EP0000557B1

EP0000557B1 - Semiconductor laser device

Info

Publication number: EP0000557B1
Application number: EP78100461A
Authority: EP
Inventors: Michiharu Nakamura; Shigeo Yamashita; Takao Kuroda; Junichi Umeda
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1977-08-01
Filing date: 1978-07-20
Publication date: 1981-12-30
Also published as: DE2861465D1; EP0000557A1; US4257011A; CA1105598A

Description

This invention relates to a semiconductor laser device with a first semiconductor layer and with a second and a third semiconductor layer which sandwich said first semiconductor layer therebetween, which are greater in the band gap and lower in the refractive index than said first semiconductor layer and which have conductivity types opposite to each other, with a fourth semiconductor layer rendering the effective complex refractive index for the laser beam discontinuous in a direction perpendicular to the travelling direction of the laser beam, and with periodic corrugations formed in at least one interface of the semiconductor layers constituting an optical wave guide of said semiconductor laser device in a manner to intersect orthogonally to said travelling direction of the laser beam.
Semiconductor laser devices have many merits, such as small size, operation at high efficiency, and capability of direct modulation by a drive current. They are therefore hopeful as light sources for optical communication, optical information processing, etc.
A distributed-feedback semiconductor laser device having the above features is known from U.S. patent specification 4 023 993. Such a distributed-feedback semiconductor laser achieves mode stabilization by exploiting the sharp oscillation mode selectivity which is produced by the light diffraction effect based on the corrugated surface.
Further distfibuted-feedback semiconductor laser devices have been reported in detail in "Applied Physics Letters", vol. 27, pages 403 to 405, October 1975, by M. Nakamura et al., and in "IEEE Journal of Quantum Electronics", vol. QE-12, pages 597 to 603, October 1976, by K. Aiki et al.
Figs. 1 and 2 of the accompanying drawing show an example of the distributed-feedback semiconductor laser device. Fig. 1 is a sectional view taken orthogonally to the travelling direction of light, while Fig. 2 is a sectional view taken along the travelling direction of light. Here, numeral 1 designates an n-GaAs substrate, numeral 2 an n-Ga,__xAl_xAs layer (x ≃ 0.3), numeral 3 a GaAs active layer, numeral 4 a p-Ga_1-xAI_xAs layer, and numeral 5 periodic corrugations which are provided at the boundary between the active layer 3 and the semiconductor layer 4. Numerals 6 and 7 indicate ohmic electrodes. The layers 2, 3 and 4 constitute an optical waveguide. Light given forth in active layer 3 is guided centering around the active layer 3, and is subjected to the Bragg reflection of 180° by the periodic corrugations 5 formed at the boundary between the layers 3 and 4.
The distributed-feedback semiconductor laser device oscillates at a single longitudinal mode and exhibits a spectral width of 0.05 mm (0.5 A) or less, so that it is excellent in monochromaticity. In addition, the temperature-dependency of the oscillation wavelength is low.
Even in the distributed-feedback semiconductor laser device controlled to the single longitudinal mode, however, the generation of excess optical noise especially for a modulated signal due to transverse mode instability is not avoided yet.
This invention has for its object to provide a novel semiconductor laser device in which a longitudinal and transverse mode is stabilized singly and in which any excess optical noise component for a modulated signal is not generated by mode competition.
In order to accomplish this object, the semiconductor laser device of the type initially referred to is characterized, in accordance with the present invention, in that said second and/or said third semiconductor layer have a protuberant region or are bordered on their side remote from said first semiconductor layer by a fifth semiconductor layer with a protuberant region, that said protuberant region having a width able to stabilize a transverse mode of the laser oscillation extends in said-travelling direction of the laser beam and has the shape of a stripe on its surface side remote from said first semiconductor layer, that said fourth semiconductor layer has portions on both sides of said protuberant region, and that said periodic corrugations are formed to include at least a region overlying said protuberant region.
Advantageous developments of the invention are called for in claims 2 to 11.
The features of the characterizing part of claim 2 and a semiconductor laser which consists of five semiconductor layers made of Ga_1-xAl_xAs and of a GaAs substrate are known from "IEEE Journal of Quantum Electronics", vol. QE-13, No. 4, April 1977, pages 233 to 253.
From "Applied Physics Letters", vol. 30, No. 10, May 1977, pages 538 to 540, a semiconductor laser is known, which is equipped with a first semiconductor layer and with a second and a third semiconductor layer which sandwich the first semiconductor layer therebetween, said second semiconductor layer having a protuberant region on both sides of which a semiconductor layer is disposed. The protuberant region extends in the travelling direction of the laser beam and has the shape of a stripe on its surface side remote from the first semiconductor layer. In these known devices, the protuberant region is used to confine the current through the active layer.
From U.S. patent specification 3,978,428, a semiconductor laser device is known, which is equipped with a first semiconductor layer and with a second and third semiconductor layer which sandwich said first semiconductor layer therebetween, with a semiconductor layer rendering the effective complex refractive index for the laser beam discontinuous in a direction perpendicular to the travelling direction of the laser beam, and with a substrate having an elongated groove extending in the travelling direction of the laser beam through a pump current confining layer, a central portion of the first semiconductor layer being bowl-shaped. This structure is used to adapt the cross section of the emitted laser beam to round lenses.
Preferred embodiments of the invention will now be described with reference to the drawings, in which:

