Polycrystalline Silicon Wafers Research Articles

Carbon concentration variation in polycrystalline silicon wafers

Carbon concentration variation in polycrystalline silicon wafers

Fracture Behavior of Silicon Cut with a High Power Laser

AbstractThe fracture twist test is used to obtain the statistical fracture strength distribution for 10-cm square single crystal and polycrystalline silicon wafers cut with a high-power Nd:YAG laser. Tensile wafer edge stresses at fracture are calculated using nonlinear finite element analysis, and the model results are used to examine the limitations of linear torsion and plate theories. The basic hypothesis is that fracture strength of laser-cut wafers is limited by microcracks formed by large residual tensile stresses produced in the cut edge upon cooling after cutting. Differences are found between single crystal CZ and polycrystalline EFG silicon material Weibull parameters characterizing the fracture strength distribution. These indicate that there is a statistical influence of material variables on the fracture strength of the EFG silicon, which lowers its strength and increases the variance of fracture response in comparison to single crystal silicon.

Combined impurity gettering and defect passivation in polycrystalline silicon solar cells

Polycrystalline silicon wafers have been subjected to annealing (700 °C, 1 h) and to a hydrogen plasma (350 °C, 30 min) during the processing of solar cells. The annealing treatment enhances the bulk minority-carrier recombination lifetime by 19%, presumably by impurity gettering. The plasma treatment improves the lifetime by 26%; hydrogen passivation accounts for at least 2/3 of this improvement. Gettering and passivation are found to be complementary: application of both treatments results in a 43% increase in lifetime compared to standard.

Gettering in polycrystalline silicon solar cells

Polycrystalline silicon wafers were subjected to extra thermal treatments (0–120 min, 500–800 °C) during the processing of solar cells. It was found that the bulk effective minority carrier diffusion length was enhanced significantly upon annealing, provided the front and back sides of the wafers were doped with phosphorus and aluminium respectively, prior to the anneal. The optimum anneal temperature was 700 °C, yielding an increase of over 10% in diffusion length and a solar cell efficiency improvement of 0.5% absolute, compared with standard cells. Light-beam-induced current measurements have shown that the thermal treatment is most effective in regions with an initially small minority carrier diffusion length. In a separate model experiment with intentionally contaminated wafers, the concentration of metallic impurities such as gold, nickel and copper in the aluminium-doped layer was found to be 1000 times larger than the solid solubility of these metals in silicon at 700 °C. From these observations, it is concluded that gettering of impurities occurs during the annealing and that the aluminium-doped layer (produced by low cost screen-printing process) provides an effective sink for impurities.

High-temperature complex structures in solar-grade poly-Si

The influence of high-temperature thermal treatment on solar-grade polycrystalline silicon wafers was studied. It is shown that after thermal treatment at very high temperatures (about 1250 degrees C), co-precipitation of carbon and oxygen occurred, thus forming specific (C,O) complexes stable at high temperatures. The characteristic broad asymmetric peak of these complexes in an IR spectrum is similar to the respective one of SiC and therefore its formation is often attributed to pure SiC formation.

Focused Ion Beam Fabrication of Micro-Mechanical Parts — Machining Characteristics of Polycrystalline Silicon and Computer Simulation of Profile Changes of Patterns

In order to fabricate micro-mechanical parts with Au + and Ga + focused ion beams of 30 keV, machining characteristics of polycrystalline silicon and a computer simulation method for predicting profile change of patterns etched into a polycrystalline silicon wafer have been investigated. For patterns of low aspect ratio, fair coincidences between experimental and simulated resuls were obtained. From the examination of surface micro-topography of processed specimens using a profile SEM which is sensitive to the micro-topography of the surface, it becomes obvious that the specimen processed with Au + ion at ion incidence angles of 0° to 70° have fine surface. Moreover, a focused ion beam apparatus having two secondary electron detectors has been developed for the in situ observation of the surface during the fabrication of the specimen.

Microwave photoconductivity scanning microscope studies of silicon surfaces

The microwave photoconductivity scanning microscope (MPSM) is a new experimental tool for the space-resolved contact-free characterization of photoconductive materials. It allows the determination of the local photoconductivity down to a resolution determined by the width of the laser beam and the diffusion length of photogenerated carriers. The reflected laser light can be measured simultaneously, which allows one to correlate the microwave signals with the surface morphology and to correct them for the absorbed light intensity. Structural, mechanical, thermal, and chemical surface treatments of monocrystalline and polycrystalline silicon wafers have been performed to demonstrate the technical applications and the scientific value of the described experimental technique. For the first time a contact-free space-resolved characterization of a heterojunction between n-silicon and p-type polymeric film is presented. Possibilities and limitations of the technique are outlined.

