Exciton Decay Time Research Articles

Dynamics of the Nitrogen Bound Excitons in 6H and 3C SiC

AbstractWe have measured the photoluminescence decay time of the bound excitons at the neutral nitrogen donors in the 6H and 3C polytypes of SiC. At 2K the decay times are 8.0 ns, 1.8 ns and 1.5 ns, for the P, R and S bound excitons in 6H SiC. For the nitrogen exciton in 3C, we find a decay time of 160 ns. These values are faster than previously reported for shallow donors in other indirect bandgap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, is temperature independent at low temperatures but decrease when the bound excitons are thermally ionised. The decay time related to different donor levels in 6H exhibits a strong dependence on the donor binding energy. We suggest that the dominant mechanism responsible for the observed decay time is a phonon-less Auger process. In high-purity 6H samples we have also measured the free exciton decay time at low temperatures to be 12 ns.

Exciton relaxation in MOVPE grown ZnSe xTe 1- x epilayers

Exciton dynamics in ZnSe x Te 1- x epilayers (0 < x < 0.4) is studied through the emission decay kinetics at liquid helium temperature. At low concentration ( x < 0.1) the emission contains free exciton lines and several impurity-related bands. The free exciton decay times in these samples are less than 100 ps due to the fast energy transfer to defects and the efficient migration of excitons into the interface region. The increase of Se content up to x ≈ 0.2 causes gradual increase of the exciton luminescence decay times (up to 1.5ns) which exhibits strong variation across the exciton emission band contour. This indicates the onset exciton localization by the compositional fluctuations of the mixed crystal.

Radiative lifetimes of excitons in semiconductor quantum dots

A theory of intrinsic radiative lifetimes of excitons in semiconductor quantum dots is presented. Simple closed-form expressions for the decay times are obtained for quantum dots fabricated from quasi-two-dimensional systems. In particular, the intrinsic temperature-dependent photoluminescence decay time of excitons in quantum dots is found to be linear in temperature above ∼ 1 K with a positive intercept extrapolated to zero temperature, in contrast to the case of quantum wells where the extrapolated intercept is zero. Otherwise, the slope is the same as expected for quantum wells (∼17 ps K -1 for 100-Å GaAs/AlAs quantum wells). This constant offset is due to a finite spatial phase-coherence effect and is largest for quantum dots for which the lateral spatial extent is small compared with the wavelength of light in the medium corresponding to the optical transition.

Decay times of excitons in lattice-matched InGaAs/InP single quantum wells

A study of the photoluminescence decay times in lattice-matched InGaAs/InP single quantum wells grown by two different epitaxial techniques is presented. We show that these decay times can be measured directly using a nonlinear photoluminescence autocorrelation technique. A model based on the saturation of localized exciton states describes the temporal behavior and the optical nonlinearities observed very well.

X-Γ scattering in GaAs/AlAs short-period superlattices

The cw and time-resolved photoluminescence spectra of a [0 0 1]-(GaAs) 10/(AlAs) 10 short-period superlattice are studied under [0 0 1] uniaxial stress. The superlattice is of the type-II and the lowest excited state is the “X z” exciton. The stress dependence of the exciton decay time is found to be explained in terms of a thermally activated transition of excitons to the l'state. Results for the stress-induced change in the X-Γ mixing are also presented.

Femtosecond Spectroscopy of Polydiacetylene

Time evolution in polydiacetylene (PDA-3BCMU) was investigated by femtosecond spectroscopy. The relaxation of photoexcited free excitons to self-trapped excitons was observed. The broad-band absorption due to self-trapped excitons was observed below 1.86 eV. The formation and decay times of self-trapped excitons at 10 K were 150±50 fs and 2.0±0.1 ps, respectively. At short delay times several nonlinear optical processes, i.e. hole burning, Raman gain, and dynamic Stark shift, were observed

Quantum confinement of excitons and their relaxation processes in CuCl microcrystals

Abstract Semiconductor microcrystals of CuCl with different mean sizes of an order of nm are grown inside transparent matrices of NaCl. Since the effective Bohr radius of excitons in CuCl is about 0.7 nm, the excitons in such microcrystals keep their bulk-like character even under the strong influence of confinement effect and the size-quantization occurs mainly in their translational motion. The blue shift and the splitting of the exciton levels are found with a decrease of the crystal size. Although there exists some size distribution of the microcrystals, exciton luminescence under the size-selective excitation with picosecond laser light and the analysis including strong reabsorption effects makes possible the determination of the quantum efficiency of exciton radiative decay. The size-dependent radiative decay time of excitons is found to be inversely proportional to the volume of the microcrystals as long as the energy separation between the size-quantized sublevels is larger than the thermal energy. This fact is quantitatively interpreted as the result of the change in the coherence volume of excitons whose motion is restricted by the microcrystal size.

Picosecond spectroscopy in III–V compounds and alloy semiconductors

Picosecond luminescence kinetic curves and time resolved luminescence spectra have been used both to observe the characteristic lifetimes of excitons bound to residual donors in InP and GaAs, and to study the formation time of localized excitons bound to nitrogen in GaP:N, GaAs 1− x P x and Ga x In 1− x . The decay time of excitons bound to residual donors has been found to be 700 ± 100 ps in InP and of the order of 1 ns in GaAs. In the indirect range GaAs material obtained by applying a hydrostatic pressure ( P>41 kbar), the experimental lifetimes become much longer (>20 ns) which illustrates the importance of the band structure effect for shallow excitons bound to residual donors in such semiconductors. Transfer mechanisms of N-bound excitons in Ga x In 1− x P:N and GaAs 1− x P x :N alloys are studied as a function of temperature using picosecond time resolved photoluminescence. At 2 K direct exciton tunneling ocurs within and between the inhomogeneously broadened bands due to differing isolated nitrogen centers. At higher temperature, variable range hopping and multiple trapping via excitons are both active. These results are different from those seen in the binary GaP:N and thus demonstrate the importance of alloy disorder on the exciton dynamics.

