Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1998-12-03
2001-08-28
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S022000, C257S097000, C257S085000, C372S045013
Reexamination Certificate
active
06281518
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to layered semiconductor structures and more particularly, to layered III-V semiconductor structures with high nitrogen contents and to light emitting devices including the layered structures for use in laser diodes., photoreceptors and other similar devices.
2. Description of the Related Art
As communication systems have developed and both of desired and current information transmission rates have increased, more attention has been focused on the development of optical communication systems.
As presently contemplated, the optical communication system presently used in main communication lines will be extended to each subscriber's domestic line.
To implement such systems, it is indispensable to develop smaller and less expensive optical devices such as, among others, light emitting devices like laser diodes and light emitting diodes and photoreceptors.
For example, although light emitting devices such as laser diodes are conventionally accompanied by a cooling device such as a peltier element or heat sink so as to control the change in device temperatures caused by input currents, it is highly desirable to have stable laser diodes even without cooling devices in order to widely implement light emitting devices in the communication system.
Several semiconductor laser diodes have been proposed to attain improved temperature characteristics.
For example, a laser diode comprising a GaInNAs active layer disposed on a GaAs substrate has been disclosed in Japanese Laid-Open Patent Application No. 6-37355. In that disclosure, GaInAs layers having a lattice constant larger than that of GaAs have nitrogen (N) added to form GaInNAs layers with a decreased lattice constant, to thereby be lattice-matched to GaAs; and exhibit decreased bandgap energy. A result, light emissions at the wavelength region of 1.3 &mgr;m or 1.5 &mgr;m have become feasible in these devices. The GaInNAs layers were formed by metal organic chemical vapor deposition (MOCVD) using active N species.
As another example, calculated results of the energy level line-up are described by Kondow et al. in Japanese Journal of Applied Physics, Vol. 35 (1996), pages 1273-5, for a laser diode comprising a GaInNAs active layer formed on a GaAs substrate. It is described in the disclosure that, since the GaInNAs system is lattice-matched to GaAs, a large value of the conduction band discontinuity may be attained by providing cladding layers of AlGaAs rather than the materials which are lattice-matched to GaAs. This fabrication of laser diodes having improved temperature characteristics. This lattice matched GaInNAs system described just above was prepared by molecular beam epitaxy (MBE) using active N species.
As another example, according to Electronics Letters, Vol. 33, pages 1386-87, 1997, a GaInNAs laser device has been fabricated having laser emissions at 1.3 micron wavelength region. The GaInNAs system in this device was prepared by MOCVD using dimethylhydrazine (DMHy) as the nitrogen source.
In the aforementioned device fabrication methods, the growth of GaInNAs layers was carried out by continuously supplying the individual component materials Ga, In, N and As, simultaneously, to result in a constant composition throughout the thickness of the layers.
In the described GaInNAs semiconductor alloy systems, those in which N is included as a group-V element, mixed crystals are generally grown with a mixing ratio of the third additive element of only a few percent different than the stoichiometric compositions. The addition of a larger amount of the third element tends to deteriorate the quality of resulting crystals, and crystallinity of the GaInNAs layers therefore decreases with, for example increasing the percentage of N in the composition.
The inclusion of indium (In) in GaInNAs layers has been known to decrease band gap energies. Therefore, the amount of the N added also intended to decrease the band gap energies, may be decreased with an increase in the amount of In inclosed, which allows realization of laser emission in the region of 1.3 micron.
A composition described in Japanese Journal of Applied Physics, Vol. 35 (1996), pages 1273-5, has an amount of In as high as 30% which was added into GaInNAs layers to be formed with a crystallinity value as high as possible and to be used as a GaInNAs quantum well structure. In these layers, the N composition was only 0.5% and its emissions were at 1.2 micron. For the laser devices to be used in a communication system, the emission wavelengths are preferably about 1.3 micron, which requires that approximately 1% of N composition be achieved. It is noted that this 1% value for N is approximately one third of the N concentration value expected for layers containing 10% of In. Again, the N content required to achieve a desired emission wavelength can be decreased by incorporating an appropriate amount of In, which is advantageous without the deterioration of crystallinity which can be caused by the addition of larger amounts of N.
Since almost all nitrogen-containing alloy semiconductors are in the non-miscible region in the solid solubility diagram, the growth of these alloy semiconductors are generally quite difficult using conventional crystal growth methods. Therefore, only a minute amount of nitrogen can be incorporated in semiconductor crystals using non-equilibrium growth methods such as, for example, metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
For alloy semiconductors, in general, the non-miscibility increases with an increase in the number of constituent elements, and also toward the middle of the elemental composition. In other words, binary alloys can be grown most easily. This is also true for alloy semiconductors containing nitrogen as a group-V element. Alloys having an elemental composition closes to GaNAs can, therefore be grown with more ease within a GaInNAs alloy system.
The present inventor has grown GaInNAs alloy layers with varying In contents on a GaAs substrate by MOCVD, in which the source materials used were trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH
3
), and dimethylhydrazine (DMHy) as the nitrogen source, while hydrogen was used as a carrier gas. During the layer growth, the substrate temperature was at 630° C., and only the feeding rate of trimethylindium as the In source was varied.
GaInNAs alloy layers thus prepared were analyzed by secondary ion mass spectroscopy (SIMS) and the results on the N content are shown in Table 1 for the alloy layers various in In content percentages.
TABLE 1
In content (%)
N content (%)
7
2.5
13
1.7
23
0.5
28
0.3
The results in Table 1 indicates that the N content decreases with an increase in the In content. The N content in the GaInNAs alloy layers, which is necessary to bring about a certain emission wavelength, can therefore be decreased by the addition of In. However, it is known that the rate of N element to be included in the alloy system by conventional growth methods decreases with an increase in the In composition, and that crystallinity of the present GaInNAs system decreases with the increase in N composition with a greater rate with increasing the In composition.
FIG. 1
shows photoluminescence spectra for a plurality of GaAs/GaInNAs/GaAs structures, in which the curves A and B are for the GaInNAs layers with In 10% and N 1.5%, and In 30% and N 1.0%, respectively.
It is shown that photoluminescence intensity is stronger in the former (curve A) than the latter (curve B), despite the larger N content in the former. This can be considered, when the N content is approximately equal, the layer with a lower In content has a stronger photoluminescence intensity and also a higher crystallinity value, which may be related to the aforementioned miscibility gap for the semiconductor alloy systems.
In addition, a monoatomic superlattice structure has been proposed in Japanese Laid-Open Patent Application No.7-263744, to grow a semiconductor alloy having an N content higher than tho
Dickstein , Shapiro, Morin & Oshinsky, LLP
Jackson, Jr. Jerome
Ricoh & Company, Ltd.
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