Electrically Detected Magnetic Resonance Research Articles

Spin-dependent recombination affected by post-annealing of organic photovoltaic devices

Organic photovoltaic devices (OPVs) are attracting attention because of recent rapid enhancement of their power conversion efficiency. For further improvement, optimization of fabrication processes is one useful path to a solution. During OPV fabrication, particularly of the bulk heterojunction active layer, annealing treatments contribute to the device performance. Many studies have examined annealing-related properties. However, further research must clarify how paramagnetic species in the devices play their roles by annealing. Using well-known OPVs, we investigated the relation between spin-dependent recombination (SDR) current and the paramagnetic species, which vary the numbers by post-annealing with active layers consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). A simultaneous detection method of electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR), which we originally developed, was applied to OPVs for the first time ever reported. Results show that PC61BM anion radicals generated by post-annealing of P3HT:PC61BM OPVs with a lithium fluoride (LiF)/aluminum (Al) electrode do not contribute to the SDR current at the interface and that P3HT cation radicals enhance the SDR current. By contrast, devices with an Al electrode without LiF decrease the total SDR current, although the quantities of cation radical molecules do not vary. This finding suggests that changes of the hole blocking layer in the devices caused by the annealing treatment affect the size of capture cross sections of P3HT cation radicals. Our new method of quantitative observation of the EDMR changes through the ESR signals is expected to be useful for investigating the capture cross sections in OPVs.

Electrically Detected Magnetic Resonance on a Chip (EDMRoC) for Analysis of Thin-Film Silicon Photovoltaics

Electrically detected magnetic resonance (EDMR) is a spectroscopic technique that provides information about the physical properties of materials through the detection of variations in conductivity induced by spin-dependent processes. EDMR has been widely applied to investigate thin-film semiconductor materials in which the presence of defects can induce the current limiting processes. Conventional EDMR measurements are performed on samples with a special geometry that allows the use of a typical electron paramagnetic resonance (EPR) resonator. For such measurements, it is of utmost importance that the geometry of the sample under assessment does not influence the results of the experiment. Here, we present a single-board EPR spectrometer using a chip-integrated, voltage-controlled oscillator (VCO) array as a planar microwave source, whose geometry optimally matches that of a standard EDMR sample, and which greatly facilitates electrical interfacing to the device under assessment. The probehead combined an ultrasensitive transimpedance amplifier (TIA) with a twelve-coil array, VCO-based, single-board EPR spectrometer to permit EDMR-on-a-Chip (EDMRoC) investigations. EDMRoC measurements were performed at room temperature on a thin-film hydrogenated amorphous silicon (a-Si:H) pin solar cell under dark and forward bias conditions, and the recombination current driven by the a-Si:H dangling bonds (db) was detected. These experiments serve as a proof of concept for a new generation of small and versatile spectrometers that allow in situ and operando EDMR experiments.

Spin dynamics of exchange-coupled nitrogen donors in heavily dopedn-type15RSiC monocrystals: Multifrequency EPR and EDMR study

Semiconductor devices based on $15R$ silicon carbide (SiC) show improved properties compared to other polytypes. Here, we report the investigation of nitrogen-doped $15R$ SiC monocrystals with $({N}_{\mathrm{D}}--{N}_{\mathrm{A}})\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ using multifrequency electron paramagnetic resonance (EPR) and electrically detected magnetic resonance (EDMR) spectroscopic methods in the microwave (MW) frequency range from 9.4 to 328.84 GHz and in the temperature range from 4.2 to 300 K. A single intensive $S$ line with $S=1/2$, at ${g}_{\ensuremath{\perp}}=2.0026(2)$, and ${g}_{\ensuremath{\parallel}}=2.0043(2)$ dominates the EPR spectrum in $15R$ SiC at $T<160$ K and was attributed to the exchange-coupled nitrogen (N) donors substituting quasicubic ``$k1$'' site having the deeper energy level in $15R$ SiC lattice. From the analysis of the temperature behavior of the integral intensity, magnetic field position, intensity, shape, and width of the $S$ line, it was concluded that at $T<90$ K, the exchange coupling occurs between localized donor electrons, while at $T=90\ensuremath{-}150$ K the interaction between exchange-coupled N donors and conduction electrons takes place. In the temperature interval from 20 to 160 K, the $S$-line EPR width was characterized by a two-phonon Orbach relaxation process with the activation energy of $\ensuremath{\sim}6.5$ meV corresponding to the valley-orbit splitting values for N donors in $15R$ SiC. The variable-range hopping regime obeying the ${T}^{1/4}$ law was found to take place at $T<20$ K leading to the appearance of the $S$ line in the EDMR spectrum at low temperatures. The appearance of the EDMR signal from exchange-coupled N donors is caused by the EPR-induced temperature-increase mechanism and the spin-flip hops process. The results obtained from MW conductivity measurements agree with EPR and EDMR data. The temperature variation of MW conductivity was described by several processes, including the electron-hopping process between N donor impurity atoms at $T<50$ K with activation energy ${\ensuremath{\varepsilon}}_{3}=1.5$ meV, electron transitions between Hubbard bands at $T=50--100$ K; the transition of the electrons from the donor energy levels to conduction band with ionization energy ${\ensuremath{\varepsilon}}_{1}=32$ meV at $T=100--200$ K and scattering of the conduction electrons by ionized donors occurred at the temperature higher than 200 K.

