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A major component of reliability of Silicon Carbide (4H-SiC) power electronics are due to the gate dielectric. Specific aspects such as hysteresis, insulator lifetime, threshold voltage (VTH) instability and high leakage currents, remain an issue of paramount importance which hamper the uptake of 4H-SiC power MOSFETs.1 The oxide understood to have the most importance is coincidently the native oxide, SiO2, due to the suitable band offset to 4H-SiC. However, most of the SiO2/SiC interface problems, such as carbon clusters, H content and O vacancies, are directly related to the thermal oxidation process.2 Here, we show these issues can partially be avoided by using dielectric deposition instead. Atomic layer deposition (ALD) offers specific advantages such as very low deposition temperature, excellent process control and suitability for conformal deposition of gate oxides in trench structures.3 Deposited oxides still require a post-deposition anneal (PDA) in a nitrogen-containing ambient4 such as nitrous oxide (N2O) or nitric oxide (NO) [2] to passivate the defects within the stack and improve the as-deposited layer’s poor electrical quality. We present the excellent reliability performance of ALD-deposited SiO2 layers on SiC through fabrication of metal-oxide-semiconductor capacitors (MOSCAPs) and then using time-dependent dielectric breakdown (TDDB) characterisation. The distribution of interface parameters, as well as the results from TDDB, have been investigated as metrics of improvement following the N2O PDA process. For benchmarking, processes will be compared to thermally grown oxides4 and LPCVD-deposited dielectric layers to demonstrate the superior process quality. An active 10 µm thick epitaxial layer of 4 × 1015 cm-3 n-type doping is used in this study and grown on 100 mm diameter, 4° off-axis (0001) 4H-SiC wafers. Growth was performed using a 30 μm/hr growth and N2 as a dopant in an LPE ACiS M8 chemical vapour deposition (CVD) reactor. After an initial clean, a 1 µm thick field oxide was deposited and a window was opened via photolithography and reactive ion etching (RIE). Then, quarter wafers underwent one of the three oxidation routines: 1. SiO2 plasma deposition at 200 °C using bis(diethylamino)silane (BDEAS) and O2 plasma precursors in an Ultratech Fiji G2 Plasma-Enhanced ALD System. 2. SiO2 deposition at 750 °C using tetraethyl orthosilicate (TEOS) as a precursor in a Thermco LPCVD system. 3. direct thermal growth of SiO2 in a HiTech furnace at 1300 °C for 5 hrs in N2O ambient. Samples that had undergone one of the first two routines then underwent a PDA in N2O, in the HiTech furnace at 1300°C for 2 hrs. All oxidation measurements resulted in oxide thicknesses between 50 and 60 nm, which were verified using step-height AFM measurements and optical interferometry. To create the vertical MOSCAP structure samples had a 500 nm aluminium (Al) backside contact deposited, before a 1 μm Al layer was deposited on the topside of the samples by means of a liftoff process. A cross-sectional diagram of the fabricated final device structure is shown in Fig. 1 (a). Table I shows the key electrical parameters which were extracted using room temperature C-V measurements, combined with I-V and constant field TDDB measurements at 175 °C. Fig. 1 (b) shows the improvement brought about by the N2O anneal, which also results in the average flatband voltage being reduced by more than 10 V to the lowest value of the dataset (1.44 V) and frequency dispersion mostly removed, with all but the ALD as-deposited samples reaching average breakdown oxide fields of about 10 MV/cm. The most promising improvement of the ALD-deposited SiO2 can be seen observing the breakdown distribution of MOSCAPs when stressed at constant high electric field (9 MV/cm), which can be seen in Fig. 2 (a) and (b). Here, these samples reach a TDDB, 63% value of 4786 s, which represents an average increase of 28% when compared to LPCVD samples and 58% when comparing this to a purely thermal process. Further evidence will be presented that high-quality, high-reliability SiO2 layers, formed by ALD and PDA have been formed. Flatband voltage and hysteresis are reduced, compared to thermally grown oxide, and frequency dispersion in accumulation is negligible. The ALD oxide with PDA has higher reliability than LPCVD and thermal oxides in TDDB tests, offering 29% to 345% improvement. The degradation mechanisms of the ALD oxide are different to that the LPCVD and thermal oxides and will be discussed in this talk. [1] P. Moens et al, ISPSD 32, 78-71 (2020). [2] P. Fiorenza et al, Appl. Surf. Sci., 149752 (2021). [3] A. Renz et al, MSSP 122, 105527 (2021). [4] K. Tachiki et al, Appl. Phy. Expr. 13.12, 121002(2020). Figure 1

The recombination of photogenerated charge carriers at metal-semiconductor interfaces remains a major source of efficiency loss in photovoltaic cells. Here, we present SiNx and AlOx nanolayers as promising interface dielectrics to enable high efficiency hole selective passivating contacts. It is demonstrated that SiNx deposited via direct plasma enhanced chemical vapour deposition can be grown controllably at thicknesses of 2 nm. The valence band offsets between crystalline silicon and ultrathin AlOx and SiNx nanolayers are measured as 3.5 and 1.4 eV, respectively. This predicts a larger tunnelling current for holes, compared to SiOx used typically. Resistivity measurements show that SiNx and AlOx nanolayers have lower contact resistivities compared to SiOx, with values as low as 100 m omega center dot cm(2). Analysis of the current transport mechanisms confirmed that tunnelling dominates the conduction through SiNx, while a mixture of tunnelling and pinholes are present in the AlOx structure. Lifetime measurements gave initial indications of the passivation quality of the films, with just 10 cycles of AlOx achieving 260 mu s after annealing and 1.9 ms with extrinsic field effect passivation added. Finally, the intrinsic built-in charge in the dielectrics was determined using surface photovoltage measurements and simulations are used to estimate the influence of nanolayer built-in charge in both poly-Si and dopant-free passivating contacts to enable future high efficiency solar cells.

