Improved Nitrogen-Doped Diamond Photoconductivity by Electrode Design

In photoconductive semiconductor switches (PCSS), the conductance between two terminals on a semiconductor is modulated by the absorption of optical radiation in the gap between the electrodes. This switching device can be applied to pulsed power technology for high speed and jitter free switching. These may include radar, particle acceleration, and pulsed high-power lasers. More recently, as we move towards electrification of the grid, trigger isolation enabled by PCSSs may enable safer mechanisms for making and breaking high power circuits as well. However, typical materials studied for photoconductive switching, such as Si and GaAs suffer from thermal runaway effects at high power [1]. Since diamond is an ideal material for high power electronics due to its large bandgap (5.47 eV), high breakdown electric field (10 MV/cm), high carrier mobility, high thermal conductivity (10-20 W/cm.K), and low coefficient of thermal expansion (~1.1um/mK) [2], we chose to study its photoconductive properties and potential as a high power optoelectronic switch.<br/>Past studies have shown photoconductivity in intrinsic diamond by UV-band excimers in high electric fields up to 2 MV/cm [3]. In our devices, however, we utilize nitrogen doped substrates to excite carriers by sub-bandgap photon energies while maintaining the desirable properties of diamond that would facilitate higher breakdown field and better thermal management [4]. In this case, a lower energy, a below bandgap Vis-NIR excitation source is sufficient, which increases the feasibility of PCSS integration. In designing extrinsic PCSS, on/off current ratios should be maximized for high performance switching. In the off state, the material conductance primarily depends on the number of ionized dopants, which is very low in the case of nitrogen doped diamond due to the high activation energy. In the on state, the conduction is increased due to the photogeneration of free carriers through ionization of dopants and traps. In this work, we aim to increase the on-state conductivity by using different metal electrode stacks on both lateral and vertical PCSS configurations. In the vertical configuration, we also aim to demonstrate the higher power capabilities of an extrinsic diamond PCSS.<br/>Testing of the extrinsic diamond PCSS was performed by optically exciting with a frequency doubled Nd:YAG pulsed laser (532 nm) with an output energy of &lt;1 mJ and a full width half maximum (FWHM) of 10 ns. The nitrogen diamond HPHT substrates from Element Six had an average doping concentration on the order of 10¹⁹. Our experiments have shown pulsed photoresponses with FWHMs like those of the laser pulse itself, thus nanosecond order rise and fall times were observed. Two different contact stacks were tested: Ti/Pt/Au and Ti/ITO. Because a thin Ti adhesion layer was used, the Ti/ITO contact was measured to be transparent to 532 nm light. In vertical PCSS devices, the on state current was observed to be as much as 70% higher when using Ti/ITO instead of Ti/Pt/Au. This is due to the reduced shadowing effect by opaque contacts and reduced contact resistance by the photogeneration of carriers beneath the contacts. Furthermore, the devices were tested up to biases of 500 V across a drift region of 300 um, resulting in &gt;0.8 A peak photocurrent and showed no signs of breakdown. Through this study, Ti/ITO was established as an effective contact metal for high power PCSS applications, which can be further developed and optimized for higher performance.<br/><i>This work was supported by ULTRA, an Energy Frontier Research Center funded by the U.S. DOE.</i><br/>[1] S. Feng, et al., IEEE Transactions on Electron Devices, 37, 12, 2511-2516, (1990).<br/>[2] Tsao, J. Y., et al., <i>Adv. Electron. Mater.</i>, 4, 1600501 (2018).<br/>[3] Yoneda, H., et al., Appl. Opt. 40, 6733-6736 (2001)<br/>[4] Woo K., et al., Appl. Phys. Lett. 120, 112104 (2022).