Measurement Of Minority Carrier Diffusion Length Research Articles

Influence of substitutional metallic impurities on the performances of p-type crystalline silicon solar cells: The case of gold

The influence of a gold bulk contamination on the performances of boron doped p-type crystalline silicon solar cells is investigated for different base doping levels and different kinds of materials, such as float zone Si, Czochralski Si, and multicrystalline Si. Solar cells are made from intentionally contaminated silicon wafers. By monitoring the evolution of the electrically active substitutional gold concentration by means of bulk lifetime and minority carrier diffusion length measurements, this paper highlights the eventual gettering or hydrogenation effects occurring throughout the whole process but also of the danger of such an impurity in materials containing large densities of extended defects generating recombination centers by means of the impurity-defect interaction.

04/02278 Extended corresponding states model for fluidsand fluid mixtures. II. Application to mixtures and natural gas systems: Estela-Uribe, J. F. et al. Fluid Phase Equilibria, 2004, 216, (1), 59–84

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.

Mapping of minority carrier diffusion length and heavy metal contamination with ultimate surface photovoltage method

The Ultimate surface photovoltage method of minority carrier diffusion length measurements reffered to as Ultimate SPV, replaces a sequential “one wavelength at a time” approach with simultaneous illumination with the entire set of wavelengths. In this multiwavelength beam, each monochromatic component is chopped with slightly different frequency. This enables simultaneous monitoring of all component SPV signals corresponding to different wavelengths using multi-frequency signal processing. The amplitude and phase of each component signals are then analyzed and used to calculate the diffusion length and surface lifetime. In-flight Ultimate SPV measurement, whereby the wafer continuously moves under SPV probe, is used for fast whole wafer mapping. In addition to speed advantages, Ultimate SPV offers a fundemental accuracy advantage due to elimination of differences in wafer condition during sequential illumination with individual wavelengths. High-speed measurements make it possible to add additional wafer treatments and perform multi-mapping required for separation of Fe and Cu in the silicon bulk. Wafer mapping in time of 2 minutes realized with Ultimate SPV is critical for monitoring of cobalt in silicon.

Hydrogen passivation of bulk defects and surface in silicon

ABSTRACTMonocrystalline and multicrystalline silicon wafers were investigated with boron doping levels in the range 1015 cm−3 to 1016 cm−3, respectively. Hydrogenation of the samples resulted from hydrogen-rich silicon nitride deposition by plasma enhanced chemical vapour deposition on the surface of the wafers. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length (L) measurements, light beam induced current scan maps and lifetime (τ) measurements, by the contact-less phase shift technique. When applied at various excitation frequencies the phase shift technique leads evaluate the surface recombination velocity (S) and the actual bulk lifetime.It was found, in multicrystalline silicon that isolated intragrain defects are well passivated while at grain boundaries and dislocation clusters deep levels are transformed into shallow levels. As a consequence L increases up to 80 % after firing the samples which the back surface was covered by a 2 μm thick aluminium layer. S decreases below 500 cm/s at the front surface covered by the hydrogen rich silicon nitride layer.

Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps. It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.

Contactless surface charge semiconductor characterization

Surface voltage and surface photovoltage measurements have become important semiconductor characterization techniques, largely because of their contactless nature and the availability of commercial equipment. The use of these contactless measurement techniques has broadened from initial application of minority carrier diffusion length measurements to a wide variety of semiconductor characterization, including surface voltage, surface barrier height, flatband voltage, oxide thickness, oxide charge density, interface trap density, mobile charge density, oxide integrity, generation lifetime, recombination lifetime and doping density. It is likely that this range of applications will broaden further. As with all characterization techniques, there are limitations but they are frequently compensated by the contactless nature of the measurement thereby simplifying test structure fabrication.

Multicrystalline silicon prepared by electromagnetic continuous pulling: recent results and comparison to directional solidification material

The electromagnetic continuous pulling (EMCP) is a new growth process competing with directional solidification for the production of massive silicon ingots. The advantages of the EMCP are higher production rates, no crucible consumption and a more uniform crystalline structure of the wafers. The present work is devoted to the characterization of the EMCP material by means of minority carrier diffusion length measurements ( L) which allow the comparison between raw, annealed, phosphorus-diffused, annealed and gettered or hydrogenated wafers. The EMCP wafers are compared with conventionally cast wafers. It is shown that L is relatively high in the raw material, but L is degraded by annealings at temperatures higher than 600°C, while hydrogenation treatments drastically improve the material. In the present state, the EMCP silicon appears as a particular form of multicrystalline silicon materials with promising abilities. However, it requires low temperature processing steps or additional hydrogenation treatments to make low-cost and efficient solar cells.