We have studied the mechanisms underpinning effective surface passivation of silicon with hafnium oxide (HfO 2 ) thin films grown via atomic layer deposition (ALD). Plasma-enhanced ALD with O 2 plasma and a tetrakis(dimethylamido)hafnium precursor was used to deposit 12 nm thick HfO 2 films at 200 °C on high-lifetime 5 Ωcm n -type Czochralski silicon wafers. The passivation was activated by postdeposition annealing, with 30 min in air at 475 °C found to be the most effective. High-resolution grazing incidence X-ray diffraction measurements revealed the film crystallized between 325 and 375 °C, and this coincided with the onset of good passivation. Once crystallized, the level of passivation continued to increase with higher annealing temperatures, exhibiting a peak at 475 °C and yielding surface recombination velocities of 475 °C. By superacid repassivation, we demonstrated this reduction in lifetime was not because of a decrease in the bulk lifetime, but rather because of changes in the passivating films themselves. Kelvin probe measurements showed the films are negatively charged. Corona charging experiments showed the charge magnitude is of order 10 12 qcm -2 and that the reduced passivation above 475 °C was mainly because of a loss of chemical passivation. Our study, therefore, demonstrates the development of highly charged HfO 2 films and quantifies their benefits as a standalone passivating film for silicon-based solar cells.

A systematic post-deposition annealing study on Silicon Carbide (SiC) metal-oxide-semiconductor capacitors (MOSCAPs) using atomic layer deposition (ALD)-deposited silicon dioxide (SiO2) layers was carried out. Anneals were done in oxidising (N2O), inert (Ar) and reducing (H2:N2) ambients at elevated temperatures from 900°C to 1300°C for 1 hour. Electrical characterisation results show that the forming gas treatment at 1100°C reduces the flatband voltage to 0.23 V from 10 V for as-deposited SiO2 layers. The density of interface traps (DIT) was also reduced by one order of magnitude to 2×1011 cm-2 eV-1 at EC-ET = 0.2 eV. As an indicator of the improvement, characterisation by x-ray photoelectron spectroscopy (XPS) showed that silicon enrichment present in as-deposited layers was largely reduced by the forming gas anneal, improving the stoichiometry. Time-dependent dielectric breakdown (TDDB) results showed that the majority of forming gas annealed samples broke down at breakdown fields of 12.5 MV × cm-1, which is about 2.5 MV × cm-1 higher than for thermally oxidised samples.

Subnanometer-scale silicon dioxide (SiO2) films are frequently present before, during, and after silicon device processing, yet they offer minimal surface passivation and can detrimentally impact subsequent processing steps. Here we develop a process whereby the surface passivation of nanometer and subnanometer SiO2 films is enhanced by up to 2 orders of magnitude by a simple room temperature treatment using the superacid bis(trifluoromethane)sulfonimide (TFSA, sometimes TFSI). By accurately modeling the effective lifetime curves corresponding to the superacid treated SiO2 samples, we have determined that the enhanced passivation is mainly due to a reduction in the interface defect density (Dit) at the Si/SiO2 interface, with a minor contribution also arising from the presence of negative charge. X-ray photoelectron spectroscopy of the treated SiO2 films reveals the presence of fluorine, and this, along with hydrogen, is a strong candidate for the chemical passivation of defects at the Si/SiO2 interface. Post treatment, the SiO2 films show short time scale electronic instability, whereby a degradation and then recovery are observed over a period of 1–10 h which is attributed to variations in the Dit, as determined from our analysis of the injection-dependent lifetime data. Following the instability period, the surface passivation remains relatively stable for days. Nuclear magnetic resonance measurements of superacid-based solutions reveal that electron-donating solvents should be avoided, as they exacerbate surface passivation instabilities. The results presented demonstrate that simple strategies can be used to enhance the passivation properties of ultrathin films greatly, which in the age of nanotechnology could offer benefits to device performance in a range of applications including solar cells and batteries.\ud \ud

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.

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments.

Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump–probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied.

The surface properties of many inorganic electronic materials (e.g., MoS2, WSe2, Si) can be substantially modified by treatment with the superacid bis(trifluoromethane)sulfonimide (TFSI). Here we find more generally that solutions based on molecules with trifluoromethanesulfonyl groups, including TFSI, give rise to excellent room temperature surface passivation, with the common factor being the presence of CF3SO2 groups and not the solution’s acidity. The mechanism of passivation comprises two effects: (i) chemical passivation; and (ii) field effect passivation from a negatively charged thin film likely to be physically adsorbed by the surface. Degradation of surface passivation is caused by de-adsorption of the thin film from the surface, and occurs slowly in air and rapidly upon vacuum exposure. The air stability of the passivation is enhanced by the presence of droplets at the surface which act to protect the properties of the film. The finding that nonacidic solutions can provide excellent electrical passivation at room temperature opens up the possibility of using them on materials more sensitive to an acidic environment.

