Semiconductor laser device Number:6,768,755 from the United States Patent and Trademark Office (PTO) owispatent

Title: Semiconductor laser device

Abstract: A depletion enhancement layer having a striped opening on the upper surface of a ridge portion, a low carrier concentration layer and an n-type current blocking layer are successively formed on a p-type cladding layer having the ridge portion. The low carrier concentration layer has a lower carrier concentration than the n-type current blocking layer. The band gap of the depletion enhancement layer is set to an intermediate level between the band gaps of the p-type cladding layer and the low carrier concentration layer. Alternatively, a first current blocking layer having a low carrier concentration and a second current blocking layer of the opposite conduction type are formed on an n-type depletion enhancement layer, and a p-type contact layer is formed on the second current blocking layer of the opposite conduction type and another p-type contact layer.

Patent Number: 6,768,755 Issued on 07/27/2004 to Inoue,   et al.

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Inventors:	Inoue; Daijiro (Kyoto, JP), Hiroyama; Ryoji (Kyotanabe, JP), Takeuchi; Kunio (Joyo, JP), Nomura; Yasuhiko (Moriguchi, JP), Hata; Masayuki (Kadoma, JP)
Assignee:	Sanyo Electric Co., Ltd. (Moriguchi, JP)
Appl. No.:	09/746,065
Filed:	December 26, 2000

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Foreign Application Priority Data

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Dec 28, 1999 [JP]			11-374497
Mar 29, 2000 [JP]			2000-092303

Current U.S. Class:	372/43.01 ; 372/68
Current International Class:	H01S 5/223 (20060101); H01S 5/00 (20060101); H01S 5/22 (20060101); H01S 5/062 (20060101)
Field of Search:	372/43,45,46,44,50,68

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References Cited [Referenced By]

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U.S. Patent Documents

	5305341		April 1994		Nishikawa et al.
	5933443		August 1999		Mushiage et al.
	5960019		September 1999		Hayashi et al.
	5963572		October 1999		Hiroyama et al.
	6181723		January 2001		Okubo et al.

Foreign Patent Documents

	1169047		Dec., 1997		CN
	8-222801		Aug., 1996		JP

Other References

Symposium on Optical Memory (SOM'94), paper P20, Tokyo, pp. 95-96, Jul., 1994. . Notice on Office Action in the counterpart Chinese application and translation..

Primary Examiner: Ip; Paul
Assistant Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Westerman, Hattori, Daniels & Adrian, LLP

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Claims

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What is claimed is:

1. A semiconductor laser device comprising: an active layer; a first cladding layer of a first conduction type provided on said active layer; a current blocking layer of a second conduction type provided on said first cladding layer except a current injection region; a low carrier concentration layer provided on the side of said current blocking layer between said first cladding layer and said current blocking layer and having a lower carrier concentration than said current blocking layer; and a depletion enhancement layer provided on the side of said first cladding layer between said first cladding layer and said current blocking layer for inhibiting storage of camera in said low carrier concentration layer, the thickness of the depletion enhancement layer is at least 10 nm.

2. The semiconductor laser device according to claim 1, wherein the band gap of said first cladding layer is larger than that of said depletion enhancement layer, and the band gap of said depletion enhancement layer is larger than that of said low carrier concentration layer.

3. The semiconductor laser device according to claim 1, wherein said first cladding layer has a flat portion formed on said active layer and a ridge portion formed on a portion of said flat portion in said current injection region, said depletion enhancement layer is provided on said flat portion located on both sides of said ridge portion and on the side surfaces of said ridge portion, and said low carrier concentration layer and said current blocking layer are successively formed on said depletion enhancement layer.

4. The semiconductor laser device according to claim 3, wherein the thickness of said depletion enhancement layer is at least 15 nm.

5. The semiconductor laser device according to claim 1, further comprising a ridge-shaped second cladding layer of a first conduction type provided on said depletion enhancement layer in said current injection region, wherein said depletion enhancement layer is formed on said first cladding layer, and said low carrier concentration layer and said current blocking layer are successively formed on said depletion enhancement layer located on both sides of said second cladding layer and on the side surfaces of said second cladding layer.

6. A semiconductor laser device comprising: an active layer; a first cladding layer of a first conduction type provided on said active layer; a current blocking layer of a second conduction type provided on said first cladding layer except a current injection region; a low carrier concentration layer provided on the side of said current blocking layer between said first cladding layer and said current blocking layer and having a lower carrier concentration than said current blocking layer; and a depletion enhancement layer provided on the side of said first cladding layer between said first cladding layer and said current blocking layer for inhibiting storage of carriers in said low carrier concentration layer, wherein said depletion enhancement layer, said low carrier concentration layer and said current blocking layer are successively formed on said first cladding layer except said current injection region, said semiconductor laser device further comprising a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of said depletion

enhancement layer, said low carrier concentration layer and said current blocking layer and the upper surface of said first cladding layer in said current injection region.

7. The semiconductor laser device according to claim 5, wherein the thickness of said depletion enhancement layer is at least 15 nm.

8. The semiconductor laser device according to claim 7, wherein the thickness of said depletion enhancement layer is at least 20 nm.

9. The semiconductor laser device according to claim 6, wherein the thickness of said depletion enhancement layer is at least 15 nm.

10. The semiconductor laser device according to claim 9, wherein the thickness of said depletion enhancement layer is at least 20 nm.

11. The semiconductor laser device according to claim 1, wherein said depletion enhancement layer has a single-layer structure or a superlattice structure.

