Semiconductor laser elements and method for the production thereof

Abstract: A semiconductor laser element having a GaAs substrate (1) formed thereon with an active layer (4) of a strained quantom well construction provided with an In x Ga 1-x As strained quantom well layer and a GaAs barrier layer and clad layers (5a, 3a) formed above and below said active layer through epitaxial growth. A lattice mismatching rate of the clad layer with respect to the substrate is less than 10⁻³.

Description: Field of the Invention The present invention relates to semiconductor laser elements of a strained quantum well type and a method for the production of the semiconductor laser elements. Description of Prior Art A semiconductor laser element, formed on a GaAs substrate with an active layer of a strained quantum well construction provided with an In x Ga 1-x As (x=0.0 to 0.5) strained quantum well layer and a GaAs barrier layer, is expected to act as a light source of wavelength of 0.9 to 1.1 µm. In the case of semiconductor laser elements, as a clad layer for confining carrier and light in an active layer, a semiconductor should be used which has a permeability with respect to light at the oscillation wavelength, is smaller in refractive index than that of the active layer (or a layer for confining light near the active layer), and is large in energy gap. In the conventional semiconductor laser element of an In x Ga 1-x As strained quantum well type, Al w Ga 1-w As of w > 0.2 is used as a clad layer. Fig. 8 shows a conventional semiconductor laser element of an In x Ga 1-x Ga strained quantum well type. In Fig. 8, an n-type GaAs substrate 1 having approximately 350 µm of thickness is formed thereon with an n-type GaAs buffer layer 2 having approximately 0.5 µm of thickness and an n-type Al 0.3 Ga 0.7 As clad layer 3 having approximately 1.5 µm of thickness, the layers being formed by epitaxial growth such as an MBE method or MOCVD method. Further, in Fig. 8, the n-type GaAs substrate 1 is formed at a predetermined position with an essential portion 4 including an active layer provided with an In 0.35 Ga 0.65 As strained quantum well layer, a GaAs barrier layer, etc., and a light confining layer, a p-type Al 0.3 Ga 0.7 As clad layer 5 having 1.5 µm of thickness and a p-type contact layer 6 having 0.2 µm of thickness. The detail of the essential part 4 is clearly shown in Fig. 9. In Fig. 9, upper and lower GaAs light confining layers 7 have 150 nm (1500 Å) of thickness, a GaAs barrier layer 8 between these light confining layers 7 has 10nm (100Å) of thickness and an In 0.35 Ga 0.65 As strained quantum well layer 9 has 4nm (40Å) of thickness. A double hetero construction formed on the GaAs substrate 1 is formed with a current restricting layer and an electrode and is applied with microworking such as element separation to prepare a laser chip. One example of prior art has been described above. The semiconductor laser element has various modifications such as a ratio of composition of mixed crystals, the number of layers of strained quantum wells, thickness of the layers, etc. As the light confining layer, there is well known a GRINSHC construction using AlGaAs in which the ratio of the Al composition is parabolic. Next, the process for working the bridge waveguide path into a strained quantum well type semiconductor laser element will be described hereinafter with reference to Figs. 10(a) to 10(d). In the process shown in Fig. 10(a), a resist 10 is patterned on a p-type GaAs contact layer 6 e.g. by photolithgraphy. In the process shown in Fig. 10(b), the resist 10 is used as a mask. The GaAs contact layer 6 of a double hetero construction and the upper clad layer 5 are subjected to etching till the depth of the etching reaches a point about 0.2µm from the active layer 4. As etching liquids in etching AlGaAs/GaAs type semiconductor, there can be used a mixed solution of sulfuric acid and hydrogen peroxide, a mixed solution of tartaric acid and hydrogen peroxide, a mixed solution of ammonia and hydrogen peroxide or dry etching such as chlorine (for example, reactive ion beam etching). In the process of Fig. 10(c), a method such as sputtering is used to form the upper surface with an etching mask 11 in the form of a film of e.g. SiO 2 , SiN, etc. In the process of Fig. 10(d), a method such as photolithgraphy is used to form a stripe-like opening 12 in the mask 11. In the thereafter processes, electrodes are formed on both upper and lower surfaces of the laminate construction, and microworking such as element separation is applied thereto. (Task to be solved by the Invention) The technical tasks with regard to the