Minority Carrier Recombination Lifetime Research Articles

Validation of minority carrier recombination lifetimes in low-dimensional semiconductors found by analytical photoresponses

It is a formidable challenge to find the minority carrier recombination lifetime in low-dimensional devices as low-dimensionality increases the surface recombination rate and often reduces the recombination lifetime to a scale of picoseconds. In this work, we demonstrated a simple but powerful method to quantitatively probe the minority carrier recombination lifetime in silicon nanowires or microwires by fitting the experimental photoresponses with our recently established analytical photoresponse principle of photoconductors. The nanowires were passivated with small molecules and Al2O3 to suppress surface recombination, which will increase the minority recombination lifetimes. As expected, the minority carrier recombination lifetime found by this approach increases by orders of magnitude. These wires were also made into PIN diodes, the leakage of which was reduced at least 1 order of magnitude after surface passivation by Al2O3. The minority recombination lifetime found from the leakage current of these devices is largely consistent with what we found from our analytical photoresponse principle. As a further step, we performed scanning photocurrent microscopy to find the minority diffusion length from which we found that the minority recombination lifetime is close to what we found from the analytical photoresponses. In short, this work validated that our analytical response principle is a reliable method to find the minority recombination lifetime in low-dimensional semiconductors.

Utility of Shockley–Read–Hall analysis to extract defect properties from semiconductor minority carrier lifetime data

The semiconductor minority carrier lifetime contains information about several important material properties, including Shockley–Read–Hall defect levels/concentrations and radiative/Auger recombination rates, and the complex relationships between these parameters produce a non-trivial temperature-dependence of the measured lifetime. It is tempting to fit temperature-dependent lifetime data to extract the properties of the Shockley–Read–Hall recombination centers; however, without a priori knowledge of the distribution of the Shockley–Read–Hall states across the bandgap, this fit problem is under-constrained in most circumstances. Shockley–Read–Hall lifetime data are not well-suited for the extraction of Shockley–Read–Hall defect levels but can be used effectively to extract minority carrier recombination lifetimes. The minority carrier recombination lifetime is observed at temperatures below 100 K in a Si-doped n-type InGaAs/InAsSb superlattice, and deviation from its expected temperature-dependence indicates that the capture cross section of the defect associated with Si-doping has an activation energy of 1.5 meV or a characteristic temperature of 17 K. This lower temperature regime is also preferrable for the analysis of the physics of defect introduction with displacement-damage-generating particle irradiation.

Effects of 63 MeV proton-irradiation on the dark-current in III-V-based, unipolar barrier infrared detectors

Recent investigations demonstrated that III-V-based nBn infrared detectors typically experience relatively significant performance degradation under 63 MeV proton irradiation due to displacement damage, limiting their viability for certain space-based applications. Additionally, the investigations suggested, based on simplified expressions, that the nBn’s diffusion-limited dark-current would transition from a linear-dependence on proton fluence to a square-root-dependence as the minority carrier recombination lifetime decreased and the subsequently decreasing diffusion length became less than the thickness of the detector’s quasi-neutral absorbing region. Here, a closer examination of this expected behavior is reported on. Evidence from multiple nBn detector rad-tolerance experiments and simple numerical simulations indicated the behavior of the nBn’s diffusion-current under proton irradiation can be more complicated than was previously suggested. For example, in some nBn detectors proton irradiation was observed to increase the doping density of the absorber sufficient to impact the diffusion-current and lessen its power-law-dependence on the proton fluence. This increase was confirmed by analysis of in situ capacitance–voltage measurements on an irradiated nBn detector, which found an n-type carrier addition rate ∼140 cm−1. Simulations showed this level of carrier addition can lead to a ∼75% reduction in the expected diffusion-current density for an equivalent dose of 100 krad(Si). Simulations also showed the nBn diffusion-current’s instantaneous power law-dependence on proton fluence peaked well below unity and then began approaching −.5 rather than.5, which was suggested by those previous investigation. A high native defect density prior to irradiation and finite surface recombination velocity were both found to reduce the initial power-law dependence and shift its peak position.

