We report velocity and internal state distributions of nitric oxide photodesorbed from an Au(100) single crystal using 355 nm and 266 nm photons. The velocity distributions were measured in all three dimensions independently using our novel 3D-velocity map imaging setup. Combined with the internal energy distributions, we reveal two distinct desorption mechanisms for the photodesorption of NO from gold dependent on the photon wavelength. The 355 nm desorption is dominated by a non-thermal mechanism due to excitation of an electron from the gold substrate to the adsorbed NO; this leads to a super-thermal and noticeably narrow velocity distribution, and a rotational state distribution that positively correlates with the velocity distribution and can be described by a rotational temperature appreciably above the surface temperature. Desorption with 266 nm photons leads to a slower average speed and wider angular distribution, and rotational temperatures not too far off the surface temperature. We conclude that in the absence of occupied orbitals in the substrate and unoccupied orbitals on the adsorbed NO separated by 4.7 eV, corresponding to 266 nm, the shorter wavelength desorption is dominated by a thermally-activated mechanism.

Herein, advanced preparation methods of cross‐section samples suitable for site‐specific studies of alloying processes are reported. To follow alloying processes in real time with the highest possible spatial resolution, in situ heating transmission electron microscopy (TEM) is conducted. As an industrially relevant model system, aluminum (Al) on multicrystalline silicon (mc‐Si) is chosen. Despite the tremendous advantages of in situ TEM compared with ex situ techniques, the development of suitable sample preparation recipes is a challenging task. As the standard focused ion beam (FIB) lift‐out fails for this preparation, three alternative methods are implemented. They show significant improvements compared with the widely used standard lift‐out and are described and evaluated in terms of contamination and overall lamella quality. Moreover, further improvements are discussed, leading to the conclusion that the new methods allow the reproducible preparation of lamellas suitable for site‐specific, in situ TEM alloying experiments.

Carrier-selective and passivating SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO$$_{\rm x}$$ x and TiO$$_{\rm y}$$ y layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO$$_{\rm x}$$ x . However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO$$_{\rm x}$$ x layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.

No abstract

The use of highly efficient and solarblind GaN photocathodes as part of multichannel plate UV detectors for applications in astronomy would strongly benefit from the direct growth of GaN on typical window materials with high transmission down to the deep UV range. GaN growth on MgF2 substrates by plasma-assisted molecular beam epitaxy has recently been demonstrated. Here, we report an extensive scanning transmission electron microscopy study of the thin film microstructure for growth at 525 • C and 650 • C on (100) MgF2. For the lower growth temperature, cubic as well as hexagonal GaN are observed with no preferred nucleation of either phase on the substrate and typical grain size for both variants of 100-200 nm. The higher growth temperature leads to predominant growth of hexagonal GaN in two different orientations with the same range of grain size and cubic GaN with grain sizes of about 20 nm. Furthermore, in-diffusion of Mg and F into the GaN is observed, which is accompanied by the formation of cavities in the MgF2 directly at the interface.

Unlike conventional opaque solar cells, semi-transparent solar cells enable simultaneous electricity generation and light transmission. Along with solar energy harvesting, the offered multiple functionalities of these technologies, such as aesthetic appearance, visual comfort and thermal management, open diverse integration opportunities into versatile technological applications. In this work, the first demonstration of a novel semi-transparent solar cell based on ultrathin hydrogenated amorphous Si/Ge multiple quantum wells (MQW) is reported. Through optoelectronic modelling, the advantages of ultrathin MQW as photoactive material to overcome the intrinsic limitations of thin (20 nm) and ultrathin (2.5 nm) single quantum well (SQW) counterparts are explained. This allows extra degree of freedom for both optical design and bandgap engineering. Mainly, the multiplication of the QWs number in a periodic configuration, taking advantage of effective synergy between electronic and photonic confinements, leads to an improvement of photocurrent, while preserving high voltage and fill factor and ensuring significant transparency. The MQW new concept yields a boost in power conversion efficiency up to 3.4% and a considerable average visible transmission of about 33%. A light utilization efficiency above 1.1% is achieved, which can be considered as one of the highest among inorganic semitransparent solar cell technologies. The successful demonstration of ultrathin semitransparent Si/Ge MQW solar cells indicates the promising integration potential of this emerging photovoltaic technology for supplying systems in relevant applications such as in buildings, vehicles and greenhouses.

No abstract

No abstract