Semiconductor superlattice structures including alternating quantum well and quantum barrier layers are well known. In the basic unit of these structures, a thin semiconductor layer having a relatively narrow energy band gap is sandwiched between thin layers of a different semiconductor material having a relatively wide energy band gap. A quantum well is thus formed in which charge carriers can be confined and from which, under appropriate circumstances, charge carriers can escape by tunneling or another charge carrier transport mechanism. In a multiple quantum well structure, a plurality of these basic units are sequentially disposed. Superlattice structures including one or more quantum wells have found applications in numerous semiconductor devices including semiconductor lasers and other light-interactive devices.
When the different crystalline semiconductor materials used in the quantum well layers and in the quantum barrier layers have different lattice constants, an atomic mismatch between adjacent crystalline layers occurs. The lattice mismatching of the crystalline structures adjacent each of the quantum well/quantum barrier layer interfaces causes strain in the layers and affects the electrical characteristics of the structure. While considerable effort has been devoted to producing superlattice structures essentially free of strain, in recent years it has been determined that beneficial results can be achieved if some strain is present in a superlattice structure. For example, wavelength of the light produced by a semiconductor laser incorporating a so-called strained superlattice structure as the active layer can be shifted compared to a similar semiconductor laser employing a superlattice structure active layer free of strain. In addition, semiconductor lasers incorporating strained superlattice structures exhibit lower threshold currents for laser oscillation and lower operating currents than similar semiconductor lasers without superlattice structures.
While strain in a superlattice structure can produce beneficial effects in a device incorporating the strained superlattice structure, an excessive amount of strain can cause detrimental effects and even prevent device operation. Assuming each quantum well layer and quantum barrier layer interface introduces the same amount of strain into a superlattice structure, the total strain present in prior art structures is equal to the product of that strain and the number of such interfaces in the strained superlattice structure. Generally, the strain at each interface between a quantum well layer and quantum barrier layer is directly related to the difference between the lattice constants of the semiconductor materials employed in the quantum well and quantum barrier layers. When the total strain in a superlattice structure exceeds a critical value, the dislocation density in the semiconductor layers increases significantly, resulting in a severe deterioration of the crystallinity of subsequently grown semiconductor layers that adversely affect the characteristics of a device, such as a laser, incorporating the structure.
An example of a semiconductor laser incorporating a strained superlattice structure as an active layer was described by Shieh et al in Electronics Letters, Volume 18, Number 25, 1989, pages 1226-1228. In that laser structure, cladding layers of Al.sub.0.2 Ga.sub.0.8 As sandwiched light guide layers of gallium arsenide (GaAs) which, in turn, sandwiched a strained superlattice structure active layer. The well layers in the strained superlattice structure were Ga.sub.0.8 In.sub.0.2 As and the barrier layers were GaAs. Those authors prepared a number of such semiconductor laser structures including 1 and 3-6 quantum wells, respectively.
In FIG. 4(a), an idealized energy band structure of the strained superlattice laser described by Shieh is schematically shown. Quantum barrier layers 13 sandwich quantum well layers 14 of the strained superlattice structure 15. The outermost quantum well layers lie adjacent to the respective GaAs light guide layers 17. The light guide layers are, in turn, sandwiched by the Al.sub.0.2 Ga.sub.0.8 As cladding layers 12 and 16.
FIG. 4(b) is a graph of the degree of lattice mismatch, .DELTA.a/a, as a function of position within the Shieh laser structure where a is the lattice constant of GaAs. The graph of FIG. 4(b) is plotted as the percent of lattice mismatch. Thus, in the cladding layer 16 and the light guide layer 17 adjacent cladding layer 16, the lattice mismatching is zero because aluminum arsenide (AlAs) and GaAs have essentially the same lattice constant. In the strained superlattice structure 15, the lattice mismatch with the cladding and light guide layers is about plus 1.4 percent for the quantum well layers and zero percent for the quantum barrier layers. In other words, as the strained superlattice structure 15 is grown on the light guide layer 17, the strain increases, i.e., accumulates, with each quantum well layer and quantum barrier layer that is grown. The total strain within the laser structure constantly increases with additional superlattice layers so that the advantages of the strained superlattice structure gradually decline with an increasing number of quantum wells. In fact, as shown in Shieh's FIG. 2, threshold current density increases with increasing quantum wells and increases unacceptably in a structure including six quantum wells. At that point, the total strain has become so large that it totally overwhelms the improvements achieved by employing the strained superlattice structure.
Because of the increasing strain and the adverse effects of strain within a strained superlattice structure as the number of quantum wells is increased, the performance improvement that can be achieved in light-interactive semiconductor devices incorporating strained superlattice structures is limited. These strain limitations prevent the use of an arbitrary number of quantum wells in a strained superlattice structure and limit the practical realization of light-interactive devices incorporating strained superlattice structures.