JP2002084036A - Nitride semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser element free of peeling of an electrode with high reliability by forming a resonance surface of a nitride semiconductor laser by cleavage. SOLUTION: This nitride semiconductor laser element has a striped positive electrode and a striped waveguide on a nitride semiconductor laser corresponding to the position of the positive electrode, the resonance surface of the waveguide is formed by cleavage of the substrate, and the positive electrode end surface on the side of a cleaved surface is inside the cleaved surface. Therefore, the shock caused by breaking at the cleavage time is not transmitted to the electrode end surface, and the electrode does not peel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride semiconductor (In).
_{X Al Y Ga 1-XY N} , 0 ≦ X, 0 ≦ Y, a laser element made of X + Y ≦ 1).

[0002]

2. Description of the Related Art A nitride semiconductor is known as a material for a laser device that emits light in the ultraviolet to blue region, and the present applicant has recently used this material to generate a laser oscillation of 410 nm at room temperature under pulse current. Announced (for example, Jpn.J.
Appl. Phys. Vol 35 (1996) pp. L74-76). The published laser is of a so-called electrode stripe type, in which laser oscillation is performed with the stripe width of the nitride semiconductor layer including the active layer set to several tens μm. FIG. 4 is a perspective view showing the shape of the laser element. This laser device has a nitride semiconductor layer 12 having a double hetero structure in which an active layer is sandwiched between an n layer and a p layer on a substrate 11, and a nitride including the p layer and the active layer. The semiconductor layer is etched into a stripe shape having a width of several tens of μm, and a stripe-shaped positive electrode 13 is formed on the uppermost p layer.
Is formed, and a striped negative electrode 14 is also formed on the n-layer exposed by the etching. The waveguide of the active layer is formed in the active layer below the positive electrode 13.

In order to form an active layer serving as a waveguide, a resonance surface is required on an end face of the nitride semiconductor layer. Generally, a nitride semiconductor is often grown on a substrate that is difficult to cleave, such as sapphire. Therefore, the resonance surface is formed by etching. The resonance surface of the laser device shown in FIG. 4 is formed by etching. As shown in this figure, when the resonance surface is formed by etching, the etching horizontal surface of the nitride semiconductor layer is exposed outside the resonance surface.
This etching horizontal surface has a disadvantage that the laser beam emitted from the resonance surface is reflected by the surface, and the spot shape of the laser beam is changed.

A nitride semiconductor is a semiconductor material used for other light emitting devices, for example, GaAs, Si, GaP, InA
Compared to lGaP or the like, the crystal has a higher hardness, and thus has a property of being difficult to be etched. Therefore, a resonance surface formed by cleavage is more likely to obtain a smooth surface than a resonance surface formed by etching. Therefore, an attempt to create a resonance surface by cleaving sapphire by changing the growth surface of the sapphire substrate has been made, for example, in Japanese Patent Application Laid-Open No. Hei 8-15 / 15.
No. 3931.

[0005]

When fabricating a stripe waveguide type semiconductor laser, the width of the stripe corresponding to the waveguide region is adjusted to several tens μm or less as described above. That is, the waveguide region of the active layer is manufactured by setting the stripe width of the uppermost layer electrode to several tens of μm. Therefore, the resonance surface is formed on the cleavage plane by cleaving the nitride semiconductor wafer having the stripe-shaped electrodes formed on the uppermost layer. However, the nitride semiconductor is a heteroepitaxial growth grown on a substrate made of a material different from the nitride semiconductor, and is also a very hard material as described above. When the resonance surface is formed, the electrode formed on the end surface of the cleavage surface tends to float or peel off due to the impact at the time of cleavage. Particularly, since the stripe width of the positive electrode is very narrow, the tendency is strong. Such a state becomes an obstacle when forming a mirror on the end face of the cleavage plane, and may cause a short circuit between the electrodes.

Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to form a resonance surface of a nitride semiconductor layer by cleavage, and to provide a reliable semiconductor device without peeling of electrodes. An object of the present invention is to provide a laser element having high performance.

