Marcel Michling

Experimental electronic structure of In2O3 and Ga2O3

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

Resonant Photoemission at the O1s threshold to characterize β-Ga2O3 single crystals

We report on spectroscopic investigations on β-Ga2O3 single crystals. We focus on the detailed analysis of the O1s resonance profile by resonant photoelectron spectroscopy. We analyze the electronic structure and assign the O2p to build the valence band and both, O2p and Ga4s4p states to contribute to the conduction band. We determine the partial density of states for the valence and conduction bands and find a strong hybridization of O2p and Ga4s states. This is deduced from constant final state spectra on the O-KLL-Auger over the O K-edge and Ga L2,3-edge and a comparison to the corresponding X-ray absorption spectroscopy data. We also identify several types of defects. A broad band of oxygen derived defects is identified that extends throughout the gap. Small polarons are attributed to cause an anti-resonance in the valence states around the O1s threshold. In addition, a separate Auger decay at resonance indicates the existence of localized charge transfer states which involves localized Ga4sp states.

In situ study of the atomic layer deposition of HfO2 on Si

The authors investigated in situ the initial stages of the atomic layer deposition (ALD) growth of HfO2 on Si(001)/SiO2 substrates by using tetrakis-di-methyl-amino-Hf and H2O as precursors. The surface morphology and the chemical and electronic properties of HfO2 ultrathin films were studied after each ALD cycle by surface-sensitive techniques. Atomic force microscopy image analysis was performed by analyzing the height–height correlation function (HHCF), the root mean square surface roughness, and the surface fractal dimension, as function of the number of ALD cycles. Parameters directly related to HHCF, e.g., surface width, correlation length, local slope, and roughness exponent, were calculated and used for determination of scaling exponents. A complex behavior of all parameters up to the eighth ALD cycle was evidenced. High-resolution synchrotron radiation photoemission spectroscopy was applied to characterize the chemical nature of Si/SiO2/HfO2 interface. Changes arising in the Si 2p, O 1 s, and Hf 4f core level lines after each ALD cycle up to the complete formation of two layers of HfO2 were observed. The thickness of the growing HfO2 layer was calculated to estimate the growth per cycle to approximately 0.1 nm/cycle. By means of ultraviolet photoemission spectroscopy, variations of valence band maximum and secondary electron cutoff after each ALD cycle were observed and the presence of an interfacial dipole was pointed out. Finally, the loss function onset of electron energy loss spectroscopy changed during ALD because of bandgap variations from SiO2 to HfO2. By combining all experimental results a new and fully comprehensive growth model of ALD during the initial stages was developed.The authors investigated in situ the initial stages of the atomic layer deposition (ALD) growth of HfO2 on Si(001)/SiO2 substrates by using tetrakis-di-methyl-amino-Hf and H2O as precursors. The surface morphology and the chemical and electronic properties of HfO2 ultrathin films were studied after each ALD cycle by surface-sensitive techniques. Atomic force microscopy image analysis was performed by analyzing the height–height correlation function (HHCF), the root mean square surface roughness, and the surface fractal dimension, as function of the number of ALD cycles. Parameters directly related to HHCF, e.g., surface width, correlation length, local slope, and roughness exponent, were calculated and used for determination of scaling exponents. A complex behavior of all parameters up to the eighth ALD cycle was evidenced. High-resolution synchrotron radiation photoemission spectroscopy was applied to characterize the chemical nature of Si/SiO2/HfO2 interface. Changes arising in the Si 2p, O 1 s, and Hf ...