Patent Publication Number: US-4929571-A

Title: Method of making a buried crescent laser with air gap insulator

Description:
This application is a division of U.S. patent application Ser. No. 130,426, filed Dec. 9, 1987,  now U.S. Pat. No. 4,847,945, issued July 11, 1989. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a semiconductor laser and, more particularly, to an improvement in the response speed thereof. 
     BACKGROUND ART 
     An InGaAsP system embedded type semiconductor laser is generally used as a light source in light communication for long distance and large capacity transmission. FIG. 6 shows a cross-section of a prior art embedded type semiconductor laser which is called a Buried Crescent (hereinafter referred to as BC) laser. The BC laser is described in detail in the IEEE Journal of Light-Wave Technology, Volume LT-3, page 978, 1985. 
     In FIG. 6, the reference numeral 1 designates a substrate comprising p type InP. The numeral 2 designates a first current blocking layer comprising n type InP. The numeral 3 designates a second current blocking layer comprising p type InP. The numeral 4 designates a first cladding layer comprising p type InP. The numeral 5 designates an active layer. The numeral 6 designates a second cladding layer comprising n type InP. The numeral 7 designates a p type electrode. The numeral 8 designates an n type electrode. The numeral 9 designates a pn junction region. 
     The semiconductor laser of FIG. 6 is produced in a sequence of steps. 
     A first current blocking layer 2 and a second current blocking layer 3 are successively grown on the substrate 1. Next, a groove having an arrowhead configuration is produced in the (011) direction, usually by photolithography and chemical etching. Hydrochloric acid is used for the etching. The width of the groove is established at a value less than 2 micrometers so that the transverse oscillation may be a single fundamental mode. 
     Next, the first cladding layer 4 and the active layer 5 are successively grown on the substrate 1 in the groove, and thereafter a second cladding layer 6 is grown on the active layer 5 and the second current blocking layer 3. 
     After the second crystal growth is concluded, a p electrode 7, such as AuZn or Au, is deposited on the substrate 1 opposite blocking layer 2, and an n electrode 8, such as AuGe or Au, is deposited on the second cladding layer 6 opposite blocking layer 3 to complete a BC laser as shown in FIG. 6. 
     When a bias voltage is applied to the electrodes so that the n electrode 8 is negative relative to electrode 7, the pn junction produced by the active layer 5 (which is normally p type) and the second cladding layer 6 becomes forward biased. The pn junction region 9 produced by the first current blocking layer 2 and the second current blocking layer 3 becomes reverse biased. Accordingly, the path of the curent flowing through the chip is restricted to the substrate 1, the first cladding layer 4, the active layer 5, and the second cladding layer 6. Current does not flow through any region other than active layer 5. As a result, almost all the current inside the chip is concentrated on the active layer 5, thereby contributing to the laser oscillation. Thus, a low threshold current, such as lower than 20 mA, is realized in the BC laser. 
     In the BC laser described above, reverse bias junctions are provided on both sides of the active layer 5 in order to concentrate current flow in a so-called embedded type low threshold current semiconductor laser. That construction has a large capacitive component from the reverse bias junctions. Since the RC time constant of the laser is large because this capacitive component functions as a parasitic capacitance, the frequency response of a semiconductor laser employing this structure has been restricted to a value lower than a few hundred MHz. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved semiconductor laser capable of oscillating at a low threshold current value and having a high speed response. 
     Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     According to an aspect of the present invention, current is concentrated in an active layer by blocking current flow with a gap void of solid material or such a gap partially or entirely filled with current blocking mterial having only negligible parasitic capacitance so that the parasitic capacitance is small and high speed response at a low current threshold is achieved. 
