Abstract:
A laser diode for emitting a coherent beam of light in the blue and/or green portions of the spectrum. The laser diode includes a plurality of layers of II-VI semiconductor forming a pn junction, including at least a first light-guiding layer. A short-period strained-layer superlattice (SPSLS) CdZnSe quantum well active layer is positioned within the pn junction. The layers of II-VI semiconductor are supported by a substrate. First and second electrodes on opposite sides of the layers of II-VI semiconductor couple electrical energy to the laser diode.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 08/139,614, filed Oct. 20, 1993 and entitled II-VI Laser Diodes With Quantum Wells Grown By Atomic Layer Epitaxy And Migration Enhanced Epitaxy now U.S. Pat. No. 5,395,791, which is a continuation of abandoned U.S. patent application 07/887,541, filed May 22, 1992 and entitled II-VI Laser Diodes With Quantum Wells Grown By Atomic Layer Epitaxy And Migration Enhanced Epitaxy now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Research undertaken by 3M in St. Paul, Minn., culminated in the demonstration of the world&#39;s first laser diodes fabricated from II-VI semiconductor materials. These devices emit coherent radiation at 490 nm in the blue-green portion of the spectrum. They are disclosed generally in Haase et al. article, Short Wavelength II-VI Laser Diodes, Conference Proceedings for Gallium Arsenide and Related Compounds, 1991 Institute of Physics Conference Series, No. 120, pp 9-16. 
     The light-generating (active) layers in the above-described laser diodes include strained Cd x  Zn 1-x  Se single quantum wells grown by conventional molecular beam epitaxy (MBE) techniques. Unfortunately, the composition and thickness of the random CdZnSe alloy is difficult to control by this process. Luminescence efficiency is also relatively poor. These characteristics limit the overall efficiency of the devices. 
     It is evident that there is a continuing need for improved laser diodes. For wide spread commercial viability, the devices must be capable of efficiently generating high intensity beams of light at room temperatures. Fabrication techniques for laser diodes having these characteristics are also needed. 
     SUMMARY OF THE INVENTION 
     The present invention is a II-VI compound semiconductor electroluminescent device capable of efficiently generating high intensity beams of light. One embodiment of the device includes a plurality of layers of II-VI semiconductor forming a pn junction, and a short-period strained-layer superlattice (SPSLS) active layer within the pn junction. The SPSLS active layer has a form described by the notation [(X) m  (Y) n  ] p , where X and Y are binary II-VI semiconductor compounds and m, n and p are integers. The layers of II-VI semiconductor are supported by a substrate. Electrical energy is coupled to the device by first and second electrodes on opposite sides of the pn junction. 
     In another embodiment, the device is a laser diode with an SPSLS quantum well active layer. The SPSLS quantum well layer can be of a form described by the notations [(CdSe) m  (ZnSe) n  ] p , [(ZnTe) m  (ZnSe) n  ] p  and [(ZnSe) m  ] p , where m, n and p are integers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view (not to scale) illustrating the structure of a II-VI semiconductor laser diode in accordance with the present invention. 
     FIG. 2 is a graph illustrating the product of the loss coefficient (α) and the full width at half maximum (FWHM) intensity of the optical mode as a function of the thickness of the light-guiding layers, for laser diodes of the type shown in FIG. 1. 
     FIG. 3 is an illustration of a molecular beam epitaxy (MBE) system that can be used to fabricate laser diodes in accordance with the present invention. 
     FIG. 4 is a detailed cross sectional view of the quantum well layer shown in FIG. 1. 
     FIG. 5 is a graph of the shutter sequence by which the MBE system, shown in FIG. 3, is operated to fabricate the active layer of laser diodes in accordance with the present invention. 
