JPH11312841A - Nitride semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease the threshold of a laser element for use of a nitride semiconductor laser element at high output. SOLUTION: On a first main surface of a nitride semiconductor substrate 1 comprising first and second main surfaces, at least an n-type nitride semiconductor layer, an active layer 5, and a p-type nitride semiconductor layer are sequentially laminated, a p-electrode 20 is formed on the p-type nitride semiconductor layer side, and an n-electrode 21 is formed on the second main surface side of the nitride semiconductor substrate 1. Here, the p-type nitride semiconductor layer comprises a p-side clad layer 8 and a p-side contact layer 9 provided on the p-side clad layer 8, and a stripe-like waveguide region formed by removing a part of the nitride semiconductor from the p-side contact layer 9 side is formed, while the flat surface of nitride semiconductor continuous with both side surfaces of the stripe positioned closer to substrate side by at least about 0.2 μm in the p-side contact layer 9 direction from the lower end surface, in the film thickness direction of the p-side clad layer 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride semiconductor (In _a A
_{l b Ga 1-ab N,} 0 ≦ a, 0 ≦ b, relates to a laser element consisting of a + b ≦ 1), in particular a nitride semiconductor on the structure of the laser element to the substrate.

[0002]

2. Description of the Related Art We have fabricated a nitride semiconductor laser device including an active layer on a nitride semiconductor substrate, and have achieved the world's first continuous oscillation of more than 10,000 hours at room temperature (ICNS). '97 Proceedings, October 27-31, 1997, P444-446,
And Jpn.J.Appl.Phys.Vol.36 (1997) pp.L1568-1571, Pa
rt2, No. 12A, 1 December 1997). The basic structure is SiO ₂ partially formed on the sapphire substrate.
On the nitride semiconductor substrate made of n-GaN grown in the lateral direction on the SiO ₂ film via the film,
A plurality of nitride semiconductor layers forming a laser element structure are stacked. (Refer to Jpn.J.Appl.Phys.Vol.36 for details)

FIG. 3 is a schematic sectional view showing one structure of a conventional laser device. This figure is almost the same as the figure shown in the above-mentioned JJAP. As shown in this figure, in the conventional laser device, a ridge stripe corresponding to a waveguide region is provided above a p-side cladding layer having a superlattice structure of p-Al _0.14 Ga _0.86 N / GaN. Over both side surfaces of the p-side cladding layer and S
consisting iO ₂ insulating film is formed, p-GaN layer and electrically connected to the p-electrode through the insulating film is formed.

[0004]

In the conventional laser device, since the nitride semiconductor substrate is grown laterally on the SiO ₂ film, threading dislocations extending in the vertical direction from the interface with the sapphire substrate are formed. And the crystallinity is very small and the crystallinity is good. Furthermore, since the nitride semiconductor layer grown on the nitride semiconductor substrate inherits the properties of the substrate, crystal defects are reduced as a whole. Long life.

[0005] However, even if a substrate having good crystallinity is obtained and continuous oscillation is performed for a long time, it is still in the state of 2 mW output. In order to use a laser as a writing light source, it is necessary to realize continuous oscillation of 5,000 hours or more at an output of 10 times this. In general, the life of a laser element depends mostly on the current and voltage at the threshold,
It is very important to reduce the current and voltage at the threshold. Therefore, an object of the present invention is to reduce the threshold value of a nitride semiconductor laser device in order to use the laser device at a high output.

