Radiative Recombination Of Electrons Research Articles

A novel pseudomorphic (GaAs1−xSbx-InyGa1−yAs)/GaAs bilayer-quantum-well structure lattice-matched to GaAs for long-wavelength optoelectronics

Two types of quantum well (QW) structures grown lattice matched on (100) GaAs have been studied. The first type of structure consists of pseudomorphic GaAsxSb1-x/GaAs (x≤0.3) SQWs which show emission wavelengths longer than those reported for pseudomorphic InyGa1−yAs/GaAs QWs. However, the attractive emission wavelength of 1.3 μm has not been achieved. To reach this goal, a novel type of bilayer QW (BQW) has been grown consisting of a stack of two adjacent pseudomorphic layers of GaAsxSb1−x and In Ga1-y As embedded between GaAs confinement layers. In this BQW, a type-II heterojunction is formed between GaAsxSb1−x and InyGa1−yAs, resulting in a spatially indirect radiative recombination of electrons and holes at emission wavelengths longer than those achieved in the GaAsxSb1−x/GaAs and IiyGa1−yAs/GaAs SQWs. The longest 300K emission wavelength observed so far was 1.332 μm.

Band-tail photoluminescence in polycrystalline silicon thin films

We have found a new photoluminescence (PL) band with a maximum at 0.9 eV and a halfwidth of 0.1 eV at 4.2 K in polycrystalline Si thin films deposited on glass at 625 °C. The PL band strongly shifts toward low energy with increasing the temperature (1.3 meV/K) and toward high energy with increasing the excitation intensity. Hydrogenation of polycrystalline Si enhances the PL intensity by factor of 3 to 5. The luminescence characteristics are consistent with radiative recombination of electrons and holes trapped in tail states of the conduction and the valence band, respectively. Excellent agreement is achieved between the 0.9 eV band shape and theoretical calculations based on a band-tail recombination. It is also argued that a corresponding luminescence spectroscopy provides a new possibility for band-tail diagnostics in polycrystalline Si thin films.

Thermoluminescence and defects of Sr 1− xEu xF 2+ x solid electrolytes

Thermoluminescence (TL) studies of the Sr 1 − x Eu x F 2 + x mixed system have revealed three groups of TL glow peaks in the temperature ranges (340, 360–370 K), (460–475, 540–575 K) and (615–635, 680–720 K). The TL emission spctrum of SrF 2 consists of two emission bands at 475 and 700 nm, whereas in Sr 1 − x Eu x F 2 + x additional TL emission bands are obtained at 418, 500, 580 and 745 nm. Electron-beam-irradiated Sr 1 − x Eu x F 2 + x showed optical absorption bands at 265, 464 and 500 nm ( x = 0), while in other compositions these are suppressed, resulting in additional bands at 226, 328, 345, 355, 365 and 385 nm. The intensity of the 370 K glow peak increases with increasing europium concentration. Room-temperature annealing also caused an increase in the intensity of the 370 K glow peak. Upon comparison of TL, TL emission, optical absorption and room-temperature annealing studies, the 370, 460 and 550 K glow peaks were attributed to radiative impurity luminescent centers and radiative recombination of electrons with holes being deexcited from two stages of F-centers. The corresponding TL emissions are at 475 and 700 nm. The TL emission bands of Sr 1 − x Eu x F 2 + x at 418 nm and at 500, 550, 580 and 745 nm are the characteristic emissions of Eu 2+ and Eu 3+, respectively, and a tentative energy-level diagram is proposed.

Carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum-well photoluminescence

A quantum-mechanical calculation of the carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum wells is performed, under steady-state optical excitation conditions and in the high-temperature regime. The variables are the continuous-wave (cw) laser intensity, well widths, and acceptor distribution in the well. Radiative recombination of electrons with free holes and holes bound at neutral acceptors are considered. Our calculations for the dependence of the electron density on laser intensity are in quantitative agreement with recent experimental results for multiple asymmetric coupled quantum wells at T=300 K and for intermediate excitation. Also, results for the carrier-density-dependent e-h recombination decay time at T=155 K are in good agreement with recent experimental data in semiconductor quantum wells.

