The proposed project involves research on novel multi-quantum-well (MQW) concepts, intraband processes and IR detector operating leading to significant improvements in focal plane quality, and to potential applications in high-performance infrared imaging systems. Under the previous NSF program, emphasis was placed on photo-emissive homojunction detector formats for detection in the far infrared (FIR). These studies led to the successful development of the new family of FIR detectors, with record values of wavelength cutoff. For optimum operation at shorter wavelengths (LWIR-VLWIR), quantum-well IR photodetectors (QWIP's), which can be bandgap engineered for more selective spectral response, are preferred.
Recent analyses summarized in this proposal have demonstrated that when operated in the steady-state photoconductive mode, IR QWIP detectors undergo an initial transient during which extensive redistribution and depletion of carriers in the well may occur. The number of wells contributing signal current decreases rapidly and responsivity and detectivity fall significantly, so that the full potential of such detectors cannot be realized. In collaboration with scientists at other universities (Cornell, NRC and UCLA) and national laboratories (JPL and U.S. Army Research lab) this problem will be addressed by exploring operation of the detector in a transient rather than a steady-state mode. Our initial thrust will be to experimentally study and model the transient IR behavior of prototype LWIR-VLWIR QWIP detectors, and to established the time-dependent operational physics of IR response in the steady-state mode. The role of intraband levels, well-doping, barrier parameters, emitter/collector structures, etc. in influencing transient values of IR response and dark current will be investigated. Time-varying operational bias conditions for re-fill of wells and periodic extraction of signal current will be devised. Extending on recent analysis of transient modes of detection and of space-charge distribution in multi-layer GaAs/AlGaAs structures, studies on the frequency and bias dependence of sensor response will be continued, with the aim of establishing optimum operation parameters. Optimized designed features, well-doping profiles, energy-selective QW barriers, emitter/collector barriers, etc. will be modeled and incorporated. These efforts should lead to the achievement of QWIP device architectures and focal plane drive and sensing circuits yielding significantly enhanced performance over all spectral ranges accessible to QWIP detectors.
Our studies of spontaneous pulsing in MQW structures and the frequency response to IR radiation will also be continued and extended, in order better to assess the potential of this operation mode in configuring higher performance detectors and focal planes.
The high national importance of this cutting-edge technology, and the close involvement of highly qualified staff and students at Georgia State, Cornell and UCLA will continue to impact significantly on advancing knowledge, and physics and engineering education making a significant contribution to the nation's science and technology base by supplying the scientific community with leading young scientists. The collaborations will also ensure effective transfer to emerging high-tech applications both in commercial and government laboratories. The results of the proposed research will vastly improve the understanding of detectors and form the basis for the next generation of detectors. In addition, realization of the proposed structures, will have an immediate impact on areas such as ultrafast electronic devices for logic, long wavelength IR digital data transmission or oscillator applications and in imaging applications. ***
