Spyros Diplas

Surface passivation and interface reactions induced by hydrogen peroxide treatment of n-type ZnO (0001¯)

X-ray photoemission spectroscopy and electrical measurements have been employed to study O-face (0001¯) n-type ZnO samples treated by hydrogen peroxide (H2O2). A highly resistive and oxygen-rich surface layer is revealed, presumably caused by a high concentration of zinc vacancies and/or adsorbed O2 molecules. As a result, the surface exhibits upward energy band bending (∼0.4 eV) promoting the formation of high barrier Schottky contacts and suppressing the surface leakage current. Furthermore, after Pd deposition an enhanced formation of PdO is found at the Pd/ZnO interface for the H2O2-treated samples, and this is also expected to increase the resulting Schottky barrier height (∼0.6 eV), which yields up to seven orders of magnitude in current rectification between forward and reverse bias voltage.

Elemental distribution and oxygen deficiency of magnetron sputtered indium tin oxide films

The atomic structure and composition of noninterfacial ITO and ITO-Si interfaces were studied with transmission electron microscopy and x-ray photoelectron spectroscopy (XPS). The films were deposited by dc magnetron sputtering on monocrystalline p-type (100) Si wafers. Both as deposited and heat treated films consisted of crystalline ITO. The ITO/Si interface showed a more complicated composition. A thin layer of SiOx was found at the ITO/Si interface together with In and Sn nanoclusters, as well as highly oxygen deficient regions, as observed by XPS. High energy electron exposure of this area crystallized the In nanoclusters and at the same time increased the SiOx interface layer thickness.

An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films

Crystalline and amorphous nanoparticles of silicon in thin silica layers were examined by transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy (XPS). We used XPS data in the form of the Auger parameter to separate initial and final state contributions to the Si2p energy shift. The electrostatic charging and electron screening issues as well as initial state effects were also addressed. We show that the chemical shift in the nanocrystals is determined by initial state rather than final state effects, and that the electron screening of silicon core holes in nanocrystals dispersed in SiO2 is inferior to that in pure bulk Si.

A study of the alloying behaviour of Ni–B amorphous catalysts using Auger parameter measurements, and primary and secondary features of the XPS spectrum

Electrodeposited Ni–B alloys of nominal compositions 30 at.% B and 50 at.% B, which were developed as catalysts for the electrolytic hydrogen production process, were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The Ni–B alloys were found to be amorphous. Changes of Auger parameter (AP) values and information from the primary and secondary structure of the XPS spectrum were employed to probe the electronic changes occurring upon alloying. It is suggested that the main electronic change occurring is hybridisation of the Ni occupied states with the B sp empty states and an apparent increase of the electron density around the Ni sites. The electron affinities of the two elements contradict their electronegativity tendencies. It is suggested that the contradiction may be resolved by considering the increase in the electron density at the Ni sites in the Ni–B alloys to be a result of shortening of the interatomic distances via the formation of hybridized bonds between Ni and B atoms.

Combination of characterization techniques for atomic layer deposition MoO3 coatings: From the amorphous to the orthorhombic α-MoO3 crystalline phase

Thin films of MoO3 deposited on Si(111) and Al2O3(001) substrates by atomic layer deposition have been investigated by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectroscopy for detailed characterization of composition and morphology. Comparison of angle resolved x-ray photoelectron spectroscopy (ARXPS) and XPS depth profiles based on Ar+ sputtering is reported. Sputtering induces a reduction of molybdenum in MoO3 from +IV to metallic Mo as the interface toward Si is approached, whereas ARXPS on a 10 nm thin film shows that Mo(VI) remains outside the interface toward Si where lower valent molybdenum compounds are formed. Upon annealing, the as-deposited amorphous thin films of MoO3 crystallize into β- or α-MoO3 as identified by x-ray diffraction. The current study provides a convenient route toward formation of metastable β-MoO3 and a full crystallization pathway from amorphous to crystalline α-MoO3. Combined AFM and Raman analysis have been performed on thin films o...

