Quenching of electroluminescence and charge trapping in high-efficiency Ge-implanted MOS light-emitting silicon diodes

Combined measurements of charge trapping and electroluminescence intensity as a function of injected charge and current have been carried out with the aim of clarifying the mechanisms of electroluminescence (EL) quenching in Ge-implanted ITO-SiO₂-Si light-emitting silicon diodes. Good correlation between the negative charge capture in traps of small effective capture cross-sections (σ_t1 ^e=1.7×10^-19 cm² and σ_t2 ^e=4.8×10^-20 cm²) located in SiO₂, and the quenching of the asymmetrical EL line with a maximum intensity at 400 nm has been observed. Similar correlation between the electron capture in traps with extremely small effective capture cross-section (σ_t3 ^e=5×10^-21 cm²) and the quenching of the EL line at 637 nm has been established. A quantitative model for the EL quenching has been developed, which takes into account the modification of the luminescent centers with subsequent electron capture at the newly generated traps. The model shows good agreement between simulation and experimental data. It also demonstrates that small effective capture cross-sections for electron charging during the EL quenching are determined by the probability of the luminescence centers (LCs) being disrupted, and enables one to estimate the Ge concentration associated with the EL at 400 nm. PACS 72.20.Jv; 73.40.Qv; 73.50.Gr     

Evolution of the Electroluminescence Spectra and the Acoustic Emission of the Epitaxial Structures of GaAsP

The dependence of the spectral position of the electroluminescence bands of epitaxial light-emitting diode n ⁺–n–p structures (GaAs_0.15P_0.85) on the density of a direct heterojunction current at different successive instants of time — before an acoustic emission and after it — has been revealed. The shifts of the electroluminescence bands accompanied by acoustic emission can be divided into three types according to the density of the current: (1) short-term shifts due to relaxation of the defect structure of a sample — at relatively low currents, (2) the magnitude of the reverse shift being determined by the current density — at large currents, and (3) formation of an IR band (1.5–1.1 eV) with irreversible degradation changing in the red (1.75 eV) and green (2.19 eV) bands of the electroluminescence spectrum — at ultrahigh currents (100–200 A/cm²).     

High-Efficiency Blue Electroluminescence Based on Coumarin Derivative 3-(4-(anthracen-10-yl)phenyl)-benzo[5,6]coumarin

The electroluminescent (EL) properties of a new coumarin derivative, 3-(4-(anthracen-10-yl)phenyl)-benzo[5,6]coumarin (APBC), were investigated. The results show that the EL devices comprised of vacuum vapor-deposited films using the derivative as dopant exhibited blue emission that is identical to the photoluminescence of the thin film. The electroluminescence device of ITO/2-TNATA (5?nm)/NPB (40?nm)/CBP : APBC (1.0?wt%, 30?nm)/PBD (30?nm)/LiF (1?nm)/Al (100?nm) gives a maximum luminous efficiency of 2.3?cd/A at the current density of 20?mA/cm², and maximum luminance of 5169?cd/m² at 16?V. The external quantum efficiency of the device is 1.85?%.     

Color tunable high efficiency microcavity organic light-emitting diodes

Color tunable microcavity organic light-emitting diodes (OLEDs) with structure of distributed Bragg reflectors (DBR)/indium-tin-oxide (ITO)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃)/LiF/Al were fabricated. Orange red and green light emissions with full width at half maximum (FWHM) of less than 20 nm were obtained through simply changing the thickness of NPB layer. Furthermore, due to the effective modification of the spontaneous emission within microcavity, the brightness and electroluminescent (EL) efficiency of the microcavity OLEDs were significantly enhanced. The maximum brightness and current efficiency, respectively, reached 31000 cd/m² at a current density of 480.0 mA/cm² and 8.3 cd/A at a current density of 110.0 mA/cm² for green devices, and 9700 cd/m² at a current density of 180.0 mA/cm² and 6.6 cd/A at a current density of 36.4 mA/cm² for red devices, which are over 1.5 times higher than those of noncavity OLEDs.      

Change of the dominant luminescent mechanism with increasing current density in molecularly doped organic light-emitting devices

We have fabricated and measured a series of electroluminescent devices with the structure of ITO/TPD/Eu(TTA)₃phen (x):CBP/BCP/ALQ/LiF/Al, where x is the weight percentage of Eu(TTA)₃phen (from 0% to 6%). At very low current density, carrier trapping is the dominant luminescent mechanism and the 4% doped device shows the highest electroluminescence (EL) efficiency among all these devices. With increasing current density, Förster energy transfer participates in EL process. At the current density of 10.0 and 80.0 mA/cm², 2% and 3% doped devices show the highest EL efficiency, respectively. From analysis of the EL spectra and the EL efficiency-current density characteristics, we found that the EL efficiency is manipulated by Förster energy transfer efficiency at high current density. So we suggest that the dominant luminescent mechanism changes gradually from carrier trapping to Förster energy transfer with increasing current density. Moreover, the conversion of dominant EL mechanism was suspected to be partly responsible for the EL efficiency roll-off because of the lower EL quantum efficiency of Förster energy transfer compared with carrier trapping.     

