Photoelectric Properties Research On Surface And Interface Of HgCdTe

Compound semiconductor material(Hg1-xCdxTe or HgCdTe) has high quantumefficiency and adjustable band gap which covers the entire infrared band, and manyother advantages. It has become the most important material for fabricating infrareddetectors. But HgCdTe’s band gap is narrow and the Hg-Te bond of its crystalstructure is very weak, the mercury atoms can escape easily. Thus material’s surfaceseverely affects prepared HgCdTe detectors, even to a large extent determinesphotoelectric performance of detectors. By far the most common solution is to usesurface passivation. In order to further improve the performance of HgCdTe detectors(mainly for photoconductive detectors), this paper takes HgCdTe’s surface as abreakthrough point and makes an relatively in-depth research on HgCdTe’s surface,HgCdTe/passivation film interface and micro-optical elements on HgCdTe surface.Firstly, this paper introduces in HgCdTe surface control device—gate-controlledphotoconductive detector to simulate the effect of detector surface/interface state onphotoelectric performance of device. Surface carrier concentration, mobility andeffective minority carrier lifetime are controlled continuously and quantitatively byelectric field produced by applying a gate voltage. Based on the multilayerdistribution model of surface layer, this paper establishes a comprehensive andsystematic physical model of HgCdTe gate-controlled photoconductive detector.After simulating n-type HgCdTe long wavelength photoconductive detector, we findthat, conventional anodic oxide/ZnS double passivation will cause excessiveaccumulation of surface carriers of HgCdTe. On one hand, it improves effectiveminority carrier lifetime. On the other hand, it will also reduce surface resistance ofdevice, which will leads to decreasement of performance of detectors. Thus theperformance of conventional photoconductive detector can still be improved.In order to furtherly study surface passivation, two most commonly used growingmethods for ZnS film, thermal evaporation and magnetron sputtering, are studied.ZnS films grown on n-type HgCdTe by these two methods are evaluated detailly.Morphology, element composition, crystal structure and optical properties of ZnSfilms are tested and analysed by SEM, EDX, XRD and FTIR, respectively.Interface electrical properties of HgCdTe and different double-layer passivationfilms are studied by MIS device. There are three kinds of n-HgCdTe MIS devicesusing double-layer passivation as insulating layer: anode oxide/ZnS (thermal evaporation), anode oxide/ZnS (magnetron sputtering) and CdTe/ZnS (thermalevaporation). I-V testing system on low temperature probe station andvariable-temperature C-V testing system are built independently. The leakage currentwithout bias voltage of MIS device with evaporated ZnS film is below10-13A, whilethat of magnetron sputtered ZnS film is big and easy to breakdown. Fixed chargedensities of three kinds of double-layer passivations obtained by C-V test are about1.091012cm-2,1.211012cm-2and-3.01011cm-2, with slow interface densities of1.271010cm-2eV-1,1.321010cm-2eV-1and1.071011cm-2eV-1. High-frequency C-Vcharacteristics is observed in anode oxide/ZnS (thermal evaporation) passivationsfrom the frequency of4MHz, which corresponds to fast interface state density of4.21011cm-2eV-1.Based on the results of device simulation and study of passivation films,n-Hg1-xCdxTe(x=0.23) gate-controlled long-wavelength photoconductive detectorsare successfully fabricated. SdH oscillation test shows that surface electron density ofdevice is about1.931012cm-2, which experimentally proves existence of thephenomena of surface excessive accumulation. Photoelectric properties ofgate-controlled device under different gate voltages are tested by improvedphotoconductor testing system. Responsivity changes similarly with photoelectricvoltage. The peak resposivity of device is792.7V/W which is obtained at-8.4V,while498.5V/W in condition of floating gate, which means that traditional operatingcondition does not maximize device performance. It is found that, additional devicenoise caused by gate voltage is connected with surface carrier concentration. Noisefrequency spectrum analysis shows that, the additional noise comes from thenon-basic1/f noise caused by fluctuation of HgCdTe’s surface carrier concentration.Finally, this paper makes some preliminary explorations on fabrication ofmonolithic integrated micro-lenses on surface of HgCdTe device. This paper presentsan ICP-RIE dry etching combined with chemical wet etching method to preparerefractive micro-lens on CdZnTe substrate. Linear micro-lenses with a depth of60μmand continuous deep relief structure are successfully fabricated on CdZnTe substrate,which have uniform radius of curvature, high crown, small F number, strongconvergence effect of infrared illumiation and good compatibility with fabricationprocess of HgCdTe detectors.