aforementioned semiconductor laser element include oxidation of Al, compressive stress from a substrate lattice and defective etching process, which twill be described hereinafter. (Oxidation of Al) Main uses of a laser having 0.9 - 1.1 µm of wavelength are excitation of a fiber in which rare earth such as Er is doped, or a visual light source in combination with SHG. In case of these semiconductor products, a prolonged service life is required at high output in excess of scores of mW. However, the conventional strained quantum well type semiconductor laser cannot fulfill such a requirement as described above since it uses the aforementioned AlGaAs as a clad layer, the reason of which is as follows. Among elements (for example, In, Al, Ga, As, P, Sb, etc.) constituting a compound semiconductor, Al is an element which tends to be oxidized most easily. For example, when a regrowth surface is made by an embedding growth method during fabrication of a laser chip, oxidation of Al tends to occur. Such an Al oxidation results in the occurrence of non-light emitting centers and degradation of crystallization, failing to obtain a semiconductor laser excellent in laser characteristics. Furthermore, in the case where a plait surface is used as a laser end, oxidation of the end progresses during use of the laser to bring forth a lowering of its refractive index and an increase of absorption, so deteriorating the laser characteristics. The semiconductor laser increases its temperature particularly during laser operation at high current, so that the progress of oxidation is sped up. Methods have been proposed to remove oxygen and water content in the process for the fabrication of a laser, in order to suppress oxidation of Al. However, this requires much labor and a device for protection of the end of the laser should be made. Because of this, in the case of the prior art, it is not possible to easily obtain a laser having a long service life when used under high output conditions. (Compressive stress from substrate lattice) In a double hetero construction for laser diode using a conventional IN x Ga 1-x As/GaAS strained quantum well construction as an active layer, semiconductors having a larger lattice constant than that of the GaAs substrate are laminated to the substrate. That is, the lattice constant of the GaAs substrate is 0.565 nm (5.65 Å) whereas Al w Ga 1-w As has a large lattice constant, 0.14 w% and In x Ga 1-x As has a large lattice constant, 7.3 x%. Incidentally, assume that the compositions of In x Ga 1-x As and Al w Ga 1-w As are x=0.35 and w=0.3, respectively, their lattice non-matching rates with respect to GaAs are +2.65 % and +0.04 %, respectively. In this case, the Al w Ga 1-w As layer is small in the lattice non-matching rate with respect to GaAs but is thick, 3 µm (about ten thousand atom layers), and the In x Ga 1-x As layer is thin, 12 nm (120 Å) (about 40 atom layers) but the lattice non-matching rate with respect to GaAs is large and therefore, a laminate of the Al w Ga 1-w Ga layer and In x Ga 1-x Ga layer receives a compressive stress from the substrate. This compressive stress causes the occurrence of transition and slip in the active layer of the strained quantum well construction during high current and laser operation at high power. As a result, the semiconductor laser element tends to give rise to DLD (dark line defect), lowering the laser oscillation life. (Defective etching process) In order to control a lateral mode, the semiconductor laser element needs to employ a waveguide mechanism of either gain waveguide type or refractive index waveguide type. With respect to these waveguide types, various laser element constructions have been proposed. In the case of the strained quantum well type semiconductor laser element, a lattice mismatching is present between a well and a barrier, and therefore, an active layer is subjected to mesa etching to form a stripe configuration, after which a layer is embedded therein and grown. However, in the laser element such as BH construction, a defect such as transition tends to occur in the embedded growth layer near the active layer, making it difficult to obtain a semiconductor laser element having a long life. On the other hand, in the ridge waveguide laser element, the element can be produced leaving an active layer flat, which is therefore one of the laser elements suitable for the strained quantum well type. This ridge waveguide laser element can be produced by a method illustrated in Fig. 10. Among the steps shown in Figs. 10(a) to 10(b), the mesa forming process in Fig. 10(b) is important, in order to control the lateral mode, to accurately control