Non-destructive diagnostic techniques for oxygen precipitates in Czochralski silicon

This work investigates the determination of oxygen precipitate density in silicon using non-destructive methods, based essentially on electrical and optical techniques. Regarding electrical techniques, minority carrier diffusion length measurements by means of surface photo voltage or electrolytical metal tracer methods to determine the density of oxygen precipitates in p-type silicon wafers is evaluated, taking advantage of the activity of oxygen precipitates as recombination centres for minority carriers. Regarding optical techniques, the scanning infra-red microscope is considered, which can detect oxygen precipitates in silicon exploiting their effectiveness as light scattering centres. These techniques are compared with the traditional etching technique, considering samples in a wide range of precipitate density (5×106–1011cm−3) and submitted to a wide variety of thermal treatments. Correlations are discussed with the support of transmission electron microscopy, X-ray diffraction and infrared spectroscopy observations, which provide complementary information on size, morphology and matrix stress of the oxygen precipitates.

Comparison of minority carrier diffusion length measurements in silicon solar cells by the photo-induced open-circuit voltage decay (OCVD) with different excitation sources

The photoinduced open-circuit voltage decay technique was used to investigate the minority carrier lifetime in crystalline and polycrystalline silicon solar cells. This convenient investigation technique allows a fast determination of the diffusion length of minority carriers in semiconductor materials and is an important technique to predict the solar cell performance. The decay curves were obtained with different excitation sources, a xenon stroboscope lamp and a Nd:YAG laser, and the results were compared.

Alkaline cleaning of silicon wafers: additives for the prevention of metal contamination

The paper presents a new strategy for cleaning of silicon wafers. A novel class of chelating agents added to alkaline cleaning mixtures provides promising performance without negative effects such as metal redeposition due to residual metal contamination of the cleaning solution. The superior capability of the new cleaning process is confirmed by the results obtained from wafer surface metal analysis as well as from minority carrier lifetime and diffusion length measurements and gate oxide integrity tests. Particle densities and surface roughness are not influenced by the presence of the chelating agents in the cleaning solution. TOF–SIMS measurements do not indicate any deposition of chelating agent on the wafer surface. With this type of modified SC-1 cleaning procedure the acid SC-2 step used in conventional RCA cleans to remove the metals deposited in the preceding SC-1 step is unnecessary resulting in substantial cost savings with respect to chemicals, waste, equipment and space.

Thin p/p + epitaxial layer characterization with the pulsed MOS capacitor

The defect properties of thin p/p + wafer are not as easily characterized as those of bulk wafers. Recombination lifetime and minority carrier diffusion length measurements do not work well for thin layers, but generation lifetime characterization is eminently suitable. However, for thin epitaxial layers it is difficult to determine the correct generation lifetime because the linear portion of the Zerbst plot is too small to extract the generation lifetime when the applied gate voltage confines the space-charge region (scr) to the lightly doped epitaxial layer. For low gate voltages, moreover, the Zerbst slope is higher then that at high gate voltages. A sufficiently high gate voltage extends the scr to the out-diffused p + region. However, if the scr reaches the p + region, there is a drop off in the Zerbst plot. We have developed a mathematical analysis for this region. We have also extended the analysis to the measurement of possible p/p + interface imperfections, e.g. misfit dislocations.

Iron detection in polished and epitaxial silicon wafers using generation lifetime measurements

For iron detection in silicon, minority carrier diffusion length measurements are often used. These methods analyze only the bulk properties of the wafer and therefore failed in the case of epitaxial wafers due to the disturbing influence of the highly doped substrate. A technique is presented for the determination of the iron content in epitaxial as well as in polished wafers using generation lifetime measurements on metal-oxide-silicon capacitors. The influence of interstitial iron and iron-boron pairs on generation lifetime was investigated for samples diffused with iron in boron-doped silicon. The difference in reciprocal generation lifetime before and after a thermal anneal of 200 °C for three minutes was calibrated using deep level transient spectroscopy to detect the iron-boron pair concentration. For a typical metal-oxide-silicon capacitor fabrication process this means a detectability limit for iron-boron concentrations of about 8×1010 at/cm3. The advantage of this method is that it detects iron in a surface near region in contrast to the often used diffusion length measurements. This gives us the opportunity of controlling the iron content in epitaxial wafers as well as in wafers after an internal gettering process. In addition, the iron-boron pairing reaction was investigated in the temperature range between 21 and 50 °C. The activation energy for this reaction was determined to be 0.70 eV.

Precipitate recognition and recombination strength in annealed Czochralski silicon wafers

Two experimental methods have been used to reveal the presence of precipitates and to evaluate their recombination strength in Czochralski grown silicon wafers. One method uses scanning infrared microscopy, the other light beam induced current mapping. These two non-destructive techniques have been associated with Fourier transform infrared spectroscopy and with minority carrier diffusion length measurements. After two step annealing, scanning infrared microscopy reveals the presence of precipitates, while light beam induced current mapping displays a ringlike distribution of recombination centres and the minority carrier diffusion length decreases drastically to 2 μm. Copper contamination does not modify these observations even when precipitates are formed. Phosphorus diffusion near the surface decreases the size of the precipitates revealed by scanning infrared microscopy but does not suppress the ringlike light beam induced current maps, and the minority carrier diffusion length increases slightly. It is concluded that the recombination strength of precipitates does not depend on metal decoration but more probably on interfacial states or on associated extended defects.

