Extraordinary Carrier Diffusion on CdTe Surfaces Uncovered by 4D Electron Microscopy

Herein, we describe the selective mapping of surface charge carriers, achieved using scanning ultrafast electron microscopy, for visualization of light-triggered carrier dynamics on the nanometer scale, diffusing on the surface of CdTe, a leading semiconductor in commercial photovoltaics. We reveal a novel diffusion model of charge carriers on surfaces. The nature of charge diffusion varies significantly, from superior diffusion to virtually trapped, depending on the crystal orientation. For instance, carriers on the (110) crystal orientation displayed a diffusion coefficient of ∼40,000 cm²·s⁻¹ (i.e., ∼10⁴ times larger than measured in the bulk). In contrast, the (211) polar facets formed oxidative layers that, in turn, formed mid-surface-trap states that nearly arrested the charge diffusion completely. Complementary techniques and theoretical modeling explain the acute sensitivity of charge carriers to surface orientation and termination. Our findings reveal the enormous untapped potential for improved CdTe-based devices and optoelectronic materials in general through facet management.