Immeasurably low surface recombination of crystalline-silicon wafers is demonstrated with an oxide-nitride-oxide (ONO) corona charged dielectric stack. We detail experimental variations to each layer of the dielectric stack to establish a procedure which provides outstanding passivation properties on textured and planar silicon wafers. We demonstrate surface recombination velocities of

Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (

This letter reports on initial investigation results on the material quality and device suitability of a homo-epitaxial 3C-SiC growth process. Atomic force microscopy surface investigations revealed root-mean square surface roughness levels of 163.21 nm, which was shown to be caused by pits (35 μm width and 450 nm depth) with a density of 1.09 × 105 cm−2 which had formed during material growth. On wider scan areas, the formation of these were seen to be caused by step bunching, revealing the need for further epitaxial process improvement. X-ray diffraction showed good average crystalline qualities with a full width of half-maximum of 160 arcseconds for the 3C-SiC (002) being lower than for the 3C-on-Si material (210 arcseconds). The analysis of C–V curves then revealed similar interface-trapped charge levels for freestanding 3C-SiC, 3C-SiC on Si and 4H-SiC, with forming gas post-deposition annealed freestanding 3C-SiC devices showing D IT levels of 3.3 × 1011 cm−2 eV−1 at E C−E T = 0.2 eV. The homo-epitaxially grown 3C-SiC material’s suitability for MOS applications could also be confirmed by leakage current measurements.

This letter reports on the improvement of a SiO2 layer formed by atomic layer deposition on 4H-SiC, using a post-deposition anneal in forming gas ambient. Capacitance–voltage measurements revealed good electrical properties, compared to a thermal oxide which was grown in N 2 O, with flatband voltage values averaging at -0.29 V and a low positive mobile ion charge density in the order of 1010 cm−2. XPS analysis revealed the FG annealed sample to have the most Si rich interface comparatively to other PDAs, with a C:Si ratio of 0.72, allowing more Si bonds to be terminated. SIMS analysis identified an increase in hydrogen near the interface of the FG-annealed sample with a peak concentration of 2.12 × 1021 cm−3. It is concluded that the improvement in electrical performance is due to the hydrogen passivating trap states at the SiO2/4H-SiC interface.

Float-zone (FZ) silicon is usually assumed to be bulk defect-lean and stable. However, recent studies have revealed that detrimental defects can be thermally activated in FZ silicon wafers and lead to a reduction of carrier lifetime by up to two orders of magnitude. A robust methodology which combines different characterization techniques and passivation schemes is used to provide new insight into the origin of degradation of 1 Ω·cm n-type phosphorus doped FZ silicon (with nitrogen doping during growth) after annealing at 500 °C. Carrier lifetime and photoluminescence experiments are first performed with temporary room temperature surface passivation which minimizes lifetime changes which can occur during passivation processes involving thermal treatments. Temperature- and injection-dependent lifetime spectroscopy is then performed with a more stable passivation scheme, with the same samples finally being studied by deep level transient spectroscopy (DLTS). Although five defect levels are found with DLTS, detailed analysis of injection-dependent lifetime data reveals that the most detrimental defect levels could arise from just two independent single-level defects or from one two-level defect. The defect parameters for these two possible scenarios are extracted and discussed.

Gallium doped silicon is an industrially viable alternative to boron doped silicon for photovoltaics, and is assumed to be immune from light-induced degradation. We have studied light soaking for >1000 h of industrially fabricated passivated emitter and rear cell (PERC) devices formed from monocrystalline gallium and boron doped substrates, with cell properties monitored using a non-contact photoluminescence imaging proxy method. As-fabricated stabilised boron doped cells did not degrade or underwent a slight improvement, whereas as-fabricated gallium doped cells which had not been intentionally stabilised experienced a slight (~5%) deterioration which then recovered. When PERC devices were subjected to a 200–300 °C dark anneal before light soaking, significant differences in the cell degradation signatures were observed. Degradation characteristic of light and elevated temperature induced degradation (LeTID) was observed for boron and gallium PERC solar cells, with the onset of degradation taking longer, and the severity being less, with gallium. Investigation of stripped gallium PERC devices with room temperature surface passivation revealed bulk lifetime degradation correlates with the cell-level degradation. When the cells were stripped and passivated with aluminium oxide, a complete change in the degradation behaviour was observed, with no degradation occurring in the gallium case and boron-oxygen-like degradation observed for boron. This indicates that dielectric passivation is not suitable for lifetime degradation diagnosis in stripped cells. Gallium retains advantages over boron doping with stabilisation processes not generally required, but manufacturers need to be aware of possible low-temperature lifetime instabilities when developing future fabrication processes, such as for passivated contact structures.

Chemical treatments play an essential role in the formation of high quality interfaces between materials, including in semiconductor devices, and in the functionalisation of surfaces. We have investigated the effects of hydrogen and fluorine termination of (100)-orientation silicon surfaces over a range of length scales. At the centimetre scale, lifetime measurements show clean silicon surfaces can be temporarily passivated by a short treatment in both HF(2%) : HCl(2%) and HF(50%) solutions. The lifetime, and hence surface passivation, becomes better with immersion time in the former, and worse with immersion time in the latter. At the nanometre scale, X-ray photoelectron spectroscopy and atomic force microscopy show treatment with strong HF solutions results in a roughened fluorine-terminated surface. Subsequent superacid-derived surface passivation on different chemically treated surfaces shows considerably better passivation on surfaces treated with HF(2%) : HCl(2%) compared to HF. Lifetime data are modelled to understand the termination in terms of chemical and field effect passivation at the centimetre scale. Surfaces passivated with Al2O3 grown by atomic layer deposition behave similarly when either HF(2%) : HCl(2%) or HF(50%) are used as a pre-treatment, possibly because of the thin silicon dioxide interlayer which subsequently forms. Our study highlights that chemical pre-treatments can be extremely important in the creation of high quality functionalised surfaces.

