Abstract:
An intersubband quantum cascade laser structure includes multiple coupled laser stages, wherein each stage has a multilayer structure including an electron injector, an active region with at least one quantum well, and an electron reflector. Electrons injected from the injector into the active region at a high energy level relax to a lower energy level with the emission of a photon at, for example, mid-infrared wavelengths. The reflector reflects electrons at the higher energy level at which they were injected and transmits electrons from the lower energy level after emission of a photon. Multiple layers of semiconductor are formed on each side of the multistage structure to provide conduction across the device and to provide optical confinement of the photons emitted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/276,712, filed Mar. 10, 2006, which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains generally to the field of semiconductor lasers and, particularly, to quantum cascade lasers. 
     BACKGROUND OF THE INVENTION 
     Semiconductor lasers are formed of multiple layers of semiconductor materials. The conventional semiconductor diode laser typically includes an n-type layer, a p-type layer and an undoped layered active structure between them such that when the diode is forward biased electrons and holes recombine within the active structure with the resulting emission of light. The layers adjacent to the active structure typically have a lower index of refraction than the active structure and form cladding layers that confine the emitted light to the active structure and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge-emitting or surface-emitting. 
     A semiconductor laser that emits photons as electrons from within a given quantized energy band, wherein the electrons relax within that band by decaying from one quantized energy level to another, rather than emitting photons from the recombination of electrons and holes, has been reported. Since the radiative transitions are very inefficient the electrons are recycled by using multiple stages. See, J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho,  Science , Vol. 264, pp. 553, et seq., 1994. This device, referred to as a quantum cascade laser (QCL), is the first reported implementation of an intersubband semiconductor laser. The basic light-generation mechanism for this device involves the use of 25 active regions composed of 3 quantum wells each. Injection by resonant tunneling occurs in the energy level (level 3) of the first, narrow quantum well. A radiative transition occurs from level 3, in the first well, to level 2, the upper state of the doublet made by two coupled quantum wells. Quick phonon-assisted relaxation from level 2 to 1 insures that level 2 is depleted so that population inversion between levels 3 and 2 can be maintained. Electrons from level 1 then tunnel through the passive region between active regions, which is designed such that, under bias, it allows such tunneling to act as injection into the next active region. Further developments of this type of device are reported in F. Capasso, J. Faist, D. L. Sivco, C. Sirtori, A. L. Hutchinson, S, N. G. Chu, and A. Y. Cho, Conf. Dig. 14th IEEE International Semiconductor Laser Conference, pp. 71-72, Maui, Hi. (Sep. 19-23, 1994); J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett., 66, 538, (1995); J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S, N. G. Chu, and A. Y. Cho, “Continuous wave quantum cascade lasers in the 4-10 μm wavelength region,” SPIE, Vol. 2682, San Jose, pp. 198-204, 1996; and J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Room temperature mid-infrared quantum cascade lasers,” Electron. Lett., Vol. 32, pp. 560-561, 1996. More recently continuous wave (CW) operation has been achieved at 300 K, but with very low power conversion efficiency (&lt;2.5%) and only at wavelengths between 4.8 and 9 μm. See M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior,  Science , Vol. 295, pp. 301-305, 2002; and A. Evans, J. S. Yu, S. Slivken, and M. Razeghi, “Continuous-wave operation at λ˜4.8 μm quantum-cascade lasers at room temperature,”  Appl. Phys. Lett ., Vol. 85, pp. 2166-2168, 2004, Despite this rapid improvement in the performance capabilities of GaInAs/InP-based quantum cascade lasers, it is unlikely that they will ever be able to operate CW at 300 K with high power-conversion efficiency (&gt;10%) at wavelengths of interest in the mid-infrared (3 to 5 μm) and far-infrared (8 to 12 μm) wavelength ranges due primarily to the fact that their electro-optical characteristics are extremely temperature sensitive near and at 300 K. One approach to obtaining efficient room temperature CW operation of intersubband semiconductor lasers in the mid-infrared (3 to 5 μm) and far-infrared (8 to 12 μm) ranges involves the use of two-dimensional arrays of quantum boxes, with each quantum box incorporating a single-stage, intersubband transition structure. See C-F Hsu, J-S. O, P. Zory and D. Botez, “Intersubband Quantum-Box Semiconductor Lasers,” IEEE J. Select. Topics Quantum Electron., Vol. 6, 2000, pp. 491-503; U.S. Pat. No. 5,953,356 entitled “Intersubband Quantum Box Semiconductor Laser.” 
