K. Fleischer

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures.

Magnesium, nitrogen codoped Cr2O3: A p-type transparent conducting oxide

A p-type transparent conductive oxide (TCO) was synthesised by codoping of poorly conducting chromium oxide (Cr2O3) with magnesium and nitrogen. We demonstrated that the codoping produced a TCO with good figure of merit despite using spray pyrolysis for deposition. The nitrogen enhances the specular transparency of the films in the visible range (400-700 nm), and Mg improves the conductivity while retaining the p-type character of the material. Co-doping with both elements produces a p-type oxide with a low resistivity of 3 Ω cm and transmission up to 65% for a 150 nm thick film. A comparison with other known p-type materials is given.

GaAs(001): Surface Structure and Optical Properties

The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.

Synthesis of nanocrystalline Cu deficient CuCrO2 – a high figure of merit p-type transparent semiconductor

The delafossite structured CuCrO2 system is well known as one of the best performing p-type transparent conducting oxides. In this paper the details of a low temperature facile growth method for CuCrO2 is described. The dependence of the growth on the precursors, the temperature and oxygen partial pressure are examined. The decomposition routes are critical to obtain the best performing films. The thermopower and electrical measurements indicate p-type films with conductivity ranging from 1–12 S cm−1 depending on the growth conditions. This p-type conductivity is retained despite the nanocrystallinity of the films. The figure of merit of these films can be as high as 350 μS, which is the best performing p-type TCO by solution methods to date. The optical properties are also investigated using ellipsometry and UV-Vis spectroscopy.

Atomic structure and optical anisotropy of III–V(001) surfaces

The optical anisotropy of materials with isotropic bulk crystal structure depends to a large extent on the surface atomic structure. For instance, data obtained by reflectance anisotropy spectroscopy (RAS) on (001) surfaces of zinc blende semiconductors such as InP and GaAs, have a fingerprint character for the various surface reconstructions. Here we present RAS spectra for GaAs(001) and InP(001) recorded at room temperature and at low temperature. We show that by comparison with a theoretical analysis based on ab initio density functional theory in local-density approximation calculations the origin of characteristic spectral features can be identified and thus RAS spectra utilized to discriminate between different competing structural models. We identify contributions related to electronic transitions between surface states as well as features arising from surface perturbed bulk wave functions. We explain the high sensitivity of RAS to the surface structure and chemistry as due to the surface state rel...

Spray pyrolysis growth of a high figure of merit, nano-crystalline, p-type transparent conducting material at low temperature

In this letter, we demonstrate a low temperature (≈345 °C) growth method for Cu deficient CuCrO2 performed by spray pyrolysis using metal-organic precursors and a simple air blast nozzle. Smooth films were grown on glass substrates with a highest conductivity of 12 S/cm. The most conductive samples retain transparencies above 55% resulting in a figure of merit as high as 350 μS, which is the best performing p-type transparent conducting material grown by solution methods to date. Remarkably, despite the nano-crystallinity of the films, properties comparable with crystalline CuCrO2 are observed. No postannealing of the films is required in contrast to previous reports on crystalline material. The low processing temperature of this method means that the material can be deposited on flexible substrates. As this is a solution based technique, it is more attractive to industry as physical vapour deposition methods are slow and costly in comparison.

Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays

Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphology of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to extract detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphological dispersion addressed.

Self-assembled broadband plasmonic nanoparticle arrays for sensing applications

Highly ordered noble metal nanoparticle (NP) arrays are produced using a glancing angle deposition on stepped substrates. The versatility of the technique is demonstrated by depositing different metals, resulting in shifts of the resonance positions. The behaviour of the NP arrays grown is predicted by a dipolar model, and it is measured using reflectance anisotropy spectroscopy (RAS). Fine tuning of the resonances can be finally realised by selecting the deposition parameters. The combined application of both RAS and deposition at glancing angles provides a unique tool to grow NP arrays with the tunable plasmonic resonances in the entire visible range.

Conducting mechanism in the epitaxial p-type transparent conducting oxide Cr2O3:Mg

Epitaxial p-type transparent conducting oxide (TCO) Cr2O3:Mg was grown by electron-beam evaporation in a molecular beam epitaxy system on c-plane sapphire. The influence of Mg dopants and the oxygen partial pressure were investigated by thermoelectric and electrical measurements. The conduction mechanism is analyzed using the small-polaron hopping model, and hopping activation energies have been determined, which vary with doping concentration in the range of 210–300 ± 5 meV. Films with better conductivity were obtained by postannealing. The effect of postannealing is discussed in terms of a crystallographic reordering of the Mg dopant. The highest Seebeck mobilities obtained from thermoelectric measurements are of the order of 10−4 cm2V−1s−1. We investigate the fundamental properties of a Mg dopant in a high crystalline quality epitaxial film of a binary oxide, helping us understand the role of short range crystallographic order in a p-type TCO in detail.

Aluminium doped Zn1−xMgxO—A transparent conducting oxide with tunable optical and electrical properties

A ternary mixed oxide Zn1−xMgxO has been doped with aluminium to create a range of transparent conducting oxides with tunable refractive index as well as work function. Conductive material was synthesised up to a magnesium concentration of x = 0.45, although the conductivity is reduced compared to standard ZnO:Al. The changes in band gap, work function, and conductivity have been attributed to a modified band structure and energetic position of the aluminium induced donor state.