Persistent Hot Carrier Diffusion in Boron Arsenide Single Crystals Imaged by Ultrafast Electron Microscopy

Cubic boron arsenide (BAs) is promising for microelectronics thermal management because of its high thermal conductivity. Recently, its potential as an optoelectronic material is also being explored. However, it remains challenging to measure its photocarrier transport properties because of small sizes of available high-quality crystals. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the diffusion of photocarriers in BAs single crystals. SUEM integrates the temporal resolution of femtosecond lasers with the spatial resolution of scanning electron microscopes (SEMs). The change in local secondary electron (SE) yield as a result of the optical excitation is measured and used to form contrast images. Given the shallow escape length of SEs (a few nanometers), SUEM is highly sensitive to surface charge dynamics and has been used to study photocarrier diffusion. We observed ambipolar diffusion at low optical fluence with persistent hot carrier dynamics for above 200 ps, which can likely be attributed to the large frequency gap between acoustic and optical phonons, the same feature that is responsible for the high thermal conductivity. At higher optical fluence, we observed spontaneous electron-hole separation. Our results show BAs is an attractive optoelectronic material combining high thermal conductivity and excellent photocarrier transport properties. Our study also demonstrates the capability of SUEM to probe photocarrier transport in emerging materials.<br/><br/>The work conducted at the University of California, Santa Barbara, is based on research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under award DE-SC0019244 (for the development of SUEM) and by the U.S. Army Research Office under award W911NF-19-1-0060 (for studying photocarrier dynamics in emerging materials). The growth of high-quality BAs crystals at the University of Houston was supported by the U.S. Office of Naval Research under Multidisciplinary University Research Initiative grant N00014-16-1-2436. D.-S.Y. acknowledges the support by the R. A. Welch Foundation (E-1860).