12. The semiconductor laser device according to claim 1, wherein said active layer includes a layer made of (Al.sub.x1 Ga.sub.1-x1).sub.y1 In.sub.1-y1 P, said depletion enhancement layer is made of (Al.sub.x2 Ga.sub.1-x2).sub.y1 In.sub.1-y2 P or Al.sub.x2 Ga.sub.1-x2 As, said low carrier, concentration layer is made of (Al.sub.x3 Ga.sub.1-x3).sub.y3 In.sub.1-y3 P or Al.sub.x3 Ga.sub.1-x3 As, said current blocking layer is made of (Al.sub.x4 Ga.sub.1-x4).sub.y4 In.sub.1-y4 P or Al.sub.x4 Ga.sub.1-x4 As, and said x1, said x2, said x3, said x4, said y1, said y2, said y3 and said y4 are at least zero and not more than 1 respectively.

13. The semiconductor laser device according to claim 1, wherein said active layer includes a layer made of Al.sub.x1 Ga.sub.1-x1 As, said depletion enhancement layer is made of Al.sub.x2 Ga.sub.1-x2 As, said low carrier concentration layer is made of Al.sub.x3 Ga.sub.1-x3 As, said current blocking layer is made of Al.sub.x4 Ga.sub.1-x4 As, and said x1, said x2, said x3 and said x4 are at least zero and not more than 1 respectively.

14. The semiconductor laser device according to claim 1, wherein said active layer is made of In.sub.x1 Ga.sub.1-x1 N, said depletion enhancement layer is made of Al.sub.x1 Ga.sub.1-x2 N, said low carrier concentration layer is made of Al.sub.x3 Ga.sub.1-x3 N, said current blocking layer is made of Al.sub.x4 Ga.sub.1-x4 N, and said x1, said x2, said x3 and said x4 are at least zero and not more than 1 respectively.

15. The semiconductor laser device according to claim 1, wherein said active layer includes a layer made of (Al.sub.x1 Ga.sub.1-x1).sub.y1 In.sub.1-y1 P, said depletion enhancement layer is made of (Al.sub.x2 Ga.sub.1-x2).sub.y2 In.sub.1-y2 P, said low carrier concentration layer is made of Al.sub.x3 Ga.sub.1-x3 As, said current blocking layer is made of Al.sub.x4 Ga.sub.1-x4 As, said x1, said x2, said x3, said x4, said y1 and said y2 are at least zero and not more than 1 respectively. and said first conduction type is the p type, and said second conduction type is the n type.

16. A semiconductor laser device comprising: an active layer; a first cladding layer of a first conduction type provided on said active layer; a first current blocking layer provided on said first cladding layer except a current injection region, said first current blocking layer having a carrier concentration; a second current blocking layer of a second conduction type provided on said first current blocking layer, said second current blocking layer having a carrier concentration; and a depletion enhancement layer formed between said first cladding layer and said first current blocking layer for inhibiting storage of carriers in said first current blocking layer, wherein said depletion enhancement layer has a thickness of at least 10 nm, and has an energy level in band gap supplying second conduction type carriers to compensate for first conduction type carriers supplied from said first cladding layer due to a modulation doping effect; and further wherein the first current blocking layer has a lower carrier concentration than the second current blocking layer.

17. The semiconductor laser device according to claim 16, wherein said first current blocking layer has a narrower band gap than said first cladding layer.

18. The semiconductor laser device according to claim 16, wherein said energy level in band gap has such density that substantially all said band-to-band levels ionize under a condition applying no bias voltage.

19. The semiconductor laser device according to claim 16, wherein said energy level in band gap is formed by doping with a second conduction type impurity.

20. The semiconductor lasor device according to claim 16, wherein the material of said depletion enhancement layer is the same as the material of said first current blocking layer.

21. The semiconductor laser device according to claim 16, wherein said first cladding layer has a larger band gap than said depletion enhancement layer, said semiconductor laser device further comprising an intermediate band gap layer provided between said first cladding layer and said depletion enhancement layer and having a band gap smaller than the band gap of said first cladding layer and larger than the band gap of said depletion enhancement layer.

22. The semiconductor laser device according to claim 16, wherein said depletion enhancement layer has a bend gap smaller than the band gap of said first cladding layer and larger than the band gap of said first current blocking layer.

23. The semiconductor loser device according to claim 16, wherein said first cladding layer has a flat portion formed on said active layer and a ridge portion formed on a portion or said flat portion in said current injection region, said depletion enhancement layer is provided on said flat portion located on both sides of said ridge portion and on the side surfaces of said ridge portion, and said first current blocking layer is formed on said depletion enhancement layer.

24. The semiconductor laser device according to claim 16, wherein said depletion enhancement layer and said first current blocking layer are successively formed on said first cladding layer except said current injection region, said semiconductor laser device further comprising a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of said depletion enhancement layer and said first current blocking layer and the upper surface of said first cladding layer in said current injection region.

25. The semiconductor laser device according to claim 16, wherein said depletion enhancement layer is formed on a region excluding said current injection region.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device having a current blocking layer.

2. Description of the Prior Art

A refractive index guided semiconductor laser device supplied with refractive index difference in a direction parallel to an active layer for forming a light guide is developed in general. FIG. 34 is a typical sectional view showing a conventional semiconductor laser device 120 described in Japanese Patent Laying-Open No. 8-222801 (1996).

In the semiconductor laser device 120 shown in FIG. 34, an n-type cladding layer 122, an active layer 123, a p-type cladding layer 124 and a p-type contact layer 127 are successively formed on an n-type substrate 121, and the p-type contact layer 127 and the p-type cladding layer 124 are etched for forming flat portions on a ridge portion and on both sides of the ridge portion.