Simulation Study of Front-Lit Versus Back-Lit Si Solar Cells

Continuous efforts are being made to improve the efficiency of Si solar cells, which is the prevailing technology at this time. As opposed to the standard front-lit solar cell design, the back-lit design suffers no shading loss because all the metal electrodes are placed on one side close to the pn junction, which is referred to as the front side, and the incoming light enters the denuded back side. In this study, a systematic comparison between the two designs was conducted by means of computer simulation. Medici, a two-dimensional semiconductor device simulation tool, was utilized for this purpose. The 0.6 μm wavelength, the peak value for the AM-1.5 illumination, was chosen for the incident photons, and the minority-carrier recombination lifetime (τ), a key indicator of the Si substrate quality, was the main variable in the simulation on a p-type 150 μm thick Si substrate. Qualitatively, minority-carrier recombination affected the short circuit current (Isc) but not the opencircuit voltage (Voc). The latter was most affected by series resistance associated with the electrode locations. Quantitatively, when τ ≥ 500 μs, the simulation yielded the solar cell power outputs of 20.7 mW·cm−2 and 18.6 mW·cm−2, respectively, for the front-lit and back-lit cells, a reasonable 10 % difference. However, when τ < 500 μs, the difference was 20 % or more, making the back-lit design less than competitive. We concluded that the back-lit design, despite its inherent benefits, is not suitable for a broad range of Si solar cells but may only be applicable in the high-end cells where float-zone (FZ) or magnetic Czochralski (MCZ) Si crystals of the highest quality are used as the substrate.

Empirical Study of the Disparity in Radiation Tolerance of the Minority-Carrier Lifetime Between II–VI and III–V MWIR Detector Technologies for Space Applications

The degradation of the minority-carrier recombination lifetime of various III–V nBn and II–VI HgCdTe midwave-infrared space detector materials under stepwise 63-MeV proton irradiation up to fluence of 7.5 × 1011 cm−2 and above has been measured using time-resolved photoluminescence while samples were held at 120 K to limit thermal annealing. As expected, the recombination rate of each sample was found to increase with proton fluence at a nearly constant rate, implying a near-linear increase in defect concentration. The rate of change of the carrier recombination rate, herein called the minority-carrier lifetime damage factor, was then plotted as a function of the initial recombination rate for each sample. Juxtaposing the III–V and II–VI results revealed a distinct disparity, with the incumbent detector material HgCdTe being roughly an order of magnitude more radiation tolerant to displacement damage from proton irradiation than any of the nBn materials. The results for the latter also suggest some degree of interrelation between the damage factor and initial lifetime. The behavior of the lifetime of each material under annealing revealed that HgCdTe exhibited nearly 100% recovery at 295 K whereas III–V materials recovered to only about 50% under the same conditions.

Electronic Quality Improvement of Highly Defective Quasi‐Mono Silicon Material by Phosphorus Diffusion Gettering

Quasi‐mono silicon (QM‐Si) attracts interest as a substrate material for silicon device processing with the promise to yield single‐crystalline silicon quality with multicrystalline silicon cost. A significant barrier to widespread implementation of QM‐Si is ingot edge‐contamination caused by the seed material and crucible walls during crystal growth. This work aims to recover the scrap material in QM‐Si manufacturing with a process easily adaptable to semiconductor device manufacturing. A phosphorus diffusion process at 870 °C for 60 min significantly improves the electronic quality of a QM‐Si wafer cut from a contaminated edge brick. The harmonic minority carrier recombination lifetime of the wafer, a key predictor of ultimate device performance, experiences a tenfold increase from 17 to 178 µs, which makes the scrap QM‐Si material usable for device fabrication. Local areas with suboptimal (&lt;50 µs) lifetimes remaining can be further improved by a high temperature anneal before the phosphorus diffusion process.

Simulated Study on the Effects of Substrate Thickness and Minority-Carrier Lifetime in Back Contact and Back Junction Si Solar Cells

