Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-03-26
2004-07-20
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S011000, C257S013000, C257S096000, C257S101000, C257S102000, C257S103000
Reexamination Certificate
active
06765232
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to the technology of optical telecommunication and more particularly to an optical semiconductor device such as a laser diode used in optical telecommunication systems. Especially, the present invention is related to a surface-emission laser diode and fabrication process thereof, a growth apparatus and a growth process used for forming such a surface-emission laser diode, as well as optical transmission modules, optical transceiver modules and optical telecommunication systems that use such a surface-emission laser diode.
With wide spread use of Internet, there is is going on some kind of explosion of information handled by telecommunication systems. In view of this situation, optical fibers are now being deployed not only in trunk lines but also in subscriber lines or LANs (local area networks) located near the side of users. Further, optical fibers are introduced also for interconnection of various apparatuses or for interconnection inside an apparatus. Thus, the importance of large-capacity optical telecommunication technology is increasing evermore.
In order to realize inexpensive long-range optical telecommunication networks or large-capacity optical telecommunication networks, it is advantageous to use a vertical-cavity surface-emission laser diode (VCSEL, referred to hereinafter simply as “surface-emission laser diode”), in which an optical cavity is provided in a direction perpendicular to the epitaxial layers constituting the laser diode. In view of minimum optical loss of silica-based optical fibers in the wavelength band of 1.3 &mgr;m and 1.55 &mgr;m, the surface-emission layer diode for use in such an optical telecommunication system is required to oscillate at the wavelength band of 1.3-1.55 &mgr;m. Here, the use of a surface-emission laser diode is particularly advantageous in view of its low cost, low power consumption, compact size and easiness of forming a two dimensional array.
There already exists a surface-emission laser diode constructed on a GaAs substrate and operable in the wavelength band of 0.85 &mgr;m. Thus, such a surface-emission laser diode is used in a high-speed LAN such as 1 Gbit/s Ethernet.
In the 1.3 &mgr;m band, on the other hand, a semiconductor material of InP has been used commonly in the conventional edge-emission type laser diodes. On the other hand, conventional laser diode of the 1.3 &mgr;m band thus constructed on the InP substrate has suffered from the problem of large increase of drive current, by the factor of three times or more, when the environmental temperature has increased from room temperature to 80° C. Further, there has been a problem in that no satisfactory distributed Bragg reflector could be constructed on an InP substrate, and thus, it has been difficult to construct a surface-emission on such an InP substrate.
In view of the foregoing difficulty, there has been a proposal to bond an active structure of a surface-emission laser diode including an InP substrate and an active layer on a distributed Bragg reflector of an AlGaAs/GaAs stacked structure formed on a GaAs substrate (V. Jayaraman, J. C. Geske, M. H. MacDougal, F. H. Peters, T. D. Lowes, and T. T. Char, Electron. Lett., 34, (14), pp.1405-1406, 1998).
However, such a construction becomes inevitably expensive and has an obvious problem of poor efficiency of production.
In view of the foregoing problems, efforts are being made to construct a surface emission laser diode operable in the wavelength band of 1.3 &mgr;m on a GaAs substrate, by using (Ga)InAs quantum dots for the active layer, or a compound semiconductor material such as GaAsSb or GaInNAs for the active layer. Reference should be made to Japanese Laid-Open Patent Publication 6-37355. Particularly, GaInNAs is expected as being a semiconductor material capable of minimizing the temperature dependence of the laser diode.
A GaInNAs laser diode constructed on a GaAs substrate has an advantageous feature of reduced bandgap for the active layer as a result of incorporation of N in the active layer, and thus, the laser diode becomes operable in the wavelength band of 1.3 &mgr;m even in the case the laser diode is constructed on a GaAs substrate. When the In content is 10%, for example, the wavelength band of 1.3 &mgr;m is realized by introducing N into the active layer with a concentration of about 3%.
FIG. 1
shows the relationship between the threshold current density of laser oscillation and the N content in the active layer for a laser diode having a GaInNAs active layer, wherein the vertical axis represents the threshold current density while the horizontal axis represents the N content in terms of percent.
Referring to
FIG. 1
, it can be seen that there occurs a steep increase in the threshold current with the N content in the active layer, wherein it is believed that the relationship of
FIG. 1
reflects the situation in which the degree of crystallization of the GaInNAs active layer is deteriorated with increase of the N content in the active layer.
Thus, the growth process of high quality GaInNAs layer becomes the key issue in the fabrication of such a surface-emission laser diode operable at the wavelength band of 1.3-1.55 &mgr;m.
Generally, a GaInNAs layer can been grown by an MOCVD (metal organic chemical vapor deposition) process or MBE (molecular beam epitaxy) process, wherein MOCVD process, in which the supply of source material is controlled by controlling a gas flow rate of the source material, is thought more advantageous and suitable for mass production of the laser diode as compared with MBE process, in which the supply of the source material is controlled solely by the control of the temperature of source cells, in view of the fact that the MOCVD process does not require a highly vacuum environment such as the one needed in the case of an MBE process, and a large growth rate is achievable easily. Thereby, the throughput of device production can be increased. In fact, mass production of the surface-emission laser diodes of the 0.85 &mgr;m band is achieved already by using an MOCVD process.
FIG. 2
shows the construction of a typical MOCVD apparatus used for growing group III-V semiconductor layers.
Referring to
FIG. 2
, the MOCVD apparatus is generally formed of a source gas supply system A for supplying a source gas, a susceptor B for supporting the substrate S and an evacuation unit C such as a vacuum pump for evacuating gases that have caused a reaction.
Generally, the substrate S is first loaded in a load/unload chamber
11
and is then transported to a growth chamber (reaction chamber)
12
after evacuating the air in the load/unload chamber
11
by driving the evacuation unit C.
Typically, the growth chamber is controlled to have an internal pressure of 50-100 Torr, and one or more of the metal organic sources such as TMG (trimethyl gallium), TEG (trimethyl gallium), TMA (trimethyl aluminum), TMI (trimethyl indium), and the like, are introduced into the growth chamber
12
as the source of group III element, together with a source of group V element. As a group V source, a hydride gas or organic compound such as AsH
3
, TBA (tertiary butyl arsine), PH
3
, TBP (tertiary butyl arsine), and the like is used.
These gaseous source are transported to the growth chamber by a hydrogen carrier gas, wherein the hydrogen carrier gas is generally used after removing impurity by passing through a purifier
13
.
For the source of nitrogen, organic compounds such as DMHy (dimethyl hydrazine), MMHy (monomethyl hydrazine), and the like, are used, although there are possibilities of using other materials.
In the case of using a liquid or solid source material, the source material is held in a bubbler
14
and the vapor of the source material or source gas formed as a result of bubbling in the bubbler
14
by the carrier gas, is supplied. The hydride gas, on the other hand, is held in a gas cylinder
15
.
In the example of
FIG. 2
, two bubblers #
1
and #
2
are provided for holding two difference source materials and two gas
Itoh Akihiro
Jikutani Naoto
Kaminishi Morimasa
Sato Shun'ichi
Takahashi Takashi
Dickstein , Shapiro, Morin & Oshinsky, LLP
Nelms David
Ricoh & Company, Ltd.
Tran Mai-Huong
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