[Show abstract][Hide abstract] ABSTRACT: In this paper, an alternative method to characterize the interface between 4H polytype of Silicon Carbide (4H-SiC) and passivating dielectric layers is established. The studies are made on dielectric-semiconductor test structures using Al2O3 as dielectric on 4H-SiC n-type epitaxial layers. Samples with different pre- and post-dielectric deposition preparations have been fabricated on epilayers of varying thicknesses. Effective lifetimes (τeff) of all the samples were measured by an optical pump-probe method utilizing free carrier absorption (FCA) to analyse the influence of the 4H-SiC/dielectric interface on charge carrier recombination. The relative contribution to τeff from the surfaces increases with decreasing epilayer thickness, and by analysing the data in combination with numerical modelling, it is possible to extract values of the surface recombination velocities (SRVs) for interfaces prepared in different ways. For instance, it is found that SRV for a standard cleaning procedure is 2 × 106 cm/s compared to a more elaborate RCA process, yielding a more than 50 times lower value of 3.5 × 104 cm/s. Furthermore, the density of interface traps (Dit) is extracted from capacitance-voltage (CV) measurements using the Terman method and a comparison is made between the SRV extracted from FCA measurements and Dits extracted from CV measurements on the same structures fabricated with metal contacts. It is observed that the SRV increase scales linearly with the increase in Dit. The strong qualitative correlation between FCA and CV data shows that FCA is a useful characterization technique, which can also yield more quantitative information about the charge carrier dynamics at the interface.
Journal of Applied Physics 03/2015; 117(10):105309. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experiments are presented that reveal an efficient optical energy conversion from the visible to the infrared wavelengths range as a result of photo-acoustic response (PAR) after light pulse incites onto the free surface of TlGaSe2 crystal. Excitation was carried out with a tunable wavelength of ns-pulse laser and the PAR was detected laterally with a focused cw- probe. The observed properties can be related to variety of successive factors: high electron- hole-phonon deformation potential, a high factor of refraction coefficient dependency on pressure, the absence of surface recombination and the band filling effect, in relation with low absorption coefficient due to the forbidden direct-band optical transition in TlGaSe2. All these ensure that the acoustic energy remain well confined under a wide pulse power and energy range suggesting that TlGaSe2 is a promising material for dynamic optical energy conversion.
IOP Conference Series Materials Science and Engineering 11/2014; 68(1). DOI:10.1088/1757-899X/68/1/012004
[Show abstract][Hide abstract] ABSTRACT: The results of low-temperature photoluminescence characterization of single silicon nanocrystals prepared from highly doped silicon-on-insulator wafers are presented. The effect of B, P, As, and Sb impurities on ensemble as well as individual emission spectra are determined by comparison with the line shapes of undoped nanocrystals. From the statistical analysis of the luminescence spectra, the donor ionization energies for nanocrystals emitting in the range of 1.5–2.0 eV are estimated to be 140–200 meV, while the exciton-impurity binding energy for As- and Sb-doped nanocrystals is found to be about 40–45 meV.
[Show abstract][Hide abstract] ABSTRACT: Optical transitions in silicon nanocrystals with different surface passivations were probed at low temperatures on a single-particle level. A type of quasidirect recombination process, different from the quantum-confined exciton transition, is identified. The luminescence spectra have different emission energies, but the contribution of a no-phonon transition is significantly higher than expected from the quantum-confinement model. Its relative strength was found to be temperature dependent, suggesting spatial localization of excitons as a possible origin.
[Show abstract][Hide abstract] ABSTRACT: Residual stress and carrier lifetime variation have been measured in free-standing n-type 3C-SiC wafer grown on undulated Si substrate. We identify extended regions of residual stress that lie parallel to epilayer surfaces. The opposite polarity of stress is identified toward the interface and toward the top surface. Integrated carrier lifetime has been determined by random defect density distribution which is enhanced in the areas of double-positioning boundary defects. It is shown that carrier lifetimes are severely reduced by the presence of residual stress towards epilayer surfaces. In this way lifetime depth-distribution can be mistakenly attributed to enhanced surface recombination.
[Show abstract][Hide abstract] ABSTRACT: A 3D X-ray detector for imaging of 30–200 keV photons is described. It comprises a stack of semitransparent structured scintillators, where each scintillator is a regular array of waveguides in silicon, and with pores filled with CsI. The performance of the detector is described theoretically and explored in detail through simulations. The resolution of a single screen is shown to be determined only by the pitch, at least up to 100 keV. In comparison to conventional homogenous screens an improvement in efficiency by a factor 5–15 is obtainable. The cross-talk between screens in the 3D detector is shown to be negligible. The concept of such a 3D detector enables ray tracing and super resolution algorithms to be applied. Realized pore geometries have a lower aspect ratio than used in simulations and the roughness of the pore walls gives a 13% decrease in waveguide efficiency. Compared to currently used regular scintillators with similar resolution an efficiency increase by a factor 4 has been found for the structured scintillator.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 08/2009; DOI:10.1016/j.nima.2009.03.139 · 1.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two-photon absorption (TPA) in GaSe is detected in the narrow (about kT) spectral range above the indirect exciton energy for E⊥c light polarization. The appearance of TPA enhancement within narrow spectral region links to the resonant Raman scattering and anti-Stockes photoluminescence previously identified in GaSe. Extracted TPA coefficient is of the order of 10 cm/W and 5–10 times exceeds ones obtained previously for lower quanta energies.
