JP3682827B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a nitride semiconductor (In _X Al _Y Ga _1-XY N, 0.ltoreq.X, 0.ltoreq.Y, X + Y.ltoreq.1), and more particularly, to a laser element having good condensing property of emitted laser light.
[0002]
[Prior art]
In recent years, various researches and developments have been actively conducted on nitride semiconductor laser diodes (LDs), and practical LDs have also been developed.
[0003]
The present applicant has proposed a nitride semiconductor laser element capable of continuous oscillation of laser light having a short wavelength of 410 nm as a nitride semiconductor laser element. For example, Appl. Lett. 69 (1996) 3034, Appl. Phys. Lett. 69 (1996) 4056.
The nitride semiconductor laser device proposed by the present applicant is capable of emitting short-wavelength laser light and is very useful for increasing the density and capacity of optical memories.
[0004]
[Problems to be solved by the invention]
However, in the LD proposed by the present applicant, since the light confinement in the light confinement layer (cladding layer) formed with the active layer sandwiched is not complete, a part of the laser light leaks out. The reflected light is reflected by the surface of the sapphire substrate used as a substrate, guided by a nitride semiconductor layer (for example, a contact layer) formed between the cladding layer and the sapphire substrate, Radiated from the end face, there is a tendency to disturb the far field pattern and the near field pattern.
In addition to the laser waveguide centered on the active layer, the disturbance such as the far-field pattern is sandwiched between the n-conducting contact layer and the low-refractive-index n-conducting clad layer. Therefore, the light leaked from the main laser waveguide (main waveguide) propagates in the multi-mode to the contact layer on the n conductive side, and is emitted from the laser waveguide centering on the active layer. This is because it overlaps the far field pattern of the laser beam. That is, the vicinity of the active layer is a main waveguide, but has a two-story structure in which the contact layer on the n conductive side is a sub-waveguide.
[0005]
Incidentally, the refractive index of the nitride semiconductor layer constituting the nitride semiconductor laser element is, in descending order, InGaN (for example, active layer), GaN (for example, guide layer, contact layer), AlGaN (for example, clad layer), substrate (for example, Sapphire, spinel). That is, if there is a contact layer made of GaN having a relatively high refractive index between the cladding layer and the substrate, light leaking from the main waveguide is guided by the contact layer and emitted from the end face of the n-side layer. In this way, if there is a sapphire substrate with a low refractive index, the light leaking from the main waveguide is less likely to be diffused from the substrate through the n-side layer, and is emitted from the end surface of the n-side layer to form a far field pattern. Disturb.
Further, if the substrate is made of a material having a low refractive index other than sapphire, the same problem as in the case of sapphire occurs.
Furthermore, even when a GaN substrate is used, the light leaking from the laser waveguide passes through the GaN substrate, but the light that has passed through the GaN substrate is reflected by metal or air in contact with the GaN substrate, and the GaN substrate passes through the sub-waveguide. And is emitted from the end face of the GaN substrate. As a result, the far field pattern tends to be disturbed.
[0006]
Therefore, an object of the present invention is to suppress the propagation of light other than a laser waveguide such as a contact layer on the n conductive side or a GaN substrate even if sapphire having a low refractive index is used as a substrate, It is an object of the present invention to provide a nitride semiconductor laser device that has a good near field pattern.
[0007]
[Means for Solving the Problems]
That is, the object of the present invention can be achieved by the following configurations (1) to (4).
(1) A first nitride semiconductor layer having a higher refractive index than the different substrate on a different substrate different from the nitride semiconductor, and a second nitride semiconductor having a lower refractive index than the first nitride semiconductor layer on the first nitride semiconductor layer. In a nitride semiconductor laser device having a structure in which a layer and an active layer having a refractive index higher than that of the second nitride semiconductor layer are stacked thereon, the second nitride semiconductor layer and the dissimilar substrate A light absorption layer made of a material capable of absorbing light leaking from the laser waveguide including the active layer is provided, The second nitride semiconductor layer is provided on the light absorption layer formation via the first nitride semiconductor layer. A nitride semiconductor laser device characterized by the above.
(2) A first nitride semiconductor layer having a higher refractive index than that of the different substrate on a different substrate different from the nitride semiconductor, and a second nitride semiconductor having a lower refractive index than that of the first nitride semiconductor layer thereon. In a nitride semiconductor laser device having a structure in which a layer and an active layer having a refractive index higher than that of the second nitride semiconductor layer are stacked thereon, leakage from a laser waveguide including the active layer on the heterogeneous substrate A nitride semiconductor laser element characterized by having a light absorptivity capable of absorbing the emitted light.
(3) In a nitride semiconductor laser element formed by forming an element structure having an active layer on a GaN substrate, an active layer is included on the surface of the GaN substrate that does not have an element structure facing the element structure forming surface. A nitride semiconductor laser element comprising a light absorption film capable of absorbing light leaking from a laser waveguide.
(4) The nitride semiconductor laser element described above further has the following configuration. The substrate is characterized in that the substrate has light absorptivity. Further, a light absorption layer is provided between the second nitride semiconductor layer and the substrate. The light absorption layer is a layer formed by adding a new component so that light can be absorbed by the nitride semiconductor layer on the n conductive side having the same composition as the active layer, or the n conductive side layer forming the element structure. It is characterized by becoming. The first nitride semiconductor layer is GaN. Further, the second nitride semiconductor layer is made of a nitride semiconductor containing at least Al.
[0008]
That is, according to the present invention, the laser element is provided with a function capable of absorbing light leaking from the main laser waveguide centering on the active layer, so that the light leaking from the laser waveguide is coupled to the second nitride semiconductor layer. It is possible to prevent the first nitride semiconductor layer (for example, contact layer) between the different types of substrates from being guided, and also to guide the GaN substrate. Thereby, it is possible to suppress light from being emitted from the end surface on the n conductive side such as the first nitride semiconductor layer or the end surface of the GaN substrate, and a good far field pattern or the like can be obtained.
In the present invention, it is needless to say that the heterogeneous substrate, the first nitride semiconductor layer, the second nitride semiconductor layer, and the active layer may not be formed in contact with each other.
[0009]
In the case of a conventionally known LD such as red, the far field pattern of the LD such as red has a unimodal elliptical shape. This is because a conventionally known red LD uses, for example, an n-type GaAs substrate, and the GaAs substrate absorbs light, so that no light is emitted from the end face other than the laser waveguide.
On the other hand, in the case of a nitride semiconductor laser element, a material having a low refractive index such as sapphire is used for the substrate, and furthermore, since the nitride semiconductor is transparent, it transmits light. As a result, it is considered that light from a sub-waveguide other than the main waveguide near the active layer is emitted and disturbs the far field pattern of the laser light.
[0010]
On the other hand, the present inventor was able to solve the above problems by providing the nitride semiconductor laser element with a function capable of absorbing light as described above. In addition, the present invention can improve the far field pattern and the like without deteriorating the performance of the nitride semiconductor laser device.
[0011]
First, according to the present invention, when a laser element is formed using a dissimilar substrate having a low refractive index such as sapphire, a light absorption layer is provided between the second nitride semiconductor layer and the substrate, or light is applied to the dissimilar substrate. By providing absorbency, conventional problems can be solved.
When forming a laser element using a GaN substrate, the above problem can be solved by providing a light absorption film on the surface of the GaN substrate opposite to the surface on which the element structure is formed.
Further, in the present invention, a combination of providing a light absorption layer between the second nitride semiconductor layer and the substrate, providing the substrate with light absorption, and providing a light absorption film on the GaN substrate is performed. Also good.
[0012]
In the present invention, the first nitride semiconductor layer has a higher refractive index than the substrate and the second nitride semiconductor layer, and the light leaking from the laser waveguide is guided in the first nitride semiconductor layer. For example, a contact layer, and may be one or more layers and one or more layers.
In the present invention, the second nitride semiconductor layer is formed on the substrate side of the active layer and is smaller than the refractive index of the active layer and the first nitride semiconductor layer. For example, the second nitride semiconductor layer emits light emitted from the active layer. For example, a clad layer for confinement. The second nitride semiconductor layer may be one or more and one or more layers.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a nitride semiconductor laser element with a function of absorbing light leaking from a laser waveguide so that the far field pattern of the laser light is not disturbed.
1 to 8 are schematic cross-sectional views showing an embodiment in which a nitride semiconductor laser element has light absorption, and a method for forming a protective film as a light absorption layer in stages. It is a schematic cross section which shows one embodiment. However, the present invention is not limited to this.
[0014]
In FIG. 1, a first nitride semiconductor layer 101 having a higher refractive index than that of the different substrate 1 and a second nitride semiconductor layer 102 having a lower refractive index than that of the first nitride semiconductor layer 101 are formed on the different substrate 1. A laser waveguide 103 including an active layer having a higher refractive index than that of the second nitride semiconductor layer 102, and a nitride semiconductor laser having a heterogeneous substrate in which one or more p-conductivity type nitride semiconductor layers 104 are sequentially stacked. It is a schematic cross section of an element. Further, the nitride semiconductor laser element of FIG. 1 is provided with a light absorption function by forming a light absorption layer 105 so that light leaking from the laser waveguide 103 including the active layer can be absorbed.
