Free Carrier Lifetime Research Articles

Contact‐Induced Mechanisms in Organic Photovoltaics: A Steady‐State and Transient Study

The role of the contacts in thin‐film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole‐collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open‐circuit voltages (VOC) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting VOC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.

Measuring femtosecond lifetimes of free charge carriers in gallium arsenide

Peculiarities of the measurement of femtosecond lifetimes of free charge carriers in gallium arsenide grown by the method of molecular beam epitaxy are considered. The carrier lifetime is determined by a variant of the pump-probe method in which the pumping and probing radiation beams are incident onto the semiconductor sample surface at the Brewster angle. The proposed variant ensures the detection of a change in the induced refractive index even in the case of very small response signals, thus providing for a high sensitivity of measurements. It is shown that the relative signal changes near the Brewster angle reach a maximum level.

On-the-surface photoconductive response of pelletized thin In2S3 nanosheets

In2S3 nanosheets were synthesized using a low temperature tetraethyleneglycol (TEG) mediated polyol route and polyvinylpyrrolidone (PVP) as the capping agent. Thin In2S3 nanosheet of thickness of 8–10nm was obtained under optimized PVP concentration of 0.5g. The thin nanosheets were formed of 10nm sized crystalline tetragonal-shaped In2S3 nanocrystals assemblying into the nanosheet structures. The dimensions and the crystallinity of the nanosheets were dependent on the concentration of the PVP in the reaction medium. The nanosheets were cold-pressed into dense nanostructured pellets to measure their room temperature on-the-surface photoresponse characteristics. The nanostructured devices showed enhancement in conductivity under light as compared to that in the dark and demonstrated photoswitching behavior. Superior electrical conductivity (144% higher) and higher life time of free carriers (20.9±2.5s) observed from the optimized sample suggested its application as a low-cost photodetector.

On Photoelectrical Properties of 6H-SiC Bulk Crystals PVT-Grown on 6H- and 4H-SiC Substrates

Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN0 from 1017 cm-3 to 1019 cm-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy Ea = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN0 1019 cm3), both crystals revealed the bipolar diffusion coefficient Da 4 cm2/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

High speed optical modulation of terahertz waves using annealed silicon wafer

Modulation properties of terahertz waves going through a light excited high resistivity silicon wafer are analyzed and measured. Free carrier lifetime of the silicon wafer affects the modulation depth and speed of the terahertz wave. The lifetime is reduced to less than 1 μs by thermal processing for high speed modulation. Experimental results show that the response time and modulation depth of the proposed modulating structure are close to 1 μs and 51%, respectively.

Carrier dynamics and photoelectrical parameters in highly compensated sublimation grown 3C-SiC layers studied by time-resolved nonlinear optical techniques

Excess carrier dynamics in compensated n-type and p-type 3C-SiC layers grown by sublimation epitaxy and doped with nitrogen and aluminum has been studied using time-resolved optical pump-probe techniques. We show that carrier recombination pathway in both layers is strongly affected by the optical recharge of compensating aluminum impurities (Al−) through trapping of holes and their thermal release, providing recovery to the initial state within 0.1 ms to 10 ns in the 80–300 K range. The dynamics of aluminum charge states and hole density were also analysed using numerical modelling, which provided a hole capture cross-section for aluminum sh(Al) = (1.0 ± 0.3)×10−15 cm−2 and its activation energy Ea(Al) = (176 ± 4) meV in both layers. Free carrier lifetimes of 0.5 and 2 ns (for the p- and n-type layer, respectively) were measured at the injected carrier densities (ΔN0) well above that of Al. We also observed a strong impact of the compensating Al impurities on carrier diffusivity at room temperature, which dropped to negligible values of D < 0.1 cm2 s−1 at injection levels below the aluminum trap density (ΔN0 < [Al−]eq) and reached its typical value of D = 2 cm2 s−1 only at ΔN0 ≥ 1019 cm−3.

Dynamics of optical nonlinearities in GaN

The dynamics of optical nonlinearities in bulk GaN crystal are investigated by a modified time-resolved pump-probe system with phase object at 532 nm under picosecond pulse excitation. The sample's optical nonlinearities can be determined by measuring the normalized transmittance with/without an aperture in the far field. The different nonlinear mechanisms in GaN are separated. Experimental results show that the nonlinear response of GaN is a combination of bound-electronic and free-carrier nonlinearities mechanisms. With the theory of two-photon-induced free-carrier nonlinearity, the related optical nonlinear parameters, including free-carrier nonlinearities and free-carrier life time which are difficult to be determined, are obtained.

Efficient CW Four-Wave Mixing in Silicon-on-Insulator Micro-Rings With Active Carrier Removal

Silicon-on-insulator rib waveguides with passive loss of -0.74 dB/cm and free-carrier lifetime reduction via reverse biased p-i-n diodes are used to show efficient continuous wave four-wave mixing with 2 × reduction in required pump power for comparably high conversion efficiency of -8.2 dB, similar to best reports. A conversion efficiency of -13.4 dB is also subsequently achieved using 20 μm radius micro-ring resonators at significantly reduced pump power.

