JP3443241B2 - Semiconductor laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly reliable and high performance semiconductor laser device used as a light source for a high speed optical disk.

[0002]

2. Description of the Related Art A semiconductor laser which is being developed as a light source for a high-speed optical disk is required to have (1) high output and high reliability, and (2) narrow vertical beam divergence angle.

In order to realize high output and high reliability of the semiconductor laser, it is generally required to raise the COD (instantaneous optical damage) level. The COD level is a level at which a sudden oscillation stop phenomenon occurs in a high output operation region of a semiconductor laser. FIG. 3 schematically shows this phenomenon with a current-light output characteristic diagram and a flow chart of the mechanism. The horizontal axis of the figure is the current value injected into the semiconductor laser, and the vertical axis is the optical output emitted from the laser.

As shown here, when the current exceeds the oscillation threshold current and the current is increased, the light output sharply increases in proportion to it. However, when the current exceeds a certain value, the light output suddenly increases. It is observed that the phenomenon that the oscillation is stopped and the oscillation is stopped. When the semiconductor laser device that has stopped oscillation of this kind is examined by a scanning electron microscope, it is observed that a melted portion is generated at the end face of the active layer.

The mechanism by which such a phenomenon occurs is as follows:
It is usually thought as follows. When a current is injected into the semiconductor laser device, heat generation due to non-radiative recombination of carriers in the vicinity of the end face of the active layer increases, and the temperature of such part rises. This temperature rise works in the direction of reducing the energy band gap of the active layer, further increases the light absorption at the end face of the active layer, accelerates the temperature rise, and finally causes melting of the end face.

Typical methods for increasing the COD level are (1) a method of reducing light absorption at the end face of the active layer and (2) a method of increasing the light emitting area.

In the former case, the material near the end surface of the laser active layer is made to have a larger energy band gap than the central portion of the active layer, and a structure transparent to the laser light is used to absorb light near the end surface of the active layer. This is a method called a window structure that attempts to reduce Although this method has a great effect on reducing light absorption at the end face, it has a problem in reproducibility because the device structure and the device manufacturing process become complicated.

The latter is intended to expand the light emitting area to reduce the light density per unit area and to reduce the temperature rise of the element. This is to thin the active layer to direct the laser light to the cladding layer. The method is roughly classified into two methods, that is, a method of exuding much and a method of providing light guide layers on both sides of the active layer.

FIG. 4 shows an example of a conventional structure of a semiconductor laser device in which the laser light is exuded from the active layer to the clad layer by the above method, and FIG. 4 (a) shows a direction perpendicular to the resonator. 4B is a view showing the refractive index distribution and the light intensity distribution seen from the AA ′ cross section, FIG. 4C.
[FIG. 3] is a schematic view of a band structure in the AA ′ cross section.

In the figure, 21 is an n-type GaAs substrate, 22 is an n-type Al _x Ga formed on this n-type GaAs substrate 11.
_1-x As clad layer (x = 0.5, layer thickness 1.6 μm, S
i-dope). On the n-type clad layer 22, A
l _y Ga _1-y As quantum well layer (y = 0.05, layer thickness 100
Å) 23a and Al _z Ga _1-z As optical guide layer (z = 0.3
5, the single quantum well active layer 23 is formed by stacking the layer thickness of 450 Å) 23b.

On the active layer 23, p-type Al _p Ga _{1-p is formed.}
The As (p = 0.5, layer thickness 1.6 μm) clad layer 24 is formed. On the p-type cladding layer 24, a contact layer 25 made of p-type GaAs (layer thickness 0.2 μm).
Are formed. On the upper surface of this contact layer 25,
The p-type ohmic electrode 26 made of Cr-Au is
An n-type ohmic electrode 27 made of Cr—Sn—Au is formed on the lower surface of the type GaAs substrate 21. The element is
It has a junction down structure in which the Cr-Au electrode 26 side is mounted on a heat sink.

