Zhengning Gao

Standing and sitting adlayers in atomic layer deposition of ZnO

The extent of reactivity of diethyl zinc (DEZ) with a hydroxylated surface during atomic layer deposition (ALD) of ZnO using DEZ and water is measured. Two adlayer configurations of DEZ are possible. The “standing” adlayer releases one ethyl group from DEZ. The “sitting” adlayer releases both ethyl groups, thus forming a Zn bridge between two O anions. Density functional theory calculations suggest the sitting configuration is more stable than the standing configuration by 790 meV. In situ quadrupole mass spectroscopy of by-product ethane generated in ALD half cycles indicate that ∼1.56 OH sites react with a DEZ molecule resulting in 71.6% of sitting sites. A simple simulation of a “ball-and-stick” DEZ molecule randomly collapsing on a neighboring site remarkably captures this adlayer behavior. It is concluded that DEZ fraction sitting is a competitive process of a standing DEZ molecule collapsing onto an available neighboring hydroxyl site, as sites vie for occupancy via adsorption and surface diffusion.

Atmospheric pressure chemical vapor deposition of methylammonium bismuth iodide thin films

We demonstrate the atmospheric pressure chemical vapor deposition of methyl ammonium bismuth iodide ((CH3NH3)3Bi2I9 or MA3Bi2I9) films. MA3Bi2I9 possesses an indirect optical bandgap of 1.80 eV and a room temperature excitonic peak at 511 nm. In contrast to recent reports, the films are n-type semiconductors with a room temperature carrier concentration of 3.36 × 1018 cm−3 and a Hall mobility of 18 cm2 V−1 s−1, which are superior to those of solution-processed, undoped films. The precursors used for the deposition are methylammonium iodide and bismuth iodide which are co-sublimated at 199 °C and 230 °C, respectively, in an Ar flow inside a tube furnace with a variable temperature profile. The substrate temperature is set at 160 °C, and dense polycrystalline films (∼775 nm thick) are deposited. Extensive characterization combined with first-principles density functional theory calculations unravels the synthesis–structure–property relationship in these films. The degradation of properties under ambient conditions results from film oxidation with a characteristic bi-exponential decay in resistivity, signifying a fast surface oxidation followed by a slower oxidation of the bulk.

Unraveling delocalized electrons in metal induced gap states from second harmonics

Second harmonic generation from Au-Al2O3 interfaces is analyzed to estimate the density of delocalized electrons occupying metal induced gap states (MIGS). Laser light of wavelength 810 nm is incident on an Au substrate and the second harmonic at 405 nm is monitored, where the area fraction of Al2O3 coverage on Au is precisely controlled via atomic layer deposition—from no coverage to full coverage. Extensive electromagnetic simulations are performed using a phenomenological model containing a dimensionless MIGS factor “α,” to represent the strength of the delocalized electrons in MIGS in attenuating the second harmonic signal. By fitting the model to experimental data, an α = 0.13 is obtained leading to a room temperature, areal density of delocalized electrons of (3.53 ± 0.4) × 1014 cm−2 for the Au-Al2O3 interface and representing a 44% occupancy of MIGS.