[0015]
As a function capable of absorbing light, as shown in FIG. 1, a light absorption layer (a protective film 105 described later is shown as a light absorption layer in FIG. 1) between the second nitride semiconductor layer 102 and the heterogeneous substrate 1 is used. It may be formed and / or colored by doping light-absorbing impurities into the heterogeneous substrate 1 or the like.
[0016]
Hereinafter, the formation of a light absorption layer between the second nitride semiconductor layer 102 and the heterogeneous substrate 1 will be described as a light absorption function.
In the present invention, the light absorption layer may be formed anywhere between the heterogeneous substrate 1 and the second nitride semiconductor layer 102. The light absorbing layer is not particularly limited as long as it can absorb light leaking from the laser waveguide without degrading the performance of the laser element. For example, the light absorption function of the protective film 105 used when forming a nitride semiconductor layer (GaN underlayer) with a low crystal defect by lateral growth of a nitride semiconductor [Lateral over growth (LOG), lateral growth]. Or a semiconductor layer capable of absorbing light between the second nitride semiconductor layer 102 and the heterogeneous substrate 1 may be used.
In the present invention, the lateral growth method is not particularly limited, and examples thereof include a method performed using a protective film 105 described later. The protective film 105 may be any material as long as the nitride semiconductor is difficult to grow and can absorb light leaking from the laser waveguide.
[0017]
In the present invention, when the light absorbing layer is a light absorbing semiconductor layer, a light absorbing semiconductor layer that does not deteriorate the performance of the laser element is preferable. For example, the light absorbing layer has the same composition as the active layer. Examples thereof include a layer formed by adding a new component so that light can be absorbed in the n-conductivity-side nitride semiconductor layer or the n-conductivity-side layer forming the element structure.
[0018]
In the present invention, the case where the light absorption layer is a protective film 105 used for growth of a nitride semiconductor provided with a light absorption function will be described below.
In the present invention, the protective film 105 used includes the protective film 105 used in the lateral growth method of nitride semiconductor. As the protective film material, a material having a property that the nitride semiconductor does not grow or hardly grow on the surface of the protective film 105 and has a light absorption property is used.
As the protective film 105 used in the lateral growth method, for example, silicon oxide (SiO _X ), Silicon nitride (Si _X N _Y ), Titanium oxide (TiO _X ), Zirconium oxide (ZrO) _X In addition to oxides and nitrides such as), and multilayer films of these, metals having a melting point of 1200 ° C. or higher can be used. These protective film materials can withstand the nitride semiconductor growth temperature of 600 ° C. to 1100 ° C., and the nitride semiconductor does not grow or hardly grow on the surface thereof. However, all of these protective film materials can be used as protective films used for lateral growth, but there are materials that do not exhibit light absorption and materials that exhibit light absorption. As described above, when a material that does not exhibit light absorption is used, a protective film that can absorb light can be obtained by using in combination with a protective film material that exhibits light absorption or another material that exhibits light absorption. The protective film material is appropriately selected according to the wavelength of light leaking from the laser waveguide.
[0019]
Among the protective film materials described above, for example, titanium oxide (TiO _X ) Is a protective film material exhibiting light absorption, and acts as a protective film and a light absorption layer. When such a light-absorbing protective film material is used, the device manufacturing process can be simplified.
Further, among the above protective film materials, examples of materials that do not exhibit light absorption include silicon oxide (SiO 2). _X ), Silicon nitride (Si _X N _Y ), Zirconium oxide (ZrO) _X ) Etc. When such a protective film material that does not absorb light is used, it cannot be used alone as a protective film as described above. For example, as shown in FIG. 1, it may be on a heterogeneous substrate 1 (may be in contact with the heterogeneous substrate 1). The material that absorbs light, such as Si shown below, is formed on silicon oxide (SiO2). _x 1), the light leaking from the laser waveguide 103 having the active layer can be absorbed by forming the protective film 105 (light absorption layer) having a two-layer structure as shown in FIG. At the same time, it functions as a protective film in the lateral growth method.
As materials other than the above-described light-absorbing protective film material used when a protective film material that does not exhibit light absorption is used, a material having a light-absorbing property that does not adversely affect the performance of the element is used. For example, Si etc. are mentioned.
[0020]
In the present invention, the thickness of the protective film formed on the substrate is a thickness that can absorb extra light that disturbs the far-field pattern and the like, and a nitride semiconductor can be favorably grown in the nitride semiconductor growth method. Film thickness is preferred. The thickness of the protective film is, for example, 0.1 to 10 μm, preferably 0.5 to 8 μm, and more preferably 1 to 5 μm. When the thickness of the protective film is within this range, the growth of the nitride semiconductor can be improved, and the light leaking from the laser waveguide can be absorbed well, and the disturbance of the far field pattern and the like can be prevented.
When a protective film material that does not exhibit light absorption and a protective film material that has light absorption or other materials are used in combination, the material having light absorption and the total film thickness that does not exhibit light absorption are within the above range. It is preferable that For example, the film thickness of the light-absorbing material is 0.3 to 10 μm, preferably 0.4 to 4 μm, more preferably 0.5 to 2 μm, and light leaking from the laser waveguide is within this range. Can be absorbed well. The film thickness of the material that does not exhibit light absorption is 1 μm or less, preferably 0.5 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less. The lower limit is not particularly limited, but one molecule or What is necessary is just to be laminated | stacked on the surface of the material which has a light absorptivity so that the nitride semiconductor may not grow on a protective film with the thickness more than the magnitude | size of 1 atom.
[0021]
In order to form the protective film material on the surface of the nitride semiconductor, for example, vapor deposition techniques such as vapor deposition, sputtering, and CVD can be used.
[0022]
Although it does not specifically limit as a lateral growth method, One Embodiment using the protective film which has a light absorptivity below is shown.
Here, in the present invention, the heterogeneous substrate may have light absorption, and a protective film may be formed on the heterogeneous substrate to perform lateral growth of the nitride semiconductor. In the case of showing, the protective film may not have light absorption.
[0023]
As a first method of lateral growth of a nitride semiconductor using a protective film, the surface of the nitride semiconductor layer is grown after or before the growth of the nitride semiconductor on a different substrate made of a material different from the nitride semiconductor. Alternatively, a protective film having a property in which a nitride semiconductor does not easily grow in the vertical direction is formed on the surface of a different substrate, for example, in a stripe shape, a dot shape, a grid shape, etc., and the nitride semiconductor is formed on the protective film. This is a method of growing the surface in the horizontal direction. In the first method, when the protective film is formed, when the protective film forming area and the exposed area (window part) are compared, the area of the thick film with many regions having few crystal defects is reduced when the area of the window part is reduced. A nitride semiconductor layer (underlayer) tends to be obtained.
[0024]
As a second method of growing a nitride semiconductor using a protective film, an uneven part is formed on the surface of a nitride semiconductor grown on a different substrate, and the protective film is formed on the flat surface of the convex part and the concave part. Then, lateral growth is performed from the nitride semiconductor exposed on the side surface, and the nitride semiconductors grown in the lateral direction are connected to the upper portion of the protective film.
[0025]
In any of the above methods, by forming the protective film, it is possible to stop the crystal defects of the nitride semiconductor that are caused by factors such as irregular lattice constants and differences in thermal expansion coefficients between the heterogeneous substrate and the nitride semiconductor. In other words, a nitride semiconductor formed on a heterogeneous substrate made of a material different from a nitride semiconductor and grown laterally on a stripe-shaped protective film having a property that the nitride semiconductor is difficult to grow in the vertical direction is grown. Initially, it has a region with many crystal defects and a region with few crystal defects. This is because when a nitride semiconductor is grown again on the protective film and the window portion (the portion where the protective film is not formed) after the protective film is formed, the nitride semiconductor is nitrided laterally from the nitride semiconductor under the window portion. This is because the growth of the nitride semiconductor is promoted to grow the nitride semiconductor to the top of the protective film. Crystal defects generated from the interface between the heterogeneous substrate and the nitride semiconductor layer tend to dislocation at the upper part of the window, but most of them do not dislocation in the thickness direction at the upper part of the protective film. The crystal defects of the GaN underlayer thus obtained are, for example, 1 × 10 6 at the upper part of the window. ⁸ Piece / cm ² For example, in the upper part of the protective film, 1 × 10 ⁷ Piece / cm ² The following conditions are satisfied, and under preferable conditions, 5 × 10 ⁶ Piece / cm ² Hereinafter, in more preferable conditions, 1 × 10 ⁶ Piece / cm ² Hereinafter, under the most preferable conditions, 5 × 10 ^Five Piece / cm ² It is desirable that
[0026]
For example, when a stripe-shaped protective film is formed, in the lateral growth of the nitride semiconductor, the nitride semiconductor grows from both sides (stripe width direction) on the protective film and is connected, for example, at the center of the stripe. In the GaN underlayer formed in this way, the number of crystal defects in the initial stage of growth is remarkably different between the upper part of the window part and the upper part of the stripe-shaped protective film. That is, in this GaN underlayer, many crystal defects in the initial stage of growth are generated in the upper part of the window. For example, the number of crystal defects at the top of the window is 1 × 10 ⁸ Piece / cm ² Above, 1 × 10 above the protective film ⁷ Piece / cm ² It becomes the following. The preferred number with few crystal defects is as described above. This crystal defect can be measured, for example, by measuring the number of etch pits exposed on the etched surface when a nitride semiconductor is dry etched. In the nitride semiconductor laser device of the present invention, when the protective film is used, there are many crystal defects in the window, that is, in the early stage of growth, and most of the crystal defects are located above the portion where dislocation is interrupted during the growth. It is preferable to reduce the area of the active layer, and in particular, to provide a laser oscillation region above the region with few crystal defects without providing an oscillation region in this portion.
