Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-03-15
2003-04-01
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06542531
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to vertical cavity surface emitting lasers (VCSELs), and particularly to electrically pumped, long wavelength VCSELs and multiple wavelength VCSEL arrays, and a method of fabrication thereof.
BACKGROUND OF THE INVENTION
A VCSEL is a semiconductor laser including an active region sandwiched between mirror stacks that can be semiconductor distributed Bragg reflectors (DBRs) [N. M. Margalit et al., “Laterally Oxidized Long Wavelength CW Vertical cavity Lasers”, Appl. Phys. Lett., 69 (4), Jul. 22 1996, pp. 471-472], or a combination of semiconductor and dielectric DBRs [Y. Oshio et al., “1.55 &mgr;m Vertical-Cavity Surface-Emitting Lasers with Wafer-Fused InGaAsP/InP-GaAs/AlAs DBRs”, Electronics Letters, Vol. 32, No. 16, Aug. 1, 1996]. One of the mirror stacks is typically partially reflective so as to pass a portion of the coherent light that builds up in a resonating cavity formed by the mirror stacks sandwiching the active region. The VCSEL is driven by a current forced through the active region. Mirror stacks are typically formed of multiple pairs of layers formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers.
VCSELs are well adapted as preferred light sources for communication applications, due to the following advantageous features: a single mode signal from a VCSEL is easily coupled into an optical fiber, has low divergence, and is inherently single frequency in operation.
One of important requirements for the operation of a VCSEL is to compensate for the small amount of gain media which is typical for VCSELs due to the compact nature thereof. This is associated with the fact that, in order to reach the threshold for lasing, the total gain of a VCSEL must be equal to the total optical loss of the VCSEL. To compensate for the small amount of gain media, and to enable reaching and maintaining the lasing threshold, it is known to use wafer fusion of one or both of the mirror stacks, with reflectivity values exceeding 99.5%, to the active region. Wafer fusion is a process by which materials of different lattice constant are atomically joined by applying pressure and heat to create a real physical bond.
VCSELs emitting light having a long wavelength are of great interest in the optical telecommunication industry. A long wavelength VCSEL can be obtained by using a VCSEL having an InGaAs/InGaAsP active cavity material, in which case an InP/InGaAsP material system must be used for the mirror stacks in order to achieve a lattice match to the InP. In this system, however, it is practically impossible to achieve DBR based mirrors with high enough reflectivity because of the small difference in the refractive indices in this material system. Many attempts have been made to address this problem including a wafer fusion technique in which a DBR mirror is grown on a separate substrate and fused to the active region.
Another important requirement for fundamental mode operation of a VCSEL and light coupling into a single mode fiber, is current and optical confinement. In order to reduce the light emitting area of the VCSEL (practically to about 5-10 &mgr;m), the opening of current flow (current aperture) is restricted through lateral oxidation of Al-containing layers which also creates a lateral refractive index variation for fundamental optical mode operation of these devices. In such a lateral oxidation technique, a mesa is etched into the top surface of the VCSEL wafer, and the exposed sidewalls of an Al-containing layer (typically AlGaAs layer) are exposed to water vapor at elevated temperature. Water vapor exposure causes conversion of the AlGaAs to AlGaO
x
, some distance in from the sidewall toward the central vertical axis depending on the duration of oxidation. Formation of the current aperture defines the active region of the device which includes the active cavity material where there is a current flow and the light is generated, while lateral refractive index variation allows to control the mode structure of the emitted light. This approach has been used for practically all short-wavelength AlGaAs/Ga(In)As(P) VCSELs (i.e., emitting at 0.65-1.1 &mgr;m) and is also applied to long wavelength VCSELs (i.e., emitting at 1.25-1.65 &mgr;m) that may comprise DBR mirrors grown in the same material system as the active region [S. Rapp et al., “Near-Room-Temperature Continuous-Wave Operation of Electrically Pumped 1.55 &mgr;m Vertical cavity Lasers with InGaAsP/InP Bottom Mirror”, Electronic Letters, Vol. 35, No. 1, Jan. 7, 1999], and AlGaAs based DBRs that are as-grown [W. Yuen et al., “High Performance 1.6 &mgr;m Single-Epitaxy Top-Emitting VCSEL, Electronic Letters, Vol. 36, No. 13, Jun. 22, 2000] or wafer-fused on the active cavity material grown on InP (as in the above-indicated article of N. M. Margalit et al.). However, this approach leads to a non-planar structure, since mesa etches are required, resulting in a complicated processing scheme and low yield. The lateral oxidation is very sensitive to various factors like temperature, surface quality and defects, and does not allow obtaining current apertures with a precise size, and, above all, uniform enough to be used in the fabrication of multiple wavelength arrays by cavity length engineering. In case of lateral oxidized devices, it is quite difficult to use high performance AlAs/GaAs DBRs with highest refractive index contrast and best thermal characteristics, as compared to other AlGaAs/GaAs DBRs.
The use of the wafer fusion technique allows for obtaining both current and optical confinement during the fusion of the p-type GaAs-based DBRs to the p-side of the active cavity material grown on InP wafers. To this end, a special structuring of one of two contacting wafers is performed. The structured surface consists of a central mesa surrounded by shallow etched regions and a large area of unetched semiconductor. The fusion front in the central mesa and the large area of the unetched semiconductor are in the same plane. The current confinement is obtained by placing a native oxide layer at the fused interface outside the central mesa [A. V. Syrbu, V. P. Iakovlev, C. A. Berseth, O. Dahaese, A. Rudra, E. Kapon, J. Jacquet, J. Boucart, C. Stark, F. Gaborit, I. Sagnes, J. C. Harmand and R. Raj, “30° CW Operation of 1.52 &mgr;m InGaAsP/AlGaAs Vertical Cavity Lasers with in situ built-in lateral current confinement by localized fusion”, Electronic Letters, Vol. 34, No. 18, Sep. 3, 1998; or by placing a proton implanted region at the fused interface outside the central mesa (U.S. Pat. No. 5,985,686). This approach, however, suffers from the following drawbacks: the fused p-GaAs-based and p-InP-based interfaces are normally highly resistive resulting in a substantial heating of the device; and it is very difficult to optimize p-AlGaAs/GaAs DBRs for long wavelength VCSELs to have both high reflectivity (low absorption) and low resistivity.
According to a different approach of the long wavelength VCSEL fabrication technique, tunnel junctions can be used to inject holes into the active region, allowing using n-type DBRs on both sides of the active cavity material. In U.S. Pat. WO 98/07218, the p-side of a InP-based active cavity material is fused to the p-GaAs side of an AlGaAs/GaAs based structure including the n-type DBR stack and the n
++
/p
++
tunnel junction. Standard mesa etching and AlGaAs wet oxidation are performed for lateral optical and current confinement in these devices. Besides the above-mentioned drawbacks related to this particular lateral confinement technique and to highly resistive p-GaAs/p-InP fused junctions, this approach suffers from the known difficulty in obtaining low resistive
Iakovlev Vladimir
Kapon Elyahou
Rudra Alok
Sirbu Alexei
Ecole Polytechnique Federale de Lausanne
Leung Quyen
Moetteli John
Moetteli & Assoc.
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