Excess Carrier Concentration Research Articles

Photoexcited Muon Spin Spectroscopy: A New Method for Measuring Excess Carrier Lifetime in Bulk Silicon.

We have measured excess carrier lifetime in silicon using photoexcited muon spin spectroscopy. Positive muons implanted deep in a wafer can interact with the optically injected excess carriers and directly probe the bulk carrier lifetime while minimizing the effect from surface recombination. The method is based on the relaxation rate of muon spin asymmetry, which depends on the excess carrier density. The underlying microscopic mechanism has been understood by simulating the four-state muonium model in Si under illumination. We apply the technique to different injection levels and temperatures, and demonstrate its ability for injection- and temperature-dependent lifetime spectroscopy.

A novel CMOS spectrometer based on wavelength absorption

This article reports on a novel spectrometer without dispersing elements fabricated in standard CMOS technology. The spectrum detection principle is based on the wavelength absorption mechanism in silicon. A finite element model confirms the excess holes’ detection principle as a function of depth where moving holes’ trajectory is deviated under the Lorentz force towards a set of collectors. In the case of high excess carrier concentration, experimental results confirm the theoretical analysis that wavelength becomes indistinguishable because the Auger recombination mechanism is dominant, which should be avoided to realize a spectrometer. For the low excess carrier concentration case, the concentration profile is determined by the incident irradiance and the wavelength and can be additive since the Shockley-Reed-Hall recombination mechanism prevails, where the excess carrier life time is constant, and hence suitable for wavelength discrimination. In order to realize a spectrometer, a light spectrum detection method is developed, which requires a linear equation set where coefficients of the matrix coming from the measurement of the current density as a function of the irradiance for different wavelengths and magnetic fields. The proposed miniature and integrated spectrometer with a pixel array can be used as a spectral imager.

Amorphous nickel incorporated tin oxide thin film transistors

Nickel as a dopant has been proposed to suppress excess carrier concentration in n-type tin oxide based thin film transistors (TFTs). The influences of Ni content on nickel doped tin oxide (TNO) thin films and their corresponding TFTs were investigated with experimental results showing that the TNO thin films are amorphous. Through the comparison of the transfer characteristic curves of the TNO TFTs with different Ni contents, it was observed that Ni doping is useful to improve the performance of SnO2-based TFTs by suppressing the off-state current and shifting the threshold voltage to 0 V. The amorphous TNO TFT with 3.3 at.% Ni content shows optimum performance, with field effect mobility of 8.4 cm2 V−1 s−1, saturation mobility of 6.8 cm2 V−1 s−1, subthreshold swing value of 0.8 V/decade, and an on-off current ratio of 2.1 × 107. Nevertheless, the bias stress stability of SnO2-based TFTs deteriorate.

Effect of hydrogen on the device performance and stability characteristics of amorphous InGaZnO thin-film transistors with a SiO2/SiNx/SiO2 buffer

We investigate the influence of the multi-layered buffer consisting of SiO2/SiNx/SiO2 on amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). The multi-layered buffer inhibits permeation of water from flexible plastic substrates and prevents degradation of overlying organic layers. The a-IGZO TFTs with a multi-layered buffer suffer less positive bias temperature stress instability compared to the device with a single SiO2 buffer layer after annealing at 250 °C. Hydrogen from the SiNx layer diffuses into the active layer and reduces electron trapping at loosely bound oxygen defects near the SiO2/a-IGZO interface. Quantitative analysis shows that a hydrogen density of 1.85 × 1021 cm−3 is beneficial to reliability. However, the multi-layered buffer device annealed at 350 °C resulted in conductive characteristics due to the excess carrier concentration from the higher hydrogen density of 2.12 × 1021 cm−3.

