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United States Patent [191 4,612,644 United States Patent [191 [11] Patent Number: 4,612,644 Partin [45] Date of Patent: Sep. 16, 1986 [54] LEAD-ALLOY-TELLURIDE by Molecular Beam Epitaxy”, Journal of Electronics HETEROJUNCI‘ION SEMICONDUCTOR Materials, vol. 13, No. 3, 1984. LASER Weber et al., “Waveguide and Luminescent Properties [75] Inventor: Dale L. Partin, Sterling Heights, of Thin Film Pb-Salt Injection Lasers”, Journal Applied Mich. Physics, vol. 44, No. 11, Nov. 1973, pp. 4991-5000. [73] Assignee: General Motors Corporation, Detroit, Primary Examiner-James W. Davie Mich. Assistant Examiner—-Georgia Y. Epps Attorney, Agent, or Firm—Randy W. Tung [21] Appl. No.: 754,171 [57] ABSTRACT [22] Filed: Jul. 12, 1985 A double heterojunction lead salt infrared diode laser [51] Int. Clx‘ ....................... .. H018 3/19; H01L 33/00 having an active region layer of a lead salt semiconduc [52] US. Cl. ...................................... .. 372/44; 357/16; tor of a given lattice constant, energy band gap, and 357/17; 357/61; 372/45 index of refraction. The active region layer is sand [58] Field of Search .................... .. 372/44, 45; 357/ 17, wiched between two lead salt semiconductor layers 357/61, 16 containing calcium and one element selected from the [56] References Cited group consisting of europium and strontium that are mutually of opposite conductivity type and have sub PUBLICATIONS stantially the same lattice constant as the active region Partin et al., “Wavelength coverage of Lead-Europi layer. In addition, the two outside layers have an energy um-Selenide-Telluride Diode Lasers”, Applied Physics band gap greater than the active region layer and an Letters, 45(3), Aug. 1, 1984, pp. 193-195. index of refraction less than the active region layer. The Par-tin, “Single Quantum Well Lead-Europi resulting laser has lattice matching, as well as enhanced um-Selenide-Telluride Diode Lasers”, Applied Physics carrier con?nement and optical con?nement. Letters, 45(5), Sep. 1; 1984, pp. 487-489. Partin, "Lcad-Europium-Selenide-Telluride Grown 3 Claims, 1 Drawing Figure (pa JUNCTION \ U.S. Patent Sep. 16,1986 _ 4,612,644 4,612,644 1 2 LEAD-ALLOY-TELLURIDE HETEROJ UNCI‘ ION _ OBJECTS AND SUMMARY OF THE SEMICONDUCTOR LASER INVENTION It is therefore a principal object of this invention to FIELD OF THE INVENTION provide a lead salt semiconductor diode laser that can This invention generally relates to improved double be operated at relatively high operating temperatures. heterojunction lead salt semiconductor infrared diode Another object of the invention is to provide an im lasers. It more particularly involves long wavelength proved lead-alloy chalcogenide salt heterojunction in infrared lasers having a lead chalcogenide active layer frared diode laser. sandwiched between two lead chalcogenide layers that Still another object of the invention is to provide a contain calcium/europium or calcium/strontium and heterojunction diode laser having a lead-telluride, lead that are lattice matched and of higher band gap energy. tin-telluride, lead-europium-calcium-telluride, or lead strontium-calcium-telluride active region and lead BACKGROUND OF THE INVENTION europium-calcium-telluride or lead-strontium-calcium A semiconductor diode laser is a monocrystalline pn 15 telluride con?ning regions. junction device. In one form of such a device, the pn In substance, this invention recognizes that the inclu junction is a plane disposed in an active region between sion of small amounts of europium/calcium or stronti two parallel rectangular faces of a monocrystalline um/calcium in a lead or lead-tin-telluride composition semiconductor body. Two mutually parallel re?ective can signi?cantly increase the energy band gap and de crease the index of refraction. This invention also rec faces that are perpendicular to the pn junction form a ognizes that small amounts of europium and/or stron laser cavity. Lasing action is produced by applying a tium increase the crystal lattice constant. This invention forward voltage across the pn junction. The forward further recognizes that calcium reduces the crystal lat bias injects electrons and holes across the pn junction. tice constant without reducing energy band. Moreover, Electrons and holes recombine in the active region to 25 this invention recognizes that even though appreciable cause stimulated emission of the radiation. Above a proportions of lead and/or tin are replaced by euro given level of electron injection, called the threshold pium, strontium or calcium, the resultant semiconduc current (1771), emitted radiation is collected and ampli tor can still be heavily doped to both n-type and p-type ?ed in the active region. The ampli?ed radiation exits conductivity. In addition, abrupt heterojunctions can be the active region parallel the pn junction as a mono 30 made because europium, calcium, and strontium have chromatic beam. low diffusion constants in lead telluride. These attri A problem is that electrons and holes can be injected butes are very important to a