Semiconductor laser and photo module using the same

Details Semiconductor laser and photo module using the same Semiconductor laser and photo module using the same

2002-11-05

2004-04-27

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010

Reexamination Certificate

active

06728283

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser which applies to and is suitable for optical communication and a photo module which uses the semiconductor laser.
A long-wavelength-band semiconductor laser which emits laser beam in a wavelength band of 1.3 &mgr;m or more is a principal device of optical communication, and, at present, is chiefly constituted by forming a compound semiconductor layer made of a material, such as InGaAsP, InAlGaAs, or InAsP, on an InP substrate. Because the InP substrate is expensive and the substrate size is difficult to increase, however, the semiconductor laser which uses the InP substrate was forced to become expensive.
On the contrary, a GaAs substrate is comparatively inexpensive and the substrate size is easy to increase. There was a problem, however, that the material which can be formed on the GaAs substrate is limited in terms of lattice strain and a semiconductor laser of high practicality in a long wavelength band is difficult to obtain. Because the GaAs substrate has the aforementioned features, however, the research and development of the long-wavelength-band semiconductor laser which uses GaAs in a substrate is advancing powerfully. Besides, if a long-wavelength-band vertical cavity surface emitting laser which uses the GaAs substrate can be realized, the laser can be combined with a GaAs/AlAs semiconductor multiple layer mirror and enables more miniaturization and realization of lower cost.
GaInNAs, GaAsSb, and InAs quantum dots are accepted as active layer materials which can be fabricated on the GaAs substrate and emit laser beam in a long wavelength which exceeds 1.3 &mgr;m. The lattice strain in each active layer material is approximately 2%, approximately 2.6%, and approximately 7% in order. Because the lattice strain (lattice-mismatch between the substrate and the active layer) is large as much as 2% or more in all active layer materials, such a problem that the life of a device is short may possibly occur.
On the other hand, there is an active layer which uses a type-II heterojunction structure as the active layer material which can be fabricated on the GaAs substrate in the same manner and emit laser beam in a long wavelength band which exceeds 1.3 &mgr;m. In a type-I heterojunction structure adopted in many usual semiconductor lasers, a semiconductor layer which forms a quantum well in a conduction band forms the quantum well also in a valence band and emission occurs in the same material. On the contrary, in the type-II, as described later, the layer adjacent to the semiconductor layer which forms the quantum well in the conduction band forms the quantum well in the valence band and the emission occurs between different materials. In the active layer which uses the type-II heterojunction structure, there is such an advantage that the degree of design freedom of energy band structure and lattice strain is high.
A laser device which uses the type-II heterojunction structure based on GaAsSb/InGaAs grown epitaxially on the GaAs substrate in the active layer is disclosed in a US document “Journal of Vacuum Science and Technology” B18 (published in 2000), on pages 1,605 to 1,608, for example. In this example, the active layer is fabricated using one layer of GaAsSb/InGaAs respectively.
Further, a laser device structure, in which a type-II superlattice constituted of the GaAsSb layer and the InGaAs layer which are thin as many as 1 to 10 molecular layers is used in the active layer, is disclosed in Japanese Patent Laid-open (Kokai) No. 2000-164990.
SUMMARY OF THE INVENTION
In the active layer which uses the type-II heterojunction structure, there is the aforementioned advantage that the degree of design freedom of the energy band structure is high, but there is the following problem and it was difficult to put the active layer to practical use.
FIG. 9
shows the energy band structure in an example of the aforementioned laser having the type-II heterojunction structure in which one layer of the GaAsSb layer and the InGaAs layer is used respectively. The horizontal axis of
FIG. 9
shows a semiconductor layer which is grown in the right direction from a substrate in order. With respect to the vertical axis, the right-side axis which shows distribution of electrons (holes) is used for a wave function and the left-side axis which shows energy is used for another function. A bottom barrier layer
57
a
, an InGaAs layer
55
, a GaAsSb layer
56
, and a top barrier layer
57
b
are formed from the substrate in order, and the InGaAs layer
55
and the GaAsSb layer
56
become active layers.
In
FIG. 9
, the energy band structure is constituted of an energy of conduction band edge
51
and an energy of valence band edge
52
. A wave function
53
of an electron (a quantized electron's (hole's) energy state
58
of a conduction band) and a wave function
54
of a hole (a quantized electron's (hole's) energy state
59
of a valence band), which contribute to emission, overlap in the physical relationship of a vertical direction only for extremely in part (a range is shown in the drawing by an arrow), and the distribution of electrons (holes) of the overlapped part (rise of a wave function to the upper part) is small. Accordingly, there is a problem that emission efficiency is exceedingly low.
On the other hand, the aforementioned another laser device structure in which the type-II superlattice constituted of the GaAsSb layer and the InGaAs layer which are thin as many as 1 to 10 molecular layers becomes the energy band structure shown in FIG.
10
. In this case, because the InGaAs layer
55
and the GaAsSb layer
56
are exceedingly thin, subbands
62
,
63
are formed, and the structure becomes the energy band structure just like a type-I quantum well used in a usual quantum well laser. As a result, high emission efficiency is obtained.
In this case, however, the subband
62
at the side of the conduction band is formed at the side of higher energy than the energy of conduction band edge of the InGaSb layer
55
and the subband
63
at the side of the energy of valence band is formed at the side of lower energy than the energy of valence band of the GaAsSb layer
56
. Accordingly, there is a problem that an emission wavelength is shifted to a short wavelength. To enable realization of a long wavelength, the In composition of InGaAs and the Sb composition of the GaAsSb layer need be increased. Because the increase of these types of composition results in the increase of lattice strain, however, the lattice strain which exceeds 2.3% need be introduced to realize the emission wavelength of the 1.3 &mgr;m band in this structure. In general, a device in which the lattice strain which exceeds 2% is introduced into an active layer may cause a problem in terms of life and reliability, and it is difficult to put the device to practical use.
An object of the present invention is to provide a semiconductor laser which has an active layer of a lattice strain of less than 2% on an average on a GaAs substrate and can be used in a long wavelength band of a 1.3 &mgr;m band or more and a photo module which uses the semiconductor laser.
To attain this and other objects, a semiconductor laser device of the present invention has a first semiconductor layer and second semiconductor layers, both of the first layer and the second layers becoming an active layer on a semiconductor substrate, and makes the first semiconductor layer and the second semiconductor layers adjacent to each other and laminates them. The semiconductor laser device forms a type-II heterojunction structure in which an energy of conduction band edge of the first semiconductor layer is larger than an energy of conduction band edge of the second semiconductor layers and an energy of valence band edge of the first semiconductor layer is larger than an energy of valence band edge of the second semiconductor layers, and has third semiconductor layers on top and bottom of the active layer, of which the energy of conduction band edge is larger

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