     FIG. 6 is a high resolution transmission electron micrograph of a cross-section of the quantum well of a laser diode fabricated in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A II-VI compound semiconductor laser diode 10 (i.e., an electroluminescent device) in accordance with the present invention is illustrated generally in FIG. 1. Laser diode 10 includes a short-period strained-layer superlattice (SPSLS) quantum well layer 12 surrounded by a ZnSe pn junction formed by n-type ZnSe light-guiding layer 14 and p-type ZnSe light-guiding layer 16. As described in greater detail below, quantum well layer 12 is a high efficiency active layer grown by atomic layer epitaxy (ALE) and/or migration enhanced epitaxy (MEE). Laser diode 10 is fabricated on an n-type GaAs substrate 18 and includes an n-type ZnSe ohmic contact layer 19 between the substrate and guiding layer 14. A p-type ZnSe ohmic contact layer 20 overlays p-type guiding layer 16. A polyimide insulating layer 22 covers the surface of ohmic contact layer 20 opposite light-guiding layer 16. 
     Electrical contact to p-type ohmic contact layer 20 is made by Au electrode 24, which is formed in an open stripe in insulating layer 22. A thin Ti layer 26 and a final Au layer 28 are applied over insulating layer 22 to facilitate lead bonding. Electrical contact to the lower side of laser diode 10 is made by an In electrode 30 on the surface of substrate 18 opposite n-type ohmic contact layer 19. 
     Light-guiding layer 14 and contact layer 19 are both doped n-type with Cl in prototypes of laser diode 10. Light-guiding layer 16 and ohmic contact layer 20 are doped p-type with N in these prototypes. The net donor concentration to which the lower light-guiding layer 14 is doped is 1×10 17  cm -3 , while the upper light-guiding layer 16 is doped to a net acceptor concentration of 2×10 17  cm -3 . Ohmic contact layers 19 and 20 are both deposited to a thickness of 0.1 μm in the prototype devices. The lower contact layer 19 is doped n-type to a net donor concentration of 1×10 18  cm -3 . The upper contact layer 20 is doped p-type to a net acceptor concentration of 1×10 18  cm -3 . 
     Light generated in quantum well active layer 12 is guided within light-guiding layers 14 and 16, clad only by the GaAs substrate 18 and the Au electrode 24. Good optical confinement and sufficiently low loss are obtained in laser diode 10 without the need for II-VI semiconductor cladding layers. Computer modeling is used to select appropriate thicknesses for light-guiding layers 14 and 16. This modeling approach takes into account the ZnSe waveguide formed by light-guiding layers 14 and 16, as well as the complex indices of refraction of GaAs substrate 18 and Au electrode 24. Modeling methods of this type are generally known and disclosed, for example, in M. R. Ramdas et al., Analysis of Absorbing and Leaky Planar Waveguides: A Novel Method, Optics Letters, Vol. 14, p. 376 (1989) and the references cited therein. 
     FIG. 2 is a graph illustrating the product of the loss coefficient (α) and the full width at half maximum intensity (FWHM) of the desired optical mode (TE polarized for the prototypes described herein) as a function of the thickness of the ZnSe layers 14 and 16 (FIG. 1). To minimize the threshold current density of the device, this product should be minimized. Using this design criterion and the information shown in FIG. 2, the thickness of the waveguide (i.e., the combined thicknesses of light-guiding layers 14 and 16) is approximately 3.5 μm in the prototype laser diode 10. In this embodiment, n-type light-guiding layer 14 has a thickness of 2.0 μm, while p-type light-guiding layer 16 has a thickness of 1.5 μm. The loss due to free-carrier absorption and scattering is estimated to be 8 cm -1  in this embodiment. Quantum well layer 12 has only a relatively small effect on the loss and optical confinement characteristics of the device, and its presence is neglected during the design procedure described above. Theory suggests that total waveguide thicknesses less than 2.0 μm result in excessive absorption losses in substrate 18 and electrode 24. At a thickness of 2.5 μm, the substrate and electrode absorption losses are 11.7 cm -1 . On the other hand, the FWHM of the optical mode is found to be almost exactly half of the waveguide thickness. Therefore, for thicknesses greater than about 6 μm the optical confinement is so poor that the single quantum well layer 12 cannot practically supply enough gain to overcome the losses. The maximum modal gain is inversely proportional to the FWHM of the waveguide mode. For a waveguide that is 6 μm thick, the FWHM is about 3 μm, and the modal gain from a single quantum well can be estimated to be 12 cm -1 . See, e.g., N. K. Dutta, Applied Physics Letters, vol. 53, p. 72 (Nov. 1982). 
