Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-29
2003-04-29
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06556605
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for fabricating a buried heterostructure semiconductor laser used as a light source for optical fiber communications.
BACKGROUND OF THE INVENTION
Laser diodes are widely used as optical sources for optical fiber communications mainly because they are capable of modulating a signal at high speed. In particular, buried heterostructure semiconductor laser diodes have superior characteristics in that they have a low oscillatory threshold value and a stable oscillation transverse mode, as well as a high quantum efficiency and high characteristic temperature. This is because, in the buried heterostructure laser diodes, a current blocking layer can be formed on both sides of an active layer formed between two clad layers having a large energy gap and a small refractive index. This way, current leakage during operation is substantially reduced, if not prevented.
A conventional method for the fabrication of semiconductor laser diodes having a semi-insulating buried ridge is exemplified in
FIGS. 1-7
and described bellow.
Referring to
FIG. 1
, the process steps for fabricating a laser diode with a buried ridge begin with the formation of a multi layered structure
100
on an n-InP substrate
10
. The multi layered structure
100
is formed of a first clad layer
12
of n-InP, an active layer
14
, a second clad layer
16
of p-InP, and a layer
18
of selective area growth using a quaternary material (SAC-Q), layers that are sequentially formed and successively epitaxially grown to complete a first crystal growth. The active layer
14
could be, for example, a multiple quantum well (MQW) structure formed of undoped InGaAs/InGaAsP pairs and formed by a Metal Organic Chemical Vapor Deposition (MOCVD) or Metal Organic Vapor Phase Epitaxy (MOVPE).
Next, as shown in
FIG. 2
, a SiO
2
or Si
3
N
4
mask
20
is formed into a stripe on the upper surface of layer
18
. Subsequently, the multi layered structure
100
is selectively etched down to the n-InP substrate
10
to produce a mesa stripe
50
, as illustrated in FIG.
3
. The mesa stripe
50
, which has the mask
20
on top, is then introduced into a liquid phase epitaxial growth system or a MOCVD growth system, so that a p-InP current blocking layer
32
and an n-InP current blocking layer
34
are subsequently formed, as shown in FIG.
4
. Current blocking layers
32
and
34
surround mesa stripe
50
and form a second crystal growth.
The first current blocking layer
32
may be doped with impurity ions, such as iron (Fe) or titanium (Ti), to form a semi-insulating (si) InP(Fe) blocking layer
32
. The addition of Fe-impurity ions increases the resistivity of the first current blocking layer
32
and reduces the leakage current that typically occurs at the interface between the substrate
10
and the first current blocking layer
32
. Similarly, the second current blocking layer
34
may be doped with impurity ions, such as silicon (Si), sulfur (Su) or tin (Sn), to form an n-type InP-doped blocking layer
34
.
Referring now to
FIG. 5
, after removal of the mask
20
and the optional removal of the SAC-Q layer
18
, a third crystal growth is performed on the upper surfaces of the second current blocking layer
34
and the SAC-Q layer
18
. Thus, a p-InP burying layer
42
(also called a third clad layer) and a p-InGaAsP or a p-InGaAs ohmic contact layer
44
are further grown to form a buried heterostructure. The burying layer
42
may be also doped with p-type impurity ions, such as zinc (Zn), magnesium (Mg), or berilium (Be), to form a p-type InP-doped burying layer
42
. Since Zn is the most commonly used p-type dopant, reference to the burying layer
42
will be made in this application as to layer InP(Zn)-doped.
Next, as illustrated in
FIG. 6
, an n-type electrode
62
is formed on the lower surface of semiconductor substrate
10
and a p-type electrode
64
is formed on the upper surface of the ohmic contact layer
44
. Thus, a buried heterostructure laser diode is fabricated in accordance with the above described method.
A problem that occurs in the method of fabricating the above structure is the iron-zinc (Fe—Zn) interdiffusion at the interface between the semi-insulating p-InP (Fe) first current blocking layer
32
and the p-InP(Zn) burying layer
42
. The problem arises because the Fe-doped InP current blocking layer
32
, which was initially covered by the mask
20
, comes in contact with the Zn-doped InP burying layer
42
after the removal of the mask
20
. The contact region is exemplified in
FIGS. 5 and 6
as regions D, situated on lateral sides of the mesa stripe
50
. The dissociation of Fe and Zn atoms at the regions D, and their consequent interdiffusion, can significantly increase the leakage current and degrade the device, leading to a poor manufacturing yield. In addition, if the active layer
14
has a multiple quantum well (MQW) structure, the Zn impurities in the Zn-doped InP burying layer
42
can enter the active layer
14
to form mixed crystals therein and practically reduce the quantum effect to zero.
In an effort to suppress the interdiffusion of dopant atoms, such as those of Zn and Fe, different techniques have been introduced in the IC fabrication. For example, one technique of the prior art, exemplified in
FIG. 7
, contemplates the insertion of an intrinsic or undoped InP layer
70
between the Fe-doped InP current blocking layer
32
and the Zn-doped InP burying layer
42
, to prevent the contact between the InP(Fe) layer and InP(Zn) layer and to eliminate the iron-zinc interdiffusion and the consequent leakage current. This technique, however, has a major drawback in that it affects the p-n junction between the n-InP second current blocking layer
34
and the p-InP burying layer
42
. Specifically, the addition of an intrinsic InP layer modifies the p-n junction that should be in the active region of devices like laser, and creates instead a p-i-n junction that alters the device characteristics altogether.
Accordingly, a method for forming a mesa stripe for buried heterostructure laser diodes, which is inexpensive to implement, and capable of decreasing the leakage current and the interdiffusion of dopant atoms, is needed. There is also a need for such a semiconductor device having good operating characteristics with reduced impurity atoms interdiffusion, reduced leakage current, and which has improved accuracy and operation reliability.
SUMMARY OF THE INVENTION
The present invention provides a method for reducing the interdiffusion between doped regions of semi-insulating buried ridge structures of forward biased devices, such as lasers and optical amplifiers.
The present invention utilizes a double dielectric mask that can be selectively etched. The mesa is undercut and an InP(Fe) layer grown. Next, the first mask is partially etched and a Si-doped InP layer is selectively grown. The second mask is subsequently etched and an InP(Zn) clad layer, along with a Zn-doped InGaAs contact layer, are grown. This way, no contact between the InP(Zn) clad layer and the InP(Fe) layer is formed, and the Zn—Fe interdiffusion is suppressed.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiment which is provided in connection with the accompanying drawings.
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patent: 5805628 (199
Chakrabarti Utpal Kumar
Joyner Charles H.
Lentz Charles William
Ougazzaden Abdallah
Shtengel Gleb E.
Nguyen Tuan
Triquent Technology Holding, Co.
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