JPH09321381A - Nitride semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable continuous oscillation at a room temperature by lowering Vf of a laser element composed mainly of a nitride semiconductor, and reducing the quantity of heat to be generated by the laser element. SOLUTION: This laser element has a stripelike n-like layer, an active layer 5, and a p-type layer formed in order on the upper part of a substrate 1, and a part of the p-type layer is etched and the surface of the n-type layer is exposed, and on the surface of the p-type layer a stripelike p electrode 11 is formed. On the other hand, stripelike n electrodes 10 are formed on the surface of the exposed n-type layer. Since the n electrodes 10 are provided approximately symmetrically with respect to the p electrode 11, in this case, the contact resistances of the n electrodes 10 lower, and each current flow becomes uniform. Accordingly, Vf lowers.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a nitride semiconductor (I
n _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, 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 disclosed laser device is a so-called electrode stripe type laser device, in which the stripe width of the nitride semiconductor layer including the active layer is set to several tens of μm, and laser oscillation is performed. This laser element is provided with one positive and one negative striped electrode on the same surface side.
It has almost the same shape as the laser element disclosed in Japanese Patent No. 4867.

[0003]

As described above, the nitride semiconductor laser developed by the present inventors realized the laser oscillation of 410 nm for the first time in the world, but it is still only the pulse oscillation and at the time of oscillation. Forward voltage (Vf)
Is 20 V or more, and the threshold current density is 10 kA / cm ²
As described above, it is difficult to realize continuous oscillation with this laser device.

When a laser device having a short wavelength is realized by continuous oscillation, the capacity of a storage medium such as a CD, a CD-ROM, or a DVD, which has been put into practical use, can be dramatically increased. Therefore, early realization is desired. For that purpose, first, Vf at the threshold of the laser element
It is necessary to reduce the heat generation amount of the laser element by decreasing

Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is mainly to reduce the Vf of a laser element made of a nitride semiconductor to lower the heat generation amount of the laser element. Is aimed at continuous oscillation at room temperature.

[0006]

A nitride semiconductor laser device of the present invention has a striped n-type layer, an active layer and a p-type layer on a substrate, and a part of the p-type layer is etched. The surface of the n-type layer is exposed, and a striped p-electrode is formed on the surface of the p-type layer, while the exposed n-type layer is exposed.
In a nitride semiconductor laser device in which a striped n-electrode is formed on the surface of a mold layer, the n-electrode is provided substantially symmetrically with respect to the p-electrode.

Further, the stripe width of the n-type layer is 100 μm.
m or less. The n-type layer corresponds to the stripe width of the active layer, the n-type layer or the like, and the threshold current and Vf can be further reduced by adjusting the stripe width within the above range.

Further, in a preferred aspect of the laser device of the present invention, the distance between the end face of the n-electrode on the side closer to the p-electrode and the end face of the n-type layer etched is 0.1 μm or more and 100 μm
It is characterized by being in the range of m or less. More preferably, the p-type layer of this laser device has a ridge shape. Furthermore, when the area of the n-electrode is 30% or more of the area of the exposed n-type layer, the contact resistance of the n-electrode decreases, so that V
f can be reduced.

The laser device of the present invention is characterized in that it is mounted face-up on a support and all electrodes are wire-bonded. That is, since the substrate side is connected to a support such as a heat sink or a submount, the contact area with the support is increased, so that the heat dissipation of the chip is improved and the reliability of the element is improved. Further, when all the electrodes are wire-bonded, short-circuiting between the electrodes is reduced as compared with face-down direct bonding, so that reliability is improved.

Since the substrate of the laser element is mounted by being embedded in a conductive adhesive, heat dissipation can be further enhanced.

Further, a preferable mode of the laser device of the present invention is characterized in that the p-type layer has a stripe-shaped ridge shape. When the p-type layer has a ridge shape, light is concentrated in the active layer under the ridge, so that the threshold current of the device can be reduced.

[0012]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention. This figure shows a cross-sectional view when the element is cut in a direction perpendicular to the stripe electrode. As a basic structure, an n-type layer such as an n-type contact layer 2, an n-type light confinement layer 3, and an n-type light guide layer 4, an active layer 5, and a p-type light guide layer 6 are formed on a substrate 1. , P-type optical confinement layer 7, p-type contact layer 8, etc.
It has a double hetero structure in which mold layers are laminated in order. 10
Is an n-electrode provided on the surface of the n-type contact layer 2, 11
Is a positive electrode provided on the surface of the uppermost p-type contact layer 8.

