C. Lei, H. Deng, J.J. Dudley, S-F. Lim,
B. Liang, M. Tashima, and R. W. Herrick
Fiber Optic Communications Division,
370 W. Trimble Road, MS 90UB,
San Jose, CA 95131
After the successful market introduction of proton implant confined vertical cavity surface emitting laser (VCSEL) based 5 V gigabit transceiver modules a few years ago,1 oxide confined VCSELs arc being developed for 3.3 V gigabit transceiver modules. An oxide confined VCSEL is desirable for 3.3 V transceiver applications due to its higher slope efficiency and lower operating voltage compared to proton implant confined VCSEL2:. Here we report the commercialization of oxide confined VCSELs at Hewlett Packard.
The VCSELs are grown using a multi-wafer organometallic vapor- phase epitaxy (OMVPE) reactor. The 850 nm oxide VCSEL structure consists of a lower 40 pair AlGaAs DBR, a full wave cavity with three GaAs quantum wells in the center, followed by a thin A10.97Ga0.03As layer for oxidation, and an upper 25 pair AlGaAs DBR and capped by a heavily doped GaAs contact layer. The DBR interfaces are graded to reduce the series resistance of the devices.
After epitaxial growth, the VCSEL wafers are first characterized by reflectivity measurements to determine layer thickness and by Polaron capacitance-voltage measurements to determine doping level. The VCSEL devices are then formed by oxidation and front and back metallization. After the wafer fabrication, the VCSELs are first tested in wafer form for DC performance, and the wafers are then scribed and broken, and the lasers are mounted on TO-46 headers for burn-in and life test.
The oxide VCSEL performance yield per product specification is largely determined by the control of the uniformity of the epitaxial layer thickness, doping and the oxide VCSEL emission aperture size. Typically, the Fabry-Perot wavelength uniformity can be controlled to within ±0.6% over 80% area of a 2-inch diameter wafer, which translates into a lasing wavelength range of around 10nm. The oxide VCSEL emission window is defined by the lateral wet oxidation process of the buried Al0.97Ga0.03As layer and the aperture size is controlled by the combination of oxidation rate and oxidation time. The oxide aperture size map across a 2-inch wafer is shown in Fig. 1. The oxide aperture is measured at ¼ intervals across the wafer for a total of 37 evenly spaced points. The aperture size across the wafer is 15.8 ± 0.3µm where ± 0.3mm (± 0.2%) is the standard derivation of the aperture size variation.
Figure 1. Oxide VCSEL aperture size map across a 2-inch wafer.
Figure 2 shows oxide VCSEL performance at the 37 evenly spaced points across a 2-inch wafer. The performance variation across the wafer is contributed by the combined variation of the layer thickness, doping and the aperture size. Excellent device performance has been achieved with low operating voltage, high power, and fast modulation speed. The operating voltage across the wafer at2 mW power is below 2 V, which is suitable for 3.3 V transceiver module applications.
Figure 2. Oxide VCSEL performance at the 37 evenly spaced points across a 2-inch wafer.
One of the greatest concerns of developing oxide VCSELs is its reliability due to the strain and defects introduced in the VCSEL structure during oxidation. Preliminary reliability of the oxide VCSELs is investigated by studying both the random failure rate and the wearout lifetime. The random failure rate is established by stressing the devices at a constant output power of 2mW at 70ºC ambient. Four hundred sixty units were stressed up to 6000 hours to achieve over 2.2 million cumulative device hours. One device failed at 5000 hours and all the rest units remain stable. The long-term wearout tests are conducted at various currents and temperatures to determine the activation energy. The stress results show that the activation energy and the wearout lifetime of oxide VCSEL are similar to that of implant VCSEL emitting the same amount of output power.
In summary, we have developed a commercially manufacturable oxide VCSEL process. Good uniformity control in epitaxial thickness and oxide aperture is found to be important in achieving high yield. The manufactured VCSELs have superior performance with operating voltage less than 2V. Preliminary results show that oxide VCSEL reliability is similar to that of proton-implanted VCSELs.
1. C. Lei, L.A. Hodge, J.J. Dudley, M.R. Keever, B. Liang, J.R. Bhagat and A. Liao, “High performance vertical-cavity surface emitting lasers for product applications,” in Proc. SPIE Conf. Vertical Cavity Surface-Emitting Lasers, vol. 3003, pp. 28-33, San Jose, California, 1997.
2. K.L. Lear, K.D. Choquettee, R.P. Schneider Jr., S.P. Kolcoyne, and K.M. Geib, “Selectivity oxidised vertical cavity surface emitting lasers with 50% power conversion efficiency,” Electron. Lett., vol 31, pp. 208-290, 1995.
Tapered-apertures for high-efficiency miniature VCSELs
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