Ultrafast dynamics of free carriers in low-temperature grown GaAs was studied using time-domain terahertz emission spectroscopy. The subpicosecond free-carrier lifetime was determined for a set of annealed samples with different growth temperatures ͑175-250°C͒, the carrier mobility was also estimated. The influence of the growth temperature on the ultrafast carrier trapping is discussed.

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We study the tuneability of nanoholes created by local droplet etching of AlGaAs surfaces with Al droplets at varied coverage θ of the deposited droplet material and process temperature T. For the contact angle of the as-grown droplets a mean value of 66° is determined, which depends neither on θ nor on T. The experimental results on the hole structural characteristics are interpreted in terms of scaling models yielding a general form f(θ,T)∝θqexp(E/[kBT]), with constants q and E and Boltzmann's constant kB. In detail, the hole density varies from N = 4.0 × 106 up to 1.5 × 108 cm−2, and the scaling parameters are q = 0 and E = EN = 2.46 eV. The hole diameter varies from 50 up to 190 nm with scaling parameters q = 1/3 and E = −EN/3. Moreover, the hole depth varies from 9 up to 125 nm with q = 2/3 and E = −1.73 eV. Furthermore, a threshold coverage of at least 0.2 monolayers (ML) must be deposited before hole formation takes place. In situ electron diffraction indicates that these 0.2 ML are consumed for a surface reconstruction change from (3 × 1) to (2 × 1). For coverages above 2.0 ML holes with a bimodal depth distribution are observed.

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