[0006]

According to a nitride semiconductor laser device of the present invention, at least an n-type nitride semiconductor is provided on a first main surface of a nitride semiconductor substrate having a first main surface and a second main surface. A nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer are sequentially stacked, a p-electrode is formed on the p-type nitride semiconductor layer side, and an n-electrode is formed on the second main surface side of the nitride semiconductor substrate A p-type nitride semiconductor layer, comprising: a p-side cladding layer; a p-side contact layer on the p-side cladding layer; To form a striped waveguide region formed by removing a part of the nitride semiconductor, and a plane of the nitride semiconductor continuous with both side surfaces of the stripe is formed in the thickness direction of the p-side cladding layer. , 0.2 from bottom face to p-side contact layer It characterized in that on the substrate side than m. The p-side cladding layer is a layer that acts as carrier confinement or light confinement. In the case of a nitride semiconductor laser device, the p-side cladding layer is formed of a layer having at least a nitride semiconductor layer containing Al, for example, AlGaN / G
It can be composed of a superlattice such as aN or AlGaN / InGaN. The p-side contact layer is a p-electrode forming layer for injecting carriers, is formed on the uppermost layer of the laser element, and can be composed of, for example, GaN, InGaN, or the like, and has a plurality of layers having different carrier concentrations. In some cases.

Preferably, the plane of the nitride semiconductor continuous with both side surfaces of the stripe is closer to the substrate than the lower end surface of the p-side cladding layer. Note that the lower end surface of the p-side cladding layer indicates an underlayer on which the p-side cladding layer is formed and an interface with the p-side cladding layer. Further, 0.2 μm in the direction from the lower end surface to the p-side contact layer refers to a state where 0.2 μm of the p-side cladding layer remains from the interface.

Further, an insulating film is formed on the stripe side surface of the stripe-shaped waveguide region and on the nitride semiconductor plane continuous with the stripe side surface.
The electrode is formed through the insulating film and is electrically connected to the p-side contact layer.

In the laser device according to the present invention, the p-electrode is bonded by a metal wire, and the bonding position is not directly above the stripe.
Au, Ag, Pt, Cu, Al or the like can be used as the metal wire, but Au is generally used.

[0010]

FIG. 1 is a schematic sectional view showing one structure of a laser device according to the present invention. The device is arranged in a direction perpendicular to the longitudinal direction of a striped waveguide, that is, in a direction parallel to a resonance surface. FIG. 2 shows a view when the slicing is cut. Basically, on a substrate 1 made of GaN, a crack prevention layer 2 made of InGaN, an n-side cladding layer 3 made of an AlGaN / GaN superlattice, an n-side light guide layer 4 made of GaN,
An active layer 5 having an InGaN barrier / InGaN well multiple quantum well structure, a cap layer 6 made of AlGaN, Ga
It has a structure in which a p-side light guide layer 7 made of N, a p-side cladding layer 8 made of an AlGaN / GaN superlattice, and a p-side contact layer 9 made of GaN are sequentially stacked. Further, a stripe-shaped waveguide region is formed from the p-side contact layer side until the surface of the n-side cladding layer is exposed,
Part of the nitride semiconductor layer has been removed. By the etching, a stripe-shaped waveguide region is formed on the n-side cladding layer 3. In the present invention, a layer on the substrate side from the active layer with the active layer as the center is called an n-side, and a layer on the p-electrode side is called a p-side nitride semiconductor layer.

Further, an insulating film 100 made of ZrO ₂ is formed on the side surface of the stripe and the plane of the n-side cladding layer 3 continuous with the side surface so as to expose the uppermost layer of the p-side contact layer 9. On the side contact layer 9, a p-electrode 20 electrically connected to the contact layer is provided with an insulating film 10.
0 is formed. The material of the insulating film is Si
O ₂ is frequently used, but in the present invention, rather than SiO ₂ ,
An oxide thin film containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta;
It is preferable to use at least one of SiC and AlN. Among them, oxides of Zr and Hf, BN, S
iC is used. In addition, SiC is CVD such as sputtering and vapor deposition.
Is an insulator because it is amorphous, and SiC that does not contain n and p-type impurities is also an insulator.