[0007]

According to the present invention, there is provided a nitride semiconductor laser device having a stripe-shaped positive electrode and a stripe-shaped waveguide in a nitride semiconductor layer corresponding to the position of the positive electrode. In the laser device, a resonance surface of the waveguide is formed by cleavage of a substrate, and a positive electrode end surface on a cleavage surface side is located inside the cleavage surface.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a laser device according to the present invention, sapphire (Al ₂ O ₃ ), SiC, spinel (M
gAl ₂ O ₄ ), ZnO, Si and the like, which are conventionally known, can be used. The laser device of the present invention is not limited to a substrate having almost no cleavage such as sapphire,
The present invention is also applicable to a laser device in which a nitride semiconductor is stacked on a substrate having a cleavage property such as spinel, Si, or SiC.

For example, the following method is preferable for cleaving a wafer on which a nitride semiconductor is stacked to form a resonance surface of the nitride semiconductor. It is a nitride semiconductor

[Outside 1] In this method, a plane (hereinafter, referred to as an M plane) is used as a cleavage plane. Although the nitride semiconductor has a rhombohedral structure,
Can be approximated by a hexagonal system as shown in FIG. The nitride semiconductor is M
C-axis oriented nitride semiconductor is grown on the substrate, and the substrate is cleaved so that the M-plane of the nitride semiconductor is exposed. A laser element having a surface can be manufactured. The M plane can be represented by six types of plane orientations along each side surface of the hexagonal prism shown in the figure.
It is assumed that the plane represents all plane directions.

FIG. 2 shows a method of using a sapphire substrate as a means of forming a resonance surface by cleavage, for example. In FIG. 2, the M-plane of the nitride semiconductor is (0001) plane of the sapphire substrate = (C plane) So that it is parallel to the nitride semiconductor

[Outside 2] Surface (hereinafter referred to as surface A), and a sapphire substrate

[Outside 3] When cleavage is performed on a plane (hereinafter referred to as an R plane), another M plane of the nitride semiconductor at a position similar to the plane orientation of the R plane of the sapphire substrate is cleaved, so that the M plane of the nitride semiconductor resonates. A laser element having a surface can be manufactured.

Although this method has been described for a sapphire substrate, the other substrates are similarly cleaved so that the M-plane of the nitride semiconductor is exposed to the resonance surface side. Can be manufactured.

FIG. 3 is a perspective view showing the structure of a laser device according to the present invention, wherein 1 is a substrate, 2 is a nitride semiconductor layer having a double hetero structure in which an active layer is sandwiched between an n-type layer and a p-type layer. Numeral 3 is a stripe-shaped positive electrode provided on the p-type layer which is the uppermost layer of the nitride semiconductor layer, and numeral 4 is a negative electrode provided on the n-type layer exposed by etching. The waveguide is provided on the active layer corresponding to the position of the positive electrode 3.

As shown in FIG. 3, in the laser device of the present invention, the positive electrode is formed such that the stripe end face of the positive electrode 3 on the resonance surface side is inside the cleavage plane.
As described above, since the positive electrode end face is formed inside the cleavage plane, the peeling of the positive electrode due to the impact at the time of cleavage can be prevented. Also, at the time of cleavage, p
Even if chipping or cracking occurs at the corners of the mold layer, the electrodes are not formed at the corners, and there is no risk of short-circuiting due to current injection, so that a highly reliable laser element can be provided. A preferable distance between the end face of the positive electrode 3 and the cleavage plane is adjusted to 10 μm or less. When the distance is more than 10 μm, the threshold value tends to increase. It should be noted that there is no particular problem even if the negative electrode 4 is formed up to a position substantially coincident with the end face of the n-type layer.

[0014]

[Embodiment 1] FIG. 5 is a schematic sectional view showing a structure of a laser device according to an embodiment of the present invention, in which the device is cut in a direction perpendicular to a stripe-shaped electrode. Is shown. Hereinafter, a specific example of the present invention will be described with reference to FIG. In the embodiment, the laser element is manufactured by MOVPE (metal organic chemical vapor deposition), but not only MOVPE, but also, for example, MBE (molecular beam chemical vapor deposition), HDVP
It can also be grown using other known nitride semiconductor vapor phase growth methods such as E (halide vapor phase epitaxy).

A sapphire substrate 31 having A surface as a main surface and an orientation flat (orientation flat) surface as a C surface.
Is placed in the reaction vessel of the MOVPE apparatus, and then a buffer layer 32 of GaN is formed on the surface of the sapphire substrate 1 at a temperature of 500 ° C. with a thickness of 200 Å using TMG (trimethylgallium) and ammonia as source gases. Let it grow. The buffer layer 32 has an effect of alleviating lattice mismatch between the substrate and the nitride semiconductor.
It is also possible to grow AlGaN or the like. It is known that by growing this buffer layer, the crystallinity of the n-type nitride semiconductor grown on the substrate is improved. In particular, when GaN or AlGaN is used, the crystallinity of the nitride semiconductor is improved. The M-plane of the nitride semiconductor to be grown tends to be parallel to the C-plane of sapphire.

