A. M. Hartel

Charge transport in Si nanocrystal/SiO2 superlattices

Size-controlled silicon nanocrystals in silicon oxynitride matrix were prepared using plasma-enhanced chemical vapor deposition following the superlattice approach. A combination of current transport and charge trapping studies is carried out on a number of samples with varied structural configuration. We demonstrate that at low electric fields, trapping of injected carriers dominates, if the coupling between the silicon nanocrystals is strong. In contrast, we show that at higher electric fields, the charge distribution within the films is essentially governed by charge separation within the superlattice. This effect can be well explained by a two-step electric field ionization of silicon nanocrystals that proceeds via defect-assisted band-to-band tunneling of silicon valence electrons to the conduction band and is mediated by silicon surface dangling bonds. The defects are dominating the charge transport even if the defect density is reduced to a minimum by efficient hydrogen passivation.

Doping efficiency of phosphorus doped silicon nanocrystals embedded in a SiO2 matrix

Strongly size controlled silicon nanocrystals in silicon oxynitride matrix were prepared using plasma enhanced chemical vapor deposition following the superlattice approach. Doping was achieved by adding diluted phosphine as a precursor gas. Phosphorus quantification was done by secondary ion mass spectrometry. A model based on Poissonian distributions of interface defects and dopants is proposed to calculate the defects and the dopants per silicon nanocrystal as a function of phosphorus concentration. The model requires the comparison between the photoluminescence spectra from passivated and unpassivated samples. Finally, the doping efficiency of silicon nanocrystals embedded in silicon oxynitride is estimated to be >20%.

Homoepitaxial branching: an unusual polymorph of zinc oxide derived from seeded solution growth

The development of hydrothermal synthesis has greatly promoted bottom-up nanoscience for the rational growth of diverse zinc oxide (ZnO) nanostructures. In comparison with normal ZnO nanowires, ZnO nanostructures with a larger surface area, for instance, branched nanowires, are more attractive in the application fields of catalysis, sensing, dye-sensitized solar cells etc. So far the ZnO branched nanowires achieved by either one-step or multistep growth always present a boundary-governed nonepitaxial branch/stem interface. In this report, seeded growth of single-crystalline ZnO hexabranched nanostructures was achieved by selecting polyethylene glycol (PEG) as capping agent based on a low-temperature, laterally epitaxial solution growth strategy. We investigated the generality of this PEG-assisted growth process using different ZnO seed layers including continuous film, patterned dots, and vertically aligned nanowire arrays. It was revealed that PEG is a distinctive c-direction inhibitor responsible for the lateral growth and subsequent branching of ZnO due to its nonionic and nonacidic feature and weak reactivity in the solution system. All the obtained branched nanostructures are of single crystallinity in nature, which is methodologically determined by the homoepitaxial growth mode. This PEG-assisted process is versatile for diameter tuning and branch formation of ZnO nanowires by secondary growth. Our proof-of-concept experiments demonstrated that the ZnO hexabranched nanostructures presented superior photocatalytic efficiency for dye degradation relative to the normal ZnO nanowires.

Improved optical properties of ZnO thin films by concurrently introduced interfacial voids during thermal annealing

We report the influence of size controlled interfacial voids on the optical properties of non-epitaxial polycrystalline ZnO films grown by atomic layer deposition. Interfacial voids generated due to nanoscale Kirkendall effect [A. D. Smigelskas and E. O. Kirkendall, Trans. AIME 171, 130, (1947)], along the ZnO-Al2O3 interface, enhanced the ratio of near band edge to defect luminescence of ZnO films up to eight folds. This improvement is attributed to stress relaxation caused by formation of interfacial voids that allowed greater grain growth within the polycrystalline ZnO films upon annealing. Larger crystals have a lower surface-to-volume ratio profile which lessens the defect emission by decreasing surface effects on the optical properties of ZnO.

