Transverse-mode stabilized laser diode

Abstract: A transverse mode stabilized AℓGaInP visible light laser diode in accordance with the present invention has a characteristic in that, instead of forming a built-in refractive index distribution in an active layer (14) along a junction plane by means of the absorption loss of the oscillated light in the conventional transverse mode stabilized AℓGaInP laser diodes, an optical guide layer (15) having a ridge stripe portion is formed on the active layer (14), and a refractive index distribution is formed by the thickness difference of the optical guide layer (15). In particular, the present laser diode employs as the current blocking layer (18) to be formed on the sides of the mesa, an AℓXGal-XAs layer which does not give rise to an absorption loss of the oscillated light.

Description: The present invention relates to transverse-mode stabilized laser diodes, and more particularly to ridge stripe AℓGaInP visible light laser diodes with oscillation wavelength not greater than 680 nm. AℓGaInP visible light laser diodes are attractive light sources for optical information processing systems such as laser printers, barcode readers, and optical disc systems. A conventional transverse-mode stabilized InAℓP/InGaP visible light laser diode is shown in Fig. 1 as reported in the Abstract of Papers for '87 Fall Meeting of the Japan Society of Applied Physics, p. 764, 19a-ZR-5. A conventional transverse-mode stabilized InGaAℓP/InGaP laser is disclosed in Japanese Journal of Applied Physics, Vol.28, No.11, November 1989, pages L2089 - L2091. Referring to Fig. 1, an n-GaAs buffer layer 2 is formed on an n-GaAs substrate 1. On top of the buffer layer 2 there is formed a double heterojunction structure consisting of an n-InAℓP cladding layer 3, a InGaP active layer 4, a ridge stripe p-InAℓP cladding layer 5, a p-GaAs layer 6, a pair of n-GaAs current blocking layers 7 and a p-GaAs contact layer 8. Here, the fabrication step of the conventional structure shown in Fig. 1 will be described. First, a five-layer structure from the n-GaAs buffer layer 2 to the p-GaAs cap layer 6 is formed sequentially by a first vapor phase growth. Subsequently, a stripe-shaped mask using a 5 µm-width SiO₂ film is formed on the cap layer 6 by photoetching, and a ridge stripe structure is formed by etching the p-InAℓP cladding layer 5 halfway. Next, the n-GaAs current blocking layers 7 are formed selectively on the mesa part except for the stripe-shaped SiO₂ film mask by a second vapor phase growth. Then, after the removal of the SiO₂ film mask, the p-GaAs contact layer 8 is grown all-over the surface by a third vapor phase growth, a p-side ohmic electrode 9 of AuZn/Au is formed on the contact layer 8 and an n-side ohmic electrode 10 of AuGe/Au is formed on the GaAs substrate 1. In this prior art structure, the current contraction is achieved by the n-GaAs current blocking layers 7. Further, the p-GaAs cap layer 6 plays the role of preventing the increase of the electrical resistance caused by the band discontinuity between the p-InAℓP cladding layer 5 and the p-GaAs contact layer 8. On the other hand, the transverse-mode stabilization is achieved by the formation of a refractive index distribution in the active layer 4 along a function plane in the direction perpendicular to an optical axis of the laser diode by utilizing the light absorption loss caused by the n-GaAs current blocking layers 7 at thinned peripheral portions of the p-InAℓP cladding layer 5. In recent years there are increasing demands for refractive index guided type laser diodes that can be operated in the fundamental transverse mode and has a low oscillation threshold and a low astigmatism. However, in the conventional refractive index guided type laser diodes as described above, since the built-in effective refractive index distribution in the active layer 4 along a junction plane is formed by the light absorption loss caused by the n-GaAs current blocking layer 7, the oscillation threshold current is high and the astigmatism is a relatively large value of 10 - 13 µm, leading to a drawback that it is not easy to narrow down the laser beam to a minute spot. Moreover, due to the necessity for making the thickness of the P-InAℓP cladding layer 5 at both sides of the ridge or mesa portion to a small value in the range of 0.1 to 0.2 µm, the active layer 4 under both sides of the mesa portion suffers thermal damage during the formation of the n-GaAs current blocking layer 7 by a second vapor phase growth, and thus the life-time of the laser diodes is shortened. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide highly reliable transverse mode stabilized AℓGaInP visible light laser diodes with satisfactory characteristics. According to the invention there is provided a transverse mode stabilized visible light laser diode comprising a substrate of GaAs having a first conductivity type (n); a first cladding layer of AlGalnP formed on said substrate and having said first conductivity type; an active layer formed on said first cladding layer; an optical guide layer of AlGalnP formed on said active layer and having a second conductivity type (p); said optical guide layer having a ridge stripe structure, whose thickness is larger in a central ridge portion than in flat portions on both sides of the ridge portion; a cap layer of GalnP formed on said ridge portion and having said