JP2921746B2 - Nitride semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride semiconductor (In _X
The present invention relates to a laser device composed of Al _Y Ga _{1 -XYN} , 0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and particularly to an electrode stripe type laser device.

[0002]

2. Description of the Related Art A nitride semiconductor has a band gap of 1.9.
Since it is 5 eV to 6.0 eV and is a direct transition type material, it has been attracting attention as a material for semiconductor laser devices ranging from ultraviolet to red.

A typical conventional nitride semiconductor laser device is
FIG. 3 is a schematic sectional view showing the structure. This laser element
The element shows an electrode stripe type structure. fundamentally
Is an n-type cladding layer 32 on the surface of a sapphire substrate 31.
An active layer 33 and a p-type cladding layer 34 are sequentially stacked.
It has a bull hetero structure. p-type cladding layer 34, active
Layer 33 and part of the n-type cladding layer 32 are striped.
And the horizontal surface of the n-type cladding layer 32 is
Is exposed. The horizontal plane of the n-type cladding layer 32
A lip-shaped negative electrode 41 is formed, and the p-type
A positive electrode 42 in the form of a stripe is also desired to be formed on the doped layer 34.
It is a so-called flip chip system. In addition, positive
A current confinement layer is provided between the pole 42 and the p-type cladding layer 34.
And SiO _TwoAn insulating layer 35 made of
A structure in which current is concentrated on the active layer 33 in the layer 35 to cause oscillation
It is built.

[0004]

However, as shown by the arrow in FIG. 3, in the conventional laser device, the current confined by the insulating layer 35 is generated in the p-type cladding layer 34 or the active layer 3.
3 and it was difficult to partially concentrate the current on the active layer 33. Since the current cannot be concentrated, the active layer 33 emits light uniformly, and in the conventional structure, an LED is used.
In fact, it showed only the characteristics as

Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a nitride semiconductor laser device which oscillates a laser by concentrating a current in an active layer. .

[0006]

A nitride semiconductor laser device according to the present invention comprises an n-type GaN or n-type AlG
an n-type contact layer made of aN, a first n-type clad layer made of n-type AlGaN, a second n-type clad layer made of n-type InGaN or n-type GaN, a single quantum well structure or a multiple quantum well An active layer having a structure;
a first p-type cladding layer made of p-type AlGaN, and a p-type contact layer made of p-type GaN or p-type AlGaN in order, and further until the plane of the n-type contact layer is exposed, To the first n-type cladding layer are etched in a stripe with the same stripe width, and a stripe-shaped positive electrode is formed on the outermost surface of the p-type contact layer so as to be in contact with the same width as the stripe width. It is characterized by being.

FIG. 1 is a perspective view showing the shape of a laser device according to one embodiment of the present invention, and FIG. 2 is a schematic sectional view of the perspective view of FIG. 1 cut in a direction perpendicular to the stripes. As shown in these figures, in the laser device of the present invention, a positive electrode having a width substantially equal to the stripe width of the p-layer etched in a stripe shape is formed in direct contact with the p-layer, thereby increasing the spread of current. Instead, the current is directly concentrated on the active layer. A preferred stripe width that allows the current to be concentrated on the active layer without spreading the current is 50 μm or less, more preferably 30 μm or less, and most preferably 1 μm or less.
0 μm or less. If the thickness is larger than 50 μm, the threshold current of laser oscillation becomes high, and the laser oscillation tends to stop. In the present invention, substantially the same width means that the electrode width is close to the width of the p-type layer within a width of -10% or less.

FIG. 2 shows the basic structure of the laser device as a double hetero structure in which an active layer is sandwiched between n-type and p-type nitride semiconductor layers. Is recommended.

FIG. 2 shows an n-type contact layer 2, a first n-type cladding layer 3, a second n-type cladding layer 4, an active layer 5, a first p-type cladding layer 6, 1 shows a so-called separated confinement type double hetero structure in which a mold contact layer 7 is sequentially stacked. The p-type contact layer 7, the first p-type cladding layer 6, the active layer 5, the second n-type cladding layer 4, the first n-type cladding layer 3, and a part of the n-type contact layer 2 are etched. Striped negative electrode 1
The positive electrode 1 having a width substantially equal to the stripe width of the p-type contact layer 7 is formed on the surface of the striped p-type contact layer 7 left by etching.
2 are formed in direct contact.

The substrate 1 includes sapphire (including C-plane, R-plane, and A-plane), SiC (including 6H and 4H), Si, and Z.
Although nO and GaAs can be used, sapphire or SiC is generally used.

