JPH04309278A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To prevent a high-frequently current from leaking out through the junction capacitance of a current blocking section when high-frequency modula tion is performed on a semiconductor laser used for an optical disk device, etc., for preventing the generation of noise. CONSTITUTION:Crystals of an n-Al0.5Ga0.5As clad layer 2 (1.5mum), undoped Al0.14Ga0.86As active layer 3 (0.04mum), p-Al0.5Ga0.5As clad layer 4 (1.5mum), and p-GaAs contact layer 5 (0.3mum) are successively grown on an n-GaAs substrate 1. Then the layers 4 and 5 are processed to a ridge-like shape and a clock layer composed of an n-GaAs layer 6 (0.8mum) and undoped GaAs layer 7 (0.3mum) is formed. Finally, the structure shown in Fig. 1 is formed by growing a p-GaAs buried layer 8 (1.2mum). The laser chip of this semiconductor laser is formed by providing Au electrodes 9 on the surface and rear of the semiconductor wafer thus manufactured.

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

[Prior Art] Fig. 9 shows a conventional semiconductor laser for an optical recording device.
Shown below. This structure is based on the Proceedings of the International Laser Conference 1
990, page 270 (Proc. of 12th
IEEE international semi
onductor laser conference
(1990) 278P), n-GaA
n-AlGaAs cladding layer 2, AlGa
As active layer 3, p-AlGaAs cladding layer 4, p-G
After sequentially forming the aAs contact layer 5, a striped insulating film of SiO2 or the like is formed (not shown), and using this film as a mask, chemical etching is performed up to the p-AlGaAs layer 4. Next, while leaving this SiO2 film, n-GaA
Selective growth of the s-current blocking layer 6 is performed. Finally, a p-GaAs buried layer 8 is grown to reduce contact resistance.

0003

[Problems to be Solved by the Invention] The above-mentioned 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 between the n-GaAs current blocking layer 6 in the current blocking section, the p-GaAs buried layer 8 and the p-AlGaAs cladding layer 4 located above and below it. There is a problem that the noise reduction effect cannot be obtained by modulation.

[0004] The purpose of the present invention is to solve this problem.

[0005]

[Means for Solving the Problems] The above object is to provide an impurity concentration in at least one of the interfaces between the n-type current blocking layer and the p-type semiconductor layers located above and below the n-type current blocking layer in the above-mentioned p and n layers. This can be achieved by providing a semiconductor layer with a low impurity concentration that is smaller than that of the semiconductor layer.

That is, the above object is to provide a semiconductor laser having a PN junction current blocking structure for supplying current only to the striped region in the active layer, which is at least compatible with the current blocking layer which is a component of the PN junction current blocking structure. This can be achieved by forming a PN junction with a layer on the active layer side, and forming a low impurity concentration semiconductor layer having a lower impurity concentration than the layer constituting the PN junction at the interface of at least one of the PN junctions. .

[0007] Furthermore, the thickness and impurity concentration of the low impurity concentration semiconductor layer can be adjusted so that the high frequency current applied when using a semiconductor laser effectively contributes to light emission, and the current component with a frequency higher than the above high frequency current contributes to light emission. By setting the voltage so as not to occur, damage to the semiconductor laser due to surge can be prevented.

[0008]

[Operation] By forming a structure in which a low impurity concentration semiconductor layer is sandwiched between a p-type semiconductor layer and an n-type semiconductor layer, the depletion layer can easily extend, and the junction capacitance can be significantly reduced. As a result, leakage of high frequency current can be reduced, and a noise reduction effect due to high frequency modulation can be obtained.

Since the junction capacitance between 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.

Incidentally, an equivalent circuit for high frequency current of a semiconductor laser having such a current block structure is shown in FIG. The current passing through resistor R2 is the effective current, and the capacitance C
The current flowing through it is the leakage current. Based on this equivalent circuit and using the actually measured R1 and R2 of the semiconductor laser, the relationship between the capacitance C and the light emission contribution rate of the high frequency current is calculated as shown in FIG. In conventional semiconductor lasers, since the junction capacitance was 200 pF or more, more than half of the high frequency current became leakage current. From FIG. 8, it can be seen that in order for more than half of the high frequency current to be used effectively, C needs to be 100 pF or less. Further, from FIG. 2, it can be seen that such a low capacitance can be obtained by setting the impurity concentration of the low impurity concentration semiconductor layer to 1×10 17 cm −3 or less. Current confinement is performed by a current blocking layer, similar to conventional semiconductor lasers. The conductivity type of the low impurity concentration semiconductor layer is arbitrary. Further, the prohibited band width may be any value as long as it functions as a laser. If the same material as the current block layer or the layer adjacent thereto is selected for the low impurity concentration semiconductor layer, the impurity concentration can be controlled very easily. Further, in that case, the low impurity concentration semiconductor layer plays the same role as the current blocking layer or a layer adjacent thereto.

