Cross-sectional scanning tunneling microscopy and spectroscopy of semimetallic ErAs nanostructures embedded in GaAs

The growth and atomic/electronic structure of molecular beam epitaxy-grown ErAs nanoparticles and nanorods embedded within a GaAs matrix are examined for the first time via cross-sectional scanning tunneling microscopy and spectroscopy. Cross sections enable the interrogation of the internal structure and are well suited for studying embedded nanostructures. The early stages of embedded ErAs nanostructure growth are examined via these techniques and compared with previous cross-sectional transmission electron microscopy work. Tunneling spectroscopy I(V) for both ErAs nanoparticles and nanorods was also performed, demonstrating that both nanostructures are semimetallic. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3547713

Controlling n-Type Carrier Density from Er Doping of InGaAs with MBE Growth Temperature

International audienceUnder certain growth conditions in molecular beam epitaxy, erbium, indium, gallium, and arsenic form a two-phase composite, consisting of ErAs nanoparticles embedded in dilute Er-doped In0.53Ga0.47As. This paper further explores the effect of growth conditions, specifically growth temperature, on the nanostructure of this material and the resulting thermal and electrical transport properties. For a set of samples grown with substrate temperatures varying from 430C to 525C, we find that the thermal conductivity decreases slightly with increasing growth temperature (from 4.8 W/m K to 4.1 W/m K) while the electrical conductivity decreases dramatically with increasing growth temperature (from 2100 S/cm to 110 S/cm), which is largely due to decreasing carrier concentration. At higher growth temperatures, more erbium precipitates out of solution and the size and density of the ErAs nanoparticles increase, as characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), while the total erbium concentration does not change with growth temperature, as characterized by Rutherford backscatter spectrometry (RBS). Measurement of the erbium concentration by secondary-ion mass spectrometry suggests that the Er bonding configuration changes with growth temperature. These results indicate that increasing the ratio of solute Er atoms in the In0.53Ga0.47As host to precipitated Er atoms in ErAs particles increases the carrier density and electrical conductivity of the total composite material

Thermoelectric properties of epitaxial TbAs:InGaAs nanocomposites

International audienceInGaAs lattice-matched to InP was grown by molecular beam epitaxy with randomly distributed TbAs nanoparticles for thermoelectric power generation applications. TbAs:InGaAs is expected to have a large thermoelectric figure of merit, ZT, particularly at high temperatures, owing to energy band alignment between the nanoparticles and their surrounding matrix. Here, the room temperature thermoelectric properties were measured as a function of TbAs concentration, revealing a maximum thermoelectric power factor of 2.38 W/mK2 and ZT of 0.19 with 0.2% TbAs. Trends in the thermoelectric properties closely resemble those found in comparable ErAs:InGaAs nanocomposite materials. However, nanoparticles were not observed by scanning transmission electron microscopy in the highest ZT TbAs:InGaAs sample, unlike the highest ZT ErAs:InGaAs sample (0.2% ErAs) and two higher concentration TbAs:InGaAs samples examined. Consistent with expectations concerning the positioning of the Fermi level in these materials, ZT was enhanced by TbAs incorporation largely due to a high Seebeck coefficient, whereas ErAs provided InGaAs with higher conductivity but a lower Seebeck coefficient than that of TbAs:InGaAs. Thermal conductivity was reduced significantly from that of intrinsic thin-film InGaAs only with TbAs concentrations greater than ∼1.7%