Studies On Surface Properties Of InAs/GaSb Type-? Superlattice Infrared Photodetectors

InAs/GaSb type ? superlattice is an emerging material for the third generation of infrared focal plane arrays?IRFPAs?for its adjustable energy band,high absorption to vertical incident light,wide absorption band covering from 3 to 30?m and the maturity in theory,fabrication equipment and techniques of ?I-V semiconductor materials.With years of development,type ? superlattice has been used to make IRFPAs with various wavelength covering short wavelength,middle wavelength?MW?,long wavelength?LW?,and very long wavelength region and with different chromatography including daul-color and multi-color design.The more advanced IRFRAs may require smaller pixel size,larger focal plane size,higher operating temperature and higher device performance.However,surface leakage current was gradually found to be the main factor restricting the electrical performance of type ? superlattice devices,especially in the long wavelength?LW?range.The quality of passivation layer is of great importance for suppressing leakage current.Degradation will happen in the following fabrication process if poor passivation layer is formed,leading to surface leakage channel and poor electrical performance of infrared detector.Although a lot of work has been done on the surface passivation of optoelectronic devices,the surface properties and passivation of superlattice infrared detectors need to be further studied,to achieve the repeatable passivation process for superlattice focal plane devices with different wavelength.Analysis of the passivation is also necessary for specific material structure and process conditions.In this thesis,the surface charge,surface leakage current composition and passivation layer stability of the superlattice mesa structure were studied with suitable characterization technology.The specific contents are as follows:1.The effect of different surface treatment on Si N_x passivated long wavelength devices was studied.Treatments such as O₂ plasma,acid etching solution,Na Cl O solution,N₂O plasma and ammonium sulfide solution were performed.And their influence on the side wall passivation and leakage current as well as the thermal stability of the devices after rapid thermal annealing at 250?for 1 minute were investigated.The results showed that the plasma oxidant can inhibit the side wall leakage current of devices with_%=10?m.The N₂O plasma treatment with additional annealing process was found to elevate R₀A from 2.72?cm² to 25.25?cm²,namely 9.3 times increasing.2.The N₂O plasma surface treatment and annealing process were studied by variable area diode array and XPS.The side leakage current extracted from the current-voltage characteristics of variable area diode array showed obvious linear characteristics,which confirmed the existence of side parallel resistance.By fitting of the sidewall current,the parallel resistivity of the device was estimated to increase from17.9?cm to 297.6?cm after the N₂O treatment and annealing process;however,the surface charge concentration Q_s of the sample increased from 2.33×10¹¹cm^-2 to3.72×10¹¹cm^-2.XPS results showed that Sb₂O₅ was highly related to the surface parallel resistivity,the content decreased from 46.3%to 18.4%after optimization,while the surface Sb content was related to the surface charge concentration,whose content increased from 2.41%to 8.99%.3.The gate-controlled structure was used to study type-? superlattice devices.It was proved that the effectiveness of gate-controlled structure is highly related to the band-structure.The gate-controlled structure can effectively control the p-i-n MW devices,but not for p Bi Bn LW devices with barrier.The transfer characteristics showed that passivation of both Si N_x and Si O₂ can make the surface fermi level of MW devices close to flat-band and obtain better leakage current inhibition.However,annealing changed the interface charge,resulting in a decline in the dark current suppression of MW devices.The large flat-band voltage offset of Si N_x passivated device corresponded to the positive charge density larger than 1.4×10¹²/cm²,which lead to surface inversion and tunneling,as well as the severe decreasing of the R₀A for the device by 4 orders of magnitude.Similarly,the R₀A of the Si O₂ passivated device decreased by half an order of magnitude.However,there was no significant change in the LW devices for small reverse bias condition under the control of gate voltage.4.Characterization was implemented on the surface of MW and LW superlattices using Metal-Insulator-Semiconductor?MIS?structure.The MIS structure can effectively extract the carrier doping and interface traps for MW material;however,its capability is limited by a lot of non-ideal conditions for LW material.For the first time,varying temperature CV tests were performed to obtain the trend with temperature of carrier concentration,interface trap concentration and mobile charges in dielectric layer for MW material.It was found that the influence of surface contamination and annealing mainly affected the fixed charge in the interface of MW mterial.Due to the narrow band gap and surface tunneling,the device with type ? superlattice LW materials still exhibited low frequency CV characteristics at 1MHz frequency.The results showed that there were traps with concentration of 4.2×10¹²/?cm²·e V?with near-uniform energy distribution and an additional 4×10¹⁰/cm² single level traps in LW materials,resulting in W-shaped CV curves.5.The TCAD simulation model of type ? superlattice was established.The TCAD deck was utilized to extract the fixed charge,carrier concentration and trap distribution for non-ideal MIS structure.More importantly,it could realize the electrical simulation of gate-controlled structure and the barrier structures,verifying the different applicability of gate-controlled structure to MW and LW materials.