JP7104408B2 - Nitride semiconductor substrate manufacturing method and nitride semiconductor substrate - Google Patents

Description

The present invention relates to a method for manufacturing a nitride semiconductor substrate having a polarity reversal structure and a nitride semiconductor substrate.

In recent years, research and development of an ultraviolet light source (wavelength 200 nm to 400 nm) using AlN (aluminum nitride), GaN (gallium nitride), AlGaN (aluminum gallium nitride), etc. as a light emitting material has been active (for example, Patent Documents 1 and 2). reference).

Further, the nitride semiconductor substrate having the polarity inversion layer can be used for the SHG element (Second Harmonic Generation). That is, if an InGaN blue laser device is formed on a nitride semiconductor substrate having a polarity inversion layer and the polarity inversion layer is configured as an optical system that generates a second harmonic as a nonlinear optical crystal, ultraviolet light having high coherency is obtained. It can be used as an SHG element for generating.

There are the following prior arts regarding the polarity reversal layer. Non-Patent Document 1 discloses that the mixing of magnesium during the molecular beam epitaxy growth of wurtzite-type GaN reverses the Ga polar (0001) plane to N polarity. Non-Patent Document 2 discloses that the polarity of GaN grown by plasma molecular beam epitaxy is reversed from N polarity to Ga polarity by inserting an intermediate layer in which aluminum nitride / aluminum oxide is mixed.

Japanese Unexamined Patent Publication No. 2017-055116 Japanese Unexamined Patent Publication No. 2008-30137

V. Ramachandran et al., "Inversion of wurtzite GaN (0001) by exposure to magnesium" Published in Appl. Phys. Lett. 75, 808 (1999) Man Hoi Wong et al., "Polarity inversion of N-face GaN using an aluminum oxide obtained" Journal of Applied Physics 108, 123710 (2010), Published Online: 28 December 2010

However, according to the prior art, there is a problem that it is difficult to improve the crystallinity of the nitride semiconductor substrate having the polarity reversal structure.

For example, when the GaN layer is formed with the polarity reversed by the method disclosed in Non-Patent Document 1, there is a problem that the crystallinity deteriorates because it contains an intermediate layer made of magnesium nitride. Further, when the GaN layer having the polarity reversed by the method disclosed in Non-Patent Document 2 is formed, there is a problem that the crystallinity deteriorates because it contains an intermediate layer made of aluminum oxide.

An object of the present invention is to provide a method for manufacturing a nitride semiconductor substrate having improved crystallinity of a nitride semiconductor substrate having a polarity reversal structure, and to provide a nitride semiconductor substrate.

In order to solve the above problems, in the method for manufacturing a nitride semiconductor substrate according to one aspect of the present invention, a first aluminum nitride layer is formed on a sapphire substrate by sputtering aluminum as a target in an atmosphere containing nitrogen gas. The first aluminum nitride layer comprises a first step of forming and a second step of forming a second aluminum nitride layer on the first aluminum nitride layer by sputtering with aluminum nitride as a target . The second aluminum nitride layer has a nitrogen polarity, and in the sputtering in the first step and the second step, the temperature of the substrate is about 500 to 650 ° C., and in the apparatus by sputtering. Is supplied with a gas containing nitrogen .

The nitride semiconductor substrate according to one aspect of the present invention is a nitride semiconductor substrate, which is formed on the substrate, the first nitride layer formed on the substrate, and the first nitride layer. The first nitride layer has a nitrogen polarity, and the second nitride layer has a polarity obtained by reversing the nitrogen polarity of the first nitride layer. The film thickness is 50 nm or more, and the nitride semiconductor substrate constitutes an SHG (Second Harmonic Generation) element .

According to the method for manufacturing a nitride semiconductor substrate of the present invention, the crystallinity of a nitride semiconductor substrate having a polarity reversal structure can be improved, and the manufacturing cost can be significantly reduced.

Further, according to the nitride semiconductor substrate of the present invention, the crystallinity of the nitride semiconductor substrate having a polarity reversal structure can be improved.

FIG. 1A is a cross-sectional view showing a configuration example of a nitride semiconductor substrate according to the embodiment. FIG. 1B is a cross-sectional view showing a more specific example of the nitride semiconductor substrate according to the embodiment. FIG. 2 is a schematic view showing a configuration example of the sputtering apparatus according to the embodiment. FIG. 3 is a flowchart showing an example of a method for manufacturing a nitride semiconductor substrate according to the embodiment. FIG. 4A is a diagram showing a micrograph of an aluminum nitride layer formed by targeting aluminum and examining the polarity. FIG. 4B is a diagram showing a micrograph of the polarity of the second aluminum nitride layer formed on the first aluminum nitride layer formed on the aluminum nitride as a target and examined on the polarity of the second aluminum nitride layer formed on the aluminum nitride as a target. FIG. 5 is a diagram showing the characteristics of the samples shown in FIGS. 4A and 4B. FIG. 6 is a diagram showing a micrograph of the polarity of the second aluminum nitride layer formed by targeting aluminum on the first aluminum nitride layer formed by targeting aluminum nitride. FIG. 7 is a diagram showing the characteristics of the sample shown in FIG. FIG. 8A is a diagram showing a configuration example of an SHG element formed on the nitride semiconductor substrate 1. FIG. 8B is a diagram showing the relationship between the waveguide width and the effective refractive index of the SHG element using the nitride semiconductor substrate 1. FIG. 9 is a diagram showing the electric field distribution of the SHG element formed on the nitride semiconductor substrate 1. FIG. 10A is a diagram showing an electron micrograph of the nitride semiconductor substrate 1. FIG. 10B is an enlarged view of the partial image P1 in FIG. 10A. FIG. 10C is an enlarged view of the partial image P2 in FIG. 10A. FIG. 10C is an enlarged view of the partial image P3 in FIG. 10A. FIG. 11 is a diagram showing measurement results of a nitride semiconductor substrate by secondary ion mass spectrometry. FIG. 12 is a cross-sectional view showing a method of manufacturing a nitride semiconductor substrate according to a comparative example. FIG. 13 is a diagram showing an electron micrograph showing a state in which the Group III polarity and the nitrogen polarity are reversed in the nitride semiconductor substrate. FIG. 14 is an explanatory diagram showing the definition of the polarity of the aluminum nitride layer formed on the sapphire substrate.

(Knowledge that became the basis of the present invention)
The present inventors have found that the nitride semiconductor substrate having the polarity reversal structure described in the "Background Techniques" column has problems of crystallinity and manufacturing cost.

Since the problem of crystallinity has already been described with reference to Non-Patent Documents 1 and 2, the problem of manufacturing cost will be mainly described here.

First, as a comparative example, a method for manufacturing a nitride semiconductor substrate according to the findings of the present inventors will be described.

FIG. 12 is an explanatory diagram showing a method of manufacturing a nitride semiconductor substrate having a polarity reversal structure according to a comparative example.

First, as shown in FIG. 12A, a step of forming an AlN buffer layer 3a on the sapphire substrate 2a is performed. The AlN buffer layer 3a is a precursor of the aluminum nitride crystal layer 3, and is a layer that becomes the aluminum nitride crystal layer 3 by being heat-treated in a later configuration. The AlN buffer layer 3a is formed into a film by a sputtering method under the conditions of, for example, 700 W and 600 ° C. The AlN buffer layer 3a generated at this time has a thickness of, for example, 200 nm. A substrate on which the AlN buffer layer 3b is formed on the sapphire substrate 2b is also manufactured in the same manner. For example, the sapphire substrate 2a and the sapphire substrate 2b are different wafers that are processed in the same process at the same time.

