Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-09-03
2004-09-28
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S044010, C372S045013, C372S046012, C372S096000
Reexamination Certificate
active
06798806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to VCSEL suitable for use at long wavelengths.
2. Discussion of the Related Art
VCSELs represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the material systems can be tailored to produce laser wavelengths such as 1550 nm, 1310 nm, 850 nm, 780 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and electrical contacts. Because of their complicated structure and material requirements, VCSELs are usually fabricated using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1
illustrates a typical VCSEL
10
. As shown, a substrate
12
has an n-type electrical contact
14
. An n-doped lower mirror stack
16
(a DBR) is on the substrate
12
, and an n-doped graded-index lower spacer
18
(a confinement layer) is disposed over the lower mirror stack
16
. An active region
20
, beneficially having a number of quantum wells, is formed over the lower spacer
18
. A p-doped graded-index top spacer
22
(another confinement layer) is disposed over the active region
20
, and a p-doped top mirror stack
24
(another DBR) is disposed over the top spacer
22
. Over the top mirror stack
24
is a p-doped conduction layer
9
, a p-doped cap layer
8
, and a p-doped electrical contact
26
.
Still referring to
FIG. 1
, the lower spacer
18
and the top spacer
22
separate the lower mirror stack
16
from the top mirror stack
24
such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack
24
includes an insulating region
40
that provides current confinement. The insulating region
40
is usually formed either by implanting protons into the top mirror stack
24
or by an oxide layer. The insulating region
40
defines a conductive annular central aperture
42
that forms an electrically conductive path though the insulating region
40
.
In operation, an external bias causes an electrical current
21
to flow from the p-doped electrical contact
26
toward the n-doped electrical contact
14
. The insulating region
40
and the conductive central aperture
42
confine the current
21
such that the current flows through the conductive central aperture
42
to the active region
20
. Some of the electrons in the current
21
are converted into photons in the active region
20
. Those photons bounce back and forth (resonate) between the lower mirror stack
16
and the top mirror stack
24
. While the lower mirror stack
16
and the top mirror stack
24
are very good reflectors, some of the photons leak out as light
23
that travels along an optical path. Still referring to
FIG. 1
, the light
23
passes through the p-doped conduction layer
9
, through the p-doped cap layer
8
, through an aperture
30
in the p-doped electrical contact
26
, and out of the surface of the VCSEL
10
.
It should be understood that
FIG. 1
illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-doped substrate
12
), a wide range of material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added. However, the VCSEL
10
beneficially illustrates a useful, common, and exemplary VCSEL configuration. Therefore, the fabrication of the VCSEL
10
will be described in more detail.
The substrate
12
effectively controls the bottom DBR and the active region. This is because overall lattice matching is extremely important since the active region
20
is intolerant of lattice defects. In practice, a semiconductor epitaxy is formed on the substrate
12
to improve lattice matching. The particular semiconductor epitaxy used is wavelength and device dependent. For 1310 nm emissions from GaAs substrates the semiconductor epitaxy is usually AlGaAs and/or InGaAsN and/or InGaAsNSb of varying compositions. For 1550 nm emissions from InP substrates the semiconductor epitaxy is usually AlGaAsSb and/or AlGaInAs and/or InGaAsP and/or InP. For 1550 nm emissions from GaAs, the semiconductor epitaxy is usually AlGaAs and/or InGaAsNSb.
With the substrate
12
and the semiconductor epitaxy in place, the lower mirror stack
16
is fabricated. For 1310 nm or 1550 nm emissions from GaAs substrates
12
the lower mirror stack
16
is beneficially comprised of alternating compositions of Al(x)Ga(1−x)As that produce high and low index layers. For emissions at 1550 nm from InP substrates
12
the lower mirror stack
16
is beneficially comprised of alternating compositions of InGaAsP/InP, of AlGaInAs/InP, of AlGaAsSb/InP, or of AlGaPSb/InP. The compositional and doping grades of the layers should be selected to improve electrical properties (such as series resistance).
After the lower mirror stack
16
is grown, the lower spacer
18
, the active region
20
, and the top spacer
22
are fabricated. The lower spacer
18
and the top spacer
22
are beneficially comprised of low-doped layers having compositional grades. The active region
20
beneficially includes a plurality of quantum wells (say 3 to 9) that are sandwiched between higher bandgap energy semiconductor barrier layers.
The top mirror stack
24
having the insulating region
40
having the conductive central aperture
42
is then fabricated over the top spacer
22
. The top mirror
24
is beneficially formed (described in more detail subsequently) from similar layers as the lower mirror stack
16
. Then, the p-doped conduction layer
9
, the p-doped cap layer
8
having the aperture
30
, and the p-doped electrical contact
26
are fabricated.
Still referring to the fabrication of the top mirror stack
24
, if an oxide layer is used to form the insulating region
40
the top mirror stack
24
includes a high aluminum content layer that is bounded by lower Al content layers. A trench is then formed to the high aluminum content layer. The high aluminum content layer is then oxidized via the trench to produce the insulating region
40
. Alternatively, if ion implantation is used to form the insulating region
40
, then such ions are implanted into the top mirror stack
24
. The incoming ions damage the top mirror structure so as to form the insulating region
40
. In either event the top mirror stack
24
must be thick enough for adequate current spreading. Ion-implanted VCSELs have demonstrated greater reliability than those that use oxidized layers, but oxide VCSEls have advantages of higher speed and higher efficiency.
While generally successful, VCSELs are not without problems. In particular, VCSELs used at long wavelengths, such as 1550 nm or 1310 nm, are currently significantly less than optimal. However, long-wavelength VCSELs (1.3 &mgr;m-1.6 &mgr;m) are well suited for next generation data communication and telecommunication applications.
A major problem with long wavelength VCSELs is fabricating their top mirror stacks
24
. This is partially because the refractive index contrasts of the materials used in the top mirror stacks
24
are relatively small, which means that the top mirror stacks
24
must have many contrasting layers. This makes the top mirror stacks
24
relatively thick. Compounding the thickness problem is that long wavelength operation makes otherwise comparable structures thicker simply because of the longer wavelengths. Thus, long-wavelength VCSELs tend to have very thick top mirror stacks
24
. But, thick mirror stacks are difficult to ion implant pro
Johnson Ralph H.
Wang Tzu-Yu
Al-Nazer Leith A
Finisar Corporation
Nydegger Workman
Wong Don
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