Towards Electrically Injected Semiconductor Nanowire Lasers

Abstract

We are at a tipping point of the next industrial revolution, which will change or day-to-day lives. Increased needs around communication and data storage will require networks that can handle around 175 zettabyte by the end of 2025. Optical interconnect technology is a promising candidate, as it can transmit data at lightning speeds within chips and boards in efficient way. Laser is the backbone of such optical circuits and is often integrated with its electrical counterpart for power inputs. With the advances in fabrication techniques, micron-sized III-V semiconductor lasers are used in data communication, medicine, robotics, green energy, military etc. However, there is still a strong need to make these lasers even smaller and more energy efficient for optical interconnect technology. Nanowires are a suitable candidate for such devices due to their large surface area-to-volume ratio, confinement of photons in two dimensions and ease in integration with other substrates. However, research of III-V semiconductor nanowire Fabry-Perot cavity lasers is still in the early stages. A lot more research need to be done to realise devices, in particular those that are electrically powered, that are manufacturable for practical applications.

In this thesis, in-depth theoretical and experimental studies to fabricate nanowire lasers are presented. First, numerical modelling to optimise the dimensions of nanowires are performed to achieve low threshold lasing. Following this, epitaxial growth optimisation is carried out to achieve the desired dimensions of nanowires. Subsequently, fabrication of two different types of devices, single nanowire and array of nanowires is done. For single nanowire devices, an InP p-i-n axial structure is explored. These devices display light emitting diode (LED) characteristics, but unfortunately, they fail at higher injected current before lasing is observed. The potential causes of degradation of devices are high metal absorption, lower gain due to smaller active region and high free carrier absorption. To overcome high metal absorption and lower gain, radial p-n junction structures are investigated and significant improvement in the device performance is observed. However, the devices also fail at higher current injection levels prior to lasing threshold.

To overcome high free carrier absorption, transparent conducting oxides (TCOs) are used as a dopant layer forming a heterojunction with InP nanowire arrays as well as a contact layer. TCOs exhibit metal-like conductivity but with a high degree of transparency. Simulations to compute the optimum nanowire dimensions to obtain lasing, such as diameter and length, are carried out. For n-type TCOs, ZnO and SnOx are explored as potential materials, while for p-type TCOs, SnxNiyOz is evaluated. Optical, compositional and electrical properties of the TCOs are investigated at various deposition conditions. Junction properties as well as band alignment at the TCO-InP interface are studied to have insight of carrier injection and transport across the heterojunction. Finally, electroluminescence characteristics are measured and all the devices show promising LED behaviour, but fail to lase due to excessive heating at higher current injection levels due to carrier crowding, non-uniformity and shifting of the recombination region. For ease of integration, cost effectiveness and minimising the shift of the recombination region, flexible nanowire array devices are also demonstrated, which provide LED characteristics, but still fail to lase. Potential reasons and steps towards improvement such as modified fabrication process to mitigate recombination inside the substrate, proper heat sinking and incorporation of quantum wells and quantum dots to enhance gain are investigated. Nevertheless, the knowledge and understanding gained from devices fabricated in this thesis are promising steps towards realising electrically injected III-V semiconductor nanowire lasers.