| dc.description.abstract | Mid-infrared (IR) optoelectronic devices form the basis for many practical applications such as thermophotovoltaic (TPV) energy conversion, gas sensing, thermal imaging, medical diagnostics, free-space communications, infrared countermeasures and IR illumination. The mid-IR device family based on interband cascade (IC) structures includes IC lasers (ICLs), ICTPV cells and IC infrared photodetectors (ICIPs). These are special types of multistage devices whose operation is made possible by the unique properties of the 6.1 Å material system: InAs, GaSb and AlSb, and their related alloys. One of the key properties is the type-II broken-gap alignment between InAs and GaSb.
In multistage ICTPV cells and ICIPs, electrons must undergo multiple interband excitations in order to travel between the electrical contacts. This means that the transport of a single electron requires multiple photons, which reverses the situation in ICLs where a single electron can generate multiple photons. Counterintuitively, this transport feature in ICTPV cells and ICIPs is conducive to improving device performance by enhancing the open-circuit voltage in ICTPV cells and suppressing the noise in ICIPs. Furthermore, the collection efficiency of photo-generated carriers in multistage IC devices can be significantly improved by thinning the absorbers in individual stages. Collectively, these advantages make IC structures an attractive choice for narrow bandgap optoelectronic devices, especially for operation at high temperatures. One focus of this dissertation is to outline and demonstrate the advantages provided by IC structures, both in theory and experiment. Another focus of this dissertation is to obtain a better understanding of the physics of IC devices and gain insights into their operation.
Theoretical studies of single-absorber and multistage ICTPV cells are presented. The limitations in efficiency are understood by considering several important practical factors. These factors are identified to be closely associated with a short carrier lifetime, high dark saturation current density, small absorption coefficient, and limited diffusion length. The multistage IC architecture is shown to be able to overcome the diffusion length limitation that is responsible for the low quantum efficiency (QE) in single-absorber TPV cells. This ability of the IC architecture offers the opportunity to enhance conversion efficiency by about 10% for wide ranges of aL (product of absorption coefficient and diffusion length) and bandgaps, resulting in a particle conversion efficiency approaching 100%.
The illustrated theoretical advantage of multistage IC structures is confirmed experimentally in a comparative study of three fabricated TPV devices, one with a single absorber and two that are multistage IC structures. The bandgap of the InAs/GaSb type-II superlattices (T2SLs) in the three devices is close to 0.2 eV at 300 K. The extracted collection efficiency is considerably higher in multistage IC devices than in the single-absorber device. To further investigate the prospects of IC TPV cells, detailed characterization and performance analyses of two sets of four IC devices with similar bandgaps are performed. The four different configurations enable a comparative study that shows how device performance is affected by material quality variations, as well as by current mismatch between stages and collection efficiency.
The carrier lifetime advantage of IC devices over another family of cascade devices, namely quantum cascade (QC) devices, is manifested in the saturation current density (J0). The values of J0 extracted using a semi-empirical model, are more than one order of magnitude lower in IC devices than in QC devices. The significance of J0 on the performances of IR detectors and TPV cells is apparent in a comparison of the measured detectivity (D*) and the estimated open-circuit voltage (Voc). To extract the carrier lifetime in IC devices, a simple and effective electrical method is developed. This method is more generally applicable and considers the parasitic shunt and series resistances found in practical devices. It provides a simple way to extract the carrier lifetime in InAs/GaSb T2SLs in a wide range of operating temperatures.
The effect of current mismatch on the performance of ICIPs is investigated using two sets of devices with current-matched and noncurrent-matched configurations. It is shown that current matching is necessary to achieve maximum utilization of absorbed photons for an optimal responsivity. The detectivities of both sets of devices are comparable largely due to the occurrence of a substantial electrical gain in noncurrent-matched ICIPs. The electrical gain is shown to be a ubiquitous property for noncurrent-matched ICIPs through the study of another three devices. To unlock the mechanism underlying electrical gain, a theory is developed for a quantitative description and the calculations are in good agreement with the experimental results. | en_US |