Time‐resolved free carrier lifetime microscopy in bulk GaN

AbstractA novel setup for lifetime microscopy measurements was designed and applied for carrier lifetime mapping in a bulk GaN. Photoexcitation by a picosecond UV pump and detection of time‐resolved free carrier absorption (FCA) images on a CCD camera enabled the mapping of carrier lifetime distribution with a spatial resolution of 5 μm. The spatial variation of lifetime in the bulk HVPE‐grown GaN revealed the presence of different‐size crystalline grains, with lifetime peaking up to 70 ns in the centers of the largest grains (∼20 μm in diameter) and dropping to 10 ns in the small ones, while the spatially averaged lifetime was 40 ns. The inhomogeneity was ascribed to the interplay of nonradiative diffusion‐limited recombination at grain boundaries and a bulk lifetime in the crystallite centers. The numerical solution of spatially‐resolved carrier decay rate in the crystallite centers at high injection levels and comparison with experimental data provided a bulk nonradiative recombination time of ∼70 ns. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Schottky MSM junctions for carrier depletion in silicon photonic crystal microcavities

Collection of free carriers is a key issue in silicon photonics devices. We show that a lateral metal-semiconductor-metal Schottky junction is an efficient and simple way of dealing with that issue in a photonic crystal microcavity. Using a simple electrode design, and taking into account the optical mode profile, the resulting carrier distribution in the structure is calculated. We show that the corresponding effective free carrier lifetime can be reduced by 50 times when the bias is tuned. This allows one to maintain a high cavity quality factor under strong optical injection. In the fabricated structures, carrier depletion is correlated with transmission spectra and directly visualized by Electron Beam Induced Current pictures. These measurements demonstrate the validity of this carrier extraction principle. The design can still be optimized in order to obtain full carrier depletion at a smaller energy cost.

Influence of boron on donor–acceptor pair recombination in type IIa HPHT diamonds

We report on the investigation of donor–acceptor pair (DAP) and free carrier recombination in HPHT IIa type diamonds and determination of boron concentration by differential transmittivity (DT) technique. Photoluminescence and photoluminescence excitation spectra were measured in 8–300K temperature range and provided a broad (~0.67eV) Gaussian DAP band which peaked at 2.2eV at low temperatures, while above 200K it sharply shifted to 2.5eV and became more intense. Thermoluminescence measurements also demonstrated a similar tendency. This peculiarity was explained by DAP recombination between the nitrogen and the boron, the latter being in the ground and the excited states at low and high temperatures, respectively. A zero phonon line position coincided with the calculated one, when using nitrogen and boron activation energies. Scanning of DT across the sample at different delay times revealed the fast (200–500ns) free carrier lifetime and the slow recovery time (of optically recharged boron to its initial state). The temperature dependence of the slow component decay time provided the boron activation energy of 360meV. Saturation of the boron-related DT signal in the samples and the determined boron ionization cross section at 1064nm (σB=3.3×10−17cm2) provided the boron density in 1014–16cm−3 range and revealed its strongly inhomogeneous distribution across the HPHT layers. The B density was found much lower than the density of nitrogen donors (~1017cm−3), which were distributed in the layers much more homogeneously.

Dynamic Characterization and Impulse Response Modeling of Amplitude and Phase Response of Silicon Nanowires

Amplitude and phase dynamics of silicon nanowires were measured using time-resolved spectroscopy. Time shifts of the maximum phase change and minimum amplitude as a function of pump power due to saturation of the free-carrier density were observed. A phenomenological impulse response model used to fit the experimental data indicated that the free-carrier lifetime was between 7.5 ≤ τfc ≤ 16.2 ns, and the two-photon absorption coefficient and the Kerr coefficient were 3 × 10-12 m.W-1 and 4 ×10-18 m-18.W-1, respectively, for silicon nanowires with lengths varying from 3.6 to 14.9 mm.

Investigation of the photoionization of Ce3+ ions in a YAG crystal by microwave resonance technique

The kinetic and spectral characteristics of the complex dielectric constant of a Ce: YAG crystal under laser irradiation in 250–275 nm spectral range are investigated. The lifetimes of free charge carriers and charge carriers, localized at the lattice defects (color centers), are estimated. It was established that photoconductivity signal of the sample is essentially caused by one-photon ionization processes from the 2 F 5/2 ground state of Ce3+ ions.

Electric control of pulse reshaping using an intracavity silicon waveguide

Electric control of the pulse reshaping and compression is demonstrated using the two-photon absorption (TPA) and TPA-induced free-carrier absorption in an intracavity silicon waveguide. A forward bias is applied to the silicon waveguide to tune the free-carrier lifetime and density, hence enhancing the pulse compression effect. A pulse compression of about 30% is experimentally verified using a 3.3 V forward bias.

