Osamu Nakada

Very‐Low‐Current Operation of Mesa‐Stripe‐Geometry Double‐Heterostructure Injection Lasers

Mesa‐stripe‐geometry double‐heterostructure lasers which operate at low‐current level have been fabricated. Lasers of this geometry are made by etching the heterostructure layers, leaving a stripe region with a width ranging from 10 to 40 μm. The current‐spreading effect inherent in stripe‐geometry lasers is eliminated in this structure. As a result of the small active region and the low‐threshold current density, a significant reduction of total threshold current has been realized. The lowest‐threshold current is 50 mA in pulsed operation, and 75 mA in dc. The thermal resistance of the diode of this structure is nearly as low as that of the stripe‐geometry laser.

Degradation sources in GaAs-AlGaAs double-heterostructure lasers

Several sources of the dark-line defect (DLD) that causes rapid degradation of GaAs-AlGaAs double-heterostructure (DH) lasers have been identified by means of photoluminescence (PL) topography and a laser-induced degradation technique. All the sources that have been identified correspond to crystal defects, among which dark-spot defects (DSD) that are native to as-grown wafers are found to be most important. The growth and propagation processes of DLDs and DSDs have also been investigated. These defects are found to be highly mobile under high-intensity laser pumping. The correlation between the substrate dislocations and the DSDs has been examined by etching and X-ray topography. Although most DSDs correspond to etch-pits in epilayers, they are not always correlated with substrate dislocations.

Mesa-stripe-geometry double-heterostructure injection lasers

Fabrication and characteristics of mesa-stripe-geometry double-heterostructure injection lasers are described. Two types of lasers have been prepared: 1) low mesa-type lasers in which mesa etching is stopped just above the active layer and 2) high mesa-type lasers in which mesa etching is effected up to the first epitaxial layer (n-GaAlAs). The stripe widths of these lasers can be narrowed below 30 μm without an appreciable increase of the threshold current density. As a result, very low current operation has been realized. The thermal resistances of these lasers are nearly as low as those of stripe-geometry lasers. This property along with the low threshold current density renders it easy to operate diodes continuously at and above room temperature. Near-single-mode operation has been observed in low mesa-type lasers. The laser emissions display TE polarization in most cases; however, diodes with thick active layers have often been observed to show TM polarization.

Limitations of power outputs from continuously operating GaAs‐Ga1−xAlxAs double‐heterostructure lasers

Reported here are the results of measurements of several factors limiting the maximum output from GaAs‐Ga1−xAlxAs double‐heterostructure injection lasers under continuous operation at room temperature. The factors are (i) catastrophic damage to the mirror facets and (ii) saturation of the output power due to self‐heating. Threshold optical density for catastrophic damage is 1.1 MW/cm2, and 390 mW is obtained from one mirror of a DH laser with a 80‐μ‐wide stripe under continuous operation. It is emphasized that DH lasers with a very thin active layer are attractive in order to achieve high‐power operation.

X‐ray topographic study of dark‐spot defects in GaAs‐Ga1−xAlxAs double‐heterostructure wafers

Dark‐spot defects (DSD) in a GaAs‐Ga1−xAlxAs double‐heterostructure (DH) wafer are studied by x‐ray topography. Such DSD’s are one of the primary sources of the dark‐line defects (DLD) that cause rapid degradation of GaAs‐Ga1−xAlxAs DH lasers. By using x‐ray topography almost all DSD’s observed by photoluminescence topography are correlated with dislocations in a GaAs substrate. In addition, it is observed that some dislocations in the substrate are not sources of DSD’s when the dislocation axes are nearly parallel to the surface of the substrate.

Effects of composition profile on characteristics of GaAs–Ga1−xAlxAs double‐heterostructure lasers

Electron probe x‐ray microanalysis was used to measure the composition profile in the active layers of GaAs–Ga1−xAlxAs double‐heterostructure wafers along the growth direction. The threshold current density and the lasing wavelength of the double‐heterostructure laser were found to be consistently affected by the composition profile in the active layer.

X-Ray Topographic Study of Lattice Defects Related with Degradation of GaAs-Ga1-xAlxAs Double-Heterostructure Lasers

Dark-spot defects (DSD) which are primary sources of the rapid degradation of GaAs-Ga1-xAlxAs double-heterostructure (DH) lasers are studied by X-ray topography. X-ray topographs of the defective parts of a DH wafer were obtained through the use of micro-X-ray topography. This technique is effective for small specimens of 0.01~1 mm2. In addition, it is suggested that elimination of a dislocation from a DH wafer is possible, not by introducing a misfit dislocation, but by using a dislocation-free GaAs substrate.