Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-04-28
2003-04-01
Wilczewski, M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C117S952000, C372S050121, C372S102000, C438S042000, C438S043000, C438S930000, C438S032000
Reexamination Certificate
active
06541297
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor optical integrated device, a method for fabricating a semiconductor laser device, a semiconductor optical integrated device, and a semiconductor laser device.
2. Description of the Related Art
In a semiconductor optical integrated device, a light emitting element (e.g., a semiconductor laser and a light emitting diode), an optical waveguide element (including an optical divider, an optical coupler, a filter, a modulator, and the like), and a light receiving element (e.g., a photodiode) are integrated on one semiconductor substrate. Such a semiconductor optical integrated device is important for realizing a small, inexpensive, highly functional optical device.
A method for forming the above elements monolithically on a semiconductor substrate by one crystal growth step is known. This method is advantageous over a fabrication method in which the respective elements are arranged by alignment, in that since an alignment process is not necessary, production yield improves, the cost can be reduced, and the size of the entire device can be easily reduced.
In a semiconductor optical integrated device, in order to increase the optical coupling efficiency among a light emitting element, an optical waveguide element, and a light receiving element, core layers of the respective elements are preferably positionally aligned to continue in series. Moreover, in order to avoid light emitted from the light emitting element being subjected to excessive absorption loss when the light propagates in the optical waveguide element, the optical waveguide element must be transparent to light emitted from the light emitting element. For this purpose, the forbidden bandwidth of the core layer of the optical waveguide element needs to be wider than that of the core layer (active layer) of the light emitting element.
As a method for fabricating a semiconductor optical integrated device which satisfies the above requirements, partly disordering a core layer composed of a quantum well structure is proposed. For example, Japanese Laid-Open Publication No. 3-89579 discloses a method for integrating a semiconductor laser element having a multiple quantum well (MQW) as an active layer (a distributed feedback (DFB) laser) and an optical waveguide element (an optical modulator) having a disordered MQW as a core layer, as well as a device obtained by this integration method. Hereinbelow, the disclosed method, as well as the construction of the device, will be described with reference to
FIGS. 15A
to
15
C.
First, referring to
FIG. 15A
, a p-type InGaAsP optical waveguide layer
19
, a MQW core layer
14
, an n-type InP upper cladding layer
15
, and an n-type InGaAsP contact layer
16
are sequentially formed by metal organic vapor phase epitaxy (MOVPE) on a p-type InP substrate
11
on which a diffraction grating
20
is partly formed. The MQW core layer
14
is composed of ten periods of an InGaAs well layer (thickness: 8 nm) and an InGaAsP barrier layer (corresponding to &lgr;
g
=1.3 &mgr;m, thickness: 11 nm).
Referring to
FIG. 15B
, a region M of the resultant structure which is to be a DFB laser is covered with a dielectric film
41
such as an SiO
2
film, while in a region N which is to be an optical modulator, sulfur is diffused so as to be distributed from the surface of the contact layer
16
to the core layer
14
. The MQW structure of the core layer
14
is destroyed due to the sulfur diffusion, turning the core layer
14
into a disordered layer
42
.
Thereafter, referring to
FIG. 15C
, a buried structure for controlling the transverse mode is formed in the following manner. First, the resultant structure is etched to form a mesa along the center in the length direction of the structure. Fe-doped high-resistance InP buried layers
43
are grown by MOVPE on both sides of the mesa along the length of the structure. Electrodes
17
and
18
are formed on both surfaces of the resultant structure, and then a separation groove
25
is formed by etching to separate the two regions. The resultant wafer is cleaved to obtain the semiconductor optical integrated device as shown in FIG.
15
C.
The above conventional fabrication method and construction of the optical integrated device have a feature that the DFB laser region M includes an active layer composed of an MQW structure and the optical modulator region N includes a semiconductor layer in which the same MQW structure is disordered (disordered layer). When a particular impurity is introduced into an MQW structure by diffusion, ion implantation, or the like, the MQW structure is disordered, changing into a bulk semiconductor layer having an average composition of the MQW structure, and thus slightly increasing the forbidden bandwidth. In other words, the optical modulator region N becomes transparent to light emitted from the DFB laser. The optical modulator which is transparent to the laser light allows the light to propagate therein without inducing light loss. Thus, when an electric field is applied to the electrodes of the optical modulator, the laser light propagating in the optical modulator can be modulated with high efficiency.
In the above conventional method for fabricating a semiconductor optical integrated device, the light emitting element, the light receiving element, and the optical waveguide element can be fabricated simultaneously without the necessity of a complicated processing step. In this conventional method, the core layers of the respective elements, i.e., the light emitting layer of the light emitting element, the light absorption layer of the light receiving element, and the optical guide layer of the optical waveguide element, are simultaneously formed by crystal growth as one continuous layer. Accordingly, the semiconductor optical integrated device fabricated by this method has no displacement or seam between the layers and thus provides a large optical coupling efficiency.
The technique of partly disordering a quantum well core layer used in the above conventional method is also utilized in methods for fabricating other types of semiconductor lasers. For example, Y. Suzuki et al., Electronics Letters, Vol.20 (1984) pp.383-384 describes that high output power can be realized for a semiconductor laser having an MQW active layer by introducing impurities only into end faces of the semiconductor laser to disorder the end faces, forming “window-stripes” which do not absorb light.
Japanese Laid-Open Publication No. 7-106697 discloses a gain-coupled distributed feedback semiconductor laser having an absorptive diffraction grating for periodically changing the absorption amount, which is obtained by periodically disordering an absorptive MQW guide layer disposed immediately above an active layer.
Japanese Laid-Open Publication No. 3-49285 discloses a gain-coupled distributed feedback semiconductor laser having a gain diffraction grating for periodically changing the gain of an active layer, which is obtained by periodically disordering the MQW active layer.
In the conventional fabrication method described with reference to
FIGS. 15A
to
15
C, impurities are introduced at a high concentration by a technique such as heat diffusion or combination of ion implantation and annealing.
In such heat diffusion and annealing, a resultant substrate after the completion of crystal growth needs to be kept at a high temperature for an extended period of time. For example, J. J. Coleman et al, Appl. Phys. Lett., Vol.40 (1982) p.904 describes a disordering method by Si ion implantation and annealing at a temperature as high as 675° C. for a period of time as long as four hours. When an p-n junction device such as a semiconductor laser is subjected to such a high-temperature, long-time heat treatment, impurities which should not be diffused, such as impurities doped in a cladding layer during crystal growth, are also diffused. This results in autodoping of impurities to the active layer, thereby greatly reducing t
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
Wilczewski M.
LandOfFree
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