Contactless Lifetime Research Articles

Selectivity of TiOx‐Based Electron‐Selective Contacts on n‐Type Crystalline Silicon and Solar Cell Efficiency Potential

The selectivity parameter S 10 of titanium oxide (TiO x )‐based selective contacts on n‐type silicon is experimentally extracted through measurements of the specific contact resistivity ρ c and the recombination current density prefactor J 0, both parameters measured on fully Al‐metallized samples. The contact resistivity ρ c is determined applying the Cox and Strack method and the J 0 values are extracted using dynamic infrared lifetime mapping, allowing for contactless lifetime measurements on fully metallized silicon samples in contrast to conventionally applied lifetime measurement techniques. The highest selectivity after annealing of an n‐Si/SiO y /TiO x /Al contact is determined to be S 10 = 11.6, which corresponds to a maximum achievable efficiency of 24.1% for this type of electron‐selective contact. The maximum selectivity is achieved after low‐temperature annealing at 500 °C or through a contact‐firing step at a set‐peak temperature of 820 °C in an industrial conveyor‐belt furnace, as used in today's commercial solar cell production.

Theoretical and experimental comparison of contactless lifetime measurement methods for thick silicon samples

Simulations of time dependent carrier profiles for thick as-grown silicon samples (e.g. ingots) were computed for the excitation conditions of two different transient photoconductance lifetime measurement methods. The simulations were performed using a partial differential equation system that allows computing also non-steady state conditions. The specific effective lifetimes for different measurement conditions can be extracted and compared. Simulation results and measurement results for μPCD (microwave detected photoconductivity decay), a non-steady state method and MDP (microwave detected photoconductivity), which operates typically with a steady state photo generation, were simulated and measured. It was found that the effective lifetimes measured at thick samples with each method may differ strongly. This discrepancy can be attributed to the different penetration depths of the laser light and microwave, but first and foremost to a varying light pulse length and its influence on the developing carrier profile. Altogether the MDP measurements or methods with a steady state photo generation in general are less prone to the surface impact and accordingly better suited for investigating the bulk properties of silicon samples.

FeB and CrB pair reassociation kinetics in imperfect Si controlled by contactless lifetime scan maps

The kinetics of metal-acceptor pairing in boron-doped multicrystalline samples are investigated by means of contactless lifetime measurement. The case of FeB and CrB pairs is discussed and the influence of extended defect is evidenced for FeB. It is found that the reassociation of the metal-acceptor pairs is fast immediately after the dissociation and then slows down for several days. The results allow the identification of mixed contamination by Cr and Fe.

Iron detection in crystalline silicon by carrier lifetime measurements for arbitrary injection and doping

An existing technique for accurate measurement of iron in silicon, which was previously restricted to low injection and a narrow doping range, has been extended to arbitrary injection and doping levels. This allows contactless lifetime measurement techniques to be used for very sensitive and rapid iron detection under a wide range of conditions. In addition, an easily measured and unambiguous “fingerprint” of iron in silicon has been identified. It is based on the invariant nature of the excess carrier density at which the injection-dependent lifetime curves, measured before and after iron–boron pair dissociation, cross over. This characteristic crossover point lies in the narrow range of 1.4 to 2.0×1014 cm−3, provided only that the boron concentration is below 5×1016 cm−3. To demonstrate the value of these techniques, they have been applied to photovoltaic-grade cast multicrystalline silicon wafers.

Behaviour of metallic impurities at grain boundaries and dislocation clusters inmulticrystalline silicon wafers deduced from contactless lifetime scan maps

By means of a contactless microwave phase shift technique, the minority carrierlifetime and surface recombination velocity were measured in multicrystallinesilicon wafers containing iron and chromium. The bulk lifetime can bededuced, leading to the determination of the concentration of interstitial iron[Fei ] associated with boron, after pair dissociation annealing at210 °C. It is foundthat [Fei ] is higher in the vicinity of extended defects but decreases at the defects because the ironsegregates irrespective of boron concentration and does not form pairs.

Minority carrier lifetime scan maps applied to iron concentration mapping in silicon wafers

Mapping of iron concentration is obtained in p-type monocrystalline and multicrystalline silicon by means of contactless lifetime measurements. The recombination rate induced by interstitial iron (Fe i ) and iron boron pairs (FeB), which is very sensitive to injection level, is appreciated thanks to comparison between deep level transient spectroscopy (DLTS) and lifetime measurement by the micro-wave phase-shift technique (μW-PS). Calibration is given for samples doped in the range 5×10 13–3×10 16 cm −3.

High-resolution lifetime mapping using modulated free-carrier absorption

Modulated free-carrier absorption (MFCA) is introduced as a novel contactless lifetime mapping technique with high spatial resolution. The advantages of MFCA mapping as compared to other methods are discussed. Examples of lifetime maps on multicrystalline silicon are presented. Solar cells fabricated from identical wafers have been investigated using light beam induced current mapping. Excellent agreement between structures obtained in the measured lifetime and in the current maps was observed. In addition, a quantitative comparison with diffusion lengths determined by local internal quantum efficiency evaluations is presented.

