Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1997-12-23
2001-03-13
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
Reexamination Certificate
active
06200826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device and method of fabricating the same, and more particularly to a reverse mesa ridge waveguide type laser diode and method of fabricating the same.
2. Discussion of Related Art
A laser diode, which receives current and outputs laser light, is used as an optical signal generation source in optical communications systems, and as a light source for an instrumentation equipment, information processing apparatus and pointer.
FIG. 1
is a cross-sectional view showing a conventional forward mesa ridge waveguide type laser diode. As shown in
FIG. 1
, a N-type buffer layer
2
, a active layer
3
, a P-type waveguide layer
4
, an etch stop layer
5
, a P-type clad layer
6
and a P-type contact layer
7
are sequentially formed on a N-type substrate
1
by metal organic chemical vapor deposition (MOCVD) technique. The contact layer
7
and the clad layer
6
are etched to form a forward mesa ridge. An oxide layer
8
acting as a protection layer is formed on the entire surface of the substrate uniformly so as to expose the upper surface of the forward mesa ridge. A P-type ohmic metal layer
9
is formed on the substrate such that it contacts with the exposed surface of the forward mesa ridge. A N-type ohmic metal layer
10
is then formed beneath the substrate
1
.
Although it is easy to fabricate the above-described forward mesa ridge waveguide type laser diode, the laser of multiple mode may be generated from it due to its wide waveguide width. To overcome this problem, a reverse mesa ridge waveguide type laser diode has been proposed.
FIG. 2
is a cross-sectional view showing the conventional reverse mesa ridge waveguide type laser diode.
As shown in
FIG. 2
, a N-type buffer layer
12
, an active layer
13
, a P-type waveguide layer
14
, an etch stop layer
15
, a P-type clad layer
16
and a P-type contact layer
17
are sequentially formed on a N-type substrate
11
by MOCVD technique. The contact layer
17
and the clad layer
16
are etched to form a reverse mesa ridge. An oxide layer
10
acting as a protection layer is formed on the substrate uniformly so as to expose the upper surface of the reverse mesa ridge. A polyimide layer
19
is filled in the etched portions at both sides of the reverse mesa ridge. A P-type ohmic metal layer
18
is formed on the entire surface of the substrate such that it contacts with the exposed surface of the reverse mesa ridge. A N-type ohmic metal layer
21
is formed beneath the substrate
11
.
Since the waveguide width in the laser diode as shown in
FIG. 2
is narrow due to the reverse mesa ridge, it is possible to generate a single-mode laser. Furthermore, the contact resistance and serial resistance are reduced due to the wide upper width of the ridge. However, to decrease the waveguide width with increasing the upper width of the ridge, it is preferable that the clad layer
16
is thick. In result, the laser diode as showing in
FIG. 2
has a serial resistance higher than that of a planar buried heterostructure (PBH) laser diode.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a reverse mesa ridge waveguide type laser diode capable of generating easily a laser of a single-mode and decreasing a contact resistance and a serial resistance.
Furthermore, another object of the present invention is to provide a method of fabricating the above laser diode.
To achieve the one object, a laser diode according to the present invention includes : a compound semiconductor substrate of a first conductivity type having an upper surface and a lower surface opposite the upper surface; a buffer layer of the first conductivity type, an active layer and a waveguide layer of a second conductivity type which are sequentially formed on the upper surface of the substrate; a waveguide control layer of the second conductivity type formed on the waveguide layer and having a predetermined width; a clad layer of the second conductivity type and a contact layer of the second conductivity type sequentially formed on the waveguide control layer and having a shape of a reverse mesa ridge whose lower portion has wider width than width of the waveguide control layer; a protection layer formed on the upper surface of the substrate, exposing the contact layer in an upper portion of reverse mesa ridge and protecting the reverse mesa ridge; a polyimide layer formed on the protection layer and filling both side portions of the reverse mesa ridge; an ohmic metal layer of the second conductivity type formed on the substrate and contacted with the exposed portion of the contact layer; and an ohmic metal layer of the first conductivity type formed on the lower surface of the substrate.
Furthermore, to achieve the another object, a laser diode according to the present invention is fabricated by the following processes. First, a buffer layer of a first conductivity type, an active layer, a waveguide layer of a second conductivity type, a waveguide control layer of the second conductivity type, a clad layer of the second conductivity type, and a contact layer of the second conductivity type are formed on the upper surface of a compound semiconductor substrate of the first conductivity type having an upper surface and a lower surface opposite the upper surface, in sequence. Next, the contact layer and the clad layer to form a reverse mesa ridge whose upper and lower portions have a predetermined widths are etched. The waveguide control layer is then selectively etched such that it has the width narrower than the upper portion of the reverse mesa ridge. Thereafter, a protection layer is formed on the substrate to protect the reverse mesa ridge. A polyimide layer is then formed on the protection layer at both sides of the reverse mesa ridge to fill the etched portions at both sides of the reverse mesa ridge. Next, the protection layer is partially removed to expose the contact layer of the upper reverse mesa ridge. Afterward, an ohmic metal layer of the second conductivity type is formed on the lower surface of the substrate to contact with the exposed portion of the contact layer and an ohmic metal layer of the first conductivity type is then formed on the lower surface of the substrate.
According to the present invention, since the waveguide width is selectively controlled by the waveguide control layer which is formed thereon, the upper and lower portions of the reverse mesa ridge have wide width. In result, it is easy to generate a single mode laser and a contact resistance and. In addition, a serial resistance of the laser diode are reduced.
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“Inp-Based Reversed-Mesa Ridge-Waveguide Structure For High-Performance Long-Wavelength Laser Diodes” by Aoki et al, IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, No. 2, Apr. 1977 pp. 672-683.
Hyundai Electronics Industries Co,. Ltd.
Mulpuri Savitri
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