Electronic structure of thermoelectric Zn–Sb

The electronic structures of the two main compounds of the binary zinc antimonides that are stable at room temperature, Zn(1)Sb(1) and β-Zn(4)Sb(3), were probed with x-ray photoelectron spectroscopy. Additionally, electron energy loss measurements and density functional theory calculations are presented. The compounds are found to share a very similar electronic structure. They both feature only small charge transfers and differ moderately in their screening potentials. These results are in line with recent theoretical works on the Zn-Sb system and are discussed in light of the reported thermoelectric performance of the materials.

Hydrogen Release and Defect Formation During Heat Treatments of SiNx:H/a-Si: H Double Passivation Layer on C-SI Substrate

The quality and temperature stability of surface passivation of silicon by a double layer consisting of a hydrogenated amorphous silicon thin film capped by a silicon nitride anti-reflection coating are studied. It is established that the passivation effect of the double layer can be significantly enhanced after short annealing for temperatures up to about 500degC, whereas annealing at higher temperatures results in degradation of the passivation properties. It is found that the increased effective recombination lifetime after annealing at temperatures below 500degC results from hydrogen redistribution in the interface region. Furthermore, presence of interfacial structural defects formed due to hydrogen release at temperatures around 600degC, is believed to be the cause of the lifetime decrease after heat treatments at higher temperatures

Rearrangement of the oxide-semiconductor interface in annealed Al2O3∕4H-SiC structures

Al2O3 films with different thicknesses have been deposited on n-type (nitrogen-doped) 4H-SiC(0001) epitaxial samples by atomic layer chemical vapor deposition at 300°C and subsequently annealed in Ar atmosphere at temperatures up to 1000°C. The Al2O3∕4H-SiC structures were analyzed by x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). The XPS and SIMS results indicate that the average composition in the wider interface area does not significantly change due to the annealing. However, as revealed by the TEM investigations in combination with XPS, the as-grown samples exhibit a double interface created by an intermediate suboxide SiOx layer (x<2). After annealing, this intermediate suboxide layer breaks up and transforms into SiO2 islands, resulting in a rather rough interface region and a high concentration of pure Si in the Al2O3 film. Furthermore, a pronounced accumulation of H is found in the rough interface region and this may pla...

Initial stages of ITO/Si interface formation: In situ x-ray photoelectron spectroscopy measurements upon magnetron sputtering and atomistic modelling using density functional theory

Initial stages of indium tin oxide (ITO) growth on a polished Si substrate upon magnetron sputtering were studied experimentally using in-situ x-ray photoelectron spectroscopy measurements. The presence of pure indium and tin, as well as Si bonded to oxygen at the ITO/Si interface were observed. The experimental observations were compared with several atomistic models of ITO/Si interfaces. A periodic model of the ITO/Si interface was constructed, giving detailed information about the local environment at the interface. Molecular dynamics based on density functional theory was performed, showing how metal-oxygen bonds are broken on behalf of silicon-oxygen bonds. These theoretical results support and provide an explanation for the present as well as previous ex-situ and in-situ experimental observations pointing to the creation of metallic In and Sn along with the growth of SiOx at the ITO/Si interface.

Electronic structure studies of Ni–X (X: B, S, P) alloys using x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy and density functional theory calculations

The electronic structure of Ni-X (X = B, S, P) alloys was studied using x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy and density functional theory. The spectroscopic data in the form of the Ni 2p shake-up satellite and the Ni 2p LMM, P 2p KLL and S 2p KLL Auger parameters combined with density of states (DOS) and charge difference plots suggest an overall charge transfer from the Ni sites towards the alloying addition sites. However, this is masked, with intra-atomic charge redistribution leading to an increased occupancy of the Ni 3d states in the alloys. The Ni 3d DOS shows strong similarity to that of Pt which is the best catalyst for hydrogen evolution.