In this rapid-scan regime, B 1 and d B 0/d t can be selected to achieve improved signal-to-noise ratios (SNRs) relative to those attained by conventional CWEPR. In particular, “rapid-scan” refers to the regime originally defined by Weger in terms of the incident MW field B 1, the magnetic-field scan rate d B 0/d t (for field-swept RSEPR) and the relaxation times T 1 and T 2 : This demand could potentially be met by the emerging rapid-scan (RS) technique, where resonance is passed on a time scale that is short with respect to the electron-spin relaxation times. These challenges faced by CWEPR create a need for alternate EPR detection schemes, which both enable spin quantitation and improve the sensitivity. g., the case for DB defects in a-Si:H), rendering quantitative CWEPR measurements substantially difficult. Especially spin species present at low concentrations frequently exhibit long relaxation times (as it is, e. The sensitivity of CWEPR is further limited in the presence of slow electron-spin relaxation: under these conditions, the spin system is readily saturated, which restricts the applicable incident microwave (MW) power-and hence the measurable signal intensity-to a low level.
2 With increasing electronic quality, defect densities in state-of-the-art TFS materials are approaching this range. For typical TFS samples, an absolute spin sensitivity of 10 12 spins corresponds to a concentration sensitivity of about 10 14 spins per cm 3. Quantitative EPR experiments thereby contribute to reveal the impact of defect states on electronic device performance. Due to the paramagnetic nature of many of these defects, EPR is routinely employed to quantitate defect concentrations. Such defects can act as recombination centers or trap states for charge carriers, thus impairing the electronic transport. g., dangling Si-Si bonds (DBs) in hydrogenated amorphous silicon ( a-Si:H). 1 Despite this already high sensitivity, many examples exist where the number of spins present is close to or even below this detection limit.Ī case in point are defect states in thin-film silicon (TFS) solar-cell materials, e. Present X-band CWEPR spectrometers typically achieve spin sensitivities of about 10 12 spins per mT line width. The most common application fields for quantitative EPR include radiation dosimetry, archaeological and geological dating, food analysis, environmental research and modern electronics, such as thin-film solar-cell materials. For more than four decades, continuous-wave (CW) EPR has been utilized to quantitate the concentration of paramagnetic states in various branches of both science and industry.
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