Demonstration of omnidirectional photoluminescence (ODPL) spectroscopy for precise determination of internal quantum efficiency of radiation in GaN single crystals

For rating unambiguous performance of a light-emitting semiconductor material, determination of the absolute quantum efficiency (AQE) of radiation, which is basically a product of internal quantum efficiency (IQE) and light-extraction efficiency, is the most delightful way. Here, we propose the use of omnidirectional photoluminescence (ODPL) spectroscopy for quantifying AQE of the near-band-edge (NBE) emission, in order to evaluate bulk GaN crystals and wafers. When the measurement was carried out in the air, the AQE showed a continuous decrease most likely due to the formation of extrinsic nonradiative recombination channels at the surface by photo-pumping. However, such an influence was suppressed by measuring ODPL in an inert ambient such as nitrogen or in vacuum. Consequently, AQE was revealed to depend on the photo-pumping density. The increase in AQE of the NBE emission caused by the increase in the excess carrier concentration was significant, indicating gradual saturation of nonradiative recombination centers in the bulk of GaN. The highest AQE value (8.22%) ever reported for the NBE emission of GaN at room temperature, which corresponds to IQE of 70.9%, was eventually obtained from the GaN wafer grown by hydride vapor phase epitaxy on a GaN seed crystal manufactured by the acidic ammonothermal method, when the cw photo-pumping density was 66 W/cm2.

Carrier Lifetimes of Iodine-Doped CdMgTe/CdSeTe Double Heterostructures Grown by Molecular Beam Epitaxy

Iodine-doped CdMgTe/CdSeTe double heterostructures (DHs) have been grown by molecular beam epitaxy and studied using time-resolved photoluminescence (PL), focusing on absorber layer thickness of 2 μm. The n-type free carrier concentration was varied to ∼7 × 1015 cm−3, 8.4 × 1016 cm−3, and 8.4 × 1017 cm−3 using iodine as dopant in DHs. Optical injection at 1 × 1010 photons/pulse/cm2 to 3 × 1011 photons/pulse/cm2, corresponding to initial injection of photocarriers up to ∼8 × 1015 cm−3, was applied to examine the effects of excess carrier concentration on the PL lifetimes. Iodine-doped DHs exhibited an initial rapid decay followed by a slower decay at free carrier concentration of 7 × 1015 cm−3 and 8.4 × 1016 cm−3. The optical injection dependence of the carrier lifetimes for DHs was interpreted based on the Shockley–Read–Hall model. The observed decrease in lifetime with increasing n is consistent with growing importance of radiative recombination.

Kinetics of carrier-induced degradation at elevated temperature in multicrystalline silicon solar cells

The degradation kinetics of multicrystalline silicon solar cells and wafers at elevated temperature (often termed “LeTID”) depend on the specific temperature and injection conditions. We apply different forward biases in the dark at a constant temperature of ~75°C to industrial passivated emitter rear contacted (PERC) solar cells fabricated on p-type multicrystalline wafers from a variety of material producers and determine the degradation rate constant in dependence of the excess carrier density at the p-n junction. We find that whereas the specific material properties influence the degradation extent, the degradation rate constant is comparable for all materials but depends on the excess carrier concentration. This implies involvement of one electron in the rate-limiting step of LeTID defect formation. The result not only is an important contribution to elucidate the physical mechanism underlying LeTID, but can also be used as a guideline for devising degradation tests of multicrystalline silicon wafers and solar cells.

An analytical study of the minority carrier distribution and photocurrent of a p–i–n quantum dot solar cell based on the InAs/GaAs system

An analytical study has been carried out on the InAs/GaAs p+–i–n+ quantum dot solar cell, taking into consideration the contributions of each region of the cell to the total photocurrent. The expressions for the excess minority carrier concentration and photocurrent from the front and the rear regions of the device have been obtained and their variations with different device parameters have been studied. Also, based on the investigations reported by some researchers earlier, the photocurrent contribution from the intrinsic region of the solar has been studied, taking into account the quantum dot ensemble absorption coefficient, which depends significantly on the quantum dot size and size dispersion. It is observed that all the three regions of the cell contribute to the overall internal quantum efficiency (IQE) of the cell. The contribution of each region of the solar cell to the total IQE has been shown graphically. From these studies it is observed that the incorporation of the quantum dots in the intrinsic region enhance the photocurrent density and hence the IQE of such solar cell, as it absorbs low energy photons, which are beyond the absorption range of GaAs. Finally, the fill factor of the solar cell has been calculated.

