Transverse-mode oscillation semiconductor laser device

Abstract: A visible-light wavelength continuous oscillation laser (10) is disclosed which has an N type GaAs substrate (12), an N type GaAℓAs cladding layer (14) formed on the substrate, and a non-doped GaAℓAs active layer (16) formed on the cladding layer. The laser further comprises two N type GaAℓAs layer (22), a P type GaAℓAs optical guide layer (20), a P type cladding layer (18) consisting of upper and lower portions (18a, 18b), and a ridge section (32). The N type GaAℓAs layers (22) contact both sides of the ridge section (32). The opti­ cal guide layer (20) is sandwiched between the upper and lower portions (18a, 18b) of the P type cladding layer (18), is located within the ridge section (32), and has a refractive index greater than that of the P type clad­ ding layer (18). The distance between the active layer (16) and the optical guide layer (20) does not depend on the condition in which etching is performed to form the ridge section (32), but depends basically on the time required to grow crystal.

Description: The present invention relates to solid-state light-emitting devices and, more particular, semiconductor laser devices that are preferably used as solid-state light sources in an optical data-processing apparatus, an optical measuring apparatus, or the like. In recent years, solid-state light-emitting devices are necessarily used in processing optical data, optical measuring of physical values, and similar technical fields. A semiconductor laser, for example, is often used as a light source for writing data onto, or reading data from, a disk-shaped optical data storage media. In such cases, it should be required that the semiconductor laser absolutely has excellent basic beam characteristics. To meet the technical requirement, the semiconductor laser must be satisfied with the following specifications: it exhibits a stable fundamental transverse mode oscillation throughout its high power output range, while the beam astigmatism is kept little. With the presently available manufacturing technology, it is by no means easy to manufacture laser diodes with fundamental transverse-mode oscillation, for the following reasons. Whether or not a semiconductor laser device can perform fundamental transverse-mode oscillation greatly depends on the structural dimensions, such as shape, size, or the like, (in particular, the width W) of the stripe-shaped light-guiding groove (ridge waveguide) portion of the laser device. The ridge waveguide portion can be formed by existing etching process to have a sufficiently precise width W if the mask used in the etching process has accurate dimensions. However, the thickness of the cladding layer, which is located just beneath the waveguide portion, cannot be satisfactorily controlled unless the etching conditions are minutely and precisely adjusted. To adjust these conditions precisely in accordance with varying factors concerning the manufacture of the semiconductor laser device, it is necessary for the manufacturer to employ high technology which has not been put to general use. Introducing such highly advanced, special technology would be a great risk to the semiconductor manufacturers, from a commercial point of view. "Extended Abstract of 17th Conference on Solid State Devices and Materials," Tokyo (1985), pp. 67-70, discloses an m-ECO type semiconductor laser. This device exhibits fundamental transverse-mode oscillation within its optical output range. Reportedly, the device is excellent in beam characteristics such as beam astigmatism characteristic. The effective refractive index within the stripe-shaped, grooved portion of this laser device is different from that outside the grooved portion, so that the device can perform stable transverse-mode oscillation. In addition, since the current-blocking layer of the laser device is made of gallium-aluminum-arsenic (GaAℓAs), the optical loss outside the grooved portion is small, resulting in reduction of astigmatism. The semiconductor laser disclosed in the above-identified document is disadvantageous, however, in that the crystalline quality of the interface of the stripe-shaped, grooved portion is deteriorated. More specifically, the laser has been made by forming, among other components, an optical guide layer on the stripe-shaped etched