Light-induced metastable defects in a-Si:H as elucidated by optically detected magnetic resonance measurements at 2K

We observed a gradual increase of a nonradiative recombination channel in a-Si:H by strong light exposure at 2K, i.e., an increase in the intensity of the quenching optically detected magnetic resonance (ODMR) line at g=2.005 and no change of the other quenching lines at g=2.004 and g=2.013. The sample with previous light soaking at 300K still showed a degradation of photoluminescence (PL) efficiency as much as that for the sample without previous light soaking, but no obvious change of the quelching ODMR lines. An increase of silicon dangling bond (db) density and the other nonradiative recombination process are suggested to explain the experimental results.     

ODMR study of recombination processes in ionic crystals and silicon carbide

The paper presents the results of optically detected magnetic resonance (ODMR) study of tunneling and photostimulated recombination processes in irradiated ionic crystals, observation of resonance effects using optically pumped F-centres, and investigation of spin-dependent recombination processes in silicon carbide doped with boron, aluminium, gallium and scandium.     

Radiative and nonradiative recombination processes in hydrogenated amorphous silicon as elucidated by optically detected magnetic resonance

Radiative and nonradiative recombination processes have been investigated by measurements of optically detected magnetic resonance at 2 K in hydrogenated anorphous silicon. Relevant processes are discussed on the basis of the experimental results.     

Optical detection of magnetic resonance on anion antisites in indium phosphide

The paper describes the application of optically detected magnetic resonance (ODMR) methods to the identification of antisite defects in the III–V semiconductor InP. In III–V semiconductors the high abundance of nuclei with large nuclear spins greatly limits the resolution of magnetic resonance techniques. In particular, ODMR signals characteristic of the P_In antisite in InP with slightly differentg-values and hyperfine splitting have been observed. The corresponding centres cannot be distinguished with confidence without the higher resolution of electron-nuclear double resonance (ENDOR). Magnetic resonance signals of the antisite in InP are too weak to allow such measurements in the conventional detection mode, but using optically detected ENDOR (ODENDOR) in absorption as well as in emission, the isolated P_In antisite and at least one other antisite defect were identified. Moreover, the energy position of the P _In ⁺ /P _In ⁺⁺ level in the bandgap was determined from the optical transitions involved.     

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically and optically detected magnetic resonance technique [ED electron spin resonance (EDESR) and ODMR], which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without application of an external cavity, as well as a high frequency source and recorder, and with measuring only the magnetoresistance (EDESR) and transmission (ODMR) spectra within the frameworks of the excitonic normal-mode coupling caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes, respectively, between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.     

Polaron spin-lattice relaxation time in pi-conjugated polymers from optically detected magnetic resonance

We describe a method for obtaining the polaron spin-lattice relaxation time T{SL} in pi-conjugated polymers by measuring the optically detected magnetic resonance (ODMR) dynamics as a function of microwave power and laser intensity. The peculiar ODMR dynamics is well described by a spin dependent recombination model where both recombination and spin relaxation rates determine together the response dynamics. We apply this method to the spin 1/2 ODMR in films of pristine 2-methoxy-5-(2{'}-ethylhexyloxy) phenylene vinylene [MEH-PPV] polymer, as well as MEH-PPV doped with various concentrations of radical impurities. We obtained T{SL} approximately 30 micros in pristine MEH-PPV, but substantially shorter when the magnetic impurities are added.     

ODMR Study of Zn1−x Mn x Se/Zn1−y Be y Se and (Cd1−x ,Mn)Te/Cd1−y Mg y Te Diluted Magnetic Semiconductor Quantum Wells

The effects of microwave pumping with a frequency of 60 GHz on the magneto-optical properties of diluted magnetic semiconductors (DMSs) are studied in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Resonant heating of the Mn²⁺ ions in the electron spin resonance conditions leads to an increase in the Mn-spin temperature, which exceeds the bath temperature by up to 5.2 K, as detected by the shift of exciton emission line and decrease of its integral intensity. Nonresonant heating mediated by free carriers is also observed through variation of the polarization degree of emission. Direct measurements of spin–lattice relaxation times for both materials using time-resolved optically detected magnetic resonance (ODMR) technique have been performed. The mechanisms of ODMR in nanostructures of DMSs are discussed.     

