Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

The electronic structure of the Pu-based superconductor PuCoGa5 and the Pauli paramagnet UCoGa5 is investigated using photoemission spectroscopy. The photoemission data of PuCoGa5 reveal features at the Fermi energy EF and about 1-1.5 eV below EF indicative of itinerant and localized f-electrons, respectively. Angle-resolved spectra of UCoGa5 show two peaks at similar energies that are highly dispersive, providing evidence for itinerant character of the f-electrons in this material. A comparison of the PuCoGa5 and UCoGa5 data to the spectra of α-Pu and δ-Pu serves to place PuCoGa5 within the context of the more general electronic structure problem in elemental Pu.

The thermal emission rate of dominant traps in molecular beam epitaxial n- and p-type AlGaAs subjected to Ar-ion beam etching has been studied by deep level transient spectroscopy. Emission signatures were determined and compared with results obtained by other authors for irradiation induced and grown-in defects in GaAs and AlGaAs. The most significant result of this study is the observation that the process-induced defects in n- as well as p-type AlGaAs exhibit emission signatures, which are characteristic of native defects found in GaAs. The effect is discussed in terms of a compensation effect and related band bending.

We have developed the deposition and studied the electrical characteristics and thermal reliability of Ru-based contacts on n- type GaN as well as on AlGaN/GaN heterostructure. Amorphous, conducting and transparent RuSiO4 contacts with their extremely low reverse currents and thermal stability up to 900°C, show great potential for use as Schottky contacts to n-type GaN and gate electrodes for AlGaN/GaN HEMT in high temperature, high power applications as well as in UV detectors.

High resolution photoelectron spectroscopy (PES) studies were conducted on a δ-phase Plutonium sample cleaned by laser ablation and gas dosed with O2 and H2. The measurements were made with an instrument resolution of 60 meV and with the sample at 77 K. The PES data strongly support a model with Pu2O3 growth on the metal and then PuO2 growth on the Pu2O3 layer at this temperature. In vacuum, the PuO2 reduces to Pu2O3 at room temperature with a pressure of 6×10−11 Torr. In the case of H2 dosing the hydrogen appears to penetrate the surface and disrupt the valence band as evidenced by a drop in intensity of the peak at EF which is not accompanied by a drop in the main 5f manifold at ∼2eV.

UAsSe and USb2 single crystals were studied at 15K by angle-resolved photoemission spectroscopy (ARPES) in the photon energy range between 20 eV and 110 eV. The high energy and momentum resolution (24 meV and 0.09Å−1, respectively) allows observation of very narrow and dispersive photoemission features within 100 meV of the Fermi level. The natural linewidth of the near EF feature of USb2 was found to be less than 10 meV with the dispersion of 10 meV observed in normal emission spectra. The near EF feature in UAsSe is slightly broader, situated closer to the Fermi edge, and exhibits 20 meV dispersion in the normal emission spectra. It gives evidence that neither UAsSe nor USb2 have purely 2D electronic structure. Both these compounds should be treated as quasi-2D materials.

The electronic structure of δ-phase Pu metal and the Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Excellent agreement between calculation and experiment defines a path forward for understanding electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. A comparison is made between the photoemission data and five computational schemes for δ-Pu. The results for δ-Pu and PuCoGa5 indicate 5f electron behavior on the threshold between localized and itinerant and a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.

Electronic structure of Mn/ZnO system has been investigated by synchrotron radiation photoemission. Manganese vacuum deposition was done at room temperature onto a ZnO(0001) single crystal for coverage ΘMn < 4 ML. Photoemission spectra taken near Mn3p-Mn3d absorption edge after each deposition step show resonant enhancement of Mn3d states within 10 eV of the Fermi level. The experimentally deduced partial Mn3d density of states for Θ > 1.2 ML shows at least three features: a major Mn3d structure at 3.8–4.5 eV below the Fermi energy, a valence structure at lower binding energy (1–3 eV) and a broad satellite in the 5.5 – 9 eV range. The branching ratio of satellite/main structure increases with deposition from 0.33 for 0.4 ML to 0.65 for 4 ML. After annealing up to 500 C the satellite/main ratio decreases to 0.43 indicating a high degree of hybridization between the Mn3d states and valence band of ZnO. After annealing no manganese cap layer was found at the crystal surface as was confirmed by the lack of metallic Fermi edge in photoemission spectra and by scanning Auger spectroscopy experiment. The photoemission Mn3p core level spectra taken after annealing consist of two components separated by about 4eV. It is evidence that at least two manganese states are observed in the Mn-ZnO interface region.

