Dependence Of Quantum Efficiency Research Articles

Optical concentration in fully delineated mid-wave infrared T2SL detectors arrays

The dependence of quantum efficiency (QE) on fill factor and pixel pitch is studied theoretically and experimentally in fully delineated type-II superlattice (T2SL) detectors. Theoretically, a 2-dimensional simulation model is used to compute the absorption in the array geometry, which shows an insensitivity of the optical response to the fill factor. This is a result of the photodiode array (PDA) geometry concentrating the light in the pixel area. QE measurements on PDAs with varying pixel pitch (from 225 to 10 μm) and fill factors (from 98% to 64%) confirm this independence of the QE on the fill factor and results in a 50% increase in the photocurrent density in 10 μm pitch PDAs compared to 225 μm pitch PDAs. Furthermore, measurements of the dark current density vs pixel size revealed an absence of surface leakage in these PDAs, which, combined with the increased photocurrent density results in an improved signal-to-noise ratio when reducing the pitch in these T2SL detectors. Finally, this result resolves the QE-modulation transfer function trade-off, as the electrical isolation of the pixel is carried out without impacting the QE of the array.

Unassisted solar water splitting by dual Cu2O–based tandem device with complementary wavelength–dependent quantum efficiency and antipodal conductivity

Unprecedented standalone tandem cell consisted of dual Cu2O, of which the complementary wavelength–dependent quantum efficiency is in favor of the full utilization of the broadband sunlight with the photon energy above their characteristic band gap and the striking difference in the Fermi level as a result of their distinct p–type and n–type conductivities turns into a strong photovoltage, is developed in the present contribution to carry out unassisted photoelectrochemical water splitting using energy exclusively from the sun. Further deposition of the n–type TiOx overlayer on p–type Cu2O engineered into a wire–like nanostructure, forming the core–shell p–n heterojunction to provide additional photovoltage to accelerate the hydrogen evolution reaction, and the circumvention of the severe charge accumulation at the Schottky Cu/n–Cu2O back contact well relax the associated kinetic overpotentials. The synergistic effect renders the solar–to–hydrogen efficiency of the tandem n–TiOx/p–Cu2O//n–Cu2O device amounting to 0.32%, which is among the highest performance reported to date for the Cu2O–based tandem PEC//PEC cells in the literature.

Modulating the Quantum Efficiency of Sb2S3‐Based Photodiodes Based on Conventional and Inverted Structures

AbstractAntimony sulfide (Sb2S3), a binary chalcogenide, has recently emerged as a promising candidate for photovoltaics due to its high absorption coefficient, facile processing, and lower toxicity. However, high‐performance photodiodes based on Sb2S3 have not been achieved and systematically investigated. Herein, Sb2S3‐based photodiodes based on conventional and inverted structures are developed, and their charge transport and spectral dependent quantum efficiency of these two types of devices are carefully modulated. After optimization, the conventional photodetectors achieve broadband responsivity up to 0.3 A W−1, high specific detectivity of 1.25 × 1012 Jones, and fast response speed of 6.4 µs with a small bias voltage of −0.5 V. On the other hand, the inverted devices exhibit imbalanced charge transport and color discrimination, which is promising for narrowband photodetection. These distinct device performances are highly dependent on the device architecture, which is crucial for designing efficient chalcogenide‐based optoelectronic devices.

Trap-assisted tunneling current and quantum efficiency loss in InGaAsSb short wavelength infrared photo detectors

We report the trap-assisted tunneling current and quantum efficiency (QE) loss in short wavelength infrared In0.22Ga0.78As0.2Sb0.8 photo detectors. Combining experiment data with a current–voltage model, we found that the trap-assisted tunneling current was boosted by increasing Beryllium (Be) doping in active region at room temperature. However higher Be doping level imposes no negative impacts on QE. Four traps with energy levels located at 49 , 60 , 155 and 199 below conduction band minimum in a In0.22Ga0.78As0.2Sb0.8 alloy are extracted from the fitting of I–V curves. Transparency measurement of an un-intentional doped In0.22Ga0.78As0.2Sb0.8 sample yields an absorption coefficient of 5191 cm−1 at 2.25 . Combining with the measured value of absorption coefficient, the QE dependence on diode length of In0.22Ga0.78As0.2Sb0.8 photo detectors is presented. Finally, by fitting quantum efficiencies of In0.22Ga0.78As0.2Sb0.8 photo detectors, we obtained that the minority electrons diffusion length is larger than 4 and the minority holes diffusion length is 0.2 . QE loss occurs at top N region of In0.22Ga0.78As0.2Sb0.8 photo detectors due to a short holes diffusion length.

