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The purest substance in the world: silicon. Silicon is the purest substance that humans have ever obtained; the purest silicon available today has a purity of 99.99999999999999%. I imagine readers won’t be able to count all those 9s, so let me tell you – it’s 16 nines. However, although pure silicon also has semiconductor properties, it is a semiconductor with little practical use. To actually create usable semiconductor devices, including solar cells, it is necessary to add certain impurities to them; the most common ones are phosphorus and boron. There are also gallium, arsenic, aluminum, and some other elements. Generally speaking, the role of impurities is to adjust the energy levels of silicon atoms. Those who have studied semiconductor or solid-state physics know that, due to the crystal structure, the various energy levels of all atoms in a solid form bands; silicon can typically be divided into three bands: the conduction band at the top, the band gap in between, and the valence band at the bottom. If we use a train as a metaphor, then the conduction band is the train, the valence band is the platform, and the band gap is the space between the platform and the train. If all the free electrons are in the valence band, then this solid is an insulator; it’s like a person standing on a platform who cannot go anywhere else ； If all the free electrons are on the conduction band, then the solid is a conductor; it’s like a person getting on a train and being able to travel across the whole country. The free electrons in semiconductors are usually located in the valence band, but when subjected to certain stimuli such as heat, light, or electrical excitation, some of these free electrons can move into the conduction band, thereby exhibiting conductive properties; this is why they are called semiconductors. Silicon is such a semiconductor, but because the distance between its conduction band and valence band in pure silicon is too large (also known as a wide band gap), it’s akin to the platform being too far away from the train; it’s difficult for ordinary electrons to jump from the platform onto the train. As a result, only a very small number of electrons can be excited from the valence band to the conduction band, so the semiconducting properties of pure silicon are relatively weak, and it cannot be used directly. Impurities in silicon (I): Useful and essential impurities. To address this issue, scientists came up with the idea of adding impurities that create impurity energy levels between the conduction band and the band gap. These impurity energy levels are either very close to the conduction band (such as phosphorus); in such cases, they provide electrons and are known as donor levels ； Either they are close to the valence band (such as boron) and accept electrons, being known as acceptor levels. In this way, very small stimuli can make silicon conductive. It’s like adding some stepping stones between the station and the platform; those that are closer to the platform represent the acceptor energy levels, while those that are closer to the train represent the donor energy levels. Impurities that can provide donor levels or acceptor levels are referred to as donor impurities and acceptor impurities respectively, and these are certainly useful impurities. A typical representative of donor impurities is phosphorus, while a typical representative of acceptor impurities is boron. In my opinion, these two types of impurities are the most commonly used semiconductor impurities because their segregation coefficient in silicon is closest to 1; in other words, after doping, they allow for a uniform concentration distribution to be achieved during the growth of single crystals. The reason their segregation coefficient in silicon can be as close to 1 as it is is that their properties are most similar to those of silicon. But it is precisely for this reason that, during the purification process using physical methods, boron and phosphorus become the most difficult elements to remove. Useful impurities also need to be present in a moderate amount; too little of them results in no significant effect, while too many of them lead to excessive conductivity, making it difficult to control them and turning them into waste. Generally, the applications of different semiconductors require different ranges of impurities. For solar cell applications, the bulk density of electrons or holes should be around 1017/cm3; one can calculate the corresponding impurity concentration by themselves. Silicon doped with acceptor impurities becomes P-type; silicon doped with boron is common. Silicon doped with donor impurities is called N-type; phosphorus-doped silicon is common. For solar cells, P-type silicon is more common, as mentioned earlier, because the segregation coefficient of boron is 0.8, and it is easier to dope single-crystal materials uniformly with boron. For a solar cell to generate electricity, it needs a PN junction, which allows positive and negative