JP3264163B2 - Nitride semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a nitride semiconductor (In _X A).
_{l 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 has been studied as a semiconductor material that oscillates in a green to ultraviolet range. A nitride semiconductor laser device is disclosed in, for example, Japanese Patent Application Laid-Open No. 6-152072. This publication discloses a laser having a double hetero structure in which an active layer is sandwiched between cladding layers lattice-matched, and a gain guided laser such as an electrode stripe type, a mesa stripe type, or a hetero isolation stripe type as an element structure. And a buried hetero-stripe type index guided laser.

In general, in a gain-guided laser device, a current spreads in a cladding layer, so that a transverse mode laser beam is controlled to obtain a single mode stable transverse mode beam and to reduce astigmatism. For the purpose of reducing the size, a refractive index waveguide type laser element in which a portion corresponding to a lateral direction of the active layer, that is, a direction parallel to the laser resonance direction is sandwiched by a material having a lower refractive index than the active layer is employed. The refractive index waveguide type laser device disclosed in the above publication also has an i-type InAl
Although sandwiched by GaN, a quaternary mixed crystal nitride semiconductor has a disadvantage that it is very difficult to grow a crystal, and for example, it is difficult to grow a nitride semiconductor having a thickness of several μm.

[0004]

In order to confine the laser light in the lateral direction of the nitride semiconductor laser device, it is desirable to select a material that is more realistic and effectively confine the light. . Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to obtain a stable laser device by performing lateral mode light confinement of an active layer of a nitride semiconductor laser device. is there.

[0005]

According to the present invention, there is provided a laser device comprising:
an active layer that oscillates a laser between the n-type nitride semiconductor and the p-type nitride semiconductor layer, wherein the n-type nitride semiconductor layer, the active layer, and the p-type A dielectric thin film is formed on the surface of the nitride semiconductor layer, a metal thin film is formed on the surface of the dielectric thin film, and a second dielectric thin film is further formed on the surface of the metal thin film. I do.

The metal thin film is made of Al, Ag, Ni,
It is one selected from Cr and Pt.

[0007]

In the laser device of the present invention, a dielectric thin film and a metal thin film are formed on the surfaces of an n-type nitride semiconductor, an active layer and a p-type nitride semiconductor in a direction parallel to the laser resonance direction. In other words, since the side surface of the active layer in the direction parallel to the resonance direction of the laser light is covered with the dielectric thin film and the metal thin film, light in the transverse mode of the laser light can be confined. Easy to obtain laser light.

Further, if the metal thin film is exposed on the surface, the positive electrode and the negative electrode may be short-circuited via the conductive material. Therefore, it is necessary to further form a second dielectric thin film on the surface of the metal thin film. Thereby, a short circuit between the electrodes can be prevented.

[0009]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to the present invention, and FIG. 2 is a perspective view showing the shape of the laser device shown in FIG. FIG. 1 is a cross-sectional view when the laser device of FIG. 2 is cut in a direction parallel to a resonance surface. The basic structure of this laser device is an insulating substrate 1
An n-type contact layer 2, n made of a nitride semiconductor
It has a laminated structure of a p-type light confinement layer 3, an n-type light guide layer 4, an active layer 5, a p-type light guide layer 6, a p-type light confinement layer 7, and a p-type contact layer 8. Further, a stripe-shaped etching is performed from the side of the uppermost p-type contact layer 8 to the n-type contact layer 2 to reduce the width of the active layer so that current is concentrated on the active layer. A positive electrode 30 is provided on the p-type contact layer 8 of the resonator formed into a stripe by etching, and a negative electrode 20 is provided on the n-type contact layer 2 in a direction parallel to the positive electrode 30.

The substrate 1 is made of sapphire (Al ₂ O ₃ , A surface, C
Surface, R surface), and an insulating substrate such as spinel (MgAl ₂ O ₄ , 111 surface).
A conventionally known substrate made of a single crystal such as O, Si, and ZnO is used. In this figure, an insulating substrate is used.