groove, a cladding layer on the optical guide layer, and a contact layer on the cladding layer, by means of crystal growth. Hence, semiconductor crystals grow further on the aluminum-containing layer included in the grooved portion, as an electric current flows through the grooved portion while the laser is emitting a beam. The more aluminum the cladding layer contains, the more the crystalline quality is deteriorated. Therefore, the m-ECO type semiconductor laser cannot be modified into indium-gallium-aluminum-phosphorus (InGaAℓP) semiconductor lasers. Another document, "Extended Abstract of 17th Conference on Solid State Devices and Materials," Tokyo (1986), pp. 153-156, discloses a ridge-stripe type InGaAℓP laser which has been designed to solve the problem inherent in the m-ECO type semiconductor laser. Since this InGaAℓP laser has been made by a method which includes no steps of re-growing crystals on the aluminum-containing semiconductor layer, the interface of the stripe-shaped, grooved portion has good crystalle quality. However, this laser will have a great optical loss if the stripe-shaped ridge section has a small width W in order to improve the mode stability in high-power oscillating operation. Inevitably, the astigmatism characteristic of the laser will be deteriorated, though the laser performs a stable transverse-mode oscillation. Obviously, the laser can neither perform a stable transverse-mode oscillation nor have good beam characteristic, if designed to emit a high optical output. Either of the conventional laser devices, described above, has a transverse-mode characteristic which greatly depends on the width W of the stripe-shaped portion. Hence, the stripe-shaped portion must have a desired width W. To form the a stripe-shaped portion having the desired width W, the manufacturer of the laser device need to employ high technology which has not been put to general use. Since such high technology is too expensive to utilize, it remains practically difficult to manufacture a semiconductor laser device which has a good beam characteristic and performs a stable transverse-mode oscillation, to emit a high optical output. It is therefore an object of the present invention to provide a new and improved semiconductor laser device. It is another object of the invention to provide a new and improved semiconductor laser device which is excellent in its basic characteristics and can be successfully manufactured by means of the existing technology. With regard to the structures of laser devices according to the present invention as hereinafter defined, it can be noted that JP-A-56-054083 disclosed a semiconductor laser with a ridge structure and a layer of high refractive index. Moreover, in US-A-4615032 is described a semiconductor laser with a metal covered ridge structure for horizontal light confinement. This device comprises a "large optical cavity" optical guide layer to enhance output. Electronics letters, vol. 23, no. 18, 1987, pp. 938-939 discloses a semiconductor laser device comprising a substrate, a first cladding layer, an active layer, and a second cladding layer, the first and second cladding layers comprising InGaAlP. The active layer and the first and second cladding layers constitute a double-heterostructure. Light-confining means for defining an elongate light-confining region for guiding light oscillations to propagate along the region are arranged on the second cladding layer, and semiconductor buried layers contact side walls of the light-confining means. In accordance with the above objects, the present invention in its various aspects is defined by, respectively claims 1 and 5. The invention, and its objects and advantages will become more apparent from the following detailed description of preferred embodiments. In the detailed description of preferred embodiments of the present invention, presented below, reference is made to the accompanying drawings, in which: Figure 1 is a sectional view schematically showing an InGaAlP semiconductor laser according to a first embodiment of the invention; Figures 2 and 2B are graphs representing the current-voltage characteristics of the laser shown in Figure 1; Figure 3 is a sectional view illustrating a modification of the laser shown in Figure 1; Figure 4 is a sectional view schematically showing an InGaAlP semiconductor