研究実績 2006年度

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

Results obtained in this research are as follows:

(1) A GaInAsP/InP quantum wire DFB laser with the active region width of 30 nm in the period of 240 nm was realized by an electron beam lithography, CH₄/H₂-reactive ion etching and two-step organometallic vapor-phase- epitaxial growth processes. High-mesa structure of this laser was fabricated by using wet chemical etching to obtain low-damage interfaces at the sidewalls. The spontaneous emission efficiency of this quantum wire DFB laser was almost comparable to that of a quantum-film laser fabricated by one-step growth. This indicates not only that this laser had a low-damage feature at the ultra fine structures but also there was little nonradiative recombination of the etched/regrown interfaces. By adopting low-damage fabrication processes for high-mesa stripe structures, a threshold current as low as 2.1 mA, which corresponds to a threshold current density of 176 A/cm², and a differential quantum efficiency of 16 %/facet were obtained for the stripe width of 3.4 mmand the cavity length of 350 mm under RT-CW condition. A single mode operation with the sub-mode suppression-ratio (SMSR) as high as 50 dB (a bias current of twice the threshold) was also achieved in the lasing wavelength of 1542 nm.

(2) A single mode operation and a high characteristic temperature operation of 1590 nm GaInAsP/InP quantum-wire DFB lasers were achieved by adopting the Bragg wavelength detuning from the gain peak of a 37 nm quantum wire active regions with a period of 247.5 nm. A single mode operation with the sub-mode suppression-ratio (SMSR) as high as 42 dB (a bias current of 20% above the threshold) was also achieved at the lasing wavelength of 1587 nm. The characteristic temperature for threshold current density (evaluated between 293 K and 353 K) of the quantum-wire DFB laser with the Bragg wavelength detuning was as high as 95 K, which is 1.6 times higher than 57 K of the DFB laser without detuning. Furthermore, the temperature dependence of the differential quantum efficiency was improved in the quantum-wire DFB laser with the Bragg wavelength detuning which was estimated to be 243 K, about 3 times higher than that of the DFB laser without the detuning (77 K).

(3) Further improvement of temperature dependences of GaInAsP/InP DFB lasers with wire-like active regions was obtained by implementing Bragg wavelength detuning. A double quantum-well GaInAsP/InP wire-like active region DFB laser with the active region width of 82 nm in the period of 248.75 nm was realized. Although the Bragg wavelength was detuned as large as 54 nm to longer wavelength side of the EL peak wavelength, a single-mode operation with a lasing wavelength of 1599 nm and a sub-mode suppression-ratio (SMSR) of 51 dB at a bias current of twice the threshold were obtained under RT-CW condition. A fixed single-mode operation without a mode hopping over a temperature range between 10°C and 85°C was achieved. The minimum threshold current density of 520 A/cm² was obtained at 50°C. Moreover, the changes of threshold current densities and differential quantum efficiencies as low as ±19% and 24%, respectively, were obtained.

(4) The investigation of optical properties of arbitrary shaped quantum structures from single quantum well GaInAsP/InP fabricated by an improved process was carried out. Optical properties of the GaInAsP/InP single quantum wire with various lateral widths down to 6 nm with wire length of 910 mm by Electron beam lithography, Metal-mask liftoff, CH₄/H₂-reactive ion etching and two-step organometallic vapor-phase-epitaxial growth processes were observed. Therein, lateral quantum confinement energies up to 90 meV were clearly and systematically observed through photoluminescence (PL) spectra, confirming high quality of etched surfaces. High homogeneity of wire sizes was observed via uniform spectra over 30 mm along the quantum wires. Moreover, various types of quantum structures, for example, quantum-dashes, quantum-dots, L-shaped quantum structures and quantum-rings, were realized.

(5) Distributed reflector (DR) laser, which consists of the active DFB and passive DBR sections with quantum-wire structure, was studied. DFB and DBR sections are integrated by using energy blue shift due to lateral quantum confinement effect. For DR laser with low-threshold and high-efficiency operation, a high reflection DBR mirror is required. From the theoretical and experimental investigations of DBR reflectivity, DBR section with the reflectivity of over 90% was confirmed. DR lasers with low threshold, high efficiency and stable single-mode operation have been realized using this high-reflective DBR section. For further improvement in lower threshold current operation, a DR laser with phase-shifted DFB section was realized. Phase-shifted grating can be fabricated easily by changing the EB lithography patterns. From the theoretical investigation of the grating structure, it was found that the lowest threshold current can be obtained by adopting l/8-shifted grating. As a result, threshold current as low as 1.2 mA and an external differential quantum efficiency of 13% from the front facet were obtained under RT-CW condition. Lasing mode exists inside the stopband due to the phase shift. A stable single-mode operation with an SMSR of 49 dB was obtained at a bias current of twice the threshold. The maximum differential quantum efficiency at the front facet was realized to be 36%. Recently, sub-mA operation of DR laser has been realized with higher refractive index coefficient of 450 cm^-1 utilizing a deep DFB grating region in the active section. A minimum threshold current of 0.8 mA (threshold current density of 180 A/cm² has been realized.

