The Research On Laser Beam Induced Current Spectra Of HgCdTe Photovoltaic Detectors

Infrared radiation is invisible radiation It is electromagnetic wave lying between visible light and microwave in nature. The intensity of infrared radiation signal can be measured by infrared detectors. Detection results of infrared detectors are hugely influenced by its photoelectric properties. As one of the most popular materials chosen for infrared detectors, HgCdTe material plays an irreplaceable role in the area of infrared detection as well as Si does in the area of microelectronics. However, we don’t know characteristics of HgCdTe clearly. Nowadays, HgCdTe detectors are developing towards the directions of high integration and multi-color. But it is quite difficult to gain focal plane arrays with high quality and large arrays. If performances of devices can be detected and characterized in time during the early process, it will be helpful to improve the process and device performance. For HgCdTe infrared photovoltaic detectors, the direct electric contact will make damage to p-n junction of detectors. Laser beam induced current (LBIC) is a destructive and high resolution optical method and widely used in HgCdTe material and devices. In this paper, we made a research on the photoelectric characteristics of HgCdTe photovoltaic infrared detectors by combining LBIC and numerical simulation method. The main contents are as follows:(1) The study of the dependence of LBIC spectra on the temperature for n+-on-p HgCdTe photovoltaic infrared detectors is performed. It is found that the measured LBIC profile of the HgCdTe device is asymmetric and the symmetry of LBIC profile varies with the temperature. Below the temperature of170K, LBIC profile becomes more asymmetric as the temperature increases. However, LBIC profile tends to be more symmetric with the increasing temperature above170K. A leakage current model with a localized leakage along the lateral p-n junction direction is used to prove the occurrence of asymmetric LBIC spectra for HgCdTe infrared detectors. The simulated results show that the LBIC profile of a device with an asymmetric leakage is asymmetric. When the localized leakage is located at the right side of the lateral junction center, the shift of LBIC signal in the device is negative. However, the shift of the LBIC signal is positive if the leakage is located on the left of the lateral junction center. Furthermore, the larger the leakage is farther away from the center of the lateral junction, the larger the vertical shift is. The mechanisms influencing the symmetry of LBIC spectra at high temperature and low temperature are analyzed respectively. Below170K, the trap-assisted tunneling current is a crucial contributor. As the temperature increases the leakage is more severe. Nevertheless, the diffusion current component becomes dominant above170K and reduces with the increasing temperature.(2) The influence of geometrical sizes of the junction on LBIC of n+-on-p ideal HgCdTe infrared photovoltaic detectors is numerically investigated and the diffusion length of minority carriers in the p-type region is extracted by exponential decay fitting model. It is found that the simulated LBIC spectra varying with the junction depth are in good agreement with experimental results. The LBIC signal of HgCdTe photovoltaic detectors is sensitive to the change of junction depth and junction length. Furthermore, the LBIC peak has a linear relationship with both the junction depth and junction length. The LBIC shape in the n-type region becomes more flat with the increasing junction depth. The competition model of lateral current or external circuit current and vertical current or inner circulatory current is proposed to explain the dependence. For HgCdTe infrared detectors with deeper junction, when the laser is incident on the n-type region, much more carriers diffuse to the vertical junction and a larger inner circulatory current is formed causing weaker LBIC signal in the n-type region. However, when the laser illuminates on the surface of p-type region and as the junction depth increases, more photo-generated electrons can be swept into the junction in time. The instant separation of carriers reduces the recombination rate of electron-hole pairs and the formation of inner circulatory current. A larger LBIC signal is given rise to at two remote contacts. For HgCdTe infrared detector with shorter junction length, the inner circulatory current is larger, leading to smaller LBIC signal. (3) The LBIC spectra of dry etched p-type HgCdTe sample are measured. It is found that the LBIC spectra of etched sample behave evident phenomenon with two peaks and two valleys. Etch-induced p-to-n type conversion model and light intensity change rule beside mesa sidewalls are used to explain the phenomenon. The appearance of inner peak and valley is caused by the p-n junction formed by the etch-induced p-to-n type conversion region and the p-type HgCdTe mesa without etching. In LBIC spectra measurement, the special mesa structure varies the light energy profiles of focused laser beam. At the relatively higher mesa region, the light focus becomes divergent and the light intensity is weaker. Then the measured LBIC spectra become smaller, causing the formation of external peak and valley. Comparing the simulated LBIC spectra by changing the light intensity near the HgCdTe mesa with the experimental LBIC spectra, it can be seen clearly that the simulated results and the experimental results are in good agreement. The lateral size of etch induced mesa damage is about2.4μm by corresponding the LBIC spectra and the SEM image of the etched sample.