Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-09-25
2001-09-04
Font, Frank G. (Department: 2877)
Coherent light generators
Particular active media
Semiconductor
C372S096000, C372S045013
Reexamination Certificate
active
06285698
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 electrically injected semiconductor lasers. A light-emitting diode is a relatively simple semiconductor device which emits light when an electric current passes through a junction of the light-emitting diode. As shown in
FIG. 1
, a light-emitting diode
100
includes a back-to-back sandwich of a p-type
110
semiconductor material and an n-type
120
semiconductor material, i.e., a p-n junction, characterized by a bandgap E
g
130
. The bandgap
130
determines 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. Conversely, the bandgap
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
in the valence band
140
. When 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
110
. 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 hv=E
g
. These semiconductor devices contain only one junction, the p-n junction, in a single material and are referred to as homojunction structures.
In order to obtain more efficient lasers, 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 double heterojunction lasers, depending on the number of wide bandgap layers formed.
The wavelength, and thus the color of light emitted by an LED or laser diode, depends on the bandgap E
g
. LEDs or laser diodes that emit light in the red region of the visible spectrum have been available since the early 1990's. There has been great difficulty in developing 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. Shorter-wavelength laser diodes 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 and read out at higher densities.
SUMMARY OF THE INVENTION
This invention provides group III-V nitride films formed on substrates usable in short-wavelength visible light-emitting optoelectronic devices, including light-emitting diodes (LEDs) and diode lasers.
The invention provides corrugated waveguiding structures using diffraction gratings or grooves, the grating or grooved structures release the strain in the active layer, allowing In segregation in the quantum wells, and providing optical feedback. The In rich regions thus provide a bandgap suitable for emission of light in the blue region of the spectrum.
Group III-V nitrides include elements from groups III and V 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 affects their emission quality is lattice matching between each of the layers.
The group III-V nitride semiconductors, GaN, AlN and InN, and their alloys are used as active materials for optoelectronic device applications because these materials share the characteristic wide bandgap 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 electric 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 bandgaps. In addition, the band discontinuity between layers with different bandgaps provides for carrier confinement, whereas the difference in refractive index provides optical 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. Since the bandgap of GaN is 3.4 eV and the bandgap of InN is 1.9 eV, an alloy group III-V with an In composition of about 20% is required to obtain blue-light emission.
Growing InGaN alloys with such a high In content on GaN has heretofore been difficult, if not impossible, 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 nonuniform composition causes the electroluminescence (EL) to be spectrally broad, i.e., the range of the wavelengths of the emitted light is broad. For instance, while the spectral emission widths of violet LEDs (390-420 nm.), corresponding to 10-20% In content, may be as narrow as 12-15 nm, the spectral emission width increases to 20-30 nm for blue LEDs (430-460 nm.), corresponding to a 20-30% In content, and 40-50 nm for green LEDs (500-530 nm.), corresponding to a 40-50% 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 pallette of colors. Likewise, such broad spectral emission widths also translate into broad gain spectra for laser diode structures. When the gain spectrum becomes broad, the peak gain is reduced, so that it becomes difficult to reach lasing threshold. For this reason, when formed using these conventional techniques, the performance of blue and green group III-V nitride laser diodes is expected to be poor compared to violet-emitting group III-V laser diode devices.
In order to improve the spectral purity of blue and green LEDs, and to promote the development of true blue or green group III-V nitride laser diodes, the growth of high-indium-content InGaN alloys is desirable. The alloy segregation problems must be overcome, so that the alloy content remains uniform, even when the indium content approaches 50%.
However, GaN and InN have a very large lattice mismatch which causes reduced miscibility. Thus, it has heretofore been difficult to form a group III-V nitride alloy having an In content higher than 10% using conventional growth techniques. Thus, constructing effective blue light emitting structures using InGaN grown over group III-V nitride layers has proven very difficult.
The inventors have determined that these problems are most likely caused by the lattice mismatch of over 10% between GaN and InN, which can cause reduced miscibility. The higher vapor pressure of InN may also play a role in the non-uniform alloy content of known devices formed using these conventional techniques. Although blue lasing in InGaN has already been achieved, there remain major issues concerning defect-free MOCVD growth of these materials. Also, the formation of laser mirrors for blue light lasing is difficult compar
Hofstetter Daniel
Paoli Thomas L.
Romano Linda T.
Flores Ruiz Delma R.
Font Frank G.
Oliff & Berridg,e PLC
Xerox Corporation
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