the distance between the mesa bottom and the active layer and, in order not to produce scattering of light, to finish the mesa bottom flat. However, severe control is required with respect to concentration of temperature of etching liquid in accurately controlling the depth of etching, and similarly, concentration and temperature of etching liquid should be maintained uniformly so that the depth of etching is not uneven within the etching surface even when etching is carried out. Therefore, the technical difficulty in the important process increases. As a result, it is difficult to produce a semiconductor laser element having an excellent laser characteristic with good reproduceability, so lowering the yield of good products. In view of the aforesaid technical task, the present invention provides a semiconductor laser element which exhibits an excellent laser characteristic for a long period of time and a method for the production of the semiconductor laser element. SUMMARY OF THE INVENTION The Journal of Applied Physics, Vol. 57 No. 1, January 1985, at pages 33 to 38, discloses a laser element of the type shown in Figures 8 and 9, namely comprising a GaAs substrate, an active layer of In x Ga 1-x AS/GaAs strained quantum well construction, and clad layers of Al w Ga 1-w As disposed below and above the active layer. FR-A-2628575 discloses a laser having an bulk GaInAsP active layer and GaInP clad layers. JP-A-63019824 discloses a laser in which InGaP and GaAs layers act as etching stop layers, each with respect to the other, when selected etching solutions are used. In accordance with the present invention, there is provided a semiconductor laser element, comprising a GaAs substrate formed thereon with an active layer of a strained quantum well construction, and clad layers disposed below and above said active layer, said layers being formed by vapour phase epitaxial growth and said active layer comprising an In x Ga 1-x As, 1>x>0, strained quantum well layer and at least a GaAs barrier layer, characterised in that each said clad layer is formed of In z Ga 1-z As y P 1-y , 1>z>0, or InGaP. In a semiconductor laser element, in order that the clad layers above and below the active layer will sufficiently confine light and carriers in the active layer, it is necessary for the clad layers to be lattice matched with the GaAs substrate, the refractive index of the clad layers (with respect to light of 0.9 to 1.1µm wavelength) is comparable to that of Al w Ga 1-w As (w = 0.5-0.6) and that the energy gap of the clad layers is equal to or larger than Al w Ga 1-w As (w = 0.5-0.6). The choice of In z Ga 1-z As y P 1-y (1>z>0) or InGaP, for the clad layers, fulfils the above conditions. Further, the clad layers do not contain Al, and avoid the problems caused by Al oxidation. Preferably Al is not used in any of the layers of the semiconductor laser element of this invention. The clad layers can be made with a lattice mismatching rate, with respect to the substrate, of less than 10 -3 . Preferably a stress relieving layer of In 1-z Ga z P is interposed between the active layer and each clad layer. Preferably z≥0.51. The lattice constant a of In 1-z Ga z P is given by the following formula (1) according to the Vegad rule. When z=0.51, lattice matching with GaAs is obtained, and when z > 0.51, the lattice constant is smaller than that of GaAs. a = 5.869 - 0.42 x z It is desired that the In 1-z Ga z P stress relieving layer is designed so that an average lattice constant a of an epitaxial layer given by the following formula (2) is equal to the lattice constant of the GaAs substrate. a = Σa i t i / Σt i a i : lattice constant of each epitaxial layer t i : thickness of each epitaxial layer Since the thus designed epitaxial layer is lattice-matched to the substrate, stress in an interface between the substrate and the epitaxial layer hardly occurs, and occurrence of transition at the interface can be suppressed. In a preferred embodiment of double hetero construction of laser having an active layer of a strained quantum well construction with a ridge waveguide type, the upper clad layer is formed of InGaP, and an GaAs etching stop layer is inserted into the InGaP clad layer at a controlled depth. Therefore, when such a double hetero construction is subjected to selective etching processing, it is possible to accurately control the distance between the mesa bottom and the active layer. InGaP is lattice-matched to GaAs and can be subjected to selective etching at different speeds. This invention also provides a method for the production of a semiconductor laser element which comprises a GaAs substrate formed thereon with