Effect Of Auger Recombination Research Articles

Investigation of theoretical efficiency limit of hot carriers solar cells with a bulk indium nitride absorber

Theoretical efficiencies of a hot carrier solar cell considering indium nitride as the absorber material have been calculated in this work. In a hot carrier solar cell highly energetic carriers are extracted from the device before thermalisation, allowing higher efficiencies in comparison to conventional solar cells. Previous reports on efficiency calculations approached the problem using two different theoretical frameworks, the particle conservation (PC) model or the impact ionization model, which are only valid in particular extreme conditions. In addition an ideal absorber material with the approximation of parabolic bands has always been considered in the past. Such assumptions give an overestimation of the efficiency limits and results can only be considered indicative. In this report the real properties of wurtzite bulk InN absorber have been taken into account for the calculation, including the actual dispersion relation and absorbance. A new hybrid model that considers particle balance and energy balance at the same time has been implemented. Effects of actual impact ionization (II) and Auger recombination (AR) lifetimes have been included in the calculations for the first time, considering the real InN band structure and thermalisation rates. It has been observed that II-AR mechanisms are useful for cell operation in particular conditions, allowing energy redistribution of hot carriers. A maximum efficiency of 43.6% has been found for 1000 suns, assuming thermalisation constants of 100 ps and ideal blackbody absorption. This value of efficiency is considerably lower than values previously calculated adopting PC or II-AR models.

Optoelectronic Properties of the p-MnZnO/n-Si Structure Photodiodes in a Strong Magnetic Field

Photodiodes with a p-MnZnO/n-Si substrate structure were fabricated. Mn-doped p-type ZnO (MnZnO) films were deposited by ultrasonic spray pyrolysis on a (100)-oriented silicon substrate. A dark current and a photocurrent of ∼4.31×10-7 and 6.95×10-4 A, respectively, were measured at a reverse bias of 2 V, and a photocurrent-to-dark current contrast ratio of almost four orders of magnitude was observed. When a p-MnZnO/n-Si structure photodiode was applied a strong magnetic field of 0.5 T, the photocurrent slightly increases to ∼1.24×10-3 A at a reverse bias of 2 V. This may be attributed to the Auger recombination effect due to the applied magnetic field.

Gain peak–cavity mode alignment optimisation in buried tunnel junction mid-infrared GaSb vertical cavity surface emitting lasers using hydrostatic pressure

High-pressure techniques are used to investigate and optimise the design of GaInAsSb/AlGaAsSb vertical cavity surface emitting lasers (VCSELs) emitting at 2.4 µm. From measurements on edge emitting lasers it is found that non-radiative Auger recombination accounts for up to 85% of the threshold current at room temperature (RT) and is responsible for their strong temperature sensitivity. The effect of Auger recombination in VCSELs may be mitigated through judicious design of the gain peak–cavity mode (CM) alignment which may be investigated using high pressure. Results show that temperature insensitivity over the range −10 to +30°C may be obtained around RT if the gain peak is offset by approximately 10 meV higher in energy from the VCSEL CM.

Effect of carrier spillover and Auger recombination on the efficiency droop in InGaN-based blue LEDs

We have investigated efficiency droop in InGaN-based blue LEDs by considering radiative, nonradiative, and carrier spillover processes in the context of internal quantum efficiency (IQE) vs. injection current. If relied on fitting only, both the Auger recombination and an empirical formula for carrier spillover are consistent with experiments. However, the dependence of IQE on quantum well parameters and lack of droop in optical pumping experiments support the notion that carrier spillover is the main mechanism in play.

Carrier relaxation dynamics in InAs∕InP quantum dots

The electronic properties of InAs∕InP(113)B double-cap quantum dots (QDs) emitting around 1.55μm are investigated. The carrier dynamics in QDs is studied by nonresonant time-resolved photoluminescence experiments. This analysis reveals the electronic structure of the QDs and the transient filling of the confined levels. Under low excitation densities, the spontaneous exciton lifetime is estimated and compared to time-resolved experiments. Under high excitation density, a direct Auger recombination effect is identified. The temperature analysis enables us to distinguish Auger and phonon-assisted relaxation processes.