Further, a first current blocking layer 125 having a low carrier concentration is formed on the flat portions of the p-type cladding layer 124 located on both sides of the ridge portion, and an n-type current blocking layer 126 is formed on the first current blocking layer 125 having a low carrier concentration. A p-type contact layer 128 is formed on the p-type contact layer 127 and the n-type current blocking layer 126.

When the semiconductor laser device 120 is driven, a reverse bias voltage is applied to a p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124. Thus, the n-type current blocking layer 126 cuts off a current so that the current is injected into the ridge portion in a narrowed state.

In general, a p-n junction formed between an n-type current blocking layer and a p-type cladding layer has large electric capacitance, and hence serves as a factor inhibiting high-speed operation of a semiconductor laser device. The electric capacitance of the p-n junction is increased as the carrier concentration in this p-n junction is increased.

Therefore, the semiconductor laser device 120 shown in FIG. 34 is provided with the current blocking layer 125 having a low carrier concentration, in order to reduce the electric capacitance in the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124.

This current blocking layer 125 has a lower carrier concentration than the n-type current blocking layer 126. Therefore, the current blocking layer 125 having a low carrier concentration defines a depletion region in the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124, for reducing the electric capacitance. Thus, the semiconductor laser device 120 is enabled to operate at a high frequency.

In the semiconductor laser device 120 having the current blocking layer 125 of a low carrier concentration having a narrower band gap than the p-type cladding layer 124, however, valence bands of the p-type cladding layer 124 and the current blocking layer 125 of a low carrier concentration have energy band structures shown in FIG. 35.

FIG. 35 is a model diagram showing the energy band structures of the valence bands of the p-type cladding layer 124 and the current blocking layer 125 having a low carrier concentration. As shown in FIG. 35, the band gap of the current blocking layer 125 having a low carrier concentration is sufficiently smaller than the band gap of the p-type cladding layer 124, and hence carriers are readily injected from the p-type cladding layer 124 into the current blocking layer 125 having a low carrier concentration and stored therein. Consequently, since depletion of the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124 is inhibited, electric capacitance between the current blocking layer 125 having a low carrier concentration and the p-type cladding layer 124 is increased. Therefore, the operating speed of the semiconductor laser device 120 cannot be sufficiently increased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor laser device sufficiently increased in operating speed.

A semiconductor laser device according to an aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a current blocking layer of a second conduction type provided on the first cladding layer except a current injection region, a low carrier concentration layer provided on the side of the current blocking layer between the first cladding layer and the current blocking layer and having a lower carrier concentration than the current blocking layer and a depletion enhancement layer provided on the side of the first cladding layer between the first cladding layer and the current blocking layer for inhibiting storage of carriers in the low carrier concentration layer.

In the semiconductor laser device, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in a depleted state. Therefore, electric capacitance between the current blocking layer and the first cladding layer is kept small for sufficiently increasing the operating speed of the semiconductor laser device.

The band gaps of the first cladding layer, the depletion enhancement layer and the low carrier concentration layer may be reduced in this order.

Thus, the depletion enhancement layer having an intermediate band gap is provided between the first cladding layer having a large band gap and the low carrier concentration layer having a small band gap.

In this case, the band offset between the first cladding layer and the depletion enhancement layer is smaller than the band offset between the first cladding layer and the low carrier concentration layer, whereby carriers are hardly injected from the first cladding layer into the depletion enhancement layer while carriers are more hardly injected into the low carrier concentration layer. Further, the carriers are injected from the first cladding layer into both of the low carrier concentration layer and the depletion enhancement layer in a divided manner, whereby the quantity of carriers stored in the low carrier concentration layer is reduced. Thus, storage of carriers in the low carrier concentration layer can be inhibited by the simple structure of setting the band gap of the depletion enhancement layer to the intermediate level between the low carrier concentration layer and the first cladding layer.

The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the low carrier concentration layer and the current blocking layer may be successively formed on the depletion enhancement layer.

In this case, the depletion enhancement layer inhibits storage of carriers from the flat portion of the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state, and the electric capacitance between the flat portion of the first cladding layer and the current blocking layer is kept small.

The thickness of the depletion enhancement layer is preferably at least 10 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.

The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.

The semiconductor laser device may further comprise a ridge-shaped second cladding layer of a first conduction type provided on the depletion enhancement layer in the current injection region, the depletion enhancement layer may be formed on the first cladding layer, and the lower carrier concentration layer and the current blocking layer may be successively formed on the depletion enhancement layer located on both sides of the second cladding layer and on the side surfaces of the second cladding layer.

In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state and the electric capacitance between the first cladding layer and the current blocking layer is kept small.

The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.

The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.

The depletion enhancement layer, the low carrier concentration layer and the current blocking layer may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer, the low carrier concentration layer and the current blocking layer and the upper surface of the first cladding layer in the current injection region.

In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state and the electric capacitance between the first cladding layer and the current blocking layer is kept small.

The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.

The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.

The depletion enhancement layer may have a single-layer structure or a superlattice structure.

The active layer may include a layer made of (Al.sub.x1 Ga.sub.1-x1).sub.y1 In.sub.1-y1 P, the depletion enhancement layer may be made of (Al.sub.x2 Ga.sub.1-x2).sub.y2 In.sub.1-y2 P or Al.sub.x2 Ga.sub.1-x2 As, the low carrier concentration layer may be made of (Al.sub.x3 Ga.sub.1-x3).sub.y3 In.sub.1-y3 P or Al.sub.x3 Ga.sub.1-x3 As, the current blocking layer may be made of (Al.sub.x4 Ga.sub.1-x4).sub.y4 In.sub.1-y4 P or Al.sub.x4 Ga.sub.1-x4 As, and x1, x2, x3, x4, y1, y2, y3 and y4 may be at least zero and not more than 1 respectively.