     Room temperature intersubband emission has been reported for single-stage, unipolar devices only from InP-based structures at wavelengths as short as 7.7 μm. C. Gmachl, et al., “Non-Cascaded Intersubband Injection Lasers at λ=7.7 μm,” Appl. Phys. Lett., Vol. 73, 1998, pp. 3822-3830. For 30- to 40-stages, GaAs—AlGaAs quantum cascade lasers at room temperature, intersubband emission wavelengths shorter than 8 μm cannot be achieved, since at higher transmission energies, the active-region upper level is apparently depopulated via resonant tunneling between the X valleys of the surrounding AlGaAs barriers. C. Sirtori, et al., “GaAs—AlGaAs Quantum Cascade Lasers: Physics, Technology and Prospects,” IEEE J. Quantum Electron., Vol. 38, 2002, pp. 547-558. Optimization studies of GaAs-based devices have shown that for thin barriers between the injector region and the active region, two effects occur which cause significant decreases in the upper level injection efficiency: (1) a diagonal radiative transition from injector-region ground level, g, to an active region lower level, and (2) severe carrier leakage from the level g to the continuum. S. Barbieri, et al., “Design Strategies for GaAs-based unipolar lasers: optimum injector-active region coupling via resonant tunneling,” Appl. Phys. Lett., Vol. 78, 2001, pp. 282-284. In addition to these limitations, quantum cascade lasers are conventionally formed of three regions, a superlattice injector, an active region, and a superlattice reflector/transmitter, functioning as an electron Bragg reflector, which is identical in structure to the superlattice injector. This fact severely restricts the device design. Furthermore, for such devices the necessary impurity doping in the superlattice injectors causes a significant increase in the room-temperature threshold-current density due to excited carriers from the doped injector region that fill the lower levels of prior active regions, thus reducing the population inversion. This phenomenon, called carrier backfilling, is the main cause for the extreme temperature sensitivity of the devices characteristics which leads to thermal runaway and very low power-conversion efficiencies. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, semiconductor lasers are formed to provide highly efficient emission at selected wavelengths, which may lie in the mid- to far-infrared range. Such semiconductor lasers constructed to emit in the infrared range provide efficient conversion of electrical energy to electromagnetic energy at infrared wavelengths, and thus may be used as compact, efficient infrared sources for a variety of applications, such as spectroscopy, measurement of gases and liquids for process control and pollution monitoring, infrared signaling, and the like. 
     In the quantum cascade lasers of the present invention, the optical gain required for laser action is provided by the radiative transition of electrons within a given energy band from a higher to a lower energy level. These transitions take place in a plurality of coupled laser stages, each of which has an internal active region in which electrons make a transition from a higher to a lower energy level with the consequent emission of photons at a wavelength related to the change in energy. Depending on the particular materials and dimensions of the laser stages, a range of emission wavelengths of may be achieved using the present devices. For example, in some embodiments emission wavelengths from 3 to 5 μm (e.g., 4.8 μm) may be produced. In other embodiments emission wavelengths from 6 to 12 (e.g., 8 to 12 μm) may be produced. 
     Each stage of the quantum cascade laser (QCL) of the present invention has a multiple layer structure that includes an electron injector, an active region coupled to the electron injector, and an electron reflector coupled to the active region, opposite the electron injector. As used herein, the phrase “coupled to” is used broadly to indicate that at least some electrons are able to pass from one section (e.g., electron injector, active region or electron reflector) of the structure to another. In some instances the coupled sections will be directly adjacent. 
