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

Abstract

Rare-earth materials epitaxially codeposited with III-V semiconductors form small, spherical rare-earthmonopnictide 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.

Determining the band alignment of TbAs:GaAs and TbAs:In0.53Ga0.47As

“…1(a) was used to fit the low fluence data where A 0 , A 1 , and A 2 are amplitude coefficients, providing information on the percentage of carriers participating in each relaxation process, and s 0 , s 1 , and s 2 are the relaxation times for each process. Consistent with previous work on TbAs:GaAs, 15 s 0 has been assigned as relaxation into the TbAs nanoparticle, s 1 as emptying the nanoparticle, and s 2 as recombination across the InGaAs band gap. At higher fluences, the saturation of traps and neighboring states reduces the fraction of carriers participating in the fastest processes (s 0 and s 1 ) to a negligible level and double or single exponential fits are used.…”

“…14 Our recent carrier dynamics study shows that TbAs nanoparticles in GaAs are saturable, in direct contrast to ErAs nanoparticles, and likely have a band gap. 15 Although TbAs nanoparticles are of interest for a variety of device applications, many details of their electronic structure in III-V semiconductors are presently unknown. In this paper we propose and systematically justify a type I heterojunction for the TbAs:GaAs system and a type II heterojunction for the TbAs:InGaAs system.…”

“…Details on the growths can be found elsewhere. 2,15 The resulting TbAs:GaAs and TbAs:InGaAs films are 930 nm and 1:1 lm thick, respectively. Rutherford backscattering measurements reveals TbAs concentrations of 2.12% and 1.3%, respectively.…”

“…Optical-pump THz-probe transient absorption spectroscopy was used to determine the carrier dynamics, including saturability, of TbAs nanoparticles epitaxially embedded in InGaAs; measurement details can be found elsewhere. 15 Briefly, carriers are generated by an optical pulse and subsequently relax into the TbAs nanoparticle or back across the bulk band gap. The rate of the carrier relaxation is measured by the rate of decay in the absorption of a THz probe.…”

Carrier transfer from InAs quantum dots to ErAs metal nanoparticles

“…We have previously shown that changes in carrier lifetime as a function of excitation fluence originate from trap state saturation, which can depend critically on the electronic structure of the traps. 15,16 If ErAs had a band gap and discrete states, we would expect to see evidence of a phonon bottleneck and slow carrier recombination that would result in saturation of the ErAs electronic states as increasing pump fluence increased the number of carriers populating the ErAs states. 17 The absence of any saturation in the QD PL decay time ( Figure 5) allows us to conclude that carrier relaxation within the ErAs MNPs is extremely fast, consistent with a continuous density of states in semi-metallic ErAs MNPs.…”