The BCBJ (Back Contact and Back Junction) or back-lit solar cell design eliminates shading loss by placing the pn junction and metal electrode contacts all on one side that faces away from the sun. However, as the electron-hole generation sites now are located very far from the pn junction, loss by minority-carrier recombination can be a significant issue. Utilizing Medici, a 2-dimensional semiconductor device simulation tool, the interdependency between the substrate thickness and the minority-carrier recombination lifetime was studied in terms of how these factors affect the solar cell power output. Qualitatively speaking, the results indicate that a very high quality substrate with a long recombination lifetime is needed to maintain the maximum power generation. The quantitative value of the recombination lifetime of minority-carriers, i.e., electrons in p-type substrates, required in the BCBJ cell is about one order of magnitude longer than that in the front-lit cell, i.e., 5 × 10−4 sec vs. 5 × 10−5 sec. Regardless of substrate thickness up to 150 μm, the power output in the BCBJ cell stays at nearly the maximum value of about 1.8 × 10−2 W·cm−2, or 18 mW·cm−2, as long as the recombination lifetime is 5 × 10−4 s or longer. The output power, however, declines steeply to as low as 10 mW·cm−2 when the recombination lifetime becomes significantly shorter than 5 × 10−4 sec. Substrate thinning is found to be not as effective as in the front-lit case in stemming the decline in the output power. In view of these results, for BCBJ applications, the substrate needs to be only mono-crystalline Si of very high quality. This bars the use of poly-crystalline Si, which is gaining wider acceptance in standard front-lit solar cells.

A Simulated Study of Silicon Solar Cell Power Output as a Function of Minority-Carrier Recombination Lifetime and Substrate Thickness

A Simulated Study of Silicon Solar Cell Power Output as a Function of Minority-Carrier Recombination Lifetime and Substrate Thickness

Proton Irradiation Effects on the Performance of III-V-Based, Unipolar Barrier Infrared Detectors

An examination of the collective results from recent experiments quantifying the performance degradation rates of III-V-based, unipolar barrier infrared detectors with various designs and materials, cutoff wavelengths and operating conditions due to 63 MeV proton irradiation is presented. Empirical relationships were established between the radiation damage factors for dark current density, lateral optical collection length, and quantum efficiency and the inverse product of the detectors' cutoff wavelength and operating temperature. Fitting the dark current density damage factor's empirical relationship reflected these detectors' tendency to remain diffusion-limited during irradiation, which was previously established using Arrhenius-analysis of the post-irradiation, temperature-dependent dark current measurements on each. Collectively, the results affirmed the performance degradation stemmed from a reduction of the minority carrier recombination lifetime via generation of additional defects by proton-induced displacement damage. For comparing detector's radiation-tolerance, the results indicated that damage factors alone were not ideal, but their empirical relationships would serve as heuristics in this role.

Impact of tungsten contamination on the sensing margin of a CMOS image sensor cell

We investigated the impact of tungsten contamination on minority-carrier recombination lifetime, the photodiode dark/photo current and sensitivity of a pinned photodiode, and the sensing margin of a CMOS image sensor (CIS) cell. After an intentional tungsten contamination and followed by driving at 800 °C for 30 min, tungsten contaminant were located from the surface to p- and n-type regions of a photodiode. The tungsten contamination degraded minority-carrier recombination life-time and the dark current and sensitivity of a pinned photodiode; i.e., there was a good correlation between the minority recombination lifetime and the sensitivity of a photodiode. As a result, tungsten contamination directly degraded the sensing margin of a CIS cell photodiode; i.e., it decreased with increasing tungsten contaminant concentration.

Effect of nanohole structure on pyramid textured surface on photo-voltaic performance of silicon solar cell

We investigated how the nanohole structure on a {111} pyramid textured surface affected the photovoltaic performance of silicon solar cells by varying nanohole depth. During the effective minority carrier recombination lifetime, the surface reflectance decreased with increasing nanohole depth, showing a trade-off relationship. The power conversion efficiency (PCE) of silicon solar cells with the {111} pyramid textured surface peaked at a specific nanohole depth, i.e., 0.43% PCE enhancement (3.32% enhancement relative to a reference cell) was obtained at the nanohole depth of 94.8 nm.

Design of Extremely Proximity Gettering Using Hydrogen Ion Implantation for Si CMOS Image-Sensor