laser conducting to a second embodiment of the present invention; Figure 5 is a sectional view schematically showing an InGaAlP semiconductor laser according to a modification of the second embodiment of this invention; Figure 6 is a diagram for explaining the basic concept of a semiconductor laser not in accordance with the present invention; Figures 7 and 8 are graphs showing the voltage-drop characteristics of the laser shown in Figure 6; Figure 1 is a section view illustrating the main part of a semiconductor laser 300 (not drawn to scale) which is the first embodiment of the invention. As is clearly shown in Figure 1, the laser 300 has an N type GaAs substrate 302. Sequentially formed on this substrate 302 are a first conductivity type cladding layer 304, an active layer 306, and a second conductivity type cladding layer 308 are formed sequentially. Cladding layer 304 may be an N type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P layer which is formed to typically 1 micrometer thickness, having a donor concentration of 4 x 10 17 atoms per cubic centimeter; active layer 306 is an non-doped In 0.5 Ga 0.5 P layer of typically 0.06 micrometers; cladding layer 308 is a P type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P layer formed to be typically 0.2 micrometers thick, having an acceptor concentration of of 5 × 10 17 atoms per cubic centimeter. As is shown in Figure 1, a third cladding layer 310, which is made of N type In 0.5 Aℓ 0.5 P, is formed on the second cladding layer 308. The third cladding layer 310 has a stripe-shaped groove 312 (hereinafter called "stripe-groove section"). It has a thickness of typically 0.6 micrometers and a donor concentration of 4 × 10 17 atoms per cubic centimeter. An optical guide layer 314 is formed on the third cladding layer 310, including the stripe-groove section 312. The layer 314 is a P type Ga 0.4 Aℓ 0.6 As thin film having a thickness of 0.15 micrometers and an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. A fourth cladding layer 316, which is made of P type Ga 0.3 Aℓ 0.7 As, is formed on the optical guide layer 314, filling the stripe-groove section 312. The fourth cladding layer 316 is relatively thick, having a thickness of typically 1 micrometer, and has an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. A contact layer 318 is formed on the top surface of the fourth cladding layer 316. The contact layer 318 is made of P type GaAs, has a thickness of typically 2 micrometers, and has an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. A metal layer 320 is formed on the lower surface of the semiconductor substrate 302, and functions as an N-side electrode. A metal layer 322 is formed on the contact layer 318, and functions as a P-side electrode. The laser 300 illustrated in Figure 1 is characterized in that the optical guide layer 314 has a refractive index greater than that of the third cladding layer 310 of N type. Therefore, the layer 314 has an aluminum mixture ratio of 0.6, whereas the third cladding layer 310 has an aluminum mixture ratio of 1.0 (i.e., In 0.5 (Ga 0.0 Aℓ 1.0 ) 0.5 P). The inventors manufactured a laser having the structure shown in Figure 1 , and evaluated the operating characteristics of the laser thus made. They found that the difference Δn in effective refractive index between the stripe-groove section 312 the layer section outside the stripe-groove portion was sufficiently great. More specifically, the difference was: Δn > 5 × 10 -3 Owing to this great difference Δn, the laser could performed stabilized transverse-mode oscillation. The width W of the stripe-groove section 312 was 2 micrometers or less, and the laser performed a stable fundamental transverse-mode oscillation, emitting a beam having a small aspect ratio of 1.5 or less. In addition, astigmatism was greatly suppressed, to 5 micrometers or less. The valence band at the heterojunction between the cladding layer 308, made of P type In 0.5 (Ga 1-z Aℓ z ) 0.5 P, and the optical guide layer, made of P type Ga 1-v Aℓ v As, is discontinuous. This band discontinuity acts as a barrier to holes, inevitably causing a voltage drop and, hence, increasing the operating voltage of the laser 300. The smaller the aluminum composition ratio of the cladding layer 308, and the greater the aluminum composition ratio of the optical guide