Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-03-07
2003-07-22
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S080000, C257S093000, C257S098000, C257S099000, C257S103000, C438S022000, C438S024000, C438S036000, C438S037000, C438S046000, C438S047000
Reexamination Certificate
active
06597017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a semiconductor device including a pseudo lattice matched layer formed with a lattice mismatched semiconductor crystal. More particularly, the present invention relates to electronic devices such as: a light emitting devices, for example a surface emitting semiconductor laser and an edge emitting semiconductor laser used as a light source of a laser printer, a DVD device or a display; light receiving devices, for example a solar cell and a light quantity measuring sensor; and a transistor and so on.
2. Description of the Prior Art
Conventionally, layers constituting a semiconductor laser have basically been formed by stacking lattice matched materials except for a thin quantum well layer, a thin barrier layer and so on. For example, in a case of an AlGaAs based laser, AlAs (AlGaAs) lattice matched to and having a bandgap larger than GaAs and GaAs are employed to form heterojunctions and produce a band structure including a valence band and a conduction band of a laser. Further, in a case of GaInP based laser, crystals of Ga
0.51
In
0.49
P and (AlGa)
0.51
In
0.49
P whose bandgap is larger than that of Ga
0.51
In
0.49
P are employed to form heterojunctions and produce a band structure of a laser, wherein the crystals are both lattice matched to GaAs.
In a case of a nitride semiconductor that has recently drawn attention as a material for a blue semiconductor laser, there is almost no material lattice matched to GaN that is used as a substrate. Further, a mixed crystal of Al
0.18
In
0.82
N, which is one of a few lattice matched materials, has a problem because of a bandgap smaller than GaN and poor crystallinity, whereby there has been found almost no way for the mixed crystal to be used as a device constituent layer. That is, in the case of a nitride semiconductor material, neither an edge emitting laser nor a surface emitting laser has been able to be produced from a lattice matched material Hence, lattice mismatched materials have been used for the purpose despite easy occurrence of crystal defects or the like.
For example, a DBR mirror of a surface emitting semiconductor laser has been formed with a multilayer having lattice mismatched materials (Al
x
Ga
1−x
N/GaN) (see Japanese Journal of Applied Physics Vol. 37 (1998), pp. L1412 to L1426). However, in the multilayer, if a composition x of Al
x
Ga
1−x
N is set as high as Al
0.34
Ga
0.66
N, crystal defects such as cracks and misfit dislocations are produced because of a large difference in in-plane lattice constant between Al
x
Ga
1−x
N and GaN, leading to poor crystallinity. Further, if a composition x of Al
x
Ga
1−x
N is set low, though the lattice constants are closer to each other, a difference in refractive index between AlGaN and GaN is small, thereby reducing a reflectance of light and narrowing a wavelength range of reflection, which are both problematic.
There has also been proposed a nitride semiconductor laser in which a DBR mirror of a surface emitting semiconductor laser is constituted of a multilayer having dielectric different from the nitride semiconductor (see JP 98-308558 A). In this case, since a nitride semiconductor cannot directly be grown on a dielectric DBR mirror, the dielectric DBR is patterned by etching or the like such that part of a GaN crystal is exposed and a crystal of the nitride semiconductor is then laterally grown over the dielectric DBR starting from the exposed area of the GaN crystal, whereby a spacer layer, an active region and so on of the surface emitting semiconductor laser are formed. However, in this method, a cavity (resonator) including a spacer layer, active layer and so on formed on a dielectric DBR mirror has to be grown in a lateral direction over a length equal or more than 10 to 20 &mgr;m while restricting a thickness of the cavity within 3 to 4&lgr; (equal to or less than about 600 nm, wherein &lgr; is a wavelength). That is, as a result, a very thin flat crystal having opposite major surfaces parallel to each other is to be formed, thereby making fabrication of the laser difficult. Further, since this lateral growth is generally a case of a facet growth in which a crystallographic plane is preferentially grown, there is a fault that the growth speed is slow: the lateral growth over 10 &mgr;m or more requires a long time, which makes the method not suitable for mass fabrication.
Also in a case of a surface emitting semiconductor laser of a long wavelength range such as for use in communications, a problem exists since there are few kinds of material lattice matched InP and a DBR mirror formed with an AlGaInAsP based material lattice matched to InP cannot have a high refractive index, thereby making it impossible to fabricate a practical surface emitting semiconductor laser. Hence, conventionally, a surface emitting semiconductor laser has been fabricated, for example, in such a way that a multilayer DBR mirror of AlAs/GaAs, which is a GaAs based material, having a good reflection characteristic is separately prepared and an active region layer of a long wavelength AlGaInAsP based laser is stuck on the mirror; since in-plane lattice constants are largely different from each other, direct growth cannot be performed. The fabrication process, though possible, is problematically cumbersome, making it impractical.
Further, while there can be exemplified a nitride semiconductor laser in which the cladding layers of an edge emitting semiconductor laser are fabricated with a lattice mismatched materials (Al
x
Ga
1−x
N/GaN), in this case, if a composition x of Al
x
Ga
1−x
N is set high, crystal defects such as cracks and misfit dislocations are produced and contrary to this, if a composition x of Al
x
Ga
1−x
N is set low, there also arises a problem since optical confinement is insufficient and in turn laser beam is leaked to the substrate side, followed by other inconveniences.
While as described above, in a case of the nitride semiconductor, a semiconductor multilayer has been formed using a lattice mismatched material with GaN, there has been arisen a problem, since crystal defects such as cracks and misfit dislocations are produced due to a difference in lattice constant, thereby making it difficult to fabricate a semiconductor multilayer of good crystallinity. Further, there has arisen a problem since if a material of a lattice constant closer to that of GaN is used for forming a semiconductor multilayer on GaN, a reflection characteristic of the semiconductor multilayer is degraded, which in turn makes it impossible to use the semiconductor multilayer as a DBR mirror of a surface emitting semiconductor laser or a cladding layer of an edge emitting semiconductor laser. Still further, there has arisen a problem in a case of InP that is used in a long wavelength range laser such as for use in communications as well, since a semiconductor multilayer formed using an AlGaInAsP based material having a lattice matching performance has a poor reflection characteristic and thereby, when the semiconductor multilayer is employed as a DBR mirror of a surface emitting semiconductor laser or a cladding layer of an edge emitting semiconductor laser, no sufficient light emission characteristic is achieved.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances and accordingly provides a semiconductor device having a pseudo lattice matched layer of good crystallinity, formed with lattice mismatched materials. The present invention further provides a semiconductor device having a pseudo lattice matched layer of a good reflection characteristic, formed with lattice mismatched materials. The present invention still further provides a good surface emitting semiconductor laser and a good edge emitting semiconductor laser.
According to one aspect of the present invention, a semiconductor device includes: a semiconductor base layer made of a semiconductor crystal whose in-plane lattice constant is a
0
; a pseudo lattice ma
Sakamoto Akira
Seko Yasuji
Fuji 'Xerox Co., Ltd.
Louie Wai-Sing
Oliff & Berridg,e PLC
Pham Long
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