Next, as shown in FIG. 12B, a semiconductor substrate having an AlN buffer layer 3a formed on the sapphire substrate 2a and a semiconductor substrate having an AlN buffer layer 3b formed on the sapphire substrate 2b are divided into two. A semiconductor substrate set is formed as a set of sheets, and the surfaces (AlN surfaces) on which the AlN buffer layers 3a and 3b are arranged in each semiconductor substrate are brought into close contact with each other.

Next, as shown in FIG. 12 (c), heat treatment is performed on the semiconductor substrate assembly in which the AlN surfaces are opposed to each other and brought into close contact with each other. The heat treatment is a heat treatment step using an electric furnace or the like. For example, heat treatment is performed at a temperature of 1700 ° C. for 3 hours. The gas does not flow substantially on the surface facing the main surface of the semiconductor substrate group and its surroundings, and the AlN component is suppressed from being dissociated and escaped during the heat treatment, and the surface is flat and the AlN crystal layer is of high quality. The nitride semiconductor substrate 1 in which the above is formed is manufactured.

Next, as shown in FIG. 12D, a process of peeling off the sapphire substrate 2b is performed on one of the two semiconductor substrates. The nitride semiconductor substrate from which the sapphire substrate 2b has been peeled off has a configuration in which an Al-polar AlN crystal layer 3a is arranged on the sapphire substrate 2a and a nitrogen-polar AlN crystal layer 3b is arranged on the AlN crystal layer 3a. There is. In this way, two AlN crystal layers whose polarities are inverted can be formed.

Next, FIGS. 12 (e) and 12 (f) are steps of processing the semiconductor substrate obtained as described above as a device. A brief description will be given using the SHG element as an example.

First, as shown in FIG. 12 (e), the + cAlN crystal layer 3a and the −cAlN crystal layer 3b laminated on the sapphire substrate 2a are subjected to a general method in a semiconductor processing process such as lithography and dry etching. It is formed in the pattern of the waveguide core layer. After that, as shown in FIG. 12 (f), the clad layer 7 is formed as a protective layer for confining light in the waveguide core layer.

According to the method for manufacturing a nitride semiconductor substrate shown in the comparative example of FIG. 12, a nitride semiconductor substrate having a polarity reversal structure can be manufactured with good crystal quality. However, in order to produce the SHG element, the sapphire substrate 2b must be peeled off ((d) in FIG. 12), which requires a complicated process. Further, it is necessary to consider the positional deviation and the angular deviation of the two substrates in the semiconductor substrate assembly, and there is a problem that precision requires equipment cost and time cost for positional alignment.

Therefore, an object of the present invention is firstly to provide a method for manufacturing a nitride semiconductor substrate having improved crystallinity of a nitride semiconductor substrate having a polarity reversal structure and a nitride semiconductor substrate. Secondly, it is an object of the present invention to provide a method for manufacturing a nitride semiconductor substrate and a nitride semiconductor substrate that reduce the manufacturing cost.

In order to achieve the above object, the method for manufacturing a nitride semiconductor substrate according to one aspect of the present invention forms a first aluminum nitride layer on a substrate by sputtering aluminum as a target in an atmosphere containing nitrogen gas. It has a first step and a second step of forming a second aluminum nitride layer on the first aluminum nitride layer by sputtering with aluminum nitride as a target.

According to this, since the polarity reversal structure is formed without interposing an intermediate layer between the first nitride layer and the second nitride layer, the crystallinity can be improved. Further, since the sputtering of the first step and the second step can be continuously performed, the steps of laminating and peeling the nitride semiconductor substrates are not required, so that a complicated manufacturing apparatus is not required, and the manufacturing apparatus can be used. By reducing the cost and shortening the manufacturing time, the manufacturing cost can be significantly reduced.

Further, the nitride semiconductor substrate according to one aspect of the present invention includes a substrate, a first nitride layer formed on the substrate, and a second nitride layer formed on the first nitride layer. The first nitride layer has a nitrogen polarity, the second nitride layer has a polarity obtained by reversing the nitrogen polarity, and the thickness of the first nitride layer is 50 nm or more.

According to this, the crystallinity can be improved by the polarity reversal structure without interposing the intermediate layer.

Supplementally, the present inventors have found a method for reversing the polarity by controlling the sputtering conditions of the nitride semiconductor for the above problem. That is, it was clarified that when an aluminum solid is used as the target, an N-polar aluminum nitride layer is formed, and when an aluminum nitride sintered body is used as the target, an Al-polar aluminum nitride layer is formed. This makes it possible to invert the polarity from N polarity to Al polarity by switching the target of sputtering. Therefore, since it does not have an intermediate layer, the crystallinity can be improved, and the complicated process of peeling the sapphire substrate can be eliminated, and the manufacturing cost can be significantly reduced.

Prior to the description of the specific embodiment, a configuration example of the nitride semiconductor substrate 1 having a polarity reversal structure will be described.

FIG. 13 is a diagram showing an electron micrograph showing a state in which the Group III polarity and the nitrogen polarity are reversed in the nitride semiconductor substrate. 13 (a) and 13 (b) are electron micrographs showing a state in which the Group III polarity (Al polarity in the figure) and the nitrogen polarity of the nitride semiconductor substrate are inverted in the Group III crystal layer. .. More specifically, the electron micrographs shown in FIGS. 13 (a) and 13 (b) are nitrides obtained by HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy). It is an observation image (STEM image) of a semiconductor substrate. In FIGS. 13A and 13B, the sapphire substrate of the nitride semiconductor substrate, the + cAlN crystal layer showing positive polarity when one direction of the sapphire substrate in the c-axis direction is positive, and the c-axis direction. A -cAlN crystal layer showing a negative polarity when the direction opposite to the positive direction of is negative has been observed. The portion where the polarity is reversed, that is, the interface between the + cAlN crystal layer and the −cAlN crystal layer is the interface when the substrate sets are bonded together as shown in FIG. 12 (b) in the comparative example. Note that FIG. 13B shows the result of measuring a part of the vicinity of the interface between the + cAlN crystal layer and the −cAlN crystal layer in the semiconductor substrate shown in FIG. 13A at a high magnification of 15 million times. ing.

In the STEM image, heavier atoms appear brighter. From this, it can be seen that in (b) of FIG. 13, the white dot indicates an Al atom heavier than the N atom, and the N atom exists in the portion extending like the tail of the comet. .. The + cAlN crystal layer and the -cAlN crystal layer were originally two independent AlN substrates, but a clear atomic image was seen at the interface, indicating that they show extremely high crystallinity. .. The polarity reversal from the + cAlN crystal layer to the −cAlN crystal layer occurs in the monatomic layer, and realizes a steep polarity reversal that was difficult with the conventional crystal growth method. Here, the disorder of the atomic structure is 1 nm or less, the + cAlN crystal layer and the -cAlN crystal layer are completely bonded at the level of 1 to 2 atoms, and the + cAlN crystal layer and the -cAlN crystal layer It can be read from the STEM image that the amorphous layer does not exist at the interface.

Further, when the interatomic distance of Al atoms was extracted from the STEM image, it was 2.8 Å at the bonding interface and 2.5 Å at a location away from the bonding interface. Since the interatomic distance is about 10% larger at the bonding interface, it is considered that the bonding interface contains impurities such as oxygen and carbon, and the atoms are bonded through these impurities.

It should be noted that all of the embodiments described below show a preferred specific example of the present invention. Numerical values, shapes, materials, components, arrangement positions and connection forms of components, steps, order of steps, etc. shown in the following embodiments are examples, and are not intended to limit the present invention. The present invention is specified by the scope of claims. Therefore, among the components in the following embodiments, the components not described in the independent claims are described as arbitrary components.

(Embodiment 1)
[1. Nitride semiconductor substrate configuration]
The nitride semiconductor substrate 1 according to the present embodiment will be described with reference to FIGS. 1A and 1B. FIG. 1A is a cross-sectional view showing a configuration example of the nitride semiconductor substrate 1 according to the embodiment. FIG. 1B is a cross-sectional view showing a more specific example of the nitride semiconductor substrate 1 according to the embodiment.