electrodes to be formed under light exposure. For P-type semiconductors, an N-type junction is formed by creating a thin layer of phosphorus on the surface of the silicon wafer through a diffusion process. The relationship between impurity concentration and resistivity in pure silicon: In semiconductor-grade silicon materials, silicon is usually first purified to a very high degree of purity, such as around 11N or 10N, before doping is carried out; as a result, the impurities present in the material are relatively simple. For example, the polycrystalline silicon material used in the production of monocrystalline silicon solar cells typically has a silicon purity of 9N; boron is then doped at a level of around 1 ppma, while all other impurities are present in amounts less than 1 ppb. (I will explain later why these three elements—C, O, and N—need to be removed.) In this case, if there are changes in the impurity concentration of boron – for example, if the doping ratio is incorrect or if the crystallization process is not ideal and results in differences across different parts – it is actually possible to determine the distribution of boron concentration without taking samples from various sections of the single-crystal silicon rod. The method is simple: by measuring the distribution of resistivity, it is possible to determine the boron content in various areas. Because the boron concentration represents the concentration of charge carriers and is directly proportional to conductivity, the boron concentration at various locations is inversely related to resistivity. Similarly, for pure N-type semiconductors, the concentration distribution of phosphorus can also be determined from the distribution of resistivity. Impurity compensation and PN transition. However, in materials that contain both phosphorus and boron, such as in silicon wafers on which PN junctions have been created (in recent years, due to a shortage of silicon material, many companies import recycled silicon, leading to this situation quite frequently), there is a presence of both boron and phosphorus in the area near the PN junction. If this material has been subjected to high-temperature treatments such as annealing, the material near the PN junction will diffuse into the deeper regions on the other side, resulting in the P-type region containing phosphorus and the N-type region containing boron. At this point, the so-called “compensation” phenomenon occurs. What is compensation? To use an analogy, the boron atoms in P-type material carry a positive charge (they act as holes), while the phosphorus atoms in N-type material carry a negative charge. If these two types of impurities are present in silicon, electrons and holes will fill each other’s places, resulting in a loss of electrical conductivity; consequently, on a macroscopic level, the resistivity increases. This is the “compensation” phenomenon of donor impurities and acceptor impurities. For example, if it is originally a P-type material with a boron concentration of 1 ppma and a resistivity of 5 ohm-cm, then if 0.5 ppma of phosphorus is doped in, it will counteract the conductive effect of that 0.5 ppma of boron. The overall conductivity of the silicon material will behave as though there is only 0.5 ppma of boron present, and the resistivity may increase to 10 ohm-cm. The higher the phosphorus concentration, the more significant the offset effect; when the phosphorus concentration reaches 1 ppm, the behavior of the silicon material will be similar to that of pure silicon free from impurities, with a resistivity of several hundred or even thousands of ohms per centimeter. However, if the phosphorus concentration continues to increase, the conductivity of electrons will overcome that of holes, and N-type characteristics begin to appear. At this point, the material changes from P-type to N-type, and its resistivity begins to decrease again. As the phosphorus concentration increases, the electrical conductivity rises as well, resulting in an even lower resistivity. This is what is known as the “transformation” phenomenon during the drawing of single-crystalline silicon. The P-type material made by doping pure silicon with boron, and the N-type material made by doping pure silicon with phosphorus, are placed together in a crucible to be melted and from which single crystals are drawn. Assuming that the atomic density of boron in the P-type material is similar to that of phosphorus in the N-type material, since the segregation coefficient for boron is 0.8, which is close to 1, boron will be distributed more evenly throughout the head and tail of the single crystal rod. The segregation coefficient for phosphorus, on the other hand, is 0.36; therefore, phosphorus is present in lower concentrations at the head of the rod while its concentration is higher at the tail. As a result, overall, the head of the single crystal rod is P-type due to the higher presence of boron compared to phosphorus, whereas the tail is N-type due to the higher presence of phosphorus compared to