MSE PhD Final Oral Defense of Wei Zhang

2:00 pm on Thursday, March 6, 2014
4:00 pm on Thursday, March 6, 2014
TITLE: Development of Aluminum Gallium Nitride Based Optoelectronics Devices Operating in Deep UV and Terahertz Spectrum Range.

ABSTRACT: In this research project I have investigated the AlGaN alloys and their quantum structures for applications as emitters in the deep UV as well as for applications to optoelectronic devices in the terahertz spectral regions. The motivation for developing the deep UV emitters is a host of potential industrial applications. Such include, for example, non-line-of-sight free space communications, water and air disinfection, food and surfaces sterilization and a number of diagnostic and therapeutic medical applications. Similarly, the development of optoelectronic devices (emitters and detectors) operating at the terahertz are expected to find applications in product quality control, biomedical imaging, detection of explosives and bio-agents, material characterization and telecommunications.
For the deep UV emitter applications the AlGaN alloys and their quantum wells were grown by rf plasma-assisted molecular beam epitaxy on 4H-SiC, 6H-SiC of various miscuts and c-plane sapphire substrates. The surface morphology of the investigated materials was studied using scanning electron and atomic force microscopy. X-ray diffraction and transmission electron microscopy were employed to study the structure and microstructure of the films. The optical and recombination properties were investigated by photoluminescence and cathodoluminescence measurements as a function of temperature and the optical gain was determined by femtosecond optical pumping.
On the SiC substrates the investigated structures had the form of AlGaN /AlN multiple quantum wells, whose well and barrier thicknesses as well their total numbers were adjusted, based on Monte Carlo simulations, for electron beam excitation at an acceleration voltage of 10 KV. In these structures the AlGaN wells were grown under excess Ga, far beyond than what is required for the growth of stoichiometric AlGaN films. In this growth mode, the arriving active nitrogen and Al species dissolve first into the liquid Ga, covering the growing surface, prior to their incorporation into the AlGaN film. Thus, this growth mode is a liquid phase epitaxy rather than physical vapor phase epitaxy, which is associated with the traditional molecular beam epitaxy method. Due to the statistical variations of the excess Ga in the surface of the growing film we found that this growth mode leads to films with lateral variations in the composition and thus band structure potential fluctuations. Transmission electron microscopy shows that the well in such structures are not homogeneous but have the appearance of quantum dots. The degree of inhomogeneities in these samples was determined by cathodoluminescence mapping. If in addition we employ indium as a surfactant during growth the resultant wells are homogeneous. We find by temperature dependent photoluminescence measurements that the multiple quantum wells with band structure potential fluctuations emitting at 240 nm have a room temperature internal quantum efficiency as high as 68%. This high internal quantum efficiency is attributed to the localization of the injected excitons in the deep band structure potential fluctuations from where they recombine radiatively instead of diffusing and recombining non-radiatively in extended and point defects. Furthermore, the same multiple quantum wells were found to have a maximum net modal optical gain of 118 cm-1 at a transparency threshold corresponding to 1.4 x 1017 cm-3 excited carriers. We attribute this low transparency threshold to population inversion of only the regions of the potential fluctuations rather than of the entire matrix. Some prototype deep UV emitting LED structures were also grown by the same method on sapphire substrates.
Optoelectronic devices, based on GaN /AlGaN quantum cascade structures were designed for emission or light detection in the terahertz spectral region, were grown on single crystal c-plane GaN substrates. Such substrates were first planarized by inductively-coupled chlorine plasmas to remove a network of scratches associated with mechanical polishing. Growth conditions such the ratio of group III to active nitrogen fluxes, which determines the appropriate Ga-coverage for atomically smooth growth without requiring growth interruptions were employed. Transmission electron microscopy was used to ascertain that the interfaces in these structures are atomically smooth and the thicknesses of the individual layers are consistent with the design of the devices. Emitters designed in the quantum cascade structure were fabricated into vertical devices and the I-V characterization at 20 K indicates sequential tunneling with electroluminescence emission at about 10 THz. Similarly, Far-infrared photoconductive detectors based on intersubband transitions in III-nitride semiconductor quantum wells, designed to eliminate the internal fields associated with polarization effects, were grown by the same method and fabricated into devices. Photocurrent spectra centered at a wavelength of 23 �m (13 THz ) are resolved up to 50 K, with responsivity of approximately 7 mA/W.

COMMITTEE: Theodore Moustakas, MSE/ECE; Roberto Paiella, MSE/ECE; Karl Ludwig, MSE/Physics; Luca Dal Negro, MSE/ECE; Chair, TBA