Ramunas Aleksiejunas

Investigation of nonequilibrium carrier transport in vanadium-doped CdTe and CdZnTe crystals using the time-resolved four-wave mixing technique

We present a novel way to determine the type of dominant carrier photoexcited from deep traps in a photorefractive semiconductor. A numerical analysis of a picosecond free-carrier grating dynamics has revealed an excitation intensity dependent grating diffusive decay time τD as well as effective carrier diffusion coefficient D, when the intensity varied in the range below that required to create a bipolar carrier plasma. According to the numerical analysis, an increase or decrease of effective diffusion coefficient D with excitation can be used as a criterion to distinguish the type of photogenerated carrier. We have verified this method experimentally by measuring dependences of effective D versus excitation density in a number of vanadium-doped and shallow-impurity codoped CdTe and ZnCdTe crystals, using for excitation a picosecond YAG:Nd laser (hν = 1.17 eV). The results were found to be in good agreement with predictions, based on carrier transport peculiarities in photorefractive crystals, and correlated well with the secondary ion mass spectroscopy data for each crystal.

Optical and electron beam studies of carrier transport in quasibulk GaN

Variable temperature light-induced transient grating technique combined with electron beam-induced current measurements in situ in a scanning electron microscope were employed for carrier transport studies in quasibulk hydride-vapor phase epitaxy grown undoped GaN layers. Diffusion length of carriers independently determined from both techniques was found to increase with temperature in the range from 70 to 400 K. This increase was attributed to the temperature-induced growth of carrier lifetime, as was confirmed by light-induced transient grating measurements below 300 K and by cathodoluminescence above room temperature.

Picosecond dynamics of spin-related optical nonlinearities in InxGa1−xAs multiple quantum wells at 1064 nm

Light-induced absorption and diffraction measurements of resonantly excited 10-nm-width InGaAs multiple quantum wells have been carried out, using circularly polarized beams at 1064 nm. Spin relaxation time τs≃280±15 ps has been determined by monitoring dynamics of light absorption bleaching at 0.04 mJ/cm2, while a nonlinearly compressed spin component in diffraction varied from 220±20 ps to 115±15 ps with excitation. The kinetics of carrier grating with randomized spins allowed the determination of the bipolar diffusion coefficient D=11.5 cm2/s, hole mobility of 230 cm2/V s, and carrier lifetime τR=0.73–1 ns.

Spin and carrier relaxation in resonantly excited InGaAs MQWs

Time-resolved absorption bleaching and four-wave mixing techniques have been used for studies of spin relaxation and nonequilibrium carrier dynamics in resonantly excited InGaAs/GaAs multiple quantum well structures. Kinetics of absorption bleaching at 1064 nm provided spin relaxation times of 240–280 ps and carrier lifetimes of 750–1000 ps at low excitations. At higher excitations, nearly three-fold increase of spin-relaxation rate has been measured by the FWM technique, while the decrease of carrier lifetime was much less pronounced. The kinetics of nonequilibrium carrier grating with randomized spins allowed determination of the in-plane bipolar diffusion coefficient in the wells D = 11.5 cm2 s−1 and hole mobility of 230 cm2 V−1 s−1.

All-optical analysis of carrier and spin relaxation in InGaAs∕GaAs saturable-absorber structures

Results of an all-optical analysis of basic semiconductor parameters such as carrier mobilities, lifetimes, and electron spin relaxation time of implanted In0.25Ga0.75As∕GaAs multiple quantum well saturable-absorber structures for the 1060nm spectral range are presented. These parameters are determined in a wide range of optical excitation, even at the practical operation point of such devices. This is accomplished by the application of polarization-resolved pump-probe and four-wave-mixing spectroscopies. The all-optical approach allows the determination of mobilities and spin relaxation time from the same experiments and points to the D’yakonov-Perel mechanism to govern the electron spin relaxation at room temperature.

Diffusion Enhancement in Highly Excited MAPbI3 Perovskite Layers with Additives

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH3NH3PbI3 (MAPbI3) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement-carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

Carrier transport and recombination in resonantly excited InGaAs/GaAs MQWs

We present nonlinear optical studies of the non‐equilibrium carrier dynamics in InGaAs/GaAs quantum well structures. We exploit the free‐carrier and electron spin‐governed nonlinearities in pump‐probe and four‐wave mixing experiments to measure the carrier lifetime, spin relaxation time, and in‐well mobilities of electrons and holes. A saturation of excitonic nonlinearity at 0.08 ± 0.02 mJ/cm2 was determined by four‐wave mixing.

Dislocation‐density Dependent Carrier Lifetime and Stimulated Recombination Threshold in GaN

Correlation between the carrier lifetime τR and threading dislocation density NTD has been studied and provided the relationship τR ∝ NTD−1/2 in a wide range of NTD (from 1010 to 106 cm−2). The conditions for stimulated recombination under picosecond optical pumping were found more favorable in the layers with higher dislocation density and attributed to effect of heterogeneous potential barriers at dislocations.

Hot-carrier transport governed optical nonlinearities in dc-biased GaAs crystals

Hot carrier dynamics and related optical nonlinearities, which arise in dc-biased GaAs under spatially varying optical illumination, have been investigated using light diffraction on transient grating technique. Under dc-bias, time resolved four-wave mixing measurements had evidenced an oscillatory behavior and increased efficiency of light diffraction. The effect was found the largest at illumination intensity corresponding to the electron density between 1015 cm-3 and 1016 cm-3. Numerical modeling of nonlinear transport at various applied dc-voltages and photoexcitation levels revealed conditions for an efficient and fast refractive index modulation by a transient high-field domain grating. Experimental obervations have been explained in terms of nonuniform carrier heating and formation of transient Gunn-domain grating in the region of negative differential resistance. Our results open the possibility of measuring hot-carrier picosecond dynamics and predict a novel way of fast refractive index modulation in dc-biased compound semiconductors.