Furthermore, according to the present structure, by appropriately setting C, the high frequency modulation current is made to contribute effectively to light emission, and the surge current harmful to the laser is designed to flow through C. It is also possible. Semiconductor laser is activated by pulsed current.
The optical output at which the optical destruction phenomenon occurs is 1/2 of the pulse width.
It is known that the pulse width is inversely proportional to the 1/4th power, and when the amount of charge discharged by a surge current is constant, the smaller the pulse width, the more harmful it becomes. Therefore, it is desirable that current components having a frequency higher than that required when using a semiconductor laser do not contribute to light emission. According to the present invention, since leakage current in areas other than the stripe region can be controlled, current components having a frequency higher than a required frequency can be easily nullified.

0012

EXAMPLES Example 1 Example 1 of the present invention will be described with reference to FIG. This structure first consists of n-Al0.5Ga0.5A on an n-GaAs substrate 1.
s cladding layer 2 (1.5 μm thick), undoped Al0.
14Ga0.86As active layer 3 (0.04 μm thick), p
-Al0.5Ga0.5As cladding layer 4 (1.5μm
(thickness), p-GaAs contact layer 5 (0.3 μm thick) is successively crystal-grown. Next, a striped pattern of SiO2 film is formed, and using this pattern as a mask, the p-Al0.5Ga0.5As cladding layer 4 and the p-
The GaAs contact layer 5 is processed and further n-GaAs
6 (0.8 μm thick) and undoped GaAs7 (0.3 μm thick)
A current blocking layer having a thickness of .mu.m is formed. At this time,
MOCVD without crystal growth on SiO2 pattern
Due to the characteristics of the method, no block layer grows on the top of the ridge. Finally, after removing the SiO2 pattern, p-
A GaAs buried layer 8 (1.2 μm thick) is grown to form a structure as shown in FIG. After providing Au electrodes 9 on the front and back surfaces of the semiconductor wafer created as described above, the thickness of the electrodes is 600 μm in the stripe direction and 300 μm in the direction orthogonal to the stripes.
The wafer is then cleaved into laser chips. The impurity concentration of the undoped GaAs 7 is less than 5 x 1016 cm~3, and the junction capacitance 23 of the block layer portion is less than 50 pF from FIG. The equivalent circuit for high frequency current of such an element is as shown in FIG. 3, where R1 and R2 are 4Ω and 2Ω, respectively. Therefore, the light emission contribution rate of the 700 MHz high frequency current is 70%.

Embodiment 2 A second embodiment of the present invention will be explained with reference to FIG. In this structure, first, n-Al0.5Ga0.
5As cladding layer 2 (1.5 μm thick), quantum well structure active layer 12 (0.04 μm thick), p-Al0.5Ga0.
A 5As cladding layer 4 (1.5 μm thick) and a p-GaAs contact layer 5 (0.3 μm thick) are successively crystal-grown. Next, a striped pattern of SiO2 film is formed, and using this pattern as a mask, p-Al0.5Ga0
．． The 5As cladding layer 4 and the p-GaAs contact layer 5 are processed, and undoped GaAs7 (0.1 μm
A current blocking layer made of n-GaAs6 (1.1 μm thick) is formed. At this time, due to the characteristics of the MOCVD method in which crystal growth does not occur on the SiO2 pattern, no block layer grows on the upper part of the ridge. Finally, Si
After removing the O2 pattern, p-GaAs buried layer 8 (1
．． 2 μm thick) to form a structure as shown in FIG. Au was applied to the front and back sides of the semiconductor wafer prepared as described above.
After providing the electrodes 9, the wafer is cleaved to 600 μm in the stripe direction and 300 μm in the direction perpendicular to the stripes to form laser chips. MOCVD group V source and III
By optimizing the supply amount of the group source, the impurity concentration of the undoped GaAs7 is reduced to 1×10 16 cm −3 or less. When the impurity concentration of the undoped GaAs layer 7 is low, the entire 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 expressed as <permittivity of GaAs>X<chip area>/<thickness of undoped layer>, and is approximately 100 pF in the case of this element. on the other hand,
The equivalent circuit for high frequency current of such an element is as shown in FIG. 3, where R1 and R2 are 4Ω and 2Ω, respectively. Therefore, the light emission contribution rate of the 700 MHz high frequency current is 60%. Moreover, in this structure, high frequency components of 1 GHz or higher leak through the junction capacitance of the current blocking layer, so they do not contribute to light emission, and there is an advantage that surge damage is less likely to occur.