     In sharp contrast to conventional QCL devices the QCL of the present invention has the electron injector separated from the electron reflector. The electron injector, which may be comprised of multiple semiconductor layers, has two minibands, the lower of which allows for the transmission of electrons into an upper energy level of the active region. The active region includes at least one quantum well, having associated therewith an upper energy level and a lower energy level, characterized in that electrons injected into the upper energy level from the miniband of the preceding electron injector undergo a radiative transition to emit a photon as they decay from the upper energy level to the lower energy level. The electron reflector, which may be comprised of multiple semiconductor layers, has a minigap that acts as a reflector for electrons in the upper energy level of the preceding active region and a lower miniband that acts as a transmitter of electrons that have decayed to the lower energy level of the preceding active region into the miniband of an adjacent electron injector. It should be understood that the electrons may undergo one or more phonon-assisted transitions to energy levels lower than the “lower energy level” of the active region subsequent to their initial radiative decay to that lower energy level. Thus, the electrons referred to in the phrase “electrons that have decayed to the lower energy level” include electrons that have undergone phonon-assisted transitions to lower energy levels subsequent to a radiative transition from the upper to the lower energy level in the active region. For typical semiconductor material systems, each one of the laser stages will have transverse dimensions no greater than about 800 angstroms (Å) and preferably no greater than about 600 (Å). Typically, the quantum cascade lasers will include at least ten adjacent laser stages (e.g., 25 or more stages). 
     In the quantum cascade lasers the electron reflector of a first stage (the “up-stream stage”) is adjacent to the electron injector of a the following stage (the “down-stream stage”). This design is advantageous because it allows for increased separation between the injector of the down-stream stage and the active region of the up-stream stage, resulting in significantly decreased electron backfilling and substantially improved power-conversion efficiency. In addition, this design allows the structures of the electron injector and the electron reflector to be optimized independently for improved performance. 
     The active region of each stage has multiple semiconductor layers, including at least one, and desirably more than one, quantum well defined by a semiconductor well layer sandwiched between two semiconductor barrier layers. The quantum wells in the active region are desirably “deep” quantum wells. A deep quantum well in an active region is defined as a quantum well having a well bottom that is lower in energy than the bottoms of the quantum wells in the adjacent electron injector. 
     The quantum wells of the active region and/or the injection barrier of the electron injector may be composite structures. For example, a composite injector barrier layer may comprise two semiconductor layers, the second semiconductor layer having a higher bandgap than the first. The second semiconductor layer of the composite injector barrier being sufficiently thin to prevent scattering to the X valleys during tunneling. A composite quantum well may comprise two adjacent semiconductor well layers, the second semiconductor well layer providing a deeper well than the first. This combination of a composite injector barrier layer and a composite well layer provides for good wavefunction overlap between the E 3  and E 2  levels in the active region and poor wavefunction overlap between the ground state of the electron-injector miniband and the E 2  level in the active region, resulting in improved tunneling efficiency. 
     By way of illustration only, the active region of a laser stage may include a quantum well (or multiple wells, if desired) formed of a layer of InGaAs between layers of AlGaAs, the electron injector may be formed of a superlattice composed of alternating layers of GaAs and AlGaAs, and the reflector may be a Bragg mirror formed from a superlattice composed of alternating layers of GaAsP and InGaAs. 
     The semiconductor structure of the quantum cascade laser of the invention includes layers on each side of the plurality of adjacent laser stages (the “laser core”) to provide conduction across the structure, and a transverse optical waveguide composed of optical-confinement layers and cladding layers on either side of the laser core to provide optical waveguiding of the photons generated in the structure. Electrodes may be formed on the top and bottom surfaces of the multistage structure to allow connection to an external circuit to provide current flow across the structure. For example, for structures based on GaAs material systems, layers adjacent to the multistage structure and the cladding layers may be GaAs layers. Alternatively, the cladding layers may be AlGaAs layers. 
     The semiconductor laser of the invention can be formed of material systems, and on substrates, such as gallium arsenide (GaAs), that are compatible with further semiconductor circuit processing. The semiconductor lasers may be formed on the (100) crystal face of GaAs, but may also be grown on other miscut faces, including the (111) face and the (411) face. 
     A variety of material systems in addition to GaAs, such an indium phosphide (InP), may also be utilized which can similarly be formed to have appropriate intersubband transitions. 
     The semiconductor lasers of the invention are also well suited to being produced using production techniques compatible with a large scale processing, such as metal-organic chemical vapor deposition (MOCVD). 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional view through a quantum cascade laser in accordance with the present invention. 
         FIG. 2  is a simplified cross-sectional view of the multilayer structure of one stage of the quantum cascade laser of  FIG. 1 . 
         FIG. 3  shows the conduction band energy diagram for the multilayer structure of  FIG. 2 . 