layer, the less prominent the discontinuity of the valence band. The cladding layer 308, made of P type In 0.5 (Ga 1-z Aℓ z ) 0.5 P, must have an aluminum composition ratio z of 0.5 or more. It would otherwise fail to provide a potential barrier strong enough to confine carriers. The inventors manufactured semiconductor lasers, each comprising cladding layer 308, made of P type In 0.5 (Ga 0.5 Aℓ 0.5 ) 0.5 P, and a P type Ga 1-v Aℓ v As optical guide layer having a different aluminum composition ratio v. Then, the inventors evaluated the current-voltage characteristics of these lasers. The results were as is shown in Figure 2A. As is evident from Figure 2A, the laser can have a sufficiently low operating voltage when the Ga 1-v Aℓ v As optical guide layer 314 has an aluminum composition ratio v of 0.4 or more. Therefore, in order to have good current-voltage characteristic, a laser comprising cladding layer 308, made of P type In 0.5 (Ga 0.5 Aℓ 0.5 ) 0.5 P, must have a P type Ga 1-v Aℓ v As optical guide layer 314 which has an aluminum composition ratio of v 0.4 or more. The inventors made semiconductor lasers having the structure shown in Figure 1 , each comprising the cladding layer, made of P type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P, and a P type Ga 1-v Aℓ v As optical guide layer having a different aluminum composition ratio v. Then, they evaluated the current-voltage characteristics of these lasers. The results were as is shown in Figure 2B. As is evident from Figure 2B, a laser having the structure of Figure 1 can have a sufficiently low operating voltage when the P type Ga 1-v Aℓ v As optical guide layer 314 has an aluminum composition ratio v of 0.6 or more. In fact, the laser whose P type Ga 1-v Aℓ v As optical guide layer had an aluminum composition ratio v of 0.6 could be driven by 2.5V or less when the input current was J to 2 kA/cm 2 . In addition, since the P type Ga 0.4 Aℓ 0.6 As optical guide layer 314 and the P type Ga 0.3 Aℓ 0.7 As cladding layer 316 had a resistivity of 0.1 to 0.2 ohm-cm, which is one order of magnitude less than the resistivity of the P type InGaAℓP layers, the operating voltage of the laser was about 0.3 volts lower than that of the conventional laser having P type InGaAℓP optical guide layer, when the input current was nearly equal to the oscillation threshold value. The inventors made semiconductor lasers, each comprising P type Ga 0.3 Aℓ 0.7 As cladding layer 316 and N type In 0.5 Aℓ 0.5 P cladding layer 310. The layer 316 had an aluminum composition ratio of 0.7 so as to have substantially the same refractive index as the cladding layer 310. As a result, a difference in effective refractive index, which was great enough to stabilize the transverse-mode oscillation, was secured. The amount of heat, which the laser radiated while operating at high current density to output high-power beam, was minimized, and the laser could generated a CW optical output of 20 mW or more, without a decrease in the differential quantum efficiency. Further, in this laser, the third cladding layer 310, which is made of N type In 0.5 Aℓ 0.5 P, functioned as a current-blocking layer, allowing a current to flow through the stripe-shaped section only. Therefore, the laser exhibited an oscillation current threshold value as small as 30 mA or less. The laser 300 of Figure 1 may be modified as shown in Figure 3 , wherein some of the components of the modified laser 350, which are identical to those shown in Figure 1 , are designated by the same reference numerals and will not be described in detail, only for the purpose of simplification of description. An optical guide layer 352 is formed on an N type cladding layer 310 having a stripe-groove section 312, filling the stripe-groove section 312. The layer 352 is relatively thick, and its top surface is flat. It is made of P type Ga 0.35 Aℓ 0.65 As, which has a thickness of 1.2 micrometers, and an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. A contact layer 354 is formed on the top surface of the optical guide layer 352. The layer 354 has a thickness of typically 1.2 micrometers and an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. A P type electrode layer 322 is formed on the contact layer 354. With the laser 350, its refraction section of the optical guide layer 352 has a refractive index greater