Charge trapping phenomena in high-efficiency metal-oxide-silicon light-emitting diodes with ion-implanted oxide

This work is a comparative study of the processes of charge trapping in silicon dioxide layers doped with different rare-earth (RE) impurities (Gd, Tb, Er) as well as with Ge. Diode SiO₂-Si structures incorporating such oxide layers exhibit efficient electroluminescence (EL) in the spectral range of UV to IR. Ion implantation was performed over a wide dose range with the implant profiles peaking in the middle of the oxide. Charge trapping was studied using an electron injection technique in constant current regime with simultaneous measurements of the EL intensity (ELI). High-frequency C/V characteristics were used to monitor the net charge in the oxides.Analysis of the charge trapping and the variation of the EL intensity during electron injection shows that the current density range can be divided in three portions: (i) low injection level, where electron/hole capture at traps with large capture cross-sections and low ELI occurs; (ii) medium injection level corresponding to the main operation mode of the devices (odd hole trapping depending on the injected current level is observed); and (iii) high injection level (electrical quenching of the EL that correlates with electron capture at traps of extremely small capture cross-sections takes place). The nature of specific hole trapping at the medium injection level in RE-doped devices is discussed. Mechanisms of EL quenching at the high injection level are proposed.     

V:YAG – a new passive Q-switch for diode-pumped solid-state lasers

gsa =3.0×10^-18 cm² and σ_esa=1.4×10^-19 cm² at 1064 nm, and σ_gsa=7.2×10^-18 cm² and σ_esa=7.4×10^-19 cm² at 1342 nm. Q-switched operation was demonstrated at 1064 nm and 1342 nm from a Nd:YVO₄ microchip laser, producing pulses as short as 9.3 ns at 1342 nm with peak powers of 350 W. Received: 17 March 1998/Revised version: 8 June 1998     

Distribution of deep levels in Si:Au by spectral analysis of deep-level transient spectroscopy

-l eV and Δσ=1.4×10^-15 cm²) around their mean values (E₀=0.47 eV and σ₀=5.0×10^-15 cm²). No broadening for the other levels is observed in the emission rate spectrum. Accepted: 13 October 1997     

Assessment of persistent-photoconductivity centers in MBE grown Al x Ga1-x as using capacitance spectroscopy measurements

Deep-level transient spectroscopy and thermally stimulated capacitance measurements were used to investigate the properties of deep traps in Si-dopedn-Al _x Ga_1–xAl layers grown by molecular beam epitaxy. Two electron traps at electron emission activation energies of 0.44 and 0.57 eV have been detected. Both traps were studied in detail and found to be the origin of the persistent-photo-conductivity phenomenon in this material. The nature of both traps is the same as of the DX center in liquid phase epitaxial material reported by Lang et al. The electron capture cross-sections are _n 1 = _n 2=8.3×10^–22cm² atT=205K. Activation energies ofE 1= 0.33eV andE 2=0.37eV at temperatures higher than 125 K were determined by DLTS measurements and by direct measurements of the capture transient. In order to allow for the variation of the free-electron concentration during the capture process, a new method for the evaluation of the electron capture crosssection was developed.     

Anisotropy of nonlinear absorption in Co<Superscript>2+</Superscript>:MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal

Anisotropy of the nonlinear absorption of Co²⁺ ions in MgAl₂O₄ single crystal at the wavelengths of 1.35 and 1.54 μm has been experimentally demonstrated. The experimental data are analyzed in the framework of a phenomenological model when the Co²⁺ ions are described as three sets of linear dipoles oriented along the crystallographic axes. Ground-state and excited state absorption cross-sections at 1.35 and 1.54 μm are evaluated to be σ_gsa=(4.0±0.3)×10^-19, σ_esa=(3.6±0.4)×10^-20 cm² and σ_gsa=(5.1±0.3)×10^-19, σ_esa=(4.6±0.4)×10^-20 cm², respectively. PACS 42.55.Rz; 71.20.Be     

Growth,spectroscopic and laser properties of Yb<Superscript>3<Emphasis Type="Bold">+</Emphasis></Superscript>-doped GdAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a candidate for infrared laser crystal

