Surface emission type semiconductor 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, the layers including 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 layers. An active layer (106) of multi-quantum well structure is further formed on the lower layer section of the cladding layer having the column-like portion. If a plurality of column-like semiconductor layers are formed, these column-like semiconductor layers are separated from one another by a separation groove terminating short of the active layer, the II-VI group compound semiconductor epitaxial layer being buried in the separation groove. Particularly, when it is desired to produce a phase synchronization type semiconductor laser, a waveguide layer is formed below the active layer, the waveguide layer adapted to propagate light rays in a direction parallel to the active 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. 12, 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 close 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 is 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. 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. Surface emission type semiconductor lasers for emitting a laser beam in a direction perpendicular to a semiconductor substrate in which an active layer of a multi-quantum well structure is employed are disclosed in the documents JP-A-61 079 280 and Japanese Journal of Applied Physics, Extended Abstracts of the 22nd (1990) International Conference of Solid State Devices and Materials, Sendai, 1990, pages 521 - 524. 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 appropriately selecting the material of the buried layer and by improving the structure of the active layer and also which can be produced in very simple manner. Another object of the present invention is to provide a surface emission type semiconductor laser which includes a plurality of light emitting portions closed to one another and can synchronize laser beams from the light emitting portions in phase and which can be continuously driven in room temperature. 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 bears 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 layers. It is preferred that the semiconductor layer defining the resonator is a III-V group compound semiconductor epitaxial layer of GaAs, GaAlAs, GaAsP, InGaP, lnGaAsP, 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. In addition, the active layer of multi-quantum well (MQW) structure also serves to reduce the generating threshold current. By the appropriate selection of the buried material and the modification of the active layer, 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. If the active layer is of MQW structure, its gain is increased with the light output being increased. If the material of the active layer is changed from one to another, the wavelength of generation also is naturally varied. However, the present invention performs the change of the wavelength by varying the MQW structure while using the same material. Furthermore, the II-VI group compound semiconductor epitaxial layer is formed by a vapor-phase epitaxy method, resulting in improvement of the yield in forming the column-like semiconductor layers. Since 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. 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 cross-section of the column-like semiconductor layer parallel to the semiconductor substrate is circular or regular 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 (Near Field Pattern) mode becomes 0-order basic mode. Since the active layer of MQW structure is very thin, it is preferred that a waveguide layer is provided to propagate the light rays in a direction parallel to the active layer. If the optical resonator has a single column-like semiconductor layers, the reflecting mirror on the exit side thereof may be formed at a position opposite to the end face of the column within the range of said end face. In this case, a refractive index waveguide structure of rib waveguide type may be provided by the active layer of MQW structure. If the column-like semiconductor layers include an active layer of MQW structure, a refractive index waveguide structure of buried type may be realized. 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. When the active layer of MQW structure is located not to reach the separation groove, the light emitting portions will be influenced by each other such that the light rays from the respective light emitting portions are synchronized with each other in phase. Even though a plurality of column-like semiconductor layers arranged in a two-dimensional array are formed within a finely small area, the present invention can reduce the reactive current such that the surface emission type semiconductor laser can be continuously driven in room temperature. In order to increase the advantage of the phase synchronization, a waveguide layer is preferably located at the lower layer section of the active layer of MQW structure. In this case, the influence between the light emitting portions will be emphasized to facilitate the phase synchronization, for example, even if the respective light emitting portions are spaced farther away from one another. 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 reflecting mirror is formed through a region opposite to the end face of each of the column-like layers and the II-VI group compound semiconductor epitaxial 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. 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. 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 an enlarged cross-sectional view of the active MQW layer in the semiconductor laser shown in Fig. 1. Fig. 3 is a cross-sectional view illustrating the process of making the semiconductor laser shown in Fig. 1. Fig. 4 is a graph illustrating the relationship between the drive current and the generated light output 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 schematic cross-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. 7 is a cross-sectional view of the semiconductor laser shown in Fig. 6, illustrating the process of making it. Fig. 8 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. 6: Fig. 8(a) shows the shape of the surface emission type semiconductor laser of the prior art on the exit side thereof; Fig. 8(b) shows an intensity profile in the near field pattern of the semiconductor laser shown in Fig. 8(a); Fig. 8(c) shows the shape of the semiconductor laser of the present embodiment at the exit side; and Fig. 8(d) shows an intensity profile of the near field pattern of the semiconductor laser shown in Fig. 8(c). Figs. 9 (a) to (m) schematically illustrate various shapes of surface emission type semiconductor lasers constructed according to further embodiments of the present invention at the exit sides thereof. Figs. 10(a) to (d) schematically illustrate various shapes of surface emission type semiconductor lasers constructed according to still further embodiments of the present invention at the exit sides thereof. Figs. 11(a) to (c) schematically illustrate various shapes of surface emission type semiconductor lasers constructed according to other embodiments of the present invention at the exit sides thereof. Fig. 12 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 multi-quantum well structure 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. 