Persistent hot carrier diffusion in boron arsenide single crystals imaged by ultrafast electron microscopy

TL;DR

This study uses ultrafast electron microscopy to visualize hot carrier diffusion in boron arsenide crystals, revealing persistent diffusion and electron-hole separation, highlighting BAs's potential for optoelectronic applications.

Contribution

First direct visualization of photocarrier diffusion in boron arsenide using SUEM, demonstrating its promising optoelectronic properties and the technique's effectiveness.

Findings

Observed ambipolar diffusion with persistent hot carriers for over 200 ps.

Detected spontaneous electron-hole separation at higher optical fluence.

Confirmed BAs's potential for high thermal conductivity and efficient photocarrier transport.

Abstract

Cubic boron arsenide (BAs) is promising for microelectronics thermal management due to 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 due to small sizes of available high-quality crystals. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the diffusion of photoexcited charge carriers in BAs single crystals. Surprisingly, we observed ambipolar diffusion at low optical fluence with persistent hot carrier dynamics for above 200 picoseconds, which can 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.