J. Tolev

Infrared lock-in carrierography (photocarrier radiometric imaging) of Si solar cells

Modulated photocarrier radiometric (PCR) imaging (lock-in carrierography) of multicrystalline (mc) Si solar cells is introduced using a near-infrared (NIR) InGaAs camera and a spread superband gap laser beam as an optoelectronic source at low modulation frequencies (<10 Hz) or point-by-point scanning PCR imaging with a focused laser beam at high (kilohertz) frequencies. PCR images are supplemented by quantitative PCR frequency scans and compared to NIR optical reflectance, modulated electroluminescence (MEL) and modulated photovoltage (MPV) images. Noncontact PCR imaging is controlled by the photoexcited carrier diffusion wave and exhibits very similar images to contacting MEL and MPV. Among these methods it exhibits the highest contrast and sensitivity to mechanical and crystalline defects in the substrate at lock-in image frequencies in the range of the inverse recombination lifetime in the quasineutral region (bulk).

Nonlinear Dependence of Photocarrier Radiometry Signals from p- Si Wafers on Optical Excitation Intensity

The dependence of the photocarrier radiometric (PCR) signal on the intensity of exciting superbandgap laser radiation was investigated. It was shown that the amplitude of the PCR signal exhibits a supralinear dependence on laser intensity I β 0 , with nonlinearity coefficient/exponent β such that 1 ≤ β ≤ 2. The power dependence of the amplitude is an important indicator of the photoexcited carrier recombination physics in semiconductors ranging between monopolar (β = 1) and bipolar (β = 2) limits. The study was made with laser beams of varying wavelength, power, and spotsize and with semiconductor silicon wafers with different transport parameters, especially recombination lifetime. One-dimensional and three-dimensional models of the nonlinear PCR signal dependence on β vs modulation frequency were developed. It was found that the conventional linear approach using β = 1 is not always consistent with experimental slopes of amplitude vs power and it may yield erroneous values of the electronic transport properties. Consideration of the fundamental and second harmonic amplitudes and phases of the PCR signal showed that the physical origin of the nonlinear dependence of the PCR amplitude on laser intensity is consistent with high-optical-injection of free-carriers in the semiconductor. The value of β can also be determined by the second harmonic-to-fundamental-amplitude ratio and is controlled by the carrier relaxation time dependence on the optically injected excess diffusive photocarrier density wave.

H+ ion-implantation energy dependence of electronic transport properties in the MeV range in n-type silicon wafers using frequency-domain photocarrier radiometry

Industrial n-type Si wafers (resistivity of 5–10Ωcm) were H+ ion implanted with energies between 0.75 and 2.00MeV, and the electronic transport properties of the implanted layer (recombination lifetime, carrier diffusion coefficient, and front-surface and implanted-interface recombination velocities s1 and s2) were studied using photocarrier radiometry (PCR). A quantitative fitting procedure to the diffusing photoexcited free-carrier density wave was introduced using a relatively simple two-layer PCR model in lieu of the more realistic but substantially more complicated three-layer model. The experimental trends in the transport properties of H+-implanted Si layers extracted from the PCR amplitude and phase data as functions of implantation energy corroborate a physical model of the implanted layer in which (a) overlayer damage due to the light H+ ions decreases with increased depth of implantation at higher energies, (b) the implanted region damage close to the interface is largely decoupled from the ove...

Infrared photocarrier radiometry, modulated photovoltage and electrical characteristics of polycrystalline Si solar cells

Laser-induced infrared photocarrier radiometry (PCR) was used to characterize industrial polycrystalline silicon solar cells. The ac photovoltage was measured simultaneously with the PCR signal. The PCR and ac photovoltage signals were investigated as functions of modulation frequency, excitation intensity, external dc illumination and load resistance. The interrelation and interpretation of PCR signal, ac photovoltage and static (dc) electrical parameters of solar cells are discussed.

