Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Having graded composition
Reexamination Certificate
2000-06-14
2002-07-30
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Having graded composition
C257S101000
Reexamination Certificate
active
06426522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a doped semiconductor material and, in particular, to a doped semiconductor material having a good electrical conductivity and a large forbidden bandgap. The present invention also relates to a method of manufacturing such a doped semiconductor material, and to a semiconductor device incorporating a layer of this material.
2. Description of the Related Art
One well-known compound semiconductor system is the (Al,Ga,In)P system. A compound belonging to the (Al,Ga,In)P system has the general formula (Al
x
Ga
1−x
)
1−y
In
y
P where both x and y are between 0 and 1.
The (Al,Ga,In)P system is widely used in the fabrication of semiconductor layer structures including, for example, optoelectronic devices such as semiconductor laser devices. One advantage of this semiconductor system is that it is lattice-matched to a gallium arsenide substrate when the indium mole fraction, y, is equal to 0.48.
As is well-known, compound semiconductors such as (Al,Ga,In)P can be “doped” by intentionally introducing impurities into the semiconductor. These intentional impurities, known as “dopants”, generate free charge carriers in the semiconductor material, and thus change its electrical properties. In a “n-doped” semiconductor material the majority of the free charge carriers are electrons, whereas in a “p-doped” material the majority of the free charge carriers are holes.
Laser devices or laser diodes (LDs) fabricated in the (Ga,Al,In)P system which emit light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer systems. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering 5 mW power output up to a temperature of 60° C. The next generation of semiconductor lasers will need an even greater maximum power output up to the same or higher (e.g. 70° C.) operating temperatures.
FIG. 1
is a schematic view of a semiconductor laser device. The laser device
1
consists of a substrate
2
, on which is disposed, in sequence, an n-doped cladding layer
3
, a waveguide
4
, an active region
5
, another waveguide
6
, and a p-doped cladding layer
7
. In this laser device, light is generated in the active region
5
. Light generated in the active region
5
is confined in the vertical direction in
FIG. 1
by the waveguides
4
,
6
. This is done, for example, by ensuring that the refractive index of the waveguides
4
,
6
is greater than the refractive index of the active region
5
.
The substrate
2
primarily serves to provide mechanical strength for the laser diode. The laser diode is produced by depositing the layer
3
-
7
sequentially on the substrate
2
. This can be done in principle by any conventional semiconductor growth technique, although it is preferred to use MBE (Molecular Beam Epitaxy) or CVD (Chemical Vapour Deposition) since these methods produce materials having high purity and well-defined geometry.
The upper and lower cladding layers
3
,
7
serve to confine carriers within the active region. It is therefore necessary for the cladding layers
3
,
7
to have a greater forbidden bandgap than the active region
5
. The efficiency of the confinement of carriers in the active region improves as the difference in bandgap between the cladding layers and the active region increases, so that it is desirable to maximise the difference between the bandgap of the cladding layers and the bandgap of the active layer.
A principal limitation of current (Al,Ga,In)P LDs is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperatures. It is generally believed that this is caused by electron leakage from the active layer of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
When a laser diode having the structure shown in
FIG. 1
is fabricated in the (Al,Ga,In)P system, in one example, the active region
5
is a (Ga,In)P active region, the optical guiding layers
4
,
6
are (Al
0.5
Ga
0.5
)
0.52
In
0.48
P, and the cladding layers
3
,
7
are (Al
0.7
Ga
0.3
)
0.52
In
0.48
P. Typical ranges for the thicknesses of the layers are 0.5-4 &mgr;m for the cladding layers
3
,
7
, 0.005-1 &mgr;m for the waveguides
4
,
6
, and 0.001-1 &mgr;m for the active region
5
. The cladding layers
3
,
7
are doped, with the upper cladding layer being p-type and the lower cladding layer being n-type. Thus, in order to fabricate a laser diode having the structure shown in
FIG. 1
in the (Al,Ga,In)P system, it is necessary to produce doped (Al,Ga,In)P layers to serve as the upper and lower cladding layers
3
,
7
. As indicated above, it is essential that these cladding layers have a greater forbidden bandgap than the active region
5
, so as to confine carriers in the active region thereby enabling generation of light to occur, and it is desirable to maximise the difference between the bandgap of the cladding layer and the bandgap of the active layer.
One problem encountered in fabricating a laser diode in the (Al,Ga,In)P system is that it is energetically favourable for an (Al,Ga,In)P material to exhibit atomic ordering. This is disclosed in “Nature and Origin of Atomic Ordering in III-V Semiconductor Alloys” by A. G. Norman et al., Inst. Phys. Conf. Ser. No. 134, Section 6, pp. 279-290, 1993. It is known that this ordering causes a reduction in the forbidden bandgap of (Al,Ga,In) P materials. For example, a reduction in the forbidden bandgap of Ga
0.5
In
0.5
P is disclosed in Applied Physics Letters Vol. 50 (11), 1987, pp. 673-675. Atomic ordering in the (Al,Ga,In)P system may also cause structural degradation of the material, and thus lead to device failure. It is therefore desirable to reduce the amount of atomic ordering that occurs in the material by as much as possible.
Atomic ordering may occur in any semiconductor system where more than two elements are competing for the same atomic site.
Atomic ordering occurs in the (Al,Ga,In)P system because the size of an In atom differs from the size of either a Ga atom or an Al atom. The presence of two differently sized sites for group III atoms leads to a segregation of In atoms from Al and Ga atoms. This leads to formation of the following:
Anti phase domains;
Anti phase domain boundaries; and
Platelets that are In-rich.
The In-rich platelets locally reduce the forbidden bandgap of the semiconductor layer. The platelets have dimensions typically in the range 10-1,000 Å(1-100 nm).
One approach to reducing atomic ordering is growing an (Al,Ga,In)P layer on an “off-axis” substrate. An “off-axis” substrate is a substrate in which the surface of the substrate on which the semiconductor layer is grown is slightly misoriented from the crystal plane. For example, rather than the growth surface being the (100) crystal plane, it could be misoriented by a few degrees towards the (111) crystal plane.
Another prior art method of reducing atomic ordering in a semiconductor material is to anneal the material. Annealing a semiconductor material, at a temperature typically in the range of 500° C. to 1,000° C., reduces atomic ordering by causing redistribution of atoms within the lattice. This redistribution of atoms is promoted if the material contains impurities which become mobile during the annealing, since such impurities will aid the redistribution of atoms.
Although it is relatively straightforward to reduce atomic ordering in an undoped semiconductor material by annealing it, difficulties occur in annealing a doped semiconductor layer such as a doped (Al,Ga,In)P semiconductor layer. One common p-type dopant for (Al,Ga,In)P is beryllium, but beryllium is known to diffuse through a host lattice at temperatures of greater than around 500° C. Thus, annealing a beryllium-doped (Al,Ga,In)P layer will cause the beryllium to diffuse through the semiconductor layer. If the doping concentration is not const
Kean Alistair Henderson
Takiguchi Haruhisa
Le Bau
Nelms David
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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