Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-19
2004-12-07
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010, C372S042000, C372S046012
Reexamination Certificate
active
06829273
ABSTRACT:
BACKGROUND OF THE INVENTION
Nitride semiconductor devices made from Group III-nitride semiconductor materials have substantial promise as electro-optical devices such as lasers and light-emitting diodes. The general formula of these materials is Al
x
Ga
1-x-y
In
y
N, where Al is aluminum, Ga is gallium, In is indium, N is nitrogen, and x and y are compositional ratios. For instance, these materials have been reported as being used to fabricate lasers that generate light in the blue through ultra-violet part of the spectrum. However, a major practical problem with conventionally-structured lasers fabricated using nitride semiconductor materials is that the far-field pattern of the light generated by such lasers exhibits more than one peak. The far-field pattern is the Fourier transform of the near-field pattern of the light. See, for example, D. Hofstetter et al.,
Excitation of a Higher Order Transverse Mode in an Optically
-
Pumped
In
0.15
Ga
0.85
N/In
0.05
Ga
0.95
N
Multi Quantum Well Laser Structure
, 70 APPL. PHYS. LETT., 1650 (1997). A laser that generates light having a far-field pattern that exhibits multiple peaks has substantially fewer practical applications than a laser that generates light having a far-field pattern that exhibits a single peak.
The far-field pattern of the light generated by such a conventional laser exhibits multiple peaks, rather than the desired single peak, because the optical confinement structure formed by an optical waveguide layer and an underlying cladding layer in the laser provides insufficient optical confinement. The optical confinement structure allows light to leak from the optical waveguide layer into the contact layer underlying the cladding layer. The contact layer then acts as a parasitic optical waveguide that causes spurious laser oscillation in a high-order mode. The contact layer is used to inject current into the active layer of the laser.
Attempts to achieve sufficient optical confinement have included using a cladding layer having an increased thickness, and increasing the refractive index difference between the semiconductor materials of the optical waveguide layer and the cladding layer. However, when implemented conventionally, these measures increase the incidence of cracks in the laser structure. This significantly reduces the production yield.
FIG. 1
is a schematic side view of the structure of the conventional nitride semiconductor layer structure
10
from which conventional nitride semiconductor lasers can be made. In all of the Figures in this disclosure that depict semiconductor layer structures and semiconductor lasers, the thicknesses of the layers are greatly exaggerated relative to their widths, and the thicknesses of the thinner layers are exaggerated more than those of the thicker layers, to enable the layers to be shown clearly.
The nitride semiconductor layer structure
10
is composed of the buffer layer
12
of low-temperature-deposited GaN, the n-contact layer
13
of n-type GaN, the cladding layer
14
of n-type AlGaN, the optical waveguide layer
15
of n-type GaN, the active layer
16
of GaInN, the electron blocking layer
17
of p-type AlGaN, the optical waveguide layer
18
of p-type GaN, the cladding layer
19
of p-type AlGaN and the p-contact layer
20
of p-type GaN. The layers
12
through
20
are successively formed on the substrate
11
. The material of the substrate is sapphire, SiC, spinel, MgO, GaAs, silicon, or some other suitable material.
The buffer layer
12
is a 35 nm-thick layer of GaN deposited at a temperature of 550° C., i.e., at a temperature below that at which single-crystal growth occurs. The n-contact layer
13
is a 4 &mgr;m-thick layer of GaN doped n-type with about 1×10
18
cm
−3
of Si. The cladding layer
14
is a 600 nm-thick layer of AlGaN having an AlN molar fraction of 0.06 and doped n-type with about 1×10
18
cm
−3
of Si. The optical waveguide layer
15
is a 100 nm-thick layer of GaN doped n-type with about 1×10
18
cm
−3
of Si. The active layer
16
is composed of five pairs of sub-layers. Each pair of sub-layers is composed of a 3 nm-thick sub-layer of GaInN having an InN molar fraction of 0.1 and a 6 nm-thick sub-layer of GaInN having an InN molar fraction of 0.03. The electron blocking layer
17
is a 15 nm-thick layer of AlGaN having an AlN molar fraction of 0.15 and doped p-type with about 5×10
19
cm
−3
of Mg. The optical waveguide layer
18
is a 100 nm-thick layer of GaN doped p-type with about 5×10
19
cm
−3
of Mg. The cladding layer
19
is a 500 nm-thick layer of AlGaN having an AlN molar fraction of 0.06 and doped p-type with about 5×10
19
cm
−3
of Mg. The p-contact layer
20
is a 100 nm-thick layer of GaN doped p-type with about 1×10
20
cm
−3
of Mg.
An improved nitride semiconductor layer structure from which can be made nitride semiconductor lasers that generate light having a far-field pattern that exhibits a single peak is described in Japanese Patent Application no. H10-313993 (the prior application). The prior application is assigned to the assignee of this application and was filed on Oct. 16, 1998. An English language version of the prior application is published as International Application no. WO 00/24097. The prior application is incorporated into this disclosure by reference.
The nitride semiconductor layer structure disclosed in the prior application lacks the electron blocking layer
17
of the layer structure shown in
FIG. 1
, but includes an additional buffer layer located between the n-type GaN contact layer
13
and the n-type AlGaN cladding layer
14
. The additional buffer layer is a layer of low-temperature-deposited semiconductor material that includes AlN. The additional buffer layer enables the n-type AlGaN cladding layer
14
grown on it to have an increased thickness and a lower incidence of cracks. In the nitride semiconductor layer structure disclosed in the prior application, either or both of the thickness of the cladding layer and the AlN molar fraction of the cladding layer can be adjusted to make the light emitted by a laser fabricated using the layer structure to have a far-field pattern, i.e., the intensity distribution in the far field, that exhibits a single peak.
Although the layer structure disclosed in the prior application can be used to fabricate lasers that generate light whose far-field pattern exhibits a single peak and that have a greater efficiency than conventional nitride semiconductor lasers, the need exists for high-quality lasers having a simpler structure.
Thus, what is needed is a nitride semiconductor layer structure that has a simple structure and from which a nitride semiconductor laser can be fabricated that generates light whose far-field pattern exhibits a single peak, and that has a low threshold current and a low power consumption.
What is also needed is a nitride semiconductor layer structure capable of providing increased optical confinement, that has a reduced manufacturing cost and that provides improved performance in opto-electric devices, other waveguide structures and other semiconductor devices.
SUMMARY OF THE INVENTION
The invention provides a nitride semiconductor layer structure that comprises a buffer layer and a composite layer on the buffer layer. The buffer layer is a layer of a low-temperature-deposited nitride semiconductor material that includes AlN. The composite layer is a layer of a single-crystal nitride semiconductor material that includes AlN. The composite layer includes a first sub-layer adjacent the buffer layer and a second sub-layer over the first sub-layer. The single-crystal nitride semiconductor material of the composite layer has a first AlN molar fraction in the first sub-layer and has a second AlN molar fraction in the second sub-layer. The second AlN molar fraction is greater than the first AlN molar fraction.
The invention additionally provides a nitride semiconductor laser that comprises a portion of the above-described nitride semiconductor layer str
Akasaki Isamu
Amano Hiroshi
Kaneko Yawara
Takeuchi Tetsuya
Watanabe Satoshi
Agilent Technologie,s Inc.
Hardcastle Ian
Nguyen Dung
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