Plasma-volume generation of H<Superscript>−</Superscript> ions in a low-voltage xenon-hydrogen discharge. II

A low-voltage xenon-hydrogen discharge is considered theoretically at an interelectrode distance of L = 1 cm and cathode emission current densities of j _s = 2–20 A/cm². Basic parameters of the discharge plasma, in particular, the total hydrogen and xenon densities, are optimized to attain the maximum possible density of negative hydrogen ions \(N_{H^ - } (L)\) at the plasma-anode boundary. The distributions of the plasma parameters over the discharge gap are calculated for optimized regimes. According to calculations, at intermediate cathode emission current densities (j _s ≈ 5–10 A/cm²) in optimized discharge regimes, the density of negative hydrogen ions in the anode region of the plasma is \(N_{H^ - } (L)\) ≈ (1.5–2.5) × 10¹² cm^?3 and the total plasma pressure is p ₀ = 0.5–0.6 Torr.     

Photoluminescence of Ca(Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>S<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript> compounds

Photoluminescence (PL) observed in solid solutions of Ca(Al _x Ga_1–x )₂S₄:Eu²⁺ (x = 0.1–0.3) is studied. It is shown that the increase in emission intensity by 18% is caused by changes in the x values and electronic 5d → 4 f transitions in Eu²⁺ ions. A change in the position of the emission spectrum or its partial shift toward shorter wavelengths is due to an increase in the aluminum concentration and a decrease in the crystal field energy. The energy of the zero phonon line E ₀, redshift D, and the Stokes shift ΔS are determined. A decrease in the photoluminescence intensity maximum and an increase in the half-width of the spectrum are found in the temperature range of 10–300 K. The efficiency of emission at temperatures of 20 and 300 K is almost independent of the excitation power density of up to ~10⁴ W/cm². The luminescence lifetime of Eu²⁺ ions was 383, 357, 346, and 333 ns for x = 0, 0.1, 0.2, and 0.3, respectively.     

Mechanism of charge transfer in <Emphasis Type="Italic">n</Emphasis>-CdS/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

The capacitance-voltage and current-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. Analysis of these characteristics demonstrates that the CdTe_1?x S _x solid solution formed at the n-CdS/p-CdTe heterointerface is inhomogeneous in both the conductivity and composition. The thickness of solid solutions is estimated from the capacitance-voltage characteristics. It is shown that, for the n-CdS/p-CdTe heterosystem, the current-voltage characteristic in the current density range 10^?8-10^?5 A cm^?2 is governed by the thermal electron emission, whereas the current in the heterostructure at current densities in the range 10^?4-10^?2 A cm^?2 is limited by recombination of charge carriers in the electroneutral region of the CdTe_1?x S _x solid solution. The lifetime and the diffusion length of minority charge carriers in the CdTe_1?x S _x solid solution and the surface recombination rate at the interface between the CdS layer and the CdTe_1?x S _x solid solution are determined. It is demonstrated that the n-CdS/p-CdTe heterostructure operates as a p-i-n structure in which CdTe is a p layer, CdTe_1?x S _x is an i layer, and CdS is an n layer.     

Features of current carrier scattering in epitaxial films

The properties of epitaxial cadmium selenide films obtained by condensation in a vacuum on mica substrates under almost equilibrium conditions are investigated. The temperature dependences of the conductivity and current carrier mobility and concentration are studied. The electron concentration in the films depended on the gas phase composition (coevaporation of CdSe + Se or CdSe + In) and varied between 5·10¹⁰cm^–3 and 3.5·10¹⁸. It is shown that the current carrier scattering mechanism depends on their concentration and production conditions. For n₁ 10¹⁶ cm^–2 (T_S520C),n₂ < 10¹⁵ (T_S=630C), scattering on intercrystallite barriers predominated. For n₁ and n₂ greater than the quantities mentioned, scattering by ionized defects becomes dominant. It is established that the magnitude of the intercrystallite barrier in films with 10¹⁵ < n < 10¹⁶ cm^–3 is comparatively small and does not exceed 4·10^–3 eV, whereupon scattering at the barriers is not explicitly manifest. Concentrations of the ionized centers, magnitudes of the intercrystallite barriers, and ionization energies of the donor levels are determined for films obtained under different conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 98–103, September, 1977.     

