JP5227870B2 - Epitaxial substrate, semiconductor element structure, and method for manufacturing epitaxial substrate - Google Patents

Description

  The present invention relates to an epitaxial substrate used as a base substrate for forming a semiconductor device.

  In recent years, devices using Group III nitrides have been developed and put into practical use, and they are mainly formed with a C plane, that is, a (0001) plane, on which a high quality crystal can be produced as a growth plane. However, in a device formed with the C-plane as the growth surface (device on the C-plane), an internal electric field is generated in the c-axis direction due to spontaneous asymmetry or piezo-polarization due to the asymmetry of atomic arrangement, which may affect device characteristics. is there. For example, in the case of a light-emitting device, the wave function of electrons and holes is spatially separated in the light-emitting layer, resulting in a decrease in light emission efficiency, and a phenomenon in which the light emission wavelength shifts to the short wavelength side with increasing current (quantum There are problems such as the confined Stark effect.

  Further, a technique for growing a GaN film having a non-polar plane A-plane as a growth plane on an R-plane sapphire substrate is already known (see, for example, Patent Document 1 and Patent Document 2).

JP 2007-243006 A JP 2008-42076 A

  Research and development of devices using nonpolar surface nitrides having a small internal electric field have been conducted in place of the above-described C-plane devices that cause problems. For example, a group III nitride formed with a growth plane such as an A plane that is a plane perpendicular to the C plane, that is, a (11-20) plane, or an M plane, that is, a (1-100) plane, has an internal electric field in the growth direction. Is known to be small. Further, it is theoretically known that the internal electric field of AlGaN greatly decreases as the distance from the C-plane increases, that is, the closer to the A-plane or M-plane. However, the practical use of devices using these nonpolar surface nitrides has many problems such as ensuring crystal quality and surface flatness.

  Patent Document 1 discloses a method for producing a GaN film having a surface roughness (RMS value) of 5 nm or less. However, there is no disclosure regarding ensuring the surface flatness of A-plane AlGaN (particularly AlN) having a high Al composition. Patent Document 1 also states that the disclosed invention has an effect when the substrate off-angle of R-plane sapphire is within a range of ± 5 ° or less.

  Patent Document 2 discloses an invention that realizes formation of a high-quality A-plane GaN film by growing GaN on an A-plane AlN / R-plane sapphire having an uneven surface. However, Patent Document 2 does not show any feasibility of AlGaN having a high Al composition (particularly, AlN).

  That is, an epitaxial substrate on which an AlGaN layer having a high Al composition including AlN and having a surface with a small internal electric field and excellent flatness has not been realized so far, and of course, a semiconductor using the same. No device is available.

  The present invention has been made in view of the above problems, and an object thereof is to provide an epitaxial substrate having a small internal electric field and excellent flatness and a device using the same.

In order to solve the above-mentioned problem, the invention of claim 1 is an epitaxial substrate, wherein an inclination angle of the substrate surface with respect to the R plane takes a positive value when the substrate surface moves away from the C plane, and the substrate When the surface is defined to take a negative value when approaching the C-plane, the sapphire single crystal substrate having an inclination angle of −25 ° or more and −15 ° or less of the substrate surface, and the sapphire single crystal substrate And a substrate surface layer made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0) having a surface roughness of 3 nm or less.

The invention according to claim 2 is the epitaxial substrate according to claim 1, wherein the r-axis direction of the sapphire single crystal base material is in the a-axis direction of Al _x1 Ga _1-x1 N constituting the substrate surface layer. It is characterized by being substantially parallel.

The invention according to claim 3 is the epitaxial substrate according to claim 1 or 2, wherein the substrate surface layer is made of Al _x1 Ga _1-x1 N (0.8 ≦ x1 ≦ 1.0). Features.

  A fourth aspect of the invention comprises the epitaxial substrate according to any one of the first to third aspects, and a functional layer made of a group III nitride formed on the epitaxial substrate. To do.

  The invention according to claim 5 is the semiconductor element structure according to claim 4, wherein the functional layer includes an n-type conductive layer and a light emitting layer.

The invention according to claim 6 takes a positive value when the substrate surface moves away from the C-plane, and takes a negative value when the substrate surface approaches the C-plane. When defined as above, it is formed on the sapphire single crystal substrate having an inclination angle of −25 ° or more and −15 ° or less, and the surface roughness is 3 nm or less. A base layer made of Al _x1 Ga _1-x1 N (0.8 ≦ x1 ≦ 1.0), an n-type conductive layer, and a light emitting layer.

  The invention according to claim 7 is the semiconductor element structure according to claim 5 or 6, wherein the light emitting layer has a multiple quantum well structure in which unit light emitting layers and unit barrier layers are alternately and repeatedly stacked. It is characterized by that.

The invention according to claim 8 is a method for producing an epitaxial substrate, wherein an inclination angle of the substrate surface with respect to the R plane takes a positive value when the substrate surface moves away from the C plane, and the substrate surface is a C plane. When defining to take a negative value when approaching, a step of preparing a sapphire single crystal substrate having an inclination angle of the substrate surface of −25 ° or more and −15 ° or less, A substrate surface layer made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0) having an a-axis direction substantially parallel to the r-axis direction of the sapphire single crystal substrate is formed on the top by MOCVD. And a process.

