Decrease In Minority Carrier Lifetime Research Articles

Light absorption enhancement and radiation hardening for triple junction solar cell through bioinspired nanostructures

Multi-junction solar cells constitute the main source of power for space applications. However, exposure of solar cells to the space radiation environment significantly degrades their performance across the mission lifetime. Here, we seek to improve the radiation hardness of the triple junction solar cell, GaInP/Ga(In)As/Ge, by decreasing the thickness of the more sensitive middle junction. Thin junctions facilitate the collection of minority carriers and show slower degradation due to defects. However, thinning the junction decreases the absorption, and consequently, the expected photocurrent. To compensate for this loss, we examined two bioinspired surface patterns that exhibit anti-reflective and light-trapping properties: (a) the moth-eye structure which enables vision in poorly illuminated environments and (b) the patterns of the hard cell of a unicellular photosynthetic micro-alga, the diatoms. We parametrize and optimize the biomimetic structures, aiming to maximize the absorbed light by the solar cell while achieving significant reduction in the middle junction thickness. The density of the radiation-induced defects is independent of the junction thickness, as we demonstrate using Monte Carlo simulations, allowing the direct comparison of different combinations of middle junction thicknesses and light trapping structures. We incorporate the radiation effects into the solar cell model as a decrease in minority carrier lifetime and an increase in surface recombination velocity, and we quantify the gain in efficiency for different combinations of junction thickness and the light-trapping structure at equal radiation damage. Solar cells with thin junctions compensated by the light-trapping structures offer a promising approach to improve solar cell radiation hardness and robustness, with up to 2% higher end-of-life efficiency than the commonly used configuration at high radiation exposure.

Passivation of thermally-induced defects with hydrogen in float-zone silicon

In this study, passivation of thermally-activated recombination centers with hydrogen in n-type float zone (FZ) Si containing nitrogen has been investigated. Prior to hydrogenation samples were heated to 550 °C using rapid thermal annealing and conventional furnaces. A large decrease in minority carrier lifetime occurred upon the heat-treatments confirming previous reports. A sequence of electron traps created in this process have been detected in the deep level transient spectra and characterized. Significant changes in the spectra have occurred after treatments in remote hydrogen plasma and subsequent annealing of the hydrogenated samples in the temperature range 100 °C–400 °C. A total elimination of electrical activity of the thermally induced defects has been observed in the hydrogenated samples subjected to annealing in the temperature range 150 °C–300 °C. The results obtained suggest a simple way for an effective cure of the degraded FZ-Si-based solar cells. Possible defect reactions occurring in the FZ-Si crystals and the role of nitrogen and carbon upon the performed treatments are discussed.

Spatial dependence of carrier localization in InAsSb/InSb digital alloy nBn detector

Recently we have demonstrated a novel method of extending the cut-off wavelength of InAsSb nBn detectors, by incorporating a series of monolayers of InSb, which we have termed a digital alloy. In recent work we have shown that while increasing temperature from 15 K to 40 K we observe a 14 meV blue shift of the photoluminescence peak energy and a decrease in minority carrier lifetime. We have hypothesized this effect is due to carrier localization caused by macro-scale fluctuations in layer thickness or alloy composition. Here we study the spatial variation of this effect over the surface of a sample, demonstrating significant variation with position. We discuss implications of this for the carrier localization hypothesis.

Wet-chemical bromination of Ge (100): A facile surface passivation tool

We demonstrate that wet-chemical surface bromination is an effective and a simple etching method for Ge surface oxide removal, providing excellent reoxidation resistance. Oxide removal and halide passivation for n-type Ge (100) were investigated using time-resolved photoluminescence and X-ray photoemission spectroscopy (XPS). In contrast to HCl, HBr treated Ge surfaces show a strong decrease in minority carrier lifetime, pointing to a surface state spectrum modification. The results from XPS using in situ sample preparation confirm that HBr effectively removes GeO2 and suboxides, providing an air stable surface. Isopropyl alcohol rinsing after Br passivation maintains the chemical surface composition and the electronic structure. In contrast, during H2O treatment in an Ar atmosphere, the brominated Ge surface is unstable, evidenced by emerging Ge-OH groups. The distinct observed upward shift of the surface Fermi level indicates an e- donating behavior of H2O.