The n-type contact layer 2 is made of In _x Al _Y Ga
It can be composed of _1-XYN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1). In particular, by being composed of GaN, InGaN, and particularly of GaN doped with Si, an n-type layer having a high carrier concentration can be formed. As a result, a preferable ohmic contact with the negative electrode 20 is obtained, so that the threshold current of the laser element can be reduced. The material of the negative electrode 20 is A
Metals or alloys such as l, Ti, W, Cu, Zn, Sn, and In can provide an ohmic. Not only GaN but also nitride semiconductors have the property of being n-type due to nitrogen vacancies formed in the crystal even when they are non-doped (in a state where impurities are not doped). However, donor impurities such as Si, Ge, and Sn are added during crystal growth. By doping, a nitride semiconductor having a high carrier concentration and exhibiting preferable n-type characteristics can be obtained.

The n-type optical confinement layer 3 is made of an n-type nitride semiconductor containing Al, and preferably has a binary or ternary mixed crystal of Al _Y Ga ₁ -YN (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 the longitudinal mode of laser light. This layer is usually preferably grown to a thickness of 0.1 μm to 1 μm. When the thickness is less than 0.1 μm, it does not easily function as a light confinement layer, and when the thickness is more than 1 μm, cracks are liable to be formed in the crystal, and it tends to be difficult to produce an element.

The n-type light guide layer 4 is composed of an n-type nitride semiconductor containing In or n-type GaN, preferably a ternary mixed crystal or a binary mixed crystal of In _x Ga ₁ -xN (0 ≦ 0). X <
1). This layer is typically 100 Angstroms to 1
It is desirable to grow with a film thickness of μm, especially InGa
By using N and GaN, the next active layer 5 can easily have a quantum well structure.

The active layer 5 contains In as described above.
Ternary mixed crystal In_X
Ga_1-XN (0 <X <1). Ternary mixed crystal InGaN
Can be obtained with better crystallinity than quaternary mixed crystals
Thus, the light emission output is improved. Among them, the activity is particularly preferable.
Insulating layer_XGa_1-XN well layer and more than the well layer
A barrier layer made of a nitride semiconductor having a large band gap;
Quantum well structure (MQW: Multi-quantum-
well). Similarly, the barrier layer is made of ternary mixed crystal In. _{X '}Ga
_{1-X '}N (0 ≦ X ′ <1, X ′ <X) is preferable, and well + barrier
+ Well + ... + barrier + well layer
Construct a quantum well structure. Thus, the active layer is formed of InG
Assuming that MQW has a stacked aN, about 3
Realization of high-output LD between 65 nm and 660 nm
Can be. Furthermore, it consists of InGaN on the well layer
When the barrier layers are stacked, the barrier layer made of InGaN becomes Ga
The crystal is softer than the N and AlGaN crystals. for that reason
Since the thickness of the AlGaN cladding layer can be increased, the laser
Oscillation can be realized. In addition, InGaN and GaN
Different crystal growth temperatures. For example, in the MOVPE method, In
GaN grows at 600-800 ° C.,
GaN is grown at a temperature higher than 800 ° C. Therefore,
After growing a well layer made of InGaN,
If you try to grow a barrier layer, increase the growth temperature
I need to do it. When the growth temperature is raised, it grows first
Wells with good crystallinity because the InGaN well layer is decomposed
It is difficult to get a door layer. Furthermore, the thickness of the well layer is several tens
And the well layer of the thin film decomposes to M
It becomes difficult to produce QW. In contrast, in the present invention
The well layer and the barrier layer are made of InGaN because the barrier layer is also made of InGaN.
Can be grown at the same temperature. Therefore, the well layer formed earlier
Form MQW with good crystallinity because it does not decompose
be able to. This shows the most preferred aspect of MQW
In addition, the well layer is made of InGaN, and the barrier layer is made of G.
Band of barrier layer rather than well layer like aN, AlGaN
Any composition if the gap energy is increased
good.

The total thickness of the active layer 5 having a multiple quantum well structure is 1
It is preferable to adjust the thickness to 00 angstrom or more.
If the thickness is less than 100 angstroms, the output does not increase sufficiently, and laser oscillation tends to be difficult. If the thickness of the active layer is too large, the output tends to decrease, and it is desirable that the thickness be adjusted to 1 μm or less, more preferably 0.5 μm or less. If the thickness is more than 1 μm, the crystallinity of the active layer deteriorates, or the laser light spreads in the active layer, and the threshold current tends to increase.

