JP3179511B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used as a light source for an optical recording device such as a rewritable optical disk.

[0002]

2. Description of the Related Art A conventional semiconductor laser for an optical recording apparatus is shown in FIG.
Shown in This structure is described in Proc. Of 12th IEEE international semiconductor laser 1990, page 270 (Proc. Of 12th IEEE International Laser Conference).
aser Conference (1990) 278P).
N-AlGaAs cladding layer 2 on an As substrate 1
aAs active layer 3, p-AlGaAs cladding layer 4, p-
After the GaAs contact layers 5 are sequentially formed, a striped insulating film such as SiO ₂ is formed (not shown), and the p-AlGaAs layer 4 is chemically etched using this film as a mask. Next, n-Ga is left while the SiO ₂ film is left.
The selective growth of the As current block layer 6 is performed. Finally, a p-GaAs buried layer 8 is grown to reduce the contact resistance.

[0003]

SUMMARY OF THE INVENTION The above conventional semiconductor laser
When a semiconductor laser is used in an optical disk device or the like, high-frequency modulation is performed to prevent noise. But,
The high frequency current leaks through the junction capacitance of the n-GaAs current block layer 6 in the current block portion and the p-GaAs buried layer 8 and the p-AlGaAs clad layer 4 located above and below the current block layer. There is a problem that the noise reduction effect by modulation cannot be obtained.

An object of the present invention is to solve this problem.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide the above-mentioned prior art, wherein the impurity concentration is at least one of the interfaces between the n-type current block layer and the p-type semiconductor layers located above and below the n-type current block layer. This can be achieved by providing a low-impurity-concentration semiconductor layer that is smaller than the above.

That is, an object of the present invention is to provide a semiconductor laser having a PN junction current block structure for energizing only a stripe region in an active layer. On the other hand, it can be achieved by forming a PN junction with the layer on the active layer side and forming a low impurity concentration semiconductor layer having an impurity concentration lower than that of the PN junction constituent layer at at least one interface of the PN junction. By the way, JP-A-61-9
No. 6791 discloses a p-type cladding layer bonded to an active layer,
Due to the impurity compensation effect of the n-type current confinement layer joined thereto
To reduce the acceptor concentration and increase the resistance
Bonding the p-type cladding layer and the n-type current confinement layer
A non-doped semiconductor layer at the part of the interface facing the active layer
Is disclosed. However, the present invention
It has a different purpose from the invention disclosed in the above publication.
Thus, a current is supplied to the active layer as in the semiconductor laser shown in FIG.
Having a strip-shaped portion for feeding
A second semiconductor forming a PN junction with the first semiconductor layers 4 and 5
The upper surface of the layer 6 is more impure than the first and second semiconductor layers.
The third semiconductor layer 7 having a low material concentration is used as the first semiconductor layer.
Even if it is formed to be bonded to the side of the stripe (third
Do not form the semiconductor layer 7 at the PN junction facing the active layer.
At least), the object of the present invention can be achieved.

The high-frequency current applied when the semiconductor laser is used effectively contributes to light emission, and the current component having a frequency higher than the high-frequency current contributes to light emission, depending on the thickness and impurity concentration of the low impurity concentration semiconductor layer. By setting not to do so, it is possible to prevent surge damage of the semiconductor laser.

[0008]

The structure in which the low-impurity-concentration semiconductor layer is sandwiched between the p-type semiconductor layer and the n-type semiconductor layer makes it easy for the depletion layer to extend, and can greatly reduce the junction capacitance. As a result, leakage of the high-frequency current can be reduced, and a noise reduction effect by high-frequency modulation can be obtained.

Since the junction capacitance between the semiconductor layers is expressed by the following equation, the relationship between the impurity concentration of the low impurity concentration semiconductor layer and the junction capacitance is as shown in FIG.

[0010] FIG. 3 shows an equivalent circuit for a high-frequency current of a semiconductor laser having such a current block structure. Resistance current through R ₂ is the active current is a current leakage current flowing through the capacitor C. On the basis of this equivalent circuit, the capacitance C ₁ is calculated using R ₁ and R ₂ of the semiconductor laser actually measured.
FIG. 8 shows the relationship between the light emission contribution rate of the high-frequency current and the light emission contribution rate. In a conventional semiconductor laser, since the junction capacitance was 200 pF or more, more than half of the high-frequency current was a leakage current. From FIG. 8, it is understood that C must be 100 pF or less in order to effectively use more than half of the high-frequency current. FIG. 2 shows that such a low capacitance reduces the impurity concentration of the low impurity concentration semiconductor layer to 1 ×.
It can be seen that it can be obtained by setting it to 10 ¹⁷ cm ~ ³ or less. The current confinement is performed in the current block layer as in the conventional semiconductor laser. The conductivity type of the low impurity concentration semiconductor layer is arbitrary. The forbidden band width may be any value as long as it functions as a laser. If the same material as the current blocking layer or a layer adjacent thereto is selected as the low impurity concentration semiconductor layer, the control of the impurity concentration becomes very easy. In that case, the low impurity concentration semiconductor layer plays the same role as the current blocking layer or a layer adjacent thereto.