Au/P polysilicon Schottky diodes

Schottky barrier diodes were fabricated by evaporation of gold layers onto chemically etched polycrystalline silicon wafers. The wafers are p type (resistivityρ⊥=160 Ωcm, the grains are columnar shape with some orientation). An average potential barrier of 0.087 eV appears to exist across the grain boundary between columns. Ohmic back contacts were made from a Ga-Al alloy. Diodes were investigated with the aid of (a)I-V characteristics at different temperatures, (b)C-V characteristics, (c) spectral response and (d) Fowler's plot of photoelectric measurements. The best diodes had characteristics similar to data typical of monocrystalline silicon. The low series columnar resistance (0.67Ω) of the cells is neglected in the analysis. The barrier height, fromI-V data (including temperature variation) was 0.67 to 0.73 eV. This is much lower than that obtained from Fowler's plot (0.9 eV). However, the barrier height obtained from theC-V graph is in agreement with the value of 0.9 eV. The diffusion potential is 0.4 eV. The value of the diode ideality factor (1.5 to 2.8) indicates that recombination generation processes play a dominant role. The normalized photovoltaic spectral response in the wavelength range 600 to 1300 nm was presented with a comparison from the theory taking into account bulk and surface recombination.

Systematic study of the process parameters affecting hydrogen plasma passivation of polycrystalline silicon and polycrystalline silicon solar cells

We report detailed studies of hydrogen passivation of polycrystalline silicon wafers by RF plasma and the effect of various process parameters on the extent of passivation as seen through electrical measurements. Similar studies made on polysilicon solar cells are also reported. The need for such studies is emphasized via some interesting results which show an excessive dependence of effective passivation on the process conditions such as processing time, substrate temperature, gas pressure and RF power which seems to have been overlooked by earlier workers. Our results reveal that there is an optimum for almost all the above parameters which yield the most effective passivation of the grain boundaries in polycrystalline silicon. Physically, therefore, it implies that optimum hydrogen incorporation in a particular bonding configuration is a necessary condition to achieve best passivation. Results of samples passivated for several hours indicate the presence of yet another process along with the normal hydrogen diffusion. Possibilities of bond breaking or hydrogen incorporation in different bonding configurations cannot be completely ruled out. The dependence of effective passivation on these process conditions is also revealed by the systematic studies of variation in average carrier concentration, charge carrier mobility and resistivity with substrate temperature, RF power and gas pressure. Polycrystalline silicon solar cells passivated with the optimum process conditions show significant improvement in the efficiency and fill factor. Presence of hydrogen was confirmed by quadrupole mass spectrometry studies by heating the passivated samples.

Improvement of electron diffusion lengths in polycrystalline silicon wafers by aluminium

In order to improve the electron diffusion lengths Ln in P-type large grained polycrystalline silicon wafers, an aluminium layer was deposited on the back surface. The structure is then annealed at 450 °C during 2h. It was found that Ln is increased, provided the initial value was smaller than 50 µm, and tht the mean density of dislocations is larger than 5 x 104 cm-2. Backside gettering appears to be the most likely mechanism to explain the results and it is assumed that dissolved oxygen may be involved.

Nondestructive Evaluation of Dislocation Subgrain Structures in Silicon Using Thermal Wave Imaging Techniques

ABSTRACTChemical sensors are being developed for in-dwelling biomedical applications. As sensor technologies progress, there is a tendency to fabricate more complex devices in a smaller area, thus enabling multiple sensors to be placed within a single in-dwelling catheter. As the active area of the detector is decreased, the defect structure of both the substrate and the various layers that make up the device plays an increasingly significant role in determining the ultimate sensitivity and reliability. A simple screening test to evaluate the viability of the substrate and the interfaces would aid in the initial evaluation of the fabricated device. To this end, thermal-wave imaging techniques have been applied to the study of a dislocation subgrain structure in a polycrystalline silicon wafer. Comparisons have been made between the results obtained using thermal-wave imaging, defect etching studies, and EBIC measurements.

