Semiconductor lasers have long served as light sources in fiber-optic communication systems, but are not without shortcomings. Low-cost vertical cavity surface-emitting lasers (VCSELs) are favored in these systems, but they consume more power than necessary. Their gain sections are longer than needed because difficulties of the fabricating method—selective oxidation—impair the reliability of small gain regions.
This fabrication problem can be remedied by ion implantation—a technique that selective oxidation has largely supplanted—but the method has its own issues. While ion-implanted VCSELs have excellent reliability, they suffer from large variations in important parameters, such as output power, that ultimately lower manufacturing yield.
We show here that both of these problems can be addressed by using optical lithography, which is able to define a small gain region with great accuracy and reliability. This promising method, based on a mature technology, would enable very low VCSEL power consumption. In our design,1 we use optical lithography to pattern the laser cavity, which lies between the mirrors and includes the gain section. We then etch away the cavity outside the gain section and replace it with epitaxial semiconductor layers that funnel the drive current into the gain region.
This design was evaluated in VCSEL-like structures (see Figure 1) consisting of a low-reflectivity distributed Bragg reflector situated below a cavity region containing three very thin semiconductor quantum well layers that provide the optical gain and light amplification. The lower part of the cavity was n-type doped, and the upper part was p-type doped. Outside the gain region, the upper cavity was etched away and replaced by a semiconductor layer structure with a p-type doped lower layer, an n-type doped middle layer, and a p-type doped upper ‘p-contact’ layer.
Figure 1. This schematic shows the semiconductor layer structure used to demonstrate the use of optical lithography to make small gain regions. DBR: distributed Bragg reflector.
The layered structure was grown to funnel current into the gain region by orienting the region sides along certain directions relative to the semiconductor crystal. The funneling occurs when a bias is applied between contacts on the p-contact layer and the lower n-type doped cavity portion. Placing the contacts inside the cavity allowed us to deposit a low-loss dielectric mirror on top of the cavity to create a high-efficiency laser structure.
Using this approach, we fabricated gain regions with sides ranging from 1 to 10 μm in length and able to convert up to 70% of the electrical current to photons. The remaining current fraction was not converted because the topmost quantum well was damaged during the fabrication process. This type of damage, however, is not inherent to the process and can be avoided in future work.
We have shown that it is possible to exploit optical lithography's precision to create very small gain regions that other methods cannot reliably manufacture. Future work will focus on fabricating a full VCSEL structure to evaluate the properties of our gain region design in a laser application.