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. Substrate 12 has on its tin concentration and is smaller than 6.460 A. In a p-type doping of about 2X 1019 atom per cubic centi this case, the buffer and con?nement layers would have meter. As is normal for such compositions, the crystal additional calcium so that their lattice constants were structure is face centered cubic and the lattice constant equal to that of the active layer. Alternatively, for a is about 6.460 angstroms. The drawing shows a frag— Pb1_ZSnzTe active layer (Pb1_ZSnz)1_x_yEuxCayTe or ment of substrate 12 in section across mesas on its sur~ (Pb1_ZSn2)1_x_ySrxCayTe could be used in the buffer face. The fragment shown includes an entire mesa in a 20 and con?nement layers with approximately the same central portion and portions mesas 32 and 34 on each ratio of En (or Sr) to Ca concentrations used for lattice side of the central mesa. The central mesa contains a matching to lead telluride substrates. laser cavity, and is as hereinafter described. As is usual, The lower portion 16a of semiconductive layer 16 is side mesas 32 and 34 are only present because they are doped to p-type conductivity, having a p-type impurity incidentally formed in the laser element manufacturing 25 concentration of about l><1017 to 1X 1018 atoms per process. Thus, they are no more important to this inven cubic centimeter. The upper portion 16b of layer 16 is tion than they are to any other semiconductor diode doped to n-type conductivity, having an n-type impu laser structure. rity concentration of about l X 1017 to l X 1018 atoms per The structure in the drawing is made by epitaxially cubic centimeter. The interface between the n-type depositing a blanket semiconductive layer 14 of layer upper portion 16b and the p-type layer lower Pb1_x_yEuxCayTe or Pb1_ x_ ySrXCayTe onto the lead portion 16!: forms a pn junction 16c. Layer 16 thus telluride substrate 12. The values of x and y for comprises the laser active region. For ease of identi?ca Pb1_x_yEuxCayTe or Osemiconductors are Dopants used in the substrate 12 and the various not particularly heat conductive. In view of this, one layers 14, 1,6, 18 and 20 can be the same as those conven ordinarily wishes to have junction 160 be as close to the tionally used in making any lead salt semiconductor upper surface 26 of the laser element 10 as is practical, diode laser. As for example, one can dope to n-type because a heat sink (not shown) is af?xed to that sur conductivity by using an excess of lead in the semicon face. ductor composition or by including appropriate quanti In addition, in order to insure that a durable, low ties of bismuth, copper or cadmium atoms in the layer. resistance ohmic contact can be obtained on the upper They can be included as grown, by ion implantation, or surface of the device, a ?nal upper semiconductive by diffusion. Analogously, p-type doping can be done layer 20 is epitaxially deposited onto the con?nement by adding an excess of tellurium or by including thal layer 18. The upper layer 20 is of lead telluride (PbTe). lium, silver, sodium or potassium in the semiconductor It is a semiconductive monocrystalline layer having the composition. Normally, one would want to use a dop same face centered cubic crystal structure and lattice ant that has a very low diffusion constant to insure that constant of 6.460 angstroms as the underlying layers. It the pn junction 16c is not only abrupt as formed but is heavily doped to n-type conductivity, by having an remains so during use. Accordingly, in most instances, it n-type impurity concentration of about 1X 1019 atoms is preferred to dope the various layers as formed rather per cubic centimeter. 20 than subsequently by diffusion. I prefer to use bismuth Layer 20 has a thickness of about 0.05-2 micrometers. as the n-type dopant and thallium as the p-type dopant. If this layer 20 is to be used at all, i.e., to enhance A signi?cant feature of my invention is that it can be contact information, it should have a thickness at least used to form a large optical cavity single quantum well large enough to provide a continuous coating. On the laser with continuous wave (CW) mode operation well other hand, it should not be so thick as to provide a 25 over 100 K. and pulsed operation over 200 K. These signi?cant thermal resistance between the pn junction lasers may have about 200-1000 angstrom regions dis 16c and the upper surface 26 of the laser element, where posed within about a 3 micron wide optical cavity. heat is ordinarily withdrawn to cool the laser. Accord Hence, single or multiple quantum well devices with ingly, I would not want the layer 20 to be appreciably large optical cavities or graded index optical con?ne more than 2 micrometers thick. 