Amplification Path Length Dependence Studies of Stimulated Emission from Optically Pumped InGaN/GaN Multiple Quantum Wel
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grown by Nichia Chemical Industries" we see the possibility that some of the varied results reported in the literature may stem from slightly different experimental conditions, which are shown here to result in significant changes in the SE behavior. We report the results of a detailed study of the SE behavior of these two SE peaks as a function of excitation length (Lexc) and excitation density (Iexc) and illustrate dramatically different SE behavior in InGaN MQWs for relatively small changes in the experimental conditions. The observation of these two distinct SE peaks from InGaN/GaN MQWs grown under different conditions by separate research groups suggests this SE behavior is a general property of present stateof-the-art InGaN based blue laser diodes. As such, a better understanding of the SE and lasing behavior of these structures is important for the development and optimization of future laser diode structures.
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Wavelength (nm) Figure 1: 10 K stimulated emission spectra (solid lines) from an InGaN/GaN MQW subjected to several 2 excitation densities, where I = 100 kW/cm . The low power PL (dashed line) and PLE (dotted line) spectra are also shown for comparison. The SE spectra have been normalized and displaced vertically for clarity.
EXPERIMENT The InGaN/GaN MQWs used in this study were grown by metalorganic chemical vapor deposition (MOCVD) on 1.8 [tm thick GaN buffer layers grown on (0001) oriented sapphire substrates. The active regions were made up of 12 quantum wells consisting of 3 nm thick Ino.2Ga 05.N wells and 4.5 nm thick GaN barriers. The structures were capped by 0.1 lam thick A10. 07 Ga0 .93 N layers. A detailed description of the growth conditions is given elsewhere.' 5 The InGaN MQWs were optically pumped by the third harmonic of an injection seeded Nd:YAG laser (355 nm, 30 Hz, - 6 ns pulse width). The excitation beam was focused to a line on the sample using a cylindrical lens and the excitation length was varied using a mask connected to a computer controlled stepper motor. The emission was collected from one edge of the sample, coupled into a 1-meter spectrometer, and spectrally analyzed using an optical multi-channel analyzer. RESULTS Typical power dependent emission spectra at 10 K are shown in Fig. I for Lexc = 1300 rim. At low Iex.,we observe a broad spontaneous emission peak centered at - 441 nm, consistent with low power cw photoluminescence (PL) spectra. As Lxc is increased, a new peak emerges at - 428 nm [designated here as SE peak (1)] and grows superlinearly with increasing 1.... If we continue to increase I ,. we observe another new peak at - 433 nm [designated here as SE peak (2)] which also grows superlinearly with increasing 1,,,. SE peak (1) is observed to be the statistical distribution of a multitude of narrow (< 0.1 nm) emission lines. No significant broadening of these emission lines was observed as the temperature was tuned from 10 K to over 500 K. This is illustra
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