Solar cells from metallurgical silicon zone melted in polycrystalline silicon tubes

The results of a new approach to utilize metallurgical grade (MG) silicon powder to obtain polycrystalline silicon wafers for the fabrication of solar cells are reported. A polycrystalline silicon tube is filled with MG silicon powder and is used as a consumable container which facilitates the float zone melting of the powder. This leads to the dilution of impurities in the melt; an in situ purification of the melt also takes place during solidification. This method has given an air mass one conversion efficiency of about 4.3% in cells 2 cm 2 in area and there is significant scope for improvement in both the efficiency and the cell area.

Fabrication of P+-N-N+ silicon solar cells by simultaneous diffusion of boron and phosphorus into silicon through silicon dioxide

A process of fabrication of P+-N-N+ silicon solar cells by simultaneous diffusion of boron and phosphorus into N-type silicon through a thermally grown silicon dioxide layer ∼0.1 μm thick is described. The inherent problems of cross doping and usefulness of silicon dioxide in minimizing them are briefly discussed. The process is well suited for large-scale production of P+-N-N+ back surface field silicon solar cells. With this process, the cells fabricated from single-crystalline silicon wafers of 100 Ω cm resistivity have shown open-circuit voltages ∼0.6 V at 28 °C under 100 mW/cm2 (AM1) intensity, and the cells fabricated from polycrystalline silicon wafers of 1–2 mm in grain size and 7–27 Ω cm resistivity have shown conversion efficiencies between 10.4 and 11.4% (AM1) at 28 °C.

DC plasma etching of silicon by sulfur hexafluoride. Mass spectrometric study of the discharge products

Polycrystalline silicon wafers were etched in dc discharges of SF6. SFx species were extracted from the discharges and measured with a mass spectrometer. A systematic procedure was used to measure the SF x + signals such that they are indicators of events in the discharge close to the sample undergoing etching. The picture that emerges is remarkably simple and shows the relative stability of several SFx species including SF6, SF4, SF2, and SF which are shown to be extracted from the discharge both in the presence and absence of the silicon sample. When silicon is being etched on the cathode of the discharge cell, the only significant additional products are SiF4 and S2F2. A comparison of blank and sample data for opposite substrate polarities shows that there is only a small cation-assisted etching effect and suggests that ions do not play an important role in the etching of silicon by SF6 discharges.

Secondary Ion Images of Impurities at Grain Boundaries in Polycrystalline Silicon

ABSTRACTA CAMECA IMS-3F secondary ion microscope has been used to investigate the presence of impurities in cast polycrystalline silicon wafers and to determine their lateral distribution. Cesium and oxygen ion bombardment were used to selectively detect potential impurities and extract their lateral distribution with submicron lateral resolution.Images of H, C, O, P, and Cu segregated in grain boundaries have been obtained from polycrystalline silicon.

Thermal Oxidation of Silicon and Iron‐Coated Silicon in the Temperature Range 692°–1003° C

Polycrystalline silicon wafers, both clean and with a thin coating of metallic iron, have been oxidized in steam and in moist air in the temperature range 692°–1003°C. Data were analyzed in terms of a linear‐parabolic rate equation. For clean silicon oxidized in steam, the parabolic rate constant displays an activation energy for the process of 26.9 kcal/mole below 903°C, and 16 kcal/mole above this temperature. The iron coating was assumed to oxidize and diffuse homogeneously in the oxide film within a relatively short period of oxidation; no evidence to the contrary could be found. In steam, the effect of iron was to extend the “high temperature” Arrhenius behavior to lower temperatures. In moist air, a single Arrhenius line corresponding to the “low temperature” behavior of clean silicon was seen. The activation energies of the “high temperature” and “low temperature” regions of silicon oxidation correspond to those reported for wet and dry oxidation, respectively. An interpretation of these phenomena in terms of a short range order transition in the vicinity of 900°C is postulated.

The Effect of Sodium Oxide on the Thermal Oxidation of Silicon

Polycrystalline silicon wafers coated with were oxidized in ambient air with in the temperature range 702°–1052°C. Initial oxidation followed cubic kinetics due to the linear change in composition of the oxide film with oxidation. At longer times, the oxidation became parabolic as the concentration of and, therefore, diffusivity of hydroxyl in the oxide became nonlinear with the sodium concentration in the film. This occurs at lower concentration of sodium, where the presence of sodium is manifested in the formation of hydroxyl by an autocatalytic reaction. Activation energies for both cubic and parabolic regions were 40.8 kcal/mole; the cubic pre‐ exponentialconstant was , and the parabolic pre‐exponential constant was . Untreated specimens oxidized as controls during these experiments showed similar behavior due to vapor phase transport of .