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United States Patent (19) 11) Patent Number: 4,722,087 Partin 45) Date of Patent: Jan

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United States Patent (19) 11) Patent Number: 4,722,087 Partin 45) Date of Patent: Jan United States Patent (19) 11) Patent Number: 4,722,087 Partin 45) Date of Patent: Jan. 26, 1988 (54) LEAD-STRONTUM-CHALCOGENIDE Grown by Molecular Beam Epitaxy"; Journal of Elec DODE LASER tronic, vol. 13, No. 3, pp. 493-504, May 1984. (75 Inventor: Dale L. Partin, Sterling Heights, Primary Examiner-William L. Sikes Mich. Assistant Examiner-Bertha Randolph 73) Assignee: General Motors Corporation, Detroit, Attorney, Agent, or Firm-Randy M. Tung Mich. 57 ABSTRACT (21) Appl. No.: 895,286 A double heterojunction lead salt infrared diode laser 22) Filed: Aug. 11, 1986 having an active region layer of a lead salt semiconduc (51) Int. Cl."................................................ H01S 3/19 tor of a given lattice constant, energy band gap, and 52) U.S. C. ........................................ 372/44; 372/45; index of refraction. The active region layer is sand 357/16; 357/17; 357/61 wiched between two lead salt semiconductor layers (58) Field of Search ...................... 372/44, 45; 357/16, containing strontium and selenium that are mutually of 357/63, 17, 61 opposite conductivity type and have substantially the same lattice constant as the active region layer. In addi (56) References Cited tion, the two outside layers have an energy band gap U.S. PATENT DOCUMENTS greater than the active region layer and an index of 4,608,694 8/1986 Partin .................................... 372/44 refraction less than the active region layer. The result 4,612,644 9/1986 Partin .................................... 372/44 ing laser has lattice matching, as well as enhanced car OTHER PUBLICATIONS rier confinement and optical confinement. D. L. Partin; "Lead-Europium-Selenide-Telluride 3 Claims, 1 Drawing Figure on Junction sS.s U.S. Patent Jan. 26, 1988 4,722,087 y 2,A., , / / 7 /% 1 / / / 2/ s t / 5 2 Y 2.22 / 221 23 4,722,087 1. 2 I have also previously filed a U.S. patent application LEAD-STRONTIUM-CHALCOGENDE DODE Ser. No. 754,171 entitled "Lead-Alloy-Telluride LASER Heterojunction Semiconductor Laser' which describes two quaternary compositions, lead-europium-calcium FIELD OF THE INVENTION telluride and lead-strontium-calcium-telluride, either of This invention generally relates to improved double which compositions may be used to make diode lasers heterojunction lead salt semiconductor infrared diode which operate at high temperatures. However, both of lasers. More particularly, it involves long wavelength these alloys contain calcium (Ca), and the energy band infrared lasers having a lead chalcogenide active layer gap of these alloys (and hence the laser emission wave sandwiched between two lead chalcogenide layers that 10 lengths of diode lasers made from these alloys) depend contain strontium and selenium and that are lattice very sensitively on the calcium concentration of these matched and of higher band gap energy. alloys. It is more difficult to measure the amount of Ca BACKGROUND OF THE INVENTION very accurately that is put into these alloys because Ca 15 is a relatively light atomic species. The technique I used A semiconductor diode laser is a monocrystalline pn for measuring alloy composition during growth by mo junction device. In one form of such a device, the pn junction is in a plane disposed in an active region be lecular beam epitaxy involves weighing vacuum depos tween two parallel rectangular faces of a monocrystal ited thin films of each film constituent separately as I line semiconductor body. Two mutually parallel reflec have described in a publication (see D. L. Partin, J. tive faces that are perpendicular to the pnjunction form 20 Electronic Materials, vol. 13, pp. 493-504, 1984). Thus, a laser cavity. Lasing action is produced by applying a if the relative weights of the appropriate molecules or forward voltage across the pn junction. The forward atoms used in the various alloys I have grown are con bias injects electrons and holes across the pn junction. sidered, they are (rounded off to nearest whole num ber): PbTe (335), PbSe (286), GeTe (200), Te2 (255), Eu Electrons and holes recombine in the active region to 25 cause stimulated emission of the radiation. Above a (152), Yb (173), Ba (137), Sr. (88) and Ca (40). Thus, Ca given level of electron injection, called the threshold is more than twice as light as the next heavier specie, current (ITH), emitted radiation is collected and ampli and in practice, I found this to cause a fair amount of fied in the active region. The amplified radiation exits difficulty, especially because the energy band gap of the active region parallel the pn junction as a mono 30 Pb1-CaTe is a sensitive function of Ca concentration. chromatic beam. Thus, I find an advantage in controlling the energy A problem is that electrons and holes can be injected band gap of Pb-Sr.SeTe1-y compared to other al