The effects of core structure on radiative and non-radiative recombinations at metal ion substituents in semiconductors and phosphors

Carrier binding and recombination processes at transition metal (TM) and post-transition metal impurities in solids are reviewed. The presence of low-lying empty core orbitals in these impurities introduces novel binding and recombination phenomena and leads naturally to a classification of impurity centres as ‘simple’ or ‘structured’. ‘Simple’ impurities, generally the main group elements, introduce only effective-mass-like states into the forbidden gap of the host lattice, whereas ‘structured’ impurities show localized atomic-like transitions below the lattice absorption edge. The fact that donor or acceptor centres generated by TM ions in semiconductors are usually deep is discussed with reference to a simple oxidationstate correlation diagram based on the variable chemical valency characteristic of these ions. Many metal ion impurities in solids generate iso-electronic centres however, and evidence is assembled to show how the normally accepted range of iso-electronic centres can be extended to include such impurities. It is emphasized that the formation of bound states at these ‘structured’ cationic iso-electronic centres is fairly common, in contrast to the situation normally found for anionic substituents in semiconductors. A possible explanation for this general property of structured iso-electronic centres is given in terms of the low-lying core levels which they may introduce in the band gap and their enhanced polarizability. Simple arguments then suggest that those centres showing electron ionization thresholds or atomic inter-configurational transitions just below E _g should be hole-attractive, whereas those showing charge transfer transitions should be electron-attractive. It is shown that two kinds of defect Auger recombination (DAR) may be significant for excitons localized at structured impurities. The first involves hole ionization at a deep neutral acceptor, and the conditions which must be met to make this process significant at room temperature are reviewed. The second DAR process involves the transfer of carrier recombination energy to the core electrons of the structured impurity. A simple model developed for this energy transfer process predicts that the coupling between exciton and core states will be larger if the impurity shows strong, broad dipole-allowed transitions quasi-resonant with the lattice absorption edge. Throughout the paper we stress the importance of recombination processes at structured impurities to the problems of phosphor activation and of shunt recombination mechanisms in semiconductor LEDs. In the final sections this general framework of ideas is used to explain the relative cathodoluminescence efficiencies of different rare-earth activators in crystals of Y₃Al₅O₁₂, with good agreement between the theoretical predictions and the experimental observations. Possible evidence for the occurrence of bound exciton states at structured TM impurities is found in the appearance of anomalously weak zero-phonon lines in the near-edge luminescence of GaP crystals prepared under special conditions.     

High temperature (T>300 K) light emitting diodes for 8–12 μm spectral range

Contrary to conventional light emitting diodes for visible and very near infrared utilizing interband (ω>E_g) luminescence, the longer infrared emitting diodes (LIREDs) we describe here utilize intraband (ω<E_g) electron transitions and emit beyond the fundamental absorption range of the material used. Made of indirect band gap semiconductors (like Ge, Si) and, therefore, free from the Auger recombination impact, LIREDs efficiently operate at higher temperatures (T>300 K) in the longer wavelength atmospheric window (8–12 μm). Electrically modulated power emitted is comparable to that for black body sources whereas shorter rise–fall times are due to recombination processes (200 μs) and not dependent on pixel thermal mass and thermal conduction. LIREDs can be made of different semiconductor structures provided the controllable modulation of free carrier concentration in the device base is achieved. The main parameters of Ge based LIREDs under injection mode are reported.     

Photo- and thermostimulated processes in α-Al2O3

We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al₂O₃ crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al₂O₃ crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al₂O₃ crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al₂O₃ in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.     

Recombination in narrow-gap semiconductors

This review is intended to be an introduction to the recombination mechanisms important for narrow-gap semiconductors. The study of recombination mechanisms gives information on the various interactions the free carriers happen to meet in crystals. On the other hand recombination mechanisms are limiting the performance of all the modern solid state electronic devices. It was recently observed that the dominant recombination processes in narrow-gap semiconductors are different from those important for the classical semiconductors. Due to the narrow band gap the intrinsic carrier density is rather large and in addition the band gap energy is of the same order of magnitude as the energy of elementary excitations as LO-phonons or plasmons, and typical cyclotron-resonance energies. Accordingly processes as the Auger effect or recombination by emitting single phonons or single plasmons become highly probable. These mechanisms have been observed recently and also a new magnetic quantum effect was discovered. The new processes have proved to be important for devices like infrared detectors and infrared solid state lasers, but revealed also some fundamental properties of semiconductors.     