The insulating film 100 has side surfaces of the stripe waveguide,
By forming the nitride semiconductor layer on the plane of the nitride semiconductor layer continuous with the side surface, a substantially buried laser element can be manufactured. Further, the surface area of the p-electrode 20 is increased to facilitate bonding on the p-electrode, and a short circuit between the p- and n-electrodes is prevented. The etch stop is p
If it is above the lower end face of the side cladding layer, a leak current is likely to occur in the device due to factors such as extremely fine pits and defects formed in the stacked nitride semiconductor layers. If there is a leakage current at first, the laser oscillation is continued, so that the leakage current gradually increases, and eventually a short circuit occurs between the electrodes. Therefore, in the most preferable state of the present invention, if the etching stop is below the lower end surface of the p-side cladding layer, the leakage current is almost eliminated, and short-circuiting is hardly caused. In order to reduce the refractive index of the p-side cladding layer and increase the band gap energy, a nitride semiconductor containing Al such as AlGaN is used. Crystalline growth is more difficult than that of a semiconductor, and defects, pits, and the like tend to occur inside. When the etch stop is located below the lower end surface of the p-side cladding layer, p
This is because there are few defects in the nitride semiconductor layer on the layer side, so that there is no leakage current and no short circuit between the electrodes. Therefore, a laser element having excellent reliability can be obtained.

When a stripe-shaped waveguide is formed, the width of the stripe is adjusted to 4 μm to 0.5 μm, more preferably 3 μm to 1 μm. If the width is larger than 4 μm, the transverse mode tends to be multi-mode. If the width is smaller than 0.5 μm, it is difficult to form a stripe and the contact area with the electrode is small, so that the threshold value tends to increase.

In the laser device of the present invention, the p-electrode 20
By removing the bonding position of the wire 22 from immediately above the stripe, the stress at the time of bonding is not directly related to the active layer, so that the crystallinity of the active layer is not deteriorated. A device having excellent reliability during oscillation can be realized.

[0015]

[Embodiment 1] Hereinafter, a specific example of obtaining the laser device of FIG. 1 will be described. However, the structure of the laser device of the present invention is not necessarily limited to the device of FIG.

(Nitride semiconductor substrate 1) By MOVPE method
GaN on a 2 inch φ sapphire substrate at 600 ° C
200 angstrom thick first buffer layer made of
Undoped GaN at 1050 ° C
A second buffer layer is grown by 5 μm. 2nd bag
After the growth of the wafer layer, the substrate is taken out of the reaction
And a stripe width of 10 on the second buffer layer.
μm, SiO with stripe interval (window part) 2 μm _TwoMore
Forming a protective film. Protective film formation word, MOV the substrate again
Transferred to PE equipment and undoped GaN at 1050 ° C
Is grown from the window to the upper part of the protective film.
so that aN grows and connects laterally above the protective film
To After that, the wafer is subjected to HVPE (hydride
Phase growth method), and transfer the raw materials to Ga metal and HCl gas.
Si, 2 x 1
0 ¹⁸/cm^ThreeNitride semiconductor substrate made of doped GaN
1 is grown to a thickness of 200 μm. Thus nitride
To make a semiconductor substrate, a heterogeneous group such as sapphire
After growing a nitride semiconductor on a plate, the nitride semiconductor
Nitride semiconductor such as SiO2 partially on the surface
Almost film that does not grow or hardly grows
Is partially formed, and a nitride semiconductor is formed through a window of the protective film.
Is grown in the lateral direction so as to have a thickness of 100 μm or more.
Then, a nitride semiconductor substrate having few crystal defects can be obtained.

After the growth of the nitride semiconductor substrate made of Si-doped GaN, the substrate is taken out of the HVPE apparatus, and the sapphire substrate, the first and second buffer layers, the protective film, and the undoped GaN layer are removed using a polishing machine. To obtain a nitride semiconductor substrate 1 having an asgrown side as a first main surface and a sapphire substrate polishing side as a second main surface.

(Crack prevention layer 2) MOVP is formed on the first main surface of the nitride semiconductor substrate 1 obtained as described above.
Using the E method, a crack preventing layer 2 made of In _0.06 Ga _0.94 N is grown at 800 ° C. to a thickness of 0.15 μm. Incidentally, when the anti-crack layer is grown to a thickness of 0.5 μm or less using GaN, InGaN, or the like, Al
Although a clad layer containing a nitride semiconductor layer containing, becomes easier to grow, it can be omitted.