[0027]
FIGS. 2 to 4 show a case in which a nitride semiconductor thick film layer (GaN substrate or GaN underlayer) is formed using a protective film on a different kind of substrate using a nitride semiconductor by the first method. It is typical sectional drawing which shows the structure of a physical semiconductor wafer. In these figures, 1 is a heterogeneous substrate, 2 is a first GaN layer, 3 is a second GaN layer, and 11 is a protective film (a protective film having a two-layer structure is illustrated). The second GaN layer 3 is the GaN underlayer. Based on these drawings, an example of a method for producing a GaN underlayer using a light-absorbing protective film will be described.
[0028]
As shown in FIG. 2, the first GaN layer 2 is grown on the surface of the heterogeneous substrate 1 with a film thickness of, for example, 10 μm or less. The first GaN layer is a layer grown directly on the substrate or via a buffer layer, and crystal defects are present in all cross sections, for example, 1 × 10 6. ⁸ Piece / cm ² Because of the above, it cannot be a GaN substrate or a GaN underlayer. The heterogeneous substrate 1 uses sapphire described later.
Before the first GaN layer 2 is grown, a low temperature growth buffer layer, such as GaN or AlN, having a lower temperature than the growth temperature of the first GaN layer is grown on the heterogeneous substrate 1 to a thickness of 0.5 μm or less. You can also.
[0029]
Next, a protective film 11 having a property that the nitride semiconductor does not grow in the vertical direction or is difficult to grow on the first GaN layer 2 is formed in a stripe shape, for example. When the stripe width is larger than the exposed portion of the first GaN layer, that is, the portion where the protective film is not formed (window portion), the second GaN layer 3 with fewer crystal defects grows when the area of the protective film is made larger. This is convenient for setting the laser oscillation portion.
As the material of the protective film 11, the above-described materials are used. When the material of the protective film is light-absorbing, the protective film material is formed as a single layer. On the other hand, when the protective film material does not have light absorption, a material other than the light absorption protective film material or the light absorption protective film material is placed on the substrate (it may not be in contact with the substrate). Then, a protective film material having no light absorption property is formed thereon to form a protective film having a two-layer structure as shown in FIG. For example, silicon oxide (SiO _X ) As a protective film material, titanium oxide (TiO 2) exhibiting light absorption on the substrate. _X ) Or Si, etc., and silicon oxide (SiO _X ) To form a protective film 11 having a two-layer structure as shown in FIG.
Here, in the present invention, when the protective film has light absorption, silicon oxide (SiO 2) _X ) Cannot absorb light and is not used alone as a material for a protective film having light absorption. However, when a protective film is formed on this substrate by providing a different substrate with light absorption, silicon oxide (SiO 2) having no light absorption as the protective film is used. _X ) May be used alone, or a light-absorbing material may be used.
[0030]
FIG. 2 shows a partial cross-sectional view when a stripe-shaped protective film is formed on the first GaN layer 2 and the wafer is cut in a direction perpendicular to the stripe. 1 schematically shows a thin line shown inside the GaN layer 2. As shown in this figure, innumerable crystal defects are generated in the first GaN layer 2 almost uniformly, so that it is impossible to form a GaN substrate or a GaN underlayer. The stripe width of this protective film is adjusted to 1 μm or more, more preferably 2 μm or more, and most preferably 5 μm or more. If it is smaller than 1 μm, the region having few crystal defects tends to be small, and it tends to be difficult to secure the laser oscillation region on the region having few crystal defects. The upper limit of the stripe width is not particularly limited, but it is usually desirable to adjust it to 100 μm or less.
[0031]
A second GaN layer 3 is further grown on the wafer on which the protective film 11 is formed. As shown in FIG. 3, when the second GaN layer 3 is grown on the first GaN layer 2 on which the protective film 11 is formed, a GaN layer grows on the first protective film 11 first. Instead, the second GaN layer 3 is selectively grown on the first GaN layer 2 in the window. FIG. 3 shows this state, and shows that a large amount of GaN grows in the window portion at the initial stage of growth and hardly grows on the first protective film 11.
[0032]
However, if the growth of the second GaN layer 3 is continued, the second GaN layer 3 grows laterally on the first protective film 11 and is connected by the adjacent second GaN layers 3. As shown in FIG. 4, the state is as if the second GaN layer 3 has grown on the protective film 11. The second GaN layer 3 grown in this manner has a tendency to reduce crystal defects as the film thickness increases, and crystal defects (penetration transition) appearing on the surface are the same as the conventional one without a protective film. Compared to very few. However, the number of crystal defects in the upper part of the window part and the upper part of the protective film at the initial growth stage of the second GaN layer 3 is significantly different. That is, the portion of the second GaN layer 3 grown on the portion (window) where the first protective film 11 is not formed on the upper portion of the different substrate is crystallized from the interface between the different substrate 1 and the GaN layer 2. Although the defects tend to dislocation, the portion of the second GaN layer 3 grown on the protective film 11 has few crystal defects dislocations in the vertical direction. In FIG. 4, the plurality of thin lines shown from the substrate toward the surface of the first nitride semiconductor layer schematically show crystal defects as in FIGS. That is, most of the crystal defects grown from the window part are dislocated toward the surface of the second GaN layer 3 in the initial stage of the growth, but dislocations in the surface direction are continued as the growth of the second GaN layer 3 is continued. The number of crystal defects tends to decrease, and the number of crystal defects that dislocation to the surface is very small. Therefore, there are 10 crystal defects at the top of the window at the beginning of growth. ⁸ Piece / cm ² In contrast to the above, it is 10 above the protective film. ⁷ Piece / cm ² It becomes smaller as below.
[0033]
5 and 6 show a method for producing a GaN foundation layer by the second method. In this method, unevenness is provided on the surface of the first GaN layer 2 grown directly on a different substrate or via a low-temperature growth buffer layer. Then, as shown in FIG. 5, when the protective films 11 and 11 ′ are formed on the flat portion of the concavo-convex portion and the second GaN layer 3 is grown, the first GaN exposed on the end face as shown in FIG. From the layer 2, the second GaN layer grows in the lateral direction and is connected to the upper part of the protective film 11 ′, and then grows upward from there. Furthermore, it is considered that the second GaN layer grew in the lateral direction on the protective film 11 and connected to form the GaN foundation layer 3. In the case of the second method, since the second GaN layer 3 is grown from the side surface portion of the first GaN layer 2, it is clearly divided into a region with many crystal defects and a region with few crystal defects as in the first method in the initial stage of growth. The total number of crystal defects tends to be smaller than that of the first method. However, the manufacturing method of the GaN underlayer described above is merely an example, and the GaN underlayer of the laser element of the present invention when the protective film is used is not restricted by the above two manufacturing methods. Either one of the protective films 11 and 11 ′ may exhibit light absorption, and preferably both of them exhibit light absorption.
[0034]
FIG. 7 shows a more preferable manufacturing method of the GaN underlayer. After the growth of the second GaN layer 3 shown in FIG. A second protective film 12 is formed so as to cover the corresponding surface and crystal defects appearing on the surface, and a third GaN layer 4 is grown laterally on the protective film. By doing so, further dislocation of crystal defects appearing on the surface of the second GaN layer 3 can be prevented, and re-dislocation of crystal defects to the active layer or the like that causes deterioration of the light emitting element can be prevented. Thus, a GaN underlayer having fewer crystal defects than the second GaN layer 3 is obtained.
At this time, the area of the second protective film 12 is larger than the area of the window of the first protective film 11, and preferably larger than the area of the first protective film. Further, one of the protective films 11 and 12 only needs to exhibit light absorption, and preferably both exhibit light absorption.
[0035]
Thus, when growing a nitride semiconductor using lateral growth of a nitride semiconductor on a heterogeneous substrate, by using a protective film having a light absorption property as a protective film, the nitride having few crystal defects In addition to being able to form a device structure with few crystal defects on the semiconductor underlayer, the protective film can absorb the light leaking from the laser waveguide, so that the far field pattern, etc. is good without degrading the performance of the laser device Is possible.
[0036]
Here, the protective film used in the present invention may be a lateral growth of a nitride semiconductor by forming a protective film on a heterogeneous substrate having a light absorption property. It does not have to have light absorption.
[0037]
Next, a description will be given of a case where the light absorbing layer is provided with a light absorbing property that allows the different substrate 1 to absorb light leaking from the laser waveguide.
Any method can be used as long as the heterogeneous substrate 1 can absorb light, as long as the heterogeneous substrate 1 can absorb light. For example, an impurity is added to the heterogeneous substrate 1 to give the light absorption property. It is done.
