Nitride semiconductor laser device

Details Nitride semiconductor laser device Nitride semiconductor laser device

2000-01-27

2003-02-18

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C372S045013, C372S046012

Reexamination Certificate

active

06522676

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to laser devices comprising a nitride semiconductor, and more particularly to nitride semiconductor laser devices adapted for self-pulsation.
BACKGROUND OF THE INVENTION
Semiconductors laser devices have found wide application to optical communication, optical recording and various other fields and are prevalent used especially in the field of optical recording chiefly in optical disk systems. In the case where semiconductor laser devices are used as light sources for optical disk systems, the diameter of the spot of light focused on the optical disk by an objective lens is directly proportional to the lasing wavelength of the semiconductor laser device, so that the shorter the lasing wavelength, the smaller the spot diameter on the optical disk is and the greater the recording capacity of the disk is. While AlGaInP semiconductor laser devices heretofore in actual use are about 650 nm in lasing wavelength, nitride semiconductor laser devices are as short as about 400 nm in lasing wavelength. Accordingly progress has been made in developing nitride semiconductor lasers to achieve increased recording capacities in optical disk systems.
Already known as semiconductor laser devices having a double heterojunction structure are the ridge stripe semiconductor laser device shown in
FIG. 11
, and the self-aligned semiconductor laser device shown in FIG.
12
.
With reference to
FIG. 11
, the ridge stripe semiconductor laser device comprises an n-type cladding layer
3
, active layer
4
, p-type cladding layer
561
, n-type current blocking layer
611
and p-type contact layer
7
which are superposed on a substrate
10
, for example, of sapphire. The p-type cladding layer
561
has an upwardly projecting ridge stripe portion
50
.
As shown in
FIG. 12
, the self-aligned semiconductor laser device comprises an n-type cladding layer
3
, active layer
4
, p-type first cladding layer
57
, n-type current blocking layer
62
, p-type second cladding layer
58
and p-type contact layer
7
which are superposed on a substrate
10
, for example, of sapphire. The p-type second cladding layer
58
has a downwardly projecting ridge stripe portion
60
.
The ridge stripe semiconductor laser device is fabricated by the process shown in
FIG. 13
, (a) to (d). First as seen in
FIG. 13
, (a), an n-type contact layer
2
, n-type cladding layer
3
, active layer
4
and p-type cladding layer
59
are formed as superposed on a substrate
10
, and a mask
15
is then formed on the surface of the p-type cladding layer
59
. Next as shown in
FIG. 13
, (b), the p-type cladding layer
59
is dry-etched with the mask
15
provided thereon to form a ridge stripe portion
50
. Subsequently as seen in
FIG. 13
, (c), an n-type current blocking layer
611
is selectively grown utilizing the mask
15
remaining on the ridge stripe portion
50
, the mask
15
is thereafter removed, and a p-type contact layer
7
is grown as shown in
FIG. 13
, (d).
In fabricating AlGaAs infrared semiconductor laser devices or AlGaInP red semiconductor laser devices, the ridge stripe portion
50
can be formed with high accuracy by inserting in advance into the p-type cladding layer an etching stop layer which is more resistant to a specified etchant than the p-type cladding layer. However, since no suitable etchant has been found for use in fabricating nitride semiconductor laser devices, it is difficult to form the ridge stripe portion by such chemical etching. It is therefore conventional practice to form the ridge stripe portion
50
by dry-etching the p-type cladding layer
59
as seen in
FIG. 13
, (a) and (b).
The self-aligned semiconductor laser device is fabricated by the process shown in
FIG. 14
, (a) to (c). First as seen in
FIG. 14
, (a), an n-type contact layer
2
, n-type cladding layer
3
, active layer
4
, p-type first cladding layer
57
and n-type current blocking layer
63
are formed as superposed on a substrate
10
, and masks
16
,
16
are formed on the surface of the n-type current blocking layer
63
. Next as shown in
FIG. 14
, (b), the n-type current blocking layer
63
is dry-etched through the masks
16
,
16
to locally remove the current blocking layer
63
to a depth where the p-type first cladding layer
57
becomes exposed. Subsequently as shown in
FIG. 14
, (c), the masks
16
,
16
are removed to grow a p-type second cladding layer
58
and p-type contact layer
7
.
However, the fabrication of the ridge stripe semiconductor laser device requires three crystal growth steps. It is also required to control the thickness t of the p-type cladding layer
561
beneath the n-type current blocking layer
611
by dry etching, whereas difficulty is encountered in controlling this thickness. If the thickness of the cladding layer
561
is smaller than the proper value, it is difficult to effect the self-pulsation to be described below, while if the thickness of the cladding layer
561
is greater than is proper, the n-type current blocking layer
611
to be grown subsequently will have a smaller thickness, failing to effectively block the current.
On the other hand, two crystal growth steps suffice to fabricate the self-aligned semiconductor laser device. Furthermore, the thickness of the p-type cladding layer
57
beneath the n-type current blocking layer
62
can be controlled by crystal growth. With this type of laser device, nevertheless, dry etching for locally removing the current blocking layer is likely to cause damage to the light-emitting portion and current injection region, and such damage is likely to produce an adverse effect on the emission of light.
It is required that the optical disk player comprising a semiconductor laser device as its optical component be low in noise against optical feedback from the optical disk. Accordingly it is practice to obtain multimode operation by superposing high-frequency waves of about several hundreds of megahertz to about 1 GHz on the drive signal for a semiconductor laser device of the single-mode pulsation type, or to cause semiconductor laser devices to undergo self-pulsation (see, for example, “Theoretical Analysis of Self-Pulsation Phenomena in Semiconductor Lasers,” Technical Report of Society of IEICE, OQE92-16).
The method of obtaining a multimode by the superposition of high-frequency waves requires an external circuit, so that the method of effecting self-pulsation is desirable from the viewpoint of compacting the device and cost reductions. Self-pulsation occurs by forming a light absorbing region termed a saturable absorbing region in the active layer around the region thereof where current is injected (active region).
Accordingly used in fabricating ridge stripe or self-aligned semiconductor laser devices is a method of giving a spot width greater than the width of the current injection region to effect self-pulsation, by optimizing device structure parameters such as the stripe width W, the thickness t of the p-type cladding layer
561
beneath the n-type current blocking layer
611
and the thickness d of the active layer
4
.
Also used is a method of causing side portions of the current injection region within the active layer to act as a saturable absorbing region by giving the active layer an increased thickness.
With the ridge stripe semiconductor laser device shown in
FIG. 11
, however, the ridge stripe portion
50
flares downward in cross section, so that current is injected as spread out along this configuration into the active layer
4
. As a result, a current injection region is provided which spreads out beyond the width W of the stripe portion
50
at the position of its lower end, and the width of the current injection region becomes approximately equal to the spot width.
Further with the self-aligned semiconductor laser device of
FIG. 12
wherein the ridge stripe portion
60
is tapered downward in cross section, current is injected as confined in this configuration into the active layer
4
. Consequently, a current injection region is provided which is narrower than th

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