Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
2001-10-31
2003-11-25
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S079000, C257S103000, C257S613000, C257S615000, C372S043010
Reexamination Certificate
active
06653662
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light-emitting device which provides improved noise characteristics when operating at low output powers, and to a method for fabricating the light-emitting device, and a method for driving the same.
FIG. 14
shows a prior-art refractive index guided semiconductor laser device disclosed in Japanese Journal of Applied Physics by A. Kuramata et al., 37(1998), L1373.
For example, as shown in
FIG. 14
, on a substrate
101
of sapphire, formed are the following layers, each of which is formed of a III-V compound semiconductor. That is, grown by crystallization on the substrate
101
are an n-type semiconductor portion
102
containing an n-type contact layer, an active layer
103
, and a p-type semiconductor portion
104
containing a p-type contact layer.
The upper portion of the p-type contact layer in the p-type semiconductor portion
104
has a ridge portion patterned in the shape of a stripe, where a p-side electrode
105
is formed on the entire surface of the ridge portion. In this structure, a region of the active layer
103
underlying the p-side electrode
105
acts as a cavity in which lasing takes place.
The n-type contact layer of the n-type semiconductor portion
102
is exposed on one side region of the p-side electrode
105
, where an n-side electrode
106
is formed substantially on the entire surface of the exposed surface.
A forward drive current is applied from the p-side electrode
105
to the n-side electrode
106
. When the drive current has exceeded a predetermined lasing current threshold, a laser beam is launched from one facet of the active layer
103
.
Suppose that a semiconductor laser device like the one shown in
FIG. 14
is used to perform a write operation on an optical disc such as a high-density digital versatile (or video) disc (HD-DVD). To use a purple laser beam in this operation, it is necessary to deliver an output of 30 mW or more. In contrast to this, it is necessary to make the output of the purple laser beam as low as 1 mW for the read operation.
However, in the read operation, there is a problem that the prior-art semiconductor laser device causes the relative intensity of noise to increase as the output decreases even when a high frequency is superimposed on the drive current. This is because a current approximately equal in magnitude to the lasing current threshold is injected to allow lasing to take place, thereby causing the relative intensity of noise to increase due to the effect of relaxation oscillation in the lasing.
In addition, lasing at approximately the same injected current as the lasing current threshold causes a characteristic of the single mode to be degraded and multi-mode components to develop, thereby increasing the relative intensity of noise.
To reduce the relative intensity of noise, it is necessary to increase the frequency of relaxation oscillation. As one of the methods that are applicable to the reduction, it is conceivable to increase the differential gain. To increase the differential gain of lasing, an optical absorption region may be formed to increase the lasing threshold.
Alternatively, the slope efficiency (differential efficiency) may be reduced to increase the current required to deliver a lasing output of approximately 1 mW, thereby setting the operating current to a value greater than the lasing threshold.
Incidentally, the facets of a cavity could be increased in reflectivity to reduce the noise of the semiconductor laser device. However, this would cause the output (optical output) of the laser beam to be reduced as well. As described above, lased light of a high output power is required for the HD-DVD device to carry out the write operation. Accordingly, this makes it impossible to employ the means for increasing the reflectivity of the facets, which leads to a reduction in optical output.
On the other hand, to allow the semiconductor laser device to provide self-pulsation, it is necessary to provide a semiconductor optical absorption layer in or near the active layer
103
.
However, this raises a problem that such an optical absorption layer provided in the semiconductor laser device itself would make it difficult to provide a high output power.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor light-emitting device which solves the aforementioned prior-art problems and provides a reduced relative intensity of noise even when operating at low output powers.
To achieve the aforementioned object, the present invention provides a semiconductor laser device which is provided with the p-side or n-side electrode divided to apply a drive current to only part of a divided electrode when the laser device is required to operate at low output powers for the read operation.
More specifically, a semiconductor light-emitting device according to the present invention comprises a first semiconductor layer of a first conductivity type formed substantially in a uniform thickness on a substrate and a second semiconductor layer of a second conductivity type formed in a uniform thickness on the first semiconductor layer. The light-emitting device also comprises an active layer, formed in a uniform thickness between the first semiconductor layer and the second semiconductor layer, for generating emission light. The light-emitting device further comprises a first electrode for supplying a drive current to the first semiconductor layer and a second electrode for supplying a drive current to the second semiconductor layer. The light-emitting device is adapted such that the first electrode or the second electrode is a divided electrode comprising a plurality of conductive members spaced apart from each other.
According to the semiconductor light-emitting device of the present invention, a drive current is applied to all the divided electrodes for the operation at high output powers. On the other hand, for the operation at low output powers, a drive current is applied to part of the divided electrodes to inject the drive current nonuniformly into the active layer, thereby forming an optical absorption region in the active layer. This causes the lasing current threshold to increase and the differential gain of lasing to thereby increase, thus making it possible to reduce the relative intensity of noise during the operation at low output powers.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is provided on a principal surface, having said active layer formed thereon, of the substrate.
In the semiconductor light-emitting device according to the present invention, it is preferable that the second electrode has a stripe pattern for forming a cavity in the active layer, and the divided electrodes are adapted to sit on the sides of an emitting facet and a reflecting facet of the cavity, respectively.
In the semiconductor light-emitting device according to the present invention, it is preferable that the first electrode and the second electrode are formed on the principal surface, having the active layer formed thereon, of the substrate.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is a p-side electrode for injecting holes into the active layer.
In this case, it is preferable that the p-side electrode has a stripe pattern formed on the second semiconductor layer, and the plurality of conductive members of the p-side electrode are spaced apart approximately 10 &mgr;m or less from each other.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is an n-side electrode for injecting electrons into the active layer.
In this case, it is preferable that the n-side electrode is formed on a region of the first semiconductor layer, the region being exposed on one side of the p-side electrode, and the plurality of conductive members of the n-side electrode are spaced apart approximately 5 &mgr
Hasegawa Yoshiaki
Kawaguchi Yasutoshi
Otsuka Nobuyuki
Sugahara Gaku
Lee Eddie
McDermott & Will & Emery
Nguyen Joseph
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