Surface emission type semicondutor laser

Abstract: A surface emission type semiconductor laser includes a plurality of semiconductor layers defining at least one resonator in a direction perpendicular to the semiconductor substrate (102) of the laser, at least a cladding layer (107) in the semiconductor layers being formed into at least one column-like semiconductor layers extending in a direction perpendicular to the semiconductor substrate, and a II-VI group compound semiconductor epitaxial layer (109) buried around the column-like semiconductor layer. The semiconductor laser also includes an exit-side eletrode (110) which is formed in contact with the contact layer (108) in the column-like semiconductor layers and has an opening (115) used to assure the exit port. The opening is formed in the exit-side electrode at a position including the geometric center of the contact layer and ranged between 10% and 90% of the surface area of the contact layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention : The present invention relates to a surface emission type semiconductor laser adapted to emit a laser beam in a direction perpendicular to the substrate thereof. Description of the Related Art : A surface emission type laser including a resonator disposed in a direction perpendicular to the substrate thereof is disclosed in Lectures of the 50-th Meeting of Applied Physics in Japan (1989), Vol. 3, p. 909, 29a-ZG-7. In accordance with the prior art, as shown in Fig. 11, there is first provided an n-type GaAs substrate 602 on which an n-type AlGaAs/AlAs multi-layer film 603, an n-type AlGaAs cladding layer 604, a p-type GaAs active layer 605 and a p-type AlGaAs cladding layer 606 are sequentially grown and formed. The multi-layered structure is then etched while leaving a column-like part at the top thereof. Each of the left column-like part is enclosed by a buried layer which is formed by sequentially growing a p-type layer 607, n-type layer 608, p-type layer 609 and p-type layer 610 all of which are of AlGaAs in liquid phase epitaxy method. Thereafter, a multi-layer dielectric film 611 is deposited on the cap layer of p-type AlGaAs 610 at the top thereof. Finally, p- and n-type ohmic electrodes 612 and 601 are formed respectively on the top and bottom end faces of the structure thus formed. In such a manner, a surface emission type semiconductor laser will be completed. The buried layer (607 - 608) formed in the above manner defines a p-n junction which is used as means for preventing current from leaking to layer sections other than the active layer section. However, such a p-n junction is difficult to provide a sufficient current restriction and cannot suppress any reactive current perfectly. Due to generation of heat in the component, therefore, the surface emission type semiconductor laser constructed in accordance with the prior art is impractical in that it is difficult to perform a continuous generating drive in room temperature. It is thus important to restrict the reactive current in the surface emission type semiconductor laser. Where the buried layer is of a multi-layered structure to form a p-n junction as in the prior art, the p-n interface in the buried layer should be positioned in consideration of a position of interface between each adjacent column-like grown layers. It is difficult to control the thickness of each layer in the multi-layered structure. It is therefore very difficult to produce surface emission type semiconductor lasers in a good reproductiveness. If a buried layer is formed around the column by the liquid phase epitaxy method as in the prior art, there is a high risk of breaking-off of the column-like part, leading to a reduced yield. The prior art was thus subject to a structural limitation in improving the characteristics. The prior art raises further problems even when it is applied to various other devices such as laser printers and the like. For example, laser printers can have an increased freedom of design as in simplifying the optical system or in decreasing the optical path, since the source of light (semiconductor laser and so on) has a relatively large size of light spot equal to several tens µm and if a light emitting element having an increased intensity of light emission is used in the laser printers. With the surface emission type semiconductor laser constructed according to the prior art, the optical resonator is entirely buried in a material having a refractive index higher than that of the resonator. Light rays are mainly guided in the vertical direction. As a result, a spot of light emission in the basic generation mode will have a diameter equal to about 2 µm even if the shape of the resonator is modified in the horizontal direction. It has been proposed that the light spots are closed to each other up to about 2 µm and that a plurality of light sources are used to increase the size of a spot. From the standpoint of reproductiveness and yield, however, the prior art is very difficult to bury a plurality of resonators spaced away from one another by several microns in LPE method. Even if such a burying can be successfully carried out, the spots cannot be united into a single spot since the transverse leakage of light is little. It is also necessary that a plurality of light spots are formed into a single beam of light and that the laser beams each consisted of plural spots are in phase to increase the intensity of light emission. However, the prior art could not produce a surface emission type semiconductor laser which emits a plurality of laser beams close to one another up to a distance by which one of the laser beams are influenced by the other, in order to synchronize the laser beams in phase. The document US-A-4,949,351 discloses a surface-emitting semiconductor laser for emitting a laser beam in a direction perpendicular to the semiconductor substrate, comprising an optical resonator means