Optical lasing from InGaN QWs
The laser structures for optical pumping were grown on HVPE bulk substrates with a dislocation density 106-107 cm~2. The structures consist of 0.5-pm Al0.08Ga0.92N bottom cladding followed by a 400-nm GaN waveguide. The active region located in the middle of the waveguide consists of one or three InKGai_KN QW with a thickness from 2.2 nm to 3.5 nm and 10 nm In0 06Ga0 94N barriers. The upper Al0 08Ga0 92N cladding have a thickness of 0.3 pm, and structures were capped by 5-nm GaN layer. The optical lasing was obtained from cleaved laser stripes with length L = 0.4-1.2 mm excited by third harmonics (355 nm) of a Nd:YAG laser beam with an aperture size of 0.25 mm; see inset to Fig. 2.22 for experimental configuration details. The laser pulses had a duration of 5 ns and a repetition rate of 20 Hz.
In Fig. 2.22 we compare optical spectra below and above the lasing threshold for two structures, with QWs optimized for an emission around 420 nm and 495 nm. We achieved optically pumped lasing from these structures at 409 nm and 472 nm, respectively. The structure which lased at 409 nm was grown with N flux 4.1 nm/min, and that lasing at 472 nm with higher N flux 6.5 nm/min, using otherwise nominally identical growth conditions. The lasing emission intensity as a function of the pumping power is shown in Fig. 2.23. It is clear that
Fig. 2.22. The photoluminescence and lasing spectra of two laser structures optimized for lasing at 409 nm and 472 nm.
Fig. 2.23. The light-power characteristics for structures lasing at 409 nm and 472 nm.
we are able to achieve lasing at 409 nm much earlier than at 472 nm. Although better optical efficiency of QW for the 409-nm laser cannot be ruled out, we attribute this effect primarily to the superior light confinement in the waveguide at a shorter wavelength (same design for both lasers).
The largest available active nitrogen flux for the nitrogen plasma source initially mounted on our MBE system was 6.5 nm/min, sufficient to grow the structure lasing at 472 nm. After upgrading the MBE system with a second RF-plasma source, the maximum nitrogen flux of 14 nm/min became available. This allowed us to further shift the PL emission towards the green region while maintaining a high-growth temperature of InGaN. In Fig. 2.24 we demonstrate spontaneous and stimulated emission from two SQW laser structures with quantum well widths of 2.2 nm and 3.5 nm, respectively. The growth conditions for both structures were nominally identical, and other than QW thickness the design was the same, with the active region consisting of 10 nm InGaN barriers containing 6% In and SQW with an In content of about 25%. As seen in Fig. 2.24, the maximum of the spontaneous emission is at a longer wavelength for the structure with wider QW: 545 nm vs. 525 nm. We attribute this shift to the stronger QCSE expected for the wider well. Note that for each laser structure we show stimulated emission from two laser bars of different length, and in both cases the stimulated emission from longer bar is red-shifted compared to that from the shorter bar. We were able to shift stimulated emission from 487 nm to 494 nm for the laser structure with the well width d = 2.2 nm, and from 488 nm to 501 nm for the one with the well width d = 3.5 nm, by increasing the laser bar length from 0.7 nm to 1.2 nm for both cases. This effect is very general. Indeed, for all the investigated laser structures we have found that lasing at longer wavelengths can be realized with a longer resonator length L.
Fig. 2.24. The photoluminescence and lasing spectra around 500 nm for two SQW structures with thickness d = 2.2 nm (a) and d = 3.5 nm (b), respectively.