(N-side cladding layer 3) Subsequently, at 1050 ° C.
A layer made of undoped Al _0.16 Ga _0.84 N is grown to a thickness of 25 Å, then TMA is stopped, silane gas is flown, and a layer made of n-type GaN doped with 1 × 10 ¹⁹ / cm ^{3 of} Si is formed. It is grown to a thickness of 25 Å. These layers are alternately stacked to form a superlattice layer, and an n-side cladding layer 3 composed of a superlattice having a total film thickness of 1.2 μm is grown. The n-side cladding layer 3 is a nitride semiconductor layer containing Al, preferably Al _x Ga ₁ -xN (0 <x
It is desirable to have a superlattice structure including <1), and more preferably a superlattice structure in which GaN and AlGaN are stacked. In the case of a superlattice, if one of the layers is heavily doped with impurities and so-called modulation doping tends to improve the crystallinity, both may be doped in the same manner. When the n-side cladding layer 3 has a superlattice structure, the Al mixed crystal ratio of the entire cladding layer can be increased, so that the refractive index of the cladding layer itself decreases and the bandgap energy increases. It is very effective in lowering. Further, the use of the superlattice reduces the number of pits generated in the cladding layer itself as compared with the non-superlattice, thereby reducing the probability of short-circuit. The oscillation wavelength is 430 to 550, which is a long wavelength.
In a laser device of nm, this cladding layer may be GaN doped with an n-type impurity.

(N-side light guide layer 4) Subsequently, the silane gas is stopped, and the n-side light guide layer 4 made of undoped GaN is grown at 1050 ° C. to a thickness of 0.1 μm. The n-side light guide layer 4 can be formed of a layer containing a nitride semiconductor having a smaller band gap energy than AlGaN of the n-side cladding layer 3, and for example, GaN or InGaN can be grown. The n-side light guide layer 4 may be doped with an n-type impurity. Note that the oscillation wavelength is 430 to 55, which is a long wavelength.
In a 0 nm laser device, this guide layer may be a superlattice layer containing InGaN.

[0021] Next (active layer 5), at 800 ° C., a barrier layer made of Si-doped In _0.01 Ga _0.95 N is grown to the thickness of 100 Å, followed at the same temperature, an undoped In _0.2 Ga _{0 .8} N A well layer is grown to a thickness of 40 Å. A barrier layer and a well layer are alternately laminated twice, and an active layer 5 (MQW) having a multiple quantum well structure having a total thickness of 380 Å and ending with the barrier layer is grown. The active layer may be undoped as in this embodiment, or may be doped with an n-type impurity and / or a p-type impurity. The impurity may be doped into both the well layer and the barrier layer, or may be doped into either one.
Note that if only the barrier layer is doped with an n-type impurity, the threshold value tends to decrease.

(P-side cap layer 6) Next, at 1050 ° C.,
p-type Al _0.3 G doped with Mg at 1 × 10 ²⁰ / cm ³ , which has a larger band gap energy than the p-side light guide layer 7.
A p-side cap layer made of a _0.7 N is grown to a thickness of 300 Å.

(P-side light guide layer 7) Subsequently, at 1050 ° C.
Then, a p-side light guide layer 7 made of undoped GaN having a band gap energy smaller than that of the p-side cap layer 10 is grown to a thickness of 0.1 μm. p-side light guide layer 7
Also, the p-side cladding layer 8 can be formed of a layer containing a nitride semiconductor having a smaller band gap energy than AlGaN, and for example, GaN or InGaN can be grown.
The p-side light guide layer 8 may be doped with a p-type impurity. In a laser device having a long oscillation wavelength of 430 to 550 nm, the guide layer may be a superlattice layer containing InGaN.

