InGaN multiple quantum well (MQW) laser diodes were fabricated on (0001)Si oriented 6H–SiC substrate using low-pressure metal organic vapor phase epitaxy (LP-MOVPE). The laser oscillation was observed above the threshold current of 800 mA at a peak wavelength of 414.3 nm under pulsed current injection at room-temperature. The pulse duration was 300 ns and the repetition frequency was 1 kHz. The threshold current density and differential efficiency were estimated to be 16 kA/cm2 and 0.03 W/A, respectively. The full width at half maximum (FWHM) of the lasing emission lines was between 0.03 nm and 0.21 nm. Streak-shaped far field patterns were clearly observable. The lifetime of the laser diode was more than 5 hours.
We demonstrated for the first time room-temperature continuous wave operation of
InGaN laser diodes with a vertical conducting structure fabricated on a SiC substrate.
The threshold current and voltage were 84 mA and 12 V, respectiely, under pulsed
operation. The threshold current corresponds to a threshold current density of 5.6 kA/cm2,
which is the lowest ever reported with InGaN laser diodes on a SiC substrate.
Under continuous wave operation, the threshold current and voltage were 115 mA and
10.5 V, respectiely. The peak lasing wavelength was 408.2 nm. Longitudinal modes
of the optical cavity were clearly obsered. The laser oscillation was obsered up to
40°C under CW operation.
A simulation of current flow for an InGaN/GaN/AlGaN multiquantum well (MQW) laser showed that generation of the optical gain is inhomogeneous across the MQW due to inhomogeneous carrier injection. Laser diodes with three and five wells in MQW were compared to experimentally investigate inhomogeneous carrier injection. We found that external quantum efficiency was significantly improved by a reduction in the number of wells from five to three. The result was explained quantitatively by the fact that the five-well laser had a larger internal loss and a poorer internal quantum efficiency due to the inhomogeneous carrier injection.
Current flow through an InGaN/GaN/AlGaN multi-quantum well (MQW) laser diode is simulated. We found that electron overflow to the AlGaN p-cladding layer is very large, which prevents the current injection into the MQW layers. We clarified that the electron overflow occurs easily in nitride lasers because of three intrinsic reasons; poor hole injection due to the small hole mobility and thermal velocity, the small conduction band offset for InGaN/GaN, and the high threshold carrier density. We show that the Al composition and the p-doping of the AlGaN p-cladding layer is of critical importance to obtain laser oscillation by current injection.
We demonstrated continuous wave operation at room temperature of InGaN laser
diodes fabricated on 4H-SiC substrates. The threshold current and voltage were 60 mA
and 8.3 V, respectively. The threshold current corresponds to a threshold current
density of 4 kA/cm2. No difference in the threshold current or voltage was observed
between 4H-SiC and 6H-SiC substrates. The laser oscillation was observed up to 80°C
for CW operation. The peak lasing wavelength was 404.4 nm. The lifetime was 57 hours
under automatic power controlled conditions with a constant output power of 1 mW at 25°C.
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