     FIG. 3 is an illustration of a molecular beam epitaxy (MBE) system used to fabricate the prototypes of laser diode 10 described above. The MBE system includes a chamber 54 with a high energy electron gun 58, a phosphorus screen 60, a substrate heater 90 and a flux monitor 62. MBE chambers such as 54 are generally known and commercially available. 
     Prototypes of laser diode 10 are fabricated on Si-doped N +  -type GaAs substrates 18 having a (100) crystal orientation. Substrates of this type are commercially available. Substrate 12 is cleaned and prepared using conventional or otherwise known techniques, and mounted to a Molybdenum sample block (not shown in FIG. 3) by In solder before being positioned within chamber 54. 
     Chamber 54 includes a Zn effusion cell 70, cracked-Se effusion cell 72, Cd effusion cell 76 and a standard Se (i.e., primarily Se 6 ) effusion cell 79. As shown, cracked-Se effusion cell 72 includes a bulk evaporator 84 and high temperature cracking zone 82, and provide a source of cracked Se (including Se 2  and other Se molecules with less than 6 atoms). The bulk evaporator 84 and high temperature cracking zone 82 used to produce the prototype laser diodes 10 are of a custom design, the details and capabilities of which are described in the Cheng et al. article, Molecular-Beam Epitaxy Growth of ZnSe Using a Cracked Selenium Source, J. Vac. Sci. Technol., B8, 181 (1990). Cl effusion cell 78 which utilizes ZnCl 2  source material provides the Cl n-type dopant. The p-type dopant is provided by N free-radical source 80. Free-radical source 80 is connected to a source 86 of ultra-pure N 2  through leak-valve 88. The free-radical source 80 used in the fabrication of laser diodes 10 is commercially available from Oxford Applied Research Ltd. of Oxfordshire, England (Model No. MPD21). The beam exit plate at the end of the source is made of pyrolyric boron nitride (PBN) and has nine 0.2 mm diameter holes through it. This source is mounted on a standard port for an effusion cell through a 10&#34; extension tube. N 2  source 86 used to fabricate laser diodes 10 is of research purity grade. The pressure at the inlet of the leak-valve of source 86 is 5 psi. 
     MBE chamber 54 is operated in a manner described in the Cheng et al. article Growth of p- and n-type ZnSe by Molecular Beam Epitaxy, J. Crystal Growth 95, 512 (1989) using the Se 6  source 79 as the source of Se to grow the n-type contact and light-guiding layers 19 and 14, respectively, of the prototype laser diode 10. 
     SPSLS quantum well layer 12 is grown on the light-guiding layer 14 of laser diode 10 using atomic layer epitaxy (ALE) and/or migration enhanced epitaxy (MEE). Using these techniques, which are generally known, quantum well layer 12 is formed as a series of overlaying single crystal thickness layers (i.e., monolayers) of Cd, Zn and Se. A detailed illustration of quantum well layer 12 is shown in FIG. 4. In this embodiment, quantum well layer 12 includes adjacent monolayers of Cd and Se between a pair of adjacent monolayers of Zn and Se. This structure can be described generally by the following notation: 
     [(CdSe) m  (ZnSe) a  ] p   
     where: m, n and p are integers. 
     In the embodiment illustrated in FIG. 4, m=1, n=2, and p=1. In other embodiments (not shown) m=1, n=1-5 and p=1-5. The equivalent Cd concentration in quantum well layer 12 is determined by the ratio of the number of CdSe layers to the total number of layers (including both ZnSe and CdSe) in the quantum well layer. Total thickness of quantum well layer 12 is given by the number of monolayers grown times the thickness of each monolayer. FIG. 6 is a high resolution transmission electron micrograph analysis of a cross section of a prototype laser diode 10 having a SPSLS quantum well with a period (p) equal to 6, clearly representing the monolayer structure of the quantum well. 
