研究実績 1997年度

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

(1) Drive current of a directly modulated semiconductor laser for optical parallel interconnection was investigated by taking into account of both the threshold current and the differential quantum efficiency. As the results, it was found that the laser cavity design which yields the minimum drive current is almost the same condition to achieve the minimum threshold current whereas the cavity design for higher differential quantum efficiency is required when the total system loss is much higher. When the cavity length is 100mm and the rear mirror reflectivity R_r is 0.99, the front mirror reflectivity R_f which gives the minimum drive current was obtained to be 0.94, 0.91, and 0.8 for the total system loss of 6dB, 10dB, and 20dB, respectively.

(2) The photon recycling effect in quantum film (Q-film), quantum wire (Q-wire), and a quantum box (Q-box) has been theoretically investigated using rate equation analysis and the density-matrix method. The threshold reductions due to photon recycling in Q-film, Q-wire, and Q-box, in the case of lattice matched (LM) Ga_0.47In_0.53As/InP are 40%, 24%, and 0%, respectively, for a fixed cavity loss of lasing mode normalized by the optical confinement factor h_L/h_L=50 cm^-1. The estimation also shows that photon recycling is more effective in compressively-strained (CS) Ga_0.18In_0.82As_0.73P_0.27/InP quantum structures than in LM quantum structures. In both cases, the threshold reduction due to photon recycling is larger in the Q-film than in Q-wire and Q-box.

(3) Polarization dependence of 1.5mm wavelength GaInAsP/InP quantum-wire structures fabricated by EB lithography and two-step OMVPE growth were measured and compared with quantum-film structures fabricated on the same wafer. As the result, clear anisotropic PL properties of the quantum-wire structure corresponding to the wire width (25nm and 35nm) were confirmed. This anisotropy was observed to be a little stronger in 1% compressively-strained quantum-wire structure than that in lattice-matched one with the same wire width (35nm).

(4) Temperature dependence of GaInAsP/InP compressively-strained single-quantum-well lasers with quantum-wire (Q-Wire) size active region were measured and compared with those of quantum-film lasers. Lower threshold current as well as higher differential quantum efficiency operation of Q-Wire laser than those of Q-Film laser at a temperature below 200K were obtained. Threshold current density of Q-Wire laser was 34A/cm² which was almost a half that of Q-Film laser being 68A/cm² at T = 90K.

(5) Gain spectra of 1.5mm wavelength GaInAsP/InP quantum-wire lasers (W=20, 25nm) were measured and compared with quantum-film lasers prepared on the same wafer. As a result, narrower gain spectra of the quantum-wire lasers were obtained at T = 100K.

(6) 50nm-period GaInAsP/InP quantum-wire lasers (W〜20nm) were fabricated using EB lithography and wet etching followed by OMVPE embedding growth, and measured the cavity length dependence of the differential quantum efficiency. As the result, x_i〜1.0 and k_WG〜4.5cm^-1 were obtained at T=90K.

(7) Realization of room temperature operation of Multiple Microcavity laser consisting of l/4-air gap (= 0.39mm) and semiconductor reflectors was fabricated using two steps organometallic vapor-phase epitaxy (OMVPE) and two steps wet chemical etching. The threshold current was measured to be 78mA (J_th = 1.25kA/cm²) for a total cavity length 164mm (Pitch: 20mm; 8 elements) and stripe width of 40mm, and an effective power reflectivity of this cavity structure was estimated to be higher than 92%.

Semiconductor directional-coupler-type all-optical switching devices with tapered-shape structures were proposed and analyzed by numerical simulations. Moreover a GaInAs/InP multiple-quantum-well directional-coupler-type all-optical switch was fabricated and its low switching power operation was demonstrated.

Results obtained in this research are as follows:

(1) A semiconductor directional-coupler-type all-optical switch with tapered-shape structures, based on the nonlinear refractive index variation induced by photoexcited carriers, was proposed and its taper-shape dependence was investigated. By the numerical simulation, it is shown that an introduction of tapered-shape structures is effective for the improvement of the extinction ratio. It is also found that this device can drive the signal light of about 17dB higher power than the switching power.

(2) A GaInAs/InP multiple-quantum-well (MQW) directional-coupler-type all-optical switch utilizing the carrier-induced nonlinearity was fabricated and its low switching power operation was demonstrated at 1.55mm wavelength. Extinction ratios at each output port of 2.3dB and 2.6dB were obtained with low switching light power of 5.9mW at the input fiber end.

(3) Multiple-quantum-well directional-coupler-type all-optical switches having various waveguide widths were fabricated using CH₄/H₂ RIE dry etching and FIB (Focused Ion Beam) techniques. Comparison between the theory and experimental results were carried out by Crosstalk measurement on the waveguide. The result shows that the permissible fabrication error of the waveguide should be less than ±160nm and also a high value of Crosstalk up to 29dB were experimentally confirmed.

Study of nanometer structure fabrication technology is important for the realization of quantum effect devices such as quantum-wire, or -box devices and ballistic electron devices based on wave characteristics of electrons.

Results obtained in this research are as follows:

(1) Sidewall recombination velocity of GaInAsP/InP wire structures fabricated by CH₄/H₂-ECR-RIBE was estimated from PL intensity dependence on the wire width. As the result, a reduction of surface damage was observed.

(2) In EB lithography, the size distribution of dense multiple wire patterns (period: 50 nm) formed on EB-resist (ZEP-520) was measured and its deviation was reduced by proximity effect correction.

(3) A 25-nm pitch GaInAs/InP buried structure was fabricated by electron beam lithography with Calixarene resist and two-step wet chemical etching. The fabrication of 20nm pitch would be possible by reduction of stage instability of system that is in progress.

(4) Si delta-doping by organometallic vapor phase epitaxy (OMVPE) onequidistant monolayer steps was studied towards the dopant-ordering in semiconductors. Growth condition conserving monolayer steps after the delta-doping was investigated in InP with respect to disilane flow and growth temperature. Transport anisotropy in the delta-doped layer is studied by measuring sheet resistance along parallel and perpendicular to the monolayer step.

(5) Si delta-doping by organometallic vapor phase epitaxy (OMVPE) onequidistant monolayer steps was studied towards the dopant-ordering in semiconductors. Growth condition conserving monolayer steps after the delta-doping was investigated in InP with respect to disilane flow and growth temperature. Transport anisotropy in the delta-doped layer is studied by measuring sheet resistance along parallel and perpendicular to the monolayer step.