High nonlinear figure-of-merit amorphous silicon waveguides

The nonlinear response of amorphous silicon waveguides is reported and compared to silicon-on-insulator (SOI) samples. The real part of the nonlinear coefficient γ is measured by four-wave-mixing and the imaginary part of γ is characterized by measuring the nonlinear loss at different peak powers. The combination of both results yields a two-photon-absorption figure of merit of 4.9, which is more than 7 times higher than for the SOI samples. Time-resolved measurements and simulations confirm the measured nonlinear coefficient γ and show the absence of slow free-carrier effects versus ns free-carrier lifetimes in the SOI samples.

Optical pulse compression on a silicon chip—Effect of group velocity dispersion and free carriers

The effect of group velocity dispersion and free carrier lifetime on pulse quality and nonlinear phase acquisition is studied in a two-stage pulse compressor on a silicon chip. Appropriately, designing the dispersion in the nonlinear medium enables compressed pulses with more than 90% of the pulse energy confined in the main lobe to be generated. Free carrier lifetime is observed to impact nonlinear phase acquisition in the single pulse regime. The pulse compression factor decreases monotonically as the pulse repetition rate increases. Conditions for efficient compression at high repetition rates in the absence of free carrier quenching techniques are discussed.

Silicon coding-decoding photonic device by electron irradiation and light down conversion

We propose and demonstrate a coding-decoding procedure as an important step to realize one more Si-based photonic device. Low-fluence (&amp;lt;1014 e/cm2) high-energy (1 MeV) electron irradiation of a bulk Si matrix is used to code an information by forming local regions with lower free carrier lifetime that are hidden under the surface and invisible to the eye. Short-wavelength (&amp;lt;1 μm) free carrier generation stands for multiple, remote, and nondestructive decoding process, which makes it easy to dynamically (ms range) visualize a code by capturing two-dimensional pattern of thermal emission in the longer-wavelength (3–12 μm) band (light down conversion).

Frequency Tuning and Bandwidth Enhancement of Raman Amplification in SiG-OI Waveguides

In this paper, some patents on silicon Raman amplifiers are investigated. These patents emphasize on the length reduction of amplifiers compared with the silica ones. However, the bandwidth of the amplifier in these patents is very low. In addition, while the pump wavelength is constant, the central frequency remains unchanged. The germanium doping leads to tuning of central frequency and bandwidth enhancement of Raman gain spectrum. Thus, the effect of germanium doping on the characteristics of major patents on silicon Raman amplifiers is investigated in this paper. Amplifier gain depends on various parameters such as the Raman gain coefficient. The effects of germanium doping on the impressive parameters of the silicon Raman amplifiers are also presented. By usage of the variable germanium concentration waveguides, the bandwidth of amplifiers is enhanced which has a great effect on communication systems. Furthermore, the characteristics of waveguide are estimated in order to achieve a flat gain amplification in silicon germanium on insulator (SiGe-OI) waveguides. Keywords: Continuous wave pumping, free carrier absorption, free carrier lifetime, pulsed pumping, Raman amplification, ring resonator, silicon germanium (SiGe), two photon absorption, SiG-OI Waveguides, Frequency, SiGe Raman Amplifiers, Amplifiers, resonator, insulator, waveguides

Spectral distribution of photoelectrical current and free carrier life time of Tex(Sb10Se90)100−x thin films

The present work studies the effect of Te additions to Sb10Se90 films on the measurements of steady state “DC” and transient “AC” photoconductivity. The steady state photoconductivity was measured in temperature range 300–400K. The spectral distribution of photocurrent was measured in the wavelength range 400–900nm at different level of illumination and applied electric filed. The dependence of photocurrent (Iph) on light intensity (G) follows the power low (IphGγ) where the exponent γ determines the nature of the recombination process. Double beam spectrophotometer was used to measuring the film transmittance T (λ) nm at normal incidence of Tex(Sb10Se90)100−x (with x=0%, 5%, 10% and 15%) films in the wavelength range 400–2500nm. Based on the envelope method, the film thickness and the complex index of refraction were determined with accuracy better than 1%. Ac photoconductivity measurements revealed that, the carrier lifetime decrease with increasing relative light intensity and the applied electric filed. It was found that, the replacement of Sb and Se by Te atoms leads to the decrease in the activation energy for electrical conduction in the dark ΔEdc and the optical band gap Egoptor Egph, while the refractive index and the width of localized states were found to be increases.

Self-sustained gigahertz electronic oscillations in ultrahigh-Qphotonic microresonators

We report on theoretical and experimental observations of self-sustained fast [gigahertz (GHz)] electronic oscillations resulting from coupled electron-photon dynamics in ultrahigh-$Q$ Si microdisk resonators with cw pumping. Our theoretical analysis identifies conditions for generating steady-state GHz oscillations while suppressing thermal oscillations [megahertz (MHz)] with submilliwatt input laser power. Such fast oscillations are tunable via changing the free-carrier (FC) lifetime of the resonator. Integrating a $p$-$i$-$n$ diode with these high-$Q$ resonators for controlling the FC lifetime promises the realization of an integrated voltage-controlled oscillator (VCO) in a silicon photonics chip.