Injection-level-dependent recombination velocities at the Si-SiO2 interface for various dopant concentrations

Measurements of the dependence of the effective surface recombination velocity Seff at the Si-SiO2 interface on light-induced carrier concentration are presented. Contactless lifetime measurements by modulated free-carrier IR absorption with additional bias light and by photoconductance decay measurements of samples immersed in HF were used to extract Seff for varying illumination levels. For various dopant concentrations the measured surface recombination velocities are compared with theoretical predictions calculated using an extended Shockley–Read–Hall formalism for Si-SiO2 interfaces. Good agreement was obtained. The consequences for solar cell performance and lifetime measurement techniques are discussed.

15.9% efficiency solar cells on electromagnetic cold crucible cast multicrystalline silicon

Hydrogen passivation is found very effective in improving the quality of electromagnetically cased (EMC) multicrystalline silicon from Osaka Titanium Co. (OTC), now Sumitomo Sitix Corp.), due to its small grain size and low oxygen content. A top efficiency of 15.9% is achieved for a 4 cm2 solar cell fabricated on hydrogen passivated EMC-OTC multicrystalline silicon. Such a result is comparable to the top efficiencies achieved on high quality conventionally casted material and represents the maximum efficiency ever reported for EMC multicrystalline silicon. Fourier transform infrared measurements confirm the low oxygen content in the EMC-OTC material and spectral response as well as contactless lifetime measurements confirm the role of the plasma hydrogen passivation in substantially improving the diffusion length in this material.

An instrument for contactless lifetime measurements in semiconductor layers of silicon-on-insulator (SOI) materials

The construction and operation of an instrument is described that measures excess-carrier lifetimes (1 mu s to 400 mu s) in semiconductors by generating electron-hole pairs with a pulsed flashlamp and contactless probing of the photoconductivity decay with microwaves. This instrument was especially designed for measurements in the thin top layer of SIMOX structures. For this purpose UV radiation is required with a short penetration depth. This made the construction of a special UV filter necessary that transmits below 400 nm and blocks all longer wavelength radiation up to the silicon band gap wavelength.

Contactless lifetime measurement in processed and preprocessed high-purity silicon radiation sensors

Abstract A contactless microwave method is presented for the measurement of the recombination lifetime of free carriers in high-ohmic silicon wafers (4 kω cm), which are used for the processing of (nuclear) radiation sensors. It is shown that the bulk lifetime can be measured following processing steps independent of surface parameters, when the intensity of the light source is kept low. The results of the measurements carried out by this method are compared with the results obtained by leakage current measurements. Good agreement is observed between the results.

Some investigations on photovoltaic mechanism in silicon ribbon

Laser scanning technique combined with microwave contactless lifetime measuring method was used to investigate characteristics of ribbon cells which were prepared by the capillary method. Lasers of two wavelengths, 633 nm (He-Ne) and 1060 nm (YAG) were used to examine local variation in characteristics. Short circuit current (Isc) was measured during traverse of ribbon stage. Isc excited by He-Ne laser (I633) represented surface information of ribbon while Isc excited by YAG laser (I633) represented that of bulk. Areas were classified in accordance with the nature of Isc into the following four categories: 1) high (I633), high I633, 2) high I633, low I1060, 3) low I633, high I1060 and 4) low I633, low I1060. Samples were then subjected to lifetime measurement where bulk lifetime (τB) and surface recombination velocity (S) were separated from observed lifetime (τM) by calculating the deviation from the exponential curve. According to the natures of τB and S, areas were also classified into four classes as follows: A) high τB, small S, B) high τB, large S, C) low τB, small S and D) low τB, large S. Correspondence between the lifetime classification and photoresponse categories were discussed concerning the identification of crystallographic defects. A model was proposed to explain the influence of inclusions on cell efficiency. Characterisitics calculated from this model are consistent with the results obtained in the present investigation and with experience in ribbon cell manufacturing.

New principle of contactless lifetime determination in semiconductor wafers

The principle of contactless determination of excess carrier lifetime or diffusion length in semiconductor wafers employs the magnetic flux created by the photomagnetoelectric (PME) current. Under certain conditions, the measured signal is nearly proportional to the lifetime according to a simple theory in which one-dimensional diffusion is considered. Experimental verification of the theory is presented. A sample region of a few square millimeters is sufficient for lifetime determination and lifetime up to 10−7 s or less is measurable by the method.

The Influence of Lighting Area for Minority Carrier Injection in Contactless Lifetime Measurement

The Influence of Lighting Area for Minority Carrier Injection in Contactless Lifetime Measurement