Electron beam induced excess carrier concentration

The maximum excess carrier concentration in Si under the electron beam irradiation is calculated as a function of beam energy and diffusion length. The values obtained are compared with approximated ones. It is shown that the Berz and Kuiken expression can be used for the excess carrier concentration estimation. A possibility to reach the low excitation conditions in EBIC and CL measurements on Si is demonstrated.

On the c-Si/SiO2 interface recombination parameters from photo-conductance decay measurements

The recombination of electric charge carriers at semiconductor surfaces continues to be a limiting factor in achieving high performance optoelectronic devices, including solar cells, laser diodes, and photodetectors. The theoretical model and a solution algorithm for surface recombination have been previously reported. However, their successful application to experimental data for a wide range of both minority excess carrier concentrations and dielectric fixed charge densities has not previously been shown. Here, a parametrisation for the semiconductor-dielectric interface charge Qit is used in a Shockley-Read-Hall extended formalism to describe recombination at the c-Si/SiO2 interface, and estimate the physical parameters relating to the interface trap density Dit, and the electron and hole capture cross-sections σn and σp. This approach gives an excellent description of the experimental data without the need to invoke a surface damage region in the c-Si/SiO2 system. Band-gap tail states have been observed to limit strongly the effectiveness of field effect passivation. This approach provides a methodology to determine interface recombination parameters in any semiconductor-insulator system using macro scale measuring techniques.

Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity

Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.

Effect of free-carrier concentration and optical injection on carrier lifetimes in undoped and iodine doped CdMgTe/CdSeTe double heterostructures grown by molecular beam epitaxy

Time-resolved and time integrated photoluminescence (PL) studies are reported for undoped and doped CdMgTe/CdSeTe double heterostructures (DHs) grown by molecular beam epitaxy. Undoped DHs are studied with absorber layer thickness varying from 0.5 to 2.5 µm. The n-type free-carrier concentration is varied ~7 × 1015, 8.4 × 1016, and 8.4 × 1017 cm−3 using iodine as a dopant in different absorber layer thicknesses (0.25–2.0 µm). Optical injection is varied from 1 × 1010 to 3 × 1011 photons/pulse/cm2, corresponding to the initial injection of photo-carriers up to ~8 × 1015 cm−3, to examine the effects of excess carrier concentration on the PL lifetimes. Undoped DHs exhibit an initial rapid decay followed by a slower dependence with carrier lifetimes up to ~485 ns. The dependence of carrier lifetimes on the thickness of the absorber layers (0.5–2.5 µm) suggests interface recombination velocities () ~ 1288 and 238 cm s−1 in the initial and later decay times, respectively, corresponding to high and low photo-carrier concentrations. The Shockley–Read–Hall model is used to describe the results in which variations are observed in for undoped DHs. The lifetimes of doped DHs show a consistent trend with thickness. The ~ 80–200 cm s−1 is estimated for doping n ~ 7 × 1015 cm−3 and 240–410 cm s−1 for n ~ 8.4 × 1016 cm−3. The observed decrease in carrier lifetimes with increasing n is consistent with growing importance of the radiative recombination rate due to the excess carrier concentration. The effect of carrier concentration on the PL spectrum is also discussed.