Further, according to this structure, by appropriately setting C, the high-frequency modulation current is effectively contributed to the light emission, and a design is made such that a surge current harmful to the laser flows through C. It is also possible. The optical output at which the optical destruction phenomenon of the semiconductor laser occurs due to the pulse current is 1/1 of the pulse width.
It is known that it is inversely proportional to the power of 2 to 1/4.
If the amount of charge discharged by the di-current is constant, the smaller the pulse width, the more harmful. Therefore, it is desirable that a current component having a frequency higher than a frequency required when using a semiconductor laser does not contribute to light emission. According to the present invention, since the leakage current outside the stripe region can be controlled, a current component having a frequency higher than a required frequency can be easily nullified.

[0012]

Embodiment 1 Embodiment 1 of the present invention will be described with reference to FIG. This structure is first, n-Al on the n-GaAs substrate 1 _0.5 Ga _0.5 As cladding layer 2 (1.5 [mu] m thick), and - flop Al _{0. 14} Ga
_0.86 As active layer 3 (0.04 μm thick), p-Al _0.5 G
a _0.5 As clad layer 4 (1.5 μm thick), p-GaAs
Crystal growth of the s-contact layer 5 (thickness: 0.3 μm) is sequentially performed. Next, a stripe-shaped pattern of an SiO ₂ film is formed, and p-Al _0.5
The Ga _0.5 As clad layer 4 and the p-GaAs contact layer 5 were processed, and further, n-GaAs 6 (0.8 μm
) And a current blocking layer made of undoped GaAs 7 (0.3 μm thick). At this time, the growth of the block layer does not occur on the ridge due to the characteristics of the MOCVD method in which no crystal growth occurs on the SiO ₂ pattern. Finally, after removing the SiO ₂ pattern, a p-GaAs buried layer 8 (1.2 μm thick) is grown to obtain a structure as shown in FIG. After providing the Au electrodes 9 on the front and back surfaces of the semiconductor wafer prepared as described above, 600 μm in the stripe direction.
The wafer is cleaved to 300 μm in the direction perpendicular to the m stripes to form a laser chip. The impurity concentration of the undoped GaAs 7 is 5 × 10 ¹⁶ cm ³ or less, and the junction capacitance 23 of the block layer portion is 50 pF or less from FIG. FIG. 3 shows an equivalent circuit of such an element for high-frequency current, where R ₁ and R ₂ are 4Ω and 2Ω, respectively. Therefore, the emission contribution ratio of the 700 MHz high-frequency current is 70%.

Second Embodiment A second embodiment of the present invention will be described with reference to FIG. The present structure is as follows. First, n-Al _0.5 Ga _0.5 As is formed on an n-GaAs substrate 1.
Cladding layer 2 (1.5 μm thick), quantum well structure active layer 12
(0.04 μm thickness), p-Al _0.5 Ga _0.5 As clad layer 4 (1.5 μm thickness), p-GaAs contact layer 5
(Thickness of 0.3 μm) is sequentially grown. Next, SiO ₂
Forming a pattern of stripes of film, processing a p-Al _0.5 Ga _0.5 As cladding layer 4 and the p-GaAs contact layer 5 in a ridge shape this pattern as a mask, further and - flop GaAs7 (0.1μm thickness) And n-GaAs
A current block layer made of s6 (1.1 μm thick) is formed. At this time, the growth of the block layer does not occur on the ridge due to the characteristics of the MOCVD method in which no crystal growth occurs on the SiO ₂ pattern. Finally, after removing the SiO ₂ pattern, a p-GaAs buried layer 8 (1.2 μm thick) was grown to obtain a structure as shown in FIG. After providing the Au electrodes 9 on the front and back surfaces of the semiconductor wafer prepared as described above,
The wafer is cleaved to 600 μm in the stripe direction and 300 μm in the direction perpendicular to the stripe to form a laser chip. By optimizing the supply amounts of the group V source and the group III source in MOCVD, the impurity concentration of the undoped GaAs 7 is 1 × 10 ¹⁶ cm ³ or less. When the impurity concentration of the undoped GaAs layer 7 is low, the entire area of the undoped layer is depleted, so that the junction capacitance is determined by the thickness of the undoped layer. The junction capacitance in this case is represented by <dielectric constant of GaAs> X <chip area> / <thickness of undoped layer>, and is about 100 pF in the case of this element. on the other hand,
FIG. 3 shows an equivalent circuit of such an element for a high-frequency current, where R ₁ and R ₂ are 4Ω and 2Ω, respectively.
Therefore, the emission contribution ratio of the 700 MHz high-frequency current is 60%. In addition, this structure has the advantage that high-frequency components of 1 GHz or more leak through the junction capacitance of the current blocking layer and thus do not contribute to light emission, so that surge damage hardly occurs.

