Carrier Capture Research Articles

Charge deep-level transient spectroscopy of SiO2 and Al2O3 layers with embedded Ge nanocrystals

The present study of deep level transient spectroscopy (DLTS) is focused on a comparison of the trap states in two types of Ge nanocrystallites (NCs)-insulator composites. The investigated systems were the dielectric matrices Al2O3 and SiO2 in which the Ge NCs were embedded. We have found couples of traps with related values of activation energies in both the Ge:Al2O3 and the Ge:SiO2 films. In the films with a relatively low Ge content (where only small NCs sized 3–5 nm could have been detected by means of Raman spectroscopy), we observed traps with an energy level ∼50 meV in the Ge:Al2O3 films and 120 and 50 meV in the Ge:SiO2 films. In both systems, we found that the electron traps have a small carrier capture cross-section (10−21–10−23 cm2). We have identified the levels of the traps to be the quantum confinement levels in the small Ge NCs. For samples of higher Ge contents, where the NC size reaches about 20 nm and where an appreciable portion of the dielectric matrix consists of amorphous Ge (α-Ge), we found traps with an energy of 0.22–0.24 eV in the Ge:Al2O3, and 0.26–0.27 eV in the Ge:SiO2 samples. We suggest that this peak in the charge DLTS (Q-DLTS) spectra is associated with a trap at the Ge-NC/α-Ge interface. We have also identified the energy position of a defect level in the Ge:Al2O3 layers, which lies 0.46–0.49 eV below the conduction-band edge of the Si substrate.

RF linewidth in passively mode locked quantum well lasers

Investigation of the stability of pulse repetition rate emitted with semiconductor lasers in the regime of passive mode locking has been conducted. It was shown experimentally that the decrease of an overlap integral of the quantum-sized active layer with the waveguide mode and the increase of the time of the carrier capture on the emitting level resulted in narrowing of the radio-frequency linewidth.

Dependence of diode sensitivity on the pulse rate of delivered radiation

It has been reported that diode sensitivity decreases by as much as 2% when the average dose rate set at the accelerator console was decreased from 600 to 40 MU∕min. No explanation was given for this effect in earlier publications. This work is a detailed investigation of this phenomenon: the change of diode sensitivity versus the rate of delivery of dose pulses in the milliseconds and seconds range. X-ray beams used in this work had nominal energies of 6 and 15 MV and were generated by linear accelerators. The average dose rate was varied from 25 to 600 MU∕min, which corresponded to time between microsecond-long dose pulses of 60-2.7 ms, respectively. The dose-per-pulse, dpp, was changed by positioning the detector at different source-to-detector distance. A variety of diodes fabricated by a number of manufacturers were tested in this work. Also, diodes in three different MapCHECKs (Sun Nuclear, Melbourne, FL) were tested. For all diodes tested, the diode sensitivity decreases as the average dose rate is decreased, which corresponds to an increase in the pulse period, the time between radiation pulses. A sensitivity decrease as large as 5% is observed for a 60-ms pulse period. The diode sensitivity versus the pulse period is modeled by an empirical exponential function. This function has a fitting parameter, t(eff), defined as the effective lifetime. The values of t(eff) were found to be 1.0-14 s, among the various diodes. For all diodes tested, t(eff) decreases as the dpp decreases and is greater for 15 MV than for 6 MV x rays. The decrease in diode sensitivity after 20 s without radiation can be reversed by as few as 60 radiation pulses. A decrease in diode sensitivity occurs with a decrease in the average dose rate, which corresponds to an increase in the pulse period of radiation. The sensitivity decrease is modeled by an empirical exponential function that decreases with an effective lifetime, t(eff), of 1.0-14 s. t(eff) varies widely for different diodes, dpp, and x-ray energy. It is hypothesized that the capture of excess minority carriers by charge traps, cause the observed decrease in diode sensitivity. Also, it is hypothesized that the slow reopening of these traps occurs in the hundreds of milliseconds to seconds range at ambient temperature and this underlies the slow decrease in the diode sensitivity. Calibration of a diode is best done at the average dose rate with which it will be used. This is easily accomplished for radiation deliveries in which the average dose rate is a constant. However, for a VMAT delivery the average dose rate is a variable. For measurements made under these conditions diodes can be calibrated with a median or average dose rate which splits the difference in diode sensitivity that is known to occur with changes in average dose rate.