The active layer may include a layer made of Al.sub.x1 Ga.sub.1-x1 As, the depletion enhancement layer may be made of Al.sub.x2 Ga.sub.1-x2 As, the low carrier concentration layer may be made of Al.sub.x3 Ga.sub.1-x3 As, the current blocking layer may be made of Al.sub.x4 Ga.sub.1-x4 As, and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.

The active layer may be made of In.sub.x1 Ga.sub.1-x1 N, the depletion enhancement layer may be made of Al.sub.x2 Ga.sub.1-x2 N, the low carrier concentration layer may be made of Al.sub.x3 Ga.sub.1-x3 N, the current blocking layer may be made of Al.sub.x4 Ga.sub.1-x4 N, and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.

The active layer preferably includes a layer made of (Al.sub.x1 Ga.sub.1-x1).sub.y1 In.sub.1-y1 P, the depletion enhancement layer is preferably made of (Al.sub.x2 Ga.sub.1-x2).sub.y2 In.sub.1-y2 P, the low carrier concentration layer is preferably made of Al.sub.x3 Ga.sub.1-x3 As, the current blocking layer is preferably made of Al.sub.x4 Ga.sub.1-x4 As, x1, x2, x3, x4, y1 and y2 are preferably at least zero and not more than 1 respectively, and the first conduction type is preferably the p type, and the second conduction type is preferably the n type.

In this case, improvement of the high-frequency characteristic resulting from the depletion enhancement layer inhibiting storage of carriers from the first cladding layer into the low carrier concentration layer is particularly remarkable.

A semiconductor laser device according to another aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a first current blocking layer having a low carrier concentration provided on the first cladding layer except a current injection region and a depletion enhancement layer formed between the first cladding layer and the first current blocking layer for inhibiting storage of carriers in the first current blocking layer, while the depletion enhancement layer has an energy level in band gap supplying second conduction type carriers to compensate for first conduction type carriers supplied from the first cladding layer due to a modulation doping effect.

The first current blocking layer having a low carrier concentration is an undoped layer or a layer doped with a low density of impurity In a range capable of blocking a current.

In the semiconductor laser device, the depletion enhancement layer formed with the energy level in band gap supplying the second conduction type carriers is formed between the first cladding layer and the first current blocking layer.

In this case, the second conduction type carriers supplied from the energy level in band gap of the depletion enhancement layer compensate for the first conduction type carriers supplied from the first cladding layer. Therefore, storage of carriers can be prevented in the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer is kept in a depleted state. Therefore, electric capacitance generated between the first current blocking layer and the first cladding layer can be reduced and the operating speed of the semiconductor laser device can be sufficiently increased.

At this point, the first current blocking layer has a narrower band gap than the first cladding layer. When the first current blocking layer has a narrower band gap than the first cladding layer, carriers are readily injected from the first cladding layer into the first current blocking layer and stored therein. In this case, however, the depletion enhancement layer formed between the first cladding layer and the first current blocking layer can inhibit storage of carriers in the first current blocking layer.

The energy level in band gap preferably has such density that substantially all energy level in band gap ionize under a condition applying no bias voltage voltage. In this case, it is possible to effectively compensate for the first conduction type carriers supplied from the first cladding layer. Therefore, storage of carriers in the first current blocking layer having a low carrier concentration can be more effectively inhibited.

The energy level in band gap may be formed by doping with a second conduction type impurity. In this case, the depletion enhancement layer provided with the energy level in band gap can be readily formed.

The material of the depletion enhancement layer may be the same as the material of the first current blocking layer. In this case, the band gap width of the depletion enhancement layer and the first current blocking layer are equalized with each other.

The first cladding layer may have a larger band gap than the depletion enhancement layer, and the semiconductor laser device may further comprise an intermediate band gap layer provided between the first cladding layer and the depletion enhancement layer and having a band gap smaller than the band gap of the first cladding layer and larger than the band gap of the depletion enhancement layer.

In this case, carriers are hardly injected from the first cladding layer into the depletion enhancement layer and hardly injected into the first current blocking layer having a low carrier concentration either due to the intermediate band gap layer provided between the first cladding layer and the depletion enhancement layer. In this case, further, the carriers are injected into both of the depletion enhancement layer and the intermediate band gap layer in a divided manner, and hence hardly injected into the first current blocking layer.

Thus, storage of carriers in the first current blocking layer is further inhibited.

Further, the ranges of the thickness and the carrier concentration of the depletion enhancement layer capable increasing the operating speed of the semiconductor layer device are widened by providing the intermediate band gap layer in the aforementioned manner. Therefore, the thickness and the carrier concentration of the depletion enhancement layer can be readily set so that the depletion enhancement layer can be readily prepared.

The depletion enhancement layer may have a band gap smaller than the band gap of the first cladding layer and larger than the band gap of the first current blocking layer. In this case, the depletion enhancement layer serves as the aforementioned intermediate band gap layer, thereby further inhibiting storage of carriers in the first current blocking layer.

Also in this case, the ranges of the thickness and the carrier concentration of the depletion enhancement layer capable of increasing the operating speed of the semiconductor laser device are widened. Thus, the thickness and the carrier concentration of the depletion enhancement layer can be readily set so that the depletion enhancement layer can be readily prepared.

The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the first current blocking layer may be formed on the depletion enhancement layer. In this case, a ridge guided semiconductor laser device improved in operating speed is implemented.

The depletion enhancement layer and the first current blocking layer may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer and the first current blocking layer and the upper surface of the first cladding layer in the current injection region. In this case, a self-aligned semiconductor laser device improved in operating speed is implemented.

