Disclosure of Invention
The present inventors have made studies in accordance with the newly disclosed technology, and have made it possible to particularly effectively improve the emission efficiency of a semiconductor light emitting device applied to the green region.
In order to solve the above problems, an object of the present invention is to manufacture a usable GaN substrate, and an epitaxial substrate and a semiconductor light emitting device using the same, so as to design a semiconductor light emitting device capable of improving emission efficiency.
In the GaN substrate according to the present invention, the growth plane is oriented off-axis according to the m-plane or the a-plane.
In the GaN substrate, the growth plane is either an m-plane or an a-plane, either of which is off-oriented (misorint). Since the m-plane and the a-plane are nonpolar planes, the semiconductor light emitting device is manufactured using the GaN substrate, avoiding the influence of the piezoelectric field, making it possible to achieve high emission efficiency. The inventors then have newly found that by providing off-axis angles based on the m-plane or a-plane, a high quality crystal structure can be achieved. As a result of these advantages, the manufacture of a semiconductor light emitting device using a GaN substrate can further improve emission efficiency.
In one aspect, the off-axis angle may be within 1.0 degree. This embodiment allows a higher quality crystal structure to be realized, enabling further improvement in the emission efficiency of the semiconductor light-emitting device.
In another aspect, the off-axis angle is in the range of 0.03 to 0.5 degrees. This embodiment enables higher emission efficiency.
In another aspect, the off-axis orientation may be tilted in the <0001> direction.
In yet another aspect, the growth surface is a plane oriented off-axis with respect to the m-plane, wherein the off-axis orientation may be tilted in the <11-20> direction. In another aspect, the growth surface is a plane oriented off-axis with respect to the a-plane, wherein the off-axis orientation may be tilted in the <1-100> direction.
The epitaxial substrate according to the present invention is characterized in that an epitaxial layer is deposited on a growth surface which is a GaN substrate surface oriented off-axis with respect to an m-plane or an a-plane.
In the epitaxial substrate, an InGaN layer is deposited on a GaN substrate, in which the growth plane is either an m-plane or an a-plane, either of which is off-oriented. Since the m-plane and the a-plane are nonpolar planes, the semiconductor light emitting device is manufactured using the epitaxial substrate, avoiding the influence of a piezoelectric field, making it possible to achieve high emission efficiency. The inventors then have newly found that providing off-axis angles with respect to the m-plane or a-plane allows for high quality crystal structures. As a result of these advantages, the semiconductor light emitting device is manufactured using the epitaxial substrate, and the emission efficiency can be further improved.
The present invention relates to a semiconductor light emitting device in which an emission layer comprising InGaN is formed on a growth surface which is a GaN substrate surface oriented off-axis with respect to an m-plane or an a-plane.
In the semiconductor light emitting device, an emission layer is deposited on a GaN substrate, wherein the growth plane is either an m-plane or an a-plane, either of which is off-oriented. Since the m-plane and the a-plane are nonpolar planes, with the semiconductor light emitting device, the influence of a piezoelectric field is avoided, and thus high emission efficiency is achieved. The present inventors then have newly discovered that high quality crystal structures can be achieved by providing off-axis angles based on either the m-plane or the a-plane. As a result of these advantages, the semiconductor light emitting device further improves emission efficiency.
The present invention makes it possible to design a semiconductor light emitting device that improves emission efficiency, as well as an epitaxial substrate and a semiconductor light emitting device that utilize the substrate.
The above and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, which is to be read in conjunction with the accompanying drawings.
Detailed Description
Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the accompanying drawings. It should be understood that the same or equivalent elements are denoted by the same reference numerals, and if the description is redundant, the description is omitted.
A process for preparing a GaN substrate utilized in manufacturing a semiconductor light emitting device according to the present invention will be described below. The GaN substrate was prepared by the HVPE reactor shown in fig. 1.
Referring next to FIG. 1, a view of an atmospheric pressure HVPE reactor 10 is shown. The reactor comprises: a reaction chamber 15 having a first gas introduction port 11, a second gas introduction port 12, a third gas introduction port 13, and an exhaust port 14; and a resistance heater 16 for heating the reaction chamber 15. Further, a Ga metal source boat 17 and a rotary support 19 supporting a GaAs substrate 18 are provided inside the reaction chamber 15.