Combining electrically detected magnetic resonance techniques to study atomic-scale defects generated by hot-carrier stressing in HfO2/SiO2/Si transistors

This work explores the atomic-scale nature of defects within hafnium dioxide/silicon dioxide/silicon (HfO2/SiO2/Si) transistors generated by hot-carrier stressing. The defects are studied via electrically detected magnetic resonance (EDMR) through both spin-dependent charge pumping and spin-dependent tunneling. When combined, these techniques probe defects both at the Si-side interface and within the oxide-based gate stack. The defects at the Si-side interface are found to strongly resemble Pb-like defects common in the Si/SiO2 system. The defect within the gate stack has not been positively identified in the literature thus far; this work argues that it is a Si-dangling bond coupled to one or more hafnium atoms. The use of EDMR techniques indicates that the defects detected here are relevant to electronic transport and, thus, device reliability. This work also highlights the impressive analytical power of combined EDMR techniques when studying complex, modern materials systems.

A new approach to electrically detected magnetic resonance: Spin-dependent transient spectroscopy

Electrically detected magnetic resonance (EDMR) is arguably the most sensitive method available to study electrically active point defects in semiconductor devices. Most EDMR studies have utilized spin-dependent recombination current and, thus, require p–n junctions or a photoconductive structure. Some time ago, Chen and Lang proposed and demonstrated EDMR via spin-dependent deep level transient spectroscopy in metal–oxide–semiconductor capacitors. We report on a similar and significantly simpler technique: spin-dependent transient spectroscopy (SDTS). We show that the sensitivity of this technique is independent of the resonance field and frequency. Through capacitance–voltage analysis, combined with our SDTS results, this technique can (crudely) provide information about the density of states of defects with a broad distribution of energy levels. In addition, we show that SDTS can be readily adapted to near-zero-field magnetoresistance effect measurements.

Intermediate spin pair relaxation through modulation of isotropic hyperfine interaction in frequency-swept spin-dependent recombination in 4H–SiC

Electrically detected magnetic resonance (EDMR) measurements have been extended to sub-mT magnetic fields through utilization of frequency sweeping of the oscillating magnetic field, where conventional electron paramagnetic resonance-based measurements traditionally utilize magnetic field magnitude ramping. In spin-dependent transport measurements in devices, an oftentimes pervasive near-zero field magnetoresistance effect overwhelms the sub-mT regime. This magnetoresistance effect is independent of the RF drive. Thus, by utilizing a constant DC magnetic field and a frequency sweep of the RF magnetic field, the magnetoresistance effect is not detected, leaving only the EDMR response. Interesting EDMR-based phenomena emerge at sub-mT fields when the oscillating field magnitude approaches the static field, such as multiple-photon transitions caused by the emergence of Floquet spin states and Bloch–Siegert shifts. A spectral-narrowing effect also emerges as the static field is reduced. In this work, we show that the narrowing of the frequency-swept EDMR response with static field can be modeled by changes in intermediate spin-pair relaxation through modulation of hyperfine fields caused by stochastic perturbations from the environment. We utilize recently developed theory to model the relaxation of spin pairs and show that stochastic interactions of the electron spin with the environment yield both Floquet spin states and changes in intermediate spin-pair relaxation.

