Shinichi Nakatsuka

Room-Temperature Pulsed Operation of GaInNAs Laser Diodes with Excellent High-Temperature Performance

We have successfully operated GaInNAs laser diodes with a pulsed current at room temperature. The lowest threshold current density was about 0.8 kA/cm2, and the lasing wavelength was about 1.2 µ m. Characteristic parameters such as internal quantum efficiency and the gain constant were measured, and excellent high-temperature performance was observed. The characteristic temperature was 127 K in the temperature range from 25 to 85° C.

High‐power operation of index‐guided visible GaAs/GaAlAs multiquantum well lasers

Stable transverse mode operation has been realized for the first time in visible (780 nm) multiquantum well lasers composed of seven 3‐nm‐thick GaAs wells separated by six 5‐nm‐thick Ga0.8Al0.2As barriers grown by metalorganic chemical vapor deposition. A self‐aligned structure with a built‐in optical waveguide to stabilize the transverse mode is fabricated by a two‐step epitaxial technique. Low threshold current (35 mA), high output power (up to 40 mW) in the fundamental transverse mode, and a very low degradation rate at 70 °C have been confirmed.

High-power 780 nm AlGaAs quantum-well lasers and their reliable operation

A high-power laser in the 780-nm wavelength region was realized by introducing an AlGaAs ternary alloy quantum-well structure. Since there is no axial structure, high reproducibility is expected. By applying the quantum-well structure to the AlGaAs ternary alloy wells, a reduction in the operation current of a laser which oscillates in the 780-nm wavelength region can be achieved. A reduction of the optical power density in the direction perpendicular to the junction plane is examined to lessen facet degradation. Investigations focus on the characteristic temperature and catastrophic optical damage (COD) level. It is found that a triple quantum-well (TQW) active layer structure has superior characteristics for high-power operation. Stable operation for over 1000 h under 50 degrees C and 60 mW conditions was achieved. A relatively high characteristic temperature of around 150 K was also obtained. The results of an aging test are provided. >

A New Self-Aligned Structure for (GaAl)As High Power Lasers with Selectively Grown Light Absorbing GaAs Layers Fabricated by MOCVD

A new transverse-mode stabilizing self-aligned structure for (GaAl)As laser diodes by metal organic chemical vapor deposition (MOCVD) is developed. This structure is characterized by an n-GaAs layer grown selectively outside of a mesa stripe using an SiO2 mask. To control the distance between active and n-GaAs layers a highly selective (GaAl)As etching technique by boiled HCl is used. Furthermore, the laser diodes can operate in the fundamental transverse-mode at light output power up to 70 mW with a threshold current of 60 mA under CW operation at lasing wavelengths of 750–760 nm.

Crystallographic microstructure and electrical characteristics of Au/Pt/Ti/Ni ohmic contacts on p‐type (001) ZnTe layers

The crystallographic microstructure and electrical characteristics of Au/Pt/Ti/Ni ohmic contacts on p‐type (001) ZnTe layers are investigated as a function of annealing temperature, by using the transmission line model method, cross‐sectional transmission electron microscopy, and Auger electron spectroscopy. The specific contact resistance decreases when the annealing temperature is increased and reaches a minimum at 300 °C. A minimum value of 1.1×10−6 Ω cm2 is obtained for a hole concentration of 3×1019 cm−3. The epitaxial NiTe2 that formed at the metal/semiconductor interface due to annealing is considered to play an important role in lowering the contact resistance. The excess Zn atoms created by the reaction between Ni and ZnTe are found to diffuse upward and to segregate at the Pt/Ni interface. A contact stability test performed at 102 °C suggests that these ohmic contact structures are stable even under high‐current injection.

Electrical properties and microstructures of Au/Pt/Ti/Ni ohmic contacts to p‐type ZnTe

Electrical properties and microstructures of Au/Pt/Ti/Ni ohmic contacts to p‐type ZnTe were investigated using the transmission line model method and cross‐sectional transmission electron microscopy. The specific contact resistance decreases when the annealing temperature is increased and reaches a minimum at 300 °C. The formation of NiTe2 from the reaction between Ni and ZnTe plays an important role in lowering the contact resistance. A contact stability test performed at 102 °C suggests that these ohmic contacts are stable even under high‐current injection.

Reliability of 780-nm High-Power Laser Diodes with Thin Quantum Well Active Layer

The reliability of 780-nm-(AlGa)As high-power laser diodes with a thin triple-quantum-well or a single-quantum-well active layer is investigated. The thin active layers achieve high catastrophic optical damage levels above 170 mW. In these laser diodes, a gradual increase of the operation current limits the lifetime of the laser diode. The gradual degradation rate is higher in the single-quantum-well laser diodes than to the triple-quantum-well laser diodes, and is also higher in shorter-cavity-length laser diodes. The power dependence of the gradual degradation rate is very small compared with the dependence on cavity length or active layer structure. The gradual degradation mainly depends on the threshold current density per active layer thickness. As a result, 60-mW reliable operation for more than 1000 hours was achieved at 50°C in 600-µm-length triple-quantum-well lasers.

Fundamental Lateral-Mode Operation in Broad-Area Lasers Having Built-in Lenslike Refractive Index Distributions

High power fundamental lateral-mode operation is achieved in an (AlGa)As broad-area laser diode. This diode has built-in lens-like refractive index distributions in the stripe. These distributions effectively suppress higher order lateral-mode generation by expanding the laser beam in the direction parallel to the junction. As a result, fundamental lateral-mode operation of up to 80 mW for CW operation is attained with this laser.

DISORDERING OF THE ZNCDSE SINGLE QUANTUM WELL STRUCTURE BY CD DIFFUSION

The effects of annealing on a ZnCdSe single quantum well (SQW) structure with ZnCdSSe/ZnSSe superlattice optical guiding layers are investigated. X‐ray diffraction and photoluminescence (PL) measurements showed disordering of a ZnCdSSe/ZnSSe superlattice after annealing at about 500 °C. The PL peak energy of the SQW shifted to the higher energy side, and the linewidth narrowed in the sample annealed at 300 °C. Cadmium diffusion was confirmed by secondary ion mass spectrometry. We found that the disordering of the ZnCdSSe/ZnSSe superlattice and the changes in the emissions from the SQW were due to the Cd diffusion.

INDEX-GUIDE GAINNAS LASER DIODE FOR OPTICAL COMMUNICATIONS

An AlGaAs/GaAs/GaInNAs single-quantum-well real-index-guide laser diode with a ridged waveguide structure was fabricated. A threshold current of 24 mA under room-temperature continuous-wave operation was attained with this structure. Obtained device parameters show that this device shows promise for application in optical communication system.