Subsequently, the temperature was raised to 1050 ° C., and TMG, ammonia and SiH 4 as donor impurities were added to the source gas.
An n-type contact layer 33 made of Si-doped n-type GaN is grown to a thickness of 4 μm using (silane) gas.
The n-type contact layer is made of In _X Al _Y Ga _{1 -XYN} (0 ≦ X, 0
≦ Y, X + Y ≦ 1), especially GaN,
By using InGaN, in particular, Si-doped GaN, an n-type layer having a high carrier concentration can be obtained,
Also, a favorable ohmic contact with the negative electrode is obtained,
The threshold current of the laser device can be reduced.
As the material of the negative electrode, Al, Ti, W, Cu, Zn, S
A metal or alloy such as n or In can provide an ohmic.

Next, the temperature is lowered to 750 ° C., and TMG, TMI (trimethylindium) and ammonia are used as source gases, silane gas is used as impurity gas, and Si-doped In0.
A crack prevention layer 34 of 1Ga0.9N is grown to a thickness of 500 angstroms. This crack prevention layer 3
4 is an n-type nitride semiconductor containing In, preferably InG
By growing with aN, it becomes possible to grow the n-type optical confinement layer 35 made of a nitride semiconductor containing Al to be grown next with a thick film. In the case of LD, it is necessary to grow a layer to be a light confinement layer and a light guide layer with a thickness of, for example, 0.1 μm or more. Conventionally, GaN, AlG
When a thick AlGaN was grown directly on the aN layer, cracks were formed in the AlGaN grown later, making it difficult to fabricate the device. It can be prevented from entering. In addition, even if the optical confinement layer to be grown next is grown as a thick film, it can be grown with good film quality. Preferably, the crack prevention layer 34 is grown to a thickness of 100 Å or more and 0.5 μm or less. If the thickness is less than 100 Å, it does not easily act as a crack preventing layer as described above. If the thickness is more than 0.5 μm, the crystals themselves tend to turn black. Although the crack prevention layer 34 can be omitted depending on the growth method and the growth apparatus, it is desirable to grow the layer in order to manufacture a laser element.

Next, the temperature is increased to 1050 ° C., and TEG, TMA (trimethylaluminum) and ammonia are used as source gases, and silane gas is used as an impurity gas.
An n-type optical confinement layer 35 of type Al0.3Ga0.7N is grown to a thickness of 0.5 .mu.m. The n-type optical confinement layer is A
and an n-type nitride semiconductor containing l, preferably binary mixed crystal or ternary mixed crystal _{Al Y Ga 1-Y N (} 0 <Y ≦ 1)
By doing so, a material having good crystallinity can be obtained, and the difference in refractive index from the active layer is increased, which is effective for confining laser light in the vertical direction. This layer is usually preferably grown to a thickness of 0.1 μm to 1 μm. If the thickness is less than 0.1 μm, it is difficult to function as a light confinement layer. If the thickness is more than 1 μm, even if AlGa grown on the crack prevention layer
Even with N, cracks tend to be easily formed in the crystal, and it tends to be difficult to produce an element.

Subsequently, TMG, ammonia,
Si-doped n-type GaN using silane gas as impurity gas
The n-type light guide layer 36 is grown to a thickness of 500 angstroms. The n-type light guide layer includes n containing In.
-Type nitride semiconductor or n-type GaN, preferably ternary or binary mixed crystal In _x Ga ₁ -xN (0 ≦ X ≦ 1)
And This layer is typically 100 Å to 1 μm
It is desirable to grow with a film thickness of InGaN,
By using GaN, the next active layer can easily have a quantum structure.

Next, an active layer 37 is grown using TMG, TMI, and ammonia as source gases. The active layer has a temperature of 7
While maintaining the temperature at 50 ° C., first, a well layer made of non-doped In0.2Ga0.8N is grown to a thickness of 25 Å. Next, a barrier layer made of non-doped In0.01Ga0.95N was deposited at the same temperature by changing only the molar ratio of TMI.
It is grown to a thickness of 0 Å. This operation 5
This is repeated twice, and finally, a well layer is grown to grow an active layer 37 having a multiple quantum well structure with a total thickness of 375 Å.

