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     

Influence of the electric characteristics of II–VI semiconductor material on the electroluminescence of lanthanide complex

The organic-inorganic combined structural device (ITO/PVK:Eu/ZnS/Al) is fabricated based on layered optimization scheme. II–VI semiconductor material ZnS is acted as an electron function (transporting and acceleration) layer. The hot electrons which have been accelerated in the ZnS layer directly impact excitation europium ions through resonant energy transfer and then recombine with injected holes to form excitons in PVK or EuTTA₂(N-HPA)Phen. Europium (Eu) ions may also be excited by intramolecular energy transfer from ligands. There are two kinds of excitation mechanisms: impacted excitation and injected recombination for the combined structural device. The electroluminescence (EL) intensity of the combined structural device is strongly improved and reaches up to 381 cd/m² at 20 V compared with the pure organic structural device. It may be an effective method to improve the EL intensity of the lanthanide complex by using electric characteristics of inorganic semiconductor materials.     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

On the recombinational enhancement of ionic lines in the afterglow of a pulsed noble-gas hollow-cathode discharge

Strong recombination peaks were observed at He-, Ne-, and Ar-ion lines in the afterglow of a pulsed, helical hollow-cathode discharge excited by 10 Hz, 0.5–100 µs duration and 1–23 A rectangular current pulses. The intensity of the peaks depended strongly on the high-voltage leading part of the exciting pulse. An optimum was found in the peak intensity as a function of pulse length. The peaks could not be observed at pulses exceeding a certain duration. From results obtained using double-pulse excitation, the peaks are considered to be due to recombination of doubly ionized atoms produced by the high-voltage leading edge of the exciting pulse with thermal-energy electrons. Line intensity investigations using an optical resonator indicate the possibility of gain at the He II 468.6 nm transition.     

Pulse characteristics of Hg0.8Cd0.2Te n +-p junctions

The pulse characteristics of Hg_0.8Cd_0.2Te n ⁺-p junctions are investigated. It is shown that the shape of the voltage pulse appearing in a junction on passage of a forward (reverse) current is determined by the recombination (generation) of nonequilibrium electrons in the hole region. An increase in the current pulse causes the appearance of an electric field, which draws electrons into the interior of the base region, and leads to variation of their lifetime because of the complex structure of the n ⁺-p junction. Zh. Tekh. Fiz. 67, 130–133 (July 1997     

Modeling of transient electroluminescence overshoot in bilayer organic light-emitting diodes using rate equations

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time t_m and intensity I_m corresponding to the peak of EL overshoot, total EL intensity I_t and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio I_m/I_s decreases with increasing value of the applied pulse voltage because I_m increases linearly with the amplitude of applied voltage pulse and I_s increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τ_t of electrons at the interface decreases with increasing temperature whereby the dependence of τ_t on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.     

Recombination radiation of ZnTe crystals with ap-n junction formed by the laser doping technique

Radiative recombination is studied in ZnTe-based diodes produced on the basis of a p–n junction formed by laser doping of crystals with an Al donor impurity. The luminescence spectra obtained in the case of excitation of the diodes by direct current with a density of 3 A/cm² and the photoluminescence spectra at 80 and 300 K are measured. A comparison of the photoluminescence spectra of the p-ZnTe substrate and the electroluminescence spectra at 80 K suggests that radiative recombination in the diodes occurs in the Al-doped region of the crystal. At 80 K in the electroluminescence spectra of the diodes, a band, unknown previously, with the position of the energy maximum at 2.276 eV was observed. This band can be assigned to radiative transitions between the donor level of the al atom and the valence band. For the first time, long-wave bands assigned to participation of deep, compensatory centers were not found in radiative recombination of ZnTe-based diodes. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 382–386, May–June, 1998.     

The electrical characteristics of MBE n-Hg1–xCdxTe (x?=?0.29–0.31) MIS structures with sharp inhomogeneities in composition

The effect of regions with periodic sharp 48–54 nm thick inhomogeneities in composition on the electrophysical characteristics of MIS structures based on graded-gap n-Hg_1–x Cd _x Te (x = 0.29–0.31) grown by molecular-beam epitaxy is studied. It is found that major electro-physical and photo-electrical characteristics are qualitatively similar for MIS structures based on n-Hg_1–x Cd _x Te with sharp inhomogeneities in composition (barriers) and without “barriers”. It is shown that the electrical characteristics are mostly affected by the “barrier regions” located close to the insulator – semiconductor interface. This effect is manifested in an increase of the effective dielectric thickness, which can be due to the fact that the regions of enhanced composition form potential barriers for electrons, and in a decrease in the relaxation time of non-equilibrium carriers due to recombination at the boundaries of the regions with sharp changes in composition.     

