Notes: We describe the first results of hybrid mode locking combining both active and passive mode locking of a semiconductor laser. These functions are integrated into a monolithic device with a 1.3 μm GaInAsP gain region, an active waveguide, and a saturable absorber. The devices have low threshold currents, and exhibit hysteresis in their light/current characteristics. The long integrated waveguides allow mode locking at a repetition rate of 15 GHz without the need for an external cavity. Pulse widths as short as 1.4 ps have been demonstrated using the combined effects of active and passive mode locking.

Notes: We report the high-power operation of λ=1.3 μm InGaAsP double-channel planar buried-heterostructure lasers with asymmetric mirror coatings. A stack of four dielectric layers is used to raise the reflectivity of one facet to over 80%, and the thickness of a single layer coating on the output facet is chosen to reduce the reflectivity to about 4%. The resulting lasers are characterized by a low threshold current of 25 mA, slope efficiency as high as 50%, and a power output of as much as 150 mW (at 5 °C) at a current of less than 300 mA. The lasers operate in a single transverse mode over the entire current range and as much as 45 mW of power could be coupled into a lensed single-mode fiber. Preliminary high-power aging data show excellent device reliability.

Notes: Very low resistance nonalloyed ohmic contacts of Pt/Ti to 1.5×1019 cm−3 Zn-doped In0.53Ga0.47As have been formed by rapid thermal processing. These contacts were ohmic as deposited with a specific contact resistance value of 3.0×10−4 Ω cm2. Cross-sectional transmission electron microscopy showed a very limited interfacial reacted layer (20 nm thick) between the Ti and the InGaAs as a result of heating at 450 °C for 30 s. The interfacial layer contained mostly InAs and a small portion of other five binary phases. Heating at 500 °C or higher temperatures resulted in an extensive interaction and degradation of the contact. The contact formed at 450 °C, 30 s exhibited tensile stress of 5.6×109 dyne cm−2 at the Ti/Pt bilayer, but the metal adhesion remained strong. Rapid thermal processing at 450 °C for 30 s decreased the specific contact resistance to a minimum with an extremely low value of 3.4×10−8 Ω cm2 (0.08 Ω mm), which is very close to the theoretical prediction.

Notes: An InP/InGaAsP laser-monitor hybrid structure which demonstrates the use of channeled-substrate buried-heterostructure lasers or broad area double heterostructure devices as "on-board'' edge-detecting back-face monitors is presented. Devices with and without antireflection facet coatings are used as monitors. A linear relationship between photocurrent and light output from the laser is observed for all monitor types. A photocurrent of 17.6 μA/mW is obtained with a facet-coated, broad area monitor (at a monitor-laser separation of ≈100 μm). The sensitivities obtained are in close agreement with those predicted assuming close to unity quantum efficiency in the junction region. Improved chip placement precision could permit a possible doubling of the monitor sensitivity.

Notes: Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak inI(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–type sidewall interfaces between the Cd-diffused p-InP and liquid-phase-epitaxial-grown n-InP buffer inside the groove. These dislocation loops subsequently grow out of the interfaces into the n-InP buffer region in the direction of minority-carrier injection, indicating a nonradiative recombination-assisted defect growth process. For those loops which enter the quaternary active region near the tip of the active crescent, the growth rate along the (001) and (010) planes is greatly enhanced and the loops eventually cut across the active stripe and become dark-line defects, as confirmed by electroluminescence. Nucleation of dislocation loops is not observed along the {111} p-p–type sidewall interfaces above the active stripe. The fact that the dislocation loops are all extrinsic in nature implies that the {111} sidewall interfaces as well as the quaternary active region contain a high density of interstitials. The possible causes for the generation and growth of the dislocation loops and the high density of point defects are discussed. The rapid degradation mode of the CSBH laser is characterized by a sudden drop in light intensity during the aging process. The associated defect mechanism starts with localized melting at the mirror facet or inside the lasing cavity. A metal-rich droplet subsequently forms which propagates along the center of the active stripe in the direction towards the cavity center via a meltback-regrowth process; i.e., material melts in front of the droplet and regrows after it propagates by. The nonideal condition of regrowth results in the formation of a wormlike defect composed of a cylinder of defective materials bounded by an off-stoichiometric interface. The wormlike defect is dark under electroluminescence. Complicated dislocation structures can also be grown from the wormlike defect under a nonradiative recombination-assisted defect growth process. These phenomena are presented and discussed.

