Solar energy is radiant light and heat from the Sun that is harnessed using a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, solar architecture, molten salt power plants and artificial photosynthesis.
Four pieces of 100 mmx 100 mm large area heterogeneous substrate crystalline silicon thin film solar ENERGY that have undergone uniform recrystallization can be placed side by side to prepare a 400 mm wide solar cell module.The larger area of 200 mmx 200 mm ceramic substrate crystalline silicon thin film solar cell also has better performance m ( see section 2.3.5 ).
We have discussed in detail the relationship between temperature gradient, growth morphology and defect density in ZMR process.According to these discussions, the precise control of the temperature gradient on the crystal surface is very important for the growth of high-quality ZMR films.
Although it is not easy to directly measure the value of the temperature gradient, it is possible to indirectly measure the temperature gradient by changing the reflectivity of molten Si.Robinson and Miaou LIS used numerical simulation to calculate the relationship between the temperature gradient on the crystal surface and the width of the melting zone [ 86 ]: The smaller the melting zone, the lower the temperature gradient.This conclusion is consistent with the experimental observation of the crystal surface morphology [ 87 〃 4 〃 Therefore, image analysis can detect the width of the melting region and thus realize ZMR process control.
Wong and Miaou LIS first reported that this method was used to control the recrystallization of Ga films, and their system realized the control of the two parameters of crystal plane position and crystal plane pattern.Although the control of crystal plane position has been successfully confirmed, the system is subject to unstable illumination conditions and relatively long sampling time.
Kawa Ma in mitsubishi electric, Japan, etc. realized the process control of ZMR Si thin film growth using CCD camera and commercial width analyzer.2］。Their experiments confirmed that the width of the melting region is a useful parameter for controlling the morphology of the crystal surface, and claimed that the width of the melting region can be stabilized within 10 % ( usually about 0.1 MnI ) [ 64 ].
More advanced ZMR process control is implemented in the equipment described in 220.127.116.11 Section [ 76 ].Images can be obtained by several small CCD cameras.The application software realizes the process control, including the analysis of the width of the melting zone and the closed - loop feedback to control the linear tungsten halogen lamp.
For standard substrates, automatic control can keep the width of the melting zone within 5 % of the ideal value, and ZMR process control can successfully reach a scanning speed of up to 400 mm / min [ 98 ], the ZMR results have already been discussed in previous chapters.At this scanning speed, manual control can no longer be used.
Automatic Control Data Recording of MM Length Zn4R Process Samples.If the wafer is not protected, the width of the melting region will decrease at the edge, forming an oval ZMR seed layer.
Actual. 16 In the automatic control ZMR process, the power of the heater at the top of the linear tungsten halogen lamp and the width of the melting zone are a function of the position of the melting zone.A wider gray line is the ideal value for the width of the melting zone, while a thinner black curve is a measure of the width of the melting zone.The PID controller can adjust the power of the top heater to reduce the difference between the ideal value and the measured value of the width of the melting zone.
Zone melting recrystallization on ceramic substrates
The discussion in the previous chapter shows that even if monocrystalline silicon or polycrystalline silicon wafer is used as the model substrate, the quality of Si thin film grown by zone melting recrystallization ZMR also depends on several parameters of the substrate.If a non-ideal ceramic substrate is used, ZMR will be more affected by the poor characteristic parameters of the ceramic substrate.Different from the Si model substrate, the following aspects of the inherent material properties of the ceramic substrate are of great significance to the preparation of heterogeneous substrate crystalline silicon thin film solar ENERGY by ZMR process:
Surface Roughness: Si substrate can be smoothed by simple isotropic etching to a microscopic level. The etching solution can be CP 133, prepared from HF ( 50 % ), HNOS and CHS COOH in a ratio of 1: 3: 3.However, the surface of the ceramic substrate is usually not changed by chemical etching and will remain in the state reached after sintering.The surface roughness of the ceramic substrate depends on the substrate material and production process' average value is in the o 110 mm range, and the surface roughness of the ceramic that can realize cost-effectiveness usually needs to be higher than 2mm.The minimum thickness of the seed layer should be 3 times greater than the value of the surface roughness.
