Results 2004

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) 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 more than 17,600 hours.

(2) 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. A 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.

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

(4) GaInAsP/InP quantum-wire distributed feedback lasers with the active region width of 24 nm were realized by electron beam lithography, CH₄/H₂-reactive ion etching and two-step organometallic vapor-phase- epitaxial growth processes for the first time. A threshold current as low as 2.7 mA (threshold current density=270 A/cm2) and differential quantum efficiency of 19 %/facet were achieved for the stripe width of 3.0 mm and the cavity length of 330 mm under a RT-CW condition. A single-mode operation with the sub-mode suppression ratio (SMSR) as high as 51 dB (injection current is twice the threshold) was also obtained in the lasing wavelength of 1541 nm. From the lasing spectrum, the stopband width was observed to be 4.8 nm which corresponds to the index-coupling coefficient (ki) of 180 cm-1.

(5) Investigations of polarization anisotropy for compressively strained GaInAsP/InP quantum-wire (Q-Wire) structures fabricated by electron beam lithography, dry etching and double-step organometallic vapor-phase-epitaxial growth processes were carried out via experimental evaluation of photoluminescence (PL) and lasing characteristics. From PL measurement, parallel transverse-electric field (TE) peak intensity to the Q-Wire direction was measured to be 1.4-1.6 times stronger than perpendicular TE peak intensity to the Q-Wire direction for the wire widths of 24-45 nm, respectively. Furthermore, 2-type of Q-Wire lasers with the wire width of 35 nm were fabricated, i.e., quantum-wire directions are perpendicular and parallel to the laser cavity; Q-Wire_^ and Q-Wire//, respectively. As a result, although the spontaneous emission efficiency of both lasers was almost the same, the threshold current density of Q-Wire_^ was much lower compared with that of Q-Wire//. From the gain spectral measurement with Hakki-Paoli method, it was demonstrated that the differential gain for Q-Wire_^ is 5 times higher than that for Q-Wire//.

(6) We have been studying distributed reflector (DR) laser, which consists of the active DFB and passive DBR sections with quantum-wire structure, by using energy blue shift due to a lateral quantum confinement effect. For DR laser with low-threshold and high-efficiency operation, a high reflection DBR is required. We estimated DBR reflectivity experimentally from the comparison between Fabry-Perot (FP) lasers and DBR lasers consisting of a quantum-film section with DBR section on a single side. As a result, the differential quantum efficiency of DBR laser was almost two times higher than that of FP laser, the reflectivity for the DBR section of over 90% was confirmed.

(7) Low threshold operation of DR laser was realized by adopting narrow stripe geometry by the combination of wet chemical and dry etchings. Lowest threshold current of 1.1 mA (threshold current density of 160 A/cm2) was obtained for the stripe width of 2.1 mm, the active section length of 330 mm, and the passive DBR section length of 110 mm. External differential quantum efficiencies from the front and the rear facets were 13% and 0.4%, respectively. A good single-mode operation with an SMSR of over 40 dB was achieved.

(8) A DR laser with phase-shifted DFB section was realized for lower threshold current operation. Phase-shifted grating can be fabricated easily by changing the EB lithography patterns. Form 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.

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 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 an SMSR 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 RT-CW optical pumping. The corresponding threshold for current injection was roughly estimated to be 27 mA.

(2) Phase shifted membrane BH-DFB laser with a cavity length of 50 mm was fabricated and characterized. A threshold pump power of 3.1mW, which corresponds to a threshold current I_th of around 70 mA, and an SMSR of 35dB were achieved under RT-CW optical pumping. 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, an index-coupling coefficient was estimated to be 710 cm-1.

(3) 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. The 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.

(4) In order to realize high reflective cavity, asperity corrugation structure was investigated. Asperity corrugation structure was made with controlling InP regrowth time. Membrane BH-DFB laser with asperity corrugation structure was realized with a cavity length of 80 mm and a threshold pump power of 1.3 mW at RT-CW condition. An index-coupling coefficient was estimated to be over 2000 cm-1.

(5) The lasing properties such as stripe-width dependence of an index-coupling coefficient were evaluated using narrow stripe membrane BH-DFB lasers. The lowest threshold pump power was obtained with the stripe width of 1.2 mm. A single transverse-mode operation was obtained for the stripe width less than 1mm.