Photoluminescent and gas-sensing properties of ZnO nanowires prepared by an ionic liquid assisted vapor transfer approach

In this work, the ionic liquid assisted technique was used to control the growth characteristic of ZnO nanowires (NWs). The major change after adding ionic liquid into the growth system was the change in NW growth orientation, which was shifted from polar c- to non-polar a-orientation. Room temperature photoluminescence demonstrates a big reduction of the green luminescence which implies an annihilation of deep level emission. We propose two possible mechanisms responsible for the reduction of the green emission: The first mechanism is the passivation of ZnO NWs surface by fractions of ionic liquid employed for the growth, which further reduces the green emission. The second mechanism is the reduction of the defect density by changing the growth orientation. By using a semi-empirical Austin Model 1 method, the formation energy of oxygen vacancies in c- and a-oriented ZnO NWs has been simulated and compared. Accordingly, the gas-sensor constructed from ionic liquid assisted ZnO nanowires does not response ...

Silicon nanocrystals prepared by plasma enhanced chemical vapor deposition: Importance of parasitic oxidation for third generation photovoltaic applications

We report on an in-situ oxidation effect during annealing of SiO2/SiO1.0N0.23 multilayers prepared by plasma enhanced chemical vapour deposition (PECVD). This in-situ oxidation leads to an undesired growth of the tunneling oxide and also affects the silicon nanocrystal (SiNC) size control, i.e., a NC shrinkage. The origin of this oxidation is identified to be a “quasi-wet” oxidation by O–H groups incorporated in the PECVD-SiO2 barrier layers. By varying the thickness of the PECVD-SiO2 layer underneath a single SiO1.0N0.23 layer, the extent of NC oxidation is tuned. The shrinkage of SiNCs is proven by a blueshift of the photoluminescence peak position as well as by transmission electron microscopy.

Superior Functionality by Design: Selective Ozone Sensing Realized by Rationally Constructed High‐Index ZnO Surfaces

A new technique is reported for the transformation of smooth nonpolar ZnO nanowire surfaces to zigzagged high-index polar surfaces using polycrystalline ZnO thin films deposited by atomic layer deposition (ALD). The c-axis-oriented ZnO nanowires with smooth nonpolar surfaces are fabricated using vapor deposition method and subsequently coated by ALD with a ZnO particulate thin film. The synthesized ZnO-ZnO core-shell nanostructures are annealed at 800 °C to transform the smooth ZnO nanowires to zigzagged nanowires with high-index polar surfaces. Ozone sensing response is compared for all three types of fabricated nanowire morphologies, namely nanowires with smooth surfaces, ZnO-ZnO core-shell nanowires, and zigzagged ZnO nanowires to determine the role of crystallographic surface planes on gas response. While the smooth and core-shell nanowires are largely non-responsive to varying O(3) concentrations in the experiments, zigzagged nanowires show a significantly higher sensitivity (ppb level) owing to inherent defect-rich high-index polar surfaces.

Photovoltaic properties of silicon nanocrystals in silicon carbide

Silicon nanocrystal quantum dots in a dielectric matrix form a material with higher band gap than silicon, but still compatible with silicon technology. So far, devices using silicon nanocrystals have been realized either on silicon wafers, or using in-situ doping in the superlattice deposition which may hinder the nanocrystal formation. In this paper, a vertical PIN device is presented which allows to investigated the electrical and photovoltaic properties of nanocrystal quantum dot layers. The device structure circumvents any influence of a substrate wafer or dopants and provides full flexibility in the material choice of both, i.e. electron and hole, contacts. Furthermore, not-high-temperature stable contact materials can be applied. Devices have been realized using SiC/Si nanocrystal multilayers as the i-region and doped a-SixC1-x:H layers as electron and hole contacts. First devices show open-circuit voltage of up to 400mV.

Effect of temperature and excitation intensity on photoexcited charge carrier dynamics in Si-NCs/SiO2 superlattices

An experimental study of the temperature dependence of photoluminescence time decay in size-controlled silicon nanocrystals in silicon nanocrystal/SiO2 superlattices is reported. The samples were prepared using thermal evaporation and subsequent thermally induced phase separation. The slow (microseconds) decay line shape is described well by a stretched exponential. The temperature dependence of the photoluminescence dynamics can be understood in terms of thermal activation of recombination processes, including hopping of carriers between localized states. Additional hydrogen treatment causes an increase in both parameters of the stretched exponential function. This behavior is interpreted as a consequence of H2-passivation of dangling bonds defects.