Deep Ultraviolet LEDs

After the successful commercialization for InGaN/GaN blue light-emitting diodes that are used to generate white light, the development of AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs) promises the complete replacement for mercury-based fluorescence light tubes and this guarantees that the Minamata Convention on Mercury can be fulfilled by the end of the year of 2020. Therefore, developing high-efficiency AlGaN based DUV LEDs is regarded as the next revolutionary event for solid-state lighting. In this book, we will review the current status and summarize the challenges for DUV LEDs. Meanwhile, we also suggest the research spots that are worth investigating for DUV LEDs.

The roadmap for AlGaN based DUV LEDs is similar to that for InGaN based visible LEDs, such that the success of achieving high crystalline-quality epilayers is the precondition for fabricating high-brightness DUV LEDs. This chapter will review the most adopted technologies for growing high-quality Al-rich AlGaN films, which is regarded as the milestone for making high-efficiency DUV LEDs.

After the crystalline quality for Al-rich AlGaN layer is significantly improved, it is then the time to design novel DUV LED structures. DUV LEDs are driven electrically which get carrier transport and current injection involved. One of the challenges is the current crowding effect, which easily occurs in the DUV LEDs. Hence, it is important to show people physical images on the underlying reason for the current crowding and the solution proposals for current spreading.

The very low doping efficiency for the p-type Al-rich AlGaN layers indicates that the hole injection capability for DUV LEDs can be poor. Therefore, we ought to investigate the approaches to enable high-efficiency hole injection. In this chapter, we propose novel DUV LED architectures to make “hot” holes, increase the hole concentration in the p-type layer, and reduce the hole blocking effect that arises from the p-type electron blocking layer (p-EBL).

The unbalanced carrier injection for DUV LEDs illustrates that the electron tends to overflow from the active region. The underly mechanism arises from three aspects: (1) electrons cannot be consumed by forming electron-hole pairs and recombine radiatively in the active region, which is due to the insufficient hole injection, (2) the electron have larger mobility and are more mobile, (3) The conduction band offset between the AlGaN based quantum barrier and quantum well decreases, which correspondingly reduces the conduction band barrier height, enabling the active region to lose the effective confinement capability for electrons. In this chapter, we propose different methods for increasing the electron injection efficiency, and specifically, we demonstrate novel designs to reduce the electron drift velocity and hence the energy, so that the quantum wells have more chances of capturing the electrons.

This chapter discusses and presents different designs to screen the polarization level in the quantum wells for [0001]-oriented DUV LEDs. By doing so, the quantum confined Stark effect (QCSE) can be decreased. We suggest a simple way to reduce the QCSE by adopting Si-doped quantum barriers. Meanwhile, we also find that DUV LEDs are very sensitive to the polarization polarity, such that if nonpolar, semipolar and nitrogen-polar DUV LED structures are grown, we shall avoid using the p-AlGaN/p-GaN hole injection layer. The p-AlGaN/p-GaN hole injection layer can have remarkably hole depletion effect at the interface for those growth orientations except the [0001] orientation.

DUV LEDs possess very huge heating issue. On one hand, the sapphire substrate has a poor thermal conductivity, and on the other hand, DUV photons are easily absorbed by the absorptive p-GaN layer and the metal contact in the way of free carrier absorption, which further increases the self-heating effect for DUV LEDs. This chapter briefs readers on the currently adopted technologies for better thermal management.

DUV LEDs have very low light extraction efficiency (LEE), which is caused by the unique optical polarization and the optically absorptive semiconductor and metal layers. This chapter reviews and analyzes the approaches that have ever been used to improve the LEE. This chapter also points out that, the removal of the p-GaN layer can yield a high LEE without guaranteeing the enhanced wall plug efficiency in the same time. Thus, even more effort shall be made to achieve excellent ohmic contact for DUV LEDs.

This chapter summarizes the content for this book and suggests the future research outlooks for DUV LEDs.