Interaction of iron with a grain boundary in boron-doped multicrystalline silicon

The article presents a study of the interaction of iron with a grain boundary in boron-doped multicrystalline silicon. The sample was intentionally contaminated with iron to a few 1014 cm−3 and investigated by the electron-beam-induced-current technique (measurement of minority-carrier diffusion length, quantitative imaging) in the temperature range 80–300 K. The measurements were carried out for two different states of iron in the sample: (i) iron paired with boron, i.e., as FeB, and (ii) iron as interstitial iron Fei. The differences between diffusion lengths for these two states were used to estimate the iron concentration. The analysis of the data revealed a pronounced iron profile around the grain boundary, indicating gettering of about 4×1011 iron atoms per cm2. The recombination velocity of the grain boundary is about 5×105 cm s−1 at 300 K and is not changed by the FeB destruction treatment. The temperature dependence of the iron-related diffusion length components is discussed and found to be in satisfactory agreement with what is expected from Shockley–Read–Hall theory. Further, the diffusion length analysis revealed also a strong recombination channel of unidentified origin.

Aluminium Gettering in Silicon Wafers

The effect of an evaporated thick aluminium paper on electrical properties of multicrystalline and gold contaminated FZ monocrystalline silicon wafers was investigated. By means of minority carrier diffusion length measurements and Deep Level Transient Spectroscopy, it was deduced that the material improvements observed after annealing at 900 °C are due to gettering of metallic impurities in the Al-Si alloyed layer.

Measurement of minority-carrier diffusion length and composition of Al xGa 1−xAs alloys by a microwave contactless method

Measurement of minority-carrier diffusion length and composition of Al xGa 1−xAs alloys by a microwave contactless method

Comments on ‘‘Measurements of minority-carrier diffusion length in n-CuInSe2 by electron-beam-induced current method’’ [J. Appl. Phys. 6 6, 5412 (1989)

The recent findings of Scheer, Wilhelm, and Lewerenz[J. Appl. Phys. 66, 5412 (1989)] regarding the application of the electron-beam-induced current technique in both the vertical and planar configurations to determine the minority-carrier diffusion length in low-diffusion-length material are compared to the results of an earlier analysis [J. Appl. Phys. 57, 1978 (1985)] of the same subject. The differences are briefly discussed.

Surface recombination effects on minority carrier diffusion length measurements using electron beam-induced current

Measurement of the minority carrier diffusion length (L) can be performed by measurement of the rate of decay of excess minority carriers with the distance (x) of an electron beam excitation source from a p-n junction or Schottky barrier junction perpendicular to the surface in an SEM. In an ideal case, the decay is exponential according to the equation, I = Ioexp(−x/L), where I is the current measured at x and Io is the maximum current measured at x=0. L can be obtained from the slope of the straight line when plotted on a semi-logarithmic scale. In reality, carriers recombine not only in the bulk but at the surface as well. The result is a non-exponential decay or a sublinear semi-logarithmic plot. The effective diffusion length (Leff) measured is shorter than the actual value. Some improvement in accuracy can be obtained by increasing the beam-energy, thereby increasing the penetration depth and reducing the percentage of carriers reaching the surface. For materials known to have a high surface recombination velocity s (cm/sec) such as GaAs and its alloys, increasing the beam energy is insufficient. Furthermore, one may find an upper limit on beam energy as the diameter of the signal generation volume approaches the device dimensions.

Thermal annealing effects on grain boundary recombination activity in silicon

We have studied the influence of conventional and rapid thermal treatments at 850°C for 30 min and 10 s, respectively, on the recombination activity of thee9,e13 (P-type) ande25 (N-type) grain boundaries in silicon. The analyses were made by scanning electron microscopy (SEM) working in the electron beam induced current mode (EBIC) and completed by minority carrier diffusion length measurements. The main result obtained from this study shows the importance of the rapid thermal process as a suitable thermal treatment for polycrystalline materials.

Diffusion length measurements in a-Si:H using the steady state Photocarrier grating technique

The new Steady State Photocarrier Grating technique for minority carrier diffusion length measurements is briefly reviewed. The uncertainty about the magnitude of the surface recombination velocity is shown to introduce an error of less than 5% in the results. The ratio between the electron and hole drift mobilities in several samples of a-Si:H is determined by a simultaneous measurement of photoconductivity and the diffusion length on the same sample and it is found that there is no correlation high values of the photoconductivity and high values of minority carrier diffusion length.