Laser Induced Infrared Radiometry For Thermal Property And Defect Characterization Of Thin Materials

In modern emerging industries and scientific researches, thin materials are extensively employed, whose thermophysical properties are different from that of the bulk materials. For now, thermal property parameters of thin materials are obtained by actual measurements instead of finding from rare existed literatures. However, the present methods for thermal property mesurements of thin materials can be only used in the laboratory, because of their high requirements in samples and equipments. Therefore, there is an urgent need to develop a non-contact, convenient and accurate method for thermal property determinations of thin materials. On the other hand, the study of the relationship between the thermophysical properties of thin materials and their microstructures has become a hotspot. Laser-induced infrared radiometry(LIIR, also called photothermal radiometry) is a new non-destructive testing technique based on photoacoustic and photothermal effect. It has been increasingly applied in the field of thermal property determinations because of its advantages, such as non-contact and high sensitivity. In my work, the traditional transmission LIIR was simplified and improved, by which the thermal diffusivity of thin metallic materials was determined, and amorphous silicon thin film solar cell defects were investigated. 1. The mechanism of LIIR for thermal diffucivity determinationThermal diffusivity determines the process of heat transfer in material, which affects the temperature distribution of the material, as well as the middle-and-far infrared induced by nonradiative recombination of heat carriers. The middle-and-far infrared from the transmissive surface was measured to obtain the frequency dependence of phase-shift of LIIR signal. A three-dimensional theoretical model was built, by which the expressions of temperature distribution and infrared radiation were derived when the material was irradiated by an intensity-modulated laser beam. The distribution of temperature rise and the infrared radiation from the surface of the material were numerical calculated, by which the laser parameters and material parameters dependence of the laser-induced infrared radiation signal could be analized. Most unknown parameters were reduced by means of normalization and experimental measurements. Thus the thermal diffusivity of the material could be precisely determined by fitting between the theoretical curve of frequency dependence of LIIR phase-shift and the experimental measured curve. 2. The mechanism of LIIR for defect investigationInternal defects might affect the heat transfer process in matrial, while surface and sub-surface defects affect the absorption and emission characteristics of the material. All of them would influence the LIIR signal from the surface of the material. The defects could be investigated and analyzed by detecting the distribution of LIIR signal. Specifically, the LIIR signal from a semiconductor material might also include near infrared induced by radiative recombination of photo-induced carriers. Therefore, the defects in a semiconductor material could be complementarily analyzed by scanning both the middle-and-far infrared and the near infrared radiation distribution. 3. The simplifications and improvments of LIIR systemSimplifications were carried out based on the traditional transmissive LIIR system. The system was simplified for easier debuging and operation on one hand, the theoretical model was simplified and the amount of calculation was decreased by reducing the impact of cyclical expansion on the infrared radiation measurement. A two-dimensional electric scanning device together with corresponding automatic control and measurement system make the simplified LIIR system able to detect the infrared radiation distribution. The HgCdZnTe infrared detector with variable gap has a wide band, which enables the LIIR system to detect both near infrared and middle-and-far infrared just by changing optical filters. A fast scanning LIIR system was achieved by replacing the lock-in amplifier with a computer internal sound card, which can detect the LIIR signal continuously. Also the signal to noise ratio and the accuracy of the system were improved by means of electromagnetic isolation and option of system parameters. 4. The thermal diffusivity determination of thin metallic material by LIIR systemThe thermal diffusivity values of thin copper and aluminum with different thickness were determined by the simplified LIIR system, while the thermal diffusivity of corresponding bulk material was measured by photoacoustic piezoelectric(PAPE) technology. The measured values by the two systems were compared and verified to each other. They were also evaluated by corresponding theories, by which dependence of thermal property on the material microstructure was further analyzed. 5. The defect investigation of amorphous silicon thin film solar cell by LIIR systemA piece of amorphous silicon thin film solar cells was scanned by the simplified scanning LIIR system to get its middle-and-far infrared distribution induced by nonradiative recombination of heat carriers, by which the characteristics of various types of defects were analyzed, as well as the influence of modulation frequency on resolution; Changing the optical filter allowed the LIIR system to investigate the near infrared distribution induced by radiative recombination of photo-induced carriers in the solar cell. The defects in semiconductor material could be investigated by the two radiation mechanisms complementarily. The solar cell was scanned via the fast sanning LIIR system, by which rapid defect detection was done. The spectrum analysis of the infrared radiation signal obtained from the quick scan showed the influence of defects on the LIIR signal.In this paper, a three-dimensional model of simplified LIIR was built; Corresponding theoretical expression of the LIIR signal was derived; The thermal diffusivity values of thin metallic materials were determined by the simplified LIIR system; The application of the LIIR system was extended to defect investigation of amorphous silicon thin film solar cells. The results show that the simplified LIIR system is easier for debugging, and less amount of theoretical calculations than the traditional LIIR, while the measurement error is reduced; The simplified LIIR is available for thermal property determinations of thin materials, and study of the relationship between the thermal property and microstructure of material; The simplified scanning LIIR system can be applied to detect investigation and analyzation of thin semiconductor materials based on both mechanisms of radiative recombination of photo-induced carriers and nonradiative recombination of heat carriers; The defects can be effectively detected by the fast scanning LIIR system, which is capable of rapid defect investigation and analysis of thin materials because of the atvantages of non-contact, low cost, high sensitivity and efficiency, etc.