Effect of Auger recombination on the thermal stability of high-voltage high-power semiconductor diodes

An analytical model that accounts for the effect of Auger recombination on the position of inversion points in the current-voltage characteristics of high-voltage semiconductor diodes is suggested. It is shown that Auger recombination not only shifts the position of the inversion points in the current-voltage characteristics of the diodes, but also changes the number of possible inversion points in the structures. Because the existence and position of the inversion points largely determine the thermal stability of the diodes, especially in the current-surge mode, the predictions of the model appear to be practically important. To verify the conclusions from the analytical model, a numerical simulation was performed using the ISSLEDOVANIE software. The results of the numerical calculations are in full agreement with the conclusions made based on the model.

Large nonlinear refraction in InSb at 10 μm and the effects of Auger recombination

Narrow bandgap semiconductors exhibit very large optical nonlinearities in the infrared owing to large two-photon absorption that scales as the inverse cube of the bandgap energy and the large losses and refraction from two-photon generated free carriers. Except for extremely short pulses, the free-carrier effects dominate the nonlinear losses and nonlinear refraction. Here we develop a method for the calculation of the free-electron refraction cross section in InSb. We also calculate the Auger recombination coefficient in InSb and find it to be in good agreement with existing experimental data. In all the calculations we rely on Fermi-Dirac statistics and use a four-band k⋅p theory for band structure calculations. Experiments on the transmission of submicrosecond CO2 laser pulses through InSb produce results consistent with the calculated parameters.

Effect of Auger recombination on the performance of p-doped quantum dot lasers

Experimental results on spontaneous emission rates from InGaAs quantum dot lasers that can be explained theoretically by considering the influence of nonradiative mixed state recombinations in the quantum dot-wetting layer system are presented. Our model qualitatively explains the experimental results such as an increase in the threshold current density, temperature stability, and a narrower gain spectrum due to doping the quantum dot active region with the acceptors. Our model also predicts that moderate acceptor concentrations can improve the laser performance at higher carrier injection densities; but high acceptor concentrations deteriorate the laser performance due to the nonradiative Auger recombination that counteracts the benefits of increased spontaneous emission rates.

Correlation of theory with experimental SOAs

Predictions for the near-traveling wave amplifier (NTWA) with multiple-quantum-well structures have been developed. The continuity equation for quantum wells (QWs) with high carrier densities is combined with the amplifier TW gain equation expressed in terms of stimulated lifetime. This formulation allows the signal gain to be related to the bias current and the optical input signal through Fermi energies. The charge neutrality condition also plays an important role for high carrier density QW amplifiers. Auger recombination and heating effects are incorporated as essential components of the model. Experimental measurements of gain versus bias current and output power for both /spl lambda/= 850- and 1550-nm devices are found to be very well matched by the calculated results.

3.3 [micro sign]m W quantum well light emitting diode

Recent studies suggest that the radiative conversion efficiency of mid-infrared semiconductor devices is limited by non-radiative Auger mechanisms. Band structure engineering techniques, such as the introduction of strain or the use of type-II band offset materials, have been shown to reduce the effect of Auger recombination. Results from light emitting diodes (LEDs) with an active region consisting of ten InAs/GaInSb/InAs/AlGaAsSb type-II ‘W’ quantum wells grown by molecular beam epitaxy (MBE) on GaSb substrates are described. At room temperature, the device was characterised by a slope efficiency of 98 µW/A at low currents, which dropped at higher currents due to heating. This corresponded to an internal efficiency of approximately 2.6%.

Hot phonons and Auger related carrier heating in semiconductor optical amplifiers

We have directly measured the carrier temperature in semiconductor optical amplifiers (SOAs) via spontaneous emission and we demonstrate an unexpectedly high carrier temperature. The direct correlation of the temperature increase with the carrier density suggests Auger recombination as the main heating mechanism. We have developed a model based on rate equations for the total energy density of electrons, holes, and longitudinal-optical phonons. This model allows us to explain the thermal behavior of carrier and phonon populations. The strong heating observed is shown to be due to the combined effects of hot phonon and Auger recombination in the valence band. We also observe an evolution of the Auger process, as the density is increased, from cubic to square dependence with coefficients C/sub 3/ = 0.9 10/sup -28/ cm/sup 6/ s/sup -1/ and C/sub 2/ = 2.4 10/sup -10/ cm/sup 3/ s/sup -1/. This change is explained by the hole quasi-Fermi level entering the valence band.