boron ； Starting from the head, the resistivity exhibits a ‘human-shaped’ distribution that increases from low to high, reaches a very high value, and then gradually decreases. If a PN-type tester is used for testing, it will be found that the area with the highest resistivity is the place where the transition from P-type to N-type occurs. The above describes the case of pure silicon doped only with boron and phosphorus, with no other impurities present. The situation with UMG is that for polysilicon purified by physical methods, things become much more complex because in addition to boron and phosphorus being present simultaneously, metal impurities such as iron, aluminum, and calcium are also present. Even for polysilicon produced by chemical methods, the same complex situations arise, as impurities inevitably get mixed in during the processing and crystal pulling steps. In cases where electronic-grade recycled materials (secondary materials, heavily doped materials), ends and tails of single-crystal wafers, bottom residues from melting processes, as well as scraps from single-crystal and polycrystalline wafers are mixed with virgin polysilicon – which is what is referred to in today’s market as polysilicon of 9N grade or higher produced by the Siemens process – the composition of impurities becomes even more complex. Due to the currently high prices of polysilicon, I purchased monocrystalline silicon wafers produced by several major domestic companies for use in batteries and analyzed them. It turned out that in China today, no solar company seems to use pure, virgin polysilicon to manufacture solar cells; instead, all wafers are made from polysilicon that has been doped before being crystallized. It seems that the analysis of impurities in silicon is not only relevant to polycrystalline silicon produced by physical methods. At present, polycrystalline silicon that has been well purified using physical methods typically contains metal impurities in the polycrystalline silicon at levels of 0.1 ppm or less; the only metals that remain at such high levels are iron, aluminum, and calcium. In polycrystalline silicon that has not been properly purified, in addition to these three metals, there are also titanium, manganese, tungsten, cobalt, vanadium, chromium, and others. A major difference between solar cells and semiconductors is that solar cells are larger in size. A solar cell of 125x 125 mm, with an area of nearly 150 square centimeters, constitutes a large PN junction. This is unimaginable in integrated circuits. Currently, in ULSI, the size of PN junctions has been reduced to 50 nm – nanometers, that is! In fact, it should be said that the latter is the one that is unimaginable. That’s why there are great difficulties in big situations. As the PN junction becomes larger, requirements for the uniformity of the material arise. With such a large sheet, even a tiny crack that allows current to flow between the two sides will render it unusable. Silicon wafers made of polycrystalline silicon are becoming increasingly thin; the thinnest ones are said to be 180 microns thick. (A few days ago someone mentioned 120 microns, but I think that must refer to single-crystal wafers.) There are gaps between the grains in polycrystalline silicon, and if there are many metal impurities in these gaps, it can easily lead to conductivity between the two sides of the wafer during cleaning, diffusion, and sintering processes – a phenomenon commonly known as \"burnthrough.\" Therefore, metal impurities are very harmful. However, the harms of impurities go far beyond these. Later, the effect of metal impurities in physical polysilicon on the properties of silicon will be discussed. Impurities in silicon (II): Metal impurities and deep levels. Originally, pure silicon should also be used in the production of solar cells, with boron or phosphorus added for doping. But now, due to the severe shortage of silicon materials, many companies are first using recycled materials and scraps in combination, in order to reduce costs while addressing the issue of insufficient raw materials. When there isn’t enough refined grain, one has to settle for some coarse grain. As the number of manufacturers of polysilicon produced by physical methods increases, polysilicon manufactured through these physical processes has gradually become one of the main raw materials used for the production of single crystals in solar cells. Physical polysilicon, also known as UMG, contains relatively more impurities. At present, among the international manufacturers capable of achieving 5N or higher, the impurities present, aside from phosphorus and boron, are mainly metal impurities such as iron, aluminum, and calcium. In his book \"Solar Cell Materials\", Professor Yang Deren analyzed the metal impurities in single-crystalline silicon and polycrystalline silicon. The analysis is very thorough. However, the analysis in this book is based on the assumption that the atomic concentration of metallic impurities in silicon is around 10 to the 15th power