     Control over the composition and thickness of the Cd, Zn and Se monolayers of quantum well layer 12 are accurately achieved by ALE and/or MEE. Through use of these techniques, monolayer growth is controlled primarily by the sequence and timing by which the shutters (not separately shown) of the Cd, Zn and Se effusion cells 76, 70, and 72, respectively, are opened and closed. A portion of the effusion cell shutter sequence used to grow the quantum well layer 12 illustrated in FIG. 4 is shown in FIG. 5. A characteristic delay time is introduced between the sequential pulses of the reactant species to allow for the reevaporation of excess reactant. 
     Prototype laser diodes 10 including SPSLS quantum well layers such as that shown in FIG. 4 have been grown at temperatures of 150° C. and using the thermally cracked Se (Se 2 ) effusion cell 76. The shutter sequence begins with the Se shutter open. The Se shutter is closed after depositing at least one monolayer (about 5 seconds). The Zn shutter is then opened after a slight delay (about 2 seconds) to allow excess Se to evaporate. Next, the Zn shutter is closed after depositing at least one monolayer of Zn (about 4 seconds). A slight delay (about 1 second) is included between the closing of the Zn shutter and the reopening of the Se shutter to allow time for the evaporation of any excess Zn. Growth continues by alternately opening and closing the shutters to sequentially deposit overlaying layers of Cd, Se and Zn. The Cd shutter is opened for about 4 seconds, followed by a delay of about 1 second before the Se shutter is reopened. The sequence beginning with the open Se shutter is then repeated to complete quantum well layer 12. Other operating parameters of MBE chamber 54 used to produce the quantum well layer 12 of the prototype laser diodes 10 are as follows: 
     Cd beam equivalent pressure: 1.0×10 -7  Torr* 
     Zn beam equivalent pressure: 1.0×10 -7  Torr* 
     Se cracking zone temperature: 600° C.* 
     Se bulk evaporator temperature: 250° C.* 
     * parameters dependent upon specific MBE system configuration and plasma source 
     Prototype laser diodes 10 having SPSLS quantum well layers 12 grown in the manner described above at 150° C. have exhibited the highest quantum efficiency. However, quantum well test wafers with SPSLS quantum well layers grown at temperatures up to 235° C. have exhibited good characteristics. Acceptable characteristics have been observed in test wafers with SPSLS quantum well layers grown at temperatures as high as 300° C. It is also anticipated that devices having desirable characteristics can be grown at temperatures less than 150° C. 
     MBE chamber 54 is operated in a manner described in the Park et al. U.S. Pat. No. 5,248,631 entitled Doping of IIB-VIA Semiconductors During Molecular Beam Epitaxy Using Neutral Free Radicals, and in the Park et al. article, P-type ZnSe by Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth, Appl. Phys. Lett. 57, 2127 (1990), using the Se 6  source 79 to grow the p-type light-guiding layer 16. The disclosures contained in the above-referenced Park et al. U.S. Patent and article are incorporated herein by reference. 
