Abstract
Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) InGaN films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with %. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.
Highlights
With a direct bandgap which can be tuned from ultraviolet (3.5 eV) to infrared (0.7 eV) [1] with increasing indium fraction, Inx Ga1− x N is a semiconducting III-V materials system of great importance to the optoelectronics industry
It is worth noting that to facilitate observation of the features associated with the valleys between the dislocations, Figure 1d,h are presented at a different lateral scale to the other parts of the figure
The main observation was consistently lower current measured on ridges around defects in the 5–12% InN samples, with transmission electron microscopy (TEM) suggesting this effect is related to sub-surface indium-deficient regions present around V-pits, which form due to lower indium incorporation on pit facets during growth
Summary
With a direct bandgap which can be tuned from ultraviolet (3.5 eV) to infrared (0.7 eV) [1] with increasing indium fraction, Inx Ga1− x N is a semiconducting III-V materials system of great importance to the optoelectronics industry. A recent cathodoluminescence study on thick InGaN layers has demonstrated that the core region of such threading dislocations is associated with enhanced light emission [7]; the authors propose this is due to indium concentrating in the dislocation strain field, thereby localising carriers in the vicinity of the dislocation core. This surprising result has implications for the role of defects in InGaN-based light emitters, and warrants further investigation.
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