Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-21
2003-09-09
Davie, James (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06618413
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the threshold voltage for a nitride based laser diode structure and, more particularly, to graded semiconductor layers for reducing the threshold voltage of a nitride based laser diode structure.
Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers. The active layers of the monolithic multi-layered laser structure are bounded at their side ends by cleaved surfaces that act as mirrors. Optical feedback is provided by the cleaved mirrors and a standing wave is formed between the mirrors in the laser resonator with a wave front parallel to the mirrors. If the optical gain produced in the active layers exceeds the optical loss in the laser structure amplified stimulated emission is produced and coherent laser light is emitted through the mirrored edges of the semiconductor laser structure.
The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. These layers with excess electrons are said to be n-type, i.e. negative. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. These layers with excess holes are said to be p-type, i.e. positive.
An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to “inject” them into the active layers, where electrons recombine with holes to produce light.
The interface between adjacent semiconductor layers in the multi-layered structure can present barriers to the electrical potential. Threshold voltage is the minimum voltage applied between the two electrodes causing the active layers to emit light.
Nitride based semiconductors, also known as group III nitride semiconductors or Group III-V semiconductors, comprise elements selected from group III, such as Al, Ga and In, and the group V element N of the periodic table. The nitride based semiconductors can be binary compounds such as gallium nitride (GaN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium aluminum nitride (InGaN), and quarternary alloys such as indium gallium aluminum nitride (InGaAlN). These materials are deposited on substrates to produce layered semiconductor structures usable as light emitters for optoelectronic device applications. Nitride based semiconductors have the wide bandgap necessary for short-wavelength visible light emission in the green to blue to violet to the ultraviolet spectrum.
These materials are particularly suited for use in short-wavelength light emitting devices for several important reasons. Specifically, the InGaAlN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under the high electric current and the intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
Semiconductor laser structures comprising nitride semiconductor layers grown on a sapphire substrate will emit light in the ultra-violet to visible spectrum within a range including 280 nm to 650 nm.
The shorter wavelength violet of nitride based semiconductor laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for high-resolution or high-speed laser printing operations and high density optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers.
A prior art nitride based semiconductor laser structure
100
of
FIG. 1
has a sapphire (Al
2
O
3
) substrate
102
on which is epitaxially deposited a succession of semiconductor layers. The sapphire substrate
102
typically has a thickness of 200 micron to 1000 micron.
The prior art laser structure
100
includes an n-type III-V nitride nucleation layer
104
formed on the sapphire substrate
102
. Typically, the nucleation layer
104
is undoped GaN and has typically a thickness in, the range between 10 nm and 30 nm.
A III-V nitride contact layer
106
is formed on the nucleation layer
104
. The III-V nitride layer
106
is an n-type GaN:Si layer acting as a lateral n-contact and current spreading layer. The contact and current spreading layer
106
typically has a thickness of from about 1 &mgr;m to about 20 &mgr;m.
A III-V nitride cladding layer
108
is formed over the contact layer
106
. The III-V nitride layer
106
is an n-type AlGaN:Si cladding layer. The cladding layer
106
typically has a thickness of from about 0.2 &mgr;m to about 2 &mgr;m.
On top of the III-V nitride cladding layer
108
, a III-V nitride waveguide layer
110
is formed followed by the III-V nitride quantum well active region
112
. The n-type GaN:Si waveguide layer
110
typically has a thickness of from about 50 nm to about 200 nm. The quantum well active region
112
is comprised of at least one InGaN quantum well. For multiple-quantum well active regions, the individual quantum wells typically have a thickness of from about 10 Å to about 100 Å and are separated by InGaN or GaN barrier layers which have typically a thickness of from about 10 Å to about 200 Å.
A III-V nitride tunnel barrier layer
114
is formed over the quantum well active region
112
. The p-type AlGaN:Mg tunnel barrier layer
114
serves as electron blocking layer and carrier confinement layer to keep electrons from leaking out of the active region
112
and has a thickness of from 15 nm to 20 nm.
A III-V nitride waveguide layer
116
is formed over the tunnel barrier layer
114
. The p-type GaN:Mg layer
116
serves as a waveguide layer and has a thickness of from about 50 nm to about 200 nm.
A III-V nitride cladding layer
118
is formed over the waveguide layer
116
. The p-type AlGaN:Mg layer
118
serves as a cladding and current confinement layer. The III-V nitride cladding layer
118
typically has a thickness of from about 0.2 &mgr;m to about 1 &mgr;m.
A III-V nitride contact layer
120
is formed over the cladding layer
118
. The p-type GaN:Mg layer
120
forms a p-contact layer for the minimum-resistance metal electrode to contact the p-side of the laser heterostructure
100
. The III-V nitride contact layer
120
typically has a thickness of from about 10 nm to 200 nm.
The laser structure
100
can be fabricated by a technique such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy as is well known in the art.
Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE (reactive ion beam etching) in an Ar/Cl
2
/BCl
3
gas mixture is used to etch the prior art laser structure
100
down to the GaN:Si current-spreading layer
106
.
An n-type Ti/Al electrode
122
is formed on the etched, exposed n-current-spreading layer
106
of the laser
100
, which is functioning as a lateral contact layer. A p-type Ni/Au electrode
124
is formed on the p-contact layer
120
of the laser
100
.
The threshold voltage to cause light emission for nitride based semiconductor lasers is relatively high. AlGaInN laser devices have threshold currents in the order of 50 mA and operating voltages of 5 V, compared to about 15 mA and 2.5 V for AlGaAs red laser devices.
This problem is related to carrier injection across internal heterostructure semiconductor layer interfaces where an electrical potential barrier is created within the valence band or conduction band at the interface between nitride based semicon
Bour David P.
Kneissl Michael A.
Davie James
Prupp William
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