Figs. 1 and 2, which have been referred to above, are sectional views of a distributed-feedback semiconductor laser device of a prior art structure, which are respectively taken perpendicularly to, and along, the travelling direction of light;
Fig. 3 is a perspective view of a semiconductor laser device according to this invention;
Fig. 4 is a graph showing the influence of the groove width of the protuberant region and the distance from the active layer to a light absorptive region on the laser oscillation.
Figs. 5 to 8 are diagrams showing the states of generation of excess optical noise ascribable to mode competition in various semiconductor laser devices;
Figs. 9 to 10 are sectional views of a device of another embodiment of this invention, which are respectively taken perpendicularly to, and along, the travelling direction of light;
Fig. 11 is a graph showing the light output versus current characteristics of one embodiment of the present invention;
Fig. 12 is a graph showing far-field intensity distributions in the junction plane;
Fig. 13 shows the lasing spectra of one embodiment of the present invention operating under d.c. bias;
Fig. 14 shows the pulse response of one embodiment of the present invention at different excitation levels; and
Figs. 15 and 16 are sectional views of devices of further embodiments of this invention as taken perpendicularly to the travelling direction of light.

This invention will be described in detail with reference to a typical example. Fig. 3 is a perspec_- tive view of the typical example of this invention.
On an n-GaAs substrate 1 as the fourth semiconductor layer formed with a groove 9; an n-Ga,_ _xAl_xAs (x ≃ 0.3) layer 2 as the second semiconductor layer, a GaAs active layer 3 as the first semiconductor layer and a p-Ga_1-yAI_yAs (y ≃ 0.3) layer 4 as the third semiconductor layer are successively formed by the well-known liquid-phase epitaxy. The upper surface of the p-Ga_1-yAl_yAs layer 4 is formed" with periodic corrugations 8. Numerals 6 and 7 designate ohmic electrodes.
In the present structure, light generated in the active layer 3 is confined in the vertical direction around the active layer by the double-heterostructure. Part of the light evanesces to the GaAIAs layers 2 and 4 on both the sides. The light having evanesced to the N-Ga_1-xAl_xAs layer 2 reaches the GaAs substrate 1 in regions on both the sides of the groove 9 because the n-Ga_1-xAl_xAs layer 2 in these regions is thin. In consequence, the complex refractive-index for the light becomes different between the region corresponding to the groove 9 and the regions corresponding to both the sides of the groove 9. For this reason, any higher-order mode oscillation spreading to outside of the groove and the deviation of the oscillation region are suppressed, and the effect that the light is stably confined only to the region of the groove 9 in the lateral direction is produced.
On the other hand, the periodic corrugations 8 are provided at the interface between the semiconductor layers 4 and 5. Therefore, the effective complex refractive-index n_. for the light given forth in the active layer 3 varies periodically in the travelling direction of the light. n_e can approximately be expressed as follows:
where I = 1, 2, 3, ...; no and Δn^(I) are constants.
The travelling direction of the light is taken in the z-direction.
Therefore, the laser light is diffracted, and
where λ: wavelength of the light, n : effective refractive index of waveguide. A: period of the corrugations and I: order of the diffraction. By fulfilling the condition, the light is subjected to the Bragg reflection at 180°. Accordingly, the light is confined in the waveguide, and the laser oscillation becomes possible.
In the above, the fundamental operation of the semiconductor laser of this invention has been explained. In order to realize the semiconductor laser intended by this invention wherein a longitudinal and transverse mode is stabilized, especially any excess optical noise component for a modulated signal is not generated by mode competition, a construction as described below is required.
The width of the stripe-shaped light non-absorptive protuberant region corresponding substantially to the radiation region is made 2 µm to 8 µm. With widths below 2 µm; especially the threshold current density (the lowest current density necessary for attaining the laser oscillation) increases rapidly. With widths above 8 µm, especially the instability of the transverse mode increases.
