Ioana Pintilie

Formation of the Z1,2 deep-level defects in 4H-SiC epitaxial layers: Evidence for nitrogen participation

As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z1,2 defect with energy levels normally detected at about EC−0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2–3) in the gas phase, was used to prepare the samples. The Z1,2 defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5–2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z1,2 defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z1,2 defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z1,2 deep-level defect...

The influence of Cu doping on opto-electronic properties of chemically deposited CdS

Cadmium sulphide (CdS) thin films were deposited on glass substrate by precipitation from aqueous solution technique. The films were doped with copper using the direct method consisting in the addition of a copper salt (CuCl2) in the deposition bath of CdS. The doped films were annealed in air, at 300°C, for 1 h. We report some structural, optical, electrical and photoelectrical properties of CdS thin films before and after Cu doping, correlated with investigation of trapping levels by Thermally Stimulated Currents (TSC) method.

Polarization-control of the potential barrier at the electrode interfaces in epitaxial ferroelectric thin films.

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

Radiation induced point- and cluster - related defects with strong impact to damage properties of silicon detectors

This work is focusing on the investigation of those radiation induced defects causing degradation effects of Silicon detector performance. Comparative studies of the defects induced by irradiation with Co60- γ rays, 23 GeV protons and 1 MeV equivalent reactor neutrons revealed the existence of some point defects and cluster related centers having a strong impact to damage properties of Si diodes. The detailed relation between the “microscopic” reasons as based on defect analysis and their “macroscopic” consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposing to high levels of Co60- γ irradiation can be completely understood by the microscopically investigated formation of two point defects: i) a defect formed via a second order process (Ip) that can be associated with the long searched for V2O complex or with a Carbon related center and is the cause for the observed type inversion effect in Oxygen lean material; ii) a bistable donor (BD) created during irradiation that is strongly generated in Oxygen rich material, associated with one of the earlier thermal donors in Si. It is the cause for the observed positive space charge induced by irradiation in oxygenated Si diodes. Specific for hadron irradiation are the annealing effects which decrease resp. increase the originally observed damage effects as seen by the changes of the depletion voltage (effects known as “beneficial” and “reverse” annealing, respectively). A group of four cluster related defects proved to be responsible for these annealing effects. Their formation is not affected by the Oxygen content or Si growth procedure suggesting that they are complexes of multi-vacancies located inside extended disordered regions.

Investigation of X-ray induced radiation damage at the Si-SiO2 interface of silicon sensors for the European XFEL

Experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors which can withstand X-ray doses up to 1 GGy. For the investigation of X-ray radiation damage up to these high doses, MOS capacitors and gate-controlled diodes built on high resistivity n-doped silicon with crystal orientations and produced by two vendors, CiS and Hamamatsu, have been irradiated with 12 keV X-rays at the DESY DORIS III synchrotron light source. Using capacitance/conductance-voltage, current-voltage and thermal dielectric relaxation current measurements, the surface densities of oxide charges and interface traps at the Si-SiO2 interface, and the surface-current densities have been determined as function of dose. Results indicate that the dose dependence of the surface density of oxide charges and the surface-current density depend on the crystal orientation and producer. In addition, the influence of the voltage applied to the gates of the MOS capacitor and the gate-controlled diode during X-ray irradiation on the surface density of oxide charges and the surface-current density has been investigated at doses of 100 kGy and 100 MGy. It is found that both strongly depend on the gate voltage if the electric field in the oxide points from the surface of the SiO2 to the Si-SiO2 interface. Finally, annealing studies have been performed at 60°C and 80°C on MOS capacitors and gate-controlled diodes irradiated to 5 MGy and the annealing kinetics of oxide charges and surface current determined.

Growth and characterization of PbS deposited on ferroelectric ceramics

The growth and properties of PbS thin films, chemically deposited on bulk lead titano‐zirconate ceramics, are investigated. Scanning electron microscopy photographs reveal that the medium size of crystallites is much larger for PbS films grown on poled ferroelectric substrates than for the films grown onto glass or onto unpoled ferroelectric substrates. In the first case the medium size of crystallites is about 1–1.5 μm while in the second case it is less than 0.3 μm. Spectral distribution of photoconductivity and thermally stimulated currents (TSC) measurements were performed on all samples. Experimental results indicate that the properties of the PbS films can vary, depending on the substrate nature and in the case of poled ferroelectric substrates, on the polarity. The photoconductive signal is usually larger, the dark resistivity is smaller, and the TSC current value is higher for PbS films deposited on the positive face of the ferroelectric substrate than for those deposited on the negative face.

Electrical properties of Al2O3∕4H‐SiC structures grown by atomic layer chemical vapor deposition

Al2O3 films have been deposited on n-type and p-type 4H‐SiC by atomic layer chemical vapor deposition using trimethylaluminum as a precursor for aluminum and both H2O and O3 as an oxidant. After oxide deposition, annealing at different temperatures (800, 900, 1000°C) in argon atmosphere for different durations (1, 2, 3h) was performed. Bulk and interface properties of the oxide films were studied by capacitance-voltage, current-voltage, deep level transient spectroscopy, and thermally dielectric relaxation current (TDRC) measurements. The results reveal a decreasing flatband voltage with increasing annealing time, suggesting decrease of oxide charges and deep interface traps. After 3h annealing at 1000°C of the n-type samples, the flatband voltage is reduced to 6V compared to a value in excess of 40V for as-deposited samples. The TDRC measurements on annealed Al2O3∕SiC (n-type) capacitors showed substantially different spectra relative to conventional SiO2∕4H‐SiC control samples; in the former ones no sig...

Trapping levels in Bi12SiO20 crystals

Localized levels play a major role in the electro‐optic properties of Bi12SiO20 (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2–1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.

Comparison of near-interface traps in Al2O3∕4H-SiC and Al2O3∕SiO2∕4H-SiC structures

Aluminum oxide (Al2O3) has been grown by atomic layer deposition on n-type 4H-SiC with and without a thin silicon dioxide (SiO2) intermediate layer. By means of capacitance-voltage and thermal dielectric relaxation current measurements, the interface properties have been investigated. Whereas for the samples with an interfacial SiO2 layer the highest near-interface trap density is found at 0.3eV below the conduction band edge Ec, the samples with only the Al2O3 dielectric exhibit a nearly trap-free region close to Ec. For the Al2O3∕SiC interface, the highest trap density appears between 0.4 and 0.6eV below Ec. The results indicate the possibility for SiC-based metal-oxide-semiconductor field-effect transistors with Al2O3 as the gate dielectric layer in future high performance devices.

Influence of Sb3+ ions on photoconductive properties of chemically deposited PbS films

Abstract PbS films were obtained on glass substrates using the chemical bath deposition (CBD) method. The infra red (IR) photosensitivity increases of about 1000 times when a reducing agent (hydroxylamine hydrochloride NH 2 OHHCl) is added in the deposition bath. A further enhancement was tried by adding small amounts of antimony chloride (SbCl 3 ) solution in the deposition bath. The result was just opposite. It was found that the photosensitivity decreases with increasing the Sb salt content. The changes in PbS photoconductive properties are correlated with the changes in film morphology due to the two compounds added in the bath — the reducing agent and the Sb salt.