second conductivity type; a second cladding layer of AlGalnP formed between said ridge portion and said cap layer and of said second conductivity type; a current blocking layer formed commonly over said flat portions and the side surfaces along the laser beam propagation direction of said ridge portion of said optical guide layer and the side surfaces of said second cladding layer and cap layer and having said first conductivity type; a contact layer of GaAs formed commonly over the top surface of said cap layer and the surface of said current blocking layer and having said second conductivity; a first electrode layer formed on said contact layer; and second electrode layer formed on the bottom surface of said substrate; and said current blocking layer is made of AlGaAs, the composition of said current blocking layer is such that its energy band gap is larger than that of said active layer so as not to absorb light generated by said active layer, and its refractive index is selected to be smaller than that of said optical guide layer so as to trap the generated laser light into said optical guide layer; and the thickness of said flat portions of said optical guide layer is 0.3 to 0.5µm. Preferably, the laser diode in accordance with the present invention employs an Al x Ga 1-x As (x ≧ 0.7) current blocking layers formed on the sides of the mesa or ridge portion. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a transverse sectional view showing an example of the prior art laser diodes. Fig. 2 is a transverse sectional view showing a first embodiment of the AlGalnP laser diode of the present invention. Fig. 3 is a transverse sectional view showing a second embodiment of the AlGalnP laser diode of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) First, referring to Fig. 2, by a low pressure metalorganic vapor phase epitaxy (MOVPE) method that uses group III organic metals (trimethylindium, triethylgallium and trimethylaluminum) and group V hydrides (PH₃ and AsH₃) as the raw materials there is formed a double heterowafer by sequentially forming, on an n-GaAs substrate 11 (with electron concentration of 2 x 10¹⁸ cm⁻³) with (100) surface orientation, a 0.5 µm-thick n-GaAs buffer layer 12 (with electron concentration of 1 x 10¹⁷ cm⁻³), a 1 µm-thick n-(Aℓ 0.6 Ga 0.4 ) 0.5 In 0.5 P cladding, layer, a 0.06 µm-thick Ga 0.5 In 0.5 P active layer 14, a 1 µm-thick p-(Aℓ 0.4 Ga 0.6 ) 0.5 In, 0.5 P optical guide layer 15 (with hole concentration of 3 x 10¹⁷ cm⁻³), a 0.7 µm-thick p-(Aℓ 0.6 Ga 0.4 ) 0.5 In 0.5 P cladding layer 16 (with hole concentration of 3 x 10¹⁷ cm⁻³) and a 0.05 µm-thick p-Ga 0.5 In 0.5 P cap layer 17 (with hole concentration of 1 x 10¹⁸ cm⁻³ Subsequently, a 5 µm-wide stripelike SiO₂ mask film (not shown) is formed on the cap layer 17 by photoetching. Next, as shown in Fig. 2, the optical guide layer 15 is etched up to a halfway by the use of a mixed solution of H₃PO₄, H₂O₂ and H₂O, and an etchant H₂SO₄ to control the thickness of the optical guide layer 15 on both sides of the stripe-shaped mesa portion to be 0.3 to 0.5 µm. Then, a 0.6 µm-thick (at the flat portion) n-Aℓ 0.7 Ga 0.3 As current blocking layers 18 (with electron concentration of 1 x 10¹⁸ cm⁻³) is selectively grown on the mesa part except for the stripe-shaped SiO₂ film mask by a second MOVPE growth. Next, after removal of the SiO₂ film mask, a 3 µm-thick p-GaAs contact layer 19 (with hole concentration of 5 x 10¹⁸ cm⁻³) is grown all-over the surface by a third MOVPE growth, and a laser diode is completed by forming a p-side ohmic electrode 20 of AuZn/Au on the contact layer 19 and an n-side ohmic electrode 21 of AuGe/Au on the n-GaAs substrate 11, and by forming reflecting end faces by cleavage. It should be noted here that the composition of the n-Aℓ 0.7 Ga 0.3 As current blocking layer 18 is regulated such that its energy band gap is larger than that of the active layer 14 so as not to cause absorption loss of light generated from the active layer, and its refractive index is selected to be smaller than that of the optical guide layer 15 so as to trap the light into the optical guide layer 15. Accordingly, the n-Aℓ 0.7 Ga 0.3 As current blocking layers 18 act mainly for current contraction for carrying out an efficient current injection to the ridge stripe mesa portion. In addition, the current blocking layers have the same function of the p-(Aℓ 0.6 Ga 0.4 ) 0.5 In 0.5 P cladding layer 16 in view of trapping light within the optical guide layer 15. To this end, refractive index of the current blocking layers 18 is selected to be substantially equal to that of the cladding layer 16. For these reasons, Aℓ composition rate x of the current blocking layers is selected to be 0.7 or more. Moreover, there can be obtained a distribution of effective refractive index difference with real number component which is free from absorption loss since the built-in effective refractive index difference in the active layer 14 along the junction plane is determined by the thickness difference of the optical guide layer 15. The thickness of the mesa portion is selected to be about twice of that of both sides of the mesa portion so as to obtain the effective refractive index difference of 6 x 10⁻³ to 2 x 10⁻². Therefore, it becomes possible to realize a refractive index guided type transverse mode stabilization which has a low oscillation threshold current and a low astigmatism. For