     According to another aspect of the present invention, a second cladding layer is grown on a mask layer and on an active layer. Thereafter, the mask layer is removed by etching, thereby producing a gap or space between the second cladding layer and the current blocking layer. Thus, a semiconductor laser of the above-described structure can be easily obtained with good reproducibility. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a semiconductor laser according to an embodiment of the present invention; 
     FIGS. 2(a)-2(d) are diagrams showing a method of producing the semiconductor laser of FIG. 1; 
     FIG. 3 and FIG. 4 are cross-sectional views showing still other embodiments of semiconductor lasers according to the present invention; 
     FIG. 5 is a cross-sectional view showing another semiconductor laser embodiment according to the present invention; and 
     FIG. 6 is a cross-sectional view showing a prior art semiconductor laser. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the same reference numerals designate the same elements as those shown in FIG. 6. The reference numerals 6a and 6b designate respectively a current concentration region and a current dispersion region of the second cladding layer 6. The reference numeral 10 designates a current blocking layer comprising n type InP. The reference numeral 11 designates a gap that is void of solid material. 
     FIGS. 2(a) to 2(d) show a method of producing the semiconductor laser of FIG. 1. 
     In FIGS. 2(a)-2(d), the same reference numerals designate the same elements as those shown in FIG. 1. The reference numeral 12 designates a mask layer comprising InGaAsP, the reference numeral 13 designates a longitudinal window in mask 12, and the reference numeral 14 designates a longitudinal groove in substrate 1 and layer 10. 
     Current blocking layer 10 and a selectively removable mask layer 12 are successively grown on the substrate 1. Their thicknesses are about 2 micrometers and 1 micrometer, respectively. 
     Next, a photoresist (not shown) is deposited on the mask layer 12, and a longitudinal window of about 2 micrometers in width is produced in the (011) direction on the photoresist. Thereafter, a longitudinal window 13 is produced in the mask layer 12 with the photoresist as a mask by etching with a solution of sulfuric acid, hydrogen peroxide, and water or hydrochloric acid, acetic acid, and hydrogen peroxide (refer to FIG. 2(a)). 
     Next, a longitudinal groove 14 reaching the substrate 1 is formed with layer 12 as a mask by etching, such as with hydrochloric acid. Hydrochloric acid is suitable as an etchant for selectively etching InP with InGaAsP as a mask because hydrochloric acid only etches InP and does not etch InGaAsP (refer to FIG. 2(b)). 
     Next, layer 4 is grown on the substrate 1 in the groove 14 by a conventional method. The first cladding layer 4 and the active layer 5 are successively grown. The second cladding layer 6 is grown on the active layer 5 and on the mask layer 13 (refer to FIG. 2(c)). 
     Next, the structure is selectively etched with a solution of sulfuric acid, hydrogen peroxide, and water or a solution of potassium ferrocyanide, potassium hydroxide, and water, both of which only etch InGaAsP and not InP. The etchant cuts into the wafer from both sides and etches only the mask layer 12 comprising InGaAsP. Etching automatically stops when it has advanced to the second cladding layer 6 on the active layer 5. At the conclusion of the etching, a gap 11 void of solid material is produced that extends close to the active layer 5 (refer to FIG. 2(d)). 
     Gap 11, as a matter of course, functions as an insulating layer so that current flowing directly from the substrate 1 through the current blocking layer 10 is completely cut off from reaching the current dispersion region 6b at the upper portion of the second cladding layer 6. As a result, the current flows to the current dispersion region 6b only through the active layer 5 and the current concentration region 6a, thereby attaining a reduced threshold value. 
     Furthermore, the capacitance due to gap 11 is negligibly small for the high frequency component, and the limitation of modulation speed by the RC time constant is negligible. Therefore, a high speed response of about 10 GHz is achieved. 
     In the above-described production method, the material for the mask layer 12, which is to be removed by etching, must be different from the materials of the current blocking layer 10 and the second cladding layer 6 surrounding the mask layer 12. The selective etching of mask layer 12 is automatically concluded after the second cladding layer 6 overlying the active layer 5 is reached. Accordingly, there is no necessity of employing high technology in the production process, and the gap 11 is produced with good reproducibility. 