Embodiment 3 Embodiment 3 of the present invention will be explained with reference to FIG. In this structure, first, a current blocking layer made of undoped GaAs 8 and n-GaAs 7 is formed on a p-GaAs substrate 13. Next, a striped groove 14 is formed in a part of the current blocking layer using a normal photolithography technique. Next, a p-Al0.5Ga0.5As cladding layer 15, an undoped Al0.14Ga0.86As active layer 16, and an n-Al0.5Ga0.5As cladding layer 17 are formed by liquid layer growth.
, the n-GaAs contact layer 18 is successively crystal-grown to form a structure as shown in FIG. After providing Au electrodes 9 on the front and back surfaces of the semiconductor wafer prepared as described above, 300 μm in the direction perpendicular to the 600 μm stripes in the stripe direction.
The wafer is cleaved to 0 μm to form a laser chip. The impurity concentration of the undoped GaAs is 5x1016 cm~3 or less, and the junction capacitance 23 of the current blocking layer portion with this structure is 50 pF or less as shown in FIG. The equivalent circuit for high frequency current of such an element is as shown in FIG. 3, where R1 and R2 are 4Ω and 2Ω, respectively. Therefore, the light emission contribution rate of the 700 MHz high frequency current is 70%.

Embodiment 4 Embodiment 4 of the present invention will be explained with reference to FIG. In this embodiment, the structure of the present invention is applied to an AlGaInP semiconductor laser. First, an n-(Al0.
5Ga0.5)0.5In0.5P cladding layer 19(1
．． 5 μm thick), undoped Ga0.5In0.5P active layer 20 (0.04 μm thick), p-(Al0.5Ga0.5P active layer 20 (0.04 μm thick),
5) A 0.5In0.5P cladding layer 21 (1.5 μm thick) and a p-GaAs contact layer 22 (0.3 μm thick) are successively crystal-grown. Next, a striped pattern of SiO2 film is formed, and using this pattern as a mask, the p-(Al0.5Ga0.5)0.5In0.5P cladding layer 21 and the p-GaAs contact layer 22 are processed into a ridge shape. Furthermore, a current blocking layer made of n-GaAs6 (0.8 μm thick) and undoped GaAs7 (0.3 μm thick) is formed. At this time, the current blocking layer does not grow on the upper part of the ridge due to the characteristics of the MOCVD method in which no crystal growth occurs on the SiO2 pattern. Finally, SiO2
After removing the pattern, p-GaAs buried layer 8 (1.2
.mu.m thick) to form a structure as shown in FIG. After providing Au electrodes 9 on the front and back surfaces of the semiconductor wafer prepared as described above, the wafer was cleaved to 600 μm in the stripe direction and 300 μm in the direction orthogonal to the stripes.
Let's call it The Chip. The impurity concentration of the undoped GaAs7 is less than 5x1016cm￣3, and the junction capacitance 23 of the current blocking layer portion with this structure is 50pF from Figure 2.
The following is true. The equivalent circuit for high frequency current of such an element is as shown in FIG. 3, where R1 and R2 are 8Ω and 12Ω, respectively. Therefore, the light emission contribution rate of the 700 MHz high frequency current is 70%.

Example 5 Example 5 of the present invention will be explained with reference to FIG. This structure first consists of n-Al0.5Ga0.5A on an n-GaAs substrate 1.
s cladding layer 2, undoped Al0.14Ga0.86
An As active layer 3, a p-Al0.5Ga0.5As cladding layer 4, and a p-GaAs contact layer 5 are successively crystal-grown. Next, a striped pattern of SiO2 film is formed,
Using this pattern as a mask, apply p-Al0.5G in a ridge shape.
The a0.5As cladding layer 4 and the p-GaAs contact layer 5 are processed, and a current blocking layer made of n-GaAs6 and undoped GaAs7 is formed. At this time, MOCV where no crystal growth occurs on the SiO2 pattern.
Due to the characteristics of the D method, no current blocking layer is grown above the ridge. Finally, the SiO2 pattern is removed to obtain a structure as shown in FIG. After providing Au electrodes 9 on the front and back surfaces of the semiconductor wafer prepared as described above, the wafer is cleaved to a length of 600 μm in the stripe direction and 300 μm in the direction perpendicular to the stripes to obtain laser chips. The impurity concentration of the undoped GaAs7 is less than 5x1016cm￣3, and the Schottky junction capacitance of the current blocking layer portion with this structure is 2.
3 is 50 pF or less. The equivalent circuit for high frequency current of such an element is as shown in FIG. 3, where R1 and R2 are 4Ω and 2Ω, respectively. Therefore, 700MHz
The light emission contribution rate of the high frequency current is 70%.

