Lock-In Carrierography Of Semiconductor Materials

Since the mid-20th Century the electronics industry has enjoyed phenomenal growth and is now the largest industry in the world. The foundation of the electronics industry is the semiconductor materials and devices. Thorough understanding of the relationship between structure, processing, physical properties, and how they are linked to device performance is the key to innovation in materials science and engineering. Since reliable and precise measurements are needed at all stages of R&D and production, it is crucial to develop real-time, detailed non-destructive and quantitative characterization methods for quality control and property evaluation through all phases of material processing, from raw wafer fabrication to final device integration.Carrier transport properties, including lifetime, diffusivity, and (front-and back-) surface recombination velocities (SRVs), are among the most important parameters of semiconductor materials and devices. They determine the transport behavior of electrons in semiconductors across device structures, thus having direct impact on device performance. Although there are many techniques capable of determining carrier transport parameters, to-date none of them can simultaneously meet the following four requirements:1) measurement precision and accuracy; 2) non-contact character; 3) lateral resolution (imaging); and 4) depth-selection.Photocarrier Radiometry (PCR), a form of frequency-domain photoluminescence, spectrally gated to block out thermal infrared photon emission from photoexcited semiconductors, capable of all optical, non-contact, and non-destructive measurement of carrier transport properties, has been assigned "PACS 78.56.Cd" by American Institute of Physics (AIP). Lock-in carrierography (LIC), the camera-based imaging extension of the single-element detector PCR, has been introduced in 2010.PCR and LIC had the following imperfection and/or bottleneck:1. the theory of PCR remains incomplete in terms of signal nonlinearity which tends to introduce measurement errors; 2. LIC was only a qualitative imaging methodology, and its quantitative imaging capability of carrier transport properties had not yet been realized; 3. due to the sampling speed limitation of the InGaAs camera, it was impossible to realize high-frequency LIC imaging, which prevents not only resolving the bulk and surface recombination lifetimes but also realizing depth-selective imaging which should be a unique advantage of frequency-domain imaging techniques.This thesis is dedicated to improving the PCR methodology and realizing quantitative LIC imaging of Si wafers and solar cells. The main contributions include three parts as follows. 1. Completed the PCR theoryA more rigorous PCR theoretical model has been established. The characteristic of the nonlinearity of PCR signal was analyzed, and two nonlinear theoretical models are given:power model and n-p product model. While power model is an empirical description of the experimentally determined PCR amplitude vs. laser intensity nonlinearity, the n-p product model is based on the widely accepted and more rigorous n-p product expression (electron-hole radiative recombination). Accordingly, the interpretation of the signal nonlinearity by the n-p product model has a natural and physically meaningful theoretical basis, and was validated by experiments.Moreover, an analytical expression linking the effective carrier lifetime with the four carrier transport parameters was derived, which allows for comparison of measurement results between different techniques. Based on the measured carrier transport parameters of a crystalline Si wafer, the effective carrier lifetime was estimated and compared to that measured by μ-PCD, and excellent agreement was found. 2. Established the quantative homodyne LIC imaging methodologyA phase correction method has been established in the camera-based LIC system. The raw data recorded by the camera shows that there is always a systematic phase added into the real phase signal. Using white papers as a zero-phase-lag reflector, the amplitude and phase information with different optical-density neutral filter in front of the camera were recorded and analyzed. By deriving the correction function of every camera pixel through fitting, a correction matrix for the camera phase channel was found. Experimental tests show that the correction method can accurately recover the real phase signal.Quantitative LIC effective carrier lifetime imaging of crystalline Si wafers has been realized. Based on the foregoing correction method, LIC phase images at different frequencies were obtained. Using the frequency-domain effective carrier lifetime model, by fitting the phase-frequency dependence of each camera pixel, the effective carrier lifetime image of the wafer was obtained. 3. Achieved quantitative heterodyne LIC imagingBy introducing the heterodyne method into the LIC imaging, the sampling speed limitation of the NIR camera was overcome, and high-frequency heterodyne LIC imaging has been realized. Based on the natural nonlinearity of the carrier radiative recombination signal, using the nonlinear frequency mixing mechanism, a low frequency component carrying high frequency information was constructed which can be well recorded by the camera, thus realizing heterodyne LIC imaging of crystalline Si wafers from 100 Hz to 10 kHz.A heterodyne PCR theory was established, and the heterodyne signal generation mechanism was interpreted. By best fitting the theoretical model to the experimental data, the carrier transport parameters were extracted. Good agreement was found by comparison between the heterodyne and the homodyne results.LIC imaging of multicrystalline Si solar cells from 10 Hz to 20 kHz was achieved by the LIC imaging system in homodyne and heterodyne modes. The PCR theory of Si solar cell under open circuit condition was established analytically, and the camera heterodyne amplitude-frequency dependence data were successfully fitted, allowing for measurement of the bulk lifetime of solar cell base by PCR and heterodyne LIC.The thesis lays theoretical and experimental foundation of quantitative LIC imaging methodology, which not only has potential to create a fundamental innovation in semiconductor materials science and device physics by providing a novel characterization tool of fundamental optoelectronic processes, but also has excellent industrial application prospects for in-line and off-line quality control during device processing.