Optically detected electron resonance in phosphorus-doped ZnTe

Samples of ZnTe showing near gap edge luminescence predominantly due to exciton recombination at shallow neutral acceptors and donor- acceptor pair recombination have been investigated using optically detected magnetic resonance (ODMR). Emission polarization changes at 2.318 eV were observed due to magnetic resonance of electrons at g_e = + 0.401 ± 0.004. The observations are consistent with the donor trapped electron resonance resulting from microwave induced changes in donor-acceptor pair photoluminescence.     

ODMR study of the nature of iron-red photoluminescence in ZnS crystals

Basing upon the ODMR (optically detected magnetic resonance) measurements on the so-called iron-red (Fe-R) luminescence in ZnS it is shown that the emission is due to the radiative capture of either free or weakly bound electron by substitutional Fe³⁺ associated with an unknown defect residing in n.n. Zn position.     

Spin resonance spectroscopy of grown-in defects in Ga(In)NP alloys

We employ the optically detected magnetic resonance (ODMR) technique to study and identify important grown-in defects in Ga(In)NP grown by molecular-beam epitaxy (MBE). Several types of defects were revealed from ODMR studies. The dominant defects were found to be related to Ga interstitials, evident form their characteristic hyperfine interaction arising from the spin interaction between the electron and the Ga nucleus. Some other as yet unidentified intrinsic defects were also found to be commonly present in the alloys. The effects of growth conditions (ion bombardment, N₂ gas flow, etc.) and post-growth rapid thermal annealing on the formation of these defects were studied in detail, shedding light on the formation mechanism of defects.     

Exciton-electron interaction in quantum wells with a two dimensional electron gas of low density

II–VI quantum-well structures containing a 2DEG of low density have been investigated by means of polarized photoluminescence, photoluminescence excitation and reflectivity in external magnetic fields up to 20 T. The spin splittings of the exciton X and the negatively charged exciton X ⁻ are measured as a function of the magnetic field strength. The behavior of the magnetic-field-induced polarization degree of the luminescence line related to X ⁻ demonstrates the formation process of negatively charged excitons from excitons and free carriers polarized by the external magnetic field. We have determined the binding energies of the trion formed either with the heavy-hole or the light-hole exciton. The optically detected magnetic resonance (ODMR) technique was applied for the first time to study the optical transition processes in a nanosecond timescale. The electron ODMR was observed with the detection on either the direct exciton or the negatively charged exciton X. Further evidence for the interaction of excitons with the electrons of the two-dimensional gas are demonstrated by a combined exciton-cyclotron resonance line observed in reflectivity and luminescence excitation, shake-up processes observed in photoluminescence, as well as inelastic and spin-dependent scattering processes. Fiz. Tverd. Tela (St. Petersburg) 41, 831–836 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Features of High-Frequency EPR/ESE/ODMR Spectroscopy of NV-Defects in Diamond

Physics of the Solid State - The methods of high-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE) and optically detected magnetic resonance (ODMR) are used to study the...     

Electronic states at dislocations and metal silicide precipitates in?crystalline silicon and their role in?solar cell materials