The invention according to claim 9 is the method for producing an epitaxial substrate according to claim 8, wherein the substrate surface layer is made of Al _x1 Ga _1-x1 N (0.8 ≦ x1 ≦ 1.0). And

According to the inventions of claims 1 to 3 and claims 8 to 9, on the sapphire single crystal substrate that satisfies the condition that the inclination angle of the substrate surface normal is -25 ° or more and -15 ° or less, Epitaxial substrate provided with a substrate surface layer having a surface with a small internal electric field and excellent flatness by epitaxially forming a substrate surface layer made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0) Can be obtained.

According to the invention of claim 4 to claim 7, on the sapphire single crystal substrate that satisfies the condition that the inclination angle of the substrate surface normal is not less than −25 ° and not more than −15 °, Al _x1 Ga _1-x1 N A semiconductor device structure having excellent characteristics can be obtained by forming a predetermined functional layer on an epitaxial substrate formed by epitaxially forming a substrate surface layer or a base layer (0 <x1 ≦ 1.0). it can.

  In particular, according to the inventions of claims 5 to 7, a light emitting element structure having excellent luminous efficiency can be obtained.

It is a figure for demonstrating the definition of an inclination angle. It is a figure which shows the orientation relationship of the substrate surface normal line of a sapphire single crystal substrate and crystal axis which are used as the base material 1 in 1st Embodiment. 1 is a schematic cross-sectional view showing the structure of an epitaxial substrate 10 according to a first embodiment. 2 is a diagram showing a relationship between an inclination angle θ and an angle α formed by an r-axis direction of a sapphire single crystal constituting a substrate 1 and an a-axis direction of AlN constituting a substrate surface layer 2. FIG. It is a figure which shows the relationship between inclination | tilt angle (theta) of the base material 1, and the surface roughness of the substrate surface layer. It is a figure which shows the relationship between inclination | tilt angle (theta) of the base material 1, and the surface roughness of the substrate surface layer. It is a figure which shows the relationship between inclination | tilt angle (theta) of the base material 1, and the surface roughness of the substrate surface layer. It is a schematic cross section which shows the light emitting element structure 20 which concerns on 2nd Embodiment. It is a schematic cross section which shows the light emitting element structure 30 which concerns on 3rd Embodiment. It is a figure which shows typically the structure of the light emitting element 100 which concerns on 4th Embodiment. It is a figure which lists and shows integral intensity ratio _I300K / _I30K about each light emitting element structure obtained in Example 4-Example 7. FIG.

<Definition of tilt angle>
FIG. 1 is a diagram for explaining the definition of the tilt angle in the embodiment of the present invention. In the following embodiments, regarding the hexagonal sapphire single crystal substrate, an angle formed by the R plane, that is, the (1-102) plane and the substrate surface is referred to as an inclination. And as shown to Fig.1 (a), when the base-material surface inclines with respect to R surface so that it may distance from C surface, ie, (0001) surface (in other words, it approaches M surface, ie, (1-100) surface). In this case, the inclination angle is defined so that a positive value is taken in the case of (in case) and a negative value is taken when the substrate surface is inclined with respect to the R-plane so as to approach the C-plane. FIG. 1B shows the relationship between the tilt angle and the crystal axis of each crystal plane. That is, when the substrate surface normal direction of the sapphire single crystal substrate is between the r-axis direction, that is, the [1-102] axis direction and the m-axis direction, that is, the [1-100] axis direction, the tilt angle is positive. It is assumed that the tilt angle is negative when the surface normal direction is between the r-axis direction and the c-axis direction, that is, the [0001] axis direction.

<First Embodiment>
FIG. 2 is a diagram showing the orientation relationship between the substrate surface normal of the sapphire single crystal substrate and the crystal axis used as the base material 1 in the first embodiment of the present invention. FIG. 3 is a schematic cross-sectional view showing the structure of epitaxial substrate 10 according to the first embodiment of the present invention.

  In the present embodiment, a sapphire single crystal substrate that satisfies the requirement that the tilt angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 ° is used as the base material 1. Thus, there is an orientation relationship as shown in FIG. 2 between the substrate surface normal in the sapphire single crystal and the main crystal axis. The thickness of the base material 1 is not particularly limited in terms of material, but for the convenience of processing and handling, a thickness of several hundred μm to several mm is preferable.

The epitaxial substrate 10 is formed by epitaxially forming a substrate surface layer 2 made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0) on such a base material 1. The substrate surface layer 2 is formed to have a thickness of the order of several microns from the submicron order. For example, it is a preferable example that it is formed to have a thickness of about 1 μm.

  FIG. 4 shows different inclination angles in the sapphire single crystal substrate that is the base material 1 and the inclination angle θ in the base material 1 when the substrate surface layer 2 is formed of AlN (when x1 = 1.0). 2 is a diagram showing a relationship between an angle α formed by an r-axis direction of a sapphire single crystal constituting a base material 1 and an a-axis direction of AlN constituting a substrate surface layer 2. FIG. Note that the angle α formed by both axial directions is obtained from an electron diffraction pattern of the epitaxial substrate 10. The results shown in FIG. 4 are plotted such that the angle α formed when the a-axis direction of AlN is between the r-axis direction and the m-axis direction of the sapphire single crystal is positive.

  According to the results shown in FIG. 4, in the range where the inclination angle θ of the substrate 1 is −10 ° to 0 ° (the absolute value is less than 10 °), the larger the absolute value of the inclination angle θ, the larger the angle α formed in both directions. On the other hand, in the range where the inclination angle θ is smaller than −10 ° (the absolute value is larger than 10 °), the angle α formed by both directions decreases and tends to settle around −1 ° to 0 °. I understand that there is. Such knowledge has been revealed for the first time by the inventors of the present invention.

