Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Mesa formation
Reexamination Certificate
2002-11-06
2004-08-31
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Mesa formation
C438S046000
Reexamination Certificate
active
06784010
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a nitride-based semiconductor laser device and a method for the production thereof, and more specifically to a nitride-based semiconductor laser device having an operation voltage controlled to be a desired value and having excellent lateral mode stability, and a method for the production of a nitride-based semiconductor laser device having an operation voltage controlled to be a desired value and having excellent lateral mode stability, in which the production process is simplified.
BACKGROUND ART
A GaN-based semiconductor laser device having a stacked structure of GaN-based compound semiconductor layers formed on a sapphire substrate or a GaN substrate is evoking much interest as a light-emitting device that emits light in a short wavelength region from an ultraviolet region to green.
The constitution of a GaN-based semiconductor laser device
100
disclosed in JP-A-2000-196201 will be explained below with reference to
FIG. 10
showing a schematic partial cross-sectional view of a conventional index-guide type GaN-based semiconductor laser device.
The GaN-based semiconductor laser device
100
disclosed in JP-A-2000-196201 has a stacked structure in which, on a substrate
12
made, for example, of a sapphire substrate having a c-surface as a main surface, a first contacting layer
14
made of n-type GaN, a first cladding layer
16
made of n-type AlGaN, a first light-guiding layer
18
made of n-type InGaN, an active layer
20
having a multiple quantum well structure of GaN/InGaN, a degradation-preventing layer
21
made of AlGaN for preventing the degradation of the active layer
20
, a second light-guiding layer
22
made of p-type InGaN, a second cladding layer
24
made of p-type AlGaN and a second contacting layer
26
made of p-type GaN are consecutively stacked. There are many cases where a buffer layer (not shown) made of GaN is grown on the substrate
12
at a low temperature, a substratum layer (not shown) made of GaN is laterally grown on the buffer layer, and then, the first contacting layer
14
is grown. There are also some cases where the first light-guiding layer
18
and the second light-guiding layer
22
are not provided, nor is the degradation-preventing layer
21
provided.
The upper layer
24
B of the second cladding layer
24
and the second contacting layer
26
have, for example, a ridge structure extending unidirectionally in the form of a stripe. Further, part of the first contacting layer
14
, the first cladding layer
16
, the first light-guiding layer
18
, the active layer
20
, the degradation-preventing layer
21
, the second light-guiding layer
22
and the lower layer
24
A of the second cladding layer
24
have, for example, a mesa structure extending in the form of a stripe and in the same direction as the extending direction of the ridge structure. That is, the thus-structured GaN-based semiconductor laser device
100
satisfies W′
1
>W′
2
wherein W′
1
is a width of the mesa structure and W′
2
is a width of the ridge structure.
The ridge structure, the mesa structure and portions of the first contacting layer
14
positioned on both sides of the mesa structure are covered with a protection layer
28
made of SiO
2
except for a second opening portion
28
A formed on the topmost surface of the ridge structure (i.e., top surface of the second contacting layer
26
) and a first opening portion
28
B formed on part of the first contacting layer
14
. On the second contacting layer
26
positioned in a bottom of the second opening portion
28
A, a second electrode
30
having a multi-layered structure of Ti/Au (Ti forms a lower layer and Au forms an upper layer) is provided as an ohmic contact electrode. In the explanation of the multi-layered structure, a material before “/” forms a lower layer and a material after “/” forms an upper layer, and “/” will be used in this sense hereinafter. Further, on the first contacting layer
14
positioned in a bottom of the first opening portion
28
B, a first electrode
32
having a multi-layered structure of Ti/Al is provided as an ohmic contact electrode. In addition, provided on the second electrode
30
and the first electrode
32
are a second pad electrode
34
and a first pad electrode
36
that are electrically connected to the second electrode
30
and the first electrode
32
, respectively, as leading electrodes. The second pad electrode
34
extends from the second electrode
30
to the top surface of the protection layer
28
.
In the above-structured GaN-based semiconductor laser device
100
disclosed in JP-A-2000-196201, the upper layer
24
B of the second cladding layer
24
and the second contacting layer
26
have the ridge structure, so that the current passage of electric current injected is limited to decrease the operation current, and that the lateral mode is controlled by means of an effective refractive index difference &Dgr;n in a lateral direction. The effective refractive index difference &Dgr;n refers to a difference (&Dgr;n=n
EFF1
−n
EFF2
) between an effective refractive index n
EFF1
obtained by measurement along the line A—A in FIG.
10
and an effective refractive index n
EFF2
obtained by measurement along the line B—B in FIG.
10
.
The above GaN-based semiconductor laser device disclosed in JP-A-2000-196201 has the following problems.
The first problem is that the operation voltage of the GaN-based semiconductor laser device
100
comes to be higher than a desired value or a designed value.
The second problem is as follows. The lateral mode is controlled by means of the effective refractive index difference &Dgr;n in a lateral direction. However, it is difficult to increase the thickness of the upper layer
24
B of the second cladding layer
24
and it is difficult to decrease the thickness of the lower layer
24
A of the second cladding layer
24
, so that the effective refractive index difference &Dgr;n in a lateral direction is small, and that the stability of the lateral mode is therefore poor. When the upper layer
24
B of the second cladding layer
24
is increased in thickness and when the lower layer
24
A thereof is decreased in thickness, leak current may flow through the protection layer
28
and the lower layer
24
A of the second cladding layer
24
.
The third problem is that the process which follows the formation of the stacked structure of GaN-based epitaxial growth layers is complicated and includes many steps, so that it is difficult to improve the productivity. After the formation of the stacked structure, for example, the process requires the steps of forming an etching mask made of SiO
2
, etching the second contacting layer
26
and further etching an upper portion of the second cladding layer
24
to form the ridge structure; the steps of forming a ZrO
2
film on the entire surface and removing the ZrO
2
film on the etching mask by removing the etching mask to expose the second contacting layer
26
; the steps of forming an etching mask made of SiO
2
again on the top surface the ridge structure, etching the lower layer of the second cladding layer and each layer positioned below said layer in the stacked structure to form the mesa structure, further, exposing the first contacting layer
14
and then removing the etching mask; and the step of forming the second electrode
30
on the exposed second contacting layer
26
.
JP-A-2000-307184 discloses another method of producing a GaN-based semiconductor laser device. In the second embodiment of the method of producing a GaN-based semiconductor laser device disclosed in the above JP-A-2000-307184, after the formation of a stacked structure of GaN-based epitaxial growth layers, first, the stacked structure is etched to form a mesa structure. Then, a protection layer is formed on the entire surface, an opening portion is formed in the protection layer, an second electrode is formed on the top surface of the second contacting layer positioned in a bottom portion of the opening portion, and then the protection
Ansai Shinichi
Kijima Satoru
Kobayashi Takashi
Kobayashi Toshimasa
Tojo Tsuyoshi
Picardat Kevin M.
Sony Corporation
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