Float-zone (FZ) silicon often has grown-in defects that are thermally activated in a broad temperature window (≈300–800 °C). These defects cause efficient electron-hole pair recombination, which deteriorates the bulk minority carrier lifetime and thereby possible photovoltaic conversion efficiencies. Little is known so far about these defects which are possibly Si-vacancy/nitrogen-related (VxNy). Herein, it is shown that the defect activation takes place on sub-second timescales, as does the destruction of the defects at higher temperatures. Complete defect annihilation, however, is not achieved until nitrogen impurities are effused from the wafer, as confirmed by secondary ion mass spectrometry. Hydrogenation experiments reveal the temporary and only partial passivation of recombination centers. In combination with deep-level transient spectroscopy, at least two possible defect states are revealed, only one of which interacts with H. With the help of density functional theory V1N1-centers, which induce Si dangling bonds (DBs), are proposed as one possible defect candidate. Such DBs can be passivated by H. The associated formation energy, as well as their sensitivity to light-induced free carriers, is consistent with the experimental results. These results are anticipated to contribute to a deeper understanding of bulk-Si defects, which are pivotal for the mitigation of solar cell degradation processes.

Minimizing carrier recombination at interfaces is of extreme importance in the development of high-efficiency photovoltaic devices and for bulk material characterization. Here, we investigate a temporary room temperature superacid-based passivation scheme, which provides surface recombination velocities below 1 cm/s, thus placing our passivation scheme amongst state-of-the-art dielectric films. Application of the technique to high-quality float-zone silicon allows the currently accepted intrinsic carrier lifetime limit to be reached and calls its current parameterization into doubt for 1 Ω·cm n-type wafers. The passivation also enables lifetimes up to 65 ms to be measured in high-resistivity Czochralski silicon, which, to our knowledge, is the highest ever measured in Czochralski-grown material. The passivation strategies developed in this work will help diagnose bulk lifetime degradation under solar cell processing conditions and also help quantify the electronic quality of new passivation schemes.

Photomodulators for mm-wave and THz radiation are an essential component for many imaging and signal processing applications. While a myriad of schemes have been devised to enhance photomodulation by enhancing the light-matter interaction, there has been less focus on the photoconductive materials themselves, which are often the limiting factor. Here, we present an approach to increase the photomodulation efficiency of silicon by orders of magnitude, using post treatment of off-the-shelf silicon wafers. The increase in efficiency removes the need for bulky and costly amplified laser sources, and creates the potential for compact and cost-effective modulators for real-world applications. By passivating the surfaces of long bulk-lifetime silicon wafers with Al2O3, the recombination of the photoexcited carriers at the surfaces is mostly eliminated. This results in vastly longer excess carrier lifetimes (up to ~50 ms), with corresponding increases in photoconductivity. The resulting modulators are highly efficient, with the transmission through them being reduced from ~90% to −2, whilst modulation factors of greater than 80% can be achieved over a broad band with similar intensities. We also discuss the limitations of such long-lifetime modulators for applications where the switching speed or spatial resolution of a modulator may be critical.

For photovoltaics, switching the p‐type dopant in silicon wafers from boron to indium may be advantageous as boron plays an important role in the light‐induced degradation mechanism. With the continuous Czochralski crystal growth process it is now possible to produce indium doped silicon substrates with the required doping levels for solar cells. This study aims to understand factors controlling the minority carrier lifetime in such substrates with a view to enabling the quantification of the possible benefits of indium doped material. Experiments are performed using temperature‐dependent Hall effect and injection‐dependent carrier lifetime measurements. The recombination rate is found to vary linearly with the concentration of un‐ionized indium which exists in the sample at room temperature due to indium's relatively deep acceptor level at 0.15 eV from the valence band. Lifetime in indium doped silicon is also shown to degrade rapidly under illumination, but to a level substantially higher than in equivalent boron doped silicon samples. A window of opportunity exists in which the minority carrier lifetime in degraded indium doped silicon is higher than the equivalent boron doped silicon, indicating it may be suitable as the base material for front contact photovoltaic cells.

Optical excitation is a useful means by which to modulate mm-wave and THz radiation. However, most photoconductive materials require intense photoexcitation to provide sufficient modulation, often relying on hugely expensive light sources such as amplified femtosecond lasers. Here we show that, through surface passivation, one can engineer silicon to yield a highly sensitive photomodulator with an efficiency that is three orders of magnitude greater than “off-the-shelf” wafers. Moreover, by using a layer of indium tin oxide to create a cavity with an un-doped silicon wafer, the modulation is increased by up to a factor of 7 for relatively low illumination powers, without compromising switching time or spatial resolution.

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.\ud \ud

Float-zone silicon has been long assumed to be bulk defect free and stable. Nevertheless, recently it was found that upon annealing between 450 °C to 700 °C detrimental defects can be activated in this material. Previous studies via deep level transient spectroscopy have identified several defect levels. However, it is still not clear which of these levels have a substantial impact on the minority carrier lifetime. In this study, we determine the recombination parameters of the dominant defect level using a combination of deep level transient spectroscopy and temperature and injection dependent lifetime spectroscopy. Additionally, we investigated the effect of hydrogenation on the thermally activated defects in ntype float-zone silicon. © 2019 American Institute of Physics Inc. All rights reserved.