         FIG. 4  is a diagram showing the calculated transmission probability as a function of electron energy for the electron injector of the laser stage shown in  FIG. 2 . This figure clearly shows a miniband (high transmission, &gt;5×10 −1 ) for electrons in the E 1  energy level of the active region, and a minigap (low transmission, ≈10 −3 ) for electrons in the E 3  energy level of the active region. 
         FIG. 5(   a ) shows the conduction band energy diagram for a multilayer structure for a laser stage having a single-phonon resonance structure and incorporating an electron injector with a composite injection barrier that includes a tensilely-strained layer to provide strain-compensation for an active layer with compressively strained quantum wells. 
         FIG. 5(   b ) shows a simplified cross-sectional view through the multilayer structure of  FIG. 5(   a ). 
         FIG. 6(   a ) shows the conduction band energy diagram for a laser stage having a double-phonon resonance structure and incorporating an electron injector that includes tensilely-strained barrier layers to provide strain-compensation for an active layer with compressively strained quantum wells. 
         FIG. 6(   b ) is a simplified cross-sectional view of the multilayer structure of the stage of the quantum cascade laser of  FIG. 6(   a ). 
         FIG. 7(   a ) shows the conduction band energy diagram for a multilayer structure for a laser stage having an active region with a thin well layer and a thin barrier layer in the active region for improved tunneling injection efficiency. 
         FIG. 7(   b ) is a simplified cross-sectional view through the multilayer structure of  FIG. 7(   a ). 
         FIG. 8  shows the optical intensity profile and refractive index profile for a transverse waveguide for a quantum cascade laser in accordance with the present invention. 
         FIG. 9  shows a cross-section view through a buried heterostructure semiconductor laser in accordance with the present invention. 
         FIG. 10(   a ) shows the conduction band energy diagram for a multilayer structure for a laser stage having an active region with deep quantum wells that includes semiconductor well layers having a high indium content. 
         FIG. 10(   b ) is a simplified cross-sectional view through the multilayer structure of  FIG. 10(   a ). 
         FIG. 11(   a ) shows the conduction band energy diagram for a multilayer structure for a laser stage having an active region with deep quantum wells that includes semiconductor well layers comprising a nitrogen-containing semiconductor. 
         FIG. 11(   b ) is a simplified cross-sectional view through the multilayer structure of  FIG. 11(   a ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, an example of an implementation of the intersubband quantum cascade laser of the present invention is illustrated generally at  20  in  FIG. 1 , a simplified cross-sectional view through the multi-layer semiconductor structure. For purposes of illustration, the semiconductor laser structure  20  is shown with a top electrode layer  21  and a bottom electrode layer  22 , both of which may be formed on the outer faces of the semiconductor structure of, e.g., conducting metal such as aluminum, gold, etc. The structure further includes top and bottom cladding layers  24  and  25 , respectively, for example, formed of n-type semiconductor. Adjacent the layers  24  and  25  are semiconductor layers  27  and  28 , respectively, which are selected to provide appropriate electrical conduction across them and to have an appropriate index of refraction so as to cooperate with the layers  24  and  25  to provide optical confinement of the emitted light to the region between the layers  24  and  25 . 
     A cross-sectional view of a particular stage  30  of a quantum cascade laser, with exemplary compositions (first column) and thicknesses (second column) of the various layers, is shown in  FIG. 2 . This stage comprises multiple layers of semiconductor material forming an electron injector  34 , an active region  35 , and a distributed Bragg mirror  36  functioning as an electron reflector and transmitter. A conduction band energy diagram for the electron injector  74 , the active region  76  and the electron reflector  78  of stage  30  is illustrated in  FIG. 3 , which also shows the moduli squared of the wavefunctions for energy level E 3    66 , energy level E 2    68 , energy level E 1    70  and the g state  72 , the ground state of the electron injector miniband. 
     As shown in  FIG. 2 , the active region  35  of the laser stage  30  may be composed of InGaAs well layers  38 ,  40  under biaxial compression, and AlGaAs barrier layers  44 ,  46  which are lattice-matched to a GaAs substrate. In the embodiment shown in  FIG. 2 , a composite injection barrier  48  and a composite quantum well  50  are used to optimize the electron tunneling injection efficiency from the electron injector  34  into the active region  35 . 