double heterojunction into the active region without stimulating emission lead salt infrared diode laser such as that shown in the therein. For example, they can escape outside the active drawing. region to adjacent portions of the semiconductor body, where they recombine without contributing to laser BRIEF DESCRIPTION OF THE DRAWING emission. Analogously, photons produced in the active Other objects, features and advantages of the inven region can escape from the active region by radiation in tion will become more apparent from the following a direction not parallel the pn junction. In addition, it is description of preferred embodiments thereof and from possible for electrons to disappear within the active the drawing which shows a fragmentary sectional view region without producing the desired emission of radia of a lead salt semiconductor diode laser element made in tion, such as by combining with holes at crystal defects. accordance with this invention. All such losses reduce laser ef?ciency, i.e., output power. One can resist escape of injected electrons and DESCRIPTION OF THE PREFERRED holes and stimulated photons from the active region by 45 EMBODIMENTS sandwiching the active region between two contiguous The invention comprehends an infrared double layers of monocrystalline semiconductive material hav heterojunction lead salt diode laser having a lead ing a larger energy band gap and a lower index of re europium-calcium-telluride, or a lead-strontiurn-calci fraction than the active region. Such layers serve to um-telluride con?nement region layer. In a speci?c con?ne electrons, holes and photons to the active re 50 example one can also use such compositions, with low gion. On the other hand, the active region, and as a to moderate europium/calcium or strontium/calcium practical matter the two contiguous layers must be of a content in the active region layer. In all cases the most very high monocrystalline quality. This requires that abundant constituents of these lead-metal element-calci these layers and the active region be closely matched um-tellurides are lead and tellurium. Alternatively, I not only in crystal structure but also in crystal lattice 55 may also add tin in the active region layer to make a size. Moreover, one of the sandwiching layers must be longer emission wavelength laser. The active region doped to n-type conductivity and the other to p-type layer is sandwiched between an upper n-type con?ne conductivity. Such a structure is referred to herein as a ment layer and a lower p-type buffer layer, both of double heterojunction semiconductor diode laser. which are lattice-matched lead-europium-calcium-tellu Lead chalcogenide double heterojunction semicon ride, or lead-strontium-calcium-telluride layers. The ductor diode lasers which operate at high temperatures buffer layer is also a con?nement layer, as is usual in a have been dif?cult to make. By high temperature I double heterojunction diode laser structure. The con mean higher than about 100 K. under continuous wave ?nement and buffer layers have a larger concentration (CW) operation. I have previously ?led US. patent of europium/calcium or strontium/calcium than the application Ser. No. 565,397 on a quaternary semicon 65 active region layer, and therefore, have a larger energy ductive diode laser system based on an alloy of lead band gap and a lower index of refraction. However, europium selenide-telluride which permits such lasers both the con?nement layer and the buffer layer have an to be made with relative ease. identical crystal structure to that of the active region 4,612,644 3 4 layer and substantially the same lattice constant. In a a lattice constant of about 6.460 angstroms. The concen preferred example, all three layers are epitaxially grown trations of europium, strontium, or calcium in layer 16 is on a high quality lead-telluride substrate and are of approximately two-thirds that of the same in buffer substantially the same lattice constant. They are cov layer 14 and con?nement layer 18. This change in con ered with an epitaxially deposited contact layer of lead centration produces a change in energy band gap and telluride, which is of the same lattice constant. In using index of refraction hereinbefore referred to. this invention to make a relatively long wavelength Additions of Eu or Sr and Ca increase the energy embodiment, emitting light at wavelengths longer than band gap of PbTe. However, the ratio of Eu to Ca or Sr approximately 6 microns, the substrate, active region to Ca concentrations must be adjusted to maintain the layer, and contact layer are of lead-tin telluride lattice same crystal lattice constant as the lead telluride sub matched to the con?nement and buffer layers. strate has (i.e., 6.460 A). If a lead-tin-telluride active The drawing illustrates a semiconductor diode laser layer is to be used for a longer emission wavelength element 10 made on a 0.5 millimeter thick monocrystal laser, it will have a lattice constant which is dependent line lead telluride (PbTe) substrate 12.
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