Role of energy transfer on the luminescent performance of UV light driven Er3+/Dy3+ co-activated fluoroborosilicate glasses

In this work, authors systematically investigates the role of Er3+→Dy3+ energy transfer (ET) on the luminescent characteristics of UV-excited Er3+/Dy3+ co-activated fluoroborosilicate glasses via spectral overlying diagram, photoluminescence excitation, emission and decay analyses, temperature dependent luminescence and quantum efficiency measurements. The concentration dependent quenching of Er3+ luminescence intensity with respect to the enhancement in Dy3+ emission bands envisages the ET from Erbium to Dysprosium under 379 nm excitation and is further affirmed through the increased ET efficiency (49.6%) estimated from the lifetime values. Evaluated CIE colorimetry analyses recommend the as-prepared material for photonic applications.

Investigation of the parameters of a single photon detector for quantum communication

Nowadays the best single photon detectors from a practical view are those based on InGaAs/InP avalanche photodiodes, operating at a wavelength of 1.55 μm. The dependence of quantum efficiency and noise levels on the temperature and bias voltage of avalanche photodiodes were carried out.

The potential of SWCNTs to extend the IR-absorption of silicon solar cells

Single wall carbon nanotubes (SWCNTs) have long been proposed as a candidate to enhance existing photovoltaic materials by providing an extension to the light absorbed in the infrared. In this work, we discuss the validity of this statement for silicon solar cells and couple a bifacial silicon solar cell to an organic SWCNT cell in a 4-terminal stack. Single chiral species of (6,5), (7,6), and (10,3) are used in the organic cell and the overall contribution to photocurrent from the SWCNTs is discussed in terms of the diameter dependent quantum efficiency, film thickness, the transmittance of silicon, the spectral overlap of the SWCNT's absorbance with the solar irradiance spectrum and the maximum obtainable current density in the infrared.

Experimental demonstration of energy-transfer ratchet intermediate-band solar cell

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediate-band solar cells (IBSCs) under maximum sunlight concentration. However, an actual IBSC device with an efficiency larger than the Shockley-Queisser (SQ) limit has so far not been reported. The main difficulties lie in realizing an efficient sequential two-photon absorption (STPA) which requires a sufficiently long lifetime intermediate state or intermediate band. In this article, we propose the concept of a ratchet type IBSC, utilizing a long lifetime of rare-earth ion luminescence centers in Erbium-doped GaAs. The temperature dependent differential external quantum efficiency reveals a significant STPA contribution originating from the Er3+ luminescence center. All the results were modeled and interpreted by integrating the ratchet effect with up-conversion along with a density functional theory (DFT) simulation. Our work demonstrates that the long lifetime energy-transfer mechanism in Er3+ centers contributes directly to the formation of a ratchet type IB.

Developing of NbN films for superconducting microstrip single-photon detector

We optimized NbN films on a Si substrate with a buffer SiO2 layer to produce superconducting microstrip single-photon detectors with saturated dependence of quantum efficiency (QE) versus normalized bias current. We varied thickness of films and observed the maximum QE saturation for device based on the thinner film with the lowest ratio R S 300/R S 20.

Physical–Mathematical Model Of The Internal Quantum Efficiency Dependence On The Current Of Leds With Quantum Wells

A physical-mathematical model of dependence of internal quantum efficiency on current for LED structures with quantum wells has been developed. The volt-ampere characteristic is modelled with the involvement of Shockley, Noyce, Sah recombination theory, supplemented by the quantum wells distribution function. In order to obtain dependence of internal quantum efficiency of LEDs on current, model of rate of ABC recombination in quantum wells is used. The developed model was tested with variations of quantum wells parameters and external impact conditions.

