Photogenerated Carrier Density Research Articles

Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells

We address the claim that the dependence of the “corrected photocurrent” (defined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanism in an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density. We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities. We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is p...

Measurement of coherent tunneling between InGaAs quantum wells and InAs quantum dots using photoluminescence spectroscopy

We present a spectroscopic manifestation of the intermediate coherent tunneling regime between a quantum-dot (QD) and a quantum-well (QW) layer. We observe a nontrivial dependence of the resonant QD photoluminescence excitation (PLE) signal as a function of dot-well barrier thickness. For thick barriers and resonant QW excitation, the photogenerated QD carrier density increases exponentially with increasing coupling strength. For separations of a few nanometers only, however, we observe an anomalous decrease in the PLE signal. This behavior is defined by subpicosecond resonant coherent tunneling dynamics between the QW and QD. Our results shed light on the intermediate coherent tunneling regime of relevance in a variety of functional nanostructures such as charge or spin injectors or photovoltaic devices.

Tunneling-barrier controlled excitation transfer in hybrid quantum dot-quantum well nanostructures

A systematic spectroscopic study of the carrier transfer between quantum dot (QD) and quantum well (QW) layers is carried out in a hybrid dot-well system based on InAs QDs and InGaAs QWs. We observe a strong dependence of the QD and QW photoluminescence (PL) both on the dot-well barrier thickness and height. For thick (or high) barriers QD and QW systems accumulate independently sufficient photogenerated carrier densities to be seen in PL even at low nonresonant excitation power. For thin (or low) barriers it is impossible to detect the PL signal from QW at low excitation densities due to effective carrier transfer from QW to QDs. Strong state-filling effects of the excited QD states influence the carrier transfer efficiencies. By investigating the carrier dynamics using time-resolved spectroscopy and the state-filling effects in the continuous wave excitation regime the basic characteristics of interlevel, intersublevel, and dot-well relaxation are determined. The mechanisms of the dot-well coupling are discussed.

Device physics of highly sensitive thin film polyfluorene copolymer organic phototransistors

We report on solution processed, highly light sensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Transistors without heat treatment showed the highest saturation mobility, while devices annealed at 280°C showed the highest drain current. The latter annealed transistors were found to give highly stable and reproducible performance over many light cycles. Measurements were carried out using an inorganic light emitting diode (LED) light source with a peak wavelength of 465nm and 19nm bandwidth from 0to400μW∕cm2 light intensity on TFTs with an F8T2 film thickness of 30nm. The TFT OFF current was found to increase both with light intensity and gate bias. The bulk photogenerated carrier density was calculated to change from 5×1011to1×1013cm−3 over the measured light intensity range. The TFT saturation mobility did not change with light intensity, remaining constant at 1.2×10−4cm2∕Vs. The TFT ON current instead increased due to a shift in the turn-on voltage VT. This changed from −27to−20V over the measured light intensity range, initially changing rapidly but then saturating at higher intensity values. Contact resistance RC measurements showed large values in the dark. RC rapidly decreases with increasing light intensity, again saturating at higher values. From these results, we propose a phototransistor model in which illumination varies the device performance by effecting injection. By considering this shift in RC as photoassisted barrier lowering which additionally varies the width of the region depleted of carriers between the injecting interface and the channel, it is possible to explain the observed shift in VT as a change in the fraction of the gate bias dropped across the contact capacitance CC. By operating the phototransistor at a value of Vg=−5V (below VT), it was possible to achieve a highly linear response of the photocurrent with light intensity. Alternatively, by operating at a value of Vg=−40V (above VT), it was possible to maximize the photoresponsivity within the measured range. A photoresponsivity of 18.5A∕W at 5μW∕cm2 light intensity was achieved.

Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3--1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective $g$ factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

Contactless Measurement of Carrier Lifetime on Silicon Ingots and Bricks

AbstractHigh efficiency silicon solar cells demand the use of high lifetime silicon wafers. Characterization of boules and bricks before wafering allows poor quality material to be rejected before expensive processing steps. This paper extends simulation techniques previously used in quasi-steady-state-photoconductance to transient photoconductance decay measurements of high lifetime bulk samples. Simulated photogenerated carrier density profiles allow estimation of the bulk lifetime of a thick silicon sample with high surface recombination velocity.

Multiple charge domains model for the lock-on effect in GaAs power photoconductive switches

This paper reports that the lock-on field of semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) was measured under different bias voltages. Based on the experimental results and the transferred-electron effect, a model for the lock-on effect in GaAs PCSSs is proposed. It is shown that the charge domain with an ultrahigh electric field is due to a high photogenerated carrier density, which gives rise to intensive impact ionization accompanied by electron–hole recombination radiation within the domain. Since new domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation, the forefront domain crosses the switch at a speed alternating between the photon velocity and the carrier saturated drift velocity, which makes the observed velocity of carriers larger than the saturated drift velocity. The lock-on field results from the fixed number of a moving train of avalanching charge domains, the steady-state domains electric fields and the steadfast external electric field of the domains. The recovery of the lock-on effect is caused by domain quenching. The calculations agree with the experimental results. Moreover, the analytical results indicate that SI-GaAs PCSS is essentially a type of photo-activated charge domain device.