an active layer of a strained quantum well construction, clad layers disposed below and above said active layer, and a GaAs contact layer on the upper clad layer, said layers being formed by epitaxial growth, and said active layer comprising an In x Ga 1-x As strained quantum well layer and at least one GaAs barrier layer, characterised in that the upper clad layer comprises InGaP and has a GaAs etching stop layer inserted therein, and the method comprises the steps of etching the GaAs contact layer to the InGaP clad layer using an etching liquid of sulphuric acid mixed with hydrogen peroxide, tartaric acid mixed with hydrogen peroxide, or ammonia mixed with hydrogen peroxide, then etching the InGaP clad layer to the etching stop layer using an etching liquid comprising hydrochloric acid but no hydrogen peroxide. In the case of one of said etching liquids containing hydrogen peroxide, the etching speed varies depending on concentration and temperature. However, the etching speed with respect to GaAs is normally 20 times or more the etching speed with respect to InGaP. In the case of the other etching liquid not containing hydrogen peroxide, the etching speed with respect to GaAs is normally one ten-thousandth of the etching speed with respect to InGaP. Accordingly, in the mesa forming step of the above-defined method according to this invention, first the aforementioned mixed etching liquid containing hydrogen peroxide can be used to selectively etch the GaAs layer alone. Subsequently, the other etching liquid (free of hydrogen peroxide) can be used to selectively etch the InGaP clad layer up to the GaAs etching stop layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a double hetero type layer construction as a first embodiment of a semiconductor laser element according to the present invention; Fig. 2 is an enlarged view of essential parts of the semiconductor laser shown in Fig. 1; Fig. 3 is a sectional view showing an embedded type layer construction as a second embodiment of a semiconductor laser element according to the present invention; Fig. 4 is a sectional view showing a double hetero type layer construction as a third embodiment of a semiconductor laser element according to the present invention; Fig. 5 is a sectional view showing a ridge waveguide layer construction as a fourth embodiment of a semiconductor laser element according to the present invention; Fig. 6 is a sectional view showing a double hetero type layer construction as a fifth embodiment of a semiconductor laser element according to the present invention; Fig. 7 is a sectional view showing an SBA type layer construction as a sixth embodiment of a semiconductor laser element according to the present invention; Fig. 8 is a sectional view showing a double hetero type layer construction as a conventional semiconductor laser element; Fig. 9 is an enlarged view of essential parts of a semiconductor laser element shown in Fig. 8; and Fig. 10(a) to 10(b) are explanatory views showing the steps of preparing a conventional double hetero construction DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for the production of a semiconductor laser element according to the present invention will be described in connection with the embodiments shown. Figs. 1 and 2 show a first embodiment of a semiconducto: laser element according to the present invention. A semiconductor laser element of the first embodiment has a structure which will be described below. In Fig. 1, an n-type GaAs substrate 1 having about 350 µm of thickness is formed thereon with a n-type GaAs buffer layer 2 having about 0.5 µm of thickness and an n-type In 0.49 Ga 0.51 As clad layer 3a having about 1.5 µm of thicknes through epitaxial growth such as an MBE method or MOCVD method and further at a predetermined position thereof with a main portion 4 including an active layer provided with an In 0.35 Ga 0.65 As strained quantum well layer, a GaAs barrier layer and the like and a light confining layer, a p-type In 0.49 Ga 0.51 As clad layer 5a having about 1.5 µm of thickness and a p-type contact layer 6 having about 0.2 µm of thickness. With the structure of the main portion 4 clearly shown in Fig. 2, two upper and lower GaAs light confining layers 7 have 150 nm (1500 Å) of thickness, a GaAs barrier layer 8 between the light confining layers 7 has 10 nm (100 Å) of thickness, and an In 0.35 Ga 0.65 As distortion quantum well layer 9 has 4 nm (40 Å) of thickness. The double hetero construction uniformly formed on the GaAs substrate 1 is formed with a current restricting layer and an electrode as is known, and is applied with microworking such as element separation to prepare