Multiparticle interactions and stimulated emission in chemically synthesized quantum dots

We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.

Absorption and emission spectra of InAs/Ga 1− xIn xSb/AlSb nanostructures for infrared applications

We report an empirical pseudopotential study of the optical properties of several InAs/GaSb superlattice structures, some of these additionally containing Ga 1− x In x Sb and AlSb layers, designed for infrared (IR) applications. The optical absorption and emission spectra of these nanostructures are modelled, with a view to establishing a quantitative link with the microscopic signature of the interface. The emission spectra of the structures having laser applications are investigated for various population inversions. We gauge the role of atomic disorder and determine the degree of alloy layer disorder for which the virtual crystal (VC) approximation provides an adequate description of the overall lineshape. An analogous study was carried out for structures having applications as detectors. We find good agreement between the experimental absorption spectra and our computer model, and investigate the effects of Auger recombination on device performance.

Effect of Auger recombination on thermal processes in InGaAs and InAsSbP IR-emitting diodes

We investigated IR-emitting diodes (IREDs) based on the lattice-mismatched structures with stressed InGaAs/InAs layers and on near-matched InAsSbP/InAs structures. The emission wavelength range was from 2.5 to 5.0 μm. Both the processes of excess energy relaxation and mechanisms responsible for the active area overheating were studied. For In 1− x Ga x As-based IREDs, it was shown that the active area overheat temperature, Δ T, is related to the Auger recombination processes. When x is increased from 0 to 0.09, the Auger recombination efficiency decreases, thus, favoring an abrupt drop of Δ T. At x=0.09–0.22, the efficiency of CHHS Auger processes exponentially decreases. However, owing to an increase in the dislocation density (due to a considerable, ∼6.9%, lattice mismatch parameter Δa/ a), the Δ T value increases slowly and less markedly. At x>0.2, the Δ T value becomes constant. For the InAsSbP-based IREDs, the active area overheat due to the current flow was less pronounced.

Carrier lifetime measurements using free carrier absorption transients. II. Lifetime mapping and effects of surface recombination

A novel noncontact technique for semiconductor wafer mapping of the charge carrier lifetime is reviewed. The principle is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. The technique is demonstrated here for Si wafer lifetime mapping, both for homogenous wafers as well as for wafers having pn junctions, and the performance and limitations are addressed. In a companion article J. Appl. Phys. 84, 275 (1998), (part I), the injection dependence is treated in more detail. We demonstrate the broad range of accessible injections, allowing on-wafer lifetime mapping both of the minority carrier lifetime and of the high injection lifetime as well as of Auger recombination effects. Furthermore, problems of surface recombination for bare Si surfaces are elucidated while pn junctions or other barriers are shown to suppress diffusion of carriers to surfaces. Also, a scheme for low temperature surface passivation is demonstrated and the resulting surface recombination velocity is derived. Finally, a few examples of characteristic lifetime maps from processed samples are discussed. We conclude that the technique is one of the few available for monitoring of the carrier lifetime during a full device process sequence.

Investigation into the effect of Auger recombination on charge carrier transport and static characteristics of silicon multilayer structures

A new model for charge carrier transport through a silicon multilayer structure at high current density is presented. This model accounts for a number of nonlinear physical phenomena (electron-hole scattering, Auger recombination, high doping effects) which become of significance at high current density. The injected charge carrier distribution in the lightly doped layer of the structure at high current density was investigated on the basis of this model and previously predicted phenomenon of injected carrier saturation is confirmed. The dependence of injected carrier limiting density on the electrophysical parameters of the silicon structure was investigated. Within the framework of the suggested model the current-voltage characteristics of a p +- n- n + structure was studied. It is demonstrated that injected carrier saturation phenomenon results in linear current-voltage characteristics at high current densities. A characteristic ratio ( W L ) c (where W is the width of the n-base layer and L is the ambipolar diffusion length of charge carriers in the n-base layer) was found to divide the diode structures into two groups. In the first group with W L <( W L ) c the recombination of injected carriers in the n-base layer at high current density is provided by Auger processes only and therefore the current-voltage characteristics does not depend on lifetime τ, conditioned by the Shockley-Read-Hall recombination processes. The second group of structures with W L >( W L ) c retains a dependence on τ at all current densities. Experimental data presented in the last section of the article confirm the results of device modeling.