Regarding the narrow vertical beam divergence angle, generally, the beam divergence angle (θ⊥) in the direction perpendicular to the active layer of a semiconductor laser having a light output of several tens of mW is 30 degrees or less, preferably a lens. It is said that it is desirable to set it to about 20 degrees in consideration of the coupling efficiency of. This is because in the optical system, the spot diameter of the laser beam is limited to the effective area of the lens, and the narrow vertical beam divergence angle allows the laser beam to be used more efficiently. This is because the angle ratio (aspect ratio) needs to be as close to 1 as possible.

In the laser structure shown in FIG. 4, in order to achieve a beam divergence angle of about 20 degrees in the vertical direction in the active layer, an extremely thin active layer is grown, and the active layer is symmetrical with respect to the junction in the vertical direction. It forms a simple waveguide structure. Since this structure forms a symmetrical refractive index distribution around the active layer, the light intensity distribution of the laser light confined by it is also symmetrical. Since the width of light seeping out from the active layer to the left and right clad layers is about 1.5 μm on one side, the p-side and n-side clad layer thicknesses are also required to be 1.5 μm or more.

[0014]

However, the AlG
In the case of an aAs-based infrared semiconductor laser, the Al composition of the clad layer is about 0.5, so the thermal conductivity is significantly smaller than that of GaAs. Therefore, with respect to the heat generated in the active layer during high-power operation, the heat dissipation in the clad layer is poor, so that there is a problem that the temperature rise of the junction becomes large and the device characteristics and reliability are deteriorated.

The present invention has been made in view of the above-mentioned problems, and has a small beam divergence angle in the direction perpendicular to the active layer and a highly reliable semiconductor laser device capable of suppressing an increase in temperature at the junction. The purpose is to provide.

[0016]

In a semiconductor laser device of the present invention, a clad layer of the first conductivity type formed on a substrate of the first conductivity type, an active layer, and the first conductivity type are provided. In a semiconductor laser device in which a second-conductivity-type clad layer having an opposite conductivity type and the second-conductivity-type contact layer are stacked in this order, with the active layer as a center, an upper layer above the active layer The waveguide structure is asymmetric with the lower layer.

In this case, since either one of the cladding layers can be thinned, the heat generated in the active layer can be efficiently dissipated, and the temperature characteristics can be improved. Further, the higher the output power of the laser element or the thinner the active layer, the smaller the beam divergence angle in the vertical direction and the larger the amount of light seeping out. As a result, the heat generated in the active layer is released more efficiently, and the effect of improving the temperature characteristics is increased.

In particular, it is characterized in that the light generated in the active layer is exuded to the first conductivity type cladding layer side more than to the second conductivity type cladding layer side.

In this case, the thickness of the second-conductivity-type cladding layer can be reduced, so that the heat generated in the active layer can be efficiently dissipated.

Further, the semiconductor laser device of the present invention is characterized in that it is attached to a heat sink with the contact layer side being the lower surface.

Also in this case, the heat generated in the active layer can be efficiently released to the heat sink, so that the temperature characteristics can be improved.

Further, the semiconductor laser device of the present invention is characterized in that the second conductivity type clad layer has a thermal conductivity higher than that of the first conductivity type clad layer.

Also in this case, the heat generated in the active layer can be efficiently released to the heat sink, so that the temperature characteristics can be improved.

Further, the semiconductor laser device of the present invention is characterized in that the layer thickness of the second conductivity type cladding layer is thinner than the layer thickness of the first conductivity type cladding layer.

Also in this case, since the heat generated by the Joule heat in the cladding layer is proportional to the thickness of the cladding layer, the total amount of heat generated can be reduced and the heat generated in the active layer can be efficiently released to the heat sink. The temperature characteristic can be improved.

