Charge carrier relaxation processes in TbAs nanoinclusions in GaAs measured by optical-pump THz-probe transient absorption spectroscopy

Rare-earth materials epitaxially codeposited with III-V semiconductors form small, spherical rare-earth-monopnictide nanoparticles embedded within the III-V host. The small size of these particles (approximately 1.5 nm diameter) suggests that interesting electronic properties might emerge as a result of both confinement and surface states. However, ErAs nanoparticles do not exhibit any signs of quantum confinement or an emergent band gap, and these experimental observations are understood theoretically. We use ultrafast pump-probe spectroscopy to investigate the electronic structure of TbAs nanoparticles embedded in a GaAs host, which were expected to be similar to ErAs. We study the dynamics of carrier relaxation into the TbAs states, which essentially act as traps, using optical-pump terahertz-probe transient absorption spectroscopy. By analyzing how the carrier relaxation rates depend on pump fluence and sample temperature, we conclude that the TbAs states are saturable. Saturable traps suggest the existence of a band gap for TbAs nanoparticles, in sharp contrast with the results for ErAs.

Figure 1

Schematic showing TbAs nanoparticles within a GaAs host matrix, formed by codepositing terbium, gallium, and arsenic.

Figure 2

Optical pump creates electron-hole pairs. THz probe is absorbed only when electrons are in the conduction band or holes are in the valence band.

Figure 3

(a) For larger delay times (50–400 ps), both a biexponential (red dashed line) and a triexponential equation (green dashed-dotted line) fit the data with good agreement. (b) For shorter delay times (0–50 ps), a biexponential equation does not accurately reflect the relaxation dynamics.

Figure 4

Charge carriers can relax via three processes: into the TbAs trap states ($\tau 0$), via trap states emptied by electron-hole annihilation within the traps ($\tau 1$), or radiative recombination across the bulk GaAs ($\tau 2$).

Figure 5

The relaxation lifetime $\tau 0$ of carriers into the TbAs traps (a) shows no statistically significant fluence dependence. The fraction of electrons entering (b) and exiting (d) the TbAs traps decreases with increasing pump fluence, indicating trap saturation. The relaxation lifetime $\tau 1$ of carriers out of the TbAs traps (c) increases with fluence, which suggests that recombination between carriers in the conduction and valence bands of the GaAs matrix and those in the TbAs nanoinclusions is a significant nonradiative recombination pathway. The relaxation lifetime $\tau 2$ of carriers across the bulk band gap (e) increases with fluence, consistent with trap saturation. The fraction of electrons relaxing across the bulk band gap (f) increases with fluence, which indicates that at high pump fluences the carrier population exceeds the saturation threshold and all additional carriers must relax via bulk radiative decay.

Figure 6

For a $2\mu$W/${\text{cm}}^2$ pump fluence, the fraction of carriers entering the TbAs traps [$A/(A+B+C)$, black squares] and the fraction leaving the traps [$B/(A+B+C)$, red circles] do not show any statistically significant dependence on temperature.