Probing carrier populations in ZnO quantum wells by screening of the internal electric fields

We investigate the carrier relaxation from (ZnMg)O barrier layers into a ZnO quantum well (QW) by following the dynamic screening of its built-in electric fields. The respective emission lines shift in energy as the carriers populate the QW, spectrally shifting the time-resolved photoluminescence. At low temperatures, the carrier capture into the QW is found to occur on the same or an even faster time scale than the carrier-trapping processes within the barriers.

Photon correlation studies of charge variation in a single GaAlAs quantum dot

Complex charge variation processes in low-density, direct-type GaAlAs quantum dots embedded in a type-II GaAs/AlAs bilayer are studied by single-photon correlation measurements. Two groups of excitonic transitions are distinguished in the single quantum dot (QD) photoluminescence spectra, namely due to recombination of neutral and charged multiexcitonic complexes. The radiative cascades are found within each group. Three characteristic time scales are identified in the QD emission dynamics. The fastest one (of the order of 1 ns) is related to excitonic radiative recombination. The two remaining ones are related to the QD charge state variation. The one of 100-ns range (typical blinking time scale) corresponds to random capture of single carriers under a quasiresonant excitation. The slowest processes, in the range of seconds, are related to charge fluctuations in the surrounding of the dot.

Electron spin resonance of paramagnetic defects and related charge carrier traps in complex oxide scintillators

AbstractIn the family of application‐important complex, oxide single crystal scintillators based on tungstates, aluminum perovskites, and yttrium (lutetium) orthosilicates, selected results of electron spin resonance study are presented. They are focused on various point defects, which participate in the processes of charge carriers transfer and capture. Particular attention is paid to the most natural defects inevitably present in oxide materials such as anion and cation vacancies, antisite defects, self‐trapped electron, and hole states. Current understanding of the nature of such charge trapping states and mechanisms of their creation in the selected oxide scintillation materials are discussed as well.

Barrier capacitance characteristics of CdS–Cu2S junction structures

Cu2S–CdS junctions between polycrystalline layers formed by substitution technique have been examined by combining the standard current–voltage and capacitance–voltage methods together with the barrier evaluation by linearly increasing pulsed voltage technique. Measurements of the capacitance characteristics enabled us to reveal three types of heterojunctions differing in density of dopants and carrier capture centers. Different species of CuxS precipitates were detected in these three types of structures by using X-ray diffraction analysis. It is shown that the layer deposition temperature and duration have to be optimized to prevent the copper diffusion into CdS layer, to increase the free carrier density, and to diminish the density of deep recombination centers.

Nonexponential photoluminescence transients in a Ga(NAsP) lattice matched to a (001) silicon substrate

Ga(NAsP) grown lattice-matched on (001) silicon substrate is a very promising material for future integrated, electrically pumped lasers on silicon. Here, we present experimental and theoretical studies of the time-resolved photoluminescence in Ga(NAsP)/Si quantum well structures. The experimental results obtained at 10 K show a strong nonexponential transient behavior for the photoluminescence signal. A detailed comparison with theoretical calculations based on rate equations and on straightforward Monte Carlo simulations reveals that this effect is controlled by an interplay between the fast capture of carriers on nonradiative centers and the slow radiative recombination via localized states. We demonstrate that the measurement of the time-resolved photoluminescence can serve as a convenient tool for estimating the relative concentrations of nonradiative and radiative centers in compound materials.

Combined influence of carrier-phonon and Coulomb scattering on the quantum-dot population dynamics

We have investigated the impact of the self-consistent coaction of carrier-phonon and Coulomb interactions on the scattering efficiency in a quantum-dot system within a quantum kinetic theory. While there is a substantial effect on the scattering between discrete quantum-dot states due to the interplay of both interactions, the capture of carriers from a wetting layer is only slightly enhanced by the coaction. For the carrier capture due to the electron-phonon interaction, results of the quantum kinetic treatment are compared to those of a perturbative approach.

Electrical Characteristics of Surface-Stoichiometry-Controlled p-GaN Schottky Contacts

Experimental results of Au/Ni Schottky contacts formed on three kinds of p-GaN layers, which were grown at different turn-off temperatures (300, 600, and 900 °C) of NH3 gas supply (TNH3) upon cooling at the end of growth to control the surface stoichiometry are reported. In the internal photoemission results, the Schottky barrier heights of all the samples were as large as around 2.2 eV, and a slight increase of 0.1 eV was observed when TNH3 decreased from 900 to 300 °C. At the same time, carrier capture and emission from acceptor-like midgap-level defects decreased as TNH3 decreased. The N-rich cooling condition tends to passivate the acceptor-type defects or create donor-type defects for compensation, and the pinning position at the interface might be moved slightly toward the conduction band edge.

