Biomedical Engineering Reference
In-Depth Information
Proton
implantation
Light output
N-contact
Substrate
(a)
Active
region
DERs
Substrate
Oxide
aperture
Oxide
aperture
Heat-spreader
Substrate
(b)
(c)
P-contact
FIGURE 2.42
Three common types of VCSEL structures: (a) a top-emitting structure with proton implan-
tation to confine the current, (b) a selectively oxidized top-emitting structure to confine the
optical modes and/or the current, and (c) a mounted bottom-emitting selectively oxidized
structure. (Courtesy of Princeton Optronics [PO] and Dr. Chuni Ghosh. With permission.)
In contrast, edge emitters are made up of cleaved bars diced from the
wafers. Because of the high index of refraction contrast between air and
the semiconductor material, the two cleaved facets act as mirrors. Hence,
in the case of an edge emitter, the light oscillates parallel to the layers and
escapes sideways. This simple structural difference between the VCSEL and
the edge emitter has important implications.
Since VCSELs are grown, processed, and tested while still in the wafer
form, there is significant economy of scale resulting from the ability to con-
duct parallel device processing, whereby equipment utilization and yields
are maximized, and setup times and labor content are minimized. In the
case of a VCSEL (see Figure 2.41), the mirrors and active region are sequen-
tially stacked along the Y -axis during epitaxial growth. The VCSEL wafer
then goes through etching and metallization steps to form the electrical con-
tacts. At this point, the wafer goes to test where individual laser devices are
characterized on a pass-fail basis. Finally, the wafer is diced and the lasers
are binned for either higher level assembly (typically >95%) or scrap (typi-
cally <5%). Figure 2.43a shows a single high-power VCSEL device (>2 W out-
put power) packaged on a high-thermal conductivity submount. Figure 2.43b
shows the L-I characteristics of a 5 W VCSEL device.
In a simple Fabry-PĂ©rot edge emitter, the growth process also occurs along
the Y -axis, but only to create the active region, as mirror coatings are later
applied along the Z -axis. After epitaxial growth, the wafer goes through the
metallization step and is subsequently cleaved along the X -axis, forming a
series of wafer strips. The wafer strips are then stacked and mounted into a
coating fixture. The Z -axis edges of the wafer strips are then coated to form
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