As the pixel size in CMOS image-sensor has scaled down less than 1 μm, photo sensitivity which is a ratio of photo and dark current has been important. To improve photo sensitivity in CMOS image-sensor, the achievement of lower dark current is essential. Leakage current is influenced by crystallographic, trapped charge at the SiO2/Si interface, structural defects, and especially metallic contaminant impurities. Also, these contaminant impurities degrade the minority-carrier recombination lifetime. Thus, we investigated the dependency of the properties of CMOS image-sensor on metallic contamination. Standard aqueous solutions of metallic ion were dropped on the surface of silicon wafers and the wafers were spin dry. They were heated to drive the metallic ion contaminants into the silicon bulk. After that, we fabricated 4-Tr CMOS image-sensor cells. In silicon bulk wafer, the minority-carrier recombination lifetime decreased with increasing a metallic contaminant concentration. These contaminations result in degradation of dark current, sensitivity of photodiode and sensing margin of CMOS image-sensor. These results indicate that metallic contaminants are critical for increasing dark current because metallic contaminants remain inside the photo-diode region, ~3 μm. In our study, we applied a proximity relaxation-type gettering method. We formed the nano-cavities close to the depletion region using hydrogen ion implantation, as shown in Fig 1. These cavities getter metallic impurities on the cavities wall. Sensing margin of CMOS image-sensor implanted by hydrogen ion was dramatically improved, as shown in Fig 2. In addition, although metallic contaminant concentration increased, the device performances of CMOS image-sensor were constant compare with that of bulk wafer which were not implanted (Fig. 3). These results indicates that proximity gettering by using hydrogen ion implantation was effective in metal gettering for image-sensor. We investigated the correlation of metallic contaminant and sensing margin of CMOS image-sensor. Finally, we will suggest an extremely proximity gettering method for removing metallic contaminants of CMOS image-sensor.* This work was financially supported by the Brain Korea 21 Plus Program in 2013 and SiWEDS (Silicon Wafer Engineering and Defect Science).

Minority carrier recombination lifetimes in n-type CdMgSe mixed crystals measured by means of the photothermal infrared radiometry

AbstractMinority recombination lifetimes of n-type CdMgSe mixed crystals were estimated by using infrared photothermal radiometry (PTR) amplitude and phase frequency spectra. The results obtained by the PTR method indicate that the lifetimes of optically generated carriers in CdSe and CdxMg1−xSe crystals are about 0.1 μs. The diffusion length of minority carrier in n-type CdSe single crystal was found to be 4.42 μm and it is in a good agreement with the literature value. It was found that with the increasing thermal-to-plasma component coefficient A the carrier concentration increases as expected from PTR theory.

Correlation of Fe-Rich Defect Centre and Minority Carrier Lifetime in p-Type Multicrystalline Silicon

A comparative study has been made to analyze the impact of interstitial iron in minority carrier lifetime of multicrystalline silicon (mc-Si). It is shown that iron plays a negative role and is considered very detrimental for minority carrier recombination lifetime. The analytical results of this study are aligned with the spatially resolved imaging analysis of iron rich mc-Si.

Effects of metallic contaminant type and concentration on photovoltaic performance degradation of p-type silicon solar cells

We investigated the effects of the metallic contaminant type and concentration (Al, Cu, Ni, and Fe) on the minority-carrier recombination lifetime and photovoltaic performance degradation of p-type silicon solar cells. For all contaminants, the lifetime after annealing at 900 °C for 15 min decreased with increasing concentration. The sequence of higher lifetime degradation induced by metallic contamination was Al (highest), Cu, Ni, and Fe (lowest), mainly determined by causing the diffusivity length and the solubility of the metallic contaminant in silicon. The sequence of higher lifetime degradation sensitivity induced by metallic contamination was Fe, Ni, Cu, and Al, as mainly determined by the trap energy level of the metallic contaminant in silicon. The contamination degraded the power-conversion efficiency (PCE) due to both the short-circuit-current and the fill-factor degradation. The degree and sensitivity of the PCE degradation depended on the contaminant type and concentration. The degree was the highest for Al, followed by Cu, Ni, and Fe, while the sensitivity was the highest for Fe, followed by Ni, Cu, and Al.

Solar cell implemented with silicon nanowires on pyramid-texture silicon surface

We investigated how silicon nanowires affected the photovoltaic performance of silicon solar cells with a pyramid textured surface. The diameter and height of silicon nanowires increased and reached saturation with electro-less etching time after Ag nanoparticle deposition. As a result, the amount of light absorbance increased with electro-less etching time while the effective minority-carrier recombination lifetime decreased with increasing electro-less etching time. The dependencies of light absorbance amount and effective minority-carrier recombination lifetime on electro-less etching time meant that a maximum power-conversion efficiency (PCE) was achieved at a specific electro-less etching time, ∼2min, to produce silicon nanowires ∼52μm in diameter and ∼221μm in height.