As the n-type contact layer 2, GaN, Al
A binary or ternary mixed crystal semiconductor layer such as GaN having good crystallinity can be obtained. Particularly when GaN is used, a negative electrode material and a preferable ohmic can be obtained. To make the semiconductor layer n-type, the semiconductor layer is doped with a donor impurity such as Si, Ge, or S. Further, a buffer layer of GaN, AlN, or the like may be formed between the substrate 1 and the n-type contact layer 2 in order to reduce lattice constant irregularities.

Next, the first n-type cladding layer 3 is formed of a nitride semiconductor having a larger band gap than that of the second n-type cladding layer.
GaN has good crystallinity and a semiconductor layer with a large band gap can be obtained.

The next second n-type cladding layer 4 forms a nitride semiconductor layer having a larger band gap than the active layer 5,
Particularly, n-type InGaN or n-type GaN doped with the donor impurity is preferable. As will be described later, the second n-type cladding layer 4 is preferably made of InGaN or GaN even in combination with the active layer 5. For laser oscillation, the second n-type cladding layer 4 made of InGaN or GaN is preferably used. Forming is particularly preferred.

The next active layer 5 is made of non-doped InGaN.
Then, the band between about 635 nm to 365 nm
Light emission is obtained. Preferably, the molar ratio of indium is
InGaN, which is less than half that of
And the life of the laser element is long. Also, dozens of active layers
A multilayer film consisting of two or more layers of gström,
A multiple quantum well structure may be used. Single quantum well structure, multiple
In any active layer of the quantum well structure, the active layer is n
Type or p-type, but especially non-doped (no addition)
The interband emission and exciton emission with narrow half width
Light or quantum well level emission can be obtained,
It is particularly preferable for realizing an LD element. Single volume of active layer
SQW (single quantum well) structure or
Multi quantum well (MQW) structure
Then, a light-emitting element having a very high output can be obtained. SQW, M
QW is the emission between quantum levels of undoped InGaN
Refers to the structure of the active layer from which light can be obtained.
Layer with a single composition of In_XGa_{1 -X}Consists of N (0 ≦ X <1)
Layer, and In_XGa_1-X100 angstrom of N film thickness
Tromes or less, more preferably 70 angstroms
Strong emission between quantum levels can be obtained by
You. In addition, MQW has different composition ratios of In_XGa_1-XN (this
(Including X = 0 and X = 1)
And As described above, the active layer is made of SQW or MQW.
To emit light between quantum levels from about 365 nm to 660 nm.
Light emission is obtained. As the thickness of the quantum well layer
Should be less than 70 Angstroms as described above
No. In the multiple quantum well structure, the well layer is In. _XGa_1-XIt is N
And the barrier layer is also In_YGa_1-YN (Y <X, in this case
(Including Y = 0). Especially preferred
The same temperature when the well layer and the barrier layer are formed of InGaN
Therefore, an active layer having good crystallinity can be obtained. barrier
Layer thickness less than 150 angstroms, more preferred
Higher output light emission at less than 120 Angstroms
An element is obtained. Further, donor impurities and
It may be doped with an acceptor impurity. Impure
The crystallinity of the active layer doped with
If doped with donor impurities, it becomes non-doped
The inter-band emission intensity can be further increased
You. Doping with an acceptor impurity causes inter-band emission
0.5 eV lower peak energy than peak wavelength
Although the wavelength can be brought, the half width becomes wider.
Doping acceptor impurity and donor impurity simultaneously
And the emission intensity of the active layer doped only with acceptor impurities
The degree can be further increased. Especially acceptors
When implementing an active layer doped with impurities,
The electric type is n-type by simultaneously doping donor impurities such as Si.
Is preferred. The active layer 5 is, for example, several angstrom.
Can be grown with a film thickness of about 0.5 μm.

Next, the first p-type cladding layer 6 is
Formed from a nitride semiconductor having a larger band gap than
Particularly preferably, p-type A doped with an acceptor impurity
With lGaN, a semiconductor layer having good crystallinity and a large band gap can be obtained. After doping the acceptor impurity, annealing may be performed at 400 ° C. or higher for the purpose of making the resistance p-type. Examples of the acceptor impurities include Group II elements such as Zn, Mg, and Cd, and C (carbon).