(P-side cladding layer 8) Subsequently, a layer made of Mg-doped Al _0.16 Ga _0.84 N is grown at 1050 ° C. to a thickness of 25 Å, and then undoped GaN
Is grown to a thickness of 25 Å,
P-side cladding layer 8 composed of a superlattice layer having a total thickness of 0.6 μm
Grow. The preferred configuration of the p-side cladding layer 8 is the same as that of the n-side cladding layer 3 and will not be described. In a laser device having a long oscillation wavelength of 430 to 550 nm, this cladding layer is made of GaN doped with a p-type impurity.
But it is good.

(P-side contact layer 9) Finally, 1050
On the p-side cladding layer 8, Mg was added at 1 × 10 ²⁰ / cm
^The p-side contact layer made of ^3- doped p-type GaN is
It is grown to a thickness of 50 angstroms. The p-side contact layer is a p-type In _x Al _Y Ga _{1 -XYN} (0 ≦ X, 0 ≦
Y, X + Y ≦ 1), preferably Mg
GaN or InGaN doped with P, the p-electrode 20
And the most preferable ohmic contact is obtained. Since the contact layer 13 is a layer for forming an electrode, 1 × 10 ¹⁷ / cm
It is desirable to have a high carrier concentration of ³ or more. 1x1
If it is lower than 0 ¹⁷ / cm ^3, it tends to be difficult to obtain an electrode and a preferable ohmic. Further, the composition of the contact layer is GaN, InGaN, or GaN, InGaN.
When a superlattice containing is used, it is easy to obtain an electrode material and a preferable ohmic.

The wafer on which the nitride semiconductor has been grown as described above is taken out of the reaction vessel, and a 1.5 μm-wide stripe is formed on the surface of the uppermost p-side contact layer 9 through a mask having a predetermined shape. A protective film made of SiO ₂ is formed. After the formation of the protective film, etching is performed by RIE (reactive ion etching) using SiCl ₄ gas until the surface of the n-side cladding layer 3 is exposed as shown in FIG. Form a wave path.

After forming the stripe waveguide, an insulating film 100 made of ZrO ₂ is formed on the surface of the nitride semiconductor layer while keeping the SiO ₂ mask. After the formation of the insulating film 100, it is immersed in buffered hydrofluoric acid to dissolve and remove the SiO ₂ formed on the p-side contact layer,
Along with O ₂ , ZrO ₂ on the p-side contact layer is removed. As described above, the protective film for forming the waveguide region is formed of SiO ₂ , an insulating film made of a material different from SiO ₂ such as ZrO ₂ is formed thereon, and the insulating film on the contact layer is formed by the lift-off method. By removing only the film, a highly insulating film having a uniform thickness can be formed on the side surface of the stripe waveguide and on the surface of the nitride semiconductor layer continuous with the side surface.

After the formation of the insulating film 100, a p-electrode 20 made of Ni / Au is formed so as to obtain a good ohmic contact with the p-side contact layer 9 via the insulating film 100 as shown in FIG. On the other hand, on the second main surface side of the GaN substrate, Ti /
An n electrode 21 made of Al is formed on the entire surface.

After the formation of both the p and n electrodes, the GaN substrate 1 is cleaved at the M plane of the GaN substrate 1 (the surface corresponding to the side of the hexagonal prism when the nitride semiconductor is represented by a hexagonal prism), and the cleavage is performed. A resonator is made on the surface. It is needless to say that, when the stripe waveguide is formed beforehand, the cleavage plane is determined in advance, and the stripe direction is designed to be substantially perpendicular to the cleavage plane. Then, the laser chip is cut in a direction parallel to the stripe.

After the fabrication of the laser chip, the n-electrode 21 side of the GaN substrate is placed on a metalized heat sink, and an Au wire is wire-bonded to a position not directly above the stripe of the p-electrode 20 as shown in FIG. Laser element. When laser oscillation of this laser element was attempted at room temperature, the oscillation wavelength was 400 to 420 nm, and the threshold current density was 1.8.
Room temperature continuous oscillation was observed at kA / cm ² , and even when the current-voltage characteristics were measured, almost no initial leak current occurred. Increase the output by further increasing the current value, 40mW
However, no short circuit occurred in the element itself, and continuous oscillation continued for 50 hours or more.