3(a)). For example, the active MQW layer 106 comprises three well layers 106a, as shown in Fig. 2. Each of the well layers 106a is sandwiched between a pair of upper and lower barrier layers 106b. Each of the well layers 106a may be formed as of an i-type GaAs film having a thickness equal to 120 Angstroms while each of the barrier layers 106b may be formed as of an i-type Ga 0.65 Al 0.35 As film having a thickness equal to 150 Angstroms. The MOCVD process 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 TMA1(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 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. 3(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. 3(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. 3(d)). 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 deposited on all the top face except 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. 3(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 G Ω and there is no injection current into the buried layer. In this embodiment, further, the active MQW layer 106 contributes to the reduction of generating threshold current. 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. 4 shows the relationship between the drive current and the generated light output in the surface emission type semiconductor laser according to this embodiment. In this embodiment, the continuous generation of laser beam can be accomplished in room temperature with a threshold level as very low as 10 µA by using the buried layer of II-VI group compound semiconductor 109 and the active MQW layer 106. If any active MQW layer is not used, the continuous generation of laser beam can be attained, but the threshold current will be as large as one mA. The active MQW layer serves to provide a light output at least five times higher than a case where no active layer of MQW structure is taken, for example, 25 mW or higher. Furthermore, the external differential quantum efficiency is increased and the characteristics of the laser is improved by restricting the reactive current. If the cross-sectional shape of the column-like portion in the surface emission type semiconductor laser according to this embodiment of the present invention is of a circle or regular polygon such as square or regular octagon, a finely circular spot of beam can be provided. However, if this cross-sectional shape is of any configuration other than the above ones, such as rectangle or trapezoid, the cross-section of the laser beam will be ellipse or multi-mode. This is not desirable in applying the semiconductor laser to the devices. Diameter of Column Cross-Section Mode of Near Filed 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 is carried out in the basic mode. 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. The present invention can be applied to a buried type refractive index waveguide structure as shown in Fig. 5, rather than the aforementioned rib waveguide type refractive index waveguide structure. In this case, the column-like semiconductor layers are formed to extend to the underlying cladding layer 105. As a result, the active MQW layer 106 will also be formed into a column-like configuration and enclosed by the buried layer 109. Referring next to Figs. 6 and 7, 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 includes 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, a waveguide layer 315, an active layer of multi-quantum well structure 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. 7(a)). At this time, for example, the active MQW layer 306 comprises three well layers 306a, each of which is sandwiched between a pair of upper and lower barrier layers 306b, as shown in Fig. 2. Each of the well layers 306a may be formed of an i-type Ga 0.65 Al 0.35 As film having a thickness equal to 80 Angstroms while each of the barrier layers 306b is made of an i-type Ga 0.95 Al 0.05 As film having a thickness equal to 60 Angstroms. The waveguide layer 315 has a composition of Al ranged between those of the well and barrier layers 306a, 306b and may be formed of an n-type epitaxial layer of Ga 0.75 Al 0.25 As. Such a waveguide layer has a refractive index lower than the equivalent refractive index of the active MQW layer 306 but higher than the refractive index of the underlying cladding layer 305. The MOCVD process may be carried out as 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, AsH ₃ as V-group material, H ₂ 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. 7(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. 7(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 µA/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 MQW layer 306 such that a part of the light rays can be transmitted in the horizontal direction within the active layer. Also in this embodiment, the propagation of light in the horizontal direction may be assured by the waveguide layer 315. If the RIBE process 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. 7(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 mirror 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. 7(e)). The reflectivity of the multi-layered dielectric film mirror 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. 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 N₂ to complete the surface emission type semiconductor laser 300 (Fig. 7(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. As a result, the generating threshold current can be reduced. In addition, the active MQW layer 306 also serves to reduce the generating threshold current, for example, to a level of about ten µ A. Furthermore, the light emitting portions 320 separated from each other by the separation groove 314 can be influenced by each other through the active and waveguide layers 306 and 315. Thus, light rays from the respective light emitting portions 320 can be synchronized with each other in phase, resulting in generation of a laser beam having an increased diameter and an emphasized intensity. 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. 8 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. 8(a) shows that the resonators 620 of the prior art surface emission type semiconductor laser 600 shown in Fig. 12 are arranged close 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 µ m. 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. 8 for clear illustration. Fig. 8(b) shows an intensity profile of NFP between points a and b in Fig. 8(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. 8(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 buried 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. 8(d) shows NFP between points c and d in Fig. 8(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 Filed 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 10 µm. 