per cubic centimeter, which means less than 0.1 ppma. Therefore, despite the fact that the conclusions and analyses presented in the book are very valuable, they are not entirely suitable for addressing the issue of metallic impurities in polycrystalline silicon produced by physical methods. This is because the metal impurity content in UMG is usually above a few ppm; in terms of atomic concentration, it is 10 to the 16th power, or even 10 to the 17th power, per cubic centimeter. In fact, based on investigations, there is currently no unified understanding regarding the behavior of metal impurities in UMG. This is illustrated by Xu Huabi, a doctoral student of Professor Shen Hui from Sun Yat-sen University, who provided a fairly comprehensive overview of international academic research on impurities in polycrystalline silicon produced by physical methods at the conference held in Changzhou on September 20, 2008. The author believes that the presence of metal impurities is the necessary condition for the degradation of the fabricated solar cells. The currently popular view internationally is that it is due to the presence of boroxine complexes, but the author does not agree with this; the reasons for this will be discussed in more detail in a separate article following a thorough analysis with relevant experts. Metal impurities in silicon create deep energy levels, that is, energy levels that are far away from both the conduction band and the valence band. Using the train as an analogy again, the platform represents the price band, the train represents the conduction band, and the gap between the platform and the train is the forbidden band. If the band gap is wide enough that one person cannot jump over it, then some “stepping stones” can be placed in the middle, allowing people to jump over by stepping on them. But if the gap is too large, and only one stake is placed between the train and the platform, with that stake still far away from either side, then if someone stands on that stake, they may find themselves in a difficult situation, unable to either get onto the train or back to the platform. The situation with metal impurities in silicon is similar; these impurities create deep energy levels in silicon, which are located far away from both the conduction band and the valence band. As a result, not only do these impurity-related energy levels have no effect on improving electrical conductivity, but also when carriers from other shallow energy levels (such as those from phosphorus or boron) encounter such deep-level impurities, they become \"trapped\" and find it more difficult to make transitions – either to the conduction band or back to the valence band – thereby losing their function as carriers of electricity. This is what is known as the combined effect of deep levels on carriers; the locations where these deep-level impurities are found are called “deep-level recombination centers”. The presence of composite centers reduces the lifetime of minority carriers, thereby lowering the efficiency of solar cells. If this combined effect occurs slowly under light, it leads to the so-called photo-degradation phenomenon of solar cells. In addition to photoinduced attenuation, an excessive amount of metal impurities can also lead to an increase in leakage current. Near the PN junction of a solar cell, there is a space charge region. Under normal conditions, the current in this region should be the photoelectric current, that is, the current generated by the transition of carriers upon exposure to light. However, when there are too many metal impurities, the electrons surrounding the atoms of these metal impurities are free electrons; as a result, leakage currents are generated. If these leakage currents become excessive, they can cause the PN junction to become conductive. Currently, many experts at home and abroad believe that the energy levels of aluminum are not deep-level ones. Moreover, since aluminum is an element in Group III, the same group as boron, it can also be used as a P-type doping element. In fact, in batteries made of N-type materials, aluminum is indeed used to form a PN junction through P-type junction diffusion. In fact, during purification by physical methods, aluminum is one of the more difficult metal impurities to remove. Since the segregation coefficient of aluminum in silicon is around 0.1, which is much higher than that of other metals such as iron, segregation has a relatively limited effect on aluminum. Therefore, in physically metallurgically produced silicon, aluminum is often one of the last metal impurities to be removed. If aluminum is present in silicon at concentrations of 0.1 ppm or higher, it will contribute to the decrease in resistivity, just like boron does. If silicon contains 0.3 ppm of boron and has a resistivity of 0.5 ohms per centimeter, and at the same time contains 0.3 ppm of aluminum, this may cause the resistivity to drop below 0.1 ohms per centimeter. However, it remains to be studied whether the mobility of the carriers (holes) generated