As to the quantity of evanescence of the light to the regions which exist on both sides of the light non-absorptive protuberant region and which make the complex refractive index for the laser beam discontinuous, at least 3 x 10^-2% of the whole quantity of the light, preferably 5 x 10-²% to 5 x 10^-1 % of the same is appropriate. Thus, the absolute value of the index different |Δn| in both the regions is preferably made 10-³ to 10-². When the quantity of evanescence of the light is too small, especially the effect of confinement in the lateral direction is unsatisfactory. On the other hand, when the quantity of evanescence of the light is too large, the quantity of absorption of the light increases, and the increase of the threshold current density is incurred. Therefore, the thickness d of the active layer 3 and the thickness t of the n-Ga_1-xAl_xAs layer 2 need to be selected. The thickness of the active layer is ordinarily set at 0.05 µm to 0.15µm. When the thickness d of the active layer is 0.1 µm, it is favorable to make the thickness t at most 0.5 µm; when the thickness d is 0.15 µm, it is favorable to make the thickness t at most 0.2 µm; when the thickness d is 0.05 µm, it is favorable to make the thickness t at most 0.7 µm. Regarding any intervening thickness of the active layer, the thickness of the semiconductor layer 2 may be set by the interpolation with the aforecited values.
On the other hand, the distance c between the active layer 3 and the periodic corrugations 8 which are provided at the interface between the semiconductor layers 4 and 5 is made at most 1 µm, preferably at most 0.5 um, and at least 0.03 µm. When the distance c is below 0.03 µm, the carrier confinement effect owing to the p-Ga_1-yAI_yAs layer 4 becomes insufficient, and an increase of the threshold current density is incurred. When the distance c is above 1 µm, the degree to which the radiation in the active layer senses the periodic corrugations decreases abruptly, so that the laser oscillation of the distributed feedback type does not take place.
The depth L of the corrugations is desirably selected in a range of 0.01 µm to 0.5 ,um. When the depth L is less than 0.01 µm, the Bragg reflection of the light owing to the periodic corrugation structure becomes insufficient, and the laser oscillation of the distributed feedback type becomes difficult. Even when the depth L is selected to be greater than 0.5 ,um, the light is distributed in a limited range centering around the active layer, so that the intensity of the Bragg reflection becomes substantially constant and the effect based on the increase of the depth L diminishes.
In the above, the fundamental concept of this invention has been described in connection with the typical example of this invention illustrated in Fig. 3.
Various modifications can be contrived as to how the light non-absorptive protuberant region in the vicinity of the active layer and the regions situated on both the sides of the non-absorptive protuberant region for making the complex refractive index for the laser beam discontinuous are provided at which interface among the stacked semiconductor layers the periodic corrugation is provided at, etc. By way of example, the semiconductor layer corresponding to the semiconductor substrate 1 in Fig. 3 may consist of a plurality of layers. It is also allowed to form a separate semiconductor layer on the semiconductor substrate and to provide a recess in the separate semiconductor layer, the recess being used as the groove 9 in Fig. 3. The purpose can be achieved even when a discontinuity in only the refractive index (corresponding to the real part of the complex refractive index) is produced. In any case, however, the technical items previously described may be conformed with.
Although, in examples to be stated later, a GaAsGaAIAs conductor will be referred to as a semiconductive material, it is obvious that the present invention concerns the property of a laser resonator including an optical waveguide and that it is independent of materials. This invention is accordingly applicable, not only to semiconductor lasers employing the above-mentioned semiconductor, but also to semiconductor lasers employing e.g. a ternary system compound semiconductor such as GalnP, GaAsP and GaAISb and a quaternary system compound semiconductor such as GalnAsP and GaAIAsSb.
Hereunder, this invention will be described more in detail in connection with examples.