including a pair of reflecting mirrors and a plurality of semiconductor layers between said reflecting mirrors, the semiconductor layers including a cladding layer being formed into a column-like portion which is surrounded by a buried layer of insulating material used for preventing current from leaking into portions other than the column-like portion. A surface emitting semiconductor laser structured similar to the afore mentioned one is disclosed in the document Conference Digest of the 11th International Semiconductor laser Conference, August 29 September 1, 1988, pages 164 - 165, Boston, MASS. US.. This known semiconductor laser comprises on top of the column-like portion a 20 µm/10µm ring electrode. The document FR-A-2 606 223 discloses a semiconductor layer with a rib-shaped cladding layer and a II-VI group compound semiconductor buried layer of ZnSe formed on both sides of the rib-shaped cladding layer for preventing current leakage. The lattice constant of the buried layer is similar to that of the cladding layer. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high-efficiency surface emission type semiconductor laser which is of a complete current restrictable structure provided by improving the buried layer in material and which can be produced in very simple manner and yet vary the magnitude of a light spot by changing the configuration and area of the opening in the exit-side electrode. Another object of the present invention is to provide a surface emission type semiconductor laser which includes a plurality of light emitting portions close to one another and can synchronize laser beams from the light emitting portions in phase. Still another object of the present invention is to provide a surface emission type semiconductor laser which can emit a single laser beam formed by the in-phase laser beams from the light emitting portions, said single laser beam having a relatively large light spot and an reduced angle of radiation. These objects are achieved with a surface emission type semiconductor laser as claimed. The II-VI group compound semiconductor epitaxial layer may be formed of a combination of three or four elements which include II-group elements such as Zn, Cd and Hg and VI-group elements such as O, S, Se and Te. It is also desirable that the lattice constant of the II-VI group compound semiconductor epitaxial layer is equal to that of the column-like semiconductor layer. It is preferred that the semiconductor layer defining the resonator is a III-V group compound semiconductor epitaxial layer of GaAs, GaAlAs, GaAsP, InGaP, InGaAsP, InGaAs, AlGaAsSb or the like. Since the II-VI group compound semiconductor epitaxial layer has a high resistance, the buried layer formed by this high-resistance layer can prevent a leakage of Incoming current thereinto. This can attain very effective current restriction. Furthermore, the threshold level of current can be decreased since the reactive current is reduced. As a result, the present invention can provide a surface emission type semiconductor laser which less generates heat and can continuously perform the generation in room temperature. Since the buried layer is not multi-layered, it can be easily formed with a good reproductiveness. Furthermore, the II-VI group compound semiconductor epitaxial layer formed by a vapor-phase epitaxy method, resulting in improvement of the yield in forming the column-like semiconductor layers. If the vapor-phase epitaxy method is used, a buried layer can be reliably formed to arrange a plurality of column-like semiconductor layers closer to one another, even if a space in which the buried layer is to be formed is small. The opening of the exit-side electrode in which the exit-side reflecting mirror is to be located is formed at a region in which the highest light emission efficiency can be obtained, that is, within a range including the geometric center of the contact layer. If the cross-sectional area of the opening is larger than 90% of the surface area of the contact layer, it becomes difficult to perform the continuous generation in room temperature since the resistance is increased. On the contrary, if the cross-sectional area of the opening is less than 10% of the surface area of the contact layer, the opening is too small to provide a necessary light output. Therefore, the opening area should be ranged between 10% and 90% of the surface area of the contact layer. If so done, the configuration and magnitude of the light spot may be changed without any change of the cross-sectional shape of the column-like semiconductor layers parallel to the semiconductor substrate depending on the shape and cross-sectional area of the opening. If the cross-sectional shape of the opening in the exit-side electrode is circular, there may be provided a finely circular spot beam. Even if that cross-sectional shape is regularly polygonal, a near-circular beam can be obtained. If the cross-sectional configuration of the column-like semiconductor layers parallel to the semiconductor substrate is circular or polygonal, it can provide a fairly circular spot beam. If the diameter or diagonal of the cross-section just mentioned is equal to or less than 10 µm, NFP mode becomes 0-order basic mode. If the thickness of the semiconductor contact layer on the exit side of the optical resonator is equal to or less than 3.0 µm, the light absorption can be reduced in the contact layer. If the optical resonator has a single column-like semiconductor layers, the exit-side electrode will have one opening formed therein at a position opposite to one column-like semiconductor layers. In this case, the refractive index waveguide structure may be either of rib waveguide type or buried type. In this surface emission type semiconductor laser, the optical resonator may include separation groove(s) for separating one of the column-like semiconductor layers from the other adjacent one. The