After growing the active layer 37, the temperature is raised to 1050 ° C., and TMG, TMA, ammonia, Cp2Mg (cyclopentadienylmagnesium) as an acceptor impurity source, and a p-type cap made of Mg-doped p-type Al0.2Ga0.8N. Layer 38 is grown to a thickness of 100 Å. This p-type cap layer 38 is 1 μm or less, more preferably 10 Å or more, and 0.1 μm or less.
The growth with the following thickness acts as a cap layer for preventing the active layer made of InGaN from being decomposed, and a p-type cap layer made of a p-type nitride semiconductor containing Al is formed on the active layer. By growing 38, the light emission output is significantly improved. Conversely, p in contact with the active layer
If the layer is made of GaN, the output of the device will be reduced to about 1/3. This is presumed to be because AlGaN is more likely to be p-type than GaN and has an effect of suppressing the decomposition of InGaN during growth of the p-type cap layer 38, but the details are unknown. If the thickness of the p-type cap layer 38 is greater than 1 μm, cracks tend to occur in the layer itself, which tends to make element fabrication difficult. Note that the p-type cap layer 38 can also be omitted.

Next, while maintaining the temperature at 1050 ° C.,
Mg-doped p-type G using MG, ammonia, Cp2Mg
A p-type light guide layer 39 made of aN is grown to a thickness of 500 Å. The p-type light guide layer is also made of a p-type nitride semiconductor containing In or p-type GaN, preferably binary or ternary mixed crystal of In _x Ga ₁ -xN (0 ≦ X ≦
1) grown, usually 100 Å to 1 μm
It is desirable to grow with a film thickness of InGaN,
By using GaN, the next p-type optical confinement layer can be grown with good crystallinity.

Subsequently, TMG, TMA, ammonia, C
Using p2Mg, a p-type optical confinement layer 40 of Mg-doped Al0.3Ga0.7N is grown to a thickness of 0.5 .mu.m. The p-type optical confinement layer is composed of a p-type nitride semiconductor containing Al, and is preferably a binary mixed crystal or a ternary mixed crystal Al
By setting YGa1-YN (0 <Y ≦ 1), a material having good crystallinity can be obtained. Like the n-type optical confinement layer, the p-type optical confinement layer is preferably grown to a thickness of 0.1 μm to 1 μm. By forming the p-type nitride semiconductor containing Al, such as AlGaN, with the active layer, By increasing the refractive index difference, it effectively acts as a vertical light confinement layer for laser light.

Subsequently, TMG, ammonia, Cp2Mg
Is used to grow a p-type contact layer 41 of Mg-doped p-type GaN with a thickness of 0.5 μm. The p-type contact layer is p-type In _x Al _Y Ga _{1 -XYN} (0 ≦ X, 0 ≦ Y, X +
Y ≦ 1), especially InGaN, Ga
N, and among them, p-type GaN doped with Mg,
A p-type layer having the highest carrier concentration is obtained, good ohmic contact with the positive electrode is obtained, and the threshold current can be reduced. Ni, Pd, I
A metal or alloy having a relatively high work function, such as r, Rh, Pt, Ag, or Au, can easily obtain an ohmic.

When the wafer on which the nitride semiconductors are stacked as described above is taken out of the reaction vessel and one end of the wafer is observed with a microscope, hexagonal columnar nitride semiconductors are growing and the sapphire orientation flat surface (C surface) )
One side (M plane) of the hexagon was parallel. In addition,
Since the thickness of the buffer layer tends to be thinner at the edge of the wafer, the shape of the nitride semiconductor crystal tends to be easily observed.

Next, the wafer is selectively etched from the uppermost p-type contact layer 41 by a reactive ion etching (RIE) apparatus to expose the surface of the n-type contact layer 33 where the negative electrode 43 is to be formed. . The etched shape is the direction of the resonator to be formed later (M-plane of the nitride semiconductor)
To the shape of a stripe perpendicular to.

Next, selective mesa etching is performed by RIE from above the p-type contact layer 41, and the p-type contact layer 41 and the p-type optical confinement layer 40 are etched into a stripe shape having a width of 3 μm to form a ridge. By forming the p-type layer above the active layer in a ridge shape, a waveguide of the active layer can be formed and the threshold value of the laser device can be reduced.

