JP4032836B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a laser element made of a nitride semiconductor (InXAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1) and a method for producing a resonance surface of the laser element.
[0002]
[Prior art]
A nitride semiconductor is known as a material of a laser element that emits light in the ultraviolet to blue region, and the present applicant recently announced laser oscillation at room temperature in a pulse current using this material (for example, Jpn. J.Appl.Phys. Vol35 (1996) pp.L74-76). The announced laser element is a so-called electrode stripe type laser element, and the resonance surface of the laser is formed by etching. The method of producing the resonance surface by etching has an advantage that the resonance surface can be easily formed, but has a disadvantage that unevenness is likely to occur on the surface etched by the etching means and it is difficult to obtain surfaces parallel to each other. The resonant surface of the laser is most preferably formed by cleaving, as is often used in infrared and red semiconductor lasers. For example, in the case of an infrared semiconductor laser using a GaAs substrate, the resonance surface is a cleavage surface utilizing the cleavage property of the substrate.
[0003]
Nitride semiconductors are often grown on a sapphire substrate, and sapphire has a hexagonal system (exactly rhombohedral, but is approximated by a hexagonal system) and thus has little cleavage. For this reason, it has been very difficult to cleave the nitride semiconductor layer grown on the sapphire substrate using the cleavage property of the substrate.
[0004]
[Problems to be solved by the invention]
The resonant surface of the semiconductor laser needs to be a flat surface that has very little unevenness, that is, a mirror surface. Accordingly, an object of the present invention is to provide a nitride semiconductor laser element in which a resonant surface close to a mirror surface is obtained by cleavage, and a method for manufacturing the resonant surface of the laser element.
[0005]
[Means for Solving the Problems]
Conventionally, nitride semiconductors grown on sapphire have been believed to have no hexagonal nitride semiconductor layer because sapphire is not cleaved. Has newly found that the nitride semiconductor layer is cleaved when the nitride semiconductor layer is divided in a specific plane orientation regardless of the substrate, and the present invention has been achieved. That is, the nitride semiconductor laser device of the present invention is any one of [Outside 1], [Outside 2], [Outside 3], [Outside 4], [Outside 5], and [Outside 6] of the nitride semiconductor. In a nitride semiconductor laser device in which a cleavage plane along one kind of plane orientation is at least one resonance surface, an n-type nitride semiconductor, an active layer having a quantum well structure, and a p-type nitride semiconductor are formed on a GaN substrate. And the n-type nitride semiconductor is in contact with the GaN substrate and contains In _x Al _y Ga _1-xy N (0 ≦ x, 0 ≦ y, x + y ≦ 1) layers, An In _x Ga _1-x N (0 ≦ x <1) layer in contact with the active layer, a positive electrode on the p-type nitride semiconductor, and a back surface of the GaN substrate Has a negative electrode. The active layer includes InGaN or GaN.
[0006]
The resonance surface of the laser element of the present invention is any one of the [Outside 1] to [Outside 6] surfaces of the uppermost nitride semiconductor after a plurality of nitride semiconductor layers are grown on the substrate. It can be manufactured by providing a groove on a surface corresponding to the type, cleaving the nitride semiconductor from the groove, and using the cleaved surface of the cleaved nitride semiconductor layer as at least one resonance surface of the laser element.
[0007]
Furthermore, in the laser element of the present invention, when the substrate on which the nitride semiconductor is grown is a nitride semiconductor single crystal, after growing a plurality of nitride semiconductor layers on the substrate made of the nitride semiconductor single crystal, A groove is provided on a surface corresponding to any one of [Outside 1] to [Outside 6] of the substrate facing the surface on which the nitride semiconductor layer is grown, and the nitride semiconductor is cleaved from the groove, It can be produced by a method in which the cleaved surface of the cleaved nitride semiconductor layer is at least one resonance surface of the laser element.
[0008]
[Action]
FIG. 1 is a unit cell diagram showing the plane orientation of a nitride semiconductor single crystal. Although the nitride semiconductor has a rhombohedral structure precisely, it can be approximated by a hexagonal system as shown in this figure. In the laser element of the present invention, the cleavage plane along any one of the surface orientations of [Outside 1] to [Outside 6] shown in the unit cell diagram is at least one resonance surface. This plane orientation corresponds to the side surface indicated by the hexagonal column, and the nitride semiconductor layer surface cleaved by these surfaces provides a surface close to a mirror surface. For example, the hatched portion shown in this figure indicates the [Outside 3] surface and the [Outside 6] surface, and it is most preferable to form the opposing resonance surfaces by cleavage, but both are necessarily formed by cleavage. There is no need, and one may be formed by cleavage and the other may be formed by other means such as etching.