II-VI group compound semiconductor epitaxial layer is buried in the separation groove and a light emitting portion is formed on each of the column-like semiconductor layers. The separation groove should not reach the active layer in the semiconductor layers defining the optical resonator. In such a manner, the respective light emitting portions are influenced by each other through the active layer, such that the lights from the light emitting portions will be in phase. If the separation groove has side walls perpendicular to the semiconductor substrate, all of light rays slantingly entering the separation groove can be perfectly reflected due a differential refractive index, this providing an increased light confinement. If a plurality of such exit-side electrodes are independently formed one for each column-like semiconductor layer, a laser beam emitted from each of the light emitting portions may be independently controlled with respect to ON, OFF and modulation. When it is desired to increase the light emission spot, a II-VI group compound semiconductor epitaxial layer which is transparent for the wavelength of the exit laser beam may be buried in the separation groove. The exit-side electrode is formed to have an opening at a position opposite to the exit-side end face of the corresponding one of the column-like semiconductor layers and also to the epitaxial II-VI group compound semiconductor layer buried in the separation groove. Thus, a region sandwiched between each adjacent light emitting portions also serves as a vertical resonating structure. Light leaked into such a region effectively contributes to the laser generation to increase the light emission spot in size. Since the synchronized laser beams are superimposed one over another, the light output increases and the angle of radiation decreases. Even if a laser beam is to be generated from a plurality of column-like semiconductor layers in the above manner, any desired light spot may be provided simply by changing the configuration and cross-sectional area of the openings in the respective exit-side electrodes. This is very advantageous in that the configuration and magnitude of the light spot can be varied without any change in the cross-sectional shape and spacing of the column-like semiconductor layers. With a GaAs laser generally used as a semiconductor layer of a resonator, the II-VI group compound semiconductor epitaxial layer transparent for the wavelength of the laser beam therefrom may be made of either ZnSSe, ZnCdS or CdSSe. If the separation groove is perpendicular to the semiconductor substrate, light rays slantingly entering the separation groove can be totally reflected to increase the confinement of light, utilizing a differential refraction. If the cross-section of the separation groove parallel to the semiconductor substrate has a width ranged between 0.5 µm and 10 µm, the order of the transverse generation mode measured from NFP becomes 0-order basic mode. If a plurality of optical resonators each consisting of plural column-like semiconductor layers are formed on the semiconductor substrate with one of the exit-side electrodes each having a single opening being independently formed for each resonator, a plurality of laser beams each having an increased light spot may be independently controlled with respect to ON, OFF and modulation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view, partially in section, of a light emitting portion of one embodiment of a semiconductor laser constructed in accordance with the present invention. Fig. 2 is a cross-sectional view of the semiconductor laser shown in Fig. 1, illustrating the process of making it. Fig. 3 is a graph illustrating the relationship between the drive current and the generated light output in the semiconductor laser shown in Fig. 1. Figs. 4(a) - (d) illustrate various configurations which can be taken in the opening of the exit-side electrode in the semiconductor laser shown in Fig. 1. Fig. 5 is a perspective view, partially in section, of a light emitting portion of another embodiment of a semiconductor laser constructed in accordance with the present invention. Fig. 6 is a cross-sectional view of the semiconductor laser shown in Fig. 5, illustrating the process of making it. Fig. 7 is a perspective view, partially in section, of a light emitting portion of a surface emission type semiconductor laser constructed in accordance with the present invention, the semiconductor laser adapted to generate laser beams synchronized in phase. Fig. 8 is a cross-sectional view of the semiconductor laser shown in Fig. 6, illustrating the process of making it. Fig. 9 illustrates differences in shape and near field pattern between the surface emission type semiconductor laser constructed according to the prior art and the semiconductor laser of Fig. 7: Fig. 9(a) shows the shape of the surface emission type semiconductor laser of the prior art on the exit side thereof; Fig. 9(b) shows an intensity profile in the near field pattern of the semiconductor laser shown in Fig. 9(a); Fig. 9(c) shows the shape of the semiconductor laser of the present embodiment at the exit side; and Fig. 9(d) shows an intensity profile of the near field pattern of the semiconductor laser shown in Fig. 9(c). Figs. 10(a) - (d) illustrate various configurations which can be taken in the opening of the exit-side electrode in the semiconductor laser shown in Fig. 9. Fig. 11 is a perspective view of a surface emission type semiconductor laser constructed according to the prior art, illustrating the light emitting portion thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1 and 2, there is shown a semiconductor laser 100 constructed in accordance with one embodiment of the present invention. The semiconductor laser 100 comprises a substrate of n-type GaAs 102 over which a buffer layer of n-type GaAs 103 is formed. Over the buffer layer 103, there are