(6) For even lower threshold current operation of DR laser, narrow stripe buried heterostructure (BH) has been proposed to adopt. Mass transport technique has been taken into consideration to cover the sidewall of the active grating region of the high mesa stripe with InP so that surface recombination leakage current can be reduced and as a consequence, threshold current will be reduced. At the same time narrower stripe width around 1 mm will be fabricated.

(7) Antireflection coating has been performed on the fabricated DR laser. Al₂O₃ (n=1.74) has been used for low residual reflectivity to improve the differential quantum efficiency and single-mode operation. As a result, threshold current was reduced and differential quantum efficiency was increased reproducibly which can be attributed from the facet phase of the DFB section. A reduction of threshold current from 4 mA to 1.4 mA and increase of differential quantum efficiency from 19%/facet to 27%/facet have been obtained. A detail investigation of facet phase of DR laser is undergoing.

(8) Monolithic integration of DR laser with electro-absorption modulator (EAM) and front side power monitor (PM) has been fabricated for the first time utilizing quantum wire like structures using the fabrication method including EB lithography, CH₄/H₂-reactive ion etching and OMVPE regrowth,. The wire width has been modulated at the different sections of the device and thereby controlling the transition energy. The wire width of EAM section and PM was 40 nm with a period of 100 nm. A DC extinction ratio of 5.5 dB has been obtained for 11V EAM bias voltage. Better performance can be achieved by fabricating narrower wire width. Front power monitor showed linear relationship of photo current with laser output light.

(9) The isolation resistance between active DFB LD region and passive device (EAM/PM) region requires high electrical isolation which was realized by deep groove etching beyond active layer. A high isolation resistance of 60 MW was realized with a 500 nm wide and 3.8 mm deep groove where the optical coupling was estimated to be 95%.

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 membrane lasers have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) Novel semiconductor laser structure, such as a membrane laser, which has the Benzocyclobutene (BCB) cladding layers, enables to increase optical confinement into the active layer due to a large refractive-index difference between the active layer and cladding layers. A RT-CW operation of membrane BH-DFB laser, consisting of deeply etched single-quantum-well wirelike active regions, was already demonstrated. In order to realize high reflective cavity, surface corrugation structure was investigated. Narrow stripe membrane BH-DFB laser array using surface corrugation was fabricated. A single-transverse mode and 68 nm large stop-band operation was realized with 0.6mm-stripe membrane BH-DFB laser using surface corrugation. This stopband width corresponds to the index-coupling coefficient ofk_i=2950 cm^-1, which is two times larger than conventional (flat surface) membrane laser of 2.0 mm stripe width. In addition, we fabricated a short-cavity membrane DFB laser with a 40-nm-deep surface corrugation structure. A threshold optical pump power of as low as 0.34 mW was realized for a 2.0-mm-wide and 80-mm-long device under RT-CW conditions.

(2) Strongly index-coupled GaInAsP/InP membrane DFB laser, consisting of a flat single-quantum-well active region, was realized by adopting a surface corrugation structure. A threshold optical pump power as low as 1.1 mW was achieved under RT-CW condition for the stripe width of 2.0 mm and the cavity length of 60 mm. The index-coupling coefficient was estimated to be 3200 cm^-1 for the surface corrugation depth of 50 nm.

(3) Membrane laser structure using Benzocyclobutene (BCB) for cladding layer offers negative temperature coefficient of refractive index which is in contrast with the semiconductor material. Therefore athermal waveguide can be designed 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 5.26×10^-2nm/K and 2.45×10^-2 nm/K, respectively where the later one is 20 % of that of typical semiconductor DFB lasers.

(4) Though thermal characteristics of membrane laser were considered to be disadvantageous due to the thermal conductivity of BCB which is about 200 times lower than InP, high temperature (85°C) continuous wave operation of an optically pumped membrane BH-DFB laser using polymer cladding was obtained using Bragg wavelength detuning technique for its low threshold operation. The thermal resistance of this laser was estimated to be 2.3×10⁴K/W.

(5) We fabricated novel membrane BH-DFB lasers with an air-bridge structure which is more robust than the conventional one and is suitable for large scale wafer fabrication. The minimum threshold pump power P_th of 4.3 mW was obtained at 20°C. Continuous wave operations up to moderately high temperature (80°C) were achieved under an optical pumping. The thermal resistance was estimated to be 11 K/mW, which is half that of membrane BH-DFB lasers fabricated by bonding on BCB coated InP substrate.

(6) Wafer bonding technology was investigated to integrate active photonic devices on a silicon on insulator (SOI) wafer for highly compact photonic- integrated circuits. A single-quantum-well (SQW) GaInAsP/InP membrane structure bonded onto an SOI wafer was successfully obtained by a direct bonding method with a thermal annealing at 300-450 ^oC under H₂ atmosphere. The PL intensity of the SQW membrane structure did not degrade after this direct bonding process and its spectral shape was not broadened.

(7) A room-temperature continuous-wave operation under optical pumping was demonstrated with GaInAsP/InP buried heterostructure membrane distributed feedback laser fabricated on an SOI by the direct wafer bonding. A threshold pump power of 2.8 mW and a sub-mode suppression ratio of 28 dB were obtained with a cavity length of 120 mm and a stripe width of 2 mm.