Radiative lifetimes, quasi-Fermi-levels, and carrier densities in GaAs-(Ga,Al)As quantum-well photoluminescence under steady-state excitation conditions

A quantum-mechanical calculation of the carrier densities, electron and hole quasi-Fermi-levels, and various radiative decay times in GaAs-(Ga,Al)As quantum wells is performed, under steady-state excitation conditions, as functions of the cw laser intensity, temperature, well widths, and acceptor distribution in the well. We consider the radiative recombination of electrons with free holes and with holes bound at neutral acceptors. Our calculations---which have no free parameters---are in quantitative agreement in the intermediate laser-intensity regime at T=300 K with the results by Ding et al. [Appl. Phys. Lett. 60, 2051 (1992)], who obtained the carrier density for multiple asymmetric coupled quantum wells through a fitting procedure that reproduced the total experimental photoluminescence intensity. Results for the carrier-dependent e-h recombination decay time are in good agreement with experimental data by Bongiovanni and Staehli [Phys. Rev. B 46, 9861 (1992)].

Magneto-optics of acceptor-doped GaAs/Ga1−xAlxAs heterostructures in the quantum Hall regime: Resonant magnetoexcitons and many-electron effects

The effect of carrier density, magnetic field, and final-state interactions on the radiative recombination of electrons with holes localized on acceptors in GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As heterostructure is investigated. In the high-density regime, strong oscillations of the position and intensity of the emission spectrum as a function of the magnetic field are observed at even filling factors. In the low-density regime, an almost perfect cancellation of many-body effects in emission from the first subband is observed accompanied by oscillations of the emission intensity of the second subband magnetoexcitons. The peaks in the intensity of the second subband emission occur at the integer values of the ratio of the subband separation to the cyclotron energy. These results are explained by the calculated self-consistent subband structure and emission spectrum including shake-up and excitonic effects.

Many-electron effects in acceptor-related radiative recombination of quasi-two-dimensional electrons.

The radiative recombination of electrons from a quasi-two-dimensional electron gas with holes localized on acceptors is investigated. The emission spectra for a single heterojunction are calculated including screening and the dynamical response of the Fermi sea. The line-shape analysis provides unambiguous identification of the Fermi-edge singularity in the emission spectra of the two-dimensional electron gas.

Excitonic effects in optical spectra of a quasi-one-dimensional electron gas

The radiative recombination of electrons in a quasi-one-dimensional gas with valence band holes localized by potential fluctuation or acceptors is investigated. For holes localized by potential fluctuations the emission spectrum contains the singularity at low energy due to the divergence in single particle density of states and a singularity at the Fermi level (Fermi Edge Singularity) due to the electron-hole pair excitations. For holes localized on acceptors only the Fermi Edge Singularity survives as the dominant structure in the spectrum.

Quasi-Fermi-levels in quantum-well photoluminescence.

The nonequilibrium quasi-Fermi-levels of electrons and holes in quantum wells are calculated during photoluminescence. It is assumed the electrons and holes are created by continuous laser excitation. Various recombination processes are included: electron radiative recombination with holes bound at neutral acceptors, electron radiative recombination with free holes, hole trapping at ionized acceptors, and Auger decay. A numerical example is presented for acceptors in GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells.

ChemInform Abstract: Electroluminescence at n‐SiC/Electrolyte Interface Under Cathodic Polarization. Observation of EL Transients in a Short Time Scale and Further Evidence for a Donor‐Acceptor Transition

Electroluminescence (EL) at electrolyte junction was studied under cathodic polarization. Various redox electrolytes including current doubling reagents like persulfate were employed to understand the nature of EL. When the cathodic bias is higher than the flatband potential of electrode, species like persulfate are reduced by the conduction band electrons to form highly oxidizing intermediates which inject holes into the valence band to produce light . In the case of redox electrolytes like , , etc., the EL emission is weak and occurs at a higher cathodic bias, i.e., when the cathodic bias exceeds the bandgap energy of . Here, an electron transfer occurs from the valence band to the redox electrolyte. This indicates that the minority carrier injection (hole) is an important process in obtaining high EL intensities. The emission appears greenish‐yellow and the peak energy of the spectrum is smaller by 1.1 eV than the bandgap of (3.2 eV, calculated from photoresponse measurements), suggesting that the radiative recombination occurs through the impurity luminescent centers. Even under continuous polarization the EL intensity is steady for longer time duration and the electrodes are highly stable. The spectral distribution and increase in the EL intensity with different cathodic pulsed bias potentials were observed and explained by a donor‐acceptor (D‐A) mechanism. The charge transfer at the interface and the radiative recombination of electrons and holes inside the semiconductor are explained from the characteristics of EL and current intensities vs. time obtained under various pulsed polarized conditions. Moreover, the electrodes are highly stable under cathodic steady‐state polarization for several hours. Time resolved measurements (using our present system) seem to indicate the EL transients are limited by the current flow.