  In the present embodiment, the surface normal direction of the substrate surface layer 2 is controlled based on such knowledge. That is, as described above, by using a sapphire single crystal substrate having a substrate surface normal inclination angle θ of −25 ° ≦ θ ≦ −15 ° as the base material 1, r of the sapphire single crystal substrate that is the base material 1 is used. The substrate surface layer 2 is formed so that the angle formed by the axial direction and the a-axis direction of the substrate surface layer 2 is within a range of ± 2 ° or less. In other words, in the epitaxial substrate 10 according to the present embodiment, the r-axis direction of the substrate 1 and the a-axis direction of the substrate surface layer 2 are substantially parallel (within the thickness range of the substrate surface layer 2). The substrate surface layer 2 is formed so as to be substantially parallel. Thereby, the surface of the substrate surface layer 2 is a nonpolar surface.

5, FIG. 6, and FIG. 7 show that the substrate surface layer 2 is made of AlN (x1 = 1.0), Al _0.9 Ga _0.1 N (x1 = 0.9), and Al _0.8 Ga _0.2 N (x1 = It is a figure which shows the relationship between the inclination | tilt angle (theta) of the base material 1 at the time of forming in 0.8), and the surface roughness (RMS value) of the substrate surface layer 2 in the range of 5 micrometers x 5 micrometers calculated | required from the AFM image. .

  The results shown in FIGS. 5, 6, and 7 show that when -25 ° ≦ θ ≦ −10 °, the substrate surface layer 2 having a good surface flatness with a surface roughness of 3 nm or less is formed. Indicates that In particular, when the substrate surface layer 2 is formed of AlN, as shown in FIG. 5, the substrate surface layer 2 having a surface with a surface roughness of about 1 nm or less and a substantially flat surface at the atomic level is formed. I understand that. Although illustration is omitted, when the inclination angle θ is −10 °, it has been confirmed that pits having a size of about 1 μm are present on the surface of the substrate surface layer 2 at intervals of several μm.

Such a result is obtained by using, as the substrate 1, a sapphire single crystal substrate satisfying the requirement that the inclination angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 °, and Al _x1 Ga _{1− having} excellent surface flatness. _This means that the substrate surface layer 2 made of _x1 N (0 <x1 ≦ 1.0) can be formed. Preferably, 0.8 ≦ x1 ≦ 1.0. Moreover, it can be said that the results shown in FIGS. 5, 6, and 7 indicate that the epitaxial substrate 10 whose inclination angle θ is not in the above range does not have good surface flatness.

  The substrate surface layer 2 having the above-described characteristics can be formed by, for example, MOCVD (metal organic chemical vapor deposition). The formation temperature may be appropriately determined according to the composition within the range of about 1000 ° C to 1200 ° C. Alternatively, another known epitaxial formation method such as MBE method (molecular beam epitaxy method) or HVPE method (vapor phase epitaxial growth method using hydride) may be used.

As described above, according to the present embodiment, the sapphire single crystal substrate that satisfies the condition that the tilt angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 ° is used as a base material, By epitaxially forming a substrate surface layer made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0), an epitaxial layer provided with a substrate surface layer having a nonpolar surface and an excellent flat surface A substrate can be obtained.

<Second Embodiment>
The epitaxial substrate 10 according to the first embodiment described above can be suitably used as a base substrate (base template) when manufacturing various semiconductor devices. In such a case, various functional layers that exhibit characteristics as devices are formed on the epitaxial substrate 10. In this embodiment, as an example, a mode of forming a light-emitting element structure which is one of semiconductor element structures will be described.

FIG. 8 is a schematic cross-sectional view showing a light-emitting element structure 20 according to the second embodiment of the present invention. The light-emitting element structure 20 is formed of the Al _x1 Ga _1-x1 N (0 <x1) on the epitaxial substrate 10 according to the first embodiment, that is, the base material 1 having the inclination angle θ of −25 ° to −15 °. An epitaxial substrate 10 formed by epitaxially forming the substrate surface layer 2 of ≦ 1.0) is used as a base substrate, and the base layer 3, the n-type conductive layer 4, and the light emitting layer 5 are stacked thereon in this order. It has a formed structure. That is, the light emitting element structure 20 corresponds to a single light emitting layer (SQW) structure.

  The underlayer 3, the n-type conductive layer 4, and the light emitting layer 5 are formed by the same method as the substrate surface layer 2. That is, for example, it can be formed by MOCVD. Or the aspect using other well-known epitaxial formation methods, such as MBE method and HVPE method, may be sufficient. In addition, as an aspect for manufacturing the light emitting element structure 20, the epitaxial substrate 10 prepared in advance may be prepared, and the above-described layers may be formed thereon, or the epitaxial substrate 10 is configured. A substrate 1 is prepared, and a substrate surface layer 2, an underlayer 3, an n-type conductive layer 4, and a light emitting layer 5 are continuously formed thereon, so that the light emitting element structure 20 is produced. It may be. In this case, the substrate surface layer 2 can be regarded as the first underlayer, and the underlayer 3 can be regarded as the second underlayer.

The underlayer 3 is a layer formed of a group III nitride having a composition of Al _x2 Ga _1-x2 N (0 ≦ x2 ≦ x1) so as to have a thickness of about several hundred nm. The specific composition of the underlayer 3 may be determined according to the composition of the light emitting layer 5. However, the underlayer 3 is intentionally formed so as not to contain impurities.

