Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-05-05
2001-01-30
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06181723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a III-V group compound semiconductor light emitting device such as a semiconductor laser device or a light emitting diode (LED) device, and a method for producing the same.
2. Description of the Related Art
A III-V group compound semiconductor light emitting device such as a semiconductor laser device or an LED device generally has at least one (typically a plurality of) crystal growth layer of the p-type conductivity (hereinafter, referred to also as the “p-type layer”) and at least one (typically a plurality of) crystal growth layer of the n-type conductivity (hereinafter, referred to also as the “n-type layer”). These crystal growth layers are typically formed by using a crystal growth method such as a liquid phase epitaxial (LPE) method, a molecular beam epitaxial (MBE) method, or a metalorganic chemical vapor deposition (MOCVD) method, which is excellent in mass productivity and allows a very thin film to be grown.
In the LPE method, although a high quality semiconductor crystal can be formed with a relatively simple apparatus, it is difficult to produce a uniform crystal over a large area. On the other hand, the MBE method and the MOCVD method are more suitable processes for mass production, and are widely used at present. The MBE method is a process in which solid elements forming a compound semiconductor are heated in a high vacuum and a substrate is irradiated with beams of the evaporated elements. With this method, a pure crystal can be relatively easily obtained. In the MOCVD method, under an atmospheric pressure or a pressure depressurized to about 1/10 atm, elements forming a compound semiconductor are carried in a gaseous form such as an organic compound or a hydrogen compound, so as to be chemically reacted on a substrate, thereby forming an intended compound semiconductor.
FIG. 1
is a cross-sectional view illustrating a typical structure of a III-V group compound semiconductor laser device produced by using the MOCVD method.
In the cross-sectional view of
FIG. 1
, an n-type GaAs buffer layer
1
, an n-type AlGaAs cladding layer
2
, an AlGaAs active layer
3
, a p-type AlGaAs first cladding layer
4
, a p-type GaAs etching stop layer
5
, a p-type AlGaAs second cladding layer
6
and a p-type GaAs protective layer
7
are deposited in this order on an n-type GaAs substrate
14
, thereby forming a layered structure. In this layered structure, the layers above the p-type GaAs etching stop layer
5
form a stripe-shaped mesa structure (a mesa stripe). An n-type AlGaAs current blocking layer
8
, an n-type GaAs current blocking layer
9
and a p-type GaAs planarizing layer
10
are buried on both sides of the mesa stripe.
Moreover, a p-type GaAs contact layer
11
is formed on the p-type GaAs protective layer
7
and p-type GaAs planarizing layer
10
. A p-side metal electrode
12
and an n-side metal electrode
13
are respectively formed on the p-type GaAs contact layer
11
and on the reverse surface of the n-type GaAs substrate
14
by, for example, a vapor deposition method.
FIG. 2
is a cross-sectional view illustrating another typical structure of a III-V group compound semiconductor laser device produced by using the MOCVD method.
In the cross-sectional view of
FIG. 2
, a Se-doped n-type GaAs buffer layer
22
, a Se-doped n-type AlGaAs cladding layer
23
, an undoped AlGaAs active layer
24
, a Zn-doped p-type first cladding layer
25
and a Se-doped n-type AlGaAs current blocking layer
26
are formed in this order on an n-type GaAs substrate
21
. A portion of the n-type current blocking layer
26
is removed in a stripe-shaped pattern, thereby forming a current path
17
. A Zn-doped p-type second cladding layer
28
and a Zn-doped p-type contact layer
29
are formed on the n-type current blocking layer
26
including the stripe-shaped portion
17
. A p-side electrode
18
and an n-side electrode
19
are respectively formed on the p-type contact layer
29
and on the reverse surface of the n-type GaAs substrate
21
.
FIG. 3
is a diagram schematically illustrating a structure of a vapor deposition apparatus of a depressurized horizontal RF heating furnace type which can be used for growing the semiconductor layers included in the semiconductor laser device illustrated in
FIG. 1
or
2
.
In the apparatus illustrated in
FIG. 3
, trimethylgallium (TMGa), trimethylaluminum (TMAl) or trimethylindium (TMIn) is used as a III-group material compound; arsine (AsH
3
) or phosphine (PH
3
) as a V-group material compound; monosilane (SiH
4
), disilane (Si
2
H
6
) or hydrogen selenide (H
2
Se) as an n-type dopant material; and diethylzinc (DEZn), dimethylzinc (DMZn) or trimethylarsenic (TMAs) as a p-type dopant material. Carbon tetrachloride (CCl
4
) may also be used as a carbon source.
In a crystal growth process, a substrate is placed inside a reaction chamber (growth chamber)
30
, the internal pressure of the reaction chamber
30
is set to a predetermined value (e.g., about 76 Torr), and the substrate temperature is set to a predetermined value (e.g., about 700° C.) using an RF coil
31
. Then, mass flow controllers (MFCs) and valves are appropriately controlled to appropriately select, and set the flow rate of, the respective materials from material sources
32
to
38
and hydrogen supplied from a hydrogen source through a line
39
so as to supply them into the reaction chamber
30
through respective supply lines
40
to
43
, thereby growing an intended semiconductor layer on the substrate. Any unwanted gas which may exist in the reaction chamber
30
is exhausted through a line
44
.
For example, when forming an n-type AlGaAs layer, AsH
3
, TMGa, TMAl and an appropriate n-type dopant material are supplied onto the substrate. When forming a Zn (zinc) doped p-type AlGaAs layer, AsH
3
, TMGa, TMAl and DMZn or DEZn are supplied onto the substrate. When forming a C (carbon) doped p-type AlGaAs layer, TMAs, AsH
3
. TMGa and TMAl are supplied onto the substrate.
In the MOCVD method, it is likely that organic matter or hydrogen generated after a chemical reaction may be introduced as an impurity into a growing compound semiconductor layer. Particularly, carbon (C) contained in organic matter, when introduced into the compound semiconductor layer, may act as a p-type dopant. Thus, it is likely that a certain amount of carbon may be present in the grown compound semiconductor layer even when carbon tetrachloride (CCl
4
) is not supplied.
For example, when forming a p-type AlGaAs layer with a high concentration of zinc added thereto, the crystal growth process is performed while reducing the substrate temperature from about 700° C. to about 600° C. during the growth process. Then, a certain amount of carbon atoms are introduced into an AlGaAs crystal layer obtained through the crystal growth process by the MOCVD method. As the Al mole fraction increases carbon atoms are increasingly likely to be introduced. Therefore, carbon atoms, though at a concentration lower than the predetermined zinc atom concentration, will be present in the grown p-type AlGaAs layer.
When forming a p-type AlGaAs layer with a high concentration of carbon added thereto, the crystal growth process is performed while reducing the supply ratio between the V-group material and the III-group material (V/III ratio) from about 60 to about 2 during the growth process. Also in this case, carbon atoms will be present at a concentration of, for example, about 1×10
17
cm
−3
in the adjacent n-type AlGaAs layer to which carbon is not intended to be added.
Moreover, as can be seen from the above description of the structure of the apparatus illustrated in
FIG. 3
, in the formation of a compound semiconductor layer by the MOCVD method, Se or Si is used as the n-type dopant element, and Zn, Mg, C, or the like, is typically used as the p-type dopant element. These dopants should be controlled so as to be present at a predetermined concentration in the intended crystal growth layer. When one of the dopant
Akagi Yoshiro
Ishida Masaya
Konushi Fumihiro
Okubo Nobuhiro
Watanabe Masanori
Arroyo Teresa M.
Leung Quyen P.
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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