Extraction of dipolar coupling constants from low-frequency electrically detected magnetic resonance and near-zero field magnetoresistance spectra via least squares fitting to models developed from the stochastic quantum Liouville equation

We report low-frequency electrically detected magnetic resonance (EDMR) and near-zero field magnetoresistance (NZFMR) measurements observed through spin-dependent trap-assisted tunneling on unpassivated 28Si/28SiO2 metal–insulator–semiconductor (MIS) capacitors. This study both experimentally and theoretically explores the effects of the low-frequency EDMR response and the mechanisms responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions. Previous reports that utilized high-frequency EDMR and NZFMR on these devices indicated that the observed trap-assisted tunneling spectra are dominated by silicon dangling bonds back bonded to silicon at the Si/SiO2 interface, Pb0 and Pb1 centers. These previous results also suggest that the rate limiting step in trap-assisted tunneling is the interface to an oxide trapping event. In this work, we extend the theory to show the explicit connection of the defects observed between the NZFMR response and the EDMR, which has not yet been demonstrated. We also extend a theoretical approach to the analysis of both the EDMR and NZFMR spectra and match the theory to experimental observations made in 28Si/28SiO2 MIS capacitors. The method utilizes a least squares fitting algorithm of models developed from the stochastic quantum Liouville equation. We find that we can extract a dipolar coupling constant by fitting both the NZFMR and EDMR spectra. Our experimental results and resulting fitted spectra from our quantitative model suggest the mechanism responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions, is predominately magnetic dipolar interactions between Pb centers at the interface.

A quantitative model for the bipolar amplification effect: A new method to determine semiconductor/oxide interface state densities

We report on a model for the bipolar amplification effect (BAE), which enables defect density measurements utilizing BAE in metal–oxide–semiconductor field-effect transistors. BAE is an electrically detected magnetic resonance (EDMR) technique, which has recently been utilized for defect identification because of the improved EDMR sensitivity and selectivity to interface defects. In previous work, BAE was utilized exclusively in EDMR measurements. Although BAE EDMR improves the sensitivity of EDMR in studies of semiconductor/oxide interface defects, an understanding of BAE in both electrical measurements and EDMR has not yet been investigated. In this work, we develop a BAE theory based on a modified Fitzgerald–Grove surface recombination methodology, which, in theory, may be utilized to fine-tune conditions for EDMR measurements. BAE may also now be utilized as an analysis tool in purely “electronic” measurements. The model presented here may ultimately prove useful in the development of resonance-based theories of BAE EDMR.

Electrically detected magnetic resonance and near-zero field magnetoresistance in 28Si/28SiO2

We report on electrically detected magnetic resonance (EDMR) and near-zero-field magnetoresistance (NZFMR) measurements observed through spin-dependent trap-assisted-tunneling on unpassivated Si/SiO2 metal–insulator–semiconductor capacitors comparing those containing silicon of natural isotopic abundance and silicon depleted of 29Si. Although our measurements involve monitoring the spin-dependence of the trap-assisted-tunneling current responsible for leakage across the oxide, the EDMR spectra resemble that of a combination of Pb0 and Pb1 silicon dangling bonds sites at the Si/SiO2 interface. Additionally, we observe a substantial narrowing of the NZFMR response with the removal of 29Si nuclei. The breadth of the NZFMR response is strongly influenced by hyperfine interactions. Since superhyerfine interactions between 29Si nuclei and silicon dangling bonds at the Si/SiO2 interface are a full order of magnitude stronger than such interactions involving silicon dangling bonds defects (E′ centers) within the oxide, the NZFMR results also strongly suggest a response dominated by Si/SiO2 interface trap defects. These results collectively suggest very strongly that the leakage currents that we observe involve tunneling from Si/SiO2 Pb dangling bonds to defects within the oxide. Our results thus offer fundamental insight into technologically important phenomena involving oxide leakage currents in metal–oxide–semiconductor devices such as stress induced leakage currents and time dependent dielectric breakdown.