In the semiconductor laser device of the present invention, the second conductivity type clad layer is composed of a plurality of second conductivity type clad layers having different bandgap energies or layer thicknesses, and the first conductivity type clad layer. And a plurality of the second
The band gap energy of the conductivity type cladding layer, respectively, E _x and _{E y1, E y2, ...,} E ym, ..., E yn (2 ≦
m <n; m and n are natural numbers, and are set as 1, 2, ..., M, ..., N from the first conductivity type cladding layer side), E _y1 <E _x ≦ E _ym <E _yn , And, the first conductivity type cladding layer and the second
Corresponding layer thicknesses of the conductivity type cladding layers are given as d ₁ and d _y1 , d
_{y2, ..., d ym, ...} , when the _{d yn (2 ≦ m ≦ n} ), characterized in that it is a _{(d y1 + ... + d yn} ) <d 1.

In this case as well, by forming a waveguide structure in which the upper layer and the lower layer of the active layer are asymmetrical with the active layer at the center, the light generated in the active layer is caused to flow through the second conductivity type clad. Since the second conductivity type clad layer can be extruded more to the first conductivity type clad layer side than to the layer side, and the second conductivity type clad layer can be thinned, the heat generated in the active layer is efficiently released to the heat sink, The characteristics can be improved.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Al which is an embodiment of the present invention
A GaAs semiconductor laser device will be described with reference to FIG.
Incidentally, FIG. 1A is a cross-sectional structural view in a direction perpendicular to the resonator, FIG. 1B is a view showing a refractive index distribution and a light intensity distribution seen from the AA ′ cross section, and FIG. It is a band structure schematic diagram in an AA 'cross section.

In FIG. 1A, 1 is an n-type GaAs substrate, 2
Is an n-type Al _x G formed on the n-type GaAs substrate 1.
a _1-x As clad layer (x = 0.45 in the present embodiment, layer thickness 2.2 μm, Si-doped).

Al _y Ga is formed on the n-type cladding layer 2.
_1-y As quantum well layer (y = 0.05, layer thickness 1 in this embodiment)
00 Å) 3a and an Al _z Ga _1-z As optical guide layer (z = 0.35 in this embodiment, layer thickness 450 Å) 3b, the active layer 3 having a single quantum well layer structure is formed.

On the active layer 3, p-type Al _p Ga _{1 -p} A is formed.
s (p = 0.4 in this embodiment, layer thickness 0.2 μm) First layer 4
A p-type clad layer 4 is formed by laminating a and a second layer 4b of p-type Al _q Ga _1-q As (q = 0.5, layer thickness 0.7 μm in this embodiment).

On the p-type cladding layer 4, p-type GaA is formed.
The contact layer 5 made of s (layer thickness 0.2 μm in this embodiment) is formed. On the upper surface of this contact layer 5,
A p-type ohmic electrode 6 made of Cr-Au is provided on the lower surface of the n-type GaAs substrate 1 and made of Cr-Sn-Au.
A type ohmic electrode 7 is formed. The element has a so-called junction-down structure with the contact layer 6 on the lower surface, and is mounted on the heat sink.

In the laser device according to the embodiment of the present invention, since the upper layer and the lower layer of the active layer form an asymmetrical waveguide structure with the active layer 3 as the center, an asymmetrical refractive index distribution is obtained. be able to. Light bleeding from the active layer 3 in the direction perpendicular to the resonator is biased toward the n-type Rad layer 2 side having a large refractive index, so that the light bleeding to the contact layer 5 side is suppressed, and the p-type cladding layer is suppressed. The film thickness of 4 can be set thin. As a result, the heat generated in the active layer 3 can be efficiently released to the heat sink side, and the temperature characteristics can be improved.

In one embodiment of the present invention, the leakage of light from the active layer is about 2.2 μm toward the n-type cladding layer 2 side.
(About 1.6 μm in the conventional structure) and about 0.9 μm toward the p-type cladding layer 4 side (about 1.6 μm in the conventional structure).

Therefore, the heat generated in the active layer efficiently escapes to the heat sink side through the p-type AlGaAs cladding layer 4 which is 0.7 μm thinner than that of the conventional structure, so that the effect of reducing the temperature rise at the junction is expected. it can.

The thermal effect in one embodiment of the present invention is evaluated by calculation. Here, the calculation model of "Optical communication element optics-Light emitting / light receiving element-" (written by Hiroo Yonezu, published in 1984, Engineering Book Co., Ltd., pp. 115-121) is used.

