Ph.D Thesis

Ph.D StudentVardi Alon
SubjectUnipolar Electro-Optical Devices Based on Nanometric
Structures in the III-N Material System
DepartmentDepartment of Electrical and Computer Engineering


Unipolar photodetectors relying on ISB transitions in QWs and QDs were successfully implemented and characterized. Most work in this field is performed on polar nitride structure, in which a strong internal polarization field up to 6 MV/cm exists in the growth direction. This field, combined with the large lattice mismatch between GaN and AlN, dictates entirely different design considerations for ISB devices. Devices that are based on vertical transport in the continuum, e.g. QWIPs, are unlikely to operate due to the alternating nature of the polarization fields. Our first solution was to relay on QD layers. In those devices, the carriers in the QDs are 3-D confined and, unlike QW structures, lateral devices may be implemented.  Intraband detector based S-Pz transition in nitride was demonstrated. We found out that, unlike other lateral QD intraband detectors, the in-plane transport of photoexcited carriers in the QD layer does not occur in the wetting layer.  Rather, it is due to various types of hopping mechanisms, depending on temperature. The existence of such mechanisms is related to the large band offset between GaN and AlN, and to the high density of QDs (up to 1012 cm-2). Thus, internal transitions within the QDs conduction band may result in photocurrent. We observed and characterize the S-Px,y transitions in GaN QDs, which were not previously characterized.

A novel photovoltaic (PV-QWIP) Quantum Cascade Detector (QCD) was designed and implemented. It is composed of a series of detection stages. Each stage has an absorbing well (active region) and an extractor region which are designed to extract carriers from the excited state of one stage to the ground state of the next. Unlike conventional QC structures, in our original design the extractor wells are of equal width. The internal field breaks the alleged miniband.  This design resulted in the best speed and responsivity performances reported so far in nitride-based NIR detectors. The ultrafast carrier dynamic in the QCD was investigated using a time-resolved bias-lead monitoring technique. It is demonstrated that the intrinsic speed limitation (carrier transit time) is smaller than 1 ps corresponding to a cut-off frequency above 200 GHz.

We designed and implemented a novel unipolar device which is hybridization of the lateral QD structure and the vertical QCD. An extractor region is used to extract carriers from the excited state of the QDs into a high mobility channel, where lateral transport takes place.