Above Unity External Quantum Efficiency Photoelectrochemical Solar to Hydrogen Conversion

Here we show that a p-silicon-insulator-graphene photocathode (i) increases the photon-to-hydrogen external quantum efficiency (EQE) above 1, and (ii) reduces the onset potential for the hydrogen evolution reaction via an applied voltage across the graphene-silicon junction. These devices were measured in an acidic solution of 0.5 M H2SO4 using a dual potentiostat setup, allowing independent control over solution potential, graphene potential, and silicon potential. An AM1.5G solar simulator was used to carry out solar illumination measurements. In comparison to a p-Si-insulator control sample, the addition of an insulator/graphene layer increases the saturation photocurrent from ~35 mA/cm2 to ~80 mA/cm2, respectively, indicating that a carrier multiplication process occurs due to the addition of graphene. In this way, the graphene/oxide behaves similarly to an electrocatalyst, where at a given overpotential, the current density is increased. However, in this case, the current density is driven by the incident photon flux and enhanced via a carrier multiplication process. To further understand this effect, a laser excitation source was used to measure power dependent quantum efficiency. The external quantum efficiencies of the SIG devices exhibit a strong dependence on power density, increasing to from EQE~1.5 to EQE~7 for low power densities of ~0.1 mW/cm2. Current component resolved measurements enabled us to separately measure the currents flowing in the graphene, silicon, and solution. During measurements, we show that the measured photoelectrochemical coccurs due to carriers in the silicon directly driving the hydrogen generation, not graphene. This unique result highlights that the graphene/silicon tunnel junction causes carrier multiplication at the silicon surface, which then drives hydrogen generation. This can occur due to electrons tunneling from silicon directly through the graphene and driving the hydrogen evolution reaction. When comparing the control and graphene photocathodes, it is shown that the turn-on voltages are nearly identical, highlighting that the carrier multiplication process does not require additional voltage input and instead is driven by the intrinsic electric field. Finally, we show that applying a bias across the silicon-graphene junction reduces the turn-on voltage for hydrogen evolution reaction by nearly 400 mV for current densities of 40 mA/cm2. It is expected that this applied bias increases the field across the oxide, increasing the total carrier multiplication rate at a given solution bias. Thus p-silicon-insulator-graphene photocathodes are shown to demonstrate: (i) EQE greater than 1, and (ii) a reduction in the voltage needed to drive the electrochemical reaction when a bias is applied across the graphene-silicon junction.

The investigations to eliminate the bias dependency of quantum efficiency of InGaAsSb nBn photodetectors for extended short wavelength infrared detection

We report our investigations to eliminate the bias dependency of quantum efficiency of nBn devices based on InGaAsSb bulk alloy aimed for extended short wavelength detection. Both a p-type doped AlGaSb barrier and a p-type doped top contact layer were employed. The 9 × 1015cm-3p-type doping of AlGaSb barrier slightly improves the bias dependency of quantum efficiency of nBn devices. The using of p-type doped top contact layer completely eliminated the band offset induced bias dependency of quantum efficiency and a 2.2% variation of quantum efficiency as applied reverse bias increases was discussed in terms of the effect of surface depletion region and junction depletion region.

Size dependence of quantum efficiency of red emission from GaN:Eu structures for application in micro-LEDs.

GaN-based micro-LEDs typically suffer from a size-dependent efficiency due to the relatively long carrier lifetime and sidewall-related recombination effects. We demonstrate that for red-emitting Eu-doped GaN, sidewall-related recombination is hardly an issue for emission efficiency. We determine the photoluminescence quantum efficiency (PL QE) of Eu-related emission as a function of the size of square structures ranging from 3 to 192 µm. With the support of finite-difference time-domain simulations, we show that the light extraction efficiency and material losses are responsible for the decrease in PL QE for large sizes. For sizes smaller than 24 µm, there is an influence of the sidewall-related non-radiative recombination of carriers on the PL QE; however, it is only minor as a result of the limited carrier diffusion lengths in the Eu-doped material. These properties combined with the high efficiency of luminescence indicate the potential of this material for micro-LED applications.

Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

Color Determination from a Single Broadband Organic Photodiode

AbstractEstablishing the color of incident light is essential for many applications, such as machine vision, but generally requires either a dispersive component or multiple spectrally selective photodetectors. In contrast, here an incident spectrum is parametrized using a single broadband organic photodiode (OPD). This is achieved by exploiting the incident wavelength dependence of charge extraction caused by optically induced trap states in a metal oxide electron extraction layer, which results in an atypical spectral dependence of the reverse bias photocurrent density vs voltage (J–V) characteristics. Such dependence is augmented by confining the active layer within an optical microcavity to influence the light absorption profile and thus metal oxide trap state density. The average wavelength of an (approximately normally distributed) incident spectrum is then calculated to within ≈5 nm by algorithmically minimizing the difference between a measured J–V curve and one determined from the overlap integral of a trial spectrum with previously acquired voltage bias dependent external quantum efficiency (EQE) spectra.

