Development Of Mid- And Far-infrared Photodetectors And Study Of Photoelectric Performances

Infrared(IR)and terahertz(THz)detectors are highly desirable for astronomy,military,and national economy fields.Although IR detectors were born over 200 years ago and THz detectors have been developed for decades,there are still many challenges to be overcome,and two of them are as follows.(1)Increasing the working temperature of detectors is one of the important development directions of the IR detection technology.The ideal operating temperature is room temperature,such that not only is the usage cost of the detection technology lowered,but also the size and the weight of the detection system are reduced,with the usage convenience improved.However,most IR photodetectors cannot perform well at room temperature with either slow response speeds or low sensitivities,failing to meet the demands from advanced detection systems.(2)The development of IR and THz photodetectors in the astronomical observation is slow in China.Blocked-impurity-band(BIB)detectors are the preferred type of photodetectors for astronomical observation(in long-wavelength,very-long-wavelength IR and THz bands).BIB detectors have been studied for 20 to 30 years in Europe,America and Japan,and have been applied in many large-scale astronomical projects.In China,however,the development of BIB detectors is still in its infancy.Almost all research work has been focused on Si-based BIB detectors,and those on Ge-based BIB detectors are far from enough.Focusing on the above two challenges,this thesis presents the scientific research advance,mainly including the creative work on the development of room-temperature mid-IR(MIR)photodetectors and low-temperature Ge-based BIB detectors.The innovative research results are as follows:1.Based on a two-dimensional electron gas(2DEG)at the interface of CdTe/PbTe(111)heterojunctions,a novel type of room-temperature MIR detectors was developed.The detectors possess identical asymmetric structures and operate without external biased voltage.Based on the unique energy-band alignment of the heterojunction interface,the IR response originates from the intrinsic transition of the PbTe layer.The response mechanism of the detectors is lateral photovoltaic effect,and thus the detectors are called lateral photovoltaic MIR detectors(LPVMIRDs).Following the working principle of the LPVMIRDs,an analytical model was established where device performances were predicted theoretically.The proposed model can be extended to other heterojunction systems based on a 2DEG.Furthermore,a series of LPVMIRDs with different structural sizes were developed experimentally,and their response characteristics verify the reliability of the model.The cutoff wavelength of the response spectra is approximately 4 ?m,corresponding to the bandgap of PbTe at room temperature.At a wavelength of 3?m,the highest detectivity of the fabricated LPVMIRDs reaches 1.3 × 1010 Jones.Moreover,the measured impulse responses demonstrate an extremely fast speed.The novel high-sensitivity and high-speed detectors have an application prospect in the room-temperature IR detection field.2.By implanting sulfur(S)ions into high-purity germanium(Ge)substrates,a new multiband IR detector,i.e.Ge:S planar BIB detector,was developed.The photocurrent spectrum at 20 K shows that there are three response bands with peak wavelengths at about 1.4 ?m(band#1),1.6 ?m(band#2)and 6.5 ?m(band#3),respectively.The study revealed that the three bands are attributed to the electronic transitions from the valence band to the conduction band at ? and L points,and from the impurity band to the conduction band,respectively.At 4 K,band#2 disappears and the peak detectivities of bands#1 and#3 are calculated to be 2.1 × 1011 Jones and 1.1 × 1011 Jones,respectively.The development of this detector provides a new route for realizing multiband IR detectors.In addition,two operating modes,i.e.forward bias mode(FBM)and reverse bias mode(RBM),for the impurity-band response of the Ge:S BIB detectors with different blocking layer(BL)lengths were further studied.Under different modes,the optimum bias and the maximum peak photocurrent are different due to different electric field intensity distributions in both absorbing layer(AL)and BL.As BL length increases,the ratio of the maximum peak photocurrent under RBM to that under FBM rises,because there are a small but not-negligible number of impurities in BL.3.Phosphorus-doped Ge(Ge:P)planar BIB IR/THz detectors were also developed using ion implantation,the response spectra of which show ultra-broadband and multiband characteristics.There are three response bands in the spectra,ranging in 3?4.2 ?m(MIR band),4.2?28 ?m(IR band),and 40?165 ?m(THz band),respectively.MIR and IR bands originate from the defect bands of the vacancy-P atom(V-P)pair and the Ge self-interstitial,respectively,and THz band arises from the P impurity band.The peak detectivities of the three bands are 2.9 × 1012 Jones(@3.9 ?m,MIR band),6.8 × 1012 Jones(@16.3 ?m,IR band),and 9.9 × 1012 Jones(@116 ?m,THz band),respectively.Compared with other multiband(broadband)IR or THz photodetectors,the Ge:P BIB detectors display a significant advantage of high sensitivity.It can be predicted that this novel low-cost ultra-broadband multiband detectors with high sensitivity will gain extensive attention and important applications in the IR&THz detection fields.In addition,the relationship bewteen the impurity-band response(i.e.THz band)and the structural parameters,such as AL and BL lengths,was also studied.By measuring the blackbody response,the impact of the structural parameters on the response of the developed detectors was explored,and besides,the internal mechanism of the impact was understood by calculating the electric field intensity distributions in AL and BL.The optimization of the structural parameters is of great significance to improve the response characteristics for BIB detectors.