Thin film silicon carbide for electroluminescent devices

In this research, the optoelectronic and structural properties of thin film silicon carbide (SiC) prepared by plasma enhanced chemical vapour deposition and excimer (ArF) laser crystallisation are presented. These materials have been utilised as p-i-n electroluminescent devices, including development of various novel device structures. A wide-ranging series of experiments aimed at optimising the deposition conditions of amorphous and microcrystalline SiC films are described. Dark conductivity, photoconductivity, photoluminescence, optical absorption by Swanepoel's method and constant photocurrent method (CPM), scanning and transmission electron microscopy, infrared spectroscopy, and elastic recoil detection analysis were employed to characterise the films. Absorption spectra and the density of states profile of amorphous silicon carbide as found by CPM are reported. As the carbon content increases, the valence band tail becomes broader. At the same time, the deep defect density of states increases and also becomes broader. The CPM data also verified that the band gap widening is due to the conduction band shifting with increasing carbon content. It is shown that H₂ dilution leads to an improvement of electronic properties via a decrease in the density of localised states. A novel method has been developed to prepare highly conductive and wide band gap doped microcrystalline silicon carbide (μc-SiC) by excimer (ArF) laser crystallisation. After crystallisation, this material has Tauc gap of around 2.0 eV and exhibits a dark conductivity as high as 20 (Ωcm)^-1, more than ten orders of magnitude higher than before the laser irradiation. This is shown to be mainly correlated to structural change. The dopant concentration plays a dominant role in the electrical transport properties of μc-SiC, regardless of type of dopant and carbon concentration up to 30 at.%. Laser crystallised μc-SiC can be utilised not only as the carrier injection layer in a-SiC:H based electroluminescent devices, but also as a luminescent layer. EL devices fabricated with μc-SiC as a hole injector possess the highest electroluminescent intensity, the most stable emission and the longest operating life-time among all the investigated device structures. The electroluminescence from these devices is possibly related to the formation of some form of porous SiC by laser crystallisation.