Next, the p-type light guide layer 6 is made of a nitride semiconductor containing In or GaN, preferably a binary or ternary mixed crystal of In _Y Ga _{1 -Y} N (0 <Y ≦ 1). To grow). The light guide layer 6 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 7 can be grown with good crystallinity. Note that a p-type nitride semiconductor can be obtained by doping an acceptor impurity such as Zn, Mg, Be, Cd, or Ca during crystal growth, and Mg exhibits the most preferable p-type characteristics. After crystal growth, at 400 ° C. in an inert gas atmosphere.
By annealing as described above, a lower resistance p
You can get the mold.

The p-type confinement layer 7 is made of a p-type nitride semiconductor containing Al, and preferably has a binary or ternary mixed crystal of Al _Y Ga ₁ -YN (0 <Y ≦ 1). By doing so, one with good crystallinity can be obtained. This p-type optical confinement layer is n
As in the case of the type optical confinement layer, it is desirable to grow the layer to a thickness of 0.1 μm to 1 μm. By using a p-type nitride semiconductor containing Al such as AlGaN, the refractive index difference from the active layer can be increased. Effectively function as a light confinement layer for longitudinal mode laser light.

The p-type contact layer 8 is made of p-type In _x Al _Y Ga
_1-XYN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1). In particular, when InGaN, GaN, and particularly p-type GaN doped with Mg, p-GaN having the highest carrier concentration is used.
A mold layer is obtained, good ohmic contact with the positive electrode 30 is obtained, and the threshold current can be reduced. Materials of the positive electrode 30 include Ni, Pd, Ir, Rh, Pt,
A metal or alloy having a relatively high work function, such as Ag or Au, can easily obtain an ohmic.

Next, in the laser device of the present invention, as shown in FIG. 1, the p-type contact layer 8, the p-type optical confinement layer 7, and the p-type
The light guide layer 6, the active layer 5, the n-type light guide layer 4, the n-type light confinement layer 3, and the n-type contact layer are etched in a stripe shape. The light emitted from the active layer 5 resonates in the length direction of the stripe and oscillates in a laser. Light in the vertical direction is controlled by n-type and p-type light confinement layers. The light in the lateral direction is confined by the dielectric thin film 40 and the metal thin film 50 formed on the etched end face in the direction parallel to the resonance direction of the laser light.

Although the stripe width of the p-type contact layer 8 is not particularly limited, if it is adjusted to 10 μm or less, more preferably 5 μm or less, and most preferably 3 μm or less,
The astigmatism of the laser is reduced and the threshold current is also reduced. The etching means preferably uses dry etching, for example, reactive ion etching, ion milling,
ECR etching, focused ion beam etching, ion beam assisted etching, or the like can be used.

Next, in order to form the dielectric thin film 40, commonly used gas-phase film forming means such as plasma CVD, sputtering and molecular beam deposition can be used. Examples of the material include SiOx, SiN, High dielectric materials such as AlN and Al ₂ O ₃ can be used. Further, these dielectrics may be formed by laminating a plurality of thin films to form a dielectric multilayer film that reflects light in the lateral direction of the active layer 5. The dielectric thin film is, for example, 0.0
It can be formed with a film thickness of about 1 μm to 50 μm.

Further, in the laser device of the present invention, a metal thin film 50 for reflecting the light of the active layer is formed on the dielectric thin film 40. By forming the metal thin film 50, light in the lateral direction of the active layer 5 can be completely confined. As a material of the metal thin film, a nitride semiconductor laser beam of Al, Ag, Ni, Cr, Pt or the like (for example, 380 nm to 550 nm)
It is desirable to select a material having a high reflectance for m).

FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention, showing a state in which the laser device is connected to a heat sink via a conductive material 90 such as solder. I have. FIG. 3 differs from FIGS. 1 and 2 in that a second dielectric thin film 41 is further formed on a metal thin film 50.

The second dielectric thin film 41 is formed on the surface of the metal thin film 50 and functions to prevent a short circuit between the electrodes.
In other words, heat sink the laser chip face down,
When connected to a base such as a submount, the conductive material such as solder connecting the electrode and the base touches the metal thin film,
If the positive and negative electrodes are connected, a short circuit between the electrodes may occur. Therefore, by further covering the surface of the metal thin film 50 with the second dielectric thin film 41, a short circuit between the electrodes can be prevented. Particularly, as shown in this figure, it is more effective to form the second dielectric thin film 41 continuously so as to cover the interface between the previously formed dielectric thin film 40 and the metal thin film 50.

