Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2001-06-07
2004-11-23
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S483000
Reexamination Certificate
active
06821806
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for forming a compound semiconductor layer, and more specifically to a method for forming a group III-V compound semiconductor layer containing at least nitrogen and arsenic as a group V element.
BACKGROUND ART
Recently, as group III-V compound semiconductor materials having a significantly wider field of use as optoelectronics materials, group III-V compound semiconductor materials containing arsenic as a group V element (GaAs, GaInAs, etc.) and nitrogen mix-crystallized therewith have been proposed.
Japanese Laid-Open Publication No. 6-37355 (first conventional example) discloses Ga
1−y
In
y
N
s
As
1−x
-based compound mix crystal semiconductor materials (z=about 0.04) as new semiconductor materials which are lattice-matched to a GaAs substrate. It is shown that use of such semiconductor materials allows a semiconductor laser for emitting light in a long wavelength band (1.3 to 1.55 &mgr;m) to be produced on a low-cost GaAs substrate, which is conventionally impossible.
PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 4, April 1998, page 487 (second conventional example) discloses producing a semiconductor laser structure on a GaAs substrate. The semiconductor laser structure includes an active layer formed of a quantum well layer which is formed of Ga
0.7
In
0.3
N
0.01
As
0.99
and a guide layer, and the active layer is held between upper and lower cladding layers formed of Al
0.3
Ga
0.7
As. It is reported that such a semiconductor laser realizes continuous oscillation for light having a wavelength of 1.31 &mgr;m at room temperature. This is the first report that such a continuous oscillation is realized by a semiconductor laser formed of materials lattice-matched to a GaAs substrate.
For crystal growth of these new semiconductor materials, a molecular beam epitaxy (MBE) method or an metal organic chemical vapor deposition (MOCVD) method is used. Usable nitrogen source materials include, for example, dimethylhydrazine (DMeHy) and nitrogen gas (N
2
) activated by plasma. Crystal growth is conducted by concurrently supplying Ga, In and As source materials and the nitrogen source material(s) described above.
Such group III-V compound crystal semiconductor materials containing a group III-V compound semiconductor having arsenic as a group V element and also containing nitrogen as a group V element mix-crystallized therewith have not been actively studied until recently. The reason is that it is difficult to grow crystals of such semiconductor materials.
For example, GaAsN is considered to be a mix crystal of GaN containing only N as a group V element and GaAs containing only arsenic as a group V element. This mix crystal system have a very large immiscible region (misciblity gap). Therefore, it is difficult even to introduce only several percent of N with GaAs. Thus, it is necessary to carefully select a method and conditions for crystal growth. It is reported that especially introducing nitrogen with GaAs is significantly influenced by a substrate temperature during crystal growth. As a substrate temperature for such crystal growth, about 500° C. is usually selected. The temperature of 500° C. is relatively low as a crystal growth temperature of a group III-V compound semiconductor.
Jpn. J. Appl. Phys. Vol. 36, No. 12A, December 1997, page L1572 (third conventional example) shows correlation between the substrate temperature during crystal growth and a nitrogen-mix crystal ratio in the crystal in the case where GaAsN containing monomethylhydrazine (MMeHy) as an N source material in crystal-grown. When the substrate temperature is lower than 500° C., MMeHy is not sufficiently thermally decomposed. Therefore, only a small amount of nitrogen is introduced. By contrast, when the substrate temperature is higher than 500° C., the nitrogen source material is thermally evaporated significantly, such that nitrogen is not introduced into GaAs. It is reported that N can be introduced into the crystal most efficiently at a substrate temperature of about 500° C. for these reasons. In the second conventional example, plasma-decomposed N
2
is used as a nitrogen source material. In this example also, about 500° C. is selected as a crystal growth temperature.
Novel compound semiconductor materials containing nitrogen mix-crystallized with, for example, GaAs or GaInAs are used for an active layer of a semiconductor laser. One such example is described above, in which a GaInNAs layer is used for an active layer of a semiconductor laser. A semiconductor laser using such a compound semiconductor material does not necessarily provide superior light emission characteristics over an equivalent structure using a compound semiconductor material not containing nitrogen. For example, in the publication showing the above-described second conventional example, semiconductor lasers having structures similar to one another are produced. One of these semiconductor lasers uses GaInAs not containing nitrogen for an active layer (quantum well), and the other semiconductor laser uses GaInNAs containing nitrogen. It is reported that when 1% of nitrogen is contained, the oscillation threshold current becomes four times larger and the light emission efficiency is reduced to about ⅔. It is also reported that when a small amount of nitrogen is contained, the light emission efficiency is drastically reduced.
As one cause of reduction in the light emission efficiency, it can be pointed out that the crystal growth temperature is too low according to the conventional crystal growth method and therefore crystals having sufficient crystallinity are not obtained.
For example, in the case of GaAsN, a crystal is produced by introducing N into GaAs by causing crystal growth to proceed in a state of non-equilibrium at a low growth temperature (about 500° C.). Such a crystal cannot be produced in a state of thermal equilibrium. GaAsN can be considered to be a mix crystal of GaAs and GaN. The optimum growth temperature of GaAs is 600° C. to 750° C., and the optimum growth temperature of GaN is 900 to 1000° C. As compared to these temperatures, about 500° C. cannot be considered to be the optimum growth temperature for GaAsN-based compound mix crystal semiconductor materials.
It is assumed, for example, that in a semiconductor laser including an active layer and upper and lower cladding layers sandwiching the active layer, the active layer is formed of GaInNAs and the upper and lower cladding layers are formed of, for example, AlGaAs, GaInP, InGaAsP or AlGaInP. For producing such a semiconductor laser, the crystal growth temperature for the upper and lower cladding layers formed of Al
h
Ga
i
In
1−h−i
As
j
P
1−j
(h≧0,i>0,j≧0) is usually set to be a low substrate temperature (about 500° C.) in conformity with the crystal growth temperature for the GaInNAs active layer. As described above, the cladding layers crystal-grown at such a low substrate temperature do not have sufficient crystallinity. Unless the lower cladding layer formed of Al
h
Ga
i
In
1−h−i
As
j
P
1−j
(h≧0,i >0, j≧0) which acts as an underlying layer of the GaInNAs active layer has sufficient crystallinity, the crystal defect of the lower cladding layer is transferred to the GaInNAs active layer which is crystal-grown on the lower cladding layer. Accordingly, when a laser structure is produced at such a low temperature, satisfactory light emission characteristics cannot be provided, and the laser device deteriorates quickly. Such a conventional low temperature crystal growth method is considered to be performed in order to meet a requirement for provision of a novel material by introducing nitrogen rather than a requirement for improvement in the light emission characteristics by growth of GnAsN or GaInNAs at a high temperature.
As an attempt to improve light emission characteristics, there is a report on the effect of heat treatment performed after the crystal growth. The abstract of Jpn. J. Appl. Phys. Spring 1998, 28p-ZM-12 reports th
Kawanishi Hidenori
Takahashi Koji
Morrison & Foerster / LLP
Picardat Kevin M.
Sharp Kabushiki Kaisha
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