As shown in FIG. 1, in the laser device of the present invention, the nitride semiconductor layer below the p-type contact layer 8 is etched in stripes to expose the n-type contact layer 2, and the exposed portion. The n-type contact 2 is provided with a striped n-electrode 10. On the other hand, the uppermost p-type contact layer 8 is also provided with a striped p-electrode 11, and the striped n-electrode 10 is
Two places are provided symmetrically with respect to the p-electrode 11.
FIG. 2 is a perspective view showing the shape of the laser device of FIG.
By providing the stripe-shaped n-electrodes symmetrically with respect to the p-electrode in this manner, it is possible to eliminate the bias of the current relating to the active layer, so that the Vf of the laser element can be reduced.

Further, the stripe width a of the n-type layer is 100.
It is desirable that it is not more than μm. FIG. 3 is a graph showing the relationship between the stripe width a of this n-type layer and Vf. This figure shows Vf at a pulse current of 100 mA of a laser device in which the stripe width b of the ridge of the structure shown in FIGS. 1 and 2 is set to 3 μm and the stripe width a of the n-type layer is appropriately changed. is there. As shown in FIG. 3, as the stripe width a of the n-type layer is increased, Vf tends to increase, and when it exceeds 120 μm, Vf tends to decrease due to heat generation of the element itself. Therefore, a value is 100μ
The thickness is preferably m or less, more preferably 60 μm or less. Also, the stripe width b of the ridge
Is adjusted to 10 μm or less, more preferably 8 μm or less, and most preferably 6 μm or less.

FIG. 4 shows the n-electrode 10 shown in FIGS. 1 and 2.
It is a perspective view which expands and shows the circumference | surroundings. In the laser device of the present invention, the distance c between the end surface of the n-electrode 10 on the side closer to the p-electrode 11 and the etching end surface of the n-type layer is 0.1 μm.
As described above, it is desirable that the thickness is in the range of 100 μm or less. Table 1 shows the relationship between the length of a and Vf. This laser device is the laser device of the structure shown in FIGS. 1 and 2, in which the stripe width b of the ridge is set to 3 μm and the stripe width a of the n-type layer is set to 10 μm. Has a stripe length of 650 μm and a stripe width of 130 μm. The area of the n-electrode 10 corresponds to 50% of the area of the exposed n-type contact layer. Table 1 shows the relationship between the length of c and Vf when a pulse current of 20 mA, 50 mA, and 100 mA was applied to this laser element.

[0016]

[Table 1]

As shown in Table 1, the distance between the end faces of the n-electrode and
When the distance c from the etching end face is 100 μm or less,
Vf can be reduced. It can be seen that Vf is remarkably reduced especially at a current value of 100 mA. Therefore, the distance c is preferably 100 μm or less, more preferably 50 μm or less, and most preferably 2 μm.
Adjust to 0 μm or less. On the other hand, the lower limit value is 0.1 μm or more. If the thickness is smaller than 0.1 μm, the electrode is the active layer, p
This is because the layers are likely to come into contact with each other and the reliability of the device is reduced.

Next, FIG. 5 is a perspective view when the laser element 100 of the present invention is mounted on a heat sink as a support body to actually form a laser diode. The two n-electrodes 10 are wire-bonded to the heat sink side, and the p-electrodes are wire-bonded to the metal post side. Since the electrodes are mounted on the support in a face-up state and all the electrodes are wire-bonded as described above, short-circuiting between electrodes is reduced and element reliability is improved as compared with face-down bonding. Also laser device 1
Since the entire substrate surface of No. 00 is in contact with the heat sink, the contact area with the heat sink is large and the heat dissipation is also improved.

