Diffusion-Related Lifetime of Photoexcited Carriers in Ultrapure Diamond

As one of the emerging ultra-wide-bandgap semiconductors, diamond has superior properties for applications to radiation detectors, single-photon sources, deep ultraviolet light-emitting diodes, and transistors for power electronics [1]. We have been investigating the impacts of impurity doping on the carrier recombination dynamics in various types of synthetic diamonds [2,3]. In the purest chemical-vapor-deposition diamond available today (Element Six), we found that the carrier dynamics are not affected by unintentionally incorporated dopants [2] but governed by recombination at dislocations after diffusion [4,5]. Meanwhile, when charge carriers are photoexcited in diamond, some are bound by the Coulomb force to form excitons and give a significant impact on the transport and optical properties of charge carriers [5,6].<br/><br/>We measured the temperature dependence of the exciton lifetime based on time-resolved photoluminescence from the purest diamond samples with no effective impurity traps. A clear inverse correlation was found between the exciton lifetime and the measured diffusion coefficient. We propose that the exciton lifetime is governed by nonradiative recombination after arrival at dislocations due to diffusion as the coexistent gas of excitons and charge carriers. The extracted dislocation distances (typically 0.1 mm) in several diamond samples were consistent with the magnitude of the small local strain (~10^-5) evaluated by birefringence. Our systematic measurements enabled the extraction of the radiative recombination time of excitons in diamond to be t_rad=1.5 µs, slightly shorter than the time t_disl for dislocation recombination at 80 K. The internal quantum efficiency of excitonic emission in diamond was obtained as η=1/(1+ t_rad/t_disl), reaching approximately 60 % under some best conditions.<br/><br/>The temperature dependence of the charge-carrier lifetime, determined by the time-resolved cyclotron resonance method, was also successfully modeled by the diffusion-related lifetime [5]. Combined with the carrier mobility obtained from the inverse width of the cyclotron resonance, we could extract the mobility-lifetime product of electrons down to a cryogenic temperature of 2 K, at which the conventional Hecht analysis is difficult. We have found an inverse temperature dependence for the mobility-lifetime product, as expected when dislocation recombination is dominant. The highest recorded mobility-lifetime product was 0.2 cm²/V at 2 K and approximately 100 times the room-temperature value.<br/>The proposed model for the diffusion-related lifetime is considered to be generally applicable to other materials when the impurity concentrations are controlled to be sufficiently low. We believe that our deep understanding of the fundamental properties of photoexcited carriers in diamond is beneficial as the basis for versatile applications of ultra-wide-bandgap materials.<br/><b>Acknowledgments</b><br/>Element Six Ltd. is gratefully acknowledged for providing samples. This work was partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grants No. 17H02910 and No. 19K21849, JSPS bilateral Project No. 120209919, and Swedish Research Council Grant No. 2018-04154.<br/><b>References</b><br/>[1] N. Donato, N. Rouger, J. Pernot, G. Longobardi, and F. Udrea, J. Phys. D: Appl. Phys. <b>53</b>, 093001, (2020).<br/>[2] Takaaki Shimomura, Yoshiki Kubo, Julien Barjon, Norio Tokuda, Ikuko Akimoto, and Nobuko Naka, Phys. Rev. Mater. <b>2</b>, 094601 (2018).<br/>[3] Yoshiki Kubo, Solange Temgoua, Riadh Issaoui, Julien Barjon, and Nobuko Naka, Appl. Phys. Lett.<b> 114</b>, 132104 (2019).<br/>[4] K. Konishi, I. Akimoto, J. Isberg, and N. Naka, Phys. Rev. B<b> 102</b>, 195204 (2020).<br/>[5] K. Konishi, I. Akimoto, H. Matsuoka, V. Djurberg, S. Majdi, J. Isberg, and N. Naka, Appl. Phys. Lett. <b>117</b>, 212102 (2020).<br/>[6] T. Ichii, Y. Hazama, N. Naka, and K. Tanaka, Appl. Phys. Lett. <b>116</b>, 231102 (2020).<br/>[7] Kazuki Konishi and Nobuko Naka, Phys. Rev. B <b>104</b>, 125204 (2021).