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Notes: The paper reviews the electrical and optical mechanisms at work in sulfide-based thin-film electroluminescence display devices within the framework of general semiconductor physics. The electrical problem is twofold: (i) charge carriers are sourced at high electric field in a nominally insulating material, the carrier density increasing by almost eight orders of magnitude; (ii) the carriers are transported at high field, with an average energy largely exceeding the thermal one. (i) Carrier sourcing is best understood from direct-current-driven ZnS films, and is ascribed to partly filled deep donors transferring electrons to the conduction band by Fowler–Nordheim tunneling. The deep donors also act as carrier sinkers, and evidence for space charge is afforded by small-signal impedance analysis disclosing a markedly inductive behavior. The conduction picture obtained from dc-driven films is then used to clarify the operation of alternating-current electroluminescence structures where the sulfide is sandwiched between two blocking oxide layers. The electrostatics of the ac structure is investigated in detail including space charge and field nonuniformity, and external observables are related to internal quantities. The simple model of interfacial carrier sourcing and sinking is examined. (ii) High-field electronic transport is controlled by the electron-phonon interaction, and the modeling resorts to numerical simulations or the lucky-drift concept. At low electron energies the interaction with phonons is predominantly polar, while at optical energies it proceeds via deformation potential scattering.In spite of the uncertainties in transport models in that range, it is likely that ∼50% of the electrons overtake 2 eV at the usual operating fields in ZnS. Light emission is associated with impurity luminescence centers embedded in the sulfide host. They are excited while current is flowing, and the ensuing relaxation is partly radiative. We describe the two ways in which an impurity may be excited electrically, namely, impact excitation (internal promotion of the center to a state of higher energy) or impact ionization (with an electron released to the host conduction band). The actual excitation mechanism depends on the position of the impurity excited level relative to the host energy bands. A calculation of the excitation yield (number of excited centers per transferred electron) is detailed in the case of impact excitation. Lastly, a phenomenological description of the various relaxation channels is given in terms of formal kinetics, and the relative importance of radiative relaxation is assessed by means of the deexcitation yield (fraction of centers decaying radiatively), which is defined in the case of the impulse response.

Notes: In this paper the influence of electron multiplication in ZnS-type electroluminescence ac devices is studied in a model where the ZnS layer is free from bulk defects. The holes are assumed to recombine with electrons at the interfaces: The two limiting cases of slow and fast hole-electron recombination rate are treated in some detail. The kinetic equations for the electric field and the filling level of interface electrons are established then the charge-voltage characteristics are obtained numerically. An anomalously steep charge-voltage characteristic may be observed for high multiplication rates and slow recombination. In all cases the field is shown to be delayed relative to the current: This phase relationship is related to bulk ideality and allows for qualitative comparisons with experimental data.

Notes: In this paper, a simple model for the charge transfer in thin-film, ac-driven electroluminescent structures of doped ZnS type is proposed. We first review the minimal assumptions needed to account for the carrier emission under high-field conditions and its subsequent feedback on the field strength; it opens the way to a quantitative description of the field variation with time and the conduction current under arbitrary low-frequency drive conditions and without using any adjustable parameter. The charge-voltage relationships are also examined in detail. Conduction in the phosphor layer is assumed to originate from deep levels (traps) in the phosphor forbidden band gap, located at the insulator-phosphor interface. These levels act for charge storage, too. When a discrete trap level is considered, field clamping in the active layer is obtained; when a smooth interface-state energy density is assumed, deviations from the field clamping are possible and simply related to the interface parameters. Most of the work is analytical and the model is shown to exhibit the main qualitative features of nonmemory devices.

Notes: Electron transport in a solid subjected to an arbitrary electric field is analyzed in the energy-position manifold instead of phase space. In the absence of deeply inelastic electron-lattice scattering, the spectral carrier density satisfies a differential equation of the Fokker–Planck type. Subsumption of deeply inelastic events results in an integrodifferential equation which agrees exactly with Monte Carlo simulations in the limit of a vanishing drift-to-instantaneous velocity ratio. Reasonable agreement is found for ratios as high as 0.4, enabling the augmented Fokker–Planck equation to tackle a number of transport issues at a much lower cost than Monte Carlo simulations. © 1999 American Institute of Physics.

Notes: In this paper time-resolved measurements of Mn2+ luminescence in ZnS electroluminescent layers grown by atomic layer epitaxy are reported. The Mn concentration ranges from 0.03 to 1.4 mol %. The intensity of the luminescence versus time is cast in the form of a kinetic equation instead of an algebraic formula. This allows one to determine on a purely phenomenological basis the kinetic rates of mono- and bimolecular processes responsible for the relaxation. We show by a Laplace transform analysis technique that the long-time behavior is not dominated by the lifetime of the single Mn2+ ion in the host matrix. The influence of the electrical properties is evidenced on the second-order kinetic rate. The methods used provide a general framework for investigating complex decay processes.

Notes: In this paper, a simple model is proposed for the impact excitation of luminescence centers in insulating layers under a high electric field (ZnS-type electroluminescence). We first examine the available descriptions of high-field electronic transport in wide-gap semiconductors and the lucky-drift model is chosen for our purpose. As for the luminescence center, the impact excitation cross section in the Born approximation is used. Next, the impact probability of an electron and the impact excitation rate are calculated analytically. The results are compared with existing experimental data: Special attention is paid to the well-known ZnS:Mn case, for which good agreement is obtained. The general picture is that the quantum yield is weak and that the electrons responsible for the impact phenomenon are drifting in the field, not ballistic. Conclusions are drawn for blue-emitting centers and new phosphor materials. It is further suggested that impact excitation can, as well as impact ionization, serve as a test of high-field transport theories.

Notes: The current-voltage characteristic of a semiinsulating, monocrystalline ZnS film grown by metalorganic vapor-phase epitaxy is reported. High-field conduction is found to occur in the same field range (1.0–1.5 MV/cm) as in highly defected, electroluminescent material. It is concluded that the conduction mechanism underlying the operation of ZnS-based electroluminescent devices is bulk controlled. © 1995 American Institute of Physics.

Notes: The effect of hydrogen on donors (N) and acceptors (Al, B) in 6H-SiC crystals has been evidenced by electron spin resonance and transport measurements. Typical passivation (i.e., complexing with H) levels of 75% have been obtained by annealing in a H2 atmosphere, and a corresponding decrease in free-carrier density has been observed. © 1995 American Institute of Physics.

Notes: Electrical characteristics of ac-driven, undoped ZnS electroluminescent layers are reported over a large temperature range. They show that, contrary to the currently assumed model of charge transfer wherein electrons are exchanged between the two ZnS interfaces, the operation involves an appreciable bulk charge evolving through capture and recombination of incident carriers. The meaning of the basic electrical observables is revisited. The results have relevance to the operation of blue emitting SrS-based electroluminescent devices.

Notes: Measurements of the excitation yield of ZnS:Mn electroluminescent devices are reported over a Mn concentration range of 0.2,...,1.1 mol %. They show a continuous increase in nonlinearity as the concentration is increased. This is attributed to the field dependence of the hot-electron impact efficiency and to the dependence of the nonuniform field profile on Mn content. This is directly relevant to the light-emitting performance of the devices.