Sub-2 cm/s passivation of silicon surfaces by aprotic solutions

Minimizing recombination at semiconductor surfaces is required for the accurate determination of the bulk carrier lifetime. Proton donors, such as hydrofluoric acid and superacids, are well known to provide highly effective short-term surface passivation. We demonstrate here that aprotic solutions based on bis(trifluoromethanesulfonyl)methane (TFSM) in hexane or pentane can also result in excellent passivation of (100)-orientation silicon surfaces. We show that the optimized TFSM-pentane passivation scheme can measure effective lifetimes up to 20 ms, with a surface recombination velocity of 1.7 cm s1 at an excess carrier density of 1015 cm3 . Fitting injection-dependent lifetime curves requires chemical passivation and field effect passivation from a negatively charged layer with a charge density of 1010–1011 q cm2 . The slightly higher recombination velocity of 2.3 cm s1 measured with TFSM-hexane can be explained by a lower charge density in the passivating layer, suggesting that the steric hindrance associated with the solvent size could play a role in the passivation mechanism. Finally, phosphorus nuclear magnetic resonance experiments confirm that TFSM-based solutions have Lewis acidity without being superacids, which opens up opportunities for them to be used in materials systems sensitive to superacidic environments

Low-temperature saw damage gettering to improve minority carrier lifetime in multicrystalline silicon

The minority carrier lifetime in multicrystalline silicon − a material used in the majority of today's manufactured solar cells − is limited by defects within the material, including metallic impurities which are relatively mobile at low temperatures (≤700 °C). Addition of an optimised thermal process which can facilitate impurity diffusion to the saw damage at the wafer surfaces can result in permanent removal of the impurities when the saw damage is etched away. We demonstrate that this saw damage gettering is effective at 500 to 700 °C and, when combined with subsequent low-temperature processing, lifetimes are improved by a factor of more than four relative to the as-grown state. The simple method has the potential to be a low thermal budget process for the improvement of low-lifetime “red zone” wafers

Small molecule screening in zebrafish: an in vivo approach to identifying new chemical tools and drug leads

In the past two decades, zebrafish genetic screens have identified a wealth of mutations that have been essential to the understanding of development and disease biology. More recently, chemical screens in zebrafish have identified small molecules that can modulate specific developmental and behavioural processes. Zebrafish are a unique vertebrate system in which to study chemical genetic systems, identify drug leads, and explore new applications for known drugs. Here, we discuss some of the advantages of using zebrafish in chemical biology, and describe some important and creative examples of small molecule screening, drug discovery and target identification

Iodine-ethanol surface passivation for measurement of millisecond carrier lifetimes in silicon wafers with different crystallographic orientations

To improve silicon device fabrication processes it is necessary to monitor bulk minority carrier lifetimes accurately, and this requires surface recombination to be well controlled and, ideally, minimized. Good surface passivation can result from thermal oxidation or by deposition of dielectrics (e.g. Al2O3, SiNx, amorphous Si), but these forms of passivation can modify the lifetime of the material under investigation. Various schemes can passivate surfaces on a temporary basis without modifying the bulk, and, in this paper, the virtues of the iodine‐ethanol temporary surface passivation scheme are explored. A procedure for preparing the wafer surfaces prior to passivation is developed. For the optimised pre‐treatment, a series of experiments on 3–5 Ωcm float‐zone wafers cut from the same ingot with different thicknesses is conducted. This enables the material's bulk lifetime to be measured at 1015 cm−3 injection as ≈46 ms, with the surface recombination velocity being 6.5 ± 0.3 cm s−1. Iodine‐ethanol passivation is then compared to a recently developed superacid‐derived temporary passivation scheme. Although the latter is superior on (100)‐orientation substrates, iodine‐ethanol performs much better on (111)‐orientation substrates, making it a better choice for (111)‐orientation wafers, such as those used for power devices

Influence of annealing and bulk hydrogenation on lifetime-limiting defects in nitrogen-doped floating zone silicon

A recombination active defect is found in as-grown high-purity floating zone n-type silicon wafers containing grown-in nitrogen. In order to identify the properties of the defect, injection dependent minority carrier lifetime measurements, secondary ion mass spectroscopy measurements, and photoluminescence lifetime imaging are performed. The lateral recombination center distribution varies greatly in a radially symmetric way, while the nitrogen concentration remains constant. The defect is shown to be deactivated through high temperature annealing and hydrogenation. We suggest that a nitrogen-intrinsic point defect complex may be responsible for the observed recombination

Permanent annihilation of thermally activated defects which limit the lifetime of float-zone silicon