[0009]
FIG. 2 is a schematic plan view showing an enlarged crystal structure of a nitride semiconductor grown on a substrate. Nitride semiconductors are thus grown in a turtle shell shape with unit crystals of hexagonal columns. In the method for producing a resonance surface according to the present invention, a surface corresponding to any one of the [Outside 1] to [Outside 6] surfaces of the nitride semiconductor of the uppermost layer, for example, on the extension line II-II shown in this figure When a groove is provided on the surface of the nitride semiconductor layer in FIG. 2 and the nitride semiconductor is cleaved by a line indicated by II-II from the groove, and the cleaved surface of the cleaved nitride semiconductor layer is the resonance surface of the laser element, A resonance surface close to is obtained. In other words, the groove is a scratch for cleaving, and can be formed by, for example, a ruled needle having diamond at the blade edge, or by etching. When manufacturing a laser element, an electrode layer is usually formed on the uppermost nitride semiconductor layer, but a groove for cleavage is provided in the uppermost nitride semiconductor layer through the electrode layer. This is also within the scope of the present invention.
[0010]
Furthermore, when the substrate is a nitride semiconductor single crystal, the cleavage property of the substrate itself can be used, so there is no need to scratch the nitride semiconductor layer grown on the substrate, and the nitride semiconductor layer is grown. A surface corresponding to any one of [Outside 1] to [Outside 6] on the surface opposite to the substrate that has been made may be scratched and cleaved from the scratch.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The structure of the laser element of the present invention is not limited as long as the laser element has a cleavage plane as a resonance surface, and the element structure is not limited. For example, examples of the gain waveguide stripe laser include an electrode stripe type, a mesa stripe type, and a heteroisolation type. In addition, examples of the stripe type laser having a built-in waveguide mechanism include a buried hetero type, a CSP type, and a rib guide type. In the laser element having these structures, a waveguide is formed in the active layer, a resonance surface is formed at a position corresponding to the end face of the waveguide, and the light emission of the active layer is resonated between the resonance surfaces. Oscillate along. Further, in order to confine light on the resonance surface, a dielectric multilayer film as usual may be formed.
[0012]
[Example 1]
FIG. 3 is a schematic cross-sectional view showing the structure of the laser device according to one embodiment of the present invention, and shows a view when cut in a direction parallel to the resonance surface. Hereinafter, Example 1 is demonstrated based on this figure.
[0013]
After the substrate 10 made of C-axis oriented GaN having a thickness of 500 μm is placed in the reaction vessel of the MOVPE apparatus, TMG (trimethyl gallium) is used as a source gas, ammonia, and SiH 4 (silane) gas is used as a donor impurity. An n-type contact layer 11 made of doped GaN was grown to a thickness of 4 μm.
[0014]
In the laser device of the present invention, the substrate is not particularly limited, and can be applied to all the substrates conventionally used and proposed for growing a nitride semiconductor. For example, in addition to GaN, oxide substrates such as sapphire, spinel and zinc oxide, and semiconductor substrates such as Si and SiC are known, and can be applied to any of them in the present invention. Particularly preferably, a single crystal substrate made of a nitride semiconductor such as GaN or AlGaN is used. The reason is that by using a nitride semiconductor single crystal substrate as described above, the resonant surface can be formed by using the cleavage property of the substrate. This is because a groove can be provided and cleaved from the groove according to the [Outside 1] to [Outside 6] surfaces. In addition, since the substrate is lattice-matched with the nitride semiconductor, crystals with very good film quality can be grown.
[0015]
The n-type contact layer 11 can be made of InXAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1), and in particular, by being made of GaN, InGaN, and especially GaN doped with Si, Since an n-type layer having a high concentration can be obtained and a preferable ohmic contact with the negative electrode can be obtained, the threshold current of the laser element can be reduced.
[0016]
The temperature was maintained at 1050 ° C., TMG and ammonia were used as the source gas, and Cp 2 Mg was used as the impurity gas, and a current blocking layer 20 made of Mg-doped p-type GaN was grown to a thickness of 0.5 μm. This current blocking layer later acts to concentrate the current on the active layer to produce a waveguide.
