Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-10-06
2003-05-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010
Reexamination Certificate
active
06563850
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light-emitting device such as, for example, a semiconductor laser or a light emitting diode, and a method for fabricating the same. Alternatively, the present invention relates to a semiconductor laser usable as a light source in the fields of, for example, optical disks, laser beam printers and optical transmission, and especially relates to a semiconductor laser having an active layer of a multiple quantum well structure.
2. Description of the Related Art
As a conventional semiconductor laser, a quantum well type laser, including a quantum well layer as an active layer acting as a light emitting section, is known in the art. The quantum well type laser has various advantages including a lower operating current and improved noise characteristic. The quantum well type laser can have a separate confinement heterostructure (hereinafter, referred to as the “SCH structure”) for enhancing light confinement into the active layer.
In general, the forbidden band width a compound semiconductor layer and the refractive index thereof are in reverse proportion to each other. On the other hand, the Al mole fraction of a compound semiconductor layer containing Al and the forbidden band width thereof are typically in proportion to each other. Accordingly, the band diagram of the active layer and the vicinity thereof of a quantum well type laser having the SCH structure is, for example, as shown in FIG.
23
.
A semiconductor laser having the band diagram shown in
FIG. 23
includes a multiple quantum well (hereinafter, referred to as “MQW”) active layer
1501
which includes a plurality of quantum wells
1510
and a plurality of barrier layers
1511
, and also includes a first optical guide layer
1502
and a second optical guide layer
1503
which interpose the active layer
1501
therebetween. Each of the first optical guide layer
1502
and the second optical guide layer
1503
has a larger forbidden band width than that of the quantum well layers
1510
. The semiconductor laser further includes an n-type first cladding layer
1504
and a p-type second cladding layer
1505
which interpose the first and second optical guide layer
1502
and
1503
therebetween. Each of the n-type first cladding layer
1504
and the p-type second cladding layer
1505
have a larger forbidden band width than that of the first and second optical guide layers
1502
and
1503
. In the semiconductor laser having such a structure, carrier confinement is established by the quantum well layers
1510
, while light confinement is established by the first optical guide layer
1502
and the second optical guide layer
1503
.
Such a semiconductor laser having the SCH structure is disclosed in, for example, Japanese Patent Publication for Opposition No. 4-67354 and Japanese Laid-Open Patent Publication No. 6-252508. The semiconductor laser disclosed in Japanese Patent Publication for Opposition No. 4-67354 contains impurities in the entire optical guide layers, while the semiconductor laser disclosed in Japanese Laid-Open Patent Publication No. 6-252508 contains no impurities in the optical guide layers.
The semiconductor laser disclosed in Japanese Patent Publication for Opposition No. 4-67354 includes an MQW active layer including a plurality of quantum well layers each having a thickness of no greater than the de Brogli wavelength of electrons, i.e., a thickness of about 20 nm or less. Referring to
FIG. 24
, such a semiconductor laser
1700
includes an n-type GaAs buffer layer
1702
, an n-type AlGaAs cladding layer
1703
, an n-type AlGaAs guide layer
1704
, an MQW active layer
1705
, a p-type AlGaAs guide layer
1706
, a p-type AlGaAs cladding layer
1707
, and a p-type GaAs cap layer
1708
, which are formed on an n-type GaAs substrate
1701
in the above order.
FIG. 25
is an energy band diagram of the MQW active layer
1705
and the vicinity thereof. As shown in
FIG. 25
, the MQW active layer
1705
includes a plurality of GaAs quantum well layers
1710
and a plurality of AlGaAs quantum barrier layers
1711
each interposed between two adjacent quantum well layers
1710
. In the example shown in
FIG. 25
, the MQW active layer
1705
includes three GaAs quantum well layers
1710
and two AlGaAs quantum barrier layers
1711
provided alternately.
The forbidden band width of each of the n-type AlGaAs guide layer
1704
and the p-type AlGaAs guide layer
1706
is set to be equal to the forbidden band width of the quantum barrier layers
1711
. By setting the forbidden band widths of the AlGaAs guide layers
1704
and
1706
and the quantum barrier layers
1711
at the same value, the quantum well layers
1710
are all interposed between two semiconductor layers having the same width forbidden band width. Thus, the dispersion of the quantization level is reduced among the quantum well layers
1710
, which leads to a narrower light emitting spectrum. Accordingly, the threshold current is lowered.
Recently, a further reduction in the threshold current of semiconductor lasers has been demanded. In order to further reduce the threshold current in the semiconductor laser
1700
, the light confinement ratio into the quantum well layers
1710
of the MQW active layer
1705
is required to be raised. In the SCH structure, the light confinement ratio can be raised to reduce the threshold current by increasing the thickness of the optical guide layers
1704
and
1706
. Accordingly, it is desirable to increase the thickness of the optical guide layers in order to reduce the threshold current.
However, thicker optical guide layers cause the following problems.
In general, a dopant concentration of the optical guide layer is set to be significantly lower than that of the cladding layer (alternatively, no doping is performed into the optical guide layer) in order to suppress the dopant diffusion from the optical guide layer to the MQW active layer. Accordingly, thicker optical guide layers increase the resistance of the semiconductor laser in the optical guide layer, resulting in an increased operating voltage. As can be appreciated, the thicker optical guide layer according to the conventional technology causes the device characteristics to be deteriorated due to an increased operating voltage, while a reduced threshold current can be realized thereby.
In the semiconductor laser
1700
, the forbidden band width of the optical guide layers
1704
and
1706
is set to be equal to the forbidden band width of the quantum barrier layers
1711
. Such setting corresponds to setting the Al mole fraction of the optical guide layers
1704
and
1706
to substantially as high as the Al mole fraction of the quantum barrier layer
1711
. Accordingly, the dopant in the cladding layers
1703
and
1707
may be diffused to the MQW active layer
1705
, whereby the Al mole fraction of the quantum well layers
1710
is likely to change. As a result, the oscillating wavelength is shifted from the designed value, resulting in difficulty in controlling the oscillating wavelength.
In the case of the semiconductor laser disclosed in Japanese Laid-Open Patent Publication No. 6-252508 which does not contain impurities in the optical guide layer, the thicker optical guide layer causes an increased resistance thereof. In addition, there occurs a potential barrier between the cladding layer and the optical guide layer, thereby raising the operating voltage.
In the case of the semiconductor laser
1700
disclosed in Japanese Patent Publication for Opposition No. 4-67354 containing impurities in the optical guide layers
1704
and
1706
, the impurities are diffused from the optical guide layers
1704
and
1706
to the MQW active layer
1705
during activation of the semiconductor laser
1700
. Accordingly, a non-emission recombination center is formed in the MQW active layer
1705
, resulting in an inner absorption loss. Thus, characteristics of the semiconductor laser
1700
are deteriorated.
Moreover, the details of influences on the laser charac
Matsumoto Mitsuhiro
Tatsumi Masaki
Ip Paul
Jackson Cornelius H
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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