III-Nitride Nanocrystal Based Green and Ultraviolet Optoelectronics

Abstract

Extensive research efforts have been devoted to III-nitride based solid-state lighting since the first demonstration of high-brightness GaN-based blue light emitting diodes (LEDs). Over the past decade, the performance of GaN-based LEDs including external quantum efficiency (EQE), wall-plug efficiency, output power and lifetime has been improved significantly while the cost of GaN substrate has been reduced drastically. Although the development of blue and near ultraviolet (UV) LED is mature, achieving equally excellent performance in other wavelengths based on III-nitrides is still challenging. Especially, the significant efficiency droop in the green wavelength, known as “green gap” and the extremely low EQE in the UV regime, known as “UV threshold”, have become two most urgent issues. Green LEDs emit light that is most sensitive to human eye, implicating its importance in a variety of applications such as screen- and projection-based displays. UV light sources have a variety of applications including water and air purification, sterilization/disinfection of medical tools, medical diagnostics, phototherapy, sensing, which make solid-state deep UV (DUV) light sources with compactness, low operating power and long lifetime highly desirable. The deterioration of performance with green LEDs originates from increased indium content of the active region, which could degrade material quality and increase quantum confined Stark effect due to the high polarization fields in c-plane InGaN/GaN quantum wells (QWs). Meanwhile, limiting factors in III-nitride UV LEDs include low internal quantum efficiency due to large densities of dislocations, poor carrier injection efficiency and low light extraction efficiency. In this dissertation, we have investigated the molecular beam epitaxial growth, structural characterization, and electrical and optical properties of low-dimensional III-nitride nanocrystals as potential solutions to above-mentioned issues. Through a combination of theoretical calculation and experimental investigation, we show that defects formation in AlN could be precisely controlled under N-rich epitaxy condition. With further optimized p-type doping, AlN nanowire-based LEDs emitting at 210 nm were fabricated. We report DUV excitonic LEDs with the incorporation of monolayer GaN with emission wavelengths of ~238 nm, and exhibit suppressed Auger recombination, negligible efficiency droop and a small turn on voltage ~5 V. To enhance the light extraction efficiency of AlGaN nanowires grown on Si substrate, we demonstrated epitaxy of AlGaN nanowires on Al coated Si(001) substrate wherein Al film functions as a UV light reflective layer to enhance the light extraction efficiency. AlGaN nanowire-based DUV LEDs on Al film were successfully grown and fabricated and measured with a turn-on voltage of 7 V and an electroluminescence emission at 288 nm. Green-emitting InGaN/GaN nanowire LEDs on Si(001) substrate were demonstrated, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers. The incorporation of planar p-GaN layer significantly reduces the fabrication complexity of nanowire-based devices and improves the robustness of electrical connection, leading to a more stable device operation. We also demonstrated micrometer scale InGaN photonic nanocrystal green LEDs with ultra-stable operation. The emission features a wavelength of ~548 nm and a spectral linewidth of ~4 nm, which is nearly five to ten times narrower than that of conventional InGaN QW LEDs in this wavelength range. Significantly, the device performance, in terms of the emission peak and spectral linewidth, is nearly invariant with injection current. Work presented in this thesis provides a new approach for achieving high-performance green and DUV LEDs by using III-nitride nanostructures.