Measuring the Magnetic Field Amplitude of rf Radiation by the Quasistatic Magnetic Field Effect in Organic Light-Emitting Diodes

Electron paramagnetic resonance (EPR) is a versatile tool to probe spin physics in organic semiconductor materials. A common method used to detect the spin-\textonehalf{} paramagnetic resonance in organic light-emitting diodes (OLEDs) is to measure the device resistance under EPR conditions, i.e., to record electrically detected magnetic resonance (EDMR). Here, we present ultralow-frequency EDMR experiments on OLEDs that exhibit a qualitatively new line shape because of a quasistatic magnetic field effect: the modulation of the static ultrasmall field-effect magnetoresistance arising from the magnetic field amplitude ${B}_{1}$ of the radio frequency (rf) radiation. The disappearance of spin-\textonehalf{} Zeeman resonances of individual charge carriers in the OLED, i.e., the resonances at magnetic fields where the Zeeman splitting matches the photon energy of the incident rf radiation, coincides with the emergence of the quasistatic effect. We discuss the origin of this quasistatic magnetic field effect, its characteristic line shape in terms of the magnetic field dependence, the influence of experimental parameters, and the application potential with regards to EDMR experiments. The EDMR line shape can be inferred numerically from the magnetoresistance measurements. This approach enables a unique means of determining the drive-field strength ${B}_{1}$ in EDMR under driving conditions where alternative methods employing an analysis of the Zeeman resonance---such as power broadening and Rabi flopping---are not applicable.

Ultra-low field frequency-swept electrically detected magnetic resonance

We have developed a new ultra-low field frequency-swept (FS) electrically detected magnetic resonance (EDMR) spectrometer to perform sensitive EDMR measurements of 4H-silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors at sub-millitesla (mT) magnetic fields. The new spectrometer design enables the detection of so-called ultra-strong coupling effects such as multiple-photon transitions and Bloch–Siegert shifts. In this paper, we present a new spectrometer design and discuss ultra-low field FS-EDMR sensitivity to both multiphoton transitions and Bloch–Siegert shifts of the FS-EDMR response. FS-EDMR effectively eliminates the interference of the sub-mT EDMR response from a near-zero field magnetoresistance (NZFMR) phenomenon that pervades the sub-mT regime in a magnetic field-swept EDMR scheme. We discuss an automatic power leveling scheme, which enables frequency sweeping. We also present results illustrating the Bloch–Siegert shift of the FS-EDMR response. Finally, we study the two-photon transition line shape in the 4H-SiC transistor as a function of the static field, in which we observe a collapse of the two-photon linewidth with decreasing static field and compare our results to the theory of two-photon absorption in EDMR.

Photogenerated carrier dynamics of TIPS-pentacene films as studied by photocurrent and electrically detected magnetic resonance.

The carrier generation process and spin dynamics through photoexcitation in the vacuum vapour deposition film of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were investigated by temperature dependence measurements of photocurrent and electrically detected magnetic resonance (EDMR). The EDMR signal was constructed from two components and showed a maximum at approximately 200 K. The temperature dependence was analysed using quantum mechanical simulation, assuming the carrier dynamics of the weakly coupled electron-hole pair (e-h pair). In addition, the analytical formula of photocurrent generation and EDMR signal intensity were also derived based on classical rate equations and used to understand the carrier dynamics. Through phase-shift analysis in quadrature detection of the EDMR signals, one of the two components was well analysed by using a narrow Lorentzian shape, and the other was by using a broad Gaussian.

A technique to measure spin-dependent trapping events at the metal–oxide–semiconductor field-effect transistor interface: Near zero field spin-dependent charge pumping

We discuss a new technique to measure spin-dependent trapping events at the metal–oxide–semiconductor field-effect transistor (MOSFET) channel/gate dielectric interface. We call this technique near zero field spin-dependent charge pumping (NZF SDCP). It is based on a powerful MOSFET interface trap characterization measurement called charge pumping and related to an electrically detected magnetic resonance (EDMR) technique called SDCP. NZF SDCP and EDMR SDCP measurements are made on 4H-SiC MOSFETs, and we find that the introduction of nitrogen to the MOSFET interface can have a profound impact on the NZF SDCP response, which suggests that NZF SDCP may be useful to get atomic scale information about MOSFET interfaces such as defect identification. We also find that the NZF SDCP amplitude appears to saturate as a function of charge pumping frequency in most cases but not all. We make model calculations to explain this behavior. We also find that the NZF SDCP spectrum broadens with increasing charge pumping frequency, which may be an inherent NZF SDCP phenomenon. We hypothesize that NZF SDCP may also allow for experimental exploration of some magnetoresistance theories regarding interaction times between charge carriers and traps.

