Results 2002

GaInAsP/InP strained-quantum-film, -wire and -box lasers have been studied. Distributed feedback (DFB) lasers consisting of wirelike active regions fabricated by the same fabrication process as Quantum-Wire lasers have been also studied.

Results obtained in this research are as follows:

(1) 1.5-µm-wavelength partially strain-compensated GaInAsP/InP 5-layered quantum-wire lasers with the wire width of 23 nm in the period of 80 nm were realized for the first time by electron beam lithography, CH₄/H₂-reactive ion etching and organometallic vapor-phase-epitaxial regrowth. The threshold current density of 774 A/cm² and differential quantum efficiency of 40 % were obtained under a pulsed condition at room temperature. From measurement of spontaneous emission spectra, the blue shift at the peak wavelength was 38 meV, which was much larger than a calculated value, and the spontaneous emission spectral width was almost constant at temperatures between 103 K and 253 K, indicating a lateral quantum confinement effect. Finally, the spontaneous emission efficiency below the threshold was almost comparable to that of the Q-Film lasers up to 85°C, that revealed low-damage property of the etched/regrown interfaces.

(2) GaInAsP/InP partially strain-compensated multiple-quantum-wire lasers with the wire widths of 18 nm and 27 nm in the period of 80 nm were also realized. Size fluctuations of these quantum-wire structures were measured by scanning electron microscope views, from which the standard deviation was obtained to be less than 2 nm. The differential quantum efficiencies of these quantum-wire lasers were almost the same as that of the 5-quantum-well lasers at room temperature. From EL spectra of various wire widths lasers, a larger energy blue shift than that from a simple analysis model was observed, which can be attributed to residual compressive strain between the active region and surrounding InP layer.

(3) Wire width dependence of the large energy blue shift in GaInAsP/InP partially strain-compensated vertically-stacked multiple-quantum-wire structures is accurately explained for the first time using an 8 band k・p theory without any fitting parameter. Variations of energy levels due to a non-uniform strain profile in stacked quantum-wires are calculated to be less than 2.4 meV. It is found that unlike quantum films, the energy-band structures of strained quantum-wires depend on the amount of strain-compensation in barrier regions and on the number of wire layers in the vertical stack.

(4) A RT-CW operation of GaInAsP/InP quantum-wire lasers (23 nm wide, 5 stacked quantum-wires) and quantum-wirelike lasers (43 nm wide, 5 stacked wires) fabricated by electron beam lithography, CH4/H2-reactive ion etching and 2-step organometallic vapor-phase-epitaxial growth processes was realized for the first time. Lifetime measurement of this quantum-wire laser was also carried out at RT-CW condition, and no noticeable degradation in light output was observed even after 2,000 hours.

(5) High-performance 1.55 µm wavelength GaInAsP/InP strongly index-coupled and gain-matched distributed feedback lasers with periodic wirelike active regions were fabricated by electron beam lithography, CH4/H2-reactive ion etching, and organometallic vapor-phase epitaxial regrowth. This type of DFB laser with wirelike active regions can operate at very low threshold because of its strong index coupling and the reduction of the active medium volume. However, this type of DFB laser consists of a larger portion of etched/regrowth interfaces than conventional DFB laser. Therefore the reliability test of lasing characteristics is very important. A CW life test was carried out. No degradations in lasing characteristics were observed after an aging time of 8200 hours at a bias current of around 10 times the threshold.

(6) By using a lateral quantum confinement effect, a new type of distributed reflector laser consisting of a wirelike active section and a passive DBR section with quantum-wire structure was demonstrated for the first time. In theoretical analysis, a waveguide loss of a DBR structure increases by only 1 cm-1 compared with the value in case of no active layers. Using this waveguide structure as a passive DBR section, a maximum reflectivity of 97 % would be obtained for the wire width of 40 nm and DBR section length of 200 µm. Threshold current of 15.4 mA, which corresponds to the threshold current density of 320 A/cm2, was obtained for the active section length of 240 µm, the passive DBR section length of 440 µm and the stripe width of 20 µm with both facets cleaved. The differential quantum efficiency from the front facet was 16.2 % and the rear facet was 0.57 %, hence an asymmetric output ratio of 28 was realized. This strong asymmetric output characteristic is a specific property of the DR laser. For a lower threshold and a single-mode operation, narrow stripe DR laser was also fabricated. As a result, Threshold current of 7.6 mA and differential quantum efficiency from the front facet of 5.1 % were obtained under RT-CW condition for the active section length of 310 µm, the passive section length of 270 µm and the stripe width of 3 µm. A single-mode operation with sub-mode suppression ratio (SMSR) of 40 dB was achieved at relatively low bias level (I=1.2Ith).

Semiconductor lasers with low threshold current, high efficiency, and single wavelength operation are very attractive for optical interconnection and a number of optoelectronics applications. New types of semiconductor lasers, such as Distributed Reflector (DR) lasers and Membrane lasers have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) 1.3µm-wide narrow mesa stripe DR lasers consisting of first-order vertical grating (VG)-DFB and first-order deeply etched DBR mirrors were realized for the first time by one-step epitaxy and fine vertical etching processes. A threshold current of 3.6mA for the active region length of 210µm and an SMSR=35dB were obtained.

(2) By use of vertical grating DFB structure, it is clarified that structural birefringence can be drastically reduced. The grating coupling coefficient can also be made polarization insensitive by adjusting the grating depth.

(3) Novel semiconductor laser structure, such as, membrane laser which has the Benzocyclobutene (BCB) cladding layers, enables to increase optical confinement into active layer due to a large refractive index difference between active layer and cladding layers. A room temperature continuous wave operation of membrane DFB laser consisting of deeply etched single-quantum-well wirelike active regions was already demonstrated. In order to realize single mode and low threshold operation of membrane DFB laser, buried heterostructure (BH) was innovated by slightly changing the fabrication process. A threshold pump power of 1.5 mW and a sub-mode suppression-ratio of 42 dB were obtained for a 142 nm-thick semiconductor membrane core layer with a cavity length of 120 µm and a stripe width of 2 µm under room-temperature continuous wave optical pumping. The corresponding threshold for current injection was roughly estimated to be 27 µA.

(4) We have realized membrane BH-DFB laser arrays by arranging the laser cavities (10 µm spaced 15 elements with 5 different grating periods). A total wavelength span of 72 nm was achieved with a small lasing wavelength fluctuation of up to ±1.2 nm at RT-CW condition under optical pumping. From this value, membrane thickness fluctuation was estimated to be ±0.4 nm. Threshold pump power of 3.4 mW and SMSR of 45 dB were achieved in a typical device.

(5) Membrane BH-DFB laser arrays with different grating periods and different stripe width were successfully fabricated using EB lithography, CH4/H2-RIE and OMVPE. The possibility for a laser array covering a wide wavelength range of 51 nm with a wavelength controllability of less than 0.8 nm (100GHz) was demonstrated. This multi-wavelength laser array may be a candidate for a coarse WDM system or a wavelength conversion device between LAN and MAN.