Open porosity and pore diameter: - Some ceramic types, especially cast ceramics, have an open porosity of 30 % or more.Open porosity refers to the accumulation of pores to form an open path through the entire ceramic substrate.If the density of the intermediate layer is not large, the liquid phase Si may be lost in the ZMR process by entering the substrate through those open pores, i.e. Si in the seed layer is completely absorbed by the ceramic substrate with high open porosity.What is more important than open porosity is ceramic benefit?The average pore diameter of the bottom.Because of the high surface tension of liquid phase, the tendency of absorption by ceramic substrate decreases with the decrease of pore diameter.However, a suitable intermediate layer can shield all open pores on the surface of the ceramic substrate.Make ZMR more reliable.
Thermal expansion coefficient tec: ceramic 7tc is usually different from si, and depending on the type of ceramic, the tec of a12o3 ceramic is even twice as large as si.Even if the substrate materials such as SiC or Si3N4 ceramics are more compatible with the TEC of Si, there is a considerable deviation from the TEC of Si.In order to avoid cracks or bubbles, the difference in length caused by the 7Tc difference between the seed layer and the ceramic substrate is less than 0.1 % of the total length over the entire temperature range.
Thermal conductivity: due to the difference in composition ( porosity is the most serious problem ), the thermal conductivity of ceramic substrates will be different locally.This will cause the temperature difference of recrystallization surface and non-ideal ZMR characteristics in ZMR process.
In addition to the inherent effects of these materials, geometric problems can also limit the results of ZMR.Warping of the ceramic substrate can occur before or during ZMR, the seed layer will deviate from the focus of the linear halogen lamp, changing the power distribution and lateral uniformity, and the thickness variation of the ceramic substrate may cause the same effect as the aforementioned local thermal conductivity variation.
Here, some experimental results of ZMR process on suitable ceramic substrates and unsuitable ceramic substrates will be illustrated.
The SiC interlayer was prepared by plasma enhanced chemical vapor deposition PECVD on the reaction bonded SiC Rb SiC ceramic substrate by tape casting, and the ZMR 400 CON formed Si seed layer.The maximum grain size is several mm wide and the local length is almost the same as that of the substrate.TEC is more matched, warpage is lower than critical value, and the influence on ZMR is limited.Only an edge width of about 1cm is affected and cut off after ZMR.
ZMR was carried out on the reaction - bonded silicon nitride RBS in ceramics prepared by hot pressing with a thickness of 1 mm, and the ZMR results appeared to be perfect at first sight, with very large crystal grains and almost ideal structure.The reason for this negative effect is the slight difference between the ceramic substrate and the Si layer TEC.Although the crack is small, it has a great influence on the performance of the heterogeneous substrate crystalline silicon thin film solar cell ( see section 2.5.3 ).
The ZMR process and epitaxially grown Si film on the thick cast Si3N4 ceramic substrate and Ono interlayer form a melting region with non-uniform width.In the area surrounded by white lines, the power distribution is high enough to melt Si parasitic deposition on the back surface of the sample.Because of the low temperature gradient formed, the crystalline structure of the recrystallized Si film in this region has the characteristics of small grains.Small plane growth was observed in some areas of the sample, and crystal growth had a certain crystal orientation.In this case, the large grains are several mm wide and several cm long.Non - uniform thermal performance results in non-uniform ZMR results in two aspects:
Incomplete mixing of the initial powder and non-uniform sintering conditions results in non-uniform tape casting of Si3N4;Serious warpage makes the back surface of the sample only partially contact with the Shi Ying plane carrier during ZMR process, and the overheating area formed melts the parasitic deposition on the back surface.
To sum up, in order to successfully realize ZMR process on ceramic substrate, the substrate / interlayer system needs to be reasonably selected.In this way, ZMR can form Si thin films with sufficient grain size and defect density, so that the conversion efficiency of the heterogeneous substrate crystalline silicon thin film solar cell can reach far more than 10 %.