30 ment may be constructed. In these lasers, a 600 ang Ordinarily, a laser such as shown in the drawing is strom thick layer of lead telluride forms the active re fabricated by initially blanket depositing its various gion. A pn junction separated opposite faces of this layers onto a substrate and then surface etching the layer. The active region layer is sandwiched between resultant product to de?ne the lasing mesa, i.e., the mutually opposite conductivity type Pb1_x_yEux central mesa in the drawing. I contemplate making my 35 CayTe or Pb1_x_ySrxCayTe layers lattice-matched to laser similarly. For example, after layers 14, 16, 18 and PbTe. They each are about 1-2 micrometers thick. The 20 are epitaxially deposited, parallel grooves 22 and 24 interface of these layers with the active region layer are etched into the upper surfaces 26 of the wafer, pref de?ned the quantum well. These layers are referred to erably down through all of the layers and into the sur herein as carrier con?nement layers. They were, in face of the substrate 12. The length, width and depth of 40 turn, sandwiched between about 1.5-2 micrometers the grooves, as well as their side wall angle with surface thick mutually opposite conductivity type 26, are no more material to this invention than they are Pb1 _ x_yEuxCayTe or Pbl _x_ySrxCayTe layers lattice to any other double heterojunction semiconductor in matched to PbTe. These latter two layers serve to con frared diode laser. For purposes of providing a speci?c ?ne photons. The interface of these optical con?nement example, they should be wide enough apart to provide 45 layers with the carrier con?nement layers de?ned an a width on upper surface 26 of the central mesa of about optical cavity. In the completed device the optical con 3-40 micrometers, preferably 3-15 micrometers. The ?nement layers are, in turn, sandwiched between a length of upper surface 26 should be about 150-600 PbTe substrate and a covering PbTe layer, of mutually micrometers, preferably 400 micrometers. The width of opposite conductivity type. Thus, the structure is analo grooves 22 and 24 is not especially critical either. Then gous to that shown in the drawing but contains still the entire upper surface 26 is anodized, including the another pair of con?nement layers, speci?cally for opti surfaces within the grooves 22 and 24, to form a native cal con?nement. oxide coating 28. The embodiments of the invention in which an exclu A rectangular contact window 30, extending substan sive property or privilege is claimed are defined as tially along the entire length of the upper surface 26, is 55 follows: opened in anodic oxide coating 28 by photoetching 1. In a double heterojunction semiconductor lead salt techniques, so that a low resistance ohmic contact can infrared diode laser, the improvement wherein a semi be made along the entire upper surface 26 of the upper conductive monocrystalline lead salt active region layer layer 20. The coated substrate is then out along the side of given energy band gap, index of refraction, crystal mesas 32 and 34 to provide an elongated body having a structure and lattice constant is sandwiched between central mesa for lasing. The ends of the substrate 12 are two semiconductive monocrystalline lead salt layers of then cleaved in the normal and accepted manner, to mutually opposite conductivity type containing calcium provide a laser cavity in the central mesa. Low resis and one element selected from the group consisting of tance electrical contacts can be made to n-type surface europium and strontium and having an energy band gap 26, and to the p-type backside (not shown) of substrate 65 signi?cantly higher than, an index of refraction signi? 12 in any normal and accepted manner for making cantly lower than, a crystal structure the same as, and a contacts to PbTe. For example, they can be made in the lattice constant substantially equal to the active region same manner previously described in Wayne Lo’s US. layer, effective to increase laser performance by provid 4,612,644 7 8 ing lattice matching among said layers as well as pro one element selected from the group consisting of euro viding at least carrier con?nement with respect to the pium and strontium that has an energy band gap greater active region layer. than, an index of refraction smaller than, and a lattice 2. In a large optical cavity quantum well semiconduc constant substantially equal to said predetermined val tor infrared diode laser exhibiting current carrier and ues. optical con?nement for its active region layer but also 3. In a double heterojunction semiconductor long exhibiting increased operating temperature due to close wavelength infrared diode laser, the improvement lattice matching of face centered cubic monocrystalline wherein a lead-tin telluride monocrystalline semicon» layers forming the double heterojunctions, said laser ductive active region layer of given energy band gap, comprising a monocrystalline buffer layer of a given index of refraction, and lattice constant is sandwiched conductivity-type lead salt semiconductor containing between two lead-metal element-calcium-telluride calcium and an element selected from the group consist monocrystalline semiconductor layers that may also ing of europium and strontium that has an energy band contain tin, that are of mutually opposite conductivity gap greater than, an index of refraction lesser than, and type, and have an energy band gap signi?cantly higher a lattice constant substantially equal to predetermined 15 than, and a lattice constant substantially equal to the values, a monocrystalline active region layer on the lead-tin telluride active layer, effective to increase laser buffer layer of a lead salt semiconductor containing a pn performance by providing lattice matching among said junction that has said predetermined energy band gap, layers as well as providing carrier and optical con?ne index of refraction and lattice constant, and a con?ne ment to the active region layer, said metal element is ment layer on the active region layer of an opposite 20 selected from the group consisting of europium and conductivity type lead salt semiconductor containing strontium. lesser amounts and smaller proportions of calcium and ?= 5* 1* it *

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