Characteristics of HfO2 and SiO2 on p-type silicon wafers using terahertz spectroscopy

The effect of high-κ dielectric HfO2 films on 200 mm diameter p-type silicon substrates was investigated and compared with conventional dielectric material, SiO2. We employed all-optical characterization methods using terahertz (THz) time-domain spectroscopy and visible cw pump/THz probe spectroscopy. Measurements were performed on two sets of samples, each set containing both HfO2 and SiO2 coated wafers with varying thickness of oxide layer. One set had a protective coating of either photoresist or Si3N4 deposition above the oxide layer which in turn was on a heavily doped p+ layer on p-type silicon. The samples exhibited similar linear transmission in the THz frequency range studied. However, when using an Ar+-ion laser as an optical pump source, differential measurements (with and without the pump source) showed that the transmission was far smaller for the high-κ dielectric HfO2 films than its conventional dielectric SiO2 counterparts, indicating the presence of large photo-generated carrier density in the p-type substrate.

Comparative study of terahertz radiation from n-InAs and n-GaAs

We have studied terahertz (THz) emissions from n-InAs and n-GaAs using an ensemble Monte Carlo method. Our simulations indicate that higher amplitude THz waves from n-InAs, compared with those from n-GaAs, result from the difference in the radiation mechanisms between these two samples and are not completely dependent on the most commonly recognized fact: lighter electron effective mass in n-InAs. The excitation-wavelength-dependent and doping-level-dependent THz emissions from n-InAs are found to be quite different from those from n-GaAs. The corresponding mechanisms are analyzed by the introduction of a weighted electric field, which is weighted by the photogenerated carrier density in a semiconductor. The simulated results are in good qualitative agreement with experimental observations from other authors.

Temporal evolution of effects of ultrafast carrier dynamics in In0.33Ga0.67N: above and near the bandgap

Ultrafast carrier dynamics in the In0.33Ga0.67N epilayer were investigated in detail, using femtosecond transient differential non-degenerate optical absorption measurements. Following an excitation at 400 nm with fluence ranging from 25 µJ cm−2 to 3000 µJ cm−2, probing was carried out above and near the bandgap using different wavelengths generated from a super continuum source. We have found that bandgap renormalization plays a key role when probing at photon energies well above the bandgap and it is clearly distinct from other effects at the lowest fluence. The critical carrier density for the onset of noticeable bandgap renormalization effects in this material when probing well above the bandgap is approximately 5 × 1018 carriers cm−3. We have observed a decrease in the energy loss rate of this material as a function of photogenerated carrier density which is attributed to phonon bottleneck effect. For the lowest carrier density, we have extracted an optical phonon lifetime to be approximately 45 ± 9 fs.

The effects of large signals on charge collection in radiation detectors: Application to amorphous selenium detectors

Analytical and numerical models for studying the effects of large signals on charge collection efficiency in radiation detectors are described by considering bimolecular recombination between drifting charge carriers, carrier trapping, and space charge effects. First, an analytical solution is obtained by assuming that the field remains relatively uniform. Then the continuity equations for both holes and electrons, and Poisson’s equation across the photoconductor for a short pulse irradiation are simultaneously solved by the finite difference method, without any assumptions. There is a very good agreement between the approximate analytical and numerical solutions for the charge collection efficiency. The numerical results are also compared with Monte Carlo simulations of carrier transport. The charge collection efficiency model is applied to amorphous selenium x-ray image detectors. The bimolecular-recombination-limited charge collection efficiency depends on the total photogenerated carrier density rather than on its spatial distribution. It is found that the recombination plays practically no role up to the total instantaneous carrier generation Q0 of 109EHPs∕cm2 at the applied electric field of 10V∕μm. The effect of recombination on charge collection increases with decreasing applied electric field strength. For high carrier generation (e.g., Q0 of 1012EHPs∕cm2 for an applied field of 10V∕μm), the electric field distributions vary widely across the photoconductor thickness during the travel of charge carriers towards the electrodes. However, the effect of bimolecular recombination on charge collection efficiency is almost independent of bias polarity and the field distribution. The numerical results are also compared with recent experimental data available in the literature.

Study on p-type δ-doping GaAs under optical-excitation

The electronic structure in p-type δ-doping GaAs is calculated self-consistently by including the contribution of the photogenerated carrier. The theoretical results indicate that the band bending around the δ-doping region is induced by the injection of photogenerated holes into the potential well of the δ-doping GaAs. Moreover the band bending results in two photogenerated carrier-induced wells for electrons. The potential of the two wells is deepened with the increase of the photogenerated carrier density. The photoluminescence (PL) properties are explained in terms of the dependence of the confined electronic states and the Fermi level of the two-dimensional hole gas (2DHG) on the concentration of photogenerated carrier, which is in good agreement with experimental observation of the variation of peak position and intensity with the excitation light intensity.

