Diffusion-Driven Charge Transport in III–V Optoelectronic Devices

Optoelectronics that seamlessly convert electrical energy to optical energy and vice versa have irreversibly changed our everyday lives through, e.g., the ubiquitous light-emitting diodes (LEDs) and increasingly important solar cells. The LEDs have revolutionized the lighting industry with improved efficiency and are also used to illuminate virtually every modern display, from smartphones and laptops to televisions. In parallel, solar cells generate increasing amounts of renewable energy, a critical component for global environmental initiatives. Despite recent advances in LEDs and solar cells, their core design principle has remained static: the active region (AR), where energy conversion mainly occurs, is placed between n- and p-doped high bandgap materials. However, emerging and more demanding solid-state fields have begun to expose limitations inherent to this core design. These limitations mainly comprise (1) resistive losses and (2) current crowding that can strongly increase heat generation at high powers, as well as (3) contact shading, which can notably reduce the illumination of solar cells. This thesis explores the possibilities of diffusion-driven charge transport (DDCT) within III–V compound semiconductor optoelectronics. In contrast to conventional LEDs and solar cells, the AR of the DDCT devices does not have to be placed between the n- and p-doped materials, as the AR can be excited via diffusion currents. This design allows near-surface active regions and opens the door for using III–V materials to fabricate interdigitated back-contact (IBC) structures that have historically been possible only for state-of-the-art silicon solar cells. The primary objectives of this thesis are to explore the general requirements, limits, and possible advantages of the DDCT devices through drift-diffusion simulations and to develop a fabrication process to demonstrate these unconventional laterally-doped gallium arsenide (GaAs) based structures also in practice. The simulation results suggest that the proposed DDCT structure can allow efficient and nearly resistance-free LEDs with fully exposed front surfaces for optimizing light extraction. Similarly, the results show that the structure is reciprocal and allows IBC designs for solar cells, which are also suitable for concentration photovoltaics. As such, DDCT devices promise to eliminate contact shading and mitigate current crowding challenges in GaAs-based LEDs and solar cells. To demonstrate the viability of these devices in practice, a method for fabricating them using selective area diffusion doping was developed. The first generation of DDCT prototypes was successfully demonstrated using thermal annealing to selectively redistribute dopant atoms incorporated in the device structure. These devices exhibited promising current-voltage and optical characteristics, suggesting that the developed annealing process holds great potential for fabricating DDCT devices.