The objective of this research is to investigate quantum-dots grown within semiconductor nanowires for terahertz lasers that operate at room-temperature. Semiconductor quantum-dot materials are excellent candidates for intersublevel detectors and emitters because of their dramatic suppression of electronic energy relaxation assisted by optical-phonon scattering. A collaborative research effort will proceed in three stages: (i) rational growth of axial and core/shell heterostructures in nanowires for the formation of coupled-quantum dots, (ii) measurement and characterization of nanowire intersublevel absorption using infrared spectroscopy, and (iii) investigation of nanowire quantum-dot superlattices for electroluminescence and stimulated emission.
The intellectual merit is the use of nanowire quantum-dots for intersublevel cascade lasers, which are expected to solve a fundamental limitation of conventional planar THz quantum-cascade lasers, where non-radiative phonon-assisted relaxation prevents room temperature operation. Selective-area MOCVD epitaxy without metal catalysts will be used to grow high-aspect ratio InGaAs/GaAs semiconductor nanowires using lithographically defined oxide growth masks, with embedded axial quantum dots for efficient dot-to-dot tunneling transport.
The broader impacts are the development of a class of low-dimensional III-V semiconductor nanomaterials with highly engineerable quantum-electronic properties. This work will advance terahertz materials and technology, where sources and detectors are desired for sensing, imaging, and spectroscopy. The program will also integrate education and research in training of undergraduate/graduate students through laboratory research participation, complementary coursework development, and scientifically focused community K?12 involvement. A plan is in place for focused recruiting of undergraduate student researchers through student underrepresented minority societies and the UCLA Center for Engineering Excellence and Diversity.
The research objective of this project was to investigate quantum dots grown within semiconductor nanopillars for the ultimate goal of achieving terahertz lasers that operate at room-temperature. Sources of terahertz radiation are desired for a wide variety of applications, including security screening, spectroscopy, chemical and gas sensing, astrophysical measurements of the interstellar medium, high speed secure communications, and various biomedical imaging. However, above 1 THz room temperature sources are extremely limited in power, or are extremely large, or require cryogenic temperatures to operate. The terahertz quantum-cascade laser is an excellent candidate source, if only it could be improved to operate at room temperature, rather than its current maximum of 200K. A quantum cascade laser is based upon creating extremely thin layers of different semiconductors (i.e. heterostructures) in order to engineer the quantum mechanical electron states and wavefunctions. The semiconductor layers produce quantum confinement in one direction, but the electrons are still free to move in the other "in-plane" directions. A fundamental limitation of conventional THz quantum-cascade lasers is non-radiative relaxation of electrons by scattering with longitudinal optical phonons (a type of vibration of the crystal). This mechanism reduces the laser gain and performance at higher temperatures as the electrons gain kinetic energy moving "in-plane" within the quantum wells. We proposed a new material system based on quantum dots growth within semiconductor nanopillars as the foundation for a new type of cascade laser: the quantum-dot cascade laser. The quantum dots would quantum mechanically confine the electrons in all 3 dimensions, and create discrete states that would strongly reduce the likelihood of nonradiative phonon scattering. Hence, room temperature is expected to be possible. Our new approach is based upon the growth of semiconductor catalyst-free nanopillars using selective area epitaxy. These cylindrical nanopillars can be grown with extremely small diameters (< 50 nm), with large heights (several microns), in large, dense arrays. This approach avoids some of the difficulties that have stymied other approaches. For example, unlike the alternate approach of etching nanopillars from the top down, our approach is growth using selective area epitaxy from the "bottom up", and avoids issues of etch damage on the surfaces. Many challenges are required to be overcome in order to make this material system suitable for demonstrating quantum-cascade devices. The materials growth challenges included growing a series of atomically sharp axial heterostructures along the length of the nanowire in a deterministic fashion, grown shell heterostructures to passivate the surface, growing nanopillars over large areas with few defects and high uniformity, and controlling the crystal phase. Our collaborative research effort proposed to include three stages of effort: First, the rational growth of axial and core/shell heterostructures in nanopillars for the formation of coupled-quantum dots. Second, the measurement and characterization of nanopillar intersublevel absorption using infrared spectroscopy. Third, the investigation of nanopillar quantum dot superlattices for electroluminescence and stimulated emission, that would form the basis of THz quantum-dot cascade lasers. During our program, a great deal of progress was made in the materials growth of nanopillar heterostructure quantum dots. Several material system combinations were investigated for suitability for nanopillar growth: GaAs/InGaAs heterostructures, GaAs/GaAsP heterostructures, and InAs/InP heterostructures. Optimum growth conditions were identified for these systems. The best results appear to be in the InAs/InP material system, which resulted in high aspect ration nanopillars with highly abrupt heterostructure interfaces. Double-barrier resonant-tunneling-diode type structures were grown and tested electrically, and shown to exhibit a negative differential resistance characteristic – highly suggestive (but 100% conclusive) of sublevel quantization. Also, significant work went to develop a theoretical and experimental understanding of the growth mechanisms necessary to create high aspect ratio nanopillars in which growth takes place selectively upon the end of the pillar and sidewall growth is suppressed. However, this proved to be a project with very significant materials challenges and the majority of the effort was placed in materials development. At this time intersubband absorption or emission has not yet been definitely observed in nanopillar quantum dots or wells, which is the key next step towards developing quantum-cascade devices. Further research will be needed achieve the goal of nanopillar quantum dot cascade lasers.