a laser chip. The semiconductor laser element of the present invention illustrated in Figs. 1 and 2 is different from that shown in Figs. 8 and 10 in that one clad layer 3a is formed of the p-type In 0.49 Ga 0.51 As and the other clad layer 5a is formed of the n-type In 0.49 Ga 0.51 As. The front electrode type semiconductor laser element having the construction as described above has 140 A/cm 2 of oscillation threshold current density during the pulse drive under room temperature. This value is equal to that in which both clad layers are formed of Al 0.50 Ga 0.5 As. Fig. 3 shows a second embodiment of a semiconductor laser element according to the present invention, and an embedded type semiconductor laser element of the second embodiment has the structure which will be described below. In Fig. 3, an n-type GaAs substrate 11 is formed thereon with a main portion 4 including an n-type GaAs buffer layer 12, an n-type InGaP clad layer 13, an active layer 14 of a strained quantum well construction and including a light confining layer, a p/n junction comprising a p-type InGaP blocking layer 15 and an n-type InGaP blocking layer 16, a p-type InGaP clad layer 17 and a p-type GaAs cap layer 18, all formed by the aforementioned epitaxial growth and etching, and further on the upper side thereof with a p-type electrode 19 of AuZn type, the n-type GaAs substrate 11 being formed on the under surface thereof with an n-type electrode 20 of AuGeNi type. In Fig. 3, the main portion 14 including an active layer of distortion quantum well construction and a light confining layer has 1.5 µm of width and 600 µm of length of resonance unit, and a cleavage surface thereof is in the form of a mirror without a protective film. When the semiconductor laser element of the second embodiment shown in Fig. 3 is operated under room temperature, CW oscillation is obtained at a current of 7 mA, and output of 100 mW is obtained at a current of 600 mA. After laser drive 50 mW and 200 hours, the I-L characteristic remains unchanged. A long service life was maintained under the high output state. While in the second embodiment, InGaP has been used as a clad layer, it is to be noted that even if InGaAsP having 1.5 eV or more of energy gap is used as a clad layer, light and carrier can be confined. Since InGaP as well as InGaAsP contain no Al, oxidation of the regrowth interface and cleavage surface is unlikely to occur and accordingly a laser of long service life can be prepared. With respect to InGaAsP, lattice matching with the GaAs substrate may be provided to a degree that transition does not occur. In the second embodiment, the film thickness of the layers, composition, and the number of strained quantum well layers are not limited to the aforementioned contents and the illustrated examples. In the second embodiment, the GRIN construction in which the composition of InGaAsP is stepwisely changed is sometimes employed as a light confining layer. In case of the semiconductor laser element of the second embodiment illustrated in Fig. 3, the cleavage surface is sometimes subjected to coating for the purpose of improving low threshold, efficiency, and output. Fig. 4 shows a third embodiment of a semiconductor laser element according to the present invention. The semiconductor laser element of the third embodiment has the structure described below. In Fig. 4, an n-type GaAs substrate 31 having 350 µm of thickness is formed thereon with an n-type GaAs buffer layer 32 having 0.5 µm of thickness, an n-type In 0.49 Ga 0.51 As clad layer 33 having 1.5 µm of thickness, a p-type In 0.47 Ga 0.53 P stress relieving layer 41 having 0.12 µm of thickness, a main portion 40 including an active layer and a light confining layer, an n-type In 0.47 Ga 0.53 P stress relieving layer 42 having 0.12 µm of thickness, a p-type In 0.49 Ga 0.51 As clad layer 35 having 0.5 µm of thickness, and a p-type contact layer 36 having 0.2 µm of thickness, these layers being formed by the aforementioned epitaxial growth method. The structure of the main portion 40 in Fig. 4 is the same as that mentioned in Fig. 2. Parts constituting the main portion 40 are indicated by reference numerals with ( ) in Fig. 2. That is, in Fig. 2 also showing the main portion 40 of Fig. 4, two GaAs light confining layers 37, 38 have 150 nm (1500 Å) of thickness, each GaAs barrier layer between these light confining layers 37, 38 has 10 nm (100 Å) of thickness, and each In 0.35 Ga 0.65 As strained quantum well layer 39 has 4 nm (40 Å) of thickness. An average lattice constant between an In 0.35 Ga 0.65 As strained quantum well layer 39, having 4 nm (40 Å) thickness and +2.5 % of lattice mismatching