K. Jarasiunas

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.

Mapping of GaAs and Si wafers and ion-implanted layers by light-induced scattering and absorption of IR light

The methods of light-induced absorption and diffraction have been used to investigate the uniformity of Si and GaAs wafers and ion-implanted Si. Metastable annealing of defects, lifetime modification by swirl and radiation defects and anomalous depth penetration of ion-implantation-created defects have been demonstrated in Si. The mapping of GaAs wafers has been performed with picosecond and nanosecond light pulses and light absorption mechanisms are discussed.

Role of the charge state of deep vanadium impurities and associations of defects in photoelectric and optical properties of semi-insulating CdTe crystals

Photoexcited carrier and space charge field subnanosecond dynamics has been investigated in bulk vanadium-doped and Cl- (or As-) co-doped CdTe crystals by using a degenerate four-wave mixing technique. Time-resolved measurements of picosecond grating decay at various illumination intensities revealed peculiarities of defect transformation with doping or under illumination. Novel features in carrier generation and dynamics were observed for the first time and allowed parameters of the defects to be extracted. A fast electron capture and its saturation with increasing intensity of illumination were observed in CdTe:V and attributed to a Cd divacancy, which is known as a deep double acceptor. Numerical modelling by using the two-deep-trap model allowed us to extract the concentration and recombination activity of Cd divacancies. An enhanced electron generation rate by factor of 3-4 was found in Cl-co-doped CdTe:V crystals in spite of a decreased density of donor vanadium states after the co-doping. We attribute this additional channel of electron generation to photoexcitation of the DX centre, which activation energy of 1-1.2 eV is close to a quantum energy of the YAG:Nd laser used ( eV).

Characterisation of bulk crystals and structures by light-induced transient grating technique

Electronic and optical properties of bulk crystals and epistructures have been studied via light interaction with inherent growth defects, deep impurities and defect complexes. Excitation by light interference pattern allowed us to monitor spatially-modulated carrier generation, recombination and transport in subnanosecond time domain, determine photogenerated carrier density, lifetime, diffusion coefficient and study defect-related features in an all-optical way.

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.

Dislocation density-dependent photorefractive effect in (001)-cut GaAs.

We present studies of the photorefractive effect in nonphotorefractive orientations of liquid-encapsulated Czochralski-grown GaAs crystals. Picosecond diffraction experiments conducted in different samples show that a forbidden photorefractive signal correlates well with dislocation density, which points out that the effect arises from strain fields and growth defects.

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.

Effects of photo-quenching-buildup and saturation of the induced impurity grating in ZnSe:Cr crystals

A model is proposed and calculations are performed to elucidate the effects of anomalous kinetics and dynamics of transient light self-diffraction signals (“a dip and a buildup” upon the termination of the grating generation pulse and saturation effects) observed in experiments on four-wave mixing in crystals doped with deep impurities. These observations are explained by the effects of optical depletion of local sites by the same grating generation pulse with only the subsequent buildup of the impurity grating through the capture of band carriers by these sites to form the previously created light-dynamic grating.

Characterization of Proton-Irradiated InGaAs/GaAs Multiple Quantum Well Structures by Nonresonant Transient Four-Wave Mixing Technique

Effects of proton irradiation and thermal annealing on free carrier lifetime and transport in a InGaAs/GaAs multiple quantum well (MQW) structure and its substrate have been studied by using the nonresonant transient four-wave mixing technique. Under the excitation of the surface region of the structure by 0.53 µm wavelength, we determined the carrier lifetime τ=150–600 ps, bipolar diffusion coefficient D=24–26 cm2 s-1, and mobility-lifetime product µτ≈(2–6)×10-7 cm2 V-1 in 3 MeV proton irradiated and annealed MQW samples. At excitation by 1.06 µm wavelength, we studied the full proton penetration range, including the substrate. We found an unexpected twofold increase in carrier bulk lifetime with increasing irradiation dose. Features of this effect at low photoexcitation and its absence at high photoexcitation are attributed to carrier separation by potential fluctuations in the vicinity of irradiation-created deep defects. These features indicate that proton-irradiated semi-insulating GaAs exhibits properties of mesoscopic pointlike defects, similar to those previously reported in nonstoichiometric crystals of GaAs. This result is important to understand the mechanism of carrier compensation in proton-irradiated semiconductors.

Laser-induced free carrier and photorefractive nonlinearities in semiconductors with deep traps

Light induced changes of optical properties are in close relation with photoelectrical properties of semiconductors. From the dynamics of nonlinear optical response, not only electronic parameters of crystals but also the channels of energy relaxation, role of defects and mechanisms of light interaction with matter may be studied. Light diffraction on laser induced transient gratings has proved to be a very powerful technique for these fundamental swdies of semiconductors as well as for their applications in opto- and microelectronics. Photorefractive Ill-V and il-VI semiconducting compounds reveal optical nonlinearities of different origins under excitation by short las pulses or by CW lasers [1,2]. The contributions to refractive index modulation by free carriers and by space charge electric fields coexist at pulse excitation while solely linear electrooptic effect is responsible for optical gain in a quasi-stationary case of excitation. Carrier plasma kinetic coefficients, as ambipolar and monopolar mobiities, carrier concentrations and lifetimes, defect concentration and other parameters may be obtained from light diffraction on transient grating experiments [3-7]. In addition, at short pulse excitation a number of novel effects have been obsved: light induced space-charge (SC) field enhancement [8], fast oscillations of diffraction efficiency and multiexponential grating decay [9,10]. These effects probably arise from simultaneous presence of two refractive index modulation mechanisms and their interaction. Indeed, the diffusive decay of free carrier grating leads to the build-up of space charge electric fields, which in turn involve electron and hole sal separation, screening of space charge fields, and affect the decay processes of electronic gratings. A significant step towards the understanding of the temporal behavior of nonlinearities involved as well as possibilities of those optical techniques for semiconductor research may be obtained by help of a detail analysis of carrier and field dynamics in different experimental situations.