FIG. 6 is a schematic cross-sectional view showing in detail the mount portion of the laser element shown in FIG. 5, and shows a view when cut in a direction perpendicular to the stripe direction of the electrodes. As shown in this figure, the laser device 100 has a conductive adhesive 50.
The board side is embedded and mounted inside. As the conductive adhesive, materials that are usually used for bonding the semiconductor and the support, such as silver paste, In paste, In solder, and Sn solder, can be used. By mounting the laser element so that the substrate side is embedded in the conductive adhesive, the contact area between the substrate and the adhesive increases,
The heat dissipation of the laser element can be improved. In FIG. 6, 22 is an insulating film made of SiO ₂ , Al ₂ O ₃ , TiO ₂ or the like formed continuously on the surfaces of the p-type layer and the n-type layer, and 33 is the insulating film 22 and the p-electrode. 11 is a pad electrode continuously formed on the surface of 11. The pad electrode 33 is p
Since the area of the electrode 11 is small and it is difficult to wire-bond directly to the p-electrode 11, it has an effect of increasing the area of the p-electrode for wire-bonding.
Therefore, the pad electrode 33 does not need to make ohmic contact with the p-type layer, and any metal material may be used. Further, the insulating film 22 prevents the pad electrode 33 from coming into contact with the n-type layer and short-circuiting between the electrodes, and also prevents the surface of the nitride semiconductor layer from being scratched during bonding, thereby improving the reliability of the device. I am raising.

Further, as shown in FIGS. 1, 2, 4 and 6, in the laser element of the present invention, the p-type layer has a ridge shape, so that light is emitted to the active layer corresponding to the lower portion of the ridge. By concentrating and stabilizing the transverse mode light, the threshold current of the laser element decreases.

[0021]

EXAMPLE A substrate 1 whose main surface is the sapphire A-side is prepared, and this substrate 1 is placed in a reaction vessel of a MOVPE apparatus. Then, TMG (trimethylgallium) and ammonia are used as source gases and a temperature of 500 is used. A buffer layer made of GaN is grown on the surface of the sapphire substrate 1 at a temperature of 200 ° C. to a thickness of 200 Å. The buffer layer has a function of alleviating the lattice mismatch between the substrate and the nitride semiconductor.
It is also possible to grow N, 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, but the buffer layer may not be grown depending on the growth method, the type of the substrate and the like. Therefore, it is not particularly shown in FIG. As the substrate, sapphire having R-face and C-face as main faces can be used in addition to sapphire A-face, and in addition, spinel (MgAl ₂ O ₄ , 111 face), SiC, MgO, Si,
A conventionally known substrate made of a single crystal such as ZnO or GaN is used.

Then, the temperature is raised to 1050 ° C.
TMG, ammonia, SiH as a donor impurity
_FourConsisting of Si-doped GaN using (silane) gas
The n-type contact layer 2 is grown to a film thickness of 4 μm. n-type
Contact layer 2 is In_XAl_YGa _1-XYN (0 ≦ X, 0 ≦
Y, X + Y ≦ 1), especially Si
High carrier concentration
A good n-type layer is obtained, and a favorable ohmic contact with the negative electrode is obtained.
As the touch can be obtained, the threshold current of the laser element is reduced.
Can be made. As the material of the negative electrode, Al, Ti,
Metals or alloys such as W, Cu, Zn, Sn, In are preferable.
A good ohmic is obtained.

Next, the temperature is set 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 of 1 Ga 0.9 N is grown to a thickness of 500 Å. The crack prevention layer is formed by growing an n-type nitride semiconductor containing In, preferably InGaN, so that the n-type optical confinement layer 3 made of a nitride semiconductor containing Al to be grown next is prevented from cracking. It is possible to grow a film, which is very preferable. The crack preventing layer is preferably grown to a thickness of 100 Å or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to act as a crack preventive as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black. Since this crack prevention layer may be omitted depending on the growth method and the growth apparatus, it is preferable to grow it when an LD is prepared, although not shown.

Next, using TMG, TMA (trimethylaluminum), ammonia as a raw material gas, and silane gas as an impurity gas, an n-type optical confinement layer 3 made of Si-doped n-type Al0.3Ga0.7N having a thickness of 0.5 μm is formed. Grow thick. The n-type optical confinement layer 3 is made of an n-type nitride semiconductor containing Al, and is preferably a binary mixed crystal or a ternary mixed crystal of A.
By setting l _Y Ga _{1 -Y} N (0 <Y ≦ 1), good crystallinity can be obtained, and the refractive index difference from the active layer can be increased to effectively confine the longitudinal mode of laser light. is there. This layer is usually preferably grown to a thickness of 0.1 μm to 1 μm.

Then, TMG, ammonia, and
Si-doped n-type GaN using silane gas as impurity gas
The n-type light guide layer 4 is grown to a film thickness of 500 angstroms. The n-type light guide layer 4 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, it becomes possible to easily form the next active layer 5 with a quantum structure.

