Influence of V-pits on the electro-optical properties of high-periodicity InGaN MQWs

We investigated the influence of V-pits on the turn-on voltage of GaN-based high periodicity multiple quantum wells (MQWs) solar cells with different thickness of the p-GaN layer. Experimental current-voltage characteristics indicate that the sample with the thinnest p-GaN layer presents an early turn-on, which is not present for samples with a thicker p-GaN layer. Focusing on the V-defects analysis, through scanning electron microscopy (SEM) we found no difference in the density and dimensions of V-pits between sample with different p-GaN thickness. Through TCAD Synopsys Sentaurus simulations, the main non-illuminated current-voltage characteristics are reproduced considering V-defects. The results indicate that V-pits play a dominant role in current conduction, especially for the devices with the thinnest p-GaN layer due to the insufficient V-pit planarization. For such devices V-pits penetrate the junctions, and locally put the MQWs region in closer connection to the p-side contact, resulting in the formation of localized short circuit paths. Finally, a Gaussian distribution of V-pits dimensions and depth is considered to reach a good matching of experimental data. Based on combined electrical analysis, microscopy investigation and 2D simulations, results provide insight on the role of V-pits on the electrical performance of GaN-based MQWs solar cells. The outcome of this work will be useful for the design of future high-periodicity quantum wells devices, ensuring the desired turn-on voltage and showing the existence of a tradeoff between the need of a thin p-GaN (to increase short-wavelength efficiency) and a thicker p-GaN, to avoid insufficient V-pit planarization.