The impact cross section of Er3+ in ZnS

Four kinds of Er³⁺ centers in ZnS:Er³⁺ thin films have been distinguished by means of laser selective excitation. Their impact cross sections in electroluminescence (EL) and absorption cross sections in photoluminescence (PL) have been compared with each other. The average value of the impact cross section of Er³⁺ obtained by comparing the EL intensity of Er³⁺ with that of Mn²⁺ in ZnS:ErF₃, Mn²⁺ thin films is about 2×10⁻¹⁶ cm².     

DD reaction enhancement and X-ray generation in a high-current pulsed glow discharge in deuterium with titanium cathode at 0.8–2.45 kV

The DD reaction yield (3-MeV protons) and the soft X-ray emission from a titanium (Ti) cathode surface in a periodic pulsed glow discharge in deuterium were studied at a discharge voltage of 0.8–2.45 kV and a discharge current density of 300–600 mA/cm². The electron screening potential U_e = 610 ± 150 eV was estimated in the range of deuteron energies 0.8 keV < E_d < 2.45 keV from an analysis of the DD reaction yield as a function of the accelerating voltage. The obtained data show evidence for a significant enhancement of the DD reaction yield in Ti in comparison to both theoretical estimates (based on the extrapolation of the known DD reaction cross section for E_d ≥ 5 keV to low deuteron energies in the Bosch-Halle approximation) and the results of experiments using accelerators at the deuteron energies E_lab ≥ 2.5 keV and current densities 50–500 μ A/cm². Intense emission of soft X-ray quanta (10¹³–10¹⁴ s^?1 cm^?2) was observed at an average energy of 1.2–1.5 keV. The X-ray emission intensity and the DD reaction yield enhancement strongly depend on the rate of deuterium diffusion in a thin subsurface layer of Ti cathode.     

Facile Construction of 3D Reduced Graphene Oxide Wrapped Ni3S2 Nanoparticles on Ni Foam for High‐Performance Asymmetric Supercapacitor Electrodes

3D reduced graphene oxide (rGO)‐wrapped Ni₃S₂ nanoparticles on Ni foam with porous structure is successfully synthesized via a facile one‐step solvothermal method. This unique structure and the positive synergistic effect between Ni₃S₂ nanoparticles and graphene can greatly improve the electrochemical performance of the NF@rGO/Ni₃S₂ composite. Detailed electrochemical measurements show that the NF@rGO/Ni₃S₂ composite exhibits excellent supercapacitor performance with a high specific capacitance of 4048 mF cm^?2 (816.8 F g^?1) at a current density of 5 mA cm^?2 (0.98 A g^?1), as well as long cycling ability (93.8% capacitance retention after 6000 cycles at a current density of 25 mA cm^?2). A novel aqueous asymmetric supercapacitor is designed using the NF@rGO/Ni₃S₂ composite as positive electrode and nitrogen‐doped graphene as negative electrode. The assembled device displays an energy density of 32.6 W h kg^?1 at a power density of 399.8 W kg^?1, and maintains 16.7 W h kg^?1 at 8000.2 W kg^?1. This outstanding performance promotes the as‐prepared NF@rGO/Ni₃S₂ composite to be ideal electrode materials for supercapacitors.     

Red electroluminescent process excited by hot holes in SrGa2S4:Ce, Mn thin film

This paper reports the first observation of red electroluminescence (EL) in SrGa₂S₄:Ce, Mn thin film. The EL spectrum consists of single broad emission band having a peak wavelength of 665 nm. The dominant EL decay time was 31 μs. The relationship between the applied voltage and the EL waveform was measured in single insulating thin film electroluminescent (TFEL) devices. An asymmetric EL waveform was observed in SrGa₂S₄:Ce, Mn TFEL devices under a rectangular applied voltage. The polarity of the EL waveform in these devices was different from the waveform in manganese-activated zinc sulfide ZnS:Mn devices. This indicates that hot holes excite the Mn²⁺ ions to cause the red EL.     