Then, using a GaAs (111) A substrate having a diameter of about 50 to 150mm (2 to 6 inches) as the GaAs substrate 18, the temperature of the GaAs substrate 18 was increased and maintained at about 450 to 530 ℃ by the resistance heater 16, wherein 4X 10 was introduced through the second gas introduction port 12-4atm to 4X 10-3Gaseous hydrogen chloride (HCl) at atm pressure to the Ga metal source boat 17. This step generates gallium chloride (GaCl) by the reaction of Ga metal and hydrogen chloride. Next, ammonia (NH) gas having a pressure of 0.1atm to 0.3atm is introduced through the first gas introduction port 113) To make NH3And GaCl react near the GaAs substrate 18 to produce gallium nitride (GaN).
It is understood that hydrogen (H)2) Is introduced into the first gas introduction port 11 and the second gas introduction port 12 as a carrier gas, while hydrogen (H) gas2) Is introduced separately into the third gas introduction port 13. GaN was grown under conditions like this for an interval of about 20 to about 40 minutes, and a 5mm thick GaN layer was thick film deposited on top of the GaAs substrate to form a GaN ingot 20 as shown in fig. 2.
Then, the GaN ingot 20 obtained in the manner just described is sliced approximately perpendicularly with respect to the c-plane as a growth plane to cut the GaN substrate 30 utilized in the manufacture of the semiconductor light emitting device of the present embodiment. In this process, dicing is performed so as to expose m-planes (i.e., (1-100) planes), which are planes perpendicular to the c-plane as shown in FIG. 3, so that a GaN substrate in which the m-plane is a growth plane can be obtained. Also, dicing is performed so that the a-plane (i.e., (11-20) plane) is exposed, the a-plane being a plane perpendicular to the c-plane, making it possible to obtain a GaN substrate in which the a-plane is a growth plane. Since the m-plane and the a-plane are nonpolar, in an embodiment in which a semiconductor light emitting device is manufactured using a GaN substrate in which the m-plane or the a-plane is a growth plane, the influence of a piezoelectric field can be avoided, thereby enabling high emission efficiency to be achieved.
However, in cutting the GaN substrate from the GaN ingot 20, the ingot is cut in such a manner as to produce the GaN substrate 30(30A, 30B) having a predetermined off-orientation angle larger than 0 degree. Here, the GaN substrate 30A is a substrate whose growth surface is a plane oriented off-axis (> 0 degrees) with respect to the m-plane, and the GaN substrate 30B is a substrate whose growth surface is a plane oriented off-axis (> 0 degrees) with respect to the a-plane.
The GaN substrate 30A has a rectangular shape of 5mm × 20mm as shown in fig. 4. Its growth face 30a is then a plane oriented off-axis with respect to the m-plane. It should be understood that the off-axis orientation is tilted in the <0001> direction or the <11-20> direction, which are perpendicular to each other.
The GaN substrate 30B is, as shown in fig. 5, a rectangular shape of 5mm × 20mm, as is the GaN substrate 30A. Its growth face 30a, then, is a plane oriented off-axis with respect to the a-plane. It should be understood that the off-axis orientation is tilted in the <0001> direction or the <1-100> direction, which are perpendicular to each other.
Next, an epitaxial layer 32 is deposited on the growth face 30a of the GaN substrate 30 obtained in the above manner, forming an epitaxial substrate as shown in fig. 6. Epitaxial layer 32 is composed of AlGaN and is deposited using well-known thin film deposition equipment (e.g., MOCVD reactors).
Further, as shown in FIG. 7, an n-GaN buffer layer 42, an InGaN/InGaN n-MQW (multiple quantum well) emission layer 44, a p-AlGaN layer, and a p-GaN layer 48 are sequentially deposited on an epitaxial substrate 40, and then an n-electrode 50A and a p-electrode 50b are prepared to complete the fabrication of a semiconductor light emitting device 60(LED) according to the present invention. Since the semiconductor light emitting device 60 has the emission layer 44 including InGaN, it emits light of a green region of a longer wavelength than a blue region.
