研究実績 1998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) 1.5mm-wavelength GaInAsP/InP quantum-wire lasers with the wire width of 20nm in 50nm period and 25nm in 70nm period were fabricated by electron-beam lithography, wet-chemical etching and 2-step organometallic vapor phase epitaxial growth. Temperature dependencies of various characteristics of these lasers were measured and compared with those of quantum-film lasers fabricated on the same wafer. As a result, better lasing properties of the quantum-wire laser over the quantum-film laser, i.e., lower threshold current and higher differential quantum efficiency operation, were confirmed for the first time at temperatures below 200K. An internal quantum efficiency of the quantum-wire laser was evaluated to be almost 100% up to 200K from the cavity length dependence of differential quantum efficiency. The possibility of a complex-coupled distributed feedback laser consisting of quantum-wire active region as the grating structure was also confirmed.

(2) Anisotropic polarization properties of photoluminescence intensity from GaInAsP/InP quantum-wire structures due to the lateral quantum confinement effect were observed. Furthermore, gain spectra of quantum-wire lasers as well as quantum-film lasers were measured. As the result, narrower material gain spectrum of quantum-wire lasers was observed at T = 100K.

(3) Synchrotron x-ray diffractometry has been used to investigate GaInAsP quantum-wire structures on InP with a quantum-well layer between the substrate and the wire. The lateral periodicity was determined with high accuracy. An elastic stress relaxation, which occurs near the free surface of the sidewalls, was observed. It results in deformation gradients in the wires, which influence the distribution of the diffracted intensity in reciprocal space.

(4) 1.5mm-wavelength GaInAsP/InP multiple-quantum-wire (2-layers) lasers with the wire width of 20nm in 100nm period were fabricated by using CH₄/H₂ RIE dry etching and wet-chemical cleaning process. Fundamental lasing characteristics of these lasers were almost the same as those fabricated by wet-chemical etching process.

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 Multiple-Micro-Cavity (MMC) lasers and Distributed Feedback (DFB) lasers with corrugated active region, have been studied both theoretically and experimentally.

Results obtained in this research are as follows:

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

(2) A room temperature operation of an MMC laser having a groove width of 3l/4 (=0.70mm, filled with BCB) was fabricated using CH₄/H₂-RIE process. The threshold current was measured to be as low as 18mA for a total cavity length of L = 200mm (pitch: L=20mm; 10 elements) for a stripe width of W_s = 5mm, and effective reflectivity was estimated to be 94%.

(3) A narrow vertical groove with high aspect ratio was fabricated using Electron Beam (EB) lithography and CH₄/H₂-RIE followed by O₂ ashing. The groove width L_L and the facet angle were measured to be 147nm and 0.3^o, respectively. The groove depth was 2.6mm and an aspect ratio reached to 17.7. The roughness of the etched facet was measured using a field emission electron probe surface roughness analyzer and found to be same as cleaved.

(4) Low temperature operation of l/4-groove (filled with BCB) MMC laser was achieved. For a temperature range of 100K to 150K, the threshold current as low as 10mA to 16mA (L = 200mm, pitch:L= 20mm, = 0.23mm, and W_s = 5mm) was obtained. A stable single-mode operation was confirmed for a wide temperature range (100K to 200K) with the temperature coefficient of 0.06nm/K.

(5) Fairly low threshold current density operation (330A/cm²) of 1.55mm wavelength GaInAsP/InP MQW-DFB (5-quantum-well) lasers with rectangular-shaped periodic active regions was achieved by CH₄/H₂ RIE and OMVPE regrowth process.

Semiconductor directional-coupler type optical switching device was investigated both theoretically and experimentally.

Results obtained in this research is as follows:

(1) Multiple-quantum-well directional-coupler-type all optical switches were fabricated by CH₄/H₂ RIE and FIB techniques. From the measurement of cross-talk characteristics of devices with various waveguide width, a permissible fabrication error of the width was obtained to be less than 160nm. A cross-talk up to 29dB was obtained with this device.

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 device based on wave characteristics of electrons.

Results obtained in this research are as follows:

(1) A 25 nm pitch InP pattern formed by using calixarene was buried in a GaInAs structure by organometallic vapor phase epitaxy (OMVPE). The introduction of tertiarybutylphosphine as the phosphorus source prevented from deforming the fine structure when the temperature was raised and a 25 nm pitch periodic structure was buried successfully.

(2) GaInAsP/InP wire structures was fabricated by CH₄/H₂-reactive ion etching and OMVPE regrowth. A product of sidewall recombination velocity and carrier lifetime was estimated from photoluminescence intensity dependence on the wire width. Low damage nano-structure fabrication was realized.

(3) The nonradiative recombination velocity at the sidewall of GaInAsP/InP quantum-well lasers with narrow wire-like active region, which were fabricated by wet etching and OMVPE regrowth, was estimated from the active region width dependence of spontaneous emission efficiency.

(4) To fabricate fine refractory metal structure, metal-stencil liftoff , in which gold/chromium and SiO₂ replace conventional resist to prevent thermal deformation in liftoff process and 20 nm width tungsten wire was fabricated by proposed metal-stencil liftoff.

(5) For buried OMVPE growth of tungsten wires, growth conditions of OMVPE were studied. As a result, tungsten stripes with 1 mm width could be buried by 1 mm thick InP layer without void.

(6) To fabricate fine periodic electrode by lift-off, new double-layer resist consisting of ZEP-520 incorporating C60 and PMMA was developed and Au/Cr electrode with 80 nm width and 40 nm height was formed on InP substrate.