Example 1

Description will be made with reference to Fig. 3. In an n-GaAs substrate 1 (Te-doped, electron concentration n 1 x 10¹⁸/cm³) having the (100) face as its surface, a groove 9 having a depth of 1.5 µm and a desired width in the range of 2 µm to 8 µm was formed in the (011) orientation. In the formation, the conventional photolithography may be employed. As an etching mask, photoresist was directly used. The chemical etching was conducted at 20°C for about 140 seconds with a mixed solution which contained phosphoric acid, hydrogen peroxide solution and ethylene glycol at 1:1:3.
On the resultant substrate 1, an n-Ga_1-xAI_xAs layer 2 as the second semiconductor layer (x ≃ 0.3, electron concentration n _- 5 x 10¹⁷ cm^-3) being 0.3 µm thick, a GaAs active layer 3 as the first semiconductor layer being 0.1 µm thick, and a p-Ga_1-yAI_yAs layer 4 as the third semiconductor layer (y ≃ 0.3, hole concentration p = 5 x 10¹⁷ cm-³) being 0.2 µm thick, were continuously grown by the conventional liquid-phase epitaxy employing a slide boat.
As regards the GaAlAs system material, it is common to employ Ga_1-zAl_zAs (0 5 z ≤ 0.3) for the first semiconductor layer, Ga_1-xAl_xAs (0.1 ≦ x ≦ 0.9) for the second semiconductor layer and Ga_1-yAl_yAs (0.1 ≦ y < 0.9) for the third semiconductor layer, where x, y, > z; r > z; and y ≠ r.
Subsequently, corrugations 8 having a period of 370 nm (3 700 A) and a depth of 150 nm (1 500 A) were formed in the surface of the p-Ga_1-yAl_yAs layer 4. In forming the corrugations, holographic photolithography employing a laser beam and chemical etching were used. More specifically, a' film of the positive type photoresist as was 80 nm (800 A) thick was formed on the surface of the semiconductor layer 4. Subsequently, using an Ar laser at a wavelength of 457.9 nm (4 579 A), an interference fringe was formed on the photoresist. After completing exposure, development was carried out for about 1 min with a mixed solution consisting of a developer and water at 1:1. In this way, a diffraction grating made of a photoresist was formed. The diffraction grating made of the photoresist was used as a mask, and a mixed solution consisting of phosphoric acid, a solution of hydrogen peroxide and ethylene glycol at 1:1:8 was used as an etchant. Periodic corrugations 8 having a depth of 0.15 µm were formed by the etching at 20°C for 80 seconds. This method is disclosed in Japanese Laid-Open Patent Application No. 111344/1976.
Subsequently, a p-Ga_1-νAl_ν As layer 5 as the sixth semiconductor layer (ν≃0.1 and in general, 0.05 ≦γ ≦ 0.09; hole concentration p - 5 x 10¹⁷ cm^-3) was formed to a thickness of 2.0 µm by employing the conventional liquid-phase epitaxial growth again. Zn was diffused into a desired region of the p-side surface of the specimen thus formed, whereupon Cr and Au were deposited by vacuum-evaporation so as to form an electrode. The substrate side was lapped down to about 150 µm, whereupon Au-Ge-Ni was brought into close contact to form an electrode. The laser length was made 300 µm.
As a result, when the groove width was 7 µm, the laser device oscillated at a threshold value of 110 mA and a wavelength of 830 nm (8 300 A), and each of longitudinal and transverse modes was single and stable. Any excess optical noise otherwise generated by mode competition was not noted.
