Studies Of The Mid-wavelength Interband Cascade Infrared Photodetector

One of the important development trends of infrared detection is"SWaP",which means smaller size,lighter weight and lower power consumption.The infrared photodetectors generally work at the temperature of liquid nitrogen due to the nature of narrow bandgap semiconductor,and the cooler system is the main reason for the size and power consumption of the detection system.Therefore,increasing the operating temperature of the infrared detector can reduce the power consumption and volume of the cooler system,and promote the applications of infrared detection technology in miniaturized devices such as portable handle equipment.Infrared detectors face two main problems at high operating temperatures:one the one hand,the dark current of the detector is an exponential function of temperature,and the dark current increases sharply as the operating temperature rises.On the other hand,the diffusion length of minority carriers decreases with temperature,and the quantum efficiency decreases.This problem can be overcome by the interband cascade structure.The interband cascade infrared photodetector(ICIP)is a multiple-absorption cascade structure based on antimonide superlattice materials,which can achieve the directional transport of photogenerated carriers through relaxation and tunneling region,and avoids the formation of PN junction which can suppresses the dark current.At the same time,the cascade structure can increase the device resistance,so that the device can match the amplifier or output circuit better.The cascade structure with short absorption thickness can reduce the recombination of photogenerated carriers,and can effectively collect photogenerated carriers even when the diffusion length is very short,thereby the detectivity can improved at high operating temperatures.This paper mainly focuses on the development of mid-wavelength interband cascade infrared photodetectors grown on InAs substrates,and studies the detectivity and response time of interband cascade detectors.The main contents of this paper are as follows:(1)Structure design for the interband cascade detector.The interband cascade detector can overcome the limitation of diffusion length to the detection performance of the detector,and the detectivity can be improved using the multiply stage structure including the number of stage and absorber thickness.Proper structure design is very important for the interband cascade detector,and the relationships between the quantum efficiency and detectivity of the ICIP is derived.The maximum detectivity of ICIP is calculated for different diffusion lengths,and the thickness of the absorber thickness for each stage is calculated for the photocurrent-matching architecture.The diffusion length of the mid-wavelength InAs/GaAsSb superlattice absorber is fitted according to the responsivity at different temperatures.For the operating temperature of 300 K,the ICIP structures with different stages are designed according to the fitted diffusion length.(2)Characterization on the material interface of interband cascade detectors.In the ICIP structures,precise information of the energy levels is in particular important as the transport process of the photogenerated carriers heavily relies on proper energy level alignments.A crucial parameter in the design of the ICIP is the accurate control of the composition of the alloy in each layer.Using molecular beam epitaxy system,mid-wavelength ICIPs was grown on an InAs substrates and Scanning transmission electron microscopy(STEM)analyzation is performed.The interface structures and chemical composition profiles of the InAs/GaAsSb superlattice absorber,InAs/AlAsSb relaxation region and GaAsSb/AlAsSb tunneling region are characterization.Combined with the high-resolution X-ray diffraction results,the atom composition distributions of the absorber,relaxation region and tunneling region are fitted.The energy band calculation model is improved by using the interface composition profile.The cutoff wavelengths of InAs/GaAsSb superlattices with different InAs thicknesses are calculated considering the interface,and are in good agreement with the experimental results.(3)The fabrication and performance measurement for the ICIPs.After the material growth,the interband cascade detectors are fabricated,and the electro-optical performance of ICIPs with different stages and absorber thicknesses are measured and analyzed.For one-stage and three-stage interband cascade detectors,the responsivity of the device starts to decrease from the temperature of 220K,while the responsivity of the ten-stage device is nearly unchanged even at high temperature due to the shorter absorber thickness.At high operating temperatures,the responsivity of the one-stage ICIP is reduced due to the decrease of diffusion length,while the responsivity of ten-stage device is nearly not affected by the diffusion length,so that the detectivity can be improved.At the temperature of 300K,the detectivity of one-stage,three-stage,five-stage,and ten-stage ICIPs with front-illuminated architecture are:4.5×10⁸,4.9×10⁸,7.6×10⁸,8.0×10⁸ cm·Hz^1/2/W,respectively.The ICIPs with immersion lens is fabricated by the InAs substrates into lens for the back-illuminated devices to improve the responsivity of the detector.The responsivity of the device is increased by about 7.5times by the means of immersion lens and anti-reflection film.The ten-stage interband cascade detector with immersion lens has a detectivity of 1.4×10¹⁰ cm·Hz^1/2/W at the temperature of 220 K and wavelength of 5?m,and detectivity of 4.7×10⁹ cm·Hz^1/2/W at the temperature of 300 K.(4)Characterization on the response time of interband cascade detectors.The response time for the interband cascade detectors with two-stage thermoelectric cooling,is analyzed.By comparing the response time of the device with different sizes,stages and bias,the it was found that the device is mainly limited by the RC time and the diffusion time of the photogenerated carrier.The response time of the device with a shorter absorber thickness and reverse bias can be reduced.The rise time of the ten-stage ICIP at the bias of-1.3 V is 0.28 ns,and the fall time is 0.51 ns.