In the present invention, the heterogeneous substrate 1 has a lower refractive index than the first nitride semiconductor layer, which can cause the problems described in the above-mentioned problems of the present invention, and cannot absorb light leaking from the laser waveguide. A dissimilar substrate 1 made of a material, for example, sapphire or spinel (MgA1) having a C-plane, R-plane or A-plane as a main surface. ₂ O _Four ), An insulating substrate such as ZnS, ZnO, or the like.
A material that can absorb light leaking from the laser waveguide, for example, a material having a color, as the heterogeneous substrate of nitride semiconductor is less likely to cause the problem described in the subject of the present invention. The present invention is effective in solving the problem that occurs when using a different substrate that has a low refractive index and does not have light absorption.
[0038]
The impurity added to the material of the different substrate is used together with the material of the different substrate at the time of forming a substance to be the different substrate, and is taken into the formed different substrate. Any impurity may be used as long as it is added to a different substrate and has a function of absorbing light. For example, a metal (including ions), specifically chromium (for example, Cr) ³⁺ ) And titanium.
Further, the added amount of the impurities may be added to such an extent that the substrate exhibits light absorption, and specifically, 0.01 to 0.15% by weight, preferably 0.03 to 0.10. % By weight, more preferably 0.04 to 0.07% by weight. It is preferable for the amount of impurities added to be in the above range because light can be absorbed well.
[0039]
Next, in the nitride semiconductor laser device having the device structure 202 including the active layer on the substrate (GaN substrate) 201 made of GaN using the schematic cross-sectional view of the nitride semiconductor laser device shown in FIG. The formation of the light absorption film 203 capable of absorbing the light generated and leaked in the active layer on the surface having no element structure 202 of 201 will be described.
When the GaN substrate 201 is used as the substrate, the refractive index of the semiconductor layer forming the element structure 202 is approximately the same as the refractive index of the GaN substrate, so that light leaking from the laser waveguide passes through the GaN substrate 201. . However, leakage from the laser waveguide was caused by metal (not shown) or air provided in contact with the GaN substrate surface opposite to the surface having the element structure 202 (hereinafter referred to as the opposite surface of the GaN substrate). The light is reflected, guided through the GaN substrate 201, and emitted from the end face of the GaN substrate 201 to disturb the far field pattern.
On the other hand, the present invention can solve the problem by forming a light absorption film 203 capable of absorbing light that has passed through the GaN substrate 201 on the opposite surface of the GaN substrate 201.
[0040]
The light absorbing film 203 formed on the opposite surface of the GaN substrate 201 may be made of a material that can absorb at least light, and is preferably a colored film or an opaque film having a higher refractive index than the GaN substrate. Examples of the material of the light absorption film include GaAs, SiC, Si, and TiO. ₂ , Carbon and the like. The light absorption film 203 is formed by appropriately selecting a material that easily absorbs light generated by the active layer.
As a method of forming the light absorption film 203, there is a so-called wafer bonding method in which the bonding surface of the light absorption film 203 and the bonding surface of the GaN substrate are used as mirror surfaces and the mirror surfaces are bonded to each other and then thermocompression bonded.
The film thickness of the light absorption film 203 is 0.1 μm or more, preferably 0.4 μm or more. The upper limit of the film thickness of the light absorption film 203 is not particularly limited, but the cost and the size of the apparatus are taken into consideration. Therefore, it is preferably about 10 μm or less. When the film thickness of the light absorption film 203 is within this range, the light generated and leaked in the active layer can be absorbed well, and the heat dissipation is also good. In addition, depending on the material of the light absorption film, there are a material that has a color even if it is a thin film, and a material that does not have a color unless it has a certain thickness, and the film thickness is appropriately adjusted depending on the type of material.
[0041]
In the present invention, the GaN substrate may be formed by any method. For example, a thick nitride semiconductor is formed by using a method of forming a thick nitride semiconductor layer by forming a protective film on the heterogeneous substrate. It is obtained by forming a layer and then removing the foreign substrate and the protective film. Here, when a protective film is used when forming a GaN substrate provided with a light absorption film on a surface opposite to the surface having the element structure, the protective film may not have light absorption. Therefore, the protective film that can be used is appropriately selected from materials having and not having light absorption.
[0042]
In the present invention, a nitride having a heterogeneous substrate having a light absorbing function, a light absorbing layer formed between the second nitride semiconductor layer and the heterogeneous substrate, and a light absorbing film on the opposite surface of the GaN substrate The layer structure of the nitride semiconductor to be a semiconductor laser element is not particularly limited, and any layer structure may be used, and the shape of the element is not particularly limited. Also, there are no particular limitations on the electrodes other than the layer configuration, such as the electrodes formed on the laser element and the electrode formation positions.
[0043]
The layer structure constituting the nitride semiconductor device of the present invention is preferably a superlattice-structured cladding layer (n-conducting side and p-conducting side cladding layers are shown in contact with or away from the active layer. The nitride semiconductor layer 2 is an n-conducting cladding layer).
As the superlattice structure cladding layer, a superlattice layer in which a nitride semiconductor layer having a large band gap energy and a nitride semiconductor layer having a smaller band gap energy than a nitride semiconductor layer having a large band gap energy are stacked. The nitride semiconductor layer having a large band gap energy and the nitride semiconductor layer having a small band gap energy have different n-type impurity concentrations.
[0044]
The superlattice layer may be formed in a layer other than the cladding layer. For example, in the case of a photoelectric conversion element such as a light emitting element or a light receiving element, a buffer layer and an n electrode formed in contact with the substrate are formed. The n-side contact layer, the n-side cladding layer for carrier confinement, and the n-side light guide layer for guiding the light emission of the active layer, etc., and the p-layer side The p-side contact layer on which the p-electrode is formed, the p-side cladding layer as carrier confinement, and the p-side light guide layer that guides the light emission of the active layer.
[0045]
Further, the n-type impurity or p-type impurity of the superlattice layer may be heavily doped on the nitride semiconductor layer side having a large band gap energy, or may be heavily doped on the nitride semiconductor layer side having a small band gap energy. .
[0046]
When doping a nitride semiconductor layer having a large band gap energy with a large amount of n-type impurities, the n-type impurity concentration of the nitride semiconductor layer having a large band gap energy is 1 × 10 ¹⁷ /cm ^Three ~ 1x10 ²⁰ /cm ^Three And the n-type impurity concentration of the nitride semiconductor layer having a small band gap energy is 1 × 10 ¹⁹ /cm ^Three Hereinafter, the impurity concentration is in the relationship of a nitride semiconductor having a large band gap energy> a nitride semiconductor having a small band gap energy. The nitride semiconductor layer having a small band gap energy is preferably 1 × 10. ¹⁸ /cm ^Three Or less, more preferably 1 × 10 ¹⁷ /cm ^Three In the following, it is most preferable to undo, that is, a state in which impurities are not intentionally doped.
[0047]
In addition, when the nitride semiconductor layer having a large band gap energy is doped with a large amount of p-type impurities, the p-type impurity concentration of the nitride semiconductor layer having a large band gap energy is 1 × 10. ¹⁸ /cm ^Three ~ 1x10 ^{twenty one} /cm ^Three And the p-type impurity concentration of the nitride semiconductor layer having a small band gap energy is 1 × 10 ²⁰ /cm ^Three Hereinafter, the impurity concentration is in the relationship of a nitride semiconductor having a large band gap energy> a nitride semiconductor having a small band gap energy. A nitride semiconductor layer having a small band gap energy is 1 × 10. ¹⁹ /cm ^Three Or less, more preferably 1 × 10 ¹⁸ /cm ^Three In the following, it is most preferable to undo, that is, a state in which impurities are not intentionally doped.
[0048]
Further, the impurity doped in the nitride semiconductor layer having a large band gap energy has a large impurity concentration near the center of the semiconductor layer and a small impurity concentration near both ends in the thickness direction.
[0049]
When the nitride semiconductor layer having a large band gap energy is doped with a small amount of n-type impurities, the n-type impurity concentration of the nitride semiconductor layer having a large band gap energy is 1 × 10. ¹⁹ /cm ^Three The n-type impurity concentration of the nitride semiconductor layer having a small band gap energy is 1 × 10 ¹⁷ /cm ^Three ~ 1x10 ²⁰ /cm ^Three (However, the impurity concentration is in the relationship of a nitride semiconductor having a large band gap energy <a nitride semiconductor having a small band gap energy). The nitride semiconductor layer having a large band gap energy is preferably 1 × 10. ¹⁸ /cm ^Three Or less, more preferably 1 × 10 ¹⁷ /cm ^Three In the following, it is most preferable to undo, that is, a state in which impurities are not intentionally doped.
[0050]
Further, when the nitride semiconductor layer having a large band gap energy is doped with a small amount of p-type impurities, the p-type impurity concentration of the nitride semiconductor layer having a large band gap energy is 1 × 10 5. ²⁰ /cm ^Three The p-type impurity concentration of the nitride semiconductor layer having a small band gap energy is 1 × 10 ¹⁸ /cm ^Three ~ 1x10 ^{twenty one} /cm ^Three (However, the impurity concentration is in the relationship of a nitride semiconductor having a large band gap energy <a nitride semiconductor having a small band gap energy). A nitride semiconductor layer having a large band gap energy is 1 × 10 ¹⁹ /cm ^Three Or less, more preferably 1 × 10 ¹⁸ /cm ^Three In the following, it is most preferable to undo, that is, a state in which impurities are not intentionally doped.