     In order to achieve efficient lasing for each laser stage in a quantum cascade laser, it is desirable to optimize the tunneling injection efficiency from the ground state of the miniband of the electron injector and the upper energy level (E 3 ) of the active region to close to 100% (e.g., about 90-95%), and minimize incoherent tunneling from the ground state of the miniband of the injector to the lower energy level, E 2 , of the active region. In accordance with these principles, the active region  35  of  FIG. 2  achieves efficient lasing by employing a composite quantum well  50  and a composite injector barrier  48  adjacent to the composite quantum well  50 . The composite injector barrier  48  comprises two adjacent semiconductor layers  42 ,  52  the second semiconductor layer having a higher bandgap than the first. The second semiconductor layer of the composite injector barrier is sufficiently thin to prevent scattering to the X valleys during tunneling. Similarly, the composite quantum well  50  comprises two adjacent semiconductor layers  38 ,  54 , the second semiconductor layer providing a deeper well bottom than the first. This combination of a composite injector barrier and a composite quantum well provides for good wavefunction overlap between the E 3  and E 2  levels in the active region and poor wavefunction overlap between the ground state of the electron injector miniband and E 2  in the active region. As a result, tunneling efficiencies of about 95%, or even higher may be achieved. In the illustrative laser stage depicted in  FIGS. 2 and 3  the composite injector barrier is composed of a first layer of Al 0.3 Ga 0.7 As and a second layer of Al 0.7 Ga 0.3 As and the composite quantum well is composed of a first layer of GaAs and a second layer of InGaAs. 
     The electron injector  34 , allows for high injection efficiency into the upper energy level of the active region. In the illustrative embodiment shown in  FIG. 2 , the electron injector  34  is a superlattice composed of alternating layers of AlGaAs and GaAs. The electron injector is n-type doped (4×10 17  cm −3 ) over the range 80 indicated in  FIG. 3 , and corresponds to a donor sheet density of 4.6×10 11  cm −2  in the superlattice section.  FIG. 4  shows the calculated transmission probability as a function of electron energy for the electron injector of the laser stage shown in  FIG. 2 . This figure clearly shows a miniband (high transmission, ˜10 −1 ) for electrons in the E 1  energy level of the active region, and a minigap (low transmission, ≈10 −3 ) for electrons in the E 3  energy level of the active region. 
     The electron reflector  36 , a superlattice formed of multiple alternating layers of GaAsP and InGaAs, acts as a Bragg reflector for electrons being reflected at energy level E 3  from the active region, and acts as a resonant tunneling structure for electrons escaping from energy level E 1  into the electron injector of an adjacent stage. Similar Bragg reflectors are used for the embodiments shown in  FIGS. 5 and 6 , discussed below. 
     In some embodiments of the laser stages, including the embodiment depicted in  FIG. 2 , the quantum wells in the active region are compressively strained. In such embodiments, strain compensation is desirable. In previous semiconductor lasers, strain compensation was achieved by incorporating tensilely-strained barrier layers between compressively-strained well layers.  FIGS. 5(   a ) and  6 ( a ) show the conduction band energy diagrams for alternative designs that provide strain-compensation for an active region having compressively strained quantum wells. These figures show the composition and thicknesses of the semiconductor layers in some illustrative embodiments. 
     Like the structure shown in  FIG. 3 , the structure shown in  FIG. 5  is a single-phonon resonance structure. In  FIG. 5  strain compensation is provided by using a tensilely-strained layer  502  as the first barrier layer of the electron injector  504  along with a tensilely-strained layer  506  as the first layer of the composite injection barrier  508 . Tensilely-strained layer  506  acts as an intermediate—height barrier layer that decouples the wavefunctions of the ground state of the electron injector miniband and a lower energy level of the active region, but still allows good coupling between the ground state of the electron injector miniband and an upper energy level of the active region. In the particular embodiment shown in  FIG. 5 , tensilely-strained layers  502  and  506  are GaAs 0.62 P 0.38  layers, the other layers making up electron injector  504  are alternating layer of GaAs and Al 0.7 Ga 0.3 As. The active region  510  is composed of a first composite quantum well  512  having a GaAs layer adjacent to a In 0.4 Ga 0.6 As layer and a second quantum well  514  of In 0.4 Ga 0.6 As sandwiched between layers of Al 0.7 Ga 0.3 As. The electron reflector  516  is a superlattice of alternating layers of In 0.3 Ga 0.7 As and GaAs 0.5 P 0.5 . The structure of  FIG. 5  is designed for emission at 4.8 μm. The optical-confinement layers, cladding layers and substrate may be as shown in  FIG. 1 . The tables in  FIG. 5(   b ) show the composition and thickness of each layer represented in the conduction band energy diagram of  FIG. 5(   a ). In the tables, the middle column indicates the Ga fraction for Al 1-x Ga x As or In 1-x Ga x As or the As fraction for GaAs x P 1-x . The far right column in the tables provides the thickness (in Å) of each layer. 