[0017]
After growing the current blocking layer 20, the wafer was taken out of the reaction vessel, and the current blocking layer 20 was mesa-etched in a V-groove shape with a depth reaching the n-type contact layer 11, as shown in FIG. The width of the V groove was 5 μm, and the surface of the current blocking layer 20 was formed in a stripe shape with a depth of 2.5 μm. In the V-groove stripe, the n-type light confinement layer 12, the n-type light guide layer 13, the active layer 14, the p-type light guide layer 15, and the p-type light confinement layer 16 enter the V-groove. It becomes easy. Laser light emitted from the active layer is controlled by a light confinement layer whose vertical direction is above and below the active layer. When the V-groove is formed, the lateral light confinement layer sandwiching the active layer 15 enters the groove, so that the lateral direction of the laser light can be controlled and the light can be confined in the groove. Can be lowered. In other words, a refractive index guided laser element can be obtained in which the lateral active layer is sandwiched between the clad layers having different refractive indexes by the V groove. Furthermore, since the V-groove has a mesa shape, the nitride semiconductor layer grown on the V-groove can be grown with a uniform film thickness.
[0018]
Next, the wafer is transferred again to the reaction vessel, the temperature is set to 750 ° C., TMG, TMI (trimethylindium), ammonia, silane gas is used as the impurity gas, and Si-doped In0.1Ga0.9N is prevented from cracking. The layer was grown to a thickness of 300 Angstroms. Although this crack prevention layer is not particularly illustrated, an n-type optical confinement layer 12 made of an n-type nitride semiconductor containing In, preferably InGaN, which is grown next by InGaN, is made of In nitride. A thick film can be grown. In the case of an LD, it is necessary to grow layers that become a light confinement layer and a light guide layer with a film thickness of, for example, 0.1 μm or more. Conventionally, when a thick AlGaN film was grown directly on the GaN or AlGaN layer, it was difficult to fabricate the device because cracks occurred in the AlGaN grown later. It is possible to prevent the layer from cracking. Moreover, even if the light confinement layer 3 to be grown next is grown as a thick film, it can be grown with good film quality. 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 can be omitted depending on the growth method and the growth apparatus.
[0019]
Next, an n-type optical confinement layer 12 made of Si-doped n-type Al0.3Ga0.7N is grown to a thickness of 0.5 μm using TEG, TMA (trimethylaluminum) as source gas, and silane gas as impurity gas. I let you. The n-type optical confinement layer 12 is made of an n-type nitride semiconductor containing Al, and is preferably made of a binary mixed crystal or a ternary mixed crystal AlYGa1-YN (0 <Y ≦ 1). A good material can be obtained, and the refractive index difference with the active layer is increased to be effective for confining the laser beam in the vertical direction. This layer is preferably grown with a thickness of usually 0.1 μm to 1 μm. If it is thinner than 0.1 μm, it will be difficult to act as a light confinement layer, and if it is thicker than 1 μm, cracks are likely to occur in the crystal, and device fabrication tends to be difficult.
[0020]
Subsequently, TMG and ammonia were used as the source gas and silane gas was used as the impurity gas, and an n-type light guide layer 13 made of Si-doped n-type GaN was grown to a thickness of 500 angstroms. The n-type light guide layer 13 is composed of an n-type nitride semiconductor containing In or n-type GaN, and is preferably ternary mixed crystal or binary mixed crystal InXGa1-XN (0 ≦ X <1). This layer is usually preferably grown to a thickness of 100 Å to 1 μm. In particular, by using InGaN or GaN, the next active layer can be easily formed into a quantum well structure.
[0021]
Next, the active layer 14 was grown using TMG, TMI, and ammonia as source gases. The active layer is maintained at a temperature of 750 ° C., and a well layer made of non-doped In 0.2 Ga 0.8 N is first grown to a thickness of 25 Å. Next, a barrier layer made of non-doped In0.01Ga0.95N is grown to a thickness of 50 angstroms at the same temperature only by changing the molar ratio of TMI. This operation was repeated 13 times. Finally, a well layer was grown to grow an active layer 14 having a multiple quantum well structure with a total film thickness of 0.1 μm.
[0022]
After growing the active layer 14, the temperature is set to 1050 ° C., TMG, TMA, ammonia, Cp2Mg (cyclopentadienylmagnesium) is used as an acceptor impurity source, and a p-type cap layer made of Mg-doped p-type Al0.2Ga0.8N is made 100 Grown with angstrom thickness. Although this p-type cap layer is not particularly shown, it is preferably formed of AlYGa1-YN (0 <Y <1), and is grown to a thickness of 1 μm or less, more preferably 10 Å or more and 0.1 μm or less. As a result, the active layer made of InGaN serves as a cap layer that prevents the active layer from decomposing, and the p-type cap layer made of a p-type nitride semiconductor containing Al is grown on the active layer to emit light. The output is greatly improved. On the other hand, if the p layer in contact with the active layer is GaN, the output of the device is reduced to about 1/3. This is presumed to be because AlGaN tends to be p-type compared to GaN and has the effect of suppressing decomposition of InGaN during the growth of the p-type cap layer, but the details are unknown. If the thickness of the p-type cap layer is greater than 1 μm, the layer itself tends to crack, and device fabrication tends to be difficult. This p-type cap layer can also be omitted.