Spin-Dependent Charge-Carrier Recombination Processes in Tris (8-Hydroxyquinolinato) Aluminum

We have studied the nature and dynamics of spin-dependent charge carrier recombination in Tris(8-hydroxyquinolinato) aluminum (Alq$_3$) films in light emitting diodes at room temperature using continuous wave and pulsed electrically detected magnetic resonance (EDMR) spectroscopy. We found that the EDMR signal is dominated by an electron-hole recombination process, and another, weaker EDMR signal whose fundamental nature was investigated. From the pulsed EDMR measurements we obtained a carrier spin relaxation time, $T_2 = 45\pm 25$ ns which is much shorter than $T_2$ in conjugated polymers, but relatively long for a molecule containing elements with high atomic number. Using multi-frequency continuous wave EDMR spectroscopy, we obtained the local hyperfine field distributions for electrons and holes, as well as their respective spin-orbit coupling induced g-factor and g-strain values.

Modeling of Near Zero-Field Magnetoresistance and Electrically Detected Magnetic Resonance in Irradiated Si/SiO2 MOSFETs

Near zero-field magnetoresistance (NZFMR) spectroscopy has the potential to provide chemical and physical information on radiation damage in 3-D integrated circuits. Electrically detected magnetic resonance (EDMR) has provided chemical and physical information for several decades but is limited in applicability due to the need for an RF electromagnetic field. We model NZFMR and EDMR in Si/SiO2 metal–oxide–semiconductor field-effect transistors (MOSFETs) by including spin-dependent recombination and hyperfine interactions in stochastic Liouville equations. We find that to accurately describe both NZFMR and EDMR, both the defect spin and conduction spin has to interact with a bath of nuclear spins, which are assumed to be composed of combinations of 29Si and hydrogen.

Electrically detected magnetic resonance of organic Schottky barrier diodes

The results of electrically detected magnetic resonance (EDMR) experiments on organic Schottky barrier diode based on regioregular poly(3-hexylthiophene-2,5-diyl) [P3HT] are presented. The existence of positively charged trapped polarons and bipolaron formation process with mobile polarons of the same kind are responsible for the EDMR signal. Variable frequency EDMR measurements indicate that spin-orbit coupling effects on the EDMR linewidth.

Electron-spin-resonance and electrically detected-magnetic-resonance characterization on PbC center in various 4H-SiC(0001)/SiO2 interfaces

We characterized an intrinsic interface defect, called the “PbC center,” formed at 4H-SiC(0001)/SiO2 interfaces by means of electron-spin-resonance (ESR) and electrically detected-magnetic-resonance (EDMR) spectroscopies. The formation of the PbC center was observed with a spin density of 3–4 × 1012 cm−2 after standard thermal oxidation. This center could be effectively removed by the NO post-oxidation-anneal (POA) process or ultra-high-temperature oxidation and could be passivated by H atoms via the H2 POA process. There was a clear correlation between the PbC center and field-effect mobility (μFE) of 4H-SiC(0001) metal–oxide–semiconductor field effect transistors (MOSFETs). The PbC center decreased μFE because this center acts as electron traps, reducing the free-carrier density in the inversion channel of 4H-SiC(0001) MOSFET. We also examined the counter doping effect of NO POA by introducing 15N impurities; however, the counter doping of 15N donors was not detectable by ESR (much lower than 2 × 1011 cm−2). Highly sensitive EDMR measurements revealed that the PbC center has two isotropic hyperfine (HF) interactions at 1.3 and 6.8 mT and suggested that its main 13C HF interaction should be larger than 14 mT. Based on the present experimental data, the origin of the PbC center was ascribed as a carbon-related interface defect that forms a C–H bond after hydrogen passivation. This feature is similar to that of the porous-PbC centers (carbon dangling-bond centers) found in porous-SiC/SiO2 systems. However, their HF signatures indicated that the PbC center at 4H-SiC(0001)/SiO2 interfaces and the porous-PbC centers in porous-SiC should be different centers associated with different wave functions.