The basic process for preparing the heterogeneous substrate crystalline silicon thin film solar cell is to deposit Si layer, while the chemical vapor deposition CVD process using Si HCl 3 as precursor gas is a better method.We will first introduce the advantages of Si deposition CVD process, then explain the basic chemical principle of Si HCl 3 _ H2 system, then discuss the development of real reaction chamber and process, finally explain the application of the process to ceramic substrate and ZMR growth of Si layer, and focus on the influence of inappropriate substrate on deposition quality.
Requirements for silicon deposition process
Almost all the research and development efforts in the solar energy industry are focused on reducing the unit evaluation and cost.Cost - effectiveness is also the most important requirement for future Si deposition equipment and processes.Through simple calculation, the upper limit of the cost of depositing Si layer is 30 / m2, and the deposition process is required to realize high-quality Si thin film in order to prepare a heterogeneous substrate crystalline silicon thin film solar cell with great commercial value ( see Section 2.4.3 ).The basic requirements of Si deposition equipment and process are:
High production rate ( 510m2 / h );
High growth rate of 05 mm / min );
the Si precursor has high chemical yield;Rectangular or square wafers can be prepared;
can be deposited on rough and porous ceramic substrates;
Sufficient film quality ( the diffusion length of the epitaxial layer exceeds 23 times the film thickness );controllable doping distribution;
Simple device configuration.
Of all the deposition methods?The atmospheric pressure chemical vapor deposition APCVD with Si HCl 3 as precursor gas can best meet the above requirements.Chemical vapor deposition ( CVD ) can decompose the source gas containing Si on the heated sample surface so that Si atoms can grow the required thin film.CVD can operate at different process temperatures and pressures.The deposition temperature of apcvd is up to 1300 c, which is suitable for mass transport and reaches a deposition rate of 10 mm / mm ..Generally, Si HCl 3 highly diluted in H2 is used as a typical S source gas. In - line doping can be realized by adding doping gas, while B2H6 and PH3 diluted in H2 can realize P - type doping and N - type doping, respectively.Apcvd process is more suitable for epitaxial deposition. higher process temperature allows deposition atoms to be optimally arranged in the crystal matrix and crystals with particularly low defect density can be grown.The Si epitaxial growth of APCVD requires a temperature range of 95012250 C ..Compared with other deposition technologies with high vacuum and complex system, APCVD technology at atmospheric pressure is simpler and can realize continuous on-line system.
In the microelectronics industry, CVD is a key technology and widely used in Si epitaxial deposition.In this field, good crystallization quality, high purity, thickness uniformity and doping uniformity in the range of 2 % are required [ 1 7 \ CVD in the microelectronics industry will use a batch system ( cylindrical reaction chamber ) or a single wafer reaction chamber to operate at a high gas flow rate to achieve the required thickness uniformity and doping uniformity, but the chemical yield and production rate are low and the cost per m2 is high.Even with high-cost deposition techniques, Si deposition accounts for only a small portion of the total cost of the final microelectronic device.From this point of view, the development of Si deposition reaction chambers with high production rate and low cost will not arouse the interest of the microelectronics industry.On the contrary, the Si deposition process accounts for a large part of the final cost of the heterogeneous substrate crystalline silicon thin film solar cell, and the development of a cost-effective Si deposition reaction chamber is the main problem to be studied.At present, the most commonly used technology for preparing Si layer of heterogeneous substrate crystalline silicon thin film solar cell is APCVD using Si HCl 3 as precursor gas.
Characteristics of atmospheric chemical vapor deposition
The Si growth process of atmospheric chemical vapor deposition ( APCVD ) is a complex problem, which requires coupling gas transport phenomenon and chemical reaction on the substrate surface and gas phase.A general model describing the entire deposition process requires solving several coupled partial differential equations.Because of the huge complexity of this problem, many researchers ignored gas-phase fluid mechanics and confined their analysis to a simplified model.The development of improved models and simulation tools is still the current research topic [ 1 81W ].
The Si deposition of heterogeneous substrate crystalline silicon thin film solar ENERGY has two key characteristic parameters that affect the production rate and cost of the deposition process: growth rate and chemical yield.These two parameters need further discussion.