As shown in FIG. 1A, the nitride semiconductor substrate 1 includes a substrate 2, a first nitride layer 3 having a nitrogen polarity on the substrate 2 side, and a second nitride layer 4 having a group III polarity on the substrate 2 side. Has. The nitride semiconductor substrate 1 is, for example, a nitride semiconductor substrate used for a polarity reversal type SHG element or the like.

The substrate 2 is a substrate made of at least one of sapphire, silicon carbide and aluminum nitride.

The nitride semiconductor substrate 1 is a crystal of a group III nitride semiconductor represented by AlxGayIn (1-xy) N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, (x + y) ≦ 1) on the surface of the substrate. It has a group III nitride semiconductor crystal layer composed of aggregates of grains. The nitride semiconductor substrate 1 has a first nitride layer 3 having a nitrogen polarity on the substrate 2 side and a second nitride layer 4 having a group III polarity on the substrate 2 side as a group III nitride semiconductor crystal layer. However, it has a polarity reversal layer structure with Group III polarity and nitrogen polarity.

As a specific example, as shown in FIG. 1B, the nitride semiconductor substrate 1 includes a sapphire substrate 2 which is an example of the substrate and an aluminum nitride layer (that is, the first nitride layer 3) having a nitrogen polarity on the substrate 2 side. , An aluminum nitride layer having Al polarity (that is, a second nitride layer 4) is provided on the substrate 2 side. Hereinafter, the substrate 2, the first nitride layer 3, and the second nitride layer 4 may be referred to as a sapphire substrate 2, a first aluminum nitride layer 3, and a second aluminum nitride layer 4, respectively, when a specific example is shown.

The first aluminum nitride layer 3 and the second aluminum nitride layer 4 formed on the sapphire substrate 2 have spontaneous polarization or piezo polarization which are antiparallel to each other along the c-axis direction of the sapphire substrate 2. Here, paying attention to the bond parallel to the c-axis among the Al—N bonds in the wurtzite type crystal, the case where Al is located on the sapphire substrate side is the Al polarity, and the case where N is located is the nitrogen polarity crystal orientation. It is common to define. FIG. 14 is an explanatory diagram showing the definition of the polarity of the aluminum nitride layer formed on the sapphire substrate. In the figure, (a) schematically shows aluminum polarity (Al polarity), and (b) shows nitrogen polarity (N polarity). The broken line frame in FIG. 14A corresponds to the partial image P131 in FIG. The broken line frame in FIG. 14B corresponds to the partial image P132 in FIG.

In FIG. 1B, the first aluminum nitride layer 3 is nitrogen polar. The second aluminum nitride layer 4 is Al-polar. In the following, with respect to the aluminum nitride layer arranged on the sapphire substrate 2, the aluminum nitride layer having Al polarity is also referred to as a + cAlN crystal layer. Further, the aluminum nitride layer having nitrogen polarity is also referred to as a -cAlN crystal layer. In the present embodiment, the first aluminum nitride layer 3 is a −cAlN crystal layer and is the first nitride layer 3. The second aluminum nitride layer 4 is a + cAlN crystal layer and is a second nitride layer 4.

Here, a structure in which the first nitride layer 3 and the second nitride layer 4 in which the Al polarity and the nitrogen polarity are reversed are laminated is referred to as a polarity inversion structure. That is, the structure in which the −cAlN crystal layer and the + cAlN crystal layer are laminated is called a polarity inversion structure. Further, in the polarity inversion structure of the first nitride layer 3 and the second nitride layer 4, the layer in which the Al polarity and the nitrogen polarity are inverted is defined as the polarity inversion layer. The polarity inversion layer has a structure within a thickness of 1 to 2 atoms, and has an aspect of an interface between the first nitride layer 3 and the second nitride layer 4. Further, in the nitride semiconductor substrate 1, the structure in which the + cAlN crystal layer is laminated on the −cAlN crystal layer is defined as the polarity reversal structure in which the interface between the −cAlN crystal layer and the + cAlN crystal layer is parallel to the surface of the sapphire substrate 2. It is configured to be placed. That is, in the nitride semiconductor substrate 1, the polarity inversion layer is parallel to the surface of the sapphire substrate 2 as shown in FIGS. 1A and 1B.

Since the nitride semiconductor substrate 1 is schematically represented in FIG. 1, the sapphire substrate and the first nitride layer 3 and the second nitride layer 4 appear to have the same thickness, but in reality, sapphire The substrate 2 is about 200 μm or more and 1000 μm or less, and the first nitride layer 3 is about 50 nm or more and about 200 nm or less. The total film thickness of the first nitride layer 3 and the second nitride layer 4 is 1000 nm or less. The nitride semiconductor substrate 1 shown in FIG. 1A has a polarity inversion layer near the center of the thickness, which is the interface between the first nitride layer 3 and the second nitride layer 4. However, the first nitride layer 3 and the second nitride layer 4 do not have to have the same thickness. Therefore, the polarity inversion layer is not necessarily formed near the center of the total thickness of the crystal layer 3 and the second nitride layer 4, and can be formed according to the characteristic requirements of the device obtained by processing the nitride semiconductor substrate 1. The position of the polarity reversal layer can be freely set.

[2. Nitride semiconductor substrate manufacturing equipment]
Next, the configuration of a sputtering device as a manufacturing device for the nitride semiconductor substrate 1 having a polarity reversal structure will be described.

FIG. 2 is a schematic view showing a configuration example of the sputtering apparatus 10 according to the embodiment. As shown in the figure, the sputtering apparatus 10 includes a chamber 100, an intake pipe 101, an exhaust pipe 102, a valve 103, an exhaust pump 104, a substrate holder 105, a permanent magnet 108, and a high-voltage power supply 109.

The chamber 100 is a substantially closed room in which the substrate 2 and the target 107, which is the raw material of the first nitride layer 3, are held so as to face each other, and the internal pressure and the temperature can be arbitrarily set.

The intake pipe 101 is an intake pipe for introducing the inert gas supplied from the outside into the chamber 100. The inert gas is nitrogen gas, argon gas or the like.

The exhaust pipe 102 is an exhaust pipe for exhausting the gas inside the chamber 100 to the outside.

The valve 103 adjusts the exhaust flow rate of the exhaust pipe 102.

The exhaust pump 104 is a pump for exhausting the gas inside the chamber 100 to the outside through the exhaust pipe 102 and the valve 103.

The substrate holder 105 holds the substrate 2 in the state of a wafer substrate. The substrate holder 105 may hold a plurality of substrates 2 that are simultaneously formed into a film.

The target 107 is held in the target holder. The target holder may have a configuration in which a plurality of types of targets are held and one target to be sputtered can be switched. In the present embodiment, the target holder holds at least two targets, that is, a solid aluminum and a sintered body of aluminum nitride, and can be selectively switched as a sputtering target while being held in the chamber 100. do.

The permanent magnet 108 forms a magnetic field for guiding the atoms of the target 107 ionized by sputtering to the substrate 2.

The high voltage power supply 109 applies a high frequency voltage between the substrate 2 and the target 107. The high frequency voltage is, for example, an RF (Radio Frequency) voltage biased by a DC (Direct Current) voltage. The RF voltage component of the high frequency voltage plasmas the inert gas between the substrate 2 and the target 107. The plasma-ized inert gas collides with the target 107 due to the electric field generated by the DC voltage component, and ejects (sputters) the atoms on the surface of the target 107. The ejected atoms fly toward the substrate 2 and adhere to the substrate 2 according to the electric field of the DC voltage component. As a result, a film made of the target 107 as a raw material is formed on the substrate 2. The high frequency voltage may be, for example, several hundred volts, and the high frequency voltage frequency may be several tens of MHz.