Next, the active layer 5 is grown using TMG, TMI, and ammonia as source gases. The active layer 5 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 film thickness of 0 angstrom. This operation 5
This is repeated a number of times, and finally a well layer is grown to grow an active layer 5 having a multiple quantum well structure with a total film thickness of 400 Å.

After the growth of the active layer 5, the temperature is set to 1050 ° C. and TMG, TMA, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) as an acceptor impurity source.
Is used to grow a p-type cap layer (not shown) made of Mg-doped p-type Al0.2Ga0.8N to a film thickness of 100 angstrom. This p-type cap layer is 1 μm or less,
More preferably 10 angstroms or more, 0.1 μ
When grown to a film thickness of m or less, it has a function as a cap layer for preventing decomposition of the active layer made of InGaN, and a p-type nitride semiconductor containing Al on the active layer, preferably Al _Y. By growing the p-type cap layer made of Ga _{1 -Y} N (0 <Y <1), the light emission output is remarkably improved. If the film thickness of this p-type cap layer is thicker than 1 μm, the layer itself tends to be cracked, and it tends to be difficult to manufacture an element. Since the p-type cap layer can be omitted depending on the growing method, the growing apparatus, etc., it is preferable to grow the active layer containing indium, though not shown in the drawing.

Next, while maintaining the temperature at 1050 ° C., T
Mg-doped p-type G using MG, ammonia, and Cp2Mg
The p-type light guide layer 6 made of aN is grown to a film thickness of 500 angstrom. This p-type light guide layer 6 is also made of In
P-type nitride semiconductor or p-type GaN, preferably a binary mixed crystal or a ternary mixed crystal of In _X Ga _1-X N (0 ≦ X
≦ 1) is grown. It is desirable that the optical guide layer is normally grown to a film thickness of 100 angstrom to 1 μm.
The type light confinement layer 7 can be grown with good crystallinity.

Then, TMG, TMA, ammonia, C
The p-type optical confinement layer 7 made of Mg-doped Al0.3Ga0.7N is grown to a thickness of 0.5 μm using p2Mg.
The p-type optical confinement layer 7 is made of a p-type nitride semiconductor containing Al, and is preferably a binary or ternary mixed crystal A.
By setting l _Y Ga _{1 -Y} N (0 <Y ≦ 1), a crystal having good crystallinity can be obtained. Like the n-type light confinement layer 3, the p-type light confinement layer 7 is preferably grown to have a film thickness of 0.1 μm to 1 μm, and contains p such as AlGaN containing Al.
By using the type nitride semiconductor, the difference in refractive index from the active layer is increased, and it effectively acts as a light confinement layer.

Then, TMG, ammonia, Cp2Mg
Using, the p-type contact layer 8 made of Mg-doped p-type GaN is grown to a thickness of 0.5 μm. The p-type contact layer 8 is a p-type In _X Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, X +
Y ≦ 1), and in particular, when p-type GaN doped with Mg is obtained, a p-type layer having a high carrier concentration is obtained, good ohmic contact with the positive electrode is obtained, and the threshold current is increased. Can be reduced. As the material of the positive electrode, a metal or alloy having a relatively high work function, such as Ni, Pd, Ir, Rh, Pt, Ag, or Au, can easily obtain an ohmic.

A wafer in which a nitride semiconductor is laminated on a sapphire substrate is taken out from the reaction container, a mask made of SiO ₂ having a stripe shape with a width of 3 μm is formed on the uppermost p-type contact layer 8, and RIE ( P-type contact layer 8 and p
The mold light confinement layer 7 is etched to form a ridge having a stripe width b of 3 μm as shown in FIG.

After forming the ridge, a stripe mask made of SiO ₂ is again formed on the exposed surface of the nitride semiconductor layer, and this time, the nitride semiconductor layer is etched in stripes until the n-type contact layer 2 is exposed. By this etching, the stripe width a of the active layer 5, the n-type light guide layer 4 and the n-type light confinement layer 3 is adjusted to 10 μm.

Next, the surface of the uppermost p-type contact layer 8 having a stripe width of 3 μm and the surface of the n-type contact layer 2 having a stripe width of 300 μm exposed symmetrically with respect to the p-type contact layer 8. Striped n
The electrode 10 and the p-electrode 11 are formed. p electrode 11 is N
It is made of an alloy containing i and Au and is formed on almost the entire surface of the p-type contact layer 8. On the other hand, the n-electrode 10 is made of an alloy containing Ti and Al and has a stripe width of 200 μm, and the distance c between the end face of the n-electrode 10 and the etched n-type optical confinement layer 3 is 30 μm.