Third Embodiment A third embodiment of the present invention will be described with reference to FIG. In this structure, first, a current blocking layer composed of undoped GaAs 8 and n-GaAs 7 is formed on a p-GaAs substrate 13. Next, a stripe-shaped groove 14 is formed in a part of the current block layer by using a normal photolithography technique. Next, the p-Al _0.5 Ga _0.5 As clad layer 15, the undoped Al _0.14 Ga _0.86 As active layer 16, the n-Al _0.5
Ga _0.5 As clad layer 17, n-GaAs contact layer 1
8 are sequentially crystal-grown to have a structure as shown in FIG. After the Au electrodes 9 are provided on the front and back surfaces of the semiconductor wafer prepared as described above, the wafer is cleaved to 300 μm in the direction perpendicular to the stripe in the direction of 600 μm in the stripe direction to form laser chips. And - the impurity concentration of the flop GaAs has a 5x10 ¹⁶ cm ~ ³ or less, the junction capacitance 23 of the current blocking layer portion according to the present structure is as follows 50pF than FIG. FIG. 3 shows an equivalent circuit of such an element for high-frequency current, where R ₁ and R ₂ are 4Ω and 2Ω, respectively. Therefore, the emission contribution ratio of the 700 MHz high-frequency current is 70%.

Embodiment 4 Embodiment 4 of the present invention will be described with reference to FIG. In this embodiment, the structure of the present invention is applied to an AlGaInP-based semiconductor laser. First, an n- (Al _0.5
Ga _0.5 ) _0.5 In _0.5 P clad layer 19 (1.5 μm
Thickness), undoped Ga _0.5 In _0.5 P active layer 20 (0.04
μm _{thickness), p- (Al 0.5 Ga 0.5} ) 0.5 In 0. 5 P cladding layer 21 (1.5 [mu] m thickness), p-GaAs contact layer 22
(Thickness of 0.3 μm) is sequentially grown. Next, SiO ₂
A stripe-shaped pattern of the film is formed, and p- (Al _0.5 Ga _0.5 ) _0.5 In _0.5 is formed in a ridge shape using the pattern as a mask.
The P cladding layer 21 and the p-GaAs contact layer 22 are processed, and a current block layer made of n-GaAs 6 (0.8 μm thick) and undoped GaAs 7 (0.3 μm thick) is formed. At this time, the current blocking layer does not grow on the ridge due to the characteristics of the MOCVD method in which no crystal growth occurs on the SiO ₂ pattern. Finally SiO
After removing the _two patterns, the p-GaAs buried layer 8 (1.
(2 μm thick) to form a structure as shown in FIG. After the Au electrodes 9 are provided on the front and back surfaces of the semiconductor wafer prepared as described above, the wafer is cleaved to 300 μm in the direction perpendicular to the stripe in the direction of 600 μm in the stripe direction to form laser chips. And - the impurity concentration of the flop GaAs7 has become a 5x10 ¹⁶ cm ~ ³ or less, the junction capacitance 23 of the current blocking layer portion according to the present structure is as follows 50pF than FIG. FIG. 3 shows an equivalent circuit of such an element for a high-frequency current, where R ₁ and R ₂ are 8Ω and 12Ω, respectively.
Ω. Therefore, the emission contribution ratio of the 700 MHz high-frequency current is 70%.

Embodiment 5 Embodiment 5 of the present invention will be described with reference to FIG. In this structure, first, an n-Al _0.5 Ga _0.5 As clad layer 2, an undoped Al _0.14 Ga _0.86 As active layer 3, a p-Al _0.5 Ga _0.5 As clad layer 4, a p-GaAs
The s-contact layer 5 is crystal-grown sequentially. Next, SiO ₂
Forming a pattern of stripes of film, the pattern processing the p-Al _0.5 Ga _0.5 As cladding layer 4 and the p-GaAs contact layer 5 in a ridge shape as a mask, further n-GaAs6 and and - consisting flop GaAs7 A current blocking layer is formed. At this time, the current blocking layer does not grow on the ridge due to the characteristics of the MOCVD method in which no crystal growth occurs on the SiO ₂ pattern. Finally, the SiO ₂ pattern is removed to obtain a structure as shown in FIG.
After the Au electrodes 9 are provided on the front and back surfaces of the semiconductor wafer prepared as described above, the wafer is cleaved to 300 μm in the direction perpendicular to the stripe in the direction of 600 μm in the stripe direction to form laser chips. And - the impurity concentration of the flop GaAs7 has become a 5x10 ¹⁶ cm ~ ³ or less, the Schottky current blocking layer portion according to the structure - junction capacitance 23 5
0 pF or less. Equivalent circuit for high-frequency current of such a device is as shown in FIG. 3, R ₁ and R ₂ each 4
Ω and 2Ω. Therefore, the emission contribution ratio of the 700 MHz high-frequency current is 70%.