FIG. 2 shows a calculation model of thermal resistance. This is an example of a junction down structure in which the contact layer side is the heat sink surface. Here, P _j is the amount of heat generated in the active layer 11, P _cl is the amount of heat generated by Joule heat in the cladding layer 12, P _cont is the amount of heat generated by Joule heat in the contact layer 13, and P _c is the amount of heat generated by the ohmic electrode 14. Heat generation due to resistivity, d _j is the thickness of the active layer 11, and d _cl is the p-type cladding layer 1.
2 of thickness, the film thickness of d _cont the contact layer 13, R _THJ the thermal resistance of the active layer _11, R thcl the thermal resistance of the p-type cladding layer 12, the thermal resistance of R _Thcont the contact layer 13, D is the light emitting Is the diameter.

In the calculation, approximately all the energy consumed in the light emitting region becomes heat in the light emitting region, and the active layer 1
1, the heat generated in the clad layer 12 and the contact layer 13 is localized at the center of each layer thickness, and the generated heat is entirely generated in the heat sink 1.
Suppose it flows to the 5 side. Further, assuming that the heat sink has an infinite size, the temperature at infinity is T _M , the average temperature of the light emitting region is T _j , and the temperature rise of the light emitting region is ΔT _j .

T _j = T _M + P _j (R _thj + R _thcl + R _thcont
+ R _thHS ) + P _cl [(R _thcl / 2) + R _thcont +
R _thHS ] + P _cont [(R _thcont / 2) + R _thHS ] + P _c
R _thHS ΔT _j ≡T _j −T _M = P _j · R _thj + [P _j + (P _cl / 2)] · R _thcl + [P _j
+ P _cl + (P _cont / 2)] ・ R _thcont + (P _j + P _cl
+ P _cont + P _c ) · R _thHS P _cl = I ² · (ρ _cl · d _cl ) / [π · (D / 2) ² ] R _thcl = (1 / κ _cl ) / {d _cl / [2π (D /
2) ² ]} Here, ρ _cl and κ _cl are the specific resistance and thermal conductivity of the p-type cladding layer, respectively.

A broad area single quantum well laser (with a stripe width of 150 μm according to an embodiment of the present invention
m, resonator length 600 μm, end face reflectivity 5% on front face, rear face 95%), operated with optical output 2 W (operating current 3.2 A, heat generation amount about 5 W).

The thermal conductivity of the p-type cladding layer is 0.086 W.
/ (Cm · ° C.), the total thermal resistance and the total temperature rise of the p-type cladding layer of the laser in the embodiment of the present invention are 1.4 ° C./W and 7 ° C., respectively. On the other hand, the thermal resistance and temperature rise of the p-type cladding layer of the conventional structure laser shown in FIG. 4 are 2.4 ° C./W and 12 ° C., respectively. Therefore, the laser according to the embodiment of the present invention can reduce the temperature rise of the junction by about 5 ° C. as compared with the conventional structure.

The optical confinement coefficient Γ in the embodiment of the present invention is 0.01 which is almost the same as that of the conventional structure.
Calculated as 7. Therefore, the problem of increase in operating current does not occur, and the improvement of the above-mentioned temperature characteristics can improve the reliability.

By the way, in the above embodiment, two layers of p
Although an example having a type clad layer has been described, even in the case of having a plurality of p type clad layers of two or more layers, the band gap energy and the layer thickness are increased stepwise from the p type clad layer on the side closer to the active layer, The band gap energy of the n-type cladding layer is in the middle of the band gap energy of the p-type cladding layer, and the total film thickness of the p-type cladding layer is n.
The same effect can be obtained by setting the thickness so that it is thinner than the thickness of the mold cladding layer.

Although the broad area type laser device has been described in the above embodiment, the present invention can be applied to a semiconductor laser device having a current block layer having a stripe ridge portion or a cutout portion.

Further, in the above embodiment, the active layer has a single quantum well structure, but it may have a multiple quantum well structure, or may be an active layer which is a non-quantum well layer.