by aluminum is the same as that of boron; therefore, the presence of aluminum complicates the behavior of the material. Furthermore, so-called holes or electrons can exist only when aluminum is completely dissolved in silicon in the form of a solid solution. If the aluminum concentration exceeds the solubility limit, aluminum precipitation will occur. The effect of this precipitation on the material is manifested entirely in the form of defects; at this point, the metallic properties of aluminum themselves become apparent, leading to even more complex situations. It is certain that these situations will not improve in any positive way. Since no one in the international community has conducted in-depth research on this yet, it is still advisable to remove aluminum as much as possible. As for iron, being a transition metal, it shows no apparent beneficial effects at all. According to research conducted by Pro in collaboration with several domestic universities, iron in silicon forms similar complexes with boron as well, which leads to a reduction in the lifetime of minority carriers. Moreover, the relative interaction between iron and boron can vary depending on light exposure or temperature; this phenomenon has also been confirmed through further experiments on physically grown polycrystalline silicon. However, the mechanisms and physical models behind it are still under investigation. Based on preliminary analysis, the role of ferroboron appears to be a better explanation for the photodegradation of polycrystalline silicon by physical methods than the boroxine complex theory. Iron has a very small segregation coefficient; therefore, it is relatively easy to remove through directional solidification. The reason it becomes a relatively difficult-to-remove impurity in physical-method polysilicon is mainly due to its high content in the raw materials (usually greater than 1000 ppm) and its susceptibility to contamination during the purification process. Impurities in silicon (III): Can directional solidification completely eliminate metallic impurities? When it comes to the removal of metal impurities from silicon, many people who have worked on purifying polysilicon using metallurgical or physical methods believe that directional solidification can effectively \"eliminate\" these metal impurities, which is a good approach. However, “daijin” means “almost none left.” It is worth examining in detail to what extent this character “dai” actually refers to. If the metal impurities are reduced from 2000 ppm to 10 ppm, leaving only one hundredth of a percent, it can be said that they are essentially absent in a normal sense, but this does not meet the requirements of solar cells. If it is reduced to 1 ppm, it can be said that it is practically absent; yet in reality, even trace amounts of metal impurities as low as 0.2 ppm can prevent the material from meeting the requirements of standard solar cells. Therefore, relying solely on directional solidification, the removal of metal impurities is limited. Many people believe that by performing directional solidification several times, metal impurities can be completely removed. In fact, even after 100 times of directional solidification, metal impurities cannot be reduced indefinitely. This is similar to the case in chemical reactions: when the content of impurities is low enough, the reactions that should occur often do not take place, and similarly, the effect of fractional crystallization is not as pronounced. If readers are patient enough to analyze it from the perspectives of chemical kinetics and quantum mechanics, they will be able to understand why this is the case. Physical and chemical reactions during vacuum melting. The main difference between physical purification methods and chemical ones is that in the chemical method, trichlorosilane is purified as a gas through distillation, while silicon is purified in its solid or liquid state. Whether solid or liquid, both are in a condensed state. The interactions between atoms in a condensed state are much stronger and more complex than those between molecules or atoms in a gas; the chemical bonds involved are also more stable. It is not easy to break these bonds. When the content of impurities is very low, it is not possible for complete chemical bonds to form in the same way as in ordinary molecules, which makes purification even more difficult. It is usually desirable to use certain additives to ‘seize’ the impurities from their bond with silicon, turning them into new compounds that are more volatile or precipitable, thereby making it easier to separate them from silicon. This is the case during secondary refining outside the furnace, and it is the same during vacuum melting. In vacuum melting, since precipitates are difficult to remove, a gas is usually introduced to react with the impurities and then cause them to volatilize. The gas reactions that occur during vacuum melting are essentially redox reactions that convert impurities into those that are more volatile. But the effect of doing