Investigation Of The Mechanism Of Pulsed Laser Ablation And The Properties Of Plasma For HgCdTe

HgCdTe is one of the important materials utilized in infrared detectors. As a key component in varied photoconductive apparatuses, it has been successfully used in fields such as aviations, space flight and military. If the pulsed laser with high power density irradiates the surface of Hg_0.8Cd_0.2Te wafer, the thermal stresses, evaporation wave and laser supported detonation wave (LSDW) exerting on the target surface can damage the wafer. Therefore, it is significant to investigate the mechanism of the damage of Hg_0.8Cd_0.2Te material irradiated by a high power pulsed laser.When the power density of the incident laser pulse exceeds the damage threshold of the material, the surface of the irradiated material instantaneously reaches a temperature higher than the material's vaporization temperature due to photon and multi-photon absorption and other absorption mechanisms. Thus the material will be melted, vaporized, even ionized within several nanoseconds, causing explosion of the surface and formation of the dense plasma. The laser-induced plasma ejecting from the target surface carries both mass and energy that can be used to grow thin films. At present, pulsed laser deposition (PLD) has been one of the most popular methods employed to prepare high-quality thin films, such as oxides, nitrides, semiconductors and superconductors. Although Hg_0.8Cd_0.2Te thin films had been fabricated in 1982, the quality of them has not been satisfactory. In order to find the optimum experimental condition and to control the process of the growth of thin films, it is necessary to investigate the properties of the plasma. Unfortunately, very limited works have been focused on the study of fundamental laser plasma parameters, such as the electron density, the electron temperature, the plasma velocity, changing with the delay time and laser power density, and the ejceting particles spatial distribution. In fact, these results are not only essential for understanding the laser material interaction process and clarifying the local behavior of the expanding plasma, but also enabling us to optimize the condition of PLD.It is believed that the structure of the thin film deposited by pulsed laser depends largely on the deposition process. The chemical and physical properties of the deposited films are also dependent on the plasma parameters, although the exact relationship has not yet been established. Many approaches can be adopted to detect the ejecting particles generated by laser ablation. Among them, optical emission spectroscopy is the most popular one. It allows us to monitor the ablation process real-time and in-situ without introducing disturbance to the plasma at all. In our experiment, we used the optical emission spectroscopy to quantify some important plasma parameters, such as the electron density, the electron temperature, and the velocity of Hg_0.8Cd_0.2Te plasma.A systematical investigation on the damage of Hg_0.8Cd_0.2Te induced by the pulsed laser with a wavelength of 1064 nm and a pulse width of 10 ns has been done, getting the main damage mechanism. Meanwhile, spectral diagnosis was used to detect the plasma, obtaining the time-resolved and time-of-flight spectra, further acquiring the electron density, the electron temperature, and the velocity of Hg_0.8Cd_0.2Te plasma. Besides, a study on the emission mechanism of laser plasma and the law of its evolution has been done, and the line spectral shift has been quantitatively analyzed as well. This thesis contains mainly two parts: one is the investigation on the mechanism of Hg_0.8Cd_0.2Te damage induced by a pulsed laser and the ablation rate. Another is the study on the emission mechanism of laser plasma and the law of its evolution, and also on the evolution of the electron density, electron temperature and plasma velocity applying the spectral diagnosis. The main work and results are as follows.1. A analytical expression of the melting threshold and vaporizing threshold of Hg_0.8Cd_0.2Te crystal irradiated by pulsed laser were theoretically deduced based on the one-demission thermal conduction theory. The calculated results consist well with the experimental data. The vaporizing threshold of Hg_0.8Cd_0.2Te crystal irradiated by pulsed laser with a wavelength of 1064 nm and a pulse width of 10 ns was measured for the first time, which value is 5.5×107 W/cm2 or 0.55 J/cm2. Further, the vaporizing temperature was calculated based on the law of energy conservation, which value is 1741 K. In addition, a diagram of vaporization time versus incident power density was drawn.2. The study on the force acting onto the surface of Hg_0.8Cd_0.2Te crystal irradiated by a pulsed laser was completed. At first, the maximum stress was calculated when the Hg_0.8Cd_0.2Te crystal was irradiated by a pulsed laser. Secondly, the recoil pressure onto the surface by the vaporizing wave, which resulted from the pulsed laser irradiating the Hg_0.8Cd_0.2Te crystal, was computed applying the vaporization model. Thirdly, the recoil pressure onto the surface by the LSDW was computed using the LSDW model. Through comparing these values and combining the SEM microphotograph, the main reasons for the surface damage are the thermal stress and recoil pressure of vaporization wave. The cracks in the Hg_0.8Cd_0.2Te surface of wafer were originated from the thermal stress before the wafer's melt. The small drops of liquid in the surface of Hg_0.8Cd_0.2Te wafer were originated mainly from the recoil pressure of vaporizing wave.3. A theoretical analysis of the ablation rate versus incident laser power density was made. Meanwhile, its change with incident angle was also investigated. The result is that the ablation rate increases with the incident laser power density increasing. When the angles range from 0 toπ/4, the ablation rate was high and stable. So the incident angle is generally chosen in the range in PLD process.4. The study on the emission mechanism of laser plasma and the law of its evolution with the delay time was done. The intensity of continuum spectra in short-band decreased more rapidly than that in long-band. This is because the mechanism of the continuum spectra is bremsstrahlung and recombination emission. But the bremsstrahlung dominates only in high temperature. The emission mechanism of line spectrum is collision excitation, especially the excitation of high power electron.5. From theoretical analysis and Gaussian fit, as well as Lorentz fit of the line spectra of Hg_0.8Cd_0.2Te plasma, it is confirmed that the line broadening results mainly from Stark broadening mechanism. The change of electron density and temperature with the delay time was computed. They all decreased quickly with the time delay at initial stage and decreased quite slowly after 200 ns. Furthermore, the electron temperature decreased more rapidly at the pressure of 1 atm than that at 3 Pa. Also the line shift observed in our experiment was explained with the shielding effect of the plasma. 6. Based on the experimental data, the spatial angular distribution of the ejecting plasma was theoretically obtained for the first time. The velocity of the plasma plume was got from the time-of-flight spectra. Their quantity magnitude was 1.0×103 m/s and 1.0×104 m/s at background pressures of 1 atm and 3 Pa respectively. Also the velocity decreased more rapidly at the pressure of 1 atm than that at 3 Pa. The velocity was affected greatly by the ambient pressure, contrasting with the effect of the laser energy. Then the velocity of each particle of Hg_0.8Cd_0.2Te plasma was computed. The velocity of Cd was the largest and that of Hg was the smallest, which is good agreement with our experimental result. However, because the ambient pressure in our experiment is 3 Pa, the calculation corresponds vacuum, therefore the calculated value of velocity is larger than the corresponding that of experiment.