Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2002-09-26
2004-04-06
Dang, Phuc T (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S201000
Reexamination Certificate
active
06717175
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a semiconductor laser device in which an active layer having a quantum well structure is sandwiched between a lower optical confinement layer and an upper optical confinement layer, and a method of manufacturing the semiconductor laser device by using the metalorganic chemical vapor deposition method (MOCVD method). More particularly, the present invention relates to a semiconductor laser device in which the composition of the semiconductor material constituting the upper and lower optical confinement layers is continuously changed in a thickness direction of the layers, and a method of manufacturing the semiconductor laser device by using the MOCVD method.
2. Prior Art
In a laser device in which a multiple quantum well (MQW) structure is adopted in the active layer, a threshold current is at a low level, and high optical power operation is possible. And in general, the upper optical confinement layer and the lower optical confinement layer each having the SCH structure (separate-confinement-heterostructure) are provided on both (upper and lower) surfaces of the active layer by means of the heterojunction, thereby improving the carrier injection efficiency and the confinement effect of laser light oscillated in the active layer. Thus, the external differential quantum efficiency of the laser device is enhanced and the high optical power operation can be achieved.
As SCH structures for use in such a case, the following structures are designed. That is, the SCH structure obtained by stacking the layers made of the same kind of semiconductor materials having different composition ratios by means of the heterojunction while changing the composition ratios by stages, and the SCH structure obtained by stacking the same kind of semiconductor materials while continuously changing the composition ratios thereof have been designed.
Of these SCH structures, if the SCH structure in which the composition of the material thereof is continuously changed in the thickness direction is employed, the carrier injection efficiency to the active layer is enhanced, and larger optical confinement effect can be obtained. In addition, it is known that since the optical confinement layer does not contain the heterojunction interface causing the crystal degradation, the advantages that the reliability at the time of the high optical power operation can be improved can be obtained.
A laser device A serving as an example of the laser device in which the lower optical confinement layer and the upper optical confinement layer with the latter SCH structure are formed is shown in FIG.
1
. Also, a diagram representing a conventional energy band in a layered structure C in the laser device A is shown in FIG.
2
.
In this laser device A, a lower cladding layer
2
A with a thickness of 500 nm and made of n-InP is stacked on a substrate
1
made of, for example, n-InP. On the lower cladding layer
2
A, a lower optical confinement layer
3
A made of InGaAsP, an active layer
4
with the MQW structure made of InGaAsP/InGaAs, an upper optical confinement layer
3
B made of InGaAsP, and an upper cladding layer
2
B with a thickness of 500 nm and made of p-InP (all of them will be described later) are sequentially stacked to form the layered structure C.
Note that a current blocking layer
6
consisting of a p type layer
6
B and an n type layer
6
A sequentially stacked is formed on both sides of the layered structure C.
Then, an upper cladding layer
2
C is formed so as to bury the layered structure C and the current blocking layer
6
, and a cap layer
5
made of p-InGaAsP with a thickness of 50 nm is further stacked thereon. An upper electrode
7
B is formed on the cap layer
5
and a lower electrode
7
A is formed on the rear surface of the substrate
1
.
In the layered structure C described above, the active layer
4
is designed in the following manner.
That is, in the active layer
4
, a well layer
4
A is constituted of an InGaAsP layer with a thickness of 4 nm, a barrier layer
4
B with a thickness of 10 nm is formed of InGaAsP with a composition having a bandgap wavelength of 1.2 &mgr;m, and a total of five quantum wells are provided (refer to FIG.
2
).
On the other hand, the lower optical confinement layer
3
A and the upper optical confinement layer
3
B are designed in such a manner as follows.
That is, the thickness of the lower optical confinement layer
3
A and the upper optical confinement layer
3
B is set at 40 nm. With respect to the lower optical confinement layer
3
A, a heterojunction part (1) with the lower cladding layer
2
A is made of InGaAsP with a composition having a bandgap wavelength of 0.92 &mgr;m, and a heterojunction part (2) with the first well layer
4
A of the active layer
4
is made of InGaAsP with a composition having a bandgap wavelength of 1.2 &mgr;m.
Furthermore, in the region between the part (1) and the part (2), the bandgap wavelength is sequentially increased from 0.92 &mgr;m to 1.2 &mgr;m. More specifically, this part of the layer is formed by the sequential stack of the InGaAsP with such a composition that the bandgap energy is sequentially decreased and the refraction index is sequentially increased.
Thus, as shown in
FIG. 2
, the lower optical confinement layer
3
A is formed of InGaAsP with such a graded composition that the bandgap wavelength thereof is linearly increased from the lower cladding layer
2
A to the first well layer
4
A of the active layer
4
.
Also, the configuration of the upper optical confinement layer
3
B is designed to be reversal to that of the lower optical confinement layer
3
A with respect to the active layer
4
serving as the center thereof.
That is, the heterojunction part with the last well layer
4
A of the active layer
4
is formed of InGaAsP with a composition having a bandgap wavelength of 1.2 &mgr;m, and the heterojunction part with the upper cladding layer
2
B is formed of InGaAsP with a composition having a bandgap wavelength of 0.92 &mgr;m. Thus, the layer between the parts is formed of InGaAsP with such a graded composition that the bandgap wavelength is sequentially and linearly decreased.
In the manufacture of the above-mentioned laser device, the MOCVD method is usually employed. For example, TMIn (trimethylindium) is used as In source, TMGa (trimethylgallium) is used as Ga source, AsH
3
(arsine) is used as As source, and PH
3
(phosphine) is used as P source. Then, these gas sources are diluted with H
2
to a predetermined concentration, and these gas sources are subjected to accurate flow rate control and time control by means of the mass flow controller in accordance with the kind of the semiconductor layers to be formed, then they are supplied to a reactor, and thus, sequentially forming predetermined semiconductor layers.
For example, the above-described lower optical confinement layer
3
A in the layered structure C can be formed in such a manner as follows.
After the process of forming the lower cladding layer made of n-InP by the use of TMIn (In source), PH
3
(P source), and n type impurity gas source, the supply of the n impurity gas source is stopped. Next, while maintaining the supply of the In source and the P source, the mass flow controller of TMGa (Ga source) and that of AsH
3
(As source) are opened to start the supply of the Ga source and the As source to the reactor.
Then, the openings of the valves of the mass flow controllers of the In source and the P source are controlled to gradually reduce the supply flow rate thereof, and the supply flow rates of the Ga source and the As source are gradually increased from 0 by controlling the valves of the mass flow controllers thereof. Note that the supply flow rates of these gas sources are controlled to a certain value so that the composition of the InGaAsP layer formed at each time can be equal to the composition having a designed bandgap wavelength shown in FIG.
2
.
Through the operations as described above, the lower optical confinement layer
3
A made of InGaAsP, in which the comp
Minato Ryuichiro
Ono Takahiro
Dang Phuc T
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
The Furukawa Electric Co. Ltd.
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