Predominant dislocation types in solar silicon are dissociated into 30°- and 90°-partials with reconstructed cores. Besides shallow 1D-band localized in their strain field and a quasi-2D band at the stacking fault connecting the two partials, the existence of several intrinsic core defects with deep lying levels has been demonstrated by electron spin resonance. The majority of core defects occur in nonequilibrium situations and, with the exception of a small EPR-signal assigned to a reconstruction defect, vanish after careful annealing above 800°C. There is good evidence now that part of deep levels observed in dislocated silicon is associated with impurities, especially with transition metal impurities. Electron-hole-pair recombination at a dislocation mainly runs via its shallow bands and is strongly increased by impurities bound to its core or in the strain field. The concentration of these impurities can be reduced by gettering processes to such a low level that radiative recombination at dislocations yields a luminescence efficiency of 0.1% at room temperature. A quite coherent picture has emerged for metal impurity precipitation in silicon. Early stages of precipitation in defect-free silicon are characterised by kinetically selected metastable defects forming as a result of large chemical driving forces for precipitation. Such defects are associated with deep level spectra which show the properties of extended multielectron defects. The evolution of the system to energetically more favourable configurations proceeds via ordinary particle coarsening but also via internal ripening, a process reminiscent of the above-mentioned metastable defects. Electronically, the defects evolve into metal-like inclusions which in general seem to act as strong recombination centers for minority carriers. In the presence of dislocations metastable defects quickly transform into equilibrium structures in the course of precipitation or do not form at all. In the presence of several metal impurities silicide precipitates which can be described as solid solutions of the respective metal atoms are observed, which is at least qualitatively in accord with ternary phase diagrams. Like single-metal silicide precipitates, strong minority carrier recombination is also typical for those multi-metal silicide particles.     

Defects in Nanodiamonds: Application of High-Frequency cw and Pulse EPR,ODMR

Different aspects of applications of electron paramagnetic resonance (EPR) based techniques including high frequency (HF) electron spin echo (ESE), electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) approaches to study diamond nanostructures are examined.     

Application of optically detected magnetic resonance to low temperature solid state photoreactions of halogenated coumarines

Optically activated solid state reactions are observed in doped molecular crystals of p-dibromobenzene. The photoreactions result in the formation of stable photoproducts. These products are identified and characterized by a combination of optically detected magnetic resonance methods such as ODMR and ODNQR. Especially optically detected nuclear quadrupole resonance on chlorine and bromine nuclei demonstrates the possibilities to reveal structural information concerning the reaction cage with high sensitivity and spectral resolution.     

Neutron transmutation doping of GaP: optical studies

An unambiguous proof for successful neutron transmutation doping (NTD) of GaP is presented on the basis of optically detected magnetic resonance (ODMR). GaP:S samples grown by the liquid encapsulated Czochralski method were irradiated with thermal neutrons and subsequently annealed at 800°C. In the ODMR experiments the transmuted Ge substitutional on Ga sites was detected. The NTD process was also found to create deep acceptors, the nature of which will be tentatively discussed.     

Temperature-scanned magnetic resonance and the evidence of two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-N pairs in diamond

New method for the detection of magnetic resonance signals versus temperature is developed on the basis of the temperature dependence of the spin Hamiltonian parameters of the paramagnetic system under investigation. The implementation of this technique is demonstrated on the nitrogen-vacancy (NV) centers in diamonds. Single NV defects and their ensembles are suggested to be almost inertialess temperature sensors. The hyperfine structure of the ¹⁴N nitrogen nuclei of the nitrogen-vacancy center appears to be resolved in the hyperfine structure characteristic of the hyperfine interaction between NV and an N _s center (substitutional nitrogen impurity) in the optically detected magnetic resonance spectra of the molecular NV-N _s complex. Thus, we show that a direct evidence of the two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-N _s pairs was observed. It is shown that more than 3-fold enhancement of the NV optically detected magnetic resonance signal can be achieved by using water as a collection optics medium.     

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

It is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–10⁴ times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.     

Effect of substrate temperatures and annealing on optically detected magnetic resonance in glow-discharge amorphous silicon

Optically detected magnetic resonance experiments have been performed at 2 K in glow-discharge amorphous silicon with different substrate temperatures. Effects of annealing and illumination on ODMR have been examined. A broad line is interpreted in terms of a three-centre bond model.     

Determination of nonradiative decay rate in electron-hole drops in Ge at 1.6°K

We show that microwave photoconductivity measurements of optically excited carriers in Ge at 1.6°K can be used to determine the importance of nonradiative recombination within electron-hole liquid drops. Our results show that the nonradiative lifetime is 80 μsec from which we calculate a radiative efficiency of 0.5 ± 0.1 for the condensed phase.