Diamond-like carbon films for electroluminescent applications

Diamond-like carbon (DLC) films grown by plasma-enhanced chemical vapor deposition are employed for the first time as an active layer in an alternating current (a.c.) thin-film electroluminescent (ACTFEL) device configuration. The ACTFEL device configuration consists of the DLC active layer sandwiched between two insulating layers which are contacted by a reflecting and transparent electrode. A large a.c. waveform with a small duty cycle is applied with a sufficiently large amplitude so that the DLC layer electrically breaks down; the insulators prevent this breakdown from being catastrophic. Electroluminescence arises from radiative recombination of electrons and holes via bandtail-to-bandtail states. This type of luminescence is inherently very broad band and the electroluminescence appears white. We have been able to produce electroluminescent emission well into the UV region of the electromagnetic spectrum so that these devices are potentially useful for visible and UV applications. The brightness and efficiency of our devices are very low to date: 0.5 ft lambert and 0.002 lm W -1.

Electroluminescence at n ‐ SiC / Electrolyte Interface under Cathodic Polarization: Observation of EL Transients in a Short Time Scale and Further Evidence for a Donor‐Acceptor Transition

Electroluminescence (EL) at electrolyte junction was studied under cathodic polarization. Various redox electrolytes including current doubling reagents like persulfate were employed to understand the nature of EL. When the cathodic bias is higher than the flatband potential of electrode, species like persulfate are reduced by the conduction band electrons to form highly oxidizing intermediates which inject holes into the valence band to produce light . In the case of redox electrolytes like , , etc., the EL emission is weak and occurs at a higher cathodic bias, i.e., when the cathodic bias exceeds the bandgap energy of . Here, an electron transfer occurs from the valence band to the redox electrolyte. This indicates that the minority carrier injection (hole) is an important process in obtaining high EL intensities. The emission appears greenish‐yellow and the peak energy of the spectrum is smaller by 1.1 eV than the bandgap of (3.2 eV, calculated from photoresponse measurements), suggesting that the radiative recombination occurs through the impurity luminescent centers. Even under continuous polarization the EL intensity is steady for longer time duration and the electrodes are highly stable. The spectral distribution and increase in the EL intensity with different cathodic pulsed bias potentials were observed and explained by a donor‐acceptor (D‐A) mechanism. The charge transfer at the interface and the radiative recombination of electrons and holes inside the semiconductor are explained from the characteristics of EL and current intensities vs. time obtained under various pulsed polarized conditions. Moreover, the electrodes are highly stable under cathodic steady‐state polarization for several hours. Time resolved measurements (using our present system) seem to indicate the EL transients are limited by the current flow.

X-ray excited and thermoluminescence of gadolinium activated KCaF 3

X-ray excited luminescence of KCaF 3:Gd is studied in the temperature range of 300K down to 10K. The luminescence spectra shows two emission bands at 360 and 446 nm, in addition to gadolinium characteristic emission lines around 313 nm. As the temperature is lowered to 10K, the 446 nm emission is increased while the 360 nm band gets quenched. The two emission bands at 360 and 446 nm have been ascribed to native impurities and radiative recombination of electrons with V k- centers respectively. The thermoluminescence of X-irradiated KCaF 3:Gd showed three glow peaks around 365, 400 and 540K, the origin of these glow peaks has been discussed and attributed to impurities, F and F-aggregate centres, respectively.

Dissociation-width-dependent radiative recombination of electrons and holes at widely split partial dislocations in silicon.

