Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-07-16
2004-11-02
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06813297
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed towards the field of lasers, and more specifically, towards alloys that can be used in the active region of a laser.
BACKGROUND OF THE INVENTION
Vertical cavity surface emitting lasers (VCSELs) are commonly used as light sources in optical communication systems.
FIG. 1
shows a diagram of a prior art VCSEL
101
, based on a gallium arsenide (GaAs) substrate
102
. VCSEL
101
emits light at 850 nm. Two mirror stacks
103
, one adjacent to the substrate
102
and one at the top of the VCSEL
101
, reflect the laser light within the VCSEL
101
. The mirror stacks
103
are typically Distributed Bragg Reflectors (DBRs) made of alternating layers of Al
x
Ga
1−x
As and Al
y
Ga
1−y
As, where “x” and “y” denote the molecular fractions of Al in high and low refractive index layers, respectively. A cladding layer
107
is adjacent to each mirror stack
103
. Although each cladding layer
107
is illustrated as a single layer, it may be composed of many different layers. The cladding layer
107
may also be called a spacer, and is used to pad the size of an active region
109
so that the VCSEL
101
will work properly. Sandwiched between the mirror stacks
103
and cladding layers
107
is the active region
109
, comprising interleaved layers of quantum wells
111
and barrier layers
113
. The quantum wells
111
have a width w. The quantum wells
111
are typically GaAs, and the barrier layers
113
are typically AlGaAs. Hereinafter, VCSELs shall be referred to by the composition of their active region. Therefore, the VCSEL
101
can be identified as a GaAs/AlGaAs VCSEL, or alternatively as a VCSEL with a GaAs/AlGaAs active region.
FIG. 2
shows an energy-band diagram identifying selected band parameters for an active region of a laser such as the VCSEL
101
shown in FIG.
1
. The conduction band is labeled E
c
and the valence band is labeled E
v
. The difference between the conduction band E
c
and the valence band E
v
is known as a band gap. The band gap of the quantum well
111
is labeled Eg
QW
. The band gap of the barrier layer
113
is labeled Eg
B
. The difference between the conduction bands E
c
of the quantum well
111
and the barrier layer
113
is known as the conduction band offset, labeled &Dgr;E
c
. The difference between the valence bands E
v
of the quantum well
111
and the barrier layer
113
is known as the valence band offset, labeled &Dgr;E
v
. Electrons and holes (collectively known as carriers) are injected into the quantum well
111
and confined by the barrier layers
113
when the VCSEL is forward biased. Ideally, the materials used in the quantum wells
111
and barrier layers
113
have a relatively large &Dgr;E
c
and &Dgr;E
v
to provide effective carrier confinement in the quantum well
111
. In a typical GaAs/AlGaAs VCSEL
101
, &Dgr;E
c
≈150 meV and &Dgr;E
v
≈75 meV. Note that &Dgr;E
c
is twice &Dgr;E
v
; a 2:1 ratio between &Dgr;E
c
and &Dgr;E
v
is often considered indicative of a well-balanced material system.
Carriers inside the quantum well
111
actually acquire a slight amount of energy as a result of their confinement, effectively increasing the quantum well bandgap Eg
QW
by the energy of quantum confinement dE
qc
(not shown). dE
qc
is a function of the quantum well width w, increasing as w is decreased. When the active region
109
is not lattice-matched to the substrate
102
, the carriers within the quantum well acquire an additional energy due to lattice strain dE
strain
(not shown). Although the band parameters described above refer specifically to the active region
109
of the VCSEL
101
, the terms are equally applicable to the active region of any laser.
Light is emitted from the quantum well
111
when electrons drop from the conduction band E
c
to the valence band E
v
. The wavelength of light emitted is determined approximately by the formula:
λ
um
≈
1.24
eV
E
g
QW
+
dE
qc
+
dE
strain
(
Equation
1
)
In Equation 1, E
g
QW
is the greatest contributing factor in determining the wavelength, as it is typically much larger than dE
qc or
dE
strain
. The material used for the quantum well
111
should be selected to have a band gap E
g
QW
that will produce light within the desired range of wavelengths. The quantum well width w and lattice strain on the substrate
102
will also be a consideration because of dE
qc
and dE
strain
.
GaAs/AlGaAs is ideal for the active region in a GaAs-substrate VCSEL for several reasons. First, GaAs and AlGaAs can be used to implement both the mirror stacks
103
and the active region
109
, thus simplifying the manufacturing process because there is no need to change the growth conditions. Second, mirror stacks
103
using AlGaAs/AlGaAs can be epitaxially grown on the GaAs substrate
102
, which results in a VCSEL that is entirely grown epitaxially. Since fully-epitaxial VCSELS are easier to manufacture and process, they are preferred over VCSELS having mirror stacks formed with other methods such as fusion bonding or deposition. Third, GaAs/AlGaAs VCSELs can be oxidized. Oxidized layers are often used in a VCSEL to electrically confine carriers and optically confine the laser beam, which leads to improved electro-optical performance of the device.
One final reason that GaAs/AlGaAs VCSELs work well is due to their low sensitivity to temperature. A VCSEL typically has to maintain performance within an operating temperature range between 0-100° C. One parameter used to measure temperature sensitivity is known as the characteristic temperature T
0
. T
0
is usually determined for broad area lasers (also known as edge-emitting lasers), not for VCSELs. However, the T
0
of an edge-emitting laser built with a given active region is still a useful indicator of how that same active region will perform with temperature changes in a VCSEL. A high characteristic temperature T
0
is preferable because it means the laser is less sensitive to temperature fluctuations. An edge-emitting laser built with a GaAs/AlGaAs active region typically has a characteristic temperature T
0
around 150K, which is relatively high. The characteristic temperature T
0
is also related to &Dgr;E
c
and &Dgr;E
v
—an active region with large &Dgr;E
c
and &Dgr;E
v
will likely exhibit high T
0
and low threshold current density, provided that the material quality is good.
The light emitted from a GaAs/AlGaAs VCSEL typically has a wavelength around 850 nm, which allows for a transmission range of 200-500 m in multimode fiber, depending on the speed of the optical link. Currently, the challenge facing the optical communications industry is creating a VCSEL capable of emitting light with a longer wavelength, which can travel longer distances along a single-mode optical fiber. The preferable target wavelength range is between 1.2 um and 1.4 um, or more specifically, 1260-1360 nm, which would produce transmission ranges of 2-40 km. The ideal long-wavelength VCSEL would possess the same qualities as a GaAs/AlGaAs VCSEL (i.e. epitaxially grown mirrors, active regions that are lattice matched to the substrate, good carrier containment, low sensitivity to temperature changes, etc.) except with a longer wavelength of emitted light.
Several material systems have been proposed that would emit light within the target range. One approach is using InGaAsN/GaAs or InGaAsN/GaAsN (hereinafter collectively referred to as InGaAsN/GaAs(N)) in the active region on a GaAs substrate. InGaAsN/GaAs(N) has acceptable performance over the desired temperature range. Unfortunately, although InGaAsN/GaAs(N) can be epitaxially grown on the GaAs substrate, the lattice structure does not match well to the GaAs substrate and introduces a compressive strain of 3% or more. Such a large strain may cause undesirable reliability problems in a VCSEL.
Another approach to long-wavelength VCSELs involves using a substrate of indium phosphide (InP). InP has been researched extensively as a VCSEL substrate, and many materials have been identified that
Agilent Technologie,s Inc.
Nguyen Tuan N.
Shie Judy I.
Wong Don
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