[0023]
Next, while maintaining the temperature at 1050 ° C., a p-type light guide layer 15 made of Mg-doped p-type GaN using TMG, ammonia, and Cp 2 Mg was grown to a thickness of 500 Å. The p-type light guide layer 15 is made of a nitride semiconductor containing In or GaN, and preferably grows binary mixed crystal or ternary mixed crystal InYGa1-YN (0 <Y ≦ 1). The light guide layer is preferably grown with a thickness of usually 100 Å to 1 μm. In particular, by using InGaN or GaN, the next p-type light confinement layer 16 can be grown with good crystallinity.
[0024]
Subsequently, a p-type optical confinement layer 16 made of Mg-doped Al0.3Ga0.7N was grown to a thickness of 0.5 μm using TMG, TMA, ammonia, and Cp2Mg. This p-type optical confinement layer 16 is made of a p-type nitride semiconductor containing Al, and preferably has a crystallinity by being made of binary mixed crystal or ternary mixed crystal AlYGa1-YN (0 <Y ≦ 1). Good thing is obtained. As with the n-type optical confinement layer 12, the p-type optical confinement layer 16 is desirably grown to a thickness of 0.1 μm to 1 μm. By using a p-type nitride semiconductor containing Al, such as AlGaN, the active layer It effectively acts as a light confinement layer.
[0025]
Subsequently, a p-type contact layer 17 made of Mg-doped p-type GaN was grown to a thickness of 0.5 μm using TMG, ammonia, and Cp 2 Mg. The p-type contact layer 17 can be composed of p-type InXAlYGa1-X-YN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and in particular, InGaN, GaN, and especially p-type GaN doped with Mg, A p-type layer with the highest carrier concentration can be obtained, good ohmic contact with the positive electrode can be obtained, and the threshold current can be lowered. The composition ratio X values of the general formula AlxGa1-XN of the n-type layer and AlxGa1-XN of the p-type layer described above merely show general formulas, and X of the n-type layer and X of the p-type layer And do not indicate the same value. Similarly, the Y values used in other general formulas do not indicate the same value in the same general formula.
[0026]
Next, a positive electrode was formed on almost the entire surface of the uppermost p-type contact layer 17, and an opposing negative electrode was formed on the surface of the substrate 10 on which no nitride semiconductor was grown.
[0027]
After the electrode is formed, a scratch of about 5 mm is made on the edge of the wafer corresponding to [External 1] of the nitride semiconductor single crystal substrate from above the negative electrode on the substrate side using a diamond point cutter of a scriber. The wafer was cleaved by external force along the scratches. Naturally, the cleavage direction is a direction perpendicular to the V-groove described above. By cleaving the wafer having the nitride semiconductor layer in this manner, a semiconductor bar in which the cleavage plane of the active layer corresponding to the [Outside 1] surface and the [Outside 4] surface was exposed was produced. By this operation, the resonance surfaces facing each other were formed by the cleavage property of the nitride semiconductor. In this example, the wafer is cleaved with scratches on the substrate side, but it goes without saying that the wafer may be cleaved with scratches on the nitride semiconductor layer on the positive electrode side.
[0028]
A reflective mirror made of a dielectric multilayer film is formed on the cleavage plane of the semiconductor bar obtained as described above by using a sputtering apparatus, and then the bar is cut in a direction perpendicular to the cleavage plane by dicing to obtain a 0.8 mm × A 0.5 mm square laser element was obtained. The cavity length of this laser element is 0.8 mm. Further, when this laser device was placed on a heat sink and pulsated at room temperature, a laser oscillation of 410 nm was exhibited at a threshold current density of 1 kA / cm 2.
[0029]
[Example 2]
FIG. 4 is a schematic cross-sectional view showing the structure of a laser device according to another aspect of the present invention, and this figure also shows a view when cut in a direction parallel to the resonance surface, and is the same as FIG. Indicates the same member. Such a structure is often found in laser elements using a substrate made of an insulator.
[0030]
In Example 1, spinel (MgAl2O4) having a crystal growth surface (111 plane) of 1 inch φ and a thickness of 500 μm is used for the substrate 30, TMG and ammonia are used as source gases, and the surface of the substrate 30 is made of GaN. The resulting buffer layer was grown at 500 ° C. with a thickness of 200 Å. This buffer layer has the effect of relaxing the lattice mismatch between the substrate and the nitride semiconductor, and it is also possible to grow AlN, AlGaN, etc., but it grows depending on the type of substrate as in the first embodiment. This buffer layer is not specifically shown because it may not be.
