Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-09-09
2001-10-02
Leung, Quyen (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06298079
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a semiconductor light emitting device such as a semiconductor laser. More particularly, the present invention relates to a semiconductor light emitting device which consists of a semiconductor of gallium nitride type compound and is suitable for emitting blue light.
The semiconductor of gallium nitride (GaN) type compound described here is referred to as a semiconductor comprising a GaN compound which consists of Ga of group III element and N of group V element, or III-V compound in which part of Ga of GaN is substituted by other group III element such as Al or In and/or part of N of GaN is substituted by other group V element such as P or As.
In the past, there has been extensively spread the technology for manufacturing a semiconductor laser which emits infrared rays or red rays using GaAs. In contrast, there has been desired a semiconductor laser which emits in a visible ray area blue ray of shorter wave length than that of the foregoing semiconductor laser, by using a semiconductor of GaN type compound, it has become possible to manufacture a light emitting diode (hereinafter referred to as LED) which emits blue light, while the development of a semiconductor laser which emits blue light is under way. In an LED which uses the semiconductor of gallium nitride type compound, during epitaxial growth of a p-type layer, comprising Ga and N, since a p-type dopant Mg or Zn is easily coupled with H atoms in the carrier gas such as H
2
or the reactant gas such as NH
3
and the p-type dopant does not fully fulfill the functions thereof, the grown p-type layer is activated by annealing in order to reduce the resistance by separating the coupling with Mg or Zn and H.
On the other hand, in such a semiconductor laser as this which uses a semiconductor of GaN type compound, it is difficult to manufacture such a semiconductor laser which corresponds to a GaAs type compound semiconductor laser of refractive index guiding structure which contain both the light absorption layer and the current blocking layer. In other words, in order to make the current blocking layer as the absorption layer, the band gap energy of the current blocking layer must be made to be equal or smaller than that of the active layer, therefore, if a semiconductor of InGaN type compound is used for the active layer, a semiconductor material having large composition ratio of In must be used for the current blocking layer. However, when the composition ratio of In increases, the atom of In easily evaporates at the normal growth temperature, and it is difficult to deposit a film of compound semiconductor, while controlling the composition ratio.
Generally, as a wave guide structure of a semiconductor laser, a refractive index guiding structure and a gain guiding structure are known. The refractive index guiding structure is provided with a difference of refractive index in parallel direction with respect to the active layer so as to confine and direct the light, and thus it is possible to obtain a single lateral mode oscillation up to a high output operation, but the coherence possibility is high and the noise induced by the return light (optical feedback noise) is apt to generate. On the other hand, the gain guiding structure is one which does not have a difference of refractive index in lateral direction, in which the lateral mode is unstable and kink is apt to occur, but the optical feedback noise is low because the longitudinal multiple mode is oscillated.
As a structure of a semiconductor laser suitable for the gain guiding structure which uses a semiconductor of GaN type compound, one shown in
FIG. 16
or
FIG. 17
can be considered. To describe the one shown in
FIG. 16
, on a sapphire substrate
1
are laminated in order a buffer layer
2
consisting of GaN, a lower cladding layer
3
consisting of Al
z
Ga
1−z
N (0<z<1), an active layer
4
consisting of In
x
Ga
1−x
N (0<x<1), an upper cladding layer
5
consisting of Al
z
Ga
1−z
N, and a contact layer
8
, and further an upper electrode
9
in the form of a stripe is provided thereon. Further, part of the layers laminated are removed until part of the lower cladding layer
3
or the buffer layer
2
is exposed, and a lower electrode
10
is provided on the exposed surface. In this case, when voltage is applied between the upper and lower electrodes
9
and
10
, electric current flows only to part of the central portion in the active layer
4
according to the shape of the upper electrode
9
so as to make such part into an active area where laser light is generated. However, in a semiconductor laser of such a structure as this, it becomes difficult to control the electric current to be injected into the active area.
With respect to
FIG. 17
in which portions corresponding to the same portions shown in
FIG. 16
are provided with the same numerals, for the purpose of leaving the upper electrode
9
and the semiconductor layers thereunder in the form of a stripe, the portion of both sides thereof is etched and removed from the top halfway down the upper cladding layer
5
, and made into a mesa-type shape. In accordance with such structure as this, the control of the electric current to be injected becomes easy as compared to the structure shown in
FIG. 16
, but the control of dimensions in manufacturing is difficult, the side wall of the stripe-like portion to be removed and exposed by etching is susceptible to damage by the etching, and a semiconductor laser with good quality cannot be obtained.
On the other hand, an example of a conventional semiconductor laser of the refractive index guiding structure which uses a semiconductor of GaAs type compound is shown in FIG.
18
. In
FIG. 18
, the numeral
21
represents a semiconductor substrate consisting, for example, of an n-type GaAs or the like, on which are laminated in order a lower cladding layer
22
consisting, for example, of an n-type Al
v
Ga
1−v
As (0.35≦v≦0.75), an active layer
23
consisting, for example, of a non-doping type or n-type or p-type Al
w
Ga
1−w
As (0<w<0.7, w<v) a first upper cladding layer
24
consisting of a p-type Al
v
Ga
1−v
As, a current blocking layer
25
consisting of an n-type GaAs, a second upper cladding layer
26
consisting of a p-type Al
v
Ga
1−v
As, and a contact layer
27
consisting of a p-type GaAs, on the top surface and the bottom surface are respectively provided a p-side electrode
28
and an n-side electrode
29
, and a chip of a semiconductor laser is formed. In this structure, the current blocking layer
25
consisting of the n-type GaAs is a conductive type layer which is different from the p-type cladding layer in the neighborhood, wherein the gap energy of a pn junction is utilized to block electric current, injection current is restricted to the stripe-like active area of width W, and by absorbing the light generated in the active layer (that is, as a light absorption layer), a difference of refractive index is provided inside and outside of the stripe. Therefore, the light is confined in the lateral direction, and is used as a semiconductor laser of refractive index guiding structure of red ray or infrared ray which stably directs the wave in a stripe-like width W of the active layer
23
a.
In this structure, by using a material which does not absorb the light as the current blocking layer
25
, and by keeping away the current blocking layer
25
from the active layer
23
at the distance, a semiconductor laser of gain guiding structure can be obtained, but in a semiconductor laser which uses a semiconductor of GaN type compound in particular, when GaN is used as the current blocking layer
25
and the distance from the active layer is kept away to obtain the gain guiding structure, leakage current increases, so that a suitable material as the current blocking layer
25
is desired.
Further, an example of a semiconductor laser of the gain guiding structure which uses a semiconductor of a conventional GaAs type compound is shown in FI
Shakuda Yukio
Tanaka Haruo
Arent Fox Kintner & Plotkin & Kahn, PLLC
Leung Quyen
Rohm & Co., Ltd.
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