The n-type conductive layer 4 contains Si as an n-type dopant in an atomic concentration of about 1 × 10 ¹⁸ / cm ^{3 to} 3 × 10 ¹⁹ / cm ^{3 and is} composed of Al _x3 Ga _1-x3 N (0 ≦ x3 ≦ x1). This is a layer exhibiting n-type conductivity by being formed to have a thickness of submicron order to several μm in the group III nitride. The specific composition of the n-type conductive layer 4 may be determined according to the composition of the light emitting layer 5.

The light emitting layer 5 is a layer responsible for light emission in the light emitting element structure 20, that is, a layer that causes carrier recombination. The light emitting layer 5 is formed of a group III nitride having a composition of Al _x4 Ga _1-x4 N (0 ≦ x4 ≦ x3) so as to have a thickness of about several nm to several tens of nm. The specific composition of the light emitting layer 5 is determined according to the wavelength of light to be emitted. For example, in the case of emitting ultraviolet light having a wavelength of 265 nm, the light emitting layer 5 may be formed to have a thickness of 10 nm with Al _0.49 Ga _0.51 N (x4 = 0.49). In this case, the n-type conductive layer 4 may be formed of Al _0.6 Ga _0.4 N (x3 = 0.6) while doping Si so as to contain about 2 × 10 ¹⁸ / cm ³ in atomic concentration.

By having the configuration as described above, the light emitting element structure 20 emits light having a wavelength corresponding to the composition of the light emitting layer 5. This can be confirmed by performing, for example, cathodoluminescence (CL) measurement on the light emitting element structure 20. Note that a cap layer 6 is preferably provided on the light emitting layer 5 in the cathodoluminescence (CL) measurement. The cap layer 6 is a layer formed of a group III nitride having a composition of Al _x5 Ga _1-x5 N (0.8 ≦ x5 ≦ 1.0) so as to have a thickness of about several nm. If the light emitting layer 5 is formed to have a light emission wavelength of 265 nm as described above, the cap layer 6 is formed to have a thickness of 5 nm using, for example, AlN (x5 = 1.0). Is a suitable example.

  As a result of cathodoluminescence (CL) measurement by the present inventors, the light emitting device structure 20 according to the present embodiment uses an epitaxial substrate that does not satisfy the requirement of −25 ° ≦ θ ≦ −15 ° and has poor surface flatness. It was confirmed that the internal quantum efficiency (carrier recombination efficiency in the light emitting layer 5) is higher than that of the light emitting element structure manufactured under the same conditions (see Examples). Since the difference between the two is the surface flatness of the epitaxial substrate, the formation of the light-emitting element structure 20 using the epitaxial substrate having the excellent surface flatness as in the present embodiment realizes excellent light emission efficiency. Can be said to be effective. This is because the surface flatness of the epitaxial substrate is excellent, the light emitting layer 5 having a steep interface is formed, and a high carrier confinement effect is realized.

<Third Embodiment>
In the present embodiment, the epitaxial substrate 10 according to the first embodiment will be described with reference to another mode used as a base substrate.

  FIG. 9 is a diagram showing a configuration of a light emitting element structure 30 according to the third embodiment of the present invention. FIG. 9A is a schematic cross-sectional view showing the configuration of the light emitting element structure 30. As shown in FIG. 9A, the light emitting element structure 30 includes a light emitting layer 7 having a multiple quantum well structure (MQW) instead of the light emitting layer 5 of the light emitting element structure 20 according to the second embodiment. Has a structure. Therefore, detailed description of the epitaxial substrate 10, the base layer 3, the n-type conductive layer 4, and the cap layer 6 included in the light emitting element structure 30 is omitted.

  FIG. 9B is a schematic cross-sectional view showing a detailed configuration of the light emitting layer 7. As shown in FIG. 9B, the light emitting layer 7 is a layer formed by repeatedly laminating a unit light emitting layer 7a and a unit barrier layer 7b each having a thickness of about several nanometers to several tens of nanometers in this order. is there. The light emitting layer 7 can also be formed by, for example, the MOCVD method. Or the aspect using other well-known epitaxial formation methods, such as MBE method and HVPE method, may be sufficient.

The unit light emitting layer 7a is formed of a group III nitride having a composition of Al _x6 Ga _1-x6 N (0 ≦ x6 <x7). The specific composition of the unit light emitting layer 7a is determined according to the wavelength of light to be emitted. The unit barrier layer 7b is formed of a group III nitride having a composition of Al _x7 Ga _1-x7 N (x6 ≦ x7 ≦ x3). The specific composition of the unit light emitting layer 7a is determined according to the composition of the unit light emitting layer 7a.

For example, if ultraviolet light having a wavelength of 265 nm is to be emitted, formation of a unit light-emitting layer 7 a having a thickness of 5 nm having a composition of Al _0.49 Ga _0.51 N (x6 = 0.49) and Al _0.58 Ga _0.42 The formation of the 5 nm unit barrier layer 7b having a composition of N (x7 = 0.58) may be repeated four times. The underlayer 3, the n-type conductive layer 4, and the cap layer 6 may be formed in the same manner as in the second embodiment.

  With the above configuration, the light emitting element structure 30 emits light having a wavelength corresponding to the composition of the unit light emitting layer 7a. This can be confirmed by performing, for example, cathodoluminescence (CL) measurement on the light emitting element structure 30.

  As a result of the cathode luminescence (CL) measurement by the present inventors, the light emitting element structure 30 according to the present embodiment also does not satisfy the requirement of −25 ° ≦ θ ≦ −15 ° with respect to the inclination angle θ, and the surface flatness is inferior. It has been confirmed that the internal quantum efficiency is higher than that of a light emitting device structure manufactured under the same conditions except that an epitaxial substrate is used (see Examples). That is, also from this embodiment, it can be said that forming a light emitting element structure using an epitaxial substrate having excellent surface flatness is effective in realizing excellent light emission efficiency.