     Furthermore, the selective etching proceeds from both sides of the wafer. Therefore, the transverse width of the current dispersion region 6b at the upper portion of the second cladding layer 6 above the active layer 5 can be wider than the active region so that a good ohmic contact is easily obtained on the second cladding layer 6. 
     FIG. 3 shows a cross-section of another semiconductor laser embodiment according to the present invention. In FIG. 3, the same reference numerals as those shown in FIG. 1 designate the same elements, and the reference numeral 6c designates a p type semiconductor layer integrated with the second cladding layer 6. 
     In the above-described first embodiment, a mask layer 12 is produced by an initial crystal growth, and thereafter a longitudinal groove 14 is produced by etching. In the embodiment of FIG. 3, a p type semiconductor layer 6c comprising, for example, p type InP is further provided on the mask layer 12 in the initial crystal growth. 
     In the embodiment of FIG. 3, since p type semiconductor layer 6c has a different conductivity type from that of the second cladding layer 6, the electrical isolation between the current dispersion region 6b and the current blocking layer 10 is improved relative to the embodiment of FIG. 1. 
     Furthermore, the semiconductor lasers shown in FIG. 1 and FIG. 3 include a slender region comprising the first cladding layer 4, the active layer 5, and the current concentration region 6a at the lower portion of the second cladding layer 6. The slender portion supports the relatively wide current dispersion region 6b of the wide second cladding layer 6, raising concern that the structure is mechanically weak. Those concerns are avoided in the embodiment of the present invention shown in FIG. 4. There, a p type semiconductor layer 6c, which functions as current blocking material having a negligible parasitic capacitance, may be provided at spaced locations between current blocking layer 10 and current dispersion region 6b. Alternatively, an insulator 15 such as SiO 2  or high molecular material such as polyimide may be provided at spaced locations between the current blocking layer 10 and the current dispersion region 6b of the second cladding layer 6, thereby enhancing the strength of a chip. 
     In FIG. 5, the same reference numerals designate the same elements as those shown in FIG. 1, and the reference numeral 15 designates an insulating layer. 
     In the semiconductor laser of FIG. 5, the insulation between the current blocking layer 10 and the current dispersion region 6b above the second cladding layer 6 is provided by an insulating layer 15. A parasitic capacitance seldom arises and a high speed response is achieved as in the embodiment of FIG. 1. 
     Insulating layer 15 can be deposited directly on the current blocking layer 10 after the initial crystal growth. Insulator 15 can also be produced by inserting a resin into the gap 11 of FIGS. 1 and 2. 
     While the novel semiconductor laser of the InP series is described aboe, the present invention may be applied to semiconductor lasers of other materials such as GaAs/AlGaAs. 
     The present invention can also be applied to a semiconductor laser having a structure of reverse conductivity types to those of the above-described embodiments. 
     As is evident from the foregoing description, according to an aspect of the present invention, a semiconductor laser includes a semiconductor substrate, a longitudinal groove formed in the substrate in the resonator direction, a current blocking layer disposed on the region of the semiconductor substrate where a groove is not provided, a first cladding layer disposed on the semiconductor substrate in the groove, an active layer disposed on the first cladding layer, and a second cladding layer disposed directly on the active layer and on the current blocking layer via a gap or via a gap and current blocking material having negligible parasitic capacitance, whereby the device is capable of a high speed response at a low threshold value. 
     According to another aspect of the present invention, a method of producing a semiconductor laser is provided which comprises depositing a current blocking layer and a selectively removable mask layer successively on a first conductivity type semiconductor substrate, forming a longitudinal window on the mask layer in the resonator direction, etching a longitudinal groove through the window extending to the semiconductor substrate through the mask layer, growing a first cladding layer and an active layer successively on the semiconductor substrate in the groove, and thereafter growing a second cladding layer on the active layer and on the mask layer, and removing the mask layer. By this method, a semiconductor laser of the above-described structure can be easily obtained with good reproducibility.