[0051]
Further, the impurity doped in the nitride semiconductor layer having a small band gap energy has a large impurity concentration near the center of the semiconductor layer and a small impurity concentration near both ends in the thickness direction.
[0052]
The thickness of the nitride semiconductor layer having a large band gap energy and the nitride semiconductor layer having a small band gap energy constituting the superlattice layer is 100 angstroms or less, more preferably 70 angstroms or less, and most preferably 10 to 40 angstroms. Adjust to. When it is thicker than 100 angstroms, the nitride semiconductor layer having a large band gap energy and the nitride semiconductor layer having a small band gap energy have a film thickness exceeding the elastic strain limit, and tend to have minute cracks or crystal defects in the film. It is in. The lower limit of the thickness of the nitride semiconductor layer having a large band gap energy and the nitride semiconductor layer having a small band gap energy is not particularly limited, and may be one atomic layer or more, but is most preferably 10 angstroms or more as described above. .
The nitride semiconductor layer having a large band gap energy is a nitride semiconductor containing at least Al, preferably Al _X Ga _1-X It is desirable to grow N (0 <X ≦ 1). On the other hand, the nitride semiconductor having a small band gap energy may be any nitride semiconductor having a lower band gap energy than a nitride semiconductor having a large band gap energy, but preferably Al. _Y Ga _1-Y N (0 ≦ Y <1, X> Y), In _Z Ga _1-Z Binary mixed crystal and ternary mixed crystal nitride semiconductors such as N (0 ≦ Z <1) are easy to grow, and those with good crystallinity are easily obtained. Among them, a nitride semiconductor having a large band gap energy is particularly preferably Al containing substantially no In or Ga. _X Ga _1-X N (0 <X <1), and a nitride semiconductor having a small band gap energy is substantially free of Al. _Z Ga _1-Z N (0 ≦ Z <1), and for the purpose of obtaining a superlattice excellent in crystallinity, Al having an Al mixed crystal ratio (Y value) of 0.3 or less _X Ga _1-X The combination of N (0 <X ≦ 0.3) and GaN is most preferable.
[0053]
When forming a clad layer as an optical confinement layer and a carrier confinement layer, it is necessary to grow a nitride semiconductor having a larger band gap energy than the well layer of the active layer. The nitride semiconductor layer having a large band gap energy is a nitride semiconductor having a high Al mixed crystal ratio. Conventionally, when a nitride semiconductor having a high Al mixed crystal ratio is grown as a thick film, cracks are easily generated, and thus crystal growth is very difficult. However, when a superlattice layer is formed as in the present invention, even if the single layer constituting the superlattice layer is a layer having a slightly higher Al mixed crystal ratio, it is grown with a film thickness less than the elastic critical film thickness, so that cracks are not easily generated. . Therefore, since a layer having a high Al mixed crystal ratio can be grown with good crystallinity, the optical confinement effect and the carrier confinement effect are enhanced, and the threshold voltage can be lowered.
[0054]
In addition, if the impurity concentration of the nitride semiconductor layer having a large band gap energy of the cladding layer is different from that of the nitride semiconductor layer having a low band gap energy, so-called modulation doping, the impurity concentration of one layer is reduced, preferably doped with impurities. If the other is doped at a high concentration in a state where it is not (undoped), the threshold voltage can be lowered. This is because when a layer having a low impurity concentration is present in the superlattice layer, the mobility of the layer is increased, and a layer having a high impurity concentration is also present at the same time, so that the superlattice remains at a high carrier concentration. By being able to form a layer. In other words, a layer having a high impurity concentration and a high mobility and a layer having a high impurity concentration and a high carrier concentration are present at the same time, so that a layer having a high carrier concentration and a high mobility becomes a cladding layer. Is estimated to decline.
[0055]
When a nitride semiconductor layer having a large band gap energy is doped with a high concentration of impurities, this modulation doping generates a two-dimensional electron gas between the high impurity concentration layer and the low impurity concentration layer. It is presumed that the resistivity decreases due to the influence.
For example, in a superlattice layer in which a nitride semiconductor layer having a large band gap doped with an n-type impurity and an undoped nitride semiconductor layer having a small band gap are stacked, a layer doped with an n-type impurity and an undoped layer The barrier layer side is depleted at the heterojunction interface, and electrons (two-dimensional electron gas) accumulate at the interface around the thickness of the layer side having a small band gap. Since the two-dimensional electron gas can be generated on the side having a small band gap, the electrons are not scattered by impurities when they travel, so that the mobility of electrons in the superlattice increases and the resistivity decreases.
In the case of the p layer, AlGaN has a higher resistivity than GaN. Therefore, since the resistivity is lowered by doping a large amount of p-type impurities into AlGaN, the substantial resistivity of the superlattice layer is lowered. Therefore, when an element is manufactured, the threshold tends to be lowered. Inferred.
[0056]
When the p-side cladding layer 17 has a superlattice structure, the action of the superlattice structure on the light emitting element is the same as that of the n-side cladding layer 12, but in addition to the case where it is formed on the n-layer side, the following is performed. There is an effect. That is, the resistivity of the p-type nitride semiconductor is usually two orders of magnitude higher than that of the n-type nitride semiconductor. Therefore, when the superlattice layer is formed on the p layer side, the threshold voltage is significantly reduced. More specifically, it is known that a nitride semiconductor is a semiconductor in which p-type crystals are very difficult to obtain. A technique for removing hydrogen by annealing a nitride semiconductor layer doped with a p-type impurity to obtain a p-type crystal is known (Japanese Patent No. 2540791). However, even if p-type is obtained, the resistivity is several Ω · cm or more. Therefore, by using this p-type layer as a superlattice layer, the crystallinity is improved and the resistivity is lowered by one digit or more, so that the threshold voltage is likely to be lowered.
[0057]
When the nitride semiconductor layer having a small band gap energy is doped with an impurity at a high concentration, it is presumed that the following effects are obtained.
For example, when the AlGaN layer and the GaN layer are doped with the same amount of Mg, the AlGaN layer has a large Mg acceptor level depth and a low activation rate. On the other hand, the acceptor level of the GaN layer is shallower than the AlGaN layer, and the activation rate of Mg is high. For example, Mg is 1 × 10 ²⁰ /cm ^Three 1 × 10 for GaN even if doped ¹⁸ /cm ^Three AlGaN has a carrier concentration of about 1 × 10 ¹⁷ /cm ^Three Only a moderate carrier concentration can be obtained. Therefore, in the present invention, a superlattice with a high carrier concentration can be obtained by forming a superlattice with AlGaN / GaN and doping more impurities into the GaN layer that can obtain a high carrier concentration. In addition, since the superlattice is used, carriers move through the AlGaN layer having a low impurity concentration due to the tunnel effect, so that the carriers are not substantially affected by the AlGaN layer, and the AlGaN layer functions as a cladding layer having a high band gap energy. Therefore, even if the nitride semiconductor layer having the smaller band gap energy is doped with a large amount of impurities, it is very effective in reducing the threshold value of the laser device. Although this description has been given of an example in which a superlattice is formed on the p-type layer side, the same effect can be obtained when a superlattice is formed on the n-layer side.
[0058]
When doping a nitride semiconductor layer having a large band gap energy with a large amount of n-type impurities, a preferable doping amount to the nitride semiconductor layer having a large band gap energy is 1 × 10 ¹⁷ /cm ^Three ~ 1x10 ²⁰ /cm ^Three More preferably 1 × 10 ¹⁸ /cm ^Three ~ 5x10 ¹⁹ /cm ^Three Adjust to the range. 1 × 10 ¹⁷ /cm ^Three Is less than the nitride semiconductor layer having a small band gap energy, it tends to be difficult to obtain a layer having a high carrier concentration. ²⁰ /cm ^Three If it is more, the leakage current of the element itself tends to increase. On the other hand, the n-type impurity concentration of the nitride semiconductor layer having a small band gap energy should be less than that of the nitride semiconductor layer having a large band gap energy, and preferably 1/10 or less. Most preferably, when undoped, a layer with the highest mobility is obtained, but since the film thickness is thin, there is an n-type impurity diffused from the side of the nitride semiconductor having a large band gap energy, and the amount is 1 × 10 ¹⁹ /cm ^Three The following is desirable. As the n-type impurity, elements of Group IVB and VIB of the periodic table such as Si, Ge, Se, S, and O are selected. Preferably, Si, Ge, and S are n-type impurities. This effect is the same when the nitride semiconductor layer having a large band gap energy is doped with a small amount of n-type impurities and the nitride semiconductor layer having a small band gap energy is doped with a large amount of n-type impurities.