Embodiment An embodiment of the present invention will be described below with reference to FIGS. Spinel (MgAl ₂ O ₄ ,
After the substrate 1 composed of (111 surfaces) is placed in a reaction vessel of a MOVPE apparatus, a buffer layer composed of GaN is formed on the surface of a sapphire substrate at a temperature of 500.degree. The thickness was grown. This buffer layer has an effect of reducing lattice mismatch between the substrate and the nitride semiconductor,
Alternatively, AlN, AlGaN, or the like can be grown. It is known that the growth of the buffer layer improves the crystallinity of the n-type nitride semiconductor grown on the substrate, but the buffer layer may not be grown depending on the growth method, the type of the substrate, and the like. .

Subsequently, the temperature was raised to 1050 ° C., and TMG, ammonia, and SiH as donor impurities were added to the source gas.
_{4 An} n-type contact layer 2 made of Si-doped GaN was grown to a thickness of 4 μm using (silane) gas.

Next, the temperature was lowered to 750 ° C., and TMG, TMI (trimethylindium) and ammonia were used as raw material gases, and silane gas was used as impurity gas.
A crack preventing layer made of 1Ga0.9N was grown to a thickness of 500 Å. Although this crack prevention layer is not specifically shown, by growing the n-type nitride semiconductor containing In, preferably InGaN, the n-type optical confinement layer 3 made of the nitride semiconductor containing Al to be grown next is formed. It becomes possible to grow with a thick film. In the case of an LD, the layers serving as the light confinement layer and the light guide layer are, for example, 0.1 mm.
It is necessary to grow with a film thickness of 1 μm or more. Conventionally G
If a thick AlGaN is grown directly on the aN, AlGaN layer, cracks will be formed in the AlGaN grown later, which makes it difficult to fabricate the device. Cracks can be prevented. In addition, even if the optical confinement layer 3 to be grown next is grown as a thick film, it can be grown with good film quality. In addition, this crack prevention layer is 100 angstroms or more,
It is preferable to grow the film with a thickness of 0.5 μm or less. 1
When the thickness is less than 00 Å, it is difficult to act as a crack prevention as described above, and when the thickness is more than 0.5 μm,
The crystals themselves tend to turn black. The crack prevention layer may be omitted depending on the growth method and the growth apparatus.

Next, the temperature was raised to 1050 ° C., and TEG, TMA (trimethylaluminum) and ammonia were used as source gases, and silane gas was used as an impurity gas.
The n-type optical confinement layer 3 made of Al0.3Ga0.7N
It was grown to a thickness of 5 μm.

Subsequently, TMG, ammonia,
Si-doped n-type GaN using silane gas as impurity gas
An n-type light guide layer 4 was grown to a thickness of 500 Å.

Next, the active layer 5 was grown using TMG, TMI and ammonia as the source gas. The active layer has a temperature of 75
While maintaining the temperature at 0 ° 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 is 1
Repeat three times and finally grow the well layer to a total thickness of 0.1μ
An active layer having a multiple quantum well structure having a thickness of m was grown.

After the active layer 5 is grown, the temperature is raised to 1050 ° C., TMG, TMA, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) as an acceptor impurity source.
Was used to grow a p-type cap layer made of Mg-doped p-type Al0.2Ga0.8N to a thickness of 100 Å. Although this p-type cap layer is not shown,
m or less, more preferably 10 angstrom or more,
By growing the film to a thickness of 0.1 μm or less, InG
The active layer made of aN acts as a cap layer for preventing the decomposition of the active layer. By growing a p-type cap layer 48 made of a p-type nitride semiconductor containing Al on the active layer, the light emission output can be reduced. Dramatically improved. Conversely, if the p-layer in contact with the active layer is GaN, the output of the device will be reduced to about 1/3. This is because AlGaN is more likely to be p-type than GaN, and when growing a p-type cap layer, InGa
It is presumed that this has the effect of suppressing the decomposition of N, but the details are unknown. If the thickness of the p-type cap layer is larger than 1 μm, cracks tend to be formed in the layer itself, which tends to make element fabrication difficult. Note that this p-type cap layer can also be omitted.