The precursor of atmospheric chemical vapor deposition apcvd is si HCl 3 - Hz system, and the corresponding growth rate, i.e. deposition rate, depends on deposition temperature, vapor composition and total gas pressure, which can be easily verified by experiments.In order to determine the optimum working point and the optimum geometric design of the reaction chamber, it is inevitable to establish an analytical model for the deposition reaction.With regard to the Si deposition process of decomposing Si HCl 3 at high temperature in a transverse atmospheric reaction chamber, HA Buka has established a theoretical growth model and has been verified by experiments.The chemical reaction to form Si deposition has two reaction steps:
Si HCl 3 molecules are chemisorbed after colliding with the substrate surface to produce SiC L2 and volatile HC1.SiC L2 molecules react with H2 to produce HC1 and solid Si that forms crystals.
In a simplified form, the overall chemical reaction can be described as:
A simple calculation can give the expression of growth rate, depending on the concentration of & HC13 and H2, and also related to the reaction rate of chemisorption and decomposition.
The main parameter describing the reaction is the C1 / H ratio, which is determined by the composition of the process gas and remains unchanged during the reaction.The influence of temperature and C1 / H ratio on growth rate can be deduced.
The growth rate curve depending on the C1 / H ratio can be divided into three characteristic ranges.
When the ratio of C1 / H is less than 0.5 %, the growth rate increases rapidly with the increase of C1 / H ratio.A smaller change in gas composition will cause a larger change in growth rate.
With the further increase of C1 / H ratio, the growth rate reached a certain saturation level.The increase of C1 / H ratio will only make a slight change in the growth rate.
Finally, the growth rate even decreases with the increase of C1 / H ratio.
The applicable conditions for this ha buka model include a wide temperature range of 800 - 1120' c and a certain gas molecular weight ( 2.7x10 - 3 - 11 xlcr 3 kg / mol ), covering the operating range of typical industrial apcvd processes.
A similar model describes the growth rate of Si HCl 3 as a precursor gas in a lateral single wafer reaction chamber.The predicted growth rate was also successfully confirmed by experiments.
The experimental results of APCVD and the simulation of growth rate can be concluded:
When Si HCl 3 is used as Si precursor gas, the C1 / H ratio needs to be higher than 2 % to realize the deposition process with high growth rate and better tolerance for parameter change.The average deposition rate of more than 5 mm / min can be achieved only when the temperature is required to be > 1150 c.
The chemical yield of atmospheric pressure chemical vapor deposition APCVD, i.e. the conversion efficiency of A, is defined as the conversion efficiency of initial vapor phase Si to solid phase Si.Comparing the initial Si / Cl molar ratio with the final Si / Cl ratio at the thermal equilibrium deposition temperature, the theoretical consumption of Si HCl 3 in Si deposition reaction can be given.The initial Si / Cl ratio using SiH CLA as the precursor gas was 0.33.If the final Si / Cl ratio exceeds this value, Si etching occurs instead of Si deposition.If the final Si / Cl ratio is less than 0.33, Si deposition occurs.The final Si / Cl ratio depends on the process temperature and the initial gas composition, i.e. C1 / H ratio.Chemical yield is defined as:
In the formula, ( Si / Cl ), and ( Si 8: L ) F are the initial and final Si / Cl ratios, respectively.
Assuming that in the heat balance state, the chemical yield can be calculated by the & and C1 partial pressure with respect to temperature and C1 / h ratio.The effect of temperature and C1 / H ratio on chemical yield can be evaluated by heat balance state calculation [ 113 ].The ratio of C1 / H and temperature determine the maximum chemical yield to reach chemical equilibrium.The lower the C1 / H ratio, the greater the chemical yield at a given temperature.When the temperature rises, the chemical reaction accelerates and the chemical yield increases.High process temperature and low C1 / H ratio can achieve the maximum chemical yield.The growth rate requires a minimum C1 / H ratio of 2 %, and then the maximum chemical yield for chemical vapor deposition CVD process to reach chemical equilibrium can reach 70 %.The actual parameter selection for preparing Si layer also depends on the cost of Si precursor and H2.