As a result of extensive studies, the present inventors confirmed by the following examples that improved emission efficiency can be effectively designed by using the above-described GaN substrate 30 in the fabrication of such a semiconductor light emitting device 60.
Examples
Hereinafter, the present invention will be described in further detail based on examples thereof, but the present invention is not limited to these examples.
Example 1
First, GaN substrate samples 1 to 14, which are the same as or equivalent to the above-described GaN substrate 30A — GaN substrate 5mm × 20mm, were prepared according to the same procedure as the above-described embodiment mode except for off-axis angles with respect to the m-plane as in table I below. Specifically, among samples 1 to 14, the off-orientation axis of samples 1 to 7 was the <11-20> direction, and in samples 8 to 14, the off-orientation axis was the <0001> direction. It should be noted that the orientation of the crystal plane (off-axis angle) of the GaN substrate was characterized by X-ray diffraction, and the off-axis angle measurement accuracy was ± 0.01 degrees.
TABLE I
| Off-axis angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <11-20>Direction | Sample 1 | Sample 2 | Sample 3 | Sample No. 4 | Sample No. 5 | Sample No. 6 | Sample 7 |
| <0001>Direction | Sample 8 | Sample 9 | Sample 10 | Sample 11 | Sample 12 | Sample 13 | Sample 14 |
Then, an MOCVD reactor was used to form an epitaxial layer on the growth surface of each of the above samples 1 to 14, thereby manufacturing an LED having a layered structure shown in fig. 7. The surface roughness of the sample was then measured in a 50 μm by 50 μm measurement zone using an Atomic Force Microscope (AFM), and the measurement results are listed in Table II below.
TABLE II
| Off-axis angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <11-20>Direction | Sample 1 | Sample 2 | Sample 3 | Sample No. 4 | Sample No. 5 | Sample No. 6 | Sample 7 |
| Roughness (Ra) | 17nm | 9nm | 7nm | 4nm | 5nm | 7nm | 11nm |
| <0001>Direction | Sample 8 | Sample 9 | Sample 10 | Sample 11 | Sample 12 | Sample 13 | Sample 14 |
| Roughness (Ra) | 16nm | 8nm | 5nm | 3nm | 4nm | 6nm | 10nm |
As is evident from the measurements listed in table II, the surface roughness of samples 1 and 8 is considerable, exceeding 15 nm. It is therefore clear that in embodiments where the epitaxial layer is grown on a growth plane (m-plane) that is 0 degrees off orientation, the planarity of the surface proves to be poor. When the surfaces of sample 1 and sample 8 were actually inspected, the wavy form shown in fig. 8A was observed.
It is also evident from the measurement results of table II that the roughness is slight, less than 15nm, except for samples 1 and 8.
In particular, it was found that surface planarity proved to be extremely satisfactory when an epitaxial layer was grown on a growth plane having an off-axis angle in the range of 0.03 to 1.0 degrees, as in the case of samples 2-6 and samples 9-13. When the surfaces of samples 2 to 6 and samples 9 to 13 were actually inspected, an extremely flat morphology or conversely a shallow step-like morphology as shown in fig. 8B was observed.
Meanwhile, in the embodiment in which the epitaxial layer is grown on the growth plane having an off-axis angle of 2.0 degrees, as in the case of samples 7 and 14, although the surface flatness is satisfactory, when the surfaces of samples 7 and 14 are actually inspected, a deep step-like morphology as shown in fig. 8C is observed. The cause of the stepped morphology is considered to be unevenness (scratches) of the GaN substrate growth surface.
In summary, it is apparent from the measurement results in table II that the surface flatness becomes better and better as the off-axis angle increases from the case where the angle is 0 degrees, and the best flatness is obtained with an off-axis angle of about 0.3 degrees. Then as the off-axis angle further increases from 0.3 degrees, the surface flatness decreases (the middle step spacing decreases, the step slope amplifies). An off-axis angle of 1.0 degree or less is suitable because increasing the off-axis angle makes it impossible to maintain the flatness level required for the semiconductor light emitting device.