[0059]
A preferable doping amount when doping a nitride semiconductor layer having a large band gap energy with a large amount of p-type impurities is 1 × 10 ¹⁸ /cm ^Three ~ 1x10 ^{twenty one} /cm ^Three More preferably 1 × 10 ¹⁹ /cm ^Three ~ 5x10 ²⁰ /cm ^Three Adjust to the range. 1 × 10 ¹⁸ /cm ^Three If it is less, the difference from the nitride semiconductor layer having a small band gap energy is also reduced, and a layer having a high carrier concentration tends to be hardly obtained. ^{twenty one} /cm ^Three When the amount is more than 1, the crystallinity tends to deteriorate. On the other hand, the p-type impurity concentration of the nitride semiconductor layer having a small band gap energy should be lower than that of the nitride semiconductor layer having a large band gap energy, and preferably 1/10 or less. Most preferably, when undoped, a layer with the highest mobility is obtained, but since the film thickness is thin, there is a p-type impurity diffused from the side of the nitride semiconductor having a large band gap energy, the amount of which is 1 × 10 ²⁰ /cm ^Three The following is desirable. As the p-type impurities, Group IIA and IIB elements of the periodic table such as Mg, Zn, Ca and Be are selected, and preferably Mg, Ca and the like are used as p-type impurities. This effect is the same when the nitride semiconductor layer having a large band gap energy is doped with a small amount of p-type impurities and the nitride semiconductor layer having a small band gap energy is doped with a large amount of p-type impurities.
[0060]
Furthermore, in the nitride semiconductor layer constituting the superlattice, the layer doped with impurities at a high concentration has a large impurity concentration near the center of the semiconductor layer and a small impurity concentration near both ends in the thickness direction ( Preferably, it is undoped. More specifically, for example, when a superlattice layer is formed of AlGaN doped with Si as an n-type impurity and an undoped GaN layer, since AlGaN is doped with Si, electrons are emitted to the conduction band as donors. Electrons fall into the low-potential GaN conduction band. Since the GaN crystal is not doped with a donor impurity, it is not subject to carrier scattering by the impurity. Therefore, the electrons can easily move in the GaN crystal, and the substantial mobility of electrons increases. This is similar to the effect of the two-dimensional electron gas described above, and the substantial mobility in the lateral direction of the electron increases and the resistivity decreases. Further, the effect is further enhanced when the n-type impurity is doped at a high concentration in the central region of AlGaN having a large band gap energy. That is, depending on the electrons moving in GaN, the n-type impurity ions (Si in this case) contained in AlGaN are somewhat scattered. However, if both ends are undoped with respect to the thickness direction of the AlGaN layer, it is difficult to receive Si scattering, and the mobility of the undoped GaN layer is further improved. Although the operation is slightly different, there is a similar effect when a superlattice is formed by the first nitride semiconductor layer and the second nitride semiconductor layer on the p-layer side. It is desirable that the central region is doped with a large amount of p-type impurity and both ends are reduced or undoped. On the other hand, a layer doped with a large amount of n-type impurities in a nitride semiconductor layer with a small bandgap energy can be configured with the impurity concentration, but in a superlattice doped with a large amount of impurities in a region with a smaller bandgap energy, The effect tends to be small.
[0061]
As described above, the n-side cladding layer and the p-side cladding layer have been described as superlattice layers. However, in the present invention, the superlattice layer also includes an n-side buffer layer as a contact layer, an n-side light guide layer, p The side cap layer, the p-side light guide layer, the p-side contact layer, and the like can have a superlattice structure. That is, any layer away from the active layer, in contact with the active layer, or any layer can be a superlattice layer. In particular, if the n-side buffer layer on which the n-electrode is formed is a superlattice, an effect similar to the HEMT is likely to appear.
[0062]
The impurity (in this case, n-type impurity) concentration is 1 × 10 5 between the n-side cladding layer made of a superlattice layer and the active layer. ¹⁹ /cm ^Three It is preferable that an n-side light guide layer adjusted as follows is formed. The preferred impurity concentration is 1 × 10 ¹⁸ /cm ^Three Or less, more preferably 1 × 10 ¹⁷ /cm ^Three Hereinafter, it is most preferably undoped. Even when undoped, there is a possibility that n-type impurities may diffuse from other layers and enter this light guide layer. ¹⁹ /cm ^Three Was the upper limit. The n-side light guide layer is preferably composed of a nitride semiconductor containing In or GaN.
[0063]
Further, the impurity (in this case, p-type impurity) concentration is 1 × 10 5 between the p-side cladding layer made of a superlattice layer and the active layer. ¹⁹ /cm ^Three The p-side light guide layer adjusted as follows is formed. Similarly, the preferred impurity concentration is 1 × 10 ¹⁸ /cm ^Three Hereinafter, it is most preferably undoped. In the case of a nitride semiconductor, if it is undoped, it usually shows n conductivity, but in the case of this p-side guide layer, there is a possibility that p-type impurities diffuse from other layers and enter this p-side light guide layer. The conductivity type is not limited to n, p, etc., and is referred to as a p-side light guide layer. This p-side light guide layer is also preferably composed of a nitride semiconductor containing In or GaN.
[0064]
The reason why it is preferable to have an undoped nitride semiconductor between the active layer and the cladding layer is as follows. That is, in the case of a nitride semiconductor, the light emission of the active layer is usually designed for the purpose of 380 to 520 nm, particularly 400 to 450 nm. The undoped nitride semiconductor has a lower absorptance at the wavelength than the nitride semiconductor doped with n-type impurities and p-type impurities. Therefore, since the undoped nitride semiconductor is sandwiched between the active layer that emits light and the clad layer as the light confinement layer, the light emission of the active layer is rarely extinguished. Can be realized, and the threshold voltage decreases.
[0065]
Therefore, as a preferable combination, there is a cladding layer having a superlattice structure in which impurities are modulation-doped at a position apart from the active layer, and the impurity concentration is low between the cladding layer and the active layer, preferably undoped. It is a light emitting element which has a guide layer.
[0066]
Further, it is made of a nitride semiconductor having a film thickness of 0.1 μm or less and having a band gap energy larger than the band gap energy of the well layer of the active layer and the interface of the p side guide layer between the p side guide layer and the active layer. A p-side cap layer is formed, and the impurity concentration of the p-side cap layer is 1 × 10 ¹⁸ /cm ^Three It is preferable to adjust as described above. The thickness of the p-side cap layer is adjusted to 0.1 μm or less, more preferably 500 angstroms or less, and most preferably 300 angstroms or less. This is because if the film is grown to a thickness greater than 0.1 μm, cracks are likely to occur in the p-side cap layer, and a nitride semiconductor layer with good crystallinity is difficult to grow. Thus, by forming a layer having a large band gap energy in contact with the active layer and forming a thin film of 0.1 μm or less, the leakage current of the light emitting element tends to be reduced. This is because electrons injected from the n-layer side accumulate in the active layer due to the energy barrier of the cap layer, and the probability of recombination of electrons and holes increases, so that the output of the element itself is also improved. The impurity concentration is 1 × 10 ¹⁸ /cm ^Three It is necessary to adjust above. This cap layer is a layer having a high Al mixed crystal ratio, and a layer having a high Al mixed crystal ratio tends to have a high resistance. For this reason, unless the carrier concentration is increased by doping impurities to lower the resistivity, this layer becomes a high-resistance i layer, which has a pin structure and poor current-voltage characteristics. It is because it tends to become. The cap layer on the p side may be formed on the n side. When forming on the n-side, it may or may not be doped with n-type impurities.
[0067]
【Example】
Although one Example of this invention is shown below, this invention is not limited to this.
[Example 1]
FIG. 9 is a perspective view showing the structure of a nitride semiconductor laser device according to an embodiment of the present invention. Thereafter, a thick nitride semiconductor layer (GaN underlayer) is formed on the sapphire substrate using a light-absorbing protective film as shown in FIGS. 2 to 4, and the nitride semiconductor laser shown in FIG. A case of creating an element will be described.
[0068]
(GaN underlayer)
A sapphire substrate 1 having a 2 inch φ and C-plane as a main surface is set in a reaction vessel, and a buffer layer made of GaN is grown on the sapphire substrate 1 to a thickness of 200 Å at 500 ° C. The first GaN layer 2 made of GaN is grown to a thickness of 5 μm at a temperature of 1050 ° C. This first GaN layer has an Al mixed crystal ratio X value of 0.5 or less. _X Ga _1-X It is desirable to grow N (0 ≦ X ≦ 0.5). If it exceeds 0.5, the crystal itself tends to crack rather than a crystal defect, so that the crystal growth itself tends to be difficult. Further, it is desirable to grow the film thickness to be thicker than the buffer layer and adjust the film thickness to 10 μm or less. In FIG. 2, the buffer layer is not particularly shown.
[0069]
After the growth of the first GaN layer 2, the wafer is taken out of the reaction vessel, a striped photomask is formed on the surface of the first GaN layer 2, and a stripe width of 20 μm and a stripe interval (window portion) of 5 μm are obtained by a CVD apparatus. After forming Si to a thickness of 1 μm, SiO ₂ A protective film 11 having a two-layer structure is formed. FIG. 2 is a schematic cross-sectional view showing a partial wafer structure when cut in a direction perpendicular to the major axis direction of the stripe.