       FIGS. 6(   a ) and ( b ) show the conduction band energy diagram and a simplified cross-sectional view of a device with a double-phonon resonance structure having strain-compensation. In this device, strain compensation is provided by incorporating semiconductor layers that are in tensile strain  602  between well layers  604  and their associated barrier layers  606  in the superlattice of the electron injector  608  (i.e., outside of the active region) of the laser stage. This structure has three deep quantum wells in the active region  610 . The first quantum well is a composite quantum well  613  that includes a layer of tensilely-strained semiconductor  614 . Layer  614  also serves to decouple the wavefunctions of the ground state of the electron injector miniband and a lower energy level of the active region (level E 3  in  FIG. 6) , but still allows good coupling between the wavefunctions of the ground state of the electron injector miniband and an upper energy level of the active region (level E 4  in  FIG. 6 ). In the illustrative embodiment shown in  FIG. 6 , tensilely-strained layers  602  are GaAsP (GaAs 0.60 P 0.40 ), well layers  604  are GaAs or InGaAs (In 0.12 Ga 0.88 As) layers and the barrier layers  606  are AlGaAs layers. The optical-confinement layers, cladding layers and substrate may be as shown in  FIG. 1 . 
       FIGS. 7(   a ) and ( b ) show the conduction band energy diagram and a simplified cross-sectional view an alternative device design for which close to 100% tunneling injection efficiency from the ground state of the injector to the upper energy level of the active region is realized by using a composite injection barrier  702  and a thin well layer  704  (e.g., having a thickness of no more than about 3 nm, and desirably no more than about 2.4 nm) and a thin barrier layer  706  (e.g., having a thickness of no more than about 2 nm and desirably no more than about 1.5 nm) in the active region  708  adjacent to the composite injection barrier  702 . 
     In the laser stage shown in  FIG. 2 , carrier tunneling through the X valley of the Al 0.7 Ga 0.3 As barrier layer  46  of active region  35  may lead to carrier leakage. Therefore, in some embodiments of the present semiconductor lasers it is desirable to include an exit barrier in the active region that prevents such carrier leakage. This exit barrier may be composed of a layer of direct bandgap material, as such materials have X valleys situated in energy much above the F valley. AlInP is an example of a direct bandgap material that may be used as an exit barrier layer in the active region. Alternatively, the exit barrier layer may be a composite exit barrier composed of a layer of indirect bandgap material adjacent to one or two layers of direct bandgap material, wherein the F valleys of the direct bandgap materials have a higher conduction band energy than the X valley of the indirect bandgap material. One non-limiting example of such a composite exit barrier is a layer of Al 0.40 Ga 0.60 As placed before a layer of AlAs. Another example is a layer of AlAs sandwiched between two layers of Al 0.40 Ga 0.60 As. 
     It is noted that for prior QCL devices, a single multi-quantum well (MQW) structure is used as both an electron injector and an electron reflector (Bragg mirror). In the present invention, because the electron injector and the electron reflector are separate elements, the electron injector  34  and the electron reflector  36  can be designed independently for improved efficiency. For example the electron reflector may be composed of undoped semiconductor layers, thus significantly reducing electron backfilling into the preceding active region. 
     The following summarizes the actions of the single-phonon resonance structure of  FIGS. 1-3 : 
     (a) Electrons, after being accelerated under bias across the electron injector  34 , are injected into energy level E 3    66  of the active region  35  via resonant tunneling. They cannot escape to the continuum, because, for a properly designed reflector  36 , the transmission is practically zero for the energy level E 3 . 
     (b) Electrons from the E 3    66  level undergo radiative decay to level E 2    68  and subsequently undergo a non-radiative transition (i.e., phonon-assisted transition) to E 1    70 . 