Photovoltaic properties and high efficiency of preferentially doped polysilicon solar cells

Making use of new solutions for the photocurrent and dark current, developed earlier, the photovoltaic properties of a preferentially doped n + p solar cell are examined taking into consideration Shockley-Read-Hall recombination, Auger recombination and heavy doping effects. Numerical calculations have been carried out for the analysis of the effects of the doping concentration, grain size, preferential doping penetration depth along the grain boundaries, and grain boundary recombination velocity on the photovoltaic parameters of the cell. The results reveal that, in order to obtain a high cell efficiency, the emitter doping level should not exceed 5 × 10 19 cm −3 and the base must be doped at an optimum level. It is found that the optimum base doping depends on the grain width and grain boundary recombination velocity and is practically unchanged with preferential doping. The results also show that a conversion efficiency exceeding 20% under AM1 conditions is obtained, when the grain is preferentially doped, and when the grain boundaries and the top and back contact surface are passivated.

Effect of trap location and trap-assisted Auger recombination on silicon solar cell performance

Model calculations were performed to investigate and quantify the effect of trap location and trap-assisted Auger recombination on silicon solar cell performance. Trap location has a significant influence on the lifetime behavior as a function of doping and injected carrier concentration in silicon. It Is shown in this paper that for a high quality silicon (/spl tau/=10 ms at 200 ohm-cm, no intentional doping), high resistivity (/spl ges/200 ohm-cm) is optimum for high efficiency one sun solar cells if the lifetime limiting trap is located near midgap. However, if the trap is shallow (E/sub t/-E/sub v//spl les/0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 /spl mu/s at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be /spl sim/0.2 ohm-cm, regardless of the trap location. It is shown that the presence of a shallow trap can significantly degrade the performance of a concentrator cell fabricated on high-resistivity high-lifetime silicon material because of an undesirable injection level dependence in the carrier lifetime. The effect of trap assisted Auger recombination on the cell performance has also been modelled in this paper. It is found that the trap-assisted Auger recombination does not influence the one sun cell performance appreciably, but can degrade the concentrator cell performance if the trap-assisted Auger recombination coefficient value exceeds 2/spl times/10/sup -14/ cm/sup 3//s. Therefore, it is necessary to know the starting lifetime as well as trap location in order to specify base resistivity in order to predict or achieve the best cell performance for a given one sun or concentrator cell design.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of thin silicon solar cells for high efficiency

A comprehensive theoretical analysis taking into account the contribution from both the emitter and base regions having finite surface recombination velocity has been developed for computing short-circuit current, open-circuit voltage, and efficiency of thin AR coated thin silicon solar cells with textured front surface. The dependence of efficiency on the front surface and back surface recombination velocities and on the cell parameters have been investigated in details for varying cell thickness considering the effects of bandgap narrowing and Auger recombination in the material. It is shown that efficiency exceeding 24% can be attained with silicon solar cells having thickness as low as 25 μm provided both front and back surfaces are well passivated (S < 103cm/s) and the doping concentration in the base and emitter are in the range of 5 × 1016 to 1017cm−3 and 1018 to 5 × 1018cm−3, respectively. It is also shown that an efficiency of about 23% can be obtained for thin cells of 25 μm thickness with a much inferior quality materials having diffusion length of about 40 μm.

Optimum strain for the suppression of Auger recombination effects in compressively strained InGaAs/InGaAsP quantum well lasers

The primary mechanism that leads to the suppression of Auger recombination effects in compressively strained quantum wells is found due to the existence and the appropriate positioning of negative-curvature zones in the first valence subbands (zones in k space where the radial curvature of the first valence subband is negative). By studying previous Monte Carlo calculation results, strong correlations between the negative-curvature zones and its adverse effects on Auger recombinations are observed, thereby confirming this finding. Through this understanding, it is inferred that compressive strain is most effective for reduction of Auger recombination effects when the amount of strain is no more than 1.0%. Beyond 1.0%, the gain in device performance with respect to more compressive strain is expected to be greatly diminished.