formed 30 pairs of distribution reflection type multi-layer film mirror 104 which include an n-type Al 0.7 Ga 0.3 As layer and an n-type Al 0.1 Ga 0.9 As layer and have a reflectivity equal to or higher than 98% against light rays having their wavelength substantially equal to 870 nm. On the multi-layer film mirror 104, there are sequentially formed a cladding layer of n-type Al 0.4 Ga 0.6 As 105, an active layer of p-type GaAs 106, another cladding layer of p-type Al 0.4 Ga 0.6 As 107 and a contact layer of p-type Al 0.1 Ga 0.9 As 108, utilizing the epitaxial growth in the MOCVD process (see Fig. 2(a)). At this time, for example, the formation of these layers was carried out under such a condition that the temperature on growth is 700° C and the pressure on growth is 150 Torr. and used organic metals such as TMGa (trimethyl gallium) and TMAl (trimethyl aluminum) as III- group materials, AsH₃ as V-group material, H₂ Se as n-type dopant and DEZn (diethyl zinc) as p-type dopant. After growth, an Si0₂ layer 112 is formed on the top of the multi-layered structure by the thermal CVD process. By the use of the reactive ion beam etching process (RIBE process), the multi-layered structure is etched up to the middle of the cladding layer of p-type Al 0.4 Ga 0.6 As 107, leaving a column-like light emitting portion covered with a hard baked resist layer 113 (Fig. 2(b)). The etching gas is a mixture of chlorine with argon under a pressure of 1 × 10⁻³ Torr.. The leading voltage used therein is equal to 400 V. The purpose of the etching of the cladding layer 107 up to the middle thereof is to provide a rib waveguide type refraction waveguide structure which can confine injected carriers and light rays in the active layer. After the resist 113 has been removed, a layer of ZnS 0.06 Se 0.94 109 which is in lattice alignment with GaAs is then formed by causing this layer to grow around the column-like portion using the MBE or MOCVD process (Fig. 2(c)). Four pairs of multi-layered dielectric film mirror of SiO ₂ / α-Si 111 are then formed on the top of the column-like portion by the use of electron beam deposition. Wet-etching is then used to remove a part of the mirror 111, leaving a region slightly smaller than the diameter of the light emitting portion (Fig. 2(d)). The exit-side mirror 111 can be formed to have a surface area ranged between 10% and 90% of the surface area of the contact layer 108. The reflectivity of the multi-layered dielectric film mirror 111 is 94% for wavelength of 870 nm. Thereafter, a p-type ohmic electrode 110 is then deposited on all the top face except the multi-layered dielectric film mirror 111. Thus, an opening 115 is formed along the profile of the multi-layered dielectric film mirror 111. An n-type ohmic electrode 101 is further deposited over the bottom face of the semiconductor substrate 102. In an atmosphere of N ₂ , the entire structure is alloyed at 420 ° C to provide a surface emission type semiconductor laser (Fig. 2(e)). The surface emission type semiconductor laser thus formed provides a very effective current restriction since the buried layer 109 of ZnS 0.06 Se 0.94 has a resistance equal to or higher than one GΩ and there is no injection current into the buried layer. Since it is not required that the buried layer is any multi-layered structure, it can be more easily grown with an increased reproductiveness from one batch to another. The utilization of the rib waveguide structure using the ZnS 0.06 Se 0.94 layer having its refractive index sufficiently smaller than that of GaAs can realize a more effective light confinement. Fig. 3 shows the relationship between the drive current and the generated light output in the surface emission type semiconductor laser according to this embodiment. It will be apparent from this graph that the continuous generation can be accomplished at room temperature and the threshold is very low, or equal to one mA. Furthermore, the external differential quantum efficiency is increased and the characteristics of the laser is improved by restricting the reactive current. Considering the configuration of a generated laser beam, it will be apparent that the present embodiment does not depend on the cross-sectional shape of the light emitting portions since the shape and magnitude of the laser beam is determined depending on the configuration of the opening 115 in the p-type ohmic electrode 110, that is, the configuration of the multi-layered di-electric film mirror 111 formed in the opening 115. As shown in Figs. 4(a) and 4(b), if the opening 115 is of a circular cross-section, a finely circular laser beam can be obtained. The diameter of the laser beam shown in Fig. 4(a) is larger than that of the laser beam shown in Fig. 4(b). In other words, a laser beam having its desired cross-sectional shape and diameter may be provided simply by changing the cross-sectional shape and area of the opening 115. As will be apparent from Figs. 4(c) and 4(d), a near circular beam may be obtained even if the opening 115 is of a regular polygon. Next, the cross-sectional configuration of the column-like portion in the surface emission type semiconductor laser constructed in accordance with this embodiment will be considered below. Diameter of Column Cross-Section Mode of Near FIELD Pattern 2 µm Zero-Order Basic Mode 5 µm Zero-Order Basic Mode 10 µm Zero-Order Basic Mode 12 µm First-Order Mode 15 µm First-Order Mode Table 1 shows the relationship of near field pattern relative to the diameter of the cross-section of the column-like portion in the surface emission type semiconductor laser according to this embodiment of the present invention. It will be apparent therefrom that if the diameter is equal to or less than 10 µm, the generation was carried out in the basic mode. If the diameter is larger than 10 µm, the generation was made in one-order or more mode. If the cross-section of the column-like portion is regularly