[0017]

According to the present invention, it is possible to prevent leakage of high-frequency current components, which was a problem in conventional semiconductor lasers, and to provide a semiconductor laser in which the effect of high-frequency modulation for preventing noise generation functions well. The result is obtained. In addition, by appropriately selecting the impurity concentration and thickness of the low impurity concentration semiconductor layer, it is possible to release current with frequency components higher than the modulation frequency.
A semiconductor laser that is less susceptible to diode damage can be obtained. Moreover, by introducing a semiconductor layer with a low impurity concentration, it is possible to prevent the early deterioration phenomenon caused by current leakage in the current blocking layer that was observed in conventional semiconductor lasers, and it is possible to reduce the defective rate of semiconductor lasers. It's also effective.

[Brief explanation of the drawing]

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

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

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

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

FIG. 5 is a cross-sectional structural diagram of a semiconductor laser according to Example 3 of the present invention.

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

FIG. 7 is a cross-sectional structural diagram of a semiconductor laser according to Example 5 of the present invention.

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

FIG. 9 is a cross-sectional structural diagram of a conventional semiconductor laser.

[Explanation of symbols]

1...n-GaAs substrate, 2...n-Al0.5Ga0.5
As cladding layer, 3... undoped Al0.14Ga0.
86As active layer, 4...p-Al0.5Ga0.5As cladding layer, 5...p-GaAs contact layer, 6...n-G
aAs, 7... Undoped GaAs, 8... p-GaAs buried layer, 9... Au electrode, 12... quantum well structure active layer, 13
...p-GaAs substrate, 14...stripe-shaped groove, 15...
p-Al0.5Ga0.5As cladding layer, 16... undoped Al0.14Ga0.86As active layer, 17...n
-Al0.5Ga0.5As cladding layer, 18...n-G
aAs contact layer, 19...n-(Al0.5Ga0.
5) 0.5In0.5P cladding layer, 20... undoped Ga0.5In0.5P active layer, 21...p-(Al0.
5Ga0.5)0.5In0.5P cladding layer, 22...
p-GaAs contact layer.

Claims (5)

[Claims] 1. A semiconductor laser having a PN junction current blocking structure for conducting current only to a striped region in an active layer, wherein the current blocking layer, which is a component of the PN junction current blocking structure, has a current blocking layer that is a component of the PN junction current blocking structure. A PN junction is formed with a layer on the active layer side, and a low impurity concentration semiconductor layer having a lower impurity concentration than the PN junction constituting layer is formed at the interface of at least one of the PN junctions. semiconductor laser. 2. A semiconductor laser according to claim 1, wherein said low impurity concentration semiconductor layer is formed at an interface of a PN junction on said active layer side. 3. The low impurity concentration semiconductor layer is formed at the interface of the PN junction on the opposite side to the active layer.
The semiconductor laser described above. 4. The semiconductor laser includes an n-AlGaAs cladding layer, an undoped Al0.14Ga0.86As active layer, and a p-type Al0.14Ga0.86As active layer, which are formed in this order on an n-GaAs substrate.
- has an Al0.5Ga0.5As cladding layer,
The p-Al0.5Ga0.5As cladding layer has a ridge-like stripe, and the p-Al0.5Ga0.5As cladding layer on both sides of the stripe has an n-G
4. A semiconductor laser according to claim 2, wherein the aAs current blocking layer and the undoped GaAs low impurity concentration semiconductor layer are formed in this order. 5. The thickness and impurity concentration of the low impurity concentration semiconductor layer are such that a high frequency current applied when using a semiconductor laser effectively contributes to light emission, and a current component with a higher frequency than the high frequency current contributes to light emission. 5. The semiconductor laser according to claim 1, wherein the semiconductor laser is set not to contribute.