<Fourth embodiment>
In this embodiment, a light-emitting element manufactured using the light-emitting element structure 20 described in the second embodiment will be described. FIG. 10 is a diagram schematically showing the structure of a light emitting device 100 according to the fourth embodiment of the present invention. FIG. 10A is a top view of the light emitting element 100, and FIG. 10B is a cross-sectional view taken along line A-B in the top view. As shown in FIG. 10B, in the light emitting device 100, the intermediate layer 11, the cladding layer 12, the carrier supply layer 13, and the contact layer 14 are formed adjacent to each other in this order on the light emitting device structure 20. Have a laminated structure. That is, the light emitting element 100 is formed of Al _x1 Ga _1-x1 N (0 <<) on the epitaxial substrate 10 according to the first embodiment, that is, the base material 1 having the inclination angle θ of −25 ° to −15 °. In this device, an epitaxial substrate 10 formed by epitaxially forming the substrate surface layer 2 of x1 ≦ 1.0) is used as a base substrate, and the respective layers are sequentially stacked thereon. Instead of the light emitting layer 5, the light emitting layer 7 having a multiple quantum well structure (MQW) may be formed as in the third embodiment.

  The intermediate layer 11, the cladding layer 12, the carrier supply layer 13, and the contact layer 14 are formed in the same manner as the substrate surface layer 2. That is, for example, it can be formed by MOCVD. Or the aspect using other well-known epitaxial formation methods, such as MBE method and HVPE method, may be sufficient.

  Further, a part of the n-type conductive layer 4 is exposed, and a cathode electrode pad 15 is provided on the exposed portion. The cathode electrode pad 15 is made of Ti / Al / Ni / Au.

  Further, an anode electrode layer 16 is provided on the contact layer 14. The anode electrode layer 16 is made of Ni / Au.

  The light emitting element 100 emits excitation light emitted by recombination of carriers in the light emitting layer 5 generated by applying a predetermined voltage between the cathode electrode pad 15 and the anode electrode layer 16 toward the outside of the element. is there.

  The clad layer 12 is a layer provided for the purpose of enhancing the carrier confinement effect in the light emitting layer 5. Accordingly, the light emitting layer 5 is formed of a group III nitride having a larger band gap than the group III nitride constituting the light emitting layer 5.

When the light emitting layer 5 is formed of a group III nitride having a composition of Al _x4 Ga _1-x4 N (0 ≦ x4 ≦ x3), the cladding layer 12 has a further Al-rich Al _x8 Ga _1-x8 N (x4). It is formed by doping a group III nitride <x8 ≦ 1.0) with a P-type dopant such as Mg. More preferably, at least in the vicinity of the junction with the light emitting layer 5, it is formed of a group III nitride of Al _x8 Ga _{1 -x8} N (x4 <x8 ≦ 1.0 and 0.6 ≦ x8 ≦ 1.0). The This is because the light emitting characteristics of the light emitting element 100 are further improved.

On the other hand, the carrier supply layer 13 is a layer provided so that carriers are efficiently supplied to the light emitting layer 4. As described above, the light emitting layer 5 is formed of a group III nitride having a composition of Al _x4 Ga _1-x4 N (0 ≦ x4 ≦ x3), and the cladding layer 12 is formed of Al _x8 Ga _1-x8 N (x4 <x8). ≦ 1.0), when the carrier supply layer 13 is formed of a group III nitride of Al _x9 Ga _1-x9 N (0 ≦ x9 <x8), a P-type such as Mg It is formed by doping with a dopant.

For example, if it is intended to emit ultraviolet light with a wavelength of 265 nm, the light emitting layer 5 is formed of a group III nitride of Al _0.49 Ga _0.51 N (x4 = 0.49) as described above. For the cladding layer 12, a layer made of Al _0.6 Ga _0.4 N containing about 1 × 10 ¹⁹ / cm ³ of Mg atoms as a P-type dopant is formed to a thickness of about several tens of nanometers. For the carrier supply layer 13, A preferred example is to form a layer made of Al _0.3 Ga _0.7 N containing about 3 × 10 ¹⁹ / cm ³ of Mg atoms as a P-type dopant to a thickness of about several tens of nanometers.

  Note that the carrier supply layer 13 does not need to have a single composition, and may have a superlattice structure or a composition gradient structure. Such a structure plays a role of improving the P-type carrier concentration and reducing the resistance.

  The contact layer 14 is formed between the carrier supply layer 13 and the anode electrode part 16 in order to obtain good ohmic contact with the anode electrode part 16.

  Contact layer 14 has P-type conductivity. The contact layer 14 is formed by doping a group III nitride having a band gap smaller than that of the group III nitride constituting the carrier supply layer 13 with a P-type dopant such as Mg. For example, GaN is a suitable example.

  The contact layer 14 may be formed to have an appropriate thickness as long as its function is ensured. For example, it can be formed to have a thickness of 100 nm or more. When the contact layer 14 is formed to be thicker than 100 nm, since the three-dimensional nucleus growth stage shifts to the two-dimensional growth stage, the surface is formed as a flat crystal layer, and the specific resistance can be reduced. It is a suitable example that it is provided at about 200 nm.

  Note that the contact layer 14 may be formed to have a multilayer structure including a plurality of layers having different group III nitride compositions and impurity concentrations. For example, it is possible to reduce the contact resistance with the P-type electrode by modulating the Al, Ga, In composition or making a superlattice structure, or increasing the P-type dopant concentration near the surface.