The depletion enhancement layer may be formed on a region excluding the current injection region. In this case, a current is quickly injected into the current injection region provided with no depletion enhancement layer of the opposite conduction type.

The semiconductor laser device may further comprise a second current blocking layer of a second conduction type provided on the first current blocking layer.

A semiconductor laser device according to another aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a first current blocking layer having a low carrier concentration provided on the first cladding layer except a current injection region and a depletion enhancement layer formed between the first cladding layer and the first current blocking layer for inhibiting storage of carriers in the first current blocking layer.

In the semiconductor laser device, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in a depleted state. Therefore, electric capacitance between the first current blocking layer having a low carrier concentration and the first cladding layer is kept small for sufficiently increasing the operating speed of the semiconductor laser device.

At this point, the first current blocking layer having a low concentration has a narrower band gap than the first cladding layer. When the first current blocking layer having a low carrier concentration has a narrower band gap than the first cladding layer, carriers are readily injected from the first cladding layer into the first current blocking layer having a low carrier concentration and stored therein. In this case, however, the depletion enhancement layer formed between the first cladding layer and the first current blocking layer having a low carrier concentration can inhibit storage of carriers in the first current blocking layer having a low carrier concentration.

The band gaps of the first cladding layer, the depletion enhancement layer and the first current blocking layer having a low carrier concentration may be reduced in this order.

Thus, the depletion enhancement layer having an intermediate band gap is provided between the first cladding layer having a large band gap and the first current blocking layer having a low carrier concentration having a small band gap.

In this case, the band offset between the first cladding layer and the depletion enhancement layer is smaller than the band offset between the first cladding layer and the first current blocking layer having a low carrier concentration, whereby carriers are hardly injected from the first cladding layer into the depletion enhancement layer while carriers are more hardly injected into the first current blocking layer having a low carrier concentration. Further, the carriers are injected from the first cladding layer into both of the first current blocking layer having a low carrier concentration and the depletion enhancement layer in a divided manner, whereby the quantity of carriers stored in the low carrier concentration layer is reduced. Thus, storage of carriers in the first current blocking layer having a low carrier concentration can be inhibited by the simple structure of setting the band gap of the depletion enhancement layer to the intermediate level between the first current blocking layer having a low carrier concentration and the first cladding layer.

The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the first current blocking layer having a low carrier concentration may be formed on the depletion enhancement layer.

In this case, the depletion enhancement layer inhibits storage of carriers from the flat portion of the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state, and the electric capacitance between the flat portion of the first cladding layer and the first current blocking layer having a low carrier concentration is kept small.

The semiconductor laser device may further comprise a ridge-shaped second cladding layer of a first conduction type provided on the depletion enhancement layer in the current injection region, the depletion enhancement layer may be formed on the first cladding layer, and the first current blocking layer having a lower carrier concentration may be formed on the depletion enhancement layer located on both sides of the second cladding layer and on the side surfaces of the second cladding layer.

In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration layer. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state and the electric capacitance between the first cladding layer and the first current blocking layer having a low carrier concentration is kept small.

The depletion enhancement layer and the first current blocking layer having a low carrier concentration may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer and the first current blocking layer having a low carrier concentration layer and the upper surface of the first cladding layer in the current injection region.

In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state and the electric capacitance between the first cladding layer and the first current blocking layer having a low concentration is kept small.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is an energy band diagram of valence bands of a p-type cladding layer, a depletion enhancement layer and a low carrier concentration layer in the semiconductor laser device shown in FIG. 1;

FIGS. 3 to 5 are typical sectional views showing steps in a method of fabricating the semiconductor laser device shown in FIG. 1;

FIG. 6 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of the depletion enhancement layer in the semiconductor laser device shown in FIG. 1;

FIG. 7 illustrates an effect of improving the cutoff frequency by doping the depletion enhancement layer of the semiconductor laser device according to the first embodiment;

FIG. 8 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to a second embodiment of the present invention;

FIG. 9 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to a third embodiment of the present invention;

FIG. 10 is a typical sectional view of a semiconductor laser device according to a fourth embodiment of the present invention;

FIGS. 11 to 13 are typical sectional views showing steps in a method of fabricating the semiconductor laser device shown in FIG. 10;

FIG. 14 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in the semiconductor laser device according to the fourth embodiment;

FIG. 15 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to a fifth embodiment of the present invention;

FIG. 16 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to a sixth embodiment of the present invention;

FIG. 17 is a typical sectional view of a semiconductor laser device according to a seventh embodiment of the present invention;

FIGS. 18 and 19 are typical sectional views showing steps of a method of fabricating the semiconductor laser device shown in FIG. 17;

FIG. 20 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in the semiconductor laser device according to the seventh embodiment;

FIG. 21 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to an eighth embodiment of the present invention;

FIG. 22 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in a semiconductor laser device according to a ninth embodiment of the present invention;

FIG. 23 is a typical sectional view of a semiconductor laser device according to each of tenth to twelfth embodiments of the present invention;

FIGS. 24(a) and 24(b) are energy band diagrams of a p-type cladding layer, a n-type depletion enhancement layer and a first current blocking layer having a low carrier concentration in the semiconductor laser device shown in FIG. 23;

FIGS. 25 to 27 are typical sectional views showing steps in a method of fabricating the semiconductor laser device shown in FIG. 23;

FIG. 28 is a typical sectional view of a semiconductor laser device according to a thirteenth embodiment of the present invention;

FIG. 29 is a typical sectional view of a semiconductor laser device according to a fourteenth embodiment of the present invention;

FIGS. 30 and 31 are typical sectional views showing steps in a method of fabricating the semiconductor laser device shown in FIG. 29;

