Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-07-28
2002-02-05
Healy, Brian (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010, C372S050121, C257S014000, C359S199200, C359S344000
Reexamination Certificate
active
06345063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to the field of semiconductors. More particularly, the invention is directed to group III-V nitride semiconductor films usable in blue light emitting devices.
2. Description of Related Art
The light-emitting diode is the basic component for electronic lighting technology. A light-emitting diode is a relatively simple semiconductor device which emits light when an electric current passes through a p-n junction of the light-emitting diode. As shown in
FIG. 1
, a light-emitting diode
100
includes a p-type
110
semiconductor material adjacent to an n-type
120
semiconductor material, i.e., a p-n junction, characterized by a bandgap energy E
g
130
. The bandgap energy
130
is the minimum energy required to excite an electron
160
from a valence band
140
to a conduction band
150
, where the electron
160
becomes mobile. Likewise, the bandgap energy
130
also determines the energy of a photon produced when the electron
160
in the conduction band, i.e., a conduction electron, recombines with a hole
170
, i.e., an unoccupied electronic state, in the valence band
140
. When forward current passes through the diode
100
, the electrons
160
in the conduction band
150
flow across the junction from the n-type material
120
, while the holes
170
from the valence band
140
flow from the p-type material
120
. As a result, a significant number of the electrons
160
and the holes
170
recombine in the p-n junction, emitting light with an energy E
photon
=E
g
. These semiconductor devices, comprising a p-n junction, in a single material, and are referred to as homojunction diodes.
In order to obtain more efficient LEDs and laser diodes, in particular, lasers that operate at room temperature, it is necessary to use multiple layers in the semiconductor structure. These devices are called heterojunction or heterostructure LEDs or lasers.
The wavelength, and thus the color of light emitted by an LED or laser diode, depends on the bandgap energy Eg. LEDs or laser diodes that emit light in the red-to-yellow spectrum have been available since the 1970's. There has been great difficulty, however, in developing efficient LEDs that emit light at shorter wavelengths. Extending LED light sources into the short-wavelength region of the spectrum, the region extending from green to violet, is desirable because LEDs can then be used to produce light in all three primary colors, i.e., red, green, and blue. Shorter-wavelength laser diodes will likewise enable full-color projection displays; and they will also permit the projection of coherent radiation to focus laser light into smaller spots. That is, in the optical diffraction limit, the size of the focused spot is proportional to the wavelength of the light. Reducing the wavelength of the emitted light allows optical information to be stored at higher densities and read out more rapidly.
FIG. 2
shows a conventional LED structure
200
in which an InGaN active layer
230
is formed over a group II-V nitride layer
220
. Specifically, as shown in
FIG. 2
, the conventional LED
200
includes a substrate
205
, which may, for example, be formed of sapphire or silicon carbide. A buffer layer
210
is formed on the substrate
205
. The group III-V nitride layer
220
is then formed on the buffer layer
210
. The group III-V nitride layer
220
is typically GaN. The InGaN active layer
230
is formed on the group III-V nitride layer
220
. A second group III-V nitride layer
240
is then formed on the InGaN active layer
230
. A third group III-V nitride layer
250
is formed on the second group V layer
240
. The first group III-V nitride layer
220
is n-type doped. The second and third group III-V nitride layers
240
and
250
are p-type doped. A p-electrode
260
is formed on the third group III-V nitride layer
250
. An n-electrode
270
is formed on the first group III-V nitride layer
220
.
SUMMARY OF THE INVENTION
This invention provides group III-V nitride films formed on substrates usable to form short-wavelength visible light-emitting optoelectronic devices, including light-emitting diodes (LEDs) and diode lasers.
This invention provides a method for growing light-emitting device heterostructures over a thick InGaN layer that provides a suitable bandgap for blue, green, or even red light emission.
The invention provides a stable InGaN structure that avoids lattice mismatch.
The invention provides other electronic devices, such as transistors, which can incorporate InGaN with other group III-V semiconductors.
Group III-V nitrides include elements from groups III, i.e., gallium, indium, and aluminum, and V, i.e., nitrogen, of the periodic table. These materials are deposited over substrates forming layered structures for optoelectronic devices, including LEDs and laser diodes. The resulting devices can emit visible light over a wide range of wavelengths.
The performance of the optoelectronic devices depends on the quality of the group III-V nitride films formed over the substrates. An important structural characteristic of the group III-V nitride films, which effects their emission quality, is lattice matching between each of the layers. In particular, lattice mismatch occurring between dissimilar materials may produce crystal defects, such as dislocations, cracks, or alloy inhomogeneity, which degrade the optoelectronic quality of the material.
The group III-V nitride semiconductors, GaN, AlN and InN, are used in visible light emitters because these materials are characterized by a wide bandgap energy, as is necessary for short-wavelength visible light emission. Group III-V nitrides also form strong chemical bonds, which make the material very stable and resistant to degradation under high electrical current densities and intense light illumination.
Most optoelectronic devices based on the group III-V nitride compounds require growth of a sequence of layers with different bandgap energies and refractive indices. The bandgap energy of the active layer determines the wavelength of light emitted from a light-emitting diode or laser. In addition, the energy band and refractive index discontinuities between layers of different composition provides for optical and carrier confinement. To obtain layers with the bandgap around 2.7 eV, which will produce light in the blue region of the spectrum, InGaN alloys can be used. The bandgap energy of GaN is 3.4 eV, while the bandgap energy of InN is 1.9 eV. Therefore, In
x
Ga
1−x
N alloys span the visible spectrum, in which case an estimated In composition x of about 30%, i.e., In
0.3
Ga
0.7
N, is required to obtain blue-light emission, 50% for green emission, and 100%, i.e., InN, for red emission.
Growing InGaN alloys with such a high In content on GaN has heretofore been, if not impossible, difficult using conventional techniques, such as metal-organic chemical vapor deposition (MOCVD). Specifically, when using these conventional techniques, the InGaN alloy active region tends to segregate. As the indium content is increased to produce longer-wavelength emission, the InGaN alloy becomes unstable. As a result of this instability, the InGaN alloy separates, or segregates, into In-rich regions and Ga-rich regions, so that the InGaN alloy composition, and therefore the active region bandgap energy, is no longer uniform.
This inhomogeneous composition causes the electroluminescence (EL) to be spectrally broad, i.e., where a broad range of wavelengths is emitted. For instance, while the spectral emission widths of violet LEDs (390-420 nm.), corresponding to 10-20% In content, may be as narrow as 10-15 nm, the spectral emission width increases to 20-30 nm for blue LEDs (430-470 nm.), corresponding to a ~30% In content, and 40-50 nm for green LEDs (500-530 nm.), corresponding to a ~50% In content.
The poor spectral purity of green LEDs limits their application in full-color displays, where pure colors are needed to generate, by additive mixing, a broader palette of colors. Likewise, such broad spectral emission w
Bour David P.
Kneissl Michael A.
Romano Linda T.
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