In the past year or so, gallium doped silicon wafers have become a mainstream substrate for solar cell production in China [1], and hence for the world. They offer intrinsically better carrier lifetime stability than boron doped substrates [2] without requiring post-cell production stabilisation processes while requiring only minimal changes to established cell lines. Gallium doped monocrystalline silicon is likely to account for the majority of passivated emitter and rear cell (PERC) production in the coming years. High purity gallium doped silicon has been the subject of much less study than some other silicon material types, and the limits of its performance are not yet well established. In terms of effective carrier lifetime – one of the most important properties for solar cells – the highest reported values are typically around 1 ms. We have conducted a study to evaluate the performance limit of modern Ga doped silicon materials with high quality surface passivation. We find effective lifetime to vary with resistivity and measure effective lifetimes > 9 ms in 11.2 Ωcm material, which is much higher than reported previously. Furthermore, the injection dependence is less strong than in the boron case which may result in a higher fill factor and favourable cell performance in operation. One of Ga doped silicon’s perceived advantages is its stability under illumination relative to B doped silicon which suffers heavily from boron-oxygen light induced degradation if not mitigated. We have monitored the stability of commercial gallium doped PERC cells under illumination (> 3000 h in some cases) using a photoluminescence imaging proxy method. We have shown however that dark annealed Ga doped PERC cells can degrade when exposed to light, albeit on a much smaller scale than B doped PERC, and this degradation occurs due to a reduction in bulk lifetime [3]. We will now present our latest results which show inconsistencies in degradation behaviour between solar cell batches. Surprisingly, cells from one ingot exhibit degradation, then recovery behaviour when annealed at 300 °C, but near stability when not annealed, but, for another ingot, the opposite is observed. We have also investigated the effect of stabilisation processes of the kind typically used to mitigate boron-oxygen degradation. We find these processes make essentially no difference to the degradation behaviour in Ga doped PERC cells. Our investigations using secondary ion mass spectrometry (SIMS) find that commercial Ga doped PERC cells contain detectable levels of boron, and this could play a role in the degradation processes observed. In summary, our work generally confirms the positive effects of a transition from B to Ga doping for solar cells. Any degradation in cell performance under illumination is fairly small, but understanding the reasons for this is one of the most important questions in high purity silicon research at present. We will highlight urgent research in this area necessary to mitigate instability issues which may arise in newly manufactured modules featuring Ga doped silicon solar cells. References 1. P. P. Altermatt, Y. Yang, Y. Chen, X. Zhang, D. Chen, G. Xu, Z. Feng, "Requirements of the Paris Climate Agreement for the coming 10 years on investments, technical roadmap, and expansion of PV manufacturing". 37th European Photovoltaic Solar Energy Conference (Online, 2020), pp. 1999-2004, doi: 10.4229/EUPVSEC20202020-7CP.1.2. 2. S. W. Glunz, S. Rein, J. Knobloch, W. Wettling, T. Abe, "Comparison of boron- and gallium-doped p-type Czochralski silicon for photovoltaic application", Progress in Photovoltaics: Research and Applications 7, 463-469 (1999), doi: 10.1002/(SICI)1099-159X(199911/12)7:63.0.CO;2-H. 3. N. E. Grant, J. R. Scowcroft, A. I. Pointon, M. Al-Amin, P. P. Altermatt, J. D. Murphy, "Lifetime instabilities in gallium doped monocrystalline PERC silicon solar cells", Solar Energy Materials & Solar Cells 206, 110299 (2020), doi: 10.1016/j.solmat.2019.110299.

Czochralski‐grown gallium‐doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron‐doped silicon. Second, the stability of gallium‐doped PERC cells are monitored under illumination (>3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature‐induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID‐like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron–oxygen degradation does not influence any cells that are studied. Secondary‐ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light‐induced degradation, Ga‐doped silicon is still more stable than unstabilized B‐doped silicon under illumination.

The reduction of parasitic recombination processes commonly occurring within the silicon crystal and at its surfaces is of primary importance in crystalline silicon devices, particularly in photovoltaics. Here we explore a simple, room temperature treatment, involving a nonaqueous solution of the superacid bis(trifluoromethane)sulfonimide, to temporarily deactivate recombination centers at the surface. We show that this treatment leads to a significant enhancement in optoelectronic properties of the silicon wafer, attaining a level of surface passivation in line with state-of-the-art dielectric passivation films. Finally, we demonstrate its advantage as a bulk lifetime and process cleanliness monitor, establishing its compatibility with large area photoluminescence imaging in the process.

A method of fabricating anodic aluminium oxide (AAO) with the capability of manipulating its stored charge is reported. This method involves the use of a pulsed current source to anodise aluminium layers instead of the typically used constant current/voltage source, with the test structures experiencing positive and negative cycles periodically. By tuning the positive cycle percentage, it is demonstrated that the effective stored charge density can be manipulated in a range from −5.2 × 1011 to 2.5 × 1012 q/cm2 when the AAO is formed over a 12 nm SiO2 layer. An investigation of the stored charge distribution in the dielectric stacks indicates a positive fixed charge at the SiO2/Si interface, a negative fixed charge at the AAO/SiO2 interface and a positive bulk charge within the AAO layer. The effective stored charge density and interface states were found to be affected by annealing conditions and it is suggested that oxygen annealing can reduce the bulk positive charge while post-metallisation anneal is most effective in reducing silicon interface defects. Charge manipulation using pulsed anodisation is shown to reduce carrier recombination on boron-diffused silicon surfaces highlighting the potential of the process to be used to tune the electrical properties of dielectric layers so that they can reduce surface recombination on silicon surfaces having different dopant polarity and concentrations.

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 s^-1 at an excess carrier density of 10^15 cm^-3. Fitting injection-dependent lifetime curves requires chemical passivation and field effect passivation from a negatively charged layer with a charge density of 10^10 –10^11 q cm^-2. The slightly higher recombination velocity of 2.3 cm s^-1 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.

A central quantity to assess the high quality of monocrystalline silicon (on scales beyond mere purity) is the minority charge carrier lifetime. We demonstrate that the lifetime in high purity float zone material can be improved beyond existing observations, thanks to a deeper understanding of grown-in defects and how they can be permanently annihilated. In a first step we investigate the influence of several process sequences on the lifetime by applying a low temperature superacid passivation treatment. We find that a pre-treatment consisting of an oxidation at 1050 °C followed by a POCl3 diffusion at 900 °C can improve the lifetime by deactivating or eliminating grown-in defects. Then, pre-treated wafers of different float zone materials are passivated with three state-of-the-art layer stacks. Very high effective lifetime values are measured, thereby demonstrating the high quality of the surface passivation schemes and the pre-treated silicon wafers. The measured effective lifetimes exceed previous records, and we report an effective lifetime of 225 ms measured on a 200 µm thick 100 O cm n-type silicon wafer symmetrically passivated with a layer stack of a thin thermally grown oxide and a polycrystalline layer (the TOPCon layer stack).

