The 40th Electronic Materials Symposium

Online (2021)

Waveguide loss measurements in III-nitride laser structures

Kenta Ogasawara¹, Shigeta Sakai²,Tadashi Okumura², Koichi Naniwae² and Atsushi A. Yamaguchi¹

¹Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa, 921-8501, Japan

²Ushio Inc., 1-90 Komakado, Gotemba, Shizuoka, 412-0038, Japan

　Various types of III-nitride laser diodes (LD) have been developed recent decades, and some of them are now commercially available. InGaN alloy materials are used for active layers in visible-light emitting LDs, and AlGaN alloy materials are usually used for UV-light emitting LDs. It has been pointed out that optical properties of InGaN and AlGaN are peculiar compared with  other  III-V  compound  semiconductor  alloy  materials  such  as AlGaAs, AlInGaP,  and InGaAsP. Especially, it is well known that InGaN quantum well (QW) active layers have very large alloy compositional fluctuation and that density of states (DOS) has a tail with large localization energy. Optical gain characteristics are largely affected by these effects. It has been reported  that  the  gain  peak  is  broaden  and  differential  gain  decreased  by  the  fluctuation. However, there is onlya few reports on waveguide loss spectrum in such LD structures.

　In this study, we have measured waveguide loss spectrum for an InGaN-QW LD structure. The  loss  spectrum  for  a  conventional  AlInGaP  red-light-emitting  LD  structure  was  also measured for comparison. Figure 1 shows a schematic diagram to explain the principle of the waveguide loss measurement [1, 2]. When excitation light is focused on the sample surface by object  lens  at  the position  where  the  distance  from  the  sample  edge  is R,  emission  light propagates in the waveguide by distance R and emits from the edge. Since a part of the light is absorbed  in  the  waveguide,  the  edge  emission  intensity  and  spectrum  will  depend  on  the distance R.

where I is the light intensity of edge emission, and α is the absorption (or loss) coefficient of the waveguide. In this measurement, edge emission intensity & spectrum were measured as functions of distance R, and absorption (loss) coefficient was calculated by Eq. (1) for each wavelength in the QW emission peak.

Figures 2 and 3 show measured waveguide loss spectra (red circles) for a red-light-emitting AlInGaP LD and an InGaN-QW LD. The photoluminescence (PL) spectra from sample surface (green solid line) and sample edge [R = 200 μm] (violet broken line) are also potted.It is observed that PL peakposition difference between the surface and edge emissions is very large (~ 200 meV) in the InGaN LD sample while there is almost no difference in the AlInGaP LD sample. In addition, the absorption coefficient gradually rises from an energy much lower than the PL peak energy of surface emission in the InGaN LD sample while the coefficient sharply rises at nearly PL peak energy in the AlInGaP LD sample. These results show that the InGaN QW has a large DOS tailing while the AlInGaP QW has a step-like DOS. 

 

References

[1]P. C. Mogensen, P. M. Smowton, and P. Blood, "Measurement of optical mode loss in visible emitting lasers",Appl. Phys. Lett.71,(1997)1975.
[2]Dmitry S. Sizov, Rajaram Bhat, Albert Heberle, Kechang Song, and Chung-en Zah, "Internal Optical Waveguide Loss and p-Type Absorption in Blue and Green InGaN Quantum Well Laser Diodes", Appl. Phys. Express 3(2010) 122104.