Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-09-27
2003-07-22
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06597716
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor laser formed of a III group nitride semiconductor.
BACKGROUND ART
FIG. 1
is a schematic cross-sectional view showing a conventional ridge waveguide type III group nitride semiconductor laser. A semiconductor laser of
FIG. 1
has a stack structure including a GaN buffer layer
202
, an n-GaN contact layer
203
, an n-GaN buffer layer
204
, an n-AlGaN cladding layer
205
, an n-GaN guiding layer
206
, an InGaN-MQW active layer
207
, a p-AlGaN cap, layer
208
, a p-GaN guiding layer
209
, a p-AlGaN cladding layer
210
, and a p-GaN contact layer
211
on a sapphire substrate
201
. Since the sapphire substrate is insulative, a portion of the stack structure is etched down to the n-type contact structure
203
in order to expose a region to which an n-type electrode is attached. Furthermore, a portion of a mesa structure is etched down to the p-type cladding layer
210
in order to form a ridge waveguide. In these processes, a dry etching method is employed, and an SiO
2
protect film
214
is added for protecting the etched portion.
FIG. 2
shows a relationship between the thickness of the residual p-cladding layer and an effective refractive index difference between the inside and the outside of a stripe (a ridge portion) (a curved line of the conventional example shown in FIG.
2
). In the conventional ridge waveguide type III group nitride semiconductor, by utilizing the refractive index difference caused by the difference in thickness of the p-AlGaN cladding layer
210
between inside and outside the ridge portion as shown in
FIG. 2
, an effective refractive distribution in a (A) portion and a (B) portion is formed, thereby controlling a transverse mode. The control for the effective refractive index in the (B) portion of
FIG. 1
is conducted by regulating a film thickness T of the p-AlGaN cladding layer
210
which has been left unetched.
Thus, optical characteristics wherein a light-emitting angle in the vertical direction is 34° and a light-emitting angle in the horizontal direction is 7° are obtained under the CW operation at a room temperature. Furthermore, a device duration under the CW operation at a room temperature is about 35 hours.
FIG. 3
shows a variation of an operation current of the conventional ridge waveguide type III group nitride semiconductor laser under the CW operation at a room temperature.
However, in the conventional ridge waveguide type III group nitride semiconductor laser as shown in
FIG. 1
, there was a problem that fabricating a semiconductor laser having a uniform transverse characteristic with a high yield is extremely difficult. Dry etching such as RIE, RIBE or the like is employed for etching because no suitable chemical etchant exists for the III group nitride semiconductor, and the control of film thickness for a P—AlGaN layer
210
of a portion (B) in
FIG. 1
is conducted by time control because no suitable etching stop layer exists. However, time control or else employ a less precise technique. As a result, a film thickness of the P—AlGaN layer
210
varies between plural lots or in the same wafer, whereby controllability of the transverse mode is considerably damaged, and the production yield deteriorates.
Another problem is short lifetime under the CW condition at a room temperature. The inventor of the present application has discovered that this results from using dry etching as a processing method for forming a stripe-shaped ridge shape. More specifically, the above problem results from side surfaces and a bottom surface of a semiconductor to be etched being damaged by an etching treatment, thereby causing a crystal defect, and pinholes being present in SiO
2
of an SiO
2
protection film covering a p-AlGaN cladding layer on the side surface of the ridge and outside the ridge, whereby the crystal surface in fact cannot be sufficiently protected.
The present invention is made in light of the above conditions, and an object thereof is to provide a semiconductor laser having a single transverse mode characteristic, which can be fabricated with high production yield.
DISCLOSURE OF INVENTION
A compound semiconductor laser of a III group nitride semiconductor according to the present invention includes a first cladding layer of a first conduction type formed on a substrate, an active layer formed on the first cladding layer; a second cladding layer of a second conduction type formed on the active layer; and a buried layer formed on the second cladding layer, the buried layer having an opening portion for constricting a current in a selected region of the active layer, wherein an upper portion of the second cladding layer has a ridge portion, the ridge portion residing in the opening portion of the buried layer, and the buried layer does not substantially absorb light output from the active layer, and the buried layer has a refractive index which is approximately identical with that of the second cladding layer, whereby the above object is achieved.
In one embodiment, a light guiding layer of the second conduction type having a refractive index of a higher value than that of the second cladding layer, a third cladding layer of the second conduction type, and a contact layer of the second conduction type are sequentially formed in this order on the upper portion of the second cladding layer.
In one embodiment, the light guiding layer is made of InGaAlN.
In one embodiment, the buried layer is a dielectric film including at least one or more types of compounds among a group including TiO
2
, ZrO
2
, HfO
2
, CeO
2
, In
2
O
3
, Nd
2
O
3
, Sb
2
O
3
, SnO
2
, Ta
2
O
5
, and ZnO.
In one embodiment, the buried layer is made of a ZnMgCdSSe compound semiconductor.
In one embodiment, the buried layer is made of a semiconductor whose composition is approximately identical with that of the second cladding layer.
In one embodiment, the buried layer is insulative or of the first conduction type.
In one embodiment, a contact layer of the second conduction type is formed on the upper portion of the second cladding layer.
A compound semiconductor laser of a III group nitride semiconductor according to the present invention includes a first cladding layer of a first conduction type formed on a substrate, an active layer formed on the first cladding layer, a second cladding layer of a second conduction type formed on the active layer, and a reflection layer formed on the second cladding layer, the reflection layer having an opening portion for constricting a current in a selected region of the active layer, wherein a layer of a semiconductor of the second conduction type, whose composition is approximately identical with that of the second cladding layer, is formed in the opening portion of the reflection layer, and the reflection layer has a refractive index of a lower value than that of the second cladding layer, whereby the above object is achieved.
In one embodiment, the reflection layer is made of InGaAlN.
In one embodiment, the reflection layer is insulative or of the first conduction type.
In one embodiment, a third cladding layer of the second conduction type and a contact layer of the second conduction type are formed on the reflection layer.
Hereinafter, the function of the present invention will be described.
The present invention enables to provide a device structure in which a transverse mode does not vary against a variation of the amount of etching, and to efficiently fabricate a ridge waveguide type III group nitride semiconductor laser with uniform characteristics.
Furthermore, a device with significantly improved operating lifetime is realized in which a crystal defect caused by damage generated in an etching process is prevented from propagating to an active layer under the CW operation by providing a dielectric layer with smaller pinholes outside the ridge-shaped stripe formed by etching or a structure in which a semiconductor layer is formed to be thick, thereby substantially burying a ridge-shaped stripe.
Furthermore, a device with significantly improved operatin
Leung Quyen
Morrison & Foerster / LLP
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