into the active region without stimulating emission loys containing Ca. therein. For example, they can escape outside the active region to adjacent portions of the semiconductor body, OBJECTS AND SUMMARY OF THE where they recombine without contributing to laser 35 INVENTION emission. Analogously, photons produced in the active It is therefore a principal object of this invention to region can escape from the active region by radiation in provide a lead salt semiconductor diode laser that can a direction not parallel the pnjunction. In addition, it is be operated at relatively high operating temperatures. possible for electrons to disappear within the active Another object of the invention is to provide an im region without producing the desired emission of radia proved lead-strontium chalcogenide heterojunction tion, such as by combining with holes at crystal defects. infrared diode laser. All such losses reduce laser efficiency, i.e., output Still another object of the invention is to provide a power. One can resist escape of injected electrons and heterojunction diode laser having a lead-telluride, lead holes and stimulated photons from the active region by 45 tin-telluride or lead-strontium-selenide-telluride active sandwiching the active region between two contiguous region and lead-strontium-selenide-telluride or lead-tin layers of monocrystalline semiconductive material hav strontium-selenide-telluride confining regions. ing a larger energy band gap and a lower index of re In substance, this invention recognizes that the inclu fraction than the active region. Such layers serve to sion of small amounts of strontium in a lead or lead-tin confine electrons, holes and photons to the active re 50 gion. On the other hand, the active region, and as a telluride composition can significantly increase the en practical matter the two contiguous layers, must be of a ergy band gap and decrease the index of refraction. This very high monocrystalline quality. This requires that invention also recognizes that small amounts of stron these layers and the active region be closely matched tium increase the crystal lattice constant. This invention not only in crystal structure but also in crystal lattice 55 further recognizes that the inclusion of small amounts of size. Moreover, one of the sandwiching layers must be selenium reduces the crystal lattice constant and the doped to n-type conductivity and the other to p-type index of refraction without significantly changing the conductivity. Such a structure is referred to herein as a energy band gap. Moreover, this invention recognizes double heterojunction semiconductor diode laser. that even though appreciable proportions of lead and Lead chalcogenide double heterojunction semicon 60 /or tin are replaced by strontium, and an appreciable ductor diode lasers which operate at high temperatures proportion of tellurium is replaced by selenium, the have been difficult to make. By high temperature I resultant semiconductor can still be heavily doped to mean higher than about 100 K. under continuous wave both n-type and p-type conductivity. In addition, abrupt (CW) operation. I have previously filed U.S. patent heterojunctions can be made because strontium and application Ser. No. 565,397 on a quaternary semicon 65 selenium have low diffusion constants in lead telluride. ductive diode laser system based on an alloy of lead These attributes are very important to a double hetero europium-selenide-telluride which permits such lasers junction lead salt infrared diode laser such as that to be made with relative ease. shown in the drawing. 4,722,087 r 3 4. This can be accomplished by maintaining the ratio of x BRIEF DESCRIPTION OF THE DRAWING to y at 1.67. If a Pb-SnTe active region 16, contact Other objects, features and advantages of the inven layer 20 and substrate 12 were to be used, the lattice tion will become more apparent from the following matching condition changes to maintaining the value of description of preferred embodiments thereof and from x (in Pbi-Sr.SeTe1-) equal to 1.67y-0.66z. Alter the drawing which shows a fragmentary sectional view natively, if a Pb1-2SnTe substrate 12, active region 16, of a lead salt semiconductor diode laser element made in and contact layer 20 is used, the buffer layer 14 could accordance with this invention. consist of (Pb1-2)1-Sr.SeTe1-, which will be lattice DESCRIPTION OF THE PREFERRED matched to Pb1-2SnTe if the ratio of x to y equals 1.67. 10 In all cases, the preferred range of the strontium con EMBODIMENTS centration is approximately given by 0.02sXS0.10, and The invention comprehends an infrared double the tin concentration range is given by OS ZS0.32. For heterojunction lead salt diode laser having a lead-stron large tin concentrations (approximately in the range tium-selenide-telluride confinement region layer which 0.10szs0.32) and moderately large strontium concen may also contain tin if the laser active region is lead-tin 15 trations (approximately in the range 0.05SxS 0.10), use telluride.
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