[0031]
Thereafter, the n-type contact layer 11 to the p-type contact layer 17 are sequentially laminated in the same manner as in Example 1, and then the wafer on which the nitride semiconductor is laminated is taken out from the reaction vessel and is reacted with a reactive ion etching (RIE) apparatus. Then, selective etching was performed from the uppermost p-type contact layer 17 to expose the plane of the n-type contact layer 11 where the negative electrode is to be formed.
[0032]
Next, a positive electrode was formed on almost the entire surface of the uppermost p-type contact layer 17, and a striped negative electrode was formed on the surface of the n-type contact layer 11 exposed by etching. After forming the electrode, the wafer is transferred to a polishing apparatus, the spinel substrate is polished and thinned to a thickness of 50 μm, and then the nitride semiconductor layer corresponding to the outer layer of the nitride semiconductor layer from above the electrode. Using a diamond point cutter at the edge of the wafer, a scratch having a length of about 5 mm was made in the same manner as in Example 1, and the wafer was cleaved along with the scratch by an external force. The cleavage direction is a direction perpendicular to the above-described V-groove, that is, a direction perpendicular to the striped negative electrode. Note that in the case where a material different from a nitride semiconductor is used for the substrate, such as a spinel substrate, it is desirable to polish the substrate to a thickness of 100 μm or less, more preferably 80 μm or less before cleaving. This is because if the thickness of the substrate is greater than 100 μm, the nitride semiconductor layer is caught in the direction in which the substrate breaks at the time of cleavage, and the nitride semiconductor tends to be difficult to break due to its own cleavage property.
[0033]
Thereafter, in the same manner as in Example 1, after forming a reflecting mirror made of a dielectric multilayer film on the cleavage surface of the semiconductor bar using a sputtering apparatus, the bar was cut by dicing in the direction perpendicular to the cleavage surface. When an 8 mm × 0.5 mm square laser device was used, a pulse oscillation of 410 nm was exhibited at a threshold current density of 2 kA / cm 2. The resonator length of this laser element is also 0.8 mm.
[0034]
[Example 3]
A laser device was manufactured in the same manner as in Example 2 except that sapphire (C-plane, 0001) was used as the substrate 30 and the substrate was polished to 20 μm at the time of substrate polishing. The laser element shown can be produced.
[0035]
[Example 4]
A laser element was fabricated in the same manner as in Example 2 except that SiC (111 surface) was used for the substrate 30 and the substrate was polished to 80 μm at the time of substrate polishing. A device was fabricated.
[0036]
【The invention's effect】
As described above, according to the present invention, since the resonance surface can be formed by using the cleavage property of the nitride semiconductor, a resonance surface close to a very smooth mirror surface can be produced. For this reason, the threshold current of the laser element can be lowered, which is very significant in practical use of the nitride semiconductor laser element.
[Brief description of the drawings]
FIG. 1 is a unit cell diagram showing the plane orientation of a nitride semiconductor single crystal.
FIG. 2 is a schematic plan view showing an enlarged crystal structure of a nitride semiconductor grown on a substrate.
FIG. 3 is a schematic cross-sectional view illustrating a structure of a laser element according to one embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view showing the structure of a laser device according to another aspect of the present invention.
[Explanation of symbols]
10, 30 ... substrate 11 ... n-type contact layer 20 ... p-type current blocking layer 12 ... n-type optical confinement layer 13 ... n-type light guide layer 14 ... .. Active layer 15... P-type light guide layer 16... P-type optical confinement layer 17.

Claims (2)

Nitride semiconductor [Outside 1]
(1-100)
[Outside 2]
(10-10)
[Outside 3]
(01-10)
[Outside 4]
(-1100)
[Outside 5]
(-1010)
[Outside 6]
(0-110)
In the nitride semiconductor laser element in which the cleavage plane along any one of the plane orientations is at least one of the resonance surfaces,
On a GaN substrate, an n-type nitride semiconductor, an active layer having a quantum well structure, a p-type nitride semiconductor,
The n-type nitride semiconductor is in contact with the GaN substrate and in contact with the active layer and an In _x Al _y Ga _1-xy N (0 ≦ x, 0 ≦ y, x + y ≦ 1) layer containing Si. An In _x Ga _1-x N (0 ≦ x <1) layer,
A positive electrode on the p-type nitride semiconductor;
A nitride semiconductor laser device comprising a negative electrode on the back surface of the GaN substrate.   The nitride semiconductor laser element according to claim 1, wherein the active layer includes InGaN or GaN.

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