Although the sputtering apparatus 10 of FIG. 2 shows an example of so-called RF sputtering using a high frequency voltage, DC sputtering using a DC voltage of several hundred volts may be used.

Further, in the sputtering apparatus 10 of FIG. 2, a configuration example of a sputtering-up type (or face-down type) in which the substrate 2 is arranged facing above the target 107 has been described, but the substrate 2 is below the target 107. It may be a spatter-down type (face-up type) arranged so as to face the target 107, or a side sputter type (side face type) in which the substrate 2 is arranged so as to face the side of the target 107.

[3. Nitride semiconductor substrate manufacturing method]
Next, a method for manufacturing the nitride semiconductor substrate 1 will be described with reference to the flowchart of FIG.

As shown in FIG. 3, the method for manufacturing the nitride semiconductor substrate 1 is roughly divided into steps S21 to S27.

In step S21, the substrate 2 is prepared in the substrate holder 105 in the sputtering apparatus 10. The substrate 2 may be in the state of a wafer substrate, for example, a wafer made of sapphire. The substrate holder 105 may have a configuration capable of holding four 2-inch wafer substrates, for example.

In step S22, a solid aluminum is prepared as the target 107, which is a film forming material. Specifically, first, the target holder is made to hold a solid aluminum and a sintered aluminum nitride. Next, a solid aluminum object is placed at a position exposed to plasma discharge as a target for sputtering. At this time, the sintered body of aluminum nitride that is not the target of sputtering is arranged at a position where it is not exposed to plasma discharge so that it is not affected by the shutter or the like.

In step S23, the first nitride layer 3 (here, the AlN layer) containing the composition of the target material is formed on the substrate 2 by sputtering the aluminum solid which is the target 107. As a result, an aluminum nitride layer having nitrogen polarity is formed on the sapphire substrate 2 side. At this time, the solid aluminum held in the target holder is sputtered, and the sintered aluminum nitride held in the target holder is not sputtered.

More specifically, the internal pressure of the chamber 100 is adjusted by the exhaust pump 104 and the valve 103 so as to be 0.5 Pa or less. A heater (not shown) is installed in the vicinity of the sapphire substrate 2 of the chamber 100, and the temperature of the sapphire substrate 2 is maintained in the range of about 500 to 650 ° C, for example, about 600 ° C. Further, cooling water is drawn in the vicinity of the target 107 and the permanent magnet 108 by a pipe, and the temperature in the vicinity of the target 107 is suppressed in the range of 100 to 200 ° C. Only nitrogen (N2) gas is supplied from the intake pipe 101 as an inert gas. The flow rate of nitrogen gas is, for example, several tens of sccm (standard Cubic Centimeter per Minute). The unit sccm is a standardized unit at 0 ° C. and 1 atm. The high frequency voltage of the high voltage power supply 109 is several hundred volts, and the frequency of the high frequency voltage is 13.56 MHz. The sputtering time may be determined according to the desired film thickness of the first nitride layer 3 to be formed.

The film thickness of the first nitride layer 3 may be 50 nm or more and 200 nm or less from the viewpoint of forming a polarity reversal structure. Further, the larger the film thickness of the first nitride layer 3, the smaller the internal pressure of the chamber 100 may be.

Sputtering is stopped after the sputtering time corresponding to the desired film thickness of the first nitride layer 3 has elapsed. That is, the generation of the high frequency voltage by the high voltage power supply 109 is stopped. At this time, the state of the chamber 100 (temperature, supply of inert gas, sputtering pressure, etc.) other than the high frequency voltage may be maintained.

In step S24, a sintered body of aluminum nitride is prepared as the target 107, which is a film forming material. Specifically, among the solid aluminum and aluminum nitride sintered bodies held in the target holder, the aluminum nitride sintered body is placed at a position exposed to plasma discharge as a target for sputtering. At this time, the aluminum solid that is not the target of sputtering is arranged at a position where it is not exposed to plasma discharge so that it is not affected by the shutter or the like.

In step S25, the second nitride layer 4 (here, the AlN layer) containing the composition of the target material is formed on the first nitride layer 3 by sputtering the sintered body of aluminum nitride which is the target 107. .. As a result, an aluminum nitride layer having Al polarity is formed on the sapphire substrate 2 side. At this time, the sintered body of aluminum nitride held in the target holder is sputtered, and the solid aluminum held in the target holder is not sputtered.

The sputtering setting conditions may be the same as in step S23. However, the sputtering time may be determined according to the desired film thickness of the second nitride layer 4 to be formed. The film thickness of the second nitride layer 4 may be 1000 nm or less in combination with the film thickness of the first nitride layer 3 from the viewpoint of forming the polarity reversal structure. The film thickness of the second nitride layer 4 may be 640 nm or less from the viewpoint of suppressing the occurrence of cracks, and the larger the film thickness of the second nitride layer 4, the smaller the internal pressure of the chamber 100. You may. Here, the internal pressure is 0.5 Pa, preferably 0.2 Pa.

In step S26, the sapphire substrate 2 on which the first nitride layer 3 and the second nitride layer 4 are formed is annealed at 1400 ° C. or higher, preferably 1650 ° C. or higher and 1750 ° C. or lower.

More specifically, first, the substrate 2 on which the first nitride layer 3 and the second nitride layer 4 are formed by step S25 is arranged inside the annealing device. The annealing device may be any device capable of annealing, and may be a device different from the sputtering device 10 or a sputtering device 10. The substrate 2 is arranged inside the annealing device as follows. That is, an airtight state is provided in which the main surface of the second nitride layer 4 is covered with a cover member for suppressing the dissociation of the nitride semiconductor component from the main surface of the formed second nitride layer 4. Here, "dissociation" means that the components (nitrogen, aluminum, gallium, indium, etc.) are separated from the seed plane of the second nitride layer 4 and escape, and includes sublimation, evaporation, and diffusion. Further, the "main surface" of a semiconductor (or substrate) refers to the surface on the side to be laminated (formed) when another material is laminated (or formed) on the semiconductor (or substrate).

Next, gas replacement is performed by exhausting the inside of the annealing device to create a vacuum and then inflowing an inert gas or a mixed gas. After that, the first nitride layer 3 and the second nitride layer 4 arranged in an airtight state are annealed. At this time, the temperature of the substrate 2 on which the first nitride layer 3 is formed is 1650 ° C. or higher and 1750 ° C. or lower, and an inert gas such as nitrogen, argon, or helium or an inert gas is mixed with ammonia gas. Anneal in a gas atmosphere.

The pressure of the inert gas or mixed gas in the annealing device is in the range of 0.1 to 10 atm (76 to 7600 Torr) where the annealing effect can be expected, but it is 0 due to the explosion-proof strength at high temperature. It is set to about 5 to 2 atmospheres. In principle, the higher the pressure of these gases, the better the crystallinity and surface roughness of the AlN buffer layer 3 can be expected, but the pressure of the gases may be set to around 1 atm. Here, the relationship of the pressure unit is 1 atm = 101,325 Pa (Pascal) = 760 Torr.

By such annealing, the penetrating dislocation density of the first nitride layer 3 and the second nitride layer 4 can be reduced to improve the crystallinity.

In step S27, the nitride semiconductor substrate 1 after annealing is cooled by leaving it until it reaches room temperature.

The annealing device is a heating container having a constant volume, and has a function of controlling the substrate temperature at 500 ° C. to 1800 ° C., and an inert gas and a mixed gas for introducing and replacing the substrate in the device. Anything that has a function of controlling pressure and flow rate may be used. Next, the airtight state in step S26 will be described.