than that of the cladding layer 310. In order to attain the numerical setup, the aluminum composition ratio of the optical guide layer 352 is set at 0.65, while that of the cladding layer 310 is 1.0 (i.e., In 0.5 (Ga 0.0 Aℓ 1.0 ) 0.5 P). The inventors manufactured a laser having the structure shown in Figure 3, and evaluated the operating characteristics of the laser thus made. The inventors found that the difference Δn in effective refractive index between the stripe-groove section 312 and layer section positioned outside the stripe-groove section 312 is sufficiently great. More specifically, the difference was: Δn > 1 × 10 -3 Owing to this great difference Δn, the laser could performed stable transverse-mode oscillation. The width W of the stripe-groove section 312 was less than 2 micrometers or less, and the laser performed a stable fundamental transverse-mode oscillation, emitting a beam having a small aspect ratio of 2.5 or less. In addition, astigmatism was greatly suppressed, to 5 micrometers or less. The inventors found that since the optical guide layer 352 was made of P type Ga 0.35 Aℓ 0.65 As, the laser 350 could be efficiently driven with a low voltage of 2.5 volts or less when the input current was J to 2 kiloamperes per square centimeter. The laser 350 could generate a CW optical output of 20 mW or more, without a decrease in the differential quantum efficiency. Further, since the cladding layer 310 was made of N type In 0.5 Aℓ 0.5 P, the laser 350 exhibited an oscillation threshold value as small as 30 milliamperes or less. A semiconductor laser device in accordance with the second embodiment of the invention is generally designated by "400" in Figure 4 (not drawn to scale). As is clearly shown in Figure 4, the laser 400 has a GaAs substrate 402 of N type. It further comprises a cladding layer 404, an optical guide layer 406, an active layer 408, and a cladding layer 410. The cladding layer 404, which is made of N type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P, is formed on the upper surface of the substrate 402, has a thickness of typically 1 micrometer, and has a donor concentration of 4 × 10 17 atoms per cubic centimeter. The optical guide layer 406, made of N type In 0.5 (Ga 0.5 Aℓ 0.5 ) 0.5 P, is formed on the cladding layer 404, has a thickness of typically of 0.5 micrometer, and has an acceptor concentration of 4 × 10 17 atoms per cubic centimeter. The active layer 408, which is made of non-doped In 0.5 Ga 0.5 P, is formed on the optical guide layer 406, and has a thickness of typically 0.06 micrometers. The cladding layer 410, which is made of P type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P, is formed on the active layer 408 and has a thickness of typically 0.8 micrometers and an acceptor concentration of 5 × 10 17 atoms per cubic centimeter. These layers 404, 406, 408, and 410 compose a double-heterojunction. The laser 400 further comprises a current-blocking layer 412 formed on the P type cladding layer 410, and a contact layer 416 formed on the current-blocking layer 412. The current-blocking layer 412 is made of N type GaAs, has a thickness of 0.7 micrometers, and a donor concentration of 2 × 10 18 atoms per cubic centimeter. The layer 412 has a stripe-shaped groove 414 (hereinafter referred to as "stripe-groove section"). The contact layer 416 fills the stripe-groove section 414 of the current-blocking layer 412. It is made of p type GaAs, has a thickness of 3 micrometers, and an acceptor concentration of 2 × 10 18 atoms per cubic centimeter. Further, a metal layer 418 is formed on the lower surface of the substrate 402, and functions as an N-side electrode. A metal layer 420 is formed on the contact layer 416, and functions as a P-side electrode. The inventors hereof made semiconductor lasers having the structure shown in Figure 4 , and evaluated the characteristics of these lasers. Since the optical guide layer 406, which contacted the active layer 408, had a refractive index greater than those of the cladding layers 404 and 410, the oscillated laser light was guided mainly through the optical guide layer 406. The density of light in the active layer 408 was about 40% less than that of the ordinary double hetero-junction which has no optical guide layers. The catastrophic optical damage (COD) of these lasers was about 1.6 times greater