Yb³⁺:GdAl₃(BO₃)₄ (hereafter Yb³⁺:GAB) crystals with large sizes and good optical quality have been grown by the top-seed solution growth (TSSG) method. The polarized absorption and emission spectra have been investigated at room temperature. For the σ-polarization, the intensities of both absorption and emission spectra are stronger than those for the π-polarization, the σ-absorption cross section of Yb³⁺ in GAB being 3.43×10^-20 cm² at 977 nm, and the σ-emission cross section being 0.98×10^-20 cm² at 1045 nm. The fluorescence lifetime of the ² F _5/2→² F _7/2 transition was measured to be 800 μs in the 5% doped sample used for our laser experiments, 993 μs in a 10% doped sample and 569 μs in a 0.5% doped sample. The laser parameters were estimated as: β_min=0.022, I_sat=10.4 kW/cm² and I_min=0.23 kW/cm². About 0.4 W laseroutput at the wavelength of 1043 nm was achieved when the Yb³⁺:GAB crystal was pumped by a 974 nm laser diode, with 27.4% slope efficiency. PACS 42.55.-f; 42.70.Hj; 78.20.-e; 81.10.Dn     

Non-steady-state photo-EMF in nanostructured GaN and polypyrrole within porous matrices

We report an experimental investigation of the non-steady-state photoelectromotive force in nanostructured GaN within porous glass and polypyrrole within chrysotile asbestos. The samples are illuminated by an oscillating interference pattern created by two coherent light beams and the alternating current is detected as a response of the material. Dependences of the signal amplitude versus temporal and spatial frequencies, light intensity, and temperature are studied for two wavelengths λ=442 and 532 nm. The conductivity of the GaN composite is measured: σ=(1.1–1.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=442 nm, I ₀=0.045–0.19 W/cm², T=293 K) and σ=(3.5–4.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=532 nm, I ₀=2.3 W/cm², T=249–388 K). The diffusion length of photocarriers in polypyrrole nanowires is also estimated: L _D=0.18 μm.     

Energy scaling of quasi-monoenergetic electron beams from laser wakefields driven by 40-TW ultra-short pulses

By focusing 40-TW, 30-fs laser pulses to the peak intensity of 10¹⁹ W/cm² onto a supersonic He gas jet, we generate quasi-monoenergetic electron beams for plasma density in the specific range 1.5×10¹⁹ cm^-3≤n_e≤3.5×10¹⁹ cm^-3. We show that the energy, charge, divergence and pointing stability of the beam can be controlled by changing n_e, and that higher electron energies and more stable beams are produced for lower densities. The observed variations are explained physically by the interplay among pump depletion and dephasing between accelerated electrons and plasma wave. Two-dimensional particle-in-cell simulations support the explanation by showing the evolution of the laser pulse in plasma and the specifics of electron injection and acceleration. An optimized quasi-monoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of n_e≃1.5×10¹⁹ cm^-3. PACS 52.35.-g; 52.38.Hb; 52.38.Kd; 52.65.-y     

Relaxation of the conductivity of CsI-Tl after excitation by subnanosecond electron pulses

The pulsed conductivity is investigated for a CsI-Tl crystal having a Tl⁺ concentration N=8×10¹⁷cm⁻³ and excited by an electron beam (0.2 MeV, 50 ps, 10²–10⁴ A/cm ²). It is shown that the amplitude of the conduction current pulse is almost an order of magnitude lower than for “pure” CsI crystals irradiated under like conditions. The conduction current relaxation time is preserved up to τ=100 ps in this case. Under the experimental conditions, therefore, the lifetime of electrons in the conduction band is controlled by trapping at Tl⁺ centers. The electron capture cross section at a Tl⁺ center is determined: σ=7×10⁻¹⁶ cm², which agrees in order of magnitude with estimates of the capture cross section for a neutral trapping center. Fiz. Tverd. Tela (St. Petersburg) 40, 66–67 (January 1998)     

Absolute cross sections of electron attachment to molecular clusters. Part II: Formation of (H2O) N? , (N2O) N? , and (N2) N?

