Optimization And Performance Limitation Of Quantum Well Infrared Photodetecotrs

Quantum Well Infrared Photodetectors (QWIPs) which are based onintersubband transition, have the advantages in mature processing,material technology. QWIPs have been successfully applied in the highspeed measurements and focal plane arrays. Optimizing the performanceof QWIPs is an interesting discussion on the relationship between thestructure parameters and working conditions. The optimizations of thebackground limited performance (BLIP) temperature and the detectivity(D*), which is one of the figures of merit of QWIPs, are investigatedbased on typical design of GaAs/AlxGaAs1-xQWIPs. Under differentoperating conditions, the performance limitations of QWIPs are alsoanalyzed.By analyzing the influence of doping concentration on backgroundlimited performance temperature (TBLIP) of QWIPs, our theoreticalcalculation shows that TBLIPincreases monotonously with decreasingdoping density, taking into account the temperature dependence of theFermi energy EF, a factor neglected in previous analyses for typicaldesigns. In contrast to the generally accepted optimal doping condition EF =kBTBLIP, in which kBis the Boltzmann constant. There is no limitation onthe maximum BLIP temperature with decreasing the doping density. TheBLIP temperature measurements were performance on a series of QWIPsof typical design for9μm peak wavelength with different doping values.The numerical modeling results are agreed well with the experimentmeasurements. Based on the Ershov gain model, the operating conditionsto improve the TBLIPare investigated. The BLIP temperatures can be madelarger by increasing the applied field to reduce the capture probability,number of periods and doping density.In addition, analysis and measurements of detectivity wereperformance on a series of typical designed9μm QWIPs. Based on thegeneral expression of the Fermi energy which taking into account thetemperature and doping effect, the optimized doping density to achievemaximum D*is given by EF=1.37kBT, a revision to the previous EF=2kBT condition. Theoretically, the amount of dark current limited peak D*is expected to increase by2.8%when the ratio EF/kBT changes from1.37to2. In order to investigate the influence of the doping profile, bysolving a self-consistent the Schrodinger equation combine with thePoisson equation, we can get more accurate energy band profile and theFermi energy level. To investigate the influence of the capture probabilityand the number of periods on the D*, different optimizations are providedfor the dark current limited and the background limited D*, respectively. A trade-off operating condition was analyzed between the two aboveoperating condition. Optimization the D*is desirable for small apertureand low signal flux condition in FPA applications. However, if theperformance of FPA is not limited by the D*but by the quantumefficiency, higher doping is desirable.The strong signal flux photocurrent can completely suppress thedark current noise and enhance the signal-to-noise ratio when the QWIPis illuminated by laser (e.g., laser spectroscopy), the performance of thedevice is determined by the signal radiation noise and we call itphoton-noise limited performance (PLIP). The Fermi energy dependencesof doping densities are obtained of various structural and physicsparameters at different temperatures, based on2D+3D electrons densityof states model. The dark current measurement results show the validityof the Fermi energy model. This model is especially important for QWIPsworking close to near room temperature regime. The threshold conditionsare also investigated to achieve the near-room-temperaturephoton-noise-limited performance of QWIPs, in particular the laserpower density requirement for different peak wavelength detection. At250K, which is easily reachable with a TEC cooler, ideally the PLIP canbe reached up to12μm for typical design QWIPs. At200K, which isclose to the lowest temperature a TEC cooler can reach, it is shown thatthe ideal performance can be reached to a detection wavelength regime 3~15μm. Therefore, QWIPs are suitable for laser spectroscopy and tracegas detection with compact devices operating near room temperature.