Results 2000

GaInAsP/InP strained-quantum-film, -wire, and -box lasers have been studied both theoretically and experimentally. Distributed feedback (DFB) lasers consisting of narrow wirelike active regions fabricated by the same fabrication process as Quantum-Wire lasers have been also studied.Results obtained in this research are as follows:

(1) 1.5mm GaInAsP/InP lasers with narrow wirelike (43nm and 70nm) active regions, which consist of strain-compensated five-quantum-well structure, were realized for the first time by using EB lithography, CH₄/H₂ reactive-ion-etching (RIE), and organo-metallic-vapor-phase-epitaxy (OMVPE) embedding growth. As the result, lower threshold current density (318A/cm²) than that of planar 5MQW lasers (550A/cm²) prepared on the same wafer was obtained at room temperature. Moreover, lower threshold current density and higher differential quantum efficiency than those of quantum-film lasers were obtained up to 85℃. The occurrence of non-radiative recombination traps at the etched/regrown interfaces was successfully suppressed.

(2) In order to evaluate the quality of etched/regrown interfaces of above mentioned GaInAsP/InP wirelike lasers, the temperature dependence of the spontaneous emission efficiency of samples with wire widths of 43nm and 70nm was compared to that of quantum-well lasers. As the result, the product of the surface recombination velocity and the carrier lifetime S×t at the etched/regrown interfaces was evaluated to be less than 2nm at room temperature. No degradation in the spontaneous emission efficiency was observed in the temperature range between 25℃ and 85℃ for both samples. Therefore, these results indicate that high quality etched/regrown interfaces can be obtained with GaInAsP/InP fine structures.

(3) By using the same fabrication method mentioned above, low threshold current density 1.5mm wavelength DFB lasers with deeply etched wirelike active regions have been demonstrated. A high index-coupling coefficient of 360cm^-1 was obtained and a record low threshold current density J_th= 94 A/cm² operation was achieved with DFB lasers consisting of double layered wirelike active regions.

(4) A submilliampare operation of 1.55mm GaInAsP/InP BH-DFB lasers with deeply etched wire-like active regions was successfully obtained. Threshold current as low as I_th = 0.7mA (J_th= 150A/cm²) and an external differential quantum efficiency of h_d=23 %/facet were obtained with a stable single-mode operation (SMSR = 36 dB @I=2.6I_th) for the cavity length of 200mm and the stripe width of 2.3mm under a RT-CW condition. Concerning the reliability of these lasers, for the 240-mm-long BH-DFB laser having a threshold current of I_th= 0.85mA, a RT-CW test is being done without bonding on a heatsink (just pin-clipped). No degradation is observed after 800 hours at an output power of 1mW (@I = 8.75mA, = 10.3I_th).

(5) Single mode operation characteristics of DFB lasers with deeply etched wirelike active regions have been studied both theoretically and experimentally. Experimentally, lasing modes for all measured samples were observed at a longer-wavelength-side of the stop-band of the grating. It was theoretically explained in terms of so called “a gain matching effect,” where the standing wave profile of longer-wavelength-side modes match gain regions while those of shorter-wavelength-side modes take peaks between gain regions.

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, Deeply Etched Distributed Bragg Reflector (DBR) lasers, Coupled Cavity (CC) lasers, Distributed Reflector (DR) lasers, and Membrane lasers have been studied both theoretically and experimentally.Results obtained in this research are as follows:

(1) MMC lasers consisting of narrow and deep grooves were analyzed by an improved perturbation feedback theory and transfer matrix method. It was shown that the attainable effective reflectivity is limited by the diffraction loss in the grooves. By filling the grooves with the polymer Benzocyclobutene (BCB), the diffraction loss can be reduced. An increasing number of periods will lead to a larger reflectivity. For 8 elements of l/(4nL)-long BCB filled grooves the reflectivity is estimated to be 98% and a threshold current can be reduced to 1mA for a microcavity length of 4.9mm and stripe width of 1mm.

(2) Highly uniform 1.55mm wavelength lasers with high reflective deeply etched semiconductor/BCB DBR structures were realized. Low threshold current of 7.2mA and high differential quantum efficiency of 50% from the front facet were demonstrated (L = 160mm, W_S = 5mm) with rather high yield. The reliability of such polymer-buried DBR lasers was tested for the first time and a lifetime over 5,000 hours at CW condition and constant output power of 2.5mW was achieved. Double sided-DBR laser having 15-DBRs on the rear and 5-DBRs on the front side showed a threshold current as low as 5.0mA (L = 160mm, W_s= 5mm).

(3) A new type of a single-mode laser consisting of a deeply etched DBR and several small cavities was fabricated and analyzed. Single-mode operation was achieved for different number of cavities. However, increasing the number of cavities will decrease the quantum efficiency drastically. By analysis with the transfer matrix method and by experiment it was found that the CC laser with only two cavities is the best according to high efficiency and low threshold current. A threshold current under room temperature CW condition as low as 11mA (L₁=150mm and L₂=40mm) with a submode-suppression-ratio (SMSR) of 36 dB (@I = 1.8I_th) was achieved for a 5-mm-wide stripe laser.

(4) A new type of DR laser consisting of high reflective DBR and a laser cavity with vertical grating at the sidewalls was proposed and demonstrated. Single-mode operation (SMSR = 35 dB) with high differential quantum efficiency (h_d= 42% from the front facet) was achieved.

(5) In order to realize high performance semiconductor lasers, we proposed membrane DFB lasers with wirelike active regions. For the cladding layer, the polymer Benzocyclobutene (BCB) was used to achieve a high optical confinement. Our model calculation reveals that threshold current as low as 10mA for a 50mm-long and 1mm-wide device can be expected.

(6) Membrane DFB laser strucutres were fabricated. Photoluminescence (PL) spectra successfully showed the stop-band due to DFB structure. The stop-band width was measured to be about 30 nm, and the index-coupling coefficient k_i was estimated to be 965cm^-1. The equivalent refractive index of the membrane waveguide was estimated to be 2.96, which is approximately 10% lower than that of conventional laser structure.

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) Buried growth of a GaAs layer over a tungsten stripe by organometallic vapor phase epitaxy was studied. Triethylgallium (TEG) was compared with trimethylgallium (TMG) from the viewpoint of migration length. A 70-nm-wide tungsten stripe was buried by a 0.77-mm-thick layer of GaAs with a flat surface using TMG.

(2) To realize ohmic contact to n-GaAs by very shallow doped layer, insertion of ultrathin Ga_0.5In_0.5As layer was evaluated theoretically and experimentally. Theoretical current-voltage characteristics by using self-consistent potential calculation and field emission current by WKB method shows 10^-6Wcm² as contact resistivity by 10-nm-thick GaInAs layer with 2×10¹⁹cm^-3 as carrier concentration and 10-nm-thick GaAs layer with 8×10¹⁸cm^-3 as carrier concentration. TLM measurement of fabricated structure by OMVPE shows 1.7×10^-5Wcm² as contact resistivity.