It is as previously stated that, in the present structure, the width W of the groove 9, the distance t of the active region from the light absorptive region, the thickness d of the active layer 3, the distance C between the active layer and the periodic corrugations, the depth L of the periodic corrugations, etc. have influence on the oscillation characteristics.
Fig. 4 shows the characteristics of semiconductor lasers employing various combinations between the width W of the groove 9 and the distance t of the active region from the light absorptive region. While values of 0.05 µm to 0.15 µm are often employed as the thickness d of the active layer 3, a value of 0.1 pm is given as a typical example. The depth L of the periodic corrugations is 150 µm (1 500 A). Mark 0 indicates an example in which the laser device oscillated in a single mode longitudinally and transversely without any optical noise, mark A an example in which the laser device oscillated in a single longitudinal mode, but excess optical noise was generated by mode competition, and mark X an example in which the laser device did not reach the continuous oscillation on account of the increase of the threshold current density.
From the results, it is understood that the width W of the groove 9 capable of achieving the object is 2 µm to 8 µm. Regarding the distance t of the active region from the light absorptive region, a value of 0.05 µm or less is the limit to which the layer can be stably fabricated in the actual process, while a value of 0.45 µm or greater falls in a region in which a desired light absorption was not attainable.
Figs. 5 to 8 illustrate the states of generation of excess optical noise attributed to mode competition. A square wave signal current having a pulse width of 8 ns, a pulse height of 160 mA (1.25 times a threshold value) and a recurrence frequency of 62.5 MHz was caused to flow through a semiconductor laser device having a threshold current of 130 mA, and the optical output of the semiconductor laser was observed. The optical output waveform is a result obtained by conversion into an electric signal with a photodiode of which the wavelength band was the oscillation wavelength ±0.1 nm ( ±1 A) and the frequency band was 1 MHz to 0.8 GHz. Fig. 5 shows an example of the optical output waveform of the distributed-feedback semiconductor laser device exemplified in Fig. 1. Fig. 6 shows an example of the optical output waveform of a prior art buried heterostructure semiconductor laser device, Fig. 7 shows an example of the optical output waveform of the semiconductor laser device of this invention, and Fig. 8 shows an example of the optical output waveform of a semiconductor laser device which has a a structure similar to that of this invention but whose stripe-shaped non-absorptive region is as broad as 12 µm. It is apprehended from the observation of the optical output waveforms that the semiconductor laser device of this invention is excellent.
The semiconductor laser device of the structure of this example is advantageous in that the deterioration is especially little and that the transverse mode is more easily stabilized. This originates in that the periodic corrugations can be formed after forming the active layer.

