Properties of molecular beam epitaxially grown ScAs:InGaAs and ErAs:InGaAs nanocomposites for thermoelectricapplications
Introduction
Semiconductors embedded with semimetallic nanoparticles are promising materials for thermoelectric applications outside the optimal temperature range of Bi2Te3 [1]. The figure of merit for thermoelectric materials has the form of ZT=S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. The embedded semimetallic nanoparticles can provide carrier doping to increase σ, electron energy filtering to increase S [2], and phonon scattering to decrease κ [3]. When these interrelated parameters are optimized through careful materials selection and growth design, the ZT of the resulting nanocomposites can be enhanced with respect to the original semiconductor matrix. Recent study has been concentrated on ErAs:InGa(Al)As nanocomposites, and a thermoelectric power generator module with an output power of 6.3 W has been fabricated [4].
However, the growth and resulting property change of embedding other rare-earth pnictides into III–Vs remains largely unexplored. Among all the rare-earth elements, Sc has the smallest atomic number and Sc mono-pnictides have the smallest lattice parameters [5]. This gives the latter an important role in the engineering of thermoelectric materials, since the incorporation of Sc-containing nanoparticles may cover a unique wavelength range of phonon scattering. Specifically, ScAs is also a semimetal with a similar band structure to the widely studied ErAs [6], [7], [8]. The solubility limit of Sc in GaAs is estimated to be on the order of 1017 cm−3 [9], which is similar to that of Er [10], [11]. Sc0.32Er0.68As films have been grown lattice matched on (0 0 1) GaAs [5]. Here, we report the molecular beam epitaxy (MBE) growth and the comparative systematic study of ScAs:In0.53Ga0.47As and ErAs:In0.53Ga0.47As nanocomposites in order to illustrate their electrical and thermoelectric properties.
Section snippets
Experimental procedure
The ScAs:InGaAs samples were grown on semi-insulating (0 0 1)Fe:InP substrates in a VG V80H MBE system. Sc was evaporated from a solid-source high-temperature effusion cell with a tungsten crucible. Before growth the substrate was heated to 530 °C under As to desorb oxide. The temperature was subsequently lowered to 460 °C to grow a 270 nm In0.53Ga0.47As buffer layer that is lattice matched to the InP substrate. The Sc cell was then opened for the codeposition of ScAs with InGaAs. The InGaAs growth
Results and discussion
Fig. 1 plots the (a) Seebeck coefficient (S), (b) power factor (S2σ), (c) electrical conductivity (σ), (d) electron mobility (μn), and (e) electron concentration (n) versus the ScAs and ErAs volume concentrations (ScAs% and ErAs%) for a series of ScAs:InGaAs and ErAs:InGaAs nanocomposites. The typical properties of unintentionally doped In0.53Ga0.47As films grown in our V80H and Gen II systems are summarized in Table 1. Since the solubility limits of Sc and Er in GaAs are on the order of 1017 cm
Conclusions
The electrical and thermoelectric properties of the MBE grown ScAs:InGaAs and ErAs:InGaAs nanocomposites are studied systematically and comparatively. ScAs:InGaAs may be used as an alternative thermoelectric material to ErAs:InGaAs, although the peak room-temperature power factor of the former is only 38% of the latter. The carrier concentration change of the nanocomposites versus the ScAs and ErAs incorporation levels is explained as due to the formation of nanoparticles with different sizes
Acknowledgements
We would like to thank T. Mates for making the SIMS measurements and M. Hashimoto for making the STEM measurements. This work is supported by the Center for Energy Efficient Materials (CEEM), an Energy Frontier Research Center (EFRC) funded at UCSB by the Office of Basic Energy Sciences of the US Department of Energy.
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