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Study On The Fabrication And Dark Current Characteristics Of Spaceborne Mercury Cadmium Telluride Infrared Photodetectors

Posted on:2017-03-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:H QiaoFull Text:PDF
GTID:1108330503964287Subject:Microelectronics and Solid State Electronics
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Study has been performed in this dissertation on the fabrication and dark current characteristics of Hg CdTe photovoltaic detector which is one of the most important photodevices for aerospace applications currently. The content and purpose of the study have been closely related with the need and necessity confronted in the aerospace application field. Dark current characteristic is a critical impacting element for the performance of HgCdTe photovoltaic detectors. Analysis has been carried out on surface recombination velocity and p type doping concentration in the optimum design of diffusion current limited detectors based on the discussion of physical mechanism of dark current.In order to resolve the problem of large dark current related with the depletion region in the fabrication of large area photodiodes, sub-pixel structure has been applied in the fabrication of short- and long- wavelength photodiodes with large areas. It was found that the sub-pixel structure could be beneficial in the decrease of dark current if the dark current was limited by bulk current, and a new concept of leakage bulk has been introduced to evaluate the effect among different sub-pixel structures. However, when the detector was dominated by surface leakage current which was caused by the surface borders, the sub-pixel structure would play a negative role and lead to the greater dark current since more borders were introduced by this structure.Discussion has been carried out on the metal-HgCdTe contact characterisitics. It was found that different characteristics would appear with the same metal-HgCdTe structure at different operating temperatures for short-wavelength detectors which had a larger energy band gap, and LBIC method has been applied to measure the change of photocurrent near the p-contact area at different temperatures. Through theoretical analysis it has been verified that the ohmic contact of p electrode arises from the recombination current in the depletion region together with thermionic emission current, which is different from the n contct results from the field emission effect. The further improvement could be obtained in the following aspcts: increase of electron density in the n contact region could be applied to form ohmic characteristic n contact, and there are two ways for p contact, one is to insert a region with high hole concentration and the other is to decrease the recombination lifetime in the depletion region.Background radiation could play an important role in the readout of photocurrent for long-wavelength detector since its cutoff wavelength is near to the peak emission wavelength of background radiation. Large number of carriers generated by background radiation would affect the performance of the detector which was manifested by the decreased signal and increased noise. A new model of thermal equivalence for photo-excited excess carriers has been brought forward to explain the generation-recombination current at low bias. And impact ionization multiplication model has been used to interpret the current at large bias in the current-voltage curve. These negative effects would be improved when the background radiation was coldly limited.Spectroscopy ellipsometry has been applied to evaluate the subsurface damage caused by polishing process in the bulk material HgCdTe photovoltaic detectors, and it has been found that a subsurface damage layer of around 15 micros could be induced by chemistry mechanical polishing(CMP), however, only several micros for bromine polishing process.Surface/interface characteristics has been evaluated by the fabrication of MIS devices with different polishing processes, and it was found that the fixed charge density were 1.2×1012cm-2 and 2.4×1011cm-2 for chemistry mechanical polishing and bromine polishing respectively. The density of slow interface state was 3.76×1010cm-2 eV-1 and 1.65×1010cm-2 eV-1 for two different processes.Variable-area diode array has been fabricated to evaluate the impact on the dark current performance of different polishing processes. It was found that the surface leakage current was dominant for the detectors fabricated by CMP process, and the detectors fabricated by bromine polishing was dominated by bulk effect. The detectors fabricated by bromine polishing shared a lower dark current with the same implanted area, and showed a lower noise accordingly manifested by noise spectrum measurement.Study on the physical mechanism of decreased dark current by hydrogenation for HgCdTe photodiodes. A decreased electron density in the n-HgCdTe and an increased hole density in the p-HgCdTe have been observed after hydrogenation. It was also found that hydrogenation performed out of the implanted area would do good to the dark current performance. The fitting results showed that the carrier generation-recombination lifetime in the depletion region was improved by hydrogenation and a lower dark current was attained accordingly.Dynamic γ irradiation effects on the mid-wavelength HgCdTe photodiodes have been studied, and two typical irradiation effects were observed: ionization effect and displacement effect. The former was reflected by the generation of photocurrent similar current at zero bias in the radiation process, and the latter was manifested by an increased series resistance by γ radiation which has been interpreted by the two-energy-level model induced by γ radiation. It was also found that hydrogenation could be used to partly eliminate the displacement effect, but it could do nothing to the ionization effect.Monolithic mid-wavelength HgCdTe single element photodiode operating at near room temperature integrated with immersion lens has been fabricated successfully. The immersion lens with a diameter of 1.5mm and a height of 0.97 mm has been fabricated by single point diamond turning method on the CdZnTe substrate. The detectivity of detectors integrated with immersion lens was about 4 times bigger than those without lens. A photo response area of 1300μm×1300μm was achieved evaluated by LBIC for the immersed detectors with an electrically active area of approximate 180μm×180μm.
Keywords/Search Tags:HgCdTe photovoltaic detector, dark current, subsurface damage, γ irradiation effect, immersion lens
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