Activities
Low Dimensional Quantum Structure Lasers
Staffs:　Y. Suematsu　S. Arai　T.Maruyama　S. Tamura
Visiting Researcher:　A. Haque
Students:　H. Yagi　K. Ohira　T. sano　T. Murayama　D. Plumwongrot　M. Hirose　K. Miura
　

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. A new type of DR (distributed reflector) lasers fabricated by the same fabrication process as that of quantum-wire lasers and distributed feedback (DFB) lasers with wirelike active regions have been also studied.

Results obtained in this research are as follows:
　

(1) GaInAsP/InP multiple-quantum-wire structures with the wire widths of 18 nm and 27 nm in the period of 80 nm were fabricated by electron beam lithography, CH4/H2-reactive ion etching and two-step organometallic vapor-phase-epitaxial growth. Size distributions of these quantum-wire structures were measured by scanning electron microscope views and the standard deviation was estimated to be less than ±2 nm. From EL spectra at 103 K, the full-width at half maximum of these quantum-wire structures was almost comparable to that of the quantum-film structure fabricated from the same initial quantum-well.

(2) 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.

(3) A RT-CW operation of GaInAsP/InP quantum-wire lasers (23 nm wide, 5 stacked quantum-wires) and wirelike lasers (43 nm wide, 5 stacked wires) fabricated by electron beam lithography, CH4/H2-reactive ion etching and organometallic vapor-phase-epitaxial regrowth was realized for the first time. Lifetime measurement of this quantum-wire laser was also carried out at RT-CW condition, and no noticeable performance degradation was observed even after 9,500 hours.

(4) GaInAsP/InP strain-compensated 5-stacked compressively strained quantum-wire lasers with the wire width of 14 nm in the period of 80 nm were realized by electron beam lithography, CH4/H2-reactive ion etching and organometallic vapor-phase-epitaxial regrowth. By adopting completely strain-compensating barriers, a smaller energy blue shift at the peak wavelength in EL spectra than that in the case of a partial strain-compensation was observed that indicates the suppression of the strain relaxation effect during etching and InP regrowth process. Lateral quantum confinement effect in this quantum-wire laser could be also observed via sharper shape of the EL spectrum than that of quantum-film lasers in the higher transition energy region.

(5) GaInAsP/InP multiple-quantum-wire lasers (wire widths of 19 nm and 27 nm in a period of 100 nm) with SiO2/semiconductor reflector by electron beam lithography, CH4/H2-reactive ion etching and two-step organometallic vapor-phase-epitaxial growth. As a result, oscillations from the ground levels could be obtained at RT. In addition, the threshold current densities of these quantum-wire lasers were lower than and differential quantum efficiencies were comparable to those of quantum-film lasers at RT.

(6) Low threshold operation of 1.55 mm wavelength GaInAsP/InP strongly index-coupled and gain-matched distributed feedback lasers with periodic wirelike active regions, which was fabricated by electron beam lithography, CH4/H2-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, were demonstrated and their reliability was also investigated to date. As a result, no degradations in lasing characteristics were observed after an aging time of 8200 hours at a bias current of around 10 times the threshold. In the experiment, all measured device of this laser oscillated on the long-wavelength side of the stopband. We investigated the single-mode operation of DFB laser with wirelike active regions by using coupled-mode theory. As a result, it was theoretically demonstrated that DFB lasers with wirelike active regions oscillated in the long-wavelength side mode of the stopband by considering the gain-matching effect. The facet phases of two cleaved facets were also investigated. Lasing modes exist on the long-wavelength side for any facet combination.

(7) We have been studying a new type of distributed reflector laser consisting of a wirelike active section and a passive DBR section with quantum-wire structure by using a lateral quantum confinement effect. To evaluate the reflectivity of the DBR, we fabricated Fabry-Perot type lasers with DBR section on a single side. The reflectivity was estimated from the output ratio from the front to the rear facet. The reflectivity higher than 90 % was achieved at a DBR length of 200 mm for the wire width of 60 nm and 350 mm for 30 nm, where cleaved facet reflectivity of Rf = 0.3 and the refractive index difference of Δn = 0.03 were assumed. Using this DBR structure, DR laser was fabricated with planer type structure by BCB polymer. As a result, threshold current of 3.2 mA was obtained for active section length of 260 mm, passive DBR section length of 130 mm and stripe width of 4.3 mm with both facets cleaved. The differential quantum efficiency from the front facet was 19.2 % and the rear facet was 1.7 %, hence an asymmetric output ratio of 11 was realized. A single-mode operation with sub-mode suppression ratio (SMSR) of 46.6 dB was achieved at a bias current of 3.75 times the threshold.

