titanium (Ti) layer and an aluminum (Al) layer may also be used. The p-electrode 7 may be formed, for example, of a laminated structure (Pd/Pt) containing a palladium (Pd) layer and a platinum (Pt) layer, or the like. In addition, a silver (Ag) layer may also be used.
The ScAlMgO4 10a formed on the side of the Group III nitride crystal layer 5 opposite to the crystal growth surface is then removed. Specifically, a blade is attached to the notch 2b formed on the side portion of the ScAlMgO4 substrate 10, and a force is applied in the cleavage direction of the ScAlMgO4 substrate 10, thereby separating the ScAlMgO4 substrate 10 into the ScAlMgO4 substrate 10b containing the device structure formed of the Group III nitride crystal layer 5, the electrodes, and the like shown in
While the embodiments 1 and 2 describe the substrate obtained from a ScAlMgO4 single crystal as a substrate formed of a single crystal represented by the general formula RAMO4, the embodiments are not limited thereto. Specifically, the substrate represented by the ScAlMgO4 substrate of the embodiments is constituted by a substantially sole crystal material represented by the general formula RAMO4. In the general formula, R represents one or a plurality of trivalent elements selected from Sc, In, Y, and a lanthanoid element (atomic number: 67 to 71) , A represents one or a plurality of trivalent elements selected from Fe (III) , Ga, and Al, and M represents one or a plurality of divalent elements selected from Mg, Mn, Fe (II), Co, Cu, Zn, and Cd. The substantially sole crystal material means a crystalline solid, in which the material contains 90% by atom or more of the RAMgO4 constituting the epitaxial growth surface, and in terms of an arbitrary crystal axis, the direction of the crystal axis is not changed in any part on the epitaxial growth surface. However, a material having a crystal axis that is locally changed in direction thereof and a material containing local lattice defects are handled as a single crystal. O represents oxygen. As described above, it is particularly preferred that R is Sc, A is Al, and M is Mg.
The Group III nitride crystal grown on the substrate formed of the single crystal is also not limited to GaN, an n-type nitride semiconductor layer, an active layer, a p-type nitride semiconductor layer, and the like. While the Group III metal element constituting the Group III nitride is most preferably gallium (Ga), examples of the Group III metal element also include aluminum (Al), indium (In), and thallium (Tl), and the Group III nitride may contain only one kind of the Group III metal element or two or more kinds thereof in combination. For example, at least one selected from the group consisting of aluminum (Al), gallium (Ga), and indium (In) may be used as the Group III metal element. In this case, the composition of the Group III nitride crystal thus produced is represented by AlsGatIn(1−(s+t))N (wherein 0≦s≦1, 0≦t≦1, and s+t≦1), and in the embodiment 1, GaN is preferably used as the Group III nitride. Examples of the ternary or higher nitride crystal produced with two or more kinds of the Group III metal elements include a crystal of GaxIn1-xN (wherein 0<x1) .
In the production of an LED device by growing an LED light emitting layer on a substrate by MOCVD vapor phase epitaxial growth, the use of the substrate of the embodiments can prevent light emission unevenness and reduction in luminance of the LED device, while enhancing the material efficiency of the material for the seed substrate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-061043 | Mar 2016 | JP | national |
| 2016-210878 | Oct 2016 | JP | national |