We have observed very large changes in the minority carrier lifetime when high purity float-zone (FZ) silicon wafers are subject to heat-treatments in the range of 200– 1100˚C. Recombination centres were found to become activated upon annealing at 450–700˚C, causing significant reductions in the bulk lifetime, detrimental for high efficiency solar cells and stable high powered devices. Photoluminescence imaging of wafers annealed at 500˚C revealed concentric circular patterns, with lower lifetimes occurring in the centre, and higher lifetimes around the periphery. Deep level transient spectroscopy measurements on samples extracted from the centre of an n-type FZ silicon wafer annealed at 500˚C revealed a large variety of defects with activation energies ranging between 0.16– 0.36eV. Our measurements indicate that vacancy related defects are causing the severe degradation in lifetime when FZ wafers are annealed at 450–700˚C. Upon annealing FZ silicon at temperatures >800°C, the lifetime is completely recovered, whereby the defect-rich regions vanish and do not reappear (permanently annihilated). Our results indicate that, in general, as-grown FZ silicon should not be assumed to be defect lean, nor can it be assumed that the bulk lifetime will remain stable during thermal processing, unless annealed at temperatures >1000°C

Light-induced activation and deactivation of bulk defects in boron-doped float-zone silicon

In this paper, we present new insight in the degradation and subsequent recovery of charge carrier lifetime upon light soaking at 75 °C observed in float-zone silicon wafers. Variations of doping type, dielectric passivation schemes and thermal treatments after layer deposition were performed. The degradation was only observed for p-type float-zone silicon wafers passivated with passivation schemes involving silicon nitride layers. An influence of thermal treatments after deposition was found. N-type wafers did not degrade independent of their passivation scheme. Room temperature re-passivation experiments showed the degradation to affect the wafer bulk, and photoluminescence studies demonstrated fine lateral striations of effective lifetime. We conclude that the degradation is caused by bulk defects that might be related to hydrogen complexes

Computation in Classical Mechanics

There is a growing consensus that physics majors need to learn computational skills, but many departments are still devoid of computation in their physics curriculum. Some departments may lack the resources or commitment to create a dedicated course or program in computational physics. One way around this difficulty is to include computation in a standard upper-level physics course. An intermediate classical mechanics course is particularly well suited for including computation. We discuss the ways we have used computation in our classical mechanics courses, focusing on how computational work can improve students' understanding of physics as well as their computational skills. We present examples of computational problems that serve these two purposes. In addition, we provide information about resources for instructors who would like to include computation in their courses.Comment: 6 pages, 3 figures, submitted to American Journal of Physic

Room temperature enhancement of electronic materials by superacid analogues

Treatment with the superacid bis(trifluoromethanesulfonyl)amide (sometimes known as TFSA, TFSI, or HNTf2) enhances the properties of a wide range of optoelectronic materials, resulting in longer effective carrier lifetimes and higher photoluminescence quantum yields. We have conducted a multimaterial study treating both crystalline silicon and transition metal dichalcogenide (TMDC) monolayers and few-layer flakes with solutions formed from TFSA and a range of compounds with related chemical structures with different Lewis acidities, in order to elucidate the factors underpinning the TFSA-related class of enhancement treatments. We adopt dichloromethane (DCM) as a common solvent as it provides good results at room temperature and is potentially less hazardous than TFSA-dichloroethane (DCE) heated to ∼100 °C, which has been used previously. Kelvin probe experiments on silicon demonstrate that structurally similar chemicals give passivating films with substantially different charge levels, with the higher levels of charge associated with the presence of CF3SO2 groups resulting in longer effective lifetimes due to an enhancement in field-effect passivation. Treatment with all analogue solutions used results in enhanced photoluminescence in MoS2 and WS2 compared to untreated controls. Importantly we find that MoS2 and WS2 can be enhanced by analogues to TFSA that lack sulfonyl groups, meaning an alternative mechanism to that proposed in computational reports for TFSA enhancement must apply

The association of genetic predisposition to depressive symptoms with non-suicidal and suicidal self-Injuries

Non-suicidal and suicidal self-injury are very destructive, yet surprisingly common behaviours. Depressed mood is a major risk factor for non-suicidal self-injury (NSSI), suicidal ideation and suicide attempts. We conducted a genetic risk prediction study to examine the polygenic overlap of depressive symptoms with lifetime NSSI, suicidal ideation, and suicide attempts in a sample of 6237 Australian adult twins and their family members (3740 females, mean age\ua0=\ua042.4\ua0years). Polygenic risk scores for depressive symptoms significantly predicted suicidal ideation, and some predictive ability was found for suicide attempts; the polygenic risk scores explained a significant amount of variance in suicidal ideation (lowest p\ua0=\ua00.008, explained variance ranging from 0.10 to 0.16\ua0%) and, less consistently, in suicide attempts (lowest p\ua0=\ua00.04, explained variance ranging from 0.12 to 0.23\ua0%). Polygenic risk scores did not significantly predict NSSI. Results highlight that individuals genetically predisposed to depression are also more likely to experience suicidal ideation/behaviour, whereas we found no evidence that this is also the case for NSSI