High-efficiency silicon light emitting diodes

Silicon has been regarded as a notoriously poor emitter of light fundamentally due to its indirect bandgap. However, as an elemental rather than a compound semiconductor, it has the advantage of fewer background defects as well as well-developed approaches to interface passivation. By minimising parasitic optical absorption and non-radiative bulk and surface recombination, and by enhancing the effective optical photon generation volume, respectable silicon light emission efficiencies are demonstrated. These are within the range of direct gap III–V semiconductors and higher than any at low powered densities. Possible applications are also discussed.     

Photoconductivity of semiconductors at low temperatures

Low-temperature oscillations in the photoconductivity of semiconductors are discussed. It is shown that oscillations will be observed if the probability for energy relaxation with optical phonons exceeds both the probability of carrier recombination and the energy-relaxation time for acoustic phonons. The oscillation shape is studied for momentum relaxation with acoustic phonons and with ionized impurities for the case in which interelectronic collisions can be neglected and when an account of these collisions leads to the establishment of an electron temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 85–92, September, 1968.     

Infrared absorption by pairs of group III and V impurities in silicon

Infrared absorption bands, shifted to regions of lower energies relative to narrow lines of transitions of impurities to excited states, are investigated in silicon doped with group III and V impurities in concentrations above 10¹⁶ cm^?3. It is found that the band structure is peculiar to each of the investigated impurities but independent of their concentrations, and the absorption coefficients in the bands increase approximately quadratically with concentration. This leads one to infer that the bands are caused by the excitation of charge carriers bound on impurity pairs localized within several Bohr radii.     

Absorption cross-sections and lifetimes as a function of size in Si nanocrystals embedded in SiO2

The photoluminescence (PL) emission yield of Si nanocrystals embedded in SiO₂ depends on their size and on Si–SiO₂ interface passivation. In this work we aim at clarifying the relative importance of both contributions by studying lifetimes and absorption cross-sections as a function of size, for samples with and without passivation in forming gas. We find that while the PL lifetime increases steadily (quasi-linear dependence), the radiative lifetime increases exponentially with the nanocrystal size. Thus, as expected, radiative oscillator strengths are much smaller for large nanocrystals, but this reduction is partially compensated by a less effective quenching at interfacial non-radiative states. The absorption cross-section per nanocrystal rises as the nanocrystal size decreases, for all excitation wavelengths, implying that the variation of oscillator strength dominates over the reduction of the density of states. Passivation processes do not affect the emission mechanism and increase the emission yield while reducing the density of non-radiative recombination centers at the Si–SiO₂ interface (P_b centers).     

Electronic charge distribution of the polarizable O2− ion in MgO and CaO in contrast to the F− ion in NaF

The electronic charge densities of NaF, MgO, and CaO are obtained by self-consistent band structure calculations using the augmented plane wave (APW) method. The fact that F^– is stable, whilst O^2– is unstable as a free ion affects the radial charge density of fluorides and ionic oxides significantly. The Watson sphere model can simulate the stabilizing crystalline environment of an O^2– ion and provides an ionic density which, when superposed with the cation density, leads to a radial charge density in excellent agreement with the one obtained by our APW calculations. It is therefore meaningful to speak of an O^2– ion, although the corresponding wave functions are more extended for O^2– than for F^–. Furthermore, the Watson sphere model can account for the main differences of the oxygen radial density between MgO and CaO and demonstrates that the polarizable O^2– ion is strongly affected by its environment.     

Exciton binding energies in II–VI compound strained layer superlattices

II–VI strained-layer superlattices are very efficient emitters of visible light. The dependence of the luminescence intensity on the excitation power density allows us to characterise the recombination processes involved in the emission. At low temperatures excitonic processes are dominant whereas electron-hole recombinations feature at room temperature. No special evidence of the dual nature of the emission is observed at intermediate temperatures because the optical transitions are broadened by well-width fluctuations. In spite of this we may estimate the exciton binding energy from the temperature dependence of the photoluminescence intensity, as long as the photoluminescence remains excitonic. This is the case for narrow wells in CdS---ZnS superlattices over the temperature range zero to room temperature. The estimated exciton binding energy measured in this way approaches the two dimensional limit but does not exceed it.     

