Long Wavelength Laser Diode Reliability and Lattice Imperfections
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gh operating temperature and stresses, are much less sensitive to the variations in the subsequent processing and packaging processes than the lower crystalline quality (weak) laser chips, and therefore a higher production yield can be expected. For the normal (mediocre) laser chips, their sensitivity to the subsequent processes usually results in a large fluctuation in the production yield. So, to achieve a reproducible high yield of reliable laser devices, the requirement for a high crystalline quality laser chip is absolutely necessary. Figure la shows schematically the structure of a single-mode distributedfeedback buried heterostructure (DFB) laser commonly used in high bit rate optical fiber communication systems operating in the 1.30-1.55 /j-m wavelength regime.56 The actual epitaxial layer structure of the grating, waveguide, and the active regions of an as-grown base wafer is shown in a transmission electron micrograph in Figure lb. The active region of the device is a low-energy bandgap (1.30 fim emitting wavelengths) InGaAsP stripe buried in a high-energy bandgap lattice grown on a (100) InP substrate for both optical and carrier confinements. A grown-in grating right underneath the active stripe, separated by a waveguide layer, provides a distributed feedback of the laser light for single-mode operation. A more advanced structure uses thin quantum wells for both the active medium and grating.7 Typically, three epitaxial growth steps5 are required for the structure shown in Figure 1, and two of the growth steps involve growth over non-
planar surfaces, i.e., the growth of waveguide layer over the etched grating and the subsequent growth of a current blocking structure around the etched mesa to form a buried stripe geometry of the active region. Although today's state-of-the-art epitaxial growth techniques can produce perfect atomic layers on planar substrates, the growth of high-quality epitaxial layers over nonplanar surfaces, such as gratings and stripe mesas, is difficult. The quality of the epitaxial layer depends not only on the control of growth over a multifaceted surface but also on the perfection and cleanliness of the etched surface. Since it is virtually impossible to precisely characterize beforehand the crystalline quality of all the as-grown layers and interfaces in such complex structures, an accelerated purge test is designed to purge out the weak devices from the major population by operating the devices at a high temperature and current for a fixed period of time. The locations of the lattice defects generated in the purge-failed devices, later identified by failure mode analysis, helps to pin down the weak links of the
Figure 1. (a) Schematic diagram of a 1.30-1.55 ]xm wavelength distributcdfeedback buried-heterostructure laser diode, and (b) a corresponding cross-sectional TEM micrograph of a high-quality as-grown base epitaxial structure showing a sinusoidal grating/waveguide interface and the active region of the laser.
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Long Wavelength Laser Diode Reliability and Lattice Imper
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