Journal of Physics Conference Series 03/2008; 100(4):042008. DOI:10.1088/1742-6596/100/4/042008
[Show abstract][Hide abstract] ABSTRACT: Periodic deflections of infrared probe beam in TlGaSe2 crystal after its lateral excitation by laser pulse with photon energy near the band gap are observed. Such deflections arise from travelling of the acoustic pulse within sample which, in turn, is produced by optical pump pulse through photoacoustic effect. The photoacoustic pulse is generated within thin region near the crystal excited face, much shorter then the light penetration depth. In the case of volume excitation photoacoustic pulse is also generated in the region near back face as well. Tentative explanation of the generation mechanism is discussed.
Journal of Physics Conference Series 03/2008; 100(4):042007. DOI:10.1088/1742-6596/100/4/042007
[Show abstract][Hide abstract] ABSTRACT: Fundamental band edge absorption is investigated in nominally undoped (n<1014 cm−3) and heavily doped (n ∼ 8×1018 cm−3) 4H‐SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500 cm−1) at temperatures from 75 to 450 K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2 K. Exciton binding energy of 30±10 meV is derived from fitting the data at 75 K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.
[Show abstract][Hide abstract] ABSTRACT: A newly developed fabrication method makes the formation of deep structured scintillator screens possible. We demonstrate that electrochemical etching in silicon can be used to produce regular arrays of 120 μm deep pores with a 4 μm pitch. A layer of SiO2 is grown on the pore walls and CsI:Tl is melted into the pores, resulting in a structure with a high refractive index core surrounded by a quartz cladding, providing efficient light guiding. The efficiency and radiation hardness of the scintillator is evaluated in realistic environment at beamline ID15 at the ESRF synchrotron. The efficiency is measured to be a factor two higher than a planar YAG: Ce scintillator of equal thickness, while radiation damage is found to be neglectable for doses up to at least 2×104 Gy.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 06/2007; 576(1):52-55. DOI:10.1016/j.nima.2007.01.119 · 1.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on electronic properties of single- and double-layer stacking faults in 4H-SiC and provide an insight into apparent distinctions of recombination-enhanced defect reactions at these faults. Photoluminescence imaging spectroscopy and deep-level transient spectroscopy experiments reveal key constituents of radiative recombination and also provide firm evidence of nonradiative centers at E-V+0.38 eV responsible for recombination-enhanced mobility of silicon-core partial dislocations. A comprehensive energy level model is proposed allowing for a qualitative description of recombination activity at different types of stacking faults and the corresponding bounding partial dislocations.
Physical Review B 12/2006; 74(23):233203. DOI:10.1103/PhysRevB.74.233203 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An experimental technique using optical excitation by a YAG laser pulse for studying avalanche injection in power devices is demonstrated. This technique enables the creation of high uniform excess carrier concentrations in an optically defined device volume, involving very little heating. A method for determining the onset of avalanche multiplication, by studying the time integral of the reverse recovery current, is proposed. A PiN diode is observed to turn off from avalanching at a dissipated power density of more than 200 kW/cm2.
Physica Scripta 12/2006; 1997(T69):134. DOI:10.1088/0031-8949/1997/T69/023 · 1.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Individual silicon quantum dots were fabricated by electron-beam lithography, plasma etching and a two-step oxidation process. This enables photoluminescence (PL) from individual dots at various temperatures to be detected and spectrally resolved using a sensitive charge-coupled device camera-imaging system, as reported previously. The regular array-like arrangement of oxidized pillars containing individual nanocrystals, in principle, enables combined transmission electron microscopy (TEM) and low-temperature PL characterization of the same Si quantum dot. To this end, a technique employing focused ion beam was developed for preparation of the pillar/nanocrystal of interest for TEM. It is shown that silicon quantum dots of several nanometers in size can be characterized using such a method.
Journal of Luminescence 12/2006; 121(2):353-355. DOI:10.1016/j.jlumin.2006.08.061 · 2.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For determination of the effective surface recombination velocity (SRV) at the Si–SiO2 interface, a new method based on depth- and time-resolved free-carrier absorption measurements is put forward. The proposed technique is more advantageous than conventionally used methods and makes it possible to obtain the SRV and its injection-level dependence on a particular sample surface without additional knowledge of other silicon parameters. The experimental measurements have been performed on n–Si with polished facets and a cleaved edge. All surfaces were covered by thermally grown SiO2. The result is that the SRV increases with the injection level followed by saturation. The computer simulation of the excess carrier decay has shown that, for polished surfaces, the value of the SRV should be reduced in the high injection region and not only does depend on the injected level, but also on the initial excitation. It is proposed that temporal charging of the Si–SiO2 interface can be responsible for such behavior.
Physica Scripta 11/2006; 1999(T79):322. DOI:10.1238/Physica.Topical.079a00322 · 1.30 Impact Factor