example, conventional threshold current of 50 mA is decreased to about 40 mA and low astigmatism of 2 µm or less can be achieved. Furthermore, the life time of the laser diode can be extended due to large thickness of the optical guide layer at flat peripheral region. [Embodiment 2] Fig. 3 is a transverse sectional view of a second embodiment of the present invention. The growth of a double hetero-structure by an MOVPE method is carried out by the same method as in the first embodiment described above. This embodiment differs from the first embodiment in that after growing a p-(Aℓ 0.4 Ga 0.6 ) 0.50 In 0.5 P optical guide layer 15 with thickness of 0.3 to 0.4 µm on an active layer 14 by a first MOVPE method, a 4 nm (40Å)-thick p-Ga 0.5 In 0.5 P etching stopper layer 22 (with hole concentration 3 x 10¹⁷ cm⁻³) is grown, and the optical guide layer 15 is formed to a thickness of 0.7 µm. In other respects it is the same as in the first embodiment. This embodiment has an advantage that the thickness of the light guide layer 15 on both sides of the mesa portion can be formed with high controllability by the formation of the etching stopper layer 22. Here, the thickness of the etching stopper layer 22 is made thin to be 4nm (40Å) so as not to subject the oscillated light to an absorption. As described in the above, in accordance with the present invention the built-in effective refractive index difference in the active layer 14 is realized by the thickness of the optical guide layer of the mesa portion and its both sides, so that a refractive index distribution with real number component having no absorption loss can be obtained. Therefore, it is possible to realize a transverse mode stabilized laser diode that has a low oscillation threshold and a low astigmatism and can be operated at the transverse fundamental mode. Moreover, since it is possible to design the thickness of the optical guide layer 15 on both sides of the mesa portion to have a value of 0.3 to 0.5 µm which is larger than that of the conventional device, there can be formed a highly reliable laser diode having an active layer 14 that has suffered a less thermal damage during the growth of the current blocking layer 18 by a second MOVPE growth.

Claims: A transverse mode stabilized visible light laser diode comprising: a substrate (11) of GaAs having a first conductivity type (n); a first cladding layer (13) of AlGalnP formed on said substrate (11) and having said first conductivity type; an active layer (14) formed on said first cladding layer; an optical guide layer (15) of AlGalnP formed on said active layer (14) and having a second conductivity type (p); said optical guide layer (15) having a ridge stripe structure, whose thickness is larger in a central ridge portion than in flat portions on both sides of the ridge portion; a cap layer (17) of GalnP formed on said ridge portion and having said second conductivity type; a second cladding layer (16) of AlGalnP formed between said ridge portion and said cap layer (17) and of said second conductivity type; a current blocking layer (18) formed commonly over said flat portions and the side surfaces along the laser beam propagation direction of said ridge portion of said optical guide layer (15) and the side surfaces of said second cladding layer (16) and cap layer (17) and having said first conductivity type; a contact layer (19) of GaAs formed commonly over the top surface of said cap layer (17) and the surface of said current blocking layer (18) and having said second conductivity; a first electrode layer (20) formed on said contact layer (19); and a second electrode layer (21) formed on the bottom surface of said substrate (11); and said current blocking layer (18) is made of AlGaAs, the composition of said current blocking layer (18) is such that its energy band gap is larger than that of said active layer (14) so as not to absorb light generated by said active layer (14), and its refractive index is selected to be smaller than that of said optical guide layer (15) so as to trap the generated laser light into said optical guide layer (15); and the thickness of said flat portions of said optical guide layer (15) is 0.3 to 0.5µm. The transverse mode stabilised laser diode as claimed in Claim 1, further comprising an etching stopper layer (22) provided on said optical guide layer (15) except for said ridge stripe portion thereof and extending along said active layer (14) such that said ridge stripe portion is separated from the flat portions of the optical guide layer (15) and said current blocking layer (18) is spaced apart from the flat portions of said optical guide layer. The transverse mode stabilized laser diode as claimed in Claim 1, wherein the thickness of said optical guide layer (25) at said ridge stripe portion is about twice that of the flat side portion thereof. A visible light laser diode as claimed in Claim 1, wherein the composition of said current blocking layer 18 is represented by Al x Ga 1-x As, where x ≧ 0.7. A visible light laser diode as claimed in Claim 4, wherein said active layer is (Al y Ga 1-y ) 0.5 In 0.5 P (0 ≦ y ≦ 1), said first cladding layer is (Al z Ga 1-z ) 0.5 In 0.5 P (0 ≦ z ≦ 1) and said optical guide layer is (Al w Ga 1-w ) 0.5 In 0.5 P (0 ≦ w ≦ 1), there being satisfied the relationship z ≧ w ≧ y.

Inventors: KAWANO HIDEO, KAWANO, HIDEO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H01S3
IPC codes: H01S5