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. Although this embodiment has been described in connection with the structure shown in Fig. 6 and the light emitting portion shown in Fig. 8(c), the present invention is not limited to such an arrangement. Figs. 9 to 11 show the other embodiments of the present invention in which various configurations and arrangements of optical resonators and associated separation grooves in a plane parallel to the substrate as viewed from the exit side are schematically illustrated. Figs. 9(a) - (j) and (m) represent line symmetry arrangements in which a plurality of column-like semiconductor layers each having a circular or regularly polygonal cross-section parallel to the substrate are formed. In any event, the light emitting spot formed by any one of such arrangements can have a dimension larger than that of a light emitting spot formed by a single light emitting portion. When it desired to provide a single circular cross-sectional laser beam having a relatively large diameter from the respective light emitting portions and separation groove, the cross-section of each of the light emitting portions may be any configuration other than circle or regular polygon. The essential requirement in the concept of the present invention is that a non-circular or non-polygonal line joining the outer edges of the light emitting portions arranged in line symmetry approximate to a circular or regularly polygonal configuration. So the configurations shown in Figs. (k) and (l) are suitable for generating the laser beam having larger diamiter. Each of embodiments shown in Figs. 10(a) - (d) and 11(a) - (c) includes light emitting portions of n in number and is advantageous in that it can produce a light emitting spot formed into any desirable size and form, in addition to the same advantages as in the embodiment of Fig. 6. In all the embodiments shown in Figs. 10 and 11, a line beam may be provided by disposing a plurality of light emitting portions in row and/or column on a two-dimensional plane parallel to the substrate. In the embodiment shown in Fig. 6, there may be produced a semiconductor laser which comprises a plurality of spaced p-type ohmic electrodes 310 equal in number to the light omitting 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 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 0, S, Se and Te, and an active layer (106) of multi-quantum well structure is formed at the lower layer section of said cladding layer (107) having the column-like structure, said active layer including a plurality of well layers (106a) each of which is sandwiched between barrier layers (106b). A semiconductor laser as defined in claim 1 wherein the cross-section of said column-like portion parallel to said semiconductor substrate (102) is of a circular or regularly polygonal configuration. A semiconductor laser as defined in claim 2 wherein said column-like portion have either of a diameter or diagonal lines equal to or less than 10 µm in planes parallel to said semiconductor substrate (102). A semiconductor laser as defined in any one of claims 1 to 3 wherein a semiconductor contact layer (108) of said optical resonator means on the exit side thereof has a thickness equal to or less than 3.0 µm. A semiconductor laser as defined in any one of claims 1 to 4 wherein said optical resonator means includes one column-like portion and wherein one of said reflecting mirrors (111) on the exit side is formed opposite to the end face of said column-like portion within the range of said end face. A semiconductor laser as defined in claim 5 wherein said optical resonator means has a rib waveguide type refractive index waveguide structure, said rib waveguide being formed by said active layer (106) of multi-quantum well structure. A semiconductor laser as defined in claim 5 wherein said column-like portion includes said active layer (106) of multi-quantum well structure to form a buried type refractive index waveguide structure. A semiconductor laser as defined in any one of claims 1 to 4 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 active layer (306) is formed as a common layer for all the light emitting portions, whereby light beams from said light emitting portions can be synchronized with one another in phase. A semiconductor laser as defined in claim 8 wherein said separation groove means (314) has side walls extending perpendicular to said semiconductor substrate. A semiconductor laser as defined in claim 8 or 9 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 reflecting mirror (311) on the exit side is formed over a region 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 10 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 10 or 11 wherein the width of said separation groove means (314) in a direction parallel to said semiconductor substrate is equal to or more than 0.5 µm and less than 10 µm. A semiconductor laser as defined in any one of claims 10 to 12 wherein each of said column-like portions has a circular or regularly polygonal cross-section in a two-dimensional plane parallel to said semiconductor substrate and wherein a plurality of said column-like portions are arranged in line symmetry in said two-dimensional plane to emit a laser beam having a circular cross-section. A semiconductor laser as defined in any one of claims 10 to 12 wherein each of said column-like portions has a non-circular or non-polygonal cross-section in a two-dimensional plane parallel to said semiconductor substrate and wherein the outer edges of said column-like portions are arranged to form substantially a circular or regularly polygonal profile, whereby a laser beam having a circular cross-section can be emitted from the semiconductor laser. A semiconductor laser as defined in any one of claims 10 to 12 wherein a plurality of said optical resonator means each defined by a plurality of said column-like portions are formed on said semiconductor substrate to provide an independent electrode in each of said optical resonator means on the exit side, whereby a laser beam emitted from each of said optical resonator means and having a circular cross-section capable of forming an increased light emitting spot can be independently controlled in ON, OFF and modulation. A semiconductor laser as defined in any one of claims 10 to 12 wherein a plurality of said column-like portions are equidistantly arranged in a row and/or column to provide a laser beam emitted therefrom in the form of a line beam. A semiconductor laser as defined in any one of claims 1 to 16, further comprising a waveguide layer (315) formed at the lower layer section of said active layer (306) of multi-quantum well structure, said waveguide layer adapted to propagate light rays in a direction parallel to said active layer. A semiconductor laser as defined in claim 17 wherein the refractive index of said waveguide layer (315) is lower than the equivalent refractive index of said active layer (306) but higher than the refractive index of the cladding layer below said waveguide layer.

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