than that of the conventional lasers having the ordinary double heterojunction. In the second embodiment (Figure 4), light has tendency of leaking from the active layer 408 through the optical guide layer 406 and farther into the N type cladding layer 404. To minimize the loss due to the light absorption in the substrate 402, it is required that the total thickness of the layers 404 and 406 be 1.5 micrometers or more. However, the N type InGaAℓP layers 404 and 406 have resistivity which was about 10% of that of the P type InGaAℓP layer 410. Hence, even if the layers 404 and 406 are 1.5 micrometers or more thick, the series resistance of the device due to these N type layers is negligible low, or far from causing a reduction in the maximum oscillation temperature. In addition, the hand-gap difference between the active layer 408 and the optical guide layer 406 is sufficiently great to confine carriers; there is no risk that the maximum oscillation temperature of the laser 400 is lower. In fact, the lasers having the structure shown in Figure 4 exhibited the maximum oscillation temperature of 70°C or more. This value is sufficient in practice. The more light leaks from the active layer 408 into the underlying N type cladding layer 404, the less light leaks from the active layer 408 into the overlying P type cladding layer 410. Therefore, even if the active layer 408 is made thinner (for example, 0.04 micrometers), the P type InGaAℓP cladding layer 408 which is higher in resistivity than N type InGaAℓP layer need not be made thicker. Hence, even if the active layer 408 is thin, the optical output characteristic of the laser 400 can be sufficient without degrading the temperature characteristics thereof. The laser 400 shown in Figure 4 may be modified into a laser 430 shown in Figure 5, which is a transverse-mode stabilized laser having a stripe-shaped ridge. Some components of the laser 430 are identical to those shown in Figure 4 , and the same reference numerals are used therefor. The laser 430 is characterized in that a cladding layer 432, which is made of P type In 0.5 (Ga 0.3 Aℓ 0.7 ) 0.5 P, is formed on a P type cladding layer 408. The P type cladding layer 432 has a thickness of typically 0.8 micrometers and an acceptor concentration of 4 × 10 17 atoms per cubic centimeter. The layer 432 has been etched, and consisting of flat side portions and a mesa-shaped center portion 434 (hereinafter called "ridge section"). The side portions have a thickness of about 0.2 micrometers. Two current-blocking layers 436, both made of N type GaAs, are formed on the left side portion of the layer 432 and the left slope of the ridge section 434, and on the right side portion of the layer 432 and the right slope of the ridge section 434, respectively. Both current-blocking layers 436 have a thickness of 1 micrometer and a donor concentration of 2 × 10 18 atoms per cubic centimeter. A contact layer 416 and two metal layers 418 and 420 are formed in the same way as in the laser 400 (Figure 4). The applicants manufactured lasers having the structure shown in Figure 5, and evaluated the charac-. teristics of these lasers. These lasers exhibited an COD output of about 1.5 times greater than that of the conventional lasers having the ordinary double heterostructure, and produced a maximum output of 20 mw or more. Further, they exhibited the maximum oscillation temperature of 80°C or more. In the laser 430 having the stripe-shaped ridge section 434, too, the current-blocking layers 436 are formed above the N type substrate 402 so as to have the N type conductivity type, for the following reason. Within the N type GaAs layer, the diffusion length of minor carriers (holes, in this case) generated by the light absorbed therein is less than the thickness of the current blocking layer. Hence, no minority carriers move from the current-blocking layers 436, and the voltage applied on these layers remains sufficiently high, whereby the layers 436 can block a current to a desired degree. Since the oscillation current is confined in the ridge section 434 composed of P type cladding layer 432 having a resistivity higher than that of the corresponding cladding layer incorporated in the conventional lasers. The fact that the light density in the optical guide layer 406 formed on the N type cladding layer 404 reduces the