Breaking the efficiency barrier of GaN laser diodes by nanoporous engineering

Yale University, New Haven CT

Investigators

Abstract

Title: BREAKING THE EFFICIENCY BARRIER OF GAN LASER DIODES BY NANOPOROUS ENGINEERING Abstract Nontechnical description: In this project we will study the possibility of dramatically improving the performance (efficiency) of GaN blue laser diodes. GaN-based blue laser diodes are widely used in optical information storage, in display, and in automobile lighting. However, to this day all the commercial blue laser diodes have relatively low efficiencies; in the best scenario only 1/3 of the input electricity is converted to optical laser output. A major and frequently overlooked issue is that the optical waves (or modes) within these laser diodes are not adequately confined for laser operation. In order to confine the optical modes effectively, layers with a sufficiently low optical index are needed to clad the emitted light, which has been very difficult to achieve. We propose to use a porous GaN layer, with a foam- or cheese-like nanostructure, as an effective confinement layer in laser diodes. Simulation results indicate that, due to a much-enhanced optical field inside the laser cavity, the threshold of blue laser diodes can be reduced by a factor of 3, and the overall efficiency can be boosted up to 60% or higher. Such an improvement will greatly extend the usage of blue (and near UV) laser diodes in applications such as chemical sensing, theater projections, and high-intensity lighting. Technical description: The purpose of this proposal is to provide a detailed analysis of GaN laser diodes (LDs), to overcome inherent material constraints through innovative nanoporous (NP) engineering, and to break the efficiency barrier of the state of the art (SOTA) GaN LDs. The efficiency barrier of the SOTA blue LDs (~30%) originates from fundamental constraints of the III-Nitride material system. In the III-Nitride material system, forming an effective waveguide with AlGaN cladding layers has proven to be a daunting task. The difficulties stem from (1) large lattice mismatches among III-N compounds for heteroepitaxy, (2) a low contrast in optical indices among III-nitride compounds, and (3) the tendency for layers to become electrically insulating as the Al-content increases. In this proposal a new, nanoporous form of GaN is exploited to address the aforementioned difficulties simultaneously. By using the NP GaN as cladding layers with an unprecedented tunability in optical index, a new degree of freedom becomes possible in optical waveguide design in an edge-emitting laser diode. The goal of this project is to demonstrate clear technological pathways to high performance blue laser diodes. The use of NP GaN introduces a new paradigm in LD design with (1) an enhanced optical confinement and therefore stimulated emission efficiency, (2) a reduced electrical impedance and an improved Joule efficiency, and (3) an enhanced light extraction efficiency. In principle, the enhancement of these three efficiency components will advance the state-of-the-art (SOTA) LD to a power conversion efficiency of greater than 60%.