MgSiN2-based nitride materials for ultraviolet optoelectronic applications

Abstract

Ultraviolet light-emitting diodes (LEDs) have a range of potential applications including water purification, disinfection, UV curing, document authentication, phototherapy and medical diagnostics. In the case of water purification, UV-LEDs operating in the deep UV (< 300 nm) offer several benefits over the mercury-based UV lamps that predominate, including lower toxicity, higher energy efficiency, longer device lifetimes, more constant light intensity, and more easily controllable heat and temperature output. In water purification, UV light attacks viruses, spores and bacteria, disrupting chemical bonds in their DNA and RNA so that they are prevented from replicating and become inactive and an emission wavelength of 265 nm is optimum for disinfection overall. However, state of the art UV-LEDs operating in the deep UV based on wurtzite-structure III nitrides have maximum external quantum efficiencies (EQEs) of approximately 1-6 %, since it is not possible using either pure III-nitrides or their alloys to achieve lattice and polarisation-matched quantum well (QW) heterostructures with appropriate band gaps and band offsets. This motivates a search for alternative wide band gap nitride materials that could introduce additional degrees of freedom for UV-LED device design. II-IV nitride ternary semiconductors have recently emerged as promising alternatives to III-nitrides for use in optoelectronic applications. The II-IV nitride MgSiN2 is a candidate for UV emission whose band gap could lie in the deep UV. MgSiN2 can be viewed as an AlN derivative, where equal parts Mg and Si atoms are substituted for the Al atoms. The resulting chemical and structural similarities to AlN (Mg, Si and Al lie in the 3rd period) suggest the possibility of creating compatible lattice- and polarisation-matched heterostructures for use in devices and imply that Al-doping could be used to tailor band structure and band gap. However, there are conflicting reports concerning the crystal structure and band structure of MgSiN2, thin-film growth of MgSiN2 has not been reported in the literature, and the Al-doping of MgSiN2 has also not been reported. Therefore, this study investigates the crystallographic and electronic properties of MgSiN2 and Al2xMg(1-x)Si(1-x)N2 alloy powders and outlines a phenomenological model for interpreting partial ordering in III2xII(1-x)IV(1-x)N2 alloys. Studies in this thesis indicate that MgSiN2 has a large indirect band gap that is close to the direct gap of AlN and that its hexagonal-equivalent a lattice parameter lies between that of GaN and AlN, whilst the electronic structure is strongly related to AlN. Al-doping is found to modify the band gap substantially, without significantly changing the lattice parameters or compromising thermodynamic stability, and with many of the alloys having band gaps in the region suitable for UV emission in germicidal applications (210-280 nm). Moreover, partial long-range ordering appears to emerge in Al2xMg(1-x)Si(1-x)N2 alloys in order to satisfy local valence requirements through preservation of the octet rule.