light density in the active layer 408, without increasing the thickness of the P type cladding layers 432, greatly serves to improve the temperature characteristic and to produce a great output. Two regions having high carrier concentration, which have a thickness of 40 to 80 nanometers and a carrier concentration of 5 × 10 17 , may be formed between those portions of the P type InGaAℓP cladding layer 432 and contact the P type GaAs layer 416. If this is the case, the ohmic contact between the P type cladding layers 432, on the one hand, end the P type contact layer 416, on the other, can be improved. With such an arrangement, it is possible to prevent an undesirable rise of the operation voltage of the laser and a degradation of the thermal characteristic thereof. The high carrier concentration regions will be analyzed, with reference to Figures 6 , 7 and 8 . In Figure 6, numeral "450" designates an In 1-x-y Ga y Aℓ x P layer, numeral 452 denotes a high carrier concentration region formed in the InGaAℓP layer 450, and numeral 454 represents a GaAs layer. Both layers 450 and 454 are of the same conductivity type, i.e., P type. Since the laser is a light-emitting device, it is particularly required that the InGaAℓP layer 450 be lattice-matched to the GaAs layer 454. Otherwise, the layer 450 could not be epitaxially grown without impairing its crystalline quality. The values of x and y which meet this requirement are: x + y = 0.5 The results of the experiments the inventors conducted show that the current-voltage characteristic of the heterojunction between the InGaAℓP layer 450 and the GaAs layer 454 largely depends on the carrier concentration p and thickness d of the region 452. Figure 7 is a graph representing the relation which the voltage drop in the interface between the region 452 and the GaAs layer 454 has with the carrier concentration p of the region 452 when a current is supplied to the heterojunction in the density of 1 kA per square centimeter. As is evident from Figure 7, the voltage drop does not appear at all when p ≧ 5 × 10 17 atoms per cubic centimeter, and neither the increase in the operating voltage nor the increase in the amount of heat radiated makes a problem. When p > 3 × 10 19 atoms per cubic centimeter, however, defects in the high carrier concentration region 452 abruptly increases. The inventors made two lasers, both having the structure shown in Figure 5, the first one having regions 452 having carrier concentration of more than 3 × 10 19 atoms per cubic centimeter, and the second one having regions 452 having carrier concentration of 3 × 10 19 atoms per cubic centimeter or less. The reliability of the first laser was impaired after a 100-hour continuous operation at 50°C and 3 mW, while the reliability of the second laser remained sufficient up until the laser was continuously operated at 50°C and 3 mW for 1000 hours or more. The inventors found that such a voltage drop occurred when the high carrier concentration regions 452 were very thin. Figure 8 is graph showing the relationship between the voltage drop between layers 454 and 450 and the thickness d of the regions 452 having carrier concentration of 1 × 10 18 atoms per cubic centimeters. As can be understood from Figure 19, the voltage drop abruptly increases when the thickness d is less than 40 nanometers. Even when the thickness d is equal to or greater than 40 nanometers, the voltage drop gradually increases with the thickness, due to the increasing series resistance. The current-voltage characteristic of the heterojunction between the InGaAℓP layer 450 and the GaAs layer 454 is greatly influenced by the carrier concentration p and thickness d of the regions 452. To have good current-voltage characteristic, these regions 452 must have a thickness of 40 nanometers or more when its carrier concentration is 5 × 10 17 atoms per cubic centimeter or more. The regions 452 must have carrier concentration ranging from 5 × 10 17 atoms per cubic centimeter to 3 × 10 19 atoms per cubic centimeter; if the carrier concentration is greater than 3 × 10 19 atoms per cubic centimeter, the laser cannot operate reliably for a long period of time. Still further, the present invention is not limited to the embodiments described above. Various changes and modifications may be made without departing the scope of the invention.

