Recently, there has been interest in a new type of light emitting device called a vertical cavity surface emitting laser (VCSEL). Conventional VCSELs have several potential advantages, such as a planar construction, emitting light perpendicular to the surface of the die, and the possibility of array fabrication.
Typically, VCSELs include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
The use of mirror stacks in VCSELs is well established in the art. Typically, mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an Al.sub.x1 Ga.sub.1-x1 As.backslash.Al.sub.x2 Ga.sub.1-x2 As material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content x1 and x2 in the layers, more particularly the aluminum content x1 ranges from 0% to 50% and the aluminum content of x2 ranges from 50% to 100%. In conventional devices, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
In conventional VCSELs, conventional material systems perform adequately. However, new products are being developed, such as light sources for gigabit/sec ethernet and fiber channel applications, that require VCSELs to operate at a higher speed. Accordingly, there exists several problems related with the conventional VCSELs operating at such high speed, such as the limitation of the VCSEL speed due to lateral carrier diffusion when modulated at GHz frequency, of which such carrier diffusion causes long optical decay tails following the switch-off of the driving source.
These problems are generally caused by the way the carrier is injected into the active area. In that the carrier distribution area is larger than the optical mode area, the carriers outside of the optical mode region in the active area will backfill the area overlapped with the optical mode, thereby causing optical tailing effects when the driving current is switched off.
It can readily be seen that conventional VCSELs and the conventional methods of fabricating them will not be able to achieve performance levels in terms of high speed operation, and the like required for today's marketplace. Thus, there is a need for developing a reliable, stable and cost effective vertical cavity surface emitting laser (VCSEL) for use in high speed operations, that includes a shorter carrier lifetime so as to reduce device turn-off tails, thus allowing operation of the device at higher speeds.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. Accordingly, it is an object of the present invention to provide a new and improved VCSEL for use in high speed operations.
Another object of the invention is to provide a reliable high speed VCSEL.
And another object of the immediate invention is to provide for a shorter carrier lifetime in a high speed VCSEL, thereby reducing device turn-off tails.
Yet another object of the invention is to provide for a highly manufacturable high speed VCSEL.