Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-03-24
2003-10-28
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06639926
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a semiconductor light-emitting device and, more particularly, to a semiconductor light-emitting device having a ridge waveguide type stripe structure, which is suitable for a semiconductor laser device, and a manufacturing method for this semiconductor light-emitting device.
DESCRIPTION OF THE RELATED ART
A structure so-called as a ridge waveguide type is frequently used to produce semiconductor light-emitting devices without difficulties.
FIG. 2
shows a manufacturing method for such a structure. First, a first conductivity type cladding layer
202
, an active layer
203
, a second conductivity type cladding layer
204
, and a second conductivity type contact layer
205
are grown on a substrate
201
. A photoresist
211
having stripe openings as a pattern made by photolithography is formed on a wafer surface to form a stripe-shaped ridge
209
by a wet etching process using the photoresist
211
as a mask so that the second conductivity type cladding layer remains with a prescribed thickness. An insulating protective film
206
such as SiNx is subsequently formed on the whole surface on an epitaxial side, and only the protective film at the top of the ridge is removed by etching using a photoresist having stripe openings as a pattern made by photolithography. This structure prevents a current from flowing through portions other than the top of the ridge. Another layer of protective film may further be formed on the ridge side surface. Then, an epitaxial side electrode
207
and a substrate side electrode
208
are formed.
According to this structure, currents are injected into the active layer
203
after injected through the ridge portion
209
of the cladding layer. Currents are thus concentrated into the active layer region under the ridge portion
209
, thereby generating light having a wavelength corresponding to the band gap of the active layer. At that time, the band gap of the active layer is ordinarily smaller than those of the upper and lower cladding layers, and the refractive index of the active layer is larger than those of the upper and lower cladding layers, so that carriers and light can be confined effectively in the active layer. Because the protective film
206
having a smaller refractive index than the semiconductor portions is formed at a non-ridge portion
210
, the effective refractive index of the active layer region under the non-ridge portion
210
becomes smaller than that of the ridge portion
209
. Consequently, the generated light is confined in the active layer region under the ridge portion
209
. This structure thus can stabilize the transverse mode for laser oscillation and can reduce the threshold currents.
With such a conventional manufacturing method for ridge waveguide type semiconductor light-emitting device, the ridge portion is formed by the etching process, so that it is difficult to accurately control the thickness of the cladding layer at the non-ridge portion
210
. As a result, the effective refractive index at that portion largely varies due to slight differences of the thickness of the cladding layer at the non-ridge portion, thereby deviating the laser characteristics of the semiconductor light-emitting device, and rendering product yields hardly improve. Where a laser device of a single transverse mode is produced, a very highly accurate alignment technique is required, because it is difficult to use process simplifying techniques such as a self-alignment in the conventional manufacturing method, though the top width of the ridge portion is at most about several microns. Such a complicated, fine photolithographic technology makes device production steps complicated and device production yields reduced. If a SiNx film is formed on the ridge side wall, a deletion layer of about 0.1 micron may be formed on the surface side of the ridge side wall to narrow the effective current channel width, thereby raising a problem that the pass resistance becomes larger.
Meanwhile, as a light source for information processing to improve the recording density, visible laser devices (ordinarily 630 to 690 nm) using AlGaInP basis in lieu of conventional AlGaAs basis (wavelength about 780 nm) are put to practical use, but the following researches have been made to realize shorter wavelength, lower threshold, and high temperature operation.
In a production of an AlGaInP/GaInP based visible laser device, use of a substrate having an off-angle from the (100) plane toward the [011] direction (or [0-1-1] direction) allows the band gap from narrowing due to formation (ordering) of natural super lattices, thereby rendering the wavelength shorter readily, facilitating high concentration doping of p-type dopants (e.g., Zn, Be, and Mg), and improving the oscillation threshold current of the device by enhancement of the hetero-barrier and temperature characteristics. If the off-angle is too small, step bunching appears outstandingly, and large undulations are formed at the hetero-boundaries, so that a shift amount in which the PL wavelength (or oscillation wavelength) is shortened by quantum effects to the bulk active layer may be smaller than the designed amount where a quantum well structure (GaInP well layer of about 10 nm or less) is manufactured. If the off-angle is made larger, the step bunching is reduced, and the hetero-boundaries become flat, thereby making the wavelength shorter by the quantum effect as designed. Thus, a substrate having an off-angle of 6 to 16 degrees from the (100) plane toward the [011] direction (or [0-1-1] direction) is generally used to suppress formation of natural super lattices and generation of step bunching, which impede the wavelength from becoming shorter, as well as to suppress the oscillation threshold current from increasing due to shortened wavelength from p-type high concentration doping and impairment of temperature characteristics. A proper off-angle should be selected in consideration of thickness of the GaInP well layer and the stress amount depending on the targeted wavelength such as 650 nm or 635 nm.
When natural super lattice is formed in the active layer, it is deformed to be mixed crystal during current injection whereby problems may be raised such that oscillation wavelength or emission wavelength is changed and device properties are impaired. Natural super lattice is easily formed in an active layer made of materials including In and Ga as constituent elements such as GaInAs, AlGaInAs, InGaAsP as well as the above-mentioned GaInP and AlGaInP. Use of an off-angle substrate suppresses formation of natural super lattice and effectively solves the problems.
To reduce waveguide loss and mirror loss, a resonator is formed in extending in a striped shape as much as vertical to the off-angled direction of the substrate. FIGS.
3
(
a
) and
3
(
b
) show cross sections of conventional ridge and groove type inner stripe structures made of a semiconductor using a current block layer. In FIGS.
3
(
a
),
3
(
b
), numeral
301
is a substrate; numeral
302
is a first conductivity type cladding layer; numeral
303
is an active layer; numeral
304
is a second conductivity type cladding layer; numeral
305
is a first conductivity type current block layer; numeral
306
is a second conductivity type contact layer; numeral
307
is an epitaxial side electrode; numeral
308
is a substrate side electrode; numeral
311
is a substrate; numeral
312
is a first conductivity type cladding layer; numeral
313
is an active layer; numeral
314
is a second conductivity type first cladding layer; numeral
315
is a first conductivity type current block layer; numeral
316
is a second conductivity type second cladding layer; numeral
317
is a second conductivity type contact layer; numeral
318
is an epitaxial side electrode; and numeral
319
is a substrate side electrode. In this situation, because the shape of the ridge or groove may become horizontally asymmetric or the optical density profile may become horizontally asymmet
Fujii Katsushi
Goto Hideki
Nagao Satoru
Shimoyama Kenji
Armstrong Westerman & Hattori, LLP
Leung Quyen
Mitsubishi Chemical Corporation
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