rate, and In 0.47 Ga 0.53 P stress relieving layers 41 and 42, having 120 nm (200 Å) thickness and -0.13 % of lattice mismatching rate, is substantially equal to that of the GaAs substrate 31. Let Δ a/ao be the lattice mismatching rate of the epitaxial growth layer with respect to the substrate 31, this value is very small as shown by the formula (3) below. Δa/ a o = 1.2 x 10 -6 As a comparative example 1, as shown in Figs. 8 and 9, on an n-type GaAs substrate 1 having 350 µm of thickness were laminated, in order, an n-type GaAs buffer layer 2 having 0.5 µm of thickness, an n-type Al 0.3 Ga 0.7 As clad layer 3 having 1.5 µm of thickness, a main portion 4 including an active layer and a light confining layer, a p-type Al 0.3 Ga 0.7 As clad layer 5 having 1.5 µm of thickness, and a p-type contact layer having 0.2 µm of thickness) to prepare a predetermined specimen. In this case, the GaAs light confining layer 7 of the main portion 4 has 150 nm (1500 Å)of thickness, the GaAs barrier layer 8 between the light confining layers 7 has 10 nm (100 Å) of thickness, and the In 0.35 Ga 0.65 As strained quantum well layer 9 has 4 nm (40 Å) of thickness. Each double hetero construction of the third embodiment and comparative example 1 was worked into a complete electrode type laser element having 300 µm of cavity length and they were pulse oscillated under room temperature. The oscillation threshold current density was measured to obtain results given in Table 1 below. Fig. 5 shows a fourth embodiment of a semiconductor laser element according to the present invention. The semiconductor laser element of the fourth embodiment shown in Fig. 5 is provided with a SiO 2 insulating film 43 and alloy electrodes 44 and 45 while, in contrast to the third embodiment, the element is prepared in the form of ridge wave guide type strained quantum well type. In Fig. 5, reference numeral 46 denotes a flow of current. In Comparative Example 2 to be compared with the fourth embodiment, the element of the Comparative Example 1 is formed into the strained quantum well type of the ridge wave guide type similarly to that of the fourth embodiment. With respect to the elements of the fourth embodiment and Comparative Example 2, the threshold current density and life were measured to obtain the result shown in Table 2 below. The life in Table 2 was determined according to 10 % (30°C) rise of drive current by APC of 50mW. It is found from the above measured results that the threshold current and life of the strained quantum well type semiconductor laser element according to the present invention were remarkably improved. In case of the third and fourth embodiments, as the clad layer, Al w Ga 1-w As of w > 0.3 is sometimes used. In the third and fourth embodiment, it is desired that the average lattice constant of the epitaxial layer is equal to that of the GaAs substrate. However, an allowable range of the mismatching rate is approximately |Δa/ao| < 1.2 x 10 -4 . Thickness of layers, composition and number of strained quantum well layers are not limited thereto but the GRIN construction is sometimes employed as a light confining layer. Fig. 6 shows a fifth embodiment of a semiconductor laser element according to the present invention. The semiconductor laser element of the fifth embodiment has the structure described below. In Fig. 6, an n-type GaAs substrate 51 having 350 µm of thickness is formed thereon with an n-type GaAs buffer layer 52 having 0.5 µm of thickness, an n-type In 0.49 Ga 0.51 As clad layer 53 having 1.5 µm of thickness, a main portion 60 including an active layer and a light confining layer, a p-type In 0.51 Ga 0.49 P clad layer 55a having 0.2 µm of thickness and a p-type In 0.51 Ga 0.49 P clad layer 55b having 1.3 µm of thickness, a p-type GaAs etching stop layer 61 interposed between both the p-type clad layers 55a and 55b, and a p-type GaAs contact layer 56 having 0.2 µm of thickness, the layer being formed by the aforementioned epitaxial growth means. The structure of the main portion 60 in Fig. 6 is the same as that mentioned in Fig. 2. Parts constituting the main portion 60 are indicated by reference numerals with [ ] in Fig. 2. That is, in Fig. 2 also showing the main portion 60 in Fig. 6, two GaAs light confining layers 57, 58 have 150 nm (1500 Å) of thickness, each GaAs barrier layer between the light confining layers 57, 58 has 10 nm has (100 Å) of thickness, and each In 0.35 Ga 0.65 As distortion quantum well layer 59 has 4 nm (40 Å). It is desired that the thickness of the etching stop layer 61 be less than 0.2 µm so as not to impair a light confining effect caused by the clad layer. As a modified form of