Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-12-28
2004-11-09
Philogene, Haissa (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S049010, C372S045013
Reexamination Certificate
active
06816526
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
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 current confinement structures.
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.
VCSELs include semiconductor active regions, which can be fabricated from a wide range of material systems, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1
illustrates a typical VCSEL
10
. As shown, an n-doped gallium arsenide (GaAS) substrate
12
has an n-type electrical contact
14
. An n-doped lower mirror stack
16
(a DBR) is on the GaAS substrate
12
, and an n-type lower spacer
18
is disposed over the lower mirror stack
16
. An active region
20
, usually having a number of quantum wells, is formed over the lower spacer
18
. A p-type top spacer
22
is disposed over the active region
20
, and a p-type top mirror stack
24
(another DBR) is disposed over the top spacer
22
. Over the top mirror stack
24
is a p-type conduction layer
9
, a p-type GaAs cap layer
8
, and a p-type 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 such that resonance occurs 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 for current confinement. The insulating region
40
is usually formed either by implanting protons into the top mirror stack
24
or by forming an oxide layer. In either case, the insulating region
40
defines a conductive annular central opening
42
. Thus, the central opening
42
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-type electrical contact
26
toward the n-type electrical contact
14
. The insulating region
40
and the conductive central opening
42
confine that current such that it flows through the conductive central opening
42
to the active region
20
. Some of the electrons that form 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-type conduction layer
9
, through the p-type GaAs cap layer
8
, through an aperture
30
in the p-type electrical contact
26
, and out of the surface of the vertical cavity surface emitting laser
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-type substrate
12
), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. For example, to some extent the insulating region
40
is less than optimal. As previously noted the insulating region
40
and the central opening
42
form a current confinement region that guides current into the active region. Also as noted, the insulating region is usually produced either by implanting protons or by forming an oxide layer. Proton implantation is described by Y. H. Lee et al., Electron Letters, Vol. 26, No. 11, pp. 710-711 (1990) and by T. E. Sale, “Vertical Cavity Surface Emitting Lasers,” Research Press Ltd., pp. 117-127 (1995), both of which are incorporated by reference. Oxide layers are taught by D. L. Huffaker et al., Applied Physics Letters, Vol. 65, No. 1, pp. 97-99 (1994) and by K. D. Choquette et al., Electron Letters, Vol. 30, No.24, pp. 2043-2044 (1994), both of which are incorporated by reference.
Ion-implanted VCSELs are typically formed by a single energy proton implant in the form of an annular ring. Proton implantation creates structural defects that produce a relatively high resistance structure having an annular conductive region. This implanted region, or gain guide, is disposed such that the peak concentration of defects lies above the active region
20
, but below the conduction region
9
. The defect density does not drop to zero immediately above and below the implantation peak, rather it gradually drops from the peak value, so that a very small, but finite number of these defects occur even in the active region
20
. While the relatively high resistance structure effectively steers current through the annular conductive region and into the active region, ion implantation does not produce significant optical guiding. Thus, ion implantation does not strongly impact the optical modes of the VCSEL. However, ion implantation defects can form non-radiative recombination centers in an active layer's P-N junction quantum wells. Such non-radiative recombination centers can cause the junction area under the implant to not emit light. Furthermore, the P-N junction under the implant has a lower forward voltage at constant current density than the P-N junction at the center of the cavity. In conjunction with the distributed nature of the series resistance of the P-N junction, the ion implanted non-radiative recombination centers cause the P-N junction current density to be highest in the center of the cavity. Until and unless other effects become large enough to counter this behavior, this makes the optical gain highest in the center of the cavity, which discourages the formation of higher order optical modes.
In contrast, VCSELs that use oxide current confinement regions can be made relatively small, which decreases threshold and operating currents. Additionally, since an oxide current confinement region has an optical index of refraction that is about half that of the region before oxidation, an oxide current confinement region forms a refractive optical index guide, which leads to transverse mode confinement, and which can further reduce operating current. The electrical properties of oxide current confinement VCSELs are very desirable. They can have higher bandwidths and lower lasing current thresholds (when compared to ion-implanted VCSELs). However, oxidation does not introduce non-radiative centers in the PN junction. Because of the distributed nature of the series resistance, oxide VCSELs have the highest P-N junction current density and the highest optical gain at the edge of the cavity. This current distribution tends to encourage the formation of higher order optical modes, particularly at large bias currents.
Oxide VCSELs (those that use oxide current confinement) typically include an AlGaAs layer having a high aluminum content (over 95%, and typically in the range of 97-98%) that is used to form the current confinement region. Such a high aluminum content structure tends to oxidize much more rapidly than the material layers used to form a P-type DBR mirror (which in this case might be 90% Al and 10% Ga). To fabricate the oxide current confinement, reactive ion etching is used to form trenches to the edge of the high Al content layer. Oxidation then typically proceeds to form a 10-micron deep oxide layer
Biard James R.
Guenter James K.
Finisar Corporation
Nguyen Dung T
Philogene Haissa
Workman Nydegger
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