Absolute cross sections σ⁻(E, N) of electron attachment to clusters (H₂O) _N , (N₂O) _N , and (N₂) _N for varying electron energy E and cluster size N are measured by using crossed electron and cluster beams in a vacuum. Continua of σ⁻(E) are found that correlate well with the functions of electron impact excitation of molecules’ internal degrees of freedom. The electron is attached through its solvation in a cluster. In the formation of (H₂O) _N ⁻ , (N₂O) _N ⁻ , and (N₂) _N ⁻ , the curves σ⁻(N) have a well-defined threshold because of a rise in the electron thermalization and solvation probability with N. For (H₂O)₉₀₀, (N₂O)₃₅₀, and (N₂)₂₆₀ clusters at E = 0.2 eV, the energy losses by the slow electron in the cluster are estimated as 3.0 × 10⁷, 2.7 × 10⁷, and 6.0 × 10⁵ eV/m, respectively. It is found that the growth of σ⁻ with N is the fastest for (H₂O) _N and (N₂) _N clusters at E → 0 as a result of polarization capture of the s-electron. Specifically, at E = 0.1 eV and N = 260, σ⁻ = 3.0 × 10⁻¹³ cm² for H₂O clusters, 8.0 × 10⁻¹⁴ cm² for N₂O clusters, and 1.4 × 10⁻¹⁵ cm² for N₂ clusters; at E = 11 eV, σ⁻ = 9.0 × 10⁻¹⁶ cm² for (H₂O)₂₀₀ clusters, 2.4 × 10⁻¹⁴ cm² for (N₂O)₃₅₀ clusters, and 5.0 × 10⁻¹⁷ cm² for (N₂)₂₆₀ clusters; finally, at E = 30 eV, σ⁻ = 3.6 × 10⁻¹⁷ cm² for (N₂O)₁₀ clusters and 3.0 × 10⁻¹⁷ cm² for (N₂)₁₂₅ clusters. Original Russian Text ? A.A. Vostrikov, D.Yu. Dubov, 2006, published in Zhurnal Tekhnicheskoĭ Fiziki, 2006, Vol. 76, No. 12, pp. 1–15.     

Investigation of cross sections for the formation and destruction of negative boron ions

The electron loss and electron capture cross sections σ _i,i+m and σ _i,i−m for boron ions and atoms traveling at the velocities V=1.19 and 1.83 a.u. in H₂, He, N₂, Ne, Ar, and Xe are measured. The known experimental data on these cross sections at velocities near the cross-section maximum are analyzed. It is found that the electron loss cross sections can be described by a formula which was previously derived in the free-collision approximation and takes into account features of both the ions and the ambient atoms. As the nuclear charge Z _t of the ambient atoms increases, the cross sections vary nonmonotonically, increasing on average as Zt _t ^1/2 . A formula based on the model of independent electrons is proposed for electron capture by ions with small values of the charge i. It describes the dependence of the electron capture cross section σ _i,i−1 on the mean binding energy of an electron in an ion with the charge i−1. The total electron capture cross section σ _i,i−1 is proportional to the number of vacancies in the unfilled electron shell nearest the nucleus. The cross sections 0σ _i,i−1 exhibit substantially nonmonotonic variation with Z _t, increasing on average as Z _t ^1/3 . Zh. éksp. Teor. Fiz. 116, 1539–1550 (November 1999)     

Nonlinear optical studies in tetraphenyl-porphyrin-doped boric acid glass using picosecond pulses

We report the results of our studies on the nonlinear optical response of thin films of tetraphenyl porphyrin doped in boric acid glass for picosecond-duration pulses. The fluence-dependent transmission measurements show that the sample exhibits reverse saturation behavior at 532 nm and saturable absorber action at 683 nm. By fitting the nonlinear transmission data at these wavelengths to a rate equation model, values for σ^S _e and σ_L the excited singlet and ground state cross sections were obtained. The large value (≈8) for σ^S _e/σ_L makes it a potential material for optical limiting at 532 nm, whereas at 683 nm the ratio σ_L/σ^S _e≈4.3. Intensity dependence of degenerate forward four-wave mixing at 683 nm was also investigated to establish the order and magnitude of the nonlinearity. The forward four-wave mixing signal at 683 nm showed a cubic dependence on the input intensity, establishing the third-order nonlinearity. A value of 4×10^-12 esu for the third-order susceptibility |χ⁽³⁾| was estimated from the measured efficiency. The time response of the nonlinearity was measured by the pump-probe transmission experiment, resulting in a decay constant of about 1.8 ns for the repopulation of the ground singlet state at 683 nm. Received: 13 April 2000 / Published online: 20 September 2000     