Localized indirect excitons in a short-period GaAs/AlAs superlattice.

We have studied the optical properties of a short-period superlattice composed of 20.4-A\r{} GaAs and 14.7-A\r{} AlAs layers. The superlattice behaves as an indirect-gap material. A slow and nonexponential decay of the luminescence can be interpreted as the emission from the \ensuremath{\Lambda} indirect excitons localized at the GaAs/AlAs interfaces. The temperature dependence of the exciton decay time can be explained in terms of a transition by phonon-assisted tunneling, followed by a nonradiative transition.

Photoluminescence lifetimes of bound excitions in ZnSe

The lifetimes of both the donor and acceptor bound exciton photoluminescence lines in ZnSe have been determined. The trend of bound exciton decay time vs. exciton localization energy follows that expected for purely radiative lifetimes, indicating that the Auger process is not an important decay mechanism of bound excitons in ZnSe. A series of time-resolved spectra reveal a longer lived, perturbed acceptor bound exciton.

Temperature Dependence of Time Decay of Photoluminescence in GaP:N

We report measurements in the temperature range 10-300 K of the decay time of excitons bound to isolated nitrogen atoms (A-line) and to pairs of nitrogen atoms with different internuclear distances (NN-lines). It is observed that the decay time of the A-line is about 45 ns up to 20 K, drops to about 4 ns at 40 K and then increases rapidly. The miminum at 40 K allows of the estimate 3 × 10-14 cm2 of the cross-section for capture of excitons by NN pair centres. Above 70 K the measured decay curves for the A-line and NN1-line indicate that thermal equilibrium is established in the system after a few times the decay time of the A-line.

Short-pulse optical studies of exciton relaxation andF-center formation in NaCl, KCl, and NaBr

Using short-pulse laser techniques, the formation time and absolute production efficiency of $F$ centers in KCl have been investigated on the time scale of ${10}^{\ensuremath{-}11}$ sec. The temperature-dependent yield of $F$ centers observable at 46 psec following two-photon band-gap excitation has been determined over the range 12-880 K in crystalline KCl. The yield of $F$ centers per ionizing event approaches unity near the melting point. The observation of transient 532-nm absorption resulting from ultraviolet pulse irradiation of molten KCl is reported. The formation time, production efficiency, and room-temperature decay time of self-trapped excitons in the lowest triplet state in NaCl have been investigated, as has the onset of 532-nm absorption in NaBr. The observations, in general, place upper limits of a few picoseconds on the time for capture of an electron by a hole in these alkali halides when carrier densities are in the range of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Recently proposed mechanisms of $F$-center formation are discussed in light of the present results. We develop mathematically convenient treatments for convolution of pulse shapes and intrinsic photochemical response in several kinetic models.

Diffussion of Singlet Excitons in Anthracene Crystals

The temperature dependence of the decay time of singlet excitons in anthracene crystals was measured under no suffering of reabsorption effect. It was clarified that the excitons were populated on the two Davydov states in quasi-thermal equilibrium before emission process. Furthermore, probabilities of energy transfer from the exciton to doped acceptors were measured and the values of the diffusion coefficient D of the exciton was estimated as 10 -1 ∼1 cm 2 /sec which was much larger than that obtained by many researchs. A similar measurement to that by Simpson was carried out by using a pure perylene film instead of the tetracene doped film in the earlier measurement, giving D as 1.5 cm 2 sec -1 . Furthermore, the drift time due to the diffusion of the exciton through a thin anthracene layer was directly observed, giving D as 1.3 cm 2 sec -1 .

Optical Properties of Excitons Bound to Neutral Acceptors in GaP

Luminescence of excitons bound to five point defects and one complex neutral acceptor has been identified in GaP. These excitons are weakly bound, and have weak no-phonon lines because of the symmetry of the electron in the exciton. The high degeneracy of this electron in GaP, resulting from the multivalleyed conduction band, together with the allowed states for the combination of two $j=\frac{3}{2}$ holes, gives 12 possible distinct no-phonon lines in the exciton transition, many of which have been seen for the shallower acceptors. These bound excitons obey a modified form of Haynes's rule. This fact, the magnitude of the exciton localization energies and the magneto-optical properties of these excitons firmly establish the nature of these transitions. Mirror symmetry is exhibited between the absorption and luminescence spectra of the Zn exciton, and the time for radiative decay has been determined. This is 500 times longer than the measured luminescence decay time, because of the predominance of the Auger recombination channel. The radiative decay times of these shallow excitons are nearly independent of the acceptor. The dependence of the Auger life-times ${\ensuremath{\tau}}_{E}$ on the binding energies of the exciton complexes have been established for the first time. It is shown that ${\ensuremath{\tau}}_{E}\ensuremath{\propto}{E}_{A}^{n}$, where $n\ensuremath{\sim}\ensuremath{-}4.5$, close to the value (-4.0) predicted from a hydrogenic model with a single spherical valence band. It is clear that deeper (neutral) acceptors form highly nonradiative recombination centers for excitons, with very short decay times. Strong complex satellites in the acceptor-exciton luminescence may involve phonons associated with the dynamic Jahn-Teller effect. Weaker "two-hole" luminescence transitions have been tentatively identified for the Zn and Cd acceptors. If this identification is correct, different types of final hole states are seen for these two acceptors.