OE-14 Laser Technology
Unit 4
Part 4
Master of Technology
Semester I
Dr. Vivek Kant Jogi
LASER STRUCTURE
Homojunction laser
: which uses a single junction. These are fabricated of a
single junction between
two direct-bandgap materials
of the same type, one p-type
and one n-type, that is called a homojunction since both materials are of the same
type. Light is emitted by electron–hole pair recombination's in the thin active region
formed by the junction of the two materials (the depletion region).
Mainly gallium arsenide (GaAs) is used, with each part of the device doped slightly
differently: one part with an electron donor and one part with an electron acceptor.
Mirrors for the laser cavity are fabricated simply by cleaving the crystal at right angles
to the laser axis. Having an index of refraction of 3.7, the reflectivity of each mirror
may be calculated to be 33% by using the Fresnel equations. This represents a large
loss in the cavity; Improved performance may be achieved by fabricating a single
dielectric mirror, composed of alternating quarter-wavelength-thick layers of high-
and low-index-of-refraction materials, at the HR end of the laser diode.
Heterojunction Lasers
: two interfaces of different indexes of refraction,
one on top and one below the active region, so two junctions are formed in what
is called a heterostructure laser diode, or a double heterostructure, since there are
two
confining
interfaces.
The
double-heterostructure
arrangement
confines
intracavity light in only one direction (top and bottom) of the GaAs layer, further
improvement in performance can be made by manufacturing the device so that a
confining dielectric interface exists on all four sides of the active region in a
buried
heterostructure laser
Single Heterojunction Laser under
forward bias
Double Heterojunction Laser under
forward bias
Use of single Heterojunction for carrier
confinement
AlGaAs
Heterojunction
grown on thin p-type GaAs layer
A double-heterojunction laser structure ,
multi layers used confine injected carriers
and provide wave guiding for light.
A strip geometry designed to
restrict the current injection to a
narrow stripe along the lasing
direction.
One
of
many
methods
for
obtaining the strip geometry , this
example is obtain by proton
bombardment
of
the
shaded
region which converts the GaAs
and AlGaAs to semi-insulating
form.
In order to fabricate a double heterostructure, it is necessary to find materials that
can be grown on these substrates, which have bandgaps different from the substrate
material, and which have a lattice constant compatible with the substrate.
The simplest example is
AlxGa1-xAs
, which consists of two group III elements (Al and
Ga) and one group V element.
The bandgap increases with the Al fraction (x), but the
lattice constant remains nearly unchanged
. A constant lattice constant is important in
order to avoid the formation of defects during the growth of the material. A double
heterostructure semiconductor laser is formed by having an active layer with a low Al
content, and confinement layers with a high Al content.
The difference in Al content
must be sufficiently high to ensure sufficient carrier confinement.
The photon energy
for the laser is slightly higher than the bandgap of the active layer, and by changing
the Al fraction in the active layer, this photon energy can be changed, resulting in
lasers with wavelengths in the 800 – 900 nm range.
Because the photon energy is higher than the bandgap of the substrate (GaAs), the
lower cladding layer must be sufficiently thick to avoid absorption losses in the
substrate.
More design flexibility is possible by using two group III and two group V materials.
The prime example is In1-xGaxAsyP1-y. Lattice matching to an InP substrate is
achieved by having x ~ 0.47y and the second degree of freedom in the composition
can be used to vary the bandgap, and hence the wavelength.
This makes it possible to
fabricate lasers with wavelengths from around 1100 nm to nearly 1700 nm
, covering
the important ‘telecommunications’ wavelengths around 1300 and 1550 nm.
The most commonly used technique for growth of heterostructures is metal-organic
chemical vapour phase epitaxy (
MOVPE
). Hydrides such as arsine (AsH3), phosphine
(PH3) and organometallics such as tri-methyl-gallium Ga(CH3)3 and tri-ethylindium
In(CH3)3 are carried by hydrogen and react on the surface of the wafer. The material
composition is controlled by adjusting the flow rate of the various sources. Large
wafers can be grown, and some reactors allow multi-wafer handling, making this
technique suitable for large volume manufacturing. MOVPE requires very stringent
safety measures due to the toxicity of the hydrides.
Micrograph of widely tunable laser from Bookham Technology. This particular structure,
known as the ‘
digital supermode structure
’ has multiple individually contacted gratings
in the front section. Courtesy Bookham Technology.
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