Further, the emission-spectrum Electroluminescence (EL) intensities of the LEDs manufactured using the above-described samples 1 to 14 were measured at a peak wavelength of 450nm, and the measurement results are shown in table III below. It should be understood that the EL intensity measurements in table III give relative intensities such that the EL intensity for samples 4 and 11 is 1at 0.3 degrees off-axis angle.
TABLE III
| Separation deviceAxial angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <11-20>Direction | Sample 1 | Sample 2 | Sample 3 | Sample No. 4 | Sample No. 5 | Sample No. 6 | Sample 7 |
| EL intensity | 0.3 | 0.8 | 0.90 | 1 | 0.95 | 0.8 | 0.5 |
| <0001>Direction | Sample 8 | Sample 9 | Sample 10 | Sample 11 | Sample 12 | Sample 13 | Sample 14 |
| EL intensity | 0.2 | 0.85 | 0.95 | 1 | 0.98 | 0.8 | 0.6 |
As is apparent from the measurement results in table III, high EL intensities were obtained with samples 2 to 7 and samples 9 to 14, whereas sufficiently high EL intensities were not obtained with samples 1 and 8. The results are considered to be derived from the crystallization properties of the samples, with reference to table II measurements. That is, samples 2 to 7 and samples 9 to 14, since good crystal growth can be performed, the crystallinity of the epitaxial layer proves to be superior, and therefore, the surface flatness is considered to be superior, and high-level EL intensity is obtained. Samples with off-axis angles of 0.03 to 0.5 degrees gave particularly high levels of EL intensity. In contrast, samples 1 and 8, since satisfactory crystal growth was not performed, the crystallinity of the epitaxial layer proved to be poor, and thus it was considered that the surface flatness proved to be poor, which decreased the EL intensity.
From the above tests, it was confirmed that in the manufacture of a semiconductor light emitting device, a GaN substrate using a plane that is misaligned from the m-plane by a predetermined angle (preferably 1.0 degree or less, more preferably 0.03 to 0.5 degrees) as its growth plane can achieve superior emission efficiency.
Example 2
In a manner similar to example 1, GaN substrate samples 15 to 28, which are the same as or equivalent to the above-described GaN substrate 30B — GaN substrate 5mm × 20mm, as shown in table IV below, except for the off-axis angle with respect to the a-plane, were prepared according to the same procedure as the previously explained example mode. Specifically, the off-orientation axis of sample 15-21 was <1-100> among samples 15-28, and the off-orientation axis was <0001> direction in sample 22-28. It should be noted that the crystal plane orientation (off-axis angle) of the GaN substrate was characterized by X-ray diffraction, and the off-axis angle measurement accuracy was ± 0.01 degrees.
TABLE IV
| Off-axis angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <1-100>Direction | Sample 15 | Sample 16 | Sample 17 | Sample 18 | Sample 19 | Sample 20 | Sample 21 |
| <0001>Direction | Sample 22 | Sample 23 | Sample 24 | Sample 25 | Sample 26 | Sample 27 | Sample 28 |
Then, an epitaxial layer was formed on the growth surface of each of the above samples 15 to 28 using an MOCVD reactor, thereby manufacturing an LED having a layered structure shown in fig. 7. The surface roughness of the samples was then measured in a 50 μm by 50 μm measurement zone using an Atomic Force Microscope (AFM), and the measurement results are listed in Table V below.
TABLE V
| Off-axis angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <1-100>Direction | Sample 15 | Sample 16 | Sample 17 | Sample 18 | Sample 19 | Sample 20 | Sample 21 |
| Roughness (Ra) | 18nm | 9nm | 7nm | 4nm | 5nm | 7nm | 12nm |
| <0001>Direction | Sample 22 | Sample 23 | Sample 24 | Sample 25 | Sample 26 | Sample 27 | Sample 28 |
| Roughness (Ra) | 17nm | 8nm | 5nm | 3nm | 4nm | 6nm | 11nm |
As is evident from the measurement results listed in table V, the surface roughness of sample 15 and sample 22 is large, exceeding 15 nm. Thus, it is clear that in embodiments where the epitaxial layer is grown on a growth plane (a-plane) that is 0 degrees off orientation, the planarity of the surface proves to be poor. When the surfaces of sample 15 and sample 22 were actually inspected, the undulating pattern shown in fig. 8A was observed.
