Method of fabricating a reverse mesa ridge waveguide type laser diode

The present invention provide a reverse mesa ridge waveguide type laser diode and a method of fabricating the same. 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.

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.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
 Hereinafter, a preferred embodiment of the present invention will be
 explained in more detail with reference to the accompanying drawings.
 FIGS. 3A to 3E are cross-sectional views showing a method of fabricating a
 reverse mesa ridge waveguide type laser diode according to an embodiment
 of the present invention. The laser diode according to the present
 embodiment is in an InP group having an oscillation wavelength of 1.3
 .mu.m.
 As shown in FIG. 3A, a N-type buffer layer 32 is grown on the upper surface
 31a of a N.sup.+ -InP substrate 31 having an upper surface 31a and a lower
 surface 31b opposite the upper surface 31a and an U-InGaAsP non-buried
 active layer 33 having an oscillation wavelength of 1.3 .mu.m is grown
 thereon. A P-InP waveguide layer 34 is grown on the U-InGaAsP active layer
 33 and a P-InGaAsP waveguide control layer 35 for controlling selectively
 the waveguide width of the waveguide layer 34 is then formed thereon. The
 waveguide control layer 35 is an InP group and its material wavelength is
 1.1 .mu.m to 1.3 .mu.m in case of the oscillation wavelength of 1.3 .mu.m.
 Next, a P-InP clad layer 36 and P.sup.+ -InGaAs contact layer 37 are
 sequentially grown on the P-InGaAsP waveguide control layer 35. These
 compound semiconductor layers are grown by MOCVD technique.
 As shown in FIG. 3B, a silicon oxide layer pattern 38 is formed on the
 P.sup.+ -InGaAs contact layer 37 so as to act an etch mask during etching
 process for forming a reverse mesa ridge. As shown in FIG. 3C, the P.sup.+
 -InGaAs contact layer 37 is selectively etched by wet etching using the
 silicon oxide layer pattern 38 as an etch mask. The wet etching is carried
 out using a mixed solution of H.sub.3 PO.sub.4, H.sub.2 O.sub.2 and
 H.sub.2 O. The P-InP clad layer 36 existing under the P.sup.+ -InGaAs
 contact layer 37 is selectively etched by a wet etching using a mixed
 solution of HBr and H.sub.2 O, thereby forming a reverse mesa ridge.
 Afterward, the P-InGaAsP waveguide control layer 35 is selectively etched
 such that its width is narrower than that of the lower portion of the
 P-InP clad layer 36. Here, the width of the P-InGaAsP waveguide control
 layer 35 is narrow enough to generate a single mode of laser. More
 specifically, since the width of the waveguide of the waveguide layer 34
 is selectively controlled by overlying the P-InGaAsP waveguide control
 layer 35, it is possible to widely form the upper and lower portions of
 the reverse mesa ridge regardless of the waveguide width. Then, the
 silicon oxide layer pattern 38 is removed by a well-known method, as shown
 in FIG. 3D.
 Referring to FIG. 3D, a protection layer 39 is then formed on the surface
 so as to protect the reverse mesa ridge. The protection layer 39 is,
 preferably, silicon oxide layer. A polyimide layer 40 is formed on the
 protection layer 39 at both sides of the reverse mesa ridge so as to fill
 the etched portions at both sides of the reverse mesa ridge. The polyimide
 layer 40 prevents a break of a P-type ohmic metal layer which will be
 formed later. Next, the protection layer 39 is removed so as to expose the
 P.sup.+ -InGaAs contact layer 37a in the upper portion of the reverse mesa
 ridge.
 As shown in FIG. 3E, a P-type ohmic metal layer 41 is formed to contact
 with the exposed P.sup.+ -InGaAs contact layer 37. The P-type ohmic metal
 layer 41 is made of a stacked layer of Ti layer, Pt layer and Au layer,
 and Au plating layer is formed thereon. A N-type ohmic metal layer 42 is
 then formed on the lower surface 31b of the substrate 31. The N-type ohmic
 metal layer 42 is made of a stacked layer of AuGe layer, Ni layer and Au
 layer, and Au plating layer is formed thereon.
 According to the above described embodiment, since the waveguide width is
 selectively controlled by the P-InGaAsP waveguide control layer 36a which
 is formed thereon, the upper and lower widths of the reverse mesa ridge is
 formed widely. In result, it is easy to generate a single mode laser and
 to reduce a contact resistance and a serial resistance of the laser diode.
 Therefore, heat emission during operating of the laser diode decreases to
 thereby enable high level ouput. Furthermore, since it is possible that
 the laser diode can be operated at high temperature, available temperature
 range is widened and frequency property is improved. In addition, the
 threshold current of the laser diode decreases, to thereby improve the
 reliability of the laser diode.
 Although the above embodiment is described with reference to the laser
 diode of InP group having oscillation wavelength of 1.3 .mu.m, the present
 invention can be applied to a laser diode having oscillation wavelength of
 1.55 .mu.m or a laser diode of GaAs group. In case of the oscillation
 wavelength of 1.55 .mu.m, the waveguide control layer is formed of InGaAsP
 of which material wavelength is 1.33 .mu.m or InGaAsP having a short
 wavelength less than 1.55 .mu.M. Furthermore, in case of the laser diode
 of GaAs group, the waveguide control layer is formed of a material having
 a refractive index lower than that of the active layer and having band gab
 energy higher than the active layer.
 While this invention has been described with reference illustrative
 embodiments, this description is not intended to be construed in a
 limiting sense. Various modifications of illustrative embodiments, as well
 as other embodiments of the invention, will be apparent to persons skilled
 in the art upon reference to this description. It is therefore
 contemplated that the appended claims will cover any such modifications of
 embodiments as falling within the true scope of the invention.