[Embodiment 2] In the step of fabricating the laser device of Embodiment 1, the surface of the p-side contact layer 9 is coated with Si.
After forming a mask made of O ₂ , the etching depth is set to p
A laser device was fabricated in the same manner except that the side cladding layer was left at a film thickness of 0.2 μm. As a result, the threshold current density was increased to 2.0 kA / cm ² , and slight leakage occurred. No short circuit occurred in 50 hours.

FIG. 2 shows the laser device of the embodiment,
The graph shows the relationship between the threshold current density of the laser element and the etching depth when etching is performed from the side contact layer 9 side and the etching stop is performed for each nitride semiconductor layer. A is 0.1 μm from the upper end surface of the p-side cladding layer 8
In the figure, B is Example 2, C is the center of the p-side light guide layer 7, D is the center of the n-side light guide layer 4, E is Example 1 (the center of the n-side cladding layer 3), and F is the substrate. 0.1 μm from top
Shows where you entered. As shown in FIG.
In order to obtain a threshold current density of kA / cm ² or less, it is desirable to etch deeper than point B. 2.0kA /
When the threshold value is higher than cm ² , the laser element tends to be broken when continuous oscillation is performed at a high output for 500 hours or more.

[0033]

As described above, in the laser device of the present invention in which a nitride semiconductor is used as a substrate, the position where the waveguide stripe is formed is closer to the substrate than the predetermined position. A low and highly reliable laser element can be obtained. In addition, an insulating film is formed on the side surface of the stripe, and a large area p having a good ohmic through the insulating film is obtained.
Since the electrode is formed, a large bonding area can be obtained when wire bonding is performed on the p-electrode, so that an element which is very preferable in production technology can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention.

FIG. 2 is a schematic sectional view showing a structure of a laser device according to another embodiment of the present invention.

FIG. 3 is a schematic sectional view showing the structure of a conventional laser device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Dissimilar board | substrate 2 ... Underlayer 3 ... Protective film for nitride semiconductor substrate growth 1 ... Nitride semiconductor substrate 2 ... Crack prevention layer 3 ... n side cladding layer 4. ..N-side light guide layer 5 ... active layer 6 ... p-side cap layer 7 ... p-side light guide layer 8 ... p-side cladding layer 9 ... p-side contact layer 20 ... p electrode 21 ... n electrode 22 ... wire 100 ... insulating film

Claims (4)

[Claims] At least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer are provided on a first main surface of a nitride semiconductor substrate having a first main surface and a second main surface. Are sequentially stacked, a p-type electrode is formed on the p-type nitride semiconductor layer side, and an n-type electrode is formed on the second main surface side of the nitride semiconductor substrate, The p-type cladding layer and its p-type cladding layer
A p-side contact layer on the side cladding layer, and a striped waveguide region formed by removing a part of the nitride semiconductor from the p-side contact layer side; A nitride semiconductor laser device, wherein a plane of the nitride semiconductor continuous with the surface is closer to the substrate than 0.2 μm in a p-side contact layer direction from a lower end face in a thickness direction of the p-side cladding layer. 2. The nitride semiconductor laser device according to claim 1, wherein a plane of the nitride semiconductor continuous with both side surfaces of the stripe is closer to the substrate than a lower end surface of the p-side cladding layer. 3. An insulating film is formed on a stripe side surface of the stripe-shaped waveguide region and a nitride semiconductor plane continuous with the stripe side surface, and the p-electrode is formed via the insulating film. 3. The nitride semiconductor laser device according to claim 1, wherein the nitride semiconductor laser device is electrically connected to a p-side contact layer. 4. The nitride according to claim 1, wherein said p-electrode is bonded by a metal wire, and a bonding position thereof is not immediately above said stripe. Semiconductor laser device.