polygonal, the length of a diagonal line corresponding to each of the diameters shown in Table 1 provides the same result. It is preferred that the contact layer 108 in the surface emission type semiconductor laser according to this embodiment of the present invention is of a thickness equal to or less than 3.0 µm. This is because of reduction of the light absorption in the contact layer. Such a thickness is more preferably less than 0.3 µm because the element resistance is reduced and the external differential quantum efficiency is increased. Referring now to Figs. 5 and 6, there is shown a semiconductor laser 200 constructed in accordance with another embodiment of the present invention. The semiconductor laser 200 comprises a substrate of n-type GaAs 202 over which a buffer layer of n-type GaAs 203 is formed. Over the buffer layer 203, there are formed 30 pairs of distribution reflection type multi-layer film mirror 204 which include an n-type AlAs layer and an n-type Al 0.1 Ga 0.9 As layer and have a reflectivity equal to or higher than 98% against light rays having their wavelength substantially equal to 870 nm. On the multi-layer film mirror 204, there are sequentially formed a cladding layer of n-type Al 0.4 Ga 0.6 As 205, an active layer of p-type GaAs 206, another cladding layer of p-type Al 0.4 Ga 0.6 As 207 and a contact layer of p-type Al 0.1 Ga 0.9 As 208, utilizing the epitaxial growth in the MOCVD process (see Fig. 6(a)). At this time, for example, the formation of these layers was carried out under such a condition that the temperature on growth is 700° C and the pressure on growth is 150 Torr. and used organic metals such as TMGa (trimethyl gallium) and TMAl(trimethyl aluminum) as III- group materials, AsH₃ as V-group material, H₂ Se as n-type dopant and DEZn (diethyl zinc) as p-type dopant. After growth, an SiO₂ layer 212 is formed on the top of the multi-layered structure by the thermal CVD process. By the use of the reactive ion beam etching process (RIBE process), the multi-layered structure is etched up to the middle of the cladding layer of n-type Al 0.4 Ga 0.6 As 205, leaving a column-like light emitting portion covered with a hard baked resist layer 213 (Fig. 6(b)). The etching gas is a mixture of chlorine with argon under a pressure of 1 × 10⁻³ Torr.. The leading voltage used therein is equal to 400 V. After the resist 213 has been removed, a layer of ZnS 0.06 Se 0.94 209 which is in lattice alignment with GaAs is then formed by causing this layer to grow around the column-like portion using the MBE or MOCVD process (Fig. 6(c)). Four pairs of multi-layered dielectric film mirror of SiO ₂ /α-Si 211 are then formed on the top of the semiconductor layers by the use of electron beam deposition. Wet-etching is then used to remove a part of the mirror 211, leaving a region slightly smaller than the diameter of the light emitting portion (Fig. 6(d)). The area of the exit-side mirror 211 can be ranged between 10% and 90% of the surface area of the contact layer 208. The reflectivity of the multi-layered dielectric film mirror 211 is 94% for wavelength of 870 nm. Thereafter, a p-type ohmic electrode 210 is deposited on all the top face except the multi-layered dielectric film mirror 211. Thus, an opening 215 is formed along the profile of the multi-layered dielectric film mirror 211. An n-type ohmic electrode 201 is further deposited over the bottom face of the semiconductor substrate. In an atmosphere of N ₂ , the entire structure is alloyed at 420 ° C to provide a surface emission type semiconductor laser (Fig. 6(e)). The surface emission type semiconductor laser thus formed provides a very effective current restriction since the buried layer of ZnS 0.06 Se 0.94 has a resistance equal to or higher than one GΩ and there is no injection current into the buried layer. Since it is not required that the buried layer is any multi-layered structure, it can be more easily grown with an increased reproductiveness from one batch to another. The utilization of the rib waveguide structure using the ZnS 0.06 Se 0.94 layer having its refractive index sufficiently smaller than that of GaAs can realize a more effective light confinement with the covering type refraction waveguide structure in which the active layer is buried. The cross-sectional shape of the laser beam may be changed depending on the cross-sectional shape and area of the opening 215 as in the previous embodiments. The active GaAs layer may be replaced by an active layer made of AlGaAs with the equivalent advantage. Even if the column-like portion is formed of any one of the other III-V group compound semiconductors, the equivalent advantage can be provided by selecting any suitable one of the II-VI group compound semiconductor to form the buried layer. Referring next to Figs. 7 and 8, there is shown still another embodiment of a phase-synchronization type semiconductor laser 300 constructed in accordance with the present invention, which can increase the dimension of the emission spot. The semiconductor laser 300 comprises a substrate of n-type GaAs 302 over which a buffer layer of n-type GaAs 303 is formed. Over the buffer layer 303, there are formed 25 pairs of distribution reflection type multi-layer film mirror 304 which include an n-type Al 0.9 Ga 0.1 As layer and an n-type Al 0.2 Ga 0.8 As layer and have a reflectivity equal to or higher than 98% against light rays having their wavelength substantially equal to 780±30 nm. On the multi-layer film mirror 304, there are sequentially formed a cladding layer of n-type Al 0.5 Ga 0.5 As 305, an active layer of p-type Al 0.13 Ga 0.87 As 306, another cladding layer of p-type Al 0.5 Ga 0.5 As 307 and a contact layer of p-type Al 0.15 Ga 0.85 As 308, utilizing the epitaxial growth in the MOCVD process (see Fig. 8(a)). At this time, for example, the formation of these layers was carried out, for example, under such a condition that