In this paper, we focus on the electrical transport properties of undoped, and carbon doped b-Ga2O3 thin-films. For this purpose, b-Ga2O3 samples were grown on sapphire substrates by plasma assisted pulsed laser deposition. The samples were characterized by Raman scattering and temperature dependent dark conductivity measurements. Raman scattering shows that all samples are polycrystalline. The dark conductivity exhibits two distinctly different ranges in the measured temperature range. In the relaxed state the conductivity is activated with an energy of 0.68 eV for T > 250 K. For T < 250 K the conductivity approaches a constant value of 6x10-12 Scm-1. After illumination with sub band-gap light the dark conductivity increases up to 3 orders of magnitude. This is accompanied by a decrease of the activation energy to 0.36 eV. For C-doped samples the activation energy decreases. The state with the smaller activation energy is metastable . The time and temperature dependence of this relaxation reveals two process and can be described with two stretched exponential decays. From the temperature dependence of the time constants the energies of the traps are determined.
Recent developments in infrared (IR) microfluidics for sensitive monitoring of molecular adsorption at solid–liquid interfaces are briefly reviewed. A microfluidic platform is presented that uses a metallic island film for surface enhanced IR absorption (SEIRA) coupled to IR spectroscopies for bio-sensing and vibrational investigations of molecular monolayers and their adsorption kinetics. Exemplarily, IR spectral monitoring of the monolayer formation of 4-mercaptobenzonitrile (4-MBN) in liquid environment is discussed as a Langmuir isotherm. Adsorption isotherms of specific molecular vibrations are analyzed from the time-dependent evolution of band amplitudes and peak areas during adsorption. Given the detection limit of 0.03 nmol/cm2 , the isotherms of 4-MBN, gluthathione (GSH) monolayer formation, and the sensing of 4-nitrobenzylmercaptan (4-NBM) by the MP/graphene surface are compared. Potential applications are bio- and bio-medical sensing as well as the study of processes, e. g., enzymatic reactions, chemical or catalytic reactions, receptor–ligand interactions, and structural changes of molecules due to environmental stimuli.
Advances in polarimetric techniques are of high interest in multiple scientific fields. Polarimetry characterizes optical and material properties such as anisotropy and structure, which relate to physical, chemical and functional properties of materials used in optoelectronic, polymer, plasmonic, bio-related and pharmaceutical applications. Laser-based infrared (IR) spectroscopic methods beyond classical Fourier-transform infrared (FTIR) spectroscopy enable previously impossible polarimetric investigations from macroscopic to nanoscopic length scales. This contribution focuses on new polarimetry techniques for detailed analyses of structural and material properties of thin films and surfaces. Specifically, we show applications of two laboratory-based instruments that employ tunable quantum cascade lasers (QCLs) from Daylight Solutions as brilliant light sources. Regarding far-field IR spectroscopic measurements, a novel laser ellipsometer (built in cooperation with Sentech Instruments) simultaneously measures spectral amplitude and phase information via single-shot detection of four different polarization states. The device reaches temporal resolutions in the μs to ms range at high spectral (< 0.5 cm−1 ) and lateral resolutions (≤ 125μm). At the nanoscale, photothermal atomic force microscopy (AFM)-IR (Anasys, Bruker), based on a QCL and a polarizing unit, enables polarimetry with lateral resolutions of a few 10 nm.
In this paper, we present an experimental study of hydrogen (H) and deuterium (D) in single crystal and polycrystalline b-Ga2O3. H or D was introduced into the specimens using ampoule passivation at elevated temperatures. The samples were characterized with infrared absorption and gas effusion measurements. As-grown single crystal b-Ga2O3 contains a residual H concentration of about 1019 cm-3 and exhibits a local vibrational mode at 3437.6 cm-1 that has been attributed to O – H. Polycrystalline b-Ga2O3 thin-films were grown by pulsed laser deposition. Hydrogen effusion measurements show that these samples contain H concentrations of up to about 1020 cm-3. From the hydrogen effusion spectra, the H chemical potential was determined as a function of the H concentration, which can be related to the H density-of-states distribution.
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