Reduction of Threshold Current Density in UV-C LDs Fabricated on AlN Substrates

Laser diodes (LDs), which operate at emission wavelengths around 270 nm, are attracting attention for healthcare applications such as virus inactivation and sterilization because DNA absorbs this wavelength range. Deep UV LDs are also expected to be used in various applications, including industrial applications such as detection of fine particles, high-precision distance sensors, and light sources for semiconductor lithography equipment [1]. In LDs using AlGaN materials that can realize these wavelength ranges, room-temperature pulsed lasing in the UV-C [2,3] and UV-B [4] bands has been achieved by improving the quality of AlGaN crystals, increasing the hole concentration in AlGaN with the high AlN mole fraction by using distributed polarization doping (DPD) [5] technology, and achieving low optical loss. However, the threshold current density is still higher than that of other nitride-based LDs, and further improvement is needed to realize continuous operation LDs.<br/>For low-threshold LDs, it is important to design an appropriate stacking structure and to improve the LD crystal quality. In particular, for the latter, crystal defects such as dislocations and point defects cause low emission efficiency in the active layer and high internal loss due to absorption and scattering of light, so it is key to minimize the introduction of defects as much as possible. In this paper, we report on the observation and suppression of non-uniform emission patterns in UV-C LDs fabricated on AlN substrates by electroluminescence (EL).<br/>The LDs that we had fabricated in the initial stage contained linear dark lines in the EL emission pattern observed from the back side. To investigate the cause of this, cathodoluminescence (CL) observation was performed on the area where the LD fabrication process was carried out in the same way without electrodes. As a result, in the CL images, linear dark lines similar to those in the EL image were observed near the mesa structure fabricated for n-electrode contact formation. This dark line was not observed at all in the region away from the mesa. Based on these results, we fabricated LDs with a new electrode design. The threshold current density of re-designed LD was about 12 kA/cm²[6]. From the above results, we have succeeded in significantly reducing the threshold current density from 25 kA/cm²[2] at the time of the first report.<br/><b>Acknowledgments</b><br/>The authors would like to thank Mr. Kazuhiro Nagase, Dr. Masato Toita, and Dr. Naohiro Kuze of Asahi Kasei Corporation for their invaluable discussion and considerable support. This work was supported by KAKENHI (21H04560).<br/><b>References</b><br/>1) M. Kneissl, T. Y. Seong, J. Han, and H. Amano, Nat. Photonics <b>13, </b>233 (2019).<br/>2) Z. Zhang, M. Kushimoto, T. Sakai, N. Sugiyama, L. J. Schowalter, C. Sasaoka, and H. Amano, Appl. Phys. Express <b>12</b>, 124003 (2019).<br/>3) T. Sakai, M. Kushimoto, Z. Zhang, N. Sugiyama, L. J. Schowalter, Y. Honda, C. Sasaoka, and H. Amano, Appl. Phys. Lett. <b>116</b>, 122101 (2020).<br/>4) K. Sato, S. Yasue, K. Yamada, S.Tanaka, T. Omori, S.Ishizuka, S.Teramura, Y. Ogino, S. Iwayama, H. Miyake, M. Iwaya, T. Takeuchi, S. Kamiyama and I. Akasaki, Appl. Phys. Express <b>13</b>, 031004 (2020).<br/>5) J. Simon, V. Protasenko, C. Lian, H. Xing, and D. Jena: Science <b>327</b>, 60 (2010).<br/>6) M. Kushimoto, Z. Zhang, Y. Honda, L. J. Schowalter, C. Sasaoka, and H. Amano: Appl. Phys. Express <b>14</b>, 051003 (2021).