The airtight state is a state realized in the annealing device, and is a cover member for suppressing the dissociation of the component from the main surface of the second nitride layer 4, and the main surface of the second nitride layer 4 is covered. It is in a covered state. That is, the airtight state suppresses the dissociation of the component from the main surface of the second nitride layer 4 by a physical method. In this state, the gas stays between the cover member and the main surface of the first nitride layer 3 so that the gas does not substantially flow. By annealing the nitride semiconductor substrate in such an airtight state, it is possible to prevent the main surface from becoming rough due to the dissociation of its components from the main surface of the second nitride layer 4. Further, annealing at a higher temperature becomes possible, and a nitride semiconductor substrate 1 having a flat surface and a high-quality second nitride layer 4 formed is realized.

The substrate 2 prepared in step S21 is not limited to sapphire, and may be a substrate composed of at least one of sapphire, silicon carbide (SiC), and aluminum nitride (AlN).

The inert gas in the sputtering of step S25 is not limited to nitrogen gas, but may be a mixed gas of nitrogen gas and argon gas.

In addition, S26 and S27 may be carried out during S25 of step S24. That is, the crystallinity may be improved by annealing after the first nitride layer 3 is formed into a film, and the crystallinity may be improved by annealing after forming the second nitride layer 4 on the film.

According to the method for manufacturing the nitride semiconductor substrate 1 shown in FIG. 3, when an aluminum solid is used as the target, an N-polar aluminum nitride layer is formed, and when an aluminum nitride sintered body is used as the target, Al-polar nitride is used. An aluminum layer is formed. This makes it possible to invert the polarity from N polarity to Al polarity by switching the target of sputtering. Therefore, since it does not have an intermediate layer, the crystallinity can be improved, and the complicated process of peeling the sapphire substrate can be eliminated, and the manufacturing cost can be significantly reduced.

Further, the annealing treatment can reduce the through-dislocation density of the first nitride layer 3 and the second nitride layer 4 to improve the crystallinity. Further, by setting the sputtering pressure to 0.5 Pa or less, even if the film thicknesses of the first nitride layer 3 and the second nitride layer 4 are large, the generation of cracks due to the annealing treatment can be suppressed.

Subsequently, the result of evaluating the sample produced by the method for producing the nitride semiconductor substrate 1 according to the embodiment will be described.

FIG. 4A is a diagram showing a micrograph of an aluminum nitride layer formed by targeting aluminum and examining the polarity. Sample 4A in the figure was taken out from the sputtering apparatus 10 for evaluation immediately after the first nitride layer 3 was formed into a film of 200 nm in step S23 of FIG.

The upper image of FIG. 4A is an image obtained by observing the surface state of sample 4A with an atomic force microscope. The vertical direction of the image is the [1-100] direction of AlN, and the horizontal direction of the image is the [11-20] direction of AlN. The measurement is performed so as to be. In the 2 × 2 μm range, the Rq value indicating the root mean square roughness of the surface was 0.41 nm. Generally, if the surface roughness is 1 nm or less, joining is possible, and the joining is sufficiently flat.

The lower image of FIG. 4A is an image of observing the surface state of sample 4A after etching it with an aqueous solution of potassium hydroxide (KOH) for 10 seconds at room temperature in order to confirm the polarity of sample 4A. The Rq value indicating the average roughness of the surface after etching was 29.1 nm. The lower image of FIG. 4A has considerably larger irregularities than the upper image, and the blackish image is a state in which Al atoms sufficiently larger than N atoms are visible on the surface. That is, since the KOH aqueous solution has a property of dissolving only nitrogen-polar AlN, it can be seen that the surface side has Al polarity (the substrate 2 side has N polarity).

FIG. 4B is a diagram showing a micrograph of the polarity of the second aluminum nitride layer formed on the first aluminum nitride layer formed on the aluminum nitride as a target and examined on the polarity of the second aluminum nitride layer formed on the aluminum nitride as a target. In the sample 4B of the figure, immediately after the first nitride layer 3 was formed by 20 nm in step S23 of FIG. 3 and the second nitride layer 4 was formed by 180 nm in step S25, the sample 4B was evaluated from the sputtering apparatus 10 for evaluation. It was taken out.

The upper image of FIG. 4B is an image obtained by observing the surface state of the sample 4B with an atomic force microscope, and the Rq value indicating the average roughness of the surface was 1.59 nm in the range of 2 × 2 μm.

The lower image of FIG. 4B is an image of observing the surface state of sample 4B after etching it with an aqueous solution of potassium hydroxide for 30 seconds at room temperature in order to confirm the polarity of sample 4B. The Rq value indicating the average roughness of the surface after etching was 1.26 nm. The lower image of FIG. 4B has slightly smaller irregularities than the upper image, and the image is not blackish because N atoms are visible on the surface. That is, it can be seen that the surface side has N polarity (the substrate 2 side has Al polarity).

FIG. 5 is a diagram showing the characteristics of the samples 4A and 4B shown in FIGS. 4A and 4B. In FIG. 5, the polarity determined from the etching result and the full width at half maximum of the diffraction peak obtained by the XRC (X-ray locking curve) measurement on the (0002) plane (FWHM: Full Width at Half Maximum, hereinafter simply referred to as the full width at half maximum). ), The full width at half maximum on the (10-12) plane, and the lattice constant in the c-axis direction. The smaller the full width at half maximum of XRC, that is, the sharper the obtained diffraction peak, the better the crystallinity. The unit of the half width of XRC is arcsec ("), which represents an angle.

As shown in FIG. 5, sample 4A has N polarity on the substrate 2 side, the XRC half width of the (0002) plane is 11 arcsec, the XRC half width of the (10-12) plane is 319 arcsec, and the c-axis lattice constant. Is 4.989 Å and the a-axis lattice constant is 3.098 Å.

Sample 4B has Al polarity on the substrate 2 side, the XRC half width of the (0002) plane is 13 arcsec, the XRC half width of the (10-12) plane is 255 arcsec, the c-axis lattice constant is 4.988 Å, and the a-axis. The lattice constant is 3.099 Å.

From FIG. 5, the polarities of the first aluminum nitride layer formed by targeting aluminum and the second aluminum nitride layer formed by targeting aluminum nitride are reversed, and the polarity inversion structure is targeted. It was confirmed that it can be formed by switching. Further, it has high quality crystallinity from the full width at half maximum and the lattice constant of FIG. 5, and for example, the first N-polar aluminum nitride layer can be used as a high quality template.

Next, the result of evaluating the sample produced by a procedure different from the method for producing the nitride semiconductor substrate 1 according to the embodiment will be described.

FIG. 6 is a diagram showing a micrograph of the polarity of the second aluminum nitride layer formed by targeting aluminum on the first aluminum nitride layer formed by targeting aluminum nitride. Sample 6 in the figure is prepared by exchanging the order of steps S23 and S25 in FIG.

The upper image of FIG. 6 is an image obtained by observing the surface state of the sample 6 with an atomic force microscope, and the Rq value indicating the average roughness of the surface was 1.64 nm in the range of 2 × 2 μm.

The lower image of FIG. 6 is an image of observing the surface state of the sample 6 after etching it with an aqueous solution of potassium hydroxide for 2.5 minutes under heating at 80 ° C. in order to confirm the polarity of the sample 6. The Rq value indicating the average roughness of the surface after etching was 0.85 nm. The lower image of FIG. 6 has slightly smaller irregularities than the upper image, and the image is not blackish because N atoms are visible on the surface. That is, it can be seen that the surface side has N polarity (the substrate 2 side has Al polarity).

FIG. 7 is a diagram showing the characteristics of the sample 6 shown in FIG. The figure shows the same items as in FIG.

Sample 6 has Al polarity on the substrate 2 side, the XRC half width of the (0002) plane is 20 arcsec, the XRC half width of the (10-12) plane is 325 arcsec, the c-axis lattice constant is 4.990 Å, and the a-axis. The lattice constant is 3.099 Å.

