Charge-carrier thermalization in bulk and monolayer CdTe from first principles

TL;DR

This study uses first-principles calculations to reveal that thinning CdTe to a monolayer significantly accelerates charge carrier thermalization, with unique energy-independent electron relaxation times due to altered electronic structure.

Contribution

It provides the first detailed ab-initio analysis of charge-carrier thermalization in monolayer CdTe, highlighting drastic reductions in thermalization times and novel energy-independent behavior.

Findings

Thermalization time decreases by one to three orders of magnitude in monolayer CdTe.

Electron thermalization time becomes independent of excess energy up to 0.5 eV.

Results may inform the design of advanced CdTe-based optoelectronic devices.

Abstract

While cadmium telluride (CdTe) thin films are being used in solar cell prototyping for decades, the recent advent of two-dimensional (2D) materials challenges the fundamental limit for thickness of conventional CdTe layers. Here, we report our theoretical predictions on photocarrier dynamics in an ultimately thin (about 1 nm) CdTe slab. It corresponds to a layer that is just a single unit cell thick, when the bulk parent crystal in the zinc blende phase is cleaved along the [110] facet. Using an \textit{ab-initio} method based on density functional theory (DFT) and the Boltzmann equation in the relaxation time approximation (RTA), we determine the thermalization time for charge carriers excited to a certain energy for instance through laser irradiation. Our calculations include contributions arising from all phonon branches in the first Brillouin zone (BZ), thus capturing all relevant inter- and intraband carrier transitions due to electron-phonon scattering. We find that the photocarrier thermalization time is strongly reduced, by one order of magnitude for holes and by three orders of magnitude for electrons, once the CdTe crystal is thinned down from the bulk to a monolayer. Most surprisingly, the electron thermalization time becomes independent of the electron excess energy up to about 0.5~eV, when counted from the conduction band minimum (CBM). We relate this peculiar behavior to the degenerate and nearly parabolic lowest conduction band that yields a constant density of states (DOS) in the 2D limit. Our findings may be useful for designing novel CdTe-based optoelectronic devices, which employ nonequilibrium photoexcited carriers to improve the performance.