Pulsed EDMR study of charge trapping at Pb centers

Journal ArticleLow temperature pulsed electrically detected magnetic resonance (pEDMR) measurements of charge trapping and recombination transitions involving Pb centers at the c-Si (111) /Si02 interface are presented. The results of these experiments show that when a conduction electron is trapped, it forms a strongly coupled spin pair with the defect electron prior to its readjustment into the charged Pb" ground state. The data reveals that the Lande factors of the two electrons within these pairs are almost identical (difference <0.002) and that they are, within the measurement accuracy, identical to the Lande factor of the uncharged, singly occupied Pb center, from % it is concluded that trapping and recombination at Pb defects is dominated by direct charge capture and not by tunneling or hopping transitions from other localized states Different cross anions attributed in previous studies to different interface defects at the c-Si/Si02 interface can be explained by readjustment out of different spin configurations of the charged Pb detect

Electrical detection of spin coherence in silicon

Journal ArticleExperimental evidence is presented showing that photocurrents in silicon can be used as highly sensitive readout probes for coherent spin states of localized electrons, the prime candidates for quantum bits in various semiconductor based quantum computer concepts. Conduction electrons are subjected to fast Rabi oscillation induced by means of pulsed electron spin resonance. The collective spin motion of the charge carrier ensemble is reflected by a spin-dependent recombination rate and therefore by the sample conductivity. Because of inhomogeneities, the Rabi oscillation dephases rapidly. However, a microwave induced rephasing is possible causing an echo effect whose intensity contains information about the charge carrier spin state and the coherence decay

High Resolution Microimaging with Pulsed Electrically-Detected Magnetic Resonance

The investigation of paramagnetic species (such as point defects, dopants, and impurities) in solid-state electronic devices is significant because of their effect on device performance. Conventionally, these species are detected and imaged using the electron spin resonance (ESR) technique. In many instances, ESR is not sensitive enough to deal with miniature devices having small numbers of paramagnetic species and high spatial heterogeneity. This limitation can in principle be overcome by employing a more sensitive method called electrically-detected magnetic resonance, which is based on measuring the effect of paramagnetic species on the electric current of the device while inducing electron spin-flip transitions. However, up until now, measurement of the current of the device could not reveal the spatial heterogeneity of its paramagnetic species. We provide here, for the first time, high resolution microimages of paramagnetic species in operating solar cells obtained through electrically-detected magnetic resonance. The method is based on unique microwave pulse sequences for excitation and detection of the electrical signal under a static magnetic field and powerful pulsed magnetic field gradients that spatially encode the electrical current of the sample. The approach developed here can be widely used in the nondestructive three-dimensional inspection and characterization of paramagnetic species in a variety of electronic devices.Comment: 19 pages, 4 figures +S

Enhancing solar cells with photochemical upconversion

Triplet-triplet annihilation in organic molecules increases the photocurrent of thin-film solar cell

Rapid-scan electron paramagnetic resonance using an EPR-on-a-Chip sensor

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we present an improved design of a miniaturized EPR spectrometer implemented on a silicon microchip (EPR-on-a-chip, EPRoC). In place of a microwave resonator, EPRoC uses an array of injection-locked voltage-controlled oscillators (VCOs), each incorporating a 200 μm diameter coil, as a combined microwave source and detector. The individual miniaturized VCO elements provide an excellent spin sensitivity reported to be about 4 × 109spins/√Hz, which is extended by the array over a larger area for improved concentration sensitivity. A striking advantage of this design is the possibility to sweep the MW frequency instead of the magnetic field, which allows the use of smaller, permanent magnets instead of the bulky and powerhungry electromagnets required for field-swept EPR. Here, we report rapid scan EPR (RS-EPRoC) experiments performed by sweeping the frequency of the EPRoC VCO array. RS-EPRoC spectra demonstrate an improved SNR by approximately two orders of magnitude for similar signal acquisition times compared to continuous wave (CW-EPRoC) methods, which may improve the absolute spin and concentration sensitivity of EPR-on-a-Chip at 14 GHz to about 6 × 107 spins/√Hz and 3.6 nM⁄√Hz, respectively

A 14-channel 7 GHz VCO-based EPR-on-a-chip sensor with rapid scan capabilities

This paper presents a VCO-based EPR-on-a-chip (EPRoC) sensor for portable, battery-operated electron paramagnetic resonance (EPR) spectrometers. The proposed chip contains an array of 14 injection-locked VCOs as the sensing element for an improved sensitive volume and phase noise performance. By cointegrating a high-bandwidth PLL, the presented design allows for continuous-wave and rapid-scan EPR experiments with a minimum number of external components. The active loop filter introduces an assisted replica charge pump that mitigates the slewing requirements on the loop-filter amplifier. The measured spin sensitivity of 2×10 9 spins/Hz−−−√ together with the large active volume of 210 nl lead to an 8-fold improvement in concentration sensitivity compared to the state-of-the-art in EPRoC detectors

Valence band offset in heterojunctions between crystalline silicon and amorphous silicon (sub)oxides (a-SiOx:H, 0 < x < 2)

The heterojunction between amorphous silicon (sub)oxides (a-SiOx:H, 0 < x < 2) and crystalline silicon (c-Si) is investigated. We combine chemical vapor deposition with in-system photoelectron spectroscopy in order to determine the valence band offset ΔEV and the interface defect density, being technologically important junction parameters. ΔEV increases from ≈0.3 eV for the a-Si:H/c-Si interface to >4 eV for the a-SiO2/c-Si interface, while the electronic quality of the heterointerface deteriorates. High-bandgap a-SiOx:H is therefore unsuitable for the hole contact in heterojunction solar cells, due to electronic transport hindrance resulting from the large ΔEV. Our method is readily applicable to other heterojunctions

Microwave field mapping for EPR-on-a-chip experiments

Electron paramagnetic resonance–on-a-chip (EPRoC) devices use small voltage-controlled oscillators (VCOs) for both the excitation and detection of the EPR signal, allowing access to unique sample environments by lifting the restrictions imposed by resonator-based EPR techniques. EPRoC devices have been successfully used at multiple frequencies (7 to 360 gigahertz) and have demonstrated their utility in producing high-resolution spectra in a variety of spin centers. To enable quantitative measurements using EPRoC devices, the spatial distribution of the B1 field produced by the VCOs must be known. As an example, the field distribution of a 12-coil VCO array EPRoC operating at 14 gigahertz is described in this study. The frequency modulation–recorded EPR spectra of a “point”-like and a thin-film sample were investigated while varying the position of both samples in three directions. The results were compared to COMSOL simulations of the B1-field intensity. The EPRoC array sensitive volume was determined to be ~19 nanoliters. Implications for possible EPR applications are discussed

The Energy Materials in-Situ Laboratory Berlin (EMIL) at BESSY II

The Helmholtz Center Berlin (HZB) and the Max-Planck Society (MPG) strengthen their research in renewable energies with the implementation of the joint Energy Material in-Situ Lab Berlin (EMIL) at the third generation light source BESSY II. The new facility is dedicated to the in-situ and in-system x-ray analysis of materials and devices for photovoltaic applications, (photo-) catalytic processes, energie conversion and storage. To obtain a comprehensive understanding of the involved materials, spectroscopic methods with x-rays from the soft- up to the hard x-ray regime reveal an almost complete picture of their chemical and electronic properties. The contribution presents the layout of the x-ray beamlines and their performance in terms of photon flux, energy resolution and spot sizes