  In the light emitting element 100 according to the present embodiment, an intermediate layer 11 is further interposed between the light emitting layer 5 and the clad layer 12 which is a P-type conductive portion.

The intermediate layer 11 is provided in the light emitting device 100 for the purpose of favorably generating light emission at a desired light emission wavelength (target light emission wavelength) from the light emitting layer 5. The intermediate layer 11 is preferably a group III nitride of Al _x10 Ga _1-x10 N (0.8 ≦ x10 ≦ 1.0), more preferably AlN, with a thickness of 5 nm or less, preferably 1 nm. It is formed to a thickness of about. It is not preferable to form the intermediate layer 11 thicker than 5 nm because it is necessary to increase the rising voltage.

  Particularly when the intermediate layer 11 is formed using AlN, the thickness of the intermediate layer 11 is set within a range in which the tunnel current is dominant so that carrier injection from the cladding layer 12 to the light emitting layer 5 is not inhibited. It is desirable.

  The intermediate layer 11 is inserted for the purpose of improving the light output with respect to the applied current and favorably producing only light emission at the target light emission wavelength. If Mg atoms doped in the cladding layer 12 and the carrier supply layer 13 diffuse into the light emitting layer 5 in the process of manufacturing the light emitting device 100, light emission other than the intended light emission wavelength may occur due to this. By providing the layer 11, diffusion of Mg atoms into the light emitting layer 5 is prevented, and such unnecessary light emission is suppressed. This effect is remarkable in the case of AlN having a strong bonding force between atoms and the smallest lattice constant.

  The light emitting element 100 having the above configuration emits light having a wavelength corresponding to the composition of the light emitting layer 5 with excellent light emission efficiency. In addition, the light emitting element 100 emits light at the target light emission wavelength without shifting the light emission wavelength as the current increases. This is an effect of using the epitaxial substrate 10 according to the first embodiment to form the light emitting element 100 as a nonpolar plane nitride laminated structure having a small internal electric field.

Example 1
In this example, a plurality of types of sapphire single crystal substrates having a 2-inch diameter and a thickness of 400 μm and various values within a range of an inclination angle θ of −30 ° ≦ θ ≦ + 5 ° are prepared as substrates. An epitaxial substrate was obtained by forming an AlN layer as the surface layer. Specifically, an epitaxial substrate was obtained by the following procedure.

  First, each substrate was placed in a MOCVD reactor and then heated so that the substrate temperature was 1200 ° C. Thereafter, the substrate temperature was set to 1200 ° C. until the AlN layer was formed. Hydrogen gas was introduced into the MOCVD reactor and a hydrogen cleaning process was performed. At that time, the atmospheric pressure was 10 Torr. Subsequently, while maintaining the temperature at 1200 ° C., ammonia gas was supplied together with the hydrogen carrier gas into the MOCVD reactor to perform nitriding treatment on the substrate surface.

  Then, TMA (trimethylaluminum) and ammonia were supplied to the MOCVD reactor at flow rates of 34 μmol / min and 4.5 mmol / min, respectively, to form 1 μm thick AlN on the substrate. Hydrogen gas was used as the carrier gas. Thus, an epitaxial substrate was obtained.

  About each obtained epitaxial substrate, the measurement in a 5 micrometer x 5 micrometer visual field by AFM was performed, and the RMS (Root mean square) value of surface unevenness | corrugation was calculated | required as the surface roughness value of an AlN layer. FIG. 5 is a graph plotting the relationship between the inclination angle θ and the surface roughness value.

  As shown in FIG. 5, the surface roughness is about 1 nm only when the inclination angle θ is in the range of −25 ° ≦ θ ≦ −10 °, and the surface of the AlN layer as the substrate surface layer has excellent flatness. Was confirmed. However, it was confirmed that many triangular pits were formed on the surface of the AlN layer only on the epitaxial substrate having the inclination angle θ of −10 °.

  From this example, a substrate surface layer made of AlN is epitaxially formed on a sapphire single crystal substrate that satisfies the condition that the tilt angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 °. Thus, it was confirmed that an epitaxial substrate provided with a substrate surface layer having a surface with excellent flatness can be obtained.

(Example 2)
An epitaxial substrate was obtained in the same procedure as in Example 1 except that an Al _0.9 Ga _0.1 N layer (x1 = 0.9) was formed as the substrate surface layer. However, the atmospheric pressure in the hydrogen cleaning process was 15 Torr.

When forming the Al _0.9 Ga _0.1 N layer, TMA, TMG (trimethylgallium), and ammonia were supplied into the MOCVD reactor at flow rates of 34 μmol / min, 34 μmol / min, and 4.5 mmol / min, respectively.

  About each obtained epitaxial substrate, the measurement in a 5 micrometer x 5 micrometer visual field by AFM was performed, and the RMS value of the surface unevenness | corrugation was calculated | required as the surface roughness value of an AlN layer. FIG. 6 is a graph plotting the relationship between the inclination angle θ and the surface roughness value.

As shown in FIG. 6, the surface roughness is about 2 nm or less only when the inclination angle θ is in the range of −25 ° ≦ θ ≦ −10 °, and the surface of the Al _0.9 Ga _0.1 N layer as the substrate surface layer is excellent and flat. It was confirmed to have sex.

From this example, a substrate surface layer made of Al _0.9 Ga _0.1 N is used as a base material on a sapphire single crystal substrate that satisfies the condition that the tilt angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 °. It was confirmed that an epitaxial substrate provided with a substrate surface layer having a surface with excellent flatness can be obtained by epitaxially forming.