(8) The device mentioned above, however, could not operate with high performance theoretically indicated. This result was caused by damage of dry etching to form the stripe. For higher efficiency, wet chemical etching was used to form the laser stripe instead of dry etching. As a result of fabricating the DR laser by wet chemical etching, threshold current of 2.5 mA, which corresponds to the threshold current density of 189 A/cm2, was obtained for the active section length of 300 mm, the passive DBR section length of 210 mm and the stripe width of 4.4 mm. The active and passive sections consist of 90-nm-wire with the period of 240.0 nm and 40-nm-wire with the period of 241.25 nm, respectively. The differential quantum efficiency from the front facet was 35.6 % and the rear facet was 0.54 %. An asymmetric output ratio of 66 was realized. A single-mode operation with sub-mode suppression ratio (SMSR) of 53.7 dB was achieved at a bias current of twice the threshold.
　
New Types of Semiconductor Lasers for Photonic Integration
Staffs: Y. Suematsu　S. Arai　S. Tamura
Students: H.-C. Kim　T. Okamoto　Y. Onodera　H. Kanjo　T. Hasegawa　S. Sakamoto　T. Yamazaki
　
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) 1.3mm-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 2.8mA for the active region length of 150mm and an SMSR=44dB were obtained.

(2) In case of TE mode, a narrow stripe waveguide exhibits relatively low coupling coefficient even for deep grating. 1.5mm vertical grating distributed feedback lasers with tensile quantum well were realized single mode operation of TM polarized VG-DFB lasers. A threshold current of 2.6mA and SMSR=50dB were obtained.

(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 mm and a stripe width of 2 mm under room-temperature continuous wave optical pumping. The corresponding threshold for current injection was roughly estimated to be 27 mA.

(4) We have realized membrane BH-DFB laser arrays by arranging the laser cavities (10 mm 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.

(6) Low threshold operation of membrane buried heterostructure distributed feedback (BH-DFB) laser arrays. 45 devices were fabricated on a wafer covering a wide wavelength range of 75 nm. The lowest threshold pump power as low as 0.64 mW along with the sub-mode suppression-ratio (SMSR) of over 30 dB at 2 times the threshold were successfully obtained at RT-CW condition.

(7) Phase shifted membrane BH-DFB laser with short cavity length of 50 mm was fabricated and characterized. Threshold pump power of 3.1mW and SMSR of 35dB were achieved with RT-CW condition under optical pumping. Laser cavities were operated under optical pumping at RT-CW using micro PL setup. Stripe width was 2 mm, cavity length was 50 mm, and grating period was 310 nm, respectively. Threshold pump power Pth was 3.1 mW which corresponded to a threshold current Ith of around 70 mA (estimated by assuming an absorption coefficient as 10000 cm-1). An emission wavelength of 1549 nm was observed. Considering the lasing mode as the Bragg wavelength, equivalent refractive index was estimated to be around 2.50 which agreed well with the value calculated from the cross sectional waveguide structure. From the stop band width of 38 nm, index coupling coefficient was estimated to be 710 cm-1.

(8) Benzosyclobutene (BCB) used for cladding layer of membrane laser structure has negative temperature coefficient of refractive index which is opposing value of semiconductor material. So athermal waveguide can be designed with controlling the thickness of membrane core layer. Membrane BH-DFB lasers with membrane core thickness of 150nm and 65nm were fabricated. Slope of lasing wavelength dependences on temperature were measured to be 0.0526nm/K and 0.0245nm/K, respectively. The value for membrane thickness of 65nm was one in five of typical semiconductor DFB lasers.