Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-02-20
2001-02-06
Sanghavi, Hemang (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S045013, C372S050121
Reexamination Certificate
active
06185238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device using nitride compound semiconductors and, in particular, to a gallium nitride blue-violet semiconductor laser and its manufacturing method.
2. Description of the Related Art
Galium nitride compound semiconductors, such as GaN, InGaN, GaAlN and InGaAlN, are recently remarked as materials of short-wavelength semiconductor lasers for use in high-density optical disc systems because the laser light with a short wavelength can be tightly focused and is therefore effective for increasing the recording density.
In general, in order to ensure the emission of semiconductor lasers with a low threshold current, carrier injection to the active layer must be performed effectively. For this purpose, it is important to form a hetero-structured p-n junction and a current confining structure. Additionally, semiconductor lasers for use as a light source of a pickup-head in an optical disc system must be regulated in oscilation mode. Therefore, a structure is required to obtain a stable emission in the fundamental oscilation mode by increasing the differences in refractive index between the active layer and the neighboring layers, and by appropriately absorbing laser light swerving from the active layer to decrease gains of higher-order modes.
However, in the case of nitride compound semiconductor lasers, there has been no report except for so-called gain-guiding lasers in which a ridge structure is employed only for a stripe-shaped electrode or a p-type semiconductor layer. On the other hand, it is known from semiconductor lasers using InGaAlP or other conventional materials that an active layer of a multi-quantum well structure is better than a bulk active layer to decrease the threshold current density. As for nitride compound semiconductor lasers, however, although multi-quantum well structures have been originally used as the active layer, power consumption is still large because they need a high threshold current and a high operation voltage. It results in too short laser lives and too large noise of oscillated laser light to use the lasers in optical disc systems. Moreover, since the maximum optical output of conventional gain-guided semiconductor lasers is on the order of mW, they are insufficient for use as a high-power light source available for 30 mW required for deletion and recording in optical disc systems.
This is caused by the difficulty in carrier injection into the quantum well active layer with a high density and a high efficiency, the difficulty in making a structure controllable in the fundamental transverse mode for low noise characteristics, and the difficulty in integrating a plurality of semiconductor lasers. That is, it has been difficult to inject carriers into the active layer with a high current density necessary for the fundamental transverse mode by confining and guiding the current from the electrode into the active layer without deviation from the active layer while making an appropriate absorption region for obtaining only the fundamental transverse mode. Moreover, integration of a plurality of nitride compound semiconductor lasers has been difficult due to the difficulty in alternately forming current confining regions and absorption regions.
As discussed above, for actual application of lasers to optical disc systems, or the like, it is necessary to realize a highly reliable gallium nitride blue-violet semiconductor laser promising continuous emission in the fundamental transverse mode with a low threshold current and low voltage.
Conventional semiconductor lasers using nitride compound semiconductors are not good in noise characteristics because of the difficulty in making a current confining structure controllable in the fundamental transverse mode. Additionally, because of the difficulty of high-density carrier injection into the active layer, it has been difficult to realize a device for a high optical output with a low threshold current and a low operation voltage.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a nitride compound semiconductor laser operative with a low threshold current and a low voltage and having low noise characteristics and a high reliability, which makes it easy to form a structure controllable in the fundamental transverse mode and includes a current confining structure ensuring high-density carrier injection into the active layer.
According to the invention, there is provided a nitride compound semiconductor laser comprising:
a multi-layered structure made of gallium nitride compound semiconductors (Ga
x
In
y
Al
z
N where x+y+z=1, 0≦x≦1, 0≦y≦1 and 0≦z≦1), including, at least, a first cladding layer of a first conduction type, an active layer overlying the first cladding layer, and a second cladding layer of a second conduction type overlying the active layer; and
current blocking layers made by forming at least two grooves extending from the major surface of the multi-layered structure closer to the second cladding layer and by filling the grooves with an insulator.
According to another aspect of the invention, there is provided a nitride compound semiconductor laser comprising:
a multi-layered structure made of gallium nitride compound semiconductors (Ga
x
In
y
Al
z
N=1, where 0≦x≦1, 0≦y≦1 and 0≦z≦1), including, at least, a first cladding layer of a first conduction type, an active layer overlying the first cladding layer, and a second cladding layer of a second conduction type overlying the active layer; and
current blocking layers made by forming at least two grooves extending from the major surface of said multi-layered structure closer to the second cladding layer and by filling the grooves with a gallium nitride compound semiconductor of the first conduction type.
In preferable versions of the invention,
(1) the current blocking layers may be deep enough to reach the first cladding layer, the second cladding layer or a contact layer;
(2) the insulator forming the current blocking layers may be an organic insulator such as polyimide, oxide or nitride;
(3) the active layer may have a multi-quantum well structure;
(4) the relation of W1≦4·W2 is established where the W1 is the distance between every adjacent two current blocking layers and W2 is the thickness of one of semiconductor layers in the multi-layered structure, having the first conduction type and being in contact with electrode metal;
(5) the nitride compound semiconductor laser may include at least three current blocking layers;
(6) these three or more current blocking layers are unequally distant; and/or
(7) the distance between every two adjacent current blocking layers may be 10 &mgr;m or less.
According to another aspect of the invention, there is provided a method for manufacturing a nitride compound semiconductor laser comprising the steps of:
forming a multi-layered structure of nitride compound semiconductors including, at least, a first cladding layer of a first conduction type, an active layer and a second cladding layer of a second conduction type;
selectively forming a stripe-shaped electrode for a semiconductor layer of the second conduction type on the multi-layered structure;
applying an etching mask on the multi-layered structure, the etching mask having apertures at opposite sides of the stripe-shaped electrode;
etching the multi-layered structure by using the etching mask and the electrode as a mask to form grooves in self-alignment at opposite sides of the stripe-shaped electrode; and
filling the grooves to form current blocking layers.
In preferable versions of the invention,
(1) the method may further include the step of covering the electrode with a mask before forming the grooves;
(2) the step of forming the current blocking layers may include the step of filling the grooves with polyimide.
According to the invention, since the nitride compound semiconductor laser has the mesa-type current injection layer above the active layer and the current bloc
Hatakoshi Gen-ichi
Ishikawa Masayuki
Nunoue Shin-ya
Onomura Masaaki
Cushwu Benjamin
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sanghavi Hemang
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