Development of Equipment and Technology
Development of Reaction Chamber
Epitaxial reaction chambers used in microelectronics are not suitable for preparing heterogeneous substrate crystalline silicon thin film solar ENERGY due to cost reasons ( see section 2.4.2 ).The equipment research of Si deposition for heterogeneous substrate crystalline silicon thin film solar ENERGY focuses on:
Large - area batch reaction chamber:
Continuous on-line reaction chamber with high production rate.
Several research teams have developed reaction chambers for heterogeneous substrate crystalline silicon thin film solar ENERGY, so that the S deposition process requirements in Section 2.4.1 can be met.The technology is the SiH CLA - H, system atmospheric pressure chemical vapor deposition APCVD.
Kunz et al reported an optically heated epitaxial reaction chamber that can deposit wafers with an area of 40 cm x 40 cm in batches.The substrate is placed downward in the inclined stainless steel cube cavity parallel to the larger surface.Both sides of the substrate carrier of the reaction chamber have Shi Ying windows, which are at a certain distance from the substrate.The heater is a halogen crane lamp.The process gas enters near the lower Shi Ying window, is parallel to the substrate surface, and flows down the Shi Ying window.When the gas flow is heated, it rotates at the lower end of the reaction gas and flows to the gas outlet located at the upper end of the same cube surface of the gas population.Due to convection, the gas flow now flows directly below the surface of the substrate.In the process of gas flowing toward the outlet, Si deposition occurs on the surface of the substrate.Since the stainless steel cube reaction chamber is inclined, the mechanism that can avoid the depletion effect o of various gases in principle is called convection coil.So far, no further experimental results have been reported.
Rodriguez et al [ 115 ] studied batch-type APCVD reaction chambers, each batch can deposit 60 substrates of 10 cm x 10 cm at the same time.The substrate was mounted on an inclined vertical side wall with a 50 cm long graphite square as the base.Six such bases are mounted in parallel arrays with each other and are directly electrically heated.The process gas enters the reaction chamber from below the pedestal array, flows upward perpendicular to the surface of the array, passes through the gap between the pedestals, Si deposition occurs on the substrate surface, and finally exits from the top of the reaction chamber.The gas depletion effect and the temperature effect need to be balanced by the configuration, especially the inclination of the side wall of the base.In order to reduce the process cost, a feedback loop is designed to reuse the process gas.
The third published study of high production rate reaction chamber comes from FHG - ISE of Hoff Solar System Research Institute in Flawn, Germany.Hurr Le et al [ 116 ] reported the CVD reaction chamber with high production rate. This project has advanced equipment configuration in cooperation with German Centro Therm:
In CON CVD with continuous chemical vapor deposition, the continuous movement of the substrate ensures a high production rate.N2 gas curtain separates the reaction chamber atmosphere from the laboratory environment;
Resistance heating to form high temperature uniformity and optimized power utilization;It operates in a depleted state and has a high chemical yield.
In such CON CVD, the sample carrier is continuously sent into the cavity, and continuous on-line deposition with high production rate can be realized.The reaction chamber in which deposition occurs includes a frame and front and back baffles respectively connecting the gas inlet and the gas outlet.Together with the moving sample, a closed reaction chamber is formed to allow Si deposition of the injected process gas on the substrate.After Si deposition, the sample enters the cooling area and cools down before leaving the reaction chamber.The maximum width of the sample suitable for CON CVD process is 200 mm, and the film of almost any thickness can be grown by adjusting the transport speed.
The movement of the sample causes the deposition process to run under depletion conditions, resulting in a higher chemical yield, thus achieving a higher thickness uniformity of the Si film.The CON CVD reaction chamber can achieve better basic characteristic parameters:
The average deposition rate was 5
The length of the reaction chamber was 400 mm;
the width of that substrate is 200 mm;
For the Si film of 20, the production rate is 2.1M2 / h.
Among the above three kinds of reaction chamber concepts, only CON CVD has obtained the experimental data of Si deposition [ 116 ]:
The optimum chemical yield is 30 % ( the theoretical limit is close to 90 % according to C1 / H ratio );The uniformity of transport direction is close to 100 %.