Highly efficient greenish blue-emitting organic diodes based on pyrene derivatives

We report the synthesis of pyrene derivatives as the light emissive layer for highly efficient organic electroluminescence (EL) diodes. Multilayer devices were fabricated with pyrene derivatives (ITO/NPB (50 nm)/blue material (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al). By using 1,1′-dipyrene (DP) and 1,4-dipyrenyl benzene (DPB), the devices produced the blue EL emissions with 1931 Commission International de L’Eclairage coordinates of (x=0.21, y=0.35) and (x=0.19, y=0.25), respectively. The device with DPB shows a maximum brightness of 42,445 cd/m² at 400 mA/cm² and the luminance efficiency of 8.57 cd/A and 5.18 lm/W at 20 mA/cm².     

注硅半绝缘GaAs的深能级

用深能级瞬态谱(DLTS)详细研究了硅离子注入Liquid-encapsulated Czochralski(缩写为LEC)半绝缘GaAs的深中心。结果表明,在注硅并经高温退火的有源区中观测到4个多子(电子)陷阱,E₀₁,E₀₂,E₀₃和E₀₄。它们的电子表观激活能分别为0.298,0.341,0.555和0.821eV。其中E₀₄与EL2有关,但不是EL2缺陷。E₀₄的电子 关键词：     

Characterisation of laser-produced tungsten plasma using optical spectroscopy method

This paper describes results of spectroscopic investigation of laser-produced tungsten plasma. The laser intensity on the target surface reached up to 30 GW/cm² depending on the focusing conditions. Optical spectra emitted from plasma plumes which were formed under vacuum conditions in front of the tungsten target due to the interaction of Nd-YAG laser pulses (1.06 μm, 0.5 J), were characterised by means of an optical spectrometer (λ/Δλ= 900) in the wavelength range from 300 to 1100 nm. The spectra were recorded automatically with the use of a CCD detector with exposition time varied from 100 ns to 50 ms. On the basis of WI and WII lines it was possible to estimate electron temperature and electron density which corresponded to the expansion phase of the plasma. T_e and N_e were measured as 1.1 eV and 8×10¹⁶ cm^-3, respectively. The spectra collected by the ion energy analyser showed that the plasma included tungsten ions up to 6+ ion charge. Signals from the ion collector allowed to estimate the average value of ion energy of tungsten as 4.6 keV. Basing on this value the electron temperature corresponding to the initial stage of the plasma formation was estimated to be about 320 eV. Optical microscope investigation showed that laser irradiation caused structural changes on the surface of the target.     

D.C. conductivity of transparent conductive ZnO:Al films in the temperature range 80–360 K

An experimental study of the temperature dependence of the d.c. conductivity σ as a function of temperature T in the range from 80–360 K on nanocrystalline ZnO:Al films (Al³⁺ 2%) of thickness 500 nm prepared on glass microscope slides by a dip — coating method is presented. The electrical conductivity σ, which at room temperature varied between 0.1 to 2.7 S/cm, increased almost linearly with T for all the samples. Measurements of the Hall coefficient at room temperature and in a magnetic field of 1.2 T, gave R_H=0.53 cm³C⁻¹, from which a carrier concentration of n=1.18×10¹⁹ cm⁻³ and a carrier mobility of μ=1.40 cm²/Vs were deduced. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14–18, 2004.     