Example 2

Description will be made with reference to Figs. 9 and 10.
In the (011) orientation of an n-GaAs substrate 21 (Te-doped, electron concentration n n ≃ 1 x 10¹⁸/cm³) having the (100) face as its surface, a groove 29 having a depth of about 1.5µm and a desired width in a range of 2 µm to.8 µm was formed by the conventional photolithography and chemical etching. Photoresist was used for an etching mask. The chemical etching was conducted at 20°C for about 140 seconds with a mixed solution which consisted of phosphoric acid, hydrogen peroxide solution and ethylene glycol at 1:1:3.
On the resultant substrate 21, an n-Ga_1-sAl_sAs layer 30 as the fifth semiconductor layer (s = 0.07, and in general, 0.1 ≦ s ≦ 0.9; Sn-doped; electron concentration n ≃ 5 x 10¹⁷/cm³) was grown to a thickness of 2.0 µm so as to fill up the groove flatly (to the extent that the thickness was about 0.5 µm outside the groove) by the conventional liquid-phase epitaxial growth employing a slide boat.
Subsequently, the surface of the grown layer was subjected to the chemical etching until the substrate 21 was exposed on both sides of the groove. The chemical etching was carried out at 20°C for about 70 seconds by the use of phosphoric acid, hydrogen peroxide solution and ethylene glycol at 1:1:3.
Subsequently, corrugations 28 having a period of 375 nm (3 750 A) were formed in the direction orthogonal to the groove (in the (011) orientation) by the holographic photolithography employing a laser beam and the chemical etching. As a mask at this time, photoresist being about 80 nm (800 A) thick was used. A mixed solution consisting of phosphoric acid, a solution of hydrogen peroxide and ethylene glycol at 1:1:8 was employed as at etchant, and the etching was conducted at 20°C for 80 seconds. As a result, the corrugations 28 being 0.15 pm deep were formed in the crystal surface.
Subsequently, an n-Ga_1-xAl_xAs layer 22 (x ≃ 0.3; Sn-doped; electron concentration n - 5 x 10" cm-³) being 0.4 um thick, an n-Ga_1-zAl_zAs active layer 23 (z ≃ 0.05; undoped; n = 1 x 10¹⁶ cm-³) being 0.1 µm a p-Ga_1-xAl_xAs layer 24 (x ≃ 0.3, Ge-doped; hole concentration p = 10¹⁷ cm^-3) being 2 µm thick, and a p-GaAs layer 25 (Ge-doped; hole concentration p = 5 x 10¹⁷ cm-³) being 1 µm thick were successively grown by employing the conventional liquid-phase epitaxial growth with the slide boat again.
Here, the AI contents of the respective layers must be so set that, with respect to light produced in the active layer 23, the substrate 21 becomes an absorptive region, while the stripe-shaped buried n-Ga_1-sAl_sAs layer 30 becomes a non-absorptive region. The composition ratio between the n-Ga_1-sAl_sAs layer 30 and the Ga_1-zAl_zAs active layer 23 is made z < s, and it is desirable that (s-z) is approximately 0.01 or greater. On the other hand, the AI content s of the n-Ga_1-sAl_sAs layer 30 may, in principle, be made 0.01 ≦ s ≦ 0.9. However, s < 0.1 must be held in order that the crystal may be smoothly grown on this layer by the liquid-phase epitaxial growth generally employed. When s _> 0.1, the normal liquid-phase growth becomes difficult in practice. As described above, (s-z) should desirably be approximately 0.01 or greater. Therefore, even in case where the active layer 23 is made a GaAs (z = 0 in Ga_1-zAl_zAs) layer it is necessary that s > 0.01, and the lower limit value of s becomes 0.01.
Zn was diffused into the p-side surface of this specimen by approximately 0.1 pm, whereupon Cr and Au were deposited by vacuum-evaporation so as to form an electrode 27. The substrate side was lapped down to approximately 150 µm, whereupon Au-Ge-Ni was evaporated to form an ohmic electrode 26.
Fig. 11 is a graph showing the light output versus current characteristics of this embodiment, while Fig. 12 is a graph showing the far-field intensity distributions in the junction plane. The results were obtained in an example in which the semiconductor laser device had a length of 300 µm and a groove width of 7µm. The threshold current value was 100 mA at room temperature, and the external differential quantum efficiency was about 35%. Fig. 11 corresponds to the light output versus current characteristics at this time. As apparent from the Figure, the laser oscillated in the fundamental transverse mode, and the transverse mode was stable up to above double the threshold value.
Fig. 13 shows the lasing spectra of the embodiment. The oscillation arose at a wavelength of 836 nm (8 360 A), and even when the current value was increased double the threshold value, no change was noted. Fi. 14 shows the pulse responses at the time when current pulses having a width of 7 ns were impressed on the present laser device. Since the transverse mode was stabilized, thé relaxation oscillation to become an optical noise as observed in the prior art distributed-feedback semiconductor laser was not noted.