Improved hot-zone for manufacturing low-oxygen silicon ingots for passivated emitter and rear cell

In the silicon wafers, interstitial oxygen and vacancies make oxygen precipitation in the cell process. The oxygen precipitates become combination centers for minority carriers, and it is resulted in decrease of minority carrier life-time (MCLT) and cell efficiency. In order to grow ingots with low oxygen concentration, we locally optimized hot-zone in the ingot grower. We have designed new hot-zone for commercial 8.3-in. diameter ingot growth using 24-in. quartz crucible and old grower. The average oxygen reduction is about 3 ppma at the top position of ingot, which is similar to the simulation result of oxygen distribution. Despite the inside temperature of grower rises, the concentration of oxygen was suppressed to 15 ppma or less as in a normal heater. As a result, it is possible to grow a low-oxygen ingot that can suppress the light induced degradation (LID), which can fundamentally help improve the efficiency of a high efficiency passivated emitter and rear cell (PERC).

Effects of crystal defects and their interactions with impurities on electrical properties of multicrystalline Si

We investigated the effects of different crystal defects and their interactions with impurities on the electrical properties of multicrystalline Si (mc-Si) using samples with unique defect patterns and impurities. By using the floating cast method, a single grain boundary (GB), identified as a Σ27 boundary, was first formed with a high density of impurities from atmosphere, leading to an inefficient external gettering of impurities during phosphorus (P) diffusion. During crystal growth, the Σ27 GB splits into the Σ3 and Σ9 GBs with accompanying generation of dislocations and reduction in the density of impurities. The external gettering of impurities became efficient for removing impurities as evidenced by an increase in average minority carrier lifetime. At the final stage of crystal growth, the decrease in minority carrier lifetime was significant, which could not be improved by phosphorus diffusion because of the high densities of segregated impurities and crystal defects originating from the strong contact with the crucible. The increase in Σ number was found to result in more enhanced precipitation of impurities, which led to the poor gettering effect of P diffusion. These results further confirmed the importance of the reduction in the densities of impurities and dislocations for the quality and yield improvement of mc-Si ingots for solar cells.

Decrease in Minority Carrier Lifetime of Crystalline Silicon Caused by Rapid Heating

ABSTRACTChange in the light-induced minority carrier effective lifetime τeff of crystalline silicon caused by rapid laser heating is reported. The top surface of n- and p-type silicon substrates with thicknesses of 520 μm coated with thermally grown SiO2 layers were heated by a 940 nm semiconductor laser for 4 ms. τeff was measured by a method of microwave absorption caused by carriers induced by 620 nm light illumination at 1.5 mW/cm2. τeff for light illumination of the top surfaces was decreased to 1.0x10-5 and 4.8x10-6 s by laser heating at 5.0x104 W/cm2 for n- and ptype 520-μm-thick silicon substrates, respectively. The decrease in τeff resulted from the generation of defect states associated with the carrier recombination velocity at the top surface region, Stop. Laser heating increased Stop to 6000 and 10000 cm/s for n- and p-type silicon samples, respectively. Heat treatment at 400oC for 4h markedly decreased Stop to 21 and 120 cm/s, respectively, for n- and p-type silicon samples heated at 5.0x104 W/cm2. Laser heating at 4.0x104 W/cm2 for 4 ms was also applied to samples treated with Ar plasma irradiation at 50 W for 60 s, which decreased τeff (top) to 2.0x10-5 s and 3.9x10-6 s for n- and p-type silicon samples, respectively. Laser heating successfully increased τeff (top) to 2.8x10-3 and 4.1x10-4 s for n- and p-type samples, respectively. Laser irradiation at 4x104 W/cm2played a role of curing recombination defect sites.