Improvement of color conversion and efficiency droop in hybrid light-emitting diodes utilizing an efficient non-radiative resonant energy transfer

Blue InGaN/GaN nanohole light-emitting diodes have been fabricated by soft UV-curing nanoimprint lithography, filling with CdSe/ZnS core/shell nanocrystals (NCs) as color conversion mediums. The excitonic recombination dynamics of hybrid nanohole light-emitting diodes were investigated by time-resolved photoluminescence, observing a significant reduction in the decay lifetime of excitons as a result of an efficient non-radiative resonant energy transfer, which leads to the improvement of color conversion and efficiency droop in these hybrid nanohole light-emitting diodes compared to hybrid nanocrystals/standard planar light-emitting diodes. The color-conversion efficiency and effective quantum yield of hybrid nanohole light-emitting diodes were nearly twice as much as those of hybrid standard light-emitting diodes. A model on the excitonic recombination process was proposed to explore this situation, explaining the advantages of non-radiative resonant energy transfer that avoiding energy loss associated with the intermediate light emission and conversion steps and transferring energy non-radiatively and resonantly to NCs with a higher quantum yield. The efficiency droop of hybrid nanohole light-emitting diodes was validly suppressed compared to the bare ones, even better than that of hybrid standard light-emitting diodes. It mainly results from the extraction of excess carrier concentrations in InGaN/GaN multiple quantum wells via the rapid non-radiative resonant energy transfer process under the higher injection condition, revealing a great potential to realize efficient white light emitters in the future.

Modeling and Analysis of Lifetime Curve of Amorphous Silicon/Crystalline Silicon Heterostructure Solar Cell

The modeling and analysis of recombination properties is one of the critical aspects in the design and development of high efficiency amorphous silicon (a-Si)/crystalline silicon (c-Si) heterostructure solar cell (SH-SC). In this paper, we have developed a recombination model pertinent for a-Si/c-Si SH-SC. Based on the model, an analytical expression for the relationship between effective carrier lifetime ( $\tau _{\mathrm {eff}}$ ) and excess carrier concentration ( $\Delta n$ ), commonly termed as lifetime curve, is derived and the results are validated with the experimental data. The results show that the inverse of $\tau _{\mathrm {eff}}$ , after correcting for the contributions of Auger and radiative recombination in silicon, when plotted as a function of $\Delta n$ , shows a linear characteristic, with each of the slope and the intercept, providing definitive details about the recombination processes occurring in the device.

Extraction of Recombination Properties from Lifetime Data

Extraction of recombination properties like the recombination pre-factor J0 and the Shockley-Read-Hall base lifetime from photoconductance data on test structures and half-fabricates of photovoltaic cells is not always straightforward and unambiguous. In this paper the well-known “slope method” of Kane and Swanson will be compared to the method offered by the Quokka code. The Quokka code numerically solves the distribution of the excess carrier concentration over the thickness of the wafer at several injection levels. In this way artefacts due to transport limitations are avoided and the analysis does not rely on data at a single injection level. This gives more reliable results for J0 and the base lifetime. A method to the determine the base lifetime from the implied VOC at 1 Sun illumination values is also presented.

Suppression of compensating native defect formation during semiconductor processing via excess carriers.

In many semiconductors, compensating defects set doping limits, decrease carrier mobility, and reduce minority carrier lifetime thus limiting their utility in devices. Native defects are often responsible. Suppressing the concentrations of compensating defects during processing close to thermal equilibrium is difficult because formation enthalpies are lowered as the Fermi level moves towards the majority band edge. Excess carriers, introduced for example by photogeneration, modify the formation enthalpy of semiconductor defects and thus can be harnessed during crystal growth or annealing to suppress defect populations. Herein we develop a rigorous and general model for defect formation in the presence of steady-state excess carrier concentrations by combining the standard quasi-chemical formalism with a detailed-balance description that is applicable for any defect state in the bandgap. Considering the quasi-Fermi levels as chemical potentials, we demonstrate that increasing the minority carrier concentration increases the formation enthalpy for typical compensating centers, thus suppressing their formation. This effect is illustrated for the specific example of GaSb. While our treatment is generalized for excess carrier injection or generation in semiconductors by any means, we provide a set of guidelines for applying the concept in photoassisted physical vapor deposition.