After forming the electrodes, the p-electrode 11 is formed as shown in FIG.
An insulating film 22 made of SiO ₂ is formed on the surface of the nitride semiconductor layer exposed between the n-electrode 10 and the n-electrode 10, and a pad electrode 33 made of Au is further formed on the insulating film 22.

Then, the wafer is cleaved along the R plane of the sapphire substrate to form a semiconductor bar having a cavity length of 650 μm,
A resonance plane is formed on the cleavage plane. This resonance plane is in a direction perpendicular to the electrode stripe. After polishing the resonance surface, the bar is cut at a position parallel to the stripe electrode to manufacture a laser device as shown in FIG. In this laser device, a positive electrode 11 is provided on almost the entire surface of a p-type contact layer 8 having a stripe width of 3 μm. The n-type contact layer 2 is exposed symmetrically with respect to the positive electrode 11 with a stripe width of 300 μm.
N electrode 1 having a stripe width of 200 μm on the surface of
0 is provided in parallel to the p electrode 11 with a length of 650 μm. The distance c between the n-electrode 10 and the etched n-type optical confinement layer 3 is 30 μm.

Laser element 10 manufactured as described above
0 is die-bonded on a heat sink made of AlN via In solder. At the time of die bonding, the amount of In solder is set to be larger than that of a normal die bond, and as shown in FIG.
The substrate is embedded in In solder. Next, as shown in FIG. 5, the n-electrode 10 and the p-electrode 11 are wire-bonded with a gold wire to form a laser diode. And
When this laser diode was oscillated with a pulse current of 150 mA, Vf was 9.0 V,
The life in mA was more than twice that of the conventional n-electrode provided on only one side, which was a very excellent characteristic.

[0037]

In the laser device of the present invention, since the n electrode is arranged symmetrically with respect to the p electrode, the current flow in the active layer becomes smooth and Vf is lowered. Therefore, the amount of heat generated by the laser element itself is reduced, and the life is greatly improved. As described above, the successful reduction of Vf by the laser device of the present invention is very important for practical use of a nitride semiconductor laser device that oscillates at a short wavelength of 500 nm or less.

[Brief description of 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 diagram showing a relationship between a stripe width a of an n layer and a forward voltage of a laser element.

FIG. 4 is an enlarged perspective view showing the vicinity of an n-electrode of the laser device of FIG.

FIG. 5 is a perspective view showing a structure of a laser diode mounted with a laser element of the present invention.

FIG. 6 is a schematic cross-sectional view showing in detail a mount portion of the laser element shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... N-type contact layer 3 ... N-type optical confinement layer 4 ... N-type optical guide layer 5 ... Active layer 6 ... P-type light Guide layer 7 ... P-type optical confinement layer 8 ... P-type contact layer 10 ... N electrode 11 ... P electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Nakamura 491, Oka, Kaminaka-cho, Anan City, Tokushima Prefecture Nichia Kagaku Kogyo Co., Ltd.

Claims (6)

[Claims] 1. A stripe-shaped n-type layer, an active layer, and a p-type layer are sequentially provided on a substrate, and a part of the p-type layer is etched to expose the surface of the n-type layer. In a nitride semiconductor laser device in which a striped p-electrode is formed on the surface of a mold layer and a striped n-electrode is formed on the surface of an exposed n-type layer, the n-electrode is a p-electrode.
A nitride semiconductor laser device, which is provided substantially symmetrically with respect to an electrode. 2. The nitride semiconductor laser device according to claim 1, wherein the stripe width of the n-type layer is within a range of 100 μm or less. 3. The distance between the end surface of the n-electrode on the side closer to the p-electrode and the etching end surface of the n-type layer is in the range of 0.1 μm or more and 100 μm or less. The nitride semiconductor laser device described in 1 .. 4. The nitride according to any one of claims 1 to 3, wherein the laser device is mounted face-up on a support, and all electrodes are wire-bonded. Semiconductor laser device. 5. The nitride semiconductor laser element according to claim 4, wherein the laser element is mounted by embedding the substrate side in a conductive adhesive. 6. The nitride semiconductor laser device according to claim 1, wherein the p-type layer has a striped ridge shape.