Accurate measurements of bulk minority carrier lifetime are essential in order to determine the true limit of silicon's performance and to improve solar cell production processes. The thin film which forms when silicon wafers are dipped in solutions containing superacids such as bis(trifluoromethane)sulfonimide (TFSI) has recently been found to be effective at electronically passivating the silicon surface. In this paper we first study the role of the solvent in which TFSI is dissolved for the passivation process. We study ten solvents with a wide range of relative polarities, finding TFSI dissolved in hexane provides improved temporal stability, marginally better passivation and improved solution longevity compared to dichloroethane which has been used previously. Sample storage conditions, particularly humidity, can strongly influence the passivation stability. The optimised TFSI-hexane passivation scheme is then applied to a set of 3 Ω cm n-type wafers cut from the same float-zone ingot to have different thicknesses. This enables the reproducibility of the scheme to be systematically evaluated. At 1015 cm−3 injection the best case effective surface recombination velocity is 0.69 ± 0.04 cm/s, with bulk lifetimes measured up to the intrinsic lifetime limit at high injection and > 43 ms at lower injection. Immersion of silicon in superacid-based ionic solutions therefore provides excellent surface passivation, and, as it is applied at room temperature, the effects on true bulk lifetime are minimal.

Float zone silicon (FZ-Si) is typically assumed to be an extremely high quality material, with high minority carrier lifetimes and low concentrations of recombination active defects. However, minority carrier lifetime in FZ-Si has previously been shown to be unstable following thermal treatments between 450 and 700 °C, with a range of unidentified deep level states being linked to reduced carrier lifetime. There are suspicions that nitrogen doping, which occurs from the growth atmosphere, and intrinsic point defects play a role in the degradation. This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type FZ-Si samples. We find that nitrogen-doped samples experience increased degradation due to higher concentrations of deep level defects during thermal treatments compared to nitrogen-lean samples. In an attempt to explain this difference, in-diffusion of nickel has been used as a marker to demonstrate the existence of higher vacancy concentrations in the nitrogen-doped samples. The origin of the recombination active defects responsible for the thermally induced lifetime degradation in FZ-Si crystals is discussed.Float zone silicon (FZ-Si) is typically assumed to be an extremely high quality material, with high minority carrier lifetimes and low concentrations of recombination active defects. However, minority carrier lifetime in FZ-Si has previously been shown to be unstable following thermal treatments between 450 and 700 °C, with a range of unidentified deep level states being linked to reduced carrier lifetime. There are suspicions that nitrogen doping, which occurs from the growth atmosphere, and intrinsic point defects play a role in the degradation. This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type FZ-Si samples. We find that nitrogen-doped samples experience increased degradation due to higher concentrations of deep level defects during thermal treatments compared to nitrogen-lean samples. In an a...

We investigate the recombination activity of a bulk silicon defect limiting the lifetime of high qualityn-type float-zone (FZ) silicon wafers. By isochronal annealing between 200 and 1100 °C, a defect was found to become activated upon annealing at 450–700 °C, causing an order of magnitude reduction in the bulk lifetime. From photoluminescence imaging, it was evident that recombination active circular patterns were present in these low lifetime samples, suggesting the defect (s) originates from the growth conditions of the ingot. When the samples were passivated by SiNx:H films, a substantial improvement in the bulk lifetime resulted, which we postulate occurred due to hydrogenation of the bulk defects. In contrast, when the samples were annealed at high temperatures (800–1100 °C), the circular recombination active patterns were removed, and the bulk lifetime improved, with the highest lifetime achieved at an annealing temperature of 1100 °C. The experimental results suggest that the defect limiting the lifetime in this FZ material is related to a lattice-impurity defect, which can be permanently annihilated upon annealing at >1100 °C.

This paper introduces a method to manipulate stored charge density and polarity in anodic aluminium oxide (AAO) for silicon solar cell passivation. The method involves the use of a pulse current source to anodise aluminium layers instead of the typically used constant current/voltage source, with the test structures experiencing positive and negative cycles periodically. By tuning the positive cycle percentage, it is demonstrated that the effective stored charge can be manipulated in a range from -5.2×10 11 to 2.5×10 12 q/cm 2 when the AAO is formed over a thin silicon dioxide layer. Photoconductance decay measurements on symmetrical n-type test structures with ∼ 150 Ω/□ boron-diffused surfaces were used to estimate the recombination current density associated with the diffused surface ( J op+ ), which was then correlated to the stored charge. As the effective stored charge changed from positive towards negative values, the J op+ was reduced from 210 fA/cm 2 to 100 fA/cm 2 . The stored charge in various test structures was monitored constantly to study the stability. It was found that the positive stored charge deteriorated with time while the negative charge was relatively stable.

