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Vertical-cavity surface-emitting laser employing native oxide layers in the distributed Bragg reflector mirrors and confinement apertures

Technical Challenge:

Vertical-cavity surface-emitting lasers (Vessels) offer several manufacturing and performance advantages over conventional edge-emitting diode lasers. But two areas of VCSEL manufacture-both of which affect VCSEL efficiency and performance-that challenge device designers and fabricators are (1) the formation of distributed Bragg reflector (DBR) mirrors, and (2) the provision of transverse electrical and optical confinement. Of particular interest is incorporation of these two device features into VCSEL structures lattice matched to the Group III-V semiconductor material InP and operating at the technologically important 1.3-µm and 1.55-µm wavelengths.

Distributed Bragg reflectors comprise alternating layers of two materials transparent to the VCSEL emission wavelength and having different refractive indices: the larger the difference in refractive index between the two materials, the smaller is the number of alternating layers required to achieve high reflectivity. For manufacturing efficiency, DBRs comprising alternating layers of semiconductor materials lattice matched to the substrate material on which the VCSEL is based are most desirable, allowing "monolithic" formation of the entire structure during a single epitaxy stage of device fabrication. However, the difference in refractive index between such similar materials, particularly materials lattice matched to InP, is small, requiring a large number of alternating layers-and hence thick DBR structures-to achieve high reflectivity. Various techniques have been devised to overcome this disadvantage, such as hybrid dielectric and wafer bonding schemes. But these methods require multiple epitaxy steps and other time-consuming, complicated processing that increase the cost of VCSEL production.

The fabrication of DBRs--particularly those employed in InP-based VCSELs--could be simplified if the refractive indices of the two materials that compose the alternating layers were significantly different. The DBR thus formed would require significantly fewer layers. For instance, the wet thermal oxide of AlxIn(1-x)As latticed matched to InP (x approximately equal to 0.48) has a refractive index significantly lower than that of semiconductor materials lattice matched to InP. But, while an efficient DBR comprising alternating layers of this oxide and a Group III-V semiconductor material lattice matched to InP would require only around seven pairs of layers, the wet thermal oxidation rate of Al0.48In0.52As is slow, rendering it unsuitable for DBR formation in InP-based VCSEL production.

A challenge equal to DBR formation is the provision of transverse electrical and optical confinement for the purpose of enhancing VCSEL efficiency and performance. Post-epitaxy ion implantation has been widely used in commercial VCSEL production to provide electrical confinement. But this technique does not provide optical confinement, which can enhance lasing efficiency. Recently, a wet thermal selective oxidation technique has been employed in commercial VCSEL production to form a "buried" oxide aperture, affording both transverse electrical and optical confinement in AlxGa(1-x)As-based VCSELs lattice matched to GaAs substrates. The enabling feature of this technique is incorporation into the VCSEL structure of a high aluminum content AlxGa(1-x)As layer that is oxidized using a wet thermal process. This technique is successful because AlxGa(1-x)As (1) is nearly lattice matched to the GaAs substrates on which the devices are based and (2) can be rapidly and controllably oxidized using a wet thermal oxidation process. The low refractive index and the electrical insulating properties of the buried oxide aperture thus formed provide index guiding for photons and dielectric guiding for current carriers, imparting transverse definition to the laser cavity. But transferring this technique to the fabrication of VCSELs lattice matched to commercially available substrates other than GaAs (particularly InP) has been hindered by the apparent nonexistence of material that is lattice matched to those other substrate materials and that is amenable to wet thermal oxidation in a rapid, well-controlled manner-a circumstance similar to that which hinders the formation of oxide-based DBRs, as mentioned in the previous paragraph.

In summary, the manufacturing and performance efficiency of VCSELs-particularly those lattice matched to InP substrates-could be enhanced by employing oxide layers in the formation of DBRs and confinement apertures. But the nonexistence of suitable materials for oxide formation hinders progress in this area.

Description:

The technology is a vertical-cavity surface-emitting laser (VCSEL), the design of which employs native oxide layers in the distributed Bragg reflector (DBR) mirrors and in the optical and current confinement aperture. The native oxide layers are formed by wet thermal oxidation of a strain-compensated superlattice comprising Group III-V semiconductor materials.

Demonstration Capability:

There is no demonstration of the invention. A key component of the invention, native oxide layers formed by the wet thermal selective oxidation of the superlattice, can be demonstrated using electron microscope images and performance data recorded from devices that incorporate that technology.

Potential Commercial Application(s):

Laser manufacturing.

Patent Status:

Issued - United States Patent Number 6,493,366

Reference Number: 1183-3

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National Security Agency
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Fort George G. Meade, Maryland 20755-6541