Absorption and emission spectroscopic characterisation of F<Subscript>3</Subscript><Superscript>-</Superscript> colour centers in a LiF crystal

For a LiF crystal containing F₃ (R₁, R₂), F₂, F₃ ⁺, F₄ (N₁, N₂), F₃ ^-, and F₂ ^- colour centers the absorption and emission spectroscopic behaviour of the F₃ ^- centers is studied. The absorption cross-section spectrum and the number density of F₃ ^- centers are extracted from saturable absorption and transmission measurements. Additionally fluorescence quantum distribution measurements and fluorescence lifetime measurements are carried out to determine the stimulated emission cross-section spectrum and the fluorescence quantum yield of the F₃ ^- centers. Fluorescence excitation spectroscopy reveals the presence of an absorption band with its absorption peak around 700 nm (called R ₁ band) in addition to the ground-state to first excited-state F₃ ^- center absorption band centered at 800 nm (called R ₂). PACS 61.72.Ji; 78.40.Ha; 78.55.Fv     

Temperature and injection current dependent electroluminescence study of GaInP/AlGaInP quantum well laser diode grown using tertiarybutylarsine and tertiarybutylphosphine

GaInP/AlGaInP triple quantum well (TQW) lasers, grown by metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP), were fabricated with a pulsed anodic oxidation (PAO) process. The devices worked at room temperature (RT) with the lowest threshold current density (J_th) of 1.5 kA/cm² ever reported for GaInP/AlGaInP lasers grown using TBAs and TBP. Temperature dependent (35–250 K) electroluminescence (EL) study of the GaInP/AlGaInP laser diode showed almost the same luminescence quenching behavior at a high temperature region (120–250 K), independent of the injection current (100–150 mA). A model involving a nonradiative recombination mechanism was presented to interpret the EL quenching behavior over the experimental temperature range. The nonradiative recombination centers in the Al-containing barrier or cladding layer are believed to contribute to the loss of carriers via nonradiative recombination. PACS 78.60.Fi; 71.20.Nr; 78.67.De; 81.15.Gh; 42.55.Px     

Spectroscopic diagnostics of a laser erosion plasma formed from CuSbS<Subscript>2</Subscript> polycrystals

The spectra and dynamics of emission from regions of a laser plasma torch located at different distances from a polycrystalline CuSbS₂ target irradiated by a neodymium laser (W=(3–5)×10⁸ W/cm², ?=20 ns, f=12 Hz, 73x03BB;=1.06μm) were investigated. The emission data were used to estimate the average temperature (≤0.82 eV) and the electron density ((1.82?1.92)×10¹⁶ cm^?3) in the laser torch and the recombination times of ions (t _r(S²⁺)=15 ns, t _r(Cu⁺)=65?85 ns), as well as to analyze the efficiency of filling of excited atomic levels. A model describing the target destruction and the evolution of the processes accompanying spread of the laser plasma is proposed.     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Ionization mechanisms of aluminum acceptor impurity in silicon

Processes of ionization of shallow acceptor centers (ACs) in silicon are studied. In crystalline silicon samples with phosphorus (1.6×10¹³, 2.7×10¹³, and 2.3×10¹⁵cm^?3) and boron (1.3×10¹⁵cm^?3) impurities, _μAl impurity atoms were produced by implantation of negative muons. It is found that thermal ionization is the main mechanism for ionizing the Al acceptor impurity in both p-type and n-type silicon with an impurity concentration of ?10¹⁵cm^?3 at T>45 K. The thermal ionization rate of Al ACs in Si varies from ～10⁵ to ～10⁶s^?1 in the temperature range 45–55 K.     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

Room temperature emission from the Pr3+ 1 S 0-level in PrF3

PrF₃ as well as PrF₃-LiF and PrF₃-MgF₂ mixtures were investigated by means of high-energetic excitation with synchrotron radiation. In the PrF₃ emission spectrum bands due to the intra-configurational 4f²4f² transitions originating from the ¹S₀ level of the Pr³⁺ ion have been identified. The emission from the ³P₀ multiplet is very weak due to non-radiative decay by cross-relaxation processes. Therefore, PrF₃ is not an efficient direct cascade emitter. In the PrF₃ excitation spectra in the ultraviolet/vacuum-ultraviolet spectral range, the ³H₄¹S₀ transition at 46858 cm^-1 as well as broad bands due to the inter-configurational 4f²4f5d transitions are observed. By comparison with data of Pr³⁺ doped YF₃, the branching ratios of the emission transitions for PrF₃ from the ¹S₀ were determined; these are 0.008, 0.075, 0.262, 0.023 and 0.629 for transitions ¹S₀³H₄, ³F₄, ¹G₄, ¹D₂ and ¹I₆, respectively. For the polycrystalline PrF₃-LiF and PrF₃-MgF₂ samples investigated by us, the measured emission and excitation spectra are nearly identical to those of PrF₃. For polycrystalline PrF₃-KMgF₃ the observed spectra are superpositions of the PrF₃ and Pr³⁺:KMgF₃ spectra. PACS 42.70.-a; 78.55.-m; 78.55.Hx