Notes: Nonalloyed ohmic contacts of Pt/Ti to 5×1018 cm−3 doped p-InGaAsP (λg =1.3 μm) have been fabricated by rapid thermal processing of sputtered and e-gun-deposited metallizations. While the former as-deposited had a rectifying characteristic, the latter showed ohmic behavior prior to any heat treatment, with a specific contact resistance of 4×10−3 Ω cm2. Rapid thermal processing at temperatures higher than 400 °C caused the formation of ohmic contacts for the sputtered metals also, but with the evaporated metals producing slightly lower contact resistance. The lowest specific contact resistance values of 3.6–5.5×10−4 Ω cm2 for evaporated and sputtered metallizations, respectively, were achieved in both cases as a result of heating at 450 °C for 30 s. These heating conditions produced only a limited reaction at the Ti/InGaAsP interface, which was sharper for the e-gun-deposited contact, but had a significant effect on the stresses in the Ti/Pt bilayer. In both the sputtered and electron gun evaporated samples, the stresses were inverted from tensile as-deposited to compressive with values of 2.4×109 and 1.0×109 dyn cm−2, respectively, as a result of the heat treatment.

Notes: The microstructure of semiconductor laser diodes is studied using a combination of focused ion beam sputtering, electroluminescence imaging, and cross-sectional transmission electron microscopy. Careful control of focused ion beam sputtering allows fabrication of high quality thin membranes for transmission electron microscope imaging, which can be located to submicron accuracy at a given position on the laser active stripe. By correlation with electroluminescence imaging, the membrane may then be positioned at an optically degraded region of the active stripe. In addition, imaging of the complete cross-sectional laser structure, from substrate to surface contact layers is possible. The applications of these techniques to studies of laser degradation mechanisms are demonstrated and discussed.

Notes: The problem of hydrogenation of InP without surface degradation has been surmounted by exposure of the InP surface to a hydrogen plasma through a thin SiNx(H) cap layer. This layer is H permeable at the hydrogenation temperature of 250 °C, but P or PH3 impermeable thus minimizing PH3 loss and the attendant In droplet formation. In contrast to our results for this type of plasma exposure of GaAs, we find that shallow acceptors in InP are heavily passivated, whereas shallow donors are only very weakly affected. For example, p+-InP(Zn) of 3×1018 cm−3 has its residual hole concentration reduced to ≤3×1014 cm−3 over a depth of 1.3 μm by a 250 °C, 0.5 h deuteration. The presence of acceptors impedes H (or D) indiffusion, as indicated by D diffusion under the same conditions occurring to depths of 18 and 35 μm in p-InP (Zn, 2×1016 cm−3) and n-InP (S or Sn), respectively. Annealing for 1 min at 350 °C causes the acceptor passivation to be lost and the hole concentration to be returned to its prehydrogenation level, indicating that the passivation has similar thermal stability to that of acceptors in GaAs, but lower than that of donors.

Notes: We report the results of the measurement of radius of curvature of 1.3 and 1.5 μm wavelength GaInAsP-InP channeled substrate buried heterostructure lasers. The objective of this investigation is to quantify the macroscopic stress present in the device and correlate it with device reliability. The change in dc threshold current (ΔIth) after an accelerated aging test was used as a measure to access device reliability, with high ΔIth indicating decreased reliability. Changes were made in the p-side metallization to bring about a change in either ΔIth or radius of curvature and they included two different contact widths and different thicknesses of the Au bonding pad. It is observed that no correlation between device curvature and ΔIth exists even though the modifications in the p metallization caused significant changes in both quantities. It is suggested that it is not the macroscopic device stress that is measured by the radius of curvature but localized stresses that may exist in the vicinity of the lasing active layer which would affect device reliability. It is surmised that the most important role of stress is its effect on the direction of defect migration with the principal driving force coming from the nonradiative electron-hole recombination occurring in the vicinity of the active layer.