the temperature on growth is 720° C and the pressure on growth is 150 Torr. and used organic metals such as TMGa (trimethyl gallium) and TMAl (trimethyl aluminum) as III- group materials, AsH₃ as V-group material, H₂ Se as n-type dopant and DEZn (diethyl zinc) as p-type dopant. After the growth, a SiO₂ layer is formed on the top of the multi-layered structure in the atmospheric pressure by the thermal CVD process. A photoresist is then applied over the SiO₂ layer and baked at a raised temperature to form a hard baked resist. A further SiO₂ layer is formed over the hard baked resist by the EB deposition. The respective layers formed on the substrate are then etched by the use of reactive ion etching process (RIE process). The SiO₂ layer formed on the hard baked resist 313 is first subjected to the conventional photolithograph to form a necessary resist pattern. This resist pattern is then used as a mask to perform the RIE process against the SiO₂ layer. For example, the RIE process may be carried out by using CF₄ gas under a pressure of 4.5 Pa and an input RF power of 150 W and by controlling the sample holder at 20° C. This SiO₂ layer is then utilized as a mask to etch the hard baked resist 313 by the RIE process which uses, for example, O₂ gas under a pressure of 4.5 Pa and an input power of 150 W and controls the sample holder at 20° C. At the same time, the resist pattern initially formed on the SiO₂ layer is also etched. In order to etch both the SiO₂ layer left in the pattern and the SiO₂ layer 312 formed on the epitaxial layer simultaneously, the etching is again performed by the use of CF₄ gas. By using the thin SiO₂ layer as a mask and performing the RIE process which is one of the dry etching processes against the hard baked resist 313, the latter may include side walls perpendicular to the substrate while maintaining the necessary pattern (Fig. 8(b)). The hard baked resist 313 having such vertical side walls is used as a mask in the reactive ion beam etching (RIBE) process so that the cladding layer of p-type Al 0.5 Ga 0.5 As 307 is etched up to its middle, leaving a plurality of column-like light emitting portions (Fig. 8(c)). The etching gas used herein is a mixture of chlorine with argon under a pressure equal to 5×10⁻⁴ Torr. and a plasma generating voltage equal to 400 V. The RIBE process is carried out at the current density of ion equal to 400 µAF/cm² on the etching sample while maintaining the sample holder at 20 ° C. The purpose of etching the cladding layer 307 up to its middle is to provide a refraction waveguide type rib waveguide structure for confining the horizontal injection carriers and light rays in the active layer such that a part of the light rays can be transmitted in the horizontal direction within the active layer. If the RIBEprocess in which an ion beam is irradiated perpendicular to the hard baked resist 313 having its vertical side walls and the etching sample to etch them is used, the light emitting portions 320 arranged closed to each other can be separated from each other by a separation groove 314 and at the same time it is possible to produce a vertical light resonator which is required to Improve the characteristics of the surface emission type semiconductor laser. After the hard baked resist 313 has been removed, the MBE or MOCVD process is used to grow a layer of ZnS 0.06 Se 0.94 309 around the light emitting portions, such a buried layer serving as a II-VI group compound epitaxial layer which is in lattice alignment with Al 0.5 Ga 0.5 As (Fig. 8(d)). This buried layer 309 is transparent for the generation wavelength of the surface emission type semiconductor laser 300. Next, the SiO₂ layer and polycrystalline ZnSSe produced thereon are removed. Thereafter, four pairs of multi-layered dielectric film mirrors 311 made of SiO₂ / α-Si are formed on the top of the multi-layered structure by means of electron beam deposition. Dry etching is then used to remove a part of the mirror 311 (Fig. 8(e)). At this time, the multi-layered dielectric film mirror 311 is formed at a position including the geometric center of the contact layer 308 in each of the light emitting portions 320 and ranged between 10% and 90% of the surface area of that contact layer 308. The reflectivity of the multi-layered dielectric film mirror 311 at wavelength of 780 nm is 95% or more. Since the multi-layered dielectric film mirror 311 is also formed over the separation groove 314 buried with ZnSSe, a vertical resonator structure also is formed at the region between the adjacent light emitting portions. As a result, light rays leaked into the separation groove 314 effectively contributes to the laser generation. Since the leaked light rays are utilized, the emitted light can be synchronized with the phase at the light emitting portions 320. Thereafter, a p-type ohmic electrode 310 is deposited on the top face except the multi-layered dielectric film mirror 311. Thus, an opening 315 is formed along the profile of the multi-layered dielectric film mirror 311. An n-type ohmic electrode 301 is deposited on the bottom face of the substrate. The structure thus formed is alloyed at 420 ° C in the atmosphere of N ₂ to complete the surface emission type semiconductor laser 300 (Fig. 8(f)). The n-type ohmic electrode 310 on the exit side is formed to connect with the contact layer 308 in each of the light emitting portion. Since the surface emission type semiconductor laser produced according to this embodiment utilizes the epitaxial ZnSSe layer 309 as a buried layer, it can have a resistance equal to or higher than one GΩ, which is higher than that of the prior art blocking structure using a counter bias at the p-n junction in the AlGaAs layer. This provides an optimum current blocking structure. Moreover, the light leaked from the light emitting portion 320 can be effectively utilized since the buried layer is made of a transparent material having