Example 3

Fig. 15 concerns another embodiment of this invention, and is a sectional view taken perpendicularly to the travelling direction of light within a laser. In the present element, corrugations 28 were formed on only the surface of the fifth semiconductor layer 30 grown in a groove. The other features were the same as in the case of Fig. 9. In this example, the feedback of light owing to the diffraction effect is selectively attained only in the upper part of the groove, and the lasing occurs concentratedly in this part. Therefore, the transverse mode is further stabilized. In case where the thickness of the active layer 23 was 0.1 µm and where the thickness of the second semiconductor layer 22 was 0.3 pm, the laser oscillated in the fundamental transverse mode up to triple the threshold value. Any excess optical noise for a modulated signal at the time of modulation as otherwise generated by mode competition was not noted.

Example 4

Fig. 16 is a sectional view showing another embodiment of this invention. In the present laser device, an n-Ga_1-yAl_yAs layer 33 as the fifth semiconductor layer, was grown on the whole surface of an n-GaAs substrate 21 in a manner to fill up the groove of the substrate, and corrugations 34 were formed thereon. The other features were the same as in Example 1. In this case, in order for light to evanesce to the substrate on the lateral outer sides, the sum of the thickness of the layer 33 and the layer 22 needed to be made 0.6 µm or less when the thickness of the active layer was 0.1 µm. By way of example, in case of a laser wherein the thickness of the fifth semiconductor layer 33 outside the groove was 0.2 pm, the thickness of the second semiconductor layer 22 was 0.2 pm, the thickness of the active layer was 0.1 µm, the groove width was 7 µm and the length was 300 µm, the threshold current value of the oscillation was 100 mA and each of the longitudinal and transverse modes was single and stable up to above double the threshold value.
As regards the arrangement of the periodic corrugations and the means for bestowing a difference on the complex refractive index, only examples in which such means exists on the substrate side have been explained above. This invention, however, is not restricted to such arrangement, but can adopt different constructions. For instance, a layer overlying an active layer is formed with periodic corrugations, whereupon a layer is formed which is provided with a protuberance corresponding substantially to a radiation portion. The protuberant layer is made a light non-absorptive layer. On this layer, a layer serving as a light absorptive layer is formed. With such a structure, the same effects as in the foregoing can be achieved.

Claims

1. A semiconductor laser device with a first semiconductor layer (3; 23) and with a second (2; 22) and a third semiconductor layer (4; 24) which sandwich said first semiconductor layer therebetween, which are greater in the band gap and lower in the refractive index than said first semiconductor layer and which have conductivity types opposite to each other, with a fourth semiconductor layer (1; 21) rendering the effective complex refractive index for the laser beam discontinuous in a direction perpendicular to the travelling direction of the laser beam, and with periodic corrugations (8; 28; 34) formed in at least one interface of the semiconductor layers constituting an optical waveguide of said semiconductor laser device in a manner to intersect orthogonally to said travelling direction of the laser beam, characterized in that said second (2; 22) and/or said third semiconductor layer (4; 24) have a protuberant region (at 9 in Fig. 3) or are bordered on their side remote from said semiconductor layer by a fifth semiconductor layer (30; 33) with a protuberant region, that said protuberant region having a width able to stabilize a transverse mode of the laser oscillation extends in said travelling direction of the laser beam and has the shape of a stripe on its surface side remote from said first semiconductor layer, that said fourth semiconductor layer (1, 21) has portions on both sides of said protuberant region, and that said periodic corrugations (8; 28; 34) are formed to include at least a region overlying said protuberant region.