Simulated Optimum Substrate Thicknesses for the BC-BJ Si and GaAs Solar Cells

In crystalline solar cells, the substrate itself constitutes a large portion of the fabrication cost as it is derived from semiconductor ingots grown in costly high temperature processes. Thinner wafer substrates allow some cost saving as more wafers can be sliced from a given ingot, although technological limitations in slicing or sawing of wafers off an ingot, as well as the physical strength of the sliced wafers, put a lower limit on the substrate thickness. Complementary to these economical and techno-physical points of view, a device operation point of view of the substrate thickness would be useful. With this in mind, BC-BJ Si and GaAs solar cells are compared one to one by means of the Medici device simulation, with a particular emphasis on the substrate thickness. Under ideal conditions of 0.6 <TEX>${\mu}m$</TEX> photons entering the 10 <TEX>${\mu}m$</TEX>-wide BC-BJ solar cells at the normal incident angle (<TEX>${\theta}=90^{\circ}$</TEX>), GaAs is about 2.3 times more efficient than Si in terms of peak cell power output: 42.3 <TEX>$mW{\cdot}cm^{-2}$</TEX> vs. 18.2 <TEX>$mW{\cdot}cm^{-2}$</TEX>. This strong performance of GaAs, though only under ideal conditions, gives a strong indication that this material could stand competitively against Si, despite its known high material and process costs. Within the limitation of the minority carrier recombination lifetime value of <TEX>$5{\times}10^{-5}$</TEX> sec used in the device simulation, the solar cell power is known to be only weakly dependent on the substrate thickness, particularly under about 100 <TEX>${\mu}m$</TEX>, for both Si and GaAs. Though the optimum substrate thickness is about 100 <TEX>${\mu}m$</TEX> or less, the reduction in the power output is less than 10% from the peak values even when the substrate thickness is increased to 190 <TEX>${\mu}m$</TEX>. Thus, for crystalline Si and GaAs with a relatively long recombination lifetime, extra efforts to be spent on thinning the substrate should be weighed against the expected actual gain in the solar cell output power.

Large surface photovoltages observed at methyl-terminated silicon surfaces synthesised through a two-step chlorination-alkylation method

Methylation of the silicon surface through a chlorination-alkylation method has been used to improve the electronic properties of silicon. Upon alkylation of the surface, an increase in the minority carrier recombination lifetime and the surface photovoltage is observed, in line with an increase in surface charge. A likely explanation of this unusually large band bending is charge accumulation during the removal of chlorine from the surface.

Non-destructive infrared optoelectronic lock-in carrierography of mc-Si solar cells

A novel non-destructive, non-contact laser-induced infrared lock-in carrierographic imaging technique based on photocarrier radiometry (PCR) was introduced to characterize industrial multi-crystalline silicon solar cells. The modulated photovoltage (MPV) was measured simultaneously with the PCR signal in point-by-point imaging using focused laser scanning. The PCR and MPV signals were also investigated as functions of modulation frequency and load resistance at fixed coordinate points in order to better understand and interpret the physical optoelectronic origins of the carrierographic imaging contrast. A InGaAs-camera based carrierograhic lock-in imaging technique with a spread laser-beam illumination of solar cells was also introduced and was shown to exhibit contrast based on p-n junction RC time constants and on quasi-neutral bulk minority carrier recombination lifetimes.

Investigating minority-carrier lifetime in small spherical Si using microwave photoconductance decay

We have studied minority-carrier recombination lifetime in small spherical Si (&amp;lt;1mm in diameter) using microwave photoconductance decay (μPCD). It was tested that 0.01molar quinhydrone in methanol solution has the best passivation effect, which results in perfect exponential photoconductance decay curve and the highest average lifetime. However, the average lifetime of single crystalline spheres that were formed by polishing single crystalline Czochralski bulks was only 4.8μs, and that of tear-drop-like spheres that were formed by a dropping method was 2.6μs. These values are significantly lower compared with crystalline Si wafers. It has been found that the low lifetime is induced by the small size, and the spherical geometry does not have influence on the lifetime value. The surface recombination velocity becomes more dominant compared to bulk recombination as the size of sphere becomes smaller. Therefore, the principal of surface equivalent lifetime in the sphere is essentially different from the case of the planar wafer.