Semiconductor light emitting device

Details Semiconductor light emitting device Semiconductor light emitting device

2000-03-23

2003-06-17

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013, C372S046012, C372S043010

Reexamination Certificate

active

06580736

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting device.
2. Description of the Prior Art
A semiconductor light emitting device such as a semiconductor laser device or a light emitting diode employing a group III nitride semiconductor (hereinafter referred to as a nitride based semiconductor) such as GaN, GaInN, AlGaN or AlGaInN is expected for application to a light emitting device emitting light over the visible to ultraviolet regions. Such a semiconductor light emitting device is formed on a (0001) plane of a substrate of sapphire, silicon carbide or the like by MOVCD (metal-organic chemical vapor deposition) or MBE (molecular beam epitaxy).
In a GaN based semiconductor light emitting device formed by successively stacking an n-type semiconductor layer, an emission layer and a p-type semiconductor layer on a substrate, an n-type current blocking layer is generally formed in the p-type semiconductor layer. This current blocking layer performs transverse mode control of the semiconductor light emitting device.
FIG. 7
is a typical sectional view showing an exemplary conventional GaN based semiconductor laser device
200
.
In the semiconductor laser device
200
shown in
FIG. 7
, a buffer layer
82
of undoped AlGaN, an n-contact layer
83
of n-GaN, an n-light cladding layer
84
of n-AlGaN, an n-light guide layer
85
of n-GaN, an emission layer
86
, a p-cap layer
87
of p-AlGaN, a p-light guide layer
88
of p-GaN and an n-current blocking layer
89
having an opening are successively formed on a sapphire substrate
81
. A p-light cladding layer
90
of p-AlGaN is formed in the opening of the n-current blocking layer
89
. A p-contact layer
91
of p-GaN is formed on the p-light cladding layer
90
and the n-current blocking layer
89
.
Partial regions of the layers from the p-contact layer
91
to the n-contact layer
83
are etched so that an n-type electrode
50
is formed on the exposed part of the n-contact layer
83
. A p-type electrode
51
is formed on the p-contact layer
91
.
In the semiconductor laser device
200
, the n-current blocking layer
89
narrows current injected from the p-type electrode
51
. Thus, the opening of the n-current blocking layer
89
defines a current injection region while an emission part is formed in the region of the emission layer
86
located under the current injection region.
The material for the current blocking layer
89
may be n-AlGaN, n-InGaN or the like.
When prepared from n-AlGaN, the n-current blocking layer
89
has a small refractive index due to Al contained therein. The region of the emission layer
86
located under the n-current blocking layer
89
having a small refractive index exhibits a smaller effective refractive index as compared with the region of the emission layer
86
located under the opening. Light is horizontally confined in the emission layer
86
due to such distribution of the refractive index. This device structure confining light by the difference in refractive index is referred to as a real refractive index guided structure.
When prepared from n-InGaN, on the other hand, the n-current blocking layer
89
having a smaller band gap than the emission layer
86
absorbs light of a higher mode generated in the emission layer
86
. Thus, light is concentrated to the region of the emission layer
86
located under the opening of the current blocking layer
89
, and horizontally confined in the emission layer
86
. This device structure confining light by light absorption is referred to as a loss guided structure.
As described above, transverse mode control is performed in the semiconductor laser device
200
due to the current narrowing in the n-current blocking layer
89
and confinement of light in the emission layer
86
.
In the semiconductor laser device
200
having the n-current blocking layer
89
of n-AlGaN, the electron concentration in the n-current blocking layer
89
is generally extremely increased to 10
19
to 10
20
cm
−3
, thereby suppressing leakage of current in the n-current blocking layer
89
and reducing current which does not contribute to the laser oscillation.
In order to increase the effect of transverse mode control in the semiconductor laser device
200
, the Al composition of the n-current blocking layer
89
made of n-AlGaN is preferably increased. When the Al composition is increased, the refractive index of the n-current blocking layer
89
is further reduced thereby increasing the difference in refractive index of the emission layer
86
along the horizontal direction. Thus, light is effectively confined.
When having a large Al composition, the n-current blocking layer
89
exhibits a lattice constant smaller than that of the n-contact layer
83
, to generate an electric field (piezoelectric field) as a result of piezoelectric effect caused by tensile strain. However, electrons of a high concentration are injected into the n-current blocking layer
89
and hence the piezoelectric field is reduced by movement of the electrons. Thus, the energy band is inhibited from bending caused by the piezoelectric effect.
FIG. 8
is a model diagram showing the energy band structure of the semiconductor laser device
200
having the n-current blocking layer
89
of n-AlGaN in a section taken along the line B—B in FIG.
7
. Referring to
FIG. 8
, positive bias applied between the p-type electrode
51
and the n-type electrode
50
is zero.
As shown in
FIG. 8
, the energy band of the n-current blocking layer
89
is flat since the high-concentration electrons suppress the piezoelectric effect in the n-current blocking layer
89
.
When applying positive bias between the p-type electrode
51
and the n-type electrode
50
of the semiconductor laser device
200
, a quasi Fermi level lowers on the side of the p-contact layer
91
and rises on the side of the n-contact layer
83
as shown by arrows in FIG.
8
. Thus, the energy band of the n-current blocking layer
89
is inclined toward the upper right. When applying higher positive bias, the inclination of the energy band of the n-current blocking layer
89
is so increased that holes can move from the p-contact layer
91
to the p-light guide layer
88
through the n-current blocking layer
89
due to tunnel effect if the n-current blocking layer
89
has a small thickness. Consequently, current which does not contribute to the laser oscillation is increased. Further, holes falling to the level in the n-current blocking layer
89
cause recombination different from desirable emission, to increase the current which does not contribute to the laser oscillation.
Such current that does not, contribute to the laser oscillation can be suppressed by increasing the thickness of the n-current blocking layer
89
. When thickly growing the n-current blocking layer
89
having a high Al composition, however, cracking results from strain caused by lattice incommensurateness with the n-contact layer
83
. Therefore, it is difficult to increase the thickness of the n-current blocking layer
89
of n-AlGaN.
Transverse mode control can be performed with the n-current blocking layer
89
whose thickness is small to some extent. When the n-current blocking layer
89
has a small thickness, however, it is difficult to suppress leakage of current caused by the aforementioned tunnel effect and hence the current which does not contribute to the laser oscillation is increased. Thus, operating current and threshold current of the semiconductor laser device
200
are increased to reduce the luminous efficiency.
Also in a semiconductor laser device of the loss guided structure having an n-current blocking layer of n-InGaN, current which does not contribute to the laser oscillation is increased if the n-current blocking layer has a small thickness and hence operating current and threshold current of the semiconductor laser device are increased to reduce the luminous efficiency similarly to the semiconductor laser device
200
having the current blocking layer
89
of n-AlGaN.
SUMMARY OF THE

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