2. A semiconductor laser device according to claim 1, characterized in that the protuberant region is provided by said second (2) or by said third semiconductor layer (4), that said third or, respectively, said second semiconductor layer having no protuberant region is formed with said periodic corrugations (8) at its surface remote from said first semiconductor layer (3), and that a sixth semiconductor layer (5) is disposed on a surface of said periodic corrugations. (Fig. 3)

3. A semiconductor laser according to claim 1, characterized in that said periodic corrugations (28) are formed at the interface between said second (22) and/or said third semiconductor layer (24), respectively, and the bordering fifth semiconductor layer (30) as well as the bordering fourth semiconductor layer (21). (Figs. 9, 10; 15)

4. A semiconductor laser according to claim 1, characterized in that said fifth semiconductor layer (30) includes said protuberant region, and that said periodic corrugations (34) are formed at an interface between said second (22) and/or said third semiconductor layer (24) and the bordering fifth semiconductor layer. (Fig. 16)

5. A semiconductor laser device according to any of claims 1 to 4, characterized in that the absolute value of the difference between the effective complex refractive indices discontinuous for the laser beam is 10-³ to 10-².

6. A semiconductor laser device according to any one of claims 1 to 5, characterized in that the width of the protuberant region (33) or said fifth semiconductor layer (30) is made 2 µm to 8 pm, and the distance from said first semiconductor layer (3; 23) to said periodic corrugations (8; 28; 34) is made .0.03 pm to 1 µm.

7. A semiconductor laser device according to any of claims 1 to 6,.characterized in that the thickness of said first semiconductor layer (3; 23) is 0.05 µm to 0.15 µm.

8. A semiconductor laser device according to any of claims 1 to 7, characterized in that said fourth semiconductor layer (1; 21) is made of a semiconductor substrate which has a groove (9; 2.9) extending in said travelling direction of the laser beam and in the shape of said stripe.

9. A semiconductor laser device according to claim 8 as depending on claim 2, characterized in that said semiconductor substrate (1) is a GaAs substrate, said first semiconductor layer (3) is made of Ga_1-zAl_zAs (0 ≦ z ≦ 0.3), said second semiconductor layer (2) is made of Ga_1-xAl_xAs (0.1 ≦ x ≦ 0.9), said third semiconductor layer (4) is made of Ga_1-yAl_yAs (0.1 ≦ y ≦ 0.9), and said sixth semiconductor layer (5) is made of Ga_1-νAl_νAs (0.05 < γ < 0.9) (where x, y > z; and y = y). (Fig. 3)

10. A semiconductor laser device according to claim 8 as depending on claim 3, characterized in that said semiconductor substrate (21) is a GaAs substrate, said first semiconductor layer (23) is made of Ga_1-zAl_zAs (0 ≦ z ≦ 0.3), said second semiconductor layer (22) is made of Ga_1-xAl_xAs (0.1 ≦ x < 0.9), said third semiconductor layer (24) is made of Ga_1-yAl_yAs (0.1 ≦ y ≦ 0.9), and said fifth semiconductor layer (3'0) is'made of Ga_1-sAl_sAs 0.01 ≦ s ≦ 0.9) (where x, y > z; s > z; and x ≠ s). (Figs. 9, 10; 15).

11. A semiconductor laser device according to claim 8 as depending on claim 4, characterized in that said semiconductor substrate (21) is a GaAs substrate, said first semiconductor layer (23) is made of Ga_1-zAl_zAs (0 ≦ z 5 0.3), said second semiconductor layer (22) is made of Ga_1-xAl_xAs (0.1 < x < 0.9), said third semiconductor layer (24) is made of Ga_1-yAl_yAs (0.1 ≦ y ≦ 0.9), and said fifth semiconductor layer (33) is made of Ga_1-νAl_νAs (0.1 ≦ ν ≦ 0.9) (where x, y > z; y > z; and x ≠ ν). (Fig. 16).