the fifth embodiment, in the same construction as that of the fifth embodiment, only the thickness of the etching stop layer 61 is set to 5 nm (50 Å), and an absorption end of the etching stop layer 61 is sometimes shifted to a short wave side by the quantum effect. In the method for production of a semiconductor laser element according to the present invention, the step of mesaforming a semiconductor laser element of the fifth embodiment is executed as described hereinafter. First, when a GaAs contact layer 56 is subjected to etching, a mixed liquid of sulfuric acid and hydrogen peroxide is used. The etching solution of the mixed solution varies with the conditions such as temperature, mixing ratio, stirring state of the solution. However, since the etching speed of the GaAs by the mixed solution is 20 times or more of InGaP, such a mixed solution is used whereby only the GaAs can be etched. Next, when an InGaP clad layer 55b is subjected to etching till reaching the etching stop layer 51, hydrochloric acid of 36 % (weight percentage) is used. The etching speed of InGaP by hydrochloric acid at 20°C is approximately 0.1 µm/sec., and an InGaP layer having 1.3 µm of thickness can be etched in 13 + 2 seconds using the hydrochloric acid. On the other hand, the etching speed of GaAs by hydrochloric acid is so small that it cannot be measured, such as 1 nm/sec (0.1 Å/sec.). For example, in case of GaAs layer of 5 nm (50 Å), even if the etching time of 10 minutes has passed, it cannot be etched. In this manner, by carrying out etching processing for 15 seconds using hydrochloric acid, the etching depth of the InGaP clad layer 55b is less than a single atomic layer ( ) (3 Å). Furthermore, if the aforementioned double hetero construction is prepared by a precise crystal growth method such as MCVD method and MBE method, a very flat film is obtained. Incidentally, in the case where a device size is about 300 µm square, a flatness of atomic layer level can be obtained, and an error in film thickness is less than 1%. Accordingly, in the mesa forming step of the present method, if such a double hetero construction is used, the depth of mesa can be designed within the range of an error less than 1%, and a ridge waveguide laser element of a strained quantum well construction having a flatness in which an etching bottom is at a level of an atomic layer can be easily prepared. Double hetero constructions of the aforementioned fifth embodiment, the modified form and the prior art were prepared three times by use of the MOCVD method, and 100 ridge waveguide laser elements having a strained quantum well construction were prepared from wafers thereof. With respect to the semiconductor laser elements of the fifth embodiment, the modified example and prior art, the oscillation threshold currents in 0.93 ± 0.101 µm of oscillation wavelength were measured to obtain the results shown in Tables 3-1, 3-2 and 3-3 below. As will be apparent from the above tables, the double hetero construction according to the embodiments of the present invention is superior in the average value of the threshold current to that of prior art. Particularly, in the modified example of the fifth embodiment, since the absorption of the etching stop layer at 0.93 µm of oscillation wavelength is small, the threshold value is smaller than that of the fifth embodiment. Moreover, the double hetero construction in the embodiment of the present invention shows favorable results as compared with prior art with respect to irregularities of thresholds, between batches as well as chips. While in the above-described embodiment, the ridge waveguide laser has been employed, it is to be noted that as a sixth embodiment, an etching stop layer can be provided also in the laser construction of SAB type illustrated in Fig. 7. In Fig. 7, an n-type GaAs substrate 71 is formed thereon with an n-type InGaP clad layer 73, an etching stop layer 78, a p-type InGaP blocking layer 77, an active layer 74, an n-type InGaP clad layer 75 and an n-type GaAs contact layer 76. In the etching step for preparing a SAB type semiconductor laser element of Fig. 7, as an etching liquid for selectively etching InGaP, a mixed solution of hydrochloric acid and phosphoric acid or hydrochloric acid and acetic acid is sometimes used. The semiconductor laser element according to the present invention has the following effects. According to a first effect, in a semiconductor laser element having a GaAs substrate formed thereon with an active layer of a strained quantum well construction provided with an In x Ga 1-x As strained quantum well layer and a GaAs barrier layer, and clad layers arrange up and down