Numerical evaluation of Auger recombination coefficients in relaxed and strained germanium

The potential applications of germanium and its alloys in infrared silicon-based photonics have led to a renewed interest in their optical properties. In this letter, we report on the numerical determination of Auger coefficients at T = 300 K for relaxed and biaxially strained germanium. We use a Green's function based model that takes into account all relevant direct and phonon-assisted processes and perform calculations up to a strain level corresponding to the transition from indirect to direct energy gap. We have considered excess carrier concentrations ranging from 1016 cm−3 to 5 × 1019 cm−3. For use in device level simulations, we also provide fitting formulas for the calculated electron and hole Auger coefficients as functions of carrier density.

Characteristic of Bismuth-Doped Tin Oxide Thin-Film Transistors

The influences of bismuth doping contents on the structure, morphology and electrical properties of amorphous bismuth-doped tin oxide (a-TBO) thin films have been investigated. With the increase of Bi content, the resistivity of TBO films monotonously increases due to the decreased oxygen vacancies. The thin-film transistors (TFTs) with a-TBO films as channel layers have been prepared on SiO2–Si substrate. Through the comparison of the transfer characteristic curves of the TFTs with different Bi contents, it is observed that Bi doping is useful to suppress the excess carrier concentration and improve the performances. TBO-TFTs with Bi content 2.7 at.% show the optimum performances, with a field effect mobility of 4.7 cm $^{2}\text{V}^{-1}\text{s}^{-1}$ , a threshold voltage of −3.9 V, a subthreshold swing value of 1.3 V/decade, and an ON–OFF current ratio of $2.3 \,\, \times \,\, 10^{6}$ . In addition, the bias stress stabilities of intrinsic SnO2 TFTs and a-TBO TFTs were compared and analyzed.

Nonradiative lifetime extraction using power-dependent relative photoluminescence of III-V semiconductor double-heterostructures

A power-dependent relative photoluminescence measurement method is developed for double-heterostructures composed of III-V semiconductors. Analyzing the data yields insight into the radiative efficiency of the absorbing layer as a function of laser intensity. Four GaAs samples of different thicknesses are characterized, and the measured data are corrected for dependencies of carrier concentration and photon recycling. This correction procedure is described and discussed in detail in order to determine the material's Shockley-Read-Hall lifetime as a function of excitation intensity. The procedure assumes 100% internal radiative efficiency under the highest injection conditions, and we show this leads to less than 0.5% uncertainty. The resulting GaAs material demonstrates a 5.7 ± 0.5 ns nonradiative lifetime across all samples of similar doping (2–3 × 1017 cm−3) for an injected excess carrier concentration below 4 × 1012 cm−3. This increases considerably up to longer than 1 μs under high injection levels due to a trap saturation effect. The method is also shown to give insight into bulk and interface recombination.

A Possible High Critical Temperature and Intergranular Current Density in Bi2Sr2CaY x Cu2 O y Superconductors

We report here the structural and superconducting properties of Bi2Sr2CaYxCu2Oy superconductors with various x values (0.00 ≤x≤ 0.50) by using the resistivity and ac susceptibility measurements. It is found that addition of Y 3+ does not influence the phase purity of Bi:2212, while c-parameter and orthorhombic distortion are affected. Furthermore, the critical temperatures Tc(R= 0) are increased from 92 K for Y = 0 up to 106 K by 0.15 of Y, followed by a decrease with increasing Y up to 0.50. It is interesting to note the resistivity of the samples with Y = 0.075 and 0.15 reaching to true zero value only at low dc currents up to 2 mA. Interestingly, the onset temperature of diamagnetism obtained from susceptibility data remains nearly unchanged at 94 K up to Y = 0.30, flowed by a decrease to 86 K at Y = 0.50. The superconducting volume fraction and critical currents are improved by Y addition up to 0.15, followed by a decrease with increasing Y up to 0.50. These results are explained in terms of the effects of excess oxygen, secondary phases, and carrier concentration per Cu ions which are produced by Y addition in Bi:2212 system.