less absorption for the generation wavelength of 780 nm. Fig. 9 shows the arrangements of the surface emission type semiconductor lasers constructed respectively in accordance with the prior art and the present invention at the exit sides thereof and intensity profiles of NFP when the laser beam is generated. Fig. 9(a) shows that the resonators 620 of the prior art surface emission type semiconductor laser 600 shown in Fig. 11 are arranged closed to one another up to a distance by which the resonators can be fully covered by the epitaxial layers of GaAlAs 607 and 608 connected with each other at the n-p junction, that is, a distance equal to about 5 µm. Although the exit face of the laser actually includes the multi-layered dielectric film mirror and the p-type ohmic electrode formed thereon, they are omitted in Fig. 9 for clear illustration. Fig. 9(b) shows an intensity profile of NFP between points a and b in Fig. 9(a). The prior art surface emission type semiconductor laser only provides a plurality of adjacent light spots even if a plurality of light emitting portions 620 are arranged closed to each other. Fig. 9(c) shows the arrangement of the exit end of the surface emission type semiconductor laser constructed according to this embodiment wherein the separation groove is filled with a ZnS 0.06 Se 0.94 layer 309 which formed by the vapor-phase epitaxy method. Thus, the minimum width of the separation groove can be equal to one µm. Fig. 9(d) shows NFP between points c and d in Fig. 9(c). It will be apparent from this NFP that the light emission spot is enlarged since light rays exit also from above the separation groove 314. Since the adjacent laser beams are synchronized with each other in phase, the light output can be increased with an angle of radiation being equal to or less than one degree. Table 2 shows the relationship between the width of the separation groove of the surface emission type semiconductor laser 300 and the order of transverse generation mode measured from NFP. Width of Separation Groove Mode of Near FIELD Pattern 0.5 µm Zero-Order Basic Mode 1.0 µm Zero-Order Basic Mode 5.0 µm Zero-Order Basic Mode 10 µm First-Order Mode 20 µm Higher-Order Mode If the width of the separation groove is less than 10µm, the transverse generation mode of the laser synchronized in phase is in the basic mode. If the width is equal to or more than 10 µm, the laser will be generated in an order equal to or higher than one. In this case, the laser beam will be of an elliptic configuration with its increased angle of radiation. This is undesirable in all the applications. If the separation groove has a width less than 0.5 µm, the laser beam will not be circular. Although the embodiments have been described as to a single optical resonator including a plurality of light emitting portions spaced away from one another, a plurality of such optical resonators may be formed on the same semiconductor substrate. If each of the optical resonators has a p-type ohmic electrode at its exit side, a laser beam from each optical resonator may be independently controlled with respect to ON, OFF and modulation. Although the embodiments have been described as to the surface emission type semiconductor laser made of GaAlAs materials, the other III-V group compounds may be equivalently used in the present invention. Particularly, the generation wavelength may be varied by changing the composition of Al in the active layer of GaAlAs. There will now be described the configuration of the opening 315 formed over the light emitting portions 320 and the buried layer 309 therebetween. Figs. 10(a) - (d) exemplify various configurations which can be taken in an opening 315 formed at a position opposed to four light emitting portions 320 and a buried layer 309 therebetween. Figs. 10(a) and 10(b) show circular openings 315, the opening shown in Fig. 10(a) providing a laser beam having a circular cross-section with its diameter larger than that of the laser beam shown in Fig. 10(b). Fig. 10(c) shows a square opening 315 which can provide a laser beam having its near circular cross-section. The magnitude of the square opening 315 may be changed to generate a laser beam having a desired diameter in cross-section. The opening 315 may be formed into any polygonal cross-sectional configuration other than the square. Fig. 10(d) exemplifies an opening 315 formed over the four light emitting portions 320 of circular cross-section and the buried layer 309 therebetween. In any event, the opening 315 is formed at a position including the geometric center of the corresponding one of the four light emitting portions 320 and ranged between 10% and 90% of the surface area of the exit-side end face of the respective light emitting portion 320. The number and arrangement of the light emitting portions 320 may be suitably selected other than those of the aforementioned embodiments. For example, a plurality of light emitting portions 320 may be equidistantly arranged in row and/or column and an opening 315 may be formed at a position opposite to the respective one of the light emitting portions 320 and the adjacent buried layer 309. In such a manner, a line beam can be obtained. In the embodiment shown in Fig. 7, there may be produced a semiconductor laser which comprises a plurality of spaced p-type ohmic electrodes 310 equal in number to the light emitting portions 320, these electrodes 310 being connected with the contact layer 308. In such a case, each of the light emitting portions will be generate a beam having a circular cross-section which can be independently controlled in ON, OFF and modulation, these beams being synchronized with one another in phase. It is to be understood that the surface emission type semiconductor laser of the present invention may be equivalently applied to any desirable light source in various devices such as printer, copying machine, facsimile, display and so on.