Quasi-planar GaAs heterojunction bipolar transistor device entirely grown by chemical beam epitaxy

Quasi-planar GaAs/GaInP heterojunction bipolar transistor (HBT) structures, fabricated with selective regrowth of an improved collector contact, are reported. Such devices present a planar surface topology which should allow large scale integration. The initial growth of the new HBT structure and the selective regrowth of the collector contact are performed by chemical beam epitaxy (CBE). In the case of the high C base doping level, the high temperature regrowth process induces some degradation of the HBT current gain which is analysed in terms of a decrease in minority carrier lifetime. Despite this effect, promising microwave performances are obtained with a cut-off frequency and maximum oscillation frequency of 30 and 25 GHz, respectively.

The role of nitrogen on the degradation behavior in LPE and VPE GaP:N light emitting diodes

Light output degradation of liquid-phase epitaxy (LPE) and vapor-phase epitaxy (VPE) GaP:N light emitting diodes (LED's) was studied by measuring electroluminescence efficiency, minority-carrier lifetimes, and effective nitrogen concentration during the degradation process. In LPE diodes the degradation is related to a decrease in minority-carrier lifetime, whereas in VPE diodes additionally a decrease in the effective nitrogen concentration was observed. This different degradation behavior is explained within a thermodynamic model which analyses the formation process of nitrogen atoms at substitutional and interstitial lattice sites under the different growth conditions of LPE and VPE.

A Comparison of Fast Neutron Irradiation Effects in Photoconductive and Photovoltaic InSb Infrared Detectors

The relative neutron irradiation hardness of photoconductive and photovoltaic InSb infrared detectors is examined for various operating modes and conditions. It is found that photoconductors are much less vulnerable to fast neutrons at 78K than are photovoltaic detectors. The damage mechanisms for photovoltaic detectors are increased leakage currents coupled with a decrease of minority carrier lifetime in the bulk region with a damage constant of 1.5 × 10-14 ?m-2 n-1 cm2, while a carrier removal rate of 1.1 cm-1 is established for 14 MeV neutron-irradiated p-type InSb with an initial carrier concentration of ~3 × 1014 cm-3. It is shown that these changes in material parameters can be used to predict fast neutron damage under all realistic operating conditions. The effects of neutron irradiations on signal, noise, current-voltage characteristics and spectral response are discussed in detail.

Photoluminescence of ion-implantation-damaged n-type GaAs

The effect of neon implants on the photoluminescence spectrum of heavily doped n-type GaAs has been determined. A large decrease in the photoluminescence efficiency is observed upon implantation, the magnitude of which depends on the ion dose and excitation wavelength. A model which explains the decrease in minority carrier lifetime and, therefore, the bulk quantum efficiency, is presented. The wavelength dependence is explained by the degree of overlap between the optical absorption length and the ion range.

Fast Neutron Effects on GaAsP Schottky Barrier Diodes and Hall Effect Devices

GaAsP Schottky barrier diodes and Hall effect devices were exposed to fast neutron radiation at room temperature at fluences ranging from 1013 neutrons cm-2 (E > 10 keV) to 8.9 × 1016 neutrons cm-2 (E > 10 keV). The carrier removal rate for the n-type material with a nominal doping of 5 × 1015 cm-3 and irradiated at a fluence of 9.7 × 1014 neutrons cm-2 (E > 10 keV) was determined from the capacitance-voltage characteristics of the diodes and was found to range between 5 and 6 cm-1. The change in conductivity of the GaAsP with neutron radiation measured on the Hall effect devices agrees very well with the diode measurements, while the change in effective carrier concentration appears to be slightly greater in the diodes than in the Hall devices. The Hall effect device measurements showed that the GaAsP material tends to become intrinsic after irradiation at high neutron fluence levels. The decrease in minority carrier lifetime after neutron irradiation was observed from the transient photocurrent response of the neutron-irradiated diodes generated by an ionizing X-ray pulse. Isochronal annealing of the neutron-irradiated diodes showed approximately a 40 percent recovery in carrier concentration over a wide temperature range starting at 225°C and continuing up to the maximum annealing temperature of 580°C.