From FIGS. 6 and 7, both the 20 nm aluminum nitride layer of the first layer and the 180 nm aluminum nitride layer of the second layer of the sample 6 have N polarity on the surface side (Al polarity on the substrate 2 side), and the polarity is reversed. It turned out that the structure could not be formed. In other words, it was found that when the order of step S23 and step S25 in FIG. 3 was changed, a nitride semiconductor substrate having a polarity reversal structure could not be manufactured.

Next, a configuration example of the SHG element using the nitride semiconductor substrate 1 will be described.

FIG. 8A is a schematic view showing a configuration example of an SHG element using the nitride semiconductor substrate 1. FIG. 8A (a) is a schematic cross-sectional view of the SHG device 300. Further, FIG. 8A (b) is a perspective view of the SHG element 300.

The SHG element 300 has a waveguide 301 and a clad layer 302 made of sapphire. The waveguide 301 has a −cAlN crystal layer 303 and a + cAlN crystal layer 304. The -cAlN crystal layer 303 and the + cAlN crystal layer 304 have a polarity reversal structure produced by the above-mentioned production method of FIG. The −cAlN crystal layer 303 and the + cAlN crystal layer 304 correspond to the first nitride layer 3 and the second nitride layer 4.

Here, in the waveguide 301, the nitride semiconductor substrate 1 is used to form an AlN crystal layer having a polarity reversal structure into a waveguide having a waveguide width w, a waveguide thickness h, and a waveguide length l. It is a thing. This AlN crystal layer is a crystal layer composed of a first nitride layer 3 and a second nitride layer 4. At this time, the waveguide width w, the waveguide thickness h, and the waveguide length l are the incident of the laser beam incident on the direction of the waveguide length l, that is, the y-axis direction shown in FIG. 8A (a), as described later. Calculated based on the wavelength.

Here, the mechanism of the second harmonic generation centering on the polarity inversion layer will be described with reference to FIG. 8A (a).

The waveguide 301 composed of the −cAlN crystal layer 303 and the + cAlN crystal layer 304 has optical non-linearity. In order to obtain a second harmonic (SH wave) by the waveguide 301 having optical non-linearity, it is necessary to satisfy the phase matching condition. That is, since the light (fundamental wave) input to the waveguide 301 and the generated light (SH wave) travel at different speeds in the crystal, they cancel each other out when the phases of the light are different by π. Therefore, in the waveguide 301, it is common to perform phase matching by utilizing the birefringence of the anisotropic crystal. That is, the refractive indexes of the fundamental wave and the SH wave are matched by adjusting the angle of incidence on the anisotropic crystal. As a result, the phase matching condition is satisfied in the waveguide 301, so that the SH wave can be efficiently generated.

Here, since the AlN crystal layer requires a large cost for producing a self-supporting substrate, the conventional birefringence phase matching method that requires an AlN crystal of several mm square is not practical. Moreover, since the birefringence is weak, phase matching using birefringence is impossible in the deep ultraviolet wavelength region. Therefore, pseudo-phase matching using an AlN crystal layer (thin film) whose polarity has been inverted is used. The output from the SHG element 300 needs to satisfy the phase matching in the y-axis direction (propagation direction) and the z-axis direction (vertical direction) as shown by the following (Equation 1). At this time, the phase matching in the y-axis direction utilizes the mode dispersion in the waveguide, and the phase matching in the z-axis direction utilizes the polarity inversion of AlN. In (Equation 1), l is the waveguide length of the waveguide extending in the y-axis direction, k is the wave number of light, and d33 is the nonlinear optical coefficient.

First, the term related to the y-axis can be expressed as (Equation 2).

In (Equation 2), λω indicates the wavelength of the fundamental wave, nω indicates the effective refractive index of the fundamental wave, and n2ω indicates the effective refractive index of the SH wave. When the effective refractive indexes of the fundamental wave and the SH wave match, Δk becomes 0, and the first term shows 1 as a sinc function, so that high SHG efficiency can be obtained. Here, the general birefringence is not used, and the phase matching condition is satisfied by using the mode dispersion as described above. That is, by using the higher-order mode in which the node of the electric field distribution exists in the center of the layer of the waveguide for the SH wave, the matching of the effective refractive index that does not match between the base-order modes is realized.

FIG. 8A (a) is a side view of the waveguide 301, and FIG. 8A (b) is a perspective view of the waveguide 301. In FIG. 8A (a), the electric field distribution (TM00 Ezω) of the fundamental wave propagating in the waveguide 301 formed by the AlN crystal layer and the electric field distribution (TM01 Ez2ω) of the SH wave are shown by solid lines. The electric field distribution diagram shown in FIG. 9A is a plot of the electric field distribution of TM00 Ezω and the electric field distribution of TM01 Ez2ω in FIG. 9B on the xz plane. In FIGS. 9A and 9B, the distribution indicated by Bl in the field shows a positive value, and the distribution indicated by Rd shows a negative value.

Here, TM means a Transfer magic mode, and in FIG. 8A (a), it means an electromagnetic wave in which a magnetic field component exists only in the x-axis direction. Further, the subscripts i and j of TMij represent the number of nodes of the electric field distribution in the x-axis direction and the z-axis direction, respectively. In FIG. 8A (a), TM00 Ezω has no knots, but TM01 Ez2ω has one knot in the center. Taking advantage of the fact that AlN has a high refractive index and sapphire (Al2O3) has a low refractive index, the effective refractive indexes of both can be adjusted by adjusting the electric field distribution in each material. In addition, in FIG. 8A (a), the broken line described in the vicinity showing the curve of TM00 Ezω and TM01 Ez2ω indicates the position of the electric field 0.

For example, the electric field distribution can be adjusted by appropriately adjusting the thickness h and the waveguide width w. Although the sapphire substrate 302 is left in FIG. 8A (b), as described in the stage of FIG. 5 (d), the substrate can be completely peeled off and another clad layer can be applied all around the circumference. In the case of an SHG element, if silicon oxide (SiO2) is used, the light confinement effect can be further improved and the wavelength conversion efficiency can be improved.

However, since TM01 Ez2ω has positive and negative electric fields, there is a problem that the overlap integral of the phase matching term becomes 0 in the case of an AlN film having a normal single polarity. Therefore, it is necessary to perform the above-mentioned polarity inversion and invert the code of the nonlinear optical coefficient d33 (z) at the film thickness corresponding to the node of the SH wave electric field distribution. As a result, the integral term of (Equation 1) becomes a non-zero value, and SHG light is output. Through these efforts, the phase matching conditions in the y-axis direction and the z-axis direction are finally satisfied, and highly efficient SHG output can be realized.

In this way, if an optical system is assembled using an existing InGaN blue laser as a light source and an SHG element that generates a second harmonic by using the AlN crystal layer of the nitride semiconductor substrate 1 as a nonlinear optical crystal, coherency is achieved. Can generate high ultraviolet light.

The advantages of using the AlN crystal layer as a nonlinear optical crystal include the following three points.
(1) Since the absorption edge wavelength of the AlN crystal layer is 210 nm, it is transparent in a wide ultraviolet region.
(2) It has a nonlinear optical coefficient d33 higher than that of the existing nonlinear optical crystals BBO (barium borate) and CLBO (cesium lithium borate).
(3) The AlN crystal layer is a chemically and mechanically stable material, and is not deliquescent and toxic like BBO and CLBO.

Next, a design example of the waveguide 301 according to the present embodiment will be described with reference to FIG. 8B. Below,
FIG. 8B is a diagram showing the relationship between the waveguide width w and the effective refractive index according to the design example of the SHG element using the nitride semiconductor substrate 1. The horizontal axis of the graph shown in the figure shows the width of the waveguide, and the vertical axis shows the effective refractive index of the fundamental wave (incident wavelength λ1 = 532 nm) and the SH wave (emission wavelength λ2 = 266 nm). The film thickness of the AlN crystal layer constituting the waveguide 301 was h = 110 nm, and the waveguide length was l = 1 mm.

