Relaxation Processes Of Photoexcited Carriers Research Articles

Observation of photoinduced polarons in semimetal 1T-TiSe2

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron–phonon coupling in 1T-TiSe2. The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron–phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe2, and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.

Excitation-density and excess-energy dependence of ultrafast dynamics of photoexcited carriers in intrinsic bulk CdTe

In this study, ultrafast dynamics of photoexcited carriers in an intrinsic CdTe film are investigated deeply and systematically as a function of carrier density and excess energy using femtosecond-resolved pump–probe transient differential transmission spectroscopy. A dynamic model is developed based on an interband transition absorption model in direct bandgap semiconductors by introducing the dynamics of thermalization, cooling and recombination of photoexcited carriers into the interband transition absorption model. Then, the model is used to best fit the measured ultrafast dynamics to retrieve the initial temperature of thermalized electrons and time constants of the thermalization, cooling and recombination dynamic processes as a function of carrier density and excess energy. The increase with carrier density and the decrease with excess energy of thermalization time constant reveal that thermalization process of photoexcited carriers is dominated by carrier-phonon scatterings. We find for the first time that cooling of thermalized carriers depend strongly on carrier density and excess energy, and cooling time constant varies in a wide range from 0.2 to 5 ps. It is also found that carrier recombination in the intrinsic CdTe film is dominated by the radiative recombination of electron-hole pairs, rather than Auger recombination as reported in doped CdTe films. These new results are very important to understanding of microscopic mechanism of relaxation processes of photoexcited carriers and applications of CdTe-based optoelectronic devices.

Ultrafast carrier dynamics in a GaN/Al0.18Ga0.82N superlattice

Relaxation processes of photoexcited carriers in a $\mathrm{GaN}/{\mathrm{Al}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$ superlattice are studied in femtosecond spectrally resolved reflectivity measurements at ambient temperature. The transient reflectivity reveals electron trapping into defect states close to the conduction-band minimum with a 150--200 fs time constant, followed by few-picosecond carrier cooling. A second slower trapping process into a different manifold of defect states is observed on a time scale of approximately 10 ps. Our results establish the prominent role of structural defects and disorder for ultrafast carrier dynamics in nitride semiconductor structures.

Ultrafast dynamics of the Mn d-d transition on hexagonal HoMnO3

The photoinduced changes in reflectivity (ΔR/R) at energies closed to the Mn d-d optical transition of HoMnO3 single crystal have been systematically measured from 300 K to 30 K by time-resolved optical pump-probe spectroscopy. The oscillation induced by the propagation of strain pulse was clearly observed in the ΔR/R signal with various probing wavelength. Moreover, the different relaxation processes of photoexcited carriers with in various wavelengths may be led by the blue shift of Mn3+ d level.

Influence of oxygen vacancies on optical properties of anatase TiO 2 thin films

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO 2 thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed.

Relaxation process of photoexcited carriers in GaAs structures with low-temperature-grown layers

Spatial relaxation processes of photoexcited carriers in GaAs structures are studied by means of photoluminescence spectroscopy. A single GaAs∕Al0.3Ga0.7As quantum well and a low-temperature-grown GaAs (LT-GaAs) layer containing a high concentration of excess arsenic are placed in a GaAs structure as optical markers; the former serves as the radiative recombination site, while the latter as the trapping site of photoexcited carriers. The photoluminescence intensity from the quantum well is significantly reduced by the presence of a LT-GaAs layer immediately next to a barrier layer. The effect of the LT-GaAs layer is exponentially enhanced as a thickness of the barrier layer decreases. The results suggest that once an excess As point defect is placed within an extent of a wave function of a photoexcited carrier, trapping of the photoexcited carrier occurs at an extremely fast rate. In a structure where a LT-GaAs is placed at a distant location from the quantum well, the photoluminescence intensity from the quantum well is weakly dependent on the location of the LT-GaAs layer as expected from thermal diffusion of photoexcited carriers to trap sites as semiclassical particles.

Photoexcited carrier dynamics in potassium tantalate

Relaxation processes of photoexcited carriers have been studied in KTaO 3. The excitation above the absorption band edge generates a broad luminescence with a large Stokes shift, suggesting the formation of self-trapped excitons. The luminescence decay is found to be non-exponential, indicating the broad distribution of the carrier lifetime. The decay obeys power law after 10 ms, whose exponent becomes steeper as the temperature increases. The mean lifetime of the luminescence is as long as 10 ms at 4.6 K, and becomes faster at high temperatures. The temperature dependence of the lifetime should come from the thermally activated hopping motion of localized electrons and holes. Localized carriers may play an important role in the giant photoinduced dielectricity in KTaO 3.

Relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown by molecular beam epitaxy at low temperatures

The relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown at low temperatures by molecular beam epitaxy were studied by a tunable single-beam femtosecond pump–probe method. Concentrations of singularly ionized antisite arsenic ions, AsGa+, in the quantum wells, which were considered as traps of photoexcited carriers, were estimated from flux conditions and substrate temperatures in the growth. Transient transmittivity of the structures were measured by varying the pump–probe photon energy. The trapping rate of photoexcited carriers, which corresponded to the reciprocal of the carrier lifetime, was derived from the relaxation profile at the pump–probe photon energy close to the exciton resonant excitation energy for each structure. The trapping rate was found to increase linearly with AsGa+ in a lower concentration range and superlinearly in a higher concentration range. Photoluminescence and absorption spectra were observed at room temperature and their correlation to the carrier lifetimes were investigated.

気相合成ダイヤモンド単結晶薄膜における短パルスレーザー励起キャリアの輸送特性

In this study, we have measured photocurrents and time-resolved photoluminescence spectra of high-quality CVD diamond UV detectors using a short pulse UV laser (wavelength : 220 nm). Photo-excited carrier transport properties measured for CVD diamond films under electric field (E) demonstrate that the photocurrent peak intensity at the shortest period after the pulse excitation proportionally increased with increasing E in the range of E below ≈1 × 103 V/cm while it was proportional to E1/2 in the range of E above ≈ 1 × 103 V/cm. This well corresponds to changes of carrier drift speeds theoretically obtained for sufficiently high quality diamond. On the other hand, the photocurrent intensity integrated in a constant time period (below 360 ns) is almost proportional to E in the whole E regions measured (to 1× 104 V/cm). On the contrary, photoluminescence (PL) intensity only slowly decreased with increasing E while the PL decay constants were independent of E. Furthermore, the non-radiative relaxation processes of photo-excited carriers were almost independent of E in the examined field region.

Ultrafast Carrier Thermalization in Hydrogenated Amorphous Silicon

AbstractWe present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ∼ 150 fs time scale and rapid phonon equilibration on a ∼ 230 fs time scale.

Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.

Relaxation processes of photoexcited carriers in CuCl nanocrystals embedded in NaCl

In a CuCl nanocrystal embedded in a NaCl single crystal, we observed a time response of exciton and biexciton luminescence intensities under strong pulsed light excitation by using the up-conversion method at 2 and 77 K. It was found that the rise of the biexciton luminescence is delayed compared to that of the exciton luminescence at 77 K. This is assumed to be because the biexciton is not directly created from electron-hole $(e\ensuremath{-}h)$ pairs but through exciton formation. On the basis of the cascade relaxation model, the formation time of an $e\ensuremath{-}h$ pair to an exciton is estimated to be 1.6 ps. We also observed a time response of luminescence intensities of the bound exciton, free biexciton, and bound biexciton at 2 K. These time profiles are interpreted by the cascade relaxation process. The formation time of the $e\ensuremath{-}h$ pair is estimated to be 1.3 ps, and the trapping lifetimes of the exciton to a surface impurity center and of the $e\ensuremath{-}h$ pair to the bound exciton are estimated to be 1.4 and 1.8 ps, respectively.

Optical studies of infrared active electronic defects in neutron irradiated silicon after annealing at 450°C

Using Fourier transform infrared absorption spectroscopy and Hall effect measurements mainly the infrared active electronic defects in fast neutron irradiated float-zone silicon and Czochralski silicon after annealing at 450 °C are investigated. Introducing thermal donors (TD) to alter the Fermi level, the defect associated with the higher-order bands (HOB) is analyzed, which is proposed to have at least three charge states within the band gap. The defect level giving rise to the HOB is located slightly below Ec −0.15 eV, and another level possibly exists near the bottom of the conduction band. Moreover, the measurement results indicate that the characteristic of photoexcitation and decay of the HOB is associated with the slow relaxation process of photoexcited carriers.

USING OPTICALLY DETECTED CYCLOTRON RESONANCE TO STUDY ELECTRON-EXCITON AND ELECTRON-PHONON INTERACTIONS IN SEMICONDUCTORS

We review our optically detected cyclotron resonance (ODCR) studies extending over Ge, Si and ZnSe. The following three subjects are taken up: (1) Impact dissociation of excitons by free carriers, which is dominated by electron-electron and hole-hole interactions rather than by electron-hole interaction. (2) The hot-carrier state produced in Ge and Si by cyclotron resonance is discussed in terms of hot-carrier-distribution function, effective carrier temperature, and carrier kinetics. (3) The relaxation process of photoexcited carriers in ZnSe is discussed. This relaxation process is found to be dominated by electron-polar-optical-phonon interaction, which gives rise to the polaron effect and to resonant formation of free excitons following one-LO-phonon emission. Thermally detected cyclotron resonance (TDCR) is also introduced.

Femtosecond luminescence measurements in GaAs

We report luminescence measurements in GaAs with femtosecond time resolution. This represents more than an order-of-magnitude improvement over previous luminescence measurements and allows us to study initial relaxation processes of photoexcited carriers. We show that the relaxation of photoexcited carriers to the band extrema is delayed by 750 fsec when the excitation density is reduced from 3 × 10 18 cm -3 to 1 × 10 17 cm -3. We have measured luminescence rise and decay curves at various photon energies ranging from 1.45 eV to 1.70 eV. These results are discussed in terms of carrier-carrier and carrier-phonon interactions.