Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-11
2003-04-01
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S044010
Reexamination Certificate
active
06542528
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to light-emitting semiconductor devices and more particularly to a light-emitting semiconductor device including a laser diode that produces visible optical radiation in the wavelength band of red color.
The system of AlGaInP is a III-V material having a direct-transition type band structure and provides the bandgap of as much as about 2.3 eV (540 nm in terms of optical wavelength), which is the largest bandgap value of the III-V material system except for the system of AlGaInN or the III-V material that contains B as the group III element. Thus, AlGaInP has been a target of intensive investigation in relation to high-power light emitting diode that produces visible optical radiation of green or red color. Such a high-power light emitting diodes of green to red color wavelength band has its application in color display devices. Further, the system of AlGaInP has been studied in relation to visible-wavelength laser diode for use in laser printers or in optical recording of information, such as compact disk players or DVD players.
In the laser diode designed for producing red color wavelength radiation, it has been practiced to use a material system that achieves a lattice matching with respect to a GaAs substrate. In the art of high-density recording of information, in particular, there is a demand of a high-power laser diode that operates stably even in a high-temperature or unregulated temperature environment.
In a laser diode, laser oscillation is caused as a result of stimulated emission taking place in an active layer of the laser diode, and as a result of stimulated emission, an optical beam is produced in the active layer. In order to achieve such a laser oscillation efficiently, it is necessary to confine the carriers and further the optical radiation thus produced in the active layer effectively, and for this purpose, a cladding layer having a larger bandgap energy than the active layer is provided in such a manner that the cladding layer is disposed adjacent to the active layer.
In an ordinary laser diode having a double-heterostructure, it has been practiced to use AlGaInP containing Al for the active layer in order to reduce the wavelength of the produced optical beam to the visible wavelength band. It should be noted that Al thus added has an effect of increasing the bandgap energy of the active layer.
On the other hand, Al is a very reactive element and easily forms a deep impurity level in the active layer by reacting with oxygen, which may exist in the atmosphere used for growing the III-V epitaxial layer(s) constituting the laser diode. Further, such oxygen impurity may also be contained in the source material of the III-V crystal, although with a trace amount.
In view of the problems noted above, it is preferable to reduce the Al content in the epitaxial layer constituting the active layer of the laser diode as much as possible. Thus, there is a proposal to use a GaInP quantum well for the active layer and sandwich the foregoing GaInP active layer vertically by a pair of optical waveguide layers of AlGaInP. Such a laser structure is called SCH-QW (separate confinement heterostructure quantum well) structure. Further, in relation to the SCH-QW laser diode, there is a proposal, as in the Japanese Laid-Open Patent Publication 6-77592, to apply a strain to such a quantum well layer constituting the active layer of the laser diode so as to decrease the threshold of laser oscillation further. In the case of such a laser diode using a strained quantum well structure for the active layer, the thickness of the quantum well layer is set to be smaller than a critical thickness above which lattice relaxation takes place in the active layer by creating dislocations therein. It should be noted that such a strained quantum well layer is formed by choosing a material having a lattice constant different from that of the substrate, for the quantum well layer. In the case of the visible-wavelength laser diode that oscillates at the visible wavelength such as the wavelength of 635 nm, it is indicated, in the Japanese Laid-Open Patent Publication 6-275915, that a tensile strain is more effective than a compressive strain. In the case of a quantum well layer formed on a GaAs substrate under tensile strain, the quantum well layer can take a composition of GaInP, which is closer to the GaP composition as compared with the substrate composition of GaAs. Thereby, it should be noted that the quantum well layer has an increased bandgap energy, and a quantum well layer having a suitable thickness can be used for the strained quantum well layer. In such a construction in which a sufficient thickness is secured for the quantum well layer, the adversary effect of interface defects is successfully avoided. On the other hand, the laser diode that uses the quantum well layer under tensile strain for the active layer operates in the TM-mode, and the optical beam produced by such a laser diode has a plane of polarization which is 90° rotated as compared with the case of usual laser diode operating in the TE-mode.
As noted before, the SCH-QW construction, which uses an optical waveguide layer typically having a composition of (Al
x
Ga
1−x
)
0.5
In
0.5
P, successfully achieves a desired optical confinement in the optical waveguide layer. On the other hand, such an optical waveguide layer, containing a large amount of Al (x~0.5) therein, has a drawback in that a damaging is tend to be caused at the free edge surface of the laser cavity as a result of recombination of the carriers associated with the Al-induced defects contained in the optical waveguide layer. Thus, such a laser diode has a drawback in that high-power operation is difficult. Further, such a laser diode has a drawback in that the reliability is degraded substantially when operated for a long period of time.
Further, such a SCH-QW has a drawback, particularly in the case it contains a heterostructure of the AlGaInP system, in that the band offset is small in the side of the conduction band. More specifically, such a structure is characterized by a small band discontinuity (&Dgr;E
c
) in the conduction band between the active layer and the cladding layer, and the carriers (electrons in particular) injected into the active layer from the cladding layer easily cause an overflow. Thereby, the laser diode shows a heavy temperature dependence in the laser threshold characteristic, particularly the threshold current, and expensive temperature regulation has been needed for stable operation thereof. This problem of poor temperature characteristic of the laser oscillation becomes rapidly worse with decreasing oscillation wavelength. For example, the temperature dependence of the laser oscillation characteristic for the laser diode operating at the wavelength of 635 nm is much worse than that of the laser diode operating at the wavelength of 650 nm.
In order to overcome the foregoing problem of temperature dependence of the laser diode, there is a proposal, as in the Japanese Laid-Open Patent Publication 4-114486, to enhance the carrier confinement efficiency by providing a multiple quantum barrier (MQB) structure between the active layer and the cladding layer, wherein the MQB structure includes a stacking of a number of extremely thin layers. This proposal, however, is turned out to be not realistic because of its structural complexity and the difficulty of thickness control of each thin layer of the MQB structure. In order to obtain the desired effect according to this conventional approach, it is necessary to form the individual layers of the MQB structure to be flat with the precision of atomic layers.
Thus, there has been a distinct limitation in the improvement of temperature dependence and further in the decrease of laser oscillation wavelength, as long as the laser diode is constructed on a conventional GaAs substrate. More specifically, it has been not possible to realize a laser diode, according to such a conventional approach, that operates at the wavelength o
Jikutani Naoto
Sato Shun'ichi
Takahashi Takashi
Menefee James
Ricoh & Company, Ltd.
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