[0017]

According to the present invention, the leakage of the high-frequency current component, which is a problem in the conventional semiconductor laser, can be prevented, and the effect of high-frequency modulation for preventing the generation of noise can function well. You get the. In addition, by appropriately selecting the impurity concentration and thickness of the low impurity concentration semiconductor layer, a current having a frequency component higher than the modulation frequency can be released.
A semiconductor laser which is less likely to cause breakdown is obtained. In addition, the introduction of the low-impurity-concentration semiconductor layer can prevent the early deterioration phenomenon caused by the current leak of the current block layer seen in the conventional semiconductor laser, and reduce the defective rate of the semiconductor laser. There is also an effect.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an impurity concentration of a low impurity concentration GaAs layer and a junction capacitance.

FIG. 3 is an equivalent circuit of a semiconductor laser having a current blocking layer.

FIG. 4 is a sectional structural view of a semiconductor laser according to a second embodiment of the present invention.

FIG. 5 is a sectional structural view of a semiconductor laser according to a third embodiment of the present invention.

FIG. 6 is a sectional structural view of a semiconductor laser according to a fourth embodiment of the present invention.

FIG. 7 is a sectional structural view of a semiconductor laser according to a fifth embodiment of the present invention.

FIG. 8 is a diagram showing a relationship between a junction capacitance and a light emission contribution ratio of the semiconductor laser of the present invention.

FIG. 9 is a sectional structural view of a conventional semiconductor laser.

[Explanation of symbols]

1 ... n-GaAs _{substrate, 2 ... n-Al 0.5 Ga} 0.5 As cladding layer, 3 ... and - flop Al _0.14 Ga _0.86 As active _{layer, 4 ... p-Al 0.5 Ga} 0.5 As cladding layer, 5 ... p-
GaAs contact layer, 6 n-GaAs, 7 undoped GaAs, 8 p-GaAs buried layer, 9 Au electrode, 12 quantum well structure active layer, 13 p-GaAs substrate,
14 ... striped _{groove, 15 ... p-Al 0.5 Ga} 0.5 As cladding layer, 16 ... and - flop Al _0.14 Ga _0.86 As active _{layer, 17 ... n-Al 0.5 Ga} 0.5 As cladding layer, 18 ... n-
GaAs contact layer, 19 ... n- (Al _0.5 Ga _0.5 )
_0.5 an In _0.5 P cladding layer, 20 ... and - flop Ga _0.5 an In
_0.5 P active layer, 21... P- (Al _0.5 Ga _0.5 ) _0.5 In _0.5
P clad layer, 22 ... p-GaAs contact layer.

Continuing on the front page (72) Inventor Kenji Uchida 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Takeo Takahashi 15 Asahidai, Moroyama-cho, Iruma-gun, Saitama Prefecture Inside Hitachi Eastern Semiconductor Co., Ltd. (72) Inventor Kenichi Uejima 190 Oji Kashiwagi, Komoro City, Nagano Prefecture Hitachi, Ltd. ) Inventor Kazunori Saito 15 Asahidai, Moroyama-cho, Iruma-gun, Saitama Prefecture Inside Hitachi Eastern Semiconductor Co., Ltd. (56) References JP-A-3-149887 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) H01S 5/00-5/50 JICST file (JOIS)

Claims (2)

(57) [Claims] An active layer, a first semiconductor layer joined to the active layer and having a stripe-shaped ridge formed on the side opposite to the active layer, and formed on both sides of the ridge of the first semiconductor layer. A second semiconductor layer that is formed and forms a PN junction with the first semiconductor layer; and a third semiconductor layer that is bonded on the semiconductor layer of the second semiconductor layer and bonded to a ridge side surface of the first semiconductor layer. A semiconductor layer, wherein the impurity concentration of the third semiconductor layer is lower than the impurity concentration of the first and second semiconductor layers and 1 × 10 ¹⁷ cm
A semiconductor laser characterized by having a value of ^-3 or less. 2. The semiconductor laser according to claim 1, wherein
A semiconductor laser, wherein the third semiconductor layer is made of GaAs.