FIG. 8B shows a curve in which the effective refractive indexes neff, 1 and neff, 2 of TM00 Ezω and TM01 Ez2ω change depending on the waveguide width w of the waveguide 301. When the waveguide width w of the waveguide 301 is set to the waveguide width w = 1.94 μm, which is the intersection of the two curves, the difference in the effective refractive index becomes zero, and the wavelength conversion efficiency becomes maximum. In this design, a wavelength of λ1 = 532 nm was used as the fundamental wave, because the SH wave of the YAG laser is used for the convenience of the measurement system. When performing wavelength conversion at a shorter wavelength, if the design is performed at a wavelength near λ1 = 450 nm, phase matching can be satisfied with the SH wave of λ2 = 225 nm.

Here, in the above-mentioned design example, when the incident wavelength is λ1 and the wavelength of the output light output from the SHG device is λ2 = λ1 / 2, the refractive index at the incident wavelength of the crystal layer is n1 and the refraction at the emission wavelength. When the ratio is n2 and the film thickness (wavelength path thickness) h of the AlN crystal layer is fixed at h = 110 nm and the value of the waveguide width w is changed, the value of the waveguide width w such that n1 = n2. Is sought. The tolerance between n1 and n2 is preferably 0.1% or less, more preferably ± 0.005% when calculated by (n1-n2) / n1. The design of the waveguide 301 is not limited to this, and for example, a graph in which the value of the waveguide width w is fixed and the film thickness h of the AlN crystal layer is changed can be used.

As described above, when the incident wavelength is λ1 and the wavelength of the output light of the nitride semiconductor substrate 1 is λ2, the refractive index at the incident wavelength of the AlN crystal layer is n1 and the refractive index at the emission wavelength is n2. Alternatively, when one of the values of the thickness h is fixed and then the width w or the unfixed value of the thickness h is changed, the value of w or h when n1 = n2 is calculated. Will be done.

The full width at half maximum (FWHM) of the locking curve of at least one of the X-ray diffraction (10-12) of the first nitride layer 3 and the second nitride layer 4 in FIG. 1A shall be 1000 arcsec or less. Is desirable.

Subsequently, using FIGS. 10A to 10D, an example of an electron micrograph of the nitride semiconductor substrate 1 produced by the method for producing the nitride semiconductor substrate 1 according to the present embodiment is shown.

FIG. 10A is a diagram showing an electron micrograph of the compound semiconductor substrate 1, and is a diagram showing an example of an observation image (STEM image) of the nitride semiconductor substrate 1 by HAADF-STEM. FIG. 10B is a diagram showing an enlarged image of the partial image P1 in FIG. 10A. FIG. 10C is a diagram showing an enlarged image of the partial image P2 in FIG. 10A. Further, FIG. 10D is a diagram showing an enlarged image of the partial image P3 in FIG. 10A.

The nitride semiconductor substrate 1 shown in FIG. 10A has an Al target layer of about 100 nm formed on the substrate 2 (sapphire substrate) by sputtering targeting aluminum, and a film formed by sputtering targeting aluminum nitride. Includes a 100 nm AlN target layer. The Al target layer generally corresponds to the first nitride layer 3 in FIG. 1B. The AlN target layer generally corresponds to the second nitride layer 4 in FIG. 1B.

In FIG. 10A, the portion of the AlN target layer on the substrate 2 side immediately after the start of film formation has a nitrogen polarity (−cAlN polarity) instead of an aluminum polarity, and then reverses to an aluminum polarity (+ cAlN polarity). .. It is considered that this is because the nitrogen polarity (-cAlN polarity) is obtained in the transitional period immediately after the target is switched from the Al solid to the AlN sintered body.

In FIG. 10B, the lower portion of the partial image P1 in FIG. 10A has a nitrogen polarity (−cAlN polarity) on the substrate 2 side. Further, the upper part of the partial image P1 has Al polarity (+ cAlN polarity) on the substrate 2 side. That is, it was confirmed that the polarity reversal structure was formed.

In FIG. 10C, a portion of the partial image P2 in FIG. 10A in which the shade appears is further enlarged and is shown as the partial image P21. In the partial image P21, although shades appeared in the image, it was confirmed that all of them had nitrogen polarity (−cAlN polarity) in the first nitride layer 3.

In FIG. 10D, a partial image P31 and a partial image P33, which are a partially enlarged part of the partial image P3 in FIG. 10A, are supported. Further, the partial image P32 in FIG. 10D shows a portion in which a stacking defect is particularly generated. The partial image P33 below the partial image P33 having a stacking defect has Al polarity (+ cAlN polarity) on the substrate 2 side. Further, it is difficult to clearly determine the polarity of the partial image P31 on the upper side of the partial image P33 having a stacking defect.

As shown in FIGS. 10A to 10D, it was confirmed that the nitride semiconductor substrate 1 as shown in FIG. 1B has a polarity reversal structure, although there are portions of unknown polarity and stacking defects in the electron micrographs.

Next, the results of secondary ion mass spectrometry (abbreviation: SIMS) performed to confirm that the nitride semiconductor substrate 1 of the present invention contains a polarity reversal structure will be described with reference to FIG. .. SIMS is one of the types of ionization methods in mass spectrometry.

FIG. 11 is a diagram showing the measurement results of the nitride semiconductor substrate by the secondary ion mass spectrometry method, that is, the distribution of the impurity concentration in the depth direction. In FIG. 11, the left side of the horizontal axis is the surface side of the second nitride layer 4 of the nitride semiconductor substrate 1, and the right end side of the horizontal axis is the sapphire substrate side. The density of impurities on the surface side is 7 × 10 ²⁰ cm ^-3 for oxygen (O), 2 × 10 ¹⁹ cm ^-3 for carbon (C), and 5 × 10 ¹⁸ cm ^-3 for silicon (Si). On the other hand, in the vicinity of the film depth of 0.5 μm (500 nm), oxygen (O) is 4 × 10 ¹⁹ cm ^-3 , carbon (C) is 1 × 10 ¹⁹ cm ^-3 , and silicon (Si) is 4 × 10 ¹⁶ . cm ^-3 . This is considered to suggest that the −cAlN polarity is reversed to the + cAlN polarity when the oxygen concentration in the membrane is high. In other words, since the oxygen concentration of the aluminum nitride layer, which is the second nitride layer 4, is 10 times or more higher than the oxygen concentration of the aluminum nitride layer, which is the first nitride layer 3, it is polar to the nitride semiconductor substrate 1. It is considered that an inverted structure is formed.

As described above, the method for manufacturing a nitride semiconductor substrate according to the present embodiment is a first step of forming a first aluminum nitride layer on a substrate by sputtering aluminum as a target in an atmosphere containing nitrogen gas. A second step of forming a second aluminum nitride layer on the first aluminum nitride layer by sputtering the aluminum nitride as a target.

According to this, since the polarity inversion layer is formed without interposing an intermediate layer between the first aluminum nitride layer and the second aluminum nitride layer, the crystallinity can be improved. Further, since the sputtering of the first step and the second step can be continuously performed, the step of laminating the nitride semiconductor substrates is not required, so that a complicated manufacturing device is not required, and the cost of the manufacturing device can be reduced. By shortening the manufacturing time, the manufacturing cost can be significantly reduced.

Here, (the first aluminum nitride layer may have a nitrogen polarity, and the second aluminum nitride layer may have a polarity conversion from a nitrogen polarity to an aluminum polarity.

According to this, by sequentially forming the nitrogen-polarity first aluminum nitride layer and the aluminum-polarity second aluminum nitride layer, the polarity reversal layer can be easily formed at low manufacturing cost and the crystallinity can be improved.

