Molecular Beam Epitaxial Growth and Characterization of InAs Nanowires

Abstract

InAs is an important III-V material for mid-infrared applications due to its unique properties such as direct narrow bandgap (E_g ~0.36 eV at 300 K), high electron mobility (~33,000 cm²/Vs at 300 K), and low ohmic contact resistivity. In nanowire (NW) geometry, it can be monolithically integrated onto the mature, commercially dominant Si CMOS platform, showing great promise as building blocks for next generation mid-infrared electronic and optoelectronic devices. This thesis mainly investigates the molecular beam epitaxial (MBE) growth and optical properties of InAs-based nanowires. Two types of catalyst-free NWs were grown by MBE: selective area InAs NWs on Si, utilizing an electron-beam lithographically patterned silicon nitride mask; and randomly-positioned InAs NWs on un-patterned Si substrates. Details of both growth techniques were presented. A diameter series of selective-area InAs NWs, and a diameter series, a length series of randomly-nucleated InAs/InAlAs core shell NWs were fabricated for subsequent optical measurements.

The development of functional NW devices requires a thorough knowledge in the NW carrier recombination dynamics. However, there is no publication which thoroughly resolved the three recombination processes−Shockley-Read-Hall (SRH), radiative and Auger−in InAs-based core-shell NWs. To fill this gap, ultrafast pump probe spectroscopy and external quantum efficiency measurement were performed on selective-area InAs/InAlAs core-shell NWs grown on Si, from which their SRH, radiative and Auger coefficients were extracted. It is discovered that the InAs/InAlAs NWs have a very low Auger rate (ten-fold smaller than planar zincblende InAs) and a high radiative rate, which results in a high estimated 77 K peak internal quantum efficiency of 22%. This suggests that InAs-based NWs show promise as high efficiency mid-infrared emitters.

The Shockley-Read-Hall defect assisted recombination rates from different parts of the InAs-based NWs have never been completely separated out. To address this problem, ultrafast measurements were performed on two sets of randomly-nucleated InAs/InAlAs NWs grown on Si, independently varying the NW length and diameter. The carrier recombination rates at various NW regions: end facets, sidewall, and interior were disentangled. It is discovered that the carrier recombination in the NW interior is non-trivial compared to the surface recombination, especially at 293 K. Surface recombination is dominated by carrier recombination on the InAs/InAlAs NW sidewall, while contributions from the impure, highly-strained base are negligible.

The surface and interior recombination rates in the selective-area InAs NWs grown on lithographically patterned Si substrates were also resolved. We found that even without an InAlAs shell, the 77 K surface recombination velocity was slightly smaller than that of the randomly-nucleated InAs/InAlAs core-shell NWs. In addition, an exceptionally long InAs NW interior minority carrier lifetime of 8.7 ns was measured. Transmission electron microscopy showed a high density of stacking fault defects within the NWs, suggesting that interior recombination lifetime can be further prolonged by improving NW interior crystal quality.

Finally, to explore the material properties of the GaInAsSb quaternary alloy, two GaInAsSb detectors with 2.6 µm cutoff wavelength, and p-doped to different levels of 3 × 10¹⁶ cm⁻³ and 3 × 10¹⁷ cm⁻³ were grown by MBE, fabricated and characterized. In the limit of infinite mesa area, R0A was extrapolated to be 21.1 Ω-cm² for p⁻³ × 10¹⁶ cm⁻³ detector, and 95.4 Ω-cm² for p⁻³ × 10¹⁷ cm⁻³ detector. Measurements showed that detector p-doped to 3 × 10¹⁷ cm⁻³ have overall better performance than detector p-doped to 3 × 10¹⁶ cm⁻³, which makes it a more preferable absorber doping level choice for the 2.6 µm GaInAsSb detector.