Nitride based laser diodes utilize as an active medium extremely strained InGaN quantum wells. As the nitride materials are piezoelectric in their nature, this strain is reflected in a strong piezoelectric field. This tilts the energy bands and shifts the emission spectrum position through the Quantum Confined Stark Effect (QCSE). As the laser diode is operated at elevated currents, the built-in electric field is reduced due to the screening by injected carriers. This leads to the increase of emission energy (blue-shift). It has been a subject of many discussions whether the field is preserved at lasing, or is it completely screened.
In this work we compare the emission wavelength shift of nitride laser diodes and superluminescent diodes having different QW compositions. The superluminescent diodes allow us to study the emission spectrum at higher carrier densities than for laser diodes. In laser structures, we clearly see the saturation of the blue-shift at threshold current. While, on the other hand, we see the continuous shift of the emission wavelength in case of superluminescent diodes. This suggests, that the piezoelectric fields are not fully screened at threshold current. We also see that for UV laser diodes the emission line shift is much smaller than for blue wavelength devices. This implies practically complete screening of the electric field for UV laser while the lasing of blue laser diodes occurs at high electric field conditions.
Anna Kafar, Szymon Stanczyk, Katarzyna Pieniak, Tadek Suski, and Piotr Perlin, "Carrier screening of built-in electric fields in nitride laser diodes and superluminescent diodes (Conference Presentation)," Proc. SPIE 10532, Gallium Nitride Materials and Devices XIII, 1053215 (Presented at SPIE OPTO: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291532.5751529784001.
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