Exciton emission from forward-biased zinc selenide Schottky diodes

Exciton emission has been observed in the forward-biased electroluminesence of ZnSe Schottky diodes free of intentionally added luminescent centres. The emission has been studied from ≈ 95° K to room temperature and is attributed to the recombination of free excitons (zero-LO phonon line). No LO phonon side bands were observed. As with exciton photoluminescence in CdS the half-width of the emission line, although a linear function of temperature, is greater than that predicted by theory. At temperatures below ≈150°K pair emission was also observed.     

Spin accumulation in forward-biased MnAs/GaAs Schottky diodes

We describe a new means for all-electrical generation of spin polarization in semiconductors. In contrast with spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe accumulation at the metal-semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the nonequilibrium carrier polarization.     

Surface photovoltage in exciton absorption range in CdS

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120–160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.     

Barrier height modification in Schottky MIS diodes

Expressions for the potential barrier height of Schottky MIS diodes having gaussian doping profiles are derived and solved numerically both at thermal equilibrium and in the presence of applied voltages. The possibility of barrier height modification using a thin highly doped surface layer is discussed.     

The MIS structure of selenium diodes

It is shown that a selenium diode biased in the reverse direction is characterized by several features special to MIS structures. The effect of ionic processes in the MIS structure on the volt-amp characteristics is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 7–11, November, 1973.     

Optical activity of CdS crystals in exciton spectral region

Natural optical activity of wurtzite CdS crystals has been studied for the first time both theoretically and experimentally in the spectral region of the B_n=1 exciton resonance. It has been shown that the observed optical activity is due to mixing the longitudinal Γ_5L exciton state with the transverse Γ₁ state by the energy terms linear in wave vector.     

Resonant Brillouin scattering near the a exciton in CdS

We have studied resonant Brillouin scattering in CdS by tuning a narrow-band cw dye laser through the A-exciton transition at helium temperatures. We observed besides the resonance enhancement a pronounced asymmetry of the Stokes and anti-Stokes shift and a dramatic increase of the Brillouin shifts as a function of laser frequency. These observations are consistent with a model of exciton-polariton scattering by longitudinal acoustic phonons. For incident frequencies higher than the longitudinal exciton frequency additional features are found which cannot be explained by the two-branch model for exciton-polaritons.     

Phonon replica of excited level of free exciton in CdS

We report here the first experimental observation of the flourescence of a LO phonon replica from the A_n=2 excitonic level in cadmium sulfide. A very simple theoretical interpretation is given. We present also experimental data about phonon replicas in the case of forbidden transitions.     

Gamma emission in the pre-equilibrium exciton model

The pre-equilibrium exciton model of nuclear reactions is extended to include gamma emission. Possible types of gamma transitions are considered for which densities of final accessible states are derived. The computed gamma spectra in the reaction (n, γ) with E_n = 14.1 MeV on ⁹³Nb and ¹³⁹La are compared with experiment.     

Triplet-to-singlet exciton formation in poly(p-phenylene-vinylene) light-emitting diodes

The triplet to singlet exciton formation ratio in a MEH-PPV light-emitting diode is measured by comparing the triplet-induced absorptions with optical and electric excitations at the same singlet exciton density. The ratio is a strong universal decreasing function of the averaged electric field. Using 4 ns for singlet to triplet intersystem crossing time, the ratio is significantly larger than the spin-independent value 3 at intermediate field but is reduced to about 2 for higher field.     

Effects of optical activity on exciton luminescence of CdS

The luminescence spectra have been investigated in the vicinity of the B_n=1 exciton resonance in CdS at T = 2 K. The effect of “weak” optical activity on luminescence has been observed for the first time. The observed spectra are compared with the frequency dependences of the polariton transmission coefficients.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

Mixing of singlet and triplet exciton states in highly excited CdS

Fluorescence spectra due to the free excitons have been studied in CdS at 4.2 K under various excitation levels. It has been found that the triplet-exciton emission intensity relative to the singlet line is enhanced remarkably with the excitation power density. This effect is reasonably explained by the mixing of the singlet with the triplet through the many body interaction. The density of optically generated excitons is determined from the magnetic field dependence of the triplet emission intensity.     

Free and bound exciton decay in indium phosphide

Excitonic recombination and photoconductivity near the energy gap of vapour grown InP epitaxial layers are investigated. Besides the free exciton several bound exciton complexes are observed and studied as a function of temperature. A new value of the band gap is derived (E_g = 1.424 ± 0.001 meV).     

Stimulated self-trapped exciton emission in CsI:Pb

Defects of the type of V_K and Pb⁺ centres were created in CsI:Pb under the 4.03 eV XeCl laser line irradiation at 10 K. After irradiation, the self-trapped and localized exciton emission excited by the same XeCl laser line was observed as a result of the recombination of electrons, optically released from Pb⁺, with the V_K centres. A strongly superlinear dependence of the emission intensity on the excitation intensity was found for the 3.65 eV emission of the self-trapped exciton. A much weaker superlinearity was observed for the visible localized exciton emission. Optical amplification of the exciton emission was considered as the most probable reason of the observed phenomenon. At 10 K, optical gain G=3.74 was calculated for the self-trapped exciton emission.     

Singlet and triplet exciton formation rates in conjugated polymer light-emitting diodes

By applying a molecular orbital perturbation approach, we calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. It is found that the interchain bond-charge correlation has a strong influence on the relative probabilities for generating singlet and triplet excitons. Most importantly, application of our approach to a model system for poly-(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.