     (c) Electrons from E 1    70  tunnel through the miniband in the adjacent electron reflector  36  and then into the miniband of the electron injector of a down-stream laser stage. 
     In the illustrative laser stage of  FIG. 2 , the GaAs/AlGaAs superlattice of the electron injector was designed for tunneling to level n=3 of the active region at a field of 60 kV/cm. The double quantum well of the active region was designed for a vertical radiative transition of 190 meV (i.e., λ=6.5 μm). The energy splitting at resonance between the g level (of the electron injector) and the n=3 state is about 6.1 meV, which leads to strong coupling. 
     The following summarizes the actions of the double-phonon resonance structure of  FIG. 6 : 
     (a) Electrons, after being accelerated under bias across the electron injector  608 , are injected into energy level E 4    620  of the active region  610  via resonant tunneling. They cannot escape to the continuum, because, for a properly designed reflector  612 , the transmission is practically zero for the energy level E 4 . 
     (b) Electrons from the E 4    620  level undergo radiative decay to level E 3    622  and subsequently undergo a non-radiative transition (i.e., phonon-assisted transition) to E 2    624 . 
     (c) Electrons from E 2    624  undergo a non-radiative transition (i.e., phonon-assisted transition) to E 1    626 . 
     (d) Electrons from E 2    624  and E 1    626  tunnel through the miniband in the adjacent electron reflector  612  and then into the miniband of the electron injector of a down-stream laser stage. 
     In the illustrative laser stage of  FIG. 6 , the (In)GaAs/AlGaAs superlattice with GaAsP strain-compensation layers of the electron injector was designed for tunneling to level n=4 of the active region at a field of 60 kV/cm. The double quantum well of the active region was designed for a vertical radiative transition of 263 meV (i.e., λ=4.72 μm). 
     In some embodiments, the active region includes at least one deep quantum well that includes a semiconductor well layer having a high indium content. For example, such deep quantum wells may comprise a semiconductor well layer containing at least 30 atomic percent indium. This includes semiconductor well layers comprising at least about 50 percent indium (e.g., about 50 to about 60 atomic percent indium). For example, the at least one deep quantum well may include a well layer of In 0.5 Ga 0.5 As. This well layer may be disposed between barrier layers composed of, for example, AlGaAs (e.g., Al 0.7 Ga 0.3 As). Due to the increased strain in the high indium content well, structures that contain the high indium content semiconductors should be grown at lower temperatures (e.g., ≦550° C.). Alternatively, the at least one deep quantum well may include a semiconductor well layer comprising a nitrogen-containing semiconductor, such as InGaAsN (e.g., In 0.4 Ga 0.6 As 0.995 N 0.005 ). This well layer may be disposed between two barrier layers of, for example, AlGaAs (e.g., Al 0.8 Ga 0.2 As) or GaAsP. 
     The high-indium content deep quantum wells and the nitrogen-containing deep quantum wells, described above, are under compressive strain. Therefore it is desirable to provide strain compensation. One way to achieve strain compensation is by employing an electron reflector composed of a superlattice with tensilely-strained quantum wells adjacent to and down-stream from active regions that incorporate such deep quantum wells. One example of a suitable strain-compensating superlattice includes alternating layers of AlGaAs and GaAsN. Another example of a suitable strain-compensating superlattice includes alternating layers of InGaAs and GaAsP. 
       FIGS. 10(   a ) and ( b ) show the conduction band energy diagram and a simplified cross-sectional view of a device that includes an active region  1000  that includes two deep quantum wells made from high indium content well layers  1002 ,  1004 . Well layer  1002  is disposed between well layer  1006  and barrier layer  1008 . Well layer  1004  is disposed between barrier layer  1008  and barrier layer  1010 . The optical-confinement layers, cladding layers and substrate may be as shown in  FIG. 1 . 
       FIGS. 11(   a ) and ( b ) show the conduction band energy diagram and a simplified cross-sectional view of a device that includes an active region  1100  that includes two deep quantum wells made from well layers  1102 ,  1104  comprising nitrogen-containing semiconductors (InGaAsN). Well layer  1102  is disposed between well layer  1106  and barrier layer  1108 . Well layer  1104  is disposed between barrier layer  1108  and barrier layer  1110 . The optical-confinement layers, cladding layers and substrate may be as shown in  FIG. 1 . 
