Takeshi Kitatani

GaInNAs: A Novel Material for Long-Wavelength-Range Laser Diodes with Excellent High-Temperature Performance.

We propose a novel material, GaInNAs, that can be formed on GaAs to drastically improve the temperature characteristics (T0) in long-wavelength-range laser diodes. The feasibility of our proposal is demonstrated experimentally.

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 /spl mu/m and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T/sub 0/=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 /spl mu/m.

1.3-μm continuous-wave lasing operation in GaInNAs quantum-well lasers

A 1.3-/spl mu/m continuous wave lasing operation is demonstrated, for the first time, in a GaInNAs quantum-well laser at room temperature. This lasing performance is achieved by increasing the nitrogen content (up to 1%) in GaInNAs quantum layer. It is thus confirmed that this type of laser is suitable for use as a light source for optical fiber communications.

GaInNAs-GaAs long-wavelength vertical-cavity surface-emitting laser diodes

Vertical-cavity surface-emitting laser diodes with GaInNAs-GaAs quantum-well (QW) active layers are demonstrated for the first time. GaInNAs permits the realization of a long-wavelength vertical-cavity laser grown directly on a GaAs substrate. Room-temperature (RT) pulsed operation is achieved, with an active wavelength near 1.18 /spl mu/m, threshold current density of 3.1 kA/cm/sup 2/, slope efficiency of /spl sim/0.04 W/A, and output power above 5 mW for 45-/spl mu/m-diameter devices. Laser oscillation is observed for temperatures at high as 95/spl deg/C.

Room-Temperature Pulsed Operation of GaInNAs Laser Diodes with Excellent High-Temperature Performance

We have successfully operated GaInNAs laser diodes with a pulsed current at room temperature. The lowest threshold current density was about 0.8 kA/cm2, and the lasing wavelength was about 1.2 µ m. Characteristic parameters such as internal quantum efficiency and the gain constant were measured, and excellent high-temperature performance was observed. The characteristic temperature was 127 K in the temperature range from 25 to 85° C.

Extremely large N content (up to 10%) in GaNAs grown by gas-source molecular beam epitaxy

Abstract GaNAs GaP strained single quantum wells are fabricated on GaP wafers by gas-source molecular beam epitaxy in which a nitrogen radical is used as the nitrogen source. The structure and luminescence properties of the quantum wells are investigated by transmission electron microscopy and photoluminescence measurements. The N content in the GaNAs quantum wells was estimated to be about 10%, which is about one order of magnitude larger than previously reported and more than sufficient for fabricating laser diodes on a Si wafer.

Mechanism analysis of improved GaInNAs optical properties through thermal annealing

We investigated the mechanisms of improved GaInNAs optical properties by thermal annealing. The absorption spectra measured for the bulk layer indicated that the large shift in the PL wavelength was probably caused by a bandgap shift in the GaInNAs itself. The cathodeluminescence measurements revealed that the enhancement of the PL intensity was generated by uniform emission from the entire region; in comparison, nonuniform dot-like regions exist in an as-grown GaInNAs layer. These analyses, which is peculiar to this type of material system, should be helpful for further improving the crystal quality, thus helping to enable semiconductor lasers with excellent high-temperature performance.

Gas-source MBE of GaInNAs for long-wavelength laser diodes

We have grown GaInNAs using gas source molecular beam epitaxy (GS-MBE) in which a nitrogen radical was used as the nitrogen source. This growth method produces more reactive-N and fewer reactions between the sources. Excellent crystalline quality and luminescence properties of the GaInNAs/GaAs quantum well were confirmed by transmission electron microscopic observation and photoluminescence measurement. We have also succeeded in applying GaInNAs to long-wavelength laser diodes that lased under room temperature continuous wave operation in the 1.3 μm wavelength range suitable for optical fiber communications. Thus, we have experimentally demonstrated that GS-MBE is a suitable method to grow GaInNAs with sufficient quality for device applications.

40-Gb/s Direct Modulation With High Extinction Ratio Operation of 1.3-

Direct modulation at 40 Gb/s of a 1.3-mum InGaAlAs distributed feedback ridge waveguide laser is experimentally demonstrated. By combination of the high differential gain of an InGaAlAs multiquantum well active layer, a short cavity length of 100 mum, and a low-resistance notch-free grating, it achieves high bandwidth of 29 GHz and high-extinction ratio of 5 dB at 40-Gb/s modulation. Moreover, the laser operates at a record maximum ambient temperature of 60degC under 40-Gb/s directly modulation. It also achieves 40-Gb/s modulated transmission over 2 km with a low power penalty of 0.25 dB at 25degC .

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We have obtained a high characteristic temperature (T0) of 215 K from a 1.3 µm GaInNAs/GaAs single-quantum-well laser under pulsed operation at 20°C to 80°C. To our knowledge, this T0 is the highest yet reported for 1.3 µm band edge emitters suitable for optical-fiber communication systems. The use of GaInNAs as an active layer is, therefore, very promising for the fabrication of long-wavelength laser diodes with excellent high-temperature performance.