Photogenerated carriers in μc-Si:H/a-Si:H multi-layers

μc-Si:H/a-Si:H multi-layers were measured by time-of-flight, photo and equilibrium charge carrier extraction by linearly increasing voltage methods to investigate the density of localized states, the equilibrium carrier location, and the relaxation of photogenerated carrier density and mobility. We observed no influence of the interfaces between μc-Si:H and a-Si:H on photogenerated charge carrier transport. Values of the equilibrium charge carrier density and photogenerated charge carrier lifetime similar to μc-Si:H can be achieved in multi-layers which offer a possibility of avoiding columnar growth and anisotropy of transport properties.

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.

Picosecond time-resolved photoluminescence at detection wavelengths greater than 1500 nm

We report what is to our knowledge the first application of high-efficiency InGaAs/InP photon-counting diode detectors in time-resolved photoluminescence measurements at wavelength greater than 1500 nm. When they were cooled to 77 K and used in conjunction with the time-correlated single-photon counting technique, the detectors were capable of an instrumental response of 230 ps and a noise equivalent power of 2x10(-17)W Hz(-1/2) . Preliminary measurement of a semiconductor heterostructure indicates sensitivity at photogenerated carrier densities as low as 10(14)cm (-3) . This development facilitates the detailed characterization of dominant recombination mechanisms in semiconductor optoelectronic materials and devices designed to operate in the third telecommunications spectral window.

Experimental investigation on carrier dynamics in SCH-MQW waveguide saturable absorber of passively mode-locked monolithic laser diode

Decay process of photo-generated carrier density in a saturable absorber (SA) section made of a separate confinement hetrostructure (SCH) waveguide with multiple quantum wells for passively mode-locked monolithic laser diode (MLLD) has been investigated experimentally by means of the pulse mixing method. The measured sample is a realistic SA device, which was cleaved off from the wafer used for the MLLD fabrication. It was found out that the hole accumulation at a SCH interface plays a major role in the dynamics. While the backward and forward carrier flows from the accumulation layer are critical issues, the electric field screening effect on the carrier dynamics is rather small. Numerical simulation based on a new SA model containing these carrier flows and three time constants was performed and was found out to agree quite well with results of the pulse mixing experiment, indicating the appropriateness of the new model.

Piezoelectric-induced quantum-confined Stark effect in self-assembled InAs quantum dots grown on (N11) GaAs substrates

We investigate the optical properties of InAs self-assembled quantum dots grown on (N11)A/B GaAs substrates, by means of cw photoluminescence under different excitation power densities. We observe a sizeable blue-shift of photoluminescence band induced by increasing the photogenerated carrier density. The shift depends on the substrate orientation and exhibits a strong asymmetric dependence on the substrate termination. We attribute the photoluminescence blue-shift to a reverse quantum confined Stark shift of ground state transition energies in the quantum dots. This effect arises from the photogenerated charge screening of the built-in piezoelectric field present in such strained structures grown on high index planes.

00/00912 A general multivariate qualitative model for sizing stand-alone photovoltaic systems

We report on investigations of the ambipolar diffusion process in n-i-p diodes and n-i-p-i doping superlattices performed by a new all-optical pump-and-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investigation of the stationary distribution of the optically-induced excess carriers. The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 104cm2s−1. The ambipolar diffusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongly affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in the small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carrier density is taken into account.

Extremely fast ambipolar diffusion in n-i-p-i doping superlattices investigated by an all-optical pump-and-probe technique

We report on investigations of the ambipolar diffusion process in n-i-p diodes and n-i-p-i doping superlattices performed by a new all-optical pump-and-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investigation of the stationary distribution of the optically-induced excess carriers. The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 104cm2s−1. The ambipolar diffusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongly affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in the small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carrier density is taken into account.

Subpicosecond carrier lifetime in beryllium-doped InGaAsP grown by He-plasma-assisted molecular beam epitaxy

The carrier dynamics and absorption edges of InGaAsP samples grown by He-plasma-assisted molecular beam epitaxy and doped with various concentrations of beryllium are investigated via pump-probe experiments and Fourier transform infrared (FTIR) absorption spectroscopy. Carrier lifetimes from 10 to <1 ps are obtained for samples of increasing doping concentrations. The reduced in carrier lifetimes are attributed to Be compensation of the deep donor levels introduced by the He plasma. The carrier lifetime increases with photogenerated carrier density due to trap saturation. The FTIR results reveal sharp absorption edges in this material for doping concentrations up to 1×1018 cm−3. The fast carrier dynamics and the steep absorption edge make this material very attractive for ultrafast optical switching devices for use in high-speed time-division-multiplexing fiber communication systems.