of said active layer through epitaxial growth means, said clad layer is formed of In z Ga 1-z As y P 1-y and therefore strained quantum well type semiconductor laser element having a large output and a long service life can be obtained. In this case, if the lattice mismatching rate of the clad layer with respect to the substrate is less than 10 -3 , a more favorable effect is obtained. According to a further effect, in the aforementioned double hetero construction, since a stress relieving layer formed of In 1-z Ga z P is provided adjacent to the active layer, a strained quantum well type semiconductor laser element having a long service life can be obtained in view of the foregoing. According to another effect, in a semiconductor laser element having a GaAs substrate formed thereon, by epitaxial growth, with an active layer of a strained quantum well construction comprising an In x Ga 1-x As strained quantum well layer and a GaAs barrier layer and clad layers arranged above and below said active layer, said clad layers are formed of InGaP, and a GaAs etching stop layer is inserted into said upper clad layer, and therefore, such a double hetero construction is applied with etching processing whereby the distance between the mesa bottom and the active layer can be accurately controlled, and as a semiconductor, an irregularity of threshold current is reduced and product yield is enhanced. Also in this case, if the lattice mismatching rate of the clad layer with respect to the substrate is less than 10 -3 a more favorable effect is obtained. Moreover, according to the method for the production of the aforementioned semiconductor laser element according to the present invention, the method comprises the steps of etching the GaAs cap layer to the InGaP clad layer with an etching liquid containing either sulfuric acid, tartaric acid or ammonia and hydrogen peroxide and etching the InGaP layer to the GaAs etching stop layer with an etching liquid containing hydrochloric acid but not containing hydrogen peroxide. Therefore, the first etching liquid containing hydrogen peroxide can be used to selectively etch only the GaAs cap layer, and then, said other etching liquid not containing hydrogen peroxide is used to selectively etch the InGaP clad layer up to the GaAs etching stop layer whereby a semiconductor having an excellent characteristic can be easily prepared.

Claims: A semiconductor laser element, comprising a GaAs substrate (1) formed thereon with an active layer (4) of a strained quantum well construction, and clad layers (3a,5a) disposed below and above said active layer (4), said layers being formed by vapour phase epitaxial growth and said active layer (4) comprising an In x Ga 1-x As, 1>x>0, strained quantum well layer (9) and at least a GaAs barrier layer (8), characterised in that each said clad layer (3a,5a) is formed of In z Ga 1-z As y P 1-y , 1>z>0, or InGaP. A semiconductor laser element according to claim 1, characterised in that a stress relieving layer (41,42) of In 1-z Ga z P is interposed between the active layer (4) and each clad layer (33,35). A semiconductor laser element according to claim 1, characterised in that the upper clad layer (55a,55b) is formed of InGaP and has a GaAs etching stop layer (61) inserted therein. A semiconductor laser element according to any preceding claim, characterised in that a lattice mismatching rate of the clad layers with respect to the substrate (1) is less than 10 -3 . A method for the production of a semiconductor laser element which comprises a GaAs substrate formed thereon with an active (60) layer of a strained quantum well construction, clad layers (53,55a and 55b) disposed below and above said active layer (60), and a GaAs contact layer (56) on the upper clad layer, said layers being formed by epitaxial growth, and said active layer (60) comprising an In x Ga 1-x As, 1>x>0, strained quantum well layer (59) and at least one GaAs barrier layer, characterised in that the upper clad layer (55a,55b) comprises InGaP and has a GaAs etching stop layer (61) inserted therein, and the method comprises the steps of etching the GaAs contact layer (56) to the InGaP clad layer (55a) using an etching liquid of sulphuric acid mixed with hydrogen peroxide, tartaric acid mixed with hydrogen peroxide, or ammonia mixed with hydrogen peroxide, then etching the InGaP clad layer (55a) to the etching stop layer (61) using an etching liquid comprising hydrochloric acid but no hydrogen peroxide.

Inventors: IJICHI TETURO, OKAMOTO HIROSHI, IJICHI, TETURO, OKAMOTO, HIROSHI
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1990
Main classification: H01S3
IPC codes: H01S5