Between the first p-type cladding layer 6 and the active layer 5, a second p-type layer having a larger band gap than the active layer 5 and a smaller band gap than the first cladding layer 6 is formed.
A mold cladding layer may be inserted. The second p-type cladding layer is preferably p-type InGaN doped with an acceptor impurity. Here, in the combination of the second n-type cladding layer 4, the active layer 5, and the second p-type cladding layer (the second p-type cladding layer does not need to be particularly grown), the active layer 5 is formed. As a single quantum well structure or a multiple quantum well structure, an elastic strain is generated at the interface with the second n-type cladding layer 4 by reducing the thickness of the nitride semiconductor layer constituting the active layer. Since a laser device having a strained quantum well structure is realized, laser oscillation is facilitated. In particular, the elastic strain is such that the active layer 5 is made of InGaN, and the second n-type clad layer 4 is made of n-type InG having a larger band gap than the active layer 6.
It tends to occur when aN or n-type GaN is used.

Next, as in the first p-type cladding layer 7, the p-type contact layer 8 is made of p-type doped with an acceptor impurity.
A semiconductor layer of binary or ternary mixed crystal such as p-type GaN or p-type AlGaN having good crystallinity can be obtained. Especially Ga
If N, a positive electrode material and a preferable ohmic can be obtained.

As means for etching the nitride semiconductor,
Although there are both dry etching and wet etching means, dry etching is preferable for forming a stripe with an etched end face vertical. In dry etching, for example, means such as reactive ion etching, ion milling, ion beam assisted etching, and focused ion beam etching can be used.

FIG. 4 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention. The difference from the cross-sectional view of FIG. An insulating film 20 is formed on the p-type layer, and a stripe-shaped positive electrode 12 in contact with the stripe width of the p-type layer is formed in contact with the p-type layer.
That is, it is formed from the mold layer to the surface of the insulating film 20. That is, when performing wire bonding to the positive electrode 12, it is almost impossible to perform wire bonding because the width of the stripe-shaped positive electrode 12 as shown in FIG. 2 is narrow. Therefore, in order to increase the width of the positive electrode 12, an insulating film 20 made of an insulator such as SiO ₂ is newly formed on the side surface of the p-type layer, and the surface of the insulating film 20 is electrically connected to the p-type layer. The positive electrode 12 which is electrically connected is formed. In the structure as shown in FIG. 1, the electrodes cannot be wire-bonded and the structure is a face-down structure. However, when the structure is as shown in FIG. 4, the structure can be a face-up structure.
The chip size can be reduced.

[0020]

In the laser device of the present invention, the stripe width of the p-type layer is reduced so that the current is concentrated on the active layer. That is, when a positive electrode having substantially the same stripe width is formed on the p-type layer etched in a stripe shape, the current density of the active layer increases easily because the stripe width is narrow even if the current spreads in the p-type layer. Laser oscillation becomes easy. Also, with respect to light emitted from the active layer in a direction parallel to the substrate, the stripe width of the active layer is narrow, so that the distance between the nitride semiconductor and the atmosphere having a large refractive index difference is short, and light is confined in this narrow region. Laser oscillation easily.

The laser device according to the present invention does not require a process of forming an insulating layer for current confinement on the surface of the p-type layer as in the prior art, so that the manufacturing process can be shortened.

[0022]

【Example】

[Example 1] On a sapphire substrate 1 having a thickness of 350 μm,
200 angstrom buffer layer made of GaN,
N-type contact layer 2 made of Si-doped n-type GaN
n-type cladding layer 3 made of Si-doped n-type Al0.3Ga0.7N
The second n-type cladding layer 4 made of 0.99 N is 500 Å, the active layer 5 made of non-doped In 0.08 Ga 0.92 N is 100 Å, and Mg-doped p-type Al.
0.1 μm of a p-type cladding layer 6 made of 0.3Ga0.7N;
A wafer is prepared by sequentially growing a p-type contact layer 7 made of Mg-doped p-type GaN with a thickness of 0.5 μm.

Next, the p-type contact layer 7 of this wafer
After a mask is formed in a predetermined shape on the surface of the nitride semiconductor layer, the nitride semiconductor layer is removed by RIE (reactive ion etching).
Etching is performed with a stripe width of 0 μm. After the etching, a negative electrode 11 of Ti / Al is formed in a stripe shape with a width of 20 μm on the exposed n-type contact layer 3.
Ni / Au is formed on the entire surface of the striped p-type contact layer 7.
The positive electrode 12 is formed.

Next, the surface of the sapphire substrate 1 on which the nitride semiconductor layer is not formed is polished by a polishing machine to a thickness of 80 μm. After polishing, the polished surface of the sapphire substrate is scribed with a scriber. The scribe direction is a line perpendicular to the stripe electrode, and the other scribe line is a direction parallel to the electrode. After forming the scribe line, the wafer was cracked with a roller, and the length of the nitride semiconductor layer cleaved in the direction perpendicular to the electrodes was 500 μm.
m laser chip.