     Low resistivity p-type ZnSe ohmic contact layer 20 has been achieved by growing the contact layer at low temperature within MBE chamber 54 utilizing the cracked Se source 72 (i.e., cracking zone 82 and evaporator 84), while at the same time doping the semiconductor material of the contact layer p-type. This low temperature growth and doping technique is described in greater detail in the DePuydt et al. U.S. Pat. No. 5,274,269. The semiconductor body with layers 19, 14, 12, and 16 on substrate 18 is heated to a temperature less than 250° C. but high enough to promote crystalline growth of the ZnSe doped with the N p-type dopants to a net acceptor concentration of at least 1×10 17  cm -3 . A net acceptor concentration of 1-10 18  cm -3  was achieved in the ohmic contact layer 20 of prototype laser diodes 10, when grown at a substrate temperature of about 150° C. However, it is anticipated that ohmic contact layers 20 with acceptable characteristics can be achieved at other growth temperatures down to at least 130° C. Other operating parameters of MBE chamber 54 used to produce the ohmic contact layer 20 of the prototype laser diodes 10 are as follows: 
     Zn beam equivalent pressure: 1.0-10 -7  Torr* 
     Se cracking zone temperature: 600° C.* 
     Se bulk evaporator temperature: 250° C.* 
     Growth rate: 0.3-0.6 μm/hr 
     Surface reconstruction: Zn-stabilized 
     Nitrogen pressure in chamber: &gt;3.5×10 -7  Torr* 
     rf power: 150-250 W * 
     * parameters dependent upon specific MBE system configuration 
     Following the deposition of contact layer 20, the as yet incomplete laser diode 10 is removed from MBE chamber 54. Electrode 24 includes Au which is vacuum evaporated onto contact layer 20 and patterned into a stripe (typically about 20 μm wide) using conventional photolithography and lift-off. An insulating layer 22 is then applied over electrode 24 and the exposed surface of contact layer 20. For an insulator that can be applied at low temperatures, polyimide photoresist is preferred. Probimide 408 from Ciba-Geigy Corp. was used to produce the prototype laser diodes 10. A stripe (about 20 μm wide) of the polyimide layer 22 directly above electrode 24 is removed by UV exposure through a photomask and development using the manufacturer&#39;s recommended processing recipe, except for the post-development cure. To cure the developed polyimide, the device was flood exposed to 1 J/cm 2  of UV light from a mask aligner, and baked at 125° C. on a hot plate in air for 3 minutes. Ti-Au layer 26 was then evaporated on the exposed surface of the Au electrode 24 and polyimide layer 22 to facilitate lead-bonding. The In used for MBE substrate bonding also served as electrode 30 on substrate 18. Opposite ends of the device were cleaved along (110) planes to form facet mirrors. The facets were coated with a total of four alternating quarter wavelength stacks of MgF 2  and ZnSe to provide a 90% reflectivity. Cavity length of the prototype devices 10 is about 1000 μm. Laser diodes 10 were then bonded p-side up to ceramic sample holders with silver-filled epoxy. 
     Laser diodes 10 in accordance with the present invention offer considerable advantages. The ALE/MEE techniques for growing the quantum well layer enables better composition control, better control of the quantum well thickness and luminescence efficiency than random alloy quantum wells grown by conventional MBE. These characteristics are evidenced by increased room temperature photoluminescence and electroluminescence intensities, and lower laser threshold currents. Although the room temperature threshold currents in these prototype devices (as low as 1030 A/cm 3 ) are not as low as in devices with cladding layers, these devices offer the advantages of simpler construction and lower operating voltages (about 13 V). 
     The embodiments described above include light guiding layers that are not lattice matched to the substrate. It is anticipated that increased device lifetime and improved performance will be exhibited by devices in which the lattice parameters of the light guiding layers are matched to that of the substrate. The first and second light guiding layer may include ZnSo 0 .06 Se 0 .94 or Cd 0 .43 Zn 0 .57 S on GaAs substrates or ZnSe light guiding layers on In 0 .04 Ga 0 .96 As or In 0 .52 Ga 0 .48 P substrates. The light guiding layers may also be comprised of ZnS x  Se 1-x , Cd x  Zn 1-x  S, ZnS 1-x  Te x , Zn 1-x  Cd x  Se, Zn 1-x  Mg x  S y  Se 1-y  or Cd x  Zn 1-Y  Mg y  S layers lattice matched to substrates such as GaAs, AlAs, GaP, Al x  Ga 1-x  As, In x  Ga 1-x  As, In x  Al 1-x  As, In x  Ga 1-xP , In x  Al 1-x  P, GaAs 1-x  P x , In x  Ga 1-x-y  Al y  As, In x  Ga 1-x-y  Al y  P, ZnSe or Zn 1-x  Cd x  S. 
     Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, it is expected that the inventive concepts used to fabricate the prototype laser diodes disclosed herein are well suited to the fabrication of laser diodes with ALE/MEE active layers and/or guiding layers from a wide variety of other II-VI semiconductors and alloys. These include ZnSe, ZnTe, ZnSeTe, CdS, CdZnSeTe, MgZnSe, CdZnS, ZnSTe and CdZnTe.