(Example 3)
An epitaxial substrate was obtained in the same procedure as in Example 2 except that an Al _0.8 Ga _0.2 N layer (x1 = 0.8) was formed as the substrate surface layer. However, the temperature of the substrate was 1150 ° C.

  About each obtained epitaxial substrate, the measurement in a 5 micrometer x 5 micrometer visual field by AFM was performed, and the RMS value of the surface unevenness | corrugation was calculated | required as the surface roughness value of an AlN layer. FIG. 7 is a graph plotting the relationship between the tilt angle θ and the surface roughness value.

As shown in FIG. 7, the surface roughness is about 3 nm or less only when the inclination angle θ is in the range of −25 ° ≦ θ ≦ −10 °, and the surface of the Al _0.8 Ga _0.2 N layer as the substrate surface layer is excellent and flat. It was confirmed to have sex.

From this example, a substrate surface layer made of Al _0.8 Ga _0.2 N is used on a sapphire single crystal substrate that satisfies the condition that the inclination angle θ of the substrate surface normal is −25 ° ≦ θ ≦ −15 °. It was confirmed that an epitaxial substrate provided with a substrate surface layer having a surface with excellent flatness can be obtained by epitaxially forming.

Example 4
In this example, a part of the epitaxial substrate (template substrate) produced in Example 1 was used as a base template, and an epitaxial film was grown on each of them by MOCVD method to produce a plurality of types of light emitting device structures.

  Specifically, the following layers were formed sequentially.

(1) A ground layer made of Al _0.6 Ga _0.4 N having a thickness of 0.5 μm.

(2) An n-type conductive layer made of n-type Al _0.6 Ga _0.4 N having a thickness of 1.0 μm and a Si atom concentration of about 2 × 10 ¹⁸ / cm ³ .

(3) A light emitting layer made of Al _0.49 Ga _0.51 N having a thickness of 10 nm.

  (4) A cap layer made of AlN having a thickness of 5 nm.

  Cathodoluminescence (CL) measurement was performed on the obtained light-emitting element structure. The electron acceleration voltage condition was 3 kV, and the measurement temperature was set in two ways: 30K and 300K. As a result of the measurement, light emission having a peak wavelength of 265 nm in the wavelength profile was confirmed for all samples.

(Example 5)
A light emitting device structure was fabricated in the same procedure as in Example 4 except that a light emitting layer having a multiple quantum well (MQW) structure was formed instead of the light emitting layer in the light emitting device structure of Example 4. Specifically, a light emitting layer was formed by alternately repeating a light emitting layer made of 5 nm thick Al _0.49 Ga _0.51 N and a barrier layer made of 5 nm thick Al _0.58 Ga _0.42 N for four cycles.

  Cathodoluminescence (CL) measurement was performed on the obtained light-emitting element structure under the same conditions as in Example 4. As a result of the measurement, light emission having a peak wavelength of 265 nm in the wavelength profile was confirmed for all samples.

(Example 6)
A light emitting element structure was fabricated in the same procedure as in Example 4 except that a part of the epitaxial substrate fabricated in Example 2 was used as the base template.

  Cathodoluminescence (CL) measurement was performed on the obtained light-emitting element structure under the same conditions as in Example 4. As a result of the measurement, light emission having a peak wavelength of 265 nm in the wavelength profile was confirmed for all samples.

(Example 7)
A light emitting device structure was fabricated in the same procedure as in Example 4 except that a part of the epitaxial substrate fabricated in Example 3 was used as the base template.

  Cathodoluminescence (CL) measurement was performed on the obtained light-emitting element structure under the same conditions as in Example 4. As a result of the measurement, light emission having a peak wavelength of 265 nm in the wavelength profile was confirmed for all samples.

(Relationship between epitaxial substrate and emission state)
FIG. 11 shows a list of peak integrated intensity ratios I _300K / I _30K having a peak wavelength of 265 nm in the wavelength profiles of 30K and 300K for the respective light emitting device structures obtained in Examples 4 to 7. FIG. It has been confirmed in advance that the integrated intensity ratio I _300K / I _30K is substantially equal to the internal quantum efficiency in the light emitting layer.

  As shown in FIG. 11, in any of the examples, the light emitting device structure in which the tilt angle θ of the sapphire single crystal substrate is in the range of −25 ° ≦ θ ≦ −15 ° is compared to the other light emitting device structures. It can be seen that the integrated intensity ratio is about one order higher. That is, it can be seen that high internal quantum efficiency is obtained.

  As shown in Examples 1 to 3, the epitaxial substrate using the sapphire single crystal substrate having the tilt angle θ in such a range as the base material has excellent surface flatness, and the result shown in FIG. Thus, it can be said that by using such a base template having excellent surface flatness, a light emitting element structure having excellent light emission efficiency can be obtained.

(Example 8)
In this example, the light emitting device 100 according to the fourth embodiment was manufactured. In addition, as the base material 1, the sapphire single crystal substrate whose inclination | tilt angle (theta) is -20 degrees was used. The same procedure as in Example 4 was performed until the light emitting layer 5 was formed. Thereafter, the following layers were successively formed by MOCVD.

  (1) An intermediate layer 11 made of AlN having a thickness of 1 nm.

(2) The clad layer 12 made of p-type Al _0.6 Ga _0.4 N having a thickness of 25 nm and a Mg atom concentration of about 1 × 10 ¹⁹ / cm ³ .

(3) The carrier supply layer 13 made of p-type Al _0.3 Ga _0.7 N having a thickness of 25 nm and a Mg atom concentration of about 3 × 10 ¹⁹ / cm ³ .