this is actually limited. Taking phosphorus as an example, its boiling point is just over 200 degrees, and its saturated vapor pressure at 1420 degrees is 30,000 pascals. According to conventional chemical principles, applying a slight vacuum at high temperatures should be sufficient to evaporate all of the phosphorus completely. In reality, from the perspective of stoichiometric chemistry, phosphorus is indeed \"clean\", as its concentration is only a few ten-thousandths. However, in terms of the impurity content in semiconductor materials, phosphorus is the most difficult impurity to remove, and its residual concentration is often unacceptably high (above 10 ppm). Therefore, with phosphorus having such a high saturated vapor pressure, it is already difficult to remove it from silicon through vacuum melting; finding compounds that are more volatile for boron or other impurities is an even greater challenge. Similarly, when it comes to metal impurities, vacuum melting cannot either be analyzed using conventional chemical thinking and logic. In that case, it often leads people down the wrong path and wastes effort for nothing. In silicon, in addition to metallic impurities, there are also non-metallic impurities. Typically, the elements that remain in large amounts in silicon are oxygen, carbon, and nitrogen. The presence of these impurities in silicon has a profound impact on the properties of silicon materials. Starting with oxygen, in addition to that contributed by metallic silicon, quartz crucibles also play a significant role in contributing oxygen to silicon. The silica in quartz reacts with liquid silicon, producing silicon monoxide while also allowing oxygen to continuously enter the silicon melt. After solidification, due to the prolonged periods of crystal growth, annealing, and cooling, oxygen can combine with vacancies to form microdefects; it can also aggregate to form oxygen clusters, or it can lead to the formation of oxygen precipitates that introduce induced defects. All of these factors have an impact on the performance of solar cells. Since polycrystalline silicon crucibles usually have a silicon nitride coating, and there is no mechanical convection caused by the rotation of the crucible during ingot casting, the oxygen content is generally much lower than that in monocrystalline silicon. Therefore, in polycrystalline silicon, oxygen has a less significant impact on the material compared to monocrystalline silicon, especially high-purity monocrystalline silicon. However, when oxygen precipitates, it can remove some metal impurities and reduce the impurities and defects in single-crystalline silicon; therefore, at certain concentrations, oxygen can be considered a beneficial impurity. However, due to the complex composition of impurities in polysilicon, many of these impurities undergo various reactions with oxygen. The boroxine complex is more widely known. This is currently considered by mainstream experts to be the main cause of battery degradation in polysilicon cells manufactured using metallurgical methods, but I personally do not agree with this view. Oxygen reacts with iron and aluminum, forming carrier recombination centers; or it causes precipitation due to complexes formed with certain impurities, leading to lattice defects that affect the lifetime of minority carriers. These factors increase the likelihood of decay. The use of an annealing process can reduce the adverse effects of oxygen; it is generally believed that the formation of oxygen precipitates reduces the solubility of oxygen in silicon, thereby lowering its concentration as well. However, the actual mechanism should await more detailed analysis. However, studies have shown that as long as the oxygen concentration is below 15 ppmw, no oxygen precipitates will form in silicon. Since oxygen has only two electrons in its outer shell, it is theorized that oxygen is also a donor element, and thermal donors can be effectively generated within certain temperature ranges. However, although the donor impurity energy levels of oxygen thermal donors were observed in the experiments, the atomic structure and morphology underlying the formation of these thermal donors remain completely unresolved. There are now many hypothetical models, such as the 4-interstitial oxygen aggregation model, the vacancy oxygen model, the self-interstitial silicon atom-oxygen model, the diatomic oxygen model, etc., but these all still require further verification. Oxygen is currently considered to be the main culprit behind the degradation of solar cells made from polycrystalline silicon produced by metallurgical methods. The mechanism is the action of boroxine complexes bound to boron. The reasoning behind this theory is that oxygen-free or low-oxygen silicon materials do not exhibit attenuation, nor do N-type (whether doped with phosphorus or gallium) oxygen-containing silicon materials; attenuation is observed only in materials that contain both boron and oxygen. So it must be the effect of the boroxine complex. The most ardent