Kinetics of indirect photoluminescence in GaAs/AlxGa1−x As double quantum wells in a random potential with a large amplitude

The kinetics of indirect photoluminescence of GaAs/Al_xGa_1−x As double quantum wells, characterized by a random potential with a large amplitude (the linewidth of the indirect photoluminescence is comparable to the binding energy of an indirect exciton) in magnetic fields B≤12 T at low temperatures T≥1.3 K is investigated. It is found that the indirect-recombination time increases with the magnetic field and decreases with increasing temperature. It is shown that the kinetics of indirect photoluminescence corresponds to single-exciton recombination in the presence of a random potential in the plane of the double quantum wells. The variation of the nonradiative recombination time is discussed in terms of the variation of the transport of indirect excitons to nonradiative recombination centers, and the variation of the radiative recombination time is discussed in terms of the variation of the population of optically active excitonic states and the localization radius of indirect excitons. The photoluminescence kinetics of indirect excitons, which is observed in the studied GaAs/Al_xGa_1−x As double quantum wells for which the random potential has a large amplitude, is qualitatively different from the photoluminescence kinetics of indirect excitons in AlAs/GaAs wells and GaAs/Al_xGa_1−x As double quantum wells with a random potential having a small amplitude. The temporal evolution of the photoluminescence spectra in the direct and indirect regimes is studied. It is shown that the evolution of the photoluminescence spectra corresponds to excitonic recombination in a random potential. Zh. éksp. Teor. Fiz. 115, 1890–1905 (May 1999)     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Charge transfer mechanism in high-purity silicon-basedn +-π-p + structures

Results of experimental investigations into current-voltage characteristics of n⁺-π-p⁺ structures based on high-purity silicon doped with boron are presented. It is shown that the I–V characteristic at high injection levels is described by Stafeev's theory, when the thickness d of the π-region is smaller than or close to three ambipolar diffusion lengths L_a. For a current density J above (3–50) A/cm² at varying temperatures, the I–V characteristic obeys a relation usually occurring in the case where charge-carrier recombination in the n⁺ and p⁺-regions dominates that found in the π-region. The effect of mutual electron-hole scattering on the behavior of the I–V characteristic is evident at J>630 A/cm³. For d/L_a=9, the I–V characteristic at high injection levels is treated by the theory of double carrier injection into a semiconductor with consideration for diffusion corrections. In the temperature range below 200–276 K, the I–V curve for all samples studied exhibits a linear dependence on J followed by a portion corresponding to a maximum occupancy of recombination levels by injected electrons. Here the Fe or Au donor levels presumably act as recombination levels. The electron and hole capture areas (cross sections) by recombination centers are roughly estimated. Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 35–45, July 2000.     

Influence of Nonlinear Gain and Nonradiative Recombination on the Quantum Noise in InGaAsP Semiconductor Lasers

Effects of nonlinear gain and nonradiative recombination on characteristics of intensity and phase noises of InGaAsP lasers emitting in a wavelength of 1.55 Μm are investigated. The investigations are performed based on a self consistent numerical approach that takes into account the cross correlation of fluctuations among the injected electron number, photon number and the optical phase. Time variations of the fluctuations of the intensity and the shift of the lasing frequency are statistically analyzed. The corresponding frequency dependencies of intensity and frequency noises are characterized. The results show that both intensity and frequency noises around the relaxation frequency are suppressed when counting the nonlinear gain in the rate equations. The intensity noise is enhanced in the low frequency regime by increasing the nonradiative recombination. The results fit well with those predicted by the small-signal analysis.     

