Exploring ultra-wide bandgap ambipolar transparent conducting semiconductors for deep ultraviolet optoelectronic devices

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Semiconductor laser diodes are an important component in a wide range of consumer and industrial products. The basic structure of a laser diode contains p- and n-type semiconductor thin film layers. Majority carriers in n- and p-type semiconductors are electrons and holes, which can be regarded as negatively and positively charged particles, respectively. Once the device is connected with an energy source such as a battery, the electrons and holes meet in the junction and recombine to emit light. Semiconductor lasers emitting light with a wavelength in the visible and infrared ranges have been widely available. In contrast, semiconductor lasers with shorter wavelength in the ultraviolet-B and ultraviolet-C bands (specifically with wavelength less than 315 nanometers) are severely underdeveloped. This is because the semiconductors commonly known as ultra-wide bandgap semiconductors such as aluminum gallium nitride, which are supposed to emit light in these spectral bands, have not been made with strong p-type material reliably. The proposed research addresses this challenge by developing a new ultra-wide bandgap semiconductor magnesium gallium oxide, which will be doped into both n-type and p-type. The PI and students will grow, fabricate and characterize light emitting devices and lasers based on these novel materials with a goal of demonstrating deep ultraviolet semiconductor laser devices with emission wavelengths between 200 and 270 nanometers. These deep ultraviolet semiconductor lasers have important applications in deep space communication, environmental monitoring, missile and flame detection, information storage and recording, virus disinfection and water purification, photodynamic medical diagnosis, therapy, and surgery, and so on. As a part of the effort, this project will train graduate and undergraduate students and involve these from underrepresented minorities to engineering. In addition, learning sessions on the subject closely related to the project will be planned for local high school students during their visit to the annual event “Discovery Day” at the College of Engineering, UC Riverside, and summer research opportunities will be provided to some high schoolers to prepare them for science fairs. This project seeks to demonstrate the first ambipolar ultra-wide bandgap transparent conducting semiconductor with bandgap of larger than 4.9 eV for deep-ultraviolet photonics. The research is planned based on the PI group’s recent finding that ultra-wide bandgap magnesium gallium oxide (MgGaO) is a strong p-type transparent conducting oxide semiconductor. The project will comprehensively study ultra-wide bandgap MgGaO, controllable p- and n-type doping of MgGaO, MgGaO/XGaO (X: Mg, Al) heterostructures, and their optoelectronic devices. The project will synthesize these semiconductors using molecular beam epitaxy and will elucidate their structural, electrical and optical properties using various characterization techniques. The origin of the p- and n-type doping of MgGaO will be revealed through detailed characterizations including Hall effect, photoluminescence, x-ray photoelectron spectroscopy, etc. Deep-ultraviolet semiconductor lasers, light emitting diodes (LEDs) and photodetectors with wavelengths less than 270 nm will be fabricated and characterized. The result will be an important step across the current boundary for the emission wavelength of semiconductor waveguide lasers, currently 375 nm in commercial use – an important advance that will enable significant economic opportunities in many technology-based domains.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.