The thickness deviation of the substrate width in the vertical transport direction is less than 20 %;The average deposition rate was 3.3 PTM / min.
The solar cell fabricated by depositing a 20 - thick Si film on a highly doped polysilicon substrate has the highest conversion efficiency of 12.
The heterogeneous substrate crystalline silicon thin film solar cell requires two different types of Si deposition processes.The first kind of deposition was prepared on the substrate / intermediate layer, and the grain size of the polycrystalline silicon thin film was on the order of magnitude.The subsequent zone melting recrystallization ZMR process requires such a Si layer to satisfy:
Thickness 15 FZM;
The thickness uniformity is 90 % 95 %;
Good structural integrity, i.e. no whiskers or voids.
A high doping concentration of 〉 1019 / cm3 is usually required to realize the back surface field BSF of the heterogeneous substrate crystalline silicon thin film solar cell.If the SiO _ 2 interlayer is prepared by plasma enhanced chemical vapor deposition PECVD, the process temperature for ZMR and Si deposition needs to be less than 1000 C to prevent SO2 from being decomposed by H2 and the dielectric layer from being damaged.The deposition at low temperature is determined by reaction kinetics, and the growth rate of Si HCl 3 is relatively low ( about 1 mm / min ).In the batch reaction chamber, the flow rate of precursor gas is low, which can avoid depletion of process gas and achieve higher thickness uniformity.
After ZMR, the coarse-grained Si seed layer needs a second Si deposition process, i.e. epitaxial thickening at a low doping concentration level, in order to prepare the absorption layer of the heterogeneous substrate crystalline silicon thin film solar cell.The high and low junctions formed on the back surface of the epitaxial layer can reduce the surface recombination of the back surface photogenerated electron-hole pair.Because the epitaxial layer determines the characteristics of the crystalline silicon thin film solar cell with heterogeneous substrate, its electrical quality and crystalline quality are more important.
The crystal quality of the epitaxial layer depends on the substrate material, cleanliness before epitaxial deposition and process conditions. The epitaxial layer has a higher defect density than the standard silicon wafer, but the typical O concentration is lower than that of monocrystalline silicon [ 118 ] The defects of the epitaxial layer can be classified into two categories according to their source:
Defects related to the substrate: grain boundaries or other defects in the crystal on the substrate surface will continue to appear in the epitaxial layer;Defects related to process conditions: stacking faults and punctures in epitaxial layers are common.
Of all defect types, stacking faults have the most harmful effect on the electrical quality of epitaxial wafers.The stacking faults can generally be attributed to organic impurities or metal impurities present on the substrate surface [ 12.Incomplete cleaning before epitaxial growth and unclean process conditions ( wafer transfer, laboratory cleanliness, process gas purity, etc. ) will cause contamination of the sample surface.
The process development of epitaxial layers needs to focus on both the economic benefits of the deposition process ( chemical yield, production rate, etc. ) and the electrical quality of epitaxial layers, and usually takes a trade-off between the two requirements.The wet chemical cleaning of the sample is an example of trade - off: the standard three-step RCA cleaning method is relatively time - consuming, and the surface after cleaning the monocrystalline silicon substrate has the lowest defect density.However, the one-step etching and cleaning method of KOH or CP 133 will only result in slightly inferior crystal quality, while the performance of the prepared heterogeneous substrate crystalline silicon thin film solar cell is still better [ 121 ].The effect of epitaxial growth rate on solar cell performance has also been similar [ 2 ].The growth rates of 5 mm / mm and 9 mm / min were compared.Although the 9pm / min growth rate process will result in higher defect density, the corresponding solar cell conversion efficiency is only 5 % less than that of the 5 ptm / min growth rate process.In terms of economic benefits, this process with a production rate twice as high will be adopted.These two examples show that reasonable trade-offs are feasible in many cases.Moreover, for heterogeneous substrate crystalline silicon thin film solar ENERGY, the quality of epitaxial layer is very dependent on the quality of seed layer, so considerable research and development efforts focus on ZMR process.
Silicon deposition on ceramic substratesPhotocatalytic water treatment: solar energy applications
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