Efficiency enhancement of III-nitride light-emitting diodes with strain-compensated thin-barrier InGaN/AlN/GaN multiple quantum wells

We introduced strain-compensated thin-barrier indium gallium nitride (InGaN)/aluminum nitride (AlN)/gallium nitride (GaN) multiple quantum wells (MQWs) to replace thin-barrier InGaN/GaN MQWs. The AlN insert layers would effectively compensate the strain of the thin-barrier InGaN/GaN MQWs to improve the opto-electrical properties of light-emitting diodes (LEDs). The 120-mA light output power of thin-barrier InGaN/GaN MQW LEDs could be improved from 31.9 mW to 35.3 mW by introducing 20-s-growth AlN insert layers, possibly reaching almost the same 120-mA light output power of traditional thick-barrier InGaN/GaN MQWs. Moreover, the current dependent external quantum efficiency (EQE) of the thin-barrier InGaN/AlN/GaN MQW LEDs with 20-s-growth AlN insert layers also indicated the largest peak EQE, showing high efficiency in low current injection. The severe carrier overflow effect that degrades the light output efficiency of the thin-barrier InGaN/GaN MQW LED in high current injection can be suppressed by introducing thin-barrier InGaN/AlN/GaN MQW with 20-s-growth AlN insert layers.

Effect of Surface Recombination on the Parameters of Photodiodes Based on HgCdTe Semiconductor Structures

The surface recombination rates for p-type HgCdTe layers with different dopant concentrations and trap densities Nt are calculated. It is shown that, at the given initial parameters, the surface recombination rate Smax lies in the range of 10–104 cm/s. The current sensitivity for p-type HgCdTe is simulated using the dependence of quantum efficiency in the approximation of large lifetimes τn0 and large diffusion lengths Ln of minority charge carriers, taking into account the effect of the surface recombination rate.

Quantum efficiency of femtosecond-laser sulfur hyperdoped silicon solar cells after different annealing regimes

Annealing dependent absorption and quantum efficiency spectra are determined on femtosecond-laser, sulfur hyperdoped silicon solar cells in a spectral range from 300 to 2500 nm. Although the samples, which are rapidly quenched after annealing at T = 1250 °C, do not feature the highest sub-bandgap absorption, the highest sub-bandgap quantum efficiency is measured on the same level as on non-annealed samples, featuring the highest sub-bandgap absorption. Findings on annealing dependent absorption are carried over, in order to explain the measured quantum efficiency spectra. In the sub-bandgap spectral range conversion is more efficient, when deep sulfur centers are obtained from annealing at higher temperatures at preferably rapid quenching. For those annealing regimes the above-bandgap quantum efficiency is decreased, which is ascribed to less shallow sulfur donors. Lower temperatures or slow cooling rates results in shallow donors, featuring a more efficient conversion in the above-bandgap spectral range. This is on the expense of deep sulfur centers and the corresponding sub-bandgap conversion ability. Therewith it is concluded, that the main absorption occurs only in the laser-induced sulfur emitter layer. Furthermore, to some extent a prediction is proposed on sulfur hyperdoped silicon prepared by ion implantation and subsequent pulsed laser melting, featuring rapid cooling from the melt.

NbN single-photon detectors with saturated dependence of quantum efficiency

The possibility of creating NbN superconducting single-photon detectors with saturated dependence of quantum efficiency (QE) versus normalized bias current was investigated. It was shown that the saturation increases for the detectors based on finer films with a lower value of Rs300/Rs20. The decreasing of Rs300/Rs20 was related to the increasing influence of quantum corrections to conductivity of superconductors and, in turn, to the decrease of the electron diffusion coefficient. The best samples have a constant value of system QE 94% at Ib/Ic ∼ 0.8 and wavelength 1310 nm.

Dependence of InGaAs/InP avalanche photodiode based single photon detector’s noise characteristics on the photodiode’s active area

Nowadays the best single photon detectors from a practical view are those based on InGaAs/InP avalanche photodiodes, operating at a wavelength of 1.55 μm. In this work the characteristics of such a detector of our development are studied, while operating in Geiger mode with a synchronization frequency of 312.5 MHz. The dependence of quantum efficiency and noise levels on the active area are studied.