Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-11-29
2001-11-13
Leung, Quyen (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06317446
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a vertical resonator laser diode (VCSEL vertical-cavity surface-emitting laser), in which an active layer sequence for generating laser radiation is disposed between a first Bragg reflector layer sequence and a second Bragg reflector layer sequence, each of which has a plurality of mirror pairs. The two Bragg reflector layer sequences form a laser resonator and are disposed, together with the active layer sequence, between a first and a second electrical contact layer. One of the two Bragg reflector layer sequences is partially transmissive or semitransparent for the laser radiation generated in the active layer sequence. On at least one of the two Bragg reflector layer sequences, a current constriction device, called “current aperture” below, is provided which serves to concentrate the operating current lowing through the active layer sequence during operation of the laser diode and thus to limit the cross section of the pumped active region of the active layer sequence. The current aperture has at least one aperture layer disposed between one of the electrical contact layers and the active layer sequence. Furthermore, the invention relates to a method for producing such a vertical resonator laser diode.
Such vertical resonator laser diodes with a current aperture
119
are disclosed in U.S. Pat. No. 5,493,577. One example of such a vertical resonator laser diode is illustrated schematically in FIG.
3
. In this example, the current constriction (the current flow is indicated by the arrows
130
) to the desired pumped region
121
of the active layer sequence
103
is effected through the use of two thin AlAs or AlGaAs aperture layers
105
, which are oxidized except for the respective current passage opening
116
.
The two aperture layers
105
are disposed on mutually opposite sides of the active layer sequence
103
, in each case between the active layer sequence
103
and the Bragg reflector layer sequence
102
,
104
. The Bragg reflector layer sequences
102
,
104
each essentially include a plurality of mirror pairs, each of which has two AlGaAs layers having different band gaps. The Bragg reflector layer sequences
102
is disposed on a substrate
120
. The bottom of the substrate
120
is provided with a contact layer
114
.
In the oxidized annular regions
122
, which define the size and form or shape of the current passage openings
116
, the AlAs or AlGaAs aperture layers
105
have a very high electrical resistance. As a result, the pump current
130
, which is supplied through a contact layer
115
, essentially flows only in the region of the current passage openings and thus in the desired pumped region
121
through the active layer sequence
103
.
With regard to a simple production of such vertical resonator laser diodes, the AlAs or AlGaAs aperture layers
105
have a higher Al content than the AlGaAs layers of the mirror pairs of the Bragg reflector layer sequences
102
,
104
.
This is because the lateral oxidation rate of Al
x
Ga
1−x
As layers during a heat treatment in a humid atmosphere depends on the Al content (the higher the Al content, the higher the oxidation rate). For this reason, the AlAs or AlGaAs aperture layers
105
as described in the U.S. Pat. No. 5,493,577, can be produced in a simple manner after the production of the laser diode layer structure with the active layer sequence
103
, the Bragg reflector layer sequences
102
,
104
and the AlAs or AlGaAs layers for the aperture layers
105
by an oxidation of the entire layer structure in a humid atmosphere.
Furthermore, the thin AlAs or AlGaAs aperture layers
105
cause only minor optical losses in the laser resonator. Consequently, it is possible to produce vertical resonator laser diodes having a very high efficiency and small threshold currents.
A disadvantage of the above-described structure of vertical resonator laser diodes, however, is that a severe current crowding occurs at the edges of the current passage openings
116
during operation. This current crowding causes a severe local heating of the Bragg reflector layer sequences
102
,
104
and of the active layer sequence
103
in the region of these edges, which accelerates the ageing of the laser diode component. Furthermore, there is also the risk of the locally very high current density leading to the generation and migration of crystal defects, which reduces the lifetime and reliability of the vertical resonator laser diodes.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a vertical resonator laser diode which overcomes the above-mentioned disadvantages of the heretofore-known vertical resonator laser diodes of this general type and in which local current crowding is minimized. It is a further object of the invention to provide a simple method for manufacturing a vertical resonator laser diode of this type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a vertical resonator laser diode, including a laser resonator having a first Bragg reflector layer sequence and a second Bragg reflector sequence, the first and second Bragg reflector layer sequences each having a plurality of mirror pairs; an active layer sequence disposed between the first and second Bragg reflector layer sequences for generating a laser radiation, the active layer sequence having a pumped active region and having a side, one of the first and second Bragg reflector layer sequences being partially transmissive for the laser radiation; a first electrical contact layer and a second electrical contact layer, the first and second Bragg reflector layer sequences and the active layer sequence being disposed between the first and second electrical contact layers; and a current aperture provided at least on the side of the active layer sequence for limiting the pumped active region of the active layer sequence by concentrating an operating current flowing through the active layer sequence during a laser operation, the current aperture including aperture layers disposed between one of the first and second electrical contact layers and the active layer sequence, the aperture layers being formed with current passage openings having different sizes, the sizes of the current passage openings increasing in a direction away from the active layer sequence toward one of the first and second electrical contact layers.
Accordingly, in the case of the vertical resonator laser diode of the type mentioned above, it is provided that the current aperture is formed by a plurality of individual, mutually separated or isolated aperture layers having current passage openings of different sizes, which are disposed between one of the electrical contact layers and the active layer sequence, and that the sizes of the current passage openings of these aperture layers increase (in steps) in the direction away from the active layer sequence toward the electrical contact layer. In particular, the sizes of the current passage openings, in the case of more than two aperture layers, permanently increase proceeding from a smallest passage opening, adjacent to the active layer sequence, toward a largest current passage opening, adjacent to the electrical contact layer.
In this vertical resonator laser diode, the first aperture layer, as seen from the active layer sequence, having the smallest current passage opening determines the size of the pumped active region. The aperture layer or layers disposed downstream of this first aperture layer, as seen from the active layer sequence, and having a larger current passage opening, reduce the effect of the current crowding in the vertical resonator laser diode because the current
13
, before reaching the smallest current passage opening, is already pushed in a step-wise manner toward the optical axis (beam axis) of the laser diode, which runs through the pumped active region. The current crowding is thus reduced by gradually compacting the current
13
or in other words gradual
Greenberg Laurence A.
Lerner Herbert L.
Leung Quyen
Siemens Aktiengesellschaft
Stemer Werner H.
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