Next, a mask of a predetermined shape is formed on the nitride semiconductor layer on the surface, and a positive electrode 42 containing Ni and Au is formed on the surface of the striped p-type contact layer. As shown in FIG. 3, the shape of the electrode is such that the end face of the stripe of the positive electrode is separated from the cleaved end face by 5 μm.

On the other hand, the previously exposed n-type contact layer 3
3, a continuous striped negative electrode 43 made of Ti and Al is formed.

The sapphire substrate of the wafer as described above is polished to a thickness of 80 μm. After polishing, a scratch is formed on the polished surface at a position corresponding to the R surface of the sapphire substrate by a diamond point cutter, and the wafer is cleaved from the scratch to produce a bar-shaped wafer. In this way, a break occurs (breaks) from the back side of the sapphire substrate
Thereby, the nitride semiconductor can be cleaved from the M-plane without peeling. Preferably, sapphire is polished to a thickness of 150 μm or less and 10 μm or more to reduce the thickness. If the thickness is less than 10 μm, the wafer tends to crack during polishing, and if the thickness is more than 150 μm, cleavage tends to be difficult. The M-plane is exposed on the surface corresponding to the position of the active layer on the nitride semiconductor surface cleaved as described above, and is substantially a mirror surface serving as a laser resonance surface and perpendicular to the substrate. , Planes parallel to each other were obtained.

Thereafter, according to a conventional method, a dielectric multilayer film is formed on the opposing resonance surfaces, and the wafer is divided at a position parallel to the stripe-shaped electrodes to form a 500 μm square laser chip. None of the positive electrodes 41 were peeled off. Further, no short circuit was observed between the pn junctions.

[Embodiment 2] Spinel (MgA)
A nitride semiconductor laser device structure is manufactured in the same manner as in Example 1 except that (111 plane) of (1 2 O 4) is used. Similarly, after the p-type contact layer 41 and the p-type optical confinement layer 40 are formed in a ridge shape, the positive electrode 42 and the negative electrode 43 are formed.

Next, after the substrate is polished to 100 μm, the substrate is cleaved in a direction in which the M plane of the nitride semiconductor layer is exposed to form a resonance surface. The distance between the end face of the cleavage plane and the end face of the positive electrode is 3 μm.

Similarly, no end of the positive electrode was peeled off from the p-type layer in this laser element, and there was no defect due to a short circuit inside the element.

Example 3 A laser device structure of a nitride semiconductor was manufactured in the same manner as in Example 1 except that the (0001) plane of 6H-SiC was used for the substrate 31, and a p-type contact layer 41 was formed. After the p-type light confinement layer 40 and the ridge shape are formed, a striped positive electrode 42 and a negative electrode 43 are formed.

Next, after the substrate is polished to 100 μm, the substrate is cleaved in a direction in which the M-plane of the nitride semiconductor layer is exposed to form a resonance surface. The distance between the end face of the cleavage plane and the end face of the positive electrode is 7 μm.

Similarly, in this laser element, no end of the positive electrode was peeled off from the p-type layer, and there was no defect due to a short circuit inside the element.

[0038]

As described above, in the laser device of the present invention, since the distance between the end face of the positive electrode and the end face of the cleavage plane on the resonance plane is large, the electrode is peeled off by the impact due to the break during cleavage. And a highly reliable laser device without any problems. Also, since the resonance surface is formed by cleavage, unlike the cleavage surface formed by etching, there is no nitride semiconductor surface protruding on the laser light emission side, so that the laser light is reflected by the protrusion surface and changes its shape. Nothing.

[Brief description of the drawings]

FIG. 1 is a unit cell diagram showing the plane orientation of sapphire and a nitride semiconductor single crystal.

FIG. 2 is a schematic view showing a plane orientation of a nitride semiconductor grown on a sapphire A plane.

FIG. 3 is a perspective view showing the structure of the laser device of the present invention.

FIG. 4 is a perspective view showing the structure of a conventional laser element.

FIG. 5 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Nitride semiconductor layer 3 ... Positive electrode 4 ... Negative electrode

Claims (1)

[Claims] 1. A nitride semiconductor laser device having a stripe-shaped positive electrode and a stripe-shaped waveguide in a nitride semiconductor layer corresponding to the position of the positive electrode, wherein the resonance surface of the waveguide is formed by cleavage of a substrate. Wherein the end face of the positive electrode on the cleavage plane side is inside the cleavage plane.