We investigate the surface passivation attained by a stack consisting of anodically grown silicon dioxide and silicon nitride. A very low surface saturation current density $J_{{\rm os}}$ of 3 fA/cm2 is attained after the silicon wafers are submersed in nitric acid under a constant bias for >30 min, followed by a silicon nitride deposition and subsequent annealing in forming gas (FG) at 400 °C. We examine $J_{{\rm os}}$ as a function of the anodic SiO2 thickness ( $d_{{\rm ox}}$ ) and show that for $d_{{\rm ox}}$ between 7 and 36 nm, $J_{{\rm os}}$ decreases monotonically from 32 to 3 fA/cm2 , respectively. From capacitance–voltage (CV) and conductance measurements, we show that this reduction in $J_{{\rm os}}$ with an increase in oxide thickness primarily results from a reduction in both the interface defect density ( $D_{{\rm it}}$ ) and the hole capture cross section (σ p ). For the lowest $J_{{\rm os}}$ of 3 fA/cm2, a low $D_{{\rm it}}$ of $\sim 2.0 \times 10^{10}$ cm−2·eV−1 and a low $\sigma _{{\rm p}}$ of $\sim 1\times 10^{-16}$ cm−2 are determined. When the anodic SiO2 films are capped by a silicon nitride film, negligible surface passivation degradation occurs, in comparison with uncapped anodic SiO2 films. Finally, we demonstrate that thermally induced bulk silicon defects can be eliminated by replacing high temperature (1000 °C) oxidations with a room-temperature anodic oxidation technique.

A recombination of active defect in very high lifetime Czochralski grown n-type silicon wafers, which can be thermally deactivated at 150 °C, is described. In addition, the existence of a recently measured defect, which is deactivated at 350 °C, is confirmed. Both defects are found to significantly degrade the lifetime of millisecond-range Czochralski-grown n-type silicon wafers: a material widely used for high-efficiency solar cells. The observed deactivation temperature suggests that it may be caused by vacancy-phosphorus pairs. The deactivation temperature of the second defect is consistent with the presence of vacancy-oxygen (V-O) pairs.

Electronic surface passivation of p-type crystalline silicon by anodic silicon dioxide (SiO2) was investigated. The anodic SiO2 was grown by light-induced anodisation (LIA) in diluted sulphuric acid at room temperature, a process that is significantly less-expensive than thermal oxidation which is widely-used in silicon solar cell fabrication. After annealing in oxygen and then forming gas at 400 °C for 30 min, the effective minority carrier lifetime of 3–5 Ω cm, boron-doped Czochralski silicon wafers with a phosphorus-doped 80 Ω/□ emitter and a LIA anodic SiO2 formed on the p-type surface was increased by two orders of magnitude to 150 μs. Capacitance–voltage measurements demonstrated a very low positive charge density of 3.4 × 1011 cm−2 and a moderate density of interface states of 6 × 1011 eV−1 cm−2. This corresponded to a silicon surface recombination velocity of 62 cm s−1, which is comparable with values reported for other anodic SiO2 films, which required higher temperatures and longer growth times, and significantly lower than oxides grown by chemical vapour deposition techniques. Additionally, a very low leakage current density of 3.5 × 10−10 and 1.6 × 10−9 A cm−2 at 1 and −1 V, respectively, was measured for LIA SiO2 suggesting its potential application as insulation layer in IBC solar cells and a barrier for potential induced degradation.

The interdigitated back contact (IBC) solar cells developed at the Australian National University have resulted in an independently confirmed (Fraunhofer Institut fur Solare Energiesysteme (ISE) CalLab) designated-area efficiency of 24.4 ± 0.7%, featuring short-circuit current density of 41.95 mA/cm2, open-circuit voltage of 703 mV and 82.7% fill factor. The cell, 2 × 2 cm2 in area, was fabricated on a 230 µm thick 1.5 Ω cm n-type Czochralski wafer, utilising plasma-enhanced chemical vapour deposition (CVD) SiNx front-surface passivation without front-surface diffusion, rear-side thermal oxide/low-pressure CVD Si3N4 passivation stack and evaporated aluminium contacts with a finger-to-finger pitch of 500 µm. This paper describes the design and fabrication of lab-scale high-efficiency IBC cells. Characterisation of optical and electronic properties of the best produced cell is made, with subsequent incorporation into 3D device modelling used to accurately quantify all losses. Loss analysis demonstrates that bulk and emitter recombination, bulk resistive and optical losses are dominant and suggests a clear route to efficiency values in excess of 25%. Additionally, laser processing is explored as a means to simplify the manufacture of IBC cells, with a confirmed efficiency value of 23.5% recorded for cells fabricated using damage-free deep UV laser ablation for contact formation. Meanwhile all-laser-doped cells, where every doping and patterning step is performed by lasers, are demonstrated with a preliminary result of 19.1% conversion efficiency recorded. Copyright © 2014 John Wiley & Sons, Ltd.

In this study, we investigate the nature of some recombination active defects limiting the lifetime in Czochralski (CZ) silicon wafers, in the millisecond range. Due to their low concentrations, the observed defects are unlikely to be identified through Deep-Level Transient Spectroscopy (DLTS) or Electron Paramagnetic Resonance (EPR), hence we use lifetime spectroscopy combine with several annealing steps to help identify the defect. We demonstrate that the defect can be deactivated by annealing above 300°C. Our experimental findings suggest that vacancy-related pairs incorporated during ingot growth may be responsible for the decreased minority carrier lifetime.

Photoconductance (PC) measurements of the diffused-region recombination-current pre-factor J 0d make the approximation of excess-carrier Δ n density uniformity. That is, from the front illuminated surface to the rear the Δ n is constant. Kane and Swanson outlined acceptable sample parameters such that the error caused by this approximation is small when performing transient J 0d measurements. For quasi-steady state (QSS) PC measurements, where there is generation, the uniform carrier-density approximation leads to a larger J 0d under-estimation compared to the transient approach. We avoid this approximation by numerically solving the J 0d accounting for depthwise generation, recombination and carrier diffusion in the quasi-neutral bulk that occurs during a QSS PC measurement. We demonstrate the application of the technique to samples with surface-diffusion sheet resistances 10–140 Ω/sq., formed by etch back, and a sample where the surface passivation degrades with damp heat exposure. We find, for the samples tested, the direct application of the Kane and Swanson method: i) under-estimates the J 0d by 2% to 8% when the sample bulk is lightly doped (boron doping of 1.3×10 14 cm -3 ), and ii) under-estimates the J 0d by 10% to 80% when the wafer is moderately doped (phosphorus doping of 1.5×10 15 cm -3 ), for samples with J 0d 350 to 2200 fA/cm 2 ; compared with an experimental uncertainty of 6%. For both samples sets the under-estimation increases as J 0d increases.

