Dark Current Analysis Of Long Wavelength HgCdTe Infrared Detectors On Alternative Substrates

In order to increase the detection distance and achieve high-temperature sensitivity detection,the development of long-wave,large-scale focal plane detector technology is one of the important technical features of the third-generation focal plane arrays?IRFPAs?.The infrared focal plane technology of mercury cadmium telluride on alternative substrates have successively been developed by many research institutions such as RVS in the United States,and its advantages such as low cost and high reliability have been fully applied to medium-and short-wave large-scale staring detectors.Based on the in-situ p⁺-on-n structure on the alternative substrate,the development of a long-wavelength?cut-off wavelength up to 11.7?m?mercury cadmium telluride infrared focal plane device has also received widespread attention.However,the large dark current and low detectivity of the long-wavelength devices on alternative substrates caused by the high density of dislocations have also been big problems.Based on these problems,the dark current mechanism and the in-situ doping p⁺-on-n device technology of the long-wavelength HgCdTe devices were studied to explore the ways to inhibit the dark current.It is of great significance to promote the development of high-performance and low-cost long-wave mercury cadmium telluride detector technology in China.Based on MBE in-situ doping growth of the p⁺-on-n heterojunction and ion implantation of the n⁺-on-p junction on long-wavelength HgCdTe materials on different substrates,the performance of the devices was compared and verified.The R-V curve was simulated by I-V test analysis,and the parameters were extracted by fitting.The dominant mechanism of the dark current of four devices at different temperatures is clarified and compared.The study shows that for the n⁺-on-p junctions,the performance of the Si-based HgCdTe devices?cutoff wavelength is about 9?m?above 80K is the same as that of the Cd Zn Te-based HgCdTe devices.However,as the temperature goes down to below 60K,dislocations up to 10⁶cm^-3inside the Si-based HgCdTe device contribute a lot to the trap concentration.The dislocations also introduce the SRH recombination centers that significantly reduce the minority lifetime in the depletion region.Therefore,the zero-bias dynamic resistance of the Si-based HgCdTe device below 60K is 2 to 3 orders of magnitude lower than that of the Cd Zn Te-based device.The p⁺-on-n mesa junction device,due to the low carrier concentration,minority mobility and relatively higher minority lifetime of the absorbing layer,reaches a better performance than the n⁺-on-p junction.The zero-bias dynamic resistance of the p⁺-on-n junction is one order of magnitude higher above 80K,and 2 to 3 orders of magnitude higher below 60K,than that of the n⁺-on-p junction.The p⁺-on-n junctions on the alternative substrates with various arsenic doping concentrations were also compared.The result shows that the high As doping concentration in the p-region can introduce high concentration of traps,which lead to the rapid increase of the trap-assisted tunneling current and affect the device performance seriously.Improving the crystal quality of the absorbing layer and controlling the doping concentration of As to restrain the trap-assisted tunneling current are the keys to the preparation of high-performance long-wavelength in-situ doped p⁺-on-n devices.In this paper,the in-situ arsenic incorporation and activation process of MBE-growth HgCdTe were also studied.The results show that the arsenic incorporation in single-crystal HgCdTe material is influenced by both the substrate temperature and the ratio of Hg flux.The steady temperature and appropriate Hg flux are the keys to the steady incorporation of arsenic.The comparison of materials with various Hg/Te indicates that the Hg vacancy blocks the As incorporation,experimentally explaining that the higher the Cd composition is,the higher the As concentration will be.The arsenic doping uniformity of the 3-inch MBE epitaxial materials was also studied.By comparing the distribution uniformity of As-doped Cd Te,and As-doped HgCdTe with different Hg/Te,the key factors of the As doping uniformity were clarified.An appropriate Hg flux is of great importance to ensure the uniformity of As distribution.In terms of the arsenic activation,annealing at 300?/16h+420?/1h under Hg saturated vapor pressure can significantly increase the As activation rate of long-wavelength HgCdTe to higher than80%.The electrical properties of the long-wavelength p-type As-doped HgCdTe materials with different activation processes were compared and characterized by LBIC and minority carrier lifetime tests.With the same p-type carrier concentration,the diffusion length and minority carrier lifetime are positively correlated with the activation rate.Based on the above-mentioned researches on the dark current mechanism,the p⁺-on-n devices were designed and tested.The zero-bias dynamic resistance of the Si-based long-wave unit device with a cut-off wavelength of 9.66?m is comparable to that of the foreign n⁺-on-p Cd Zn Te-based ones.The detection rate(1.7×10¹¹cm Hz^1/2/W)and the noise equivalent temperature difference?27.8m K?of the 128×128 small-scale Si-based 8.8?m long-wavelength focal plane achieve the same level as those of the Cd Zn Te-based devices with the equivalent cut-off wavelength.These results show that the preliminary verification of material structure and device performance of HgCdTe in-situ doping heterojunction on the alternative substrate has been completed.