Claims: A surface emission type semiconductor laser for emitting a laser beam in a direction perpendicular to a semiconductor substrate (102), comprising an optical resonator means including a pair of reflecting mirrors (104, 111) having different reflectivities, a plurality of III-V semiconductor layers (105, 106, 107) between said reflecting mirrors, the semiconductor layers including at least a contact layer (108) and a cladding layer (107) being formed into at least one column-like portion, and a buried layer (109) for current restriction formed around said column-like portion, characterized in that said buried layer is a vapor phase grown II-VI group compound semiconductor epitaxial layer of three or four elements selected from II group elements Zn, Cd and Hg and VI group elements O, S, Se and Te, the surface of the buried layer being flush with that of the contact layer, and an exit-side electrode (110) is formed in contact with a part of said contact layer (108), said electrode having an opening formed therein at a position including the geometric center of said contact layer, said opening facing 10% to 90% of the surface area of said contact layer, and being formed to receive the exit-side reflecting mirror (111) of said pair of reflecting mirrors. A semiconductor laser as defined in claim 1 wherein the semiconductor contact layer (108) of said optical resonator means on the exit side thereof has a thickness equal to or less than 3.0µm. A semiconductor laser as defined in any one of claims 1 or 2 wherein said optical resonator means includes one column-like portion and wherein said exitside electrode (110) has one opening formed therein at a position opposite to the end face of said column-like portion. A semiconductor laser as defined in claim 3 wherein said column-like portion does not include any active layer and wherein said column-like portion defines a rib waveguide type refraction waveguide structure. A semiconductor laser as defined in claim 3 wherein said column-like portion includes an active layer (206) and wherein said column-like portion defines a buried type refraction waveguide structure. A semiconductor laser as defined in any one of claims 1 to or 2 wherein said optical resonator means includes separation groove means (314) for separating a plurality of said column-like portions from one another, said II-VI group compound semiconductor epitaxial layer (309) being buried in said separation groove means to form a light emitting portion on each of said column-like portions, and wherein said separation groove means does not reach the active layer (306) in said optical resonator means, whereby light beams from said light emitting portions can be synchronized with one another in phase. A semiconductor laser as defined in claim 6 wherein said separation groove means (314) has side walls extending perpendicular to said semiconductor substrate (302). A semiconductor laser as defined in claim 6 or 7 wherein a plurality of said exit-side electrodes are independently formed one for each column-like portion, each of said exit-side electrodes having one opening formed therein at a position opposite to the corresponding end face of said column-like portion. A semiconductor laser as defined in claim 6 or 7 wherein said separation groove means (314) is buried with said II-VI group compound semiconductor epitaxial layer (309) transparent for the wavelength of the emitted laser beam and wherein the exit-side electrode (310) includes an opening formed therein at a position opposite to the end faces of said column-like portions and to said II-VI group compound semiconductor epitaxial layer buried in said separation groove means. A semiconductor laser as defined in claim 9 wherein said II-VI group compound semiconductor epitaxial layer is formed of any one selected from a group consisting of ZnSSe, ZnCdS and CdSSe. A semiconductor laser as defined in claim 9 or 10 wherein the width of said separation groove means (314) in a direction parallel to said semiconductor substrate (302) is equal to or more than 0.5 µm and less than 10µm. A semiconductor laser as defined in any one of claims 9 to 11 wherein a plurality of said optical resonators each consisting of a plurality of said column-like portions are formed on said semiconductor substrate and wherein a plurality of said exit-side electrodes each having said opening formed therein are independently formed one for each optical resonator. A semiconductor laser as defined in any one of claims 1 to 12 wherein the cross-sectional shape of said opening in a direction parallel to said semiconductor substrate is either circular or regularly polygonal. A semiconductor laser as defined in any one of claims 1 to 13 wherein the cross-sectional shape of said column-like portion(s) in a direction parallel to said semiconductor substrate is either circular or regularly polygonal and wherein the diameter or diagonal length thereof is equal to or less than 10µm.

Inventors: ASAKA TATSUYA, IWANO HIDEAKI, MORI KATSUMI, ASAKA, TATSUYA, IWANO, HIDEAKI, MORI, KATSUMI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1991
Main classification: H01S3
IPC codes: H01S5, H01L33