[0070]
After the protective film 11 is formed, the wafer is set again in the reaction vessel, and Si is 1 × 10 ° C. at 1050 ° C. ¹⁸ /cm ^Three A second GaN layer 3 made of doped GaN is grown to a thickness of 6 μm (FIGS. 3 and 4). The preferred growth thickness of the second GaN layer 3 varies depending on the thickness and size of the protective film 11 formed previously, but the second GaN layer 3 is grown so as to cover the surface of the protective film 11. The size of the protective film 11 is not particularly limited, but it is very preferable to make the area of the protective film 11 larger than the area of the window portion in order to obtain a GaN substrate with few crystal defects.
[0071]
(Layer structure of nitride semiconductor laser element)
As shown in FIG. 9, the following layers are grown using the second GaN layer 3 as a GaN foundation layer 50.
[0072]
(Second buffer layer 71)
A wafer whose main surface is the GaN foundation layer 50 is set in a reaction vessel, and Si is deposited on the GaN foundation layer 50 at 1050 ° C. ¹⁸ /cm ^Three A second buffer layer 71 made of doped GaN is grown to a thickness of 4 μm. The second buffer layer 71 is a nitride semiconductor single crystal layer that is grown at a high temperature of 900 ° C. or higher. In order to alleviate lattice mismatch between the conventionally grown substrate and the nitride semiconductor, the second buffer layer 71 is grown next. It is distinguished from a buffer layer grown at a lower temperature than a semiconductor. The thickness of the buffer layer is preferably 2 to 6 μm. A film thickness within this range is preferable in terms of crystallinity and heat dissipation of the buffer layer.
In the case of manufacturing a laser element, the second buffer layer 71 is a strained superlattice formed by stacking nitride semiconductors having different thicknesses of 100 angstroms or less, more preferably 70 angstroms or less, and most preferably 50 angstroms or less. A layer is preferred. When the strained superlattice layer is used, the crystallinity of the single nitride semiconductor layer is improved, so that a high-power laser element can be realized.
[0073]
(Crack prevention layer 72)
Next, Si is 5 × 10 ¹⁸ /cm ^Three Doped In _0.1 Ga _0.9 A crack prevention layer 42 made of N is grown to a thickness of 500 angstroms. The crack prevention layer 72 can be prevented from cracking in the nitride semiconductor layer containing Al by growing it with an n-type nitride semiconductor containing In, preferably InGaN. The crack prevention layer is preferably grown with a film thickness of 100 Å or more and 0.5 μm or less. If it is thinner than 100 angstroms, it is difficult to act as a crack prevention as described above, and if it is thicker than 0.5 μm, the crystal itself tends to turn black. The crack prevention layer 72 can be omitted.
[0074]
(N-side cladding layer 73)
Next, Si is 5 × 10 ¹⁸ /cm ^Three Doped n-type Al _0.2 Ga _0.8 A superlattice structure with a total film thickness of 0.4 μm is formed by alternately laminating 100 first layers made of N, 20 Å, and second layers made of undoped GaN, 20 Å. The n-side cladding layer 73 functions as a carrier confinement layer and an optical confinement layer, and is desirably a nitride semiconductor containing Al, preferably a superlattice layer containing AlGaN, and the total thickness of the superlattice layer is 100 angstroms or more. It is desirable to grow at 2 μm or less, more preferably 500 Å or more and 1 μm or less. When the superlattice layer is used, a carrier confinement layer having good crystallinity without cracks can be formed. In addition, when a superlattice layer is formed, if a nitride semiconductor layer having different band gap energies is stacked and modulation doping is performed so that one of the impurity concentrations is increased and the other is decreased, the threshold value is lowered. It tends to be easy.
[0075]
(N-side light guide layer 74)
Then, Si is 5 × 10 ¹⁸ /cm ^Three An n-side light guide layer 74 made of doped n-type GaN is grown to a thickness of 0.1 μm. The n-side light guide layer 74 acts as a light guide layer of the active layer, and it is desirable to grow GaN and InGaN. Usually, the n-side light guide layer 74 is grown to a thickness of 100 angstroms to 5 μm, more preferably 200 angstroms to 1 μm. desirable. The n-side light guide layer 74 is usually doped with an n-type impurity such as Si or Ge so as to have an n-type conductivity type, but can be undoped in particular. In the case of a superlattice, at least one of the first layer and the second layer may be doped with an n-type impurity or may be undoped.
[0076]
(Active layer 75)
Next, undoped In _0.2 Ga _0.8 N well layer, 25 Å, and undoped In _0.01 Ga _0.99 An active layer 75 of a multiple quantum well structure (MQW) having a total film thickness of 175 Å formed by alternately stacking N barrier layers and 50 Å is grown.
[0077]
(P-side cap layer 76)
Next, the band gap energy is larger than that of the p-side light guide layer 77 and larger than that of the active layer 75. ²⁰ /cm ^Three Doped p-type Al _0.3 Ga _0.7 A p-side cap layer 76 made of N is grown to a thickness of 300 angstroms. Although this p-side cap layer 76 is p-type, it may be i-type in which the carrier is compensated by doping n-type impurities or undoped because the film thickness is thin, and most preferably a layer doped with p-type impurities. And The film thickness of the p-side cap layer 76 is adjusted to 0.1 μm or less, more preferably 500 angstroms or less, and most preferably 300 angstroms or less. This is because if the film is grown to a thickness greater than 0.1 μm, cracks are likely to occur in the p-side cap layer 76 and a nitride semiconductor layer with good crystallinity is difficult to grow. When the AlGaN having a larger Al composition ratio is formed thinner, the LD element tends to oscillate. For example, Al with a Y value of 0.2 or more _Y Ga _1-Y If N, it is desirable to adjust to 500 angstroms or less. The lower limit of the thickness of the p-side cap layer 76 is not particularly limited, but it is desirable to form the p-side cap layer 76 with a thickness of 10 Å or more.
[0078]
(P-side light guide layer 77)
Next, the band gap energy is smaller than the p-side cap layer 76, and Mg is 1 × 10. ²⁰ /cm ^Three A p-side light guide layer 77 made of doped p-type GaN is grown to a thickness of 0.1 μm. This layer acts as a light guide layer of the active layer, and is preferably grown of GaN and InGaN as with the n-side light guide layer 44. This layer also functions as a buffer layer when the p-side cladding layer 78 is grown, and functions as a preferable light guide layer by growing it at a film thickness of 100 angstroms to 5 μm, more preferably 200 angstroms to 1 μm. . This p-side light guide layer is usually doped with a p-type impurity such as Mg to have a p-type conductivity, but it is not particularly necessary to dope the impurity. The p-side light guide layer can be a superlattice layer. In the case of a superlattice layer, at least one of the first layer and the second layer may be doped with a p-type impurity or may be undoped.
[0079]
(P-side cladding layer 78)
Next, Mg is 1 × 10 ²⁰ /cm ^Three Doped p-type Al _0.2 Ga _0.8 A first layer of N, 20 Å, and 1 × 10 Mg ²⁰ /cm ^Three A p-side cladding layer 78 made of a superlattice layer having a total thickness of 0.4 μm is formed by alternately laminating a second layer made of doped p-type GaN and 20 Å. This layer, like the n-side cladding layer 73, acts as a carrier confinement layer, and acts as a layer for reducing the resistivity on the p-type layer side by adopting a superlattice structure. The thickness of the p-side cladding layer 78 is not particularly limited, but it is desirable that the p-side cladding layer 78 be grown at a thickness of 100 angstroms or more and 2 μm or less, more preferably 500 angstroms or more and 1 μm or less. In particular, when a nitride semiconductor layer having a superlattice structure is used as a cladding layer, providing a superlattice layer on the p-layer side is more effective in reducing the threshold current. As with the n-side cladding layer, when a superlattice layer is formed, a nitride semiconductor layer having different band gap energies is stacked, and either one is increased in impurity concentration and the other is decreased in modulation doping. If done, the threshold tends to decrease.
[0080]
In the case of a nitride semiconductor device having a double hetero structure having an active layer having a quantum well layer, a nitride containing Al having a film thickness of 0.1 μm or less in contact with the active layer and having a band gap energy larger than that of the active layer A cap layer made of a semiconductor is provided, a p-side light guide layer having a band gap energy smaller than that of the cap layer is provided at a position farther from the active layer than the cap layer, and the p-side light guide layer is separated from the active layer. It is very preferable to provide a p-side cladding layer made of a superlattice layer containing a nitride semiconductor containing Al having a larger band gap than the p-side light guide layer. In addition, since the band gap energy of the p-side cap layer is increased, electrons injected from the n layer are blocked by this cap layer, so that electrons do not overflow the active layer, and the leakage current of the device is reduced. .
[0081]
(P-side contact layer 79)
Finally, Mg 2 × 10 ²⁰ /cm ^Three A p-side contact layer 79 made of doped p-type GaN is grown to a thickness of 150 Å. When the thickness of the p-side contact layer is adjusted to 500 angstroms or less, more preferably 400 angstroms or less, or 20 angstroms or more, the p-layer resistance is reduced, which is advantageous in reducing the threshold voltage.