FIG. 32 is a diagram showing results of measurement of the relation between a cutoff frequency and the thickness of a depletion enhancement layer in the semiconductor laser device according to the fourteenth embodiment;

FIGS. 33(a) and 33(b) are diagrams for illustrating the principle and the function of the present invention;

FIG. 34 is a typical sectional view showing the structure of a conventional semiconductor laser device; and

FIG. 35 is an energy band diagram of valence bands of a p-type cladding layer and a current blocking layer having a low carrier concentration in the semiconductor laser device shown in FIG. 34.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) First Embodiment

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

In the semiconductor laser device shown in FIG. 1, a cladding layer 2 of n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P having a thickness of 1500 nm and an emission layer 14 described later are successively formed on an n-GaAs substrate 1. A cladding layer 2 of p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P having a thickness of 1500 nm and a contact layer of p-Ga.sub.0.5 In.sub.0.5 P having a thickness of 200 nm are successively formed on the emission layer 14. The p-type cladding layer 6 and the p-type contact layer 7 are etched to define a ridge portion.

The carrier concentration of the n-GaAs substrate 1 is 1.times.10.sup.18 cm.sup.-3, the carrier concentration of the n-type cladding layer 2 is 3.times.10.sup.17 cm.sup.-3, and the carrier concentration of the p-type contact layer 7 is 2.times.10.sup.18 cm.sup.-3 respectively.

Further, a depletion enhancement layer 8 of a thickness t having a striped opening on the upper surface of the ridge portion is formed on the p-type cladding layer 6. A low carrier concentration layer 9 of GaAs of 1000 nm in thickness having a striped opening on the upper surface of the ridge portion is formed on the depletion enhancement layer 8. An n-type current blocking layer 10 of n-GaAs of 500 nm in thickness having a striped opening on the upper surface of the ridge portion is formed on the low carrier concentration layer 9. The carrier concentration of the n-type current blocking layer 10 is 8.times.10.sup.17 cm.sup.-3. The carrier concentration of the low carrier concentration layer 9 is lower than that of the n-type current blocking layer 10.

A contact layer 11 of p-GaAs having a thickness of 3000 nm is formed on the p-type contact layer 7 located in the striped opening of the n-type current blocking layer 10 and on the n-type current blocking layer 10. The carrier concentration of the p-type contact layer is 3.times.10.sup.19 cm.sup.-3. A p-electrode 12 having a thickness of 300 nm is formed on the p-type contact layer 11. An n electrode 13 having a thickness of 300 nm is formed on the back side of the n-GaAs substrate 1.

The emission layer 14 includes a guide layer 3 of (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P having a thickness of 30 nm formed on the n-type cladding layer 2, a quantum well active layer 4 formed on the guide layer 3 and a guide layer 5 of (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P having a thickness of 30 nm formed on the quantum well active layer 4.

The quantum well active layer 4 has a superlattice structure formed by alternately stacking a plurality of quantum well layers 15 of Ga.sub.0.5 In.sub.0.5 P each having a thickness of 5 nm and a plurality of barrier layers 16 of (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P each having a thickness of 5 nm. For example, the number of the barrier layers 16 is 2, and the number of the quantum well layers 15 is 3.

Table 1 shows the aforementioned structure.

TABLE 1 Carrier Thick- Concen- Composition and ness tration Name of Layer Name of Layer (nm) (cm.sup.-3) Numeral n-GaAs Substrate 1 .times. 10.sup.18 1 Cladding Layer 1500 3 .times. 10.sup.17 2 of n- (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P Emis- Guide Layer of 30 3 sion (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P Layer Quantum Quantum Well 5 15 Well Layer of Active Ga.sub.0.5 In.sub.0.5 P Layer Barrier Layer of 5 16 (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P Guide Layer of 30 5 (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P Cladding Layer 1500 3 .times. 10.sup.17 6 of p- (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P Contact Layer of p- 200 2 .times. 10.sup.18 7 (Ga.sub.0.5 In.sub.0.5 P Depletion Enhance- t 8 ment Layer of Ga.sub.0.5 In.sub.0.5 P Low Carrier Con- 1000 9 centration Layer of GaAs Current Blocking 500 8 .times. 10.sup.17 10 Layer of n-GaAs Contact Layer of 3000 3 .times. 10.sup.19 11 p-GaAs p-Electrode 300 12 n-Electrode 300 13

FIG. 2 typically shows an energy band diagram of valence bands of the p-type cladding layer 6, the depletion enhancement layer 8 and the low carrier concentration layer 9 of the semiconductor laser device shown in FIG. 1.

As shown in FIG. 2, the band gaps of the p-type cladding layer 6, the depletion enhancement layer 8 and the low carrier concentration layer 9 are reduced in this order. Therefore, the band offset between the p-type cladding layer 6 and the depletion enhancement layer 8 in contact therewith is reduced as compared with the band offset between the p-type cladding layer 6 and the low carrier concentration layer 9 so that carriers are hardly injected from the p-type cladding layer 6 into the depletion enhancement layer 8 and hardly injected into the low carrier concentration layer 9 either. Consequently, the quantity of carriers stored in the low carrier concentration layer 9 is reduced. Further, carriers are injected into both of the low carrier concentration layer 9 and the depletion enhancement layer 8 in a divided manner, whereby the quantity of the carriers stored in the low carrier concentration layer 9 is reduced.

The low carrier concentration layer 9, storing a small quantity of carriers, is kept in a depleted state, whereby the electric capacitance between the n-type current blocking layer 10 and the p-type cladding layer 6 is kept small for sufficiently increasing the operating speed of the semiconductor laser device.

