Studies On Long-wavelength InAs/GaSb Superlattice Infrared Detectors Grown By MOCVD

InAs/GaSb type-? superlattices(T2SLs)are considered as the most potential candidates for long-wavelength infrared(LWIR)detectors due to their low Auger recombination rate,good material uniformity and low manufacturing cost.After decades of development,great progress has been made in InAs/GaSb SL detectors.Up to now,all the InAs/GaSb T2SL detectors and focal plane arrays(FPAs)have been exclusively grown by molecular beam epitaxy(MBE).On the other hand,as the commonly industrial epitaxy technique,metalorganic chemical vapor deposition(MOCVD)has the advantages of mass throughput production,easy maintenance process and flexible production configurations.On the basis of our group growing high-quality InAs/GaSb T2SL on InAs substrate by MOCVD,an Al-free single heterojunction LWIR detector structure was proposed,named "PNn".After theoretical calculation and experiments,a set of high performance LWIR detectors are demonstrated.Besides,the first MOCVD-grown T2SL FPA was fabricated and successfully imaged.The contents of this dissertation are as follows:1.An Al-free single heterostructure detector structure which can be grown by MOCVD was proposed,named "PNn",in which P represents p-type doped InAs/GaSb mid-wavelength superlattice(MWSL)as contact layer,N represents n-type doped InAs/GaSb MWSL as barrier layer,and n represents n-type doped InAs/GaSb LWSL superlattice as absorber.By adjusting the appropriate doping level and thickness of the barrier layer,the depletion region of the device completely falls into the wide-bandgap MWSL layer,so as to suppress the generation-recombination(G-R)and tunneling current while the quantum efficiency of the device is not affected.Theoretically,the PNn detectors can achieve the diffusion-limited performance.2.The PNn detectors with high-performance were demonstrated.The structure of the device was grown by MOCVD,and the doping level of each layer was calibrated by hall measurement.All the material characterizations indicate high crystal quality of the MOCVD-grown SL.For device fabrication,a shallow-etched technique was employed to suppress the surface leakage currents.At 77 K and-0.1 V,the detectors exhibit a dark current density of 2.4 × 10-5 A/cm2,a 50%cut-off wavelength of 8?m,a 100%cut-off wavelength of 10 ?m,a peak responsivity of 2.1 A/W at 6.5 ?m,a quantum efficiency of 41.2%and a peak detectivity of 7.3×1011 cm·Hz1/2/W,which is comparable to those of MBE-grown detectors at similar cut-off wavelength.Besides,PNn detectors have shown a diffusion-limited behavior above 75 K by temperature-dependent dark current measurement.3.The PNn device performance was improved by optimizing the doping level of the barrier layer.If the barrier layer is doped too high,although the dark current of detectors can be reduced,the transport of the photocurrent will also be hindered.Only a higher bias is applied,the detectors would work again.If the doping level of the barrier layer is too low,the depletion region of the device will enter the narrow-bandgap absorber from the wide-bandgap barrier,resulting in the increase of generation-recombination(G-R)and tunneling current.Only when the barrier doping level is appropriate,the depletion region of the device just falls in the wide-bandgap barrier region,and the absorber maintains a flat band.Thus,the G-R current and tunneling current can be suppressed without affecting the photocurrent transport.At 77 K and-0.1 V,the optilized device exhibited a dark current density of only 4.5×10-4 A/cm2,a RA of 399 2·cm2,a 50%cut-off wavelength of 12 ?m,a peak responsivity of 1.01 A/W and a peak detectivity of 1×1011 cm·Hz1/2/W at 8 ?m.The detectivity of the device still remains 5×1010 cm·Hz1/2/W from 10?m to 12 ?m.The high performance of PNn devices grown by MOCVD is comparable with that of Al-containing detectors grown by MBE.4.The lateral diffusion length of MOCVD-grown LWIR InAs/GaSb SL detectors was measured.At 80 K and-0.1 V,the deeply-etched devices exhibit a diffusion-limited performance,with a dark current density of only 9.1×10-6 A/cm2,and a 50%cut-off wavelength of 10.1 ?m.In shallow-etched pixels with a common absorber,the photocurrent density and dark current density of the shallow-etched device shows a size-dependent behavior.The diffusion length estimated from the two methods are 211?m and 251 ?m respectively,which is much longer than those in MBE-grown SLs.5.The first MOCVD-grown InAs/GaSb T2SL LWIR FPA was demonstrated.A 320 x 256 MOCVD-grown T2SL FPA with 30 ?m pixel pitch was fabricated without thinning down the InAs substrate and no antireflection coating was applied.At 80 K and bias voltage of-0.1 V,the FPA shows an average operability of 96.96%,a non-uniformity of 4.97%,a noise equivalent temperature difference of 51.1 mK,a responsivity of 1.0×108 V/W and a peak detectivity of 2.3×1010 cm·Hz1/2/W with integration time of 1.9 ms and a lens F-number of 2.0.Although the performance of this first MOCVD-grown T2SL FPA is inferior to those of the state-of-art MBE-grown FPAs,it definitely demonstrates the potential of MOCVD-grown T2SL materials for high-quality infrared imaging.