Materials Characterization on Optically Pumped InGaN/GaN Lasers by Farfield Measurements and Fourier Analysis of the Emi
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Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
processing which is necessary to fabricate an electrically pumped device such as a light emitting diode or a laser diode nor the effort to prepare a TEM sample. Nevertheless, it reveals some very important features of the quality of the optical waveguide and is therefore expected to become a suitable diagnostic tool for improving device performance. EXPERIMENTAL For these experiments, we used C-face sapphire (A120 3) wafers as substrate material. Growth was performed in an OMVPE system. On the sapphire, we grew 4 gm of GaN, followed by a 500 nm thick Al0 .oGao. 9N lower cladding layer, a 240 nm thick GaN/InGaN waveguide, and a 50 nm thick Al 0.1Ga6 .gN upper cladding layer. Because of the relatively low carrier diffusion length in nitride films, the thin upper cladding layer was required for optical pumping of the MQWs. The active region consisted of five 25 A wide Ino.j Ga 0.85N QWs separated by 70 A thick ln0.0 sGa0 .95N barrier layers. This MQW stack was sandwiched between two 100 nm thick GaN waveguide layers. The processing of the 2 mm long laser bars was straightforward and included sawing of 2.2 mm long and 10 mm wide pieces, subsequent polishing of both facets, and high reflection (HR) coating of one facet. For this purpose. we used 5 pairs of A14 thick Si0 2/TiO 2 layers with a final reflectance of more than 90 %. Since the optical output fiom the HR-coated facet was only 5 % compared to that of the uncoated one, we used the latter for the succeeding measurements. Optical pumping was carried out using a pulsed 337 nm N2 laser (rvl,, = 5 Hz, Ppeak = 250 kW, Wput. = 75 p J) whose light was focused to a 100 pm wide and approximately 4 mm long stripe using both a spherical and a cylindrical lens. In order to attenuate the pump intensity, we inserted an appropriately chosen number of 1 mm thick glass slides into the pump 0 4 beam; these glass plates acted as 120 neutral density filters for the UV 0,8 emission of the nitrogen laser (optical Lt:l•f ••---rwt ---- op -.
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[a.u.] Pump intensity Fig. 1 - Laser output intensity and linewidth versus pump intensity for a 2 mm long bar pumped with a pumped 37nmnitrogenmmlongar, pfor pulsed 337 nm nitrogen laser.
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density - 0.05 per slide). The output intensity of the semiconductor laser was collected by an optical multimode fiber and fed into a grating spectrometer (A = 1200 lines/mm, d 10,, = 0. 1 m). The latter allowed the simultaneous measurement of the laser spectrum and output intensity. Although the spectral resolution of the spectrometer was only around I nm and therefore not sufficient to see single Fabry-Pmrot modes in the laser spectrum, we used this spectrometer the determination of the linewidth of the blue laser emission.
The measurement of the far field was accomplished by placing a CCD camera without objective lens in firont of the laser facet. The distance between laser and camera was chosen to be well outside the Rayleigh range for our 5 p
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