Thus, the high-frequency characteristic of the semiconductor laser device shown in FIG. 1 is improved through the simple structure of setting the band gap of the depletion enhancement layer 8 to the intermediate level between those of the low carrier concentration layer 9 and the p-type cladding layer 6.

FIGS. 3, 4 and 5 are typical sectional views showing steps in a method of fabricating the semiconductor laser device shown in FIG. 1.

As shown in FIG. 3, the cladding layer 2 of n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P, the guide layer 3 of (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P, the quantum well active layer 4, the guide layer 5 of (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P, the cladding layer 6 of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P and the contact layer 7 of p-Ga.sub.0.5 In.sub.0.5 P are successively grown on the n-GaAs substrate 1 by MOCVD (metal-organic chemical vapor deposition).

As shown in FIG. 4, an SiO.sub.2 film is formed on the p-type contact layer 7 and patterned for forming a striped SiO.sub.2 film 17. Thereafter the p-type contact layer 7 and the p-type cladding layer 6 are partially removed by etching through the SiO.sub.2 film 17 serving as a mask, for forming the ridge portion.

As shown in FIG. 5, the depletion enhancement layer 8 of Ga.sub.0.5 In.sub.0.5 P, the low carrier concentration layer 9 of GaAs and the current blocking layer 10 of n-GaAs are successively grown on the p-type cladding layer 6 by MOCVD through the SiO.sub.2 film 17 serving as a selective growth mask.

The SiO.sub.2 film 17 is removed and thereafter the contact layer 11 of p-GaAs is formed on the n-type current blocking layer 10 and on the p-type contact layer 7 by MOCVD as shown in FIG. 1, while the p-electrode 12 of Cr/Au is formed on the surface of the p-type contact layer 11 and the n electrode 13 of AuGe/Ni/Au is formed on the back side of the n-GaAs substrate 1.

FIG. 6 is a diagram showing the results of measurement of a cutoff frequency of the semiconductor laser device shown in Table 1 with variation of the thickness t of the depletion enhancement layer 8. The cutoff frequency is such a frequency that the amplitude of a laser beam superposed with a sine wave output from the object semiconductor laser device is reduced by 3 dB as compared with the case of superposing a low frequency (the superposed frequency is not more than 10 MHz in this example). Referring to FIG. 6, .largecircle. denotes a case of employing a depletion enhancement layer 8 of Ga.sub.0.5 In.sub.0.5 P having a single-layer structure, .quadrature. denotes a case of employing a depletion enhancement layer 8 of a superlattice structure alternately having (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P barrier layers and Ga.sub.0.5 In.sub.0.5 P well layers (the thickness t is the sum of the thicknesses of the well layers), and .DELTA. denotes a case of employing a depletion enhancement layer 8 of Al.sub.0.45 Ga.sub.0.55 As having a single-layer structure respectively.

The cutoff frequency, 200 MHz when the semiconductor laser device is formed with no depletion enhancement layer 8, is improved when the thickness t of the depletion enhancement layer 8 is increased, remarkably improved when the thickness t of the depletion enhancement layer 8 exceeds 10 nm, and substantially saturated when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating improvement of the cutoff frequency. When the thickness t of the depletion enhancement layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency characteristic can be sufficiently improved.

FIG. 7 illustrates an effect of improving the cutoff frequency by doping the depletion enhancement layer 8 of the semiconductor laser device shown in FIG. 1. The horizontal axis shows the ratio of the carrier concentration of the depletion enhancement layer 8 to the carrier concentration of the p-type cladding layer 6, and the vertical axis shows the cutoff frequency. The depletion enhancement layer 8 of this semiconductor laser device is made of p-type GaInP, and the thickness t thereof is 25 nm.

As shown in FIG. 7, the effect of improving the cutoff frequency is small when the carrier concentration of the depletion enhancement layer 8 is higher than that of the p-type cladding layer 6, while a large effect of improving the cutoff frequency is attained when the carrier concentration of the depletion enhancement layer 8 is lower than that of the p-type cladding layer 6. Therefore, the carrier concentration of the depletion enhancement layer 8 is preferably lower than that of the p-type cladding layer 6.

(2) Second Embodiment

A semiconductor laser device according to a second embodiment of the present invention is now described.

The structure of the semiconductor laser device according to the second embodiment is similar to that shown in FIG. 1, while the materials, thicknesses and carrier concentrations of respective layers are different from those in the first embodiment. Table 2 shows the materials, thicknesses and carrier concentrations of the respective layers forming the semiconductor laser device according to this embodiment.

TABLE 2 Carrier Thick- Concen- Composition and ness tration Name of Layer Name of Layer (nm) (cm.sup.-3) Numeral n-GaAs Substrate 1 .times. 10.sup.18 1 Cladding Layer of 1500 3 .times. 10.sup.17 2 n-Al.sub.0.45 Ga.sub.0.55 As Emis- Guide Layer of 30 3 sion Al.sub.0.35 Ga.sub.0.65 As Layer Quantum Quantum Well 5 15 Well Layer of Active Al.sub.0.1 Ga.sub.0.9 As Layer Barrier Layer of 5 16 Al.sub.0.35 Ga.sub.0.65 As Guide Layer of 30 5 Al.sub.0.35 Ga.sub.0.65 As Cladding Layer of 1500 1 .times. 10.sup.18 6 p-Al.sub.0.45 Ga.sub.0.55 As Contact Layer of 200 4 .times. 10.sup.18 7 p-GaAs Depletion Enhance- t 8 ment Layer of Al.sub.0.25 Ga.sub.0.75 As Low Carrier Con- 1000 9 centration Layer of GaAs Current Blocking 500 5 .times. 10.sup.17 10 Layer of n-GaAs Contact Layer of 3000 3 .times. 10.sup.19 11 n-GaAs p-Electrode 300 12 n-Electrode 300 13

FIG. 8 illustrates results of measurement of a cutoff frequency of the semiconductor laser device shown in Table 2 with variation of the thickness t of a depletion enhancement layer 8. Referring to FIG. 8, .largecircle. denotes a case of employing a depletion enhancement layer 8 of Al.sub.0.25 Ga.sub.0.75 As having a single-layer structure, and .quadrature. denotes a case of employing a depletion enhancement layer 8 of a superlattice structure alternately having Al.sub.0.45 Ga.sub.0.55 As barrier layers and Al.sub.0.25 Ga.sub.0.75 As well layers (the thickness t is the sum of the thicknesses of the well layers).