Carrier dynamics in modulation-doped InAs/GaAs quantum rings

Carrier dynamics in undoped and modulation-doped self-assembled InAs/GaAs quantum rings was comprehensively examined through time-resolved photoluminescence techniques at room temperature. After photoexcitation, the rates of carrier capture and relaxation in the ground state and excited states were equally fast in both the undoped and charged quantum rings. Instead of cascading down through the excited states, the carriers excited in the undoped quantum rings relaxed through the quasi-continuum states via coupling with acoustical phonons and eventually thermalized to the ground state by emitting longitudinal optical phonons. The dynamics of carrier thermalization was governed by cold carriers in the charged quantum rings. In carrier recombination, the undoped quantum rings had a shorter PL decay time and lower luminescence intensity than the charged quantum rings because radiative recombination was suppressed by defects in the matrix and the absence of cold carriers.

Optimal Color Stability for White Organic Light-Emitting Diode (WOLED) by Using Multiple-Ultra-Thin Layers (MUTL)

The work demonstrates the improvement of color stability for white organic light-emitting diode (WOLED). The devices were prepared by vacuum deposition on ITO-glass substrates. These guest materials of 5,6,11,12-tetraphenylnaphthacene (Rubrene) were deposited in 4,4′-bis(2,2-diphenyl vinyl)-1,1′-biphenyl (DPVBi), resulting in an emitting layer. Experimental results reveal that the properties in the multiple-ultra-thin layer (MUTL) are better than those of the emitting layer with a single guest material, reaching the commercial white-light wavelength requirement of 400–700 nm. The function of the MUTL is as the light-emitting and trapping layer. The results show that the MUTL has excellent carrier capture effect, leading to high color stability of the device at various applied voltages. The Commissions Internationale De L’Eclairage (CIE) coordinate of this device at 3~7 V is few displacement and shows a very slight variation of (0.016, 0.009). The CIE coordinates at a maximal luminance of 9980 cd/m2are (0.34, 0.33).

Experimental Evidence of the Impact of Nitrogen on Carrier Capture and Escape Times in InGaAsN/GaAs Single Quantum Well

We report our experimental results and theoretical analysis on carrier escape time in In0.4Ga0.6As1 -yNy/GaAs (y = 0; 0.005) ridge waveguide single-quantum-well (QW) lasers with N-contents of 0% and 0.5%. The experiments were carried out by using novel time-resolved two-color pump-probe transmission measurements. Our results show a significant decrease of carrier escape time with nitrogen incorporation in the InGaAsN QW, which agrees well with the values obtained from the theoretical calculation based on thermally activated hole leakage. The measurement results provide experimental supports for hole leakage theory.

Multiphonon ionization of traps formed in hafnium oxide by electrical stress

AbstractWe have investigated behavior of traps formed in hafnium oxide (HfO2) by electrical stress and their influence on the charge carrier transport through Si/SiO2/HfO2/poly‐Si nanostructures. The traps govern the transport process assuming a capture of charge carriers followed by their ionization via the multiphonon transition mechanism. The multiphonon transitions via the Poole–Frenkel effect or electron tunneling as well as the multiphonon tunneling ionization of neutral traps have been carefully considered for charged traps. We also provide a set of parameters including the trap concentration, ionization energy, the frequency factor, the effective mass of charge carriers, optical energy, and phonon energy in order to reproduce and reasonably fit available experimental data.

Charging behavior of silicon nitride based non-volatile memory structures with embedded semiconductor nanocrystals

The charging behavior of MNS (metal-nitride-silicon) and MNOS (metal-nitride-oxide-silicon) structures containing Si or Ge nanocrystals were studied by capacitance–voltage (C–V) and memory window measurements and by simulation. Both the width of hysteresis of C–V characteristics and the injected charge exhibited exponential dependence on the charging voltage at moderate voltage values, while at high voltages the width of hysteresis of C–V characteristics and the injected charge exhibited saturation. The memory window for reference MNS structure without nanocrystals was wider than that for reference MNOS structures. The presence of nanocrystals enhanced the charging behavior of MNOS structures, but in MNS structures nanocrystals exhibited the opposite effect. The main conclusion is that the presence of nanocrystals or other deep levels close to the Si surface enhances the charge injection properties due to the increased tunneling probability, but nanocrystals or other deep levels located far from the Si surface in the nitride layer do not enhance, but even can degrade the charging behavior by the capture of charge carriers.

