R. L. Hartman

Defect structure introduced during operation of heterojunction GaAs lasers

The nature and origin of the defects responsible for the rapid degradation of stripe geometry GaAs–GaAlAs double‐heterostructure lasers have been identified by transmission electron microscopy. These defects are formed by a three‐dimensional dislocation network which originates at a dislocation crossing the GaAlAs and GaAs epilayers. The propagation of the dislocation network takes place by a climb mechanism induced by the operation of the device.

Reliability of DH GaAs lasers at elevated temperatures

Data from accelerated aging tests on continuously operating stripe−geometry double−heterostructure GaAs lasers are presented. By extrapolating data obtained in dry−nitrogen ambients at temperatures of 90, 70, and 50 °C, it is concluded that continuous room−temperature operation of these devices as lasers with power outputs exceeding 1 mW per laser face for times in excess of 100000 h is possible.

Improved light‐output linearity in stripe‐geometry double‐heterostructure (Al,Ga)As lasers

Nonlinearities in the optical‐power‐output–versus–current characteristics of (Al,Ga)As stripe‐geometry double‐heterostructure lasers are found to be associated with excessive spectral broadening, asymmetries in the outputs from the two mirrors, and spatial movements of the lasing mode within the width of the stripe. Manifestations of these nonlinearities are severely deleterious in a variety of optical fiber communication applications. It is shown that the impact of these defects may be significantly reduced, with little lasing‐threshold‐current penalty, by fabricating devices with narrower stripe widths.

Strain‐induced degradation of GaAs injection lasers

This letter shows that strain is a controlling factor in the rapid degradation of GaAs double‐heterostructure junction lasers. A birefringence study of these lasers reveals extensive strain fields introduced during the bonding procedure resulting from the different thermal‐expansion coefficients of the materials involved. A new bonding procedure was developed which does not introduce significant strains. Lasers fabricated with the old and new bonding procedures were subjectively categorized as low, moderate, or high strained. A direct correlation was measured between low‐strain and room‐temperature cw laser lifetime. Lasers fabricated with the new bonding procedure have operated cw for as long as 880 h.

Continuously operated (Al,Ga)As double‐heterostructure lasers with 70 °C lifetimes as long as two years

Lifetimes longer than two years, and decreases in light output power less than 15% at constant current after one year, both at 70 °C, are reported for selected continuously operated double‐heterostructure (Al,Ga)As lasers. Also, a median 70 °C lasing lifetime of 4500 h is reported for 100 lasers chosen randomly from 10 slices. This median lifetime is thought to correspond to 3.0×105 h (34 yr) of continuous operation had the devices been operated at 22 °C. The corresponding mean time to failure is 1.3×106 h (≳100 yr).

Statistical characterization of the lifetimes of continuously operated (Al,Ga)As double‐heterostructure lasers

The statistical distribution of lifetimes of routinely grown and fabricated continuously operated (Al,Ga)As double‐heterostructure lasers is presented and discussed. The 90 typical devices studied were operated as lasers in a dry‐nitrogen elevated‐temperature ambient (70 °C) until failure. The resulting median life, τm=750 h, and mean life, 〈τ〉=1370 h, extrapolate to τm=5.7 years and 〈τ〉=10.5 years at room temperature (22 °C) using a 0.7‐eV activation energy. The observed lifetimes are consistent with a model in which 17% of the lasers die prematurely as infant mortalities while 83% die by a mechanism well characterized by a lognormal distribution. The value of the standard deviation (σ=1.1) in lnτ is typical of other semiconductor devices.

Continuous room‐temperature operation of GaAs‐AlxGa1−xAs double‐heterostructure lasers prepared by molecular‐beam epitaxy

The continuous (cw) operation at temperatures as high as 100 °C of stripe‐geometry GaAs‐AlxGa1−xAs double‐heterostructure lasers fabricated by molecular‐beam epitaxial (MBE) techniques has been achieved. Improved MBE laser performance was the result of the extensive efforts to eliminate hydrocarbon and water vapor from the growth apparatus. For 12‐μm‐wide stripe‐geometry lasers with 380‐μm‐long cavities, the cw threshold currents varied between 163 and 297 mA at room temperature.

Threshold reduction by the addition of phosphorus to the ternary layers of double‐heterostructure GaAs lasers

By the addition of phosphorus to the ternary layers of standard double‐heterostructure GaAs injection lasers, significant reductions in lasing current threshold and increases in differential quantum efficiency have been obtained. Additional measurements suggest that the improvements are connected with a reduction in optical scattering loss.

Pulsations and absorbing defects in (Al,Ga)As injection lasers

Forty‐seven strip‐buried‐heterostructure (Al,Ga)As lasers have been examined for pulsations and absorbing defects in the active volume. All 18 pulsating lasers were found to have defects while only one of the 29 nonpulsating lasers had a visible defect. These results show conclusively that absorbing defects can cause pulsations in double‐heterostructure injection lasers and support a recently published model for the origin of these pulsations. The presence of absorbing defects and pulsations also correlates with structure in the functional dependence of −I2 d2V/dI2 upon I above threshold. Lasers not exhibiting pulsations were aged, and the same correlation was found between the development of defects (particularly mirror darkening), the change in −I d2V/dI2, and the development of pulsations.

Nature of optically induced defects in Ga1−xAlxAs–GaAs double‐heterojunction laser structures

It is shown by transmission electron microscopy (TEM) investigation that the dark line defect (DLD) structure induced by optical pumping of undoped Ga1−xAlxAs–GaAs double‐heterostructure (DH) laser material is identical to the DLDs which develop during laser diode operation of devices fabricated from similar p‐n structure material. This confirms the DLD generation, and subsequent failure of cw laser diodes can be a growth‐related property of Ga1−xAlxAs–GaAs DH laser material and is not necessarily associated with p‐n junction dopants or contact metallization technology.