Low-magnetic field effect and electrically detected magnetic resonance measurements of photocurrent in vacuum vapor deposition films of weak charge-transfer pyrene/dimethylpyromellitdiimide (Py/DMPI) complex

Magnetic field effect (MFE) and electrically detected magnetic resonance (EDMR) measurements of photocurrent have been conducted to clarify the excited-state dynamics in films of an organic weak charge-transfer (CT) complex, Pyrene/Dimethylpyromellitdiimide (Py/DMPI), fabricated by vacuum vapor deposition. Low-field MFE measurements of the photocurrent were carried out using an interdigitated platinum electrode made on a quartz substrate as well as the re-examination of the photocurrent and MFE in the range of 3-200 mT. The spin-dependent carrier dynamics leading to the low-field MFE are reasonably simulated as the low-field effect due to the hyperfine mechanism in the radical-pair intersystem crossing, which was solved through the Liouville equations of the density matrix for the stepwise hopping model in the doublet electron-hole pair (DD pair mechanism). Single-crystal time-resolved electron spin resonance measurement was also carried out to justify the MFE mechanism. The averaged trap depth (Etrap) of the triplet exciton was estimated to be +640 ± 89 cm-1 (Etrap/kB = +921 ± 128 K) by the temperature dependence of the signal intensity. This finding gave confidential experimental evidence for the majority of the trapped triplet exciton (3ext). The EDMR experiment directly revealed the evidence of the weakly coupled electron-hole pairs. The effective activation energies (ΔE) for the separation from the photoinduced CT state to the mobile carries are 1200-1900 cm-1 (ΔE/kB = 1700-2700 K). A systematic protocol to clarify the photo-generated carrier dynamics in weak CT complexes is demonstrated, and our findings from this method give not only further support for the two types of collision mechanisms assumed in our previous work but also the detailed information of the carrier dynamics of the weak CT complex, including the activation energy and trapping/detrapping process, which have significant influence on the performance of the organic devices.

Electron–Hole Pairs Generated in the Crystalline Phase of Polymer Diodes Studied by Electrically Detected Magnetic Resonance Techniques

The properties of electron–hole (e–h) pairs generated in a working poly(3-hexylthiophene) (P3HT) diode are investigated by electrically detected magnetic resonance (EDMR) techniques. The EDMR intensity is shown to increase with increasing density of injected electrons for a given hole density, demonstrating that the EDMR signal arises from an e–h pair. The EDMR spectrum consists of two Gaussian curves, one of which gives a g-value very similar to that of hole carriers, suggesting that the EDMR spectrum is given by the sum of electron spin resonance (ESR) spectra from hole and electron carriers forming e–h pairs. Bias-dependent correlation between carriers and e–h pairs under diode operation is examined directly from independent measurements of near-infrared (NIR) spectroscopy for carriers and EDMR for e–h pairs. When the bias is increased to above a threshold, the EDMR signal is strongly reduced despite a gradual increase in the NIR signals, providing evidence that the e–h pairs are dissociated by an elec...

Electrically detected magnetic resonance study of barium and nitric oxide treatments of 4H-SiC metal-oxide-semiconductor field-effect transistors

We report on the effects of barium interfacial layer (IL) deposition and nitric oxide (NO) anneals on interface/near-interface defects in 4H-SiC metal-oxide-semiconductor field-effect transistors utilizing electrically detected magnetic resonance (EDMR). The 4H-SiC/SiO2 interface has a large number of electrically active defects that reduce the effective channel mobility. Various passivation schemes have been utilized to decrease the interface defect density and thus increase mobility. Two passivation schemes of great interest are postoxidation annealing in nitric oxide (NO) and deposition of a barium interfacial layer (IL) before oxide growth. Our measurements compare the chemical nature of defects very near the 4H-SiC/SiO2 interface in devices utilizing both passivation schemes and nonpassivated devices. Both the NO anneal and the barium IL greatly reduce the interface region EDMR response, which corresponds to a large improvement in mobility. However, the EDMR response in devices subjected to the two passivation processes is somewhat different. We present results that suggest spin lattice relaxation times are longer in samples that received a barium IL than in samples with NO annealing; this result suggests a lower level of local strain within the vicinity of defects very near the 4H-SiC/SiO2 interface in barium treated samples over NO annealed samples.