Effect of etching with cysteamine assisted phosphoric acid on gallium nitride surface oxide formation

Abstract

In-situ functionalization of polar GaN was performed by adding cysteamine to a phosphoric acid etchant in order to study its effect on photoluminescence and oxide formation on the surfaces. The functionalization was characterized by atomic force microscopy, x-ray photoelectron spectroscopy, photoluminescence (PL), and water contact angle measurements. Two sets of polar GaN samples with different dislocation densities were evaluated, thin GaN layers residing on sapphire and thick free-standing GaN separated from sapphire substrate aiming to reveal the effect of material quality on in-situ functionalization. The addition of cysteamine to the phosphoric acid solution was found to result in: (i) decreased surface roughness, (ii) no change to hydrophobicity, (iii) decreased oxygen content especially at high-temperature treatments. The effect of the in-situ functionalization on the PL efficiency was more pronounced in the free-standing sample than in the film residing on the sapphire, which was attributed to a higher crystal quality free from strain.

A review of in situ surface functionalization of gallium nitride via beaker wet chemistry

“…49,50 For thiol functionalization, the water contact angle response varied more by the different etchant temperatures used rather than the presence or lack thereof of cysteamine. 41 The decrease in water contact angle is more sensitive to the surface topography (the quantity and size of surface defects), which was seen at higher temperatures. Although cysteamine presence on the surface was identified, the surface coverage was not uniform.…”

“…From the atomic ratios, it was found that the addition of cysteamine to polar bulk and thin layer samples resulted in an increase in the nitrogen content on the surface. 41 The bulk nonpolar samples showed an increase in environmental carbon contamination when any of the etching processes were performed. 50 The nonpolar samples provided insight into the oxide sensitivity of the bulk and thin layer samples, as mentioned previously.…”

“…The qualitative analysis provides an efficient way of comparing the effect of various treatments on GaN. 41,50 It has been identified that the lower oxide content with functionalization is associated with poorer binding and lower surface coverage. This was also observed with the long-chained phosphonic acids, and particularly for 1H,1H,2H,2H-perfluorooctane phosphonic acid (PFOPA), and 11-mercaptoundecylphosphonic acid (MUDPA).…”

“…47 Surface morphology alteration of bulk (low-defectdensity) GaN and of thin layer (high-defect-density) GaN upon various adsorbates and variable etchant temperature was studied by AFM. 41 The surface topography and water contact angle are closely linked, hence the increase in surface roughness (as determined by root mean square, RMS, parameter) resulted in lower water contact angle due to more potential sites to hydrate. Thin-layered GaN, having a greater degree of surface defects demonstrated higher sensitivity to etching, especially at higher temperatures.…”

“…In this way, a completely pristine surface is not a requirement. 41 Another potential adsorbate for GaN surface modification is phosphonic acid, a derivative of phosphoric acid, that can bind either directly onto low valence metal atoms or pre-existing oxide layers. 42 Specific reaction conditions such as solvent, time, and temperature can be varied to graft organophosphoric acid with the aid of Ga 2 O 3 prior to its complete dissolution from the surface.…”