Claims: A semiconductor laser device (400, 430) comprising a semiconductor substrate (402), a first cladding layer (404) of the same conductivity type as the substrate, said first cladding layer (404) being disposed on said substrate, a semiconductor active layer (408) on said first cladding layer, a second cladding layer (410, 432) arranged on said active layer (408) and being different in conductivity type from said first cladding layer (404), said first and second cladding layers (404, 410, 432) comprising indium-gallium-aluminium-phosphorous (InGaAlP), said active layer and said first and second cladding layers constituting a double-heterostructure, light-confining means (414, 434) arranged on said second cladding layer (432) for defining, in said second cladding layer, an elongate light-confining region for guiding light oscillations to propagate along the region, and semiconductor buried layers (412, 436) contacting side walls of said light-confining means, characterized in that an optical guide layer (406) is provided between said first cladding layer (404) and said active layer (408), said optical guide layer (406) being of the same conductivity type as said first cladding layer (404), and having a lower refractive index than that of said active layer (408) and greater than that of said first cladding layer (404); said optical guide layer (406) comprises indium-gallium-aluminium-phosphorus (InGaAlP), and in that said first cladding layer (404) and said optical guide layer (406) have N-type conductivity. A laser device according to claim 1, further characterized in that said second cladding layer (410) is less in thickness than said first cladding layer. A laser device according to any preceding claim, further characterized in that said light-confining means comprises a semiconductor layer (412) arranged on said second cladding layer having a conductivity type opposite to that of said second cladding layer and having a groove (414) having an elongate trapezoidal profile. A laser device according to any preceding claim, further characterized in that said second cladding layer has a ridge section (434) having an elongate trapezoidal profile, and said light-confining means comprises a semiconductor layer (436) contacting both sides of said ridge section and having a conductivity type opposite to that of said second cladding layer. A semiconductor laser device (300) comprising a semiconductor substrate (302), a first semiconductor cladding layer (304) on said substrate, said first cladding layer comprising indium-gallium-aluminium-phosphorous (InGaAlP), a semiconductor active layer (306) on said first cladding layer, a second semiconductor cladding layer (308) arranged on said active layer and being different in conductivity type from said first cladding layer, said second cladding layer comprising indium-gallium-aluminium-phosphorous (InGaAlP), a light confinement means (310, 312) arranged on or above said second cladding layer for defining an elongate light-confining region for guiding light oscillations to propagate along the region, and semiconductor buried layers (310) contacting side walls of said light-confinement means, characterized in that said second cladding layer (308) has a refractive index greater than that of said buried layers (310), and in that said light confinement means includes a groove section (312) defined on said second cladding layer (308), a said semiconductor buried layer (310) which is made from indium-aluminium-phosphorous (InAlP) being same in conductivity type as said first cladding layer (304), and is contacted with the side walls of said groove section (312), and a high-refractive-index layer (314, 352) overlying said groove section (312) said high-refractive-index layer being made from gallium-aluminium-arsenic (GaAlAs) and having a greater refractive index than that of said buried layers, and in that said second cladding layer (308) and said high-refractive-index layer (314, 352) have P-type conductivity. A light emitting device according to claim 5, characterized in further comprising a further semiconductor cladding layer (316) above said second semiconductor cladding layer (308), said semiconductor high-refractive-index layer (314) being formed between said fourth semiconductor cladding layer and said second semiconductor cladding layer, and said semiconductor high-refractive-index layer (314) having a refractive index higher than that of said second cladding layer. A light emitting device according to claim 5, further characterized in that said buried layer serves as a current-blocking layer (310). A light emitting device according to claim 6, further characterized in that said buried layer (310) has a conductivity type opposite to that of said second cladding layer (308) and functions as a current-blocking layer.

Inventors: HATAKOSHI GENICHI, ISHIKAWA MASAYUKI, MOTEGI NAWOTO, OKAJIMA MASAKI, UEMATSU YUTAKA, HATAKOSHI, GENICHI, ISHIKAWA, MASAYUKI, MOTEGI, NAWOTO, OKAJIMA, MASAKI, UEMATSU, YUTAKA, HATAKOSHI, GENICHI, C/O INTELLECTUAL PROPERTY DIV., ISHIKAWA, MASAYUKI, C/O INTELLECTUAL PROPERTY DIV., MOTEGI, NAWOTO, C/O INTELLECTUAL PROPERTY DIV., OKAJIMA, MASAKI, C/O INTELLECTUAL PROPERTY DIV., UEMATSU, YUTAKA, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01S5
IPC codes: H01S5