The cutoff frequency, 400 MHz when the semiconductor laser device is formed with no depletion enhancement layer 8, is improved when the thickness t of the depletion enhancement layer 8 is increased, remarkably improved when the thickness t of the depletion enhancement layer 8 exceeds 10 nm, and substantially saturated when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating improvement of the cutoff frequency. When the thickness t of the depletion enhancement layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency characteristic can be sufficiently improved.

(3) Third Embodiment

A semiconductor laser device according to a third embodiment of the present invention is now described.

The structure of the semiconductor laser device according to the third embodiment is similar to that shown in FIG. 1, while the materials, thicknesses and carrier concentrations of respective layers are different from those in the first embodiment. Table 3 shows the materials, thicknesses and carrier concentrations of the respective layers forming the semiconductor laser device according to this embodiment.

TABLE 3 Carrier Thick- Concen- Composition and ness tration Name of Layer Name of Layer (nm) (cm.sup.-3) Numeral n-GaN Substrate 1 .times. 10.sup.18 1 Cladding Layer of 1000 3 .times. 10.sup.17 2 n-Al.sub.0.15 Ga.sub.0.85 N Emis- Guide Layer of GaN 30 3 sion Quantum Quantum Well 5 15 Layer Well Layer of Active In.sub.0.15 Ga.sub.0.85 N Layer Barrier Layer 5 16 of In.sub.0.05 Ga.sub.0.95 N Guide Layer of GaN 30 5 Cladding Layer of 1000 2 .times. 10.sup.17 6 p-Al.sub.0.15 Ga.sub.0.85 N Contact Layer of 200 3 .times. 10.sup.17 7 p-GaN Depletion Enhance- t 8 ment Layer of Al.sub.0.07 Ga.sub.0.93 N Low Carrier Con- 800 9 centration Layer of n-GaN Current Blocking 200 5 .times. 10.sup.17 10 Layer of n-GaN Contact Layer of 3000 8 .times. 10.sup.17 11 p-GaN p-Electrode 300 12 n-Electrode 300 13

FIG. 9 illustrates results of measurement of a cutoff frequency of the semiconductor laser device shown in Table 3 with variation of the thickness t of a depletion enhancement layer 8. Referring to FIG. 9, .largecircle. denotes a case of employing a depletion enhancement layer 8 of Al.sub.0.07 Ga.sub.0.93 N having a single-layer structure, and .quadrature. denotes a case of employing a depletion enhancement layer 8 of a superlattice structure alternately having Al.sub.0.15 Ga.sub.0.85 N barrier layers and Al.sub.0.07 Ga.sub.0.93 N well layers (the thickness t is the sum of the thicknesses of the well layers).

The cutoff frequency, 320 MHz when the semiconductor laser device is formed with no depletion enhancement layer 8, is gradually improved when the thickness t of the depletion enhancement layer 8 is increased, remarkably improved when the thickness t of the depletion enhancement layer 8 exceeds 10 nm, and substantially saturated when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating improvement of the cutoff frequency. When the thickness t of the depletion enhancement layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency characteristic can be sufficiently improved.

(4) Fourth Embodiment

FIG. 10 is a typical sectional view showing a semiconductor laser device according to a fourth embodiment of the present invention.

In the semiconductor laser device shown in FIG. 10, respective layers 2 to 5 are formed on an n-GaAs substrate 1, similarly to the semiconductor laser device shown in FIG. 1.

A cladding layer 61 of p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P having a thickness of 200 nm and a depletion enhancement layer 62 of Ga.sub.0.5 In.sub.0.5 P are successively formed on the guide layer 5. The carrier concentration of the p-type cladding layer 61 is 3.times.10.sup.17 cm.sup.-3.

A cladding layer 63 of p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P of 1300 nm in thickness having a ridge shape is formed on the depletion enhancement layer 62. The carrier concentration of the p-type cladding layer 63 is 3.times.10.sup.17 cm.sup.-3. A contact layer 7 of p-Ga.sub.0.5 In.sub.0.5 P is formed on the upper surface of the p-type cladding layer 63.

A low carrier concentration layer 9 of GaAs of 1000 nm in thickness having a striped opening on the upper surface of the p-type contact layer 7 is formed on portions of the depletion enhancement layer 62 located on both sides of the p-type cladding layer 63 and on the side surfaces of the p-type cladding layer 63.

Further, a current blocking layer 10 of n-GaAs of 500 nm in thickness having a striped opening on the upper surface of the ridge portion is formed on the low carrier concentration layer 9. A p-type contact layer 11 is formed on the p-type contact layer 7 and the n-type current blocking layer 10.

Table 4 shows the aforementioned structure.

TABLE 4 Carrier Thick- Concen- Composition and ness tration Name of Layer Name of Layer (nm) (cm.sup.-3) Numeral n-GaAs substrate 1 .times. 10.sup.18 1 Cladding Layer 1500 3 .times. 10.sup.17 2 of n