Next, while maintaining the temperature at 1050 ° C., the TM
Mg-doped p-type Ga using G, ammonia, Cp2Mg
A p-type light guide layer 6 made of N was grown to a thickness of 500 Å. As described above, by using InGaN or GaN for the second p-type light guide layer, the following light confinement layer containing Al can be grown with good crystallinity.

Subsequently, TMG, TMA, ammonia, C
Using p2Mg, a p-type optical confinement layer 7 made of Mg-doped Al0.3Ga0.7N was grown to a thickness of 0.5 .mu.m.

Subsequently, TMG, ammonia, Cp2Mg
Was used to grow a p-type contact layer 8 of Mg-doped p-type GaN with a thickness of 0.5 μm.

The wafer on which the nitride semiconductor is laminated as described above is taken out of the reaction vessel, and the uppermost p-type contact layer 8 is formed by a reactive ion etching (RIE) apparatus.
Selective etching was performed from the side to expose the surface of the n-type contact layer on which the negative electrode was to be formed, and the p-type contact layer 8 to the n-type contact layer 2 were etched in a stripe shape. The stripe width was 10 μm.

A mask is applied to a portion of the nitride semiconductor wafer on which the positive electrode and the negative electrode are to be formed after the etching, and
Further, a dielectric multilayer film 40 made of SiO ₂ and TiO ₂ is formed on the above-mentioned etched end face by a plasma CVD apparatus with a total film thickness of 1 μm.
It was formed at 0 μm. Needless to say, the dielectric multilayer film is designed to reflect the emission wavelength of the active layer. Although this dielectric thin film is a multilayer film, for example, Al
It may be formed of a single film such as ₂ O ₃ or SiO ₂ .

Next, a metal thin film 50 made of Al was formed on the dielectric thin film 40 to a thickness of 0.1 μm by vapor deposition.

Further, SiO ₂ is deposited on the metal thin film 50.
A second dielectric thin film 41 made of
It was formed to a thickness of 1 μm by the method.

A striped positive electrode 30 made of Ni and Au is formed on the p-type contact layer 8 via the insulating film 11, and Ti and Al are formed on the n-type contact layer 2 previously exposed.
A striped negative electrode 20 was formed.

The wafer as described above is first divided at a position parallel to the stripe-shaped electrodes, and then divided in a direction perpendicular to the electrodes, and the divided surfaces divided in the vertical direction are polished to a mirror surface. did. A reflecting mirror was formed on the resonance surface according to a conventional method to form a laser chip. As shown in FIG. 3, this laser chip was placed on a heat sink on which the electrode pattern 100 had been formed in advance, and pulse oscillation was performed at room temperature. As a result, laser oscillation of 410 nm was shown at a threshold current density of ² kA / cm ² .

[0041]

As described above, in the laser device of the present invention, the active layer and the cladding layer in the direction parallel to the resonance direction of the laser beam are covered with the dielectric thin film and the metal.
Complete transverse mode light confinement can be achieved, and stable laser light can be obtained. Further, a dielectric thin film, a metal thin film and the like can be formed more easily than a nitride semiconductor. Therefore, if a nitride semiconductor is to be formed, after the cladding layer and the active layer are etched in a stripe shape, a mask must be formed and selective growth must be performed at a high temperature. Even if it is not a nitride semiconductor growth apparatus such as MOCVD and MBE, the film can be formed by another simple CVD apparatus, which is very useful in production technology.

[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 perspective view showing the shape of the laser device of FIG.

FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... N-type contact layer 3 ... N-type light confinement layer 4 ... N-type light guide layer 5 ... Active layer 6 ... P-type light Guide layer 5 p-type optical confinement layer 6 p-type contact layer 40 dielectric thin film 50 metal thin film 41 second dielectric thin film

Claims (2)

(57) [Claims] 1. An n-type nitride semiconductor layer having an active layer that oscillates laser between an n-type nitride semiconductor and a p-type nitride semiconductor layer, wherein the n-type nitride semiconductor layer is parallel to a resonance direction of laser light. A dielectric thin film is formed on the surface of the layer and the p-type nitride semiconductor layer, a metal thin film is formed on the surface of the dielectric thin film, and a second dielectric thin film is further formed on the surface of the metal thin film. A nitride semiconductor laser device. 2. The metal thin film is made of Al, Ag, Ni, C
A nitride semiconductor laser device, which is one selected from r and Pt.