     As shown in  FIG. 1 , the plurality of adjacent laser stages, which define a “laser core”  56  may be formed between a GaAs upper optical-confinement layer  27  and a GaAs lower optical-confinement layer  28 . (For purposes of simplicity, only three of the laser stages  30 ,  58 ,  60  are shown in  FIG. 1 ) An upper cladding layer  24  of GaAs may be formed over the upper confinement layer  27  and a lower cladding layer  25  of GaAs may be formed between a GaAs substrate  23  and the lower confinement layer  28 . Current confinement is provided by contact metal stripes (e.g., 8 μm wide) as a top electrode  21 , and a bottom electrode  22  may be formed on the bottom face of the substrate  23  so that current can flow between the electrodes transversely through the device. 
       FIG. 8  shows the optical intensity profile  802  and refractive index profile  804  for an alternative waveguide structure. This waveguide, which surrounds an active laser core  803 , includes an Al 0.9 Ga 0.1 As layer  806  sandwiched between two lightly n-type (Si) doped (4×10 16  cm −3 ) GaAs layers  808  and  810 . An outer layer of heavily Si doped (5×10 18  cm −3 ) GaAs n ++   812  completes the optical confinement. For purposes of illustration, the thicknesses of layers  806 ,  808 ,  810  and  812  are approximately 0.5 μm, 0.8 μm, 1 μm and 1 μm, respectively. A thicker high aluminum-content cladding layer tends to improve the confinement, however, this material also has poor conductivity. Therefore, in order to minimize resistance, the cladding layer should not be too thick and should be moderately doped. In some preferred embodiments, the Al 0.9 Ga 0.1 As layer has a thickness of about 0.3 to about 0.8 μm and it is doped about 5×10 17  to 10 18  cm −3 . 
     Another aspect of the invention provides a buried heterostructure semiconductor laser. These lasers are grown on GaAs substrates and use semi-insulating AlGaAs as a burying material. This laser includes a laser core comprising at least one laser stage having an active region containing at least one quantum well. The laser core is sandwiched between upper and lower optical-confinement layers. The upper and lower optical-confinement layers are themselves sandwiched between upper and lower cladding layers. Suitable substrates, cladding layers, confinement layers and active regions are described herein. In one illustrative embodiment, shown in  FIG. 9 , the substrate  902  and cladding layers  904 ,  906  comprise n + -GaAs layers and the confinement layers  908 ,  910  comprise n − -GaAs layers. The laser transverse waveguide can also be as shown in  FIG. 8 . The active region  912  includes at least one laser stage having at least one quantum well. In some embodiments, the laser core include a plurality of coupled active stages, such that the semiconductor laser is a quantum cascade laser. Suitable laser stages are described at length herein. After the semiconductor laser structure is grown on a GaAs substrate, the laser structure is etched as a mesa ridge waveguide. This may be done using a conventional etching process with dielectric masks. The waveguide is then covered with a layer of SiO 2  to prevent AlGaAs growth on top of the upper cladding layer. Layers of semi-insulating Al 0.95 Ga 0.05 As  914  and  915  are then grown on either side of the waveguide to provide a buried heterostructure. The SiO 2  layer is then removed and a thin insulating layer of Si 3 N 4    916 , defining aperture  918 , is deposited (e.g., evaporated) over upper cladding layer  906 . Alternatively the high Al-content Al x Ga 1-x As grown material can also be partially and controllably removed through a simple etch process using aqueous HCl or HF, which will stop at the GaAs-topped buried ridges. Finally, a layer of metal  920  is deposited over insulating material  916  and upper cladding layer  906 . This buried heterostructure significantly improves heat removal from the laser core and reduces lateral waveguide losses. 
     The semiconductor structures represented in the figures may be produced by conventional semiconductor processing techniques, and do not require the use of molecular beam epitaxy for crystal growth. For example, by utilizing a state-of-the-art MOCVD reactor, high-quality semiconductor multi layers can be fabricated. 
     Various semiconductor material systems may be utilized in the present invention. In general, GaAs-based structures grown on GaAs substrates are preferred. Nonetheless, the invention may be implemented using the InP-based material system as well as others. 
     It is understood that the invention is not confined to the particular embodiments set forth herein as illustrative, but embraces all such modified forms thereof as come within the scope of the following claims.