Study On HgCdTe Loophole Photodiodes

This dissertation mainly focuses on the study of Hg Cd Te loophole photodiode. Based on the research of speed rate and profile in Hg Cd Te etching process, loophole technology is achieved. By studying the electrical properties in ion-beam-etched Hg Cd Te, the structure of pn junction was built. The key factors which affecting the size of photosensitive area were studied and provided useful reference for design of the device. The performance of loophole photodiodes was characterized, and especially the performance on high temperature and avalanche was preliminary explored. The main research listed as following:1. Achieve the key technology of Hg Cd Te loophole photodiode. In the etching rate experiments, the inherited correlation between etching rate and other etching parameters(high ion energy and ion beam density) meets the Smoekh model, namely. And the etching rate of Hg Cd Te may achieve the maximum when the ion beam incident angle is set to 20o; In the experiment of outline etching, it is found that the sidewall angle depends strongly on the mask profile and selection ratio, therefore the thick photosensitive resist is more suitable for loophole channel etching; The phenomenon of bottom trenching can be eliminated when the ion beam incident angle is set to 30o, which is significant for the fabrication on interconnection of the loophole channel. After comparing the etching rates between glue and metal which were got by different ion beams(O+ and Ar+), an etching solution has been achieved. Finally, interconnect resistance is measured to be 39W, which guarantees an effective interconnection of the loophole channel.2. Study on the electrical properties in ion-beam-etched Hg Cd Te. With mobility spectrum analysis technique, the electrical properties in ion-beam-etched Hg1-xCdxTe(x=0.236) was studied, it suggests that p-Hg Cd Te can be converted to n+-n structure which includes both a damaged surface electron layer with 1.5′104cm2/V×s and a bulk electron layer with 1.0′105cm2/V×s after ion etching. The mobility spectra at different temperature(77K, 150 K and 230K) show that the mobility of surface electrons is nearly invariable in current temperature range while the bulk electrons depend strongly on temperature. In addition, the mobility of bulk electron at the high temperature follows the T-2.0 law which is consistent with a classical n-Hg Cd Te. The Hall data shows that the electrical properties of the bulk layer are constant no matter how thickness it is. Otherwise, the concentration of surface electron layer ranges from 1′1016cm-3 to 1.3′1017cm-3 and is about 2~3 order higher than that of bulk electron layer.3. Study on the key factor to affect the size of the photosensitive area. Width of n-type layer is measured by laser beam inducing current measurement. With the same etching condition, the width of n-type layer depends on both of the mercury vacancy concentration and the volume of the etched-Hg Cd Te. After different loopholes are produced by Ar+ ion-beam in p-Hg Cd Te, the result shows that the width of n-type layer linearly decreases with an increase of the mercury vacancy concentration with the same volume of the etched-Hg Cd Te. On the other, the width of n-type layer linearly increases with the volume of the etched-Hg Cd Te increasing if the mercury vacancy concentration keeps a constant. According to the experiment, it is found that the calculation of diffusion length of minority carrier from present material parameters is always an order of magnitude lower than the data achieved in the test. That indicates that because of many uncertain factors such as technology and test, it is hard to make an accurate judgment for minority carrier diffusion in advance. Through the extraction of minority carrier diffusion length in n-type layer, the research illustrates a practicable way to determinate the size of photosensitive area under different conditions. Finally, the calculation method of the photosensitive area is given by analyzing the test results of LBIC.4. Study on the performance of Hg Cd Te loophole diodes. The thickness consistency between the calculations from value of the peak and testing results indicated that there was perhaps interference phenomenon of multiple reflections light in the device, which can explain well the multiple-peak phenomenon in the spectrum. The effects of the series resistance and temperature on the I-V curve were analyzed. Series resistance increases the zero resistance and has a little effect on cut-in voltage of diode, but it results in the loss of the photocurrent. Cut-in voltage decreases with the rise of the temperature. In addition, under different temperature, the characteristic of I-V curve under reverse bias is variable because of the different dark current dominant mechanisms. Test result shows that Hg Cd Te loophole photodiodes can achieve a performance around 1′1011cm HZ1/2W-1, and the best performance of the detectors is: the peak responsivity is 4.6mm@85K,the peak detectivity is 1.83×1011cm HZ1/2W-1, which is closed to the level of a planar junction. The mechanism of dark current under different biases is studied through numerical calculation. It is found that surface leakage current accounting for 80% of the dark current in zero bias is the main factor to hinder the performance of the detector. When operating in the high temperature, the responsivity of the loophole detector reduces a little, up to 30%, that result is significantly better than the performance of planar junction. Furthermore, the performance of the loophole detector at 230 K is 2 times higher than the planar detector fabricated by LPE material, which indicates that the loophole detector has an obvious advantage in high temperature operating. For avalanche performance of the loophole detector, it may achieve the maximum gain of 2100@10.6V in linear mode, whereas dark current is also multiplication.