Phonon-Kick Mechanism for Defect Reactions in Semiconductors

The possibility of a feedback mechanism between nonradiative carrier captures by a deep-level defect and induced transient lattice vibrations is discussed using appropriate configuration coordinate diagrams for many carriers. By treating the lattice motion classically, we self-consistently simulate the time evolution of the interaction mode and a series of athermal captures of electron(s) and hole(s). When both the activation energies Eeact and Ehact are low, a series of successive nonradiative carrier-captures are possible for high carrier densities. However, we find that the possibility of inflation in the amplitude of lattice vibration critically depends on minority carrier capture rate and the relative width of the phonon frequency distribution. Referring to the obtained results, we discuss the phonon-kick mechanism for defect reactions.

Phonon-Kick Mechanism for Defect Reactions in Semiconductors

The possibility of a feedback mechanism between nonradiative carrier captures by a deep-level defect and induced transient lattice vibrations is discussed using appropriate configuration coordinate diagrams for many carriers. By treating the lattice motion classically, we self-consistently simulate the time evolution of the interaction mode and a series of athermal captures of electron(s) and hole(s). When both the activation energies Eeact and Ehact are low, a series of successive nonradiative carrier-captures are possible for high carrier densities. However, we find that the possibility of inflation in the amplitude of lattice vibration critically depends on minority carrier capture rate and the relative width of the phonon frequency distribution. Referring to the obtained results, we discuss the phonon-kick mechanism for defect reactions.

Study of deep level characteristics in the neutrons irradiated Si structures by combining pulsed and steady-state spectroscopy techniques

The standard methods, such as capacitance deep level transient spectroscopy (C-DLTS) and thermally stimulated current (TSC) techniques are unsuitable for the analysis of heavily irradiated devices. In this work, therefore, several steady-state and pulsed techniques have been combined to comprehensively evaluate parameters of radiation defects and functional characteristics of the irradiated Si pin detectors. In order to understand defects created by radiation and evaluate their evolution with fluence, C-DLTS and TSC techniques have been employed to make a baseline identification of the radiation induced traps after irradiation with a rather small neutron fluence of 1012 cm−2. The steady-state photo-ionization spectroscopy (PIS) technique has been involved to correlate thermal- and photo- activation energies for definite radiation defects. A contactless technique for simultaneous measurements of the carrier lifetime and the parameters of deep levels based on microwave probed pulsed photo-conductivity (MW-PC) spectroscopy has been applied to correlate carrier capture cross-sections and densities of the identified different radiation defects. A technique for spectroscopy of deep levels in junction structures (BELIV) based on measurements of barrier capacitance charging current transient changes due to additional spectrally resolved pulsed illumination has been applied to evaluate the functional characteristics of the irradiated diodes. Pulsed spectroscopic measurements were implemented by combining the analysis of generation current and of barrier capacitance charging transients modified by a single fs pulse of illumination generated by an optical parametric oscillator of varied wavelength in the range from 0.5 to 10 μm. Several deep levels with activation energy in the range of 0.18–0.8 eV have been resolved from spectral analysis in the samples of Si grown by magnetic field applied Czochralski (MCz) technology.

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

AbstractAuger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Conductivity of Se95As5 chalcogenide glassy semiconductor layers containing the EuF3 rare-earth impurity in high electric fields

By investigating the I–V characteristics of an Al-Se95As5:EuF3-Te structure, it is established that, upon the application of a positive potential to Te, the current flows in it by the mechanism of space-charge-limited currents (SCLCs) of unipolar-injection, and the N-type I-V characteristic is observed for the opposite polarity. It is shown that these are processes of the thermal-field ionization of neutral and negatively charged U− centers, which play a dominant role in the current-flow mechanism in the investigated structures upon the application of an electric field with intensities exceeding 105 V/cm. These are also processes of the electron-hole recombination and capture of charge carriers at U− centers. The energy position and concentration of the local states, which correspond to the indicated centers and characterize the effect of the electric field—activation length are determined. It is established that it is the EuF3 impurities that predominantly affect the local-state concentration.