[0082]
After completion of the reaction, the wafer is annealed in a nitrogen atmosphere at 700 ° C. in the reaction vessel to further reduce the resistance of the p layer. After annealing, the wafer is taken out of the reaction vessel, and as shown in FIG. 9, the uppermost p-side contact layer 79 and p-side cladding layer 78 are etched by an RIE apparatus to obtain a ridge shape having a stripe width of 4 μm. To do. The ridge formation position is formed in a direction parallel to the stripe of the protective film, and the striped crystal defect region in the GaN underlayer is removed.
[0083]
That is, the GaN layer formed on the protective film having a width of 20 μm and a window portion of 5 μm has a region with many crystal defects in the window portion at the initial stage of growth, so that the ridge does not cover this region, that is, a stripe. It is designed to be located immediately above the protective film. By designing in this way, since the active layer existing under the stripe-shaped ridge corresponds to the laser oscillation region, the laser oscillation region can be prevented from covering a region having many crystal defects. In the element of FIG. 9, a ridge is provided and light emission is concentrated on the active layer below the ridge to produce a laser oscillation region. In addition to this, for example, an insulating layer is formed on the uppermost layer of the p layer. The laser oscillation region can also be provided in the active layer by a method of providing a thin stripe width electrode capable of current confinement, a method of forming a current confinement layer in the nitride semiconductor layer, or the like. In such a case as well, the active layer in the upper part of the region having many crystal defects is shifted from the laser oscillation region.
[0084]
After the ridge is formed, as shown in FIG. 9, the p-side cladding layer 77 exposed on both sides of the ridge stripe is etched with the ridge stripe as the center so that the surface of the n-side cladding layer 71 on which the n-electrode 82 is to be formed is etched. Expose. The surface on which the n electrode 82 is formed may be the surface of the n-side cladding layer 71 as shown in FIG. 9 or the surface of the GaN foundation layer 50, but the surface of the n-type nitride semiconductor layer having the higher carrier concentration. It is desirable to expose.
[0085]
Next, a p-electrode 80 made of Ni / Au is formed on the entire surface of the ridge. Next, as shown in FIG. 9, the surfaces of the p-side cladding layer 78 and the p-side contact layer 79 excluding the p-electrode 80 are made of SiO. ₂ An insulating film 83 is formed, and a p-pad electrode 81 electrically connected to the p-electrode 80 through the insulating film 83 is formed. On the other hand, an n-electrode 82 made of W and Al is formed on the surface of the n-side cladding layer 71 exposed earlier.
[0086]
After electrode formation, only the sapphire substrate of the wafer is polished to a thickness of 50 μm, and then the sapphire substrate 1 is cleaved in a direction perpendicular to the stripes of the striped p-electrode 80 and n-electrode 82 to resonate the cleavage plane of the active layer. A surface. The shape of the laser element after cleavage is shown in FIG. Thus, in the structure of the laser element in which the n-electrode and the p-electrode are provided on the same surface side, the active layer is provided on the upper layer of the nitride semiconductor having a region with few crystal defects and a region with many crystal defects. In this case, the exposed area of the nitride semiconductor layer not including the active layer provided with the n-electrode is made larger than the active layer area on the side having the active layer, so that the active layer where heat is concentrated is destroyed by crystal defects. Therefore, an element with high reliability and long life can be realized. When this laser device was oscillated at room temperature, the threshold current density was 2.0 kA / cm. ² In addition, since continuous oscillation with an oscillation wavelength of 405 nm was confirmed at a threshold voltage of 4.0 V, a lifetime of 1000 hours or more was exhibited, and the protective film was light-absorbing, so that light from other than the laser waveguide could be a far field of laser light. Good laser light was obtained without disturbing the pattern and near-field pattern.
[0087]
[Example 2]
In Example 1, the sapphire substrate 1 was made of chromium ions (Cr ³⁺ ) Is added at 0.05% by weight to make the substrate light-absorbing, and on this substrate, the temperature is set to 510 ° C., the carrier gas is hydrogen, the source gas is ammonia (NH _Three ) And TMG (trimethylgallium), and a buffer layer made of GaN is grown to a thickness of about 200 angstroms. Subsequently, the second buffer layer 71 is sequentially stacked in the same manner as in Example 1 to form a nitride. A semiconductor laser element was obtained.
When the obtained laser element was oscillated at room temperature, good continuous oscillation was confirmed in substantially the same manner as in Example 1, and the far-field pattern and near-field pattern of the laser beam were also good.
[0088]
[Example 3]
In Example 3, a nitride semiconductor laser device using a GaN substrate was prepared as shown in FIG.
In Example 3, the second GaN layer 3 obtained by forming a protective film on the sapphire substrate in Example 1 and further forming a thick nitride semiconductor layer is used as a GaN substrate (GaN underlayer 50). went. Similar to the first embodiment, each layer of the laser element structure is laminated on the GaN foundation layer 50, and the p-side contact layer 79 and the p-side cladding layer 78 are etched to form a ridge. Then, a p-electrode 80 is formed, and then an insulating film 83 and a p-pad electrode 81 are formed as shown in FIG. Thereafter, the protective film 11 and a part of the GaN foundation layer 50 are polished and removed from the sapphire substrate 1 of the wafer to expose the surface of the GaN foundation layer 50. The exposed GaN foundation layer 50 is made into a mirror surface, and a light absorption film 203 made of Si having a film thickness of 3 μm and bonded to the entire mirror surface is bonded by wafer bonding. Further, an n-electrode 82 made of W / Al is formed on the surface of the light absorption film 203 with a film thickness of 0.5 μm, and then the GaN foundation layer 50 is cleaved in the direction perpendicular to the stripe-shaped p-electrode 80 and cleaved. The surface is a resonance surface. The shape of the laser element after cleavage is shown in FIG. The obtained laser element oscillated as well as in Example 1, and the far field pattern and the like were also good.
The n-electrode 82 can also be formed on the second buffer layer 71 as shown in FIG.
[0089]
【The invention's effect】
As described above, the present invention forms a light absorbing layer in the element structure, makes the substrate light absorbing, and forms a light absorbing film on the surface opposite to the element structure forming surface of the GaN substrate. It is possible to prevent light other than the laser waveguide from being emitted without impairing the performance of a long-life and highly reliable laser device, etc., and to prevent disturbance of the far-field pattern and near-field pattern. An improved nitride semiconductor laser device can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an LD device as an embodiment showing a nitride semiconductor laser device structure of the present invention.
FIG. 2 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 3 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 4 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 5 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 6 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 7 is a schematic cross-sectional view showing one structure of a nitride semiconductor wafer obtained by producing a GaN underlayer using a protective film.
FIG. 8 is a schematic cross-sectional view of an LD element according to an embodiment showing a nitride semiconductor laser element structure of the present invention.
FIG. 9 is a schematic cross-sectional view showing the structure of an LD element according to an embodiment of the present invention.
FIG. 10 is a schematic cross-sectional view showing the structure of an LD element according to an embodiment of the present invention.
[Explanation of symbols]
1 ... Different substrates
2. First GaN layer
3 ... Second GaN layer
11, 11 '... protective film
50 ... GaN underlayer
71: Second buffer layer
72 ... Crack prevention layer
73 ... n-side cladding layer
74: n-side light guide layer
75 ... Active layer
76 ... p-side cap layer
77 ... p-side light guide layer
78 ... p-side cladding layer
79 ... p-side contact layer
101... First nitride semiconductor layer
102: Second nitride semiconductor layer
103 ... Laser waveguide including active layer
104... P-conducting nitride semiconductor layer
105 ... Protective film (light absorption layer)
201 ... GaN substrate
202 ... Element structure
203 ... Light absorption film

Claims (7)

A first nitride semiconductor layer having a higher refractive index than that of the different substrate, and a second nitride semiconductor layer having a lower refractive index than that of the first nitride semiconductor layer; In the nitride semiconductor laser device having a structure in which an active layer having a refractive index higher than that of the second nitride semiconductor layer is stacked thereon, an active layer is provided between the second nitride semiconductor layer and the dissimilar substrate. A light absorption layer made of a material capable of absorbing light leaked from the laser waveguide containing the second nitride semiconductor layer is formed on the light absorption layer formation via the first nitride semiconductor layer. nitride semiconductor laser device, characterized in that provided Te. "   A nitride semiconductor laser device in which an element structure having an active layer is formed on a GaN substrate, and a laser waveguide including an active layer on a surface not having the element structure of the GaN substrate facing the element structure forming surface A nitride semiconductor laser device, wherein a light absorption film capable of absorbing light leaking from the substrate is formed.   The nitride semiconductor laser element according to claim 1, wherein the substrate has a light absorption property.   4. The nitride semiconductor laser device according to claim 3, wherein a light absorption layer is provided between the second nitride semiconductor layer and the substrate.   The light absorption layer is a layer formed by adding a new component so that light can be absorbed by the nitride semiconductor layer on the n conductive side having the same composition as the active layer, or the n conductive side layer forming the element structure. The nitride semiconductor laser device according to claim 1, 3 to 4.   6. The nitride semiconductor laser element according to claim 1, wherein the first nitride semiconductor layer is GaN.   The nitride semiconductor laser element according to claim 1, wherein the second nitride semiconductor layer is made of a nitride semiconductor containing at least Al.

Images