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (>23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of ~70 nm within ~30 min, comparable to the growth rate of a thermal oxide at 1000 °C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J0 of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J0 of

We determine uncertainty in photoconductance (PC) measurements of the emitter saturation current density (J0e). A Monte Carlo method is used to calculate the impact of uncertainty from the input parameters including the test equipment calibration, the intrinsic-recombination model, and the measured sample's thickness, doping, and optics. The uncertainty in the measurement of J0e is calculated, where we find that the sensitivity to input uncertainty depends on the measurement mode, transient decay, or quasi-steady-state PC. For quasi-steady-state measurements, the uncertainty in J0e is largely affected by thickness and generation uncertainty. For transient measurements, thickness uncertainty dominates the uncertainty in J0e. The wafer doping and measured voltage data has little impact on the resultant J0e uncertainty. We find, in our case study, that measurements of J0e to be accurate within 3%-6% when using the transient mode, and 4%-7% using the quasi-steady-state mode.

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 0.16–0.36 eV. 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.

A procedure to measure the bulk lifetime (100 µsec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (10(12) cm(-3)).

In this paper, we present a new method to determine the simultaneous injection and temperature dependence of the sum of the majority and minority carrier mobilities in silicon wafers. The technique is based on combining transient and quasi-steady-state photoconductance measurements. It does not require a full device structure or contacting but only adequate surface passivation. The mobility dependence on both carrier injection level and temperature, as measured on several test samples, is discussed and compared with well-known mobility models. The potential of this method to measure the impact of dopant concentration, compensation ratio, injection level, and temperature on the mobility is demonstrated.

High efficiency silicon solar cells require substrates with carrier lifetimes well into the millisecond range. High bulk lifetimes can be extremely difficult to measure accurately because of recombination at the sample’s surfaces. Although it is possible, in principle, to passivate the surface well by oxidation or by deposition of a dielectric, such surface passivation approaches often change the bulk lifetime which is being measured. This can be because: (i) the passivation occurs at elevated temperature hence creating or destroying bulk defects; (ii) hydrogen enters the bulk and passivates recombination centres; or (iii) impurities are gettered to the passivating film at the sample’s surface. It has recently been discovered that the thin film which forms on the silicon surface when samples are immersed in ionic solutions containing superacids can provide exceptional surface passivation [1-3]. As this passivating layer is formed at room temperature, the bulk lifetime of the silicon is not modified and hence the artefacts identified above are avoided. In this talk it will be shown how surface recombination velocities of 0.7 cm/s can be achieved using room temperature ionic passivation, and it will be demonstrated how this can be used to measure extremely long bulk lifetimes (>43 ms) [3]. Results of experiments which aim to establish the mechanism by which passivation occurs will be reported, and factors such as the solvent polarity, humidity and solution storage conditions will be reviewed. The talk will then highlight three applications of the ionic passivation for improving the performance of silicon materials. The first will be in the development of high efficiency interdigitated back contact (IBC) solar cells, in which ionic surface passivation is used to understand process-induced bulk lifetime degradation [4]. The second will be in the understanding of light-induced degradation which has recently been found to occur in high performance p-type float-zone silicon [5]. The final example will involve using the superacid-based passivation to measure bulk lifetimes up to and beyond the current intrinsic lifetime limit [6]. It will be shown that the true limit of the bulk lifetime of silicon must be higher than previously assumed, and results from a study which aims to re-evaluate the fundamental limit of silicon’s performance will be presented. References: 1. J. Bullock, D. Kiriya, N. Grant, A. Azcatl, M. Hettick, T. Kho, P. Phang, H. C. Sio, D. Yan, D. Macdonald, M. A. Quevedo-Lopez, R. M. Wallace, A. Cuevas, A. Javey, ACS Applied Materials & Interfaces 8, 24205 (2016), doi: 10.1021/acsami.6b07822. 2. N. E. Grant, T. Niewelt, N. R. Wilson, E. C. Wheeler-Jones, J. Bullock, M. Al-Amin, M. C. Schubert, A. C. van Veen, A. Javey, J. D. Murphy, IEEE Journal of Photovoltaics 7, 1574 (2017), doi: 10.1109/JPHOTOV.2017.2751511. 3. A. I. Pointon, N. E. Grant, E. C. Wheeler-Jones, P. P. Altermatt, J. D. Murphy, Solar Energy Materials & Solar Cells, in press (2018), doi: 10.1016/j.solmat.2018.03.028. 4. T. Rahman, A. To, M. E. Pollard, N. E. Grant, J. Colwell, D. N. R. Payne, J. D. Murphy, D. M. Bagnall, B. Hoex, S. A. Boden, Progress in Photovoltaics: Research and Applications 26, 38 (2018), doi: 10.1002/pip.2928. 5. T. Niewelt, M. Selinger, N. E. Grant, W. M. Kwapil, J. D. Murphy, M. C. Schubert, Journal of Applied Physics 121, 185702 (2017), doi: 10.1063/1.4983024. 6. A. Richter, S. W. Glunz, F. Werner, J. Schmidt, A. Cuevas, Physical Review B 86, 165202 (2012), doi: 10.1103/PhysRevB.86.165202.