Laser Stripe Structures The simplest semiconductor laser stripe structure is
called an
oxide stripe laser
. The metallic contact on the n - doped side of a
semiconductor laser is normally
applied with no definition for current confinement
;
current confinement is introduced on the p side of the device . For a wide-stripe laser
, a dielectric coating (usually SiO 2 or Si3N4 ) is evaporated on the p side of the laser .
Contact openings in the dielectric are made through photolithography combined with
etching of the dielectric . The p metallic contact is then applied across the whole
device , but makes electrical contact only at the dielectric openings . since the active
region extends outside of the stripe , there is no mechanism to prevent optical leakage
in a contact-stripe laser . Lasers like this , which provide electrical confinement , but
no optical confinement are called
gain - guided lasers
.
Gain-guide laser
is an ion bombardment stripe. The material outside the stripe is
made highly resistive by ion bombardment or implantation which produces lattice
defects . Implantation causes optical damage , so implantation should not be heavy
enough to reach the active region .
Complicated stripe structure with electrical and optical confinement is required for
an efficient narrow-stripe laser . A number of structures which accomplish the
necessary confinement have been developed. These structures are called
index -
guided lasers
, since optical confinement is achieved through a change in refractive
index. The
buried heterostructure laser (BH),
a planar laser structure is first grown .
Stripe mesas of the laser structure are formed by photolithography combined with
etching For a GaAs-based BH laser , AlGaAs is then regrown around the lasing stripe .
Since the active region is completely surrounded by AlGaAs , a BH has tight optical
confinement . If the regrown layers are doped to produce a reverse-biased junction or
are semi-insulating , a BH laser can also provide good current confinement There are
many variations on the BH structure . In some cases the active region is grown in the
second growth step. The tight optical confinement of BH lasers allows practical
fabrication of very narrow stripes , on the order of 1 to 2 μm.
other stripe structures that provide weaker optical confinement than a buried
heterostructure . One of the simplest and most widely used of these is the
ridge
waveguide laser (RWG)
. After epitaxial growth , most of the p -cladding layer is
etched away , leaving a mesa where the lasing stripe will be . Only this mesa is
contacted , which provides electrical confinement . Another type of laser stripe is one
in which confinement is provided by the p – n junction . The best-known laser of this
type is
the transverse junction stripe
. In order to fabricate a TJS laser , both cladding
layers are grown as n -AlGaAs. Zn diffusion is then used to create a p - n junction and
contacts are applied on either side of the junction. In this laser the current flows
parallel to the substrate rather than perpendicular to it . In a TJS laser the active
region is limited to the small region of GaAs in which the Zn diffusion front ends.
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