As is evident from the measurements listed in table V, the roughness is slight, less than 15nm, except for sample 15 and sample 22.
In particular, it should be recognized that surface planarity proves to be extremely good when epitaxial layers are grown on growth planes having off-axis angles in the range of 0.03 to 1.0 degrees, as is the case with samples 16-20 and samples 23-27. When the surfaces of samples 16 to 20 and samples 23 to 27 were actually inspected, an extremely flat morphology or a shallow step-like morphology as shown in fig. 8B was observed.
Meanwhile, in the embodiment in which the epitaxial layer is grown on the growth plane having an off-axis angle of 2.0 degrees, as in the case of samples 21 and 28, although the surface flatness is satisfactory, when the surfaces of samples 21 and 28 are actually inspected, a deep step-like morphology as shown in fig. 8C is observed. This stair-step morphology is thought to be due to unevenness (scratches) in the growth plane of the GaN substrate.
In summary, it is apparent from the measurement results in table V that as the off-axis angle increases from the case where the angle is 0 degrees, the surface flatness becomes better and better, and the best flatness is obtained with an off-axis angle of about 0.3 degrees. Then as the off-axis angle further increases from 0.3 degrees, the surface flatness decreases (the spacing between steps decreases and the slope of the steps increases). Since enlarging the off-axis angle means that the level of flatness required for the semiconductor light emitting device cannot be maintained, an off-axis angle of 1.0 degree or less is most suitable for this case.
Further, the emission-spectrum Electroluminescence (EL) intensities of the LEDs manufactured using the above-described samples 15 to 28 were measured at a peak wavelength of 450nm, and the measurement results are shown in table VI below. It should be noted that the EL intensity measurements in Table VI give relative intensities such that the EL intensity for samples 18 and 25 is 1at 0.3 degree off-axis angles.
TABLE VI
| Off-axis angle | 0.00 | 0.03 | 0.1 | 0.3 | 0.5 | 1.0 | 2.0 |
| <1-100>Direction | Sample 15 | Sample 16 | Sample 17 | Sample 18 | Sample 19 | Sample 20 | Sample 21 |
| EL intensity | 0.3 | 0.8 | 0.90 | 1 | 0.95 | 0.8 | 0.5 |
| <0001>Direction | Sample 22 | Sample 23 | Sample 24 | Sample 25 | Sample 26 | Sample 27 | Sample 28 |
| EL intensity | 0.2 | 0.85 | 0.95 | 1 | 0.98 | 0.8 | 0.6 |
As is apparent from the measurement results in Table VI, high EL intensities can be obtained with samples 16 to 21 and samples 23 to 28, whereas sufficiently high EL intensities cannot be obtained with samples 15 and 22. The results are measured with reference to table V and are believed to be derived from the crystallization properties of the samples. That is, with samples 16 to 21 and samples 23 to 28, since good crystal growth can be performed, the crystallinity of the epitaxial layer proves to be superior, wherein it can be considered that, as a result, the surface flatness proves to be superior, and high-level EL intensity can be obtained. In particular, samples having off-axis angles of 0.03 to 0.5 degrees gave high levels of EL intensity. In contrast, with samples 15 and 22, since satisfactory crystal growth was not performed, the crystallinity of the epitaxial layer proved to be poor, wherein it can be considered that, as a result, the surface flatness proved to be poor, thus decreasing the EL intensity.
From the above tests, it was confirmed that in the manufacture of a semiconductor light emitting device, using a GaN substrate having as its growth plane a plane that is off-oriented from the a-plane by a predetermined angle (preferably 1.0 degree or less, more preferably 0.03 to 0.5 degrees) makes it possible to achieve superior emission efficiency.
The present invention is not limited to the above-described embodiments, in which various modifications can be made. For example, the semiconductor light emitting device is not limited to an LED having an MQW emission layer, but may be an LED, a laser diode, and the like having different light emitting structures.