Elucidating the role of non-radiative processes in charge transfer of core–shell Si–SiO2 nanoparticles

Crystalline silicon is the most commonly used material in photovoltaics but has limitations due to its high cost and non-tunable band gap. A new approach of using inexpensive, non-toxic materials with layers that have different band gaps which absorb a wide range of the solar spectrum has the potential to dramatically increase the efficiencies and lower the costs. Core–shell Si–SiO₂ nanoparticles are ideally suited for the photovoltaic application and have been synthesised by different groups in an array of sizes allowing for absorption in a wide spectral range. A theoretical investigation of fundamental charge transfer processes in these systems can potentially lead to improved devices. Calculations on a model core–shell interface with the formula Si₂₆₄O₁₆₀ which features a silicon layer sandwiched between two SiO₂ layers were performed using the Vienna ab initio software package. The Perdew–Burke–Ernzerhof functional in the basis of plane waves was used along with pseudopotentials to simulate electronic structure. The nuclear motion was considered using ab initio molecular dynamics. The density of states, absorption spectrum, partial charge densities, and radiative recombination lifetimes have been calculated. This interface shows quantum confinement behaviour similar to a particle in a box. The role of non-radiative recombination was also determined by relaxation dynamics.     

Kinetics of the behavior of photosensitive impurities and defects in high-purity semi-insulating silicon carbide

The kinetics of the behavior of photosensitive impurities and defects in the high-purity semi-insulating material 4H-SiC has been studied both theoretically and experimentally using electron paramagnetic resonance (EPR) under photoexcitation and optical admittance spectroscopy. The rate equations describing the processes of recombination, trapping, and ionization of nonequilibrium charge carriers bound dynamically to shallow donors and acceptors (nitrogen and boron), as well as of charge carrier transfer from the shallow nitrogen donor to deep levels of intrinsic defects, have been solved. A comparison of the calculations with the experimental curves plotting the decay of admittance conductance and EPR signal intensities due to nitrogen and boron after termination of photoexcitation has revealed that the probabilities of hole trapping by an ionized acceptor and the rate of ionization of a neutral boron acceptor are two orders of magnitude higher than those of similar processes in a system of donor levels. The latter is dominated by cascade electron transitions between levels in the band gap, as well as by electron-hole recombination.     

Lifetime of charge carriers in Hg1-xCdxTe (x=0.20) crystals

We examine the mechanisms for Auger recombination in narrow-gap semiconductors of the Hg_1-xCd_xTe type, connected with collision between two electrons in the E_c band followed by transition of one of them to the E_v1 band and collision between two holes in the E_v1 band followed by transition of one of them to the E_v2 band. In analyzing the contributions from different recombination mechanisms over broad concentration and temperature range, we used the models of P. E. Petersen and B. L. Gel'mont, taking into account the specific characteristics of the band structure of Hg_1-xCd_xTe crystals. We determined the temperature and concentration ranges for n- and p-type semiconductors in which different recombination mechanisms are realized, including a radiative mechanism. We compare the experimental data on charge carrier lifetime with the calculation results using different recombination models in the crystals under study.V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 99–104, February, 1994.     

The effect of Mn-doped ZnSe passivation layer on the performance of CdS/CdSe quantum dot-sensitized solar cells

ZnSe as a surface passivation layer in quantum dot-sensitized solar cells plays an important role in preventing charge recombination and thus improves the power conversion efficiency(PCE).However, as a wide bandgap semiconductor, ZnSe cannot efficiently absorb and convert long-wavelength light.Doping transition metal ions into ZnSe semiconductors is an effective way to adjust the band gap, such as manganese ions.In this paper, it is found by the method of density functional theory calculation that the valence band of ZnSe moves upward with manganese ions doping, which leads to acceleration of charge separation, wider light absorption range, and enhancing light harvesting.Finally, by using ZnSe doped with manganese ions as the passivation layer, the TiO_2/CdS/CdSe co-sensitized solar cell has a PCE of 6.12%, and the PCE of the solar cell increases by 9% compared with the undoped one(5.62%).     