Here, the film thickness of the first aluminum nitride layer is 50 nm or more, the film thickness of the first aluminum nitride layer is 200 nm or less, and the total film thickness of the first and second aluminum nitride layers is 1000 nm or less. It may be.

According to this, it is possible to secure a film thickness that can be used for the polarity inversion layer as a nonlinear optical system that generates a second harmonic.

Here, in the sputtering in the first step and the second step, the temperature of the substrate is about 500 to 650 ° C., and a gas containing nitrogen may be supplied into the apparatus by sputtering.

According to this, by heating the substrate, the migration of atoms on the surface of the substrate is promoted, and high-quality film formation becomes possible. Further, by inflowing a gas containing nitrogen, N atoms constituting AlN can be supplied.

Here, after the second step, there may be a third step of annealing the substrate on which the first and second aluminum nitride layers are formed at a temperature of 1400 ° C. or higher and 1750 ° C. or lower.

According to this, since it is annealed at a relatively high temperature, the crystallinity of the first and second aluminum nitride layers can be improved, and thus the crystallinity of the polarity inversion layer can be improved.

Further, the nitride semiconductor substrate according to one aspect of the present invention includes a substrate, a first nitride layer formed on the substrate, and a second nitride layer formed on the first nitride layer. The first nitride layer has the nitrogen polarity, the second nitride layer has a polarity obtained by reversing the nitrogen polarity, and the thickness of the first nitride layer is 50 nm or more.

According to this, it is possible to have a polarity inversion layer without an intermediate layer and improve its crystallinity.

Here, the substrate may be composed of at least one of sapphire, silicon carbide, silicon, and aluminum nitride.

According to this, the substrate made of sapphire or silicon carbide is relatively inexpensive, so that the cost can be reduced.

Here, the first nitride layer is an aluminum nitride layer containing a nitrogen polarity, the second nitride layer is an aluminum nitride layer containing an aluminum polarity, and oxygen in the aluminum nitride layer which is the second nitride layer. The concentration may be 10 times or more higher than the oxygen concentration of the aluminum nitride layer which is the first nitride layer.

According to this, the polarity of the aluminum nitride layer can be arbitrarily controlled by appropriately adjusting the oxygen concentration in the film.

Here, the full width at half maximum (FWHM) of the locking curve of at least one of the first nitride layer and the second nitride layer of X-ray diffraction (10-12) may be 1000 arcsec or less.

According to this, it is possible to secure a film thickness that can be used for the polarity inversion layer as a nonlinear optical system that generates a second harmonic.

Here, the film thickness of the first nitride layer may be 200 nm or less, and the total film thickness of the first nitride layer and the second nitride layer may be 1000 nm or less.

According to this, the manufacturing cost of the SHG element having improved crystallinity can be reduced. Here, the nitride semiconductor substrate may constitute an SHG (Second Harmonic Generation) element.

According to this, as a nitride semiconductor device having a nonlinear optical crystal, for example, an optical second harmonic generation element (SHG element) is realized, and output light having a wavelength of 1/2 of the incident laser light wavelength is efficiently generated. Obtainable.

Here, the nitride layer composed of the first nitride layer and the second nitride layer is formed in a waveguide having a thickness h, a width w, and a length l, and the thickness h, width w, The length l may be calculated based on the incident wavelength of the laser beam incident in the l direction.

Here, when the incident wavelength is λ1 and the wavelength of the output light of the nitride semiconductor substrate is λ2, the refractive index at the incident wavelength of the nitride layer is n1 and the refractive index at the emission wavelength is n2. After fixing w or one of the values of the thickness h, when the unfixed value of the width w or the thickness h is changed, the value of w or h when n1 = n2 becomes It may be calculated.

The present invention includes nitride semiconductor substrates having a polar inversion layer on the substrate, such as lighting, sterilization, photolithography, laser processing machines, medical equipment, DNA analyzers, phosphor light sources, spectral distribution analysis, ultraviolet curing, and the like. It can be used for an ultraviolet light emitting element used as a light source. In addition to optical devices, it can be expected to be applied to various electronic components.

1 Nitride semiconductor substrate 2 Substrate 3 First nitride layer 4 Second nitride layer 10 Sputtering device 100 Chamber 101 Intake pipe 102 Exhaust pipe 103 Valve 104 Exhaust pump 105 Board holder 107 Target 108 Permanent magnet 109 High-pressure power supply 300 SHG device ( Nitride semiconductor device)
301 Waveguide 303-cAlN crystal layer 304 + cAlN crystal layer

Claims (10)

The first step of forming the first aluminum nitride layer on the sapphire substrate by sputtering the aluminum as a target in an atmosphere containing nitrogen gas, and
It has a second step of forming a second aluminum nitride layer on the first aluminum nitride layer by sputtering with aluminum nitride as a target.
The first aluminum nitride layer has nitrogen polarity and has a nitrogen polarity.
The second aluminum nitride layer has aluminum polarity and has aluminum polarity.
In the sputtering in the first step and the second step,
The temperature of the substrate is about 500 to 650 ° C., and a gas containing nitrogen is supplied into the apparatus by sputtering.
A method for manufacturing a nitride semiconductor substrate. The film thickness of the first aluminum nitride layer is 50 nm or more, and the film thickness is 50 nm or more.
The film thickness of the first aluminum nitride layer is 200 nm or less, and the film thickness is 200 nm or less.
The method for manufacturing a nitride semiconductor substrate according to claim 1, wherein the total film thickness of the first and second aluminum nitride layers is 1000 nm or less. The nitride semiconductor according to claim 1 or 2 , further comprising a third step of annealing the substrate on which the first and second aluminum nitride layers are formed at a temperature of 1400 ° C. or higher and 1750 ° C. or lower after the second step. Substrate manufacturing method. Nitride semiconductor substrate
With the board
The first nitride layer formed on the substrate and
A second nitride layer formed on the first nitride layer is provided.
The first nitride layer has nitrogen polarity and has a nitrogen polarity.
The second nitride layer has a polarity that is the reverse of the nitrogen polarity.
The film thickness of the first nitride layer is 50 nm or more, and the film thickness is 50 nm or more.
The nitride semiconductor substrate constitutes an SHG (Second Harmonic Generation) element.
Nitride semiconductor substrate. The nitride semiconductor substrate according to claim 4 , wherein the substrate is made of at least one of sapphire, silicon carbide, silicon, and aluminum nitride. The first nitride layer is an aluminum nitride layer containing nitrogen polarity, and is
The second nitride layer is an aluminum nitride layer containing aluminum polarity, and is
The nitride semiconductor substrate according to claim 5. The nitride semiconductor substrate according to claim 6 , wherein the rocking curve full width at half maximum (FWHM) of at least one of the first nitride layer and the second nitride layer of X-ray diffraction (10-12) is 1000 arcsec or less. The film thickness of the first nitride layer is 200 nm or less, and the film thickness is 200 nm or less.
The nitride semiconductor substrate according to claim 7 , wherein the total film thickness of the first nitride layer and the second nitride layer is 1000 nm or less. The nitride layer composed of the first nitride layer and the second nitride layer is formed in a waveguide having a thickness h, a width w, and a length l.
The nitride semiconductor substrate according to claim 4 , wherein the thickness h, the width w, and the length l are calculated based on the incident wavelength of the laser beam incident in the l direction. When the incident wavelength is λ1 and the wavelength of the output light of the nitride semiconductor substrate is λ2, the refractive index at the incident wavelength of the nitride layer is n1, the refractive index at the emission wavelength is n2, and the width w or the above. After fixing one of the values of the thickness h, when the width w or the unfixed value of the thickness h is changed, the value of w or h when n1 = n2 is calculated. The nitride semiconductor substrate according to claim 9 .

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