Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2003-03-07
2004-10-05
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S092000, C372S039000
Reexamination Certificate
active
06801559
ABSTRACT:
This is a patent application based on Japanese patent applications No. 2002-063811 and No. 2003-040462, which were filed on Mar. 8, 2002 and Feb. 19, 2003, respectively, and which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor laser. Especially, the present invention relates to a group III nitride compound semiconductor laser having a ridge type of a resonator.
2. Description of the Related Art
A semiconductor laser which has a cladding layer and an active layer and so on made of a group III nitride compound semiconductor (Al
x
Ga
y
In
1−x−y
N, 0≦x≦1, 0≦y≦1 and 0≦x+y≦1) has been known. The conventional semiconductor laser is a semiconductor diode having multi-layer structure with multiple group III nitride compound semiconductor layers, which are formed on a sapphire substrate in sequence. A conventional example of the semiconductor diode, disclosed by the same applicant in Japanese Patent Laid-open No. 2000-261105, is shown in FIG.
3
.
A semiconductor laser
900
shown in
FIG. 3
has the following eight layers formed on a sapphire substrate
91
in sequence: a buffer layer
92
; an n-layer
93
; an n-cladding layer
94
; an n-guide layer
95
; an active layer
96
made of multiple quantum well (MQW) layer; a p-guide layer
97
; a p-cladding layer
98
; and a p-contact layer
99
. As shown in
FIG. 3
, a cavity or a resonator part (a ridged resonator cavity of part) A is formed by using, e.g., photoresist and etching, and a positive electrode
901
and a negative electrode
902
are formed on the upper surface of the p-contact layer
99
and the etched surface of the n-layer
93
, respectively.
The active layer
96
, comprising a multiple quantum well (MQW) layer, is a semiconductor layer which functions as a main layer to oscillate laser. Each carriers (holes and electrons) injected from the positive electrode
901
and the negative electrode
902
combine in the active layer
96
, that causes laser oscillation. The n-guide layer
95
and the p-guide layer
97
function to confine carriers into the active layer
96
. Also, the n-cladding layer
94
and the p-cladding layer
98
function to confine laser light. And the n-layer
93
and the p-contact layer
99
are semiconductor layers which are formed in order that carriers can be injected smoothly from the negative electrode
902
and the positive electrode
901
to the layers existing between the n-cladding layer
94
and the p-cladding layer
98
, respectively.
In order that the semiconductor laser made of group III nitride compound semiconductor can oscillate laser efficiently, the cross-section of electric current path of the semiconductor laser is, for example, narrowed by decreasing the contact area of electrodes, or by decreasing the width w of the positive electrode
901
. In addition, the above-mentioned Japanese Patent Laid-open No. 2000-261105 suggests forming a deep ridged hole injection part B. That is, a boundary between a ridged cavity part A and the ridged hole injection part B is regarded as a boundary between the p-guide layer
97
and the p-cladding layer
98
.
When forming the ridged hole injection part B, however, it is not easy for all the semiconductor lasers formed on a wafer that a boundary between the ridged resonator part A and the ridged hole injection part B functions as a boundary between the p-guide layer
97
and the p-cladding layer
98
. The reason is that each one of group III nitride compound semiconductor layers formed on one wafer has different thickness according to the portion on which the layer is formed. So, as discloses in the above-mentioned official gazette, the applicant of the present invention suggests completely etching the p-cladding layer
98
even if a portion of the p-guide layer
97
is etched.
The thickness of the p-guide layer
97
, however, is extremely thin, e.g.,. about 100 nm. So when 200 nm in thickness of p-contact layer
99
and approximately 500 nm in thickness of p-cladding layer
98
are completely etched, the p-guide layer
97
may be damaged considerably, which may deteriorate its device characteristic as a semiconductor laser.
SUMMARY OF THE INVENTION
An object of the present invention is to form a ridged carrier injection part in a ridge type of a group III nitride compound semiconductor laser, especially in a process of manufacturing the ridge type of a group III nitride compound semiconductor laser, so as to obtain a structure which hardly damages a guide layer. Another object of the present invention is that the cross sectional shape of oscillated laser beam becomes closer to a perfect circle by forming a part of cladding layer in the ridged cavity part and controlling its thickness becomes easy.
To achieve the above object, a first aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a laser cavity and multiple layers which are made of group III nitride compound semiconductors and formed on a substrate. The group III nitride compound semiconductor laser comprises: a first layer, which functions as a guide layer and actually confines carriers to an active layer which functions as a main layer oscillating laser; a second layer having smaller refractive index compared with the first layer, which is formed above or on the first layer and mainly confines light to the active layer and the first layer; and a third layer which is formed between the first layer and the second layer or formed into the second layer and has larger composition of aluminum (Al) in group III elements compared with the second layer. Here forming the third layer into the second layer represents that the second layer comprises two layers and that the third layer is formed between the upper second layer and the lower second layer. Composition of the upper second layer and the lower second layer may be equivalent or not equivalent. In order that the third layer comes in the scope of the present invention, aluminum (Al) composition of the third layer may be larger than that of at least one layer of upper and the lower second layers.
The second aspect of the present invention is that aluminum (Al) composition of group III elements in the second layer is larger than that in the first layer.
The third aspect of the present invention is that the second layer functions as a cladding layer.
The fourth aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a laser cavity. The laser cavity is formed by removing multiple layers, which are made of group III nitride compound semiconductors and formed on a substrate, except the width of the laser cavity part. A carrier injection part is formed contacting to the laser cavity part by removing at least all layers on the third layer except the area corresponding to the width of an electrode formed above the second layer.
The fifth aspect of the present invention is that the electrode is a positive electrode.
The sixth aspect of the present invention is that aluminum (Al) composition of the third layer is larger than that of the second layer by 10% or more. That is represented by the formula x
3
≧x
2
+0.1 when x
3
and x
2
(0≦x
3
, x
2
≦1) are aluminum (Al) compositions of the third and the second layers in all the group III elements. The seventh aspect of the present invention is that the third layer is thinner than the first layer.
By forming the third layer having larger aluminum (Al) composition in group III elements compared with that of the second layer between the first layer which functions as a guide layer and actually confines carriers to an active layer functioning as a main layer to oscillate laser and the second layer which mainly confine light to the active layer which functions as a main layer oscillating laser and the first layer, the third layer can protect the first layer in an etching process. That is because etching rate of a group III nitride compound semiconductor
Hatano Takashi
Iwayama Sho
Koike Masayoshi
Jr. Leon Scott
Toyoda Gosei Co,., Ltd.
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