(4) The contact layer 14 made of p-type GaN having a thickness of 200 nm and an Mg atom concentration of about 1 × 10 ²⁰ / cm ³ .

A part of the Al _0.6 Ga _0.4 N layer that becomes the n-type conductive layer 4 was exposed to the obtained multilayer structure using a photolithography process and an RIE method. The approximate size of the non-etched area was 0.5 mm × 0.5 mm.

Next, as an activation treatment of Mg ions in the Al _0.6 Ga _0.4 N layer serving as the cladding layer 12, the layer comprising Al _0.3 Ga _0.7 N serving as the carrier supply layer 13, and the GaN layer serving as the contact layer 14, in a nitrogen atmosphere The heat treatment at 800 ° C. was performed for 25 minutes.

Subsequently, a Ti / Al / Ni / Ai film serving as the cathode electrode pad 15 is formed to 15 nm on the exposed portion of the Al _0.6 Ga _0.4 N layer to be the n-type conductive layer 4 by using a photolithography process and a vacuum deposition method. , 200 nm, 20 nm, and 100 nm in thickness. Thereafter, in order to improve the ohmic contact characteristics, a heat treatment at 900 ° C. in a nitrogen atmosphere was performed for 30 seconds.

  Further, a Ni / Au film to be the anode electrode layer 16 was patterned to a thickness of 6 nm and 100 nm on the uppermost surface of the GaN layer to be the contact layer 14 by using a photolithography process and a vacuum deposition method, respectively. Thereafter, a heat treatment at 600 ° C. in a nitrogen atmosphere was performed for 30 seconds in order to improve the ohmic contact characteristics. Thus, the light emitting element 100 is obtained.

  When a positive bias was applied to the light emitting element 30 between the anode electrode layer 16 and the cathode electrode pad 15, ultraviolet light emission with a wavelength of 265 nm was confirmed. The light output from the ultraviolet light emitting substrate 1 side was 23.5 μW at an input current of 60 mA. Further, the wavelength shift accompanying the increase in current as observed in the light emitting device formed with the C-plane as the growth surface was not observed. This is a result indicating that the internal electric field in the light emitting element 30 is small.

DESCRIPTION OF SYMBOLS 1 Base material 2 Substrate surface layer 3 Underlayer 4 N-type conductive layer 5, 7 Light emitting layer 6 Cap layer 7a Unit light emitting layer 7b Unit barrier layer 10 Epitaxial substrate 11 Intermediate layer 12 Cladding layer 13 Carrier supply layer 14 Contact layer 20, 30 Light emitting element structure 100 Light emitting element

Claims (9)

An epitaxial substrate,
When the inclination angle of the substrate surface with respect to the R plane is defined to take a positive value when the substrate surface moves away from the C plane and to take a negative value when the substrate surface approaches the C plane, A sapphire single crystal substrate having an inclination angle of −25 ° or more and −15 ° or less of the substrate surface;
A substrate surface layer made of Al _x1 Ga _1-x1 N (0 <x1 ≦ 1.0) having a surface roughness of 3 nm or less, formed on the sapphire single crystal substrate;
An epitaxial substrate comprising: The epitaxial substrate according to claim 1,
An epitaxial substrate, wherein the r-axis direction of the sapphire single crystal base material is substantially parallel to the a-axis direction of Al _x Ga _1-x N constituting the substrate surface layer. The epitaxial substrate according to claim 1 or 2, wherein
An epitaxial substrate, wherein the substrate surface layer is made of Al _x Ga _1-x N (0.8 ≦ x ≦ 1.0). The epitaxial substrate according to any one of claims 1 to 3,
A functional layer made of a group III nitride formed on the epitaxial substrate;
A semiconductor device structure comprising: The semiconductor device structure according to claim 4,
The functional layer is
an n-type conductive layer;
A light emitting layer;
A semiconductor device structure comprising: When the inclination angle of the substrate surface with respect to the R plane is defined to take a positive value when the substrate surface moves away from the C plane and to take a negative value when the substrate surface approaches the C plane, A sapphire single crystal substrate having an inclination angle of −25 ° or more and −15 ° or less of the substrate surface;
An underlayer made of Al _x Ga _1-x N (0.8 ≦ x ≦ 1.0) having a surface roughness of 3 nm or less, formed on the sapphire single crystal substrate;
an n-type conductive layer;
A light emitting layer;
A semiconductor device structure comprising: A semiconductor device structure according to claim 5 or 6, wherein
A semiconductor device structure, wherein the light emitting layer has a multiple quantum well structure in which unit light emitting layers and unit barrier layers are alternately and repeatedly stacked. An epitaxial substrate manufacturing method comprising:
When the inclination angle of the substrate surface with respect to the R plane is defined to take a positive value when the substrate surface moves away from the C plane and to take a negative value when the substrate surface approaches the C plane, A step of preparing a sapphire single crystal substrate having an inclination angle of −25 ° or more and −15 ° or less of the substrate surface;
From Al _x Ga _1-x N (0 <x ≦ 1.0) in which the a-axis direction is substantially parallel to the r-axis direction of the sapphire single crystal substrate on the sapphire single crystal substrate by MOCVD. Forming a substrate surface layer comprising:
A method for producing an epitaxial substrate, comprising: A method for producing an epitaxial substrate according to claim 8,
A method for manufacturing an epitaxial substrate, wherein the substrate surface layer is made of Al _x1 Ga _{1 -x1} N (0.8 ≦ x1 ≦ 1.0).

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