Exciton–dopant and exciton–charge interactions in electronically doped OLEDs

The electronic dopants, like tetrafluorocyanoquinodimethane (F₄–TCNQ) molecules, used for p-doping of hole transport layers in organic light-emitting diodes (OLEDs) are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We observed EL quenching in OLED with F₄-TCNQ doped N,N′-diphenyl-N′N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine hole transport layer at large dopant concentrations, >5%. To separate the effects of exciton–dopant quenching, from exciton–polaron quenching we have intentionally doped the emissive layer of (8-tris-hydroxyquinoline) with three acceptors (A) of different electron affinities: F₄-TCNQ, TCNQ, and C₆₀, and found that C₆₀ is the strongest EL-quencher, while F₄-TCNQ is the weakest, contrary to intuitive expectations. The new effects of charge transfer and usually considered energy transfer from exciton to neutral (A) and charged acceptors (A⁻) are compared as channels for non-radiative Ex–A decay. At high current loads the EL quenching is observed, which is due to decay of Ex on free charge carriers, hole polarons P⁺. We consider contributions to Ex–P⁺ interaction by short-range charge transfer and describe the structure of microscopic charge transfer (CT)-processes responsible for it. The formation of metastable states of ‘charged excitons’ (predicted and studied by Agranovich et al. Chem. Phys. 272 (2001) 159) by electron transfer from a P to an Ex is pointed out, and ways to suppress non-radiative Ex–P decay are suggested.     

Electron–phonon-induced spin relaxation in InAs quantum dots

We have calculated spin-relaxation rates in parabolic quantum dots due to the phonon modulation of the spin–orbit interaction in the presence of an external magnetic field. Both deformation potential and piezoelectric electron–phonon coupling mechanisms are included within the Pavlov–Firsov spin–phonon Hamiltonian. Our results have demonstrated that, in narrow gap materials, the electron–phonon deformation potential and piezoelectric coupling give comparable contributions to spin-relaxation processes. For large dots, the deformation potential interaction becomes dominant. This behavior is not observed in wide or intermediate gap semiconductors, where the piezoelectric coupling, in general, governs the spin-relaxation processes. We have also demonstrated that spin-relaxation rates are particularly sensitive to the Landé g-factor.     

Impurity recombination of hot current carriers with the participation of optical phonons

In the framework of a modified cascade capture theory, this work examines the recombination of hot current carriers at impurity attractive centers in semiconductors with, in the first stage, optical phonon capture assisting in conditions of the carrier distribution for scattering by zero-point lattice vibrations. Calculation of the effect on carrier recombination of the excited states of the impurity centers with binding energy is less than that given in the Lax model and leads to a more distinct negative differential resistance. The quantitative results obtained are evaluated and an estimate is made of the magnitudes of the processes hindering observation of recombination with the assistance of optical phonons.Translated from Izvestiya Vysshikh Uchebnikh Zavedenii, Fizika, No. 7, pp. 10–15, July, 1986.     

High power non-linear magnetophotoconductivity in n-GaAs using the UCSB free electron laser

The kinetics of electrons bound to shallow donor impurities in n-GaAs was investigated by saturation spectroscopy using the University of California at Santa Barbara free electron laser. The resonant photothermal conductivity from 1s–2p⁺ transitions was measured at intensities greatly exceeding previous studies. Saturation of bound-to-free photoionization transitions was measured from 0 to 4 Tesla. The 1s–2p⁺ resonant photoconductive signal shows a distinct intensity dependence caused by the competing bound-to-free transitions which saturate differently. Evaluation of the electron recombination kinetics allows us to calculate the transition time of electrons from the 2p⁺ level to the ground state, the recombination time of free electrons, and the thermal ionization probability of the 2p⁺ state.     

Modelling the thermal quenching mechanism in quartz based on time-resolved optically stimulated luminescence

This paper presents a new numerical model for thermal quenching in quartz, based on the previously suggested Mott-Seitz mechanism. In the model electrons from a dosimetric trap are raised by optical or thermal stimulation into the conduction band, followed by an electronic transition from the conduction band into an excited state of the recombination center. Subsequently electrons in this excited state undergo either a direct radiative transition into a recombination center, or a competing thermally assisted non-radiative process into the ground state of the recombination center. As the temperature of the sample is increased, more electrons are removed from the excited state via the non-radiative pathway. This reduction in the number of available electrons leads to both a decrease of the intensity of the luminescence signal and to a simultaneous decrease of the luminescence lifetime. Several simulations are carried out of time-resolved optically stimulated luminescence (TR-OSL) experiments, in which the temperature dependence of luminescence lifetimes in quartz is studied as a function of the stimulation temperature. Good quantitative agreement is found between the simulation results and new experimental data obtained using a single-aliquot procedure on a sedimentary quartz sample.