Further, although the AlGaAs semiconductor laser device has been described above, other material systems such as Al can be used.
It can be appropriately applied to a semiconductor laser device using a GaInP system, a GaInAsP system, or the like.

[0047]

The vertical beam divergence angle of the laser is as small as about 20 degrees, that is, centering on a very thin active layer.
By forming an asymmetrical waveguide structure in a direction perpendicular to the resonator, light leakage from the active layer to the contact layer side is suppressed. As a result, by reducing the thickness of the clad layer on the contact layer side, the heat generated in the active layer is efficiently dissipated to the heat sink side, the temperature rise at the junction is suppressed, and the temperature characteristics are improved. The semiconductor laser device can be provided.

[Brief description of drawings]

FIG. 1A is a schematic sectional view of a semiconductor laser device according to an embodiment of the present invention, and FIG. 1B is a refractive index distribution of the semiconductor laser device according to an embodiment of the present invention. FIG. 3C is a diagram showing a light intensity distribution, and FIG. 6C is a schematic band structure diagram of a semiconductor laser device according to an embodiment of the present invention.

FIG. 2 is a diagram showing a thermal resistance calculation model.

FIG. 3 is a current-light output characteristic diagram for explaining the principle of COD and a flowchart diagram showing a COD generation mechanism.

4A is a schematic sectional view of a conventional semiconductor laser device, FIG. 4B is a diagram showing a refractive index distribution and a light intensity distribution of a conventional semiconductor laser device, and FIG. FIG. 6 is a schematic view of a band structure of a conventional semiconductor laser device.

[Explanation of symbols]

1 n-type GaAs substrate 2 n-type clad layer 3 Active layer 3a Single quantum well layer 3b light guide layer 4 p-type clad layer 5 p-type contact layer 6 p-type ohmic electrode 7 n-type ohmic electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (4)

(57) [Claims] 1. A clad layer of the first conductivity type formed on a substrate of the first conductivity type, an active layer, and a second conductivity type clad layer having a conductivity type opposite to the first conductivity type. And a contact layer of the second conductivity type are stacked in this order, the clad layer of the second conductivity type has a bandgap energy.
Or a plurality of the second conductivity type clad layers having different layer thicknesses
And a plurality of second conductivity type cladding layers and a plurality of second conductivity type cladding layers.
The band gap energies of the Rad layers are respectively E
_x and E _y1 , E _y2 , ..., E _ym , ..., E _yn (2 ≦ m <n;
m and n are natural numbers, 1, 2 from the first conductivity type cladding layer side,
, M, ..., N), E _y1 <E _x ≦ E _ym <E _yn , and the first conductivity type cladding layer and the second
Corresponding layer thicknesses of the conductivity type cladding layers are given as d ₁ and d _y1 , d
_{y2, ..., d ym, ...} , when the _{d yn (2 ≦ m ≦ n} ), the semiconductor laser device which is a _{(d y1 + ... + d yn} ) <d 1. 2. A front substrate formed on a first conductivity type substrate.
A clad layer of the first conductivity type, an active layer, and the first conductivity type.
A second conductivity type cladding layer having a conductivity type opposite to the conductivity type;
The second conductivity type contact layer is laminated in this order.
In the semiconductor laser device, the second conductivity type clad layer is sequentially laminated on the active layer.
A first conductive type clad layer and a second conductive type clad.
The bandgap energies of the first and second layers of the layer are
Respectively, when the E _x and E _y1, E _y2, a _{E y1 <E x <E y2} , and said first conductivity type cladding layer and the second
The thicknesses of the first layer and the second layer of the conductivity type cladding layer are set to d ₁ and
When the d _y1, d _y2, the semiconductor laser device which is a _{(d y1 + d y2) <} d 1. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is mounted on a heat sink with the contact layer side facing down. 4. The semiconductor laser device according to claim 1, wherein the second conductivity type clad layer has a thermal conductivity higher than that of the first conductivity type clad layer. .