Highly conductive ZnO films were fabricated on p-GaN in a two-step process. First, zinc was thermally evaporated on p-GaN. Next, zinc film was oxidized in oxygen flow. To increase the conductivity of ZnO, nitrogen was introduced into zinc during its deposition. The above procedure proved successful in fabricating ZnO of the resistivity of ~1x10-3Ωcm and resulted in ohmic contacts of resistivity ~1x10-2Ωcm2 to low-doped p-GaN, and light transmittance of ~75% in the wavelength range of 400-700 nm.

Ni/Si-based ohmic contact scheme for GaN, based on the solid-phase regrowth (SPR) mechanism have been developed. Using Mg and Si as dopant species, ohmic contacts with a resistivity of ∼1*10-3Ωcm2 to p-GaN (p≈3*1017 cm-3) and n-GaN (n≈2*1017cm-3), respectively, have been obtained. SIMS, XRD, and RBS analysis show in as-deposited contacts, an initial reaction at GaN/Ni interface, leading to the formation of an Ni-Ga-N layer. The ohmic behavior of contacts, observed after annealing at 400°C, is accompanied by structural transformations in the contact region: i) the decomposition of Ni-Ga-N layer and ii) the growth of NiSi compound.

The formation of n-GaN/Ti ohmic contacts with TiN diffusion barriers has been investigated by electrical measurements, x-ray diffraction and SIMS. It has been shown that the onset of the ohmic behaviour is associated with the thermally induced phase transformation of Ti into TiN at the GaN/Ti interface. It is suggested that the process is accompanied by an increase in the doping level in the semiconductor subcontact region. The presence of a TiN barrier is found to inhibit excessive decomposition of GaN and to confine the reaction between n-GaN and Ti.

Annealing behavior of Au, AuZn, and AuSb metallization on GaSb have been investigated by the combined use of RBS, XRD, TEM, and I-V characterization. The results give evidence that the thermally activated contact reaction strongly depends on the particular elements incorporated in the Au layer. Pure Au reacts with GaSb at 100°C. The addition of Zn to Au metallization increases the thermal stability of the metallization/semiconductor system to 200°C. Antimony, forming with gold the AuSb2 phase in metallization, provides the most stable ohmic contact system.

We report on the use of AuGeNi/W and AuZn/W metallizations and their further processing by rapid thermal nitridation. Nitrided tungsten serves as a cap suppressing the sublimation of As. To study the effect of annealing conditions on the properties of the W cap, comparative experiments involving conventional annealing in H2, and RTA in N2 or O2 flow, have been performed. TEM, XRD, RBS, and SIMS have been employed to investigate contact microstructures. The obtained results prove the superiority of the contacts with nitrided W cap over conventional insulating cap. No W-N phase is observed in nitrided W layers, which is consistent with the model of a "stuffed" barrier by Nicolet.

We report on the formation of AuZn/p-GaAs ohmic contacts by short time annealing either in open or closed configuration with a dielectric or metallic capping layer. Quantitative determination of the amount of arsenic losses was of primary concern. This was studied using the Cr-collector method, applying three types of collectors all of them using 100 nm thick Cr film for seizing evaporative As losses. In the first type, the Cr film was deposited onto oxidized Si substrate. In the second type - quartz substrate was used for the deposition of chromium film while the third collector used natural mica as a substrate for the Cr film. The obtained results show that the loss of arsenic is reduced below 2×1014 atom/cm2 using thin 20 nm Au/10 nm Zn/60 nm Au metallization annealed with a capping layer at 440°C for 15 sec. Electrical measurements indicate that under these conditions good ohmic contacts are obtained.

The formation of ohmic contacts to p-GaAs based on Au-Zn system comprising a TiN diffusion barrier has been investigated using 2 MeV He+ RBS and the specific contact resistance measurements. It has been proved that TiN films deposited by reactive RF bias magnetron sputtering serves two purposes. First it suppresses the arsenic evaporation and thus confines the reaction between AuZn and GaAs. Second, it prevents intermixing between p-GaAs/Au(Zn) ohmic contact and an overlayer of Au.