advocate of this theory is a foreign expert named SCHMIDT. He used quasi-steady-state photoconductivity technology to study the relationship between illumination and minority carrier lifetime, and found that the defect concentration is proportional to the oxygen concentration in a nearly quadratic manner; therefore, he concluded that the boron-oxygen complex consists of X atoms of boron and 2Y atoms of oxygen. ADEY also supports this theory and conducted theoretical calculations. But in fact, with a little careful analysis, it becomes clear that this conclusion is wrong. OHSHITA in Japan demonstrated that boroxine complexes cannot exist stably in silicon. After conducting attenuation tests on Pro’s physical method polycrystalline silicon solar cells and performing a thorough analysis, Professor Chen Chao from Xiamen University concluded that boron-iron complexes play a key role in the light-induced attenuation, especially during the recovery process after exposure to light. Professor Cui Rongqiang from Shanghai Jiao Tong University also raised doubts about the theory of boron-oxygen complexes once while traveling back to Shanghai from Changzhou in a car with the author. He said that in polycrystalline silicon produced by solid-state metallurgical methods, the boron concentration is only 1 ppm, while the oxygen concentration is 10 pmm. In the solid state, there are millions of silicon atoms surrounding each boron atom; for this boron atom to combine with an oxygen atom, it must traverse dozens of atoms, and there is also an issue related to directionality. The attenuation test shows that the minority carrier lifetime increases again when annealed at 200 degrees. The boroxine complex theory suggests that boron and oxygen separate again. How can boron and oxygen combine and separate so freely time and again under the constraints of a solid crystal lattice? I firmly believe the old gentleman’s questions are valid. But despite the doubts, finding a theory that can provide an explanation is the correct path in scientific research. In my opinion, the contribution of oxygen to attenuation lies in the lattice defects caused by its own precipitation and changes in the chemical bonds formed with silicon, and these defects can be restored during annealing. A more in-depth exploration is not carried out here due to its high theoretical nature. Impurities in silicon (IV): What is a “monkey butt”? After the previous blog post was published, someone asked what a monkey butt is Let me add a response here. “\"Monkey butt\" refers to the scan distribution graph of the minority carrier lifetime for silicon wafers. According to SEMILAB’s testing method, colors are used to indicate the length of the minority carrier lifetime: the red end represents a shorter minority carrier lifetime, while the blue end represents a longer one. The “monkey butt” phenomenon in physically deposited polycrystalline silicon refers to circular red areas in the middle of the silicon wafer, which resemble a monkey’s buttock, hence the name. I heard this term for the first time from Mr. Zheng Zhixiong, the owner of Nan’an Sanjing in Fujian Province. I think if this name has any intellectual property rights, they probably belong to him. The reason for the monkey butt phenomenon is the high level of impurities in metallurgical silicon. When growing single crystals from silicon materials produced by metallurgical methods, the edges of the silicon rod crystallize first compared to the center (on the plane where the silicon wafers are cut). Due to the segregation effect, impurities accumulate in the central area, resulting in a higher concentration of impurities in the middle of each silicon wafer. Therefore, the minority carrier lifetime in the silicon wafer is shorter in the center while it is longer at the edges; as a result, a \"monkey butt\" shape is formed on the minority carrier lifetime scan graph, as shown in Figure 1. Figure 1: “Monkey butt” The impurities that cause the “monkey butt” phenomenon are likely to be metal impurities, especially iron, aluminum, calcium, and phosphorus. For polysilicon produced by chemical methods, this phenomenon is not apparent because there are fewer impurities, so it does not occur. However, it was inaccurate for Zheng Zhixiong to say at the time that \"monkey butts\" were an inevitable phenomenon in the metallurgical method for producing polysilicon. In fact, in the silicon material produced by Pro in June, this phenomenon no longer existed after crystal pulling and slicing, as shown in Figure 2. It can be seen that physical polysilicon can also avoid the \"monkey butt\" phenomenon; the key lies in whether the impurities can be thoroughly removed. Figure 2: Scanning plot of minority carrier lifetime for the polycrystalline silicon produced by Pro’s metallurgical method. Carbon in silicon – There are two sources of carbon in silicon; one comes from metallic silicon. If oxygen blowing is insufficient during the production of metal silicon, some carbon elements may be introduced into the