Recombination processes in europium-doped cadmium iodide crystals

Results of comprehensive research into optical and luminescent-kinetic characteristics of europium-doped cadmium iodide crystals excited by nitrogen laser radiation, α-particles, and x-rays are presented. Crystals under study have been grown by the Bridgman–Stockbarger method. The doping EuCl₃ admixture was introduced into the charge in quantities of about 0.05 and 1.0 mol%. Impurity absorption detected in the near-edge region of the crystals is interpreted as part of the Eu²⁺ ion long-wavelength band associated with f–d-transitions. The cation impurity and matrix defects in CdI₂:Eu²⁺ crystals create complex centers responsible for emission with a maximum in the 580–600-nm region. The short component in the luminescence decay kinetics of weakly-doped crystal excited by α-particles and x-ray photons is due to the exciton emission characteristic of CdI₂. The slow component in the scintillation pulse results from recombination of charge carriers followed by creation of exciton-like states on the defect-impurity centers. Laser or x-ray excitation induces light-sum accumulation on the trapping levels at a depth of 0.2–0.6 eV that is mainly related to matrix microdefects. Trapping centers associated with the chlorine impurity are observed in the heavily-doped crystal. Photostimulated luminescence at 85 K arising at the electron stage of the recombination process is caused by recombination of electrons released from F-type centers with holes localized near the activator. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 358–364, May–June, 2009.     

Recombination processes in structures with GaN/ALN quantum dots

Mechanisms of the generation and the radiative and nonradiative recombination of carriers in structures with GaN quantum dots in the AlN matrix are studied experimentally and theoretically. Absorption, stationary and nonstationary photoluminescence of quantum dots at different temperatures are investigated. It is found that the photoluminescence intensity considerably decreases with the temperature while the photoluminescence kinetics weakly depends on the temperature. The photoluminescence kinetics is shown to be determined by radiative recombination inside quantum dots. A mechanism of nonradiative recombination is proposed, according to which the main reason for the thermal quenching of photoluminescence is nonradiative recombination of charge carriers, generated by optical transitions between quantum dots and wetting layer states.     

The homogeneity of a stabilized discharge-pumped XeCl* laser

* laser arrangement. The special pumping technique of this laser discharge is based on the additional use of a stabilizing low-current preliminary discharge. The model takes into account the time-dependent electron Boltzmann equation including electron–electron interaction for the determination of the electron kinetics, an extensive reaction kinetics involving various heavy particles and photons, and the relevant electrical circuit equations. The study has shown that density perturbations of preionization electrons lead to the inhomogeneity of the discharge plasma and the laser output. Furthermore, the impact of the spatial distribution of preionization electrons, of the HCl portion of the gas mixture, and of a low-current preliminary discharge for the discharge operation is discussed. Received: 10 September 1996/Revised version: 21 July 1997     

Features of charge-carrier recombination in amorphous molecular semiconductors doped with polymethine dyes

The photogeneration and recombination of charge carriers in poly-N-epoxypropylcarbazole films with additions of a polymethine dye are investigated irradiation of films with blocking contacts by light both within and outside the absorption range of the dye. The kinetics of the accumulation and relaxation of electron-hole pairs, whose lifetimes exceed tens and hundreds of seconds, are studied. It is postulated that an increase in the recombination luminescence intensity occurs in an electric field as a result of an increase in the efficiency of the bimolecular radiative recombination stimulated by trapped electrons from photogenerated excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 629–635 (April 1998)     

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)     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

Effects of electrode film modifications on the open-circuit photovoltage in enhanced dye-sensitized solar cells

In this study, the open-circuit photovoltage (V _oc) decay technique was used to investigate the relationship between the electrode film morphology and the open-circuit photovoltage. Results indicate that dye-sensitized solar cells (DSCs) based on ordered arrays of TiO₂ nanostructures (100 nm external diameters and 20–50 nm internal diameters) generally show higher open-circuit photovoltage (V _oc) values than those based on sintered TiO₂ nanoparticles (20–40 nm diameters). In particular, cells based on thick nanotubules (wall thickness ≥ 45 nm in our research) and on nanorods (100 nm diameters) show particularly high V _oc values, indicating slow recombination kinetics under open-circuit conditions. It can be argued that the nanorods and the thick nanotubules act like singles crystals and therefore the injected electrons in the inner TiO₂ molecules are shielded from holes in the electrolyte under open-circuit conditions. The open-circuit recombination time constant of electrons accumulated in the TiO₂ conduction band is therefore prolonged and resulting in high V _oc values.