Analyse de la structure fine des bandes de photoluminescence D5 et D5' montrant que ces bandes sont formees par la superposition d'au moins onze spectres de «raies», dues vraisemblablement a la recombinaison radiative sur des dislocations partielles de type Shockley a differentes distances de separation

Excitonic radiative recombination of electrons and holes in semiconductors

The radiative excitonic recombination coefficient Bx in semiconductors for the process e + h → X* + photon is calculated. Electrons and holes are considered in a parabolic-band approximation. The exciton’s internal state is taken as hydrogenic. The exciton’s center-of-mass energy spectrum is found to be modified Maxwellian with a sharper tail at low energies. Numerical calculations are presented for gallium arsenide. The value of Bx is about 3 × 10−9 cm3 sec−1 at 45 K. Bx decreases rapidly with increasing temperature and approaches the value of 3 × 10−10 cm3 sec−1 at temperatures when excitons are unstable against thermal decomposition, which is comparable with a direct radiative recombination coefficient at room temperature. The possibility for a stimulated recombination process is also discussed.

A new (non-copper-induced) 1.35 eV emission band in n-type GaAs. Origin and characteristics

A new (non-copper induced) emission band peaked at hνm(77 K) = 1.35 eV is observed in n-type GaAs. Pronounced features of this emission are: it disappears as copper atoms are introduced into GaAs and appears again as the copper atoms are afterwards removed from the GaAs interior. Evidence is presented that the free electron transitions to acceptor-type (VGaTeAs) VAScomplexes are responsible for the non-trivial emission observed. Temperature and doping variations of the peak positions, intensities, and half-widths of the new 1.35 eV emission band are presented. An analysis of these results shows that the energy levels of the (VGaTeAs) VAS complexes are localized not far (e = 0.1 eV) from the top of the valence band; these complexes readily capture both, free electrons (capture coefficient cn = 10−11cm3/s) and holes (capture coefficient cn= 10−8cm3/s); rather strong electron-phonon coupling exists in (VGaTeAs) VAScomplexes – phonons are effectively emitted during the free electron radiative recombination. [Russian Text Ignored].

Annealing behavior of the 0.8 eV luminescence in undoped semiinsulating GaAs

The annealing behavior of the 0.8 eV luminescence band in undoped semiinsulating GaAs has been investigated. It is found to be fully analogous to that of the AsGa antisite electron-paramagnetic-resonance signal. The radiative recombination of electrons with the doubly ionized AsGa double donor is discussed as the origin for the 0.8 eV band.

A tunneling model for the decay of luminescence in inorganic phosphors: The case of Zn2SiO4:Mn

We have investigated phosphorescence, thermoluminescence, and photostimulated luminescence in manganese activated zinc silicate phosphors, both with and without arsenic doping. Both phosphorescence and photostimulated luminescence intensities are found to decay as the reciprocal of the time, a result which requires a different interpretation from that given by the usual model of electron release from a distribution of trap levels. To account for these results we propose a new model based on the radiative recombination of electrons and holes through tunneling from shallow traps or from excited states of the deeper traps. A simple expression is derived which describes the decay of both types of luminescence in phosphors under very general conditions for which tunneling is the dominant recombination mechanism. Excellent agreement between theory and experiment is found.

Some fundamental measurements of the atmospheric-pressure, microwave-induced discharge

Measurements of several fundamental parameters in a microwave-induced atmospheric-pressure flowing plasma are presented. Optical and electrical measurements were performed on argon and argon/nitrogen plasmas in the region 1–7 cm outside the cavity, as the applied microwave power, and plasma composition were varied. The stability of the plasma, atomic emission from the argon support gas, and emission from the analyte species, are proportional to the electron density. The observed electron density was varied when the power was changed,-when an electrophilic species was added, and as the observation zone was moved relative to the microwave field. In all cases, the change in the emission signal followed the change in electron density. The electron temperature, as measured by the double-probe method, is related to the kinetic energy of the fastest-moving electrons in the plasma. It is unchanged by variations in power, plasma gas composition, flow rate, and is independent of the location of the probes relative to the cavity. The spectroscopic and electrical data are consistent with excitation by ion—electron radiative recombination.

A new interimpurity recombination in GaP; revised values for acceptor binding energies

We have observed the radiative recombination of electrons, attached to nitrogen pairs, with holes on neutral acceptors. The acceptor binding energies are found to be 102.5±1 meV for Cd, 70.1±1 meV for Zn, and 54.7±1 meV for C, 8.3 meV larger than previously accepted values. This result implies corresponding increases in the band gap and exciton binding energy, to 2.350±0.002 eV and 22±2 meV, respectively, at 2 K.