Optimal thickness of hole transport layer in doped OLEDs

Current-voltage (I–V) and electroluminescence (EL) characteristics of organic light-emitting devices with N,N’-Di-[(1-naphthalenyl)-N,N’-diphenyl]-(1,1’-biphenyl)-4,4’-diamine (NPB) of various thicknesses as the hole transport layer, and tris(8-hydroxyquinoline)aluminum (Alq₃) selectively doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) as the electron transport layer, have been investigated. A trapped charge induced band bend model is proposed to explain the I–V characteristics. It is suggested that space charge changes the injection barrier and therefore influences the electron injection process in addition to the carrier transport process. Enhanced external quantum efficiency of the devices due to the electron blocking effect of an inserted NPB layer is observed. The optimal thickness of the NPB layer is experimentally determined to be 12±3 nm in doped devices, a value different from that for undoped devices, which is attributed to the electron trap effect of DCM molecules. This is consistent with the result that the proportion of Alq₃ luminescence in the total electroluminescence (EL) spectra increases with NPB thickness up to 12 nm under a fixed bias. PACS 72.80.Le; 85.60.Jb     

Electron screening in backward elastic scattering

Abstact: The elastic scattering cross sections, σ (E,θ), for the systems He+Ta and He+W have been measured at θ_lab=165° and E _lab=76.1 keV to 3.988 MeV using targets with a thickness of a few atomic layers. The results are smaller than the results given by the Rutherford scattering law, σ_R(E,θ), due to the effects of electron screening and can be described by σ(E,θ)/σ_R(E,θ)=(1+U_e/E)⁻¹, where U _e is an atomic screening potential energy. The deduced average value, U _e=28 ± 3 keV, is consistent with the Moliére- and Lenz-Jensen-models as well as electron binding energies. Received: 25 May 1998     

Influence of Ar, Ne, Xe, and CCl4 impurities on luminescence kinetics of Cd II ions (5s22D5/2-5p2P3/2 transition, λ = 441.6 nm) in He-Cd mixture

The luminescence kinetics of the Cd II ion at a wavelength of 441.6 nm has been studied experi-mentally in a high-pressure He-Cd mixture in the presence of Ar, Ne, Xe, and CCl₄ impurities. Cadmium ions were excited through the bombardment of a cadmium foil heated up to 240°C by a pulsed electron beam with an electron energy of 150 keV, a pulse duration of 3 ns, and a current of 500 A. The constants of collisional quenching of the Cd II 5s ^{2 2} D _5/2 level by Ar, Ne, and Xe atoms and CCl₄ molecules and the integral luminescence quenching constants of this level in the helium medium by these impurity gases have been determined. The constants of collisional quenching appeared to be 8.1 × 10⁻¹² (Ar), 1.2 × 10⁻¹² (Xe), 1.5 × 10⁻¹³ (Ne), and 1.8 × 10⁻¹⁰ cm³/s (CCl₄, for λ = 325 nm), while the integral constants were found to be, respectively, 4.1 × 10⁻¹¹, 3.4 × 10⁻¹¹, 9.5 × 10⁻¹², 1.4 × 10⁻⁹ cm³/s for Ar, Ne, Xe, and CCl₄ at a buffer gas pressure of 1 atm. Original Russian Text ? A.I. Miskevich, Liu Tao, 2009, published in Optika i Spektroskopiya, 2009, Vol. 107, No. 1, pp. 45–49.     

Nonlinear optical properties of a new two-photon-absorption organic dye: HEASPS

Trans-4-[p-(N-hydroxyethyl-N-ethylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HEASPS) is a two-photon-absorption (TPA) dye newly synthesized by our research group. It possesses a much larger TPA cross-section and much stronger upconversion fluorescence emission than those of common organic dyes (such as rhodamine) when excited with near-infrared (IR) radiation. TPA spectrum and upconversion efficiency spectrum of HEASPS solution at different wavelengths have been measured. The largest molecular TPA cross-section σ₂ ^′ is measured to be 2.06×10^-47 cm⁴ s/photon at 930 nm. At 1064 nm, σ₂ ^′ is 2.71×10^-48 cm⁴ s/photon, which is only one-ninth of that at 930 nm. The upconverted lasing efficiency spectrum has been measured at different wavelengths. The highest efficiency is 5.1% at 1020 nm, whereas it is 3.5% at 1064 nm. Its optical-power-limiting properties at 930 nm have also been illustrated. Received: 30 November 2000 / Published online: 27 April 2001