Two-beam cross-modulation photocarrier radiometry: principles and contrast amplification in semiconductor subsurface imaging

A two-beam photo-carrier radiometry (PCR) technique of semiconductors has been developed. The technique operates on the superposition of superband-gap and subband-gap laser beams which results in the cross-modulation of the backscattered subband-gap laser intensity by the harmonically varying free-carrier-wave density-dependent infrared absorption coefficient. A theory of this two-beam cross-modulation approach and various experimental configurations applied to the imaging of electronic contamination and defects in silicon wafers are presented. Owing to the nonlinear interaction of the two beams, the configuration revealed a new optoelectronic effect, the decrease of the residual subband-gap absorption coefficient due to the decreased carrier capture cross-section brought about by the depletion of occupied band-gap states in the presence of photons produced by radiative recombination. Quantitative values of the optoelectronic constant B associated with the rate of depletion of free-carrier capture cross-section with superband-gap intensity, as well as of IeR, the intensity of radiative recombination emissions, were obtained. These values cannot be measured by conventional PCR or other single-ended optoelectronic techniques. The theory explains the experimental dependence of electronic transport properties on the intensity of the subband-gap beam and accounts for optoelectronic imaging contrast amplification in contaminated or defect semiconductors. The two-beam cross-modulation PCR was further shown to enhance the imaging contrast of a certain electronic contamination type (Fe in p-Si). A dramatic phase contrast enhancement of subsurface defects made by low-dose proton implantation was demonstrated at superband-gap laser intensity levels one order of magnitude lower than possible with single-ended optoelectronic imaging methodologies. This is tentatively attributed to relatively low-injection trap-filling well below optoelectronic trap saturation.

Non-destructive infrared optoelectronic lock-in carrierography of mc-Si solar cells

A novel non-destructive, non-contact laser-induced infrared lock-in carrierographic imaging technique based on photocarrier radiometry (PCR) was introduced to characterize industrial multi-crystalline silicon solar cells. The modulated photovoltage (MPV) was measured simultaneously with the PCR signal in point-by-point imaging using focused laser scanning. The PCR and MPV signals were also investigated as functions of modulation frequency and load resistance at fixed coordinate points in order to better understand and interpret the physical optoelectronic origins of the carrierographic imaging contrast. A InGaAs-camera based carrierograhic lock-in imaging technique with a spread laser-beam illumination of solar cells was also introduced and was shown to exhibit contrast based on p-n junction RC time constants and on quasi-neutral bulk minority carrier recombination lifetimes.

Photothermal radiometry parametric identifiability theory for reliable and unique nondestructive coating thickness and thermophysical measurements

In this paper, we present a detailed reliability analysis of estimated parameters to a three-layer theoretical model of photothermal radiometry frequency domain signals by applying parameter identifiability conditions from two steel samples coated with ∼10 μm and 20 μm thick ceramic coating, to measure the thermophysical parameters of the coating, such as thermal diffusivity, thermal conductivity, and coating thickness. The three parameters are unique only when their sensitivity coefficients are linearly independent over the range of measurements. The study demonstrates the complexity of the identifiable experimental conditions through identifiability maps (calculated nonidentifiable locations) and sensitivity coefficient plots, even when the three separated parameters are grouped into two parameters. The validation of the reliability analysis theory by comparing the independently measured, with the fitted thicknesses of two coatings under random and optimized conditions, underscore the great importance o...

Laser photothermal non-destructive metrology of cracks in un-sintered powder metallurgy manufactured automotive transmission sprockets

A non-contact and non-intrusive method of revealing crack presence in un-sintered (green) automotive transmission parts (sprockets), manufactured by means of a powder metallurgy technology based on analysis of photo-thermal radiometric (PTR) signals and their statistical analysis was developed. The inspection methodology relies on the interaction of a modulated laser generated thermal wave with the potential crack and the resulting change in amplitude and phase of the detected signal [1-5]. The crack existence at points in high stress regions of a group of green (unsintered) sprockets was evaluated through frequency scans. The results were validated by independent destructive cross-sectioning of the sprockets following sintering and polishing. Examination of the sectioned sprockets under a microscope at the locations where signal changes was used for correlation with the PTR signals. Statistical analysis confirmed the capabilities of the method to detect the presence of hairline cracks (~5 − 10 μm size) with excellent sensitivity (91%) and good accuracy (78%) and specificity (61%). This measurement technique and the associated statistical analysis can be used as a simple and reliable on-line inspection methodology of industrial powder metallurgy manufactured steel products for non-destructive quality and feedback control of the parts forming process.