Next, after a mask is applied to the nitride semiconductor layer surface of the laser chip, a dielectric multilayer film made of SiO ₂ and ZrO ₂ is formed on the cleavage plane by a sputtering device. This dielectric multilayer film has an effect of reflecting the wavelength of the active layer by 90% or more.

After the laser chip thus obtained was placed face down (the electrodes face the heat sink surface) and placed on a heat sink, laser oscillation was attempted at room temperature. The threshold current density was 1.0 kA / Laser oscillation with an oscillation wavelength of 440 nm was confirmed at cm ² or more.

Example 2 A laser device was fabricated in the same manner as in Example 1, except that the stripe width was changed to 30 μm at the time of etching the nitride semiconductor layer. The threshold current density at room temperature was 3.0 kA / cm. Laser oscillation was confirmed at ² or more.

Example 3 A laser device was fabricated in the same manner as in Example 1 except that the stripe width was changed to 50 μm during the etching of the nitride semiconductor layer. The threshold current density was 5.0 kA at the temperature of liquid nitrogen. Laser oscillation was confirmed at / cm ² or more.

Example 4 A laser device was fabricated in the same manner as in Example 1 except that the stripe width was changed to 60 μm during the etching of the nitride semiconductor layer. The threshold current density was 6.0 kA at the temperature of liquid nitrogen. Laser oscillation was confirmed at / cm ² or more, but the element was immediately destroyed.

Fifth Embodiment In the first embodiment, the active layer 5
The composition of the well layer made of non-doped In0.08Ga0.92N is 25 Å, and the non-doped In0.01Ga0.92N
A barrier layer of 99N is grown to a thickness of 50 Å. This operation was repeated 13 times, and finally, a well layer was laminated to grow an active layer 6 having a total thickness of 1000 Å. Thereafter, when laser oscillation was attempted at room temperature in the same manner as in Example 1, the threshold current density was 1.0 kA /
Laser oscillation with an oscillation wavelength of 440 nm was confirmed at cm ² or more.

[Comparative Example] A laser device was fabricated in the same manner as in Example 1 except that the stripe width was changed to 100 μm at the time of etching of the nitride semiconductor layer. The device has been destroyed.

[0032]

As described above, in the laser device of the present invention, the positive electrode which is in contact with the stripe with the same width as the stripe is formed on the surface of the p-type layer which is etched in the stripe shape. It acts as a stenosis. That is, p
Even if the current spreads in the mold layer, it has a stripe width enough to increase the current density for laser oscillation, so that light can be easily confined and oscillation occurs at room temperature. Further, it is not necessary to form a current confinement layer with an insulator on the surface of the p-type layer as in the related art. Therefore, the technique of fine mask alignment at the time of forming the insulator is not required, and the production yield is improved. As described above, the realization of a laser device in a short wavelength region at room temperature using a nitride semiconductor has dramatically improved the recording density as a light source for writing and a light source for compact discs.
Its industrial utility value is very large.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the shape of a laser device according to one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of the laser device of FIG. 1 cut in a direction perpendicular to a stripe.

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

FIG. 4 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.

[Explanation of symbols]

 1 ... substrate 2 ... n-type contact layer 3 ... first n-type clad layer 4 ... second n-type clad layer 5 ... active layer 6 ... · First p-type cladding layer 7 ··· p-type contact layer 12 ··· positive electrode 11 ··· negative electrode

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-64912 (JP, A) JP-A-8-56046 (JP, A) JP-A-8-83956 (JP, A) 97502 (JP, A) JP-A-8-97514 (JP, A) JP-A-8-264891 (JP, A) JP-A-7-176826 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/18 H01L 33/00

Claims (2)

(57) [Claims] An n-type GaN or an n-type Al is provided on a substrate.
An n-type contact layer made of GaN, a first n-type clad layer made of n-type AlGaN, a second n-type clad layer made of n-type InGaN or n-type GaN, a single quantum well structure or a multiple quantum well An active layer having a structure, a first p-type cladding layer made of p-type AlGaN,
a p-type contact layer made of p-type GaN or p-type AlGaN, and stripes having the same stripe width from the p-type contact layer to the first n-type cladding layer until the plane of the n-type contact layer is exposed. A nitride semiconductor laser device characterized in that a stripe-shaped positive electrode is formed on the outermost surface of the p-type contact layer and is in contact with the stripe width substantially equal to the stripe width. 2. The nitride semiconductor laser device according to claim 1, wherein said stripe width is 50 μm or less.