silicon. Additionally, in polycrystalline silicon and monocrystalline silicon furnaces, graphite heating elements and carbon fiber insulation are commonly used, which leads to the volatilization of carbon vapor at high temperatures; this vapor then enters the silicon, increasing its carbon content as well. However, since the segregation coefficient of carbon is only 0.07, during directional solidification, carbon will accumulate at the top of the silicon ingot or at the bottom of the single-crystal silicon crucible. Carbon is also a group IV element, in the same group as silicon; therefore, C does not exert a donor or acceptor effect on silicon. However, the presence of carbon also affects the properties of silicon. Generally, during the growth of Czochralski single crystals and polycrystalline silicon ingots, carbon itself finds it difficult to form precipitates, and it is also hard for it to react with oxygen to produce oxygen precipitates or carbon-oxygen complexes. However, when annealing is carried out from high temperature to low temperature and then back to high temperature, both the carbon concentration and the oxygen concentration in silicon change. Therefore, some experts speculate that during the annealing process, carbon and oxygen may combine with each other or facilitate the formation of oxygen precipitates, as carbon atoms can often serve as the core for such precipitates, leading to the formation of primary oxygen precipitates. However, this precipitation is unstable; at high temperatures, it dissolves again, causing the carbon and oxygen concentrations to rise once more. Although there is a theory suggesting that, due to their small atomic radius, carbon atoms can easily cause lattice distortion, leading to the preferential accumulation of oxygen atoms in the vicinity and the formation of heterogeneous core sites of oxygen precipitates, which can have a positive effect on the material. However, if there is an excess of carbon, it will react with silicon to produce a certain amount of silicon carbide. The deposition of silicon carbide leads to lattice dislocations, which create recombination centers for deep-level carriers, thereby affecting the lifetime of these minority carriers. This negative impact may be much greater than the positive impact of elemental carbon atoms. Nitrogen in silicon: The presence of nitrogen in silicon seems to bring more benefits than drawbacks. Nitrogen can increase the mechanical strength of silicon materials, suppress microdefects, and promote oxygen precipitation. Mr. Que Duanlin from the **Key Laboratory of Silicon Materials at Zhejiang University was the first to develop single-crystal pulling in a nitrogen atmosphere, by taking advantage of these properties of nitrogen. However, in the physical method for producing polycrystalline silicon, since nitrogen is often used as a protective gas, and the silicon nitride in the crucible coating reacts partially with silicon at high temperatures, or silicon nitride particles enter the silicon melt directly, this leads to the formation of fine grains, an increase in the number of grain boundaries, and ultimately affects the performance of solar cells. During the crystallization of polysilicon, nitrogen can also react with oxygen to form nitroxygen complexes, affecting the electrical properties of the material. However, since the nitrogen-oxygen complex is a shallow energy level and the solubility of nitrogen is very low, its impact on the material is not significant. Generally speaking, if the impurity concentration of elements such as C, O, and N can be kept below 10–20 ppm, then there are no adverse effects for silicon materials used in solar applications. This conclusion may differ from that of certain “authorities,” but it is drawn from practice. I believe that the technical managers at many solar cell factories and single-crystal factories are well aware of this. Reducing these elements to below 20 ppm is not a very difficult task. It is mainly because these elements are quite reactive and tend to form compounds, which are then removed from the silicon material. Other impurities in silicon include titanium, tungsten, manganese, etc. Due to their own properties, these impurities combine with elements such as oxygen, hydrogen, and nitrogen, which thus leads to relatively complex situations. In summary, the effect of metallic impurities in silicon materials is a new issue arising from physical polysilicon, and it is also a problem worth studying. Research and analysis of these phenomena are worth the effort, whether by companies using physical purification methods or by relevant research institutions. Most importantly, it is still necessary to remove metal impurities as thoroughly as possible. This issue is not a problem in the purification process of the Siemens method, nor should it always be an issue with polycrystalline silicon produced by physical methods. Moreover, both theoretically and in terms of practical processing, it is possible to purify metal impurities to a level where no side effects occur.